Plastic responses of neonatal 5‐hydroxytryptamine 1B receptors to 5,7‐dihydroxytryptamine lesions mapped by quantitative autoradiography

We previously found different effects on behavior, serotonin (5‐HT) concentrations, 5‐HT uptake sites, and 5‐HT 1A binding sites of neonatal 5,7‐dihydroxytryptamine (5,7‐DHT) lesions depending on the route of 5,7‐DHT injection. To study the impact of early lesions on 5‐HT 1B sites as putative 5‐HT terminal autoreceptors, we labelled them autoradiographically with [ 3 H]5‐HT 4 months after intraperitoneal (i.p.) or intracisternal (i.c.) 5,7‐DHT injection during the first postnatal week and quantitated specific binding in 22 brain regions. Changes were confined to the subiculum and substantia nigra, regions with the most 5‐HT 1B ‐specific binding and projection areas of structures with high mRNA expression. Both routes of 5,7‐DHT injection were associated with increases in specific binding in subiculum (24% for i.p. and 47% for i.c. route). In contrast, there was a 32% increase in specific binding in the substantia nigra in rats with lesions made i.c. but not i.p. No significant differences were found in nucleus accumbens, caudate‐putamen or other brain areas. In saturation homogenate binding studies of 5‐HT 1B sites using [ 125 I]iodocyanopindolol 1 month after i.p. injections, neonatal 5,7‐DHT lesions did not significantly alter B max or K d in the neocortex, striatum, diencephalon or brainstem. These data indicate the differential effects of the route of neonatal 5,7‐DHT injections on plasticity of 5‐HT 1B receptor recognition sites and suggest the presence of a subpopulation of post‐synaptically located 5‐HT 1B sites which increases in response to denervation. The data also suggest that sprouting of 5‐HT neurons after neonatal 5,7‐DHT lesions does not involve 5‐HT 1B sites.