ACTIVATION OF GALANIN PATHWAYS ACROSS PUBERTY IN THE MALE-RAT - ASSESSMENT OF REGIONAL DENSITIES OF GALANIN BINDING-SITES

Galanin-like immunoreactivity and galanin messenger RNA levels increas e across puberty in neurons of gonadal steroid-dependent brain nuclei. We hypothesized that this activation and the associated increase in e ndogenous galanin release would result in changes across puberty in bo th galanin binding density and the level of receptor occupancy. Here w e have assessed the density of galanin binding sites in several brain regions of prepubertal and adult male rats with or without GTP to indu ce dissociation of endogenous galanin from its binding sites. The deve lopmental changes in the level of receptor occupancy were used as an i ndirect measure of changes in neuropeptide release from galanin expres sing neurons. In standard binding conditions (buffer preincubation), I -125-labeled galanin binding showed a generalized decline in adult bra ins (34-68%) compared with prepubertal levels in most regions of the t elencephalon and diencephalon. Following preincubation with 10(-5) M G TP, galanin binding showed a dramatic increase in most regions of the adult (152-504%) and several regions of the prepubertal brain (132-245 %) over their standard binding levels. However, this increase was grea test in adult animals. Finally, although preincubation of brain slices with GTP eliminated most of the apparent age-related differences obse rved in standard binding conditions, several brain regions of the adul t brain continued to show a significant reduction (38-76%) in I-125-la beled galanin binding compared with prepubertal animals. Only one regi on, the lateral preoptic area, exhibited enhanced I-125-labeled galani n binding in adult (160%) compared with prepubertal brain after GTP pr eincubation. The results support our hypothesis that pubertal activati on of galanin pathways is associated with increased galanin release an d enhanced galanin receptor occupancy by endogenous ligand in the adul t brain. However, the persistent age-associated differences in I-125-l abeled galanin binding sites density found in several brain regions af ter unmasking of binding sites suggest that, in addition to a higher l evel of receptor occupancy by endogenous ligand, an actual change in g alanin binding sites number and/or affinity occurs in certain brain re gions across puberty in the male rat.