LIGHT RESPONSIVENESS OF THE SUPRACHIASMATIC NUCLEUS - LONG-TERM MULTIUNIT AND SINGLE-UNIT RECORDINGS IN FREELY MOVING RATS

The suprachiasmatic nuclei (SCN) of the hypothalamus contain a pacemak er that generates circadian rhythms in many functions. Light is the mo st important stimulus that synchronizes the circadian pacemaker to the environmental cycle. In this paper we have characterized the baseline neuronal firing patterns of the SCN as well as their response to ligh t in freely moving rats. Multiunit and single-unit recordings showed t hat SCN neurons increase discharge during daytime and decrease dischar ge at night. Discharge levels of individual neurons that were followed throughout the circadian cycle appeared in phase with the population and were characterized by low discharge rates (often below 1 Hz), with a twofold increase during the day. The effect of light on the multiun it response was dependent on the duration of light exposure and on lig ht intensity, with light thresholds of similar to 0.1 lux. The light r esponse level showed a strong dependency on time of day, with large re sponsiveness at night and low responsiveness during day. At both phase s of the circadian cycle, the response level could be raised by an inc rease in light intensity. Single-unit measurements revealed that the t ime-dependent light response of SCN neurons was present also at the le vel of single units. The results show that the basic light response ch aracteristics that were observed at the multiunit level result from an integrated response of similarly behaving single units. Research at t he single-unit level is therefore a useful approach for investigating the basic principles of photic entrainment.