Binding of 3H-clonidine to an alpha-adrenoceptor in membranes of guinea-pig ileum

Food deprivation increases alpha(2)-adrenoceptor-mediated modulation of jejunal epithelial transport in young and adult rats

This study examined the effect of food deprivation on the jejunal response to alpha(2)-adrenoceptor activation in young (20-d-old) and adult (60-d-old) rats, using short-circuit (I(sc)) measurements in the absence or presence of furosemide (1 mmol/L). The effect of alpha(2)-adrenoceptor stimulation by 5-bromo-N:-(4, 5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304; 0.3-3000 nmol/L) was a concentration-dependent decrease in I(sc) with similar half-maximal effective concentration (EC(50); 12.3 +/- 1.1 vs. 9.6 +/- 1.1 nmol/L) and maximal effect (E(max); 70.6 +/- 6.9 vs. 80.6 +/- 4.5% of reduction) values in adult food-deprived and fed rats. The effect of UK 14,304 on I(sc) in fed and food-deprived rats was markedly (P: < 0.05) attenuated by furosemide (1 mmol/L). E(max) values for UK 14,304 in 20-d-old food-deprived rats were higher (P: < 0.05) than those observed in fed rats (93.3 +/- 3.3 vs. 67.0 +/- 11.3% of reduction), without differences in EC(50) values. The effect of UK 14,304 on I(sc) in 20-d-old fed rats was completely abolished by furosemide (1 mmol/L). In food-deprived young rats, the effect of UK 14,304 was also markedly (P: < 0.05) antagonized by furosemide, but not completely abolished. Specific [(3)H]-rauwolscine binding in membranes from jejunal epithelial cells revealed the presence of a single class of binding sites, with an apparent K:(D) in the low nmol/L range. In 20-d-old food-deprived rats, specific [(3)H]-rauwolscine binding was markedly increased, and this was reversed by refeeding. Na(+),K(+)-ATPase activity in isolated jejunal epithelial cells from 60-d-old fed rats was twice that in 20-d-old fed rats [117 +/- 14 vs. 52 +/- 5 nmol free inorganic phosphorus/(mg protein.min)]. Food deprivation in adult rats, but not in 20-d-old rats, was accompanied by a significant decrease in Na(+),K(+)-ATPase activity. In both young and adult rats (fed and food-deprived), UK 14,304 did not affect Na(+),K(+)-ATPase activity. In conclusion, food deprivation in 20-d-old rats enhanced the response to alpha(2)-adrenoceptor stimulation. This effect, which depends primarily on the stimulation of a furosemide-sensitive antisecretory mechanism, is suggested to result from increases in the number of jejunal epithelial alpha(2)-adrenoceptors.

Modulation of enteric cholinergic neurons by hetero- and autoreceptors: cooperation among inhibitory inputs

In the guinea-pig colon, acetylcholine (ACh) release from intrinsic cholinergic motor neurons is inhibited by adrenoceptors, opioid and muscarinic receptors. Chronic sympathetic denervation resulted in supersensitivity to the inhibitory effect of DAMGO (mu-opioid agonist) on ACh release and on the peristaltic reflex. After chronic treatment with naltrexone (NTX) supersensitivity to DAMGO and subsensitivity to UK14,304 (alpha2-adrenoceptor agonist) developed for both functional parameters. The facilitatory effect of scopolamine on ACh release remained unchanged after chronic NTX treatment, whereas it was potentiated after chronic sympathetic denervation. These data suggest the existence of a functional interaction between different inhibitory pathways modulating cholinergic motor neurons in the guinea-pig colon. Namely, chronic manipulation of an inhibitory pathway may entail adaptive sensitivity changes in another inhibitory pathway so that homeostasis can be maintained.

Luminal adrenergic agents modulate ileal transport: discrimination between alpha 1 and alpha 2 receptors

Luminal alpha-adrenergic agonists alter ileal water, ion, and glucose transport by a local mechanism. This study tested the hypothesis that luminal adrenergic agents modulate ileal transport selectively, via specific alpha 1 and alpha 2 receptors. Absorption studies (n = 72) were performed on dogs with 25-cm ileal Thiry-Vella fistulas (TVF). Perfusion with (14C) polyethylene glycol was used to calculate absorption of water, ions, and glucose from the TVF. Experiments included four 1-hour periods. Agonists used were phenylephrine (alpha 1), clonidine (alpha 2), and norepinephrine (alpha 1 > alpha 2 and beta). Antagonists used were terazosin (alpha 1) and yohimbine (alpha 2). Phenylephrine and norepinephrine caused significant increases in water and ion absorption (p < 0.05). Clonidine caused significant decreases in water, ion, and glucose absorption (p < 0.05). Terazosin and yohimbine had no effect alone. Terazosin prevented the proabsorptive effect of phenylephrine and norepinephrine, and yohimbine blocked the prosecretory effect of clonidine. Yohimbine significantly increased the norepinephrine-induced proabsorptive effect. Luminal alpha-adrenergic agents selectively modulate ileal transport. Alpha 1-receptor activation causes a proabsorptive response, whereas alpha 2-receptor activation causes a prosecretory response. The combination of a luminally administered mixed alpha- and beta-adrenergic agonist (norepinephrine) with alpha 2 receptor blockade (yohimbine) may prove useful in pathologic secretory states such as intestinal transplants, diabetic diarrhea, or diarrhea-associated endocrinopathies.

Human brain imidazoline receptors: further characterization with [3H]clonidine

The aim of the present study was to further characterize [3H]clonidine binding in the ventrolateral medulla of the human brainstem, the region involved in the vasodepressor effect of imidazoline drugs of the clonidine type. Under basal conditions, [3H]clonidine can bind both to the imidazoline receptors and to the alpha-adrenoceptors. The latter represent only a small part of the total [3H]clonidine binding with a Bmax of 61 +/- 13 fmol/mg proteins and a KD of 4.9 +/- 2.2 nM. Most of the binding was associated with imidazoline receptors with a KD of 67 +/- 13 nM and a Bmax of 677 +/- 136 fmol/mg protein. alpha-Adrenoceptor binding of [3H]clonidine could be completely prevented when membranes were either treated during preparation with the aIkylating agent phenoxybenzamine or incubated in the presence of 30 microM (-)-noradrenaline or in the presence of the non-hydrolysable analogue of GTP, guanylyl imidodiphosphate (Gpp(NH)p). When the alpha-adrenoceptors binding was prevented, we demonstrated the insensitivity of [3H]clonidine binding to Gpp(NH)p and showed that the competition between clonidine and idazoxan for imidazoline receptors was insensitive to Gpp(NH)p suggesting that imidazoline receptors are not G protein coupled receptors. The specificity of [3H]cloniding binding to imidazoline receptors in the human ventrolateral medulla indicates that these receptors are different from imidazole receptors as defined with p-aminoclonidine in the bovine brainstem.

Effect of alpha-methylnorepinephrine, an alpha 2-adrenergic agonist, on jejunal absorption in neurally intact conscious dog

Although alpha 2-adrenergic agonists stimulate absorption in the mammalian small and large intestine in vitro, the possibility of central neural effects have confounded interpretation of in vivo studies. Our aim was to assess the effects of intravenous administration of alpha-methylnorepinephrine (MNE), an alpha 2-adrenergic agonist that does not cross the blood-brain barrier, on net jejunal absorption of water and electrolytes in the neurally intact, conscious dog. Absorption from a 30-cm proximal jejunal segment was studied using a triple-lumen perfusion technique in seven dogs. A warmed, isosmolar, balanced electrolyte solution containing [14C]polyethylene glycol was infused at 5 ml/min. Net jejunal fluxes of water and electrolytes were determined before, during, and after a 1.5-hr infusion of MNE (900 nmol/kg/hr). MNE increased net jejunal water absorption (from 12.9 +/- 1.8 to 22.5 +/- 1.5 microliters/cm/min, P < 0.05). Peripheral alpha 2-adrenergic receptors mediate a net proabsorptive response in the neurally intact canine jejunum in vivo independent of direct central neural effects.

A detailed autoradiographic mapping of histamine H3 receptors in rat brain areas

[3H](R)alpha-methylhistamine, a selective histamine H3-receptor ligand, was used to perform binding studies with membranes and generate light microscopic autoradiograms in sections of the rat brain. High densities of H3 receptors were found in membranes from the anterior part of the cerebral cortex, the accumbens nucleus, the striatum, the olfactory tubercles and the substantia nigra. Autoradiography of sagittal and frontal sections evidenced specific labelling in a number of gray matter areas over a very low background, as determined using thioperamide, a selective H3-receptor antagonist, as competing drug. Labelled areas were identified by comparison with adjacent Nissl-stained sections and their labelling was rated visually. H3 receptors are heterogeneously distributed among areas known to receive histaminergic projections. In the cerebral cortex, H3 receptors are present in all areas and layers, with a rostrocaudal gradient and a higher density in deep layers (laminae IV-VI). In the hippocampal formation, H3 receptors are the most abundant in the dentate gyrus and the subiculum. In the amygdaloid complex, the highest densities are found in the central, lateral and basolateral groups of nuclei. In the basal forebrain, the accumbens nucleus, the striatum, the olfactory tubercles and the globus pallidus are highly labelled. In the thalamus in which histaminergic fibres are scarce, H3 receptors are present in a rather high density, particularly in the midline, median and intralaminar groups of nuclei. In the hypothalamus, where the densest network of histaminergic fibres is found, H3 receptors occur in moderate density, being slightly more abundant in the anterior and medial part. They are also present at the level of the tuberomammillary nuclei where they may reside on histaminergic perikarya. In mesencephalon and lower brainstem, H3 receptors are abundant in the reticular part of the substantia nigra and central gray. They are present in low density in areas of noradrenergic and serotoninergic perikarya and in the spinal cord, where a faint specific labelling is detected in the gray matter, particularly in the external layers of the dorsal horn. In the cerebellum and pituitary gland, H3 receptors are scarce. Kainic acid infusions into the striatum were followed by marked local decreases in H3 receptors evidenced in both membrane binding and autoradiographic studies. Unilateral interruption of the ascending histaminergic pathways via electrocoagulation of the lateral hypothalamic area was followed by ipsilateral increase in striatal [3H](R)alpha-methylhistamine binding, a process consistent with denervation up regulation of postsynaptic H3 receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrahypothalamic injections of norepinephrine facilitate feline affective aggression via alpha 2-adrenoceptors

The purpose of this study was to investigate the effects of noradrenergic agents infused into the anterior hypothalamus on feline affective defense responses elicited by electrical stimulation of the ventromedial hypothalamus. Anterior hypothalamic sites which are known to receive inputs from both the ventromedial hypothalamus and ascending noradrenergic pathways were selected for pharmacological analysis. Intracerebral infusions of NE (1.2-2.4 nmol) into the anterior hypothalamus significantly reduced the threshold current required to elicit the hissing component of affective defense via electrical stimulation of the ventromedial hypothalamus. Maximal threshold reductions (17 +/- 3% to 20 +/- 3%) were observed 30 min following infusion. Anterior hypothalamic infusions of clonidine facilitated feline affective defense by reducing hissing current thresholds by 18 +/- 4%. Clonidine-induced changes in response thresholds parallel those obtained with NE. Both NE-induced and clonidine-induced reductions in current thresholds were reversible by pre- and post-treatment of the anterior hypothalamic sites with yohimbine. These results demonstrate that the reductions in response thresholds are mediated by post-synaptic alpha 2-adrenoceptors located within the anterior hypothalamus. Thus the noradrenergic system may play an important role in the regulation of affective aggressive behavior.

Clinical trial of clonidine hydrochloride as an antisecretory agent in cholera

Clonidine hydrochloride (an alpha 2-adrenoceptor agonist) was tested for antisecretory effects in patients with cholera in a randomized controlled trial. Nineteen adults with diarrhea due to Vibrio cholerae were treated with clonidine (0.9 mg/24 h orally for 72 h) and 18 served as controls. During the first 24 h of treatment and for 24 h afterwards, the mean +/- SD concentrations of sodium (in millimoles per liter) in the stools of clonidine-treated patients were 120.6 +/- 10.9 and 112.3 +/- 11.9, which were significantly lower than 135.5 +/- 17.1 and 125.0 +/- 16.4 in the controls (p less than 0.01). Stool chloride concentrations (in millimoles per liter) were also significantly less in the clonidine group during the same periods: 82.1 +/- 16.8 and 62.4 +/- 19.4 vs. 92.1 +/- 18.3 and 78.0 +/- 23.0, respectively (p less than 0.05). Concentrations of potassium but not bicarbonate were also significantly reduced in the stools of clonidine-treated patients (p less than 0.05). However, there were no significant differences in the mean +/- SD stool volumes (in liters) between the clonidine and the control group in any of the six 12-h periods after treatment or in the cumulative volumes in 72 h (24.2 +/- 10.6 and 22.9 +/- 8.3, respectively). We conclude that clonidine causes modest reduction of stool electrolyte loss but does not significantly reduce fecal fluid loss in patients with cholera.

Amphetamine antagonizes the presynaptic inhibitory effect of clonidine through an interaction at the level of the alpha 2-adrenoceptors

1. In the whole rat vas deferens 20 microM noradrenaline (NA) and 0.011 microM clonidine decreased (36 +/- 7.4% and 80 +/- 6.0% respectively) the motor response induced by hypogastric nerve stimulation. These effects were reverted by 1 microM yohimbine. Amphetamine 5.4 microM failed to antagonize the inhibitory effect of NA and attenuated clonidine effect. 2. The effect of amphetamine was not altered by preincubation with either cocaine 1 microM, (-)-propranolol 0.3 microM or cocaine plus prazosin 0.028 microM. 3. In reserpine pretreated animals amphetamine 5.4 microM shifted to the right the concentration-response curve (CRC) to clonidine 0.62 +/- 0.05 log units with a KB value of 1.83 +/- 0.30 microM. 4. Binding of [3H]clonidine and [3H]prazosin were inhibited by amphetamine. Amphetamine was 90 times more potent to inhibit [3H]clonidine binding. 5. The results obtained suggest a possible direct interaction between clonidine and amphetamine on alpha-adrenoceptor.

Effects of histamine H2 receptor antagonists on acid secretion stimulated by imidazoline derivatives in isolated parietal cells

Both the effect of imidazoline derivatives on acid secretion and the mechanism of this effect were studied in the parietal cells isolated from guinea pigs. Clonidine and tolazoline both stimulated the parietal cells to elevate the concentration of cyclic AMP and the accumulation of [14C]aminopyrine concentration dependently, although these imidazoline derivatives are known as alpha 2-adrenoceptor agonist or antagonist. These stimulatory effects were inhibited by famotidine, ranitidine and cimetidine, histamine H2 receptor antagonists. However, [3H]clonidine binding to the membrane preparations of parietal cells was not affected by these H2 antagonists and yohimbine but was inhibited by imidazoline derivatives. These results suggest that imidazoline derivatives may bind to the specific binding sites (different from H2 receptor or alpha 2-adrenoceptor) and stimulate the acid secretion of parietal cells with an increase of cyclic AMP, and that H2 antagonists may not only compete with the agonists for receptor binding but also interfere with the receptor adenylate cyclase system.

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic muscarinic and alpha-adrenoceptor-mediated regulation of GABA release from myenteric neurones of the guinea-pig small intestine

The effects of cholinomimetic and sympathomimetic drugs on the release of [3H]-gamma-aminobutyric acid ([3H]-GABA) evoked by high K+ from the isolated small intestine of the guinea-pig were investigated, in the presence of tetrodotoxin. Acetylcholine and oxotremorine, at concentrations ranging from 10(-9) to 10(-6) M inhibited the evoked release of [3H]-GABA in a concentration-dependent manner, while nicotine was without effect. Scopolamine and pirenzepine inhibited the effect of oxotremorine, while hexamethonium had no effect. The IC50 values for scopolamine and pirenzepine of the oxotremorine (3 X 10(-8) M)-induced inhibition were 1.02 X 10(-9) M and 9.78 X 10(-10) M, respectively. Noradrenaline, but not isoprenaline inhibited the evoked release of [3H]-GABA. Clonidine (10(-10)-10(-6) M) reduced the evoked release of [3H]-GABA in a concentration-dependent manner, but phenylephrine had no effect. The inhibitory effect of clonidine was antagonized by yohimbine but not by prazosin. These findings provide evidence for the localization of M1-muscarinic and alpha 2-adrenoceptors on GABAergic nerve terminals and their involvement in the presynaptic control of the release of GABA from the guinea-pig small intestine.

Alpha 2-adrenoceptor-mediated contractile response to catecholamines in smooth muscle strips isolated from rainbow trout stomach (Salmo gairdneri)

The type of adrenoceptor involved in the contractile response to catecholamines in smooth muscle strips isolated from rainbow trout stomach was determined. Noradrenaline (10 nM-10 microM) and adrenaline (10 nM-3 microM) caused non-sustained contractions which were markedly decreased by phentolamine (5.4 microM) but not by carteolol (5 microM). Phenylephrine (1 microM-1 mM) was less effective in causing muscle contraction and methoxamine produced no contraction. Clonidine (100 nM-300 microM) caused no mechanical response but inhibited the contraction to noradrenaline or adrenaline but not acetylcholine or 5-hydroxytryptamine. Yohimbine (10 nM-1 microM) decreased the contraction induced by noradrenaline or adrenaline but prazosin (1 microM) did not. Tetrodotoxin (780 nM) partially reduced the contraction induced by noradrenaline or adrenaline but atropine (500 nM) did not. In the presence of atropine (1 microM), electrical transmural stimulation caused frequency-dependent, tetrodotoxin-sensitive contractions. These results suggest that the contractile response induced by noradrenaline or adrenaline is mediated by alpha 2-adrenoceptors. It is also suggested that noradrenaline and adrenaline contract the smooth muscle by direct action and by indirect action through the non-cholinergic excitatory nerve.

Characterization of postsynaptic alpha 1-adrenoceptors in the rabbit iris dilator smooth muscle

Interactions of several alpha-adrenoceptor agonists and antagonists with their receptors were studied in rabbit and guinea pig iris dilator smooth muscle and rabbit aortic strips using pharmacological procedures. In rabbit iris dilators and aortic strips, noradrenaline acted as a full agonist, while oxymetazoline, clonidine and tizanidine acted as partial agonists. The dissociation constants of full and partial agonists in the dilators, calculated after irreversible blockade of a proportion of the active receptors with phenoxybenzamine, were similar to those in the aortic strips. Furthermore, the relative intrinsic efficacies of partial agonists were practically equal in the two tissues, suggesting that these drugs act on the same alpha-adrenoceptors. Since the alpha 2-agonists clonidine and tizanidine had low affinity in the rabbit dilators, the alpha-adrenoceptors in this tissue appear to be of alpha 1-type. These results were further supported by the fact that the pA2-value of prazosin, an alpha 1-antagonist, was approximately 2 log units higher than that of yohimbine, an alpha 2-antagonist. However, pA2-values of four quinazolines (prazosin, bunazosin, SM911 and SM2470) and two yohimbine alkaloids (yohimbine and corynanthine) were significantly lower in the rabbit dilator muscle than in rabbit aortic strips. Two imidazoline antagonists (phentolamine and tolazoline) and a phenethanolamine (labetalol) acted on the alpha 1-adrenoceptors in the two tissues nonselectively. These results suggest that alpha 1-adrenoceptors in the rabbit dilator muscle and aortic strips may not be identical and that both selective and nonselective antagonists which act on these receptor sites exist.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of yohimbine on exploratory and locomotor behaviour are attributable to its effects at noradrenaline and not at benzodiazepine receptors

A recent study has shown that the alpha 2-adrenoceptor antagonist, yohimbine, has an additional action, in micromolar concentrations, to inhibit the binding of [3H]benzodiazepines to their receptors in the CNS. An important question raised by this finding is to what extent the behavioural effects of yohimbine can be attributed to this action. Yohimbine (1.25-2.5 mg/kg) produced a dose-related decrease in exploratory head-dipping and locomotor activity in the holeboard test. The alpha 2-adrenoceptor agonist clonidine, in small doses (0.01-0.025 mg/kg), antagonized the reduction in exploratory head-dipping and locomotor activity produced by yohimbine (2.5 mg/kg). The benzodiazepine chlordiazepoxide (5-10 mg/kg), which reduces the activity of noradrenergic neurones, antagonized the effects of yohimbine less effectively. The inability of flumazepil (10-20 mg/kg; Ro 15-1788, a benzodiazepine receptor antagonist) to reverse the effects of yohimbine suggested that the low-affinity effect of yohimbine to displace the binding of benzodiazepines from their receptors, is not important in its behavioural effects in the holeboard, but that these effects are attributable to the alpha 2-antagonist action of yohimbine. These conclusions are consistent with previous results in an animal test of anxiety.

Differential sensitivity to prazosin and yohimbine blockade of tyramine and noradrenaline

Previous researches demonstrated that in the rat vas deferens the effects of noradrenaline released by tyramine were more easily affected than those induced by exogenous noradrenaline by the non selective alpha-blockers, phentolamine and dihydroergocristine. The investigation has now been extended to the effects of selective alpha-blockers, prazosin and yohimbine with the aim to see whether the major sensitivity of tyramine to alpha-blockade correlates with the type of receptor activated. The results obtained with the two antagonists which selectively block alpha 1 and alpha 2 receptors strongly resemble each other and those previously obtained with the non selective alpha-adrenoceptor blockers. Thus, the peculiar sensitivity to alpha-blockade of noradrenaline released by tyramine with respect to exogenous noradrenaline does not seem to be dependent on the type (alpha 1 or alpha 2) of receptor involved.

The properties of the clonidine withdrawal response of guinea-pig isolated ileum

The dependence-inducing effects of clonidine were investigated on the guinea-pig isolated ileum. Clonidine produced relaxation of the ileum with a threshold concentration between 0.01 and 0.1 mumol 1(-1). Washout of clonidine did not induce a withdrawal contraction. Following 2 min contact of the ileum with clonidine, 1 mumol 1(-1), addition of phentolamine, 5 mumol 1(-1), induced a contracture. The phentolamine-precipitated withdrawal contracture did not increase in height with a longer period of contact (32 min) of the ileum with clonidine. The phentolamine-precipitated withdrawal contracture following 2 min contact of ileum with clonidine was abolished by atropine, 5 mumol 1(-1), and substance P (SP) antagonists, (D-Pro2,D-Phe7, D-Trp9)-SP and spantide, 10 mumol 1)-1). [Met5]enkephalin, 1 mumol 1(-1), abolished the withdrawal response to clonidine and clonidine, 1 mumol 1(-1), abolished the withdrawal response to [Met5]enkephalin. Following 2 min contact of the ileum with noradrenaline, 5 mumol 1(-1), washout or addition of phentolamine or yohimbine, 5 mumol 1(-1), also induced a withdrawal response. The noradrenaline washout withdrawal response was abolished by atropine, 5 mumol 1(-1), and spantide, 10 mumol 1(-1). Since clonidine dependence may be induced as rapidly as opiate dependence in the ileum and the pharmacology of the withdrawal responses is similar, it is suggested that they both induce the same post-receptor neuronal feedback disturbance in which substance P neurones play a major role.

Subsensitivity of enteric cholinergic neurones to alpha 2-adrenoceptor agonists after chronic sympathetic denervation

The concentration-effect relationships of noradrenaline, dopamine and clonidine in inhibiting resting and stimulated acetylcholine output have been studied in intact and in sympathetically denervated preparations of guinea pig isolated distal colon. The order of potencies for the inhibition of resting acetylcholine release in intact preparations was clonidine greater than dopamine greater than noradrenaline while the order of intrinsic activities was noradrenaline greater than dopamine greater than clonidine. Sympathetic denervation was able to modify the potency of either clonidine, dopamine and noradrenaline. Noradrenaline was 6 times more potent in inhibiting resting acetylcholine release in denervated than in intact preparations, while clonidine and dopamine underwent a 18-fold and a 11-fold decrease in potency after denervation. The potency of clonidine relative to noradrenaline was 110 in intact preparations and only 1.2 in denervated organs. The intrinsic activities of noradrenaline, dopamine and clonidine were almost unchanged in denervated organs. A dose-dependent facilitatory effect of yohimbine on both the resting acetylcholine output and the peristaltic reflex could be observed in intact but not in sympathetically denervated preparations at concentrations ranging from 2.5 X 10(-8) M to 2.5 X 10(-7) M. Yohimbine was able to counteract the inhibitory effect of dopamine and to remove the inhibitory effect of periarterial nerve stimulation on both acetylcholine release and the peristaltic reflex. Our results are consistent with the existence of a tonic physiological modulation of enteric cholinergic neurones by postganglionic sympathetic fibres. The order of potencies of adrenoceptor agonists and the antagonism by yohimbine is consistent with such a modulation being entirely carried out through alpha 2-heteroceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative analysis of rat brain alpha 2-receptors discriminated by [3H]clonidine and [3H]rauwolscine

Quantitative analysis of direct ligand binding of both [3H]clonidine and [3H]rauwolscine to the rat cerebral cortex alpha 2-receptors indicates the existence of two affinity states of the same receptor populations. In the presence of Mn2+, the high affinity state of [3H]clonidine binding was increased, whereas the high affinity state of [3H]rauwolscine binding was reduced. By contrast, GTP in micromolar ranges caused a decrease of the agonist high affinity state and an increase of the antagonist high affinity state. The total receptor sites and the respective separate affinities for both radioligands were approximately equal to their control values under all conditions, indicating that Mn2+ and GTP modulate the proportion of the two affinity states of the receptor. These results can be incorporated into a two-step, ternary complex model involving a guanine nucleotide binding protein (N protein) for the agonist and antagonist interaction with the alpha 2-receptor. Furthermore, the effects of GTP on the interaction of both ligands with the two affinity states can be mimicked by EDTA. It is suggested that divalent cations induce the formation of the receptor-N protein binary complex showing high affinity for agonists and low affinity for antagonists.

Alpha-2 receptors in the gastrointestinal system: a new therapeutic approach

Alpha-2 receptor activation mediates the inhibition of a number of gastrointestinal functions including gastric and intestinal secretions. Alpha-2 receptors are located in the brain and presynaptically on cholinergic nerve terminals; activation of either inhibits vagus nerve activity. Intestinal secretions are inhibited by postsynaptic alpha-2 receptors located on intestinal epithelial cells. Agents which selectively activate alpha-2 receptors in the gut may therefore be beneficial in treating gastric ulcers and diarrheal states. Two such agents which activate alpha-2 receptors in the gut are WHR-1370A [1-n-butoxy-3-(2,6-dimethylphenylcarbamoyl) guanidine hydrochloride] and lidamidine. WHR-1370A is a potent gastric antisecretory and antiulcer agent which inhibits the release of acetylcholine from the vagus nerve. WHR-1370A's activity is blocked by yohimbine. Lidamidine is a clinically effective antidiarrheal agent. Lidamidine's response is partially inhibited by yohimbine in animal diarrheal models. Alpha-2 agonists represent a new class of drugs which have a promising future in the treatment of gastrointestinal disorders.

The binding of [3H]prazosin and [3H]clonidine to rat jejunal epithelial cell membranes

The binding of [3H]prazosin and [3H]clonidine to rat jejunal epithelial cell membranes has been studied. The membrane preparation was enriched in baso-lateral components as determined by Na+, K+ ATPase and alkaline phosphatase activities. The membranes possessed two saturable specific binding sites for [3H]prazosin, a high affinity (Kd 0.17 nM) low capacity (Bmax 27.3 fmole bound per mg protein) and a low affinity (Kd 5.0 nM) high capacity (Bmax 276 fmole bound per mg protein) site. The specificity of both sites was similar and was related to alpha 1-adrenoceptors. [3H]Clonidine bound to the membranes in a saturable fashion (Kd 7.3 nM). The specificity of this site was related to alpha 2-adrenoceptors. The [3H]clonidine binding site was present in the membranes in much lower density (Bmax 22.8 fmole bound per mg protein) suggesting that alpha 1-adrenoceptors predominate in this tissue.

Stereochemical requirements of alpha 2-adrenergic receptors for alpha-methyl substituted phenethylamines

The alpha 1- and alpha 2-adrenergic effects of the stereoisomers of alpha-methyldopamine were evaluated in guinea pig aorta and field-stimulated guinea pig ileum, respectively, in order to establish the stereochemical requirements of these receptors for alpha-methyl substituted phenethylamines. The alpha 1-adrenergic receptor did not distinguish between the stereoisomers of alpha-methyldopamine which is in marked contrast to the alpha 2-adrenergic receptor where a dramatic stereochemical preference for the 2S(+)-isomer was observed. In addition, 2R(-)-alpha-methyldopamine displayed no alpha-receptor subtype specificity whereas 2S(+)-alpha-methyldopamine was highly selective (23 fold) for the alpha 2-adrenergic receptor. These results indicate that the alpha 2-adrenergic receptor can recognize and accept methyl substituents at the alpha-carbon atom of phenethylamines when correctly oriented, while the alpha 1-adrenergic receptor cannot. Thus, the alpha-carbon atom is a major determinant of the alpha 2-adrenergic effects of phenethylamines, and plays an important role in determining alpha-receptor subtype specificity. It is hypothesized that the alpha 2-adrenergic receptor (but not alpha 1) has an additional recognition site which will accommodate alpha-substituted phenethylamines.

Facilitation, and muscarinic and alpha-adrenergic inhibition of the secretion of 3H-acetylcholine and 3H-noradrenaline from guinea-pig ileum myenteric nerve terminals

The transmitter stores of cholinergic and noradrenergic neurons in guinea-pig ileum myenteric plexus were labelled by preincubation with 3H-choline or 3H-noradrenaline (3H-NA), respectively. Secretion of transmitter was evoked by electrical field stimulation. In the presence (but not in the absence) of eserine (10 microM) the secretion of 3H-acetylcholine (3H-ACh) per shock increased with the frequency of stimulation. Half maximal secretion was obtained at 0.7 Hz (apparent Km Freq). Variation in number of shocks per train did not influence the secretory response per shock (at 6 Hz). The secretion of 3H-NA per shock also increased with the frequency of stimulation (Km Freq = 1.5 Hz). Comparison of the inhibitory effects of oxotremorine and of exogenous NA, on the secretory responses to stimulation at frequencies close to Km Freq, showed that the secretory mechanisms of both neurons are 13-17-fold more sensitive to the inhibitory effect of their 'own', than to that of 'foreign', agonist. In both types of neuron the inhibitory effects of oxotremorine and of NA were competitively antagonized by atropine and by yohimbine, respectively. Dissociation constants for the respective antagonist were essentially the same, irrespectively of the type of neuron, indicating that the properties of 'presynaptic' muscarinic or alpha-receptors are independent of whether they occur on cholinergic or on noradrenergic terminals.

Evidence that phentolamine is not an inhibitor of extraneuronal uptake

1. Tracheal segments from guinea-pigs pretreated with 6-hydroxydopamine were incubated in isoprenaline at 37 degrees C for 5 min in the absence or presence of phentolamine. Catechol-O-methyl transferase was inhibited by 100 mumol l-1 U-0521. Tissues were prepared for fluorescence histochemistry and accumulated isoprenaline in trachealis smooth muscle cells (fluorescence) was measured by microphotometry. 2. Phentolamine, in concentrations up to 100 mumol l-1, had no effect on isoprenaline fluorescence. 3. It is concluded that phentolamine does not inhibit extraneuronal uptake in concentrations used to block alpha-adrenoceptors in isolated tissue experiments. Thus, is can be present in experiments designed to examine the effects of extraneuronal uptake inhibitor drugs on beta-adrenoceptor-mediated responses.

Effect of yohimbine and its diastereoisomers on clonidine-induced depression of exploration in the rat

The effects of yohimbine, rauwolscine and corynanthine on clonidine-induced depression of exploration (ambulation and rearing) in the rat were investigated. Yohimbine and rauwolscine antagonized clonidine-induced hypoactivity in a dose-range of 0.3 - 3 mg/kg i.p. By contrast, corynanthine was found to lack such an effect up to ten times higher doses. These results are in agreement with the reported differential affinity of these drugs for the alpha 2-adrenoceptors respectively and suggest the involvement of alpha 2-receptors in the mediation of depressant effect of clonidine. The antagonism of clonidine-induced hypoactivity appears, therefore, to provide a suitable in vivo measure of preferential alpha 2-adrenoceptor blocking activity of drugs.

Alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal fluid accumulation

The effects of adrenoceptor agonists and antagonists on the cholera-toxin-induced intestinal fluid accumulation and the mucosal levels of cAMP were investigated in vivo. Cholera toxin produced a marked fluid accumulation. Adrenaline inhibited the effect of the toxin in a dose-dependent manner. An alpha 2-adrenoceptor blocking agent yohimbine antagonized the effect of adrenaline. The alpha 1-adrenoceptor blocking agents prazosin and phenoxybenzamine failed to antagonize the effect of adrenaline. A high dose of a beta-adrenoceptor blocking agent pindolol did not antagonize the effect of adrenaline. Yohimbine or pindolol alone did not produce any effects on the toxin-induced fluid accumulation. However, prazosin and phenoxybenzamine per se inhibited the toxin-induced fluid accumulation. An alpha 2-selective agonist clonidine was slightly more potent than adrenaline, and was about 100-fold more potent than the alpha 1-selective agonist methoxamine in inhibiting the cholera-toxin-induced intestinal secretion. Clonidine, adrenaline and methoxamine failed to reduce the mucosal levels of cAMP, while these alpha-adrenoceptor agonists inhibited the toxin-induced fluid accumulation in the same preparations. These results suggest that the stimulation of alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal secretion without reducing the whole mucosal levels of cAMP.

Characterization and localization of [3H]-clonidine binding in membranes prepared from guinea-pig spleen

1. [3H]-Clonidine binds to membranes prepared from guinea-pig spleen with high affinity. 2. Kinetic experiments indicated that [3H]-clonidine associates rapidly to the binding site and that the binding is reversible. A study of the dissociation of [3H]-clonidine from splenic membranes revealed two components. The slowly dissociating component corresponded to a high affinity process (Kd = 2.1 nmol/l) in good agreement to that obtained by saturation analysis. 3. Over the concentration range used, saturation experiments revealed only a single population of sites with a dissociation constant (Kd) of 2.4 nmol/l and a density of 5.1 pmol/g wet weight tissue. 4. Examination of the relatively potency of a series of alpha-adrenoceptor agonists and antagonists indicates that [3H]-clonidine binding is to alpha 2-adrenoceptors. 5. High levels of binding were obtained to lymphocytes prepared from guinea-pig spleen and to membranes from the splenic capsule. Pretreatment of animals with 6-hydroxydopamine produced changes in apparent affinity of binding with little change in the number of receptor sites. 6. It is concluded that [3H]-clonidine labels is a site resembling the alpha 2-adrenoceptor in guinea-pig spleen. Few if any of these sites are located prejunctionally and a significant fraction are associated with lymphocytes.

Clonidine activates membrane potassium conductance in myenteric neurones

1 Intracellular recordings were made from neurones in the myenteric plexus of the ileum removed from guinea-pigs. The effects of clonidine and adrenaline on membrane potential and resistance were observed.2 Clonidine (100 pM-30 nM) caused a concentration-dependent membrane hyperpolarization associated with a fall in neurone input resistance.3 The amplitude of the clonidine hyperpolarization, but not the conductance increase, was greater in cells with lower resting potentials and smaller in more polarized neurones. In a given cell, membrane hyperpolarization decreased and membrane depolarization increased the clonidine effect.4 Low potassium solutions enhanced and high potassium solutions reduced the hyperpolarizing action of clonidine but did not significantly change the conductance increase caused by clonidine.5 The concentration-effect curve for clonidine was displaced to the left when the extracellular calcium concentration was reduced. Conversely, clonidine was almost ineffective in elevated calcium concentrations. This was true for both the hyperpolarization and the conductance increase.6 It is suggested that clonidine activates a potassium conductance by causing an elevation in the free intracellular calcium concentration.7 Clonidine reversibly depressed the amplitude of the nicotinic fast excitatory postsynaptic potential and the noncholinergic slow excitatory postsynaptic potential.8 All the effects of clonidine were shared by adrenaline and the actions of both were reversed or prevented by phentolamine (100 nM-1 muM).

Clonidine dependence in the guinea-pig isolated ileum

1 Compared with the response of preparations incubated in solutions without clonidine, a three to four fold increase in the magnitude of the contracture of the longitudinal muscle to challenge with phentolamine (1.0 mum) was induced by incubating the guinea-pig isolated ileum at 22 degrees C for 24 h with clonidine (1.0 mum) in Krebs solution containing hexamethonium (70 mum). Incubation of the ileum with clondine (1.0 mum) for 0.5 h at 37 degrees C did not increase responsiveness to phentolamine.2 The increase in responsiveness to phentolamine was directly related to the clonidine concentration in the incubation fluid over the range 0.01 to 1.0 mum.3 The magnitude of the contracture to phentolamine of ilea incubated with clonidine (1.0 mum) (withdrawal contracture) was directly related to the challenge dose of phentolamine over the range 0.3 to 1.0 mum.4 Yohimbine (1.0 mum) or piperoxane (1.0 mum) elicited a response comparable to that elicited by phentolamine but propranolol (1.0 mum) was inactive.5 Addition of phentolamine (1.0 mum) to clonidine (1.0 mum) in the incubation fluid abolished the increased response of the preparation to subsequent challenge with phentolamine.6 Addition of hyoscine (0.5 mum) immediately after challenge with phentolamine restored the tension of the withdrawal contracture to its resting level.7 Tetrodotoxin (3.0 mum) given before challenge, prevented phentolamine from eliciting a withdrawal contracture.8 Ileal segments incubated with clonidine (1.0 mum) were unresponsive to challenge with naloxone (100 nm); and segments incubated with normorphine (1.0 mum) were unresponsive to phentolamine (1.0 mum), although responsive to naloxone.9 Normorphine (1.0 mum) restored to resting level the tension of the clonidine withdrawal contracture; and clonidine (0.1 mum) restored to resting level the tension of the contracture to naloxone in ileal segments incubated with normorphine.10 These experiments indicate that incubation with clonidine induces, in the final cholinergic motor neurones of the myenteric plexus of the isolated ileum, a dependence the withdrawal from which is expressed as a contracture in response to alpha-adrenoceptor antagonists.11 Although opiate receptors are not involved in clonidine dependence nor alpha-adrenoceptors in opiate dependence, the findings that normorphine suppresses the clonidine withdrawal-contracture and that clonidine suppresses the contracture of opiate-dependent ileum to naloxone, suggest that the withdrawal effect studied in both clonidine and normorphine dependence in this preparation is mediated by release of acetylcholine from the final motor neurone.

Functional characterization of central alpha-adrenoceptors by yohimbine diastereomers

Rat occipital cortex slices were preincubated with [3H]noradrenaline and then superfused with medium containing 30 micrometer cocaine. They were stimulated electrically at 3 Hz. Unlabelled noradrenaline (0.1 micrometer), alpha-methyl-noradrenaline (0.01-0.1 micrometer), xylazine (0.1-10 micrometer) and guanabenz (0.01 micrometer) decreased, whereas yohimbine (0.01-1 micrometer), rauwolscine (0.01-1 micrometer), corynanthine (10 micrometers), tolazoline (0.1-10 micrometers) and azapetine (0.1-1 micrometer) increased the stimulation-evoked overflow of tritium without a change in basal outflow. Pseudoyohimbine and prazosin at up to 0.1 micrometer did not change the evoked overflow, and a higher concentrations enhanced the basal outflow of tritium. In vivo, yohimbine 10 mg/kg and rauwolscine 10 mg/kg markedly, and corynanthine 10 mg/kg slightly accelerated the alpha-methyltyrosine-induced disappearance of noradrenaline from rat whole brain. Yohimbine and rauwolscine but not corynanthine also accelerated the alpha-methyltyrosine-induced depletion of dopamine. The results add four compounds to the list of drugs with alpha-adrenoceptor affinity which inhibit (agonists) or facilitate (antagonists) action-potential-evoked release of noradrenaline in rat brain cortex. The presynaptic receptors are of the alpha 2-type. The receptors which control the activity of noradrenaline and dopamine neurons in vivo also appear to be alpha 2.

Mini-review. The postsynaptic alpha 2-adrenoreceptor

The discovery of postsynaptic alpha 2-adrenoreceptors with the drug specificities of presynaptic alpha 2-adrenoreceptors has contributed to a refinement of the classification of alpha-adrenoreceptors. postsynaptic alpha 2-adrenoreceptors have been identified by pharmacological means and with the aid of direct radioligand-receptor binding studies. The evidence for the existence of this class of alpha 2-adrenoreceptors in the brain and in vascular smooth muscle is particularly strong. Central postsynaptic alpha 2-adrenoreceptors play a major part in the hypotensive action of centrally acting antihypertensive drugs such as clonidine and alpha-methyl-DOPA. Vascular smooth muscle cells contain postsynaptic alpha 2-adrenoreceptors which mediate vasoconstriction, like the more classical alpha 1-adrenoreceptors. The simultaneous occurrence of contractile alpha 1- and alpha 2-adrenoreceptors in vascular smooth muscle offers a simple model for the characterization of alpha-adrenoreceptor agonists and antagonists. At present, highly selective agonists of alpha 2-adrenoreceptors have been found. These new compounds may be useful for the classification of other alpha-adrenoreceptor populations. It has been suggested that the vascular postsynaptic alpha 2-adrenoreceptor might be located at extrasynaptic sites. Accordingly, adrenaline released by the adrenal medulla would be the endogenous stimulant. Finally, observations have been made in vivo indicating that a transmembrane influx of calcium ions is necessary for linking the drug-induced activation of these alpha 2-adrenoreceptors to vasoconstriction.