RET RECEPTOR TYROSINE KINASE IMMUNOREACTIVITY IS ALTERED IN GLIAL-CELL LINE-DERIVED NEUROTROPHIC FACTOR-RESPONSIVE NEURONS FOLLOWING LESIONS OF THE NIGROSTRIATAL AND SEPTOHIPPOCAMPAL PATHWAYS

Glial cell line-derived neurotrophic factor was initially identified a s a survival factor for developing midbrain dopamine neurons (for revi ews, see Refs 17 and 19). Subsequent studies have demonstrated a more widespread role for glial cell line-derived neurotrophic factor in the developing and adult CNS.(6,9,11,14,18,27,32) In the adult rat brain, for instance, prior administration of glial cell line-derived neurotr ophic factor protects nigrostriatal dopamine neurons from 6-hydroxydop amine-induced damage.(14,32) When given several weeks after 6-hydroxyd opamine injection, glial cell line-derived neurotrophic factor also re stores the function of these neurons.(6,11,18) Glial cell line-derived neurotrophic factor attenuates excitotoxin-induced cell death in the striatum(3,24) and hippocampal formation(20) and protective effects of glial cell line-derived neurotrophic factor following axotomy have be en reported for spinal motor neurons and basal forebrain cholinergic n eurons.(17,36) These findings suggest that glial cell line-derived neu rotrophic factor may be a protective/restorative agent for a diverse p opulation of neurons and imply that it may be a useful therapeutic too l for a variety of neurodegenerative diseases including Parkinson's, H untington's and Alzheimer's diseases. The potential receptor mediating the pleiotropic effects of glial cell line-derived neurotrophic facto r has been characterized only recently as a novel glycosyl-phosphatidy linositol-linked protein, GDNFR-alpha.(13,33) Because GDNR-alpha is a cell surface receptor, an additional protein(s) was thought to be invo lved in the glial cell line-derived neurotrophic factor signalling cas cade.(8,13,33) The identity of the likely candidate, ret, was inferred initially from indirect evidence, Not only vi ere there remarkable si milarities in the distribution of glial cell line-derived neurotrophic factor and the proto-oncogene r.et in the developing rat and mouse br ain,(23,31,35) but also in the phenotype of glial cell line-derived ne urotrophic factor knockout mice and mice with r.et mutations.(8,22,25, 26,28) Mice with either mutation exhibited pronounced renal and enteri c abnormalities, implicating the receptor tyrosine kinase protein prod uct of the ret proto-oncogene as the glial cell line-derived neurotrop hic factor signalling protein.(8,22,25,26,28) More conclusive evidence showing that activation of GDNFR-alpha by glial cell line-derived neu rotrophic factor induces phosphorylation of ret has confirmed ret as a signalling protein for glial cell line-derived neurotrophic factor.(1 3,33) Preliminary results showing that 6-hydroxydopamine lesions of th e substantia nigra markedly reduced ret messenger RNA expression,(34) established its localization to presumably glial cell line-derived neu rotrophic factor-responsive dopamine neurons in the nigrostriatal path way. In contrast, it is not clear whether other glial cell line-derive d neurotrophic factor-responsive neurons in the CNS, such as the basal forebrain cholinergic neurons and striatal neurons, also express ret, nor is it evident whether levels of the protein are regulated by disr uption of the respective pathways. The present study shows that dense networks of ret immunoreactivity are distributed throughout the nigros triatal pathway, with lower densities of staining in other brain regio ns, including the septohippocampal pathway, Following extensive unilat eral 6-hydroxydopamine lesions of the medial forebrain bundle, ret imm unoreactivity in the substantia nigra and striatum was reduced signifi cantly, to a similar extent as tyrosine hydroxylase immunoreactivity. In contrast, excitotoxic lesions of the striatum, achieved by intrastr iatal quinolinic acid injections, resulted in increased, et staining i n this brain region. In addition, marked decrements in septal fet immu noreactivity were consequent to complete transections of the fimibriaf ornix. (C) 1997 IBRO. Published by Elsevier Science Ltd.