Evidence for dopamine D₁ receptor involvement in the stimulus selection task: overshadowing in the rat

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Abstract

Rationale. A number of lines of evidence suggest that dopamine might play a role in stimulus selection, the process whereby specific cues are selected to guide action.

Objectives. In order further to define the potential role for dopamine in stimulus selection, the present series of studies examined whether dopaminergic drugs modulate overshadowing, a paradigm that involves stimulus selection in rats. Overshadowing is where preferential learning occurs to one (usually the more salient) element of a stimulus compound.

Methods. Overshadowing was measured in rats using a thirst motivated conditioned emotional response paradigm (CER). Two simultaneously presented stimuli (light and tone) were paired with an aversive unconditioned stimulus (mild footshock); overshadowing is observed when learning to the less salient stimulus is weaker than learning to the same stimulus when it is conditioned alone.

Results.d-Amphetamine sulphate (1 mg/kg, IP) was found selectively to disrupt overshadowing, without affecting the CER in control animals. The dopamine (DA) D₂ receptor antagonists, haloperidol (0.2 mg/kg, IP) or raclopride (0.5 mg/kg, IP), failed to reverse amphetamine-induced disruption of overshadowing. In contrast, the selective DA D₁ antagonist SCH 23390 (0.05 mg/kg, IP) reversed amphetamine-induced disruption of overshadowing. The partial DA D₁ agonist SKF 38393 (5 mg/kg, IP) was found to abolish overshadowing when given alone.

Conclusion. These data indicate a modulatory role for the DA D₁ receptor in the expression of stimulus selection and suggest that the DA D₁ receptor might play a role in salience allocation aspects of learning.