Modulation of 5-hydroxytryptamine1A receptor density by nonhydrolyzable GTP analogues

Acute flesinoxan treatment produces a different effect on rat brain serotonin synthesis than chronic treatment: an alpha-methyl-l-tryptophan autoradiographic study

5-HT(1A) receptor agonists display anxiolytic and anti-depressant properties in clinical studies. In this study, we used the alpha-[(14)C]methyl-l-tryptophan (alpha-MTrp) autoradiographic method to evaluate the effects of the 5-HT(1A) agonist, flesinoxan, on regional 5-HT synthesis in the rat brain, following acute or a 14-day continuous treatment. In the first series of experiments, flesinoxan (5mg/kg; i.p.) was administered 40min before the alpha-MTrp. It resulted in a significant increase of the arterial blood oxygen partial pressure (pO(2)) and a reduction of the regional rate of 5-HT synthesis throughout the brain, with the exception of a few regions (medial geniculate body and thalamus). In the second series of experiments, flesinoxan (5mg/kgday) was administered for 14 days, using an osmotic minipump implanted subcutaneously. When compared to rats treated with saline, there was an overall significant (p<0.05) reduction in the synthesis (one-sample two-tailed t-test). However, there was no significant influence on the 5-HT synthesis rate in the dorsal and median raphe nuclei and the majority of their projection areas. A significant (p<0.05) reduction was observed in the nucleus raphe magnus, medial caudate, ventral thalamus, amygdala, ventral tegmental area, medial forebrain bundle, nucleus accumbens, medial anterior olfactory nucleus and superior olive. The unaltered 5-HT synthesis rates in a large majority of regions following the 14-day treatment of flesinoxan may reflect the normalization (implies to not be different from salne treated control) of synthesis due to a desensitization of 5-HT(1A) autoreceptors on the cell body of 5-HT neurons as well as at postsynaptic sites, which is known to occur following long-term treatment with 5-HT(1A) agonists. It is of some importance to note that the normalization of the synthesis occurred in the majority of the brain limbic structures, the brain areas implicated in affective disorders and the corresponding successful treatments, as well as in the cortical regions, which are implicated in mood. However, there were some terminal regions (e.g., accumbens, anterior olfactory, lateral thalamus, raphe magnus and obscurus) in which the chronic flesinoxan treatment resulted in a significant reduction of synthesis, suggesting that there was not a full desensitization across the brain of the receptors controlling 5-HT synthesis.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

5-HT1A receptor-mediated G protein activation assessed by [35S]GTPgammaS binding in rat cerebral cortex

To date, 5-hydroxytryptamine1A (5-HT1A) receptor-mediated functional assays (adenylyl cyclase inhibition, high-affinity GTPase activity and [35S]guanosine-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding) have been performed mainly in hippocampal membranes. In the current study, 5-HT-stimulated G protein activation assays were carried out in rat cerebral cortical membranes. High-affinity GTPase activity was stimulated by 5-HT, but not by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). By contrast, 5-HT- and 8-OH-DPAT-stimulated [35S]GTPgammaS binding displayed sufficient dynamic range enough to warrant further pharmacological analysis. Under standard conditions, which were determined precisely in terms of the concentrations of GDP, MgCl2 and NaCl, the profile of 5-HT-stimulated [35S]GTPgammaS binding investigated using a series of 5-HT receptor agonists and antagonists clearly indicated the involvement of the 5-HT1A receptor subtype. There appeared to be no evidence supporting the presence of regional heterogeneity in coupling efficiency between 5-HT1A and G proteins in the hippocampus or cortex. This method is a useful tool for investigating functional coupling between postsynaptic 5-HT1A receptors and G proteins in cerebral cortical membranes.

Effect of Mg2+ on guanine nucleotide sensitivity of ligand binding to serotonin1A receptors from bovine hippocampus

The serotonin1A (5-HT1A) receptor is an important member of the superfamily of seven transmembrane domain G-protein coupled receptors (GPCRs). We report here that guanine nucleotide sensitivity of agonist binding to hippocampal 5-HT1A receptors is dependent on the concentration of Mg2+. Our results show that agonist binding to 5-HT1A receptors is relatively insensitive to guanine nucleotides in the absence of Mg2+. In contrast to this, the specific antagonist binding is insensitive to guanine nucleotides, even in the presence of Mg2+. These results point out the requirement of an optimal concentration of Mg2+ which could be used in assays toward determining guanine nucleotide sensitivity of ligand binding to GPCRs such as the 5-HT1A receptor. Our results provide novel insight into the requirement and concentration dependence of Mg2+ in relation to guanine nucleotide sensitivity for the 5-HT1A receptor in particular, and GPCRs in general.

Evaluation of EMD 128 130 occupancy of the 5-HT1A and the D2 receptor: a human PET study with [11C]WAY-100635 and [11C]raclopride

The use of so-called, atypical antipsychotic medication is becoming more widespread in the treatment of psychotic disorders. EMD 128 130 is a novel compound acting as an agonist at the 5-HT1A receptor, and as an antagonist at the dopamine-2 (D2) receptor. This dual action may confer additional benefits over selective D2 antagonists in the treatment of psychotic disorders. In this study, we investigated the occupancy of EMD 128 130 in vivo at the human D2 and 5-HT1A receptors with positron emission tomography using the radiotracers [11C]raclopride and [11C]WAY-100635. Seven healthy volunteers were examined before and after 5 days of treatment with EMD 128 130, administered in an incremental dose building up to 50 mg, b.d. A significant occupancy was demonstrated at the human D2 receptor (40% following a dose of 50 mg, b.d.) while there was no consistent effect observed at the 5-HT1A receptor, despite a similar affinity of EMD 128 130 for cloned human D2 and 5-HT1A receptors, and the presence of typical, central 5-HT1A agonist side-effects. The differential effects of EMD 128 130 at the D2 and the 5-HT1A receptor (antagonist at D2 receptor, agonist at the 5-HTIA receptor) may explain the differences in occupancy observed.

Pindolol augmentation of antidepressant treatment: recent contributions from brain imaging studies

Preclinical studies suggest that augmentation of selective serotonin (5-HT) reuptake inhibitors by the 5-HT(1A) receptor agent pindolol might reduce the delay between initiation of treatment and antidepressant response, an effect largely mediated by blockade of 5-HT(1A) autoreceptors in the dorsal raphe nuclei. Although some controlled clinical trials suggest that pindolol might reduce latency to selective serotonin reuptake inhibitor response in acute depressive episodes, the effect is moderate and highly variable. Recent positron emission tomography studies investigating the occupancy of 5-HT(1A) receptors in humans by pindolol have shown that at the dose used most often in clinical trials the occupancy is low and variable, which might explain the inconsistent clinical results. Positron emission tomography studies also suggest that pindolol might be more potent at blocking 5-HT(1A) autoreceptors than at blocking postsynaptic receptors, a property that may be useful in this pharmacologic strategy. Thus, the positron emission tomography data support the potential of pindolol to augment the antidepressant response of selective serotonin reuptake inhibitors, but also imply that this potential has not been fully evaluated. Here we review the clinical trials, the positron emission tomography studies, and the possible mechanisms of pindolol augmentation. It is also suggested that positron emission tomography may be used to define therapeutic dosing early on in the process of clinical evaluation of new treatment strategies.

Differential receptor-G-protein coupling evoked by dissimilar cannabinoid receptor agonists

Multiple affinity states are revealed by agonist competition for radioantagonist [3H]SR141716A binding to rat brain CB1 cannabinoid receptors. Desacetyllevonantradol (DALN), a tricyclic cannabinoid, and WIN55212-2, an aminoalkylindole, both bound in two discrete affinity states (30% high affinity), but the ratios of the IC50 revealed distinct differences. Other affinity-state differences between the agonists were: Na+ reduced the affinity for the membrane-bound receptor by 10-fold for DALN but minimally for WIN55212-2; a nonhydrolysable GTP analogue decreased the fraction of high-affinity WIN55212-2 binding but not that of DALN unless Na+ was also present. Detergent solubilisation increased the fraction of high-affinity binding for both agonists but eliminated any effect of Na+ on the agonist affinities. In detergent solution, the GTP analogue reduced the WIN55212-2 high-affinity fraction but not that of DALN, even though the IC50 values increased for both DALN and WIN55212-2. The differential modulation of CB1 receptor-G-protein coupling by Na+ and guanine nucleotides is dependent upon the cannabimimetic agonist bound.

Metal ion and guanine nucleotide modulations of agonist interaction in G-protein-coupled serotonin1A receptors from bovine hippocampus

1. The serotonin type 1A (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to GTP-binding regulatory proteins (G-proteins). We have studied the modulation of agonist binding to 5-HT1A receptors from bovine hippocampus by metal ions and guanine nucleotide. 2. Bovine hippocampal membranes containing the 5-HT1A receptor were isolated. These membranes exhibited high-affinity binding sites for the specific agonist [3H]OH-DPAT. 3. The agonist binding is inhibited by monovalent cations Na+, K+, and Li+ in a concentration-dependent manner. Divalent cations such as Ca2+, Mg2+, and Mn2+, on the other hand, show more complex behavior and induce enhancement of agonist binding up to a certain concentration. The effect of the metal ions on agonist binding is strongly modulated in the presence of GTP-gamma-S, a nonhydrolyzable analogue of GTP, indicating that these receptors are coupled to G-proteins. 4. To gain further insight into the mechanisms of agonist binding to bovine hippocampal 5-HT1A receptors under these conditions, the binding affinities and binding sites have been analyzed by Scatchard analysis of saturation binding data. Our results are relevant to ongoing analyses of the overall regulation of receptor activity for G-protein-coupled seven transmembrane domain receptors.

Pharmacological characterization of recombinant human 5-hydroxytryptamine1A receptors using a novel antagonist radioligand, [3H]WAY-100635

The ligand binding characteristics of the recombinant human 5-HT1A receptor stably expressed in a Chinese Hamster Ovary (CHO) cell line are described using a selective agonist, [3H]8-OH-DPAT, and a novel antagonist radioligand, [3H]WAY-100635. The association of [3H]WAY-100635 was a time- and temperature-dependent process. Mn2+ > Ca2+ > Mg2+ reduced the specific [3H]WAY-100635 binding in a concentration-dependent manner, whereas Na+ and K+ were ineffective. Scatchard analyses revealed a homogeneous population of [3H]WAY-100635 recognition sites (Kd = 0.32 nM; Bmax = 162 fmol/mg of protein). Addition of divalent cations to the incubation medium produced a two-fold decrease in the binding affinity of [3H]WAY-100635 with no significant change in Bmax; GTP gamma S had no effect on Kd or Bmax parameters. [3H]WAY-100635 displayed a higher affinity (2-3 fold) for the 5-HT1A site when compared with [3H] 8-OH-DPAT binding under similar incubation conditions. Furthermore, [3H] 8-OH-DPAT labelled approximately 53-61% of total 5-HT1A sites recognised by [3H]WAY-100635. The competition binding profiles of [3H]WAY-100635 and [3H]8-OH-DPAT were highly correlated and consistent with the recognition of 5-HT1A receptors. Agonist competition curves with [3H]WAY-100635 were best-resolved into high- and low-affinity binding states, whereas partial agonist and antagonist curves were best-fit to one-site binding models. A significant correlation between the respective affinities of a range of agonists and antagonists at recombinant human and rodent hippocampal 5-HT1A binding sites (previously published) was also observed using [3H]WAY-100635 (r = 0.92; P < 0.0005) and [3H]8-OH-DPAT (r = 0.96; P < 0.0005). The availability of a novel, high-affinity antagonist radioligand, [3H]WAY-100635, will provide a useful tool for the further characterisation of 5-HT1A receptor pharmacology.

Resolution of multiple affinity states of 5-hydroxytryptamine-1A receptors labelled by [3H]-8-hydroxy-2-(di-n-propilamino) tetralin in rat hippocampal membranes

1. We examined the binding of [3H]-8-hydroxy-2-(DI-n-propilamino)tetralin ([3H]-8OH-DPAT) to 5-hydroxytriptamine-1A (5-HT1A) receptors in rat hippocampal membranes. 2. Computer analysis of [3H]-8OH-DPAT displacement curves in the absence and in the presence of 100 microM guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) were best fitted with a three-site model with apparent dissociation constants (Kd) of 0.45, 2.8 and 30 nM; the corresponding binding capacity (Bmax) existing in the three affinity states were 4, 2 and 12 pmol/g of tissue (wet weight), respectively. 3. These results suggest that [3H]-8OH-DPAT binding can be resolved as complex isotherms and we provided evidence that [3H]-8OH-DPAT labels 2 high-affinity GTP gamma S-sensitive and one low-affinity GTP gamma S-sensitive state of 5-HT1A receptors.

BIMT 17, a 5-HT2A receptor antagonist and 5-HT1A receptor full agonist in rat cerebral cortex

In the search for antidepressant agents with a rapid onset of action, we have found that compound BIMT 17 (1-[2-[4-(3-trifluoromethylphenyl)piperazin-1- yl]ethyl]benzimidazol-[1H]-2-one) shows a good affinity for cerebral cortical 5-HT1A (pKi = 7.72) and 5-HT2A (pKi = 6.90) receptors, with no appreciable affinity for the other 5-HT receptor subtypes, including 5-HT2C. BIMT 17 reduced forskolin-stimulated cAMP accumulation in the cerebral cortex (pEC50 = 6.09) and in the hippocampus (pEC50 = 6.50), and antagonized 5-HT-induced phosphatidylinositol turnover (pKi = 6.96) in the cerebral cortex. The effect on cAMP accumulation was blocked by the 5-HT1A receptor antagonist tertatolol. Buspirone, 8-OH-DPAT and S 14671 (1-[2-(2-thenoylamino)ethyl]- 4[1-(7-methoxynaphtyl)]-piperazine), claimed to be 5-HT1A receptor agonists, did not reduce forskolin-stimulated cAMP formation in the cerebral cortex. On the basis of these data, it was concluded that BIMT 17 was the only compound that behaved as a full agonist with respect to the cAMP response in the cortex, while exerting concurrent agonism at 5-HT1A receptors and antagonism at 5-HT2A receptors. These characteristics might explain the peculiar behavior of BIMT 17 in mimicking the inhibitory action of 5-HT on the basal firing rate of the cortical neurons (see accompanying paper).

Quantitative autoradiographic characterisation of the binding of [3H]WAY-100635, a selective 5-HT1A receptor antagonist

The binding characteristics of [3H]WAY-100635 ([O-methyl 3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride), a potent and selective 5-HT1A antagonist radioligand, were examined in the rat brain using in vitro quantitative receptor autoradiography. The regional distribution of specific [3H]WAY-100635 binding sites was heterogeneous and demonstrated a strong correlation with that of [3H]8-OH-DPAT binding. The highest concentrations of [3H]WAY-100635-labelled sites were found in the lateral septal areas, dorsal raphe n., entorhinal cortex and the hippocampal formation (CA1, CA3 and dentate gyrus). Scatchard transformation of saturation isotherms revealed saturable [3H]WAY-100635 binding sites of high-affinity: in the hippocampal formation, Kd was approximately 1 nM and Bmax ranged between 187 and 243 fmol/mg tissue wet weight, in the entorhinal cortex, Kd = 0.44 nM and Bmax = 194 fmol/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., Kd = 0.52 nM and Bmax = 157 fmol/mg tissue wet weight. The affinity of [3H]WAY-100635 for the 5-HT1A binding site tended to be higher in the dorsal raphe n. and entorhinal cortex compared with that of the hippocampal formation. In contrast, the binding affinity of [3H]8-OH-DPAT in the hippocampal formation was between 1.1 and 2.3 nM and the Bmax was 137 to 183 fmoles/mg tissue wet weight; in the entorhinal cortex, Kd = 3.2 nM and Bmax = 141 fmoles/mg tissue wet weight, and in the rostral portion of the dorsal raphe n., Kd = 3.4 nM and Bmax = 163 fmol/mg tissue wet weight.(ABSTRACT TRUNCATED AT 250 WORDS)

5-HT1A receptor-mediated activation of high-affinity GTPase in rat hippocampal membranes

GTP hydrolyzing activity was assayed by measuring the amount of 32P(i) released from 0.3 microM [gamma-32P]GTP in the membranes prepared from rat brain. 5-Hydroxytryptamine (5-HT) stimulated the high-affinity GTPase activity in hippocampus, but not in striatum, in a concentration-dependent manner with an EC50 value of 18 nM and maximal percent stimulation of 13.9%. This response was mimicked by (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin [(+/-)-8-OH-DPAT], but not by (+/-)-1-(2,5-dimethoxy-4-iodophyenyl)-2-aminopropane [(+/-)-DOI]. These results suggest that 5-HT-stimulated high-affinity GTPase activity of the GTP-binding protein(s) is mediated via 5-HT1A receptor subtype in the rat hippocampus.

Socially defeated male rats display a blunted adrenocortical response to a low dose of 8-OH-DPAT

The study examined in male Wistar rats the influence of social defeat on the neuroendocrine stress response system using injection of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), as the pharmacological challenge. Social defeat was defined by the submissive postures displayed by the Wistar rats which were threatened and attacked by Tryon Maze Dull S3 rats for 10 min. 18-20 h after social defeat, the defeated rats were injected intravenously (i.v.) with a low and high dose of 8-OH-DPAT in their home cages. Blood samples were withdrawn from the freely moving cannulated rats for determination of plasma corticosterone and catecholamines. The corticosterone response to the low dose of 8-OH-DPAT (0.05 mg/kg, i.v.) was significantly diminished in the defeated rats as compared to the controls, but this dose failed to affect catecholamine concentrations. The high dose of 8-OH-DPAT (0.15 mg/kg, i.v.) significantly elevated corticosterone and adrenaline levels in defeated and control rats to the same extent, whereas no effect on noradrenaline was found. The present data thus indicate that social defeat blunts 5-HT1A receptor-mediated adrenocortical activation probably via a decrease in the sensitivity of a population of postsynaptic 5-HT receptors.

Differential effects of guanine nucleotides on kainic acid binding and on adenylate cyclase activity in chick optic tectum

In G protein-coupled receptors, neurotransmitter-induced binding of GTP to G proteins triggers the activation of effector systems while simultaneously decreasing the affinity of the transmitter for its specific binding site within the receptor-G protein complex. In the present study we show that, in the chick optic tectum, guanine nucleotides inhibit the binding of the glutamate analog, kainate, and activate adenylate cyclase by different mechanisms and acting on different sites. GMP-PNP, a non-hydrolyzable analog of GTP, binds tightly to G proteins so that the binding is stable even after exhaustive washing. By use of this property, we have prepared membrane samples in which G protein GTP-binding sites are pre-saturated with GMP-PNP. Experiments carried out with these membranes show that GMP-PNP, GDP-S and GMP inhibit the binding of [3H]kainate by interacting with site(s) unrelated to G proteins, whereas GMP-PNP activates adenylate cyclase activity by binding to G proteins.

Heterogeneity of cortical and hippocampal 5-HT1A receptors: a reappraisal of homogenate binding with 8-[3H]hydroxydipropylaminotetralin

The selective serotonin (5-HT) agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT) has been extensively used to characterize the physiological, biochemical, and behavioral features of the 5-HT1A receptor. A further characterization of this receptor subtype was conducted with membrane preparations from rat cerebral cortex and hippocampus. The saturation binding isotherms of [3H]8-OH-DPAT (free ligand from 200 pM to 160 nM) revealed high-affinity 5-HT1A receptors (KH = 0.7-0.8 nM) and low-affinity (KL = 22-36 nM) binding sites. The kinetics of [3H]8-OH-DPAT binding were examined at two ligand concentrations, i.e., 1 and 10 nM, and in each case revealed two dissociation rate constants supporting the existence of high- and low-affinity binding sites. When the high-affinity sites were labeled with a 1 nM concentration of [3H]8-OH-DPAT, the competition curves of agonist and antagonist drugs were best fit to a two-site model, indicating the presence of two different 5-HT1A binding sites or, alternatively, two affinity states, tentatively designated as 5-HT1AHIGH and 5-HT1ALOW. However, the low correlation between the affinities of various drugs for these sites indicates the existence of different and independent binding sites. To determine whether 5-HT1A sites are modulated by 5'-guanylylimidodiphosphate, inhibition experiments with 5-HT were performed in the presence or in the absence of 100 microM 5'-guanylylimidodiphosphate. The binding of 1 nM [3H]8-OH-DPAT to the 5-HT1AHIGH site was dramatically (80%) reduced by 5'-guanylylimidodiphosphate; in contrast, the low-affinity site, or 5-HT1ALOW, was seemingly insensitive to the guanine nucleotide. The findings suggest that the high-affinity 5-HT1AHIGH site corresponds to the classic 5-HT1A receptor, whereas the novel 5-HT1ALOW binding site, labeled by 1 nM [3H]8-OH-DPAT and having a micromolar affinity for 5-HT, may not belong to the G protein family of receptors. To further investigate the relationship of 5-HT1A sites and the 5-HT innervation, rats were treated with p-chlorophenylalanine or with the neurotoxin p-chloroamphetamine. The inhibition of 5-HT synthesis by p-chlorophenylalanine did not alter either of the two 5-HT1A sites, but deafferentation by p-chloroamphetamine caused a loss of the low-affinity [3H]8-OH-DPAT binding sites, indicating that these novel binding sites may be located presynaptically on 5-HT fibers and/or nerve terminals.

8-[3H]hydroxy-2-(di-n-propylamino) tetralin binding sites in goldfish retina

The binding sites of 8-[3H]hydroxy-2-(di-n-propylamino)tetralin ([3H]DPAT) were characterized in the retina of goldfish in order to evaluate the selectivity of the ligand for serotonin1A (5HT1A) receptors. Specificity of the binding was performed in the presence of serotonergic and dopaminergic agonists and antagonists. Buspirone, spiroxatrine and 5-methoxy-N,N-dimethyltryptamine were potent inhibitors, followed by propranolol, citalopram, imipramine and desipramine. Serotonin was not a potent inhibitor, and its interaction with the binding sites of [3H]DPAT was complex. Nomifensine displayed an important inhibition, however, other dopamine uptake blockers, such as bupropion and GBR-12909, were less potent. Haloperidol was also a good inhibitor, but the D1 receptor agonist, SKF-38393, the D2 receptor antagonist, sulpiride, and dopamine did not inhibit the binding. GppNHp inhibited the binding in the micromolar range. The analysis of saturation experiments by isotopic dilution, using buspirone to determine nonspecific binding, revealed two sites. The number of binding sites defined by buspirone were higher than the ones defined by nomifensine. The specific binding, using buspirone for definition, was reduced by the intraocular injection of 6-hydroxydopamine. This investigation demonstrates that [3H]DPAT labels 5HT1A receptors in goldfish retina, but also interacts with a non-5HT receptor site. These receptors seem to be localized in dopaminergic neurons.

The effect of chronic imipramine administration on the densities of 5-HT1A and 5-HT2 receptors and the abundances of 5-HT receptor and transporter mRNA in the cortex, hippocampus and dorsal raphe of three strains of rat

We have recently demonstrated that the LEW/N rat contains lower concentrations of cortical and hippocampal 5-HT1A receptors compared with the F344/N and out-bred HSD rats. To further characterize these strains, we investigated the effect of chronic (8 wk) imipramine administration (5 mg/kg/day) on 5-HT1A and 5-HT2 receptor densities and mRNA in the cortex and hippocampus and 5-HT transporter mRNA in the dorsal raphe of LEW/N, HSD, and F344/N rats, using quantitative autoradiography and in situ hybridization histochemistry. After imipramine treatment, a significant increase in the levels of hippocampal 5-HT1A receptors, but not mRNA, was observed in LEW/N rats while the abundance of hippocampal 5-HT1A receptor mRNA, but not 5-HT1A receptor densities, decreased in F344/N rats. Cortical and hippocampal 5-HT2 receptor densities, but not mRNA, significantly decreased after imipramine administration in all three strains. Finally, 5-HT1A receptor densities and the abundance of mRNAs encoding the 5-HT1A receptor and 5-HT transporter in the dorsal raphe remained unaltered after imipramine administration in all three strains. The effects of imipramine on the levels of cortical and hippocampal 5-HT1A and 5-HT2 receptors and their transcripts, therefore, appear to be strain-dependent. The implications of these findings are discussed.

Species-selective binding of [3H]-idazoxan to alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in the central nervous system

1. We have used the imidazoline derivative [3H]-idazoxan to define alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in cerebral cortex membranes of calf, mouse, rat, guinea-pig and man. 2. Competition experiments using the selective alpha-adrenoceptor drugs, rauwolscine and corynanthine, indicated that [3H]-idazoxan bound to a single population of sites in the calf and mouse membranes. However, [3H]-idazoxan also labelled non-adrenoceptor, imidazoline binding sites in the rat (15%), guinea-pig (30%) and human (40%) cerebral cortex membranes. 3. Competition experiments with adrenaline and cirazoline in the guinea-pig cortex, verified [3H]-idazoxan binding to both alpha 2-adrenoceptors and to non-adrenoceptor, imidazoline binding sites. 4. It has been postulated by several groups that [3H]-idazoxan may possess partial agonist activity. To investigate this further, saturation experiments were performed in the cerebral cortex membranes of all five species in the absence and presence of 300 microM guanosine triphosphate (GTP). GTP had no effect on [3H]-idazoxan binding in guinea-pig cerebral cortex; in both rat and mouse membranes 300 microM GTP increased the dissociation constant for [3H]-idazoxan by 2-3 fold without significantly affecting the Bmax. GTP reduced the Bmax by approximately 30% and 60% in calf and human cerebral cortex membranes, respectively, without significantly altering the Kd. 5. Saturation experiments were performed in the calf cerebral cortex membranes in the absence and presence of 300 microM GTP with the selective alpha 2-adrenoceptor agonist [3H]-clonidine and the selective muscarinic antagonist [3H]-quinuclidinyl benzilate (QNB). GTP reduced the Bmax for [3H]-clonidine without altering the Kd, but failed to affect either the Bmax or the Kd for [3H]-QNB.6. Saturation experiments were performed in human cerebral cortex membranes in the presence of alpha2-adrenoceptor blockade with and without GTP. GTP 300 microM reduced the Bmax for [3H]-idazoxan at the non-adrenoceptor, imidazoline binding sites, without affecting the Kd. GTP did not affect [3H]-QNB binding to muscarinic sites.7. Thus, there is a need to investigate further the pharmacological actions of [3H]-idazoxan in view of its ability to recognise both alpha2-adrenoceptors and non-adrenoceptor, imidazoline binding sites and because it might possess agonist activity at some of these sites.

Characterization of a cloned rat serotonin 5-HT1A receptor expressed in the HeLa cell line

We have previously isolated the rat serotonin (5-HT)1A receptor gene (G21Y2) and now report the expression and characterization of this receptor. The BamHI/Xbal fragment of this gene was cloned into Rc/RSV and stably transfected into HeLa cells by the calcium phosphate method. For determination of specific 5-HT1A receptor binding, [3H]8OH-DPAT was used as the radioligand and incubated with HeLa cell membranes. The cells expressed specific and saturable binding of [3H]8OH-DPAT with a Kd value of 0.3 nM and a Bmax value of 2 pmol/mg protein. GTP (50 microM) added to the incubation mixture increased the Kd value to 3 nM indicating that the expressed receptor is coupled to a G protein. The specific binding was inhibited by selective 5-HT1A partial agonists, such as buspirone, ipsapirone, gepirone, tandospirone, zalospirone and SUN8399 with Ki values of 1-30nM, whereas other neurotropic drugs except for spiperone (Ki = 46 nM) and nemonapride (Ki = 2.3 nM) were effective only at concentrations of more than 100 microM. The potencies of these compounds to inhibit [3H]8OH-DPAT from its specific binding sites were similar to their affinities determined in rat hippocampus binding studies. These data suggest that the expressed receptor is a 5-HT1A-type similar to 5-HT1A receptors in the rat hippocampus.

Further evidence for differential affinity states of the serotonin1A receptor in rat hippocampus

The binding profile of [3H]8-hydroxy-2-(di-N-propylamino)-tetralin ([3H]8-OH-DPAT) to serotonin1A (5-HT1A) sites in rat hippocampal, frontocortical and striatal membranes has been compared. In these regions, [3H]8-OH-DPAT labels both a high and a low-affinity binding site; the affinity values for each of the two sites are comparable in the different brain regions, but have different maximal capacity. By modifying the experimental conditions in a series of hippocampal membrane preparations, reciprocal changes in the proportion of the two sites were observed suggesting that they represent, at least in this region, different conformations or affinity states of a single receptor protein. In contrast to the lower affinity state, it appears that the high-affinity state is stabilized by coupling with a G-protein. Evidence supporting this statement is provided by addition of the guanine nucleotide Gpp(NH)p, breakage of labile disulfide bonds using N-ethylmaleimide and increasing membrane rigidity with ascorbate-induced lipid peroxidation, conditions which all reduced the density of receptors in the high-affinity state. Moreover, the high-affinity state appears to be stabilized at the expense of the lower affinity state in the presence of Mn2+. On the other hand, a complete shift to the low-affinity binding state was observed after a 24 h in vivo treatment with inhibitors of monoamine oxidase A (phenelzine or clorgyline) but not of monoamine oxidase B (deprenyl). This disappearance of the high-affinity state with a concomitant increase in the binding capacity of the low-affinity state was reproduced by inhibiting monoamine oxidase A in vitro, as well as by reducing preincubation washout periods. Also, competitors of the [3H]8-OH-DPAT binding site, such as serotonin and unlabelled 8-OH-DPAT, display two affinity states while others like (+/-)-propranolol, tryptamine and spiperone recognize a single affinity component. These results suggest that the 5-HT1A binding site may exhibit at least two different affinity states depending upon its microenvironment and the intrinsic activity of the ligand used.

Characterization of [3H]CP-96,501 as a selective radioligand for the serotonin 5-HT1B receptor: binding studies in rat brain membranes

3-(1,2,5,6-Tetrahydro-4-pyridyl)-5-n-propoxyindole (CP-96,501) was found to be more selective ligand at the serotonin 5-HT1B receptor than the commonly used 5-HT1B agonist, 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-methoxyindole (RU 24969). In rat brain membranes, the tritiated derivative, [3H]CP-96,501, was found to bind with a high affinity (KD, 0.21 nM) to a single binding site (nH, 1.0). The receptor density of this site (Bmax, 72 fmol/mg of protein) matched that of the 5-HT1B receptor determined with [3H]5-HT. Competition curves of 16 serotonergic compounds in [3H]CP-96,501 binding also indicated a single binding site. The rank order of their binding affinities with this new radioligand showed a high degree of correlation with their affinities at the 5-HT1B receptor determined with [3H]5-HT or [125I]iodocyanopindolol. Serotonergic compounds displayed competitive inhibition of [3H]CP-96,501 binding. In the presence of 5'-guanylylimidodiphosphate [Gpp(NH)p], [3H]CP-96,501 to displace [125I]iodocyanopindolol binding was also decreased. These findings are consistent with the agonist nature of CP-96,501. The results of this study suggest that [3H]CP-96,501 is a useful agonist radioligand for the 5-HT1B receptor.

Preservation of 5-hydroxytryptamine1A receptor-G protein interactions in the cerebral cortex of patients with Alzheimer's disease

The coupling of 5-hydroxytryptamine1A (5-HT1A) receptors to guanine nucleotide binding (G) proteins was investigated in membranes prepared from frontal and parietal cortices of control and Alzheimer's disease brains by characterising the effect of guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) on [3H]8-hydroxy-2-(di-n-propylamino)-tetralin ([3H]8-OH-DPAT) binding parameters. In the absence of guanine nucleotides, [3H]8-OH-DPAT bound to a single high affinity binding site in all membrane types. The number of [3H]8-OH-DPAT binding sites was significantly decreased in the parietal cortex of Alzheimer's disease samples compared with controls, whereas in the frontal cortex the number of binding sites remained unchanged. Gpp[NH]p reduced the [3H]8-OH-DPAT binding affinity and the number of binding sites to the same degree in both regions in control and Alzheimer's disease cases. [3H]8-OH-DPAT binding was inhibited in a concentration dependent manner with an IC50 value of approximately 1 microM in all cases. These results suggest that the 5-HT1A receptor-G protein complex is functionally intact in these regions in Alzheimer's disease brain.

The GTP-insensitive component of high-affinity [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding in the rat hippocampus corresponds to an oxidized state of the 5-hydroxytryptamine1A receptor

Previous studies on central 5-hydroxytryptamine1A (5-HT1A) receptors have consistently shown the existence of a GTP-insensitive component of agonist binding, i.e., binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) that persists in the presence of 0.1 mM GTP or guanylylimidodiphosphate (GppNHp). The molecular basis for this apparent heterogeneity was investigated pharmacologically and biochemically in the present study. The GppNHp-insensitive component of [3H]8-OH-DPAT binding increased spontaneously by exposure of rat hippocampal membranes or their 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate-soluble extracts to air; it was reduced by preincubation of solubilized 5-HT1A binding sites in the presence of dithiothreitol and, in contrast, reversibly increased by preincubation in the presence of various oxidizing reagents like sodium tetrathionate or hydrogen peroxide. In addition, exposure of hippocampal soluble extracts to short-cross-linking reagents specific for thiols produced an irreversible increase in the proportion of GppNHp-insensitive over total [3H]8-OH-DPAT binding. The pharmacological properties of this GppNHp-insensitive component of [3H]8-OH-DPAT binding were similar to those of 5-HT1A sites in the absence of nucleotide. Sucrose gradient sedimentation of solubilized 5-HT1A binding sites treated by dithiothreitol or sodium tetrathionate showed that oxidation prevented the dissociation by GTP of the complex formed by the 5-HT1A receptor binding subunit (R[5-HT1A]) and a guanine nucleotide-binding protein (G protein). Moreover, the oxidation of -SH groups by sodium tetrathionate did not prevent the inactivation of [3H]8-OH-DPAT specific binding by N-ethylmaleimide, in contrast to that expected from an interaction of both reagents with the same -SH groups on the R[5-HT1A]-G protein complex. These data suggest that the appearance of GTP-insensitive [3H]8-OH-DPAT specific binding occurs as a result of the (spontaneous) oxidation of essential -SH groups (different from those preferentially inactivated by N-ethylmaleimide) on the R[5-HT1A]-G protein complex.

Structural determinants of 5-HT1A versus 5-HT1D receptor binding site selectivity

A structure-activity analysis was used to identify selective 5-HT1A versus 5-HT1D receptor agents. An analysis of published data identified 13 drugs which display nanomolar affinity for the 5-HT1A receptor and that have been analyzed at 5-HT1D receptor binding sites. Four agents display greater than or equal to 100-fold selectivity for the 5-HT1A receptor. Two structural features were identified which hypothetically result in selectivity for 5-HT1A versus 5-HT1D binding sites. The linkage of an indole ring to a basic nitrogen atom via the 4 position on the indole ring or the absence of an indole ring are two features which lower the affinity for the 5-HT1D receptor, but do not necessarily lower the affinity for the 5-HT1A receptor. A series of 7 agents (5 indoles, 2 quinolines) was identified which met these hypothetical selectivity criteria. These compounds were then analyzed in radioligand binding studies. These 7 agents display affinities of 1.3-170 nM for the 5-HT1A receptor binding site, and 1,800-13,000 nM for the 5-HT1D receptor binding site. All 7 agents display greater than or equal to 47-fold selectivity for the 5-HT1A versus 5-HT1D site and 4 of the agents are greater than 100-fold selective. Compound No. 1 (N,N'-bis[3-(4-indolyloxy)-2-hydroxypropyl]-(Z)-1,8-diamino-p-meth ane) and compound No. 2 (N8-[3-(4-indolyloxy)-2-hydroxypropyl]-N1-(propioloyl)-(Z)-1 ,8-diamino-p-methane) are the most selective agents yet described for 5-HT1A versus 5-HT1D receptor binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

The 5-HT receptor--G-protein--effector system complex in depression. I. Effect of glucocorticoids

Hormonal modulation of neurotransmission emerged as a concept from the recognition that adrenocortical steroids exert profound effects at the level of receptors, G-proteins and effector units. G-proteins, a family of guanine nucleotide binding regulatory components that couple neurotransmitter receptors to various types of intracellular effector systems, appear to be a key target of glucocorticoid (GC) action in the CNS. It is thought that Gs/Gi mediates stimulation/inhibition of adenylate cyclase (AC system), which forms cyclic AMP as second messenger, while receptors stimulating phospholipase C do so through Go to produce two second messengers, inositol 1,4,5-triphosphate and diacylglycerol (PI system). Recent evidence suggests that GC increase Gs alpha-and decrease Gi alpha-protein subunit expression without affecting Go alpha. Activation of central pre- and postsynaptic 5-HT1A receptors which are linked to the Gi-AC complex, induces hypothermia and ACTH/cortisol release in rodents and humans. Compared with controls, patients with a major depressive disorder exhibit increased basal cortisol secretion associated with decreased hypothermic and ACTH/cortisol responses. The attenuated neuroendocrine and thermoregulatory response to 5-HT1A receptor activation may reflect a GC-dependent feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) system and subsensitivity of the presynaptic 5-HT1A-Gi-AC complex function. Differential regulation of 5-HT1A and 5-HT2 function leading to a relative 5-HT2-Go-PI complex supersensitivity may maintain HPA hyperactivity during the course of depression. These findings corroborate recent reports that GC, via GC-GC receptor (GR) complex activated promotion of gene transcription, modify the expression 5-HT1A-coupled Gi (but not 5-HT2-coupled Go) resulting in altered sensitivity of 5-HT1A-mediated signal transduction and further support the hypothesis of a differential regulation of 5-HT1A and 5-HT2 receptor function and a GC-GR/5-HT1A-G-protein--effector system-related abnormality in depression.

Antagonism by 8-hydroxy-2(di-n-propylamino)tetraline and other serotonin agonists of muscarinic M1-type receptors coupled to inositol phospholipid breakdown in human IMR-32 and SK-N-MC neuroblastoma cells

IMR-32 and SK-N-MC cells were found to contain [3H]quinuclidinyl benzilate specific binding sites inhibited by pirenzepine in a manner suggesting the presence of both M1-type and M2-type muscarinic receptor recognition sites. Neither cell had detectable [3H]8-OH-DPAT binding sites. Carbachol stimulated the rate of inositol phospholipid breakdown in IMR-32 and SK-N-MC human neuroblastoma cells with an EC50 value of about 50 microM in both cases. Pirenzepine inhibited the carbachol (100 microM)-stimulated inositol phospholipid breakdown in both cells with Hill slopes of unity and IC50 values of 15 nM (IMR-32) and 12 nM (SK-N-MC). The 5-HT1A receptor agonist 8-OH-DPAT competitively inhibited carbachol-stimulated inositol phospholipid breakdown with pA2 values of 5.78 (IMR-32) and 5.61 (SK-N-MC). These values are consistent with the inhibitory potency of 8-OH-DPAT towards [3H]quinuclidinyl benzilate binding in these cells. The 5-HT agonists 5-MeODMT and buspirone at micromolar concentrations inhibited carbachol-stimulated breakdown in IMR-32 cells. The inhibition by 8-OH-DPAT and 5-MeODMT was not affected by preincubation with (-)alprenolol. 5-HT (10-100 microM) was without effect on either basal or carbachol-stimulated breakdown. It is concluded that IMR-32 and SK-N-MC neuroblastoma cells express muscarinic M1-type but not serotoninergic receptors coupled to phosphoinositide-specific phospholipase C. 8-OH-DPAT acts as a weak antagonist at these muscarinic receptors.