CYP2D6 phenotype explains reported yohimbine concentrations in four severe acute intoxications.

The indole alkaloid yohimbine is an alpha-2 receptor antagonist used for its sympathomimetic effects. Several cases of yohimbine intoxication have been reported and the most recent one involved four individuals taking a yohimbine-containing drug powder. All individuals developed severe intoxication symptoms and were admitted to the hospital. Even though all individuals were assumed to have taken the same dose of the drug powder, toxicology analyses revealed yohimbine blood concentrations of 249-5631 ng/mL, amounting to a 22-fold difference. The reason for this high variability remained to be elucidated. We used recently reported knowledge on the metabolism of yohimbine together with state-of-the art nonlinear mixed-effects modelling and simulation and show that a patient's cytochrome P450 2D6 (CYP2D6) phenotype can explain the large differences observed in the measured concentration after intake of the same yohimbine dose. Our findings can be used both for the identification of safe doses in therapeutic use of yohimbine and for an explanation of individual cases of overdosing.

PEEK tube-based online solid-phase microextraction-high-performance liquid chromatography for the determination of yohimbine in rat plasma and its application in pharmacokinetics study.

Yohimbine is a novel compound for the treatment of erectile dysfunction derived from natural products, and pharmacokinetic study is important for its further development as a new medicine. In this work, we developed a novel PEEK tube-based solid-phase microextraction (SPME)-HPLC method for analysis of yohimbine in plasma and further for pharmacokinetic study. Poly (AA-EGDMA) was synthesized inside a PEEK tube as the sorbent for microextraction of yohimbine, and parameters that could influence extraction efficiency were systematically investigated. Under optimum conditions, the PEEK tube-based SPME method exhibits excellent enrichment efficiency towards yohimbine. By using berberine as internal standard, an online SPME-HPLC method was developed for analysis of yohimbine in human plasma sample. The method has wide linear range (2-1000 ng/mL) with an R2 of 0.9962; the limit of detection was determined and was as low as 0.1 ng/mL using UV detection. Finally, a pharmacokinetic study of yohimbine was carried out by the online SPME-HPLC method and the results have been compared with those of reported methods.

Effects of three antagonists on selected pharmacodynamic effects of sublingually administered detomidine in the horse.

OBJECTIVE: To describe the effects of alpha2 -adrenergic receptor antagonists on the pharmacodynamics of sublingual (SL) detomidine in the horse. STUDY DESIGN: Randomized crossover design. ANIMALS: Nine healthy adult horses with an average age of 7.6 ± 6.5 years. METHODS: Four treatment groups were studied: 1) 0.04 mg kg(-1) detomidine SL; 2) 0.04 mg kg(-1) detomidine SL followed 1 hour later by 0.075 mg kg(-1) yohimbine intravenously (IV); 3) 0.04 mg kg(-1) detomidine SL followed 1 hour later by 4 mg kg(-1) tolazoline IV; and 4) 0.04 mg kg(-1) detomidine SL followed 1 hour later by 0.12 mg kg(-1) atipamezole IV. Each horse received all treatments with a minimum of 1 week between treatments. Blood samples were obtained and plasma analyzed for yohimbine, atipamezole and tolazoline concentrations by liquid chromatography-mass spectrometry. Behavioral effects, heart rate and rhythm, glucose, packed cell volume (PCV) and plasma proteins were monitored. RESULTS: Chin-to-ground distance increased following administration of the antagonists, however, this effect was transient, with a return to pre-reversal values as early as 1 hour. Detomidine induced bradycardia and increased incidence of atrioventricular blocks were either transiently or incompletely antagonized by all antagonists. PCV and glucose concentrations increased with tolazoline administration, and atipamezole subjectively increased urination frequency but not volume. CONCLUSIONS AND CLINICAL RELEVANCE: At the doses administered in this study, the alpha2 -adrenergic antagonistic effects of tolazoline, yohimbine and atipamezole on cardiac and behavioral effects elicited by SL administration of detomidine are transient and incomplete.

The effects of yohimbine on the pharmacokinetic parameters of detomidine in the horse.

OBJECTIVE: To describe the pharmacokinetics of detomidine and yohimbine when administered in combination. STUDY DESIGN: Randomized crossover design. ANIMALS: Nine healthy adult horses aged 9 ± 4 years and weighing of 561 ± 56 kg. METHODS: Three dose regimens were employed in the current study. 1) 0.03 mg kg(-1) detomidine IV (D), 2) 0.2 mg kg(-1) yohimbine IV (Y) and 3) 0.03 mg kg(-1) detomidine IV followed 15 minutes later by 0.2 mg kg(-1) yohimbine IV (DY). Each horse received all three dose regimens with a minimum of 1 week in between subsequent regimens. Blood samples were obtained and plasma analyzed for detomidine and yohimbine concentrations by liquid chromatography-mass spectrometry. Data were analyzed using both non-compartmental and compartmental analysis. RESULTS: The maximum measured detomidine concentrations were 76.0 and 129.9 ng mL(-1) for the D and DY treatments, respectively. Systemic clearance and volume of distribution of detomidine were not significantly different for either treatment. There was a significant increase in the maximum measured yohimbine plasma concentrations from Y (173.9 ng mL(-1)) to DY (289.8 ng mL(-1)). Both the Cl and V(d) for yohimbine were significantly less (6.8 mL minute(-1) kg(-1) (Cl) and 1.7 L kg(-1) (V(d) )) for the DY as compared to the Y treatments (13.9 mL minute(-1) kg(-1) (Cl) and 2.7 L kg(-1) (V(d))). Plasma concentrations were below the limit of quantitation (0.05 and 0.5 ng mL(-1)) by 18 hours for both detomidine and yohimbine. CONCLUSION AND CLINICAL RELEVANCE: The Cl and V(d) of yohimbine were affected by prior administration of detomidine. The elimination half life of yohimbine remained unaffected when administered subsequent to detomidine. However, the increased plasma concentrations in the presence of detomidine has the potential to cause untoward effects and therefore further studies to assess the physiologic effects of this combination of drugs are warranted.

Effect of yohimbine on detomidine induced changes in behavior, cardiac and blood parameters in the horse.

OBJECTIVE: To describe selected pharmacodynamic effects of detomidine and yohimbine when administered alone and in sequence. STUDY DESIGN: Randomized crossover design. ANIMALS: Nine healthy adult horses aged 9 ± 4 years and weighing 561 ± 56 kg. METHODS: Three dose regimens were employed in the current study. 1) 0.03 mg kg(-1) detomidine IV, 2) 0.2 mg kg(-1) yohimbine IV and 3) 0.03 mg kg(-1) detomidine IV followed 15 minutes later by 0.2 mg kg(-1) yohimbine IV. Each horse received all three treatments with a minimum of 1 week between treatments. Blood samples were obtained and plasma analyzed for detomidine and yohimbine concentrations by liquid chromatography-mass spectrometry. Behavioral effects, heart rate and rhythm, glucose, packed cell volume and plasma proteins were monitored. RESULTS: Yohimbine rapidly reversed the sedative effects of detomidine in the horse. Additionally, yohimbine effectively returned heart rate and the percent of atrio-ventricular conduction disturbances to pre-detomidine values when administered 15 minutes post-detomidine administration. Plasma glucose was significantly increased following detomidine administration. The detomidine induced hyperglycemia was effectively reduced by yohimbine administration. Effects on packed cell volume and plasma proteins were variable. CONCLUSIONS AND CLINICAL RELEVANCE: Intravenous administration of yohimbine effectively reversed detomidine induced sedation, bradycardia, atrio-ventricular heart block and hyperglycemia.

Glutaminyl cyclase contributes to the formation of focal and diffuse pyroglutamate (pGlu)-Aß deposits in hippocampus via distinct cellular mechanisms.

In the hippocampal formation of Alzheimer's disease (AD) patients, both focal and diffuse deposits of Aß peptides appear in a subregion- and layer-specific manner. Recently, pyroglutamate (pGlu or pE)-modified Aß peptides were identified as a highly pathogenic and seeding Aß peptide species. Since the pE modification is catalyzed by glutaminyl cyclase (QC) this enzyme emerged as a novel pharmacological target for AD therapy. Here, we reveal the role of QC in the formation of different types of hippocampal pE-Aß aggregates. First, we demonstrate that both, focal and diffuse pE-Aß deposits are present in defined layers of the AD hippocampus. While the focal type of pE-Aß aggregates was found to be associated with the somata of QC-expressing interneurons, the diffuse type was not. To address this discrepancy, the hippocampus of amyloid precursor protein transgenic mice was analysed. Similar to observations made in AD, focal (i.e. core-containing) pE-Aß deposits originating from QC-positive neurons and diffuse pE-Aß deposits not associated with QC were detected in Tg2576 mouse hippocampus. The hippocampal layers harbouring diffuse pE-Aß deposits receive multiple afferents from QC-rich neuronal populations of the entorhinal cortex and locus coeruleus. This might point towards a mechanism in which pE-Aß and/or QC are being released from projection neurons at hippocampal synapses. Indeed, there are a number of reports demonstrating the reduction of diffuse, but not of focal, Aß deposits in hippocampus after deafferentation experiments. Moreover, we demonstrate in neurons by live cell imaging and by enzymatic activity assays that QC is secreted in a constitutive and regulated manner. Thus, it is concluded that hippocampal pE-Aß plaques may develop through at least two different mechanisms: intracellularly at sites of somatic QC activity as well as extracellularly through seeding at terminal fields of QC expressing projection neurons.

Pharmacokinetics of yohimbine following intravenous administration to horses.

Yohimbine is an alpha 2 adrenergic receptor antagonist used most commonly in veterinary medicine to reverse the effects of the alpha 2 receptor agonists, xylazine and detomidine. Most notably, yohimbine has been shown to counteract the CNS depressant effects of alpha 2 receptor agonists in a number of species. The recent identification of a yohimbine positive urine sample collected from a horse racing in California has led to the investigation of the pharmacokinetics of this compound. Eight healthy adult horses received a single intravenous dose of 0.12 mg/kg yohimbine. Blood samples were collected at time 0 (prior to drug administration) and at various times up to 72 h post drug administration. Plasma samples were analyzed using liquid chromatography-mass spectrometry (LC-MS) and data analyzed using both noncompartmental and compartmental analysis. Peak plasma concentration was 114.5 + 31.8 ng/mL and occurred at 0.09 + 0.03 h. Mean ± SD systemic clearance (Cls) and steady-state volume of distribution (Vdss) were 13.5 + 2.1 mL/min/kg and 3.3 + 1.3 L/kg following noncompartmental analysis. For compartmental analysis, plasma yohimbine vs. time data were best fitted to a two compartment model. Mean ± SD Cls and Vdss of yohimbine were 13.6 ± 2.0 mL/min/kg and 3.2 ± 1.1 L/kg, respectively. Mean ± SD terminal elimination half-life was 4.4 ± 0.9 h following noncompartmental analysis. Immediately following administration, two horses showed signs of sedation, while the other six appeared behaviorally unaffected. Gastrointestinal sounds were moderately increased compared to baseline while fecal consistency appeared normal.

Pharmacokinetics and pharmacodynamics of three intravenous doses of yohimbine in the horse.

Yohimbine is an alpha 2 adrenergic receptor antagonist, which has been shown to counteract the CNS depressant effects of alpha 2 receptor agonists in a number of species. Recently, our laboratory identified yohimbine in the absence of detectable concentrations of an alpha 2 agonist in a regulatory sample collected from a horse racing in California. This coupled with anecdotal reports of CNS stimulation and documented reports of cardiovascular changes when administered in conjunction with an agonist led us to investigate the pharmacokinetics and pharmacodynamics of yohimbine when administered alone. Nine healthy adult horses received a single intravenous dose of 0.1, 0.2, and 0.4 mg/kg yohimbine. Blood samples were collected at time 0 (prior to drug administration) and at various times up to 24 h postdrug administration. Plasma samples were analyzed using liquid chromatography-mass spectrometry (LC-MS), and resulting data analyzed using both noncompartmental and compartmental analysis. Peak plasma concentrations were 106.0 ± 28.9, 156.7 ± 34.3, and 223.0 ± 44.5 ng/mL for doses of 0.1, 0.2, and 0.4 mg/kg, respectively. Immediately following administration, two horses showed signs of sedation, one horse appeared excited, while the other six appeared behaviorally unaffected. Episodes of tachycardia were noted within minutes of administration for all horses at all doses; however, there was no correlation between behavioral responses and episodes of increased heart rate. Sixty-three percent of the horses (8, 6, and 4 of the 9 horses in the 0.1, 0.2, and 0.4 mg/kg dose groups, respectively) exhibited second-degree atrial-ventricular conduction blocks and bradycardia prior to drug administration that transiently improved or disappeared upon administration of yohimbine. Gastrointestinal sounds were transiently increased following all doses.

Oral Yohimbine as a New Probe Drug to Predict CYP2D6 Activity: Results of a Fixed-Sequence Phase I Trial.

OBJECTIVE: Yohimbine pharmacokinetics were determined after oral administration of a single oral dose of yohimbine 5 mg and a microdose of yohimbine 50 µg in relation to different cytochrome P450 (CYP) 2D6 genotypes. The CYP2D6 inhibitor paroxetine was used to investigate the influence on yohimbine pharmacokinetics. Microdosed midazolam was applied to evaluate a possible impact of yohimbine on CYP3A activity and the possibility of combining microdosed yohimbine and midazolam to simultaneously determine CYP2D6 and CYP3A activity. METHODS: In a fixed-sequence clinical trial, 16 healthy volunteers with a known CYP2D6 genotype [extensive (10), intermediate (2) and poor (4) metaboliser] received an oral dose of yohimbine 50 µg, yohimbine 5 mg at baseline and during paroxetine as a CYP2D6 inhibitor. Midazolam (30 µg) was co-administered to determine CYP3A activity at each occasion. Plasma concentrations of yohimbine, its main metabolite 11-OH-yohimbine, midazolam and paroxetine were quantified using validated liquid chromatography-tandem mass spectrometry assays. RESULTS: Pharmacokinetics of yohimbine were highly variable and a CYP2D6 genotype dependent clearance was observed. After yohimbine 5 mg, the clearance ranged from 25.3 to 15,864 mL/min and after yohimbine 50 µg, the clearance ranged from 39.6 to 38,822 mL/min. A more than fivefold reduction in clearance was caused by paroxetine in CYP2D6 extensive metabolisers, while the clearance in poor metabolisers was not affected. Yohimbine did not alter CYP3A activity as measured by microdosed midazolam. CONCLUSIONS: The pharmacokinetics of yohimbine were highly correlated with CYP2D6, which was further supported by the clearance inhibition caused by the CYP2D6 inhibitor paroxetine. With these data, yohimbine is proposed to be a suitable probe drug to predict CYP2D6 activity. In addition, the microdose can be used in combination with microdosed midazolam to simultaneously evaluate CYP2D6 and CYP3A activity without any interaction between the probe drugs and because the microdoses exert no pharmacological effects. CLINICAL TRIAL REGISTRATION: EudraCT2017-001801-34.

Detection of alpha2-adrenergic receptors in brain of living pig with 11C-yohimbine.

UNLABELLED: There have been few radiotracers for imaging adrenergic receptors in brain by PET, but none has advanced for use in human studies. We developed a radiosynthesis for the alpha(2)-adrenergic antagonist (11)C-yohimbine and characterized its binding in living pigs. As a prelude to human studies with (11)C-yohimbine, we determined the whole-body distribution of (11)C-yohimbine and calculated its dosimetry. METHODS: Yorkshire x Landrace pigs weighing 35-40 kg were used in the study. Baseline and postchallenge PET recordings of (11)C-yohimbine in pig brain were conducted for 90 min, concurrent with arterial blood sampling, and with yohimbine and RX821002 as pharmacologic interventions. (15)O-Water scans were performed to detect changes in cerebral perfusion. The PET images were manually coregistered to an MR atlas of the pig brain. Maps of the (11)C-yohimbine distribution volume ([V(d)] mL g(-1)) in brain were calculated relative to the arterial input function. RESULTS: Whole-body scans with (11)C-yohimbine revealed high accumulation of radioactivity in kidney, intestine, liver, and bone. The estimated human dose was 5.6 mSv/GBq, a level commonly accepted in human PET studies. Brain imaging showed baseline values of V(d) ranging from 1.9 in medulla, 3.0 in cerebellum, and to 4.0 in frontal cortex. Coinjection with nonradioactive yohimbine (0.07 mg/kg) reduced V(d) globally to approximately 1.5-2 mL g(-1). A higher yohimbine dose (1.6 mg/kg) was without further effect on self-displacement. Very similar results were obtained by displacement with the more selective alpha(2)-adrenergic antagonist RX821002 at doses of 0.15 and 0.7 mg/kg. Cerebral blood flow was globally increased 43% after administration of RX821002. Notable features of (11)C-yohimbine are a lack of plasma metabolism over 90 min and a rapid approach to equilibrium binding in brain. CONCLUSION: The new radiotracer (11)C-yohimbine seems well suited for PET investigations of alpha(2)-adrenergic receptors in brain and peripheral structures, with the caveat that displaceable binding was present in cerebellum and throughout the brain.

Dopamine activates noradrenergic receptors in the preoptic area.

Dopamine (DA) facilitates male sexual behavior and modulates aromatase activity in the quail preoptic area (POA). Aromatase neurons in the POA receive dopaminergic inputs, but the anatomical substrate that mediates the behavioral and endocrine effects of DA is poorly understood. Intracellular recordings showed that 100 microm DA hyperpolarizes most neurons in the medial preoptic nucleus (80%) by a direct effect, but depolarizes a few others (10%). DA-induced hyperpolarizations were not blocked by D1 or D2 antagonists (SCH-23390 and sulpiride). Extracellular recordings confirmed that DA inhibits the firing of most cells (52%) but excites a few others (24%). These effects also were not affected by DA antagonists (SCH-23390 and sulpiride) but were blocked by alpha2-(yohimbine) and alpha1-(prazosin) noradrenergic receptor antagonists, respectively. Two dopamine-beta-hydroxylase (DBH) inhibitors (cysteine and fusaric acid) did not block the DA-induced effects, indicating that DA is not converted into norepinephrine (NE) to produce its effects. The pK(B) of yohimbine for the receptor involved in the DA- and NE-induced inhibitions was similar, indicating that the two monoamines interact with the same receptor. Together, these results demonstrate that the effects of DA in the POA are mediated mostly by the activation of alpha2 (inhibition) and alpha1 (excitation) adrenoreceptors. This may explain why DA affects the expression of male sexual behavior through its action in the POA, which contains high densities of alpha2-noradrenergic but limited amounts of DA receptors. This study thus clearly demonstrates the existence of a cross talk within CNS catecholaminergic systems between a neurotransmitter and heterologous receptors.

Yohimbine: a clinical review.

Although yohimbine (YOH) has been available for the treatment of male erectile dysfunction (ED) for longer than Viagra, there is a perception that little is known about the clinical performance of the drug. This review attempts, by comprehensive analysis of the literature, to cover the clinical, pharmacological, and therapeutic profiles of YOH, relevant to its potential utility in the management of patients with ED. Relatively few well-designed studies have been completed. From these, however, it can be concluded that YOH as monotherapy possesses only modest efficacy in ED patients. In acute and chronic (long-term) studies, YOH has been found to be relatively free of side effects over the dose range predicted to be effective in ED. At much higher doses, the most frequently observed effects, consistent with the primary pharmacological action of the drug, are elevation of blood pressure, a slight anxiogenic action, and increased frequency of urination. These side effects are all easily reversible on termination of YOH therapy. There is increasing evidence that the erectogenic action of YOH can be augmented by concomitant administration of agents that augment the release and/or action of nitric oxide in the corpus cavernosum. YOH has yet to be studied in female sexual dysfunction. Overall, the benefit risk profile of YOH would indicate that it has potential, more probably as part of a combination strategy, e.g., with a drug that enhances the nitric oxide pathway, in the treatment of ED.

Decreased in vivo α2 adrenoceptor binding in the Flinders Sensitive Line rat model of depression.

Depression is a debilitating heterogeneous disorder and the underlying mechanisms remain elusive. Alterations in monoaminergic neurotransmission, including noradrenergic, have been implicated in the etiology of depression. Although depression is difficult to model in animals, the availability of animal models with face, predictive and construct validity permits more in-depth investigations resulting in a greater understanding of the disease. We investigated the role of noradrenaline (NA) and α2 adrenoceptors in vivo in a genetic model of depression, the Flinders Sensitive Line (FSL) rat. We determined baseline differences in NA receptor volume of distribution to α2 adrenoceptors in FSL, in comparison with two routinely used controls, Flinders Resistant Line (FRL) and Sprague-Dawley (SD) rats using positron emission tomography (PET) imaging and the carbon-11 labeled radioligand yohimbine. We demonstrate a 42-47% reduction in the binding of the tracer in the cortex, striatum, cerebellum, thalamus and pons of FSL rats compared to the two control groups. Our results suggest that the behavioral deficits expressed in the FSL depression model are associated with functional over-activity of the NA system.

A Synthetic Heterobivalent Ligand Composed of Glucagon-Like Peptide 1 and Yohimbine Specifically Targets ß Cells Within the Pancreas.

PURPOSE: ß Cell specificity for a heterobivalent ligand composed of glucagon-like peptide-1 (GLP-1) linked to yohimbine (GLP-1/Yhb) was evaluated to determine its utility as a noninvasive imaging agent. PROCEDURES: Competition binding assays were performed on ßTC3 cells and isolated rat islets. Immunostaining for insulin was used to co-localized intravenously injected Cy5-labeled GLP-1/Yhb in ß cells of Sprague-Dawley rats. Rats were intravenously injected with In-111-labeled GLP-1/Yhb to determine clearance rates and tissue biodistribution. Tissue-specific binding was confirmed by competition with pre-administration of unlabeled GLP-1/Yhb and in Streptozotocin-induced diabetic rats. RESULTS: In ßTC3 cells, high affinity binding of GLP-1/Yhb required interactions with both receptors because monovalent competition or receptor knockdown with RNAi lowered specificity and avidity of the heterobivalent ligand. Binding specificity for isolated islets was 2.6-fold greater than that of acinar tissue or islets pre-incubated with excess unlabeled GLP-1/Yhb. Immunofluorescent localization of Cy5-labeled GLP-1/Yhb was restricted to pancreatic islets. Within 30 min, ~90% of the In-111-labeled GLP-1/Yhb was cleared from blood. Tissue-specific accumulation of radiolabeled ligand was apparent in the pancreas, but not in other tissues within the abdominal imaging field. Pancreas specificity was lost in Streptozotocin-induced diabetic rats. CONCLUSIONS: The GLP-1/Yhb exhibits high specificity for ß cells, rapid blood clearance rates, and low non-specific uptake by other tissues within the abdominal imaging field. These characteristics of GLP-1/Yhb are desirable for application to ß cell imaging in vivo and provide a basis for developing additional multivalent ß cell-specific targeting agents to aid in the management of type 1 diabetes.

Alpha1- and alpha2-adrenoceptor control of sodium transport reverses in developing hypertension.

Alpha-Adrenergic receptor (AR) activation enhances sodium retention in certain forms of hypertension. The objective of the present study was to understand the role of alpha-ARs in regulating sodium transport by distal tubules (DT). DT cells were isolated from kidneys of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 6 weeks, when hypertension is developing, or at 12 weeks, when hypertension is established. The alpha1-AR agonist phenylephrine increased 22Na uptake by 50% into DT cells of 6-week SHR; no effect was observed with WKY cells. The alpha2-AR agonist B-HT 933 increased uptake by only 10%. At 12 weeks, the pattern of alpha-AR regulation was reversed: alpha1-AR-induced sodium uptake was only 15%, whereas alpha2-AR activation increased sodium uptake by 35% in SHR and WKY cells. alpha1-AR-induced sodium uptake in 6-week SHR cells was abolished by prazosin; alpha2-AR-stimulated sodium uptake was blocked by yohimbine in 12-week SHR and WKY. Competitive binding studies were performed with [3H]prazosin and alpha1A-, alpha1B-, and alpha1D-selective antagonists with DT cell membranes from 6- and 12-week SHR and WKY. alpha2-AR subtypes were determined with [3H]rauwolscine and alpha2A- and alpha2B-selective antagonists. Expression of alpha1B-ARs was increased 4-fold in DT cells during the developing phase of hypertension in SHR. No change was detected in alpha2-AR expression. DT cells transiently increase [Ca2+]i in response to alpha1-AR agonists from 6-week but not 12-week SHR. Conversely, alpha2-AR agonists increase [Ca2+]i at 12 weeks. In summary, during developing hypertension, alpha1-ARs increase sodium uptake and [Ca2+]i in SHR cells. Expression of alpha1B-ARs is selectively upregulated during developing hypertension. In established hypertension (and normotension), alpha2-ARs regulate sodium transport and [Ca2+]i in DT cells. We conclude that a molecular switch of alpha1-AR and alpha2-AR signaling occurs in DT cells during the development of hypertension.

Platelet alpha 2 adrenoceptors in human and canine narcolepsy.

We have recently established that canine narcolepsy (an autosomal recessive genetic model of the human disorder) is dramatically improved by treatment with alpha 2 antagonists such as yohimbine (Nishino et al: J Pharmacol Exp Ther 253:1145-1152, 1990). To further investigate the role of alpha 2 adrenoceptors in narcolepsy, receptors labeled with [3H] yohimbine were examined on platelets from human and canine narcoleptic subjects. Twenty-eight Doberman pinschers were studied, 7 controls (C), 7 heterozygous (Hz), and 14 narcoleptics (N) (age and sex matched), including eight animals born in a backcross setting (narcoleptic x heterozygous; 5 narcoleptics and 3 heterozygous). The Kd and Bmax of each group respectively, were as follows: C, Kd = 2.86 +/- 0.76 nmol/L, Bmax = 295.78 +/- 31.89 fmol/mg protein; Hz, Kd = 2.06 +/- 0.23 nmol/L, Bmax = 307.02 +/- 22.21 fmol/mg protein; and N, Kd = 2.72 +/- 0.45 nmol/L, Bmax = 267.52 +/- 19.47 fmol/mg protein. No statistical differences were found between groups using nonparametric (Kruskall-Wallis) statistical procedures, and there were no correlations between any binding parameter and symptom severity within the narcoleptic group. Platelet alpha 2 receptor affinity and density also did not differ between narcoleptic and heterozygous dogs in the backcross litter (N [n = 5], Kd = 1.94 +/- 0.59 nmol/L, Bmax = 290.6 +/- 64.7 fmol/mg protein; Hz [n = 3], Kd = 2.83 +/- 0.47 nmol/L, Bmax = 294.2 +/- 42.9 fmol/mg protein). Fourteen human subjects, seven control and seven narcoleptic patients (age and sex matched), were included in the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic function in generalized anxiety disorder, major depressive disorder, and healthy subjects.

Plasma norepinephrine (NE), free 3-methoxy-4-hydroxyphenethylene glycol (MHPG), and binding of tritiated yohimbine to platelet membranes were measured in 14 patients with generalized anxiety disorder (GAD), who were matched for age and sex with 14 patients with unipolar major depressive disorder (MDD) and 14 normal subjects. Plasma NE and MHPG levels were increased and the number of alpha2-adrenoreceptors (Bmax) was decreased in GAD patients compared with MDD and normal subjects. No differences were found between MDD patients and normal subjects for plasma NE, MHPG, and alpha2-adrenoreceptor binding. Plasma NE and MHPG were significantly correlated in MDD patients and tended toward a significant positive correlation in GAD patients. Plasma MHPG and affinity of binding platelet alpha2-adrenoreceptors (Kd) were significantly correlated in normal subjects. Thus, noradrenergic activity seems to be increased in patients with GAD, but not in patients with MDD. In GAD patients, higher levels of catecholamines may lead to a down-regulation of presynaptic alpha2-adrenoreceptors.

Adrenergic status in anxiety disorders: platelet alpha 2-adrenergic receptor binding, blood pressure, pulse, and plasma catecholamines in panic and generalized anxiety disorder 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.

Alpha 2-adrenergic receptor sensitivity in depressed patients: relation between 3H-yohimbine binding to platelet membranes and clonidine-induced hypotension.

Alpha 2-adrenergic receptor changes during antidepressant treatment were studied using 3H-yohimbine binding to human platelet membranes and clonidine-induced hypotension. Twenty-six patients with major depressive disorder (MDD) participated for 4-6 weeks in a trial of imipramine (2.5 mg/kg/day), tyrosine (100 mg/kg/day), or placebo. Alpha 2-adrenergic receptors measured by 3H-yohimbine binding were not significantly changed following antidepressant treatment. Similarly, clonidine-induced hypotension did not differ significantly following treatment. No measure of alpha 2-adrenergic receptor sensitivity was significantly correlated with clinical improvement. The correlation between platelet receptor binding and clonidine-induced hypotension was not statistically significant, even though both tests are considered to be measures of alpha 2-adrenoceptor sensitivity.

Decreased platelet alpha-2 adrenoceptor density in major depression: effects of tricyclic antidepressants and fluoxetine.

BACKGROUND: It has been suggested that major depression is accompanied by a subsensitivity of central alpha 2-adrenoceptors (alpha 2-ARs) and, consequently, by an impaired negative feedback on the presynaptic catecholaminergic neuron, which, in turn, may induce a disinhibition of noradrenergic output and norepinephrine release in response to any activation. METHODS: The maximum number of platelet binding sites (Bmax) and their affinity for [3H]-rauwolscine, a selective alpha 2-AR antagonist, were measured in unmedicated and medicated major depressed patients and in normal volunteers. Specific binding was defined as that inhibited by idazoxan, another alpha 2-AR antagonist. RESULTS: Unmedicated major depressed patients had significantly decreased platelet [3H]-rauwolscine binding Bmax values compared to normal volunteers. [3H]-rauwolscine binding Kd values did not differ significantly between unmedicated major depressed patients and normal controls. [3H]-rauwolscine binding Kd values were significantly higher in depressed patients treated with tricyclic antidepressants than in unmedicated patients. Subchronic treatment with fluoxetine did not significantly alter either [3H]-rauwolscine binding Bmax or Kd values. [3H]-rauwolscine binding Bmax values were significantly greater in men than in women. CONCLUSIONS: The results suggest that i) major depression is accompanied by decreased platelet alpha 2-AR density; and that ii) subchronic treatment with tricyclic antidepressants, but not fluoxetine, results in a decreased affinity of rauwolscine for platelet alpha 2-ARs.