Effects of nicotinic antagonists on working memory performance in young rhesus monkeys.

Acetylcholine plays a pivotal neuromodulatory role in the brain, influencing neuronal activity and cognitive function. Nicotinic receptors, particularly α7 and α4ß2 receptors, modulate firing of dorsolateral prefrontal (dlPFC) excitatory networks that underlie successful working memory function. Minimal work however has been done examining working memory following systemic blockade of nicotinic receptor systems in nonhuman primates, limiting the ability to explore interactions of other neuromodulatory influences with working memory impairment caused by nicotinic antagonism. In this study, we investigated working memory performance after administering three nicotinic antagonists, mecamylamine, methyllycaconitine, and dihydro-ß-erythroidine, in rhesus macaques tested in a spatial delayed response task. Surprisingly, we found that no nicotinic antagonist significantly impaired delayed response performance compared to vehicle. In contrast, the muscarinic antagonist scopolamine reliably impaired delayed response performance in all monkeys tested. These findings suggest there are some limitations on using systemic nicotinic antagonists to probe the involvement of nicotinic receptors in aspects of dlPFC-dependent working memory function, necessitating alternative strategies to understand the role of this system in cognitive deficits seen in aging and neurodegenerative disease.

Nicotinic aspects of the discriminative stimulus effects of arecoline.

Despite the evidence that the muscarinic agonist arecoline is a drug of abuse throughout Southeast Asia, its stimulus characteristics have not been well studied. The goal of this work was to understand more about the mediation of discriminative stimulus effects of arecoline. Arecoline (1.0 mg/kg s.c.) was trained as a discriminative stimulus in a group of eight rats. The ability of various cholinergic agonists and antagonists to mimic or antagonize the discriminative stimulus effects of arecoline and to modify its rate-suppressing effects was evaluated. A muscarinic antagonist, but neither of two nicotinic antagonists, was able to modify the discriminative stimulus effects of arecoline, suggesting a predominant muscarinic basis of arecoline's discriminative stimulus effects in this assay. However, both nicotine itself and two nicotine agonists with selective affinity for the α4ß2* receptor (ispronicline and metanicotine) produced full arecoline-like discriminative stimulus effects in these rats. The discriminative stimulus effects of the selective nicotine agonists were blocked by both the general nicotine antagonist mecamylamine and by the selective α4ß2* antagonist, dihydro-beta-erythroidine (DHßE). Surprisingly, only DHßE antagonized the rate-suppressing effects of the selective nicotine agonists. These data indicate a selective α4ß2* nicotine receptor component to the behavioral effects of arecoline. Although the nicotinic aspects of arecoline's behavior effects could suggest that abuse of arecoline-containing material (e.g. betel nut chewing) is mediated through nicotinic rather than muscarinic actions, further research, specifically on the reinforcing effects of arecoline, is necessary before this conclusion can be supported.

Nicotine-like discriminative and aversive effects of two α4ß2-selective nicotine agonists, ispronicline and metanicotine.

An attempt to determine the receptor selective nature of some of nicotine's behavioral effects was undertaken through the evaluation of the ability of two nicotinic α4ß2*-selective receptor agonists to produce nicotine-like effects and modify rates of responding in a discrimination assay and in an aversive stimulus assay. A group of eight rats was trained to discriminate the presence of 1 mg/kg nicotine base. Another group of 4-6 rats was trained to report the aversive effects of nicotine by selecting a lever that produced one food pellet over a second lever that produced two food pellets and an intravenous injection of nicotine. Ispronicline and metanicotine, two α4ß2*-selective receptor agonists, increased selection of the nicotine-appropriate lever in a dose-related manner, up to a maximum of approximately 75%. The α4ß2*-selective receptor antagonist, dihydro-beta-erythroidine blocked both the discriminative stimulus effects and the rate-suppressing effects of ispronicline, metanicotine, and small, but not large doses of nicotine. The nonselective antagonist, mecamylamine, antagonized the discriminative stimulus effects of each of the three nicotine agonists as well as the rate-decreasing effects of nicotine and metanicotine. Mecamylamine did not modify the rate-decreasing effects of ispronicline. Both ispronicline and metanicotine as well as nicotine were avoided in the drug + food vs. food choice situation. The receptor-selective nature of ispronicline and metanicotine was hereby confirmed in a behavioral assay, as were earlier reports that the discriminative stimulus effects of relatively small doses of nicotine are likely mediated by activity at the α4ß2* nicotine receptor.

Nicotine Enhances Object Recognition Memory via Stimulating α4ß2 and α7 Nicotinic Acetylcholine Receptors in the Medial Prefrontal Cortex of Mice.

Nicotine has been known to enhance recognition memory in various species. However, the brain region where nicotine acts and exerts its effect remains unclear. Since the medial prefrontal cortex (mPFC) is associated with memory, we examined the role of the mPFC in nicotine-induced enhancement of recognition memory using the novel object recognition test in male C57BL/6J mice. Systemic nicotine administration 10 min before training session significantly enhanced object recognition memory in test session that was performed 24 h after the training. Intra-mPFC infusion of mecamylamine, a non-selective nicotinic acetylcholine receptor (nAChR) antagonist, 5 min before nicotine administration blocked the effect of nicotine. Additionally, intra-mPFC infusion of dihydro-ß-erythroidine, a selective α4ß2 nAChR antagonist, or methyllycaconitine, a selective α7 nAChR antagonist, significantly suppressed the nicotine-induced object recognition memory enhancement. Finally, intra-mPFC infusion of nicotine 1 min before the training session augmented object recognition memory in a dose-dependent manner. These findings suggest that mPFC α4ß2 and α7 nAChRs mediate the nicotine-induced object recognition memory enhancement.

Nicotine stimulates ion transport via metabotropic ß4 subunit containing nicotinic ACh receptors.

BACKGROUND AND PURPOSE: Mucociliary clearance is an innate immune process of the airways, essential for removal of respiratory pathogens. It depends on ciliary beat and ion and fluid homeostasis of the epithelium. We have shown that nicotinic ACh receptors (nAChRs) activate ion transport in mouse tracheal epithelium. Yet the receptor subtypes and signalling pathways involved remained unknown. EXPERIMENTAL APPROACH: Transepithelial short circuit currents (ISC ) of freshly isolated mouse tracheae were recorded using the Ussing chamber technique. Changes in [Ca2+ ]i were studied on freshly dissociated mouse tracheal epithelial cells. KEY RESULTS: Apical application of the nAChR agonist nicotine transiently increased ISC . The nicotine effect was abolished by the nAChR antagonist mecamylamine. α-Bungarotoxin (α7 antagonist) had no effect. The agonists epibatidine (α3ß2, α4ß2, α4ß4 and α3ß4) and A-85380 (α4ß2 and α3ß4) increased ISC . The antagonists dihydro-ß-erythroidine (α4ß2, α3ß2, α4ß4 and α3ß4), α-conotoxin MII (α3ß2) and α-conotoxin PnIA (α3ß2) reduced the nicotine effect. Nicotine- and epibatidine-induced currents were unaltered in ß2-/- mice, but in ß4-/- mice no increase was observed. In the presence of thapsigargin (endoplasmatic reticulum Ca2+ -ATPase inhibitor) or the ryanodine receptor antagonists JTV-519 and dantrolene there was a reduction in the nicotine-effect, indicating involvement of Ca2+ release from intracellular stores. Additionally, the PKA inhibitor H-89 and the TMEM16A (Ca2+ -activated chloride channel) inhibitor T16Ainh-A01 significantly reduced the nicotine-effect. CONCLUSION AND IMPLICATIONS: α3ß4 nAChRs are responsible for the nicotine-induced current changes via Ca2+ release from intracellular stores, PKA and ryanodine receptor activation. These nAChRs might be possible targets to stimulate chloride transport via TMEM16A.

Repeated phencyclidine disrupts nicotinic acetylcholine regulation of dopamine release in nucleus accumbens: Implications for models of schizophrenia.

Dopaminergic dysregulation in nucleus accumbens has been implicated in the origin of schizophrenia. Accumbal cholinergic interneurons exert powerful modulatory control of local dopamine function, through nicotinic receptors located on dopamine terminals. Fast-scan cyclic voltammetry in rat brain slices in vitro was used to measure dopamine release evoked by high-frequency electrical stimulation, mimicking phasic dopamine activity. We investigated whether cholinergic regulation of stimulated dopamine release was disrupted by pretreatment with phencyclidine, a non-competitive NMDA receptor antagonist, which provides a well validated animal model of schizophrenia. Dihydro-ß-erythroidine, an antagonist at ß2-subuit containing nicotinic receptors, caused a concentration-dependent enhancement of stimulated dopamine release, indicating cholinergic inhibitory control over dopamine release. The agonist, nicotine, also caused concentration-dependent increases in release, consistent with rapid desensitisation of the receptors previously described. In slices taken from animals pretreated with phencyclidine, the augmentation of electrically-stimulated dopamine release elicited by both drugs was attenuated, particularly when each drug was applied at high concentration. In addition, the concentration-dependence of each drug effect was lost. Taken together these findings indicate that pretreatment with phencyclidine causes changes in acetylcholine systems modulating dopamine release in accumbens. Since phencyclidine treatment was terminated at least a week before the slices were taken, the effects are due to long-term changes in function resulting from the treatment, rather than from transient changes due to the presence of the drug at test. Such enduring dysregulation of cholinergic control of phasic dopamine release could account for deficits in behaviours mediated by accumbal dopamine seen in schizophrenia, and may provide a route for novel therapeutic strategies to treat the disease.

The discriminative stimulus effects of epibatidine in C57BL/6J mice.

The α4ß2* nicotinic acetylcholine receptor (nAChR) subtypes are targeted for the development of smoking cessation aids, and the use of drug discrimination in mice provides a robust screening tool for the identification of drugs acting through nAChRs. Here, we established that the α4ß2* nAChR agonist epibatidine can function as a discriminative stimulus in mice. Male C57BL/6J mice discriminated epibatidine (0.0032 mg/kg, subcutaneously) and were tested with agonists varying in selectivity and efficacy for α4ß2* nAChRs. The discriminative stimulus effects of epibatidine were characterized with the nonselective, noncompetitive nicotinic antagonist mecamylamine, with the selective ß2-substype-containing nAChR antagonist dihydro-ß-erythroidine hydrobromide (DHßE), and the α7 antagonist methyllycaconitine (MLA). Nicotine (0.32-1.0 mg/kg, subcutaneously), the partial nAChR agonist cytisine (1.0-5.6 mg/kg, subcutaneously), and the α7 nAChR agonist N-[(3R)-1-azabicyclo[2.2.2]oct-3-yl]-4-chlorobenzamide (10-56 mg/kg, intraperitoneally) produced no more than 33% epibatidine-appropriate responding. The partial α4ß2* nAChR agonists varenicline and 2'-fluoro-3'-(4-nitro-phenyl)deschloroepibatidine produced 61 and 69% epibatidine-appropriate responding, respectively. DHßE and mecamylamine, but not MLA, significantly antagonized the discriminative stimulus effects of epibatidine. These results show that epibatidine may be trained as a discriminative stimulus in mice and has utility in elucidating the in-vivo pharmacology of α4ß2* nAChR ligands.

Unexpected loss of sensitivity to the nicotinic acetylcholine receptor antagonist activity of mecamylamine and dihydro-ß-erythroidine in nicotine-tolerant mice.

OBJECTIVES: There is a long-standing interest in developing nicotinic acetylcholine receptor (nAChR) antagonists for concomitant use with nAChR agonists (e.g., nicotine replacement) as complementary smoking cessation aids. Previous studies demonstrate that daily nicotine treatment confers tolerance to some effects of nicotine, as well as cross-tolerance to other nAChR agonists. The current study assessed the extent to which antagonism of nicotine varies as a function of daily nicotine treatment. METHODS: Schedule-controlled responding and hypothermia were selected for study because they have been previously used to examine the pharmacology of nicotine, and both are sensitive to the development nicotine tolerance. The rate-decreasing and hypothermic effects of nicotine, as well as antagonism of those effects, were examined in C57BL/6J mice before, during treatment with, and after discontinuation of three daily injections of 1.78 mg/kg nicotine. The nonselective nAChR antagonist mecamylamine and the ß2 nAChR antagonist dihydro-ß-erythroidine (DHßE) were studied in combination with nicotine. RESULTS: The ED50 values of nicotine to produce rate-decreasing and hypothermic effects were, respectively, 0.44 and 0.82 mg/kg prior, 1.6 and 3.2 mg/kg during, and 0.74 and 1.1 mg/kg after discontinuation of daily nicotine treatment. Prior to daily nicotine treatment, mecamylamine decreased response rate and rectal temperature. However, during daily nicotine, mecamylamine (up to 5.6 mg/kg) only decreased rectal temperature. DHßE (up to 5.6 mg/kg) when studied prior to daily nicotine decreased rectal temperature, but that decrease was abolished during chronic nicotine treatment. Mecamylamine and DHßE antagonized the rate-decreasing and hypothermic effects of nicotine before and after daily nicotine; however, during daily nicotine, mecamylamine and DHßE antagonized only the hypothermic effects of nicotine. CONCLUSIONS: The differential antagonism of rate-decreasing and hypothermic effects implicates differential involvement of nAChR subtypes. The decreased capacity of mecamylamine and DHßE to antagonize nicotine during chronic nicotine treatment may indicate that their effectiveness as smoking cessations might vary as a function of nicotine tolerance and dependence.

Nicotine Induced Neurocognitive Protection and Anti-inflammation Effect by Activating α 4ß 2 Nicotinic Acetylcholine Receptors in Ischemic Rats.

INTRODUCTION: The main objective of this study was to explore the mechanism of nicotine improving cognitive impairments in ischemic rats. METHODS: Twenty adult male Sprague-Dawley (SD) rats underwent ischemic model surgery by injecting endothelin-1 into the left thalamus, which were classified into four different groups with different intervention: nicotine (1.5 mg/kg/d), dihydro-ß-erythroidine (DHßE; 3 mg/kg/d), nicotine (1.5 mg/kg/d) + DHßE (3 mg/kg/d), or saline, after ischemic model surgery. Another five male SD rats also underwent same surgery, while not injecting endothelin-1 but saline, as the control group. Morris water maze (MWM) test was adopted to assess the cognition. All the rats underwent the MWM test, micro positron emission tomography imaging with 2-[18F]-A-85380, and messenger RNA (mRNA) test of α 4 nicotinic acetylcholine receptor (nAChR), ß 2 nAChR, tumor necrosis factor-alpha (TNF-α), IL-1ß, and IL-6. RESULTS: The MWM test showed the rats given nicotine showing better memory than ischemic rats (p < .05), whereas the rats given DHßE or both nicotine and DHßE did not show any statistical difference from the ischemic rats (p > .05). Micro positron emission tomography imaging showed higher uptake of tracer in the left thalamus and whole brain in rats given nicotine than in ischemic rats, but the rats given DHßE or both nicotine and DHßE did not. By real-time PCR test, the mRNA of α 4 nAChR and ß 2 nAChR in rats given nicotine increased significantly compared with ischemic rats and decreased TNF-α, IL-1ß, and IL-6 mRNA (all ps < .05). CONCLUSIONS: By activating α 4ß 2 nAChRs, nicotine plays a role in inhibiting the inflammatory factors, which contributes to improving cognitive impairment in ischemic rats. IMPLICATIONS: It is well acknowledged that vascular cognitive impairment (VCI) is the second most common cause of dementia after Alzheimer's disease. Cholinergic agents have potential for the symptomatic treatment of the cognitive symptoms of dementia, but the exact mechanism still remains unclear. There are potential complex associations and interactions between VCI and inflammation. This study showed that nicotine had anti-inflammatory potency, which is most likely because of the activation of the nAChRs. By activating α4ß2 nAChRs, nicotine played a role in inhibiting the inflammatory factors, which contribute to improving cognitive impairment in ischemic rats.

HPLC-DAD-SPE-NMR isolation of tetracyclic spiro-alkaloids with antiplasmodial activity from the seeds of Erythrina latissima.

Seven tetracyclic spiro-alkaloids, i.e. glucoerysodine (1), erysodine (2), epi-erythratidine (3), erysovine (4), erythratidine (5), erysotrine (6) and erythraline (7) were isolated from the seeds of Erythrina latissima by means of conventional separation methods and HPLC-DAD-SPE-NMR. Their structures were elucidated by spectroscopic means. This is the first report on the isolation of compounds 3, 5 and 6 from this plant. Antiplasmodial activity against the chloroquine-resistant strain Plasmodium falciparum K1 and cytotoxicity against MRC-5 cells (human fetal lung fibroblast cells) was assessed in vitro. Erysodine (2) and erysovine (4) showed moderate activity (IC50 6.53 µM and 4.05 µM, respectively), compared with the standard chloroquine (IC50 = 0.14 µM). No cytotoxicity was observed in a concentration up to 64.0 µM.

α4ß2 nicotinic acetylcholine receptors intrinsically influence body weight in mice.

Desensitization of the nicotinic acetylcholine receptor (nAChR) containing the ß2 subunit is a potentially critical mechanism underlying the body weight (BW) reducing effects of nicotine. The purpose of this study was a) to determine the α subunit(s) that partners with the ß2 subunit to form the nAChR subtype that endogenously regulates energy balance and b) to probe the extent to which nAChR desensitization could be involved in the regulation of BW. We demonstrate that deletion of either the α4 or the ß2, but not the α5, subunit of the nAChR suppresses weight gain in a sex-dependent manner. Furthermore, chronic treatment with the ß2-selective nAChR competitive antagonist dihydro-ß-erythroidine (DHßE) in mice fed a high-fat diet suppresses weight gain. These results indicate that heteromeric α4ß2 nAChRs play a role as intrinsic regulators of energy balance and that desensitizing or inhibiting this nAChR is likely a relevant mechanism and thus could be a strategy for weight loss.

Brainstem cholinergic pathways diminish cardiovascular and neuroinflammatory actions of endotoxemia in rats: Role of NFκB/α7/α4ß2AChRs signaling.

Nicotine improves endotoxic manifestations of hypotension and cardiac autonomic dysfunction in rats. Here, we test the hypothesis that brainstem antiinflammatory pathways of α7/α4ß2 nicotinic acetylcholine receptors (nAChRs) modulate endotoxic cardiovascular derangements. Pharmacologic and molecular studies were performed to determine the influence of nicotine or selective α7/α4ß2-nAChR ligands on cardiovascular derangements and brainstem neuroinflammation caused by endotoxemia in conscious rats. The i.v. administration of nicotine (50 µg/kg) abolished the lipopolysaccharide (LPS, 10 mg/kg i.v.)-evoked: (i) falls in blood pressure and spectral measure of cardiac sympathovagal balance (ratio of the low-frequency to high-frequency power, LF/HF), (ii) elevations in immunohistochemical protein expressions of NFκB and α4ß2-nAChR in medullary neurons of the nucleus tractus solitarius (NTS) and rostral ventrolateral medulla (RVLM), and (iii) decreases in medullary α7-nAChR protein expression. These favorable nicotine influences were replicated in rats treated intracisternally (i.c.) with PHA-543613 (selective α7-nAChR agonist) or 5-iodo-A-85380 (5IA, selective α4ß2-nAChRs agonist). Measurement of arterial baroreflex activity by the vasoactive method revealed that nicotine, PHA, or 5IA reversed the LPS depression of reflex bradycardic, but not tachycardic, activity. Moreover, the counteraction by nicotine of LPS hypotension was mostly inhibited after treatment with i.c. methyllycaconitine (MLA, α7-nAChR antagonist) in contrast to a smaller effect for dihydro-ß-erythroidine (DHßE, α4ß2-nAChR antagonist), whereas the associated increases in LF/HF ratio remained unaltered. The data signifies the importance of brainstem α7, and to a lesser extent α4ß2, receptors in the nicotine counteraction of detrimental cardiovascular and neuroinflammatory consequences of endotoxemia.

Enantioselective Total Synthesis of (+)-Dihydro-ß-erythroidine.

Erythrina alkaloids represent a rich source of complex polycyclic, bioactive natural products. In addition to their sedative and hypotensive effect, their curare-like activity and structural framework have made them attractive targets for synthetic and medicinal chemists. (+)-Dihydro-ß-erythroidine (DHßE), the most potent nicotine acetylcholine receptor antagonist (nAChR) of the Erythrina family, is synthesized for the first time in 13 steps from commercially available material.

Molecular dynamics simulations of dihydro-ß-erythroidine bound to the human α4ß2 nicotinic acetylcholine receptor.

BACKGROUND AND PURPOSE: The heteromeric α4ß2 nicotinic acetylcholine receptor (nAChR) is abundant in the human brain and is associated with a range of CNS disorders. This nAChR subtype has been recently crystallised in a conformation that was proposed to represent a desensitised state. Here, we investigated the conformational transition mechanism of this nAChR from a desensitised to a closed/resting state. EXPERIMENTAL APPROACH: The competitive antagonist dihydro-ß-erythroidine (DHßE) was modelled by replacement of the agonist nicotine in the α4ß2 nAChR experimental structure. DHßE is used both in vitro and in vivo for its ability to block α4ß2 nAChRs. This system was studied by three molecular dynamics simulations with a combined simulation time of 2.6 µs. Electrophysiological studies of mutated receptors were performed to validate the simulation results. KEY RESULTS: The relative positions of the extracellular and transmembrane domains in the models are distinct from those of the desensitised state structure and are compatible with experimental structures of Cys-loop receptors captured in a closed/resting state. CONCLUSIONS AND IMPLICATIONS: Our model suggests that the side chains of α4 L257 (9') and α4 L264 (16') are the main constrictions in the transmembrane pore. The involvement of position 9' in channel gating is well established, but position 16' was only previously identified as a gate for the bacterial channels, ELIC and GLIC. L257 but not L264 was found to influence the slow component of desensitisation. The structure of the antagonist-bound state proposed here should be valuable for the development of therapeutic or insecticide compounds.

The Place of Erythroidines in the History of Neuromuscular Blockers.

Between 1938 and 1951 erythroidine derivatives were seriously considered as alternatives to curare for the provision of muscle relaxation. This has been overlooked in the published history of anaesthesia. The first publication on the paralysing effect of an extract of Erythrina americana was in 1877, but this was in a Mexican journal, which was not widely read. Sixty years later erythroidine was isolated, and in 1938 it was first used clinically to treat spastic dystonia, preceding the use of Intocostrin for this purpose. By 1943 dihydro-ß-erythroidine was prepared in crystalline form, which was equipotent with curarine and of acceptable duration; it was used in clinical anaesthesia in 1946. In the 1940s curare was presented in solutions with potency stated in units, determined by bioassay, which was a disadvantage compared with the straightforward mg of dihydro-ß-erythroidine. However, by the early 1950s, improvement in the pharmaceutical presentation of d-tubocurarine and new neuromuscular blockers, displaced the erythroidines.

Inhibition of Nigrostriatal Dopamine Release by Striatal GABAA and GABAB Receptors.

Nigrostriatal dopamine (DA) is critical to action selection and learning. Axonal DA release is locally influenced by striatal neurotransmitters. Striatal neurons are principally GABAergic projection neurons and interneurons, and a small minority of other neurons are cholinergic interneurons (ChIs). ChIs strongly gate striatal DA release via nicotinic receptors (nAChRs) identified on DA axons. Striatal GABA is thought to modulate DA, but GABA receptors have not been documented conclusively on DA axons. However, ChIs express GABA receptors and are therefore candidates for potential mediators of GABA regulation of DA. We addressed whether striatal GABA and its receptors can modulate DA release directly, independently from ChI regulation, by detecting DA in striatal slices from male mice using fast-scan cyclic voltammetry in the absence of nAChR activation. DA release evoked by single electrical pulses in the presence of the nAChR antagonist dihydro-ß-erythroidine was reduced by GABA or agonists of GABAA or GABAB receptors, with effects prevented by selective GABA receptor antagonists. GABA agonists slightly modified the frequency sensitivity of DA release during short stimulus trains. GABA agonists also suppressed DA release evoked by optogenetic stimulation of DA axons. Furthermore, antagonists of GABAA and GABAB receptors together, or GABAB receptors alone, significantly enhanced DA release evoked by either optogenetic or electrical stimuli. These results indicate that striatal GABA can inhibit DA release through GABAA and GABAB receptors and that these actions are not mediated by cholinergic circuits. Furthermore, these data reveal that there is a tonic inhibition of DA release by striatal GABA operating through predominantly GABAB receptors.SIGNIFICANCE STATEMENT The principal inhibitory transmitter in the mammalian striatum, GABA, is thought to modulate striatal dopamine (DA) release, but definitive evidence for GABA receptors on DA axons is lacking. Striatal cholinergic interneurons regulate DA release via axonal nicotinic receptors (nAChRs) and also express GABA receptors, but they have not been eliminated as potentially critical mediators of DA regulation by GABA. Here, we found that GABAA and GABAB receptors inhibit DA release without requiring cholinergic interneurons. Furthermore, ambient levels of GABA inhibited DA release predominantly through GABAB receptors. These findings provide further support for direct inhibition of DA release by GABA receptors and reveal that striatal GABA operates a tonic inhibition on DA output that could critically influence striatal output.

Blockade of nicotinic acetylcholine receptor enhances the responsiveness to bupropion in the mouse forced swim test.

The objective of the present study is to investigate the role of α4, α5, α6 or ß2 nAChR subunits in the antidepressant-like effect of bupropion. Adult male mice were treated with subcutaneous acute doses of bupropion (3 and 10 mg/kg) 30 min before the forced swim test (FST) in α4, α5, α6, or ß2 nAChR subunit knockout (KO) and wild-type (WT) mice. In addition, the effects of ß2* antagonist dihydro-ß-erythroidine (DHßE, 3 mg/kg) on antidepressant-like effects of bupropion in C57BL/6 J mice were assessed. Our results showed that baseline immobility and climbing time did not differ between KO and corresponding WT mice except for ß2 KO. Bupropion significantly decreased immobility time and increased climbing time in the α4, α6 and ß2 nAChR KO mice in comparison to WT littermates, indicating that lack of these nAChR subunits enhanced antidepressant effects of bupropion. On the contrary, the α5 nAChR subunit deletion did not alter the FST behavior in the bupropion-treated mice. Not only in the transgenic mice, bupropion also showed antidepressant-like effects in the WT mice. In addition, DHßE pretreatment before bupropion administration resulted in decreased immobility time and increased climbing time. Taken together, the present study provides evidence on the involvement of α4*, α6*, and ß2* (* indicates possible presence of other subunits) nAChRs in the antidepressant-like effects of bupropion in the FST.

Nicotine enhances responding for conditioned reinforcement via α4ß2 nicotinic acetylcholine receptors in the ventral tegmental area, but not the nucleus accumbens or the prefrontal cortex.

Nicotine enhances the conditioned reinforcing properties of reward-paired cues. We investigated the role of the ventral tegmental area (VTA), nucleus accumbens (NAcc), and prelimbic (PrL) and infralimbic (IL) cortices in mediating this enhancement. Male Long-Evans rats were implanted with bilateral guide cannulae aimed at either the VTA, NAcc, PrL or IL cortex. Next, rats underwent 12 sessions of Pavlovian conditioning. Each session consisted of 30 trials wherein a 5 s conditioned stimulus (CS) was paired with water (0.05 ml). Tests of responding for conditioned reinforcement were conducted during which presentation of the CS, now acting as a conditioned reinforcer (CRf), was contingent upon pressing one of two levers (CRf lever). Pressing the other lever had no consequences (NCRf lever). To determine if nicotinic acetylcholine receptors (nAChRs) in the VTA, NAcc, PrL cortex, or IL cortex mediate nicotine-enhanced responding for a CRf, the α4ß2 nAChR antagonist Dihydro-Beta-Erythroidine (DHßE; 10 nmol/0.5 µL) was infused into the respective areas prior to a systemic nicotine injection (0.2 mg/kg; SC). DHßE infused into the VTA, NAcc, or IL cortex, but not PrL cortex, attenuated nicotine-enhanced responding for a CRf. Next, to confirm that nAChRs in the VTA, NAcc, or IL cortex mediate this enhancement, nicotine (8, 16, or 32 nmol/0.5 µL) was infused into the respective areas. Nicotine infused into the VTA, but not NAcc or IL cortex, enhanced responding for a CRf. These findings suggest that nicotine primarily acts on α4ß2 nAChRs in the VTA to potentiate the conditioned reinforcing properties of reward-related cues.

A behavioral economic analysis of the value-enhancing effects of nicotine and varenicline and the role of nicotinic acetylcholine receptors in male and female rats.

Reinforcement value enhancement by nicotine of non-nicotine rewards is believed to partially motivate smoking behavior. Recently, we showed that the value-enhancing effects of nicotine are well characterized by reinforcer demand models and that the value-enhancing effects of the smoking-cessation aid bupropion (Zyban) are distinct from those of nicotine and differ between the sexes. The present study evaluated potential sex differences in the enhancement effects of nicotine and varenicline (Chantix) using a reinforcer demand methodology. The role of α4ß2* and α7 nicotinic acetylcholine receptors (nAChRs) in the enhancing effects of nicotine and varenicline is also evaluated. Male and female rats (n=12/sex) were trained to lever press maintained by sensory reinforcement by visual stimulus (VS) presentations. Changes in the VS value following nicotine and varenicline administration were assessed using an established reinforcer demand approach. Subsequently, the effects of antagonism of α4ß2* and α7 nAChRs on varenicline and nicotine-induced enhancement active lever-pressing were assessed using a progressive ratio schedule. Nicotine and varenicline enhanced VS demand equivalently between the sexes as evaluated by reinforcer demand. However, α4ß2* receptor antagonism attenuated value enhancement by nicotine and varenicline in females, but only of nicotine in males.

Erysodine, a competitive antagonist at neuronal nicotinic acetylcholine receptors, decreases ethanol consumption in alcohol-preferring UChB rats.

Alcohol abuse is a worldwide health problem with high economic costs to health systems. Emerging evidence suggests that modulation of brain nicotinic acetylcholine receptors (nAChRs) may be a therapeutic target for alcohol dependence. In this work, we assess the effectiveness of four doses of erysodine (1.5, 2.0, 4.0 or 8.0 mg/kg/day, i.p.), a competitive antagonist of nAChRs, on voluntary ethanol consumption behavior in alcohol-preferring UChB rats, administered during three consecutive days. Results show that erysodine administration produces a dose-dependent reduction in ethanol consumption respect to saline injection (control group). The highest doses of erysodine (4 and 8 mg/kg) reduce (45 and 66%, respectively) the ethanol intake during treatment period and first day of post-treatment compared to control group. While, the lowest doses of erysodine (1.5 and 2 mg/kg) only reduce ethanol intake during one day of treatment period. These effective reductions in ethanol intake were 23 and 29% for 1.5 and 2 mg/kg erysodine, respectively. Locomotor activity induced by a high dose of erysodine (10 mg/kg) was similar to those observed with saline injection in control rats, showing that the reduction in ethanol intake was not produced by hypolocomotor effect induced by erysodine. This is the first report showing that erysodine reduces ethanol intake in UChB rats in a dose-dependent manner. Our results highlight the role of nAChRs in the reward effects of ethanol and its modulation as a potentially effective pharmacological alternative for alcohol dependence treatment.