Abstract
Behavioural inhibition is the ability to suppress irrelevant behaviours that are either unsafe or are inappropriate for the task at hand. Neuropsychological studies have linked behavioural inhibition primarily to the actions of the medial prefrontal cortex (mPFC), the hippocampus (HPC), as well as several additional regions of the brain. The current investigation addressed the role of the mPFC-hippocampal pathway on behavioural inhibition within the place-preference task. The task required rats to enter and wait for two seconds within a defined region within a circular maze for food reward. Muscimol, a GABA-A agonist, was used to create a transient block of the prelimbic cortex (PL) within the mPFC to examine its effects on both behaviour and on hippocampal electrophysiology. Recording electrodes were surgically implanted into the dorsal and ventral hippocampal CA1. The rationale for this line of inquiry stems from a growing body of research purporting a link between the mPFC and the HPC in terms of theta-coordinated communication between the two regions during mnemonic tasks. When muscimol was infused into the PL, rats had a markedly decreased ability to remain within a circularly-defined goal region for the two-seconds required to obtain a food reward. The question that arose from this finding was whether or not transient lesions to the PFC resulted in the observed behaviour because the PFC was acting through non-hippocampal regions to inhibit animals’ movement within the goal region, or was a signal sent from the mPFC so as to influence the dorsal and/or ventral HPC via their inputs so as to disrupt behaviour through hippocampal mechanisms. To address this query, data was analyzed in two ways. The first method was to look at two second blocks of time that corresponded to only correct responses across trial number, time segment and effect of drug on theta amplitude and frequency. The second method focused on the first one second of entry into the goal region to look at the effects of lesions to the PL for both correct responses and incorrect responses. Contrary to expectations, inhibiting the mPFC with muscimol did not result in significant changes to either the power of theta or the frequency within both the dorsal and ventral CA1 in the first method of analysis. In contrast, ANOVA analyses on the one second blocks of data revealed that a decrease in dorsal CA1 theta frequency was associated with a subsequent incorrect response. Correlating hippocampal theta to behaviour produced mixed results that were inconclusive. The current investigation demonstrates that the PL plays a key role in behavioural inhibition. However, it does not provide conclusive evidence that the HPC has a role in behavioural inhibition, at least with regard to the completion of the goal in the place preference task, if theta is a marker for behavioural inhibition.