Josiane Feldman

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.

The nucleus reticularis lateralis: A region highly sensitive to clonidine

Slow bilateral microinjections of a low dose of clonidine (75 ng/kg) in the cats nucleus reticularis lateralis (NRL) lead to significant hypotension and bradycardia. This finding confirms the existence of a ventromedullary highly sensitive site of action of clonidine. It is suggested that clonidine inhibits some vasopressive and cardioacceleratory structures within the NRL region.

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 μg 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.

Rilmenidine selectivity for imidazoline receptors in human brain

The selectivity of three centrally acting antihypertensive agents for the medullary imidazoline-preferring receptors (IPR) versus cortical alpha-adrenoceptors was investigated in human brain. [3H]Clonidine binding was studied in various membrane preparations. Competition experiments were performed. Cortical membrane preparations were used as they mainly contained classical alpha-adrenoceptors whereas medullary membrane preparations from the nucleus reticularis lateralis contained only IPR insensitive to catecholamines. Rilmenidine, a new antihypertensive agent, appeared 2.5 and 3.5 times more selective than clonidine and guanfacine, respectively, for medullary IPR sites than for cortical alpha-adrenoceptors, thus providing a possible explanation for the low sedative effects of this new molecule.

Drugs Acting on Imidazoline Receptors A Review of Their Pharmacology, Their Use in Blood Pressure Control and Their Potential Interest in Cardioprotection

Drugs acting within the autonomic nervous system 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. Several classes of drugs acting within the central, as well as the peripheral, autonomic nervous system are very efficient in treating hypertensive disease. One class — the second generation of a group of centrally acting drugs selective for imidazoline receptors — has proved beneficial in this respect, because drugs in this class are well tolerated and have interesting additional effects such as their antiarrhythmic action. Rilmenidine and moxonidine are the lead compounds of this class of drugs. Rilmenidine and moxonidine both proved more selective for cerebral imidazoline receptors than the reference drug, clonidine. It was suggested that this selectivity, attributable to their lower affinity for α2-adrenoceptors, explains the low incidence of adverse effects (including sedation) associated with these drugs. In addition, potentially beneficial actions on cardiac dysrythmias and congestive heart failure enlarge the therapeutic potential of the second generation of imidazoline-related drugs. This review focuses on the main pharmacological and clinical properties of rilmenidine and moxonidine, paying particular attention not only to their efficacy in hypertension but also to other potential cardiovascular indications.

evidence for a neuromodulatory role of GABA at the first synapse of the baroreceptor reflex pathway

SummaryMicroinjections of GABA and of the specific agonist of GABA receptors, muscimol, in the intermediate nucleus tractus solitarii (NTS) of pentobarbitone anaesthetized cats produced hypertension and tachycardia. The GABA receptor antagonist, bicuculline, had opposite effects and prevented those of muscimol. Therefore, a GABAergic system appears to modulate the cardiovascular regulation within the NTS. d,l-Baclofen also increased blood pressure and heart rate when injected into the same region, but this effect was not antagonized by bicuculline. The mechanism of this action of baclofen is discussed.

An endogenous, non-catecholamine clonidine antagonist increases mean arterial blood pressure

We report here that topical application of clonidine displacing substance (CDS), an endogenous brain extract, directly in the nucleus reticularis lateralis (NRL) region of anaesthetized cats regularly produced hypertension. CDS (5 units) increased the mean blood pressure by 40 +/- 8%. Pretreatment of anaesthetized rabbits with intracisternal CDS (500 units) shifted to the right the dose-response curve obtained with clonidine alone injected the same way. This brain extract might be considered as an endogenous antagonist for the hypotensive effects of clonidine at least in the NRL region.

Role of the ventral surface of the brain stem in the hypotensive action of clonidine

The areas S of the ventral surface of the brain stem and the immediately surrounding zone were superficially destroyed by the means of electro-coagulation, in 14 cats. This destruction produced a drop in blood pressure, which was transient in 9 and definitive in 4 animals; in one cat only the arterial pressure did not change after the destruction. In 6 animals which have been sham-operated, clonidine (15 mug/kg, i.v.) always induced a marked fall in blood pressure whereas in 10 animals which had maintained or recovered a normal blood pressure after the destruction of the area S, clonidine (15 mug/kg) injected intravenously no longer produced any decrease of the arterial pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at the level of the ventral surface of the brain stem, through inhibition of a vasopressive structure.

Respective contributions of α-adrenergic and non-adrenergic mechanisms in the hypotensive effect of imidazoline-like drugs

The hypotensive effect of imidazoline‐like drugs, such as clonidine, was first attributed to the exclusive stimulation of central α2‐adrenoceptors (α2ARs). However, a body of evidence suggests that non‐adrenergic mechanisms may also account for this hypotension. This work aims (i) to check whether imidazoline‐like drugs with no α2‐adrenergic agonist activity may alter blood pressure (BP) and (ii) to seek a possible interaction between such a drug and an α2ARs agonist α‐methylnoradrenaline (α‐MNA). We selected S23515 and S23757, two imidazoline‐like drugs with negligible affinities and activities at α2ARs but with high affinities for non‐adrenergic imidazoline binding sites (IBS). S23515 decreased BP dose‐dependently (−27±5% maximal effect) when administered intracisternally (i.c.) to anaesthetized rabbits. The hypotension induced by S23515 (100 μg kg−1 i.c.) was prevented by S23757 (1 mg kg−1 i.c.) and efaroxan (10 μg kg−1 i.c.), while these compounds, devoid of haemodynamic action by themselves, did not alter the hypotensive effect of α‐MNA (3 and 30 μg kg−1 i.c.). Moreover, the α2ARs antagonist rauwolscine (3 μg kg−1 i.c.) did not prevent the effect of S23515. Finally, whilst 3 μg kg−1 of S23515 or 0.5 μg kg−1 of α‐MNA had weak hypotensive effects, the sequential i.c. administration of these two drugs induced a marked hypotension (−23±2%). These results indicate that an imidazoline‐like drug with no α2‐adrenergic properties lowers BP and interacts synergistically with an α2ARs agonist.

The central hypotensive action of baclofen in the anaesthetized cat.

In the pentobarbital-anaesthetized cat, the intracerebroventricular administration of 20 microgram/kg d,1-baclofen led to marked hypotension and bradycardia, whereas the same dose of d-baclofen (i.c.v.) had no cardiovascular effect. When the drug was prevented from reaching the brain stem structures by collecting perfusate through a catheter lodged in the Aqueduct of Sylvius, the efficacy of d,1-baclofen was increased. Bicuculline did not antagonize the central cardiovascular effects of d,1-baclofen at doses which inhibit the cardiovascular effects of GABA agonists. However, both glutamic acid and kainic acid prevented and reversed the central hypotension and bradycardia produced by d,1-baclofen. These effects of baclofen may therefore be mediated by a selective inhibition of the release of the excitatory neurotransmitter, glutamate, within forebrain structures involved in central cardiovascular regulation.