Spexin in the European sea bass, Dicentrarchus labrax: Characterization, brain distribution, and interaction with Gnrh and Gnih neurons

Spexin (Spx) is a recently characterized neuropeptide implicated in multiple physiological processes in vertebrates, including reproduction, food intake, and regulation of anxiety and stress. Two orthologs (Spx1 and Spx2) are present in some nonmammalian vertebrates, including teleosts. However, information on the distribution of Spx in the brain and its interactions with other neuroendocrine systems in fish is still scarce. In this work, we cloned and sequenced the sea bass (Dicentrarchus labrax) Spx1, which included a 27 aa signal peptide and a mature peptide of 14 aa that is C‐terminal amidated. spx1 transcripts were higher in the diencephalon/caudal preoptic area/hypothalamus and medulla but were also detected in the olfactory bulbs, telencephalon/rostral preoptic area, optic tectum/tegmentum, cerebellum/pons, and pituitary. The immunohistochemical study revealed Spx1‐immunoreactive (ir) cells in different nuclei of the preoptic area, habenula, prethalamus, mesencephalic tegmentum and in the proximal pars distalis (PPD) and pars intermedia of the pituitary. Spx1‐ir fibers were widely distributed throughout the brain being particularly abundant in the midbrain and hindbrain, in close contact with tegmental gonadotropin‐releasing hormone 2 (Gnrh2) cells and isthmic gonadotropin‐inhibitory hormone (Gnih) cells of the secondary gustatory nucleus. Moreover, Gnih fibers were observed innervating Spx1‐ir cells lying in several subdivisions of the magnocellular preoptic nucleus and in the lateral nucleus of the valvula, whereas ventrolateral prethalamic Spx1‐ir cells received immunopositive Gnrh2 fibers. In the pituitary, Gnrh1‐ir fibers were observed closely associated with Spx1‐ir cells of the PPD. These results suggest that Spx1 could be involved in both reproductive and nonreproductive (i.e., food intake, behavior) functions in sea bass.