Opposing effects of behavioural activity and light on neurons of the suprachiasmatic nucleus

The mammalian circadian pacemaker is located in the suprachiasmatic nuclei. It can be shifted in phase by photic cues and by the behavioural activity of the animal. When presented together, light and behavioural activity atte nuate each others' phase-shifting effect. Still unclear is how behavioural activity affects the suprachiasmatic nuclei and how it interacts with photi c information. Previously, we reported the occurrence of behaviourally indu ced suppressions of neuronal activity. The present study investigates the c haracteristics of these suppressions as a function of circadian time and, a dditionally, in the presence of photic cues. We performed long-term multiun it activity recordings of neurons in freely moving rats and found that thes e suppressions of neuronal firing in the suprachiasmatic nucleus occurred a t every phase of the circadian cycle. The magnitude of the suppressions sho wed a circadian variation, with larger suppressions during subjective day. When a light pulse was applied during a suppression, light and activity app eared to oppose each others' effects within the recorded population of neur ons. The resulting discharge level appeared to be the sum of both responses . The opposing effects of light and activity were also found in single unit recordings, indicating that photic and behavioural stimuli interact at the level of a single neuron.