Effect of noradrenergic drugs on response of posterior hypothalamic neurons to stimulation of the locus coeruleus (LC) and the central nucleus of amygdala (ACE) in rats.

Abstract

Physiological and pharmacological natures of neuronal responses in posterior hypothalamus (HPA) elicited by stimulation of LC and ACE were investigated in rats. Most extracellularly recorded HPA neurons (186 units) remained unaffected by LC stimulation: 4 showed spike discharges at latencies of 5 to 16 msec, 1 gave rise to an antidromic spike at latencies of 3 to 4 msec, 22 showed inhibition extending from 12 to 40 msec after LC stimulation. When applied iontophoretically, phenylephrine slightly intensified the inhibition by LC stimulation, E-643 partly blocked the inhibition with lower ejection currents and increased it with higher ejection currents, Clonidine intensified the inhibition for the most part, and other three drugs, isoproterenol, dopamine and acetylcholine, hardly produced any effect on inhibition. In 4 of 22 units which snowed inhibition to LC stimulation, conditioning stimulation of LC failed to produce any effect on discharge patterns of HPA neurons elicited by ACE stimulation. Only in 1 unit, however, it produced a significant reduction in latency and spike number. An iontophoretic application of the above-mentioned 6 drugs did not mimic the inhibitory effect of conditioning stimulation of LC on the discharge patterns elicited by ACE stimulation. It is likely that an ascending NA pathway from LC to HPA exerts some inhibitory effect on the input from ACE to HPA, but the pharmacological nature of inhibition remains to be clarified.