Action potential-triggered somatic exocytosis in mesencephalic trigeminal nucleus neurons in rat brain slices

The neurons in the mesencephalic trigeminal nucleus (MeV) play essential roles in proprioceptive sensation of the face and oral cavity. The somata of MeV neurons are generally assumed to carry out neuronal functions but not to play a direct role in synaptic transmission. Using whole-cell recording and membrane capacitance (C _m) measurements, we found that the somata of MeV neurons underwent robust exocytosis (C _m jumps) upon depolarization and with the normal firing of action potentials in brain slices. Both removing [Ca ²⁺] _o and buffering [Ca ²⁺] _i with BAPTA blocked this exocytosis, indicating that it was completely Ca ²⁺ dependent. In addition, an electron microscopic study showed synaptic-like vesicles approximated to the plasma membrane in somata. There was a single Ca ²⁺-dependent releasable vesicle pool with a peak release rate of 1912 fF s ^-1. Importantly, following depolarization-induced somatic exocytosis, GABA-mediated postsynaptic currents were transiently reduced by 31%, suggesting that the somatic vesicular release had a retrograde effect on afferent GABAergic transmission. These results provide strong evidence that the somata of MeV neurons undergo robust somatic secretion and may play a crucial role in bidirectional communication between somata and their synaptic inputs in the central nervous system.