Expression of relaxin mRNA and relaxin receptors in postnatal and adult rat brains and hearts. Localization and developmental patterns.

Relaxin is a polypeptide hormone best known for its role in parturition. However, high affinity relaxin receptors have been localized in the rat brain and heart in addition to the uterus. Several lines of evidence also suggest that relaxin may be involved in the regulation of blood pressure, heart rate, and the release of oxytocin and vasopressin. We now show by Northern analysis that a 1-kilobase relaxin transcript is detected in the rat brain as well as the ovary of pregnant rats. Using in situ hybridization, relaxin mRNA is localized in discrete regions of the male and female brains, including the anterior olfactory nucleus, tenia tecta, pyriform cortex, neocortex, and hippocampus. Developmental studies show that relaxin mRNA is present in the 1-day postnatal brain, while relaxin receptors are not detectable until 7 days after birth. The relaxin receptor binding affinity was similar in the developing brains, but there was a steady increase in relaxin binding sites during postnatal days 7 to 29, suggesting that relaxin may play a role in brain maturation. While relaxin mRNA is not detected in the heart, high levels of relaxin receptors are detected in the cardiac atrium as early as 1 day after birth. These atrial receptors remained at similar levels throughout postnatal development, suggesting an important role for relaxin in cardiovascular function.