Serotonin modulates photic responses in the hamster suprachiasmatic nuclei.

The aim of the present study was to examine the effects of serotonin agonists on three elements of the photic response in the hamster suprachiasmatic nuclei (SCN). Both serotonin and the selective 5-HT1A agonist 8-OH-DPAT inhibited field potentials recorded in the SCN in response to optic nerve stimulation in the hypothalamic slice preparation. The effects of both drugs were dose related over a concentration range of 1-50 microM, and, in both cases, a maximal inhibition of approximately 60% was achieved at a concentration of 25-50 microM. Systemic administration of 8-OH-DPAT inhibited light-stimulated Fos expression in the SCN. A regionally selective pattern of inhibition was observed, with decreases restricted predominately to the ventral and dorsal borders of the SCN. Finally, systemic administration of 8-OH-DPAT was found to dose-dependently attenuate light-induced phase shifts of the free-running activity rhythm. The effects of 8-OH-DPAT on light-induced phase advances were dose dependent. Injection of 8-OH-DPAT at a dose of 0.5 mg/kg caused 57% inhibition of light-induced phase advances, while a dose of 5 mg/kg inhibited the phase advance by 82%. Injection of 0.05 mg/kg 8-OH-DPAT did not significantly inhibit light-induced phase advances. Injection of 5 mg/kg 8-OH-DPAT alone did not significantly alter the phase of the activity rhythm. Similarly, injection of 5 mg/kg 8-OH-DPAT 30 min prior to light stimulation at CT14 completely inhibited light-induced phase delays, while this dose of the drug did not alter the phase of the activity rhythm when administered alone.(ABSTRACT TRUNCATED AT 250 WORDS)