The effect of preganglionic nerve stimulation on the accumulation of certain analogues of choline by a sympathetic ganglion

1. Cat superior cervical ganglia were perfused with a Krebs solution containing 10(-6) M [3H]homocholine (2-hydroxypropyl-trimethylammonium) or 10(-5) M [14C]triethylcholine (2-hydroxyethyl-triethylammonium). Preganglionic nerve stimulation (20 Hz) increased the accumulation of homocholine (3-2-fold) and of triethylcholine (2-1-fold). This increased accumulation during stimulation was not the result of increased metabolism. 2. The increased accumulation of homocholine or triethylcholine induced by pregnaglionic nerve stimulation was not reduced by tubocurarine or by atropine, but it was blocked by choline and by hemicholinium. These results suggested that preganglionic nerve stimulation increased choline analogue accumulation into cholinergic nerve terminals. 3. The increased accumulation of homocholine or of triethylcholine induced by preganglionic nerve stimulation was reduced when the Ca2+ concentration was reduced and was abolished in the absence of Ca2+. However, changes in the Mg2+ concentration which depressed acetylcholine (ACh) release by amounts comparable to those induced by altered Ca2+ concentrations did not alter the uptake of homocholine or triethylcholine. It is concluded that the uptake of choline analogues is not regulated by transmitter release but that stimulation increases the uptake of the choline analogues by a Ca2+-dependent mechanism. 4. The accumulation of ACh by ganglia perfused with a Krebs solution containing choline and high MgSO4 (18 mM) was measured. The ACh content of these ganglia did not increase, although choline transport presumably exceeded that necessary for ACh synthesis to replace released ACh. It is concluded that choline transport does not limit ACh synthesis in ganglia.

Characterization of the neuromuscular block produced by clindamycin and lincomycin

The site of neuromuscular blockade induced by clindamycin and lincomycin was studied on isolated nerve and nerve-muscle preparations. Clindamycin (3.6 X 10(-3) M) but not lincomycin (up to 1.5 X 10(-2) M) had a local anaesthetic effect on a frog desheathed nerve preparation. Clindamycin (8 X 10(-4) M) and lincomycin (4 X 10(-3) M) depressed the response of the rat diaphragm to nerve stimulation and to direct muscle stimulation in parallel. This indicated that the predominant neuromuscular blocking effect of these antibiotics was due to an effect on the muscle. Clindamycin was fivefold more potent than lincomycin in this effect, and the unionized form of both drugs was the active form. Lincomycin (4 X 10(-3) M) but not clindamycin (8 X 10(-4) M) also had some depressant effect on nerve-muscle transmission as indicated by the interaction of the effects of the antibiotics and d-tubocurarine. The significance of these findings is discussed in relation to the acute clinical toxicity of these antibiotics.

The site of the neuromuscular block produced by polymyxin B and rolitetracycline

The site of neuromuscular blockade induced by polymyxin B and rolitetracycline was studied on isolated nerve and nerve-muscle preparations. Polymyxin B (1.8 X 10(-4) M) was equipotent to lidocaine as a local anaesthetic on a frog desheathed nerve preparation, while rolitetracycline (up to 3.6 X 10(-3)M) had no local anaesthetic effect. Polymyxin B (6 X 10(-5) M) and rolitetracycline (7 X 10(-4) M) blocked by 50% the response of rat diaphragm induced by phrenic nerve stimulation, but did not decrease the amount of acetylcholine (ACh) released from this preparation during nerve stimulation. Both antibiotics depressed the response of the rat diaphragm to inject ACh, and this response was more sensitive to inhibition by the drugs than was the response to nerve stimulation. With rolitetracycline, a concentration that blocked the response to nerve stimulation by 50% inhibited the response to injected ACh by 85%, and this relationship was similar to that with d-tubocurarine; however, polymyxin B was relatively more effective than d-tubocurarine in inhibiting the effect of ACh. Polymyxin B (1-1.5 X 10(-4) M) but not rolitetracycline (1 X 10(-3) M) depressed the response of the diaphragm to direct muscle stimulation. It is concluded that polymyxin B and rolitetracycline block neuromuscular transmission predominatly by an effect to depress the muscle's sensitivity to ACh; polymyxin B probably acts by an effect similar to that of local anaesthetics, while rolitetracycline probably acts by an effect similar to that of d-tubocurarine.

The effect of atropine upon acetylcholine release from cat superior cervical ganglia and rat cortical slices: measurement by a radio-enzymic method

Atropine is known to increase the release of acetylcholine (ACh) from cerebral cortex, and the present experiments tested the effect of this drug upon ACh release in the superior cervical ganglion of the cat. The release of ACh was measured by a radio-enzymic method, which was shown to provide an estimate of the ACh content of samples collected from perfused ganglia that was similar (102%) to that obtained by the method of bioassay more usually used . Atropine (3 X 10(-6) M) increased (3.5 to 4-fold) the amount of ACh released by rat's sliced cerebral cortex incubated in a high (23 mM) potassium medium. However atropine (3 X 10(-6)-3 X 10(-5) M) did not change the amount of ACh released by ganglia during preganglionic nerve stimulation (5-10 Hz). It is concluded that cholinergic nerve terminals in different tissues appear to have different pharmacological properties.

Studies upon the mechanism by which acetylcholine releases surplus acetylcholine in a sympathetic ganglion

1. Acetylcholine (ACh) releases surplus ACh from the superior cervical ganglion of the cat and the experiments described in this paper tested whether this results from exchange of endogenous ACh with exogenous ACh; the experiments also attempted to characterize pharmacologically the mechanism of this action of ACh. 2. The surplus ACh in the ganglion was radioactively labelled by perfusion of the ganglion with [3H]-choline-Krebs solution containing diisopropylphosphofluoridate, and the release of surplus [3H]-ACh by [14C]-ACh injected close arterially to the ganglion measured. The amount of [3H]-ACh released by [14C]-ACh was 33 +/- 5 times greater than was the amount of [14C]-ACh accumulated by ganglia. The amount of exogenous ACh accumulated by ganglia that had first formed surplus ACh was not different from exogenous ACh accumulation by ganglia that had not formed surplus ACh. Thus, it is concluded that surplus ACh release by ACh is not the result of ACh exchange. 3. In other experiments, surplus [3H]-ACh was accumulated in ganglia exposed to physostigmine. Nicotine, pilocarpine or ACh released surplus ACh; the effect of both nicotine and ACh was blocked by hexamethonium; atropine blocked the effect of ACh but not that of nicotine. It is concluded that both nicotinic and muscarinic receptors can be involved in the release of surplus ACh by cholinomimetic agonists.

Acetylcholine synthesis from recaptured choline by a sympathetic ganglion

1. The recapture and re-use of choline formed by the hydrolysis of released acetylcholine (ACh) was studied in the superior cervical ganglion of the cat using radioactive tracer techniques. The ganglion's ACh store was labelled by perfusion, during preganglionic nerve stimulation, with Krebs solution containing [(3)H]choline.2. Preganglionic stimulation (5 Hz for 20 min) of ganglia containing [(3)H]ACh released similar amounts of radioactivity when perfusion was with neostigmine-choline-Krebs or with hemicholinium-Krebs. This indicated that neostigmine does not increase transmitter release.3. The amount of radioactivity collected from stimulated ganglia during perfusion with choline-Krebs was 39% of the amount of radioactivity collected during perfusion with medium containing neostigmine or hemi-cholinium. This difference in release was almost (85%) accounted for at the end of the experiment by extra radioactive ACh in the ganglia perfused with choline-Krebs. It is concluded that during preganglionic nerve stimulation approximately 50-60% of endogenously produced choline is recaptured for ACh synthesis; thus, during activity preganglionic nerve terminals appear selectively to accumulate choline.4. However, chronically decentralized ganglia accumulated as much choline as did acutely decentralized ganglia, and this was interpreted as indicating that at rest preganglionic nerve terminals do not selectively accumulate choline.5. Increased exogenous choline concentration increased the amount of radioactivity collected during nerve stimulation in the absence, but not the presence, of an anticholinesterase agent. The spontaneous efflux of radioactivity was little affected by changes in external choline levels. It is concluded that exogenous choline and choline made available from released transmitter compete for uptake into nerve terminals.

The accumulation of hemicholinium by tissues that transport choline

Cat superior cervical ganglia accumulated 14C-hemicholinium (HC-3), but the amount of drug taken up was not changed by nerve stimulation; accumulated HC-3 was not released by subsequent nerve stimulation. Choline and HC-3 were accumulated by a crude preparation of synaptosomes; the uptake of choline was blocked by HC-3, but the uptake of HC-3 was not blocked by choline. Choline, but not HC-3, was transported by red blood cells. It is concluded that HC-3 is not transported by the choline transport mechanism, and that in concentrations that are usually used in pharmacological experiments, HC-3 appears not to act as, or to form, a false cholinergic transmitter.

The synthesis, turnover and release of surplus acetylcholine in a sympathetic ganglion

1. Surplus acetylcholine (ACh) is the extra ACh that accumulates in cholinergic nerve endings when they are exposed to an anticholinesterase agent. The synthesis and turnover of this ACh was examined in the cat's superior cervical ganglion.2. Surplus ACh did not accumulate in chronically decentralized ganglia perfused with eserine-choline-Locke solution, and this shows that it is stored in presynaptic nerve terminals.3. Surplus ACh accumulated more rapidly in ganglia perfused with eserine than in ganglia perfused with neostigmine or with ambenonium; accumulation was delayed by 45-60 min when a quaternary anticholinesterase was used. However, the release of ACh upon preganglionic nerve stimulation was the same during perfusion with eserine, neostigmine or ambenonium. It is concluded that intracellular acetylcholinesterase normally destroys surplus ACh, whereas extracellular enzyme destroys released ACh.4. When ganglia were perfused with [(3)H]choline and eserine, the surplus ACh that accumulated was labelled but its specific radioactivity was only 38% of that of the choline added to the perfusion fluid.5. Surplus ACh was not released by nerve stimulation and was not mobilized for release during, or after, prolonged nerve stimulation. It is concluded that ACh released by nerve impulses is replaced by synthesis at the site of ACh storage and not by movement of ACh from the surplus pool.6. The accumulation of surplus ACh no more than doubled the total ACh content of ganglia, but turnover of ACh continued when the total amount was constant. Surplus ACh may contribute to spontaneous ACh output from eserinized preparations.7. When ganglia were perfused with a medium containing high K(+) (56 mM), surplus ACh was released.

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

1. 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 (0.15-15 mug) or by carbachol (1-10 mug) when these drugs were injected close to the ganglion. Perfusion with low or moderate concentrations of ACh (0.15-5 mug/ml) also failed to release ACh, but high concentrations (15-50 mug/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 (3)H-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 (0.5-15 mug/ml), or injection of carbachol (0.5-2.5 mug) 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.

The preferential release of newly synthesized transmitter by a sympathetic ganglion

1. The acetylcholine (ACh) store of cat's superior cervical ganglia was replaced with radioactive ACh by perfusion, during stimulation, with [(3)H]choline-Locke solution. Perfusion was continued with Locke containing unlabelled choline (Ch) (in physiological concentration) and the release of labelled and unlabelled ACh was measured.2. Electrical stimulation of the preganglionic sympathetic nerve (20/sec or 5/sec), or stimulation by perfusing with raised K, released ACh that had a lower specific radioactivity than ganglionic ACh. The proportion of released ACh that was labelled was slightly higher when stimulation was at lower frequency or by K.3. Preganglionic nerve stimulation released, in the first few minutes, ACh that had a specific activity 70-80% of ganglionic ACh, but after 5 min the proportion of label in the released ACh fell to 35-45% of that in the ganglion.4. It is concluded that newly synthesized ACh is released before equilibration with preformed stores, and the significance of this to the mechanism of transmitter release is discussed.

The source of choline for acetylcholine synthesis in a sympathetic ganglion

Choline (Ch) and acetylcholine (ACh) uptake and release were measured by a combination of tracer and bioassay techniques in perfused superior cervical ganglia of the cat during rest and repetitive preganglionic stimulation. The uptake of labelled ACh as such was small; but when Ch (methyl-3H labelled) was present at physiological concentration (1.5 μg/ml) in the perfusion fluid, its incorporation into the ACh and free Ch pools of the ganglion proceeded linearly in the absence of stimulation, was accelerated by stimulation, and was inhibited by hemicholinium but not by hexamethonium. Up to 85% of ganglionic ACh could be replaced by labelled ACh during a 1-h stimulation period. On subsequent perfusion with fluid containing unlabelled Ch, labelled Ch was lost but labelled ACh was retained and could be released by stimulation, either as ACh in the presence or as Ch in the absence of eserine. The output of label during stimulation was approximately doubled by eserine but was unaffected by hemicholinium, which, however, prevented the formation and release of newly synthesized ACh. It appears that about half the Ch formed from released ACh is immediately recaptured and resynthesized into ACh. Newly synthesized ACh rapidly gains access to the releasable transmitter pool and may be preferentially released.

The metabolism of choline by a sympathetic ganglion

Cat's superior cervical ganglia were perfused with Locke's solution containing choline (Ch) at physiological concentration but labelled (methyl-3H), and the radioactive products in extracts of such ganglia were identified by a combination of selective precipitation and chromatographic tests. Ch was incorporated into acetylcholine (ACh), into phosphorylcholine (PCh), and into phospholipid. The rate of formation of PCh and phospholipid from Ch was measured to be about 2 ng/min of each, and this rate was unaffected by activity or by hemicholinium. Free Ch liberated by PCh or phospholipid turnover is unlikely to be an important source of Ch for ACh synthesis under physiological conditions.

The central release of acetylcholine during consciousness and after brain lesions

1. Acetylcholine (ACh) has been collected from the visual cortex of anaesthetized rabbits during stimulation of the lateral geniculate body and after cutting central nervous pathways. ACh has also been collected from the visual cortex of conscious, free-moving rabbits.2. After a unilateral ;vertical' lesion separating the geniculate body from more centrally situated nuclei, ACh release evoked from the contralateral cortex by geniculate body stimulation was abolished but evoked release from the ipsilateral cortex was only reduced.3. After a bilateral, ;horizontal' lesion separating the thalamic nuclei from the reticular formation, unilateral geniculate stimulation gave an increased ACh release from the ipsilateral but not from the contralateral visual cortex.4. The ;vertical' and ;horizontal' lesions had no permanent effect on the spontaneous release of ACh from the visual cortex.5. Unilateral destruction of the geniculate body reduced the spontaneous release of ACh from the ipsilateral cortex but did not affect the contralateral release.6. The spontaneous and directly evoked ACh release from chronically undercut areas of cortex was found to be considerably lower than from intact areas of cortex.7. A high output of ACh was obtained from the visual cortex of conscious, free-moving rabbits. The rate of ACh release was closely related to the activity and state of arousal of the animals.8. These results support an earlier suggestion that two major ascending cholinergic systems exist in the rabbit brain. One pathway is the non-specific reticulo-cortical tract responsible for cortical arousal and the other is the more specific thalamo-cortical pathway associated with augmenting and repetitive after-discharge responses. The functional significance of these two cholinergic pathways and their role in the conscious animal are discussed.

The central release of acetylcholine during stimulation of the visual pathway

1. In rabbits anaesthetized with Dial ACh has been collected from the surface of the cerebral cortex during stimulation of the visual pathways.2. The spontaneous release of ACh from the visual and non-visual areas of the cortex was found to be similar.3. Stimulation of the retinae by diffuse light produced a large increase in ACh release from the primary visual receiving areas (4.3 times the spontaneous release) and a smaller increase (1.9 times the spontaneous release) from other parts of the cortex.4. Direct unilateral electrical stimulation of the lateral geniculate body evoked a large increase in ACh release (3.4 times the spontaneous release) from the ipsilateral visual cortex and a smaller increase (1.7 times the spontaneous release) from the contralateral visual area and other regions of the cerebral cortex. The evoked increase from the contralateral cortex was not mediated by transcallosal pathways.5. The increase in ACh release evoked from the visual cortex by stimulation of the ipsilateral lateral geniculate body was dependent on the frequency of stimulation. The evoked release was smallest at low stimulus frequencies and increased to a maximum at 20 stimuli/sec. The evoked ACh release from other areas of the cortex was independent of the frequency at which the lateral geniculate body was stimulated.6. The possible central nervous pathways associated with the spontaneous release of ACh and the release evoked by stimulation of the eyes by light and by direct stimulation of the lateral geniculate body are discussed.7. It is concluded that two ascending cholinergic systems may be involved; the non-specific reticulo-cortical pathways responsible for the e.e.g arousal response, and the more specific thalamo-cortical pathways associated with augmenting and repetitive after-discharge responses. The first system is thought to be concerned with the small but widespread increase in ACh release from the cortex following stimulation of the visual pathway while the second system could give rise to the larger increases evoked from the primary receiving areas of cortex. The spontaneous release of ACh from the surface of the brain may be the result of contributions from both systems.