Biomedical subjects

M J Marks

Publications and source records attributed to M J Marks.

At least 37 records · Page 2Linked to original sources

PubMed1996

Inbred mouse strains vary in oral self-selection of nicotine.

Inbred mouse strains differ in sensitivity to a first dose of nicotine and in the development of tolerance to nicotine. The experiments reported here used six inbred mouse strains (A, BUB, C3H, C57BL/6, DBA/2, ST/b) that differ in sensitivity to an acute challenge dose of nicotine to determine whether differences in oral self-selection of nicotine exist. Animals were presented with solutions containing nicotine or vehicle (water or 0.2% saccharin) and their daily intake of the two fluids was measured for 4 days starting with a 10 micrograms/ml nicotine solution. This was followed by sequential 4-day testing with 20, 35, 50, 65, 80, 100, 125, 160 and 200 micrograms/ml nicotine solutions. The strains differed dramatically in their self-selection of nicotine and in maximal daily dose (mg/kg); the rank order of the strains was C57BL/6 > DBA > BUB > A > or = C3H > or = ST/b for both the tap water and 0.2% saccharin choice experiments. Correlations between nicotine consumption and sensitivity to nicotine, as measured by a battery of behavioral and physiological responses, were also calculated. Strain differences in nicotine intake were highly correlated with sensitivity to nicotine-induced seizures. As sensitivity to nicotine-induced seizures increases, oral self-selection of nicotine decreases. This finding may suggest that this toxic action of nicotine serves to limit intake.

Animals↗

PubMed1996

ST/b and DBA/2 mice differ in brain alpha-bungarotoxin binding and alpha 7 nicotinic receptor subunit mRNA levels: a quantitative autoradiographic analysis.

Inbred mouse strains vary in sensitivity to a number of behavioral and physiological effects produced by nicotine. Differences in sensitivity to nicotine are correlated with variance in the number of brain nicotinic receptors as measured in regionally dissected brain tissue. The studies reported here used quantitative autoradiography and in-situ hybridization methods to measure regional levels of alpha-bungarotoxin (alpha BTX) binding and alpha 7 mRNA levels. Two inbred mouse strains, ST/b and DBA/2, were compared because these strains differ maximally in sensitivity to nicotine-induced seizures and in alpha BTX binding measured in regional brain homogenates. The binding of alpha BTX was significantly greater in the St/b strain in 42 of 127 brain regions that were analyzed, and a trend towards increased binding was seen in many additional brain regions. The most consistent strain differences were found in hippocampal, thalamic and pontine nuclei. Strain differences in alpha 7 mRNA levels were also detected, but these were not as widespread as were the alpha BTX binding differences. The alpha 7 mRNA levels were significantly correlated with alpha BTX binding in both mouse strains which suggests that the strain differences in binding are related, in part, to the levels of alpha 7 mRNA.

Animals↗

PubMed1996

Genetic correlation of inhibitory gating of hippocampal auditory evoked response and alpha-bungarotoxin-binding nicotinic cholinergic receptors in inbred mouse strains.

One function of the hippocampus is to ascertain the novelty of incoming sensations and encode significant new information into memory. The regulation of response to repeated stimuli may prevent overloading of this function by redundant sensory input. Recent pharmacological studies implicate the role of alpha-bungarotoxin-sensitive nicotinic cholinergic receptors in the inhibition of hippocampal response to repeated auditory stimuli. The number of hippocampal alpha-bungarotoxin-sensitive receptors has a major genetic determinant, as demonstrated by a significant variance between different inbred mouse strains. The purpose of the present study was to determine whether there was a related genetic correlation for the gating of auditory response. Nine inbred mouse strains, representing a continuum of hippocampal alpha-bungarotoxin binding, were tested for the electrophysiological response to repeated auditory stimulation, followed by whole hippocampus membrane alpha-bungarotoxin binding studies. Several parameters of the auditory evoked response showed significant genetic variance over the nine strains, and a significant correlation was found between hippocampal alpha-bungarotoxin binding and both the amplitude of the initial evoked response and its inhibition to repeated auditory stimuli. There was no correlation of the auditory evoked response with high-affinity nicotine binding. These data further support the hypothesis that alpha-bungarotoxin-sensitive nicotinic receptors are involved in the regulation of hippocampal response to repeated auditory stimuli and suggest that this function is genetically controlled.

Acoustic Stimulation↗

PubMed1996

Thoracic disease in children with AIDS.

Knowledge of common and uncommon thoracic pathologic conditions in children with acquired immunodeficiency syndrome (AIDS) can expedite disease management. Chest radiography, computed tomography (CT), and magnetic resonance (MR) imaging are useful in cases involving possible complications of thoracic AIDS. Lymphocytic interstitial pneumonitis (LIP) is generally seen on plain radiographs and CT scans as a diffuse, symmetric, reticulonodular or nodular pattern, occasionally associated with mediastinal or hilar adenopathy. Chronic consolidations and bronchiectasis may be observed in pediatric AIDS patients with no evidence of previous LIP. Bacterial pneumonia, a frequent initial manifestation of AIDS, appears as lobar or segmental consolidations on radiographs. Radiographic findings of Pneumocystis carinii pneumonia, the most common infection, include rapidly progressive increased air-space opacity with air bronchograms. Lymphoma often appears as a mediastinal or hilar mass, often without involvement of the lung parenchyma. Thoracic smooth muscle tumors have also been observed in children with AIDS. Multilocular thymic cysts have low attenuation on CT scans and increased signal intensity on T2-weighted MR images. Most pediatric AIDS patients with cardiac disease have cardiomegaly, often associated with pulmonary edema, at chest radiography. An esophagogram may show ulceration, plaque formation, mucosal edema, and dysmotility in patients with candidal esophagitis.

AIDS-Related Opportunistic Infections↗

PubMed1996

Nicotinic agonists differ in activation and desensitization of 86Rb+ efflux from mouse thalamic synaptosomes.

The effects of the nicotinic agonists acetylcholine, (+)-anatox in-a, carbachol, cytisine, dimethylphenylpiperazinum, (+)-epibatidine, (-)-epibatidine, methylcarbachol, D-nicotine, L-nicotine, and tetramethylammonium on 86Rb+ efflux from mouse thalamic synaptosomes were investigated. All 11 agonists evoked a concentration-dependent stimulation of 86Rb+ efflux as well as a time- and concentration-dependent reduction of response (desensitization). The agonists varied widely in potency, efficacy and rate of desensitization. (+)-Epibatidine was the most potent agonist (EC50 = 10 nM), whereas tetramethylammonium was the least potent (EC50 = 65 microM). The agonists containing a quaternary ammonium group were generally more efficacious than the other agonists, except for both of the enantiomers of epibatidine, which stimulated 86Rb+ efflux at least as well as acetylcholine. Cytisine was the least efficacious compound tested with a maximal response approximately 10% that of (-)-epibatidine. Exposure of the thalamic synaptosomes to agonist concentrations that generally stimulated little or no efflux reduced in a concentration-dependent manner a subsequent response to 10 microM nicotine. The IC50 values for this functional blockade (desensitization) were highly correlated with the Ki values for the inhibition of [3H]nicotine binding. Furthermore, exposure of the thalamic synaptosomes to 300 nM L-nicotine reduced the responses evoked by a subsequent exposure to a stimulating concentration of all 11 agonists. The observation of desensitization by both stimulating and substimulating concentrations of each agonist is consistent with the predictions of the two-state model of Katz and Thesleff.

Acetylcholine↗

PubMed1996

Chronic nicotine and mecamylamine treatment increase brain nicotinic receptor binding without changing alpha 4 or beta 2 mRNA levels.

It has recently been shown using regionally dissected tissue that both chronic mecamylamine treatment and chronic nicotine treatment increase the number of [3H]-nicotine binding sites in mouse brain, and that additive increases in the number of binding sites is observed after cotreatment with the two drugs. The studies reported here extend these findings by using quantitative autoradiographic methods to analyze the brains of chronically treated mice at a finer anatomical level of resolution. DBA/2 mice were chronically infused (i.v.) with saline, 4.0 mg/ kg/hr nicotine, 4.0 mg/kg/hr mecamylamine or both drugs for 7 days. The brains of these mice were subsequently analyzed to determine the effects of chronic drug treatment on [3H]-nicotine binding and on the levels of mRNA encoding the nicotinic receptor subunits, alpha 4 and beta 2. [3H]-Nicotine binding was increased in 37 of 46 brain regions after chronic nicotine treatment (average increase = 66.5%), in 41 of 46 regions after chronic mecamylamine treatment (average increase = 56.5%), and in 45 of 46 brain regions after chronic treatment with both drugs (average increase = 107.1%). The changes in [3H]-nicotine binding produced by the two drugs across brain regions were highly (r = 0.98) and significantly correlated. These results are consistent with the proposal that the same receptors are affected by agonist or antagonist treatment. Chronic treatment had no significant effect on the levels of alpha 4 or beta 2 mRNA as measured by in situ hybridization. Therefore, the increases in [3H]-nicotine binding do not arise from changes in the steady-state levels of either alpha 4 or beta 2 mRNA which are believed to encode the subunits of this nicotinic receptor subtype.

Animals↗

PubMed1995

Sodium channel blockers partially inhibit nicotine-stimulated 86Rb+ efflux from mouse brain synaptosomes.

The possible contribution of non-nicotinic cation channels to nicotine-stimulated 86Rb+ efflux was investigated. Dendrotoxin, charybdotoxin and apamin did not affect nicotine-stimulated 86Rb+ efflux. Although 4-aminopyridine and tetraethyl-ammonium completely inhibited nicotine-stimulated 86Rb+ efflux, the concentrations required to achieve this inhibition differed markedly from those required to inhibit K(+)-stimulated 86Rb+ efflux. Cs+ reduced basal 86Rb+ efflux, but did not affect nicotine-stimulated efflux. Tetrodotoxin completely inhibited veratridine-stimulated 86Rb+ efflux (IC50 = 3.4 nM) and partially inhibited (about 42%) nicotine-stimulated efflux (IC50 = 1.3 nM). Saxitoxin also completely inhibited veratridine-stimulated efflux (IC50 = 19.5 nM) and partially inhibited (about 47%) nicotine-stimulated efflux (IC50 = 5.4 nM). Tetrodotoxin had no significant effect on the EC50 for nicotine, but decreased the efflux rate and reduced modestly the rate of desensitization measured with prolonged exposure to nicotine. Tetrodotoxin (50 nM) also inhibited maximal nicotine-stimulated 86Rb+ efflux to approximately the same extent in each of ten brain regions examined. Nicotine-stimulated 86Rb+ efflux measured in the presence or absence of 100 nM tetrodotoxin is correlated with the density of [3H]nicotine, but not [3H]saxitoxin, binding sites in 12 regions, suggesting that the density of nicotinic receptors, not the number of Na+ channels, is the rate-limiting step for nicotine-stimulated 86Rb+ efflux. Therefore, activation of Na+ channels secondary to the stimulation of nicotinic receptors seems to contribute to the nicotine-stimulated 86Rb+ efflux observed in mouse brain synaptosomes. In contrast, no evidence for K+ channel modulation of nicotine-stimulated 86Rb+ efflux from thalamic synaptosomes was obtained.

Animals↗

PubMed1994

ABT-200, a norepinephrine uptake inhibitor, blocks Ca2+ signals in chromaffin cells.

We previously described inhibition by racemic (+/-)-(1'R*,3R*)-3-phenyl-1- [1',2',3',4'-tetrahydro-5',6'-methylenedioxy-1'- napthalenyl-methyl]-pyrrolidine methanesulfonate (ABT-200), and its two constituent enantiomers, SS,ABT-200 and RR,ABT-200, of nicotine-stimulated but not histamine-stimulated catecholamine release from bovine adrenal chromaffin cells. To test the hypothesis that this inhibition reflects a blockade of Ca2+ influx, we used fura-2 loaded chromaffin cells to investigate cytosolic Ca2+ signals. We found that SS,ABT-200 inhibited nicotine- and K(+)-stimulated Ca2+ signals, both of which depend on Ca2+ influx. However, the early phase of the histamine-stimulated Ca2+ signals, which depends on Ca2+ mobilization from intracellular stores, was unaffected. We also examined ion flux through the nicotinic receptor by measuring 86rubidium+ (86Rb+) efflux from preloaded mouse midbrain synaptosomes. We found that SS,ABT-200 partially inhibited nicotine-stimulated 86Rb+ efflux, suggesting that it blocks ion flux through the nicotinic receptor directly. These data support a model in which ABT-200 blocks nicotine-stimulated catecholamine release by inhibiting cation flux through multiple channels.

Adrenal Glands↗

PubMed1994

An analysis of response to nicotine infusion using an automated radiotelemetry system.

Previous studies from our laboratory have demonstrated that chronic nicotine infusion evokes tolerance to nicotine injected IP several hours after withdrawal from chronic infusion. This method may introduce problems related to withdrawal reactions and to stress associated with handling of the animals. The studies reported here measured tolerance to nicotine in mice using an automated radiotelemetry system. DBA/2 mice were infused intravenously with saline for 4 days followed by infusion of a 4 mg/kg per h dose of nicotine for 7 days. After the nicotine treatment, the mice were infused with saline for 7 days. The nicotine was infused continuously or in four 1 mg/kg pulses, two 2 mg/kg pulses or one 4 mg/kg pulse each hour. Home cage activity and body temperature were measured throughout the treatment periods using a radiotelemetry system. Nicotine infusion produced an abrupt decrease in body temperature and activity, but this effect was totally reversed within 12 h in the continuously infused and four infusions/h treatment groups. Mice that received one or two infusions/h also showed a rapid response to nicotine that was reversed as treatment proceeded, but nicotine continued to produce a measurable effect for several days. After nicotine withdrawal, temperature and activity returned to predrug infusion values in all of the groups except those infused once per hour. This group showed depressed activity for a minimum of 3 days after nicotine treatment stopped. Thus, the kinetics of nicotine administration affected the intensity of response during continued treatment as well as activity after cessation of chronic treatment.

Animals↗

PubMed1994

Individual differences in sensitivity to nicotine in mice: response to six generations of selective breeding.

Four hundred seventeen heterogeneous stock mice were tested for their relative sensitivity to a low dose of nicotine (0.75 mg/kg) using activity in an automated Y-maze and body temperature as response measures. A wide spectrum of individual responsiveness to nicotine, ranging from complete suppression of activity to stimulation above baseline activity, was found. Replicate measures taken 1 week later on the same animals showed the responses to nicotine to be reliable and reproducible. Activity levels and body temperatures following nicotine administration were highly correlated (r = 0.60, df = 415). From analysis of between-litter proportions of variance, the heritability of nicotine-influenced activity was estimated to be 0.12, indicating that selective breeding for differential responsiveness to nicotine would be possible. The 10 most activated and 10 most depressed male and female mice were chosen as breeders for replicate nicotine activated (NA) and nicotine depressed (ND) lines, respectively. The selection criterion was nicotine-induced activity corrected for baseline activity using regression residuals. After six generations of selective breeding a good response to selection was obtained, although the response was better for the ND than for the NA lines. Realized heritability for responsiveness to nicotine calculated from the six selected generations was found to be 0.20, or slightly greater than that estimated from the foundation population. There were no significant differences in response to selection between the replicate NA or ND lines. Nicotine-induced body temperature was measured as a correlated response to selection, and was found to remain highly correlated with nicotine-induced locomotor activity. The response was more robust for the ND lines than it was for the NA lines. In contrast to the large differences between the ND and NA lines in locomotor activity and body temperatures following nicotine administration, mean baseline activities and body temperatures remained nearly identical throughout. This indicates that selection acted specifically on nicotine-induced responses, and not on baseline measurements, as predicted for response to a selection criterion based on regression residuals.

Animals↗

PubMed1994

Effects of chronic nicotine infusion on kinetics of high-affinity nicotine binding.

It is well established that chronic nicotine treatment produces a dose-dependent increase in high-affinity L-[3H]nicotine binding. This increase may be due to chronic desensitization of the receptor. Sophisticated kinetic analyses of high-affinity nicotine binding to rat brain have demonstrated that the association rate is biphasic; the fast phase may represent binding to a high-affinity predesensitized state and the slow phase may represent binding to a lower affinity ground state that then isomerizes to form the high-affinity binding site. This isomerization presumably leads to receptor desensitization. The studies reported here assessed whether binding to mouse brain nicotinic receptors shows these same properties and whether chronic intravenous infusion of nicotine results in changes in these kinetic properties. The results obtained indicate that mouse brain nicotine binding also shows biphasic association kinetics and uniphasic dissociation kinetics, which supports the assertion that the receptor exists in two interconvertible states. However, unlike other results obtained with rat brain, the rate of the slow association process did not change with ligand concentration. Chronic infusion resulted in a dose-dependent increase in L-[3H]nicotine binding, but the ratio of fast/slow phases of binding was not changed by these treatments. These results suggest that chronic infusion does not alter measurably the kinetics of nicotinic receptor binding when measured in vitro.

Animals↗

PubMed1994

Desensitization of nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes.

Potential desensitization of brain nicotinic receptors was studied using a [3H]dopamine release assay. Nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes was concentration-dependent with an EC50 of 0.33 +/- 0.13 microM and a Hill coefficient of 1.44 +/- 0.18. Desensitization by activating concentrations of nicotine had a similar EC50 and a half-time of 35 s. Concentrations of nicotine that evoked little release also induced a concentration-dependent desensitization (EC50 = 6.9 +/- 3.6 nM, t1/2 = 1.6-2.0 min, nH = 1.02 +/- 0.01). Both types of desensitization produced a maximum 75% decrease in [3H]dopamine release. Recovery from desensitization after exposure to low or activating concentrations of nicotine was time-dependent with half-times of 6.1 min and 12.4 min, respectively. Constants determined for binding of [3H]nicotine to striatal membrane at 22 degrees C included a KD of 3.7 +/- 0.5 nM, Bmax of 67.5 +/- 2.2 fmol/mg, and Hill coefficient of 1.07 +/- 0.06. Association of nicotine with membrane binding sites was biphasic with half-times of 9 s and 1.8 min. The fast rate process contributed 37% of the total reaction. Dissociation was a uniphasic process with a half-time of 1.6 min. Comparison of constants determined by the release and binding assays indicated that the [3H]-nicotine binding site could be the presynaptic receptor involved in [3H]dopamine release in mouse striatal synaptosomes.

Animals↗

PubMed1994

Desensitization of nicotine-stimulated 86Rb+ efflux from mouse brain synaptosomes.

The desensitization of nicotine-stimulated 86Rb+ efflux from synaptosomes prepared from C57BL/6 mouse brain was investigated. Nicotine stimulated a saturable, concentration-dependent efflux of 86Rb+ from synaptosomes (EC50 = 0.60 microM), but the response decreased with time of exposure to nicotine. The rate of decrease of the response (desensitization) increased as the nicotine concentration was increased (EC50 = 0.35 microM; maximal rate of desensitization = 1.1 min-1). Desensitization of nicotine-stimulated 86Rb+ efflux was also observed when synaptosomes were exposed to low (1-200 nM) concentrations of nicotine that caused little or no stimulation of efflux (EC50 = 13 nM). The rate of desensitization observed with low nicotine concentrations (0.30 min-1) was less than that measured at stimulating concentrations. Desensitization was not fully reversible for synaptosomes exposed to nicotine concentrations between 10 nM and 10 microM: Only 60-40% of the control response was regained after a 10-min washout period. The kinetics of functional desensitization were compared with the kinetics of [3H]nicotine binding. [3H]Nicotine binding to midbrain particulate fractions displayed both a fast and a slow phase. The EC50 values for these two phases were 2.6 and 14 nM, respectively. Data obtained from functional desensitization and ligand binding experiments were analyzed using a two-state model. The kinetic constants obtained from the analyses of these two processes were very similar. Overall, the results suggest that nicotinic receptor function measured with ion flux desensitizes when exposed to either stimulating or nonstimulating concentrations of nicotine. In addition, the kinetic properties calculated for the functional desensitization are comparable to those for [3H]nicotine binding.

Animals↗

PubMed1994

Sensitivity to nicotine and brain nicotinic receptors are altered by chronic nicotine and mecamylamine infusion.

DBA/2 mice were chronically infused with saline (control), 4 mg/kg/hr of nicotine, 4 mg/kg/hr of mecamylamine or nicotine+mecamylamine for 7 days. The binding of L-[3H]nicotine was significantly increased in seven of eight brain regions dissected from mice 5 hr after cessation of drug treatment. Similarly, [3H]nicotine binding was increased in six of eight regions by chronic mecamylamine treatment. Treatment with both drugs led to increases in [3H]nicotine binding that were at least the sum of the those observed with either drug alone. These increases were partially reversed 48 hr after cessation of treatment. No significant effects of nicotine or mecamylamine treatment on alpha-[125I]bungarotoxin binding were observed 5 hr after treatment was stopped when assayed using a single ligand concentration. However, saturation analyses of the cerebral cortex detected small increases from control after treatment with nicotine or both drugs. No effects of chronic treatment on alpha-[125I]bungarotoxin binding were evident 48 hr after treatment was stopped. Mice treated with nicotine, mecamylamine or both drugs tested 5 hr after withdrawal were less sensitive to acute injection with nicotine than were saline-infused mice. However, the effects of mecamylamine may have been influenced by continued presence of this drug at the time of testing. Mice tested 48 hr after cessation of treatment were more sensitive to nicotine than those tested after 5 hr, but some tolerance to the effects of nicotine persisted in the nicotine-treated mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Downregulation of nicotinic receptor function after chronic nicotine infusion.

Chronic nicotine treatment generally results in tolerance to several actions of nicotine and a paradoxical increase in brain nicotinic receptor numbers. Receptor upregulation, it has been argued, arises as a consequence of functional desensitization. In the studies reported here, mice were chronically infused with saline (control) or one of five doses of nicotine (0.25-4.0 mg/kg/hr) for 10 days. This treatment resulted in a dose-dependent tolerance to nicotine-induced decreases in body temperature as well as decreases in locomotor and rearing activities in a Y-maze. The anticipated increase in [3H]nicotine binding was also observed. To assess functional status of the nicotinic receptors, nicotine-stimulated release of [3H]dopamine from striatal synaptosomes and 86Rb+ efflux from cortical and midbrain synaptosomes were also measured. Chronic nicotine infusion resulted in an infusion dose-dependent decrease in [3H]dopamine release from striatum and 86Rb+ efflux from midbrain; cortical 86Rb+ efflux was not affected by chronic nicotine treatment. Dose-response analyses of the release and efflux assays demonstrated that chronic nicotine infusion evoked decreases in the maximal effects of nicotine on the functional assays; potency was not altered by chronic drug treatment. These results are consistent with the hypothesis that behavioral tolerance to nicotine is a consequence of down-regulation of brain nicotinic receptor function.

Animals↗

PubMed1993

A comparison of the effects of chronic nicotine infusion on tolerance to nicotine and cross-tolerance to ethanol in long- and short-sleep mice.

Several recent studies suggest that common genes regulate sensitivity to an acute dose of ethanol and nicotine. The studies reported here attempted to determine whether common genes regulate the development of cross-tolerance between these drugs. Long-sleep (LS) and short-sleep (SS) mice, which were selectively bred for differential sensitivity to ethanol as measured by duration of ethanol-induced anesthesia (sleep time), were used in this study. The mice were infused i.v. with saline (control) or one of several doses of nicotine (0.5-4.0 mg kg-1 hr-1) for 10 days. The LS mice were more sensitive to the acute actions of nicotine than were the SS and they developed more tolerance to nicotine. LS mice were tolerant to nicotine as measured by all four of the behavioral and physiological tests used but this tolerance was readily seen only after treatment with the highest infusion dose. The SS mice developed some tolerance to nicotine but this effect was less than that seen in the LS, was restricted to two of the tests (Y-maze crosses and body temperature) and was seen only after treatment with the 4.0-mg kg-1 hr-1 dose of nicotine. The LS mice developed cross-tolerance to ethanol as measured by the Y-maze crosses and the heart rate and body temperature tests. Cross-tolerance to ethanol was not seen in the LS mice for the Y-maze rears and sleep-time tests. Almost no evidence for cross-tolerance to ethanol was seen in nicotine-infused SS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Nicotinic receptor function determined by stimulation of rubidium efflux from mouse brain synaptosomes.

The ability of nicotinic agonists to activate ion channels resulting in Na+ and K+ fluxes has been used to develop a functional assay by using mouse brain synaptosomes. Synaptosomes prepared using Percoll gradients were enriched in binding sites for [3H]nicotine and were capable of accumulating the K+ analog, 86Rb+. The efflux of 86Rb+ from the synaptosomes was subsequently monitored using continuous superfusion at 21 degrees C. Ion flux was stimulated in a concentration-dependent manner by several nicotinic agonists, including L-nicotine, acetylcholine, N-methylcarbamylcholine and dimethylphenylpiperazinium. The process was stereoselective: L-nicotine was 30-fold more potent than D-nicotine. Cytisine stimulated ion flux at low concentrations, but this drug was less efficacious than most other agonists tested. Anabasine was also less efficacious than the other agonists. The EC50 values for agonist-stimulated efflux correlated closely to the IC50 values for inhibition of [3H]nicotine binding, but concentrations required to inhibit binding were lower than those required to stimulate ion flux. Nicotine-induced 86Rb+ efflux was blocked by several nicotinic antagonists including mecamylamine, D-tubocurarine, hexamethonium and decamethonium. Mecamylamine was approximately 50 times as potent as hexamethonium. Neither alpha-bungarotoxin nor atropine were effective antagonists and neuronal-bungarotoxin was a relatively ineffective inhibitor. The amount of nicotine-induced efflux varied among brain regions with midbrain (thalamus and mesencephalon) having the largest response and cerebellum the smallest. The magnitude of the ion flux correlated closely with the amount of [3H] nicotine binding in each brain region. The results indicate that a nicotinic-receptor-mediated ion flux can be measured in brain tissue and that the ion flux may serve as a useful functional assay for nicotinic receptors in the central nervous system. Furthermore, it is postulated that the nicotinic-agonist stimulated ion flux may be mediated by receptors measured by high affinity [3H]nicotine binding.

Acetylcholine↗

PubMed1992

Characterization of nicotinic receptor-mediated [3H]dopamine release from synaptosomes prepared from mouse striatum.

This study establishes that presynaptic nicotinic receptors modulate dopamine release in the mouse striatum. Nicotinic agonists elicit a dose-dependent increase in the release of [3H]dopamine from synaptosomes prepared from mouse striatum. At low concentrations, this release is Ca2+ dependent, whereas at higher concentrations Ca(2+)-independent, mecamylamine-insensitive release was also observed. The Ca(2+)-dependent nicotine-evoked release was not blocked by alpha-bungarotoxin but was effectively blocked by neuronal bungarotoxin as well as several other nicotinic receptor antagonists. The relationship between potency for stimulation of release for agonists and potency for inhibition of release for antagonists was compared to the affinity of these compounds for the [3H]nicotine binding site. The overall correlation between release and binding potency was not high, but the drugs may be classified into separate groups, each of which has a high correlation with binding. This finding suggests either that more than one nicotinic receptor regulates dopamine release or that not all agonists interact with the same receptor in an identical fashion.

Animals↗