Recent advances in imidazoline receptor research: ligands--localization and isolation--signaling--functional and clinical studies

A case report of clonidine induced syncope: a review of central actions of an old cardiovascular drug

Background

Clonidine is an imidazoline sympatholytic, acting on both α₂-adrenergic and imidazoline receptors in the brainstem to induce antihypertensive and negative chronotropic effects in the vasculature and heart respectively.

Case presentation

A 69-year-old gentleman with hypertension presented to the emergency department after multiple syncopal episodes over the past 12 months. Electrocardiogram demonstrated sinus bradycardia with a heart rate of 42 beats per minute. It was hypothesized that the antihypertensive agent clonidine was responsible for inducing symptomatic bradycardia. Clonidine was thus gradually tapered and then discontinued over five days restoring normal sinus rhythm rates while avoiding hypertensive rebound related to sympathetic surge. His heart rate and blood pressure remained within normal limits after the clonidine taper and subsequent adjustments to his other hypertensive medications and he was discharged.

Conclusions

While clonidine has fallen out of favor for its indication as an antihypertensive, it remains a viable option for the use of opioid withdrawal, chronic pain, and smoking cessation, necessitating the appropriate clinical and pharmacological competencies for a physician to prescribe. A discussion of the clinical effects of clonidine brainstem receptor activation follows.

Comparison of agmatine with moxonidine and rilmenidine in morphine dependence in vitro: role of imidazoline I(1) receptors

Moxonidine and rilmenidine are classical imidazoline I(1) receptor agonists, and used as anti-hypertension drugs in clinical practice. Agmatine is an imidazoline I(1) receptor endogenous ligand as well as its agonist, but more and more evidences suggest it has no influence on blood pressure. In the present study we compared the effects of moxonidine, rilmenidine and agmatine in the development of morphine dependence, and investigated the role of imidazoline I(1) receptor in the effects of these agents. Chinese hamster ovary cells co-expressing mu opioid receptor and imidazoline receptor antisera-selected protein (IRAS), the strong candidate for imidazoline I(1) receptor, were used as the cell line. cAMP overshoot, which represents an opioid dependent state in vitro, was measured to study the effects on morphine dependence. siRNA against IRAS was carried out to investigate the role of imidazoline I(1) receptor. Moxonidine and rilmenidine (0.01-10 microM) were ineffective on cAMP level in the cells when given alone, and failed to inhibit chronic morphine exposure, naloxone-precipitated cAMP overshoot when co-pretreated with morphine. Agmatine (0.01-10 microM) by itself was ineffective but co-pretreated with morphine concentration-dependently inhibited chronic morphine exposure, naloxone-precipitated cAMP overshoot in the cells. Furthermore, we found that the inhibitory effect of agmatine (100 nM and 1 microM) on cAMP overshoot was significantly reduced by siRNA against IRAS. This study indicates that agmatine can inhibit the development of morphine dependence in vitro, whereas moxonidine and rilmenidine have no the effect. Imidazoline I(1) receptor plays an important role in agmatine inhibiting morphine dependence.

Effects of the beta-carbolines, harmane and pinoline, on insulin secretion from isolated human islets of Langerhans

It is well known that certain imidazoline compounds can stimulate insulin secretion and this has been attributed to the activation of imidazoline I(3) binding sites in the pancreatic beta-cell. Recently, it has been proposed that beta-carbolines may be endogenous ligands having activity at imidazoline sites and we have, therefore, studied the effects of beta-carbolines on insulin secretion. The beta-carbolines harmane, norharmane and pinoline increased insulin secretion two- to threefold from isolated human islets of Langerhans. The effects of harmane and pinoline were dose-dependent (EC(50): 5 and 25 microM, respectively) and these agents also blocked the inhibitory effects of the potassium channel agonist, diazoxide, on glucose-induced insulin release. Stimulation of insulin secretion by harmane was glucose-dependent but, unlike the imidazoline I(3) receptor agonist efaroxan, it increased the rate of insulin release beyond that elicited by 20 mM glucose (20 mM glucose alone: 253+/-34% vs. basal; 20 mM glucose plus 100 microM harmane: 327+/-15%; P<0.01). Stimulation of insulin secretion by harmane was attenuated by the imidazoline I(3) receptor antagonist KU14R (2 (2-ethyl 2,3-dihydro-2-benzofuranyl)-2-imidazole) and was reduced when islets were treated with efaroxan for 18 h, prior to the addition of harmane. The results reveal that beta-carbolines can potentiate the rate of insulin secretion from human islets and suggest that these agents may be useful prototypes for the development of novel insulin secretagogues.

Effect of Clonidine on Cardiac Norepinephrine Spillover in Isolated Rat Heart

The purpose of this study is to determine the effect of clonidine on cardiac norepinephrine spillover utilizing an isolated rat heart preparation with attached cardiac sympathetic nerves. Following a 20-minute stabilization period, the sympathetic ganglion for each heart preparation was electrically stimulated with 10V and 2 Hz for 30 seconds (S1: 60 pulses). Heart rate, left ventricular developed pressure, and coronary perfusion pressure was allowed to return to baseline and the perfusate was randomly switched to Krebs buffer containing one of two treatments: placebo or clonidine (1 microM). After 10 minutes of treatment, the sympathetic ganglion was again electrically stimulated with 10V and 2 Hz for 30 seconds (S2: 60 pulses). The perfusate exiting the heart before, during, and after each electrical stimulation was collected for the determination of cardiac norepinephrine spillover. Clonidine administration significantly reduced cardiac norepinephrine spillover by approximately 50% (P < 0.05) and was associated with a 36% reduction in heart rate (P < 0.05). These findings provide evidence that clonidine can directly suppress NE spillover from cardiac sympathetic nerve terminals. Thus, suppression of cardiac NE by clonidine may be due to stimulation of presynaptic alpha2-adrenergic receptors or imidazoline subtype I receptors located on cardiac sympathetic nerve terminals. Results from our study demonstrate a reduction in cardiac NE spillover by clonidine and provide additional evidence that it can directly suppress peripheral sympathetic activity in that our results were obtained utilizing an isolated perfused heart preparation with attached cardiac sympathetic nerves devoid of any CNS input.

Clonidine toxicity revisited

The incidence of clonidine overdose is increasing, yet there is a paucity of new information regarding treatment options for clonidine toxicity. Reported treatment approaches vary widely, demonstrating the lack of science on which current treatment is based. Available research needs to be reassessed. Neurotransmitters, receptors, endogenous opioids, and baseline sympathetic tone determine the clinical response to clonidine as well as the potential response to drug therapy following clonidine overdose. This article reviews aspects of clonidine toxicity that need to be further investigated. Multicenter research trials will be required to evaluate new treatment options.

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.