Zheng, S.; Focke, C. M. B.; Young, C. K.; Tripp, I.; Ganeshan, D.; Power, E.; Schwenke, D. O.; Herbison, A.; Kim, J. S.; Iremonger, K. J.

The stress axis is always active, even in the absence of any threat. This manifests as hourly pulses of corticosteroid stress hormone secretion over the day. Corticotropin-releasing hormone neurons in the paraventricular nucleus of the hypothalamus (CRHPVN) control both the neuroendocrine stress axis as well as stress-associated behaviours. However, it is currently unclear how the resting activity of these neurons is coordinated with both spontaneous behaviour and ultradian pulses of corticosteroid secretion. To investigate this, we performed fiber photometry recordings of CRHPVN neuron activity in Crh-Ires-Cre mice and a newly generated line of Crh-Ires-Cre rats. In both mice and rats, CRHPVN neurons displayed an ultradian rhythm of activity with reoccurring upstates of activity approximately once per hour over the 24-hour day. Upstates in activity were coordinated with increases in animal activity/arousal. Chemogenetic activation of CRHPVN neurons was also sufficient to induce behavioural arousal. In rats, increases in CRH neural activity preceded some pulses of corticosteroid secretion but not others. Thus, while CRHPVN neurons display an ultradian rhythm of activity over the 24-hour day that is coordinated with behavioural arousal, the relationship between CRHPVN activity and pulses of corticosteroid secretion is not one-to-one.