A FOREBRAIN CALYX-OF-HELD–LIKE SYNAPSE INTEGRATES FAST SYNAPTIC DRIVE AND SLOW MODULATION FOR VISCERAL STATE CONTROL

How fast ionotropic synaptic transmission and slow neuromodulatory signaling are structurally integrated to enable dynamic control of internal state remains largely unknown. Here, we report the discovery of a calyx-of-Held–like synapse in the forebrain, revealing a novel circuit mechanism that unifies fast and slow neurotransmission within a single synaptic structure. Using confocal microscopy, RNAscope multiplex in situ hybridization, electrophysiology, viral tracing, juxtacellular labeling, and correlated ultrastructural approaches, including focused ion beam scanning electron microscopy and freeze-fracture replica immunolabeling, we identified a giant axo-somatic terminal in the extended amygdala. This synapse originates from a discrete population of PACAP-expressing neurons in the pontine Kolliker-Fuse nucleus and selectively envelops PKCdelta/GluD1 neurons in the capsular central amygdala and the oval bed nucleus of the stria terminalis. Within a single presynaptic element, the terminal co-packages glutamate, acetylcholine, and multiple neuropeptides, forming multiple synaptic specializations reminiscent of the classical auditory calyx of Held. To assess circuit engagement, we examined neuronal activation during acute hypotension. Fos mapping revealed rapid recruitment of the Kolliker-Fuse-extended amygdala pathway during the early compensatory phase, preceding delayed activation of hypothalamic magnocellular neuroendocrine neurons. Most early-activated amygdala neurons were directly contacted by PACAP⁺/VGluT1⁺ calyceal terminals (image), linking this synapse to fast autonomic responses, while slower endocrine mechanisms emerged later. Together, these findings identify a previously unrecognized forebrain calyx-like synapse that implements dual-time-scale signaling, providing a structural and functional framework for integrating rapid synaptic drive with slower modulatory control in limbic–autonomic circuits.