Abstract
The emotional learning such as fear conditioning is likely to be inhibited during the lactation period. We examined whether conditioned taste aversion (CTA), a kind of the emotional learning, is also inhibited during the lactation period or not. When 0.2% sodium saccharin as a conditioned stimulus (CS) was followed by an i.p. injection of 0.15 M LiCl as an unconditioned stimulus (US), the acquisition of CTA memory (CTAM) of lactating mice was comparable to those of naive female mice. However, the extinction of the lactating mice significantly delayed more than that of the naive mice. In the two-bottle preference test, mice during the lactation period preferred water rather than high concentration of saccharin, whereas naive and post-lactating female mice preferred saccharin over all the concentration tested. In contrast, all the mice preferred sucrose much to water. Oxytocin, a nonapeptide hormone, which is produced in the hypothalamus and secreted from the posterior pituitary gland during the late pregnancy or lactation period, is thought to contribute to the social behavior such as maternal behavior. Therefore, we tried to inject oxytocin to the lateral ventricle. Both acquisition and retention of CTAM in the oxytocin-injected mice were markedly raised more than those of the sham-operated mice, suggesting that oxytocin secreted during the lactation period affects the acquisition and/or retention mechanisms of the CTAM. [J Physiol Sci. 2008;58 Suppl:S92]
