Adrenergic receptors in frontal cortex in human brain

Distribution of the Noradrenaline Innervation and Adrenoceptors in the Macaque Monkey Thalamus

Noradrenaline (NA) in the thalamus has important roles in physiological, pharmacological, and pathological neuromodulation. In this work, a complete characterization of NA axons and Alpha adrenoceptors distributions is provided.

NA axons, revealed by immunohistochemistry against the synthesizing enzyme and the NA transporter, are present in all thalamic nuclei. The most densely innervated ones are the midline nuclei, intralaminar nuclei (paracentral and parafascicular), and the medial sector of the mediodorsal nucleus (MDm). The ventral motor nuclei and most somatosensory relay nuclei receive a moderate NA innervation. The pulvinar complex receives a heterogeneous innervation. The lateral geniculate nucleus (GL) has the lowest NA innervation.

Alpha adrenoceptors were analyzed by in vitro quantitative autoradiography. Alpha-1 receptor densities are higher than Alpha-2 densities. Overall, axonal densities and Alpha adrenoceptor densities coincide; although some mismatches were identified. The nuclei with the highest Alpha-1 values are MDm, the parvocellular part of the ventral posterior medial nucleus, medial pulvinar, and midline nuclei. The nucleus with the lowest Alpha-1 receptor density is GL. Alpha-2 receptor densities are highest in the lateral dorsal, centromedian, medial and inferior pulvinar, and midline nuclei.

These results suggest a role for NA in modulating thalamic involvement in consciousness, limbic, cognitive, and executive functions.

Noradrenergic blockade and numeric working memory in humans

To investigate the noradrenergic modulation of working memory in humans single doses of two beta-blockers [either 25 mg of propranolol (lipophilic) or 50 mg of atenolol (hydrophilic)] or placebo were administered to young healthy volunteers (16 subjects per drug condition) performing a numerical working memory task that requires either short-term maintenance or maintenance plus manipulation of visually presented four-number sequences. Higher manipulation costs (i.e. process-specific slowing of reaction times in the manipulation conditions compared to the control condition) were observed after propranolol but not after atenolol. The propranolol effect was mainly observed in subjects with low emotional arousal (i.e. low state anxiety rating at baseline). Because both beta-blockers induced a comparable decrease of blood pressure and pulse, the propranolol effect on the 'working component' of working memory is considered to be a central, presumably prefrontal one.

Characterization of two novel sigma receptor ligands: antidystonic effects in rats suggest sigma receptor antagonism

The novel sigma receptor ligands, N(-)[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD1047) and 1(-)[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine (BD1063), were characterized in rats using binding assays and behavioral studies. In radioligand binding studies, the novel ligands showed marked selectivity for sigma binding sites, generally having a 100-fold or better affinity for sigma sites compared to nine other tested receptors (opiate, phencyclidine, muscarinic, dopamine, alpha 1-, alpha 2-, beta-adrenoceptor, 5-HT1, 5-HT2); the only exception was the affinity of BD1047 for beta-adrenoceptors. Competition assays further revealed that the drugs interacted with both sigma 1 and sigma 2 binding sites. Although both drugs had preferential affinities for sigma 1 sites, BD1047 exhibited a higher affinity for sigma 2 sites than BD1063. In behavioral studies, BD1047 and BD1063 had no effects on their own when unilaterally microinjected into the red nucleus of rats, but both compounds attenuated the dystonia produced by the high affinity sigma ligands, di-o-tolylguanidine (DTG) and haloperidol. BD1047 and BD1063 dose-dependently attenuated the dystonia produced by DTG, suggesting a receptor-mediated mechanism, and the dose curve for DTG was shifted to the right in the presence of the novel ligands. BD1047 and BD1063 appear to act as antagonists at sigma sites and may represent promising new tools for probing other functional effects associated with sigma binding sites.

Human brain imidazoline receptors: further characterization with [3H]clonidine

The aim of the present study was to further characterize [3H]clonidine binding in the ventrolateral medulla of the human brainstem, the region involved in the vasodepressor effect of imidazoline drugs of the clonidine type. Under basal conditions, [3H]clonidine can bind both to the imidazoline receptors and to the alpha-adrenoceptors. The latter represent only a small part of the total [3H]clonidine binding with a Bmax of 61 +/- 13 fmol/mg proteins and a KD of 4.9 +/- 2.2 nM. Most of the binding was associated with imidazoline receptors with a KD of 67 +/- 13 nM and a Bmax of 677 +/- 136 fmol/mg protein. alpha-Adrenoceptor binding of [3H]clonidine could be completely prevented when membranes were either treated during preparation with the aIkylating agent phenoxybenzamine or incubated in the presence of 30 microM (-)-noradrenaline or in the presence of the non-hydrolysable analogue of GTP, guanylyl imidodiphosphate (Gpp(NH)p). When the alpha-adrenoceptors binding was prevented, we demonstrated the insensitivity of [3H]clonidine binding to Gpp(NH)p and showed that the competition between clonidine and idazoxan for imidazoline receptors was insensitive to Gpp(NH)p suggesting that imidazoline receptors are not G protein coupled receptors. The specificity of [3H]cloniding binding to imidazoline receptors in the human ventrolateral medulla indicates that these receptors are different from imidazole receptors as defined with p-aminoclonidine in the bovine brainstem.

Quantitative autoradiography of alpha 1- and alpha 2-adrenergic receptors in the cerebral cortex of controls and suicide victims

Alterations 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 alpha 1- and alpha 2-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 alpha 1-adrenergic receptors in the temporal cortex (TC) of suicide victims and controls matched for postmortem delay, age, side of brain and sex. We found that: (1) the laminar patterns of alpha 1-adrenergic receptors in the PFC (n = 20) and the TC (n = 16) were different (P = 0.022); (2) there was a 37% increase in alpha 1-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 alpha 1-adrenergic binding sites than the PFC across all cortical layers (P = 0.006); (4) alpha 2-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 alpha 2-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 alpha 1- or alpha 2-adrenergic sites. The increase in [3H]prazosin (to alpha 1-adrenergic receptors) but not in [3H]-p-aminoclonidine (to alpha 2-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.

Quantitative light microscopic autoradiographic localization of alpha 2-adrenoceptors in the human brain

In the present work the anatomical distribution of alpha 2-adrenoceptors in the human central nervous system was studied in detail by quantitative autoradiography using the selective alpha 2 agonist [3H]bromoxidine ([3H]UK-14304) as a ligand. Only postmortem tissues from subjects free of neurological disorders were used in this study. Very high or high densities of alpha 2-adrenoceptors were found along layers I and III in non-visual neocortex, layers III and IVc of the visual cortex, CA1 field--stratum lacunosum-moleculare--and dentate gyrus--stratum granularis--at the hippocampal formation, nucleus arcuatus at the hypothalamus, locus ceruleus, nucleus dorsalis of vagus and at the stratum granularis of the cerebellar cortex. Relevant densities of alpha 2-adrenoceptors were also observed along the remaining layers of neocortex, nuclei centralis, medialis and corticalis at the amygdala, anterior thalamic group and rotundocellularis nuclei, paraventricular and ventromedial hypothalamic nuclei, substantia innominata, superior colliculus--stratum zonale--and lateral periaqueductal area at the midbrain, nucleus tractus solitarii and dorsal horn--substantia gelatinosa--of the spinal cord. [3H]Bromoxidine specific binding was very low or negligible in the remaining brain areas. Although a general parallelism between the distribution of these receptors could be observed for the rat and human brain, dramatic species differences in the level of alpha 2-receptors were found in several brain areas, such as thalamus, amygdala or cerebellar cortex. In general, the distribution of alpha 2-adrenoceptors in the human brain found here was parallel to that described for the noradrenergic presynaptic terminals in the mammalian central nervous system, lending some weight to the proposed predominant presynaptic localization of these receptors. The relevance of the anatomical distribution of alpha 2-adrenoceptors in the human brain for a better knowledge of the neurochemistry of neuropsychiatric disorders is discussed.

Distribution of beta-adrenergic receptor subtypes in human post-mortem brain: alterations in limbic regions of schizophrenics

The distribution of the beta 1 (beta 1) and beta 2 (beta 2) subtypes of the beta-adrenergic receptor was examined in rat and nondiseased control human tissue. The distribution of the beta 1 and beta 2 receptors was also examined in schizophrenic cases, with additional studies in schizophrenic suicide and nonschizophrenic suicide cases. Scatchard analysis of the binding of [125I]iodopindolol (IPIN) to cortical membranes showed a similar Kd in human (177 pM) and rat (161 pM), but a lower maximum binding site (Bmax) in the human tissue (18.7 fmol/mg protein and 55.6 fmol/mg protein). For the autoradiographic studies [125I]IPIN was used to visualize both subtypes (total) or was displaced with the selective beta 1-receptor antagonist ICI-89,406 to visualize beta 2 sites, or with the selective beta 2-receptor antagonist ICI-118,551 to visualize beta 1 sites. Important differences in the regional distribution of the two subtypes of the beta-adrenergic receptors were noted between rat and human. In the nucleus accumbens and ventral putamen (ventral striatum), a patchy distribution of beta 1 receptors was observed that was not evident in the rat. These patches were aligned with markers of the matrix compartment of the striatum. The schizophrenic cases showed significant increases in the labeling of the beta 1-receptor patches with [125I]IPIN. In contrast to the frontal cortex of the nondisease controls, the parietal and temporal cortex showed a high ratio of beta 1 to beta 2 receptors and a highly laminar organization of the subtypes. [125I]IPIN binding to beta 1 receptors was highest in the external laminae with the reverse gradient for the beta 2 subtype. The medial temporal cortex displayed an alteration in the ratio of the 2 subtypes of the beta-adrenergic receptor, with the parahippocampus and hippocampus of the human, in contrast to the rat brain, predominantly expressing the beta 2 receptor. Moreover, there were consistently higher densities of beta 2 receptors in the hippocampus of the right hemisphere than the left hemisphere of the nondisease controls. There was not a left and right hemispheric asymmetry of beta 2 receptors in the hippocampus of elderly schizophrenics or in young schizophrenics who committed suicide. The asymmetry was evident in nonschizophrenic suicides, suggesting that the lack of asymmetry in the hippocampus of schizophrenics is evident early in the disease process. Thus limbic structures show alterations in the patterning of beta 1 and beta 2 receptors in the schizophrenic cases.

Coupling of human brain cerebral cortical alpha 2-adrenoceptors to GTP-binding proteins in Alzheimer's disease

The coupling of alpha 2-adrenoceptors to guanine nucleotide binding G-proteins was investigated in cerebral cortical membranes from control and Alzheimer's disease brain by characterizing the effects of MnCl2 and Gpp[NH]p on [3H]clonidine binding. The manganese induced elevation of [3H]clonidine binding was apparent in both control and Alzheimer's disease samples and the effect showed no significant difference between the two groups in the frontal cortex. However, the MnCl2 concentration dependence curves for control and Alzheimer's disease samples were significantly different from one another in the temporal cortex, whereas the pattern of stimulation by MnCl2 remained the same. The guanine nucleotide analogue, Gpp[NH]p inhibited [3H]clonidine binding in a concentration-dependent manner, the profiles of inhibition showing no significant differences between control and Alzheimer's disease samples. Analysis of the effect of Gpp[NH]p on [3H]clonidine saturation binding curves showed no significant differences between control and Alzheimer's disease samples in either frontal (Kd = 9.68 +/- 1.38, 9.1 +/- 2.6 nM; Bmax = 40.23 +/- 4.33, 44.3 +/- 9.4 fmol/mg, control and Alzheimer's disease values, respectively), or temporal (Kd = 11.61 +/- 4.04, 5.38 +/- 2.5 nM; Bmax = 52.0 +/- 14.0, 31.07 +/- 8.00 fmol/mg control and Alzheimer's disease values, respectively) cortices.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic receptor subtypes in the basal ganglia of patients with Huntington's chorea and Parkinson's disease

The density of [125I]iodo-cyanopindolol binding to beta-1 and beta-2 adrenergic receptors was studied in post mortem basal ganglia samples of Huntington's chorea and Parkinson's disease patients using autoradiography. Whereas no significant changes were observed in sections from Parkinson's and Huntington's chorea grade 2 patients, a nearly complete loss of beta-1 binding sites was observed in the basal ganglia of Huntington patients at later stages of the disease. The concentration of beta-2 receptors was increased by a factor 2 in the posterior putamen of all choreic cases. These results are consistent with the view that beta-1 receptors are predominantly located on a subpopulation of neurons which degenerate at late stages of Huntington's chorea, while beta-2 receptors are present mainly on glial elements.

[3H]rauwolscine behaves as an agonist for the 5-HT1A receptors in human frontal cortex membranes

The alpha 2 adrenergic antagonist [3H]rauwolscine binds with comparable nanomolar affinity to alpha 2 adrenoceptors and the nonadrenergic 5-HT1A receptors sites in human frontal cortex membranes. Addition of 0.5 mM GTP into the incubation medium produces a significant decrease in the amount of [3H]rauwolscine binding sites (Bmax = 230 +/- 16 and 115 +/- 11 fmol/mg protein in the absence and presence of GTP, respectively). The affinity for [3H]rauwolscine remains unchanged (i.e. KD = 40 +/- 0.9 nM and 4.1 +/- 1 nM). This effect of GTP can be attributed to decreased binding of the radioligand to the 5-HT1A receptors. GTP decreases binding of [3H]rauwolscine to nearly the same level as the one corresponding to the alpha 2 adrenoceptors in membranes from both the human frontal cortex and hippocampus. The venom of the marine cone snail, Conus tessulatus, preferentially inhibits [3H]rauwolscine binding to 5-HT1A receptors as compared with the alpha 2 adrenoceptors. Following complete masking of the 5-HT1A receptors by this venom. GTP no longer affects the saturation binding characteristics of [3H]rauwolscine for the remaining alpha 2 adrenoceptors. Nucleotides decrease the binding of [3H]rauwolscine to the 5-HT1A receptors with an order of potencies (i.e. GTP gamma S greater than GPP(NH)P much greater than GDP greater than GTP much greater than ATP) that is typical for nucleotide-mediated receptor-G protein dissociation. This suggests that [3H]rauwolscine is a 5-HT1A receptor agonist and this conclusion is compatible with earlier functional studies, indicating that rauwolscine (as well as yohimbine) has agonistic properties at the level of 5-HT autoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic receptor binding in frontal cortex of suicide victims

The high-affinity binding of the beta-adrenergic receptor antagonist, [3H]dihydroalprenolol, was measured in homogenates of frontal cortex (Brodmann's areas 8 and 9) of suicide victims and matched controls. Suicides were classified as violent if gunshot, hanging, or jumping was the cause of death and as nonviolent if carbon monoxide poisoning or drug overdose was the cause of death. No significant difference were found between controls and nonviolent or violent suicide victims with regard to the number of beta-adrenergic receptors (Bmax), or the binding affinity (Kd) of the receptor. Beta-Adrenergic receptor binding was not significantly affected by sex, age, race, or postmortem interval. Serotonin-2 receptor binding (Bmax) in homogenates from the same tissue specimens was previously reported to be significantly increased in violent suicides (Arora and Meltzer 1989). In these sample groups, suicide by violent means appears to be associated with an increase in the number of serotonin-2, but not beta-adrenergic, receptors in frontal cortex.

Characterization of [125I]HEAT binding to alpha 1-receptors in human brain: assessment in aging and Alzheimer's disease

The binding of [125I]2-(beta-4-hydroxyphenylethylamino-ethyltetralone ([ 125I]HEAT), an alpha 1-adrenergic receptor antagonist, to human brain membranes was characterized and the binding assessed in tissue from subjects with Alzheimer's disease (AD) and aging controls. Under Na+-K+ phosphate buffer conditions, [125I]HEAT bound to a single class of binding sites in prefrontal cortex (Brodmann area 10) with a Kd of about 120 pM. High binding capacities of [125I]HEAT were evident in the hippocampus and neocortex but were low in subcortical areas and cerebral microvessels comparable to the regional distribution of [3H]prazosin binding reported previously. Displacement of [125I]HEAT by various adrenergic drugs was consistent with its binding to alpha 1-adrenergic receptors. The specific binding was not affected by postmortem delay between death and freezing of tissue at autopsy. There was no correlation of [125I]HEAT binding with age of subjects. In AD subjects, the binding was significantly decreased in prefrontal cortex by about 25% but not changed in hippocampus, putamen or cerebellum compared to age-matched controls. The reduced binding of [125I]HEAT in prefrontal cortex may reflect a region-specific change in alpha 1-adrenergic receptors associated with neuronal loss in AD.

Beta-adrenoceptors in human brain labelled with [3H]dihydroalprenolol and [3H]CGP 12177

beta-Adrenoceptor binding sites were characterised and quantitated in post-mortem human brain with [3H]dihydroalprenolol ([3H]DHA) and [3H]CGP 12177. In cerebral cortex, isoprenaline and propranolol displaced both radioligands with uniform affinity. Practolol and CGP 20712A (selective beta 1-adrenoceptor antagonists) displaced with high affinity from a greater proportion of sites than ICI 118,551 and IPS 339 (selective beta 2-adrenoceptor antagonists). In cerebellum, propranolol displaced both radioligands with uniform affinity. ICI 118,551 displaced with high affinity from a greater proportion of sites than CGP 20712A. The density of total beta-adrenoceptors (defined with isoprenaline) and of beta 1- and beta 2-adrenoceptors (defined with CGP 20712A and ICI 118,551 respectively) was studied by saturation binding of both radioligands in 13 brain areas. beta-Adrenoceptor density was higher in caudate, putamen and nucleus accumbens (100-120 fmol/mg protein) than cortex (50-70 fmol/mg protein) and densities were lowest in hypothalamus and cerebellum (27-38 fmol/mg protein). The proportion of beta 1-adrenoceptors (as a % of total beta-adrenoceptors) was high in caudate (80%), putamen (80%) and cortex (60-70%) and lower in hippocampus (40%) and cerebellum (30%). Both radioligands labelled a very similar number of beta-adrenoceptors in all brain regions studied.

Characterization and regional distribution of alpha 2-adrenoceptors in postmortem human brain using the full agonist [3H]UK 14304

The full agonist [3H]UK 14304 [5-bromo-6-(2-imidazolin-2-yl-amino)-quinoxaline] was used to characterize alpha 2-adrenoceptors in postmortem human brain. The binding at 25 degrees C was rapid (t1/2, 4.6 min) and reversible (t1/2, 14.1 min), and the KD determined from the kinetic studies was 0.48 nM. In frontal cortex, the rank order of potency of adrenergic drugs competing with [3H]UK 14304 or [3H]clonidine showed the specificity for an alpha 2A-adrenoceptor: UK 14304 approximately equal to yohimbine approximately equal to oxymetazoline approximately equal to clonidine greater than phentolamine approximately equal to (-)-adrenaline greater than idazoxan approximately equal to (-)-noradrenaline greater than phenylephrine greater than (+/-)-adrenaline much greater than corynanthine greater than prazosin much greater than (+/-)-propranolol. GTP induced a threefold decrease in the affinity of [3H]UK 14304, with no alteration in the maximum number of binding sites, suggesting that the radioligand labelled the high-affinity state of the alpha 2-adrenoceptor. In the frontal cortex, analyses of saturation curves indicated the existence of a single population of noninteracting sites for [3H]UK 14304 (KD = 0.35 +/- 0.13 nM; Bmax = 74 +/- 9 fmol/mg of protein). In other brain regions (hypothalamus, hippocampus, cerebellum, brainstem, caudate nucleus, and amygdala) the Bmax ranged from 68 +/- 7 to 28 +/- 4 fmol/mg of protein. No significant changes in the KD values were found in the different regions examined. The binding of [3H]UK 14304 was not affected by age, sex or postmortem delay.(ABSTRACT TRUNCATED AT 250 WORDS)

[Central effects of five beta-adrenergic receptor blockers in healthy volunteers: a quantitative EEG study]

The effects of five beta blockers on the central nervous system of healthy subjects was studied by computerized EEG analysis. All subjects underwent continuous recording with a Holter magnetic type recorder during the experimental period. For 10 consecutive days, five groups of subjects received alternately placebo and the beta blockers acebutolol 600 mg, carteolol 20 mg, metoprolol 200 mg, pindolol 30 mg and sotalol 320 mg. EEG recordings (C4/P4, P4/02 and C3/P3, P3/01) lasting 5 min were made between 8.30 and 9.30 a.m. Subjects were at rest with eyes closed and there was no vigilance control. The signal was recorded on a magnetic tape recorder and then processed by Nicolet MED 80 system. Comparisons of absolute and relative powers and of average frequencies were then made between the different sequences and groups. The possible correlations between the changes observed in the power spectrum and the clinical, pharmacological and pharmacokinetic specific properties of each beta blocker are discussed.

Alpha 2-adrenoceptors in human forebrain: autoradiographic visualization and biochemical parameters using the agonist [3H]UK-14304

The regional distribution and biochemical parameters of alpha 2-adrenoceptors in post-mortem human brain tissue were analyzed in autoradiographic and membrane binding studies, using the full agonist, [3H]UK-14304. Autoradiographic visualization of these receptors in the forebrain revealed a heterogeneous anatomical distribution with high levels in the neocortex, ventral hypothalamus, hippocampus and some thalamic nuclei, among others. In membrane binding studies, analyses of saturation curves indicated the presence of a single population of sites for [3H]UK-14304 both in the human and rat brain. There was a good correlation between the density of alpha 2-adrenoceptors obtained by both procedures. [3H]UK-14304 is a feasible ligand to localise and quantify alpha 2-adrenoceptors in human post-mortem material by autoradiographic and membrane binding techniques.

Specific [3H]UK 14,304 binding in human cortex occurs at multiple high affinity states with alpha 2-adrenergic selectivity and differing affinities for GTP

[3H]UK 14,034 is a full agonist at alpha 2-adrenergic receptors. Although the characteristics of the binding of the partial alpha 2-adrenergic agonists in postmortem human brain were known, the binding of [3H]UK 14,304 had not been studied in this tissue. Multi-site binding of this radiolabel had been reported in other tissues and guanosine triphosphate (GTP) had been shown to reduce [3H]UK 14,304 binding. We now report that [3H]UK 14,304 labels at least 2 specific binding sites in human brain that both have the characteristics of an alpha 2-adrenergic binding site. GTP decreases agonist binding at both of these sites, but with different potencies at each site.

Increased alpha 2-adrenoceptor density in the frontal cortex of depressed suicide victims

The density of brain alpha 2-adrenoceptors, quantitated by means of the binding of the agonist [3H]clonidine, was studied in post-mortem cortical membranes of matched control subjects and depressed suicide victims. In the depressed suicide group, the specific high affinity binding of [3H]clonidine was found to be significantly increased (Bmax, 72% greater; p less than 0.01) without significant changes in the KD value for the radioligand. These preliminary results indicate that alpha 2-adrenoceptor density in the high affinity state (alpha 2H) is increased in the brain of depressed patients and add strong support to the hypothesis that endogenous depression is related to supersensitive alpha 2-adrenoceptors.

Evaluation of studies on platelet alpha 2 adrenoreceptors in depressive illness

Discrepant results have been reported from at least ten laboratories regarding the status of platelet alpha 2 adrenoreceptors in depressed patients. Using a statistical test to combine those studies which utilized radioligand binding techniques, we find the overall data support an elevation in density of platelet alpha 2 adrenoreceptors from drug-free depressed patients (p less than 0.05) and suggest a normalization to lower binding values following antidepressant drug treatment (0.05 less than p less than 0.10). However, these positive results are attributable to highly significant findings by only three laboratories. Much of the discrepancy may be attributable to numerous methodological variables which distinguish the studies. Foremost amongst these variables are the use of different platelet size populations, the use of different medium, and the choice of radioactive ligand and competitor (non-radioactive ligand) in the assay. We present a rationale for the proper choice of each methodological condition used in the clinical assessment of platelet alpha 2 adrenoreceptor status, hoping that improved experimental designs will resolve the current controversy.

[3H]clonidine binds at multiple high affinity states in human prefrontal cortex

[3H]Clonidine binds at particulate membrane fractions of human prefrontal cortex in a process that demonstrates high affinity, saturability, reversibility, alpha 2-adrenergic selectivity and the existence of multiple affinity states. At 37 degrees C maximal specific [3H]clonidine binding was briefly attained at 10 and lasted only until 15 min, while at 21 degrees C maximal binding was maintained from 20 to 90 min. At 21 degrees C, rate dissociation studies and saturation analyses were at least biphasic, and adrenergic competitors decreased [3H]clonidine binding with Hill coefficients less than 0.70. Analysis of these data showed at least two affinity states with apparent KDs of 0.34 and 6.0 nM, and the order in which ligands decreased [3H]clonidine binding was clonidine greater than (-)-epinephrine greater than (-)-norepinephrine greater than yohimbine greater than (+)-norepinephrine greater than (+/-)-isoproterenol greater than prazosin greater than serotonin.

Cellular localization of adrenergic receptors in rat and human brain

The localization of adrenergic receptors in the central nervous system was studied in two physiological conditions of noradrenergic denervation, a 6-hydroxydopamine-induced lesion of the locus coeruleus in newborn rat, and a pathological related degeneration of the locus coeruleus in man, Parkinson's disease. The localization of these receptors in the synapse has been studied with the technique of subcellular fractionation by differential centrifugation. In lesioned rats, an increase in the density of alpha 1 and beta 1 receptors was observed in several brain regions, in contrast to alpha 2 receptors which were not modified. Subcellular fractionation in lesioned rats showed an increase in alpha 1 and beta 1 receptors in synaptosomal fractions. Similar results were found in parkinsonian patients: alpha 1 receptors increased in the synaptosomal fraction; beta receptors increased in synaptosomal and microsomal fractions. These results suggest that alpha 1 and beta 1 receptors may be located on non-noradrenergic nerve terminals in mammalian brain. alpha 2 and beta 2 receptors may be situated on glial cells or neuronal elements unrelated to noradrenergic input.

Beta-adrenoceptor subtypes in the human brain: autoradiographic localization

The distribution and characteristics of beta-adrenoceptors in postmortem human brain was studied using quantitative autoradiographic techniques. 125I-Cyanopindolol was used as a ligand. High densities of beta-adrenoceptors were found in the caudate, putamen, different cortical areas and layers and the hippocampal formation. Low densities were present in other areas such as the thalamus, hypothalamus, midbrain and cerebellar cortex. Specific beta 1 and beta 2 antagonists were used to visualize and quantify separately the two subtypes of beta-adrenoceptors. Computer analysis of the competition curves obtained revealed that the putamen was enriched in beta 1 sites while the cerebellum contained predominantly beta 2 adrenoceptors. The regional distribution of beta-adrenoceptor subtypes was found to be similar to that seen in the rat brain.