Posterior hypothalamic neurons

The diffuse part of the TMN consists of a small group of neurons scattered within the lateral hypothalamic area, the posterior hypothalamic region, the perifornical area, the supramammillary nucleus, and the dorsomedial hypothalamic nucleus. 

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. 

Oxytocin (Pitocin, Syntocinon) is a cyclic 8-amino acid peptide that is synthesized in the paraventricular nucleus of the hypothalamus and transported within hypothalamic neurons (in association with neurophysin) to the posterior pituitary for storage. Its mechanism of action involves the direct stimulation of oxytocin receptors found on the myometrial cells. Oxytocin circulates unbound in the plasma, where it has a half-Ufe of approximately 15 minutes. It is primarily inactivated in the kidneys and liver. 

Anti-diuretic hormone (arginine vasopressin) is a nine-amino-acid peptide secreted from the posterior pituitary. Within hypothalamic neurons, the hormone is packaged in secretory vesicles together with a carrier protein called neurophysin both are released upon secretion. 

Oxytocin in a nine amino acid peptide that is synthesized in hypothalamic neurons and transported down through axons of the posterior pituitary for secretion into blood. Oxytocin has three major physiological effects stimulation of miUc ejection, stimulation of uterine smooth muscle contraction at birth, and establishment of maternal behavior. 

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). 

The hypothalamus contains a high density of nerve cell bodies clustered into nuclei or areas. Neurons in each of these nuclei tend to send their axons to the same regions in the form of traets. These nuclei innervate the median eminence, other hypothalamic nuclei, the posterior pituitary, and various structures in the extrahy-pothalamic central nervous system. All of the hypothalamic neurons are presumably monoaminergic (i.e., they... 

Hypothalamic neurones synthesize arginine vasopressin (AVP) and oxytocin which pass along axonal nerve fibres in the pituitary stalk to the posterior pituitary where they are stored in granules in the terminal bulbs of nerves in close proximity to systemic veins. 

Peptide-secreting cells of the hypothalamic-hypophyseal circuits originally were described as neurosecretory cells, receiving synaptic information from other central neurons, yet secreting transmitters in a hormone-like fashion into the circulation. The transmitter released from such neurons was termed a neurohormone, i.e., a substance secreted into the blood by a neuron. These hypothalamic neurons also may form traditional synapses with central neurons, and cytochemi-cal evidence indicates that the same substances that are secreted as hormones from the posterior pituitary (oxytocin, arginine-vasopressin see Chapters 29 and 55) mediate transmission at these sites. Thus, the designation hormone relates to the release at the posterior pituitary and does not necessarily describe all actions of the peptide. 

The posterior lobe is composed of nerve fibres and endings of hypothalamic neurones (Fig. 250), capillaries (Figs. 251,254, and 255) and characteristic as-troghal cells, the pituicytes. In rapidly frozen, freeze-substituted neural lobes of male albino rats, pituicytes occupied close to 60% of the basal lamina at the neurohaemal contact zone, while axons occupied about 20% (Tian et al. 1991). Takei et al. (1980) classified the human pituicytes into five classes based on their ultrastructural features ... 

Krilowicz, B. L., Szymusiak, R., McGinty, D. (1994). Regulation of posterior lateral hypothalamic arousal related neuronal discharge by preoptic anterior hypothalamic warming. Brain Res. 668, 30-8. 

FIGURE 14-2 The histaminergic system of the rat brain. (A) Frontal sections through the posterior hypothalamus showing the location of histaminergic neurons. Arc, arcuate nucleus DM, dorsomedial nucleus LM, lateral mammillary nucleus MM, medial mammillary nucleus MR, mammillary recess PM, premammillary nucleus 3V, third ventricle VMH, ventromedial hypothalamic nucleus. (Modified with permission from reference [5].)... 

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. 

The neurotransmitter histamine (HA) exerts several functions in the hypothalamus [1-2] including an involvement in the neuroendocrine regulation of pituitary hormone secretion [3]. HA has no effect directly at the level of the pituitary gland, but influences the secretion of anterior pituitary hormones either by an exerted e.g. in the paraventricular nucleus (PVN) on other central transmitters or hypothalamic regulating factors, which subsequently regulate the release of anterior pituitary hormones. In addition, HA acts on the supraoptic nucleus (SON) in the hypothalamus where the posterior pituitary hormones are synthesized and thereby exerts a direct effect on the release of the posterior pituitary hormones. Immunohistochemical studies have revealed that the histaminergic neurons, which originate in the tuberomammillary nuclei of the posterior hypothalamus, densely innervate most of the hypothalamic areas involved in the neuroendocrine control of pituitary hormone secretion [4-5]. Within the last two decades the effect of HA on pituitary hormone secretion have been explored in several studies and it has been... 

The modulation of the N-type Ca2+ channels has been shown for some presynaptic receptors to be the mechanistic basis for the inhibition of Ca2+ influx [29]. In 1989 Takemura et al. [30] reported on the effective inhibition of histamine release from rat hypothalamic slices by the N-type Ca2+ -channel blocker ca-conotoxin. In addition Endou et al. [23] showed that to-conotoxin greatly potentiated the modulatory effect of (R)a-methylhistamine on cardiac adrenergic responses. Yang and Hatton [31] provided direct evidence for an H3 receptor-mediated modulation of ion permeability of neurons. They showed that in magnocellular histaminergic neurons from the rat posterior hypothalamus, H3... 

The hypothalamic control of the posterior pituitary is quite different than that of the anterior and intermediate lobes. Specific neurons have their cell bodies in certain hypothalamic nuclei. Cell bodies in the paraventricular nuclei manufacture oxytocin, whereas the supraoptic nuclei contain cell bodies that synthesize ADH. The axons from these cells extend downward through the infundibulum to terminate in the posterior pituitary. Hormones synthesized in the hypothalamic cell bodies are transported down the axon to be stored in neurosecretory granules in their respective nerve terminals (located in the posterior pituitary). When an appropriate stimulus is present, these neurons fire an action potential, which causes the hormones to release from their pituitary nerve terminals. The hormones are ultimately picked up by the systemic circulation and transported to their target tissues. 

There is evidence that the histaminergic tract that passes from the posterior hypothalamus to the cortex via the thalamus is inhibited by a GABAergic pathway. It is now known that H3 receptors act as autoreceptors on histaminergic neurons and that agonists of H3 receptors augment slow-wave sleep. In addition, histamine can increase cortical arousal by enhancing excitatory cholinergic neurons from the basal forebrain and also inhibits the hypothalamic pre-optic area which normally promotes sleep. With respect to the control of the circadian rhythm, histamine has... 

There have been several studies characterizing the stimulatory effects of neurotensin on PHDA neurons terminating in the intermediate lobe of the posterior pituitary. Central (icv) administration of neurotensin increases the activity of PHDA neurons and this causes a concomitant decrease in concentration of aMSH in plasma (Pan et al., 1992). Neurotensin stimulates DA release in vitro from explants containing PHDA neurons, but not from isolated neurointermediate lobes, which is consistent with a hypothalamic site of action of neurotensin in stimulating the activity of PHDA neurons (Davis and Kilts 1987a,b). Neurotensin causes a sustained excitation of DA release from PHDA neurons with little or no tolerance as compared with short-lived transient responses of mesolimbic DA neurons to neurotensin (Davis and Kilts, 1987b). 

The posterior pituitaiy (ncurnhypophysis) is the source of vasoprc.ssin. oxytocin, a- and /8-MSH. and coherin. The synthesis, transport, and release of these hormones have been reviewed by Brownstein.- - Va.sopressin and oxytocin ore synthesized and relea.sed by neurons of the hypothalamic-neurohypophyseal system. Thc.se peptide hormones, and their respective neurophysin carrier proteins, are synthesized as structural components of separate precursor proteins, and these proteins appear to be partially degraded into... 

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