Dopamine suppresses stimulus-induced field potentials in layer III of rat medial entorhinal cortex

We studied the effect of dopamine (DA) on field potential responses in layer III of the medial entorhinal cortex (mEC) evoked by synaptic stimulation of superficial layers of the lateral EC either by single or paired-pulses. The maximum amplitude of the field response was reduced to 50.6+/-2.6% by DA (500 microM) bath-applied for 5 min. Both the D1 antagonist SCH 23390 (20 microM) and the D2 antagonist sulpiride (20-50 microM) reduced the effect of DA to the same extent, namely 18% less reduction in their presence. The D2 agonist quinelorane (500 microM) reduced the field response to 75+/-4.0%, whereas the D1 agonist SKF38393 was without significant effect. The paired-pulse ratio increased significantly during DA application. These results suggest predominant D2 receptor involvement in the decrement of synaptic transmission in layer III by dopamine. Paired-pulse experiments may be indicative of a presynaptic mechanism of action.