Glucocorticoid receptors and serotonin N-acetyltransferase activity in the fish pineal organ

Abstract

This study aimed to determine whether glucocorticoid receptors are expressed in the photosensitive trout pineal organ, and whether glucocorticoids modulate melatonin secretion. On Western blots from pineal extracts, an antibody directed against trout glucocorticoid receptor labeled a single band at the expected size (approximately 100 kDa). Dexamethasone inhibited pineal arylalkylamine N-acetyltransferase activity (AANAT2; serotonin --> N-acetylserotonin) in a dose-dependent manner after 6 h of culture in the dark (IC50 2.10(-8) M). RU486 (10(-7) M) alone had a partial agonistic activity, whereas it antagonized the effects of 10(-8) M dexamethasone. Hydroxyindole-O-methyltransferase activity (N-acetylserotonin --> melatonin) remained unaffected. This is the first demonstration that glucocorticoid receptors are present in the pineal organ and that glucocorticoids modulate melatonin production.

Stress inhibition of melatonin synthesis in the pineal organ of rainbow trout (Oncorhynchus mykiss) is mediated by cortisol.

“…A previous in vitro study on the hormonal regulation of fish pineal melatonin synthesis revealed the presence of glucocorticoid receptors in trout pineal organ, and when assayed in vitro with the glucocorticoid analogue dexamethasone, decreased AANAT2 activity was observed (Benyassi et al, 2001), which is consistent with our results from the pineal organ in vitro assay showing decreased melatonin release in pineal organs exposed to cortisol in darkness. Also, previous research found decreased AANAT activity in cultured pineal organ of the North African catfish (Clarias gariepinus) after millimolar corticoid treatment, an effect similar to that reported recently in cultured pineal organs of tilapia with nanomolar cortisol concentrations (Nikaido et al, 2010;Yanthan and Gupta, 2007).…”

“…Little is known regarding the effects of those hormones in the trout pineal organ, but previous studies have not found any effect of catecholamines in this tissue location, in contrast to that in other teleosts for which a regulatory role has been proposed (Falcón et al, 1991). Melatonin synthesis in trout pineal organ was reported to be influenced by glucocorticoid hormones (Benyassi et al, 2001), with cortisol being a serious candidate as a mediator of such an effect.…”

“…Roles have been suggested for prolactin (De Vlaming and Olcese, 1981), estrogens (Bégay et al, 1994;Forlano et al, 2005), glucocorticoids (Falcón, 1999;Benyassi et al, 2001) and catecholamines (Martin and Meissl, 1992;Samejima et al, 1994;Ekström and Meissl, 1997). Cortisol is a glucocorticoid synthesized in the interrenal tissue of fish.…”

“…Based on findings that: (1) cortisol levels show daily variations; (2) there is interaction within several neurohumoral signals and melatonin production; (3) cortisol circulating levels increase right Stress inhibition of melatonin synthesis in the pineal organ of rainbow trout (Oncorhynchus mykiss) is mediated by cortisol after fish are exposed to stress (Wendelaar Bonga, 1997;Mommsen et al, 1999); and (4) glucocorticoids exert an inhibitory effect on AANAT activity of cultured pineal organs in trout (Benyassi et al, 2001;Yanthan and Gupta, 2007), we hypothesized that stress negatively affects melatonin synthesis in pineal organ of rainbow trout with cortisol mediating such inhibitory effect, in the same way as has been described for other teleost species such as tipalia, Oreochromis mossambicus (Nikaido et al, 2010), and that previously suggested for trout (Larson et al, 2004).…”

Interplay between the endocrine and circadian systems in fishes

“…In this sense, cortisol has been identified as a component of a systemic signalling pathway that is required for circadian cell cycle rhythmicity in zebrafish (Dickmeis et al 2007) and appears to be involved in the regulation of the melatonergic system rhythmicity in rainbow trout (Benyassi et al 2001, Ceinos et al 2005, catfish (Clarias gariepinus; Yanthan & Gupta 2007) and Mozambique tilapia (Nikaido et al 2010).…”