Peggie J. Hollingsworth

α2-Adrenoreceptors in rat brain are decreased after long-term tricyclic antidepressant drug treatment

After two weeks of twice-daily administration of amitriptyline to rats, the binding of [3H]clonidine to presynaptic alpha 2-adrenoreceptors was decreased in membranes isolated from 5 areas of the rat brain. After one day of treatment, binding did not differ from saline treated controls. In vitro, a high concentration of amitriptyline caused a competitive inhibition of [3H]clonidine binding but did not alter the number of binding sites. The decrease in the number of alpha 2-adrenoreceptor binding sites after two weeks of amitriptyline treatment would explain the subsensitivity of these receptors which occurs after prolonged administration of antidepressant drugs.

α2-adrenoreceptors on human platelets: Selective labelling by [3H]clonidine and [3H]yohimbine and competetive inhibition by antidepressant drugs☆

[3H]Clonidine and [3H]yohimbine have been used to characterize alpha 2-adrenoreceptors on human platelets. At 25 degrees C binding was rapid (t 1/2 of association, 1.8 and 2.7 min) and reversible (t 1/2 of dissociation, 0.5 and 8.2 min). The binding sites for [3H]clonidine and ]3H]yohimbine showed the specificity required for an alph 2-adrenoreceptor. The rank order of potency of inhibitors of [3h[clonidine binding was clonidine greater than yohimbine greater than phenylephrine greater than prazosin and of [3H]yohimbine binding was yohimbine greater than clonidine greater than phenylephrine greater than prazosin. Scatchard analysis of [3H]yohimbine binding indicated the existence of a single population of noninteracting sites (KD = 3.0 nM; Bmax = 188 fmol/mg protein). The high-affinity binding of [3H-clonidine had a lower affinity and a lower number of sites (KD = 5.0 nM; Bmax = 35 fmol/mg protein). [3H]Clonidine binding also showed evidence of a second site of much lower affinity and greater number (KD = 18.6 nM; Bmax = 77 fmol/mg protein) in 40% of the normal population. In vitro, antidepressant drugs competed with [3H]clonidine and [3H]yohimbine for the platelet alpha 2-adrenoreceptor. The rank order of potency of inhibitors of [3H]clonidine binding was mianserin greater than amitriptyline greater than iprindole greater than desipramine and of [3H]yohimbine binding was mianserin greater than amitriptyline greater than desipramine greater than iprindole. The inhibition constants (Ki) of adrenergic drugs and of various antidepressant drugs in competing with [3H]clonidine were correlated with the inhibition constants of these drugs in competing with [3H]yohimbine (r = 0.970; P less than 0.001) which suggests that both radioligands labelled the same alpha 2-adrenoreceptor on the human platelet. The inhibition of binding induced by all antidepressant drugs was competitive. In contrast, long-term administration of tricyclic antidepressant drugs to patients was recently found to be associated with a decrease in the number of binding sites for [3H]clonidine on platelet membranes. The present results indicate that both [3H]clonidine and [3H]yohimbine are useful tools for the quantification of alpha 2-adrenoreceptors on blood platelets and suggests that the specific binding of radiolabelled alpha 2-adrenoreceptor ligands to human platelet membranes might be used to monitor changes in alpha 2-adrenoreceptors during tricyclic antidepressant drug treatment.

Adrenergic status in anxiety disorders: Platelet alpha2-adrenergic receptor binding, blood pressure, pulse, and plasma catecholamines in panic and generalized anxiety disorders patients and in normal subjects

In order to evaluate adrenergic function in anxiety disorders, platelet alpha 2-adrenergic binding parameters and supine and standing blood pressure, pulse, and venous plasma epinephrine and norepinephrine were determined in patients with panic attacks or generalized anxiety disorder and in normal subjects. The maximum number of binding sites (Bmax) for the partial agonist tritiated clonidine was significantly lower for both patient groups than for normal subjects, and the Bmax for the antagonist tritiated yohimbine was significantly lower for panic patients. There were no other substantive differences across groups. Prior exposure to psychotropic drugs might account for the results for clonidine binding, but not for yohimbine. The Bmax for clonidine was correlated with norepinephrine increases upon standing and, for panic patients, with the severity of full unexpected panic attacks. These data provide further evidence of adrenergic receptor abnormalities in people with anxiety disorders.

Platelet alpha2 adrenoreceptors are decreased in number after antidepressant therapy

Specific binding of 3H-clonidine to alpha 2 adrenoreceptors upon human blood platelet membranes is increased in patients with major depressive disorder (endogenous depression). Specific binding of 3H-yohimbine to the platelet adrenoreceptor is not altered in endogenously depressed patients. Other psychiatric disorders are not associated with alterations in the specific binding of either 3H-clonidine or 3H-yohimbine. In patients with severe congestive heart failure or with symptomatic coronary artery disease the number of platelet alpha 2 adrenoreceptors is actually decreased. Treatment of endogenously depressed patients with tricyclic antidepressants, lithium salts or electroconvulsive therapy results in a decrease in the number of alpha 2 adrenoreceptors on blood platelet membranes. These studies suggest that a supersensitivity of the alpha 2 adrenoreceptor might exist in patients with endogenous depression and that effective forms of therapy lead to a decrease in the number of neural alpha 2 adrenoreceptors which is reflected by a decrease in the number of these receptors upon blood platelet membranes.

Changes in α-adrenoceptor number and function in brains of morphine-dependent rats

The effects of long-term treatment of rats with morphine sulfate were assessed upon the specific binding of [3H]clonidine to α2-adrenoceptors on neural membranes isolated from various brain areas and upon the function of presynaptic α2-adrenoceptors during field stimulation of hippocampal slices. Rats were injected with morphine every 8 h for 14 days with doses which started at 10 mg/kg per injection i.p., and which increased every 3 days to a final dose of 100 mg/kg per injection on the last 2 days. At 8 and 32 h after the last injection the Bmax for [3H]clonidine binding to neural membranes from various brain areas was significantly decreased. At the same times, the fractional release of [3H]noradrenaline during field stimulation of hippocampal slices was increased and the sensitivity of the hippocampal slice to clonidine was reduced which indicated the development of a functional subsensitivity of the presynaptic α2-adrenoceptor. These changes in receptor function persisted at 72 h after the last morphine injection although at this time there were marked increases over control values in [3H]clonidine binding to membranes from all rat brain areas except the caudate nucleus. These findings suggest that changes in α2-adrenoceptor number and function which develop during long-term morphine administration might play an important role in opiate dependence.

Intravenous Yohimbine Selective Enhancer of Norepinephrine and Cortisol Secretion and Systolic Blood Pressure in Humans

Yohimbine hydrochloride was administered intravenously to nine normal volunteers to assess alpha 2-adrenergic receptor function. Plasma catecholamines, cortisol, and psychophysiological parameters (blood pressure, pulse, somatic symptoms checklist, and visual analogue scales assessing mood) were used as dependent variables. Plasma norepinephrine and plasma cortisol increased significantly after yohimbine administration, while epinephrine remained unchanged. Systolic blood pressure and somatic symptoms were also significantly increased by yohimbine. These findings suggest that low-dose intravenous yohimbine is an effective probe for alpha 2-adrenergic receptors and the hypothalamic-pituitary-adrenal axis.

PLATELET ALPHA 2-ADRENORECEPTORS, CATECHOLAMINES, HEMODYNAMIC VARIABLES, AND ANXIETY IN PANIC PATIENTS AND THEIR ASYMPTOMATIC RELATIVES

The objectives of this study were to a) replicate our prior finding of a decreased number (Bmax) of platelet alpha2-adrenoreceptors in panic disorder, b) determine if binding is also decreased in asymptomatic first-degree relatives of panic patients (known to be at increased risk for developing panic), and c) evaluate the effect of treatment on the presumptive decrease in binding (ie, is the decrease a state or a trait marker for panic?). Panic patients had clonidine and yohimbine platelet-binding assays, symptom ratings, and measurement of lying and standing plasma epinephrine, norepinephrine, systolic and diastolic blood pressure, and heart rate before treatment, after approximately 2 months of medication (fluoxetine, tricyclics, or alprazolam) and/or cognitive behavioral treatment, and after symptom remission while drug free; normal subjects had determinations of the same measures at approximately the same time intervals. Relatives of both groups had one determination only of all measures. Tritiated clonidine binding was decreased and lying heart rate was increased in patients before treatment. Magnitude of binding decrease was correlated with symptom severity and standing norepinephrine. No binding abnormality was seen in first-degree relatives of patients. Treatment increased clonidine binding in patients. Both patients and relatives of patients showed significantly increased standing plasma norepinephrine in comparison to controls. There is a state-related decrease in binding, associated with symptom severity and norepinephrine, in panic disorder. Abnormal reactivity of norepinephrine to standing might be a marker for increased likelihood of panic development in individuals at risk.

Comorbidity of panic disorder and major depressive disorder: effects on platelet alpha2 adrenergic receptors

Adrenergic receptor dysregulation has been described as occurring in both major depressive disorder (MDD) and panic disorder. Measurements of platelet alpha2 adrenergic receptors in these patients may be confounded by the coexistence or comorbidity of both diagnoses in the same patient. To explore this possibility, we measured platelet alpha2 adrenergic receptors (3H‐clonidine and 3H‐yohimbine binding) in 3 groups of patients (MDD only, panic disorder only, and those showing comorbidity of MDD and panic) and normal controls. Patients with comorbidity of MDD and panic disorder had significantly lower agonist binding (3H‐clonidine).

Chronic treatment with cholinesterase inhibitors increases α2-adrenoceptors in rat brain

The specific binding of [3H]clonidine to a2-adrenoceptors on neural membranes isolated from various brain areas was determined with rats treated for 7-14 days with the cholinesterase inhihitors neostigmine, triorthocresyl phosphate (TOCP), diisopropylfluorophosphate (DFP) and paraoxon, or with vehicle. Treatment with all four inhibitors increased the number of clonidine binding sites in various brain areas. In those areas which demonstrated significant increases in [3H]clonidine binding, there was also a significant inhibition of acetylcholinesterase activity. The possibility is discussed that increases in brain a2-adrenoceptors are related to the alterations in mood seen in individuals chronically exposed to organophosphorus cholinesterase inhibitors. a2-Adrenoceptors; rase inhibition (chronic); [3H]Clonidine binding; Brain; (Rat)

Effects of muscarinic receptor agonists and antagonists on α2-adrenoceptors in rat brain

Abstract The specific binding of [ 3 H]clonidine to α 2 -adrenoceptors on neural membranes isolated from six brain areas was determined with rats treated for various periods of time with the muscarinic agonists, oxotremorine or pilocarpine, or with the muscarinic antagonists atropine, atropine methyl nitrate, scopolamine and scopolamine methyl bromide. Administration of pilocarpine, 10 mg/kg, twice daily i.p. for 1 and 14 days increased markedly the number of α 2 -adrenoceptors on neural membranes from all six brain areas. In contrast, oxotremorine, 0.3 mg/kg, twice daily i.p., for 7 days decreased the number of α 2 -adrenoceptors on membranes from all brain areas except the brainstem and caudate nucleus. Both atropine and scopolamine increased the density of α 2 -adrenoceptors in specific brain areas. Neither atropine methyl nitrate nor scopolamine methyl bromide had an appreciable effect upon the specific binding of [ 3 H]clonidine to neural membranes from most brain areas.