Stress-induced neuronal hypertrophy decreases the intrinsic excitability in stress habituation
A rapid activation of the hypothalamic-pituitary-adrenal (HPA) axis is a hallmark stress response to an imminent threat, but its chronic activation can be detrimental. Thus, the long-term survival of animals requires experience-dependent fine-tuning of the stress response. However, the cellular mechanisms underlying the ability to decrease the stress responsiveness of the HPA axis remain largely unsolved. Using a stress habituation model in male mice and slice patch-clamp electrophysiology, we studied hypothalamic corticotropin-releasing hormone neurons that form the apex of the HPA axis. We found that the intrinsic excitability of these neurons substantially decreased after daily repeated restraint stress in a time course that coincided with their loss of stress responsiveness in vivo. This plasticity of intrinsic excitability co-developed with an expansion of surface membrane area, resulting in an increase in input conductance with minimal changes in conductance density. Moreover, multi-photon and electron microcopy data found that repeated stress augmented ruffling of the plasma membrane, suggesting an ultrastructural plasticity that efficiently accommodates membrane area expansion with proportionally less expansion of gross cell volume. Overall, we report a novel structure-function relationship for intrinsic excitability plasticity that correlates with habituation of the neuroendocrine stress response.\n\nSignificance statementThe long-term survival of animals requires experience-dependent fine-tuning of stress response. Using a mouse model of repeated stress that develops habituation of the hypothalamic-pituitary-adrenal (HPA) axis, our study demonstrates a robust decrease in the intrinsic excitability of the output neuroendocrine neurons of the HPA axis. Mechanistically, we show that repeated stress increases the cell size of these neurons (i.e. surface membrane area). This cell-size change increases input conductance, and hence decreases excitability. Our findings challenge a conventional view that plasticity of intrinsic excitability relies on changes on membrane excitability resulting from up- and down-regulation of various voltage-gated ion channels. Our study reports a novel structure-function relationship for intrinsic excitability plasticity that correlates with habituation of the neuroendocrine stress response.