Ontogeny of Psychomotor and Sensory Functions in the Rat: Effects of Sexual Dimorphism

Abstract

Background: We studied the development of eight (8) different psychomotor and sensory functions in male and female rats, from postnatal day 10 to 45, with the aim of determining whether the ontogenesis of these functions was subject to sexual dimorphism. Methods: Wistar rats bred according to standard conditions in our laboratories were put into reproduction. Ten days after whelping, male and female pups were identified and subjected to a battery of behavioral tests on postnatal days 10, 15, 20, 25, 30 and 45, to assess the development of the following psychomotor and sensory functions: Exploratory activity, locomotor activity, emotional defecation, hind paws lifting reflex latency, wire-grasping time, Latencies of execution of crawling along the wire and of leap onto the ground, nociception (tail flick) and body weight. Results: Only complex brain functions generated by cerebral cortex activities, i.e. exploratory activity and leap execution latency, do not undergo differential development sex-dependent. However, voluntary motor functions initiated in the motor cortex, and requiring high peripheral muscle performance such as crawling execution latency and wire-grasping time developed more rapidly in males than in females. Correlatively, body weight i.e. muscle mass index increased more speedily in males than in females. On the other hand, studies of automatic motor functions such as locomotor activity, and reflex motor functions i.e . hind paws lifting reflex latency and tail flick latency showed earlier development in females than in males. In addition, the study of emotional response, an emanation of limbic structures, showed prodigious development in females compared to males. Conclusion: Our studies have shown that there is a developmental sexual dimorphism of the central nervous system in the rat. Indeed, studies of automatic and reflex motor functions, whose activities are essentially linked to the spinal cord and brainstem, indicated that hindbrain areas develop more speedily in females than in males. Likewise, study of the emotional response emanating from diencephalic limbic structures, in particular the hypothalamus, showed a prodigious and early development in females compared to males. Taken together, our studies indicate that the vast majority of brain structures and functions reached maturation earlier in females than in males. Estrogen is the trigger hormone for early maturation of the female brain.