Facilitatory influence of noradrenergic afferents on the excitability of rat paraventricular nucleus neurosecretory cells.

The role of the A1 and A2 noradrenergic cell groups of the caudal medulla in regulating the activity of paraventricular nucleus neurosecretory cells was examined with electrophysiological methods in anaesthetized male Sprague-Dawley rats. Antidromically identified neurosecretory cells were classified as vasopressin or oxytocin secreting on the basis of spontaneous firing patterns and responsivity to baroreceptor activation. The effect on cell firing of single pulses (25-200 microA) delivered to either the A1 or A2 cell group areas was then examined using peri-stimulus histograms. Stimulation of the A1 region enhanced the activity of 78% of putative vasopressin-secreting neurones tested (n = 18), but failed to affect the activity of the majority (73%) of putative oxytocin-secreting units (n = 15). A2 stimulation enhanced the firing rate of both putative vasopressin- (60%, total n = 14) and putative oxytocin-secreting (70%, total n = 27) neurones. Destruction of the paraventricular nucleus catecholamine terminal plexus by pre-treatment with the neurotoxin 6-hydroxydopamine abolished the facilitatory effects of both A1 and A2 stimulation. These findings suggest that noradrenergic afferents of medullary origin facilitate the activity of paraventricular nucleus neurosecretory cells. The role of the projection from the A1 cell group appears to differ from that of the A2 group, however, in that its effects are specific to putative vasopressin-secreting units.