Cholinergic modulation of dopamine overflow in the rat neostriatum: A fast cyclic voltammetric study in vitro

Stimulus-evoked dopamine overflow in rat neostriatal slices was determined using fast cyclic voltammetry. The dopamine efflux induced by intrastriatal stimulation increased with stimulus intensity and was found to be enhanced by more than 100% upon application of the dopamine uptake inhibitor nomifensine. The acetylcholine esterase inhibitor eserine concentration-dependently and reversibly depressed stimulus-induced dopamine overflow. This effect was mediated by both, muscarinic and nicotinic cholinergic receptors: the action of eserine was mimicked by cholinergic agonists (muscarine and nicotine) and the effects of these agonists were blocked by muscarinic and nicotinic antagonists (atropine and dihydro-β-erythroidine). These experiments suggest that endogenous acetylcholine exerts an inhibitory control on stimulus-evoked (i.e. phasic) dopamine overflow in vitro by affecting striatal dopaminergic nerve terminals

Ascorbic acid: A useful reductant to avoid oxidation of catecholamines in electrophysiological experiments in vitro?

The actions of the reductant ascorbic acid on rat neocortical neurons in vitro was investigated by means of intracellular recordings. At a concentration (500 μM), which reduced the magnitude of dopamine degradation in oxygen-saturated saline solutions by about 50%, ascorbic acid reversibly depressed synaptic potentials and enhanced direct excitability of cortical neurons. The latter effect was not reversible within the observation period. Ascorbic acid did not alter membrane potential and input resistance of the neurons. On the basis of our results we conclude that ascorbic acid is not a useful reductant to avoid oxidation of catecholamines in oxygen-saturated solutions used in electrophysiological experiments in vitro

EPSPs in rat neocortical neurons in vitro. II. Involvement of N-methyl-D-aspartate receptors in the generation of EPSPs

1. Intracellular recordings were obtained from neurons in layer II/III of rat frontal cortex. Single-electrode current- and voltage-clamp techniques were employed to compare the sensitivity of excitatory postsynaptic potentials (EPSPs) and iontophoretically evoked responses to N-methyl-D-aspartate (NMDA) to the selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-2-APV). The voltage dependence of the amplitudes of the EPSPs before and after pharmacologic changes in the neuron's current-voltage relationship was also examined. 2. NMDA depolarized the membrane potential, increased the neuron's apparent input resistance (RN), and evoked bursts of action potentials. The NMDA-induced membrane current (INMDA) gradually increased with depolarization from -80 to -40 mV. The relationship between INMDA and membrane potential displayed a region of negative slope conductance in the potential range between -70 and -40 mV which was sufficient to explain the apparent increase in RN and the burst discharges during the NMDA-induced depolarization. 3. Short-latency EPSPs (eEPSPs) were evoked by low-intensity electrical stimulation of cortical layer IV. Changes in the eEPSP waveform following membrane depolarization and hyperpolarization resembled those of NMDA-mediated responses. However, the eEPSP was insensitive to D-2-APV applied at concentrations (up to 20 microM) that blocked NMDA responses. 4. EPSPs with latencies between 10 and 40 ms [late EPSPs (lEPSPs)] were evoked by electrical stimulation using intensities just subthreshold to the activation of IPSPs. The amplitude of the lEPSP increased with hyperpolarization and decreased with depolarization. 5. The lidocaine derivative QX-314, injected intracellularly, suppressed sodium-dependent action potentials and depolarizing inward rectification. Simultaneously, the amplitude of the eEPSP significantly decreased with depolarization. Neither the amplitude of a long-latency EPSP nor the amplitude of inhibitory postsynaptic potentials (IPSPs) was significantly affected by QX-314. 6. Cesium ions (0.5-2.0 mM) added to the bathing solution reduced or blocked hyperpolarizing inward rectification. Under these conditions, the amplitude of the eEPSP increased with hyperpolarization. The amplitude of the lEPSP was unaltered or enhanced. 7. The lEPSP was reversibly blocked by D-2-APV (5-20 microM), although the voltage-dependence of its amplitude did not resemble the action of NMDA on neocortical neurons

Presynaptic M1 muscarinic cholinoceptors mediate inhibition of excitatory synaptic transmission in the hippocampus in vitro

The effects of the cholinoceptor agonist, carbachol (CCh), were examined in the rat hippocampal slice preparation. Intracellular recordings from CA1 pyramidal neurones revealed that CCh (1–3 μM) inhibited excitatory postsynaptic responses evoked by stimulation of the Schaffer collateral/commissural pathway while, at the same time, direct excitability was enhanced. Extracellularly, CCh produced a concentration-dependent reduction of the amplitude of the field excitatory postsynaptic potential (field EPSP) recorded in the CA1 apical dendritic region. The muscarinic receptor antagonist, pirenzepine, competitively antagonized the effects of CCh on the field EPSP with a pA2 of 7.4. These results confirm earlier reports of a presynaptic inhibitory action of CCh in the hippocampal CA1 region and provide strong evidence that this effect is mediated by muscarinic receptors of the M1 subtype

EPSPs in rat neocortical neurons in vitro. I. Electrophysiological evidence for two distinct EPSPs

1. To investigate excitatory postsynaptic potentials (EPSPs), intracellular recordings were performed in layer II/III neurons of the rat medial frontal cortex. The average resting membrane potential of the neurons was more than -75 mV and their average input resistance was greater than 20 M omega. The amplitudes of the action potentials evoked by injection of depolarizing current pulses were greater than 100 mV. The electrophysiological properties of the neurons recorded were similar to those of regular-spiking pyramidal cells. 2. Current-voltage relationships, determined by injecting inward and outward current pulses, displayed considerable inward rectification in both the depolarizing and hyperpolarizing directions. The steady-state input resistance increased with depolarization and decreased with hyperpolarization, concomitant with increases and decreases, respectively, in the membrane time constant. 3. Postsynaptic potentials were evoked by electrical stimulation via a bipolar electrode positioned in layer IV of the neocortex. Stimulus-response relationships, determined by gradually increasing the stimulus intensity, were consistent among the population of neurons examined. A short-latency EPSP [early EPSP (eEPSP)] was the response with the lowest threshold. Amplitudes of the eEPSP ranged from 4 to 8 mV. Following a hyperpolarization of the membrane potential, the amplitude of the eEPSP decreased. Upon depolarization, a slight increase in amplitude and duration was observed, accompanied by a significant increase in time to peak. 4. The membrane current underlying the eEPSP (eEPSC) was measured using the single-electrode voltage-clamp method. The amplitude of the eEPSC was apparently independent of the membrane potential in 8 of 12 neurons tested. In the other 4 neurons, the amplitude of the eEPSC increased with hyperpolarization and decreased with depolarization. 5. Higher stimulus intensities evoked, in addition to the eEPSP, a delayed EPSP [late EPSP (lEPSP)] in greater than 90% of the neurons tested. The amplitude of the lEPSP ranged from 12 to 20 mV, and the latency varied between 20 and 60 ms. The amplitude of the lEPSP varied with membrane potential, decreasing with depolarization and increasing following hyperpolarization. The membrane current underlying the lEPSP (lEPSC) displayed a similar voltage dependence. 6. At stimulus intensities that led to the activation of inhibitory postsynaptic potentials (IPSPs), the lEPSP was no longer observed

Long-term potentiation in frontal cortex: Role of NMDA-modulated polysynaptic excitatory pathways

The present study examined the role of N-methyl-D-aspartic acid (NMDA) receptors in synaptic plasticity in regular-spiking cells of rat frontal cortex. Intracortical stimulation, at levels subthreshold for elicitation of action potentials, evoked a late excitatory postsynaptic potential (EPSP) in layer II III neurons that was sensitive to the selective NMDA antagonist -2-amino-5-phosphonovaleric acid (APV). This late EPSP showed marked short-term frequency-dependent depression, suggesting that it is polysynaptic in origin. Polysynaptic late EPSPs were selectively enhanced following high-frequency stimulation. This sustained increase in synaptic efficacy, or long-term potentiation, was expressed in regular spiking cells and appeared to result from activation of NMDA receptors on excitatory interneurons. These data demonstrate the existence of an NMDA-modulated polysynaptic circuit in the neocortex which displays several types of use-dependent plasticity

Excitatory postsynaptic potentials in rat neocortical neurons in vitro. III. Effects of a quinoxalinedione non-NMDA receptor antagonist

1. Intracellular microelectrodes were used to obtain recordings from neurons in layer II/III of rat frontal cortex. A bipolar electrode positioned in layer IV of the neocortex was used to evoke postsynaptic potentials. Graded series of stimulation were employed to selectively activate different classes of postsynaptic responses. The sensitivity of postsynaptic potentials and iontophoretically applied neurotransmitters to the non-N-methyl-D-asparate (NMDA) antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was examined. 2. As reported previously, low-intensity electrical stimulation of cortical layer IV evoked short-latency early excitatory postsynaptic potentials (eEPSPs) in layer II/III neurons. CNQX reversibly antagonized eEPSPs in a dose-dependent manner. Stimulation at intensities just subthreshold for activation of inhibitory postsynaptic potentials (IPSPs) produced long-latency (10 to 40-ms) EPSPs (late EPSPs or 1EPSPs). CNQX was effective in blocking 1EPSPs. 3. With the use of stimulus intensities at or just below threshold for evoking an action potential, complex synaptic potentials consisting of EPSP-IPSP sequences were observed. Both early, Cl(-)-dependent and late, K(+)-dependent IPSPs were reduced by CNQX. This effect was reversible on washing. This disinhibition could lead to enhanced excitability in the presence of CNQX. 4. Iontophoretic application of quisqualate produced a membrane depolarization with superimposed action potentials, whereas NMDA depolarized the membrane potential and evoked bursts of action potentials. At concentrations up to 5 microM, CNQX selectively antagonized quisqualate responses. NMDA responses were reduced by 10 microM CNQX. D-Serine (0.5-2 mM), an agonist at the glycine regulatory site on the NMDA receptor, reversed the CNQX depression of NMDA responses

Transient and selective blockade of adenosine A1-receptors by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) causes sustained epileptiform activity in hippocampal CA3 neurons of guinea pigs

The effects of endogenously released adenosine on the excitability of hippocampal neurons were studied using the novel and highly selective adenosine A1-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Extra- and intracellular recordings performed in area CA1 and CA3 of the guinea pig hippocampal slice preparation revealed that a transient suppression of an inhibitory purinergic tonus by DPCPX leads to sustained interictal-like epileptiform activity arising in area CA3. Once induced, the spontaneous burst discharges were apparently irreversible within the observation period, even after prolonged washout (2–3h) in normal solution. In contrast, the hyperpolarizing action of exogenous adenosine, which was substantially reduced by DPCPX, recovered within 30–60 min of drug washout, indicating that DPCPX was not irreversibly bound to the A1-receptor

Effects of cromakalim (BRL 34915) on potassium conductances in CA3 neurons of the guinea-pig hippocampus in vitro

The action of the potassium channel activator, cromakalim (BRL 34915), on membrane potential, input resistance and current-voltage-relationship of CA3 neurons in a slice preparation of the guinea-pig hippocampus was investigated by means of intracellular recordings. In the presence of tetrodotoxin, cromakalim (30–100 mol/l) produced a hyperpolarization up to 4 mV associated with a decrease in input resistance up to 10 MOhms. Determination of the equilibrium potential of the cromakalim action revealed that the hyperpolarization is due to the activation of a potassium conductance. This cromakalim-activated potassium conductance was voltage-dependent, i.e. it increased with hyperpolarization. Among a number of potassium channel blockers tested, only Cs+ (2 mmol/l) and Ba2+ (0.5 mmol/1) were able to inhibit the cromakalim-induced effects. Simultaneously, both cations suppressed the hyperpolarizing inward rectification (anomalous rectification) in these neurons, indicating that cromakalim activated or potentiated an inwardly rectifying potassium conductance. In addition, cromakalim slightly enhanced both amplitude and duration of afterhyperpolarizations following single calcium-dependent action potentials, suggesting that cromakalim might have a weak facilitatory effect on calcium-dependent potassium conductances