Synaptic stimulation of nicotinic receptors in rat sympathetic ganglia is followed by slow activation of postsynaptic potassium or chloride conductances

Two slow currents have been described in rat sympathetic neurons during and after tetanization of the whole preganglionic input. Both effects are mediated by nicotinic receptors activated by native acetylcholine (ACh). A first current, indicated as IAHPsyn, is calcium dependent and voltage independent, and is consistent with an IAHP-type potassium current sustained by calcium ions accompanying the nicotinic synaptic current. The conductance activated by a standard synaptic train was approximately 3.6 nS per neuron; it was detected in isolation in 14 out of a 52-neuron sample. A novel current, IADPsyn, was described in 42/52 of the sample as a post-tetanic inward current, which increased in amplitude with increasing membrane potential negativity and exhibited a null-point close to the holding potential and the cell momentary chloride equilibrium potential. IADPsyn developed during synaptic stimulation and decayed thereafter according to a single exponential (mean tau = 148.5 ms) in 18 neurons or according to a two-exponential time course (tau = 51.8 and 364.9 ms, respectively) in 19 different neurons. The mean peak conductance activated was approximately 20 nS per neuron. IADPsyn was calcium independent, it was affected by internal and external chloride concentration, but was insensitive to specific blockers (anthracene-9-carboxylic acid, 9AC) of the chloride channels open in the resting neuron. It is suggested that gADPsyn represents a specific chloride conductance activatable by intense nicotinic stimulation; in some neurons it is even associated with single excitatory postsynaptic potentials (EPSCs). Both IAHP and IADPsyn are apparently devoted to reduce neuronal excitability during and after intense synaptic stimulation.     

Slow postsynaptic potentials in neurones of submucous plexus of guinea-pig caecum and their mimicry by noradrenaline and various peptides

Intracellular recordings of membrane potential and membrane currents were made from neurones in the submucous plexus of the guinea-pig caecum in vitro. Fast and slow excitatory postsynaptic potentials and slow inhibitory postsynaptic potentials were recorded from the majority of neurones following focal stimulation of presynaptic fibres in the plexus. The slow inhibitory postsynaptic potential was associated with an increase in membrane conductance and reversed its polarity at -90 mV; it was reversibly blocked by yohimbine. The slow excitatory postsynaptic potential and its underlying current was associated with a decrease in membrane conductance. Two kinds of voltage-dependence both of the slow excitatory postsynaptic potential and current were observed; in 80% of cells, the excitatory postsynaptic potential and current became smaller with membrane hyperpolarization and reversed polarity at -90 mV (reversing type) but in 20% of cells both the excitatory postsynaptic potential and current simply disappeared when the membrane potential reached -70 mV (non-reversing type). The effects of acetylcholine, adenosine 5'-triphosphate, bombesin, 5-hydroxytryptamine, neurotensin, noradrenaline, substance P and vasoactive intestinal polypeptide were examined. The only substance which mimicked the slow inhibitory postsynaptic potential was noradrenaline; brief applications of noradrenaline caused hyperpolarizations which had the same time-course, reversal potential and sensitivity to yohimbine as the slow inhibitory postsynaptic potential. The non-reversing type of slow excitatory postsynaptic potential was mimicked only by adenosine 5'-triphosphate. The reversing type of slow excitatory postsynaptic potential was mimicked by bombesin, neurotensin, substance P and vasoactive intestinal polypeptide. 5-Hydroxytryptamine and vasoactive intestinal polypeptide (in some neurones) caused a depolarization with an increase in membrane conductance. All three synaptic potentials were reversibly depressed by superfusion of noradrenaline but noradrenaline did not affect the potential changes evoked by brief application of exogenous acetylcholine or substance P. It is concluded that, in guinea-pig submucous plexus neurones, the slow inhibitory postsynaptic potential is mediated by noradrenaline and results from a potassium conductance increase.(ABSTRACT TRUNCATED AT 400 WORDS)     

Deficits in Behavioral and Neuronal Pattern Separation in Temporal Lobe Epilepsy

In temporal lobe epilepsy, the ability of the dentate gyrus to limit excitatory cortical input to the hippocampus breaks down, leading to seizures. The dentate gyrus is also thought to help discriminate between similar memories by performing pattern separation, but whether epilepsy leads to a breakdown in this neural computation, and thus to mnemonic discrimination impairments, remains unknown. Here we show that temporal lobe epilepsy is characterized by behavioral deficits in mnemonic discrimination tasks, in both humans (females and males) and mice (C57Bl6 males, systemic low-dose kainate model). Using a recently developed assay in brain slices of the same epileptic mice, we reveal a decreased ability of the dentate gyrus to perform certain forms of pattern separation. This is because of a subset of granule cells with abnormal bursting that can develop independently of early EEG abnormalities. Overall, our results linking physiology, computation, and cognition in the same mice advance our understanding of episodic memory mechanisms and their dysfunction in epilepsy.SIGNIFICANCE STATEMENT People with temporal lobe epilepsy (TLE) often have learning and memory impairments, sometimes occurring earlier than the first seizure, but those symptoms and their biological underpinnings are poorly understood. We focused on the dentate gyrus, a brain region that is critical to avoid confusion between similar memories and is anatomically disorganized in TLE. We show that both humans and mice with TLE experience confusion between similar situations. This impairment coincides with a failure of the dentate gyrus to disambiguate similar input signals because of pathologic bursting in a subset of neurons. Our work bridges seizure-oriented and memory-oriented views of the dentate gyrus function, suggests a mechanism for cognitive symptoms in TLE, and supports a long-standing hypothesis of episodic memory theories.     

Roles of glutamate transporters in shaping excitatory synaptic currents in cerebellar Purkinje cells

Several subtypes of glutamate transporters are abundantly expressed near the excitatory synapses on cerebellar Purkinje cells. We investigated the roles of the glutamate transporters in shaping the excitatory postsynaptic currents (EPSCs) and regulating the levels of extracellular glutamate in the mouse cerebellum using a potent blocker of glutamate transporters, dl-threo-beta-benzyloxyaspartate (dl-TBOA). This drug markedly prolonged AMPA receptor-mediated EPSCs in Purkinje cells evoked by stimulating both parallel fibres and climbing fibres. The decay phase of the prolonged EPSCs was fitted by double exponentials, of which the slower component was preferentially inhibited by a low-affinity competitive antagonist of AMPA receptors, gamma-d-glutamyl-glycine, indicating that the slow component induced by dl-TBOA was the AMPA receptor-mediated current activated by lower concentrations of glutamate than those contributing to the peak of the EPSC. This result suggests that dl-TBOA prolongs the stay of synaptically released glutamate in the synaptic cleft and also induces glutamate spillover to extrasynaptic targets as well as neighbouring synapses. Furthermore, high concentrations of dl-TBOA in the presence of cyclothiazide generated a continuous inward current in Purkinje cells, of which the amplitude reached the peak level of the climbing-fibre EPSC. This continuous inward current was abolished by the blocker of AMPA receptors, indicating that the strong inhibition of glutamate uptake causes the rapid accumulation of glutamate in the extracellular space. These results highlight the importance of glutamate transporters in maintaining the proper glutamatergic transmission in Purkinje cell synapses.     

Synaptic plasticity in rat subthalamic nucleus induced by high-frequency stimulation

The technique of deep brain stimulation (DBS) has become a preferred surgical choice for the treatment of advanced Parkinson's disease. The subthalamic nucleus (STN) is presently the most promising target for such DBS. In this study, whole-cell patch-clamp recordings were made from 46 STN neurons in rat brain slices to examine the effect of high-frequency stimulation (HFS) of the STN on glutamatergic synaptic transmission in STN neurons. HFS, consisting of trains of stimuli at a frequency of 100 Hz for 1 min, produced three types of synaptic plasticity in 17 STN neurons. First, HFS of the STN induced short-term potentiation (STP) of evoked postsynaptic current (EPSC) amplitude in four neurons. STP was associated with a reduction in the EPSC paired-pulse ratio, suggesting a presynaptic site of action. Second, HFS of the STN generated long-term potentiation (LTP) of EPSC amplitude in eight neurons. Although the EPSC paired-pulse ratio was reduced transiently in the first 2 min following HFS, ratios measured 6-20 min after HFS were unchanged from control. This suggests that LTP is maintained by a postsynaptic mechanism. Third, HFS produced long-term depression (LTD) of EPSC amplitude in five STN neurons. LTD was associated with a significant increase in EPSC paired-pulse ratios, indicating a presynaptic site of action. These results suggest that HFS can produce long-term changes in the efficacy of synaptic transmission in the STN. HFS-induced synaptic plasticity might be one mechanism underlying the effectiveness of DBS in the STN as a treatment of advanced Parkinson's disease.     

Long-term potentiation of glutamatergic synaptic transmission induced by activation of presynaptic P2Y receptors in the rat medial habenula nucleus

A novel form of long-term potentiation of glutamatergic synaptic transmission is described in the rat medial habenula nucleus. It occurs when uridine 5'-triphosphate is bath applied at low micromolar concentrations and is prevented by Reactive Blue 2, suggesting that it is mediated by P2Y4 receptors. Uridine 5'-diphosphate can also cause such a Reactive Blue 2-sensitive potentiation, but at higher concentrations (200 microm), suggesting that this might also be an effect on the relatively uridine 5'-diphosphate-insensitive P2Y4 receptor. The potentiation is due to an increase in presynaptic release probability. It requires neither depolarization nor calcium influx postsynaptically and is thus probably non-Hebbian. When potentiation due to low concentrations of uridine 5'-triphosphate is inhibited in the presence of Reactive Blue 2, uridine 5'-triphosphate causes instead a significant inhibition of glutamate release. We suggest that this inhibition may be mediated by a Reactive Blue 2-insensitive P2Y2-like receptor. At higher concentrations of uridine 5'-triphosphate (200 micro m), the inhibitory effect dominates such that even in the absence of Reactive Blue 2 no potentiation is seen.     

Convolution of mini distributions for fitting evoked synaptic amplitude histograms

According to a basic formulation of the quantal model, evoked synaptic currents are made up of a linear summation of uniquantal synaptic currents, which in turn are equivalent to the spontaneous miniature synaptic currents (‘minis’) that often persist when evoked neurotransmitter release is blocked. Here I describe a convolution method for calculating linear summations of the ‘mini’ amplitude distribution, which can then be fitted to the measured amplitude distribution for evoked synaptic currents. Provided certain conditions are satisfied, this method can give information about the statistics of neurotransmitter release even when clear quantal peaks are not apparent in the evoked amplitude distribution. The method is illustrated by an experiment in which the appropriate minis are identified with the asynchronous excitatory postsynaptic currents that follow synaptic stimulation when the cell is bathed in strontium. Finally, I discuss the assumptions behind the convolution method, and the conditions under which the properties of the minis are likely to be appropriate for an analysis of this type.     

The glutamate receptor GluN2 subunit regulates synaptic trafficking of AMPA receptors in the neonatal mouse brain

The N‐methyl‐d ‐aspartate receptor (NMDAR) plays various physiological and pathological roles in neural development, synaptic plasticity and neuronal cell death. It is composed of two GluN1 and two GluN2 subunits and, in the neonatal hippocampus, most synaptic NMDARs are GluN2B‐containing receptors, which are gradually replaced with GluN2A‐containing receptors during development. Here, we examined whether GluN2A could be substituted for GluN2B in neural development and functions by analysing knock‐in (KI) mice in which GluN2B is replaced with GluN2A. The KI mutation was neonatally lethal, although GluN2A‐containing receptors were transported to the postsynaptic membrane even without GluN2B and functional at synapses of acute hippocampal slices of postnatal day 0, indicating that GluN2A‐containing NMDARs could not be substituted for GluN2B‐containing NMDARs. Importantly, the synaptic α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionic acid receptor (AMPAR) subunit GluA1 was increased, and the transmembrane AMPAR regulatory protein, which is involved in AMPAR synaptic trafficking, was increased in KI mice. Although the regulation of AMPARs by GluN2B has been reported in cultured neurons, we showed here that AMPAR‐mediated synaptic responses were increased in acute KI slices, suggesting differential roles of GluN2A and GluN2B in AMPAR expression and trafficking in vivo. Taken together, our results suggest that GluN2B is essential for the survival of animals, and that the GluN2B–GluN2A switching plays a critical role in synaptic integration of AMPARs through regulation of GluA1 in the whole animal.     

Ibogaine and a Total Alkaloidal Extract of Voacanga africana Modulate Neuronal Excitability and Synaptic Transmission in the Rat Parabrachial Nucleus In Vitro

Ibogaine is a natural alkaloid of Voacanga africana that is effective in the treatment of withdrawal symptoms and craving in drug addicts. As the synaptic and cellular basis of ibogaine’s actions are not well understood, this study tested the hypothesis that ibogaine and Voacanga africana extract modulate neuronal excitability and synaptic transmission in the parabrachial nucleus using the nystatin perforated patch-recording technique. Ibogaine and Voacanga africana extract dose dependently, reversibly, and consistently attenuate evoked excitatory synaptic currents recorded in parabrachial neurons. The ED₅₀ of ibogaine’s effect is 5 μM, while that of Voacanga africana extract is 170 μg/ml. At higher concentrations, ibogaine and Voacanga africana extract induce inward currents or depolarization that are accompanied by increases in evoked and spontaneous firing rate. The depolarization or inward current is also accompanied by an increase in input resistance and reverses polarity around 0 mV. The depolarization and synaptic depression were blocked by the dopamine receptor antagonist haloperidol. These results indicate that ibogaine and Voacanga africana extract 1) depolarize parabrachial neurons with increased excitability and firing rate; 2) depress non-NMDA receptor-mediated fast synaptic transmission; 3) involve dopamine receptor activation in their actions. These results further reveal that the Voacanga africana extract has one-hundredth the activity of ibogaine in depressing synaptic responses. Thus, ibogaine and Voacanga africana extract may produce their central effects by altering dopaminergic and glutamatergic processes.