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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Pharmacol Exp Ther. Author manuscript; available in PMC Mar 3, 2011.
Published in final edited form as:
PMCID: PMC3048052
NIHMSID: NIHMS214167
GABAergic Modulation of Ethanol-Induced Motor Impairment
GERALD D. FRYE and GEORGE R. BREESE
Center for Alcohol Studies (G.D.F., G.R.B.), Biological Sciences Research Center (G.D.F., G.R.B.), Departments of Psychiatry (G.D.F., G.R.B.) and Pharmacology (G.R.B), University of North Carolina School of Medicine, Chapel Hill, North Carolina
Send reprint requests to: Dr. Gerald D. Frye, Center for Alcohol Studies (207-H), University of North Carolina, Chapel Hill, NC 27514.
Direct or indirect pharmacological manipulation of γ-aminobutyric acid (GABA) receptor activity was examined in relation to the motor incoordinating actions of ethanol in the rat. Ethanol (1.13–3.0 g/kg i.p.) caused a dose-dependent increase in the height of aerial righting. This motor impairment was increased selectively by intracisternal injection of the GABA agonists muscimol (0.10 μg), 4,5,6,7-tetrahydroisoxazole(5,4-c) pyridin(3-ol) (1.0 μg) and GABA (1000 μg). The GABA antagonist, bicuculline (1.0 and 5.0 μg intracisternally), reduced impairment. Thus, direct manipulation of GABA receptor activity modulated motor incoordination caused by ethanol. In addition, indirect-acting GABA-mimetics, such as γ-acetylenic GABA (100 mg/kg i.p.), aminooxyacetic acid (50 mg/kg i.p.), ethanolamine-O-sulfate (250 mg/kg i.p.) and L-2,4-diaminobutyric acid (600 mg/kg i.p.) all potentiated the increase in the height of aerial righting caused by ethanol treatment. Failure of ethanol to modify the binding of [3H]muscimol to cerebral cortical membranes in vitro suggested there was no direct competition for GABA binding sites or facilitation of the binding of GABA to these sites by ethanol. Also, no simple relationship was observed between the degree of motor impairment caused by either ethanol or γ-acetylenic GABA and changes in GABA concentration in three brain areas. Although GABAergic neurons may be involved in the mechanism underlying ethanol-induced depression of motor coordination, the interaction does not involve a direct activation of GABA receptors by ethanol.
Impairment of motor coordination is a well known result of the action of ethanol on CNS (Wallgren and Barry, 1970; Ritchie, 1980). Since the initial proposal that GABA serves as a neurotransmitter in the brain, it has become apparent that GABA is involved in the CNS control of motor coordination (Grimm et al., 1975; Wachtel and Andén, 1978; Robin et al., 1980). As suggested by several observations, GABA also might play a role in the CNS depressant actions of ethanol. Acute ethanol treatment may alter the steady-state concentrations of GABA in the brain (Sutton and Simmonds, 1973; Sytinsky et al., 1975; Volicer and Klosowicz, 1979; Frye, 1982). Furthermore, GABA accumulation after the inhibition of GABA-transaminase is diminished by acute ethanol treatment, suggesting that ethanol may reduce GABA turnover in the CNS (Wixon and Hunt, 1980; Supavilai and Karobath, 1980). Also, Ticku and Burch (1980) have indicated that acute ethanol treatment in vivo may modify [3H]GABA binding in vitro and could influence the homeostasis of CNS GABAergic neurons by altering GABA receptor function.
Only a few studies have attempted to evaluate whether a functional relationship exists between changes in GABA neurotransmission and changes in motor coordination during ethanol intoxication. Häkkinen and Kulonen (1976) found that administration of the GABA receptor antagonist, bicuculline, diminished ethanol-induced impairment of performance on a tilting plane task in rats. They also found that treatment of rats with AOAA which increased whole-brain GABA concentrations, enhanced ethanol-induced impairment. Similarly, Leitch et al. (1977) showed that picrotoxin, an antagonist of the GABA receptor, reduced impairment of aerial righting due to ethanol. Inhibition of GABA-transaminase with AOAA increased this action of ethanol. Thus, these experiments suggest that facilitation of GABAergic transmission can enhance the motor incoordinating action of ethanol. The present studies were designed to systematically evaluate whether direct or indirect pharmacological manipulation of the activity of GABA receptors in vivo alters the motor incoordinating actions of acute ethanol administration.
Animals
Male Sprague-Dawley rats, Crl:CD(SD)BR (150–175 g), were purchased from Charles River Laboratories (Somerville, MA) and housed for several days in groups of five animals under environmentally controlled conditions (7:00 A.M. light-7:00 P.M. dark cycle; 22–25°C) before use. Wayne Blox Rodent Laboratory Chow and water were freely available during this time.
Drug treatments
All drugs were dissolved in sterile saline as a vehicle (pH 7.0) when possible (exceptions are discussed below). Ethanol was administered i.p. as a 0.1 g/ml solution to minimize tissue irritation (Wallgren and Barry, 1970). Saline volumes for controls were equivalent to the largest volume of ethanol administered. AOAA hemihydrochloride, L-2,4-DABA (Sigma Chemical Co., St. Louis, Mo.) and EOS (Aldrich Chemical Co., Milwaukee, WI, purified by recrystallization) were administered (2 ml/kg) by i.p. injection. GAG (a gift from Centre de Recherche Merrell Internationale, Strasbourg Cedex, France) also was administered (10 ml/kg) by i.p. injection. GABA, muscimol and bicuculline (Sigma Chemical Co.) and THIP (a gift from Lundbeck and Co A/S, Copenhagen, Denmark) were administered i.c. in 10-μl volumes. Bicuculline was dissolved in sterile water with the addition of a small quantity of 0.01 M HC1. After the appropriate dilutions were made in sterile water, each solution of bicuculline was titrated with 0.01 M NaOH toward the desired pH of 7.0. Solutions were used for injection at the pH closest to 7.0 that did not result in any bicuculline precipitation.
Intracisternal injections were made in unanesthetized, restrained rats, as previously described by Cooper et al. (1979), with the following modifications. Landmark features of the calvarium were more readily recognized if the dorsal neck area was shaved. Also, the exposed length of the 30-gauge needle used for the injection was reduced by shielding it with a shortened plastic sheath. The plastic needle sheath supplied with the needle was cut off at the closed end to expose a 5 mm length of the needle. This needle length was used routinely for i.c. injections in rats weighing between 150 to 200 g; this dramatically reduced the incidence of needle-induced spinal cord damage previously described by Cooper et al. (1979).
Measurement of motor coordination
Changes in the aerial righting reflex of rats were used to evaluate ethanol-induced changes in motor coordination as previously described (Frye et al., 1980, 1981). A meter stick mounted vertically above a foam rubber pad served as the test apparatus. Rats were held by the back of the neck and base of the tail in an inverted position at specified heights above the pad and released. Successful righting required that all four feet of the rat were in flat contact with the foam rubber pad at the time of first contact on two of three consecutive releases. The minimum height necessary for successful righting was used as an index of motor impairment. Successful righting in saline or untreated rats usually required 5.0 cm. Animals that received sedative doses of ethanol were not released from heights greater than 55 cm.
Blood and brain ethanol measurement
The concentration of ethanol in venous blood collected from the tip of the tail or in brain tissue was determined as previously described (Frye et al., 1981). Blood samples (20 μl) were collected and immediately diluted with 180 μl of ice-cold, glass-distilled water containing 0.3 mg/ml of terf-butanol as an internal standard. Samples, stored tightly capped at −20°C, were stable for at least 11 days, but were usually assayed within 24 hr. Brain tissue was rapidly excised from the skull and homogenized in ice-cold distilled water (2 μl/mg). Samples were tightly capped, centrifuged (20,000 × g for 20 min) and 20-μl aliquots of the supernatant were diluted in 180 μl of distilled water containing the internal standard. One- to five-μl aliquots of diluted sample were injected into a 2400 Varian Aerograph gas chromatograph (Varian Associates, Instrument Division, Palo Alto, CA) equipped for flame-ionization detection. A glass column (2 mm inside diameter × 1.8 m length) packed with 80/100 mesh Carbopack C coated with 0.2% Carbowax 1500 (Supelco, Inc., Bellefonte, PA) was used to separate ethanol and tert-butanol. Chromatographic conditions were He = 30 ml/min; H2 = 30 ml/min; air = 250 ml/min; injector = 155°C; column = 130°C (isothermal) and detector = 185°C.
Measurement of brain GABA
Rats were rapidly sacrificed by delivering an intense pulse of focused microwave radiation (Gerling-Moore Metabostat, 3.5 kw, 2450 MHz, 1.8 sec) to the head. After decapitation and cooling on ice, the brain was removed from the skull, weighed, rapidly frozen on Dry Ice and stored at −70°C for no longer than 1 week. Frozen tissue was homogenized (10 μl/mg) in ice-cold ethanol (12.2 M) and centrifuged (20,000 × g for 20 min). Aliquots of sample supernatants or standard solutions of GABA were spiked with 5-AVA (5 μg/ml) as an internal standard. GABA and 5-AVA were determined by high-performance liquid chromatography with fluorescence detection as previously described (Frye, 1982). The mobile phase, which contained 0.1 M NaH2PO4 (pH 6.0) (52% by volume) plus high-performance liquid chromatography grade methanol (48% by volume) was prepared from glass-distilled water, filtered (0.45-μm filter, Gelman Sciences, Ann Arbor, MI) and degassed under vacuum with ultrasonic agitation. GABA and 5-AVA were derivatized by reacting equal aliquots of sample or standard solutions with an OPT reagent for 1 minute. The OPT reagent was prepared by dissolving 250 mg of OPT (Sigma Chemical Co.) in 2 ml of absolute ethanol. This was then combined with 46 ml of a 0.4 M boric acid solution previously adjusted to pH 10.4 with 10 M KOH. Two milliliters of 2-mercaptoethanol (Calbiochem, San Diego, CA) were added and the OPT reagent was stored at 4°C in an opaque glass bottle. Ten- to 50-μl aliquots of the reaction mixture were injected into the chromatograph. The mobile phase was delivered to the column by a Constametric IIG pump (Laboratory Data Control, Riviera Beach, FL) at a flow rate of 1.5 ml/min. Fluorometric monitoring of column effluents was accomplished using an FS970 liquid chromatography fluorometer (Kratos/Schoeffel Instruments, Westwood, NJ). The wave length of the excitatory light beam was 330 nm; emission wavelengths greater than 418 nm were monitored after passing a Corning 7–54 primary filter. Standard and unknown GABA peaks were measured manually from strip chart recorder tracings and evaluated by peak-height analysis as ratios based on the internal standard.
[3H]Muscimol binding in vitro
Rats were sacrificed by decapitation and the cerebral cortices collected, rinsed, homogenized in 0.32 M sucrose and frozen at −70°C for 24 to 72 hr. Evaluation of the binding of [3H]muscimol to cerebral cortical tissue homogenates was performed according to the method of Williams and Risley (1979). Frozen sucrose homogenates were thawed and centrifuged (20,000 kg, 20 min). The pellets were then resuspended in 50 mM Tris-citrate buffer (pH 7.1) containing Triton X-100 (0.025%; Packard Inc., Downers Grove, IL) and incubated for 30 min at 37°C. The homogenates were then resuspended in buffer and centrifuged three times to remove the Triton X-100 before final resuspension at a tissue concentration of 1.25 mg/ml. Each tube contained 1 mg of tissue, [3H]muscimol (8 nM) and the appropriate concentration of drug to be examined for binding displacement in a total volume of 1 ml. Incubations were carried out for 30 min at 0°C. The contents of each tube was rapidly filtered through 25 mm glass fiber filters (Gelman Instrument Co., Ann Arbor, MI) and washed twice with 5 ml of buffer. The amount of radioactive compound bound to the filters was determined by liquid scintillation spectroscopy.
Statistical analysis
Data were expressed as the mean ± S.E.M. Experiments evaluating the effects of drugs on the height of aerial right were analyzed with the Mann-Whitney U test. Data resulting from GABA and blood ethanol measurements were evaluated using the two-tailed Student's t test.
Effect of L-2,4-DABA or GABA-transaminase inhibitors on ethanol-induced aerial righting impairment
In agreement with previous reports (Leitch et al., 1977; Frye et al., 1980, 1981), ethanol caused a dose-dependent increase in the height necessary for successful aerial righting by rats dropped from an inverted position (table 1). In light of the report by Leitch et al. (1977), an attempt was made to examine further the effect of indirectly activating GABA receptors with several different drugs on ethanol-induced impairment of aerial righting. First, L-DABA (600 mg/kg i.p.), an inhibitor of the neuronal uptake of GABA from the synaptic cleft (Sutton and Simmonds, 1974), was administered and followed immediately by ethanol (2.25 g/kg i.p.). This treatment significantly increased the height of aerial righting relative to ethanol treatment alone (table 2). When given alone, this dose of DABA had no effect on the height of aerial righting. No change in ethanol-induced impairment was observed after treatment with a smaller amount of DAB A (300 mg/kg). Interestingly, tandem treatments with DABA (600 mg/kg i.p.) and ethanol (3.0 g/kg i.p.) resulted in a slightly lower BEC than the combination of saline with this dose of ethanol (table 3). However, no significant difference was observed in brain ethanol concentrations.
TABLE 1
TABLE 1
Effect of ethanol on the height of aerial righting
TABLE 2
TABLE 2
Effect of ethanol and indirect-acting GABA-mimetics on the height of aerial righting
TABLE 3
TABLE 3
Effect of GABA-mimetics on BEC and brain ethanol concentrations
Rats treated with ethanol (1.13 or 2.25 g/kg i.p.) and one of three different GABA transaminase inhibitors also exhibited significant increases in the height of aerial righting relative to that of animals receiving the same ethanol treatment and saline (table 2). None of the GABA-transaminase inhibitors significantly increased the height of aerial righting when administered alone at doses that potentiated the motor impairment caused by ethanol (table 2) except 200 mg/kg of GAG (table 5). Neither BEC nor brain ethanol concentrations was altered by combined administration of ethanol (3.0 g/kg i.p.) with any of the GABA transaminase inhibitors we studied (table 3).
TABLE 5
TABLE 5
Effect of ethanol and GAG on the height of aerial righting and brain GABA concentration
Effect of ethanol and GAG on aerial righting and brain GABA concentrations
Because indirect pharmacological manipulation of GABA receptors was found to enhance ethanol-induced motor impairment, we next examined the relationship between the concentration of GABA in brain tissue and the degree of motor impairment after various treatments. The concentration of GABA in brain tissue has been reported to be increased in animals sacrificed by decapitation when compared to animals sacrificed by rapid freezing or microwave irradiation of the brain (Balcom et al., 1975). The data in table 4 replicated this observation and indicated that ethanol did not alter the concentration of GABA in brain tissue from rats sacrificed by decapitation or microwave irradiation. An analysis of the GABA concentration in three brain regions (table 5) also showed no significant increase in GABA concentration in any area after ethanol treatment.
TABLE 4
TABLE 4
Effect of ethanol and microwave irradiation on whole-brain GABA concentration
A comparison of the effects of ethanol and GAG on the GABA concentrations of these brain areas and the degree of motor impairment revealed no clear relationship between the two (table 5). For example, ethanol (2.25 g/kg i.p.) and GAG (200 mg/kg i.p.) both caused a small, but significant increase in the height of aerial righting (table 5). However, only GAG (200 mg/kg) markedly increased the concentration of GABA in the cerebellum (850% of control) as well as in cerebral cortex (498% of control) and brainstem (550% of control). Combined ethanol and GAG (200 mg/kg) treatment resulted in brain GABA concentrations which were not significantly different from those after GAG alone, yet the aerial righting reflex of ethanol-GAG-treated rats was completely abolished. The sum of the total height necessary for aerial righting after ethanol alone (22.5 ± 2.1 cm) or GAG (200 mg/kg) alone (15.8 ± 3.3 cm) was less than that for combined ethanol + GAG treatment (55.0 ± 0.0 cm), the maximum value that could be assigned, suggesting a synergistic rather than additive interaction. The increased depressant action of combined ethanol + GAG treatment relative to ethanol treatment alone (table 5) was not explained by differences in the concentration of ethanol present in blood at the time of testing (BEC, milligrams per milliliter; ethanol + saline = 2.06 ± 0.16, ethanol + GAG (200 mg/kg) = 2.19 ± 0.12; P > .05).
Effect of i.c. GABA, muscimol and THIP on ethanol-induced aerial righting impairment
The findings that several different indirect-acting GABA-mimetics all potentiated the depressant actions of ethanol (table 2) suggested that activation of GABA receptors probably was responsible for the modulation of this action of ethanol. To further test this hypothesis, the action of several drugs which directly activate GABA receptors was examined next. To minimize peripheral effects of these drugs they were injected into the cisterna magna in a small amount of sterile saline. Administration of saline (i.c.) 10 min before testing was found to have no observable effect on the height of aerial righting when compared with i.p. saline treatment (ethanol (2.25 g/kg) + saline (10 ml/kg i.p.) = 26.9 ± 1.6 cm; ethanol (2.25 g/kg) + saline (10 μl i.c.) = 25.3 ± 1.7; P > .05). Intracisternal administration of GABA (300 or 1000 μg in 10 μl of sterile saline) 10 min before testing had no effect on the height of aerial righting in saline-pretreated rats. As expected, ethanol (2.0 g/kg i.p.) administered 50 min before 10 μl of saline significantly increased the height of aerial righting when compared with saline i.p. and saline i.c. pretreatment. GABA (30, 100 or 300 μg i.c.) did not change the height of aerial righting from that caused by ethanol. However, administration of 1000 μg of GABA with ethanol significantly increased the height of aerial righting relative to ethanol treatment alone (saline + ethanol = 16.5 ± 1.7; GABA + ethanol = 37.5 ± 5.6; P < .05). Thus, GABA was found to enhance the impairment of the aerial righting reflex caused by ethanol, but only when administered in an exceedingly large dose. This suggested two possibilities. Either the activation of GABA receptors was having little or no effect on this action of ethanol or that the exogenously applied GABA was being very rapidly inactivated and was unavailable for receptor interaction. Because GABA is known to be taken up into neurons and glial cells, a process which would markedly reduce access to GABA receptors (Iversen, 1978), the effects of muscimol and THIP, two GABA receptor agonists with a longer duration of action in vivo than GABA, were studied.
Intracisternal administration of muscimol (0.03–0.3 μg) 50 min after ethanol (2.25 g/kg i.p.) produced a dose-dependent increase in the height of aerial righting (fig. 1) when this reflex was tested 10 min later. Muscimol (0.1 μg) selectively increased the motor impairment of ethanol-treated rats, but had no effect on the height of aerial righting when given to saline-treated animals. The largest dose of muscimol (0.3 μg) which potentiated the ethanol-induced motor impairment also caused a small increase in the height of aerial righting of saline-treated rats (fig. 1). Immediately after i.c. administration of this dose of muscimol (0.3 μg), many of the rats extended their limbs and exhibited a side-to-side rocking motion, leaning first to one side then shifting to the other as if postural control were impaired. Postural impairment lasted 1 to 5 min and was absent at the time animals were tested for righting 10 min after the i.c. injection.
Fig. 1
Fig. 1
Effect of muscimol and ethanol on the height of aerial righting of rats. Ethanol (2.25 g/kg i.p.) administration was followed 50 min later by muscimol (10 μl i.c.). The height of aerial righting was measured 60 min after ethanol or 10 min after (more ...)
Like muscimol, THIP (1.0 and 3.0 μg i.c.) increased the height of aerial righting of rats pretreated with ethanol (2.25 g/kg i.p.; fig. 2); a smaller dose of THIP (0.3 μg) was without effect. Although THIP (3.0 μg i.c.) potentiated the action of ethanol, it also caused a significant increase in the height of aerial righting when given alone. Furthermore, this dose of THIP produced postural impairment similar to that caused by muscimol, suggesting that GABA receptor activation was involved.
Fig. 2
Fig. 2
Effect of THIP and ethanol on the height of aerial righting in rats. Ethanol (2.25 g/kg i.p.) administration was followed 50 min later by THIP (10 μl i.c.). The height of aerial righting was measured 60 min after ethanol or 10 min after THIP treatments. (more ...)
Effect of ethanol and bicuculline on aerial righting
The effect of GABA receptor blockade on ethanol-induced impairment of aerial righting was examined next. Rats given i.c. injections of bicuculline (1.0 or 5.0 μg), 10 min before the aerial righting test, required less height for aerial righting 60 min after a 2.25 g/kg ethanol injection than ethanol-treated rats given saline (10 μl i.c.; fig. 3). Larger or smaller doses of bicuculline did not significantly alter ethanol-induced motor impairment. No dose of bicuculline caused a significant increase in the height of aerial righting of saline-pretreated rats. However, there was an obvious increase in response variability with the largest dose of bicuculline (10 μg), which also rapidly evoked transient facial seizure activity within 1 to 2 sec after treatment, lasting up to 1 min. Larger doses of bicuculline (25 and 50 μg) caused prolonged seizure activity that continued up to 15 min. This action of bicuculline appeared to be due to the well recognized convulsant action of the drug (Buckett, 1981), rather than the low pH of the more concentrated drug solutions (bicuculline, 0.5 and 1.0 μg = pH 7.0; 5 μg = pH 4.4; 10 μg = 3.4; and 25 μg = pH 3.0) that were required in order to maintain bicuculline in solution. Intracisternal injection of saline (pH 3.5) alone did not cause seizures or increase the height of aerial righting (saline, pH 7.0 = 6.4 ± 0.7 cm; and pH 3.5 = 6.0 ± 1.0 cm). Furthermore, it is unlikely that antagonism of ethanol-induced motor impairment caused by bicuculline (1.0 and 5.0 μg) was due to the lower pH of these solutions because 10 μl of saline (pH 2.0) did not reduce this action of ethanol (2.25 g/kg; saline, pH 2.0 = 27.5 ± 1.4 cm; and saline, pH 7.0 = 27.6 ± 1.9).
Fig. 3
Fig. 3
Effect of bicuculline and ethanol on the height of aerial righting of rats. Ethanol (2.25 g/kg i.p.) administration was followed 50 min later by bicuculline (10 μl i.c.). The height of serial righting was measured 60 min after ethanol or 10 min (more ...)
Effect of ethanol on [3H]muscimol binding
Inasmuch as pharmacological manipulation of GABA receptor activity altered ethanol-induced motor impairment, it was possible that ethanol might directly activate GABA receptors or enhance the binding of GABA to its receptor. This was explored indirectly by examining the effect of ethanol on the binding of [3H] muscimol to membranes isolated from the cerebral cortex of rats. Addition of ethanol (1 × 10−5 to 1 × 10−1 M) to the incubation medium had no effect on the binding of [3H]muscimol to cerebral cortical membranes in vitro. However, as expected, [3H]muscimol binding was altered by addition of unlabeled GABA (IC50 = 2.2 × 10−6 M), muscimol (IC50 = 4.5 × 10−7 M) or bicuculline (IC50 = 6.3 × 10−5 M) to the incubation medium. Thus, ethanol did not directly compete for binding at the receptor labeled by [3H]muscimol, a GABA receptor agonist, or increase the binding of the [3H]muscimol to its receptor, receptor.
The present findings clearly demonstrated that the administration of drugs which directly activate or inhibit GABA receptors can cause a predictable modulation of the motor impairing action of moderate doses of ethanol in the rat. For example, direct activation of GABA receptors with GABA, muscimol or THIP potentiated ethanol-induced impairment. Furthermore, drugs such as GAG, AOAA, EOS or DABA that are thought to enhance the activity of GABA receptors indirectly by blocking the catabolism of endogenous GABA (Schechter et al., 1977; Wallach, 1961; Fowler and John, 1972) or by inhibiting GABA uptake from the synaptic cleft (Sutton and Simmonds, 1974), all enhanced the depression of aerial righting after ethanol. This action was observed with doses which alone had no significant effect on the ability of rats to right themselves.
Also important was the finding that bicuculline, a specific GABA receptor antagonist, significantly reduced this action of ethanol. This finding extends that of Leitch et al. (1977) who found that picrotoxin, a drug which probably antagonizes the combined functioning of the GABA receptor-chloride iontophore complex (Ticku et al., 1978), also reduced the impairment of aerial righting in ethanol-intoxicated rats. In addition, our findings parallel the results of Häkkinen and Kulonen (1976), who found that AOAA pretreatment increased, whereas bicuculline treatment decreased the impairment caused by ethanol in rats performing a tilting plane test.
A previous report showed that the effects of ethanol were antagonized by naloxone (Vogel et al., 1981). However, this action required doses of naloxone (30–60 mg/kg) far in excess of those necessary for the antagonism of opiate or endorphin receptor activity. Naloxone has previously been reported to exert a GABA-antagonistic action (Billingsley and Kubena, 1978; Dingledine et al., 1978). The present work with bicuculline is consistent with the hypothesis that large doses of naloxone reduced ethanol-induced impairment via a GABAergic antagonistic mechanism (Vogel et al., 1981).
Certain “GABA-like” agents including AOAA have been shown to inhibit the locomotor stimulant action of ethanol in mice (Cott et al., 1976). It was suggested that these drugs might unmask the depressant action of ethanol as a result of selectively eliminating ethanol-induced stimulation (Cott et al., 1976). The present investigation further suggests that activation of GABA receptors modulates the depressant effects of ethanol on motor function. Thus, it is possible that a synergistic potentiation of the depressant actions of ethanol might occur after the activation of GABA receptors and this could mask the stimulant effects of ethanol.
Data in table 5 failed to establish a clear relationship between the concentration of GABA in three brain areas that play important roles in motor function and the magnitude of aerial righting impairment after ethanol or GAG treatments. For example, doses of ethanol (2.25 g/kg) or GAG (200 mg/kg) which alone caused minor impairment of the height of aerial righting had markedly different effects on GABA concentrations in the cerebellum, cortex and brainstem (table 5). Ethanol exerted no effect on GABA concentrations, whereas GAG increased them several-fold over control levels. These findings are in sharp contrast to those of Grimm et al. (1975) who have suggested that a direct relationship exists between the effects of AOAA and di-n-propyl-acetate to increase cerebral cortical and cerebellar GABA concentrations and the ability of these drugs to impair motor performance. The results of a multitude of studies on seizure mechanisms now suggest that a rough correlation exists between the concentration of GABA in brain tissue and susceptibility to seizures. Reduction of brain GABA concentrations frequently has been observed to occur coincident with increased susceptibility to seizures (Tapia, 1975), whereas increased GABA concentrations often accompany anticonvulsant drug action (Wood and Peesker, 1975). However, many exceptions to these data indicate that there exists no simple relationship between the concentration of GABA in brain tissue and seizure susceptibility (Wood, 1975). Recently, Wood et al. (1979) have shown that the concentration of GABA in nerve endings, in contrast to that in unfractionated brain tissue, may more consistently be correlated with the onset of seizure susceptibility. In addition, two reports suggest that ethanol causes dose-dependent changes in the rate of GABA accumulation after GABA transaminase inhibition (Wixon and Hunt, 1980; Supavilai and Karobath, 1980). Similar analysis dealing more specifically with changes in neuronal GABA may lead to a clearer understanding of the relationship between motor impairment due to ethanol and changes in brain GABA homeostasis.
Although the present results clearly indicate that activation or inhibition of GABA receptors can modulate ethanol-induced impairment of motor coordination, conclusive evidence for the hypothesis that GABA mediates this action of ethanol was not obtained. However, when these results are considered along with neurophysiological results suggesting that ethanol potentiates GABAergic hyperpolarization of cortical neurons (Nestoros, 1980), binding data suggesting an in vivo ethanol-induced change in GABA receptor kinetics (Ticku, 1980; Ticku and Burch, 1980; Reggiani et al., 1980) and observed changes in measures of GABA turnover (Wixon and Hunt, 1980; Supavilai and Karobath, 1980), a direct involvement of GABA in the actions of ethanol, seems likely. Future work should attempt to resolve the molecular basis of this potential interaction.
Acknowledgments
The excellent technical assistance provided by Robert Considine, David Knight and Herbert Lea in the conduct of these experiments as well as the skilled word processing support provided by Faygele ben Miriam during the writing of this manuscript are most gratefully acknowledged.
This work was supported by U.S. Public Health Service Grant AA-02334 and by the North Carolina Alcoholism Research Authority Grants 7920 and 8019.
ABBREVIATIONS
CNScentral nervous system
GABAγ-aminobutyric acid
AOAAaminooxyacetic acid
DABAdiaminobutyric acid
EOSethanolamine-O-sulfate
GAGγ-acetylenic GABA
THIP4,5,6,7-tetrahydroisoxazole (5,4-c)pyridin(3-ol)
i.c.intracisternol
5-AVA5-aminovaleric acid
OPTo-phthaldehyde
BECblood ethanol concentration

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