Mecamylamine is a selective non-competitive antagonist of N-methyl-D-aspartate- and aspartate-induced currents in horizontal cells dissociated from the catfish retina

The α4 Nicotinic Acetylcholine Receptor Is Necessary for the Initiation of Organophosphate-Induced Neuronal Hyperexcitability

Acute intoxication with organophosphorus (OP) cholinesterase inhibitors can produce seizures that rapidly progress to life-threatening status epilepticus. Significant research effort has been focused on investigating the involvement of muscarinic acetylcholine receptors (mAChRs) in OP-induced seizure activity. In contrast, there has been far less attention on nicotinic AChRs (nAChRs) in this context. Here, we address this data gap using a combination of in vitro and in vivo models. Pharmacological antagonism and genetic deletion of α4, but not α7, nAChR subunits prevented or significantly attenuated OP-induced electrical spike activity in acute hippocampal slices and seizure activity in mice, indicating that α4 nAChR activation is necessary for neuronal hyperexcitability triggered by acute OP exposures. These findings not only suggest that therapeutic strategies for inhibiting the α4 nAChR subunit warrant further investigation as prophylactic and immediate treatments for acute OP-induced seizures, but also provide mechanistic insight into the role of the nicotinic cholinergic system in seizure generation.

Central cholinergic transmission affects the compulsive-like behavior of mice in marble-burying test

The presence of the cholinergic system in the brain areas implicated in the precipitation of obsessive-compulsive behavior (OCB) has been reported but the exact role of the central cholinergic system therein is still unexplored. Therefore, the current study assessed the effect of cholinergic analogs on central administration on the marble-burying behavior (MBB) of mice, a behavior correlated with OCB. The result reveals that the enhancement of central cholinergic transmission in mice achieved by intracerebroventricular (i.c.v.) injection of acetylcholine (0.01 µg) (Subeffective: 0.1 and 0.5 µg), cholinesterase inhibitor, neostigmine (0.1, 0.3, 0.5 µg/mouse) and neuronal nicotinic acetylcholine receptor agonist, nicotine (0.1, 2 µg/mouse) significantly attenuated the number of marbles buried by mice in MBB test without affecting basal locomotor activity. Similarly, central injection of mAChR antagonist, atropine (0.1, 0.5, 5 µg/mouse), nAChR antagonist, mecamylamine (0.1, 0.5, 3 µg/mouse) per se also reduced the MBB in mice, indicative of anti-OCB like effect of all the tested cholinergic mAChR or nAChR agonist and antagonist. Surprisingly, i.c.v. injection of acetylcholine (0.01 µg), and neostigmine (0.1 µg) failed to elicit an anti-OCB-like effect in mice pre-treated (i.c.v.) with atropine (0.1 µg), or mecamylamine (0.1 µg). Thus, the findings of the present investigationdelineate the role of central cholinergic transmission in the compulsive-like behavior of mice probably via mAChR or nAChR stimulation.

Further pharmacological characterization of a preclinical model of the early development of nicotine withdrawal

BACKGROUND

Establishing preclinical models of the development of nicotine withdrawal following acute nicotine exposure could inform tobacco addiction-related research, treatment, and policy. To this end, this lab has previously reported that rats exhibit withdrawal-like elevations in intracranial self-stimulation (ICSS) thresholds (anhedonia-like behavior) following acute nicotine exposure. The goal of this study was to provide further pharmacological characterization of ICSS as a measure of spontaneous and antagonist-precipitated withdrawal from acute nicotine.

METHODS AND RESULTS

Rats exhibited a small increase in ICSS thresholds over time following a single nicotine injection (1.0 mg/kg, s.c.), suggesting a modest spontaneous withdrawal effect (Experiment 1). In Experiment 2, the antidepressant bupropion (5.0 mg/kg, i.p.), which is used to treat tobacco addiction and attenuates nicotine withdrawal in both humans and rodents, blocked elevations in ICSS thresholds induced by a single injection of nicotine (0.5 mg/kg, s.c.) followed ≈ 2 h later by the non-selective, non-competitive nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (3.0 mg/kg, s.c.). In Experiment 3a, s.c. administration of the competitive, relatively selective α4ß2 nAChR antagonist dihydro-beta-erythroidine (DHßE) (5.6 mg/kg, but not 3.0 mg/kg) following each of 5 daily injections of nicotine (0.5 mg/kg, s.c.) elevated ICSS thresholds. Mecamylamine (3.0 mg/kg, s.c.) also elevated ICSS thresholds when administered following all 5 daily nicotine injections (0.5 mg/kg, s.c., Experiment 3b).

CONCLUSIONS

These findings provide further characterization of elevations in ICSS thresholds as a measure of withdrawal from acute nicotine exposure. Further use of these models may be useful for understanding the early development of nicotine withdrawal.

Enhanced analgesic cholinergic tone in the spinal cord in a mouse model of neuropathic pain

Endogenous acetylcholine (ACh) is an important modulator of nociceptive sensory processing in the spinal cord. An increased level of spinal ACh induces analgesia both in humans and rodents while interfering with cholinergic signaling is allodynic, demonstrating that a basal tone of spinal ACh modulates nociceptive responses in naïve animals. The plasticity undergone by this cholinergic system in chronic pain situation is unknown, and the mere presence of this tone in neuropathic animals is controversial. We have addressed these issues in mice through behavioral experiments, histology, electrophysiology and molecular biology, in the cuff model of peripheral neuropathy. Our behavior experiments demonstrate the persistence, and even increased impact of the analgesic cholinergic tone acting through nicotinic receptors in cuff animals. The neuropathy does not affect the number or membrane properties of dorsal horn cholinergic neurons, nor specifically the frequency of their synaptic inputs. The alterations thus appear to be in the neurons receiving the cholinergic signaling, which is confirmed by the fact that subthreshold doses of acetylcholinesterase (AChE) inhibitors in sham animals become anti-allodynic in cuff mice and by the altered expression of the β2 nicotinic receptor subunit. Our results demonstrate that endogenous cholinergic signaling can be manipulated to relieve mechanical allodynia in animal models of peripheral neuropathy. Until now, AChE inhibitors have mainly been used in the clinics in situations of acute pain (parturition, post-operative). The fact that lower doses (thus with fewer side effects) could be efficient in chronic pain conditions opens new avenues for the treatment of neuropathic pain. SIGNIFICANCE STATEMENT: Chronic pain continues to be the most common cause of disability that impairs the quality of life, accruing enormous and escalating socio-economic costs. A better understanding of the plasticity of spinal neuronal networks, crucially involved in nociceptive processing, could help designing new therapeutic avenues. We here demonstrate that chronic pain modifies the spinal nociceptive network in such a way that it becomes more sensitive to cholinergic modulations. The spinal cholinergic system is responsible for an analgesic tone that can be exacerbated by acetylcholinesterase inhibitors, a property used in the clinic to relief acute pain (child birth, post-op). Our results suggest that lower doses of acetylcholinesterases, with even fewer side effects, could be efficient to relieve chronic pain.

Genetic Differences in Dorsal Hippocampus Acetylcholinesterase Activity Predict Contextual Fear Learning Across Inbred Mouse Strains

Learning is a critical behavioral process that is influenced by many neurobiological systems. We and others have reported that acetylcholinergic signaling plays a vital role in learning capabilities, and it is especially important for contextual fear learning. Since cholinergic signaling is affected by genetic background, we examined the genetic relationship between activity levels of acetylcholinesterase (AChE), the primary enzyme involved in the acetylcholine metabolism, and learning using a panel of 20 inbred mouse strains. We measured conditioned fear behavior and AChE activity in the dorsal hippocampus, ventral hippocampus, and cerebellum. Acetylcholinesterase activity varied among inbred mouse strains in all three brain regions, and there were significant inter-strain differences in contextual and cued fear conditioning. There was an inverse correlation between fear conditioning outcomes and AChE levels in the dorsal hippocampus. In contrast, the ventral hippocampus and cerebellum AChE levels were not correlated with fear conditioning outcomes. These findings strengthen the link between acetylcholine activity in the dorsal hippocampus and learning, and they also support the premise that the dorsal hippocampus and ventral hippocampus are functionally discrete.

Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

Self-administration of the synthetic cathinone MDPV enhances reward function via a nicotinic receptor dependent mechanism

Methylenedioxypyrovalerone (MDPV) is an addictive synthetic drug with severe side effects. Previous studies have shown that MDPV has positive reinforcing properties. However, little is known about the effect of MDPV self-administration on the state of the brain reward system and the neuronal mechanisms by which MDPV mediates its effects. The goal of the present studies was to determine the effect of MDPV self-administration on reward function and the role of cholinergic neurotransmission in the reinforcing effects of MDPV. To study the effect of MDPV self-administration on the brain reward system, rats were prepared with intravenous catheters and intracranial self-stimulation electrodes (ICSS). For 10 days, the reward thresholds were assessed immediately before (23 h post prior session) and after 1 h of MDPV self-administration. The reward thresholds were decreased immediately after MDPV self-administration, which is indicative of a potentiation of brain reward function. The reward thresholds 23 h after MDPV intake gradually increased over time, which is indicative of anhedonia. Pretreatment with the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine decreased the self-administration of MDPV and completely prevented the decrease in reward thresholds. A control study with palatable chocolate pellets showed that responding for a natural reinforcer does not affect the state of the brain reward system. Furthermore, mecamylamine did not affect responding for food pellets. In conclusion, the self-administration of MDPV potentiates reward function and nAChR blockade prevents the reward enhancing effects of MDPV self-administration. Preventing the MDPV-induced increase in cholinergic neurotransmission might be a safe approach to diminish MDPV abuse.

Differential Modulation of GABAA and NMDA Receptors by an α7-nicotinic Acetylcholine Receptor Agonist in Chronic Glaucoma

Presynaptic modulation of γ-aminobutyric acid (GABA) release by an alpha7 nicotinic acetylcholine receptor (α7-nAChR) agonist promotes retinal ganglion cell (RGC) survival and function, as suggested by a previous study on a chronic glaucomatous model from our laboratory. However, the role of excitatory and inhibitory amino acid receptors and their interaction with α7-nAChR in physiological and glaucomatous events remains unknown. In this study, we investigated GABA_A and N-methyl-D-aspartate (NMDA) receptor activity in control and glaucomatous retinal slices and the regulation of amino acid receptor expression and function by α7-nAChR. Whole-cell patch-clamp recordings from RGCs revealed that the α7-nAChR specific agonist PNU-282987 enhanced the amplitude of currents elicited by GABA and reduced the amplitude of currents elicited by NMDA. The positive modulation of GABA_A receptor and the negative modulation of NMDA receptor (NMDAR) by PNU-282987-evoked were prevented by pre-administration of the α7-nAChR antagonist methyllycaconitine (MLA). The frequency and the amplitude of glutamate receptor-mediated miniature glutamatergic excitatory postsynaptic currents (mEPSCs) were not significantly different between the control and glaucomatous RGCs. Additionally, PNU-282987-treated slices showed no alteration in the frequency or amplitude of mEPSCs relative to control RGCs. Moreover, we showed that expression of the α1 subunit of the GABA_A receptor was downregulated and the expression of the NMDAR NR2B subunit was upregulated by intraocular pressure (IOP) elevation, and the changes of high IOP were blocked by PNU-282987. In conclusion, retina GABA_A and NMDARs are modulated positively and negatively, respectively, by activation of α7-nAChR in in vivo chronic glaucomatous models.

Multiple Nicotinic Acetylcholine Receptor Subtypes in the Mouse Amygdala Regulate Affective Behaviors and Response to Social Stress

Electrophysiological and neurochemical studies implicate cholinergic signaling in the basolateral amygdala (BLA) in behaviors related to stress. Both animal studies and human clinical trials suggest that drugs that alter nicotinic acetylcholine receptor (nAChR) activity can affect behaviors related to mood and anxiety. Clinical studies also suggest that abnormalities in cholinergic signaling are associated with major depressive disorder, whereas pre-clinical studies have implicated both β2 subunit-containing (β2*) and α7 nAChRs in the effects of nicotine in models of anxiety- and depression-like behaviors. We therefore investigated whether nAChR signaling in the amygdala contributes to stress-mediated behaviors in mice. Local infusion of the non-competitive non-selective nAChR antagonist mecamylamine or viral-mediated downregulation of the β2 or α7 nAChR subunit in the amygdala all induced robust anxiolytic- and antidepressant-like effects in several mouse behavioral models. Further, whereas α7 nAChR subunit knockdown was somewhat more effective at decreasing anxiety-like behavior, only β2 subunit knockdown decreased resilience to social defeat stress and c-fos immunoreactivity in the BLA. In contrast, α7, but not β2, subunit knockdown effectively reversed the effect of increased ACh signaling in a mouse model of depression. These results suggest that signaling through β2* nAChRs is essential for baseline excitability of the BLA, and a decrease in signaling through β2 nAChRs alters anxiety- and depression-like behaviors even in unstressed animals. In contrast, stimulation of α7 nAChRs by acetylcholine may mediate the increased depression-like behaviors observed during the hypercholinergic state observed in depressed individuals.

Evidence for the involvement of NMDA receptors in the antidepressant-like effect of nicotine in mouse forced swimming and tail suspension tests

RATIONALE

The antidepressant action of acute nicotine administration in clinical and animal studies is well recognized. But the underlying mechanism for this effect has not been carefully discovered.

OBJECTIVES

We attempted to evaluate the possible role of N-Methyl-D-aspartate (NMDA) receptors in the antidepressant-like effect of nicotine.

METHODS

After the assessment of locomotor activity in the open-field test, forced swimming test (FST) and tail suspension test (TST) were used to evaluate the antidepressant-like effect of nicotine in mice. We performed intraperitoneal administration of nicotine at different doses and periods before the tests. To assess the possible involvement of NMDA receptors, non-effective doses of NMDA antagonists and an NMDA agonist were obtained and were administered simultaneously with the non-effective and effective doses of nicotine, respectively.

RESULTS

Nicotine (0.2 mg/kg, 30 min before FST/TST) significantly reduced the immobility time of mice similar to fluoxetine (20 mg/kg). Nicotine did not affect the locomotor behavior of mice in open-field test. Co-administration of non-effective doses of NMDA receptor antagonists, ketamine (1 or 0.3 mg/kg), MK-801 (0.05 or 0.005 mg/kg), and magnesium sulfate (10 or 5 mg/kg) with nicotine (0.1 or 0.03 mg/kg) had remarkable synergistic antidepressant effect in both FST and TST. Also, non-effective NMDA (75 or 30 mg/kg) reversed the anti-immobility effect of nicotine (0.2 mg/kg) on mouse FST and TST.

CONCLUSIONS

Our study has for the first time confirmed that the antidepressant-like effect of nicotine on mice is NMDA-mediated, and nicotine presumably exerts this effect by antagonizing the glutamatergic NMDA receptors.

The discriminative stimulus effects of mecamylamine in nicotine-treated and untreated rhesus monkeys

The extent to which chronic nicotine treatment can alter the effects of the nicotinic acetylcholine receptor antagonist mecamylamine, and whether those effects can be attenuated by nicotine have not been clearly established in the literature. Here, the discriminative stimulus effects of mecamylamine were compared between one group of rhesus monkeys receiving a continuous infusion of nicotine base (5.6 mg/kg/day subcutaneously) and another group of monkeys not receiving nicotine treatment. Both groups responded under a fixed ratio 5 schedule of stimulus-shock termination. Stimulus control was obtained at doses of 1.78 mg/kg mecamylamine in monkeys receiving continuous nicotine and 5.6 mg/kg mecamylamine in monkeys not receiving continuous nicotine treatment. Nicotine did not attenuate the discriminative stimulus effects of mecamylamine in either group. Discontinuation of continuous nicotine produced responding on the mecamylamine lever within 24 h in some but not all monkeys. This may indicate a qualitative difference in the discriminative stimulus effects of mecamylamine between groups, perhaps reflecting antagonism of nicotine and nicotine withdrawal in monkeys receiving continuous nicotine. The failure of nicotine to reverse the effects of mecamylamine is consistent with a noncompetitive interaction at nicotinic acetylcholine receptors and indicates that mecamylamine-induced withdrawal cannot be readily modified by nicotine.

Potential substrates for nicotine and alcohol interactions: a focus on the mesocorticolimbic dopamine system

Epidemiological studies consistently find correlations between nicotine and alcohol use, yet the neural mechanisms underlying their interaction remain largely unknown. Nicotine and alcohol (i.e., ethanol) share many common molecular and cellular targets that provide potential substrates for nicotine-alcohol interactions. These targets for interaction often converge upon the mesocorticolimbic dopamine system, where the link to drug self-administration and reinforcement is well documented. Both nicotine and alcohol activate the mesocorticolimbic dopamine system, producing downstream dopamine signals that promote the drug reinforcement process. While nicotine primarily acts via nicotinic acetylcholine receptors, alcohol acts upon a wider range of receptors and molecular substrates. The complex pharmacological profile of these two drugs generates overlapping responses that ultimately intersect within the mesocorticolimbic dopamine system to promote drug use. Here we will examine overlapping targets between nicotine and alcohol and provide evidence for their interaction. Based on the existing literature, we will also propose some potential targets that have yet to be directly tested. Mechanistic studies that examine nicotine-alcohol interactions would ultimately improve our understanding of the factors that contribute to the associations between nicotine and alcohol use.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

Morphine dependence and withdrawal induced changes in cholinergic signaling

Cholinergic signaling is thought to be involved in morphine dependence and withdrawal, but the specific mechanisms involved remain unclear. The current study aimed to identify alterations in the cholinergic system that may contribute to the development of morphine dependence and withdrawal. Acetylcholinesterase (AChE) activity and [³H]-epibatidine binding were evaluated in order to determine if morphine dependence and withdrawal induces alterations in cholinergic signaling or expression of high affinity nicotinic acetylcholine receptors (nAChRs) in the midbrain (MB), medial habenula (MHb) and interpeduncular nucleus (IPN). The effect of cholinergic signaling through nAChRs on morphine-withdrawal induced jumping behavior was then determined. Lastly, the contribution of β4-containing nAChRs receptors in the MHb to morphine-withdrawal induced jumping behavior and neuronal activity as indicated by c-fos expression was assessed. Chronic morphine administration decreased AChE activity in MB and MHb, an effect that was no longer present following precipitated withdrawal. Morphine dependent mice showed increased nicotinic acetylcholine receptor (nAChR) levels in MB. Further, nicotine (0.4 mg/kg) and lobeline (3 mg/kg) decreased jumping behavior while mecamylamine (1 mg/kg) had no effect. Knock-down of β4 subunit-containing nAChRs in the MHb attenuated c-fos activation, but did not decrease morphine withdrawal-induced jumping. Thus, morphine withdrawal induces cholinergic signaling in the MHb, but this does not appear to be responsible for the effects of cholinergic drugs on somatic signs of opiate withdrawal, as measured by jumping behavior.

Acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in mice: effects of exposure to stress and modulation by mecamylamine

Nicotinic acetylcholine receptors mediate some of the rewarding and motivational effects of ethanol, including relapses. Relapses are common in drug addicts during abstinence when exposure to any stressor ensues. However, the role of nicotinic acetylcholine receptors in the ethanol- and stress-induced reinstatement of ethanol-induced conditioned place preference has not yet been explored. Therefore, the present study investigated the influence of mecamylamine, a nicotinic acetylcholine receptors antagonist on acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in adult male Swiss mice. The results revealed that mecamylamine (0.1-10 µg/mouse, intracerebroventricularly) dose dependently prevented the development, expression, and reinstatement of ethanol-induced conditioned place preference. Further, acute treatment with mecamylamine blocked the restraint stress and forced swim stress-induced reinstatement of ethanol-induced conditioned place preference. All of these treatments had no influence on the locomotor activity. Therefore, it is concluded that mecamylamine blocks the acquisition, expression and reinstatement of conditioned reinforcing effects of ethanol without per se reinforcing or aversive influence. This ability of mecamylamine might be a potential advantage in the treatment of alcoholism.

Effects of calcium channel antagonists on the motivational effects of nicotine and morphine in conditioned place aversion paradigm

The motivational component of drug withdrawal may contribute to drug seeking and relapse through the negative reinforcement-related process; thus, it is important to understand the mechanisms that mediate affective withdrawal behaviors. The present study was undertaken to examine the calcium-dependent mechanism of negative motivational symptoms of nicotine and morphine withdrawal using the conditioned place aversion (CPA) paradigm. Rats were chronically treated with nicotine (1.168 mg/kg, free base, s.c., 11 days, three times daily) or morphine (10 mg/kg,s.c., 11 days, twice daily). Then, during conditioning, rats pre-treated with nicotine or morphine received a nicotinic receptor antagonist mecamylamine (3.5 mg/kg) or an opioid receptor antagonist naloxone (1 mg/kg) to precipitate withdrawal in their initially preferred compartment, or saline in their non-preferred compartment. Our results demonstrated that after three conditioning sessions, mecamylamine induced a clear place aversion in rats that had previously received nicotine injections, and naloxone induced a significant place aversion in rats that had previously received morphine injections. Further, the major findings showed that calcium channel antagonists, i.e., nimodipine, verapamil and flunarizine (5 and 10 mg/kg, i.p.), injected before the administration of mecamylamine or naloxone, attenuated nicotine or morphine place aversion. As an outcome, these findings support the hypothesis that similar calcium-dependent mechanisms are involved in aversive motivational component associated with nicotine a morphine withdrawal. We can suggest that calcium channel blockers have potential for alleviating nicotine and morphine addiction by selectively decreasing the incentive motivational properties of both drugs, and may be beneficial as smoking cessation or opioid dependence pharmacotherapies.

Effects of mecamylamine on flash-evoked potentials, body temperature, and behavior in Long-Evans rats

This experiment examined the effects of mecamylamine, a nicotinic acetylcholine receptor antagonist, on flash-evoked potentials (FEPs) recorded from the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats, and on body temperature and open field behavior. FEPs were recorded at 20 and 35 min following intraperitoneal injections of saline, and of 0.3, 3.0, and 10.0 mg/kg mecamylamine on separate days. The 0.3 mg/kg dose did not produce significant effects. The amplitude of VC components N₃₀, P₄₈, and P₈₇ increased, N₁₅₀ and P₂₃₁ decreased, and P₂₃, N₄₀, N₅₈, and N₆₈ were unchanged following administration of the 10.0 mg/kg dose. In the SC, component P₂₈ was unaffected, P₃₉ was reduced, and N₄₉ was augmented by the 10.0 mg/kg dose. All component peak latencies were increased by the 3.0 and 10.0 mg/kg doses. Significant hypothermia was also produced by the 3.0 and 10.0 mg/kg doses, suggesting that this was the basis for the increased latencies. The 10.0 mg/kg dose produced a significant decrease in movement during the recording sessions. In subsequent open field observations, both line crossings and rearings were reduced by the 3.0 and 10.0 mg/kg doses. The results suggest that endogenous acetylcholine acting on nicotinic acetylcholine receptors plays at most a modest role in producing FEPs recorded from the VC and SC.

Nicotinic acetylcholine receptors are required for the conditioned reinforcing properties of sucrose-associated cues

RATIONALE

We recently demonstrated that blocking specific nicotinic acetylcholine receptors (nAChRs) abolishes the conditioned reinforcing properties of ethanol-associated cues in rat, suggesting nAChRs as promising pharmacological targets for prevention of cue-induced relapse.

OBJECTIVES

The present study investigated the involvement of nAChR subtypes in the conditioned reinforcing properties of stimuli associated with a natural reward (sucrose).

METHODS

Water-deprived rats were trained to associate a tone + light stimulus (CS) with the presentation of a 0.1 M sucrose solution for 10 consecutive days. On the subsequent day, the animals were tested on the stringent acquisition of a new instrumental response with conditioned reinforcement, following a systemic injection of the nonselective nAChR antagonist mecamylamine (MEC) or the selective α7 and α6/α3β2β3* nAChR antagonist methyllycaconitine (MLA). At testing, the rats were presented with two novel levers. Responding on the lever assigned as active (CR lever) resulted in a presentation of the CS alone, while pressing the inactive lever (NCR lever) had no programmed consequences.

RESULTS

Control animals pressed the CR lever significantly more than the NCR lever, demonstrating that the CR had acquired conditioned reinforcing properties. Systemic MEC as well as MLA reduced the CR lever responses to the same level as for the NCR lever.

CONCLUSIONS

These results demonstrate a role for the α7 and/or α6/α3β2β3* nAChRs in conditioned reinforcement to a natural reward and suggest neuronal nAChRs as common mediators of the impact of cues on incentive processes.

Inhibitory influence of mecamylamine on ethanol withdrawal-induced symptoms in C57BL/6J mice

Several reports show the involvement of neuronal nicotinic acetylcholine receptors (nAChRs) in the behavioral effects of ethanol, including ethanol drinking and relapse. Therefore, this study evaluated the effects of mecamylamine, a nAChR antagonist, on ethanol withdrawal signs. Ethanol dependence was induced in C57BL/6J mice by ethanol liquid diet administration. Animals were provided with nutritionally balanced control liquid diet (600 kcal/l) as their sole nutrient source on day 0; from days 1 to 4, 3% v/v of ethanol, followed by 6% v/v of ethanol (from days 5 to 7), and 10% v/v of ethanol (from days 8 to 10) were incorporated into the liquid diet. On day 11, ethanol liquid diet was replaced with nutritionally balanced control liquid diet, and ethanol withdrawal-induced physical signs were recorded. Results showed that acute administration of mecamylamine (1-4 mg/kg, intraperitoneally) dose-dependently attenuated ethanol withdrawal-induced signs, and these effects were comparable with those of diazepam (1-2 mg/kg, intraperitoneally). In addition, chronic administration of mecamylamine into ethanol diet-fed mice markedly attenuated the ethanol withdrawal sign scores, thus supporting the contention that nAChR is involved in ethanol dependence. In conclusion, our results suggest that mecamylamine exhibited inhibitory effects on ethanol withdrawal signs which could be mediated through nAChR.

Inhibitory influence of mecamylamine on the development and the expression of ethanol-induced locomotor sensitization in mice

Several evidences have indicated the involvement of neuronal nicotinic acetylcholine receptors (nAChR) in behavioral effects of drugs of abuse, including ethanol. nAChRs are implicated in ethanol-induced behaviors as well as neurochemical responses to ethanol. Recently, it is demonstrated that mecamylamine, a nAChR antagonist blocks cocaine-, d-amphetamine-, ephedrine-, nicotine-, and methylphenidate-induced psychomotor sensitization. However, no reports are available on its role in ethanol-induced psychomotor sensitization. Therefore, an attempt was made to evaluate its effect on ethanol-induced locomotor sensitization using a model previously described by us. The results revealed that acute administration of mecamylamine (1 and 2mg/kg, i.p.) blocked the acute stimulant effect of ethanol (2.0g/kg, i.p.). In addition, treatment with mecamylamine (0.5-2.0mg/kg, i.p.), 30min prior to the challenge dose of ethanol (2.0g/kg, i.p.) dose dependently attenuated expression of sensitization to locomotor stimulant effect of ethanol. Moreover, administration of mecamylamine (1 and 2mg/kg, i.p.) during development (prior to each ethanol injection on days 1, 4, 7, and 10) blocked acquisition as well as expression (day 15) of sensitization to locomotor stimulant effect of ethanol. Mecamylamine per se did not affect locomotor activity. Further, it also did not influence blood ethanol levels and rotarod performance in mice. These results support the hypothesis that neuroadaptive changes in nAChRs may participate in the development and the expression of ethanol-induced locomotor sensitization.

Bupropion and nicotine enhance responding for nondrug reinforcers via dissociable pharmacological mechanisms in rats

RATIONALE

Nicotine serves as a primary reinforcer but also potently enhances responding for nonnicotine stimuli with reinforcing properties. One of the most successful pharmacotherapies for smoking cessation, bupropion, also increases responding for nondrug reinforcers such as food and brain stimulation rewards.

OBJECTIVE

The present studies investigated whether treatment with bupropion and nicotine had similar effects on responding for a reinforcing visual stimulus (VS). They also investigated whether the effects of bupropion and nicotine depended on common pharmacological substrates.

RESULTS

Nicotine (0.4 mg/kg base) enhanced responding for the VS, and this enhancing effect increased across testing sessions, replicating our previous findings. Bupropion (3, 10, and 30 mg/kg salt) dose-dependently increased responding for the VS. Treatment with 10 and 30 mg/kg bupropion resulted in a profile similar to nicotine; operant responding increased over repeated drug treatments. The reinforcement enhancing effect of nicotine, but not bupropion, was blocked by pretreatment with the nicotinic acetylcholine receptor antagonist mecamylamine. In contrast, the reinforcement enhancing effect of bupropion, but not nicotine, was blocked by pretreatment with the alpha noradrenergic antagonist prazosin.

CONCLUSION

The reinforcement enhancing effects of nicotine and bupropion increased over time and repeated treatments suggesting a shared mechanism of action. However, the reinforcement enhancing effects of nicotine are mediated by nicotinic acetylcholine receptors, whereas the reinforcement enhancing effects of bupropion were mediated by alpha noradrenergic receptors.

Antidepressant-like effects of nicotinic acetylcholine receptor antagonists, but not agonists, in the mouse forced swim and mouse tail suspension tests

Current literature suggests involvement of nicotinic acetylcholine receptors (nAChRs) in major depression. However, it is controversial whether the antidepressant-like effect of nAChR modulation is induced by activation, desensitization or inhibition of central nAChRs. In addition, the specific nAChR subtype/s involved remains unknown. In this study, we systematically compared the effects of non-selective and selective nicotinic agonists and antagonists in two different tests for antidepressant effects in mice: the tail suspension test and the forced swim test. Compounds: nicotine, RJR-2403 (alpha4beta2-selective agonist), PNU-282987 (alpha7-selective agonist), mecamylamine (non-selective antagonist), dihydro-beta-erythroidine (DHbetaE; alpha4beta2-selective antagonist), methyllycaconitine (MLA; alpha7-selective antagonist) and hexamethonium (non-brain-penetrant non-selective antagonist). All compounds were tested in a locomotor activity paradigm to rule out non-specific stimulant effects. The data show that blockade of nAChRs with mecamylamine, or selective antagonism of alpha4beta2 or alpha7 nAChRs with DHbetaE or MLA, respectively, has antidepressant-like effects. These effects were not confounded by motor stimulation. Hexamethonium did not show antidepressant-like activity, supporting the involvement of central nAChRs. At the dose levels tested, none of the nAChR agonists displayed antidepressant-like profiles. In conclusion, antagonism of central alpha4beta2 and/or alpha7 nAChRs induced antidepressant-like effects in mice. A strategy involving antagonism of central nAChRs could potentially lead to the development of novel antidepressant therapeutics.

Modulation of ethanol drinking-in-the-dark by mecamylamine and nicotinic acetylcholine receptor agonists in C57BL/6J mice

RATIONALE

Recent reports describe a restricted access ethanol consumption paradigm where C57Bl/6J mice drink until intoxicated. Termed "drinking in the dark" (DID), this paradigm has been used as a model of binge drinking. Although neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in alcohol drinking in rats pre-trained to self-administer ethanol, their role in binge-like ethanol consumption is unknown.

OBJECTIVES

To determine if nAChRs are involved in binge drinking as measured by the DID assay in C57Bl/6J mice.

MATERIALS AND METHODS

Adult male C57Bl/6J mice were injected i.p. with nicotinic receptor antagonists including mecamylamine, hexamethonium, dihydro-beta-erythroidine, and methyllycaconitine. Immediately following injection, mice were presented with 20% ethanol for 2 h in the DID assay to measure ethanol consumption. Nicotinic agonists including cytisine and nicotine were also evaluated. The effects of mecamylamine and nicotine on ethanol-induced dopaminergic neuronal activation in the VTA were evaluated via immunohistochemistry.

RESULTS

Mecamylamine dose dependently reduced ethanol consumption; whereas, the peripheral antagonist hexamethonium had no significant effect. Nicotinic agonists, cytisine and nicotine, reduced ethanol consumption. None of the effective nicotinic receptor drugs reduced sucrose drinking. Mecamylamine blocked ethanol activation of dopaminergic neurons while nicotine alone activated them without additional activation by ethanol.

CONCLUSIONS

Neuronal nAChRs are involved in ethanol consumption in the DID paradigm. The effects of mecamylamine, nicotine, and cytisine on ethanol intake appear to be specific because they do not reduce sucrose drinking. Mecamylamine reduces alcohol consumption by blocking activation of dopaminergic neurons; whereas, nicotinic agonists may activate the same reward pathway as alcohol.

Effects of mecamylamine on alcohol consumption and preference in male C57BL/6J mice

Alcohol and nicotine (in the form of tobacco) are 2 commonly used recreational drugs and studies show a high correlation between tobacco use and alcohol consumption. In the present study, using C57BL/6J mice, we investigated the ability of mecamylamine (a nicotinic antagonist) to reduce alcohol consumption and alcohol preference with free 24-hour access using a 2-bottle choice test drinking procedure. Male C57BL/6J mice were individually housed and acclimatized to 10% alcohol. Immediately following the last day of alcohol acclimatization, the mice (n = 5/group) received subcutaneous injections of mecamylamine (0.5, 1 and 2 mg/kg) or saline consisting of either intermittent (3 injections given every other day) or daily (injections on all 5 days) exposures. Fluid consumption (alcohol and water) was recorded daily. The results showed that mecamylamine significantly reduced alcohol consumption and alcohol preference in both phases of intermittent and daily drug exposures, while the total fluid consumption was unchanged. These results provide further support that mecamylamine is effective in reducing alcohol consumption and preference, and nicotinic-receptor-based drugs could further be explored as potential treatments for alcoholism.

Cholinergic direct inhibition of N-methyl-D aspartate receptor-mediated currents in the rat neocortex

Acetylcholine (ACh) and N-methyl-D aspartate receptors (NMDARs) interact in the regulation of multiple important brain functions. NMDAR activation is indirectly modulated by ACh through the activation of muscarinic or nicotinic receptors. Scant information is available on whether ACh directly interacts with the NMDAR. By using a cortical brain slice preparation we found that the application of ACh and of other drugs acting on muscarinic or nicotinic receptors induces an acute and reversible reduction of NMDAR-mediated currents (I(NMDA)), ranging from 20 to 90% of the control amplitude. The reduction displayed similar features in synaptic I(NMDA) in brain slices, as well as in currents evoked by NMDA application in brain slices or from acutely dissociated cortical cells, demonstrating its postsynaptic nature. The cholinergic inhibition of I(NMDA) displayed an onset-offset rate in the order of a second, and was resistant to the presence of the muscarinic antagonist atropine (10 microM) in the extracellular solution, and of G-protein blocker GDP(beta)S (500 microM) and activator GTP(gamma)S (400 microM) in the intracellular solution, indicating that it was not G-protein dependent. Recording at depolarized or hyperpolarized holding voltages reduced NMDAR-mediated currents to similar extents, suggesting that the inhibition was voltage-independent, whereas the reduction was markedly more pronounced in the presence of glycine (20 microM). A detailed analysis of the effects of tubocurarine suggested that at least this drug interfered with glycine-dependent NMDAR-activity. We conclude that NMDAR-mediated current scan be inhibited directly by cholinergic drugs, possibly by direct interaction within one or more subunits of the NMDAR. Our results could supply a new interpretation to previous studies on the role of ACh at the glutamatergic synapse.

Effects of subunit selective nACh receptors on operant ethanol self-administration and relapse-like ethanol-drinking behavior

RATIONALE AND OBJECTIVES

The sensitivity to ethanol central effects is partially determined by the subunit composition of brain nicotinic acetylcholine receptors (nAChRs). Thus, the effects of intraventral tegmental area (VTA) administration of the nicotinic subunit-specific antagonist, alpha-conotoxin MII (alphaCtxMII, alpha(3)beta(2)*, beta(3)*, alpha(6)*), were compared to those of systemic mecamylamine (MEC, an allosteric negative modulator of the nAChR), dihydro-beta-erythroidine (DHbetaE, alpha(4)beta(2)*), and methyllycaconitine (MLA, alpha(7)*) to elucidate involvement of different subunits of nAChRs in operant ethanol self-administration and relapse-like activation of ethanol consumption after ethanol deprivation in rats.

METHODS

The effects of drugs were studied in rats trained for operant oral self-administration of ethanol (FR = 1). For ethanol deprivation, trained animals were subjected to a period of alcohol deprivation for 10 days. alphaCtxMII was given directly into the VTA through implanted permanent intracranial cannulae, whereas MEC, DHbetaE, and MLA were administered systemically.

RESULTS

alphaCtxMII reduced operant ethanol self-administration and blocked the deprivation-induced relapse-like ethanol consumption. MEC reduced operant ethanol self-administration and inhibited the deprivation-induced increase in alcohol consumption. DHbetaE did not alter ethanol self-administration in the lower-dose range but inhibited ethanol intake at a higher dose (4 mg/kg), although this effect might have been nonspecific. MLA failed to block self-administration of ethanol and relapse-like drinking after deprivation.

CONCLUSIONS

Our results indicate that nAChRs are involved in the modulation of operant alcohol self-administration and relapse-like alcohol drinking behavior in rats. Our observations support the working hypothesis that systemically active selective ligands for nAChR alpha(3)beta(2)*, beta(3), and/or alpha(6)* receptor subunits might be of therapeutic value for the treatment of alcoholism.

Desensitization of nicotinic acetylcholine receptors as a strategy for drug development

The specific pharmacological response evoked by a nicotinic acetylcholine receptor (nAChR) agonist is governed by the anatomical distribution and expression of each receptor subtype and by the stoichiometry of subunits comprising each subtype. Contributing to this complexity is the ability of agonists that bind to the orthosteric site of the receptor to alter the affinity state of the receptor and induce desensitization and the observation that, at low doses, some nAChR antagonists evoke agonist-like nicotinic responses. Brain concentrations of nicotine rarely increase to the low-mid micromolar concentrations that have been reported to evoke direct agonist-like responses, such as calcium influx or neurotransmitter release. Low microgram per kilogram doses of nicotine administered to humans or to nonhuman primates to improve cognition and working memory probably result only in low nanomolar brain concentrations--more in line with the ability of nicotine to induce receptor desensitization. Here we review data illustrating that nicotine, its major metabolite cotinine, and two novel analogs of choline, JWB1-84-1 [2-(4-(pyridin-3-ylmethyl)piperazin-1-yl)ethanol] and JAY2-22-33, JWB1-84-1 [2-(methyl(pyridine-3-ylmethyl)amino)-ethanol], improve working memory in macaques. The effectiveness of these four compounds in the task was linearly related to their effectiveness in producing desensitization of the pressor response to ganglionic stimulation evoked by a nAChR agonist in rats. Only nicotine evoked an agonist-like action (increased resting blood pressure). Therefore, it is possible to develop new chemical entities that have the ability to desensitize nAChRs without an antecedent agonist action. Because these "silent desensitizers" are probably acting allosterically, an additional degree of subtype specificity could be attained.

Interactions between age and the aversive effects of nicotine withdrawal under mecamylamine-precipitated and spontaneous conditions in male Wistar rats

RATIONALE

Adolescent onset of smoking is associated with a rapid progression to dependence. Although adolescents may exhibit a greater susceptibility to nicotine addiction, relatively little is known about the influence of the aversive effects of nicotine withdrawal in maintaining smoking behavior.

OBJECTIVES

The present study investigated age differences in the motivational effects of mecamylamine-precipitated and spontaneous nicotine withdrawal in adolescent and adult rats using the conditioned place aversion procedure (CPA).

MATERIALS AND METHODS

In experiment 1, adolescent (postnatal day (PD) 28) and adult (PD60) male Wistar rats chronically treated with nicotine (3 or 6 mg/kg/day, s.c.) received mecamylamine (1 mg/kg, s.c.), a nicotinic receptor antagonist, or vehicle prior to place conditioning; physical withdrawal signs were also measured. Experiment 2 was conducted to increase nicotine levels in which adolescents were treated with 4.5 or 9 mg/kg/day nicotine. In experiment 3, age differences in spontaneous nicotine withdrawal were evaluated.

RESULTS

Nicotine-treated adults developed a CPA to the mecamylamine-associated compartment and expressed significant physical withdrawal signs, whereas similarly treated adolescents did not. Increasing nicotine exposure levels did not modify the adolescent response to mecamylamine-precipitated withdrawal. Spontaneous nicotine withdrawal produced similar physical withdrawal signs in adolescents and adults, but did not elicit CPA.

CONCLUSIONS

The current study indicates that adolescent rats are less responsive to the aversive effects of mecamylamine-precipitated, but not spontaneous, nicotine withdrawal compared to adult rats. These findings suggest that adolescents and adults may exhibit similar sensitivity to the affective and physical effects of withdrawal following smoking cessation.

Cholinergic innervation of the zebrafish olfactory bulb

A number of fish species receive forebrain cholinergic input but two recent reports failed to find evidence of cholinergic cell bodies or fibers in the olfactory bulbs (OBs) of zebrafish. In the current study we sought to confirm these findings by examining the OBs of adult zebrafish for choline acetyltransferase (ChAT) immunoreactivity. We observed a diffuse network of varicose ChAT-positive fibers associated with the nervus terminalis ganglion innervating the mitral cell/glomerular layer (MC/GL). The highest density of these fibers occurred in the anterior region of the bulb. The cellular targets of this cholinergic input were identified by exposing isolated OBs to acetylcholine receptor (AChR) agonists in the presence of agmatine (AGB), a cationic probe that permeates some active ion channels. Nicotine (50 microM) significantly increased the activity-dependent labeling of mitral cells and juxtaglomerular cells but not of tyrosine hydroxlase-positive dopaminergic neurons (TH(+) cells) compared to control preparations. The nAChR antagonist mecamylamine, an alpha7-nAChR subunit-specific antagonist, calcium-free artificial cerebrospinal fluid, or a cocktail of ionotropic glutamate receptor (iGluR) antagonists each blocked nicotine-stimulated labeling, suggesting that AGB does not enter the labeled neurons through activated nAChRs but rather through activated iGluRs following ACh-stimulated glutamate release. Deafferentation of OBs did not eliminate nicotine-stimulated labeling, suggesting that cholinergic input is primarily acting on bulbar neurons. These findings confirm the presence of a functioning cholinergic system in the zebrafish OB.

Nicotinic and dopamine D2 receptors mediate nicotine-induced changes in ventral tegmental area neurotensin system

Neuropeptides have been implicated in the psychopathology of stimulants of abuse. Neurotensin is a neuropeptide associated with the regulation of the nigrostriatal and mesolimbic dopamine pathways. In addition, the ventral tegmental area, a midbrain region implicated in the rewarding effects of most, if not all, addictive drugs, appears to be a particularly critical target for nicotine action. Because neurotensin has been linked with both mesolimbic and mesocortical dopamine function, we examined the impact of nicotine treatment on central nervous neurotensin systems by measuring changes in neurotensin tissue content because it has been shown that such changes reflect alterations in release and activity of this peptide system. Male Sprague-Dawley rats received multiple administrations of (+/-) nicotine 4.0 mg/kg/day (0.8 mg/kg, i.p.; 5 x 2-h intervals) in the presence or absence of selective dopamine receptor antagonists (dopamine D(1); SCH 23390 or dopamine D(2); eticlopride) or two doses of the non-selective nicotinic acetylcholine receptor antagonist (mecamylamine; 3.0 and 6.0 mg/kg, s.c.). The nicotine treatment significantly decreased neurotensin-like immunoreactivity content in the ventral tegmental area, as well as related regions such as prefrontal cortex, substantia nigra, and anterior striatal region 12-18 h after drug treatment, but not the nucleus accumbens. The nicotine-mediated decrease in the neurotensin-like immunoreactivity of the ventral tegmental area was selectively blocked by a specific dopamine D(2), but not a dopamine D(1), receptor antagonist, while mecamylamine attenuated at the low (3.0 mg/kg) and completely blocked at high (6.0 mg/kg) dose this nicotine effect. These findings with previous studies, suggest that nicotine-mediated dopamine release activates D(2) receptors which in turn increases neurotensin release, turnover and acutely reduces tissue levels in the ventral tegmental area and other limbic and basal ganglia structures.

The nicotinic acetylcholine receptor antagonist mecamylamine prevents escalation of cocaine self-administration in rats with extended daily access

RATIONALE

Escalation from moderate to excessive drug intake is a hallmark of human addiction that can be modeled in rats by giving them longer daily access time to self-administer cocaine. Nicotine and cocaine are commonly coabused drugs in humans and recent work in animals suggests that activation of nicotinic acetylcholine receptors (nAChR) can increase cocaine self-administration.

OBJECTIVES

Determine the role of nAChR in the escalation of cocaine self-administration.

METHODS

Control rats self-administered cocaine (0.75 mg/kg/infusion) for either 1 or 6 h per day. Experimental groups had the nAChR antagonist mecamylamine (MEC) added to the cocaine solution for 5 days after the transition from short (1 h per day) to long access (6 h per day) for cocaine self-administration. After 5 days, MEC was removed from the cocaine solution.

RESULTS

Control rats and rats that received a low dose of MEC (7 microg/infusion) with cocaine increased their average hourly intake over 5 days of 6 h per day cocaine access. Rats that received a higher dose of MEC (70 microg/infusion) did not increase their intake of cocaine during 6 h access but continued to self-administer cocaine. When MEC was removed, this group showed an escalation in cocaine self-administration. MEC did not alter cocaine intake in a group that had continuous 1 h access.

CONCLUSIONS

Antagonism of nAChRs during the initial exposure to extended cocaine self-administration access time prevented escalation of, but did not eliminate, drug intake. These findings indicate that MEC-sensitive nAChRs are critical for determining cocaine intake as a function of longer access time.

Nicotine-induced phosphorylation of ERK in mouse primary cortical neurons: evidence for involvement of glutamatergic signaling and CaMKII

Extracellular signal-regulated kinase (ERK) is activated in vivo in a number of brain areas by nicotine and other drugs of abuse. Here we show that nicotine stimulation of cultured mouse cortical neurons leads to a robust induction of ERK phosphorylation that is dependent on nicotine concentration and duration of exposure. Calcium/calmodulin-dependent protein kinase II activity is necessary for nicotine-induced ERK phosphorylation and neither cAMP-dependent protein kinase or protein kinase C appear to be involved. Activity of glutamate receptors, L-type voltage-gated calcium channels, and voltage-gated sodium channels are also required for nicotine-induced ERK phosphorylation. Nicotine-induced ERK phosphorylation was inhibited by high concentrations of mecamylamine, however it was not blocked by other broad nicotinic acetylcholine receptor (nAChR) inhibitors (including hexamethonium and chlorisondamine) or nAChR subtype selective inhibitors (such as methyllycaconitine, alpha-bungarotoxin, dihydro-beta-erythroidine, and alpha-conotoxin Au1B). In accord with these pharmacological results, nicotine-induced ERK phosphorylation was normal in primary cultures made from beta2 or alpha7 nAChR subunit knockout mice. The alpha3/beta4 nAChR agonist cytisine did not induce ERK phosphorylation suggesting that alpha3/beta4 nAChRs were not involved in this process. Taken together, these data define a necessary role for glutamatergic signaling and calcium/calmodulin-dependent protein kinase II in nicotine-induced ERK phosphorylation in cortical neurons and do not provide evidence for the involvement of classical nAChRs.

Regulation of synaptic transmission and plasticity by neuronal nicotinic acetylcholine receptors

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

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.

Evidence of reuptake inhibition responsible for mecamylamine-evoked increases in extracellular serotonin

The present report using microdialysis approach investigates the neurochemical mechanism of mecamylamine in the regulation of extracellular serotonin in the dorsal raphe nucleus of freely behaving rats. These results suggest that mecamylamine may block serotonin reuptake, the effect consistent with its efficacy of antidepressant.

Dynorphin A (2-13) improves mecamylamine-induced learning impairment accompanied by reversal of reductions in acetylcholine release in rats

Accumulating evidence indicates that the endogenous opioid peptides dynorphin A (1-17) and synthetic dynorphin A (1-13) interact not only with opioid receptors but also with as yet poorly characterized non-opioid binding sites. Dynorphin A (1-13) improved impairments of learning and memory via not only kappa-opioid receptor-mediated, but also 'non-opioid' mechanisms. In the present study, the effects of des-tyrosine(1) dynorphin A (2-13) as a non-opioid metabolite of dynorphin A, and dynorphin A (1-13) on mecamylamine-induced impairment of the acquisition of learning in rats were investigated using a step-through type passive avoidance task. Further, hippocampal acetylcholine release was examined using in vivo microdialysis. Mecamylamine significantly shortened the step-through latency when given 30 min before the acquisition trial. Not only dynorphin A (1-13) but also dynorphin A (2-13) attenuated the mecamylamine-induced impairment of the acquisition of learning. The effect of dynorphin A (2-13) was not blocked by pre-treatment with nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist. Dynorphin A (2-13) completely abolished the decrease in the extracellular acetylcholine concentration induced by mecamylamine and this effect was not blocked by nor-BNI. Taken together with our previous findings, the present results may indicate that dynorphin A (2-13) improves impairment of learning and/or memory in 'non-opioid' mechanisms and dynorphin A (1-13) ameliorates impairment of the acquisition of learning via not only kappa-opioid receptor-mediated mechanisms but also 'non-opioid' mechanisms, by regulating the release of extracellular acetylcholine.

Inhibition of human alpha 7 nicotinic acetylcholine receptors by open channel blockers of N-methyl-D-aspartate receptors

1. Human alpha 7 nicotinic acetylcholine (ACh) receptors were expressed in Xenopus oocytes and the effects of the N-methyl-D-aspartate (NMDA) receptor open channel blockers memantine and cerestat on this receptor were examined using two-electrode voltage-clamp recordings and 125I-alpha-bungarotoxin (125I-alpha-bgtx) binding. 2. Memantine and cerestat produced complete inhibition of ACh-induced inward currents with affinities similar to that reported for native NMDA receptors. Cerestat, IC50 1.7 (-1; +2) microm, was more potent than memantine, IC50 5 (-3;+8) microM, and the effects of both drugs were fully and rapidly reversible. 3. Inhibition of alpha 7 receptor function was voltage-independent, and it occurred at concentrations far lower than those needed to inhibit (never completely) binding of 125I-alpha-bgtx to alpha 7 receptors, suggesting that the effects of memantine or cerestat are noncompetitive. 4. These results provide evidence that human alpha 7 receptors are inhibited by memantine and cerestat and suggest that caution should be applied when using these compounds to study systems in which NMDA and nACh receptors co-exist.

Epibatidine induces long-term potentiation (LTP) via activation of alpha4beta2 nicotinic acetylcholine receptors (nAChRs) in vivo in the intact mouse dentate gyrus: both alpha7 and alpha4beta2 nAChRs essential to nicotinic LTP

Activation of nicotinic acetylcholine receptors (nAChRs) induces nocotinic long-term potentiation (LTPn) in vivo in the mouse dentate gyrus. We have found that alpha4beta2 nAChRs activated by epibatidine induce LTPn, the full size of which requires the involvement of alpha4beta2 and alpha7 nAChRs, in the intact mouse dentate gyrus using extracellular recording techniques. Intraperitoneal application of epibatidine, a potent alpha4beta2 nAChR agonist, at 0.3 - 3.0 mug/kg induced a long-lasting increase similar to LTPn induced by choline, a selective alpha7 nAChR agonist, and at 10 mug/kg caused a transient increase followed by a depression. The LTPn induced by epibatidine at 3.0 mug/kg or choline at 30 mg/kg was significantly suppressed by pre-treatment but not post-treatment with mecamylamine (0.5 mg/kg, i.p.), a non-selective neuronal nicotinic antagonist. Post-application of nicotine at 3.0 mg/kg enhanced epibatidine-induced LTPn to the same level of nicotine-induced LTPn, but post-application of epibatidine had no effect on nicotine-induced LTPn. Epibatidine-induced LTPn was additionally increased by post-application of choline, and vice versa, reaching the same level of nicotine-induced LTPn. The present study revealed that epibatidine induced the LTPn via alpha4beta2 nAChRs and that both alpha7 and alpha4beta2 nAChRs were essential for full-sized LTPn, suggesting that both nAChRs play an important role in synaptic plasticity.

Activation of cholinergic and adrenergic receptors increases the concentration of extracellular adenosine in the cerebral cortex of unanesthetized rat

Adenosine is an inhibitory neuromodulator in the CNS. For extracellular adenosine to play a physiological role in the brain, it must be present at effective concentrations. Acetylcholine and noradrenaline are known to play an important role in modulating the activity of cortical neurons, and they might have a role also in the release of adenosine in the cerebral cortex in vivo. We examined whether activation of cholinergic and adrenergic receptors affects extracellular adenosine levels in the cerebral cortex of unanesthetized rats using in vivo microdialysis. All drugs were administered locally within the cortex by reverse dialysis. Both acetylcholine and the acetylcholinesterase inhibitor neostigmine increased extracellular adenosine levels, and the effect of neostigmine was blocked by the nicotinic receptor antagonist mecamylamine. Both nicotine and the nicotinic receptor agonist epibatidine increased the concentration of extracellular adenosine. Activation of muscarinic receptors using the nonselective agonist oxotremorine and a selective M1 receptor agonist also increased extracellular adenosine levels. Noradrenaline and the noradrenergic reuptake inhibitor desipramine increased extracellular adenosine levels. The alpha(1)-adrenergic receptor agonist phenylephrine and the beta-adrenergic agonist isoproterenol increased extracellular adenosine levels, whereas the alpha(2)-adrenergic receptor agonist clonidine did not have an effect. These findings indicate that activation of specific cholinergic and adrenergic receptors can increase extracellular levels of adenosine in the cortex, and suggest that cholinergic and adrenergic receptor-mediated regulation of adenosine levels may represent a mechanism for controlling the excitability of cortical neurons.

Cholinergic modulation of the cortical neuronal network

Acetylcholine (ACh) is an important neurotransmitter of the CNS that binds both nicotinic and muscarinic receptors to exert its action. However, the mechanisms underlying the effects of cholinergic receptors have still not been completely elucidated. Central cholinergic neurons, mainly located in basal forebrain, send their projections to different structures including the cortex. The cortical innervation is diffuse and roughly topographic, which has prompted some authors to suspect a modulating role of ACh on the activity of the cortical network rather than a direct synaptic role. The cholinergic system is implicated in functional, behavioural and pathological states including cognitive function, nicotine addiction, Alzheimer's disease, Tourette's syndrome, epilepsies and schizophrenia. As these processes depend on the activation of glutamatergic and GABAergic systems, the cholinergic terminals must exert their effects via the modulation of excitatory and/or inhibitory neurotransmission. However, the understanding of cholinergic modulation is complex because it is the result of a mixture of positive and negative modulation, implying that there are various types, or even subtypes, of cholinergic receptors. In this review, we summarize the current knowledge on central cholinergic systems (projections and receptors) and then aim to focus on the implications for ACh in the modulation of cortical neuronal activity.

Role of different nicotinic acetylcholine receptors in mediating behavioral and neurochemical effects of ethanol in mice

Ethanol and nicotine are the most abused drugs, and it is well known that co-abuse of ethanol and nicotine is frequent in human beings. We have previously obtained results indicating that the ethanol-induced stimulation of both the mesolimbic dopamine system and locomotor activity may involve activation of central nicotinic acetylcholine receptors (nAChRs), especially those located in the ventral tegmental area. Different subpopulations of nAChRs have been identified, and, in the present series of experiments, we have studied the effects of various nAChR antagonists on the stimulation of dopamine overflow in the nucleus accumbens and on locomotor activity induced by ethanol in male mice. Ethanol (2.0 g/kg, i.p.) enhanced dopamine overflow in the nucleus accumbens by approximately 40%, measured by means of in vivo microdialysis in awake, freely moving mice. Mecamylamine (negative allosteric modulator of nAChR; 2.0 mg/kg, i.p.) blocked the ethanol-induced stimulation of both locomotor activity and accumbal dopamine overflow. Methyllycaconitine citrate (alpha(7) antagonist; 2.0 mg/kg, i.p.) and dihydro-beta-erythroidine (competitive and selective alpha(4)beta(2) antagonist; 0.5 mg/kg, s.c.), in doses that had no marked effects per se, did not significantly reduce the behavioral and neurochemical stimulation caused by ethanol. The present results support the suggestion that the stimulatory effects of ethanol on locomotor activity and dopamine release do not involve the alpha(4)beta(2) or alpha(7) subunit compositions of the nAChR and that the effects of mecamylamine are mediated through a site not directly associated with the alpha(4)beta(2) or alpha(7) nAChR subunits.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.

Antagonist of nicotinic acetylcholine receptors (nAChR) enhances formalin-induced nociception in rats: tonic role of nAChRs in the control of pain following injury

Following tissue injury, spinal neurons increase in spontaneous activity and in responsiveness to peripheral stimulation. These changes in spinal neurons may underlie abnormal pain behavior. Nicotinic acetylcholine receptor (nAChR) agonists are analgesic when evaluated in animal models of pain, but it is not known if the nAChRs differentially modulate acute and tonic pain. To test this, mecamylamine, a non-subtype selective nAChR antagonist, was systemically injected into rats prior or after hind paw injection of formalin. Formalin injection results in biphasic pain-related behaviors, characterized by a first phase (i.e. acute pain) immediately following formalin injection, then by a second phase (i.e. tonic pain) 15-60 min after formalin injection. Either pre- or post-formalin treatment with mecamylamine decreased phase 1 behaviors and significantly increased phase 2 pain behaviors in a dose-dependent manner. These results suggest that nAChRs may exert opposing effects on acute versus tonic pain and, as such, may have implications for the potential development of nAChR ligands for the treatment of pain.

Effect of subtype selective nicotinic compounds on attention as assessed by the five-choice serial reaction time task

Nicotine can improve attentional functioning in humans, and a number of studies have recently demonstrated that under specific task conditions, nicotine can also improve attention in the rat. Neuronal nicotinic receptors comprise combinations of alpha(2-9) and beta(2-4) subunits, arranged to form a pentameric receptor, with the principal CNS subtypes currently believed to be alpha(4)beta(2) and a homomeric alpha(7) receptor. In the present studies, we attempted to delineate the particular nicotinic receptor subtype(s) contributing to the effects of nicotine on attention by assessing various nicotinic ligands on performance in the five-choice serial reaction time task (5-CSRTT). In rats performing below criterion (<80% correct, >20% omissions to a 1-s visual stimulus), subchronic dosing with nicotine (0.2 mg/kg sc) and the alpha(4)beta(2) agonist SIB 1765F (5 mg/kg sc) increased correct responding and decreased response latencies across the treatment week; whereas the alpha(7) agonist AR-R 17779 (20 mg/kg sc) was without effect. In subjects meeting the criterion, the competitive high affinity (including alpha(4)beta(2)) nicotine receptor antagonist DHbetaE (1-10 mg/kg sc) and the alpha(7) antagonist methyllycaconitine (MLA: 5-10 mg/kg i.p.) did not disrupt performance, whereas at the highest dose, the non-competitive antagonist mecamylamine (0.3-3 mg/kg sc) decreased accuracy and increased response latencies. These changes bore some similarities to those of pre-feeding and the non-competitive NMDA antagonist dizocilpine (0.03-0.06 mg/kg sc), suggesting that mecamylamine-induced performance disruption may relate to non-nicotinic receptor effects. In subjects chronically treated with nicotine, acute nicotine challenge (0.4 mg/kg sc) significantly increased accuracy whilst having no effect on any other performance measures. Finally, in these same nicotine pre-treated rats, the decrease in latency and increase in premature responses induced by nicotine (0.2 mg/kg sc) to a target stimulus of 150 ms was fully antagonised by DHbetaE (3 mg/kg sc) but not MLA (5 mg/kg i.p.). These results suggest that alpha(7) receptors do not play a role in any of the behavioural effects of nicotine observed in the 5-CSRTT, whereas a high affinity site, perhaps alpha(4)beta(2), is more likely involved.

Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

The effects of nicotine on 5-hydroxytryptamine (5-HT) release from serotonergic nerve endings in rat dorsal hippocampal slices were studied. Nicotine (50-500 microM:) caused a concentration-dependent increase in 5-HT release. This effect was antagonised by mecamylamine (0.5 microM:), indicating an action at nicotinic receptors. Nicotine-evoked 5-HT release was not affected by tetrodotoxin (3 microM:), cadmium chloride (0.1 mM:), or the absence of Ca(2+) or Na(+) in the superfusion medium. Unexpectedly, higher concentrations of mecamylamine alone (1-50 microM:) increased 5-HT release. This suggested the presence of inhibitory input to 5-HT neurones and that these inhibitory neurones possess tonically active nicotinic receptors. The effect of mecamylamine (50 microM:) on 5-HT release was reduced by the muscarinic M(1) receptor agonist, McN-A-343 (100 microM:), but pirenzepine (0.005-1 microM:), which blocks M(1) receptors, alone increased 5-HT release. Hippocampal serotonergic neurones are known to possess both excitatory nicotinic receptors and inhibitory M(1) receptors. Although there may be several explanations for our results, one possible explanation is that nicotine stimulates 5-HT release by activating nicotinic heteroreceptors on 5-HT terminals. Mecamylamine (0.5 microM:) antagonises this effect, but higher concentrations increase 5-HT release indirectly by blocking the action of endogenous acetylcholine on nicotinic receptors situated on cholinergic neurones that provide muscarinic inhibitory input to 5-HT neurones.