Quantitative localization of [H]TCP binding in rat brain by light microscopy autoradiography

The anatomical localization of phencyclidine (PCP)/σ-opiate receptors in rat brain was determined by quantitative light microscopy autoradiography using the new ligand N-(1-[2-thienyl]cyclohexyl)[³H]TCP). TCP is a potent analog of PCP which possesses a higher affinity for PCP/σ-opiate receptor than those PCP itself. The higest of [³H]TCP binding was detected in the hippocampus. Intermediate levels were found in frontal cortex, striatum, amygdala and cerebellum. Specific [³H]TCP binding was undetectable in anterior commissure and corpus callosum. The distribution pattern of [³H]TCP binding sites is similar to the pattern obtained with [³H]PCP but more sharply defined. On the basis of its greater potency and specificity, [³H]TCP may prove superior to [³H]PCP as a molecular probe for the study of brain sigma opiate/phencyclidine receptors.     

The novel anticonvulsant MK-801: a potent and specific ligand of the brain phencyclidine/gd-receptor

MK-801 (5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine maleate) is a novel anticonvulsant agent reported to antagonize certain N-methyl-spd-aspartate (NMDA)-mediated effects non-competitively. The question arises of the mechanism underlying the anti-NMDA and anticonvulsant effects of MK-801. In the present study MK-801 is shown to be an extremely potent inhibitor of the binding of N-[³H](1-[2-thienyl]cyclohexy)piperidine) ([s3H]TCP to brain phencyclidine (PCP)/δ-receptors. It is IC₅₎ value of 3.8 ± 0.8 nM in this assay ranks it as the most potent known ligand of brain PCP/δ-receptors. Addition of MK-801 altered the apparent K_d but not the apparent B_max values for [³H]TCP binding, indicating a competitive interaction. The specificity of action of MK-801 is supported by the finding that MK-801 strongly inhibited the binding of (+)-N-[³H]allylnormetazocine ((+)-[³H]SKF 10.047) to the PCP/δ-receptor but its effect on (+)-[³H]SKF 10.047 binding to the non-PCP, haloperidol-sensitive δ-binding site was weaker by several orders of magnitude. Furthermore, MK-801 exerts PCP-like antagonistic effects upon NMDA-induced [³H]norepinephrine release. These findings support the concept that the anticonvulsant and anti-NMDA effects of MK-801 result from its being the most potent known ligand of PCP/δ-receptors.     

3H]TCP: a new tool with high affinity for the PCP receptor in rat brain

PCP binding sites have previously been demonstrated in the central nervous system with [3H]PCP. We now describe the binding properties to rat brain membranes of [3H]TCP, a PCP derivative. It is very advantageous to use [3H]TCP instead of [3H]PCP for the 3 following reasons: (i) it has a better affinity (Kd = 7.4 nM) for PCP binding sites than PCP itself; (ii) it dissociates slowly from its binding sites (t 1/2 = 20 min); (iii) the non-specific binding component obtained with [3H]TCP is much lower than that found with [3H]PCP.     

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.     

Distribution of α-adrenergic, β-adrenergic and dopaminergic receptors in discrete hypothalamic areas of rat

Catecholamine receptor binding sites were measured in discrete hypothalamic nuclei or regions as well as in certain extrahypothalamic areas of the adult male rat. For each assay, discrete areas were microdissected from frozen tissue sections and pooled from several animals. Specific high affinity binding sites were assessed at fixed ligand concentrations for [³H]p-aminoclonidine (PAC) and [³H](2-C 2′,6′-(CH₃O)₂ phenoxyethylamino)-methylbenzodioxan (WB-4101) for α-adrenergic receptor sites, for [³H]dihydroalprenolol (DHA) for β-adrenergic receptor sites, and for [³H]2-amino-6, 7-dihydroxy-1,2,3,4-tetrahydronaphtalene (ADTN) and [³H]spiroperidol in the presence of cinanserin for dopaminergic receptor sites.Regional variations in [³H]WB-4101 binding were relatively small in magnitude, with most hypothalamic and extrahypothalamic areas possessing between 60 and 90% of the binding in frontal cortex. [³H]PAC binding showed a wider range of binding density across brain areas than did [³H]WB-4101, but, in general, variations in [³H]PAC binding paralleled those in [³H]WB-4101 binding. In hypothalamus, binding was characterized as being predominantly to α₁-receptors in the of [³H]WB-4101 and to α₂-receptors in the case of [³H]PAC. The medial hypothalamic areas exhibited a somewhat higher density of these α-adrenergic sites than did the lateral hypothalamus (perifornical hypothalamus and medial forebrain bundle). Also, the ratio of [³H]PAC to [³H]WB-4101 binding differed in different hypothalamic areas, ranging from 1.5:1 to 4:1. The median eminence was exceptional in that it contained appreciable [³H]PAC but no significant [³H]WB-4101 binding sites at the radioligand concentrations tested. Binding of [³H]DHA to β-adrenergic receptors varied over approximately a 3-fold range in the different hypothalamic areas, with binding highest in the medial forebrain bundle and the medial preoptic area, and lowest in the periventricular, dorsomedial and posterior hypothalamic nuclei, the median eminence and the zona incerta. The ratio of β-adrenergic to α-adrenergic binding sites was generally lower in the medial than in the lateral hypothalamic areas and higher in the extrahypothalamic areas examined than in the hypothalamus. With regard to [³H]spiroperidol and [³H]ADTN binding to dopaminergic sites, the striatum, nucleus accumbens and olfactory tubercle showed a greater density of [³H]spiroperidol than of [³H]ADTN sites, in contrast to the hypothalamus where [³H]ADTN binding was more predominant. Within the hypothalamus, [³H]ADTN binding was relatively uniform, while [³H]spiroperidol binding was quite high in four hypothalamic areas (lateral perifornical area, medial forebrain bundle, paraventricular and dorsomedial nuclei), intermediate in the median eminence and arcuate nucleus, and low or not detectable in all other hypothalamic areas.     

RTI-4793-14, a new ligand with high affinity and selectivity for the (+)-MK801-insensitive [3H]1-[1-(2 thienyl)cyclohexyl]piperidine binding site (PCP site 2) of guinea pig brain

[³H]TCP, an analog of the dissociative anesthetic phencyclidine (PCP), binds with high affinity to two sites in guinea pig brain membranes, one that is MK-801 sensitive and one that is not. The MK-801-sensitive site (PCP site 1) is associated with NMDA receptors, whereas the MK-801-insensitive site (PCP site 2) may be associated with biogenic amine transporters (BAT). Although several “BAT ligands” are known that bind selectively to PCP site 2 and not to PCP site 1 (such as indatraline), these compounds have low affinity for site 2 (K_i values > 1 μ). Here we demonstrate that the novel pyrrole RTI-4793-14 is a selective, high affinity ligand for PCP site 2. We determined the IC₅₀ values of RTI-4793-14 and several reference compounds [PCP, (+)-MK801 and indatraline] for PCP site 1 (assayed with [³H](+)-MK801), PCP site 2 (assayed with [³H]TCP in the presence of 500 nM (+)-MK801) and a variety of BAT-related measures ([³H]CFT binding to the DA transporter, [³H]nisoxetine binding to the norepinephrine transporter, [³H]dopamine uptake, [3H]serotonin uptake). In addition, we determined the ability of RTI-4793-14 to block NMDA responses in cultured hippocampal neurons under voltage clamp. (+)-MK801 had high affinity for PCP site 1 (4.6 nM) and potently inhibited NMDA-induced responses, but was much less potent in the BAT-related measures (IC₅₀s > 10 μ). PCP had high affinity at PCP site 1 (IC₅₀ = 92 nM) and PCP site 2 (IC₅₀ = 117 nM), and was moderately potent in all BAT-related measures except [³H]nisoxetine binding. Indatraline was potent in BAT-related measures (IC50s, 2 to 5 nM), but weak in other measures (IC₅₀s > 1 μ). In contrast, RTI-4793-14 had high affinity for PCP site 2 (38 nM), low affinity for PCP site 1 (> 36 μ), moderate IC₅₀s for all BAT-related measures, and negligible activity at NMDA receptors. Viewed collectively, these data indicate that RTI-4793-14 binds with high affinity and selectivity to PCP site 2 and provide further support for an association between PCP site 2 and the BATs. © 1994 Wiley-Liss, Inc.     

Coupling between hypothalamic α2-adrenoceptors and [H]mazindol binding site in response to several hyperglycaemic stimuli in mice

The hypothalamic response to circulating glucose and insulin levels was studied in the mouse by differentially attenuating glucose-homeostasis. The administration of glucose, 2-deoxyglucose or the α₂-adrenoceptor agonist UK 14.304 was accompanied by a persistent hyperglycaemia; however, an increase in insulin levels was obtained with glucose and a decrease with the other two manipulations. Both α₂-adrenoceptors (labeled with [³H]idazoxan) and the anorectic recognition site (labeled with [³H]mazindol) were upregulated by the three treatments. A good correlation was obtained between circulating glucose levels and either hypothalamic [³H]mazindol binding (r = 0.70, P < 0.001) or [³H]idazoxan binding (r = 0.63, P < 0.001), as well as between the two binding sites (r = 0.88, P < 0.001). No correlation was obtained between circulating insulin levels and these binding sites (r = 0.18, r = 0.26, P =n.s. for [³H]mazindol and [³H]idazoxan binding, respectively). It is suggested the α₂-adrenoceptors and the anorectic binding sites are associated in their response to glucose as part of a hypothalamic center involved in the regulation of feeding mechanisms.     

Characterization of adenosine receptors in brain using N cyclohexyl [H]adenosine

The presence of distinct binding sites for adenosine in both the CNS and PNS has been proposed in numerous studies. The recent availability of stable adenosine analogues such as cyclohexyladenosine, 2-chloroadenosine and diethylphenylxanthine has made the characterization of such a receptor feasible. In the present report the binding of N⁶ cyclohexyl [³H]adenosine ([³H]CHA) to rat brain synaptosomal membranes is characterized. [³H]CHA binding is saturable and exhibits a biphasic kinetic saturation profile characteristic of 2 binding sites. The high affinity site has a K_d of 0.7 nM and the low affinity site 2.4 nM. The respective B_max values are 230 and 120 fmol/mg protein in rat forebrain. The highest density of binding sites is found in the hippocampus and subcellular distribution studies indicate that the [³H]CHA site is predominantly synaptosomal. [³H]CHA binding is highly dependent on the presence of adenosine deaminase since only 30% of the binding capacity is observed in synaptosomal membranes not treated with this enzyme. Of the many cations and anions tested only copper and zinc have effects on [³H]CHA binding. Both metals are potent inhibitors of binding with copper having an IC₅₀ of 30 μM and zinc 150 μM. Sulfhydryl reducing and alkylating agents also inhibit binding indicating that the binding site is a sulfhydryl-dependent protein.     

Differential labeling of dopamine and sigma sites by [H]nemonapride and [H]raclopride in postmortem human brains

The difference between the binding of [³H]nemonapride and [³H]raclopride has been used to quantify dopamine D₄ receptors in postmortem schizophrenic brain studies. Recent work, however, has suggested that at least part of the differential between [³H]nemonapride and [³H]raclopride binding may represent σ rather than D₄ receptor sites. We applied the nemonapride-raclopride subtraction method to postmortem, non-schizophrenic human striatum to examine the variation in dopaminergic receptor binding labeled by these ligands. Variation in σ receptor binding labeled by [³H]nemonapride was studied in frontal cortex, striatum and cerebellum. Specific binding was defined by sulpiride (dopamine receptor ligand), PPAP (σ receptor ligand) and haloperidol (mixed dopaminergic/σ agent), respectively. Haloperidol defined a combination of sites, which were approximately the sum of the dopaminergic and σ components defined by sulpiride and PPAP, respectively. Significant inter-individual variation in the amount of specific binding for dopaminergic and σ receptor sites was observed. However, no significant nor consistent observation of striatal dopamine D₄ receptors or D₄-like binding sites was observed in the striatum even though two independent sets of tissues, with different dissections were used. The inconsistencies in some previous postmortem studies appear to be at least partially explained by the inclusion of both σ and dopaminergic components in [³H]nemonapride binding and the inherent high inter-individual variability of the different components.     

Blockade ofCa influx through theN-methyl-d-aspartate receptor ion channel by the non-psychoactive cannabinoid HU-211

The effects of the synthetic non-psychoactive cannabinoid(+)-(3S,4S)-7-hydroxy-Δ⁶-tetrahydrocannabinol 1,1-dimethylheptyl (HU-211) on the activity of theN-methyl-d-aspartate (NMDA) receptor/ion channel were examined. HU-211 non-competitively blocks the increase in binding of[³H]N-[1-(2-thienyl)-cyclohexyl]piperidine ([³H]TCP) induced by the polyamines spermine and spermidine or by glutamate and glycine. HU-211 does not, however, affect the direct binding of [³H]glycine and [³H]glutamate to their binding sites on the NMDA receptor, which suggests that the effects of HU-211 are not mediated via the binding sites of glutamate-, glycine- and phencyclidine-like drugs or of polyamines. HU-211 can also block⁴⁵Ca²⁺ uptake through the NMDA-receptor/ion channel in primary cell cultures of rat forebrain. All of the above inhibitory effects of HU-211 on the NMDA-receptor/ion channel activity are stereospecific, since the(−)(3R,4R) enantiomer (HU-210) is ineffective.     

Alteration of nicotinic cholinergic agonist binding sites in hippocampus after fimbria transection

Nicotinic cholinergic agonist binding sites were studied in rat hippocampus by the binding of [³H]acetylcholine in the presence of 1.5 μM atropine sulfate. Following transection of the fimbria/fornix there was a 49% increase in the binding of [³H]acetylcholine reflecting an increase in the affinity of the receptor binding site from K_d = 18.82 ± 3.6 nM in control animals to K_d = 9.06 ± 1.2 nM in experimental tissue. Chronic administration of the agonist nicotine (4 mg/kg/day) by osmotic minipumps produced an increased in the binding of 10 nM [³H]acetylcholine after 14 days (49% increase over control) and after 28 days (141% increase over controls). These data are consistent with the suggestion that [³H]acetylcholine labels a nicotinic cholinergic receptor in rat brain. Further they support the notion that some of the termination sites of the septal-cholinergic projection to the hippocampus are nicotinic.     

Specific [H]strychnine binding associated with glycine receptors in bovine retina

Saturable, specific [³H]strychnine binding can be demonstrated in homogenates of bovine retina. Scatchard plots revealed only one set of binding sites with a dissociation constant (K_d) of about 60 nM and a maximal number of binding sites of about 1.5 pmol/mg protein. The structural specificity of [³H]strychnine binding sites in bovine retina parallels the properties found for [³H]strychnine binding sites in the spinal cord of several vertebrates. Thus, the data do not give any evidence that specific [³H]strychnine binding in bovine retina labels taurine rather than glycine receptors and favors glycine rather than taurine as inhibitory neurotransmitter in bovine retina. The subcellular distribution of specific [³H]strychnine binding in bovine retina parallels that of sodium-dependent, high-affinity uptake of glycine and taurine. All 3 parameters are mainly found in the P₂ fractions of bovine retina homogenates, containing conventional synaptosomes, most abundant in the inner plexiform layer, but can also be found in the P₁ fractions, containing large synaptosomes from the photoreceptor cell layer.     

Autoradiographic localization of muscarinic agonist binding sites in the rat central nervous system with (+)-cis-[H]methyldioxolane

(+)-cis-[³H]Methyldioxolane ((+)-[³H]CD), a potent muscarinic agonist, was used to label high-affinity agonist states of muscarinic receptors in thin tissue sections of the rat central nervous system. Light microscopic autoradiography of atropine-sensitive (+)-[³H]CD binding sites revealed regions of dense labeling (superior colliculus, inferior colliculus, lateral geniculate body, hypoglossal (XII) nucleus, facial (VII) nucleus, tractus diagonalis) and regions of sparse labeling (hippocampus, dentate gyrus). The inverse regional correlation between high-affinity (+)-[³H]CD states and binding sites for the muscarinic antagonists [³H]pirenzepine (r = −0.79) and (-)-[³H]quinuclidinyl benzilate (r = −0.30) underscores potentially important differences between agonist and antagonist binding to CNS tissue slices.     

Characterisation of dopamine receptors in insect (Apis mellifera) brain

In vitro binding experiments using the vertebrate D₁ dopamine receptor ligand [³H]SCH23390 and the vertebrate D₂ dopamine receptor ligand [³H]spiperone were conducted on membrane preparations of honey bee (Apis mellifera) brain. Specific binding of [³H]SCH23390 was saturable and reversible. Analysis of saturation data gave an apparent K_d of 6.3 ± 1.0 nM and B_max of 1.9 ± 0.2 pmol/mg protein for a single class of binding sites. The specificity of high affinity [³H]SCH23390 binding was confirmed in displacement experiments using a range of dopaminergic antagonists and agonists. The rank order of potency for antagonists was: R(+)-SCH23390 > cis-(Z)-flupentixol ≥ chlorpromazine > fluphenazine> S(+)-butaclamol > spiperone. R(±)-SKF38393 and dopamine were the most effective agonists tested. [³H]SCH23390 labels a site in bee brain that is similar, but not identical to the vertebrate D₁ dopamine receptor subtype. [³H]Spiperone also bound with high affinity to bee brain homogenates. Scatchard analysis of [³H]spiperone saturation data revealed a curvilinear plot suggesting binding site heterogeneity. The high affinity site had a apparent K_d of 0.11 ± 0.02 nM and B_max of 9.2 ± 0.5 fmol/mg protein. The calculated values for the low affinity site were a K_d of 19.9 nM and B_max of 862 fmol/mg protein. Kinetic analyses also indicated that [³H]spiperone recognises a heterogeneous population of sites in bee brain. Furthermore, agonist competition studies revealed a phenolaminergic as well as a dopaminergic component to [³H]spiperone binding in bee brain. The rank order of potency of dopaminergic antagonists in competing for [³H]spiperone binding was: spiperone > fluphenazine> S(+)-butaclamol > domperidone> R(+)-SCH23390 > S(−)-sulpiride.     

[H]Nemonapride Binding in Human Caudate and Putamen

The binding of [³H]nemonapride to human postmortem caudate and putamen tissue was autoradiographically investigated using several antipsychotic drugs. Saturation experiments revealed a single population of binding sites (dissociation constant (K_D) 0.38 ± 0.01 nM, and total binding capacity (B_MAX) 55 fmol/TE). Prototypic dopamine (DA) receptors antagonists displaced [³H]nemonapride in a monophasic manner. The order of displacement potency was expected for DA D₂-like receptors: spiperone > (+)butaclamol ≥ chlorpromazine > (−)sulpiride > ketanserin. Displacement with serotonergic antagonists suggests that in human caudate and putamen tissue [³H]nemonapride may have a very low affinity serotonergic component. However, [³H]nemonapride displays a high affinity and selectivity for DA D₂-like receptors and should make it a preferred compound for tritium-based autoradiography.     

5-Hydroxytryptamine-sensitive [H]imipramine binding of protein nature in the human brain.: II. Effect of normal aging and dementia disorders

Recently, a high-affinity [³H]imipramine binding site of protein nature that appeared related to the 5-hydroxytryptamine (5-HT, serotonin) uptake mechanism was demonstrated in the rat brain. In a preceding paper a similar [³H]imipramine binding site of protein nature and displaceable by 5-HT was demonstrated in the human brain. Most previous [³H]imipramine binding studies of the human brain have used desipramine-sensitive binding, which appears to contain a significant amount of additional binding not related to 5-HT neurons. Therefore this study of the human brain in the normal aging, in Alzheimer's disease/senile dementia of Alzheimer type (AD/SDAT) and in multiinfarction dementia (MID) presents data on 5-HT-sensitive [³H]imipramine binding. The influence of normal aging (17–100 years) was studied in the frontal and cingulate cortices, in the putamen, caudate nucleus, amygdala and in the hippocampus. An age-related change in 5-HT-sensitive [^{3H]imipramine binding was only noted in the cingulate cortex with a 50% loss in B_max over the adult age range. In contrast, desipramine-sensitive [3H]imipramine binding studied in the frontal cortex and in the putamen showed marked increases in B_max with age which correlated with increases in K_d}. It is suggested that these increases are related to an increased binding to lipophilic membrane components not related to 5-HT neurons. The 5-HT-sensitive [³H]imipramine binding ( B_max) was reduced to 60% of control in the cingulate cortex and to 50% in the putamen in AD/SDAT. In MID there was a 50% loss of [³H]imipramine binding sites ( B_max) in the putamen, but a 30% loss in the cingulate cortex did not reach statistical significance. It is concluded that the losses of 5-HT-sensitive [³H]imipramine binding sites reflect a corresponding loss of 5-HT neurites.     

Development of a micromethod to study the Na-independentl-[H]glutamic acid binding to rat striatal membranes. II. Effects of selective striatal lesions and deafferentations

An apparent single class of Na⁺-independentl-[³H]glutamate (l-[³H]Glu) binding sites was biochemically and pharmacologically identified on rat striatal tissue. The K_d value was 1.75 μM and the B_max3.89 nmol/g protein. In order to further elucidate the putative physiological role of these binding sites and to valid our binding assay, experiments were conducted to determine the anatomical location of thel-[³H]Glu binding sites in the striatum. Local injection of the neurotoxin, kainic acid into the striatum caused degeneration of target cells in the structure followed by an important decrease (-37%) in the number of these binding sites, with no significant change in the affinity constant. Lesions of the cortical frontal and parietal areas projecting to the striatum via putative glutamatergic fibers led, on the other hand, after 3 weeks to one month, to a significant increase (+23%) in the number ofl-[³H]Glu binding sites. The K_d value does not significantly change after decortication. Finally, suppression either of the nigrostriatal dopaminergic input or of the partially cholinergic thalamostriatal tract do not affect [³H]Glu binding to striatal tissue. These results suggest that about 40% of our [³H]Glu binding sites are located on striatal target cell sensitive to the neurotoxic effect of kainic acid. Therefore, they seem to be partly postsynaptic. The existence of a relation between these binding sites and the corticostriatal glutamatergic input was shown by the development of a supersensitivity response after suppression of this cortical input. These sites might therefore constitute one of the receptor subclass for Glu linked to the excitatory action of the corticostriatal afferent. Since degeneration of either the nigrostriatal dopaminergic or the thalamostriatal fibres did not affect the binding ofl-[³H]Glu characterized in our study, the binding sites will probably not occur at the presynaptic level on these nerve terminals in the striatum.     

Quantitative localization of [3H]TCP binding in rat brain by light microscopy autoradiography

The anatomical localization of phencyclidine (PCP)/sigma-opiate receptors in rat brain was determined by quantitative light microscopy autoradiography using the new ligand N-(1-[2-thienyl]cyclohexyl) [3H]piperidine ([3H]TCP). TCP is a potent analog of PCP which possesses a higher affinity for PCP/sigma-opiate receptor than does PCP itself. The highest level of [3H]TCP binding was detected in the hippocampus. Intermediate levels were found in frontal cortex, striatum, amygdala and cerebellum. Specific [3H]TCP binding was undetectable in anterior commissure and corpus callosum. The distribution pattern of [3H]TCP binding sites is similar to the pattern obtained with [3H]PCP but more sharply defined. On the basis of its greater potency and specificity, [3H]TCP may prove superior to [3H]PCP as a molecular probe for the study of brain sigma opiate/phencyclidine receptors.     

Frontal decortication decreases the affinity ofN-(1-[2-thienyl]cyclohexyl)[H]piperidine binding to rat striatum Frontal decortication; Striatum

Bilateral ablation or transection of the corticostriatal pathways of the rat causede a reduction inN-(1-[2-thienyl]cyclohexyl)-[³H]piperidine ([³H]TCP) binding to the striatum at 5–7, but not 3 and 14–28, days postlesion with a persistent decrease in striatal glutamate content 3–28 days after the decortication. This reduction was found to be due to an increase inK_d without changes inB_max of the [³H]TCP binding. The present binding suggest that interruption of striatal glutamatergic transmission following frontal decortication may produce a temporal reduction in the affinity of phencyclidine receptor in th striatum.     

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.