Indirect evidence for an association of 5-HT1B binding sites with retinal and geniculate axon terminals in the rat suprachiasmatic nucleus

The purpose of the present study was to investigate the possible cellular l ocation of 5-HT1B receptors on retinal and geniculate afferents in the rat suprachiasmatic nucleus (SCN). Biocular enucleation significantly decreased 5-HT1B binding site labeling (35%), specifically in the ventral part of th e SCN, while monocular enucleation produced a decrease of smaller magnitude (12%), limited to the ventral Dart of the contralateral SCN, these results being consistent with the known distribution of retinal afferents in the n ucleus. By contrast, bilateral geniculate lesion did not induce any signifi cant variation of 5-HT1B binding site labeling in the SCN. Previously, we r eported that serotonin (5-HT) synthesis inhibition by parachlorophenylalani ne increases 5-HT1B binding site labeling in the SCN. Using saturation stud ies, we have now demonstrated that this upregulation reflected an increase in the total number of 5-HT1B binding sites (+41% in the dorsal and +67% in the ventral part of the SCN). Furthermore, we evaluated the effects of bil ateral geniculate lesion after 5-HT stores depletion in order to overcome p roblems of technical resolution limits. The magnitude of upregulation was s ignificantly decreased (27%) after bilateral geniculate lesion, suggesting that part of the 5-HT1B receptor population was located on geniculate axon terminals within the SCN. The possible involvement of 5-HT1B receptors, acc ording to their cellular locations evidenced in the present study, in photi c and nonphotic entrainment of the circadian clock is discussed. Synapse 33 :314-323, 1999 (C) 1999 Wiley-Liss, Inc.