半胱胺耗竭体内生长抑素及其机制

半胱胺,又称β巯基乙胺,相当于半胺氨酸的脱羧产物,是辅酶A分子的组成部分,在体内有重要的生理意义。临床上用于治疗放射病、胱氨酸病、扑热息痛中毒和四乙基铅中毒。Selye等发现半胱胺可诱发大鼠十二指肠溃疡。在研究其致溃疡机制过程中,Szabo     

Gonadotropin-releasing hormone release from the mediobasal hypothalamus of fetal baboons

Pulsatile luteinizing-hormone releasing hormone (LHRH) secretion was measured from the mediobasal-suprachiasmatic-preoptic (MBH-SCN-POA) region of the hypothalamus from fetal baboons (Papio anubis) at midgestation (day 100; term = day 184). The entire MBH-SCN-POA (48 ± 5 mg) was obtained between 1100 and 1200 hours and was immediately placed in icecold phosphate buffer (pH 7.4). The MBH-SCN-POA units were halved at the midline and superfused in parallel at 37.5°C for 5 hours. Then 500 μl of superfusate was collected at 10-minute intervals, and LHRH concentration was measured by radioimmunoassay using the Chen-Ramirez antibody. In fetuses of untreated baboons (N = 3), LHRH pulse amplitude (mean ± SE) was 16.0 ± 4.2 pg, with a period of 30 ± 1 minute; the average 10-minute output of LHRH was 9.4 ± 2.0 pg. In fetal baboons in which the hormonal milieu in the mother was modulated by androstenedione treatment of midpregnancy (N = 3), average LHRH pulse amplitude was 1.7 ± 0.3 pg, with a period of 33.5 ± 4.9 minutes; the average 10-minute output of LHRH was 1.2 ± 0.2 pg. Collectively, these data suggest that as early as midgestation, fetal baboons secrete LHRH in vitro in a pulsatile fashion and with a periodicity of 30–35 minutes. In addition, the decrease in LHRH pulse amplitude and the average 10-minute LHRH output (P < .01, P < .05) in tissue from fetal baboons of mothers in which the normal pattern of steroidogenesis is altered suggest that the output of LHRH systems in the fetus is sensitive to changes in the maternal hormonal milieu.     

Growth hormone release inhibiting hormone: actions on thyrotrophin and prolactin secretion after thyrotrophinreleasing hormone.

The hypothalamic tetradecapeptide growth hormone release inhibiting hormone (GH-RIH) blocked the thyrotrophin response to thyrotrophin-releasing hormone (TRH) in normal people and in patients with primary hypothyroidism. This inhibition was dose related. The TRH-induced prolactin release was not affected by GH-RIH. This dissociation of the thyrotrophin and prolactin responses to TRH by GH-RIH suggests that there are different mechanisms for release of thyrotrophin and prolactin and that only the former is affected by GH-RIH.     

Prolonged pulsatile release of gonadotropin-releasing hormone from the guinea pig hypothalamus in vitro

The sexually mature mammal secretes luteinizing hormone in a pulsatile fashion. This is presumed to depend on the intermittent release of hypothalamic gonadotropin- releasing hormone (GnRH). The isolated guinea pig hypothalamus has been studied because, in this species, as in primates, the pulse generator appears to reside within the medial basal hypothalamus. The basal 2 mm of guinea pig hypothalami were rapidly removed and perifused at 37 degrees C with Krebs-Ringer solution containing 20 mM bacitracin gassed with 95% O2, 5% CO2. The eluates were sampled at 15 and 5 min intervals and pulsatile patterns of GnRH were consistently observed for periods up to 72 h. There was no difference in GnRH levels from hypothalami of intact and ovariectomized animals. Simultaneous measurement of TRH and somatostatin disclosed independent pulses of both neurohormones which did not coincide with GnRH, indicating that the peaks were secretory episodes not artefacts generated by varying perifusion rates. The hypothalami disclosed no histologic evidence of necrosis when examined after 20 h perifusion.     

The medial basal hypothalamus and luteinizing hormone release in the rat: Where are the LH-RH neurons responsible for tonic gonadotropin secretion?

Careful review of the literature demonstrates conflicting results concerning the ability of the deafferented medial basal hypothalamus to support gonadotropin release in the rat and thus one may question the existence of LH-RH neurons in the medial basal hypothalamus. The direct search for the LH-RH perikarya in the rat hypothalamus has not settled the question of whether these releasing hormone neurons are located in the medial basal hypothalamus. Most investigators do agree that following complete hypothalamic deafferentation there is a reduction of the immunoassayable LH-RH in the medial basal hypothalamus; however, these results do not necessarily prove that LH-RH originates outside the hypothalamus. It is argued that the completely deafferented medial basal hypothalamus may be so altered by the deafferentation procedure that it may be inadequate as a means to study neuroendocrine function.     

Ascorbic acid enhances the release of luteinizing hormone-releasing hormone from the mediobasal hypothalamus in vitro

Ascorbic acid is frequently used in in vitro studies of neurotransmitter-evoked release of luteinizing hormone-releasing hormone (LHRH) from hypothalamic fragments. Although it is assumed that ascorbate merely prevents the oxidative degradation of catecholamines, we have discovered that ascorbic acid itself produces significant increases in the release of LHRH. Our studies showed that ascorbic acid, at concentrations below 1 mM, produced a dose-dependent release of LHRH from incubated rat mediobasal hypothalamus (MBH). The magnitude of the ascorbate-induced release was in the range of 100-200% above controls; significant amounts of LHRH were released only if the MBH were incubated with ascorbate for time periods longer than 30 minutes. We also found that ascorbate-induced increases in LHRH were equivalent to those produced by another LHRH secretagogue, naloxone, and that the combined effects of the two substances were additive in nature. Although the mechanisms underlying this effect are not fully understood, nonspecific chemical reduction is probably not a factor since sodium metabisulfite did not induce the release of LHRH. It seems probable that ascorbate may enhance the activity of endogenous norepinephrine in the MBH and, thereby, lead to increased release of LHRH.     

Glucose-sensing neurons of the hypothalamus

Specialized subgroups of hypothalamic neurons exhibit specific excitatory or inhibitory electrical responses to changes in extracellular levels of glucose. Glucose-excited neurons were traditionally assumed to employ a 'beta-cell' glucose-sensing strategy, where glucose elevates cytosolic ATP, which closes KATP channels containing Kir6.2 subunits, causing depolarization and increased excitability. Recent findings indicate that although elements of this canonical model are functional in some hypothalamic cells, this pathway is not universally essential for excitation of glucose-sensing neurons by glucose. Thus glucose-induced excitation of arcuate nucleus neurons was recently reported in mice lacking Kir6.2, and no significant increases in cytosolic ATP levels could be detected in hypothalamic neurons after changes in extracellular glucose. Possible alternative glucose-sensing strategies include electrogenic glucose entry, glucose-induced release of glial lactate, and extracellular glucose receptors. Glucose-induced electrical inhibition is much less understood than excitation, and has been proposed to involve reduction in the depolarizing activity of the Na+/K+ pump, or activation of a hyperpolarizing Cl- current. Investigations of neurotransmitter identities of glucose-sensing neurons are beginning to provide detailed information about their physiological roles. In the mouse lateral hypothalamus, orexin/hypocretin neurons (which promote wakefulness, locomotor activity and foraging) are glucose-inhibited, whereas melanin-concentrating hormone neurons (which promote sleep and energy conservation) are glucose-excited. In the hypothalamic arcuate nucleus, excitatory actions of glucose on anorexigenic POMC neurons in mice have been reported, while the appetite-promoting NPY neurons may be directly inhibited by glucose. These results stress the fundamental importance of hypothalamic glucose-sensing neurons in orchestrating sleep-wake cycles, energy expenditure and feeding behaviour.     

Electrophysiological effects of MCH on neurons in the hypothalamus

Melanin concentrating hormone (MCH) has been implicated in many brain functions and behaviors essential to the survival of animals. The hypothalamus is one of the primary targets where MCH-containing nerve fibers and MCH receptors are extensively expressed and its actions in the brain are exerted. Since the identification of MCH receptors as orphan G protein coupled receptors, the cellular effects of MCH have been revealed in many non-neuronal expression systems (including Xenopus oocytes and cell lines), however, the mechanism by which MCH modulates the activity in the neuronal circuitry of the brain is still under investigation. This review summarizes our current knowledge of electrophysiological effects of MCH on neurons in the hypothalamus, particularly in the lateral hypothalamus. Generally, MCH exerts inhibitory effects on neurons in this structure and may serve as a homeostatic regulator in the lateral hypothalamic area. Given the contrast between the limited data on cellular functions of MCH in the hypothalamus versus a fast growing body of evidence on the vital role of MCH in animal behavior, further investigations of the former are warranted.     

Histamine-immunoreactive neurons in the hypothalamus of cats

The localization of histaminergic neurons in the cat brain was determined immunohistochemically with an antibody against histamine. We found that histamine-immunoreactive neurons are observed exclusively in the posterior hypothalamus of colchicine treated cats. The larger group of neurons was found in the ventrolateral part of the posterior hypothalamus, including the tuberomammillary nucleus. Histamine-positive neurons were also observed in the supramammillary area and adjacent posterior hypothalamic area, as well as in the peri- and premammillary regions. In addition, numerous histamine immunoreactive fibers were detected, not only in the posterior hypothalamus, but also in other brain areas, such as the preoptic area of the anterior hypothalamus.     

Ethanol inhibits the naloxone-induced release of luteinizing hormone-releasing hormone from the hypothalamus of the male rat

It has been inferred that ethanol suppresses the secretion of luteinizing hormone (LH) in the male by depressing the release of LH-releasing hormone (LH-RH) from the hypothalamus. Direct support for this inference has been difficult to obtain, however, because of significant technical difficulties in measuring LH-RH release under in vivo conditions. To circumvent these problems, we made use of the opiate antagonist naloxone, as a neuroendocrine probe, to elicit the release of LH-RH under in vivo conditions. We found that ethanol was a potent suppressor of the increase in serum LH levels evoked by naloxone at extremely low blood ethanol concentrations ( less than 60 mg/dl). Furthermore, we observed that the antagonism between ethanol and naloxone appeared to be competitive in nature since a fixed dose of ethanol (1 g/kg, blood ethanol concentration 60 mg/dl) shifted the naloxone dose-response curve significantly to the right and high doses of the antagonist overcame ethanol's effects. Finally, we found that the interaction between ethanol and naloxone took place at the level of the hypothalamus. Our results, therefore, seem to provide the first in vivo evidence supporting the widely-held hypothesis that ethanol reduces serum LH levels by depressing the hypothalamically-medicated release of LH-RH. The mechanisms underlying ethanol's depression of naloxone-induced increases in the release of LH-RH are not fully understood at this time, but one prominent possibility is that ethanol enhances the synthesis or release of endogenous opioids which in turn override naloxone's effects.     

CRF-containing neurons of the rat hypothalamus

Summary The immunoreactive CRF-neurons of the rat hypothalamus have been examined immunohistochemically employing anti-rat CRF serum. These neurons are confined to the paraventricular nucleus, dorsomedial-lateral hypothalamic area, and suprachiasmatic nucleus, and are, respectively, also immunoreactive to anti-Met-enk, -alpha-MSH, and -VIP sera. Intraventricular administration of colchicine (50 g/5 l/rat) induces a dramatic enhancement of the immunostainability of the cell somata, and also accelerates the development of immunoreactivity of other stored peptides, especially in the paraventricular nucleus.The CRF-neurons respond to adrenalectomy by showing increased immunoreactivity and an increase in the number of cell bodies; in the dorsomedial-lateral area and suprachiasmatic nucleus, there is also an enhanced immunoreactivity for alpha-MSH and VIP, respectively. CRF-cells in the paraventricular nucleus become markedly hypertrophied, but do not show any enhanced immunoreactivity for Met-enk. Since the axons of the paraventricular neurons run to the median eminence, it is probable that they are involved with the endocrine control of hypophysial ACTH release. It is concluded that the CRF-containing neurons in rat hypothalamus consist of three types which are functionally and morphologically different.     

Leptin sensitive neurons in the hypothalamus.

A major paradigm in the field of obesity research is the existence of an adipose tissue-brain endocrine axis for the regulation of body weight. Leptin, the peptide mediator of this axis, is secreted by adipose cells. It lowers food intake and body weight by acting in the hypothalamus, a region expressing an abundance of leptin receptors and a variety of neuropeptides that influence food intake and energy balance. Among the most promising candidates for leptin-sensitive cells in the hypothalamus are arcuate nucleus neurons that co-express the anabolic neuropeptides, neuropeptide Y (NPY) and agouti-related peptide (AGRP), and those that express proopiomelanocortin (POMC), the precursor of the catabolic peptide, alphaMSH. These cell types contain mRNA encoding leptin receptors and show changes in neuropeptide gene expression in response to changes in food intake and circulating leptin levels. Decreased leptin signaling in the arcuate nucleus is hypothesized to increase the expression of NPY and AGRP. Levels of leptin receptor mRNA and leptin binding are increased in the arcuate nucleus during fasting, principally in NPY/AGRP neurons. These findings suggest that changes in leptin receptor expression in the arcuate nucleus are inversely associated with changes in leptin signaling, and that the arcuate nucleus is an important target of leptin action in the brain.     

Localization of alpha 2-adrenergic agonist sensitive area in the hypothalamus for growth hormone release in the rat

To determine the localization of the clonidine sensitive area responsible for GH release, a minute amount of the alpha 2-agonist (67 ng/0.2 microliter) was injected into the hypothalamus and vicinity of adult male conscious rats. The animals were chronically implanted with double metal cannulae fixed on the skull for clonidine microinjection and with silastic tubing into the right atria for collecting blood samples. Ten hr prior to the microinjection, alpha-methyl-p-tyrosine (250 mg/kg body weight) was intraperitoneally injected to prevent spontaneous pulsatile GH release. Localization of the microinjection was assessed by histological examination after the experiment. Clonidine microinjection into the amygdala nucleus had no effect on GH release, while the injection into the preoptic and anterior hypothalamic area (PO/AH) significantly stimulated GH release by causing it to begin 30 min earlier. However, the paraventricular nucleus, the dorsomedial nucleus, the lateral hypothalamus and the ventromedial hypothalamus areas did not respond to the injection, although the latter nucleus has been shown to be a specific locus sensitive to electrical stimulation of release. In the area from the posterior hypothalamus to the mammillary body, several injections stimulated GH release (6/15), but the stimulatory effect was statistically insignificant when comparison was made with the mean (+/- SE) for all 15 rats. These findings suggest that the alpha 2-agonist acts on the PO/AH to induce an increase in GH release in alpha-methyl-p-tyrosine-pretreated rats, probably mediating the inhibitory input to somatostatinergic neurons which reside in the periventricular nucleus of the PO/AH area.     

Growth hormone-release inhibiting hormone (GH-RIH) pathway of the rat hypothalamus revealed by the unlabeled antibody peroxidase-antiperoxidase method

Utilizing the unlabeled antibody enzyme method, we investigated the distribution of hypothalamic elements immunoreactive with antibodies to growth hormone-release inhibiting hormone (GH-RIH). Immunostained elements, resembling neural processes, are distributed along a pathway corresponding to a portion of the tuberoinfundibular tract. However, GH-RIH fibers are caudal, dorsal and medial to LH-RH fibers detected by the same technique. Similar topographic arrangements are noted in coronal and sagittal sections. Comparable results were obtained with two different preparations of antisera to GH-RIH. No cell bodies specifically stained by anti-GH-RIH were detected. Our data agree with those of other investigators using immunohistochemical techniques.