Unit activity indicators of a catecholamine role in expression of morphine effects

1. We tested the hypothesis that morphine effect on unit activity in certain limbic system areas is primarily dependent on morphine's action on noradrenergic function. Unit activity (UA) was recorded in two limbic areas rich in noradrenergic projections and in a control area (caudate). We compared the influence of morphine on the UA of these areas in intact rats and in rats that were lesioned with intraventricular 6-OH dopamine to destroy adrenergic input. 2. The results showed that: a) morphine alone depressed UA in the caudate, while causing a mild increase in the hippocampus and cingulate gyrus, b) lesioning of animals with 6-OH dopamine resulted in suppressed morphine response in the noradrenergically innervated limbic areas, and c) the caudate was depressed similarly by morphine whether or not there was 6-OH dopamine lesioning. 3. Thus we conclude that the full expression of morphine effects in these two limbic areas seems to require intact noradrenergic input.