Evidence for non-cholinergic, non-adrenergic transmission in the guinea-pig ileum

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Abstract
To elucidate the innervation of the longitudinal and circular muscle cells of the guinea-pig ileum, junction potentials elicited by field stimulation were recorded from both muscle layers in the presence and absence of atropine (1-2 .mu.M) with guanethidine (10 .mu.M) at 36.degree. C. In longitudinal muscle cells, a single stimulus elicited an atropine-sensitive transient depolarization (cholinergic ejp [excitatory junctional potential]), whereas in circular muscle cells, a transient hyperpolarization (non-cholinergic [non-ch] non-adrenergic [non-adr] ijp [inhibitory junctional potential]) was elicited. With stimulation of nerves in the presence of atropine and guanethidine, responses of circular muscle cells to nerve stimulation were preserved while the longitudinal muscle cells generated depolarization (non-ch., non-adr. ejp) hyperpolarization (non-ch., non-adr. ijp) or both depolarization and subsequent hyperpolarization. These potential changes ceased with application of TTX [tetrodotoxin] or excess Mg2+. The latency for junction potentials recorded from longitudinal muscles after field stimulation was in the following order; non-ch., non-adr. ejp < cholinergic ejp < non-ch., non-adr. ijp. The cholinergic ejp had a lower, and non-ch., non-adr. ijp a higher threshold. At low frequencies of stimulation (below 1 Hz) amplitudes of successively generated cholinergic ejp and non-ch., non-adr. ijp were gradually reduced, but at higher frequencies (2-20 Hz) of stimulation they were summated. The amplitudes of non-ch., non-adr. ejp were not affected at low frequencies of stimulation (up to 0.5 Hz) and were summated at higher frequencies (> 1 Hz) of stimulation. Reversal potentials for non-ch., non-adr. ejp and ijp estimated from the amplitude of junction potentials and membrane potential were -27 and -80 mV, respectively. The reversal potential for non-ch., non-adr. ejp was higher (more negative) than that for cholinergic ejp. Generation of ijp in longitudinal muscle cells elicited by repetitive stimulation was followed by a rebound depolarization on which was superimposed a burst of spikes. During repetitive stimulation, the amplitude of hyperpolarization was gradually reduced but the rebound depolarization and spike discharge were enhanced. While the rebound depolarization was related to the amplitude of the preceding hyperpolarization it was more related to the duration of the stimulation. Longitudinal muscles of the guinea-pig ileum show evidence of having in addition to cholinergic and adrenergic innervation, both excitatory and inhibitory non-ch., non-adr. innervation. In the longitudinal muscle layer of the ileum, the non-ch., non-adr. excitatory fibers are more densely distributed in the terminal rather than in the proximal region, while in the case of non-ch., non-adr. inhibitory fibers, the distribution is reversed. Circular muscle cells are homogenously innervated by non-ch., non-adr. inhibitory nerves.