Dopamine inhibits excitatory synaptic responses in layer I of the rat parasubiculum

The parasubiculum is a component of the hippocampal formation that projects to the entorhinal cortex and plays an important role in spatial navigation. Dopamine has marked effects on excitatory synaptic transmission in the hippocampus and entorhinal cortex, and the present study investigated the effects of dopamine on evoked field excitatory postsynaptic potentials (fEPSP) in layer I of the rat parasubiculum in vitro. Application of 50 μM dopamine for 15 min resulted in a reduction in the amplitude of fEPSPs to 71 ± 5 % of baseline that was reversed following washout of dopamine. A lower concentration of 10 μM dopamine had no effect. Application of the dopamine D₁-like receptor antagonist SCH23390 failed to block the reduction in fEPSP amplitude induced by dopamine, but the D₂-like receptor antagonist sulpiride prevented significant reductions in fEPSPs. Application of sulpiride alone facilitated fEPSP amplitude to 110 ± 3 % of baseline. These findings suggest that strong activation of dopaminergic inputs to the parasubiculum likely results in reduced excitatory synaptic activation of parasubicular neurons which may attenuate activity in their outputs to the entorhinal cortex.