Distribution, physiology and pharmacology of relaxin-3/RXFP3 systems in brain

Abstract

Relaxin-3 is a member of a superfamily of structurally-related peptides that includes relaxin and insulin-like peptide hormones. Soon after the discovery of the relaxin-3 gene, relaxin-3 was identified as an abundant neuropeptide in brain with a distinctive topographical distribution within a small number of GABAergic neuron populations that is well conserved across species. Relaxin-3 is thought to exert its biological actions through a single class-A GPCR - relaxin-family peptide receptor 3 (RXFP3). Class-A comprises GPCRs for relaxin-3 and insulin-like peptide-5 and other peptides such as orexin and the monoamine transmitters. The RXFP3 receptor is selectively activated by relaxin-3, whereas insulin-like peptide-5 is the cognate ligand for the related RXFP4 receptor. Anatomical and pharmacological evidence obtained over the last decade supports a function of relaxin-3/RXFP3 systems in modulating responses to stress, anxiety-related and motivated behaviours, circadian rhythms, and learning and memory. Electrophysiological studies have identified the ability of RXFP3 agonists to directly hyperpolarise thalamic neurons in vitro, but there are no reports of direct cell signalling effects in vivo. This article provides an overview of earlier studies and highlights more recent research that implicates relaxin-3/RXFP3 neural network signalling in the integration of arousal, motivation, emotion and related cognition, and that has begun to identify the associated neural substrates and mechanisms. Future research directions to better elucidate the connectivity and function of different relaxin-3 neuron populations and their RXFP3-positive target neurons in major experimental species and humans are also identified.

Linked articles: This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Figure 1

The nucleus incertus and its relaxin‐3 neurons are similarly located in the midline periventricular central grey of non‐human primate (macaque), rat and mouse brain and are conserved across species (adapted from Ma et al., 2007; 2009b; Smith et al., 2010). Nucleus incertus (NI) is the primary source of neurons expressing relaxin‐3 mRNA and abundant relaxin‐3 immunoreactivity, which are located in the midline periventricular central grey at the base of the fourth ventricle (4V) of (A) macaque, (B) rat and (C) mouse. Abbreviations: CGM, mid central grey; DTg, dorsal tegmental nucleus; LC, locus coeruleus; Me5, mesencephalic trigeminal nucleus; mlf, medial longitudinal fasciculus; PDTg, posterodorsal tegmental nucleus; scp, superior cerebellar peduncle. Scale bars, (A) Nissl, 0.6 mm, inset, 80 μm, relaxin‐3, 0.2 mm; (B) Nissl, 0.3 mm, relaxin‐3, 0.1 mm; (C) 0.2 mm.

Figure 2

Schematic illustration of some of the major downstream neural targets of relaxin‐3 neurons and the likely ‘tested’ () or putative untested () functional roles of relaxin‐3/RXFP3 signalling in the coordinated regulation of modalities including cognition, arousal, motivation, anxiety, mood, pain and oculomotor control. Abbreviations: ACC, anterior cingulate cortex; BF, basal forebrain; CeA, central amygdala; DR, dorsal raphe; Hip, hippocampus; IC, inferior colliculus; IO, inferior olive; LH, lateral hypothalamus; LS, lateral septum; MeA, medial amygdala; MR, median raphe; MS, medial septum; PAG, periaqueductal grey; PFC, prefrontal cortex; PVN, paraventricular hypothalamic nucleus; SC, superior colliculus.

Figure 3

Activation of RXFP3 receptors excites or inhibits intergeniculate leaflet neurons, depending on their neurochemical nature (adapted from Blasiak et al., 2013). (A) A zero current‐clamp recording illustrating the depolarising effect of bath‐applied RXFP3 agonist, R3/I5 (100 nM, horizontal bar). Upwards deflections represent truncated action potentials present on top of calcium spikes evoked by membrane potential recovery from hyperpolarisation induced by current injection (downward deflections), and a confocal projection image of the neuron depolarised by R3/I5 stained for biocytin injected into the neuron (red) and neuropeptide Y (NPY) immunoreactivity (yellow) revealing the NPY nature of the neuron recorded. Scale bar, 10 μm. (B) A zero current‐clamp recording illustrating the hyperpolarising effect of bath‐applied R3/I5 (100 nM, horizontal bar) on the membrane potential and firing properties of another intergeniculate leaflet neuron, and a confocal projection image of the neuron hyperpolarised by R3/I5 stained for biocytin (red) and NPY immunoreactivity (yellow) revealing the NPY‐negative nature of the neuron recorded. Scale bar, 10 μm.

Figure 4

Variations in the response of different types of nucleus incertus neurons to CRF in vivo (adapted from Ma et al., 2013). Extracellular recording and juxtacellular‐filling of nucleus incertus neurons in rat revealed that (A) relaxin‐3 neurons increased firing in response to i.c.v. administration of CRF, whereas (B) some non‐relaxin‐3 neurons exhibited decreased firing in response to CRF, suggesting specific but complex responses to the stress hormone.

Figure 5

‘Whole‐of‐life’ deletion (knockout) of the relaxin‐3 or the Rxfp3 gene results in circadian hypoactivity in adult mice (adapted from Smith et al., 2012; Hosken et al., 2015). The distance travelled on home‐cage voluntary running wheels by male and female Rln3 (Smith et al., 2012) and Rxfp3 (Hosken et al., 2015) gene knockout mice is markedly less than their wildtype littermates, possibly reflecting a reduced level of sustained attention or motivation. Distance is shown as m·h^–1 during an average 24 h day. Grey shading indicates the dark phase.