Do rats really express neophobia towards novel objects? Experimental evidence from exposure to novelty and to an object recognition task in an open space and an enclosed space

Abstract

Three set of experiments were performed in an enclosed space (open-field) and in an open space (elevated platform). The surface of the open-field and the elevated platform were divided in nine equal squares. Rats were exposed (without previous habituation) in a unique session (experiment 1) or three consecutive sessions (experiment 2) either to an open-field (enclosed space) or to an elevated platform (open space) with and without an object on the centre of the field. In experiment 3, rats were exposed (without previous habituation) either to an enclosed or an open space on five consecutive sessions, one session a day. They were tested in an object recognition test in sessions 1, 3 and 5. In sessions 2 and 4, no objects were present. In experiment 1, we recorded the latency, frequency and duration of entries into different areas of the field. In experiment 3, we recorded the latency, frequency and duration of contacts with objects in addition to entries into different areas of the field.

The first experiment demonstrates that rats exposed for the first time to an enclosed or an open space do not express neophobia toward novel objects in the absence of walls that surround an open-field. They crossed frequently into and spent more time in areas occupied with an object than in unoccupied areas. After two sessions of habituation to an empty open space or an empty enclosed space, the latency of first approach to a novel object is reduced while the frequency and duration of approaches are increased. The third experiment on object recognition confirmed that rats do not avoid novel objects; they made frequent visit and spent more time in the corner of the field occupied with an object than in empty corners. Recording of crossings provided detailed information about the patterns of exploratory behavior of rats but failed to reveal discrimination between novel and familiar objects which was evident in both open and enclosed space with recording of contacts with objects on the fifth exposure.

Introduction

Exploration of novelty in an open-field has been extensively exploited in neuroscience studies of behavior and brain functions in rats and mice. It is used in the study of anxiety [25], [49], [76], [79], [86], [140], reward and addiction [18], [4], [6], [20], [56], [136] as well as in the study of memory [7], [38], [39], [40], [47], [48], [55], [81], [103], [133]. It has been thought to provide evidence of neophobia and anxiety and it has been used in drug screening and in the genetic dissection of the molecular targets for novel anxiolytic drugs [29], [53], [75], [82], [117], [130], [142]. It has also been thought to provide evidence for novelty seeking and it has been used in the study of the reward system and the pharmacological actions of addictive substances [10], [21], [121], [129], [139]. Additionally, it has been considered to provide measure of discrimination and recognition memory, and it has been used in drug and lesion studies and in screening for genes and other molecular targets responsible for learning and memory processes [14], [30], [38], [39], [40], [54], [62], [65], [64], [87], [90], [100], [102], [103], [106], [141].

In order to study anxiety, animals are directly exposed to novelty in an open-field with or without an object in the central area whereas in order to study memory animals are introduced to an open-field with or without object for several sessions of habituation before the start of the object recognition test. In most learning and memory studies the number and duration of habituation sessions are determined a-priori and vary considerably between laboratories. In a previous report, we suggested that one can take advantage of the habituation phase and record the reaction of animal to unfamiliarity and novelty [49]. In the present report, we suggest the omission of the habituation phase and to directly assess anxiety and memory simultaneously. It is likely that during the first sessions of the test, anxiety is predominant and prevents object recognition whereas after repeated exposures, anxiety would subside and animals would be able to recognize between a novel and a familiar object. The reason for the omission of the habituation phase is that genetic factors and pretreatment applications may either reduce or prolong animal emotionality which would have subsequent effects on learning and memory performance. One strain of animals may require more sessions of habituation than another strain to start freely moving around and adventuring into the arms of a maze or approaching an object in an open-field. A drug pretreatment or a lesion application which take place several days before habituation may prolong the emotionality of animals which would persist during the first training sessions of a learning and memory test. It is also possible that a drug with potential anxiolytic and/or nootropic properties may facilitate object recognition in the first sessions after direct exposure to the test.

In most studies of the effects of stress on behavior and cognition, very often animals are exposed to a stressful stimulus then transferred to a behavioral test where their learning and memory performance is assessed [2], [9], [41], [42], [46], [69], [96], [108], [147], see: Refs. [11], [78], [97], [99], [126], [132]. The stress stimuli used can be in the form of an administration of an electric shock [89], physical restraint [5], [33], [80], [92], cold stress [80], [88], [138], forced swim [46], [104], or exposure to a predator or to a predator odor [23], [43], [45], [108]. Except in shuttle-box escape and avoidance tasks [3], [8], [93], [113], [120], [143], the environments in which stressors are administrated are very often totally different from the environment of the test in which cognition is assessed. The effects of a stressful episode may not be always transferable to other conditions and it is questionable whether cognitive processes involved during and those involved after exposure to a stressful episode are modulated by comparable neurobiological mechanisms. In the real world cognition is an ongoing process which might be required at its optimum under conditions of stress. Therefore, understanding the state and function of cognitive processes under stress is relevant to various human and animal circumstances. Indeed, efficient learning and memory processes may be required to overcome immediate emotionally challenging circumstances.

The purpose of the present study is to examine how rats would respond to novelty exposure and perform an object recognition test on first and subsequent exposures to an enclosed space and an open space using the same experimental test settings and conditions that induce anxiety and involve discrimination between a novel and a familiar object. The rationale for this study is that numerous tasks in human everyday life are performed under stress or in an anxiogenic environment and that exposure to novelty is thought to induce neophobia which is used as an indicator of anxiety. Therefore, animals exposed to an unfamiliar environment are expected to show neophobic responses towards novel objects and poorer performances in an object recognition task. In order to evaluate the discriminant validity of the measure of neophobic or anxiogenic responses animals were tested in an open space and in an enclosed space. In a previous report, we suggested that an enclosed space involves fear which induces escape or avoidance and that an open space involve fear which induces anxiety. We expect that object recognition is less affected in the former than in the latter. We also used two methods for recording in the object recognition task, one based on crossings into areas and another method based on contacts/approaches to objects to examine which of the two methods is appropriate to reveal discrimination between objects.