Gender and cognitive aspects of neonatal and juvenile neuromuscular locomotor development of F1 hybrid mice in swim tests

Abstract

F1 hybrid pups from crosses between the strains 129SvEv/Crl and C57BL/6/Crl were subjected to an analysis of the development of adult swimming pattern (from the day of birth until 21 days old) to study the potential gender difference in neuromuscular development of neonatal and juvenile mice and the cognitive component in the development of swimming skills. Swimming as a parameter of scrutiny was chosen because it requires total coordination of the body's muscles, and we have previously demonstrated that the gradual change from a neonatal to an adult swimming pattern follows a fixed pattern, that can be scored objectively. Five different parameters were scored: the position of the head in the water, the use of front legs, the use of hind legs, the use of the tail as a rudder and whether or not the animals are able to maintain a straight course in the water. Each parameter could be objectively scored as 0 (neonatal), 1 (juvenile) or 2 (adult) level of development. There was no significant difference between development of locomotor skills in female and male pups. The maximum score obtained at any given day of development was not altered by learning from the previous daily swimming experiences. However, in individual swimming sessions, the time span between exposure to the water and display of maximum swimming score for the age was significantly shortened by daily exposure to water and swimming, indicating habituation to submersion in water. Startle reactions to water exposure could be minimized and finally eliminated by daily swimming sessions. This suggests a cognitive component limited, however, by the physical maturation of the nervous system and muscles, thus not resulting in acceleration of the development of swimming skills.