Distribution of relaxin‐3 and RXFP3 within arousal, stress, affective, and cognitive circuits of mouse brain

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Abstract
Relaxin‐3 (RLN3) and its native receptor, relaxin family peptide 3 receptor (RXFP3), constitute a newly identified neuropeptide system enriched in mammalian brain. The distribution of RLN3/RXFP3 networks in rat brain and recent experimental studies suggest a role for this system in modulation of arousal, stress, metabolism, and cognition. In order to facilitate exploration of the biology of RLN3/RXFP3 in complementary murine models, this study mapped the neuroanatomical distribution of the RLN3/RXFP3 system in mouse brain. Adult, male wildtype and RLN3 knock‐out (KO)/LacZ knock‐in (KI) mice were used to map the central distribution of RLN3 gene expression and RLN3‐like immunoreactivity (‐LI). The distribution of RXFP3 mRNA and protein was determined using [35S]‐oligonucleotide probes and a radiolabeled RXFP3‐selective agonist ([125I]‐R3/I5), respectively. High densities of neurons expressing RLN3 mRNA, RLN3‐associated β‐galactosidase activity and RLN3‐LI were detected in the nucleus incertus (or nucleus O), while smaller populations of positive neurons were observed in the pontine raphé, the periaqueductal gray and a region adjacent to the lateral substantia nigra. RLN3‐LI was observed in nerve fibers/terminals in nucleus incertus and broadly throughout the pons, midbrain, hypothalamus, thalamus, septum, hippocampus, and neocortex, but was absent in RLN3 KO/LacZ KI mice. This RLN3 neural network overlapped the regional distribution of RXFP3 mRNA and [125I]‐R3/I5 binding sites in wildtype and RLN3 KO/LacZ KI mice. These findings provide further evidence for the conserved nature of RLN3/RXFP3 systems in mammalian brain and the ability of RLN3/RXFP3 signaling to modulate “behavioral state” and an array of circuits involved in arousal, stress responses, affective state, and cognition. J. Comp. Neurol. 518:4016–4045, 2010.

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