Effects of fear on exploratory behaviour in the rat

Social isolation and adult handling are supposed to alter emotionality in rats. In order to investigate the hypothesis that dark preference in albino rats is dependent on their emotional level, one group of male, 60-day-old Wistar rats was housed in pairs, another group was housed individually, both for 30 days. After this period one half of each group was handled for 7 days. The animals were tested on several consecutive days in a light-dark choice situation. Handled animals stayed significantly less time in the dark than non-handled ones. Isolation on the other hand, increased time spent in the dark. Groups not only differed in the mean times they stayed in the dark, but also showed different forms of curves over the days. These results could be replicated in a second experiment. Data seem to support the hypothesis of dark preference as an emotional response. Dark preference is a phenomenon which has often been reported in investigations on albino rats. Lockard (1963) gives an extended review of light-dependent behaviour in rodents. During recent years, dark preference has been frequently used in research on biochemical transfer of learning, because it has been con­ sidered a very stable and consistent trait (Gay & Raphelson 1967; Ungar, Desiderio & Parr 1972). An animal is said to have this property if in a light-dark choice situation, it goes preferentially to the dark. In these transfer experiments it is assumed that injections of extracts from the brains of rats who had learned to avoid the dark, reduce dark preference in recipient rats. We tried to replicate some of these findings in our laboratory, but soon started to doubt whether dark preference is in fact a consistent trait in albino rats. It seemed to us that animals which we thought to be fearful, preferred the dark part of a two-compartment box more than animals which showed fewer signs of fear. In order to investigate this sug­ gestion we planned to manipulate the emotional level by means of handling and social isolation. Adult handling is considered to make rats less fearful and more exploratory (Winokur, Stern & Taylor 1959; Meyers 1965; Joffe & Levine 1973), while social isolation is expected to have the opposite effect (Ader & Friedman 1964; Archer 1969; Morrison 1969). Both treatments were chosen because they are in­ volved implicitly in many investigations. In most experiments, for example, animals get some amount of handling while being picked up by the experimenter. There is, however, often a lack of an appropriate control for possible effects of handling. From the hypothesis that dark preference in albino rats is dependent on the emotional state of the animals, the prediction was made that handling would reduce dark preference and that social isolation would increase it. Methods Experiment 1 Eighty, male, random-bred albino rats from the Wistar-Glaxo strain, bred in our laboratory, were housed after weaning at 28 days in groups of four in Macrolon cages (38 χ 75 χ 15 cm). Food and water were available ad libitum. The light period lasted from 08.00 to 20.00 hours. Once a week the animals were picked up from their cages and put into clean ones. At 60 days of age they were randomly assigned to two groups containing forty animals each. All of them were rehoused in an automated animal care system (UNO), with cage walls made of metal. The cages measured 32 X 26 χ 19 cm. Cage cleaning and handling in connection with it were from then on unnecessary. The animals of one group were housed individually (isolated group), those of the other group were housed in pairs (non-isolated group). After 30 days in these conditions both groups were subdivided randomly into two subgroups. In both housing conditions the animals of one subgroup were handled, those of the other subgroup remained undisturbed. Handling con­ sisted of removing an animal from its cage, holding it in one hand for 5 min and stroking its back with the thumb of the other hand about sixty times a minute. This handling treatment was given on seven consecutive days. After this the animals were again rehoused in Macrolon