Modulatory Role for Substance P on the Regulation of Luteinizing Hormone Secretion by Cultured Porcine Gonadotrophs1
Substance P (SP) is an undecapeptide of the tachykinin family, which was originally isolated from bovine hypothalamus and has been shown to be widely distributed throughout the peripheral and central nervous systems. In addition to its classical role in the processes of nociception and of cardiovascular control, increasing evidence has led to the concept that SP may function as a hypophysiotropic factor (for review see ). In particular, information suggesting that SP participates in the regulation of the hypothalamo-pituitary-gonadal axis has been offered by a number of reports. Thus, SP-containing neurons innervating GnRH-containing neurons have been observed in the septopreoptic area of the rat. In the pig, radioimmunoassayable SP has been shown to be present at high levels in various regions of the hypothalamus, including the preoptic and suprachiasmatic areas, as well as the medial basal hypothalamus, where GnRH is also present at high levels. More recently, SP has been immunolocal-ized in perivascular nerve fibers of the median eminence, and in perikarya of the arcuate nucleus of the immature female pig hypothalamus. In addition, SP has been also found to occur in the pituitary, thus suggesting a putative paracrine role for this peptide. Indeed, earlier reports described high concentrations of SP in the porcine pituitary. buy asthma inhalers
In rats, immunocytochemical studies have revealed the existence of SP within various pituitary cell types including gonadotrophs, lactotrophs, somatotrophs, and thy-rotrophs; and cell immunoblot studies indicate that SP can be released by pituitary cells, specifically, by a subset of somatotrophs and thyrotrophs. Both the hypothalamus and the pituitary also contain binding sites for SP, and gonadotrophs and lactotrophs have been shown to bind and internalize iodinated SP. Recently, the SP-specific tachykinin receptor NK1 has been detected in the rat pituitary by reverse transcription-poly-merase chain reaction (RT-PCR). When viewed as a whole, the above findings, together with the sexually dimorphic distribution of SP in the pituitary and the ability of gonadal steroids to regulate both hypothalamic and pituitary SP levels, constitute strong evidence that SP plays a physiological role in the control of gonadotroph function.