Neurons dissociated from rat myenteric plexus retain differentiated properties when grown in cell culture. III. Synaptic interactions and modulatory effects of neurotransmitter candidates.

We have used intracellular recordings to study synaptic interactions between myenteric neurons grown in dissociated cell culture. Intracellular stimulation of individual myenteric neurons caused several types of synaptic effects in nearby neurons: (1) fast excitatory synaptic potentials mediated by nicotinic acetylcholine receptors; (2) slow, non-cholinergic synaptic potentials; (3) dual transmission having both fast cholinergic and slow non-cholinergic components and (4) inhibition of spontaneously occurring fast nicotinic synaptic potentials. Fast nicotinic synaptic potentials were elicited by about 40% of neurons tested and often occurred spontaneously. The fast synaptic potentials were similar to those that have been studied in other autonomic neurons with respect to their estimated reversal potential and their sensitivity to cholinergic antagonists. The amplitudes of the fast synaptic potentials declined if evoked at frequencies greater than 0.5 Hz. Potentiation of the fast synaptic potentials was observed following high-frequency stimulation of presynaptic neurons. Several transmitter candidates modulated fast cholinergic transmission. Substance P and vasoactive intestinal peptide promoted nicotinic transmission by causing increased amplitudes of evoked and spontaneous fast synaptic potentials and an increased frequency of spontaneous synaptic potentials. γ-Aminobutyrate and [Met]enkephalin both caused decreased amplitudes and frequency of nicotinic synaptic potentials. Serotonin depressed synaptic potentials in some neurons while enhancing them or having no effect in others. Slow, non-cholinergic, synaptic potentials were elicited by about 10% of neurons tested. These synaptic effects lasted 15–300 s, caused depolarizations of 3–15 mv and were accompanied by increased neuronal input resistance. The transmitter(s) causing these slow synaptic potentials has not yet been identified. When stimulated at 10–20 Hz, a small proportion of cholinergic neurons caused slow non-cholinergic synaptic potentials in the same postsynaptic cells in which they evoked fast nicotinic potentials. These 5–12-mv depolarizations lasted tens of seconds and were not reduced even by very high concentrations of both nicotinic and muscarinic antagonists. The transmitters causing them have not yet been identified. The only type of inhibitory interaction observed between the cultured neurons was an indirect one. When stimulated at 5–20 Hz, about 5% of cells tested caused a diminution or cessation of spontaneously occurring nicotinic synaptic potentials in nearby neurons. We conclude that myenteric neurons grown in dissociated cell cultures can form several types of synaptic connections. These synapses share many properties with those that have been observed in myenteric neurons in vivo, including the ability to be modulated by enteric neurotransmitter candidates. Thus dissociated cell cultures of myenteric neurons will permit detailed biophysical and pharmacological studies of enteric neurotransmission and of its modulation.