With THE advent of RIAs to measure low concentrations of circulating hormones in the early 1970s, endocrine investigators were able to recognize that many hormones are secreted in an episodic fashion. The methodology of the RIA permitted repeated measures of a particular hormone in individual subjects over extended intervals, e.g. 24 h. When serial hormone concentrations are plotted over time, the resultant time series exhibits visually distinct regions in which a relative increase is followed by a subsequent decrease in hormone concentrations. These discrete events have been termed pulses. The pulsatile secretion of endocrine products is based upon a complex interaction of peptides, glycoproteins, and steroid hormones termed the neuroendocrine axis. This merging of the neural and endocrine systems begins in the brain where neurons located in the hypothalamus, under the influence of largely unknown suprahypothalamic neurons, secrete peptides termed hypothalamic releasing factors (e.g. GnRH and TRH), which travel to the pituitary gland via the hypophyseal-portal microvascular system. Such releasing factors stimulate specific pituitary cells to release one or more (glyco)protein hormones (e.g. LH and FSH) into the bloodstream. The circulatory compartment then conveys this pituitary signal to the various target sites; viz. cells of the respective endocrine end organs (e.g. ovary, thyroid, adrenal gland). The end organ responds to the trophic signal by secreting its relevant hormone (e.g. estrogen, T4, cortisol) into the bloodstream. The glandular products in turn act through feedback mechanisms to modulate secretion of (neuro)hormones at the hypothalamic and pituitary levels.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism