Dynamic synapses in the hypothalamic-neurohypophyseal system.

The release of vasopressin and oxytocin from the posterior pituitary is tightly coupled to the activity of magnocellular neurosecretory cell (MNC) bodies in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. These cell groups exhibit distinct patterns of activity which are regulated by a combination of synaptic inputs and intrinsic properties. The postsynaptic currents (intrinsic properties) that shape these bursts have been described extensively but examinations of the contributions of synaptic input to activity patterns in these cells are relatively sparse. Although the synaptic release of glutamate is necessary to initiate and sustain bursting, precisely how a brief depolarization associated with a synaptic current would ignite such a prolonged postsynaptic discharge is not clear. Here, we review recent work from our laboratory showing that unlike the majority of synapses in the brain, glutamate synapses onto MNCs release transmitter in an asynchronous fashion following a presynaptic action potential. This input is integrated by the postsynaptic neuron and may serve to activate the postsynaptic conductances necessary for the induction of patterned activity.