Adolescent intermittent alcohol exposure: persistence of structural and functional hippocampal abnormalities into adulthood.

BACKGROUND: Human adolescence is a crucial stage of neurological development during which ethanol (EtOH) consumption is often at its highest. Alcohol abuse during adolescence may render individuals at heightened risk for subsequent alcohol abuse disorders, cognitive dysfunction, or other neurological impairments by irreversibly altering long-term brain function. To test this possibility, we modeled adolescent alcohol abuse (i.e., intermittent EtOH exposure during adolescence [AIE]) in rats to determine whether adolescent exposure to alcohol leads to long-term structural and functional changes that are manifested in adult neuronal circuitry. METHODS: We specifically focused on hippocampal area CA1, a brain region associated with learning and memory. Using electrophysiological, immunohistochemical, and neuroanatomical approaches, we measured post-AIE changes in synaptic plasticity, dendritic spine morphology, and synaptic structure in adulthood. RESULTS: We found that AIE-pretreated adult rats manifest robust long-term potentiation, induced at stimulus intensities lower than those required in controls, suggesting a state of enhanced synaptic plasticity. Moreover, AIE resulted in an increased number of dendritic spines with characteristics typical of immaturity. Immunohistochemistry-based analysis of synaptic structures indicated a significant decrease in the number of co-localized pre- and postsynaptic puncta. This decrease is driven by an overall decrease in 2 postsynaptic density proteins, PSD-95 and SAP102. CONCLUSIONS: Taken together, these findings reveal that repeated alcohol exposure during adolescence results in enduring structural and functional abnormalities in the hippocampus. These synaptic changes in the hippocampal circuits may help to explain learning-related behavioral changes in adult animals preexposed to AIE.

Adolescent alcohol exposure alters GABAA receptor subunit expression in adult hippocampus.

BACKGROUND: The long-term consequences of adolescent alcohol abuse that persist into adulthood are poorly understood and have not been widely investigated. We have shown that intermittent exposure to alcohol during adolescence decreased the amplitude of GABAA receptor (GABAA R)-mediated tonic currents in hippocampal dentate granule cells in adulthood. The aim of this study was to investigate the enduring effects of chronic intermittent alcohol exposure during adolescence or adulthood on the expression of hippocampal GABAA Rs. METHODS: We used a previously characterized tissue fractionation method to isolate detergent resistant membranes and soluble fractions, followed by Western blots to measure GABAA R protein expression. We also measured mRNA levels of GABAA R subunits using quantitative real-time polymerase chain reaction. RESULTS: Although the protein levels of α1-, α4-, and δ-GABAA R subunits remained stable between postnatal day (PD) 30 (early adolescence) and PD71 (adulthood), the α5-GABAA R subunit was reduced across that period. In rats that were subjected to adolescent intermittent ethanol (AIE) exposure between PD30 and PD46, there was a significant reduction in the protein levels of the δ-GABAA R, in the absence of any changes in mRNA levels, at 48 hours and 26 days after the last ethanol (EtOH) exposure. Protein levels of the α4-GABAA R subunit were significantly reduced, but mRNA levels were increased, 26 days (but not 48 hours) after the last AIE exposure. Protein levels of α5-GABAA R were not changed by AIE, but mRNA levels were reduced at 48 hours but normalized 26 days after AIE. In contrast to the effects of AIE, chronic intermittent ethanol (CIE) exposure during adulthood had no effect on expression of any of the GABAA R subunits examined. CONCLUSIONS: AIE produced both short- and long-term alterations of GABAA R subunits mRNA and protein expression in the hippocampus, whereas CIE produced no long-lasting effects on those measures. The observed reduction of protein levels of the δ-GABAA R, specifically, is consistent with previously reported altered hippocampal GABAA R-mediated electrophysiological responses after AIE. The absence of effects of CIE underscores the emerging view of adolescence as a time of distinctive vulnerability to the enduring effects of repeated EtOH exposure.

Adolescent intermittent ethanol (AIE) produces lasting, sex-specific changes in rat body fat independent of changes in white blood cell composition.

Early initiation of alcohol use during adolescence, and adolescent binge drinking are risk factors for the development of alcohol use disorder later in life. Adolescence is a time of rapid sex-dependent neural, physiological, and behavioral changes as well as a period of heightened vulnerability to many effects of alcohol. The goal of the present studies was to determine age-related changes in blood (leukocyte populations) and body composition across adolescence and early adulthood, and to investigate whether adolescent intermittent ethanol (AIE) exposure would alter the trajectory of adolescent development on these broad physiological parameters. We observed significant ontogenetic changes in leukocyte populations that were mirrored by an age-related increase in cytokine expression among mixed populations of circulating leukocytes. Despite these developmental changes, AIE did not significantly alter overall leukocyte numbers or cytokine gene expression. However, AIE led to sex-specific changes in body fat mass and fat percentage, with AIE-exposed male rats showing significantly decreased fat levels and female rats showing significantly increased fat levels relative to controls. These changes suggest that while AIE may not alter overall leukocyte levels, more complex phenotypic changes in leukocyte populations could underlie previously reported differences in cytokine expression. Coupled with long-term shifts in adipocyte levels, this could have long-lasting effects on innate immunity and the capacity of individuals to respond to later immunological and physiological threats.

Adolescent intermittent ethanol exposure enhances adult stress effects in male rats.

Binge patterns of alcohol use, prevalent among adolescents, are associated with a higher probability of developing alcohol use disorders (AUD) and other psychiatric disorders, like anxiety and depression. Additionally, adverse life events strongly predict AUD and other psychiatric disorders. As such, the combined fields of stress and AUD have been well established, and animal models indicate that both binge-like alcohol exposure and stress exposure elevate anxiety-like behaviors. However, few have investigated the interaction of adolescent intermittent ethanol (AIE) and adult stressors. We hypothesized that AIE would increase vulnerability to restraint-induced stress (RS), manifested as increased anxiety-like behavior. After AIE exposure, in adulthood, animals were tested on forced swim (FST) and saccharin preference (SP) and then exposed to either RS (90 min/5 days) or home-cage control. Twenty-four hours after the last RS session, animals began testing on the elevated plus maze (EPM), and were re-tested on FST and SP. A separate group of animals were sacrificed in adulthood after AIE and RS, and brains were harvested for immunoblot analysis of dorsal and ventral hippocampus. Consistent with previous reports, AIE had no significant effect on closed arm time in the EPM (anxiety-like behavior). However, in male rats the interaction of AIE and adult RS increased time spent in the closed arms. No effect was observed among female animals. AIE and RS-specific alterations were found in glial and synaptic markers (GLT-1, FMRP and PSD-95) in male animals. These findings indicate AIE has sex-specific effects on both SP and the interaction of AIE and adult RS, which induces a propensity toward anxiety-like behavior in males. Also, AIE produces persistent hippocampal deficits that may interact with adult RS to cause increased anxiety-like behaviors. Understanding the mechanisms behind this AIE-induced increase in stress vulnerability may provide insight into treatment and prevention strategies for alcohol use disorders.

Adolescent intermittent ethanol exposure reduces astrocyte-synaptic proximity in the adult medial prefrontal cortex in rats: Reversal by gabapentin.

Alcohol consumption in adolescence causes multiple acute negative changes in neural and behavioral function that persist well into adulthood and possibly throughout life. The medial prefrontal cortex (mPFC) and dorsal hippocampus are critical for executive function and memory and are especially vulnerable to adolescent ethanol exposure. We have reported that astrocytes, particularly in the mPFC, change both in morphology and synaptic proximity during adolescence. Moreover, adolescent intermittent ethanol (AIE) exposure produces enduring effects on both astrocyte function and synaptic proximity in the adult hippocampal formation, and the latter effect was reversed by the clinically used agent gabapentin (Neurontin), an anticonvulsant and analgesic that is an inhibitor of the VGCC α2δ1 subunit. These findings underscore the importance of investigating AIE effects on astrocytes in the mPFC, a region that undergoes marked changes in structure and connectivity during adolescence. Using astrocyte-specific viral labeling and immunohistochemistry, mPFC astrocytic morphology and colocalization with AMPA-(α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) glutamate receptor 1 (GluA1), an AMPA receptor subunit and established neuronal marker of excitatory synapses, were assessed to quantify the proximity of astrocyte processes with glutamatergic synaptic puncta. AIE exposure significantly reduced astrocyte-synaptic proximity in adulthood, an effect that was reversed by sub-chronic gabapentin treatment in adulthood. There was no effect of AIE on astrocytic glutamate homeostasis machinery or neuronal synaptic proteins in the mPFC. These findings indicate a possible glial-neuronal mechanism underlying the effects of AIE on frontal lobe-mediated behaviors and suggest a specific therapeutic approach for the amelioration of those effects.

Sex differences in the effects of adolescent intermittent ethanol exposure on exploratory and anxiety-like behavior in adult rats.

Adolescent intermittent ethanol (AIE) exposure in rodents has been shown to alter adult behavior in several domains, including learning and memory, social interaction, affective behavior, and ethanol self-administration. AIE has also been shown to produce non-specific behavioral changes that compromise behavioral efficiency. Many studies of these types rely on measuring behavior in mazes and other enclosures that can be influenced by animals' activity levels and exploratory behavior, and relatively few such studies have assessed sex as a biological variable. To address the effects of AIE and its interaction with sex on these types of behavioral assays, male and female adolescent rats (Sprague Dawley) were exposed to 10 doses of AIE (5 g/kg, intra-gastrically [i.g.]), or control vehicle, over 16 days (postnatal day [PND] 30-46), and then tested for exploratory and anxiety-like behaviors on the novelty-induced hypophagia (NIH) task in an open field, the elevated plus (EPM) maze, and the Figure 8 maze. AIE reduced activity/exploratory behaviors in males on the anxiety-producing NIH and EPM tasks, but reduced activity in both males and females in the Figure 8 maze, a task designed to create a safe environment and reduce anxiety. Independent of AIE, females engaged in more rearing behavior than males during the NIH task but less in the EPM, in which they were also less active than males. AIE also increased EPM open arm time in females but not in males. These findings demonstrate previously unrecognized sex differences in the effects of AIE on activity, exploratory behavior, and anxiety-like behavior; additionally, they underscore the need to design future behavioral studies of AIE using sex as a variable and with rigorous attention to how AIE alters these behaviors.

The novel micro-opioid receptor antagonist, [N-allyl-Dmt(1)]endomorphin-2, attenuates the enhancement of GABAergic neurotransmission by ethanol.

AIMS: We investigated the effects of [N-allyl-Dmt(1)]endomorphin-2 (TL-319), a novel and highly potent micro-opioid receptor antagonist, on ethanol (EtOH)-induced enhancement of GABA(A) receptor-mediated synaptic activity in the hippocampus. METHODS: Evoked and spontaneous inhibitory postsynaptic currents (eIPSCs and sIPSCs) were isolated from CA1 pyramidal cells from brain slices of male rats using whole-cell patch-clamp techniques. RESULTS: TL-319 had no effect on the baseline amplitude of eIPSCs or the frequency of sIPSCs. However, it induced a dose-dependent suppression of an ethanol-induced increase of sIPSC frequency with full reversal at concentrations of 500 nM and higher. The non-specific competitive opioid receptor antagonist naltrexone also suppressed EtOH-induced increases in sIPSC frequency but only at a concentration of 60 microM. CONCLUSION: These data indicate that blockade of micro-opioid receptors by low concentrations of [N-allyl-Dmt(1)]endomorphin-2 can reverse ethanol-induced increases in GABAergic neurotransmission and possibly alter its anxiolytic or sedative effects. This suggests the possibility that high potency opioid antagonists may emerge as possible candidate compounds for the treatment of ethanol addiction.

Changes in Neuroimmune and Neuronal Death Markers after Adolescent Alcohol Exposure in Rats are Reversed by Donepezil.

Adolescent intermittent ethanol (AIE) exposure diminishes neurogenesis and dendritic spine density in the dentate gyrus. The cholinesterase inhibitor, donepezil (Aricept), reverses AIE effects on dendritic spines, possibly by interacting with inflammatory and/or epigenetic mediators after AIE exposure. This study tests the hypothesis that donepezil reverses AIE-induced neuroimmune, and epigenetic changes in the adult dentate gyrus. Adolescent Sprague-Dawley male rats (PD30-43) were given 10 intermittent, intragastric doses of ethanol (5.0 g/kg) or isovolumetric water (AIW). Twenty-one days later half of the animals from each group were treated with either donepezil or isovolumetric water (i.g.) once daily for four days. Two hours after the last donepezil or water dose animals were sacrificed and brains prepared for immunohistochemical analyses. AIE reduced immunoreactivity for doublecortin (DCX) and increased immunoreactivity for activated caspase-3 and death receptor-3 in adulthood, suggesting an enduring attenuation of neurogenesis and an increase in progenitor death. These effects were reversed by donepezil treatment in adulthood. AIE also increased immunoreactivity for the inflammatory signaling molecules HMGB1 and RAGE, as well as the activated phosphorylated transcription factor pNFκB p65, and the gene silencing marker dimethylated histone H3K9. All of these AIE effects were also reversed by donepezil, with the exception of HMGB1.

Adolescent Ethanol Exposure Enhances NMDA Receptor-Mediated Currents in Hippocampal Neurons: Reversal by Gabapentin.

Adolescent intermittent ethanol (AIE) exposure compromises neural function into adulthood. We have reported that astrocyte-secreted thrombospondins, and their target neuronal receptors (α2δ-1) are upregulated in the hippocampus in adulthood after AIE, suggesting aberrant excitatory synaptogenesis and hyperexcitability in memory-related circuits. Gabapentin antagonizes the interaction of thrombospondins (TSPs) with the α2δ-1 receptor, and thus may reverse or ameliorate the effects of AIE on hippocampal function. Adolescent rats were exposed to AIE or vehicle. In adulthood, hippocampal slices were prepared. Half of the slices from each animal were pre-incubated in normal artificial cerebrospinal fluid (aCSF) while half were pre-incubated in aCSF containing gabapentin. Whole-cell voltage clamp recordings were then made from CA1 pyramidal cells in normal aCSF. Evoked, N-methyl-D-aspartate (NMDA) receptor-mediated currents were recorded at baseline, and after application of the GluN2B antagonist, RO25-6981. Current amplitudes were higher in neurons from AIE-exposed animals. However, no amplitude increase was observed in neurons from slices that had been pre-incubation in gabapentin. GluN2B antagonism reduced NMDA receptor-mediated currents more efficaciously in cells from AIE-exposed animals, an effect that was also reversed by pre-incubation in gabapentin. These findings identify a mechanism underlying the enduring effects of AIE, and a clinically-utilized agent that may ameliorate those effects.

Behavioral Inefficiency on a Risky Decision-Making Task in Adulthood after Adolescent Intermittent Ethanol Exposure in Rats.

Adolescence is a period of development in neural circuits that are critical for adult functioning. There is a relationship between alcohol exposure and risky decision-making, though the enduring effects of adolescent ethanol exposure on risky decision-making in adulthood have not been fully explored. Studies using positive reinforcement have shown that adolescent intermittent ethanol (AIE) exposure results in higher levels of risky decision-making in adulthood, but the effects of AIE on punishment-mediated decision-making have not been explored. Adolescent rats were exposed to AIE or saline vehicle across a 16-day period, and then allowed to mature into adulthood. They were then trained to lever press for food reward and were assessed for risky decision-making by pairing increased levels of food reward with the probability of footshock punishment. AIE did not alter punishment-mediated risky decision-making. However, it did result in a significant increase in the delay to lever pressing. This finding is consistent with previous reports, using other behavioral tasks, which show decreased behavioral efficiency in adulthood after AIE. These findings indicate that AIE increases behavioral inefficiency, but not punishment-mediated risk-taking, in adulthood. Thus they contribute to a more nuanced understanding of the long-term effects of AIE on adult behavior.

Differential effects of delta9-THC on learning in adolescent and adult rats.

Marijuana use remains strikingly high among young users in the U.S., and yet few studies have assessed the effects of delta9-tetrahydrocannabinol (THC) in adolescents compared to adults. This study measured the effects of THC on male adolescent and adult rats in the Morris water maze. In Experiment 1, adolescent (PD=30-32) and adult (PD=65-70) rats were treated acutely with 5.0 mg/kg THC or vehicle while trained on the spatial version of the water maze on five consecutive days. In Experiment 2, adolescent and adult rats were treated acutely with 2.5 or 10.0 mg/kg THC or vehicle while trained on either the spatial and non-spatial versions of the water maze. In Experiment 3, adolescent and adult rats were treated with 5.0 mg/kg THC or vehicle daily for 21 days, and were trained on the spatial and then the non-spatial versions of the water maze task four weeks later in the absence of THC. THC impaired both spatial and nonspatial learning more in adolescents than in adults at all doses tested. However, there were no long-lasting significant effects on either spatial or non-spatial learning in rats that had been previously exposed to THC for 21 days. This developmental sensitivity is analogous to the effects of ethanol, another commonly used recreational drug.

Antagonism by d-amphetamine of trimethyltin-induced hyperactivity evidence toward an animal model of hyperkinetic behavior.

In male rats of the Long-Evans strain, either 7.0 mg/kg of trimethyltin (TMT) or 0.9% NaCl was administered by intragastric gavage. After a period of recovery from the typical signs of trimethyltin toxicity, each rat was tested at 72-hr intervals for its locomotor activity in an open field apparatus, the floor of which was divided into square grids. The baseline activity of each of the trimethyltin-treated rats was significantly greater than the saline-treated controls. d-Amphetamine, injected intraperitoneally in a dose of 0.5 or 2.0 mg/kg, augmented the hyperactivity of the trimethyltin-treated animals. However, a 4.0 mg/kg dose of d-amphetamine markedly attenuated the hyperactivity of trimethyltin-treated rats while elevating that of the controls. Since trimethyltin produced an autism-like behavioral disorder involving hyperactivity, preservation, aggressiveness and impairment in problem-solving and memory function, the placating effect of amphetamine supports the proposition that the pathology due to trimethyltin may represent an experimental analogue to the hyperkinetic syndrome in children.

The drug MK-801 attenuates the development, but not the expression, of long-term potentiation and stimulus train-induced bursting in hippocampal slices.

Recent studies have demonstrated that (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), a use-dependent blocker of N-methyl-D-aspartate (NMDA)-activated membrane channels, attenuates the development of long-term potentiation in vitro and kindling in vivo. Both of these phenomena are manifestations of physiological plasticity related to behavioural states and the results of these studies add to the gathering evidence for the involvement of the NMDA receptor/channel system in such processes. In the present experiment, slices of hippocampus, prepared from rats, were electrically stimulated to produce either long-term potentiation of the CA1 population spike or stimulus train-induced epileptiform bursting in area CA3. At 10 microM, MK-801 attenuated the development of long-term potentiation, but had no attenuating effect upon the previously-potentiated population spike. Similarly, 10 microM MK-801 attenuated the development of epileptiform activity in area CA3, but had little or no effect on the previously-established bursting in area CA3. These data support the suggestion that MK-801 exerts an antiepileptogenic, but not an anticonvulsant effect, at concentrations which also inhibit long-term potentiation.

Ethanol and magnesium ions inhibit N-methyl-D-aspartate-mediated synaptic potentials in an interactive manner.

The role of magnesium ions in the inhibitory effect of ethanol on NMDA receptor-mediated population synaptic potentials (pEPSPs) in area CA1 of the hippocampus of the adult rat, was studied. The excitatory amino acid (non-NMDA) receptor antagonist, DNQX and the GABAA channel antagonist, picrotoxin, were used to pharmacologically isolate NMDA-mediated pEPSPs. In the presence of a physiological concentration of magnesium (1.0 mM), ethanol (25-100 mM) inhibited NMDA-mediated pEPSPs, with an apparent EC50 of approximately 50 mM. The ability of ethanol to inhibit NMDA-mediated pEPSPs was reduced when the slices were incubated in the absence of magnesium. Concentrations of ethanol, in the range of 50-200 mM (apparent EC50 100 mM), were required to inhibit NMDA-mediated pEPSPs, in the absence of added magnesium. Combination studies of these two antagonists indicated that the sensitivity of NMDA-mediated pEPSPs to one antagonist was not altered by the presence of the other. This finding suggests that the affinity of each antagonist binding site is not affected by the presence of the other antagonist. In the case of ethanol, its low maximum antagonist efficacy may require larger concentrations of ethanol to inhibit NMDA-mediated pEPSPs, in the absence of other non-competitive antagonists such as magnesium.

Age-related differences in neurosteroid potentiation of muscimol-stimulated 36Cl(-) flux following chronic ethanol treatment.

Alcoholism and alcohol abuse create costly social and economic problems in many nations. Recent studies indicate that alcohol exposure during adolescence may convey unique risks for subsequent neurocognitive deficits and problem drinking. Although GABA(A) receptor function is one of the principle neurochemical targets of ethanol action in the adult brain, little is known about the effects of alcohol on this system during adolescence. Adolescent (30-day-old) and adult (90-day-old) male rats were intermittently exposed to ethanol for 1 month. At various times after the end of the exposure period, synaptoneurosomes were prepared from their cerebral cortices. GABA(A) receptor-mediated 36Cl(-) influx was measured in the absence and presence of the neurosteroid 3alpha,21-dihydroxy-5alpha-pregnan-20-one (THDOC). In tissue from ethanol-exposed animals, sensitization to the potentiating effects of the neurosteroid was apparent 5 and 12 days after ethanol withdrawal. This sensitization was more apparent at the low concentrations of THDOC in animals pretreated with ethanol as adolescents. Sensitization to the potentiating effects of a neurosteroid is an enduring phenomenon, persistent long after the acute phase of ethanol withdrawal, and may be indicative of long-term changes in GABA(A) receptor function. Enhanced neurosteroid sensitization in animals pretreated as adolescents is consistent with the notion that adolescence is a period of unique sensitivity to the effects of ethanol. This uniqueness may now be extended to the chronic effects of ethanol.

Hippocampal long-term potentiation and spatial learning in the rat: effects of GABAB receptor blockade.

This series of experiments assessed the role of GABAB receptors in the induction of long-term potentiation in the dentate gyrus in vivo, and spatial learning and memory in three different tasks. In urethane-anesthetized rats, the GABAB receptor antagonist CGP 46381 was injected intraperitoneally at a dose which effectively suppressed GABAB-mediated paired pulse disinhibition. Theta-burst stimulation reliably produced long-term potentiation in control rats. However, GABAB receptor blockade significantly suppressed the induction of long-term potentiation in the dentate gyrus. To compare the results of the long-term potentiation experiments with behavior, we assessed the performance of rats on several spatial learning and memory tasks in the presence of CGP 46381. We found that the working memory performance of highly trained rats on the eight-arm radial maze was unaffected by CGP 46381. There was also no effect of GABAB receptor blockade on learning in the eight-arm maze using a five-trial repeated acquisition paradigm. However, when we tested spatial learning in naive rats using a mildly stressful water maze task, we found that CGP 46381 substantially impaired both the latency to find the platform and the path-length travelled in the maze during acquisition. CGP 46381-treated rats took longer to learn the location of the escape platform and travelled a greater distance over the acquisition trials. These data demonstrate that GABAB receptor blockade results in a suppression of hippocampal long-term potentiation in vivo and impairs spatial learning in a task where stress may be a component of performance.

Acute treatment with trimethyltin alters alcohol self-selection.

Male rats of the Long-Evans strain were divided into two equal groups of nine each and given either 7.0 mg/kg trimethyltin (TMT) or 0.9% saline by intragastric gavage. The pattern of self-selection of alcohol in concentrations of 3%--30% was examined in both groups at 21 and 150 days following the gavage. The TMT-treated rat consistently drank less alcohol than did the controls at every concentration of alcohol. This difference in alcohol intake was equally significant when the rats were tested in a food-contingent, schedule-induced polydipsia situation. Further, although the TMT-lesioned animal consumed fewer calories per day in the form of alcohol, their overall daily caloric intakes were slightly higher than those of the controls. These results are interpreted as a consequence of damage to structures of the forebrain and as part of a syndrome of behavioral and neurological pathology.

Potential interactions between GABAb and cholinergic systems: baclofen augments scopolamine-induced performance deficits in the eight-arm radial maze.

Sixteen male Fischer-344N rats were trained on a eight-arm radial maze task for food reinforcement. The effects of various doses of baclofen (1.25 or 2.50 mg/kg) and scopolamine (0.188, 0.375, and 0.750 mg/kg) were determined alone and in combination. Relative to vehicle controls, baclofen alone did not affect performance in the radial arm maze (number correct in the first eight responses, total errors) or the time required to complete the maze. Scopolamine alone decreased the number of correct responses in the first eight arm choices, while increasing both the number of errors and the time necessary to complete the maze. When the two drugs were co-administered, baclofen had no effect on the number of errors or time required to complete the maze in the presence of scopolamine; however, in combination with the high dose of scopolamine, it significantly increased the number of errors made during the first eight choices. Baclofen thus can exacerbate some radial arm maze dificits produced by an anticholinergic drug. In a subsequent experiment to test the interaction between scopolamine and baclofen using a nonlearned behavior, baclofen (1.25 and 2.5 mg/kg) did not affect motor activity, whereas all doses of scopolamine (0.188-0.75 mg/kg) increased activity. The higher dose of baclofen attenuated scopolamine-induced hypermotility by 50%, but the lower dose of baclofen was not effective. These data demonstrate pharmacological interactions between baclofen, a drug used clinically for spaticity, and a drug having anticholinergic properties.

Baclofen disrupts passive avoidance retention in rats.

Baclofen (Lioresal, Ciba-Geigy) is an analog of the inhibitory neurotransmitter GABA and is used clinically to control spasticity. Recent studies have demonstrated that this compound produces a marked inhibition of synaptically evoked responses in area CA3 of the hippocampal slice, suggesting that this drug could influence behavior mediated by the limbic system. In the present study, male rats of the Fischer-344 strain were trained on a one-trial passive avoidance task and tested for retention 1 week later. After the training trial, separate groups of rats received either 5 or 10 mg/kg/4 ml IP of baclofen or the distilled H2O vehicle immediately, 10 min, or 60 min after training. One week later, the rats that received baclofen immediately after training reentered the test chamber with a significantly higher frequency than controls, although no differences in vacillatory responses were observed between groups. Similar effects were observed following posttrial administration of chlordiazepoxide. In a separate experiment rats were tested for locomotor activity after receiving the same doses of baclofen. Although baclofen decreased activity during a 30-min period after dosing, rats exposed to baclofen showed no significant change in activity relative to controls 1 week later. These data are consistent with the interpretation that baclofen may interfere with memory consolidation or retention.

Ethanol inhibition of NMDA mediated depolarizations is increased in the presence of Mg2+.

The inhibitory potency of ethanol upon excitatory amino acid induced depolarizations of rat hippocampal CA1 pyramidal cells was assessed in the presence and absence of magnesium (Mg2+) using the grease-gap technique. Ethanol shifted the N-methyl-D-aspartate (NMDA) dose-response curves to the right in a non-parallel manner. In the presence of Mg2+, ethanol appeared to be a more effective NMDA antagonist (IC50 47 mM) than in the absence of Mg2+ (IC50 107 mM). The IC50 for ethanol upon non-NMDA mediated CA1 pyramidal cell depolarizations was in excess of 170 mM. These results strongly suggest a preferential inhibitory action of ethanol against NMDA, rather than non-NMDA, mediated responses. Experiments in which ethanol and Mg2+ were covaried indicated that these substances act by two distinct mechanisms to antagonize the action of NMDA. These effects of ethanol, at concentrations which elicit intoxication (less than 50 mM) but not anesthesia, suggest that the NMDA receptor complex may play an important role in the acute effects of ethanol.