D2 dopamine receptor-mediated mechanisms in the medial preoptic-anterior hypothalamus regulate effective defense...

The role of the dopaminergic innervation of the medial preoptic-anterior hypothalamus (mPO-AH) in regulating the expression of affective defense behavior in the cat has been investigated in the present study. Feline affective defense behavior, characterized mainly by autonomic arousal, ear retraction, growling, hissing and paw striking, was elicited by electrical stimulation of the ventromedial hypothalamic nucleus (VMH). Following the establishment of a stable threshold current for eliciting the hissing response of the behavior, the effect of injecting various DAergic agonists and antagonists into the mPO-AH on the hissing threshold was determined. The microinjection of the non-selective DA agonist apomorphine (0.03, 0.16, 0.33, 0.66, 1.56 and 3.3 nmol) into the mPO-AH facilitated hissing in a time- and dose-dependent manner. This effect was mimicked by the D2-selective agonist LY 171555 (0.2 and 1.0 nmol) but not by the D1-selective agonist SKF 38393 (1.7 and 17 nmol), and was blocked by the non-selective and the D2-selective antagonists haloperidol (1.3 nmol) and sulpiride (14.5 nmol), respectively. The injection of the D1-selective antagonist SCH 23390 (0.3 nmol), however, did not inhibit apomorphine-induced facilitation of hissing. In addition, the injection of haloperidol (1.3 nmol) and sulpiride (14.5 nmol), but not SCH 23390 (0.3 nmol), alone inhibited the behavior. It was therefore concluded that dopaminergic stimulation of the mPO-AH may facilitate the expression of affective defense behavior in the cat via a D2 receptor-mediated mechanism. The physiological significance of this effect and the interaction between dopaminergic, noradrenergic and serotonergic innervation of the mPO-AH in modulating the expression of affective defense behavior in response to threatening stimuli are discussed.