Autoradiographic distribution of serotonin transporters and receptor subtypes in human brain

Several neurochemical in vitro and in vivo imaging studies have been aimed at characterizing the localization of serotonin receptors and transporters in the human brain. In this study, a detailed comparison of the distribution of a number of 5-HT receptor subtypes and the 5-HT transporter was carried out in vitro using human postmortem brain tissue. Anatomically adjacent whole hemisphere sections were incubated with specific radioligands for the 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(4) receptors and the 5-HT transporter. The autoradiograms revealed different laminar and regional distribution patterns in the isocortex, where 5-HT(1A) and 5-HT(4) receptor binding showed highest densities in superficial layers and 5-HT(2A) receptor binding was most abundant in middle layers. In cortical regions, 5-HT transporters were concentrated to several limbic lobe structures (posterior uncus, entorhinal, cingulate, insular and temporal polar regions). 5-HT(1A) receptor densities were also high in limbic cortical regions (hippocampus, posterior entorhinal cortex, and subcallosal area) compared to the isocortex. Subregionally different distribution patterns were observed in the basal ganglia with a trend toward higher levels in ventral striatal (5-HT(1B) receptors) and pallidal (5-HT transporters and 5-HT(1B) receptors) regions. The localization in regions belonging to limbic cortico-striato-pallido-thalamic circuits is in line with the documented role of 5-HT in modulation of mood and emotion, and the suggested involvement of this system in pathophysiology of various psychiatric disorders. The qualitative and quantitative information reported in this study might provide important complements to in vivo neuroimaging studies of the 5-HT system.     

Mapping of serotonin 5-HT(4) receptor mRNA and ligand binding sites in the post-mortem human brain

The anatomical localization of 5-HT(4) receptor mRNA and 5-HT(4) receptor protein was examined in sections of post-mortem human brain by in situ hybridization histochemistry and radioligand receptor autoradiography. In the in situ hybridization study, the highest levels of 5-HT(4) receptor mRNA were found in caudate nucleus, putamen, nucleus accumbens, and in the hippocampal formation. No 5-HT(4) receptor mRNA was detected in globus pallidus and substantia nigra. For receptor autoradiography, two new and highly selective radioligands were compared: [(3)H]prucalopride, which preferentially labels the G-protein coupled fraction of receptors, and [(3)H]R116712, which labels the entire receptor population at subnanomolar concentrations. [(3)H]Prucalopride and [(3)H]R116712 binding was performed on human brain hemisphere sections. The highest densities for both radioligands were found in the basal ganglia (caudate nucleus, putamen, nucleus accumbens, globus pallidus, substantia nigra). Moderate to low densities were detected in the hippocampal formation and in the cortical mantle. Mismatches between 5-HT(4) receptor mRNA and binding sites in the globus pallidus and the substantia nigra suggested that the binding sites may be localized on axonal projections originating from the striatum. To compare densities of binding sites, concentration binding curves with [(3)H]prucalopride, [(3)H]R116712 and [(3)H]GR113808 were performed on membranes from homogenates of several human brain regions. Comparison of B(max)-values obtained with [(3)H]prucalopride and [(3)H]R116712 indicated that the G-protein coupled fraction of 5-HT(4) receptors in the substantia nigra was exceptionally high (54%) in comparison with percentages (16-27%) found in the frontal cortex, the striatum and the hippocampus. Such a high percentage (40%) of [(3)H]prucalopride vs. [(3)H]R116712 binding was also observed in the substantia nigra in the receptor autoradiography experiments. The [(3)H]prucalopride binding was GppNHp-sensitive, whereas [(3)H]R116712 and [(3)H]GR113808 was not. These data indicate that in the substantia nigra 5-HT(4) receptors are more strongly coupled to their signal transduction pathway than in other brain regions.     

Autoradiographic mapping of 5-HT(1B) and 5-HT(1D) receptors in the post mortem human brain using [(3)H]GR 125743

The distribution of 5-HT(1B) and 5-HT(1D) receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [(3)H]GR 125743. [(3)H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [(3)H]GR 125743 binding was observed, reflecting low densities of 5-HT(1B/1D) receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT(1D) receptors and the second using SB 224289 to inhibit 5-HT(1B) receptor binding. The autoradiograms indicated that in the human brain, the 5-HT(1B) receptor is much more abundant than the 5-HT(1D) receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [(3)H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT(1B) receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT(1D) receptor in the human brain.     

Binding characteristics of the 5-HT2A receptor antagonists altanserin and MDL 100907

To study the 5-HT(2A) receptors in the living human brain, using positron emission tomography (PET), two selective radiotracers are currently in use: [(18)F]altanserin and [(11)C]MDL 100907. It is, however, currently unknown to what extent data obtained with either tracer are directly comparable. The aim of this study was to compare binding characteristics of these two radiotracers in rat brain with respect to affinity (K(d)), receptor binding density (B(max)), binding potential (BP), and nonspecific binding. Further, binding kinetics, sensitivity towards competition with the endogenous transmitter serotonin, and the competitive/noncompetitive interaction between the two radioligands were evaluated. In addition, the selectivity of [(18)F]altanserin for the 5-HT(2A) receptor was assessed.The K(d) value of [(18)F]altanserin and [(3)H]MDL 100907 was in the order of 0.3 nM. B(max) in frontal cortex was 523 and 527 fmol/mg protein, respectively. The binding of [(18)F]altanserin was not influenced by blocking either the 5-HT(2B/2C) or the alpha(1)-adrenergic receptors. At 37 degrees C the association t(1/2) was 2.8 and 2.7 min and the dissociation t(1/2) was 11 and 13.5 min for [(18)F]altanserin and [(3)H]MDL 100907, respectively.Both radioligands were displaced by 5-HT, only at high concentrations; the K(i) value of 5-HT ranging between 650 and 3,300 nM. This indicates that binding of both radioligands in PET studies is not directly influenced by changes in endogenous 5-HT.Overall, the binding of [(18)F]altanserin and [(3)H]MDL 100907 to the 5-HT(2A) receptor was very comparable, showing selective high affinity binding in the subnanomolar range.     

Reduction in 5-HT1A receptor density, 5-HT1A mRNA expression, and functional correlates for 5-HT1A receptors in genetically defined aggressive rats

The present experiments tested the hypothesis that one of the critical mechanisms underlying genetically defined aggressiveness involves brain serotonin 5-HT1A receptors. 5-HT1A receptor density, the receptor mRNA expression in brain structures, and functional correlates for 5-HT1A receptors identified as 8-OH-DPAT-induced hypothermia and lower lip retraction (LLR) were studied in Norway rats bred for 59 generations for the lack of aggressiveness and for high affective aggressiveness with respect to man. Considerable differences between the highly aggressive and the nonaggressive rats were shown in all three traits. A significant decrease in B(max) of specific receptor binding of [3H]8-OH-DPAT in the frontal cortex, hypothalamus, and amygdala and a reduction in 5-HT1A receptor mRNA expression in the midbrain of aggressive rats were found. 5-HT1A receptor agonist 8-OH-DPAT (0.5 mg/kg, i.p.) produced a distinct hypothermic reaction in nonaggressive rats and did not affect significantly the body temperature in aggressive rats. Similar differences were revealed in 8-OH-DPAT-induced LLR: LLR was expressed much more in nonaggressive than in aggressive animals. Additionally, 8-OH-DPAT (0.5 mg/kg i.p.) treatment significantly attenuated the aggressive response to man. The results demonstrated an association of aggressiveness with reduced 5-HT1A receptor expression and function, thereby providing support for the view favoring the idea that brain HT1A receptor contributes to the genetically defined individual differences in aggressiveness.     

Altered expression and functions of serotonin 5-HT1A and 5-HT1B receptors in knock-out mice lacking the 5-HT transporter

By taking up serotonin (5-hydroxytryptamine, 5-HT) released in the extracellular space, the 5-HT transporter (5-HTT) regulates central 5-HT neurotransmission. Possible adaptive changes in 5-HT neurotransmission in knock-out mice that do not express the 5-HT transporter were investigated with special focus on 5-HT1A and 5-HT1B receptors. Specific labelling with radioligands and antibodies, and competitive RT-PCR, showed that 5-HT1A receptor protein and mRNA levels were significantly decreased in the dorsal raphe nucleus (DRN), increased in the hippocampus and unchanged in other forebrain areas of 5-HTT-/- vs. 5-HTT+/+ mice. Such regional differences also concerned 5-HT1B receptors because a decrease in their density was found in the substantia nigra (-30%) but not the globus pallidus of mutant mice. Intermediate changes were noted in 5-HTT+/- mice compared with 5-HTT+/+ and 5-HTT-/- animals. Quantification of [35S]GTP-gamma-S binding evoked by potent 5-HT1 receptor agonists confirmed such changes as a decrease in this parameter was noted in the DRN (-66%) and the substantia nigra (-30%) but not other brain areas in 5-HTT-/- vs. 5-HTT+/+ mice. As expected from actions mediated by functional 5-HT1A and 5-HT1B autoreceptors, a decrease in brain 5-HT turnover rate after i.p. administration of ipsapirone (a 5-HT1A agonist), and an increased 5-HT outflow in the substantia nigra upon local application of GR 127935 (a 5-HT1B/1D antagonist) were observed in 5-HTT+/+ mice. Such effects were not detected in 5-HTT-/- mice, further confirming the occurrence of marked alterations of 5-HT1A and 5-HT1B autoreceptors in these animals.     

Autoradiographic localization of 5-HT(2A) receptors in the human brain using [(3)H]M100907 and [(11)C]M100907

M100907 (MDL 100907, R-(+)-alpha-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol++ +) is a new selective antagonist of 5-HT(2A) receptors. The compound has been labeled with (11)C and proved useful for in vivo studies of 5-HT(2A) receptors using positron emission tomography (PET). In the present study the distribution of 5-HT(2A) receptors was examined in the postmortem human brain using whole hemisphere autoradiography and [(3)H]M100907 and [(11)C]M100907. The autoradiograms showed very dense binding to all neocortical regions, whereas the hippocampus was only weakly labeled with [(3)H]M100907. Other central brain regions, such as the basal ganglia and thalamus, showed low [(3)H]M100907 binding, reflecting low densities of 5-HT(2A) receptors. The cerebellum or structures of the brain stem were virtually devoid of 5-HT(2A) receptors. [(11)C]M100907 gave images qualitatively similar to those of [(3)H]M100907, although with lower spatial resolution. The labeling of human 5-HT(2A) receptors with [(3)H]M100907 was inhibited by the addition of the 5-HT(2A) receptor blockers ketanserin or SCH 23390 (10 microM), leaving a very low background of nonspecific binding. The 5-HT(1A) receptor antagonist WAY-100635 and the D(2)-dopamine receptor antagonist raclopride had no effect on the binding of [(3)H]M100907. The selective labeling of 5-HT(2A) receptors with [(3)H]M100907 clearly shows that this compound is suitable for further studies of the human 5-HT(2A) receptor subtype in vitro. The in vitro autoradiography of the distribution of 5-HT(2A) receptors obtained with radiolabeled M100907 provides detailed qualitative and quantitative information on the distribution of 5-HT(2A)-receptors in the human brain as well as reference information for the interpretation of previous initial results at much lower resolution in humans in vivo with PET and [(11)C]M100907.     

Development of 5-HT1A receptor radioligands to determine receptor density and changes in endogenous 5-HT

[(18)F]FCWAY and [(18)F]FPWAY, analogues of the high affinity 5-HT(1A) receptor (5-HT(1A)R) antagonist WAY100635, were evaluated in rodents as potential radiopharmaceuticals for determining 5-HT(1A)R density and changes in receptor occupancy due to changes in endogenous serotonin (5-HT) levels. The in vivo hippocampus specific binding ratio [(hippocampus(uptake)/cerebellum(uptake))-1] of [(18)F]FPWAY was decreased to 32% of the ratio of [(18)F]FCWAY, indicating that [(18)F]FPWAY has lower affinity than [(18)F]FCWAY. The 5-HT(1A)R selectivity of [(18)F]FPWAY was confirmed using ex vivo autoradiography studies with 5-HT(1A)R knockout, heterozygous, and wildtype mice.Pre- or post-treatment of awake rodents in tissue dissection studies with paroxetine had no effect on hippocampal binding of [(18)F]FCWAY or [(18)F]FPWAY compared to controls, indicating neither tracer was sensitive to changes in endogenous 5-HT. In mouse ex vivo autoradiography studies in which awake mice were treated with fenfluramine following the [(18)F]FPWAY, a significant decrease was not observed in the hippocampus specific binding ratios. In rat dissection studies with fenfluramine administered following [(18)F]FPWAY or [(18)F]FBWAY ([(18)F]-MPPF) in awake or urethane-anesthetized rats, no significant differences in the specific binding ratios of the hippocampus were observed compared to their respective controls. [(18)F]FPWAY and [(18)F]FBWAY uptakes in all brain regions were increased variably in the anesthetized group (with the greatest increase in the hippocampus) vs. the awake group, but were decreased in the fenfluramine-treated anesthetized group vs. the anesthetized group. These data are best explained by changes in blood flow caused by urethane and fenfluramine, which varies from region to region in the brain.     

11C]NNC 112 selectivity for dopamine D1 and serotonin 5-HT(2A) receptors: a PET study in healthy human subjects.

The dopamine D(1) receptor antagonist radioligand [(11)C]NNC 112 has previously been reported to have 100-fold selectivity for the D(1) receptor compared with the serotonin 5-HT(2A) receptor. In this study, we tested the selectivity by scanning seven healthy human research volunteers with [(11)C]NNC 112 before and after 2 mg of the antipsychotic drug risperidone, a dose that putatively blocks all 5-HT(2A) receptors with negligible effect on D(1) receptors. We found that specific binding in cortical regions was reduced by 20% to 30%, whereas the striatum showed no change. Based on the known relative densities of these receptors in humans, our results suggest 5- to 10-fold selectivity of [(11)C]NNC 112 for D(1) versus 5-HT(2A) as opposed to 100-fold selectivity. These results suggest caution in interpreting data from studies using this tracer to measure cortical D(1) receptors as well as the need for more selective radioligands to assess cortical D(1) receptors.     

A possible mechanism of the nucleus accumbens and ventral pallidum 5-HT1B receptors underlying the antidepressant action of ketamine: a?PET study with macaques

Ketamine is a unique anesthetic reagent known to produce various psychotic symptoms. Ketamine has recently been reported to elicit a long-lasting antidepressant effect in patients with major depression. Although recent studies provide insight into the molecular mechanisms of the effects of ketamine, the antidepressant mechanism has not been fully elucidated. To understand the involvement of the brain serotonergic system in the actions of ketamine, we performed a positron emission tomography (PET) study on non-human primates. Four rhesus monkeys underwent PET studies with two serotonin (5-HT)-related PET radioligands, [¹¹C]AZ10419369 and [¹¹C]DASB, which are highly selective for the 5-HT1B receptor and serotonin transporter (SERT), respectively. Voxel-based analysis using standardized brain images revealed that ketamine administration significantly increased 5-HT1B receptor binding in the nucleus accumbens and ventral pallidum, whereas it significantly reduced SERT binding in these brain regions. Fenfluramine, a 5-HT releaser, significantly decreased 5-HT1B receptor binding, but no additional effect was observed when it was administered with ketamine. Furthermore, pretreatment with 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX), a potent antagonist of the glutamate α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor, blocked the action of ketamine on the 5-HT1B receptor but not SERT binding. This indicates the involvement of AMPA receptor activation in ketamine-induced alterations of 5-HT1B receptor binding. Because NBQX is known to block the antidepressant effect of ketamine in rodents, alterations in the serotonergic neurotransmission, particularly upregulation of postsynaptic 5-HT1B receptors in the nucleus accumbens and ventral pallidum may be critically involved in the antidepressant action of ketamine.     

Dopamine neurotransmission is involved in the attenuating effects of 5-HT3 receptor antagonist MDL 72222 on acute methamphetamine-induced locomotor hyperactivity in mice

We have previously shown that 5-HT3 receptors are involved in the development and expression of methamphetamine (MAP)-induced locomotor sensitization in mice. In the present study, we further examined whether the dopaminergic system is involved in the attenuating effects of MDL 72222, a 5-HT3 receptor antagonist, on acute MAP-induced locomotor hyperactivity. For this, we examined alterations of dopamine (DA) in the form of D1 receptor, D2 receptor, and dopamine transporter (DAT) binding labeled with [3H]SCH23390 for D1, [3H]raclopride for D2, and [3H]mazindol for DAT binding in the mouse brains with acute MAP exposure or pretreatment of MDL 72222 with MAP. No significant differences were detected in the D1 receptor, D2 receptor, or DAT binding between any of the groups studied. Interestingly, we found increased DA levels in the striatum following acute MAP exposure; these increased levels were reversed by pretreatment with MDL 72222, but did not affect 5-HT levels in the dorsal raphe. Overall, our results suggest that dopamine neurotransmission plays an important role in the attenuating effects of 5-HT3 receptor antagonist MDL 72222 on acute MAP-induced locomotor hyperactivity in mice.     

Mapping of 5-HT2A receptors and their mRNA in monkey brain: [3H]MDL100,907 autoradiography and in situ hybridization studies

The anatomic distribution of serotonin 5-HT2A receptors visualized with [3H]MDL100,907 and of their mRNA detected by in situ hybridization were studied in monkey brain. Both autoradiographic patterns of signal showed heterogeneous distributions and were in general in good agreement in the majority of brain regions. In most neocortical areas, [3H]MDL100,907 presented a four-banded pattern with layers I and III-IV more intensely labeled and layers II and V-VI showing weaker labeling. 5-HT2A receptor mRNA was detected in layers III and IV, and in some cases also in layers II and V. In intra- and extra-calcarine areas of striate cortex a five-banded pattern was distinguished, with layers III and IVc-V showing the highest densities of [3H]MDL100,907 labeling. These two areas showed the highest neocortical hybridization signal. An unexpected finding was the presence of low densities of [3H]MDL100,907 labeling and 5-HT2A receptor mRNA in choroid plexus. Comparison of the distribution of [3H]MDL100,907 and [3H]ketanserin binding sites in monkey brain regions with high nonspecific [3H]ketanserin binding (caudate, putamen, substantia nigra, inferior olive) revealed specific binding of [3H]MDL100,907 with very low nonspecific binding. Some differences were noted between the distribution of [3H]MDL100,907-labeled 5-HT2A receptors in monkey brain and the previously reported distribution of these receptors in human brain: absence of striosome labeling in monkey striatum and different patterns of neocortical labeling. The present results provide the first detailed comparison of 5-HT2A receptor and mRNA distribution in primate brain. The observed species differences in 5-HT2A receptor distribution should be considered when extrapolating results among different species.     

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors

The distribution of serotonin-1 (5-HT1) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with [3H]serotonin (5-[3H]HT), 8-hydroxy-2-[N-dipropylamino-3H]tetralin (8-OH- [3H]DPAT), [3H]LSD and [3H]mesulergine, and the densities quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Competition experiments for 5-[3H]HT binding by several serotonin-1 agonists led to the identification of brain areas enriched in each one of the three subtypes of 5-HT1 recognition sites already described (5-HT1A, 5-HT1B, 5-HT1C). The existence of these 'selective' areas allowed a detailed pharmacological characterization of these sites to be made in a more precise manner than has been attained in membrane-binding studies. While 5-[3H]HT labeled with nanomolar affinity all the 5-HT1 subtypes, the other 3H-labeled ligands labeled selectively 5-HT1A (8-OH-[3H]DPAT), 5-HT1C ([3H]mesulergine) and both of them ([3H]LSD). Very high concentrations of 5-HT1 receptors were localized in the choroid plexus, lateroseptal nucleus, globus pallidus and ventral pallidum, dentate gyrus, dorsal subiculum, olivary pretectal nucleus, substantia nigra, reticular and external layer of the entorhinal cortex. The different fields of the hippocampus (CA1-CA4), some nuclei of the amygdaloid complex, the hypothalamic nuclei and the dorsal raphé, among others, also presented high concentrations of sites. Areas containing intermediate densities of 5-HT1 receptors included the claustrum, olfactory tubercle, accumbens, central grey and lateral cerebellar nucleus. The nucleus caudate-putamen and the cortex, at the different levels studied, presented receptor densities ranging from intermediate to low. Finally, in other brain areas--pons, medulla, spinal cord--only low or very low concentrations of 5-HT1 receptors were found. From the areas strongly enriched in 5-HT1 sites, dentate gyrus and septal nucleus contained 5-HT1A sites, while globus pallidus, dorsal subiculum, substantia nigra and olivary pretectal nucleus were enriched in 5-HT1B. The sites in the choroid plexus, which presented the highest density of receptors in the rat brain, were of the 5-HT1C subtype. The distribution of 5-HT1 receptors reported here is discussed in correlation with the distribution of serotoninergic neurons and fibers, the related anatomical pathways and the effects which appear to be mediated by these sites.     

In vitro characterization of SLV308 (7-[4-methyl-1-piperazinyl]-2(3H)-benzoxazolone, monohydrochloride): a novel partial dopamine D2 and D3 receptor agonist and serotonin 5-HT1A receptor agonist

Present Parkinson's disease treatment strategies are far from ideal for a variety of reasons; it has therefore been suggested that partial dopamine receptor agonism might be a potential therapeutic approach with potentially fewer side effects. In the present study, we describe the in vitro characterization of the nonergot ligand SLV308 (7-[4-methyl-1-piperazinyl]-2(3H)-benzoxazolonemonohydrochloride). SLV308 binds to dopamine D(2), D(3), and D(4) receptors and 5-HT(1) (A) receptors and is a partial agonist at dopamine D(2) and D(3) receptors and a full agonist at serotonin 5-HT(1) (A) receptors. At cloned human dopamine D(2,L) receptors, SLV308 acted as a potent but partial D(2) receptor agonist (pEC(50) = 8.0 and pA(2) = 8.4) with an efficacy of 50% on forskolin stimulated cAMP accumulation. At human recombinant dopamine D(3) receptors, SLV308 acted as a partial agonist in the induction of [(35)S]GTPgammaS binding (intrinsic activity of 67%; pEC(50) = 9.2) and antagonized the dopamine induction of [(35)S]GTPgammaS binding (pA(2) = 9.0). SLV308 acted as a full 5-HT(1) (A) receptor agonist on forskolin induced cAMP accumulation at cloned human 5-HT(1) (A) receptors but with low potency (pEC(50) = 6.3). In rat striatal slices SLV308 concentration-dependently attenuated forskolin stimulated accumulation of cAMP, as expected for a dopamine D(2) and D(3) receptor agonist. SLV308 antagonized the inhibitory effect of quinpirole on K(+)-stimulated [(3)H]-dopamine release from rat striatal slices (pA(2) = 8.5). In the same paradigm, SLV308 had antagonist properties in the presence of quinpirole (pA(2) = 8.5), but the partial D(2) agonist terguride had much stronger antagonistic properties. In conclusion, SLV308 combines high potency partial agonism at dopamine D(2) and D(3) receptors with full efficacy low potency serotonin 5-HT(1) (A) receptor agonism and is worthy of profiling in in vivo models of Parkinson's disease.     

[3H]Paroxetine and [3H]citalopram as markers of the human brain 5-HT uptake site: a comparison study

Summary The binding of [3H]paroxetine and [3H]citalopram to the human brain serotonin (5-HT) uptake site has been characterized and compared. Our results reveal that the binding exclusively involved with the 5-HT uptake site is identical for both [3H]ligands. The selective 5-HT uptake inhibitor citalopram displays the highest affinity for this uptake site, as compared with the affinities obtained for desipramine and norzimeldine, which is in accordance with their respective blockage of 5-HT uptake. Similar Bmax values were obtained for both radioligands in the brain regions studied, indicating their binding to the same presynaptic membrane protein. Together these findings suggest that both [3H]paroxetine and [3H]citalopram are good markers of the 5-HT transporter as both bind selectively and with high affinity to the serotonin uptake sites. However, the higher affinity of [3H]paroxetine confirms that this compound is the best radioligand for the 5-HT uptake site available today.     

5-HT(1A) receptor imaging in the human brain: effect of tryptophan depletion and infusion on [(18)F]MPPF binding

The 5-HT(1A) receptor has been implicated in a variety of physiological processes, psychiatric disorders, and neurodegenerative disorders. [(18)F]MPPF is a useful radioligand for quantitative imaging of 5-HT(1A) receptors in human subjects. Previous studies have shown that the binding of some radioligands is sensitive to changes in neurotransmitter concentration, whereas in other cases, binding is not affected. In the present study we investigated if [(18)F]MPPF binding to the 5-HT(1A) receptor is sensitive to changes in 5-HT. Changes in 5-HT levels were achieved by influencing its synthesis through tryptophan depletion, including a tryptophan-free amino acid drink 4.5 h prior to the PET scan and tryptophan infusion (10 mg/ml, 50 mg/kg, 30 min, starting 60 min prior to the PET scan). Binding of [(18)F]MPPF in the brain of six healthy, male volunteers was compared in these two conditions. Mean binding potentials in the medial temporal cortex, cortical regions, and raphe nucleus did not significantly differ between the two conditions. The results of the study show that, under the experimental conditions used, [(18)F]MPPF binding was not affected. It is hypothesized that the increases in 5-HT levels needed to produce a measurable effect on [(18)F]MPPF binding would be significantly greater than that possible with tryptophan manipulation. Therefore, in pathological conditions, where such large increases in 5-HT levels are not expected, [(18)F]MPPF seems a useful ligand to measure 5-HT(1A) receptor distribution without the interference of endogenous 5-HT.     

The 5-HT(2A) receptor and serotonin transporter in Asperger's disorder: A PET study with [¹¹C]MDL 100907 and [¹¹C]DASB

Evidence from biochemical, imaging, and treatment studies suggest abnormalities of the serotonin system in autism spectrum disorders, in particular in frontolimbic areas of the brain. We used the radiotracers [(11)C]MDL 100907 and [(11)C]DASB to characterize the 5-HT(2A) receptor and serotonin transporter in Asperger's Disorder. Seventeen individuals with Asperger's Disorder (age=34.3 ± 11.1 years) and 17 healthy controls (age=33.0 ± 9.6 years) were scanned with [(11)C]MDL 100907. Of the 17 patients, eight (age=29.7 ± 7.0 years) were also scanned with [11C]DASB, as were eight healthy controls (age=28.7 ± 7.0 years). Patients with Asperger's Disorder and healthy control subjects were matched for age, gender, and ethnicity, and all had normal intelligence. Metabolite-corrected arterial plasma inputs were collected and data analyzed by two-tissue compartment modeling. The primary outcome measure was regional binding potential BP(ND). Neither regional [11C]MDL 100907 BP(ND) nor [11C]DASB BP(ND) was statistically different between the Asperger's and healthy subjects. This study failed to find significant alterations in binding parameters of 5-HT(2A) receptors and serotonin transporters in adult subjects with Asperger's disorder.     

Serotonin receptors in the human brain. I. Characterization and autoradiographic localization of 5-HT1A recognition sites. Apparent absence of 5-HT1B recognition sites

The presence, pharmacological properties and anatomical distribution of serotonin-1A and serotonin-1B receptor subtypes were studied in the human brain by both radioligand binding assays and autoradiographic procedures. Frontal cortices and hippocampi from human brains obtained at autopsy without evidence of neurological disease were used in this study. [3H]5-HT was used to label both 5-HT1A and 5-HT1B receptor subtypes. 5-HT1A receptors were selectively labeled by [3H]8-hydroxy-2[di-N-propylamino]tetralin, while 5-HT1B receptors were labeled by (-)-[125I]iodocyanopindolol ([125I]CYP) in the presence of 30 microM isoprenaline. The pharmacological profile of 5-HT1A receptors in human brain tissue was very similar to those previously found in rat and pig brain tissues. The general anatomical distribution of these sites was also similar to that found in the rat brain, although some differences were observed when analyzed at the microscopic level. In contrast to 5-HT1A receptors, it was not possible to identify 5-HT receptors having the pharmacological properties of 5-HT1B sites in the human brain, using either [3H]5-HT or [125I]CYP as ligands. The absence of identifiable 5-HT1B receptors in human brain preparations, a fact previously found in pig brain tissue, is discussed in terms of the existence of species differences in brain serotonin receptors.     

Higher postmortem prefrontal 5-HT2A receptor binding correlates with lifetime aggression in suicide.

BACKGROUND: In vivo studies find altered serotonin function associated with aggressive and suicidal behaviors. Postmortem studies also reveal serotonergic alterations in suicide subjects but have not reported on the relationship between aggression and the serotonin system. We measured 5-hydroxytryptamine 2A (5-HT(2A)) receptor binding in prefrontal cortex of suicide and nonsuicide subjects and explored the relationship between 5-HT(2A) receptor binding, lifetime aggression, and suicide. METHODS: The 5-HT(2A) receptor binding in coronal sections of prefrontal cortex was quantified by autoradiography with [(3)H] ketanserin in 37 suicide subjects and 73 nonsuicide subjects. The relationship between [(3)H] ketanserin binding and lifetime aggression, rated on the Brown-Goodwin Aggression History Scale, was assessed controlling for age and sex. RESULTS: In suicide subjects, lifetime aggression scores correlated positively with [(3)H] ketanserin binding in all prefrontal Brodmann areas examined, after adjusting for age and sex. This was not the case in nonsuicide subjects. We found no significant differences in aggression scores or [(3)H] ketanserin binding between the suicide subjects and nonsuicide subjects. CONCLUSIONS: The relationship between aggression and 5-HT(2A) receptor binding in suicide subjects, but not in nonsuicide subjects, may reflect differences in the regulation of the 5-HT(2A) receptor related to suicidal behavior and perhaps other proaggressive changes in brains of suicide subjects.