Activation Of Presynaptic Muscarinic Receptors Research Articles

Regional differences in neostigmine-induced contraction and relaxation of stomach from diabetic guinea pig

Delayed gastric emptying and autonomic neuropathy have been documented in patients with diabetes mellitus. Some medications used to treat delayed gastric emptying enhance release of acetylcholine from autonomic neurons to strengthen gastric contractions. Autonomic coordination among gastric regions may be altered in diabetes resulting in poor outcomes in response to prokinetic drugs. Fundus, antrum, and pylorus from STZ or control guinea pigs were treated with neostigmine to mimic release of acetylcholine from autonomic neurons by prokinetic agents. In diabetic animals, neostigmine-induced contractions were weaker in fundus and pylorus but similar in antrum. The muscarinic receptor antagonist 4-DAMP or the nicotinic receptor antagonist hexamethonium reduced neostigmine-induced contractions. Activation of presynaptic muscarinic receptors on nitrergic neurons was impaired in fundus and antrum from diabetic animals. Nerve-stimulated contractions and relaxations, number of nNOS myenteric neurons, and tissue choline content were reduced in fundus from diabetic animals. Despite reduced number of myenteric neurons, tissue choline content was increased in antrum from diabetic animals. Since cholinergic motility of each gastric region was affected differently by diabetes, prokinetic drugs that nondiscriminately enhance acetylcholine release from autonomic neurons may not effectively normalize delayed gastric emptying in patients with diabetes and more selective medications may be warranted.

What Causes Muscle Atonia in REM?

What Causes Muscle Atonia in REM?

Cholinergic mechanism underlying prepulse inhibition of the startle response in rats

Startle responses are attenuated by prepulse inhibition (PPI), which is considered to reflect a sensorimotor gating mechanism and is impaired in patients suffering from schizophrenia. A midbrain circuit that mediates PPI in rats has been proposed and behavioral experiments have indicated an important role of acetylcholine and GABA in inhibiting startle. We here test the hypothesis that activation of the midbrain neurons can inhibit startle signaling through a cholinergic mechanism. We have developed a brain slice that comprises startle mediating giant pontine neurons as well as midbrain mesopontine neurons required for PPI. Patch clamp recordings of startle mediating brainstem neurons combined with stimulation of sensory afferents within the startle pathway and activation of mesopontine neurons revealed a delayed inhibition of synaptic transmission 300 ms and 1 s after midbrain activation. The latter was reversed by the muscarinic antagonist scopolamine. Further, there was a shift in the paired pulse ratio 1 s but not 300 ms after midbrain stimulation. Our results show that there is a direct cholinergic projection from the proposed PPI midbrain circuit to startle mediating neurons in the brainstem and that this projection inhibits synaptic transmission in the startle pathway in a distinct time window through the activation of presynaptic muscarinic receptors. Moreover, there is indication for a different receptor that mediates inhibition through this projection in a shorter time window and is located postsynaptically. Our results contribute to the understanding of mechanisms underlying PPI, which is important for developing new targets in the treatment of disorders accompanied with pre-attentive cognitive deficits.

Presynaptic inhibition of Schaffer collateral synapses by stimulation of hippocampal cholinergic afferent fibres.

It has been known for decades that muscarinic agonists presynaptically inhibit Schaffer collateral synapses contacting hippocampal CA1 pyramidal neurons. However, a demonstration of the inhibition of Schaffer collateral synapses induced by acetylcholine released by cholinergic hippocampal afferents is lacking. We present original results showing that electrical stimulation at the stratum oriens/alveus with brief stimulus trains inhibited excitatory postsynaptic currents evoked by stimulation of Schaffer collaterals in CA1 pyramidal neurons of rat hippocampal slices. The increased paired-pulse facilitation and the changes in the variance of excitatory postsynaptic current amplitude that paralleled the inhibition suggest that it was mediated presynaptically. The effects of oriens/alveus stimulation were inhibited by atropine, and blocking nicotinic receptors with methyllycaconitine was ineffective, suggesting that the inhibition was mediated via the activation of presynaptic muscarinic receptors. The results provide a novel demonstration of the presynaptic inhibition of glutamatergic neurotransmission by cholinergic fibres in the hippocampus, implying that afferent cholinergic fibres regulate the strength of excitatory synaptic transmission.

Dual Cholinergic Control of Fast-Spiking Interneurons in the Neostriatum

GABAergic interneurons appear to play a fundamental role in the functioning of the neostriatum by modulating the spiking of striatal projection neurons with great efficacy. The powerful and strongly divergent output of the GABAergic interneurons neurons suggests that modulation of their activity may be particularly effective at controlling the functioning of the entire neostriatal circuitry. Acetylcholine is one of the main modulators of striatal functioning. The effects of acetylcholine on fast-spiking (FS) GABAergic interneurons were studied with whole-cell recording in an in vitro slice preparation. Acetylcholine exerted two distinct effects on fast-spiking interneurons. Acetylcholine directly depolarized FS interneurons by acting on nondesensitizing soma-dendritic nicotinic receptors. In addition, acetylcholine attenuated the GABAergic inhibition of projection neurons by fast-spiking interneurons through activation of presynaptic muscarinic receptors. It is suggested that the nicotinic excitation of FS interneurons may play an important role in translating the effect of the brief behaviorally contingent cessation of firing of the tonically active cholinergic interneurons to the output neurons of the neostriatum. In contrast, the muscarinic presynaptic inhibitory mechanism may be engaged primarily during longer-lasting elevations of extracellular acetylcholine levels.

Muscarinic inhibition of quantal transmitter release from the magnesium-paralysed frog sartorius muscle

The existence of presynaptic muscarinic acetylcholine receptors on motor nerve terminals of the isolated frog sartorius muscle was investigated. The modulatory role of these receptors was studied by observing the effects of muscarinic ligands on the frequency of miniature endplate potentials and on the quantal content of endplate potentials. The agonist oxotremorine reduced in concentration-dependent fashion the frequency of spontaneous potentials and the amplitude of evoked potentials. Also, high concentrations of oxotremorine depolarized the postsynaptic membrane and reduced the amplitude of the miniature endplate potentials. The depolarizing action of the drug was blocked by d-tubocurarine. The muscarinic antagonist atropine attenuated agonist-induced reductions in endplate potential amplitude and miniature endplate potential frequency but did not affect the depression in amplitude of the spontaneous potentials evoked by oxotremorine. It is concluded that activation of presynaptic muscarinic receptors inhibits the release of acetylcholine from motor nerve terminals. Atropine itself had no effect on the quantal content of evoked potentials or on the frequency of spontaneous potentials suggesting that the nerve terminal is not affected by non-quantal acetylcholine.

Muscarinic inhibition of [3H]‐noradrenaline release on rabbit iris in vitro: effects of stimulation conditions on intrinsic activity of methacholine and pilocarpine

1. Rabbit isolated irides were loaded with [3H]-noradrenaline and superfused with Tyrode solution. The inhibition by the muscarinic agonists (+/-)-methacholine and pilocarpine of the [3H]-noradrenaline overflow into the superfusate evoked by field stimulation (pulses of 1 ms duration, 75 mA) was measured as an index of activation of presynaptic muscarinic receptors. 2. The fractional rate of release per pulse during the first stimulation period (S1) was low with 360 pulses at 3 Hz, intermediate with 360 pulses at 10 Hz and high with 1200 pulses at 10 Hz. Upon repetitive stimulation (7 periods at 20 min intervals), the fractional rates of release per pulse during S7 no longer differed, suggesting a 'long-term' regulation of [3H]-noradrenaline release depending on the stimulation conditions. 3. The evoked [3H]-noradrenaline overflow was depressed by (+/-)-methacholine in a concentration-dependent manner. The EC50 ranged from 0.29 to 0.42 microM. Methacholine nearly abolished the transmitter release evoked at 3 Hz but reduced that induced at 10 Hz by only 50%. Under the latter condition the methacholine concentration-inhibition curve was bell-shaped and no muscarinic inhibition was observed in the presence of methacholine 30 microM. After washout of methacholine the evoked [3H]-noradrenaline release was temporarily enhanced. 4. Atropine 0.1 microM enhanced the [3H]-noradrenaline overflow (evoked by stimulation with 360 or 1200 pulses at 10 Hz), probably antagonizing a presynaptic inhibition by endogenous acetylcholine. The inhibition by methacholine was competitively antagonized by atropine 0.1 microM (apparent -log KB = 8.5-9.0). 5. Depending on the concentration, pilocarpine reduced the [3H]-noradrenaline overflow evoked by 360 pulses at 3 Hz up to 63%. However, at 10 Hz stimulation frequency the compound was inactive as an agonist but competitively antagonized the presynaptic inhibition induced by methacholine. The KB under the latter condition (0.95 microM) was very close to the EC50 value determined at 3 Hz (0.85 microM). 6. The results demonstrate a muscarinic inhibition of noradrenaline release from the rabbit isolated iris. The activation by pilocarpine of the presynaptic receptors provides an alternative explanation for the miosis induced in the rabbit in vivo, which might be the result of a decreased sympathetic tone in the iris dilator muscle.

Modulation of stimulation-evoked release of newly formed acetylcholine from mouse hemidiaphragm preparation.

A radioisotope method has been developed for measuring the stimulation-evoked release of acetylcholine without the use of cholinesterase inhibitors from the mouse hemidiaphragm preparation which had been loaded with 3H-choline. Evidence has been obtained that 3H-choline was taken up by and released from both innervated and non-innervated mouse hemidiaphragm preparations. However, it was released in the form of 3H-acetylcholine in response to electrical field stimulation only from the innervated preparations. Long lasting (51 min) S1 stimulation of the preparations exhausted the radioactive acetylcholine stores to the extent that S2 did not evoke any release of 3H. These data suggest that when the labelled acetylcholine stores become exhausted, the labelled choline, still present in the tissue, cannot be released by electrical stimulation. Tetrodotoxin (1 mumol/l) administration and Ca withdrawal inhibited, 20-100 mumol/l 4-aminopyridine enhanced the release of 3H-acetylcholine in response to electrical stimulation. Activation of the presynaptic muscarinic receptors by the agonist oxotremorine (50 mumol/l) decreased the liberation of 3H-acetylcholine. The muscarinic antagonist atropine (1 mumol/l) abolished the inhibitory effect of oxotremorine and by itself increased the evoked release of the newly formed 3H-acetylcholine. Adenosine (50 mumol/l) reduced the evoked release of radioactivity. Theophylline (30 mumol/l) prevented the inhibitory effect of adenosine and itself enhanced the release. Xylazine (1 mumol/l), an alpha 2-adrenoceptor agonist did not affect the release. It is concluded that the stimulation-evoked release of 3H-acetylcholine from the mouse phrenic nerve hemidiaphragm preparation preloaded with 3H-choline is derived from the motor nerves. The release of acetylcholine is modulated by activation of presynaptic muscarinic and adenosine receptors.

Activation of central muscarinic receptors inhibit Ca 2+/Mg 2+ ATPase and ATP-dependent Ca 2+ transport in synaptic membranes

Preparations of lysed synaptosomes exhibit a high affinity Ca 2+/Mg 2+ ATPase and ATP-dependent Ca 2+ accumulation activity, with a K m forCa 2+ ⋍ 0.5 μM, close to the cytosolic concentration of Ca 2+. When these membrane suspensions were incubated with cholinergic agonists muscarine or oxotremorine (1–20 μM), both Ca 2+/Mg 2+ ATPase and ATP-dependent Ca 2+ uptake were inhibited in a concentration-dependent fashion. Atropine alone (0.5–1.0 μM) had no effect on either enzyme or uptake activity, but significantly inhibited the actions of both muscarine and oxotremorine. No significant effects by cholinergic agonists or antagonists were seen on fast or slow phase voltage-dependent Ca 2+ channels or Na +-Ca 2+ exchange. These results suggest that activation of presynaptic muscarinic receptors produce inhibition of two processes required for the buffering of optimal free Ca 2+ by the nerve terminal. Activation of presynaptic muscarinic receptors have been reported to reduce the release of ACh from nerve terminals. Alterations in intracellular free Ca 2+ may contribute to a reduction in transmitter (ACh) release seen following activation of cholinergic receptors.

Affinity and efficacy of racemic, (+)-, and (-)-methacholine in muscarinic inhibition of [3H]-noradrenaline release.

The right postganglionic sympathetic nerves of rat isolated perfused hearts (previously loaded with [3H]-noradrenaline) were stimulated electrically with 10 trains of 10 pulses at 10 Hz. The inhibition by methacholine of stimulation-evoked [3H]-noradrenaline overflow into the perfusate (determined in the presence of corticosterone, desipramine, phentolamine, and propranolol) was taken as a measure for activation of presynaptic muscarinic receptors. The evoked [3H]-noradrenaline overflow was inhibited by (+)-, racemic, and (-)-methacholine in a reversible and concentration-dependent manner. The concentration causing 50% inhibition (IC50) was 0.1, 0.26, and 65 microM, respectively, resulting in an isomeric potency ratio IC50 (+)/IC50(-) of 650. The dissociation constant KA of the (+/-)- or (+)-methacholine-presynaptic receptor complex was determined after fractional receptor inactivation according to Furchgott & Bursztyn (1967) with phenoxybenzamine or propylbenzilylcholine mustard as irreversible antagonists of muscarinic receptors. KA for (-)-methacholine was estimated according to Mackay (1966). KA of (+)-, (+/-)-, and (-)-methacholine were 2.5, 4 and 440 microM, resulting in an isomeric affinity ratio KA (+)/KA(-) of 180. The discrepancy between the isomeric IC50 ratio and the isomeric KA ratio is explained by a higher intrinsic efficacy of the (+)-enantiomer compared to the (-)-enantiomer. Thus, (+)-methacholine has to occupy fewer receptors to induce a given inhibition of release than its antipode as revealed by a plot of fractional receptor occupancy vs response. The results show that, in the effector system of presynaptic muscarinic inhibition, methacholine enantiomers differ greatly not only in affinity for the receptor, but also to some extent in the efficiency of signal transmission, and both parameters contribute to the high isomeric potency ratio. The activity of the racemate is fully accounted for by the activity of the (+)-enantiomer.

Presynaptic muscarinic receptor activation enhances striatal dopamine release evoked by depolarization but not that induced by non-depolarizing stimuli

The release of [ 3H]dopamine stimulated by depolarization with 15 mM KCl of superfused rat striatal synaptosomes was potentiated by acetylcholine through the activation of presynaptic muscarinic receptors. In contrast, acetylcholine did not potentiate the release of [ 3H]dopamine elicited by d-amphetamine nor that caused by the calcium ionophore A23187. The dopamine carrier blocker nomifensine prevented the releasing action of amphetamine but not that of acetylcholine. The results suggest that the activation of muscarinic receptors on dopamine terminals in the rat corpus striatum selectively affects the calcium-dependent depolarization-induced release of the [ 3H]catecholamine. Moreover, the [ 3H]dopamine release caused by acetylcholine seems to occur independently of the membrane dopamine carrier.

A calcium requirement for the phosphatidylinositol response following activation of presynaptic muscarinic receptors

The labelling in vitro by [ 32P]phosphate of phosphatidic acid and phosphatidylinositol in synaptosomes from guinea-pig brain was studied. Acetylcholine increased the labelling and evidence is provided that pre-synaptic muscarinic receptors are involved. The increase was not seen in the presence of EGTA. Experiments with various calcium buffers indicated that concentrations of greater than 10 −7 M free Ca 2+ are required in the incubation medium for this phospholipid effect. A similar muscarinic effect in parotid gland is unaffected by EGTA. It is suggested that, in the parotid, postsynaptic receptors mediate increased phosphatidylinositol labelling and increased availability of Ca 2+ for stimulus-secretion coupling. Presynaptic receptors similarly mediate increased labelling, which differs in being sensitive to EGTA and associated with decreased availability of Ca 2+.

Muscarinic inhibition of endogenous myocardial catecholamine liberation in the dog.

The release of endogenous catecholamine (CA) into the coronary sinus of the anesthetized dog was investigated under basal sympathetic tone and during sympathetic and vagal electrical stimulation. Under basal conditions, right distal cervical vagal stimulation (1, 2, 4, 8, 16 Hz) did not significantly alter the output of CA into the conorary sinus although important increases in aortic CA levels were observed. Stimulation of the right cardioaccelerator nerves (10 Hz) caused a marked and reproducible increase in coronary sinus CA levels but this effect could be progressively attenuated by superposing vagal stimulation at increasing frequencies. A maximal inhibition (66%) of CA liberation was observed at a vagal stimulation frequency of 16 Hz. Atropine (2 μg/kg) blocked selectively the effect of low frequency vagal stimulation. Methacholine (3 μg/kg per minute) infusion mimicked vagal inhibition on CA liberation. It is suggested that a functional vagal inhibition of sympathetic CA liberation could be mediated through activation of presynaptic muscarinic receptors located on the adrenergic fibers within the left ventricle of the dog; moreover, the excellent correlation observed between the decrease in CA liberation and the first derivative of left ventricular pressure (dp/dt) suggests that this inhibition is of physiological significance. Such mechanisms could be implicated in the local fine adjustments of neurotransmitter liberation and in the functional peripheral integration of both components of the autonomic nervous system.