Posterior lobe

The posterior lobe of the pituitary, ie, the neurohypophysis, is under direct nervous control (1), unlike most other endocrine organs. The hormones stored in this gland are formed in hypothalamic nerve cells but pass through nerve stalks into the posterior pituitary. As early as 1895 it was found that pituitrin [50-57-7] an extract of the posterior lobe, raises blood pressure when injected (2), and that Pitocin [50-56-6] (Parke-Davis) causes contractions of smooth muscle, especially in the utems (3). Isolation of the active materials involved in these extracts is the result of work from several laboratories. Several highly active posterior pituitary extracts have been discovered (4), and it has been deterrnined that their biological activities result from peptide hormones, ie, low molecular weight substances not covalendy linked to proteins (qv) (5). 

In addition to the weU-defined opioid systems in the central nervous system, the three opioid peptides and their precursor mRNA have also been identified in peripheral tissues. ( -Endorphin is most abundant in the pituitary, where it exists in corticotroph cells with ACTH in the anterior lobe and in melanotroph cells with MSH in the intermediate lobe (59). Enkephalin and pre-pro-enkephalin mRNA have been identified in the adrenal medulla (60) and this has been the source of material for many studies of pro-enkephalin synthesis and regulation. Pre-pro-enkephalin mRNA has also been identified in the anterior and posterior lobes of the pituitary (61). mRNA for all three opioid precursors has been identified in the reproductive system (62—64). POMC... 

Concerning other endocrine activities of the pituitary gland, including the posterior lobe, not enough data appear to be available to... 

Antidiuretic hormone (ADH) and oxytocin are synthesized in the supraoptic and paraventricular nuclei in the brain and are transported in secretory granules through axons to the posterior lobe. These hormones are cyclic peptides of eight amino acids. Each is synthesized as a larger precursor, which is processed into the hormone plus a protein that binds the hormone, called neuro-physin. ADH and oxytocin have different amino acids at positions 3 and 8. 

The correlation of the hypothalamus and its hormones with the hormones of the anterior pituitary gland is summarized in figure 5.2. There is no direct vascular connection between the hypothalamus and the posterior lobe of the pituitary that would correspond to the portal vein system for the anterior lobe of the gland. Not all hypothalamo-pituitary hormones will be discussed in the subsequent sections. Only those that are well-defined chemical entities or have a direct connection with drug action are considered. 

The antidiuretic hormone is an octapeptide released from the posterior lobe of pituitary gland. It is used in the treatment of diabetes insipidus. ADH reduces the total urine volume and absence of this hormone cause diabetes insipidus. ADH acts on collecting duct cells to increase their water permeability. It acts on V2 receptors in collecting duct and regulate their water permeability through cAMP production. 

The pituitary gland is situated in sella turcica or hypophyseal fossa of the sphenoid bone attached to the brain by a stalk which is continuous with the part of brain i.e. hypothalamus and there is a communication between the hypothalamus and the pituitary gland by means of nerve fibres and a complex of blood vessels. Pituitary gland consists of three parts - anterior lobe or adenohypophysis, posterior lobe or neurohypophysis and middle lobe or pars intermedia. 

The anterior lobe secretes various trophic hormones, the posterior lobe is responsible for the secretion of oxytocin and antidiuretic hormone (vasopressin) and middle lobe secretes melanocyte-stimulating hormone (MSH) which may affect the synthesis of melanin. 

The posterior lobe secretes two hormone namely oxytocin and antidiuretic hormone (ADH or vasopressin). 

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 posterior lobe hormones are synthesized in the hypothalamus and transported via the neurosecretory fibers in the stalk of the pituitary to the posterior lobe, from which they are released into the circulation. 

The pituitary gland is composed of anterior and posterior lobes that are attached to the hypothalamus via the infundibulum or hypophyseal stalk, which includes the portal vessels. Processes from GnRH cells in the hypothalamus terminate on or in the vicinity of the portal vessels that convey blood to the anterior lobe of the pituitary. The... 

Figure 30-1 (A) Median sagittal section of the human brain. From Maya Pines.15 (B) Drawing illustrating the synthesis of peptide hormones in the hypothalamus and transport via portal blood vessels into the anterior lobe of the pituitary gland or via nerve tracts into the posterior lobe.16...

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. 

OXYTOCIN. A polypeptide hormone which is secreted by the posterior lobe of the pituitary gland of mammals and other vertebrates. Oxytocin exerts a stimulating effect upon the muscles of the breast (milk-ejection) and those of the uterus of mammals. It is sometimes used medically to stimulate labor in cases of difficult childbirth and to time the onset of labor. 

Vasopressin and oxytocin are peptide hormones secreted from the posterior lobe of the pituitary gland. They function primarily to raise blood pressure (vasopressin), as antidiuretic (vasopressin), and to promote contraction of uterus and lactation muscies (oxytocin). The isolation, identification, and synthesis of these hormones was accomplished by Vincent du Vigneaud, for which he was awarded the Nobel Prize in chemistry in 1955. 

The pituitary can be subdivided into an anterior, an intermediate, and a posterior lobe. These subdivi-... 

Posterior lobe Antidiuretic hormone [ADH] Oxytocin T renal reabsorption of water t uterine contraction t milk ejection during lactation... 

Posterior Lobe. The posterior pituitary, or neuro-hypophysis, secretes two hormones antidiuretic hormone (ADH) and oxytocin.2 12 ADH exerts its effect... 

Vasopressin Hypophysis Isolated posterior lobe (rat P2Y) Sperlagh et al. 1999... 

The result of this anatomical characteristic of endothelial cells in the CNS is an increased resistance to water-soluble and ionized drugs entering the brain, and cerebrospinal fluid (CSF), from capillary blood. However, in a few areas of the brain the barrier is absent. These areas include the lateral nuclei of the hypothalamus, the area postrema of the fourth ventricle, the pineal body, and the posterior lobe of the hypophysis. Highly lipophilic compounds can cross the barrier. Tranquilizers such as diazepam and its analogs are known to gain access rapidly to the CSF with a half-life (tm) entry time of less than 1 minute. 

Enkephalins are found in varying amounts in nearly all regions of the nervous system, in the posterior lobe of the pituitary, and in the adrenal cortex. They play a role in pain transmission in that they act as transmitters for the pain-inhibiting neurons in the spinal cord. 

The pituitaries of male Sprague-Dawley rats weighing about 100 g are quickly removed after decapitation. The posterior lobe is discarded and the anterior lobe is divided into two halves by a midsagittal cut. Five bisected hemipituitaries are incubated in plastic vials containing 4 ml TCM 199 with 0.1 % BSA, 15 pg/ml penicillin and 25 fig/ml streptomycin. The vials are gassed with 95 % O2 and 5 % C02. After 30 min of control incubation, the medium is changed and various doses of standard and test substances are added for an incubation of 90 min. GH content in the medium and in the pituitary tissue after incubation is determined by a specific radioimmunoassay (Schalch and Reichlin 1966). Other of hormones may be tested in the same procedure, for example prolactin. 

In h)rpopituitarism there is a partial or complete deficiency of hormones secreted by the anterior lobe of the pituitary. The posterior lobe hormones (see below) may also be deficient in a few cases, e.g. when a tumour has destroyed the pituitary. Patients suffering from hypopituitarism may present in coma, in which case treatment is as for a severe acute adrenal insufficiency. Maintenance therapy is required, using hydrocortisone, th5Toxine, oestradiol and progesterone (in women) and testosterone (in men). For growth hormone see above. 

---