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

Dexmedetomidine stops benzodiazepine-refractory nerve agent-induced status epilepticus

Nerve agents are highly toxic chemicals that pose an imminent threat to soldiers and civilians alike. Nerve agent exposure leads to an increase in acetylcholine within the central nervous system, resulting in development of protracted seizures known as status epilepticus (SE). Currently, benzodiazepines are the standard of care for nerve agent-induced SE, but their efficacy quickly wanes as the time to treatment increases. Here, we examine the role of the α2-adrenoceptor in termination of nerve agent-induced SE using the highly specific agonist dexmedetomidine (DEX). Adult male rats were exposed to soman and entered SE as confirmed by electroencephalograph (EEG). We observed that administration of DEX in combination with the benzodiazepine midazolam (MDZ) 20 or 40 min after the onset of SE stopped seizures and returned processed EEG measurements to baseline levels. The protective effect of DEX was blocked by the α2-adrenoceptor antagonist atipamezole (ATI), but ATI failed to restore seizure activity after it was already halted by DEX in most cases, suggesting that α2-adrenoceptors may be involved in initiating SE cessation rather than merely suppressing seizure activity. Histologically, treatment with DEX + MDZ significantly reduced the number of dying neurons as measured by FluoroJade B in the amygdala, thalamus, and piriform cortex, but did not protect the hippocampus or parietal cortex even when SE was successfully halted. We conclude that DEX serves not just as a valuable potential addition to the anticonvulsant regimen for nerve agent exposure, but also as a tool for dissecting the neural circuitry that drives SE.

Mangiferin, a natural xanthone, accelerates gastrointestinal transit in mice involving cholinergic mechanism

AIM: To investigate the effects of mangiferin on gastrointestinal transit (GIT) in normal and constipated mice, together with the possible mechanism.

METHODS: Intragastrically-administered charcoal meal was used to measure GIT in overnight starved Swiss mice. In the first experiments, mangiferin (3 mg/kg, 10 mg/kg, 30 mg/kg, and 100 mg/kg, po) or tegaserod (1 mg/kg, ip) were administered 30 min before the charcoal meal to study their effects on normal transit. In the second series, mangiferin (30 mg/kg) was tested on delayed GIT induced by several different pharmacological agonists (morphine, clonidine, capsaicin) or antagonists (ondansetron, verapamil, and atropine) whereas in the third series, mangiferin (30 mg/kg, 100 mg/kg and 300 mg/kg) or tegaserod (1 mg/kg) were tested on 6 h fecal pellets outputted by freely fed mice. The ratio of wet to dry weight was calculated and used as a marker of fecal water content.

RESULTS: Mangiferin administered orally significantly (P < 0.05) accelerated GIT at 30 mg/kg and 100 mg/kg (89% and 93%, respectively), similarly to 5-hydroxytryptamine₄ (5-HT₄) agonist tegaserod (81%) when compared to vehicle-treated control (63%). Co-administered mangiferin (30 mg/kg) totally reversed the inhibitory effect of opioid agonist morphine, 5-HT₃-receptor antagonist ondansetron and transient receptor potential vanilloid-1 receptor agonist capsaicin on GIT, but only to a partial extent with the GIT-delay induced by α₂-adrenoceptor agonist clonidine, and calcium antagonist verapamil. However, co-administered atropine completely blocked the stimulant effect of mangiferin on GIT, suggesting the involvement of muscarinic acetylcholine receptor activation. Although mangiferin significantly enhanced the 6 h fecal output at higher doses (245.5 ± 10.43 mg vs 161.9 ± 10.82 mg and 227.1 ± 20.11 mg vs 161.9 ± 10.82 mg of vehicle-treated control, at 30 and 100 mg/kg, P < 0.05, respectively), the effect of tegaserod was more potent (297.4 ± 7.42 mg vs 161.9 ± 10.82 mg of vehicle-treated control, P < 0.05). Unlike tegaserod, which showed an enhanced water content in fecal pellets (59.20% ± 1.09% vs 51.44% ± 1.19% of control, P < 0.05), mangiferin evidenced no such effect, indicating that it has only a motor and not a secretomotor effect.

CONCLUSION: Our data indicate the prokinetic action of mangiferin. It can stimulate the normal GIT and also overcome the drug-induced transit delay, via a cholinergic physiological mechanism.

Effects of clonidine and sumatriptan on postprandial gastric volume response, antral contraction waves and emptying: an MRI study

Gastric emptying (GE) may be driven by tonic contraction of the stomach ('pressure pump') or antral contraction waves (ACW) ('peristaltic pump'). The mechanism underlying GE was studied by contrasting the effects of clonidine (alpha(2)-adrenergic agonist) and sumatriptan (5-HT(1) agonist) on gastric function. Magnetic resonance imaging provided non-invasive assessment of gastric volume responses, ACW and GE in nine healthy volunteers. Investigations were performed in the right decubitus position after ingestion of 500 mL of 10% glucose (200 kcal) under placebo [0.9% NaCl intravenous (IV) and subcutaneous (SC)], clonidine [0.01 mg min(-1) IV, max 0.1 mg (placebo SC)] or sumatriptan [6 mg SC (placebo IV)]. Total gastric volume (TGV) and gastric content volume (GCV) were assessed every 5 min for 90 min, interspersed with dynamic scan sequences to measure ACW activity. During gastric filling, TGV increased with GCV indicating that meal volume dictates initial relaxation. Gastric contents volume continued to increase over the early postprandial period due to gastric secretion surpassing initial gastric emptying. Clonidine diminished this early increase in GCV, reduced gastric relaxation, decreased ACW frequency compared with placebo. Gastric emptying (GE) rate increased. Sumatriptan had no effect on initial GCV, but prolonged gastric relaxation and disrupted ACW activity. Gastric emptying was delayed. There was a negative correlation between gastric relaxation and GE rate (r(2 )=49%, P < 0.001), whereas the association between ACW frequency and GE rate was inconsistent and weak (r2=15%, P = 0.05). These findings support the hypothesis that nutrient liquid emptying is primarily driven by the 'pressure pump' mechanism.

Distribution of mRNA that codes for subtypes of adrenergic receptors in the gastrointestinal tract of dairy cows

OBJECTIVE

To describe the distribution of mRNA that codes for 9 subtypes of adrenergic receptors in the digestive tract of dairy cows.

SAMPLE POPULATION

Fresh full-thickness wall specimens from the abomasum (fundus, corpus, and antrum), ileum, cecum, proximal loop of ascending colon, and 4 locations of the spiral colon collected from 10 healthy cows at slaughter.

PROCEDURE

Concentrations of mRNA that code for 9 subtypes of adrenergic receptors in the bovine gastrointestinal tract (alpha1A, alpha1B, alpha1D, alpha2AD, alpha2B, alpha2C beta1, beta2, and beta3) were measured by use of a quantitative real-time reverse transcription-polymerase chain reaction assay. Results were reported in relation to mRNA expression of the housekeeping gene glyceraldehyde phosphate dehydrogenase (GAPDH).

RESULTS

Mean mRNA contents of adrenergic receptors in the bovine digestive tract were low (range, 0.00006% to 5.04% of GAPDH). Distribution of receptor subtypes was similar in all tissues, with lowest expression of alpha1D receptors, followed by alpha2B, alpha2C, beta3, alpha1B, alpha1A, beta1, and beta2 in the abomasum, whereas alpha2AD and beta2 in the intestines were highest. In comparison with the intestines, relative concentrations of mRNA for receptors beta2 and beta3 were significantly lower in the abomasum.

CONCLUSIONS AND CLINICAL RELEVANCE

Relative concentrations of mRNA that code for adrenergic receptors differed among receptor subtypes and among locations in the bovine gastrointestinal tract. Comparison of these values established in healthy cattle with results for cows with motility disorders, such as abomasal displacement and cecal dilatation, may lead to improved therapeutic or prophylactic approaches for these diseases.

Restraint stress delays solid gastric emptying via a central CRF and peripheral sympathetic neuron in rats

Central corticotropin-releasing factor (CRF) delays gastric emptying through the autonomic nervous system. CRF plays an important role in mediating delayed gastric emptying induced by stress. However, it is not clear whether a sympathetic or parasympathetic pathway is involved in the mechanism of central CRF-induced inhibition of solid gastric emptying. The purpose of this study was to investigate whether 1) CRF inhibits solid gastric emptying via a peripheral sympathetic pathway and 2) stress-induced inhibition of solid gastric emptying is mediated via a central CRF and peripheral sympathetic pathways. Using male Sprague-Dawley rats, CRF was injected intracisternally with or without various adrenergic-blocking agents. To investigate whether central CRF-induced inhibition of solid gastric emptying is mediated via a peripheral sympathetic pathway, rats underwent celiac ganglionectomy 1 wk before the gastric emptying study. After solid meal ingestion (90 min), gastric emptying was calculated. To investigate the role of endogenous CRF in stress-induced delayed gastric emptying, a CRF type2 receptor antagonist, astressin2-B, was intracisternally administered. Rats were subjected to a restraint stress immediately after the feeding. Intracisternal injection of CRF (0.1-1.0 microg) dose-dependently inhibited solid gastric emptying. The inhibitory effect of CRF on solid gastric emptying was significantly blocked by guanethidine, propranolol, and celiac ganglionectomy but not by phentolamine. Restraint stress significantly delayed solid gastric emptying, which was improved by astressin2-B, guanethidine, and celiac ganglionectomy. Our research suggests that restraint stress inhibits solid gastric emptying via a central CRF type2 receptor and peripheral sympathetic neural pathway in rats.

Alpha2-adrenergic regulation of NO production alters postoperative intestinal smooth muscle dysfunction in rodents

Alpha2-adrenergic receptor activation plays an important role in the development of postoperative ileus. Alpha2-adrenergic receptors also regulate nitric oxide (NO) production by the mononuclear phagocyte system. We have previously shown that intestinal manipulation leads to a significant increase in NO production by infiltrating monocytes within the intestinal muscularis. The purpose of this study was to investigate whether alpha2-adrenergic blockade with yohimbine would alter postsurgical intestinal smooth muscle dysfunction and NO production by infiltrating monocytes and macrophages within the intestinal muscularis. Rats underwent small bowel intestinal manipulation with or without yohimbine. In vivo gastrointestinal transit and in vitro jejunal circular muscle contractility was measured 24 h postoperatively. RT-PCR was used to detect inducible NO synthase (iNOS) expression. NO levels in tissue culture supernatants were measured. Immunohistochemistry was used to localize alpha2-adrenergic receptor expression in the intestinal muscularis. Yohimbine significantly decreased manipulation-induced delay in gastrointestinal transit and reversed the postoperative decrease in intestinal muscle contractility. Intestinal manipulation resulted in significant iNOS mRNA induction in the intestinal muscularis, which was markedly attenuated after yohimbine treatment. Yohimbine also significantly decreased the postoperative increase in NO released into intestinal muscularis tissue culture supernatant. Immunohistochemistry identified alpha2-adrenergic receptors on monocytes recruited postoperatively into the intestinal muscularis. This study demonstrates that alpha2-adrenergic receptor stimulation of the inflamed postoperative intestinal muscularis plays a significant role in aggravating postoperative ileus through an enhanced induction of iNOS mRNA and increased release of NO from manipulated intestinal muscularis.

Effect of clonidine on colonic motility in rats

Clonidine, an alpha(2)-adrenoceptor agonist, has been reported to inhibit gastric and small intestinal motility in rats. Whether clonidine also inhibits colonic motility is still not clear. The aim of this study was to examine the effect of clonidine on colonic motility and its possible site of action in adult Wistar rats. Colonic motilities in anesthetized rats in vivo or motilities of the isolated colon of rats in vitro were recorded. Clonidine was administered intravenously (i.v.) and intracerebroventricularly (i.c.v.) in vivo while bath administration was used in in vitro study. Clonidine i.v. or i.c.v. significantly inhibited colonic motility. This inhibitory effect was antagonized by pre-administration of yohimbine, an alpha(2)-adrenoceptor antagonist, but not by pre-administration of prazosin, an alpha(1)-adrenoceptor antagonist. Also, we have unpublished data indicating that the sympathectomy antagonized the inhibitory effect of systemically administered clonidine. A significant depression of colonic motility on the isolated colon was induced by bath administration of noradrenaline, while no such inhibition was seen by clonidine. The results of the present study suggested that clonidine inhibits colonic motility in rats through activation of central alpha(2)-adrenergic receptor.

Induction of neuronal nitric oxide synthase by sympathetic denervation is mediated via alpha 2-adrenoceptors in the jejunal myenteric plexus

Nitric oxide (NO) is an important nonadrenergic, non-cholinergic (NANC) inhibitory neurotransmitter in the gastrointestinal tract. In previous studies, neuronal nitric oxide synthase (nNOS) in the jejunal myenteric plexus, a key enzyme responsible for the release of NO, has been demonstrated to increase after splanchinic ganglionectomy (sympathetic nerve transection). The alpha2-adrenoceptor is known to be one of the most important receptors which controls intestinal motility. In the present study, we examined the effect of application of the alpha2-adrenoceptor agonist, clonidine hydrochloride, on nNOS expression in the rat jejunal myenteric plexus after splanchinic ganglionectomy. Clonidine (0.1-1 mg/kg, i.p.) or saline was administered for 5 days after the splanchinic ganglionectomy. The nNOS expression and nNOS mRNA were detected by immunohistochemistry and in situ hybridization for nNOS mRNA, respectively. In the rats treated with vehicle after the splanchinic ganglionectomy, nNOS expression in the myenteric plexus significantly increased compared with sham-operated rats. The increases in nNOS protein and mRNA after splanchinic ganglionectomy were significantly reversed by clonidine treatment. Clonidine-treated naive rats showed no difference in nNOS expression compared with sham-operated rats. These data suggest that nNOS expression in the jejunal myenteric plexus after splanchinic ganglionectomy is regulated by the alpha2-adrenoceptor and that the alpha2-adrenoceptor may play an important role in abnormal intestinal motility following splanchinic ganglionectomy in rat jejunum.

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

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

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role

Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

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.

The mechanistic pathway of gastric adaptive cytoprotection: a study on different components of the autonomic nervous system

1. The involvement of different components of the autonomic nervous system in the pathogenesis of ethanol-induced damage and the adaptive cytoprotection of mild irritants were studied in the gastric mucosa of male rats. 2. Capsaicin, yohimbine, and domperidone aggravated the 100% ethanol-induced gastric mucosal damage and attenuated the cytoprotective action of 20% ethanol, but not of 5% NaCl and 0.3 M HCl. Butoxamine and prazosin blocked the adverse actions of yohimbine and domperidone respectively. 3. Atropine, pirenzepine, and lidocaine lessened the severity of 100% ethanol-induced mucosal injury and further increased the cytoprotective action of 5% NaCl and 0.3 M HCl, but not of 20% ethanol. 4. Our results demonstrated that sensory afferent neurones, alpha 2-adrenoceptors and D2-dopaminergic receptors all play a significant role in the defensive mechanism of the gastric mucosa and the adaptive cytoprotection of 20% ethanol, while the M1- and M2-muscarinic receptors and sensory chemoreceptors on the gastric mucosa contribute only to the former action. The adverse effect of yohimbine and domperidone on lesion formation is probably mediated through the release of catecholamines, which subsequently act on the beta 2- and alpha 1-adrenoceptors respectively.

Regulation of gastrin release in the dog by alpha 2-adrenoceptors

1. The purpose of the present study was to analyse the effects of the alpha 2-adrenoceptor agonist medetomidine and the antagonist yohimbine on gastrin release in conscious dogs. 2. Gastrin secretion was investigated under both basal conditions and stimulation by 2-deoxy-D-glucose, food or bombesin. 3. Basal gastrin under fasting conditions was significantly reduced by medetomidine and increased by yohimbine. 4. 2-deoxy-D-glucose-induced gastrin increase was fully inhibited by medetomidine; this effect was antagonized by yohimbine. 5. Medetomidine significantly inhibited food-induced increase in plasma gastrin; under these conditions yohimbine was without effect per se, but significantly antagonized the inhibitory action of medetomidine. 6. Gastrin release induced by bombesin was not affected by medetomidine or yohimbine. 7. These results suggest that alpha 2-adrenoceptors play an inhibitory role under conditions in which gastrin release is mainly mediated through cholinergic and non-cholinergic nervous pathways; in contrast, they do not indicate the presence of alpha 2-adrenoceptors on G cells of the dog stomach.

Characterization and distribution of alpha 2-adrenergic receptors in the human intestinal mucosa

The subtype and the expression of the alpha 2-adrenergic receptor were investigated in the normal mucosa from human intestine by means of radioligand binding, RNase mapping, and measurement of adenylate cyclase activity. The study of the binding of the alpha 2-adrenergic antagonist, [3H]RX821002, to epithelial cell membranes indicated the existence of a single class of noninteracting sites displaying a high affinity for the radioligand (Kd = 1.1 +/- 0.5 nM). The rank order of potency of antagonists to inhibit [3H]RX821002 binding (RX821002 > yohimbine = rauwolscine > phentolamine approximately idazoxan >> chlorpromazine > prazosin) suggested that the receptor is of the alpha 2A subtype. A conclusion which is confirmed by the fact that only alpha 2C10 transcripts were found in the human intestine mucosa. Competition curves with (-)-norepinephrine demonstrated that 60% of the receptor population exhibited high affinity for agonists. This high-affinity state was abolished by the addition of GTP plus Na+ or by prior treatment of the membranes with pertussis toxin indicating it corresponded to G protein-coupled receptors. [32P]ADP-ribosylation and immunoblotting experiments identified two pertussis toxin-sensitive G proteins corresponding to Gi2 and Gi3. The study of the distribution of the receptor indicated that (a) the proximal colon is the intestine segment exhibiting the highest receptor density and (b) the receptor is predominantly expressed in crypts and is preferentially located in the basolateral membrane of the polarized cell. The distribution of the receptor along the crypt-surface axis of the colon mucosa can be correlated with a higher level of alpha 2C10-specific mRNA and a higher efficiency of UK14304 to inhibit adenylate cyclase in crypt cells.

Involvement of alpha 2-adrenoceptors in mechanism of intragastric nicotine protection against ethanol injury in rat stomach

To elucidate the role of alpha- and beta-adrenoceptors in the mechanism of intragastric nicotine protection against ethanol-induced gastric mucosal injury, the following studies were performed. At 0.5-hr prior to the injury study, rats were pretreated with: subcutaneous control, prazosin (0.5 mg/kg) or yohimbine (5 mg/kg) to block alpha 1- or alpha 2-adrenoceptors; or intraperitoneal control, metoprolol (2 mg/kg) or butoxamine (4 mg/kg) to block beta 1- or beta 2-adrenoceptors, respectively. At 1-hr intervals, rats received intragastric vehicle or nicotine (4 mg/kg) and 40% ethanol (10 ml/kg). Total lengths of the linear gastric corpus mucosal lesions were measured by an unbiased observer using a caliper. In a separate study, 0.5-hr after subcutaneous control or yohimbine (5 mg/kg), rats were treated with intragastric vehicle or nicotine (4 mg/kg). One hour later, gastric mucus volume, gastric juice volume and pH, and titratable acid in the gastric juice were measured. In the rat stomach, the intragastric nicotine protection against 40% ethanol-induced mucosal injury was not blocked by selective alpha 1-(prazosin), beta 1-(metoprolol), or beta 2-(butoxamine) adrenoceptor antagonists. The protection was significantly reduced although not completely abolished by selective alpha 2-(yohimbine) adrenoceptor antagonist. Yohimbine also significantly reduced basal and nicotine-stimulated increase in gastric mucus volume. These data suggest that alpha 2-adrenoceptors are involved in the protective effect of intragastric nicotine against 40% ethanol-induced gastric mucosal injury possibly by a mucus-dependent mechanism.

Inhibition of intestinal motility and reversal of postlaparotomy ileus by selective alpha 2-adrenergic drugs in the rat

BACKGROUND

The effects of selective alpha-agonist medetomidine and alpha 2-antagonist atipamezole on gastrointestinal motility were studied.

METHODS

The passage of intragastrically administered Evans blue in the small bowel of unanesthetized rats was followed, and the stomachs were weighted after killing the rats.

RESULTS

Subcutaneous medetomidine, 0.01-0.1 mg/kg, was found to delay small intestinal transit but not gastric emptying, with a maximal effect seen at 0.03 mg/kg. Atipamezole fully reversed the effect of 0.1 mg/kg of medetomidine with a dose of 2.5 mg/kg. Atipamezole alone did not affect small intestinal transit. Subcutaneous morphine, 6 mg/kg, delayed gastric emptying and small intestinal transit, whereas intraperitoneal morphine only delayed gastric emptying. Subcutaneous atipamezole, 0.06 mg/kg, was partially able to reverse the delayed intestinal transit but did not inhibit morphine-induced gastric retention. Subcutaneous atipamezole, 0.06 mg/kg, reversed laparotomy-induced ileus completely.

CONCLUSIONS

Atipamezole may provide a useful treatment for postlaparotomy ileus.

The effect of alpha 2 agonists and antagonists on the lower urinary tract of the rat

The pharmacologic potential of selective alpha 2 adrenergic agents in modulating vesicourethral function was evaluated in urethane-anesthetized and conscious rats. The bladder was exposed through a midline incision and intubated via a cystostomy to measure pressure. The bladder was filled with saline at 0.05 ml./min. In six animals intravenous dexmedetomidine 0.6 micrograms./kg. followed by 2 micrograms./kg. was given. In another six rats the infusion was made with atipamezole at the same concentration. The parameters obtained were pressure/volume during cystometry, detrusor voiding pressures, and evidence of urine leakage. Conscious rats were placed in a restrainer outfitted with a voided volume sensor and a data recorder. The animals were divided in three groups. In controls, group I (n = 8), the animals were injected with a subcutaneous (SC) load of 5 ml. saline in which furosemide 10.0 mg./kg. was added. In group II (n = 6), dexmedetomidine was added in group III (n = 6), atipamezole was added. Parameters obtained were volume per micturition, latency, frequency of micturition, and patterns of micturition (such as dribbling), and total volume produced after a three hour recording. Values are expressed as mean +/- standard deviation. The results show that in an anesthetized preparation, the alpha 2 agonist dexmedetomidine produced inhibitory effects on the volume evoked micturition reflex (VEMR) at a dose level of 2.0 micrograms./kg. There was a significant decrease in peak pressure from 26.4 +/- 3.6 to 6.8 +/- 2.1 cm. H2O (p < 0.01). Urine leakage was observed at the external meatus. Atipamezole, the alpha 2 agonist, gradually increased bladder baseline pressure and inhibited VEMR. At a critical pressure there was continuous leakage from the external meatus. There was a significant elevation of baseline bladder pressure by atipamezole from 6.3 +/- 1.7 to 29.8 +/- 4.6 cm. H2O (p < 0.005). In conscious rats, the alpha 2 agonist produced urinary dribbling at approximately 20 minutes after SC injection, while the antagonists produced delayed dribbling 70 minutes after SC injection. Leakage occurred in 3 of 6 rats at a dose of 10 micrograms./kg. of atipamezole and in 5 of 6 rats at a dose of 30 micrograms./kg. The alpha 2 agonist caused a diuretic effect and a dose-dependent increase in the frequency of voiding (p < 0.05), while the antagonist decreased the frequency of voiding. Bladder capacity was decreased with the alpha 2 agonist while the antagonist increased capacity.

The effect of some alpha 2-adrenoceptor agonists and antagonists on gastrointestinal transit in mice: influence of morphine, castor oil and glucose

1. The effects of graded doses of the alpha 2-adrenoceptor agonists clonidine, tizanidine and BHT-920, and the alpha 2-adrenoceptor antagonists yohimbine and idazoxan, on gastrointestinal transit were investigated in mice using the charcoal meal test. 2. The agonists produced significant and dose-dependent decreases in gastrointestinal transit, and the antagonists produced the opposite effect. In affecting the gastrointestinal transit, clonidine (1 mg/kg) was as effective as tizanidine (12 mg/kg) and BHT-920 (40 mg/kg), while yohimbine (2 mg/kg) was as effective as idazoxan (1 mg/kg). 3. Morphine (2, 4 and 8 mg/kg) significantly inhibited gastrointestinal transit. This effect was significantly reversed by the co-administration of yohimbine (2 mg/kg) and idazoxan (1 mg/kg). 4. The acute administration of glucose (5.04 g/kg, i.p.) potentiated the inhibition of gastrointestinal transit produced by clonidine (1 mg/kg) and BHT-920 (40 mg/kg). Glucose treatment, however, had no significant effect on the increase in gastrointestinal transit induced by yohimbine (2 mg/kg) or idazoxan (1 mg/kg). 5. Castor oil (0.25 mL/mouse, orally) induced diarrhoea in saline-treated animals within about 45 min. Clonidine (1 mg/kg), tizanidine (12 mg/kg) and BHT-920 (40 mg/kg) delayed the occurrence of diarrhoea to 2.1, 1.2 and 1.4 h, respectively.

Stimulation of faecal excretion in rats by alpha 2-adrenergic antagonists

The effects of several alpha-adrenoceptor antagonists on faecal output and water content in rats were investigated. Fed rats were treated either subcutaneously (s.c.) or orally with phentolamine, idazoxan, yohimbine, 1-(2-pyrimidinyl) piperazine (PmP) or prazosin. Drug potencies were compared on the basis of the dose inducing excretion of 1 g dry weight of faeces (AD1) by rats that do not normally excrete any faecal pellet during the observation time. The alpha 2-antagonist, idazoxan (AD1 = 0.25 mg kg-1, s.c.) was approximately 2.5, 4 and 8 times more potent than PmP, phentolamine and yohimbine in promoting faecal excretion. Prazosin, an alpha 1-antagonist with putative affinity for the alpha 2B-receptor subtype, was the least effective (AD1 greater than 5 mg kg-1, s.c.). The same compounds also increased the water content of faeces and had similar potencies by the oral route. Both clonidine (0.15 mg kg-1, s.c.) and atropine (0.2 mg kg-1, s.c.) significantly prevented the effects of all antagonists on faecal excretion. The present results are consistent with the view that rat colon is under tonic inhibitory control of prejunctional alpha 2-adrenergic receptors, whose blockage by specific antagonists induces faecal excretion. The alpha 2A-receptor subtype appears to be the most likely candidate for controlling faecal excretion through inhibition of acetylcholine release.

Effect of adenosine analogues on the expression of opiate withdrawal in rats

The aim of this study was to test whether convergent dependence occurs in vivo. The adenosine A1 receptor agonist N6-[(R)-1-methyl-2-phenylethyl]adenosine (R-PIA), the A2 agonist 2-(phenylamino)adenosine (CV-1808), the nonselective A1, A2 agonist (adenosine-5'-ethylcarboxamide (NECA), and the alpha 2-adrenoceptor agonist clonidine were screened (each at 30, 100, and 300 micrograms/kg, SC) for their ability to alter naloxine-precipitated withdrawal signs in morphine-dependent rats. The results indicate that there is convergent dependence involving opioid and adenosine A1 receptors on those effects expressed by withdrawal diarrhoea, paw-shakes, teeth-chattering, body-shakes, and jumping. Further, dependence expressed by body-shakes involves convergence involving A1 receptors, as well as alpha 2-adrenoceptors; while A1 receptors are involved in dependence expressed by jumping, stimulation of alpha 2-adrenoceptors augments this sign. Adenosine analogues may be of clinical value for detoxification of opiate addicts.

Cecectomized rat. A model of experimental secretory diarrhea in conscious animals

Evaluation of antisecretory antidiarrheal agents in animal models is limited primarily to extrapolations of efficacy from enteropooling studies in vivo, isolated intestinal loops in situ, and Ussing flux chamber preparations in vitro. While these standard techniques are useful, they do not mimic secretory diarrhea. Our studies indicate that in rats, the cecum may serve a "reservoir" function in response to secretagogue administration. Thus, diarrhea is not observed consistently and reliably in this species to allow valid evaluation of potential antidiarrheal agents. Therefore, we have developed a reproducible model of secretory diarrhea utilizing conscious cecectomized rats by surgical resection of the cecum, without compromising ileocecal patency, and by the use of potent intestinal secretagogues. Animals quickly recover and maintain normal growth and other physiologic parameters for as long as 60 days. After 48 hr on standard chow, secretory diarrhea can be induced by oral administration of standard intestinal secretagogues (dimethyl prostaglandin E2, cholera toxin, or carbachol). Dimethyl prostaglandin E2 (300 micrograms/kg, p.o.) induces diarrhea within 1 hr that continues for approximately 3.5 hr. Oral administration of known antidiarrheal agents chlorpromazine (10 mg/kg), clonidine (1 mg/kg), or morphine (10 mg/kg) all significantly reduce fecal output within 30-60 min following administration. These studies indicate that in the rat, the cecum may serve as a fluid reservoir during periods of small intestinal hypersecretion and that the cecectomized rat serves as a useful, accurate, and reliable tool for evaluating new compounds with proposed antidiarrheal activity.

Streptozotocin-diabetes attenuates alpha 2-adrenoceptor agonist-induced delay in small intestinal transit in mice

1. The effect of alpha 2-adrenoceptor agonists on gastrointestinal motility was assessed in normoglycaemic and streptozotocin-diabetic mice. 2. The alpha 2-adrenoceptor agonists used were: clonidine (0.1, 0.3 and 1 mg kg-1, azepexole (10, 20 and 40 mg kg-1), tizanidine (1, 3 and 10 mg kg-1) and ST-91 (10, 20 and 30 mg kg-1). 3. Acute hyperglycaemia was induced by D-(+)-glucose (5 g kg-1) and chronic hyperglycaemia by streptozotocin (200 mg kg-1) injection. 4. The gut motility was quantitated using the charcoal meal test. 5. The results indicate that in normoglycaemic and acutely hyperglycaemic mice, all of the alpha 2-adrenoceptor agonists used produced significant inhibition of meal transit. 6. However, in streptozotocin-diabetic mice, the anti-transit effect of alpha 2-adrenoceptor agonists was attenuated. 7. Since streptozotocin-induced diabetes but not acute hyperglycaemia was associated with the attenuation of anti-transit effect, elevated blood sugar is not the mechanism for the observed effect. 8. As with groups treated with clonidine, azepexole or tizanidine, the anti-transit effect of a peripherally acting alpha 2-adrenoceptor agonist, ST-91, was attenuated in streptozotocin-diabetic mice. This suggests the involvement of peripheral mechanism(s) in attenuating the anti-transit effect of alpha 2-adrenoceptor agonists. 9. These results identify the need for critical evaluation of the role and efficacy of alpha 2-adrenoceptor agonists in the therapeutic management of diabetic diarrhoea.

Cholinergic and adrenergic control of enzyme secretion in isolated rat pancreas

While cholinergic nervous control of pancreatic enzyme secretion is well documented, data concerning adrenergic regulation of the exocrine pancreas are contradictory. In the present study we attempted to elucidate the direct action of adrenergic stimulation on pancreatic enzyme secretion. Rat pancreatic segments were set up in an organ bath and superfused with modified Krebs-Henseleit solution. Electrical field stimulation (EFS) stimulated amylase release from the segments. This stimulation was subject to inhibition with atropine up to 80%. Atropine-resistant enzyme discharge in response to EFS could be blocked by propranolol. Cholinergic agonist urecholine-induced amylase release was completely blocked by atropine. Noradrenaline (NA) exhibited a biphasic effect on amylase release. It inhibited the urecholine-induced amylase release in lower concentrations (10(-8)-10(-7) M), while it stimulated basal enzyme secretion in higher concentrations (10(-5)-10(-4) M). The inhibitory effect was mimicked by phenylephrine and completely prevented by prazosin. Isoprenaline concentration dependently enhanced, while clonidine and guanfacine did not affect amylase discharge. In conclusion, in rat pancreatic acinar tissue it seems likely that acetylcholine is the main neurotransmitter. Adrenergic action can be dual, inhibitory via alpha 1-adrenoceptors or stimulatory via beta-adrenoceptors on amylase secretion.

Rational pharmacotherapy of gastrointestinal motility disorders

Nervous control of gastrointestinal motility is extremely complex, is regulated by the enteric system, the "brain of the gut", and modulated by extrinsic nerves. This system with its multiplicity of transmitters and receptors does not always allow a clear interpretation of experimental data, especially with compounds lacking specificity. In this review the complex situation is described particularly in relation to receptor populations (cholinergic, adrenergic, dopamine, histamine, 5-hydroxytryptamine, opioid, gamma-aminobutyric acid (GABA), prostanoid and dihydropyridine receptors), therapeutic aspects of drugs and their usefulness in children. Newer principles with known drugs and promising new compounds with a more appropriate kinetic or fewer side-effects, deriving from distinct pharmacological groups, as candidates for the treatment of gastrointestinal disorders are considered e.g. anticholinergics (prifinium or actilonium bromide), adrenergic alpha 2-agonists (clonidine, lidamidine) for diarrhoea in diabetic neuropathy, adrenergic beta-blockers for shortening postoperative ileus (propranolol), dopamine receptor antagonists (metoclopramide, domperidone, alizapride) and another prokinetic substance (cisapride) which may be useful for a number of applications as gastro-oesophageal reflux, gastro-paresis, intestinal pseudo-obstruction, cystic fibrosis and constipation, morphine derivatives (e.g. loperamide) for intractable diarrhoea and calcium antagonists (e.g. nifedipine) for achalasia. Increasing experience in digestive tract pharmacology and reliable clinical studies will furthermore be the basis for a more specific and better tolerated therapy of gastrointestinal motility disorders in adults and children.

Double-blind study of an alpha 2 agonist in the treatment of irritable bowel syndrome

A double-blind crossover trial of the alpha 2 agonist lidamidine hydrochloride in 72 patients with irritable bowel syndrome is reported. Lidamidine was found to have no significant effect on frequency and severity of abdominal pain or abdominal bloating. It did cause a statistically significant reduction in frequency of defaecation (P = 0.005), but this was of a degree unlikely to be of clinical importance. Although alpha 2 agonists inhibit gastrointestinal motility in animals this study suggests that lidamidine hydrochloride does not have a useful therapeutic role in irritable bowel syndrome.

Sites of clonidine action to inhibit gut propulsion in mice: demonstration of a central component

The role of central (supraspinal and spinal) and peripheral alpha-adrenoceptors in the regulation of gastrointestinal propulsion in the mouse was studied using clonidine, an alpha 2-adrenoceptor agonist. Clonidine produced a dose-dependent inhibition of propulsion when given intracerebroventricularly, intrathecally, or subcutaneously, but was most potent when given intracerebroventricularly. The antitransit effects of centrally given clonidine were antagonized by intracerebroventricular (i.c.v.) yohimbine, but higher doses were required when this antagonist was given peripherally. Whereas i.c.v. and s.c. administration of clonidine were effective in inhibiting gut transit in spinally transected mice, intrathecal (i.th.) administration of this agonist was not. A supraspinal site of clonidine action is suggested based upon (a) the higher central to peripheral potency of clonidine; (b) the greater potency of i.c.v., compared with s.c., administration of yohimbine in blocking i.c.v. clonidine; (c) the lack of effect of i.th. administration of clonidine in spinally transected mice; and (d) the reduced potency of i.c.v., but not s.c., administration of clonidine in spinally transected mice. Additionally, a peripheral site of clonidine action is suggested by (a) the lower potency of i.c.v. yohimbine in blocking s.c., compared with i.c.v., clonidine; (b) the lower potency of i.c.v. yohimbine in blocking i.c.v. clonidine in transected mice (compared with normal mice); (c) the equal potency of s.c. clonidine in slowing propulsion in normal and spinally transected mice; and (d) the equal potency of i.c.v. yohimbine in blocking s.c. clonidine in normal and spinally transected mice. These data in mice would thus support the concept that normal (peripheral) therapeutic administration of clonidine would affect gut motor function by interactions within the brain and directly at the level of the gut.

A double-blind crossover comparison of lidamidine, loperamide and placebo for the control of chronic diarrhoea

Fourteen patients with chronic diarrhoea, but no evidence of active organic disease, completed a double-blind crossover comparison of the anti-diarrhoeal effects of loperamide, placebo and the clonidine analogue, lidamidine. Failure of diarrhoea control led to early withdrawals from seven placebo- and six lidamidine-treatment periods, but there was only one early withdrawal during treatment with loperamide. Loperamide was found to be superior to lidamidine or placebo for the control of stool consistency in patients with chronic diarrhoea.

Central and peripheral inhibition of exocrine pancreatic secretion by alpha-2 adrenergic agonists in the rat

The effect of ST91, a clonidine derivative crossing poorly the blood-brain barrier, was compared to that of clonidine on exocrine pancreatic secretion in rats. The experiments were performed in anaesthetized rats after stimulation by a maximal dose of 2-deoxy-D-glucose, and in conscious rats under basal interdigestive conditions. In anaesthetized rats, the 2-deoxy-D-glucose-induced stimulation of pancreatic secretion was suppressed by clonidine but not by ST91, both injected subcutaneously. This effect of clonidine was not antagonized by prazosin, but was decreased by 70-100% (according to the variables measured) by yohimbine. The alpha-2 antagonists rauwolscine and corynanthine were less efficient than yohimbine, while idazoxan suppressed totally the effect of clonidine. In conscious rats, the basal interdigestive secretion was inhibited by ST91 and by clonidine. After sc injections, the potency of ST91 was about ten times smaller than that of clonidine, whereas after injections in the cerebral ventricles, ST91 was as potent as clonidine to inhibit pancreatic secretion. Most (70-90%) of the inhibition induced by sc ST91 and clonidine in conscious rats was suppressed by yohimbine or by prazosin. It is concluded that both ST91 and clonidine inhibit pancreatic secretion in rats, and that this effect has probably both central and peripheral components. The central effect involves alpha-2 receptors, while the peripheral effect may involve alpha-1 and alpha-2 receptors.

The effect of two alpha 2-adrenoreceptor agonists and an antagonist on gastric emptying and mouth to caecum transit time in humans

Experiments were performed to investigate the effect of two alpha 2-adrenoreceptor agonists, clonidine and lidamidine, and a specific alpha 2-adrenoreceptor antagonist, idazoxan, on gastric emptying and mouth to caecum transit time (MCTT) of a radiolabelled meal in 27 healthy male subjects. Lidamidine (20 mg p.o.) and clonidine (0.3 mg p.o.), given alone had no significant effect on gastric emptying or MCTT suggesting that the anti-diarrhoeal action of clonidine and lidamidine are unlikely to be explained by a slowing of small intestinal transit. Idazoxan (20 mg p.o.) reversed the effect of clonidine in 10 subjects, who showed a delay in MCTT after taking clonidine, but did not alter MCTT under basal conditions. These results suggest that although the sympathetic nervous system can influence upper gastrointestinal motility by an action on alpha 2-adrenoreceptors, this action does not exert a tonic influence on upper gastrointestinal motility under basal conditions.

Lidamidine inhibits intrinsic contractile patterns of the rat proximal colon

Lidamidine is a clinically effective antidiarrheal agent that inhibits intestinal secretion, reduces intestinal transit, and inhibits smooth muscle contraction. Yet, its specific effects upon colonic motility have not been thoroughly examined. The purpose of our studies was to examine lidamidine's inhibitory effects upon colonic contractile patterns in the rat and identify the responsible receptor mechanism. Fasted male rats were anesthetised and equipped with an intraluminal cannula positioned at the proximal end of a 10 cm fluid-filled segment of the ascending colon (basal pressure, 10 cm H2O). Intraluminal pressure was monitored by a transducer attached to a closed fluid-filled system. All drugs were administered intravenously by slow infusion. A regular pattern of distinct contractile complexes was observed over a 70 min period. These contractions increased intraluminal pressure to 39 +/- 1.2 cm H2O (mean +/- S.E.), occurred at a frequency of 0.3 per min and lasted from 1 to 2.5 min. Inhibition of these contractile patterns was observed with either atropine (0.1 mg/kg) or lidamidine (3.0 mg/kg). A 20 min pretreatment with idazoxan (3.0 mg/kg) antagonized lidamidine's but not atropine's effect. Trimazosin (1 mg/kg) or propranolol (0.3 mg/kg) pretreatment did not antagonize the lidamidine-generated inhibition. These results indicate that lidamidine inhibits an intrinsically generated, cholinergically controlled pattern of colonic contractions primarily by an alpha 2-receptor-mediated mechanism.

Neuropeptide Y inhibits excitatory enteric neurons supplying the circular muscle of the guinea pig small intestine

The effect of neuropeptide Y on the motor activity of the circular and longitudinal muscle of the guinea pig small intestine was investigated. Neuropeptide Y (0.3-30 nM) inhibited, in a concentration-dependent manner, the ascending enteric reflex contraction of the circular muscle; both the cholinergic and noncholinergic component of the ascending enteric reflex were suppressed. Neuropeptide Y also blocked nerve-mediated phasic contractions of the circular muscle, induced by dimethylphenylpiperazinium or ceruletide, and abolished the peristaltic movements of the small intestine. Nerve-mediated cholinergic and noncholinergic contractions of the longitudinal muscle, induced by electrical field stimulation, were only slightly depressed by neuropeptide Y. Further analysis showed that neuropeptide Y did not act directly on intestinal muscle but interrupted excitatory pathways of the enteric nervous system. The effect of neuropeptide Y did not involve activation of alpha-adrenoceptors, opioid receptors, or P1 purinoceptors or the release of endogenous factors stimulating these receptors. These findings suggest a possible physiologic role of neuropeptide Y as a nonnoradrenergic inhibitory transmitter involved in the enteric nervous control of intestinal circular muscle activity.

Central alpha 2-adrenergic control of the pattern of small intestinal motility in rats

The effects of central and peripheral administration of alpha 2-adrenoceptor agonists and antagonists on small intestinal motility were examined in conscious rats chronically fitted with electrodes implanted in the duodenojejunal wall and a cannula placed in a cerebral lateral ventricle. In fasted rats, intracerebroventricular or intraperitoneal administration of clonidine (5 micrograms) immediately disrupted the migrating myoelectric complex pattern with a total inhibition of spiking activity during the first hour, followed by a period of irregular spiking activity for 2 h. The inhibition was abolished by previous intramuscular administration of yohimbine (600 micrograms), and the period of irregular activity was suppressed by intracerebroventricular yohimbine (30 micrograms). Naphazoline, an alpha 2-agonist that poorly crosses the blood-brain barrier, only inhibited spiking activity when administered intraperitoneally (1 microgram) and induced only a period of irregular spiking activity when administered intracerebroventricularly at the same dose. In fed rats, intracerebroventricular administration of yohimbine or phentolamine (30 micrograms), and to a lesser extent prazosin, restores a migrating myoelectric complex pattern typical of the fasted state. Peripheral administration of these three antagonists at a dose 20 times higher was ineffective. Finally, both feeding and central administration of alpha 2-agonists disrupt the migrating myoelectric complex pattern. Such pharmacologic data suggest a possible role of central alpha 2-adrenoceptors in the regulation of intestinal motility in rats.

Inhibitory effects of sympathomimetic drugs on cholinergically mediated contractions of guinea-pig isolated tracheal muscle

The experiments examine the actions of sympathomimetic drugs on the responses evoked by electrical field stimulation or by acetylcholine in guinea-pig tracheal strip chains. Electrical field stimulation evoked contractions which were cholinergically mediated, in the presence of guanethidine (10 microM) and indomethacin (2 microM). All the sympathomimetic drugs tested caused a concentration-dependent reduction in the height of these contractions. Inhibitory effects of isoprenaline and terbutaline were largely prevented by propranolol (2 microM) alone, whereas those of clonidine, oxymetazoline, lidamidine and WHR1370 were prevented by yohimbine alone (2 microM). Treatments with both propranolol and yohimbine were required to prevent the inhibitory effects of noradrenaline, adrenaline and dopamine. Contractions evoked by exogenous acetylcholine (0.1-3 microM) were also inhibited by all catecholamines and terbutaline, but not by clonidine, oxymetazoline, lidamidine and WHR1370. The inhibitory effects were antagonized by propranolol (2 microM) alone. The results suggest that in guinea-pig isolated tracheal muscle, sympathomimetic drugs can inhibit cholinergic neurotransmission not only by postjunctional beta 2-adrenoceptors but also by prejunctional alpha 2-adrenoceptors.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (2). Alpha-adrenoceptor agonists

Eighteen chemically dissimilar alpha-adrenoceptor agonists were assessed in 6 different tests. The antidiarrheal activity of the compounds was highly correlated with the diuretic activity. Both activities were highly correlated with the inhibition of [3H]clonidine or [3H]idazoxan binding; the degree of correlation with inhibition of [3H]WB4101 binding or with antiptotic activity was much less. Antiptotic activity was better correlated, although still poorly, with inhibition of [3H]WB4101 binding than with inhibition of [3H]clonidine or [3H]idazoxan binding. The in vivo antiptotic/diuretic or antiptotic/antidiarrheal potency ratios both reflected the known alpha 2/alpha 1-selectivity of the compounds tested and were significantly correlated with the in vitro potency ratios between inhibition of [3H]WB4101 binding on one hand and of [3H]clonidine or [3H]idazoxan binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor agonistic activity and selectivity is discussed.

Further validation of in vivo and in vitro pharmacological procedures for assessing the alpha 2/alpha 1-selectivity of test compounds: (1). Alpha-adrenoceptor antagonists

Twenty one chemically dissimilar alpha-adrenoceptor antagonists were assessed in 7 different tests. The potencies of the compounds to protect rats from norepinephrine lethality were closely correlated with their potencies for inhibition of [3H]WB4101, but not of [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding to rat brain homogenates. The reverse was found for the potencies of the compounds to prevent the antidiarrheal effect of clonidine. The in vivo anti-norepinephrine/anti-clonidine potency ratios were highly correlated with the ratios between the in vitro potency to inhibit [3H]WB4101 binding on one hand and [3H]clonidine, [3H]idazoxan, [3H]rauwolscine or [3H]yohimbine binding on the other. The reliability of the above models for measuring alpha 1- and alpha 2-adrenoceptor antagonistic activity and selectivity is discussed.

L-654,284 a new potent and selective alpha 2-adrenoceptor antagonist

L-654,284 [(2R, 12bS)-N-(1,3,4,6,7,12b-hexahydro-2H-benzo[b]-furo[2,3-a] quinolizine-2-yl)-N-methyl-2-hydroxyethanesulfonamide) was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonist activity and compared to several reference agents. In vitro L-654,284 completed for the binding of 3H-clonidine or 3H-rauwolscine (Ki's 0.8 nM, 1.1 nM) and blocked the presynaptic effects of clonidine in the rat isolated vas deferens (pA2, 9.1). L-654,284 exhibited marked alpha 2-vs. alpha 1-adrenoceptor selectivity in vitro, inhibiting 3H-prazosin binding with a Ki of 110 nM and blocking the effects of methoxamine on the vas deferens with a pA2 of 7.5. In vivo L-654,284 at 22 nmoles/kg i.v. doubled the ED50 of clonidine to produce mydriasis in rats. Given orally, the potency of L-654,284 in this test was reduced by a factor of 5.5. L-654,284 also potently increased cerebrocortical NE turnover in the rat, another in vivo index of alpha 2-adrenoceptor blockade in the central nervous system. In the periphery, L-654,284 demonstrated alpha 2-adrenoceptor selectivity by preferentially blocking the pressor effects of UK 14304 versus those of methoxamine in the pithed rat. Overall, L-654,284 was generally a more potent alpha 2-adrenoceptor antagonist than RX 781094 with comparable alpha 2/alpha 1 selectivity and was several times more potent and alpha 2-selective than WY 26703 or yohimbine. In addition, L-654,284 had better (5-6 times) oral bioavailability than RX 781094 or WY 26703.

Reversal by alpha-2 agonists of diazepam withdrawal hyperactivity in rats

Rats were made diazepam dependent by chronic treatment with daily injections of the drug, 20 mg/kg, IP, for 3 weeks. On abrupt termination of the drug, the animals showed withdrawal hyperactivity which was indicated by increased horizontal locomotion and vertical activity, and diarrhoea. The peak effect was seen 3 days after the withdrawal of diazepam. Effects of various alpha 2 agonists, clonidine, guanfacine and B-HT 920, were studied on the diazepam withdrawal phenomena. Clonidine (100 micrograms/kg, IP) given twice a day at an interval of 12 h prevented both withdrawal-induced hyperactivity and diarrhoea. On the contrary, equimolar doses of guanfacine and B-HT 920 failed to reverse withdrawal-induced hyperactivity but attenuated the effect of diarrhoea. However, higher doses (500 micrograms/kg, IP) of guanfacine and B-HT 920 given twice a day at 12-h intervals were found to be effective. Pretreatment with yohimbine (1.5 mg/kg, IP) reversed the protective effect of clonidine, indicating the involvement of alpha 2 receptors in the action of clonidine.

Yohimbine attenuates the delayed lethality induced in mice by amitraz, a formamidine pesticide

We have found that a single dose of amitraz, a formamidine pesticide, produces death in mice 2-5 days after dosing. To further examine this phenomenon, adult albino mice of both sexes were treated with either yohimbine (10 mg/kg, i.p.) or deionized water (6 ml/kg, i.p.), immediately before an injection of amitraz (600 mg/kg, i.p.) and twice daily thereafter for 8 days. Male mice treated with water were more susceptible than water-treated females to the lethal effects of amitraz. In addition, yohimbine treatment significantly decreased the number of deaths in both sexes. These data suggest that an alpha 2-adrenergic mechanism is involved in the delayed lethality produced by amitraz.