Adolescent Intermittent Alcohol Exposure: Persistence of Structural and Functional Hippocampal Abnormalities into Adulthood

Adolescence is a critical period for cognitive, emotional, and social maturation (Choudhury et al., 2006) that is accompanied by the pruning of synapses, refinement of neural circuitry, and changes in receptor expression and sensitivity (Kilb, 2012). These processes contribute to the normal maturation of cognitive processes crucial for successful adult function, including planning, inhibitory control, and working memory (Paus, 2005). Adolescence is also a period during which alcohol consumption is often initiated and sustained at high levels (Squeglia et al., 2012). While it has become clear that adolescents respond differently than adults to the acute effects of ethanol (EtOH) on learning, sedation, and motor function (Little et al., 1996;Markwiese et al., 1998; Spear, 2000), the enduring consequences of repeated EtOH exposure during this developmental period have only recently begun to be addressed. In humans, chronic excessive alcohol use during adolescence has been associated with cognitive deficits manifesting in adulthood, particularly in the domain of memory function (Brown et al., 2000; Hanson et al., 2011). In animal models used to reflect human levels of consumption, behavioral studies have shown that adolescent intermittent EtOH (AIE) exposure in rats results in long-lasting changes in EtOH sensitivity, consumption, and aversion (Diaz-Granados and Graham, 2007; Matthews et al., 2008; Risher et al., 2013). With respect to learning and memory in particular, it has been reported that AIE, but not chronic intermittent EtOH (CIE) exposure in adulthood, results in greater sensitivity to the spatial memory-impairing effects of acute EtOH in the radial arm maze, in the absence of an effect on baseline learning ability (Risher et al., 2013; White et al., 2000). Interestingly, AIE has also been shown to reduce the efficacy of EtOH to impair spatial learning in the Morris water maze 24 hours after the last EtOH dose (Silvers et al., 2003, 2006), although it is noteworthy that the effect may have been driven by withdrawal, tolerance, or both, given the brief delay between repeated EtOH exposure and testing. Aside from the subsequent responsiveness to EtOH challenge, Sircar and Sircar (2005) reported that AIE caused deficits in Morris water maze performance up to 25 days after the last EtOH treatment, and fear retention deficits have also been observed 25 days following AIE exposure (Broadwater and Spear, 2013). At the neuronal level in similar AIE rodent models (which achieve equivalent blood EtOH concentrations [BECs]), enduring functional effects are produced that are not apparent when comparable EtOH exposure is administered in adulthood. For example, AIE has been shown to reduce A-type potassium current (IA) in GABAergic hippocampal interneurons (Li et al., 2013), GABAA receptor-mediated tonic current in dentate granule cells (Fleming et al., 2012, 2013) in adulthood, and protein levels of delta and alpha-4 GABA receptors in whole hippocampus (Centanni et al., 2014), while comparable EtOH exposure during adulthood did not produce equivalent long-lasting changes in these cellular functions or receptor proteins. Thus, not only does adolescent EtOH exposure promote long-lasting changes in hippocampal cellular function, but that adolescence is also a period of distinctive vulnerability to the long-term effects of EtOH, enduring even after an extended period of abstinence. While these findings support heightened vulnerability to the long-term consequences of repeated alcohol exposure during adolescence, the extent to which AIE alters subsequent hippocampal neuronal, synaptic, and behavioral processes is unclear. We used hippocampal slices to assess the long-term effects of AIE (an established model of intermittent EtOH exposure) on CA1 structure and function. The CA1 area of the hippocampus was selected because of its role in learning/memory processes and because it has been a focus of studies in the emerging literature on the enduring effects of AIE. Here, we assessed the effects of AIE on long-term potentiation (LTP), dendritic spine morphology (to determine whether AIE alters synaptic structure that in turn can influence synaptic function), and postsynaptic density proteins; the latter of which has been found to influence dendritic spine morphology and play a critical role in the recruitment and stabilization of excitatory synapses and synaptic function (Beique et al., 2006; El-Husseini et al., 2000; Zheng et al., 2012). This approach represents a comprehensive analysis combining physiological, biochemical, and morphological assessments that support the hypothesis that AIE results in neuronal changes that persist into adulthood, emphasizing the vulnerability of this critical developmental period to insults that can potentially alter the trajectory of brain development.