Acute and chronic effects of citalopram on 5‐HT1A receptor—Labeling by [18F]MPPF and—Coupling to receptors‐G proteins

Selective serotonin reuptake inhibitors take several weeks to produce their maximal therapeutic antidepressant effect. This delay has been attributed to the gradual desensitization of somatodendritic serotonin 5‐HT_1A autoreceptors. We evaluated adaptive changes of 5‐HT_1A receptors after acute and chronic citalopram challenges in rat. Small animal positron emission tomography trial and quantitative ex vivo autoradiography studies using [¹⁸F]MPPF were employed, as well as in vitro 8‐OH‐DPAT‐stimulated [³⁵S]‐GTPγS binding assay. Additionally, 5‐HT_1A receptor knock‐out mice were used to assess the specificity of [¹⁸F]MPPF. Acute treatment with citalopram did not alter [¹⁸F]MPPF binding in dorsal raphe nucleus (DR), frontal cortex, or hippocampus. The absence of [¹⁸F]MPPF binding in the brain of 5‐HT_1A knock‐out mice demonstrates the specificity of MPPF for 5‐HT_1A receptor brain imaging, but the high affinity of [¹⁸F]MPPF compared to 5‐HT suggests that it would only be displaced by dramatic increases in extracellular 5‐HT. Chronic citalopram did not modify 5‐HT_1A receptor density in any of the brain regions studied. In addition, this treatment did not modify 8‐OH‐DPAT‐stimulated [³⁵S]‐GTPγS binding in DR, although a significant increase was observed in frontal cortex and hippocampus. [¹⁸F]MPPF appears to be an efficient radioligand to quantify specifically 5‐HT_1A receptor density in brain imaging. The delayed therapeutic efficacy of citalopram did not appear to be linked to either a downregulation of 5‐HT_1A receptors or to a 5‐HT_1A receptor‐G protein decoupling process in serotonergic neurons, but to increased functional sensitivity of postsynaptic 5‐HT_1A receptors. Synapse 63:106–116, 2009. ©2008 Wiley‐Liss, Inc.     

Characterization of Glucocorticoid Type II Receptors in Neuronal and Glial Cultures from Rat Brain

The purpose of this study was to characterize and compare the properties of glucocorticoid Type II receptors in neuronal and astrocyte glial cultures prepared from rat brain. Type II receptors in cytosol prepared from cultured cells were labeled with [³ H]dexamethasone (DEX) at 0°C. The binding was saturable and specific, with a complete displacement by unlabeled DEX or RU 28362 (a pure glucocorticoid). Scatchard analysis of [³ H]DEX binding suggested a single class of receptors with a slightly lower dissociation constant (K_d) in neuronal (1.13 nM) versus astrocyte glial (1.64 nM) cytosol. The number of binding sites (B_max) in astrocyte glial cultures was four times that in neuronal cultures on a per milligram protein basis (120.3 versus 29.3 fmol/mg protein). The presence of Type II receptors in cultured neurons and astrocyte glia was further confirmed by immunofluorescent staining with a monoclonal antibody against this receptor (BuGR-2). The steroid specificity of Type II receptors was studied by examining the displacement of [³ H]DEX binding to cytosol with unlabeled steroids. For both types of cultures, the potency series for competition was RU 28362> DEX> corticosterone> > aldosterone. Switching cultured cells from serum-supplemented to serum-free medium reduced [³ H]DEX binding at low concentrations (0.5 to 5 nM) of the ligand in both types of culture, thus resulting in a decrease in the apparent affinity. This treatment did not, however, have any significant effect on the total number of binding sites. In summary, these results demonstrate that both neuronal and astrocyte glial cells in culture contain specific glucocorticoid Type II receptors, which resemble those seen in the brain and peripheral tissues.     

Brain 5-HT(2A) receptors in MPTP monkeys and levodopa-induced dyskinesias

Levodopa‐induced dyskinesias (LIDs) are abnormal involuntary movements induced by the chronic use of levodopa (l ‐Dopa) limiting the quality of life of Parkinson’s disease (PD) patients. We evaluated changes of the serotonin 5‐HT_2A receptors in control monkeys, in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐lesioned monkeys and in l ‐Dopa‐treated MPTP monkeys, without or with adjunct treatments to inhibit the expression of LID: CI‐1041, a selective NR1A/2B subunit antagonist of glutamate N‐methyl‐d ‐aspartic acid (NMDA) receptor, or Cabergoline, a long‐acting dopamine D₂ receptor agonist. All treatments were administered for 1 month and animals were killed 24 h after the last dose of l ‐Dopa. Striatal concentrations of serotonin were decreased in all MPTP monkeys investigated, as measured by high‐performance liquid chromatography. [³H]Ketanserin‐specific binding to 5‐HT_2A receptors was measured by autoradiography. l ‐Dopa treatment that induced dyskinesias increased 5‐HT_2A receptor‐specific binding in the caudate nucleus and the anterior cingulate gyrus (AcgG) compared with control monkeys. Moreover, [³H]Ketanserin‐specific binding was increased in the dorsomedial caudate nucleus in l ‐Dopa‐treated MPTP monkeys compared with saline‐treated MPTP monkeys. Nondyskinetic monkeys treated with CI‐1041 or Cabergoline showed low 5‐HT_2A‐specific binding in the posterior dorsomedial caudate nucleus and the anterior AcgG compared with dyskinetic monkeys. No significant difference in 5‐HT_2A receptor binding was observed in any brain regions examined in saline‐treated MPTP monkeys compared with control monkeys. These results confirm the involvement of serotonergic pathways and the glutamate/serotonin interactions in LID. They also support targeting 5‐HT_2A receptors as a potential treatment for LID.     

Hippocampal serotonin‐1A receptor function in a mouse model of anxiety induced by long‐term voluntary wheel running

We have recently demonstrated that, in C57/Bl6 mice, long‐term voluntary wheel running is anxiogenic, and focal hippocampal irradiation prevents the increase in anxiety‐like behaviors and neurobiological changes in the hippocampus induced by wheel running. Evidence supports a role of hippocampal 5‐HT_1A receptors in anxiety. Therefore, we investigated hippocampal binding and function of 5‐HT_1A receptors in this mouse model of anxiety. Four weeks of voluntary wheel running resulted in hippocampal subregion‐specific changes in 5‐HT_1A receptor binding sites and function, as measured by autoradiography of [³H] 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin binding and agonist‐stimulated binding of [³⁵S]GTPγS to G proteins, respectively. In the dorsal CA1 region, 5‐HT_1A receptor binding and function were not altered by wheel running or irradiation. In the dorsal dentate gyrus and CA2/3 region, 5‐HT_1A receptor function was decreased by not only running but also irradiation. In the ventral pyramidal layer, wheel running resulted in a decrease of 5‐HT_1A receptor function, which was prevented by irradiation. Neither irradiation nor wheel running affected 5‐HT_1A receptors in medial prefrontal cortex or in the dorsal or median raphe nuclei. Our data indicate that downregulation of 5‐HT_1A receptor function in ventral pyramidal layer may play a role in anxiety‐like behavior induced by wheel running. Synapse 67:648–655, 2013. © 2013 Wiley Periodicals, Inc.     

Translational possibility of [18F]Mefway to image serotonin 1A receptors in humans: Comparison with [18F]FCWAY in rodents

Purpose: To compare the cerebral uptake and binding potential of [¹⁸F]FCWAY and [¹⁸F]Mefway in the rodent to assess their potential for imaging serotonin 1A (5‐HT_1A) receptors. Materials and Methods: In vitro liver microsomal studies were performed to evaluate the degree of defluorination. Dynamic positron emission tomography (PET) studies were then conducted for 2 h with or without an anti‐defluorination agent. The regions of interest were the hippocampus and frontal cortex (5‐HT_1A target regions) and the cerebellum (5‐HT_1A nontarget region). The in vivo kinetics of the radioligands were compared based on the brain uptake values and target‐to‐nontarget ratio. We also performed a comparison of binding potential (BP_ND) as a steady‐state binding parameter. Finally, binding affinities to 5‐HT_1A receptors were assessed in Chinese hamster ovary cells (CHO‐K1) cells expressing human recombinant 5‐HT_1A receptors. Results: The radiochemical yield of [¹⁸F]Mefway was slightly higher than that of [¹⁸F]FCWAY (19 vs. 15%). With regard to metabolic stability against defluorination, both compounds exhibited similar stability in rat liver microsomes, but [¹⁸F]Mefway displayed higher stability in the human microsome (defluorination ratio at 30 min: 32 vs. 29 in rat liver microsomes, 31 vs. 64 in human liver microsomes for [¹⁸F]Mefway and [¹⁸F]FCWAY, respectively). There were no significant differences in brain uptake, the target‐to‐nontarget ratios, and the BP_ND (at hippocampus, peak brain uptakes: 6.9 vs. 8.5, target‐to‐nontarget ratios: 6.9 vs. 8.5, BP_ND: 5.2 vs. 6.2 for [¹⁸F]Mefway and [¹⁸F]FCWAY). The binding affinity of [¹⁸F]Mefway was considerably higher than that of [¹⁸F]FCWAY (IC₅₀ : 1.5 nM vs. 2.2 nM). Conclusion: [¹⁸F]Mefway exhibits favorable characteristics compared to [¹⁸F]FCWAY in rodents, and may be a promising radioligand for use in human subjects. Synapse 68:595–603, 2014 . © 2014 Wiley Periodicals, Inc.     

An immunocapture/scintillation proximity analysis of Gαq/11 activation by native serotonin (5‐HT)2A receptors in rat cortex: Blockade by clozapine and mirtazapine

Though transduction mechanisms recruited by heterologously expressed 5‐HT_2A receptors have been extensively studied, their interaction with specific subtypes of G‐protein remains to be directly evaluated in cerebral tissue. Herein, as shown by an immunocapture/scintillation proximity analysis, 5‐HT, the prototypical 5‐HT_2A agonist, DOI, and Ro60,0175 all enhanced [³⁵S]GTPγS binding to Gαq/11 in rat cortex with pEC₅₀ values of 6.22, 7.24 and 6.35, respectively. No activation of Go or Gs/olf was seen at equivalent concentrations of DOI. Stimulation of Gαq/11 by 5‐HT (30 μM) and DOI (30 μM) was abolished by the selective 5‐HT_2A vs. 5‐HT_2C/5‐HT_2B antagonists, ketanserin (pK_B values of 9.11 and 8.88, respectively) and MDL100,907 (9.82 and 9.68). By contrast, 5‐HT‐induced [³⁵S]GTPγS binding to Gαq/11 was only weakly inhibited by the preferential 5‐HT_2C receptor antagonists, RS102,221 (6.94) and SB242,084 (7.39), and the preferential 5‐HT_2B receptor antagonist, LY266,097 (6.66). The antipsychotic, clozapine, which had marked affinity for 5‐HT_2A receptors, blocked the recruitment of Gαq/11 by 5‐HT and DOI with pK_B values of 8.54 and 8.14, respectively. Its actions were mimicked by the “atypical” antidepressant and 5‐HT_2A receptor antagonist, mirtazapine, which likewise blocked 5‐HT and DOI‐induced Gαq/11 protein activation with pK_B values of 7.90 and 7.76, respectively. In conclusion, by use of an immunocapture/scintillation proximity strategy, this study shows that native 5‐HT_2A receptors in rat frontal cortex specifically recruit Gαq/11 and that this action is blocked by clozapine and mirtazapine. Quantification of 5‐HT_2A receptor‐mediated Gαq/11 activation in frontal cortex should prove instructive in characterizing the actions of diverse classes of psychotropic agent. Synapse 63:95–105, 2009. © 2008 Wiley‐Liss, Inc.     

Comparative assessment of 18F‐Mefway as a serotonin 5‐HT1A receptor PET imaging agent across species: Rodents,nonhuman primates,and humans

We have developed ¹⁸F‐trans‐Mefway (¹⁸F‐Mefway) for positron emission tomography (PET) imaging studies of serotonin 5‐HT_1A receptors which are implicated in various brain functions. Translation of imaging the 5‐HT_1A receptor in animal models to humans will facilitate an understanding of the role of the receptor in human brain disorders. We report comparative brain distribution of ¹⁸F‐Mefway in normal mice, rats, monkeys, and healthy human volunteers. Mefway was found to be very selective, with subnanomolar affinity for the 5‐HT_1A receptor. Affinities of >55 nM were found for all other human‐cloned receptor subtypes tested. Mefway was found to be a poor substrate (>30 μM) for the multidrug resistance 1 protein, suggesting low likelihood of brain uptake being affected by P‐glycoprotein. Cerebellum was used as a reference region in all imaging studies across all species due to the low levels of ¹⁸F‐Mefway binding. Consistent binding of ¹⁸F‐Mefway in cortical regions, hippocampus, and raphe was observed across all species. ¹⁸F‐Mefway in the human brain regions correlated with the known postmortem distribution of 5‐HT_1A receptors. Quantitation of raphe was affected by the resolution of the PET scanners in rodents, whereas monkeys and humans showed a raphe to cerebellum ratio of approximately 3. ¹⁸F‐Mefway appears to be an effective 5‐HT_1A receptor imaging agent in all models, including humans. ¹⁸F‐Mefway therefore may be used to quantify 5‐HT_1A receptor distribution in brain regions for the study of various CNS disorders. J. Comp. Neurol. 524:1457–1471, 2016. © 2015 Wiley Periodicals, Inc.     

Acute social defeat does not alter cerebral 5‐HT2A receptor binding in male Wistar rats

It has been hypothesized that effects of uncontrollable stress on serotonin receptor expression contribute to the etiology of stress‐related disorders like depression. While the serotonin‐2A receptors (5‐HT_2AR) are thought to be important in this context, only few studies examined effects of stress on this receptor subtype. In this study, we therefore assessed acute and long‐term changes in 5HT_2AR binding after social defeat stress in rats. Male Wistar rats were subjected to social defeat by placing them in the home cage of an aggressive, dominant Long Evans rat. Acute social defeat suppressed growth, but did not affect anxiety‐like behavior in an open field test. A positron emission tomography scan with the 5‐HT_2AR tracer [¹¹C]MDL 100907 1 day and 3 weeks after defeat did not show significant changes in receptor binding. To verify these results, [³H]MDL 100907 binding assays were performed in homogenates of prefrontal cortex and hippocampus, which also did not indicate any changes in B_max or K_d. These findings do not support the hypothesis that changes in 5‐HT_2AR function are a vital mechanism through which uncontrollable stress contributes to stress‐related pathologies such as depression. It remains to be determined whether effects of stress on 5HT_2AR binding depend on the nature of the stressor or on the characteristics of the rat strain. Synapse, 2014. © 2014 Wiley Periodicals, Inc.     

Receptor inhibition by immunoglobulins: Specific inhibition by autistic children,their relatives,and control subjects

Forty-two parents of children with autistic disorder, 15 children with autistic disorder, 17 siblings of children with autistic disorder, and 12 unrelated normal adult controls were studied to determine if immunoglobulins isolated from their plasma would inhibit binding of the 5HT _1A agonist, [ ³ H]-8-hydroxy-N,N-dipropyl-2-aminotetralin (DPAT) to 5HT _1A receptors in human hippocampal membranes. There were no significant differences among the means of percentage inhibition of DPAT binding of parents, children with autistic disorder, siblings, or unrelated controls. In addition, there were no differences in the proportion of subjects with >15% DPAT inhibition among autistic children, their parents, their siblings, or unrelated controls. Immunoglobulin inhibition was not specific for the 5HT _1A receptor binding site, since immunoglobulins inhibited binding to 5HT ₂ , D ₁ , D ₂ , and ₂-adrenergic binding sites. The immunoglobulins isolated from normal controls inhibited [ ³ H]-rauwolscine binding at ₂-adrenergic sites less than immunoglobulins of children with autistic disorder and their parents and siblings. This study did not support the hypothesis that autoantibodies to 5HT _1A or 5HT ₂ receptors are characteristic of autistic disorder.Presented at the annual meeting of the American Academy of Child and Adolescent Psychiatry, New York, October 12, 1989. This study was supported by the Brain Research Foundation, the Freedman-Falk Academic Psychiatry Fund, the Harris Center for Developmental Studies, and NICCHD postdoctoral training grant #HDMC 5T32 HD07307-02 HCB. Local chapters of the Autism Society of America in Chicago and Indianapolis provided assistance in subject recruitment. Elizabeth Hinkle provided assistance in organization of subject recruitment and data collection. Zheng Li and Domenico Vigilante provided expert technical assistance. Hani Salti and Jennifer Tomzak provided technical assistance. Matthew Leventhal assisted in collection of samples. Judith Wopner provided laboratory facilities for processing of samples.     

Evaluation in monkey of two candidate PET radioligands, [11C]RX‐1 and [18F]RX‐2, for imaging brain 5‐HT4 receptors

The serotonin subtype‐4 (5‐HT₄) receptor, which is known to be involved physiologically in learning and memory, and pathologically in Alzheimer's disease, anxiety, and other neuropsychiatric disorders—has few radioligands readily available for imaging in vivo. We have previously reported two novel 5‐HT₄ receptor radioligands, namely [methoxy‐¹¹C](1‐butylpiperidin‐4‐yl)methyl 4‐amino‐3‐methoxybenzoate; [¹¹C]RX‐1), and the [¹⁸F]3‐fluoromethoxy analog ([¹⁸F]RX‐2), and in this study we evaluated them by PET in rhesus monkey. Brain scans were performed at baseline, receptor preblock or displacement conditions using SB 207710, a 5‐HT₄ receptor antagonist, on the same day for [¹¹C]RX‐1 and on different days for [¹⁸F]RX‐2. Specific‐to‐nondisplaceable ratio (BP_ND) was measured with the simplified reference tissue model from all baseline scans. To determine specific binding, total distribution volume (V_T) was also measured in some monkeys by radiometabolite‐corrected arterial input function after ex vivo inhibition of esterases from baseline and blocked scans. Both radioligands showed moderate to high peak brain uptake of radioactivity (2–6 SUV). Regional BP_ND values were in the rank order of known 5‐HT₄ receptor distribution with a trend for higher BP_ND values from [¹⁸F]RX‐2. One‐tissue compartmental model provided good fits with well identified V_T values for both radioligands. In the highest 5‐HT₄ receptor density region, striatum, 50–60% of total binding was specific. The V_T in receptor‐poor cerebellum reached stable values by about 60 min for both radioligands indicating little influence of radiometabolites on brain signal. In conclusion, both [¹¹C]RX‐1 and [¹⁸F]RX‐2 showed positive attributes for PET imaging of brain 5‐HT₄ receptors, validating the radioligand design strategy. Synapse 68:613–623, 2014 . © 2014 Wiley Periodicals, Inc.     

Direct comparison of [18F]MH.MZ and [18F]altanserin for 5‐HT2A receptor imaging with PET

Imaging the cerebral serotonin 2A (5‐HT_2A) receptors with positron emission tomography (PET) has been carried out in humans with [¹¹C]MDL 100907 and [¹⁸F]altanserin. Recently, the MDL 100907 analogue [¹⁸F]MH.MZ was developed combining the selectivity profile of MDL 100907 and the favourable radiophysical properties of fluorine‐18. Here, we present a direct comparison of [¹⁸F]altanserin and [¹⁸F]MH.MZ. 5‐HT_2A receptor binding in pig cortex and cerebellum was investigated by autoradiography with [³H]MDL 100907, [¹⁸F]MH.MZ, and [¹⁸F]altanserin. [¹⁸F]MH.MZ and [¹⁸F]altanserin were investigated in Danish Landrace pigs by brain PET scanning at baseline and after i.v. administration of blocking doses of ketanserin. Full arterial input function and high performance liquid chromatography (HPLC) analysis allowed for tissue‐compartment kinetic modeling of PET data. In vitro autoradiography showed high binding in cortical regions with both [¹⁸F]MH.MZ and [¹⁸F]altanserin. Significant 5‐HT_2A receptor binding was also found in the pig cerebellum, thus making this region unsuitable as a reference region for in vivo data analysis in this species. The cortical binding of [¹⁸F]MH.MZ and [¹⁸F]altanserin was blocked by ketanserin supporting that both radioligands bind to 5‐HT_2A receptors in the pig brain. In the HPLC analysis of pig plasma, [¹⁸F]MH.MZ displayed a fast and reproducible metabolism resulting in hydrophilic radiometabolites only whereas the metabolic profile of [¹⁸F]altanserin as expected showed lipophilic radiometabolites. Due to the slow kinetics of [¹⁸F]MH.MZ in high‐binding regions in vivo, we suggest that [¹⁸F]MH.MZ will be an appropriate tracer for low binding regions where kinetics will be faster, whereas [¹⁸F]altanserin is a suitable tracer for high‐binding regions. Synapse, 2013. © 2013 Wiley Periodicals, Inc.     

Trait Openness and serotonin 2A receptors in healthy volunteers: A positron emission tomography study

Recent research found lasting increases in personality trait Openness in healthy individuals and patients after administration of the serotonin 2A receptor (5‐HT_2AR) agonist psilocybin. However, no studies have investigated whether 5‐HT_2AR availability as imaged using positron emission tomography (PET) is associated with this trait. In 159 healthy individuals (53 females), the association between 5‐HT_2AR binding in neocortex imaged with [¹⁸F]altanserin or [¹¹C]Cimbi‐36 PET and personality trait Openness was investigated using linear regression models. In these models the influence of sex on the association was also investigated. Trait Openness was assessed with the NEO Personality Inventory‐Revised. No significant associations between neocortical 5‐HT_2AR binding and trait Openness were found for [¹⁸F]altanserin (p = 0.5) or [¹¹C]Cimbi‐36 (p = 0.8). Pooling the data in a combined model did not substantially change our results (p = 0.4). No significant interactions with sex were found (p > 0.35). Our results indicate that differences in 5‐HT_2AR availability are not related to variations in trait Openness in healthy individuals. Although stimulation of the 5‐HT_2AR with compounds such as psilocybin may contribute to long‐term changes in trait Openness, there is no evidence in favor of an association between 5‐HT_2AR and trait Openness.     

Differential effects of serotonin (5‐HT)2 receptor‐targeting ligands on locomotor responses to nicotine‐repeated treatment

We verified the hypothesis that serotonin (5‐HT)₂ receptors control the locomotor effects of nicotine (0.4 mg kg⁻¹) in rats by using the 5‐HT_2A receptor antagonist M100907, the preferential 5‐HT_2A receptor agonist DOI, the 5‐HT_2C receptor antagonist SB 242084, and the 5‐HT_2C receptor agonists Ro 60‐0175 and WAY 163909. Repeated pairings of a test environment with nicotine for 5 days, on Day 10 significantly augmented the locomotor activity following nicotine administration. Of the investigated 5‐HT₂ receptor ligands, M100907 (2 mg kg⁻¹) or DOI (1 mg kg⁻¹) administered during the first 5 days in combination with nicotine attenuated or enhanced, respectively, the development of nicotine sensitization. Given acutely on Day 10, M100907 (2 mg kg⁻¹), Ro 60‐0175 (1 mg kg⁻¹), and WAY 163909 (1.5 mg kg⁻¹) decreased the expression of nicotine sensitization. In another set of experiments, where the nicotine challenge test was performed on Day 15 in animals treated repeatedly (Days: 1–5, 10) with nicotine, none of 5‐HT₂ receptor ligands administered during the second withdrawal period (Days: 11–14) to nicotine‐treated rats altered the sensitizing effect of nicotine given on Day 15. Our data indicate that 5‐HT_2A receptors (but not 5‐HT_2C receptors) play a permissive role in the sensitizing effects of nicotine, while stimulation of 5‐HT_2A receptors enhances the development of nicotine sensitization and activation of 5‐HT_2C receptors is essential for the expression of nicotine sensitization. Repeated treatment with the 5‐HT₂ receptor ligands within the second nicotine withdrawal does not inhibit previously established sensitization. Synapse 64:511–519, 2010. © 2010 Wiley‐Liss, Inc.     

Increased levels of 5‐HT1A receptor binding in ventral visual pathways in Parkinson's disease

Visual hallucinations are common in advanced Parkinson's disease (PD). The pathophysiology of visual hallucinations may involve enhanced serotonergic neurotransmission. The atypical antipsychotics clozapine and quetiapine, which have affinity for 5‐HT_2A and 5‐HT_1A receptors, are effective against visual hallucinations in PD. 5‐HT_2A receptors are increased in ventral visual pathways in PD patients with visual hallucinations, and we hypothesized that 5‐HT_1A receptors were also involved in visual hallucinations in PD. Autoradiographic binding using [³H]‐WAY‐100,635 and NAN‐190 was performed in brain sections from 6 PD patients with visual hallucinations, 6 PD patients without visual hallucinations, and 5 age‐matched controls. All PD subjects had been treated with L ‐dopa. Brain areas studied were the orbitofrontal, inferolateral temporal, and motor cortices, as well as the striatum, globus pallidus, substantia nigra, and thalamus. 5‐HT_1A‐binding levels were dramatically increased in the ventral visual pathways of all PD patients compared with controls (0 vs 11 and 0 vs 100 nmol/mg, respectively; both P < .05). There was no significant difference in 5‐HT_1A‐binding levels in PD patients with visual hallucinations compared with PD patients without visual hallucinations or with controls in any of the brain areas studied (P > .05). Gross abnormalities in 5‐HT_1A levels in ventral visual areas occurred in all PD patients exposed to L ‐dopa. However, as there was no difference in 5‐HT_1A‐binding levels between hallucinators and nonhallucinators, alterations in 5‐HT_1A receptor levels may not contribute specifically to visual hallucinations in PD. However, the discrete anatomical distribution of rises to the ventral visual areas suggests some role in predisposing to visual hallucinations. © 2012 Movement Disorder Society     

Increased 5‐HT2A receptors in the temporal cortex of parkinsonian patients with visual hallucinations

Well‐formed visual hallucinations (VH) are common in patients with Parkinson's disease (PD). The pathophysiology of VH in PD is unknown but may involve structures mediating visual processing such as the inferior temporal cortex. Serotonergic type 2A (5‐HT_2A) receptors have been linked to many psychiatric disorders, including psychosis. We hypothesized that enhanced 5‐HT_2A receptor levels may be involved in VH in PD. Autoradiographic binding using [³H]‐ketanserin and spiperone, to define 5‐HT_2A receptors, was performed in 6 PD patients with VH, 6 PD patients without VH, and 5 healthy, age‐matched controls. The cerebral regions studied included the orbitofrontal cortex, inferolateral temporal cortex, motor cortex, striatum, and substantia nigra. There was a significant (45.6%) increase in the levels of [³H]‐ketanserin binding in the inferolateral temporal cortex of PD patients with VH when compared with PD patients without VH (54.3 ± 5.2 fmol/mg vs. 37.3 ± 4.3 fmol/mg, P = 0.039). Additionally, there was a significant increase in the levels of 5‐HT_2A receptors in the motor cortex of all PD patients taken as a group when compared with controls (57.8 ± 5.7 fmol/mg vs. 41.2 ± 2.6 fmol/mg, P = 0.0297). These results suggest that enhanced 5‐HT_2A‐mediated neurotransmission in the inferolateral temporal cortex, a critical structure in visual processing, might be associated with the development of VH in PD. Our results provide new insights into the pathophysiology of VH in PD and provide an anatomical basis to explain why compounds with 5‐HT_2A antagonist activity are effective at alleviating this debilitating complication. © 2010 Movement Disorder Society     

Alterations in 5‐HT2A receptor binding in various brain regions among 6‐hydroxydopamine‐induced Parkinsonian rats

The serotonergic system has close interactions with the dopaminergic system and is strongly implicated in the pathophysiological mechanisms and therapeutic paradigms of Parkinson's disease (PD). This study aims to investigate regional changes in 5‐hydroxytryptamine (5‐HT) 2A receptors in the rat brain 3 weeks after unilateral medial forebrain bundle lesion by 6‐hydroxydopamine (6‐OHDA). 5‐HT 2A receptor distributions and alterations in the postmortem rat brain were detected by [³H]ketanserin‐binding autoradiography. In the 6‐OHDA‐induced Parkinson's rat model, nigrostriatal dopaminergic neuron loss significantly mediated the decreased [³H]ketanserin binding, predominantly in the agranular insular cortex (17.3%, P = 0.03), cingulate cortex (18.2%, P < 0.001), prefrontal cortex (8%, P = 0.043), primary somatosensory cortex (17.7%, P = 0.002), and caudate putamen (14.5%, P = 0.02) compared to controls while a profound reduction of tyrosine hydroxylase (TH) immunostaining in the striatum was also observed. Alterations in [³H]ketanserin binding in the examined brain areas may represent the specific regions that mediate cognitive dysfunctions via the serotonin system. The downregulation of 5‐HT_2A receptor binding in this study also provides indirect evidence for plasticity in the serotonergic system in the rat brains. This study contributes to a better understanding of the critical roles of 5‐HT_2A receptors in treating neurodegenerative disorders and implicates 5‐HT_2A receptors as a novel therapeutic target in the treatment of PD. Synapse 64:224–230, 2010. © 2009 Wiley‐Liss, Inc.     

Similar serotonin‐2A receptor binding in rats with different coping styles or levels of aggression

Individual differences in coping style emerge as a function of underlying variability in the activation of a mesocorticolimbic brain circuitry. Particularly serotonin seems to play an important role. For this reason, we assessed serotonin‐2A receptor (5‐HT_2AR) binding in the brain of rats with different coping styles. We compared proactive and reactive males of two rat strains, Wild‐type Groningen (WTG) and Roman high‐ and low avoidance (RHA, RLA). 5‐HT_2AR binding in (pre)frontal cortex (FC) and hippocampus was investigated using a radiolabeled antagonist ([³H]MDL‐100907) and agonist ([³H]Cimbi‐36) in binding assays. No differences in 5‐HT_2AR binding were observed in male animals with different coping styles. [³H]MDL‐100907 displayed a higher specific‐to‐nonspecific binding ratio than [³H]Cimbi‐36. Our findings suggest that in these particular rat strains, 5‐HT_2AR binding is not an important molecular marker for coping style. Because neither an antagonist nor an agonist tracer showed any binding differences, it is unlikely that the affinity state of the 5‐HT_2AR is co‐varying with levels of aggression or active avoidance in WTG, RHA and RLA. Synapse, 69:226–232, 2015 . © 2015 Wiley Periodicals, Inc.     

Acetylcholine Receptor Targets on Cortical Pyramidal Neurones as Targets for Alzheimer's Therapy

Experimental lesions using the retrogradely transported toxin, volkensin, have been used in conjunction with autoradiography to investigate the cellular localization of 5–HT_1A, muscarinic M₁and nicotinic receptors. Selective destruction of neocortical pyramidal neurones forming the corticostriatal or corticocortical pathways was achieved by intrastriatal or intracortical injection of volkensin. Selective destruction of layer V corticostriatal neurones was accompanied by loss of binding in the cortex to 5–HT_1Aand muscarinic M₁receptors, and an upregulation of [³H] nicotine binding contralateral to the pyramidal cell loss. Destruction of corticocortical neurones was accompanied by loss of binding to muscarinic and nicotinic receptors. The presence of these cholinoceptors on corticocortical neurones was confirmed by recording carbachol-induced depolarizations from a novel cortical brain slice preparation. It is proposed that cholinoceptors represent a consistent marker for neocortical pyramidal cells, and as such are viable targets for the continuing development of therapies designed to ameliorate the cortical hypoactivity observed in Alzheimer's disease. Ligands for these receptors may also be suitable for positron emission tomography to assess pyramidal neurone numbers in suspected Alzheimer's disease.     

Characterization of the peripheral-type benzodiazepine receptors in cultured astrocytes: Evidence for multiplicity

In mammalian brain peripheral benzodiazepine (PBZD) receptors are predominantly localized on astroglial cells. Previous studies utilizing whole membrane preparations from brain and peripheral organs of various species have indicated several distinctions between the drug-receptor interactions of the two prototypic PBZD receptor ligands, PK 11195 and Ro5-4864. The present study was undertaken to determine whether putative differences in the binding of PBZD receptor ligands in homogenates of primary astrocyte cultures can be interpreted as the labeling of PBZD receptor subtypes. Equilib-rium competition and saturation binding experiments in homogenate preparations of primary astrocytes from cerebral cortex of new born rats revealed that [³H]PK 11195 labels twice the number of [³H]Ro5-4864 binding sites. Unlabeled Ro5-4864 competes for [³H]PK 11195 binding in a manner suggesting the existence of multiple PK 11195 bind-ing sites. The competition binding experiments, using various benzodiazepines, indicate that one binding component of PK 11195 corresponds to Ro5-4864 binding sites, whereas the second is different. The latter binding site does not correspond to the central BZD receptor but displays the pharmacological properties of the PBZD receptor. Further differences between the binding of PK 11195 and Ro5-4864 in astrocytes were detected in the presence of ethanol which was more effective in inhibiting the binding of the latter. Subcellular distribution studies indicated, however, that the binding of both [³H]PK 11195 and [³H]Ro5-4864 is associated primarily with the mitochondrial fraction of astro-cytes. Taken together, the present study indicates the existence of non-overlapping PBZD binding sites in astrocytes and thus suggests the existence of PBZD receptor subtypes. It appears, however, that there is no distinction in the subcellular localization of the putative subtypes of the PBZD receptor. © 1993 Wiley-Liss, Inc.