Evaluation of age-related changes in serotonin 5-HT2 and dopamine D2 receptor availability in healthy human subjects

We assessed the relation between serotonin 5-HT₂ receptor availability and aging and compared it with that for dopamine D₂ receptors on 19 healthy male volunteers (age range, 21–49 years) using positron emission tomography (PET) and F-18 N-methylspiperone (NMS). 5-HT₂ Receptor availability was obtained using the ratio of the distribution volume in the region of interest to that in the cerebellum ($\text{B}{}_{\max }\text{′}\text{K}{}_{\mathrm{d}}\text{′}\text{+ 1}$). 5-HT₂ Receptor measures were obtained in frontal and occipital cortices. D₂ receptor availability in striatum was measured using the “ratio index”. 5-HT₂ Receptor availability decreased significantly with age. This effect was significantly more accentuated for 5-HT₂ receptor availability in the frontal (r = 0.92, p ≤ 0.0001) than in the occipital (r = 0.67, p ≤ 0.0016) cortex (df = l, p < 0.025). Dopamine D₂ receptors were also found to decrease significantly with age (r = 0.63, p ≤ 0.007). In a given subject, striatal D₂ receptor availability significantly correlated with 5-HT₂ receptor availability in the frontal (r = 0.51, p ≤ 0.035) but not in the occipital cortex. These results document a decline in 5-HT₂ and D₂ receptor availability with age and showed an association between frontal 5-HT₂ and striatal D₂ receptors.     

Cocaine abusers show a blunted response to alcohol intoxication in limbic brain regions

Cocaine and alcohol are frequently used simultaneously and this combination is associated with enhanced toxicity. We recently showed that active cocaine abusers have a markedly enhanced sensitivity to benzodiazepines. Because both benzodiazepines and alcohol facilitate GABAergic neurotransmission we questioned whether cocaine abusers would also have an enhanced sensitivity to alcohol that could contribute to the toxicity. In this study we compared the effects of alcohol (0.75 g/kg) on regional brain glucose metabolism between cocaine abusers (n = 9) and controls (n = 10) using PET and FDG. Alcohol significantly decreased whole brain metabolism and this effect was greater in controls (26+/-6%) than in abusers (17+/-10%) even though they had equivalent levels of alcohol in plasma. Analysis of the regional measures showed that cocaine abusers had a blunted response to alcohol in limbic regions, cingulate gyrus, medial frontal and orbitofrontal cortices. CONCLUSIONS: The blunted response to alcohol in cocaine abusers contrasts with their enhanced sensitivity to benzodiazepines suggesting that targets other than GABA-benzodiazepine receptors are involved in the blunted sensitivity to alcohol and that the toxicity from combined cocaine-alcohol use is not due to an enhanced sensitivity to alcohol in cocaine abusers. The blunted response to alcohol in limbic regions and in cortical regions connected to limbic areas could result from a decreased sensitivity of reward circuits in cocaine abusers.     

5-HT<Subscript>1A</Subscript> receptor expression in pyramidal neurons of cortical and limbic brain regions

We studied expression of the 5-HT_1A receptor in cortical and limbic areas of the brain of the tree shrew. In situ hybridization with a receptor-specific probe and immunocytochemistry with various antibodies was used to identify distinct neurons expressing the receptor. In vitro receptor autoradiography with ³H-8-OH-DPAT (³H-8-hydroxy-2-[di-n-propylamino]tetralin) was performed to visualize receptor-binding sites. In the prefrontal, insular, and occipital cortex, 5-HT_1A receptor mRNA was expressed in pyramidal neurons of layer 2, whereas ³H-8-OH-DPAT labeled layers 1 and 2 generating a columnar-like pattern in the prefrontal and occipital cortex. In the striate and ventral occipital cortex, receptor mRNA was present within layers 5 and 6 in pyramidal neurons and Meynert cells. Pyramid-like neurons in the claustrum and anterior olfactory nucleus also expressed the receptor. Principal neurons in hippocampal region CA1 expressed 5-HT_1A receptor mRNA, and ³H-8-OH-DPAT labeled both the stratum oriens and stratum radiatum. CA3 pyramidal neurons displayed low 5-HT_1A receptor expression, whereas granule neurons in the dentate gyrus revealed moderate expression of this receptor. In the amygdala, large pyramid-like neurons in the basal magnocellular nucleus strongly expressed the receptor. Immunocytochemistry with antibodies against parvalbumin, calbindin, and gamma aminobutyric acid (GABA) provided no evidence for 5-HT_1A receptor expression in GABAergic neurons in cortical and limbic brain areas. Our data agree with previous findings showing that the 5-HT_1A receptor mediates the modulation of glutamatergic neurons. Expression in the limbic and cortical areas suggested an involvement of 5-HT_1A receptors in emotional and cognitive processes.This work was supported by the German Science Foundation (SFB 406; C4 to G.F.).     

Receptor binding profile of R 41 468, a novel antagonist at 5-HT2 receptors

For a new antiserotonergic agent, R 41 468 and 13 reference compounds with alleged antiserotonergic activity, the receptor binding profile is reported, comprising K_i-values measured in ten different receptor binding models. R 41 468 appeared to be a particularly selective agent with respect to differentiation between two 5-hydroxytryptamine (5-HT) receptor models; it primarily displayed high binding affinity for 5-HT₂ receptors and was inactive at 5-HT₁ receptors. Besides showing a moderate binding affinity for histamine₁ and α₁ adrenergic receptors, the compound was very weakly active at dopamine receptors and inactive at the remaining receptors. Receptor binding profiles of the reference compounds differed widely. Apart from R 41 468 no other compound showed a similar selectivity towards 5-HT₂ receptors. Reference compounds either poorly differentiated between 5-HT₂ and 5-HT₁ receptors, showed other primary effects, or were only moderately active. In the 5-HT₂ and 5-HT₁ receptor binding models the ‘D-receptor’ antagonist phenoxybenzamine was weakly active and the ‘M-receptor’ antagonist morphine was inactive. It is concluded that R 41 468 will be a particularly suitable tool to antagonize 5-HT action mediated by 5-HT₂ receptors.     

Identification of Three Residues Essential for 5-Hydroxytryptamine 2A-Metabotropic Glutamate 2 (5-HT2A·mGlu2) Receptor Heteromerization and Its Psychoactive Behavioral Function

Serotonin and glutamate G protein-coupled receptor (GPCR) neurotransmission affects cognition and perception in humans and rodents. GPCRs are capable of forming heteromeric complexes that differentially alter cell signaling, but the role of this structural arrangement in modulating behavior remains unknown. Here, we identified three residues located at the intracellular end of transmembrane domain four that are necessary for the metabotropic glutamate 2 (mGlu2) receptor to be assembled as a GPCR heteromer with the serotonin 5-hydroxytryptamine 2A (5-HT_2A) receptor in the mouse frontal cortex. Substitution of these residues (Ala-677^4.40, Ala-681^4.44, and Ala-685^4.48) leads to absence of 5-HT_2A·mGlu2 receptor complex formation, an effect that is associated with a decrease in their heteromeric ligand binding interaction. Disruption of heteromeric expression with mGlu2 attenuates the psychosis-like effects induced in mice by hallucinogenic 5-HT_2A agonists. Furthermore, the ligand binding interaction between the components of the 5-HT_2A·mGlu2 receptor heterocomplex is up-regulated in the frontal cortex of schizophrenic subjects as compared with controls. Together, these findings provide structural evidence for the unique behavioral function of a GPCR heteromer.     

Chronic Electroconvulsive Seizures Increase the Expression of Serotonin2 Receptor mRNA in Rat Frontal Cortex

Abstract: The present study examines the influence of electroconvulsive seizure (ECS), as well as antidepressant drugs, on levels of serotonin₂ (5-HT₂) receptor mRNA in rat frontal cortex. Using a sensitive RNase protective assay, preliminary studies demonstrated the predicted regional distribution for the 5-HT₂ receptor mRNA: levels of 5-HT₂ mRNA were highest in frontal cortex (2.58 amol/μg of total RNA), intermediate in neostriatum, thalamus, and midbrain, and lowest in hippocampus, cerebellum, and choroid plexus. Chronic (10 or 14 days), but not acute (1 or 3 days), ECS treatment significantly increased levels of 5-HT₂ receptor mRNA. ECS treatment resulted in a similar time-dependent up-regulation of 5-HT₂ receptor ligand binding; chronic, but not acute, ECS treatment significantly increased levels of [³H]ketanserin ligand binding, confirming previous reports. Northern blot analysis demonstrated that 5-HT₂ receptor mRNA occurs as two bands (～5 and 6 kb in size), both of which were increased by chronic ECS treatment. The influence of antidepressant drug treatments on 5-HT₂ receptor mRNA was also examined. Chronic fluoxetine treatment increased levels of 5-HT₂ receptor mRNA, although levels of [³H]ketanserin ligand binding were not altered. In contrast, chronic administration of imipramine, mianserin, and tranylcypromine, treatments that decreased ligand binding, did not decrease levels of 5-HT₂ receptor mRNA. In fact, mianserin treatment caused a small, but significant, increase in levels of receptor mRNA. The results suggest that ECS up-regulation of 5-HT₂ receptor mRNA could underlie the increased density of 5-HT₂ receptor binding sites in response to this treatment, but that other mechanisms likely operate in the down-regulation of 5-HT₂ receptor ligand binding by antidepressant drug treatments.     

Phospholipase A2 Activity Is Selectively Decreased in the Striatum of Chronic Cocaine Users

Abstract: Dopamine-mediated stimulation of arachidonic acid metabolism, via activation of the phospholipid metabolizing enzyme phospholipase A₂ (PLA₂), has recently been implicated in dopamine neurotransmitter function. We examined the status of PLA₂ in autopsied brain of 10 chronic users of cocaine, a dopamine reuptake inhibitor. PLA₂ activity, assayed at pH 8.5 in the presence of Ca²⁺, was significantly ( p < 0.01) decreased by 31% in the putamen of cocaine users (n = 10) compared with that in controls (n = 10), whereas activity was normal in the frontal and occipital cortices, subcortical white matter, and cerebellum. In contrast, calcium-independent PLA₂ activity, assayed at pH 7.0, was normal in all brain regions examined. Our finding of altered PLA₂ activity restricted to a region of high dopamine receptor density suggests that modulation of PLA₂ may be involved in mediating some of the dopamine-related behavioral effects of cocaine and could conceivably contribute to dopamine-related processes in the normal brain.     

Characterisation of Human 5-Hydroxytryptamine2A and 5-Hydroxytryptamine2C Receptors Expressed in the Human Neuroblastoma Cell Line SH-SY5Y: Comparative Stimulation by Hallucinogenic Drugs

Abstract: Stable transfection of the human neuroblastoma cell line SH-SY5Y with the human 5-hydroxytryptamine_2A (5-HT_2A) or 5-HT_2C receptor cDNA produced cell lines demonstrating ligand affinities that correlated closely with those for the corresponding endogenous receptors in human frontal cortex and choroid plexus, respectively. Stimulation of the recombinant receptors by 5-HT induced phosphoinositide hydrolysis with higher potency but lower efficacy at the 5-HT_2C receptor (pEC₅₀ = 7.80 ± 0.06) compared with the 5-HT_2A receptor (pEC₅₀ = 7.30 ± 0.08). Activation of the 5-HT_2A receptor caused a transient fourfold increase in intracellular Ca²⁺ concentration. Whole-cell recordings of cells clamped at ?50 mV demonstrated a small inward current (2 pA) in response to 10 µM 5-HT for both receptors. There were no differences in potency or efficacy of phosphoinositide hydrolysis among four hallucinogenic [d-lysergic acid diethylamide (LSD), 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI), 5-methoxy-N,N-dimethyltryptamine, and mescaline] and three nonhallucinogenic drugs (m-chlorophenylpiperazine, quipazine, and ergotamine). Comparison of equipotent doses producing 20% of the maximal response induced by 5-HT revealed selective activation of the 5-HT_2A receptor by LSD and to a lesser degree by DOI, mescaline, and ergotamine. Quipazine and 5-methoxy-N,N-dimethyltryptamine were relatively nonselective, whereas m-chlorophenylpiperazine selectively activated the 5-HT_2C receptor. It is unlikely therefore that hallucinosis is mediated primarily by activity at the 5-HT_2C receptor, whereas activity at the 5-HT_2A receptor may represent an important but not unique mechanism associated with hallucinogenic drug action.     

Effect of chronic l-DOPA treatment on 5-HT1A receptors in parkinsonian monkey brain

After chronic use of l-3,4-dihydroxyphenylalanine (l-DOPA), most Parkinson’s disease (PD) patients suffer from its side effects, especially motor complications called l-DOPA-induced dyskinesia (LID). 5-HT_1A agonists were tested to treat LID but many were reported to worsen parkinsonism. In this study, we evaluated changes in concentration of serotonin and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) and of 5-HT_1A receptors in control monkeys, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys, dyskinetic MPTP monkeys treated chronically with l-DOPA, low dyskinetic MPTP monkeys treated with l-DOPA and drugs of various pharmacological activities: Ro 61-8048 (an inhibitor of kynurenine hydroxylase) or docosahexaenoic acid (DHA) and dyskinetic MPTP monkeys treated with l-DOPA + naltrexone (an opioid receptor antagonist). Striatal serotonin concentrations were reduced in MPTP monkeys compared to controls. Higher striatal 5-HIAA/serotonin concentration ratios in l-DOPA-treated monkeys compared to untreated monkeys suggest an intense activity of serotonin axon terminals but this value was similar in dyskinetic and nondyskinetic animals treated with or without adjunct treatment with l-DOPA. As measured by autoradiography with [³H]8-hydroxy-2-(di-n-propyl) aminotetralin (8-OH-DPAT), a decrease of 5-HT_1A receptor specific binding was observed in the posterior/dorsal region of the anterior cingulate gyrus and posterior/ventral area of the superior frontal gyrus of MPTP monkeys compared to controls. An increase of 5-HT_1A receptor specific binding was observed in the hippocampus of MPTP monkeys treated with l-DOPA regardless to their adjunct treatment. Cortical 5-HT_1A receptor specific binding was increased in the l-DOPA-treated MPTP monkeys alone or with DHA or naltrexone and this increase was prevented in low dyskinetic MPTP monkeys treated with l-DOPA and Ro 61-8048. These results highlight the importance of 5-HT_1A receptor alterations in treatment of PD with l-DOPA.     

The Serotonin 5-HT2C Receptor Is a Prominent Serotonin Receptor in Basal Ganglia: Evidence from Functional Studies on Serotonin-Mediated Phosphoinositide Hydrolysis

Abstract: Serotonin (5-hydroxytryptamine; 5-HT) 5-HT_2A and 5-HT_2C receptors belong to the class of phosphoinositide-specific phospholipase C (PLC)-linked receptors. Conditions were established for measuring 5-HT_2A-linked and 5-HT_2C-linked PLC activity in membranes prepared from previously frozen rat frontal cortex and caudate. In the presence of Ca²⁺ (300 nM) and GTPγS (1 µM), 5-HT increased PLC activity in caudate membranes. Pharmacological analysis using the selective 5-HT_2A antagonist, spiperone, and the nonselective 5-HT_2A/2C antagonist, mianserin, demonstrated that over half of the 5-HT-stimulated PLC activity was due to stimulation of 5-HT_2C receptors as opposed to 5-HT_2A receptors. Radioligand binding assays with [³H]RP 62203 and [³H]-mesulergine were used to quantify 5-HT_2A and 5-HT_2C sites, respectively, in caudate. From these data, the B_max for caudate 5-HT_2A sites and 5-HT_2C sites was 165.4 ± 9.7 fmol/mg of protein and 49.7 ± 3.3 fmol/mg of protein, respectively. In contrast to that in caudate, PLC activity in frontal cortex was stimulated by 5-HT in a manner that was inhibited by the 5-HT_2A-selective antagonists, spiperone and ketanserin. Taken together, the results indicate that 5-HT_2A- and 5-HT_2C-linked PLC activity can be discerned in brain regions possessing both receptor subtypes using membranes prepared from previously frozen tissue. More importantly, significant 5-HT_2C-mediated phosphoinositide hydrolysis was observed in caudate, despite the relatively low density of 5-HT_2C sites. The significance of these observations with respect to the physiological function of 5-HT_2C receptors is discussed.     

Convergent evidence from multimodal imaging reveals amygdala abnormalities in schizophrenic patients and their first-degree relatives

Background

Shared neuropathological features between schizophrenic patients and their first-degree relatives have potential as indicators of genetic vulnerability to schizophrenia. We sought to explore genetic influences on brain morphology and function in schizophrenic patients and their relatives.

Methods

Using a multimodal imaging strategy, we studied 33 schizophrenic patients, 55 of their unaffected parents, 30 healthy controls for patients, and 29 healthy controls for parents with voxel-based morphometry of structural MRI scans and functional connectivity analysis of resting-state functional MRI data.

Results

Schizophrenic patients showed widespread gray matter reductions in the bilateral frontal cortices, bilateral insulae, bilateral occipital cortices, left amygdala and right thalamus, whereas their parents showed more localized reductions in the left amygdala, left thalamus and right orbitofrontal cortex. Patients and their parents shared gray matter loss in the left amygdala. Further investigation of the resting-state functional connectivity of the amygdala in the patients showed abnormal functional connectivity with the bilateral orbitofrontal cortices, bilateral precunei, bilateral dorsolateral frontal cortices and right insula. Their parents showed slightly less, but similar changes in the pattern in the amygdala connectivity. Co-occurrences of abnormal connectivity of the left amygdala with the left orbitofrontal cortex, right dorsolateral frontal cortex and right precuneus were observed in schizophrenic patients and their parents.

Conclusions

Our findings suggest a potential genetic influence on structural and functional abnormalities of the amygdala in schizophrenia. Such information could help future efforts to identify the endophenotypes that characterize the complex disorder of schizophrenia.     

Acetaminophen-induced antinociception via central 5-HT2A receptors

Acetaminophen is one of the most widely used analgesic drugs. Although the mechanism of analgesic action of acetaminophen is still not known, the involvement of the central serotonin (5-hydroxytryptamine: 5-HT) system is one possibility. In the present study, we examined the antinociceptive effect of acute and chronic intraperitoneally (i.p.) administered acetaminophen by tail flick latency measurements in the rat. A significantly increased tail flick latency was observed in acute and 15-day acetaminophen-treated rats, but not in 30-day acetaminophen-treated rats, at a dose of 400 mg/kg/day. To investigate the plasticity of receptors at postsynaptic membrane, we conducted a series of experiments by radioligand binding method on frontal cortex and brainstem membrane. The technique involved radioligand binding with [phenyl-4-³H]spiperone and ketanserin for studying 5-HT_2A receptor characteristics. A significant decrease in the maximum number of 5-HT_2A binding sites (B_max) was demonstrated in all treatment groups with acetaminophen 300 and 400 mg/kg on frontal cortex membrane, whereas the value of the dissociation equilibrium constant (K_d) remained unchanged. The down-regulation of 5-HT_2A binding sites in frontal cortex was of a lesser magnitude after 30 days of treatment and the tail flick latency was as in the control animals. These results suggest that down-regulation of 5-HT_2A receptor in response to 5-HT release is a major step in the mechanism underlying analgesia produced by this agent. On the contrary, chronic use of acetaminophen may result in 5-HT depletion, which in turn produces re-adaptation of postsynaptic 5-HT_2A receptors. These data provide further evidence for a central 5-HT-dependent antinociceptive effect of acetaminophen.     

DHA prevents altered 5-HT1A, 5-HT2A,CB1 and GABAA receptor binding densities in the brain of male rats fed a high-saturated-fat diet

Low levels of docosahexaenoic acid (DHA) have been linked to a number of mental illnesses such as memory loss, depression and schizophrenia. While supplementation of DHA is beneficial in improving memory and cognition, the influence of dietary fats on the neurotransmitters and receptors involved in cognitive function is still not known. The aim of this study was to investigate serotonin receptor (5-HT_1A and 5-HT_2A), cannabinoid receptor (CB1) and gamma-aminobutyric acid type A (GABA_A) receptor binding densities in the brain of male rats fed a high-saturated-fat (HF) diet, as well as the effect of DHA supplementation on HF diet. Alterations of these receptors in the post-mortem rat brain were detected by [³H]-WAY-100635, [³H]-ketanserin, [³H]-CP-55,940 and [³H]-muscimol binding autoradiography, respectively. In the hippocampus, the 5-HT_1A, CB1 and GABA_A receptor binding densities significantly increased in response to an HF diet, while in the hypothalamus, 5-HT_1A and CB1 binding densities significantly increased in HF-fed rats. Importantly, DHA supplementation prevented the HF-induced increase of receptors binding density in the hippocampus and hypothalamus. Furthermore, DHA supplementation attenuated 5-HT_2A receptor binding density in the caudate putamen, anterior cingulate cortex and medial mammillary nucleus, which was also increased in HF group. This study showed that an HF diet increased 5-HT_1A, 5-HT_2A, CB1 and GABA_A receptor binding densities in the brain regions involved in cognitive function and that dietary DHA can attenuate such alterations. These findings provide insight into the mechanism by which DHA supplementation ameliorates reduced cognitive function associated with an HF diet.     

Reduced metabolism in brain "control networks" following cocaine-cues exposure in female cocaine abusers

Objective

Gender differences in vulnerability for cocaine addiction have been reported. Though the mechanisms are not understood, here we hypothesize that gender differences in reactivity to conditioned-cues, which contributes to relapse, are involved.

Method

To test this we compared brain metabolism (using PET and ¹⁸FDG) between female (n = 10) and male (n = 16) active cocaine abusers when they watched a neutral video (nature scenes) versus a cocaine-cues video.

Results

Self-reports of craving increased with the cocaine-cue video but responses did not differ between genders. In contrast, changes in whole brain metabolism with cocaine-cues differed by gender (p<0.05); females significantly decreased metabolism (−8.6%±10) whereas males tended to increase it (+5.5%±18). SPM analysis (Cocaine-cues vs Neutral) in females revealed decreases in frontal, cingulate and parietal cortices, thalamus and midbrain (p<0.001) whereas males showed increases in right inferior frontal gyrus (BA 44/45) (only at p<0.005). The gender-cue interaction showed greater decrements with Cocaine-cues in females than males (p<0.001) in frontal (BA 8, 9, 10), anterior cingulate (BA 24, 32), posterior cingulate (BA 23, 31), inferior parietal (BA 40) and thalamus (dorsomedial nucleus).

Conclusions

Females showed greater brain reactivity to cocaine-cues than males but no differences in craving, suggesting that there may be gender differences in response to cues that are not linked with craving but could affect subsequent drug use. Specifically deactivation of brain regions from “control networks” (prefrontal, cingulate, inferior parietal, thalamus) in females could increase their vulnerability to relapse since it would interfere with executive function (cognitive inhibition). This highlights the importance of gender tailored interventions for cocaine addiction.     

Disruption to the 5-HT<Subscript>7</Subscript> Receptor Following Hypoxia–Ischemia in the Immature Rodent Brain

It has become increasingly evident the serotonergic (5-hydroxytryptamine, 5-HT) system is an important central neuronal network disrupted following neonatal hypoxic–ischemic (HI) insults. Serotonin acts via a variety of receptor subtypes that are differentially associated with behavioural and cognitive mechanisms. The 5-HT₇ receptor is purported to play a key role in epilepsy, anxiety, learning and memory and neuropsychiatric disorders. Furthermore, the 5-HT₇ receptor is highly localized in brain regions damaged following neonatal HI insults. Utilising our well-established neonatal HI model in the postnatal day 3 (P3) rat pup we demonstrated a significant decrease in levels of the 5-HT₇ protein in the frontal cortex, thalamus and brainstem one week after insult. We also observed a relative decrease in both the cytosolic and membrane fractions of 5-HT₇. The 5-HT₇ receptor was detected on neurons throughout the cortex and thalamus, and 5-HT cell bodies in the brainstem. However we found no evidence of 5-HT₇ co-localisation on microglia or astrocytes. Moreover, minocycline treatment did not significantly prevent the HI-induced reductions in 5-HT₇. In conclusion, neonatal HI injury caused significant disruption to 5-HT₇ receptors in the forebrain and brainstem. Yet the use of minocycline to inhibit activated microglia, did not prevent the HI-induced changes in 5-HT₇ expression.     

Disruption of Potential α-Helix in the G Loop of the Guinea: Pig 5-Hydroxytryptamine2 Receptor Does Not Prevent Receptor Coupling to Phosphoinositide Hydrolysis

Abstract: Heterogeneity of the 5-hydroxytryptamine₂ (5-HT₂) receptor across species has been implicated in several pharmacological and physiological studies. Although 5-HT₂ receptors in the rat have been linked to increases in Phosphoinositide (PI) hydrolysis, little evidence exists to support the association of guinea pig 5-HT₂ receptors with Pl hydrolysis, the second messenger generally linked with 5-HT₂receptors. In the present study, we have taken a molecular and biochemical approach to determining whether species differences in brain 5-HT₂ receptors exist between rat and guinea pig. First, we isolated partial cortical 5-HT_a receptor cDNA clones that encompassed the third intracellular loop, a receptor area putatively important in receptor-effector coupling. The amino acid sequences deduced from the cDNA clones for rat and guinea pig brain 5-HT₂ receptor were 97% homologous. However, the guinea pig 5-HT₂ receptor had two tandem substitutions that disrupted a potential alpha helix in the region of the third cytoplasmic loop, which theoretically could alter the intracellular coupling of the guinea pig cortical 5-HT₂ receptor. Because of these molecular differences, we examined further the pharmacological activation of the brain 5-HT₂ receptor from guinea pig. 5-HT and the 5-HT₂ receptor agonist α-methyl-5-HT increased PI hydrolysis in guinea pig cortical slices whereas the 5-HT_1c receptor agonist 5-methyltryptamine was significantly less potent. In addition, the 5-HT₂ receptor antagonists LY53857, ketanserin, and spiperone blocked 5-HT-stimulated Pl hydrolysis. These pharmacological data suggested that activation of the 5-HT₂ receptor in guinea pig cortical slices was associated with PI hydrolysis. Thus, although areas of the guinea pig brain 5-HT₂ receptor that influence receptor-effector coupling were different from the rat, such differences were not critical to receptor-effector coupling because, as in the rat, guinea pig brain 5-HT₂ receptors were also coupled to PI hydrolysis.     

Regional Amyloid Deposition in Amnestic Mild Cognitive Impairment and Alzheimer's Disease Evaluated by [18F]AV-45 Positron Emission Tomography in Chinese Population

Background

To compare the neocortical amyloid loads among cognitively normal (CN), amnestic mild cognitive impairment (aMCI), and Alzheimer''s disease (AD) subjects with [¹⁸F]AV-45 positron emission tomography (PET).

Materials and Methods

[¹⁸F]AV-45 PET was performed in 11 CN, 13 aMCI, and 12 AD subjects to compare the cerebral cortex-to-whole cerebellum standard uptake value ratios (SUVRs) of global and individual volumes of interest (VOIs) cerebral cortex. The correlation between global cortical [¹⁸F]AV-45 SUVRs and Mini-Mental State Examination (MMSE) scores was analyzed.

Results

The global cortical [¹⁸F]AV-45 SUVRs were significantly different among the CN (1.08±0.08), aMCI (1.27±0.06), and AD groups (1.34±0.13) (p = 0.0003) with amyloidosis positivity rates of 9%, 62%, and 92% in the three groups respectively. Compared to CN subjects, AD subjects had higher SUVRs in the global cortical, precuneus, frontal, parietal, occipital, temporal, and posterior cingulate areas; while aMCI subjects had higher values in the global cortical, precuneus, frontal, occipital and posterior cingulate areas. There were negative correlations of MMSE scores with SUVRs in the global cortical, precuneus, frontal, parietal, occipital, temporal, posterior cingulate and anterior cingulate areas on a combined subject pool of the three groups after age and education attainment adjustment.

Conclusions

Amyloid deposition occurs relatively early in precuneus, frontal and posterior cingulate in aMCI subjects. Higher [¹⁸F]AV-45 accumulation is present in parietal, occipital and temporal gyri in AD subjects compared to the aMCI group. Significant correlation between MMSE scores and [¹⁸F]AV-45 SUVRs can be observed among CN, aMCI and AD subjects.