Median eminence

A further totally separate DA pathway arises from A12 in the arcuate nucleus and forms the tuberoinfundibular tract in the median eminence to the pituitary gland for controlling prolactin release. This is partly achieved by DA being released into capillaries of the hypothalamic-hypophyseal portal system and then inhibiting the prolactin releasing cells (lactotrophs) of the anterior pituitary. 

Control (inhibition) of prolactin release Tuberoinfundibular tract from A12 in the arcuate nucleus of the median eminence to pituitary Humans Hypoprolactaemia Humans Hyperprolactaemia Galactorrhoea Amenorrhoea D2 ... 

Fig. 4.6(b) GnRH +ve neurones (relative density) in the brain dot = mammalian, circle = avian, forms. Parasagittal view of distribution in Musk Shrew (Suncus murinus) — from bulb to median eminence (ME), optic X (OC), cerebral aqueduct (Aq), and preoptic area (POA) (from Schwanzel-Fukuda and Pfaff, 1994). 

Dopamine is formed from tyrosine by hyclroxylation with tyrosine hydroxylase and the removal of a CO2 group by aromatic amino acid decarboxylase. The catecholamine is found in high concentrations in parts of the brain—the caudate nucleus, the median eminence, the tuberculum olfactorium, and the nucleus accumbens. Dopamine appears to act as an inhibitory neurotransmitter. 

The hypothalamic releasing factors regulate release of the anterior pituitary trophic hormones. As summarized in Figure 52-1, the releasing factors are produced in various neuronal groups within the hypothalamus and are transported to the median eminence for release into the portal circulation to the anterior pituitary. Neurons in the hypothalamus also produce the hormones oxytocin and vasopressin, which are released by the posterior pituitary into the blood. Therefore, it is not surprising that behavior and experience, which influence the hypothalamus, sometimes alter the secretion of these hypothalamic releasing factors and hormones. 

Prevot V, Croix D, Rialas CM, Poulain P, Fricchione GL, Stefano GB, Beauvillain JC (1999) Estradiol coupling to endothelial nitric oxide stimulates gonadotropin-releasing hormone release from rat median eminence via a membrane receptor. Endocrinology 140 652-659... 

The intermediate length systems include the tuberoinfundibular system, which projects from the arcuate and periventricular nuclei into the intermediate lobe of the pituitary and the median eminence. This system is responsible for the regulation of such hormones as prolactin. The inter hypothalamic neurons send projections to the dorsal and posterior hypothalamus, the lateral septal nuclei and the medullary periventricular group, which are linked to the dorsal motor nucleus of the vagus such projections may play a role in the effects of dopamine on the autonomic nervous system. 

Dopamine is an intermediate product in the biosynthesis of noradrenaline. Furthermore it is an active transmitter by itself in basal ganglia (caudate nucleus), the nucleus accumbens, the olfactory tubercle, the central nucleus of the amygdala, the median eminence and some areas in the frontal cortex. It is functionally important, for example in the extra-pyramidal system and the central regulation of emesis. In the periphery specific dopamine receptors (Di-receptors) can be found in the upper gastrointestinal tract, in which a reduction of motility is mediated, and on vascular smooth muscle cells of splanchnic and renal arteries. Beside its effect on specific D-receptors, dopamine activates, at higher concentrations, a- and -adrenoceptors as well. Since its clinical profile is different from adrenaline and noradrenaline there are particular indications for dopamine, like situations of circulatory shock with a reduced kidney perfusion. Dopamine can dose-dependently induce nausea, vomiting, tachyarrhythmia and peripheral vasoconstriction. Dopamine can worsen cardiac ischaemia. 

Fig. 1A-D Increased expression of AVP mRNA in the PVN of mice deficient for CRHR-1 (C) relative to wild-type mice (A). Accordingly, AVP immimoreactivity is markedly enhanced in the external zone of the median eminence (ZEME) of mutants (D) compared to the wild-type (B). (Adapted from Muller et al. 2000h)...

Growth hormone releasing hormone 24 tex produces vasodilation high levels in hypothalamus and median eminence... 

A considerable body of literature has focused on the long-term consequences of maternal separation in rats (Table 9.1). Adult rats separated from their dams for 180 minutes/day on pnd 2-14 demonstrate increased CRT concentrations in the median eminence, portal blood, and CSF, as well as increased CRT mRNA ex-... 

Plotsky, P.M. and Meaney, M.J. (1993) Early, postnatal experience alters hypothalamic corticotropin-releasing factor (CRF) mRNA, median eminence CRF content and stress-induced release in adult rats. Mol Brain Res 18 195-200. 

Male New Zealand White rabbits (2200 g) were exposed to 750 ppm [3.25 g/m ] ethylbenzene for 12 h per day for seven days. Twelve or 24 h following the final day of exposure, the rabbits were killed and their brains dissected. Ethylbenzene depleted both striatal and tubero-infundibular dopamine levels (Mutti et al, 1988). In male Sprague-Dawley rats exposed to 2000 ppm [8.70 g/m ] ethylbenzene for 6 h per day for three consecutive days and killed 16-18 h following the last exposure, ethylbenzene increased dopamine and noradrenaline levels and turnover in the hypothalamus and the median eminence. Ethylbenzene exposure also reduced the secretion of prolactin and increased dopamine turnover within the dopamine-cholecystokinin-8 immunoreactive nerve terminals of the nucleus accumbens (Andersson et al, 1981). 

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