Late effect of dopamine D1/5 receptor activation on stimulus-induced BOLD responses in the hippocampus and its target regions depends on the history of previous stimulations
fMRI was used to study late effects of dopamine D1/5 receptor activation on hippocampal signal processing and signal propagation to several target regions. The dopamine D1/5 receptor agonists SKF83959 and SKF38393 were intraperitoneally applied without, immediately before or 7 days after electrical stimulation of the right perforant pathway with bursts of high-frequency pulses. Control animals received a 0.9% NaCl solution. One day after D1/5 receptor activation, the perforant pathway was stimulated and the induced BOLD responses in the right hippocampus and its target regions, left hippocampus (l-HC) and medial prefrontal cortex (mPFC), were measured. Depending on the temporal relation between dopamine receptor activation and the first perforant pathway stimulation the induced BOLD response pattern differed. When applied without concurrent perforant pathway stimulation, the agonists caused region-selective increases in the induced BOLD responses: the effect of SKF83959 was evident in the mPFC whereas that of SKF38393 was confined to the l-HC. When applied in conjunction with perforant pathway stimulation, either agonist caused increased BOLD responses in both regions. In contrast, when applied 7 days after perforant pathway stimulation, neither SKF83959 nor SKF38393 modified the BOLD responses in the mPFC or l-HC 1 day later. These findings suggest that (i) activation of dopamine D1/5 receptors alone is sufficient to modify stimulus-induced BOLD responses in target regions of the right hippocampus 24 h later, and (ii), the history of previous stimulations crucially affects the impact of dopamine receptor activation on stimulus-induced BOLD responses.