Circadian rhythms are endogenous biological clocks that regulate physiology and behaviors, such as food intake, and are synchronized by the environmental light/dark cycle. The nucleus of the solitary tract (NTS) receives excitatory glutamatergic inputs from vagal afferent neurons that innervate the gastrointestinal tract and are sensitive to the gut peptide cholecystokinin (CCK), which is released following food intake to promote satiation. Previously, we observed that NTS membrane properties, neurotransmission, and action potential firings were all under circadian control. Although it is well established that the food intake varies with the light/dark cycle, circadian variations in the cellular actions of CCK on vagal afferent neurons remain unknown. Here, we test the extent to which CCK signaling on vagal afferents and the NTS changes as a function of time of day. We used RT-qPCR and functional cellular measurements to measure gene expression and responses to CCK across the time of day in rats. Although we confirmed the presence of rhythmic clock gene expression in vagal afferent neurons, we found that CCK1 receptors did not show diurnal rhythmicity. We also observed that CCK-induced calcium responses were consistent during the day and night in dissociated vagal afferent neurons. Similarly, CCK-driven increases in spontaneous glutamate release were also constant across the light cycle at the vagal afferent-NTS synapse. We conclude that vagal afferent CCK signaling remains constant across the light cycle, and CCK1 receptors provide a consistent point of reference independent of the time of day.NEW & NOTEWORTHY Daily rhythms govern feeding and the processes of satiation conveyed by cholecystokinin acting on vagal afferent neurons. Yet, the putative changes in efficacy and direct cellular effects of CCK on vagal afferents across the time of day remain unknown. Here, we report constant CCK1 receptor expression and signaling in vagal afferents across the light cycle. This model indicates a fixed point of reference for CCK signaling in parallel to documented circadian changes in feeding neurocircuitry.
Keywords: TTFL; calcium; circadian; nucleus of the solitary tract; synaptic transmission.