Background

We designed the electric-foot defensive behavior model to discriminate individual behavioral strategy of the rats in response to stressful experiences. As generally accepted now, individuals can display two coping strategies in aversive conditions: the active and the passive one [1, 2, 4, 6, 7, 8]. The active aversive strategy was determined as attempts to escape aversive stimulation ("coping with" behavior). Contrary, passive aversive strategy was described as a long-lasting freezing duration and "refuse" to cope with source of aversive stimulation. To study different aspects of response to stress in this approach, some experimental paradigms were developed, such as the shock-probe burying test and the conditioned defensive burying test [5, 9]. In these paradigms rats diplay a behavior strategy (active or passive) which is due to the experimental procedure but not to their "personality" [1]. Differently to that, we would like to propose a model which allows to discriminate diverse individual coping strategies of rats in response to the biologically identical stressful exposure.

Apparatus

In our model a cage (48 x 24 x 22 cm) with electrified grid floor is used. The electrical scheme includes direct and alternating (50 Hz) current, and provides electrical stimulation of the animal in any possible dispositions in the cage. The power of stimulation (in mA) is regulated by an amperometer.

Procedure

The first step of our test is the evaluation for each rat of the individual threshold of electric current intensity which is able to provoke its aversive response. Therefore we put the animal into the cage and 3 min later (when the rat stops to investigate a cage) we administrate electric current (0.5 mA) through the floor, which is augmented slowly until the intensity (0.8-1.4 mA, for the majority of rats - 1.0 mA) when the rat flinches and shows running/jumping/rearing or intensive exploratory reaction (these behaviors were taken together to form "locomotor response"). This behavior is used as an indicator of active aversive strategy. If instead locomotor response rat display long-lasting freezing behavior, this is taken as a passive aversive strategy. In the current experiment rats are used after threshold evaluation at least 3 days later. We place a rat into the cage for 3 min, then we submit each animal to stimulation of intensity, which was found individually for each rat in the first step. The duration of stimulation applied in rats with active aversive strategy was taken as a sum of latency of "locomotor response" + 2 s, but no longer that 30 s, and 30 s for rats with passive aversive strategy. Furthermore, we record the latency of first movement (time between start of stimulation and first movement of head, body or paws), latency of the locomotor response, and for 3 min after switching off a current, the duration of exploring, freezing and grooming behavior, number of rearings. From our data obtained in examination of Wistar rats (n=81) in described model, 86.67% of rats displayed active aversive strategy, and 13.33% showed the passive one. These data were found to correlate to those from available literature (Bohus B., 1995). Thus, the electric-foot defensive model can provide a discrimination of individual coping strategy of rats in aversive conditions.

References

1. Boer, S.F. de; Slangen, J.L.; van der Gagten, J. (1990). Plasma catecholamine and corticosteron levels during active and passive shock-prode avoidance behavior in rats: effects of chlordiazepoxide. Physiology & Behavior, 47, 1089-1098.
2. Bohus B. et al. (1974)
3. Bohus (1995)
4. Bohus B. et al. (1995)
5. Pinel, J.P.J.; Treit, D. (1983). The conditioned defensive burying paradigm and behavioral neuroscience. In: T.E. Robinson (ed.). Behavioral Approaches to Brain Research, pp. 212-233. Oxford: Oxford University Press.
6. Rotenberg V.S. (1979)
7. Valdman A.V. (1976)
8. Valdman A.V. (1986)
9. Wiersma, A.; Bohus, B.; Koolhaas, J.M. (1996). Corticotropine-Releasing Hormone in the central amigdala enhances active behavior responses in the conditioned defensive burying paradigm. Stress, 1, 113-122.