Second-Scale Neural Dynamics Shape Hormonal Outputs in Hypothalamic CRH Neurons
The elevation of glucocorticoids is a hallmark of stress, arising from the integration of rapid neuronal signals into sustained hormonal outputs. A key interface for this neuroendocrine signal translation is corticotropin releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN), which release CRH at the median eminence (ME). We recently discovered that CRHPVN neurons exhibit a characteristic shift in firing patterns from rhythmic short-bursts (low-activity state) to tonic firing (high-activity state) in response to stress. This raised a critical question: how are these distinct firing patterns are integrated into slower, sustained hormonal outputs at neuroendocrine terminals? Here, we implemented optical approaches to detect CRH release ex vivo and in vivo. Using newly developed sniffer cells for CRH, we measured CRH release at ME ex vivo, triggered by the distinct, in vivo-like firing patterns. Our results demonstrated that the primary determinant of neuroendocrine CRH release was the firing rate sustained over the timescale of seconds, with little contribution from specific firing patterns. These results collaborated with second-scale increase in firing rate triggered by stress stimuli. Additionally, we recorded the dynamics of CRH at the ME in the freely moving mice using genetically-encoded GPCR-activation based (GRAB) sensors for CRH. Foot shock stress triggered transient, time-locked increases in CRH release on the timescale of seconds. Importantly, these second-scale CRH pulses, when elicited during repeated foot shocks, were integrated over minutes to scale downstream hormone releases. Together, our data revealed critical roles of second-scale dynamics in CRHPVN neuron activity for the neuroendocrine translation of stress signals.