Naloxone-induced modulation of feline aggression elicited from midbrain periaqueductal gray

In the present study, peripheral administration of naloxone hydrochloride (IP) was employed to identify the role of endogenous opioid peptides in the regulation of two forms of aggressive behavior in the cat--affective defense and quiet biting attack behavior. These forms of aggressive behavior were elicited by electrical stimulation of dorsal and ventral aspects of the midbrain periaqueductal gray, respectively, utilizing monopolar electrodes. Following the establishment of stable baseline thresholds for affective defense and quiet biting attack behavior, naloxone (0.5, 1.0, 4.0 and 7.0 mg/kg) and saline (vehicle control) were administered peripherally (IP). The response thresholds were tested 5-30, 30-60, 60-90, 180-210 and 1440-1470 min following naloxone administration. These results indicated that a dose level of 7.0 mg/kg of naloxone had a profound facilitatory effect on affective defense behavior. Response threshold values returned to prenaloxone baseline levels at 1440-1470 min postinjection. Administration of lower doses of naloxone (1.0 and 4.0 mg/kg) also resulted in a significant facilitation of this response but of shorter durations. Neither the lowest dose of naloxone (0.5 mg/kg) nor saline (vehicle control) were effective in modifying the threshold for affective defense behavior. In contrast, when tested for its effects upon quiet biting attack, the maximum dose utilized in this study (7.0 mg/kg) tended to suppress this response although the overall effect was not significant. The selective dose-dependent facilitatory effects of naloxone upon affective defense behavior in the cat suggests that the opioid peptide system plays a significant (inhibitory) role in the regulation of this response.