Neonatal Ethanol Exposure Impairs Object-In-Place Learning and Trace Fear Conditioning in Juvenile Rats

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University of Delaware
The purpose of this study was to extend our previous work on neonatal alcohol exposure and object recognition performance (Jablonski et al., 2013) to the object-in-place (OiP) task. This task requires memory of both object identity and object location, and therefore serves as a combination of the standard object-recognition (OR) and object-location (OL) tasks (Barker et al., 2007; Barker & Warburton, 2011; Jablonski et al., 2013). The present study utilized 4-Object and 2-Object variants of the OiP task that were modeled after other studies (Barker et al., 2007; Ainge & Langston, 2012, respectively). Rats preferentially explore novelty—in the OiP task, an object is novel if it changed locations with another object (4-Obj) or replaced another object (2-Obj) between sample and test phases. In the 4-Object variant, 4 different objects are presented during the sample phase; the locations of the 2 left or right objects are interchanged for the test phase. The 2-Object variant consists of 2 different objects during the sample phase; one of these objects is replaced with an identical copy of the remaining object for the test phase. Lesion studies have implicated roles of prefrontal cortex (PFC), hippocampus (HPC), and perirhinal cortex (PRH) in this task (Barker studies, cited above), while trace fear conditioning (TFC) is believed to engage both PFC (Gilmartin & Helmstetter, 2010) and HPC (McEchron et al., 1998). Additionally, neonatal alcohol disrupts maturation of PFC (Whitcher & Klintsova, 2008) and HPC (Marino et al., 2004). Therefore, we predicted that neonatal alcohol would disrupt OiP learning, as well as visual TFC as a “positive control” task (Schreiber et al., 2012). We report that normative PD26 rats can perform the 4-Object, but not 2-Object, variant of the OiP task, which may imply reliance of these tasks on different neural mechanisms. Neonatal ethanol exposure during postnatal days (PD) 7-9 disrupted both the 4-Object variant of OiP in PD26 rats, as well as TFC—but not background contextual conditioning during TFC—in PD30-31 rats. These findings underscore the previously supported claim that OR, OL, and OiP tasks rely on different neural regions and/or systems; furthermore, they are interesting in the context of our recent report that PD7-9 ethanol exposure does not impair OR and OL tasks (Jablonski et al., 2013). Future studies could better inform understanding of the relationship between ethanol exposure window, brain targeting, and behavioral deficits.