INTRODUCTION: The brainstem nucleus incertus (NI) plays a key regulatory role in the control of stress, anxiety, and social interactions. It is the primary source of the neuropeptide relaxin-3 (RLN3), which signals through its receptor RXFP3. The NI provides dense RLN3ergic projections to the forebrain bed nucleus of the stria terminalis (BNST), a region critically involved in the control of anxiety, stress, and social and sexual behaviours. Consistent with this connectivity, the BNST exhibits high RXFP3 expression. However, the molecular characteristics of the NI-BNST axis, particularly concerning the RLN3/RXFP3 systems and its contribution to sociosexual behaviour, remain poorly understood.

AIM(S): This study aimed to investigate the involvement of the NI in sociosexual behaviour in male and female Sprague–Dawley rats. Specifically, we aimed to: (i) assess changes in NI neuronal activity associated with sociosexual behaviour; (ii) characterise the distribution and origin of RLN3-positive fibres in the BNST; and (iii) evaluate the direct sensitivity of BNST neurons to RLN3 administration.

METHOD(S): The involvement of NI in sociosexual behaviour was assessed using c-Fos immunohistochemistry. The source of RLN3-ergic fibres innervating the BNST was identified using viral anterograde tract tracing. Ex vivo patch-clamp recordings were performed in BNST neurons to evaluate their sensitivity to RLN3.

RESULTS: Male rats that interacted with females showed a higher density of c-Fos–positive cells in the NI compared to control males, whereas no such difference was observed in females. RLN3-positive fibres originating from the NI were identified in the BNST. During patch-clamp recordings, RLN3 application elicited a whole-cell outward current in BNST neurons.

CONCLUSIONS: These findings position the NI as an important brainstem contributor to male sociosexual behaviour and identify the BNST as a functional target of RLN3/RXFP3 signalling.

FINANCIAL SUPPORT: National_Science_Centre, Poland: UMO-2023/49/B/NZ4/01885