Evidence for activation of cyclic AMP in a bulbospinal noradrenergic pathway.

Previous studies indicate that sympathetic preganglionic neurons (SPGNs) receive reciprocal innervation by two bulbospinal monoaminergic pathways, norepinephrine (NE) pathways being excitatory and 5-hydroxy-tryptamine (5-HT) pathways being inhibitory. Two phosphodiesterase inhibitors were tested to evaluate whether cyclic AMP is generated in SPGNs following excitation by NE pathways. Sympathetic discharges recorded from preganglionic rami were evoked in spinal cats by stimulating descending excitatory pathways in the cervical spinal cord or by stimulating thoracic spinal reflex pathways. Aminophylline and isobutyl-methyl-xanthine rapidly and markedly enhanced transmission through the intraspinal pathway. Enhancement was not significantly altered by depletion of 5-HT. Aminophylline also enhanced transmission through the spinal reflex pathway, but the onset of enhancement and the time to maximal enhancement were significantly delayed. The results support the proposal that cyclic AMP is generated in SPGNs in response to NE. Activation of cyclic AMP by spinal reflex pathways appears to be mediated by a different mechanism.