Re-evaluation of the effects of norepinephrine on the single-unit activity of paraventricular neurosecretory neurons.

Electrical stimulation of the A1 noradrenergic region elicited predominantly an excitatory orthodromic response from paraventricular neurosecretory neurons recorded in urethane-anesthetized male rats. In a majority of cases, locally applied norepinephrine (NE) as well as A1 region stimulation excited paraventricular neurosecretory neurons. However, in some neurons including a subpopulation of putative vasopressin neurons, iontophoresed NE was inhibitory. Both excitatory and inhibitory effects of NE were selectively blocked by the alpha-antagonist, phentolamine, but not by the beta-antagonist, timolol. In general, the results support the notion that the A1 noradrenergic input to paraventricular neurosecretory neurons is excitatory. Nevertheless, the results do not exclude the possibility that NE exerts a specific, alpha-receptor-mediated, inhibitory effect on a subpopulation of paraventricular neurons.