Reviewing the physiological roles of the novel hormone-receptor pair INSL5-RXFP4: a protective energy sensor?

There is no common consensus on the physiological role of insulin-like peptide 5 (INSL5) and its cognate receptor, relaxin family peptide receptor 4 (RXFP4). The experimental data for INSL5-RXFP4 expression and function point to a potential role of the peptide hormone and receptor pair in linking energy availability, homeostasis, and inflammation. In this review, we summarize studies on the INSL5-RXFP4 system and propose that the current findings from diverse experimental settings point broadly to a role as a protective energy sensor (PES). Specifically, we review the evidence that (1) INSL5-RXFP4 could regulate immune response by decreasing the production of proinflammatory cytokines and may be involved in the stress response via the HPA axis; (2) INSL5-RXFP4 may signal through sensory neurons on the vagus nerve, transmitting signals to the CNS; and (3) INSL5-RXFP4 could have local autocrine/paracrine roles within the intestinal tract and immune cells. Further investigation and clarification of these proposed roles of INSL5-RXFP4 may prove a greater physiological relevance for the pair and add to existing evidence of INSL5-RXFP4 role as a PES.

Repeatability of burst swimming performance in medaka (Oryzias latipes).

Burst swimming performance (Uburst) is a putative indicator of "success" in wild fish. In this study, thirty-five lab-reared medaka (Oryzias latipes) were tested for Uburst using a French press exercise system. Fish were tested once a week for four consecutive weeks and repeatability was estimated in several ways to allow comparisons between studies. Following the initial swimming tests, 50% of fish were either thermally stressed, or not, for 180 s prior to testing Uburst once a week for four consecutive weeks. Burst swimming performance was found to be 24.0 ± 6.7 (s.d.) cm s-1 and repeatability prior to the thermal stress experiment was estimated to be 0.28 (intraclass correlation coefficient) with an upper and lower limit of 0.48 and 0.12, respectively. The measured Uburst and repeatability estimate in the thermal stressor experiment did not significantly differ from the first four trials. Swimming velocities observed match what is known about medaka swimming capabilities and, interestingly, are similar to maximum current velocities observed in their native habitat. Furthermore, our repeatability estimates confirm that burst swimming performance in medaka is a repeatable trait and validate the apparatus and swimming test used.