Morphine increases locus coeruleus noradrenergic neuronal activity in vitro

Morphine produced a dose-dependent increase in the activity of norepinephrine-containing locus coeruleus neurons recorded from mouse brain slices in vitro. The response was not changed in a low calcium-high magnesium incubation medium, indicating that the observed effects are the result of a direct action of morphine on locus coeruleus neurons. When the mice were anesthetized with chloral hydrate prior to preparation of tissue slices, morphine produced a dose-dependent suppression of noradrenergic neuronal activity. These studies suggest that previous data showing a decrease in activity of locus coeruleus neurons following incubation with morphine in vitro are due to the use of anesthesia.