Neural synchrony and oscillatory markers of morphine reward in prefrontal-mesolimbic and prefrontal-hippocampal pathways.

The prelimbic cortex (PrL) plays a critical role in reward-related memory processes. While its functional interactions with the ventral tegmental area (VTA) and cornu ammonis 1 (CA1) are implicated in reward circuitry, the specific oscillatory dynamics between these regions during the expression of morphine-associated memory remain poorly understood. In this study, local field potentials (LFPs) were recorded from the PrL, CA1, and VTA to investigate the PrL-CA1 and PrL-VTA pathways in a rat model of morphine-induced conditioned place preference (CPP). Recordings were analyzed during both pre-conditioning and post-conditioning (expression) phases. Coherence and Granger causality (GC) analyses evaluated functional synchronization and directional information flow across these pathways. A comparison of relative power between saline- and morphine-treated rats during the post-conditioning phase revealed significant alterations across all LFP sub-bands in the VTA. Coherence analysis demonstrated a significant increase in PrL-VTA synchronization during the expression phase, particularly within the theta and alpha bands. In contrast, coherence within the PrL-CA1 pathway remained unaltered during either phase. Furthermore, GC analysis indicated distinct, band-specific directional changes: a significant increase in delta-band causality in both directions of the PrL-VTA pathway, alongside specific reductions in delta, beta, and low-gamma information flow from the PrL to CA1. These results indicate that the expression of morphine-associated reward memory recruits the PrL-VTA pathway through increased functional synchronization and bidirectional information flow, while overall PrL-CA1 coherence remains unaffected. These findings highlight pathway-specific oscillatory dynamics underlying reward conditioning and memory retrieval.