Mutual dependence of calcitonin-gene related peptide and acetylcholine release in neuromuscular preparations

Localization of sensory nerve terminals containing calcitonin gene-related peptide (CGRP) on striated muscle fibers in the rat esophagus: Evidence for triple innervation via motor endplates

BACKGROUND

Many esophageal striated muscles of mammals are dually innervated by the vagal and enteric nerves. Recently, substance P (SP)-sensory nerve terminals with calcitonin gene-related peptide (CGRP) were found on a few striated muscle fibers in the rat esophagus, implying that these muscle fibers are triply innervated. In this study, we examined the localization and origin of CGRP-nerve endings in striated muscles to consider their possible roles in the esophagus regarding triple innervation.

METHODS

Wholemounts of the rat esophagus were immunolabeled to detect CGRP-nerve endings in striated muscles. Also, retrograde tracing was performed by injecting Fast Blue (FB) into the esophagus, and cryostat sections of the medulla oblongata, nodose ganglion (NG), and the tenth thoracic (T10) dorsal root ganglion (DRG) were immunostained to identify the origin of the CGRP-nerve endings.

RESULTS

CGRP-fine, varicose nerve endings were localized in motor endplates on a few esophageal striated muscle fibers (4 %), most of which received nitric oxide (NO) synthase nerve terminals, and most of the CGRP nerve endings were SP- and transient receptor potential vanilloid member 1 (TRPV1)-positive. Retrograde tracing showed many FB-labeled CGRP-neurons positive for SP and TRPV1 in the NG and T10 DGR.

CONCLUSIONS

This study suggests that the CGRP-varicose nerve endings containing SP and TRPV1 in motor endplates are sensory, and a few esophageal striated muscle fibers are triply innervated. The nerve endings may detect acetylcholine-derived acetic acid from the vagal motor nerve endings and NO from esophageal intrinsic nerve terminals in the motor endplates to regulate esophageal motility.

Localization of substance P (SP)-immunoreactivity in the myenteric plexus of the rat esophagus

The present immunohistochemical study was performed to examine the number, distribution, and chemical coding of intrinsic substance P (SP) neurons and nerve fibers within the esophagus and discuss their functional roles. Many SP neurons and nerve fibers were found in the myenteric plexus, and the SP neurons gradually decreased from the oral side toward the aboral side of the esophagus. Double-immunolabeling showed that most SP neurons were cholinergic (positive for choline acetyltransferase), and few were nitrergic (positive for nitric oxide synthase). Some cholinergic SP nerve terminals surrounded cell bodies of several myenteric neurons. In the muscularis mucosa and lower esophageal sphincter, and around blood vessels, numerous SP nerve endings were present, and many of them were cholinergic. Also, SP nerve endings were found on only a few motor endplates of the striated muscles, and most of them were calcitonin gene-related peptide (CGRP)-positive. Retrograde tracing using Fast Blue (FB) showed that numerous sensory neurons in the dorsal root ganglia (DRGs) and nodose ganglion (NG) projected to the esophagus, and most FB-labeled SP neurons were CGRP-positive. These results suggest that the intrinsic SP neurons in the rat esophagus may play roles as, at least, motor neurons, interneurons, and vasomotor neurons, which are involved in local regulation of smooth muscle motility, neuronal transmission, and blood circulation, respectively. Moreover, SP nerve endings on only a minority of motor endplates may be extrinsic, derived from DRGs or NG, and possibly detect chemical circumstances within motor endplates to modulate esophageal motility.

Delayed increase of acetylcholine quantal size induced by the activity-dependent release of endogenous CGRP but not ATP in neuromuscular junctions

In mouse motor synapses tetanic neuromuscular activity (30 Hz, 2 min) led to a delayed posttetanic potentiation of amplitude and duration of spontaneous miniature endplate potentials (MEPPs). Microelectrode recordings of MEPPs before and after nerve stimulation showed an increase in MEPP amplitude and time course by 30% and 15%, respectively, without changes in their frequency. Peak effect was detected 20 min after tetanic activity and progressively faded throughout the next 40 min of recording. The revealed potentiation of MEPPs was fully preserved in preparations from pannexin 1 knockout mice. It means, that myogenic ATP released via pannexin 1 channels from contracting muscle fibers is not likely to participate in the described phenomenon. But posttetanic potentiation of MEPPs was fully prevented by competitive antagonist of calcitonin gene-related peptide (CGRP) receptors CGRP_8-37 , ryanodine receptors inhibitor ryanodine and by vesicular acetylcholine transporter inhibitor vesamicol. It is suggested that the combination of intensive synaptic and contractile activity in neuromuscular junctions is required to induce Ca²⁺ -dependent exocytosis of endogenous CGRP. The accumulation of CGRP in the synaptic cleft and its presynaptic activity may induce posttetanic potentiation of MEPP amplitude due to CGRP-stimulated acetylcholine loading into vesicles and subsequent increase of quantal size.

Ryanodine- and CaMKII-dependent release of endogenous CGRP induces an increase in acetylcholine quantal size in neuromuscular junctions of mice

OBJECTIVE

The aim of this study was to identify the mechanism responsible for an increase in miniature endplate potentials (MEPPs) amplitude, induced by ryanodine as an agonist of ryanodine receptors in mouse motor nerve terminals.

METHODS

Using intracellular microelectrode recordings of MEPPs and evoked endplate potentials (EPPs), the changes in spontaneous and evoked acetylcholine release in motor synapses of mouse diaphragm neuromuscular preparations were studied.

RESULTS

Ryanodine (0.1 μM) increased both the amplitudes of MEPPs and EPPs to a similar extent (up to 130% compared to control). The ryanodine effect was prevented by blockage of receptors of calcitonin gene-related peptide (CGRP) by a truncated peptide CGRP_8-37 . Endogenous CGRP is stored in large dense-core vesicles in motor nerve terminals and may be released as a co-transmitter. The ryanodine-induced increase in MEPPs amplitude may be fully prevented by inhibition of vesicular acetylcholine transporter by vesamicol or by blocking the activity of protein kinase A with H-89, suggesting that endogenous CGRP is released in response to the activation of ryanodine receptors. Activation of CGRP receptors can, in turn, upregulate the loading of acetylcholine into synaptic vesicles, which will increase the quantal size. This new feature of endogenous CGRP activity looks similar to recently described action of exogenous CGRP in motor synapses of mice. The ryanodine effect was prevented by inhibitors of Ca/Calmodulin-dependent kinase II (CaMKII) KN-62 or KN-93. Inhibition of CaMKII did not prevent the increase in MEPPs amplitude, which was caused by exogenous CGRP.

CONCLUSIONS

We propose that the activity of presynaptic CaMKII is necessary for the ryanodine-stimulated release of endogenous CGRP from motor nerve terminals, but CaMKII does not participate in signaling downstream the activation of CGRP-receptors followed by quantal size increase.

An expansion of Simons' integrated hypothesis of trigger point formation

Simons' integrated hypothesis proposed a model of trigger point (TrP) activation to explain known TrP phenomena, particularly endplate noise. We propose an expansion of this hypothesis to account for new experimental data and established muscle pathophysiology.

Calcitonin gene-related peptide potentiates nicotinic acetylcholine receptor-operated slow Ca2+ mobilization at mouse muscle endplates

1. The involvement of calcitonin gene-related peptide (CGRP) in the non-contractile slow Ca2+ mobilization induced by prolonged nicotinic stimulation was investigated by measurement of [Ca2+], levels in mouse single muscle cells (flexor digitorum brevis; FDB) loaded with a Ca2+ indicator fluo-3 using confocal laser scanning microscopy. 2. CGRP (3-30 nM) potentiated acetylcholine (ACh, 1 microM)-elicited slow Ca2+ mobilization in a concentration-dependent manner. 3. The potentiation by CGRP of the slow Ca2+ component was greatly depressed by a competitive nicotinic antagonist (+)-tubocurarine (5 microM). The Ca2+ channel blocker nitrendipine (1 microM) affected neither ACh responses nor the CGRP potentiation. 4. The slow Ca2+ component was completely abolished by reducing [Ca2+]0 from 2.5 to 0.25 mM whereas the fast component was not affected. The CGRP-induced potentiation of slow Ca2+ signal was also depressed by decreasing [Ca2+]0. 5. Isoproterenol (30 microM) and 8-bromo-adenosine 3',5'-cyclic monophosphate (1 mM) potentiated the ACh-elicited slow Ca2+ response. The potentiation by CGRP of the slow Ca2+ component was completely abolished by a protein kinase-A inhibitor H-89 (1 microM). 6. These findings indicate that CGRP potentiates the nicotinic ACh receptor-operated slow Ca2+ signal via the activation of protein kinase-A system at the skeletal muscle endplates.

Calcitonin gene-related peptide acts presynaptically to increase quantal size and output at frog neuromuscular junctions

1. Calcitonin gene-related peptide (CGRP) is found in dense-cored vesicles in the motor nerve terminal. 2. Exogenous CGRP increased the size of the quanta. The increase in size reached a maximum after about 40 min. The lowest effective concentration of human CGRP (hCGRP) was 0.8 nM. The action of hCGRP was antagonized by (-)-vesamicol, a drug that blocks active acetylcholine (ACh) uptake into synaptic vesicles, so it appears that hCGRP increases size by adding more ACh to the quanta. The action of hCGRP was antagonized by drugs that block the activation of protein kinase A (PKA). (In other preparations CGRP also activates PKA.) 3. The hCGRP effect was not blocked by fragment 8-37, an antagonist of one class of CGRP receptor. 4. hCGRP increases evoked quantal output and miniature endplate potential (MEPP) frequency, again by activating PKA. 5. CGRP release was measured by radioimmunoassay. Release was increased by depolarization with elevated K+, but the amounts released appear to be below those needed to affect quantal size or output. Moreover, although elevated K+ can increase quantal size it acts by a pathway that does not involve PKA. We suggest that the most likely target of endogenously released CGRP is the regulation of circulation of the muscle.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.