Neurotensin agonists block the prepulse inhibition deficits produced by a 5-HT<Subscript>2A</Subscript> and an α<Subscript>1</Subscript> agonist

Rationale Neurotensin (NT) agonists have been proposed as potential antipsychotics based exclusively upon their ability to inhibit dopamine-2 (D₂) receptor transmission. Several other pharmacological mechanisms have been implicated in enhancing the antipsychotic profile produced by D₂ inhibition alone. These include inhibition of 5-HT_2A and ₁-adrenoceptors. Recently, we reported that systemic administration of the neurotensin agonist PD149163 blocks deficits in prepulse inhibition (PPI) of the startle reflex produced by the 5-HT_2A receptor agonist DOI. This suggested that NT agonists could inhibit 5-HT_2A modulation of neurotransmission.Objective To determine if other peripherally administered NT agonists shared this effect, we examined the effects of NT69L, another NT agonist, on DOI-induced PPI deficits. In addition, to determine if NT agonists also inhibit ₁-adrenoceptor neurotransmission, we examined the effects of PD149163 and NT69L on PPI deficits induced by the ₁-adrenoceptor agonist, cirazoline.Methods In the NT69L/DOI study, rats received subcutaneous (SC) injections of NT69L (0, 0.1, 1, or 2 mg/kg) followed 30 min later by SC saline or DOI (0.5 mg/kg). In the NT agonist/cirazoline studies, animals received SC injections of either PD149163 (0, 0.01, 0.1, or 1 mg/kg) or NT69L (0, 0.01, 0.1, or 1 mg/kg) followed 30 min later by SC saline or cirazoline (0.7 mg/kg). Animals were tested in startle chambers 20 min later.Results In all three experiments the PPI disruption produced by DOI and cirazoline was blocked by the NT agonists.Conclusions These findings provide strong evidence that NT agonists inhibit 5-HT_2A and ₁-adrenoceptor modulation of neurotransmission, pharmacological effects that, in conjunction with their known inhibition of dopamine transmission, strengthen the antipsychotic potential of NT agonists.     

Yohimbine disrupts prepulse inhibition in rats via action at 5-HT<Subscript>1A</Subscript> receptors,not α<Subscript>2</Subscript>-adrenoceptors

RATIONALE: Prepulse inhibition (PPI) of the acoustic startle response is an operational measure of sensorimotor gating that can be assessed in both humans and animals. The noradrenergic system appears to play a role in PPI as the alpha1 agonist cirazoline disrupts PPI and the alpha1 antagonist prazosin blocks the disruptions in PPI produced by phencyclidine. OBJECTIVES: To better understand the role of adrenergic receptors in the modulation of PPI, we assessed the effects of the alpha2 adrenergic antagonist yohimbine (2.5, 5.0, and 7.5 mg/kg) on PPI. RESULTS: Yohimbine reduced PPI at the 5.0 and 7.5 mg/kg doses, without significantly affecting startle magnitude. In separate experiments, we examined whether adrenergic or serotonergic compounds blocked this disruption in PPI produced by yohimbine. There was a trend for the alpha2 agonist clonidine (0.01, 0.02 mg/kg) to attenuate the PPI disruption produced by yohimbine. However, other alpha2 agonists (guanfacine, medetomidine) and an alpha1 antagonist (prazosin) failed to prevent the disruption. The alpha2 antagonist atipamezole weakly decreased PPI in a narrow dose range (0.3-1.0 mg/kg). The 5-HT1A antagonist WAY100,635 (0.1, 0.3 mg/kg) significantly prevented the yohimbine-induced disruption of PPI. CONCLUSIONS: These findings indicate that (1) yohimbine disrupts PPI in rats and (2) the yohimbine-induced disruption of PPI is largely due to the 5-HT1A partial agonist properties of yohimbine.     

Anticataleptic properties of α<Subscript>2</Subscript> adrenergic antagonists in the crossed leg position and bar tests: differential mediation by 5-HT<Subscript>1A</Subscript> receptor activation

Rationale Recent studies suggest that ₂ adrenoceptor blockade may improve the antipsychotic-like effects of neuroleptics and attenuate dopamine D₂ receptor antagonist-induced catalepsy. However, several ₂ adrenergic antagonists also display serotonin 5-HT_1A receptor agonist activity, which may contribute to anticataleptic actions.Objectives In this study, we examined a series of ₂ adrenergic antagonists to determine the role of activity at serotonin 5-HT_1A receptors in their anticataleptic effects.Methods Catalepsy in rats induced by the antipsychotic haloperidol (2.5 mg/kg, SC) was measured using the cross-legged position (CLP) and bar tests. The compounds examined in this study, in decreasing rank order of ₂ adrenergic versus 5-HT_1A receptor selectivity, were atipamezole, methoxy-idazoxan (RX821002), efaroxan, idazoxan, and yohimbine. Antagonism studies were conducted using the selective 5-HT_1A receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide dihydrochloride (WAY100635).Results Idazoxan, efaroxan, and yohimbine significantly attenuated the cataleptic effects of haloperidol (2.5 mg/kg, SC) in the CLP test and the actions of their highest doses were significantly blocked by pre-treatment with WAY100635 (0.63 mg/kg, SC). In contrast to the other compounds, methoxy-idazoxan was ineffective in the CLP test. Atipamezole exhibited anticataleptic effects in the bar and CLP tests which were not blocked by WAY100635. Similarly, the anticataleptic effects of methoxy-idazoxan and idazoxan in the bar test were not blocked by WAY100635.Conclusions Serotonin 5-HT_1A receptors play a prominent role in anticataleptic effects of certain ₂ adrenergic antagonists in the CLP test, whereas ₂-adrenergic mechanisms are likely to be primarily responsible for the anticataleptic effects of these ligands in the bar test.     

Binding of [<Superscript>3</Superscript>H]prazosin to α<Subscript>1A</Subscript>- and α<Subscript>1B</Subscript>-adrenoceptors,and to a cimetidine-sensitive non-α<Subscript>1</Subscript> binding site in rat kidney membranes

[(3)H]Prazosin bound to alpha(1A)- and alpha(1B)-adrenoceptors, as well as to a cimetidine-sensitive non-alpha(1)-adrenoceptor binding site in rat kidney membranes. An experimental design is presented where the alpha(1)-adrenoceptors are selectively exposed by blocking the non-alpha(1) binding site with 60 microM cimetidine. Conversely, the non-alpha(1) binding site can be selectively exposed by blocking the alpha(1)-adrenoceptors with 600 nM metitepine. The identity of the non-alpha(1) binding site for [(3)H]prazosin in the rat kidney, herein pharmacologically characterized by 33 competing substances, is still unknown.     

Cross-talk between β<Subscript>1</Subscript>-adrenoceptors and ET<Subscript>A</Subscript> receptors in modulation of the slow component of delayed rectifier K<Superscript>+</Superscript> currents

Delayed rectifier K⁺ currents (I_K) play a critical role in determining cardiac action potential duration (APD). Modulation of I_K affects cardiac excitability critically. There are three components of cardiac delayed rectifier, and the slowly activating component (I_Ks) is influenced strongly by a variety of stimuli. Plasma levels of noradrenaline and endothelin are elevated in heart failure, and arrhythmias are promoted by such humoral abnormalities through modulation of ion channels. It has been reported that protein kinase A (PKA) and protein kinase C (PKC) modulate I_Ks from human minK in a complex manner. In the present study, we coexpressed human minK with the human ₁-adrenoceptor (h₁AR) and the endothelin receptor subtype A (hET_AR) in Xenopus oocytes and investigated the effects of receptor activation on the currents (I_Ks) flowing through the oocytes. ET-1 modulated I_Ks biphasically: a transient increase followed by a decrease. The PKC inhibitor chelerythrine completely inhibited the effects of ET-1. Intracellular EGTA abolished the transient increase by ET-1 and partially inhibited the subsequent decrease in the currents. When I_Ks was increased by 10^–6 M isoproterenol (ISO), ET-1 did not increase but rather decreased the current to an even greater extent than under control conditions. In addition, the effects of ISO on I_Ks were suppressed by ET_AR stimulation. These data indicate that I_Ks can be regulated by cross-talk between the ET_AR and ₁AR systems in addition to direct regulation by each receptor system.     

Discriminative stimulus effects of zolpidem in squirrel monkeys: role of GABA<Subscript>A</Subscript>/α<Subscript>1</Subscript> receptors

Abstract Rationale. The discriminative stimulus effects of zolpidem in squirrel monkeys trained at doses greater than or equal to 3.0 mg/kg differ from those of conventional benzodiazepines (BZs), but the extent to which these effects reflect the selectivity of zolpidem for GABA_A/α₁ receptors is not known. Objectives. The present study investigated the ability of GABA_A/α₁-preferring agonists to substitute for training doses of zolpidem greater than or equal to 3.0 mg/kg and the ability of GABA_A/α₁-preferring antagonists to block zolpidem's discriminative stimulus effects. Methods. Squirrel monkeys were trained to discriminate intravenous injections of zolpidem (3.0 or 5.6 mg/kg) from saline and tested with BZ agonists differing in selectivity and efficacy at GABA_A/α₁ receptors. Antagonism of the effects of zolpidem was studied using the GABA_A/α₁-preferring antagonists β-carboline-3-carboxylate-t-butyl ester (β-CCT) and 3-propyloxy-β-carboline (3-PBC). Results. Zolpidem and quazepam (GABA_A/α₁-preferring agonist) engendered full substitution for zolpidem, whereas CL 218,872 (GABA_A/α₁-preferring partial agonist) and the non-selective BZ agonists alprazolam and flunitrazepam engendered low and variable levels of zolpidem-lever responding (35–58%). Both β-CCT and 3-PBC antagonized the discriminative stimulus effects of zolpidem in a surmountable fashion. Conclusions. Our findings provide evidence for a key role of GABA_A/α₁ receptors in the discriminative stimulus effects of zolpidem at relatively high training doses, and suggest that selectivity and relatively high efficacy at GABA_A/α₁ receptors is required for BZ agonists to reproduce these discriminative stimulus effects. Electronic Publication     

Effect of the blockade of μ<Subscript>1</Subscript>-opioid and 5HT<Subscript>2A</Subscript>-serotonergic/α<Subscript>1</Subscript>-noradrenergic receptors on sweet-substance-induced analgesia

Rationale Sweet-substance-induced analgesia has been widely studied, and the investigation of the neurotransmitters involved in this antinociceptive process is an important way for understanding the involvement of the neural system controlling this kind of antinociception.Objective The aim of this study was to investigate the involvement of opioid and monoaminergic systems in sweet-substance-induced analgesia.Methods The present work was carried out in an animal model with the aim of investigating whether acute (24 h) or chronic (14 days) intake of a sweet substance, such as sucrose (250 g/l), is followed by antinociception. Tail withdrawal latencies in the tail-flick test were measured before and immediately after this treatment. Immediately after the recording of baseline values, independent groups of rats were submitted to sucrose or tap-water intake and, after chronic treatment, they were pretreated with intraperitoneal administration of (1) naltrexone at 0.5, 1, 2 or 3 mg/kg; (2) naloxonazine at 5, 10, 20 or 30 mg/kg; (3) methysergide at 0.5, 1, 2 or 3 mg/kg; (4) ketanserin at 0.5, 1, 2 or 3 mg/kg; or (5) physiological saline.Results Naltrexone and methysergide at two major doses decreased sweet-substance-induced analgesia after chronic intake of a sweet substance. These effects were corroborated by peripheral administration of naloxonazine and ketanserin.Conclusions These data give further evidence for: (a) the involvement of endogenous opioids and a ₁-opioid receptor in the sweet-substance-induced antinociception; (b) the involvement of monoamines and 5HT_2A serotonergic/₁-noradrenergic receptors in the central regulation of the sweet-substance-produced analgesia.     

Selective antagonism of the ataxic effects of zolpidem and triazolam by the GABA<Subscript>A</Subscript>/α<Subscript>1</Subscript>-preferring antagonist β-CCt in squirrel monkeys

Abstract Rationale. Delineation of the receptor mechanisms underlying the behavioral effects of benzodiazepines should allow for the development of drugs with improved clinical utility and reduced side effects. Objectives. The purpose of the present study was to investigate the role of GABA_A/α₁ receptors in the sedative and motor-impairing effects of benzodiazepines. Methods. Squirrel monkeys were tested with the GABA_A/α₁-preferring agonist zolpidem and the nonselective benzodiazepine agonist triazolam alone and in combination with the GABA_A/α₁-preferring antagonist β-CCt and the nonselective benzodiazepine antagonist flumazenil. During 30-min experimental sessions, all occurrences of normal behaviors like locomotion, environment- and self-directed behaviors, as well as side effects such as ataxia, rest and procumbent postures were scored. Results. Zolpidem and triazolam produced dose-dependent reductions in locomotion and environment-directed behavior and increased ataxia and procumbent posture. Triazolam, but not zolpidem, also engendered species-typical rest posture at some doses. Flumazenil antagonized all of the behavioral effects of zolpidem and triazolam, whereas β-CCt antagonized only zolpidem- and triazolam-induced ataxia. Conclusions. GABA_A/α₁ receptor mechanisms appear to play a key role in the ataxic effects of benzodiazepine agonists in squirrel monkeys, similar to recent results with transgenic mice. In contrast to the findings of these recent studies, GABA_A mechanisms other than or in addition to those mediated at the α₁ subunit may play a more important role in the sedative/hypnotic effects of benzodiazepines in squirrel monkeys. Electronic Publication     

Modulation of tnf-α-induced icam-1 expression,no and h<Subscript>2</Subscript>0<Subscript>2</Subscript> production by alginate,allicin and ascorbic acid in human endothelial cells

Plant nutrients are believed to provide protection against various diseases including inflammation. Since interactions of the cell adhesion molecules are known to play important roles in mediating inflammation, inhibiting adhesion protein upregulation is a possible therapeutic target. In this study, the interacellular adhesion molecule-1 (ICAM-1) was induced in human umbilical endothelial cells (HUVECs) after stimulation with TNF-alpha. In addition, alginate, ascorbic acid and allicin were demonstrated to inhibit the TNF-alpha induced expression of ICAM-1 on the HUVECs in a dose-dependent manner. These compounds also inhibited the production of NO and H2O2 induced by TNF-alpha, which suggests that the inhibition of ICAM-1 expression by the three compounds may be due to the modulated production of the reactive oxygen/nitrogen components. Overall, these results indicate that these dietary components have a therapeutic potential in the treatment of various inflammatory disorders associated with an increase in endothelial leukocyte adhesion molecules.     

Binding of (−)-[<Superscript>3</Superscript>H]-CGP12177 at two sites in recombinant human β<Subscript>1</Subscript>-adrenoceptors and interaction with β-blockers

To verify the hypothesis that the non-conventional partial agonist (–)-CGP12177 binds at two ₁-adrenoceptor sites, human ₁-adrenoceptors, expressed in CHO cells, were labelled with (–)-[³H]-CGP12177. We compared the binding affinity and antagonist potency of 12 clinically used -blockers against the cyclic AMP-enhancing effects of (–)-isoprenaline and (–)-CGP12177.(–)-[³H]-CGP12177 bound to a high affinity site (H; K_H=0.47 nM) and low affinity site (L); K_L=235 nM). (–)-[³H]-CGP12177 dissociated from the ₁-adrenoceptors with a fast component (k_off=0.45 min^–1), consistent with the L-site, and a slow component (k_off=0.017–0.033 min^–1), consistent with the H-site. (–)-Isoprenaline and (–)-CGP12177 caused 96-fold and 12-fold maximal increases in cyclic AMP levels with –logEC₅₀M of 8.2 and 7.6. (–)-CGP12177 antagonised the effects of (–)-isoprenaline with a pK_B of 9.9. The -blockers antagonised the effects of (–)-isoprenaline more than the effects of (–)-CGP12177 with potency ratios: (–)-atenolol 1,000, (±)-metropolol 676, (–)-pindolol 631, (–)-timolol 589, (±)-carvedilol 204, (±)-oxprenolol 138, (±)-sotalol 132, (–)-propranolol 120, (±)-bisoprolol 95, (±)-alprenolol 81, (±)-nadolol 68 and (–)-bupranolol 56. In intact cells the binding constants of -blockers, estimated from competition with 3–5 nM (–)-[³H]-CGP12177 (binding to the H-site), correlated with the corresponding affinities estimated from antagonism of the (–)-isoprenaline effects.We conclude that (–)-[³H]-CGP12177 binds at two sites in the recombinant ₁-adrenoceptor. (–)-CGP12177 is an antagonist of catecholamine effects through the H-site and a non-conventional partial agonist through the L-site. -blockers are more potent antagonists through the H-site than the L-site.     

GABA<Subscript>A</Subscript>/α<Subscript>1</Subscript> receptor agonists and antagonists: effects on species-typical and heightened aggressive behavior after alcohol self-administration in mice

Rationale The positive modulation of gamma-aminobutyric acid type-A (GABA_A) receptors is a putative mechanism via which alcohol escalates aggressive behavior. Broad-spectrum benzodiazepine antagonists block alcohol-heightened aggression in rats and monkeys. However, the degree to which GABA_A subunit composition plays a role in heightened aggressive behavior induced by self-administration of a moderate alcohol dose remains unresolved.Objective -Carboline-3-carboxylate-t-butyl ester (-CCt) and zolpidem act preferentially at GABA_A receptors containing the ₁ subunit as antagonist and agonist, respectively, and serve as useful tools to evaluate the role of GABA_A receptor subtypes in self-administered alcohol on aggression.Methods Male resident mice, housed in breeding pairs, were conditioned to nose-poke in a removable panel in their home cage, with each fifth poke being reinforced by the delivery of 0.05 ml of 6% ethanol (EtOH). After consuming EtOH, the resident mice were given the antagonists -CCt and flumazenil or agonists zolpidem and triazolam, and then confronted an intruder male in their home cage for a 5-min period.Results Following self-administration of EtOH (1.0 g/kg, 1.7 g/kg), 14 of 37 resident mice displayed unusually large increases in the frequency of attack bites and sideways threats. Flumazenil or -CCt decreased alcohol-heightened and non-heightened aggression in a dose-dependent manner. Administration of 3 mg/kg -CCt lowered the aggression-heightening effects of 1 g/kg and 1.7 g/kg EtOH, but did not antagonize the sedative effects of 3.0 g/kg EtOH. Triazolam and zolpidem decreased alcohol-heightened and non-heightened aggressive behavior, and these antiaggressive effects were accompanied by reduced motor activity, indicating sedation.Conclusions Benzodiazepine antagonists, particularly those acting preferentially at GABA_A/₁ subunit-containing receptors, decrease alcohol-heightened and species-typical aggressive behavior, but are ineffective in attenuating the sedative effects of alcohol.     

Molecular characterization of specific H<Subscript>1</Subscript>-receptor agonists histaprodifen and its N<Superscript>α</Superscript>-substituted analogues on bovine aortic H<Subscript>1</Subscript>-receptors

We determined the molecular properties of the selective and potent H(1)-receptor agonist histaprodifen and its N(alpha) substituted analogues: methyl-, dimethyl-, and imidazolylethyl-histaprodifen (suprahistaprodifen). All derivatives show high affinity for (3)H-mepyramine labeled bovine aortic H(1)-receptor binding sites with the following order of potency: suprahistaprodifen > dimethylhistaprodifen > methylhistaprodifen > histaprodifen > histamine. Suprahistaprodifen and dimethylhistaprodifen were the most potent displacers of (3)H-mepyramine binding (K(i)=4.3 and 4.9 nM, respectively). Histaprodifen, methylhistaprodifen and suprahistaprodifen binding was differentially influenced by GTP, whereas dimethylhistaprodifen was not affected. All drugs, except dimethylhistaprodifen, were activators of G-proteins. Their order of potency was suprahistaprodifen > histamine > histaprodifen > methylhistaprodifen. Their effect on G-protein activation was abolished by the addition of the H(1)-receptor antagonist triprolidine (10 microM), which given alone did not activate G-proteins. Our data suggest that histaprodifens are potent but heterogeneous H(1)-receptor ligands with diverse effects on the molecular level in our model system. While the histaprodifen, methylhistaprodifen and suprahistaprodifen data are in agreement with their agonistic nature, as shown in the functional studies performed on different species (rat and guinea pig H(1)-receptor), dimethylhistaprodifen behaved as an antagonist in our study.     

Cardiomyocyte Ca<Superscript>2+</Superscript> overload in atrial tachycardia: is blockade of L-type Ca<Superscript>2+</Superscript> channels a promising approach to prevent electrical remodeling and arrhythmogenesis?

Electrical remodeling paradigm has important implications for the understanding of atrial fibrillation (AF) and improvement of current treatment. Cardiomyocyte Ca²⁺ overload is generally accepted as the initiating signal for the tachycardia-induced changes in atrial electrical properties (electrical remodeling). The precise role of cardiomyocyte Ca²⁺ overload in AF-related ion channel alterations that contribute to AF maintenance is not fully understood. Clinically, patients with AF are often treated with Ca²⁺ channel blockers such as verapamil to control their ventricular rate and to improve the success rate of cardioversion procedures. However, verapamil may produce an increased L-type Ca²⁺ channel current (I_Ca,L) that may reinforce Ca²⁺ overload thereby promoting AF in the atrium. Ca²⁺ channel blockers which target T-type Ca²⁺ channels in addition to I_Ca,L (for instance, efonidipine) may be more efficient at preventing Ca²⁺ overload and arrhythmogenic electrical remodeling, but the potential benefits of these drugs have usually been tested in experimental models where drug administration preceded the initiation of electrical remodeling. Studies in animal models with established atrial tachycardia remodeling and in patients with AF are clearly warranted to prove the efficacy of Ca²⁺ channel blockers that additionally target T-type Ca²⁺ channels.     

Loss of serotonin 5-HT<Subscript>2A</Subscript> receptors in the postmortem temporal cortex correlates with rate of cognitive decline in Alzheimer’s disease

Rationale Previous studies have demonstrated reductions of serotonin 5-HT_2A receptors in the neocortex of Alzheimers disease (AD) patients. However, it is unclear whether such losses play a role in the cognitive decline of AD.Objectives To correlate neocortical 5-HT_2A receptor alterations with cognitive decline in AD.Methods Postmortem frontal and temporal cortical 5-HT_2A receptors were measured by [³H]ketanserin binding in aged controls as well as in a cohort of AD patients who had been longitudinally assessed for cognitive decline and behavioral symptoms.Results 5-HT_2A receptor densities in both regions were reduced in severely demented AD patients compared to age-matched controls. In the temporal cortex, this reduction also correlated with the rate of decline of Mini-Mental State Examination (MMSE) scores. The association between 5-HT_2A receptor loss and cognitive decline was independent of the effects of choline acetyltransferase (ChAT) activity and presence of behavioral symptoms.Conclusions Our data suggest that loss of neocortical 5-HT_2A receptors may predict for faster cognitive decline in AD, and point to serotomimetics as potentially useful adjuvants to cholinergic replacement therapies.     

Active cyamemazine metabolites in patients treated with cyamemazine (Tercian<Emphasis Type="Bold">®</Emphasis>): influence on cerebral dopamine D<Subscript>2</Subscript> and serotonin 5-HT<Subscript>2A</Subscript> receptor occupancy as measured by positron emission tomography (PET)

Rationale

Cyamemazine (Tercian®) is an antipsychotic agent blocking central dopamine D₂ receptors, which induces few extrapyramidal adverse effects, due to a potent antagonistic action at serotonin 5-HT_2A receptors. In vitro studies showed that the desmethyl metabolite of cyamemazine (N-desmethyl cyamemazine) has similar affinity for 5-HT_2A receptors as cyamemazine, whereas its D₂ receptor affinity is eight times lower (Benyamina et al. in Eur J Pharmacol 578(2–3):142–147, 2008). Moreover, cyamemazine sulfoxide showed modest affinity for 5-HT_2A receptors.

Objectives

The objective of this study is to measure steady-state plasma levels of N-desmethyl cyamemazine and cyamemazine sulfoxide in patients treated with clinically relevant doses of cyamemazine and correlate them with dopamine D₂ and serotonin 5-HT_2A receptor occupancies (RO) assessed by positron emission tomography (PET).

Methods

Eight patients received Tercian® 37.5, 75, 150, or 300 mg/day according to their symptoms. Dopamine D₂ and serotonin 5-HT_2A RO were assessed at steady-state cyamemazine plasma levels using [¹¹C]raclopride and [¹¹C]N-methyl-spiperone, respectively, for PET. Plasma levels of cyamemazine metabolites were determined using a validated high-performance liquid chromatography (PerkinElmer) associated with a mass spectrometry detection (API 365, PE SCIEX). The apparent equilibrium inhibition constant (K _i) was estimated by fitting RO with plasma levels of cyamemazine metabolites at the time of the PET scan.

Results

After 6 days of cyamemazine administration, plasma N-desmethyl cyamemazine reached steady-state levels at 2 to 12 times higher than those previously found for cyamemazine (Hode et al. in Psychopharmacology (Berl) 180:377–384, 2005). Plasma levels of N-desmethyl cyamemazine were closely related to striatal D₂ RO (r ²?=?0.942) and extrastriatal 5-HT_2A RO (r ²?=?0.901). The estimated K _i(app) value of N-desmethyl cyamemazine for striatal D₂ receptors was about fivefold higher than that for extrastriatal 5-HT_2A receptors (48.7 vs. 10.6 nM). Striatal D₂ RO increased with the plasma levels of N-desmethyl cyamemazine but remained below 75% even at its highest levels. At steady state, plasma cyamemazine sulfoxide levels were about double those of N-desmethyl cyamemazine. However, these cyamemazine sulfoxide levels should not contribute significantly to in vivo 5-HT_2A and D₂ receptor occupancy.

Conclusions

In patients orally given cyamemazine, N-desmethyl cyamemazine, but not cyamemazine sulfoxide, should significantly contribute to in vivo frontal 5-HT_2A and striatal D₂ receptor occupancy. The higher in vivo affinity of cyamemazine and its desmethyl metabolite for serotonin 5-HT_2A receptors compared with dopamine D₂ receptors should explain the low incidence of extrapyramidal adverse effects.

     

5-HT<Subscript>1B</Subscript> receptors, α<Subscript>2A/2C</Subscript>- and,to a lesser extent, α<Subscript>1</Subscript>-adrenoceptors mediate the external carotid vasoconstriction to ergotamine in vagosympathectomised dogs

It has previously been suggested that ergotamine produces external carotid vasoconstriction in vagosympathectomised dogs via 5-HT_1B/1D receptors and ₂-adrenoceptors. The present study has reanalysed this suggestion by using more selective antagonists alone and in combination. Fifty-two anaesthetised dogs were prepared for ultrasonic measurements of external carotid blood flow. The animals were divided into thirteen groups (n=4 each) receiving an i.v. bolus injection of, either physiological saline (0.3 ml/kg; control), or the antagonists SB224289 (300 g/kg; 5-HT_1B), BRL15572 (300 µg/kg; 5-HT_1D), rauwolscine (300 µg/kg; ₂), SB224289 + BRL15572 (300 µg/kg each), SB224289 + rauwolscine (300 µg/kg each), BRL15572 + rauwolscine (300 µg/kg each), rauwolscine (300 µg/kg) + prazosin (100 µg/kg; ₁), SB224289 (300 µg/kg) + prazosin (100 µg/kg), SB224289 (300 µg/kg) + rauwolscine (300 µg/kg) + prazosin (100 µg/kg), SB224289 (300 µg/kg) + prazosin (100 µg/kg) + BRL44408 (1,000 µg/kg; _2A), SB224289 (300 µg/kg) + prazosin (100 µg/kg)+ imiloxan (1,000 µg/kg; _2B), or SB224289 (300 µg/kg) + prazosin (100 µg/kg) + MK912 (300 µg/kg; _2C). Each group received consecutive 1-min intracarotid infusions of ergotamine (0.56, 1, 1.8, 3.1, 5.6, 10 and 18 µg/min), following a cumulative schedule. In saline-pretreated animals, ergotamine induced dose-dependent decreases in external carotid blood flow without affecting arterial blood pressure or heart rate. These control responses were: unaffected by SB224289, BRL15572, rauwolscine or the combinations of SB224289 + BRL15572, BRL15572 + rauwolscine, rauwolscine + prazosin, SB224289 + prazosin, or SB224289 + prazosin + imiloxan; slightly blocked by SB224289 + rauwolscine; and markedly blocked by SB224289 + rauwolscine + prazosin, SB224289 + prazosin + BRL44408 or SB224289 + prazosin + MK912. Thus, the cranio-selective vasoconstriction elicited by ergotamine in dogs is predominantly mediated by 5-HT_1B receptors as well as _2A/2C-adrenoceptor subtypes and, to a lesser extent, by ₁-adrenoceptors.In memoriam: Luis F. Valdivia died on 26 May 2004     

Postsynaptic 5-HT<Subscript>1A</Subscript> receptor stimulation increases motor activity in the 6-hydroxydopamine-lesioned rat: implications for treating Parkinson’s disease

RATIONALE: We have shown that the 5-HT1A agonist R-(+)-hydroxy-2-(di-n-propylamino)tetralin [R-(+)-8-OHDPAT] enhances motor activity in the monoamine-depleted rat, an acute model of Parkinson's disease. The present work extends these findings by investigating motor effects of R-(+)-8-OHDPAT in the unilateral 6-hydroxydopamine-lesioned rat, a chronic model of Parkinson's disease. OBJECTIVE: The objectives of the present study were to assess the motor response to R-(+)-8-OHDPAT in rats with unilateral destruction of the nigrostriatal dopamine system and to determine the involvement of postsynaptic 5-HT1A receptors in this response. MATERIALS AND METHODS: Rotational behavior after R-(+)-8-OHDPAT was investigated in rats that received 6-hydroxydopamine unilaterally into the median forebrain bundle 2 weeks before testing. RESULTS: A dose of 0.3 mg/kg subcutaneously (s.c.) R-(+)-8-OHDPAT induced significant ipsilateral turning in unilateral 6-OHDA-lesioned rats. Pretreatment with the selective 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxiamide maleate (WAY-100635, 0.1 mg/kg, s.c.) blocked turning. Blockade of 5-HT synthesis and 5-HT depletion caused by the tryptophan hydroxylase inhibitor DL: -p-chlorophenylalanine did not decrease R-(+)-8-OHDPAT-induced turning. Finally, a subset of animals were tested for their turning response to the dopamine agonist apomorphine after tests with R-(+)-8-OHDPAT had been completed. Correlation analysis indicated no relationship existed between the turning response to apomorphine and the turning response to R-(+)-8-OHDPAT. CONCLUSION: R-(+)-8-OHDPAT induces ipsilateral turning in unilateral 6-OHDA lesioned rats by stimulating postsynaptic 5-HT1A receptors, not by altering 5-HT synthesis and release. The mechanism underlying the motor effects of R-(+)-8-OHDPAT appears to differ from classic dopaminergic anti-parkinsonian agents suggesting that 5-HT1A agonists might prove useful adjunctive therapy in the treatment of Parkinson's disease.     

α<Subscript>1</Subscript>-Adrenergic and α<Subscript>2</Subscript>-adrenergic balance in the dorsal pons and gross behavioral activity of mice in a novel environment

Rationale Central α₁- and α₂-adrenoceptors in a number of different brain regions are known to have opposing actions on gross behavioral activity, with the former stimulating and the latter inhibiting activity. Therefore, blockade of α₁-receptors may induce inactivity by leading to unopposed α₂ activity.Objective The aim of this study was to test if central blockade of α₂-receptor function restores behavioral activity in α₁-receptor-blocked mice.Methods Dose-response studies were undertaken on the effects of α₁- and α₂-agonists and antagonists microinjected into the dorsal pons on gross behavioral activity in a novel cage test.Results The behavioral inactivity resulting from blockade of α₁-receptors in the pons with the antagonist, terazosin, was reversed by either a low dose of an α₂-antagonist, atipamezole, or a low dose of an α₂-agonist, dexmedetomidine, but was exacerbated by a high dose of the α₂-agonist.Conclusion The results support the hypothesis that blockade of α₁-receptors in the dorsal pons of mice produces inactivity by causing unopposed activity of α₂-receptors. This condition may be relevant to inactive states seen after stress or during depressive illness.     

Unique excitation–contraction characteristics of mouse myocardium as revealed by SEA0400, a specific inhibitor of Na<Superscript>+</Superscript>–Ca<Superscript>2+</Superscript> exchanger

The functional role of the sodium–calcium exchanger in mouse ventricular myocardium was evaluated with a newly developed specific inhibitor, SEA0400. Contractile force and action potential configuration were measured in isolated ventricular tissue preparations, and cell shortening and Ca²⁺ transients were measured in indo-1-loaded isolated ventricular cardiomyocytes. SEA0400 increased the contractile force, cell shortening and Ca²⁺ transient amplitude, and shortened the late plateau phase of the action potential. -adrenergic stimulation by phenylephrine produced a sustained decrease in contractile force, cell shortening and Ca²⁺ transient amplitude, which were all inhibited by SEA0400. Increasing the contraction frequency resulted in a decrease in contractile force in the absence of drugs (negative staircase phenomenon). This frequency-dependent decrease was attenuated by SEA0400 and enhanced by phenylephrine. Phenylephrine increased the Ca²⁺ sensitivity of contractile proteins in isolated ventricular cardiomyocytes, while SEA0400 had no effect. These results provide the first pharmacological evidence in the mouse ventricular myocardium that inward current generated by Ca²⁺ extrusion through the sodium–calcium exchanger during the Ca²⁺ transient contributes to the action potential late plateau, that -adrenoceptor-mediated negative inotropy is produced by enhanced Ca²⁺ extrusion through the sodium–calcium exchanger, and that the negative staircase phenomenon can be explained by increased Ca²⁺ extrusion through the sodium–calcium exchanger at higher contraction frequencies.