High concentrations (up to 600 pg/ml) of corticotropin-releasing hormone (CRH) were detected in plasma of the teleost fish Oreochromis mossambicus (tilapia) when screening peripheral tissues of tilapia exposed to stress. Notably, the plasma CRH response to stressors in tilapia is much more pronounced than that in higher vertebrates, such as rats. After characterisation by RIA, by spiking plasma with synthetic tilapia CRH and by methanol-acid extraction, it is concluded that the immunoreactive (ir) material in plasma represents tilapia CRH(1-41). Results indicate that a CRH-binding protein is absent in tilapia plasma. Unstressed fish had plasma CRH levels under the limit of detection (<2 pg/ml), but following capture stress plasma CRH levels (170-300 pg/ml) as well as plasma cortisol levels (120 ng/ml) increased rapidly to plateau levels, which were reached after approximately 5 min. Tilapia CRH(1-41) tested at concentrations between 10(-11) and 10(-7) M in vitro did not stimulate the cortisol release from interrenal tissue. Also pretreatment of interrenal tissue with 10(-9) M CRH did not sensitise the cortisol-producing cells to a subsequent ACTH challenge. Forty-eight hours of net confinement or 48 h of cortisol treatment abolished the plasma CRH response and cortisol response to capture stress. The rapidity of the plasma CRH response and its inhibition after 48 h of stress or cortisol treatment point to release by central nervous tissue. Therefore the distribution of glucocorticoid receptors (GRs) in the brain and pituitary of tilapia was investigated. Main GR-ir cell clusters were found in the medial part (Dm) and posterior part of the dorsal telencephalon, in the preoptic region, in the inferior lobe of the hypothalamus and in the cerebellum. We conclude from comparison of CRH brain contents of unstressed and stressed fish that plasma CRH was released by CRH-ir cells located in the lateral part of the ventral telencephalon (Vl), and suggest that the cortisol feedback on CRH release by Vl is mainly exerted via the forebrain Dm region. We propose that CRH is mobilised during stress to fulfil peripheral functions, such as the regulation of circulating leukocytes or of cardiac output, as CRH receptors have been reported in these organs for fish species.
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