Geoffrey White

Ethanol inhibits NMDA-activated current but does not alter GABA-activated current in an isolated adult mammalian neuron

The effects of ethanol (EtOH) on membrane ion currents activated by N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acid (GABA) were studied under voltage-clamp conditions in isolated sensory neurons within hours of being dissociated from adult rats. The amplitude of the ion current activated by NMDA was decreased in the presence of 2.5-100 mM EtOH (IC50, 10 mM or 0.05% EtOH), a concentration range that produces intoxication. The amplitude of the GABA-activated Cl- current, on the other hand, was not significantly affected by this concentration range of EtOH. The observations suggest that some of the neural and cognitive impairments associated with EtOH intoxication may result from inhibition of the NMDA-activated ion current.

Ethanol inhibition of N‐methyl‐D‐aspartate‐activated current in mouse hippocampal neurones: whole‐cell patch‐clamp analysis

1 The action of ethanol on N‐methyl‐D‐aspartate (NMDA)‐activated ion current was studied in mouse hippocampal neurones in culture using whole‐cell patch‐clamp recording. 2 Ethanol inhibited NMDA‐activated current in a voltage‐independent manner, and did not alter the reversal potential of NMDA‐activated current. 3 Concentration–response analysis of NMDA‐ and glycine‐activated current revealed that ethanol decreased the maximal response to both agonists without affecting their EC50 values. 4 The polyamine spermine (1 μM) increased amplitude of NMDA‐activated current but did not alter the percentage inhibition of ethanol. 5 Compared to an extracellular pH of 7.0, pH 6.0 decreased and pH 8.0 increased the amplitude of NMDA‐activated current, but these changes in pH did not significantly alter the percentage inhibition by ethanol. 6 The sulphydryl reducing agent dithiothreitol (2 mM) increased the amplitude of NMDA‐activated current, but did not affect the percentage inhibition by ethanol. 7 Mg2+ (10, 100, 500 μM), Zn2+ (5, 20 μM) or ketamine (2, 10 μM) decreased the amplitude of NMDA‐activated current, but did not affect the percentage inhibition by ethanol. 8 The observations are consistent with ethanol inhibiting the function of NMDA receptors by a non‐competitive mechanism that does not involve several modulatory sites on the NMDA receptor–ionophore complex.

Effects of nerve growth factor on TTX- and capsaicin-sensitivity in adult rat sensory neurons

We have investigated the effects of nerve growth factor (NGF, 2.5 ng/ml for 1-2 weeks) on enriched adult rat dorsal root ganglion (DRG) neurons maintained in cell culture in defined media. Whole-cell recordings in cells cultured in the absence and presence of NGF revealed no significant difference in resting membrane potential and input resistance. However, the threshold for spike generation was significantly lower in untreated cells than in treated cells; -25 +/- 1.1 mV vs -19 +/- 2.2 mV, respectively. The sensitivity of the Na+ spike to tetrodotoxin (TTX, 1 microM) was different in cells cultured in the absence or presence of NGF. For example, spikes were abolished by TTX in 100% of untreated cells, while in NGF-treated cells the spike was abolished in only 41% of the neurons. Chemosensitivity of DRG neurons was also different in the absence and presence of NGF. For example, the percent of neurons in which a current activated by 8-methyl-N-vanillyl-6-nonenamide (capsaicin, 500 nM) was detected, increased from 18% in untreated cells to 55% in NGF-treated cells. NGF did not influence the number of cells surviving. The results indicate that NGF can regulate TTX and capsaicin sensitivity in these adult rat sensory neurons. Our experimental protocol indicates that this effect is not mediated by a factor in the serum or released from non-neuronal cells.

Ethanol Inhibition of Neuronal Giutamate Receptor Function

Acute ethanol intoxication is associated with changes in the activity of neurons in the central nervous system. However, the cellular and molecular mechanisms underlying these changes are poorly understood. We have examined the acute effects of ethanol on excitatory synaptic mechanisms in neurons from mammalian central nervous system, and observed that intoxicating concentrations of ethanol can inhibit the ion current activated by the glutamate receptor agonist N-methyl-D-aspartate in cultured neurons from mouse hippocampus, cortex and spinal cord. This inhibition is seen under a variety of experimental recording conditions. On the other hand, ethanol is less effective in inhibiting ion current produced by activation of non-N-methyl-D-aspartate glutamate receptors. Intoxicating concentrations of ethanol also inhibit excitatory synaptic transmission mediated by N-methyl-D-aspartate receptors in hippocampal slices from adult rodents. These observations support the hypothesis that the N-methyl-D-aspartate receptor/ionophore complex is a target for the neural actions of ethanol, and that inhibition of N-methyl-D-aspartate receptor-mediated responses might contribute to acute ethanol intoxication. The possibility that other receptor-gated ion channels may also be sensitive to ethanol is discussed.

Alcohol and Anesthetic Actions on Excitatory Amino Acid—Activated Ion Channels

The actions of alcohol and anesthetics have been studied on excitatory amino acid activated ion channels in mammalian neurons. Ethanol inhibits NMDA-activated current over a concentration range that produces intoxication, and the potency of several alcohols for inhibiting the NMDA-activated current is correlated with their intoxicating potency, suggesting that alcohol-induced inhibition of responses to NMDA receptor activation may contribute to the neural and cognitive impairments associated with intoxication. Studies on the mechanism of ethanol inhibition of NMDA-activated current indicate that ethanol does not appear to block the ion channel, alter the ion selectivity of the channel, or interact with previously described binding sites on the NMDA receptor/ionophore complex. The linear relation between the potency of several alcohols for inhibiting the NMDA-activated current and the hydrophobicity of the alcohols suggests that ethanol may inhibit the NMDA-activated ion current by a novel type of interaction with a hydrophobic site associated with the NMDA channel. In addition, different types of general anesthetic agents exhibit different inhibitory actions on NMDA-, kainate-, and quisqualate-activated currents, suggesting that differences in the profile of inhibition of excitatory amino acid neurotransmission in the CNS among different classes of general anesthetics may contribute to the differences in their behavioral and physiological effects.

Post-natal development of burst firing behavior and the low-threshold transient calcium current examined using freshly isolated neurons from rat dorsal root ganglia

Burst firing was triggered by an afterdepolarizing potential (ADP) in whole-cell recordings from neurons recently isolated from dorsal root ganglia (DRG) of adult rats, less frequently in neurons from 12-day-old rats, and not at all in neurons from 1-day-old rats. Both the ADP and voltage-activated transient (T-type) Ca2+ current which generates the ADP were present in neurons at all postnatal ages; however, the amplitude of the ADP and of the T-type current were greatest in neurons from adults, smaller in neurons from 12-day animals, and smallest in neurons from 1-day animals. These observations suggest that the increase in amplitude of the T-type current with age leads to the generation of an ADP of increased amplitude and those properties of the ADP which develop with age may contribute to or generate burst firing behavior.

Inhibition of N-methyl-l-aspartate activated ion current by desmethylimipramine

The tricyclic antidepressant desmethylimipramine (DMI) interacts with the NMDA receptor/ionophore complex; however, the site of the interaction has not been clearly established. Although evidence from receptor binding assays suggests that DMI interacts with the Zn2+ binding site, other binding studies and electrophysiological studies suggest otherwise. Using the whole-cell patch clamp technique to record from cultured hippocampal neurons, we report that recovery of NMDA-activated current from block by DMI is time-dependent and this time-dependent component was not observed following preexposure of neurons to Zn2+. These observations favor the hypothesis that DMI interacts at a binding site within the NMDA receptor/complex channel pore and not at the Zn2+ binding site.

Heterogeneity in EC50 and nH of GABAA receptors on dorsal root ganglion neurons freshly isolated from adult rats

GABA activates a Cl- current through the GABAA receptor/ionophore complex that influences excitability of neurons. Studies using expression of cloned cDNAs coding for different GABAA receptor/ionophore subunits suggest that the EC50 and Hill coefficient for GABA are influenced by subunit composition. However, no direct evidence for such heterogeneity has been reported for vertebrate neurons. I have investigated the heterogeneity of EC50 and Hill coefficients (nH) of isolated dorsal root ganglion neurons using the whole-cell patch clamp technique. The EC50 for GABA varied from 26 to 107 microM among neurons. nH calculated from the logistic equation varied from 1.18 to 2.0. A negative correlation was found between the EC50 and nH (r = -0.81). Both nH and EC50 differed between some cells. However, in some instances, nH differed between cells while EC50 values were similar, and in other cells, EC50 values differed and nH was similar. In addition, when cells were categorized according to action potential shape, the EC50 and Hill coefficients differed among cell types in some instances and were similar in other instances. These findings demonstrate that different pharmacological profiles for GABA can be observed in adult mammalian neurons. Selective distribution of such pharmacological subtypes of GABAA receptors may contribute to control of neuronal excitability.

1,9-Dideoxyforskolin does not mimick all cAMP and protein kinase a independent effects of forskolin on GABA activated ion currents in adult rat sensory neurons

The effect of forskolin on GABAA receptor activated events has been the subject of recent investigations, the conclusions of which are conflicting. Forskolin can reduce current amplitude and increase the rate of decay of current activated by 100 microM GABA and these effects are not mimicked by 1,9-dideoxyforskolin (Tehrani et al., Synapse, 4 (1989) 126-131). On the other hand, both forskolin and 1,9-dideoxyforskolin inhibit 36Cl- flux induced by lower concentrations of muscimol (Heuschneider and Schwartz, Proc. Natl. Acad. Sci. USA, 86 (1989) 2938-2942). Using the whole-cell patch clamp technique to measure GABA activated current in dorsal root ganglion neurons that were freshly isolated from adult rats, we have confirmed the finding of Tehrani et al. (Synapse, 4 (1989) 126-131) using 100 microM GABA; however, the effects of forskolin that were not mimicked by 1,9-dideoxyforskolin were not blocked by the kinase inhibitor H-7 (50 microM). In contrast, at lower concentrations of GABA (10-20 microM), both forskolin and 1,9-dideoxyforskolin increased the decay rate of GABA activated current. In addition, all effects of forskolin occurred within 200 ms of application of forskolin and the effects were not blocked or occluded by H-7, 10 microM cAMP, or the active subunit of protein kinase A. We conclude that: (1) 1,9-dideoxyforskolin is not a reliable indicator of forskolin specificity in this system because its effects are dependent upon GABA concentration; and (2) the most prominent effects of forskolin on amplitude and decay time course of GABA activated ion current are not mediated by cAMP or protein kinase A (PKA).

TTX-sensitive action potentials and excitability of adult rat sensory neurons cultured in serum- and exogenous nerve growth factor-free medium.

The excitability of adult rat dorsal root ganglion (DRG) neurons cultured in the absence of serum and exogenously added nerve growth factor (NGF) was studied. Current-clamp recordings revealed the presence of tetrodotoxin (TTX)-sensitive action potentials. Voltage-clamp recordings demonstrated the presence of both inward and outward currents. The inward Na+ current had a maximal amplitude near -10 mV and was completely blocked by TTX. A sustained Ca2+ inward current and a slowly activating outward K+ current were also observed. TTX-sensitive and TTX-resistant action potentials have been observed in previous studies in DRG neurons cultured in the presence of serum. By contrast, in the study reported here, only TTX-sensitive action potentials and Na+ currents were found in the neurons cultured in the absence of serum and nerve growth factor.