Upregulation of surface alpha4beta2 nicotinic receptors is initiated by receptor desensitization after chronic exposure to nicotine.
(17/1634)
It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upregulation of nAChR number. To test this hypothesis directly, oocytes expressing alpha4beta2 receptors were chronically incubated (24-48 hr) in nicotine, and the resulting changes in specific [3H]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensitization. Four lines of evidence strongly support the hypothesis. (1) The half-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The concentration of [3H]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical desensitization models. (3) Functional desensitization of alpha3beta4 receptors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha4beta2 receptors that do not recover fully from desensitization, but not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-maximal binding of alpha4beta2 receptors in total cell membrane homogenates was 20-fold lower than that measured for surface nAChRs in intact oocytes. These data suggest that cell homogenate binding assays may not accurately reflect the in vivo desensitization affinity of surface nAChRs and may account for some of the previously reported differences in the efficacy of nicotine for inducing nAChR desensitization and upregulation. (+info)
Minor structural changes in nicotinoid insecticides confer differential subtype selectivity for mammalian nicotinic acetylcholine receptors.
(18/1634)
The major nitroimine insecticide imidacloprid (IMI) and the nicotinic analgesics epibatidine and ABT-594 contain the 6-chloro-3-pyridinyl moiety important for high activity and/or selectivity. ABT-594 has considerable nicotinic acetylcholine receptor (AChR) subtype specificity which might carry over to the chloropyridinyl insecticides. This study considers nine IMI analogues for selectivity in binding to immuno-isolated alpha1, alpha3 and alpha7 containing nicotinic AChRs and to purported alpha4beta2 nicotinic AChRs. Alpha1- and alpha3-containing nicotinic AChRs (both immuno-isolated by mAb 35, from Torpedo and human neuroblastoma SH-SY5Y cells, respectively) are between two and four times more sensitive to DN-IMI than to (-)-nicotine. With immuno-isolated alpha3 nicotinic AChRs, the tetrahydropyrimidine analogues of IMI with imine or nitromethylene substituents are 3-4 fold less active than (-)-nicotine. The structure-activity profile with alpha3 nicotinic AChRs from binding assays is faithfully reproduced in agonist potency as induction of 86rubidium ion efflux in intact cells. Alpha7-containing nicotinic AChRs of SH-SY5Y cells (immuno-isolated by mAb 306) and rat brain membranes show maximum sensitivity to the tetrahydropyrimidine analogue of IMI with the nitromethylene substituent. The purported alpha4beta2 nicotinic AChRs [mouse (Chao & Casida, 1997) and rat brain] are similar in sensitivity to DN-IMI, the tetrahydropyrimidine nitromethylene and nicotine. The commercial insecticides (IMI, acetamiprid and nitenpyram) have low to moderate potency at the alpha3 and purported alpha4beta2 nicotinic AChRs and are essentially inactive at alpha1 and alpha7 nicotinic AChRs. In conclusion, the toxicity of the analogues and metabolites of nicotinoid insecticides in mammals may involve action at multiple receptor subtypes with selectivity conferred by minor structural changes. (+info)
Is telephone counselling a useful addition to physician advice and nicotine replacement therapy in helping patients to stop smoking? A randomized controlled trial.
(19/1634)
BACKGROUND: The authors evaluated the incremental efficacy of telephone counselling by a nurse in addition to physician advice and nicotine replacement therapy in helping patients to stop smoking. METHODS: The trial was conducted at the University of Ottawa Heart Institute. A total of 396 volunteers who smoked 15 or more cigarettes daily were randomly assigned to either of 2 groups: usual care (control group) and usual care plus telephone counselling (intervention group); the groups were stratified by sex and degree of nicotine dependence. Usual care involved the receipt of physician advice on 3 occasions, self-help materials and 12 weeks of nicotine replacement therapy. Telephone counselling was provided by a nurse at 2, 6 and 13 weeks after the target quit date. Point-prevalent quit rates were determined at 52 weeks after the target quit date. RESULTS: The point-prevalent quit rates at 52 weeks did not differ significantly between the control and intervention groups (24.1% v. 23.4% respectively). The quit rates did not differ significantly at the secondary measurement points of 4, 12 and 26 weeks. INTERPRETATION: Brief physician assistance, along with nicotine replacement therapy, can help well-motivated smokers to quit. Three additional sessions of telephone counselling by a nurse were ineffective in increasing quit rates. This form of assistance may be useful in the absence of physician advice or when self-selected by patients. (+info)
Monkeys that receive chronic low-dose 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) administration have difficulty performing numerous cognitive tasks. This study further examines the extent to which chronic low-dose MPTP exposure affects performance of a visual memory task [variable delayed response (VDR)] with both attentional and short-term memory components and assesses the effects of the novel neuronal nicotinic acetylcholine receptor agonist SIB-1508Y and levodopa on cognitive task performance. Before MPTP treatment, these monkeys displayed a delay-dependent decrement in performance on the VDR task and performed well on delayed matching-to-sample and visual pattern discrimination tasks. Chronic low-dose MPTP treatment caused a shift to a delay-independent pattern of responding on the VDR task, such that short-delay trials were performed as poorly as long-delay trials. There were also deficits in performing the delayed matching-to-sample task, whereas visual discrimination performance remained intact. SIB-1508Y normalized the pattern of response on the VDR task by significantly improving performance on short-delay trials and on the delayed matching-to-sample task. These effects lasted up to 24 to 48 h after SIB-1508Y administration. Neither levodopa nor nicotine significantly improved task performance. These results suggest that chronic low-dose MPTP exposure results in a cognitive disturbance that can be corrected by the nicotinic acetylcholine receptor agonist SIB-1508Y but not by levodopa. Thus, SIB-1508Y may be useful in the treatment of the cognitive deficits in Parkinson's disease. (+info)
The alkyl chain dependence of the effect of normal alcohols on agonist-induced nicotinic acetylcholine receptor desensitization kinetics.
(21/1634)
BACKGROUND: The nAcChoR is the prototypical member of a superfamily of ligand-gated ion channels that are all relevant targets of anesthetics and undergo desensitization upon prolonged exposure to agonist. This study was designed to investigate the effects of representative normal alcohols on the apparent rate of acetylcholine-induced nAcChoR desensitization. METHODS: Nicotinic acetylcholine receptors were obtained from the electroplax organ of Torpedo nobiliana. The apparent rate of acetylcholine-induced desensitization in the presence and absence of normal alcohols was measured using stopped-flow fluorescence. RESULTS: Normal alcohols as long as octanol (the longest studied) increased the apparent rate of desensitization induced by low concentrations of acetylcholine, shifting the agonist concentration-response curve for desensitization to the left Ethanol butanol, and, to a lesser extent, hexanol increased the maximal rate of desensitization induced by high, saturating concentrations of agonist. Beyond hexanol, heptanol and octanol had no effect on this maximal apparent rate of desensitization, even at concentrations that approach those that directly induce desensitization in the absence of agonist. CONCLUSION: Normal alcohols ranging from ethanol to octanol increase the apparent affinity of nAcChoR for agonist with potencies that are proportional to their hydrophobicities. However, normal alcohol effects on the rate constant for desensitization show a cutoff beyond hexanoL This suggests that the effects of normal alcohols on the apparent agonist affinity and rate constant for desensitization of nAcChoR may be modulated by distinct sites that have different steric constraints; the site(s) responsible for increasing the maximal rate of desensitization are predicted to be smaller than those that increase the apparent agonist affinity. (+info)
Melatonin modulation of presynaptic nicotinic acetylcholine receptors located on short noradrenergic neurons of the rat vas deferens: a pharmacological characterization.
(22/1634)
Melatonin, the pineal hormone produced during the dark phase of the light-dark cycle, modulates neuronal acetylcholine receptors located presynaptically on nerve terminals of the rat vas deferens. Recently we showed the presence of high affinity nicotine-binding sites during the light phase, and low and high affinity binding sites during the dark phase. The appearance of the low affinity binding sites was due to the nocturnal melatonin surge and could be mimicked by exposure to melatonin in vitro. The aim of the present research was to identify the receptor subtypes responsible for the functional response during the light and the dark phase. The rank order of potency of agonists was dimethylphenylpiperazinium (DMPP) = cytisine > nicotine > carbachol and DMPP = nicotine = cytisine > carbachol, during the light and dark phase, respectively, due to an increase in apparent affinity for nicotine. Mecamylamine similarly blocked the DMPP response during the light and the dark phase, while the response to nicotine was more efficiently blocked during the light phase. In contrast, methyllycaconitine inhibited the nicotine-induced response only at 21:00 h. Since alpha7 nicotinic acetylcholine receptors (nAChRs) have low affinity for nicotine in binding assays, we suggest that a mixed population composed of alpha3ss4 - plus alpha7-bearing nAChR subtypes is present at night. This plasticity in receptor subtypes is probably driven by melatonin since nicotine-induced contraction in organs from animals sacrificed at 15:00 h and incubated with melatonin (100 pg/ml, 4 h) is not totally blocked by mecamylamine. Thus melatonin, by acting directly on the short adrenergic neurons that innervate the rat vas deferens, induces the appearance of the low affinity binding site, probably an alpha7 nAChR subtype. (+info)
Antagonist activities of mecamylamine and nicotine show reciprocal dependence on beta subunit sequence in the second transmembrane domain.
(23/1634)
We show that a portion of the TM2 domain regulates the sensitivity of beta subunit-containing rat neuronal nicotinic AChR to the ganglionic blocker mecamylamine, such that the substitution of 4 amino acids of the muscle beta subunit sequence into the neuronal beta4 sequence decreases the potency of mecamylamine by a factor of 200 and eliminates any long-term effects of this drug on receptor function. The same exchange of sequence that decreases inhibition by mecamylamine produces a comparable potentiation of long-term inhibition by nicotine. Inhibition by mecamylamine is voltage-dependent, suggesting a direct interaction of mecamylamine with sequence elements within the membrane field. We have previously shown that sensitivity to TMP (tetramethylpiperidine) inhibitors is controlled by the same sequence elements that determine mecamylamine sensitivity. However, inhibition by bis-TMP compounds is independent of voltage. Our experiments did not show any influence of voltage on the inhibition of chimeric receptors by nicotine, suggesting that the inhibitory effects of nicotine are mediated by binding to a site outside the membrane's electric field. An analysis of point mutations indicates that the residues at the 6' position within the beta subunit TM2 domain may be important for determining the effects of both mecamylamine and nicotine in a reciprocal manner. Single mutations at the 10' position are not sufficient to produce effects, but 6' 10' double mutants show more effect than do the 6' single mutants. (+info)
Differential blockade of rat alpha3beta4 and alpha7 neuronal nicotinic receptors by omega-conotoxin MVIIC, omega-conotoxin GVIA and diltiazem.
(24/1634)
Rat alpha3beta4 or alpha7 neuronal nicotinic acetylcholine receptors (AChRs) were expressed in Xenopus laevis oocytes, and the effects of various toxins and non-toxin Ca2+ channel blockers studied. Nicotinic AChR currents were elicited by 1 s pulses of dimethylphenylpiperazinium (DMPP, 100 microM) applied at regular intervals. The N/P/Q-type Ca2+ channel blocker omega-conotoxin MVIIC inhibited alpha3beta4 currents with an IC50 of 1.3 microM; the blockade was non-competitive and reversible. The alpha7 currents were unaffected. At 1 microM, omega-conotoxin GVIA (N-type Ca2+ channel blocker) inhibited by 24 and 20% alpha3beta4 and alpha7 currents, respectively. At 1 microM, omega-agatoxin IVA (a P/Q-type Ca2+ channel blocker) did not affect alpha7 currents and inhibited alpha3beta4 currents by only 15%. L-type Ca2+ channel blockers furnidipine, verapamil and, particularly, diltiazem exhibited a preferential blocking activity on alpha3beta4 nicotinic AChRs. The mechanism of alpha3beta4 currents blockade by omega-conotoxins and diltiazem differed in the following aspects: (i) the onset and reversal of the blockade was faster for toxins; (ii) the blockade by the peptides was voltage-dependent, while that exerted by diltiazem was not; (iii) diltiazem promoted the inactivation of the current while omega-toxins did not. These data show that, at concentrations currently employed as Ca2+ channel blockers, some of these compounds also inhibit certain subtypes of nicotinic AChR currents. Our data calls for caution when interpreting many of the results obtained in neurons and other cell types, where nicotinic receptor and Ca2+ channels coexist. (+info)