Effects of apomorphine and haloperidol on plasma cortisol levels in conscious dogs.

Cortisol levels were measured in peripheral plasma of conscious dogs after i.v. administration of apomorphine, a dopaminergic agonist, haloperidol, a dopaminergic antagonist, or apomorphine in combination with haloperidol. Apomorphine in a dose of 0.001 and 0.005 mg/kg did not cause any release of cortisol whereas 0.01 and 0.05 mg/kg caused a dose dependent and peakshaped increase of plasma cortisol levels from 20 to 30 and from 20 to 60 ng/ml respectively. Basal levels were reached again within 1 h. Haloperidol (0.1 mg/kg), when given alone, induced a continuous rise of plasma cortisol levels. After 60 min the levels were approximately 80 ng/ml and still tended to rise. If apomorphine (0.01 mg/kg) was given 30 min after the injection of haloperidol (0.5 mg/kg) no further effect of the apomorphine injections was observed. We therefore speculate that apomorphine activates central dopaminergic receptors which cause a release of ACTH from the pituitary into the circulation, which in turn stimulates the release of cortisol from the adrenals. The fact that not only apomorphine but also haloperidol stimulated cortisol release suggests a complex role or dopamine transmission involving different dopaminergic pathways as well as different dopamine receptors. It is also possible that injection of haloperidol induces a stress reaction in the dogs, which causes a release of cortisol from the adrenals.