These cells share the characteristics of both neurons and endocrine gland cells. They respond to signals in the bloodstream, as well as to neurotransmitters within the brain, in a process known as neuro secretion. The cells that produce GnRH originate from the olfactory area and migrate along cranial nerves connecting the nose and the forebrain to their primary location, the arcuate nucleus of the hypothalamus. This amazing journey accounts for Kallmann's syndrome, an association between an absence of GnRH and a defect in smell, a failure of both olfactory axonal and GnRH neuronal migration from the olfactory placode.
The half-life of GnRH is only 2-4 minutes. Because of this rapid degradation, combined with the enormous dilution upon entry into the peripheral circulation, biologically effective amounts of GnRH do not escape the portal system. Therefore, control of the reproductive cycle depends upon constant release of GnRH. This function, in turn, depends upon the complex and coordinated interrelationships among this releasing hormone, other neurohormones, the pituitary gonadotropins and the gonadal steroids. The interplay among the substances is governed by feedback effects, both positive stimulatory and negative inhibitory.
The long feedback loop refers to the feedback effects of circulating levels of target gland hormones, and this occurs both in the hypothalamus and the pituitary. The short feedback loop indicates a negative feedback of pituitary hormones on their own secretion, presumably via inhibitory effects on releasing hormones in the hypothalamus. Ultrashort feedback refers to inhibition by the releasing hormone on its own synthesis. These signals as well as signals from higher centers in the central nervous system may modify GnRH secretion through an array of neurotransmitters, primarily dopamine, norepinephrine, and endorphin but also serotonin and melatonin. .
PHYSIOLOGY OF GONADOTROPIN SURGE.
