OPPOSITE CHANGES IN STRIATAL NEUROPEPTIDE-Y IMMUNOREACTIVITY AFTER PARTIAL AND COMPLETE SEROTONERGIC DEPLETION IN THE RAT

The aim of this study was to investigate the consequences of partial v s. complete serotonergic (5-HT) depletions on the immunoreactivity of striatal interneurons containing neuropeptide Y (NPY). Taking into acc ount the plasticity of the monoaminergic neurons, the effects of vario us doses of 5,7-dihydroxytryptamine (5,7-DHT) injected into the anteri or raphe nuclei and P-chlorophenylalanine (PCPA) administration were c ompared in the dorsal (caudate-putamen) and the ventral(nucleus accumb ens) striatum. Twenty days after administering 5,7-DHT injections indu cing a substantial but partial decrease in the striatal 5-HT concentra tions (about 80%), we detected a significant decrease in the number of NPY immunoreactive cells. In contrast, the PCPA inhibition of seroton in synthesis in the neurons spared by the partial lesion or the near-c omplete neurotoxic lesion induced an increase in the number of striata l NPY neurons. These results suggest that complex adaptive mechanisms are probably responsible for the changes in striatal NPY reactivity ob served after a partial lesion and that these neurons can adapt accordi ng to the extent of 5-HT depletion. Upon comparing the NPY responses i n the dorsal and ventral components of the striatal complex, no main d ifferences were observed; while in the caudate-putamen, the changes we re primarily found to occur in the medial zone. This finding is discus sed here with reference to the topographical effects of dopaminergic o r glutamatergic deafferentation. Finally, these results suggest that a complete interruption of the 5-HT transmission may lead to an increas e in the intracellular NPY level, which may be associated with a decre ase in the release of the peptide. It can therefore be postulated that serotonergic neurons normally exert a positive influence on NPY stria tal neurons. (C) 1996 Wiley-Liss, Inc.