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.     

The effects of hydrocortisone on rat heart muscarinic and adrenergic α<Subscript>1</Subscript>, β<Subscript>1</Subscript> and β<Subscript>2</Subscript> receptors,propranolol-resistant binding sites and on some subsequent steps in intracellular signalling

Glucocorticoids affect the expression and density of neurotransmitter receptors in many tissues but data concerning the heart are contradictory and incomplete. We injected rats with hydrocortisone for 1–12 days and measured the densities of cardiac muscarinic receptors, ₁-, ₁- and ₂-adrenoceptors and propranolol-resistant binding sites (formerly assumed to be the putative ₄-adrenoceptor). Some aspects of intracellular signalling were also evaluated: we measured adenylyl cyclase activity (basal, isoprenaline- and forskolin-stimulated and carbachol-inhibited), the coupling between muscarinic receptors and G proteins and basal and isoprenaline-stimulated heart rate. The density of cardiac muscarinic receptors increased (in both the atria and the ventricles). The density of ₁-adrenoceptors increased in the atria and was little changed in the ventricles. The density of ₂-adrenoceptors increased in both the atria and the ventricles. The number of ₁-adrenoceptors decreased initially, followed by a transient increase in the atria and did not change in the ventricles. The density of propranolol-resistant binding sites first increased and then diminished in the atria and did not change in the ventricles. Although there were noticeable changes in receptor densities, the stimulatory and inhibitory effects on adenylyl cyclase, basal and isoprenaline-stimulated heart rate and the coupling between muscarinic receptors and G proteins were not significantly altered. This may indicate that changes in receptor densities might be one of the mechanisms maintaining stable functional output. Deceased     

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     

Cardiostimulant and cardiodepressant effects through overexpressed human β<Subscript>2</Subscript>-adrenoceptors in murine heart: regional differences and functional role of β<Subscript>1</Subscript>-adrenoceptors

(-)-Isoprenaline enhances cardiac contractility through beta-adrenoceptors. However, in cardiac tissue from transgenic mice with a 200-400-fold cardiac overexpression of the human beta(2)-adrenoceptor (TG4) we observed a pronounced cardiodepression at high (-)-isoprenaline concentrations. Here, we investigated the functional role of the coexisting beta(1)-, beta(2)-, and beta(3)-adrenoceptor subtypes in several regions of the TG4 heart, and in particular their contribution to the negative inotropic effect. In paced TG4 left atria, (-)-isoprenaline produced bell-shaped concentration-effect curves increasing (-logEC(50)M=9.0) and decreasing (-logIC(50)M=6.4) contractile force. These effects were unaffected by the beta(1)-selective CGP 20712A (300 nM). The beta(2)-selective inverse agonist ICI 118,551 (30-1,000 nM) antagonised in surmountable manner both the positive and negative inotropic effects of (-)-isoprenaline with similar concentration-dependence, consistent with an exclusive mediation through beta(2)-adrenoceptors. The beta(3)-adrenoceptor-selective agonist BRL37344 (1 nM-10 microM) failed to produce significant inotropic effects in TG4 left atria. Subsequently, we measured left atrial action potentials accompanying the inotropic changes induced by (-)-isoprenaline. Action potentials tended to have shorter duration in left atria from TG4 mice than from non-transgenic littermate mice. However, (-)-isoprenaline prolonged the duration of 30% repolarisation in atria from non-transgenic littermate but not from TG4 mice, while 90% repolarisation was abbreviated in both groups of atria. Negative inotropic effects of (-)-isoprenaline were also observed in right ventricular preparations. Pertussis toxin-treatment of the mice abolished the negative inotropic effects in left atria and reduced cardiodepression in right ventricle, indicating an involvement of beta(2)-adrenoceptor coupling to PTX-sensitive G-proteins. In additional experiments, designed to study the native murine beta(1)-adrenoceptor function, we used the physiological beta(1)-adrenoceptor agonist (-)-noradrenaline. In the presence of 600 nM ICI 118,551 we failed to find a functional role of the beta(1)-adrenoceptors in left atria, and detected only a marginal contribution to the positive chronotropic effect in right atria. We also investigated the effects of the non-conventional partial agonist (-)-CGP 12177 (0.2 nM-6 microM), which in wild-type mice causes tachycardia through beta(1)-adrenoceptors. In TG4 right atria, however, (-)-CGP 12177-evoked tachycardia was resistant to blockade by CGP 20712A but antagonised by ICI 118,551, consistent with mediation through human beta(2)-adrenoceptors.The results from TG4 mice suggest that the positive and negative inotropic effects of (-)-isoprenaline are mediated through human overexpressed beta(2)-adrenoceptors coupled to G(s) protein and G(i) protein, respectively. The (-)-isoprenaline-evoked shortening of the atrial action potential combined with reduced responses of L-type Ca(2+) current may contribute to the negative inotropic effects. The function of murine cardiac beta(1)-adrenoceptors is suppressed by overexpressed human beta(2)-adrenoceptors.     

Calcium channel function and regulation in β<Subscript>1</Subscript>- and β<Subscript>2</Subscript>-adrenoceptor transgenic mice

Cardiac effects of catecholamines on the L-type calcium channel depend on -adrenoceptor subtype (₁- vs. ₂-adrenoceptor). Chronic overexpression of these receptors leads to hypertrophy and early death at moderate (₁) or excessive (₂) levels of overexpression respectively. In order to examine the role of L-type calcium channels in altered cardiomyocyte calcium homeostasis found with ₁-adrenoceptor overexpression, and to understand the quantitative differences between -adrenoceptor subtypes regarding calcium channel regulation, we examined single channels in myocytes obtained from ₁- and ₂-adrenoceptor transgenic mice. The effects of the agonist isoproterenol were investigated and compared with acute receptor stimulation in the respective non-transgenic littermates.Channels from ₁-adrenoceptor transgenic mice have normal baseline activity, and channel number is not reduced. This contrasts to previous findings with ₂-adrenoceptor transgenic mice, where channel activity is depressed. Isoproterenol is unable to stimulate channel activity in both transgenic models.In conclusion, the L-type calcium channel is not likely to be involved in alterations of calcium handling of ₁-adrenoceptor transgenic myocytes. Furthermore, chronic ₁-adrenoceptor overexpression does not depress channel activity, giving another example of the difference between ₁- and ₂-adrenoceptor signal transduction.K.F. and T.K. equally contributed to this work     

Positive allosteric modulatory effects of ajulemic acid at strychnine-sensitive glycine α<Subscript>1</Subscript>- and α<Subscript>1</Subscript>β-receptors

The synthetic cannabinoid ajulemic acid (CT-3) is a potent cannabinoid receptor agonist which was found to reduce pain scores in neuropathic pain patients in the absence of cannabis-like psychotropic adverse effects. The reduced psychotropic activity of ajulemic acid has been attributed to a greater contribution of peripheral CB receptors to its mechanism of action as well as to non-CB receptor mechanisms. Loss of inhibitory synaptic transmission within the dorsal horn of the spinal cord plays a key role in the development of chronic pain following inflammation or nerve injury. Inhibitory postsynaptic transmission in the adult spinal cord involves mainly glycine. As we hypothesised that additional non-CB receptor mechanisms of ajulemic acid might contribute to its effect in neuropathic pain, we investigated the interaction of ajulemic acid with strychnine-sensitive α₁- and α₁β-glycine receptors by using the whole-cell patch clamp technique. Ajulemic acid showed a positive allosteric modulating effect in a concentration range which can be considered close to clinically relevant concentrations (EC₅₀ values: α₁ = 9.7 ± 2.6 μM and α₁β = 12.4 ± 3.4 μM). Direct activation of glycine receptors was observed at higher concentrations above 100 μM (EC₅₀ values: α₁ = 140.9 ± 21.5 μM and α₁β = 154.3 ± 32.1 μM). These in vitro results demonstrate that ajulemic acid modulates strychnine-sensitive glycine receptors in clinically relevant concentrations.     

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.     

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     

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.     

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.     

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.     

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.     

The α<Subscript>2</Subscript>-adrenoceptor antagonist atipamezole potentiates anti-Parkinsonian effects and can reduce the adverse cardiovascular effects of dopaminergic drugs in rats

The present experiments investigated the effects of the specific ₂-adrenoceptor antagonist atipamezole, alone and in combination with a dopamine agonist, on motor function in rats with a unilateral 6-hydroxydopamine lesion of the nigro-striatal pathway and on exploratory behaviour and cardiovascular function in rats equipped with telemetry transmitters. Dexmedetomidine, an ₂-adrenoceptor agonist and the ₂-adrenoceptor antagonists idazoxan and yohimbine were used as reference compounds. In the unilaterally lesioned animals, direct dopamine agonists, such as apomorphine, induce contralateral turning behaviour. Indirect agonists, such as amphetamine, induce ipsilateral circling in the animals. Atipamezole (0.3 mg/kg s.c) potentiated and dexmedetomidine (10 µg/kg s.c.) decreased contralateral circling evoked by apomorphine (50 µg/kg s.c.) and by l-3,4-dihydroxyphenylalanine (L-DOPA, 5 mg/kg i.p.). Atipamezole also prolonged the duration of action of L-DOPA. Atipamezole dose-dependently induced ipsilateral turning behaviour and potentiated turning induced by amphetamine (1 mg/kg i.p.). The ₁-adrenoceptor antagonist prazosin (0.1 mg/kg i.p.) partially antagonised the effect of amphetamine and had a strong inhibitory effect on the atipamezole-induced potentiation of the amphetamine response. Prazosin did not have any major effect on either the apomorphine response itself or on the potentiation of the apomorphine response by atipamezole. This suggests that atipamezole can modulate motor function both indirectly, by stimulating the release of noradrenaline and directly, by blocking postsynaptic ₂-adrenoceptors in neurones other than noradrenergic nerves. The ₂-adrenoceptor antagonists, when tested at comparably effective central ₂-adrenoceptor antagonising doses in a rat mydriasis model: atipamezole 0.3 mg/kg s.c., idazoxan 1 mg/kg s.c. and yohimbine 3 mg/kg s.c., all induced ipsilateral turning behaviour and potentiated apomorphine-induced contralateral circling. The effects of the ₂-adrenoceptor antagonists were in general similar in these experiments. In habituated non-lesioned rats equipped with telemetry transmitters, apomorphine (50 µg/kg s.c.) decreased blood pressure in the home cage and in an open-field test. It also decreased spontaneous motor activity in the open field. Neither atipamezole (0.3 mg/kg s.c.) nor idazoxan (1 mg/kg s.c.) had any effect on blood pressure when given alone, but reversed the apomorphine-induced decrease in blood pressure. Atipamezole also diminished apomorphine-induced sedation in the open-field test. In conclusion, atipamezole improved the efficacy of L-DOPA and apomorphine in an animal model of Parkinsons disease and also reduced adverse dopaminergic effects on vigilance and on cardiovascular function. These results suggest that an investigation of the effects of specific ₂-adrenoceptor antagonists in Parkinsons disease patients is warranted.     

A new method for evaluating σ<Subscript>2</Subscript> ligand activity in the isolated guinea-pig bladder

We demonstrated the presence of ₂ receptors in the guinea-pig ileum by saturation analysis and extended our investigation to guinea-pig bladder and rat bladder. In functional assays of the isolated tissues in organ baths, ₂ receptor ligands inhibited electrically evoked contractions in both guinea-pig bladder and ileum and a linear correlation was found between ₂ receptor affinity and ₂ receptor activity values of selected compounds. The ₂ activity of these compounds in the presence of desensitised ₁ receptors both in bladder and ileum was also tested. On the basis of our results, we propose the electrically stimulated guinea-pig bladder as a new method for evaluating ₂ activity.     

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.     

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.     

Comparison of the α-adrenoceptor-mediated effects of β<Subscript>3</Subscript>-adrenoceptor ligands in rat pulmonary artery

We have recently shown that the -adrenoceptor ligands CGP 12177, bupranolol, and SR 59230A (aryloxypropanolamines), but not BRL 37344 and CL 316243 (phenylethanolamines), exhibit significant affinity for ₁-adrenoceptors and that CGP 12177 displays partial agonist properties at -adrenoceptors in rat pulmonary artery. In this study, bupranolol and SR 59230A were further evaluated for their potential -adrenoceptor mediated effects (i.e., agonist and/or antagonist properties) in rat intralobar pulmonary artery and compared with BRL 37344 and CL 316243. Bupranolol induced a relaxation in phenylephrine-precontracted arteries, but had no effect in prostaglandin -precontracted ones. SR 59230A also elicited a relaxation in phenylephrine-precontracted arteries. In -precontracted arteries, SR 59230A induced a contractile response that was insensitive to the irreversible -adrenoceptor antagonist phenoxybenzamine. BRL 37344 at high concentrations, but not CL 316243, produced slight relaxation in both phenylephrine- and -precontracted arteries. The contractile response to phenylephrine was antagonized by bupranolol and SR 59230A in a competitive manner (pA₂: 6.38 and 7.08 respectively). The concentration–response curve to phenylephrine was also shifted to the right by BRL 37344 (mean pK_b: 4.45), but not by CL 316243 (100 M). This study indicates that the aryloxypropanolamine derivatives bupranolol and SR 59230A exhibit competitive antagonist, but no agonist properties on ₁-adrenoceptors, SR 59230A also inducing -adrenoceptor-independent contraction. Among the phenylethanolamines, BRL 37344 but not CL 316243, also exerts an antagonist effect on ₁-adrenoceptors, with a much lower potency than the aryloxypropanolamines studied.     

Indirect role of α<Subscript>2</Subscript>-adrenoreceptors in anti-ulcer effect mechanism of nimesulide in rats

Nimesulide, a non-steroidal, anti-inflammatory drug, produces ulcerogenic effects in adrenalectomized rats but is gastro-protective in intact rats. The objective of this study was to determine whether adrenal gland hormones are involved in the anti-ulcer effects of nimesulide. The results revealed that 100 mg/kg nimesulide produces gastric ulceration in adrenalectomized rats, which is prevented by prednisolone and adrenaline. The anti-ulcer effects of adrenaline and prednisolone in adrenalectomized rats were in turn antagonized by yohimbine, a selective α₂-receptor blocker, but not by doxazosine (α₁-receptor blocker) or propranolol (β-blocker). Adrenaline prevented the formation of indomethacin-induced ulcers in both adrenalectomized and intact rats, but prednisolone increased the indomethacin-induced ulcerous area in intact rats, whereas it decreased the size of the ulcers in adrenalectomized rats. In addition, prednisolone prevented ulcer formation in intact rats in which the adrenaline concentration had been decreased by metyrosine. These results suggest that glucocorticoids are anti-ulcerogenic in not only adrenalectomized rats but also in intact rats with diminished circulating levels of adrenaline. In the light of these data, the effect of nimesulide on plasma adrenaline concentrations was studied. In comparison to the adrenaline levels found in intact control rats, the administration of nimesulide at doses of 10, 20, 40 and 100 mg/kg decreased adrenaline concentrations by 12.8, 22.6, 30.4, and 58.2%, respectively, without affecting blood corticosterone concentrations. The anti-ulcer effect of nimesulide was observed to be dose-dependent, and the strength of this effect was directly correlated the decreasing concentration of adrenaline. The concentration of adrenaline was decreased by 60.9% in rats treated with 300 mg/kg metyrosine in which prednisolone produced anti-ulcer effects. In summary, we have shown that nimesulide produces its anti-ulcer effect by decreasing endogenous adrenaline concentrations and that glucocorticoids may induce anti-ulcer effects via α₂-adrenoreceptors, but not via their own receptors. This research was conducted in the Laboratory of Pharmacology at Ataturk University, Faculty of Medicine, Department of Pharmacology, 25240 Erzurum/Turkey.