Iwao Yanagiya

Acetylcholine release from the myenteric plexus of guinea-pig ileum by prostaglandin E1

Release of acetylcholine (ACh) by prostaglandin E1 from the nerve terminals of the guinea‐pig longitudinal muscle strip was studied in order to reveal the effect of PGE1 on myenteric plexus activity. The ACh released was collected in the presence of physostigmine (2·1 μg ml−1) and choline (0·1 μg ml−1) at 38° C. Five to 100 ng ml−1 PGE1 enhanced the release dose‐dependently. The effect was maintained during the presence of PGE1 in the organ bath, while rapid tachyphylaxis was observed with the ACh‐releasing action of nicotine. Tetrodotoxin or morphine almost completely inhibited the effect of PGE1 on ACh release. Hexamethonium, in a concentration which completely blocked the effect of nicotine, partially inhibited the effect of PGE1. In the late phase of nicotine action, the tissue was still sensitive to PGE1 despite the continued exposure to nicotine. These data suggest the presence in the myenteric plexus of PG receptors which can increase ACh release.

Contribution of endogenous prostaglandins to excitation of the myenteric plexus of guinea-pig ileum: Are adrenergic factors involved?

The role of endogenous prostaglandins (PGs) in the nicotine-induced contraction and release of ACh was investigated in the isolated guinea-pig ileum. A low concentration of indomethacin (IND 2.8 microM) inhibited the contraction and ACh release induced by nicotine. These inhibitory effects of IND were reversed by PGE2 at concentrations which are thought to be released spontaneously. SC-19220, a PG receptors antagonist, also inhibited the contraction and ACh release induced by nicotine. Unlike the nicotine-induced release of ACh, the potassium-induced release of ACh was unaffected by IND and SC-19220. IND was as potent in inhibiting the responses to cholinergic nerve stimulation by nicotine after treatment of the preparations with antiadrenergic agents. It is concluded that the inhibitory effect of IND does not depend on the functional integrity of adrenergic neurons and that endogenous PGs contribute directly to the modulation of myenteric plexus excitability by nicotine.

Effects of desensitization to adenosine 5‘‐triphosphate and vasoactive intestinal polypeptide on non‐adrenergic inhibitory responses of longitudinal and circular muscles in the rat ileum

Non-cholinergic, non-adrenergic inhibitory nerves are widely distributed in peripheral tissues, particularly within the gastrointestinal tracts in various species of animals (Holman et al 1965). Although there is an abundance of evidence and observations supporting a role for ATP as an inhibitory transmitter (reviewed by Burnstock 1975) many experimental data argue against such a role for ATP (Ohga & Taneike 1977; Ambach et al 1977; Crema et al 1982). Immunohistochemical techniques have demonstrated the occurrence of several biologically active polypeptides within gastrointestinal nerves, and their localization in the myenteric and submucous plexus (Besson et a1 1978; Hokfelt et all980; Schultzberg et all980). In particular, vasoactive intestinal polypeptide (VIP) has been found to have potent inhibitory effects on a variety of smooth muscles including intestine (Piper et al 1970) and so it has been claimed that VIP, rather than ATP, may function as a non-adrenergic inhibitory neurotransmitter in some tissues (Fahrenkrug et a1 1978; Goyal et a1 1980; Matsuzaki et al 1980). The present experiments were carried out to examine ATP and VIP as possible candidates for the neurotransmitter substance of non-adrenergic inhibitory nerve fibres in rat ileum. Because of the lack of specific and reliable antagonists to ATP (Baer & Frew 1979; Small & Weston 1979; Jenkinson 1981) and VIP (Bolton et al 1981) desensitization to these substances was achieved by means of continuous or repeated exposure of the preparations to the respective drugs. We assumed that desensitization to exogeneous agents may be associated with the simultaneous development of desensitization to the corresponding substances when they are released by nerve excitation.

Effect of methylmercury on brain acetylcholine concentration and turnover in mice

Effects of methylmercury chloride (MMC) on regional acetylcholine (ACh) concentrations and turnover in the mouse brain were studied and compared with those of 3′-chloro-4-stilbazole (CS) and of hemicholinium-3 (HC-3). A long-term treatment with MMC (5 mg Hg/kg/day) induced nervous signs and decreased ACh in striatum and cerebral cortex, and conversion ratios of [14C]ACh in cerebellum, striatum, and cerebral cortex of mice. A single administration of CS (200 mg/kg) decreased ACh in the cortex and the conversion ratios in the three brain regions. An intracerebral injection of HC-3 (100 μg/kg) also decreased ACh and the conversion ratios in striatum and cortex. It is postulated that ACh concentration and ACh turnover rate in brain of methylmercury-poisoned animals may be reduced.

Role of adrenergic alpha-receptors in regulation of acetylcholine release evoked by distension of guinea pig ileum.

The significance of adrenergic nerves in the regulation of acetylcholine (ACh) release evoked by distension of the guinea pig ileum was evaluated by pharmacological manipulations. ACh release was measured by bioassay. Release in response to distension was completely abolished by epinephrine and norepinephrine. Tyramine also suppressed the distension-evoked ACh release, while dopamine was ineffective. The release of ACh in the anal segment, adjacent to the distended part, was abolished by epinephrine and norepinephrine. Dibenamine and phentolamine abolished ACh release anal to the distension, but augmented release orally, while dichloroisoproterenol and propranolol were ineffective. The present results give direct evidence that adrenergic nerves modulate cholinergic transmission in the myenteric plexus through α-receptors.

Changes in cytoplasmic and vesicular acetylcholine content in insect ganglia during tetanic stimulation

Acetylcholine (ACh) content was reduced by about 30 pmol or 20% of the initial ACh content in the cockroach sixth abdominal ganglion in response to prolonged (30 min) tetanic stimulation at 40 Hz of the cercal nerves in the presence of 10−3 M hemicholinium-3 (HC-3). The reduction in ACh content in ganglia occurred in the cytoplasmic rather than the vesicular ACh fraction. The latter showed instead a transient increase followed by a gradual decrease to the previous level. Similar changes in ACh in the fractions were produced also by the stimulation, although the ACh content in ganglia did not change in a calcium-free saline, but was reduced in the presence of 50 μM dantrolene or 1–5 mM cobalt chloride. Synaptic transmission at the cercal nerve-giant nerve fiber synapses rapidly decreased and was abolished within a few minutes during tetanic stimulation at 40 Hz, but recovered on reducing the frequency to 0.1 Hz. The decline in transmission was not affected by HC-3, but a significant delay was observed in the recovery following 30 min of tetanic stimulation in the presence of HC-3. These results may suggest that the depletion of ACh as a functional store occurs in the cytoplasmic ACh fraction, rather than in the vesicular one, after prolonged stimulation in the presence of HC-3. The latter fraction shows and increase in the uptake of cytoplasmic ACh that depend on the presence of intracellular calcium ions during stimulation.