Post-learning ethanol effects on a water-finding task in rats

The entorhinal cortex, but not the dorsal hippocampus, is necessary for single-cue latent learning

Two experiments were conducted to examine the roles of the entorhinal cortex (EC), dorsal hippocampus (DH), and ventral hippocampus (VH) in a modified Latent Cue Preference (LCP) task. The modified LCP task utilized one visual cue in each compartment, compared to several multimodal cues used in a previous version. In the single-cue LCP task, water-replete rats drink water in one compartment of the LCP box on 1 day, and then have no water in a second compartment of the LCP box the following day (one training trial), for a total of three training trials. Rats are then water-deprived prior to a preference test, in which they are allowed to move freely between the two compartments with the water removed. Latent learning is demonstrated when water-deprived rats spend more time in the compartment that previously contained the water. Experiment 1 demonstrated that the single-cue LCP task results in the same irrelevant-incentive latent learning as the multicue LCP task. In addition, Experiment 1 replicated the finding that a compartment preference based on this latent learning requires a deprivation state during the preference test, while a compartment preference based on conditioning does not. Experiment 2 examined the effects of pretraining neurotoxin lesions of the EC, DH, and VH on this single-cue LCP task. Results showed that lesions of the EC and VH disrupted the irrelevant-incentive latent learning, while lesions of the DH did not. These results indicate that a latent learning task that involves one discrete compartment cue, rather than several compartmental cues, does not require the DH. Therefore, the EC appears to play a central role in single-cue latent learning in the LCP task.

Ethanol enhances reactivated fear memories

Although ethanol has been shown to impair acquisition of memory, its effect on consolidated memories is not clear. Recent reports revealed that memory retrieval converted consolidated memory into a labile state and initiated the reconsolidation process. In the present study, we have demonstrated the effect of ethanol on reactivated fear memory. We used contextual fear conditioning where rats were conditioned with mild footshock, re-exposed to the training context for 2 min, immediately injected with ethanol or saline, and finally tested 48 h after re-exposure. Ethanol-treated groups demonstrated longer freezing and the effect lasted for 2 weeks. Reactivation is necessary for this effect. Injection of ethanol itself did not induce a fearful response. Reactivated and ethanol-treated rats exhibited longer freezing than non-reactivated controls, suggesting that ethanol does not inhibit the memory decline but facilitates the fear memory. Two minute re-exposures induced no or little extinction. The effect of ethanol was specific for 2-min reactivation, which induces reconsolidation. Moreover, we found that picrotoxin inhibited the memory enhancement that was produced by ethanol administered just after the reactivation. These studies demonstrate that ethanol enhances reactivated contextual fear memories via activation of GABA(A) receptors.

Brain catalase mediates potentiation of social recognition memory produced by ethanol in mice

The involvement of catalase in ethanol-induced locomotion has been clearly proven. However, studies addressing the role of this enzyme in the effects that ethanol exerts on memory are lacking. In the present study, the social recognition test (SRT) was used to evaluate ethanol effects on memory. In this test, the reduction in investigation time of a juvenile conspecific, when this social stimulus is presented for the second time, is considered a reliable index of memory. Exploration ratios (ER) were calculated to evaluate the recognition capacity of mice. Ethanol (0.0, 0.5, 1.0 or 1.5g/kg, i.p.) was administered immediately after the first juvenile presentation, and 2h later the juvenile was re-exposed to the adult. Additionally, adult mice received aminotriazole (AT) or sodium azide (two catalase inhibitors) 5h or 30 min before juvenile presentation, respectively. Ethanol (1.0 and 1.5g/kg) was able to reduce ER, indicating an improving effect on memory. This improvement was prevented by either AT or sodium azide pre-treatment. However, neither AT nor sodium azide attenuated the memory-enhancing capacity of NMDA or nicotine, suggesting a specific interaction between catalase inhibitors and ethanol in their effects on memory. The present results suggest that brain catalase activity could mediate the memory-enhancing capacity of ethanol and add further support to the idea that this enzyme mediates some of the psychopharmacological effects produced by ethanol.

Facilitation of short-term social memory by ethanol in rats is mediated by dopaminergic receptors

Ethanol is a drug that has apparently opposite effects on memory processes depending on when it is given relative to the task, as well as the nature of the task under study. Recently, we demonstrated that acute low doses of ethanol (0.5 and 1.0 g/kg, i.p.) improve the short-term social memory in rats in a specific and time-dependent manner, and that this action is, at least in part, related to opioid, but not to muscarinic receptors. In the present study, we evaluated whether this positive effect of ethanol on the short-term memory of rats is related to a reducing impact of interference during the task through two different procedures: the introduction of an unfamiliar juvenile rat or the placing of the adult rat in the open field during the inter-exposure interval. The actions of reserpine (0.4 and 0.8 mg/kg, s.c.), haloperidol (0.05 and 0.2 mg/kg, i.p.), the D2 receptor antagonist sulpiride (20.0 and 50.0 mg/kg, i.p.) and the D1 receptor antagonist SCH 23390 (0.01 and 0.03 mg/kg, s.c.) and their interaction with ethanol (1.0 g/kg, i.p.) in relation to short-term memory were also studied. The administration of ethanol (1.0 g/kg, i.p.), immediately after the end of the first presentation, did not reduce the effect on social memory of the introduction of an unfamiliar juvenile or placing the adult rat in the open field during the inter-exposure interval. The facilitatory effect of ethanol on social memory was inhibited by the pretreatment with reserpine and it was antagonized by the administration of haloperidol or sulpiride, but not by SCH 23390. These results indicate that the facilitation of short-term social memory by ethanol is not related to a reduction in the deleterious impact of interference and that this action of ethanol is mediated, at least in part, by D2 receptors, but not by D1 dopaminergic receptors.

Ethanol improves short-term social memory in rats. Involvement of opioid and muscarinic receptors

Some human and animal studies have demonstrated enhancement of memory processes when ethanol was administered immediately after training and subjects were later tested in the drug-free state. The aim of this study was to evaluate the effect of acute ethanol administration (0.5, 1.0 and 2.0 g/kg) by intraperitoneal (i.p.) and oral route on short-term memory, using the social recognition test in rats. The actions of scopolamine (0.06 and 0.5 mg/kg, i.p.) and naloxone (1.0 mg/kg, i.p.) and their interaction with ethanol in relation to short-term memory were also studied. The doses of ethanol used did not show any sedative effect, which was assessed by measuring locomotor activity. The results indicate that acute low doses of ethanol (0.5 and 1.0 g/kg, i.p.) improve the short-term olfactory memory in rats in a specific and time-dependent manner, and that this action is, at least in part, related to opioid, but not to muscarinic receptors. In addition, these findings confirm that the social recognition test in rats is a useful and reliable model to investigate short-term memory affected by ethanol.

Effects of moderate alcohol consumption on the central nervous system

The concept of moderate consumption of ethanol (beverage alcohol) has evolved over time from considering this level of intake to be nonintoxicating and noninjurious, to encompassing levels defined as "statistically" normal in particular populations, and the public health-driven concepts that define moderate drinking as the level corresponding to the lowest overall rate of morbidity or mortality in a population. The various approaches to defining moderate consumption of ethanol provide for a range of intakes that can result in blood ethanol concentrations ranging from 5 to 6 mg/dl, to levels of over 90 mg/dl (i.e., approximately 20 mM). This review summarizes available information regarding the effects of moderate consumption of ethanol on the adult and the developing nervous systems. The metabolism of ethanol in the human is reviewed to allow for proper appreciation of the important variables that interact to influence the level of exposure of the brain to ethanol once ethanol is orally consumed. At the neurochemical level, the moderate consumption of ethanol selectively affects the function of GABA, glutamatergic, serotonergic, dopaminergic, cholinergic, and opioid neuronal systems. Ethanol can affect these systems directly, and/or the interactions between and among these systems become important in the expression of ethanol's actions. The behavioral consequences of ethanol's actions on brain neurochemistry, and the neurochemical effects themselves, are very much dose- and time-related, and the collage of ethanol's actions can change significantly even on the rising and falling phases of the blood ethanol curve. The behavioral effects of moderate ethanol intake can encompass events that the human or other animal can perceive as reinforcing through either positive (e.g., pleasurable, activating) or negative (e.g., anxiolysis, stress reduction) reinforcement mechanisms. Genetic factors and gender play an important role in the metabolism and behavioral actions of ethanol, and doses of ethanol producing pleasurable feelings, activation, and reduction of anxiety in some humans/animals can have aversive, sedative, or no effect in others. Research on the cognitive effects of acute and chronic moderate intake of ethanol is reviewed, and although a number of studies have noted a measurable diminution in neuropsychologic parameters in habitual consumers of moderate amounts of ethanol, others have not found such changes. Recent studies have also noted some positive effects of moderate ethanol consumption on cognitive performance in the aging human. The moderate consumption of ethanol by pregnant women can have significant consequences on the developing nervous system of the fetus. Consumption of ethanol during pregnancy at levels considered to be in the moderate range can generate fetal alcohol effects (behavioral, cognitive anomalies) in the offspring. A number of factors--including gestational period, the periodicity of the mother's drinking, genetic factors, etc.--play important roles in determining the effect of ethanol on the developing central nervous system. A series of recommendations for future research endeavors, at all levels, is included with this review as part of the assessment of the effects of moderate ethanol consumption on the central nervous system.

Ethanol affects context memory and long-term habituation in the crab Chasmagnathus

A shadow moving overhead acts as a danger stimulus and elicits an escape response in the crab Chasmagnathus granulatus that habituates promptly and for a long period. The effect of acute ethanol treatment on this long-term memory was analyzed. A single injection of 0.01, 0.05, or 0.1 micrograms ethanol (ET)/g given 30 min before iterated presentation of a visual danger stimulus failed to affect short-term habituation. Posttraining ethanol (0.01 to 0.1 microgram/g) produces a dose-dependent impairment of long-term habituation, but pretraining ethanol had no amnestic effect. However, a retention deficit confined to context memory was disclosed with both pre- and posttraining ethanol. Results from experiments with double injection (posttraining and pretesting injections) account for the retention impairment in terms of true amnesia (failure to acquire memory) but not due to state-dependence or retrieval deficit. The nonamnestic effect of pretraining ethanol upon long-term habituation is explained by a nonespecific depressing effect caused by interaction between iterative presentation of the danger stimulus and drug-induced internal state during training.

Effects of post-training ethanol and group housing upon memory of an appetitive task in mice

It has been shown that post-training ethanol's facilitating effects upon memory disappeared if the mice were kept isolated after training. Since ethanol-treated mice were attacked by their cagemates, it has been hypothesized that the improved retention induced by ethanol resulted from an interaction between ethanol and group housing which added aversive information to training. To investigate the correctness of this interpretation, ethanol effects upon memory of an appetitive task were studied. C57BL/6J mice (isolated the day before training) were individually trained to find a cheese pellet placed in a corner of an open-field. Mice were injected intraperitoneally immediately after training with saline, 0.5, 1.5, or 2.0 g/kg of ethanol. They were then returned to their home cage and left alone, with another mouse, or with five other mice for 2 h after training. All mice were tested 24 h later for retention. Reductions in the number of pellet approaches or in the latency to eat the pellet were taken as measures of learning. Post-training ethanol disrupted retention of the appetitive task in a dose-related manner. Moreover disruption was greater in mice group housed after training. The results support the hypothesis that ethanol's post-training facilitating effects upon aversive memory may be due to added aversive information to the stimulus complex, rather than, or in addition, to enhanced storage of memory traces.

Behavioral factors in development of tolerance to ethanol's effects

Dose-effect analyses were used to monitor the development of tolerance for ethanol's effects on FR30 operant performance in rats under different conditions of chronic ethanol exposure: (a) pre-session ethanol injections (PRE) vs. post-session ethanol injections (POST) in Experiment 1; and (b) an ethanol liquid diet (ED) vs. a control diet (CD) in Experiment 2. The PRE and ED groups developed tolerance at the conclusion of the chronic regimens, which declined by six months but not to baseline levels. These data suggest that tolerance results from learned compensatory adjustments (through intoxicated practice) to ethanol's disruptive effects. The POST, but not the CD, group developed a progressively increasing degree of tolerance after several ethanol challenge tests. These results suggest that some threshold level of passive ethanol exposure in the POST group interacted with their limited intoxicated practice. Finally, the tolerance developed under intoxicated practice conditions did not appear to reflect a generalized tolerance to rate-reducing properties of drugs, changes in ethanol kinetics, or age-related changes.