Memory facilitation by post-training injection of ethanol

The effects of passive and active administration of heroin, and associated conditioned stimuli, on consolidation of object memory

Mode of administration (i.e., active vs passive) could influence the modulatory action that drugs of abuse exert on memory consolidation. Similarly, drug conditioned stimuli modulate memory consolidation and, therefore, acquisition and extinction of this conditioned response could also be influenced by mode of drug administration. Exploring these questions in male Sprague-Dawley rats, Study 1 assessed memory modulation by post-training 0, 0.3 and 1 mg/kg heroin injected subcutaneously in operant chambers (i.e., drug conditioned context). Study 2 asked a similar question but in rats trained to self-administer 0.05 mg/kg/infusion heroin intravenously, as well as in rats that received identical amounts of intravenous heroin but passively, using a yoked design. The period of heroin exposure was followed by repeated drug-free confinement in the conditioned context, and by sessions during which responses on the active lever had no scheduled consequences. Study 2 also included a cue-induced reinstatement session during which lever responses reactivated a light cue previously paired with intravenous heroin infusions. The post-training effects of injected/self-administered/yoked heroin, extinction and reinstatement sessions on memory consolidation were tested using the object location memory task. It was found that post-sample heroin enhanced memory in injected and yoked, but not self-administering, rats. However, post-sample exposure to the heroin cues (i.e., context or/and light cue) modulated memory equally in all groups. Taken together, these data support the conclusion that mode of administration impacts the cognitive consequences of exposure to drugs but not of environmental stimuli linked to their reinforcing effects.

The Effects of Post-Learning Alcohol Ingestion on Human Motor Memory Consolidation

The neurochemical mechanisms underlying motor memory consolidation remain largely unknown. Based on converging work showing that ethyl alcohol retrogradely enhances declarative memory consolidation, this work tested the hypothesis that post‐learning alcohol ingestion would enhance motor memory consolidation. In a within‐subject and fully counterbalanced design, participants (n = 24; 12M; 12F) adapted to a gradually introduced visual deviation and ingested, immediately after adaptation, a placebo (PBO), a medium (MED) or high (HIGH) dose of alcohol. The alcohol doses were bodyweight‐ and gender‐controlled to yield peak breath alcohol concentrations of 0.00% in the PBO, ~0.05% in the MED and ~0.095% in the HIGH condition. Retention was evaluated 24 h later through reach aftereffects when participants were sober. The results revealed that retention levels were neither significantly nor meaningfully different in both the MED and HIGH conditions as compared to PBO (all absolute Cohen's d_z values < ~0.2; small to negligible effects), indicating that post‐learning alcohol ingestion did not alter motor memory consolidation. Given alcohol's known pharmacological GABAergic agonist and NMDA antagonist properties, one possibility is that these neurochemical mechanisms do not decisively contribute to motor memory consolidation. As converging work demonstrated alcohol's retrograde enhancement of declarative memory, the present results suggest that distinct neurochemical mechanisms underlie declarative and motor memory consolidation. Elucidating the neurochemical mechanisms underlying the consolidation of different memory systems may yield insights into the effects of over‐the‐counter drugs on everyday learning and memory but also inform the development of pharmacological interventions seeking to alter human memory consolidation.

Reinforcement History Dependent Effects of Low Dose Ethanol on Reward Motivation in Male and Female Mice

Alcohol use disorders (AUDs) are more prevalent in men than in women, though AUD diagnoses in women are growing rapidly, making an understanding of sex differences in alcohol-related behaviors increasingly important. The development of AUDs involves the transition from casual, low levels of alcohol drinking to higher, maladaptive levels. The ability of low dose alcohol to drive reward and drug seeking may differ in males and females, and this could underlie differences in susceptibility to AUD. In this study we sought to determine whether a history of chronic, low dose ethanol exposure (0.5 g/kg; i.p.) could drive sucrose reward seeking and motivation, and whether this differed between male and female mice. Adult mice were trained to lever press for a liquid sucrose reward on two reinforcement schedules: a random interval (RI) schedule and a variable ratio (VR) schedule. After training, mice were tested on each of these levers for reward motivation using a progressive ratio test. We found that a history of low dose ethanol exposure increased sucrose reward motivation in male mice, but only on the RI lever and only when exposure occurred proximal to learning. Female mice were more motivated for sucrose on the RI lever than the VR lever regardless of ethanol exposure condition. These findings indicate that training on different reinforcement schedules affects reward motivation. Further, we show that males are more susceptible to the effects of low dose ethanol on sucrose reward motivation than females. These data broaden our understanding of sex differences in reward seeking as a result of ethanol exposure.

Opioid withdrawal and memory consolidation

It is well established that learning and memory are central to substance dependence. This paper specifically reviews the effect of opioid withdrawal on memory consolidation. Although there is evidence that opioid withdrawal can interfere with initial acquisition and retrieval of older memories, there are several reasons to postulate a facilitatory action on the consolidation of newly acquired memories. In fact, there is substantial evidence that memory consolidation is facilitated by the release of stress hormones, that it requires the activation of the amygdala, of central noradrenergic and cholinergic pathways, and that it involves long-term potentiation. This review highlights evidence that very similar neurobiological processes are involved in opioid withdrawal, and summarizes recent results indicating that naltrexone-precipitated withdrawal enhanced consolidation in rats. From this neurocognitive perspective, therefore, opioid use may escalate during the addiction cycle in part because memories of stimuli and actions experienced during withdrawal are strengthened.

Ethanol facilitates socially evoked memory recall in mice by recruiting pain-sensitive anterior cingulate cortical neurons

Alcohol is a traditional social-bonding reinforcer; however, the neural mechanism underlying ethanol-driven social behaviors remains elusive. Here, we report that ethanol facilitates observational fear response. Observer mice exhibited stronger defensive immobility while observing cagemates that received repetitive foot shocks if the observer mice had experienced a brief priming foot shock. This enhancement was associated with an observation-induced recruitment of subsets of anterior cingulate cortex (ACC) neurons in the observer mouse that were responsive to its own pain. The vicariously activated ACC neurons projected their axons preferentially to the basolateral amygdala. Ethanol shifted the ACC neuronal balance toward inhibition, facilitated the preferential ACC neuronal recruitment during observation, and enhanced observational fear response, independent of an oxytocin signaling pathway. Furthermore, ethanol enhanced socially evoked fear response in autism model mice.

Drinkers' memory bias for alcohol picture cues in explicit and implicit memory tasks

BACKGROUND

Alcohol cues can bias attention and elicit emotional reactions, especially in drinkers. Yet, little is known about how alcohol cues affect explicit and implicit memory processes, and how memory for alcohol cues is affected by acute alcohol intoxication.

METHODS

Young adult participants (N=161) were randomly assigned to alcohol, placebo, or control beverage conditions. Following beverage consumption, they were shown neutral, emotional and alcohol-related pictures cues. Participants then completed free recall and repetition priming tasks to test explicit and implicit memory, respectively, for picture cues. Average blood alcohol concentration for the alcohol group was 74±13mg/dl when memory testing began. Two mixed linear model analyses were conducted to examine the effects of beverage condition, picture cue type, and their interaction on explicit and implicit memory.

RESULTS

Picture cue type and beverage condition each significantly affected explicit recall of picture cues, whereas only picture cue type significantly influenced repetition priming. Individuals in the alcohol condition recalled significantly fewer pictures than those in other conditions, regardless of cue type. Both free recall and repetition priming were greater for emotional and alcohol-related cues compared to neutral picture cues. No interaction effects were detected.

CONCLUSIONS

Young adult drinkers showed enhanced explicit and implicit memory processing of alcohol cues compared to emotionally neutral cues. This enhanced processing for alcohol cues was on par with that seen for positive emotional cues. Acute alcohol intoxication did not alter this preferential memory processing for alcohol cues over neutral cues.

Effect of Alcohol and Tobacco Smoke on Long-Term Memory and Cell Proliferation in the Hippocampus of Rats

INTRODUCTION

Alcohol is frequently used in combination with tobacco and few studies explore interactions between these two drugs of abuse. Here, we evaluated the effect of chronic alcohol administration and concomitant exposure to tobacco smoke on long-term memory and on cell proliferation in the hippocampus of rats.

METHODS

Forty male Wistar rats were assigned to four groups and treated with alcohol (2g/kg by gavage) and/or exposed to tobacco smoke (from six cigarettes, by inhalation) twice a day (at 9:00 AM and 2:00 PM) for 30 days. Long-term memory was evaluated in the inhibitory avoidance test and hippocampal cell proliferation was analyzed for bromodeoxyuridine immunohistochemistry.

RESULTS

Our results showed that alcohol, tobacco smoke, or their combination improved the long-term memory evaluated by the memory index in rats. Moreover, alcohol and tobacco coadministration decreased bromodeoxyuridine-labeled cells by 60% when compared to the control group, while alcohol treatment decreased labeled cells by 40%. The tobacco group showed a nonsignificant 26% decrease in labeled cells compared to the control group.

CONCLUSIONS

Chronic alcohol and tobacco coadministration improves the long-term memory in rats in the inhibitory avoidance test. However, coadministration decreases the cell proliferation in the hippocampus of rats, suggesting a deleterious effect by the combined use of these drugs of abuse.

The Impact of Caffeine on the Behavioral Effects of Ethanol Related to Abuse and Addiction: A Review of Animal Studies

The impact of caffeine on the behavioral effects of ethanol, including ethanol consumption and abuse, has become a topic of great interest due to the rise in popularity of the so-called energy drinks. Energy drinks high in caffeine are frequently taken in combination with ethanol under the popular belief that caffeine can offset some of the intoxicating effects of ethanol. However, scientific research has not universally supported the idea that caffeine can reduce the effects of ethanol in humans or in rodents, and the mechanisms mediating the caffeine-ethanol interactions are not well understood. Caffeine and ethanol have a common biological substrate; both act on neurochemical processes related to the neuromodulator adenosine. Caffeine acts as a nonselective adenosine A1 and A2A receptor antagonist, while ethanol has been demonstrated to increase the basal adenosinergic tone via multiple mechanisms. Since adenosine transmission modulates multiple behavioral processes, the interaction of both drugs can regulate a wide range of effects related to alcohol consumption and the development of ethanol addiction. In the present review, we discuss the relatively small number of animal studies that have assessed the interactions between caffeine and ethanol, as well as the interactions between ethanol and subtype-selective adenosine receptor antagonists, to understand the basic findings and determine the possible mechanisms of action underlying the caffeine-ethanol interactions.

Alcohol-induced retrograde memory impairment in rats: prevention by caffeine

RATIONALE

Ethanol and caffeine are two of the most widely consumed drugs in the world, often used in the same setting. Animal models may help to understand the conditions under which incidental memories formed just before ethanol intoxication might be lost or become difficult to retrieve.

OBJECTIVES

Ethanol-induced retrograde amnesia was investigated using a new odor-recognition test.

MATERIALS AND METHODS

Rats thoroughly explored a wood bead taken from the cage of another rat, and habituated to this novel odor (N1) over three trials. Immediately following habituation, rats received saline, 25 mg/kg pentylenetetrazol (a seizure-producing agent known to cause retrograde amnesia) to validate the test, 1.0 g/kg ethanol, or 3.0 g/kg ethanol. The next day, they were presented again with N1 and also a bead from a new rat's cage (N2).

RESULTS

Rats receiving saline or the lower dose of ethanol showed overnight memory for N1, indicated by preferential exploration of N2 over N1. Rats receiving pentylenetetrazol or the higher dose of ethanol appeared not to remember N1, in that they showed equal exploration of N1 and N2. Caffeine (5 mg/kg), delivered either 1 h after the higher dose of ethanol or 20 min prior to habituation to N1, negated ethanol-induced impairment of memory for N1. A combination of a phosphodiesterase-5 inhibitor and an adenosine A(2A) antagonist, mimicking two major mechanisms of action of caffeine, likewise prevented the memory impairment, though either drug alone had no such effect. Binge alcohol can induce retrograde, caffeine-reversible disruption of social odor memory storage or recall.

Post-training ethanol disrupts trace conditioned fear in rats: effects of timing of ethanol, dose and trace interval duration

Ethanol has complex effects on memory performance, although hippocampus-dependent memory may be especially vulnerable to disruption by acute ethanol intoxication occurring during or shortly after a training episode. In the present experiments, the effects of post-training ethanol on delay and trace fear conditioning were examined in adolescent rats. In Experiment 1, 30-day-old Sprague-Dawley rats were given delay or trace conditioning trials in which a 10s flashing light CS was paired with a 0.5 mA shock US. For trace groups, the trace interval was 10 s. On days 31-33, animals were administered ethanol once daily (0.0 or 2.5 g/kg via intragastric intubation), and on day 34 animals were tested for CS-elicited freezing. Results showed that post-training ethanol affected the expression of trace, but had no effect on delay conditioned fear. Experiment 2 revealed that this effect was dose-dependent; doses lower than 2.5 g/kg were without effect. Experiment 3 evaluated whether proximity of ethanol to the time of training or testing was critical. Results show that ethanol administration beginning 24h after training was more detrimental to trace conditioned freezing than administration that was delayed by 48 h. Finally, in Experiment 4 animals were trained with one of three different trace intervals: 1, 3 or 10s. Results indicate that post-training administration of 2.5 g/kg ethanol disrupted trace conditioned fear in subjects trained with a 10s, but not with a 1 or 3s, trace interval. Collectively the results suggest that ethanol administration impairs post-acquisition memory processing of hippocampus-dependent trace fear conditioning.

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.

Paradoxical facilitatory effect of low-dose alcohol consumption on memory mediated by NMDA receptors

Epidemiological studies have suggested a negative correlation between alcohol intake and Alzheimer's disease. In vitro, ethanol negatively modulates NMDA receptor function. We hypothesized that chronic moderate alcohol intake leads to improved memory via adaptive responses in the expression of NMDA receptors and downstream signaling. We fed liquid diets containing no, moderate, or high amounts of ethanol to control and matched rats with hippocampal knock-down of the NR1 subunit. Rats with increased hippocampal NR1 expression were also generated to determine whether they had a phenotype similar to that of ethanol-fed animals. We found that moderate ethanol intake improved memory, increased NR1 expression, and changed some aspects of neurotrophin signaling. NR1 knock-down prevented ethanol's facilitatory effects, whereas hippocampal NR1 overexpression mimicked the effect of chronic low-dose ethanol intake on memory. In contrast, high-dose ethanol reduced neurogenesis, inhibited NR2B expression, and impaired visual memory. In conclusion, adaptive changes in hippocampal NMDA receptor expression may contribute to the positive effects of ethanol on cognition.

Acute ethanol has biphasic effects on short- and long-term memory in both foreground and background contextual fear conditioning in C57BL/6 mice

BACKGROUND

Ethanol is a frequently abused, addictive drug that impairs cognitive function. Ethanol may disrupt cognitive processes by altering attention, short-term memory, and/or long-term memory. Interestingly, some research suggests that ethanol may enhance cognitive processes at lower doses. The current research examined the dose-dependent effects of ethanol on contextual and cued fear conditioning. In addition, the present studies assessed the importance of stimulus salience in the effects of ethanol and directly compared the effects of ethanol on short-term and long-term memory.

METHODS

This study employed both foreground and background fear conditioning, which differ in the salience of contextual stimuli, and tested conditioning at 4 hours, 24 hours, and 1 week in order to assess the effects of ethanol on short-term and long-term memory. Foreground conditioning consisted of 2 presentations of a foot shock unconditioned stimulus (US) (2 seconds, 0.57 mA). Background conditioning consisted of 2 auditory conditioned stimulus (30 seconds, 85 dB white noise)-foot shock (US; 2 seconds, 0.57 mA) pairings.

RESULTS

For both foreground and background conditioning, ethanol enhanced short-term and long-term memory for contextual and cued conditioning at a low dose (0.25 g/kg) and impaired short-term and long-term memory for contextual and cued conditioning at a high dose (1.0 g/kg).

CONCLUSIONS

These results suggest that ethanol has long-lasting, biphasic effects on short-term and long-term memory for contextual and cued conditioning. Furthermore, the effects of ethanol on contextual fear conditioning are independent of the salience of the context.

Early Ethanol's Anxiolytic Effects Assessed Through an Unconditional Stimulus Revaluation Procedure

BACKGROUND

Even though ethanol is considered to exert antianxiety effects, the existence of such properties during early ontogeny of the rat has rarely received attention, mainly because of the restricted behavioral repertoire of the pup. This study evaluated ethanol's anxiolytic properties in preweanling rats through an unconditional stimulus revaluation procedure.

METHODS

Optimal parameters for the acquisition of an odor avoidance response were obtained in Experiment 1. In Experiment 2 preweanlings experienced 4 conditioning trials defined by a 5 minute exposure to a lemon odor while they were intraorally infused with a sapid aversive stimulus (citric acid). Twenty-four hours later, pups were briefly exposed to the acid 5 minutes after being administered with 0.0, 0.25, 0.5, 1.25, or 2.5 g/kg of ethanol. Odor preferences were then evaluated in a 2-way odor preference test. Blood ethanol concentrations (BEC's) derived from these doses were assessed in Experiment 3. In Experiment 4, citric acid mediated-conditioning was followed by a similar odor-preference test after pups were re-exposed to the acid either 5 or 25 minutes after receiving a 2.5 g/kg ethanol dose. Ethanol's effects upon the avoidance response were also analyzed in an alternative rat strain (Experiment 5).

RESULTS

Moderate to low ethanol doses ameliorated the otherwise strong avoidance response. Further experiments: (a) replicated these phenomena, (b) established that they are correlated with BEC's in the range of 17-70 mg%, and (c) Indicated that ethanol's influence upon an avoidance response changes as a function of the temporal course of the intoxication.

CONCLUSIONS

Acute ethanol interacts with aversive nondrug-mediated learning. These findings are likely to be discussed in terms of possible negative reinforcing properties of ethanol and indicate that this drug exerts differential motivational effects during the course of the intoxication. The devaluation procedure should be considered as an alternative method to assess ethanol's motivational properties.

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.

Alcohol-induced impairment and enhancement of memory: A test of the interference theory

Many studies have found cognitive deficits related to alcohol consumption. However, few studies have studied cognitive performance when alcohol was administered after the to-be-remembered information was presented with memory testing occurring when participants are once again sober. The present study examined effects of alcohol on cognitive performance using a prose recall task during acute intoxication and a post-trial recall task for prose passages that had been presented before intoxication. Fifty-one men were given either 2.0 g/kg of 100 proof (50% absolute ethanol) vodka or a placebo. In the present study, evidence was found of acute alcohol impairment in prose memory, along with alcohol facilitation of memory on a post-trial task.

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.

Biphasic effects of ethanol on acetylcholine release in the rat prefrontal cortex

Low doses of ethanol (0.5 g/kg i.p.) increased, while higher doses (1 g/kg i.p.) reduced acetylcholine (ACh) release in the rat prefrontal cortex (PFC). Ethanol (50-300 mM) applied in the nucleus basalis through a second dialysis probe caused concentration-dependent biphasic changes in prefrontocortical ACh release. Ethanol apparently acts on cholinergic fibers to modulate ACh transmission in the PFC. These results could be of relevance for the bidirectional modulation of working memory by ethanol.

Multiple binge alcohol consumption during rat adolescence increases anxiety but does not impair retention in the passive avoidance task

We investigated the effect of binge alcohol consumption on anxiety-related behavior and memory in adolescent male Wistar rats. Three consecutive daily sessions of ethanol administration (5 g/kg) were repeated weekly for 4 weeks. The retention of passive avoidance was measured weekly, 48 h following the treatment. Three days after the last memory test a novel object exploration test was done. There was no significant difference in step-through latency between the groups, but the ethanol-treated group displayed a significantly higher incidence of defecation, and an increased number of boluses during the passive avoidance test. The latency to explore a novel object was also higher, while the duration of exploration was significantly lower. Together, these data suggest that binge alcohol consumption in adolescent rats does not impair their memory in passive avoidance tasks, but may significantly increase their anxiety.

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.

Immediate and delayed voluntary ethanol effects on motor performance, learning and inhibition in rats

The effects of prolonged voluntary ethanol consumption on psychomotor performance, operant conditioning and inhibition were examined in adult male Wistar rats. Animals were food deprived and alcohol or control solution was available 1 h/day during 15 days, with free water for the rest of the day. Then, rats were tested in a two-bottle paradigm (solution and water available) for 1 h/day during 19 days, and subjects were tested daily for psychomotor performance and operant conditioning immediately or 6 h after (delayed) the solution access. Psychomotor performance was tested in an 80 degrees -inclined screen. Successive conditioning phases were: free shaping (FS), continuous reinforcement (CRF), operant extinction (EXT), successive discrimination (DIS) and two-stimuli test (TST). Alcohol consumption deteriorated psychomotor performance and improved the animal's ability to learn simple associations between stimuli and responses (free shaping and extinction), in immediate and delayed groups. Finally, alcohol deteriorated behavioral inhibition (DIS and TST) tested immediately after drinking. Taken together, results suggest that prolonged voluntary ethanol intake could induce permanent psychomotor impairment and associative learning facilitation, and also an impairment of the inhibition related to the intoxication state.

Effects of posttraining ethanol on an appetitive task

The present study examined the effects of posttraining ethanol administration upon retention of an appetitive task using a variety of retention behaviors associated with the task. Male C57BL/6J mice were individually trained to find a cheese pellet placed in the corner of an open field. Five behavioral measures were used including locomotor activity counts, rearings, grooming episodes, approaches to the cheese pellet, and latency to consume the cheese pellet. Immediately after training, mice were injected intraperitoneally with saline or 2.0 g/kg of ethanol and then returned to their home cage in which four "intruder" mice were added for 2 h after training. On subsequent testing days (1, 6, 14, and 51 days posttraining), mice were returned to the original training environment and the five behaviors were measured. Both saline- and ethanol-treated mice habituated to the initially novel test environment at similar rates as indicated by decreased exploratory behavior (locomotor activity and rearings). In contrast, a divergence in the latency to consume the cheese pellet was observed: Saline-treated mice behaved as though the cheese was rewarding (decreased latency to eat the pellet), while the ethanol group behaved as though the cheese was aversive (increased latency to eat the pellet). Taken with previous studies, these results demonstrate that posttraining ethanol can have strikingly different effects on retention depending on the task, the measure of retention used, and the underlying neural structures involved.

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.

Context and tone conditioning are selectively impaired by ethanol in the preweanling rat: effects of dose and time of administration

Depending on dose and task requirements, ethanol can have either a facilitative or an impairing influence on learning. Some explanations for this dichotomy have considered ethanol's suppression of behavioral variability and processing of incidental stimuli (e.g., context). The present study examined the effect of ethanol on context and conditioned stimulus (CS) learning in the preweanling rat. To assess state-dependent effects, a drug dissociation design was used. Learning to both context and CS were analyzed within each dose of ethanol (0, 1.2, 1.6, or 2.0 g/kg) and a trend analysis was conducted to determine dose-response relationships as a function of train-test state. The 1.2 g/kg dose of ethanol did not affect conditioning to either the context or the CS. A 1.6 g/kg dose tended to disrupt context, but not CS, conditioning. The influence of 2.0 g/kg ethanol depended on train-test conditions. Ethanol administration prior to training resulted in the stronger impairment of CS learning while context conditioning was most disrupted if ethanol was given only prior to testing. The results suggest that ethanol selectively attenuates processing of stimuli, possibly dependent on relative saliency at the time of testing.

Enhancement of sensory preconditioning by a moderate dose of ethanol in infant and juvenile rats

Little is known regarding the effects of acute ethanol on learning and memory among infant and juvenile rats. Two experiments were designed to evaluate the influence of various doses of ethanol administration on sensory preconditioning and primary conditioning in preweanling (16-day-old) and postweanling (28-day-old) rats. Moderate doses of ethanol facilitated conditioning at both ages. In the absence of ethanol, sensory preconditioning was not statistically significant among postweanlings in Experiment 1, although the phenomenon was clear and robust among preweanlings. Sensory preconditioning was facilitated by administration of doses of 0.8 and 1.2 g/kg ethanol in preweanlings and a dose of 1.6 g/kg ethanol in postweanlings, whereas sensory preconditioning was impaired in preweanlings by a high dose (2.4 g/kg) of ethanol. This reflected a shift to the right, between the preweaning and the postweaning periods, in the dose-response curve. It was determined in Experiment 2 that the effect of ethanol on sensory preconditioning could not be explained by stimulus generalization or an effect of ethanol on first-order conditioning, confirming the effect of ethanol on learning of the odor-odor association in the preexposure phase. The basis for a dose-dependent biphasic effect of ethanol on sensory preconditioning is discussed.

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.

Genetic analyses of the biphasic nature of the alcohol dose-response curve

Ethanol (ETOH)-induced locomotor activation and depression were studied in 23 genotypes of mice. This included a diallel cross of four inbred strains tested with a range of ETOH doses from 0 to 2.75 g/kg. The diversity in shapes of the biphasic ETOH dose-response curves was both qualitative and quantitative, and additive gene action characterized the genetic control of the dose-response curve. Small dominance effects were typically directional in the direction of more activation, or resistance to sedation. No evidence was found for maternal effects, sex linkage, or epistasis. Sex differences were seen in the increased susceptibility of male mice to locomotor sedation at higher ETOH doses. In the diallel cross, there was no correlation between the degree of activation produced by low ETOH doses and sedation produced by higher doses. This indicates that while considerable genetic influences exist for both activational and sedative domains of ETOH effects, these genetic influences are relatively independent.

Posttrial treatment with ethanol enhances recall of prose narratives

A small number of studies have examined the effects of postlearning alcohol intoxication on memory for various materials. In contrast to most research examining the effects of alcohol on human memory, these studies demonstrated a facilitation of memory for information learned prior to intoxication. The present study was designed to examine the effects of alcohol on memory for two different kinds of materials. Standard word lists and narrative prose passages were employed to obtain a more detailed picture of the effects of posttrial intoxication with alcohol on memory. Intoxication with alcohol did not affect subject's ability to recall simple word lists. However, recall of prose passages was facilitated to a statistically significant degree. Results are discussed within the context of a current theory of retrograde facilitation of memory via various drugs/substances.

Different forms of post-training memory processing

Memories are not acquired in their definitive form, but can be considerably modified in the period that follows after acquisition, both quali- and quantitatively. This may happen either by a process of consolidation or strengthening of each memory trace or by the incorporation of further information to each experience. This further information may be provided by the action of drugs, including that of endogenous substances released by each training experience, and by the addition of information provided by other tasks or events. Evidence in favor of the existence of these various post-training processes, and of their importance for memory formation, is discussed. All of these processes are time dependent, all may depend on stimulus aftereffects, each is differently affected by drugs given in the post-training period, and all may interact with each other.

The effect of home or novel environment on the facilitation of passive avoidance by post-training ethanol

Passive avoidance behavior of mice is improved when mice are injected with ethanol immediately after footshock training. Further study has shown that avoidance can be affected by ethanol injections given within 1 h, but not at 90 or 180 min, after training. The present study was conducted to investigate the possibility that events which occur in the homecage during this sensitive period may influence the effect of ethanol on subsequent avoidance. Male Swiss-Webster mice were housed either singly in a novel environment for 90 min or returned to their (group) homecage following one-trial, step-through, passive avoidance training (0.1 mA footshock) and intraperitoneal injection of 3.0 g/kg ethanol (15% v/v) or saline. As in previous studies, when ethanol-treated mice were returned to their homecage, avoidance was significantly increased at 24 h compared to the behavior of saline-treated mice. However, when mice were isolated in the novel environment for 90 min immediately following treatment, the memory facilitating effects of ethanol were not observed. The avoidance behavior of mice injected with saline was the same regardless of their post-training environment. Also, the number of mice (6 or 10) housed per homecage did not significantly influence the effects of ethanol or post-training environment on avoidance. These findings indicate that environmental factors may interact with the effects of ethanol to modify avoidance behavior. The possible influence of variables such as aggression, thermoregulation, and behavioral arousal on the effects of ethanol in this paradigm are discussed.

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

Ethanol's post-training facilitation of memory was examined using a latent learning paradigm known as the "water-finding task." Rats were assigned to one of two ethanol groups (E0.75 g/kg or E1.5 g/kg) or to a control group (saline) and individually placed in a novel open field containing a drinking tube. Following this exposure, subjects were immediately administered intraperitoneal (IP) injections of either the saline or ethanol and 48 hours later, re-introduced to the field. Initial latencies to contact the tube each time were recorded. A linear regression analysis of trial 2 latencies regressed onto trial 1 latencies indicated a statistically significant effect of ethanol on the relation between initial and subsequent latencies. Though the control rats' trial 2 latencies were completely random with respect to their previous speeds (rSAL = -0.07), the ethanol rats' trial 2 latencies were positively correlated with initial speeds (rE0.75 = 0.35, rE1.5 = 0.67). These results suggest that under conditions of post-training ethanol, trial 2 behavior is more similar to, or controlled by, trial 1 behavior and are consistent with the argument that, under certain training and testing contexts, ethanol can come to exert control over a response's recurrence.

Ethanol enhancement of Pavlovian conditioning: comparison with instrumental conditioning

Yoked pairs of rabbits received conditioning and extinction of eyeblink and heart rate responses using aversive Pavlovian versus instrumental avoidance contingencies. A low dose of ethanol (375 mg/kg) increased the amplitude of conditioned eyeblink responses during training and during subsequent extinction following either ethanol or water; this effect was more pronounced in the Pavlovian than the instrumental contingency groups. Ethanol treatment did not affect the cardiac component of the Orienting Reflex to novel tones or heart rate conditioned responses to tone signals, but did bias heart rate responses to tone-shock stimulus pairs in a parasympathetic direction; this effect occurred in both contingency groups but appeared to involve associative factors. These results support the conclusion that mild ethanol intoxication enhances the acquisition of Pavlovian conditioned reflexes, and impairs the ability to modify these responses when stimulus contingencies later change.

The conditioned place preference is affected by two independent reinforcement processes

The conditioned place preference method for measuring the affective properties of reinforcing events was studied using treatments of known affective value. The size of the place aversion observed increased with dose when the reinforcer was injections of lithium chloride. The size of the place preference observed increased with concentration when the reinforcer was drinking sucrose solutions. However, when the reinforcer was solutions of saccharin (that were consumed in the same amounts as the sucrose solutions) no place preferences were observed. This finding was explained in terms of the dual reinforcement hypothesis [20] which postulates that although sucrose and saccharin both have positive affective properties (based on their tastes) only sucrose has memory improving properties (based on its post-ingestive action). It was therefore proposed that conditioned place preferences depend on the activation of both affective and memory improving processes. This hypothesis was confirmed by the observation of place preferences with a saccharin solution as the reinforcer when the pairing trials were followed by non-contingent, post-pairing injections of glucose or amphetamine (both of which are known to improve memory). Therefore, behavior in the place preference method depends upon both the affective and the memory improving properties of the reinforcers under test.

Facilitation or inhibition of memory by morphine: a question of experimental parameters

The effects of morphine on memory are highly controversial. According to some investigators post-trial injections of morphine facilitate memory. Others, however, have reported impairment of memory after morphine injections. To investigate the extent to which this may be due to different experimental parameters, foot-shock intensity and dosage of morphine were systematically varied in a passive-avoidance task. It was found that post-trial administration of medium and relatively high doses of morphine facilitate retention performance following moderate levels of foot-shock. Under other conditions of dose and shock intensity, the drug was not effective or even impaired retention.

Retrograde enhancement by alcohol of delayed free recall performance

Two experiments are reported in which retrograde enhancement of human memory by alcohol was observed. In both studies male undergraduate volunteers performed an immediate free recall task before and after consuming either alcohol (0.66 abs alc/kg) or placebo. About two hours later, many words as they could delayed free recall was tested when subjects were asked to write down as many words as they could remember from the free recall trials in the session. Subjects given alcohol recalled significantly more words from lists heard before drinking than subjects given placebo; this effect appeared more pronounced for words from the primacy portion of the lists. The possibility that this retrograde enhancement effect is due to alcohol's effects on brain reward systems is raised.

Sequence of alcohol presentation is important in the potentiation of long-term events

Animal and human studies have demonstrated that, depending upon the sequence of alcohol presentation, long-term memory of events can either be enhanced or diminished. In the present study a similar phenomenon is demonstrated in the neuronal excitability of slices of hippocampus from guinea pig brains. Alcohol given after, but not before, 3 days of pentylenetetrazol (PTZ) administration to the intact animal produced kindling equivalent to 5 days of PTZ given by itself. This effect appears to be independent of the known withdrawal effects of alcohol and lasts for at least 14 days after the alcohol and PTZ administration have been discontinued.

An analysis of some effects of ethanol on performance in a passive avoidance task

In a one-trial step-through passive avoidance task, pretraining administration of ethanol was shown to decrease the latencies to step through at both training (day 1) and testing (day 2) for both rats and mice. A detailed analysis of these effects showed that they differed from those reported previously by others. The mechanisms underlying these effects of ethanol were also examined. The decreased day 1 latency to step through seen in rats may have been caused by an ethanol-induced hypermotility. However, ethanol did not increase the locomotor activity of mice, although it also reduced the day 1 latency to step through of this species. In addition, it was found that the ethanol-induced impairment of passive avoidance responding (i.e. the decreased day 2 latency to step through) was not state-dependent and that it was unlikely that it could be explained by a drug-induced impairment of task acquisition, long-term memory formation or memory recall. It also seemed unlikely that the impairment could be explained by an ethanol-induced decrease in shock sensitivity. Other mechanisms which may be involved are discussed.

The alcohol facilitation effect on memory: a dose-response study

Sixteen normal male subjects participated in four sessions where they studied a set of pictures followed by either placebo, 0.025, 0.50, or 1.0 ml/kg alcohol. Later, when sober, recognition memory was tested. These doses resulted in peak blood alcohol concentrations (BAC) of 0.00, 0.018, 0.034, and 0.067 g/100 ml, respectively. The 1.0 and 0.50 ml/kg doses significantly improved memory for pictures studied before drinking. Alcohol appears to enhance memory directly rather than indirectly via a reduction in interference. It is suggested that a particular phase of the rising blood alcohol curve (0.02-0.03 g/100 ml) facilitates trace consolidation. The facilitating and possibly excitatory effects of alcohol may be important for understanding the rewarding aspects of drinking.

Retrograde enhancement of human memory with alcohol

In two experiments with normal male subjects, the ingestion of alcohol (1 ml/kg) immediately after learning significantly improved subsequent remembering. By comparison, marijuana (15 mg) had no significant post-acquisition effect. Facilitation of visual and verbal memory with alcohol under these conditions has implications for the interference and consolidation views of memory.