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

An antiserum to idazoxan recognizes an immunoreactive substance in human serum and cerebral spinal fluid which is not agmatine

A polyclonal antibody was generated in rabbits to an idazoxan-albumin antigen. The anti-idazoxan antiserum had high affinity for unconjugated 3H-idazoxan (Kd of 19.8 nM) in a radio-immunoassay (RIA). Of various drugs and native molecules only idazoxan potently (Ki of 24 nM) inhibited 3H-idazoxan binding to the anti-idazoxan antibody. A few drugs weakly inhibited 3H-idazoxan binding (IC50 > 605 microM) with rank order of UK 14304 > guanabenz > cirazoline > amiloride > naphazoline. Neither agmatine, an endogenous clonidine displacing substance (CDS), catecholamines or imidazoles inhibited the binding of 3H-idazoxan to the anti-idazoxan antibody. The anti-idazoxan RIA was 4-6 fold more sensitive than an antibody to para-amino clonidine. The CDS detected by ligand displacement from bovine brain dose-dependently inhibited 3H-idazoxan binding. This immunoreactive (ir-) CDS activity was present in human (0.9-4.1 U/ml) and rat sera (1-2 U/ml) and in the cerebro-spinal fluid of eight patients with serious disease of the central nervous system, but not in controls. We conclude: (1) an anti-idazoxan RIA is a sensitive, selective and clinically applicable RIA for measuring ir-CDS; (2) ir-CDS is not agmatine; (3) CDS represents a family of endogenous ligands for imidazoline receptors including ir-CDS and agmatine.

Endogenous ligands of imidazoline receptors: classic and immunoreactive clonidine-displacing substance and agmatine

1. There are several endogenous ligands that bind to I-receptors of both the I1 and I2 subclass. These include: (a) classic CDS, a partially purified entity isolated by the criteria that it displaces binding ligands to alpha 2- and I-receptors; (b) immunoreactive (ir)-CDS, a moiety that binds to antibodies raised against clonidine, para-amino-clonidine, or idazoxan; and (c) agmatine. 2. Classic-CDS, not yet defined structurally, binds to I1, I2, and alpha 2-adrenergic receptors, is neither a peptide nor a catecholamine, and has purportedly a molecular weight of 588 Da. By ligand binding assays, it was found in brain, serum, CSF, and placenta and in a neural-glial cell line. Partially purified classic CDS is bioactive. Like clonidine, it contracts aorta and vas deferens and inhibits platelet aggregation, effects largely attributable to agonism at alpha 2-adrenergic receptors. Unlike clonidine, it contracts rat gastric fundus and releases catecholamines from chromaffin cells, effects attributable to actions at I-receptors. Injected into the RVL, classic CDS alters arterial pressure, but the direction of change of pressure has differed between groups of investigators. However, in the absence of structure, it is possible that ligand binding and bioactivity may be attributable to different molecules. 3. Ir-CDS, also of unknown structure, is a material(s) that binds to antibodies raised against clonidine, PAC, or idazoxan. Ir-CDS, measured by radioimmunoassay, is unevenly distributed in brain with highest concentrations in the hypothalamus, midbrain, and dorsal medulla. It is contained in the gastric fundus, adrenal gland, heart, kidney, and serum in amounts substantially higher than found in brain. Ir-CDS may be elevated in the serum of some patients with hypertension and in the CSF of patients with structural brain disease. The concentration of ir-CDS and bioactivity on gastric fundus directly correlates, suggesting that it may share similarities with classic-CDS. However, until the structure of classic and ir-CDS is determined, the possibility that ligand binding and antibody recognition are properties of different molecules must be considered. 4. Agmatine (decarboxylated arginine) is the only endogenous molecule that, like CDS, binds to alpha 2- and I-receptors of both classes. It and its biosynthetic enzyme arginine decarboxylase are present in brain, and agmatine is widely distributed throughout the body. However, the distribution of agmatine and ir-CDS differs, whereas the biological actions of agmatine do not mimic those of classic CDS. Its presence raises the possibility of an alternative pathway for polyamine biosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Agmatine, the bacterial amine, is widely distributed in mammalian tissues

We sought to determine whether agmatine (decarboxylated arginine), a bacterial product recently discovered for the first time in mammalian brain, was contained in other organs. A method was developed for isolation of agmatine from tissue and detection by RP-HPLC following solid-liquid extraction and derivatization with o-phthalaldehyde and mercaptoethanol. Recovery was about 80% and the limit of fluorometric detection was about 10 pg on column. In male Sprague-Dawley rats agmatine was unevenly and widely distributed in many tissues confirming its presence in mammals. The highest concentration (approximately 71 ng/mg net weight) was found in stomach, with aorta and small intestine next, followed by smaller levels in spleen, adrenal, aorta, and skeletal muscle and brain. Serum concentrations were high. Agmatine in male Long Evans rats of 3, 12, and 24 months of age demonstrated similar but not identical tissue distribution without any effect of aging. Since agmatine binds to alpha 2-adrenergic and imidazoline receptors, is bioactive in a number of tissues, is contained in neurons and is found in serum and tissues, the findings are consistent with a potential role for agmatine as a neurotransmitter and/or hormone. It also raises the possibility that agmatine may, as in bacteria, serve as a polyamine precursor along metabolic pathways previously not detected in mammals.

A novel mechanism of action for hypertension control: moxonidine as a selective I1-imidazoline agonist

Sympathoadrenal inhibition by a direct action within the central nervous system is an advantageous route to blood pressure control. Stimulation of brain alpha 2-adrenergic receptors is one mechanism for sympathoadrenal suppression, but comes at the cost of nonspecific depression of CNS function, including sedation and decreased salivary flow. Evidence is accumulating for a second pathway for pharmacological control of sympathoadrenal outflow, mediated by a novel receptor specific for imidazolines. First-generation central antihypertensive agents, which are imidazolines such as clonidine, act primarily to stimulate these I1-imidazoline receptors in the rostral ventrolateral medulla oblongata (RVLM) to lower blood pressure, but have sufficient agonism at alpha 2-adrenergic receptors to produce side effects. Second-generation centrally acting antihypertensive agents, such as moxonidine and rilmenidine, are selective for I1 relative to alpha 2 receptors. The reduced alpha 2 potency of these agents correlates with reduced severity of side effects. In this study we further established the selectivity of moxonidine for I1-imidazoline sites by characterizing the direct interaction of [3H]moxonidine with these receptors in the RVLM and in adrenomedullary chromaffin cells. [3H]Moxonidine preferentially labeled I1-imidazoline sites relative to alpha 2-adrenergic sites, only a small portion of which were labeled in the RVLM. [3H]Moxonidine binding to I1-imidazoline sites was modulated by guanine nucleotides, implying that I1-imidazoline sites may be membrane receptors coupled to guanine nucleotide binding regulatory proteins (G proteins). Receptor autoradiography with [125I]p-iodoclonidine confirmed the presence of I1-imidazoline sites in the RVLM and other areas of the brainstem reticular formation. In contrast, alpha 2-adrenergic sites were mainly localized to the nucleus of the solitary tract. Moxonidine selectively displaced [125I]p-iodoclonidine binding from reticular areas, including the RVLM. In vivo studies in SHR rats confirmed the ability of moxonidine to normalize hypertension by an action within the RVLM and confirmed the correspondence of I1 binding affinity and antihypertensive efficacy. We also discuss prior literature on the cardiovascular pharmacology of imidazolines, reinterpreting previous studies that only considered alpha-adrenergic mechanisms.

Evidence for a bioactive clonidine-displacing substance in peripheral tissues and serum

Clonidine-displacing substance (CDS) from brain is biologically active in the kidney and stomach and on platelets. To determine whether CDS is contained in these and other peripheral tissues, homogenates of fresh brain, eight other organs and serum from rat were ultrafiltered (less than 10,000 mol. wt only), dried and extracted with methanol. Evaluation by radioimmunoassay (RIA) using antibodies to p-aminoclonidine showed that adrenal gland and gastric fundus (GF) contained significantly greater amounts of CDS-like radioimmunoactivity than brain; intermediate-to-low activity was present in heart, small intestine, serum, kidney and liver; lung and skeletal muscle values were near-background. RIA-positive extracts elicited well-correlated contractile activity in a GF smooth muscle bioassay; contractions persisted in the presence of antagonists of various transmitters and modulators, but were abolished by low concentrations of the calcium channel blocker verapamil. Serum levels of CDS were profoundly reduced following removal of the adrenal glands. We conclude that a CDS-like substance is present not only in brain as previously reported, but also in peripheral organs and in the circulation.

Production and characterization of an iminoimidazolidine specific monoclonal antibody using para-aminoclonidine as antigen

Para-aminoclonidine coupled to hemocyanin was used to produce mouse monoclonal antibodies directed against clonidine. The properties of one of these, called mFE7, secreted by a clone of hybrid myeloma, are described. This antibody displayed total crossreactivity with imidazolidines and no crossreactivity at all with catecholamines or other known naturally occurring substances tested. A liquid phase radioimmunoassay permitted the detection of immunoreactivity in human brain extracts. The mFE7 antibody could be useful for immunopurifying the endogenous ligand of Imidazolines Preferring Receptors (IPR) which are catecholamines insensitive.

Effect of clonidine on blood pressure and GABAergic mechanisms in spontaneously hypertensive rats

The action of clonidine on blood pressure and on the functional state of the gamma-aminobutyric acid-ergic (GABAergic) system was studied. A single injection of clonidine (1, 5, 10, 20 micrograms.kg-1) induced a dose-dependent decrease of blood pressure. Chronic administration of clonidine, 10 micrograms.kg-1, produced the maximum effect after the third injection. The effect was maintained for the duration of the study. Single or chronic clonidine injections, at the dose of 10 micrograms.kg-1 enhanced the GABA content in the brain and hypothalamus. This effect was less pronounced in the hippocampus. The drug administered according to the same regimen stimulated glutamic acid decarboxylase activity only in the hypothalamus. Clonidine caused a marked enhancement of specific [3H]GABA binding in the hypothalamus. These data suggest that the hypotensive action of clonidine is related to stimulation of the GABAergic system.

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.

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.