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Abstract
The effects of cyproheptadine, a serotonin (5-HT) antagonist and clozapine, a 5-HT/dopamine (DA) antagonist, on the agonistic, submissive, and nonagonistic social approach behaviors of aggressive and submissive hamsters were investigated. Research has indicated that 5-HT is involved in the expression of aggressive behavior, and that DA is involved in the expression of social approach behavior.
Cyproheptadine and clozapine were hypothesized to decrease the aggressive animals' aggressive behaviors, to increase nonagonistic social approach behaviors in both aggressive and submissive animals, and to decrease the submissive animals' submissive behaviors. Clozapine was hypothesized to increase the nonagonistic social approach behaviors in both aggressive and submissive animals to a greater degree than cyproheptadine because of clozapine's dopamine antagonist effect.
The study was conducted over five consecutive days, and all encounters occurred during the first two hours of the dark cycle and lasted five minutes. A resident-intruder paradigm was used to determine social status. Data was collected from the resident if it was the first to display either aggressive or submissive behavior, or from the intruder if it was the first to display aggressive or submissive behavior. For the first three days, the animals were paired with the same partner. On day 4, the partner was changed to see if the animal remained aggressive or submissive. If the status did not change, the animal had a new partner on day five. Drugs were injected intraperitoneally in a volume of 5 ml/kg. Eight mg/kg of cyproheptadine, or 0.3 mg/kg of clozapine, or 5 ml/kg of distilled water was injected thirty minutes before the social encounter on day five. Three groups each of seven aggressive and seven submissive animals were used.
Cyproheptadine decreased the aggressive animals' offensive aggressive behaviors and clozapine increased the submissive animals' reciprocated social and reciprocated approach behaviors. Internal mechanisms, operating differently in aggressive and submissive hamsters, may have altered the substrates upon which cyproheptadine and clozapine supposedly work. The repeated defeat stressor may have increased the number and sensitivity of the 5-HT receptors in the submissive animals, and could account for the differential effects of cyproheptadine and clozapine in aggressive and submissive animals.
