Department of Pharmacology, Rudolf Magnus Institue for Neurosciences, Utrecht University, Utrecht, The Netherlands
Social play is the earliest form of non-mother-directed social behaviour. This specific type of behaviour is restricted to a short period of life between weaning and sexual maturation. Social play behaviour is of major importance for the normal develop- ment of social behaviour, because rats are very susceptible to the effects of social isolation during this period of life. Such an isolation causes both short- and long-term effects, such as abnormal patterns of social behaviour. We use an automated social interaction test, capable of quantifying disturbances in social behaviour. The automated system ("EthoVision") determines the positions of the rats and can calculate the relative position to each other. The amount of approach and avoidance behaviour can be measured for each animal. The main direction of movement can be derived from the total movement towards and away from each aninmal and this parameter is used to determine the social interest of each rat.
In the present study, the role of exogenous opioids in behavioural changes observed after social isolation during the play period was investigated. Since social isolation can probably be considered as a lack of endogenous opioids released by social activity, the hypothesis was that treatment with the opiate morphine should counteract the isolation-induced changes in later social behaviour. Rats were raised during the play period (week 4 and 5 of life) under different housing conditions. They were exposed to either total social isolation (I) or socially-housed (S). The groups of animals were treated daily (subcutaneously) with either saline or morphine (1 mg/kg). During these weeks the body weights and sweet solution intake were measured and in adulthood their social behaviour was registered with the automated system.
Isolates have higher body weights and consume more sucrose solution relative to socially housed animals. Morphine treatment caused a decraese in body weights and sucrose intake in isolates, but in socially housed animals an increase in body weight and sucrose consumption. To measure social interest in adulthood, morphine-treated rats were tested versus saline-treated rats with a similar housing condition. Isolated morphine-treated rats showed an increase in a positive direction of movements (in cm) relative to saline-treated isolates. Socially housed morphine-treated rats showed a negative direction of move-ment as compared to saline controls.
These data show that the motivational state with respect to social contact in adulthood can be influenced by morphine treatment during the play period. The experiments underscore the role of opioids in the development of social behaviour.