Distribution of exponentiality in miniature endplate current decay

Exponential functions are commonly used to describe miniature endplate current (MEPC) decay; under control conditions a monoexponential is usually regarded as sufficient, whereas in the presence of some drugs a biexponential may be necessary. Using an automated fitting procedure which estimated exponential parameters and the period of decay from peak to baseline, a unimodal distribution of curvature was found for control MEPCs recorded in frog sartorius muscle. The majority of MEPCs were of monoexponential form, while the remainder were biexponential with either less or greater curvature than expected for a simple exponential (hypoexponential or hyperexponential, respectively). The proportion of MEPCs in each of the 3 groups was constant for a given endplate but varied between endplates. A possible explanation for this phenomenon could be differences in synaptioc geometry withing and between endplates. The increased curvature of individual MEPCs in the hyperexponential group was analyzed by assuming a sequencial model for agonist blockade or desensitization, and calculating closing and reopening rate concstants. These rate constants were altered by procaine and verapamil (100 μM) in a manner consistent with blockade of the acetylcholine receptor by enhancement of agonist-induced desensitization.     

Desensitization of mutant acetylcholine receptors in transgenic mice reduces the amplitude of neuromuscular synaptic currents

While the slow onset of desensitization of nicotinic acetylcholine receptors (AChRs), relative to the rate of acetylcholine removal, excludes this kinetic state from shaping synaptic responses in normal neuromuscular transmission, its role in neuromuscular disorders has not been examined. The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder caused by point mutations in the AChR subunit-encoding genes leading to kinetically abnormal (slow) channels, reduced miniature endplate current amplitudes (MEPCs), and degeneration of the postsynaptic membrane. Because of this complicated picture of kinetic and structural change in the neuromuscular junction, it is difficult to assess the importance of the multiple factors that may be responsible for the reduced endplate current amplitudes, and ultimately the clinical syndrome. In order to address this we have used a transgenic mouse model for the SCCMS that has slow AChR ion channels and reduced endplate responsiveness in the absence of any of the degenerative changes. We found that the reduction in MEPC amplitudes in these mice could not be explained by either reduced AChR number or by reduced AChR channel conductance. Rather, we found that the mutant AChRs in situ manifested an activity-dependent reduction in sensitivity that caused diminished MEPC and endplate current amplitude with nerve stimulation. This observation demonstrates that the basis for the reduction in MEPC amplitudes in the SCCMS may be multifactorial. Moreover, these findings demonstrate that, under conditions that alter their rate of desensitization, the kinetic properties of nicotinic AChRs can control the strength of synaptic responses. Synapse 27:367–377, 1997. © 1997 Wiley-Liss, Inc.     

Presynaptic calcium currents evoking quantal transmission from avian ciliary ganglion neurons

Using whole-cell patch clamp techniques, we simultaneously recorded presynaptic Ca++ current and excitatory postsynaptic currents (EPSCs) from avian neuromuscular junctions in culture. Quantal synaptic transmission was proportional to evoked presynaptic Ca++ current except with large stimuli, which evoked bursts of quanta, reflecting a shift to synchronized release. Synaptic delay, measured from the onset of presynaptic depolarization to the appearance of the first postsynaptic quantal response, was often greater than 100 msec for weak depolarizations but declined as stimulus intensity was increased. Quantal events evoked by Ca++ tail currents had a mean synaptic delay of 1.67 msec. The single type of presynaptic Ca++ current observed displayed an inactivation time constant of greater than 100 msec and tail currents well fit by a single exponential function.     

Postsynaptic block of frog neuromuscular transmission by conotoxin GI

Conotoxin GI, a peptide neurotoxin contained in the venom of the marine snail Conus geographus, was applied to the cutaneous pectoris muscle of the frog, and the effects on the postsynaptic response to acetylcholine were examined. Conotoxin GI reversibly blocked nerve-evoked muscle contractions at concentrations greater than or equal to 3 to 4 microM. Micromolar concentrations of conotoxin GI significantly reduced the amplitude of miniature endplate potentials and membrane depolarizations produced by ionophoretic application of acetylcholine, suggesting that the toxin reduced the postsynaptic sensitivity to acetylcholine. The reduction in the sensitivity of the muscle to acetylcholine was not due to changes in muscle fiber resting membrane potential or input resistance. Conotoxin GI reduced the amplitudes but did not affect the rates of decay of focal, extracellularly recorded endplate currents or miniature endplate currents, suggesting that the toxin did not affect the lifetime of ion channels opened by acetylcholine. Miniature endplate currents decay five to six times more slowly than normal when acetylcholinesterase is blocked with neostigmine methyl sulfate due to repeated binding of acetylcholine to receptors as it diffuses from the synaptic cleft. Conotoxin GI reduced the amplitude and increased the rate of decay of miniature endplate currents recorded in the presence of neostigmine methyl sulfate, suggesting that the toxin reduced the binding of acetylcholine to endplate receptors. These results are consistent with the hypothesis that conotoxin GI blocks neuromuscular transmission at the frog endplate by reducing the binding of acetylcholine to receptors.     

Direct muscarinic and nicotinic receptor-mediated excitation of rat medial vestibular nucleus neurons in vitro

We have utilized intracellular recording techniques to investigate the cholinoceptivity of rat medial vestibular nucleus (MVN) neurons in a submerged brain slice preparation. Exogenous application of the mixed cholinergic agonists, acetylcholine (ACh) or carbachol (CCh), produced predominantly membrane depolarization, induction of action potential firing, and decreased input resistance. Application of the selective muscarinic receptor agonist muscarine (MUSC), or the selective nicotinic receptor agonists nicotine (NIC) or 1,1-dimethyl-4-phenylpiperazinium (DMPP) also produced membrane depolarizations. The MUSC-induced depolarization was accompanied by decreased conductance, while an increase in conductance appeared to underlie the NIC- and DMPP-induced depolarizations. The muscarinic and nicotinic receptor mediated depolarizations persisted in tetrodotoxin and/or low Ca²⁺/high Mg²⁺ containing media, suggesting direct postsynaptic receptor activation. The MUSC-induced depolarization could be reversibly blocked by the selective muscarinic-receptor antagonist, atropine, while the DMPP-induced depolarization could be reversibly suppressed by the selective ganglionic nicotinicreceptor antagonist, mecamylamine. Some neurons exhibited a transient membrane hyperpolarization during the depolarizing response to CCh or MUSC application. This transient inhibition could be reversibly blocked by the γ-aminobutyric acid (GABA) antagonist, bicuculline, suggesting that the underlying hyperpolarization results indirectly from the endogenous release of GABA acting at GABA receptors. This study confirms the cholinoceptivity of MVN neurons and establishes that individual MVN cells possess muscarinic as well as nicotinic receptors. The data provide support for a prominent role of cholinergic mechanisms in the direct and indirect regulation of the excitability of MVN neurons.     

Comparison of the electrophysiology and morphology of layers III and II neurons of the rat medial entorhinal cortex in vitro

The basic membrane characteristics of neurons in layers II and III of the medial entorhinal cortex (MEA) were recorded using the intracellular current clamp technique in in vitro slices of the rat brain. Two types of cells were distinguished according to the presence of a time-dependent inward rectification (SAG current) with hyperpolarizing current pulses. The cells in which this inward rectification was not observed (No-SAG cells) had a larger input resistance, a more negative resting membrane potential and a more depolarized firing threshold. They more often displayed a strongly adapting firing pattern, and their action potentials had a slower decay rate and lacked a depolarizing afterpotential, compared with the SAG cells. SAG cells typically had a prominent rebound depolarization at the end of a hyperpolarizing current and membrane potential oscillations (7 Hz) upon subthreshold depolarizations. Cs⁺ blocked the time-dependent inward rectification. The rebound depolarization persisted, even in the presence of tetrodotoxin. Biocytin labelling showed that layer III consisted mainly of pyramidal-shaped cells. Most layer III cells were of the No-SAG type. All cells in layer II, stellate and pyramidal cells, were classified as SAG cells. We conclude that the cells in MEA layers II and III display different electroresponsiveness, but that this appears to be more related to the layer where they are located than to a specific morphology. As layer III consisted mainly of cells of the No-SAG type, we suggest that layer III cells are less excitable than the SAG type layer II cells.     

Concentration-dependent effects of neostigmine on the endplate acetylcholine receptor channel complex

The concentration-dependent actions of neostigmine, a carbamate anticholinesterase agent, were studied on the acetylcholine receptor channel complex in voltage-clamped twitch fibers of costocutaneous muscles of garter snakes. Low concentrations of neostigmine (10(-6) or 10(-5) M) increased miniature endplate current (MEPC) amplitude and the time constant of MEPC decay without changing the relationship between the MEPC decay time constant and membrane potential. Acetylcholine- or carbachol-induced endplate current fluctuation spectra were well fitted by a single Lorentzian curve with a characteristic frequency and single-channel conductance unaltered by low concentrations of neostigmine. Concentrations of neostigmine greater than 5 X 10(-5) M decreased MEPC amplitude and split the decay of MEPCs into two components, one faster and one slower than the control rate. These effects were both voltage and concentration dependent. Spectra of current fluctuations recorded in concentrations greater than or equal to 5 X 10(-5) M neostigmine required two time constants, one faster and one slower than the control. Two component spectra were also obtained with carbachol-induced current fluctuation spectra, indicating that these effects of neostigmine were direct and not a consequence of acetylcholinesterase inhibition. Similar results were also obtained in muscles pretreated with collagenase to remove junctional acetylcholinesterase. The fast and slow time constants obtained from current fluctuation spectra decreased and increased, respectively, with either increases in the concentration of neostigmine or membrane hyperpolarization when analyzed in the same fiber. The effects of neostigmine on channel lifetime were reversible with washing. These results indicate that the effects of neostigmine are concentration dependent. Concentrations greater than 2.5 X 10(-5) M exhibit direct effects on the endplate receptor channel complex which are unrelated to acetylcholinesterase inhibition. These actions include: a prolongation of the gating kinetics of the endplate receptor channel complex, the production of an altered state of the receptor channel complex evidenced by a high frequency component to current fluctuation spectra, and a direct action to block the acetylcholine receptor.     

Differences in a K current in schwann cells from normal and neurofibromatosis-infected damselfish

Patch clamp techniques were used to study whole cell ionic currents in Schwann cells (SC) from a tropical marine fish, the bicolor damselfish, Pomacentrus partitus. The bicolor damselfish is affected by a disease termed damselfish neurofibromatosis (DNF), being developed as an animal model of neurofibromatosis-type 1 (NF1) in humans. NF1 affects SC, fibroblasts, and perineurial cells. The sole depolarization-activated ionic current present in cultured SC from normal fish peripheral nerve and from neurofibromas of fish with induced or spontaneously occurring DNF was an inactivating K⁺ current (K current), with a strong dependence on the Nernst potential for K⁺. This K current activated at depolarizations to -40 mV and above and inactivated during a maintained test pulse (0.2-1 s), but inactivation was significantly greater in tumored SC. Both currents were inhibited by 4-aminopyridine (K_d ? 1 mM) and by dendrotoxin (15 μM) but were insensitive to extracellular tetraethyammonium (≤ 150 mM), indicating that the whole cell currents were similar pharmacologically. The currents could be distinguished on the basis of their sensitivity to depolarized holding potential, with normal cells less sensitive. Half-inactivation of the current was -32 mV in normal cells and -38 mV in tumored cells. Inactivation curves constructed from the average normalized current for many SC were significantly different in normal and tumored cells. When the depolarized holding potential was maintained between test depolarizations, greater voltage-dependent inactivation in tumored cells was apparent. Normal cells maintained an average of 36% of peak current at a holding voltage of ?40 mV, while in tumored cells this average was 12%, a significant difference. © 1994 Wiley-Liss, Inc.     

Antagonizing effect of protein kinase C activation on the mu-opioid agonist-induced inhibition of high voltage-activated calcium current in rat periaqueductal gray neuron

Opioids have been thought to induce analgesia by activating the descending pain control system, especially at the level of periaqueductal gray, and regulate the neurotransmitter release through the inhibition of calcium channel. In the present study, the modulatory effects of protein kinase C and protein kinase A on the mu-opioid agonist-induced inhibition of the high-voltage activated calcium current were examined in the acutely dissociated rat periaqueductal gray neurons with the nystatin-perforated patch-clamp technique. Among 505 neurons tested, the barium current passing through the high-voltage activated calcium channels of 172 neurons (34%) were inhibited by 32+/-3% with the application of an mu-opioid agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM). The barium currents itself and the DAMGO-induced inhibitory effects were not affected by the application of either an adenylate cyclase activator (forskolin, 1 microM) or a protein kinase inhibitor (staurosporin, 10 nM) for 2 min. The DAMGO inhibition was completely and irreversibly antagonized by the application of a protein kinase C activator, phorbol-12-myristate-13-acetate (PMA, 1 microM) for 2 min without any alteration of the barium current itself. However, the antagonizing effect of PMA was completely abolished by the application of 10 nM staurosporin for 2 min. After then, PMA did not show the antagonizing effect any more. Inversely, when staurosporin was applied before PMA, the antagonizing effect of PMA was also not shown. These results demonstrate that the mu-opioid agonist-induced inhibition of the periaqueductal gray neuronal high-voltage activated calcium current can be antagonized by protein kinase C activation. This finding may provide us a significant clue to understand the action mechanism of opioid-induced analgesia in the periaqueductal gray.     

Amyloid β‐protein suppressed nicotinic acetylcholine receptor‐mediated currents in acutely isolated rat hippocampal CA1 pyramidal neurons

Amyloid β protein (Aβ) is responsible for the deficits of learning and memory in Alzheimer's disease (AD). The high affinity between Aβ and nicotinic acetylcholine receptors (nAChRs) suggests that the impairment of cognitive function in AD might be involved in the Aβ‐induced damage of nAChRs. This study investigated the effects of Aβ fragments on nAChR‐mediated membrane currents in acutely isolated rat hippocampal pyramidal neurons by using whole‐cell patch clamp technique. The results showed that: (1) nonspecific nAChR agonist nicotine, selective α7 nAChR agonist choline, and α4β2 nAChR agonist epibatidine all effectively evoked inward currents in CA1 neurons at normal resting membrane potential, with different desensitization characteristics; (2) acute application of different concentrations (pM–μM) of Aβ25‐35, Aβ31‐35, or Aβ35‐31 alone did not trigger any membrane current, but pretreatment with 1 μM Aβ25‐35 and Aβ31‐35 similarly and reversibly suppressed the nicotine‐induced currents; (3) further, choline‐ and epibatidine‐induced currents were also reversibly suppressed by the Aβ pretreatment, but more prominent for the choline‐induced response. These results demonstrate that the functional activity of both α7 and α4β2 nAChRs in the membrane of acutely isolated hippocampal neurons was significantly downregulated by Aβ treatment, suggesting that nAChRs, especially α7 nAChRs, in the brain may be the important biological targets of neurotoxic Aβ in AD. In addition, the similar suppression of nAChR currents by Aβ25‐35 and Aβ31‐35 suggests that the sequence 31‐35 in Aβ molecule may be a shorter active center responsible for the neurotoxicity of Aβ in AD. Synapse, 2013. © 2012 Wiley Periodicals, Inc.     

Clindamycin-induced alteration of ganglionic function. II. Effect of nicotinic receptor-channel function

The postsynaptic effects of clindamycin have been analyzed in bullfrog sympathetic ganglion B cells using single electrode current and voltage clamp recordings and two electrode voltage clamp measurements. Clindamycin added to the bathing solution in the concentration range, 2.5 x 10(-4) to 5 x 10(-4) M, inhibited fast ganglionic transmission. In addition, local application of clindamycin decreased depolarizations produced by direct application of acetylcholine and decreased the amplitude of miniature excitatory postsynaptic potentials (MEPSPs) evoked by tetanic stimulation of the preganglionic trunk. In contrast, clindamycin did not change the amplitude or time course of the slow EPSP elicited by preganglionic stimulation (30 Hz for 10 s) or muscarinic depolarizations produced by local acetylcholine application to preparations pretreated with 25-50 microM (+)-tubocurarine. In voltage-clamped ganglion cells, excitatory postsynaptic current (EPSC) amplitude initially was increased and then decreased with increasing concentrations of clindamycin (0.5 x 10(-5) to 2.5 x 10(-4) M). The EPSC time course in control cells was exponential. After exposure to clindamycin, the EPSC decay was composed of two exponential components. The time constant of the fast component decreased and the time constant of the slow component increased with increasing concentrations of clindamycin. The two time constants of EPSCs obtained in clindamycin were independent of membrane voltage between -50 and -100 mV. We concluded that the block of fast ganglionic transmission is primarily due to a postsynaptic site of action, at least part of which is due to a concentration-dependent, but voltage-independent blockade of open nicotinic receptor channel complexes.     

Negative slope conductance at large depolarization in cat spinal motoneurons

The current-voltage relation of cat spinal motoneurons obtained by somatic voltage clamp exhibits a region of negative slope conductance at large depolarizations in addition to the previously reported N-shaped region at small depolarizations. The N shape at large depolarizations is caused by one outward current component which grows larger and then smaller with depolarization.     

Kainic acid on neostriatal neurons intracellularly recorded in vitro: electrophysiological evidence for differential neuronal sensitivity

The electrophysiological effects produced by different concentrations of kainic acid (KA) were studied by utilizing intracellular recordings from neostriatal slices. In most of the recorded cells (81%), concentrations of KA ranging between 10 and 300 nM produced reversible and dose-dependent membrane depolarizations. Higher concentrations of this agonist caused larger depolarizations and changes of the membrane properties of the recorded neurons not reversible during the time of recording. In a smaller percentage (19%) of the recorded cells, 10-100 nM KA did not produce significant membrane depolarizations; in these neurons, the depolarizations produced by higher concentrations of KA were small and reversible. The 2 populations of neurons showed similar electrophysiological properties and did not reveal differential sensitivity to quisqualic acid (QUIS; 10-30 microM) or to NMDA (10-30 microM). Tetrodotoxin (TTX; 1 microM) did not reduce the depolarizations produced by KA and by NMDA. Low-calcium, cobalt-containing solutions abolished the effects produced by NMDA, but not the KA-induced depolarizations. Kynurenic acid (500 microM) significantly antagonized the depolarizations produced by KA and reduced the changes of the membrane properties caused by high doses of this agonist. In several neurons, KA induced bicuculline-sensitive synaptic depolarizing potentials. Our findings suggest the presence of 2 subpopulations of neostriatal neurons showing differential postsynaptic sensitivity to KA. The differential sensitivity of neostriatal neurons to KA might influence the responses of these cells to glutamatergic cortical inputs and the degenerative changes observed in neostriatal neurons in some pathological conditions.     

Hyperpolarizing after-potentials regulate generation of long-duration plateau depolarizations in rat paraventricular nucleus neurons

Activation of N-methyl-d -aspartate (NMDA) receptors in a population of neurons of the paraventricular nucleus (PVN) results in long-duration plateau depolarizations during which the membrane rapidly depolarizes, reaching a stable plateau near ?20 mV. These responses were observed in 29% of the Type II PVN neurons tested with 1 μm NMDA agonist (n = 84). The stable plateau phase is characterized by an increase in ionic conductance, from 1.19 ± 0.11 nS to 5.24 ± 2.17 nS (n = 5). Bath application of tetrodotoxin (n = 4) or alternatively inclusion of QX-314 in the pipette solution (n = 3) prevented the generation of these events. The remaining cells tested (n = 56) also depolarized in response to NMDA agonist, but long duration plateau depolarizations were not observed. Previous evidence from hypothalamic cultures has demonstrated synaptically driven plateau potentials following the blockade of repolarizing conductances. Pharmacological blockade of the post-spike hyperpolarizing afterpotential with 4-aminopyridine (200 μm ), in cells that did not generate plateaux, resulted in the observance of long duration plateau depolarizations in response to a subsequent application of NMDA agonist (n = 4). Our results demonstrate that this 4-aminopyridine-sensitive ionic conductance plays a critical role in determining whether a cell will depolarize for a prolonged duration in response to NMDA receptor activation. As a prolonged depolarization of the postsynaptic membrane and accompanying membrane permeability changes are essential for neurotoxicity, these findings provide evidence for a potential protective mechanism that depends solely on the ability of the cell, through its ionic conductances, to control imposed changes in membrane potential.     

Bacterial lipopolysaccharide depresses spontaneous, evoked, and ionophore-induced transmitter release at the neuromuscular junction.

The neurotoxocity of RNA-free lipopolysaccharide (LPS) extracted from Salmonella Typhimurium (SR-11) was tested at the frog neuromuscular junction using intracellular recording techniques. Spontaneous miniature endplate potential (MEPP) frequency was reduced to 45% of control after 60 minutes in the presence of 10 and 50 micrograms LPS/ml Ringer's solution. Elevation of extracellular [Ca] to 10 mM converted the MEPP frequency response to a biphasic pattern of early acceleration followed by late depression. Evoked endplate potentials (EEPs) were reduced in quantal content until phasic release of transmitter was abolished, while MEPP amplitude and endplate resting potential remained constant. Effects of the potent cation ionophore X537A on MEPP frequency were blocked by 45 minutes of pre-exposure to LPS. Because of its extremely lipophilic character, LPS apparently alters the physical structure of the presynaptic terminal membrane, eventually reducing resting and phasic Ca influx, and isolating the presynaptic terminal from ionophore action.     

Kainate-induced currents in rat cortical neurons in culture are modulated by riluzole

The action of the neuroprotective and anticonvulsant agent riluzole on kainate-induced currents was studied in rat cortical neurons in primary culture by using the whole-cell configuration of the patch-clamp technique. Kainate elicited macroscopic, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive inward currents in all the patched cells and the amplitude of the current was concentration-dependent (EC50= 106 microM). Riluzole decreased the inward currents induced by 100 microM kainate at all holding potentials and the reduction was dose-dependent (IC50= 101 microM). The maximal response to kainate decreased in the presence of 50 microM riluzole, without changing its EC50, indicating a noncompetitive mechanism of inhibition. The amplitude of the responses induced by kainate under control conditions and during riluzole was a linear function of the membrane potential and the reversal potential of the currents was not significantly different in the two experimental conditions. Instead, the total conductance of the cell membrane for the currents induced by 100 microM kainate was significantly reduced in the presence of 50 microM riluzole (P < 0.05). The analysis of the kainate membrane current noise performed under control conditions and during perfusion of 100 microM riluzole revealed that riluzole reduced the probability of kainate-activated ionic channels to be in the open state. Conversely, the unitary conductance of channels, as well as their characteristic time constant, seemed to be unchanged. These results reveal an additional mechanism by which riluzole can interact with glutamatergic neurotransmission and provides further support for the idea that riluzole may prove beneficial in the treatment of central nervous system injuries involving the excitotoxic actions of glutamate.     

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.     

Differential arithmetic of shunting inhibition for voltage and spike rate in neocortical pyramidal cells

The main inhibitory neurotransmitter in the mammalian forebrain is gamma-amino butyric acid (GABA), which acts through A and B type receptors. GABAA receptors mediate inhibition via an increase in membrane conductance (shunting) and/or membrane potential hyperpolarization. Shunting inhibition is thought to decrease the gain between neural input and output, and thus to act as a divisor, but may do so only below the spike threshold. To investigate the role of shunting inhibition in neocortical neurons, whole-cell patch-clamp recordings were obtained from layer V pyramidal cells of somatosensory cortex in juvenile rats. Sub- and suprathreshold voltage responses were elicited by somatic step current injections and a shunting conductance was generated via a dynamic clamp. Increasing the dynamic clamp shunting conductance led to a parallel shift of the current-discharge curves and a reduced slope of the current-voltage relationship, i.e. a decrease of neural gain. Selective activation of GABAAA receptors with the competitive agonist isoguvacine or rises of endogenous GABA with the specific reuptake blocker nipecotic acid led to a proportional decrease of subthreshold membrane voltage, but a constant offset of discharge rates, thus acting like a shunting conductance. Similarly, isoguvacine and nipecotic acid decreased the gain of excitatory postsynaptic potentials. In all three experimental conditions, shunting inhibition divisively affected subthreshold voltages, while the time-averaged suprathreshold membrane potential was offset by a constant amount. I conclude that shunting inhibition in pyramidal cells has a dual impact on neural output: it is divisive for subthreshold voltages but subtractive for spike frequencies.     

Presynaptic effects of sodium bisulfite at the frog neuromuscular junction

Both spontaneous and evoked transmitter release from the frog neuromuscular junction can be modified by application of sodium bisulfite, a reagent specific for disulfide bonds. An increase in miniature endplate frequency is produced that is not dependent on external calcium, sodium, or presynaptic terminal depolarization. The increased release can be halted by application of the sulfhydryl oxidizing agent DTNB. The response of bisulfite can be prevented by prior treatment of the endplate with acetylcholine or an anticholinesterase. It is concluded that bisulfite produces its effects by acting on a protein in the presynaptic membrane that is involved in regulation of transmitter release.     

The membrane potential of cat hippocampal neurons recorded in vivo displays four different reaction-mechanisms to iontophoretically applied transmitter agonists

Intracellular recordings were obtained in vivo from cat hippocampal pyramidal cells and cells in deeper layers of the hippocampal formation. Five transmitter agonists were applied to these cells by microiontophoresis. Glutamate caused depolarizations with fast onset and decay, fastest reaction times being within hundreds of msec of the beginning and the of the application. These depolarizations were accompanied by a decrease of the apparent input resistance (AIR) and an increase of the mean firing rate. Carbachol, a cholinergic agonist, caused depolarizations with slow onset and decay, reaction times were in the range of tens of seconds. The depolarizations were also accompanied by an increase in the mean firing rate. These effects are thought to be muscarinic. gamma-Aminobutyric acid elicited fast hyperpolarizations, a decrease of the AIR, mean firing rate and occasionally of the amplitude of action potentials (APs). Dopamine and norepinephrine hyperpolarized the membrane potential relatively fast, reaction times being within seconds, and reduced the mean firing rate, but this was accompanied by a marked increase of the AIR, of the size of the remaining APs and fimbria-evoked EPSPs. Inhibitory slow depolarizations of the type seen in an earlier study with dopamine in the caudate were not seen on pyramidal cells and only very rarely on non-identified cells of the deeper layers. These results indicate a partially tissue-specific effect of dopamine compared to that on caudate cells.