Photic responses of suprachiasmatic area neurons in diurnal degus (Octodondegus) and nocturnal rats (Rattus norvegicus)

Photic sensitivity of cells in the suprachiasmatic nuclei (SCN), the princi pal pacemaker of the mammalian circadian system, has been documented in sev eral species. In nocturnal rodents, the majority of photically responsive S CN cells are activated by retinal illumination. One report identified mostl y photic suppressions among SCN cells in a diurnal rodent, studied under so mewhat different conditions. We examined photic sensitivity of SCN cells in a predominantly diurnal rodent, the degu, studied in vivo under identical conditions to rats, and found that a large majority of photic SCN cells wer e suppressed by light. In both rats and degus, SCN cells were more responsi ve to light during the subjective night than during the subjective day. Lig ht-responsive cells did not show a daily rhythm in baseline firing rates in either species, but rat SCN cells that did not respond to light were more active spontaneously during the subjective day. Light-unresponsive SCN cell s in degus did not show a similar pattern. There are substantial difference s in the neurophysiological activity and photic responsiveness of SCN cells in diurnal degus and nocturnal rats. (C) 1999 Elsevier Science B.V. All ri ghts reserved.