Jaap M. Koolhaas

Behavioural strategies of aggressive and non-aggressive male mice in active shock avoidance

The hypothesis, partly based on findings in social interactions, that aggressive mice generally adopt an active behavioural strategy (cf. fight-flight) in threatening situations, while non-aggressive ones generally assume a passive strategy (cf. conservation-withdrawal) was tested using a two-way active shock avoidance paradigm. Overall, aggressive mice were found to be better active shock avoiders than non-aggressive animals; a finding that is consistent with our hypothesis. However, within the non-aggressive mice a clear dichotomy in high and low avoidance individuals was found. The high intertrial activity in the superior avoidance groups and the low activity in the poor avoidance group was interpreted as another indication of an active versus passive strategy respectively. Accordingly, it was concluded that not all non-aggressive mice assume a passive strategy, but that some mice adopt an active strategy, like all aggressive males.

Male Wistar rats are more susceptible to lasting social anxiety than Wild-type Groningen rats following social defeat stress during adolescence

Adolescence is an important period for the development of adult social competences. Social stress during adolescence may contribute not only to an inadequate social development but also to adult vulnerability to social anxiety. There seems to be a clear individual differentiation, however, in the vulnerability to the long-term negative consequences of social stress. The current study further explores this individual vulnerability and is aimed at the influence of social stress during adolescence on adult social anxiety and its context specificity. Rats from different strains (Wistar and Wild-type Groningen rats) were exposed to the resident-intruder paradigm five times during 10 min each in the period between postnatal day 45 and 58. Three and 7 weeks later, the animals were re-exposed to the context in the presence of either a dominant male or an anestrous female behind a wire mesh screen. Wistar rats that were socially defeated spent less time exploring the social stimulus in comparison with socially defeated Wild-type rats and their non-defeated controls. We conclude that the stressed Wistar rat shows signs of generalized social anxiety indicating that the Wistar rat can be considered as a vulnerable phenotype to effects of adolescent social stress.

Behavioural strategies of aggressive and non-aggressive male mice in response to inescapable shock

The effect of exposure to inescapable long-duration shocks of moderate intensity on intershock activity and on subsequent escape or avoidance performance was studied in aggressive and non-aggressive male mice. The activity of the non-aggressive mice was severely suppressed during the inescapable shock session, while that of the aggressive males was hardly influenced. The decremental effect of prior shock exposure on subsequent response latency and activity in an active two-way escape or avoidance task was greater in the non-aggressive than in the aggressive mice. There was no evidence that learned inactivity or learned helplessness (an associative deficit) could explain the results. Instead, individual differences in behavioural strategy in response to threatening situations appeared to account for the effects of inescapable shock. Aggressive male mice predominantly adopted an active behavioural strategy in challenging situations, which resulted in persistent attempts to exercise control over the external situation and hence in a sustained tendency to initiate responses. Non-aggressive mice primarily assumed a passive strategy; their tendency to exercise control was low, which readily resulted in a reduced tendency to initiate responses.

Adolescent social defeat disturbs adult aggression-related impulsivity in wild-type rats

Adolescence is generally considered as a developmental period during which adverse social experiences may have lasting consequences in terms of an increased vulnerability to affective disorders. This study aimed at determining the individual susceptibility to adolescent social stress using a rat model. We used rats of the Wild-type Groningen strain, which are characterized by a broad variation in adult levels of aggression and impulsivity. We hypothesized that experience of social defeat in adolescence results in heightened aggression and impulsivity levels in adulthood. In contrast to our expectation, adolescent social defeat did not lead to a difference in the average adult level of aggression and impulsivity, but the significant correlation between offensive aggression and impulsivity found in control animals was not present in animals defeated during adolescence.