The inhibition by dopamine of cholinergic transmission in the isolated guinea-pig ileum. Mediation through alpha-adrenoceptors

Parkinson's Disease Medication Alters Small Intestinal Motility and Microbiota Composition in Healthy Rats

Parkinson’s disease (PD) is known to be associated with altered gastrointestinal function and microbiota composition. To date, the effect of PD medication on the gastrointestinal function and microbiota, at the site of drug absorption, the small intestine, has not been studied, although it may represent an important confounder in reported microbiota alterations observed in PD patients. To this end, healthy (non-PD) wild-type Groningen rats were employed and treated with dopamine, pramipexole (in combination with levodopa-carbidopa), or ropinirole (in combination with levodopa-carbidopa) for 14 sequential days. Rats treated with dopamine agonists showed a significant reduction in small intestinal motility and an increase in bacterial overgrowth in the distal small intestine. Notably, significant alterations in microbial taxa were observed between the treated and vehicle groups; analogous to the changes previously reported in human PD versus healthy control microbiota studies. These microbial changes included an increase in Lactobacillus and Bifidobacterium and a decrease in Lachnospiraceae and Prevotellaceae. Markedly, certain Lactobacillus species correlated negatively with levodopa levels in the systemic circulation, potentially affecting the bioavailability of levodopa. Overall, the study highlights a significant effect of PD medication intrinsically on disease-associated comorbidities, including gastrointestinal dysfunction and small intestinal bacterial overgrowth, as well as the gut microbiota composition. The results urge future studies to take into account the influence of PD medication per se when seeking to identify microbiota-related biomarkers for PD.

IMPORTANCE Parkinson’s disease (PD) is the second most common neurodegenerative disorder and is known to be associated with altered gastrointestinal function and microbiota composition. We previously showed that the gut bacteria harboring tyrosine decarboxylase enzymes interfere with levodopa, the main treatment for PD (S. P. van Kessel, A. K. Frye, A. O. El-Gendy, M. Castejon, A. Keshavarzian, G. van Dijk, and S. El Aidy, Nat Commun 10:310, 2019). Although PD medication could be an important confounder in the reported alterations, its effect, apart from the disease itself, on the microbiota composition or the gastrointestinal function at the site of drug absorption, the small intestine, has not been studied. The findings presented here show a significant impact of commonly prescribed PD medication on the small intestinal motility, small intestinal bacterial overgrowth, and microbiota composition, irrespective of the PD. Remarkably, we observed negative associations between bacterial species harboring tyrosine decarboxylase activity and levodopa levels in the systemic circulation, potentially affecting the bioavailability of levodopa. Overall, this study shows that PD medication is an important factor in determining gastrointestinal motility and, in turn, microbiota composition and may, partly, explain the differential abundant taxa previously reported in the cross-sectional PD microbiota human studies. The results urge future studies to take into account the influence of PD medication on gut motility and microbiota composition when seeking to identify microbiota-related biomarkers for PD.

Gastrointestinal dysfunction in Parkinson's disease: molecular pathology and implications of gut microbiome, probiotics, and fecal microbiota transplantation

Gastrointestinal symptoms and gut dysbiosis may occur before the onset of motor symptoms in Parkinson's disease (PD). Prediagnostic and prodromal features, such as constipation and α-synuclein pathology, can be detected several years before the clinical diagnosis of PD and have the potential to develop as early PD biomarkers. Environmental toxins and gut dysbiosis may trigger oxidative stress and mucosal inflammation, and initiate α-synuclein accumulation in the enteric nervous system, early in PD. Chronic gut inflammation can lead to a leaky gut and systemic inflammation, neuro inflammation, and neuro degeneration via gut-vagus-brain signaling or through blood-brain barrier permeability. Concepts regarding the gut-brain signaling in PD pathogenesis are changing rapidly and more investigation is required. The gut microbiota interacts with the human body by modulating the enteric and central nervous systems, and immune activity. Understanding the immune responses between gut microbiota and human body might help in elucidating the PD pathogenesis. As changes in gut microbiota composition might be associated with different clinical phenotypes of PD, gut microbiota-modulating interventions, such as probiotics and fecal microbiota transplantation (FMT), have the potential to restore the gut dysbiosis, reduce inflammation, and possibly modulate the clinical PD phenotype.

Contributions of Gut Bacteria and Diet to Drug Pharmacokinetics in the Treatment of Parkinson's Disease

Parkinson's disease is the second-most common neurodegenerative disorder worldwide. Besides deciphering the mechanisms that underlie the etiology of the disease, it is important to elucidate the factors that influence the efficacy of the treatment therapeutics. Levodopa, which remains the golden treatment of the disease, is absorbed in the proximal small intestine. A reduction in levodopa absorption, leads to reduction in striatal dopamine levels and, in turn, an "off"-episode. In fact, motor fluctuations represent a major problem during the progression of the disease and alteration between "on" (mobility often with dyskinesia) and "off" (immobility, akinesia) episodes contribute to a decreased quality of life. Dietary amino acids can interfere with the absorption of levodopa from the gut lumen and its transport through the blood brain barrier. In addition, higher abundance of specific gut bacteria that restrict levodopa absorption plays a significant role in motor fluctuations in a subset of Parkinson's disease patients. Here, we review the impact of factors potentially interfering with levodopa absorption, focusing on levodopa transport, diet, and gut bacterial interference with the bioavailability of levodopa.

Dopamine-induced inhibition of endogenous acetylcholine release from the isolated ileal synaptosomal preparations of guinea-pig mediated via alpha-adrenoceptors

1. The effect of exogenous dopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal guinea-pig preparations was examined by means of high pressure liquid chromatography with electrochemical detection. 2. Release of ACh was induced by substance P or by depolarization with high potassium (50 mM) in a medium containing atropine propranolol and naloxone. 3. Dopamine produced a concentration-dependent inhibition of the evoked ACh release induced by substance P or in samples depolarized by high potassium. This action of dopamine was not reversed by the dopamine receptor antagonists either for the DA2 subtype domperidone, or for the DA1 subtype, SCH23390. Fenoldopam, the agonist of dopamine DA1 receptors, or quinpirole, the agonist of dopamine DA2 receptors, reduced the evoked ACh release, although only in high, non-dopamine-specific concentrations. 4. Failure of guanethidine or desipramine to inhibit this effect of dopamine ruled out mediation by endogenous noradrenaline. 5. Idazoxan and yohimbine reversed this dopamine-induced inhibition at concentration sufficient to abolish the action of clonidine. Influx of (45)Ca stimulated by substance P or high potassium into synaptosomal preparations was attenuated in the presence of dopamine. This inhibition by dopamine was also reversed by idazoxan or yohimbine but not by dopamine receptor antagonists. Moreover, the dopamine-induced inhibitions of both the ACh release and the influx of (45)Ca disappeared in the samples treated with pertussis toxin at a dose sufficient to abolish the action of clonidine. 6. It is concluded that dopamine suppresses the influx of calcium ions into cholinergic nerve terminals via an activation of alpha2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein, resulting in the decrease of ACh release from ileal synaptosomes of guinea-pigs.

A method for simultaneous measurement of intragastric, intraduodenal, and intracolonic pressures in the bilaterally vagotomized spinal rat

A method has been developed where quantitative evaluation of intragastric (IGP), intraduodenal (IDP), and intracolonic (ICP) pressures (cm H2O) can be obtained in the bilaterally vagotomized spinal rat. The preparation is very sensitive to 5-hydroxytryptamine (5-HT) and 5-HT-agonists, carbachol, histamine, dopamine, and/or noradrenaline. The effects of drugs on IGP, IDP, and ICP (increase or decrease) can be assessed and a dose-response curve before and after antagonists can be computed. The described experimental model is simple, reliable, reproducible, and appropriate to study and differentiate the different gastrointestinal system receptors that seem to be involved in the modulation of gastrointestinal motility. The additional advantage in using this method is that drug effects on the motility of stomach, duodenum, and colon can be investigated simultaneously.

Inhibitory effect of octopamine on the release of endogenous acetylcholine from isolated myenteric synaptosomes of guinea-pig

1. The effect of octopamine on the release of endogenous acetylcholine (ACh) from isolated ileal synaptosomal preparations of guinea-pigs was examined using high pressure liquid chromatography with electrochemical detection. Release of ACh was induced by substance P or by depolarization with high potassium (50 mmol/L) in medium containing atropine, propranolol and naloxone. 2. Octopamine produced a dose-dependent inhibition of substance P-induced ACh release. A similar inhibitory action of octopamine was found in the samples depolarized by high potassium as a reference. 3. The action of octopamine was not reversed by the dopamine receptor antagonists either for the DA-2 subtype, domperidone, or for the DA-1 subtype, SCH23390, or by haloperidol. However, idazoxan and yohimbine antagonized this octopamine-induced inhibition at concentrations sufficient to abolish the action of clonidine. 4. Failure of guanethidine or nomifensine to inhibit octopamine ruled out mediation by noradrenergic neurotransmitters. 5. Octopamine decreased the influx of [45Ca] stimulated by substance P into synaptosomal preparations and this was reversed by idazoxan or yohimbine at concentrations sufficient to block the action of clonidine. 6. Pertussis toxin abolished the inhibitory action of octopamine at a dose high enough to block the action of clonidine. 7. These results indicate that octopamine suppresses the influx of calcium ions into cholinergic nerve terminals of ileal synaptosomes of guinea-pigs via an activation of alpha 2-adrenoceptors coupled with a pertussis toxin-sensitive GTP-binding protein which results in a decrease of ACh release.

Influence of fenoldopam and quinpirole in the guinea-pig stomach

1. The influence of the selective DA1-agonist fenoldopam and the selective DA2-agonist quinpirole was investigated in the guinea-pig intact stomach model and in guinea-pig gastric corpus muscle strips. 2. In the intact stomach model, quinpirole induced a relaxation from 10(-6) M on. The relaxation by quinpirole (3 x 10(-5) M) was significantly inhibited by rauwolscine (10(-7) M), yohimbine (10(-7) M) and domperidone (10(-6) M). In the presence of tetrodotoxin, quinpirole (3 x 10(-5) M) induced a contraction. 3. In the same model, fenoldopam induced a relaxation but only at 3 x 10(-5) M. The relaxation by fenoldopam (3 x 10(-5) M) was not inhibited by SCH 23390 (10(-6) M). The relaxant effect of dopamine (3 x 10(-6) M) was significantly inhibited by rauwolscine (10(-7) M), yohimbine (3 x 10(-7) M), haloperidol (10(-6) M) and domperidone (10(-6) M). 4. In circular muscle strips of the gastric corpus, the electrically induced cholinergic contractions were inhibited by dopamine but not consistently influenced by quinpirole or fenoldopam. 5. Dopamine, fenoldopam and quinpirole induced an increase in basal tone of the strips. The contraction by dopamine (10(-5) M) was significantly antagonized by prazosin and methysergide. 6. No evidence was thus found for the presence of DA1-receptors in both guinea-pig stomach models. Equally, no evidence for the presence of DA2-receptors was found when studying quinpirole in the strips. Although the relaxant effect of quinpirole in the intact stomach seems predominantly mediated via alpha 2-adrenoceptors, an involvement of DA2-receptors cannot be excluded.

Agonist and antagonist characterization of a putative adrenoceptor with distinct pharmacological properties from the alpha- and beta-subtypes

1. Experiments were done to characterize a putative adrenoceptor which functions to inhibit longitudinal muscle tension development in the guinea-pig ileum. Several phenylethylamine based agonists were investigated: BRL 37344, (-)-isoprenaline, (+)-isoprenaline, noradrenaline, adrenaline, and fenoterol. Propranolol and nadolol were tested as antagonists. Agonist-induced inhibition of the contractile response to histamine was measured under equilibrium conditions with alpha-adrenoceptors and muscarinic cholinoceptors inhibited. 2. Inhibitory responses were obtained to (-)-isoprenaline and BRL 37344 that were resistant to beta-adrenoceptor blockage with propranolol (5 microM) and nadolol (10 microM). These resistant responses were antagonized by much higher concentrations of nadolol (30 to 1000 microM) yielding apparent pA2 values for nadolol of 4.31 with (-)-isoprenaline as the agonist, and 4.68 with BRL 37344 as the agonist. Similar apparent pA2 values for nadolol at the putative adrenoceptor were obtained with noradrenaline (4.79), adrenaline (4.68), and fenoterol (4.38). 3. The order and relative potency of agonists at the putative adrenoceptor was: BRL 37344 (20) greater than (-)-isoprenaline (8) greater than noradrenaline (1) greater than adrenaline (0.5) greater than fenoterol (0.35) greater than (+)-isoprenaline (0.27). 4. The resistance to blockade by propranolol (5 microM), the low affinity of nadolol, and the order and relative potency of agonists, suggest the presence of an adrenoceptor with distinct pharmacological characteristics from currently defined alpha- and beta-adrenoceptors.

A submucosal mechanism for catecholamine-induced increases in fluid absorption in rabbit ileum in vitro

1. The effects of clonidine and dopamine on water movements across the mucosal and serosal surfaces of rabbit ileum have been investigated using a high-resolution method for monitoring water flows in vitro. 2. Theophylline (10 mM) and carbamyl choline (10 microM) caused a reduction in fluid inflow across the mucosal surface and a smaller decrease in fluid outflow across the serosal surface. Addition of the alpha 2-adrenergic agonist clonidine or dopamine fully reversed the theophylline, or carbamyl choline-induced decrease in mucosal inflow in a dose-related manner. 3. The effects of clonidine on mucosal inflow are blocked by the alpha 2-adrenergic antagonist, yohimbine. Yohimbine was much less effective than pimozide or d-butaclamol in blocking the effect of dopamine on mucosal inflow. These findings support the view that there are separate alpha 2-adrenergic and dopaminergic receptors. 4. The hydraulic conductance (Lp) of the serosal surface was measured directly from the change in serosal exit flow following addition of 2 mosmol kg-1 of polyethylene glycol (molecular mass 20,000 Da) to the serosal bathing solution. Theophylline reduced the Lp by 35%. Clonidine (1 microM) added to theophylline-treated tissues increased the Lp by 66%. This effect was prevented by yohimbine (1 microM). 5. The effects of theophylline, clonidine and dopamine on the permeability of the mucosal and serosal surfaces of the tissue to [3H]mannitol were measured. These showed that theophylline increased the rate of labelled mannitol loss across the mucosal surface but reduced the mannitol permeability across the serosal surface. This latter effect was reversed by clonidine and dopamine. 6. Changes in transepithelial electrical potential difference (PD), short-circuit current and resistance were monitored. Theophylline caused a rapid increase in PD and short-circuit current and a slower increase in resistance. Clonidine (5 microM) reversed the effects on PD and resistance but was without significant effect on short-circuit current. The results suggest that a major component of secretagogue-induced reduction in fluid transport in vitro is due to mechanical changes in the submucosa, probably induced by modulation of neurotransmitter release within the tissue.

The bindings of 3H-prazosin and 3H-yohimbine to alpha adrenoceptors in the guinea-pig stomach

Alpha adrenoceptor subtypes have been investigated by radioligand binding study in guinea-pig stomach using 3H-prazosin and 3H-yohimbine. The specific 3H-prazosin binding to guinea-pig stomach was saturable and of high affinity (KD = 1.4 nM) with a Bmax of 33 fmol/mg protein. Specific 3H-yohimbine binding to the tissue was also saturable and of high affinity (KD = 25.5 nM) with a Bmax of 150 fmol/mg protein. Adrenergic drugs competed for 3H-prazosin binding in order of prazosin greater than phentolamine greater than methoxamine greater than norepinephrine greater than clonidine greater than epinephrine greater than yohimbine. These drugs competed for 3H-yohimbine binding in order of yohimbine greater than phentolamine greater than clonidine greater than epinephrine greater than norepinephrine greater than prazosin greater than greater than prazosin greater than methoxamine. We also examined whether dopamine receptors exist in guinea-pig stomach, using radioligand binding study. Specific binding of 3H-spiperone, 3H-apomorphine, 3H-dopamine and 3H-domperidone was not detectable in the stomach. Dopaminergic drugs such as dopamine, haloperidol, domperidone and sulpiride competed for 3H-prazosin binding in order of haloperidol greater than domperidone greater than dopamine greater than sulpiride. Metoclopramide, sulpiride and dopamine competed for 3H-yohimbine binding in order of metoclopramide greater than sulpiride greater than dopamine. These results suggest that guinea-pig stomach has alpha 1 and alpha 2 adrenoceptors and has no specific dopamine receptors. It is also suggested that some dopamine receptor antagonists such as domperidone, haloperidol, sulpiride and metoclopramide have antagonistic actions on alpha adrenoceptors.

A response to isoprenaline unrelated to alpha- and beta-adrenoceptor agonism

The hypothesis that beta-adrenoceptor agonism might explain a reported lack of competitive antagonism between alpha 2-adrenoceptor antagonists and agonists of the phenylethylamine class was tested in the electrically field stimulated ileum of the guinea-pig. The beta-adrenoceptor agonist, isoprenaline, was used as the phenylethylamine and inhibition of 'twitch' response evoked by cholinergic stimulation was measured. In the presence of idazoxan (3 microM), to block inhibitory alpha 2-adrenoceptors, propranolol (0.1 to 5.0 microM) failed to act competitively toward isoprenaline. Isoprenaline responses totally resistant to inhibition by propranolol were obtained. As inhibitory alpha 1-adrenoceptors are absent from guinea-pig ileum, a recognition site distinct from the currently defined alpha- and beta-adrenoceptors is postulated. Agonism by phenylethylamine based agonists at this site may explain their inability to act competitively with alpha- and beta-adrenoceptor antagonists.

Increased gut cholinergic activity and antagonism of 5-hydroxytryptamine M-receptors by BRL 24924: potential clinical importance of BRL 24924

The mechanisms by which BRL 24924 ([(+/-)- (endo)])-4-amino-5-chloro-2-methoxy-N-(1-azabicyclo-[3.3.1]-non- 4-yl) benzamide hydrochloride stimulates gut motility and the relationships between BRL 24924 and 5-hydroxytryptamine (5-HT) receptors have been studied. In guinea-pig isolated ileum, BRL 24924 (10(-14)-10(-6) M) increased electrically-evoked, cholinergically-mediated contractions, probably by increasing acetylcholine (ACh) release. This action of BRL 24924 was prevented by the presence of high concentrations of 5-HT, but not by hexamethonium, phentolamine and propranolol, methysergide or ICS 205-930. The mechanism by which BRL 24924 can increase gut ACh release is not certain, but most likely involves activation of an enteric 5-HT receptor which differs from those 5-HT M-receptors antagonized by ICS 205-930 or by higher concentrations of BRL 24924 in other test systems. BRL 24924 antagonized 5-HT-evoked, cholinergically-mediated contractions of guinea-pig isolated ileum (pA2 = 7.56 +/- 0.12). Similar and higher concentrations of BRL 24924 did not antagonize contractions evoked by nicotinic receptor stimulation. In rabbit isolated heart, BRL 24924 1-10 nM reduced the tachycardia evoked by 5-HT. In anaesthetized rats, BRL 24924 0.3-83 nmol kg-1 i.v. antagonized the Bezold-Jarisch reflex evoked by 5-HT; the ID50 for BRL 24924 was 10.2 +/- 3.0 nmol kg-1 (3.7 +/- 1.1 microgram kg-1). A direct action of BRL 24924 on nerve function was excluded. In rat cortex, BRL 24924 10(-6) M did not displace [3H]-5-HT or [3H]-ketanserin binding to 5-HT1 and 5-HT2 receptors. The actions of BRL 24924 are discussed in terms of its potential clinical use as a stimulant of gastric motility and as a 5-HT M-receptor antagonist.

Two types of receptors for 5-hydroxytryptamine on the cholinergic nerves of the guinea-pig myenteric plexus

The effects of 5-hydroxytryptamine (5-HT) on spontaneous and electrically-evoked release of [3H]-acetylcholine (ACh) from guinea-pig myenteric plexus preparations preincubated with [3H]-choline have been investigated in the absence of cholinesterase inhibitors. 5-HT caused a transient increase in spontaneous release and an inhibition of the electrically-evoked release of [3H]-ACh. The 5-HT-induced contractions of the longitudinal muscle were clearly related to the increase in spontaneous release. The inhibitory effect was not due to activation of alpha-adrenoceptors since it was also observed in the presence of tolazoline and on strips from reserpine-pretreated guinea-pigs. After desensitization of the excitatory 5-HT receptors with 5-HT or metoclopramide the effects of 5-HT on spontaneous [3H]-ACh release were largely reduced. A variety of established antagonists at neuronal 5-HT receptors (i.e. metitepine 0.1-1 microM; methysergide 1 microM; ketanserin 0.1-1 microM; MDL 72222 0.1 microM; tropacocaine 1 microM) failed to block the excitation. The inhibition by 5-HT of the electrically evoked [3H]-ACh release was competitively antagonized by metitepine (pA2 7.6) and methysergide (pA2 7.0) but not by ketanserin. Tachyphylaxis to the inhibitory action of 5-HT did not occur. The results suggest that the excitatory 5-HT receptor ('M'-receptor) differs in its pharmacological properties from other neuronal 5-HT receptors. The presynaptically located inhibitory receptor may roughly correspond to the 5-HT1 receptor subtype but probably differs from the 5-HT autoreceptor.

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)

A comparison of the stimulatory effects of metoclopramide and cinitapride in the guinea-pig isolated ileum

The pharmacological effects of a new benzamide derivative cinitapride, have been compared to those of metoclopramide in guinea-pig isolated ileum and longitudinal smooth muscle-myenteric plexus preparations treated with propranolol (3 microM). Cinitapride (EC50 = 0.74 microM) was 6 times more potent than metoclopramide (EC50 = 4.69 microM) in enhancing the twitch response of co-axially stimulated preparations and 11 times more potent in eliciting contractions in non-stimulated tissues, their respective EC50 values being 0.58 microM and 6.52 microM. These contractile effects of cinitapride and metoclopramide amounted to approximately 25% of the maximum response of the tissues to acetylcholine (1 microM). Neither cinitapride nor metoclopramide, in concentrations up to 10 microM, significantly affected concentration-response curves to exogenous acetylcholine or 5-hydroxytryptamine but both drugs elicited a concentration-dependent potentiation of the ileum responses to a fixed concentration (10 microM) of the ganglion stimulant dimethylphenylpiperazinium (DMPP). Analysis of the twitch-enhancing and contractile effects of cinitapride using a variety of drugs suggested that a common, prejunctional locus of action upon the cell bodies or axons of postganglionic, parasympathetic neurones of the myenteric plexus is involved in both of these responses. In hexamethonium (100 microM) and methysergide (0.1 microM)-treated longitudinal smooth muscle preparations desensitization or blockade of 5-hydroxytryptamine receptors using high concentrations of the same agonist (30 microM) or quipazine (10 microM) or the putative antagonists cocaine (30 microM) or tubocurarine (10 microM) produced small inhibitions (congruent to 20%) of the contractile responses to metoclopramide and cinitapride but did not affect twitch responses to these drugs. It is concluded that cinitapride is a more potent stimulant of guinea-pig intestinal smooth muscle than metoclopramide in vitro although the mechanism of action of both drugs appears to be similar and involves a prejunctional enhancement of acetylcholine release from intramural cholinergic neurones. Attempts to implicate a prejunctional facilitatory 5-hydroxytryptamine receptor in the mediation of the stimulant effects of these drugs were not conclusive and additional studies are required to fully explore this possibility.

Peripheral receptor populations involved in the regulation of gastrointestinal motility and the pharmacological actions of metoclopramide-like drugs

This minireview is concerned with a re-examination of the locus of action and the possible peripheral mechanisms involved in the gastrointestinal (GI) stimulant effects of metoclopramide. Such a re-evaluation is opportune given the increasing use of this drug in the therapy of certain GI tract disorders. To provide an orientation on this subject the location in the GI tract and function of several relevant receptor types have been reviewed. In the past metoclopramide has been reported to enhance contractions of a variety of GI preparations to electrical stimulation, acetylcholine, carbachol and ganglion stimulants, to inhibit responses to alpha 2-adrenoreceptor agonists and 5-hydroxytryptamine, as well as blocking those to dopamine. Also in such preparations metoclopramide facilitates the release of acetylcholine to transmural stimulation. One important question is whether this effect is mediated via a specific prejunctional receptor. In this respect 2 suggestions have been made. Firstly that the drug may act as a preferential, prejunctional muscarinic antagonist thus inhibiting the negative feedback inhibition of acetylcholine release and secondly that metoclopramide may be a prejunctional agonist (partial) at 5-hydroxy-tryptamine receptors. Although the latter possibility appears most tenable at present, the involvement of a specific receptor remains to be confirmed. The important finding that dopamine receptors are probably not involved in the local stimulant effects of metoclopramide has important implications for future research orientated towards the discovery of a new generation of GI drugs lacking the side effects associated with central dopamine receptor blockade. Several compounds (cinitapride, BRL 20627A and cisapride) are now in the early stages of clinical evaluation.

The action and interaction of beta-phenethylamines and imidazolines on prejunctional alpha 2-adrenoceptors of guinea-pig ileum in the presence of the non-competitive antagonist benextramine

The effects of benextramine, a selective non-competitive, irreversible antagonist of alpha-adrenoceptors, against alpha 2-adrenoceptor agonists has been investigated in isolated field-stimulated guinea-pig ileum. Benextramine was equipotent against the imidazoline, clonidine and the thiazoloazepine B-HT920, however, higher concentrations of benextramine were required for an equivalent non-competitive antagonism of the beta-phenethylamine, alpha-methylnoradrenaline. Using benextramine to differentially block the agonist effects of clonidine but not alpha-methylnoradrenaline it was shown that clonidine can competitively antagonize the effects of alpha-methylnoradrenaline. From these and previous results on the differential effects of imidazoline-like and beta-phenethylamine-like drugs on alpha 2-adrenoceptors, it is proposed that two distinct populations of these receptors exist.

Inhibitory effect of dopamine on canine gastric fundus

The mechanism of the inhibitory effect of dopamine on canine stomach fundus was studied in longitudinal and circular muscle fundus strips, contracted by transmural electrical stimulation or by methacholine. Results obtained for longitudinal and circular strips were similar. Dopamine (1 X 10(-6)-1 X 10(-4) M) concentration-dependently inhibited frequency-response curves to electrical stimulation; these concentrations did not change the resting tone of the strips. Dopamine (1 X 10(-4) M), tested on contractions of similar amplitude induced in the same strips by electrical stimulation at 0.5 Hz and by methacholine, inhibited the electrically induced contractions but had little influence on the contractions induced by methacholine. The inhibition of the electrically induced contractions by dopamine 1 X 10(-4) M was not influenced by the presence of cocaine 3 X 10(-5) M or hydrocortisone 3 X 10(-5) M. The alpha 1- and alpha 2-adrenoceptor antagonist phentolamine and the alpha 2-adrenoceptor antagonist rauwolscine markedly antagonized the inhibitory effect of dopamine on the response to electrical stimulation at 0.5 Hz. The alpha 1-adrenoceptor antagonist prazosin and the dopamine receptor antagonists haloperidol and domperidone had no effect. The dopamine receptor antagonist metoclopramide decreased the inhibitory effect of dopamine but had a similar effect on the inhibition caused by noradrenaline. These results indicate that the inhibitory effect of dopamine in the dog gastric fundus is mainly mediated by an interaction with alpha 2-adrenoceptors on the intramural cholinergic neurons; this effect is largely direct since it was not influenced by cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

2-[125Iodo]LSD, a new ligand for the characterisation and localisation of 5-HT2 receptors

LSD was iodinated with Na125I and chloramine T, to get the radioligand [125I]LSD ( 125IOL ) and with N-I-succinimide to obtain the nonradioactive compound 2-I-LSD (IOL) for comparative pharmacological studies. The introduction of iodine in position 2 of LSD leads to an increase in selectivity for 5HT2 receptors. In rat cortex membranes, 125IOL possesses a KD = 0.9 +/- 0.1 nmol/l, Bmax = 240 +/- 20 fmoles/mg, and a nonspecific binding of 30-40% in presence of 100 nmol/l ketanserin. In competition experiments, 5HT antagonists showed monophasic displacement curves. Their KI-values correlate well with pD'2-values for inhibition of 5HT-induced contraction of canine basilar artery. It can be concluded that the sites labelled by 125IOL have pharmacological properties in common with central 5HT2 receptors, which are identical with vascular postjunctional 5HT receptors. The high specific radioactivity of 125IOL permits detection of even small 5HT2 receptor densities which exist in the guinea pig ileum. These 125IOL binding sites are pharmacologically different to those found in the brain or on the vessels and might be a special subpopulation of 5HT2 sites. For example, ketanserin has a high affinity to the sites labelled by 125IOL in the brain and a 100 times lower affinity to the sites labelled in the ileum. In a routine binding screen with various ligands, the inhibition constants of IOL for alpha 1, alpha 2, beta, histamine and muscarinic receptors are greater than 100 nmol/l with the exception for dopamine receptors, 40 nmol/l. 125IOL was employed for the autoradiographic localisation of its binding sites after in vitro labelling of microtome rat brain sections. 125IOL labelled 5HT2 sites in the cortex and dopamine receptors in the nucleus caudatus. The exposure times required were very short, compared to those of other 5HT2 ligands available.

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.

The effects of metoclopramide on acetylcholine release and on smooth muscle response in the isolated guinea-pig ileum

The effects of metoclopramide on smooth muscle contraction and on release of acetylcholine were studied in the guinea-pig myenteric plexus longitudinal muscle preparation. Acetylcholine was determined either as endogenous acetylcholine, or as labelled transmitter from strips preloaded with 3H-choline. Metoclopramide caused an increase in resting tension of longitudinal muscle as well as an increase in resting output of either endogenous or labelled acetylcholine. Tetrodotoxin abolished the metoclopramide-evoked increase in transmitter release. The increase in smooth muscle tension was clearly related to the increase in resting output. The effects of metoclopramide on both longitudinal muscle contraction and resting release of labelled acetylcholine were prevented in the presence of a concentration of 5-hydroxytryptamine (5-HT) that desensitized 5-HT receptors. This suggests that metoclopramide stimulates neuronal 5-HT receptors and, thereby, facilitates acetylcholine release. Metoclopramide augmented the twitch-like contractions induced by field stimulation at 0.1 Hz. Contractions elicited at 1 Hz were only slightly enhanced. Similarly, metoclopramide facilitated only the release of labelled acetylcholine evoked by electrical stimulation at 0.1 Hz, but not that at 1 Hz. The facilitatory effects of metoclopramide on twitch height and evoked release could not be attributed to a blockade of presynaptic inhibitory alpha-adrenoceptors, dopamine or muscarine receptors.