Patent Publication Number: US-7718382-B2

Title: Method for identifying compounds for treatment of pain

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the 371 National Phase of International Application No. PCT/CA2005/000738, filed May 13, 2005, which was published in English under PCT Article 21(2) as International Publication No. WO 2005/110490. This application further claims priority from International application No. PCT/CA2004/000726 filed May 14, 2004. This application further claims the benefit, under 35 U.S.C. 119(e), of U.S. provisional patent application Ser. No. 60/627,174 filed Nov. 15, 2004. All of these applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the modulation of activity, function and/or expression of phospholipase C gamma (PLC γ ) in a subject and uses thereof, such as for treating and preventing and pain. 
     BACKGROUND OF THE INVENTION 
     The need for new and improved methods and agents for pain treatment is a significant ongoing concern in medicine. Acute pain, e.g. related to injury or disease, can be severe and have critical effects on patient recovery. An even greater concern is chronic pain, which affects a large proportion of the population, causing not only significant discomfort, but can result in low self-esteem, depression, anger, and can interfere with or completely prevent a sufferer from typical daily activities. 
     While a number of studies have been done in this area, many mechanisms and pathways involved in pain sensation remain poorly understood. There remains a continued need to provide new strategies of therapeutic intervention for pain treatment. 
     SUMMARY OF THE INVENTION 
     The invention relates to methods of treating and/or preventing pain. The invention further relates to methods of modulating (e.g. inhibiting) activity, expression, and/or function of a phospholipase C gamma (PLC γ ) in a subject. 
     According to one aspect of the present invention, there is provided a method of treating or preventing pain in a subject, the method comprising modulating activity, function and/or expression of a PLC γ  in the subject. 
     According to another aspect of the present invention, there is provided a method of treating or preventing pain in a subject, the method comprising administering to the subject an agent capable of modulating activity, function and/or expression of a PLC γ  in the subject. 
     According to still another aspect of the present invention, there is provided a method of treating or preventing pain in a subject, comprising administering to the subject a composition comprising: an agent capable of modulating activity, function and/or expression of a PLC γ  in the subject; and a pharmaceutically acceptable carrier. 
     According to yet another aspect of the present invention, there is provided a composition for treatment or prevention of pain in a subject, comprising: an agent capable of modulating activity, function and/or expression of a PLC γ  in the subject; and a pharmaceutically acceptable carrier. 
     According to a further aspect of the present invention, there is provided a package comprising an agent capable of modulating activity, function and/or expression of a PLC γ  in a subject together with instructions for its use in the treatment or prevention of pain. 
     According to yet a further aspect of the present invention, there is provided a package comprising a composition comprising: an agent capable of modulating activity, function and/or expression of a PLC γ  in a subject; and a pharmaceutically acceptable carrier, together with instructions for its use in the treatment or prevention of pain. 
     According to still a further aspect of the present invention, there is provided a use of an agent capable of modulating activity, function and/or expression of a PLC γ  in a subject for the treatment or prevention of pain. 
     According to another aspect of the present invention, there is provided a use of an agent capable of modulating activity, function and/or expression of a PLC γ  in a subject for the preparation of a medicament for the treatment or prevention of pain. 
     According to yet another aspect of the present invention, there is provided a use, for the treatment or prevention of pain, of a composition comprising: an agent capable of modulating activity, function and/or expression of a PLC γ  in a subject; and a pharmaceutically acceptable carrier. 
     According to another aspect of the present invention, there is provided a method of identifying or characterizing an agent for treatment or prevention of pain, comprising determining PLC γ  activity, function and/or expression in the presence of a test agent. In an embodiment, the method comprises contacting the test agent with a PLC γ  or a cell having an activity, function and/or expression of a PLC γ ; and determining whether there is modulation (e.g. an inhibition) of the activity, function and/or expression of a PLC γ  in the presence of the agent; wherein the modulation is an indication that the agent may be used for treatment or prevention of pain. 
     The invention further provides a method of identifying or characterizing a compound for treatment or prevention of pain, said method comprising: (a) contacting a test compound with a cell comprising a first nucleic acid comprising a transcriptionally regulatory element normally associated with a PLC γ  gene, operably linked to a second nucleic acid comprising a reporter gene capable of encoding a reporter protein; and (b) determining whether reporter gene expression or reporter protein activity is decreased in the presence of said test compound; wherein said decrease in reporter gene expression or reporter protein activity being is an indication that said test compound may be used for treatment or prevention of pain. 
     The invention further provides a method of identifying or characterizing an agent for treatment or prevention of pain, comprising: 
     contacting the agent and a PLCγ ligand with a cell comprising a PLC γ ; and 
     determining whether there is a decrease of the binding of the ligand to the PLC γ  in the presence of the agent; 
     wherein the decrease is an indication that the agent may be used for treatment or prevention of pain. 
     The invention further provides a method of identifying or characterizing an agent for treatment or prevention of pain, comprising: 
     contacting the agent and a PLCγ ligand with a PLC γ ; and 
     determining whether there is decrease of the binding of the ligand to the PLC γ  in the presence of the agent; 
     wherein the decrease is an indication that the agent may be used for treatment or prevention of pain. 
     According to still another aspect of the present invention, there is provided a method for decreasing nociception in a subject, comprising modulating activity, function and/or expression of a PLC γ  in the subject. 
     According to yet another aspect of the present invention, there is provided a method for decreasing nociception in a subject, comprising administering to the subject an agent capable of modulating activity, function and/or expression of a PLC γ  in the subject. 
     According to a further aspect of the present invention, there is provided a method for decreasing nociception in a subject, comprising administering to the subject a composition comprising: an agent capable of modulating activity, function and/or expression of a PLC γ  in the subject; and a pharmaceutically acceptable carrier. 
     In an embodiment, the above-mentioned modulation of PLC γ  activity, function and/or expression is an inhibition of PLC γ  activity or expression. In an embodiment the above-mentioned agent capable of modulating PLC γ  activity, function and/or expression is an agent capable of inhibiting PLC γ  activity, function and/or expression. 
     In an embodiment, the agent as defined herein may be a PLC γ  inhibitor, an anti-PLC γ  antibody, an antisense molecule, a siRNA, a siRNA-like molecule, or an inhibitor of binding of a ligand or binding partner to a PLCγ. For example, the PLC γ  inhibitor may be tricyclodecan-9-yl-xanthogenate, 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphorylcholine, neomycin sulfate, spermine tetrahydrochloride, 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione, or 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-2,5-pyrrolidinedione. 
     In an embodiment, the antisense molecule is a nucleic acid that is substantially complementary to a portion of an mRNA encoding a PLC γ . In an embodiment, the antisense molecule is a nucleic acid that is substantially complementary to a nucleic acid comprising a nucleotide sequence capable of encoding a polypeptide having an amino acid sequence selected from SEQ ID NO:2 and SEQ ID NO:4. In a further embodiment, the antisense molecule is a nucleic acid that is substantially complementary to a nucleic acid comprising a nucleotide sequence selected from SEQ ID NO:1 and SEQ ID NO:3. In an embodiment, the above-noted portion of an mRNA comprises at least 5 contiguous bases. 
     In an embodiment, the siRNA or siRNA-like molecule is substantially identical to a portion of an mRNA encoding PLC γ . In an embodiment, the siRNA or siRNA-like molecule is substantially identical to a portion of an mRNA corresponding to a DNA sequence capable of encoding a polypeptide having an amino acid sequence selected from SEQ ID NO:2 and SEQ ID NO:4. In a further embodiment, the siRNA or siRNA-like molecule is substantially identical to a portion of an mRNA corresponding to a DNA sequence selected from SEQ ID NO:1 and SEQ ID NO:3. In an embodiment, the siRNA or siRNA-like molecule comprises less than about 30 nucleotides, in a further embodiment about 21 to 23 nucleotides. 
     In embodiments, the pain is pain associated with neuropathic pain and/or CNS dysfunction. 
     In a further embodiment, pain may be neuropathic pain, somatic or visceral pain. For example, the neuropathic pain may be associated with a nerve or tract injury. In yet another embodiment, pain may be chronic inflammatory pain, pain associated with arthritis, fibromyalgia, back pain, cancer-associated pain, pain associated with digestive disease, pain associated with Crohn&#39;s disease, pain associated with autoimmune disease, pain associated with endocrine disease, pain associated with diabetic neuropathy, phantom limb pain, spontaneous pain, chronic post-surgical pain, chronic temporomandibular pain, causalgia, post-herpetic neuralgia, AIDS-related pain, complex regional pain syndromes type I and II, trigeminal neuralgia, chronic back pain, pain associated with spinal cord injury or recurrent acute pain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Comparison between the 50% withdrawal threshold for pain hypersensitivity of rats treated with tricyclodecan-9-yl-xanthogenate (∘) and a vehicle (□). 
         FIG. 2 : DNA (SEQ ID NO:1,  FIGS. 2A-2D ) and polypeptide (SEQ ID NO:2,  FIG. 2E ) sequences of human phospholipase C gamma 1 (PLCG1; accession nos. NM — 002660 and NP — 002651). Coding sequence is defined by positions 122-3997 of DNA sequence. 
         FIG. 3 : DNA (SEQ ID NO:3,  FIGS. 3A-3C ) and polypeptide (SEQ ID NO:4, FIG. 3D ) sequences of human phospholipase C gamma 2 (PLCG2; accession nos. NM — 002661 and NP — 002652). Coding sequence defined by positions 153-3911 of DNA sequence. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a method for the prevention and/or treatment of pain in a subject. In embodiments, the subject may be a vertebrate or a mammal. In an embodiment, the subject is a mammal, for example a human. The method comprises inhibiting or decreasing the activity, function and/or expression of a phospholipase C gamma (PLC γ ) in a subject, e.g. in a cell or tissue of the subject, for example a central nervous system (CNS) neural cell or tissue. The method may comprise administering to the subject, systemic or local, an agent capable of modulating, e.g. inhibiting, activity, expression and/or function of a PLC γ  as a means to attenuate pain. The agent may be administered before, at about the time of, or subsequent to the onset of pain. 
     In an embodiment, the CNS neural cell in which the PLC γ  activity, function and/or expression is modulated may be located in the superficial dorsal horn or the spinal cord. In addition, the cell may also be transsynaptic to a peripheral nerve cell or sensory fiber from which a signal for pain originates. 
     The invention also relates to the treatment of acute and chronic pain, more specifically to the treatment of neuropathic pain. “Neuropathic pain”, as used herein, refers to chronic pain associated with nerve injury (e.g. following crush, transection or compression of nerves or following nerve degeneration resulting from disease). Neuropathic pain may be associated with a nerve or tract injury. Moreover, the neuropathic pain may be associated with visceral and/or somatic pain. 
     In an embodiments, the pain may be associated with a condition chosen from chronic inflammatory pain, pain associated with arthritis, fibromyalgia, back pain, cancer-associated pain, pain associated with digestive disease, pain associated with Crohn&#39;s disease, pain associated with autoimmune disease, pain associated with endocrine disease, pain associated with diabetic neuropathy, phantom limb pain, spontaneous pain, chronic post-surgical pain, chronic temporomandibular pain, causalgia, post-herpetic neuralgia, AIDS-related pain, complex regional pain syndromes type I and II, trigeminal neuralgia, chronic back pain, pain associated with spinal cord injury, recurrent acute pain, and/or any combination of the above. 
     The invention further provides a method for decreasing nociception in a subject. The method comprises modulating activity, function and/or expression of a PLC γ  in a subject to reduce nociception. “Nociception” as used herein refers to the sensory component of pain. Pain may be the result of various stimuli, including but not limited to pressure, injury, thermal stimuli or chemical (e.g. ionic) stimuli. 
     The agent includes, but is not limited to, that which directly or indirectly modifies the activity of the protein and that which modulates the production and/or stability of the protein (e.g. at the level of transcription, translation, maturation, post-translational modification, phosphorylation and degradation). For example, the agent may be a PLC γ  inhibitor. In an embodiment, such an inhibitor may be a peptide or peptide-based compound (e.g. a peptidomimetic). In further embodiments, the agent may be an anti-PLC γ  antibody which is capable of modulating the binding and/or catalytic activity of a PLC γ . Examples of anti-PLC γ  antibodies are described in for example Lee et al. (2002 , Mol. Vis.,  8: 17-25) and Buckley et al. (2004 , J. Biol. Chem.,  279: 41807-14). In a further embodiment, the agent may be an antisense molecule complementary to all or a portion of the mRNA encoding a PLC γ . In a further embodiment, the agent may be an siRNA or siRNA-like molecule. 
     In a further embodiment, the above-mentioned inhibition is effected by the inhibition of a binding domain(s) of a PLC γ . In such a case, the agent is an inhibitor of binding of a ligand or binding partner to a PLCγ. For example, binding of mitogen-activated protein kinases [e.g. extracellular signal-related kinase (ERK1 and ERK2)] to a PLCγ (e.g. to the D-domain) may be inhibited. As shown in Buckley et al., 2004 , J. Biol. Chem.,  279:41807-41814, both ERK and phospho-ERK (which, like PLC γ  are activated by BDNF binding of TrkB) bind the D domain of PLC γ  and induce phosphorylation of this isozyme. 
     In one embodiment, the agent capable of modulating activity and/or expression of a PLC γ  is a PLC γ  inhibitor. Examples of PLC γ  inhibitors include, but are not limited to, tricyclodecan-9-yl-xanthogenate, 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphorylcholine, neomycin sulfate, spermine tetrahydrochloride, 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione, or 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-2,5-pyrrolidinedione. References describing the PLC γ  inhibiting action of tricyclodecan-9-yl-xanthogenate (also referred to as D609) include: Monick, M. M. et al., 1999 , J. Immunol.  162, 3005; Maragoudakis, M. E. et al., 1993 , Kidney Int.  43, 147; Schutze, S. et al., 1992 , Cell  71, 765; Muller-Decker, K., 1989 , Biochem. Biophys. Res. Commun.  162, 198; Sauer, G. et al., 1984 , Proc. Natl. Acad. Sci. USA  81, 3263. The aboved-mentioned PLC γ  inhibitors are available from EMD Biosciences (a division of Calbiochem). 
     In another embodiment, the agent capable of modulating activity and/or expression of PLC γ  is an anti-PLC γ  antibody. Examples of anti-PLC γ  antibodies are described in for example Lee et al. (2002 , Mol. Vis.,  8: 17-25) and Buckley et al. (2004 , J. Biol. Chem.,  279: 41807-14). To prepare such an anti-PLC γ  antibody, a PLC γ  or fragment, homolog, and/or variant thereof may be used to immunize a small mammal, e.g., a mouse or a rabbit, in order to raise antibodies which recognize a PLC γ . An anti-PLC γ  antibody may be either polyclonal or monoclonal. Methods to produce polyclonal or monoclonal antibodies are well known in the art. For a review, see Harlow and Lane (1988)  Antibodies: A Laboratory Manual , Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. and Yelton et al. (1981)  Ann. Rev. Biochem.  50:657-690, both of which are herein incorporated by reference. For monoclonal antibodies, see Kohler and Milstein (1975)  Nature  256:495-497, herein incorporated by reference. 
     “PLC gamma” or “PLC γ ” refers to a polypeptide possessing an activity capable of cleaving a phosphotidylinositol 4,5 bisphosphate to yield diacylglycerol and inositol 1,4,5-triphosphate. In embodiments, the PLC γ  is selected from the PLCγ1 and PLCγ2 isoforms. Genes which encode human PLC γ  polypeptides are described for example at NCBI accession Nos. M34667, NM — 002660, NM — 002661 and NM — 182811. Examples of genes which encode human PLC γ  polypeptides as well as the polypeptides they encode are set forth in  FIGS. 2 and 3  and SEQ ID NOs:1-4. For a review of PLCγ activity see Carpenter and Ji (1999 , Experimental Cell Research,  253: 15-24). PLCγ activity may be assayed by for example electrophoretic mobility shift assays (Noh et al., 1998 , Anticancer Research,  18: 2643-8) and myo-inositol 1,4,5-triphosphate radioceptor assays (Shu et al., 2002 , J. Biol. Chem.,  277: 18447-18453). 
     As noted above, a fragment, homolog and/or variant of a PLC γ  which retains activity may also be used in the methods of the invention. Homologs include protein sequences which are substantially identical to the amino acid sequence of a PLC γ , sharing significant structural and functional homology with a PLC γ . Variants include, but are not limited to, proteins or peptides which differ from a PLC γ  by any modifications, and/or amino acid substitutions, deletions or additions. Modifications can occur anywhere including the polypeptide backbone, (i.e. the amino acid sequence), the amino acid side chains and the amino or carboxy termini. Such substitutions, deletions or additions may involve one or more amino acids. Fragments include a fragment or a portion of a PLC γ  or a fragment or a portion of a homolog or variant of a PLC γ . 
     Antibodies may be recombinant, e.g., chimeric (e.g., constituted by a variable region of murine origin associated with a human constant region), humanized (a human immunoglobulin constant backbone together with hypervariable region of animal, e.g., murine, origin), and/or single chain. Both polyclonal and monoclonal antibodies may also be in the form of immunoglobulin fragments, e.g., F(ab)′ 2 , Fab or Fab′ fragments. The antibodies may be of any isotype, e.g., IgG or IgA, and polyclonal antibodies are of a single isotype or a mixture of isotypes. Anti-PLC γ  antibodies may be produced and identified using standard immunological assays, e.g., Western blot analysis, dot blot assay, or ELISA. 
     Another class of compounds that can be used to limit PLC γ  expression are compounds that lower the level of PLC γ  transcripts. By doing so, these compounds limit the number of PLC γ  polypeptides that can be produced and can therefore be use to treat or prevent pain. These compounds include, but are not limited to, dsRNA, siRNA, siRNA-like molecule, antisense oligonucleotide or ribozyme. 
     In an embodiment, expression of a nucleic acid encoding a polypeptide of interest, or a fragment thereof, may be inhibited or prevented using RNA interference (RNAi) technology, a type of post-transcriptional gene silencing. RNAi may be used to create a pseudo “knockout”, i.e. a system in which the expression of the product encoded by a gene or coding region of interest is reduced, resulting in an overall reduction of the activity of the encoded product in a system. As such, RNAi may be performed to target a nucleic acid of interest or fragment or variant thereof, to in turn reduce its expression and the level of activity of the product which it encodes. Such a system may be used for functional studies of the product, as well as to treat disorders related to the activity of such a product. RNAi is described in for example Hammond et al. (2001), Sharp (2001), Caplen et al. (2001), Sedlak (2000) and published US patent applications 20020173478 (Gewirtz; published Nov. 21, 2002) and 20020132788 (Lewis et al.; published Nov. 7, 2002). Reagents and kits for performing RNAi are available commercially from for example Ambion Inc. (Austin, Tex., USA) and New England Biolabs Inc. (Beverly, Mass., USA). 
     The initial agent for RNAi in some systems is thought to be dsRNA molecule corresponding to a target nucleic acid. The dsRNA is then thought to be cleaved into short interfering RNAs (siRNAs) which are 21-23 nucleotides in length (19-21 bp duplexes, each with 2 nucleotide 3′ overhangs). The enzyme thought to effect this first cleavage step has been referred to as “Dicer” and is categorized as a member of the RNase III family of dsRNA-specific ribonucleases. Alternatively, RNAi may be effected via directly introducing into the cell, or generating within the cell by introducing into the cell a suitable precursor (e.g. vector encoding precursor(s), etc.) of such an siRNA or siRNA-like molecule. A siRNA may then associate with other intracellular components to form an RNA-induced silencing complex (RISC). The RISC thus formed may subsequently target a transcript of interest via base-pairing interactions between its siRNA component and the target transcript by virtue of homology, resulting in the cleavage of the target transcript approximately 12 nucleotides from the 3′ end of the siRNA. Thus the target mRNA is cleaved and the level of protein product it encodes is reduced. 
     RNAi may be effected by the introduction of suitable in vitro synthesized siRNA or siRNA-like molecules into cells. RNAi may for example be performed using chemically-synthesized RNA (Brown et al., 2002). Alternatively, suitable expression vectors may be used to transcribe such RNA either in vitro or in vivo. In vitro transcription of sense and antisense strands (encoded by sequences present on the same vector or on separate vectors) may be effected using for example T7 RNA polymerase, in which case the vector may comprise a suitable coding sequence operably-linked to a T7 promoter. The in vitro-transcribed RNA may in embodiments be processed (e.g. using  E. coli  RNase III) in vitro to a size conducive to RNAi. The sense and antisense transcripts are combined to form an RNA duplex which is introduced into a target cell of interest. Other vectors may be used, which express small hairpin RNAs (shRNAs) which can be processed into siRNA-like molecules. Various vector-based methods are described in for example Brummelkamp et al. (2002), Lee et al. (2002), Miyagashi and Taira (2002), Paddison et al. (2002) Paul et al. (2002) Sui et al. (2002) and Yu et al. (2002). Various methods for introducing such vectors into cells, either in vitro or in vivo (e.g. gene therapy) are known in the art. 
     Accordingly, in an embodiment expression of a nucleic acid encoding a PLC γ , or a fragment thereof, may be inhibited by introducing into or generating within a cell an siRNA or siRNA-like molecule corresponding to a nucleic acid encoding a polypeptide of interest, or a fragment thereof, or to an nucleic acid homologous thereto. “siRNA-like molecule” refers to a nucleic acid molecule similar to an siRNA (e.g. in size and structure) and capable of eliciting siRNA activity, i.e. to effect the RNAi-mediated inhibition of expression. In various embodiments such a method may entail the direct administration of the siRNA or siRNA-like molecule into a cell, or use of the vector-based methods described above. In an embodiment, the siRNA or siRNA-like molecule is less than about 30 nucleotides in length. In a further embodiment, the siRNA or siRNA-like molecule is about 21-23 nucleotides in length. In an embodiment, siRNA or siRNA-like molecule comprises a 19-21 bp duplex portion, each strand having a 2 nucleotide 3′ overhang. In embodiments, the siRNA or siRNA-like molecule is substantially identical to a nucleic acid encoding a PLC γ , or a fragment or variant (or a fragment of a variant) thereof. Such a variant is capable of encoding a protein having activity similar to a PLC γ . In embodiments, the sense strand of the siRNA or siRNA-like molecule is substantially identical to SEQ ID NOs: 1 or 3, or a fragment thereof (RNA having U in place of T residues of the DNA sequence). 
     In yet another embodiment, transformation of cells with antisense constructs may be used to inhibit expression of a PLC γ . Antisense constructs are nucleic acid molecules that may be transcribed to provide an antisense molecule that is substantially complementary to all or a portion of the mRNA encoding a PLC γ , so that expression of the antisense construct interferes with the expression of the PLC γ . In an embodiment, the just noted antisense molecule is antisense to a DNA sequence coding a PLC γ , in an embodiment, a human PLC γ . In some embodiments, antisense constructs of the invention may therefore encode five or more contiguous nucleic acid-residues substantially complimentary to a contiguous portion of a nucleic acid sequence encoding a PLC γ . 
     In further embodiments, polypeptides and nucleic acids which are substantially identical to those noted herein may be utilized in the context of the present invention. 
     “Homology” and “homologous” refers to sequence similarity between two peptides or two nucleic acid molecules. Homology can be determined by comparing each position in the aligned sequences. A degree of homology between nucleic acid or between amino acid sequences is a function of the number of identical or matching nucleotides or amino acids at positions shared by the sequences. As the term is used herein, a nucleic acid sequence is “homologous” to another sequence if the two sequences are substantially identical and the functional activity of the sequences is conserved (as used herein, the term ‘homologous’ does not infer evolutionary relatedness). Two nucleic acid sequences are considered substantially identical if, when optimally aligned (with gaps permitted), they share at least about 50% sequence similarity or identity, or if the sequences share defined functional motifs. In alternative embodiments, sequence similarity in optimally aligned substantially identical sequences may be at least 60%, 70%, 75%, 80%, 85%, 90% or 95%. As used herein, a given percentage of homology between sequences denotes the degree of sequence identity in optimally aligned sequences. An “unrelated” or “non-homologous” sequence shares less than 40% identity, though preferably less than about 25% identity, with any of SEQ ID NOs 1-4. 
     Substantially complementary nucleic acids are nucleic acids in which the “complement” of one molecule is substantially identical to the other molecule. Optimal alignment of sequences for comparisons of identity may be conducted using a variety of algorithms, such as the local homology algorithm of Smith and Waterman, 1981,  Adv. Appl. Math  2: 482, the homology alignment algorithm of Needleman and Wunsch, 1970 , J. Mol. Biol.  48:443, the search for similarity method of Pearson and Lipman, 1988 , Proc. Natl. Acad. Sci. USA  85: 2444, and the computerised implementations of these algorithms (such as GAP, BESTEIT, FASTA and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, Madison, Wis., U.S.A.). Sequence identity may also be determined using the BLAST algorithm, described in Altschul et al., 1990,  J. Mol. Biol.  215:403-10 (using the published default settings). Software for performing BLAST analysis may be available through the National Center for Biotechnology Information. The BLAST algorithm involves first identifying high scoring sequence pairs (HSPs) by identifying short words of length W in the query sequence that either match or satisfy some positive-valued threshold score T when aligned with a word of the same length in a database sequence. T is referred to as the neighbourhood word score threshold. Initial neighbourhood word hits act as seeds for initiating searches to find longer HSPs. The word hits are extended in both directions along each sequence for as far as the cumulative alignment score can be increased. Extension of the word hits in each direction is halted when the following parameters are met: the cumulative alignment score falls off by the quantity X from its maximum achieved value; the cumulative score goes to zero or below, due to the accumulation of one or more negative-scoring residue alignments; or the end of either sequence is reached. The BLAST algorithm parameters W, T and X determine the sensitivity and speed of the alignment. The BLAST program may use as defaults a word length (W) of 11, the BLOSUM62 scoring matrix (Henikoff and Henikoff, 1992,  Proc. Natl. Acad. Sci. USA  89: 10915-10919) alignments (B) of 50, expectation (E) of 10 (or 1 or 0.1 or 0.01 or 0.001 or 0.0001), M=5, N=4, and a comparison of both strands. One measure of the statistical similarity between two sequences using the BLAST algorithm is the smallest sum probability (P(N)), which provides an indication of the probability by which a match between two nucleotide or amino acid sequences would occur by chance. In alternative embodiments of the invention, nucleotide or amino acid sequences are considered substantially identical if the smallest sum probability in a comparison of the test sequences is less than about 1, preferably less than about 0.1, more preferably less than about 0.01, and most preferably less than about 0.001. 
     An alternative indication that two nucleic acid sequences are substantially complementary is that the two sequences hybridize to each other under moderately stringent, or preferably stringent, conditions. Hybridization to filter-bound sequences under moderately stringent conditions may, for example, be performed in 0.5 M NaHPO 4 , 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at 65° C., and washing in 0.2×SSC/0.1% SDS at 42° C. (see Ausubel, et al. (eds), 1989 , Current Protocols in Molecular Biology , Vol. 1, Green Publishing Associates, Inc., and John Wiley &amp; Sons, Inc., New York, at p. 2.10.3). Alternatively, hybridization to filter-bound sequences under stringent conditions may, for example, be performed in 0.5 M NaHPO 4 , 7% SDS, 1 mM EDTA at 65° C., and washing in 0.1×SSC/0.1% SDS at 68° C. (see Ausubel, et al. (eds), 1989, supra). Hybridization conditions may be modified in accordance with known methods depending on the sequence of interest (see Tijssen, 1993 , Laboratory Techniques in Biochemistry and Molecular Biology—Hybridization with Nucleic Acid Probes , Part I, Chapter 2 “Overview of principles of hybridization and the strategy of nucleic acid probe assays”, Elsevier, New York). Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point for the specific sequence at a defined ionic strength and pH. 
     In alternative embodiments, the invention provides antisense molecules and ribozymes for exogenous administration to bind to, degrade and/or inhibit the translation of PLC γ  mRNA. Examples of therapeutic antisense oligonucleotide applications, incorporated herein by reference, include: U.S. Pat. No. 5,135,917, issued Aug. 4, 1992; U.S. Pat. No. 5,098,890, issued Mar. 24, 1992; U.S. Pat. No. 5,087,617, issued Feb. 11, 1992; U.S. Pat. No. 5,166,195 issued Nov. 24, 1992; U.S. Pat. No. 5,004,810, issued Apr. 2, 1991; U.S. Pat. No. 5,194,428, issued Mar. 16, 1993; U.S. Pat. No. 4,806,463, issued Feb. 21, 1989; U.S. Pat. No. 5,286,717 issued Feb. 15, 1994; U.S. Pat. No. 5,276,019 and U.S. Pat. No. 5,264,423; BioWorld Today, Apr. 29, 1994, p. 3. 
     Preferably, in antisense molecules, there is a sufficient degree of complementarity to the PLC γ  mRNA to avoid non-specific binding of the antisense molecule to non-target sequences under conditions in which specific binding is desired, such as under physiological conditions in the case of in vivo assays or therapeutic treatment or, in the case of in vitro assays, under conditions in which the assays are conducted. The target mRNA for antisense binding may include not only the information to encode a protein, but also associated ribonucleotides, which for example form the 5′-untranslated region, the 3′-untranslated region, the 5′ cap region and intron/exon junction ribonucleotides. A method of screening for antisense and ribozyme nucleic acids that may be used to provide such molecules as inhibitors of the invention is disclosed in U.S. Pat. No. 5,932,435 (which is incorporated herein by reference). 
     Antisense molecules (oligonucleotides) of the invention may include those which contain intersugar backbone linkages such as phosphotriesters, methyl phosphonates, short chain alkyl or cycloalkyl intersugar linkages or short chain heteroatomic or heterocyclic intersugar linkages, phosphorothioates and those with CH 2 —NH—O—CH 2 , CH 2 —N(CH 3 )—O—CH 2  (known as methylene(methylimino) or MMI backbone), CH 2 —O—N(CH 3 )—CH 2 , CH 2 —N(CH 3 )—N(CH 3 )—CH 2  and O—N(CH 3 )—CH 2 —CH 2  backbones (where phosphodiester is O—P—O—CH 2 ). Oligonucleotides having morpholino backbone structures may also be used (U.S. Pat. No. 5,034,506). In alternative embodiments, antisense oligonucleotides may have a peptide nucleic acid (PNA, sometimes referred to as “protein nucleic acid”) backbone, in which the phosphodiester backbone of the oligonucleotide may be replaced with a polyamide backbone wherein nucleosidic bases are bound directly or indirectly to aza nitrogen atoms or methylene groups in the polyamide backbone (Nielsen et al., 1991, Science 254:1497 and U.S. Pat. No. 5,539,082). The phosphodiester bonds may be substituted with structures that are chiral and enantiomerically specific. Persons of ordinary skill in the art will be able to select other linkages for use in practice of the invention. 
     Oligonucleotides may also include species which include at least one modified nucleotide base. Thus, purines and pyrimidines other than those normally found in nature may be used. Similarly, modifications on the pentofuranosyl portion of the nucleotide subunits may also be effected. Examples of such modifications are 2′-O-alkyl- and 2′-halogen-substituted nucleotides. Some specific examples of modifications at the 2′ position of sugar moieties which are useful in the present invention are OH, SH, SCH 3 , F, OCN, O(CH 2 ) n NH 2  or O(CH 2 ) n CH 3  where n is from 1 to about 10; C 1  to C 10  lower alkyl, substituted lower alkyl, alkaryl or aralkyl; Cl; Br; CN; CF 3 ; OCF 3 ; O-, S-, or N-alkyl; O-, S-, or N-alkenyl; SOCH 3 ; SO 2  CH 3 ; ONO 2 ; NO 2 ; N 3 ; NH 2 ; heterocycloalkyl; heterocycloalkaryl; aminoalkylamino; polyalkylamino; substituted silyl; an RNA cleaving group; a reporter group; an intercalator; a group for improving the pharmacokinetic properties of an oligonucleotide; or a group for improving the pharmacodynamic properties of an oligonucleotide and other substituents having similar properties. One or more pentofuranosyl groups may be replaced by another sugar, by a sugar mimic such as cyclobutyl or by another moiety which takes the place of the sugar. 
     In some embodiments, the antisense oligonucleotides in accordance with this invention may comprise from about 5 to about 100 nucleotide units. As will be appreciated, a nucleotide unit is a base-sugar combination (or a combination of analogous structures) suitably bound to an adjacent nucleotide unit through phosphodiester or other bonds forming a backbone structure. 
     The invention also provides pharmaceutical compositions (medicaments) comprising an agent capable of modulating activity and/or expression of a PLC γ , and a pharmaceutically acceptable carrier. In an embodiment, such compositions include the agent, in a therapeutically or prophylactically effective amount sufficient to treat or attenuate pain, and a pharmaceutically acceptable carrier. 
     A “therapeutically effective amount” refers to an amount effective, at dosages and for periods of time necessary, to achieve the desired therapeutic result, such as reduction of pain. A therapeutically effective amount of an agent capable of modulating activity and/or expression of a PLC γ , may vary according to factors such as the disease state, age, sex, and weight of the individual, and the ability of the agent to elicit a desired response in the individual. Dosage regimens may be adjusted to provide the optimum therapeutic response. A therapeutically effective amount is also one in which any toxic or detrimental effects of the agent are outweighed by the therapeutically beneficial effects. A “prophylactically effective amount” refers to an amount effective, at dosages and for periods of time necessary, to achieve the desired prophylactic result, such as preventing or inhibiting onset of pain or increases in the severity of pain. A prophylactically effective amount can be determined as described above for the therapeutically effective amount. For any particular subject, specific dosage regimens may be adjusted over time according to the individual need and the professional judgement of the person administering or supervising the administration of the compositions. 
     As used herein “pharmaceutically acceptable carrier” or “excipient” includes any and all solvents, dispersion media, coatings, antibacterial and antifungal agents, isotonic and absorption delaying agents, and the like that are physiologically compatible. In one embodiment, the carrier is suitable for parenteral administration. Alternatively, the carrier can be suitable for intravenous, intraperitoneal, intramuscular, intracranial, intrathecal, sublingual or oral administration. Pharmaceutically acceptable carriers include sterile aqueous solutions or dispersions and sterile powders for the extemporaneous preparation of sterile injectable solutions or dispersion. The use of such media and agents for pharmaceutically active substances is well known in the art. Except insofar as any conventional media or agent is incompatible with the active agent, use thereof in the pharmaceutical compositions of the invention is contemplated. Supplementary active compounds can also be incorporated into the compositions. 
     Therapeutic compositions typically must be sterile and stable under the conditions of manufacture and storage. The composition can be formulated as a solution, microemulsion, liposome, or other ordered structure suitable to high drug concentration. The carrier can be a solvent or dispersion medium containing, for example, water, ethanol, polyol (for example, glycerol, propylene glycol, and liquid polyethylene glycol, and the like), and suitable mixtures thereof. The proper fluidity can be maintained, for example, by the use of a coating such as lecithin, by the maintenance of the required particle size in the case of dispersion and by the use of surfactants. In many cases, it will be preferable to include isotonic agents, for example, sugars, polyalcohols such as mannitol, sorbitol, or sodium chloride in the composition. Prolonged absorption of the injectable compositions can be brought about by including in the composition an agent which delays absorption, for example, monostearate salts and gelatin. Moreover, the agent capable of modulating activity and/or expression of a PLC γ , can be administered in a time release formulation, for example in a composition which includes a slow release polymer. The active compounds can be prepared with carriers that will protect the compound against rapid release, such as a controlled release formulation, including implants and microencapsulated delivery systems. Biodegradable, biocompatible polymers can be used, such as ethylene vinyl acetate, polyanhydrides, polyglycolic acid, collagen, polyorthoesters, polylactic acid and polylactic, polyglycolic copolymers (PLG). Many methods for the preparation of such formulations are patented or generally known to those skilled in the art. 
     Sterile injectable solutions can be prepared by incorporating the active agent (e.g. an agent capable of modulating activity and/or expression of a PLC γ ) in the required amount in an appropriate solvent with one or a combination of ingredients enumerated above, as required, followed by filtered sterilization. Generally, dispersions are prepared by incorporating the active agent into a sterile vehicle which contains a basic dispersion medium and the required other ingredients from those enumerated above. In the case of sterile powders for the preparation of sterile injectable solutions, the preferred methods of preparation are vacuum drying and freeze-drying which yields a powder of the active ingredient plus any additional desired ingredient from a previously sterile-filtered solution thereof. In accordance with an alternative aspect of the invention, an agent capable of modulating activity and/or expression of a PLC γ  in a subject, may be formulated with one or more additional compounds that enhance its solubility. 
     In another embodiment, an agent of the invention is administered such that it comes into contact with a CNS tissue or a CNS neuron. As used herein, the “central nervous system” or CNS is the portion of the nervous system comprising the brain and the spinal cord (e.g. in the lumbar region). By contrast, the “peripheral nervous system” or PNS is the portion of the nervous system other than the brain and the spinal cord. In a further embodiment, the CNS tissue is the superficial dorsal horn, in a further embodiment, a lamina I neuron. As such, in embodiments an agent of the invention can be administered to treat CNS cells in vivo via direct intracranial or intrathecal injection or injection into the cerebrospinal fluid. Alternatively, the agent can be administered systemically (e.g. intravenously, or orally) in a form capable of crossing the blood brain barrier and entering the CNS. “Neural” and “neuronal” are used herein interchangeably and both relate to neurons and the nervous system. 
     In a further embodiment of the present invention, therapeutic compositions of the present invention, comprising an agent capable of decreasing/inhibiting activity and/or expression of a PLC γ  in a CNS cell may be provided in kits/containers or packages (e.g. commercial packages) which further comprise instructions for their use for the treatment of pain. Similarly, the invention provides a package comprising an agent capable of decreasing/inhibiting activity and/or expression of a PLC γ  together with instructions for the prevention and/or treatment of pain. 
     The invention further provides a use of an agent capable of decreasing/inhibiting activity and/or expression of a PLC γ  or a composition comprising an agent capable of decreasing/inhibiting activity and/or expression of a PLC γ  for the prevention and/or treatment pain, or for the preparation of a medicament for the prevention and/or treatment of pain. 
     In another aspect, the invention relates to the use of PLC γ  as a target in screening methods that may be used for the identification and characterization of agents capable of decreasing/inhibiting activity and/or expression of a PLC γ . Therefore, the invention further provides a method of determining whether a candidate agent is capable of modulating activity and/or expression of a PLC γ  in a subject, and in turn is useful for the prevention and treatment of pain. In an embodiment, the method comprises contacting a cell (e.g. a CNS-derived cell) with the candidate agent and determining whether PLC γ  activity has been decreased or inhibited in the presence of the test agent. In a further embodiment, the assay may be carried out in a cell-free system, by providing a suitable preparation or sample of active PLC γ , contacting the PLC γ  with the candidate agent, and similarly determining whether its activity is inhibited or decreased in the presence of the test compound. In a further embodiment, the assay may comprise assessing whether a candidate compound inhibits the binding of a binding partner or ligand to a PLC γ . In such a case, the method comprises contacting a PLC γ  or a cell comprising a PLC γ  with a candidate agent and the binding partner or ligand, and determining whether the binding of the binding partner or ligand to the PLC γ  is decreased in the presence of the candidate or test compound. In embodiments, the binding partner or ligand is a mitogen-activated protein kinase or homolog or fragment thereof (such as ERK1 or ERK2, or a homolog or fragment thereof). In an embodiment, binding to the D-domain of PLC γ  is determined. 
     Inhibition or a decrease of PLC γ  activity or of the above-mentioned binding is indicative that the test agent may be used for the treatment or the prevention of pain. 
     As used herein, a “CNS-derived cell” is a cell isolated or derived from a CNS tissue, and in embodiments includes both primary neuronal cultures, immortalized neuronal cell lines, as well as accepted in vitro neuronal model systems (e.g. cells differentiated into neurons in vitro). In an embodiment, the above-mentioned cell possesses a PLC γ  activity. 
     The above-mentioned method may be employed either with a single test agent or a plurality or library (e.g. a combinatorial library) of test agents. In the latter case, synergistic effects provided by combinations of agents may also be identified and characterized. The above-mentioned agents may be used for prevention and/or treatment of pain, or may be used as lead agents for the development and testing of additional agents having improved specificity, efficacy and/or pharmacological (e.g. pharmacokinetic) properties. In an embodiment the agent may be a prodrug which is altered into its active form at the appropriate site of action, e.g. in CNS tissue (e.g. in the spinal cord). In certain embodiments, one or a plurality of the steps of the screening/testing methods of the invention may be automated. 
     The invention further relates to methods for the identification and characterization of agents capable of decreasing/inhibiting PLC γ  gene expression. Such a method may comprise determining PLC γ  gene expression in the presence versus the absence of a test agent. Such gene expression may be determined by detection of the corresponding RNA or protein, or via the use of a suitable reporter construct comprising a transcriptional regulatory element(s) normally associated with such PLC γ  gene, operably-linked to a reporter gene. A first nucleic acid sequence may “operably-linked” with a second nucleic acid sequence when the first nucleic acid sequence is placed in a functional relationship with the second nucleic acid sequence. For instance, a promoter is operably-linked to a coding sequence if the promoter affects the transcription or expression of the coding sequences. Generally, operably-linked DNA sequences are contiguous and, where necessary to join two protein coding regions, in reading frame. However, since, for example, enhancers generally function when separated from the promoters by several kilobases and intronic sequences may be of variable lengths, some polynucleotide elements may be operably-linked but not contiguous. “Transcriptional regulatory element” is a generic term that refers to DNA sequences, such as initiation and termination signals, enhancers, and promoters, splicing signals, polyadenylation signals which induce or control transcription of protein coding sequences with which they are operably-linked. The expression of such a reporter gene may be measured on the transcriptional or translational level, e.g. by the amount of RNA or protein produced. RNA may be detected by for example Northern analysis or by the reverse transcriptase-polymerase chain reaction (RT-PCR) method (see for example Sambrook et al (1989) Molecular Cloning: A Laboratory Manual (second edition), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., USA). Protein levels may be detected either directly using affinity reagents (e.g. an antibody or fragment thereof [for methods, see for example Harlow, E. and Lane, D (1988)  Antibodies: A Laboratory Manual , Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY]; a ligand which binds the protein) or by other properties (e.g. fluorescence in the case of green fluorescent protein) or by measurement of the protein&#39;s activity, which may entail enzymatic activity to produce a detectable product (e.g. with altered spectroscopic properties) or a detectable phenotype (e.g. alterations in cell growth). Suitable reporter genes include but are not limited to chloramphenicol acetyltransferase, beta-D galactosidase, luciferase, or green fluorescent protein. 
     The invention further provides a method for diagnosing or prognosticating pain associated with CNS dysfunction. In an embodiment, the pain associated with such CNS dysfunction is neuropathic pain. In an embodiment, the method comprises determining whether there is modulation of activity and/or expression of a PLC γ  in a CNS neural cell relative to a corresponding control activity and/or expression of PLC γ . In this particular method, a difference in the test level relative to a control level is an indication that the subject is experiencing pain associated with CNS dysfunction. In an embodiment, the method may comprise determining whether PLC γ  activity and/or expression is modulated relative to a control activity and/or expression. In yet another embodiment, the PLC γ  activity and/or expression can be selected from an established standard, such as a corresponding PLC γ  activity and/or expression level determined in the subject at an earlier time; a corresponding PLC γ  activity and/or expression level determined in said subject when the subject is experiencing less pain (relative to the current sensation of pain noted above) or substantially no pain; or a corresponding PLC γ  activity and/or expression level determined in a control subject experiencing less pain (relative to the current sensation of pain in the test subject noted above) or substantially no pain. In an embodiment, a subject or control subject experiencing less pain or substantially no pain presents no evident lesions to his central or peripheral nervous system (e.g. neuropathic pain) or persistent pain. 
     For example, PLC γ  activity and/or expression may be determined by administering, to a subject, an indicator compound (such as a compound indicative of activity and/or expression of PLC γ ) that is capable of contacting a CNS neural cell of that subject. Following the administration of the indicator compound, assessment of the in vivo signal associated with such indicator compound may be performed. In an embodiment, an indicator compound, such as an immunodetection-based reagent (e.g. antibody, single chain antibody or Fab fragment directed against the PLC γ  polypeptide) or a suitable substrate which yields detectable products as a result of PLC γ  activity, may be employed. Following injection of the indicator compound, an imaging technique may be performed to assess the in vivo signal associated with the indicator compound. The imaging technique may enable the assessment of the in vivo signal of the indicator compound. 
     In an embodiment, the methods of diagnosis/prognostication noted above may be performed in conjunction with the therapeutic/prophylactic methods noted above, for preventing or treating pain in a subject. Such a method thus comprises the diagnosis or prognostication of pain and modulation of activity and/or expression of a PCL γ  in the subject, thereby to prevent or treat pain. 
     Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way. Numeric ranges are inclusive of the numbers defining the range. In the claims, the word “comprising” is used as an open-ended term, substantially equivalent to the phrase “including, but not limited to”. The following examples are illustrative of various aspects of the invention, and do not limit the broad aspects of the invention as disclosed herein. 
     EXAMPLES 
     Example 1 
     Eleven adult, Sprague-Dawley rats received an experimental chronic constriction injury to the sciatic nerve (Mosconi &amp; Kruger, 1996 , Pain  64:37-57). Pain hypersensitivity, indicative of neuropathic pain, increased progressively over a two week interval and was monitored using the Von Frey technique (Chaplan et al., 1994 , J. Neurosci. Methods  53:55-63). Following the development of allodynia (characterized by a 50% withdrawal threshold (WD 50 ) of ≦2.0 g), rats were administered 500 ng tricyclodecan-9-yl-xanthogenate by intrathecal catheter, the tricyclodecan-9-yl-xanthogenate was prepared in a solution of saline (0.9% NaCl) with 10% v/v dimethyl sulfoxide. Tricyclodecan-9-yl-xanthogenate is available from EMD Biosciences (a division of Calbiochem). A solution of saline (0.9% NaCl) with 10% v/v dimethyl sulfoxide was injected into control rats as a vehicle. The volume of the vehicle administered was the same as for the PLC γ  inhibitor solution. 
     Seven allodynic rats received single intrathecal injections of a vehicle, whereas 4 rats received single injections of tricyclodecan-9-yl-xanthogenate; the mean WD 50  for both groups was 1.9±0.2 g prior to injections. In as little as 4 hours post-injection, the WD 50  of rats administered tricyclodecan-9-yl-xanthogenate was significantly different from those that received the vehicle (tricyclodecan-9-yl-xanthogenate, 8.8±2.1 g vs. vehicle, 4.1±0.7 g; p&lt;0.05), and significantly different from the initial, pre-injection WD 50  (p&lt;0.05, mixed design ANOVA). This significant reduction of pain hypersensitivity persisted for greater than 6 hours. 
     Throughout this application, various references are referred to describe more fully the state of the art to which this invention pertains. The disclosures of these references are hereby incorporated by reference into the present disclosure.