Relaxin-3 mRNA levels in nucleus incertus correlate with alcohol and sucrose intake in rats

Background: Chronic alcohol intake produces multiple neuroadaptive changes, including up- and down-regulation of neuropeptides and receptors. There are widespread projections of relaxin-3 containing neurons to, and abundant relaxin family peptide 3 receptor (RXFP3) expression within, brain regions involved in modulating alcohol intake. Recently we demonstrated the involvement of relaxin-3/RXFP3 signalling in alcohol-seeking in rats; therefore in this study we examined whether relaxin-3 and/or RXFP3 expression were altered by chronic alcohol intake in alcohol-preferring iP rats. Methods: Expression of relaxin-3 mRNA in the hindbrain nucleus incertus and RXFP3 radioligand binding levels in discrete forebrain regions were investigated following voluntary intake of alcohol or sucrose for 12 weeks, with a 2 day washout, using quantitative in situ hybridisation histochemisty and in vitro receptor autoradiography, respectively, in cohorts of adult, male iP rats. Results: Levels of relaxin-3 mRNA in the hindbrain nucleus incertus were positively correlated with the level of intake of both alcohol (r(12)= 0.59,p = 0.03) and sucrose (r(7)= 0.70,p = 0.04) in iP rats. Dense binding of the RXFP3-selective radioligand, [(125)]-R3/15, was detected in hypothalamic and extrahypothalamic sites, but no significant changes in the density of RXFP3 were observed in the brain regions quantified following chronic sucrose or ethanol intake. Conclusions: Our findings suggest high endogenous relaxin-3 expression may be associated with higher intake of rewarding substances, rather than its expression being regulated in response to their intake, consistent with an active role for the relaxin-3/RXFP3 system in modulating ingestive and alcohol-related behaviours. (C) 2014 Elsevier Ireland Ltd. All rights reserved.