Involvement of presynaptic imidazoline receptors in the alpha 2-adrenoceptor-independent inhibition of noradrenaline release by imidazoline derivatives

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Modulation of N-type calcium currents by presynaptic imidazoline receptor activation in rat superior cervical ganglion neurons

Presynaptic imidazoline receptors (R(i-pre)) are found in the sympathetic axon terminals of animal and human cardiovascular systems, and they regulate blood pressure by modulating the release of peripheral noradrenaline (NA). The cellular mechanism of R(i-pre)-induced inhibition of NA release is unknown. We, therefore, investigated the effect of R(i-pre) activation on voltage-dependent Ca(2+) channels in rat superior cervical ganglion (SCG) neurons, using the conventional whole-cell patch-clamp method. Cirazoline (30 μM), an R(i-pre) agonist as well as an α-adrenoceptor (R(α)) agonist, decreased Ca(2+) currents (I(Ca)) by about 50% in a voltage-dependent manner with prepulse facilitation. In the presence of low-dose rauwolscine (3 μM), which blocks the α(2)-adrenoceptor (R(α2)), cirazoline still inhibited I(Ca) by about 30%, but prepulse facilitation was significantly attenuated. This inhibitory action of cirazoline was almost completely prevented by high-dose rauwolscine (30 μM), which blocks R(i-pre) as well as R(α2). In addition, pretreatment with LY320135 (10 μM), another R(i-pre) antagonist, in combination with low-dose rauwolscine (3 μM), also blocked the R(α2)-resistant effect of cirazoline. Addition of guanosine-5-O-(2-thiodiphosphate) (2 mm) to the internal solutions significantly attenuated the action of cirazoline. However, pertussis toxin (500 ng ml(1)) did not significantly influence the inhibitory effect of cirazoline. Moreover, cirazoline (30 μM) suppressed M current in SCG neurons cultured overnight. Finally, omega-conotoxin (omega-CgTx) GVIA (1 μM) obstructed cirazoline-induced current inhibition, and cirazoline (30 μM) significantly decreased the frequency of action potential firing in a partly reversible manner. This cirazoline-induced inhibition of action potential firing was almost completely occluded in the presence of omega-CgTx. Taken together, our results suggest that activation of R(i-pre) in SCG neurons reduced N-type I(Ca) in a pertussis toxin- and voltage-insensitive pathway, and this inhibition attenuated repetitive action potential firing in SCG neurons.

Einfluss der Clonidin-induzierten systemischen Sympathikolyse auf die Oxygenierung und Perfusion der Leber

BACKGROUND

Increased sympathetic nervous activity which induces vasoconstriction and decreases perfusion may be an underlying mechanism behind the development of perioperative liver damage. This animal study was designed to assess how clonidine-induced systemic sympathicolysis affects liver oxygenation with respect to induced hypotension and vasodilatation under physiological conditions.

METHODS

Following ethical approval 17 anesthetized and acutely instrumented pigs were randomly assigned to 2 groups. Group 1 consisted of 8 animals receiving intravenous clonidine (2 microg x kg(-1) bolus and 2 microg x kg(-1) x h(-1) for induction of sympathicolysis and group 2 consisted of 9 animals serving as controls. After obtaining baseline values, measurements were repeated 90 and 250 min after starting to reduce systemic sympathetic nervous activity.

RESULTS

Clonidine-induced systemic sympathicolysis was associated with decreased mean arterial blood pressure, cardiac output and heart rate. Portal venous and hepatic arterial blood flow, oxygen delivery to the liver, oxygen uptake and liver tissue oxygen partial pressure remained unchanged. The plasma indocyanine green disappearance rate increased.

CONCLUSION

We concluded that despite decreased mean arterial pressure and cardiac output, clonidine-induced systemic sympathicolysis did not affect liver oxygenation or perfusion.

Effects of systemically applied clonidine on intestinal perfusion and oxygenation in healthy pigs during general anaesthesia and laparotomy 1

BACKGROUND AND OBJECTIVE

Clonidine, which is used for induction of sympatholysis and prevention or treatment of alcohol withdrawal in anaesthesia and intensive care medicine, may have deleterious effects on intestinal mucosal perfusion. This study was designed to investigate the effects of clonidine on intestinal perfusion and oxygenation.

METHODS

Following ethical approval 17 anaesthetized, and acutely instrumented pigs were randomly assigned to two groups: eight animals received intravenous clonidine (2 microg kg(-1) bolus and 2 microg kg(-1) h(-1)), nine animals served as a control group. Measurement points for systemic and regional haemodynamic and oxygenation parameters were 135 and 315 min after starting the clonidine application.

RESULTS

Clonidine elicited systemic haemodynamic changes (median [25-75th interquartile range]): heart rate (106 [91, 126] to 84 [71, 90] beats min(-1)) cardiac output (147 [123, 193] to 90 [87, 107] mL min(-1) kg(-1)) and mean arterial pressure (77 [72, 93] to 69 [61, 78] mmHg) decreased. Despite systemic haemodynamic changes, the superior mesenteric artery blood flow did not change in the clonidine group. The vascular resistance of the superior mesenteric artery decreased. The small intestinal oxygen supply, the mucosal and the serosal tissue oxygen partial pressure did not change.

CONCLUSIONS

Systemic sympatholysis induced by intravenously applied clonidine in addition to basic intravenous anaesthesia elicited a decrease in cardiac output and mean arterial pressure. However, regional macrohaemodynamic perfusion was maintained and intestinal oxygenation did not change. Clonidine does not impair intestinal mucosal and serosal oxygenation under physiological conditions.

Non-adrenergic inhibition at prejunctional sites by agmatine of purinergic vasoconstriction in rabbit saphenous artery

We investigated the effects of agmatine, clonidine, xylazine and moxonidine on the purinergic vasoconstriction induced by electrical stimulation in the rabbit isolated saphenous artery without endothelium. Transmural electrical stimulations induced reproducible responses in the arterial preparations, which were abolished by tetrodotoxin at 0.1 microM or pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt (PPADS, 30 microM), but were not affected by 1 microM prazosin. Clonidine, xylazine and moxonidine induced transient and concentration-independent vasoconstriction, with threshold concentrations of 1, 3 and 30 microM, respectively. Agmatine, in contrast, did not produce any vascular response even at 1 mM. Lower concentrations of clonidine, xylazine and moxonidine (0.01-0.3 microM) concentration-dependently decreased vasoconstrictor responses to electrical stimulation, whereas agmatine (0.1-1 mM) induced an inhibitory followed by a facilitatory effect on electrically evoked responses. Agmatine, clonidine and moxonidine but not xylazine significantly enhanced the vasoconstriction elicited by 1 mM ATP. The concentration-response curve for NA was shifted to the left slightly by 1 mM agmatine, but not affected by 0.3 microM of other three agonists. Phenoxybenzamine did not affect the vasoconstrictive responses to 1mM ATP and to electrical stimulations, but abolished those to NA. Agmatine at 1mM evoked only an inhibitory effect on electrical stimulation-induced vasoconstriction in the preparation pretreated with phenoxybenzamine, and the inhibitory action was enhanced to 38.6% from the control value (without treatment with phenoxybenzamine) of 22.5%. The non-imidazoline compound xylazine at 0.3 microM lost its inhibitory effect on the neurogenic vasoconstriction in the presence of phenoxybenzamine. In conclusion, agmatine produces a biphasic effect on the purinergic vasoconstriction induced by sympathetic nerve stimulation in the rabbit isolated saphenous artery. The monophasic inhibition of agmatine in the artery treated with phenoxybenzamine is due to an alpha-adrenoceptor-independent mechanism at prejunctional sites, and the potentiation effect of agmatine is mainly dependent on its enhancement of vasoconstriction at postjunctional sites.

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.

Synthesis and structure of novel 2,3, 5,6-tetrahydro-1H-imidazo[2,1-b][1,3,5]benzotriazepines with vasocontractile activity in rabbit aortic rings

A series of novel 2,3,5,6-tetrahydro-1H-imidazo[2,1-b][1,3,5]benzotriazepines (3a-n) and hydrochlorides (4a-b) were prepared and their structure was determined by IR and NMR spectroscopic data as well as by X-ray analysis of the hydrochloride 4a. The newly synthesized benzotriazepine 4b exhibited concentration-dependent vasocontractile activity in isolated rabbit aortic rings. Rimalkalim was found to induce a relaxation in rabbit aortic rings precontracted by 4b (3x10(-5) mol). Present results indicate that K(ATP)-dependant channels may contribute in the contractile activity of benzotriazepine 4b.

Modification of noradrenaline release in pithed spontaneously hypertensive rats by I1-binding sites in addition to alpha2-adrenoceptors

It is known that moxonidine acts as an agonist at presynaptic alpha(2)-adrenoceptors of the postganglionic sympathetic nerve terminals and leads to a reduction in noradrenaline release. In addition, it is conceivable that I(1)-binding sites located in other regions of the pre- and postganglionic sympathetic neurons are involved in this effect. Our aim was to investigate whether and to what extent activation of the I(1)-binding sites contributes to the moxonidine-induced inhibition of noradrenaline release. Noradrenaline release was induced in pithed spontaneously hypertensive rats (pretreated with phenoxybenzamine/desipramine at 10/0.5 mg/kg) by stimulation of sympathetic overflow from the spinal cord. Noradrenaline overflow was reduced using moxonidine (0.18, 0.6, and 1.8 mg/kg) by 39.4, 70.4, or 78.7%, respectively, even when all alpha(1)-/alpha(2)-adrenoceptors were blocked effectively by phenoxybenzamine. In contrast, the I(1)-antagonist efaroxan (0.1, 1, and 3 mg/kg) increased noradrenaline overflow from 453 (control) to 1710, 1999, or 2754 pg/ml, suggesting an autoreceptor-like function of I(1)-binding sites. In consequence, moxonidine (0.18, 0.6, and 1.8 mg/kg) reduced the increase in noradrenaline overflow in efaroxan-treated animals (1 mg/kg) by 22.7, 41.7, and 50.5%, respectively. Agmatine (6 and 60 mg/kg), an endogenous agonist at I(1)-binding sites, reduced noradrenaline overflow (-36 or 53%), even under alpha(2)-adrenoceptor blockade. When 2-endo-amino-3-exo-isopropylbicyclo[2.2.1]heptane (AGN192403) (10 mg/kg) was injected, a selective blocker of I(1)-binding sites, noradrenaline overflow was not influenced by agmatine. It is concluded that moxonidine reduces noradrenaline overflow by acting at I(1)-binding sites in addition to its agonistic property at alpha(2)-adrenoceptors. The exact location of the I(1)-binding sites on the pre- or postsynaptic sympathetic neurons is unknown, but the location in the pre- or postsynaptic membrane of the sympathetic ganglion is the most plausible explanation.

Presynaptic effects of moxonidine in isolated buffer perfused rat hearts: role of imidazoline-1 receptors and alpha2-adrenoceptors

Numerous studies support the concept that centrally acting antihypertensive drugs, such as imidazolines, mediate sympathoinhibition not only via activation of central nervous alpha2-adrenoceptors (alpha2-AR) but also via imidazoline-1 receptors (I1-R). An additional presynaptic involvement in sympathetic neurotransmission of imidazolines, via I1-R independent of alpha2-AR, is still controversial and remains to be clarified in the heart. Concentration response curves on endogenous norepinephrine (NE) overflow evoked by stimulation of epicardial postganglionic sympathetic nerves in isolated buffer-perfused rat hearts were performed for brimonidine, moxonidine, rauwolscine, 2-endo-amino-3-exo-isopropylbicyclo[2.2.1]heptane (AGN192403), and efaroxan. To unmask an I1-R-mediated effect of moxonidine, hearts were pre-exposed in additional experiments with brimonidine or rauwolscine with or without AGN192403 or efaroxan, respectively. Moxonidine reduced stimulated NE overflow (log EC50: -6.15 +/- 0.14). AGN192403, a selective ligand at I1-R, had no influence on the dose-response curve of moxonidine (log EC50: -6.01 +/- 0.25). After pre-exposure to brimonidine [ stimulation 1 (S1) + stimulation 2 (S2); 10(-5) M], the inhibitory action of moxonidine was potentiated compared with control (32.0 +/- 2.8 versus 73.13 +/- 3.0%) and completely abolished with AGN192403 or efaroxan. This effect was also totally inhibited by pre-exposure with indomethacin (10(-7) M) and tricyclodecan-9-yl-xanthogenate (D-609), an inhibitor of phosphatidylcholine-selective phospholipase C (PC-PLC; 10(-7) M). Conversely, moxonidine was without modulating efficacy under alpha2-AR-blockade by rauwolscine. In summary, we demonstrate that moxonidine reduces NE release independently of I1-R, demonstrating the prominent effect of alpha2-AR in cardiac tissue under basal conditions. We also demonstrate that I1-R are involved in NE release under conditions of alpha2-AR-stimulation involving both a pathway of prostaglandins and PC-PLC.

Imidazoline receptors in the heart: characterization, distribution, and regulation

Imidazoline receptors were identified in cardiac tissues of various species. Imidazoline receptors were immunolocalized in the rat heart. Membrane binding and autoradiography on frozen heart sections using 0.5 nM para-iodoclonidine (125I-PIC) revealed that binding was equally and concentration-dependently inhibited by epinephrine and imidazole-4-acetic acid (IAA), implying 125I-PIC binding to cardiac alpha2-adrenergic and I1-receptors, respectively. After irreversible blockade of alpha2-adrenergic receptors, binding was inhibited by the selective I1-agonist, moxonidine, and the I1-antagonist, efaroxan, in a concentration-dependent (10-12 to 10-5 M) manner. Calculation of kinetic parameters revealed that in canine left and right atria, I1-receptor Bmax was 13.4 +/- 1.7 and 20.1 +/- 3.0 fmol/mg protein, respectively. Compared to age-matched normotensive Wistar Kyoto rats, I1-receptors were increased in 12-week-old hypertensive rat (SHR) right (22.6 +/- 0.3 to 43.7 +/- 4.4 fmol/unit area, p < 0.01) and left atria (13.3 +/- 0.6 to 30.2 +/- 4.1 fmol/unit area, p < 0.01). Also, compared to corresponding normal controls, Bmax was increased in hearts of hamsters with advanced cardiomyopathy (13.9 +/- 0.4 to. 26.0 +/- 2.3 fmol/unit area, p < 0.01) and in human ventricles with heart failure (12.6 +/- 1.3 to 35.5 +/- 2.9 fmol/mg protein, p < 0.003). These studies demonstrate that the heart possesses imidazoline I1-receptors that are up-regulated in the presence of hypertension or heart failure, which would suggest their involvement in cardiovascular regulation.

Noradrenaline release-inhibiting receptors on PC12 cells devoid of alpha(2(-)) and CB(1) receptors: similarities to presynaptic imidazoline and edg receptors

The aim of the present study was to classify release-inhibiting receptors on rat pheochromocytoma PC12 cells. Veratridine-evoked [3H]noradrenaline release from PC12 cells was inhibited by micromolar concentrations of the imidazoline and guanidine derivatives cirazoline, clonidine, aganodine, 1,3-di(2-tolyl)guanidine, BDF6143 and agmatine, and of the cannabinoid receptor agonist WIN55,212-2 (R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-yl](1-naphthalenyl)methanone mesylate), but not by noradrenaline. The inhibitory effect of clonidine was antagonized by micromolar concentrations of rauwolscine and SR141716A (N-[piperidin-1-yl]-5-[4-chlorophenyl]-1-[2,4-dichlorophenyl]-4-methyl-1H-pyrazole-3-carboxamide). The potencies of the agonists and antagonists were compatible with an action at previously characterized presynaptic imidazoline receptors. 1-Oleoyl-lysophosphatidic acid, but not sphingosine-1-phosphate, produced an inhibition of release that was antagonized by 30 microM rauwolscine, 1 microM SR141716A and 10 microM LY320135 as well as by pretreatment of the cells with 100 microM clonidine for 72 h. Polymerase chain reaction (PCR) experiments on cDNA from PC12 mRNA suggest mRNA expression of lysophospholipid receptors encoded by the genes edg2, edg3, edg5 and edg7, but not of receptors encoded by edg1, edg4, edg6 and edg8, and not of alpha(2A(-))nd CB(1) receptors. In conclusion, PC12 cells are not endowed with alpha(2)-adrenoceptors and CB(1) cannabinoid receptors, but with an inhibitory receptor recognizing imidazolines, guanidines and WIN55,212-2 similar to that on sympathetic nerves. The PCR results and the ability of 1-oleoyl-LPA to mimic these drugs (also with respect to their susceptibility to antagonists) suggest that the release-inhibiting receptor may be an edg-encoded lysophospholipid receptor.

Imidazoline antihypertensive drugs: a critical review on their mechanism of action

It was long thought that the prototypical centrally acting antihypertensive drug clonidine lowers sympathetic tone by activating alpha(2)-adrenoceptors in the brain stem. Supported by the development of two new centrally acting drugs, rilmenidine and moxonidine, the imidazoline hypothesis evolved recently. It assumes the existence of a new group of receptors, the imidazoline receptors, and attributes the sympathoinhibition to activation of I(1) imidazoline receptors in the medulla oblongata. This review analyzes the mechanism of action of clonidine-like drugs, with special attention given to the imidazoline hypothesis. Two conclusions are drawn. The first is that the arguments against the imidazoline hypothesis outweigh the observations that support it and that the sympathoinhibitory effects of clonidine-like drugs are best explained by activation of alpha(2)-adrenoceptors. The second conclusion is that this class of drugs lowers sympathetic tone not only by a primary action in cardiovascular regulatory centres in the medulla oblongata. Peripheral presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall sympathoinhibition.

Hu 210: a potent tool for investigations of the cannabinoid system

The synthetic compound HU 210 displays a multiplicity of biochemical, pharmacological, and behavioral effects, most of which have been demonstrated to be dependent on a selective agonistic activity at CB(1) and CB(2) cannabinoid receptors and to involve the main neurotransmitter systems. Results obtained in various studies suggest a potential clinical application of this highly potent drug (e.g., as antipyretic, antiinflammatory, analgesic, antiemetic, and antipsychotic agent) as well as its usefulness in research aimed to develop a better understanding of the involvement of the endogenous cannabinoid system in a number of physiopathological functions.

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.

Presynaptic imidazoline receptors. New developments in characterization and classification

Presynaptic imidazoline receptors (IRs) which inhibit norepinephrine (NE) release from sympathetic nerve endings have been identified in cardiovascular tissue of man, rabbit, rat, and guinea pig. They do not belong to one of the classical presynaptic inhibitory receptor classes such as alpha 2-adrenoceptors or H3 histamine receptors, and there is also no relation to I1- and I2-imidazoline binding sites. Segments of human right atrial appendages preincubated with [3H]NE were used to determine unknown pharmacologic properties of the presynaptic IRs. In the presence of 1 microM rauwolscine, S23230, the (-)-enantiomer of the racemic oxazoline derivative S22687 (5-(2(methyl-phenoxy-methyl)-1,3-oxazoline-2-yl)amine) exhibited low potency in inhibiting the electrically evoked [3H]NE release (pIC30% = 4.96), whereas the (+)-enantiomer S23229 and the racemate S22687 were ineffective. The IR-mediated inhibitory effect of the imidazoline BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline) and the guanidine aganodine on evoked [3H]NE release from sympathetic nerves in human atrial appendages was counteracted by rauwolscine (with very low potency) and by the cannabinoid CB1-receptor antagonist SR141715A (N-[piperdin-1-yl]-5-[4-chlorophenyl]-2,4-dichlorophenyl]-4- methyl-1H-pyrazole-3-carboxamide). The inhibitory effect of moxonidine on evoked [3H]NE release (which is exclusively mediated via activation of alpha 2-autoreceptors) was antagonized with high potency by rauwolscine, but not by SR141716A. The cannabinoid CB1 receptor agonists CP55,940([(-)-Cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl] -trans-4- (3-hydroxy-propyl)cyclohexane]) and anandamide inhibited evoked [3H]NE release. Inhibition by CP55,940 and anandamide was abolished by 1 microM SR141716A as well as by 30 microM rauwolscine. In radioligand binding experiments on membranes from human atrial appendages (alpha 2- and sigma-binding sites were masked), cannabinoid receptor ligands and IR agonists displaced the radiolabeled guanidine derivative [3H]DTG (1,3-di-o-tolyguanidine, an agonist at presynaptic IRs) from its binding sites. Comparison of the potencies of these drugs determined in the competition experiments with [3H]DTG with those in inhibiting NE release via activation of the presynaptic IRs in the same tissue revealed a correlation. The present results suggest, e.g., that the presynaptic IRs may have certain binding domains in common with presynaptic cannabinoid receptors or that both receptors are different proteins which interact with each other in an unknown manner.

Affinity isolation of imidazoline binding proteins from rat brain using 5-amino-efaroxan as a ligand

We have employed an amino derivative of the imidazoline ligand, efaroxan, to isolate imidazoline binding proteins from solubilised extracts of rat brain, by affinity chromatography. A number of proteins were specifically retained on the affinity column and one of these was immunoreactive with an antiserum raised against the ion conducting pore component of the ATP-sensitive potassium channel. Patch clamp experiments confirmed that, like its parent compound, amino-efaroxan blocks ATP-sensitive potassium channels in human pancreatic beta-cells and can stimulate the insulin secretion from these cells. The results reveal that a member of the ion conducting pore component family is strongly associated with imidazoline binding proteins in brain and in the endocrine pancreas.

Presynaptic imidazoline receptors mediate inhibition of noradrenaline release from sympathetic nerves in rat blood vessels

In rat vena cava and aorta preincubated with [3H]noradrenaline the involvement of imidazoline receptors in modulation of [3H]noradrenaline release from sympathetic nerves was investigated. In the vena cava, the guanidine 1,3-di(2-tolyl)guanidine (DTG) inhibited the electrically evoked [3H]noradrenaline release; the inhibitory effect was more pronounced in the presence than in the absence of the alpha 2-adrenoceptor antagonist rauwolscine. The concentration-response curves of BDF 6143 [4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline], and idazoxan for their facilitatory effect on electrically evoked [3H]noradrenaline release was bell-shaped; in the presence of rauwolscine, BDF 6143 inhibited the evoked [3H]noradrenaline release, whereas idazoxan did not. After blockade of alpha 2-autoreceptors by rauwolscine, the electrically evoked [3H]noradrenaline release from vena cava was inhibited not only by DTG and BDF 6143 but also by aganodine, clonidine and cirazoline; the rank order of potency of most of the drugs was similar to that found at the presynaptic imidazoline receptors in the rabbit aorta and pulmonary artery as well as in human atrial appendages. In the presence of rauwolscine, clonidine-induced inhibition of electrically evoked [3H]noradrenaline release was counteracted by 1 microM of the selective CB1 receptor antagonist SR141716A (N-[piperidin-1-yl]-5-[4-chlorophenyl]-1-[2,4-dichlorophenyl] -4-methyl-1H-pyrazole-3-carboxamide). In the aorta, BDF 6143 and cirazoline did not modify [3H]noradrenaline release in the absence of alpha 2-adrenoceptor blockade; in the presence of rauwolscine, the electrically evoked [3H]noradrenaline release from aorta was inhibited by BDF 6143, cirazoline, aganodine and clonidine with a rank order of potency similar to that in the vena cava. SR141716A 1 microM antagonized the inhibitory effect of BDF 6143 and clonidine (in the presence of rauwolscine). In conclusion, noradrenaline release in rat vena cava and aorta is inhibited via presynaptic imidazoline receptors which appear to be related to those previously characterized in rabbit and human cardiovascular tissue.

Alpha 2-adrenoceptors and I1-imidazoline binding sites: relationship with catecholamines in women of reproductive age

A comparison is presented between plasma catecholamine concentrations and platelet [125I]-p-iodoclonidine binding sites in 16 healthy women. Blood samples were obtained at six regularly spaced intervals over two consecutive menstrual cycles from healthy women with regular menstrual periods. Although no cycle-related changes were observed per se, there were significant correlations between the platelet binding sites and plasma norepinephrine and epinephrine concentrations. The densities of platelet alpha 2-adrenoceptors were negatively correlated in an exponential fashion (r2 = 0.694, P = 0.009) with plasma epinephrine concentrations, implying agonist-induced downregulation. On the other hand, platelet I1-imidazoline binding sites were positively correlated with plasma concentrations of norepinephrine in a linear fashion (r2 = 0.326, P = 0.021). This is the first indication that I1 binding sites might be upregulated by a physiological factor. Furthermore, the data suggest that elevations in plasma norepinephrine might explain reports of upregulated I1 binding sites in depressed patients.

Presynaptic imidazoline receptors and non-adrenoceptor [3H]-idazoxan binding sites in human cardiovascular tissues

1 In segments of human right atrial appendages and pulmonary arteries preincubated with [3H]-noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, the involvement of imidazoline receptors in the modulation of [3H]-noradrenaline release was investigated. 2 In human atrial appendages, the guanidines aganodine and DTG (1,3-di(2-tolyl)guanidine) which activate presynaptic imidazoline receptors, inhibited electrically-evoked [3H]-noradrenaline release. The inhibition was not affected by blockade of alpha 2-adrenoceptors with 1 microM rauwolscine, but antagonized by extremely high concentrations of this drug (10 and/or 30 microM; apparent pA2 against aganodine and DTG: 5.55 and 5.21, respectively). 3 In the presence of 1 microM rauwolscine, [3H]-noradrenaline release in human atrial appendages was also inhibited by the imidazolines idazoxan and cirazoline, but not by agmatine and noradrenaline. The inhibitory effects of 100 microM idazoxan and 30 microM cirazoline were abolished by 30 microM rauwolscine. 4 In the atrial appendages, the rank order of potency of all guidelines and imidazolines for their inhibitory effect on electrically-evoked [3H]-noradrenaline release in the presence of 1 microM rauwolscine was: aganodine > or = BDF 6143 [4-chloro-2-(2-imidazolin-2-yl-amino)-isoindoline] > DTG > or = clonidine > cirazoline > idazoxan (BDF 6143 and clonidine were previously studied under identical conditions). This potency order corresponded to that previously determined at the presynaptic imidazoline receptors in the rabbit aorta. 5 When, in the experiments in the human pulmonary artery, rauwolscine was absent from the superfusion fluid, the concentration-response curve for BDF 6143 (a mixed alpha 2-adrenoceptor antagonist/imidazoline receptor agonist) for its facilitatory effect on electrically-evoked [3H]-noradrenaline release was bell-shaped. In the presence of 1 microM rauwolscine, BDF 6143 and cirazoline concentration-dependently inhibited the evoked [3H]-noradrenaline release. 6 In human atrial appendages, non-adrenoceptor [3H]-idazoxan binding sites were identified and characterized. The binding of [3H]-idazoxan was specific, reversible, saturable and of high affinity (KD: 25.4 nM). The specific binding of [3H]-idazoxan (defined by cirazoline 0.1 mM) to membranes of human atrial appendages was concentration-dependently inhibited by several imidazolines and guanidines, but not by rauwolscine and agmatine. In most cases, the competition curves were best fitted to a two-site model. 7 The rank order of affinity for the high affinity site (in a few cases for the only detectable site; cirazoline = idazoxan > BDF 6143>DTG> or = clonidine) is compatible with the pharmacological properties of I2-imidazoline binding sites, but is clearly different from the rank order of potency for inhibiting evoked noradrenaline release from sympathetic nerves in the same tissue. 8 It is concluded that noradrenaline release in the human atrium and, less well established, in the pulmonary artery is inhibited via presynaptic imidazoline receptors. These presynaptic imidazoline receptors appear to be related to those previously characterized in rabbit aorta and pulmonary artery, but differ clearly from I1 and I2 imidazoline binding sites.

The serotonergic and noradrenergic systems of the hippocampus: their interactions and the effects of antidepressant treatments

Previous reviews have well illustrated how antidepressant treatments can differentially alter several neurotransmitter systems in various brain areas. This review focuses on the effects of distinct classes of antidepressant treatments on the serotonergic and the noradrenergic systems of the hippocampus, which is one of the brain limbic areas thought to be relevant in depression: it illustrates the complexity of action of these treatments in a single brain area. First, the basic elements (receptors, second messengers, ion channels, ...) of the serotonergic and noradrenergic systems of the hippocampus are revisited and compared. Second, the extensive interactions occurring between the serotonergic and the noradrenergic systems of the brain are described. Finally, issues concerning the short- and long-term effects of antidepressant treatments on these systems are broadly discussed. Although there are some contradictions, the bulk of data suggests that antidepressant treatments work in the hippocampus by increasing and decreasing, respectively, serotonergic and noradrenergic neurotransmission. This hypothesis is discussed in the context of the purported function of the hippocampus in the formation of memory traces and emotion-related behaviors.

A series of novel imidazoline I2-receptor selective Schiff bases of 1-(benzylidenamino)-3,3-dimethylguanidine

A series of 12 Schiff bases of 1-(benyzylidenamino)-3,3-dimethylguanidine were evaluated for their affinities at imidazoline I2-receptors and alpha 2-adrenoceptors in the guinea pig cerebral cortex by using a 2-curve assay with [3H]idazoxan as ligand and the simultaneous computer analysis of (-)adrenaline masked and non-masked competition curves. All of the dimethylguanidines were selective for I2-receptors with the selectivity ranging 6-110-fold, and the most selective being WAP8 (2-bromobenzylidenamino-3,3-dimethylguanidine) closely followed by FLA151 (2-chlorobenzylidenamino-3, 3-dimethylguanidine). Some of the dimethylguanidines were also tested for affinity at bovine kidney cortex I2-receptors and guinea pig alpha 1-adrenoceptors. These substances were found to have similar affinities for bovine kidney I2-receptors as for guinea pig cerebral cortex I2-receptors, but their affinities for the alpha 1-adrenoceptors were considerably lower.

Effects of imidazolines on noradrenaline release in brain: an investigation into their relationship to imidazoline, alpha 2 and H3 receptors

The present study was carried out to clarify whether the imidazolines clonidine, moxonidine and cirazoline as well as the guanidine aganodine inhibit noradrenaline release in the rat and rabbit brain via imidazoline receptors, alpha 2-adrenoceptors and/or histamine H3 receptors. Slices or synaptosomes from the rat or the rabbit brain were incubated with 3H-noradrenaline and exposed to phenoxybenzamine, which irreversibly blocks presynaptic alpha 2-adrenoceptors and, at considerably lower potency, imidazoline receptors. Tritium overflow in the superfused preparations was evoked electrically (3 Hz; slices) or by K+ 15 mmol/l (synaptosomes). Noradrenaline and rauwolscine, which possess low affinity, if any, for imidazoline receptors, were used as reference drugs. The evoked overflow in rat brain cortex slices and synaptosomes and in rat medulla oblongata slices, not exposed to phenoxybenzamine, was inhibited by clonidine, moxonidine and noradrenaline. Phenoxybenzamine markedly attenuated the effect of each drug to about the same extent. In rabbit brain cortex slices, not exposed to phenoxybenzamine, the evoked overflow was inhibited by clonidine, moxonidine, aganodine and noradrenaline, facilitated by BDF 6143 (4-chloro-2-(2-imidazoline-2-yl-amino)-isoindoline), idazoxan and rauwolscine and not affected by cirazoline. In slices exposed to phenoxybenzamine, the inhibitory effects of the imidazolines, of aganodine and of noradrenaline were again attenuated by about the same high degree, the facilitatory effects of BDF 6143, idazoxan and rauwolscine were abolished and cirazoline produced a slight inhibition of the evoked overflow. The latter effect was not affected by high concentrations of rauwolscine and idazoxan (at which these drugs act antagonistic at imidazoline receptors in other models). The specific binding of 3H-N alpha-methylhistamine to H3 receptors in rat brain cortex membranes was displaced only by high concentrations of moxonidine (pKi = 6.16) and at even lower affinity by aganodine, BDF 6143, cirazoline, clonidine and idazoxan (pKi < 5). Histamine, which was used as a reference drug, proved to be very potent (pKi = 8.20). In conclusion, imidazolines affect noradrenaline release in the rat and rabbit brain cortex and medulla oblongata via alpha 2-adrenoceptors but not via imidazoline receptors resembling the presynaptic imidazoline receptors previously identified in peripheral tissues of the rabbit. In addition, the involvement of I1- or I2-imidazoline binding sites or of H3 receptors is very improbable in view of the low affinity of aganodine, moxonidine and/or clonidine for these recognition sites and/or incompatibility of the rank order of their affinities with the potencies of the drugs in inhibiting noradrenaline release.

Further biochemical characterization of imidazoline binding sites from the human brainstem

Biochemical characteristics of imidazoline specific binding sites from the human brainstem were further investigated using [3H]idazoxan as radiolabeled ligand. The study of the interaction of [3H]idazoxan binding sites with heparin and lectins (soybean and lentil lectin) confirm the heterogeneity of these sites in the human brain. In fact, about 10-15% of [3H]idazoxan binding sites were retained by each of the three supports used, leading to the hypothesis that two populations of sites, with different biochemical characteristics, coexist in this tissue. A small proportion of [3H]idazoxan binding sites was retained on an affinity chromatography support consisting of a clonidine-derived Pharmalink column. The binding activity of these clonidine-eluted sites was markedly and dose-dependently improved by the addition of 'treated fall-through' fraction from the same column. On the other hand, this 'treated fall-through' fraction inhibited the binding activity detected in the solubilized human brainstem membranes. These results also suggest the existence of heterogeneous imidazoline specific binding sites in the human brainstem and the existence of endogenous factors able to discriminate between them.

Are imidazoline receptors involved in sympathetic neurotransmission in rat vas deferens?

1. An involvement of imidazoline receptors in the modulation of neurotransmitter release was investigated in the prostatic portion of the rat vas deferens stimulated transmurally at 0.2 Hz or by single pulses. 2. Idaxozan and yohimbine induced a concentration-dependent potentiation of the contractile response to 0.2-Hz transmural stimulation in the epididymal and prostatic portion of the vas. 3. After reserpine treatment, idazoxan, but not yohimbine, still potentiated the contractile response, suggesting a possible involvement of imidazoline receptors. 4. Clonidine and rilmenidine, agonists with different affinities to alpha 2-adrenoceptors and imidazoline receptors, inhibited with the same potency the contractile responses to a single pulse transmural stimulation. 5. Yohimbine (a selective alpha 2-adrenoceptor antagonist) antagonized the inhibitory concentration effect curve to rilmenidine in a competitive manner. pA2 values for idaxozan (an antagonist to alpha 2-adrenoceptors and imidazoline receptors) were not different when noradrenaline or rilmenidine were used as agonists. Phenoxybenzamine blocked the effect of both agonists. 6. Thus, the potency relationship of agonists, as well as the effect of the antagonists, did not favor the hypothesis that imidazoline receptors are involved in the idazoxan-potentiating effect in the rat vas deferens.

Pharmacological modulation of immunoreactive imidazoline receptor proteins in rat brain: relationship with non-adrenoceptor [3H]-idazoxan binding sites

1. The densities of various imidazoline receptor proteins (with apparent molecular masses of approximately 29/30-45- and 66-kDa) were quantitated by immunoblotting in the rat cerebral cortex after various drug treatments. The modulation of these imidazoline receptor proteins was then compared with the changes in the density of non-adrenoceptor [3H]-idazoxan binding sites (I2-sites) induced by the same drug treatments. 2. Chronic treatment (7 days) with the I2-selective imidazol(in)e drugs idazoxan (10 mg kg-1), cirazoline (1 mg kg-1) and LSL 60101 (10 mg kg-1) differentially increased the immunoreactivity of imidazoline receptor proteins. The levels of the 29/30-kDa protein were increased by idazoxan and LSL 60101 (23%), the levels of the 45-kDa protein only by cirazoline (44%) and those of the 66-kDa protein only by idazoxan (50%). These drug treatments also increased the density of I2-sites (32-42%). 3. Chronic treatment (7 days) with efaroxan (10 mg kg-1), RX821002 (10 mg kg-1) and yohimbine (10 mg kg-1), which possess very low affinity for I2-imidazoline receptors, did not alter either the immunoreactivity of imidazoline receptor proteins or the density of I2-sites. 4. Chronic treatment (7 days) with the monoamine oxidase (MAO) inhibitors clorgyline (10 mg kg-1) and phenelzine (10 mg kg-1) decreased the immunoreactivity of the 29/30-kDa (17-24%), 45-kDa (19%) and 66-kDa (23-31%) imidazoline receptor proteins. The alkylating agent N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (1.6 mg kg-1, 6 h) also decreased the levels of the three imidazoline receptor proteins (20-47%). These drug treatments consistently decreased the density of I2-sites (31-57%). 5. Significant correlations were found when the mean percentage changes in immunoreactivity of imidazoline receptor proteins were related to the mean percentage changes in the density of I2-sites after the various drug treatments (r = 0.92 for the 29/30-kDa protein, r = 0.69 for the 45-kDa protein and r = 0.75 for the 66-kDa protein). 6. In the rat cerebral cortex the I2-imidazoline receptor labelled by [3H]-idazoxan is heterogeneous in nature and the related imidazoline receptor proteins (29/30-, 45- and 66-kDa) detected by immunoblotting contribute differentially to the modulation of I2-sites after drug treatment.

The actions of medetomidine may not be mediated exclusively by alpha 2-adrenoceptors in the equine saphenous vein

Spirals of endothelially denuded equine saphenous vein were used to study the pre- and post-junctional effects of medetomidine in vitro. The pD2 values were calculated for noradrenaline (6.7 +/- 0.1), phenylephrine (5.6 +/- 0.1), BHT 920 (6.2 +/- 0.2) and UK 14304 (5.7 +/- 0.2). Medetomidine produced a biphasic response, with a pD(2)1 of 8.2 +/- 0.1 and a pD(2)2 of 5.7 +/- 0.1 in the equine saphenous vein (n = 6). Prazosin (10(-7) M) significantly shifted the second phase of the medetomidine concentration-response curve to the right (pD(2)1 was 8.1 +/- 0.2 and pD(2)2 was 5.0 +/- 0.2, P < 0.05). Rings of equine saphenous vein were electrically stimulated to investigate the pre-junctional effects of medetomidine. Increasing concentrations of the alpha 2-adrenoceptor agonist BHT 920 reduced the response to electrical stimulation in a concentration dependent manner to a maximum of 40 +/- 5%, whereas medetomidine (0.1-100 nM) caused a concentration dependent enhancement to a maximum of 490 +/- 150%. These results suggest alpha 1- and alpha 2-adrenoceptors are functional in the equine saphenous vein, but that medetomidine is not acting exclusively as an alpha 2-adrenoceptor agonist.

Alpha 2 noradrenoceptors in the anterior piriform cortex decline with acute amino acid deficiency

The responses of the brain to the amino acid deficiency that occur after eating imbalanced amino acid diets (IMB) have been associated with decreased concentrations of norepinephrine (NE) and cAMP in the anterior piriform cortex (APC), an area essential for the initial feeding responses to amino acid deficiency. In addition, the anorectic responses to IMB were decreased after injections of the alpha 2 agonist, clonidine, and increased after injections of the alpha 2 antagonist, idazoxan, into the APC. Therefore, to study the role of the alpha 2-noradrenergic receptor further in this model, we measured alpha 2-noradrenergic receptor binding in the APC of rats fed two levels of threonine IMB or a low-protein basal control diet. After basal prefeeding for 10 days, rats were given either a mild IMB, a severe IMB, or the basal diet for 2.5 h. The APC, anterior cingulate cortex (AC), ventromedial hypothalamus (VMH), and lateral hypothalamus (LH) were assayed. Binding of [3H]p-aminoclonidine to alpha 2 receptors determined that alpha 2 binding was decreased the most in APC (P < 0.0003). Binding in APC was significantly correlated with food intake in the anorectic response to IMB (P < 0.001). In AC, binding was also significantly decreased, but less dramatically (P = 0.012), and was not correlated with food intake. There were no significant changes in LH or VMH, although alpha 2-noradrenergic binding in VMH tended to decrease with the severe IMB in a pattern similar to APC. Plasma glucose values did not differ after the same feeding protocol. These data support our hypothesis that NE activity in the APC plays a role in initiating the anorectic response to IMB, perhaps via the alpha 2-noradrenergic receptor.

Subtype determination of presynaptic alpha 2-autoreceptors in the rabbit pulmonary artery and human saphenous vein

The pharmacological properties of the presynaptic a2-autoreceptors mediating inhibition of noradrenaline release were investigated in human saphenous vein and rabbit pulmonary artery. Segments of these blood vessels were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing uptake1 and uptake2 blockers. The potencies of a2-adrenoceptor antagonists in facilitating (pEC40) the electrically (2 Hz) evoked tritium overflow were determined. The order of potency and potency ratios of a2-adrenoceptor antagonists obtained in our experiments were compared with the corresponding order of affinity and affinity ratios from radioligand binding studies in tissues and cells expressing only one of the alpha 2-adrenoceptor subtypes. In the rabbit pulmonary artery, oxymetazoline was a highly potent agonist at presynaptic a2-adrenoceptors, as reflected by its ability to inhibit at low concentrations the electrically evoked tritium overflow. However, in the human saphenous vein oxymetazoline behaved as a partial agonist, which, in interaction experiments with the a2-adrenoceptor agonist B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), exhibited high potency in antagonizing the inhibitory effect of the latter drug on tritium overflow. Prazosin given alone at concentrations up to 1 mumol/l did not affect tritium overflow. The data obtained with oxymetazoline and prazosin make it very improbable that the a2-autoreceptors on the sympathetic nerves in both tissues are of the a2B- or a2C-subtype. In both blood vessels, rauwolscine given alone was highly potent in facilitating the electrically evoked overflow. In agreement with this, rauwolscine exhibited high potency in antagonizing the inhibitory effect of oxymetazoline on tritium overflow in the rabbit pulmonary artery and of B-HT 920 in the human saphenous vein. The ratio phentolamine/rauwolscine calculated from their potencies in increasing the electrically evoked tritium overflow was also used to discriminate between the various a2-adrenoceptor subtypes. Comparison of this potency ratio with the corresponding affinity ratios for a2-adrenergic binding sites on HT 29 cells, human platelets, bovine pineal gland, rat submaxillary gland, and cell lines transfected with the human a2 genes indicates that in the rabbit pulmonary artery and human saphenous vein the pharmacological characteristics of the autoreceptors conform best to those of a2A-adrenoceptors. Finally, in both blood vessels the potencies of the antagonists BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), rauwolscine, corynanthine, phentolamine, idazoxan, SKF 104078 (6-chloro-9-[(3-methyl-2-butenyl) oxyl]-3-methyl-1H-2,3,4,5-tetrahydro-3-benzazepine), and/or tolazoline in facilitating evoked noradrenaline release was determined. The potencies of these drugs which can discriminate between a2A- and a2D-adrenoceptors (but not between these and a2B/2C-adrenoceptors) were correlated significantly with their affinities for a2A, but not a2D, sites in radioligand binding studies. In conclusion, the present results suggest that the sympathetic nerves of the human saphenous vein and rabbit pulmonary artery are endowed with a2-autoreceptors of the a2A subtype.

RS-45041-190: a selective, high-affinity ligand for I2 imidazoline receptors

1. RS-45041-190 (4-chloro-2-(imidazolin-2-yl)isoindoline) showed high affinity for I2 imidazoline receptors labelled by [3H]-idazoxan in rat (pKi = 8.66 +/- 0.09), rabbit (pKi = 9.37 +/- 0.07), dog (pKi = 9.32 +/- 0.18) and baboon kidney (pKi = 8.85 +/- 0.12), but had very low affinity for alpha 2-adrenoceptors in rat cerebral cortex (pKi = 5.7 +/- 0.09). 2. RS-45041-190 showed low affinity for other adrenoceptors, dopamine, 5-hydroxytryptamine, and muscarinic receptors and dihydropyridine binding sites (selectivity ratio > 1000). 3. RS-45041-190 showed moderate potency for the inhibition of monoamine oxidase A in vitro (pIC50 = 6.12), but had much lower potency for monoamine oxidase B (pIC50 = 4.47), neither of which equated with its affinity for I2 receptors. 4. RS-45041-190 (0.001 to 3 mg kg-1, i.v. and 1 ng-50 micrograms i.c.v.) had only small, transient effects on blood pressure and heart rate in anaesthetized rats. In conscious rats, RS-45041-190 had no effect on body core temperature or tail skin temperature (1 mg kg-1, s.c.) or on activity or rotarod performance (10 mg kg-1, i.p.). There were also no effects on barbiturate sleeping time in mice after doses of 1-10 mg kg-1, i.p. 5. RS-45041-190 (10 and 25 mg kg-1, i.p.) significantly increased food consumption in rats for up to 4 h after dosing, but unlike idazoxan (10 mg kg-1, i.p.) did not increase water consumption. RS-45041-190 is therefore a selective, high-affinity ligand at I2 imidazoline receptors and its hyperphagic effect may suggest a role for I2 imidazoline receptors in the modulation of appetite.However, in the absence of a selective agonist it is unclear whether this ligand is an agonist or an antagonist at I2 receptors.

Involvement of peripheral presynaptic inhibition in the reduction of sympathetic tone by moxonidine, rilmenidine and UK 14304

We studied the possibility that presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone produced by moxonidine, rilmenidine and 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK 14304). In pithed rabbits without electric stimulation, moxonidine, rilmenidine and UK 14304 caused a long-lasting, > 10 min, increase in arterial pressure. Heart rate was not changed. In pithed rabbits in which sympathetic tone was created by electric stimulation through the pithing rod (2 Hz), the same doses of moxonidine, rilmenidine and UK 14304 caused only a brief, < 10 min, blood pressure rise. Heart rate was decreased, as were the plasma concentrations of noradrenaline and adrenaline. Dose-response curves for the effects on the plasma noradrenaline concentration (stimulated pithed rabbits) were compared with previously obtained dose-response curves for depression of renal sympathetic nerve activity (conscious rabbits). For each drug, the curve describing peripheral presynaptic inhibition and the curve describing central sympathoinhibition were very similar. Both the power and the dose dependence of the peripheral inhibitory effect support its contribution to the overall decrease in sympathetic tone produced by clonidine-like drugs in intact animals. The peripheral effect in all likelihood consists in activation of presynaptic alpha 2-autoreceptors. The agreement of the dose-response curves for the peripheral and for the central effect supports the view that the central effect, like the peripheral one, is mediated through alpha 2-adrenoceptors.

Mechanism of sympathoinhibition by imidazolines

The aim of the present study is to characterize the cardiovascular effects of rilmenidine and moxonidine, two recently developed centrally acting antihypertensive drugs. Rilmenidine and moxonidine are alpha 2-adrenoceptor agonists and, in addition, possess affinity for imidazoline (I1)-receptors. To determine if alpha 2- or I1-receptors are involved in sympathoinhibition, rilmenidine and moxonidine were compared with UK 14304, an alpha 2-agonist devoid of affinity for I1-receptors, and antagonism by the "pure" alpha 2-adrenoceptor antagonists yohimbine, SK&F86466, and RX821002 was studied. When injected intravenously into conscious rabbits, rilmenidine and moxonidine, on the one hand, and UK 14304, on the other, elicited a similar pattern of effects. Thus, transient hypertension was followed by long-lasting hypotension accompanied by a decrease in heart rate, renal sympathetic nerve firing rate, and plasma norepinephrine concentration. The effects of rilmenidine, moxonidine, and UK 14304 were antagonized by intravenously administered yohimbine, SK&F86466, and RX821002. The effects of moxonidine and UK 14304 were also prevented by yohimbine injected into the cisterna magna. Altogether, the degree of antagonism of the effects of rilmenidine and moxonidine did not differ from the degree of antagonism of the effects of UK 14304. Rilmenidine, moxonidine, and UK 14304 were also given to pithed rabbits in which a constant sympathetic tone was maintained by electrical stimulation of the sympathetic nerves. At doses that caused sympathoinhibition in conscious rabbits, they lowered the plasma norepinephrine concentration markedly. Our experiments show by direct measurement of sympathetic nerve activity and plasma norepepinephrine concentration that rilmenidine, moxonidine, and UK 14304 cause sympathoinhibition. As a consequence, blood pressure and heart rate decrease. The simplest interpretation of the blockade of central sympathoinhibition by the selective alpha 2-adrenoceptor antagonists is that rilmenidine, moxonidine, and UK 14304 primarily activated alpha 2-adrenoceptors. The decrease in plasma norepinephrine in pithed rabbits indicates peripheral presynaptic alpha 2-adrenoceptor-mediated inhibition of norepinephrine release per action potential from postganglionic sympathetic axons and suggests a contribution of this mechanism to the overall reduction in sympathetic tone.

Clonidine and cirazoline inhibit activation of nicotinic channels in PC-12 cells

Clonidine and cirazoline bind with high affinity to a nonadrenergic site in the brain stem, the so-called imidazoline I1 receptor. Our aim was to determine the mechanism by which these receptors act and their possible linkage to signal-transducing heterotrimeric G-proteins. We examined the effects of clonidine and cirazoline on PC-12 cells, a neuronal cell line that is reported to possess the I1 site and have no alpha 2-adrenoceptors. In undifferentiated PC-12 cells loaded with the Ca2+ indicator dye fura-2, clonidine and cirazoline (10-100 microM) inhibited the increase in [Ca2+]i produced by nicotine (10 microM). This inhibition was not reversed by yohimbine (100 microM), and adrenaline and BHT 920 were ineffective at 100 microM. This effect was not inhibited by pretreatment with pertussis toxin (24 hours, 100 ng/ml) and not modulated by pretreatment with IBMX (100 microM). The nicotine-induced increase in [Ca2+]i is apparently due to Ca2+ entering via the intrinsic ion channel of the nicotinic acetylcholine receptor. Clonidine and cirazoline inhibited the inward current produced by nicotine (10 microM) as measured by the whole cell patch-clamp technique in differentiated PC-12 cells, recorded at a holding potential of -60 mV. In agreement with the results found with fura-2, inhibition of inward current was concentration dependent and not blocked by yohimbine (100 microM) or mimicked by adrenaline (100 microM). Pretreatment of PC-12 cells with pertussis toxin or infusion of GDP-beta-S (2 mM) via the patch pipette did not alter the inhibition of the nicotine-induced inward current by clonidine or cirazoline. Clonidine and cirazoline, but not adrenaline, displayed [3H]phencyclidine from Torpedo electroplaque membranes enriched in nicotinic acetylcholine receptors in a concentration-dependent manner (10-100 microM). Taken together, these results suggest that clonidine and cirazoline inhibit Na+ and Ca2+ entry through the nicotinic acetylcholine receptor via a nonadrenergic mechanism that is independent of G-proteins and cyclic nucleotides, presumably by direct blockade of the intrinsic ion channel of the nicotinic acetylcholine receptor.

Distribution of imidazoline receptor binding protein in the central nervous system

I-receptors can be localized immunocytochemically in rat nervous system with polyclonal antibodies to an IRBP. I-receptors are cytoplasmic and detected in neuronal perikarya, processes, and glia. Labeled neuronal perikarya in the CNS are uncommon and localized to the mesencephalic trigeminal nucleus. I-receptors are heavily represented in primary sensory systems including: somatosensory systems (spinal and trigeminal) and visceral afferent systems (NTS), in central networks subserving autonomic regulation, neuroendocrine control and emotional behaviors, in circumventricular (neurohaemal) organs and in nonneuronal cells including astrocytes with regional densities paralleling neuronal innervation. The distributions of I-receptors and alpha 2-adrenergic receptors partially differ. I-receptors in the CNS appear to relate broadly to the visceral brain and its afferent inputs, particularly pain. Its functions may relate to regulation of integrative behaviors related to stress.

Imidazoline receptors, subclassification, and drug-induced regulation

Overall, as summarized in TABLE 6, a variety of responses to chronic drug treatment were observed depending on the drug, the tissue, and the ligand. Taken together these studies support the concept that the three ligands bind to distinct sites. In addition, they suggest that idazoxan and possibly yohimbine act as agonists at the I2 site in kidney. Finally, the lack of regulation of the I1 site in hindbrain is consistent with the low incidence of withdrawal symptoms reported with imidazoline-preferring drugs.

The relationship between the adrenoceptor and nonadrenoceptor-mediated effects of imidazoline- and imidazole-containing compounds

This article brings together work on imidazoline or imidazole-containing compounds concerned with the pharmacology of alpha-adrenoceptors, principally on smooth muscle, to illustrate how imidazolines have contributed to the subclassification of alpha-adrenoceptors and how, against this background, attempts have been made to use this knowledge to uncover "nonadrenoceptor"-mediated biological effects of previously uncharacterized compounds, notably imidazole-containing dipeptides and "clonidine displacing substance" (CDS). Recent data are included on (1) the pharmacology of UK-14304, (2) nonadrenoceptor actions of phentolamine, (3) the pharmacology of tissue extracts containing imidazole-containing dipeptides and CDS activity, and (4) ligand binding data at I1 and I2 sites.

Influence of imidazolines on catecholamine release in pithed spontaneously hypertensive rats

The actions of the imidazoline derivatives clonidine, moxonidine, and rilmenidine and of the recently discovered clonidine-displacing substance agmatine on stimulation-induced norepinephrine overflow and epinephrine release were studied in pithed spontaneously hypertensive rats. All three imidazolines dose-dependently decreased norepinephrine overflow and led to an increase in epinephrine release when the highest dose of each compound was injected. The alpha 2-adrenoceptor antagonist rauwolscine shifted the dose-response curves of plasma norepinephrine concentrations to higher levels. Agmatine did not change norepinephrine overflow but increased epinephrine release into plasma after the highest dose administered. It is concluded that the investigated imidazolines decrease norepinephrine overflow via presynaptic alpha 2-adrenoceptors, whereas epinephrine release is mediated through putative imidazoline receptors on the adrenal medulla.

Possible structural and functional relationships between imidazoline receptors and alpha 2-adrenoceptors

Although it is now well established that imidazoline receptors and alpha 2-adrenoceptors are discrete entities with distinct endogenous ligands, the two receptor classes apparently have several common features. While the catecholamines stimulate alpha 2-adrenoceptors but not imidazoline receptors, agmatine, a guanidine analog that may be an endogenous imidazoline receptor ligand, can interact with both I1 and I2 imidazoline receptors as well as alpha 2-adrenoceptors, although, interestingly, other guanidines such as guanabenz are highly selective for alpha 2-adrenoceptors versus I1 receptors. Most I1 receptor agonists such as moxonidine, rilmenidine, and clonidine can also stimulate alpha 2-adrenoceptors, and the same physiological response is produced by activation of central I1 receptors and alpha 2-adrenoceptors, but their anatomical locations differ. The imidazoline idazoxan is an antagonist at I1, I2, and alpha 2-receptors, but minor structural alterations of idazoxan can result in molecules with selectivity for either alpha 2-adrenoceptors or imidazoline receptors. The precise mode of interaction of imidazoline agonists and antagonists with the alpha 2-adrenoceptor is not yet understood, and structures of the imidazoline receptors are still unknown. Nevertheless, the fact that many agents can stimulate or block both receptor classes, combined with the fact that alpha 2-adrenoceptors and I1 receptors can mediate identical physiological responses, suggests that many common structural features may be present.

Dynamic properties of I2-receptors

Evidence is presented that drug binding to I2-sites in the guinea pig kidney and liver is subjected to dynamic homeotropic allosteric regulation. Drugs have been found that are capable of inducing both positive and negative cooperativity on both kidney and liver I2-sites when studied in competition with [3H]idazoxan. Moreover the kinetics of [3H]idazoxan association with guinea pig kidney I2-sites was complex and occurred with three different rate constants. However, on dissociation of [3H]idazoxan from its binding sites in the kidney a substantial number of the slower sites had been converted to faster sites, thus supporting the idea of a dynamic interconversion of I2 sites. The guinea pig liver and kidney I2-sites appear to be similar in their pharmacological properties. They also bear some resemblance to the I2-sites of the smooth muscle of the guinea pig ileum, but they appear to be more distant to the guinea pig cerebral cortex I2-sites. Nevertheless, data indicate that considerable overall heterogeneity exists between the guinea pig kidney and liver I2-sites and the guinea pig ileum and cerebral cortex I2-sites.

Why imidazoline receptor modulator in the treatment of hypertension?

The influence of the sympathetic nervous system on blood pressure control was impressively demonstrated in 1940 by bilateral excision of sympathetic nerve fibers. Thereafter, the first generation of drugs lowering blood pressure by central modulation of the sympathetic outflow through alpha 2-adrenoceptor for stimulation, such as alpha-methyldopa, guanabenz, clonidine, and guanfacine, were marketed. However, these compounds were often tolerated poorly, because they caused orthostatic hypotension, sedation, tachycardia or bradycardia, dry mouth, and reduced cardiac output. The mode of action of the second generation centrally acting antihypertensive drugs moxonidine and rilmenidine is different from that of the first generation compounds (e.g., clonidine). Contrary to clonidine, the newer drugs bind more selectively to I1-imidazoline receptors rather than to alpha 2-adrenoceptors where first-generation drugs act. The high affinity and selectivity of these two drugs for this recently discovered new receptor class make it possible to discriminate between I1-imidazoline receptor-mediated blood pressure lowering, on the one hand, and alpha 2-adrenoceptor-mediated side effects, on the other. Discrimination of the two effects was substantiated either by studies using moxonidine alone or in interaction experiments with I1-imidazoline receptor or alpha 2-adrenoceptor antagonists. The high selectivity of moxonidine at the I1-imidazoline receptor allows discrimination between alpha 2-adrenoceptors and I1-imidazoline receptors and is reflected in man by the relatively low incidence of adverse drug events during moxonidine treatment. Concentration of endazoline, a specific mediator of I1-imidazoline receptors, is elevated in some patients with essential hypertension. Modulation of I1-imidazoline receptors by moxonidine could be interpreted as antagonism with regard to the endogenous agonistic effect of the endogenous "transmitter" endazoline. On the other hand, moxonidine acted directly as an agonist at the putative I1-imidazoline receptor. Therefore, to clear the ground, characterization as well as physiological function of the mediator for imidazoline receptors seems essential. The therapeutic relevance of using drugs selective for I1-imidazoline receptors for blood pressure reduction in hypertensive patients is substantiated by the finding that in human rostral ventrolateral medulla (RVLM), which is essential in central blood pressure regulation, the relation between alpha 2-adrenoceptors and I1-imidazoline receptors is about one to ten (1:10). Reduction of a long-lasting sympathetic overdrive may avoid the deteriorating effects on the heart and peripheral circulation. These recent findings give a rational explanation for the very low incidence of sedation and the absence of respiratory depression, orthostatic hypotension, and rebound hypertension that banned the former central acting antihypertensive drugs from first-line treatment despite the advantages of central mediated blood pressure control.

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

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

Subclassification of presynaptic alpha 2-adrenoceptors: alpha 2D-autoreceptors in mouse brain

The study was devised to classify, by means of antagonist affinities, the presynaptic alpha 2-autoreceptors in mouse cerebral cortex in terms of alpha 2A, alpha 2B, alpha 2C and alpha 2D. A set of antagonists was chosen that was able to discriminate between the four subtypes. Slices of the cortex were preincubated with 3H-noradrenaline and then superfused and stimulated electrically. The stimulation periods used (4 pulses, 100 Hz) did not lead to alpha 2-autoinhibition as shown by the lack of an increase by rauwolscine of the evoked overflow of tritium. The alpha 2-selective agonists 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) and alpha-methylnoradrenaline reduced the evoked overflow. All 10 antagonists shifted the concentration-inhibition curve of UK 14,304 to the right. Rauwolscine also shifted the concentration-inhibition curve of alpha-methylnoradrenaline to the right. pKd values of the antagonists were calculated from the shifts. The pKd values of rauwolscine against UK 14,304 and alpha-methylnoradrenaline were very similar (8.0 and 7.9, respectively). Comparison with antagonist affinities for prototypic native alpha 2 binding sites, alpha 2 binding sites in cells transfected with alpha 2 subtype genes, and previously classified presynaptic alpha 2-adrenoceptors--all taken from the literature--indicates that the alpha 2-autoreceptors in mouse brain cortex are alpha 2D. This is the first subtype determination of alpha 2-autoreceptors in the mouse. It supports the hypothesis that at least the majority of alpha 2-autoreceptors belong to the alpha 2A/D branch of the alpha 2-adrenoceptor tree.

Evidence for two different imidazoline sites on pancreatic B cells and vascular bed in rat

The relative potencies of imidazoline compounds to induce insulin secretion and vascular resistance were compared in the isolated perfused rat pancreas. On insulin secretion, only the two imidazolines, antazoline and efaroxan, induced a concentration-dependent response, antazoline being 10 times more potent than efaroxan. In contrast, idazoxan, a blocker of imidazoline I1 sites, at concentrations up to 30 microM, antagonized the insulin response to 10 microM efaroxan (IC50 approximately equal to 14 +/- 2 microM) without affecting that to 3 microM tolbutamide. On pancreatic vessels, not only antazoline and efaroxan but also idazoxan induced a concentration-dependent vasoconstriction; the rank order of agonist potency was antazoline > efaroxan > idazoxan. In addition, cimetidine, an imidazole known to bind imidazoline I1 sites, ineffective per se, partially reversed the insulin stimulatory effect of efaroxan without affecting its vasoconstrictor effect. This study demonstrates that the insulin secretory and vasoconstrictor actions of imidazolines involve different imidazoline sites in rat pancreas. The results provide evidence for an I1 type mediating insulin secretion on B cells and an I2 type mediating vasoconstriction in vessels.