Bupropion dose-dependently reverses nicotine withdrawal deficits in contextual fear conditioning

Bupropion, a norepinephrine and dopamine reuptake inhibitor and nicotinic acetylcholine receptor antagonist, facilitates smoking cessation and reduces some symptoms of nicotine withdrawal. However, the effects of bupropion on nicotine withdrawal-associated deficits in learning remain unclear. The present study investigated whether bupropion has effects on contextual and cued fear conditioning following withdrawal from chronic nicotine or when administered alone. Bupropion was administered alone for a range of doses (2.5, 5, 10, 20 or 40 mg/kg), and dose-dependent impairments in contextual and cued fear conditioning were observed (20 or 40 mg/kg). Follow-up studies investigated if bupropion disrupted acquisition or expression of fear conditioning. Bupropion (40 mg/kg) administration on training day only produced deficits in contextual fear conditioning. Alternatively, bupropion (20 or 40 mg/kg) administration during testing dose-dependently produced deficits in contextual and cued fear conditioning. To test the effect of bupropion on nicotine withdrawal, mice were withdrawn from 12 days of chronic nicotine (6.3 mg/kg/day) or saline treatment. Withdrawal from chronic nicotine disrupted contextual fear conditioning; however, 5 mg/kg bupropion reversed this deficit. Overall, these results indicate that a low dose of bupropion can reverse nicotine withdrawal deficits in contextual fear conditioning, but that high doses of bupropion produce deficits in fear conditioning.     

The duration of nicotine withdrawal-associated deficits in contextual fear conditioning parallels changes in hippocampal high affinity nicotinic acetylcholine receptor upregulation

A predominant symptom of nicotine withdrawal is cognitive deficits, yet understanding of the neural basis for these deficits is limited. Withdrawal from chronic nicotine disrupts contextual learning in mice and this deficit is mediated by direct effects of nicotine in the hippocampus. Chronic nicotine treatment upregulates nicotinic acetylcholine receptors (nAChR); however, it is unknown whether upregulation is related to the observed withdrawal-induced cognitive deficits. If a relationship between altered learning and nAChR levels exists, changes in nAChR levels after cessation of nicotine treatment should match the duration of learning deficits. To test this hypothesis, mice were chronically administered 6.3mg/kg/day (freebase) nicotine for 12 days and trained in contextual fear conditioning on day 11 or between 1 to 16 days after withdrawal of treatment. Changes in [(125)I]-epibatidine binding at cytisine-sensitive and cytisine-resistant nAChRs and chronic nicotine-related changes in α4, α7, and β2 nAChR subunit mRNA expression were assessed. Chronic nicotine had no behavioral effect but withdrawal produced deficits in contextual fear conditioning that lasted 4 days. Nicotine withdrawal did not disrupt cued fear conditioning. Chronic nicotine upregulated hippocampal cytisine-sensitive nAChR binding; upregulation continued after cessation of nicotine administration and the duration of upregulation during withdrawal paralleled the duration of behavioral changes. Changes in binding in cortex and cerebellum did not match behavioral changes. No changes in α4, α7, and β2 subunit mRNA expression were seen with chronic nicotine. Thus, nicotine withdrawal-related deficits in contextual learning are time-limited changes that are associated with temporal changes in upregulation of high-affinity nAChR binding.     

β2 subunit-containing nicotinic receptors mediate the enhancing effect of nicotine on trace cued fear conditioning in C57BL/6 mice

Rationale Previous research indicates that acute nicotine administration enhances the acquisition of contextual fear conditioning and trace cued fear conditioning. Pharmacological inhibition of α4β2 nicotinic acetylcholine receptors (nAChRs), but not α7 nAChRs, blocked the enhancing effect of nicotine on contextual fear conditioning. Similarly, genetic deletion of the β2 nAChR subunit but not the α7 nAChR subunit blocked the enhancing effect of nicotine on contextual fear conditioning. Objectives In the present study, nAChR subunit knockout mice were used to compare the involvement of β2 subunit-containing nAChRs and α7 subunit-containing nAChRs in the effects of nicotine on hippocampus-dependent trace cued fear conditioning and contextual fear conditioning. Methods β2 nAChR subunit knockout mice, α7 nAChR subunit knockout mice, and their wild-type littermates received either nicotine or saline 5 minutes before training and testing. Mice were trained using five conditioned stimulus (CS; 30 s, 85 dB white noise)—trace (30 s)—unconditioned stimulus (US; 2 s footshock) pairings. Freezing to the context and freezing to the CS were assessed 24 h later. Results Both contextual and trace cued fear conditioning were enhanced by nicotine administration in wild-type littermates and in α7 nAChR subunit knockout mice. In contrast, neither contextual fear conditioning nor trace cued fear conditioning was enhanced by nicotine administration in β2 nAChR subunit knockout mice. Conclusions These results suggest that β2 subunit-containing nAChRs but not α7 nAChR subunit-containing nAChRs are critically involved in the enhancing effect of nicotine on contextual and trace cued fear conditioning.     

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

Rationale Previous research indicates that nicotine administration enhances hippocampus-dependent forms of learning, including contextual fear conditioning. This effect is blocked by mecamylamine, a noncompetitive, broad-spectrum nicotinic receptor antagonist. Objectives The present study extends previous research by further characterizing the nicotinic acetylcholinergic receptor (nAChR) subtypes through which nicotine acts to enhance contextual fear conditioning. Methods C57BL/6J mice were trained with two conditioned stimulus (CS; 30 s, 85-dB white noise)–unconditioned stimulus (US; 2 s, 0.57-mA foot shock) pairings and tested 24 h later for contextual and cued fear conditioning. The effects of the α7 nAChR antagonist methyllycaconitine (MLA; 1.00, 10.00, and 20.00 mg/kg) and the effects of the α4β2 nAChR antagonist dihydro-beta-erythroidine (DHBE; 1.00, 3.00, and 6.00 mg/kg) on cued and contextual fear conditioning and on the enhancement of contextual fear conditioning by nicotine (0.25 mg/kg) were examined. Results We demonstrate that DHBE (all doses) administration attenuates the enhancing effect of nicotine on contextual fear conditioning, and MLA administration has no significant effect on the enhancement of contextual fear conditioning by nicotine. Conclusions The data suggest that non-α7 nAChRs (most likely α4β2 nAChRs) underlie the enhancement of contextual fear conditioning by nicotine.     

Genetic background influences the effects of withdrawal from chronic nicotine on learning and high-affinity nicotinic acetylcholine receptor binding in the dorsal and ventral hippocampus

Rationale

The effects of nicotine on cognitive processes may play an important role in nicotine addiction. Nicotine withdrawal impairs hippocampus-dependent learning and genetic factors influence this effect. However, the neural changes that contribute to these impairments are unknown. Chronic nicotine upregulates hippocampal nicotinic acetycholine receptors (nAChRs), which may contribute to cognitive deficits when nicotine administration ceases. If nAChR upregulation underlies withdrawal deficits in learning, then strains of mice exhibiting withdrawal deficits in hippocampus-dependent learning should also show upregulation of hippocampal nAChRs.

Objectives

Here, we examined the effects of nicotine withdrawal on fear conditioning and [³H]epibatidine binding in the dorsal and ventral hippocampus in two inbred mouse strains and their F1 hybrids.

Methods

Male C57BL/6NTac, 129S6/SvEvTac, and B6129SF1/Tac mice were administered chronic nicotine (18 mg/kg/day) for 12 days through osmotic pumps and then were trained and tested in fear conditioning 24 h after cessation of nicotine treatment.

Results

Nicotine withdrawal impaired hippocampus-dependent contextual conditioning in C57BL/6NTac mice but not 129S6/SvEvTac or B6129SF1/Tac mice; no changes were observed in hippocampus-independent cued fear conditioning. Upregulated [³H]epibatidine binding was found in the dorsal, but not ventral, hippocampus of C57BL/6NTac mice and in the ventral hippocampus of B6129SF1/Tac mice after chronic nicotine.

Conclusions

Upregulation of high-affinity binding sites in the dorsal hippocampus of C57BL/6NTac mice, the only strain that exhibited nAChR upregulation in this region and withdrawal deficits in contextual conditioning, suggests that upregulation of high-affinity binding sites in the dorsal hippocampus mediates, in part, nicotine withdrawal deficits in contextual conditioning and genetic background modulates these effects.     

Interactive effects of ethanol and nicotine on learning in C57BL/6J mice depend on both dose and duration of treatment

Objective and rationale Alcohol and nicotine are commonly co-abused; one possible explanation for co-abuse is that each drug ameliorates the aversive effects of the other. Both drugs have dose-dependent effects on learning and memory. Thus, this study examined the interactive effects of acute ethanol and acute, chronic, or withdrawal from chronic nicotine on fear conditioning in C57BL/6J mice. Materials and methods Conditioning consisted of auditory conditioned stimulus-foot-shock unconditioned stimulus pairings. For acute studies, saline or ethanol, then saline or nicotine was administered before training, and saline or nicotine was also administered before testing. For chronic and withdrawal studies, saline or nicotine was administered chronically, and ethanol or saline was administered before training. Results Acute nicotine (0.09 mg/kg) reversed ethanol-induced deficits (1.0 and 1.5 g/kg) in contextual and cued fear conditioning, whereas a low dose of ethanol (0.25 g/kg) reversed nicotine (6.3 mg kg⁻¹ day⁻¹) withdrawal-induced deficits in contextual conditioning. Tolerance developed for the effects of nicotine on ethanol-induced deficits in conditioning and cross-tolerance between chronic nicotine and acute ethanol was seen for the enhancing effects of ethanol on conditioning. Conclusions The complex and sometimes polar actions of ethanol and nicotine on behavior may contribute to co-abuse of these drugs. Specifically, smoking may initially reduce the aversive effects of ethanol, but tolerance develops for this effect. In addition, low doses of alcohol may lessen nicotine withdrawal symptoms.     

Effects of the beta-blocker propranolol on cued and contextual fear conditioning in humans

Rationale Beta-adrenergic receptors are involved in the consolidation of emotional memories. Yet, a number of studies using Pavlovian cued fear conditioning have been unable to demonstrate an effect of beta-adrenergic blockade on acquisition or retention of fear conditioning. Evidence for the involvement of beta-adrenergic receptors in emotional memories comes mostly from studies using fear inhibitory avoidance in rodents. It is possible that fear inhibitory avoidance is more akin to contextual conditioning than to cued fear conditioning, suggesting that context conditioning may be disrupted by beta-adrenergic blockade.Objective This study investigated the effects of the beta-adrenergic blocker propranolol on cued and contextual fear conditioning in humans.Methods Subjects were given either placebo (n=15) or 40 mg propranolol (n=15) prior to differential cued conditioning. A week later, they were tested for retention of context and cued fear conditioning using physiological (startle reflex and electrodermal activity) and subjective measures of emotional arousal.Results The results were consistent with the hypothesis. The skin conductance level (SCL) and the subjective measure of arousal suggested reduced emotional arousal upon returning to the conditioning context in the propranolol group, compared to the placebo group. The acquisition and retention of cued fear conditioning were not affected by propranolol.Conclusions These results suggest that beta-adrenergic receptors are involved in contextual fear conditioning.     

Post-Retrieval Extinction Attenuates Cocaine Memories

Recent studies have shown that post-retrieval extinction training attenuates fear and reward-related memories in both humans and rodents. This noninvasive, behavioral approach has the potential to be used in clinical settings to treat maladaptive memories that underlie several psychiatric disorders, including drug addiction. However, few studies to date have used a post-retrieval extinction approach to attenuate addiction-related memories. In the current study, we attempted to disrupt cocaine-related memories by using the post-retrieval extinction paradigm in male Sprague Dawley rats. Results revealed that starting extinction training 1 h after cocaine contextual memory was retrieved significantly attenuated cocaine-primed reinstatement of conditioned place preference (CPP) and relapse of cocaine CPP (drug-free and cocaine-primed) following 30 days of abstinence. However, animals that did not retrieve the contextual cocaine memory before extinction training, or animals that began extinction training 24 h after retrieval (outside of the reconsolidation window), demonstrated normal cocaine CPP. Conversely, animals that received additional CPP conditioning, rather than extinction training, 1 h after reactivation of cocaine memory showed enhanced cocaine CPP compared with animals that did not reactivate the cocaine memory before conditioning. These results reveal that a behavioral manipulation that takes advantage of reconsolidation and extinction of drug memories may be useful in decreasing preference for, and abuse of, cocaine.     

小鼠条件性恐惧模型的建立和品系敏感性研究

目的在敏感品系上建立稳定可靠的小鼠条件性恐惧模型。方法采用给予30 s声音刺激(85 dB,5000 Hz),后2 s伴随给予不可逃避的足底电击(0.6 mA,持续2 s),声音-电击共配对5次,每次间隔2 min的方法建立小鼠条件性恐惧模型,在ICR、DBA/2(简称DBA)和C57BL/6J(简称C57)3种品系小鼠上评价场景恐惧和声音线索恐惧获得和表达的行为特点,以确定敏感品系鼠;分别于条件性恐惧训练第2,7,14,21和28天对C57小鼠场景恐惧和声音线索恐惧进行检测,以评价该品系小鼠条件性恐惧的自然消退特点;采用30 s声音+30 s间隔+10次消退训练的模式进行声音线索恐惧消退训练,并于24 h后进行消退保持测试,以评价该品系小鼠消退训练和保持的特点。结果声音-电击配对5次可成功诱导C57小鼠形成场景恐惧和声音线索恐惧,DBA小鼠可诱导形成声音线索恐惧但无法形成场景恐惧,ICR小鼠场景恐惧和声音线索恐惧均未诱导成功,确定C57小鼠为敏感品系鼠。C57小鼠呈现时间依赖的恐惧反应自然消退,其中场景恐惧在训练后第7天僵住百分率消退至(25±12)%,声音线索恐惧训练后第28天僵住百分率维持在(48±22)%;C57小鼠经10轮30 s声音消退训练,僵住时间百分率由(55±30)%消退至(32±27)%,但24 h后消退保持测试时僵住时间百分率又回升至(47±35)%。结论以敏感品系C57小鼠为研究对象,建立了恐惧获得、表达、自然消退、消退训练和消退保持各阶段恐惧行为检测的实验方法,成功建立小鼠条件性恐惧模型。     

Adolescent nicotine exposure disrupts context conditioning in adulthood in rats

Despite the prevalence of smoking among adolescents, few studies have assessed the effects of adolescent nicotine exposure on learning in adulthood. In particular, it remains unclear whether adolescent nicotine exposure has effects on hippocampus-dependent learning that persist into adulthood. The present experiment examined whether there were effects of adolescent nicotine exposure on context conditioning, a form of learning dependent on the integrity of the hippocampus, when tested during adulthood. Rats were exposed to nicotine during adolescence (postnatal days [PD] 28-42) via osmotic minipump (0, 3.0 or 6.0 mg/kg/day). Context conditioning occurred in early adulthood (PD 65-70). Animals were exposed to an experimental context and were given 10 unsignaled footshocks or no shock. Additional groups were included to test the effects of adolescent nicotine on delay conditioning, a form of learning that is not dependent upon the hippocampus. Conditioning was assessed using a lick suppression paradigm. For animals in the context conditioning groups, adolescent nicotine resulted in significantly less suppression of drinking in the presence of context cues compared with vehicle-pretreated animals. For animals in the delay conditioning groups, there was a trend for adolescent nicotine (3.0 mg/kg/day) to suppress drinking compared to vehicle-pretreated animals. There were no differences in extinction of contextual fear or cued fear between rats previously exposed to vehicle or nicotine. The data indicate that adolescent nicotine administration impairs context conditioning when animals are trained and tested as adults. The present data suggest that adolescent nicotine exposure may disrupt hippocampus-dependent learning when animals are tested during adulthood.     

Atomoxetine reverses nicotine withdrawal-associated deficits in contextual fear conditioning.

Recent evidence suggests that the cognitive symptoms of nicotine withdrawal and the cognitive symptoms of attention deficit hyperactivity disorder (ADHD) may share neural correlates. Thus, therapeutics that ameliorate ADHD symptoms may also ameliorate nicotine-withdrawal symptoms. The present research tested this hypothesis in an animal model of nicotine withdrawal-associated cognitive deficits using atomoxetine, a norepinephrine reuptake inhibitor that is approved by the FDA to treat the symptoms of ADHD. C57BL/6 mice were prepared with osmotic minipumps that administered 6.3 mg/kg/day of nicotine or saline, and the minipumps were removed after 12 days of continuous treatment. Twenty-four hours later, mice were trained in delay fear conditioning using two paired presentations of an auditory conditioned stimulus (CS) with a footshock unconditioned stimulus. Testing for freezing in response to the training context and for freezing in response to the CS occurred the next day. Nicotine-withdrawn mice and their saline-treated counterparts received either saline or atomoxetine before training and the context test. Consistent with previous research, the results indicate that mice withdrawn from chronic nicotine demonstrated lower levels of contextual fear conditioning than mice that were not withdrawn from chronic nicotine. Atomoxetine dose-dependently reversed the deficit, suggesting that nicotine withdrawal may be associated with changes in noradrenergic function, acetylcholinergic function, and/or with changes in cell signaling cascades that are activated by both nicotine and norepinephrine. These data suggest that atomoxetine may be efficacious for treating nicotine withdrawal-associated cognitive deficits that promote relapse in abstinent smokers.     

Hippocampal nAChRs mediate nicotine withdrawal-related learning deficits

Nicotine modulation of learning may contribute to its abuse liability. The role of hippocampal nicotinic acetylcholine receptors (nAChRs) in the effects of acute, chronic and withdrawal from chronic nicotine on learning was assessed via intrahippocampal drug infusion in mice. Acute dorsal hippocampal nicotine infusion enhanced contextual fear conditioning. Conversely, chronic intrahippocampal infusion of a matched dose had no effect, and withdrawal from chronic infusion impaired learning. Thus, hippocampal functional adaptation, evidenced by learning deficits during abstinence, occurs with the transition from acute to chronic nicotine exposure. To investigate which hippocampal nAChRs mediate these adaptations, C57BL/6, β2 nAChR subunit knockout (KO), and wildtype (WT) mice treated chronically with systemic nicotine received intrahippocampal dihydro-β-erythroidine (a high affinity nAChR antagonist). Intrahippocampal dihydro-β-erythroidine precipitated learning deficits in all but the KO mice. Therefore, the action of nicotine at hippocampal β2⁎ nAChRs mediates adaptations in hippocampal function that underlie withdrawal deficits in contextual fear conditioning.     

Ethanol disrupts fear conditioning in C57BL/6 mice

Ethanol has been demonstrated to disrupt numerous forms of learning. For example, ethanol disrupts fear conditioning in rats. Surprisingly, the opposite result was reported for mice. Because of the importance of mouse models in ethanol research and the predominance of transgenic mice generated on a C57BL/6 background, the present study examined the effects of acute ethanol administration on fear conditioning in C57BL/6 mice. Fear conditioning was chosen because of the apparent contradiction in results between mice and rats, because of its popularity in assessing forebrain-dependent learning and because the task examines two types of learning: (i) the hippocampus-dependent contextual learning and (ii) the hippocampus-independent conditioned stimulus-unconditioned stimulus learning. Dose-response curves were generated for ethanol (0.5, 1.0 and 1.5 g/kg) given on either training day, testing day, or both days. Ethanol, in a dose-dependent manner, disrupted fear conditioning when given on training day or given on both training and testing days. Ethanol given on testing day only did not disrupt fear conditioning. The present results demonstrate that ethanol disrupts fear conditioning in C57BL/6 mice.     

Cannabinoid CB1 receptor antagonist rimonabant disrupts nicotine reward-associated memory in rats

Exposure to cues previously associated with drug intake leads to relapse by activating previously acquired memories. Based on previous findings, in which cannabinoid CB₁ receptors were found to be critically involved in specific aspects of learning and memory, we investigated the role of CB₁ receptors in nicotine reward memory using a rat conditioned place preference (CPP) model. In Experiment 1, rats were trained for CPP with alternating injections of nicotine (0.5 mg/kg, s.c.) and saline to acquire the nicotine-conditioned memory. To examine the effects of rimonabant on the reconsolidation of nicotine reward memory, rats were administered rimonabant (0, 0.3, and 3.0 mg/kg, i.p.) immediately after reexposure to the drug-paired context. In Experiment 2, rats were trained for CPP similarly to Experiment 1. To examine the effects of rimonabant on the reinstatement of nicotine reward memory, rimonabant (0, 0.3, and 3.0 mg/kg, i.p.) was administered before the test of nicotine-induced CPP reinstatement. In Experiment 3, to evaluate whether rimonabant itself produces a reward memory, rats were trained for CPP with alternating injections of different doses of rimonabant (0, 0.3, and 3.0 mg/kg) and saline. Rimonabant at a dose of 3.0 mg/kg significantly disrupted the reconsolidation of nicotine memory and significantly blocked the reinstatement of nicotine-induced CPP. However, rimonabant itself did not produce CPP. These findings provide clear evidence that CB₁ receptors play a role in nicotine reward memory, suggesting that CB₁ receptor antagonists may be a potential target for managing nicotine addiction.     

Ameliorating effect of histamine on impairment of cued fear extinction induced by morphine withdrawal in histidine decarboxylase gene knockout mice

Aim:

Histamine plays an important role in morphine addiction and memory-dependent behavior. However, little is known about the effect of histamine on the impairment of memory after morphine withdrawal. This study was designed to investigate the effect of histamine on memory impairment induced by morphine withdrawal in histidine decarboxylase knockout (HDC-KO) and wild-type (WT) mice.

Methods:

WT and HDC-KO mice were given subcutaneous morphine or saline twice daily for 5 consecutive days. The mice received a cued or contextual fear conditioning session 7 days after the last injection. During subsequent days, mice received 4 cued or contextual extinction sessions (one session per day). Western blot was used to assess extracellular signal-regulated kinase (ERK) phosphorylation in the amygdala and hippocampus.

Results:

Morphine withdrawal did not affect the acquisition of cued or contextual fear responses. It impaired cued but not contextual fear extinction. The acquisition of cued and contextual fear responses was accelerated in HDC-KO mice. Histamine deficiency aggravated the impairment of cued fear extinction induced by morphine withdrawal, whereas histamine (icv, 5 μg/mouse) reversed this effect. Morphine withdrawal decreased ERK phosphorylation in the amygdala after cued fear extinction, especially in HDC-KO mice.

Conclusion:

These results suggest that morphine withdrawal specifically impairs cued fear extinction and histamine ameliorates this impairment. Its action might be mediated by the modulation of ERK phosphorylation in the amygdala. Histamine should be explored for possible roles in the prevention or treatment of morphine abuse and relapse.     

MK-801 disrupts acquisition of contextual fear conditioning but enhances memory consolidation of cued fear conditioning

The effects of pre-training or post-training subcutaneous injections of multiple doses of the non-competitive NMDA-receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) on cued and contextual fear conditioning were examined in F344 rats. Pre-training injections of MK-801 (0.3 and 1.0 mg/kg) disrupted contextual fear conditioning but not cued fear conditioning. Post-training injections of MK-801 did not disrupt cued or contextual fear conditioning. In fact, the 0.3 mg/kg dose of MK-801 enhanced cued fear conditioning. Finally, rats were tested for MK-801-induced alterations in sensitivity to pain using the formalin test for nociception. MK-801 did not reduce sensitivity to pain. These results suggest that NMDA receptors are involved in acquisition of contextual fear conditioning but not in memory consolidation of the learned response.     

Post-retrieval propranolol treatment does not modulate reconsolidation or extinction of ethanol-induced conditioned place preference

The reconsolidation hypothesis posits that established emotional memories, when reactivated, become labile and susceptible to disruption. Post-retrieval injection of propranolol (PRO), a nonspecific β-adrenergic receptor antagonist, impairs subsequent retention performance of a cocaine- and a morphine-induced conditioned place preference (CPP), implicating the noradrenergic system in the reconsolidation processes of drug-seeking behavior. An important question is whether post-retrieval PRO disrupts memory for the drug-cue associations, or instead interferes with extinction. In the present study, we evaluated the role of the β-adrenergic system on the reconsolidation and extinction of ethanol-induced CPP. Male DBA/2J mice were trained using a weak or a strong conditioning procedure, achieved by varying the ethanol conditioning dose (1 or 2 g/kg) and the number of ethanol trials (2 or 4). After acquisition of ethanol CPP, animals were given a single post-retrieval injection of PRO (0, 10 or 30 mg/kg) and tested for memory reconsolidation 24 h later. Also, after the first reconsolidation test, mice received 18 additional 15-min choice extinction tests in which PRO was injected immediately after every test. Contrary to the prediction of the reconsolidation hypothesis, a single PRO injection after the retrieval test did not modify subsequent memory retention. In addition, repeated post-retrieval administration of PRO did not interfere with extinction of CPP in mice. Overall, our data suggest that the β-adrenergic receptor does not modulate the associative processes underlying ethanol CPP.     

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

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

Cholinergic effects on fear conditioning II: nicotinic and muscarinic modulations of atropine-induced disruption of the degraded contingency effect

Rationale In a companion study (Carnicella et al., 2005), we showed that the muscarinic antagonist atropine, when administered after extensive training during both conditioning and testing, affected neither cued nor contextual fear memories when both of them did not compete for the control of the overt behaviour. In contrast, atropine altered the degraded contingency effect (DCE), that is, the processes by which contextual fear memory competes with the cued one for the control of the conditioned response. Atropine-induced disruption of the DCE was fully reversed by the administration of the anticholinesterase inhibitor physostigmine, which suggests a direct cholinergic implication.Objective The present series of experiments was conducted in order to define more precisely the involvement of the cholinergic system in such an effect.Methods Oxotremorine (0.0, 0.0075, 0.015, or 0.03 mg/kg), pilocarpine (0.0, 0.3, 1, or 3 mg/kg), xanomeline (0.0, 2.5, 5.0, 10.0 or 20.0 mg/kg) and nicotine (0.0, 0.1, 0.2, or 0.4 mg/kg) were tested for reversal of the atropine-induced alteration of the DCE.Results Oxotremorine and pilocarpine did not reverse the atropine-induced alteration of the DCE. In contrast, xanomeline and nicotine reversed the effect of atropine on the DCE.Conclusion The present series of experiments reveals complex pharmacological interactions within the cholinergic system when cued and contextual fear memories interact. Results are discussed in this connection and with regard to the relation between the properties of cholinergic agonists and their therapeutic values.