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

Despite its low potency, approximately 17.6 million adult Americans consume enough alcohol on a regular basis to be considered abusers (National Institute on Alcohol Abuse and Alcoholism, 2004). Symptoms of alcohol abuse include disruptions of both physiological and psychological functioning that can lead to long-term changes in behavior. One function altered by ethanol is learning. Acute ethanol produces dose-dependent impairments in learning (for review, Ryback, 1971). These impairments have been demonstrated in many tasks, including passive avoidance, delay matching-to-position, free recall, radial maze, signal detection, and fear conditioning (Bammer and Chesher, 1982; Escher and Mittleman, 2004; Gibson, 1985; Gould, 2003; Higgins et al., 1992; Melia and Ehlers, 1989). Fear conditioning is useful for investigating the effects of ethanol on different types of learning that differentially involve the hippocampus because it tests both hippocampus-dependent contextual learning, in which contextual features are associated with an unconditioned stimulus (US) foot shock, and hippocampus-independent cued associations, in which a conditioned stimulus (CS) auditory cue is associated with the US (Logue et al., 1997; Phillips and LeDoux, 1992). Previous work has demonstrated that ethanol severely disrupts contextual learning and impairs cued learning to a lesser extent (Celerier et al., 2000; Gould, 2003; Melia et al., 1996). Although ethanol impairs both forms of fear conditioning, it should still be noted that the effects of ethanol are specific to the type of memory being tested; a dose of ethanol that impairs fear conditioning (Gould, 2003) enhances cue-based reward learning (Matthews et al., 1999). Thus, the behavioral requirements and/or underlying neural substrates of a learning task may determine the susceptibility of the task to the effects of ethanol. A number of studies have demonstrated ethanol-induced impairments in fear conditioning (Celerier et al., 2000; Gould, 2003; Melia et al., 1996) and diverse explanations for these deficits can be proposed. Ethanol could have direct effects on learning, attention, consolidation, or sensitivity to the shock stimulus. Although ethanol may decrease shock sensitivity, prior research in a passive avoidance paradigm suggests that 2.0 g/kg ethanol does not alter shock sensitivity within a range of shock intensities that include the intensity used in the current study (Bammer and Chesher, 1982). Furthermore, if ethanol was mediating learning by altering sensitivity to the shock, contextual and cued conditioning should show the same level of impairment in response to ethanol treatment. However, other studies have demonstrated that ethanol impairs contextual conditioning to a greater degree than cued conditioning (Gould, 2003; Gould and Lommock, 2003; Melia et al., 1996). Ethanol could also be altering processes involved in consolidation but prior studies have demonstrated that post-training ethanol does not affect fear conditioning (Gould and Lommock, 2003); this suggests that ethanol is not altering processes involved in consolidation but may alter processes involved in acquisition. Ethanol can impair attentional processes (Givens and McMahon, 1997; Lyon et al., 1975; Rezvani and Levin, 2003), and selective attention is necessary for acquisition of associative conditioning (Steele-Russell et al., 2006). However, it is unclear whether the effects of ethanol on attention are related to ethanol-induced impairments in contextual fear conditioning. Previous studies that examined the effects of ethanol on contextual fear conditioning have used background fear conditioning (i.e., pairing an auditory CS with the shock US), relegating the context to a background stimulus (Rescorla and Wagner, 1972). Following background fear conditioning, the tone is a more robust predictor of the shock because the tone and the shock have a consistent temporal relationship. In contrast, contextual cues are present throughout the training session and may, therefore, be a less robust predictor of the shock because greater attention may be paid to the auditory CS than to the context during background fear conditioning (Odling-Smee, 1978), although this overshadowing has not been seen in all studies (Lolordo et al., 2001). It is unclear whether the differential effects of ethanol on contextual and cued conditioning are due to an ethanol-induced decrease in attention to the background contextual cues. Conducting contextual fear conditioning in the absence of an auditory CS renders the context a foreground stimulus (Odling-Smee, 1975, 1978), which is more salient than a background stimulus. Changes in salience may alter the level of contextual fear conditioning, possibly due to the activation of divergent substrates during the encoding of background versus foreground contextual fear conditioning (Majchrzak et al., 2006; Phillips and LeDoux, 1994; Stiedl et al., 1999). Thus, examining the effects of ethanol on background and foreground contextual fear conditioning will further our understanding of the behavioral effects of ethanol by determining the importance of stimulus salience in the effects of ethanol on learning. If ethanol is affecting the acquisition of learning, as suggested by other research (Bammer and Chesher, 1982; Birnbaum et al., 1978), then the effects of ethanol may extend to both short-term and long-term memory. Studies of working memory tasks have demonstrated strong ethanol-induced deficits (Escher and Mittleman, 2004; Givens, 1995; Givens and McMahon, 1997). In addition, Sircar and Sircar (2005) have demonstrated that ethanol-treated animals perform poorly at testing sessions immediately following training, as well as at 4, 7, and 25 days post-training in a Morris Water Maze task. However, ethanol has differential effects on learning depending on the type of memory being tested, and it is unclear if ethanol differentially affects short-term and long-term memory for fear conditioning. In fear conditioning, 1.0 g/kg ethanol impairs contextual fear conditioning at 1 and 7 days post-training (Gould and Lommock, 2003), suggesting that the effects of ethanol are long-lasting. As the neural substrates underlying working memory, short-term memory, and long-term memory differ across tasks (for review, Squire and Kandel, 1999), it is important to independently determine the effects of ethanol on different forms of memory. A direct comparison of the effects of ethanol on short-term and long-term memory for fear conditioning will help to determine whether there are differences in the degree to which ethanol affects memory at diverse time points. Finally, most studies of the effects of ethanol on cognition have focused on the impairing effects that occur at high doses of the drug. These studies have demonstrated that acute ethanol impairs learning in many paradigms, usually at doses of 1.0 g/kg and greater (Bammer and Chesher, 1982; Birnbaum et al., 1978; Gould, 2003; Markwiese et al., 1998; Matthews et al., 1995; Popke et al., 2000). Far fewer studies have examined the effects of low doses of ethanol on cognition. The studies that have been conducted suggest that lower doses of ethanol may enhance learning in paradigms such as the 8-arm radial maze, serial learning during stress, and eye blink conditioning (Devenport et al., 1983; Hernandez and Powell, 1986; Hernandez and Valentine, 1990; Korman et al., 1960). To date, no studies have examined the effects of ethanol on fear conditioning using both doses that have been shown to enhance learning in other studies and doses associated with impairments. The goals of this study were to investigate the dose-dependent effects of ethanol on fear conditioning and to determine whether these effects varied with changes in the interval between training and testing and whether they depended on the salience of the context. In addition, the present study tested an extended dose range for the effects of ethanol on fear conditioning to examine if the effects of ethanol were biphasic. Based on prior ethanol studies, we predicted that ethanol would enhance contextual fear conditioning at lower doses while impairing it at higher doses, regardless of the salience of the context or the time of testing.