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At least 469 records · Page 26Linked to original sources
PubMed1994

Comparison of neuronal nicotinic receptors in rat sympathetic neurones with subunit pairs expressed in Xenopus oocytes.

1. The agonist sensitivity of nicotinic acetylcholine receptors in rat superior cervical ganglion (SCG) neurones was compared with that of cloned receptors expressed in Xenopus oocytes by pairwise injections of alpha 3-beta 2 or alpha 3-beta 4 neuronal nicotinic subunit combinations. 2. Agonist responses in rat SCG neurones indicated that cytisine was the most potent agonist and lobeline the least potent (rank order of potency: cytisine > dimethylphenylpiperazinium iodide (DMPP) > nicotine > ACh > carbachol > lobeline). 3. Receptors expressed in oocytes by injection of alpha 3 and beta 2 subunits had a relatively high sensitivity to DMPP and low sensitivity to cytisine (rank order of potency: DMPP > ACh > lobeline > carbachol > nicotine > cytisine), whereas receptors composed of alpha 3 and beta 4 subunits had a high sensitivity to cytisine and low sensitivity to DMPP (rank-order of potency: cytisine > nicotine approximately ACh > DMPP > carbachol > lobeline). 4. With the exception of responses to DMPP, agonist sensitivity measurements suggest that nicotinic receptors in the rat SCG are composed of alpha 3 and beta 4 subunits. The results are discussed in terms of the receptor subunit mRNAs known to be expressed in the rat SCG and previous evidence of functional heterogeneity of rat SCG nicotinic acetylcholine receptors.

Alkaloids↗
PubMed1995

Heterogeneity of neuronal nicotinic acetylcholine receptors in thin slices of rat medial habenula.

1. Neuronal nicotinic acetylcholine receptors in slices of rat medial habenula were studied using patch clamp recording techniques. 2. Whole cell current responses to cytisine could be blocked by hexamethonium, as expected for nicotinic receptors. The whole cell current-voltage relations were linear at negative membrane potentials, but showed strong inward rectification when chloride currents were minimized. 3. When 1 mM Ca2+ (0 mM Mg2+) was present in the external recording solution, the single channel conductances elicited by acetylcholine or nicotine in twenty patches were in the range 39-58 pS, with a mean of 47 pS. There appeared to be at least two groups of conductances. 4. In the open point amplitude distributions of three patches, the most common amplitude corresponded to 41 pS (81% of the area). In another four patches the most common amplitude corresponded to a mean conductance of 51 pS (83% of the area). Direct transitions between open levels were rare. 5. Channel closed times were not significantly different for the two conductance groups. However, for the four patches with predominantly 51 pS openings, the means of the distributions of open times longer than two filter rise times averaged 5.8 ms. Those patches with predominantly 41 pS openings averaged 14 ms. Also, for patches with predominantly 51 pS openings the overall mean burst length was 5.8 ms, whereas for patches with predominantly 41 pS openings it was 16.1 ms. 6. These observations suggest that 51 and 41 pS openings result from the activity of at least two, but possibly more, different receptor subtypes. We conclude that nicotinic receptors in the rat ventral medial habenula are heterogeneous.

Acetylcholine↗
PubMed1996

Nicotinic actions on neurones of the central autonomic area in rat spinal cord slices.

1. Nicotinic responses and actions on excitatory synaptic activity were studied in eighty-four neurones in the region dorsal to the central canal (lamina X) in transverse thoracolumbar spinal cord slices of neonate (P2-P10) rats by using the whole-cell patch-clamp technique. 2. Neurones (n = 15) labelled with Lucifer Yellow, showed the typical morphology of sympathetic preganglionic neurones (SPNs) in the central autonomic area (CA). Unlabelled neurones of comparable morphology were visually identified and recorded. 3. All neurones recorded responded to the nicotinic acetylcholine receptor (nAChR) agonist, DMPP. Under current-clamp conditions, pressure ejections of DMPP depolarized cells and induced the discharge of action potentials. Tetrodotoxin suppressed action potentials but not DMPP-induced depolarization. 4. Under voltage-clamp conditions at a holding potential (Vh) of -50 mV, DMPP induced a transient inward current (which reversed around 0 mV) and an increase in membrane current noise in 50% of the recorded neurones. In the others, DMPP increased membrane current noise without measurable inward current. The current-voltage relationship showed strong inward rectification at holding potentials more positive than 0 mV. 5. In neurones displaying a detectable current response to DMPP, the following agonist rank order potency could be established: DMPP = nicotine > cytisine > ACh. The DMPP response could be blocked by mecamylamine but was insensitive to methyllycaconite. 6. Pressure application of glutamate induced inward currents in all cells tested at a Vh of -50 mV. This response reversed at 10 mV, displayed a region of negative slope conductance at Vh more negative than -30 mV and was partially blocked by CNQX. Pressure application of DMPP transiently increased the amplitude of the glutamate-induced current in six out of nine cells tested. This potentiation persisted in the presence of tetrodotoxin. 7. Forty per cent of the recorded neurones displayed spontaneous excitatory postsynaptic currents (sEPSCs). At a Vh of -50 mV the sEPSCs had a mean amplitude of -19.3 pA and occurred at a frequency below 0.5 Hz. sEPSCs were blocked by CNQX and inverted around 0 mV. Brief application of DMPP increased the discharge frequency of sEPSCs without affecting their kinetics. Additionally, in some cells DMPP increased mean sEPSC amplitude. 8. Focal electrically evoked EPSCs reversed close to 10 mV and were sensitive to CNQX. They occurred with a constant latency, rise time and a mono-exponential decay time. Application of DMPP decreased the percentage of stimulation failures and increased the amplitude of evoked EPSCs, in all cells tested. 9. It is concluded that neurones in the CA, presumed to be SPNs, have functional nAChRs with activation having two distinct effects: firstly, a direct depolarization of the postsynaptic membrane; and secondly, a facilitation of the excitatory transmission onto these cells. This second effect is achieved by an increase of the size of the glutamate-induced current at the postsynaptic level as well as by an enhancement of the presynaptic release of glutamate.

Acetylcholine↗
PubMed1997

Recombinant nicotinic receptors, expressed in Xenopus oocytes, do not resemble native rat sympathetic ganglion receptors in single-channel behaviour.

1. In order to establish the subunit composition of neuronal nicotinic receptors in rat superior cervical ganglia (SCG), their single-channel properties were compared with those of recombinant receptors expressed in Xenopus oocytes, using outside-out excised patch recording. 2. The mean main conductance of SCG channels from adult and 1-day-old rats was 34.8 and 36.6 pS, respectively. Less frequent openings to lower conductances occurred both as isolated bursts and as events connected to the main level by direct transitions. There was considerable interpatch variability in the values of the lower conductances. 3. Nicotinic receptors from oocytes expressing alpha3beta4 and alpha4beta4 subunits had chord conductances lower than that of SCG neurones (22 pS for alpha3beta4 and 29 pS for alpha4beta4). 4. Prolonged recording from both native and recombinant channels was precluded by 'run-down', i.e. channel activity could be elicited for only a few minutes after excision. Nevertheless, SCG channel openings were clearly seen to occur as short bursts (slowest component, 38 ms), whereas recombinant channels opened in very prolonged bursts of activity, the major component being the slowest (480 ms). 5. Addition of the alpha5 subunit to the alpha3beta4 pair produced channels with a higher conductance than those observed after injection of the pair alone (24.9 vs. 22 pS), suggesting incorporation of alpha5 into the channel. Addition of the beta2 subunit did not change alpha3beta4 single-channel properties. In one out of fourteen alpha3alpha5beta4 patches, both ganglion-like, high conductance, short burst openings and recombinant-type, low conductance, slow burst openings were observed. 6. Channels produced by expression in Xenopus oocytes of neuronal nicotinic subunits present in rat SCG as a rule differ from native ganglion receptors in single-channel conductance and gross kinetics. While it is possible that an essential nicotinic subunit remains to be cloned, it is perhaps more likely that oocytes either cannot assemble neuronal nicotinic subunits efficiently into channels with the correct composition and stoichiometry, or that they produce post-translational channel modifications which differ from those of mammalian neurones.

Acetylcholine↗
PubMed2002

Autonomic function in mice lacking alpha5 neuronal nicotinic acetylcholine receptor subunit.

Neuronal acetylcholine nicotinic receptors (nAChR) are composed of 12 subunits (alpha2-10, beta2-4), of which alpha3, alpha5, alpha7, beta2 and beta4 subunits are known to exist in the autonomic nervous system (ANS). alpha5 subunits possess unique biophysical and pharmacological properties. The present study was undertaken to examine the functional role and pharmacological properties of the nAChR alpha5 subunits in the ANS using mice lacking alpha5 nAChR subunits (alpha5-/-). These mice grew to normal size showing no obvious physical or neurological deficit. They also showed normality in thermoregulation, pupil size and resting heart rate under physiological conditions. The heart rate and rectal temperature did not differ between alpha5-/- and wild-type mice during exposure to cold stress. An impairment of cardiac parasympathetic ganglionic transmission was observed during high frequency vagal stimulation, which caused cardiac arrest in all wild-type animals while alpha5-/- mice were more resistant. Deficiency of alpha5 subunits strikingly increased the sensitivity to a low concentration of hexamethonium, leading to a nearly complete blockade of bradycardia in response to vagal stimulation. Such a concentration of hexamethonium only slightly depressed the effects of vagal stimulation in control mice. Deficiency of alpha5 subunits significantly increased ileal contractile responses to cytisine and epibatidine. These results suggest that alpha5 subunits may affect the affinity and sensitivity of agonists and antagonists in the native receptors. Previous studies revealed that alpha5 subunits form functional receptors only in combination with other alpha and beta subunits. Thus, the data presented here imply that alpha5 subunits modulate the activity of nAChR in autonomic ganglia in vivo.

Alkaloids↗
PubMed2006

Roles of nicotinic acetylcholine receptor beta subunits in function of human alpha4-containing nicotinic receptors.

Naturally expressed nicotinic acetylcholine receptors (nAChR) containing alpha4 subunits (alpha4*-nAChR) in combination with beta2 subunits (alpha4beta2-nAChR) are among the most abundant, high-affinity nicotine binding sites in the mammalian brain. beta4 subunits are also richly expressed and colocalize with alpha4 subunits in several brain regions implicated in behavioural responses to nicotine and nicotine dependence. Thus, alpha4beta4-nAChR also may exist and play important functional roles. In this study, properties were determined of human alpha4beta2- and alpha4beta4-nAChR heterologously expressed de novo in human SH-EP1 epithelial cells. Whole-cell currents mediated via human alpha4beta4-nAChR have approximately 4-fold higher amplitude than those mediated via human alpha4beta2-nAChR and exhibit much slower acute desensitization and functional rundown. Nicotinic agonists induce peak whole-cell current responses typically with higher functional potency at alpha4beta4-nAChR than at alpha4beta2-nAChR. Cytisine and lobeline serve as full agonists at alpha4beta4-nAChR but are only partial agonists at alpha4beta2-nAChR. However, nicotinic antagonists, except hexamethonium, have comparable affinities for functional alpha4beta2- and alpha4beta4-nAChR. Whole-cell current responses show stronger inward rectification for alpha4beta2-nAChR than for alpha4beta4-nAChR at a positive holding potential. Collectively, these findings demonstrate that human nAChR beta2 or beta4 subunits can combine with alpha4 subunits to generate two forms of alpha4*-nAChR with distinctive physiological and pharmacological features. Diversity in alpha4*-nAChR is of potential relevance to nervous system function, disease, and nicotine dependence.

Acetylcholine↗
PubMed2002

Characterization of epibatidine binding to medial habenula: potential role in analgesia.

The objective of the present study was to characterize a recently described binding site in the habenula, which has high affinity for [(3)H]epibatidine and low affinity for nicotine and acetylcholine. We report that the extension of this binding area in coronal and horizontal sections corresponds to the anatomical extension of the medial habenula. The affinity (K(D)) of the medial habenula receptors for [(3)H]epibatidine was estimated to be 0.5 nM using an autoradiographic saturation assay, whereas the affinity of the binding site for nicotine and acetylcholine was estimated to be 5 and 8 microM, respectively. The receptor density (B(max)) in the medial habenula was estimated to be about 1100 fmol/mg wet weight using [(3)H]epibatidine. The subunit composition of the "epibatidine receptor" was investigated by the ability of different compounds with affinity to various subtypes of nicotinic receptors to displace [(3)H]epibatidine bound to the receptor. The results suggest that the receptor contains alpha3 subunits but that it is unlikely to be an alpha3beta4 nicotinic receptor. Systemic administration of epibatidine has analgesic effects in rats. Here we report that 2 x 1 microl of 10 nM epibatidine, resulting in a 2 x 10-fmol dose, administered directly to the medial habenula by bilateral stereotactic injection had an analgesic effect measured in the hot-plate test. This dose of epibatidine increased hot-plate latency significantly, whereas 2 x 2 fmol of epibatidine or 2 x 10 fmol of nicotine were without effect. This leads us to suggest that the medial habenular epibatidine binding site might be a valuable target for the development of non-opiate analgesics.

Alkaloids↗
PubMed2004

Subunit composition of nicotinic receptors in monkey striatum: effect of treatments with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or L-DOPA.

Nicotinic acetylcholine receptors (nAChRs) represent an important modulator of striatal function both under normal conditions and in pathological states such as Parkinson's disease. Because different nAChR subtypes may have unique functions, immunoprecipitation and ligand binding studies were done to identify their subunit composition. As in the rodent, alpha2, alpha4, alpha6, beta2, and beta3 nAChR subunit immunoreactivity was identified in monkey striatum. However, distinct from the rodent, the present results also revealed the novel presence of alpha3 nAChR subunit-immunoreactivity in this same region, but not that for alpha5 and beta4. Relatively high levels of alpha2 and alpha3 subunits were also identified in monkey cortex, in addition to alpha4 and beta2. Experiments were next done to determine whether striatal subunit expression was changed with nigrostriatal damage. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment decreased alpha6 and beta3 subunit immunoreactivity by approximately 80% in parallel with the dopamine transporter, suggesting that they are predominantly expressed on nigrostriatal dopaminergic projections. In contrast, alpha3, alpha4, and beta2 subunit immunoreactivity was decreased approximately 50%, whereas alpha2 was not changed. These data, together with those from dual immunoprecipitation and radioligand binding studies ([(3)H]cytisine, (125)I-alpha-bungarotoxin, and (125)I-alpha-conotoxin MII) suggest the following: that alpha6beta2beta3, alpha6alpha4beta2beta3, and alpha3beta2* nAChR subtypes are present on dopaminergic terminals and that the alpha4beta2 subtype is localized on both dopaminergic and nondopaminergic neurons, whereas alpha2beta2* and alpha7 receptors are localized on nondopaminergic cells in monkey striatum. Overall, these results suggest that drugs targeting non-alpha7 nicotinic receptors may be useful in the treatment of disorders characterized by nigrostriatal dopaminergic damage, such as Parkinson's disease.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine↗
PubMed2005

Mouse beta-TC6 insulinoma cells: high expression of functional alpha3beta4 nicotinic receptors mediating membrane potential, intracellular calcium, and insulin release.

Nicotine elicited membrane depolarization, elevation of intracellular calcium, rubidium efflux, and release of insulin from mouse beta-TC6 insulinoma cells. Such responses were blocked by the nicotinic antagonist mecamylamine but not by the muscarinic antagonist atropine. Neither the selective alpha4beta2 antagonist dihydro-beta-erythroidine nor the selective alpha7 antagonist methyllycaconitine significantly blocked the nicotine-elicited depolarization or the calcium response. The elevation of intracellular calcium did not occur in calcium-free media, indicating that the increase in intracellular calcium was due to the influx of calcium. The rank order of potency for nicotinic agonists was as follows: epibatidine > nicotine = 3-(azetidinylmethoxy)pyridine (A-85380), cytisine, dimethylphenylpiperazinium (DMPP). Cytisine and DMPP seemed to be partial agonists. The density of nicotinic receptors measured by [3H]epibatidine binding was 7-fold higher in membranes from beta-TC6 cells than in rat brain membranes. No binding of 125I-A-85380 was detected, indicating the absence of beta2-containing receptors. Reverse transcription-polymerase chain reaction analyses indicated the presence of mRNA for alpha3 and alpha4 subunits and beta2 and beta4 subunits in beta-TC6 cells. The binding and functional data suggest that the major nicotinic receptor is composed of alpha3 and beta4 subunits. The beta-TC6 cells thus provide a model system for pharmacological study of such nicotinic receptors.

Alkaloids↗
PubMed2006

alpha4beta2 nicotinic receptors with high and low acetylcholine sensitivity: pharmacology, stoichiometry, and sensitivity to long-term exposure to nicotine.

alpha4 and beta2 nicotinic acetylcholine receptor (nAChR) subunits expressed heterologously assemble into receptors with high (HS) and low (LS) sensitivity to acetylcholine (ACh); their relative proportions depend on the alpha4to beta2 ratio. In this study, injection of oocytes with 1:10 alpha4/beta2 subunit cDNA ratios favored expression of HS alpha4beta2 nAChRs, as evidenced by monophasic ACh concentration-response curves, whereas injections with 10:1 cDNA ratios favored expression of LS alpha4beta2 receptors. The stoichiometry was inferred from the shifts in the ACh EC(50) values caused by Leu to Thr mutations at position 9' of the second transmembrane domain of alpha4 and beta2. The 1:10 injection ratio produced the (alpha4)(2)(beta2)(3) stoichiometry, whereas 10:1 injections produced the (alpha4)(3)(beta2)(2) stoichiometry. The agonists epibatidine, 3-[2(S)-azetidinylmethoxy]pyridine (A-85380), 5-ethoxy-metanicotine (TC-2559), cytisine, and 3-Br-cytisine and the antagonists dihydro-beta-erythroidine and d-tubocurarine were more potent at HS receptors. TC-2559 was more efficacious than ACh at HS receptors but was a partial agonist at LS receptors. Epibatidine was more efficacious than ACh at LS receptors and a partial agonist at HS receptors. Cytisine and 5-halogenated cytisines had moderate efficacy at LS receptors but had almost no efficacy at HS receptors. By exploiting the differential effects of ACh, TC-2559 and 5-I-cytisine we evaluated the effects of long-term exposure to nicotine on HS and LS receptors expressed in Xenopus laevis oocytes after cDNA injections or microtransplantation of alpha4beta2 receptors assembled in human embryonic kidney 293 cells. We conclude that nicotine up-regulates HS alpha4beta2 receptors, probably by influencing the assembly of receptors rather than by altering the functional state of LS alpha4beta2 nAChRs.

Acetylcholine↗
PubMed2006

Characterization of nicotinic acetylcholine receptors that modulate nicotine-evoked [3H]norepinephrine release from mouse hippocampal synaptosomes.

Nicotine's modulation of hippocampal noradrenergic neurotransmission may contribute to its mnemonic properties, but the nicotinic acetylcholine receptor (nAChR) subtypes that modulate terminal release of norepinephrine are unknown. In the present study, we used a number of subtype-selective alpha-conotoxins in combination with nicotinic receptor subunit-deficient mice to characterize nAChRs that modulate [3H]nore-pinephrine release from synaptosomes. The results indicate that at least two populations of nAChRs contribute to this release: a novel alpha6(alpha4)beta2beta3beta4 subtype and an alpha6(alpha4)beta2beta3 subtype. These are distinct from subtypes that modulate synaptosomal norepinephrine release in the rat hippocampus in which an alpha6/beta2 and/or alpha6/beta4 ligand binding interface is not present. Whereas alpha-conotoxin MII fully inhibits nicotine-evoked [3H]norepinephrine release in mouse, it is ineffective in blocking [3H]norepinephrine release in rat. Block of [3H]norepinephrine release by alpha-conotoxin BuIA, a toxin that kinetically distinguishes between beta2- and beta4-containing nAChRs, was partially reversible in mouse but irreversible in rat. This indicates that in contrast to rat, mouse nAChRs are made of both beta4 and non-beta4-containing populations. Results from beta2 and beta4 null mutant mice confirmed this conclusion, indicating the presence of the beta2 subunit in all nAChRs and the presence of the beta4 subunit in a subpopulation of nAChRs. In addition, both alpha4 and beta3 subunits are essential for the formation of functional nAChRs on mouse noradrenergic terminals. Cytisine, a ligand formerly believed to be beta4-selective, was a highly effective agonist for alpha6beta2-containing nAChRs. The sum of these results suggests a possible novel nAChR subtype that modulates nor-adrenergic neurotransmission within the mouse hippocampus.

Alkaloids↗
PubMed2006

Deletion of the alpha7, beta2, or beta4 nicotinic receptor subunit genes identifies highly expressed subtypes with relatively low affinity for [3H]epibatidine.

Diversity of neuronal nicotinic acetylcholine receptor binding was measured using [3H]epibatidine after deletion of alpha7, beta2, or beta4 subunits. [3H]Epibatidine binding is distinctly biphasic. Densities of higher (Kd approximately 0.02 nM) and lower (Kd approximately 5 nM) affinity sites in whole brains of wild-type mice are very similar. Relative sensitivity to inhibition by cytisine or alpha-bungarotoxin was used to evaluate pharmacological subsets of the higher- and lower-affinity sites, respectively. Deletion of each subunit had distinct effects on the binding sites. Deletion of alpha7 did not affect higher-affinity sites but reduced the numbers of lower-affinity sites. This reduction was confined to the [3H]epibatidine binding sites sensitive to inhibition by alpha-bungarotoxin. Deletion of the beta2 subunit had the largest effect. Higher-affinity sites sensitive to inhibition by cytisine were eliminated, and cytisine-resistant sites were reduced. Deletion of the beta2 subunit also significantly reduced the number of lower-affinity sites insensitive to alpha-bungarotoxin. beta4 Gene deletion partially reduced cytisine-resistant and alpha-bungarotoxin-resistant sites with lower and higher affinity for [3H]epibatidine, respectively. Gene deletion in four brain regions (thalamus, hippocampus, superior colliculus, and inferior colliculus) elicited changes generally similar to whole brain. However, relative expression of the binding sites differed among the regions. [3H]Cytisine and 125I-alpha-bungarotoxin binding sites were eliminated by beta2 and alpha7 gene deletion, respectively. These studies establish that the lower-affinity sites represent a structurally diverse set of sites that require expression of either alpha7, beta2, or beta4 subunits and extend and confirm previous classifications of the higher-affinity [3H]epibatidine binding sites.

Alkaloids↗
PubMed2002

Protons enhance the gating kinetics of the alpha3/beta4 neuronal nicotinic acetylcholine receptor by increasing its apparent affinity to agonists.

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely distributed in the nervous system. Although there is a vast literature on the molecular, structural and pharmacological properties of neuronal nAChR, little is known of their pH regulation. Here we report that rapid acidification (pH 6.0) enhances the current through the alpha3/beta4 recombinant nAChRs expressed stably in human embryonic kidney 293 cells and accelerates its activation kinetics without altering selectivity. Acidification also strongly accelerates the decay kinetics ("desensitization") of cytisine- and nicotine-evoked currents (pK(a) approximately 6.1), but the effect is somewhat smaller with acetylcholine and carbachol (undetermined pK(a) values), suggesting that protonation of the agonist contributes to the relaxation of the current. Transient increases of [H(+)](o) from pH 7.4 to 6.0, during the time course of decay of the current, enhances the current and accelerates its decay kinetics in a manner similar to reactivation of current by higher concentrations of agonists. We suggest that protons interact with multiple extracellular sites on alpha3/beta4 nAChRs, decreasing the effective EC(50) values of the agonist and accelerating gating kinetics, in part by promoting agonist-induced block. We speculate that corelease of protons with ACh from the secretory vesicles may induce rapid and reversible conformational changes in the slowly "desensitizing" alpha3/beta4 nAChRs, leading to accelerated signaling.

Alkaloids↗
PubMed2002

First and second transmembrane segments of alpha3, alpha4, beta2, and beta4 nicotinic acetylcholine receptor subunits influence the efficacy and potency of nicotine.

The first three transmembrane segments (M1-M3) of human nicotinic acetylcholine receptors (nAChRs) have been implicated in determining the efficacy of nicotine by studies of alpha3/alpha4 subunit chimeras. Nicotine has full efficacy on the alpha4beta2 nAChR and partial efficacy on the alpha3beta2 nAChR. Now, we have exchanged individually three amino acids between the alpha4 and the alpha3 subunits at positions 226(M1), 258(M2), and 262(M2). Also, similar exchanges were made in the beta2 and beta4 subunits at positions 224(M1), 226(M1), and 254(M2) (using alpha subunit numbering). Expression of these mutated nAChRs in Xenopus laevis oocytes showed that the mutated M1 amino acids were important in influencing the potency of ACh and nicotine. It is hypothesized that these M1 amino acids affect the stability between the resting and activated states of the nAChR. M2 amino acids altered the efficacy of nicotine, usually without altering its potency. When the residue located at position 258 in the M2 region of the alpha subunit was valine (as in the alpha3 subunit), the resulting nAChR exhibited partial efficacy for nicotine that was voltage-dependent. Therefore, we believe that these M2 amino acids contribute to the formation of a binding site for nicotine in the alpha3beta2 nAChR channel, which results in a low-affinity channel block, causing the lower efficacy of nicotine on this nAChR.

Alkaloids↗
PubMed2004

Nicotine activation of alpha4* receptors: sufficient for reward, tolerance, and sensitization.

The identity of nicotinic receptor subtypes sufficient to elicit both the acute and chronic effects of nicotine dependence is unknown. We engineered mutant mice with a4 nicotinic subunits containing a single point mutation, Leu9' --> Ala9' in the pore-forming M2 domain, rendering a4* receptors hypersensitive to nicotine. Selective activation of a4* nicotinic acetylcholine receptors with low doses of agonist recapitulates nicotine effects thought to be important in dependence, including reinforcement in response to acute nicotine administration, as well as tolerance and sensitization elicited by chronic nicotine administration. These data indicate that activation of a4* receptors is sufficient for nicotine-induced reward, tolerance, and sensitization.

Alkaloids↗
PubMed1994

Isolation of three hexahydro-1,3,5-trinitro-1,3,5-triazine-degrading species of the family Enterobacteriaceae from nitramine explosive-contaminated soil.

Three species of the family Enterobacteriaceae that biochemically reduced hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) were isolated from nitramine explosive-contaminated soil. Two isolates, identified as Morganella morganii and Providencia rettgeri, completely transformed both RDX and the nitroso-RDX reduction intermediates. The third isolate, identified as Citrobacter freundii, partially transformed RDX and generated high concentrations of nitroso-RDX intermediates. All three isolates produced 14CO2 from labeled RDX under O2-depleted culture conditions. While all three isolates transformed HMX, only M. morganii transformed HMX in the presence of RDX.

Aniline Compounds↗
PubMed2004

Biodegradation of nitro-substituted explosives 2,4,6-trinitrotoluene, hexahydro-1,3,5-trinitro-1,3,5-triazine, and octahydro-1,3,5,7-tetranitro-1,3,5-tetrazocine by a phytosymbiotic Methylobacterium sp. associated with poplar tissues (Populus deltoides x nigra DN34).

A pink-pigmented symbiotic bacterium was isolated from hybrid poplar tissues (Populus deltoides x nigra DN34). The bacterium was identified by 16S and 16S-23S intergenic spacer ribosomal DNA analysis as a Methylobacterium sp. (strain BJ001). The isolated bacterium was able to use methanol as the sole source of carbon and energy, which is a specific attribute of the genus Methylobacterium. The bacterium in pure culture was shown to degrade the toxic explosives 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazene (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5-tetrazocine (HMX). [U-ring-(14)C]TNT (25 mg liter(-1)) was fully transformed in less than 10 days. Metabolites included the reduction derivatives amino-dinitrotoluenes and diamino-nitrotoluenes. No significant release of (14)CO(2) was recorded from [(14)C]TNT. In addition, the isolated methylotroph was shown to transform [U-(14)C]RDX (20 mg liter(-1)) and [U-(14)C]HMX (2.5 mg liter(-1)) in less than 40 days. After 55 days of incubation, 58.0% of initial [(14)C]RDX and 61.4% of initial [(14)C]HMX were mineralized into (14)CO(2). The radioactivity remaining in solution accounted for 12.8 and 12.7% of initial [(14)C]RDX and [(14)C]HMX, respectively. Metabolites detected from RDX transformation included a mononitroso RDX derivative and a polar compound tentatively identified as methylenedinitramine. Since members of the genus Methylobacterium are distributed in a wide diversity of natural environments and are very often associated with plants, Methylobacterium sp. strain BJ001 may be involved in natural attenuation or in situ biodegradation (including phytoremediation) of explosive-contaminated sites.

Azocines↗