Inhibitory effect of norepinephrine on the single‐unit activity of caudally projecting paraventricular neurons

The role of norepinephrine (NE) in controlling the single‐unit activity of paraventricular (PVN) neurons projecting to or passing through the caudal ventrolateral medulla (CVLM) was investigated in adult male rats anesthetized with urethane. Of 72 PVN neurons studied, 19 were antidromically activated by CVLM stimulation (Group I) and 48 were antidromically activated by posterior pituitary (PP) stimulation (Group II). The remaining 5 neurons were antidromically driven by both CVLM and PP stimulation (Group III). In 14 of the 19 Group I neurons and in all the 5 Group III neurons, iontophoretically applied NE was demonstrated to inhibitory to the single‐unit activity. No excitatory effect of NE was observed. In contrast, both excitatory and inhibitory actions of NE were observed in the Group II neurons. Of 37 Group II neurons tested, 28 were excited and 7 were inhibited by NE. The inhibitory effect of NE in Group I and Group III neurons was selectively blocked by the alpha antagonist, phentolamine, that was coiontophoresed with NE, but not by the beta antagonist, timolol (n = 9). The unit activity of Group I neurons that were inhibited by NE was not altered by an increase in arterial blood pressure (n = 3), whereas the unit activity of one NE‐insensitive Group I neuron was decreased by an increase in blood pressure. Taken together, the results suggest that NE plays an alpha‐adrenoreceptor‐mediated inhibitory role in controlling the single‐unit activity of caudally projecting PVN neurons. These neurons include a subpopulation of PVN neurons that project caudally as well as to the PP. The possible function associated with the NE‐sensitive, caudally projecting PVN neurons may be other than the regulation of blood pressure.