TFMPP, A 5HT(1B) RECEPTOR AGONIST, INHIBITS LIGHT-INDUCED PHASE-SHIFTS OF THE CIRCADIAN ACTIVITY RHYTHM AND C-FOS EXPRESSION IN THE MOUSE SUPRACHIASMATIC NUCLEUS

The hypothalamic suprachiasmatic nucleus (SCN) receives afferents from the retina and the midbrain raphe. The retinal innervation mediates p hotic entrainment of the SCN circadian oscillator whereas the serotone rgic input arising from the midbrain raphe nuclei appears to modulate retinohypothalamic neurotransmission. We hypothesized that serotonergi c innervation of the SCN may modulate retinal input by activation of 5 HT(1B) presynaptic receptors on retinal axon terminals in the SCN. We tested this hypothesis using the 5HT(1B) receptor agonist, 1-[3-(trifl uoromethyl)phenyl]-piperazine (TFMPP). Systemic administration of TFMP P prior to light stimulation significantly attenuated light-induced ph ase shifts of the circadian activity rhythm and Fos expression in the SCN. These results in the mouse support our earlier findings in the ha mster [Pickard, G.E., Weber, E.T., Scott, P.A., Riberdy, A.F. and Pea, M.A., J. Neurosci., 16 (1996) 8208-8220] and are consistent with the interpretation that 5HT(1B) presynaptic receptors participate in the r egulation of photic input to the SCN. (C) 1997 Elsevier Science Irelan d Ltd.