THE EFFECTS OF ELECTRICAL-STIMULATION OF THE OPTIC NERVES AND ANTERIOR OPTIC CHIASM ON THE CIRCADIAN ACTIVITY RHYTHM OF THE SYRIAN-HAMSTER - INVOLVEMENT OF EXCITATORY AMINO-ACIDS

The circadian pacemaker of the suprachiasmatic nuclei (SCN) is entrain ed to the environmental light-dark cycle via the retinohypothalamic tr act (RHT). It is unknown whether light activates or suppresses firing of the retinal ganglion cells which mediate photic entrainment. We the refore electrically stimulated the optic nerves and the anterior optic chiasm of hamsters with free-running activity rhythms in continuous d arkness. These electrical stimulations are thought to induce a release of neurotransmitter at the RHT terminals. Electrical stimulation mimi cked the phase dependent shifts induced by light pulses. The phase shi fts were significantly larger than the shifts induced by sham stimulat ion in the same animals or by electrical stimulation in animals with a n electrode outside the optic nerves and chiasm. Our results indicate that the retinal ganglion cells which project to the SCN are activated by light. Intraperitoneal administration of MK-801, a non-competitive antagonist of the NMDA-receptor, attenuated the phase delays induced by electrical stimulation in the early subjective night. This suggests that an excitatory amino acid mediates the effects of light upon the circadian pacemaker.