Presynaptic effects of muscarine on ACh release at the frog neuromuscular junction

Abstract

1. Presynaptic effects of muscarine on neurotransmitter release were studied at the frog neuromuscular junction, using focal depolarization of the presynaptic terminal to different levels. 2. Muscarine (10 ìÒ) had a dual effect on ACh release: concomitant inhibition and enhancement of release at the same patch of presynaptic membrane. 3. These two effects were maximal at low depolarizing pulses and diminished as depolarization increased. 4. At low depolarizing pulses, atropine (1 ìÒ) enhanced release, suggesting that ACh in the synaptic cleft causes a net tonic inhibition of ACh release. 5. In the presence of the M2 antagonist methoctramine (1 ìÒ), muscarine (10 ìÒ) enhanced ACh release. 6. In the presence of the M1 antagonist pirenzepine (10 ìÒ), muscarine (10 ìÒ) produced stronger inhibition. 7. These results show that the M2 receptor is responsible for inhibition of ACh release, while the M1 receptor is responsible for its enhancement. 8. The inhibitory effect of muscarine did not depend on extracellular [Ca¥]. Enhancement of release was abolished at low extracellular [Ca¥]. 9. The muscarine inhibitory effect was not associated with a reduction of Ca¥ current, while release enhancement was associated with an increase of Ca¥ current.8104

The Interaction between Tropomyosin-Related Kinase B Receptors and Presynaptic Muscarinic Receptors Modulates Transmitter Release in Adult Rodent Motor Nerve Terminals

“…Figure 5A shows that the partial (selective M 1 receptor block with pirenzepine or M 2 block with methoctramine) or total (atropine) block of the mAChR mechanism, the function of which is to modulate ACh release (Slutsky et al, 1999;Minic et al, 2002;Santafé et al, 2003Santafé et al, , 2006, prevents exogenous BDNF from carrying out the potentiating action (BDNF produces a change no higher than ϳ10%; p Ͼ 0.05). These data indicate the importance of the link between the ACh-mAChR pathway and the BDNFtrkB intracellular pathway.…”

“…Link between the trkB and muscarinic pathways Several developmentally regulated mAChR subtypes are known to be linked to ACh release at presynaptic sites at the NMJ (Slutsky et al, 1999;Minic et al, 2002;Santafé et al, 2006Santafé et al, , 2007. Although the precise cellular localization of the subtypes has not been fully resolved because of the low specificity of the staining antibodies (Pradidarcheep et al, 2008;Wright et al, 2009), pharmacological and electrophysiological experiments show that M 1 (pirenzepine-sensitive) and M 2 (methoctramine-sensitive) subtypes of mAChRs are involved in the enhancement and inhibition of ACh release, respectively, in the adult.…”

“…At the presynaptic level, membrane receptors for these mediators control the functional conditions of transmitter release in response to variable activity demands. In the neuromuscular synapse, muscarinic acetylcholine autoreceptors (mAChRs) (Caulfield, 1993;Slutsky et al, 1999;Minic et al, 2002;Santafé et al, 2003Santafé et al, , 2004Garcia et al, 2005), adenosine receptors (Song et al, 2000), neurotrophin receptors (Bibel and Barde, 2000;Roux and Barker, 2002;Pitts et al, 2006), and receptors for other trophic factors cooperate to produce synaptic plasticity.…”

Molecular mechanisms that control initiation and termination of physiological depolarization-evoked transmitter release

“…Much of this suggested mechanism was supported experimentally (3)(4)(5)(6)(7)(8) by using mainly the cholinergic neuromuscular junction (NMJ), where the M 2 muscarinic autoreceptor (M 2 R) controls both slow feedback inhibition (9,10) and fast ACh release (6)(7)(8). However, the relevance of this hypothesis for other NTs was not investigated.…”

Can the Ca 2+ hypothesis and the Ca 2+ -voltage hypothesis for neurotransmitter release be reconciled?

“…Accordingly, under physiological conditions of DIR, Ca 2ϩ is indeed necessary but membrane potential plays the pivotal role in release (5). In particular, we proposed that presynaptic inhibitory autoreceptors, in addition to their known role in feedback inhibition (6), are also the vehicle by which membrane potential controls initiation and termination of release. As a molecular mechanism for the calcium-voltage hypothesis we suggested that under rest conditions the release machinery is subject to a tonic block, which is attained by the association of the ligand-occupied inhibitory autoreceptor (which mediates feedback inhibition of transmitter release) with proteins of the exocytotic machinery.…”