1. An isolated and purified neurotrophic factor receptor human protein, obtained from glial cell line-derived neurotrophic factor (GDNFR) comprising an amino acid sequence as depicted in Figure 2 SEQ ID No: 2 and analogs thereof, obtained by exclusion, addition and/or substitution of amino acids in said sequence with preservation by the finite molecule of GDNFR activity, wherein the protein is capable of complexing with glial cell line-derived neurotrophic factor (GDNF) and thereby mediating cell response to GDNF. 2. A protein of Claim 1 comprising the amino acid sequence from Ser<18> to Pro<446>, as depicted in Figure 2 (SEQ ID No: 2). 3. A protein of Claim 1 comprising the amino acid sequence from Asp<25> to Leu<447>, as depicted in Figure 2 (SEQ ID No: 2). 4. A protein of Claim 1 comprising the amino acid sequence from Cys<29> to Cys<442>, as depicted in Figure 2 (SEQ ID No: 2). 5. An isolated and purified neurotrophic factor receptor rat protein, obtained from glial cell line-derived neurotrophic factor (GDNFR) comprising an amino acid sequence as depicted in Figure 4 SEQ ID No: 4 and analogs thereof, obtained by exclusion, addition and/or substitution of amino acids in said sequence with preservation by the finite molecule of GDNFR activity, wherein the protein is capable of complexing with glial cell line-derived neurotrophic factor (GDNF) and thereby mediating cell response to GDNF. 6. A protein of Claim 5 comprising the amino acid sequence from Ala<19> to Val<450>, as depicted in Figure 4 (SEQ ID No:4). 7. A protein of Claim 5 comprising the amino acid sequence from Cys<29> to Cys<443>, as depicted in Figure 4 (SEQ ID No:4). 8. A protein of any of Claims 1 or 5, which is glycosylated. 9. A protein of any of Claims 1 or 5, which is non-glycosylated. 10. A protein of any of Claims 1 to 4 or 5 to 7, which is produced by recombinant technology. 11. A protein of any of Claims 1 to 4 or 5 to 7, which is produced by chemical synthesis. 12. A pharmaceutical composition comprising a protein as claimed in any one of Claims 1 to 11 in combination with a pharmaceutically acceptable carrier. 13. An isolated nucleic acid sequence encoding a neurotrophic factor receptor human protein (GDNFR) according to Claim 1, comprising a nucleotide sequence SEQ ID No: 1, as depicted in Figure 1 or an analogue thereof, which was obtained owing to the degeneracy of the genetic code, at that said analogue can encode a protein with GDNFR human protein activity. 14. An isolated nucleic acid sequence encoding a neurotrophic factor receptor rat protein (GDNFR) according to Claim 5, comprising a nucleotide sequence SEQ ID No: 3, as depicted in Figure 3 or an analogue thereof, which was obtained owing to the degeneracy of the genetic code, at that said analogue can encode a protein with GDNFR rat protein activity. 15. A vector comprising a nucleic acid sequence according to Claim 13, operatively linked to one or more operational elements capable of effecting the amplification or expression of said nucleic acid sequence. 16. A vector comprising a nucleic acid sequence according to Claim 14, operatively linked to one or more operational elements capable of effecting the amplification or expression of said nucleic acid sequence. 17. A procariotic host cell line transformed or transfected with the vector of any of Claims 15 or16, wherein said cell line expresses GDNFR protein. 18. A mammal host cell line transformed or transfected with the vector of any of Claims 15 or16, wherein said cell line expresses GDNFR protein. 19. A host cell line of Claim 17 wherein said cells are E. coli cells. 20. A host cell line of Claim 18 wherein said cells are COS-7 cells. 21. A procariotic host cell line of Claim 17 wherein said cell line is transformed or transfected ex vivo. 22. A procariotic host cell line of Claim 18 wherein said cell line is transformed or transfected ex vivo. 23. A use of host cell line of Claim 17 for human implantation. 24. A use of host cell line of Claim 18 for human implantation. 25. A use of host cell line of any of Claim 23 or 24, characterized in that the host cell line is put into semipermeable membrane suitable for implantation. 26. A method for the production of a neurotrophic factor receptor protein comprising the steps of: (a) culturing a host cell, which expresses protein in accordance with any of Claims 1-4 under conditions suitable for the expression of said receptor protein expressed by said host cell; and (b) optionally, isolating said receptor protein expressed by said host cell. 27. A method for the production of a neurotrophic factor receptor protein comprising the steps of: (a) culturing a host cell, which expresses protein in accordance with any of Claims 5-7 under conditions suitable for the expression of said receptor protein expressed by said host cell; and (b) optionally, isolating said neurotrophic factor receptor protein expressed by said host cell. 28. A method of Claim 26 or 27, further comprising the step of refolding the isolated neurotrophic factor receptor. 29. A method of Claim 26 or 27, wherein said host cell is a prokaryotic cell. 30. A method of Claim 26 or 27, wherein said host cell is a eukaryotic cell. 31. The use of the neurotrophic factor receptor protein (GDNFR) of any of Claims 1-4 for the manufacture of a pharmaceutical composition. 32. The use of the neurotrophic factor receptor protein (GDNFR) of any of Claims 5-7 for the manufacture of a pharmaceutical composition. 33. A method of treating improperly functioning dopaminergic nerve cells by administering a neurotrophic factor receptor protein of any of Claims 1-4 or 5-7. 34. A method according to Claim 33 for treating Parkinson's disease. 35. A method according to Claim 33 for treating Alzheimer's disease. 36. A method according to Claim 33 for treating amyotrophic lateral sclerosis. 37. A polyclonal antibody, which binds a neurotrophic factor receptor protein of any of Claims 1-4 and is produced by immunizing an animal with said protein. 38. A polyclonal antibody, which binds a neurotrophic factor receptor protein of any of Claims 5-7 and is produced by immunizing an animal with said protein. 39. A device for treating nerve damage, comprising: (a) a semipermeable membrane suitable for implantation; and (b) host cells encapsulated within said membrane, wherein said cells have been transformed or transfected in order to express and secrete a neurotrophic factor receptor protein according to Claims 1-4; said membrane being permeable to the neurotrophic factor protein receptor and impermeable to materials detrimental to said cells. 40. A device for treating nerve damage, comprising: (a) a semipermeable membrane suitable for implantation; and (b) host cells encapsulated within said membrane, wherein said cells have been transformed or transfected in order to express and secrete a neurotrophic factor receptor protein according to Claims 5-7; said membrane being permeable to the neurotrophic factor protein receptor and impermeable to materials detrimental to said cells. 41. An assay device for analyzing a test sample for the presence of glial cell line-derived neurotrophic factor, comprising a solid phase containing or coated with a GDNFR protein, as claimed in Claims 1-11, wherein said GDNFR protein reacts with GDNF present in the test sample and produces a detectable reaction product indicative of the presence of GDNF. 42. A method for analyzing a test sample for the presence of glial cell line-derived neurotrophic factor, comprising: contacting the sample to an assay reagent comprising GDNFR protein, as claimed in Claims 1-11, wherein said GDNFR protein reacts with GDNF present in the test sample and produces a detectable reaction product indicative of the presence of GDNF.