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 To test the effect of bupropion on nicotine withdrawal, mice were withdrawn from 12 days of chronic nicotine (6.3 mg/kg/day) or saline treatment.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 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.     

Fear conditioning and latent inhibition in mice lacking the high affinity subclass of nicotinic acetylcholine receptors in the brain

Nicotine can enhance performance in several tests of cognition but the specific nicotinic receptor subtypes mediating these effects are largely unknown. Knock out mice lacking the β2 subunit of the nicotinic receptor were evaluated in fear conditioning and latent inhibition tasks to begin to determine which receptor subtypes mediate the cognitive effects of nicotine. Young (2–4 months) knock out and wild type mice did not differ in either contextual or tone-conditioned fear, but aged (9–20 months) knock out males were impaired in freezing to both context and tone compared to aged wild type males. No differences in fear conditioning were observed between aged knock out and wild type females. Latent inhibition of fear to a pre-exposed tone, as measured by behavioral freezing, was also assessed. Both knock out and wild type mice displayed similar levels of latent inhibition, although overall levels of freezing were lower in knock out mice. These results support a previous study showing spatial learning deficits in aged β2 subunit knock out mice [EMBO J. 18 (1999) 1235] and suggest that performance of other cognitive tasks may not be influenced by absence of β2 subunit-containing receptors.     

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.     

Prefrontocortical dopamine loss in rats delays long-term extinction of contextual conditioned fear, and reduces social interaction without affecting short-term social interaction memory.

Prefrontal dopamine loss delays extinction of cued fear conditioning responses, but its role in contextual fear conditioning has not been explored. Medial prefrontal lesions also enhance social interaction in rats, but the role of prefrontal dopamine loss on social interaction memory is not known. Besides, a role for subcortical accumbal dopamine on mnesic changes after prefrontal dopamine manipulation has been proposed but not explored. The objective was to study the involvement of dopaminergic neurotransmission in the medial prefrontal cortex (mPFC) and nucleus accumbens in two mnesic tasks: contextual fear conditioning and social interaction memory. For contextual fear conditioning, short- and long-term freezing responses after an electric shock were studied, as well as extinction retention. Regarding social interaction memory, the recognition of a juvenile, a very sensitive short-term memory test, was used. Dopamine loss was carried out by injection of 6-hydroxydopamine, and postmortem catecholamine levels were analyzed by high-performance liquid chromatography. Prefrontocortical dopamine loss (>76%) led to a reactive enhancement of accumbal dopamine content (p<0.01), supporting the hypothesis that a hyperdopaminergic tone emerges in the nucleus accumbens after prefrontocortical dopamine loss. In lesioned rats, long-term extinction of contextual fear conditioning was significantly delayed and extinction retention was impaired without changes in acquisition and short-term contextual fear conditioning and, on the other hand, acquisition and short-term social interaction memory were not affected, although time spent on social interaction was significantly reduced. Added dopamine loss in the nucleus accumbens (>76%) did not alter these behavioral changes. In summary, the results of the present study indicate that the dopaminergic network in the mPFC (but not in the nucleus accumbens) coordinates the normal long-term extinction of contextual fear conditioning responses without affecting their acquisition, and it is involved in time spent on social interaction, but not acquisition and short-term social interaction memory.     

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.     

Endocannabinoid modulation of fear responses: learning and state-dependent performance effects

Recently, disruption of the endogenous cannabinoid (endocannabinoid, eCB) system was found to impair extinction in delay and contextual fear conditioning models. However, conditioning procedures used in that work precluded investigation of possible eCB effects on acquisition of learned fear. We therefore examined the role of eCBs in modulating fear responses using multiple-trial versions of trace (hippocampal-dependent) and delay (amygdala-dependent) Pavlovian fear conditioning. By administering the CB1 receptor antagonist AM251 (5 mg/kg, i.p) to C57/Bl/6 mice at various times, we systematically identified the stages of learning and memory (i.e. acquisition, consolidation, recall and extinction) that are modulated by eCB signaling. During tone (CS) - footshock (US) conditioning, AM251 enhanced acquisition of freezing behavior for both trace- and delay-conditioning protocols. CB1 antagonism also enhanced generalized fear (baseline freezing) and cued (CS) freezing during memory recall tests in a state-dependent manner for both trace and delay conditioned animals. Furthermore, in trace-conditioned animals, AM251 impaired extinction performance of both cued and generalized fear. CB1 antagonism did not affect short-term memory (STM) or long-term memory (LTM) consolidation processes. Together, these results suggest that during acquisition and recall of aversive learning, eCBs prevent the expression and retention of inappropriate generalized and learned responses. These findings have important implications for the therapeutic use of CB1 antagonists.     

Functional Reuniens and Rhomboid Nuclei Are Required for Proper Acquisition and Expression of Cued and Contextual Fear in Trace Fear Conditioning

BackgroundThe reuniens (Re) and rhomboid (Rh) nuclei (ReRh) of the midline thalamus interconnect the hippocampus and the medial prefrontal cortex. The hippocampus and medial prefrontal cortex are both involved in the acquisition of trace fear conditioning, in which a conditioned stimulus (tone) and an aversive unconditioned stimulus (footshock) are paired but separated in time with a trace interval. Earlier, we demonstrated that ReRh inactivation during trace conditioning impaired the acquisition of cued fear. In contrast, ReRh inactivation during both conditioning and test resulted in heightened fear to tones during retrieval. Because there was a generalized contextual fear on top of heightened fear to tones in the latter experiment, here we aimed to examine the specific importance of the functional ReRh in cued fear and contextual fear through introducing prolonged contextual exposure.MethodsThe ReRh were pharmacologically inactivated with muscimol (or saline as controls) before each experimental session.ResultsWe showed that although ReRh inactivation before trace fear conditioning impaired the acquisition of cued fear, the animals still acquired a certain level of fear to the tones. However, without the functional ReRh throughout the entire behavioral sessions, these animals showed heightened contextual fear that did not decline much with the passage of time, which generalized to the other context, and fear to tones reoccurred when the tones were presented.ConclusionsOur results suggested that functional ReRh are important for proper acquisition and expression of fear to context and tones acquired under trace procedure.     

Effects of muscarinic receptor antagonism upon two forms of stress-induced analgesia

The present study assessed in rats the effects of muscarinic receptor antagonism upon analgesia induced by cold-water swims (CWS: 2 degrees C for 3.5 min) and 2-deoxy-D-glucose (2DG: 600 mg/kg). First, CWS analgesia was significantly reduced 30 min after the swim by scopolamine (0.01 and 0.1 mg/kg) and methylscopolamine (10 mg/kg) pretreatment, and was eliminated 60 min after the swim by scopolamine (0.01-10 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment. In contrast, scopolamine potentiated CWS hypothermia. Second, while scopolamine (1 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment prolonged 2DG analgesia, both antagonists dose-dependently reduced 2DG hyperphagia. Third, the changes in analgesic and hypothermic stress responses were not due to baseline shifts in jump thresholds or body temperatures. However the dose-dependent reductions by scopolamine and methylscopolamine in baseline food intake and 2DG hyperphagia were significantly correlated. Fourth, the dose-dependent reduction by scopolamine and methylscopolamine of pilocarpine analgesia differed in pattern from the other analgesic effects, suggesting heterogeneity in muscarinic receptor modulation of different analgesic responses.     

Chronic morphine selectively impairs cued fear extinction in rats: implications for anxiety disorders associated with opiate use.

Previous studies have shown that opioid transmission plays an important role in learning and memory. However, little is known about the course of opiate-associated learning and memory deficits after cessation of chronic opiate use in a behavioral animal model. In the present study, we examined the effects of chronic morphine on fear extinction, an important preclinical model for behavior therapy of human anxiety disorders. Rats were administrated subcutaneously morphine hydrochloride or saline twice per day for continuous 10 days. Rats received a cued or contextual fear conditioning session 7 days after the last morphine injection. During subsequent days, rats received four cued or contextual extinction sessions (one session per day). Percent freezing was assessed during all phases of training. Chronic morphine did not affect the acquisition of cued fear response or the initial encoding of extinction memory within each session, but produced an impairment in the between-session extinction. However, the same morphine treatment schedule did not affect the acquisition or extinction of contextual fear response. These results suggest that the effects of chronic morphine on memory for fear extinction are complex. Chronic morphine selectively impairs extinction of cued fear response. This deficit in fear extinction may be one of those critical components that contribute to the high prevalence of anxiety disorders in opiate addicts.     

Low dose quetiapine reverses deficits in contextual and cued fear conditioning in rats with excitotoxin-induced hippocampal neuropathy

Previous studies have demonstrated that adult rats with excitotoxic lesions of the hippocampus display deficits in memory-related behaviors similar to the memory deficits associated with schizophrenia. In this study, we assessed the sub-chronic effects of quetiapine, risperidone and haloperidol on performance deficits after intracerebroventricular administration of the excitotoxin, kainic acid, using paradigms for contextual and cued fear conditioning and spatial reversal learning in rats. The effects of three doses of quetiapine (5, 10 and 20 mg/kg) and single doses of risperidone (0.5 mg/kg) and haloperidol (0.15 mg/kg) were compared. Quetiapine administration at the lowest dose (5 mg/kg) reversed deficits in contextual and cued fear conditioning, but not deficits in spatial reversal learning, in kainic acid-treated animals. However, the two higher doses of quetiapine, and the single doses of risperidone and haloperidol, did not reverse any of the kainic acid-induced behavioral deficits. These results may be relevant to the effects of quetiapine and other antipsychotic drugs on memory deficits in patients with schizophrenia.     

Effects of acute and subchronic treatments with fluoxetine and desipramine on the memory of fear in moderate and high-intensity contextual conditioning

Selective serotonin and noradrenalin reuptake inhibitors such as fluoxetine and desipramine, respectively, are efficacious in the treatment of depression and chronic stress. Although they inhibit the reuptake of the biogenic monoamines soon after administration, therapeutic improvements occur only after 2 or 3 weeks. Freezing response and potentiated startle are common responses to moderate fear contextual conditioning. However, freezing but not startle is increased in rats that undergo intense fear conditioning. In this study, we evaluated the effects of acute and subchronic administration of fluoxetine and desipramine on these responses in testing sessions, as indices of fear in moderate and high fear conditioning. Fluoxetine did not show any significant effect on the moderate fear conditioning but reduced freezing and restored the startle response in rats under intense fear conditioning. In comparison, desipramine had no effect on the startle response when administered acutely or subchronically while freezing of the intense fear conditioning was reduced. Our findings indicate that intense contextual fear conditioning is sensitive to subchronic treatment with fluoxetine and resistant to desipramine. Fluoxetine appears to restore the serotoninergic function in brain areas recruited by intense contextual fear conditioning. These effects of fluoxetine may underlie its reported efficacy in the pharmacotherapy of panic disorders.     

5-HT6 receptor antagonist reversal of emotional learning and prepulse inhibition deficits induced by apomorphine or scopolamine

5-HT6 receptors have been implicated in consolidation of visuospatial and reward-based learning tasks. Since 5-HT6 receptors may be important in modulation of sensory gating which is often affected in schizophrenic patients, we tested whether Ro 4368554, a 5-HT6 selective antagonist at a dose of 10 mg/kg, could reverse the loss of prepulse inhibition from apomorphine or scopolamine. In addition, we also tested whether Ro 4368554 altered fear conditioning using fear potentiated startle, a model for emotional learning. Prepulse inhibition of startle was disrupted by apomorphine (0.5 mg/kg) when prepulse emissions were 5 dB above background but not above 15 dB, while scopolamine (0.5 mg/kg) caused disruption at both prepulse levels. Scopolamine-mediated disruption was not reversed by Ro 4368854 but apomorphine-mediated disruption was significantly ameliorated by 5-HT6 inhibition. For fear potentiated startle, scopolamine and/or Ro 4368554 were administered before two daily fear conditioning sessions; rats were tested on the following day. Rats that received scopolamine displayed no fear potentiated startle but Ro 4368554 reversed this scopolamine deficit. Additionally, we mapped Fos induction in rats treated with scopolamine and/or Ro 4368554; scopolamine increased Fos expression in the central nucleus of the amygdala and this was attenuated by Ro 4368554. In summary, we have demonstrated the efficacy of 5-HT6 antagonists in modulating sensory gating and fear conditioning, and thus may be of therapeutic use for schizophrenia-related disorders.     

Nicotine withdrawal disrupts new contextual learning

Interactions between nicotine and learning could contribute to nicotine addiction. Although previous research indicates that nicotine withdrawal disrupts contextual learning, the effects of nicotine withdrawal on contextual memories acquired before withdrawal are unknown. The present study investigated whether nicotine withdrawal disrupted recall of prior contextual memories by examining the effects of nicotine withdrawal on recall of nicotine conditioned place preference (CPP) and contextual fear conditioning. C57BL/6J mice trained in CPP exhibited a significant preference for an initially non-preferred chamber that was paired with 0.35 mg/kg nicotine. Following CPP, mice were implanted with mini-osmotic pumps containing 6.3 mg/kg/d nicotine or saline. Pumps were removed twelve days later and nicotine CPP was retested 24 h later. Mice withdrawn from chronic nicotine exhibited CPP, suggesting that older drug-context associations are not disrupted by nicotine withdrawal. One hour later, the same mice were trained in contextual and cued fear conditioning; nicotine withdrawal disrupted contextual but not cued fear conditioning. A subsequent experiment demonstrated that nicotine withdrawal did not disrupt recall of contextual or cued fear conditioning when acquisition occurred before nicotine withdrawal. These data suggest that nicotine withdrawal disrupts new contextual learning, but does not alter contextual learning that occurred before withdrawal.     

Blockade of cannabinoid CB1 receptors alters contextual learning and memory

The endocannabinoid system appears to have an important role in specific aspects of learning and memory, yet there has been no systematic study of the role of cannabinoid receptors in contextual fear conditioning. The present study examined the effects of cannabinoid CB(1) receptor blockade on the acquisition, consolidation, and expression of contextual fear using the selective cannabinoid CB(1) receptor antagonist AM251. AM251 produced a decrease in the expression of contextual fear when administered prior to training, testing, or both. This effect was observed when footshock was signaled by an auditory cue but not in an unsignaled shock version of the task. Moreover, blocking cannabinoid CB(1) receptors had no effect on consolidation of contextual memory regardless of the conditioning paradigm. These data indicate that inhibition of cannabinoid CB(1) receptors produces specific deficits in processing contextual information and that the effects of CB(1) antagonists on contextual learning may differ from effects on other types of learning.     

Antistereotype effects of ceruletide and some neuroleptics differentiated by interactions with clonazepam, muscimol, scopolamine and clonidine

Compulsive gnawing was produced in mice by administration of either methylphenidate or (after sensitizing pretreatment with the neuroleptic, tetflutixol) apomorphine. Drugs which antagonise stereotypy, such as ceruletide (CER, a sulphated decapeptide related to cholecystokinin octapeptide), haloperidol, zuclopenthixol and fluphenazine were applied in equipotent doses (reducing stereotypy by 80%). Clonazepam, muscimol, clonidine and scopolamine (but not methylscopolamine) antagonized to a different extent the antistereotype effect of ceruletide and the neuroleptics. The ED50s for clonazepam and other drugs, were determined; clonazepam had the greatest potency. Regarding the antagonism of the antistereotype effect, ceruletide was similar to but by no means congruent with haloperidol. The antagonism of the antistereotype effect was specific because other effects of ceruletide and cholecystokinin octapeptide (inhibition of exploratory rearing activity, ptosis, antinociception, hypothermia) were not antagonized by clonazepam and only weakly modified by scopolamine. Methylscopolamine was ineffective throughout, indicating a central site for the mechanism of the actions studied of scopolamine. In conclusion, the antistereotype effect of ceruletide is different from that of conventional neuroleptic drugs and functionally independent of other behavioural effects of the cholecystokinin-like peptides.     

Low Doses of 17��-Estradiol and 17��-Estradiol Facilitate, Whereas Higher Doses of Estrone and 17��- and 17��-Estradiol Impair, Contextual Fear Conditioning in Adult Female Rats

Estrogens are known to exert significant structural and functional effects in the hippocampus of adult rodents. In particular, 17β-estradiol can improve, impair, or have no effect on hippocampus-dependent learning and memory depending on dose and time of administration. The effects of other forms of estrogen, such as estrone and 17α-estradiol, on hippocampus-dependent learning have not been as thoroughly investigated. Therefore, the purpose of this study was to investigate the effects of 17β-estradiol, estrone, and 17α-estradiol at three different doses on two different tasks: hippocampus-dependent contextual fear conditioning and hippocampus-independent cued fear conditioning. Adult ovariectomized female rats were injected with one of the estrogens at one of the three doses 30 mins before conditioning to assess the rapid effects of these estrogens on acquisition. Twenty-four hours later memory for the context was examined and 1 h later memory for the cue (tone) was assessed. Levels of synaptophysin were examined in the dorsal hippocampus of rats to identify a potential synaptic correlate of hormonal effects on contextual fear conditioning. Low 17β-estradiol and 17α-estradiol enhanced, whereas high 17β-estradiol and 17α-estradiol impaired, contextual fear conditioning. Only the middle dose of estrone severely impaired contextual fear conditioning. Estrogens did not alter performance in the hippocampus-independent cued task. Synaptophysin expression was increased by estrone (at a middle and high dose) and 17β-estradiol (at a middle dose) in the CA3 region of the hippocampus and was not correlated with cognition. The results of this study indicate that estradiol can positively or negatively influence hippocampus-dependent learning and memory, whereas estrone impairs hippocampus-dependent learning and memory in a dose-dependent manner. These results have important therapeutic implications, as estrone, a main component of a widely used hormone replacement therapy, was shown to have either a negative effect or no effect on learning and memory. It may be possible to use 17α-estradiol and lower doses of estrogens as potential alternatives in hormone replacement therapies.     

Scopolamine impairs learning performance of rats in a 14-unit T-maze

To assess involvement of muscarinic cholinergic systems in performance of a shock-motivated 14-unit T-maze task, 3-month old Fischer-344 rats were given an IP injection of scopolamine (0.1, 0.3, 1.0 or 3.0 mg/kg), methylscopolamine (1.0 mg/kg), or saline 30 min prior to maze training on 2 consecutive days. Scopolamine, but not methylscopolamine, impaired all components of acquisition performance. Measures of error performance, run time, shock duration, and number of shocks received were significantly increased but only at the 1.0 and 3.0 mg/kg scopolamine doses. The cognitive component of the task, measured by error performance, appeared most affected. Cognitive performance deficits observed following scopolamine administration in the present study resembled age-related impairments in rats and mice previously observed in this task. The cholinergic hypothesis of geriatric memory dysfunction appears to be implicated by these findings; however, the degree to which memory systems are involved remains unclear. Other performance variables such as discriminative control of stimuli or mechanisms of attention are implicated and discussed.     

Nicotine ameliorates NMDA receptor antagonist-induced deficits in contextual fear conditioning through high-affinity nicotinic acetylcholine receptors in the hippocampus

NMDA glutamate receptors (NMDARs) and nicotinic acetylcholine receptors (nAChRs) are both involved in learning and synaptic plasticity. Increasing evidence suggests processes mediated by these receptors may interact to modulate learning; however, little is known about the neural substrates involved in these interactive processes. The present studies investigated the effects of nicotine on MK-801 hydrogen maleate (MK-801) and DL-2-Amino-5-phosphonovaleric acid (APV)-induced disruption of contextual fear conditioning in male C57BL/6J mice, using direct drug infusion and selective nAChR antagonists to define the brain regions and the nAChR subtypes involved. Mice treated with MK-801 showed a deficit in contextual fear conditioning that was ameliorated by nicotine. Direct drug infusion demonstrated that the NMDAR antagonists disrupted hippocampal function and that nicotine acted in the dorsal hippocampus to ameliorate the deficit in learning. The high-affinity nAChR antagonist Dihydro-β-erythroidine hydrobromide (DhβE) blocked the effects of nicotine on MK-801-induced deficits while the α7 nAChR antagonist methyllycaconitine citrate salt hydrate (MLA) did not. These results suggest that NMDARs and nAChRs may mediate similar hippocampal processes involved in contextual fear conditioning. Furthermore, these results may have implications for developing effective therapeutics for the cognitive deficits associated with schizophrenia because a large subset of patients with schizophrenia exhibit cognitive deficits that may be related to NMDAR dysfunction and smoke at much higher rates than the healthy population, which may be an attempt to ameliorate cognitive deficits.     

Pharmacological dissociation of moderate and high contextual fear as assessed by freezing behavior and fear-potentiated startle.

The amplitude of the whole-body acoustic startle response is reliably enhanced when elicited in the presence of foreground signals, such as light, previously paired with footshocks. It has been shown that this enhancement is evident by moderate fear levels, but is less affected by high fear levels. Potentiation of the acoustic startle reflex has also been reported in the presence of background cues previously associated with footshocks. However, the effects of anxiolytic drugs on different levels of fear elicited by moderate and intense contextual fear conditioning associated with startle reflex have not been examined yet. To approach this issue, we examined the effects of the anxiolytic, midazolam, on two intensities of contextual fear; freezing behavior and the startle response to loud noise. First, we compared the magnitude of the freezing behavior and the startle amplitude during the testing sessions in groups of rats submitted to fear conditioning using 0.3 and 0.6 mA as unconditioned stimuli (10 stimuli of 1 s each, intertrial interval from 60 to 180 s). Afterwards, the effects of midazolam (0.5 and 1.0 mg/kg) were assessed in these two conditions. Rats showed a potentiated startle reflex and a significant freezing behavior to moderate fear conditioning, which were both attenuated by midazolam. Higher levels of fear conditioning caused more intense freezing behavior without enhancing the startle reflex. Whereas midazolam reduced this freezing response, the startle response was unaffected. These results are indicative that anxiolytic-sensitive freezing and fear-potentiated startle are triggered by moderate contextual fear conditioning, while contextual conditioning with the use of high footshocks causes a distinct pattern of behavioral responses, which is only partially affected by midazolam. Due to the differential sensitivity to midazolam of these two patterns of startle responses generated as a function of the intensity of contextual fear conditioning, it is proposed that they represent moderate and intense aversive states that may be related to anxiety or panic/phobic conditions, respectively.