Abstract
Mammalian pituitaries exhibit a high degree of intercellular coordination; this enables them to mount large-scale coordinated responses to various physiological stimuli. This type of communication has not been adequately demonstrated in teleost pituitaries, which exhibit direct hypothalamic innervation and expression of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in distinct cell types. We found that in two fish species, namely tilapia and zebrafish, LH cells exhibit close cell-cell contacts and form a continuous network throughout the gland. FSH cells were more loosely distributed but maintained some degree of cell-cell contact by virtue of cytoplasmic processes. These anatomical differences also manifest themselves at the functional level as evidenced by the effect of gap-junction uncouplers on gonadotropin release. These substances abolished the LH response to gonadotropin-releasing hormone stimulation but did not affect the FSH response to the same stimuli. Dye transfer between neighboring LH cells provides further evidence for functional coupling. The two gonadotropins were also found to be differently packaged within their corresponding cell types. Our findings highlight the evolutionary origin of pituitary cell networks and demonstrate how the different levels of cell-cell coordination within the LH and FSH cell populations are reflected in their distinct secretion patterns. The pituitary is a master endocrine gland that integrates hypothalamic and systemic signals to produce and secrete several types of hormones; these hormones regulate a variety of physiological functions, including lacta-tion, metabolism, reproduction and stress response 1. Accumulating evidence from mammalian models indicates that several of the pituitary cell types are organized into complex three-dimensional networks that enable functional cell-cell coordination within homotypic cell populations 2,3. Pituitary cell networks have been found to be imprinted by past experience 4 , and exhibit a high degree of plasticity as they react to feedback signals to optimize their output to the changing needs of the organism 5-7. Such observations have been made for somatotropes 8 , lactotropes 4 corticotropes and gonadotropes 9 , as well as for the non-endocrine folliculostellate (FS) cells 10. The latter have been shown to form exceptionally long-range functional networks that have been postulated to act in the transduction of signals between distant endocrine cell populations 10. Apart from the direct cell-cell interactions , which are largely mediated through gap junctions 11 , a complex array of paracrine signals serve to modulate pituitary cell activity, thus presenting an additional regulatory pathway in which pituitary cells interact to produce physiologically accurate output 12. Reproduction in vertebrates is dependent upon the coordinated actions of various hormones associated with the hypothalamus-pituitary-gonadal axis. The key modulators of reproduction are the gonadotropins (GtHs) luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which are produced by the pituitary. The expression and release of GtHs from the gonadotropes is primarily regulated by the hypothalamic peptide gonadotropin-releasing hormone (GnRH) that binds to membrane receptors on the gonadotropes and triggers action potentials, a rise in cytosolic calcium, and exocytosis of GtHs into the circulation 13,14. Both GtHs are gly-coprotein hormones comprised of two subunits: a common α-subunit and a specific β-subunit that confers their biological specificity. In mammals and many other studied tetrapods, both GtHs are produced in the same cell but control distinct biological processes and hence require differential regulation and exhibit unique secretion patterns 15. The differential control of LH and FSH secretion in mammals is achieved through differential packaging