Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2D-autoreceptors and alpha 2D-adrenoceptors modulating release of acetylcholine in guinea-pig ileum

Jejunal cholinergic, nitrergic, and soluble guanylate cyclase activity in postoperative ileus

BACKGROUND

In animal models of postoperative ileus (POI), inflammation of the intestine plays an important role in the pathogenesis of POI. Changes in alpha(2)-adrenoceptors and nitrergic regulation have been proposed to be implicated. The aim of our study was to investigate the presynaptic alpha(2)-receptor-mediated control of cholinergic nerve activity, the nitrergic nerve activity, and the possible role of soluble guanylate cyclase (sGC) during the inflammatory phase of POI.

METHODS

Ileus was induced by anesthesia and manipulation of the rat jejunum. Rats were treated with the sGC inhibitors methylene blue or ODQ; nonoperated animals served as controls. After 24 h, plasma and jejunal tissue were collected for biochemical assays, nitric oxide synthase-1 (NOS-1)-immunohistochemistry, acetylcholine (Ach)-release experiments, and muscle tension experiments.

RESULTS

In all operated animal groups, myeloperoxidase activity was significantly increased, which indicates initiation of an inflammatory response. The alpha(2)-adrenoceptor agonist UK14,304 reduced electrically induced Ach-release similarly in operated and nonoperated animals. In strips of operated animals, electrically induced nitrergic relaxations were decreased, whereas relaxations induced by exogenous nitric oxide (NO) remained unchanged compared with control. The number of myenteric neurons and the percentage of NOS-1-positive neurons were not influenced. Plasmatic cyclic guanosine monophosphate (cGMP) levels were decreased in all operated groups, whereas jejunal cGMP levels were unchanged compared with nonoperated controls; treatment with sGC inhibitors did not reduce plasmatic cGMP levels.

CONCLUSIONS

This study demonstrates that presynaptic alpha(2)-receptor mediated control of intestinal cholinergic nerve activity is unchanged during manipulation-induced inflammation. However, this inflammation induces impaired nitrergic neurotransmission related to decreased NOS-1 activity in the nitrergic nerves.

Altered prejunctional modulation of intestinal cholinergic and noradrenergic pathways by alpha2-adrenoceptors in the presence of experimental colitis

1 This study investigates the influence of intestinal inflammation on: (1) the control of intestinal neurotransmission and motility by prejunctional alpha(2)-adrenoceptors and (2) the expression of intestinal alpha(2)-adrenoceptors. Experimental colitis was induced by intrarectal administration of 2,4-dinitrobenzenesulphonic acid (DNBS) to rats. 2 UK-14,304 inhibited atropine-sensitive electrically evoked contractions of ileal and colonic longitudinal muscle preparations. UK-14,304 acted with similar potency, but higher efficacy, on tissues from DNBS-treated animals; its effects were antagonized with greater potency by phentolamine than rauwolscine. 3 Electrically induced [(3)H]noradrenaline release from ileal preparations was reduced in the presence of colitis. Tritium outflow was decreased by UK-14,304 and stimulated by rauwolscine or phentolamine: these effects were enhanced in preparations from animals with colitis. 4 Reverse transcription-polymerase chain reaction and Western blot assay demonstrated the protein expression of alpha(2A)-adrenoceptors in mucosal and muscular tissues isolated from ileum and colon. The induction of colitis increased alpha(2A)-adrenoceptor expression in both ileal and colonic muscular layers, without concomitant changes in mucosal tissues. 5 Induction of colitis reduced gastrointestinal propulsion of a charcoal suspension in vivo. In this setting, the gastrointestinal transit was inhibited by intraperitoneal (i.p.) UK-14,304 and stimulated by i.p. rauwolscine. After pretreatment with guanethidine, the stimulant action of rauwolscine no longer occurred, and UK-14,304 exerted a more prominent inhibitory effect that was antagonized by rauwolscine. 6 The present results indicate that, in the presence of intestinal inflammation, prejunctional alpha(2)-adrenoceptors contribute to an enhanced inhibitory control of cholinergic and noradrenergic transmission both at inflamed and noninflamed distant sites. Evidence was obtained that such modulatory actions depend on an increased expression of alpha(2A)-adrenoceptors within the enteric nervous system.

Affinity profile at alpha(1)- and alpha(2)-adrenoceptor subtypes and in vitro cardiovascular actions of (+)-boldine

The present study examines the functional and binding affinities of the aporphine alkaloid, (+)-boldine, at different alpha(1)- and alpha(2)-adrenoceptor subtypes, namely, alpha(1A) (rat vas deferens and kidney) and its L-like state (rabbit spleen), alpha(1B) (guinea pig spleen, mouse spleen and rabbit aorta), alpha(1D) (rat aorta and pulmonary artery), at possible subtypes of prejunctional alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium, alpha(2D) in guinea pig ileum, cloned human alpha(1)-adrenoceptor subtypes A, B and D and alpha(2)-adrenoceptor subtypes A, B and C as well as rat alpha(2D)-adrenoceptors. Additionally, we investigated its Ca(2+) channel antagonism in vascular and cardiac preparations. (+)-Boldine had higher affinity at alpha(1)-adrenoceptor subtype A (pA(2)=7.46, pK(i)=7.21) compared with its L-like state (pA(2)=5.63) or subtype B (pA(2)=5.98- 6.12, pK(i)=5.79) and subtype D (pA(2)=6.18-6.37, pK(i)=6.09). Its affinities at alpha(2)-adrenoceptors in rat and rabbit vas deferens and rat atrium (pA(2)=6.02, 6.36, 6.06, respectively) were identical, but lower at guinea pig ileum alpha(2D)-adrenoceptors (pA(2)=4.38). (+)-Boldine displayed nearly undistinguishable affinity at cloned human alpha(2)-adrenoceptor subtypes A, B and C (pK(i)=6.26, 5.79 and 6.35, respectively), whereas its affinity at rat alpha(2D)-adrenoceptors was low (pK(i)=4.70). In perfused rat kidney, (+)-boldine inhibited K(+)-evoked vasoconstriction at doses 70-fold higher than diltiazem. In guinea pig Langendorff heart, (+)-boldine (10(-5) - 2 x 10(-4) M) was equieffective in increasing coronary flow and in depressing cardiac force, while lower concentrations already depressed heart rate. In papillary muscles from guinea pig, (+)-boldine (10(-6) - 10(-5) M) mainly prolonged the duration of action potential at levels >30% of repolarization. These data reveal that (+)-boldine, except for its moderate selectivity (15 to 25-fold) for alpha(1A)-adrenoceptors, does not discriminate between the alpha(1)-adrenoceptor subtypes B and D and alpha(2)-adrenoceptor subtypes A, B and C, at which the drug consistently displays micromolar affinity. In vascular and cardiac preparations, (+)-boldine, although being at least 50-fold weaker than diltiazem, shows Ca(2+) channel antagonistic properties but no specificity for coronary dilatation relative to cardiodepression.

Alpha 2-adrenoceptors in the enteric nervous system: a study in alpha 2A-adrenoceptor-deficient mice

Mammals possess three types of alpha(2)-adrenoceptor, alpha(2A), alpha(2B) and alpha(2C). Our aim was to determine the type of alpha(2)-adrenoceptor involved in the control of gastrointestinal motility. In transmitter overflow experiments, myenteric plexus longitudinal muscle (MPLM) preparations of the ileum were preincubated with [(3)H]-choline and then superfused. The alpha(2)-adrenoceptor agonist medetomidine reduced the electrically evoked overflow of tritium from preparations taken from wild type but not alpha(2A)-adrenoceptor-knockout mice. In a second series of overflow experiments, MPLM preparations were preincubated with [(3)H]-noradrenaline and then superfused. Again medetomidine reduced the electrically evoked overflow of tritium from wild type but not alpha(2A)-knockout preparations. In organ bath experiments, medetomidine reduced electrically evoked contractions of segments of the ileum from wild type but not alpha(2A)-knockout mice. In each of these three series, phentolamine antagonized the effect of medetomidine in wild-type preparations with greater potency than rauwolscine. In conscious mice, gastrointestinal transit was assessed by means of an intragastric charcoal bolus. In alpha(2A)-knockout mice, the speed of gastrointestinal transit was doubled compared to wild-type. Medetomidine, injected intraperitoneally, slowed gastrointestinal transit in wild type but not alpha(2A)-knockout mice. We conclude that the cholinergic motor neurons of the enteric nervous system of mice possess alpha(2)-heteroreceptors which mediate inhibition of acetylcholine release, of neurogenic contractions and of gastrointestinal transit. The noradrenergic axons innervating the intestine possess alpha(2)-autoreceptors. Both hetero- and autoreceptors are exclusively alpha(2A). It is the alpha(2A)-adrenoceptor which in vivo mediates the inhibition of intestinal motility by the sympathetic nervous system.

Presynaptic modulation of cholinergic neurotransmission in the human proximal stomach

1. This study investigates whether the cholinergic neurones, innervating the human proximal stomach, can be modulated by nitric oxide (NO) or vasoactive intestinal polypeptide (VIP), or via presynaptic muscarinic, alpha(2)- or 5-hydroxytryptamine(4) (5-HT(4)-) receptors. 2. Circular muscle strips, without mucosa, were incubated with [(3)H]-choline to incorporate [(3)H]-acetylcholine into the cholinergic transmitter stores. The basal and electrically-induced release of tritium and [(3)H]-acetylcholine were analysed in a medium containing guanethidine (4 x 10(-6) M), hemicholinium-3 (10(-5) M), physostigmine (10(-5) M) and atropine (10(-6) M). Tissues were stimulated twice for 2 min (S(1) and S(2): 40 V, 1 ms, 4 Hz) and drugs were added before S(2). 3. The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3 x 10(-4) M) and the NO donor sodium nitroprusside (10(-5) M), as well as VIP (10(-7) M) did not influence the basal release nor the electrically-evoked release. 4. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically-evoked release of [(3)H]-acetylcholine, and this was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2 x 10(-6) M). 5. The 5-HT(4)-receptor agonist prucalopride (3 x 10(-7) M) significantly enhanced the electrically-evoked release of [(3)H]-acetylcholine, and the 5-HT(4)-receptor antagonist SB204070 (10(-9) M) prevented this. 6. When atropine (10(-6) M) was omitted from the medium and added before the second stimulation, it significantly increased the release of [(3)H]-acetylcholine. 7. These results suggest that the release of acetylcholine from the cholinergic neurones, innervating the circular muscle in the human proximal stomach, can be inhibited via presynaptic muscarinic auto-receptors and alpha(2)-adrenoceptors, and stimulated via presynaptic 5-HT(4)-receptors. No evidence for modulation by NO or VIP was obtained.

Effects of A1-adenosine receptor antagonists on purinergic transmission in the guinea-pig vas deferens in vitro

1. Intracellularly recorded excitatory junction potentials (ej.ps) were used to study the effects of adenosine receptor antagonists on neurotransmitter release from postganglionic sympathetic nerve terminals in the guinea-pig vas deferens in vitro. 2. The A1 adenosine receptor antagonists, 8-phenyltheophylline (10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM), increased the amplitude of e.j.ps evoked during trains of 20 stimuli at 1 Hz in the presence, but not in the absence, of the alpha2-adrenoceptor antagonist, yohimbine (1 microM) or the non-selective alpha-adrenoceptor antagonist, phentolamine (1 microM). 3. Adenosine (100 microM) reduced the amplitude of e.j.ps, both in the presence and in the absence of phentolamine (1 microM). This inhibitory effect of adenosine is most likely caused by a reduction in transmitter release as there was no detectable change in spontaneous ej.p. amplitudes. 4. In the presence of phentolamine, application of the adenosine uptake inhibitor, S-(p-nitrobenzyl)-6-thioinosine (0.1 microM), had no effect on ej.p. amplitudes. 5. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (100 microM), significantly increased the amplitudes of all e.j.ps evoked during trains of 20 stimuli at 1 Hz, both in the presence and in the absence of phentolamine (1 microM). 6. These results suggest that endogenous adenosine modulates neurotransmitter release by an action at prejunctional A1 adenosine receptors only when alpha2-adrenoceptors are blocked.

Inhibition of stress-stimulated colonic propulsion by alpha 2-adrenoceptor antagonists in rats

Alpha2-adrenoceptor antagonists have been reported to stimulate colonic motor activity, but the effect on colonic motor dysfunction is unclear. We have investigated the effect of alpha 2-adrenoceptor antagonists on wrap-restraint stress-stimulated and normal colonic propulsion in rats. Colonic propulsion was evaluated by the transit of a charcoal marker along the colon. Faecal pellets output was also measured. A 30-min exposure to wrap-restraint stress starting 120 min after infusion of the charcoal marker significantly stimulated colonic transit with a concomitant increase in faecal pellets. Yohimbine and idazoxan, alpha 2-adrenoceptor antagonists, clonidine, an alpha 2-adrenoceptor agonist, and atropine suppressed wrap-restraint stress-stimulated colonic transit and faecal excretion in a dose-dependent manner. Ondansetron and YM060, 5-hydroxytryptamine3 (5-HT3) receptor antagonists, potently inhibited wrap-restraint stress-stimulated colonic transit, but only weakly inhibited faecal excretion. Neither alpha 2-adrenoceptor antagonists nor atropine had any significant effect on normal colonic transit, whereas clonidine and the 5-HT3 receptor antagonists inhibited it. alpha 2-Adrenoceptor antagonists as well as clonidine, atropine and 5-HT3 receptor antagonists inhibit the stress-induced colonic motor dysfunction in rats.

Rilmenidine reveals differences in the pharmacological characteristics of prejunctional alpha2-adrenoceptors in the guinea-pig, rat and pig

1. The alpha2A and alpha2D-adrenoceptor subtypes are thought to be species homologs most easily differentiated on the basis of the potency of antagonists. In the present study we have compared the effect of rilmenidine with two other selective alpha2-adrenoceptor agonists, UK-14304 (5-bromo-6- [2-imidazolin-2-ylamino]-quinoxaline) and clonidine, against electrically-evoked contractions in five isolated preparations from the rat, guinea-pig and pig, and, where possible, determined the receptor subtype involved. 2. UK-14034, clonidine and rilmenidine produced concentration-dependent inhibition of the electrically-evoked contractions of the rat isolated vas deferens and tail artery and the guinea-pig ileum. These inhibitory effects were reversed by the selective alpha2-adrenoceptor antagonist, RX-811058 (1 microM), except in the rat tail artery preparations where the remaining neurogenic response was inhibited; evidence for the involvement of 'innervated' alpha2-adrenoceptors. Both clonidine and UK-14304 produced concentration-dependent inhibition of responses in the porcine isolated tail artery and urinary bladder but clonidine was markedly less efficacious in these preparations. In contrast, rilmenidine failed to inhibit the neurogenic contractions in either preparation. 3. Although rilmenidine failed to elicit a detectable response in either the porcine isolated tail artery or urinary bladder, it (10 microM and 30 microM, respectively) competitively antagonised the inhibitory effects of UK-14304 with an estimated dissociation constant of (pK(B)) 5.82 and 5.93, respectively. 4. Prazosin (1 microM) failed to alter the effect of UK-14304 against neurogenic contractions in the porcine isolated urinary bladder, while rauwolscine (pK(B) 8.87) was 10 fold more potent than phentolamine (pK(B) 7.56). On the other hand, phentolamine (pK(B) 8.42) was only marginally more potent than rauwolscine (pK 8.05) against clonidine-induced inhibition of electrically-evoked contractions of the guinea-pig isolated ileum. This pharmacological evidence with antagonists supports the presence of alpha2D-adrenoceptors in the rat and guinea-pig and the alpha2A-adrenoceptors in the pig. 5. We have demonstrated that rilmenidine, unlike clonidine and UK-14304, is devoid of any agonist activity at prejunctional alpha2A-adrenoceptors in the pig, but is an efficacious agonist at alpha2D-adrenoceptors in the rat and guinea-pig.

Gene targeting--homing in on alpha 2-adrenoceptor-subtype function

The alpha-adrenoceptor was subdivided into three subtypes: alpha 2A-, alpha 2B- and alpha 2C-adrenoceptors almost ten years ago. Since then, the search has been on to discover and develop subtype-selective agonists and antagonists, but as yet no major breakthrough has been made. In the past year, several strains of genetically engineered mice have become available, either overexpressing, totally lacking or expressing heavily modified alpha 2-adrenoceptor subtypes. Ewen MacDonald, Brian Kobilka and Mika Scheinin describe how these mice may be utilized to elucidate the physiological functions of the receptor subtypes and the properties of future subtype-selective drugs.

Presynaptic alpha 2-autoreceptors in mouse heart atria: evidence for the alpha 2D subtype

Presynaptic alpha 2-autoreceptors in mouse atria were characterized in terms of the alpha 2A, alpha 2B, alpha 2C and alpha 2D subtypes. Segments of the atria were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. The affinity of up to 16 antagonists for the autoreceptors was assessed as (1) pEC30% values. i.e. concentrations that increased previously autoinhibited release of 3H-noradrenaline (120 pulses, 3 Hz) by 30%, and (2) pKd values against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under conditions of no or little autoinhibition (2 trains of 20 pulses, 50 Hz, train interval 120 s). The pKd values correlated well with the pEC30% values (r = 0.98; P < 0.001; slope of regression line 0.93), indicating that UK 14,304 and released noradrenaline modulated the release of noradrenaline through pharmacologically identical receptors. Comparison with antagonist affinities for (1) prototypic native alpha 2 radioligand binding sites, (2) radioligand binding sites in COS cells transfected with alpha 2 subtype genes, and (3) previously classified presynaptic alpha 2-autoreceptors-all taken from the literature-indicated that the mouse atrial autoreceptors corresponded to the alpha 2D subtype. For example, the pKd values at mouse atrial auto-receptors correlated closely with pKd values at native alpha 2D binding sites in the bovine pineal gland (r = 0.96; P < 0.001); with pKd values at alpha 2D binding sites in COS cells transfected with the rat alpha 2D gene (r = 0.85; P < 0.01); and with pKd values at guinea-pig cerebral and atrial and mouse cerebral alpha 2D-autoreceptors (r = 0.96-0.98; P < 0.001). The antagonist pKd values at mouse atrial autoreceptors correlated less with pKd values at alpha 2A, alpha 2B and alpha 2C sites. It is concluded that the presynaptic alpha 2-autoreceptors in mouse atria are alpha 2D. This identification supports the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D pair of orthologous alpha 2-adrenoceptors.

Determination on functional basis of presynaptic alpha 2-adrenoceptor subtypes in guinea-pig duodenum

The effects of several alpha 2-adrenoceptor agonists and antagonists were examined on the cholinergic twitch contractions evoked by electrical field stimulation of guinea-pig duodenum. Oxymetazoline, xylazine, noradrenaline, alpha-methyl-noradrenaline or medetomidine (0.01-30 microM) were nearly equieffective in inhibiting duodenal twitch responses. The effects of xylazine were competitively counteracted by antagonists tested (0.03-10 microM) with the following order of potency: RX 821002 = idazoxan > rauwolscine = yohimbine = BRL 44408 >> prazosin = ARC 239 = BRL 41992. According to the current classification, it is suggested that alpha 2-heteroadrenoceptors involved in the modulation of duodenal cholinergic neurotransmission belong to the alpha ZD subtype.

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2A-autoreceptors in rabbit atria and kidney

The study was devised to classify, by means of antagonist affinities, the presynaptic alpha 2-autoreceptors of rabbit atria and kidney in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D. A set of antagonists was chosen that was able to discriminate between the four subtypes. Small pieces of the left atrium and slices of the kidney cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. In one series of experiments, tissue pieces were stimulated by relatively long pulse trains (1 or 2 min) leading to alpha 2-autoinhibition. All 11 (atria) or 10 (kidney) antagonists increased the evoked overflow of tritium. pEC30% values (concentrations causing 30% increase) were interpolated from concentration-response curves. In a second series of experiments, tissue pieces were stimulated by brief pulse trains (0.4 s) that did not lead to alpha 2-autoinhibition, and concentration-inhibition curves of the alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) were determined. Most of the 11 (atria) or 8 (kidney) antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. pKd values of the antagonists were calculated from the shifts. pEC30% values correlated with pKd values, both in atria (r = 0.728) and in the kidney (r = 0.930). pEC30% values in atria correlated with pEC30% values in the kidney (r = 0.988) and pKd values in atria correlated with pKd values in kidney (r = 0.923). It is concluded that the alpha 2-autoreceptors in atria and the kidney are the same. Comparison with antagonist affinities for prototypic native alpha 2 binding sites, alpha 2 binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that both autoreceptors are alpha 2A. This conclusion is reached with either of the two independent estimates of autoreceptor affinity, pEC30% and pKd. The results are compatible with the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D branch of the alpha 2-adrenoceptor tree, across mammalian or at least rodent and lagomorph species.