Nicotine enhances contextual fear conditioning and ameliorates ethanol-induced deficits in contextual fear conditioning

Nicotine and ethanol are 2 commonly used and abused drugs that have divergent effects on learning. The present study examined the effects of acute nicotine (0.25 mg/kg), ethanol (1.0 g/kg), and ethanol-nicotine coadministration on fear conditioning in C57BL/6 mice. Mice were assessed for contextual and cued fear conditioning at 1 day and 1 week posttraining. Ethanol disrupted acquisition but not consolidation of contextual fear conditioning; nicotine enhanced contextual fear conditioning and ameliorated ethanol-associated deficits in contextual fear conditioning. Mecamylamine antagonized this effect. Fear conditioning was reassessed 1 week after initial testing with no drug administered. At the 1-week retest, mice previously treated with nicotine continued to show enhanced contextual fear, and mice previously treated with ethanol continued to show contextual fear deficits. Thus, nicotine both produces a long-lasting enhancement of contextual fear conditioning and protects against ethanol-associated deficits.     

Amygdala-dependent and amygdala-independent pathways for contextual fear conditioning

The basolateral amygdala (BLA), consisting of the lateral and basal nuclei, is considered to be essential for fear learning. Using a temporary inactivation technique, we found that rats could acquire a context-specific long-term fear memory without the BLA but only if intensive overtraining was used. BLA-inactivated rats' learning curves were characterized by slow learning that eventually achieved the same asymptotic performance as rats with the BLA functional. BLA inactivation abolished expression of overtrained fear when rats were overtrained with a functional BLA. However, BLA-inactivation had no effect on the expression of fear in rats that learned while the BLA was inactivated. These data suggest that there are primary and alternate pathways capable of mediating fear. Normally, learning is dominated by the more efficient primary pathway, which prevents learning in the alternate pathway. However, alternate pathways compensate when the dominant pathway is compromised.     

Ginseng enhances contextual fear conditioning and neurogenesis in rats

Panax Ginseng is a commonly used galenical known to have an enhancing effect on learning. Neurogenesis in the hippocampus has been shown to be necessary for hippocampus/amygdala-dependent learning tasks. To investigate the role of Ginseng in neurogenesis and learning of rats, we administered both Ginseng and BrdU for five consecutive days. As a result, Ginseng increased the number of BrdU-positive cells in the dentate gyrus in a dose-dependent manner. Further, we administered one dose of BrdU after Ginseng treatment for five consecutive days, and the number of BrdU-positive cells did not increase significantly. However, when one dose of BrdU was given 1 day before the following five consecutive days of Ginseng treatment, the number of BrdU-positive cells markedly increased in the hippocampus. Therefore, it is likely that Ginseng enhances not proliferation but survival of newly generated neurons in the hippocampus. Second, we administered both Ginseng and BrdU to rats for five consecutive days. One day after the last Ginseng and BrdU co-administration, contextual fear conditioning (CFC) was conducted. Ginseng in a dose-dependent manner increased the % freezing time and the number of BrdU-positive cells in the dentate gyrus of rats that received CFC. Thus, an increase in CFC-related neurogenesis may be one mechanism of Ginseng's properties to enhance learning ability.     

Nicotine enhances both foreground and background contextual fear conditioning

The present study examined if nicotine enhances contextual fear conditioning when the training context is either a background stimulus or a foreground stimulus. In the background conditioning experiment, mice were trained using two auditory conditioned stimulus (CS; 30 s, 85 dB white noise)-footshock unconditioned stimulus (US; 2 s, 0.57 mA) pairings and tested 24 h later. In the foreground conditioning experiment, mice were trained with two presentations of a footshock US (2 s, 0.57 mA) and tested 24 h later. Mice received 0.09 mg/kg nicotine before training and testing. For both the foreground and background conditioning experiments, nicotine enhanced contextual conditioning. No enhancement of the auditory CS-US association was seen. These results demonstrate that nicotine enhances contextual fear conditioning regardless of whether the context is a background stimulus or a foreground stimulus during conditioning.     

Impaired trace and contextual fear conditioning in aged rats

Trace and contextual fear conditioning were evaluated in adult (3-6 months), early middle-aged (8-12 months), late middle-aged (16-20 months), and aged (24-33 months) Sprague-Dawley rats. After trace conditioning, aged animals exhibited significantly less freezing to the tone conditioned stimulus and training context. Levels of trace-cue and context conditioning were negatively correlated with age (r = -0.56 and -0.59, respectively) and positively correlated with each other (r = +0.52). Aged rats showed robust conditioning in short- and long-delay fear paradigms, suggesting that the trace interval, rather than the use of a long interstimulus interval, is responsible for the aging-related deficits in trace fear conditioning. The authors suggest that these aging-related conditioning deficits furnish useful indices of functional changes within hippocampus or perirhinal cortex. ((c) 2006 APA, all rights reserved).     

Prior chronic nicotine impairs cued fear extinction but enhances contextual fear conditioning in rats

Clinical observations have shown a link for the high comorbid rate between smoking and psychiatric disorders, including anxiety disorders. However, little is known about the neural mechanism underlying the progression from nicotine dependence to an anxiety disorder. A deficit in fear extinction in general is considered to contribute to anxiety disorders. The aim of the present study is to investigate the effects of chronic nicotine on fear extinction in rats. Rats were administrated s.c. nicotine twice per day for 14 days. Two weeks after the last injection rats received a cued or contextual fear conditioning session. Twenty-four hours and 48 h after conditioning, rats received an extinction training session and an extinction test session, respectively. Percent freezing was assessed during all phases of training. In the cued task, prior chronic nicotine did not affect the acquisition of fear response or the within-session fear extinction, but impaired the between-session fear extinction. In the contextual task, the same nicotine treatment schedule did not affect the acquisition of fear response or the within- and between-session fear extinction, but enhanced the retention of fear conditioning. This prior chronic nicotine-induced deficit in cued fear extinction and/or enhanced fear to context may be one of the critical components that contribute to the progression from nicotine dependence to an anxiety disorder.     

Males show stronger contextual fear conditioning than females after context pre-exposure

Estradiol affects the structure and function of the hippocampus. We have found that repeated estradiol affects neurogenesis and cell death in the hippocampus of adult female, but not male rats. In the present study we sought to determine whether using the same regimen of estradiol would influence hippocampus-dependent behaviour. Adult male and female rats were given estradiol or sesame oil for 15 days, and then tested using a contextual pre-exposure paradigm in which performance depends on the hippocampus. The time spent freezing displayed by rats was scored on subsequent days in (1) the training context, (2) a novel context in which rats had never been shocked, and (3) the training context a second time. Irrespective of treatment, males showed stronger memory for the context by exhibiting greater freezing in both the training context exposures and the novel context. Previous estradiol treatment, in either sex, did not affect the ability to learn and retain information about the training context. However, female rats treated with estradiol and exposed to a novel context after fear conditioning exhibited less freezing behaviour than controls. Taken together, our results demonstrate that gonadectomized male rats outperform females, regardless of previous treatment with estradiol, on a hippocampus-contextual fear conditioning test, and that previous estradiol treatment has a subtle effect on performance in female but not male rats.     

Long-term potentiation and contextual fear conditioning increase neuronal glutamate uptake.

Induction and expression of long-term potentiation (LTP) in area CA1 of the hippocampus require the coordinated regulation of several cellular processes. We found that LTP in area CA1 was associated with an N-methyl-D-aspartate (NMDA) receptor-dependent increase in glutamate uptake. The increase in glutamate uptake was inhibited by either removal of Na+ or addition of D,L-threo-beta-hydroxyaspartate. Dihydrokainate (DHK), a specific inhibitor of the glial glutamate transporter GLT-1, did not block the increase in glutamate uptake. LTP was also associated with a translocation of the EAAC1 glutamate transporter from the cytosol to the plasma membrane. Contextual fear conditioning increased the maximum rate (Vmax) of glutamate uptake and membrane expression of EAAC1 in area CA1. These results indicate that regulation of glutamate uptake may be important for maintaining the level of synaptic strength during long-term changes in synaptic efficacy.     

REM sleep deprivation affects extinction of cued but not contextual fear conditioning

Previous research has demonstrated that rapid eye movement (REM), or paradoxical, sleep deprivation can interfere with the retention of certain types of learning tasks, particularly spatial learning. The present study investigated the effects of 6 h of REM sleep deprivation on the retention and extinction of both cued and contextual conditioning tasks in rats. Sleep-deprived animals showed normal retention of both types of conditioning tasks but retarded extinction of the cued task and a trend toward attenuated spontaneous recovery of the contextual task. The results provide further evidence for the involvement of REM sleep in learning and memory processes.     

Neurotoxic lesions of retrosplenial cortex disrupt signaled and unsignaled contextual fear conditioning

The majority of research regarding contextual learning and memory has focused on the contributions of the hippocampus and related medial temporal lobe structures. However, little is known about other possible cortical contributions to these processes. Our laboratory recently demonstrated that electrolytic lesions of the retrosplenial cortex (RSP), a posterior region of cingulate cortex, impaired contextual but not cue-specific fear conditioning. The present experiments further examined the role of RSP in contextual fear memory using fiber-sparing neurotoxic lesions and both signaled and unsignaled fear conditioning paradigms. Despite comparable acquisition of the conditioned fear response, rats with neurotoxic lesions of RSP exhibited impaired contextual memory relative to control animals in both the signaled and unsignaled paradigms. These results further suggest an important role for RSP in contextual learning and memory.     

Adenosine A1 receptor activation selectively impairs the acquisition of contextual fear conditioning in rats

Three experiments were conducted to examine the importance of adenosine A1 receptors for the acquisition and expression of hippocampal-dependent and hippocampal-independent forms of conditioned fear. In Experiment 1, the selective adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA), or saline was administered intraperitoneally to male rats 30 min prior to Pavlovian fear conditioning, which consisted of 7 tone-shock pairings. Adenosine A1 receptor activation dose-dependently and selectively disrupted the acquisition of contextual fear conditioning while sparing tone-shock associations. Experiments 2 and 3 demonstrated that CPA's selective disruption of contextual learning could not be attributed to context being weaker than tone conditioning or to state-dependent learning. Adenosine A1 receptor activation also impaired the expression of both context- and tone-elicited fear. These results suggest that endogenous adenosine modulates the acquisition and expression of emotional (fear) memories by acting on A1 receptors in brain regions underlying fear conditioning.     

Infant rats can acquire,but not retain contextual associations in object-in-context and contextual fear conditioning paradigms

Context learning in postnatal day (PD) 16–18 rats has been taken by Revillo, Cotella, Paglini, and Arias (2015, Physiology & Behavior, 148 , 6–21) to challenge the view that the ontogeny of contextual learning is related to the development of the hippocampal system (Rudy, 1993, Behavioral Neuroscience, 107 (5), 887–891; Schiffino, Murawski, Rosen, & Stanton, 2011 Neurobiology of Learning and Memory, 95 (2), 190–198). Whether context learning is “incidental” or “reinforcement-driven” may determine the ontogeny and neural systems involved (Rudy, 2009, Learning & Memory (Cold Spring Harbor, N.Y.), 16 , 573–585). However, we have shown differential ontogeny of two different forms of incidental context learning, the context pre-exposure facilitation effect (CPFE; Jablonski, Schiffino, & Stanton, 2012, Developmental Psychobiology, 54 (7), 714–722), which emerges between PD 17 and 21; and object-in-context recognition (OiC, Ramsaran, Westbrook, & Stanton, 2016, Developmental Psychobiology, 58 (7), 883–895; Ramsaran, Sanders, & Stanton, 2016, Behavioural Brain Research, 298 , 37–47), which is present on PD17. We investigated whether this task-dissociation reflects an encoding or a retention deficit, by varying the sample-to-testing intervals for both tasks. Experiment 1A found that PD17 rats were able to perform the OiC task after short (5 min) but not long (24 hr) sample-to-test intervals. Experiments 1B and 1C found that PD17 rats trained on the CPFE are able to acquire and express context-shock associations after short but not long retention intervals. These findings suggest that pre-weanling rats encode contexts but show poor consolidation or retrieval after longer retention intervals.     

Differential contribution of amygdala and hippocampus to cued and contextual fear conditioning.

The contribution of the amygdala and hippocampus to the acquisition of conditioned fear responses to a cue (a tone paired with footshock) and to context (background stimuli continuously present in the apparatus in which tone-shock pairings occurred) was examined in rats. In unoperated controls, responses to the cue conditioned faster and were more resistant to extinction than were responses to contextual stimuli. Lesions of the amygdala interfered with the conditioning of fear responses to both the cue and the context, whereas lesions of the hippocampus interfered with conditioning to the context but not to the cue. The amygdala is thus involved in the conditioning of fear responses to simple, modality-specific conditioned stimuli as well as to complex, polymodal stimuli, whereas the hippocampus is only involved in fear conditioning situations involving complex, polymodal events. These findings suggest an associative role for the amygdala and a sensory relay role for the hippocampus in fear conditioning.     

NMDA receptors and the ontogeny of post-shock and retention freezing during contextual fear conditioning

The ontogeny and NMDA-receptor (NMDAR) mechanisms of context conditioning were examined during standard contextual fear conditioning (sCFC) – involving context and context-shock learning in the same trial – as a comparison with our previous reports on the Context Preexposure Facilitation Effect (CPFE), which separates these two types of learning by 24 hr. In Experiment 1, systemic administration of the NMDAR antagonist, MK-801, prior to conditioning disrupted retention but not post-shock freezing during sCFC in PD31 rats. Experiment 2 replicated and extended this effect to PD17 versus PD31 rats. Consistent with Experiment 1, pre-training MK-801 spared post-shock freezing but impaired retention freezing in PD31 rats. In contrast, pre-training MK-801 disrupted post-shock freezing in PD17 rats, which showed no retention freezing regardless of drug. These results reveal developmental differences in the role of NMDAR activity in the acquisition versus retention of a context-shock association during sCFC in pre-weanling and adolescent rats.     

Contributions of the retrosplenial and posterior parietal cortices to cue-specific and contextual fear conditioning

The retrosplenial cortex (RSP) and the posterior parietal cortex (PPC) are the primary sources of cortical sensory input to the postrhinal cortex (POR) in rodents. Together, these areas compose a major corticohippocampal circuit that is involved in processing visuospatial information. The POR has been implicated in contextual learning and memory, consistent with the type of information presumably being processed by this region. By comparison, little is known about the role of the RSP or the PPC in contextual learning. In the present study, rats were trained either before or after surgery in a standard signaled fear conditioning task in which an auditory cue was paired with foot shock. Contextual fear and tone-specific fear were assessed in subsequent test sessions. In Experiment 1, electrolytic damage to the RSP either before or immediately after training impaired the expression of contextual fear but not tone-specific fear. In contrast, electrolytic damage to the PPC had no effect on conditional fear to the context or the tone in Experiment 2. These findings indicate that the RSP, but not the PPC, contributes to the processing of contextual information by the POR corticohippocampal processing stream.     

Involvement of medial prefrontal cortex neurons in behavioral and cardiovascular responses to contextual fear conditioning

To explore the ventral medial prefrontal cortex (vMPFC) involvement in behavioral and autonomic fear-conditioned responses to context, vMPFC synaptic transmission was temporarily inhibited by bilateral microinjections of 200 nL of the nonselective synapse blocker CoCl(2) (1 mM). Behavioral activity (freezing, motor activity and rearing) as well as evoked cardiovascular responses (arterial pressure and heart rate) was analyzed. Rats were pre-exposed to the footshock chamber (context) and shock stimulus was used unconditioned stimulus. During re-exposure to context, conditioned rats spent 80% of the session in freezing while non-conditioned rats (no shock group) spent less than 15% of the session time in freezing. Conditioned rats had significantly lower activity scores than non-conditioned animals. Exposure to context increased mean arterial pressure (MAP) and heart rate (HR) of both groups. MAP and HR of the conditioned animals were markedly increased and remained at a high and stable level, whereas MAP and HR increases in non-conditioned animals were less pronounced and declined during the session. CoCl(2) microinjected in the vMPFC significantly reduced freezing and attenuated MAP and HR increase of the conditioned group. Cobalt-induced vMPFC inhibition also significantly reduced MAP and HR increase observed in non-conditioned animals, without any behavioral changes. The effect of vMPFC acute ablation on MAP and HR did not seem to be specific to the fear response because they were also evident in non-conditioned animals. The results indicate that vMPFC integrity is crucial for expression of fear-conditioned responses to context, such as freezing and cardiovascular changes, suggesting that fear-conditioned responses to context involve cortical processing prior to amygdalar output. They also indicate a cardiovascular response observed during re-exposure of non-conditioned rats to the context is completely dependent on vMPFC integrity.     

Isolation reduces contextual but not auditory-cue fear conditioning: a role for endogenous opioids.

Isolation for several hours after fear conditioning reduces contextual but not auditory-cue fear conditioning (J. W. Rudy, 1996). This isolation effect is reversed by both, centrally and peripherally acting opioid receptor antagonists. As in isolation, systemically administered morphine given immediately after conditioning also reduces contextual fear conditioning. Morphine's effect is also reversed by both centrally and peripherally acting opioid receptor antagonists. Exposure to the conditioning context has been shown to eliminate the effect of isolation on contextual fear conditioning (J. W. Rudy, 1996). Context preexposure also eliminated the effect of morphine on contextual fear conditioning. These results imply that opioids released in the periphery play an important role in producing the isolation effect and that they do so by disrupting the postconditioning memory consolidation processes.     

Role of neuronal nicotinic receptors in the effects of nicotine and ethanol on contextual fear conditioning

Nicotine can enhance contextual learning while ethanol impairs some forms of learning. Nicotine can overcome some of the impairing effects of ethanol when the two drugs are co-administered. The specific brain nicotinic acetylcholine receptors (nAChRs) that mediate nicotine's effects on contextual learning and whether any of ethanol's actions are mediated by nAChRs are unknown. The potential roles of nAChRs in contextual and cued fear conditioning as well as the effects of nicotine, ethanol, or co-administration of nicotine and ethanol were examined in wild type and homozygous null mutant mice from alpha7, beta2, beta3, and beta4 mouse lines at 24 h after training. Nicotine was given prior to training and testing, whereas ethanol was given only before training. Nicotine enhanced contextual learning in both alpha7 wild types and mutants when mice were trained at 0.17 mA, but not 0.35 mA. Mutants lacking the alpha7 subunit were less sensitive to the memory impairing effects of ethanol trained at 0.35 mA. beta2 Null mutants receiving saline showed a small, but significant, impairment in contextual learning compared with wild type littermates when the shock stimulus was 0.35 mA. Beta2 Null mutant mice also did not respond to the cognitive enhancing effects of nicotine alone, or after ethanol administration. beta3 and beta4 null mutants did not differ from wild types either after saline or any of drug treatments. These results show that beta2-containing nAChRs, but not beta3- or beta4-containing receptors, mediate the enhancing effects of nicotine on contextual learning and confirm previous studies implicating beta2 in other forms of learning. A new role for alpha7 nAChRs in regulating sensitivity to the cognitive disrupting effects of ethanol is proposed.