Metabotropic GABAB receptors mediate GABA inhibition of acetylcholine release in the rat neuromuscular junction

Multi targets of cannabidiol (CBD) on skeletal mammalian and avian neuromuscular preparations

Cannabidiol (CBD) has been used in diseases that affect the central nervous system. Its effects on the peripheral synapses are of great interest, since endocannabinoid receptors are expressed in muscles. CBD (0.3 mM) was analysed using mammalian and avian neuromuscular preparations, through myographic techniques in complementary protocols. Mammalian cells were examined by light microscopy while exogenous acetylcholine (40 µM) and potassium chloride (100 mM) were added into avian preparations, before and at the end of experiments. Pharmacological tools such as atropine (2 µM), polyethylene glycol (PEG 400, 20 µM), Ca2+ (1.8 mM), F55-6 (20 µg/mL), and nifedipine (1.3 mM) were assessed with CBD. In mice, CBD causes a facilitatory effect and paralysis, whereas in avian, paralysis. Concluding, CBD is responsible for activated or inhibited channels, for ACh release via muscarinic receptor modulation, and by the inhibition of nicotinic receptors leading to neuromuscular blockade, with no damage to striated muscle cells.

Effects of cholesterol oxidase on neurotransmission and acetylcholine levels at the mice neuromuscular junctions

Membrane cholesterol oxidation is a hallmark of redox and metabolic imbalance, and it may accompany neurodegenerative disorders. Using microelectrode recordings of postsynaptic responses as well as fluorescent dyes for monitoring synaptic vesicle cycling and membrane properties, the action of enzymatic cholesterol oxidation on neuromuscular transmission was studied in the mice diaphragms. Cholesterol oxidase (ChO) at low concentration disturbed lipid-ordering specifically in the synaptic membranes, but it did not change markedly spontaneous exocytosis and evoked release in response to single stimuli. At low external Ca2+ conditions, analysis of single exocytotic events revealed a decrease in minimal synaptic delay and the probability of exocytosis upon plasmalemmal cholesterol oxidation. At moderate- and high-frequency activity, ChO treatment enhanced both neurotransmitter and FM-dye release. Furthermore, it precluded a change in exocytotic mode from full-fusion to kiss-and-run during high-frequency stimulation. Accumulation of extracellular acetylcholine (without stimulation) dependent on vesamicol-sensitive transporters was suppressed by ChO. The effects of plasmalemmal cholesterol oxidation on both neurotransmitter/dye release at intense activity and external acetylcholine levels were reversed when synaptic vesicle membranes were also exposed to ChO (i.e., the enzyme treatment was combined with induction of exo-endocytotic cycling). Thus, we suggest that plasmalemmal cholesterol oxidation affects exocytotic machinery functioning, enhances synaptic vesicle recruitment to the exocytosis and decreases extracellular neurotransmitter levels at rest, whereas ChO acting on synaptic vesicle membranes suppresses the participation of the vesicles in the subsequent exocytosis and increases the neurotransmitter leakage. The mechanisms underlying ChO action can be related to the lipid raft disruption.

Investigation on property differences of ginseng and American ginseng by spatial metabolomics of neurochemicals with desorption electrospray ionization mass spectrometry imaging

ETHNOPHARMACOLOGICAL RELEVANCE

The properties are the body's response to traditional Chinese medicine (TCM). The essence of traditional Chinese medicine properties are cold, hot, warm, and cool. In the theory of traditional Chinese medicine, ginseng is warm and American ginseng is cool, they present two opposite properties. The material basis of property differences and effect mechanism of property degree need further investigation.

AIM OF THE STUDY

The aim of this work was to screen out the neurochemicals related to warm and cool properties of ginseng and American ginseng, and investigate the distributions of identified neurochemicals in rat brain and the metabolic mechanism.

MATERIALS AND METHODS

Spatial metabolomics was applied to study the effects of ginseng and American ginseng on the distributions of neurochemicals in rat brain by desorption electrospray ionization mass spectrometry imaging (DESI-MSI). Based on discriminant coefficients in partial least square discriminant analysis (PLS-DA) processing, neurochemicals related to warm and cool properties were classified. In addition, the score contributions of the neurochemicals markers could be used to evaluate the warm and cool property degrees.

RESULTS

A total of 25 neurochemicals were imaged and identified in brain section. The distribution regions of main neurochemicals were consistent with in sagittal and coronal sections of brain reported in literature. 17 neurochemicals were classified as warm markers. Meanwhile, 8 neurochemicals were identified as cool markers, correlated with the cool properties of American ginseng. It demonstrated that the score contributions of the 25 neurochemicals markers could be used to evaluate the warm and cool property degrees. Based on the regulatory effects of neurochemicals, the warm markers could promote the body's energy metabolism, improve the function of endocrine system, and enhance the excitability of central nervous system. The cool property markers have reduced excitability of central nervous system, weakened metabolism and stress response ability, thus presented the biological activity of cool and cold.

CONCLUSIONS

Our findings provided a rapid and effective visualization method for the spatial distribution and metabolism of small molecular neurochemicals in rat brain. DESI-MSI was a reference methodology for evaluating the properties of TCM.

GABA in developing rat skeletal muscle and motor neurons

In recent years, considerable evidence is accumulated pointing to participation of gamma-aminobutyric acid (GABA) in intercellular signaling in the peripheral nervous system, including, in particular, neuromuscular transmission. However, where in the neuromuscular synapse GABA is synthesized remains not quite clear. We used histochemical methods to detect GABA and L-glutamate decarboxylase (GAD) in developing skeletal muscle fibers and in cultured motor neurons. We found that GABA can be detected already in myocytes, but with further muscle maturation, GABA synthesis gradually attenuates and completely ceases in early postnatal development. We found also that formation of GABA in muscle tissue does not depend on activity of GAD, but presumably proceeds through some other, alternative pathways. In motor neurons, GABA and GAD can be detected at the early stage of development (prior to synapse formation). Our data support the hypothesis that GABA and GAD, which are detectable in adult neuromuscular junctions, have neuronal origin. The mechanism of GABA production and its role in developing muscle tissue need further clarification.

Components of the GABAergic signaling in the peripheral cholinergic synapses of vertebrates: a review

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian central nervous system. Since the 1970s, many studies have focused on the role of GABA in the mammalian peripheral nervous system, and particularly in the cholinergic synapses. In this review, we present current findings for the cholinergic neurons of vegetative ganglia as well as for the neurons innervating smooth and striated muscles. Synaptic contacts formed by these neurons contain GABA and the enzyme, glutamic acid decarboxylase, which catalyzes the synthesis of GABA from glutamate. Newly formed GABA is released in the cholinergic synapses and mostly all the peripheral cholinergic synaptic contacts contain iono- and metabotropic GABA receptors. Although the underlying molecular mechanism of the release is not well understood, still, it is speculated that GABA is released by a vesicular and/or non-vesicular way via reversal of the GABA transporter. We also review the signaling role of GABA in the peripheral cholinergic synapses by modulating acetylcholine release, but its exact physiological function remains to be elucidated.

Enhancement of mouse diaphragm contractility in the presence of antagonists of GABAA and GABAB receptors

NEW FINDINGS

What is the central question of this study? Do GABA receptors play any role at the neuromuscular junction? What is the main finding and its importance? In the presence of either ionotropic or metabotropic GABA receptor antagonists, diaphragm muscle force production elicited by stimulating the motor nerve at ≥50 Hz was increased. Our data indicate the presence of GABAergic signalling at the neuromuscular junction.

ABSTRACT

Despite the signalling role of GABA in the brain and spinal cord, the role of this molecule in the peripheral nervous system and, in particular, at the neuromuscular junction remains practically unexplored. In the present work, the force of mouse diaphragm contractions was measured in the presence of blockers of metabotropic GABA_B receptors (CGP 55845) and ionotropic GABA_A receptors (picrotoxin) with various patterns of indirect and direct stimulation of muscle by trains of 40 pulses delivered at 10, 20, 50 and 70 Hz. It was found that neither blocker affected the diaphragm contractility caused by indirect stimulation through the motor nerve at 10 and 20 Hz. However, when the stimulation frequency was increased to 50 or 70 Hz, the force of subsequent contractions in the train (when compared with the amplitude of contraction in response to the first pulse) was increased by both CGP 55845 and picrotoxin. With direct stimulation of the diaphragm, no significant changes in the contraction force were detected at any frequency used. The results obtained support the following conclusions: (i) pharmacological inhibition of GABA receptors increases the contractile activity of skeletal muscle; and (ii) frequency-dependent enhancement of GABA receptor activation takes place in the region of the neuromuscular junction.

On the issue of closed versus open forms of gamma-aminobutyric acid (GABA) in water: Ab initio molecular dynamics and metadynamics studies

Gamma-aminobutyric acid (GABA), a primary neurotransmitter, accomplishes its activities by binding to different receptor sites in different conformations. It is known to have two major conformers: the closed and open forms. Earlier studies on preferred conformation of GABA in water revealed differing results with some reporting the open form while others inferring the closed form to be more stable. We found the existence of many open forms and only one closed form of GABA in water through ab initio metadynamics simulation. Some of the open conformers are equally or more stable while others are less stable than the closed form. Free energy barriers reveal that different conformers are interconvertible at room temperature in typical experimental time scales. Ab initio molecular dynamics simulations are performed to further investigate the inter-conversion of various conformers of GABA in water and their dipole moments and also to make connections to experiments on the conformation of GABA in water.

Autoregulation of Acetylcholine Release and Micro-Pharmacodynamic Mechanisms at Neuromuscular Junction: Selective Acetylcholinesterase Inhibitors for Therapy of Myasthenic Syndromes

Neuromuscular junctions (NMJs) are directly involved into such indispensable to life processes as respiration and locomotion. However, motor nerve forms only one synaptic contact at each muscle fiber. This unique configuration requires specific properties and constrains to be effective. The very high density of acetylcholine receptors (AChRs) of muscle type in synaptic cleft and an excess of acetylcholine (ACh) released under physiological conditions make this synapse extremely reliable. Nevertheless, under pathological conditions such as myasthenia gravis and congenital myasthenic syndromes, the safety factor can be markedly reduced. Drugs used for short-term symptomatic therapy of these pathological states, cause partial inhibition of cholinesterases (ChEs). These enzymes catalyze the hydrolysis of ACh, thus terminate its action on AChRs. Extension of the lifetime of ACh molecules compensates muscular AChRs abnormalities and, consequently, rescues muscle contractions. In this mini review, we will first outline the functional organization of the NMJ, and then, consider the concept of the safety factor and how it may be changed. This will be followed by a look at autoregulation of ACh release that influences the safety factor of NMJs. Finally, we will consider the morphological features of NMJs as a putative reserve to increase effectiveness of pathological muscle weakness therapy by ChEs inhibitors due to opportunity to use micro-pharmacodynamic mechanisms.

Elements of molecular machinery of GABAergic signaling in the vertebrate cholinergic neuromuscular junction

It is generally accepted that gamma-aminobutyric acid (GABA) is a signaling molecule abundant in central synapses. In a number of studies though, it has been shown that GABA signaling functions in the peripheral nervous system as well, in particular, in the synapses of sympathetic ganglia. However, there exists no firm evidence on the presence of GABAergic signaling cascade in the intercellular junctions of the somatic nerve system. By the use of immunohistochemistry methods, in the synaptic area of cholinergic neuromuscular contact in rat diaphragm, we have detected glutamate decarboxylase, the enzyme involved in synthesis of GABA, molecules of GABA, and also GAT-2, a protein responsible for transmembrane transport of GABA. Earlier we have also shown that metabotropic GABA_B receptors have overlapping localization in the same compartment. Moreover, activation of GABA_B receptors affects the intensity of acetylcholine release. These data taken together, allows us to suggest that in the mammalian cholinergic neuromuscular junction, GABA is synthesized and performs certain synaptic signaling function.