Connexin 32 mRNA levels in the rat supraoptic nucleus: up-regulation prior to parturition and during lactation.

Toward the end of pregnancy, mammalian mothers undergo a cascade of hormonal, neural and somatic events that culminate in birth and lactation. Milk ejection, the final step in lactation, is regulated by high-frequency, synchronized firing of oxytocinergic neurons in the magnocellular nuclei of the hypothalamus. This synchronization of neural activity may be regulated by an increase in the cell-to-cell coupling of magnocellular neurons by connexins. Direct intercellular channels between neurons are formed by the connexin 32 protein. The present study examined the pattern of connexin 32 mRNA levels using semiquantitative in situ hybridization histochemistry with isotopically labeled cRNA probes complementary to connexin 32 and connexin 43, found in neurons/oligodendrocytes and astrocytes, respectively. Connexin 32 mRNA levels were relatively low in the supraoptic nucleus of virgin females and were dramatically up-regulated during late pregnancy. Immediately after birth, connexin 32 mRNA levels dropped and were similar to levels in virgins. On the 13th day of lactation when dye-coupling between magnocellular neurons is high, connexin 32 message levels were again very high. Levels of the astrocytic connexin 43 were no different in lactating and virgin animals. These results demonstrate that there are two peaks of connexin 32 expression, one immediately preceding parturition and one during lactation. So far, only the second peak of connexin 32 is known to be related to an increase in dye-coupling of magnocellular neurons in the supraoptic nucleus, suggesting that in this case, the elevation of connexin 32 message levels leads to the subsequent increase in intercellular coupling.