Modulatory role of alpha adrenoceptors on the release of [3H]norepinephrine elicited by preganglionic stimulation of the cat superior cervical ganglion.

In the isolated cat superior cervical ganglion labeled in vitro with [3H]norepinephrine ([3H]NE), the overflow of radioactivity evoked by preganglionic stimulation at 10 Hz (80 V, 2 msec duration for 5 min) was reduced to 50% of control values by the alpha adrenoceptor agonists clonidine (0.001 microM) and methoxamine (12.0 microM). The alpha adrenoceptor antagonist phenoxybenzamine (2.9 microM) produced a 2-fold increase in the overflow of [3H]NE elicited by nerve stimulation. Preincubation with drugs that reduce the neuronal uptake of norepinephrine in the isolated ganglion (8.8 microM cocaine and 0.33 microM desipramine) did not modify the release of [3H]NE by preganglionic stimulation. However, a higher concentration of desipramine (3.3 microM) produced a 4-fold increase in the overflow of tritium evoked by stimulation. As this concentration of desipramine produced a shift to the right in the concentration-response curve to methoxamine in the isolated nictitating membrane of the cat, the conclusion is drawn that a feedback mechanism mediated through presynaptic alpha adrenoceptors regulates the release of [3H]NE induced by preganglionic stimulation of the cat superior cervical ganglion. In addition, it is suggested that regulatory mechanisms for norepinephrine release by nerve stimulation are not restricted to nerve terminals but are also present in dendrites of the postganglionic adrenergic neurons.