Relaxin-3/RXFP3 system in alcohol self-administration and relapse

© 2016 Dr. Hanna Erika KastmanAlcoholism is a chronic relapsing disorder, accounting for 10% of disability-adjusted life years lost in industrialized countries. Our understanding of the neurobiology of addiction is far from complete and due to high relapse rates, there is a need to identify new therapeutic targets to assist the development of better treatments. In early studies, the neuropeptide relaxin-3 was implicated in the regulation of stress responses and arousal/motivational behaviours such as feeding, as well as spatial memory. Relaxin-3 is primarily expressed in large GABAergic neurons of the nucleus incertus (NI) in the hindbrain that project topographically to forebrain regions containing neurons expressing the native relaxin-3 G-protein-coupled receptor, RXFP3, several of which are implicated in the control of drug-seeking behaviour. Therefore, the present study directly investigated the role of the central relaxin-3/RXFP3 signalling system in alcohol- and sucrose-seeking in alcohol-preferring (iP) and Wistar rats. Firstly, the effect of central antagonism of RXFP3 using a receptor-selective relaxin-3 analogue peptide on self-administration of alcohol and sucrose was investigated; and then the effect of the same antagonist treatment on cue- and stress-induced reinstatement of both sucrose- and alcohol-seeking was studied. Thereafter, the impact of RXFP3 antagonism within the stress-related bed nucleus of stria terminalis (BNST) on stress-induced reinstatement of alcohol-seeking was also examined. Central antagonism of RXFP3 reduced cue- and stress-induced reinstatement of alcohol-seeking, but did not markedly alter sucrose-seeking in either paradigm, suggesting a specific effect on alcohol-related behaviour(s). Alcohol consumption and stress-induced reinstatement were also both attenuated by local RXFP3 antagonism within the BNST. These data are the first identifying a role for relaxin-3/RXFP3 signalling in alcohol use and seeking. In light of these initial findings and the enrichment of RXFP3 in other stress-related brain areas, the ability of RXFP3 antagonism within the central amygdala (CeA) to modulate alcohol self-administration and stress-induced reinstatement of alcohol-seeking was also examined. Indeed, RXFP3 antagonism in CeA also reduced alcohol self-administration and stress-induced reinstatement of alcohol-seeking, further indicating an involvement of native relaxin-3/RXFP3 signalling in stress-induced reinstatement and potential interactions with other major peptide transmitter systems implicated in these behaviours, such as corticotropin-releasing factor (CRF) and orexin. NI relaxin-3 neurons express the CRF type 1 receptor (CRF1) and are activated by CRF; likewise there is anatomical evidence for the expression of orexin-1 (OX1) and orexin-2 (OX2) receptors in the NI. Accordingly, the effect of bilateral NI infusions of the CRF1receptor antagonist (CP376395), the OX1 receptor antagonist (SB-334867) and the OX2 receptor antagonist (TCS-OX2-29) on stress-induced reinstatement of alcohol-seeking was examined. CP376395 and TCS-OX2-29 reduced stress-induced reinstatement of alcohol-seeking, whereas SB-334867 was ineffective. These data suggest that CRF- and orexin-mediated activation of NI neurons occurs during stress-induced reinstatement, via CRF1 and OX2 receptor signalling events. Together, these data demonstrate that the relaxin-3/RXFP3 system can modulate stress-induced reinstatement of alcohol-seeking via both forebrain sites and a potential CRF- and orexin-primed activation of the NI during stress exposure. These findings have added significantly to our knowledge of the neurocircuitry that underpins stress-induced relapse-like behaviour and to our general understanding of addictive behaviours, with implications for the development of better treatment strategies and the identification of potential drug targets for treating alcoholism and other stress-related addictions