Intracellular regulation of neuronal nicotinic cholinoreceptors.

Experiments on isolated superior cervical ganglia from rats were used to study the effects of substances affecting intracellular second messengers on membrane currents evoked by iontophoretic application of acetylcholine (ACh currents) and on excitatory postsynaptic currents (EPSC) induced by single discharges of preganglionic nerve fibers. These studies showed that the adenylate cyclase activator forskolin, the phosphodiesterase inhibitor isobutylmethylxanthine (IMBX), and the protein kinase C activator phorbol ester decreased the amplitude of the ACh current. Neither IMBX nor phorbol ester had any effect on the amplitude or decay time constant of EPSC, while forskolin increased the amplitude of EPSC without altering its decay time constant. Thapsigargin, which liberates intracellular calcium, not only decreased the amplitude of the ACh current, but also decreased EPSC amplitude without affecting its decay time constant. These results suggest that intracellular signaling via protein kinases A and C may affect neuronal nicotinic cholinoceptors (nAChR) only by altering receptor desensitization and not affecting receptor sensitivity to transmitters released from nerves or the kinetics of receptor ion channels. At the same time, neuronal nAChR are influenced by intracellular calcium, which decreases their ability to be activated by exogenous (perhaps acting via desensitization) and nerve-released acetylcholine without affecting the kinetics of ion channel function.