Genetic dissection of mouse exploratory behaviour

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Abstract

A large variety of apparatus and procedures are being employed to measure mouse exploratory behaviour. Definitions of what constitutes exploration also vary widely. The present article reviews two studies whose results permit a genetic dissection of behaviour displayed in an open-field situation. The results agree that factors representing exploration and stress/fear underlie this type of behaviour. Both factors appear to be linked to neuroanatomical variation in the sizes of the hippocampal intra- and infrapyramidal mossy fibre terminal fields. Multivariate analysis of genetic correlations may render important insights into the structure of behaviour and its relations with neuroanatomical and neurophysiological systems.

Introduction

It has already been shown years ago [24] that the full genetic variation that is potentially available for a phenotype will only get expressed in ecologically meaningful situations. Therefore, if we want to understand behaviour in the context of an animal's natural habitat, then we have to attempt to study it either in the field under natural circumstances or in the laboratory under semi-natural conditions [1]. Exploratory behaviour is most often studied in the laboratory, using a large variety of methods: the field is clearly open for standardisation.

Exploration is usually evaluated by measuring the behaviour displayed by animals placed in some kind of arena. Such open-fields exist in many varieties, square, circular, or rectangular, and in many different sizes. The procedures employed are manifold, too. The exposition to the novel environment may be forced (the animal is placed in the apparatus without possibility of escape) or free (the subject is given the choice when to enter the arena). The duration of the behavioural measurement may vary from a few minutes [19] to 20 min [36] or more. Generally, only a few behavioural measures, such as activity and defecation, are taken [19], [35]. Others, however, have advocated using an ethogram to quantify exploratory activity. With the help of an ethogram, seemingly continuous behaviour is described as a sequence of successive, mutually exclusive, and distinct motor-posture patterns that represent species-specific units of behaviour which may be quantified subsequently by measuring their frequency and/or duration. Complex behavioural responses are thus regarded as organised appearances of the behavioural units. Ethograms of behaviour displayed in open fields have been devised, among others, for rodents [36] and paradise fish [21].

Section snippets

Defining exploration

Mice are attracted by novel stimuli and they spend long periods exploring when exposed to a novel environment, even when satiated in every aspect. Although seemingly simple, some confusion exists on the precise definition of exploratory behaviour. Most authors merely equate exploratory behaviour with ‘activity’, ‘open-field behaviour’, or even treat it as the opposite of ‘emotionality’ (whatever that may be). This is a moot point. Some authors feel that even the more sharply defined locomotor

Genetic correlations

A weakness inherent in correlational studies is that a phenotypical correlation between characters does not necessarily reflect a functional relationship. On the other hand, if two independent processes, one causing a positive relationship, the other causing a negative relationship, act simultaneously upon two characters, the effects may cancel each other so that no detectable correlation can emerge. These problems can to a large extent be avoided by looking at the genetic correlations, that

Hippocampal mossy fibres and exploration

Although many different theories exist that address the question of the proper function of the hippocampus, most agree, more or less, that this structure is intimately involved with the processing of information about the environment [32]. This notion is supported by evidence from lesion studies [22], pharmacogenetic findings [38], [39], and electrophysiological data, such as the observation that dentate synapses become potentiated during exploratory learning in rats [27].

The information

Genetic dissection of exploratory behaviour and hippocampal neuroanatomical variation

Two experiments have been published in which exploratory behaviour and hippocampal mossy fibres were analysed employing genetic correlations [16], [30] and in what follows their results will be briefly presented.

In the first study, Crusio et al. [16] carried out a diallel cross, in which five different inbred strains were intercrossed in all possible combinations, producing 25 genetically different populations with a total of 150 males being analysed. At the age of 3 months, all animals were

Conclusion

The studies discussed above confirm the early findings of Whimbey and Denenberg [43] that exploratory behaviour as displayed in an open-field is multifactorial, with exploration and fear/stress being the main motivational systems underlying the behavioural variation observed. It might be noted that although in the diallel cross a strong genetic correlation between locomotion and defecation was observed (data not shown), this did not appear in Roullet and Lassalle's strain study. In addition,

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