Rat nicotinic acetylcholine receptor alpha2beta2 channels: comparison of functional properties with alpha4beta2 channels in Xenopus oocytes

β2* nicotinic acetylcholine receptor subtypes mediate nicotine-induced enhancement of Pavlovian conditioned responding to an alcohol cue

Nicotine enhances Pavlovian conditioned responses to reward-associated cues. We investigated through which nicotinic acetylcholine receptor (nAChR) subtypes nicotine acts to produce this behavioral effect to an alcohol-associated cue. Male Long-Evans rats with freely available food and water were first accustomed to drinking 15% ethanol in their home cages using an intermittent access, two-bottle choice procedure. Then the rats were given 15 Pavlovian conditioning sessions in which a 15-s audiovisual conditioned stimulus (CS) predicted the delivery of 0.2 ml of ethanol, the unconditioned stimulus (US). Each session contained 12 CS-US trials. A control group received explicitly unpaired presentations of the CS and US. We measured Pavlovian conditioned approach to the site of US delivery during presentations of the CS, accounting for pre-CS baseline activity. Before each conditioning session, rats were injected subcutaneously with nicotine (0.4 mg/kg) or saline (1 ml/kg). During nAChR antagonist test sessions, rats were first injected systemically with the β2*-selective nAChR antagonist dihydro-beta-erythroidine (DHβE; 3 mg/kg) or the α7-selective nAChR antagonist methyllycaconitine (MLA; 6 mg/kg), followed by their assigned nicotine or saline injection before assessing their conditioned response to the alcohol-associated cue. Consistent with previous reports, nicotine enhanced the Pavlovian conditioned response to the alcohol-paired cue. DHβE attenuated this enhancement, whereas MLA did not. These results suggest that nicotine acts via β2*, but not α7, nAChRs to amplify Pavlovian conditioned responding to an alcohol cue. These findings contribute to a growing literature that identifies nAChRs as potential targets for pharmacological treatment of co-morbid alcohol and tobacco use disorders.

Memantine has a nicotinic neuroprotective pathway in acute hippocampal slices after an NMDA insult

Memantine is a non-competitive antagonist with a moderate affinity to the N-methyl-d-Aspartate (NMDA) receptor. The present study assessed memantine's neuroprotective activity using electrophysiology of ex-vivo hippocampal slices. Interestingly, a nicotinic component was necessary for memantine's neuroprotection (NP). Memantine demonstrated a bell-shaped dose-response curve of NP against NMDA. Memantine was neuroprotective at concentrations below 3 μM, but the NP declined at higher concentrations (>3 μM) when memantine inhibits the NMDA receptor. Additional evidence that memantine NP is mediated by an alternate mechanism independent of the inhibition of the NMDA receptor is supported by its ability to protect neurons when applied before or after the NMDA insult and in the presence of D(-)-2-Amino-5-phosphonopentanoic acid (APV), the standard NMDA receptor inhibitor. We found several similarities between the memantine NP mechanism and the neuroprotective nicotinic drug, the 4R cembranoid. Memantine's NP requires the release of acetylcholine, the activation of α4β2, and is independent of MEK/MAPK signaling. Both 4R and memantine require the activation of PI3K/AKT for NP against NMDA-mediated excitotoxicity, although at different concentrations. In conclusion, our studies show memantine is neuroprotective through a nicotinic pathway, similar to the nicotinic drug 4R. This information leads to a better understanding of memantine's mechanisms of action and explains its dose-dependent effectiveness in Alzheimer's and other neurological disorders.

Characterization of an α 4/7-Conotoxin LvIF from Conus lividus That Selectively Blocks α3β2 Nicotinic Acetylcholine Receptor

Nicotinic acetylcholine receptor (nAChR), a member of pentameric ligand-gated ion channel transmembrane protein composed of five subunits, is widely distributed in the central and peripheral nervous system. The nAChRs are associated with various neurological diseases, including schizophrenia, Alzheimer’s disease, Parkinson’s disease, epilepsy and neuralgia. Receptors containing the α3 subunit are associated with analgesia, generating our interest in their role in pharmacological studies. In this study, α-conotoxin (α-CTx) LvIF was identified as a 16 amino acid peptide using a genomic DNA clone of Conus lividus (C. lividus). The mature LvIF with natural structure was synthesized by a two-step oxidation method. The blocking potency of α-CTx lvIF on nAChR was detected by a two-electrode voltage clamp. Our results showed that α-CTx LvIF was highly potent against rα3β2 and rα6/α3β2β3 nAChR subtypes, The half-maximal inhibitory concentration (IC₅₀) values of α-CTx LvIF against rα3β2 and rα6/α3β2β3 nAChRs expressed in Xenopus oocytes were 8.9 nM and 14.4 nM, respectively. Furthermore, α-CTx LvIF exhibited no obvious inhibition on other nAChR subtypes. Meanwhile, we also conducted a competitive binding experiment between α-CTxs MII and LvIF, which showed that α-CTxs LvIF and MII bind with rα3β2 nAChR at the partial overlapping domain. These results indicate that the α-CTx LvIF has high potential as a new candidate tool for the studying of rα3β2 nAChR related neurophysiology and pharmacology.

Potentiation of (α4)2(β2)3, but not (α4)3(β2)2, nicotinic acetylcholine receptors reduces nicotine self-administration and withdrawal symptoms

The low sensitivity (α4)3(β2)2 (LS) and high sensitivity (α4)2(β2)3 (HS) nAChR isoforms may contribute to a variety of brain functions, pathophysiological processes, and pharmacological effects associated with nicotine use. In this study, we examined the contributions of the LS and HS α4β2 nAChR isoforms in nicotine self-administration, withdrawal symptoms, antinociceptive and hypothermic effects. We utilized two nAChR positive allosteric modulators (PAMs): desformylflustrabromine (dFBr), a PAM of both the LS and HS α4β2 nAChRs, and CMPI, a PAM selective for the LS nAChR. We found that dFBr, but not CMPI, decreased intravenous nicotine self-administration in male mice in a dose-dependent manner. Unlike dFBr, which fully reverses somatic and affective symptoms of nicotine withdrawal, CMPI at doses up to 15 mg/kg in male mice only partially reduced nicotine withdrawal-induced somatic signs, anxiety-like behavior and sucrose preference, but had no effects on nicotine withdrawal-induced hyperalgesia. These results indicate that potentiation of HS α4β2 nAChRs is necessary to modulate nicotine's reinforcing properties that underlie nicotine intake and to reverse nicotine withdrawal symptoms that influence nicotine abstinence. In contrast, both dFBr and CMPI enhanced nicotine's hypothermic effect and reduced nicotine's antinociceptive effects in male mice. Therefore, these results indicate a more prevalent role of HS α4β2 nAChR isoforms in mediating various behavioral effects associated with nicotine, whereas the LS α4β2 nAChR isoform has a limited role in mediating body temperature and nociceptive responses. These findings will facilitate the development of more selective, efficacious, and safe nAChR-based therapeutics for nicotine addiction treatment.

Tetrapeptide Ac-HAEE-NH2 Protects α4β2 nAChR from Inhibition by Aβ

The cholinergic deficit in Alzheimer’s disease (AD) may arise from selective loss of cholinergic neurons caused by the binding of Aβ peptide to nicotinic acetylcholine receptors (nAChRs). Thus, compounds preventing such an interaction are needed to address the cholinergic dysfunction. Recent findings suggest that the ¹¹EVHH¹⁴ site in Aβ peptide mediates its interaction with α4β2 nAChR. This site contains several charged amino acid residues, hence we hypothesized that the formation of Aβ-α4β2 nAChR complex is based on the interaction of ¹¹EVHH¹⁴ with its charge-complementary counterpart in α4β2 nAChR. Indeed, we discovered a ³⁵HAEE³⁸ site in α4β2 nAChR, which is charge-complementary to ¹¹EVHH¹⁴, and molecular modeling showed that a stable Aβ₄₂-α4β2 nAChR complex could be formed via the ¹¹EVHH¹⁴:³⁵HAEE³⁸ interface. Using surface plasmon resonance and bioinformatics approaches, we further showed that a corresponding tetrapeptide Ac-HAEE-NH₂ can bind to Aβ via ¹¹EVHH¹⁴ site. Finally, using two-electrode voltage clamp in Xenopus laevis oocytes, we showed that Ac-HAEE-NH₂ tetrapeptide completely abolishes the Aβ₄₂-induced inhibition of α4β2 nAChR. Thus, we suggest that ³⁵HAEE³⁸ is a potential binding site for Aβ on α4β2 nAChR and Ac-HAEE-NH₂ tetrapeptide corresponding to this site is a potential therapeutic for the treatment of α4β2 nAChR-dependent cholinergic dysfunction in AD.

Distinctive single-channel properties of α4β2-nicotinic acetylcholine receptor isoforms

Central nervous system nicotinic acetylcholine receptors (nAChR) are predominantly of the α4β2 subtype. Two isoforms exist, with high or low agonist sensitivity (HS-(α4β2)2β2- and LS-(α4β2)2α4-nAChR). Both isoforms exhibit similar macroscopic potency and efficacy values at low acetylcholine (ACh) concentrations, mediated by a common pair of high-affinity α4(+)/(-)β2 subunit binding interfaces. However LS-(α4β2)2α4-nAChR also respond to higher concentrations of ACh, acting at a third α4(+)/(-)α4 subunit interface. To probe isoform functional differences further, HS- and LS-α4β2-nAChR were expressed in Xenopus laevis oocytes and single-channel responses were assessed using cell-attached patch-clamp. In the presence of a low ACh concentration, both isoforms produce low-bursting function. HS-(α4β2)2β2-nAChR exhibit a single conductance state, whereas LS-(α4β2)2α4-nAChR display two distinctive conductance states. A higher ACh concentration did not preferentially recruit either conductance state, but did result in increased LS-(α4β2)2α4-nAChR bursting and reduced closed times. Introduction of an α4(+)/(-)α4-interface loss-of-function α4W182A mutation abolished these changes, confirming this site's role in mediating LS-(α4β2)2α4-nAChR responses. Small or large amplitude openings are highly-correlated within individual LS-(α4β2)2α4-nAChR bursts, suggesting that they arise from distinct intermediate states, each of which is stabilized by α4(+)/(-)α4 site ACh binding. These findings are consistent with α4(+)/(-)α4 subunit interface occupation resulting in allosteric potentiation of agonist actions at α4(+)/(-)β2 subunit interfaces, rather than independent induction of high conductance channel openings.

Understanding structure-function relationships of the human neuronal acetylcholine receptor: insights from the first crystal structures of neuronal subunits

Nicotinic ACh receptors (nAChRs) are the best studied members of the superfamily of pentameric ligand-gated ion channels (pLGICs). Neuronal nAChRs regulate neuronal excitability and neurotransmitter release in the nervous system and form either homo- or hetero-pentameric complexes with various combinations of the 11 neuronal nAChR subunits (α2-7, α9, α10 and β2-4) known to exist in humans. In addition to their wide distribution in the nervous system, neuronal nAChRs have been also found in immune cells and many peripheral tissues. These nAChRs are important drug targets for neurological and neuropsychiatric diseases (e.g. Alzheimer's, schizophrenia) and substance addiction (e.g. nicotine), as well as in a variety of diseases such as chronic pain, auditory disorders and some cancers. To decipher the functional mechanisms of human nAChRs and develop efficient and specific therapeutic drugs, elucidation of their high-resolution structures is needed. Recent studies, including the X-ray crystal structures of the near-intact α4β2 nAChR and of the ligand-binding domains of the α9 and α2 subunits, have advanced our knowledge on the detailed structure of the ligand-binding sites formed between the same and different subunits and revealed many other functionally important interactions. The aim of this review is to highlight some of the structural and functional findings of these studies and to compare them with recent breakthrough findings on other pLGIC members and earlier data from their homologous ACh-binding proteins.

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.

Orthosteric and allosteric potentiation of heteromeric neuronal nicotinic acetylcholine receptors

Heteromeric nicotinic ACh receptors (nAChRs) were thought to have two orthodox agonist-binding sites at two α/β subunit interfaces. Highly selective ligands are hard to develop by targeting orthodox agonist sites because of high sequence similarity of this binding pocket among different subunits. Recently, unorthodox ACh-binding sites have been discovered at some α/α and β/α subunit interfaces, such as α4/α4, α5/α4 and β3/α4. Targeting unorthodox sites may yield subtype-selective ligands, such as those for (α4β2)₂ α5, (α4β2)₂ β3 and (α6β2)₂ β3 nAChRs. The unorthodox sites have unique pharmacology. Agonist binding at one unorthodox site is not sufficient to activate nAChRs, but it increases activation from the orthodox sites. NS9283, a selective agonist for the unorthodox α4/α4 site, was initially thought to be a positive allosteric modulator (PAM). NS9283 activates nAChRs with three engineered α4/α4 sites. PAMs, on the other hand, act at allosteric sites where ACh cannot bind. Known PAM sites include the ACh-homologous non-canonical site (e.g. morantel at β/α), the C-terminus (e.g. Br-PBTC and 17β-estradiol), a transmembrane domain (e.g. LY2087101) or extracellular and transmembrane domain interfaces (e.g. NS206). Some of these PAMs, such as Br-PBTC and 17β-estradiol, require only one subunit to potentiate activation of nAChRs. In this review, we will discuss differences between activation from orthosteric and allosteric sites, their selective ligands and clinical implications. These studies have advanced understanding of the structure, assembly and pharmacology of heteromeric neuronal nAChRs.

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.

Potentiation of Neuronal Nicotinic Receptors by 17β-Estradiol: Roles of the Carboxy-Terminal and the Amino-Terminal Extracellular Domains

The endogenous steroid 17β-estradiol (βEST) potentiates activation of neuronal nicotinic receptors containing α4 subunits. Previous work has shown that the final 4 residues of the α4 subunit are required for potentiation. However, receptors containing the α2 subunit are not potentiated although it has these 4 residues, and only one amino acid difference in the C-terminal tail (FLAGMI vs. WLAGMI). Previous work had indicated that the tryptophan residue was involved in binding an analog of βEST, but not in potentiation by βEST. To determine the structural basis for the loss of potentiation we analyzed data from chimeric subunits, which indicated that the major factor underlying the difference between α2 and α4 is the tryptophan/phenylalanine difference, while the N-terminal extracellular domain is a less significant factor. When the tryptophan in α4 was mutated, both phenylalanine and tyrosine conferred lower potentiation while lysine and leucine did not. The reduction reflected a reduced maximal magnitude of potentiation, indicating that the tryptophan is involved in transduction of steroid effects. The regions of the α4 N-terminal extracellular domain involved in potentiation lie near the agonist-binding pocket, rather than close to the membrane or the C-terminal tail, and appear to be involved in transduction rather than binding. These observations indicate that the C-terminal region is involved in both steroid binding (AGMI residues) and transduction (W). The role of the N-terminus appears to be independent of the C-terminal tryptophan and likely reflects an influence on conformational changes caused during channel activation by agonist and potentiation by estradiol.

Analysis of free ACh and 5-HT in milk from four different species and their bioactivity on 5-HT(3) and nACh receptors

Milk is one of the most beneficial aliments and is highly recommended in normal conditions; however, in certain disorders, like irritable bowel syndrome, cow milk and dairy products worsen the gastric symptoms and their use is not recommended. Among the most recognized milk-induced gatrointestinal symptoms are abdominal pain, nausea and vomiting, which are processes controlled by cholinergic and serotonergic transmission. Whether the presence of bioavailable ACh and 5-HT in milk may contribute to normal peristalsis, or to the developing of these symptoms, is not known. In this work we attempt to determine whether the content of free ACh and 5-HT is of physiological significance in milk from four different species: cow (bovine), goat, camel and human. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was used to identify and quantify free ACh and 5-HT in milk, and activation of the serotonergic and cholinergic ionotropic receptors was investigated using electrophysiological experiments. Our principal hypothesis was that milk from these four species had sufficient free ACh and 5-HT to activate their correspondent receptors expressed in a heterologous system. Our results showed a more complex picture, in which free ACh and 5-HT and their ability to activate cholinergic and serotonergic receptors are not correlated. This work is a first step to elucidate whether 5-HT and ACh, at the concentrations present in the milk, can be associated to a direct function in the GI.

Endogenous ACh suppresses LTD induction and nicotine relieves the suppression via different nicotinic ACh receptor subtypes in the mouse hippocampus

AIMS

Studying the normal role of nicotinic cholinergic systems in hippocampal synaptic plasticity is critical for understanding how cholinergic loss in Alzheimer's disease (AD) and tobacco use affect cognitive function. However, it is largely unknown how nicotinic cholinergic systems regulate the induction of long-term depression (LTD).

MAIN METHODS

Extracellular field potential recordings were performed in hippocampal slices prepared from wild-type, α2, α7, and β2 knockout (KO) mice. Effects of nicotine and nicotinic antagonists on LTD induction in wild-type, α2, α7, and β2 KO mice were compared.

KEY FINDINGS

Activation of α7 nicotinic acetylcholine receptors (nAChRs) occurs during LTD-inducing stimulation to suppress LTD induction at CA3-CA1 synapses. Nicotine relieves this suppression, causing larger LTD. This nicotine effect was mediated by the activation of non-α7 nAChR subtypes, which were not activated by ACh released during LTD-inducing stimulation, and requires the presence of endogenous ACh-induced α7 nAChR activation. Furthermore, the effect of nicotine was prevented in the presence of mecamylamine, but not dihydro-β-erythroidine, and was still observed in both α2 KO and β2 KO mice.

SIGNIFICANCE

This is the first report to evaluate the involvement of different nAChR subtypes in LTD induction. Findings indicate the involvement of unique non-α7 nAChR subtypes, which have not been considered in the nicotinic modulation of hippocampal long-term potentiation, in the control of LTD induction. The implication of our results is that the loss of cholinergic projections to the hippocampus, which reduces ACh release as seen in AD patients, and nicotine from tobacco smoking can differentially affect LTD induction.

High affinity and low affinity heteromeric nicotinic acetylcholine receptors at central synapses

Most neuronal heteromeric nicotinic receptors seem able to adopt two different stochiometries depending on the ratio of α and β subunits. In recombinant receptors these two stoichiometries have been associated with different affinities to ACh, but it is not known which stoichiometry is present at nicotinic synapses in the nervous system. One possible clue to this identification is the speed of decay of the synaptic currents. In many ionotropic receptors this speed has been linked to the dissociation rate of the transmitter, which is itself related to its affinity. On this basis we propose that, at the synapse between motoneuron and Renshaw cells, the heteromeric nicotinic receptors are mostly low affinity receptors and suggest that, in contrast, the very slow decay of some synaptic currents recorded in other parts of the brain signs the presence of high affinity receptors rather than volume transmission.

A signal peptide missense mutation associated with nicotine dependence alters α2*-nicotinic acetylcholine receptor function

A cytosine to thymidine (C → T) missense mutation in the signal peptide (SP) sequence (rs2472553) of the nicotinic acetylcholine receptor (nAChR) α2 subunit produces a threonine-to-isoleucine substitution (T22I) often associated with nicotine dependence (ND). We assessed effects on function of α2*-nAChR ('*'indicates presence of additional subunits) of this mutation, which could alter SP cleavage, RNA/protein secondary structure, and/or efficiency of transcription, translation, subunit assembly, receptor trafficking or cell surface expression. Two-electrode voltage clamp analyses indicate peak current responses to ACh or nicotine are decreased 2.8-5.8-fold for putative low sensitivity (LS; 10:1 ratio of α:β subunit cRNAs injected) α2β2- or α2β4-nAChR and increased for putative high sensitivity (HS; 1:10 α:β subunit ratio) α2β2- (5.7-15-fold) or α2β4- (1.9-2.2-fold) nAChR as a result of the mutation. Agonist potencies are decreased 1.6-4-fold for putative LS or HS α2(T22I)β2-nAChR or for either α2*-nAChR subtype formed in the presence of equal amounts of subunit cRNA, slightly decreased for LS α2(T22I)β4-nAChR, but increased 1.4-2.4-fold for HS α2(T22I)β4-nAChR relative to receptors containing wild-type α2 subunits. These effects suggest that the α2 subunit SP mutation generally favors formation of LS receptor isoforms. We hypothesize that lower sensitivity of human α2*-nAChR to nicotine could contribute to increased susceptibility to ND. To our knowledge this is the first report of a SP mutation having a functional effect in a member of cys-loop family of ligand-gated ion channels.

Effects of neuronal nicotinic acetylcholine receptor allosteric modulators in animal behavior studies

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation-conducting transmembrane channels from the cys-loop receptor superfamily. The neuronal subtypes of these receptors (e.g. the α7 and α4β2 subtypes) are involved in neurobehavioral processes such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and a number of cognitive functions like learning and memory. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders, and behavioral studies in animals are useful models to assess the effects of compounds that act on these receptors. Allosteric modulators are ligands that bind to the receptors at sites other than the orthosteric site where acetylcholine, the endogenous agonist for the nAChRs, binds. While conventional ligands for the neuronal nAChRs have been studied for their behavioral effects in animals, allosteric modulators for these receptors have only recently gained attention, and research on their behavioral effects is growing rapidly. Here we will discuss the behavioral effects of allosteric modulators of the neuronal nAChRs.

Augmentation of cognitive function by NS9283, a stoichiometry-dependent positive allosteric modulator of α2- and α4-containing nicotinic acetylcholine receptors

BACKGROUND AND PURPOSE

Positive allosteric modulation of α4β2 nicotinic acetylcholine (nACh) receptors could add a new dimension to the pharmacology and therapeutic approach to these receptors. The novel modulator NS9283 was therefore tested extensively.

EXPERIMENTAL APPROACH

Effects of NS9283 were evaluated in vitro using fluorescence-based Ca(2+) imaging and electrophysiological voltage clamp experiments in Xenopus oocytes, mammalian cells and thalamocortical neurons. In vivo the compound was tested in models covering a range of cognitive domains in mice and rats.

KEY RESULTS

NS9283 was shown to increase agonist-evoked response amplitude of (α4)(3) (β2)(2) nACh receptors in electrophysiology paradigms. (α2)(3) (β2)(2) , (α2)(3) (β4)(2) and (α4)(3) (β4)(2) were modulated to comparable extents, but no effects were detected at α3-containing or any 2α : 3β stoichiometry nACh receptors. Native nACh receptors in thalamocortical neurons similarly displayed DHβE-sensitive currents that were receptive to modulation. NS9283 had favourable effects on sensory information processing, as shown by reversal of PCP-disrupted pre-pulse inhibition. NS9283 further improved performance in a rat model of episodic memory (social recognition), a rat model of sustained attention (five-choice serial reaction time task) and a rat model of reference memory (Morris water maze). Importantly, the effects in the Morris water maze could be fully reversed with mecamylamine, a blocker of nACh receptors.

CONCLUSIONS AND IMPLICATIONS

These results provide compelling evidence that positive allosteric modulators acting at the (α4)(3) (β2)(2) nACh receptors can augment activity across a broad range of cognitive domains, and that α4β2 nACh receptor allosteric modulation therefore constitutes a promising therapeutic approach to symptomatic treatment of cognitive impairment.

γ-Aminobutyric acid type A receptor inhibition triggers a nicotinic neuroprotective mechanism

Nicotinic acetylcholine receptor (nAChR)-mediated neuroprotection has been implicated in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases and hypoxic ischemic events as well as other diseases hallmarked by excitotoxic and apoptotic neuronal death. Several modalities of nicotinic neuroprotection have been reported. However, although this process generally involves α4β2 and α7 subtypes, the underlying mechanisms are largely unknown. Interestingly, both activation and inhibition of α7 nAChRs have been reported to be neuroprotective. We have shown that inhibition of α7 nAChRs protects the function of acute hippocampal slices against excitotoxicity in an α4β2-dependent manner. Neuroprotection was assessed as the prevention of the N-methyl-D-aspartate-dependent loss of the area of population spikes (PSs) in the CA1 area of acute hippocampal slices. Our results support a model in which α7 AChRs control the release of γ-aminobutyric acid (GABA). Blocking either α7 or GABA(A) receptors reduces the inhibitory tone on cholinergic terminals, thereby promoting α4β2 activation, which in turn mediates neuroprotection. These results shed light on how α7 nAChR inhibition can be neuroprotective through a mechanism mediated by activation of α4β2 nAChRs.

Withdrawal from chronic nicotine exposure alters dopamine signaling dynamics in the nucleus accumbens

BACKGROUND

Unaided attempts to quit smoking commonly fail during the first 2 weeks of the nicotine withdrawal syndrome. Alterations in dopamine (DA) signaling correlate with withdrawal from chronic nicotine exposure, but those changes have not been well-characterized.

METHODS

Mice were administered nicotine in their drinking water for 4 or 12 weeks. Then nicotine was withheld for 1 to 10 days while DA signaling was characterized with in vivo microdialysis or fast-scan cyclic voltammetry.

RESULTS

Upon withdrawal of nicotine, the basal DA concentration in the nucleus accumbens decreased as measured by microdialysis. The length of time that the low basal DA state lasted depended on the length of the chronic nicotine treatment. Microdialysis indicated that acute re-exposure to nicotine during withdrawal temporarily reversed this hypodopaminergic state. Voltammetry measurements supported the microdialysis results by showing that nicotine withdrawal decreased tonic and phasic DA release. The basal DA concentration and tonic DA signals, however, were disproportionately lower than the phasic DA signals. Therefore, the phasic/tonic DA signaling ratio was increased during the withdrawal period.

CONCLUSIONS

The relative increase in the sensitivity of DA release to phasic stimulation suggests an increase in the signal-to-noise relationship of DA signaling during the withdrawal period. Therefore, the DA signal produced by acute nicotine re-exposure produces a DA response that might reinforce relapse to drug use (i.e., smoking). Because the basal DA concentration is low during withdrawal, therapies aimed at elevating the background DA signal represent a reasonable treatment strategy for nicotine-dependent individuals attempting to quit.

Emerging Pharmacological Properties of Cholinergic Synaptic Transmission: Comparison between Mammalian and Insect Synaptic and Extrasynaptic Nicotinic Receptors

Acetylcholine (ACh) is probably the oldest signalling neurotransmitter which appeared in evolution before the nervous system. It is present in bacteria, algae, protozoa and plants. In insects and mammals it is involved in cell-to-cell communications in various neuronal and non-neuronal tissues. The discovery of nicotinic acetylcholine receptors (nAChRs) as the main receptors involved in rapid cholinergic neurotransmission has helped to understand the role of ACh at synaptic level. Recently, several lines of evidence have indicated that extrasynaptically expressed nAChRs display distinct pharmacological properties from the ones expressed at synaptic level. The role of both nAChRs at insect extrasynaptic and/or synaptic levels has been underestimated due to the lack of pharmacological tools to identify different nicotinic receptor subtypes. In the present review, we summarize recent electrophysiological and pharmacological studies on the extrasynaptic and synaptic differences between insect and mammalian nAChR subtypes and we discuss on the pharmacological impact of several drugs such as neonicotinoid insecticides targeting these receptors. In fact, nAChRs are involved in a wide range of pathophysiological processes such as epilepsy, pain and a wide range of neurodegenerative and psychiatric disorders. In addition, they are the target sites of neonicotinoid insecticides which are known to act as nicotinic agonists causing severe poisoning in insects and mammals.

Characterizing low affinity epibatidine binding to α4β2 nicotinic acetylcholine receptors with ligand depletion and nonspecific binding

Background

Along with high affinity binding of epibatidine (K_d1≈10 pM) to α4β2 nicotinic acetylcholine receptor (nAChR), low affinity binding of epibatidine (K_d2≈1-10 nM) to an independent binding site has been reported. Studying this low affinity binding is important because it might contribute understanding about the structure and synthesis of α4β2 nAChR. The binding behavior of epibatidine and α4β2 AChR raises a question about interpreting binding data from two independent sites with ligand depletion and nonspecific binding, both of which can affect equilibrium binding of [³H]epibatidine and α4β2 nAChR. If modeled incorrectly, ligand depletion and nonspecific binding lead to inaccurate estimates of binding constants. Fitting total equilibrium binding as a function of total ligand accurately characterizes a single site with ligand depletion and nonspecific binding. The goal of this study was to determine whether this approach is sufficient with two independent high and low affinity sites.

Results

Computer simulations of binding revealed complexities beyond fitting total binding for characterizing the second, low affinity site of α4β2 nAChR. First, distinguishing low-affinity specific binding from nonspecific binding was a potential problem with saturation data. Varying the maximum concentration of [³H]epibatidine, simultaneously fitting independently measured nonspecific binding, and varying α4β2 nAChR concentration were effective remedies. Second, ligand depletion helped identify the low affinity site when nonspecific binding was significant in saturation or competition data, contrary to a common belief that ligand depletion always is detrimental. Third, measuring nonspecific binding without α4β2 nAChR distinguished better between nonspecific binding and low-affinity specific binding under some circumstances of competitive binding than did presuming nonspecific binding to be residual [³H]epibatidine binding after adding a large concentration of cold competitor. Fourth, nonspecific binding of a heterologous competitor changed estimates of high and low inhibition constants but did not change the ratio of those estimates.

Conclusions

Investigating the low affinity site of α4β2 nAChR with equilibrium binding when ligand depletion and nonspecific binding are present likely needs special attention to experimental design and data interpretation beyond fitting total binding data. Manipulation of maximum ligand and receptor concentrations and intentionally increasing ligand depletion are potentially helpful approaches.

Allosteric modulators of the α4β2 subtype of neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are ligand-gated ion conducting transmembrane channels from the Cys-loop receptor super-family. The α4β2 subtype is the predominant heteromeric subtype of nicotinic receptors found in the brain. Allosteric modulators for α4β2 receptors interact at a site other than the orthosteric site where acetylcholine binds. Many compounds which act as allosteric modulators of the α4β2 receptors have been identified, with both positive and negative effects. Such allosteric modulators either increase or decrease the response induced by agonist on the α4β2 receptors. Here we discuss the concept of allosterism as it pertains to the α4β2 receptors and summarize the important features of allosteric modulators for this nicotinic receptor subtype.

Nicotinic excitatory postsynaptic potentials in hippocampal CA1 interneurons are predominantly mediated by nicotinic receptors that contain α4 and β2 subunits

In the hippocampus, activation of nicotinic receptors that include α4 and β2 subunits (α4β2*) facilitates memory formation. α4β2* receptors may also play a role in nicotine withdrawal, and their loss may contribute to cognitive decline in aging and Alzheimer's disease (AD). However, little is known about their cellular function in the hippocampus. Therefore, using optogenetics, whole cell patch clamping and voltage-sensitive dye (VSD) imaging, we measured nicotinic excitatory postsynaptic potentials (EPSPs) in hippocampal CA1. In a subpopulation of inhibitory interneurons, release of ACh resulted in slow depolarizations (rise time constant 33.2 ± 6.5 ms, decay time constant 138.6 ± 27.2 ms) mediated by the activation of α4β2* nicotinic receptors. These interneurons had somata and dendrites located in the stratum oriens (SO) and stratum lacunosum-moleculare (SLM). Furthermore, α4β2* nicotinic EPSPs were largest in the SLM. Thus, our data suggest that nicotinic EPSPs in hippocampal CA1 interneurons are predominantly mediated by α4β2* nicotinic receptors and their activation may preferentially affect extrahippocampal inputs in SLM of hippocampal CA1.

Substitutions of amino acids in the pore domain of homomeric α7 nicotinic receptors for analogous residues present in heteromeric receptors modify gating, rectification and binding properties

We have studied the role of different amino acids in the M2 transmembrane domain of the α7 neuronal nicotinic receptor by mutating residues that differ from the ones located at the same positions in other α (α2-α10) or β (β2-β4) subunits. Our aim was to investigate the contribution of these amino acids to the peculiar kinetic and inward rectification properties that differentiate the homomeric α7 receptor from other nicotinic receptors. Mutations of several residues strongly modified receptor function. We found that Thr245 had the most profound effect when mutated to serine, an amino acid present in all heteromeric receptors composed of α and β subunits, by dramatically increasing the maximal current, decreasing the decaying rate of the currents and decreasing receptor rectification. Some mutants also showed altered agonist-binding properties as revealed by shifts in the dose-response curves for acetylcholine. We conclude that residues in the M2 segment and flanking regions contribute to the unusual properties of the α7 receptor, especially to its characteristic fast kinetic behavior and strong inward rectification and furthermore to the potency of agonists.

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.

86Rb+ efflux mediated by alpha4beta2*-nicotinic acetylcholine receptors with high and low-sensitivity to stimulation by acetylcholine display similar agonist-induced desensitization

The nicotinic acetylcholine receptors (nAChR) assembled from alpha4 and beta2 subunits are the most densely expressed subtype in the brain. Concentration-effect curves for agonist activation of alpha4beta2*-nAChR are biphasic. This biphasic agonist sensitivity is ascribed to differences in subunit stoichiometry. The studies described here evaluated desensitization elicited by low concentrations of epibatidine, nicotine, cytisine or methylcarbachol of brain alpha4beta2-nAChR function measured with acetylcholine-stimulated (86)Rb(+) efflux from mouse thalamic synaptosomes. Each agonist elicited concentration-dependent desensitization. The agonists differed in potency. However, IC(50) values for each agonist for desensitization of (86)Rb(+) efflux both with high (EC(50) approximately 3 microM) and low (EC(50) approximately 150 microM) acetylcholine sensitivity were not significantly different. Concentrations required to elicit desensitization were higher that their respective K(D) values for receptor binding. Even though the two components of alpha4beta2*-nAChR-mediated (86)Rb(+) efflux from mouse brain differ markedly in EC(50) values for agonist activation, they are equally sensitive to desensitization by exposure to low agonist concentrations. Mice were also chronically treated with nicotine by continuous infusion of 0, 0.5 or 4.0mg/kg/h and desensitization induced by nicotine was evaluated. Consistent with previous results, chronic nicotine treatment increased the density of epibatidine binding sites. Acute exposure to nicotine also elicited concentration-dependent desensitization of both high-sensitivity and low-sensitivity acetylcholine-stimulated (86)Rb(+) efflux from cortical and thalamic synaptosomes. Although chronic nicotine treatment reduced maximal (86)Rb(+) efflux from thalamus, IC(50) values in both brain regions were unaffected by chronic nicotine treatment.

Endogenous acetylcholine modulates impulsive action via alpha4beta2 nicotinic acetylcholine receptors in rats

Nicotine has been well established as an impulsive action-inducing agent, but it remains unknown whether endogenous acetylcholine affects impulsive action via nicotinic acetylcholine receptors. In the present study, the 3-choice serial reaction time task (3-CSRTT), a simple and valid assessment of impulsive action, was employed. Male Wistar/ST rats were trained to detect and respond to 1-s flashes of light presented in one of three holes until stable performance was achieved. Following training on the 3-CSRTT, rats received intracerebroventricular injections of the preferential alpha4beta2 nicotinic acetylcholine receptor antagonist dihydro-beta-erythroidine (DHbetaE; 0, 3, 10, and 30 microg) or the selective alpha7 nicotinic acetylcholine receptor antagonist methyllycaconitine (MLA; 0, 3, 10, and 30 microg) 5 min before test sessions. Injection of 10 microg of DHbetaE significantly suppressed premature responses, an index of impulsive-like action, without changing other behavioral parameters. On the other hand, MLA infusions failed to affect impulsive-like action at any dose. These results suggest that the central alpha4beta2 nicotinic acetylcholine receptors that enable a provoking effect of endogenous acetylcholine play a critical role in impulsive action. Substances that modulate nicotinic acetylcholine receptors, especially the alpha4beta2 subtype, may be beneficial for the treatment of psychiatric disorders characterized by lack of inhibitory control.

Nicotine provokes impulsive-like action by stimulating alpha4beta2 nicotinic acetylcholine receptors in the infralimbic, but not in the prelimbic cortex

RATIONALE

Nicotine, a major addictive component of tobacco, has been suggested to provoke impulsivity by activating central alpha4beta2 nicotinic acetylcholine receptors (nAChRs). Although lesion studies have demonstrated the involvement of the medial prefrontal cortex (mPFC) in impulsive action, the precise brain sites responsible for nicotine-induced impulsive action have not been identified.

OBJECTIVES

Our goal was to determine whether alpha4beta2 nAChRs in the prelimbic cortex (PL) and/or infralimbic cortex (IL), which are subregions of the mPFC, mediate nicotine-induced impulsive-like action in the three-choice serial reaction time task (3-CSRTT).

METHODS

The 3-CSRTT is a simple version of five-choice serial reaction time task and a rodent model of impulsive action in which the animal is required to inhibit the response until a light stimulus is presented randomly in one of three holes. Following the completion of the training, rats were bilaterally injected with dihydro-beta-erythroidine (DHbetaE; 6 and 18 microg/side), a selective alpha4beta2 nAChRs antagonist, into the PL or IL before systemic injection of nicotine (0.2 mg/kg, salt, s.c.).

RESULTS

Intra-IL DHbetaE infusions dose-dependently blocked nicotine-induced impulsive-like action, while infusions of DHbetaE into the PL failed to block the effects of nicotine on impulsive-like action.

CONCLUSION

The present results suggest a critical role for alpha4beta2 nAChRs in the IL in mediating the effects of nicotine on impulsive-like action in the 3-CSRTT.

Mouse striatal dopamine nerve terminals express alpha4alpha5beta2 and two stoichiometric forms of alpha4beta2*-nicotinic acetylcholine receptors

Wild-type and alpha5 null mutant mice were used to identify nicotinic cholinergic receptors (nAChRs) that mediate alpha-conotoxin MII (alpha-CtxMII)-resistant dopamine (DA) release from striatal synaptosomes. Concentration-effect curves for ACh-stimulated release (20 s) were monophasic when wild-type synaptosomes were assayed but biphasic with synaptosomes from the alpha5 null mutant. Deleting the alpha5 gene also resulted in decreased maximal ACh-stimulated alpha-CtxMII-resistant DA release. When a shorter perfusion time (5 s) was used, biphasic curves were detected in both wild-type and alpha5 null mutants, indicative of high- and low-sensitivity (HS and LS) activity. In addition, DHbetaE-sensitive (HS) and DHbetaE-resistant (LS) components were found in both genotypes. These results indicate that alpha-CtxMII-resistant DA release is mediated by alpha4alpha5beta2, (alpha4)(2)(beta2)(3) (HS), and (alpha4)(3)(beta2)(2) (LS) nAChRs.

Hippocampal nAChRs mediate nicotine withdrawal-related learning deficits

Nicotine modulation of learning may contribute to its abuse liability. The role of hippocampal nicotinic acetylcholine receptors (nAChRs) in the effects of acute, chronic and withdrawal from chronic nicotine on learning was assessed via intrahippocampal drug infusion in mice. Acute dorsal hippocampal nicotine infusion enhanced contextual fear conditioning. Conversely, chronic intrahippocampal infusion of a matched dose had no effect, and withdrawal from chronic infusion impaired learning. Thus, hippocampal functional adaptation, evidenced by learning deficits during abstinence, occurs with the transition from acute to chronic nicotine exposure. To investigate which hippocampal nAChRs mediate these adaptations, C57BL/6, beta2 nAChR subunit knockout (KO), and wildtype (WT) mice treated chronically with systemic nicotine received intrahippocampal dihydro-beta-erythroidine (a high affinity nAChR antagonist). Intrahippocampal dihydro-beta-erythroidine precipitated learning deficits in all but the KO mice. Therefore, the action of nicotine at hippocampal beta2 nAChRs mediates adaptations in hippocampal function that underlie withdrawal deficits in contextual fear conditioning.

Pharmacological and immunochemical characterization of alpha2* nicotinic acetylcholine receptors (nAChRs) in mouse brain

AIM

alpha2 nAChR subunit mRNA expression in mice is most intense in the olfactory bulbs and interpeduncular nucleus. We aimed to investigate the properties of alpha2* nAChRs in these mouse brain regions.

METHODS

alpha2 nAChR subunit-null mutant mice were engineered. Pharmacological and immunoprecipitation studies were used to determine the composition of alpha2 subunit-containing (alpha2*) nAChRs in these two regions.

RESULTS

[(125)I]Epibatidine (200 pmol/L) autoradiography and saturation binding demonstrated that alpha2 deletion reduces nAChR expression in both olfactory bulbs and interpeduncular nucleus (by 4.8+/-1.7 and 92+/-26 fmol mg(-1) protein, respectively). Pharmacological characterization using the beta2-selective drug A85380 to inhibit [(125)I]epibatidine binding proved inconclusive, so immunoprecipitation methods were used to further characterize alpha2* nAChRs. Protocols were established to immunoprecipitate beta2 and beta4 nAChRs. Immunoprecipitation specificity was ascertained using tissue from beta2- and beta4-null mutant mice, and efficacy was good (>90% of beta2* and >80% of beta4* nAChRs were routinely recovered).

CONCLUSION

Immunoprecipitation experiments indicated that interpeduncular nucleus alpha2* nAChRs predominantly contain beta2 subunits, while those in olfactory bulbs contain mainly beta4 subunits. In addition, the immunoprecipitation evidence indicated that both nuclei, but especially the interpeduncular nucleus, express nAChR complexes containing both beta2 and beta4 subunits.

Development and optimization of a high-throughput electrophysiology assay for neuronal alpha4beta2 nicotinic receptors

Historically, the identification of alpha4beta2 nicotinic acetylcholine receptor ligands has been based on high-throughput radioligand binding, rubidium efflux assays and Ca++ flux assays using a fluorometric imaging plate reader (FLIPR). Among other approaches, low-throughput electrophysiological assays in Xenopus oocytes and two channel application "liquid filament" systems for mammalian cells have been commonly used. More recent technical innovations that have been introduced into the field of electrophysiology allow for automated simultaneous multi-channel operation. Here we report the development and optimization of a high-throughput electrophysiological assay for identifying functionally active alpha4beta2 nicotinic receptor ligands using such a system. Characterization of the test system yielded results comparable to those obtained by other investigators using conventional electrophysiological assays. For example, the concentration-response relationships obtained for alpha4beta2 receptor activation by acetylcholine and nicotine were best described by biphasic Hill equations, and the inhibition of alpha4beta2 receptor currents by the nicotinic antagonist dihydro-beta-erythroidine was consistent with previously published results. Functional up-regulation of alpha4beta2 receptors by prolonged exposure to nicotine or lower temperature was also confirmed. Using this methodology we were able to characterize the activation of alpha4beta2 receptors by multiple compounds in a mammalian cell expression system, exemplifying its utility for rapid identification of novel nicotinic ligands within a screening cascade. Our results demonstrate the utility of this electrophysiological tool for the discovery of alpha4beta2 nicotinic acetylcholine receptor ligands with potential applications in numerous clinical indications.

Varenicline attenuates nicotine-enhanced brain-stimulation reward by activation of alpha4beta2 nicotinic receptors in rats

Varenicline, a partial alpha4beta2 and full alpha7 nicotinic receptor agonist, has been shown to inhibit nicotine self-administration and nicotine-induced increases in extracellular dopamine in the nucleus accumbens. In the present study, we investigated whether varenicline inhibits nicotine-enhanced electrical brain-stimulation reward (BSR), and if so, which receptor subtypes are involved. Systemic administration of nicotine (0.25-1.0 mg/kg, i.p.) or varenicline (0.03-3 mg/kg, i.p.) produced biphasic effects, with low doses producing enhancement (e.g., decreased BSR threshold), and high doses inhibiting BSR. Pretreatment with low dose (0.03-1.0 mg/kg) varenicline dose-dependently attenuated nicotine (0.25 or 0.5 mg/kg)-enhanced BSR. The BSR-enhancing effect produced by varenicline was blocked by mecamylamine (a high affinity nicotinic receptor antagonist) or dihydro-beta-erythroidine (a relatively selective nicotinic alpha4-containing receptor antagonist), but not methyllycaconitine (a selective alpha7 receptor antagonist), suggesting an effect mediated by activation of alpha4beta2 receptors. This suggestion is supported by findings that the alpha4beta2 receptor agonist SIB-1765F produced a dose-dependent enhancement of BSR, while pretreatment with SIB-1765F attenuated nicotine (0.5 mg/kg)-enhanced BSR. In contrast, the selective alpha7 receptor agonist ARR-17779, altered neither BSR itself nor nicotine-enhanced BSR, at any dose tested. These findings suggest that: 1) varenicline inhibits nicotine-enhanced BSR, supporting its use as a smoking cessation aid; and 2) varenicline-enhanced BSR by itself and varenicline's anti-nicotine effects are mediated by activation of alpha4beta2, but not alpha7, receptors.

Sazetidine-A is a potent and selective agonist at native and recombinant alpha 4 beta 2 nicotinic acetylcholine receptors

Sazetidine-A has been recently proposed to be a "silent desensitizer" of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), implying that it desensitizes alpha4beta2 nAChRs without first activating them. This unusual pharmacological property of sazetidine-A makes it, potentially, an excellent research tool to distinguish between the role of activation and desensitization of alpha4beta2 nAChRs in mediating the central nervous system effects of nicotine itself, as well as those of new nicotinic drugs. We were surprised to find that sazetidine-A potently and efficaciously stimulated nAChR-mediated dopamine release from rat striatal slices, which is mediated by alpha4beta2(*) and alpha6beta2(*) subtypes of nAChR. The agonist effects on native striatal nAChRs prompted us to re-examine the effects of sazetidine-A on recombinant alpha4beta2 nAChRs in more detail. We expressed the two alternative stoichiometries of alpha4beta2 nAChR in Xenopus laevis oocytes and investigated the agonist properties of sazetidine-A on both alpha4(2)beta2(3) and alpha4(3)beta2(2) nAChRs. We found that sazetidine-A potently activated both stoichiometries of alpha4beta2 nAChR: it was a full agonist on alpha4(2)beta2(3) nAChRs, whereas it had an efficacy of only 6% on alpha4(3)beta2(2) nAChRs. In contrast to what has been published before, we therefore conclude that sazetidine-A is an agonist of native and recombinant alpha4beta2 nAChRs but shows differential efficacy on alpha4beta2 nAChRs subtypes.

A role for neuronal nicotinic acetylcholine receptors in ethanol-induced stimulation, but not cocaine- or methamphetamine-induced stimulation

RATIONALE

Cocaine (COC), ethanol (EtOH), and methamphetamine (MA) are widely abused substances and share the ability to induce behavioral stimulation in mice and humans. Understanding the biological basis of behavioral stimulation to COC, EtOH, and MA may provide a greater understanding of drug and alcohol abuse.

OBJECTIVES

In these studies we set out to determine if neuronal nicotinic acetylcholine receptors were involved in the acute locomotor responses to these drugs, our measure of behavioral stimulation.

METHODS

A panel of acetylcholine receptor antagonists was used to determine if nicotinic receptors were involved in EtOH- and psychostimulant-induced stimulation. We tested the effect of these drugs in genotypes of mice (FAST and DBA/2J) that are extremely sensitive to this drug effect. To determine which acetylcholine receptor subunits may be involved in this response, relative expression of the alpha3, alpha6, beta2, and beta4 subunit genes was examined in mice selectively bred for high and low response to EtOH.

RESULTS

Mecamylamine, but not hexamethonium, attenuated the acute locomotor response to EtOH. The acetylcholine receptor antagonist dihydro-beta-erythroidine and methyllycaconitine had no effect on this response. The alpha6 and beta4, but not alpha3 or beta2, subunits of the acetylcholine receptor were differentially expressed between mice bred for extreme differences in EtOH stimulation. Mecamylamine had no effect on psychostimulant-induced locomotor activity.

CONCLUSIONS

Neuronal nicotinic receptors are involved in EtOH, but not psychostimulant, stimulation. These studies suggest a lack of involvement of some nicotinic receptor subtypes, but more work is needed to determine the specific receptor subtypes involved in this behavior.

Nicotinic Receptors: Role in Addiction and Other Disorders of the Brain

Nicotine, the addictive component of cigarette smoke has profound effects on the brain. Activation of its receptors by nicotine has complex consequences for network activity throughout the brain, potentially contributing to the addictive property of the drug. Nicotinic receptors have been implicated in psychiatric illnesses like schizophrenia and are also neuroprotective, potentially beneficial for neurodegenerative diseases. These effects of nicotine serve to emphasize the multifarious roles the drug, acting through multiple nicotinic acetylcholine receptor subtypes. The findings also remind us of the complexity of signaling mechanisms and stress the risks of unintended consequences of drugs designed to combat nicotine addiction.

Hippocampal alpha4beta2 nicotinic acetylcholine receptor involvement in the enhancing effect of acute nicotine on contextual fear conditioning

Nicotine is known to enhance learning and memory in hippocampus-dependent tasks such as contextual fear conditioning. The present study was designed to directly examine whether the hippocampus plays a role in mediating this enhancement and which nicotinic acetylcholine receptor (nAChR) subtypes localized to the hippocampus are critical for enhanced learning. Contextual fear conditioning consisted of two white noise conditioned stimuli presentations, each coterminating with a 2 s, 0.57 mA footshock separated by a 120 s intertrial interval. Nicotine (0.09, 0.18, and 0.35 microg per side) was bilaterally infused into the dorsal hippocampus before training and testing. Infusions of nicotine into the dorsal hippocampus produced a dose-dependent enhancement of contextual fear conditioning. To determine which nAChRs are critical to the enhancing effect of nicotine, the preferential alpha4beta2 nAChR antagonist, dihydro-beta-erythroidine (DHbetaE) (6.00 and 18.00 microg per side), or the preferential alpha7 nAChR antagonist, methyllycaconitine (MLA) (13.50 and 27.00 microg per side), was bilaterally infused into the dorsal hippocampus before systemic injections of nicotine (0.09 mg/kg). DHbetaE infusions dose-dependently blocked the enhancement of contextual fear conditioning by nicotine, whereas MLA infusions yielded an intermediate effect. In addition, neither DHbetaE nor MLA had an effect on contextual fear conditioning in the absence of systemic nicotine. The present results suggest a critical role for alpha4beta2 nAChRs in the dorsal hippocampus for mediating the enhancing effect of nicotine on contextual fear conditioning.

Postnatal expression of alpha2 nicotinic acetylcholine receptor subunit mRNA in developing cortex and hippocampus

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated cation channels composed of alpha and beta subunits. nAChR subunit expression is highly regulated during development. Previous studies have revealed increased expression of alpha3, alpha5, alpha7, and beta4 subunit mRNAs and alpha7 binding sites during hippocampal and cortical development. Here, we examined the expression of alpha2 subunit mRNA in rat cortex and hippocampus using highly sensitive radioactive in situ hybridization. alpha2 Subunit mRNA expression was first detected at P3 in cortex and hippocampus. During postnatal development the distribution of alpha2 subunit mRNA expression was spatially similar to the one found in adult, exhibiting highly restricted expression in scattered cells mostly in cortical layer V and retrosplenial cortex, and in scattered cells in CA1/CA3 stratum oriens and CA3 stratum radiatum. However, the expression intensity and number of alpha2 positive cells strongly increased to reach peak levels in both cortex and hippocampus at P7 and decreased thereafter to moderate to low to levels. Double in situ hybridization revealed that most, but not all, alpha2 mRNA expression was located in non-pyramidal GAD-positive cortical and hippocampal interneurons. Thus, similar to other nAChR subunits, alpha2 mRNA expression is transiently upregulated during postnatal development and nAChRs containing alpha2 subunits could regulate GABAergic activity during a critical period of network formation.

5-I A-85380 and TC-2559 differentially activate heterologously expressed α4β2 nicotinic receptors

The neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunits expressed in heterologous expression systems assemble into at least two distinct subunit stoichiometries of alpha4beta2 receptor. The (alpha4)2(beta2)3 stoichiometry is about 100-fold more sensitive to acetylcholine than the (alpha4)3(beta2)2 stoichiometry. In order to investigate if agonists in general distinguish high- and low-affinity alpha4beta2 nicotinic acetylcholine receptors, we have expressed human alpha4 and beta2 nicotinic acetylcholine receptor subunits in two different expression systems. The relative amounts of alpha4beta2 nicotinic acetylcholine receptors with high- and low-affinity for acetylcholine were manipulated by (a) injecting the subunit cDNAs at different alpha:beta ratios into Xenopus oocytes and (b) by culturing HEK-293 cells stably expressing alpha4beta2 nicotinic acetylcholine receptors overnight at different temperatures. The sensitivities of the alpha4beta2 nicotinic acetylcholine receptors to the agonists acetylcholine, 5-I A-85380, and TC-2559 were investigated using the voltage-clamp technique on Xenopus oocytes and using a fluorescent imaging plate reader to measure calcium responses from HEK-293 cells. Like acetylcholine, 5-I A-85380 produced biphasic concentration-response curves and the high-affinity component became larger when the cells were manipulated to produce a greater proportion of (alpha4)2(beta2)3 nicotinic acetylcholine receptors. Interestingly, under all circumstances, TC-2559 produced monophasic concentration-response curves. In oocytes injected with alpha4 and beta2 subunits in the 1:1 ratio the maximum effect of TC-2559 was 28% of that of acetylcholine. The EC50 for TC-2559 was not changed when oocytes were manipulated to express exclusively (alpha4)2(beta2)3 nicotinic acetylcholine receptors, however, the maximum effect of TC-2559 was dramatically enhanced. These results suggest that TC-2559 is a selective agonist of the (alpha4)2(beta2)3 nicotinic acetylcholine receptor stoichiometry.

The effects of DHBE and MLA on nicotine-induced enhancement of contextual fear conditioning in C57BL/6 mice

RATIONALE

Previous research indicates that nicotine administration enhances hippocampus-dependent forms of learning, including contextual fear conditioning. This effect is blocked by mecamylamine, a noncompetitive, broad-spectrum nicotinic receptor antagonist.

OBJECTIVES

The present study extends previous research by further characterizing the nicotinic acetylcholinergic receptor (nAChR) subtypes through which nicotine acts to enhance contextual fear conditioning.

METHODS

C57BL/6J mice were trained with two conditioned stimulus (CS; 30 s, 85-dB white noise)-unconditioned stimulus (US; 2 s, 0.57-mA foot shock) pairings and tested 24 h later for contextual and cued fear conditioning. The effects of the alpha7 nAChR antagonist methyllycaconitine (MLA; 1.00, 10.00, and 20.00 mg/kg) and the effects of the alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (DHBE; 1.00, 3.00, and 6.00 mg/kg) on cued and contextual fear conditioning and on the enhancement of contextual fear conditioning by nicotine (0.25 mg/kg) were examined.

RESULTS

We demonstrate that DHBE (all doses) administration attenuates the enhancing effect of nicotine on contextual fear conditioning, and MLA administration has no significant effect on the enhancement of contextual fear conditioning by nicotine.

CONCLUSIONS

The data suggest that non-alpha7 nAChRs (most likely alpha4beta2 nAChRs) underlie the enhancement of contextual fear conditioning by nicotine.

The beta2 but not alpha7 subunit of the nicotinic acetylcholine receptor is required for nicotine-conditioned place preference in mice

RATIONALE

Tobacco use is implicated in approximately 440,000 deaths per year, making it the leading cause of preventable death in the United States. Although it is generally recognized that tobacco use is correlated with a variety of health-related complications, many smokers are unsuccessful in their efforts to stop smoking using current cessation therapies.

OBJECTIVES

Given that nicotine is the addictive component of tobacco, successful smoking cessation therapies must address the various processes, including reward, which contribute to nicotine addiction. As such, determining the nicotinic receptor subtypes involved in nicotine reward is of utmost importance to understanding how nicotine addiction progresses.

METHODS

Conditioned place preference (CPP) in three-chamber conditioning boxes was performed. For antagonist studies, drug was given on all conditioning sessions 10 min before nicotine or saline injection and placement in the boxes.

RESULTS

We have demonstrated that a pretreatment with the alpha4beta2 subunit of the nicotinic acetylcholine receptor (nAChR) antagonist dihydro-beta-erythroidine (2.0 mg/kg, s.c.) blocked nicotine (0.5 mg/kg, s.c.) CPP in wild-type mice (C57BL/6 mice). In contrast, pretreatment with an antagonist of the alpha7 subunit of the nAChR, methyllycaconitine (MLA, 5.0 or 10.0 mg/kg, s.c.), had no effect on this behavior. Finally, we showed that mice lacking the beta2 subunit of the nAChR did not exhibit nicotine CPP while alpha7 knock-out mice did.

CONCLUSION

Taken together, these data suggest that the beta2 subunit of the nAChR is critically involved in nicotine reward as measured by CPP.

Influence of melatonin on the development of functional nicotinic acetylcholine receptors in cultured chick retinal cells

The influence of melatonin on the developmental pattern of functional nicotinic acetylcholine receptors was investigated in embryonic 8-day-old chick retinal cells in culture. The functional response to acetylcholine was measured in cultured retina cells by microphysiometry. The maximal functional response to acetylcholine increased 2.7 times between the 4th and 5th day in vitro (DIV4, DIV5), while the Bmax value for [125I]-alpha-bungarotoxin was reduced. Despite the presence of alpha8-like immunoreactivity at DIV4, functional responses mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors were observed only at DIV5. Mecamylamine (100 microM) was essentially without effect at DIV4 and DIV5, while dihydro-ss-erythroidine (10-100 microM) blocked the response to acetylcholine (3.0 nM-2.0 microM) only at DIV4, with no effect at DIV5. Inhibition of melatonin receptors with the antagonist luzindole, or melatonin synthesis by stimulation of D4 dopamine receptors blocked the appearance of the alpha-bungarotoxin-sensitive response at DIV5. Therefore, alpha-bungarotoxin-sensitive receptors were expressed in retinal cells as early as at DIV4, but they reacted to acetylcholine only after DIV5. The development of an alpha-bungarotoxin-sensitive response is dependent on the production of melatonin by the retinal culture. Melatonin, which is produced in a tonic manner by this culture, and is a key hormone in the temporal organization of vertebrates, also potentiates responses mediated by alpha-bungarotoxin-sensitive receptors in rat vas deferens and cerebellum. This common pattern of action on different cell models that express alpha-bungarotoxin-sensitive receptors probably reflects a more general mechanism of regulation of these receptors.

The interactive effects of nicotinic and muscarinic cholinergic receptor inhibition on fear conditioning in young and aged C57BL/6 mice

Both normal aging and age-related disease, such as Alzheimer's disease, have diverse effects on forebrain-dependent cognitive tasks as well as the underlying neurobiological substrates. The purpose of the current study was to investigate if age-related alterations in the function of the cholinergic system are associated with memory impairments in auditory-cued and contextual fear conditioning. Young (2-3 months) and aged (19-20 months) C57BL/6 mice were administered scopolamine (0.1, 0.3, 0.5, or 1.0 mg/kg), a muscarinic cholinergic receptor antagonist, mecamylamine (1.0 and 2.0 mg/kg), a nicotinic cholinergic receptor antagonist, both scopolamine and mecamylamine (0.1 and 1.0 mg/kg, respectively), or saline prior to training. Training consisted of two white-noise CS (85 dB, 30 s)-footshock US (0.57 mA, 2 s) presentations. Testing occurred 48 h post-training. Scopolamine administration impaired contextual and cued fear conditioning in young and aged mice, although the aged mice were less sensitive to disruption by scopolamine. Mecamylamine did not disrupt conditioned fear in the young or aged mice. Scopolamine and mecamylamine co-administration, at doses sub-threshold for disrupting fear conditioning with separate administration, disrupted contextual and auditory-cued fear conditioning in the young mice, indicating that in the young mice the muscarinic and nicotinic cholinergic processes interact in the formation and maintenance of long-term memories for conditioned fear. Co-administration of both antagonists did not disrupt fear conditioning in the aged mice, indicating that age-related alterations in the cholinergic receptor subtypes may occur.

Alkaloids indolizidine 235B', quinolizidine 1-epi-207I, and the tricyclic 205B are potent and selective noncompetitive inhibitors of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are key molecules in cholinergic transmission in the nervous system. Because of their structural complexity, only a limited number of subtype-specific agonists and antagonists are available to study nicotinic receptor functions. To overcome this limitation, we used voltageclamp recordings to examine the effects of several frog skin alkaloids on acetylcholine-elicited currents in Xenopus laevis oocytes expressing major types of neuronal nicotinic receptors (alpha4beta2, alpha7, alpha3beta2, alpha3beta4, and alpha4beta4). We found that the 5,8-disubstituted indolizidine (-)-235B' acted as a potent noncompetitive blocker of alpha4beta2 nicotinic receptors (IC50 = 74 nM). This effect was highly selective for alpha4beta2 receptors compared with alpha3beta2, alpha3beta4, and alpha4beta4 receptors. The inhibition of alpha4beta2 currents by (-)-235B' was more pronounced as the acetylcholine concentration increased (from 10 nM to 100 microM). Moreover, the blockade of alpha4beta2 currents by (-)-235B' was voltage-dependent (more pronounced at hyperpolarized potentials) and use-dependent, indicating that (-)-235B' behaves as an open-channel blocker of this receptor. Several other 5,8-disubstituted indolizidines (5-n-propyl-8-n-butylindolizidines), two 5,6,8-trisubstituted indolizidines ((-)-223A and (+)-6-epi-223A), and a 1,4-disubstituted quinolizidine ((+)-207I) were less potent than (-)-235B', and none showed selectivity for alpha4beta2 receptors. The quinolizidine (-)-1-epi-207I and the tricyclic (+)-205B had 8.7- and 5.4-fold higher sensitivity, respectively, for inhibition of the alpha7 nicotinic receptor than for inhibition of the alpha4beta2 receptor. These results show that frog alkaloids alter the function of nicotinic receptors in a subtype-selective manner, suggesting that an analysis of these alkaloids may aid in the development of selective drugs to alter nicotinic cholinergic functions.