Postsynaptic Alpha-adrenoceptors Research Articles

Cooling-evoked hemodynamic perturbations facilitate sympathetic activity with subsequent myogenic vascular oscillations via alpha2-adrenergic receptors.

This study extends our previous work by examining the effects of alpha2-adrenoceptors under cold stimulation involving the increase of myogenic vascular oscillations as increases of very-low-frequency and low-frequency of the blood pressure variability. Forty-eight adult male Sprague-Dawley rats were randomly divided into four groups: vehicle; yohimbine; hexamethonium+yohimbine; guanethidine+yohimbine. Systolic blood pressure, heart rate, power spectral analysis of spontaneous blood pressure and heart rate variability and spectral coherence at very-low-frequency (0.02 to 0.2 Hz), low-frequency (0.2 to 0.6 Hz), and high-frequency (0.6 to 3.0 Hz) regions were monitored using telemetry. Key findings are as follows: 1) Cooling-induced pressor response was attenuated by yohimbine and further attenuated by hexamethonium+yohimbine and guanethidine+yohimbine, 2) Cooling-induced tachycardia response of yohimbine was attenuated by hexamethonium+yohimbine and guanethidine+yohimbine, 3) Different patterns of power spectrum reaction and coherence value compared hexamethonium+yohimbine and guanethidine+yohimbine to yohimbine alone under cold stimulation. The results suggest that sympathetic activation of the postsynaptic alpha2-adrenoceptors causes vasoconstriction and heightening myogenic vascular oscillations, in turn, may increase blood flow to prevent tissue damage under stressful cooling challenge.

Comparative pharmacodynamic analysis of imidazoline compounds using rat model of ocular mydriasis with a test of quantitative structure–activity relationships

Imidazol(in)e derivatives, having the chemical structure similar to clonidine, exert diverse pharmacological activities connected with their interactions with alpha2-adrenergic receptors, e.g. hypotension, bradycardia, sedation as well as antinociceptive, anxiolytic, antiarrhythmic, muscle relaxant and mydriatic effects. The mechanism of pupillary dilation observed after systemic administration of imidazol(in)es to rats, mice and cats depends on the stimulation of postsynaptic alpha2-adrenoceptors within the brain. It was proved that the central nervous system (CNS)-localized I1-imidazoline receptors are not engaged in those effects. It appeared interesting to analyze the CNS-mediated pharmacodynamics of imidazole(in)e agents in terms of their chromatographic and calculation chemistry-derived parameters.In the present study a systematic determination and comparative pharmacometric analysis of mydriatic effects in rats were performed on a series of 20 imidazol(in)e agents, composed of the well-known drugs and of the substances used in experimental pharmacology. The eye pupil dilatory activities of the compounds were assessed in anesthetized Wistar rats according to the established Koss method. Among twenty imidazol(in)e derivatives studied, 18 produced diverse dose-dependent mydriatic effects. In the quantitative structure–activity relationships (QSAR) analysis, the pharmacological data (half maximum mydriatic effect – ED50 in μmol/kg) were considered along with the structural parameters of the agents from molecular modeling. The theoretically calculated lipophilicity parameters, CLOGP, of imidazol(in)es, as well as their lipophilicity parameters from HPLC, logkw, were also considered. The attempts to derive statistically significant QSAR equations for a full series of the agents under study were unsuccessful. However, for a subgroup of eight apparently structurally related imidazol(in)es a significant relationship between log(1/ED50) and logkw values was obtained. The lack of “predictive” QSAR for the whole series of the structurally diverse agents is probably due to a complex mechanism of the ligand–alpha2-adrenergic receptor interactions, which are predominantly of a highly structurally specific polar nature. Such interactions are difficult to quantify with the established chemical structural descriptors, contrary to the less specific, molecular bulkiness-related interactions.

Postsynaptic alpha 1-adrenoceptor mechanisms in rat vas deferens and ageing.

Postsynaptic alpha 1-adrenoceptor mechanisms in vasa deferentia isolated from 3, 6, 18 and 40 week-old rats were studied by analysis of the concentration-response curve of noradrenaline and the Scatchard plot of specific binding of [3H]prazosin to microsomal fractions. The maximum tension developed by noradrenaline also increased with age from 3 to 18 weeks. The efficacy of noradrenaline and capacity of the maximum binding sites of [3H]prazosin increased with increasing age, while the dissociation constants of noradrenaline (KA) and prazosin (Kd) were not changed with age. The increase of the maximum tension was proportional to the increase in efficacy. The increase of efficacy for noradrenaline in the vasa deferentia from rats of different ages is due to the increase in the total concentration of postsynaptic alpha 1-adrenoceptors.

Effect of prazosin and guanfacine on stress-induced reinstatement of alcohol and food seeking in rats

Relapse to alcohol use during abstinence or maladaptive eating habits during dieting is often provoked by stress. The anxiogenic drug yohimbine, which causes stress-like responses in humans and non-humans, reliably reinstates alcohol and food seeking in a rat relapse model. Yohimibine is a prototypical alpha-2 adrenoceptor antagonist, but results from studies on noradrenaline's role in yohimbine-induced reinstatement of drug and food seeking are inconclusive. Here, we further addressed this issue by studying the effect of the alpha-1 adrenoceptor antagonist prazosin and the alpha-2 adrenoceptor agonist guanfacine on yohimbine-induced reinstatement. In exp. 1, we trained rats to self-administer alcohol (12% w/v, 1 h/day), and after extinction of alcohol-reinforced lever pressing, we tested prazosin's (0.5, 1.0, and 2.0 mg/kg, i.p.) or guanfacine's (0.125, 0.25, and 0.5 mg/kg, i.p.) effect on yohimbine (1.25 mg/kg, i.p.)-induced reinstatement; we also examined prazosin's effect on intermittent-footshock-stress-induced reinstatement. In exp. 2, we trained food-restricted rats to self-administer 45 mg food pellets and first examined prazosin's or guanfacine's effects on food-reinforced responding, and then, after extinction of lever presses, on yohimbine-induced reinstatement. Prazosin (0.5-2.0 mg/kg) blocked yohimbine-induced reinstatement of food and alcohol seeking, as well as footshock-induced reinstatement of alcohol seeking. Guanfacine attenuated yohimbine-induced reinstatement of alcohol seeking at the highest dose (0.5 mg/kg), but its effect on yohimbine-induced reinstatement of food seeking was not significant. Neither prazosin nor guanfacine affected high-rate food-reinforced responding. Results demonstrate an important role of postsynaptic alpha-1 adrenoceptors in stress-induced reinstatement of alcohol and food seeking.

ChemInform Abstract: Synthesis and Pharmacological Characterization of Some Benzazepinone Derivatives.

A series of some benzazepinone derivatives were prepared. The compounds were evaluated for inotropic, chronotropic and calcium antagonist properties. For the compounds bearing well-known alpha-blocking fragments, the antagonist potency at postsynaptic alpha-adrenoceptors was assessed. The new compounds were less effective as bradycardic agents and calcium antagonists but show remarkable negative inotropic activity.

Guanfacine Potentiates the Activation of Prefrontal Cortex Evoked by Warning Signals

Warning signals evoke an alert state of readiness that prepares for a rapid response by priming a thalamo-frontal-striatal network that includes the dorsolateral prefrontal cortex (DLPFC). Animal models indicate that noradrenergic input is essential for this stimulus-driven activation of DLPFC, but the precise mechanisms involved have not been determined. We tested the role that postsynaptic alpha(2A) adrenoceptors play in the activation of DLPFC evoked by warning cues using a placebo-controlled challenge with the alpha(2A) agonist guanfacine. Sixteen healthy young adults were scanned twice with event-related functional magnetic resonance imaging (fMRI), while performing a simple cued reaction time (RT) task following administration of a single dose of oral guanfacine (1 mg) and placebo in counterbalanced order. The RT task temporally segregates the neural effects of warning cues and motor responses and minimizes mnemonic demands. Warning cues produced a marked reduction in RT accompanied by significant activation in a distributed thalamo-frontal-striatal network, including bilateral DLPFC. Guanfacine selectively increased the cue-evoked activation of the left DLPFC and right anterior cerebellum, although this increase was not accompanied by further reductions in RT. The effects of guanfacine on DLPFC activation were specifically associated with the warning cue and were not seen for visual- or target-related activation. Guanfacine produced marked increases in the cue-evoked activation of DLPFC that correspond to the well-described actions of postsynaptic alpha(2) adrenoceptor stimulation. The current procedures provide an opportunity to test postsynaptic alpha(2A) adrenoceptor function in the prefrontal cortex in the pathophysiology of several psychiatric disorders.

Accumbal noradrenaline that contributes to the alpha-adrenoceptor-mediated release of dopamine from reserpine-sensitive storage vesicles in the nucleus accumbens is derived from alpha-methyl-para-tyrosine-sensitive pools

Alpha-adrenoceptors in the nucleus accumbens are known to inhibit accumbal dopamine release from reserpine-sensitive pools. The aim of this study was to test our previously reported hypothesis that accumbal noradrenaline that controls the dopamine release from these storage vesicles is derived from alpha-methyl-para-tyrosine-sensitive pools. The sensitivity of accumbal alpha-adrenoceptors to noradrenergic agents depends on the amount of noradrenaline that is available in the synapse. In case the synaptic noradrenaline levels decrease, the conformation of alpha-adrenoceptors changes into a state that makes these receptors more sensitive to its agonists. The effects of alpha-methyl-para-tyrosine, respectively reserpine, on the alpha-adrenoceptor-agonist-induced changes of accumbal dopamine release were investigated. Alpha-methyl-para-tyrosine, but not reserpine, made accumbal postsynaptic alpha-adrenoceptors more sensitive to phenylephrine. These results indicate that noradrenaline that inhibits the release of dopamine from reserpine-sensitive storage vesicles, via stimulation of accumbal postsynaptic alpha-adrenoceptors, is derived from alpha-methyl-para-tyrosine-sensitive pools. The clinical impact of these data is discussed.

Postsynaptic Alpha 2-Adrenoceptors Mediate Melanosome Aggregation in Melanophores of the White-Spotted Rabbitfish (Siganus canaliculatus)

The present investigation was undertaken to study the nature of neuro-melanophore junction in the white-spotted rabbit fish Siganus canaliculatus. In vitro experiments using split fin preparation indicated that melanophores of S. canaliculatus are highly responsive to potassium ions and adrenergic agonists. Potassium ions and the adrenergic agonists induced prompt melanosome aggregation that could be competitively blocked by yohimbine (alpha-2 specific adrenergic antagonist) and phentolamine (non-specific alpha adrenergic antagonist). The melanophore responses to repeated potassium stimulation (up to 20 stimuli) did not show any sign of fatigue. However, statistically significant enhancement was observed in responses to potassium that followed the first five stimulations. Adrenergic agonists acted in a time and concentration-dependent manner and their relative potency had the following rank order: clonidine (alpha-2 specific agonist) > norepinephrine (non-specific adrenergic agonist) > phenylephrine (alpha-1 specific agonist) > methoxamine (alpha-1-specific agonist). Yohimbine exerted a more potent inhibiting effect on norepinephrine induced melanosome aggregation compared to phentolamine. Prazosine (alpha-1 specific antagonist) had no effect on such aggregation. Chemically denervated melanophores displayed hypersensitivity to alpha-adrenergic agonists but were refractive to potassium ion stimulation. The refractivity of denervated melanophores to potassium indicates the effect of potassium ion is not direct on melanophores but it is rather through depolarization effect of potassium on the neuro-melanophore peripheral sympathetic fibers and hence release of norepinephrine. In denervated melanophores, similar to intact melanophores, only phentolamine and yohimbine but not prazosine, significantly inhibited melanosome aggregation effect of norepinephrine, indicating that norepinephrine effect is through postsynaptic alpha-2 adrenoceptors. The present data demonstrate that the nature of melanophore innervation in this teleost is adrenergic and neuro-melanophore signals mediating melanosome aggregation are transmitted through alpha-2 postsynaptic adrenoceptors.

Functional subtypes of renal α1-adrenoceptor in diabetic and non-diabetic 2K1C Goldblatt renovascular hypertension

This study investigates the subtypes of the alpha1-adrenoceptor mediating the adrenergically-induced renal vasoconstrictor responses in streptozotocin-induced diabetic and non-diabetic 2-kidney one clip (2K1C) Goldblatt hypertensive rats. The renal blood flow responses to renal nerve stimulation, noradrenaline, phenylephrine, and methoxamine were measured in the absence and presence of nitrendipine, 5-methylurapidil, chloroethylclonidine and BMY 7378. The renal vasoconstrictor responses were markedly attenuated by nitrendipine and 5- methylurapidil in the diabetic rats (all P< 0.05). In the non-diabetic rats, these responses were markedly attenuated by nitrendipine, 5-methylurapidil, and BMY 7378 (all P< 0.05). In both experimental groups, chloroethylclonidine markedly accentuated the renal vasoconstrictions caused by all the adrenergic stimuli (all P< 0.05). These observations indicate that alpha 1A-adrenoceptor subtypes play a major role in mediating adrenergically-induced renal vasoconstriction in the diabetic 2K1C Goldblatt hypertensive rats. In the non-diabetic 2K1C Goldblatt hypertensive rats, contributions of alpha 1A and alpha 1D-adrenoceptor subtypes were proposed. Apart from post-synaptic alpha 1-adrenoceptors, both in the diabetic and non-diabetic 2K1C Goldblatt hypertensive rats, the potential involvement of presynaptic alpha 1- adrenoceptors is also suggested.

Intermittent Hypoxia Induces Early Functional Cardiovascular Remodeling in Mice

Intermittent hypoxia, a hallmark of sleep apnea, is a major factor for hypertension and impaired vasoreactivity. To examine the temporal occurrence of these two outcomes in order to provide insight into mechanisms and early cardiovascular disease identification. Functional and structural cardiovascular alterations were assessed in C57BL6 mice exposed to intermittent hypoxia (21-4% Fi(O(2)), 30-s cycle, 8 h/d) or air for up to 35 days. Blood pressure, heart rate, and urinary catecholamines were measured at Days 1 and 14. Hindquarter vasoreactivity was assessed at Days 14 and 35, including vasoconstriction to norepinephrine, endothelium-, and non-endothelium-dependent vasodilation. Aorta, heart, and hindquarter skeletal muscles were immunostained for vascular markers PECAM-1 and collagen IV. Hemodynamic alterations occurred from Day 1, characterized by blood pressure surges with bradytachyarrhythmia driven by cyclic hypoxia. At Day 14, blood pressure at normoxia was elevated, with predominant diastolic increase. With hypoxia, vasopressive catecholamines were elevated, blood pressure surged with a lower hypoxic threshold, whereas heart rate fluctuations decreased. Histologic alterations started from Day 14, with decreased endothelial PECAM-1 expression in descending aorta and left heart. Impaired peripheral vasoreactivity occurred at Day 35, including hypervasoconstriction to norepinephrine secondary to sympathetic hyperactivity, without changes in pre- and postsynaptic alpha-adrenoceptors or in endothelium- and non-endothelium-dependent vasodilation. Intermittent hypoxia induces sequential cardiovascular events suggesting increased chemoreflex and depressed baroreflex, resulting in sympathoadrenal hyperactivity, early hemodynamic alterations with proximal histologic remodeling, and delayed changes in peripheral vasoreactivity. Such early alterations before overt cardiovascular disease strengthen the need for identifying at-risk individuals for systematic treatment.

Effects of Low and High Plasma Concentrations of Dexmedetomidine on Myocardial Perfusion and Cardiac Function in Healthy Male Subjects

Dexmedetomidine, a selective alpha2-adrenoceptor agonist, has counteracting effects on the cardiovascular system. It mediates sympatholysis by activating alpha2 adrenoceptors in the central and peripheral nervous system, and vasoconstriction and vasorelaxation by activating postsynaptic alpha2 adrenoceptors in blood vessels. The goal of this study was to determine the effects of therapeutic and high concentrations of dexmedetomidine on myocardial perfusion and cardiac function in healthy subjects. The authors studied 12 healthy young men. Myocardial blood flow (assessed with positron emission tomography), myocardial function (by echocardiography), and hemodynamic data were collected before and during low (measured mean plasma concentration, 0.5 ng/ml) and high (5 ng/ml) plasma concentrations of dexmedetomidine. The low concentration of dexmedetomidine reduced myocardial perfusion (mean difference, -27% from baseline [95% confidence interval, -31 to -23%], P < 0.001) in parallel with a reduction in myocardial oxygen demand (estimated by the rate-pressure product (-23% [-28 to -18%], P < 0.001). The high dexmedetomidine plasma concentration did not further attenuate myocardial perfusion (-3% [-12 to +6%] from low dexmedetomidine, P > 0.05; -29% [-39 to -18%] from baseline, P < 0.001) or statistically significantly affect the rate-pressure product (+5% [0 to +10%], P > 0.05). Systolic myocardial function was attenuated by sympatholysis during the low infusion rate and was further attenuated by a combination of the sustained sympatholysis and increased afterload during the high infusion rate. In healthy subjects, plasma concentrations of dexmedetomidine that significantly exceed the recommended therapeutic level do not seriously attenuate myocardial perfusion below the level that is observed with usual therapeutic concentrations and do not induce evident myocardial ischemia.

Alpha-Adrenergic agents modulate the activity of the spinal pattern generator for ejaculation.

Spinal cord transection at a thoracic level activates fictive ejaculation (FE) in the male rat. It has earlier been demonstrated that fictive motor patterns may be activated by pharmacological means and that the noradrenergic system seems to be particularly efficient in triggering locomotor fictive patterns in spinal animals. In the present study, the hypothesis was tested that the spinal noradrenergic system participates in the activation of the spinal generator for ejaculation (SGE). To this aim, the effect of the adrenergic agents, methoxamine, prazosin, clonidine, and yohimbine, upon FE was evaluated in spinal male rats using electromyographic techniques. The results obtained show that ejaculatory rhythmic patterns, accompanied by the expulsion of urethral contents and phasic penile movements, can be elicited by the intravenous (i.v.) injection of methoxamine or yohimbine. These drug-induced motor sequences appear superimposed to the intrinsic ejaculatory spinal rhythm. By contrast, i.v. injection of prazosin or clonidine blocked the expression of the spontaneous ejaculatory rhythmic pattern without inducing any other genital response. These data suggest that an increased noradrenergic tone, either by blockade of presynaptic alpha2-adrenoceptors or by stimulation of postsynaptic alpha1-adrenoceptors, results in the activation of the SGE. Present findings provide the evidence that the SGE might be importantly influenced by the noradrenergic system, which exerts a facilitatory control on the expression of the genital motor pattern of ejaculation.

High concentrations of KCl release noradrenaline from noradrenergic neurons in the rat anococcygeus muscle.

The aim of the present study was to investigate the effects of high concentrations of KCl in releasing noradrenaline from sympathetic nerves and its actions on postsynaptic alpha-adrenoceptors. We measured the isotonic contractions induced by KCl in the isolated rat anococcygeus muscle under different experimental conditions. The contractile responses induced by KCl were inhibited by alpha-adrenoceptor antagonists in 2.5 mM Ca2+ solution. Prazosin reduced the maximum effect from 100 to 53.9 +/- 10.2% (P<0.05) while the pD2 values were not changed. The contractile responses induced by KCl were abolished by prazosin in Ca2+-free solution (P<0.05). Treatment of the rats with reserpine reduced the maximum effect induced by KCl as compared to the contractile responses induced by acetylcholine from 339.5 +/- 157.8 to 167.3 +/- 65.5% (P<0.05), and increased the pD2 from 1.57 +/- 0.01 to 1.65 +/- 0.006 (P<0.05), but abolished the inhibitory effect of prazosin (P<0.05). In contrast, L-NAME increased the contractile responses induced by 120 mM KCl by 6.2 +/- 2.3% (P<0.05), indicating that KCl could stimulate the neurons that release nitric oxide, an inhibitory component of the contractile response induced by KCl. Our results indicate that high concentrations of KCl induce the release of noradrenaline from noradrenergic neurons, which interacts with alpha1-adrenoceptors in smooth muscle cells, producing a contractile response in 2.5 mM Ca2+ (100%) and in Ca2+-free solution, part of which is due to a direct effect of KCl on the rat anococcygeus muscle.

Mutation of the α2A-Adrenoceptor Impairs Working Memory Performance and Annuls Cognitive Enhancement by Guanfacine

Norepinephrine strengthens the working memory, behavioral inhibition, and attentional functions of the prefrontal cortex through actions at postsynaptic alpha2-adrenoceptors (alpha2-AR). The alpha2-AR agonist guanfacine enhances prefrontal cortical functions in rats, monkeys, and human beings and ameliorates prefrontal cortical deficits in patients with attention deficit hyperactivity disorder. The present study examined the subtype of alpha2-AR underlying these beneficial effects. Because there are no selective alpha2A-AR, alpha2B-AR, or alpha2C-AR agonists or antagonists, genetically altered mice were used to identify the molecular target of the action of guanfacine. Mice with a point mutation of the alpha2A-AR, which serves as a functional knock-out, were compared with wild-type animals and with previously published studies of alpha2C-AR knock-out mice (Tanila et al., 1999). Mice were adapted to handling on a T maze and trained on either a spatial delayed alternation task that is sensitive to prefrontal cortical damage or a spatial discrimination control task with similar motor and motivational demands but no dependence on prefrontal cortex. The effects of guanfacine on performance of the delayed alternation task were assessed in additional groups of wild-type versus alpha2A-AR mutant mice. We observed that functional loss of the alpha2A-AR subtype, unlike knock-out of the alpha2C-AR subtype, weakened performance of the prefrontal cortical task without affecting learning and resulted in loss of the beneficial response to guanfacine. These data demonstrate the importance of alpha2A-AR subtype stimulation for the cognitive functions of the prefrontal cortex and identify the molecular substrate for guanfacine and novel therapeutic interventions.

Autonomic effects of some scorpion venoms and toxins.

1. The autonomic effects of venoms and toxins from several species of scorpions, including the Indian red scorpion Mesobuthus tamulus, the Chinese scorpion Buthus martensi Karsch and the Israeli scorpion Leiurus quinquestriatus quinquestriatus, all belonging to Buthidae, and the Asian black scorpions Heterometrus longimanus and Heterometrus spinifer, belonging to Scorpionidae, are reviewed. 2. The effects of the venoms of M. tamulus and L. q. quinquestriatus on noradrenergic and nitrergic transmission in the rat isolated anococcygeus muscle revealed that both venoms mediated their pharmacological effects via a prejunctional mechanism involving the activation of voltage-sensitive sodium channels with consequent release of neurotransmitters that mediate target organ responses, similar to the effects mediated by other alpha-scorpion toxins. 3. Two new toxins, Makatoxin I and Bukatoxin, were purified to homogeneity from the venom of B. martensi Karsch. Determination of their complete amino acid sequences confirmed that both toxins belonged to the class of alpha-scorpion toxins. The effects of both toxins on noradrenergic and nitrergic transmission in the rat anococcygeus muscle provided firm evidence that their pharmacological actions also closely resembled those mediated by other alpha-scorpion toxins on neuronal voltage-sensitive sodium channels. 4. The venoms of H. longimanus and H. spinifer were found to have high concentrations of noradrenaline (1.8 +/- 0.3 mmol/L) and relatively high concentrations of acetylcholine (79.8 +/- 1.7 micromol/L) together with noradrenaline (146.7 +/- 19.8 micromol/L), respectively, which can account for their potent direct cholinergic and noradrenergic agonist actions in the rat anococcygeus muscle. 5. Our studies confirmed that the rat anococcygeus muscle is an excellent nerve-smooth muscle preparation for investigating the effects of bioactive agents on noradrenergic and nitrergic transmission, as well as the direct agonist actions of these agents on post-synaptic alpha-adrenoceptors and M3 muscarinic cholinoceptors. Although many studies, including our own, have documented that scorpion venoms and toxins mediate their primary effects via a prejunctional mechanism that leads to the marked release of various autonomic neurotransmitters, our studies have shown that there are exceptions to this generally accepted phenomenon. In particular, we have provided firm evidence to show that the venoms from H. longimanus and H. spinifer do not have such a prejunctional site of action but, instead, the venoms mediate their autonomic effects through direct agonist actions on post-junctional muscarinic M3 cholinoceptors and alpha-adrenoceptors.

The potential antidiabetic activity of some alpha-2 adrenoceptor antagonists

The effect of alpha-2 adrenoceptor antagonists, yohimbine and efaroxan, on the plasma glucose and insulin levels was studied in non-diabetic control, type-I (insulin-dependent) and type-II (non-insulin-dependent) diabetic rats. Pretreatment with either yohimbine or efaroxan potentiated glucose-induced insulin release in non-diabetic control rats and produced an improvement of the oral glucose tolerance and potentiated glucose-induced insulin release in type-II but not in type-I diabetic rats. Treatment with either yohimbine or efaroxan reduced the plasma glucose level and increased the plasma insulin level of non-diabetic control and type-II diabetic rats but not of type-I diabetic rats. Effects of efaroxan were more marked. Pretreatment of non-diabetic control and type-II diabetic rats with either yohimbine or efaroxan inhibited clonidine-induced hyperglycaemia and suppressed or reversed clonidine-induced hypoinsulinaemia. Also, pretreatment of these animals with either yohimbine or efaroxan enhanced the hypoglycaemic and insulinotropic effects of glibenclamide. The combination of glibenclamide and efaroxan led to a synergistic increase in insulin secretion, while that of glibenclamide and yohimbine led to an additive increase. The hyperglycaemic effect of diazoxide in non-diabetic control and type-II diabetic rats was inhibited by pretreatment with either yohimbine or efaroxan. The hypoinsulinaemic effect of diazoxide in these animals was antagonized and reversed by pretreatment with yohimbine and efaroxan, respectively. In type-I diabetic rats, there was no change in the plasma glucose and insulin levels induced by the treatment of animals with each of clonidine or diazoxide alone or in combination with either yohimbine or efaroxan. Glibenclamide produced a slight decrease in the plasma glucose level of type-I diabetic rats, at the end of the 120 min period of investigation but there was no change in the plasma insulin level. Pretreatment of these animals with either yohimbine or efaroxan produced no change in glibenclamide effects. Additionally, bath application of efaroxan or glibenclamide inhibited the relaxant effects of different concentrations of diazoxide on the isolated norepinephrine-contracted aortic strips, while the application of yohimbine produced insignificant changes. The combination of glibenclamide and efaroxan led to complete inhibition of the relaxant effects of different concentrations of diazoxide, while that of glibenclamide and yohimbine did not produce such an effect. It is concluded that yohimbine, via blockade of postsynaptic alpha-2 adrenoceptors, and efaroxan, via blockade of postsynaptic alpha-2 adrenoceptors and adenosine triphosphate-sensitive potassium channels in the pancreatic beta-cell membrane, produce insulinotropic and subsequent hypoglycaemic effects.

Modulation of MK-801-induced behaviour by noradrenergic agents in mice.

Inhibition of glutamatergic N-methyl-D-aspartate (NMDA) receptors following the administration of NMDA receptor antagonists results in psychotic-like behaviour. Whereas it is known that pharmacological manipulation of dopaminergic and serotonergic pathways affect this drug-induced psychosis, a role for noradrenaline has not yet been clearly defined. Thus, in the present study, we assessed a possible role for noradrenaline in the behavioural response to the non-competitive NMDA receptor anatgonist, MK-801, in male CD-I mice. MK-801 (0.02-1.28 mg/kg; ED50 0.2 mg/kg; s.c.) induced a dose-dependent increase in locomotor, stereotypic and ataxic behaviours. Pre-treatment with the noradrenaline re-uptake inhibitors, desipramine (10 mg/kg; i.p.) and reboxetine (20 mg/kg; i.p.), attenuated the locomotor, stereotypic and ataxic response to MK-801 (0.2 mg/kg; s.c.). The noradrenergic system was lesioned with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4, 50 mg/kg; i.p., 7 and 4 days prior to challenge) to reduce noradrenaline concentrations in the cortex by 70%-80%. Whereas DSP-4 lesioning had little effect on the response to MK-801, it completely reversed the attenuating effects of reboxetine. Pre-treatment with the alpha2 adrenoceptor agonist, clonidine (0.2 mg/kg; i.p.), and the antagonist, yohimbine (2 mg/kg; i.p.), attenuated and potentiated the response to MK-801, respectively. Pre-treatment with the alpha1 adrenoceptor antagonist, prazosin (2 mg/kg; i.p.), reduced the MK-801-induced response. It therefore appears that presynaptic noradrenergic alpha2 and postsynaptic alpha1 adrenoceptor stimulation exert opposing effects on the behavioural expression of MK-801 in mice.

Clostridium difficile toxin A excites enteric neurones and suppresses sympathetic neurotransmission in the guinea pig

BACKGROUND AND AIMSEvidence suggests that the intestinal actions of Clostridium difficile toxin A—stimulation of secretion and motility, and an acute inflammatory response—have a neurally mediated component.METHODSDirect intracellular electrophysiological recording of...

Effects of clonidine and alpha-adrenoceptor antagonists on motor activity in DSP4-treated mice I: dose-, time- and parameter-dependency.

In three experiments the acute effects of clonidine administration upon locomotor and rearing behaviour of mice pretreated with the selective noradrenaline (NA) neurotoxin, DSP4 (1 x 75 mg/kg, i.p.) 10-12 days previously, were studied. Clonidine (0.01, 0.05, 0.25, 1.25 and 3.0 mg/kg, i.p.) induced a dose-dependent reduction of motor activity during the initial 30 min of testing in both DSP4-treated and control mice; this effect was attenuated by DSP4 treatment in the 0.01, 0.05, 0.25 and 3.0 mg/kg dose groups. By the third 30-min period of testing (60-90 min), each clonidine dose group, except the highest (3.0 mg/kg) dose for locomotion and the two highest (1.25 and 3.0 mg/kg) doses for rearing, induced increases in motor activity in the control mice. In DSP4-treated mice, a large increase in locomotor counts was produced by the 0.05 mg/kg dose of clonidine with lesser increases induced by the 0.01 mg/kg dose group, whereas a lesser effect of the 0.05 mg/kg group (30-60 min) was obtained for rearing but a larger effect of the 0.25 mg/kg group (60-90 min). Yohimbine (0.5 mg/kg, i.p., 15 min before clonidine) attenuated the suppressive effects of clonidine (0.01 and 0.05 mg/kg) during the initial 30 min of testing and markedly increased locomotor and rearing counts, both by itself and in combination with each dose of clonidine, in both DSP4-treated and control mice over the following 90 min of testing. Yohimbine treatment attenuated the large increase in locomotor counts induced by the 0.05 mg/kg dose of clonidine in the NA-denervated mice. Dihydroergotamine (0.5 mg/kg, i.p., 15 min before clonidine) did not antagonise either the initial suppressive effect or the later supersensitivity effect of the 0.05 mg/kg dose of clonidine. DSP4 treatment by itself reduced motor activity. The effects of clonidine, dose- and time-dependently, by itself or in co-administration with alpha-adrenoceptor antagonists, in DSP4-treated or control mice displayed denervation-induced supersensitivity that appear to reflect mainly postsynaptic alpha2-adrenoceptor mediation.

Mental well-being in alcohol withdrawal: relationship to alpha2-adrenoceptor function.

The possible relationship between postsynaptic alpha2-adrenoceptor function, as assessed by the growth hormone (GH) response to clonidine (CLON) and aspects of mental well-being by self-report of mood using the Swedish Mood Adjective Check List, was investigated in alcohol-dependent patients in the early withdrawal period. Patients had blunted GH responses to CLON and worse mental well-being than control subjects immediately after the end of alcohol intake. No relation was found between mental well-being and postsynaptic alpha2-adrenoceptor function. After 1 week, the GH responses to CLON remained blunted, whereas the state of mental well-being had improved to levels similar to those of control subjects. The results further support a downregulated alpha2-adrenoceptor function during 1 week of alcohol withdrawal. Furthermore, even if subsensitive postsynaptic alpha2-adrenoceptor function was not generally related to state of mood, patients with the lowest postsynaptic alpha2-adrenoceptor function reported the highest levels in the dimensions pleasantness/unpleasantness and activation/deactivation when sober.