Impairment of serotoninergic transmission is followed by adaptive changes in 5HT1B binding sites in the rat suprachiasmatic nucleus

Serotonin1B (5-HT_1B) receptor binding in the suprachiasmatic nucleus (SCN) following impairment of serotoninergic transmission was studied by quantitative autoradiography. Serotonin (5-HT) denervation with 5,7-dihydroxytryptamine (5,7-DHT) caused a significant increase in the density of 5-HT_1B receptors in both the ventral (62%) and dorsal (53%) parts of the SCN as early as 3 days after axotomy. The magnitude of this increase did not differ 3, 15 or 21 days post-lesion. An up-regulation of 5-HT_1B receptors with similar magnitude was obtained in the two parts of the SCN after inhibition of 5-HT synthesis by chronic parachlorophenylalanine treatment. In this case, up-regulation was shown to be reversible after restoration of 5-HT synthesis with -5-hydroxytryptophan. These results indicate that 5-HT_1B receptor density in the SCN was inversely correlated with 5-HT levels. These plastic properties exhibited by 5-HT_1B receptors in the SSN are discussed in relation to the mode of 5-HT transmission and possible localization of the receptors onto the main chemically defined cell populations of the nucleus.