A positron emission tomography (PET) study of cerebral dopamine D<Subscript>2</Subscript> and serotonine 5-HT<Subscript>2A</Subscript> receptor occupancy in patients treated with cyamemazine (Tercian)

Rationale Cyamemazine (Tercian) is an antipsychotic drug with anxiolytic properties. Recently, an in vitro study showed that cyamemazine possesses high affinity for serotonin 5-HT_2A receptors, which was fourfold higher than its affinity for dopamine D₂ receptors (Hameg et al. 2003).Objectives The aim of this study is to confirm these previous data in vivo in patients treated with clinically relevant doses of Tercian.Methods Eight patients received 37.5, 75, 150 or 300 mg/day of Tercian depending on their symptomatology. Dopamine D₂ and serotonin 5-HT_2A receptor occupancies (RO) were assessed at steady-state plasma levels of cyamemazine with positron emission tomography (PET), using [¹¹C]raclopride and [¹¹C]N-methyl-spiperone, respectively. The effective plasma level of the drug leading to 50% of receptor occupancy was estimated by fitting RO with plasma levels of cyamemazine at the time of the PET scan.Results Cyamemazine induced near saturation of 5-HT_2A receptors (RO=62.1–98.2%) in the frontal cortex even at low plasma levels of the drug. On the contrary, occupancy of striatal D₂ receptors increased with plasma levels, and no saturation was obtained even at high plasma levels (RO=25.2–74.9%). The effective plasma level of cyamemazine leading to 50% of D₂ receptor occupancy was fourfold higher than that for 5-HT_2A receptors. Accordingly, individual 5-HT_2A/D₂ RO ratios ranged from 1.26 to 2.68. No patients presented relevant increased prolactin levels, and only mild extrapyramidal side effects were noticed on Simpson and Angus Scale.Conclusion This in vivo binding study conducted in patients confirms previous in vitro findings indicating that cyamemazine has a higher affinity for serotonin 5-HT_2A receptors compared to dopamine D₂ receptors. In the dose range 37.5–300 mg, levels of dopamine D₂ occupancy remained below the level for motor side effects observed with typical antipsychotics and is likely to explain the low propensity of the drug to induce extrapyramidal side effects.     

D<Subscript>2</Subscript> but not D<Subscript>1</Subscript> dopamine receptor stimulation augments brain signaling involving arachidonic acid in unanesthetized rats

Rationale and objectives Signal transduction involving the activation of phospholipase A₂ (PLA₂) to release arachidonic acid (AA) from membrane phospholipids, when coupled to dopamine D₁- and D₂-type receptors, can be imaged in rats having a chronic unilateral lesion of the substantia nigra. It is not known, however, if the signaling responses occur in the absence of a lesion. To determine this, we used our in vivo fatty acid method to measure signaling in response to D₁ and D₂ receptor agonists given acutely to unanesthetized rats. Methods [1-¹⁴C]AA was injected intravenously in unanesthetized rats, and incorporation coefficients k* for AA (brain radioactivity/integrated plasma radioactivity) were measured using quantitative autoradiography in 61 brain regions. The animals were administered i.v. the D₂ receptor agonist, quinpirole (1 mg kg⁻¹, i.v.), the D₁ receptor agonist SKF-38393 (5 mg kg⁻¹, i.v.), or vehicle/saline. Results Quinpirole increased k* significantly in multiple brain regions rich in D₂-type receptors, whereas SKF-38393 did not change k* significantly in any of the 61 regions examined. Conclusions In the intact rat brain, D₂ but not D₁ receptors are coupled to the activation of PLA₂ and the release of AA.     

Striatal dopamine D<Subscript>2</Subscript> receptors in medication-naive patients with major depressive disorder as assessed with [<Superscript>11</Superscript>C]raclopride PET

Rationale Among other monoamine neurotransmitters, dopamine is implicated in the pathophysiology of major depression. Experimental studies suggest the involvement of the mesolimbic dopamine system in the mechanism of action of antidepressant drugs. Previous in vivo imaging studies have studied striatal dopamine D₂ receptor availability in depression but the results are equivocal thus far. Objective To study the striatal and thalamic dopamine D₂ receptor availability in drug-naive patients with major depression was the aim of this study. Materials and methods Caudate, putamen, and thalamic dopamine D₂ receptor availability was estimated using positron emission tomography and [¹¹C]raclopride in 25 treatment-seeking drug-free patients (of whom 24 were drug-naive) with major depression (primary care patients) as well as in 19 demographically similar healthy control subjects. Receptor availability was expressed as the binding potential (BP_ND), and analyses were carried out based on both regional and voxel-level BP_ND estimates. Results No statistically significant differences in [¹¹C]raclopride BP_ND were observed between the groups either in the caudate nucleus (+1.7%, CI −4.8% to +8.3%), putamen (−1.0%, CI −7.2% to 5.1%), thalamus (−2.4%, CI −8.7% to 4.0%), or ventral striatum (−3.8%, CI −9.3% to +1.6%). In the patients, depressive symptoms were not associated with [¹¹C]raclopride BP_ND in any region. Conclusions The findings in this sample of treatment-seeking, drug-naive and predominantly first-episode patients with major depression do not support the involvement of striatal dopamine D₂ receptors in the pathophysiology of the illness, but do not exclude the potential importance of dopaminergic mechanisms in antidepressant drug action.     

Non-uniform blockade of intrastriatal D<Subscript>2</Subscript>/D<Subscript>3</Subscript> receptors by risperidone and amisulpride

Rationale Atypical antipsychotic drugs have been shown to preferentially affect extrastriatal (mesolimbic) D₂/D₃ receptors over those within the striatum (nigrostriatal). The striatum does not contain exclusively nigrostriatal dopamine tracts, however. The caudate nucleus and ventral parts of the striatum primarily contain limbic and associative dopamine pathways more relevant to psychosis. Objectives We tested the hypothesis that two pharmacologically distinct atypical antipsychotic drugs, amisulpride and risperidone, would preferentially occupy of D₂/D₃ dopamine receptors in limbic and associative regions of the striatum. Methods Eight amisulpride-treated patients, six risperidone-treated patients and six age- and sex-matched healthy controls were recruited. Dynamic SPET studies were performed after bolus injection of [¹²³I]epidepride. Binding potential (BP) images were generated using a modified Logan method and aligned between subjects. Regions of interest (ROIs) were placed around head of caudate and putamen bilaterally on an average BP map derived from aligned control images. These ROIs were then applied user-independently to the BP maps for each subject to calculate BP for head of caudate and putamen. Mean occupancy of D₂/D₃ receptors in each ROI was determined by reference to the drug-free healthy volunteer group. Occupancy values for head of caudate and putamen were compared using paired Student’s t test. Results D₂/D₃ receptor occupancy was 42% in caudate and 31% in putamen for risperidone (t=5.9, df=11, p=0.0001) and 51% in caudate and 37% in putamen for amisulpride (t=11.1, df=15, p<0.0001). Conclusions Amisulpride and risperidone both show selective occupancy for limbic and associative D₂/D₃ receptors within the striatum.     

The activity of pramipexole in the mouse forced swim test is mediated by D<Subscript>2</Subscript> rather than D<Subscript>3</Subscript> receptors

Rationale Recent studies have reported antidepressant-like activities of the dopamine D₂/D₃ agonist pramipexole in the chronic mild stress model and in the forced swim test, suggesting that D₃ receptor agonists may represent a new class of antidepressant drugs. However, the relative contribution of D₂ or D₃ receptors to the activity of pramipexole in these models is unclear.Objectives The aim of the current studies was to explore the role of dopamine D₂ and D₃ receptors in the activity of pramipexole in the mouse forced swim test.Methods The effect of pramipexole (0.1–3.2 mg/kg) in the mouse forced swim test was examined both in conjunction with D₂ and D₃ receptor antagonists (haloperidol (0.1–1 mg/kg) and LU-201640 (A-437203, 5.6–17.8 mg/kg), as well as in D₃ receptor knockout mice obtained on two different background strains (C57BL/6J and B6129SF2/J). Locomotor activity was also assessed following pramipexole administration.Results Pramipexole produced dose-dependent reductions in immobility in the forced swim test at doses that did not produce generalized increases in locomotor activity. LU-201640, the D₃ selective antagonist, failed to block the antidepressant-like effects of pramipexole. In contrast, the efficacy of pramipexole in the forced swim test was completely blocked by the D₂ antagonist, haloperidol. No baseline differences were observed between knockout and wild-type mice from either background strain in locomotor activity or in the forced swim test. Furthermore, in both background strains, pramipexole showed similar efficacy in the forced swim test for both wild-type and knockout mice.Conclusions Taken together, these studies suggest that the D₂ receptor rather than the D₃ receptor is important for the antidepressant-like activity observed for pramipexole in the mouse forced swim test.Portions of this work were presented at the 36th Winter Conference on Brain Research, Snowbird, UT, January 26–31, 2003.     

Distinct temporal phases in the behavioral pharmacology of LSD: dopamine D<Subscript>2</Subscript> receptor-mediated effects in the rat and implications for psychosis

Rationale The effect of LSD in humans has been described as occurring in two temporal phases. The behavioral effects in rats also occur in two temporal phases: an initial suppression of exploration followed by increased locomotor activity.Objectives We decided to investigate this phenomenon from the perspective that the pharmacology might have relevance to the neurochemical mechanisms underlying psychosis.Methods Twenty-five male Sprague–Dawley rats were trained to discriminate LSD (186 nmol/kg, 0.08 mg/kg, i.p.) with a 30-min preinjection time (LSD-30, N=12) and LSD (372 nmol/kg, 0.16 mg/kg, i.p.) with a 90-min preinjection time (LSD-90, N=13) from saline, using a two-lever, food-reinforced operant conditioning task.Results LSD (186 or 372 nmol/kg, 0.08 or 0.16 mg/kg) given 30 min prior to training produced a cue that was completely antagonized by 5-HT_2A antagonists and lasted no longer than 1 h. LSD (372 nmol/kg, 0.16 mg/kg) injected 90 min before training produced a cue that was not fully blocked by 5-HT_2A antagonists, but instead was significantly inhibited by haloperidol. In these rats, substitution no longer occurred with the 5-HT₂ agonists DOI or LSD (30 min preinjection), but full substitution was obtained with the D₂ agonists apomorphine, N-propyldihydrexidine, and quinelorane.Conclusion The discriminative stimulus effect of LSD in rats occurs in two phases, and these studies provide evidence that the later temporal phase is mediated by D₂ dopamine receptor stimulation. A second temporal phase that involves dopaminergic pathways would be consistent with the widespread belief that excessive dopaminergic activity may be an underlying cause of paranoid psychosis.     

Imaging apomorphine stimulation of brain arachidonic acid signaling <Emphasis Type="Italic">via</Emphasis> D<Subscript>2</Subscript>-like receptors in unanesthetized rats

Rationale and objective Because of the important role of dopamine in neurotransmission, it would be useful to be able to image brain dopamine receptor-mediated signal transduction in animals and humans. Administering the D₁–D₂ receptor agonist apomorphine may allow us to do this, as the D₂-like receptor is reported to be coupled to cytosolic phospholipase A₂ activation and arachidonic acid (AA) release from membrane phospholipid. Methods Unanesthetized adult rats were given intraperitoneally apomorphine (0.5 mg/kg) or saline, with or without pretreatment with 6 mg/kg intravenous raclopride, a D₂/D₃ receptor antagonist. [1–¹⁴C]AA was injected intravenously, then AA incorporation coefficients k*—brain radioactivity divided by integrated plasma radioactivity—markers of AA signaling, were measured using quantitative autoradiography in 62 brain regions. Results Apomorphine significantly elevated k* in 26 brain regions, including the frontal cortex, motor and somatosensory cortex, caudate-putamen, thalamic nuclei, and nucleus accumbens. Raclopride alone did not change baseline values of k*, but raclopride pretreatment prevented the apomorphine-induced increments in k*. Conclusions A mixed D₁–D₂ receptor agonist, apomorphine, increased the AA signal by activating only D₂-like receptors in brain circuits containing regions with high D₂-like receptor densities. Thus, apomorphine might be used with positron emission tomography to image brain D₂-like receptor-mediated AA signaling in humans in health and disease.     

The role of dopamine D<Subscript>3</Subscript> compared with D<Subscript>2</Subscript> receptors in the control of locomotor activity: a combined behavioural and neurochemical analysis with novel,selective antagonists in rats

Background The role of dopamine D₃/D₂ receptors in the control of locomotion is poorly understood.Objectives To examine the influence of selective antagonists at D₃ or D₂ receptors on locomotion in rats, alone and in interaction with the preferential D₃ versus D₂ receptor agonist, PD128,907.Methods Affinities of ligands at rat D₂ and cloned, human hD₃, hD_2S, hD_2L and hD₄ sites were determined by standard procedures. Locomotion was monitored automatically in rats pre-habituated for 30 min to an open-field environment. Extracellular levels of dopamine (DA) were determined by dialysis in the nucleus accumbens and striatum. Drugs were given acutely via the systemic route.Results PD128,907, which preferentially recognised D₃ versus D₂ sites, biphasically reduced and enhanced locomotion at low (0.01–0.63 mg/kg) and high (2.5–10 mg/kg) doses, respectively. L741,626 and S23199, which behaved as preferential D₂ versus D₃ receptor antagonists, enhanced the reduction in locomotion evoked by the low dose of PD128,907, blocked the increase provoked by the high dose and suppressed spontaneous locomotion alone. Analogous findings were obtained with haloperidol and raclopride which showed equilibrated affinity at D₂ and D₃ receptors. UH232 and AJ76, which showed a mild preference for D₃ versus D₂ sites, did not modify the effect of a low dose of PD128,907, slightly enhanced the hyperlocomotion elicited by the high dose and exerted little influence on locomotion alone. S14297 and U99194, which acted as preferential D₃ versus D₂ receptor antagonists, abolished the reduction in locomotion elicited by a low dose of PD128,907, potentiated the induction of locomotion by a high dose, and failed to influence locomotion alone. The actions of S14297 were stereoselective inasmuch as they were mimicked by the racemic form, S11566, but not by the inactive enantiomer, S17777. In contrast to S14297, S11566 and U99194, however, S33084, SB269,652, GR218,231 and N-[-4-[-(1-naphtyl)piperazine-1-yl]butyl] anthracene-2-carboxamide (NGB-1), highly selective D₃ versus D₂ receptor antagonists, were inactive under all conditions. PD128,907 (0.01–10.0 mg/kg) suppressed dialysate levels of DA in the nucleus accumbens and striatum, actions blocked by L741,626 and haloperidol, yet unaffected by S14297 and S33084.Conclusions The facilitatory influence of a high dose of PD128,907 upon locomotion is mediated by postsynaptic D₂ receptors and, possibly, countered by their D₃ counterparts. Correspondingly, selective blockade of D₂ but not of D₃ receptors alone suppresses motor function. The reduction in locomotion provoked by a low dose of PD128,907 may be mediated by D₂ autoreceptors, but a role of postsynaptic D₃ receptors cannot be excluded. Finally, mechanisms underlying the contrasting influence of chemically diverse D₃ receptor antagonists upon locomotion remain to be elucidated.     

Characterization of dopamine D<Subscript>1</Subscript> and D<Subscript>2</Subscript> receptor function in socially housed cynomolgus monkeys self-administering cocaine

Rationale Social rank has been shown to influence dopamine (DA) D₂ receptor function and vulnerability to cocaine self-administration in cynomolgus monkeys. The present studies were designed to extend these findings to maintenance of cocaine reinforcement and to DA D₁ receptors.Objective Examine the effects of a high-efficacy D₁ agonist on an unconditioned behavior (eyeblinking) and a low-efficacy D₁ agonist on cocaine self-administration, as well as the effects of cocaine exposure on D₂ receptor function across social ranks, as determined by positron emission tomography (PET).Methods Effects of the high-efficacy D₁ agonist SKF 81297 and cocaine (0.3–3.0 mg/kg) on spontaneous blinking were characterized in eight monkeys during 15-min observation periods. Next, the ability of the low-efficacy D₁ agonist SKF 38393 (0.1–17 mg/kg) to decrease cocaine self-administration (0.003–0.1 mg/kg per injection, IV) was assessed in 11 monkeys responding under a fixed-ratio 50 schedule. Finally, D₂ receptor levels in the caudate and putamen were assessed in nineteen monkeys using PET.Results SKF 81297, but not cocaine, significantly increased blinking in all monkeys, with slightly greater potency in dominant monkeys. SKF 38393 dose-dependently decreased cocaine-maintained response rates with similar behavioral potency and efficacy across social rank. After an extensive cocaine self-administration history, D₂ receptor levels did not differ across social ranks.Conclusions These results suggest that D₁ receptor function is not substantially influenced by social rank in monkeys from well-established social groups. While an earlier study showed that dominant monkeys had higher D₂ receptor levels and were less sensitive to the reinforcing effects of cocaine during initial exposure, the present findings indicate that long-term cocaine use changed D₂ receptor levels such that D₂ receptor function and cocaine reinforcement were not different between social ranks. These findings suggest that cocaine exposure attenuated the impact of social housing on DA receptor function.     

Prolactinemia is uncoupled from central D<Subscript>2</Subscript>/D<Subscript>3</Subscript> dopamine receptor occupancy in amisulpride treated patients

Rationale Atypical antipsychotic drugs are classically associated with lower propensity to extrapyramidal symptoms (EPS) and hyperprolactinemia than typical antipsychotic drugs. It has not been clarified why some atypical antipsychotic drugs, such as amisulpride, induce prolactin plasma concentration (PRL) elevation, but little EPS. Previous studies have found an association between striatal D₂/D₃ receptor occupancy and PRL in typical antipsychotic treated patients suggesting that PRL is a marker of central D₂/D₃ receptors blockade.Objective We have evaluated the relationship between PRL and central (striatum, temporal cortex and thalamus) D₂/D₃ receptor occupancy in amisulpride treated schizophrenic patients.Methods Single photon emission tomography (SPET) and [¹²³I]-epidepride were used to determine D₂/D₃ receptor occupancy in eight amisulpride treated patients. PRL was measured concurrently with the scans.Results The mean PRL was 1166 (range 499–1892 mIU/l) for a mean amisulpride dose of 406 mg/day (range 150–600 mg/day). Amisulpride plasma concentration and central D₂/D₃ receptor occupancy were positively correlated (r=0.83–0.89, df=4, P<0.05). No significant correlations were observed between PRL and amisulpride (daily dose or plasma concentration, P>0.05), or between PRL and central D₂/D₃ receptor occupancy (P>0.05).Conclusions Our findings show that amisulpride-induced hyperprolactinemia is uncoupled from central D₂/D₃ receptor occupancy. Amisulpride has poor blood–brain barrier penetration and reaches much higher concentration at the pituitary, which is outside the blood–brain barrier. Higher D₂/D₃ receptor occupancy at the pituitary gland than at central regions is a possible explanation for amisulpride PRL elevation with low EPS. Further studies evaluating pituitary D₂/D₃ receptor occupancy in vivo are necessary to confirm this hypothesis.This study was partially presented (poster) at the International Congress on Schizophrenia Research, Colorado Springs, USA, 2003 and received the Young Investigator Award.     

A PET study on regional coexpression of 5-HT<Subscript>1A</Subscript> receptors and 5-HTT in the human brain

Rationale Several lines of evidence suggest inter-dependency between the serotonin transporter (5-HTT) and the 5HT_1A receptor, two recognised targets for the treatment of anxiety and depression. Objectives to examine the correlation of regional expression levels for these two serotonergic markers in the human brain in vivo. Methods Twelve male control subjects were examined with PET twice on the same day, using the radioligands [¹¹C]WAY 100635 and [¹¹C]MADAM for quantification of the 5-HT_1A receptor and the 5-HTT, respectively. The binding potential (BP) was calculated for raphe nuclei, hippocampus and frontal cortex. Results In all regions, the BP for both [¹¹C]WAY 100635 (raphe nuclei 1.85–4.71, hippocampus 2.52–6.17, frontal cortex 2.03–3.79) and [¹¹C]MADAM (2.70–7.65, 0.47–1.76, 0.18–0.51) varied several fold between subjects. In the raphe nuclei, where the two markers are situated on the same neurons, the ratio of [¹¹C]WAY 100635 binding to [¹¹C]MADAM BP binding varied considerably (0.43–1.05). There was a positive correlation between the two markers in the raphe nuclei (r _xy = 0.68, p < 0.05) and in the hippocampus (r _xy = 0.97, p < 0.001) but not in the frontal cortex (r _xy = −0.25, p = 0.44). Conclusions The results support a correlation between density levels of the 5-HT_1A-receptor and the 5-HTT in the raphe nuclei and hippocampus but not in the frontal cortex. A suggested clinical implication is that the inter-individual variability in 5-HT_1A-receptor and 5-HTT densities, as well as the ratio of these, is of particular interest in relation to individual responses to selective serotonin reuptake inhibitor treatment.     

Dexmedetomidine inhibits muscarinic type 3 receptors expressed in <Emphasis Type="Italic">Xenopus</Emphasis> oocytes and muscarine-induced intracellular Ca<Superscript>2+</Superscript> elevation in cultured rat dorsal root ganglia cells

Dexmedetomidine, an α₂-adrenoceptor agonist, has been approved for clinical use, although the mechanism of dexmedetomidine action has not been fully elucidated. Several studies have shown that G protein-coupled receptors (GPCRs) are recognized as targets for anesthetics and analgesics. Therefore, it is of interest to determine whether dexmedetomidine affects the function of GPCRs other than the α₂-adrenoceptor. We examined the effects of dexmedetomidine on M₁, M₃, 5-HT_2C, substance P, and orexin 1 receptors in Xenopus oocytes expressing individual receptors. In addition, we investigated the effects of dexmedetomidine on muscarinic receptor-mediated changes in [Ca²⁺]_i in the dorsal root ganglia (DRG) of 3-week-old Wister rats. Dexmedetomidine did not affect the 5-HT_2C-, or substance P-induced Cl⁻ currents and had little inhibition on the orexin A-induced current in oocytes expressing the respective receptors. The compound also had little effect on the acetylcholine (ACh, 1 μM)-induced Ca²⁺-activated Cl⁻ currents in Xenopus oocytes expressing M₁ receptors. In contrast, dexmedetomidine inhibited the ACh-induced currents in Xenopus oocytes expressing M₃ receptors; 1 nM, 10 nM, 100 nM, and 1 μM dexmedetomidine reduced the current to 66.5 ± 4.8, 51.3 ± 12, 34.6 ± 11, and 26.8 ± 6.4% of the control value, respectively (EC₅₀ = 3.5 ± 0.7 nM). Dexmedetomidine reduced the ACh-induced Cl⁻ currents after treatment with the selective protein kinase C inhibitor GF109203X. Moreover, the compound inhibited the muscarinic receptor-mediated increases in [Ca²⁺]_i in cultured DRG cells in a concentration-dependent manner. Dexmedetomidine inhibits the function of M₃ receptors, in addition to its agonistic effects on α₂-adrenoceptors, which provides further insight into the pharmacological properties of dexmedetomidine.     

WAY-100635 is a potent dopamine D4 receptor agonist

Rationale and objectives WAY-100635 is a prototypical 5-HT_1A receptor antagonist and has been used widely as a pharmacological probe to investigate the distribution and function of 5-HT_1A receptors. Results from our studies suggested that WAY-100635 was potently inducing effects unrelated to its 5-HT_1A receptor affinity. In the present work, we evaluated the in vitro pharmacology of this compound at two D₂-like receptor subtypes.Method The functional properties and binding affinities of WAY-100635 were evaluated in HEK 293 cells stably expressing dopamine D_2L or D_4.4 receptors.Results Initial screens performed by the NIMH Psychoactive Drug Screening Program indicated that WAY-100635 displayed 940, 370, and 16 nM binding affinities at D_2L, D₃, and D_4.2 receptors, respectively. Subsequent saturation analyses demonstrated that the K _d of [³H]WAY-100635 at D_4.2 receptors was 2.4 nM, only tenfold higher than 5-HT_1A. WAY-100635 and its major metabolite, WAY-100634, were potent agonists in HEK-D_4.4 cells (EC₅₀=9.7±2.2 and 0.65±0.2 nM, respectively). WAY-100635 behaved as a full agonist, and WAY-100634 was a nearly full agonist. In HEK-D_2L cells, WAY-100635 weakly antagonized the effects of 300 nM quinpirole. Subsequent radioligand binding studies confirmed that WAY-100635 possesses high affinity for D_4.4 receptors but binds weakly to D_2L receptors (3.3±0.6 and 420±11 nM, respectively).Conclusions This study demonstrates that WAY-100635 is not a “selective” 5-HT_1A receptor antagonist, as previously reported, and conclusions drawn from studies that employed WAY-100635 as a selective 5-HT_1A antagonist may need to be reevaluated.     

Further characterization of the 5-HT<Subscript>1</Subscript> receptors mediating cardiac sympatho-inhibition in pithed rats: pharmacological correlation with the 5-HT<Subscript>1B</Subscript> and 5-HT<Subscript>1D</Subscript> subtypes

Serotonin (5-hydroxytryptamine; 5-HT) is capable of inhibiting the tachycardic responses elicited by sympathetic stimulation, but not by exogenous noradrenaline, in pithed rats pre-treated with desipramine. More recently, it has been shown that this cardiac sympatho-inhibitory response to 5-HT, mediated by prejunctional 5-HT₁ receptors as well as putative 5-ht_5A/5B receptors, is mimicked dose-dependently by the agonists CP 93,129 (r5-HT_1B), sumatriptan (5-HT_1B/1D) and PNU-142633 (5-HT_1D). This study analysed further the pharmacological profile of the above 5-HT₁ receptors.Continuous i.v. infusions of CP 93,129, sumatriptan or PNU-142633 (30 µg kg^–1min^–1 each) failed to modify the tachycardic responses to exogenous noradrenaline but inhibited those elicited by preganglionic (C7–T1) stimulation of the cardiac sympathetic outflow. These sympatho-inhibitory responses were unaltered after i.v. administration of physiological saline (1 ml kg^–1) or the 5-HT_1A receptor antagonist WAY 100635 (10 µg kg^–1). In contrast, the antagonist GR 127935 (5-HT_1B/1D; 100 µg kg^–1, i.v.) abolished the responses to CP 93,129, sumatriptan and PNU-142633, whilst SB224289 (5-HT_1B; 300 µg kg^–1, i.v.) abolished the responses to CP 93,129 without affecting those to sumatriptan and PNU-142633. Interestingly, BRL15572 (5-HT_1D; 300 µg kg^–1, i.v.) abolished the responses to PNU-142633 and attenuated those to sumatriptan, but not those to CP 93,129.WAY 100635, GR 127935, SB224289 and BRL15572, given alone at the above doses, failed to modify the sympathetically induced tachycardic responses. The 5-HT₁ receptors producing cardiac sympatho-inhibition in pithed rats thus display the pharmacological profile of the 5-HT_1B and 5-HT_1D receptor subtypes.     

Changes in urinary PGE<Subscript>2</Subscript> after ibuprofen treatment in preterm infants with patent ductus arteriosus

Purpose  To investigate changes in urinary PGE₂ after ibuprofen treatment in preterm infants with patent ductus arteriosus (PDA). Methods  Twenty preterm infants with a hemodynamically significant PDA (gestational age, 28.6 ± 2.3 weeks) and 20 controls (gestational age, 30.4 ± 1.5 weeks) were prospectively enrolled at 48–72 h of life. After enrollment, the former underwent conventional ibuprofen-lysine treatment. At 48–72 h (T₀) and 108–144 h of life (T₁), urine samples were noninvasively collected in both groups to measure urinary PGE₂ concentrations (enzyme immunoassay method), and renal function was investigated. Results  Urinary PGE₂ decreased significantly both in ibuprofen-treated patients (66.95 ± 16.78 vs. 27.15 ± 17.92 pg/mL, P < 0.001) and in controls (71.7 ± 16.2 vs. 53.2 ± 18.4 pg/mL, P < 0.001) from T₀ to T₁. However, urinary PGE₂ at T₁ was significantly lower (P < 0.001) in the ibuprofen group compared to the control group. Acute renal failure occurred in three ibuprofen-treated patients (15%). Conclusions  Ibuprofen markedly reduces (59.4%) urinary PGE₂ and may alter renal function in the newborn.     

The selective dopamine D<Subscript>3</Subscript> receptor antagonists SB-277011A and NGB 2904 and the putative partial D<Subscript>3</Subscript> receptor agonist BP-897 attenuate methamphetamine-enhanced brain stimulation reward in rats

Rationale We have previously reported that selective antagonism of brain D₃ receptors by SB-277011A or NGB 2904 significantly attenuates cocaine- or nicotine-enhanced brain stimulation reward (BSR). Objective In the present study, we investigated whether the selective D₃ receptor antagonists SB-277011A and NGB 2904 and the putative partial D₃ agonist BP-897 similarly reduce methamphetamine (METH)-enhanced BSR. Materials and methods Rats were trained to respond for rewarding electrical self-stimulation of the medial forebrain bundle. To assess the degree of drug-induced changes in BSR, a rate–frequency curve shift paradigm was used to measure brain-reward threshold (θ ₀). Results METH (0.1–0.65 mg/kg, i.p.) dose-dependently lowered (∼10–50%) BSR thresholds, producing an enhancement of BSR. Pretreatment with SB-277011A (12 mg/kg, but not 24 mg/kg, i.p.) significantly attenuated METH-enhanced BSR. NGB 2904 (0.1–1.0 mg/kg, but not 10 mg/kg) also attenuated METH-enhanced BSR. SB-277011A or NGB 2904 alone, at the doses tested, had no effect on BSR. Pretreatment with BP-897 (0.1–5 mg/kg) dose-dependently attenuated METH-enhanced BSR. However, when the dose was increased to 10 mg/kg, BP-897 shifted the stimulation–response curve to the right (inhibited BSR itself) in the presence or absence of METH. Conclusions Selective antagonism of D₃ receptors by SB-277011A or NGB 2904 attenuates METH-enhanced BSR in rats, while the METH-enhanced BSR attenuation produced by BP-897 may involve both D₃ and non-D₃ receptors. These findings support a potential use of selective D₃ receptor antagonists for the treatment of METH addiction.     

Functional crosstalk and heteromerization of serotonin 5-HT2A and dopamine D2 receptors

The serotonin 5-HT_2A receptor (5-HT_2AR) and dopamine D₂ receptor (D₂R) are high-affinity G protein-coupled receptor targets for two different classes of antipsychotic drugs used to treat schizophrenia. Interestingly, the antipsychotic effects are not based on the regulation of same signaling mediators since activation of the 5-HT_2AR and of the D₂R regulate G_q/11 protein and G_i/o protein, respectively. Here we use radioligand binding and second messenger production assays to provide evidence for a functional crosstalk between 5-HT_2AR and D₂R in brain and in HEK293 cells. D₂R activation increases the hallucinogenic agonist affinity for 5-HT_2AR and decreases the 5-HT_2AR induced inositol phosphate production. In vivo, 5-HT_2AR expression is necessary for the full effects of D₂R antagonist on MK-801-induced locomotor activity. Co-immunoprecipitation studies show that the two receptors can physically interact in HEK293 cells and raise the possibility that a receptor heterocomplex mediates the crosstalk observed. The existence of this 5-HT_2AR-D₂R heteromer and crosstalk may have implications for diseases involving alterations of serotonin and dopamine systems and for the development of new classes of therapeutic drugs.     

Attenuation of the effects of <Emphasis Type="Italic">d</Emphasis>-amphetamine on interval timing behavior by central 5-hydroxytryptamine depletion

Rationale  Interval timing in the free-operant psychophysical procedure is sensitive to the monoamine-releasing agent d-amphetamine, the D₂-like dopamine receptor agonist quinpirole, and the D₁-like agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzepine (SKF-81297). The effect of d-amphetamine can be antagonized by selective D₁-like and 5-HT_2A receptor antagonists. It is not known whether d-amphetamine’s effect requires an intact 5-hydroxytryptamine (5-HT) pathway. Objective  The objective of this study was to examine the effects of d-amphetamine, quinpirole, and SKF-81297 on timing in intact rats and rats whose 5-hydroxytryptaminergic (5-HTergic) pathways had been ablated. Materials and methods  Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data for derivation of timing indices (T ₅₀, time corresponding to %B = 50%; Weber fraction). The effects of d-amphetamine (0.4 mg kg⁻¹ i.p.), quinpirole (0.08 mg kg⁻¹ i.p.), and SKF-81297 (0.4 mg kg⁻¹ s.c.) were compared between intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Results  Quinpirole and SKF-81297 reduced T ₅₀ in both groups; d-amphetamine reduced T ₅₀ only in the sham-lesioned group. The lesion reduced 5-HT levels by 80%; catecholamine levels were not affected. Conclusions   d-Amphetamine’s effect on performance in the free-operant psychophysical procedure requires an intact 5-HTergic system. 5-HT, possibly acting at 5-HT_2A receptors, may play a ‘permissive’ role in dopamine release.

Locomotor and discriminative-stimulus effects of cocaine in dopamine D<Subscript>5</Subscript> receptor knockout mice

Rationale Dopamine D₁-like antagonists block several effects of cocaine, including its locomotor-stimulant and discriminative-stimulus effects. Because these compounds generally lack selectivity among the dopamine D₁ and D₅ receptors, the specific roles of the subtypes have not been determined. Objectives Dopamine D₅ receptor knockout (DA D₅R KO), heterozygous (HET) and wild-type (WT) mice were used to study the role of D₅ dopamine receptors in the effects of cocaine. In addition, effects of the D₁-like antagonist, SCH 39166 were also studied to further clarify the roles of D₁ and D₅ dopamine receptors in the discriminative-stimulus effects of cocaine. Methods DA D₅R KO, HET and WT mice were treated with cocaine (3–30 mg/kg) or vehicle and their horizontal locomotor activity was assessed. The mice were also trained to discriminate IP injections of saline from cocaine (10 mg/kg) using a two-lever food-reinforcement (FR10) procedure. Doses of cocaine (1.0–10 mg/kg) were administered 5 min before 15-min test-sessions. Results Cocaine dose-dependently stimulated activity in each genotype, with the highest level of activity induced in the DA D₅R WT mice. Both DA D₅R KO and HET mice showed reduced levels of horizontal activity compared to WT mice. All three genotypes acquired the discrimination of 10 mg/kg cocaine; doses of 1.0–10.0 mg/kg produced dose-related increases in the number of cocaine-appropriate responses. SCH 39166, at inactive to fully active doses (0.01–0.1 mg/kg) produced predominately saline-appropriate responding. SCH 39166 produced a dose-dependent rightward shift in the cocaine dose-effect curve in all genotypes, with similar apparent affinities. Conclusions The present data suggest an involvement of DA D₅R in the locomotor stimulant effects of cocaine. In addition, the data indicate that there is little involvement of the DA D₅R in the discriminative-stimulus effects of cocaine. In addition, the antagonism data suggest a role of the D₁ receptor in the behavioral effects of cocaine.