Measuring emotional memory in the elevated T-maze using a training-to-criterion procedure

Roadbumps at the Crossroads of Integrating Behavioral and In Vitro Approaches for Neurotoxicity Assessment

With the appreciation that behavior represents the integration and complexity of the nervous system, neurobehavioral phenotyping and assessment has seen a renaissance over the last couple of decades, resulting in a robust database on rodent performance within various testing paradigms, possible associations with human disorders, and therapeutic interventions. The interchange of data across behavior and other test modalities and multiple model systems has advanced our understanding of fundamental biology and mechanisms associated with normal functions and alterations in the nervous system. While there is a demonstrated value and power of neurobehavioral assessments for examining alterations due to genetic manipulations, maternal factors, early development environment, the applied use of behavior to assess environmental neurotoxicity continues to come under question as to whether behavior represents a sensitive endpoint for assessment. Why is rodent behavior a sensitive tool to the neuroscientist and yet, not when used in pre-clinical or chemical neurotoxicity studies? Applying new paradigms and evidence on the biological basis of behavior to neurobehavioral testing requires expertise and refinement of how such experiments are conducted to minimize variability and maximize information. This review presents relevant issues of methods used to conduct such test, sources of variability, experimental design, data analysis, interpretation, and reporting. It presents beneficial and critical limitations as they translate to the in vivo environment and considers the need to integrate across disciplines for the best value. It proposes that a refinement of behavioral assessments and understanding of subtle pronounced differences will facilitate the integration of data obtained across multiple approaches and to address issues of translation.

AAV-mediated overexpression of neuroserpin in the hippocampus decreases PSD-95 expression but does not affect hippocampal-dependent learning and memory

Neuroserpin is a serine protease inhibitor, or serpin, that is expressed in the nervous system and inhibits the protease tissue plasminogen activator (tPA). Neuroserpin has been suggested to play a role in learning and memory but direct evidence for such a role is lacking. Here we have used an adeno-associated virus (AAV) vector expression system to investigate the effect of neuroserpin on hippocampal-dependent learning and memory in the young adult rat. A FLAG-tagged neuroserpin construct was initially characterized by in vitro transcription/translation and transfection into HEK293 cells and shown to interact with tPA and be targeted to the secretory pathway. Targeted injection of a chimeric AAV1/2 vector expressing FLAG-neuroserpin resulted in localized overexpression in the dorsal hippocampus. Neuroserpin overexpression led to the appearance of an unstable neuroserpin:tPA complex in zymographic assays consistent with interaction with endogenous tPA in vivo. Rats overexpressing neuroserpin also showed a significant decrease in the levels of postsynaptic density protein 95, a major postsynaptic scaffolding protein. Three weeks after injection, a range of behavioural tests was performed to measure spatial and associative learning and memory, as well as innate and acquired fear. These tests provided no evidence of a role for neuroserpin in hippocampal-dependent learning and memory. In summary this study does not support a role for neuroserpin in hippocampal-dependent learning and memory in young adult rats but does suggest an involvement of neuroserpin in hippocampal synaptic plasticity.

Alcohol selectively impairs negative self-relevant associations in young drinkers

The stress-dampening effects of alcohol have been attributed to 'appraisal disruption'- decreased ability of stimuli to evoke threatening associations in memory. Appraisal disruption could apply to oneself as well as situational stimuli. This question was investigated in undergraduate drinkers (n=90/Gender) with low or high anxiety sensitivity (AS; n=90/AS Group), a trait linked with hyper-vigilance to threat. Subjects received alcohol (0.7 g/kg males; 0.63 g/kg females), placebo or soft drink and performed a speech about their appearance. Sequence of drink administration and speech advisory (threat) was manipulated between subjects: Threat before Drink, Threat after Drink, No-Threat Control. The Implicit Association Test measured self-relevant associations based upon time to classify positive and negative attribute words (e.g. Cute, Ugly) paired with self-relevant or non-self-relevant object words (e.g. Me, Them). Alcohol selectively slowed negative self-relevant decisions, regardless of other factors. Relative fluency of negative versus positive decisions (D) correlated inversely with state anxiety and systolic blood pressure immediately before speech performance, and correlated directly with severity of alcohol problems. These findings are consistent with the Appraisal Disruption hypothesis. Preferential impairment of negative self-relevant associations may decrease perceived vulnerability under alcohol and increase risk for alcohol problems in young drinkers.

Effect of repeated restraint stress and clomipramine on Na+/K+-ATPase activity and behavior in rats

Activation of the limbic-hypothalamic-pituitary-adrenal axis (LHPA) and the release of glucocorticoids are fundamental for the adaptive response and immediate survival of an organism in reaction to acute stimuli. However, high levels of glucocorticoids in the brain may produce neuronal injury and a decrease of Na(+)/K(+)-ATPase activity, with effects on neurotransmitter signaling, neural activity, as well as the whole animal behavior. Clomipramine is a tricyclic antidepressant that inhibits the reuptake of serotonin and norepinephrine by indirect actions on the dopaminergic system and LHPA axis. Its chronic use increases the body's ability to cope with stress; however, high doses can potentiate its side effects on memory, learning, and sensory motor function. The purpose of the present study was to compare the effect of repeated restraint stress and clomipramine treatment on Na(+)/K(+)-ATPase activity and on the behavior of male rats. Changes in the behavioral response were evaluated by measuring the memory, learning, anxiety, and exploratory responses. Our results showed that exposure to repeated restraint stress reduced levels of Na(+)/K(+)-ATPase in brain structures and changed short and long-term memory, learning, and exploratory response when compared to the control group. Exposure to clomipramine treatment increased anxiety levels and reduced Na(+)/K(+)-ATPase activity in the cerebral cortex as well as short term memory, learning, and exploratory response. In conclusion, the present results provide additional evidence concerning how repeated restraint stress and clomipramine chronically administered at higher dose levels affect the neural activity and behavior of male rats.

Elevated T-maze evaluation of anxiety and memory effects of NMDA/glycine-B site ligands injected into the dorsal periaqueductal gray matter and the superior colliculus of rats

Rat behaviors in the elevated T-maze (ETM) were evaluated following tectum microinjections of either glycine (GLY, 1, 10, 80 and 120 nmol) or d-serine (D-SER, 160 and 320 nmol), the putative endogenous agonists of GLY-B site at NMDA receptor, or the respective antagonist 7-chloro-kynurenic acid (7CK, 8 nmol). ETM performance was appraised by two validated scores of anxiety, i.e., the inhibitory avoidance duration (AD) and risk assessment behavior, and two scores derived from a newly developed approach to inhibitory avoidance learning curves, i.e., the learning median number of trials (T50) and avoidance variability (standard deviation of learning curve). Effects on aversive memory consolidation were assessed through changes in the AD measured 48 h after the full-acquisition of inhibitory avoidance. Drug effects were compared to those of vehicle. In most cases, microinjection of GLY-B site agonists into the dorsal periaqueductal gray (dPAG) produced increases in AD, which were compatible with an increase in anxiety. However, neither the intra-periaqueductal injection of 80 nmol GLY, nor that of 160 nmol D-SER, increased the AD. On the other hand, these microinjections invariably produced a parallel left shift in avoidance learning curves, thereby reducing the T50 but not the variability. Effects of 120 nmol GLY on AD and T50 were both antagonized by a previous microinjection of 7CK into the dPAG. The inverse relationship of AD and T50 suggests that increases in the anxiety level reduce the number of trials required for the acquisition of inhibitory avoidance. The above data also suggest the higher consistency and drug sensitivity of T50 as compared to the AD. In turn, whereas the microinjection of 120 nmol GLY into the superior colliculus (SC) did not affect the T50, it increased the AD. On the other hand, there was an increase in avoidance variability following the microinjection of either 120 nmol GLY into the SC or 8 nmol 7CK into the dPAG. Therefore, the GLY-B receptors within these structures seem to play opposite roles on avoidance variability. In contrast, neither of these treatments changed T50. Finally, whereas the risk assessment was solely decreased by the microinjection of GLY into the SC, the aversive memory was only impaired by the microinjection of 7CK into the dPAG. Overall, these data suggest that NMDA/GLY-B receptors of dPAG mediate both anxiety and aversive memory, while those in the SC are most likely involved with attention and visuomotor components of risk assessment behavior.

Electroacupuncture prevents cognitive deficits in pilocarpine-epileptic rats

Here we investigated the effects of electroacupuncture over the cognitive deficits in the pilocarpine model of epilepsy in rats. Acupuncture stimulation was provided at acupoints located in either the midline of the back and of the head [HD]: Gv-20 (Baihui), Gv-14 (Dazhui), Gv-2 (Yaoshu) and M-HN-3 (Yin Tang); or acupoints located in the limbs [LB]: St-36 (Zusanli) and Sp-6 (Sanyinjiao). In the elevated T-maze test, electroacupuncture at HD and LB acupoints produced an improvement in the acquisition and retention parameters. Retention in the inhibitory avoidance test was seen only in short-term retention and only for animals stimulated at HD. At histology it was found that electroacupuncture at HD acupoints abolished tissue shrinkage in dorsal hippocampus, basolateral nucleus of the amygdala, substantia nigra and perirhinal cortex, whereas stimulation of LB acupoints prevented tissue shrinkage in all of the above structures except dorsal hippocampus. Administration of p-chlorophenylalanine, a serotonergic releaser, abolished both behavioral and part of the histological changes in these animals. We conclude that electroacupuncture at HD and LB acupoints prevents atrophy of some limbic structures and improves cognitive deficits in pilocarpine-epileptic rats and that this effect is dependent on the serotonergic system.

Effects of pentylenetetrazole-induced status epilepticus on behavior, emotional memory and learning in immature rats

Status epilepticus (SE) can be harmful to the developing brain. Our knowledge of the emotional and behavioral consequences of generalized SE in developing animals remains limited. Therefore, we investigated the short- and long-term effects of pentylenetetrazole (PTZ)-induced SE on emotional memory and learning and behavioral parameters in immature rats. SE was induced in 16- to 20-day-old rats (P16-P20) using intraperitoneal injections of PTZ (n=21); control rats received saline (n=10). All animals were tested using an elevated T-maze and open-field test 2, 14, 30, and 180 days after SE, to evaluate emotional memory and learning and behavior. Anxiety levels decreased 2 and 14 days after SE, and conditioned learning of PTZ-treated immature rats was better than that of the control rats. These results indicate that a decreased anxiety level facilitates conditioned learning. Behavioral changes are transient, and no emotional memory or learning deficits occur following PTZ-induced SE in immature rats.

The effects of pentylenetetrazole-induced status epilepticus on behavior, emotional memory, and learning in rats

Status epilepticus (SE) can cause spatial learning, memory, and behavioral deficits; however, little information is available, especially regarding the effects of such seizures on emotional memory and learning functions. We investigated the effects of SE on emotional memory, learning, and behavior in mature rats over short and long periods. SE was induced in 50- to 60-day-old rats (P50-P60) using intraperitoneal injections of pentylenetetrazole (PTZ, n = 20); control rats received saline (n = 10). All animals were tested with elevated T-maze and open-field tests on the 1st, 7th, 14th, and 180th days after SE to evaluate emotional memory, learning, and behavior. The number of fecal boli increased, and one-way escape latency was long in a short period after SE. PTZ-induced SE causes transient memory deficits, which is related to unconditioned fear, but it did not cause any persistent abnormalities of behavior, emotional memory, and learning in mature rats.

Male and female C57BL/6 mice respond differently to diazepam challenge in avoidance learning tasks

Benzodiazepines (BZ) impair learning and memory performance of animals. The goal of this study was to examine sex differences in the effects of diazepam on learning and memory of C57BL/6 mice in avoidance paradigms. Male and female C57BL/6 mice were tested in the one-way active avoidance, step-down passive avoidance, and foot-shock pain threshold tasks, following administration of vehicle or diazepam (1 mg/kg). No substantial sex or drug effects on the threshold of the pain response to shock were found. There were no significant differences in avoidance performance between vehicle-treated male and female mice while 1 mg/kg of diazepam produced opposite effects on performance of males and females in both tasks. Diazepam-treated females learned faster in the active avoidance task and showed stronger retention in the passive avoidance task. In contrast, diazepam impaired learning of males in the active avoidance task and had no effect on their performance in the passive avoidance task. Diazepam-induced impairment in males was not due to higher sensitivity to the sedative effect of diazepam as females were more sedated than males on the first trial of the passive avoidance task. Our data showed that sedative and amnesic effects of BZs are not tightly linked. This study also suggests that cognitive effects of BZs in rodents could be sex dependent and highlight the importance of using both sexes in studies on behavioral effects of psychoactive drugs.

Elevated T-maze as an animal model of memory: effects of scopolamine

The elevated T-maze (ETM) is a putative model for the assessment of anxiety and memory in rodents. This study was designed to further evaluate the utility of the ETM in the study of memory processes. We compared the performance of rats in the ETM, the water maze (WM) and the two-way avoidance task (TWA), after pretreatment with scopolamine (SCO; 0.3 or 1.2 mg/kg i.p.). In the ETM, rats were first trained to meet the criterion of remaining inside the enclosed arm for 300 seconds. Seventy-two hours after training, a retrieval test session was performed. At the lower dose, SCO impaired performance in the retrieval session on all three tasks, whereas in the training session an effect was noted only on the WM task. At the higher dose, SCO impaired the performance of rats in the training sessions for ETM and WM, but not TWA. In a fourth experiment using the elevated plus-maze, SCO showed anxiolytic-like effects at the higher dose only. In conclusion, the effects of SCO in rats submitted to the ETM were dose dependent, with the lower dose exerting a selective effect detected only on retrieval, whereas the higher dose induced motor effects that disrupted inhibitory avoidance acquisition, resulting in impaired retrieval. The results are discussed in terms of the utility of the ETM in the study of drug effects and the neurobiological mechanisms underlying anxiety, learning and memory.

Comparison of neurokinin SP with diazepam in effects on memory and fear parameters in the elevated T-maze free exploration paradigm

The elevated T-maze was combined with a free exploration protocol, which, in contrast to the conventional procedure, dispenses with handling of the animals during the experimental sessions. This allows measurement of fear indexes derived from the elevated plus-maze as well as assessment of acquisition of open arm avoidance and open arm escape in one continuous session. Retention of the different fear-responses is measured 72 h later without drug treatment. In order to assess the effects of two known anxiolytics in this paradigm, rats received an IP injection of diazepam (1 to 4 mg/kg), substance P (5 to 500 microg/kg) or vehicle (1 ml/kg) and were tested on the T-maze for 5 min. Diazepam elevated open arm activity, indicative of an anxiolytic effect. The drug also increased the latency to escape from the open arms, but did not significantly affect acquisition of open arm avoidance. During the retention trial, diazepam in higher doses impaired the performance of both fear-responses, suggestive of an anterograde amnesic effect. Substance P did not influence acquisition and retention of open arm avoidance and escape. However, in high doses, the peptide increased the sojourn time in the central arena of the maze, indicating reduced fear and, hence, a dissociation between anxiolytic and amnesic effects. The present findings demonstrate that the elevated T-maze free exploration paradigm is sensitive to anxiolytic and memory-modulating effects of drugs.

Nitric oxide may underlie learned fear in the elevated T-maze

The present study evaluated the role of nitric oxide (NO) in learned and innate fear in rats submitted to the elevated T-maze (ETM). Learned and innate fear were evaluated through the inhibitory avoidance and escape behaviour from the open arms, respectively. Rats treated with the inhibitor of NO synthesis N(omega)-nitro-L-Arginine methyl ester (L-NAME; 5, 10, and 50 mg. kg(-1)) were able to learn the inhibitory avoidance. However, L-NAME (50 mg. kg(-1)), but not its inert isomer N(omega)-nitro-D-arginine methyl ester (D-NAME, 50 mg. kg(-1)), impaired the inhibitory avoidance 2 with no change in the baseline values, thus suggesting an anxiolytic-like effect without locomotor impairment. All treatments with L-NAME were able to induce increased mean arterial pressure (MAP), measured indirectly through the animal's tail. The treatment with L-NAME (5 and 10 mg. kg(-1)) failed to induce anxiolysis but significantly increased the MAP of the animals, which indicates that hypertension per se, did not underlie anxiolysis induced by L-NAME. L-Arginine, the precursor molecule for NO synthesis, facilitated the inhibitory avoidance and counteracted the L-NAME (50 mg. kg(-1))-induced anxiolysis. Neither previous treatment was able to change the escape behaviour. The results indicate that NO may underlie learned, but not innate, fear in the ETM.

Effects of emotional reactivity on inhibitory avoidance in the elevated T-maze

The possibility of the presence of inter-individual emotional differences and the memory performance of rats was examined in the elevated T-maze. Two kinds of aversively motivated behaviors, inhibitory avoidance and escape learning, were measured. Based on the number of trials to achieve a learning criterion, rats were divided into two subgroups with either low or high avoidance reactivity (LAR or HAR, respectively). Retention test avoidance latencies showed that HAR animals had better avoidance memory (Mann-Whitney rank sum test, P = 0.0035). No such differences were found for the escape component of this test. These data suggest that individual emotional differences affect inhibitory avoidance performance, which may help to explain the dispersion of the data observed in other studies using this paradigm.