Inhibitory Control of the Brain-Pituitary Reproductive Axis of Male European Sea Bass: Role of Gonadotropin Inhibitory Hormone

Abstract

Reproduction in vertebrates is controlled by a hierarchically organized endocrine system called the hypothalamic-pituitarygonadal (HPG) axis. In the early of 1970s, two teams, Schally's and Guillemin's, discovered gonadotropin-releasing hormone (GnRH), a hypothalamic neuropeptide that stimulates the release of both luteinizing hormone (LH) and folliclestimulating hormone (FSH) from gonadotropes in the anterior pituitary of mammals [1][2][3]. Subsequently, several GnRHs have been identified in other vertebrates [4][5][6], and it is generally accepted that GnRH is the only hypothalamic neuropeptide that regulates the release of gonadotropin in mammals and other vertebrates. Despite the divergent reproductive strategies and behaviors within this taxon, this endocrine network is remarkably well conserved across vertebrates. In response to hypothalamic GnRH, gonadotropins are secreted from the pituitary and stimulate the gonads, where they induce the synthesis and release of sex steroid hormones, which in turn elicit growth and maturation of the gonads.However, control of pituitary function by a single neuroendocrine mechanism is not always the rule in vertebrates in which dual (positive and negative) hypothalamic control has often been observed in somatotropes, thyrotropes, and lactotropes. Several investigations have challenged the dogma of single control of gonadotropes by were the first to establish that a dopaminergic inhibitor can control reproduction in goldfish Carrassius auratus. After that, the role of dopamine (DA) as a gonadotropin release-inhibiting factor was confirmed not only in teleosteans, but also in chondrostean fish [11][12][13][14][15]. The discovery of DA inhibition led to the development of a new method to induce spawning in aquaculture, using a combined treatment with a GnRH agonist and a DA-D2 type receptor antagonist. This method, called LinPe, is now widely applied in aquaculture as an alternative means of inducing spawning because it is more reliable and physiologically relevant than treatments involving pituitary extracts. However, wide variations in the intensity of DA inhibition have been noted among the species expressing the DA inhibition, indicating a major role for DA in some species, such as the cyprinids, and a minor one in others, such as the salmonids. Interestingly, some species (e.g. the Atlantic croaker Micropogonias undulatus, gilthead seabream Sparus aurata, striped bass Morone saxatilis, sea bass Dicentrarchus labrax, and red seabream Pagrus major) do not exhibit a dopaminergic inhibition of gonadotropin [16][17][18][19][20]. These variations suggest that DA neuroendocrine inhibition may have been differentially conserved and expressed among teleosts in relation to the diversity of their reproductive cycles and their dependency upon various environmental factors. However, it is also possible that other inhibitory factors may be involved in the reproduction neuroendocrine systems in fish.In 2000, a hypothalamic neuropeptide, referred to as gonadotropin inhibitory hormone (...

Comparative Gene Expression Level of Reproductive Genes Along the Brain-Pituitary-Gonad Axis in the Ripe Female of the Wild and Captive Grey Mullets; Mugil cephalus

“…The brain-pituitary-gonad (BPG) axis is a crucial endocrine system that exhibits a significant role in the regulation of reproductive functions in vertebrates. This axis involves a complex interaction of hormones and feedback mechanisms that control the development and functioning of the gonads (Yan, 2016).…”