The relaxin‐3/RXFP3 system as a peptidergic pathway to control hypothalamic neurons

Nearly every neuron in the brain secretes a bioactive peptide along with a conventional neurotransmitter such as GABA or glutamate. Peptides seem to have preceded the ‘classical’ transmitters as the nervous system developed. The simple nervous system of the cnidarian Hydra extensively uses neuropeptides to mediate all types of neuronal transmission (Grunder & Assmann, 2015). In contrast to fast acting neurotransmitters, neuropeptide release is not restricted to the synaptic active zone. It can occur extrasynaptically in the axons, but also from soma and dendrites. After release, neuropeptides may diffuse at some distance to exert a broad array of functions, mainly through G protein‐coupled receptors. Some neuropeptides, such as neuropeptide Y, are synthesized throughout the brain by neuronal populations that are unrelated to each other. They are likely to exert local paracrine actions. In contrast, other neuropeptides are synthesized only in discrete regions of the brain and project to various brain areas. This is the case for the archetypical neuropeptides oxytocin (OT) and vasopressin (VP) produced by magno‐ and parvocellular hypothalamic neurons and for relaxin‐3, mainly synthesized in the nucleus incertus (NI). In this issue of The Journal of Physiology, Kania et al. (2017) decode a peptidergic circuit whereby fibres arising from the NI impinge on neurons of the paraventricular nucleus (PVN) of the hypothalamus. NI and PVN release relaxin‐3 and OT, respectively, two neuropeptides with opposite actions on food intake.

In the PVN, VP and OT play an essential role in body fluid balance and reproductive functions. More recently, their importance in the control of food intake has been demonstrated. Both VP and OT act as strong anorexigenic factors. Other hypothalamic peptides, located in hypothalamic areas outside the PVN, also contribute to modulating energy homeostasis and food consumption. For instance, NPY and AgRP induce hyperphagia as shown by the activation of NPY/AgRP neurons with DREADD receptors, which increases feeding and reduces energy expenditure (Krashes et al. 2011). Among extrinsic inputs from other brain areas, relaxin‐3‐containing fibres arise from the NI and establish contacts with VP and OT neurons in the PVN (Kania et al. 2017). The role of relaxin‐3 on feeding has been studied extensively with acute and chronic infusion, both of which increase food intake. Detailed pharmacological analysis demonstrated the orexigenic role of the relaxin‐3–RXFP3 system (Ganella et al. 2013b). Interestingly, chronic adeno‐associated virus‐mediated activation of RXFP3 indicated that only OT/VP neurons can be considered as relaxin‐3 targets whereas other feeding peptidergic systems remain unaffected (Ganella et al. 2013a). However the mechanism of action of relaxin‐3 has remained unknown so far.

Slice recordings unambiguously demonstrate direct postsynaptic actions of relaxin‐3‐containing fibres on magno‐ and parvocellular neurons of PVN (Kania et al. 2017). Therefore, Kania and collaborators propose that relaxin‐3 and the VP/OT systems interact through a series of interconnected neurons. The question of interactions between neuropeptides is central in peptidergic transmission. In contrast to colocalized and co‐released neuropeptides with a blend of biological activities, relaxin‐3 and OT/VP participate in sequential cooperation between peptidergic systems. In this case, relaxin‐3 acts as a master regulator of the neurohormone actions, being able to switch on and off the activity of parvo‐ and magnocellular hypothalamic neurons.

The classic view that neuropeptide‐containing neurons represent a peculiar class of neurons has been replaced by the perspective that many, if not most, neurons in the brain contain neuropeptides in addition to fast‐acting neurotransmitters. Hypothalamic neurons, especially magnocellular neuroendocrine neurons, are archetypical peptidergic neurons. However, VP/OT neurons also use glutamate, a classic neurotransmitter. On the other hand, NI neurons are non‐neuroendocrine neurons also expressing neuropeptides including relaxin‐3. These neurons are classically described as GABAergic neurons. GABA fibres are found in the hypothalamus, and serial ultrathin section reconstruction of GABA‐immunogold‐labelled presynaptic boutons from the PVN demonstrated that every bouton contained at least a few dense core vesicles, suggesting that in addition to GABA, these fibres also contain neuropeptides (Decavel & Van den Pol, 1990). At least some of these fibres may well originate from the NI and contain relaxin‐3.

Classically, hypothalamic homeostatic functions are considered to be controlled by direct peptidergic transmission. Compelling evidence based on in situ hybridization data demonstrate the expression of RXFP3 postsynaptically in PVN neurons. Moreover, the relaxin‐3‐inhibitory effect is maintained in the presence of TTX and glutamate/GABA receptor antagonists (Kania et al. 2017). Altogether, these data point to a relaxin‐3/RXFP3 modulation of OT/VP neurons. However, modulation of fast amino acid synaptic activity is a key target of central nervous system neuropeptides (Van den Pol, 2012). In this view, relaxin‐3/RXFP3 may alter GABA transmission between NI fibres and hypothalamic neurons. Other neuropeptides, i.e. NPY/AgRP, have already been proposed to regulate the effects of fast‐acting neurotransmitters, especially GABA, in food intake processes (Atasoy et al. 2012). In addition, the authors do not exclude that relaxin‐3 also acts at presynaptic RXFP3 auto‐ or heteroreceptors. These various possibilities illustrate the diversity of mechanisms by which neuropeptides can control the activity of their target neurons.

A full picture of relaxin‐3/RXFP3 regulation of PVN neurons in homeostatic processes will require an understanding of their interactions with GABA at the cellular level, and with other peptidergic circuits, e.g. NPY/AgRP, at the network level. This will rely on the analysis of intracellular postsynaptic signalling cascades that are activated or modulated by relaxin‐3/RXFP3. Furthermore, it remains to be investigated how relaxin‐3/RXFP3 signalling is altered in pathological conditions.

A last important contribution of the study by Kania et al. (2017) is to highlight the potential of neuropeptide transmission at a distance from the target. The authors identified a dense network of fibres in close proximity to the PVN, but only scarce nerve endings within the nucleus although in situ hybridization demonstrates that the RXFP3 receptor is expressed by hypothalamic PVN neurons . It is therefore tempting to hypothesize relaxin‐3 is released in the periphery of the PVN and the peptide undergoes local diffusion to act on cells near the release sites. Such restricted diffusion has been proposed as a model of local volume transmission (van den Pol, 2012). By this mean, even limited numbers of relaxin‐3 fibres and release sites may be sufficient to influence the entire population of OT/VP PVN neurons and produce a coordinated behaviour.

The study of Kania et al. (2017) confirms the central role of relaxin‐3/RXFP3 signalling in the control of PVN neurons. Together with other hypothalamic peptidergic circuits (e.g. NPY/AgRP, POMC), the relaxin‐3/RXFP3 system contributes to feeding behaviours by inhibiting OT and VP neurons. PVN neurons can then be seen as a hub that controls a broad array of fundamental behaviours, including reproduction and feeding, and that is in turn influenced by various independent circuits releasing, all together, a cocktail of bioactive neuropeptides. A key question remaining is to understand how independent neuropeptide signalling pathways cooperate and interact with each other, and with fast‐acting neurotransmitter systems.

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Competing interests

None declared.