Importance of the Locus coeruleus and involvement of α-adrenergic receptors in the post-decapitation reflex in the rat

The latency, duration, hindlimb kick frequency, and total activity components of the post-decapitation reflex (PDR) were measured in the rat using a movement-sensitive transducer. Reduction of brain and spinal cord norepinephrine (NE) caused by neonatal administration of 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine, which also reduced brain serotonin, decreased all components of the PDR. Depletion of serotonin or dopamine alone reduced the vigor of the reflex, suggesting that these pathways can influence the PDR but are not essential for the response. Lesions of neurons in the Locus coeruleus, made electrolytically or with 6-OHDA, decreased the intensity of the PDR, with the 6-OHDA-induced lesion being more effective. Depletion of forebrain NE terminals with 6-OHDA did not alter the PDR, consistent with a critical involvement of spinal noradrenergic fibers. The PDR was also decreased by phentolamine and prazosin, but not by propanolol, suggesting an involvement of -adrenergic receptors in the response. This hypothesis was further supported by the finding that the efficacy of a variety of drugs (such as tricyclic antidepressants, phenothiazines, and antihypertensive compounds) for blocking the reflex was apparently related to their affinity for -adrenergic receptors. Thus, the PDR is dependent on noradrenergic fibers in the spinal cord and may provide a simple screen for drugs with suspected -adrenergic blocking properties or for agents that disrupt the function of central noradrenergic fibers.Bruce A. Pappas was a visiting Professor on sabbatical leave from the Department of Psychology, Carleton University, Ottawa, Canada K1S 5B6     

β-blockers and the risk of incident depression in the elderly

The hypothesis that β-blockers cause depression has been both confirmed and refuted in previous studies. However, in hardly any of these studies, depression was systematically and adequately assessed. The aim of this cohort study was to examine whether β-blockers, in general, highly lipid-soluble, nonselective, or serotonergic receptor-binding β-blockers, are associated with incident depression. Between 1993 and 2005, 5104 elderly persons were followed for incident depressions. Depressions were identified by regular interview and continuous monitoring of medical records. Cases were categorized as clinically relevant depressive symptoms or as depressive syndromes, the latter including Diagnostic and Statistical Manual of Mental Disorders-IV-defined depressive disorders. Pharmacies provided information on filled β-blockers. We used Cox regression with drug use as a time-dependent variable to analyze the data, adjusted for potential demographic covariates, activity of daily living, and (contra)indications for β-blockers. We found that use of β-blockers in general did not convey an increased risk of depressive symptoms (hazard ratio [HR], 0.76; 95% confidence interval [CI], 0.37-1.59) or depressive syndromes (HR, 0.99; 95% CI, 0.53-1.84). Highly lipid-soluble β-blockers, mostly propranolol in our study, were associated with depressive symptoms during the first 3 months of use (HR, 3.31; 95% CI, 1.03-10.6), but not with depressive syndromes. Nonselective or serotonergic receptor affinity was not associated with an increased risk of depressive symptoms or syndromes independent of high lipid solubility. We conclude that β-blockers in general do not convey an increased risk of depression. Lipophilic β-blockers are associated with an increased risk of depressive symptoms.     

GABAA ρ receptor mechanisms in the rat amygdala and its role in the modulation of fear and anxiety

Rationale  

Accumulating evidence for the presence of GABA_A ρ receptors within the amygdala which differ from other members of the GABA_A receptor family in both subunit composition and functional properties has been recently obtained.     

Comparison of the intestinal absorption and bioavailability of γ-tocotrienol and α-tocopherol: in vitro, in situ and in vivo studies

The aim of this work was to compare the intestinal absorption kinetics and the bioavailability of γ-tocotrienol (γ-T3) and α-tocopherol (α-Tph) administered separately as oil solutions to rats in vivo. Also, to explain the significant difference in the oral bioavailability of the compounds: (1) the release profiles using the dynamic in vitro lipolysis model, (2) the intestinal permeability and (3) carrier-mediated uptake by Niemann-Pick C1-like 1 (NPC1L1) transporter were examined. Absolute bioavailability studies were conducted after oral administration of γ-T3 or α-Tph prepared in corn oil to rats. In situ rat intestinal perfusion with ezetimibe (a NPC1L1 inhibitor) was performed to compare intestinal permeability. The in vitro interaction kinetics with NPC1L1 was examined in NPC1L1 transfected cells. While the in vitro release studies demonstrated a significantly higher release rate of γ-T3 in the aqueous phase, the oral bioavailability of α-Tph (36%) was significantly higher than γ-T3 (9%). Consequent in situ studies revealed significantly higher intestinal permeability for α-Tph compared with γ-T3 in rats. Moreover, the NPC1L1 kinetic studies demonstrated higher Vmax and Km values for α-Tph compared with γ-T3. Collectively, these results indicate that intestinal permeability is the main contributing factor for the higher bioavailability of α-Tph. Also, these results emphasize the potentially important role of intestinal permeability in the bioavailability of γ-T3, suggesting that enhancing its permeability would increase its oral bioavailability.     

A comparative study of the role of crocin and sitagliptin in attenuation of STZ-induced diabetes mellitus and the associated inflammatory and apoptotic changes in pancreatic β-islets

Type 1 diabetes mellitus (T1DM) describes a complex group of metabolic disorders associated with elevated blood glucose levels and increased risks of complications development. Exploring new drug therapies would reduce the increased diabetes-associated morbidity and mortality and will reduce the excessive health care costs. Crocin is the major active ingredient of saffron. In the current study, DM was induced by single intraperitoneal injection of streptozocin (50 mg/kg).DM progression was associated with a significant increase in blood glucose level with reduced insulin and increased glucagon secretion. Pancreatic malondialdehyde (MDA) content significantly escalated, while superoxide dismutase (SOD) activity, reduced glutathione (GSH) concentration, catalase activity, thioredoxin level and serum total antioxidant capacity significantly declined. This was associated with a significant increase in pancreatic caspase-3 contents and pancreatic infiltration with inflammatory cells in β-islets. Both sitagliptin and crocin significantly reduced blood glucose levels, enhanced pancreatic insulin expression and secretion and suppressed glucagon secretion with enhancement of anti-oxidant defenses and reduction of oxidative burden, with evident anti-inflammatory impacts. Interestingly, the effect of crocin on DM indices, inflammatory and apoptotic changes was comparable to that of sitagliptin; the standard oral hypoglycemic agent. Nevertheless, crocin had a superior effect compared to sitagliptin on blood sugar level, β-islets diameter and insulin immune-reactivity. In conclusion, crocin reduced blood glucose level mainly via reduction of oxidative burden, modulation of apoptotic pathway and attenuation of pancreatic inflammation.     

Role of the MPTP in conditioning the heart – translatability and mechanism

Mitochondria have long been known to be the gatekeepers of cell fate. This is particularly so in the response to acute ischaemia‐reperfusion injury (IRI). Following an acute episode of sustained myocardial ischaemia, the opening of the mitochondrial permeability transition pore (MPTP) in the first few minutes of reperfusion, mediates cell death. Preventing MPTP opening at the onset of reperfusion using either pharmacological inhibitors [such as cyclosporin A (CsA) ] or genetic ablation has been reported to reduce myocardial infarct (MI) size in animal models of acute IRI. Interestingly, the endogenous cardioprotective intervention of ischaemic conditioning, in which the heart is protected against MI by applying cycles of brief ischaemia and reperfusion to either the heart itself or a remote organ or tissue, appears to be mediated through the inhibition of MPTP opening at reperfusion. Small proof‐of‐concept clinical studies have demonstrated the translatability of this therapeutic approach to target MPTP opening using CsA in clinical settings of acute myocardial IRI. However, given that CsA is a not a specific MPTP inhibitor, more novel and specific inhibitors of the MPTP need to be discovered – the molecular identification of the MPTP should facilitate this. In this paper, we review the role of the MPTP as a target for cardioprotection, the potential mechanisms underlying MPTP inhibition in the setting of ischaemic conditioning, and the translatability of MPTP inhibition as a therapeutic approach in the clinical setting.

Linked Articles

This article is part of a themed section on Conditioning the Heart – Pathways to Translation. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue‐8

Abbreviations

ANT

adenine nucleotide translocase

CABG

coronary artery bypass graft

CsA

cyclosporin A

CypD

cyclophilin D

Drp1

dynamin‐related protein 1

GSK

glycogen synthase kinase

IPC

ischaemic preconditioning

IPost

ischaemic postconditioning

IRI

ischaemia‐reperfusion injury

LV

left ventricular

MI

myocardial infarct

MitoK_ATP

mitochondrial ATP‐sensitive potassium channel

MPTP

mitochondrial permeability transition pore

OMM

outer mitochondrial membrane

OPA1

optic atrophy 1

PMI

perioperative myocardial injury

PPCI

primary percutaneous coronary intervention

RIC

remote ischaemic conditioning

RISK

reperfusion injury salvage kinase

ROS

reactive oxygen species

SAFE

survivor activating factor enhancement

STEMI

ST segment elevation myocardial infarction

VDAC

voltage‐dependent anion channel

Tables of Links

Kinetics of butyrate metabolism in the normal colon and in ulcerative colitis: the effects of substrate concentration and carnitine on the β-oxidation pathway

Aliment Pharmacol Ther 2011; 34: 526–532

Summary

Background Butyrate, a colonic metabolite of carbohydrates, is considered as the major energy source for the colonic mucosa. An impaired butyrate metabolism has been reported in ulcerative colitis (UC), however, the cause still remains unknown. Aim In the present study, we investigated whether higher butyrate concentrations could normalise the oxidation rate in UC. Furthermore, it was investigated whether carnitine could enhance the butyrate oxidation. Methods Mucosal biopsies from a total of 26 UC patients and 25 controls were incubated with ¹⁴C‐labelled Na‐butyrate and the produced ¹⁴CO₂ was measured. First, the rate of oxidative metabolism was compared at three different concentrations of Na‐butyrate (0.05 mm , 1 mm and 10 mm ). Then, incubations of biopsies were performed with carnitine alone or combined with ATP. Results Overall, butyrate oxidation in UC was significantly lower than that in controls. The maximum rate of butyrate oxidation was achieved in UC and control subjects from 1 mm onwards. Increasing the butyrate concentration to a level to be present in the colonic lumen, i.e. 10 mm , did not increase the rate of butyrate oxidation in UC to the rate observed in controls. Addition of carnitine alone or combined with ATP caused no effects. Conclusions Saturation of butyrate kinetics was achieved from 1 mm in UC and control subjects. The rate of butyrate metabolism was significantly impaired in active ulcerative colitis. The addition of compounds interfering with the β‐oxidation pathway had no effect on the butyrate metabolism in UC.     

Pharmacokinetic–Pharmacodynamic Modeling of the Effectiveness and Safety of Buprenorphine and Fentanyl in Rats

Objective Respiratory depression is a serious and potentially life-threatening side-effect of opioid therapy. The objective of this investigation was to characterize the relationship between buprenorphine or fentanyl exposure and the effectiveness and safety outcome in rats. Methods Data on the time course of the antinociceptive and respiratory depressant effect were analyzed on the basis of population logistic regression PK–PD models using non-linear mixed effects modeling software (NONMEM). The pharmacokinetics of buprenorphine and fentanyl were described by a three- and two-compartment model, respectively. A logistic regression model (linear logit model) was used to characterize the relationship between drug exposure and the binary effectiveness and safety outcome. Results For buprenorphine, the odds ratios (OR) were 28.5 (95% CI, 6.9–50.1) and 2.10 (95% CI, 0.71–3.49) for the antinociceptive and respiratory depressant effect, respectively. For fentanyl these odds ratios were 3.03 (95% CI, 1.87–4.21) and 2.54 (95% CI, 1.26–3.82), respectively. Conclusion The calculated safety index (OR_{antinociception}/OR_{respiratory depression}) for fentanyl of 1.20 suggests that fentanyl has a low safety margin, implicating that fentanyl needs to be titrated with caution. For buprenorphine the safety index is 13.54 suggesting that buprenorphine is a relatively safe opioid.     

Effects of sympathectomy on the in vitro α and β-responses of the parotid gland

Summary Amylase and K⁺ release were measured in slices obtained from innervated and sympathetically denervated rat parotid glands. Amylase release induced by noradrenaline was found to be increased after chronic denervation. The postjunctional component of supersensitivity to noradrenaline was evidenced by an increase in the maximal response to this agonist. The maximal response of denervated glands was blocked by propranolol 10^–5 M. On the other hand, chronic denervation did not modify the -adrenoceptor-mediated response, K⁺ release, either in the presence or in the absence of ouabain. It is concluded that, after sympathetic denervation, postjunctional supersensitivity develops for the -adrenoceptor-mediated but not for the -adrenoceptors-mediated effects of noradrenaline.     

Effects of sympathectomy on the in vivo α and β-responses of the parotid gland

Summary Salivary secretion in response to noradrenaline and isoprenaline was measured in innervated and chronically sympathectomized parotid glands of the rat. In innervated glands, the responses to isoprenaline lasted longer than those to noradrenaline. Chronic sympathetic denervation enhanced the responses to both noradrenaline and isoprenaline. The magnitude of the supersensitivity to isoprenaline was related to the dose and the time at which the responses were analyzed. Supersensitivity was greater for the initial than for the total secretion elicited by isoprenaline. Propranolol (1 mg/kg) and phenolamine (5 mg/kg) were used in order to determine the relative participation of and -adrenoceptors in the enhanced responses to isoprenaline. The results suggest that postjunctional supersensitivity for the secretory responses of this organ to isoprenaline is mainly mediated through -adrenoceptors of the secretory cells and -adrenoceptors of the myoepithelial cells.     

Importance of sex and relative efficacy at the µ opioid receptor in the development of tolerance and cross-tolerance to the antinociceptive effects of opioids

RATIONALE: Recent studies indicate that mu opioids are generally more potent and effective as antinociceptive agents in male than female rodents. OBJECTIVES: To evaluate the influence of sex on the development of tolerance to the antinociceptive effects of morphine and cross-tolerance to the lower efficacy mu opioids buprenorphine and dezocine in F344 and Lewis rats. METHODS: Using a warm-water tail-withdrawal procedure, the antinociceptive effects of morphine, buprenorphine and dezocine were determined before and during chronic morphine (5, 10 and 20 mg/kg, b.i.d., for 7 and 14 days) administration. RESULTS: Under acute conditions, morphine was more potent in males and during chronic morphine administration tolerance development was generally greater in males. As males were more sensitive to the acute effects of morphine, the functional chronic morphine dose (i.e., chronic morphine dose/acute morphine ED50) administered to males was larger than in females. Analyses of the relationship between the functional chronic morphine dose and tolerance indicated that morphine tolerance development was comparable in males and females. Under acute conditions, buprenorphine and dezocine were more potent and effective in males. During chronic morphine administration, cross-tolerance was conferred to these opioids as evidenced by rightward, and in some cases downward, shifts in their dose-effect curves. Decreases in the maximal effects produced by buprenorphine and dezocine were more frequently observed in females. CONCLUSIONS: That comparable levels of morphine tolerance were obtained in males and females when the functional chronic morphine dose was taken into consideration suggests that the mechanism underlying tolerance is not sex-dependent. Sex differences in the effectiveness of buprenorphine and dezocine when administered acutely and during chronic morphine administration further suggest that these opioids have lower efficacy at the mu opioid receptor in females.     

Combination of α-glucosidase inhibitor and ribavirin for the treatment of dengue virus infection in vitro and in vivo

Cellular α-glucosidases I and II are enzymes that sequentially trim the three terminal glucoses in the N-linked oligosaccharides of viral envelope glycoproteins. This process is essential for the proper folding of viral glycoproteins and subsequent assembly of many enveloped viruses, including dengue virus (DENV). Imino sugars are substrate mimics of α-glucosidases I and II. In this report, we show that two oxygenated alkyl imino sugar derivatives, CM-9-78 and CM-10-18, are potent inhibitors of both α-glucosidases I and II in vitro and in treated animals, and efficiently inhibit DENV infection of cultured human cells. Pharmacokinetic studies reveal that both compounds are well tolerated at doses up to 100mg/kg in rats and have favorable pharmacokinetic properties and bioavailability in mice. Moreover, we showed that oral administration of either CM-9-78 or CM-10-18 reduces the peak viremia of DENV in mice. Interestingly, while treatment of DENV infected mice with ribavirin alone did not reduce the viremia, combination therapy of ribavirin with sub-effective dose of CM-10-18 demonstrated a significantly enhanced antiviral activity, as indicated by a profound reduction of the viremia. Our findings thus suggest that combination therapy of two broad-spectrum antiviral agents may provide a practically useful approach for the treatment of DENV infection.     

Effects of the Aconitum alkaloid mesaconitine in rat hippocampal slices and the involvement of α- and β-adrenoceptors

1. The effects of mesaconitine, the main alkaloid contained in Aconiti tuber, were investigated by use of extracellular recordings of stimulus-evoked population spikes and field excitatory postsynaptic potentials (e.p.s.ps) in the CA1 region of rat hippocampal slices.
2. At a concentration of 10 nM, mesaconitine evoked excitations, which were manifested as an increase in the amplitude of the orthodromic spike and the appearance of multiple spikes following the first postsynaptic spike, without affecting the magnitude of paired-pulse facilitation. The increase in spike amplitude was persistent and was not reversed by up to 90 min of washout. At concentrations of 30 and 100 nM, the alkaloid produced a biphasic effect, that is an excitation followed by an inhibition without having any effect upon the field e.p.s.p. At concentrations above 100 nM, mesaconitine suppressed the orthodromic population spike and the field e.p.s.p.
3. The excitatory effect was also observed when electrical stimulation was stopped completely during the application of mesaconitine (10 nM) and during the first 15 min of washout.
4. The enhancement of the population spike and the appearance of multiple spikes induced by mesaconitine (10–100 nM) were blocked by pretreatment with the β-adrenoceptor antagonists propranolol (1 μM) and timolol (1 μM), whereas the inhibitory effect was blocked by the α-adrenoceptor antagonists yohimbine (1 μM) and phentolamine (10 μM). However, when the β-adrenoceptor antagonist timolol was added 10 min after the application of mesaconitine, it failed to block the long-lasting enhancement of the spike amplitude and the appearance of multiple population spikes.
5. Application of the selective β-adrenoceptor agonist isoprenaline (500 nM) to the hippocampal slices induced an increase in the amplitude of the orthodromic population spike and elicited 2–3 additional spikes. Mesaconitine (10 nM) did not further potentiate this enhancement of the spike amplitude when added after a 15 min pretreatment with isoprenaline.
6. Perfusion of forskolin, which directly activates adenylate cyclase, enhanced the population spike. Mesaconitine had no additional effect when applied after pretreatment with forskolin.
7. It is concluded that the excitatory effects evoked by lower concentrations of the plant alkaloid mesaconitine are mediated by stimulation of β-adrenoceptors and the consequent activation of intracellular processes which lead to the long-lasting changes in excitability.

     

Care of the stroke patient—communication between the community pharmacist and prescribers in the Republic of Ireland

Objective This study sought to examine the perceptions that community pharmacists have of communication with prescribers in both primary and secondary care in Ireland, with respect to care of stroke patients. Setting Community pharmacies across Ireland, stratified into the four representative administrative regions. Method Survey using a structured postal questionnaire. Main outcome measure Perceptions of communication with prescribers based in primary and secondary care; pharmacy and pharmacy premises demographics. Results A response rate of 52% (n = 314) was achieved. Community pharmacists’ perceptions of information provision from secondary care were low, the majority (83%) never received any information from the hospital, although they would welcome it. Communication with hospital based prescribers was considered by most (93%) to be poor. The majority (greater than 75%) of respondents expressed a desire for greater information provision concerning a stroke patient’s medication and diagnostic information. Pharmacists’ perceptions of interaction with general practitioners were generally regarded as good (63%) although information provision in both directions between pharmacist and general practitioner could be improved. Conclusion The findings of this study indicated that community pharmacists perceive that there is room for improvement in the communication between themselves and prescribers in the primary and secondary care settings, concerning the care of the stroke patient. This highlights the need for the development of formal communication channels between community pharmacists and other members of the healthcare team involved in the care of the stroke patient. However, the challenges of communicating patient information across healthcare sectors are recognized.     

Studies on the antinociception effect of melatonin and its mechanism in the rat

The antinociception effect of melatonin（Mel） and its mechanisms were studied on the trauma- pain model in the rat. A trauma - pain model was established in Wistar rats by combining right - hind limb amputation with 50℃ tail - flick test. The effects of Mel on central sensitization were studied through measuring the changes of the expression of c - fos （ an indicator of nociceptive transmission at the spinal level ） , subtance P（SP） and brain - derived neurotrophic factor （BDNF） in the spinal cord using immunohistochemistry. In addition, the contents of nitric oxide （NO） in brain and spinal tissues were also observed. The results showed that the immunoreactivity of BDNF and c - fos in spinal dorsal horn in injuried rats began to increase at the first day,     

Sex differences in the potency of κ opioids and mixed-action opioids administered systemically and at the site of inflammation against capsaicin-induced hyperalgesia in rats

Rationale Sex differences in the potency of the antinociceptive effects of κ opioids have been reported in various acute pain models with evidence suggesting that these sex differences are mediated by activity in the N-methyl-d-aspartate (NMDA) system. Objectives The purpose of the present study was to evaluate sex differences in the antihyperalgesic actions of selected κ and mixed-action opioids in a persistent pain model and determine if the NMDA system modulates these effects in a sexually dimorphic manner. Methods Using gonadally intact male and female F344 rats, hyperalgesia was induced by local administration of capsaicin in the tail, after which the tail was immersed in a mildly noxious thermal stimulus (45°C water), and tail-withdrawal latency measured. Opioids were then administered systemically (s.c.) and locally (in the tail) alone, and in selected combinations with the noncompetitive NMDA antagonist dextromethorphan. Results When administered systemically and locally, the κ opioids spiradoline, U69,593 and U50,488, and the mixed-action opioids butorphanol and nalbuphine, produced dose-dependent antihyperalgesic effects. Whereas the κ opioids were generally more potent in males, sex differences were not observed with the mixed-action opioids. Peripheral receptor activity was confirmed for local administration of κ opioids by the antagonism observed after local, but not intracerebroventricular (i.c.v.), administration of the κ antagonist nor-binaltorphamine (nor-BNI). Dextromethorphan was equally potent in attenuating the antihyperalgesia induced by κ opioids in both males and females. Conclusions These findings demonstrate sex differences in κ opioid activity in a persistent pain model. Although an NMDA antagonist blocked the effects of κ opioids in this model, these effects were not sexually dimorphic as reported in most acute pain models.     

Crosstalk between the proteasome system and autophagy in the clearance of α-synuclein

Aim:

A growing body of evidence suggests that α-synuclein accumulation may play an important role in the pathogenesis of Parkinson''s disease. The aim of this study was to investigate the roles of the proteasome and autophagy pathways in the clearance of wild-type and mutant α-synuclein in PC12 cells.

Methods:

PC12 cells overexpressing either wild-type or A30P mutant α-synuclein were treated with the proteasome inhibitor epoxomicin, the macroautophagy inhibitor 3-MA and the macroautophagy activator rapamycin alone or in combination. The cell viability was assessed using MTT assay. Immunofluorescence and Western blot analysis were used to detect the level of α-synuclein, LAMP-2A, E1 activase, and E2 ligase in the cells. Chymotrypsin-like proteasomal activity was measured using a commercial kit.

Results:

When the proteasome and macroautophagy in the wild-type and mutant cells were inhibited with epoxomicin and 3-MA, respectively, the cell viability was significantly decreased, and the α-synuclein level was increased. Both epoxomicin and 3-MA activated the chaperone-mediated autophagy (CMA) by increasing the level of the CMA-limiting enzyme LAMP-2A. Furthermore, 3-MA or epoxomicin significantly decreased chymotrypsin-like proteasomal activity. 3-MA or epoxomicin did not change E1 activase expression in either mutant or wild-type cells, but increased E2 ligase expression, especially when used together. Macroautophagy inducer rapamycin increased the cell viability and reduced epoxomicin-induced α-synuclein accumulation. Interestingly, CMA was also activated by rapamycin.

Conclusion:

Our results demonstrate the existence of complex crosstalk between different forms of autophagy and between autophagy and the proteasome pathway in the clearance of α-synuclein in PC12 cells.     

Investigation of the distribution and function of ��-adrenoceptors in the sheep isolated internal anal sphincter

BACKGROUND AND PURPOSE

We have investigated the distribution of α-adrenoceptors in sheep internal anal sphincter (IAS), as a model for the human tissue, and evaluated various imidazoline derivatives for potential treatment of faecal incontinence.

EXPERIMENTAL APPROACH

Saturation and competition binding with ³H-prazosin and ³H-RX821002 were used to confirm the presence and density of α-adrenoceptors in sheep IAS, and the affinity of imidazoline compounds at these receptors. A combination of in vitro receptor autoradiography and immunohistochemistry was used to investigate the regional distribution of binding sites. Contractile activity of imidazoline-based compounds on sheep IAS was assessed by isometric tension recording.

KEY RESULTS

Saturation binding confirmed the presence of both α₁- and α₂-adrenoceptors, and subsequent characterization with sub-type-selective agents, identified them as α_1A- and α_2D-adrenoceptor sub-types. Autoradiographic studies with ³H-prazosin showed a positive association of α₁-adrenoceptors with immunohistochemically identified smooth muscle fibres. Anti-α₁-adrenoceptor immunohistochemistry revealed similar distributions of the receptor in sheep and human IAS. The imidazoline compounds caused concentration-dependent contractions of the anal sphincter, but the maximum responses were less than those elicited by l-erythro-methoxamine, a standard non-imidazoline α₁-adrenoceptor agonist. Prazosin (selective α₁-adrenoceptor antagonist) significantly reduced the magnitude of contraction to l-erythro-methoxamine at the highest concentration used. Both prazosin and RX811059 (a selective α₂-adrenoceptor antagonist) reduced the potency (pEC₅₀) of clonidine.

CONCLUSIONS AND IMPLICATIONS

This study shows that both α₁- and α₂-adrenoceptors are expressed in the sheep IAS, and contribute (perhaps synergistically) to contractions elicited by various imidazoline derivatives. These agents may prove useful in the treatment of faecal incontinence.