Neuronal and Astroglial Localization of Glucocorticoid Receptor GR in Adult Zebrafish Brain (Danio rerio)

Glucocorticoid receptor & alpha; (GR & alpha;), a ligand-regulated transcription factor, mainly activated by cortisol in humans and fish, mediates neural allostatic and homeostatic functions induced by different types of acute and chronic stress, and systemic inflammation. Zebrafish GR & alpha; is suggested to have multiple transcriptional effects essential for normal development and survival, similarly to mammals. While sequence alignments of human, monkey, rat, and mouse GRs have shown many GR & alpha; isoforms, we questioned the protein expression profile of GR & alpha; in the adult zebrafish (Danio rerio) brain using an alternative model for stress-related neuropsychiatric research, by means of Western blot, immunohistochemistry and double immunofluorescence. Our results identified four main GR & alpha;-like immunoreactive bands (95 kDa, 60 kDa, 45 kDa and 35 kDa), with the 95 kDa protein showing highest expression in forebrain compared to midbrain and hindbrain. GR & alpha; showed a wide distribution throughout the antero-posterior zebrafish brain axis, with the most prominent labeling within the telencephalon, preoptic, hypothalamus, midbrain, brain stem, central grey, locus coeruleus and cerebellum. Double immunofluorescence revealed that GR & alpha; is coexpressed in TH+, & beta;(2)-AR+ and vGLUT+ neurons, suggesting the potential of GR & alpha; influences on adrenergic and glutamatergic transmission. Moreover, GR & alpha; was co-localized in midline astroglial cells (GFAP+) within the telencephalon, hypothalamus and hindbrain. Interestingly, GR & alpha; expression was evident in the brain regions involved in adaptive stress responses, social behavior, and sensory and motor integration, supporting the evolutionarily conserved features of glucocorticoid receptors in the zebrafish brain.