Hemicholinium-3 facilitates the release of acetylcholine by acting on presynaptic nicotinic receptors at a central synapse in Aplysia

Hyperglycemia induced by intracerebroventricular choline: involvement of the sympatho-adrenal system

Intracerebroventricular (i.c.v.) injection of choline (75-300 microg) produced a dose-dependent increase in blood glucose levels. Pre-treatment with the nicotinic acetylcholine receptor antagonist, mecamylamine (50 microg, i.c.v.) blocked the hyperglycemia induced by choline (150 microg, i.c.v.), but the response was not affected by pre-treatment with the muscarinic acetylcholine receptor antagonist, atropine (10 microg, i.c.v.). Pre-treatment with the neuronal choline uptake inhibitor, hemicholinium-3 (20 microg, i.c.v.), attenuated the hyperglycemia induced by choline. The hyperglycemic response to choline was associated increased plasma levels of adrenaline and noradrenaline. The hyperglycemia elicited by choline was greatly attenuated by bilateral adrenalectomy, and entirely blocked by either surgical transection of the splanchnic nerves or by pre-treatment with the alpha-adrenoceptor antagonist, phentolamine. These data show that choline, a precursor of acetylcholine, increases blood glucose and this effect is mediated by central nicotinic acetylcholine receptor activation. An increase in sympatho-adrenal activity appears to be involved in the hyperglycemic effect of choline.

Repetitive nerve stimulation decreases the acetylcholine content of quanta at the frog neuromuscular junction

We investigated how elevated quantal release produced by motor nerve stimulation affects the size of the quanta. The motor nerve was stimulated at 10 Hz in preparations in which excitation-contraction coupling was disrupted. Two hundred stimuli reduced the size of the time integrals of the miniature endplate currents ([integral]MEPCs), measured at the same junction immediately after stimulation, by 16 %. Three thousand stimuli reduced size by 23 %. When the solution contained 10 microM neostigmine (NEO) 3000 stimuli reduced [integral]MEPCs by 60 %, because with acetylcholinesterase (AChE) inhibited, [integral]MEPC size is more sensitive to changes in acetylcholine (ACh) content. Similar decreases in miniature endplate potential size ([integral]MEPP) followed repetitive stimulation of contracting preparations. The depolarization produced by iontophoretic pulses of ACh was scarcely changed by 3000 nerve stimuli at 10 Hz, suggesting that the decreases in miniature sizes are largely due to less ACh released per quantum. Following 3000 stimuli at 10 Hz the sizes of the [integral]MEPCs increased back to pre-stimulus values with a half-time of 8-10 min. Recovery was blocked by (-)-vesamicol (VES), by hemicholinium-3 (HC3) and by nicotinic cholinergic agonists - all of which inhibit ACh loading into synaptic vesicles. The number of quanta in the total store was estimated by releasing them with carbonyl cyanide m-chlorophenylhydrazone (CCCP). CCCP releases fewer quanta after stimulation than from unstimulated controls. After resting for hours following stimulation, the releasable number increased, even when ACh loading inhibitors were present. We conclude that the inhibitors do not block a significant fraction of the ACh loading into reformed reserve vesicles and propose that ACh can be loaded in a series of steps.

The effects of choline on body temperature in conscious rats

Choline (75-300 microg) produced dose-dependent hypothermia when injected intracerebroventricularly (i.c.v.). Pre-treatment with the muscarinic receptor antagonist, atropine (10 microg, i.c.v.), blocked the hypothermic effect of choline (150 microg), but the response was only partially attenuated by pre-treatment with the nicotinic receptor antagonist, mecamylamine (20 microg, i.c.v.). Pirenzepine (25 microg), a muscarinic M1 receptor antagonist, or hexahydro-siladifenidol (HHSD) (100 microg), a muscarinic M3 receptor antagonist, also blocked choline-induced hypothermia when injected centrally. Unlike the other muscarinic receptor antagonists, M2-selective 11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyri do[2,3-b][1,4]benzodiazepin-6-one (AF-DX116) (10 microg), did not affect choline-induced hypothermia. We also found that choline-induced hypothermia was very sensitive to the ambient temperature. Similar to its effect at room temperature, choline produced dose-dependent hypothermia at 4 degrees C, but this effect was abolished at 32 degrees C. These data suggest that choline produces hypothermia and this effect is mediated by muscarinic receptors.

Effect of intracerebroventricularly injected choline on plasma ACTH and beta-endorphin levels in conscious rats

In the present study, we examined the effect of intracerebroventricularly injected choline on plasma ACTH (adrenocorticotrophin) and beta-endorphin levels in conscious rats. The intracerebroventricularly injection of choline (50-150 micrograms) elevated plasma ACTH levels in a dose-dependent manner. Plasma beta-endorphin levels were also significantly increased. Pretreatment of rats with mecamylamine (50 micrograms; intracerebroventricularly), the nicotinic receptor antagonist, completely inhibited the ACTH and beta-endorphin response to choline (150 micrograms; intracerebroventricularly). An antagonist of the muscarinic receptor, atropine (10 micrograms; intracerebroventricularly), failed to alter these effects. Pretreatment of rats with hemicholinium-3 (20 micrograms; intracerebroventricularly), a drug which inhibits the uptake of choline into cholinergic neurons, abolished the choline-induced increases in both plasma ACTH and beta-endorphin levels. These results indicate that choline can increase plasma concentrations of ACTH and beta-endorphin through the activation of central nicotinic acetylcholine receptors.

Inhibition of choline efflux results in enhanced acetylcholine synthesis and release in the guinea-pig corticocerebral synaptosomes

Synthesis and release of [3H]acetylcholine ([3H]ACh) were measured in synaptosomes from the guinea pig cerebral cortex after preloading with [3H]choline ([3H]Ch). We demonstrate here that inhibition of choline (Ch) efflux results in an increase in acetylcholine (ACh) synthesis and release. Our findings are as follows: (1) inhibition of [3H]Ch efflux by hemicholinium-3 (HC-3) (100 microM), increased the levels of both the released (116% of control) and the residing (115% of control) [3H]ACh. (2) The muscarinic agonist, McN-A-343 (100 microM), which was previously shown to inhibit Ch efflux, also increased the released (121% of control) and the residing (109% of control) [3H]ACh. (3) Omission of Na+ ions (which are required for Ch transport) from the incubation medium had similar effects to those observed with McN-A-343 and HC-3. These results suggest inverse relationships between Ch efflux on one hand, and ACh synthesis and release on the other hand. (4) Depolarization with 50 mM K+, or with the K+ channel blocker, 4-aminopyridine (100 microM), also increased the total level of [3H]ACh (113 and 107% of nondepolarized synaptosomes, respectively). However, whereas conditions that inhibit Ch transport such as HC-3, McN-A-343 and "no sodium" increased both the residing and the released [3H]ACh depolarization with high K+ or 4-aminopyridine reduced the residing (79 and 87% of control, respectively) and increased only the released [3H]ACh (182 and 148% of control, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Histamine and FLRFamide regulate acetylcholine release at an identified synapse in Aplysia in opposite ways

1. The effects of histamine and FLRFamide (Phe-Leu-Arg-Phe-NH2) on acetylcholine (ACh) release were studied in the buccal ganglion of Aplysia californica on an identified synapse (buccal ganglion inhibitory synapse, BGIS) involved in a small neuronal circuit controlling the feeding behaviour. The inhibitory postsynaptic current (IPSC) evoked by a presynaptic spike in the voltage-clamped postsynaptic neurone was decreased by histamine and increased by FLRFamide. 2. Histamine and FLRFamide modified the amplitude of the presynaptic spike. To test if these drugs acted directly on presynaptic calcium influx, we evoked transmitter release by 3 s depolarizations of the presynaptic neurone (to +10 mV) under voltage clamp to avoid modifications of presynaptic membrane polarization induced by changes in presynaptic voltage-dependent K+ and/or Na+ conductances. 3. Statistical analysis of this evoked long-duration (3 s) induced postsynaptic current (LDIPSC) allowed us to calculate the amplitude and the decay time of the miniature postsynaptic current and consequently the number of quanta released by the presynaptic terminal. 4. The amplitude of the LDIPSC decreased during the 3 s presynaptic depolarization. This was not due to a lack of available transmitter, since LDIPSC amplitude could be maintained constant by a 'clamp of the release of ACh' which adequately depolarized the presynaptic neurone, but rather to changes in the calcium influx into the presynaptic neurone. 5. FLRFamide increased more the initial portions of the LDIPSC than the final portions. This effect of FLRFamide was only reduced and delayed by atropine or curare, antagonists of muscarinic-like and nicotinic-like autoreceptors previously demonstrated to be present at the same terminal. Activation of the nicotinic-like receptors, which also increased transmitter release, induced a modification of the shape of the LDIPSC which was completely different from that due to FLRFamide. 6. Histamine decreased the amplitude of the LDIPSC. This effect was more pronounced at the beginning of the response. The effects of histamine were insensitive to curare and atropine, but were completely blocked by cimetidine, a specific histamine receptor antagonist. 7. The modifications of the shape and of the amplitude of the LDIPSC by FLRFamide and histamine suggested that these molecules alter presynaptic influx of calcium. This was confirmed by the analysis of calcium current recorded from the presynaptic neurone: the calcium inward current in the presynaptic neurone was increased by FLRFamide and reduced by histamine, whereas the activation of autoreceptors had no measurable effect on calcium current.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of protein kinase C by presynaptic FLRFamide receptors facilitates transmitter release at an aplysia cholinergic synapse

Modulation of evoked quantal transmitter release by protein kinase C (PKC) was investigated at an identified cholinergic neuro-neuronal synapse of the Aplysia buccal ganglion. Evoked acetylcholine release was increased by a diacylglycerol analog that activates PKC and was decreased by H-7, a blocker of PKC. FLRFamide facilitated evoked quantal release by increasing presynaptic Ca2+ influx. The inhibition of PKC by H-7 prevented both the increase of presynaptic Ca2+ influx and the facilitation of evoked acetylcholine release induced by the activation of presynaptic FLRFamide receptors. These results provide evidence that the activation of PKC could be a step in the intracellular pathway by which FLRFamide receptors increase evoked quantal acetylcholine release.

Receptor-mediated presynaptic facilitation of quantal release of acetylcholine induced by pralidoxime in Aplysia

1. Possible interactions of contrathion (pralidoxime sulfomethylate), a reactivator of phosphorylated acetylcholinesterase (AChE), with the regulation of cholinergic transmission were investigated on an identified synapse in the buccal ganglion of Aplysia californica. 2. Transmitter release was evoked either by a presynaptic action potential or, under voltage clamp, by a long depolarization of the presynaptic cell. At concentrations higher than 10(-5) M, bath-applied contrathion decreased the amplitude of miniature postsynaptic currents and increased their decay time. At the same time, the quantal release of ACh was transiently facilitated. The facilitatory effect of contrathion was prevented by tubocurarine but not by atropine. Because in this preparation, these drugs block, respectively, the presynaptic nicotinic-like and muscarinic-like receptors involved in positive and negative feedback of ACh release, we proposed that contrathion activates presynaptic nicotinic-like receptors. 3. Differential desensitization of the presynaptic receptors is proposed to explain the transience of the facilitatory action of contrathion on ACh release. 4. The complexity of the synaptic action of contrathion raises the possibility that its therapeutic effects in AChE poisonings are not limited to AChE reactivation.

Both presynaptic nicotinic-like and muscarinic-like autoreceptors regulate acetylcholine release at an identified neuro-neuronal synapse of Aplysia

The possible involvement of cholinergic presynaptic receptors regulating evoked quantal acetylcholine (ACh) release was investigated at an identified cholinergic neuro-neuronal synapse in the buccal ganglion of Aplysia, using cholinergic agonists (carbachol, pilocarpine, oxotremorine) and/or antagonists (curare, atropine, hexamethonium). Bath applied carbachol or pilocarpine (10(-8) M to 10(-4) M) induced a decrease in the evoked quantal release of ACh. As the effects of carbachol were prevented by atropine (5.10(-6) M) and not by curare (10(-5) M), it was concluded that carbachol activated presynaptic muscarinic-like receptors implicated in a negative feed-back on ACh release. On the contrary, oxotremorine (up to 10(-4) M) induced a potentiation of ACh release which was suppressed by curare (4.10(-6) M) or hexamethonium (10(-5) M) but not by atropine (5.10(-6) M) pointing to the activation of presynaptic nicotinic-like receptors implicated in a positive feed-back on ACh release. Moreover, in the presence of curare, oxotremorine decreased ACh release: this suggested that oxotremorine also activated the presynaptic muscarinic-like receptors. These results revealed the conjoint presence, on the same terminal, of both muscarinic-like and nicotinic-like autoreceptors.