Role of D1 dopamine receptors of the ventral pallidum in inhibitory avoidance learning.

The mesolimbic dopaminergic system (MLDS) originating from the ventral tegmental area has important role in the regulation of motivation, learning and memory. The ventral pallidum (VP), innervated by the MLDS, is involved in the regulation of adaptive behavior, but its exact role is not known in inhibitory avoidance learning. The VP contains both D1 and D2 dopamine receptors, but the density of the former subtype is more excessive. Therefore, in our present experiments, the role of D1 dopamine receptors of the VP in one trial step-through inhibitory avoidance paradigm was investigated. In the conditioning trial, animals were shocked 3 times with 0.5 mA current for 1s, and subsequently were microinjected bilaterally with D1 dopamine receptor agonist SKF38393 into the VP in three doses (0.1 μg, 1.0 μg or 5.0 μg in 0.4 μl saline). To clarify whether the agonist effect was specific, we also applied the D1 dopamine receptor antagonist SCH23390 (5.0 μg in 0.4 μl saline) 15 min prior the agonist treatment. The D1 dopamine receptor agonist, in a dose-dependent manner, significantly increased the step-through latency during the test trials: retention was significant relative to the controls even after 2 weeks of conditioning. The D1 dopamine receptor antagonist SCH23390 pretreatment eliminated SKF38393 effects in the ventral pallidum. Our results show that D1 dopamine receptor mediated mechanisms in the VP facilitate learning and memory in inhibitory avoidance paradigm and this facilitation is specific because it can be eliminated by D1 dopamine receptor antagonist.