Effects of ifenprodil and baclofen on exercise-induced increase of myocardial oxygen demand in normotensive rats

Central glutamatergic relays are known to be present in the central sympathetic pathways. Ifenprodil (an N-methyl-D-aspartate antagonist) and baclofen (a gamma-aminobutyric acid(B) agonist) are both modulators of these synapses; we previously reported their ability to reduce the cardiovascular responses induced by a central hypothalamic stimulation in rabbits. In this work, we investigated the actions of chronic treatments with these two drugs on the increase of myocardial oxygen demand induced by exercise in normotensive rats. Moreover, their effects on the baroreceptor heart rate reflex were observed. Male normotensive WKY rats were treated with placebo (two groups), baclofen, or ifenprodil for 14 days. They were then submitted to a progressively increased exercise test on a treadmill. In another three groups of animals, the same treatment was applied but, at the end, a baroreflex study was performed by the injection of phenylephrine (vagal component of the reflex) and of sodium nitroprusside (sympathetic component). Ifenprodil and baclofen reduced by nearly 50% the level of the increase of the rate x pressure product during exercise as compared with control rats. This effect appeared to be mainly due to a reduction of the hypertensive response. In the same conditions, neither baclofen nor ifenprodil significantly altered the baroreceptor heart rate reflex. The fact that these two drugs are capable of reducing the myocardial oxygen demand encourages us to test them in a model of myocardial ischemia associated with sympathetic hyperactivity.

Identification of human I1 receptors and their relationship to alpha 2-adrenoceptors

I1 imidazoline receptors (I1R) were defined as receptors insensitive to catecholamines and highly sensitive to [3H]clonidine and analogs. By contrast, the I2R subtype is more sensitive to [3H]idazoxan. [3H]clonidine and [3H]idazoxan imidazoline specific binding sites (IBS) have been detected in crude human membranes. Pharmacologic characterization by binding assays clearly differentiates IBS from alpha 2-adrenoceptors, whereas differences between [3H]clonidine and [3H]idazoxan IBS are less clear in crude preparations. In fact, only moderate affinity for [3H]clonidine was detectable in such preparations. However, purification procedures allowed detection of high affinity [3H]clonidine IBS in the human brain, corresponding to the I1R. Difficulties in the characterization of the I1R in crude membranes are due to multiple factors including heterogeneity of IBS, their low Bmax value, the existence of allosteric modulation, and possibly the presence of natural binding inhibitors. Immunologic studies with specific anti-idiotypic antibodies revealed a 43-kD protein as the best candidate for I1R as binding activity coincides with immunodetection. No cross-reaction was found with anti-monoamine oxidase (MAO) A/B antibodies and the 43-kD protein, ruling out the possibility of this protein being an MAO-associated I2R. Neither anti-alpha 2A- nor anti-alpha 2B-specific antibodies were able to immunodetect the 43-kD protein in crude membrane preparations or in purified fractions. These results and further biochemical characterization (pHi, N-glycosylation) of the 43-kD protein definitely assessed that human brain I1R and alpha 2-adrenoceptors clearly differ physically. However, coexpression of I1R and alpha 2-adrenoceptors in synaptic plasma membranes of the bovine brainstem reinforce the possibility of a functional relationship between the two types of receptor.

Participation of imidazoline receptors and alpha(2-)-adrenoceptors in the central hypotensive effects of imidazoline-like drugs

The central hypotensive effect of imidazoline-like drugs (IMs) involves non-adrenergic imidazoline receptors (IRs). IMs cause hypotension irrespective of their affinity and selectivity for one or the other alpha-adrenoceptor subtypes. LNP 509, which binds to I1Rs (Ki = 5.10(-7) M) but roughly not to alpha 2-adrenoceptors (A2Rs) (Ki > 10(-5) M), causes hypotension when injected alone into the brainstem. As far as hybrid drugs, that is, those with mixed binding profiles (I1/alpha 2), are concerned, a significant correlation was reported between their central hypotensive effect and their affinity for IRs. Imidazoline antagonists such as idazoxan competitively antagonized the centrally induced hypotensive effect of IMs. Yohimbine, an A2Rs antagonist, blocks the hypotensive effect of hybrids but usually in a noncompetitive manner. Mutation of A2Rs prevented the hypotensive effects of drugs highly selective for A2Rs, but also that of hybrids such as clonidine. These data indicate that triggering of the hypotensive effects of IMs (1) needs implication of IRs; (2) appears to be facilitated by additional activation of A2Rs; and (3) requires integrity of A2Rs along the sympathetic pathways.

[Central neurocytoma. Immunohistochemical study: MIB1, p53 and bcl-2. Report of 5 cases]

To further characterize central neurocytoma, a rare intraventricular tumour described in 1982, we analyzed six tumours by immunohistochemistry for MIB1, p53 and bcl-2. bcl-2, an inhibitor of p53-mediated apoptosis is frequently expressed in gliomas, especially in tumors with wild-type p53. Its expression in peripheral neuroblastomas suggests a down-regulation during final terminal differentiation. Six tumors from five patients (one female/four males, age ranged from 18 to 63 years) were examined. All patients were alive from 2 to 88 months after initial surgical resection. On histological sections, tumours demonstrated a typical pattern. Synaptophysin staining was seen in all cases. Proliferation index was low (< 4.5%). bcl-2 was never expressed. p53 expression varied but within low values (< 10% of cells). These latter antibodies were rarely analyzed until now in this usually benign neoplasm which represents a well differentiated variant of neuron derived tumors.

Recurrent intracranial neurenteric cysts

Neurenteric cysts are cystic lesions lined by a columnar epithelium of endodermal derivation. They are rarely located in the central nervous system. We describe the case history of two recurrent neurenteric cysts located within the posterior fossa: one in the 4th ventricle and the second in the cerebellopontine angle. The literature is reviewed and the follow-up and recurrence of such cysts are discussed.

Ligand binding to I2 imidazoline receptor: the role of lipophilicity in quantitative structure-activity relationship models

A series of 2-trans-styryl-imidazoline (tracizoline) congeners were designed and tested to develop 2-D and 3-D QSAR models for their binding to imidazoline (I2) receptor. The important role of lipophilicity was assessed by classical 2-D QSAR study (Hansch approach) and by comparative molecular field analysis (CoMFA) with the inclusion of the molecular lipophilicity potential (MLP), as an additional descriptor, besides standard steric and electrostatic fields. Results from these studies were compared to those obtained in a previous modeling study of I2 receptor ligands and integrated into a new, comprehensive model, based on about sixty I2 receptor ligands. This model revealed, at the three-dimensional level, the most significant steric, electrostatic, and lipophilic interactions accounting for high I2 receptor affinity.

Evidence for the existence of imidazoline-specific binding sites in synaptosomal plasma membranes of the bovine brainstem

Nonadrenergic imidazoline-specific binding sites were characterized pharmacologically in crude cerebral membrane preparations, but little is known about their subcellular localization in neurons. As in the brainstem these sites are involved in cardiovascular regulation and peripherally imidazolines modulate neurotransmitter release, we tried to determine a possible (pre)synaptic localization in brainstem. We found a specific enrichment in (entire) synaptosome, purified synaptosomal plasma membrane (37 fmol/mg), and mitochondrial (83 fmol/mg) fractions as compared with other membrane fractions (3-8 fmol/mg). Synaptosomes appeared to be free of postsynaptic structures, and purified synaptosomal plasma membranes were devoid of mitochondrial material, as determined by electron microscopy and by comparison with the distribution of marker enzymes such as monoamine oxidase. These results show for the first time that these extramitochondrial imidazoline-specific sites are neuronal and are located on presynaptic terminals. We found high affinities for unlabeled p-iodoclonidine (subnanomolar), clonidine (0.2 nM), and efaroxan (11 nM), but idazoxan did not compete significantly for the p-[125I]iodoclonidine binding in these membranes. Therefore, these sites can be classified as I1 imidazoline receptors. In summary, we describe for the first time that high-affinity I1 receptors of the bovine brainstem are located on (pre)synaptic membranes.

Characterization of a partial cDNA clone detected by imidazoline receptor-selective antisera

A cDNA clone has been isolated from a human hippocampal cDNA expression library by relying on the selectivity of two antisera that are specific for imidazoline binding proteins. A 1789 bp cDNA clone was sequenced and shown to contain a single open-reading frame that predicts a 66 kDa polypeptide, but it is truncated based on its lack of a stop codon and poly-A+ tail. Two regions of homology exist for the predicted amino acid sequence in common with chromogranin-A and B proteins, a zinc finger protein, and the ryanodine receptor. Northern blot analyses of poly-A+ mRNA from 36 human tissues indicated two differentially expressed transcripts of 6.0 and 9.5 kb. The 6.0 kb mRNA form was enriched in brain and endocrine tissues as compared to other tissues, but not in strict concordance with I1-imidazoline binding sites. The highest overall amounts of the combined transcripts were found in pituitary. In situ hybridization histochemistry revealed an enrichment of the message in neuronal cell bodies of the rat hippocampus and cerebellar cortex. This clone has some of the properties expected of an imidazoline receptor.

Does a second generation of centrally acting antihypertensive drugs really exist?

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem: the nucleus reticularis lateralis. After that, it was shown that imidazolines and related substances reduced blood pressure when applied in this area whereas catecholamines were not capable of producing such an effect. These data led us to suggest the existence of receptors specific for imidazoline-like compounds different from the alpha2-adrenoceptors. Soon after, the existence of imidazoline binding sites was reported in the brain and in a variety of peripheral tissues including the human kidney. As expected, these specific binding sites do not bind the catecholamines. The imidazoline binding sites are already subclassified in two groups: the I1-subtype sensitive to clonidine and idazoxan, and the I2-subtype, sensitive to idazoxan and nearly insensitive to clonidine. Functional studies confirmed that the hypotensive effects of clonidine-like drugs involved imidazoline receptors while their most frequent side effects only involved alpha2-adrenoceptors. However, recent functional evidence suggests that a cross talk between imidazoline receptors and alpha2-adrenoceptors is necessary to trigger a hypotensive effect within the ventral brainstem. Rilmenidine and Moxonidine are the leader compounds of a new class of antihypertensive drugs selective for imidazoline receptors. At hypotensive doses, these drugs are devoid of significant sedative effect. Rilmenidine evoked hypotension when injected within the nucleus reticularis lateralis region; it competed for [3H]-clonidine bound to specific imidazoline binding sites in human medullary membrane preparations but proved more selective for cerebral imidazoline receptors than clonidine. It is suggested that this selectivity might explain the low incidence of their side effects. Additional potentially beneficial actions on cardiac arrhythmias or congestive heart failure enlarge the therapeutic interest of imidazoline-related drugs. Recent binding and functional data throw a new light on the optimal pharmacological profile of this second generation of centrally acting antihypertensive drugs.

I1-imidazoline receptors: an update

BACKGROUND

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the nucleus reticularis lateralis of the rostroventrolateral part of the medulla (NRL/RVLM). It was shown that imidazolines and related substances reduced blood pressure when applied in this area whereas no catecholamine was capable of such an effect. IMIDAZOLINE-SPECIFIC BINDING: We previously suggested the existence of receptors specific for imidazoline-like compounds that differed from the alpha-adrenergic receptors. Imidazoline-binding sites were subsequently reported in the brain and in a variety of peripheral tissues, including the human kidney, and as expected these specific binding sites do not bind the catecholamines. The imidazoline-binding sites are classified into two subgroups: the I1-type, which is sensitive to clonidine and idazoxan, and the I2-type, sensitive to idazoxan and largely insensitive to clonidine. Numerous studies have confirmed the involvement of these receptors in various regulations and pathological processes, hypertension being the most notable.

DRUGS

Functional studies have confirmed that the hypotensive effects of clonidine-like drugs involve I1-imidazoline receptors while their most frequent side effects only involve alpha2-adrenergic receptors. Recent studies have shown that a contribution of both receptor types might be necessary to trigger the hypotensive effect of central origin. Rilmenidine, an oxazoline analogue to the imidazolines, has been proposed as the prototype of a new class of antihypertensive drugs selective for I1-imidazoline receptors. At hypotensive doses, this drug is devoid of any significant sedative effect. As with clonidine, it evokes hypotension when injected into the NRL region and it completely displaces the [3H]clonidine bound to specific imidazoline-binding sites in human medullary membrane preparations, but it has proved more selective for cerebral imidazoline receptors than clonidine. This selectivity might explain the low incidence of side effects evoked by rilmenidine.

CONCLUSION

Rilmenidine is the first example of a drug exhibiting a favourable selectivity between I1-imidazoline receptors and alpha2-adrenergic receptors, for example reducing blood pressure but avoiding sedation and mouth dryness.

Preventive effect of rilmenidine on the occurrence of neurogenic ventricular arrhythmias in rabbits

BACKGROUND

Centrally acting antihypertensive drugs bearing an imidazoline or a related chemical structure inhibit sympathetic nervous output to the heart and vascular beds, and enhance parasympathetic tone. Cardiac ischaemia and ventricular arrhythmias that can result from hypertension are likely to benefit from such effects.

OBJECTIVE

To investigate the effects of rilmenidine, an oxazoline with antihypertensive properties, in a model of neurogenically induced ischaemic ventricular arrhythmias.

METHODS AND RESULTS

Bicuculline, a alpha-aminobutyric acid (GABA(A)) receptor antagonist, was administered intracisternally in pentobarbitone anaesthetized rabbits; 10 microg/kg intracisternal bicuculline induced polymorphic ventricular ectopic beats and ventricular tachycardia, while blood pressure increased by about 50-60% and heart rate in sinus rhythm decreased by about 20%. Rilmenidine pretreatment (10 min), either administered intravenously (0.01, 0.1, 1 mg/kg) or intracisternally (3, 10, 30 microg/kg), dose-dependently prevented the occurrence of bicuculline-induced arrhythmias and, because of a lower baseline, the blood pressure values reached were less when compared with controls. Intracisternal idazoxan (15 microg/kg) had no significant antiarrhythmic effect but antagonized, in part, the haemodynamic and antiarrhythmic effects of rilmenidine (1 mg/kg intravenously; 30 microg/kg intracisternally).

CONCLUSION

The antiarrhythmic effects observed with rilmenidine are mainly mediated by blunting the bicuculline-induced increase in the sympathetic nervous output to the heart and the vascular beds. These effects of rilmenidine are likely to originate from action on the central as well as on the peripheral nervous systems. Direct coronary or cardiac effects might also play a role, in particular at low non-hypotensive intravenous doses.

Autonomic nervous system as a target for cardiovascular drugs

1. Drugs acting within the autonomic nervous system (ANS) are of particular interest when autonomic abnormalities are implicated in the development and maintenance of various cardiovascular pathologies. For example, it has been documented that in the early stages of hypertensive disease (i.e. hyperkinetic borderline hypertension) a sympathetic hyperactivity associated with a decreased parasympathetic activity results in increased cardiac output and heart rate. 2. Several classes of drugs acting within the central, as well as the peripheral ANS, are very efficient in treating hypertensive disease. One of these classes of drugs, the second generation of centrally acting drugs, has proved beneficial in this respect because, in addition to their therapeutic efficacy, these drugs are well tolerated. 3. The central nervous system may also be the target for drugs with the potential to treat other cardiovascular diseases. Some recent experimental and clinical data supporting such new perspectives concerning idiopathic dysrhythmias, angina pectoris and congestive heart failure will be summarized.

Co-detection by two imidazoline receptor protein antisera of a novel 85 kilodalton protein

Imidazoline receptors (I-receptors) are considered as potential therapeutic targets for a spectrum of stress-induced illnesses. Yet, I-receptors remain poorly defined at the molecular level. In this study, candidate imidazoline receptor proteins were compared using two imidazoline receptor-selective antisera of diverse origins. One antiserum was derived from affinity-purified imidazoline-binding protein. The second antiserum was produced as an anti-idiotypic antiserum, from purified IgG selective for imidazolines. Despite such diverse origins, both antisera co-identified an 85 kDa band on western blots from a variety of tissues. The integrity of the 85 kDa band was dependent on protection by eight different protease inhibitors. Other proteolytic breakdown products (obtained after homogenization with only one protease inhibitor) were comparable in size to previously reported smaller immunoreactive bands. The full-size 85 kDa band was also enriched in plasma membrane fractions and abundant in rat PC12 cells and brain regions known to be abundant in I1 binding sites. Furthermore, the immunodensity of the 85 kDa band, against anti-idiotypic antiserum, was linearly correlated with reported I1 site radioligand Bmax values (r2 = 0.8736, P = 0.0002) across nine rat tissues. Therefore, a possible candidate for the full-length imidazoline receptor(s) appears to be an 85 kDa protein.