Mechanisms of conjunctive learning in a rat model of fetal alcohol spectrum disorders

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University of Delaware
Fetal alcohol spectrum disorders (FASDs) describes a continuum of birth defects caused by maternal intake of alcohol during pregnancy. In humans, developmental alcohol exposure impairs the normal development of many brain regions, including the hippocampus and frontal cortices (Fryer et al., 2007; Norman, Crocker, Mattson, & Riley, 2009; Norman et al., 2013). Importantly, the adverse effects of alcohol are largely a result of the timing, pattern and dosage of maternal ethanol consumption (Maier & West, 2001). In rat models of FASDs, the hippocampus, for example, is particularly vulnerable to damage when alcohol is administered during the neonatal period (Livy, Miller, Maier, & West, 2003; Marino, Aksenov, & Kelly, 2004; Tran & Kelly, 2003), which is equivalent to the brain growth spurt during the third-trimester of human pregnancy (Dobbing & Sands, 1979). The context preexposure facilitation effect (CPFE) is a hippocampus-dependent variant of contextual fear conditioning in which learning about the context (preexposure) and associating the context with shock (training) occur on separate occasions. This produces conditioned freezing during testing relative to a control group not preexposed to the conditioning context. Alcohol exposure from Postnatal Day (PD) 4-9 in the rat (equivalent to human third trimester) reveals a linear relationship between alcohol dose and spatial/contextual learning impairment in the CPFE (Murawski & Stanton, 2011). This dissertation further characterizes these neonatal alcohol-induced deficits in the CPFE. The purpose of the first group of experiments was to examine the effect of a range of alcohol doses administered during a narrower window, PD7-9, than previously reported (Experiment 1) and to begin to determine which memory processes involved in this task are impaired by developmental alcohol (Experiment 2). In Experiment 1, rats pups received a single day binge alcohol dose of 2.75, 4.00, 5.25 g/kg/day or were sham-intubated (SI; Group SI) from PD7-9. Conditioned freezing during CPFE testing on PD33 was evident in all dosing groups, except for Group 5.25g. In Experiment 2, rat pups received either 5.25 g/kg/day (Group EtOH) or were sham intubated over PD7-9. During training on PD32, rats remained in the conditioning context for 5-min following immediate shock delivery, in which rats preexposed to the conditioning context showed comparable freezing levels, regardless of neonatal treatment (Group SI vs. Group EtOH). This suggests that alcohol impairs retention but not acquisition of the context-shock association and that context learning, consolidation, and retrieval is unaffected by alcohol treatment. Next, the novel object recognition (OR) and object location recognition (OL) paradigms were developed as a probe for examining the spatial learning processes affected by alcohol, without engaging the long-term consolidation, retrieval, and context-shock learning/retention components of the CPFE (Experiment 3). During testing in the OR task, a familiar object is replaced with a novel object. However, during testing in the OL task, a familiar object is placed in a novel location. In Experiment 4, we report no impairment in single-day novelty detection on PD31 in either task for either Group EtOH or Group SI. This suggests contextual/spatial learning was not impaired by alcohol, in agreement with the results of Experiment 2. Lastly, Experiment 5 examined the impact of neonatal alcohol on Egr-1 mRNA expression in the infralimbic (IL) and prelimbic (PL) subregions of the medial prefrontal cortex (mPFC), the CA1 region of dorsal hippocampus, and the lateral nucleus of the amygdala (LA), following the preexposure and training phases of the CPFE. Following context preexposure on PD31, early-growth response gene 1 (Egr-1) mRNA was elevated in both Group EtOH and Group SI compared to baseline control animals in all regions analyzed. Following both preexposure and training, Group EtOH displayed a significant decrease in mPFC Egr-1 mRNA expression compared to Group SI. However, this dissociation was greatest after training. Training day decreases in Egr-1 expression were not found in LA or CA1 in Group EtOH compared to Group SI. These studies demonstrate that deficits in CPFE performance produced by PD7-9 alcohol exposure likely reflect a failure to consolidate or retrieve a context-shock association, rather than a deficit in hippocampal conjunctive processes (context learning, consolidation, pattern completion) that occur prior to shock on the training day. Additionally, the alcohol-induced impairment of mPFC Egr-1 mRNA expression during the context-shock association may contribute to these deficits. These findings illustrate the value of the CPFE for characterizing the separable memory processes that are impaired by neonatal alcohol exposure.