(1/1182) Extra-vesicular binding of noradrenaline and guanethidine in the adrenergic neurones of the rat heart: a proposed site of action of adrenergic neurone blocking agents.
1 The binding and efflux characteristics of [14C]-guanethidine and [3H]-noradrenaline were studied in heart slices from rats which were pretreated with reserpine and nialamide. 2 Binding of both compounds occurred at extra-vesicular sites within the adrenergic neurone. After a brief period of rapid washout, the efflux of [14C]-guanethidine and [3H]-noradrenaline proceeded at a steady rate. The efflux of both compounds appeared to occur from a single intraneuronal compartment. 3 (+)-Amphetamine accelerated the efflux of [14C]-noradrenaline; this effect was inhibited by desipramine. 4 Unlabelled guanethidine and amantadine also increased the efflux of labelled compounds. Cocaine in high concentrations increased slightly the efflux of [14C]-guanethidine but not that of [3H]-noradrenaline. 5 Heart slices labelled with [3H]-noradrenaline became refractory to successive exposures to releasing agents although an appreciable amount of labelled compound was still present in in these slices. 6 It is suggested that [14C]-guanethidine and [3H]-noradrenaline are bound at a common extravesicular site within the adrenergic neurone. Binding of guanethidine to the extra-vesicular site may be relevant to its pharmacological action, i.e., the blockade of adrenergic transmission. (+info)
(2/1182) Acquisition of nicotine discrimination and discriminative stimulus effects of nicotine in rats chronically exposed to caffeine.
Caffeine and nicotine are the main psychoactive ingredients of coffee and tobacco, with a high frequency of concurrent use in humans. This study examined the effects of chronic caffeine exposure on 1) rates of acquisition of a nicotine discrimination (0.1 or 0.4 mg/kg, s.c., training doses) and 2) the pharmacological characteristics of the established nicotine discrimination in male Sprague-Dawley rats. Once rats learned to lever-press reliably under a fixed ratio of 10 schedule for food pellets, they were randomly divided into two groups; 12 animals were maintained continuously on caffeine added to the drinking water (3 mg/ml) and another 12 control rats continued to drink tap water. In each group of water- and caffeine-drinking rats, there were six rats trained to discriminate 0.1 mg/kg of nicotine from saline and six rats trained to discriminate 0.4 mg/kg of nicotine from saline. Regardless of the training dose of nicotine, both water- and caffeine-drinking groups required a comparable number of training sessions to attain reliable stimulus control, although there was a trend for a slower acquisition in the caffeine-drinking group trained with 0.1 mg/kg of nicotine. Tests for generalization to different doses of nicotine revealed no significant differences in potency of nicotine between water- and caffeine-drinking groups. The nicotinic-receptor antagonist mecamylamine blocked the discriminative effects of 0.1 and 0.4 mg/kg nicotine with comparable potency and efficacy in water- and caffeine-drinking groups. There was a dose-related generalization to both the 0.1 and 0.4 mg/kg nicotine cue (maximum average of 51-83%) in water-drinking rats after i.p. treatment with d-amphetamine, cocaine, the selective dopamine uptake inhibitor GBR-12909, apomorphine, and the selective dopamine D1 receptor agonist SKF-82958, but not in caffeine-drinking rats (0-22%). There was no generalization to the nicotine cues after i.p. treatment with caffeine or the selective D2 (NPA) and D3 (PD 128,907) dopamine-receptor agonists in water- and caffeine-drinking rats. The dopamine-release inhibitor CGS 10746B reduced the discriminative effects of 0.4 mg/kg nicotine in water-drinking rats, but not in caffeine-drinking rats. There was no evidence of development of tolerance or sensitization to nicotine's effects throughout the study. In conclusion, chronic caffeine exposure (average, 135 mg/kg/day) did not affect the rate of acquisition of the nicotine discrimination, but it did reduce the dopaminergic component of the nicotine-discriminative cue. The reduction of the dopaminergic component of the nicotine cue was permanent, as this effect was still evident after the caffeine solution was replaced with water in caffeine-drinking rats. That nicotine could reliably serve as a discriminative stimulus in the absence of the dopaminergic component of its discriminative cue may differentiate nicotine from "classical dopaminergic" drugs of abuse such as cocaine and amphetamine. (+info)
(3/1182) N-oxygenation of amphetamine and methamphetamine by the human flavin-containing monooxygenase (form 3): role in bioactivation and detoxication.
(+)- And (-)-amphetamine and methamphetamine were N-oxygenated by the cDNA expressed adult human flavin-containing monooxygenase form 3 (FMO3), their corresponding hydroxylamines. Two major polymorphic forms of human FMO3 were studied, and the results suggested preferential N-oxygenation by only one of the two enzymes. Chemically synthesized (+/-)-amphetamine hydroxylamine was also a substrate for the human FMO3 and it was converted to phenylpropanone oxime with a stereoselectivity ratio of trans/cis of 5:1. Human FMO3 also N-oxygenated methamphetamine to produce methamphetamine hydroxylamine. Methamphetamine hydroxylamine was also N-oxygenated by human FMO3, and the ultimate product observed was phenylpropanone. For amphetamine hydroxylamine, studies of the biochemical mechanism of product formation were consistent with the production of an N, N-dioxygenated intermediate that lead to phenylpropanone oxime. This was supported by the observation that alpha-deutero (+/-)-amphetamine hydroxylamine gave an inverse kinetic isotope effect on product formation in the presence of human FMO3. For methamphetamine, the data were consistent with a mechanism of human FMO3-mediated N,N-dioxygenation but the immediate product, a nitrone, rapidly hydrolyzed to phenylpropanone. The pharmacological activity of amphetamine hydroxylamine, phenylpropanone oxime, and methamphetamine hydroxylamine were examined for effects at the human dopamine, serotonin, and norepinephrine transporters. Amphetamine hydroxylamine and methamphetamine hydroxylamine were apparent substrates for the human biogenic amine transporters but phenylpropanone oxime was not. Presumably, phenylpropanone oxime or nitrone formation from amphetamine and methamphetamine, respectively, represents a detoxication process. Because of the potential toxic nature of amphetamine hydroxylamine and methamphetamine hydroxylamine metabolites and the polymorphic nature of N-oxygenation, human FMO3-mediated metabolism of amphetamine or methamphetamine may have clinical consequences. (+info)
(4/1182) The Ca2+ channel blockade changes the behavioral and biochemical effects of immobilization stress.
We investigated how the effects of chronic immobilization stress in rats are modified by Ca2+ channel blockade preceding restraint sessions. The application of nifedipine (5 mg/kg) shortly before each of seven daily 2 h restraint sessions prevented the development of sensitized response to amphetamine as well as the stress-induced elevation of the densities of L-type Ca2+ channel in the hippocampus and significantly reduced the elevation of the densities of [3H]nitrendipine binding sites in the cortex and D1 dopamine receptors in the limbic forebrain. Neither stress, nor nifedipine affected the density of alpha 1-adrenoceptors and D1 receptors in the cerebral cortex nor D2 dopamine receptors in the striatum. A single restraint session caused an elevation of blood corticosterone level that remained unaffected by nifedipine pretreatment, but the reduction of this response during the eighth session was significantly less expressed in nifedipine-treated rats. We conclude that L-type calcium channel blockade prevents development of several stress-induced adaptive responses. (+info)
(5/1182) Necessary role for ventral tegmental area adenylate cyclase and protein kinase A in induction of behavioral sensitization to intraventral tegmental area amphetamine.
In the present study, we investigated the effects of selective activation or inhibition of ventral tegmental area (VTA) adenylate cyclase (AC) and protein kinase A (PKA) on long-term sensitization induced by repeated intra-VTA or peripheral amphetamine (AMPH). Selective inhibition of AC by SQ 22,536 (9-(tetrahydro-2-furanyl)-9H-purin-6-amine; 100 nmol/side bilateral into VTA) had no effect on acute basal locomotion but attenuated the locomotor stimulation induced by acute i.p. AMPH (1.5 mg/kg). Coinjection of SQ 22,536 (100 nmol/side) fully blocked the sensitization induced by repeated intra-VTA AMPH (15 nmol/side) but had no detectable effect on the sensitization induced by repeated i. p. AMPH. Persistent activation of AC by intra-VTA cholera toxin (500 ng/side) modestly increased acute locomotion and induced a robust sensitization to i.p. AMPH challenge 10 days after the last of three repeated VTA microinjections. Selective inhibition of PKA by Rp-adenosine-3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPS; 25 nmol/side) had no effect on acute basal or AMPH-stimulated locomotion. Coinjection of Rp-cAMPS (25 nmol/side) fully blocked the sensitization induced by repeated intra-VTA AMPH but had no effect on sensitization induced by repeated i.p. AMPH. Intra-VTA microinjection of the selective PKA activator Sp-adenosine-3',5'-cyclic monophosphothioate triethylamine (Sp-cAMPS; 25-100 nmol/side) dose-dependently stimulated acute locomotion and exerted synergistic effects on locomotor activity when coinfused into the VTA with AMPH but had no detectable effect on acute i.p. AMPH-induced locomotion. Repeated intra-VTA Sp-cAMPS did not induce sensitization to AMPH challenge but potentiated the sensitization induced by repeated i.p. AMPH. These results suggest that VTA cAMP signal transduction is necessary for the induction of persistent sensitization to intra-VTA amphetamine and that peripheral and intra-VTA AMPH may not induce behavioral sensitization by identical mechanisms. (+info)
(6/1182) Modulation of amphetamine-stimulated [3H]dopamine release from rat pheochromocytoma (PC12) cells by sigma type 2 receptors.
An important regulatory mechanism of synaptic dopamine (DA) levels is activation of the dopamine transporter (DAT), which is a target for many drugs of abuse, including amphetamine (AMPH). sigma receptors are located in dopaminergic brain areas critical to reinforcement. We found previously that agonists at sigma2 receptors enhanced the AMPH-stimulated release of [3H]DA from slices of rat caudate-putamen. In the present study, we modeled this response in undifferentiated pheochromocytoma-12 (PC12) cells, which contain both the DAT and sigma2 receptors but not neural networks that can complicate investigation of individual neuronal mechanisms. We found that enhancement of AMPH-stimulated [3H]DA release by the sigma agonist (+)-pentazocine was blocked by sigma2 receptor antagonists. Additionally, the reduction in the effect of (+)-pentazocine by the inclusion of ethylene glycol bis(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid led us to hypothesize that sigma2 receptor activation initiated a Ca2+-dependent process that resulted in enhancing the outward flow of DA via the DAT. The source of Ca2+ required for the enhancement of reverse transport did not appear to be via N- or L-type voltage-dependent Ca2+ channels, because it was not affected by nitrendipine or omega-conotoxin. However, two inhibitors of Ca2+/calmodulin-dependent protein kinase II blocked enhancement in AMPH-stimulated release by (+)-pentazocine. Our findings suggest that sigma2 receptors are coupled to the DAT via a Ca2+/calmodulin-dependent protein kinase II transduction system in PC12 cells, and that sigma2 receptor antagonists might be useful in the treatment of drug abuse by blocking elevation of DA levels via reversal of the DAT. (+info)
(7/1182) Effects of atypical antipsychotic drug treatment on amphetamine-induced striatal dopamine release in patients with psychotic disorders.
Clozapine, risperidone, and other new "atypical" antipsychotic agents are distinguished from traditional neuroleptic drugs by having clinical efficacy with either no or low levels of extrapyramidal symptoms (EPS). Preclinical models have focused on striatal dopamine systems to account for their atypical profile. In this study, we examined the effects of clozapine and risperidone on amphetamine-induced striatal dopamine release in patients with psychotic disorders. A novel 11C-raclopride/PET paradigm was used to derive estimates of amphetamine-induced changes in striatal synaptic dopamine concentrations and patients were scanned while antipsychotic drug-free and during chronic treatment with either clozapine or risperidone. We found that amphetamine produced significant reductions in striatal 11C-raclopride binding during the drug-free and antipsychotic drug treatment phases of the study which reflects enhanced dopamine release in both conditions. There were no significant differences in % 11C-raclopride changes between the two conditions indicating that these atypical agents do not effect amphetamine-related striatal dopamine release. The implications for these data for antipsychotic drug action are discussed. (+info)
(8/1182) Review article: mechanisms and management of hepatotoxicity in ecstasy (MDMA) and amphetamine intoxications.
The social use of ecstasy (methylenedioxymethampheta-mine, MDMA) and amphetamines is widespread in the UK and Europe, and they are popularly considered as 'safe'. However, deaths have occurred and hepatotoxicity has featured in many cases of intoxication with amphetamine or its methylenedioxy analogues such as ecstasy. Recreational use of these drugs presents an important but often concealed cause of hepatitis or acute liver failure, particularly in young people. The patterns of liver damage and multiple putative mechanisms of injury are discussed. Recognition of the aetiological agent requires a high index of suspicion. Optimum management of the resultant liver damage, including the controversial role of liver transplantation for fulminant hepatic failure, is also discussed. (+info)