Evidence for a negative feedback in the control of eel growth hormone by thyroid hormones

Abstract

The regulation of growth hormone (GH) by thyroid hormones (THs) has been shown to present species variation. We investigated the regulation of GH in the eel, a representative of an ancient group of teleosts. In vivo administration of triiodothyronine (T 3 ) or thyroxine (T 4 ) significantly reduced pituitary and serum GH levels, as measured by homologous RIA. In order to investigate the ability of THs to regulate GH production directly at the pituitary level, we used a long-term, serum-free primary culture of eel pituitary cells. Both T 3 and T 4 inhibited GH release in a concentration-dependent manner, producing up to 50% inhibition at 10 nM, with an ED 50 of <0·2 nM, within the range of their physiological circulating levels. Other hormones also acting via the nuclear receptor superfamily, such as sex steroids (testosterone, estradiol and progesterone) and corticosteroid (cortisol), had no effect on GH release in vitro, underlining the specificity of the regulatory effect of THs on GH. Measurement of both GH release and cellular content for calculation of GH production in vitro indicated that THs not only inhibited GH release but also GH synthesis. Dot-blot assay of GH messenger RNA (mRNA) using an homologous eel cDNA probe showed a decrease in GH mRNA levels in cells cultured in the presence of T 3 , as compared with control cells. This demonstrated that the inhibition of T 3 on GH synthesis was mediated by a decrease in GH mRNA steady state levels. In conclusion, we demonstrate inhibitory regulation of eel GH synthesis and release by THs, exerted directly at the pituitary level. These data contrast with the rat, where THs are known to have a stimulatory effect and suggest that the pattern observed here in an early vertebrate and also found in birds, reptiles and some mammals including humans, may represent an ancestral and more generalized vertebrate pattern of TH regulation of pituitary GH.

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

“…Eels were transferred to the laboratory and kept in running aerated freshwater (12 ± 2°C). Fish (8 eels/group) received one perivisceral injection per week of thyroxine (T4; Sigma, Saint Quentin Fallavier, France), estradiol (E2; Sigma) or testosterone (T; Sigma) (2 µg/body weight) suspended in saline (0.15 M NaCl) or of saline alone (controls) for 3 months, according to the protocol previously developed [31, 32]. …”

“…Individual pituitaries were extracted in 0.15 M NaCl by sonication (Bioblock Scientific, Illkirch, France), as previously described [32]. LH and GH levels were measured using respectively a radioimmunoassay (RIA) for carp LHβ subunit validated for European eel LHβ [34] and a homologous RIA for European eel GH [35].…”

“…The following probes were used in dot blot analysis: FSHβ probe (425 bp [39]), LHβ probe (404 bp [40]), GPα subunit probe (371 bp [41]), TSHβ probe (448 bp [42]), and GH probe (481 bp [32]). All probes revealed single bands after northern hybridization, which made them suitable to use in dot-blot assays.…”

Proximate and Ultimate Control of Eel Continental Dispersal

“…Indeed, in both glass and yellow eels, cortisol promotes gill and intestine Na + /K + -ATPase activity, i.e., adaptation to SW residency, probably in synergy with GH (Epstein et al 1971, Butler and Carmichael 1972, Wilson et al 2004). In addition, cortisol injections decrease plasma THs levels (Redding et al 1986), while THs exert a negative feedback on GH production by the pituitary, an eel-specific regulation (Rousseau et al 2002). These crossed negative feedbacks between THs vs. cortisol-GH could mediate the swap of glass eels from FW-oriented migration to settlement induced by a lowered body condition (Fig.…”

Comparative Aspects of GH and Metabolic Regulation in Lower Vertebrates

“…In the European eel, it was demonstrated that GH was able to increase circulating T3 by stimulation of peripheral 5′-monodeiodination [104] and that both T3 and T4 could inhibit GH release and synthesis in vivo and in vitro [105]. In other teleosts, studies reported either stimulatory or no effects of TH on GH, and this species-related variation was also observed among mammals [for review, see [3]].…”