Organization of the Orexin/Hypocretin System in the Brain of Holostean Fishes: Assessment of Possible Relationships with Monoamines and Neuropeptide Y

Abstract

Holosteans form a small group of actinopterygian fishes considered the sister group of teleosts. Despite this proximity to the biggest group of vertebrates, relatively few studies have been conducted to investigate the organization of the central nervous system of this group of fishes. In this study, the neuroanatomical distribution of orexin/hypocretin-like immunoreactive (OX-ir) cell bodies and fibers was analyzed in the brain of 3 representative species of the 2 orders of extant holosteans, the spotted gar Lepisosteus oculatus, the Florida gar Lepisosteus platyrhincus, and the bowfin Amia calva. Antibodies against orexin-A (OXA) and orexin-B (OXB) were used, which labeled the same cells and fibers throughout the brain. In addition, double immunohistofluorescence was performed for the simultaneous detection of OXA and OXB with tyrosine hydroxylase, serotonin, and neuropeptide Y (NPY), in an attempt to localize the orexinergic structures precisely and study the possible interactions between these neuroactive substances. The pattern of distribution of OX-ir cells in the 3 species was largely similar, showing labeled cells in the preoptic area (POA), and the tuberal and retrotuberal hypothalamic regions, with only subtle differences between species in the density of labeled cells. OX-ir fibers were found in all main brain subdivisions of the 3 species, mostly in the ventral subpallial areas, POA, hypothalamus, posterior tubercle, thalamus, and mesencephalic tectum. Different densities of orexinergic fibers were observed in relation to catecholaminergic and serotoninergic cell groups, as well as an absence of colocalization between orexins and NPY in the same hypothalamic neurons. The comparison of these results with those obtained in other vertebrates highlights a constant pattern of distribution of this system of neurotransmission among different groups of actinopterygian fishes, especially in teleosts. Conserved features shared by all vertebrates studied were also observed, such as the presence of OX-ir cells in the basal hypothalamus, reflecting the preserved functions of these neuropeptides over the course of evolution.

Distribution of the transcription factor islet‐1 in the central nervous system of nonteleost actinopterygian fish: Relationship with cholinergic and catecholaminergic systems

“…The localization of the Isl1-ir cell populations detected in the present study was framed within the neuromeric model (Puelles & Rubenstein, 2003, adapted for the brains of nonteleost actinopterygian fishes, and is summarized in sagittal schematic drawings of the three groups of fishes (Figures 3a, 4a The nomenclature used is essentially the same followed in previous studies of the brain in nonteleost actinopterygian fishes (González et al, 2014;Jiménez et al, 2020;López et al, 2013López et al, , 2014López et al, , 2016López et al, , 2019López et al, , 2022Lozano et al, 2018Lozano et al, , 2019Lozano et al, , 2020Morona et al, 2013).…”

“…We have observed the expression of NPY in neurons of the Tu of cladistian and holostean fishes in previous works (López et al, 2014;Lozano et al, 2018), and the presence of somatostatin, NPY, GHRH, and POMC, among other neuropeptides, has been reported in the hypothalamus of the Chinese sturgeon (Xie et al, 2022). Finally in the basal hypothalamus, the double labeling of Isl1 and TH, observed in some cells of the tuberal hypothalamus of nonteleost actinopterygian fishes, has not been reported in teleost fishes (Baeuml et al, 2019), or in any other vertebrate studied, being an exclusive feature of these groups of fishes.…”

“…The nomenclature used is essentially the same followed in previous studies of the brain in nonteleost actinopterygian fishes (González et al., 2014; Jiménez et al., 2020; López & González, 2014; López et al., 2013, 2014, 2016, 2017, 2019, 2022; Lozano et al., 2018, 2019, 2020; Morona et al., 2013).…”

Orexins modulate membrane excitability in rat trigeminal motoneurons

“…Immunohistochemical studies have reported the expression of 5-HT 1A receptors in trigeminal motor nuclei and surrounding areas during the neonatal period [53]. The colocalization of 5-HT receptor subtypes and Ox has been analyzed in the hypothalamus, posterior tubercle, thalamus, and mesencephalic tectum using immunofl uorescence [54,55], but colocalization has not yet been reported in trigeminal neurons. In the present study, Ox-A enhanced bursting activities, accompanied by increased burst duration and frequency.…”

Dopamine system in the fish brain: A review on current knowledge

“…Hence attempt has been made to convey the updates on dopamine in the brain of fishes. Abundant orexinergic fibers and terminals have been observed interspersed with the TH-ir cells of the POA and hypothalamus in the L. oculatus (Lozano et al, 2018) [28] suggests the collective role of peptides in the reproduction. Super fused POA slices treated with a NPY Y 2 -receptor agonist, NPY 13-36 resulted in a significant reduction in THimmunoreactivity in NPPa and demonstrated that DA and NPY interacts in NPPa to regulate the LH release (Kumar et al, 2014) [24] .…”