Ontogeny of behavioral arousal: The role of environmental stimuli.

Abstract

During ontogenesis the developing rat has been reported to pass through a transient period of intense behavioral arousal which peaks at 15 days of age, a phenomenon that has been interpreted to reflect a sequential caudal to rostral development of excitatory and inhibitory systems in the mammalian brain. This period of intense hyperactivity occurs only when the animal is tested in an unfamiliar environment. The degree of arousal is proportional to the dissimilarity between the home cage and the test environment, and isolation per se is insufficient to elicit the arousal response. Environmental temperature has a minimal influence on the degree of behavioral arousal seen in either familiar or unfamiliar environments. Unlearned responses to pheromonal or other naturally occurring nest odors do not suppress the high levels of locomotor activity evoked by unfamiliar environments in the 15 day old rat pup. It is fear or distress evoked by the unfamiliar environment rather than curiosity that underlies this developmental phenomenon. The sequential increase and decrease in locomotor activity that occurs during ontogenesis cannot be used to support the principle of caudal to rostral development of excitatory and inhibitory centers in the CNS.