Pituitary gland Intermediate lobe

Connected to the brain by a stalk (Fig. 30-1), the pituitary gland releases at least ten peptide or protein hormones that regulate the activity of other endocrine (hormone-producing) glands in distant parts of the body. The pituitary is composed of several distinct parts the anterior lobe (adenohypophysis), a thin intermediate portion (pars intermedia), and a posterior lobe (neurohypophysis). Each has its own characteristic endocrine functions. 

Following messenger formation, the amount and types of proteins can be modulated in additional ways. The initial polypeptide can be processed in various ways so that different polypeptides or proteins are expressed in different tissues. Such a situation exists for processing the precursor polypeptide preproopiomelanocortin (see fig. 24.7). This polypeptide is processed in different ways in the anterior and intermediate lobes of the pituitary gland to give rise to different hormones in these two tissues. 

The D-2 Dopamine Receptor in the Intermediate Lobe of the Rat Pituitary Gland... 

Dopaminergic neurons synapse upon the parenchymal cells of the intermediate lobe (IL) of the rat pituitary gland. Dopamine decreases the capacity of the IL cells to synthesize cyclic AMP and inhibits the release of otMSH and other peptides from this tissue. The presence of a D-2 receptor accounts for both of these phenomena. This D-2 dopamine receptor can be studied in a binding assay using [3H]-spiroperidol, a dopamine... 

The dopamine receptor in the intermediate lobe of the pituitary gland is negatively coupled to adenylate cyclase... 

Dopamine can thus be added to the list of hormones and neurotransmitters which can stimulate or inhibit cyclic AMP formation, depending upon their tissue of action. Thus, while dopamine stimulates cyclic AMP formation in parathyroid cells, superior cervical ganglia, retina and striatal tissue (27, 58-61), it inhibits the accumulation of the cyclic nucleotide in cells of the intermediate and anterior lobes of the pituitary gland. Opposite effects on the cyclic AMP system are also found with LHRH which stimulates and inhibits cyclic AMP levels in the anterior pituitary gland (62) and ovary (63), respectively. Similarly, alpha-adrenergic agents show opposite effects on cyclic AMP formation in brain (64) and platelets (65). PGE, stimulates cyclic AMP formation in the anterior pituitary gland (62) while it inhibits the same parameter in fat cells (66). 

The above-described data show that CRF added to cells of the rat Intermediate lobe In culture causes a rapid stimulation of oe-MSH release and cyclic AMP accumulation, thus demonstrating a direct action of the peptide on pars intermedia cells (15). It is however difficult, using intact cells, to dissociate between increases in cyclic AMP levels due to stimulation of adenylate cyclase activity or to Inhibition of cyclic nucleotide phosphodiesterase or to a combination of both effects. Definitive proof of the role of adenylate cyclase In the action of CRF In the intermediate lobe of the pituitary gland is provided by the following findings of a CRF-lnduced stimulation of adenylate cyclase activity in homogenate of rat and bovine pars Intermedia cells. 

The highest density of D2 receptor, as measured by selective radiolabeled drugs or antibodies, was found in the NAc, olfactory tubercle, olfactory bulb (glomerular layer) and CPu. In the pituitary gland, D2 receptors were found to be present at a high level in the intermediate lobe. 

Cote TE, Eskay RL, Frey EA, Crewe CW, Munemura M, Stoof JC, Tsuruta K, Kebabian JW (1982) Biochemical and physiological studies of the beta-adrenoceptor and the D-2 dopamine receptor in the intermediate lobe of the pituitary gland a review. Neuroendocrinology 35 217-224. 

Holzbauer M, Racke K (1985) The dopaminergic innervation of the intermediate lobe and of the neural lobe of the pituitary gland. Med Biol 63 97-116. 

Lindley SE, Gunnet JW, Lookingland KJ, Moore KE (1990a) 3,4-Dihydroxyphenylacetic acid concentrations in the intermediate lobe and neural lobe of the posterior pituitary gland as an index of tuberoinfundibular dopaminergic neuronal activity. Brain Res 506 133-138. 

Lookingland KJ, Moore KE (1985) Differential effects of morphine on the rates of dopamine turnover in the neural and intermediate lobes of the rat pituitary gland. Life Sci 37 1225-1229. 

Smelik PG, Berkenbosch F, Vermes I, Tilders FJH (1983) The role of catecholamines in the control of the secretion of pro-opiocortin-derived peptides from the anterior and intermediate lobes and its implications in the response to stress. In Bhatnagar AS (Ed), The Anterior Pituitary Gland, pp. 113-125. Raven Press, New York. 

Von Euler G, Meister B, Hokfelt T, Eneroth P, Fuxe K (1990) Intraventricular injection of neurotensin reduces dopamine D2 agonist binding in rat forebrain and intermediate lobe of the pituitary gland. Relationship to serum hormone levels and nerve terminal coexistence. Brain Res 537 253-262. 

MSHs consist of three peptide hormones a-MSH, 3-MSH, and y-MSH, which are secreted by intermediate lobe of the pituitary gland. They are cleaved from the same precursor peptide as ACTH. Their basic function is stimulation of melanocytes to darken skin and stimulation of melanin synthesis to darken the skin and hair. They also have been found to be released in the brain affecting appetite, sexual arousal, and many other functions. 

The pituitary gland (hypophysis) is located at the base of the skull (Figure 50-1) in a bone cavity called the sella turcica (Turkish saddle). The gland is small—1 cm or less in height and width and weighs approximately 500 mg. As discussed previously, the gland is anatomically divided into the anterior (adenohypophysis) and the posterior (neurohypoph-ysis) lobes. A third lobe (the intermediate lobe) is present in most vertebrates and in the human fetus this lobe is rudimentary in the adult human. 

Some polyproteins are differentially cleaved in different tissues. An example is proopiomelanocortin, a polyprotein that is the source of several hormones synthesized in the pituitary gland. In the anterior lobe of the pituitary, the polyprotein is cleaved to release -lipotropin and adrenocorticotropic hormone (ACTH). In the intermediate lobe, a different pattern of cleavage forms jS-endorphin and a-melanotropin (Chapter 31). 

Fig. 1. Processing of proopiomelanocortin (POMC) the precursor for the melanocortins and opiates in the mammalian pituitary gland. Processing occurs by proteolytic cleavage at sites of paired amino acids, some of which are shown here as dark bands. In both the anterior and the intermediate lobes, POMC is processed into an ACTH biosynthetic intermediate and into )J-lipotropin ()3-LPH 1-91). In the anterior lobe, subsequent processing yields the two biologically important products, ACTH 1-39 and )S-MSH. In the intermediate lobe, ACTH 1-39 is further processed to yield a-MSH (ACTH 1-13) and corticotropin-like intermediate peptide (CLIP). -MSH is derived, via y-MSH, from the 16-K fragment of POMC. This hormone varies in length in different species. Also in the intermediate lobe, /3-LPH is processed to produce p-endorphin 1-31. Many other small peptide fragments, of uncertain biological properties, are also produced, (from Strand et al., 1989).

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