This dissertation examines context learning in a rodent model of Fetal Alcohol Spectrum Disorders (FASD). Alcohol exposure in the rat over postnatal days (PD)4–9 leads to significant reductions in CA1 pyramidal cells and impairs behavior on tasks thought to depend on hippocampal function. There are significant limitations in much of the literature examining hippocampal function in rodent models of developmental alcohol that complicate the interpretation of results. Some of these limitations involve the use of behavioral tasks that can be solved through multiple (non-hippocampal) strategies, the lack of proper performance and non-associative control tasks, and the limited use of multiple measures of hippocampal function. The studies in this dissertation were designed to overcome many of these limitations through the use of a behavioral task that uniquely engages hippocampal processes and is sensitive to hippocampal insult, and by examining behavioral, molecular, and anatomical effects in the same animals through a dose-response design.
Here I examine the effects of neonatal alcohol exposure on hippocampal function using a variant of the contextual fear conditioning paradigm, the context preexposure facilitation effect (CPFE). In the CPFE, rats preexposed to the testing context (Pre) show enhanced contextual conditioning to an immediate shock given on a subsequent occasion relative to rats preexposed to an alternate context (No Pre).
In Experiment 1, rats were exposed to a single binge dose of alcohol at one of three doses (2.75, 4.00, or 5.25g/kg/day), sham intubated (SI), or left undisturbed (UD) over PD4-9. Rats were preexposed to Context A (Pre) or B (No Pre) on PD31, received an immediate 1.5 mA foot shock in Context A on PD32, and were tested for contextual conditioning in Context A on PD33. UD- and SI-Pre rats showed the CPFE, i.e., context preexposure facilitated contextual conditioning, relative to their No Pre counterparts. An immediate shock deficit (no conditioning) was present in all No Pre groups, regardless of previous alcohol exposure. In Experiment 1, blood alcohol level was negatively correlated with contextual freezing. Group 2.75g-Pre did not differ from controls. Group 4.00g-Pre froze significantly less than Groups UD- and SI-Pre but more than Group 5.25-Pre, which showed the immediate shock deficit. In Experiment 2, rats were exposed to 5.25g/kg/day over PD4-6 or PD7-9, with SI and UD rats acting as controls. All groups were again run on the CPFE over PD31-33. Alcohol exposure limited to PD7-9, but not PD4-6, disrupted the CPFE.
I next examined the relationship between behavioral impairment, hippocampal cell loss, and hippocampal c-Fos expression in rats exposed to alcohol over PD4–9. Rats were sacrificed 2h following different phases of the CPFE (preexposure, training, testing) and processed for c-Fos immunohistochemistry (Exp. 3 and 4) and CA1 pyramidal cell quantification (Exp. 4). In Exp. 3, c-Fos+ cells in the DG were consistently high among normally developing rats preexposed to the testing (Pre) or an alternate (No Pre) context or sacrificed directly from their home cage (Controls) and this did not differ across CPFE phases. CA3 and CA1 c-Fos+ cells were highest during preexposure and decreased across training phases, with Group No Pre showing greater numbers of c-Fos+ cells during training than Group Pre and Controls. In Exp. 4, SI rats showed higher numbers of CA1 c-Fos+ cells following preexposure compared to Group 5.25g, with Group 4.00g not differing from either group. Exp. 4 also revealed a linear decline in CA1 pyramidal cells across treatment groups. In Exp. 3, Group Pre showed the CPFE whereas Group No Pre showed an immediate shock deficit. In Exp. 4, Group SI shows the CPFE, whereas both Groups 4.00g and 5.25g showed the immediate shock deficit.
These studies demonstrate dose-response reductions in the CPFE, behaviorally-induced CA1 c-Fos, and CA1 pyramidal cells. In addition, behavioral impairments occurred when alcohol exposure was limited to a narrow window of the neonatal period (PD7-9). The known sensitivity of the CPFE to hippocampal manipulations, normal performance on controls tasks, and reductions in both behaviorally induced c-Fos and CA1 pyramidal cells, suggests that the behavioral impairments demonstrated by alcohol-exposed rats during the CPFE reflect alcohol-induced hippocampal dysfunction. By employing the CPFE, in addition to both molecular and anatomical markers, researchers will better understand the relationship between developmental alcohol exposure and the hippocampus.