Adenohypophysis, control

The release of prolactin from the adenohypophysis is normally inhibited by prolactin-inhibiting hormone (PIH, dopamine) from the hypothalamus. Prolactin secretion is also controlled by prolactin-releasing factor (PRF). The release of PRF from the hypothalamus is mediated by reflexes elicited by suckling and breast stimulation. 

The control of metabolism, growth, and reproduction is mediated by a combination of neural and endocrine systems located in the hypothalamus and pituitary gland. The pituitary weighs about 0.6 g and rests at the base of the brain in the bony sella turcica near the optic chiasm and the cavernous sinuses. The pituitary consists of an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis) (Figure 37-1). It is connected to the overlying hypothalamus... 

The role of cyclic AMP as modulator of prolactin secretion was first suggested by the finding of a stimulatory effect of cyclic AMP derivatives (17-22) and inhibitors of cyclic nucleotide phosphodiesterase activity such as theophylline and IBMX (22-26) on the secretion of this hormone. More convincing evidence supporting a role of cyclic AMP in the action of dopamine on prolactin secretion had to be obtained, however, by measurement of adenohypophysial adenylate cyclase activity or cyclic AMP accumulation under the influence of the catecholamine. As illustrated in Fig. 1, addition of 100 nM dopamine to male rat hemipituitaries led to a rapid inhibition of cyclic AMP accumulation, a maximal effect (30% inhibition) being already obtained 5 min after addition of the catecholamine. Thus, while dopamine is well known to stimulate adenylate cyclase activity in the striatum (27, 28), its effect at the adenohypophysial level in intact cells is inhibitory. Dopamine has also been found to exert parallel inhibitory effects on cyclic AMP levels and prolactin release in ovine adenohypophysial cells in culture (29) and purified rat mammotrophs (30). Using paired hemipituitaries obtained from female rats, Ray and Wallis (22) have found a rapid inhibitory effect of dopamine on cyclic AMP accumulation to approximately 75% of control. 

The gland consists of the posterior pituitary (neurohypophysis) and the anterior pituitary (adenohypophysis). The functions of the two are not related but both are controlled by the hypothalamus. (See Figure 6.2.)... 

When Harris [1] and Brooks [2] first suggested, perhaps rather tentatively, that the adenohypophysis may be controlled by humorally transmitted stimuli from the hypothalamus, little interest was expressed in their hypothesis. This was... 

It is now firmly established that the synthesis and release of the hormones of the adenohypophysis are controlled by substances released from nerve endings in the hypothalamus and conveyed to the anterior pituitary gland via the portal vessels. The evidence which demonstrated the fundamental importance of the hypothalamus and the hypothalamo-hypophysial portal vessels came mainly from experiments which involved transection of the pituitary stalk, transplantation of the pituitary gland to a site remote from the sella turcica or electrical stimulation of the hypothalamus. 

GnRH acts directly on the adenohypophysis to stimulate the synthesis and release of both gonadotrophins [72—75]. It was originally postulated that separate hypothalamic hormones control the secretion of LH and FSH. However, attempts to isolate two such fractions from hypothalamic tissue have failed... 

Neurohumoral releasing factors stimulate the anterior pituitary cell to secrete specific tropins (LSH, FSH, TSH, and ACTH). The evidence for the hypothalamic control of ACTH release by the adenohypophysis was obtained by electrostimulation of hypothalamic nuclei, destruction of the median eminence, sectioning of the stalk, and transplantation of the pituitary at a distance from its normal anatomical location. 

Everett, J.W. Central neural control of reproductive functions of the adenohypophysis. Physiol. Rev. 44, 373-431 (1964)... 

The mechanisms of hormonal regulation provide us with many instances of insertions into new systems. The secretion of milk, due to the biochemical differentiation of one type of Mammalian cell (p. 306) is provoked and controlled by prolactin, resulting from the biochemical specialization of another type of cell, the adenohypophysis. But prolactin is secreted by the adenohypophysis of fish, amphibians and reptiles. Its intervention in the secretion of milk in mammals is thus an insertion into a new biochemical system. 

One of the important aspects of the biochemical evolution of Vertebrates has been the acquisition by the cells of the mesoderm of enzyme systems for the biosynthesis of new types of steroid (heteromorphic evolution). One of the physiological effects of this evolution is the ionic regulation brought about by the action of the corticosteroid hormones at the urinary tube. In the Amphibiae this action is established in conjunction with a pre-existing system, that of the regulation controlled by the adenohypophysis. 

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