Enkephalin and substance P modulate synaptic properties of chick ciliary ganglion neurons in cell culture

Abstract

Since enkephalin- and substance P-like immunoreactive materials have been identified in preganglionic terminals of the avian ciliary ganglion, we tested the effects of enkephalin and substance P directly on chick ciliary ganglion neurons in dissociated cell culture. Under these conditions the neurons form cholinergic synapses with each other that are spontaneously active. Both peptides modulate properties of membrane components associated with synaptic transmission between the neurons. Enkephalin causes a 60% reduction in the mean amplitude of the excitatory synaptic potentials, and the effect appears to be presynaptic in origin: enkephalin does not alter acetylcholine sensitivity on the neurons, but does inhibit Ca2+ influx as reflected by a 38% shortening of the Ca2+ component of the action potential. Both the reduction in synaptic potential amplitude and the shortening of the Ca2+ action potential produced by enkephalin are blocked by naloxone. Substance P, on the other hand, has no effect on Ca2+ action potentials but does reduce the time course of acetylcholine responses in the neurons by a mechanism consistent with enhanced receptor desensitization. Decay of the acetylcholine voltage response in the absence of substance P is described by a single exponential process with a time constant of 4-5 s. Coapplication of acetylcholine and substance P results in a second exponential decay process with a time constant of about 1 s that appears after a 200-400 ms lag period. Preincubation with substance P alone does not decrease the peak voltage response or shorten the lag, suggesting that either agonist or activated receptor is necessary for the substance P effect. These findings suggest modulatory roles for the peptides in ganglionic transmission.