IMPAIRMENT OF SEROTONINERGIC TRANSMISSION IS FOLLOWED BY ADAPTIVE-CHANGES IN 5HT(1B) BINDING-SITES IN THE RAT SUPRACHIASMATIC NUCLEUS

Serotonin1B (5-HT1B) receptor binding in the suprachiasmatic nucleus ( SCN) following impairment of serotoninergic transmission was studied b y quantitative autoradiography. Serotonin (5-HT) denervation with 5,7- dihydroxytryptamine (5,7-DHT) caused a significant increase in the den sity of 5-HT1B receptors in both the ventral (62%) and dorsal (53%) pa rts 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-regulatio n of 5-HT1B receptors with similar magnitude was obtained in the two p arts of the SCN after inhibition of 5-HT synthesis by chronic parachlo rophenylalanine treatment. In this case, up-regulation was shown to be reversible after restoration of 5-HT synthesis with L-5-hydroxytrypto phan. These results indicate that 5-HT1B receptor density in the SCN w as inversely correlated with 5-HT levels. These plastic properties exh ibited by 5-HT1B receptors in the SCN 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.