A Novel Feedback Mechanism Regulating Growth Hormone Synthesis and Secretion in Chinese Grass Carp (Ctenopharyngodon idellus)

Abstract

Gonadotropin-Ⅱ (GTH-Ⅱ) and growth hormone (GH) are the key hormones released from the anterior pituitary regulating reproductive functions and body growth, respectively. These two hormones can interact at multiple levels to modulate the important functions of the gonadotrophic and somatotrophic axes. The bony fish, or teleosts, are unique for the zonal distribution of pituitary cells in the pituitary, which is quite different from the random distribution reported in mammals. In the grass carp pituitary, somatotrophs and gonadotrophs are located exclusively in the proximal pars distalis, with small clusters of gonadotrophs embedding in a matrix of somatotrophs. This spatial relationship provides the anatomical substrate for functional interactions between the two cell types through autocrine/paracrine mechanisms. Using Chinese grass carp as a model, we have recently identified a novel intrapituitary feedback loop regulating GH release by local interactions between somatotrophs and gonadotrophs. This feedback loop has three components, namely (i) GTH-Ⅱ induction of GH release from somatotrophs, (ii) amplification of GH secretion through GH-induced GH release from somatotrophs, and (iii) GH "feedback inhibition" on GTH-II release in adjacent gonadotrophs. The autocrine/paracrine interactions between the two cell types form a local feedback loop maintaining basal GH and GTH-Ⅱ secretion at the pituitary level. To our knowledge, our study represents the first report of a functional "intrapituitary circuitry" maintaining body growth and reproductive functions in teleosts.