Abstract
Previous studies have established that the expression of defensive rage behavior in the cat is mediated over reciprocal pathways that link the medial hypothalamus and the dorsolateral quadrant of the midbrain periaqueductal gray matter (PAG). The present study was designed to determine the roles played by 5-HT 1A and 5-HT 2C receptors in the medial hypothalamus on the expression of defensive rage behavior elicited from electrical stimulation of the PAG. Monopolar stimulating electrodes were placed in the midbrain PAG from which defensive rage behavior could be elicited by electrical stimulation. During the course of this study, defensive rage was determined by measuring the latency of the 'hissing' component of this behavior. Cannula-electrodes were implanted into sites within the medial hypothalamus from which defensive rage behavior could also be elicited by electrical stimulation in order that serotonergic compounds could be microinjected into behaviorally identifiable regions of the hypothalamus at a later time. Microinjections of the 5-HT 1A receptor agonist 8-OHDPAT (0.1, 1.0 and 3.0 nmol) into the medial hypothalamus suppressed PAG-elicited hissing in a dose-dependent manner. Administration of the 5-HT 1A antagonist p-MPPI (3.0 nmol) blocked the suppressive effects of 8-OHDPAT upon hissing. The suppressive effects of 8-OHDPAT were specific to defensive rage behavior because this drug (3 nmol) facilitated quiet biting attack. Microinjections of the 5-HT 2C receptor agonist (±)-DOI hydrochloride into the medial hypothalamus (0.5, 1.0, and 3.0 nmol) facilitated the occurrence of PAG-elicited hissing in a dose-dependent manner. In turn, these facilitating effects were blocked by pretreatment with the selective 5-HT 2 antagonist, LY-53,857, which was microinjected into the same medial hypothalamic site. The findings of this study provide evidence that activation of 5-HT 1A and 5-HT 2 receptors within the medial hypothalamus exert differential modulatory effects upon defensive rage behavior elicited from the midbrain PAG of the cat.
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