Spontaneous Excitatory Synaptic Potentials Research Articles

Analogue signaling of somatodendritic synaptic activity to axon enhances GABA release in young cerebellar molecular layer interneurons.

Axons are equipped with the digital signaling capacity by which they generate and faithfully propagate action potentials (APs), and also with the analogue signaling capacity by which subthreshold activity in dendrites and soma is transmitted down the axon. Despite intense work, the extent and physiological role for subthreshold synaptic activity reaching the presynaptic boutons has remained elusive because of the technical limitation to record from them. To address this issue, we made simultaneous patch-clamp recordings from the presynaptic varicosities of cerebellar GABAergic interneurons together with their parent soma or postsynaptic target cells in young rat slices and/or primary cultures. Our tour-de-force direct functional dissection indicates that the somatodendritic spontaneous excitatory synaptic potentials are transmitted down the axon for significant distances, depolarizing presynaptic boutons. These analogously transmitted excitatory synaptic potentials augment presynaptic Ca++ influx upon arrival of an immediately following AP through a mechanism that involves a voltage-dependent priming of the Ca++ channels, leading to an increase in GABA release, without any modification in the presynaptic AP waveform or residual Ca++. Our work highlights the role of the axon in synaptic integration.

Optical Control of Endogenous Proteins with a Photoswitchable Conditional Subunit Reveals a Role for TREK1 in GABAB Signaling

Selective ligands are lacking for many neuronal signaling proteins. Photoswitched tethered ligands (PTLs) have enabled fast and reversible control of specific proteins containing a PTL anchoring site and have been used to remote control overexpressed proteins. We report here a scheme for optical remote control of native proteins usinga "photoswitchable conditional subunit" (PCS), which contains the PTL anchoring site as well as a mutation that prevents it from reaching the plasma membrane. In cells lacking native subunits for the protein, the PCS remains nonfunctional internally. However, in cells expressing native subunits, the native subunit and PCS coassemble, traffic to the plasma membrane, and place the native protein under optical control provided by the coassembled PCS. We apply this approach to the TREK1 potassium channel, which lacks selective, reversible blockers. We find that TREK1, typically considered to be a leak channel, contributes to the hippocampal GABA(B) response.

Interneuronal synapses formed by motor neurons appear to be glutamatergic

Acetylcholine release at motor neuron synapses has been long established; however, recent discoveries indicate that synaptic transmission by motor neurons is more complex than previously thought. Using whole-cell patch clamp, we show that spontaneous excitatory postsynaptic currents of rat motor neurons in primary ventral horn cultures are entirely glutamatergic, although the cells respond to exogenous acetylcholine. Motor neurons in cultures express the vesicular glutamate transporter VGlut2, and culturing motor neurons for weeks with glutamate receptors blocked upregulates glutamate signaling without increasing cholinergic signaling. In spinal cord slices, motor neurons showed no decrease in spontaneous excitatory synaptic potentials after blocking acetylcholine receptors. Our results suggest that motor neuron synapses formed on other neurons are largely glutamatergic in culture and the spinal cord.

Group II mGlu receptor agonists inhibit behavioural and electrophysiological effects of DOI in mice

It has been suggested that metabotropic glutamate (mGlu) receptor agonists selective for Group II mGlu receptors may have antipsychotic action. Therefore, we studied whether the effects, which could be related to psychotomimetic action of hallucinogenic drugs, are inhibited by Group II mGlu receptor agonists. The selective mGlu2/3 agonists LY354740 and LY379268 inhibited (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced head twitches in mice in a dose-dependent manner. Furthermore, LY379268 suppressed an increase in the frequency of spontaneous excitatory synaptic potentials induced by bath-applied DOI in layer V pyramidal cells recorded in the murine medial frontal cortex. The data indicate that Group II mGlu receptor agonists may counteract the effects of hallucinogenic drugs.

Sub-nanomolar concentrations of ciguatoxin-1 excite preganglionic terminals in guinea pig sympathetic ganglia.

The actions of low concentrations of ciguatoxin-1 (CTX-1, 0.2-0.8 nM) in guinea-pig sympathetic ganglia have been analysed using intracellular recording techniques in vitro. The effects of CTX-1 were graded with concentration but sensitivity varied markedly between neurones in the same preparation. Other than an initial transient (approximately 10 min) depolarization of some ganglion cells accompanied by an increase in input resistance, passive electrical properties did not significantly differ from controls. Amplitude and threshold of action potentials evoked by depolarizing current and threshold, latency and form of the initial responses to nerve stimulation were also not affected. Exposure to CTX-1 generated marked increases in the frequency of spontaneous excitatory synaptic potentials which often occurred in bursts (15-66 Hz) of similar amplitudes. Single stimuli to incoming nerves produced repetitive synaptic responses arising from preganglionic, but not from peripheral afferent, axons. Following brief (< 5 min) exposure to CTX-1, these effects declined over 30 min but, after longer exposure (> 15 min), they persisted for several hours despite continuous washing. All activity generated by CTX-1 was significantly reduced or abolished by d-tubocurarine (10(-5)-10(-4) M), hexamethonium (10(-5) M), tetrodotoxin (10(-7)-10(-6) M), omega-conotoxin (10(-7) M), reduced Ca2+ (0.1 mM)/raised Mg2+ (10 mM), raised Ca2+ (6 mM) or raised Mg2+ (25 mM). The data suggest that CTX-1 activates preganglionic axons by modifying the voltage sensitivity of a subpopulation of Na+ channels. Effects on these unmyelinated axons occur at much lower concentrations than have been reported to affect myelinated ones. Many of the symptoms of ciguatera poisoning might be explained by activity in autonomic and perhaps other unmyelinated nerve terminals.

Currents caused by the spontaneous release of quanta of acetylcholine onto chromaffin cells in guinea-pig adrenal medulla

Membrane potentials were recorded from chromaffin cells in isolated bisected adrenal glands from guinea-pigs. Spontaneous excitatory synaptic potentials (SESPs) were recorded whose frequency was increased following brief (up to 10 s) periods of presynaptic nerve stimulation at 10–30 Hz. The single electrode voltage-clamp method was used to record the currents underlying all but the largest SESPs. Spontaneous excitatory synaptic currents (SESCs) decayed according to a single exponential with a time constant of about 8 ms at 30°C. Thus the neuronal nicotinic receptor-channels giving rise to SESCs in chromaffin cells are probably very similar to those that are opened by quanta of acetylcholine in sympathetic ganglion cells.

Synaptic transmission from splanchnic nerves to the adrenal medulla of guinea‐pigs.

1. Membrane potentials were recorded with conventional intracellular microelectrodes from chromaffin cells in isolated, bisected adrenal glands from guinea-pigs. 2. All cells were electrically excitable and responded to depolarizing current with all-or-nothing action potentials that were blocked by tetrodotoxin. 3. Input resistance was 180 +/- 14 M omega and this was lower than that reported for isolated chromaffin cells using patch electrodes. 4. All cells responded to transmural stimulation with action potentials that arose from excitatory synaptic potentials in response to the excitation of one or more preganglionic fibres, many having strong synaptic action. Other fibres had weaker synaptic action but in all cases, maximal transmural stimulation caused depolarization well above threshold for action potential initiation. 5. Spontaneous excitatory synaptic potentials were observed whose frequency was greatly increased by repetitive stimulation at 10 or 30 Hz. 6. No evidence was found for the desensitization of nicotinic receptors in response to acetylcholine released from presynaptic nerve terminals. 7. These experiments show that there are many similarities between the responses to splanchnic nerve stimulation of guinea-pig chromaffin cells in situ and sympathetic ganglion cells from the same species.

An electrophysiological study of the in vitro, perfused brain stem-cerebellum of adult guinea-pig.

1. We describe here a technique which allows the long-term in vitro survival of the perfused isolated brain stem-cerebellum of adult guinea-pig. The viability of this preparation was assessed by comparing the electrophysiological properties of individual neurones and of neuronal pools to those obtained in vivo or in brain slices. The areas investigated included the cerebellar cortex, the inferior olive and the pontine nuclei. 2. Cerebellar field potential and intra- and extracellular single-cell recordings could be obtained for as long as 15 h after the preparation was initially isolated. The waveforms of field potentials recorded at various depths in the cerebellar cortex following surface folial stimulation were similar to those recorded in vivo. Extracellular recordings from single Purkinje cells following white matter stimulation demonstrated antidromic as well as mossy- and climbing fibre-mediated excitation. Stimulation of the cerebellar surface elicited orthodromic parallel fibre excitation of Purkinje cells and basket-stellate and Golgi cell inhibition. 3. Intrasomatic and intradendritic recordings from Purkinje cells reproduced all the phenomenology described earlier under in vivo conditions and in vitro slice preparations. In addition, spontaneous excitatory synaptic potentials generating simple spikes (mossy fibre-parallel fibre-mediated activity) and complex spikes (climbing fibre-mediated activity) were consistently observed. 4. Extracellular field potentials and extra- and intracellular recordings from inferior olive neurones were similar to those previously shown for the mammalian inferior olive. 5. Intracellular recordings were also obtained from pontine nuclei neurones, a major source of mossy fibre afferents to the cerebellum. Stimulation of the contralateral superior cerebellar peduncle produced antidromic invasion of these neurones whereas stimulation of the ipsilateral inferior cerebral peduncle resulted in their orthodromic activation. 6. The preparation responded to pharmacological challenge in a manner which demonstrated a sequential activation of sets of synaptic links in a given pathway. Thus, harmaline generated oscillations of inferior olivary neurones which were similar to those observed in vivo and which produced climbing fibre EPSPs in Purkinje cells at the same frequency as the inferior olivary oscillations. Climbing fibre activation of the Purkinje cells generated powerful inhibitory potentials in the cerebellar nuclear neurones at the same frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of neocortical neurons in vitro.

1. Intracellular recordings were obtained from neurons of the guinea pig sensorimotor cortical slice maintained in vitro. Under control recording conditions input resistances, time constants, and spiking characteristics of slice neurons were well within the ranges reported by other investigators for neocortical neurons in situ. However, resting potentials (mean of -75 mV) and spike amplitudes (mean of 93.5 mV) were 10-25 mV greater than has been observed in intact preparations. 2. Current-voltage relationships obtained under current clamp revealed a spectrum of membrane-rectifying properties at potentials that were subthreshold for spike generation. Ionic and pharmacologic analyses suggest that subthreshold membrane behavior is dominated by voltage-sensitive, very slowly inactivating conductances to K+ and Na+. 3. Action potentials were predominantly Na+ dependent under normal conditions but when outward K+ currents were reduced pharmacologically, it was possible, in most cells, to evoke a non-Na+-dependent, tetrodotoxin-(TTX) insensitive spike, which was followed by a prominent depolarizing after-potential. Both of these events were blocked by the Ca2+ current antagonists, Co2+ and Mn2+. 4. A small population of neurons generated intrinsic, all-or-none burst potentials when depolarized with current pulses or by synaptic activation. These cells were located at a narrow range of depths comprising layer IV and the more superficial parts of layer V. 5. Spontaneous excitatory synaptic potentials appeared in all neurons. Spontaneous inhibitory events were visible in only about 10% of the cells, and in those cases apparently reversed polarity at a level slightly positive to resting potential. Stimulation of the surface of the slice at low intensities evoked robust and usually concurrent excitatory and inhibitory synaptic potentials. Unitary inhibitory postsynaptic potentials (IPSPs) reversed at levels positive to rest. Stronger stimulation produced a labile, long-duration, hyperpolarizing IPSP with a reversal potential 15-20 mV negative to the resting level. 6. Neocortical neurons in vitro retain the basic membrane and synaptic properties ascribed to them in situ. However, the array of passive and active membrane behavior observed in the slice suggests that cortical neurons may be differentiated by specific functional properties as well as by their extensive morphological diversity.

Stimulation-Induced Release of Serotonin

Publisher Summary The chapter describes studies on the stimulus-induced release of exogenous 5-HT from molluscan tissues in vivo or in vitro and from mammalian brain in vitro. These data provide additional support for the hypothesis that 5-HT may act as a transynaptic mediator. Serotonin has been identified in the nervous system and other tissues in many classes of mollusks together with the enzymes for its synthesis and metabolism. 5-HT exerts a strong depolarizing and excitatory action upon specific molluscan neurons while serotonin antagonists such as lysergic acid diethylamide (LSD) and bromolysergic acid diethylamide (BOL) block the spontaneous excitatory synaptic potentials as well as the effect of iontophoretically applied 5-HT on these neurons. The chapter presents study on the sea hare, Aplysia californica, to obtain further evidence of the neurohumoral role of 5-HT in mollusks. It also presents studies on rat brain slices. The findings from these studies are similar to the results obtained with NE, and support the contention that 5-HT may serve as a synaptic mediator in the mammalian central nervous system.