A substance P-like peptide in bullfrog autonomic nerve terminals: Anatomy biochemistry and physiology

Abstract

Substance P immunoreactivity was localized to dense-cored vesicles in parasympathetic preganglionic nerve terminals in the bullfrog cardiac ganglion. Analysis with radioimmunoassay and high pressure liquid chromatography indicated that the immunoreactive substance was more similar to substance P than to substance K but was not identical to either. The bullfrog substance P was also shown to be different than kassinin, eledoisin, ranatensin, bombesin, neuromedin K and physalaemin. Although the peptide was present in most of the terminals in the ganglion and appeared to be releasable in a calcium-dependent manner, intracellular recordings from ganglion neurons during stimulation revealed no electrophysiological events that might be mediated by an endogenous peptide. In addition, the direct application of substance P to ganglion neurons generally produced no changes in membrane potential, membrane conductance, somal calcium spikes or nerve-evoked release of acetylcholine. High concentrations of substance P did cause an apparent increase in the rate of desensitization of the nicotinic receptors but there was little indication that this phenomenon would occur under physiological conditions. It is suggested that bullfrog substance P is released as a neurotransmitter but is involved in electrically silent events in the postsynaptic neurons or is acting on non-neuronal cells near the terminals. The possible implications of these results for peptidergic systems in general are discussed.