Antagonism of nicotinic receptors of rat chromaffin cells by N,N, N-trimethyl-1-(4-trans-stilbenoxy)-2-propylammonium iodide: a patch clamp and ligand binding study.
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The effect of the oxystilbene derivative F3 was tested on nAChRs of whole-cell patch-clamped rat chromaffin cells in vitro and of rat adrenal gland membranes using (125)I-epibatidine. F3 (30 nM) rapidly and reversibly blocked inward currents generated by pulse applications of nicotine, shifting the dose-response curve to the right in a parallel fashion without changing the maximum response. The action of F3 was voltage insensitive and not due to altered current reversal potential. The R isomer of F3 was more potent (IC(50) = 350+/-30 nM) than its S-enantiomer (IC(50) = 1.5+/-0.3 microM). Nicotine-evoked currents were insensitive to 10 microM alpha-bungarotoxin. Equi-amplitude currents evoked by nicotine or epibatidine were similarly antagonized by R-F3 in a reversible fashion. Epibatidine-evoked currents readily produced receptor desensitization. Adrenal membranes specifically bound (125)I-epibatidine with a single population of binding sites endowed with high affinity (K(D) = 159 pM) and B(max) of 6.5+/-1.3 fmol mg(-1) of protein. (125)I-epibatidine binding was specifically displaced by cytisine (K(i) = 68 nM) or ACh (K(i) = 348 nM). F3 specifically displaced (125)I-epibatidine binding although with lower affinity (K(i) = 29.6 microM) than in electrophysiological experiments. (125)I-epibatidine binding to rat adrenal tissue was insensitive to alpha-bungarotoxin which readily antagonized (125)I-epibatidine binding to bovine adrenal tissue. The present results suggest that F3 is a relatively potent and apparently competitive antagonist of nAChRs on rat chromaffin cells. Since previous studies have indicated that F3 targets different subtypes on chick neuronal tissue, it appears that nAChRs display interspecies differences to be considered for drug development studies. (+info)
Comparative cholinergic neurotoxicity of oral chlorpyrifos exposures in preweanling and adult rats.
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Chlorpyrifos (CPF) is a common organophosphorus (OP) pesticide. Previous studies have demonstrated that neonatal rats are more sensitive than adults to the acute toxicity of high dosages of CPF. The present study examined lethality and age-related differences in neurochemical indicators and functional signs of neurotoxicity following a broad range of acute and repeated oral CPF exposures. There was about a 9-fold difference in sensitivity to the acute-dose lethality of chlorpyrifos among neonatal (7 days-of-age) and adult (90 days-of-age) rats (LD(10): neonates = 15 mg/kg; adults = 136 mg/kg), while juvenile rats (21 days-of-age) exhibited intermediate sensitivity (LD(10) = 47 mg/kg). Neonatal and adult rats (n = 5-7/treatment/age group/time point) were given CPF (0, 0.15, 0.45, 0. 75, 1.5, 4.5, 7.5, or 15 mg/kg/day) for 14 days and sacrificed 4 h after either the first or 14th dose for neurochemical measurements (cholinesterase activity in frontal cortex, plasma and RBC, and muscarinic ([(3)H]QNB) and nicotinic ([(3)H]epibatidine) receptor binding in frontal cortex. No overt signs of functional toxicity (involuntary movements, SLUD signs) were noted in either age group by 4 h after the first dose. With repeated CPF exposures, however, signs of cholinergic toxicity were noted in both age groups at the higher dose levels [no observed effect levels (NOELs): neonate = 4.5 mg/kg/day; adult = 7.5 mg/kg/day]. Similar degrees of ChE inhibition were noted in neonatal brain and blood fractions following acute exposure, but substantial ChE inhibition was only noted in adult plasma and RBC 4 h after the first treatment. Following repeated CPF exposures, similar degrees of ChE inhibition were again noted in tissues from immature animals, but a wide range of sensitivity to inhibition was noted in adult tissues. NOELs based on ChE inhibition for adults were about 1->/=10-fold higher than in neonates with acute exposure but only 0.2-2 times higher with repeated dosing. Moreover, dose-related inhibition of brain ChE was similar between age groups, and similar reductions in both QNB and epibatidine binding were noted between the age groups after repeated dosing, even though by the end of the dosing period young animals (juveniles) were still about 3 times more sensitive than adults, based on acute lethality. We conclude that while immature animals can be markedly more sensitive to lethal effects of high doses of CPF, lesser or no age-related differences are apparent, based on non-lethal endpoints, in particular with repeated exposures. (+info)
Treatment of attention-deficit/hyperactivity disorder.
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OBJECTIVES: To determine (a) the long-term and short-term effectiveness and safety of pharmacological and nonpharmacological interventions for attention-deficit/hyperactivity disorder (ADHD) in children and adults and (b) whether combined interventions are more effective than individual interventions. SEARCH STRATEGY: MEDLINE (from 1966), CINAHL (from 1982), HEALTHStar (from 1975), PsycINFO (from 1984), EMBASE (from 1984), and the Cochrane Library searches were completed in November 1997. Reference lists of eligible studies and files of members of the research team and partner organizations were also searched. SELECTION CRITERIA: Studies were selected if they focused on the treatment of ADHD in humans and were published in any language as a full report in peer-reviewed journals. Studies including conditions other than ADHD were reported if separate subgroup analyses for patients with ADHD were provided. DATA COLLECTION AND ANALYSIS: Two reviewers independently extracted data for 41 variables on general characteristics, along with detailed information on interventions, outcomes, and tests. Differences were resolved by consensus or by a third researcher. Studies were not combined quantitatively because the quality of reporting was low and heterogeneity existed across outcome measures and tests. MAIN RESULTS: Seventy-eight studies (77 randomized controlled trials) met the inclusion criteria. Twenty-three studies compared drugs and showed few, if any, differences among methylphenidate (MPH), dextroamphetamine (DEX), and pemoline; studies comparing stimulants with tricyclic antidepressants (2) were inconclusive. Six studies compared drugs with nondrug interventions and showed consistently that stimulants, particularly MPH, may be more effective than nonpharmacological interventions. Twenty studies compared combination therapies with a stimulant or a nondrug intervention alone; no additional beneficial effects for combination therapies were shown. Nine studies compared tricyclic antidepressants with placebo and showed that desipramine may be more effective than placebo; no consistent effect was shown for imipramine. Fourteen studies (13 in school children and 1 in adults) evaluated long-term therapy (> or = 12 weeks) and showed a trend to general improvement regardless of treatment, but the length of followup was inadequate. MPH may reduce behavioral disturbance in children with ADHD while it is taken. Academic performance does not appear to be improved with stimulants. Twelve studies evaluated treatment in adults with ADHD. For MPH vs. placebo, the results were contradictory. Antidepressants may be effective in adults, but no beneficial effect was seen with pemoline, nicotine, or phenylalanine compared with placebo. Thirty-two reports (29 studies) evaluated adverse effects of drug therapy; many of the side effects associated with stimulant use appear to be relatively mild and of short duration and to respond to dosing or timing adjustments. Data are inadequate on the long-term effects and severity of adverse effects of most interventions. CONCLUSIONS: This report describes rigorous systematic reviews on the treatment of ADHD, ready for incorporation into evidence-based clinical practice guidelines or performance measures. The report also provides a detailed description of the many limitations of the evidence available and provides recommendations to fill existing knowledge gaps. Studies on ADHD have low reporting quality, methodological flaws, and heterogeneity across outcome measures and tests. A detailed description is included of the many limitations of the available evidence plus recommendations to fill existing knowledge gaps. Fulfilling such knowledge gaps will not be easy and will require genuine collaboration among decisionmakers. (+info)
GABA(B) receptor-mediated effects on vagal pathways to the lower oesophageal sphincter and heart.
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GABA(B) receptors influencing vagal pathways to the lower oesophageal sphincter and heart were investigated. In urethane-anaesthetized ferrets, the GABA(B) agonist baclofen (7 micromol kg(-1) i.v.) increased basal lower oesophageal sphincter (LOS) pressure. This was reversed by antagonism with CGP35348 (100 micromol kg(-1) i.v.). Baclofen's effect was abolished by vagotomy, suggesting a central action, yet it was ineffective when given centrally (3 - 6 nmol i.c.v.). Peripheral vagal stimulation (10 Hz, 5 s duration) caused LOS inhibition, followed by excitation, then prolonged inhibition. Bradycardia was also evoked during stimulation. Bradycardia and LOS responses were abolished after chronic supranodose vagotomy, indicating that they were due to stimulation of vagal pre-ganglionic neurones, not antidromic stimulation of afferents. Baclofen (1 - 10 micromol kg(-1)) reduced bradycardia and enhanced LOS excitation, which was also seen in animals pretreated with atropine (400 microgram kg(-1) i.v.) and guanethidine (5 mg kg(-1) i.v.), but not in those pretreated with L-NAME (100 mg kg(-1) i.v.). Effects of baclofen (7 micromol kg(-1) i.v.) on vagal stimulation-induced LOS and cardiac responses were unchanged by the GABA(B) antagonists CGP35348 or CGP36742 (up to 112 micromol kg(-1) i.v.), but were reversed by CGP62349 (ED(50) 37 nmol kg(-1) i.v.) or CGP54626 (ED(50) 100 nmol kg(-1) i.v.). Responses of isolated LOS strips to electrical stimulation, capsaicin, NK-1, NK-2 and nicotinic receptor agonists were all unaffected by baclofen (+info)
Effects of repeated nicotine administration and footshock stress on rat mesoprefrontal dopamine systems: Evidence for opioid mechanisms.
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We have examined the effects of nicotine pre-treatment on mesoprefrontal dopamine (DA) function in the presence and absence of acute stress, and the involvement of endogenous opiate peptide systems (EOPS). Acute electrical footshock stress preferentially increases DA utilization in medial prefrontal cortex (mPFC) compared to nucleus accumbens (NAS) and striatal terminal fields, and this is correlated with profound locomotor immobility. Our recent studies have demonstrated that repeated, but not acute, nicotine pre-treatment significantly reduced mPFC DA utilization and footshock stress-induced immobility responses. There is increasing evidence that the biochemical and behavioral effects of nicotine are mediated by EOPS, and we hypothesized that the stress-reducing effects of repeated nicotine administration in these studies were mediated by EOPS. Accordingly, rats pre-treated subcutaneously with repeated nicotine were given a single dose of the opiate receptor antagonist naloxone (0.1-10.0 mg/kg, i.p.) or saline as a co-treatment with nicotine or saline 10 min prior to acute footshock stress. Naloxone had no effects on non-stressed or acute footshock stress-induced mPFC DA utilization, but dose-dependently antagonized repeated nicotine's attenuation of stress-induced mesoprefrontal DA utilization and immobility responses. Furthermore, naloxone dose-dependently blocked repeated nicotine's augmentation of accumbal DA utilization. These results suggest that EOPS may be involved in mediating repeated nicotine administration effects on mesoprefrontal dopaminergic and immobility responses to acute footshock stress. (+info)
Multiple cellular mechanisms mediate the effect of lobeline on the release of norepinephrine.
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The complex effect of lobeline on [(3)H]norepinephrine ([(3)H]NE) release was investigated in this study. Lobeline-induced release of [(3)H]NE from the vas deferens was strictly concentration-dependent. In contrast, electrical stimulation-evoked release was characterized by diverse effects of lobeline depending on the concentration used: at lower concentration (10 microM), it increased the release and at high concentration (100 and 300 microM), the evoked release of [(3)H]NE was abolished. The effect of lobeline on the basal release was [Ca(2+)]-independent, insensitive to mecamylamine, a nicotinic acetylcholine receptor antagonist, and to desipramine, a noradrenaline uptake inhibitor. However, lobeline-induced release was temperature-dependent: at low temperature (12 degrees C), at which the membrane carrier proteins are inhibited, lobeline failed to increase the basal release. Lobeline dose dependently inhibited the uptake of [(3)H]NE into rat hippocampal synaptic vesicles and purified synaptosomes with IC(50) values of 1.19 +/- 0.11 and 6.53 +/- 1.37 microM, respectively. Lobeline also inhibited Ca(2+) influx induced by KCl depolarization in sympathetic neurons measured with the Fura-2 technique. In addition, phenylephrine, an alpha(1)-adrenoceptor agonist, contracted the smooth muscle of the vas deferens and enhanced stimulation-evoked contraction. Both effects were inhibited by lobeline. Our results can be best explained as a reversal of the monoamine uptake by lobeline that is facilitated by the increased intracellular NE level after lobeline blocks vesicular uptake. At high concentrations, lobeline acts as a nonselective Ca(2+) channel antagonist blocking pre- and postjunctional Ca(2+) channels serving as a counterbalance for the multiple transmitter releasing actions. (+info)
Effect of zaldaride maleate, an antidiarrheal compound, on fecal pellet output induced by hyperpropulsion in gastrointestine of rats.
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The effect of zaldaride, a calmodulin inhibitor, on fecal pellet output in rats was compared with that of loperamide, an antidiarrheal drug. 5-Hydroxytryptamine (10 mg/kg, s.c.), neostigmine (0.3 mg/kg, s.c.) and nicotine (1.0 mg/kg, s.c.) increased fecal pellet output. Zaldaride (> or = 30 mg/kg, p.o.) reduced these increases in fecal pellet outputs. Loperamide (10 mg/kg, p.o.) inhibited fecal pellet output induced by 5-hydroxytryptamine and neostigmine but not nicotine. Under normal conditions, zaldaride and loperamide did not affect fecal pellet output at doses used in these studies. In conclusion, zaldaride may inhibit increases in fecal pellet output induced by hyperpropulsion of the gastrointestinal tract without causing constipation as a side effect. (+info)
Binding properties of agonists and antagonists to distinct allosteric states of the nicotinic acetylcholine receptor are incompatible with a concerted model.
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Recent work has shown that the nicotinic acetylcholine receptor (nAChR) can be fixed in distinct conformations by chemical cross-linking with glutardialdehyde, which abolishes allosteric transitions in the protein. Here, two conformations that resemble the desensitized and the resting states were compared with respect to their affinities for different classes of ligands. The same ligands were tested for their ability to convert the nAChR from a conformation with low affinity to a conformation with high affinity for acetylcholine. As expected, agonists were found to bind with higher affinity to the desensitized state-like conformation and to induce a shift of the nAChR to this high affinity state. In contrast, although most antagonists tested bound preferentially to the desensitized receptor as well they failed to induce a change of the affinity for acetylcholine. These observations sharply contradict basic predictions of the concerted model, including the postulate of a preformed equilibrium between the different states of the nAChR in the absence of agonist. With a similar approach we could show that the non-competitive inhibitor ethidium is displaced in a non-allosteric manner by other well characterized channel blockers from the cross-linked nAChR. These results require revision of current models for the mechanisms underlying non-competitive antagonism at the nAChR. (+info)