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M J Marks

Publications and source records attributed to M J Marks.

At least 19 recordsLinked to original sources
PubMed2001

Nicotinic agonists stimulate acetylcholine release from mouse interpeduncular nucleus: a function mediated by a different nAChR than dopamine release from striatum.

Acetylcholine release stimulated by nicotinic agonists was measured as radioactivity released from perfused synaptosomes prepared from mouse interpeduncular nucleus (IPN) that had been loaded with [(3)H]choline. Agonist-stimulated release was dependent upon external calcium and over 90% of released radioactivity was acetylcholine. The release process was characterized by dose response curves for 13 agonists and inhibition curves for six antagonists. alpha-Conotoxin MII did not inhibit this release, while alpha-conotoxin AuIB inhibited 50% of agonist-stimulated release. Comparison of this process with [(3)H]dopamine release from mouse striatal synaptosomes indicated that different forms of nicotinic acetylcholine receptors (nAChRs) may mediate these processes. This was confirmed by assays using mice homozygous for the beta 2 subunit null mutation. The deletion of the beta 2 subunit had no effect on agonist-stimulated acetylcholine release, but abolished agonist-stimulated release of dopamine from striatal synaptosomes. Mice heterozygous for the beta 2 subunit null mutation showed decreased dopamine release evoked by L-nicotine with no apparent change in EC(50) value, as well as similar decreases in both transient and persistent phases of release with no changes in desensitization rates.

Acetylcholine↗
PubMed2000

UB-165: a novel nicotinic agonist with subtype selectivity implicates the alpha4beta2* subtype in the modulation of dopamine release from rat striatal synaptosomes.

Presynaptic nicotinic acetylcholine receptors (nAChRs) on striatal synaptosomes stimulate dopamine release. Partial inhibition by the alpha3beta2-selective alpha-conotoxin-MII indicates heterogeneity of presynaptic nAChRs on dopamine terminals. We have used this alpha-conotoxin and UB-165, a novel hybrid of epibatidine and anatoxin-a, to address the hypothesis that the alpha-conotoxin-MII-insensitive subtype is composed of alpha4 and beta2 subunits. UB-165 shows intermediate potency, compared with the parent molecules, at alpha4beta2* and alpha3-containing binding sites, and resembles epibatidine in its high discrimination of these sites over alpha7-type and muscle binding sites. (+/-)-Epibatidine, (+/-)-anatoxin-a, and (+/-)-UB-165 stimulated [(3)H]-dopamine release from striatal synaptosomes with EC(50) values of 2.4, 134, and 88 nM, and relative efficacies of 1:0.4:0.2, respectively. alpha-Conotoxin-MII inhibited release evoked by these agonists by 48, 56, and 88%, respectively, suggesting that (+/-)-UB-165 is a very poor agonist at the alpha-conotoxin-MII-insensitive nAChR subtype. In assays of (86)Rb(+) efflux from thalamic synaptosomes, a model of an alpha4beta2* nAChR response, (+/-)-UB-165 was a very weak partial agonist; the low efficacy of (+/-)-UB-165 at alpha4beta2 nAChR was confirmed in Xenopus oocytes expressing various combinations of human nAChR subunits. In contrast, (+/-)-UB-165 and (+/-)-anatoxin-a were similarly efficacious and similarly sensitive to alpha-conotoxin-MII in increasing intracellular Ca(2+) in SH-SY5Y cells, a functional assay for native alpha3-containing nAChR. These data support the involvement of alpha4beta2* nAChR in the presynaptic modulation of striatal dopamine release and illustrate the utility of exploiting a novel partial agonist, together with a selective antagonist, to dissect the functional roles of nAChR subtypes in the brain.

Animals↗
PubMed2000

Pharmacological and null mutation approaches reveal nicotinic receptor diversity.

We have developed an array of assays for nicotinic acetylcholine receptor binding and function. [125I]alpha-Bungarotoxin-, (-)-[3H]nicotine-, and [3H]epibatidine-binding nicotinic acetylcholine receptors were assayed in mouse brain membranes and sections. Nicotinic acetylcholine receptor function was quantified using synaptosomal [3H]dopamine, [3H]gamma-aminobutyric acid ([3H]GABA), and 86Rb(+) efflux techniques. Additionally, the effects of beta2 subunit deletion on each of the measures were assessed. Detailed pharmacological comparison revealed minimally six nicotinic binding subtypes: [125I]alpha-bungarotoxin-binding nicotinic acetylcholine receptors; beta2-subunit-dependent and -independent high-affinity (-)-[3H]nicotine-binding sites; beta2-dependent and -independent cytisine-resistant [3H]epibatidine-binding sites; and a beta2-dependent low-affinity [3H]epibatidine binding site. Comparative pharmacology suggested that [3H]GABA and dihydro-beta-erythroidine (DHbetaE)-sensitive 86Rb(+) efflux are mediated by the same (probably alpha4beta2) nicotinic acetylcholine receptor subtype, while other nicotinic acetylcholine receptor subtypes evoke [3H]dopamine and DHbetaE-resistant 86Rb(+) efflux. In whole-brain preparations, each measure of nicotinic acetylcholine receptor function was beta2 dependent. The majority of beta2-independent [3H]epibatidine binding was located in small, scattered brain nuclei, suggesting that individual nuclei may prove suitable for identification of novel, native nicotinic acetylcholine receptors.

Animals↗
PubMed2000

Nicotinic-agonist stimulated (86)Rb(+) efflux and [(3)H]epibatidine binding of mice differing in beta2 genotype.

Nicotinic acetylcholine receptor function and binding was measured in 12 brain regions from mice differing in beta2 subunit expression. Function was measured by on-line detection of (86)Rb(+) efflux stimulated under conditions that measure two pharmacologically distinct nicotinic responses: (1) stimulation with 10 microM nicotine, a response that is relatively sensitive to inhibition by the antagonist, dihydro-beta-erythroidine (DHbetaE); and (2) stimulation with 10 microM epibatidine in the presence of 2 microM DHbetaE, a response that is relatively resistant to inhibition by DHbetaE. Deletion of the beta2 subunit profoundly reduced both DHbetaE-sensitive and -resistant (86)Rb(+) efflux in each brain region and essentially eliminated activity in regions such as cerebral cortex and thalamus. However, residual activity was observed in regions such as olfactory bulbs and inferior colliculus. [(3)H]Epibatidine binding was measured under conditions that allow estimation of both high- and low-affinity sites. High-affinity sites sensitive to inhibition by the nicotinic agonist, cytisine, were virtually eliminated in every region by the beta2 null mutation. In contrast, only a subset of the high-affinity sites insensitive to inhibition by cytisine were eliminated in beta2 null mutants, suggesting receptor heterogeniety. Similarly, low affinity [(3)H]epibatidine binding was heterogeneous in that a fraction of the sites required the beta2 subunit. Many remaining sites were sensitive to inhibition by alpha-bungarotoxin indicating that a subset of the low affinity [(3)H]epibatidine binding are of the alpha7* subtype. Distinct regional variation was observed among the 12 brain regions. These studies confirm important roles for beta2-containing receptors in mediating pharmacologically distinct functions and as components of several identifiable binding sites.

Animals↗
PubMed2000

Abnormal regulation of high affinity nicotinic receptors in subjects with schizophrenia.

Previous studies have suggested that an abnormality in neuronal nicotinic acetylcholine receptor expression or function may be involved in the neuropathophysiology of schizophrenia. [(3)H]-nicotine and [(3)H]-epibatidine binding were compared in postmortem brain from control and schizophrenic subjects with varying smoking histories. In control subjects, increased receptor binding was seen in hippocampus, cortex, and caudate with increasing tobacco use. In contrast, schizophrenic smokers had reduced nicotinic receptor levels in these brain regions compared to control smokers. Chronic haloperidol and nicotine treatment, in the rat, was used to assess neuroleptic effects on receptor up-regulation by nicotine. A significant increase in cortical nicotinic receptors was seen in both nicotine treated as well as haloperidol and nicotine co-treated animals, suggesting that the abnormal regulation of high affinity neuronal nicotinic receptors in schizophrenics following nicotine use was not related to chronic neuroleptic treatment.

Aconitine↗
PubMed2000

Identification of a novel nicotinic binding site in mouse brain using [(125)I]-epibatidine.

[(125)I]-Epibatidine binds to multiple nicotinic acetylcholine receptor (nAChR) subtypes with high affinity. In this study, [(125)I]-epibatidine was used to label and characterize a novel nAChR subtype found in mouse brain inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates. Binding of [(125)I]-epibatidine was saturable and apparently monophasic in each brain region (K:(D:)=71+/-12 pM mean+/-s.e.mean across regions) but inhibition of [(125)I]-epibatidine binding (200 pM) by A85380, cytisine and (-)-nicotine was biphasic, indicating the presence of multiple binding sites. The sites with lower agonist affinity comprised 30.0+/-2.2, 58.6+/-0.1 and 48.7+/-3.3% of specific [(125)I]-epibatidine (200 pM) binding in inferior colliculus, interpeduncular nucleus, and olfactory bulb homogenates, respectively. The affinity difference between A85380-sensitive and -resistant binding sites was particularly marked (approximately 1000 fold). Thus A85380 was used to differentiate agonist-sensitive and -resistant sites. The pharmacological profiles of the A85380-resistant sites in each region were assessed with inhibition binding experiments, using 14 agonists and five antagonists. The profiles were indistinguishable across regions, implying that A85380-resistant [(125)I]-epibatidine binding sites in inferior colliculus, interpeduncular nucleus, and olfactory bulb represent a single nAChR subtype. The pharmacological profile of the A85380-resistant sites is very different from that previously reported for high affinity (-)-[(3)H]-nicotine-, [(125)I]-alpha-bungarotoxin-, or [(125)I]-alpha-conotoxin MII-binding sites, suggesting that they represent a novel nAChR population in mouse brain.

Alkaloids↗
PubMed2000

Potential role of the alpha4 and alpha6 nicotinic receptor subunits in regulating nicotine-induced seizures.

Several studies have shown that genetic factors influence sensitivity to nicotine-induced seizures in the mouse. We used recombinant inbred (RI) strains derived from the Long-Sleep (LS) and Short-Sleep (SS) mouse lines to assess the possibility that polymorphisms associated with one or more of the nicotinic receptors cosegregate with differential sensitivity to nicotine-induced seizures. Restriction fragment length polymorphisms (RFLPs) associated with the alpha2, alpha3, alpha4, alpha5, and alpha6 nicotinic receptors were identified in the LS and SS mouse lines, but the RI strains were polymorphic for only the alpha4 and alpha6 RFLPs. The RI strains were tested for sensitivity to nicotine-induced seizures. Strain and gender effects on seizure sensitivity were obtained as assessed by ED(50) values and latency to seizure. Those RI strains with the LS-like alpha4 RFLP were, on average, more sensitive to nicotine-induced seizures than were those RI strains with SS-like alpha4 RFLP. The alpha6 nicotine receptor may also play a role in modulating nicotine-induced seizures, but this effect is markedly influenced by gender. Females of the RI strains with the LS-like alpha6 RFLP were more sensitive to nicotine than were females of the strains with the SS-like alpha6 RFLP. Similar trends were seen in the males, but these trends were not significant. Thus, these strain differences may be due to polymorphisms associated with both the alpha4 and alpha6 nicotinic receptors, but gender also plays an important role in regulating sensitivity to nicotine-induced seizure.

Animals↗
PubMed2000

125I-alpha-conotoxin MII identifies a novel nicotinic acetylcholine receptor population in mouse brain.

alpha-Conotoxin MII (CtxMII), a peptide toxin from the venom of the predatory cone snail Conus magus, displays an unusual nicotinic pharmacology. Specific binding of a radioiodinated derivative ((125)I-alpha-CtxMII) was identified in brain region homogenates and tissue sections. Quantitative autoradiography indicated that (125)I-alpha-CtxMII binding sites have an unique pharmacological profile and distribution in mouse brain, being largely confined to the superficial layers of the superior colliculus, nigrostriatal pathway, optic tract, olivary pretectal, and mediolateral and dorsolateral geniculate nuclei. Expression of alpha-CtxMII binding sites in the nigrostriatal pathway, combined with evidence for alpha-CtxMII-sensitivity of nicotine-induced [(3)H]dopamine release in rodent striatal preparations indicates that (125)I-alpha-CtxMII binding nicotinic acetylcholine receptors are likely to be physiologically important. Unlabeled alpha-CtxMII potently (K(i) < 3 nM) competed for a subset of [(3)H]epibatidine binding sites in mouse brain homogenates, but weakly (IC(50) > 10 microM) interacted with (125)I-alpha-bungarotoxin and (-)-[(3)H]nicotine binding sites, confirming this compound's novel nicotinic pharmacology. Quantitative autoradiography revealed that alpha-CtxMII binds with high affinity at a subset of [(3)H]epibatidine binding sites with relatively low cytisine affinity ("cytisine-resistant" sites), resolving [(3)H]epibatidine binding into three different populations, each probably corresponding to a receptor subtype. The majority population seems to correspond to that which binds nicotine and cytisine with high affinity ("cytisine-sensitive" sites). Comparison of the cytisine-resistant population's distribution with that of alpha3 subunit mRNA expression suggests that the fractions both more and less sensitive to alpha-CtxMII probably contain the alpha3 subunit, perhaps in combination with different beta subunits.

Animals↗
PubMed1999

An autoradiographic study of the distribution of binding sites for the novel alpha7-selective nicotinic radioligand [3H]-methyllycaconitine in the mouse brain.

[3H]-Methyllycaconitine ([3H]-MLA) is a new radioligand with selectivity for alpha7-type neuronal nicotinic acetylcholine receptors (nAChRs). In our previous study [Davies, A.R.L., Hardick, D.J., Blagbrough, I.S., Potter, B.V.L., Wolstenholme, A.J. & Wonnacott, S. (1999) Neuropharmacology, 38, 679-690], this radioligand labelled a single class of site in rat brain membranes; its pharmacology and distribution in crudely dissected brain regions closely paralleled that of the well-established alpha7-ligand [125I]-alpha-bungarotoxin. However, a small population of [3H]-MLA binding sites was apparently insensitive to alpha-bungarotoxin. Here we have extended the study to mouse brain, using autoradiography to examine the distribution of [3H]-MLA and [125I]-alpha-bungarotoxin binding sites. [3H]-MLA labelled a single class of site in mouse brain membranes with a KD of 2.2 nM and a Bmax of 45.6 fmol/mg protein. Specific binding, defined by unlabelled MLA (Ki = 0.69 nM), was completely inhibited by (-)-nicotine (Ki = 1.62 microM), whereas alpha-bungarotoxin inhibited only 85% of specific binding (Ki = 3.5 nM). The distributions of [125I]-alpha-bungarotoxin and [3H]-MLA binding sites were compared by autoradiography, and binding was quantitated in 72 brain regions. Binding of both radioligands was highly correlated, with highest densities in the dorsal tegmental nucleus of the pons, colliculi and hippocampus. Serial sections labelled with [3H]-MLA in the absence or presence of unlabelled MLA or alpha-bungarotoxin provided no evidence for any alpha-bungarotoxin-resistant binding. The results are discussed in terms of binding sites that are inaccessible to alpha-bungarotoxin in membrane preparations. This study demonstrates the utility of [3H]-MLA for characterization of alpha7-type nicotinic receptors in mammalian brain, and suggests that it labels a population identical to that defined by [125I]-alpha-bungarotoxin.

Aconitine↗
PubMed1999

Two pharmacologically distinct components of nicotinic receptor-mediated rubidium efflux in mouse brain require the beta2 subunit.

Nicotinic agonist-stimulated efflux of 86Rb+ from mouse brain synaptosomes was monitored continuously by on-line radioactivity detection. The concentration-effect curve following a 5-s stimulation with acetylcholine was biphasic (EC50 = 7.2 and 550 microM). alpha-Bungarotoxin (100 nM) did not inhibit the response, but dihydro-beta-erythroidine (DHbetaE) blocked both phases with differing potency (average IC50 =.22 and 8.9 microM for responses activated by low and high acetylcholine concentrations, respectively). Differential sensitivity DHbetaE inhibition was used to measure stimulation of 86Rb+ efflux by 17 nicotinic agonists, which differed markedly in potency and efficacy. All agonists were more potent at the DHbetaE-sensitive site. Both components were inhibited by the six antagonists tested. Methyllycaconitine and DHbetaE were more potent for the DHbetaE-sensitive component, whereas hexamethonium was more potent at the DHbetaE-resistant component. Both DHbetaE-sensitive and DHbetaE-resistant responses were reduced more than 95% in beta2-null mutant mice, establishing the requirement for the beta2 subunit for both components. Both components were widely, but not identically, distributed throughout the brain. The DHbetaE-sensitive component appears to be identical with agonist-stimulated 86Rb+ efflux described previously and is likely to be mediated by alpha4beta2 receptors. The DHbetaE-resistant component is a novel, active, and widely distributed response mediated by nicotinic receptor(s) that also require the beta2 subunit.

Acetylcholine↗
PubMed1999

Desensitization of nicotinic agonist-induced [3H]gamma-aminobutyric acid release from mouse brain synaptosomes is produced by subactivating concentrations of agonists.

Several neurochemical and electrophysiological studies have shown that neuronal nicotinic receptors are desensitized by pretreatment with lower agonist concentrations than are required to activate the receptors, but the extent of desensitization and agonist concentration required to produce desensitization vary depending upon receptor subtype. Recently, we reported that nicotinic agonists will stimulate the release of [3H]gamma-aminobutyric acid (GABA) from synaptosomes prepared from mouse brain. The studies described herein evaluated desensitization of [3H]GABA release produced by pretreatment with 12 nicotinic agonists. Pretreatment produced near total desensitization that developed slowly (onset T(1/2) = 3.46 min) and was totally reversible (recovery T(1/2) = 4.95 min). Nine of the 12 compounds tested induced total or near total desensitization at concentrations that were less than those required to produce a reliably measured increase in [3H]GABA release. Nicotine produced total block with an IC(50) value of 26 nM. This value is two orders of magnitude lower than the EC(50) for nicotine-induced [3H]GABA release (1630 nM). The three compounds that showed an overlap of the desensitization and activation concentration-effect curves (cytisine, anabasine, nornicotine) are all partial agonists. Comparison of the desensitization properties of the [3H]GABA release with an ion ((86)Rb+) efflux that we have measured previously suggests that the receptor that mediates GABA release and (86)Rb(+) efflux is the same, most likely the alpha4beta2 subtype.

Animals↗
PubMed1998

Differential agonist inhibition identifies multiple epibatidine binding sites in mouse brain.

The binding of [3H]epibatidine, an alkaloid isolated from the skin of an Ecuadorean tree frog, was measured both in brain regions dissected from mouse brain and in tissue sections. Binding to each of 12 brain areas was saturable, but apparently monophasic; no indication of multiple binding sites was obtained. However, inhibition of epibatidine binding by nicotine, acetylcholine, methylcarbachol and cytisine in olfactory bulbs revealed a biphasic pattern consistent with the presence of two sites differentially sensitive to inhibition by these nicotinic agonists. Cytisine displayed the greatest difference in inhibitory potency between the two apparent sites. Subsequent analysis of the inhibition of epibatidine binding by cytisine in membranes prepared from 12 brain areas also suggested the presence of two sites in each brain region. The estimated potency of cytisine at each site was similar in each brain region. However, the proportion of [3H]epibatidine binding sites that were more sensitive to inhibition by cytisine and those sites less sensitive to inhibition by this agonist varied markedly among the brain regions. Quantitative autoradiographic analyses of mouse brain revealed pattern of [3H]epibatidine binding sites less sensitive to inhibition by cytisine that differed markedly from the pattern obtained with [3H]nicotine. Among brain regions demonstrating substantial sites less sensitive to cytisine inhibition were the accessory olfactory nucleus, medial habenula, interpeduncular nucleus, fasciculus retroflexus, superior colliculus, inferior colliculus and the pineal gland. The results indicate that epibatidine binds to at least two distinct nicotinic sites in mouse brain that may represent different nicotinic receptor subtypes, one of which appears to be identical to that measured by the binding of other agonists such as nicotine or cytisine.

Animals↗
PubMed1998

Pharmacological characterization of nicotinic receptor-stimulated GABA release from mouse brain synaptosomes.

Several recent electrophysiological studies have demonstrated that nicotinic agonists stimulate the release of gamma-aminobutyric acid (GABA) from rodent brain tissue. Our studies used a neurochemical approach to characterize nicotinic receptor-stimulated [3H]-GABA release from mouse brain synaptosomes. Nicotine increased [3H]-GABA release from synaptosomes preloaded with [3H]-GABA in a concentration-dependent manner. This release appeared rapidly, was Ca++ dependent, and was partially (about 50%) blocked by 100 nM tetrodotoxin and totally blocked by mecamylamine and dihydro-beta-erythroidine. alpha-Bungarotoxin had no effect. Twelve nicotinic agonists were compared for their effects on [3H]-GABA release. The agonists differed in potency (EC50) and efficacy (Emax). The EC50 and Emax values were significantly correlated (r = 0.95, P <.001 for EC50; r = 0.93, P <.01 for Emax) to values obtained for these same agonists when 86Rb+ efflux was determined. A significant correlation (r = 0.84, P <.01) was found when the EC50 values for agonist-stimulated [3H]-GABA release and IC50 values for agonist inhibition of [3H]-L-nicotine binding were compared. Differences in [3H]-GABA release were detected in 12 brain regions and maximal release was significantly correlated with [3H]-nicotine binding. The pharmacological and regional comparisons suggest that the nAChR that stimulates [3H]-GABA release is the one that binds [3H]-nicotine with high affinity (alpha4beta2). Unequivocal evidence that the receptor that modulates nicotine-stimulated [3H]-GABA release contains a beta2 subunit was obtained in a study using wild-type, heterozygous and homozygous beta2 null mutant mice. [3H]-GABA release and [3H]-nicotine binding decreased along with the number of copies of the null mutant gene.

Animals↗
PubMed1997

Neurosteroids modulate nicotinic receptor function in mouse striatal and thalamic synaptosomes.

Progesterone and its A-ring reduced metabolites are allosteric activators of GABA(A) receptors. The studies reported here examined the effects of these steroids on brain nicotinic receptors using an 86Rb+ efflux assay that likely measures the function of alpha4beta2-type nicotinic receptors and [3H]dopamine release, which may be modulated by an alpha3-containing nicotinic receptor. Both of the A-ring reduced metabolites of progesterone were noncompetitive inhibitors of both assays, whereas progesterone inhibited only the 86Rb+ efflux assay. The 86Rb+ efflux assay was slightly more sensitive than was the dopamine release assay to steroid inhibition. Inhibition developed slowly for both assays (t1/2 = 0.4 min) and was reversed even more slowly (t1/2 = 10-15 min). Steroid addition did not alter either the rate of association of [3H]nicotine binding to brain membranes, nor was equilibrium binding changed. These findings argue that neurosteroids are allosteric inhibitors of brain nicotinic receptors.

5-alpha-Dihydroprogesterone↗
PubMed1997

Effect of smoking history on [3H]nicotine binding in human postmortem brain.

Chronic nicotine administration in animal models evokes a dose-dependent increase in brain nicotinic receptor numbers. Genetically determined variability in nicotinic receptor number in different mouse strains has also been reported, which is thought to affect sensitivity to nicotine, as well as the development of tolerance. Humans self-administer nicotine principally in the form of cigarettes and other tobacco products. The present study compared [3H]nicotine binding in human postmortem brain from thalamus and hippocampus of nonsmoking subjects, subjects who had variable life-long smoking histories and subjects who had quit smoking. A significant increase was seen in [3H]nicotine binding in both hippocampus and thalamus of subjects with life-long smoking histories. In the hippocampus, this change resulted from a change in total receptor number (Bmax), with no change in receptor affinity (Kd). There was also a positive correlation between the degree of smoking, as measured by the average reported packs smoked per day, and the number of nicotine binding sites found in both the hippocampus and thalamus, showing that humans exhibit a dose-dependent increase in brain nicotinic receptor binding. Receptor levels in these brain regions after smoking cessation were at or below those found in the control population, which indicated that smoking-induced changes are reversible after cessation of nicotine treatment. These results suggest that increases in nicotinic receptor levels in the human brain may underlie nicotine tolerance and addiction in smokers.

Adult↗
PubMed1997

Pharmacological comparison of transient and persistent [3H]dopamine release from mouse striatal synaptosomes and response to chronic L-nicotine treatment.

L-Nicotine stimulates a biphasic release of [3H]dopamine from mouse striatal synaptosomes which does not persist after agonist is removed. Approximately 80% of the initial release is transient and disappears with a half-time of less than 1 min; the other 20% persists for several minutes (t(1/2), 5-10 min). Both the transient and persistent phases were investigated by 10-min exposures to agonists with an in vitro perfusion technique. A series of nicotinic agonists and antagonists were used to determine the pharmacological relationship of the two phases. Parameters measured included EC50 and Vmax values and desensitization rates for both phases for agonists, Ki values for antagonists and Ki values for low concentrations of agonists. The results are consistent with both phases being mediated by a single type of receptor. In addition, the effects of chronic nicotine treatment on transient and persistent [3H]DA release were measured. For both phases, release was decreased approximately 15% by chronic infusion of 4.0 mg/kg/hr L-nicotine. Correlation of the results with inactivation of a portion of the receptors rather than a reversible desensitization is discussed.

Animals↗
PubMed1997

Pharmacological investigation of (+)- and (-)-cis-2,3,3a,4,5,9b-hexahydro-1-methyl-1H-pyrrolo-[3,2-h]isoq uinoline, a bridged-nicotine analog.

We recently synthesized a bridged-nicotine (BN) analog and its enantiomers. They failed to compete for [3H]nicotine binding in rat brain homogenates, yet they produced nicotine-like effects by decreasing locomotor activity and producing antinociception in the tail-flick, hot-plate and PPQ tests in mice. Therefore, additional in vivo and in vitro studies were undertaken to determine whether these compounds are indeed acting independently of the nicotinic system. Although these analogs did not produce nicotine-like responding when evaluated in rat drug discrimination, the racemate augmented the cue when administered in conjunction with nicotine. Moreover, the antinociceptive measured in the different tests and hypothermic effects of (+)-BN, the more potent enantiomer, were not blocked by the nicotinic antagonists mecamylamine and dihydro-beta-erythroidine. Acute tolerance developed to (+)-BN-induced antinociception but not to hypothermia after subcutaneous administration. In addition, no cross-tolerance was observed between (+)-BN and nicotine in the different tests. The absence of generalization in the discrimination test suggests that the BN analogs do not possess nicotine-like activity. In addition, the failure of mecamylamine and dihydro-beta-erythroidine to antagonize the antinociceptive and hypothermic effects of (+)-BN, on one hand, and the inability of the bridge analogs to stimulate 86Rb+ efflux in brain synaptosomes, on the other hand, provide further evidence that BN analog agonist effects are not mediated by the alpha-4, beta-2 receptor subunit combination. It is unlikely that alpha-7 subunits mediate the agonists effects of BN analogs because their affinity to neuronal [125I]alpha-bungarotoxin binding sites is in the higher micromolar range. Other nicotinic receptor subtypes remain possible candidates because (+/-)-BN augments the generalization of nicotine in drug discrimination and produces some nicotine-like pharmacological effects. BN analogs could represent a novel class of nicotinic analgesics because naloxone and atropine failed to alter the antinociceptive effects of (+)-BN. Alternatively, their actions may be entirely independent of the nicotinic system.

Analgesics↗