Noradrenaline modulates transmitter release by enhancing the Ca2+ sensitivity of exocytosis in the chick ciliary presynaptic terminal

β-Adrenergic Receptors/Epac Signaling Increases the Size of the Readily Releasable Pool of Synaptic Vesicles Required for Parallel Fiber LTP

The second messenger cAMP is an important determinant of synaptic plasticity that is associated with enhanced neurotransmitter release. Long-term potentiation (LTP) at parallel fiber (PF)-Purkinje cell (PC) synapses depends on a Ca²⁺-induced increase in presynaptic cAMP that is mediated by Ca²⁺-sensitive adenylyl cyclases. However, the upstream signaling and the downstream targets of cAMP involved in these events remain poorly understood. It is unclear whether cAMP generated by β-adrenergic receptors (βARs) is required for PF-PC LTP, although noradrenergic varicosities are apposed in PF-PC contacts. Guanine nucleotide exchange proteins directly activated by cAMP [Epac proteins (Epac 1-2)] are alternative cAMP targets to protein kinase A (PKA) and Epac2 is abundant in the cerebellum. However, whether Epac proteins participate in PF-PC LTP is not known. Immunoelectron microscopy demonstrated that βARs are expressed in PF boutons. Moreover, activation of these receptors through their agonist isoproterenol potentiated synaptic transmission in cerebellar slices from mice of either sex, an effect that was insensitive to the PKA inhibitors (H-89, KT270) but that was blocked by the Epac inhibitor ESI 05. Interestingly, prior activation of these βARs occluded PF-PC LTP, while the β1AR antagonist metoprolol blocked PF-PC LTP, which was also absent in Epac2 ^-/- mice. PF-PC LTP is associated with an increase in the size of the readily releasable pool (RRP) of synaptic vesicles, consistent with the isoproterenol-induced increase in vesicle docking in cerebellar slices. Thus, the βAR-mediated modulation of the release machinery and the subsequent increase in the size of the RRP contributes to PF-PC LTP.SIGNIFICANCE STATEMENT G-protein-coupled receptors modulate the release machinery, causing long-lasting changes in synaptic transmission that influence synaptic plasticity. Nevertheless, the mechanisms underlying synaptic responses to β-adrenergic receptor (βAR) activation remain poorly understood. An increase in the number of synaptic vesicles primed for exocytosis accounts for the potentiation of neurotransmitter release driven by βARs. This effect is not mediated by the canonical protein kinase A pathway but rather, through direct activation of the guanine nucleotide exchange protein Epac by cAMP. Interestingly, this βAR signaling via Epac is involved in long term potentiation at cerebellar granule cell-to-Purkinje cell synapses. Thus, the pharmacological activation of βARs modulates synaptic plasticity and opens therapeutic opportunities to control this phenomenon.

Analysis of Neuro-Neuronal Synapses Using Embryonic Chick Ciliary Ganglion via Single-Axon Tracing, Electrophysiology, and Optogenetic Techniques

The calyx-type synapse is a giant synaptic structure in which a presynaptic terminal wraps around a postsynaptic neuron in a one-to-one manner. It has been used for decades as an experimental model system of the synapse due to its simplicity and high accessibility in physiological recording methods. In particular, the calyx of the embryonic chick ciliary ganglion (CG) has enormous potential for synapse science because more flexible genetic manipulations are available compared with other synapses. Here, we describe methods to study presynaptic morphology, physiology, and development using CGs and cutting-edge molecular tools. We outline step-by-step protocols for presynaptic gene manipulation using in ovo electroporation, preparation of isolated CGs, 3-D imaging for single-axon tracing in transparent CGs, electrophysiology of the presynaptic terminal, and an all-optical approach using optogenetic molecular reagents. These methods will facilitate studies of the synapse and neuronal circuits in the future. © 2019 by John Wiley & Sons, Inc.

GPCR-mediated rapid, non-genomic actions of steroids: comparisons between DmDopEcR and GPER1 (GPR30)

Steroid hormones classically mediate their actions by binding to intracellular receptor proteins that migrate to the nucleus and act as transcription factors to change gene expression. However, evidence is now accumulating for rapid, non-genomic effects of steroids. There is considerable controversy over the mechanisms underlying such effects. In a number of cases evidence has been presented for the direct activation of G-protein coupled receptors (GPCRs) by steroids, either at the plasma membrane, or at intracellular locations. Here, we will focus on the non-genomic actions of ecdysteroids on a Drosophila GPCR, DopEcR (CG18314), which can be activated by both ecdysone and the catecholamine, dopamine. We will also point out parallels between this system and the activation of the vertebrate GPCR, GPER1 (GPR30), which is thought to be activated by 17β-estradiol. We propose that the cellular localization and signalling properties of both DopEcR and GPER1 may be cell specific and depend upon their interactions with both accessory molecules and signalling pathways.

G-protein oestrogen receptor 1: trials and tribulations of a membrane oestrogen receptor

Oestrogens are now recognised to be able to initiate rapid, fast responses, in addition to their classical, longer-term actions. There is a growing appreciation of the potential implications of this mode of action for oestrogenic signalling in both neuronal and non-neuronal systems. As such, much effort has been made to determine the mechanisms that are critical for transducing these rapid effects into cellular responses. Recently, an orphan G-protein-coupled receptor (GPCR), termed GPR30, was identified as an oestrogen-sensitive receptor in cancer cells. This receptor, now term G-protein oestrogen receptor 1 (GPER1) has been the subject of many investigations, and a role for this receptor in the nervous system is now emerging. In this review, we highlight some of the more recent advances in our understanding of the distribution and subcellular localisation of this receptor in the brain, as well as some of the evidence for the potential role that this receptor may play in the brain. We then discuss some of the controversies surrounding the pharmacology of this receptor, and attempt to reconcile these by suggesting that the 'agonist-specific coupling' model of GPCR function may provide a potential explanation for some of the divergent reports of GPER1 pharmacology.

Role of ERβ and GPR30 in the endocrine pancreas: A matter of estrogen dose

The endocrine pancreas has emerged as a target for estrogens. The functions of pancreatic α-, β- and δ-cells are modulated by the endogenous hormone, 17β-estradiol (E2). Low physiological concentrations (100pM-1nM) of E2 rapidly decrease the activity of the ATP-sensitive potassium channel (K(ATP)) and enhance glucose-induced insulin release in β-cells in an estrogen receptor β (ERβ)-dependent manner. In addition to the insulinotropic action of ERβ, the newly described estrogen receptor, GPR30, is involved in the insulinotropic effects of high doses of E2 (100nM-5μM). The specific GPR30 agonist G1 also increases insulin secretion in β-cells. Low glucose-induced calcium oscillations and glucagon secretion are suppressed by E2. The effects on glucagon secretion may be mediated by GPR30. Somatostatin release is also decreased by E2 and G1. In this review we summarize all the data published up to date on the rapid insulinotropic effects of estrogens in the endocrine pancreas and propose a model to integrate the estrogen actions mediated through both receptors.

Esmolol modulates inhibitory neurotransmission in the substantia gelatinosa of the spinal trigeminal nucleus of the rat

Background

β₁-adrenaline receptor antagonists are often used to avoid circulatory complications during anesthesia in patients with cardiovascular diseases. Of these drugs, esmolol, a short-acting β antagonist, is also reported to exert antinociceptive and anesthetic sparing effects. This study was designed to identify the central mechanism underlying the antinociceptive effect of esmolol.

Methods

Wistar rats (7-21 d, 17-50 g) were anesthetized with ketamine (100-150 mg/kg) or isoflurane (5%) and decapitated. Horizontal slices (400-μm thick) of the lower brainstem containing the substantia gelatinosa (SG) of the caudal part of the spinal trigeminal nucleus (Sp5c), in which the nociceptive primary afferents form the first intracranial synapses, were made with a vibrating slicer. The miniature inhibitory and excitatory postsynaptic currents (mIPSCs and mEPSCs, respectively) were simultaneously recorded from visually identified SG neurons of the Sp5c in the presence of tetrodotoxin (1 μM). Additionally, mIPSCs were recorded during pharmacological isolation of GABA- and glycine-mediated mIPSCs with kynurenic acid (1 mM).

Results

Esmolol (500 μM) significantly and selectively increased the mIPSC frequency (to 214.2% ± 34.2% of the control, mean ± SEM, n = 35; P < 0.001), but not that of mEPSCs, without changing their amplitude. The increase in mIPSC frequency with esmolol was not affected by prior activation of β receptors with isoproterenol (100 μM) but it was significantly attenuated by removal of extracellular Ca²⁺.

Conclusions

These data suggest that esmolol modulates inhibitory transmitter release in the Sp5c through a mechanism involving Ca²⁺-entry but in a β₁-adrenoceptor-independent manner. The present results suggest that the facilitation of inhibitory transmitter release in the central nociceptive network underlies, at least in part, the antinociceptive effect of esmolol.

The role of estrogen receptors in the control of energy and glucose homeostasis

Estrogens have been related to energy balance and glucose metabolism for a long time; however, the mechanisms involved in their actions are now being unveiled. The development of ERalpha and ERbeta knockout mice has demonstrated the participation of these receptors in the regulation of many processes related to the control of energy homeostasis. These include food intake and energy expenditure, insulin sensitivity in the liver and muscle, adipocyte growth and its body distribution as well as the pancreatic beta-cell function. In addition, other membrane receptors unrelated to ERalpha and ERbeta function in key tissues involved in energy balance and glucose homeostasis, i.e. the islet of Langerhans and the hypothalamus. Along with naturally occurring estrogens, there are endocrine disrupters that act as environmental estrogens and can impair the physiological action of ERalpha, ERbeta and other membrane ERs. New research is revealing a link between environmental estrogenic pollutants and the metabolic syndrome.

beta-Adrenoceptor-mediated long-term up-regulation of the release machinery at rat cerebellar GABAergic synapses

Properly regulated interactions among excitatory and inhibitory synapses are critical for brain function. Compared to excitatory synapses, much less is known about the gain control mechanisms at inhibitory synapses. Herein we report a mechanism of noradrenergic long-term potentiation (LTP) at inhibitory synapses following presynaptic beta-adrenoceptor activation. Stimulation of beta-adrenoceptors elicited LTP of GABA release from terminals of cerebellar interneurones. This action was dependent on the cAMP/protein kinase A signalling cascade and independent of the beta-adrenoceptor-mediated acceleration of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel. Furthermore, the beta-adrenoceptor- and protein kinase A-mediated LTP was triggered by enhancement of the Ca2+ sensitivity of the release machinery and increase in the readily releasable pool. beta-Adrenoceptor activation also accelerated the recruitment of GABA into the releasable pool and enhanced synchronous and asynchronous release of GABA from the presynaptic terminal. Thus, the up-regulation of GABA release machinery mediated by noradrenaline and beta-adrenoceptor activation provides a likely mechanism of feedforward inhibition of the cerebellar output neurone Purkinje cell, leading to a profound effect on motor control and learning associated with the cerebellum.

Rapid activation of presynaptic nicotinic acetylcholine receptors by nerve-released transmitter

Nicotine's ability to enhance neurotransmitter release has implicated presynaptic nicotinic acetylcholine receptors (nAChRs) in synaptic modulation, but there aze few examples where presynaptic nAChRs are known to be activated by nerve-released transmitter. We searched for endogenous activation of presynaptic nAChRs in the calyceal nerve terminals of the chick ciliary ganglion by imaging presynaptic calcium transients using dextran-coupled indicator dyes. The amplitude of Ca(+)signals recorded in individual nerve terminals was frequency dependent over 2-50 Hz. Calcium transients evoked by stimulation of the preganglionic nerve were significantly reduced (approximately 10-15%) by the nonspecific nAChR antagonist d-tubocurarine (d-TC; 100 microM) and the alpha7-specific antagonist methyllycaconitine (20-50 nM) but were not affected by 10 microM dihydro-beta-erythroidine, which should inhibit several non-alpha7 nAChRs. Feedback was rapid and did not require a stimulation-dependent build-up of transmitter, as d-TC and MLA reduced the amplitude of the first calcium transient in a 2-Hz train. Choline is an agonist at alpha7 nAChRs but is not the sole agonist in this system, as inhibition of acetylcholinesterase by echothiophate failed to reduce calcium transients. These results show that nerve-released acetylcholine (ACh) feeds back onto presynaptic alpha7 nAChRs to enhance calcium signals within the terminal. This feedback may help maintain the high rate of transmission at this cholinergic synapse.

The effects of noradrenaline on the amplitude-time characteristics of multiquantum endplate currents and the kinetics of induced secretion of transmitter quanta

Experiments on frog neuromuscular junctions using a two-electrode membrane potential clamping method were used to study the effects of noradrenaline on the amplitude-time characteristics of multiquantum endplate current (EPC) parameters and the time course of secretion of transmitter quanta during the process of EPC generation. Noradrenaline (10 microM) induced significant increases in EPC amplitude (by 16%), with a decrease in the ratio of the duration of the leading front of the EPC to the duration of the leading front of the miniature endplate current (mEPC). Analysis of the time course of induced secretion, based on sequential subtraction of signals with displacement on the time scale, showed that noradrenaline induced synchronization of the process of secretion of quanta involved in generating multiquantum EPC, resulting in a 25% decrease in parameter P90, which characterizes the extent of synchronization of quantum release. The quantum composition of EPC, measured by dividing the area of induced and spontaneous signals and by analysis of the time course of the secretion of quanta, showed no changes in response to noradrenaline. Thus, in conditions in which responses to single stimuli applied to the motor nerve results in the release of several tens of quanta, noradrenaline can lead to increases in the amplitude of multiquantum EPC by increasing the level of synchronization of secretion of the transmitter quanta forming this signal.

A nonclassical estrogen membrane receptor triggers rapid differential actions in the endocrine pancreas

Glucose homeostasis in blood is mainly maintained by insulin released from beta-cells and glucagon released from alpha-cells, both integrated within the pancreatic islet of Langerhans. The secretory processes in both types of cells are triggered by a rise in intracellular calcium concentration ([Ca2+](i)). In this study, rapid effects of the natural hormone E2 on [Ca2+](i) were studied in both types of cells within intact islets using laser scanning confocal microscopy. alpha- And beta-cells showed opposite [Ca2+](i) responses when stimulated with physiological concentrations of 17beta-E2. Although the estrogen produced an increase in the frequency of glucose-induced [Ca2+](i) oscillations in insulin-releasing beta-cells, it prevented the low glucose-induced [Ca2+](i) oscillations in glucagon-releasing alpha-cells. The effects of 17beta-E2 on alpha-cells were mimicked by the cGMP permeable analog 8bromo-cGMP and blocked by the cGMP-dependent protein kinase (PKG) inhibitor KT5823. Evidence indicated that these were membrane actions mediated by a nonclassical ER. Both effects were rapid in onset and were reproduced by 17beta-E2 linked to horseradish peroxidase, a cell-impermeable molecule. Furthermore, these actions were not blocked by the specific ER blocker ICI 182,780. Competition studies performed with 17beta-E2 linked to horseradish peroxidase binding in alpha-cells supported the idea that the membrane receptor involved is neither ERalpha nor ERbeta. Additionally, the binding site was shared by the neurotransmitters epinephrine, norepinephrine, and dopamine and had the same pharmacological profile as the receptor previously described for beta-cells. Therefore, rapid estrogen actions in islet cells are initiated by a nonclassical estrogen membrane receptor.

Nongenomic actions of estrogens and xenoestrogens by binding at a plasma membrane receptor unrelated to estrogen receptor alpha and estrogen receptor beta

The molecular mechanism used by environmental chemicals to exert their hormone-like actions is still only partially resolved. Although it generally is accepted that xenoestrogens act at the genomic level by binding to intracellular estrogen receptors, we have shown here that they trigger nongenomic effects in pancreatic beta cells. Both xenoestrogens and the circulating hormone, 17beta-estradiol, bind with high affinity to a common membrane binding site unrelated to the intracellular estrogen receptors ERalpha and ERbeta. This binding site is shared by dopamine, epinephrine, and norepinephrine and has the pharmacological profile of the gammaadrenergic receptor. This study provides an outline of the membrane receptor involved in rapid xenoestrogen actions.

mu-Opioid receptor inhibits N-type Ca2+ channels in the calyx presynaptic terminal of the embryonic chick ciliary ganglion

A study was made on the mechanisms by which enkephalins inhibit synaptic transmission at calyx-type presynaptic terminals in the ciliary ganglion of chick embryos at stages 39-40. Excitatory postsynaptic currents (EPSCs) were recorded by nystatin-perforated patch clamp at low [Ca2+]o and high [Mg2+]o. [Leu5]enkephalin (L-ENK, 1-10 microM) reduced the quantal content (m) without changing the quantal size (q). This effect was antagonized by naloxone (1 microM). Similar results were observed under conventional whole-cell clamp of the postsynaptic neuron. A specific agonist of the mu-opioid receptor, [D-Ala2, M-Me-Phe4,Gly5]enkephalin-ol (DAMGO) reduced m without changing q. A specific agonist of the delta-opioid receptor, [d-Pen2, d-Pen5]enkephalin (DPDPE) also reduced m without changing q. Both L-ENK and [Met5]enkephalin (M-ENK) reduced the stimulus-dependent increment of the intraterminal Ca2+ concentration (Delta[Ca2+]t) without affecting the decay time constant of the intraterminal Ca2+ concentration and basal Ca2+ level. This effect was antagonized by naloxone. DAMGO reduced Delta[Ca2+]t more effectively than DPDPE. When extracellular Ca2+ was replaced by Ba2+, the stimulus-dependent increment of the intraterminal Ba2+ concentration (Delta[Ba2+]t) was also reduced by L-ENK or DAMGO. L-ENK reduced Delta[Ca2+]t even in the presence of 4-aminopyridine (4-AP), which blocks the transient K+ conductance during the falling phase of the presynaptic action potential. When N-type Ca2+ channels were blocked by omega-conotoxin GVIA (omega-CgTxGVIA), the Delta[Ca2+]t was no longer sensitive to L-ENK and DAMGO. It is suggested that enkephalins reduce the transmitter release through presynaptic opioid receptors. The mu-opioid receptor may suppress presynaptic Ca2+ influx by selectively inhibiting N-type Ca2+ channels.

Synchronization of evoked secretion of quanta of mediator as a mechanism facilitating the action of sympathomimetics

Experiments on frog neuromuscular junction preparations with extracellular recording of nerve terminal action potentials and single-quantum end-plate currents (EPC) were used to assess the time course of evoked quantum secretion of mediator by analyzing histograms of the distribution of true synaptic delays. These studies showed that noradrenaline, isoproterenol, and dobutamine change the kinetics of secretion of quanta, leading to synchronization of the process of mediator release; substances blocking beta-adrenoceptors (atenolol, propranolol) blocked this effect. Clonidine and phenylephrine, which activate alpha-receptors, had no effect on the kinetics of secretion, while the alpha-blocker phentolamine had no effect on the synchronizing action of noradrenaline. Reconstruction of multiquantum EPC from changes in the level of synchronization in the release of individual quanta, showed that EPC amplitude increased in response to noradrenaline by 17%, and that this was due only to alterations in the time course of secretion. These data led to the conclusion that there is a special presynaptic mechanism which facilitates the action of sympathomimetics, acting via beta-adrenoceptors.

Involvement of cGMP-dependent protein kinase in adrenergic potentiation of transmitter release from the calyx-type presynaptic terminal

I have previously reported that norepinephrine (NE) induces a sustained potentiation of transmitter release in the chick ciliary ganglion through a mechanism pharmacologically distinct from any known adrenergic receptors. Here I report that the adrenergic potentiation of transmitter release was enhanced by a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and by zaprinast, an inhibitor of cGMP-selective phosphodiesterase. Exogenous application of the membrane-permeable cGMP, 8-bromo-cGMP (8Br-cGMP), potentiated the quantal transmitter release, and after potentiation, the addition of NE was no longer effective. On the other hand, 8Br-cAMP neither potentiated the transmitter release nor occluded the NE-induced potentiation. The NE-induced potentiation was blocked by neither nitric oxide (NO) synthase inhibitor nor NO scavenger. The quantal transmitter release was not potentiated by NO donors, e.g., sodium nitroprusside. The NE-induced potentiation and its enhancement by IBMX was antagonized by two inhibitors of protein kinase G (PKG), Rp isomer of 8-(4-chlorophenylthio) guanosine-3', 5'-cyclic monophosphorothioate and KT5823. As with NE-induced potentiation, the effects of 8Br-cGMP on both the resting intraterminal [Ca2+] ([Ca2+]i) and the action potential-dependent increment of [Ca2+]i (DeltaCa) in the presynaptic terminal were negligible. The reduction of the paired pulse ratio of EPSC is consistent with the notion that the NE- and cGMP-dependent potentiation of transmitter release was attributable mainly to an increase of the exocytotic fusion probability. These results indicate that NE binds to a novel adrenergic receptor that activates guanylyl cyclase and that accumulation of cGMP activates PKG, which may phosphorylate a target protein involved in the exocytosis of synaptic vesicles.

Involvement of cGMP-dependent protein kinase in adrenergic potentiation of transmitter release from the calyx-type presynaptic terminal

I have previously reported that norepinephrine (NE) induces a sustained potentiation of transmitter release in the chick ciliary ganglion through a mechanism pharmacologically distinct from any known adrenergic receptors. Here I report that the adrenergic potentiation of transmitter release was enhanced by a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) and by zaprinast, an inhibitor of cGMP-selective phosphodiesterase. Exogenous application of the membrane-permeable cGMP, 8-bromo-cGMP (8Br-cGMP), potentiated the quantal transmitter release, and after potentiation, the addition of NE was no longer effective. On the other hand, 8Br-cAMP neither potentiated the transmitter release nor occluded the NE-induced potentiation. The NE-induced potentiation was blocked by neither nitric oxide (NO) synthase inhibitor nor NO scavenger. The quantal transmitter release was not potentiated by NO donors, e.g., sodium nitroprusside. The NE-induced potentiation and its enhancement by IBMX was antagonized by two inhibitors of protein kinase G (PKG), Rp isomer of 8-(4-chlorophenylthio) guanosine-3', 5'-cyclic monophosphorothioate and KT5823. As with NE-induced potentiation, the effects of 8Br-cGMP on both the resting intraterminal [Ca2+] ([Ca2+]i) and the action potential-dependent increment of [Ca2+]i (DeltaCa) in the presynaptic terminal were negligible. The reduction of the paired pulse ratio of EPSC is consistent with the notion that the NE- and cGMP-dependent potentiation of transmitter release was attributable mainly to an increase of the exocytotic fusion probability. These results indicate that NE binds to a novel adrenergic receptor that activates guanylyl cyclase and that accumulation of cGMP activates PKG, which may phosphorylate a target protein involved in the exocytosis of synaptic vesicles.

Noradrenaline synchronizes evoked quantal release at frog neuromuscular junctions

1. Noradrenaline (NA) increases synaptic efficacy at the frog neuromuscular junction. To test the hypothesis that one of the actions of NA is to shorten the period over which evoked quanta are released, we measured the latencies of focally recorded uniquantal endplate currents (EPCs). 2. NA shortened the release period for evoked quantal release. The interval between the time when responses with minimal delay appeared and the point at which 90 % of all latencies had occurred was shortened in the presence of 1 x 10-5 M NA by about 35 % at 20 C and by about 45 % at 8 C. Inhibitor and agonist experiments showed that NA acts on a beta-adrenoreceptor. 3. The better synchronization of release significantly increased the size of reconstructed multi- quantal EPCs. This suggests that NA facilitates synaptic transmission by making the release of quanta more synchronous. 4. The synchronizing action of NA might potentiate neuromuscular transmission during nerve regeneration, transmitter exhaustion and other extreme physiological states where the quantal content is reduced, such as survival in cold and hibernation.

Long-term use-dependent enhancement of impulse-induced exocytosis by adrenaline at frog motor nerve terminals

Adrenaline (5-20 microM) use-dependently increased end-plate potentials (EPPs) in normal Ringer solution (containing d-tubocurarine to partially block acetylcholine receptors) and a low Ca2+, high Mg2+ solution for more than several hours and decreased the coefficient of variation of EPP amplitude in the latter solution in frog neuromuscular junctions. The amplitude and frequency of miniature EPPs and impulse-induced increases in intraterminal Ca2+ concentration were unaffected. Adrenaline thus causes sustained enhancement of impulse-induced exocytosis by acting at a mechanism of exocytosis downstream to Ca2+ entry.

Protein kinase C potentiates transmitter release from the chick ciliary presynaptic terminal by increasing the exocytotic fusion probability

1. The giant presynaptic terminal of chick ciliary ganglion was used to examine how protein kinase C (PKC) modulates neurotransmitter release. Cholinergic excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp. 2. Although the EPSC was potentiated by phorbol ester (phorbol 12-myristate 13-acetate, PMA; 0.1 microM) in a sustained manner, the nicotine-induced current was unaffected. PMA increased the quantal content to 2.4 +/- 0.4 (n = 9) of control without changing the quantal size. 3. The inactive isoform of PMA, 4alpha-PMA, showed no significant effect on EPSCs. The PMA-induced potentiation was antagonized by two PKC inhibitors with different modes of action, sphingosine (20 microM) and bisindolylmaleimide I (10 microM). 4. When stimulated by twin pulses of short interval, the second EPSC was on average larger than the first EPSC (paired-pulse facilitation; PPF). PMA significantly decreased the PPF ratio with a time course similar to that of the potentiation of the first EPSC. 5. PMA did not affect resting [Ca2+]i or the action potential-induced [Ca2+]i increment in the giant presynaptic terminals. 6. The effect of PMA was less at 10 mM [Ca2+]o than at 1 mM [Ca2+]o. 7. When a train of action potentials was generated with a short interval, the EPSC was eventually depressed and reached a steady-state level. The recovery process followed a simple exponential relation with a rate constant of 0.132 +/- 0.029 s-1. PMA did not affect the recovery rate constant of EPSCs from tetanic depression. In addition, PMA did not affect the steady-state EPSC which should be proportional to the refilling rate of the readily releasable pool of vesicles. 8. These results conflict with the hypothesis that PKC upregulates the size of the readily releasable pool or the number of release sites. PKC appears to upregulate the Ca2+ sensitivity of the process that controls the exocytotic fusion probability.

Adrenaline stimulates glucagon secretion in pancreatic A-cells by increasing the Ca2+ current and the number of granules close to the L-type Ca2+ channels

We have monitored electrical activity, voltage-gated Ca2+ currents, and exocytosis in single rat glucagon-secreting pancreatic A-cells. The A-cells were electrically excitable and generated spontaneous Na+- and Ca2+-dependent action potentials. Under basal conditions, exocytosis was tightly linked to Ca2+ influx through omega-conotoxin-GVIA-sensitive (N-type) Ca2+ channels. Stimulation of the A-cells with adrenaline (via beta-adrenergic receptors) or forskolin produced a greater than fourfold PKA-dependent potentiation of depolarization-evoked exocytosis. This enhancement of exocytosis was due to a 50% enhancement of Ca2+ influx through L-type Ca2+ channels, an effect that accounted for <30% of the total stimulatory action. The remaining 70% of the stimulation was attributable to an acceleration of granule mobilization resulting in a fivefold increase in the number of readily releasable granules near the L-type Ca2+ channels.

Aminergic modulation of glycine release in a spinal network controlling swimming in Xenopus laevis

1. Neuromodulators can effect changes in neural network function by strengthening or weakening synapses between neurons via presynaptic control of transmitter release. We have examined the effects of two biogenic amines on inhibitory connections of a spinal rhythm generator in Xenopus tad poles. 2. Glycinergic inhibitory potentials occurring mid-cycle in motoneurons during swimming activity are reduced by 5-hydroxytryptamine (5-HT; serotonin) and enhanced by noradrenaline (NA). These opposing effects on inhibitory synaptic strength are mediated presynaptically where 5-HT decreases and NA increases the probability of glycine release from inhibitory terminals. 3. The amines also have contrasting effects on swimming: 5-HT increased motor burst durations while NA reduced swimming frequency. Aminergic modulation of glycinergic transmission may thus control fundamental parameters of swimming and force the spinal network to generate opposite extremes of its spectrum of possible outputs.

Opposing effects of phorbol esters on transmitter release and calcium currents at frog motor nerve endings

1. Phorbol esters activate protein kinase C (PKC) and also increase the secretion of neurotransmitter substances by an unknown mechanism. To evaluate whether the stimulatory effects of such agents on acetylcholine (ACh) secretion occur as a consequence of stimulation of Ca2+ entry, we made electrophysiological measurements of ACh secretion (i.e. endplate potentials, EPPs) and the component of the prejunctional perineural voltage change associated with nerve terminal calcium currents (perineural calcium current) at frog neuromuscular junctions. 2. In the first series of experiments, modest concentrations of K+ channel blockers were employed so that simultaneous measurements of EPP amplitudes and perineural calcium currents could be made. In these experiments, 12-O-tetradecanoylphorbol 13-acetate (TPA; 162 nM) and phorbol 12,13-dibutyrate (PDBu; 100-200 nM) each increased ACh release but simultaneously decreased the calcium component of the prejunctional perineural current TPA and PDBu also inhibited perineural calcium currents in the presence of higher concentrations of K+ channel blockers. 3. Blockade of Ca2+ channels by Cd2+ prevented the action of PKC stimulators on perineural waveforms. 4. The inactive compound 4-alpha-phorbol 12-myristate 13-acetate (150 nM) did not affect EPP amplitudes or perineural currents. 5. The extracellular [Ca2+]-ACh release relationship was increased in maximum by PDBu without any change in the potency of Ca2+ to support evoked ACh release. 6. The results demonstrate that phorbol esters increase neurotransmitter secretion whilst simultaneously decreasing the nerve ending calcium currents that promote evoked release. The results, which suggest that the optimal control point for secretion might not be the calcium channel but rather a component of the secretory apparatus, are discussed in conjunction with the possible target sites for phorbol esters in the nerve ending.

Protein kinase A-mediated enhancement of miniature IPSC frequency by noradrenaline in rat cerebellar stellate cells

1. Cellular mechanisms underlying the enhancement by noradrenaline (NA) of inhibitory postsynaptic currents (IPSCs) were studied at inhibitory synapses in the molecular layer of the cerebellum. IPSCs were obtained from stellate cells in rat cerebellar slices using tight-seal whole-cell recording. 2. Miniature IPSCs (mIPSCs) were recorded in the presence of tetrodotoxin (TTX; 100 nM). NA (10 microM) markedly increased the frequency of mIPSCs, but did not alter their mean amplitude. Bath application of the inhibitor of adenylyl cyclase 9-(tetrahydro-2'-furyl) adenine (SQ 22,536; 300 microM), of the wide spectrum protein kinase inhibitor staurosporine (1 microM), and of the Rp-diastereomer of adenosine-3',5'-cyclic monophosphothioate (Rp-cAMPS; 500 microM), a specific inhibitor of cAMP-dependent protein kinase (PKA), inhibited the mIPSC frequency increase induced by NA. 3. The increase in mIPSC frequency was not attenuated by Cd2+ (100 microM), a blocker of voltage dependent calcium channels. However, after a 12-15 min pre-incubation in Ca(2+)-free saline, the effect of NA on mIPSCs was markedly inhibited. If Ca2+ ions were readmitted in the presence of NA, enhancement of the mIPSC frequency was largely restored. 4. Application of the membrane permeant analogue of cAMP, 8-Br-cAMP (1 mM), together with the inhibitor of cAMP phosphodiesterase, 3-isobutyl-1-methylxanthine (IBMX; 100 microM), caused a frequency increase of mIPSCs. Forskolin also mimicked the stimulatory effect of NA on mIPSC frequency. The effects of both 8-Br-cAMP and forskolin persisted in Ca(2+)-free saline, suggesting that the modulation of transmitter release does not require Ca2+ influx. 5. On the whole, the results indicate that the potentiation of mIPSC frequency by NA is mediated through the sequential activation of adenylyl cyclase and protein kinase A (PKA), and that PKA modulates the vesicle release mechanism rather than Ca2+ influx. The lack of effect of NA after prolonged incubation in Ca(2+)-free solution may be due to an inhibition of adenylyl cyclase by a gradual lowering of the cytosolic presynaptic Ca2+ concentration.