Morphine decreases 3H-norepinephrine release and increases endogenous norepinephrine levels in the isolated cat superior cervical ganglion.

The isolated cat superior cervical ganglion (SCG) was labeled in vitro with either 3H-norepinephrine (3H-NE) or 3H-choline and stimulated through its preganglionic trunk. The release of 3H-NE and 3H-acetylcholine (3H-ACh) elicited by the stimulation was measured under control conditions and in the presence of drugs. The incubation during 30 min with 10 microM morphine lead to a 70% decrease in the amount of 3H-NE released in response to the preganglionic stimulation (10 Hz, 80 V, during 5 min). No further decrease in 3H-NE release was produced by a 10 times higher concentration of morphine. The reduction in 3H-NE release caused by morphine was coincident with a 60% increase in the endogenous content of NE. Both effects of morphine were entirely prevented by an antagonist of opioid receptors, 1.0 microM naltrexone. The opioid antagonist did not modify by itself either the stimulation-induced release of 3H-NE or the endogenous content of NE. The basal efflux of 3H-NE was not altered by morphine. In ganglia labeled with 3H-choline, morphine (10 and 100 microM) did not modify either the basal efflux of 3H-ACh or the release of 3H-ACh evoked by stimulation of the preganglionic trunk (5 Hz, 40 V, during 5 min). These observations suggest that in the cat SCG morphine has a direct action on the dendrites of the postganglionic neuron which store and release NE. The effects of morphine in vitro on 3H-NE release and on the tissue levels of NE may be mediated through the interaction with dendritic opioid receptors.