Improvement of shuttle-box avoidance with post-training intracranial self-stimulation, in rats: a parametric study

Rats were trained in a two-way active avoidance task followed immediately by a lateral hypothalamic intracranial self-stimulation (ICSS) treatment, during 5 consecutive days. The effects of the number of ICSS trains allowed (0, 500, 2500 or 4500) were studied upon acquisition and long-term retention (LTR, 10 and 30 days). The number of ICSS trains administered and the number of avoidances at the last acquisition session (5th) showed a positive lineal relation, that is, the more number of ICSS trains, the more number of avoidances. The level of learning achieved during the 5th session was maintained after the LTR periods in all experimental groups. It is concluded that the number of ICSS trains could be a critical parameter in the facilitatory effect of lateral hypothalamic ICSS upon learning, and it is suggested that the facilitatory effect of post-training lateral hypothalamic ICSS might be due to the activation of general activatory neural systems.     

Intracranial self-stimulation facilitates memory consolidation, but not retrieval: its effects are more effective than increased training.

To evaluate possible differential effects of lateral hypothalamic intracranial self-stimulation (ICSS) on memory consolidation and retrieval, independent groups of Wistar rats were trained in a single session of two-way active avoidance task (acquisition session) and tested 24 h later (retention session). The post-ICSS groups received an ICSS treatment immediately after the acquisition session, and the pre-ICSS groups received the same treatment immediately before the retention session. Because the ICSS effects on memory seem to be dependent on the initial performance level shown by the subjects, the possible influence of initial training (number of trials) on ICSS effects was also studied. Therefore, we used different control and experimental groups, which received either 30 or 50 trials in the acquisition session. Post-training ICSS facilitated the 24-h retention in both training conditions (30 and 50 trials). In contrast, pre-retention ICSS treatment did not facilitate performance in the retention test. We also observed that post-training ICSS was more effective for improving the 24-h retention than increasing the initial training from 30 to 50 trials. This findings confirm that ICSS treatment improves memory consolidation and suggest that it might not affect memory retrieval mechanisms.     

Involvement of the parafascicular nucleus in the facilitative effect of intracranial self-stimulation on active avoidance in rats

To evaluate whether parafascicular nucleus (PF) is involved in the facilitative effect of lateral hypothalamic intracranial self-stimulation (LH-ICSS) on two-way active avoidance acquisition (5 sessions, 10 trials each, one daily) and long-term retention (10 days), rats were lesioned bilaterally at the PF and implanted with an electrode aimed at the LH to obtain ICSS behavior. After each acquisition session rats were allowed to self-administer 2500 trains of LH-ICSS. The main results were: (1) LH-ICSS facilitated the acquisition and retention of conditioning; (2) PF lesions impaired both acquisition and retention of two-way active avoidance; (3) there was a positive relationship between PF lesions size and learning disruption, and (4) LH-ICSS failed to facilitate learning when PF was lesioned. We concluded that the lesion size is a critical variable to evaluate the effects of PF lesions on learning and memory, and that LH-ICSS treatment may exert their effects through the PF nucleus or, at least, the integrity of PF is required for LH-ICSS to improve clearly the task.     

Scopolamine blocks the effects of swim stress on memory retrieval in rats

Summary This study examined whether application of swim stress improved retrieval of a passive avoidance memory and if pretreatment with the anticho-linergic agent, scopolamine, blocked this effect on memory retrieval. Animals initially given a passive avoidance training session were subjected to either a two or four swim stress sessions (15 min each) with or without prior treatment of scopolamine (0.05 or 0.1 mg/kg). The retrieval performance in passive avoidance test and motor activity was assessed 24 hr after the last swim stress session. In an independent control experiment, the passive avoidance training and test were conducted respectively, 24 and 72 hr after the last of four swim stress sessions with or without prior injection of scopolamine (0.1 mg/kg). The results showed an enhanced performance for the passive avoidance task in rats subjected to four swim stress sessions in both experiments and scopolamine given 30 min prior to each stress session diminished this performance of animals in the passive avoidance test. Two swim stress sessions with or without scopolamine treatment caused no significant effects on the retrieval performance. Also, no significant difference was observed among the groups in motor activity following any of the stress treatments in the open field test. These results, thus suggested for the first time, a relationship among swim stress, cholinergic activity and avoidance memory processes.     

Catecholamines and conditioned blocking: effects of ventral tegmental, septal and frontal 6-hydroxydopamine lesions in rats

The performance of rats on the conditioned blocking test of learned inattention was measured in a two-way shuttle avoidance task after sham and dopamine (DA)-depleting lesions of the frontal cortex, septum and ventral tegmental area (VTA). Animals were trained on two sessions with tone and/or light as conditioned stimuli. One group was trained with both stimuli on both sessions. A second group was trained on the first session with one stimulus and on the second with both stimuli. The blocking of conditioning to the added stimulus (b) was tested by presenting the stimuli (a and b) separately and measuring the blocking ratio (avoidance to b/a + b) and response latencies. No deficits were recorded on tests of sensory and motor ability. The VTA group alone showed a hyperlocomotor response to apomorphine treatment and did not acquire the avoidance response. The appearance of blocking in the septal group was delayed until the end of the test session. Blocking was mildly attenuated in the frontal group. DA levels were depleted by about 80% and noradrenaline (NA) levels by, respectively, 20 and 50% in frontal and septal areas. This suggests that the level of DA activity or the balance between the activity of DA and NA in frontal and limbic regions can contribute to efficient associative conditioning and/or the normal ability of rats not to attend to a redundant stimulus.     

Inhibitory avoidance learning altered ensemble activity of amygdaloid neurons in rats

In this study, we examined single-unit activity in the amygdala before and after a rat had acquired an inhibitory avoidance task. Long-Evans rats with microwires chronically implanted into the central nucleus (CeA) or basolateral complex (BLC) of the amygdala were acclimatized to the apparatus of a step-through inhibitory avoidance task for three sessions. On the fourth session, rats in the experimental group received an inescapable footshock (3 mA, 1 s) as they stepped from the lit side into the dark side of the task apparatus, whereas rats in the control group received the same amount of shock on a different apparatus. All rats were tested for retention in the task apparatus 1 day after shock training. The experimental rats showed better retention than the controls as they stayed longer in the lit side. Ensemble unit activities were recorded in the amygdala nuclei from the indwelling wire bundles during the acclimation and test sessions. The data collected from well-isolated amygdala units showed that neuronal discharge habituated from the first to the third acclimation session. In the test session, the experimental group, but not the control group, showed elevated firing rates in the CeA or BLC neurons located on either side of the brain. These findings provide the first piece of evidence showing that learning of an inhibitory avoidance task leads to an increase in amygdala neuronal discharges during a retention test.     

Effects of electrical stimulation of the nucleus basalis on two-way active avoidance acquisition, retention, and retrieval

This study assessed the role of the nucleus basalis magnocellularis (NBM) in specific memory phases of two-way active avoidance conditioning. We evaluated the effects of NBM electrical stimulation applied during different phases of the avoidance task. Rats were trained in a 30-trial acquisition session, and were tested again 24 and 48 h later. NBM stimulation was applied at different stages of memory formation of the conditioning: (1) immediately before the first training session to determine the effects on acquisition of the two-way avoidance task; (2) immediately after the first training session to evaluate effects on memory consolidation; and (3) immediately before the 24-h retention session to analyze the effects on the retrieval process. NBM stimulation before training significantly improved the acquisition of the task, without affecting subsequent retention at either 24 or 48 h. Stimulation of the NBM immediately after the first training session slightly impaired performance in the 24-h retention session. Stimulation of the NBM immediately before the 24-h retention session did not affect performance in either the 24 or 48-h retention sessions. Therefore, the NBM may play a more important role in acquisition of memory in aversively motivated conditioning tasks than in consolidation or retrieval of such memories. These results are discussed in the context of attention enhancement and cortical and amygdala activation.     

Reversible lesion of the rat's orbitofrontal cortex interferes with hippocampus-dependent spatial memory

In this study, tetrodotoxin (TTX) inactivation was employed to evaluate the involvement of the rat's orbitofrontal cortex (OFC) in hippocampus-dependent spatial memory using Morris water maze (MWM) and place avoidance learning (PAL) tasks. In Experiment 1, rats trained in MWM task with two blocks of four trials per day for 3 consecutive days received bilateral injections of either TTX or saline into the OFC 60 min before each daily training session. The acquisition of spatial memory was evaluated 24h after the last training day and it was shown an impairment by the TTX. In Experiment 2, bilateral intra-OFC injections of TTX or saline were made immediately after two blocks of four trials. Testing 24h later, it was revealed that TTX also impairs spatial memory consolidation. In Experiments 3 and 4, rats were trained in a single 30-min session to avoid a 60 degrees segment of the stable circular (80-cm diameter) arena, entering which was punished by a mild shock (PAL task) and retention was tested 24h later in a 30-min extinction session. Bilateral injections of TTX or saline were made into the OFC 60 min before training or immediately after training. Again, TTX impaired the place avoidance retention when it was injected into the OFC either before (acquisition phase) or after (consolidation phase) training. These findings indicate that functional integrity of the OFC is necessary for both the acquisition and the consolidation of hippocampus-dependent spatial memory in rats.     

Hypericum extract and hyperforin: memory-enhancing properties in rodents

Effects of a Hypericum extract in therapeutic use and hyperforin sodium salt were evaluated in rat and mouse avoidance tests. In a conditioned avoidance response (CAR) test on the rat, oral daily administration of hyperforin (1.25 mg/kg/day) or of the extract (50 mg/kg/day) before the training sessions considerably improved learning ability from the second day onwards until the day 7. In addition, the memory of the learned responses acquired during 7 consecutive days of administration and training was largely retained even after 9 days without further treatment or training. The observations made using different doses indicate that these learning-facilitating and/or memory-consolidating effects by the agents follow inverse U-shaped dose-response curves in dose ranges lower than (for hyperforin) or equal to (for Hypericum extract) their effective dose in the behavioral despair test for antidepressants. In a passive avoidance response test on the mouse, a single oral dose (1.25 mg/kg) of hyperforin not only improved memory acquisition and consolidation, but also almost completely reversed scopolamine-induced amnesia. The single Hypericum extract dose tested (25 mg/kg) did not reveal any significant effects in the passive avoidance response (PAR) test on the mouse. These observations suggest that the Hypericum extract could be a novel type of antidepressant with memory enhancing properties, and indicate that hyperforin is involved in its cognitive effects. Pure hyperforin seems to be a more potent antidementia agent than an antidepressant.     

Cancer chemotherapy impairs contextual but not cue-specific fear memory

This study examined the effects of a standard breast cancer chemotherapeutic protocol on learning and memory in rats. Ovariectomized rats were treated once a week for 3 weeks with a combination of cyclophosphamide and doxorubicin prior to training in a classical fear conditioning task. Training took place 1 week after the final treatment. During the training session, an auditory stimulus (a tone) was paired with a mild foot-shock. The resulting conditioned fear to the tone (cue-specific fear) and to the training environment (contextual fear) was measured in subsequent test sessions. Chemotherapy did not affect the acquisition of the conditioned response (freezing) during the training session or the expression of fear during the tone test session. In contrast, rats treated with cyclophosphamide and doxorubicin exhibited decreased freezing during the context test session, suggestive of a specific deficit in hippocampal-related learning and memory. Together, these data indicate that administration of cyclophosphamide and doxorubicin may have toxic effects on the hippocampus and results in specific learning deficits shortly after treatment has ended.     

Activation of the phasic pontine-wave generator enhances improvement of learning performance: a mechanism for sleep-dependent plasticity

The aim of this study was to test the hypothesis that supplementary activation of the phasic pontine wave (P-wave) generator during rapid eye movement (REM) sleep enhances consolidation and integration of memories, resulting in improved learning. To test this hypothesis, two groups of rats were trained on a two-way active avoidance learning task in the morning. Immediately after training, one group of rats received a carbachol microinjection into the P-wave generator and the other group was microinjected with control saline into the same target area. After training trials and microinjections, rats were allowed a 6-h period of undisturbed sleep in the polygraphic recording chamber. At the end of 6 h of undisturbed sleep-wake recordings, rats were retested in a session of avoidance learning trials. After learning trials, the total percentage of time spent in REM sleep was significantly increased in both saline (15.36%)- and carbachol (17.70%)-microinjected rats. After learning trials, REM sleep P-wave density was significantly greater throughout the 6-h period of recordings in carbachol treated rats than in the saline treated rats. In the retrial session, the improvement in learning task performance was 22.75% higher in the carbachol-microinjected rats than in the saline-microinjected rats. These findings show that the consolidation and integration of memories create a homeostatic demand for P-waves. In addition, these findings provide experimental evidence, for the first time, that activation of the P-wave generator may enhance consolidation and integration of memories, resulting in improved performance on a recently learned task.     

Chronic administration of fluoxetine impairs inhibitory avoidance in male but not female mice

The effects of chronic administration of fluoxetine (20 mg/kg/day i.p.) on a one-trial step-through inhibitory avoidance task were investigated in male and female CD1 mice. In Experiment 1, treatment was administered for 21 days before the training session, whereas in Experiment 2, other subjects were subjected to the same treatment starting 24 h after the training session. The comparison of test versus training latencies showed memory deterioration with pre-training administration of fluoxetine (Experiment 1), which affected males but not females. Sex differences in this task were also observed in Experiment 1, with females showing a better performance. Sex differences were evident in controls as well as in treated animals. The locomotor activity of the animals was also tested in Experiment 1. Due to the absence of sex differences in the drug effects on this measure, the sex differences in the effects of fluoxetine on inhibitory avoidance were hardly attributable to non specific effects on locomotor activity. The lack of effect of post-training administration of fluoxetine (Experiment 2) constitutes additional support of the idea that the observed effect on inhibitory avoidance in Experiment 1 is specifically related to learning and memory.     

Effects of Psychotropic Drugs on Impairment of Acquisition of Shuttle Avoidance Produced by Pretreatment with Pairs of Tone Signal and Shock in Mice

Abstract: The acquisition of discrete shuttle avoidance of mouse was significantly impaired by presentation of 120 pairs of a tone signal and an unavoidable/unescapable shock at 30 sec intervals for 3 consecutive days prior to the training session. Caffeine (3 and 10 mg/kg s. c.) reduced such an impairment of the avoidance acquisition, whereas diazepam (0.5-2 mg/kg s. c.), imipramine (30 mg/kg i. p.) and chlorpromazine (2 mg/kg s. c.) enhanced it, when these drugs were administered 10 min before each shock-presenting session. Ethanol (0.8-2.4 g/kg p. o.), nicotine (0.1-1 mg/kg s. c.), methamphetamine (0.1-1 mg/kg s. c.) and morphine (1–10 mg/kg s. c.) did not modify the avoidance acquisition. These results indicate that the nature of impairment of the avoidance acquisition caused by this condition is different from learned helplessness, and elicits different responses to psychotropics.     

Cingulate cortical and anterior thalamic neuronal correlates of the overtraining reversal effect in rabbits

Multiple-unit activity of the cingulate cortex and the anteroventral (AV) nucleus of the thalamus in rabbits was recorded during reversal training, following differential conditioning of a locomotory (wheel rotation) avoidance response. The CS+ and CS? were pure tones (1 or 8 kHz) and the UCS was a footshock delivered through the grid floor of the wheel. One group of the rabbits received original training to a criterion followed immediately by reversal training. A second group received training to criterion followed by additional training sessions (overtraining), prior to reversal training. The results indicated that the overtraining reversal effect (ORE) occurred. That is, the overtrained subjects acquired the reverse discrimination in significantly fewer sessions than the non-overtrained subjects. The overall (non-discriminative) neuronal reactivity in all cingulate cortical laminae was reduced by overtraining, whereas the overall reactivity of the AV thalamus was enhanced. In addition, the non-overtrained subjects manifested a discriminative neuronal response appropriate to the original task (i.e. a greater response to the CS? than to the CS+ for reversal training), throughout the precriterial sessions of reversal training. This persistent ‘original habit effect’ occurred only at brief latencies (20–30 msec) after CS onset, and only in the deep cortical laminae (V–VI). The neuronal activity of the superficial laminae (I–IV) in non-overtrained subjects underwent a transition, in parallel with the behavior, from a discriminative response appropriate to the original task to one appropriate to the reversal task. No significant training-related changes were seen at any cortical depth in the overtrained subjects. Presentation of non-contingent footshocks during two sessions of reversal training following criterion re-enhanced the overall reactivity and the brief-latency discriminative neuronal response appropriate to the original task, which had undergone decline during reversal training, in the deep cortical laminae of the non-overtrained subjects. The enhanced overall reactivity and the discriminative activity in the superficial laminae of non-overtrained subjects had not declined during reversal training and were not altered by the non-contingent footshocks. The implications of these data are discussed in regard to the neural causation of the ORE.     

Early integrative processes physiologically observed in dentate gyrus during an olfactory associative training in rat

Modifications of synaptic efficacy in the dentate gyrus were investigated during an olfactory associative task. A group of rats was trained to discriminate between a patterned electrical stimulation of the lateral olfactory tract, used as an artificial cue, associated with a water reward, and a natural odor associated with a flash of light. Monosynaptic field potential responses evoked by single electrical stimuli to the lateral perforant path were recorded in the granular layer of the ipsilateral dentate gyrus prior to and just after each training session. An early increase in this response was observed just after the first learning session but disappeared 24 hours later. Inversely, a synaptic depression developed across sessions, becoming significant at the onset of a last (fifth) session. When a group of naive animals was pseudo-conditioned, no increase was observed and the synaptic depression was noted since the onset of the second session. In a group of rats similarly trained for only one session, and in which EPSPs were recorded throughout the 24 hours that followed, it was demonstrated that the increase lasted at least two hours, while the significant synaptic depression started after the fourth hour. These results are consistent with the early involvement of the dentate gyrus in learning the association between the cues and their respective rewards. These early integrative processes physiologically observed in dentate gyrus suggest early hippocampal processing before dentate gyrus reactivation via entorhinal cortex which will allow long-term memory storage in cortical areas once the meaning of the olfactory cues is learned.     

ERP evidence for distinct mechanisms of fast and slow visual perceptual learning

Perceptual learning (PL) occurs not only within the first training session but also between sessions. Once acquired, the learning effects can last for a long time. By examining the time course of learning-associated ERP changes, this study explores whether fast and slow visual PL contribute to long-term preservation. Subjects first participated in a visual task for three training sessions, and were then given one test session six months later. ERP results showed that fast learning effects, as reflected by the decrement of posterior N1 and increment of posterior P2 within session 1, were preserved in session 3 but not in the test session. However, slow learning effects, as reflected by the increment of posterior N1 and decrement of frontal P170 between sessions 1 and 3, were retained completely in the test session. This study indicates that PL induces different changes in the human adult brain during and after active training, and only the delayed changes of brain activity can be preserved for a long period of six months.     

Deficits in avoidance responding after paradoxical sleep deprivation are not associated with altered [3H]pirenzepine binding to M1 muscarinic receptors in rat brain

Previous work had indicated that animals that were sleep-deprived and then trained on a passive avoidance task show poor retention of the task 24 h later after being allowed to sleep freely again. Cholinergic involvement is suggested by the fact that this effect is prevented by treatment with the muscarinic agonist pilocarpine during sleep deprivation. The observation that similar deficits are observed in non-deprived rats after treatment with M1-selective antagonist compounds such as dicyclomine or pirenzepine cause similar impairments, and gave rise to the hypothesis that sleep deprivation might induce significant reductions in M1 binding in brain areas involved in learning and memory processes. Rats were deprived of sleep for 96 h and then either immediately killed, or allowed to recover sleep for 24 h before being killed. [3H]pirenzepine binding to M1 sites was examined by quantitative autoradiography in 39 different brain areas in cage controls, sleep-deprived and sleep-recovered animals (N=8 per group). No significant differences among groups were found in any brain region. A separate group of animals was subjected to the sleep deprivation procedure and then trained in a simple avoidance task. Animals were then allowed to sleep and retested 24 h later. This group showed a significant impairment in the avoidance task compared to cage controls, in agreement with previous observations. These data suggest that proactive learning/memory deficits induced by sleep deprivation cannot be attributed to altered M1 binding either immediately after deprivation (when avoidance training occurs) or after sleep has recovered (when acquisition/retention are tested). The possibility remains that alterations in M1 function occur at post-membrane second messenger systems.     

Involvement of the lateral entorhinal cortex for the formation of cross‐modal olfactory‐tactile associations in the rat

While the olfactory and tactile vibrissal systems have been extensively studied in the rat, the neural basis of these cross‐modal associations is still elusive. Here we tested the hypothesis that the lateral entorhinal cortex (LEC) could be particularly involved. In order to tackle this question, we have developed a new behavioral paradigm which consists in finding one baited cup (+) among three, each of the cups presenting a different and specific odor/texture (OT) combination. During the acquisition of a first task (Task OT1), the three cups were associated with the following OT combination: O1T1 for the baited cup; O2T1 and O1T2 for non‐baited ones. Most rats learn this task within three training sessions (20 trials/session). In a second task (Task OT2) animals had to pair another OT combination with the reward using a new set of stimuli (O3T3+, O4T3, and O3T4). Results showed that rats manage to learn Task OT2 within one session only. In a third task (Task OT3) animals had to learn another OT combination based on previously learned items (e.g. O4T4+, O1T4 and O4T1). This task is called the “recombination task.” Results showed that control rats solve the recombination task within one session. Animals bilaterally implanted with cannulae in the LEC were microinfused with d‐APV (3 µg/0.6 µL) just before the acquisition or the test session of each task. The results showed that NMDA receptor blockade in LEC did not affect recall of Task OT1 but strongly impaired acquisition of both Task OT2 and OT3. Moreover, two control groups of animals infused with d‐APV showed no deficit in the acquisition of unimodal olfactory and tactile tasks. Taken together, these data show that the NMDA system in the LEC is involved in the acquisition of association between an olfactory and a tactile stimulus during cross‐modal learning task. © 2014 Wiley Periodicals, Inc.     

Effects of ventral tegmental area stimulation on the acquisition and long-term retention of active avoidance learning

The development of avoidance learning depends on dopamine release in forebrain regions. Previous studies indicated that rewarding brain stimulation facilitated two-way active avoidance learning. However, it is not clear whether the temporal relationship of brain stimulation to the training session (before, during or after) is important. To investigate the role of stimulation condition (no stimulation, self-stimulation only, or self-stimulation plus avoidance stimulation) and sequence of self-stimulation training (before or after avoidance training), we used a 3 × 2 factorial design, in which every level of stimulation was paired with every level of sequence for a total of 6 different groups. The results suggest that self-stimulation either before or after avoidance learning improved acquisition performance, but acquisition was maximal when stimulation was also given during acquisition trials. Importantly, the sequence of self-stimulation (before or after each acquisition session) was irrelevant to this beneficial effect. However, stimulation had no apparent effect on long-term retention when tested 10 days later under conditions of no stimulation, except that the performance of the group that had previously received avoidance-contingent stimulation deteriorated over the course of 60 trials. This may reflect frustration from the omission of expected reward. These results are relevant for optimizing brain stimulation to improve learning.     

Influence of dorsal rhizotomy on a sustained avoidance response in rats

Sustained avoidance responding was studied in each hind limb of rats before and after unilateral dorsal rhizotomy (T13 through L6, left). The subjects were required to make a change in the resting posture of the unseen hind limb, and to maintain the change, to avoid shock to a sensate area of the trunk. Two dichotomous training variables were manipulated: the presence or absence of preoperative training and the order of training of the two hind limbs. Although the animals rapidly reached criterion with sensate limbs before and after surgery, acquisition with dorsal rhizotomized (DR) limbs failed to occur except when the order of postoperative training was sensate limb:DR limb. Even then, acquisition was significantly slower with DR than intact limbs, and the topography of the avoidance response differed in the two cases. It appeared that response-related changes in body parts immediately surrounding the DR limb were not critical for successful performance, and when there was failure of acquisition, the number of training sessions was not a relevant variable. It was concluded that tonic movements cannot be acquired in the absence of peripheral topographic feedback from the responding body part, unless information concerning the general response requirement is first provided to the rat via training with a sensate limb.