Patterned afferent activity and synaptic plasticity in the magnocellular neurosecretory system

The organum vasculosum of the lamina terminalis (OVLT) is a central osmoreceptor which regulates neurohypophysial hormone release through glutamatergic synapses onto hypothalamic magnocellular neurosecretory cells (MNC's). Previous studies have shown that OVLT neurons can express low threshold spikes which, in other neurons, contribute to rhythmic firing. Electrophysiological recordings were therefore obtained in the medial OVLT in superfused explants of rat hypothalamus, which was found to contain neurons projecting to MNC's in the supraoptic nucleus. The OVLT neurons recorded revealed the presence of tonic firing at varying frequencies (0-40 Hz), as well as two distinct forms of bursting activity. Among these, clustered firing featured the highest frequencies (up to 100 Hz), and was observed in 31% of the neurons studied. The effects of two conditioning protocols on synaptic function were investigated by examining excitatory postsynaptic potentials (EPSP's) in MNC's during electrical stimulation of the OVLT. Trains of 100 Hz stimulation lasting 1 s were found to induce post-tetanic potentiation and long lasting potentiation ($>$1 hr). Patterned stimulation mimicking clustered firing evoked only short-term ($<$30 s) increases in EPSP amplitude capable of increasing the probability of spike discharge in MNC's. Activity patterns adopted by OVLT neurons may therefore be important for the regulation of neurohypophysial hormone release and body fluid homeostasis in vivo