Facilitated Acquisition of the Classically Conditioned Eyeblink Response in Male Rats After Systemic IL-1?

Exposure to inescapable stressors enhances cue-dependent learning in male rats; enhanced learning is apparent as facilitated acquisition of the classically conditioned eyeblink respouse (CCER). The proinflammatory cytokines, in particular interleukin (IL)-1beta, are presumed to orchestrate a number of acute-phase stress responses in rats, most notably fever, reduced feeding, and inactivity. Little is known of the impact proinflammatory cytokines have on learning and memory processes. Here, we address the effects of IL-1beta treatment on acquisition of the classically conditioned eyeblink response 2 hours [?] after injection in male rats. Training was accomplished with a delay-type paradigm (500-ms conditional stimulus coterminating with a 10-ms periorbital unconditional stimulation). Facilitated acquisition was clearly apparent in rats treated with IL-1beta (3.0 microg/kg). In a second experiment, we compared rats treated with 3.0 microg/kg to those treated with 1.0 microg/kg. Facilitated acquisition was reproduced, but the lower dose did not appreciably affect acquisition. These data further support contentions that IL-1beta has anxiogenic properties, affecting basic new motor learning in a manner similar to that observed after exposure to stress.     

Facilitated acquisition of the classically conditioned eyeblink response in male rats after systemic IL-1β

Exposure to inescapable stressors enhances cue—dependent learning in male rats; enhanced learning is apparent as facilitated acquisition of the classically conditioned eyeblink respouse (CCER). The proinflammatory cytokines, in particular interleukin (IL)-1β, are presumed to orchestrate a number of acute-phase stress responses in rats, most notably fever, reduced feeding, and inactivity. Little is known of the impact proinflammatory cytokines have on learning and memory processes. Here, we address the effects of IL-1β treatment on acquisition of the classically conditioned eyeblink response 2 hours [?] after injection in male rats. Training was accomplished with a delay-type paradigm (500-ms conditional stimulus coterminating with a 10-ms periorbital unconditional stimulation). Facilitated acquisition was clearly apparent in rats treated with IL-1β (3.0 μg/kg). In a second experiment, we compared rats treated with 3.0 μg/kg to those treated with 1.0 μg/kg. Facilitated acquisition was reproduced, but the lower dose did not appreciably affect acquisition. These data further support contentions that IL-1β has anxiogenic properties, affecting basic new motor learning in a manner similar to that observed after exposure to stress.     

Memory as a state dependent phenomenon: role of ACTH and epinephrine

Rats received an ip injection of saline, epinephrine HCl (5.0 micrograms/kg), or ACTH1--24(0.2 microgram/kg) immediately after training in a step-down inhibitory avoidance task, and an ip injection of saline, or epinephrine (0.5, 1.0, 2.0, 3.0, 5.0, or 10.0 mu/kg), or ACTH (0.02, 0.04 0.08, 0.12, 0.2, or 0.4 microgram/kg) 6 min prior to a test session of the same task 24 hr after training. Retention was excellent in the groups treated with saline after training, and poor in those treated with ACTH or epinephrine after training and tested under saline. The amnestic effect of the drugs was counteracted by their administration prior to testing, and there was a dose-response curve for this effect: partial recovery from amnesia was obtained with 20 or 40% of the amnestic dose and full recovery was obtained with 60 to 100% of that dose when the same drug was given after training and prior to testing; if the drugs given after training and prior to testing were not the same, full recovery was obtained only with twice the amnestic dose of the latter. These findings are consistent with the hypothesis that learning depends on the relation between the endogenous levels of these two hormones in the post-training period and during testing. The animals seem capable of discriminating between the two drugs only partially.     

Brain interleukin-1 is involved in spatial memory and passive avoidance conditioning

Within the brain, the inflammatory cytokine interleukin-1 (IL-1) mediates illness-associated neural, neuroendocrine, and behavioral responses; however, its role in normal neurobehavioral processes is not clear. To examine the role of IL-1 signaling in memory, we infused Long-Evans rats intracerebroventricularly with IL-1beta (10 ng/rat), IL-1 receptor antagonist (IL-1ra, 100 microg/rat), or saline immediately following a learning task and tested memory functioning 1-8 days later. In the Morris water maze (MWM), IL-1ra caused memory impairment in the hippocampus-dependent, spatial version, whereas IL-1beta had no effect. Neither IL-1beta nor IL-1ra influenced the hippocampus-independent, nonspatial version of the MWM. In the passive avoidance response, which also depends on hippocampal functioning, IL-1ra caused memory impairment, and IL-1beta caused memory improvement. These results suggest that IL-1 signaling within the hippocampus plays a critical role in learning and memory processes.     

Response suppression by visual stimuli paired with postsession d-amphetamine injections in the pigeon

Responding of pigeons, maintained under a fixed-interval 3-minute schedule of food presentation, was decreased on days that the color of the lights illuminating the food magazine was changed and d-amphetamine (1.0 mg/kg, i.m.) was injected after the session. Responding was not decreased by keylight color changes paired with postsession d-amphetamine or by postsession injections of saline. Administration of pentobarbital (3.0 to 5.6 mg/kg), but not d-amphetamine (.3 to 3.0 mg/kg), before the session increased rates of responding suppressed by drug-paired magazine lights. Responding maintained under a fixed-ratio 30-response schedule was not decreased when differently colored magazine lights were paired with a low (.3 mg/kg) postsession dose of d-amphetamine; with high (3.0 mg/kg) postsession doses, however, responding was completely suppressed after two pairings. The effects of pairing magazine stimuli with an intermediate (1.0 mg/kg) postsession dose of d-amphetamine depended upon the magnitude of prior postsession doses. After being paired with a low dose, stimuli paired with 1.0 mg/kg did not suppress responding. After being paired with a high dose, stimuli paired with 1.0 mg/kg completely suppressed responding. The suppression of food-maintained responding by stimuli paired with postsession drug administration depends upon both behavioral and pharmacological variables.     

Pharmacological evidence of a central effect of naltrexone, morphine, and beta-endorphin and a peripheral effect of met- and leu-enkephalin on retention of an inhibitory response in mice

Immediate post-training administration of the central acting opioid receptor antagonist naltrexone (0.01-1.00 mg/kg) facilitated 48-h retention of a one-trial inhibitory avoidance task. An inverted-U dose-response curve was obtained. In this dose range naltrexone did not significantly affect response latencies of mice not given a footshock during the training. However, higher doses of naltrexone (3.0 and 10.0 mg/kg) increased latencies of both shocked and unshocked mice. The peripheral-acting opioid receptor blocker, naltrexone methyl bromide (MR 2263) (0.01-10.00 mg/kg), did not significantly influence retention latencies of either shocked or unshocked mice. Further, MR 2263 (0.1, 1.0, or 10.0 mg/kg) did not block the retention impairment produced by concurrently administered morphine (3.0 mg/kg) or beta-endorphin (0.1 microgram/kg). These findings indicate that the effect of these agonists on memory are not due to a peripheral influence. However, MR 2263 does prevent the memory-impairing effect of both metenkephalin (1.0 microgram/kg) and leu-enkephalin (0.3 microgram/kg) on retention. Those results suggest that enkephalins affect retention through influences initiated peripherally. Thus, different sites and mechanisms of action for beta-endorphin and the enkephalins are proposed.     

Effects of parafascicular excitotoxic lesions on two-way active avoidance and odor-discrimination

To investigate whether the parafascicular (PF) nucleus of the thalamus is involved in different learning and memory tasks, two experiments were carried out in adult male Wistar rats that were submitted to pre-training bilateral N-methyl-d-aspartate PF infusions (0.15M, pH 7.4; 1.2 microl/side, 0.2 microl/min). In Experiment 1, we evaluated the effects of PF lesions in two identical 30-trial training sessions, separated by a 24-h interval, of a two-way active avoidance conditioning. PF-lesioned rats exhibited impaired performance in both sessions, measured by number of avoidance responses. In Experiment 2, the effects of PF lesions were assessed in a training session (5 trials) and a 24-h retention test (2 retention trials and 2 relearning trials) of an odor-discrimination task. PF lesions did not significantly disrupt the acquisition or the first retention trial, which was not rewarded. However, lesioned animals' performance was clearly affected in subsequent trials, following the introduction of the single non-rewarded trial. Current data are discussed considering evidence that lesions of the PF nucleus affect learning and memory functions mediated by anatomically related areas of the frontal cortex and striatum.     

Choice for signalled over unsignalled shock as a function of shock intensity

Choice between a signalled shock schedule and an unsignalled one was examined at various shock intensities. Three rats were given the opportunity to change from the unsignalled schedule to the signalled one at intensity values between 0.15 mA and 1.0 mA. Steps were usually 0.15 mA and both ascending and descending series were given. For two other rats, shock intensity increased from 0.20 mA to 1.0 mA in 0.20-mA increments; for two additional rats, shock intensity was first 3.0 mA and was then reduced to 1.0 mA. Subjects tended to remain in the unsignalled schedule at the lower shock intensities, but spent most of each session under the signalled schedule at the higher intensities (1.0 mA and 3.0 mA). In addition, the time spent in the signalled schedule tended to vary systematically with shock intensity over at least part of the range of intensity values. It was concluded that the relationship between shock intensity and choice behavior is similar to the relationship between intensity and behavior in procedures involving avoidance, escape, and punishment.     

Navigation in the Morris swim task as a baseline for drug discrimination: a demonstration with morphine

A morphine versus saline discrimination was demonstrated using the Morris swim task as the behavioral baseline. The apparatus was a large circular pool filled with water made opaque by floating polypropylene pellets. Rats were placed in the tank in randomly selected locations (12 trials per session) and could escape by swimming to a platform submerged 2 cm below the surface. Morphine (5.6 mg/kg) or saline was injected prior to training sessions. The position of the platform in a given session depended on the drug condition, thus forming the basis for discriminative responding. Three of the 4 rats acquired the discrimination, as evidenced by direct swims to the condition-appropriate platform. Generalization probe sessions were conducted following acquisition. Probe sessions were preceded by injections of morphine (0, 1.0, 3.0, 5.6, or 10.0 mg/kg) and involved placing the rat in the pool for 1 min without a platform. Swim patterns revealed a gradient, with probe swimming more concentrated in the area of the morphine platform position after higher morphine doses. In addition, dose-dependent increases in the likelihood of swimming first to the morphine-associated platform location were obtained. These results illustrate the generality of drug discrimination across different behavioral procedures, and of particular interest with respect to spatial learning, demonstrate interoceptive stimulus control of navigation.     

Diazepam impairs place learning in the Morris water maze

The effect of diazepam (0.3, 1.0, and 3.0 mg/kg) on the acquisition and retention of place learning was evaluated. The analysis of escape latencies indicates that 1.0 and 3.0 mg/kg diazepam significantly impaired the retention of spatial information. When a free swim trial was carried out only control animals showed spatial bias to the target quadrant. The absence of spatial bias in the group that received 0.3 mg/kg suggests that the amnesic effect of diazepam can be seen at doses similar to or even lower than the anxiolytic ones, and that the GABA/benzodiazepine receptor complex is highly sensitive to the cognitive impairment induced by diazepam in spatial tasks.     

Active avoidance conditioning in zebrafish (Danio rerio)

The zebrafish represents a potentially useful organism for studying genes involved in learning and memory function in vertebrates, because a number of genetic techniques in zebrafish have been developed to produce a wide variety of genetic mutants. While zebrafish mutants are being developed, behavioral studies on learning and memory function in zebrafish are in urgent need. The present study investigated active avoidance conditioning in normal zebrafish. Zebrafish were trained to swim from a lighted (CS) compartment to a dark compartment to avoid an electrical body shock (US) in a shuttle-box that consisted of a water-filled tank separated by an opaque barrier into two equal compartments. By varying the number of trials per training session and the duration of the intertrial interval, Experiments 1 and 2 showed that, with the CS, US, and intertrial interval being 12s, zebrafish learned avoidance responses within a training session consisting of 30 trials and retained the avoidance responses. Experiment 3 showed that zebrafish learned avoidance responses following the association between the CS of light and the US of shock in the avoidance conditioning paradigm. Using the avoidance conditioning paradigm, Experiment 4 investigated the amnestic effects of N-methyl-D-aspartate receptor antagonist MK-801 and nitric oxide synthase inhibitor L-NAME in zebrafish. Experiment 4 showed that post-training injection of L-NAME significantly impaired retention of avoidance responses while MK-801 did not, confirming previous results with other vertebrates. The results of the present study suggest the similar involvements of neurochemicals in learning and memory among vertebrates. Thus, future studies with zebrafish mutants may identify genes involved in learning and memory in vertebrates.     

The role of nucleus accumbens and dorsolateral striatal D2 receptors in active avoidance conditioning

The role of dopamine (DA) in rewarding motivated actions is well established but its role in learning how to avoid aversive events is still controversial. Here we tested the role of D2-like DA receptors in the nucleus accumbens (NAc) and the dorsolateral striatum (DLS) of rats in the learning and performance of conditioned avoidance responses (CAR). Adult male Wistar rats received systemic, intra-NAc or intra-DLS (pre- or post-training) administration of a D2-like receptor agonist (quinpirole) or antagonist ((−)sulpiride) and were given two sessions in the two-way active avoidance task. The main effects observed were: (i) sulpiride and lower (likely pre-synaptic) doses of quinpirole decreased the number of CARs and increased the number of escape failures; (ii) higher doses of quinpirole (likely post-synaptic) increased inter-trial crossings and failures; (iii) pre-training administration of sulpiride decreased the number of CARs in both training and test sessions when infused into the NAc, but this effect was observed only in the test session when it was infused into the DLS; (iv) post-training administration of sulpiride decreased CARs in the test session when infused into the NAc but not DLS. These findings suggest that activation of D2 receptors in the NAc is critical for fast adaptation to responding to unconditioned and conditioned aversive stimuli while activation of these receptors in the DLS is needed for a slower learning of how to respond to the same stimuli based on previous experiences.     

Epinephrine modulates long-term retention of an aversively motivated discrimination

These experiments examined the effects of post-training epinephrine (Epi) on retention of an aversively motivated discrimination task. Male CFW mice were trained to escape from footshock by entering one of two alleys of a Y-maze. On a 24-h retention test (six trials) the correct alley was reversed. The findings of Experiment 1 indicate that errors on the discrimination reversal varied directly with number of trials (criterion of 0, 3, or 6 successive correct choices) on the original training. These findings indicate that errors on discrimination reversal training provide a sensitive index of retention of the original training. In Experiment 2, mice were trained to a criterion of three successive correct choices and were given post-training injections of saline or Epi (0.1, 0.3, or 1.0 mg/kg ip). On a 24-h discrimination reversal test mice given the low doses of Epi made more errors than did saline controls while mice given the high dose made fewer errors. In Experiment 3, mice trained as in Exp 2 received post-training saline or Epi (0.3 or 1.0 mg/kg) and were tested for retention either 1 week or 1 month later. At each retention interval, performance was comparable to that found with a 24-h retention interval. The findings provide additional evidence that post-training Epi produces long-lasting dose-dependent modulating effects on memory storage.     

Effects of reserpine on retention of escape reversal in mice: absence of state-dependent learning.

A discriminated escape training paradigm was used to study the effects of reserpine on learning and memory in mice. Intraperitoneal injection of reserpine before reversal training had no effect on acquisition but did produced a time-and dose-dependent impairment of retention test performance 10 days later. These results suggested that reserpine may have interfered with some aspect of memory storage. Retention impairments observed when a 2.0 mg/kg reserpine injection was given 2 hr before reversal training were not attenuated by readministering the drug before testing, a finding that provides no support for a state-dependency interpretation. Furthermore, animals treated with reserpine exhibited inferior retention of previous training, regardless of the pharmacological state present during that learning. This was interpreted as a drug-induced impairment of memory retrieval. In addition, performance during the initial discriminated escape training session suggested that reserpine may also impair acquisition under some conditions. In the last experiment, it was found that when the catecholamine precursor L-dihydroxyphenylalamine (100 mg/kg) and the indole amine precursor D,L-5-hydroxytryptophan (125 mg/kg) were both given after reserpine treatment, subsequent retention performance was not significantly impaired. The results are discussed in terms of the possible roles of biogenic amines in arousal, learning, and memory.     

Calcium channel antagonists enhance retention of passive avoidance and maze learning in mice

Although a number of studies have shown that treatment with calcium channel antagonists (CCAs) can ameliorate impairments in learning and memory in aged animals, evidence for a general nootropic effect of CCAs in neurologically normal young adult animals is ambiguous. This study attempts to resolve some of this ambiguity by comparing the effects of several CCAs on retention of passive avoidance learning and acquisition and retention of appetitively motivated spatial discrimination learning in young adult mice. Animals were trained in a step through passive avoidance apparatus and, immediately after training, injected subcutaneously with different doses of nimodipine, nifedipine, amlodipine, flunarazine, diltiazem, or verapamil. Retention was tested 24 h after training. In the maze-learning task mice were treated with the same doses of the aforementioned CCAs immediately after a brief training session in a linear maze and retention was tested 24 h after training. The most effective dose of each agent in the maze-retention experiment was administered to additional groups of animals 1 h prior to training to determine the effects of CCAs on acquisition processes. The effects of central administration of CCAs were examined by intracerebroventricular injection of different doses of amlodipine immediately after passive avoidance training. Results showed (1) all peripherally administered drugs except verapamil facilitated retention of passive avoidance training in a dose-dependent manner, (2) all drugs dose dependently facilitated retention of linear maze learning, (3) all doses of the drugs (except verapamil) which facilitated maze retention also facilitated maze learning, and (4) central administration of the dihydropyridine amlodipine produced a dose-dependent facilitation of the retention of passive avoidance learning. These data indicate that drugs which block calcium channels can enhance retention of two different types of learning in mice.     

Scopolamine amnesia of passive avoidance: a deficit of information acquisition

Despite its increasing use as an animal model of memory deficit in human dementia, relatively few studies have attempted to assess the memory processes involved in the anticholinergic-induced impairment of passive avoidance retention. In the present experiments, the influence of scopolamine administered prior to or immediately following training on 24-h retention of step-through passive avoidance was studied in NMRI mice. In low doses (0.3-3.0 mg/kg ip) pretraining administration (-5 min) of scopolamine induced a very strong amnesia. Post-training scopolamine induced a significant effect only at the highest dose tested (30 mg/kg). In a retention test of longer than normal duration (600 vs 180 s), which resulted in a more favorable comparison value in the control group, an intermediate post-training dose (10 mg/kg) induced a small effect which approached significance; a finding which may account for conflicting reports in the literature concerning the ability of scopolamine to induce a post-training deficit. The pretraining effect does not appear to have been solely the result of state-dependent learning; scopolamine (3 mg/kg) administered before both the training and test sessions induced a deficit of approximately the same magnitude as that found when administered before training or before testing only. The results indicate that scopolamine can induce a small post-trial effect, presumably through an influence on consolidation processes. The much larger effect of pretrial scopolamine, however, indicates a primary influence on processes related to information acquisition. Together with findings from the literature, the present experiments suggest that scopolamine-induced amnesia partially, but not completely, models the memory deficits of human dementia.     

Phlorizin,a Competitive Inhibitor of Glucose Transport,Facilitates Memory Storage in Mice

Posttraining intraperitoneal administration of phlorizin (3.0–300.0 μg/kg), a competitive inhibitor of glucose transport from blood to brain, facilitated 48-h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose–response curve was an inverted-U shape. Phlorizin did not increase the retention latencies of mice that had not received a foot shock during training. The effects of phlorizin (30.0 μg/kg) on retention were time dependent, and the administration of phlorizin (30.0 μg/kg) 5 or 10 min prior to the retention test did not affect the retention performance of mice given posttraining injections of saline or phlorizin (30.0 μg/kg). These findings indicate that phlorizin influenced memory storage, but not memory retrieval. Finally, the simultaneous administration of phlorizin (3.0–300.0 μg/kg, ip) antagonized, in a dose-related manner, the memory impairment induced by insulin (8 IU/kg, ip). Taken together, the results show that phlorizin enhance retention acting as a “glucose-like substance” although the mechanism(s) of this enhancement is unknown.     

Serotonin neurons and aversive conditioning in the rat

The acute and long-term effects of p -chloroamphetamine (PCA) on one-way and two-way active avoidance (AA), passive avoidance (PA) learning, fear retention (FR) and on central monoamine concentrations were examined in the male rat. Acute PCA administration (0.63–5 mg/kg i.p.), which releases presynaptic 5-HT, produced a dose-related impairment of both one-and two-way AA acquisition, AA retention, PA and fear retention. The selective serotonin (5-HT) uptake inhibitor zimelidine, but not the noradrenaline (NA) uptake inhibitor desipramine, blocked the avoidance deficit induced by acute PCA. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 × 10 mg/kg i.p.) failed to change AA and PA learning but blocked the avoidance deficit induced by acute PCA. Degeneration of locus coeruleus NA neurones with DSP4 (1 × 50 mg/kg), a selective NA neurotoxin, failed to block the acute PCA action. Thus, the acute avoidance learning impairment appears to be specifically related to the acute release of endogenous 5-HT. Both acute and long-term PCA treatment affected 5-HT neurones preferentially in the forebrain while marginal effects were observed in the midbrain and spinal cord. A marked impairment in the retention and retrieval of fear conditioning in the rat was also observed following acute PCA administration. The serotoninergic mechanisms underlying the retrieval deficit were found to be similar but not identical to those involved in AA acquisition. These results suggest an important role for central 5-HT neurones in aversive learning processes. The possible involvement of 5-HT neurones in learning, memorial and/or retrieval processes is discussed.     

Serotonin neurons and aversive conditioning in the rat

The acute and long-term effects of p-chloroamphetamine (PCA) on one-way and two-way active avoidance (AA), passive avoidance (PA) learning, fear retention (FR) and on central monoamine concentrations were examined in the male rat. Acute PCA administration (0.63–5 mg/kg i.p.), which releases presynaptic 5-HT, produced a dose-related impairment of both one-and two-way AA acquisition, AA retention, PA and fear retention. The selective serotonin (5-HT) uptake inhibitor zimelidine, but not the noradrenaline (NA) uptake inhibitor desipramine, blocked the avoidance deficit induced by acute PCA. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 × 10 mg/kg i.p.) failed to change AA and PA learning but blocked the avoidance deficit induced by acute PCA. Degeneration of locus coeruleus NA neurones with DSP4 (1 × 50 mg/kg), a selective NA neurotoxin, failed to block the acute PCA action. Thus, the acute avoidance learning impairment appears to be specifically related to the acute release of endogenous 5-HT. Both acute and long-term PCA treatment affected 5-HT neurones preferentially in the forebrain while marginal effects were observed in the midbrain and spinal cord. A marked impairment in the retention and retrieval of fear conditioning in the rat was also observed following acute PCA administration. The serotoninergic mechanisms underlying the retrieval deficit were found to be similar but not identical to those involved in AA acquisition. These results suggest an important role for central 5-HT neurones in aversive learning processes. The possible involvement of 5-HT neurones in learning, memorial and/or retrieval processes is discussed.