5-HT6 receptor binding sites in schizophrenia and following antipsychotic drug administration: autoradiographic studies with [125I]SB-258585

The 5-hydroxytryptamine (5-HT; serotonin)-6 receptor (5-HT6R) is a putative target of atypical antipsychotic drugs and its mRNA expression is altered in schizophrenia. [125I]SB-258585 is a selective 5-HT6R antagonist which has been well characterized for use in the rat brain. The present study evaluated its suitability for receptor autoradiography in the human brain and its application to quantitative studies. The affinity (K(d) approximately 1.2 nM) and relative distribution of binding sites (striatum > cortex approximately hippocampus) were similar to the rat. The distribution of [125I]SB-258585 binding in these regions was also consistent with that of 5-HT6R mRNA, determined in parallel using in situ hybridization. [125I]SB-258585 binding site densities were measured in dorsolateral prefrontal cortex of 20 patients with chronic schizophrenia and compared with 17 normal subjects. No differences were seen between groups. Neither were [125I]SB-258585 binding site densities affected in the frontal cortex or striatum of rats following 2 weeks' administration of the antipsychotic drugs haloperidol, chlorpromazine, olanzapine, risperidone, or clozapine. In summary, [125I]SB-258585 is a suitable radioligand for studies of human brain 5-HT6R binding sites and shows that their distribution is broadly similar to that of the rodent. The lack of effect of schizophrenia or antipsychotic drug administration on [125I]SB-258585 binding suggests that an altered receptor density does not contribute to any involvement which the 5-HT6R may have in the disease or its treatment.     

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. II. Serotonin-2 receptors

The distribution of serotonin-2 (5-HT2) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with four ligands: [3H]ketanserin, [3H]mesulergine, [3H]LSD and [3H]spiperone, which are reported to show high affinity for 5-HT2 receptors. Co-incubation with increasing concentrations of several well-known 5-HT2-selective drugs, such as pirenperone, cinanserin and ketanserin, resulted in an inhibition of the binding of the four 3H-labeled ligands to the same areas. However, all of them recognized, in addition to 5-HT2 sites, other populations of binding sites. Receptor densities were quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Our results reveal a heterogeneous distribution of 5-HT2 receptor densities in the rat brain. Very high concentrations were localized in the claustrum, olfactory tubercle and layer IV of the neocortex. The anterior olfactory nucleus, piriform cortex and layer I of neocortex were also rich in 5-HT2 receptors. Intermediate concentrations of receptors were found in caudate putamen, nucleus accumbens, layer V of neocortex, ventral dentate gyrus and mammillary bodies. Areas containing only low concentrations of receptors included the thalamus, hippocampus, brainstem, medulla, cerebellum and spinal cord. The specificity of the different ligands used is discussed in terms of the other populations of sites recognized by them. The distribution of 5-HT2 receptors here reported is discussed in correlation with (a) the known distribution of serotoninergic terminals, (b) the specific anatomical systems and (c) the central effects reported to be mediated by 5-HT2-selective drugs.     

Quantitative analysis of benzodiazepine receptor in temporal lobe epilepsy: [(125)I]iomazenil autoradiographic study of surgically resected specimens

PURPOSE: To evaluate the changes of the inhibitory neurotransmitter receptor system related to epileptogenesis by measuring central benzodiazepine receptors (BZDRs) in surgically resected specimens of temporal lobe epilepsy by using [(125)I]iomazenil autoradiography. METHODS: Surgically resected specimens were obtained from 66 temporal lobe epilepsy patients [51 with mesial temporal lobe epilepsy (MTLE) and 15 with non-MTLE] receiving no BZDs and seven MTLE patients receiving BZDs. BZDR densities in brain sections were measured by using [(125)I]iomazenil autoradiography. Cell densities were measured from cresyl violet-stained sections. RESULTS: Compared with non-MTLE patients, non-BZD-treated MTLE patients showed remarkable reduction of BZDR density in the pyramidal cell region of cornu ammonis (CA) 1, CA3, and CA4, and a smaller but significant reduction in CA2 and the molecular and granule cell layers of dentate gyrus (mDG). In the MTLE group, the BZDR density in the mDG correlated with that in lateral cortex. Significant correlations between BZDR density and cell density were found in all hippocampal regions. A significant difference in BZDR density/cell-density ratio was observed in CA1 region between MTLE and non-MTLE. BZD-treated patients tended to have lower BZDR densities than did non-BZD-treated patients, although the differences did not reach significance. In all MTLE cases, [(123)I]iomazenil singlephoton emission computed tomography (SPECT) showed decreased BZDR binding in MTL. CONCLUSIONS: In MTLE, BZDR densities decreased parallel to reduction in cell density in most hippocampal subfields, but BZDR density appeared to decrease in excess of neuron loss in CA1. [(125)I]iomazenil SPECT might be useful for detecting in vivo changes of BZDR density.     

Measurement of 5-HT(1A) receptor density and in-vivo binding parameters of [(18)F]mefway in the nonhuman primate

The goal of this work was to characterize the in-vivo behavior of [¹⁸F]mefway as a suitable positron emission tomography (PET) radiotracer for the assay of 5-hydroxytryptamine_1A (5-HT_1A) receptor density (B_max). Six rhesus monkeys were studied using a multiple-injection (M-I) protocol consisting of three sequential bolus injections of [¹⁸F]mefway. Injection times and amounts of unlabeled mefway were optimized for the precise measurement of B_max and specific binding parameters k_off and k_on for estimation of apparent K_D. The PET time series were acquired for 180 minutes with arterial sampling performed throughout. Compartmental analysis using the arterial input function was performed to obtain estimates for K₁, k₂, k_off, B_max, and K_Dapp in the cerebral cortex and raphe nuclei (RN) using a model that accounted for nontracer doses of mefway. Averaged over subjects, highest binding was seen in the mesial temporal and dorsal anterior cingulate cortices with B_max values of 42±8 and 36±8 pmol/mL, respectively, and lower values in the superior temporal cortex, RN, and parietal cortex of 24±4, 19±4, and 13±2 pmol/mL, respectively. The K_Dapp of mefway for the 5-HT_1A receptor sites was 4.3±1.3 nmol/L. In conclusion, these results show that M-I [¹⁸F]mefway PET experiments can be used for the in-vivo measurement of 5-HT_1A receptor density.     

Distribution of [Leu,Pro]NPY-sensitive, BIBP3226-insensitive [I]PYY(3–36) binding sites in rat brain: possible relationship to Y5 NPY receptors

Recently, using molecular cloning approaches, three new neuropeptide Y (NPY)/peptide YY (PYY) receptors have been described in rodent brain, with pharmacological profiles that differ from the three previously described Y₁, Y₂ and Y₃ NPY receptors and the Y₄ pancreatic polypeptide- (PP-) preferring receptor. Two of these new receptors are spice variants and are called Y₅ receptors, whilst a third receptor has been called Y₆ and has been suggested to be expressed only in the mouse. In the absence of a totally selective Y₅ and/or Y₆ radioligands, we have examined [¹²⁵I]PYY(3–36) binding, which binds Y₂ and Y₅/Y₆ receptors, using homogenate assays and quantitative receptor autoradiography to study the distribution of the three newly discovered Y₅/Y₆ receptors by masking binding to Y₁ receptors with high concentrations of the non-peptidergic selective Y₁ antagonist, BIBP3226, and using either [Leu³¹,Pro³⁴]NPY or human PP to mask binding to Y₅ and Y₆ receptors, leaving binding to Y₂ receptors. Using this approach, [¹²⁵I]PYY(3–36) labels a small population of Y₁ receptors and a larger population of binding sites that are insensitive to BIBP3226, human PP and [Leu³¹,Pro³⁴]NPY, presumed to be Y₂ receptors. There was also [¹²⁵I]PYY(3–36) binding to sites sensitive to NPY, human PP and [Leu³¹,Pro³⁴]NPY, but insensitive to BIBP3226, located in the hypothalamus, amygdala, hippocampus and thalamus. As one of the recently cloned Y₅ receptors is synthesized in these regions, as shown by in-situ hybridization techniques, we suggest that the small population of [¹²⁵I]PYY(3–36) binding sites which are sensitive to human PP and [Leu³¹,Pro³⁴]NPY, but insensitive to BIBP3226, may represent binding to Y₅ receptors. We have been unable, however, to visualize a smaller population of Y₆ receptors which are labelled by [¹²⁵I]PYY₃–36 and sensitive to [Leu³¹,Pro³⁴]NPY, but not to BIBP3226 and human PP, confirming that the murine Y₆ receptor does not appear to be expressed in rat brain.     

Glycine receptor distribution in mouse CNS: autoradiographic localization of [H]strychnine binding sites

Biochemical and electrophysiological studies of mammalian CNS indicate that the amino acid, glycine, is a major inhibitory neurotransmitter whose location is, for the large part, confined to the spinal cord and brain stem. In this study, autoradiographs of C57BL/6J mouse brain sections labeled with [³H] strychnine, a potent antagonist of glycine, were used to map the distribution of glycine receptors in the CNS. Autoradiographs showed highly localized areas of grain density, which confirmed the gross distributions described in homogenate binding studies and gave a more precise regional localization of glycine receptors in this animal. The highest overall labeling was observed in the spinal cord and medulla; areas of highest grain density included the dorsal horn of the spinal cord, the cranial nerve nuclei, the dorsal column nuclei and nuclei of the medullary reticular formation. A decrease in overall grain density was observed rostrally throughout the midbrain and pons; in caudal regions, however, dense labeling was observed over the trigeminal, vestibular and facial nuclei and over the major nuclei of the auditory system. In more rostral areas, the interpeduncular nucleus and the substantia nigra were also clearly delineated, as were certain thalamic nuclei. The cerebellum, cortex, hippocampus and olfactory bulbs showed only very low levels of grain density. In summary, it appears that high concentrations of glycine receptors in the brain and spinal cord may be preferentially localized to neurons involved in the processing of information originating from exteroceptive sensory mechanoreceptors.     

Selective heterologous regulation of 5-HT1A receptor-stimulated 35S GTPgammaS binding in the anterior cingulate cortex as a result of 5-HT2 receptor activation

Previous studies have shown that administration of the 5-HT(2) receptor agonist DOI to rats results in the heterologous desensitization of 5-HT(1A) receptor-mediated behavioral and neuroendocrine responses [Neuropsychopharmacology 19 (1998) 354; J. Neurosci. 21 (2001) 7919]. We hypothesized that the basis for these changes in 5-HT(1A) receptor function may involve changes in the capacity of the 5-HT(1A) receptor to activate G proteins. We examined the effect of chronic administration of DOI on the regulation of 5-HT(1A) receptor function at the level of receptor-G protein interaction using quantitative autoradiography of [(35)S]GTPgammaS binding stimulated by the 5-HT(1A) receptor agonist (+/-)8-OH-DPAT (1 microM). Repeated administration of DOI (1 mg/kg, s.c. once daily for 8 days) resulted in a marked down-regulation in 5-HT(2A) binding sites, as labeled by the antagonist radioligand [(3)H]ketanserin, throughout the cerebral cortex. Chronic DOI treatment also resulted in a significant and selective attenuation of 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in the anterior cingulate cortex (vehicle-treated: 74+/-7.7% above basal; DOI-treated: 43+/-4.6% above basal). Interestingly, 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding was not altered in the dorsal or median raphe, or in the limbic structures and other cortical regions examined. The decrease in 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in anterior cingulate cortex was not due to a decrease in 5-HT(1A) receptor number, indicating that the capacity of the 5-HT(1A) receptor to activate G proteins is attenuated in this cortical area following repeated DOI treatment. The heterologous regulation of 5-HT(1A) receptor function by chronic 5-HT(2) receptor activation in the anterior cingulate cortex raises interesting questions as to how the regulatory interaction between these serotonin receptor subtypes influences cognition, memory and emotion.     

Non 5-HT1A/5-HT1C [3H]5-HT binding sites in the hamster, opossum, and rabbit brain show similar regional distribution but different sensitivity to beta-adrenoceptor antagonists.

We have used receptor autoradiography to investigate the distribution and pharmacological profile of non 5-HT1A/5-HT1C[3H]5-hydroxytryptamine binding sites in the brain of rabbits, hamsters and opossums. These data were compared to those found under similar conditions in the brain of rats and guinea pigs, species which are known to possess 5-HT1B and 5-HT1D receptors, respectively. In the presence of 100 nM 8-OH-DPAT and mesulergine, the regional distribution of [3H]5-hydroxytryptamine binding sites was very similar in the brain of all species investigated; densest labelling was observed in the globus pallidus, substantia nigra and superior colliculus. In all species, 5-carboxamidotryptamine competed for the labelled sites in a biphasic manner and metergoline displayed a subnanomolar affinity. In contrast, the beta-adrenoceptor blocking agents (-)propranolol, (-)pindolol, and (+/-)SDZ 21009 were potent displacers only in the rat, hamster and opossum brains. These data indicate that non 5-HT1A/5-HT1C[3H]5-HT binding sites display a high affinity for these agents in a particular rodent suborder as well as in opossum, a phylogenetically unrelated species.     

Decreased muscarinic receptor binding in rat brain after paradoxical sleep deprivation: an autoradiographic study

Previous work demonstrated that paradoxical sleep deprivation (PSD) leads to a decrease in yawning behavior elicited by cholinergic agonists, suggesting that a downregulation of cholinergic muscarinic receptors may occur after PSD. More recent work using intracerebral injections of muscarinic agonists has suggested a critical role for M2 receptors in paradoxical sleep. In this study [³H]AF-DX 384 was used to investigate the effects of PSD on M2-type cholinergic receptors throughout the brain using quantitative autoradiography. After 96 h of paradoxical sleep deprivation, [³H]AF-DX 384 binding was generally reduced throughout the brain, and significantly so in the olfactory tubercle (−20%), n. accumbens (−23%), frontal caudate-putamen (−16%), islands of Callejas (−20%), piriform cortex (−24%), lateral (−26%) and medial (−24%) septum, anteromedial (−19%), ventrolateral (−22%), and lateral geniculate (−15%) nuclei of thalamus, deep layers of the superior colliculus (−15%), entorhinal cortex (−12%) and subiculum (−23%). [³H]AF-DX 384 binding was reduced in pontine structures, but not to a higher degree than in other brain areas. The observed downregulation of M2-type muscarinic receptors after PSD may be causally related to the previously reported decrease in cholinergically induced behaviors after PSD.     

Presynaptic and postsynaptic D1 dopamine receptors in the nigrostriatal system of the rat brain: a quantitative autoradiographic study using the selective D1 antagonist [H]SCH 23390

Ibotenic acid lesions of the caudate-putamen in rat brain resulted in dramatic reductions in [³H]SCH 23390 binding in both the ipsilateral caudate-putamen and substantia nigra reticulata as assessed by quantitative autoradiography. Nigral ibotenic acid and 6-hydroxydopamine lesions did not significantly alter the binding in either structure. This indicates that D₁ receptors in the caudate-putamen are postsynaptic on striatal neurons, while those in the substantia nigra reticulata are presynaptic on nerve terminals originating in the caudate-putamen.     

Effects of the selective 5-HT(7) receptor antagonist SB-269970 in animal models of psychosis and cognition

The 5-hydroxytryptamine₇ (5-HT₇) receptor is a G-protein coupled receptor for serotonin that has been implicated in the pathophysiology of psychiatric and neurological disorders including anxiety, depression and schizophrenia. A number of studies have attempted to evaluate the potential role of the 5-HT₇ receptor in schizophrenia by utilising genetic or pharmacological tools but to date these have provided conflicting results. Here we investigate the effect of a selective 5-HT₇ receptor antagonist, SB-269970, in in vivo psychosis and cognition models and relate efficacy to brain exposures of the compound. SB-269970 significantly attenuated amphetamine-induced rearing and circling in rats. A similar effect was observed in an N-methyl d-aspartic acid (NMDA) receptor antagonist driven psychosis model, where SB-269970 significantly reversed phencyclidine-induced hyperlocomotion, rearing and circling; although the effect was not as robust as with the 5-HT_2a receptor antagonist positive control, MDL100,907. SB-269970 also attenuated a temporal deficit in novel object recognition (NOR), indicative of an improvement in recognition memory. Pharmacokinetic analysis of plasma and brain samples taken after behavioural testing confirmed that efficacy was achieved at doses and pre-treatment times where receptor occupancy was substantial. These findings highlight the anti-psychotic and pro-cognitive potential of 5-HT₇ receptor antagonists and warrant further studies to explore their therapeutic potential in schizophrenia.     

Characterization and quantitative autoradiography of [H]tryptamine binding sites in rat brain

[³H]tryptamine binds with high affinity (K_d = 9.1nM, B_max= 54fmol/mg wet wt.) to tissue sections of rat brain. The binding occurs rapidly and is reversible. Low concentrations of the β-carbolines harmaline (IC₅₀ = 25nM) and tetrahydronorharman (tetrahydro-β-carboline), IC₅₀ = 50nM) inhibit [³H]tryptamine binding. Serotonin (5-HT, IC₅₀ = 2600nM) as well as the 5-HT receptor antagonists methysergide and metergoline displace [³H]tryptamine at much higher concentrations from brain slices. The distribution of [³H]tryptamine binding sites in section of rat brain has been analyzed by quantitative autoradiography. The highest density of binding sites is found in the nucleus (n.) interpreduncularis, a slightly lower one in the locus coeruleus. Moderately labelled are the n. accumbens septi, n. septi lateralis, n. medalis habenulae, n. tractus olfactorii lateralis, the central region of the amydgala, n. caudatsu/putamen, n. reuniens and the hippocampal formation. A low density of binding sites is detected in the cerebral cortex and the subiculum. Even less binding sites are found in the n. dorsalis raphe and the substantia nigra. The pattern of distribution of [³H]tryptamine binding sites differs from that of [³H]5-HT(5-HT₁), [³H]ketanserin (5-HT₂) as well as [³H]imipramine binding sites. These data suggest unique tryptamine binding sites.     

Atypical in vitro and in vivo binding of [3H]S-14506 to brain 5-HT1A receptors

Summary The tritiated derivative of the potent 5-HT_1A receptor agonist S-14506 {1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)piperazine} was tested for its capacity to selectively label the serotonin 5-HT_1A receptors both in vitro in the rat and the mouse brain, and in vivo in the mouse. In vitro studies showed that the pharmacological profile and the distribution of [³H]S-14506 specific binding sites (Kd=0.15 nM) in different brain regions matched perfectly those of the prototypical 5-HT_1A receptor ligand [³H]8-OH-DPAT. However, in the three regions examined (hippocampus, septum, cerebral cortex), the density of [³H]S-14506 specific binding sites was significantly higher (+ 66–90%) than that found with [³H]8-OH-DPAT. Whereas the specific binding of [³H]8-OH-DPAT was markedly reduced by GTP and Gpp(NH)p and increased by Mn²⁺, that of [³H]S-14506 was essentially unaffected by these compounds. In addition, the alkylating agent N-ethylmaleimide was much less potent to inhibit the specific binding of [³H]S-14506 than that of [³H]8-OH-DPAT. Measurement of in vivo accumulation of tritium one hour after i.v. injection of [³H]S-14506 to mice revealed marked regional differences, with about 2.5 times more radioactivity in the hippocampus than in the cerebellum. Pretreatment with 5-HT_1A receptor ligands prevented tritium accumulation in the hippocampus but not in the cerebellum. Autoradiograms from brain sections of injected mice confirmed the specific in vivo labeling of 5-HT_1A receptors by [³H]S-14506, therefore suggesting further developments with derivatives of this molecule for positron emission tomography in vivo in man.     

Human platelet 5-HT2 receptor binding sites re-evaluated: A criticism of recurrent techniques

Summary The human platelet 5-HT₂ receptor may resemble a peripheral model of central 5-HT₂ binding sites and has been linked to changes in 5-HT₂ receptor function in depression. Therefore, evaluation of the human platelet 5-HT₂ binding characteristics is important. Comparing [³H]ketanserin and [³H]LSD as ligands clearly indicated [³H]LSD as ligand of choice for binding studies dealing with the human platelet 5-HT₂ receptor. [³H]LSD binding was specific, saturable, and depended upon incubation time, protein concentration and previous handling of tissue, i.e., use of fresh or frozen tissue. In contrast, studies with [³H]ketanserin were unsatisfactory. Although mean receptor densities and affinities have been relatively constant between individuals and over time in healthy subjects with [³H]LSD, examination of the individual data showed considerable variations within single subjects. Thus, K_D ranged between 0.50 and 0.68 nM, and B_max was in the range of 64.9 to 97.1 fmol/mg protein in healthy individual subjects. Therefore, we recommend [³H]LSD as ligand of choice to study platelet 5-HT₂ receptor binding in humans. Furthermore, repeated measurement of individual data over time should be interpreted cautiously, especially when data from depressed patients are under examination.     

Autoradiographic mapping of 5-HT1B and 5-HT1D receptors in the post mortem human brain using [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 [³H]GR 125743. [³H]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 [³H]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 [³H]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.     

Chronic electroconvulsive shock and 5-HT autoreceptor activity in rat brain: an in vivo microdialysis study

Summary In vivo microdialysis was used to determine the effects of chronic electroconvulsive shock (ECS), given daily for 10 days, on basal 5-HT levels in rat frontal cortex and hippocampus and on the effect of systemic administration of the 5-HT-la receptor agonist, 8-OH-DPAT (0.2 mg/kg), to reduce 5-HT levels in these areas by activation of somatodendritic autoreceptors. Neither basal 5-HT levels nor the effects of 8-OH-DPAT on 5-HT levels were altered after chronic ECS. The effect of systemic administration of the 5-HT_1A and 5-HT_1B antagonist, (±)-pindolol (10mg/kg), to increase 5-HT levels in hippocampus, was also not affected by chronic ECS.     

Characterization of [3H]clozapine binding sites in rat brain

Summary We examined the characteristics of [³H]clozapine binding sites in four rat brain regions (frontal cortex, limbic area, hippocampus and striatum) in order to elucidate the pharmacological profile of this unique atypical antipsychotic drug. The specific [³H]clozapine binding was found to be saturable and reversible in all these brain regions. Scatchard analysis of the saturation data indicated that the specific binding consisted of high- and low-affinity components. Displacement experiments showed that the muscarinic cholinergic receptor represented about 50% of [³H]clozapine binding in each brain area. Serotonin 5-HT₂ and dopamine D₄ receptor binding sites could also be detected by displacement experiments using ketanserin and nemonapride, respectively, in frontal cortex and limbic area, but not in hippocampus or striatum. Alpha-1, alpha-2, histamine H₁, dopamine D₁, D₂, or D₃ receptor components could not be determined within the high-affinity [³H]clozapine binding sites in any brain region. It is possible that the atypical property of clozapine may depend on the modulatory effect on dopaminergic function via 5-HT₂ receptor blockade and/or may be mediated via D₄ receptor blockade in the mesocortical and mesolimbic area.     

Confirmation of fenfluramine effect on 5-HT(1B) receptor binding of [(11)C]AZ10419369 using an equilibrium approach

Assessment of serotonin release in the living brain with positron emission tomography (PET) may have been hampered by the lack of suitable radioligands. We previously reported that fenfluramine caused a dose-dependent reduction in specific binding in monkeys using a classical displacement paradigm with bolus administration of [(11)C]AZ10419369. The aim of this study was to confirm our previous findings using an equilibrium approach in monkey. A total of 24 PET measurements were conducted using a bolus infusion protocol of [(11)C]AZ10419369 in three cynomolgus monkeys. Initial PET measurements were performed to assess suitable K(bol) values. The fenfluramine effect on [(11)C]AZ10419369 binding was evaluated in a displacement and pretreatment paradigm. The effect of fenfluramine on [(11)C]AZ10419369 binding potential (BP(ND)) was dose-dependent in the displacement paradigm and confirmed in the pretreatment paradigm. After pretreatment administration of fenfluramine (5.0 mg/kg), the mean BP(ND) of the occipital cortex decreased by 39%, from 1.38±0.04 to 0.84±0.09. This study confirms that the new 5-HT(1B) receptor radioligand [(11)C]AZ10419369 is sensitive to fenfluramine-induced changes in endogenous serotonin levels in vivo. The more advanced methodology is suitable for exploring the sensitivity limit to serotonin release as measured using [(11)C]AZ10419369 and PET.     

The effects of manipulation of presynaptic 5-HT nerve terminals on postsynaptic 5-HT1 and 5-HT2 binding sites of the rat brain

Summary The effects of long-term treatment of rats with alaproclate and amiflamine on the number and kinetics of 5-HT₁ and 5-HT₂ binding sites were investigated usingin vitro receptor binding techniques. Some other studies have reported down-regulatory effects of alaproclate and amiflamine on 5-HT₂ binding sites in certain regions of the rat forebrain, but no such effects could be detected in the present study. Induction of a high-affinity binding site for³H-5-HT after long-term antidepressant treatment, as has been reported elsewhere, was not obtained in the present study. The results are compared to the effects obtained by treatment of rats with para-chloroarnphetamine (PCA), which depletes the presynaptic neurons of monoamines. These different types of treatment do not cause any change in the binding properties of the specific 5-HT binding sites. It is thus concluded that such manipulations of the presynaptic 5-HT neurons do not affect the postsynaptic 5-HT₁ and 5-HT₂ binding sites.     

Direct demonstration of D1 dopamine receptors in the bovine parathyroid gland using the D1 selective antagonist [125I]-SCH 23982

The presence of D1 dopamine receptors in the parathyroid gland has been proposed based on the demonstration of dopaminergic regulation of adenylate cyclase activity and parathyroid hormone release in dispersed bovine parathyroid cells. Using a radioiodinated D1 selective antagonist [125I]-SCH 23982, we have now directly labeled and characterized the D1 dopamine receptors in bovine parathyroid gland membranes. [125I]-SCH 23982 binds in a saturable manner with high affinity and low nonspecific binding to membranes prepared from bovine parathyroid glands. D1 dopamine receptors are present in this preparation at a concentration of approximately 130 fMoles/mg protein and [125I]-SCH 23982 binding increases with increasing protein concentration in a linear fashion. Determination of the Kd using the association (k1) and dissociation (k-1) rate constants revealed good agreement with the Kd determined by saturation analysis (390 pM vs. 682 pM, respectively). Inhibition of 0.3 nM [125I]-SCH 23982 binding by a series of dopaminergic antagonists verified the D1 nature of this binding site, exhibiting appropriate affinities and rank order of potency. The competition curves of all antagonists exhibited Hill coefficients that were not significantly different from 1. Inhibition of [125I]-SCH 23982 binding by dopamine and other dopaminergic agonists revealed the presence of high and low affinity agonist binding sites. Addition of 200 microM GppNHp effected a complete conversion of high affinity dopamine binding sites to a homogeneous population of low affinity dopamine sites. The D1 receptors identified in the parathyroid gland with [125I]-SCH 23982 appear to be pharmacologically identical with those previously characterized in the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)