Serotonin potentiates presynaptic calcium currents and transmitter release at cortical synapses.

Synaptic strength is dynamically modulated not only by activity-dependent processes but also by neuromodulators. These modulators can enhance or depress synaptic transmission and play essential roles in circuit function and in the pathophysiology of psychiatric disorders. Although neuromodulation is functionally important in vivo, the underlying cellular mechanisms-particularly at the presynaptic site-remain poorly understood in mammalian synapses because of the difficulty in obtaining direct access. Serotonin is a neuromodulator implicated in learning and memory, mood regulation, anxiety, and psychiatric disorders. In this study, direct presynaptic patch-clamp recordings from hippocampal mossy fiber terminals were used to examine the mechanism of presynaptic potentiation by serotonin. Serotonin potentiated both calcium currents and synaptic vesicle exocytosis, as measured by presynaptic capacitance recordings. Kinetic analysis and pharmacology of presynaptic calcium currents revealed that serotonin potentiates P/Q-type calcium channels through protein kinase A-dependent mechanisms. Simultaneous voltage-clamp recordings from pre- and postsynaptic compartments demonstrated that both calcium channel potentiation and modulation of the release machinery contribute to the enhancement of transmitter release. These findings suggest mechanistic similarities and differences between activity-dependent homosynaptic plasticity and heterosynaptic neuromodulation, suggesting complex modulation of mossy fiber synapses under diverse physiological contexts.