A hypothalamic circuit for circadian regulation of corticosterone secretion

The secretion of cortisol in humans and corticosterone (Cort) in rodents follows a daily rhythm which is important in readying the individual for daily activity. This rhythm is orchestrated by the suprachiasmatic nucleus (SCN), but how it ultimately regulates the circadian rhythm of activity of neurons in the paraventricular nucleus of the hypothalamus that produce corticotropin-releasing hormone (PVH^CRH neurons) is not known. We hypothesized that the SCN may exert this influence by projections to the subparaventricular zone (SPZ), which in turn innervates neurons in the dorsomedial nucleus of the hypothalamus (DMH) that regulate PVH^CRH neurons. First, we found that ablating SPZ^Vgat neurons eliminates the circadian rhythm of Cort secretion, but that deleting Vgat from them does not, suggesting that they predominantly use some other transmitter. Next, we found that either ablating or acutely inhibiting the DMH glutamatergic (DMH^Vglut2) neurons resulted in a 40-70% reduction in the daily peak of Cort. Deletion of the Vglut2 gene within the DMH produced a similar effect, highlighting the indispensable role of glutamatergic signaling. Chemogenetic stimulation of DMH^Vglut2 neurons led to an increase of Cort levels, and optogenetic activation of their terminals in the PVH in hypothalamic slices directly activated PVH^CRH neurons through glutamate action on AMPA receptors (the DMH^Vglut2 → PVH^CRH pathway). Similar to the disruption of DMH^Vglut2 neurons, ablating, inhibiting, or disrupting GABA transmission by DMH GABAergic (DMH^Vgat) neurons diminished the circadian peak of Cort, particularly under constant darkness conditions. Chemogenetic stimulation of rostral DMH^Vgat neurons increased Cort, although with a lower magnitude compared to DMH^Vglut2 neuron stimulation, suggesting a role in disinhibiting PVH^CRH neurons. Supporting this hypothesis, we found that rostral DMH^Vgat neurons project directly to GABAergic neurons in the caudal ventral part of the PVH and adjacent peri-PVH area (cvPVH), which directly inhibit PVH^CRH neurons, and that activating the rostral DMH^Vgat terminals in the cvPVH in brain slices reduced GABAergic afferent input onto the PVH^CRH neurons. Finally, ablation of cvPVH^Vgat neurons resulted in increased Cort release at the onset of the active phase, affirming the pivotal role of the DMH^Vgat → cvPVH^Vgat → PVH^CRH pathway in Cort secretion. In summary, our study delineates two parallel pathways transmitting temporal information to PVH^CRH neurons, collectively orchestrating the daily surge in Cort in anticipation of the active phase. These findings are crucial to understand the neural circuits regulating Cort secretion, shedding light on the mechanisms governing this physiological process and the coordinated interplay between the SCN, SPZ, DMH, and PVH.