Involvement of the opioid system in the central antisecretory action of alpha-2 adrenoceptor agonists in rat

The aim of the present study was to analyse the role of the central alpha-2 adrenoceptors in the regulation of gastric acid secretion in pylorus ligated rats. It was found that the intracerebroventricularly (icv.) injected presynaptic alpha-2 adrenoceptor agonist clonidine and the alpha-2A adrenoceptor subtype selective stimulant oxymetazoline exerted a dose dependent inhibition on gastric acid secretion. The antisecretory ED(50) values for clonidine and oxymetazoline were 20 and 7.5 nmol/rat icv., respectively. The antisecretory effect of these compounds was antagonised by the presynaptic adrenoceptor antagonist yohimbine (50 nmol/rat icv.) indicating that the action is mediated through central presynaptic alpha-2 adrenoceptors. Moreover, naloxone (50 nmol/rat icv.)--non-selective opioid antagonist--and naltrindole (0.5 nmol/rat icv.)--delta-opioid receptor selective antagonist--also decreased the antisecretory effect of clonidine and oxymetazoline suggesting that the endogenous opioid system is likely to be involved in the central antisecretory action of alpha-2 adrenoceptor stimulants.     

Adrenoceptor mechanisms in the cardiovascular effects of cocaine in conscious squirrel monkeys.

The purpose of the current experiment was to study the role of various adrenoceptor subtypes in the cardiovascular response to cocaine in conscious squirrel monkeys. A variety of adrenoceptor antagonists were administered i.v. prior to the administration of 0.3 mg/kg cocaine (i.v.). Cocaine alone produced an increase in both blood pressure and heart rate. The non-selective alpha adrenoceptor antagonist phentolamine produced a dose-dependent antagonism of the pressor effect of cocaine, as did the alpha-1 selective antagonist prazosin. The alpha-2 selective antagonist yohimbine had no effect on the pressor effect of cocaine. The non-selective beta antagonist propranolol enhanced the pressor effect of cocaine as did the beta-1 selective antagonist atenolol. However, the effect of atenolol was not dose-dependent. The beta-2 selective antagonist ICI 118,551 and labetalol, which blocks both alpha and beta adrenoceptors, did not alter the pressor effect of cocaine. Propranolol, atenolol, and labetalol all antagonized the tachycardiac effect of cocaine in a dose-dependent manner, while the beta-2 antagonist ICI 118,551 did not. Phentolamine, prazosin and yohimbine also reduced the tachycardiac effect of cocaine, although these effects were dose-dependent only for yohimbine, which also significantly elevated baseline heart rate. These results indicate that alpha-1 adrenoceptor mechanisms mediate the pressor effect of cocaine, while beta-1 adrenoceptor mechanisms are involved in the tachycardiac effect of cocaine in squirrel monkeys. Propranolol potentiated cocaine's pressor effect through beta-2 independent mechanisms. Thus, neither alpha-2 nor beta-2 adrenoceptor mechanisms appear to be involved in cocaine's cardiovascular effects.     

Alpha- and beta-adrenoceptors in the female dog urethra

The existence and subtypes of alpha- and beta-adrenoceptors in the female dog urethra were studied in vivo and in vitro by means of agonist and antagonist drugs. Noradrenaline, phenylephrine and B-HT 920, stimulants of alpha, alpha-1 and alpha-2 receptors respectively, caused an increase in the urethra tonicity. Thus indicating that the contractile activity is mediated by alpha-1 and alpha-2 adrenoceptors subtypes. On the other hand, the inhibitory urethral activity is under control of beta-adrenoceptors of beta-2 subtype, since the isoprenaline relaxing action is inhibited when beta-2 receptors are blocked, whilst this effect was not observed when beta-1 receptors were blocked. This fact was proved when beta-2 receptors were stimulated with salbutamol.     

Effects of alpha-2 adrenoreceptor blockade by yohimbine on the hormonal response to hypoglycaemic stress in normal man

In order to evaluate the effect of alpha-2 adrenoreceptor blockade on the ACTH response to insulin-induced hypoglycaemia, six normal men were studied with and without yohimbine (30 mg p.o.) premedication. Despite a similar hypoglycaemic stimulus and significant suppression of the growth hormone response (P less than 0.05), no change was observed in basal or stimulated plasma ACTH, cortisol, arginine vasopressin (AVP) or prolactin responses following yohimbine. We conclude that alpha-2 adrenoceptor blockade with yohimbine does not significantly affect the ACTH response to hypoglycaemia in man.     

Attenuation of ethanol intoxication by alpha-2 adrenoceptor antagonists

The interaction of a highly potent and selective alpha-2 adrenoceptor antagonist, atipamezole with ethanol was investigated in tests assessing a number of ethanol's behavioral effects. Atipamezole antagonized ethanol's effects on directed exploration in a holeboard test, reduced observer-rated intoxication and also reduced the duration of loss of righting reflex caused by ethanol. Similar effects were produced by another alpha-2 adrenoceptor antagonist idazoxan. The magnitude of the effects was comparable to that produced in the same animal models by the imidazodiazepine Ro 15-4513, which antagonizes ethanol by an action at central benzodiazepine receptors. Whereas Ro 15-4513 possesses marked behavioral effects on its own, atipamezole is comparatively inactive in all paradigms so far tested. The data suggest that alpha-2 adrenoceptors can play an important role in modulating the intoxicating effects of ethanol.     

Hypothalamic adrenoceptors concerned with control of temperature, elicited by chemical stimulation

Norepinephrine (NE) and its blockers (alpha-1, alpha-2, beta-1 and beta-2) were micro-injected into the anterior hypothalamus of male albino rats and the effects of these injections on rectal temperature were recorded. The results indicated that the thermoregulatory effects of NE were dependent on ambient temperature. The present study further demonstrated the specific involvement of beta-2 receptors present in the anterior hypothalamus concerned with thermoregulation.     

Inverse regulation of alpha-2 and beta-2 adrenoceptors in salt-sensitive hypertension: an hypothesis

A high salt diet leads to up-regulation of alpha-2 adrenoceptors and down-regulation of beta-2 adrenoceptors in normotensive subjects. Although the increase in blood pressure with a high salt diet is not related to the magnitude of the alpha-2 or beta-2 adrenoceptor changes alone, it is related to the increase in the ratio of the receptor changes (operative alpha/beta adrenoceptor ratio). An increase in the operative alpha/beta adrenoceptor ratio with a high salt intake results in vasoconstriction and reduced vasodilatation at resistance vessels, as well as increased renal proximal tubular sodium reabsorption. An influence of heredity on this relationship is supported by four lines of evidence: 1) salt-sensitivity of blood pressure occurs predominantly in subjects with a family history of hypertension; 2) studies in twin children document the influence of genetic variance on salt-sensitivity of blood pressure; 3) subjects with a family history of hypertension have a significantly lower salivary sodium concentration and an altered urinary sodium excretion after salt loading compared to subjects with no such history; 4) salt-sensitivity of blood pressure may be associated with specific genetic markers. On the basis of these observations, we propose the hypothesis that enhanced inverse alpha-beta-adrenoceptor regulation in response to a high salt intake may be responsible for salt sensitivity in the normal population, and may contribute to the development of essential hypertension in susceptible individuals. This alteration is likely to be genetically mediated.     

Effect of the GABAA agonist muscimol on prolactin secretion from human prolactin-secreting adenomas and GH3 rat pituitary tumour cells.

The effect of muscimol, a specific potent GABAA receptor agonist, on prolactin release from human prolactin-secreting tissue was investigated using a perifusion system. Perifusion studies on normal rat anterior pituitary tissue, which has identical GABA receptors to those found in normal human pituitary glands, show that muscimol has a specific biphasic effect on prolactin release. This is characterized by an initial transient stimulation (222.3 +/- 21.6% of basal) lasting for 5-10 min followed by a more prolonged inhibitory phase (63.9 +/- 3.1% inhibition of basal). Five human prolactin-secreting adenomas were studied, and in none of the tumours could a biphasic response be demonstrated. One of the prolactin-secreting adenomas had a blunted inhibitory response, but the other 4 showed no inhibitory effect of muscimol on prolactin release. Muscimol had no significant effect on basal or thyrotropin-releasing-hormone (TRH)-stimulated prolactin secretion from GH3 rat pituitary tumour cells. These studies suggest that the GABAergic effect on prolactin secretion is absent or altered in both rat and human prolactin-secreting tumour cells.     

Regulation of sialyltransferase activity in intestinal segments of rats

A differential distribution of sialyltransferase (ST) in different regions of intestine has been shown. Jejunum and ileum homogenates from rats showed almost exclusive presence of alpha-2-3 ST (to Gal in Gal beta-1-4GlcNAc and/or to Gal in Gal beta-1-3GalNAc). In contrast, colon homogenates showed the presence of both alpha-2-3 ST (as above) and alpha-2-6 ST. Incubation of intestinal slices in presence of heat-inactivated horse serum (HHS) showed a time- and temperature-dependent secretion of soluble ST into the medium. Both jejunum and ileum slices showed high rates of secretion of alpha-2-3 ST. Colon slices, though rich in alpha-2-6 ST, secreted only alpha-2-3 ST. Colchicine, an anti-mitotic drug, injected into rats caused about 10-fold increase of the serum ST level. Jejunum slices from colchicine-treated rats showed an increased secretion of alpha-2-6 ST, suggesting that intestine undergoes a change in the expression of normal secretion of alpha-2-3 ST to a secretion of alpha-2-6 ST. The secretion of ST from incubated intestinal slices was inhibited by heparin. Certain protein factors (anti-proteases) in HHS bind to heparin-sepharose column and these protein factors are responsible for causing the secretion of ST into the medium. It has also been found that a supernatant fraction of the colon homogenate activated ST. Gel chromatography on HPLC produced 3-4 protein fractions from the colon cytosol and one of this fraction bearing high molecular weight proteins produced the maximum activation of ST.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo and in vitro effect of naloxone on prolactin response to TRH in rat

In adult male Wistar rats submitted to a standardized noise stress, intravenous TRH induced a prolactin (PRL) secretory response. Prior IV naloxone administration not only lowered plasma PRL levels in those stressed rats but abolished also the stimulatory action of TRH. This effect was further studied by superfusion experiments on enriched PRL cell suspensions (70% lactotrophs) from female adult Wistar rats. Naloxone kept unaffected the basal PRL secretion but lowered significantly that induced by TRH. These experiments suggest a dual effect of naloxone on rat PRL secretion, one exerted on central opioid receptors lowering stress-related increased basal PRL levels, the other inhibiting the TRH-dependent PRL secretion exerted at the lactotroph level itself.     

Alpha-2 adrenergic and opioid receptor-mediated gastroprotection.

Clonidine inhibited the development of gastric mucosal lesions induced by either acidified ethanol or indomethacin. The ED50 values were: 7.1 and 5.2 microg x kg(-1) orally, respectively. The gastroprotective effect was antagonised by the pre-synaptic alpha-2 antagonist yohimbine, the more selective alpha-2 antagonist Ch-38083 and the pre-synaptic alpha-2B antagonist prazosin. Moreover, the non-selective opioid receptor antagonist naloxone, the delta receptor selective naltrindole also reversed the clonidine-induced mucosal protective action. Clonidine was also effective following intracerebroventricular administration with the ED50 of 37 ng/rat against ethanol-induced mucosal damage. Our results suggest that: 1) the gastroprotective effect of clonidine is likely to be mediated by alpha-2B adrenoceptor subtype; 2) there is an interaction between pre-synaptic alpha-2 adrenoceptors and opioid system; and 3) clonidine can induce gastroprotection by central mechanism.     

Human natural killer cells express VLA-4 and VLA-5, which mediate their adhesion to fibronectin.

Very late Ag (VLA)-3, VLA-4, and VLA-5, belonging to the beta-1 subfamily of integrins, have been recently identified as receptors for different binding regions of fibronectin (FN). We have detected VLA-4 and VLA-5, but not VLA-3, on fresh CD3-, CD16+, CD56+ human NK cells by flow cytometry and immunochemical analyses using mAb directed against beta-1, alpha-3, alpha-4, and alpha-5 subunits. Binding assays, performed on FN-coated plates, showed that NK cells specifically adhere to FN and their binding capacity is increased by MgCl2 but not by CaCl2. Using as inhibitory probes a polyclonal antibody against the beta-1 chain of the human FN receptor, the synthetic peptide GRGDSP, which is able to inhibit cellular adhesion mediated by VLA-5, the CS1 fragment, which contains the principal adhesion site in the IIICS domain recognized by VLA-4, and functional mAb directed against alpha-4 or alpha-5 subunits, we show that both VLA-4 and VLA-5 mediate the adhesion of human NK cells to FN. The expression of these integrin receptors may be relevant for NK interaction with extracellular matrix components and other cell types.     

Regulation of hypoxia-induced release of corticotropin-releasing factor in the rat hypothalamus by norepinephrine

Corticotropin-releasing factor (CRF) peptide release was activated by hypoxia in the rat hypothalamus. The mechanisms, however, of the hypoxia-induced CRF release remains unclear. In this study, we demonstrated that the norepinephrine (NE) and its receptors in the paraventricular nucleus (PVN) mediated the CRF release in a simulated altitude hypoxia. When rats were exposed to 5 or 7 km altitude of hypoxia for a short or long term: (1) NE levels in the PVN and the CeA, using the HPLC analysis, were intensity and time course dependently increased, but the increase in the PVN were potential than in the CeA. Restraint-induced NE increase was much higher in both the PVN and the CeA, compared with hypoxia-induced response. (2) Hypoxia and restraint significantly enhanced CRF release in the ME and the PVN but not in the CeA, through RIA assay, which result in stimulating corticosterone secretion. (3) Hypoxia-induced CRF release was reversed by an injection of prazosin (i.c.v.), an alpha-1 adrenoceptor antagonist, while administration of yohimbine (i.c.v.), an alpha-2 receptor antagonist, facilitated further CRF release. These data suggested that hypoxia induced NE activation centrally, via alpha-1 and -2 receptors, leading to improving hypothalamic CRF release, which in turn stimulated pituitary and adrenal cortex. Restraint presented much potential action on NE activation than hypoxia.     

The regulation of adenohypophyseal prolactin secretion: effect of triiodothyronine and methylene blue on estrogenized rat adenohypophysis

Estrogens and thyroid hormones contribute importantly to cell proliferation and tumor transformation in the pituitary gland. We found that methylene blue antagonized estrogen-promoted adenohypophyseal enlargement and the enhancement of prolactin secretion. The purpose of the present article is to provide a review about neurotransmitters and their receptors involved in estrogen-induced anterior pituitary growth and in the antagonistic effects of triiodothyronine (T3) and methylene blue (MB). Central dopaminergic and noradrenergic systems are the most important factors regulating pituitary growth and function. Recently nitric oxide (NO) was added to the list of the neurotransmitters and neuropeptides involved in the control of the anterior pituitary secretion. Our data suggest that estrogen-induced anterior pituitary growth is associated with decreased synthesis and metabolism of central catecholamines, reduction of adenohypophyseal beta-adrenergic receptors and increase of dopamine DA-2 receptors. We found that the treatment with T3 or MB prevented both estrogen-induced catecholaminergic inhibition and dopamine DA-2 receptor increment in the anterior pituitary. In contrast to T3, MB given alone also slightly decreased the anterior pituitary weight. Serum levels and anterior pituitary content of prolactin were increased after treatment with estradiol benzoate (EB), whereas T3 or MB partially attenuated prolactin hypersecretion after estrogen administration. This is in accord with the attenuation of EB-induced inhibition of dopaminergic system by T3 and MB. MB given in combination with EB also partially attenuated EB-promoted rise of adenohypohyseal NO synthase activity which plays an important role in the regulation of prolactin secretion. Further studies on central catecholaminergic systems, pituitary receptors, the nitrergic system and mechanisms of intracellular signal transduction are necessary for better understanding of pituitary tumor transformation and possibly for the discovery of new approaches towards treating patients with these diseases.     

Inheritance of alleles for glutelin alpha-2 subunit genes in rice and identification of their corresponding cDNA clone

Rice glutelins consist of acidic (alpha) and basic (beta) subunits which are further separated into three polypeptide components assigned as alpha-1, alpha-2, and alpha-3 subunit components and beta-1, beta-2 and beta-3 subunit components. Nine rice mutant lines with a decreased amount of the glutelin alpha-2 subunit component (alpha-2L) were obtained by screening about 6,800 potential mutant lines derived from the fertilized egg treatment with N-methyl-N-nitrosourea (MNU) using the SDS-PAGE method. The mutants were classified into three types of the increased alpha-1 subunit (alpha-1H/alpha-2L), the decreased beta-2 subunit (beta-2L/alpha-2L) and the increased alpha-3 subunit (alpha-3H/alpha-2L) represented by EM278, CM1707 and EM659, respectively. Iso-electric focus (IEF) analysis revealed that all of the mutants had an extremely low amount of a polypeptide with a 6.71 pI value, whereas a polypeptide with either a 6.50 pI value or with a 6.90 pI value increased significantly in alpha-1H/alpha-2L mutants or in alpha-3H/alpha-2L mutants, respectively. The beta-2L/alpha-2L mutants had a decreased amount of a basic polypeptide with a 8.74 pI value. Genetic analysis revealed that the three types of mutants were controlled by a single incomplete dominant gene respectively, and the three are alleles. The gene was temporarily named glu4, which was found to be located on chromosome 1 linked with the eg and spl6 genes. Two-dimensional electrophoresis analysis revealed that the glu4 encoded polypeptides of pI 6.71/alpha-2 and pI 8.74/beta-2. Amino acid sequence analysis suggested that the mutated acidic polypeptide was the product of a GluA subfamily gene. Northern and RT-PCR analyses revealed that glu4 corresponded to the GluA-1 gene.     

The putative dopamine autoreceptor agonist B-HT 920 decreases nigral dopamine cell firing rate and prolactin release in rat

Recent behavioural and biochemical investigations have suggested that the alpha-2 receptor agonist B-HT 920 is also a centrally acting dopamine (DA) agonist with a selectivity for autoreceptors. It is presently demonstrated that B-HT 920, in contrast to the structurally related alpha-2 agonist B-HT 933, effectively reduces the firing rate of nigral DA neurons both after intravenous and microiontophoretic administration. Furthermore, B-HT 920, but not B-HT 933, decreases plasma levels of prolactin in reserpine pretreated rats. The electrophysiological as well as the neuroendocrine effects of the drug were antagonised by DA antagonists but not by alpha-2 receptor antagonists. The data support the contention that B-HT 920 acts as an agonist at central DA autoreceptors. Furthermore, they reinforce the hypothesis that lactotroph DA receptors are more similar to DA autoreceptors than to postsynaptic DA receptors in the brain.