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Notes: Accumulating evidence suggests that both oxytocin and arginine vasopressin (AVP) are vital components in the regulation of body fluid balance. However, the physiological role of oxytocin and possible cooperative interactions between oxytocin and AVP in sodium balance remain obscure, even though recent studies using oxytocin knockout (OTKO) mice suggested that oxytocin may contribute to the regulation of salt appetite. In the present study, we examined the effects of salt loading (drinking 2% NaCl for 5 days) on the expression of the AVP gene in the paraventricular (PVN) and supraoptic nuclei (SON) of wild-type, OTKO and heterozygous littermates using in situ hybridization histochemistry. In addition, the effects of salt loading on the expression of the oxytocin gene were also examined in wild-type and heterozygous mice. Under the non salt-loaded condition, the levels of AVP mRNA in the PVN and SON of OTKO mice were significantly decreased compared to those in wild-type mice. Nevertheless, the up-regulation of the expression of the AVP gene in response to salt loading was preserved in OTKO mice. The degree of the up-regulation in OTKO mice tended to be greater compared to those in wild-type mice, suggesting compensatory up-regulation of the expression of the AVP gene in OTKO mice after salt loading. The basal levels of oxytocin mRNA in the PVN and SON of heterozygous mice were significantly lower than those in wild-type mice. Salt loading caused an increase of oxytocin mRNA levels in the PVN and SON of both wild-type and heterozygous mice. The ratios of increase of oxytocin mRNA levels were very similar between wild-type and heterozygous mice, suggesting that the single remaining oxytocin gene in heterozygous mice responds normally to an osmotic cue. Finally, salt loading tended to increase the serum concentration of sodium regardless of genotype, and there were no genotype differences in both the control and salt-loaded groups. These results suggest ways in which oxytocin may play a cooperative role together with AVP in the regulation of sodium balance.

Notes: The neuropeptide oxytocin is released not only into the blood, but also within the brain in response to various stressors. Accumulating evidence suggests that central oxytocin may play a major role in the regulation of neuroendocrine responses to stress. In the present study, using the oxytocin knockout mouse model, we tested whether oxytocin might act to attenuate stress-induced up-regulation of corticotropin-releasing hormone (CRH) mRNA expression in the brain. The expression of CRH mRNA in the paraventricular nucleus (PVN) after 4 h of restraint stress was examined in oxytocin gene-deficient (OTKO), wild-type and heterozygous male mice using in situ hybridization histochemistry. We found that basal levels of CRH mRNA were not different among the three genotypes. Although restraint stress resulted in a significant increase of CRH mRNA expression in the PVN regardless of genotype, the degree of stress induced-up-regulation was significantly higher in OTKO mice than in wild-type mice. The effects of restraint stress on the expression of the arginine vasopressin (AVP) and the oxytocin genes were also examined. Unlike CRH mRNA, basal expression (in nonstressed control groups) of AVP mRNA in OTKO mice, as well as oxytocin mRNA in heterozygous mice, was significantly lower in the PVN and the supraoptic nucleus than in wild-type mice. After restraint stress, the expression of AVP mRNA was significantly increased in the PVN of OTKO mice compared to the nonstressed control group, whereas the expression of both AVP and oxytocin mRNA were unchanged in the PVN and the supraoptic nucleus of wild-type and heterozygous mice. Finally, in a separate set of mice, restraint stress-induced Fos expression was also examined in several brain regions involved in stress response, including the lateral septum, the bed nucleus of the stria terminalis (BNST), the medial preoptic area, the PVN, the medial and central amygdala using immunohistochemistry. After 90 min of restraint stress, the number of Fos-expressing cells significantly increased in all brain regions examined regardless of genotype. However, the number of stress-induced Fos-expressing cells in the BNST and the medial amygdala of OTKO mice was significantly lower than in wild-type mice. Collectively, the findings in the present study suggest that oxytocin may regulate stress-induced CRH gene expression in the PVN. Furthermore, neuronal activity in the BNST and the medial amygdala may be involved in this neuroendocrine regulatory system.