Luminance coding in a circadian pacemaker: the suprachiasmatic nucleus of the rat and the hamster

The hypothalamic suprachiasmatic nuclei (SCN) of mammals function as a pacemaker driving circadian rhythms. This pacemaker is entrained to the daily light-dark cycle in the environment via the retina and central retinal projections to the anterior hypothalamus. We carried out a comparative study of the visual properties of rat and hamster SCN neurons. Extracellular single cell activity was recorded in the SCN of urethane-anaesthetized animals. In both species, visual SCN neurons responded to retinal illumination with a sustained increase or a sustained decrease in electrical discharge. The majority (75%) of these cells were activated by light. In both the rat and the hamster SCN, visually responsive cells altered their discharge rate as a monotonic function of luminance. The intensity-response curve could be described by a Michaelis function with a small working range between threshold and saturation (2-3 log units) and a relatively high threshold. Intensity-response curves in both species were occasionally different for increasing as opposed to decreasing luminance. Thus, hysteresis effects of illumination may occur in the SCN. The spontaneous firing rates as well as the responsiveness of visual SCN cells were subject to marked variations between and within cells. The overall photic responsiveness of SCN neurons, however, indicated that they are specialized for luminance coding in the range of light intensities naturally occurring at dawn and dusk. This property makes these cells suitable to mediate photic entrainment of circadian rhythms as well as the measurement of photoperiod.