Transmitter identification of pyloric neurons: electrically coupled neurons use different transmitters

Publisher Website
Google Scholar
Abstract
The neurotransmitters mediating the synaptic interactions among the neurons of the pyloric system of the stomatogastric ganglion (STG) of the lobster, Panulirus internaptus, were examined using a combination of electrophysiological, pharmacological and biochemical techniques. Iontophoretically applied L-glutamate inhibited all motor neurons of the pyloric system. This inhibitory response was blocked by low concentrations of picrotoxin but unaffected by atropine. The anterior burster (AB) interneuron, pyloric dilator (PD) motor neurons and ventricular dilator (VD) motor neuron were depolarized and excited by iontophoretically applied acetylcholine (ACh). The lateral pyloric (LP) and pyloric (PY) constrictor motor neurons were inhibited by ACh and by the cholinergic agonist, carbachol. These inhibitory cholinergic responses were blocked by atropine but not by picrotoxin. The inhibitory postsynaptic potentials (IPSP) evoked by the constrictor motor neurons were blocked by picrotoxin but not by atropine. The constrictor motor neurons release glutamate at both their excitatory neuromuscular junctions and their inhibitory intraganglionic junctions. The lucifer yellow photoinactivation technique was used to study the neurotransmitters released by the electrically coupled PD and AB neurons. The AB-evoked IPSP were blocked by picrotoxin but not by atropine. The PD-evoked IPSP were blocked by atropine and other muscarinic antagonists but not by picrotoxin. Somata of PD neurons contained choline acetyltransferase (CAT) activity, but somata of AB neurons contained no detectable CAT activity. The PD neurons release ACh at both their excitatory neuromuscular junctions and their inhibitory intraganglionic connections. Although the AB neuron is electrically coupled to the PD neurons, the AB neuron is not cholinergic. Glutamate is a likely transmitter candidate for the AB neuron. Electrically coupled neurons can release different transmitters. An IPSP can be the result of the combined actions of 2 different neurotransmitters, each released from a different neuron.