Ethanol impairs memory of a simple discrimination in adolescent rats at doses that leave adult memory unaffected

Abstract

Adolescent rats are less sensitive than adults to the hypothermic, anxiolytic, motor impairing, hypnotic, and lethal effects of ethanol. In vitro experiments nevertheless suggest that hippocampal neural activity is more affected by ethanol in preweanling or adolescent rats than in adults. These data are complemented by in vivo results showing that pretraining ethanol impairs learning in adolescent rats at doses that do not affect adult learning. In order to determine if posttraining ethanol affects memory differently in adolescents than in adults, Sprague–Dawley albino rats of both ages were trained in an appetitively motivated odor discrimination in which they were required to dig in scented sand for sweetened cereal reward. Immediately after training subjects received intraperitoneal injections of 0, 0.5, or 1 g/kg ethanol (12.6%). At test, 48 h later, subjects were presented with unbaited discriminanda and the time (s) spent digging in the S+ and S− was measured. Adolescents, but not adults, showed impaired discrimination performance if training was followed by ethanol. A subsequent experiment discounted the possibility that impaired adolescent performance was due to ethanol-induced conditioned taste or odor aversions. It thus appears that relative to adults, memory in adolescent rats is more strongly affected by ethanol in a test of appetitive conditioning that excludes ethanol’s effects on sensory and motivational influences during the learning experience.

Introduction

Sensitivity to the sedative and hypnotic effects of ethanol is age-dependent, with developing organisms showing less intoxication than mature animals as indexed by locomotion (Little, Kuhn, Wilson, & Swartzwelder, 1996), motor coordination (White et al., 2002), hypothermia (Silveri & Spear, 2000), mean time to lose righting reflex (Silveri & Spear, 1998) and to regain it (Little et al., 1996; but see Kelly, Bonthuis, & West, 1987) as well as sleep time (Silveri & Spear, 1998). As development progresses, sensitivity increases to these effects of ethanol (Little et al., 1996; see Spear, 2000a, Spear, 2000b, Spear, 2002 for reviews).

In contrast, sensitivity to the cognition-impairing effects of ethanol may be more pronounced in younger than in older organisms. For example, ethanol impairs conditioned suppression in 17-day-old rats, but not in adults (McKinzie, Lee, McKinzie, Spear, & Spear, 1996). Odor aversion expression is impaired in preweanling, but not adult, rats when ethanol precedes training (Lopez, Spear, & Molina, 1996), and water maze learning in adolescent rats is impaired at doses that do not affect learning in adults (Markwiese, Acheson, Levin, Wilson, & Swartzwelder, 1998). Even in humans, younger adults (21–24 years) show greater impairment in semantic and figural learning following ethanol than older adults (25–29 years; Acheson, Stein, & Swartzwelder, 1998). However, some reports indicate equivalent ethanol-induced learning impairment in adolescents and adults (Rajachandran, Spear, & Spear, 1993).

Taken together, these findings suggest that developing rats may be hypersensitive, with respect to adults, to the disruption of cognitive processing by ethanol. These behavioral effects are likely influenced by differential cellular sensitivity, as both long-term potentiation (LTP) and NMDA-receptor-dependent excitatory postsynaptic potentials (EPSPs) in vitro are more suppressed by ethanol in immature hippocampal slices than in mature ones (Swartzwelder et al., 1995a, Swartzwelder et al., 1995b). NMDA-receptor-dependent LTP has long been considered important for learning and memory, and inhibition of LTP results in attenuation or blockade of acquisition (Bliss & Collingridge, 1993).

Clearly, learning in adolescents is more sensitive to perturbations by ethanol than is learning in adults. The present experiments test the effect of ethanol on memory processes occurring following acquisition, including consolidation and maintenance of information (Spear & Riccio, 1994). In adult humans and rodents, ethanol administration following learning can modulate memory, causing either enhancement (e.g., Alkana & Parker, 1979; Lamberty, Beckwith, Petros, & Ross, 1990; Mann, Cho-Young, & Vogel-Sprott, 1984; Mueller, Lisman, & Spear, 1983) or impairment (Babbini, Jones, & Alkana, 1991; Castellano & Populin, 1990; Colbern, Sharek, & Zimmerman, 1986; Ladner, Babbini, Davies, Parker, & Alkana, 2001; but see De Carvalho, Vendite, & Izquierdo, 1978). The current experiments were undertaken to determine if ethanol modulates memory differently in adolescents than in adults.

Because ethanol more greatly inhibits in vitro hippocampal activity in adolescents than in adults (Carta, Ariwodola, Weiner, & Valenzuela, 2003; Swartzwelder et al., 1995a, Swartzwelder et al., 1995b), it is likely to selectively or preferentially impair hippocampus-dependent learning (Matthews, Simson, & Best, 1996; Matthews, Ilgen, White, & Best, 1999). Indeed, as previously mentioned, adolescents are impaired at water-maze learning by ethanol doses not affecting adult learning (Markwiese et al., 1998). However, it is also possible that ethanol may modulate memory nonselectively; that is, ethanol-induced impairments may not be restricted to hippocampus-dependent tasks. Therefore, the present experiments utilized an instance of simple discrimination learning that is well established as independent of the integrity of the hippocampus in adult rats (Bunsey and Eichenbaum, 1993, Bunsey and Eichenbaum, 1996).

Adolescents and adults were trained in an appetitive simple odor discrimination that is quickly learned (Bunsey & Eichenbaum, 1996). The present study is therefore unlike most previous tests in not only addressing the possibility of ethanol disrupting memory following learning, but also in using a test that is appetitively, rather than aversively, motivated (but see Ladner et al., 2001). A further advantage of this paradigm is that administration of ethanol following the learning episode allows for similar perceptual, motivational, and sensory experiences among the groups during the episode to be learned, and also permits training and testing in the same psychopharmacological state.

Bunsey and Eichenbaum (1996) developed a simple odor discrimination that capitalizes on the rat’s natural tendency to dig by placing sweetened cereal reward in plastic cups filled with sand substrate. The sand, scented with various odors in the form of cooking spices, serves as a discriminative stimulus and guides the operant response. Rats are trained that certain odors are paired with food reward hidden in the sand, and quickly learn to dig in the odors that predict food reward. Bunsey and Eichenbaum (1996) used a simple odor discrimination, as well as a transitive inference task, successfully in adult rats.

Here, the task has been adapted for adolescent subjects in pilot experiments and the original training protocol has been slightly modified (cf. Bunsey & Eichenbaum, 1996). An unpaired group of adolescents was included to confirm that rats of this age learn to dig preferentially in a cup scented by an odor previously paired with food reward. Food deprivation was not needed to motivate behavior, thus eliminating the problem of equating deprivation levels across age.