GnRH is secreted in the
hypophysial portal bloodstream at the
median eminence. GnRH is very low during
childhood, and is reactivated at
puberty during adolescence. During the reproductive years, pulse activity is critical for successful reproductive function as controlled by feedback loops. However, once a pregnancy is established, GnRH activity is not required. Pulsatile activity can be disrupted by hypothalamic-pituitary disease, either dysfunction (i.e.,
hypothalamic suppression) or organic lesions (trauma, tumor). Elevated
prolactin levels decrease GnRH activity. In contrast,
hyperinsulinemia increases pulse activity leading to disorderly LH and FSH activity, as seen in
polycystic ovary syndrome (PCOS). GnRH formation is congenitally absent in
Kallmann syndrome.
Control of FSH and LH At the pituitary, GnRH stimulates the synthesis and secretion of
follicle-stimulating hormone (FSH) and
luteinizing hormone (LH). There are differences in GnRH secretion between females and males. In males, GnRH is secreted in pulses at a constant frequency; however, in females, the frequency of the pulses varies during the menstrual cycle, and there is a large surge of GnRH just before ovulation. GnRH secretion is
pulsatile in all vertebrates, and is necessary for correct reproductive function. Thus, a single hormone, GnRH1, controls a complex process of
follicular growth,
ovulation, and
corpus luteum maintenance in the female, and
spermatogenesis in the male.
Neurohormone GnRH is considered a
neurohormone, a
hormone produced in a specific
neural cell and released at its
neural terminal. A key area for production of GnRH is the
preoptic area of the hypothalamus, which contains most of the GnRH-secreting neurons.
GnRH neurons originate in the nose and migrate into the brain, where they are scattered throughout the medial septum and hypothalamus and connected by very long >1-millimeter-long
dendrites. These bundle together so they receive shared
synaptic input, a process that allows them to synchronize their GnRH release. The
GnRH neurons are regulated by many different afferent neurons, using several different transmitters (including
norepinephrine,
GABA,
glutamate). For instance,
dopamine appears to stimulate LH release (through GnRH) in estrogen-progesterone-primed females; dopamine may inhibit LH release in ovariectomized females.
Kisspeptin appears to be an important regulator of GnRH release. GnRH release can also be regulated by
estrogen. It has been reported that there are
kisspeptin-producing neurons that also express
estrogen receptor alpha.
Other organs GnRH is found in organs outside of the hypothalamus and pituitary, and its role in other life processes is poorly understood. For instance, there is likely to be a role for GnRH1 in the
placenta and in the
gonads. GnRH and GnRH receptors are also found in cancers of the breast, ovary, prostate, and endometrium. ==Effects of behavior==