[3H]Para-amino-clonidine: A novel ligand which binds with high affinity to α-adrenergic receptors
Abstract
Para-amino-clonidine (PAC) is an α-adrenergic agonist with extraordinarily high potency in some peripheral tissues. We have demonstrated the labeling of α-adrenergic binding sites in central and peripheral tissues with [3H]PAC and compared properties of this binding to those of [3H]clonidine. [3H]PAC binds saturably with a dissociation constant (KD) of about 0.9 nM to rat cerebral cortex membranes. It has about 2–3 times the affinity of [3H]clonidine for α-receptor binding sites. The greater affinity is attributable mainly to a slower dissociation of [3H]PAC than [3H]clonidine from binding sites. The relative and absolute potencies of various adrenergic agonists and antagonists in competing for [3H]PAC and [3H]clonidine binding are essentially the same. [3H]PAC can also be utilized to label α-adrenergic binding sites in the kidney and spleen where the relative potencies of PAC and clonidine are the same as in the brain.
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A comparison of two subarachnoid α<inf>2</inf>-agonists, xylazine and clonidine, with respect to duration of antinociception, and hemodynamic effects in goats
2003, Small Ruminant ResearchSubarachnoid administration of clonidine and xylazine produces antinociception in several species and in humans. The present study compares these two drugs administered by the subarachnoid route to goats. Goats (n=6) were randomly assigned to three treatment groups. All animals of each group received 0.06 mg kg−1 clonidine (CLO), 0.1 mg kg−1 xylazine (XIL) and 0.9% saline solution (SS), with a minimum interval of 1 week between treatments. All injections were made into the subarachnoid space between the last lumbar vertebra and the first sacral vertebra. Analgesia, ataxia, sedation, cardiovascular and respiratory effects, and rectal temperature were evaluated at predefined regular time intervals before drug administration (baseline) and after administration. The onset of analgesia by clonidine and xylazine was observed in 6.8±1.8 and 9.5±2.6 min (mean±S.D.), respectively. Both α2-agonists produced analgesia of dorsocaudal rib areas, flanks, hind limbs, perineum and tail, sedation and ataxia. The duration of antinociception after clonidine administration was 118.8±24 min and after xylazine 88.3±15 min (mean±S.D.). Clonidine and xylazine subarachnoidally administered induced a significant (P<0.05) decrease in heart and respiratory rates and hypothermia in relation to the basal value. Neither drug significantly altered blood pressure. Both α2-agonist drugs induced frequent diuresis and an increase in salivation. We conclude that subarachnoid clonidine produces longer antinociception with less ataxia than xylazine in goats. However, the drug induced bradycardia, a decrease in the respiratory rate and hypothermia, with a small compromise in the blood pressures at the doses studied. Further studies should be done to determine whether this analgesia is sufficient for surgical procedures.
Alpha<inf>2</inf> noradrenoceptors in the anterior piriform cortex decline with acute amino acid deficiency
1996, Molecular Brain ResearchThe responses of the brain to the amino acid deficiency that occur after eating imbalanced amino acid diets (IMB) have been associated with decreased concentrations of norepinephrine (NE) and cAMP in the anterior piriform cortex (APC), an area essential for the initial feeding responses to amino acid deficiency. In addition, the anorectic responses to IMB were decreased after injections of the αa agonist, clonidine, and increased after injections of the α2 antagonist, idazoxan, into the APC. Therefore, to study the role of the α2-noradrenergic receptor further in this model, we measured α2-noradrenergic receptor binding in the APC of rats fed two levels of theonine IMB or a low-protein basal control diet. After basal prefeeding for 10 days, rats were given either a mild IMB, a severe IMB, or the basal diet for 2.5 h. The APC, anterior cingulate cortex (AC), ventromedial hypothalamus (VMH), and lateral hypothalamus (LH) were assayed. Binding of [3H]p-aminoclonidine to α2 receptors determined that α2 binding was decreased the most in APC (P < 0.0003). Binding in APC was significantly correlated with food intake in the anorectic response to IMB (P < 0.001). In AC, binding was also significantly decreased, but less dramatically (P = 0.012), and was not correlated with food intake. There were no significant changes in LH or VMH, although α2-noradrenergic binding in VMH tended to decrease with the severe IMB in a pattern similar to APC. Plasma glucose values did not differ after the same feeding protocol. These data support our hypothesis that NE activity in the APC plays a role in initiating the anorectic responses to IMB, perhaps via the α2-noradrenergic receptor.
Quantitative autoradiography of α<inf>1</inf>- and α<inf>2</inf>-adrenergic receptors in the cerebral cortex of controls and suicide victims
1993, Brain ResearchAlterations in both serotonergic and noradrenergic indices have been found in the brain of suicide victims. In order to better understand the role of the noradrenergic system in suicide, we carried out quantitative autoradiography of α1-andα2-adrenergic receptors using [3H]prazosin and [3H]-p-aminoclonidine respectively. We compared the distribution and relative density of these receptors in the prefrontal (PFC) and α1-adrenergic receptors in the temporal cortex (TC) of suicide victims and controlsmatched for postmortem delay, age, side of brain and sex. We found that: (1) the laminar patterns of α1-adrenergic receptors in the PFC (n = 20) and the TC (n = 16) were different (P = 0.022); (2) there was a 37% increase in α1-adrenergic binding corresponding to layers IV–V of PFC of suicide victims compared to controls (P = 0.029); (3) the TC had a greater density of α1-adrenergic binding sites than the PFC across all cortocal layers (P = 0.006); (4) α2-adrenergic binding sites had a specific laminar distribution in the PFC (n = 24) which did not differ in controls and suicide victims; (5) binding to α2-adrenergic sites in the PFC of suicide victims did not differ from controls; and (6) norepinephrine concentrations in the same brain areas were elevated in the suicide group compared to controls, but did not correlate with binding to α1-orα2-adrenergic sites. The increase in [3H]prazosin (to α1-adrenergic receptors) but not in [3H]-p-aminoclonidine (to α2-adrenergic receptors), and in norepinephrine concentrations in the brain of suicide victims provides further evidence for an association between suicide and altered brain noradrenergic function. Future studies must determine whether these changes in brain noradrenergic function indicate increased or decreased transmission.
Effects of chronic total sleep deprivation on central noradrenergic receptors in rat brain
1993, Brain ResearchThe effect of chronic total sleep deprivation (TSD) on the regulation of central noradrenergic receptors was evaluated. Rats were subjected to 10 days of TSD by the disk-over-water method. As in previous TSD studies, these rats showed greater increases in food intake and energy expenditure and greater eventual declines in waking body temperature than their yoked-control (TSC) rats. After sacrifice, α1-, α2-, and β-adrenoceptors were determined in 11 brain regions using radioligand binding assays with [3H]prazosin, [3H]rauwolscine, and125I-iodocyanopindolol, respectively. Adrenoceptor density and affinity values were significantly different among TSD, TSC, and normally caged control rat groups only for the cerebellum, which showed higher α2-binding density concomitant with lower affinity and lower β-binding density than cage control rats. Such changes are attributable to apparatus or stimulus effects common to TSD and TSC rats. Given the absence of firm evidence for a TSD-induced downregulation of central noradrenergic receptors, the present results offer no support for the hypothesis of Siegel and Rogawski that a major function of paradoxical sleep is to upregulate these receptors.
α<inf>2</inf>-agonist binding sites in brain: [<sup>125</sup>I]Para-iodoclonidine versus [<sup>3</sup>H]para-aminoclonidine
1993, Brain Research BulletinThe localization of α2-receptors was determined by quantitative autoradiography using [125I]para-iodoclonidine ([125I]PIC) and [3H]para-aminoclonidine ([3H]PAC). In cortical tissue, [125I]PIC and [3H]PAC were equipotent in their capacity to bind sites recognized by oxymetazoline (preferentially binds to the α2A receptor subtype). The iodinated ligand was about 10 times more potent than [3H]PAC for binding to the heterogenous receptor population labeled by ARC-239 (α2B and α2C). The density of [125I]PIC binding was found to be two-fold higher than that of [3H]PAC in many brain areas and the disparity was even greater in regions such as the dentate gyrus, stria terminalis, and granular layer of the cerebellum. By contrast, other regions of the brain such as the laterodorsal thalamic nucleus, the locus coeruleus, and several amygdaloid areas had equivalent levels of binding. These observations indicate that [3H]PAC has selectivity for the α2A receptor subtype and thus offer a restricted view of α2-adrenergic receptor distribution. The iodinated ligand provides a more complete picture of the overall α2 receptor population.
Recovery from lateralized neocortical damage: dissociation between amphetamine-induced asymmetry in behavior and striatal dopamine neurotransmission in vivo
1992, Brain ResearchIt has been hypothesized that neocortical damage is accompanied by secondary changes in other brain areas (the shock or diaschisis of von Monakow), which contributes to initial non-specific behavioral depression. The relation between behavioral changes and dopamine (DA), serotonin (5-HT), and their metabolites, measured with intracerebral microdialysis in freely moving rats and by tissue assay postmortem, was examined during postsurgical recovery from unilateral hemidecortications. Rats were tested for rotational asymmetry and extracellular concentration of DA was measured both during rest and after amphetamine (1.5 mg/kg). It was found that: (1) during the first few postsurgical days the hemidecorticate rats rotated ipsilateral to their lesions after amphetamine but thereafter on tests given up to 121 days postsurgery there was no asymmetry in rotation; (2) there were no asymmetries in the concentration of DA or its metabolites at any time after surgery; (3) the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) was elevated acutely for a few days following surgery; (4) during the first 3 postoperative days, both baseline extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) and amphetamine-induced DA release were significantly elevated bilaterally. These findings demonstrate that the acute behavioral asymmetry in rotation produced by hemidecortication is not related to unilateral changes in striatal DA activity and its metabolites. Thus, the behavioral asymmetries might be related to other striatal changes (i.e. 5-HIAA) or other damage, such as to the corticospinal projections of the lesioned hemisphere. Nevertheless, unilateral lesions did produce acute bilateral increases in DA levels, which may be a correlate of generalized neural shock produced by the lesion.