Differential sensitivity of cerebellar purkinje neurons to ethanol in selectively outbred lines of mice: maintenance in vitro independent of synaptic transmission

The Cerebellar GABAAR System as a Potential Target for Treating Alcohol Use Disorder

In the brain, fast inhibitory neurotransmission is mediated primarily by the ionotropic subtype of the gamma-aminobutyric acid (GABA) receptor subtype A (GABA_AR). It is well established that the brain's GABA_AR system mediates many aspects of neurobehavioral responses to alcohol (ethanol; EtOH). Accordingly, in both preclinical studies and some clinical scenarios, pharmacologically targeting the GABA_AR system can alter neurobehavioral responses to acute and chronic EtOH consumption. However, many of the well-established interactions of EtOH and the GABA_AR system have been identified at concentrations of EtOH ([EtOH]) that would only occur during abusive consumption of EtOH (≥40 mM), and there are still inadequate treatment options for prevention of or recovery from alcohol use disorder (AUD, including abuse and dependence). Accordingly, there is a general acknowledgement that more research is needed to identify and characterize: (1) neurobehavioral targets of lower [EtOH] and (2) associated brain structures that would involve such targets in a manner that may influence the development and maintenance of AUDs.Nearly 15 years ago it was discovered that the GABA_AR system of the cerebellum is highly sensitive to EtOH, responding to concentrations as low as 10 mM (as would occur in the blood of a typical adult human after consuming 1-2 standard units of EtOH). This high sensitivity to EtOH, which likely mediates the well-known motor impairing effects of EtOH, combined with recent advances in our understanding of the role of the cerebellum in non-motor, cognitive/emotive/reward processes has renewed interest in this system in the specific context of AUD. In this chapter we will describe recent advances in our understanding of cerebellar processing, actions of EtOH on the cerebellar GABA_AR system, and the potential relationship of such actions to the development of AUD. We will finish with speculation about how cerebellar specific GABA_AR ligands might be effective pharmacological agents for treating aspects of AUD.

Gaze-evoked nystagmus induced by alcohol intoxication

KEY POINTS

The cerebellum is the core structure controlling gaze stability. Chronic cerebellar diseases and acute alcohol intoxication affect cerebellar function, inducing, among others, gaze instability as gaze-evoked nystagmus. Gaze-evoked nystagmus is characterized by increased centripetal eye-drift. It is used as an important diagnostic sign for patients with cerebellar degeneration and to assess the 'driving while intoxicated' condition. We quantified the effect of alcohol on gaze-holding using an approach allowing, for the first time, the comparison of deficits induced by alcohol intoxication and cerebellar degeneration. Our results showed that alcohol intoxication induces a two-fold increase of centripetal eye-drift. We establish analysis techniques for using controlled alcohol intake as a model to support the study of cerebellar deficits. The observed similarity between the effect of alcohol and the clinical signs observed in cerebellar patients suggests a possible pathomechanism for gaze-holding deficits.

ABSTRACT

Gaze-evoked nystagmus (GEN) is an ocular-motor finding commonly observed in cerebellar disease, characterized by increased centripetal eye-drift with centrifugal correcting saccades at eccentric gaze. With cerebellar degeneration being a rare and clinically heterogeneous disease, data from patients are limited. We hypothesized that a transient inhibition of cerebellar function by defined amounts of alcohol may provide a suitable model to study gaze-holding deficits in cerebellar disease. We recorded gaze-holding at varying horizontal eye positions in 15 healthy participants before and 30 min after alcohol intake required to reach 0.6‰ blood alcohol content (BAC). Changes in ocular-motor behaviour were quantified measuring eye-drift velocity as a continuous function of gaze eccentricity over a large range (±40 deg) of horizontal gaze angles and characterized using a two-parameter tangent model. The effect of alcohol on gaze stability was assessed analysing: (1) overall effects on the gaze-holding system, (2) specific effects on each eye and (3) differences between gaze angles in the temporal and nasal hemifields. For all subjects, alcohol consumption induced gaze instability, causing a two-fold increase [2.21 (0.55), median (median absolute deviation); P = 0.002] of eye-drift velocity at all eccentricities. Results were confirmed analysing each eye and hemifield independently. The alcohol-induced transient global deficit in gaze-holding matched the pattern previously described in patients with late-onset cerebellar degeneration. Controlled intake of alcohol seems a suitable disease model to study cerebellar GEN. With alcohol resulting in global cerebellar hypofunction, we hypothesize that patients matching the gaze-holding behaviour observed here suffered from diffuse deficits in the gaze-holding system as well.

Response to ethanol induced ataxia between C57BL/6J and 129X1/SvJ mouse strains using a treadmill based assay

More sensitive assays of mouse motor ataxia may provide a better understanding of the pathological profile. Treadmill gait analysis using ventral imaging allows for unhindered access to the ambulating mouse. In contrast to genetic mutations or exogenous brain injury, ethanol (EtOH) allows for the detection of dose dependent changes in motor behavior, which can be used to assess an assay's detection sensitivity. EtOH induced ataxia was assessed in C57BL/6J (B6) and 129X1/SvJ (129) mice using the DigiGait imaging system. Gait was analyzed across EtOH dosage (1.75, 2.25 and 2.75 g/kg) in each strain using a linear mixed effects model. Overall, 129 mice displayed greater susceptibility to EtOH ataxia than their B6 counterparts. In both strains, hind paws exhibited greater sensitivity to EtOH dosage than fore paws. Across most variables analyzed, only a modest EtOH-induced change in motor behavior was observed in each strain with the 1.75 g/kg EtOH doses failing to elicit significant change. These data indicate the ability to detect motor differences between strains, yet only moderate ability to detect change across EtOH dosage using the automated treadmill. Rotarod assays, however, were able to detect motor impairment at lower doses of EtOH. The significant, but opposite changes in paw placement with increasing EtOH doses highlight strain-specific differences in biophysical adaptations in response to acute EtOH intoxication.

Alcohol Pathophysiology

There is no specialized alcohol addiction area in the brain; rather, alcohol acts on a wide range of excitatory and inhibitory nervous networks to modulate neurotransmitters actions by binding with and altering the function of specific proteins. With no hemato-encephalic barrier for alcohol, its actions are strongly related to the amount of intake. Heavy alcohol intake is associated with both structural and functional changes in the central nervous system with long-term neuronal adaptive changes contributing to the phenomena of tolerance and withdrawal. The effects of alcohol on the function of neuronal networks are heterogeneous. Because ethanol affects neural activity in some brain sites but is without effect in others, its actions are analyzed in terms of integrated connectivities in the functional circuitry of neuronal networks, which are of particular interest because of the cognitive interactions discussed in the manuscripts contributing to this review. Recent molecular data are reviewed as a support for the other contributions dealing with cognitive disturbances related to alcohol acute and addicted consumption.

Ethanol decreases Purkinje neuron excitability by increasing GABA release in rat cerebellar slices

Cerebellar Purkinje neurons (PNs) receive inhibitory GABAergic input from stellate and basket cells, which are located in the outer and inner portions of the molecular layer, respectively. Ethanol (EtOH) was recently shown to increase GABAergic transmission at PNs via a mechanism that involves enhanced calcium release from presynaptic internal stores (J Pharmacol Exp Ther 323:356-364, 2007). Here, we further characterized the effect of EtOH on GABA release and assessed its impact on PN excitability. Using whole-cell patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that EtOH acutely increases the frequency but not the amplitude or half-width of miniature and spontaneous inhibitory postsynaptic currents (IPSCs). EtOH significantly increased the amplitude and decreased the paired pulse ratio of IPSCs evoked by stimulation in the outer but not inner molecular layer. In current clamp, EtOH decreased both the amplitude of excitatory postsynaptic potentials evoked in PNs by granule cell axon stimulation and the number of action potentials triggered by these events; these effects depended on GABA(A) receptor activation because they were not observed in presence of bicuculline. Loose-patch cell-attached PN recordings revealed that neither the spontaneous action potential firing frequency nor the coefficient of variation of the interspike interval was altered by acute EtOH exposure. These findings suggest that EtOH differentially affects GABAergic transmission at stellate cell- and basket cell-to-PN synapses and that it modulates PN firing triggered by granule cell axonal input. These effects could be in part responsible for the cerebellar impairments associated with acute EtOH intoxication.

No effect of prenatal alcohol exposure on activity in three inbred strains of mice

AIMS

Prenatal exposure to alcohol can have adverse effects on the developing fetus. Two of the hallmarks of children exposed to alcohol prenatally are attention deficits and hyperactivity. While hyperactivity has been observed in rats following prenatal ethanol exposure, few studies have examined these effects in mice. The present study investigated the effects of prenatal ethanol exposure on activity in mice from three inbred strains: C57BL/6 (B6), Inbred Long Sleep (ILS) and Inbred Short Sleep (ISS).

METHODS

On Days 7 through 18 of gestation, mice were intragastrically intubated twice daily with either 3.0 g/kg ethanol (E) or an isocaloric amount of maltose-dextrin (MD); non-intubated control (NIC) litters were also generated. Offspring activity was monitored at 30, 60, 90 and 150 days of age.

RESULTS

While results showed no effects of prenatal ethanol exposure on any measures of activity, we did observe differences in baseline activity among the strains. ISS mice were more active than B6 and ILS for all activity measures except stereotypy; B6 mice had higher measures of stereotypy than ILS and ISS. Younger mice were more active than older mice. The only sex effects were on measures of stereotypy, where males had higher scores.

CONCLUSIONS

Mice are an excellent organism to study genetic influences on many phenotypes. However, our study and others have shown few effects of prenatal ethanol exposure on behavior in mice. It appears as if the prenatal period in mice, corresponding to organogenesis, is not a sensitive period for producing behavioral deficits following ethanol exposure. It is likely that the first 2 weeks postnatally, corresponding to the brain growth spurt, are more sensitive for producing behavioral effects.

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.

Alcohol potently modulates climbing fiber-->Purkinje neuron synapses: role of metabotropic glutamate receptors

Consumption of alcoholic beverages produces alterations in motor coordination and equilibrium that are responsible for millions of accidental deaths. Studies indicate that ethanol produces these alterations by affecting the cerebellum, a brain region involved in the control of motor systems. Purkinje neurons of the cerebellar cortex have been shown to be particularly important targets of ethanol. However, its mechanism of action at these neurons is poorly understood. We hypothesized that ethanol could modulate Purkinje neuron function by altering the excitatory input provided by the climbing fiber from the inferior olive, which evokes a powerful all-or-none response denoted as the complex spike. To test this hypothesis, we performed whole-cell patch-clamp electrophysiological and Ca2+ imaging experiments in acute slices from rat cerebella. We found that ethanol potently inhibits the late phase of the complex spike and that this effect is the result of inhibition of type-1 metabotropic glutamate receptor-dependent responses at the postsynaptic level. Moreover, ethanol inhibited climbing fiber long-term depression, a form of synaptic plasticity that also depends on activation of these metabotropic receptors. Our findings identify the climbing fiber-->Purkinje neuron synapse as an important target of ethanol in the cerebellar cortex and indicate that ethanol significantly affects cerebellar circuits even at concentrations as low as 10 mm (legal blood alcohol level in the United States is below 0.08 g/dl = 17 mm).

Chronic ethanol exposure enhances AMPA-elicited Ca2+ signals in the somatic and dendritic regions of cerebellar Purkinje neurons

Intracellular Ca2+ signals produced by the glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 5 microM) were measured in the somatic and dendritic regions of cerebellar Purkinje neurons in mature cerebellar control cultures (> or = 20 days in vitro) and cultures chronically treated with 32 mM ethanol (146 mg%; 8-11 days). Recordings were made in physiological saline without ethanol. The mean peak amplitude of the Ca2+ signal elicited by AMPA (applied by brief 1-s microperfusion) in the somatic region was enhanced 38% in chronic ethanol-treated Purkinje neurons compared with control neurons. In contrast, Ca2+ signals evoked by AMPA in the dendritic region were similar in magnitude between control and chronic ethanol-treated Purkinje neurons. When tetrodotoxin (TTX; 500 nM) was included in the bath saline to block spike activity and synaptically-generated events, the mean peak amplitude of the Ca2+ signal elicited by AMPA was enhanced 60% in both the somatic and dendritic regions of chronic ethanol-treated Purkinje neurons compared with control neurons. Thus, TTX-sensitive mechanisms (i.e., spike or synaptic activity) appear to play a role in normalizing neuronal functions involved in Ca2+ signaling in the chronic ethanol-treated neurons. In parallel current clamp experiments, the resting membrane potential of chronic ethanol-treated neurons was slightly depolarized compared with control neurons. However, no differences were found between control and chronic ethanol-treated Purkinje neurons in input resistance or the peak amplitude or duration of the depolarizations or hyperpolarizations elicited by AMPA. AMPA receptors mediate fast excitatory neurotransmission in the majority of neurons in the central nervous system (CNS) and Ca2+ signals in response to AMPA receptor activation contribute to synaptic function. Thus, our results suggest that modulation of Ca2+ signals to AMPA receptor activation (or other cellular inputs) may provide an important mechanism contributing to the actions of prolonged ethanol exposure in the CNS.

Lifelong ethanol consumption and loss of locus coeruleus neurons in AA and ANA rats

The effects of lifelong ethanol exposure and aging on the morphology of the locus coeruleus (LC) were studied in the AA (Alko, Alcohol) and ANA (Alko, Nonalcohol) rats of both sexes. The ethanol-consuming (EtOH) rats were given 12% (v/v) ethanol as the only drinking fluid from 4 to 22 months of age, whereas the young (3-month-old) and aged (24-month-old) controls had only water available. The total LC neuron numbers were obtained by using the unbiased disector method. In the AA line, as we have previously reported. the EtOH female and male rats displayed a 26-30% loss of LC neurons compared with the controls. In the ANA line, the EtOH females had 30% fewer LC neurons than the controls (EtOH 1579 +/- 377 vs. controls 2264 +/- 269, ANOVA p < 0.01), whereas the EtOH males showed no neuron loss compared to the controls (EtOH 1848 +/- 525 vs. controls 2216 +/- 152, ANOVA NS). However, taking into account (sex by line ANCOVA) the markedly higher ethanol intake of the female rats in both lines, no gender or line differences in the ethanol-induced LC degeneration were detected. Neither was there any difference in LC neuron numbers between the young and old control rats of either line of rats. In conclusion, chronic alcohol consumption, not aging per se, damages the LC neurons in experimental animals.

Modulatory effects of acute ethanol on metabotropic glutamate responses in cultured Purkinje neurons

Ethanol has been shown to affect several transmitter- and voltage-gated channels in the brain, although little attention has focused on potential interactions between ethanol and metabotropic glutamate receptors (mGluRs). This is of interest as mGluRs are now recognized to be important components of synaptically mediated responses, including short- and long-term changes in the efficacy of neurotransmission. Cerebellar Purkinje neurons are sensitive to the effects of ethanol and express high levels of mGluRs. We made extracellular recordings from cerebellar Purkinje neurons at 21-37 days in culture to examine the effect of ethanol on mGluR-mediated responses. mGluRs were activated by pressure ejection of 300 microM (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), a selective agonist of mGluRs, or 5 microM quisqualate (Quis). As Quis activates both ionotropic and metabotropic glutamate receptors, 50 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX) was used to block the ionotropic component of Quis-mediated responses. Both ACPD and Quis produced biphasic changes in firing rates consisting of an initial brief excitatory phase (5-20 s) followed by a prolonged inhibitory phase (10 s to 2.5 min), and induced the generation of bursts. Addition of 33 mM (150 mg%) ethanol to the recording medium had little effect on ACPD-mediated responses. In the presence of 66 mM (300 mg%) ethanol, however, ACPD-mediated responses exhibited an increase in the total response duration, with no change in the percent excitation or the induction of bursts as compared to controls. On the other hand, 66 mM ethanol decreased Quis-induced burst activity, while having no effect on the percent excitation or the total response duration.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Mapping quantitative trait loci for behavioral traits in the mouse

After many years of studying various behavioral characters in the mouse, it is clear that most are heritable and are specified by complexes of genes or quantitative trait loci (QTLs). In order to attain a more complete understanding of the genetic causes of individual differences in behavior, the mechanism of action of these QTLs must be elucidated. The most straightforward approach to determining the mechanism of action of a particular QTL is to identify and molecularly clone the gene; this can be done by positional cloning, which depends on precise knowledge of the genetic map position. As the genetic data base for the mouse genome continues to develop, such strategies will become increasingly easy to perform. Here we suggest a multistage strategy for QTL mapping using recombinant-inbred strains of mice: (1) characterize genomic DNA from parental strains originally used to generate the RI strains; (2) characterize the RI strains for a quantitative character and for DNA markers that differ in the parental strains; and (3) assess the quantitative character in F2 mice derived from crosses between the parental strains, then determine the genotypes of extreme F2 mice for markers that account for at least 5% of the additive genetic variance. Data from these F2 crosses can be used to test hypotheses from the analysis of RI strains, i.e., that a QTL maps to a particular region. Using data from the mouse genome data base, this strategy should allow the molecular identification of the gene based on a candidate-gene approach. We illustrate the approach with examples from our work in mapping QTLs specifying neural sensitivity to the anesthetic effects of ethanol.

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.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

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.

Differential effects of norepinephrine on phosphatidylinositol 4,5-bisphosphate stimulated hydrolysis in brains of mice genetically selected for differences in ethanol sensitivity

The effects of norepinephrine on phosphoinositide turnover were evaluated in five brain regions of the long sleep (LS) and short sleep (SS) mice. These mice were selectively bred for differences in central nervous system sensitivity to ethanol with the LS exhibiting much greater sensitivity to a hypnotic dose of ethanol than the SS, as determined by the ability of the mice to regain their righting reflex. Norepinephrine (10(-3) M, 10(-4) M, and 10(-5) M) significantly increased phosphoinositide turnover in the hippocampus, hypothalamus, locus ceruleus, cerebellum, and cortex within each line of mice. Basal and norepinephrine-stimulated phosphoinositide turnover were significantly higher in the SS mice as compared with the LS mice in the cerebellum and cortex but not the other brain regions. Incorporation of 3H-inositol into 3H-phosphatidylinositols was not different between SS and LS mice in the cerebellum and cortex. The greater norepinephrine-stimulated phosphoinositide turnover in the cerebellum and cortex of the SS versus the LS mice may contribute to the CNS sensitivity to ethanol in these two lines of mice. However, ethanol (500 mM) had no effect on basal or norepinephrine-stimulated phosphoinositide turnover in any of the five brain areas examined in the LS and SS mice.

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)

Ethanol increases the firing rate of dopamine neurons of the rat ventral tegmental area in vitro

The ventral tegmental area (VTA) is a brain region rich in dopamine-containing neurons. Since most agents which act as substrates for self-administration increase dopaminergic outflow in the mesolimbic or mesocortical areas, the VTA slice preparation may be useful for identifying drugs with potential for abuse. While ethanol (EtOH) is a drug of abuse which has been widely studied, the properties of ethanol which contribute to its abuse potential are not known. We have developed a brain slice preparation of the VTA in order to study the action of EtOH on putative dopamine neurons. Concentrations of EtOH from 20 to 320 mM produce a dose-dependent excitation of the dopamine-type neurons of the VTA. About 89% of neurons which have electrophysiological characteristics established for presumed dopamine-containing neurons were excited by ethanol in the pharmacologically relevant concentration range. This excitation persists in low-calcium, high-magnesium medium, which suggests a direct excitatory action of EtOH on dopamine-type cells in the VTA slice.

Selected mouse lines, alcohol and behavior

The technique of selective breeding has been employed to develop a number of mouse lines differing in genetic sensitivity to specific effects of ethanol. Genetic animal models for sensitivity to the hypnotic, thermoregulatory, excitatory, and dependence-producing effects of alcohol have been developed. These genetic animal models have been utilized in numerous studies to assess the bases for those genetic differences, and to determine the specific neurochemical and neurophysiological bases for ethanol's actions. Work with these lines has challenged some long-held beliefs about ethanol's mechanisms of action. For example, lines genetically sensitive to one effect of ethanol are not necessarily sensitive to others, which demonstrates that no single set of genes modulates all ethanol effects. LS mice, selected for sensitivity to ethanol anesthesia, are not similarly sensitive to all anesthetic drugs, which demonstrates that all such drugs cannot have a common mechanism of action. On the other hand, WSP mice, genetically susceptible to the development of severe ethanol withdrawal, show a similar predisposition to diazepam and phenobarbital withdrawal, which suggests that there may be a common set of genes underlying drug dependencies. Studies with these models have also revealed important new directions for future mechanism-oriented research. Several studies implicate brain gamma-aminobutyric acid and dopamine systems as potentially important mediators of susceptibility to alcohol intoxication. The stability of the genetic animal models across laboratories and generations will continue to increase their power as analytic tools.

Genetic animal models in the study of alcoholism

Several methods have been applied to the study of the genetic determinants of ethanol (EtOH) sensitivity in animals. The use of inbred strains has indicated that virtually all responses to EtOH have a significant degree of genetic determination. Studies with large batteries of inbred strains have elucidated the common genetic control of several clusters of EtOH-related variables. Studies with Recombinant Inbred strains have identified single genes that may influence EtOH withdrawal severity and EtOH preference drinking. The best developed method has been the use of selective breeding to develop lines of mice or rats differing in EtOH-related behavioral characters. Illustrative examples of potentially important research findings from experiments with LS/SS, P/NP, and WSP/WSR selected lines are discussed. Significant progress has been made in the use of genetic animal models to further our understanding of EtOH-related traits. Several avenues for further research appear to be promising, and specific directions to be pursued are suggested.

Effects of ethanol and calcium on lipid order of membranes from mice selected for genetic differences in ethanol intoxication

Fluorescent probes were used to compare the physical properties of membranes from mice selected for sensitivity (LS) and insensitivity (SS) to the hypnotic action of ethanol. Brain synaptic plasma membranes (SPM) from LS mice were more sensitive to the disordering action of ethanol than those from LS mice when probes were located near the membrane surface. However, the membrane core of membranes from the two lines was equally sensitive to ethanol. The genetic differences in ethanol sensitivity of the membrane surface were eliminated when fluorescence measurements were carried out in the presence of 2-3 mM CaCl2. Consistent with behavioral data, differential genetic sensitivity to the disordering action was not obtained with longer chain alcohols. The genetic difference in ethanol sensitivity was not detected with erythrocyte membranes or lipids extracted from SPM. These results indicate that there is a structural difference in the surface of brain membranes of LS and SS mice than may influence their sensitivity to ethanol.

Effects of ethanol in vivo and in vitro on stimulated phosphoinositide hydrolysis in rat cortex and cerebellum

The effects of ethanol on phosphoinositide (PI) hydrolysis in rat cortex and cerebellum were studied to determine if this signal transduction mechanism is a pharmacological site of action for ethanol. In addition PI responses in young adult (8 months old) and old (22 months old) rats were compared to investigate the possible interaction between chronic ethanol treatment and aging on stimulated inositide metabolism. Fischer 344 rats were maintained on a nutritionally complete liquid diet containing sucrose or ethanol for 5 months. PI hydrolysis in prelabeled cortical or cerebellar slices was determined by measuring the release of [3H]inositol phosphates in the presence of 8 mM LiCl. Neither chronic ethanol nor aging altered maximal PI responses to carbachol or submaximal responses elicited by 20 mM KCl or 30 microM A23187. The glutamate-induced response was slightly reduced in the aged rats. Concentration-effect curves for norepinephrine (NE)-stimulated PI hydrolysis were similar in sucrose- and ethanol-treated cortex and cerebellum. Ethanol in vitro inhibited NE-stimulated PI hydrolysis in cortical but not cerebellar slices. The ethanol-induced inhibition of the NE-stimulated PI response was not altered by aging or chronic ethanol treatment. These results suggest that aging or chronic ethanol treatment do not cause large changes in the responsiveness of most PI-linked receptors, and thus, any deficits caused by these conditions may not be due to functional changes in receptor-mediated PI hydrolysis.

Molecular markers for linkage of genetic loci contributing to alcoholism

Specific locus and random locus linkage approaches to identify markers for genes whose allelic variants predispose to alcoholism or for genes controlling relevant physiological and behavioral phenotypes are discussed. Sib-pair analysis is superior for the direct analysis of complex genetic traits such as alcoholism, but classic family analysis will be useful for transmission and linkage analysis for marker traits whose genetics is less complex. In mice, a large number of inbred strains, recombinant inbred and congenic strains, and specifically selected outbred strains are available. In the human, an intriguing linkage result has emerged between a brain protein variant and alcoholism accompanied by suicide. In the mouse, preliminary linkages have been established to loci controlling ethanol preference and also activation after ethanol. Large panels of random DNA and protein genetic probes and of probes for specific loci will in the future increase the probability of establishing linkage in both species.

Genetic correlation of ethanol-induced ataxia and cerebellar Purkinje neuron depression among inbred strains and selected lines of rats

In the present study, we compared phenotypic differences in behavioral and neurophysiological responses to acute ethanol administration among six inbred rat strains. Genetic variation was found both for ataxia, as measured by loss of righting response (sleep time) after a hypnotic dose of ethanol, and for the depressant action of ethanol on the spontaneous discharge of cerebellar Purkinje neurons. Results from an analysis of covariance of these phenotypes, measured among the inbred strains, provided strong evidence for a high genetic correlation between sleep time and inhibition of cerebellar Purkinje neuron discharge in response to acute ethanol administration. However, ethanol metabolism was also found to correlate with the behavioral sensitivity of rats to ethanol. Preliminary data from the third generation of replicate lines of rats currently being selectively bred for high and low acute sensitivity to ethanol shows a trend toward divergence of both ethanol sleep time and neuronal sensitivity to acute ethanol. The conclusion from these data supports the hypothesis that the cerebellum is an important locus of ethanol action, and suggests that neuronal sensitivity to ethanol will continue to diverge between these rat lines as selection for the sleep time phenotype progresses.

Effect of ethanol on motor performance and hippocampal population spikes in some standard and selectively outbred rat strains

Ethanol sensitivity of Wistar and Long-Evans rats was compared in vivo and in vitro. Ethanol was more effective in reducing motor performance in Long-Evans than in Wistar rats, as determined by the tilting plane test. In addition, ethanol produced a greater reduction in the population spikes recorded from hippocampal slices (in vitro) of Long-Evans rats compared to Wistar rats. When rats from the Wistar, Long-Evans, and Sprague-Dawley strains were crossbred and then selectively outbred for high (ANT) and low (AT) sensitivity to ethanol-induced impairment of motor performance, no differences were observed in the ethanol sensitivity of the hippocampal population spike between these two strains. These data suggest that differences in ethanol sensitivity may exist among standard laboratory rodent strains. Selective outbreeding may reduce or eliminate the differences in ethanol sensitivity of brain regions or neurons other than those directly involved in producing the selected behavior. Therefore, it may be incorrect to assume a general difference in ethanol sensitivity when these traits are not coselected during outbreeding, thus indicating different neuronal pools in terms of sensitivity to ethanol.

Mapping of a putative genetic locus determining ethanol intake in the mouse

In the mouse, there is evidence that a single genetic locus is a major determinant of differences in ethanol intake between some preferring and non-preferring inbred strains. In this report, we present evidence from two independent experiments indicating that this locus maps to chromosome 1 and that its expressed product is the abundant protein LTW-4 (a 28 kDa, pI 5.6 protein expressed in brain, liver and kidney). The genetic association was found using a panel of 14 polypeptides of mouse brain which were visualized by two-dimensional electrophoresis and which exhibit genetic variation in isoelectric point. Fifteen BXD recombinant inbred strains and the two parental strains were typed for these loci and also tested for ethanol acceptance. Strains exhibiting the basic allele showed significantly higher ethanol acceptance. When 19 distantly related inbred mouse strains were tested for ethanol acceptance and typed for LTW-4, it was again found that strains exhibiting the basic allele showed significantly higher ethanol acceptance.

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.

Use of chromosomally mapped and identified mouse brain proteins for behavioral genetic analysis of alcoholism

A logical first place to look in order to identify loci determining behavioral differences between inbred and certain outbred strains of mice is among the proteins expressed in brain. Fourteen mouse brain proteins have been demonstrated to be genetically variant, four of these have been chromosomally mapped and an additional twelve have been identified and can be simultaneously screened by two dimensional electrophoresis. Certain genetic differences in behavior relevant to alcohol consumption and the effects of alcohol occur between inbred, recombinant inbred and selectively outbred strains. Two genetic correlations are reported, one between an isoelectric point variant of A7 (a 71 kd, pI 5.4 abundant protein) and resistance to signs of ethanol withdrawal and the other between A12 (a 28 kd, pI 5.6 protein) and ethanol intake. Though tentative, these findings illustrate the power of this approach for behavioral genetic analysis and may allow the biochemical genetic bases of these traits to be understood.

Alpha-methyl-para-tyrosine effects in mice selectively bred for differences in sensitivity to ethanol

The responses of catecholamine systems in long sleep (LS) and short sleep (SS) mice to alpha-methyl-p-tyrosine (AMPT) have been examined. Marked differences were found between LS and SS mice in the dose necessary for maximal brain catecholamine depletion and in the time-course of the catecholamine depletion. Brain catecholamines in the LS mice were depleted by lower doses of AMPT and the levels remained depressed for longer periods of time in this line of mice. These differences may be explained only partially by an increased susceptibility of the LS mice to the hypothermia and toxic effects caused by AMPT administration, as they persist with non-toxic AMPT dosage regimens and under conditions where the degree of hypothermia is comparable in both lines of mice. In addition, there were no differences between the Ki values for the effect of AMPT on the tyrosine hydroxylase from striata of these mouse lines. The primary cause of the heightened response to AMPT in LS mice would appear to be pharmacokinetic in nature, as brain and plasma peak levels of AMPT in LS mice were greater and the levels remained higher for a longer time. The depletion of brain tyrosine by AMPT combined with the lower affinity of the LS striatal tyrosine hydroxylase for the substrate tyrosine may also contribute to the heightened response in LS mice.

Genetic brain polypeptide variants in inbred mice and in mouse strains with high and low sensitivity to alcohol

Twelve genetically determined brain polypeptide charge variants were identified by comparing cerebellar vermis of 7 inbred mouse strains and of mice selectively bred from 8 strains closely related to these 7 ancestral strains and one other for acute behavioral sensitivity to the sedative effects of ethanol. The selectively bred ethanol-sensitive (LS, long sleep) and insensitive (SS, short sleep) mice exhibited different allelic variants at 6 of these 12 gene loci expressed in the cerebellum. Variant polypeptide A1 (81 kdalton, pI 5.6) was shown to be associated with the membrane of synaptosomal mitochondria and to exhibit a basic variant in SS mice that is determined by a dominant allele. Other variant polypeptides showed codominant inheritance in F1 crosses. However, the phenotype of no single one of these brain polypeptides consistently correlated with the ethanol behavioral sensitivity of the 7 inbred mouse strains nor of 8 recombinant inbred (B X D, C57BL X DBA) strains. This finding supports the hypothesis that a substantial amount of inbreeding, leading to random fixation of alleles independent of selection for ethanol sensitivity, occurred during the breeding of the SS and LS mice. The present findings of a lack of a strong association between sleep time and a brain polypeptide variant do not preclude the existence of a major gene effect contributing to variation in acute sensitivity to ethanol but are consistent with reports that multiple loci are responsible for the difference in ethanol sensitivity between SS and LS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of Purkinje cell firing by systemic administration of phenylisopropyl adenosine: effect of central noradrenaline depletion by DSP4

The effect of the metabolically stable adenosine analog (-)-N6(R-phenyl-isopropyl)-adenosine (PIA) on the rate of spontaneous Purkinje cell firing was studied in anesthetized rats. In control animals, systemically administered PIA elicited only small and inconsistent changes in firing rate. However, in animals previously treated with DSP4 (50 mg/kg i.p.), which selectively lesions central noradrenergic afferents, or with the adrenergic antagonist sotalol (15 mg/kg), PIA elicited consistent decreases in firing rate. These effects were antagonized by the systemic administration of the adenosine receptor antagonist aminophylline (50-150 mumol/kg). Local administration of adenosine by pressure ejection caused a dose-dependent depression of Purkinje cell firing that was likewise inhibited by the methylxanthine. In DSP4 treated rats the depression of synaptic transmission by adenosine in rat hippocampus in vitro was unaltered, and theophylline did not cause any marked rise in Purkinje cell firing, suggesting that DSP4 does not sensitize neurons to the depressant effects of adenosine derivatives. PIA also caused a dose-dependent decrease in arterial blood pressure and a decrease in heart rate that was of equal magnitude in control and DSP4 treated rats. The results show that the central effects of systemically administered adenosine analogs are altered by procedures that disrupt the normal depressant effect of tonic noradrenergic input.

Ethanol tolerance of cerebellar purkinje neurons from selectively outbred mouse lines: in vivo and in vitro electrophysiological investigations

The electrophysiological activity of cerebellar Purkinje neurons was characterized in long sleep (LS: ethanol sensitive) and short sleep (SS: ethanol insensitive) mice made tolerant to ethanol. After 1 to 4 weeks of feeding on a liquid ethanol diet, mice of both lines were less sensitive to the sedative and ataxic effects of parenteral ethanol than were controls. In addition, cerebellar Purkinje cells in ethanol-fed LS and SS mice were less responsive than the controls to the depressant effects of ethanol applied via bath perfusion in vitro and via local pressure ejection application in vivo. Tolerance to the electrophysiological effects of ethanol were already apparent after 7 to 9 days on the ethanol diet, and the degree of tolerance did not increase significantly in either mouse line fed ethanol for an additional 1-3 weeks. Finally, the differences in ethanol sensitivities of naive mice (LS greater than SS) were maintained following the development of tolerance. We conclude that tolerance to both the cellular and behavioral depressant effects of ethanol can be observed after chronic feeding with ethanol in LS and SS mice, and that there are no significant differences in the degree of tolerance developed by these mice. In addition, our data suggest that the inherited differences in ethanol sensitivity between LS and SS mice, and the changes in ethanol sensitivity which occur in these mice with chronic exposure to this depressant agent, are mediated by different mechanisms.

Differential CNS sensitivity to PIA and theophylline in long-sleep and short-sleep mice

Long sleep (LS) and short sleep (SS) mice have a differential sensitivity to the behavioral actions of an adenosine agonist, R-phenylisopropyl-adenosine (PIA) that parallels their differential sensitivity to the soporific effects of ethanol. In addition to being more sensitive to the sedative effects of PIA, LS mice also show a greater excitatory response to an adenosine antagonist, theophylline (1,3-dimethylxanthine). The brain concentrations of both PIA and theophylline following drug administration do not differ in LS and SS mice, suggesting that the central nervous system of the LS mouse is more sensitive to both adenosine receptor agonists and antagonists. However, LS and SS mice made tolerant to ethanol did not show cross-tolerance to PIA. These results suggest that genetic selection for ethanol sensitivity has resulted in a parallel CNS sensitivity to purinergic drugs, but that acute alterations in sensitivity due to the development of ethanol tolerance do not involve changes in purinergic systems.

Effect of ethanol on tyrosine hydroxylation in brain regions of long and short sleep mice

The effect of ethanol on the in vivo rate of tyrosine hydroxylation in 6 brain regions was examined in two lines of mice selectively bred for a differential sensitivity to ethanol. The mice are designated long-sleep (LS) and short-sleep (SS) and lose their righting reflex for a duration of 100 minutes (LS) and 13 minutes (SS) following an intraperitoneal dose of ethanol of 4.0 g/kg. DOPA accumulation after NSD-1015 administration was measured in the absence and presence of ethanol (4.0 g/kg, IP) in the periods 5-35 minutes and 85-115 minutes after saline or ethanol. There were no differences between the lines in either basal catecholamine levels or basal tyrosine hydroxylation rates (as measured by DOPA accumulation) in any brain region except the cerebellum, where the norepinephrine content in the SS mice is 33% greater and the tyrosine hydroxylation rate is 25% higher than that in the LS mice. In the presence of ethanol, there was a differential effect on the in vivo tyrosine hydroxylation rate. In the cerebellum of both LS and SS mice there was a decreased rate of tyrosine hydroxylation in the early period after ethanol, but the rate in the cerebellum of SS mice returned to the control value at 85-115 min. At that time, the rate in LS mice is still decreased. In the locus ceruleus, hypothalamus and frontal cortex, ethanol has no effect on the rate of tyrosine hydroxylation in either LS or SS mice during the early period, but ethanol decreases the rate during the later period in the LS mice only.(ABSTRACT TRUNCATED AT 250 WORDS)

Norepinephrine elicits both excitatory and inhibitory responses from Purkinje cells in the in vitro rat cerebellar slice

Superfusion of Purkinje neurons in the in vitro rat cerebellar slice with norepinephrine caused increases and decreases of spontaneous Purkinje cell firing. Excitations were evoked by low concentrations of norepinephrine (0.5-10 microM) and by the beta receptor agonist isoproterenol (0.1-5 microM). These excitations were reduced by timolol (1-2 microM), a beta receptor antagonist. Perfusion with higher concentrations of norepinephrine (greater than 16 microM), caused a depression of Purkinje neuron spontaneous activity. This inhibitory response was blocked by the alpha receptor antagonist phentolamine. The alpha 1 selective agonist phenylephrine had no effect on spontaneous activity at concentrations up to 100 microM, but the alpha 2 selective agonist clonidine (1-50 microM) elicited decreases in firing rate. These responses appeared to be due to a direct action on Purkinje cells, because neither the excitation nor the depression of Purkinje neuron activity elicited by norepinephrine was substantially altered when tested in a medium which substantially blocked synaptic transmission within the slice. Under these in vitro conditions, norepinephrine appears to increase the firing rate of Purkinje neurons via an interaction with beta adrenergic receptors, while norepinephrine induced depressions may be linked to alpha adrenergic receptor interactions; both receptors appear to be located directly on the Purkinje neurons.

Electrophysiological responses to adenosine analogs in rat hippocampus and cerebellum: evidence for mediation by adenosine receptors of the A1 subtype

Adenosine has profound depressant effects upon the electrophysiological activity of the brain, but the adenosine receptor subtypes which mediate these responses are uncertain. In order to resolve this question, we have characterized the effects of two adenosine analogs which differ in their relative potencies at adenosine A1 and A2 receptors. The effects of these adenosine analogs were examined on spontaneous firing rate of Purkinje neurons in the rat cerebellum in situ, in cerebellar brain slices in vitro, and on synaptic transmission in the rat hippocampus in vitro. Although the A2 agonist appeared to be more potent with local drug application techniques in situ, our in vitro results suggest that the A1 receptor subtype is involved in the electrophysiological actions of these drugs in both rat cerebellum and hippocampus. Furthermore, these data indicate that the physical properties of some adenosine analogs may reduce apparent drug potencies when they are studied with local application techniques.

Neonatal cerebellectomy alters ethanol-induced sleep time of short sleep but not long sleep mice

The effects of neonatal cerebellectomy on ethanol-induced sleep times in long sleep (LS) and short sleep (SS) mice were investigated. Cerebellectomy did not alter the ethanol sensitivity of LS animals for loss of righting reflex. In contrast, SS mice became more sensitive to alcohol after cerebellectomy. Even so, large differences were still observed between the alcohol-induced sleep times of cerebellectomized LS and SS mice. The data indicate that, while the cerebellum must have a prominant influence on alcohol sleep time in SS animals, this brain structure is not solely responsible for the observed differences in righting reflex sensitivity to ethanol in these two mouse lines. We postulate the existence of noncerebellar central neurons with differential sensitivities to the depressant effects of ethanol in LS and SS mice.

Voltage clamp analysis of ethanol effects on pacemaker currents of Aplysia neurons

The effect of 4% ethanol on pacemaker currents of Aplysia neurons was studied under voltage clamp. In normal seawater the n-shape in the I-V disappeared and outward current increased. Ion substitution and drug blocking experiments determined that leakage current and the slow inward calcium current were decreased and that the outward currents, IA and IK, were increased. This knowledge can be used to explain ethanol effects on spontaneous firing patterns.