Developmental expression of RFamide-related peptides in the rat central nervous system.

RFamide-related peptides (RFRP-1 and RFRP-3) have been recently identified in mammals and considered to play significant functional roles in the rat brain. In this study, we report the developmental expression of RFRP mRNA and its immunoreactive neuronal cells and fibers in the rat brain. The RFRP mRNA was expressed in the brain from embryonic day 15 (E15) according to reverse transcription-polymerase chain reaction analysis. We first detected RFRP mRNA expressing neurons in the caudal portion of the hypothalamus at E16 by in situ hybridization analysis. Immunohistochemical analysis showed that RFRP-3 or RFRP-1 immunoreactive neuronal cell bodies were first detected at E16 or E17, respectively. Double-labeling fluorescent immunohistochemical analysis showed that neurons containing both RFRP-1 immunoreactivity (ir) and RFRP-3-ir were detected from E18. We also detected RFRP-1 immunoreactive nerve fiber processes in the forebrain, hypothalamus, thalamus, midbrain, pons and medulla oblongata at prenatal day and the distribution of RFRP-1 immunoreactive nerve fibers in postnatal day 0 (P0) were almost coincident with that in adult. However, localization of RFRP-3 immunoreactive nerve fibers was limited around the RFRP-3 immunoreactive neuronal cell bodies during prenatal days. The distribution of RFRP-3 immunoreactive nerve fibers was first detected in the above areas at P0. The nerve fibers containing only RFRP-3-ir in the thalamus or spinal cord were first appeared at P21 or P28. Our results show that RFRP mRNA was expressed during the neonates and the distribution of RFRP-1 or RFRP-3 immunoreactive nerve fibers would be distinctly regulated in the developing rat brain.