Disinhibition of rat cerebellar Purkinje neurons from noradrenergic inhibition during rising blood ethanol

Effect of chronic ethanol ingestion on Purkinje and Golgi cell firing in vivo and on motor coordination in mice

As motor coordination impairment is a common symptom of acute and chronic alcohol intoxication, different studies have been conducted on cerebellar Purkinje cell sensitivity to ethanol since Purkinje cell firing constitutes the final integrative output of the cerebellar cortex. However, the effects of chronic ethanol ingestion on Purkinje firing and other cerebellar neurons such as Golgi cells remain unknown. Here, we studied the extracellular discharge of Purkinje and Golgi cells in four groups of non-anesthetized mice drinking ad libitum either 0%, 6%, 12% or 18% ethanol isocallorically compensated with sucrose 25% during a 3-month period. No difference in Golgi cell firing was found with respect to ethanol consumption. The only group that presented significant differences in Purkinje cell firing compared to the other groups was the 18% ethanol-drinking group. These mice presented decreased simple spike and complex spike firing and increased complex spike duration and pause. The 18% ethanol-drinking group was also the only one to present a slight but significant motor coordination impairment (evaluated by rotarod and runway) in naïve task. No motor coordination impairment was noticed in task learned before ethanol consumption. These results suggest that chronic high doses of ethanol are necessary to produce Purkinje cell firing alterations and measurable motor coordination impairment in naïve task. These alterations in Purkinje cell firing did not affect the ability to learn or to recall a motor coordination task.

Histiotypic electrophysiological responses of cultured neuronal networks to ethanol

Embryonic murine neuronal networks cultured on substrate-integrated microelectrode arrays were used to quantify acute electrophysiological effects of ethanol by using extracellular, multichannel recording of action potentials. Spontaneously active frontal cortex cultures showed repeatable, concentration-dependent sensitivities to ethanol, with initial inhibition at 20 mM and a spike rate 50% effective concentration (EC50) of 48.8+/-5.4 mM. Ethanol concentrations of greater than 100 mM led to cessation of activity. The ethanol inhibitions up to the maximum tested 160 mM were reversible. Although ethanol did not change the shape of action potentials, unit-specific spike pattern effects were found. At 40 mM, ethanol decreased neuronal firing in 71%, increased firing in 20%, and generated no effect in 9% of all units observed (14 cultures, 200 discriminated units). The effects of combined application of ethanol and fluoxetine were additive. Excellent agreement with findings obtained from experimental studies with animals validates the use of these in vitro systems for alcohol research.

Effects of systemic and local ethanol on responses of rat cerebellar Purkinje neurons to iontophoretically applied norepinephrine and gamma-aminobutyric acid

The goal of the present study was to determine the effect of acute ethanol (ETOH), administered intraperitoneally or electro-osmotically, on norepinephrine (NE) induced increases in gamma-aminobutyric acid (GABA) mediated inhibition of single cerebellar Purkinje neurons (P-cells). Male Sprague-Dawley rats (230-370g) were anesthetized with halothane and implanted with an intraperitoneal catheter for systemic administration of ETOH (1.0-1.5 g/kg) prior to the recording session. Extracellular activity of single P-cells was recorded before and after iontophoresis of GABA and NE using five-barrel glass micropipettes. GABA was administered at the recording site by microiontophoretic pulses before, during and after continuous iontophoretic application of NE. Spontaneous discharge, GABA responses and NE-GABA interactions in P-cells were monitored for each experiment before and 1-1.5 h following systemic administration of ETOH. As in our previous reports administration of NE, at low ejection currents (10-60 nA), augmented GABA mediated suppression of P-cell spontaneous discharge. Between 10 and 60 min after injection of ETOH, this NE induced augmentation of GABA inhibition was further potentiated. This potentiation involved increases in both the magnitude and the duration of the GABA inhibition observed after NE alone. NE-induced augmentation of GABA inhibition persisted for 2-13 min longer after ETOH administration than in the pre-ETOH control period. Local electro-osmotic application of ETOH, which resulted in strong depression of spontaneous activity and caused small increases in GABA-mediated inhibition, did not directly potentiate NE-induced augmentation of GABA action. These results indicate that NE-mediated augmentation of GABA inhibition of P-cell activity is potentiated following systemic, but not local, ETOH administration.

Differential effects of ethanol on the firing rates of Golgi-like neurons and Purkinje neurons in cerebellar slices in vitro

Previous studies have demonstrated that ethanol (EtOH) inhibits the firing rate of Purkinje neurons both in vitro and in vivo. However, little is known about the response of cerebellar interneurons to EtOH. In this report, we describe the effects of locally applied EtOH on the firing of one type of cerebellar interneuron, tentatively identified as Golgi neurons, and on Purkinje cells in brain slices in vitro. The Golgi neurons were excited by EtOH, whereas EtOH depressed the firing rate of Purkinje neurons. To the best of our knowledge, this is the first report of responses of cerebellar Golgi neurons to local applications of EtOH.

Acute alcohol alters the excitability of cerebellar Purkinje neurons and hippocampal neurons in culture

Acute exposure to ethanol at 22 and 44 mM concentrations altered several features of the current-evoked voltage responses of cerebellar Purkinje neurons and hippocampal neurons studied in culture model systems. Whole cell current clamp techniques were used. At 22 mM, ethanol depressed current-evoked spiking in the hippocampal neurons but enhanced the current-evoked spiking in the Purkinje neurons. In both neuronal types, 44 mM ethanol depressed spiking, the amplitude of the afterhyperpolarization generated at the termination of a current pulse and the amplitude of the off-response generated at the termination of a hyperpolarizing pulse. Ethanol had little or no effect on resting membrane potential or the passive membrane properties measured near resting level in either neuronal type. Some changes in the current-voltage curves were observed at more depolarized or hyperpolarized potentials in both neuronal types. In the Purkinje neurons, where spontaneous activity was a prominent feature of some recordings, exposure to ethanol reduced the frequency of the spontaneous events. These results indicate that acute exposure to ethanol at intoxicating doses alters the membrane excitability of these two CNS neuronal types. The ethanol induced changes in neuronal excitability presumably contribute to the changes in firing properties observed in extracellular recordings from these neuronal types in vivo and the behavioral effects observed during alcohol intoxication in animal models.

Chronic exposure to alcohol during development alters the membrane properties of cerebellar Purkinje neurons in culture

The active and passive membrane properties of developing Purkinje neurons in control cultures and cultures chronically treated with 20 or 40 mM ethanol for 1 or 2 weeks were examined using whole-cell current-clamp techniques. The membrane properties were characterized by the features of the voltage responses evoked by intracellular current injection of a series of depolarizing and hyperpolarizing current pulses. Analysis of these responses and background spontaneous activity showed several differences between the control and ethanol-treated Purkinje neurons: (1) membrane input resistance was significantly larger in the ethanol-treated neurons; (2) the percentage of neurons exhibiting immature firing patterns was significantly higher in the ethanol-treated neurons; (3) the afterhyperpolarization following a current-evoked train of action potentials was significantly larger in the ethanol-treated neurons; (4) spontaneous activity (synaptic potentials and synaptically evoked spike events) was significantly reduced in neurons treated with 40 mM ethanol for 1 week; spontaneous activity in neurons treated with 20 mM ethanol for 1 or 2 weeks was similar to that observed in the control group. These differences indicate that ethanol exposure during development directly alters the physiological properties of this CNS neuronal type. These neuronal actions of ethanol may contribute to the behavioral deficits observed in animals models of fetal alcohol syndrome. Similar target sites of ethanol action are likely to be present in the human CNS neurons and may be involved in human fetal alcohol syndrome.

The molecular biology of addictive drugs

The additive drugs alcohol, morphine, cocaine, and amphetamine are each associated with the development of tolerance and physical dependence. Changes in gene expression occur in cell culture and in vivo with the administration of these centrally-acting drugs. This article reviews those experiments that have studied drug-induced alterations in gene transcription. Ethanol has diverse effects on the amounts of messenger RNA molecules within the central nervous system. Ion channels, neuropeptides, membrane receptors, and immediate early genes represent several regulated mRNAs. The effects are selective, however, as many other specific products are not altered. Evidence for a genetic predisposition to ethanol use reinforces the importance of the genotype. Opioids, cocaine, and amphetamine also affect gene transcription. Messenger RNAs studied have included many of those demonstrated to be altered by alcohol use. Interestingly, use of any of these drugs alters the expression of immediate early genes. These genes may represent an initial step in the pathway that leads to drug addiction. The composite of drug-induced changes in gene expression results in the cellular responses of tolerance and dependence. The characterization of these changes should provide a better understanding of the molecular mechanisms of drug addiction.

GABAergic mechanisms in the electrophysiological actions of ethanol on cerebellar neurons

We have found that the partial inverse benzodiazepine agonists Ro 15-4513 and FG 7142 antagonize the depressant electrophysiological effects of locally applied ethanol in the cerebellum. Although absolute tissue concentrations are not known, dose-response curves constructed using pressure-ejection doses as previously described we found that FG 7142 was more efficacious, but less potent than Ro 15-4513. Our observation that ethanol and inverse benzodiazepine agonists have interactions which are not competitive might suggest that these two drugs act through separate, but interactive mechanisms in order to produce the observed ethanol antagonism. If such independent interactions were mediated at different sites on a given macromolecular complex, such as the GABAa/Cl- channel, then one might expect to find allosteric interactions between those sites as well as with the functional response of the complex to GABA activation. Indeed, this hypothesis is consistent with the recent finding of Harris and collaborators that ethanol potentiates the inverse agonist actions of Ro 15-4513 and FG 7142. On the other hand, we were unable to find large ethanol-induced potentiations of GABA effects on all neurons which showed depressant responses to ethanol administration in rat cerebellum. However we did find that the GABAa antagonist, bicuculline, blocks the depressant effects of ethanol on the same neurons. We conclude that the interaction between ethanol and GABA probably does not occur directly at the GABAa receptor site, but that the GABAa mechanism does play a permissive role in the ethanol-induced depressions of cerebellar Purkinje neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

A paradigm for determination of direct drug-induced modulatory alterations in Purkinje cell activity during treadmill locomotion

This paper describes a paradigm employing chronic single unit recording techniques and videotape analysis of treadmill locomotion in order to determine drug-induced modulation of sensorimotor neuronal activity. Animals implanted with a chronic headstage microdrive unit and an indwelling jugular cannula are trained to walk on a treadmill (10 s on, 10 s off). Characteristically, cerebellar Purkinje cells recorded 1-1.5 mm from midline, exhibit increased rates of discharge in phase with movement of specific limbs during a particular stage of the step-swing cycle, as assessed by video analysis of locomotor patterns. Drug-induced alterations in this movement-correlated discharge relative to changes in the spontaneous firing rate can then be determined to assess drug-induced neuromodulatory effects beyond general non-specific excitatory or inhibitory actions.

Development of spontaneous and glutamate-evoked activity is altered by chronic ethanol in cultured cerebellar Purkinje neurons

The effects of continuous exposure to ethanol on the cytological and physiological development of a central nervous system (CNS) neuron were studied using the cultured Purkinje neuron of the rat cerebellar cortex. Purkinje neurons in fetal rat brain cultures which are established at one day before birth show development comparable to that described in vivo in other studies. In culture, Purkinje neurons progress from immature rounded cells with fine neurites to mature neurons with a branched dendritic structure. These structural changes are accompanied by an increase in the duration and complexity of the excitatory response to glutamate, by transitions in the patterns of spontaneous activity, and by an increase in mean firing rate. Our results demonstrate that chronic exposure to a low concentration of ethanol (90 mg%; 19.5 mM) during development selectively alters the electrophysiological but not the morphological properties of Purkinje neurons. Specifically, ethanol treatment reduces the responsiveness of these neurons to glutamate, delays the expected developmental transitions in patterns of spontaneous activity, and induces increased spontaneous bursting activity, particularly at the stage of dendritic formation. Impairment of responsiveness to glutamate is significant in that it may reflect the compromise by ethanol of a major excitatory pathway in the cerebellar cortex, resulting from the decreased efficacy of glutamatergic input from parallel fibers. In contrast to the results of other studies using adult neurons as a model for the effects of ethanol, our work suggests that the developing CNS neuron does not become tolerant; that is, in the continuing presence of ethanol, it does not express physiological function equivalent to that of the control.

Acute ethanol alters the firing pattern and glutamate response of cerebellar Purkinje neurons in culture

Modified explant cultures derived from the cortical region of fetal rat cerebellum, and extracellular recording techniques were used to examine the sensitivity and response of cerebellar neurons, isolated from extracerebellar afferent input, to acute ethanol (EtOH) exposure. Recordings were made from Purkinje neurons (PNs) and granule cells maintained in culture for several weeks, with the emphasis on the PN. Both the PNs and granule cells exhibited spontaneous activity in culture, but, unlike the PNs, not all of the granule cells were spontaneously active. The majority of PNs studied exhibited a high frequency, regular simple spike firing pattern, previously shown to be endogenously generated by voltage-sensitive mechanisms intrinsic to the PN. The granule cells exhibited slow, irregular patterns of activity. EtOH at doses as low as 22 mM (100 mg%), a concentration that reflects blood levels during EtOH intoxication, altered the spontaneous activity of both neuronal types, demonstrating that EtOH has direct actions on cerebellar neurons. In the PNs, acute EtOH (20-80 mM) produced an increase in the regularity of the spontaneous activity and either a transient increase or no change in firing rate. Acute EtOH also significantly altered the response of PNs to the excitatory transmitter glutamate. In the granule cells, acute EtOH altered firing pattern with small and variable effects on firing rate. These data demonstrate that there are multiple sites of EtOH action in the cerebellum and that changes in PN activity with acute EtOH exposure may occur via direct actions on the PN and indirect actions via synaptically connected cerebellar neurons. The demonstration of EtOH-sensitive sites intrinsic to the cerebellum suggests that EtOH actions at these sites contribute to alterations in PN activity that occur in vivo after acute EtOH exposure.

Ethanol effects on striatal neuron activity

Cellular effects of ethanol were investigated in the rat neostriatum with local perfusion and extracellular, single unit recording techniques. Neuronal activity was modulated specifically as a function of ethanol concentration over a wide test range. At extremely low doses (10(-9) and 10(-8) M), the neuronal responses to drug perfusion were exclusively excitatory. However, at the highest doses examined (10(-8) and 10(-4) M), the results were reversed. In the midrange, a number of apparently ineffective tests were obtained, along with bimodal (excitation followed by depression) responses. The unequivocal responsiveness to ethanol at very low concentrations raises the possibility of a physiological role for the endogenous substance.

Ethanol effects on harmaline-induced tremor and increase of cerebellar cyclic GMP

The spectra of pharmacological effects of ethanol and the benzodiazepine show a degree of overlap. Neurophysiological and neurochemical evidence indicates that both ethanol and benzodiazepines facilitate inhibitory neurotransmission mediated by GABA. Diazepam has been reported to inhibit both the tremor and mechanism of cerebellar cyclic GMP caused by harmaline by a neurotransmission in the cerebellum. Because of the similarities between ethanol and benzodiazepines, the effects of ethanol on harmaline-induced tremor and increase of cerebellar cyclic GMP were studied. Ethanol inhibited harmaline-induced tremor at doses as low as 0.1 g/kg. At this low dose, however, a dissociation between inhibition of harmaline tremor and inhibition of the harmaline-induced increase of cerebellar cyclic GMP was observed.

Ethanol-GABA interactions at the rat Purkinje cell

Single unit cerebellar Purkinje cell activity was recorded extracellularly in urethane-anaesthetised rats. An inhibition of these cells, believed to be GABA-mediated, which is produced by local surface stimulation of the cerebellar cortex was antagonized by the local, micropressure application of ethanol. The inhibition of cerebellar Purkinje cells produced by local micropressure application of GABA was similarly antagonized by a slow i.v. infusion of ethanol (1.5 g/kg over 10 min). When both ethanol and GABA were applied to the Purkinje cells by local micropressure the results were difficult to interpret due to an ethanol-induced decrease in the baseline firing rate. There was a decrease in the absolute GABA-mediated inhibition but no change in the relative inhibition. In general, it appears that ethanol produces an antagonism of GABA-mediated inhibition of cerebellar Purkinje cells.

Neurochemical correlates of tolerance and strain differences in the neurochemical effects of ethanol

The behavioral and neurochemical effects of acute and chronic ethanol administration were studied in BALB/c, C57B1/6 and DBA/2 mice. The rates of dopamine synthesis and release in the striatum were estimated by measuring the accumulation of DOPA and DOPAC, respectively, after inhibition of aromatic amino acid decarboxylase with NSD-1024. Biphasic behavioral effects were found in BALB/c and DBA/2 mice, but not in C57B1/6 mice, with low doses of ethanol producing activation and high doses, depression. Biphasic effects were also found in the dopamine response to acute doses of ethanol. The BALB/c and DBA/2 mice showed larger suppressions of DA release in the lower dose ranges of ethanol, and smaller increases at the higher doses than did the C57B1/6 mice. Ethanol stimulated dopamine synthesis in a monophasic, dose-dependent manner, and C57B1/6 mice were less sensitive to this effect of ethanol compared to the other tested strains of mice. Chronic ethanol feeding produced behavioral tolerance to the high-dose depressant effects of ethanol, but not to the low-dose activating effects. Similarly, tolerance developed in the dopaminergic responses to a higher challenge dose of ethanol (3.5 g/kg). These findings demonstrate that genetically determined differences exist in the sensitivity of the dopaminergic systems of mice to ethanol, and suggest that central dopamine neurons may be important in the behavioral effects of ethanol.

Does tolerance develop to the activating, as well as the depressant, effects of ethanol?

Genetically determined differences were demonstrated in the response of mice to low doses of ethanol. Ethanol (1.35 g/kg) produced an increase in locomotion in DBA/2 and BALB/c mice, but did not alter the locomotor activity of C57B1/6 mice. Chronic administration of ethanol produced tolerance to the sedative/hypnotic effects of high doses of ethanol in DBA/2 and BALB/c mice, but the equivalent chronic ethanol administration paradigm produced no tolerance to the activating effects of ethanol in these animals. C57B1/6 mice became tolerant to the hypnotic effects of ethanol, but no change in the behavior of these mice, given a low dose of ethanol, was noted after the mice were withdrawn from chronic feeding with ethanol-containing diets. The results indicate the presence of different mechanisms for tolerance development to the activating and depressant effects of ethanol, and indicate that strain-dependent differences in the activating effects of ethanol are not determined by an animal's greater sensitivity to the sedating effects of this drug.