Effect of quinpirole on timing behaviour in the free-operant psychophysical procedure: evidence for the involvement of D2 dopamine receptors.

RATIONALE: Operant timing behaviour is sensitive to dopaminergic manipulations. It has been proposed that this effect is mediated principally by D(2)-like dopamine receptors. However, we recently found that the effect of d-amphetamine on timing in the free-operant psychophysical procedure was mediated by D(1)-like dopamine receptors. It has not been established whether stimulation of D(2)-like receptors affects timing in this schedule.
OBJECTIVE: To examine the effects of a D(2)-like receptor agonist quinpirole on second-range timing and the ability of dopamine receptor antagonists to reverse quinpirole's effects.
MATERIALS AND METHODS: Rats responded on two levers (A and B) under a free-operant psychophysical schedule in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rates [percent responding on B (%B) vs time (t)] under each treatment; quantitative timing indices [T (50) (value of t when %B = 50) and Weber fraction] were compared among treatments.
RESULTS: Quinpirole (0.04, 0.08 mg kg(-1)) reduced T (50). This effect was attenuated by D(2)-like receptor antagonists haloperidol (0.05, 0.1 mg kg(-1)), eticlopride (0.04, 0.08 mg kg(-1)) and sulpiride (30, 60 mg kg(-1)), but not by the D(3) receptor-preferring antagonist nafadotride (0.5, 1 mg kg(-1)), the D(4) receptor antagonist L-745870 (1, 3 mg kg(-1)) or the D(1)-like receptor antagonist SKF-83566 (0.015 mg kg(-1)).
CONCLUSIONS: Results suggest that quinpirole reduced T (50) via an action at D(2) receptors. D(1)-like and D(2)-like receptors may mediate behaviourally similar but pharmacologically distinct effects on timing behaviour.