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 cel
