Elsevier

Neuroscience

Volume 93, Issue 1, June 1999, Pages 243-251
Neuroscience

High versus low reactivity to a novel environment: behavioural, pharmacological and neurochemical assessments

https://doi.org/10.1016/S0306-4522(99)00158-XGet rights and content

Abstract

Based on their rearing response to a novel open field, male Wistar rats were divided into two sub-groups with either high or low behavioural activity (high rearings, versus low rearings). These sub-groups were repeatedly exposed to the same open field and tested for behavioural habituation. Since we previously found neurochemical evidence for different cholinergic reactivities in such high rearing and low rearing rats, their behavioural responses to the muscarinic antagonist scopolamine (0.5 mg/kg) were also investigated in the open field. Additionally, they were exposed to the elevated plus-maze to test for possible differences in measures of anxiety. After behavioural testing, tissue concentrations of biogenic amines were determined in the ventral striatum (nucleus accumbens, olfactory tubercle), frontal cortex, striatum, hippocampus and amygdala. The results show that the higher rearing responses of high rearing rats in the novel open field were paralleled by higher locomotor activity. These behavioural differences between groups decreased with repeated open field exposure, an effect which was largely due to between-session habituation in high rearing rats. Thereby, high rearing rats approached the lower levels of low rearing rats, in which locomotor activity and rearings did not habituate between testing. Nevertheless, habituation was also observed in low rearing rats, especially in the measure of thigmotactic scanning, since the levels of scanning declined both between and within test sessions. The anti-cholinergic challenge with scopolamine induced a general pattern of behavioural activation. Furthermore, scopolamine partly reinstated the behavioural differences between high and low rearing rats that had been observed in the novel open field, since high rearing rats showed more rearing behaviour than low rearing rats under scopolamine. In contrast to the open field, there were no significant differences between high and low rearing rats in the plus-maze. The neurochemical analysis revealed, among others, higher dopamine levels in the ventral striatum of high rearing rats together with lower serotonin levels in the medial frontal cortex.

The current findings thus indicate that high and low rearing rats not only differ in their behavioural response to a novel environment, but also in their patterns of behavioural habituation, and with respect to behaviour induced by an anti-cholinergic challenge. These differential behavioural profiles of high and low rearing animals are discussed with respect to the role of dopaminergic mechanisms in the forebrain, and the potential impact of cholinergic mechanisms.

Section snippets

Animals

Twenty-four male Wistar rats (RCC-BLR, Füllingsdorf, Switzerland), weighing between 250–300 g at the beginning of the experiment were used. They were housed individually under standard laboratory conditions and allowed free access to food and water. All animals were handled daily for three days prior to behavioural testing.

Drugs

Scopolamine (0.5 mg/ml; Sigma) was dissolved in 0.9% saline for i.p. injection. The injection volume was 1 ml/kg.

Open field testing

The open field consisted of a black wooden chamber (60×60×40 cm)

Results

Assigning the animals to sub-groups with high and low levels of rearings in the novel open field yielded the following behavioural and neurochemical profiles.

Discussion

The present work provides a pattern of behavioural and neurochemical differences between animals with high or low rearing responses in a novel open field (defined as HR vs LR rats). Besides the number of rearings, these sub-groups of animals also differed in their levels of locomotor activity. In contrast, other behavioural parameters (thigmotaxis, centre time, faecal boli) did not indicate differences between HR and LR rats in the novel environment. With repeated testing, rearing, scanning,

Conclusions

The current findings complement and extend the existing literature on differential behavioural responses to novelty in rats. Animals that showed higher rearing responses also differed from animals with low rearings in their locomotor response to novelty, in behavioural habituation, and in scopolamine-induced rearing. Neurochemically, these differential behavioural profiles might be mediated by systematic variations in striatal DA and frontal cortical serotonin function, together with

Acknowledgements

This work was supported by grant Schw 559/2-2 from the Deutsche Forschungsgemeinschaft. R. K. W. Schwarting is a Heisenberg-fellow of the Deutsche Forschungsgemeinschaft.19.33.59.

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