The discriminative properties of the D1 dopamine agonist dihydrexidine in the rat

The objective of this study was to train rats to discriminate the interoceptive stimuli produced by a selective dopamine D₁ agonist. Fourteen male Sprague-Dawley rats acquired the discrimination of the fully effective, high potency, D₁ agonist dihydrexidine (DHX) within 20 sessions using a training dose of 3.0 mg/kg. DHX (0.75–4.5 mg/kg) dose-dependently increased DHX-appropriate responding with an ED₅₀=1.44 mg/kg. The selective D₁ agonist SKF 38398 (2.0–8.0 mg/kg) dose-responsively generalized with an ED₅₀=3.54 mg/kg; significantly less potent than DHX. The selective D₁ antagonist SCH 23390 (0.06–0.12 mg/kg) dose-responsively decreased DHX-appropriate discriminative performance. These data would indicate that DHX is a selective D₁ agonist that may allow for testing of the selectivity of other putative D₁ agonists in this experimental procedure. Administration of non-selective dopaminergically active drugs, including apomorphine, amphetamine and cocaine, were each shown to produce intermediate DHX-appropriate discriminative performance.