Relaxin peptide hormones are protective during the early stages of ischemic stroke in male rats

The pregnancy hormone relaxin protects tissue from ischemic damage. The ability of relaxin-3, a relaxin paralog, to do so has not been explored. The cerebral expression of these peptides and their receptors make them logical targets for study in the ischemic brain. We assessed relaxin peptide-mediated protection, relative relaxin family peptide receptor (RXFP) involvement and protective mechanisms. Sprague-Dawley rats receiving permanent or transient middle cerebral artery occlusions (pMCAO and tMCAO, respectively), were treated with relaxin peptides and brains collected for infarct analysis. Activation of the endothelial nitric oxide synthase (eNOS) pathway was evaluated as a potential protective mechanism. Primary cortical rat astrocytes were exposed to oxygen glucose deprivation (OGD), treated with relaxin peptides, and viability examined. Receptor involvement was explored using RXFP3 antagonist or agonist treatment and PCR. Relaxin and relaxin-3 reduced infarct size after pMCAO. Both peptides activated eNOS. As relaxin-3 has not previously been associated with this pathway and displays promiscuous RXFP binding, we explored receptor contribution. Expression of rxfp1 was greater than rxfp3 in rat brain, although peptide binding at either receptor resulted in similar overall protection after pMCAO. Only RXFP3 activation reduced infarct size after tMCAO. In astrocytes, rxfp3 was greater than rxfp1 gene expression. Selective activation of RXFP3 maintained astrocyte viability after OGD. Relaxin peptides are protective during early stages of ischemic stroke. Differential responses among treatments and models suggest that RXFP1 and RXFP3 initiate different protective mechanisms. This preliminary work is a pivotal first step in identifying clinical implications of relaxin peptides in ischemic stroke.