Antonio M. González

Autoradiographic Demonstration of Increased α2‐Adrenoceptor Agonist Binding Sites in the Hippocampus and Frontal Cortex of Depressed Suicide Victims

Abstract: To examine directly in the brain the status of α2‐adrenoceptors in major depression, the specific binding of the agonist [3H]UK 14304 was measured by quantitative receptor autoradiography in the hippocampus and frontal cortex of suicide victims (n = 17) with a retrospective diagnosis of depression (n = 7) or other psychiatric disorders (n = 10) as well as of matched control subjects (n = 9). In suicide victims, a significant increase in the number of α2‐adrenoceptors was found in the CA1 field (40%) and dentate gyrus (20%) of the hippocampus and in the external layers I (33%) and II (31%) of the frontal cortex, compared with that in matched controls. In depressed suicide victims, the increase in α2‐adrenoceptors in the CA1 field (57%) was significantly greater (24%, p < 0.05) than that observed in the group of suicide victims with other diagnoses (26%). In the same depressed suicide victims, the increase in cortical α2‐adrenoceptors was restricted to layer I (34%) and it was equivalent to that found in layer I (33%) of suicide victims with other diagnoses. The results indicate that suicide is associated with increases in the high‐affinity state of brain α2‐adrenoceptors and that there is a pronounced localized increase of this inhibitory receptor in the hippocampus of depressed suicide victims.

Quantitative light microscopic autoradiographic localization of α2-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.

Characterization of [3H]Hemicholinium‐3 Binding Sites in Human Brain Membranes: A Marker for Presynaptic Cholinergic Nerve Terminals

Abstract: We report here on the binding properties of [3H]hemicholinium‐3, a selective inhibitor of the high‐affinity choline uptake process, to human brain membranes. Under the assay conditions described, the binding of [3H]hemicholinium‐3 exhibited a dependency of physiological conditions on pH, temperature, and NaCl concentrations. Striatal binding proved to be specific, to a single site, saturable, and reversible, with an apparent KD of 10 nM and a Bmax of 82 fmol/mg of protein. [3H]Hemicholinium‐3 specific binding exhibited a pharmacological profile and an ionic dependency suggestive of physiologically relevant interactions and comparable with those reported for the high‐affinity choline uptake. Moreover, specific [3H]hemicholinium‐3 binding exhibited an uneven regional distribution: striatum ≫ nucleus basalis > spinal cord ≫ midbrain = cerebellum ≧ hippocampus > neocortex = anterior thalamus > posterior thalamus ⋙ white matter. This distribution closely corresponds to the reported activity of both enzymatic cholinergic presynaptic markers and high‐affinity choline uptake in mammalian brain. There are no significant differences between these results and those previously found in the rat brain using this radioligand. Our results demonstrate, for the first time, the presence of [3H]hemicholinium‐3 binding sites in human brain and strongly support the proposal that this radioligand binds to the carrier site mediating the high‐affinity choline uptake process on cholinergic neurons. Thus, [3H]hemicholinium‐3 binding may be used in postmortem human brain as a selective and quantifiable marker of the presynaptic cholinergic terminals.

Modification of muscarinic acetylcholine receptors in the rat brain following chronic immobilization stress: an autoradiographic study

The modifications of rat brain muscarinic acetylcholine receptors induced by chronic immobilization stress lasting 10 min/daily or 2 h/daily for 3, 7 or 21 days were analyzed by quantitative in vitro autoradiography. [3H]N-Methylscopolamine ([3H]NMS) was used as ligand. Chronic immobilization stress for 10 min/day did not produce any significant change in the properties of [3H]NMS binding sites throughout the rat brain. In contrast, 2 h/day immobilization caused a significant increase in the maximal number of muscarinic receptors (Bmax) in several brain areas such as the cortical layers, the CA1 field of the hippocampus and caudate-putamen, among others. Affinity values (Kd) were not modified. These results suggest that chronic immobilization stress induces supersensitivity of muscarinic receptors in certain cholinergic pathways in rat brain, the pattern of response being different to that previously found for acute stress.

Regionally specific age-dependent decline in α2-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.

Affinity changes in muscarinic acetylcholine receptors in the rat brain following acute immobilization stress: an autoradiographic study

The modifications in rat brain muscarinic acetylcholine receptors induced by acute immobilization stress lasting 10 min or 2 h were analyzed by quantitative in vitro autoradiography. [3H]N-Methylscopolamine ([3H]NMS) was used as a ligand. Immobilization stress for 10 min did not produce any significant change in the properties of [3H]NMS binding sites throughout the brain. In contrast, 2 h immobilization caused a significant increase in receptor affinity (Kd) without modification in the maximal number of receptors (Bmax) in several brain areas such as the caudate-putamen, cortical layers and CA1 field of the hippocampus, among others. These results, found even in animals killed immediately after the end of the immobilization sessions, suggest that immobilization stress induces supersensitivity of muscarinic receptors in certain cholinergic pathways in rat brain.

Autoradiographic distribution of [3H]hemicholinium-3 binding sites in human brain

Since previous radioligand binding studies support the evidence that [3H]hemicholinium-3 ([3H]HC-3) selectively labels the high-affinity choline uptake (HACU) process, we have studied the autoradiographic characteristics and regional distribution of [3H]HC-3 binding to post mortem human brain tissue. [3H]HC-3 specific binding was saturable, of high affinity and exhibited an uneven distribution. High densities were observed in caudate-putamen, nucleus basalis accesorius of the amygdala, hippocampal gyrus dentatus and CA3 field, locus niger, nucleus interpeduncularis and motor trigeminal and facial nuclei. Low densities were found in areas such as neocortex, thalamus, hypothalamus or cerebellum. Our results agree with those obtained in human brain membranes and are comparable to previous autoradiographic data from rat brain. Remarkably, the distribution of [3H]HC-3 binding sites closely corresponds with that of cholinergic enzymatic presynaptic markers and HACU. These findings, together with previous data from membrane studies, allow the use of [3H]HC-3 as a selective anatomical marker of cholinergic presynaptic terminals.

Autoradiographic localization of α2-adrenoceptors in chick brain

Abstract α 2 -Adrenoceptors were localized in the chick brain by ‘in vitro’ receptor autoradiography using [ 3 H]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 α 2 -adrenoceptors were detected in the paleostriatum, hippocampus and cerebellum. Our data indicate that α 2 -adrenoceptors in the chick brain present similar properties and homologous anatomical distribution to those reported in mammalian brain.

Chapter 4 – Multiple Serotonin Receptors in the Human Brain

Publisher Summary This chapter discusses multiple serotonin receptors in the human brain. Serotonin (5-HT, 5-hydroxytryptamine) in the central nervous system can be analyzed in detail with an adequate knowledge of the characteristics of 5-HT receptors. The application of radioligand binding procedures results in the generation of information on 5-HT receptor subtypes. The 5-HT1A, 5-HT1B, and 5-HT1D binding sites are linked to adenylate cyclase systems. The effects mediated by 5-HT1C and 5-HT2 recognition sites involve phosphatidylinositol hydrolysis, and 5-HT3 receptors are linked to ion channels. The quantitative autoradiographical analysis allows the identification of microscopic brain areas, which are highly enriched in subtypes of 5-HT receptor. Receptor autoradiographic helps to understand the pharmacological properties of 5-HT1A and 5-HT1B receptors. 5-HT1c receptors are present at significant densities in the basal ganglia. The globus pallidus contains the highest densities of [3H]mesulergine binding sites. The caudate, putamen, and accumbens are also moderately rich in these sites. The very high densities of 5-HT1C sites over the choroid plexus suggest a role for this receptor in regulating the production and composition of the cerebrospinal fluid. The presence of very high densities of 5-HT2 receptors in the neocortex, especially over the pyramidal cell layers, suggests their involvement in the regulation of many brain functions.

Loss of dopamine uptake sites and dopamine D2 receptors in striatonigral degeneration

To explore the mechanisms underlying L-dopa response, we studied, by postmortem autoradiography, selective makers of dopamine presynaptic terminals, [3H]WIN 35428, and dopamine D2 receptors, [3H]nemonapride, in the putamen of four Parkinsons disease (PD) and one striatonigral degeneration (SND) neuropathologically confirmed brains as compared with six matched control brains. Dopamine uptake transporter was dramatically decreased (> 90%) both in PD and SND striatum. Dopamine D2 receptors were preserved in PD, but clearly reduced (> 76%) in the SND putamen. These data confirm that L-dopa response is closely associated with the preservation of striatal dopamine D2 receptors.