Intracranial Self-Stimulation and Memory in Rats: A Sistematic Review.

BACKGROUND: Intracranial self-stimulation (ICSS) is a technique by which rats press a lever to stimulate their brains through an electrode chronically implanted in brain reward areas. Currently only two laboratories in the world, one in India and one in Spain, are intensively studying the effect of this kind of deep brain stimulation on learning and memory. This paper will present the main findings. METHODS: Different groups of young and old healthy and brain-damaged rats with electrodes implanted in the medial forebrain bundle received a treatment of ICSS after being trained in several paradigms of implicit and explicit learning. Memory was tested over short and long-term periods. Structural and molecular post-mortem analyses of their brains were examined in relation to memory results. RESULTS: ICSS enhances implicit and explicit memory, especially in animals showing poor performance in the learning tasks, such as brain-damaged subjects. At the structural and molecular level, ICSS enhances size and dendritic arborization and promotes neurogenesis in specific hippocampal areas. ICSS also regulates the expression of genes related to learning and memory. CONCLUSIONS: Through activating reward and neural plasticity mechanisms, ICSS in the medial forebrain bundle is a promising technique for memory-enhancing treatments.

Protocol to assess rewarding brain stimulation as a learning and memory modulating treatment: Comparison between self-administration and experimenter-administration.

Intracranial electrical self-stimulation (ICSS) is a useful procedure in animal research. This form of administration ensures that areas of the brain reward system (BRS) are being functionally activated, since the animals must perform an operant response to self-administer an electrical stimulus. Rewarding post-training ICSS of the medial forebrain bundle (MFB), an important system of the BRS, has been shown to consistently improve rats' acquisition and retention in several learning tasks. In the clinical setting, deep brain stimulation (DBS) of different targets is currently being used to palliate the memory impairment that occurs in some neurodegenerative diseases. However, the stimulation of the MFB has only been used to treat emotional alterations, not memory disorders. Since DBS stimulation treatments in humans are exclusively administered by external sources, studies comparing the efficacy of that form of application to a self-administered stimulation are key to the translationality of ICSS. This protocol compares self-administered (ICSS) and experimenter-administered (EAS) stimulation of the MFB on the spatial Morris Water Maze task (MWM). c-Fos immunohistochemistry procedure was carried out to evaluate neural activation after retention. Results show that the stimulation of the MFB improves the MWM task regardless of the form of administration, although some differences in c-Fos expression were found. Present results suggest that MFB-ICSS is a valid animal model to study the effects of MFB electrical stimulation on memory, which could guide clinical applications of DBS. The present protocol is a useful guide for establishing ICSS behavior in rats, which could be used as a learning and memory-modulating treatment.

Intracranial Self-Stimulation and Memory in Rats: A Sistematic Review / Autoestimulación Eléctrica Intracraneal y Memoria en Ratas: una Revisión Sistemática

Background: Intracranial self-stimulation (ICSS) is a technique by which rats press a lever to stimulate their brains through an electrode chronically implanted in brain reward areas. Currently only two laboratories in the world, one in India and one in Spain, are intensively studying the effect of this kind of deep brain stimulation on learning and memory. This paper will present the main findings. Methods: Different groups of young and old healthy and brain-damaged rats with electrodes implanted in the medial forebrain bundle received a treatment of ICSS after being trained in several paradigms of implicit and explicit learning. Memory was tested over short and long-term periods. Structural and molecular post-mortem analyses of their brains were examined in relation to memory results. Results: ICSS enhances implicit and explicit memory, especially in animals showing poor performance in the learning tasks, such as brain-damaged subjects. At the structural and molecular level, ICSS enhances size and dendritic arborization and promotes neurogenesis in specific hippocampal areas. ICSS also regulates the expression of genes related to learning and memory. Conclusions: Through activating reward and neural plasticity mechanisms, ICSS in the medial forebrain bundle is a promising technique for memory-enhancing treatments.(AU)

Antecedentes: La autoestimulación eléctrica intracraneal (AEIC) es un tipo de estimulación cerebral profunda autoadministrada a través de un electrodo implantado de forma crónica en áreas cerebrales de la recompensa. Actualmente, dos laboratorios en el mundo, uno en India y otro en España, están estudiando intensivamente el efecto de este tipo de estimulación cerebral reforzante sobre el aprendizaje y la memoria. Aquí se presentan los principales hallazgos. Métodos: Diferentes grupos de ratas sanas y con daño cerebral, jóvenes y viejas, con electrodos implantados en el haz prosencefálico medial recibieron un tratamiento de AEIC después de ser entrenados en diferentes paradigmas de aprendizaje. La memoria se evaluó a corto y largo plazo. Resultados: La AEIC mejora la memoria implícita y explícita, especialmente en animales con un bajo rendimiento o con daño cerebral. A nivel estructural y molecular, la AEIC estimula del desarrollo de la arborización dendrítica, promueve la neurogénesis en el hipocampo y regula la expresión de genes relacionados con plasticidad, aprendizaje y memoria. Conclusiones: La AEIC en el haz prosencefálico medial, al activar mecanismos de recompensa y de plasticidad neural, constituye un tratamiento prometedor para la mejora de la memoria.(AU)

Rewarding deep brain stimulation at the medial forebrain bundle favours avoidance conditioned response in a remote memory test, hinders extinction and increases neurogenesis.

Intracranial Self-Stimulation (ICSS) at the medial forebrain bundle consistently facilitates learning and memory in rats when administered post-training or when administered non-concurrent to training, but its scope regarding remote memory has not yet been studied. The present work aims to test whether the combination of these two forms of ICSS administration can cause a greater persistence of the facilitating effect on remote retention and affect neurogenesis in the dentate gyrus (DG) of the hippocampus. Rats were trained in active avoidance conditioning and tested in two retention sessions (10 and 90 days) and later extinction. Subjects received an ICSS session after each of the five avoidance acquisition sessions (post-training treatment) and half of them also received ten additional ICSS sessions during the rest period between retention tests (non-concurrent treatment). All the stimulated groups showed a higher performance in acquisition and retention sessions, but only the rats receiving both ICSS treatments showed greater resistance to extinction. Remarkably, at seven months, rats receiving the non-concurrent ICSS treatment had a greater number of DCX-positive cells in the DG as well as a higher amount of new-born cells within the granular layer compared to rats that did not receive this additional ICSS treatment. Our present findings significantly extend the temporal window of the facilitating effect of ICSS on active avoidance and demonstrate a neurogenic effect of rewarding medial forebrain bundle stimulation.

Arc protein expression after unilateral intracranial self-stimulation of the medial forebrain bundle is upregulated in specific nuclei of memory-related areas.

BACKGROUND: Intracranial Self-Stimulation (ICSS) of the medial forebrain bundle (MFB) is a deep brain stimulation procedure, which has a powerful enhancement effect on explicit and implicit memory. However, the downstream synaptic plasticity events of MFB-ICSS in memory related areas have not been described thoroughly. This study complements previous work studying the effect of MFB-ICSS on the expression of the activity-regulated cytoskeleton-associated (Arc) protein, which has been widely established as a synaptic plasticity marker. We provide new integrated measurements from memory related regions and take possible regional hemispheric differences into consideration. RESULTS: Arc protein expression levels were analyzed 4.5 h after MFB-ICSS by immunohistochemistry in the hippocampus, habenula, and memory related amygdalar and thalamic nuclei, in both the ipsilateral and contralateral hemispheres to the stimulating electrode location. MFB-ICSS was performed using the same paradigm which has previously been shown to facilitate memory. Our findings illustrate that MFB-ICSS upregulates the expression of Arc protein in the oriens and radiatum layers of ipsilateral CA1 and contralateral CA3 hippocampal regions; the hilus bilaterally, the lateral amygdala and dorsolateral thalamic areas as well as the central medial thalamic nucleus. In contrast, the central amygdala, mediodorsal and paraventricular thalamic nuclei, and the habenular complex did not show changes in Arc expression after MFB-ICSS. CONCLUSIONS: Our results expand our knowledge of which specific memory related areas MFB-ICSS activates and, motivates the definition of three functionally separate groups according to their Arc-related synaptic plasticity response: (1) the hippocampus and dorsolateral thalamic area, (2) the central medial thalamic area and (3) the lateral amygdala.

Intracranial self-stimulation also facilitates learning in a visual discrimination task in the Morris water maze in rats.

Intracranial self-Stimulation (ICSS) of the medial forebrain bundle is a treatment capable of consistently facilitating acquisition of learning and memory in a wide array of experimental paradigms in rats. However, the evidence supporting this effect on implicit memory comes mainly from classical conditioning and avoidance tasks. The present work aims to determine whether ICSS would also improve the performance of rats in another type of implicit task such as cued simultaneous visual discrimination in the Morris Water Maze. The ICSS treatment was administered immediately after each of the five acquisition sessions and its effects on retention and reversal were evaluated 72h later. Results showed that ICSS subjects committed fewer errors than Sham subjects and adopted more accurate trajectories during the acquisition of the task. This improvement was maintained until the probe test at 72h. However, ICSS animals experienced more difficulties than the Sham group during the reversal of the same learning, reflecting an impairment in cognitive flexibility. We conclude that post-training ICSS could also be an effective treatment for improving implicit visual discrimination learning and memory.

Increase in c-Fos and Arc protein in retrosplenial cortex after memory-improving lateral hypothalamic electrical stimulation treatment.

Post-training Intracranial self-stimulation (ICSS) of the lateral hypothalamus (LH), a kind of rewarding deep-brain stimulation, potentiates learning and memory and increases c-Fos protein expression in specific memory-related brain regions. In a previous study, Aldavert-Vera et al. (2013) reported that post-acquisition LH-ICSS improved 48 h retention of a delay two-way active avoidance conditioning (TWAA) and induced c-Fos expression increase in CA3 at 90 min after administration. Nevertheless, this c-Fos induction was only observed after the acquisition session and not after the retention test at 48 h, when the ICSS improving effect was observed on memory. This current study aims to examine the hypothesis that post-training ICSS treatment may stimulate c-Fos expression at the time of the TWAA retention test in retrosplenial cortex (RSC), a hippocampus-related brain region more closely related with long-lasting memory storage. Effects of ICSS on Arc protein, a marker of memory-associated synaptic plasticity, were also measured by immunohistochemistry in granular and agranular RSC. The most innovative results are that the ICSS treatment potentiates the c-Fos induction across TWAA conditions (no conditioning, acquisition and retention), specifically in layer V of the granular RSC, along with increases of Arc protein levels in the granular but not in agranular areas of RSC ipsilaterally few hours after ICSS. This leads us to suggest that plasticity-related protein activation in the granular RSC could be involved in the positive modulatory effects of ICSS on TWAA memory consolidation, opening a new approach for future research in ICSS memory facilitation.

Structural plasticity in hippocampal cells related to the facilitative effect of intracranial self-stimulation on a spatial memory task.

Posttraining intracranial self-stimulation (SS) in the lateral hypothalamus facilitates the acquisition and retention of several implicit and explicit memory tasks. Here, intracellular injections of Lucifer yellow were used to assess morphological changes in hippocampal neurons that might be specifically related to the facilitative posttraining SS effect upon the acquisition and retention of a distributed spatial task in the Morris water maze. We examined the structure, size and branching complexity of cornus ammonis 1 (CA1) cells, and the spine density of CA1 pyramidal neurons and granular cells of the dentate gyrus (DG). Animals that received SS after each acquisition session performed faster and better than Sham ones--an improvement that was also evident in a probe trial 3 days after the last training session. The neuromorphological analysis revealed an increment in the size and branching complexity in apical CA1 dendritic arborization in SS-treated subjects as compared with Sham animals. Furthermore, increased spine density was observed in the CA1 field in SS animals, whereas no effects were observed in DG cells. Our results support the hypothesis that the facilitating effect of SS on the acquisition and retention of a spatial memory task could be related to structural plasticity in CA1 hippocampal cells.

Rewarding brain stimulation reverses the disruptive effect of amygdala damage on emotional learning.

Intracranial self-stimulation (SS) in the lateral hypothalamus, a rewarding deep-brain stimulation, is able to improve acquisition and retention of implicit and explicit memory tasks in rats. SS treatment is also able to reverse cognitive deficits associated with aging or with experimental brain injuries and evaluated in a two-way active avoidance (2wAA) task. The main objective of the present study was to explore the potential of the SS treatment to reverse the complete learning and memory impairment caused by bilateral lesion in the lateral amygdala (LA). The effects of post-training SS, administered after each acquisition session, were evaluated on distributed 2wAA acquisition and 10-day retention in rats with electrolytic bilateral LA lesions. SS effect in acetylcholinestaresase (AchE) activity was evaluated by immunohistochemistry in LA-preserved and Central nuclei (Ce) of the amygdala of LA-damaged rats. Results showed that LA lesion over 40% completely impeded 2wAA acquisition and retention. Post-training SS in the LA-lesioned rats improved conditioning and retention compared with both the lesioned but non-SS treated and the non-lesioned control rats. SS treatment also seemed to induce a decrease in AchE activity in the LA-preserved area of the lesioned rats, but no effects were observed in the Ce. This empirical evidence supports the idea that self-administered rewarding stimulation is able to completely counteract the 2wAA acquisition and retention deficits induced by LA lesion. Cholinergic mechanisms in preserved LA and the contribution of other brain memory-related areas activated by SS could mediate the compensatory effect observed.

Intracranial self-stimulation facilitates active-avoidance retention and induces expression of c-Fos and Nurr1 in rat brain memory systems.

Intracranial self-stimulation (ICSS), a special form of deep brain stimulation in which subjects self-administered electrical stimulation in brain reward areas as the lateral hypothalamus, facilitates learning and memory in a wide variety of tasks. Assuming that ICSS improves learning and memory increasing the activation of memory-related brain areas, the present work examined whether rats receiving an ICSS treatment immediately after the acquisition session of a two-way active avoidance conditioning (TWAA) show both an improved retention and a pattern of increased c-Fos and Nurr1 protein expression in the amygdala, hippocampus, dorsal striatum and/or lateral hypothalamus. The response of both activity-induced IEGs to ICSS was examined not only as markers of neural activation, but because of their reported role in the neural plasticity occurring during learning and memory formation. Results showed that the TWAA conditioning alone increased the expression of the two analysed IEGs in several hippocampal areas, and TWAA retention increased Nurr1 expression in amygdala. ICSS treatment increased the number of c-Fos and Nurr1 positive cells in almost all the brain regions studied when it was measured 70min, but not 48h, after the stimulation. Post-training ICSS treatment, as expected, facilitated the 48h retention of the conditioning. It is noteworthy that in CA3 conditioning and ICSS separately increased c-Fos expression, but this increasing was greater when both, conditioning and ICSS, were combined. Present results suggest that rapid and transient increased expression of these two synaptic plasticity and memory related IEGs in some hippocampal areas, such as CA3, could mediate the facilitative effects of ICSS on learning and memory consolidation.

Intracranial self-stimulation recovers learning and memory capacity in basolateral amygdala-damaged rats.

We studied the capacity of post-training intracranial self-stimulation (SS) to reverse or ameliorate learning and memory impairments caused by amygdala damage in rats. A first experiment showed that lesions of the basolateral amygdala (BLA) slow down acquisition of two-way active avoidance conditioning (2wAA). In a second experiment we observed that a post-training SS treatment administered immediately after each 2wAA conditioning session is able to completely reverse the disruptive effects of the BLA lesions, and the facilitative effect lasts for 10days. A third experiment allowed us to differentiate the strong recuperative effects of the SS treatment from the slight effect caused by overtraining the same conditioning response. We concluded that SS is able to counteract the behavioral deficit induced by BLA damage, probably by activating alternative undamaged brain structures related to learning and memory, such as the hippocampus.

Intracranial self-stimulation improves memory consolidation in rats with little training.

Post-training intracranial electrical self-stimulation can improve learning and memory consolidation in rats. However, the molecular mechanisms involved are not known yet. Since previous paradigms of this kind of facilitation are relatively unsuitable to try a molecular approach, here we develop a single and short model of learning and memory facilitation by post-training self-stimulation that could make easier the research of its neural and molecular basis. Thus, three consecutive experiments were carried out to ascertain whether post-training self-stimulation is able to facilitate memory when learning consists of only a brief (5 trials) two-way active avoidance conditioning session. The results of Experiment 1 showed that it is actually possible, and that 48 h after the acquisition session is a very good time to observe the memory improvement. As a way to probe the retroactive effect of self-stimulation, in Experiment 2 we observed that the same self-stimulation treatment given to the subjects not post-training but 48 h before a single two-way active avoidance session does not improve the acquisition of conditioning. In Experiment 3, we showed that the SS facilitative effect observed 48 h after the acquisition session in Experiment 1 was still maintained one week later. We concluded that post-training intracranial self-stimulation can consistently improve memory consolidation even when little acquisition training is given to the animals in a single training session.

Intracranial self-stimulation after memory reactivation: immediate and late effects.

To assess whether intracranial self-stimulation (SS) given after memory reactivation could improve memory retrieval, we tested the immediate (Experiment 1) and late (24 h; Experiment 2) effects of an SS treatment on the retrieval of a two-way active avoidance conditioning in Wistar rats. Memory was reactivated 24 h after training and the reminder (Rm) used consisted of a 3 s exposure to the conditioned stimulus (a tone) in the same context as in the original learning. SS treatment (2500 trains at 100% of each rat's optimal intensity) was administered immediately afterwards. No significant differences between SS-treated and control groups were observed when the retrieval was tested immediately after the SS treatment with or without memory reactivation. However, retrieval was improved when tested 24 h after SS treatment alone or after the reminder exposure alone. The greatest improvement in avoidance was observed when both treatments were given together, that is, when the SS treatment was administered immediately after memory reactivation. Moreover, there were no significant statistical interactions between the effect of SS treatment and the ones of memory reactivation in any of both experiments. The present results show that the effect of an immediate SS treatment can be added to the ones of memory reactivation causing a strong long-term facilitation of memory retrieval.

Effects of posttraining damage to the pedunculopontine tegmental nucleus on conditioned stimulus transfer in two-way active avoidance in rats.

The effects of posttraining excitotoxic lesions of the pedunculopontine tegmental nucleus (PPTg) on two-way active avoidance after changing the conditioned stimulus (CS) used during prelesion training were examined. Prelesion training was carried out with either a tone or a light as the CS, and this CS was changed during postlesion training. Replacing the tone with a light reduced the performance of control and lesioned rats, but the degree of reduction was higher in the latter. Replacing the light with a tone had slight detrimental effects in lesioned rats but not in controls. Thus, posttraining PPTg lesions slowed down the reacquisition of shuttle-box avoidance under conditions of CS transfer, an effect that may be attributable to disruption of attention and/or gating of sensory stimuli.

Effects of pre-training pedunculopontine tegmental nucleus lesions on delayed matching- and non-matching-to-position in a T-maze in rats.

Lesions of the pedunculopontine tegmental nucleus (PPTg) can impair spatial learning tasks, but it is not clear whether those detrimental effects depend on the specific training conditions (for example, number of response choices available) or are secondary to enhanced anxiety. In the present work, rats with either bilateral excitotoxic (ibotenate) lesions of the PPTg (lesion group) or with vehicle infusions (control group) were tested in an elevated plus-maze, in order to measure anxiety-like behaviours and spontaneous locomotion. Subsequently, they were trained in a delayed matching-to-position (DMTP) task in a T-maze (a two-response choice task). After reaching a predefined learning criterion, or after a maximum of 30 training sessions, the animals were trained in a delayed non-matching-to-position (DNMTP) task. Lesioned animals made less grooming episodes, stretch-attend postures and closed arm entries than controls in the elevated plus-maze, suggesting slightly lower anxiety levels. None of the lesioned rats reached the learning criterion for the DMTP, and overall accuracy levels were significantly lower in those rats, compared to controls. In the DNMTP task, lesioned animals showed lower accuracy levels and higher side bias than controls in some of the sessions, but there were no significant differences between the two groups in the proportion of animals reaching the learning criterion. It is concluded that spatial learning deficits induced by damage to the PPTg are not secondary to enhanced anxiety. Instead, those deficits seem to be influenced by several conditions that modify task demands, the number of response choices being only one of such conditions.

Post-training intracranial self-stimulation facilitates a hippocampus-dependent task.

Previous research has shown that post-training intracranial self-stimulation facilitates implicit or procedural memory. To know whether it can also facilitate explicit memory, post-training intracranial self-stimulation was given to Wistar rats immediately after every daily session of a delayed spatial alternation task that seems to depend on the integrity of the hippocampal memory system. We tested the effects of intracranial self-stimulation in three consecutive learning phases which tried to make the task progressively more difficult: 10 s delay (D10 phase), 30 s delay (D30 phase), and inverting the starting position of the animals to make their response more dependent on allocentric cues (INV phase). Every phase finished when each rat achieved a fixed learning criterion. Intracranial self-stimulation facilitated the flexible expression of the learned response (INV phase). That is, when the starting position was randomly inverted, only the rats that received intracranial self-stimulation maintained the performance level acquired in the previous training phases. Changing the starting position reduced the correct performance of the non-treated subjects, which need more training sessions to achieve the learning criterion and made less correct responses than treated rats. These findings show that post-training intracranial self-stimulation can facilitate hippocampus-dependent memories.

Automated sleep staging in rat with a standard spreadsheet.

A new method of automated sleep-wake staging in the rat is described. Hippocampal electroencephalographic (HPC) and nuchal electromyographic signals were recorded by a digital polygraph. The HPC channel was filtered off-line to obtain the original plus theta and delta waves. Statistics of each of these four channels were obtained every 5 s and exported to a standard spreadsheet. The automated staging consisted of five steps: (1) automatic detection of waking, nonrapid eye movement sleep and rapid eye movement sleep patterns (5-s periods); (2) calculation of statistics for each vigilance state; (3) final classification of 5-s periods; (4) construction of a primary 20-s epoch hypnogram; and (5) automatic refinement of the previous hypnogram. The system includes indices about the accuracy of the staging and was validated with five recordings of 23 h each. The global agreement between human and automatic scoring in the validation recordings was 94.32%.

Parafascicular electrical stimulation attenuates nucleus basalis magnocellularis lesion-induced active avoidance retention deficit.

Previous experiments from our laboratory showed that retention of two-way active avoidance learning is improved by post-training intracranial electrical stimulation (ICS) of the parafascicular nucleus (PF) and impaired by pre-training electrolytic lesions of the nucleus basalis magnocellularis (NBM). The question investigated here was whether post-training PF ICS is able to attenuate the active avoidance retention deficit observed in rats lesioned pre-training in the NBM. To this goal, the following experimental design was used: rats bilaterally lesioned in the NBM and stimulated in the PF, rats lesioned in the NBM, rats stimulated in the PF, control rats implanted in the PF, and sham-operated rats were first trained in a shuttle-box for a single 30-trial session and tested again following two successive retention intervals (24 h and 11 days). The results showed that: (1) NBM lesions impaired the 11-day performance without affecting either the acquisition or the 24-h retention of the avoidance learning; (2) PF ICS treatment in unlesioned rats improved performance in both retention sessions only when the stimulation was applied in the posterior region of the nucleus; and (3) stimulation of the posterior PF compensated the 11-day retention impairment induced by NBM lesions. These results are discussed in relation to the interaction of arousal systems in the modulation of cognitive processes.

Improvement of shuttle-box performance by anterodorsal medial septal lesions in rats.

Septal lesions impair a variety of tasks, including inhibitory avoidance and one way active avoidance. In contrast, these lesions improve two-way active avoidance, probably by reducing anxiety. The present work aimed to study whether anterodorsal medial septal lesion (a) improves performance of two-way active avoidance task (Experiment I), as it has been observed with wider septal lesion, and (b) affect anxiety and/or locomotor activity (Experiment II). This precise region was chosen because some evidences suggest that its lesion do not lead to a reduction of anxiety. Lesioned rats tended to make a higher, but statistically non-significant (P=0.074), number of avoidances regardless of the session, being this difference statistically significant on the retention session (RT). The same lesion did not appear to have an anxiolytic effect, and did not affected basal locomotor activity. Different possible explanations of our results are discussed.