Spike and Neuropeptide-Dependent Mechanisms Control GnRH Neuron Nerve Terminal Ca2+ over Diverse Time Scales.

Fast cell-to-cell communication in the brain is achieved by action potential-dependent synaptic release of neurotransmitters. The fast kinetics of transmitter release are determined by transient Ca2+ elevations in presynaptic nerve terminals. Neuromodulators have previously been shown to regulate transmitter release by inhibiting presynaptic Ca2+ influx. Few studies to date have demonstrated the opposite, that is, neuromodulators directly driving presynaptic Ca2+ rises and increases in nerve terminal excitability. Here we use GCaMP Ca2+ imaging in brain slices from mice to address how nerve terminal Ca2+ is controlled in gonadotropin-releasing hormone (GnRH) neurons via action potentials and neuromodulators. Single spikes and bursts of action potentials evoked fast, voltage-gated Ca2+ channel-dependent Ca2+ elevations. In contrast, brief exposure to the neuropeptide kisspeptin-evoked long-lasting Ca2+ plateaus that persisted for tens of minutes. Neuropeptide-mediated Ca2+ elevations were independent of action potentials, requiring Ca2+ entry via voltage-gated Ca2+ channels and transient receptor potential channels in addition to release from intracellular store mechanisms. Together, these data reveal that neuromodulators can exert powerful and long-lasting regulation of nerve terminal Ca2+ independently from actions at the soma. Thus, GnRH nerve terminal function is controlled over disparate timescales via both classical spike-dependent and nonclassical neuropeptide-dependent mechanisms.SIGNIFICANCE STATEMENT Nerve terminals are highly specialized regions of a neuron where neurotransmitters and neurohormones are released. Many neuroendocrine neurons release neurohormones in long-duration bursts of secretion. To understand how this is achieved, we have performed live Ca2+ imaging in the nerve terminals of gonadotropin-releasing hormone neurons. We find that bursts of action potentials and local neuropeptide signals are both capable of evoking large increases in nerve terminal Ca2+ Increases in Ca2+ driven by spike bursts last seconds; however, the increases in nerve terminal Ca2+ driven by neuropeptides can persist for tens of minutes. These findings reveal new mechanisms by which neuroendocrine nerve terminal Ca2+ can be controlled in the brain.