The release of acetylcholine by acetylcholine in the cat's superior cervical ganglion

Abstract

Summary1 The experiments described in this paper tested the effect of acetylcholine (ACh), carbachol or preganglionic nerve stimulation on the release of ACh from the cat's perfused superior cervical ganglion; radioactive tracer methods were used. 2. When the ganglion's transmitter store of ACh had been labelled, radioactive ACh was released by nerve stimulation (5 Hz for 2 min), but there was no release by ACh (015-15 jg) or by carbachol (1-10 Mig) when these drugs were injected close to the ganglion. Perfusion with low or moderate concentrations of ACh (0-15-5 jug/ml) also failed to release ACh, but high concentrations (15-50 ,tg/ml) released a small amount of labelled material. There was no correlation between ganglion stimulation by ACh and release of radioactivity.3. Ganglion-blocking concentrations of ACh did not reduce the release of ACh during continuous nerve stimulation. 4. When resting (unstimulated) ganglia were perfused with 3H-choline and eserine, the extra ACh synthesized and stored by such ganglia (surplus ACh) was labelled. Preganglionic nerve stimulation (5 Hz for 2 min) did not release surplus ACh, but perfusion with ACh (05-15 pig/ml), or injection of carbachol (0O5-2-5 ytg) did. 5. Surplus ACh released by ACh or by carbachol did not contribute to the ganglion stimulating effect of either drug. 6. It is concluded that the presynaptic effects of ACh are not of physiological importance. IntroductionThe concept that the arrival of an action potential in a presynaptic nerve terminal results in the release of enough transmitter substance to effect synaptic transmission has been challenged by Koelle (1961Koelle ( , 1962). Koelle's hypothesis is that, at cholinergic synapses, a nerve impulse releases too little acetylcholine (ACh) to stimulate the postganglionic cell directly, but enough to prolong the state of depolarization of the presynaptic nerve terminals so that they discharge additional ACh which does effect transmission.This suggestion that there is a positive feed-back mechanism for release implies that exogenously applied ACh, or ACh-like agent, should release transmitter from presynaptic nerve endings, and some evidence has accumulated that this might be

Studies Upon the Mechanism by Which Acetylcholine Releases Surplus Acetylcholine in a Sympathetic Ganglion

“…The previous demonstration (Collier & Katz, 1970) (Brown & Feldberg, 1936;Feldberg, 1943) and it was assumed that 85% of the total ACh was releasable (Birks & MacIntosh, 1961;Collier & MacIntosh, 1969 Time ( The effect of acetylcholine-blocking agents upon the release of surplus acetylcholine induced by acetylcholine or by nicotine…”

“…Two experiments showed that hexamethanium (2 x 1O-4M) clearly reduced the release of ACh induced by nicotine (Figure 5a). Five experiments tested whether hexamethonium (2 x 1 0-4 M) blocked the release of surplus ACh induced by ACh; in four of these release was almost completely abolished ( Figure Sb) (Brown et al, 1970;Collier & Katz, 1970 Kuhar & Simon, 1974 (Barker, Dowdall, Essman & Whittaker, 1970;Richter & Marchbanks, 1971;Marchbanks & Israel, 1971 The results of experiments which measured the uptake of exogenous ACh by ganglia that had, or had not, first accumulated surplus ACh were consistent with the conclusion that surplus ACh release by ACh does not result from ACh exchange. If the exchange occurred, it might be expected that accumulation of exogenous ACh would have been greater in ganglia with surplus ACh.…”

“…Surplus ACh is the extra ACh that is synthesized and stored by a ganglion that is exposed to an anticholinesterase agent (Birks & MacIntosh, 1961); this ACh is not immediately available for release by nerve impulses (Birks & MacIntosh, 1961;Collier& Katz, 1971) The mechanism by which ACh releases surplus ACh is not known. It has been suggested that carbachol (McKinstry & Koelle,1 967a) and tetramethylammonium (Fellman, 1969) release ACh from ganglia by a process of cation exchange; a similar proposal was made much earlier (Renshaw, Green & Ziff, 1938) in an attempt to explain the pharmacological actions of certain choline derivatives, and it was also inferred by Collier & Katz (1970) Other experiments attempted to characterize the pharmacology of the process by which ACh releases surplus ACh by testing whether the effect of ACh could be mimicked by muscarinic or nicotinic agonists, and whether ACh-blocking agents block the effect.…”

“…Acetylcholine (ACh) and ACh-like agents such as carbachol and tetramethylammonium can release ACh from the superior cervical ganglion of the cat (McKinstry & Koelle, 1967a;Fellman, 1969; Corner & Katz, 1970). The ACh that is released by ACh is not from the ganglion's normal store of transmitter but is surplus ACh (Brown, Jones, Halliwell & Quilliam, 1970;Colier & Katz, 1970).…”

Actions of acetylcholine and carbachol on the chick biventer cervicis muscle

“…Collier & Katz (1970) have shown that acetylcholine itself may also act to release quantities of surplus endogenous acetylcholine, although they believe that the presynaptic effects of acetylcholine are not of physiological importance.…”