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

Locus Coeruleus and Noradrenergic Pharmacology in Neurodegenerative Disease

Adrenoceptors (ARs) throughout the brain are stimulated by noradrenaline originating mostly from neurons of the locus coeruleus, a brainstem nucleus that is ostensibly the earliest to show detectable pathology in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. The α₁-AR, α₂-AR, and β-AR subtypes expressed in target brain regions and on a range of cell populations define the physiological responses to noradrenaline, which includes activation of cognitive function in addition to modulation of neurometabolism, cerebral blood flow, and neuroinflammation. As these heterocellular functions are critical for maintaining brain homeostasis and neuronal health, combating the loss of noradrenergic tone from locus coeruleus degeneration may therefore be an effective treatment for both cognitive symptoms and disease modification in neurodegenerative indications. Two pharmacologic approaches are receiving attention in recent clinical studies: preserving noradrenaline levels (e.g., via reuptake inhibition) and direct activation of target adrenoceptors. Here, we review the expression and role of adrenoceptors in the brain, the preclinical studies which demonstrate that adrenergic stimulation can support cognitive function and cerebral health by reversing the effects of noradrenaline depletion, and the human data provided by pharmacoepidemiologic analyses and clinical trials which together identify adrenoceptors as promising targets for the treatment of neurodegenerative 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.

From circuits to behavior: Amygdala dysfunction in fragile X syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by a repeat expansion mutation in the promotor region of the FMR1 gene resulting in transcriptional silencing and loss of function of fragile X messenger ribonucleoprotein 1 protein (FMRP). FMRP has a well-defined role in the early development of the brain. Thus, loss of the FMRP has well-known consequences for normal cellular and synaptic development leading to a variety of neuropsychiatric disorders including an increased prevalence of amygdala-based disorders. Despite our detailed understanding of the pathophysiology of FXS, the precise cellular and circuit-level underpinnings of amygdala-based disorders is incompletely understood. In this review, we discuss the development of the amygdala, the role of neuromodulation in the critical period plasticity, and recent advances in our understanding of how synaptic and circuit-level changes in the basolateral amygdala contribute to the behavioral manifestations seen in FXS.

Neurobiology of early life adversity: A systematic review of meta-analyses towards an integrative account of its neurobiological trajectories to mental disorders

Adverse childhood experiences (ACEs) may leave long-lasting neurobiological scars, increasing the risk of developing mental disorders in later life. However, no review has comprehensively integrated existing evidence across the fields: hypothalamic-pituitary-adrenal axis, immune/inflammatory system, neuroimaging, and genetics/epigenetics. We thus systematically reviewed previous meta-analyses towards an integrative account of ACE-related neurobiological alterations. Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline, a total of 27 meta-analyses until October 2021 were identified. This review found that individuals with ACEs possess blunted cortisol response to psychosocial stressors, low-grade inflammation evinced by increased C-reactive protein levels, exaggerated amygdalar response to emotionally negative information, and diminished hippocampal gray matter volume. Importantly, these alterations were consistently observed in those with and without psychiatric diagnosis. These findings were integrated and discussed in a schematic model of ACE-related neurobiological alterations. Future longitudinal research based on multidisciplinary approach is imperative for ACE-related mental disorders' prevention and treatment.

Mapping α2 adrenoceptors of the human brain with 11C-yohimbine

A previous study from this laboratory suggested that (11)C-yohimbine, a selective α2-adrenoceptor antagonist, is an appropriate ligand for PET of α2 adrenoceptors that passes readily from blood to brain tissue in pigs but not in rodents. To test usefulness in humans, we determined blood-brain clearances, volumes of distribution, and receptor availability by means of PET with (11)C-yohimbine in healthy male adults.

METHODS

We recorded the distribution of (11)C-yohimbine with 90-min dynamic PET and sampled arterial blood to measure intact (11)C-yohimbine in plasma. For analysis, we coregistered PET images to individual MR images and automatically identified 27 volumes of interest. We used 1-tissue-compartment graphical analysis with 6 linearized solutions of the fundamental binding equation, with the metabolite-corrected arterial plasma curves as input function, to estimate the kinetic parameters of (11)C-yohimbine. With the lowest steady-state distribution volume (VT), determined in the corpus callosum, we calculated the binding potential (receptor availability) of the radioligand in other regions.

RESULTS

The linear regressions yielded similar estimates of the kinetic parameters. The cortical values of VT ranged from 0.82 mL cm(-3) in the right frontal cortex to 0.46 mL cm(-3) in the corpus callosum, with intermediate VT values in subcortical structures. Binding potentials averaged 0.6-0.8 in the cortex and 0.2-0.5 in subcortical regions.

CONCLUSION

The maps of (11)C-yohimbine binding to α2 adrenoceptors in human brain had the highest values in cortical areas and hippocampus, with moderate values in subcortical structures, as found also in vitro. The results confirm the usefulness of the tracer (11)C-yohimbine for mapping α2 adrenoceptors in human brain in vivo.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

Advances in the pharmacological treatment of Parkinson's disease: targeting neurotransmitter systems

For several decades, the dopamine precursor levodopa has been the primary therapy for Parkinson's disease (PD). However, not all of the motor and non-motor features of PD can be attributed solely to dopaminergic dysfunction. Recent clinical and preclinical advances provide a basis for the identification of additional innovative therapeutic options to improve the management of the disease. Novel pharmacological strategies must be optimized for PD by: (i) targeting disturbances of the serotonergic, noradrenergic, glutamatergic, GABAergic, and cholinergic systems in addition to the dopaminergic system, and (ii) characterizing alterations in the levels of neurotransmitter receptors and transporters that are associated with the various manifestations of the disease.

Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation

The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.

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.

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.

Oxytocin increases the density of high affinity α2-adrenoceptors within the hypothalamus, the amygdala and the nucleus of the solitary tract in ovariectomized rats

Oxytocin induces long-term changes in, for example, blood pressure, spontaneous motor activity and corticosterone levels in rats. Previous studies in male rats have suggested a role for alpha(2)-adrenoceptors within the central nervous system in these effects. The aim of the present study was to investigate if oxytocin treatment in female rats would influence alpha(2)-adrenoceptors within the hypothalamus, the amygdala and the nucleus of the solitary tract (NTS). For this purpose, female ovariectomized (OVX) rats were treated with oxytocin (1 mg/kg s.c.) or saline once a day for 10 days. Rats were decapitated 5 days after the last injection, and brains and plasma were collected. Quantitative receptor autoradiography for characterization of high affinity alpha(2)-adrenoceptor agonist binding and radioimmunoassay for corticosterone were performed. Oxytocin increased the B(max) values of the alpha(2)-adrenoceptor agonist [3H]UK14.304 binding sites significantly in all the analyzed areas (P<0.05). K(d) values were unchanged. Plasma levels of corticosterone were significantly decreased in the oxytocin-treated rats (P<0.05). These findings are in further support of an interaction between oxytocin receptors and alpha(2)-adrenoceptors and show that oxytocin treatment may increase alpha(2)-adrenoceptor recognition probably leading to an increase in alpha(2)-adrenoceptor signaling in several parts of the brain.

Aminergic receptors during the development of the human brain: the contribution of in vitro imaging techniques

The development of the human brain is a complex process and, in this regard, the maturation of neurotransmitter systems and their receptors is of special interest. The study of these systems requires methodological approaches with powerful anatomical resolution. In this paper we review the application of visualization procedures to the fine localization, pattern of appearance and functional relevance of monoaminergic receptors in postmortem human brain samples corresponding to different stages of development (fetal, neonatal, infant). Data obtained by using mostly in vitro autoradiography but also in situ hybridization and, very recently, second messenger labeling, are discussed, including the methodological limitations inherent in working with inmature human tissue. From these studies, several conclusions were made. (1) It is possible to visualize, in the human brain with high resolution, the presence of neuroreceptors at early prenatal stages. (2) The anatomical distribution of monoaminergic receptors in the developing human brain is, in general terms, comparable to that found in the adult. (3) During the developmental process, some receptors, which are early and sometimes transiently expressed, play important thophic roles in the regulation of neuronal development: this is the case with the serotonin 5-HT1A receptors, which attain peak levels of hyperexpression over the hippocampus (dentate gyrus, dendritic areas of CA fields) and the raphe nuclei and show a transient expression in the cerebellum, around the 25 week of gestational age. (4) Different patterns of ontogenetic appearance for human receptors have been identified: dopamine D2-like (caudate, putamen, nigra) and 5-HT1A receptors are good examples of prenatal development, while 5-HT1B sites (basal ganglia, neocortex) present a mainly postnatal pattern of appearance. (5) Neurotransmitter receptors at human fetal stages are already functional from the point of view of transducing response.

Increased alpha 2-adrenergic receptor binding in locus coeruleus projection areas in dementia with Lewy bodies

Clinical studies suggest involvement of brain noradrenergic systems in the pathophysiology of disruptive agitation in Alzheimer's disease (AD). This behavioral problem is even more prevalent in dementia with Lewy bodies (DLB). Here we used receptor autoradiography with [(125)I]para-iodoclonidine to estimate alpha-2 adrenergic receptor (A2R) density in locus coeruleus (LC) projection areas in postmortem brain tissue from age and gender comparable groups of DLB (n = 6), AD (n = 5) and normal (n = 7) subjects. LC neuronal loss was substantial and equivalent in DLB and AD. A2R density was greater in DLB than in normals in the deep layers of the frontal cortex. A2R density was greater in DLB than in AD in hippocampus (CA-1, CA-3 and dentate hilus) and in the granule layer of the cerebellum. Increased A2R binding in DLB is consistent with expression of presynaptic A2R on fibers from surviving LC neurons involved in reinnervation of LC projection areas. These areas develop compensatory noradrenergic hyperinnervation in a rat model of partial LC ablation. It is also consistent with upregulation of post-synaptic A2R in response to loss of LC noradrenergic innervation. Either mechanism could lower the threshold for increased agitation in response to noradrenergic outflow in these dementing disorders.

Distribution of alpha 2-adrenergic receptor binding in the developing human brain stem

Rapid and dramatic changes occur in cardiorespiratory function during early human life. Catecholamines within select brain stem nuclei are implicated in the control of autonomic and respiratory function, including in the nucleus of the solitary tract and the dorsal motor nucleus of X. Animal and adult human studies have shown high binding to alpha 2-adrenergic receptors in these regions. To determine the developmental profile of brainstem alpha 2-adrenergic binding across early human life, we studied brain stems from five fetuses at midgestation, three newborns (37-38 postconceptional weeks), and six infants (44-61 postconceptional weeks). We used quantitative tissue receptor autoradiography with [3H]para-aminoclonidine as the radioligand and phentolamine as the displacer. In the fetal group, binding was high (63-93 fmol/mg tissue) in the nucleus of the solitary tract, dorsal motor nucleus of X, locus coeruleus, and reticular formation; it was low (< 32 fmol/mg tissue) in the principal inferior olive and basis pontis. Binding decreased in all regions with age: in infancy, the highest binding was in the intermediate range (32-62 fmol/mg tissue) and was localized to the nucleus of the solitary tract and dorsal motor nucleus of X. The most substantial decrease in binding (75%-85%) between the fetal and infant periods occurred in the pontine and medullary reticular formation and hypoglossal nucleus. Binding remained low in the principal inferior olive and basis pontis. The decreases in binding with age remained significant after quench correction. These data suggest that rapid and dramatic changes occur in early human life in the brain stem catecholaminergic system in regions related to cardiorespiratory control.

Systemic oxytocin treatment modulates alpha 2-adrenoceptors in telencephalic and diencephalic regions of the rat

Systemic subchronic oxytocin treatment significantly and substantially increased the B(max) values of the alpha 2 agonist [(3)H]UK14.304 binding sites in the hypothalamus, the amygdala and the paraventricular thalamic nucleus of the rat as shown by quantitative receptor autoradiography. These results suggest that long-term modulation of autonomic and neuroendocrine functions and emotional behaviours elicited by brain oxytocin may involve enhancement of central alpha 2-adrenoceptor function.

Effects of yohimbine on cerebral blood flow, symptoms, and physiological functions in humans

OBJECTIVE

Increases in adrenergic activity are associated with stress, anxiety, and other psychiatric, neurological, and medical disorders. To improve understanding of normal CNS adrenergic function, CBF responses to adrenergic stimulation were determined.

METHODS

Using PET, the CBF changes after intravenous yohimbine, an alpha2-adrenoreceptor antagonist that produces adrenergic activation, were compared with placebo in nine healthy humans. Heart rate, blood pressure, Paco2, plasma catecholamines, and symptom responses were also determined.

RESULTS

Among nonscan variables, yohimbine produced significant symptom increases (including a panic attack in one subject), a decrease in Paco2 due to hyperventilation, increases in systolic and diastolic blood pressure, and a trend toward a significant norepinephrine increase. Among scan results, yohimbine produced a significant decrease in whole-brain absolute CBF; regional decreases were greatest in cortical areas. Medial frontal cortex, thalamus, insular cortex, and cerebellum showed significant increases after normalization to whole brain. Medial frontal CBF change was correlated with increases in anxiety. A panic attack produced an increase instead of a decrease in whole-brain CBF. Factors potentially contributing to the observed CBF changes were critically reviewed. Specific regional increases were most likely due in large part to activation produced by adrenergically induced anxiety and visceral symptoms.

CONCLUSIONS

This study supports the relationship of anxiety and interoceptive processes with medial frontal, insular, and thalamic activation and provides a baseline for comparison of normal yohimbine-induced CNS adrenergic activation, adrenergically-based symptoms, and other markers of adrenergic function to stress, emotion, and the adrenergic pathophysiologies of various CNS-related disorders.

Autoradiography of receptor-activated G-proteins in post mortem human brain

The agonist-stimulated guanosine 5'-(gamma-[(35)S]thio)triphosphate binding assay was used to anatomically localize receptor-activated G-proteins by autoradiography in post mortem human brain. The optimal conditions for guanosine 5'-(gamma-[(35)S]thio)triphosphate binding to human brain sections were established in post mortem samples of the prefrontal cortex, hippocampus, basal ganglia, brainstem and cerebellar cortex. An excess of GDP (2mM) was required to decrease basal activity and obtain effective stimulation by specific agonists. guanosine 5'-(gamma-[(35)S]Thio)triphosphate binding was increased after stimulation with specific agonists of different G-protein-coupled receptors. They include cannabinoid (WIN55212-2), mu-opioid ([D-Ala(2),N-Me-Phe(4), Gly(5)-ol]enkephalin), serotonin-1A [(+/-)-8-hydroxy-2-(di-n-propylamino)tetralin] and serotonin-1B/1D (sumatriptan), cholinergic muscarinic receptors (carbachol) and alpha(2)-adrenoceptors (UK14304). Such stimulation reached 1458%, 440%, 188%, 219%, 61% and 339%, respectively, over the basal levels. In tissue sections, the use of the above-mentioned agonists (10(-4)M) showed patterns of anatomical distribution similar to those already described by receptor autoradiography, with high densities over the hippocampus (serotonin-1A receptors), cortex (alpha(2)-adrenoceptors) and striatum (mu-opioid receptors). The highest binding levels were reached with the cannabinoid receptor agonist in most of the analysed brain regions. Carbachol produced only moderate stimulation of those same regions. The blockage of agonist-stimulated guanosine 5'-(gamma-[(35)S]thio)triphosphate binding by selective antagonists verified that the effect was receptor mediated. This technique provides a method to identify modifications of the receptor-mediated activation of G-proteins in post mortem human brain with anatomical resolution. It also provides valuable information on the level of drug efficacy in the human species.

Oxytocin/alpha(2)-Adrenoceptor interactions in feeding responses

The modulation of alpha(2)-adrenoceptor-induced food intake by oxytocin has been evaluated in studies on food intake and by quantitative receptor autoradiography in the hypothalamus and the amygdala of the rat. The effects of lateral intracerebroventricular administration of clonidine and oxytocin were evaluated on food intake in satiated animals. Food consumption was measured at 30, 90, 240 min and 22 h (1,320 min) after injection. The coinjection of oxytocin and clonidine was found to counteract the increase in food intake produced by clonidine (p < 0.001) in satiated rats. Receptor autoradiographic experiments showed that oxytocin significantly increased the K(d) values of [(3)H]p-aminoclonidine alpha(2)-agonist-binding sites in the hypothalamus. Effective oxytocin concentrations ranged between 0.3 and 1 nM (p < 0.05) with a maximal action of 250% at 1 nM. The B(max) value was significantly increased (p < 0.05) for all concentrations of oxytocin. In the amygdala, oxytocin also increased both the K(d) of [(3)H]p-aminoclonidine-binding sites by about 190% at 1 nM and the B(max) values at 1 and 3 nM (p < 0.05). Oxytocin (1 nM) also significantly and substantially (p < 0.01) increased the K(d) and B(max) values of the [(3)H]UK 14.304 alpha(2)-agonist-binding sites in the hypothalamus and amygdala in agreement with the results obtained with the other agonist of the alpha(2)-adrenoceptor [(3)H]p-aminoclonidine. This effect was partially blocked by the presence of the specific oxytocin receptor antagonist, CAP. These findings suggest the existence of an antagonistic oxytocin/alpha(2)-receptor interaction in the hypothalamus and amygdala that may be of relevance for the demonstrated modulation of alpha(2)-adrenoceptor-induced feeding responses by oxytocin.

Effects of intrathecal alpha1- and alpha2-noradrenergic agonists and norepinephrine on locomotion in chronic spinal cats

Noradrenergic drugs, acting on alpha adrenoceptors, have been found to play an important role in the initiation and modulation of locomotor pattern in adult cats after spinal cord transection. There are at least two subtypes of alpha adrenoceptors, alpha1 and alpha2 adrenoceptors. The aim of this study was to investigate the effects of selective alpha1 and alpha2 agonists in the initiation and modulation of locomotion in adult chronic cats in the early and late stages after complete transection at T13. Five cats, chronically implanted with an intrathecal cannula and electromyographic (EMG) electrodes were used in this study. Noradrenergic drugs including alpha2 agonists (clonidine, tizanidine, and oxymetazoline) and an antagonist, yohimbine, one alpha1 agonist (methoxamine), and a blocker, prazosin, as well as norepinephrine were injected intrathecally. EMG activity synchronized to video images of the hindlimbs were recorded before and after each drug injection. The results show differential effects of alpha1 and alpha2 agonists in the initiation of locomotion in early spinal cats (i.e., in the first week or so when there is no spontaneous locomotion) and in the modulation of locomotion and cutaneous reflexes in the late-spinal cats (i.e., when cats have recovered spontaneous locomotion). In early spinal cats, all three alpha2 agonists were found to initiate locomotion, although their action had a different time course. The alpha1 agonist methoxamine induced bouts of nice locomotor activity in three spinal cats some hours after injection but only induced sustained locomotion in one cat in which the effects were blocked by the alpha1 antagonist prazosin. In late spinal cats, although alpha2 agonists markedly increased the cycle duration and flexor muscle burst duration and decreased the weight support or extensor activity (effects blocked by an alpha2 antagonist, yohimbine), alpha1 agonist increased the weight support and primarily the extensor activity of the hindlimbs without markedly changing the timing of the step cycle. Although alpha2 agonists, especially clonidine, markedly reduced the cutaneous excitability and augmented the foot drag, the alpha1 agonist was found to increase the cutaneous reflex excitability. This is in line with previously reported differential effects of activation of the two receptors on motoneuron excitability and reflex transmission. Noradrenaline, the neurotransmitter itself, increased the cycle duration and at the same time retained the cutaneous excitability, thus exerting both alpha1 and alpha2 effects. This work therefore suggests that different subclasses of noradrenergic drugs could be used to more specifically target aspects of locomotor deficits in patients after spinal injury or diseases.

Aortic barodenervation up-regulates alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla: an autoradiographic study

Earlier findings have shown that alpha2-adrenoceptors in the nucleus tractus solitarius and rostral ventrolateral medulla modulate baroreflexes. The present study investigated whether attenuation of baroreflexes induced by surgical interruption of aortic baroafferents is related to an alteration of alpha2-adrenoceptor binding in these regions of the brainstem. In vitro autoradiography was utilized to assess the density and binding dissociation constant (affinity) of alpha2-adrenoceptors in the rostral ventrolateral medulla and in the middle and rostral portions of the nucleus tractus solitarius of aortic-barodenervated and sham-operated rats. Compared to sham operation, aortic barodenervation caused an acute rise in mean arterial pressure and heart rate and a significant reduction in baroreflex sensitivity. Two days later, mean arterial pressure and heart rate of conscious aortic-barodenervated rats subsided to sham-operated levels, whereas the baroreflex sensitivity remained significantly (P<0.05) reduced when measured by phenylephrine (0.55+/-0.08 vs 1.26+/-0.07 ms/mmHg) or nitroprusside (0.43+/-0.06 vs 1.01+/-0.09ms/mmHg). Examination of brainstem coronal sections obtained from separate groups of rats 48 h after surgery and preincubated with [3H]rauwolscine (0.5-16 nM) revealed that labeling of alpha2 binding sites was saturable and of high affinity. Scatchard analysis of the saturation isotherms obtained from the three brain areas of sham-operated rats showed an uneven distribution of alpha2 binding sites; the rostral nucleus tractus solitarius exhibited the highest density and lowest affinity. Aortic barodenervation caused region-dependent changes in the binding activity of alpha2-adrenoceptors. These changes comprised significant (P<0.05) increases in the density of alpha2-adrenoceptors in the middle nucleus tractus solitarius (436+/-60 vs 240+/-50 fmol/mg protein) and rostral ventrolateral medulla (350+/-67 vs 194+/-35 fmol/mg protein) compared with sham-operated rats; no significant changes occurred in the rostral nucleus tractus solitarius. The affinity of alpha2 binding sites was not changed by aortic barodenervation in any of the three brain regions. These findings suggest that attenuation of baroreflexes produced by aortic barodenervation coincides with up-regulation of alpha2-adrenoceptors in brainstem areas that play critical roles in the control of cardiovascular functions.

The neural correlates of the noradrenergic modulation of human attention, arousal and learning

The prefrontal cortex has been suggested as a site of action for the noradrenergic modulation of cognition. In healthy volunteers attentional deficits can be induced by the alpha 2 adrenoceptor agonist clonidine, without impairment of more explicit tests of frontal lobe function. It is therefore possible that the effects of noradrenaline cannot be localized to a specific brain area such as the prefrontal cortex, but instead involve structures in a more widespread attentional network. A 1.5 micrograms/kg dose of clonidine or placebo was administered to 13 healthy male volunteers performing the rapid visual information processing task, which places demands on both sustained attention and working memory. Twelve positron emission tomography measurements of regional cerebral blood flow (rCBF) were collected during performance of this task and also during a rest state. A second experiment in 12 healthy volunteers examined the effects of a 1.3 micrograms/kg dose of clonidine on the rCBF changes associated with performance of a paired associates learning task compared with passive listening to word pairs. Comparison of each of the experimental tasks with its respective control replicated previous findings. A significant drug x task interaction, common to the two studies, was found in the right thalamus. Inspection of the adjusted rCBF values showed that the effect was due to attenuation of thalamic rCBF during the control states rather than to any effects of clonidine during performance of the cognitive tasks, although the effect was stronger in the rapid visual information processing study than in the paired associates learning study. The significant effect of clonidine during the control as opposed to the "cognitive' activation state is consistent with previous findings in animals and humans demonstrating greater effects of clonidine during states of relatively low arousal. The results suggest neuroanatomical dissociation of the noradrenergic modulation of arousal (via the thalamus) and attention.

Evaluation in rat of RS-79948-197 as a potential PET ligand for central alpha 2-adrenoceptors

Tritium-labelled RS-79948-197 {(8aR,12aS,13aS)-5, 8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphon yl)-6H-iso- quino[2,1-g][1,6]naphthyridine} was evaluated in rat brain as an in vivo ligand for central alpha 2-adrenoceptors, as a preliminary step in the development of a radioligand for positron-emission tomography (PET) studies. The maximal receptor-specific signal was achieved within 90-120 min after i.v. injection of [ethyl-3H]RS-79948-197 and was selective for the alpha 2- compared with the alpha 1-adrenoceptor, with no detectable binding to the imidazoline-I2 site. Estimates for binding potential (approximating to Bmax/Kd) ranged between 3.4 in entorhinal cortex and 0.5 in medulla oblongata. The results, which indicate a similarly localised but 2-fold increase in specific binding compared with that previously demonstrated using [3H]RX 821002 (2-methoxy-idazoxan), are sufficiently encouraging as to support further investment in the development of 11C-labelled RS-79948-197, or a close structural analogue, as a ligand for clinical PET.

Laminar distribution of the multiple opioid receptors in the human cerebral cortex

Quantitative autoradiographic assessment of cerebral cortical laminar distribution of mu, delta and kappa opioid receptors was carried out in coronal sections of five post-mortem human brains obtained at autopsy. The cortical areas studied were: cingulate, frontal, insular, parietal, parahippocampal, temporal, occipitotemporal, occipital and striate area. In general, the laminar patterns of distribution for the three types of receptors are distinctive. Peak levels of delta opioid binding are in laminae I, II, and IIIa. mu-Receptors are located in lamina III followed by I and II in cingulate, frontal, insular and parietal cortices and lamina IV in temporal and occiptotemporal cortices. kappa-Receptors are found concentrated in laminae V and VI. The patterns of opioid binding in cortical laminae showed remarkable consistency in all five brains examined. In contrast to other cortical areas, the parahippocampal gyrus, at the level of the amygdaloid formation, demonstrated peak kappa receptor density in laminae I, II and III. mu-Opioid binding was undetectable in the lateral occipital cortex and in the striate area.

Alpha 2-adrenoceptor subtypes in the human brain: a pharmacological delineation of [3H]RX-821002 binding to membranes and tissue sections

In order to study the characterization and localization of [3H]RX-821002 (2-methoxy-idazoxan) binding to alpha 2-adrenoceptor subtypes in several regions of the human brain, we have carried out competition studies using both autoradiography and membrane binding assays. The alpha 2A-adrenoceptor subtype was found to be predominant in the different layers of the frontal cortex, cerebellum and hippocampal formation, while in the neostriatum it was the non-alpha 2A- (alpha 2B- and alpha 2C-) adrenoceptor subtype. In the frontal cortex, in addition to binding to the alpha 2A-adrenoceptor subtype, [3H]RX-821002 bound also to a small portion of alpha 2B- and alpha 2C-adrenoceptors in layer III, and to an unidentified binding site in the external layers. In the hippocampus, both alpha 2A- and non-alpha 2A- (alpha 2B- and alpha 2C-) adrenoceptors were labelled in the dentate gyrus and the CA1 field, together with 5-HT1A receptors. 5-HT1A receptors were labelled predominantly in the stratum pyramidale layer. These results, in addition to delineate the relative presence of alpha 2-adrenoceptor subtypes, indicate that caution is needed when analyzing RX 821002 binding to human brain tissue.

Regional brain glucose metabolism after acute alpha 2-blockade by idazoxan

BACKGROUND

Several classes of antidepressant drugs act on alpha 2-adrenergic receptors. Studies of patients with disorders responsive to treatment with these drugs report group differences in ex vivo measures of alpha 2-binding and in vivo responses mediated by alpha 2-receptors. Measurement of regional brain metabolic response to an alpha 2-antagonist may be a useful method for further definition of the role alpha 2-receptor regulation plays in the treatment of neuropsychiatric disorders.

METHODS

Regional brain glucose metabolism was measured before and after infusion with 200 micrograms/kg idazoxan with use of 18F-fluoro-2-deoxyglucose positron emission tomography in 13 healthy men. Arterial drug concentration, behavioral responses, and cardiovascular responses were also measured.

RESULTS

The absolute and normalized glucose metabolic rate significantly increased in primary visual cortex. Significant increases and decreases occurred in normalized metabolic rates in prefrontal cortical regions. Measurement of metabolic effects occurred during the peak cardiovascular response.

CONCLUSIONS

Our findings are consistent with regionally specific effects of alpha 2-blockade. This method may be useful for the study of alpha 2-receptor function in humans.

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.

Imidazoline receptors, non-adrenergic idazoxan binding sites and alpha 2-adrenoceptors in the human central nervous system

Both [3H]clonidine and [3H]idazoxan bind to alpha 2 adrenoceptors. The former also labels imidazoline receptors, and the latter non-adrenergic idazoxan binding sites. In order to investigate whether the imidazoline receptors and non-adrenergic idazoxan binding sites are identical, we compared the binding characteristics of [3H]clonidine and [3H]idazoxan to these sites by radioligand binding experiments on ultra-thin slices and homogenates of human striatum. A good correlation was found between the effect of different ions on the binding characteristics of [3H]clonidine and [3H]idazoxan, and the affinities of most competing drugs. However, clonidine and rilmenidine displayed a 100- and 10-fold lower affinity, respectively, for the idazoxan binding sites than for the imidazoline receptors. Autoradiography with [3H]clonidine showed that high densities of imidazoline receptors were present in the striatum, pallidum, gyrus dentatus of the hippocampus, amygdala, and substantia nigra. Moderate densities were found throughout the cerebral cortex, thalamus and several brainstem nuclei including the nucleus olivarius inferior. Low densities were seen in the cerebellum, spinal cord and pituitary gland. As for the non-adrenergic sites labelled by [3H]idazoxan, the imidazoline receptors can be found in all major brain areas examined. However, there are some striking differences between the concentrations of imidazoline receptors and non-adrenergic idazoxan binding sites in certain brain regions. To reconcile distribution and pharmacologic data, we propose that imidazoline receptors and non-adrenergic idazoxan binding sites represent different proteins or protein complexes and that at least in the nucleus reticularis lateralis and the striatum, imidazoline receptors and non-adrenergic idazoxan binding sites may be physically associated. The regional distribution of alpha 2 adrenoceptors within the human CNS was determined by quantitative autoradiography with [3H]RX821002. The highest densities of alpha 2 adrenoceptors were found in the cerebral and cerebellar cortex, and certain regions in the medulla oblongata (floor of the IV ventricle, reticular formation, hypoglossal nucleus and nucleus olivarius inferior). No alpha 2 adrenoceptors were detected in the pituitary gland. There exists no relationship between the distribution pattern of imidazoline receptors and alpha 2 adrenoceptors, indicating that these binding sites are independent from each other.

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.

Noradrenergic modulation of gamma-aminobutyric acid outflow from the human cerebral cortex

The noradrenergic modulation of endogenous gamma-aminobutyric acid (GABA) outflow from slices and synaptosomes prepared from human cerebral cortex biopsies has been studied. GABA outflow was responsive to depolarizing stimuli such as ouabain and high potassium. Basal GABA outflow in slices, but not in synaptosomes, appeared to be largely dependent upon neuronal activity, being prevented by tetrodotoxin (TTX). 10 mM K(+)-evoked outflow in synaptosomes also proved to be TTX sensitive. Norepinephrine (NE) concentration dependently increased basal GABA outflow both in slices and synaptosomes. This effect was alpha 1-adrenoreceptor-mediated because it was prevented by a selective antagonist of the alpha 1-adrenoreceptor class (prazosin) but not by the alpha 2 antagonist idazoxan. However, an alpha 2-mediated inhibitory modulation was also present in the preparations used, since (1) in slices, NE significantly inhibited GABA outflow in the presence of prazosin; (2) in synaptosomes, NE significantly inhibited 10 mM K(+)-evoked outflow in the presence of prazosin. Both of these effects were prevented by idazoxan. No beta-adrenoreceptor modulation could be demonstrated. A comparison between species was also conducted. The response to ouabain and to TTX proved similar in human, rat and guinea-pig cerebral cortex. In the most simple tissue preparation used (synaptosomes), a close similarity between the three species could be observed. In all species, NE stimulated basal GABA outflow, an effect prevented by prazosin. This suggests a predominant alpha 1-adrenoreceptor-mediated stimulatory effect. In a more complex preparation (slices), differences between species could be demonstrated.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo hypothalamic release and synthesis of catecholamines in spontaneously hypertensive rats

Juvenile spontaneously hypertensive rats (SHR) have higher plasma levels of catechols and markedly larger catechol responses to yohimbine than do normotensive Wistar-Kyoto rats, indicating increased sympathoadrenal outflow and increased alpha 2-adrenergic receptor-mediated restraint of peripheral catecholamine release during hypertension development in SHR. Yohimbine-induced catecholamine release and metabolism in the posterolateral hypothalamus of the brain were assessed in juvenile (6 to 7 weeks) and adult (15 to 16 weeks) SHR and Wistar-Kyoto rats. In vivo microdialysis was used to obtain samples for measurements of norepinephrine, dihydroxyphenylglycol, methoxyhydroxyphenylglycol, and dihydroxyphenylacetic acid in conscious animals before and after yohimbine injection (1 mg/kg IV) beginning 24 hours after probe implantation. Catecholamine synthesis was examined from elevations of 3,4-dihydroxyphenylalanine levels after probe perfusion with NSD-1015, an inhibitor of L-aromatic acid decarboxylase. In adults, SHR had higher dialysate norepinephrine (277 +/- 38 versus 181 +/- 35 pg/mL), dihydroxyphenylglycol (3260 +/- 509 versus 2231 +/- 201 pg/mL), methoxyhydroxyphenylglycol (2659 +/- 369 versus 1890 +/- 144 pg/mL), and dihydroxyphenylacetic acid (46,312 +/- 5512 versus 13,187 +/- 1963 pg/mL) levels and markedly larger increases in 3,4-dihydroxyphenylalanine levels after NSD-1015 than Wistar-Kyoto rats. In juveniles, SHR had larger proportionate increments in microdialysate norepinephrine levels after yohimbine than Wistar-Kyoto rats (85% versus 25%). Although juvenile SHR and Wistar-Kyoto rats had similar NSD-1015-elicited increments in 3,4-dihydroxyphenylalanine levels, systemic yohimbine enhanced the NSD-1015-elicited 3,4-dihydroxyphenylalanine elevations in juvenile SHR but not in Wistar-Kyoto rats. These findings suggest augmented norepinephrine release and catecholamine synthesis in the posterolateral hypothalamus of adult SHR and augmented alpha 2-adrenergic receptor restraint of both norepinephrine release and catecholamine synthesis in juvenile SHR.

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.