Clonidine binds to imidazole binding sites as well as alpha 2-adrenoceptors in the ventrolateral medulla

Suppression of copulatory behavior in male rats following central administration of clonidine

Adrenergic transmitters have been implicated in the regulation of male sexual behavior. In the present study the contribution of alpha 2-adrenoceptors, located within the central nervous system, was evaluated. Sexually experienced male Long-Evans rats were implanted with a permanent cannula in either the third cerebral ventricle or, unilaterally, in the medial preoptic area. Baseline mating tests were administered 4, 7 and 10 days after surgery and only males ejaculating (at least) in the 7 and 10 day tests were used. Clonidine-evoked dose-dependent decrements in the number of males mounting, intromitting and ejaculating, with administration into the medial preoptic area more effective than into the 3rd ventricle. In those animals mating, administration of 20 nmol into the 3rd ventricle was associated with decreases in the latency to ejaculation and intercopulatory interval, whereas administration of 2 nmol into the medial preoptic area was associated with increases in these parameters and decreases in the ejaculatory threshold. Administration of yohimbine into the medial preoptic area attenuated the effects of systemically-administered clonidine and the systemic administration of yohimbine completely prevented the copulatory suppression induced by administration of clonidine into the medial preoptic area. It is suggested that central alpha 2-adrenoceptors are important in the control of male sexual behavior and that alterations in adrenergic mechanisms in the medial preoptic area may underlie sexual dysfunction of various etiologies.

Presynaptic alpha 2-adrenoceptor function and sleep in man: studies with clonidine and idazoxan

The effects of an alpha 2-adrenoceptor agonist, clonidine and an antagonist, idazoxan, were studied on nocturnal sleep in man. Clonidine increased non-rapid eye movement sleep and idazoxan reduced slow wave sleep and increased awake activity. Changes in the continuity of sleep with clonidine were similar to, and those with idazoxan opposite to, the effects of maprotiline, an inhibitor of the uptake of noradrenaline, used as an active control. These findings support the previous conclusion that raised levels of noradrenaline in the synapse, after inhibition of uptake, lead to increased presynaptic inhibition of release of transmitter in man. However, all three drugs decreased rapid eye movement (REM) sleep and the ratio of REM to nonREM sleep and this is believed to be due to a non-specific upset of the balance of influences which control the appearance of REM sleep.

Rilmenidine lowers arterial pressure via imidazole receptors in brainstem C1 area

We sought to determine the site of action and receptor type responsible for the antihypertensive actions of rilmenidine, an oxazoline analogue of clonidine. In anesthetized paralyzed rats decerebration did not alter the dose dependent reductions in arterial pressure and heart rate elicited by i.v. drug. Rilmenidine microinjected bilaterally into the C1 area of the rostral ventrolateral medulla (RVL), but not nucleus tractus solitarii (NTS) nor caudal ventrolateral medulla (CVL), elicited dose-dependent falls in arterial pressure and heart rate at doses an order of magnitude less than required systemically. Prior microinjection into the C1 area of the selective alpha 2-adrenoceptor antagonist SKF-86466, even at high doses, failed to modify the hypotension to i.v. rilmenidine. However, microinjection of 3- to 10-fold lower doses of idazoxan, a ligand for imidazole as well as alpha 2-adrenoceptors, blocked the effects. Rilmenidine also competed with the clonidine analogue [3H]p-aminoclonidine ([3H]PAC) at specific binding sites in membranes of bovine ventrolateral medulla and frontal cortex. In RVL rilmenidine competed with binding to imidazole and alpha 2-adrenergic binding sites with a 30-fold selectivity for the imidazole binding sites. In frontal cortex binding was of lower affinity and restricted to alpha 2-adrenergic sites. We conclude that rilmenidine, like clonidine, acts to lower arterial pressure by an action on imidazole receptors in the C1 area of RVL. The higher selectivity of rilmenidine for imidazole to alpha 2-adrenoceptors as compared to clonidine may explain the lower sedative effects of rilmenidine.

Alpha 2-adrenoceptors not imidazole receptors mediate depression of a sacral spinal reflex in the cat

In chloralose-anaesthesized cats, clonidine, an alpha 2-adrenoceptor agonist with an imidazole ring, depressed pudendal nerve reflex activity. Clonidine's inhibitory action on this compound action potential response was mimicked by guanabenz, a non-imidazole alpha 2-adrenoceptor agonist, and was reversed by SK & F 86466, a non-imidazole alpha 2-adrenoceptor antagonist. These results imply that clonidine's action on this reflex related to urinary sphincter function is mediated by alpha 2-adrenoceptors and is not dependent on an imidazole structure.

Non-adrenergic binding sites for the "alpha 2-antagonist" [3H]idazoxan in the rabbit urethral smooth muscle. Pharmacological and biochemical characterization

In the present study, pharmacological and biochemical binding characteristics of [3H]idazoxan, an originally thought alpha 2-adrenoceptor antagonist, have been determined in smooth muscle of rabbit urethra. It is shown that [3H]idazoxan labels with high affinity non-adrenergic binding sites. Specific binding of [3H]idazoxan is inhibited by compounds possessing an imidazoline or a guanidinium moiety whereas phenylethanolamines and classical alpha 2-antagonists are ineffective competitors which suggests an imidazoline-preferring binding site. However, imidazolidines such as clonidine and paminoclonidine are poorly effective, which differs considerably from pharmacological characteristics of imidazoline binding sites previously reported in the central nervous system. In addition, it is shown that K+ and Mn2+ inhibit [3H]idazoxan binding in a competitive and non-competitive manner, respectively. Other cations such as Na+, Li+ and Mg2+ have no significant effect. It is shown that K+ accelerates the dissociation of [3H]idazoxan binding while Mn2+ does not produce any modification. These results suggest that K+ may bind to an allosteric site, while Mn2+ may bind with a membrane component susceptible to alter [3H]idazoxan binding sites.

Medetomidine stereoisomers delineate two closely related subtypes of idazoxan (imidazoline) I-receptors in the guinea pig

A number of imidazoline, imidazole and guanidinium compounds and other drugs were compared for their ability to bind to non-adrenergic idazoxan (imidazoline) I-receptors in particulate guinea pig cerebral cortex and ileum smooth muscle fractions. Radioligand binding with [3H]idazoxan was used for the experiments. Computer modelling of the binding data gave dissociation constants for drug binding to both I-receptors and alpha 2-adrenoceptors. Most drugs showed similar affinities for I-receptors in cortex and ileum. However, medetomidine stereoisomers as well as a few other drugs clearly delineated the I-receptors in cortex and ileum as different.

Imidazoline-guanidinium and alpha 2-adrenergic binding sites in basolateral membranes from human kidney

In the present study, we used [3H]idazoxan and [3H]rauwolscine to characterize the imidazoline-guanidinium receptive site (IGRS) and alpha 2-adrenoceptors in the human renal proximal tubule, respectively. In purified basolateral membranes, 11-fold enriched in Na(+)-K+ ATPase. [3H]idazoxan and [3H]rauwolscine binding was twofold higher than in homogenates ([3H]idazoxan: 87 +/- 19 vs. 45 +/- 23.3 fmol/mg protein, P less than 0.05; [3H]rauwolscine: 56.4 +/- 21.4 vs. 25.2 +/- 7.3 fmol/mg protein, P less than 0.01). In competition studies performed at saturating concentration of [3H]idazoxan (15 NM), specific binding was competed for by epinephrine and rauwolscine only by 10-15% but was completely inhibited by imidazoline and guanidinium compounds. Thus, in human renal proximal tubule. [3H]idazoxan mainly binds to an IGRS. The highest density of alpha 2-adrenoceptors in basolateral membranes and of IGRS in partially purified membrane preparations, suggests that these two binding sites have a different subcellular localization. When compared to the rabbit renal IGRS, the human [3H]idazoxan binding site displays different affinities for guanabenz, rilmenidine, clonidine, amiloride and its derivatives that persist after membrane solubilization. In contrast, the human and rabbit renal IGRS share similar regulatory properties such as the sensitivity to K+ and the insensitivity to Na+, divalent cations and 5'-guanylylimidodiphosphate (Gpp(NH)p). In conclusion, we demonstrated that, in the human renal proximal tubule, alpha 2-adrenoceptors are mainly located in basolateral membranes while IGRS appear to be associated with another cell compartment. As indicated by their common interaction with imidazoline and guanidinium derivatives and by similar regulatory properties, human and rabbit IGRS belong to the same family of membrane proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and chronic idazoxan in normal volunteers: biochemical, physiological and psychological effects

The acute and chronic effects of the selective a(2)-antagonist idazoxan were studied in 12 normal volunteers. Plasma 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), blood pressure and psychological responses to oral challenge doses of idazoxan 40 mg were measured twice, on the first and 22nd day of treatment with idazoxan 40 mg t.d.s. Changes in nocturnal melatonin output were studied on six occasions, before, during and after idazoxan treatment. Although baseline MHPG levels were significantly reduced after chronic treatment with idazoxan, idazoxan challenge did not alter MHPG concentrations on either test day. A small rise in systolic blood pressure occurred after acute but not chronic idazoxan challenge tests. Systolic blood pressure values were significantly lower during the chronic compared with the acute test. Diastolic blood pressure and heart rate were not affected by acute or chronic treatment. Subjects reported increases in self- ratings of arousal and reductions in sedation and anxiety of similar magnitude after acute and chronic idazoxan. Nocturnal plasma melatonin secretion was not altered by drug administration or withdrawal, although urinary 6-sulphatoxymelatonin excretion was significantly reduced on acute withdrawal. The increase in systolic blood pressure and arousal self-ratings after acute idazoxan are in accordance with the reported effects of other a(2)-antagonists, although we did not find increased anxiety or elevated plasma MHPG levels. Chronic idazoxan appears to reduce or normalize activity of noradrenergic systems, indicated by reduced baseline systolic blood pressure and MHPG, and loss of the pressor response to idazoxan. Withdrawal of idazoxan leads to an abrupt fall in noradrenergic activity, as demonstrated by the fall in urinary 6-sulphatoxymelatonin.

Release of contractile agents from rat vas deferens by clonidine

Clonidine induces contractile effects on the isolated rat vas deferens, but not on rat uterus or guinea-pig ileum. However, we have observed that if clonidine is incubated for about 10 min with a nutrient solution containing an isolated rat vas deferens, the resulting solution can contract an isolated rat uterus, or guinea-pig ileum indicating the involvement of a substance released from the vas. This contractile effect was partially reduced by naloxone and by serotonin antagonists, and by using a denervated vas, indicating that opioids, serotonin and eventually other substances released from nerve tissue of the vas can be involved.

Protection against ischemia-induced neuronal damage by the alpha 2-adrenoceptor antagonist idazoxan: influence of time of administration and possible mechanisms of action

The protective effect of the alpha 2-receptor antagonist idazoxan against neuronal damage in the neocortex and in the hippocampal CA1 region was studied in rats exposed to 10 min of incomplete forebrain ischemia. When administered i.v. immediately after ischemia (0.1 mg/kg) and subsequently for 6 h (10 micrograms/kg/min), idazoxan significantly reduced neuronal damage in the hippocampus (from 84 to 26%) and in the vulnerable parts of the neocortex (from 15 to 1%). The bolus dose alone provided no significant protection. When idazoxan administration was delayed for 30 min, no significant protection was noticed in the neocortex, and the effect in the hippocampus was ambiguous. A transient elevation of plasma corticosterone levels was induced during ischemia. Idazoxan administration for 2 h did not affect postischemic changes in corticosterone levels compared with saline infusion. Idazoxan (10(-7)-10(-4) M) did not influence the in vitro binding to glutamate receptors in brain slices. Thus, the protective effect of idazoxan cannot be explained by suppression of the plasma corticosteroid levels or via an antagonistic effect on glutamate receptors. Idazoxan apparently protects neurons when given during the first hours of postischemic reperfusion, while histopathological necrosis of neurons becomes visible 48-72 h after ischemia. Detrimental processes causing delayed neuronal death occur in the early postischemic phase and can be influenced by adrenoceptor ligands. Idazoxan may protect by several mechanisms but probably exerts its protective postischemic effect mainly through an increased noradrenergic neuronal activity and an elevation of extracellular noradrenaline (NA) levels in the brain. The favorable effects of NA may either be due to inhibition of excitotoxic neurotransmission or activation of survival-promoting and trophic processes.

Role of central alpha 1- and alpha 2-adrenoceptors on the dipsogenic and cardiovascular effect of angiotensin II

We investigated the effects of previous central treatment with prazosin (an alpha 1-adrenoceptor antagonist) or clonidine (an alpha 2-adrenoceptor agonist) on the dipsogenic, pressor and tachycardic responses produced by intracerebroventricular (ICV) injection of angiotensin II (AII) in conscious rats. Holtzman rats with a chronic cannula implanted in the lateral ventricle were tested for dipsogenic and cardiovascular (arterial pressure and heart rate) responses in separate experiments. Previous ICV treatment with clonidine (20, 40, 80 and 120 nmol) abolished the pressor, tachycardic and dipsogenic effects of ICV AII. After all doses of prazosin (40, 80 and 120 nmol), AII induced bradycardic responses, but only the 80 and 120 nmol doses of prazosin reduced the pressor responses to AII. Prazosin produced no alteration in the dipsogenic effect of AII. The results show that the periventricular alpha 1-adrenoceptors are involved only in the cardiovascular responses produced by central AII, whereas clonidine acting through alpha 2-adrenergic and/or imidazole receptors can modulate all actions of AII.

Alpha 2-adrenergic binding sites in the medulla oblongata of tree shrews demonstrated by in vitro autoradiography: species related differences in comparison to the rat

Alpha 2-adrenergic binding sites in the medulla oblongata of tree shrews and rats were detected and quantified by in vitro-autoradiography with the alpha 2-antagonist 3H-rauwolscine (3H-RAUW). The autoradiographic pattern of the radioligand binding in the tree shrew medulla oblongata resembles that which has been described by others for the human myelencephalon. This pattern coincides well with the occurrence of catecholaminergic structures detected by immunocytochemistry with antibodies against phenylethanolamine-N-methyltransferase and tyrosine hydroxylase. In contrast to the rat, where only the nucleus tractus solitarii and the nucleus dorsalis nervi vagi were labeled, five discrete nuclei specifically bound 3H-RAUW in tree shrews. The highest number of binding sites was detected in the nucleus dorsalis nervi vagi (nX; Bmax: 333 fmoles/mg) and the nucleus tractus solitarii (NTS; 311 fmoles/mg), followed by the nucleus nervi hypoglossi (nXII; 297 fmoles/mg), the nucleus reticularis parvocellularis (FRS; 230 fmoles/mg), and the area of the catecholamine cell groups A1 and C1 (area C1; 202 fmoles/mg). Maximal binding in the two labeled nuclei of the rat was 158 fmoles/mg. The discrete nuclei of the two species also showed different affinities for 3H-RAUW with Kd ranging from 0.17 to 0.83 nM in tree shrews and 1.80 to 1.95 nM in rats. Competition experiments revealed that the radioligand bound specifically to alpha 2-binding sites. In the tree shrew, nX, nXII and the area C1, also have a relatively high affinity for the alpha 1-antagonist prazosin which is a quality of the adrenoceptor subtype alpha 2B. Furthermore, in the area C1, 3H-RAUW binding was inhibited by the dopamine antagonist haloperidol. There are thus species related as well as regional differences with respect to the number, the affinity, and the pharmacological properties of alpha 2-binding sites in the medulla oblongata. In tree shrews, alpha 2-adrenoceptors can be autoradiographically quantified in regions which are not labeled in the rat, although former data predicted the existence of such receptors, e.g., in the area of the adrenaline cell group C1.

Evidence for the involvement of imidazoline receptors in the central hypotensive effect of rilmenidine in the rabbit

1. Rilmenidine has recently been introduced as a new centrally-acting antihypertensive agent. We examined its cardiovascular effects after intracerebral injection to anaesthetized rabbits. Cumulative doses of rilmenidine injected intracisternally (1 to 300 micrograms kg-1) led to dose-dependent decreases in arterial blood pressure and heart rate. The effective doses of rilmenidine were lower when injected centrally than when injected intravenously. 2. Pretreatment with the same dose of yohimbine or idazoxan shifted the rilmenidine dose-response curves for its hypotensive and bradycardic effects to the right. Idazoxan, which has an imidazoline structure, proved to be a more active antagonist than yohimbine of rilmenidine centrally-mediated cardiovascular effects. 3. The dose-response curve for the central hypotensive effect of rilmenidine was also shifted to the right after pretreatment with a bovine brain extract. This extract contains the endogenous ligand of the imidazoline-preferring receptors which is not a catecholamine. 4. Rilmenidine, like clonidine, proved to be active when micro-injected into the rabbit nucleus reticularis lateralis region. 5. In conclusion, rilmenidine exhibited in the rabbit a central hypotensive effect which originated in the same area as where clonidine acts. Specific imidazoline-preferring receptors appear to be involved in this hypotensive effect.

Prenatal exposure to antiadrenergic antihypertensive drugs: effects on neurobehavioral development and the behavioral consequences of enriched rearing

One of the beta blockers propranolol (5 or 20 mg/kg) or atenolol (5 or 20 mg/kg) or the alpha-2 agonist clonidine (20 or 100 micrograms/kg) was administered in the drinking water to pregnant Wistar rats over gestation days 1 to 21. Maternal weight gain, litter size or litter weight were unaffected as were a number of measures of physical and behavioral development. The rats were raised after weaning in either enriched or impoverished housing. At maturity, male but not female rats that had been prenatally exposed to either of the three drugs weighed less than the controls. Testing for cognitive function using an automated version of the Hebb-Williams maze indicated impaired spatial maze problem solving ability for female rats that had been exposed to propranolol (20 mg/kg) and raised in impoverishment. Generally, enriched-reared rats committed fewer errors on the maze problems. Prenatal exposure to the drugs did not alter the beneficial effects of enriched rearing on maze solving. However, clonidine-exposed (100 micrograms/kg) female rats that were raised in enrichment showed a significantly increased mortality rate. Thus, prenatal exposure to all of these drugs exerted effects which were latent, appearing in adulthood, and which were to some extent precipitated by environmental factors. We conclude that these drugs may be physical and behavioral teratogens for the rat.

Further characterization of the guinea pig cerebral cortex idazoxan receptor: solubilization, distinction from the imidazole site, and demonstration of cirazoline as an idazoxan receptor-selective drug

We have demonstrated previously that [3H]idazoxan, besides being able to bind to alpha 2-adrenergic receptors, may also bind to a nonadrenergic idazoxan-receptor site with high affinity. The idazoxan receptor is tightly bound to cellular membranes, and we have now developed a method to solubilize it from the guinea pig cerebral cortex by using the detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). The CHAPS-solubilized receptor retains its binding properties for drugs: the membrane-bound, as well as the solubilized, idazoxan receptor shows high affinities for a number of imidazolines (cirazoline, idazoxan, tolazoline, naphazoline, tramazoline, clonidine, and oxymetazoline), some imidazoles (medetomidine, detomidine), and guanfacine. By contrast, catecholamines (adrenaline, noradrenaline, isoprenaline, and dopamine) and a number of other neurotransmitters and neuromodulators (serotonin, histamine, glutamic acid, gamma-aminobutyric acid, glycine, and adenosine) show negligible affinities for the idazoxan receptor. Moreover, the idazoxan receptor shows grossly different binding properties for histamine, cimetidine, and imidazole-4-acetic acid compared to what has been described for the nonadrenergic imidazole site labeled by p-[3H]amino-clonidine, indicating that the two receptor sites are distinct. Radioligand binding data further indicate that cirazoline is an idazoxan receptor-selective drug (KD = 1 nM) showing a 50-210-fold selectivity for binding to the idazoxan receptor when compared to alpha 2-adrenergic receptors and an about 500-fold selectivity when compared to alpha 1-adrenergic receptors. We have also reviewed the literature for possible nonadrenergic actions of idazoxan and cirazoline, and we suggest that idazoxan receptors might be involved in the control of prolactin release from the pituitary.

Astrocytes cultured from specific brain regions differ in their expression of adrenergic binding sites

We sought to characterize regional heterogeneity of astrocytes using adrenergic receptor sites as cellular markers. Primary cultures made from 6 regions of neonatal rat brain consisted almost exclusively of astrocytes. Membranes from astrocytes cultured 1-3 weeks were prepared for radioligand binding assays of beta- and alpha 2-adrenergic sites using the ligands [3H]dihydroalprenolol and [3H]p-aminoclonidine, respectively. Receptor expression was not affected by time in culture. Astrocytes from different brain regions varied up to 3-fold with respect to number but not affinity for both classes of adrenergic binding site with a rank order of cerebral cortex = superior colliculus greater than hippocampus = ventral midbrain greater than or equal to caudate nucleus greater than or equal to hypothalamus. Binding to beta- and alpha 2-adrenergic receptors was positively correlated across brain regions. Astrocytic receptor binding in each region did not correspond to total receptor levels assessed by quantitative autoradiography. We conclude that: (a) astrocytes are markedly heterogeneous between major brain regions with respect to expression of adrenergic binding sites; (b) regional variations in the density of adrenergic binding sites in brain reflect, in part, local specialization of astrocytes; and (c) a substantial proportion of the adrenergic binding sites in some brain regions may be on astrocytes.

Discrimination between alpha 2-adrenoceptors and [3H]idazoxan-labelled non-adrenergic sites in rabbit white fat cells

Previous results indicated that the rabbit could represent a suitable model for investigations on the functional role of alpha 2-adrenoceptors in fat cells, but the characterization of these receptors was not resolved yet. In the present report, imidazoline compounds were used to attempt a better definition of rabbit adipocyte alpha 2-receptivity by means of lipolysis and binding studies. Lipolysis data showed that UK14304 is a full alpha 2-adrenoceptor agonist promoting a strong antilipolysis in rabbit fat cells. Moreover, the methoxy derivative of idazoxan, RX821002, is a more potent antagonist of UK14304-induced antilipolysis than idazoxan or yohimbine. Whereas [3H]yohimbine failed to bind at rabbit adipocyte alpha 2-adrenoceptors, [3H]UK14304 and [3H]RX821002 are valuable tools to study this receptor. Analysis of binding data demonstrated that [3H]UK14304 labels the high-affinity-state receptor while [3H]RX821002 binds to the whole alpha 2-adrenergic population. Inhibition studies of [3H]RX821002 and [3H]UK14304 binding by various compounds confirmed the alpha 2-adrenergic nature of the sites labelled by both radioligands. The other alpha 2-adrenoceptor radioligand, [3H]idazoxan, labelled binding sites which are insensitive to catecholamines. Competition studies of [3H]idazoxan binding with imidazoline derivatives revealed structure-activity relationships different from those of alpha 2-adrenoceptors. The most striking observation is that substitutions in the 2-position of idazoxan markedly reduce the affinity for the non-adrenergic sites, whereas the alpha 2-potency is increased or unchanged.

Binding of [3H]p-aminoclonidine to two sites, alpha 2-adrenoceptors and imidazoline binding sites: distribution of imidazoline binding sites in rat brain

Binding sites labeled by [3H]p-aminoclonidine [( 3H]PAC) were investigated by the competitive analysis with imidazoline and non-imidazoline derivatives. Phenylethylamine derivatives displaced only the part of specific sites for [3H]PAC, which was considered as alpha 2-adrenoceptor, whereas imidazoline derivatives, such as clonidine and tolazoline, competed for a further specific binding of [3H]PAC to the non-adrenergic sites, in addition to the alpha 2-adrenoceptor. Because the non-adrenergic sites were specific for the imidazoline structure, they were termed imidazoline sites. The imidazoline sites were not distributed uniformly among rat brain regions. In striatum, hippocampus and medulla oblongata, they occupied 39.6, 33.0 and 36.5% of the specific binding of [3H]PAC, respectively. Saturation isotherms revealed that Kd and Bmax of imidazoline sites for [3H]PAC were 3.09 +/- 0.59 nM, 27.4 +/- 1.7 fmol/mg protein and 2.23 +/- 0.29 nM, 21.0 +/- 1.5 fmol/mg protein in striatum and hippocampus, respectively. Because imidazoline binding sites also displayed weak affinities for imidazole compounds, such as histamine and cimetidine, the imidazoline site may be a subtype of histamine H2-receptor.

Alpha 2-adrenoceptor subtypes and imidazoline-like binding sites in the rat brain

1. The binding of [3H]-yohimbine and [3H]-idazoxan to rat cortex and hippocampus is rapid, reversible and of high affinity. Saturation data indicate that a single population of binding sites exist for [3H]-yohimbine in the cortex (Bmax 121 +/- 10 fmol mg-1, protein; Kd 5.2 +/- 0.9 nM) and hippocampus (Bmax 72 +/- 6 fmol mg-1 protein; Kd 5.8 +/- 0.7 nM). [3H]-idazoxan labels one site in the cortex (Bmax 87 +/- 8 fmol mg-1 protein; Kd 4.1 +/- 0.9 nM) and hippocampus (Bmax 30 +/- 6 fmol mg-1 protein; Kd 3.5 +/- 0.5 nM), when 3 microM phentolamine is used to define non-specific binding. A second distinct [3H]-idazoxan binding site (Bmax 110 +/- 21 fmol mg-1 protein; Kd 3.6 +/- 0.07 nM) is identified in rat cortex if 0.3 microM cirazoline is used to define non-specific binding and 3 microM yohimbine is included to prevent binding to alpha 2-adrenoceptors. 2. Displacement studies indicate that the alpha 1-adrenoceptor antagonist prazosin and the 5-HT1 ligands 8-OH-DPAT, RU 24969 and methysergide differentiate [3H]-yohimbine binding into two components; a high and low affinity site. In contrast the displacement of [3H]-idazoxan by each ligand was monophasic. 3. The affinities of 8-OH-DPAT, RU 24969 and methysergide determined against [3H]-idazoxan binding to the cortex and hippocampus correlate significantly with the binding site displaying low affinity for prazosin and previously designated alpha 2A. In contrast, a poor correlation exists for the high affinity site for prazosin designated alpha 2B. 4. [3H]-idazoxan, in the presence of 3 microM yohimbine, labels a site that displays high affinity towards cirazoline, naphazoline and guanabenz, but low affinity towards clonidine, p-aminoclonidine, adrenaline, noradrenaline and the alpha 2-adrenoceptor antagonists yohimbine, rauwolscine, WY 26703 and BDF 6143. 5. The results of this study indicate that [3H]-yohimbine labels two sites; the alpha 2A- and alpha 2B-adrenoceptors whereas [3H]-idazoxan labels an alpha 2-adrenoceptor with a profile consistent with the alpha 2A-adrenoceptor subtype. In addition, [3H]-idazoxan labels an imidazoline binding site in the rat cortex that is pharmacologically distinct from alpha 2-adrenoceptors. The low affinity of clonidine and p-aminoclonidine indicates that the imidazoline-like binding site in rat cortex is different from the site labelled by [3H]-clonidine and [3H]-p-aminoclonidine in human, rat and bovine brain stem, providing evidence of potential heterogeneity within this class of binding sites.

Differences in the regulation of [3H]idazoxan and [3H]yohimbine binding sites in the rabbit

In vitro studies suggest that [3H]yohimbine binds to alpha 2-adrenoceptors while [3H]idazoxan binds preferentially at a non-adrenergic site. In order to compare in vitro with in vivo effects male New Zealand White rabbits received the following treatments: 5 days idazoxan 1.1 mg/kg per h, 10 days noradrenaline 46 micrograms/kg per h (intravenous infusion), 21 days amitriptyline 30 mg/kg per day (intraperitoneally) or vehicle. The effect of these treatments on the number of [3H]yohimbine and [3H]idazoxan binding sites was examined. Ten days noradrenaline infusion and 21 days amitripytyline treatment significantly reduced [3H]yohimbine binding in kidney and hindbrain membranes respectively, but had no significant effect on [3H]idazoxan binding. Five days idazoxan infusion significantly increased [3H]yohimbine binding in the forebrain, while a significant reduction in [3H]idazoxan binding sites in the kidney was observed. Thus differential regulation of the two binding sites was observed in vivo. These alterations in binding site number are consistent with the differing affinities of noradrenaline and idazoxan for the [3H]yohimbine and [3H]idazoxan binding sites previously observed in vitro and support the hypothesis that in the rabbit idazoxan binds preferentially at non-adrenergic sites while yohimbine binds to an alpha 2-adrenergic site. The idazoxan site may be an imidazoline type of receptor but further work, including functional studies, is required to substantiate this.

The effect of clonidine and related substances on voluntary ethanol consumption in rats

Clonidine, guanfacine and tiamenidine, in equihypotensive doses, significantly reduced alcohol intake in ethanol-preferring rats having free choice between 10% ethanol and drinking water. Water intake was only slightly reduced, especially during the first hours following the administration of clonidine. Simultaneous treatment with yohimbine attenuated the clonidine-induced reduction in ethanol intake. Putative central mechanisms underlying the observed inhibitory actions of clonidine and other alpha-2 adrenoceptor agonists on oral self-administration of alcohol are discussed.

Effect of clonidine on second messenger systems in rat adrenal gland

Clonidine, an alpha 2-adrenergic agonist, also binds to non-adrenergic imidazole receptors in brain and peripheral tissues. In adrenal medulla, however, clonidine appears to bind only to imidazole receptors. To assess whether the signal transduction mechanism of imidazole receptors differs from alpha 2-adrenergic receptors, we studied the actions of clonidine on the turnover of phosphoinositide and the production of cAMP and cGMP in slices of rat adrenal gland. Clonidine did not modify basal or carbachol mediated increases in phosphoinositide turnover or production of cAMP, however it increased the production of cGMP. The increase in cGMP was slow and unaffected by the addition of the phosphodiesterase inhibitor, IBMX. We conclude that the second messenger response triggered by clonidine in adrenal differs from that usually coupled to alpha 2-adrenergic receptors. Whether the effect is mediated by cell surface imidazole receptors remains to be established.

Desensitization and down-regulation of brain alpha 2-adrenoceptors by centrally acting antihypertensive drugs

Rabbits were treated with intravenous clonidine (8 mumol kg-1 day-1), guanabenz (20 mumol kg-1 day-1), rilmenidine (80 mumol kg-1 day-1) or vehicle via osmotic minipumps. After 6 days treatment mean arterial pressure (MAP), pressor responses to intravenous alpha-methyl noradrenaline and depressor responses to intracisternal clonidine were studied, and [3H]-yohimbine binding to forebrain and hindbrain examined in vitro. Clonidine, guanabenz and rilmenidine had similar effects on MAP and caused a similar attenuation of the depressor response to intracisternal clonidine, but only guanabenz attenuated pressor responses to intravenous alpha-methyl noradrenaline. Rilmenidine had no effect on [3H]-yohimbine binding to brain membranes. Clonidine treatment decreased binding in hindbrain while guanabenz treatment decreased binding in both fore- and hindbrain. Thus, the depressor effects of chronic treatment did not correlate with the effects on [3H]-yohimbine binding sites in rabbit brain suggesting that the blood pressure lowering effects of many centrally acting antihypertensive drugs are not necessarily dependent on binding to the alpha 2-adrenoceptor site labelled by [3H]-yohimbine.

Neuroblastoma-glioma hybrid cells contain clonidine-displacing substance

Clonidine-displacing substance (CDS) isolated from bovine brain potently inhibits clonidine binding and elicits contraction of gastric smooth muscle. We sought to determine if CDS was contained in neuron-like clonal cells (neuroblastoma X glioma hybrid NG108-15). Extracts were prepared from osmotically shocked P2 fractions of NG108-15 cells. One unit of CDS, as defined by a [3H]p-aminoclonidine radioreceptor assay using bovine frontal cortex membranes, was obtained from each 1.3 million cells processed. CDS isolated from NG108-15 cells was biologically active on gastric smooth muscle. NG108-15 cells may serve as a model system for the study of this endogenous clonidine-like ligand.

The C1 area of rostral ventrolateral medulla: a central site integrating autonomic responses to hemorrhage

Activation of the sympathetic neurons and release of adrenomedullary catecholamines are the principal early reflex responses to hemorrhage. These are initiated by arterial baro- and chemoreceptors, from other cardiopulmonary receptors, and by intracerebral receptors responding to ischemia. A principal gateway for integrating the autonomic responses are a small collection of neurons in a region of the rostral ventrolateral medulla (RVL), containing a cluster of neurons of the C1 adrenergic cell group, the C1 area. Neurons in the C1 area of RVL project exclusively to autonomic nuclei of the spinal cord, are tonically active, and fire with a rhythm linked to the cardiac cycle. They are essential for maintaining resting discharge of sympathetic nerves and, consequently, arterial pressure (AP) and heart rate. They also are critical for reflex changes in AP in the baro- and chemoreceptor, somato-sympathetic (pain), and cerebral ischemic reflexes. Neurons of the C1 area are under tonic excitatory and inhibitory control by pathways from other autonomic centers. They are controlled by a range of neurotransmitters including, gamma aminobutyric acid (GABA), acetylcholine, catecholamines, enkephalin, and several neuropeptides. They also serve as a site of action for the hypotensive actions of several clinically important neurotransmitters. The C1-area of RVL may play a critical role in the autonomic responses to hemorrhage and may be an important target for drugs seeking to treat hemorrhagic shock.

[3H]-idazoxan binds with high affinity to two sites on hamster adipocytes: an alpha 2-adrenoceptor and a non-adrenoceptor site

1. [3H]-idazoxan labels a single population of high affinity sites (Kd 2.26 +/- 0.02 nM; Bmax 372 +/- 25 fmol mg-1 protein) in hamster adipocyte membranes. In the presence of 1 microM yohimbine to preclude binding to alpha 2-adrenoceptors, the density of [3H]-idazoxan binding sites was reduced (287 +/- 18 fmol mg-1 protein) without an apparent decrease in the affinity (Kd 2.19 +/- 0.24 nM) of the radioligand. 2. Displacement studies indicate that alpha-adrenoceptor ligands with an imidazoline side chain completely inhibit [3H]-idazoxan binding to hamster adipocyte membranes; in contrast, the alpha 2-adrenoceptor antagonists yohimbine, rauwolscine, BDF 6143 and phentolamine inhibited only 20-30% of the specific binding with affinity values consistent with an interaction at alpha 2-adrenoceptors. 3. The low potency of noradrenaline and adrenaline in displacing [3H]-idazoxan binding to the second site on hamster adipocyte membranes indicates that it is unlikely that this site is a type of adrenoceptor. 4. These results suggest that [3H]-idazoxan binds with high affinity to two sites in hamster adipocytes: an alpha 2-adrenoceptor and a non-adrenoceptor imidazoline site.

Dual action of clonidine on insulin release: suppression, but stimulation when alpha 2-adrenoceptors are blocked

As shown previously clonidine reduces glucose-stimulated insulin release and this effect is mediated by inhibitory postsynaptic alpha 2-adrenoceptors. The present experiments demonstrate that clonidine has the additional property to also stimulate insulin release. This became evident when the alpha 2-adrenoceptors of isolated islets were blocked by benextramine, and thus protected from being stimulated by clonidine. In the presence of benextramine, clonidine no longer reduced, but on the contrary enhanced, the release of insulin in response to glucose. In control experiments benextramine by itself failed to affect insulin release from isolated islets. These results show that the imidazoline derivative clonidine shares the property of other imidazoline compounds to enhance the insulin response to glucose. All of these agents may stimulate insulin by binding to "imidazoline-preferring" sites, that clearly differ from alpha-adrenoceptors.

New concepts on the central regulation of blood pressure. Alpha 2-adrenoceptors and "imidazoline receptors"

The most usual hypothesis to explain the central hypotensive effect of clonidine-like substances was to admit that these drugs stimulated alpha 2-adrenoceptors within the brainstem. Now it has been demonstrated that neither the endogenous ligand to the alpha-adrenoceptors, noradrenaline, nor any other catecholamine or phenylethylamine was hypotensive in the medullary nucleus reticularis lateralis, where all imidazolines proved to be such. Recently, a membrane receptor population sensitive to clonidine and insensitive to catecholamines was described within the nucleus reticularis lateralis; this subgroup of receptors represented 20 to 30 percent of the [3H]clonidine binding sites in the bovine nucleus reticularis lateralis and 100 percent within the human nucleus reticularis lateralis region. Thus, the existence of such imidazoline specific receptors was clearly established and the endogenous ligand for those receptors, which is neither a catecholamine nor likely a peptide, is under processing for purification. Therefore, it appeared that the hypotensive effect of substances with an imidazoline or imidazoline-like structure might be due to their action within medullary receptors specific for this endogenous ligand temporarily named "clonidine displacing substance." Rilmenidine, structurally close to imidazolines, also interfered with these receptors. The central component of its hypotensive effect was recently confirmed in rabbits, where its central cardiovascular effects were antagonized by "the clonidine displacing substance." Although exhibiting a lower affinity than the reference substance for these receptors, rilmenidine might have a higher selectivity, thus explaining its restricted side effects. A structure-activity study with this molecule would bring a confirmation to these first observations.

High NaCl diets increase alpha 2-adrenoceptors in renal cortex and medulla of NaCl-sensitive spontaneously hypertensive rats

Diets high in NaCl simultaneously elevate renal alpha 2-adrenoceptor binding and exacerbate hypertension in young NaCl-sensitive spontaneously hypertensive rats (SHR-S). The present study tests the hypothesis that in SHR-S on a high NaCl diet, an upregulation of renal alpha 2-adrenoceptors is present in densely innervated areas of the kidney, and this precedes the increase in blood pressure. Seven week old SHR-S fed on a high (8%) compared to basal (1%) NaCl diet for 2 weeks displayed significantly exacerbated hypertension and elevated renal alpha 2-adrenoceptor binding in both cortex and medulla. In contrast one week on the high NaCl diet did not alter renal alpha 2-adrenoceptor number or blood pressure in SHR-S. Autoradiographic experiments demonstrated that the NaCl-induced upregulation of alpha 2-adrenoceptors occurs in all areas of the renal cortex and medulla. None of these differences were observed in NaCl-resistant, Wistar-Kyoto rats (WKY). Further, the high NaCl diet did not alter renal alpha 1-adrenoceptor binding in SHR-S or WKY. Together with previous findings, these data suggest that the NaCl-induced upregulation of renal alpha 2-adrenoceptors is not specific to densely innervated regions of the kidney.

Characterization and regional distribution of alpha 2-adrenoceptors in postmortem human brain using the full agonist [3H]UK 14304

The full agonist [3H]UK 14304 [5-bromo-6-(2-imidazolin-2-yl-amino)-quinoxaline] was used to characterize alpha 2-adrenoceptors in postmortem human brain. The binding at 25 degrees C was rapid (t1/2, 4.6 min) and reversible (t1/2, 14.1 min), and the KD determined from the kinetic studies was 0.48 nM. In frontal cortex, the rank order of potency of adrenergic drugs competing with [3H]UK 14304 or [3H]clonidine showed the specificity for an alpha 2A-adrenoceptor: UK 14304 approximately equal to yohimbine approximately equal to oxymetazoline approximately equal to clonidine greater than phentolamine approximately equal to (-)-adrenaline greater than idazoxan approximately equal to (-)-noradrenaline greater than phenylephrine greater than (+/-)-adrenaline much greater than corynanthine greater than prazosin much greater than (+/-)-propranolol. GTP induced a threefold decrease in the affinity of [3H]UK 14304, with no alteration in the maximum number of binding sites, suggesting that the radioligand labelled the high-affinity state of the alpha 2-adrenoceptor. In the frontal cortex, analyses of saturation curves indicated the existence of a single population of noninteracting sites for [3H]UK 14304 (KD = 0.35 +/- 0.13 nM; Bmax = 74 +/- 9 fmol/mg of protein). In other brain regions (hypothalamus, hippocampus, cerebellum, brainstem, caudate nucleus, and amygdala) the Bmax ranged from 68 +/- 7 to 28 +/- 4 fmol/mg of protein. No significant changes in the KD values were found in the different regions examined. The binding of [3H]UK 14304 was not affected by age, sex or postmortem delay.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in serotonin metabolism in the rat raphe magnus and cardiovascular modifications following systemic administration of clonidine and other central alpha 2-agonists: an in vivo voltammetry study

By using the in vivo voltammetry, it was demonstrated that an injection of clonidine induced both cardiovascular modifications (hypotension and bradycardia) and a decrease in the level of 5-hydroxyindolacetic acid (5-HIAA) in the ventromedial B3 serotonergic (5-HT) cell bodies of the medulla oblongata of the rat. The cardiovascular effects of clonidine and of two other imidazolic compounds (detomidine and medetomidine) are likely to be related to their alpha 2 adrenoceptor agonist properties since hypotension and bradycardia were completely antagonized by idazoxan. The decrease in levels of 5-HIAA, induced by these three imidazolic compounds is likely to represent the combination of two additional mechanisms: (i) the stimulation of the alpha 2 adrenoceptors which could contribute to 55% of the decrease observed for the extracellular 5-HIAA and (ii) the interaction with a non-alpha 2 site (through a putative imidazole recognition site), corresponding to the part of the decrease (about 45%) which was not prevented by idazoxan.

Clonidine-specific antisera recognize an endogenous clonidine-displacing substance in brain

An endogenous substance in brain, clonidine-displacing substance, binds to the same receptor populations as clonidine and is biologically active. Since receptor binding sites can be modeled by using specific antiligand antibodies, we tested the hypothesis that polyclonal antibodies raised in rat and rabbit against the clonidine analog p-aminoclonidine coupled to hemocyanin would recognize compounds structurally related to clonidine, including clonidine-displacing substance. Binding to anti-p-aminoclonidine antibodies was examined by using a competitive radioimmunoassay with tritiated p-aminoclonidine as the radioligand. Central vasodepressor agents that, like clonidine, are known to bind with high affinity to both imidazole sites and alpha 2-adrenergic receptors in brain inhibited radioligand binding to anti-p-aminoclonidine antibodies. All of these agents contain imidazol(in)e and phenyl ring moieties as part of their chemical structures (e.g., oxymetazoline); a number of other compounds without one or both of these rings failed to cross-react with the antisera. Clonidine-displacing substance, partially purified from bovine brain, also inhibited specific radioligand binding to anti-p-aminoclonidine antibodies. The inhibition was dose dependent and high affinity (IC50, 4 Units). The endogenous substance had no effect on the apparent affinity of the antibodies for the radioligand, but blocked a specific number of binding sites. Immunoprecipitation experiments showed that authentic clonidine-displacing substance, that which displaces tritiated p-aminoclonidine binding to membrane receptors, is recognized by anti-p-aminoclonidine antibodies. We conclude that a unique subset of structural determinants required for ligand interaction with both imidazole and alpha 2-adrenergic receptors is critical for binding to anti-p-aminoclonidine antibodies, and that since clonidine-displacing substance is recognized by highly clonidine-specific antisera, it may also contain these determinants within its structure, namely the imidazol(in)e and phenyl ring systems.

The imidazoline preferring receptor: binding studies in bovine, rat and human brainstem

The binding of [3H]clonidine to brainstem membrane preparations was studied in an attempt to characterize imidazoline-sensitive, catecholamine-insensitive receptors. Human samples and samples from two animal species were used. [3H]Clonidine binding was always saturable, reversible and specific with a KD value of 6-7 nM. The Bmax values were 45.5 +/- 5.5, 145 +/- 34 and 65 +/- 33 fmol/mg protein in the whole rat medulla oblongata, the nucleus reticularis lateralis region of bovine and that of human, respectively. In the whole rat brainstem we could not demonstrate the presence of [3H]clonidine binding sites that were insensitive to catecholamines. In bovine and human nucleus reticularis lateralis (NRL) preparations, the amount of specifically bound labelled clonidine that was not displaced by an excess of (-)-norepinephrine was 25 and 100%, respectively. Substances that had a structure similar to that of clonidine were able to compete with [3H]clonidine binding within the human NRL. Cirazoline was the most potent to inhibit [3H]clonidine binding although yohimbine was also able to displace binding in the human NRL but with lower apparent affinity. Competition assays with idazoxan stereoisomers clearly showed that this binding was stereospecific. Therefore the human NRL region provides the first model of an homogenous population of imidazoline-preferring, non-alpha-adrenergic membrane receptors.

High affinity binding of 3H rauwolscine and 3H RX781094 to alpha 2 adrenergic receptors and non-stereoselective sites in human and rabbit brain cortex membranes

The radiolabeled antagonists 3H RX 781094 and 3H rauwolscine bind with high affinity to alpha 2 adrenergic receptors as well as to non-receptor sites in human and rabbit brain cortex membranes. These non-receptor sites form an important contaminant of the specific binding when non-specific binding is determined in the presence of 10 microM phentolamine or more. While phentolamine is no suitable ligand to discriminate both sites, (-)-epinephrine displays a sufficient affinity ratio to separate radioligand binding to these sites. When 1 microM (-)-epinephrine is used for the determination of the non-specific binding, both radioligands bind specifically to alpha 2 receptors. Under these conditions, 3H rauwolscine and 3H RX 781094 bind to the same amount of non-cooperative sites; binding isotherms for human brain are Bmax = 113 +/- 15 fmol/mg protein and Kd = 22.8 +/- 4.2 nM for 3H RX781094 and Bmax = 110 +/- 17 fmol/mg protein and Kd = 4.7 +/- 2.5 nM for 3H rauwolscine. Competition binding experiments show, for both radioligands and in both species, the typical pharmacological potency order of alpha 2 adrenergic receptors, i.e. phentolamine greater than yohimbine greater than prazosin for the antagonists and UK 14304 greater than p-aminoclonidine greater than or equal to (-)-epinephrine greater than (+)-epinephrine greater than isoproterenol for the agonists. Whereas the alpha 2 receptor sites display high affinity and stereoselectivity towards (-)-epinephrine and (+)-epinephrine, the non-receptor sites bind both epinephrine isomers with equal low affinity. Specific binding of both radioligands to these sites can be determined when total binding is performed in the presence of 1 microM (-)-epinephrine and non-specific binding the presence of 1 mM phentolamine. 3H rauwolscine binding to the non-stereoselective sites can be displaced with high affinity by 5-HT, suggesting binding to a 5-HT1-receptor. The 3H RX 781094 binding displays low affinity for most alpha adrenergic ligands and do not correspond to beta adrenergic, dopaminergic or serotonergic receptors.

Guanabenz, guanochlor, guanoxan and idazoxan bind with high affinity to non-adrenergic sites in pig kidney membranes

[3H]Idazoxan is a labelled ligand that is frequently used to study alpha 2-adrenoceptors in the central nervous system. In pig kidney membranes, [3H]idazoxan labelled high-affinity binding sites (Kd = 1.5 nM) that were not alpha 2-adrenoceptors and which recognized clonidine with low affinity. This new class of binding sites was recognized by amiloride derivatives; however, it is not likely that these sites are the well-known targets of amiloride in the kidney: the Na+/H+ exchanger and the epithelium Na+ channel. These binding sites may be the normal target of a series of imidazolidines derivatives (guanabenz, guanochlor, guanoxan), which are known for their antihypertensive properties.

[3H]idazoxan binding at non-alpha 2-adrenoceptors in rabbit adipocyte membranes

The imidazoline ligand, [3H]idazoxan, labels a large population of high-affinity binding sites in rabbit fat cell membranes (Bmax = 1370 +/- 91 fmol/mg protein; KD = 1.6 +/- 0.6 nM) when imidazoline derivatives are used for definition of non-specific binding. [3H]Idazoxan sites are not alpha 2-adrenoceptors as assessed by competition studies which showed that epinephrine, norepinephrine and yohimbine do not inhibit [3H]idazoxan binding. Naphazoline, tramazoline and the Na+/H+ exchange inhibitor, amiloride, completely inhibited [3H]idazoxan binding. The Ki values were 9, 27 and 48 nM, respectively.

Does anxiety underly imidazole-induced behavioural effects in the rat?

Since behavioural symptoms (tremor, irritability, pilo-erection and shaking) induced by imidazole (IMID) in rats suggested an underlying modification of arousal and/or emotionality, further studies were performed in order to extend the range of behavioural influences of IMID. In the open-field test, IMID (37.5-300 mg/kg IP) inhibited crossing and rearing in a dose-dependent fashion, this effect being long lasting (about 3 h at 75 mg/kg). Yohimbine (YOH) (1, 5 and 10 mg/kg IP), described as anxiogenic and fear-inducing in animals and in man, when investigated in this same test, inhibited the activity of rats similarly to IMID. Since diazepam (0.5 and 1 mg/kg) but not clonidine (0.075 and 0.150 mg/kg IP) pretreatment reversed IMID- and YOH-induced hypomotility, the hypothesis that IMID effects in the open field might reflect an anxiety-like state was investigated by means of social interaction and x-maze, two tests considered highly specific for anxiety studies. The data obtained show that IMID depresses social interactions only at doses inhibiting motor activity; YOH, in our experimental conditions, produced a similar effect. In an elevated x-maze, with alternate open and closed arms, IMID (37.5 and 75 mg/kg) decreased the proportion of open-arm entries and the time spent in them, an effect prevented by diazepam pretreatment (1 mg/kg IP). Finally, mean arterial pressure (MAP) was assessed in anesthetized rats treated with IMID and YOH at doses equivalent as regards behavioural effects. MAP was increased by IMID whether IP or IV and decreased by YOH; moreover, YOH, as expected, antagonized clonidine-induced hypotension, while IMID was ineffective.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of polyphosphoinositide turnover in rat cerebral cortex by clonidine

In the rat brain, a number of receptors are linked to phospholipase C which catalyzes the hydrolysis of membrane inositol phospholipids; stimulation of alpha 1-adrenergic receptors, for example, increases polyphosphoinositide turnover, but stimulation of alpha 2-receptors does not. The hydrolysis of inositol phospholipids in rat cortical slices was investigated using a direct assay involving prelabeling these lipids with 3H-inositol and then measuring the formation of 3H-inositol phosphates in the presence of lithium ions. As expected, clonidine, an alpha 2-agonist, did not stimulate the formation of 3H-inositol phosphates; however, clonidine antagonized the ability of noradrenaline to stimulate 3H-inositol phosphate formation. This effect was not blocked by antagonists of alpha 2, 5HT2, H2, or muscarinic receptors. Clonidine did not affect carbachol-stimulated 3H-inositol phosphate formation.

The imidazoline-preferring receptor

Evidence gathered over the past ten years supports the existence of subtypes of alpha 2-adrenoceptors. A receptor which resembles the alpha 2-adrenoceptor, called the imidazoline-preferring receptor (IPR), is virtually insensitive to catecholamines but binds selectively imidazolines and oxazolines such as idazoxan and rilmenidine. In contrast, the catecholamine-preferring alpha 2-adrenoceptor is preferentially activated by catecholamines including alpha-methylnorepinephrine and epinephrine and is antagonized selectively by rauwolscine. In addition to different pharmacological profiles to agonists and antagonists, the IPR and alpha 2-adrenoceptors show differences in anatomical distribution and molecular properties. The evidence has been drawn primarily from in vitro physiological and radioligand binding studies, but is gradually extending into in vivo and even clinical studies.