Response of phasically firing paraventricular neurons to A1 noradrenergic region stimulation and its attenuation by adrenoceptor antagonists.

In urethane-anesthetized rats electrical stimulation of the A1 noradrenergic region in the ventrolateral medulla produced excitatory (27%), excitatory-inhibitory (24%), or inhibitory (34%) responses of spontaneous activity of phasically firing units in the paraventricular nucleus (PVN) of the hypothalamus. The latency of excitatory responses was significantly shorter in those followed by inhibition, suggesting the existence of two distinct excitatory pathways. The latency of inhibitory responses, however, was not different between primary and postexcitatory inhibitory responses. Effects of alpha- and beta-adrenoceptor antagonists on responses of phasically firing PVN units to stimulation of the A1 region were tested. The excitatory responses that were not followed by inhibition were significantly attenuated by intraperitoneal phenoxybenzamine, whereas the other responses were not affected. Both primary and postexcitatory inhibitory responses were significantly attenuated by intravenous propranolol, whereas neither type of excitatory response was affected. These results suggest that the medullary (A1 region) inputs to phasically firing PVN units are mediated by alpha-adrenergic (excitatory), beta-adrenergic (inhibitory), and nonaminergic (excitatory) pathways.