Activation of TRPV1 Channels Inhibits the Release of Acetylcholine and Improves Muscle Contractility in Mice

TRPV1 represents a non-selective transient receptor potential cation channel found not only in sensory neurons, but also in motor nerve endings and in skeletal muscle fibers. However, the role of TRPV1 in the functioning of the neuromuscular junction has not yet been fully established. In this study, the Levator Auris Longus muscle preparations were used to assess the effect of pharmacological activation of TRPV1 channels on neuromuscular transmission. The presence of TRPV1 channels in the nerve terminal and in the muscle fiber was confirmed by immunohistochemistry. It was verified by electrophysiology that the TRPV1 channel agonist capsaicin inhibits the acetylcholine release, and this effect was completely absent after preliminary application of the TRPV1 channel blocker SB 366791. Nerve stimulation revealed an increase of amplitude of isometric tetanic contractions upon application of capsaicin which was also eliminated after preliminary application of SB 366791. Similar data were obtained during direct muscle stimulation. Thus, pharmacological activation of TRPV1 channels affects the functioning of both the pre- and postsynaptic compartment of the neuromuscular junction. A moderate decrease in the amount of acetylcholine released from the motor nerve allows to maintain a reserve pool of the mediator to ensure a longer signal transmission process, and an increase in the force of muscle contraction, in its turn, also implies more effective physiological muscle activity in response to prolonged stimulation. This assumption is supported by the fact that when muscle was indirect stimulated with a fatigue protocol, muscle fatigue was attenuated in the presence of capsaicin.

Synergistic Action of Presynaptic Muscarinic Acetylcholine Receptors and Adenosine Receptors in Developmental Axonal Competition at the Neuromuscular Junction

The development of the nervous system involves the initial overproduction of synapses, which promotes connectivity. Hebbian competition between axons with different activities leads to the loss of roughly half of the overproduced elements and this refines connectivity. We used quantitative immunohistochemistry to investigate, in the postnatal day 7 (P7) to P9 neuromuscular junctions, the involvement of muscarinic receptors (muscarinic acetylcholine autoreceptors and the M1, M2, and M4 subtypes) and adenosine receptors (A1 and A2A subtypes) in the control of axonal elimination after the mouse levator auris longus muscle had been exposed to selective antagonists in vivo. In a previous study we analyzed the role of each of the individual receptors. Here we investigate the additive or occlusive effects of their inhibitors and thus the existence of synergistic activity between the receptors. The main results show that the A2A, M1, M4, and A1 receptors (in this order of ability) delayed axonal elimination at P7. M4 produces some occlusion of the M1 pathway and some addition to the A1 pathway, which suggests that they cooperate. M2 receptors may modulate (by allowing a permissive action) the other receptors, mainly M4 and A1. The continued action of these receptors (now including M2 but not M4) finally promotes axonal loss at P9. All 4 receptors (M2, M1, A1, and A2A, in this order of ability) are necessary. The M4 receptor (which in itself does not affect axon loss) seems to modulate the other receptors. We found a synergistic action between the M1, A1, and A2A receptors, which show an additive effect, whereas the potent M2 effect is largely independent of the other receptors (though can be modulated by M4). At P9, there is a full mutual dependence between the A1 and A2A receptors in regulating axon loss. In summary, postnatal axonal elimination is a regulated multireceptor mechanism that involves the cooperation of several muscarinic and adenosine receptor subtypes.

Preservation of neuromuscular function in symptomatic SOD1-G93A mice by peripheral infusion of methylene blue

In mutant superoxide dismutase 1 (SOD1) mouse models of familial amyotrophic lateral sclerosis (fALS) some of the earliest signs of morphological and functional damage occur in the motor nerve terminals that innervate fast limb muscles. This study tested whether localized peripheral application of a protective drug could effectively preserve neuromuscular junctions in late-stage disease. Methylene blue (MB), which has mitochondria-protective properties, was infused via an osmotic pump into the anterior muscle compartment of one hind limb of late pre- symptomatic SOD1-G93A mice for ≥3weeks. When mice reached end-stage disease, peak twitch and tetanic contractions evoked by stimulation of the muscle nerve were measured in two anterior compartment muscles (tibialis anterior [TA] and extensor digitorum longus [EDL], both predominantly fast muscles). With 400μM MB in the infusion reservoir, muscles on the MB-infused side exhibited on average a ~100% increase in nerve-evoked contractile force compared to muscles on the contralateral non-infused side (p<0.01 for both twitch and tetanus in EDL and TA). Pairwise comparisons of endplate innervation also revealed a beneficial effect of MB infusion, with an average of 65% of endplates innervated in infused EDL, compared to only 35% on the non-infused side (p<0.01). Results suggested that MB's protective effects required an extracellular [MB] of ~1μM, were initiated peripherally (no evidence of retrograde transport into the spinal cord), and involved MB's reduced form. Thus peripherally-initiated actions of MB can help preserve neuromuscular structure and function in SOD1-G93A mice, even at late stages of disease.

Presynaptic muscarinic acetylcholine autoreceptors (M1, M2 and M4 subtypes), adenosine receptors (A1 and A2A) and tropomyosin-related kinase B receptor (TrkB) modulate the developmental synapse elimination process at the neuromuscular junction

Background

The development of the nervous system involves an initially exuberant production of neurons that make an excessive number of synaptic contacts. The initial overproduction of synapses promotes connectivity. Hebbian competition between axons with different activities (the least active are punished) leads to the loss of roughly half of the overproduced elements and this refines connectivity and increases specificity. The neuromuscular junction is innervated by a single axon at the end of the synapse elimination process and, because of its relative simplicity, has long been used as a model for studying the general principles of synapse development. The involvement of the presynaptic muscarinic ACh autoreceptors may allow for the direct competitive interaction between nerve endings through differential activity-dependent acetylcholine release in the synaptic cleft. Then, the most active ending may directly punish the less active ones. Our previous results indicate the existence in the weakest axons on the polyinnervated neonatal NMJ of an ACh release inhibition mechanism based on mAChR coupled to protein kinase C and voltage-dependent calcium channels. We suggest that this mechanism plays a role in the elimination of redundant neonatal synapses.

Results

Here we used confocal microscopy and quantitative morphological analysis to count the number of brightly fluorescent axons per endplate in P7, P9 and P15 transgenic B6.Cg-Tg (Thy1-YFP)16 Jrs/J mice. We investigate the involvement of individual mAChR M₁-, M₂- and M₄-subtypes in the control of axonal elimination after the Levator auris longus muscle had been exposed to agonist and antagonist in vivo. We also analysed the role of adenosine receptor subtypes (A₁ and A_2A) and the tropomyosin-related kinase B receptor. The data show that postnatal axonal elimination is a regulated multireceptor mechanism that guaranteed the monoinnervation of the neuromuscular synapses.

Conclusion

The three receptor sets considered (mAChR, AR and TrkB receptors) intervene in modulating the conditions of the competition between nerve endings, possibly helping to determine the winner or the lossers but, thereafter, the final elimination would occur with some autonomy and independently of postsynaptic maturation.

Capsaicin modulates acetylcholine release at the myoneural junction

Transient receptor potential (TRP) proteins are non-selective cation channel proteins that are expressed throughout the body. Previous studies demonstrated the expression of TRP Vanilloid 1 (TRPV1), capsaicin (CAP) receptor, in sensory neurons. Recently, we reported TRPV1 expression in mouse motor nerve terminals [MNTs; (Thyagarajan et al., 2009)], where we observed that CAP protected MNTs from botulinum neurotoxin A (BoNT/A). Phrenic nerve diaphragm nerve muscle preparations (NMP) isolated from isoflurane anesthetized adult mice were analyzed for twitch tension, spontaneous (mEPCs) and nerve stimulus evoked (EPCs) acetylcholine release. When acutely applied to isolated NMP, CAP produced a concentration-dependent decline of twitch tension and produced a significant decline in the amplitude of EPCs and quantal content without any effect on the mEPCs. The suppression of nerve stimulus evoked acetylcholine release by CAP was antagonized by capsazepine (CPZ), a TRPV1 antagonist. CAP did not suppress phrenic nerve stimulus evoked acetylcholine release in TRPV1 knockout mice. Also, CAP treatment, in vitro, interfered with the localization of adapter protein 2 in cholinergic Neuro 2a cells. Wortmannin, (WMN; non-selective phosphoinositol kinase inhibitor), mimicked the effects of CAP by inhibiting the acetylcholine exocytosis. Our data suggest that TRPV1 proteins expressed at the MNT are coupled to the exo-endocytic mechanisms to regulate neuromuscular functions.