A Thalamic Contribution to Arousal‐induced, Non‐photic Entrainment of the Circadian Clock of the Syrian Hamster

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Abstract
It is well established that the circadian clock of the suprachiasmatic nuclei (SCN) is entrained by light. More recently, the potent effects of arousing, non‐photic cues on the clock have been recognized. The neural mediators of non‐photic entrainment are yet to be identified. To examine the contribution of the thalamic intergeniculate leaflet (IGL) and its NPY‐immunopositive projection, the geniculo‐hypothalamic tract to non‐photic entrainment by arousal, male Syrian hamsters received lesions of the IGL (IGLX) which ablated NPY‐immunoreactivity in the SCN. Their circadian responses to both photic and non‐photic cues were then tested. Lesions resulted in a delay in the timing of activity onset following lights out, but had no effect on the behavioural or cellular circadian responses to phase‐advancing light pulses presented at circadian time (CT) CT19 (where CT12 represents the time of activity onset). Injection with a benzodiazepine (chlordiazepoxide, 100 mg/kg) at CT6 suppressed wheel‐running, increased general locomotion of intact controls and induced large phase advances of the circadian rhythm of wheel‐running. Chlordiazepoxide also inhibited wheel‐running in lesioned animals, but there was no significant increase in general locomotion and the lesioned animals did not phase advance. Serial arousal by injection of saline at intervals of 23.5 h for 6 days entrained the circadian rhythm of wheel‐running of intact hamsters and was associated with an increase in general locomotor activity. Entrainment by serial arousal was abolished by IGLX. However, the lesioned animals did show a clear behavioural response to every presentation of the non‐photic cue. These results show that the IGL is a necessary component of the neural pathways mediating both arousal‐ and benzodiazepine‐induced non‐photic entrainment.