Protein Kinase C Facilitation of Acetylcholine Release at the Rat Neuromuscular Junction

Synaptic activity-related classical protein kinase C isoform localization in the adult rat neuromuscular synapse

Protein kinase C (PKC) is essential for signal transduction in a variety of cells, including neurons and myocytes, and is involved in both acetylcholine release and muscle fiber contraction. Here, we demonstrate that the increases in synaptic activity by nerve stimulation couple PKC to transmitter release in the rat neuromuscular junction and increase the level of alpha, betaI, and betaII isoforms in the membrane when muscle contraction follows the stimulation. The phosphorylation activity of these classical PKCs also increases. It seems that the muscle has to contract in order to maintain or increase classical PKCs in the membrane. We use immunohistochemistry to show that PKCalpha and PKCbetaI were located in the nerve terminals, whereas PKCalpha and PKCbetaII were located in the postsynaptic and the Schwann cells. Stimulation and contraction do not change these cellular distributions, but our results show that the localization of classical PKC isoforms in the membrane is affected by synaptic activity.

The role of the theta isoform of protein kinase C (PKC) in activity-dependent synapse elimination: evidence from the PKC theta knock-out mouse in vivo and in vitro

PKC plays a critical role in competitive activity-dependent synapse modification at the neuromuscular synapse in vitro and in vivo. This action involves a reduction of the strength of inactive inputs to muscle cells that are activated by other inputs. A decrease of postsynaptic responsiveness and a loss of postsynaptic acetyl choline receptors account for the heterosynaptic loss in vitro. The loss is not seen in preparations in which PKC has been blocked pharmacologically. Here, we show that the loss does not occur in in vitro preparations made from animals genetically modified to lack the theta isoform of PKC. Synapse elimination in the newborn period in vivo is delayed but is eventually expressed in knock-out animals. PKC-dependent synapse reduction is suppressed in heterologous cultures combining normal nerve and PKC theta-deficient muscle, as might be expected from the postsynaptic locus of the changes that underlie the activity-dependent plasticity. Preparations in which PKC theta-deficient neurons innervated normal muscle also exhibited a marked deficit in PKC-deficient synapse reduction. The presynaptic action of PKC theta implied by this observation is blocked by TTX, and we propose that activity-related synapse strengthening is decreased by presynaptic PKC theta. Thus, PKC theta in both presynaptic and postsynaptic elements plays a critical role in activity-dependent synapse modulation and loss. We provide a model for activity-dependent synapse loss incorporating these findings.

Activation of metabotropic glutamate receptors enhances synaptic transmission at the Drosophila neuromuscular junction

We examined the effects of activation of metabotropic glutamate receptors (mGluRs) on glutamatergic synaptic transmission at the neuromuscular junction of newly hatched Drosophila larvae. In nominally Ca(2+)-free solutions puff-application of low concentrations of glutamate evoked a transient frequency increase of miniature synaptic currents (mSCs). The mean amplitude of mSCs was unaffected, suggesting that this effect was presynaptic. Similar alterations of the mSC frequency were obtained using the mGluR agonists, (S)-4C3HPG, DCG-IV, or (1S,3S)-ACPD, but not when using agonists for ionotropic glutamate receptors, NMDA, AMPA or kainate. An mGluR antagonist, MCCG-I, blocked the effect of agonists on the mSC frequency. An adenylate cyclase activator, forskolin, and a cAMP analog, CPT-cAMP, mimicked the effects of mGluR activation. Meanwhile, an adenylate cyclase inhibitor, SQ22,536, blocked the mGluR agonist-induced effects, and in rutabaga, an adenylate-cyclase-defective mutant, the effect of the agonist was greatly reduced. In the presence of external Ca2+, (S)-4C3HPG decreased the failure rate and increased the mean amplitude of stimulus-evoked SCs, while MCCG-I decreased the amplitudes. We suggest that at the larval Drosophila neuromuscular junction endogenous glutamate released at the terminal potentiates synaptic transmission via a process involving cAMP.

Miniature endplate current kinetics at the mouse neuromuscular junction: effects of temperature and medium viscosity

The time course of miniature endplate currents (MEPCs), derived by extracellular focal recording, was studied in the mouse neuromuscular junction at different temperatures and medium viscosities, and in eserine-treated endplates. At low temperatures (6-10 degrees C), almost the whole MEPC decay is exponential and the rising phase is not significantly modified by the channel closing process. At physiological temperatures (38-40 degrees C), the early part of the decay is much slower than the later part and the rising phase is made shorter and smaller by channel closing, showing that the channel opening and channel closing processes overlap remarkably. Even at physiological temperatures, however, the late part of the MEPC decay shows an exponential time course. At high temperatures the channel opening process has low temperature sensitivity and slows down when bath solution viscosity is increased, suggesting that at high temperatures channel opening kinetics may mainly be controlled by the time course of acetylcholine diffusion. The lower limit of conformational change rate leading to channel opening was estimated at 10 degrees C (4400 s-1). Experimentally recorded MEPCs were mathematically simulated to obtain a quantitative description of the processes controlling MEPC generation. Mathematical simulation further suggests that (i) acetylcholine diffusion kinetics may affect the onset rate of MEPCs without, however, being rate-limiting; and (ii) partial, transient acetylcholinesterase inhibition operated by acetylcholine may explain the low temperature sensitivity exhibited by the onset rate of MEPCs at high temperatures.

TNF-alpha increases the frequency of spontaneous miniature synaptic currents in cultured rat hippocampal neurons

Tumor necrosis factor-alpha (TNF-alpha) is a cytokine secreted by activated astrocytes and is known to alter evoked synaptic activity in slices of adult rat hippocampus. In this paper we show that TNF-alpha increases the frequency of spontaneous miniature synaptic currents in cultured hippocampal neurons, acting at nanomolar concentrations. In addition, we show that the mRNA for the 55 kDa TNF-alpha receptor (TNF-R1) is detected in embryonic rat hippocampal cultures, as well as in acutely dissected embryonic and adult rat hippocampi. Possible transduction pathways mediating the TNF-alpha effect are discussed.

The upregulation of acetylcholine release at endplates of alpha-bungarotoxin-treated rats: its dependency on calcium

1. The presynaptic component of an adaptive feedback mechanism leading to increased acetylcholine (ACh) release was studied in endplates of diaphragms from rats treated chronically with alpha-bungarotoxin (alpha BTX). 2. Quantal contents were calculated 'directly' from the amplitude of miniature endplate potentials (MEPPs) and endplate potentials (EPPs) which were recorded after mu-conotoxin treatment to prevent muscle action potentials. 3. In vitro application of the Ca2+ channel blockers nifedipine (10 microM) or omega-conotoxin (40 nM) had no significant effect on the increased quantal content of endplates from alpha BTX-treated rats. 4. At control endplates, in vitro block of presynaptic K+ channels by 5 microM 3,4-diaminopyridine did increase the quantal content to a level which was similar to that found in endplates of alpha BTX-treated rats but also induced a broadening of EPPs, which was not found at endplates after alpha BTX treatment. 5. The difference between quantal contents of alpha BTX-treated and control rats was highly dependent on the [Ca2+]o/[Mg2+]o ratio when [Mg2+]o was fixed at 1 mM. At low [Ca2+]o, the quantal content of endplates from alpha BTX-treated rats was lower than that of controls while at [Ca2+]o in the normal and high range this was reversed. However, changing the [Ca2+]o/[Mg2+]o ratio by means of [Mg2+]o, at a fixed [Ca2+]o of 2 mM, did not influence the relative increase of quantal contents at endplates from alpha BTX-treated rats. Double logarithmic plots of the 'toxin-induced' myasthenia gravis (TIMG) and control quantal content versus [Ca2+]o had an approximately linear part between 0.2 and 1.5 mM [Ca2+]o. The slopes of the TIMG and control lines were 1.81 and 0.96, indicating that the ACh release in TIMG muscles was more sensitive to changes of [Ca2+]o than controls. 6. At normal [Ca2+]o and [Mg2+]o, the depression of EPP amplitude during stimulation of the phrenic nerve at 30-50 Hz was somewhat larger at endplates from alpha BTX-treated rats than at control endplates. At low [Ca2+]o, the potentiation of EPP amplitudes during a stimulus train was much larger at endplates from alpha BTX-treated rats than from controls. 7. The results do not support the idea that the increased release of ACh is caused via regulatory effects on the presynaptic Ca2+ or K+ channels. Instead, the anomalous dependency of ACh release on Ca2+ in muscles of alpha BTX-treated rats suggests that a cytoplasmic, Ca(2+)-dependent, component is involved in the adaptive change of transmitter release.

Ephedrine: effects on neuromuscular transmission

(-)-Ephedrine has been used in the treatment of patients with myasthenia gravis. To investigate the possible effects of ephedrine on neuromuscular transmission, canine intercostal muscle endplates were studied by microelectrode techniques. At concentrations less than 10(-4) M, ephedrine had no effect on neuromuscular transmission. At a concentration of 10(-4) M, ephedrine increased the quantal content of the endplate potential by 21%. The presynaptic store of acetylcholine quanta available for immediate release was unchanged, but the probability of quantal release was increased by 16%. At this concentration, ephedrine decreased the amplitude of the miniature endplate potential by 38%. In the presence of 10(-3) M ephedrine, the miniature endplate potentials and currents became undetectable. The kinetic properties of the acetylcholine receptor channel were studied by analysis of acetylcholine-induced endplate current noise. At 10(-4) M, ephedrine reduced the channel conductance by 43% but had no effect on the open time. At 5 x 10(-4) M, ephedrine reduced the channel conductance by 84% and increased the open time by 23 percent.