SEROTONIN 1B RECEPTOR REGULATION AFTER DORSAL SUBICULUM DEAFFERENTATION

The subiculum may be the key structure in the transfer of relevant pro cessed information from the hippocampal formation to cortical areas. W e investigated the location of the serotonin 1B receptor (5-HT1B) in t he hippocampus with the specific ligand serotonin-O-carboxymethyl-glyc yl [I-25]tyrosinamide in rat brain sections using in vitro autoradiogr aphy. A high density of 5-HT,B binding sites was found in the dorsal s ubiculum (DS), in the lacunosum moleculare, and in the most dorsal lay er of the stratum oriens of the CA1 field. CA1 pyramidal neurons that contain 5-HT1B mRNA project primarily to the DS. We interrupted the py ramidal CA1 axons unilaterally by a stereotaxic knife cut. Histologica l analysis showed that the lesion was restricted to a trail of cells l ost between CA1 and DS. Specific B-HT,B binding site density was decre ased in the DS on the ipsilateral side of the lesion compared to the c ontralateral side. We conclude that 5-HT1B receptors are located on CA 1 pyramidal axon terminals in the DS. Serotonin, acting on these recep tors, should inhibit CA1 neurotransmitter release and, in this way, mo dulate subicular functions.