Behavioural differences in high and low explorers in tests

When the HE- and LE-rats were compared in various tests that have traditio-nally been used to measure different aspects of emotionality in studies on anxio-lytic and antidepressant drugs, significantly different behavioural profiles were found.

When tested in a cage similar to home cage, no differences were found between HE and LE animals (Paper I), suggesting that the HE/LE differen-tiation is specific to the exploration box and not to more familiar environments.

The important aspect of exploration box behaviour is probably the emergence of the animal from the small compartment attached to the open part of the apparatus.

The elevated plus-maze is one of the most widely used test paradigms to investigate anxiety-related behaviour (Carobrez et al., 2005). The present experiments show higher anxiety levels in elevated plus-maze in rats with low activity in exploration box test (Figure 2), and while the decrease in activity described by File (1993) on trial 2 is visible in both groups, the LE animals remain the less active group even then. Liebsch et al. (1998) have shown that in animals bred on the basis of their high or low anxiety-related behaviour in the plus-maze (HAB and LAB, respectively), the former were less active in the black-white box and open field test. It might therefore be concluded that the exploration box test and elevated plus-maze test measure partly the same construct.

No differences were observed in social activity between HE and LE animals (Paper I). Social interaction time has often been shown to be in inverse relation with anxiety levels (File and Seth, 2003), and Henniger et al. (2000) have found that rats, selectively bred for differences in anxiety-related behaviour in the elevated plus-maze, show consistent differences in social interaction test, with animals with high anxiety-related behaviour of both sexes being less active in both tests. In our previous studies we have found that sociability – measured with successive social interaction testing – is a stable trait, with an animals’

social behaviour in each single test correlating highly with its mean social behaviour level (Tõnissaar et al., 2004). The present findings suggest that different mechanisms govern anxiety-related behaviours in those two different situations that are motivating and at the same time anxiety-provoking – namely a novel environment and a novel partner.

Figure 2. Plus-maze behaviour in HE- and LE-rats (Paper I). n(LE)=18; n(HE)=14. * – P<0.05 vs HE; ¤ – p<0.05, ¤¤ – p<0.01, ¤¤¤¤ – P<0.0001 vs Day 1. HE – high exploratory behaviour rats; LE – low exploratory behaviour rats. Bars represent mean ± SEM.

In the fear-conditioning paradigm, animals relate a formerly neutral environ-ment or other cue to a stressful event and develop anxiety towards it. HE animals showed higher freezing levels on the first test day (Figure 3), sug-gesting higher proneness to develop anxiety in response to acute stressors in HE-rats. Nevertheless, the extinction rate was faster in this group as the decrease in freezing between the two test days was greater in the HE group, suggesting more active coping style. In contrast, Borta et al. (2005) have found

in previous experiments that rats selected for their low or high amount of time spent on the open arms of the elevated plus-maze also display differences in acute and conditioned responses in a fear conditioning paradigm – namely, rats with low levels of time spent in the open arms of the plus-maze showing more freezing and emitting more distress calls.

Figure 3. Freezing in fear conditioning test in HE- and LE-rats (Paper I). n(LE)=8;

n(HE)=8. * – P<0.05 vs HE; ¤ – P<0.05 vs Day 1. HE – high exploratory behaviour rats; LE – low exploratory behaviour rats. Bars represent mean ± SEM.

Taghzouti et al. (1999) have shown a significant negative correlation (r=–0.61) between novelty-related behaviour in a novel circular corridor and immobility time on the second day of the forced swimming test, with high responders to novelty being less immobile and swimming more. Our present studies give a basically similar result, with HE animals showing less immobility and more swimming on Day 2 (Figure 4), indicative of more active coping strategies in the HE group.

The sucrose preference test has been used as a measure of anhedonic tendencies in stress and depression models. In the present experiments, the LE-rats consumed more sucrose solution (Paper I). In the light of previous findings (Harro et al., 2001b), which suggest that similarly to cases of atypical depression in clinical psychiatry in which both decrease and increase in body weight are considered as symptoms of depression, the higher levels of consumption of sweet solution in LE-rats might also be interpreted as a result of a negative affect.

Nevertheless, in the second experiment on sucrose preference reported in Paper I, the differences waned off after 18 h of food and water deprivation, as the increase of sucrose intake after the fast was significantly greater in HE animals. This could be indicative of differences in reactivity to short-term stress like food deprivation between the HE- and LE-rats, or perhaps just of differences in attaining and maintaining individual energetic balance in these two groups.

Figure 4. Forced swimming test in HE- and LE-rats (Paper I). n(LE)=18; n(HE)=14. * – P<0.05 vs HE; ¤ – P<0.05, ¤¤ – P<0.01, ¤¤¤¤ – P<0.0001 vs Day 1. HE – high exploratory behaviour rats; LE – low exploratory behaviour rats. Bars represent mean ± SEM.

4.1.3. Neurobiological differences in high and low explorers