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

Noradrenergic alterations in Parkinson's disease: a combined 11C-yohimbine PET/neuromelanin MRI study

Degeneration of the noradrenergic system is now considered a pathological hallmark of Parkinson's disease, but little is known about its consequences in terms of parkinsonian manifestations. Here, we evaluated two aspects of the noradrenergic system using multimodal in vivo imaging in patients with Parkinson's disease and healthy controls: the pigmented cell bodies of the locus coeruleus with neuromelanin sensitive MRI; and the density of α2-adrenergic receptors (ARs) with PET using 11C-yohimbine. Thirty patients with Parkinson's disease and 30 age- and sex-matched healthy control subjects were included. The characteristics of the patients' symptoms were assessed using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS). Patients showed reduced neuromelanin signal intensity in the locus coeruleus compared with controls and diminished 11C-yohimbine binding in widespread cortical regions, including the motor cortex, as well as in the insula, thalamus and putamen. Clinically, locus coeruleus neuronal loss was correlated with motor (bradykinesia, motor fluctuations, tremor) and non-motor (fatigue, apathy, constipation) symptoms. A reduction of α2-AR availability in the thalamus was associated with tremor, while a reduction in the putamen, the insula and the superior temporal gyrus was associated with anxiety. These results highlight a multifaceted alteration of the noradrenergic system in Parkinson's disease since locus coeruleus and α2-AR degeneration were found to be partly uncoupled. These findings raise important issues about noradrenergic dysfunction that may encourage the search for new drugs targeting this system, including α2-ARs, for the treatment of Parkinson's disease.

Distribution of α2-Adrenergic Receptors in the Living Human Brain Using [11C]yohimbine PET

The neurofunctional basis of the noradrenergic (NA) system and its associated disorders is still very incomplete because in vivo imaging tools in humans have been missing up to now. Here, for the first time, we use [¹¹C]yohimbine in a large sample of subjects (46 healthy volunteers, 23 females, 23 males; aged 20-50) to perform direct quantification of regional alpha 2 adrenergic receptors' (α₂-ARs) availability in the living human brain. The global map shows the highest [¹¹C]yohimbine binding in the hippocampus, the occipital lobe, the cingulate gyrus, and the frontal lobe. Moderate binding was found in the parietal lobe, thalamus, parahippocampus, insula, and temporal lobe. Low levels of binding were found in the basal ganglia, the amygdala, the cerebellum, and the raphe nucleus. Parcellation of the brain into anatomical subregions revealed important variations in [¹¹C]yohimbine binding within most structures. Strong heterogeneity was found in the occipital lobe, the frontal lobe, and the basal ganglia, with substantial gender effects. Mapping the distribution of α₂-ARs in the living human brain may prove useful not only for understanding the role of the NA system in many brain functions, but also for understanding neurodegenerative diseases in which altered NA transmission with specific loss of α₂-ARs is suspected.

Cyto- and receptor architectonic mapping of the human brain

Mapping of the human brain is more than the generation of an atlas-based parcellation of brain regions using histologic or histochemical criteria. It is the attempt to provide a topographically informed model of the structural and functional organization of the brain. To achieve this goal a multimodal atlas of the detailed microscopic and neurochemical structure of the brain must be registered to a stereotaxic reference space or brain, which also serves as reference for topographic assignment of functional data, e.g., functional magnet resonance imaging, electroencephalography, or magnetoencephalography, as well as metabolic imaging, e.g., positron emission tomography. Although classic maps remain pioneering steps, they do not match recent concepts of the functional organization in many regions, and suffer from methodic drawbacks. This chapter provides a summary of the recent status of human brain mapping, which is based on multimodal approaches integrating results of quantitative cyto- and receptor architectonic studies with focus on the cerebral cortex in a widely used reference brain. Descriptions of the methods for observer-independent and statistically testable cytoarchitectonic parcellations, quantitative multireceptor mapping, and registration to the reference brain, including the concept of probability maps and a toolbox for using the maps in functional neuroimaging studies, are provided.

Receptor mapping: architecture of the human cerebral cortex

PURPOSE OF REVIEW

Cytoarchitectonical brain mapping is of growing interest as a powerful tool for localization of activated brain regions in functional neuroimaging. Mapping of neurotransmitter receptors can provide novel molecular and functionally relevant information to the available cytoarchitectonical brain maps, because receptors are key molecules of neurotransmission. This review highlights the relation between cytoarchitectonical parcellations and the regionally inhomogeneous distribution of receptors. It will demonstrate the potential of receptor mapping for novel and functionally relevant insights into the regional organization of the human cortex.

RECENT FINDINGS

Mapping of a single receptor type can already reveal borders of functionally and cytoarchitectonically distinct cortical regions. The combined mapping of various receptors in each cortical area (receptor fingerprint) represents the balance between different neurotransmitter systems and often reveals hitherto unknown parcellations. Different brain regions are identified as parts of distinct functional systems.

SUMMARY

Receptor mapping of the human brain, particularly multireceptor mapping, provides a novel and multimodal view of its anatomical, functional and molecular organization. It reveals organizational principles of the segregation of cortical and subcortical structures. It improves our understanding of the brain's architecture beyond the limits of cytoarchitectonics and serves as a basis for clinical and pharmacological studies of brain diseases.

Opposite to α2-adrenergic agonists, an imidazoline I1 selective compound does not influence reflex bradycardia in rabbits

This work aimed to study the respective effects of central alpha2-adrenergic receptors (alpha2-ARS) and I1 imidazoline receptors (I1Rs) in the facilitatory effects of imidazoline-like drugs on the reflex bradycardia (RB). Experiments were performed in anaesthetized rabbits. The reflex bradycardic response was induced by phenylephrine injected i.v. LNP 509, rilmenidine and dexmedetomidine were administered intracisternally (i.c.). LNP509 (1 mg/kg, i.c.), a ligand highly selective for I1Rs, induced hypotension (54+/-3 vs. 93+/-2 mm Hg) and bradycardia (260+/-13 vs. 322+/-13 beats/min) (p<0.05, n=5) but did not affect RB. Rilmenidine (1 microg/kg, i.c.), a hybrid ligand which binds to both I1 and alpha2-ARS, also decreased arterial pressure (61+/-2 vs. 101+/-2 mm Hg) and heart rate (260+/-4 vs. 308+/-8) (p<0.01, n=5); it potentiated the RB (maximum R-R interval: 284+/-17 vs. 196+/-6 ms) (p<0.05, n=5). Dexmedetomidine (1 microg/kg, i.c.), a ligand selective for alpha2-ARs, reduced blood pressure (53+/-3 vs. 104+/-2 mm Hg) and heart rate (246+/-4 vs. 312+/-8 beats/min) (p<0.05, n=5) and potentiated the RB (maximum R-R interval: 518+/-38 vs. 194+/-4 ms) (p<0.05, n=5). The potentiation of RB was much greater than that observed with rilmenidine and was significantly prevented by L-NNA injected centrally. This study shows that: (i) an exclusive action on I1Rs which decreases arterial pressure, does not potentiate the RB ii) activation of alpha2-ARs potentiates the RB (iii) the R-R prolongation caused by alpha2-ARs stimulation is prevented by central NOS inhibition.

Neuron specific alpha-adrenergic receptor expression in human cerebellum: implications for emerging cerebellar roles in neurologic disease

Recent data suggest novel functional roles for cerebellar involvement in a number of neurologic diseases. Function of cerebellar neurons is known to be modulated by norepinephrine and adrenergic receptors. The distribution of adrenergic receptor subtypes has been described in experimental animals, but corroboration of such studies in the human cerebellum, necessary for drug treatment, is still lacking. In the present work we studied cell-specific localizations of alpha1 adrenergic receptor subtype mRNA (alpha 1a, alpha 1b, alpha 1d), and alpha2 adrenergic receptor subtype mRNA (alpha 2a, alpha 2b, alpha 2c) by in situ hybridization on cryostat sections of human cerebellum (cortical layers and dentate nucleus). We observed unique neuron-specific alpha1 adrenergic receptor and alpha2 adrenergic receptor subtype distribution in human cerebellum. The cerebellar cortex expresses mRNA encoding all six alpha adrenergic receptor subtypes, whereas dentate nucleus neurons express all subtype mRNAs, except alpha 2a adrenergic receptor mRNA. All Purkinje cells label strongly for alpha 2a and alpha 2b adrenergic receptor mRNA. Additionally, Purkinje cells of the anterior lobe vermis (lobules I to V) and uvula/tonsil (lobules IX/HIX) express alpha 1a and alpha 2c subtypes, and Purkinje cells in the ansiform lobule (lobule HVII) and uvula/tonsil express alpha 1b and alpha 2c adrenergic receptor subtypes. Basket cells show a strong signal for alpha 1a, moderate signal for alpha 2a and light label for alpha 2b adrenergic receptor mRNA. In stellate cells, besides a strong label of alpha 2a adrenergic receptor mRNA in all and moderate label of alpha 2b message in select stellate cells, the inner stellate cells are also moderately positive for alpha 1b adrenergic receptor mRNA. Granule and Golgi cells express high levels of alpha 2a and alpha 2b adrenergic receptor mRNAs. These data contribute new information regarding specific location of adrenergic receptor subtypes in human cerebellar neurons. We discuss our observations in terms of possible modulatory roles of adrenergic receptor subtypes in cerebellar neurons responding to sensory and autonomic input signals, and review species differences in cerebellar adrenergic receptor expression.

Pharmacological characterization and autoradiographic distribution of α2-adrenoceptor antagonist [3H]RX 821002 binding sites in the chicken brain

Knowledge about the noradrenergic system in birds is very scarce even though their biological diversity and complex social behavior make them an excellent model for studying neuronal functions and developmental biology. While the role of norepinephrine has been described in depth in a large number of central and peripheral functions in mammals, reports for avian species are limited. The radioligand [(3)H]RX 821002 ([(3)H]1,4-[6,7(n)3H]-benzodioxan-2-methoxy-2-yl)-2-imidazol) has been used to map and characterize alpha(2)-adrenoceptors through the chicken brain using in vitro autoradiography and membrane homogenates binding assays. [(3)H]RX 821002 showed a saturable and high affinity binding to a site compatible with alpha(2)-adrenoceptor, and to a serotonergic component. The autoradiographic assays displayed a similar alpha(2)-adrenoceptor distribution than those previously reported in birds using other radioligands such as [(3)H]UK 14304 ([(3)H]5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) or [(3)H]clonidine. [(3)H]RX 821002 binding pharmacological characterization was carried out in different chicken brain regions using membrane homogenates for competition assays with different alpha(2)-adrenoceptor agonists and antagonists drugs (oxymetazoline, BRL 44408 [2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidazole] ARC 239 [2-(2-4-(O-methoxyphenyl)-piperazin-1-yl)-ethyl-4,4-dimethyl-1,3-(2H,4H)-isoquinolindione], prazosin, UK 14304 and RX 821002). The results showed alpha(2A) as the predominant alpha(2)-adrenoceptor subtype in the chicken brain while alpha(2B)- and/or alpha(2C)-adrenoceptor subtypes were detected only in the telencephalon. RX 821002, serotonin (5-HT) and 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] competition assays, and competition binding assays performed in the presence of serotonin demonstrated that [(3)H]RX 821002 binds with higher affinity to a serotonergic component, probably 5-HT(1A) receptors, than to the alpha(2)-adrenoceptors. Similar pharmacological properties for the alpha(2)-adrenoceptor component were observed both in rat and chicken brain. The results demonstrate that the different alpha(2)-adrenoceptor subtypes are present in chicken brain and suggest that these receptors are highly conserved through evolution.

Comparative anatomy of α2 and β adrenoceptors in the adult and developing brain of the marine teleost the red porgy (Pagrus pagrus, Sparidae): [3H]clonidine and [3H]dihydroalprenolol quantitative autoradiography and receptor subtypes immunohistochemistry

The present study aimed to determine the anatomic distribution and developmental profile of alpha(2) and beta adrenoceptors (AR) in marine teleost brain. Alpha 2 and beta adrenoceptors were studied at different developmental stages by using [(3)H]clonidine and [(3)H]dihydroalprenolol, respectively, by means of in vitro quantitative autoradiography. Furthermore, immunohistochemical localization of the receptor subtypes was performed to determine their cellular distribution. Saturation studies determined a high-affinity component of [(3)H]clonidine and [(3)H]dihydroalprenolol binding sites. High levels of both receptors were found in preglomerular complex, ventral hypothalamus, and lateral torus. Dorsal hypothalamus and isthmus included high levels of alpha(2) AR, whereas pretectum and molecular and proliferative zone of cerebellum were specifically characterized by high densities of beta AR. From the first year of life, adult levels of both AR were found in most medial telencephalic, hypothalamic, and posterior tegmental areas. Decreases in both receptors densities with age were prominent in ventral and posterior telencephalic, pretectal, ventral thalamic, hypothalamic, and tegmental brain regions. Immunohistochemical data were well correlated with autoradiography and demonstrated the presence of alpha(2A), alpha(2C), beta(1), and beta(2) AR subtype-like immunoreactivity. Both the neuronal (perikaryal or dendritic) and the glial localization of receptors was revealed. The localization and age-dependent alterations in alpha(2) and beta AR were parallel to plasticity mechanisms, such as cell proliferation in periventricular thalamus, hypothalamus, and cerebellum. In addition, the biochemical characteristics, distribution pattern, and neuronal or glial specificity of the receptors in teleost brain support a similar profile of noradrenergic transmission in vertebrate brain evolution.

[(35)S]-GTPgammaS autoradiography reveals alpha(2) adrenoceptor-mediated G-protein activation in amygdala and lateral septum

alpha(2)-adrenoceptor-mediated G-protein activation was examined by [(35)S]-GTPgammaS autoradiography. In alpha(2)-adrenoceptor-rich regions (amygdala, lateral septum), noradrenaline stimulated [(35)S]-GTPgammaS binding. These actions were abolished by the selective alpha(2) antagonist, atipamezole. Conversely, in caudate nucleus, which expresses few alpha(2) receptors, noradrenaline-induced stimulation was not inhibited by atipamezole, suggesting that it is not mediated by alpha(2)-adrenoceptors.

Pre- and post-hatching developmental changes in beta-adrenoceptor subtypes in chick brain

This study used [3H]CGP 12177 as a radioligand to determine the beta1 and beta2-adrenoceptor changes from the pre-hatching E17 stage, where the beta2 subtype is first detected, to the post-hatching P30 stage. While beta1-adrenoceptors were found to be present from E18 and were limited to cerebellum and hyperstriatum in all stages studied, beta2-adrenoceptors showed a wider distribution throughout the brain. In most of the structures analysed both beta1- and beta2-adrenoceptor binding values reached a maximum in the P2 stage, followed by a decrease over the following days. A second increase in both subtypes was detected again in the P15 and P30 stages. These results support the notion of a specific role for beta-adrenoceptors in neural plasticity in the first week after hatching and suggest that the beta2 subtype is the main adrenoceptor in chick brain throughout its development.

Synthesis and biological evaluation of [11C]MK-912 as an alpha2-adrenergic receptor radioligand for PET studies

In vitro studies showed that MK-912 ((2S, 12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octahydro -2H-benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one) is a potent alpha2-adrenergic receptor antagonist with high affinity (Ki = 0.42, 0.26 and 0.03 nM to alpha2A, alpha2B and alpha2C, respectively) and high selectivity (alpha2A/alpha1A = 240; alpha2A/D-1 = 3600; alpha2A/D-2 = 3500; alpha2A/5-HT1 = 700; alpha2A/5-HT2 = 4100). The compound was labeled with 11C and evaluated in rodents and monkey as a specific radioligand for studying alpha2-adrenergic receptors using PET. [11C]MK-912 was synthesized by methylation of its desmethyl precursor, L-668,929, with [11C]CH3I in (Bu3O)P=O at 85 degrees C for 8 min followed by purification with HPLC in 18% yield in a synthesis time of 45 min from end of bombardment (EOB). The specific activity was 0.83-0.93 Ci/micromol and the radiochemical purity was 97%. The initial uptake of [11C]MK-912 in mouse brain, heart, lung, liver and kidney was high (5%, 4%, 5%, 17% and 8% per gram of organ, respectively, at 5 min postinjection) and the activities were then slowly cleared from these organs. The uptake of [11C]MK-912 in rat olfactory tubercle, a brain region with high density of alpha2-adrenergic receptors, was reduced by 30%, and the ratio of radioactivity in olfactory tubercle/cerebellum was reduced from 2:1 to 1:1 by coinjection of [11C]MK-912 with a potent alpha2-adrenergic receptor antagonist, atipamezole (3 mg/kg), indicating that compound 2 binds to alpha2-adrenergic receptors. However, a PET study in a rhesus monkey revealed that the initial influx of [11C]MK-912 into various brain regions (cerebellum, cortex, olfactory tubercle and striatum) was high (0.02%/cc), and the radioactivity was then washed out slowly and without significantly differential retention in these brain regions. This, coupled with the fact that none of the high-density alpha2-adrenergic receptor brain regions exceeds a few millimeters in diameter, suggests that [11C]MK-912 is probably not an ideal radioligand for studying alpha2-adrenergic receptors in humans using commercially available PET.

Adrenergic receptors in Alzheimer's disease brain: selective increases in the cerebella of aggressive patients

In this study, the distribution and concentration of beta1, beta2, and alpha2 adrenergic receptors were examined in the frontal cortex, hypothalamus, and cerebellum of Alzheimer's disease (AD) and age-matched control human brains by receptor autoradiography. The purpose of this study was to detect changes in adrenergic receptor concentrations in key areas of the brain known to affect behavior. For these studies, [125I]iodopindolol ([125I]IPIN) was used to visualize total beta adrenergic sites (with ICI-89,406 and ICI-118, 551 as subtype-selective antagonists to visualize beta2 and beta1 receptors, respectively). [3H]UK-14,304 was used to localize the alpha2 sites. Essentially no significant difference in adrenergic receptor concentration was found between total AD cases taken together and control patients. It was found, however, that there were important distinctions within the AD group when cases were subdivided according to the presence or absence of aggression, agitation, and disruptive behavior. Aggressive AD patients had markedly increased (by approximately 70%) concentrations of alpha2 receptors in the cerebellar cortex compared with nonaggressive patients with similar levels of cognitive deficit. The levels of cerebellar alpha2 receptors in aggressive AD patients were slightly above the healthy elderly controls, suggesting that these receptors are preserved and perhaps increased in this subgroup of AD. beta1 And beta2 adrenergic receptors of the cerebellar cortex showed smaller but significant ( approximately 25%) increases in concentration in aggressive AD subjects versus both nonaggressive AD patients and controls. No significant differences were found in adrenergic receptor concentrations within the frontal cortex or hypothalamus. These results point out the importance of distinguishing behavioral subgroups of AD when looking for specific neurochemical changes. These autoradiographic results may reflect the importance of the cerebellum in behavioral control.

A comparative study of alpha2- and beta-adrenoceptor distribution in pigeon and chick brain

The pharmacological properties and anatomical distribution of alpha2-, beta1- and beta2-adrenoceptors in pigeon and chick brains were studied by both homogenate binding and tissue section autoradiography. [3H]Bromoxidine (alpha2-adrenoceptor-), [3H]CGP 12177 (beta-adrenoceptor) and [125I]cyanopindolol (beta-adrenoceptor) were used as radioligands. In both species, [3H]bromoxidine binding to avian brain tissue showed a pharmacological profile similar to that previously reported for alpha2-adrenoceptors in mammals. Regarding the anatomical distribution, the areas with the highest densities of alpha2-adrenoceptors in the pigeon brain included the hyperstriatum, nuclei septalis, tectum opticum and some brainstem nuclei. Most beta-adrenoceptors found in tissue membranes and sections from chick and pigeon brain were of the beta2 subtype, in contrast to what has been reported in the mammalian brain, where the beta1 subtype is predominant. A striking difference was found between the two species regarding the densities of these receptors: while pigeon brain was extremely rich in [125I]cyanopindolol binding throughout the brain (mainly cerebellum) in the pigeon, the levels of labelling in the chick brain were much lower; the exception was the cerebellum, which displayed a higher density than other parts of the brain in both species. Overall, our results support the proposed anatomical equivalences between a number of structures in the avian and mammalian encephalon.

Adrenergic receptors in the cerebellum of olivopontocerebellar atrophy

Using autoradiographic techniques we studied the changes that in adrenergic receptors occurred in the cerebellum of two olivopontocerebellar atrophy (OPCA) patients as compared with a control group. In OPCA cerebellum the densities of total beta-adrenoceptors were reduced along the cortex but increased in the white matter. Although mainly the beta 1 subtype was decreased along the cerebellar cortex, the increase of beta-receptors over the white matter was due to a selective raise in the beta 2 subtype. These findings suggest a postsynaptic neuronal location for the beta 1 subtype and a glial location for the beta 2-adrenoceptor. On the other hand, alpha 2-adrenoceptors were clearly reduced all along the cerebellar cortex of these OPCA brains, this probably being secondary to the loss of presynaptic adrenergic terminals arising from the locus coeruleus. These results help clarify both the subcellular location of adrenoceptors in human cerebellum and the neurochemical pathophysiology of OPCA.

Effect of morphine and abstinence syndrome on [3H]bromoxidine binding to alpha 2-adrenoceptors in rat brain

At 4 days after the implantation of two subcutaneous 75 mg morphine pellets in the back skin, rats were morphine-dependent. In the three layers studied in the occipital cortex we found that the values of the alpha 2-adrenergic agonist [3H]bromoxidine binding increased with respect to animals implanted with placebo pellets. Typical behavioral and physiological symptoms of the abstinence syndrome appeared 30 minutes after administration of naloxone, [3H]bromoxidine binding values being similar to those obtained in animals implanted with placebo pellets. The pattern of response of the [3H]bromoxidine binding was similar in the hippocampus and the superficial gray layer of the superior colliculus of the mesencephalon, but the differences were not statistically significant in these areas. This paper concludes that exist brain regional differences in the alpha 2-adrenoceptors response under morphine-treatment and possibly under naloxone-induced morphine abstinence syndrome.

Identification of alpha 2-adrenoceptors in rat lymph nodes and spleen: an autoradiographic study

The anatomical localization and pharmacological characteristics of alpha 2-adrenoceptors were studied in rat lymphoid tissues by quantitative autoradiography with [3H]bromoxidine as a ligand. In lymph nodes, a significant density of these receptors was found in the capsule, interfollicular cortex and medullary cords, while only very low densities were observed in the paracortex and germinal centres. In the spleen, these receptors appeared to be mainly distributed in the marginal zone of white pulp as well as in the red pulp. These results suggest a role for alpha 2-adrenoceptors in the interaction between nervous and immune systems.

Changes in aminergic receptors in a PSP postmortem brain: correlation with pathological findings

The state of different aminergic receptors was assessed, by quantitative autoradiography in tissue sections, in several representative brain regions from a typical progressive supranuclear palsy (PSP) patient and from 9 matched brains. The densities of muscarinic receptors were within control limits in most of the brain areas of this PSP brain. Serotonin1 receptors were clearly reduced only in areas with very relevant neuropathological damage, such as locus niger and globus pallidus. The density of D1 dopamine receptors in the caudate-putamen and frontal cortex of the patient was within control limits. By contrast, nigral D1 and striatal D2 dopamine receptors were dramatically reduced in the patient as compared to controls. Finally, alpha 2-adrenoceptors were clearly reduced in all the examined areas of this PSP patient as compared to control group. Both the potential role of these receptor changes in the pathophysiology of the clinical features of PSP and their correlation with the neuropathological findings of this PSP patient are discussed.

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.

Opposite age-dependent changes of alpha 2A-adrenoceptors and nonadrenoceptor [3H]idazoxan binding sites (I2-imidazoline sites) in the human brain: strong correlation of I2 with monoamine oxidase-B sites

In the postmortem human brain (27 specimens of frontal cortex, Brodmann area 9), the specific binding of the antagonists [3H]RX 821002 (2-methoxyidazoxan) to alpha 2A-adrenoceptors and that of [3H]idazoxan to I2-imidazoline sites (a nonadrenoceptor mitochondrial site) were determined in parallel to study the effect of aging (range, 4-89 years) on both brain proteins. The density of alpha 2A-adrenoceptors and age were negatively correlated (r = -0.71; p < 0.001). In contrast, the density of I2-imidazoline sites was positively correlated with aging (r = 0.59; p < 0.005). The ratio of receptor densities (alpha 2A/I2) also showed a marked negative correlation with age (r = -0.76; p < 0.001). In an age-selected group (range, 10-89 years), the density of monoamine oxidase (MAO)-B sites labeled by [3H]Ro 19-6327 (lazabemide) also showed a positive correlation with age (r = 0.80; p < 0.005). In these subjects, the density of I2-imidazoline sites correlated well with the density of MAO-B sites (r = 0.70; p < 0.005). The ratio of the density of these sites (MAO-B/I2) did not correlate with the age of the subject at death (r = -0.15). In the human frontal cortex, idazoxan displayed very low affinity (Ki = 89 microM) against the binding of [3H]Ro 19-6327 to MAO-B, which discounted a direct interaction of [3H]idazoxan with the active center of the enzyme and indicated that the I2-imidazoline site cannot be identified with MAO-B.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic demonstration of loss of alpha 2-adrenoceptors in progressive supranuclear palsy: preliminary report

We assessed, by quantitative autoradiography in tissue sections, the density of alpha 2-adrenoceptors in several representative brain regions from a typical progressive supranuclear palsy (PSP) patient and in 9 matched brains. The full agonist 3H-bromoxidine was used as a ligand. The density of alpha 2-receptors was dramatically reduced in all the examined brain areas of this PSP patient as compared to the control group. The locus ceruleus degeneration observed here is the most plausible explanation for this loss of alpha 2-receptors. Our data show that abnormalities in the noradrenergic system may justify some clinical features of the PSP clinical picture, this supporting the idea of further study of the clinical effects of noradrenergic drugs in PSP.

Loss of high-affinity alpha 2-adrenoceptors in Alzheimer's disease: an autoradiographic study in frontal cortex and hippocampus

We assessed, by quantitative autoradiography, the density of high-affinity alpha 2-adrenoceptors in hippocampus and frontal cortex sections from 18 patients dying with Alzheimer's disease (AD) in comparison with a control group of 13 matched cases. The full agonist [3H]bromoxidine (UK-14304) was used as a ligand. In AD brains, the specific binding of [3H]bromoxidine was markedly decreased both in frontal cortex, the reduction ranging from 55% on layer I (P less than 0.0005) to 40% loss on layers IV-VI (P less than 0.01), and in the hippocampus where the mean of alpha 2-receptor loss was 53% both for the CA1 (P less than 0.0005) and the dentate gyrus (P less than 0.005). This dramatic decrease in the density of functional, high-affinity alpha 2-adrenoceptors very probably reflects the neuronal loss described in locus coeruleus of AD brains. The important implications of these findings for the pharmacological treatment of AD are discussed.

Quantitative in vitro and ex vivo autoradiography of the α2-adrenoceptor antagonist [3H]atopamezole

Atipamezole is an alpha-adrenoceptor antagonist with high affinity and selectivity for the alpha 2-receptor. In vitro autoradiography of [3H]atipamezole on rat and human brain sections revealed a pattern virtually identical to the one observed using 3H-labeled antagonists such as idazoxan or methoxyidazoxan or agonists such as p-aminoclonidine and 5-bromo-6-(2-imidazolin-2-yl-amino-quinoxaline (UK-14304). In vivo studies of [3H]atipamezole demonstrated good penetration into the brain (0.3-1.8% injected dose/g at 5 min, depending upon brain region). In addition, [3H]atipamezole displayed rapid in vivo clearance of nonspecific binding such that at 1 h following an i.v. injection of the drug (100 microCi/animal, rat tail vein administration) the pattern of radioactivity in the brain correlated very well with the receptor distribution as seen by in vitro autoradiography. Atipamezole binding in vivo was displaceable by idazoxan. These results demonstrate the potential of suitably labeled atipamezole for regional studies of brain alpha 2-adrenoceptors in vitro and in vivo, in experimental animals as well as in human positron emission tomography (PET) studies.

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.

Decreased density of presynaptic alpha 2-adrenoceptors in postmortem brains of patients with Alzheimer's disease

The full agonist [3H]bromoxidine (UK 14304) was used to quantitate alpha 2-adrenoceptors in postmortem brains of patients with Alzheimer's disease. The effects of aging and human serum Cohn fraction IV on [3H]bromoxidine binding were also assessed. In patients with Alzheimer's disease, the binding capacity (Bmax) of [3H]bromoxidine was lower in the frontal cortex (37%), hypothalamus (33%), and cerebellum (52%) than in matched controls. In the hippocampus, amygdala, and head of caudate, the binding capacities (Bmax) were unchanged. Quantitative autoradiographic analyses with [3H]bromoxidine confirmed the existence of a marked reduction (55-60%) in alpha 2A-adrenoceptor density in the frontal cortex (layers I and III). In patients with dementia who did not meet neuropathological criteria for Alzheimer's disease, the density of alpha 2-adrenoceptors was unchanged. In control subjects, the density of alpha 2A-adrenoceptors in the frontal cortex showed a significant negative correlation with age at death. The inhibitory effect of human serum Cohn fraction IV on [3H]bromoxidine was very similar in control subjects and patients with Alzheimer's disease. The observed decrease in the density of brain alpha 2-adrenoceptors in Alzheimer's disease may represent direct biochemical evidence of a presynaptic location of this receptor on noradrenergic nerve terminals in the human CNS.

Regionally specific age-dependent decline in alpha 2-adrenoceptors: an autoradiographic study in human brain

The effect of sex, postmortem delay and aging on alpha 2-adrenoceptor binding was studied in tissue sections from several representative regions of the human brain from 21 subjects using [3H]UK-14304 as a ligand. Sex and postmortem delay did not influence the density of alpha 2-receptors. Aging resulted in clear decreases in most forebrain areas examined (n. basalis greater than basal ganglia greater than hypothalamus greater than fronto-temporal cortex greater than hippocampus greater than visual cortex), whereas alpha 2-receptors did not significantly change with age in the amygdala and several infratentorial areas. We conclude that age-related, regionally specific decreases in the density of alpha 2-receptors occur in the human brain. The implications of these findings for age-dependent noradrenergic degeneration are discussed.

Autoradiographic localization of alpha 2-adrenoceptors in chick brain

alpha 2-Adrenoceptors were localized in the chick brain by 'in vitro' receptor autoradiography using [3H]UK 14304 as a ligand. High or very high densities of binding sites were found in the hyperstriatum, tuberculum olfactorium, hypothalamic nuclei, tectum opticum and some medullary nuclei. Comparatively, intermediate densities were observed over the thalamic nuclei and locus ceruleus, among others. Low densities of alpha 2-adrenoceptors were detected in the paleostriatum, hippocampus and cerebellum. Our data indicate that alpha 2-adrenoceptors in the chick brain present similar properties and homologous anatomical distribution to those reported in mammalian brain.