Nicotine-evoked transmitter release from synaptosomes: functional association of specific presynaptic acetylcholine receptors and voltage-gated calcium channels

Nicotinic acetylcholine receptor redux: Discovery of accessories opens therapeutic vistas

The neurotransmitter acetylcholine (ACh) acts in part through a family of nicotinic ACh receptors (nAChRs), which mediate diverse physiological processes including muscle contraction, neurotransmission, and sensory transduction. Pharmacologically, nAChRs are responsible for tobacco addiction and are targeted by medicines for hypertension and dementia. Nicotinic AChRs were the first ion channels to be isolated. Recent studies have identified molecules that control nAChR biogenesis, trafficking, and function. These nAChR accessories include protein and chemical chaperones as well as auxiliary subunits. Whereas some factors act on many nAChRs, others are receptor specific. Discovery of these regulatory mechanisms is transforming nAChR research in cells and tissues ranging from central neurons to spinal ganglia to cochlear hair cells. Nicotinic AChR-specific accessories also enable drug discovery on high-confidence targets for psychiatric, neurological, and auditory disorders.

Functional α6β4 acetylcholine receptor expression enables pharmacological testing of nicotinic agonists with analgesic properties

The α6β4 nicotinic acetylcholine receptor (nAChR) is enriched in dorsal root ganglia neurons and is an attractive non-opioid therapeutic target for pain. However, difficulty expressing human α6β4 receptors in recombinant systems has precluded drug discovery. Here, genome-wide screening identified accessory proteins that enable reconstitution of human α6β4 nAChRs. BARP, an auxiliary subunit of voltage-dependent calcium channels, promoted α6β4 surface expression while IRE1α, an unfolded protein response sensor, enhanced α6β4 receptor assembly. Effects on α6β4 involve BARP's N-terminal region and IRE1α's splicing of XBP1 mRNA. Furthermore, clinical efficacy of nicotinic agents in relieving neuropathic pain best correlated with their activity on α6β4. Finally, BARP-knockout, but not NACHO-knockout mice lacked nicotine-induced antiallodynia, highlighting the functional importance of α6β4 in pain. These results identify roles for IRE1α and BARP in neurotransmitter receptor assembly and unlock drug discovery for the previously elusive α6β4 receptor.

Advances in smoking cessation pharmacotherapy: Non-nicotinic approaches in animal models

The landscape of worldwide tobacco use is changing, with a decrease in traditional smoking and an exponential rise in electronic cigarette use. No new nicotine cessation pharmacotherapies have come to market in the last 10 years. The current therapies that have been approved by the United States Food and Drug Administration for nicotine cessation include nicotine replacement therapy, varenicline, a nicotinic acetylcholine receptor partial agonist, and the atypical antidepressant bupropion. Nicotine replacement therapy and varenicline both act on nicotinic acetylcholine receptors. Bupropion inhibits the dopamine transporter, the norepinephrine transporter, and the nicotinic acetylcholine receptors to inhibit smoking behavior. Notwithstanding these treatments, rates of successful nicotine cessation in clinical trials remain low. Recent pharmacological approaches to improve nicotine cessation rates in animal models have turned their focus away from activating nicotinic acetylcholine receptors. The present review focuses on such pharmacological approaches, including nicotine vaccines, anti-nicotine antibodies, nicotine-degrading enzymes, cannabinoids, and metformin. Both immunopharmacological and enzymatic approaches rely on restricting and degrading nicotine within the periphery, thus preventing psychoactive effects of nicotine on the central nervous system. In contrast, pharmacologic inhibition of the enzymes which degrade nicotine could affect smoking behavior. Cannabinoid receptor agonists and antagonists interact with the dopamine reward pathway and show efficacy in reducing nicotine addiction-like behaviors in preclinical studies. Metformin is currently approved by the Food and Drug Administration for the treatment of diabetes. It activates specific intracellular kinases that may protect against the lower metabolism, higher oxidation, and inflammation that are associated with nicotine withdrawal. Further studies are needed to investigate non-nicotinic targets to improve the treatment of tobacco use disorder. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

α6-Containing Nicotinic Acetylcholine Receptor Reconstitution Involves Mechanistically Distinct Accessory Components

Acetylcholine gates a large family of nicotinic receptor cation channels that control neuronal excitation and neurotransmitter release. These receptors are key targets for neuropsychiatric disorders; however, difficulties in expressing nicotinic acetylcholine (nACh) receptors hamper elaboration of their pharmacology and obscure elucidation of their biological functions. Particularly intriguing are α6-containing nACh receptors, which mediate nicotine-induced dopamine release in striatum-nucleus accumbens. Using genome-wide cDNA screening, we identify three accessory proteins, β-anchoring and -regulatory protein (BARP), lysosomal-associated membrane protein 5 (LAMP5), and SULT2B1, that complement the nACh receptor chaperone NACHO to reconstitute α6β2β3 channel function. Whereas NACHO mediates α6β2β3 assembly, BARP primarily enhances channel gating and LAMP5 and SULT2B1 promote receptor surface trafficking. BARP knockout mice show perturbations in presynaptic striatal nACh receptors that are consistent with BARP modulation of receptor desensitization. These studies unravel the molecular complexity of α6β2β3 biogenesis and enable physiological studies of this crucial neuropharmacological target.

α-Conotoxins active at α3-containing nicotinic acetylcholine receptors and their molecular determinants for selective inhibition

Neuronal α3-containing nicotinic acetylcholine receptors (nAChRs) in the peripheral nervous system (PNS) and non-neuronal tissues are implicated in a number of severe disease conditions ranging from cancer to cardiovascular diseases and chronic pain. However, despite the physiological characterization of mouse models and cell lines, the precise pathophysiology of nAChRs outside the CNS remains not well understood, in part because there is a lack of subtype-selective antagonists. α-Conotoxins isolated from cone snail venom exhibit characteristic individual selectivity profiles for nAChRs and, therefore, are excellent tools to study the determinants for nAChR-antagonist interactions. Given that human α3β4 subtype selective α-conotoxins are scarce and this is a major nAChR subtype in the PNS, the design of new peptides targeting this nAChR subtype is desirable. Recent studies using α-conotoxins RegIIA and AuIB, in combination with nAChR site-directed mutagenesis and computational modelling, have shed light onto specific nAChR residues, which determine the selectivity of the α-conotoxins for the human α3β2 and α3β4 subtypes. Publications describing the selectivity profile and binding sites of other α-conotoxins confirm that subtype-selective nAChR antagonists often work through common mechanisms by interacting with the same structural components and sites on the receptor.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Pharmacology of dextromethorphan: Relevance to dextromethorphan/quinidine (Nuedexta®) clinical use

Dextromethorphan (DM) has been used for more than 50years as an over-the-counter antitussive. Studies have revealed a complex pharmacology of DM with mechanisms beyond blockade of N-methyl-d-aspartate (NMDA) receptors and inhibition of glutamate excitotoxicity, likely contributing to its pharmacological activity and clinical potential. DM is rapidly metabolized to dextrorphan, which has hampered the exploration of DM therapy separate from its metabolites. Coadministration of DM with a low dose of quinidine inhibits DM metabolism, yields greater bioavailability and enables more specific testing of the therapeutic properties of DM apart from its metabolites. The development of the drug combination DM hydrobromide and quinidine sulfate (DM/Q), with subsequent approval by the US Food and Drug Administration for pseudobulbar affect, led to renewed interest in understanding DM pharmacology. This review summarizes the interactions of DM with brain receptors and transporters and also considers its metabolic and pharmacokinetic properties. To assess the potential clinical relevance of these interactions, we provide an analysis comparing DM activity from in vitro functional assays with the estimated free drug DM concentrations in the brain following oral DM/Q administration. The findings suggest that DM/Q likely inhibits serotonin and norepinephrine reuptake and also blocks NMDA receptors with rapid kinetics. Use of DM/Q may also antagonize nicotinic acetylcholine receptors, particularly those composed of α3β4 subunits, and cause agonist activity at sigma-1 receptors.

Anxiolytic- and antidepressant-like effects of the methadone metabolite 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (EMDP)

The enhancement of GABAergic and monoaminergic neurotransmission has been the mainstay of pharmacotherapy and the focus of drug-discovery for anxiety and depressive disorders for several decades. However, the significant limitations of drugs used for these disorders underscores the need for novel therapeutic targets. Neuronal nicotinic acetylcholine receptors (nAChRs) may represent one such target. For example, mecamylamine, a non-competitive antagonist of nAChRs, displays positive effects in preclinical tests for anxiolytic and antidepressant activity in rodents. In addition, nicotine elicits similar effects in rodent models, possibly by receptor desensitization. Previous studies (Xiao et al., 2001) have identified two metabolites of methadone, EMDP (2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline) and EDDP (2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine), which are considered to be inactive at opiate receptors, as relatively potent noncompetitive channel blockers of rat α3β4 nAChRs. Here, we show that these compounds are likewise highly effective blockers of human α3β4 and α4β2 nAChRs. Moreover, we show that they display relatively low affinity for opiate binding sites labeled by [(3)H]-naloxone. We then evaluated these compounds in rats and mice in preclinical behavioral models predictive of potential anxiolytic and antidepressant efficacy. We found that EMDP, but not EDDP, displayed robust effects predictive of anxiolytic and antidepressant efficacy without significant effects on locomotor activity. Moreover, EMDP at behaviorally active doses, unlike mecamylamine, did not produce eyelid ptosis, suggesting it may produce fewer autonomic side effects than mecamylamine. Thus, the methadone metabolite EMDP may represent a novel therapeutic avenue for the treatment of some affective disorders.

Nicotinic acetylcholine receptors control acetylcholine and noradrenaline release in the rodent habenulo-interpeduncular complex

Background and purpose

Nicotinic acetylcholine receptors (nACh receptors) play a central role in the habenulo-interpeduncular system. We studied nicotine-induced release of NA and ACh in the habenula and interpeduncular nucleus (IPN).

Experimental approach

The habenula and IPN were loaded with [³H]-choline or [³H]-NA and placed in superfusion chambers. [³H]-ACh release was also stimulated using nicotinic agonists, electrical pulses and elevated [KCl]_o in hippocampal and cortical slices from rats, wild-type mice and mice lacking α5, α7, β2, or β4 nACh receptor subunits. Finally, we analysed nACh receptor subtypes in the IPN using immunoprecipitation.

Key results

Nicotine induced release of [³H]-ACh in the IPN of rats and mice. This release was calcium-dependent but not blocked by tetrodotoxin (TTX); moreover, [³H]-ACh release was abolished in β4-knockout mice but was unaffected in β2- and α5-knockout mice. In contrast, nicotine-induced release of [³H]-NA in the IPN and habenula was blocked by TTX and reduced in both β2-knockout and β4-knockout mice, and dose–response curves were right-shifted in α5-knockout mice. Although electrical stimuli triggered the release of both transmitters, [³H]-ACh release required more pulses delivered at a higher frequency.

Conclusions and implications

Our results confirm previous findings that β4-containing nACh receptors are critical for [³H]-ACh release in the mouse IPN. Experiments using α5-knockout mice also revealed that unlike in the hippocampus, nicotine-induced [³H]-NA release in the habenulo-interpeduncular system is altered in this knockout model. As α5-containing nACh receptors play a key role in nicotine intake, our results add NA to the list of transmitters involved in this mechanism.

L-type calcium channels in sympathetic α3β2-nAChR-mediated cerebral nitrergic neurogenic vasodilation

AIM

Nicotine stimulation of α3β2-nicotinic acetylcholine receptors (α3β2-nAChRs) located on sympathetic nerves innervating basilar arteries causes calcium-dependent noradrenaline release, leading to activation of parasympathetic nitrergic nerves and dilation of basilar arteries. This study aimed to investigate the major subtype of calcium channels located on cerebral peri-vascular sympathetic nerves, which is involved in nicotine-induced α3β2-nAChR-mediated nitrergic vasodilation in basilar arteries.

METHODS

Nicotine- and transmural nerve stimulation (TNS)-induced dilation of isolated porcine basilar arteries was examined using in vitro tissue bath. Nicotine-induced calcium influx, nicotine-induced noradrenaline release and nicotine-induced inward currents were evaluated in rat superior cervical ganglion (SCG) neurones, peri-vascular sympathetic nerves of porcine basilar arteries and α3β2-nAChRs-expressing oocytes respectively. mRNA and protein expression of Cav 1.2 and Cav 1.3 channels were detected by RT-PCR, Western blotting and immunohistochemistry.

RESULTS

Nicotine-induced vasodilation was not affected by ω-agatoxin TK (selective P/Q-type calcium channel blocker) or ω-conotoxin GVIA (N-type calcium channel blocker). The vasodilation, however, was inhibited by nicardipine (L-type calcium channel blocker) in concentrations which did not affect TNS-induced vasodilation, suggesting the specific blockade. Nicardipine concentration-dependently inhibited nicotine-induced calcium influx in rat SCG neurones and reduced nicotine-induced noradrenaline release from peri-vascular sympathetic nerves of porcine basilar arteries. Nicardipine (10 μm), which significantly blocked nicotine-induced vasorelaxation by 70%, did not appreciably affect nicotine-induced inward currents in α3β2-nAChRs-expressing oocytes. Furthermore, the mRNAs and proteins of Cav 1.2 and Cav 1.3 channels were expressed in porcine SCG and peri-vascular nerve terminals.

CONCLUSION

The sympathetic neuronal calcium influx through L-type calcium channels is modulated by α3β2-nAChRs. This calcium influx causes noradrenaline release, initiating sympathetic-parasympathetic (axo-axonal) interaction-induced nitrergic dilation of porcine basilar arteries.

Characterization of a novel α-conotoxin TxID from Conus textile that potently blocks rat α3β4 nicotinic acetylcholine receptors

The α3β4 nAChRs are implicated in pain sensation in the PNS and addiction to nicotine in the CNS. We identified an α-4/6-conotoxin (CTx) TxID from Conus textile. The new toxin consists of 15 amino acid residues with two disulfide bonds. TxID was synthesized using solid phase methods, and the synthetic peptide was functionally tested on nAChRs heterologously expressed in Xenopus laevis oocytes. TxID blocked rat α3β4 nAChRs with a 12.5 nM IC50, which places it among the most potent α3β4 nAChR antagonists. TxID also blocked the closely related α6/α3β4 with a 94 nM IC50 but showed little activity on other nAChR subtypes. NMR analysis showed that two major structural isomers exist in solution, one of which adopts a regular α-CTx fold but with different surface charge distribution to other 4/6 family members. α-CTx TxID is a novel tool with which to probe the structure and function of α3β4 nAChRs.

[Conotoxins: from the biodiversity of gastropods to new drugs]

A review describes general trends in research of conotoxins that are peptide toxins isolated from sea gastropods of the Conus genus, since the toxins were discovered in 1970th. There are disclosed a conotoxin classification, their structure diversity and different ways of action to their molecular targets, mainly, ion channels. In the applied aspect of conotoxin research, drug discovery and development is discussed, the drugs being based on conotoxin structure. A first exemplary drug is a ziconotide, which is an analgesic of new generation.

Stable respiratory activity requires both P/Q-type and N-type voltage-gated calcium channels

P/Q-type voltage-gated calcium channels (Ca(v)2.1) play critical presynaptic and postsynaptic roles throughout the nervous system and have been implicated in a variety of neurological disorders. Here we report that mice with a genetic ablation of the Ca(v)2.1 pore-forming α(1A) subunit (α(1A)⁻/⁻) encoded by CACNA1a (Jun et al., 1999) suffer during postnatal development from increasing breathing disturbances that lead ultimately to death. Breathing abnormalities include decreased minute ventilation and a specific loss of sighs, which was associated with lung atelectasis. Similar respiratory alterations were preserved in the isolated in vitro brainstem slice preparation containing the pre-Bötzinger complex. The loss of Ca(v)2.1 was associated with an alteration in the functional dependency on N-type calcium channels (Ca(v)2.2). Blocking N-type calcium channels with conotoxin GVIA had only minor effects on respiratory activity in slices from control (CT) littermates, but abolished respiratory activity in all slices from α(1A)⁻/⁻ mice. The amplitude of evoked EPSPs was smaller in inspiratory neurons from α(1A)⁻/⁻ mice compared with CTs. Conotoxin GVIA abolished all EPSPs in inspiratory neurons from α(1A)⁻/⁻ mice, while the EPSP amplitude was reduced by only 30% in CT mice. Moreover, neuromodulation was significantly altered as muscarine abolished respiratory network activity in α(1A)⁻/⁻ mice but not in CT mice. We conclude that excitatory synaptic transmission dependent on N-type and P/Q-type calcium channels is required for stable breathing and sighing. In the absence of P/Q-type calcium channels, breathing, sighing, and neuromodulation are severely compromised, leading to early mortality.

The α3β4* nicotinic acetylcholine receptor subtype mediates nicotine reward and physical nicotine withdrawal signs independently of the α5 subunit in the mouse

The 15q25 gene cluster contains genes that code for the α5, α3, and β4 nicotinic acetylcholine receptor (nAChRs) subunits, and in human genetic studies, has shown the most robust association with smoking behavior and nicotine dependence to date. The limited available animal studies implicate a role for the α5 and β4 nAChR subunits in nicotine dependence and withdrawal; however studies focusing on the behavioral role of the α3β4* nAChR receptor subtype in nicotine dependence are lacking. Because of the apparent role of the α3β4* nAChR subtype in nicotine dependence, the goal of the current study was to better evaluate the involvement of this subtype in nicotine mediated behavioral responses. Using the selective α3β4* nAChR antagonist, α-conotoxin AuIB, we assessed the role of α3β4* nAChRs in acute nicotine, nicotine reward, and physical and affective nicotine withdrawal. Because α5 has also been implicated in nicotine dependence behaviors in mice and can form functional receptors with α3β4*, we also evaluated the role of the α3β4α5* nAChR subtype in nicotine reward and somatic nicotine withdrawal signs by blocking the α3β4* nAChR subtype in α5 nAChR knockout mice with AuIB. AuIB had no significant effect on acute nicotine behaviors, but dose-dependently attenuated nicotine reward and physical withdrawal signs, with no significant effect in affective withdrawal measures. Interestingly, AuIB also attenuated nicotine reward and somatic signs in α5 nAChR knockout mice. This study shows that α3β4* nAChRs mediate nicotine reward and physical nicotine withdrawal, but not acute nicotine behaviors or affective nicotine withdrawal signs in mice. The α5 subunit is not required in the receptor assembly to mediate these effects. Our findings suggest an important role for the α3β4* nAChR subtype in nicotine reward and physical aspects of the nicotine withdrawal syndrome.

In vitro exposure to nicotine induces endocytosis of presynaptic AMPA receptors modulating dopamine release in rat nucleus accumbens nerve terminals

Here we provide functional and immunocytochemical evidence supporting the presence on Nucleus Accumbens (NAc) dopaminergic terminals of cyclothiazide-sensitive, alfa-amino-3-hydroxy-5-methyl-4-isoxazolone propionate (AMPA) receptors, which activation causes Ca²⁺-dependent [³H]dopamine ([³H]DA) exocytosis. These AMPA receptors cross-talk with co-localized nicotinic receptors (nAChRs), as suggested by the finding that in vitro short-term pre-exposure of synaptosomes to 30 μM nicotine caused a significant reduction of both the 30 μM nicotine and the 100 μM AMPA-evoked [³H]DA overflow. Entrapping pep2-SVKI, a peptide known to compete for the binding of GluA2 subunit to scaffolding proteins involved in AMPA receptor endocytosis, in NAC synaptosomes prevented the nicotine-induced reduction of AMPA-mediated [³H]DA exocytosis, while pep2-SVKE, used as negative control, was inefficacious. Immunocytochemical studies showed that a significant percentage of NAc terminals were dopaminergic and that most of these terminals also posses GluA2 receptor subunits. Western blot analysis of GluA2 immunoreactivity showed that presynaptic GluA2 proteins in NAc terminals were reduced in nicotine-pretreated synaptosomes when compared to the control. The nACh-AMPA receptor-receptor interaction was not limited to dopaminergic terminals since nicotine pre-exposure also affected the presynaptic AMPA receptors controlling hippocampal noradrenaline release, but not the presynaptic AMPA receptors controlling GABA and acetylcholine release. These observations could be relevant to the comprehension of the molecular mechanisms at the basis of nicotine rewarding.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Nerve terminal nicotinic acetylcholine receptors initiate quantal GABA release from perisomatic interneurons by activating axonal T-type (Cav3) Ca²⁺ channels and Ca²⁺ release from stores

Release of conventional neurotransmitters is mainly controlled by calcium (Ca²⁺) influx via high-voltage-activated (HVA), Ca(v)2, channels ("N-, P/Q-, or R-types") that are opened by action potentials. Regulation of transmission by subthreshold depolarizations does occur, but there is little evidence that low-voltage-activated, Ca(v)3 ("T-type"), channels take part. GABA release from cortical perisomatic-targeting interneurons affects numerous physiological processes, and yet its underlying control mechanisms are not fully understood. We investigated whether T-type Ca²⁺ channels are involved in regulating GABA transmission from these cells in rat hippocampal CA1 using a combination of whole-cell voltage-clamp, multiple-fluorescence confocal microscopy, dual-immunolabeling electron-microscopy, and optogenetic methods. We show that Ca(v)3.1, T-type Ca²⁺ channels can be activated by α3β4 nicotinic acetylcholine receptors (nAChRs) that are located on the synaptic regions of the GABAergic perisomatic-targeting interneuronal axons, including the parvalbumin-expressing cells. Asynchronous, quantal GABA release can be triggered by Ca²⁺ influx through presynaptic T-type Ca²⁺ channels, augmented by Ca²⁺ from internal stores, following focal microiontophoretic activation of the α3β4 nAChRs. The resulting GABA release can inhibit pyramidal cells. The T-type Ca²⁺ channel-dependent mechanism is not dependent on, or accompanied by, HVA channel Ca²⁺ influx, and is insensitive to agonists of cannabinoid, μ-opioid, or GABA(B) receptors. It may therefore operate in parallel with the normal HVA-dependent processes. The results reveal new aspects of the regulation of GABA transmission and contribute to a deeper understanding of ACh and nicotine actions in CNS.

Chemical synthesis and characterization of two α4/7-conotoxins

α-Conotoxins are small disulfide-constrained peptides that act as potent and selective antagonists on specific subtypes of nicotinic acetylcholine receptors (nAChRs). We previously cloned two α-conotoxins, Mr1.1 from the molluscivorous Conus marmoreus and Lp1.4 from the vermivorous Conus leopardus. Both of them have the typical 4/7-type framework of the subfamily of α-conotoxins that act on neuronal nAChRs. In this work, we chemically synthesized these two toxins and characterized their functional properties. The synthetic Mr1.1 could primarily inhibit acetylcholine (ACh)-evoked currents reversibly in the oocyte-expressed rat α7 nAChR, whereas Lp1.4 was an unexpected specific blocker of the mouse fetal muscle α1β1γδ receptor. Although their inhibition affinities were relatively low, their unique receptor recognition profiles make them valuable tools for toxin-receptor interaction studies. Mr1.1 could also suppress the inflammatory response to pain in vivo, suggesting that it should be further investigated with respect to its molecular role in analgesia and its mechanism or therapeutic target for the treatment of pain.

Quantitative analysis of the heteromeric neuronal nicotinic receptors in the rat hippocampus

The objective of this study was to identify and quantify the heteromeric neuronal nicotinic receptors (nAChRs) in the rat hippocampus. The density of nAChR subtypes was assessed by labeling them with [(3)H]epibatidine ([(3)H]EB) followed by immunoprecipitation with subunit-selective antibodies. Sequential immunoprecipitation assays were used to establish associations between two different subunits, which then allowed the full subunit composition of the receptors to be deduced. Our results show that most of the hippocampal heteromeric nAChRs contain α4 and β2 subunits. In fact, we identified two populations containing these two predominant subunits, the α4β2 and α4β2α5 subtypes which account for ∼ 40% and ∼ 35%, respectively, of the total [(3)H]EB-labeled receptors. An additional heteromeric subtype with the subunit composition of α4β2α3 represented ∼ 10% of the total nAChRs, and another 10% of the immunoprecipitated receptors contained α4 and β4 subunits, with or without the α3 subunit. To determine if α4β2 and α4β2α5 nAChR subtypes differ in their ligand binding affinities, the α3- and β4-containing receptors were first removed by immunoprecipitation and then, competition studies with acetylcholine, nicotine, cytisine and sazetidine-A against [(3)H]EB were carried out on the remaining α4β2 and α4β2α5 subtypes. Results suggested these subtypes have comparable binding affinities for the nicotinic ligands used here.

Activation of nicotinic acetylcholine receptors enhances a slow calcium-dependent potassium conductance and reduces the firing of stratum oriens interneurons

A large variety of distinct locally connected GABAergic cells are present in the hippocampus. By releasing GABA into principal cells and interneurons, they exert a powerful control on neuronal excitability and are responsible for network oscillations crucial for information processing in the brain. Here, whole-cell patch clamp recordings in current and voltage clamp mode were used to study the functional role of nicotinic acetylcholine receptors (nAChRs) on the firing properties of stratum oriens interneurons in hippocampal slices from transgenic mice expressing enhanced green fluorescent protein in a subpopulation of GABAergic cells containing somatostatin (GIN mice). Unexpectedly, activation of nAChRs by nicotine or endogenously released acetylcholine strongly enhanced spike frequency adaptation. This effect was blocked by apamin, suggesting the involvement of small calcium-dependent potassium channels (SK channels). Nicotine-induced reduction in firing frequency was dependent on intracellular calcium rise through calcium-permeable nAChRs and voltage-dependent calcium channels activated by the depolarizing action of nicotine. Calcium imaging experiments directly showed that nicotine effects on firing rate were correlated with large increases in intracellular calcium. Furthermore, blocking ryanodine receptors with ryanodine or sarcoplasmic-endoplasmic reticulum calcium ATPase with thapsygargin or cyclopiazonic acid fully prevented the effects of nicotine, suggesting that mobilization of calcium from the internal stores contributed to the observed effects. By regulating cell firing, cholinergic signalling through nAChRs would be instrumental for fine-tuning the output of stratum oriens interneurons and correlated activity at the network level.

Nicotine tolerance to PC12 cell line: acute and chronic exposures modulate dopamine D2 receptor and tyrosine hydroxylase expression

PC12 is a clonal cell line from chromaffin tumor of rat adrenal pheochromocytoma that releases catecholamine including dopamine, which via interaction with its receptor (D(1) and D(2) receptor), is known to be involved in reward and reinforcement properties of many addictive drugs like nicotine. Nicotine tolerance is the key aspect of nicotine addiction. However, nicotine tolerance on dopamine receptors in PC12 cell line is poorly understood. In this paper, we have demonstrated the tolerance to acute and chronic nicotine administrations on PC12 cell line on the basis of the expressions of dopamine receptors and tyrosine hydroxylase, the rate-limiting enzyme of dopamine biosynthesis, by Western blot, immunohistochemistry and in situ hybridization. In vitro treatment of nicotine resulted in similar expressional changes of dopamine D(2) receptor and tyrosine hydroxylase at protein and mRNA levels in dose- and time-dependent manner, whereas dopamine D(1) receptor did not reveal any positive output. Moreover, moderate to strong signals were obtained from 0.1 to 10 microM of nicotine concentrations and the signals were gradually decreased at 100 and 1000 microM nicotine concentrations relative to the untreated control cell line. Therefore, this study implied a new approach towards nicotine tolerance which is likely to be related to the modulation of dopamine D(2) receptor and tyrosine hydroxylase expressions by chronic and acute nicotine exposures in PC12 cell line.

Nicotinic acetylcholine receptor-mediated calcium signaling in the nervous system

Based on the composition of the five subunits forming functional neuronal nicotinic acetylcholine receptors (nAChRs), they are grouped into either heteromeric (comprising both alpha and beta subunits) or homomeric (comprising only alpha subunits) receptors. The nAChRs are known to be differentially permeable to calcium ions, with the alpha7 nAChR subtype having one of the highest permeabilities to calcium. Calcium influx through nAChRs, particularly through the alpha-bungarotoxin-sensitive alpha7-containing nAChRs, is a very efficient way to raise cytoplasmic calcium levels. The activation of nAChRs can mediate three types of cytoplasmic calcium signals: (1) direct calcium influx through the nAChRs, (2) indirect calcium influx through voltage-dependent calcium channels (VDCCs) which are activated by the nAChR-mediated depolarization, and (3) calcium-induced calcium release (CICR) (triggered by the first two sources) from the endoplasmic reticulum (ER) through the ryanodine receptors and inositol (1,4,5)-triphosphate receptors (IP(3)Rs). Downstream signaling events mediated by nAChR-mediated calcium responses can be grouped into instantaneous effects (such as neurotransmitter release, which can occur in milliseconds after nAChR activation), short-term effects (such as the recovery of nAChR desensitization through cellular signaling cascades), and long-term effects (such as neuroprotection via gene expression). In addition, nAChR activity can be regulated by cytoplasmic calcium levels, suggesting a complex reciprocal relationship. Further advances in imaging techniques, animal models, and more potent and subtype-selective ligands for neuronal nAChRs would help in understanding the neuronal nAChR-mediated calcium signaling, and lead to the development of improved therapeutic treatments.

Cross-regulation between colocalized nicotinic acetylcholine and 5-HT3 serotonin receptors on presynaptic nerve terminals

AIM

Substantial colocalization of functionally independent alpha4 nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors on presynaptic terminals has been observed in brain. The present study was aimed at addressing whether nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors interact on the same presynaptic terminal, suggesting a convergence of cholinergic and serotonergic regulation.

METHODS

Ca(2+) responses in individual, isolated nerve endings purified from rat striatum were measured using confocal imaging.

RESULTS

Application of 500 nmol/L nicotine following sustained stimulation with the highly selective 5-HT(3) receptor agonist m-chlorophenylbiguanide at 100 nmol/L resulted in markedly reduced Ca(2+) responses (28% of control) in only those striatal nerve endings that originally responded to m-chlorophenylbiguanide. The cross-regulation developed over several minutes. Presynaptic nerve endings that had not responded to m-chlorophenylbiguanide, indicating that 5-HT(3) receptors were not present, displayed typical responses to nicotine. Application of m-chlorophenylbiguanide following sustained stimulation with nicotine resulted in partially attenuated Ca(2+) responses (49% of control). Application of m-chlorophenylbiguanide following sustained stimulation with m-chlorophenylbiguanide also resulted in a strong attenuation of Ca(2+) responses (12% of control), whereas nicotine-induced Ca(2+) responses following sustained stimulation with nicotine were not significantly different from control.

CONCLUSION

These results indicate that the presynaptic Ca(2+) increases evoked by either 5-HT(3) receptor or nicotinic acetylcholine receptor activation regulate subsequent responses to 5-HT(3) receptor activation, but that only 5-HT(3) receptors cross-regulate subsequent nicotinic acetylcholine receptor-mediated responses. The findings suggest a specific interaction between the two receptor systems in the same striatal nerve terminal, likely involving Ca(2+)-dependent intracellular pathways that regulate these signaling systems at one or more levels.

Cholinergic signal transduction in the mouse sphenopalatine ganglion

The sphenopalatine ganglia (SPG) receive their preganglionic innervation from the ventro-lateral reticular formation and nuclei of the caudal pons, and are involved in parasympathetic control of cranial glandular and vascular components including the blood supply to specific brain areas. In 53% of all SPG neurons, a particular member (MOL2.3) of the odorant receptor superfamily is co-expressed with green fluorescent protein (GFP) in MOL2.3 transgenic mouse pups. Choline acetyltransferase and vesicular acetylcholine transporter (VAChT) could be demonstrated in 90% of the GFP-positive, and 60% of the GFP-negative cells, these cells thus representing cholinergic neurons. Some 50% of all SPG neurons were nitrergic at a high rate of VAChT co-expression, the majority of them being GFP-positive. Most SPG neurons received cholinergic innervation as demonstrated by perineuronal VAChT immunoreactive nerve terminals. To characterize cholinergic signal transduction in SPG neurons, calcium imaging experiments were performed in a SPG primary culture system containing GFP-positive and -negative neurons. Ganglionic neurons could repeatedly be activated by cholinergic stimulation in a dose-dependent manner, with calcium entering all cells from the extracellular compartment. Stimulation with specific agonists supported prevalence of nicotinic cholinergic receptors (nAChRs). Inhibition of cholinergically induced intracellular calcium signalling by various omega-conotoxins indicated functional expression of alpha 3 beta 4 and alpha 7 nAChR subtypes in murine SPG cells, which could be supported by RT-PCR analysis of the neonatal mouse SPG. With regard to secondary cholinergic activation, L- but not N-subtype voltage-gated calcium channels might represent a prime target. Nicotinic signal transduction did not prove to be different in GFP-positive as compared to-negative murine SPG neurons.

The neuronal nicotinic acetylcholine receptors alpha 4* and alpha 6* differentially modulate dopamine release in mouse striatal slices

Striatal dopamine (DA) plays a major role in the regulation of motor coordination and in the processing of salient information. We used voltammetry to monitor DA-release evoked by electrical stimulation in striatal slices, where interneurons continuously release acetylcholine. Use of the alpha6-selective antagonist alpha-conotoxin MII[E11A] and alpha4 knockout mice enabled identification of two populations of DA-ergic fibers. The first population had a low action potential threshold, and action potential-evoked DA-release from these fibers was modulated by alpha6. The second population had a higher action potential threshold, and only alpha4(non-alpha6) modulated action potential-evoked DA-release. Striatal DA-ergic neurons fire in both tonic and phasic patterns. When stimuli were applied in a train to mimic phasic firing, more DA-release was observed in alpha4 knockout versus wild-type mice. Furthermore, block of alpha4(non-alpha6), but not of alpha6, increased DA release evoked by a train. These results indicate that there are different classes of striatal DA-ergic fibers that express different subtypes of nicotinic receptors.

Presynaptic metabotropic glutamate and GABAB receptors

Glutamate and GABA, the two most abundant neurotransmitters in the mammalian central nervous system, can act on metabotropic receptors that are structurally quite dissimilar from those targeted by most other neurotransmitters/modulators. Accordingly, metabotropic glutamate receptors (mGluRs) and GABA(B) receptors (GABA(B)Rs) are classified as members of family 3 (or family C) of G protein-coupled receptors. On the other hand, mGluRs and GABA(B)Rs exhibit pronounced and partly unresolved differences between each other. The most intriguing difference is that mGluRs exist as multiple pharmacologically as well as structurally distinct subtypes, whereas, in the case of GABA(B)Rs, molecular biologists have so far identified only one structurally distinct heterodimeric complex whose few variants seem unable to explain the pharmacological heterogeneity of GABA(B)Rs observed in many functional studies. Both mGluRs and GABA(B)Rs can be localized on axon terminals of different neuronal systems as presynaptic autoreceptors and heteroreceptors modulating the exocytosis of various transmitters.

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

This review summarizes studies that attempted to determine the subtypes of nicotinic acetylcholine receptors (nAChR) expressed in the dopaminergic nerve terminals in the mouse. A variety of experimental approaches has been necessary to reach current knowledge of these subtypes, including in situ hybridization, agonist and antagonist binding, function measured by neurotransmitter release from synaptosomal preparations, and immunoprecipitation by selective antibodies. Early developments that facilitated this effort include the radioactive labeling of selective binding agents, such as [(125)I]-alpha-bungarotoxin and [(3)H]-nicotine, advances in cloning the subunits, and expression and evaluation of function of combinations of subunits in Xenopus oocytes. The discovery of epibatidine and alpha-conotoxin MII (alpha-CtxMII), and the development of nAChR subunit null mutant mice have been invaluable in determining which nAChR subunits are important for expression and function in mice, as well as allowing validation of the specificity of subunit specific antibodies. These approaches have identified five nAChR subtypes of nAChR that are expressed on dopaminergic nerve terminals. Three of these contain the alpha6 subunit (alpha4alpha6beta2beta3, alpha6beta2beta3, alpha6beta2) and bind alpha-CtxMII with high affinity. One of these three subtypes (alpha4alpha6beta2beta3) also has the highest sensitivity to nicotine of any native nAChR that has been studied, to date. The two subtypes that do not have high affinity for alpha-CtxMII (alpha4beta2, alpha4alpha5beta2) are somewhat more numerous than the alpha6* subtypes, but do bind nicotine with high affinity. Given that our first studies detected readily measured differences in sensitivity to agonists and antagonists among these five nAChR subtypes, it seems likely that subtype selective compounds could be developed that would allow therapeutic manipulation of diverse nAChRs that have been implicated in a number of human conditions.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Regulation of synaptic transmission and plasticity by neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are widely expressed throughout the central nervous system and participate in a variety of physiological functions. Recent advances have revealed roles of nAChRs in the regulation of synaptic transmission and synaptic plasticity, particularly in the hippocampus and midbrain dopamine centers. In general, activation of nAChRs causes membrane depolarization and directly and indirectly increases the intracellular calcium concentration. Thus, when nAChRs are expressed on presynaptic membranes their activation generally increases the probability of neurotransmitter release. When expressed on postsynaptic membranes, nAChR-initiated calcium signals and depolarization activate intracellular signaling mechanisms and gene transcription. Together, the presynaptic and postsynaptic effects of nAChRs generate and facilitate the induction of long-term changes in synaptic transmission. The direction of hippocampal nAChR-mediated synaptic plasticity - either potentiation or depression - depends on the timing of nAChR activation relative to coincident presynaptic and postsynaptic electrical activity, and also depends on the location of cholinergic stimulation within the local network. Therapeutic activation of nAChRs may prove efficacious in the treatment of neuropathologies where synaptic transmission is compromised, as in Alzheimer's or Parkinson's disease.

Pharmacology of alpha-conotoxin MII-sensitive subtypes of nicotinic acetylcholine receptors isolated by breeding of null mutant mice

Subtypes of nicotinic acetylcholine receptors (nAChR) containing alpha6 subunits comprise 25 to 30% of the presynaptic nAChRs expressed in striatal dopaminergic terminals in rodents and 70% in monkeys. This class of receptors, potentially important in nicotine addiction, binds alpha-conotoxin MII (alpha-CtxMII) with high affinity and is heterogeneous, consisting of several subtypes in mice, possibly an important consideration for the design of compounds that selectively activate or antagonize the alpha6 subclass of nAChRs. Selected-null mutant mice were bred to generate isolated subtypes of alpha6beta2* nAChRs expressed in vivo for assessing pharmacology of alpha6beta2* nAChRs. Binding to striatal membranes and function in synaptosomes from (alpha4-/-)(beta3+/+) and (alpha4-/-)(beta3-/-) mice were measured and compared with wild-type (alpha4+/+)(beta3+/+) mice. Gene deletions (alpha4 and beta3) decreased binding of (125)I-alpha-CtxMII without affecting affinity for alpha-CtxMII or inhibition of alpha-CtxMII binding by epibatidine or nicotine. Deletion of the alpha4 subunit substantially increased EC(50) values for both nicotine- and cytisine-stimulated alpha-CtxMII-sensitive dopamine release from striatal synaptosomes. A further increase in EC(50) values was seen upon the additional deletion of the beta3 subunit. The data indicate that one alpha-CtxMII-sensitive nAChR subtype, prevalent on wild-type dopaminergic terminals, has the lowest EC(50) for a nicotine-mediated function so far measured in mice. In conclusion, the gene deletion strategy enabled isolation of alpha6* subtypes, and these nAChR subtypes exhibited differential activation by nicotine and cytisine.

Regulation of Nicotinic Acetylcholine Receptor Desensitization by Ca2+

The relationship between the concentration of intracellular Ca2+ ([Ca2+]i) and recovery from desensitization of nicotinic acetylcholine receptors (nAChRs) in rat medial habenula (MHb) neurons was investigated using the whole cell patch-clamp techniques in combination with microfluoresecent [Ca2+]i measurements. Recovery from desensitization was assessed with a paired-pulse agonist application protocol. Application of 100 μM nicotine (5 s) caused pronounced desensitization of nAChRs, after which recovery proceeded with two components. The relative weight of the two phases of recovery was sensitive to the nature of the intracellular Ca2+ chelator, with a greater fraction of channels recovering during the fast phase in the presence of BAPTA than EGTA. Recovery was affected by differential Ca2+ buffering only when Ca2+ was present in the extracellular solution, implying that Ca2+ influx through nAChRs was responsible for slowing the recovery. Simultaneous [Ca2+]i measurements showed that recovery from desensitization was inversely correlated with the instantaneous [Ca2+]i, further supporting the suggestion that elevation of [Ca2+]i limits the return of nAChRs to the resting state. In a separate set of experiments, activation of voltage-gated Ca2+ channels during the recovery phase produced a sufficiently large increase in [Ca2+]i to reduce recovery from desensitization even in the absence of Ca2+ influx through nAChRs. Overall, it is suggested that Ca2+ entry through both nAChRs and voltage-gated Ca2+ channels exerts a negative feedback on nAChR activity through stabilization of desensitized states. The interaction of these two Ca2+ sources could form the basis of a coincidence detector under specific circumstances.

Heteromeric nicotinic acetylcholine-dopamine autoreceptor complexes modulate striatal dopamine release

In the striatum, dopamine and acetylcholine (ACh) modulate dopamine release by acting, respectively, on dopamine D(2) autoreceptors and nicotinic ACh (nACh) heteroreceptors localized on dopaminergic nerve terminals. The possibility that functional interactions exist between striatal D(2) autoreceptors and nACh receptors was studied with in vivo microdialysis in freely moving rats. Local perfusion of nicotine in the ventral striatum (shell of the nucleus accumbens) produced a marked increase in the extracellular levels of dopamine, which was completely counteracted by co-perfusion with either the non-alpha(7) nACh receptor antagonist dihydro-beta-erythroidine or the D(2-3) receptor agonist quinpirole. Local perfusion of the D(2-3) receptor antagonist raclopride produced an increase in the extracellular levels of dopamine, which was partially, but significantly, counteracted by coperfusion with dihydro-beta-erythroidine. These findings demonstrate a potent crosstalk between G protein-coupled receptors and ligand-gated ion channels in dopaminergic nerve terminals, with the D(2) autoreceptor modulating the efficacy of non-alpha(7) nACh receptor-mediated modulation of dopamine release. We further demonstrate physical interactions between beta(2) subunits of non-alpha(7) nicotinic acetylcholine receptors and D(2) autoreceptors in co-immunoprecipitation experiments with membrane preparations from co-transfected mammalian cells and rat striatum. These results reveal that striatal non-alpha(7) nicotinic acetylcholine receptors form part of heteromeric dopamine autoreceptor complexes that modulate dopamine release.

Developmental regulation of nicotinic acetylcholine receptors within midbrain dopamine neurons

We have combined anatomical and functional methodologies to provide a comprehensive analysis of the properties of nicotinic acetylcholine receptors (nAChRs) on developing dopamine (DA) neurons of Sprague-Dawley rats. Double-labeling in situ hybridization was used to examine the expression of nAChR subunit mRNAs within developing midbrain DA neurons. As brain maturation progressed there was a change in the pattern of subunit mRNA expression within DA neurons, such that alpha3 and alpha4 subunits declined and alpha6 mRNA increased. Although there were strong similarities in subunit mRNA expression in substantia nigra (SNc) and ventral tegmental area (VTA), there was higher expression of alpha4 mRNA in SNc than VTA at gestational day (G) 15, and of alpha5, alpha6 and beta3 mRNAs during postnatal development. Using a superfusion neurotransmitter release paradigm to functionally characterize nicotine-stimulated release of [(3)H]DA from striatal slices, the properties of the nAChRs on DA terminals were also found to change with age. Functional nAChRs were detected on striatal terminals at G18. There was a decrease in maximal release in the first postnatal week, followed by an increase in nicotine efficacy and potency during the second and third postnatal weeks. In the transition from adolescence (postnatal days (P) 30 and 40) to adulthood, there was a complex pattern of functional maturation of nAChRs in ventral, but not dorsal, striatum. In males, but not females, there were significant changes in both nicotine potency and efficacy during this developmental period. These findings suggest that nAChRs may play critical functional roles throughout DA neuronal maturation.

Facilitation of glutamate release by nicotine involves the activation of a Ca2+/calmodulin signaling pathway in rat prefrontal cortex nerve terminals

The effect of nicotine on evoked glutamate release from isolated nerve terminals (synaptosomes) from rat prefrontal cortex was examined. We found that nicotine significantly potentiated 4-aminopyridine (4AP)-evoked glutamate release, and this potentiatory effect was mimicked by the selective alpha7 nicotinic receptor agonist choline and was blocked by the selective alpha7 nicotinic receptor antagonist methyllycaconitine, indicating its mediation by alpha7 nicotinic receptors. Examination of the effect of nicotine on cytosolic [Ca(2+)] revealed that the potentiation of glutamate release was associated with an increase in voltage-dependent Ca(2+) influx through N- and P/Q-type Ca(2+) channels. The potentiatory effect of nicotine on Ca(2+) influx seems to be attributed to its increasing synaptosomal excitability because nicotine significantly increased depolarization-evoked increase in the intrasynaptosomal free Na(+) concentration and 4AP-evoked depolarization of the synaptosomal plasma membrane potential. Also, Ca(2+) ionophore ionomycin-induced glutamate release was enhanced by nicotine, and this action was blocked by methyllycaconitine. These results suggest that nicotine exerts its potentiatory effect presynaptically, likely through the activation of alpha7 nicotinic receptors, resulting in Na(+) influx and local depolarization, which subsequently enhances the Ca(K+) entry through voltage-dependent N-and P/Q-type Ca(2+) channels as well as the vesicular release machinery to cause an increase in evoked glutamate release from rat prefrontocortical nerve terminals. Moreover, in this release potentiation may involve an activation of Ca(2+)/calmodulin signaling pathway as nicotine-mediated potentiation of 4AP- and ionomycin-evoked glutamate release were significantly attenuated by KN62, a selective inhibitor of Ca(2+)/calmodulin-dependent kinase II.

Reinstatement of nicotine-conditioned place preference by drug priming: Effects of calcium channel antagonists

Reinstatement of drug-seeking behaviour in animals is relevant to drug relapse in humans. In the present study, we used the conditioned place preference paradigm to investigate the establishment, extinction, reinstatement and cross-reinstatement of nicotine-induced place conditioning in rats. Nicotine produced a place preference to the initially less-preferred compartment paired with its injections during conditioning (0.5 mg/kg, i.p., three drug sessions). Once established, nicotine place preference was extinguished by repeated training. Following this extinction phase, the reinstatement of place conditioning was investigated. For this purpose, nicotine-experienced rats were challenged with nicotine (0.5 mg/kg, i.p.) or morphine (10 mg/kg, i.p.). These priming injections of both drugs renewed a marked preference for the compartment previously paired with nicotine. In the second step, we examined the influence of the calcium channel antagonists, nimodipine (10 and 20 mg/kg, i.p.) and flunarizine (5 and 10 mg/kg, i.p.), on the reinstatement of nicotine-conditioned place preference induced by priming doses of nicotine and morphine. It was shown that the calcium channel blockers dose dependently attenuated the reinstatement of nicotine place preference induced by both drugs. These findings support the hypothesis that similar neural calcium-dependent mechanisms are involved in nicotine- and morphine-induced reinstatement. Finally, the conditioned place preference paradigm appears to be a useful tool for studies of the relapse of drug-seeking behaviour in laboratory animals.

Nicotine has a permissive role on the activation of metabotropic glutamate 5 receptors coexisting with nicotinic receptors on rat hippocampal noradrenergic nerve terminals

The existence of metabotropic glutamate receptors (mGluRs) on hippocampal noradrenergic nerve terminals and their interaction with coexisting nicotinic acetylcholine receptors (nAChRs) were investigated in superfused rat synaptosomes using [(3)H]-noradrenaline ([(3)H]-NA) release as a readout. The selective agonist of group I mGluRs, (S)-3,5-dihydroxyphenylglycine (DHPG), inactive on its own, acquired ability to release [(3)H]-NA when added together with (-)-nicotine. The effect of DHPG was prevented by 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective antagonist of mGluR5, but not by 7-(hydroxyimino)cyclopropane[b]chromen-1-carboxylate ethyl ester (CPCCOEt), selective antagonist of mGluR1. The [(3)H]-NA release evoked by (-)-nicotine plus DHPG was totally abrogated by the nAChR antagonist mecamylamine. Veratrine mimicked the permissive role of (-)-nicotine on the activation of mGluR5 mediating [(3)H]-NA release. The mGluR5-mediated component of the [(3)H]-NA release provoked by DHPG plus (-)-nicotine was blocked by xestospongin C, a selective antagonist of inositoltrisphosphate (IP(3)) receptors. It can be concluded that (i) release-enhancing mGluRs of subtype 5 exist on hippocampal noradrenergic axon terminals; (ii) activation of mGluR5 to mediate IP(3)-dependent NA release requires activation of depolarizing nAChRs coexisting on the same terminals.

Alpha-conotoxin Vc1.1 alleviates neuropathic pain and accelerates functional recovery of injured neurones

This paper demonstrates the capacity of the neuronal nicotinic acetylcholine receptor (nAChR) antagonist alpha-conotoxin Vc1.1 to inhibit pain responses in vivo. Vc1.1 suppressed pain behaviors when tested in two models of peripheral neuropathy of the rat sciatic nerve, the chronic constriction injury (CCI) and partial nerve ligation (PNL) models. Mechanical hyperalgesia was assessed using an Ugo Basile Analgesymeter. Vc1.1 was administered by intramuscular bolus injection near the site of injury at doses of 0.036 microg, 0.36 microg and 3.6 microg in CCI rats and at a dose of 0.36 microg in PNL rats. Vc1.1 was also administered contralaterally in CCI rats at doses of 0.36 microg and 3.6 microg. Treatment started after the development of hyperalgesia and continued for 7 days. Vc1.1 significantly attenuated mechanical hyperalgesia in both CCI and PNL rats for up to a week following cessation of treatment. Vc1.1 also accelerated functional recovery of injured neurones. A blister was raised over the footpad innervated by the peripheral terminals of the injured nerve. The ability of these terminals to mount an inflammatory vascular response upon perfusion of the blister base with substance P provided a measure of functional recovery. This study shows that alpha-conotoxin Vc1.1, a neuronal nAChR antagonist, suppressed mechanical pain responses associated with peripheral neuropathy in rats in vivo and accelerated functional recovery of the injured neurones. A role for neuronal nAChRs in the analgesic activity of Vc1.1 is proposed.

Chronic nicotine differentially affects the function of nicotinic receptor subtypes regulating neurotransmitter release

It is known that nicotine can activate several subtypes of release-regulating presynaptic nicotinic receptors (nAChRs) including those situated on central noradrenergic, dopaminergic, cholinergic and glutamatergic axon terminals. The objective of this study was to investigate the effects of chronic administration of (-)nicotine on the function of the above autoreceptors and heteroreceptors using rat superfused synaptosomes. In hippocampal synaptosomes prelabelled with [3H]noradrenaline (NA) the nicotine-evoked overflow of [3H]NA was higher in rats treated with nicotine for 10 days (via osmotic mini-pumps) than in vehicle-treated rats. In striatal synaptosomes, prelabelled with [3H]dopamine (DA), chronic nicotine did not modify the releasing effect of nicotine. No significant change was observed in experiments with synaptosomes from nucleus accumbens prelabelled with [3H]DA. Exposure of hippocampal synaptosomes prelabelled with [3H]choline to nicotine elicited release of [3H]acetylcholine; this effect was almost abolished in synaptosomes from animals administered nicotine for 10 days, suggesting down-regulation of nicotinic autoreceptors. In hippocampal synaptosomes prelabelled with [3H]D-aspartate, the releasing effect of epibatidine following chronic nicotine treatment did not differ from that in controls. The K+-evoked exocytotic release of the neurotransmitters tested was not modified by long-term nicotine administration. The results show that chronic nicotine differentially affects the function of release-regulating nAChR subtypes.

Calcium channel antagonists attenuate cross-sensitization to the rewarding and/or locomotor effects of nicotine, morphine and MK-801

The present study focused on the evaluation of behavioural cross-sensitization, particularly in locomotor activities and conditioned rewarding effects, between nicotine and morphine, cocaine, amphetamine or MK-801. Nicotine (0.5 mg kg(-1))-experienced mice manifested an enhanced locomotor response to morphine (5 mg kg(-1)) or MK-801 (0.3 mg kg(-1)). No cross-sensitization was observed between nicotine and amphetamine (2 mg kg(-1)) or cocaine (15 mg kg(-1)). Additionally, the L-type voltage-dependent calcium-channel antagonists, nimodipine and verapamil, but not diltiazem, at a dose of 20 mg kg(-1) injected before morphine or MK-801 challenge, blocked the expression of this cross-sensitization. In the second test, an enhancement of morphine place conditioning in rats pre-exposed to nicotine (0.5 mg kg(-1), injected daily for 5 days) was demonstrated. After two conditioning sessions, morphine (5 mg kg(-1)) induced a clear place preference only in animals that had previously received nicotine injections. The administration of nimodipine (10 and 20 mg kg(-1)), verapamil (10 and 20 mg kg(-1)) and diltiazem (10 and 20 mg kg(-1)) prior to nicotine dose-dependently prevented this sensitization to the rewarding effect of morphine produced by prior injections of nicotine. These findings support the hypothesis that similar neural calcium-dependent mechanisms are involved in the appetitive effects of nicotine and morphine and in the sensitized locomotor stimulant effects of nicotine and morphine or MK-801.

Functional responses and subunit composition of presynaptic nicotinic receptor subtypes explored using the novel agonist 5-iodo-A-85380

The novel compound 5-iodo-A-85380 binds with higher affinity to alpha4beta2* nicotinic acetylcholine receptors (nAChR), compared with other nAChR subtypes (Mukhin et al., 2000). In the present study, we have confirmed that in competition binding assays for three major nAChR subtypes, 5-iodo-A-85380 is 850 and 27,000-fold more potent at rat brain alpha4beta2* binding sites than at alpha3beta4 and alpha7 subtypes, respectively. In functional assays, 5-iodo-A-85380 potently activated (EC50 12-35 nM) both alpha-CTx-MII-sensitive and -insensitive components of [3H]dopamine release from rat striatal synaptosomes, corresponding to alpha6beta2* and alpha4beta2* nAChR, respectively. 5-Iodo-A-85380 was markedly less potent at eliciting [3H]ACh release from rat interpeduncular nucleus synaptosomes, [3H]noradrenaline release from rat hippocampal slices, and Ca2+ increases in a cell line expressing rat alpha3beta4 nAChR (EC50 = 5, 3.2, 1.6 microM, respectively). As predicted by ligand binding studies, 5-iodo-A-85380 is a more discriminating agonist than the parent compound epibatidine. However, it is not specific for alpha4beta2* nAChR as it also potently activates alpha6beta2* nAChR.

Nicotine exerts a permissive role on NMDA receptor function in hippocampal noradrenergic terminals

The coexistence of nicotinic cholinergic receptors (nAChRs) and of N-methyl-D-aspartate (NMDA) receptors on the same noradrenergic axon terminals and the nAChR/NMDA receptor cross-talk were investigated by monitoring the release of noradrenaline (NA) evoked in superfused rat hippocampal synaptosomes by (-)-nicotine and NMDA alone or in combination. In medium containing a physiological concentration (1.2 mM) of Mg2+, the release of [3H]NA was very slightly increased by NMDA plus glycine, whereas it was significantly enhanced by (-)-nicotine. The (-)-nicotine/NMDA combination elicited supraadditive release which was totally abolished by the nAChR blocker mecamylamine and partly prevented by selectively blocking NMDA receptors. Supraadditive [3H]NA release was also observed by exposing synaptosomes to veratrine, but not to ionomycin. The supraadditive release elicited by the (-)-nicotine/NMDA or the veratrine/NMDA combination was sensitive to the protein kinase A/C inhibitor staurosporine and the selective protein kinase A inhibitor H89, but insensitive to the protein kinase C inhibitor Ro 31-8220. It is concluded that (i) release-modulating nAChRs and NMDA receptors coexist on hippocampal noradrenergic axon terminals; and (ii) nicotine permits NMDA receptor activation in the presence of Mg2+, possibly because the nicotine-induced influx of Na+ depolarizes the nerve ending membrane sufficiently to remove the Mg2+ block.

Nicotinic acetylcholine receptors and the regulation of neuronal signalling

Neuronal nicotinic acetylcholine (nACh) receptors in the brain are more commonly associated with modulatory events than mediation of synaptic transmission. nACh receptors have a high permeability for Ca(2+), and Ca(2+) signals are pivotal in shaping nACh receptor-mediated neuromodulatory effects. In this review, we consider the mechanisms through which nACh receptors convert rapid ionic signals into sustained, wide-ranging phenomena. The complex Ca(2+) responses that are generated after activation of nACh receptors can transmit information beyond the initial domain and facilitate the interface with many intracellular processes. These mechanisms underlie the diverse repertoire of neuronal activities of nicotine in the brain, from the enhancement of learning and memory, to addiction and neuroprotection.