Central injection of relaxin-3 receptor (RXFP3) antagonist peptides reduces motivated food seeking and consumption in C57BL/6J mice

Abstract

Behavioural arousal in mammals is regulated by various interacting central monoamine- and peptide-neurotransmitter/receptor systems, which function to maintain awake, alert and active states required for performance of goal-directed activities essential for survival, including food seeking. Existing anatomical and functional evidence suggests the highly-conserved neuropeptide, relaxin-3, which signals via its cognate Gi/o-protein coupled receptor, RXFP3, contributes to behavioural arousal and feeding behaviour in rodents. In studies to investigate this possibility further, adult male C57BL/6J mice were treated with the selective RXFP3 antagonist peptides, R3(B1-22)R/I5(A) and R3(B1-22)R, and motivated food seeking and consumption was assessed as a reflective output of behavioural arousal. Compared to vehicle treatment, intracerebroventricular (icv) injection of RXFP3 antagonists reduced: (i) food anticipatory activity before meal time during food restriction; (ii) consumption of highly palatable food; (iii) consumption of regular chow during the initial dark phase, and; (iv) consumption of regular chow after mild (∼4-h) food deprivation. Effects were not due to sedation and appeared to be specifically mediated via antagonism of relaxin-3/RXFP3 signalling, as RXFP3 antagonist treatment did not alter locomotor activity in wild-type mice or reduce palatable food intake in relaxin-3 deficient (knock-out) mice. Notably, in contrast to similar studies in the rat, icv injection of RXFP3 agonists and infusion into the paraventricular hypothalamic nucleus did not increase food consumption in mice, suggesting species differences in relaxin-3/RXFP3-related signalling networks. Together, our data provide evidence that endogenous relaxin-3/RXFP3 signalling promotes motivated food seeking and consumption, and in light of the established biological and translational importance of other arousal systems, relaxin-3/RXFP3 networks warrant further experimental investigation.

Central relaxin-3 receptor (RXFP3) activation reduces elevated, but not basal, anxiety-like behaviour in C57BL/6J mice

“…Although there is a possibility that the doses of RXFP3-A2 or R3(B1-22)R were not sufficient to elicit a response in all assays, the 1 nmol dose of RXFP3-A2 was similar to that used in our rat study [33] and the 4 nmol dose of R3(B1-22)R was based on a study in mice in which a reduction in several measures of motivated food intake were observed [44], indicating that this is a physiologically active dose. Furthermore, our positive data appear comparable to findings in a study in which a cohort of rats spent an average of ~15% of the total test time in the light half of the L/D box [33], similar to the ~20% of total time spent in the light half by FG-7142/aCSF treated mice.…”

“…Analysis of innate anxiety in the behavioural tests used previously revealed that icv R3(B1-22)R (4 nmol; a dose shown to reduce motivated food seeking in mice [44]) increased anxiety-like behaviour in the EPM ( Table 2 In regard to the latter, RXFP3 mRNA is abundantly expressed in several brain areas associated with control of anxiety-like behaviours, including the well characterised extended amygdala [6,[50][51][52]. Relaxin-3 is primarily expressed in GABA neurons, as reflected by the co-expression of relaxin-3 and glutamate decarboxylase immunoreactivity in the rat nucleus incertus [18], which suggests RXFP3 activation may act to reinforce inhibitory signalling on target neurons.…”

“…Injections (1 µL) were made via an injector protruding beyond the guide cannula (see above) and into the lateral ventricle 15 min before each behavioural test. Doses used and the timing of behavioural testing (see Results) were based on studies that indicated effective doses in rats and mice and the time course of activity of these peptides post-icv injection [29,33,44]. indicate that 30-60 min post-injection is the period when anxiogenesis is highest [48].…”

“…For example, increased feeding and modest body weight gain following acute and chronic central RXFP3 activation is well characterised in the rat (see [31] for review), but icv or intrahypothalamic injection of a specific RXFP3 agonist does not induce food intake in mice [44] and studies of relaxin-3 and RXFP3 knockout (KO) mice reveal no differences in body weight relative to their wildtype (WT) littermates [45,46].…”

The role of relaxin-3 and its receptor RXFP3 in defense of elevated body weight in diet-induced obesity

“…signaling is implicated in stress and anxiety [41,[48][49][50] , feeding and metabolism [48,[51][52][53] , motivation, reward, and arousal [49,54,55] .…”