Pharmacology Biochemistry & Behavior, Vol. 16, pp. 569-571, 1982. Printed in the U.S.A. The Effects of Methadone on the Social Behavior and Activity of the Rat MARK PLONSKY AND PHYLLIS R. FREEMAN 1 Department of Psychology, State University of New York at New Paltz, New Paltz, NY 12561 Received 4 March 1981 PLONSKY, M. AND P. R. FREEMAN. The effects of methadone on the social behavior and activity of the rat. PHARMAC. BIOCHEM. BEHAV. 16(4)569-571, 1982.--Pairs of 80-day-old female rats were given SC injections of either 0, 1, 2.5, or 4 mg/kg of methadone hydrochloride on each of 6 days. Both animals of the pair received the same dose. One hour postinjection, each pair was observed in a circular arena for a five minute period during which the following dependent measures were recorded: total time in contact, latency to initial contact, frequency of aggressive grooms, and locomotor activity. The results indicated that the rats treated with methadone spent less time in contact, took longer to contact, and aggressively groomed each other less frequently than rats treated with a saline vehicle. Also, the results suggested that the disruption of social behavior produced by methadone was not a reflection of decreased activity levels. Methadone Opiates Social behavior Aggression Locomotor activity Rat METHADONE is an opiate drug commonly used for the treatment of heroin addicts. Although the purpose of admin- istering methadone to heroin addicts is to rehabilitate them both physically and socially, relatively little is known about its effects on social behavior. There is some suggestion, however, that it may decrease sociability in humans. In one study using ex-addict volunteers and a self-administration procedure, Babor et al. [1 ] found that the percentage of wak- ing time subjects spent in social isolation was greater when receiving methadone than when given no drug. While studies of methadone's effects on the social behav- ior of humans are important, there is much information to be gained from examining the effects of methadone on the social behavior of animals, primarily because there are numerous variables which cannot be manipulated in humans for ethical and/or practical reasons. Crowley et al. [2] worked with a small group of Bonnet macaques and administered a dose of about 10-20 mg/kg/day orally. They found that "the overall total of associative behaviors, including proximity, contact, grooming, and clasping, fell during methadone administra- tion" (p. 139). In addition, they noted that "a net increase in motor activity occurred despite intermittent drowsy nod- ding" (p. 141). Therefore, the possibility exists that the observed decreases in social behavior were a result of the altered motor activity, rather than being specific effects of the drug. Thus, the present study was designed to investigate the effects of methadone on the social behavior and activity of the rat. In an effort to dissociate the effects of methadone on social behavior from its effects on activity, we used several doses of methadone which were expected to influence activ- ity in opposite directions. A low dose was expected to stimu- late activity [3], and a higher dose to suppress activity (the latter had been observed during pilot work). If social behav- ITo whom reprint requests should be sent. ior is influenced in a similar fashion when activity is both stimulated and depressed, then it would be difficult to attribute the alterations produced in social behavior to changes produced in activity. METHOD Subjects Seventy-two naive female Wistar rats approximately 80 days old at the time of testing were used. They were obtained from Royalhart Laboratory Animals, Inc. (New Hampton, NY) in 2 squads of 36 each, separated by a 21 day interval. After an initial analysis of each dependent measure revealed no significant differences between squads, the squad data were combined for the final analyses. Animals were main- tained on a 12:12 hr light-dark cycle. Purina lab chow and water were available ad lib. Apparatus The apparatus was a circular arena consisting of a six beam photoactivity cage (BRS/LVE, PAC001; Fogelsville, MD) measuring 38 cm (height) × 61 cm (diameter). The in- side of the apparatus was finished flat black and a 25 watt light bulb in a conical shield was hung 61 cm above the floor. The testing room was supplied with white noise (50 dB) and the only light in the room was that of the apparatus. Procedure After extensive handling, the animals were housed indi- vidually for 8 days prior to, as well as during, the 6 days of testing. Prior to experimental manipulations, the animals were randomly assigned permanent partners and these pairs of animals were then randomly assigned to one of four drug Copyright © 1982 ANKHO International Inc.--0091-3057/82/040569-03503.00/0