1. A truncated glial cell line-derived neurotrophic factor (GDNF) protein product having an amino acid sequence  X-[Cys<41> -Cys<133>]-Y wherein [Cys<41>-Cys<133>] represents the amino acid sequence of Cys<41> through Cys<133> as depicted in Figure 1 (SEQ ID N0 2); Y represents the carboxy terminal group of Cys<133> or a carboxy-terminus amino acid residue of Ile<134>; and X represents a methionylated or nonmethionylated amine group of Cys<41> or amino-terminus amino acid residue(s) selected from the group, comprising     or an analog of said factor, having the same biological activity. 2. A truncated GDNF protein product according to claim 1, wherein X = RQAAA ANPENSRGKG (SEQ ID NO:24) or an anolog thereof. 3. A truncated GDNF protein product according to claim 1, wherein X = NPENSRGKG RRGQRGKNRG (SEQ ID NO:18) or an anolog thereof. 4. A truncated GDNF protein product according to claim 1, wherein X = PENSRGKG RRGQRGKNRG (SEQ ID NO:17) or an anolog thereof. 5. A truncated GDNF protein product according to claim 1, wherein X = SRGKG RRGQRGKNRG (SEQ ID NO:14) or an anolog thereof. 6. A truncated GDNF protein product according to claim 1, wherein X = RGQRGKNRG (SEQ ID NO:8) or an anolog thereof. 7. A truncated GDNF protein product according to claim 1, wherein X = GQRGKNRG (SEQ ID NO:7) or an anolog thereof. 8. A truncated GDNF protein product according to claim 1, wherein X = KNRG (SEQ ID NO:3) or an anolog thereof. 9. A truncated GDNF protein product according to claim 1, wherein X = NRG or an anolog thereof. 10. A truncated GDNF protein product according to claims 1 through 9, wherein said amino acid sequence is glycosylated. 11. A truncated GDNF protein product according to claims 1 through 9, wherein said amino acid sequence is nonglycosylated. 12. A truncated GDNF protein product according to claim 1, wherein said GDNF protein is conjugated to a water soluble polymer. 13. DNA encoding a truncated GDNF protein according to claim 1. 14. DNA according to claim 13, comprising the structure as set forth in Figure 1. 15. DNA according to claim 13, comprising the structure as set forth in Figure 3. 16. DNA according to claim 13, comprising the structure as set forth in Figure 4. 17. DNA according to claim 13, comprising the structure as set forth in Figure 5. 18. DNA according to claim 13, comprising the structure as set forth in Figure 6. 19. DNA according to claim 13, comprising the structure as set forth in Figure 7. 20. A vector, comprising DNA of claim 13 operatively linked to an expression control sequence. 21. A strain of prokaryotic host cell transformed or transfected with a DNA of claim 13. 22. A strain of eukaryotic host cell transformed or transfected with a DNA of claim 13. 23. A method for producing a truncated GDNF protein, comprising growing host cells of claim 21 or 22 in a suitable nutrient medium and, optionally, isolating said truncated GDNF from said cells or said nutrient medium. 24. A method according to claim 23, wherein the strain of procaryotic host cells are Escherichia coli. 25. A method according to claim 23, wherein the line of eucaryotic host cells, wherein said host cells are Chinese hamster ovary cells. 26. A method for the production of a truncated glial cell line-derived neurotrophic factor (GDNF) protein, comprising the steps of: (a) culturing a prokaryotic or eukaryotic host cell transformed or transfected with a vector of claim 20 in a suitable nutrient medium under conditions allowing the expression of truncated GDNF protein; and  (b) optionally isolating the truncated GDNF protein expressed by said host cell. 27. A truncated GDNF protein which is the recombinant expression product of a prokaryotic strain or eukaryotic host cell transformed or transfected with DNA of claim 13 or with a vector of claim 20. 28. A pharmaceutical composition comprising a truncated GDNF protein product according to claim 1 and at least one pharmaceutically acceptable vehicle. 29. A pharmaceutical composition comprising a truncated GDNF protein according to claim 27 and a pharmaceutically acceptable vehicle. 30. A method of treating Parkinson's Disease comprising administering to a patient the pharmaceutical composition of claim 28. 31. A method of treating Parkinson's Disease comprising administering to a patient DNA of claim 13 to provide in vivo production of said truncated GDNF protein. 32. A method of treating Parkinson's Disease comprising implanting in a patient a cell transformed with DNA of claim 13 to provide in vivo production of said truncated GDNF protein. 33. A GDNF geterodimer, comprising a mature and a truncated GDNF molecules of claim 1. 34. A GDNF dimer, comprising two truncated GDNF molecules of claim 1. 35. A truncated GDNF protein according to claim 1, wherein derived from a mature GDNF protein expressed by a recombinantly modified bacterial or mammalian cell. 36. A truncated GDNF protein according to claim 35, wherein X is an amino-terminus amino acid residue or amino-terminus sequences, selected from the group, comprising   37. A truncated GDNF protein according to claim 35, wherein mature GDNF protein is expressed by a recombinantly modified bacterial cell in vitro or in vivo. 38. A method of preparing a pharmaceutical composition according to claim 28, wherein a therapeutically effective amount of a truncated GDNF protein product according to claim 1 is mixed with one or more pharmaceutically acceptable vehicles. 39. The use of a truncated GDNF protein product according to claim 1 for treating diseases or injuries of the nervous system. 40. The use of a truncated GDNF protein product according to claim 39, wherein the disease of the nervous system is Parkinson's disease. 41. The use of a truncated GDNF protein product according to claim 1 for the preparation of a pharmaceutical composition for treating diseases or injuries of the nervous system.