Hypothalamic-hypophyseal portal

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. 

The adenohypophysis does not have a direct anatomical connection with the hypothalamus therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis. 

Located in close proximity to the primary capillary plexus in the hypothalamus are specialized neurosecretory cells. In fact, the axons of these cells terminate on the capillaries. The neurosecretory cells synthesize two types of hormones releasing hormones and inhibiting hormones (see Table 10.2). Each of these hormones helps to regulate the release of a particular hormone from the adenohypophysis. For example, thyrotropin-releasing hormone produced by the neurosecretory cells of the hypothalamus stimulates secretion of thyrotropin from the thyrotrope cells of the adenohypophysis. The hypo-thalamic-releasing hormone is picked up by the primary capillary plexus travels through the hypothalamic-hypophyseal portal veins to the anterior pituitary leaves the blood by way of the secondary capillary plexus and exerts its effect on the appropriate cells of the adenohypophysis. The hypophyseal hormone, in this case, thyrotropin, is then picked up by the secondary capillary plexus, removed from the pituitary by the venous blood, and delivered to its target tissue. 

Arterial blood reaches the pituitary gland via the superior hypophyseal artery, a branch of the internal carotid artery. Venous blood is supplied through a venous portal system that originates in the median eminence of the hypothalamus and ends in sinusoidal capillaries of the pituitary gland. This venous system is known as the hypothalamic-hypophyseal portal system. This system carries neurosecretory hormones from the hypothalamus to the adenohypophysis. These hypothalamic factors stimulate or inhibit the release of hormones from the adenohypophysis. Retrograde flow from the adenohypophysis to the median eminence of the hypothalamus is also believed to occur. With upstream flow, pituitary hormones can reach the hypothalamus and influence hypothalamic function through a short feedback loop. 

Figure 16.1 Hypothalamic-pituitary system. The hypothalamus receives various types of impulses and responds by secreting appropriate release and release-inhibiting factors. These migrate to the anterior or intermediate pituitary via the hypophyseal portal vein system and elicit the secretion of various tropic or non tropic hormones. For instance, when the organism is exposed to cold, blood TSH levels increase when under stress, blood ACTH levels rise. In some animals, the absence of light causes the release of a-MSH.

The hypothalamus is a small region of the brain in the ventral aspect of the diencephalon. In the adult human, it is about 2.5 cm in length and weighs about 4 g. Ventromedi-ally, it surrounds the third ventricle and is continuous with the infundibular stalk of the pituitary (hypophysis). This cone-shaped region of the hypothalamus, the median eminence, consists mainly of axonal fibers from hypothalamic neurons, which either terminate in the median eminence or continue down into the posterior lobe of the pituitary, and it is perfused by a capillary network (primary plexus) derived from the carotid arteries. Blood from the primary plexus is transported by portal vessels (hypophyseal portal vessels) to another capillary network (secondary plexus) in the anterior lobe of the pituitary (adenohypophysis) (Figure 31-1). 

In addition to the common neural mechanism, primer pheromones have a common endocrine effect expressed principally by changing prolactin secretion from the anterior pituitary. Our own studies have shown that lowering prolactin by injections of bromocriptine (a dopamine agonist) is able to reproduce the actions of all the described primer pheromones in female mice with identical timing (Keverne, 1982). Direct measurement of prolactin has also demonstrated that exposure of female mice to males results in rapid decreases of serum prolactin (Ryan and Schwartz, 1980). Such decreases in prolactin are probably brought about by increases in the release of hypothalamic dopamine from the tuberoinfundibular dopaminergic (TIDA) neurons into the hypophyseal portal circulation. Indeed, our recent studies show there to be an increase in the synthesis of dopamine in this... 

The hypothalamic releasing hormones are peptides. They reach their target cells in the AH lobe by way of a portal vascular route consisting of two serially connected capillary beds. The first of these lies in the hypophyseal stalk, the second corresponds to the capillary bed of the AH lobe. Here, the hypothalamic hormones diffuse from the blood to their target cells, whose activity they control. Hormones released from the AH cells enter the blood, in which they are distributed to peripheral organs (1),... 

The neurohumoral control of hypophyseal activity is at present perfectly established Several mediators reaching the anterior lobe through the portal vessels have been extracted from hypothalamic tissue acting selectively on the elaboration and liberation of the various pituitary hormones. 

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