Search

• NA decreases spontaneous ACh release via β adrenoreceptors and AD acts via α receptors. • The EPP quantal content of the evoked ACh quanta release is modulated by catecholamines via α adrenoreceptors. • NA desynchronizes and AD synchronizes ACh secretion via both α and β adrenoreceptors. • Activation of both α and β adrenoreceptors influence spontaneous quantal ACh secretion. Despite the long history of investigations of adrenergic compounds and their biological effects, specific mechanisms of their action in distinct compartments of the motor unit remain obscure. Recent results have suggested that not only skeletal muscles but also the neuromuscular junctions represent important targets for the action of catecholamines. In this paper, we describe the effects of adrenaline and noradrenaline on the frequency of miniature endplate potentials, the quantal content of the evoked endplate potentials and the kinetics of acetylcholine quantal release in the motor nerve endings of the mouse diaphragm. Noradrenaline and adrenaline decreased the frequency of the spontaneous release of acetylcholine quanta. The effect of noradrenaline was prevented by the β adrenoreceptor blocker propranolol, whereas the action of adrenaline was abolished by the α adrenoreceptor antagonist phentolamine. Noradrenaline did not alter the quantal content of endplate potentials, while adrenaline suppressed the evoked release of acetylcholine. Blocking the α adrenoreceptors prevented the decrease in quantal secretion caused by adrenaline. Quantal release became more asynchronous under noradrenaline, as evidenced by a greater dispersion of real synaptic delays; in contrast, adrenaline synchronized the release process. Our data suggest an involvement of α and β adrenoreceptors in the diverse modulation of the frequency of miniature endplate potentials, the quantal content of the evoked endplate potentials and the kinetics of acetylcholine quantal secretion in the mouse neuromuscular junction. Moreover, the adrenoblockers affected both the evoked and spontaneous quantal release of acetylcholine, suggesting the presence of endogenous catecholamines in the vicinity of cholinergic synapses. [ABSTRACT FROM AUTHOR]

G protein-gated inwardly rectifying potassium (GIRK) channels are one of the main regulators of neuronal excitability. Activation of GIRK channels in the CNS usually leads to postsynaptic inhibition. However, the function of GIRK channels in the presynaptic processes, notably neurotransmitter release form motor nerve terminals, is yet to be comprehensively understood. Here, using electrophysiological and fluorescent approaches, the role of GIRK channels in neurotransmitter release from frog motor nerve terminals was studied. We found that the inhibition of GIRK channels with nanomolar tertiapin-Q synchronized exocytosis events with action potential but suppressed spontaneous and evoked neurotransmitter release, as well as Ca2+ transient and membrane permeability for K+. The action of GIRK channel inhibition on evoked neurotransmission was prevented by selective antagonist of voltage-gated Ca2+ channels of L-type. Furthermore, the effects of muscarinic acetylcholine receptor activation on neurotransmitter release, Ca2+ transient and K+ channel activity were markedly modulated by inhibition of GIRK channels. Thus, at the motor nerve terminals GIRK channels can regulate timing of neurotransmitter release and be a positive modulator of synaptic vesicle exocytosis acting partially via L-type Ca2+ channels. In addition, GIRK channels are key players in a feedback control of neurotransmitter release by muscarinic acetylcholine receptors. [Display omitted] • GIRK channels modulate intensity and timing of neurotransmitter release in NMJs. • Inhibition of GIRK channels lead to suppression and synchronization of transmitter release. • These effects can be result of decrease in LTCC activity and hence [Ca2+] in transient. • M2AChRs differentially regulate GIRK channel in [Ca2+] out -dependent manner at NMJs. [ABSTRACT FROM AUTHOR]

Neurotransmitter release from the synaptic vesicles can occur through two modes of exocytosis: "full-collapse" or "kiss-and-run". Here we investigated how increasing the nerve activity and pharmacological stimulation of adrenoceptors can influence the mode of exocytosis in the motor nerve terminal. Recording of endplate potentials with intracellular microelectrodes was used to estimate acetylcholine release. A fluorescent dye FM1–43 and its quenching with sulforhodamine 101 were utilized to visualize synaptic vesicle recycling. An increase in the frequency of stimulation led to a decrease in the rate of FM1–43 unloading despite the higher number of quanta released. High frequency activity promoted neurotransmitter release via the kiss-and-run mechanism. This was confirmed by experiments utilizing (I) FM1–43 dye quencher, that is able to pass into the synaptic vesicle via fusion pore, and (II) loading of FM1–43 by compensatory endocytosis. Noradrenaline and specific α2-adrenoreceptors agonist, dexmedetomidine, controlled the mode of synaptic vesicle recycling at high frequency activity. Their applications favored neurotransmitter release via full-collapse exocytosis rather than the kiss-and-run pathway. At the diaphragm neuromuscular junctions, neuronal commands are translated into contractions necessary for respiration. During stress, an increase in discharge rate of the phrenic nerve shifts the exocytosis from the full-collapse to the kiss-and-run mode. The stress-related molecule, noradrenaline, restricts neurotransmitter release in response to a high frequency activity, and prevents the shift in the mode of exocytosis through α2-adrenoceptor activation. This may be a component of the mechanism that limits overstimulation of the respiratory system during stress. [Display omitted] • Exocytic mode in motor terminal is dependent on frequency of nerve stimulation in diaphragm. • Increased phrenic nerve activity shifts the exocytosis from full-collapse to kiss-and-run. • Noradrenaline and α2-adrenomimetic prevent the shift in the mode of exocytosis. • Noradrenaline restricts neurotransmitter release in response to high frequency activity. • The adrenergic control of exocytosis can limit overstimulation of respiratory system. [ABSTRACT FROM AUTHOR]

Abstract: For the first time the venom of recently established viper species Vipera nikolskii was fractionated and two heterodimeric phospholipases A2 (HDP-1 and HDP-2) were isolated. Isolation of HDP-1 and HDP-2 is the first indication of the presence of two heterodimeric phospholipases A2 in the venom of one viper species. When tested on the frog neuromuscular junction, isolated proteins affected neuromuscular transmission acting presynaptically. Using RP-HPLC, each heterodimer was separated into two monomeric subunits: basic phospholipase A2 (HDP-1P and HDP-2P) and acidic component without enzymatic activity (HDP-In). The complete primary structures of subunits were deduced from corresponding sequences of cDNAs. The determined amino acid sequences were homologous to those of vipoxin from Vipera ammodytes and vaspin from Vipera aspis. Similar proteins were not found earlier in the well-studied venom of Vipera berus, the species from which V. nikolskii was recently separated. Our finding supports at the biochemical level the correctness of the establishment of V. nikolskii as an independent species. The finding of similar proteins (HDPs and vipoxin) in geographically remote species (V. nikolskii and V. ammodytes) corroborates the hypothesis about the pre-existence of genes encoding these proteins in all true viper species and their expression under certain conditions. [Copyright &y& Elsevier]