Impairment of two-way active avoidance after pedunculopontine tegmental nucleus lesions: effects of conditioned stimulus duration.

It was investigated whether the disruptive effects of bilateral lesions of the pedunculopontine tegmental nucleus on two-way active avoidance might vary depending on variations of task demand. The animals were either subjected to bilateral electrolytic lesions of the pedunculopontine tegmental nucleus (Lesion groups) or were sham-operated (Control groups). All the rats were subjected to two 30-trial sessions of two-way active avoidance (separated by ten days), using either a 10-s conditioned stimulus (low task demand) or a 3-s conditioned stimulus (high task demand). The lesions induced a significant disruption of two-way active avoidance in the two conditions tested, but, in both lesioned and control rats, the number of avoidance responses was higher when the 10-s conditioned stimulus was used. In lesioned animals, the condition of high task demand was associated with a significant increase of escape failures. Lesions did not affect locomotor activity during the period of adaptation to the conditioning apparatus, but induced training-specific motor deficits (a decrease of intertrial crossings and an enhancement of escape latencies) regardless of the specific training conditions used. The results are discussed in terms of the influences of the pedunculopontine tegmental nucleus in thalamocortical and striatal systems.

The parafascicular nucleus and two-way active avoidance: effects of electrical stimulation and electrode implantation.

To evaluate whether electrical stimulation of the parafascicular nucleus (PF) can improve short-term (24 h) and/or long-term (21 days) retention of two-way active avoidance, rats were implanted with an electrode at this nucleus (experimental groups) or above it (control groups). After a single 30-trial acquisition session, experimental groups were submitted to a 10-min session of electrical stimulation. Results showed that the simple implantation of an electrode at the posterior PF enhanced by itself the acquisition of two-way active avoidance, in such a way that the subsequent stimulation of this region may have been unable to further improve the performance of the rats. On the other hand, parafascicular stimulation improved the 24-h retention of the task in a site-specific way, since this effect was mainly seen after stimulation of the central PF region. The facilitative effect on 24-h retention could also depend on the level of performance achieved during the acquisition session, because this improvement was only evidenced in poorly learning animals. No effects were found on 21-day retention. The present results confirm the involvement of the PF in learning and memory and the functional heterogeneity of this nucleus.

Facilitatory effects of thalamic reticular nucleus lesions on two-way active avoidance in rats.

Two experiments were performed in order to study the effects of lesions of the rostral thalamic reticular nucleus (Rt) on two-way active avoidance. Male Wistar rats were subjected to either a bilateral electrolytical lesion of the rostral Rt or to control procedures. After recovery, all rats were trained in either a distributed (five training sessions, ten trials each; experiment I) or a massed (a single 30-trials session; experiment II) two-way, active-avoidance task. The level of long-term retention of the task was assessed 10 days later. Lesioned rats showed an overall higher performance than control rats both in experiment I (with lesions affecting the rostral Rt and small portions of some adjacent nuclei) and in experiment II (with lesions almost restricted to the rostral Rt). In contrast, detrimental effects on other tasks have been reported in the literature. Although it cannot be ruled out that those differences might be due to methodological factors, they also might be indicative of an action of rostral Rt lesions on certain mechanisms (either indirectly or directly related to information processing) that could be differentially required depending on the kind of learning task. The latter possibility is discussed in terms of the role played by this nucleus as a modulator of thalamocortical transmission, attentional mechanisms and cortical arousal.

Effects of habenular lesions upon two-way active avoidance conditioning in rats.

To evaluate if habenular nuclei lesions improve, impair, or have no effects on two-way active avoidance acquisition and/or retention, rats in a Lesion group were subjected to bilateral electrolytical lesions of this complex, while control rats were sham-operated (Sham group). Once recovered from the stereotaxic procedures, rats were submitted to 5 training sessions (10 trials each, one session per day) of two-way active avoidance conditioning. Ten days after the last training session, another session was administered in order to test the long-term retention of the task. Results indicated that habenular lesions did not affect the overall performance of the rats during either the acquisition sessions or the retention session of two-way active avoidance. We suggest that habenular lesions can affect the acquisition of several learning tasks, probably through their role in modulating stress responses and/or arousal states. The nature of these effects (whether facilitative, detrimental, or neutral) might depend on the interaction between several factors such as the kind of task, the specific conditioning procedures (which may generate different stress levels), and the specific area destroyed by the lesion.

Effects of pretraining paradoxical sleep deprivation upon two-way active avoidance.

In order to study whether paradoxical sleep (PS) is necessary to prepare subjects for the subsequent learning of a distributed two-way active avoidance conditioning, 10 rats were subjected to 5 h of paradoxical sleep deprivation (PSD group) by means of the platform method immediately before each of 5 acquisition sessions (one daily), as well as before a long-term retention (LTR) session (14 days). Another group of rats (PSD control group; n = 10) were placed on large platforms as a control for the side effects induced by PSD platforms. Rats in the dry control group (n = 10) did not receive any treatment. The number of avoidances of the PSD group was significantly lower on the 1st, 2nd and 3rd acquisition sessions compared to the PSD control group, and on the 2nd and 3rd sessions compared to the dry control group. PSD rats made significantly less intertrial crossings than dry controls on the 2nd and 3rd acquisition sessions, but no significant correlations were found between this variable and the number of avoidances. Therefore, our results are not fully in contradiction with the hypothesis that PS previous to the training sessions might prepare the animal for subsequent learning, although the influence of locomotor changes upon the performance of PSD subjects cannot be fully rejected.

Facilitation of shuttle-box avoidance by the platform method: effects of conditioned stimulus duration.

To evaluate whether the duration of the conditioned stimulus (CS) influences the facilitatory effect of posttraining platform treatment upon the acquisition and long-term retention (LTR) of a shuttle-box conditioning, rats were assigned to one of the four following treatments: P-3 group rats were subjected to a 5-h treatment on 16 cm diameter platforms immediately after each of 5 training sessions (10 trials each separated by 24-h intervals) in which the CS consisted of a tone lasting 3 s; control-3 rats were trained the same way but were not subjected to the platform treatment; P-10 rats did also receive the immediate 5-h treatment on platforms, but the CS was a tone lasting 10 s; finally, control-10 rats did not receive any treatment and were also trained with a 10-s tone. Ten days after training, all rats were also tested for LTR (1 session of 10 trials). When the CS duration was 3 s, the platform treatment improved both the acquisition and LTR of the task, compared to control subjects, but the same treatment had no effect when the CS lasted 10 s. With the use of a 10-s CS, the level of learning achieved by both treated and untreated subjects was similar to the final level of acquisition reached by treated subjects trained with a 3-s CS. We conclude that the facilitatory effects of the platform method treatment upon the acquisition and LTR of a distributed shuttle-box avoidance depend on the difficulty of the task.

Facilitation of shuttle-box avoidance by the platform method: temporal effects.

Two experiments were carried out in order to 1) replicate a previous finding according to which the treatment on large platforms (commonly used as control for the stress induced by smaller paradoxical sleep deprivation platforms) can facilitate the acquisition and long-term retention (LTR) of a distributed shuttle-box avoidance in rats, and 2) further examine the temporal conditions in which that facilitation can be observed. The results showed that an immediate posttraining treatment lasting 6 hours induced a significant improvement of acquisition both when applied in the light (8 a.m.) and in the dark cycle (8 p.m.), while the LTR (10 and 31 days) seemed to be better preserved when the treatment was applied during the dark cycle. A shorter treatment (3 h) had no effect upon shuttle-box avoidance, regardless of whether it was applied in the dark or in the light cycle and whether it was immediate or delayed for 3 h. In summary, under certain temporal conditions, a posttraining immediate treatment on large platforms can facilitate the acquisition and/or the LTR of shuttle-box avoidance. Stress hormones and/or the enhancement of CNS arousal are suggested to be some of the mechanisms operating in this facilitatory effect.

[Short-term effects of the isolated platform on corticosterone plasma level]. / Efectos a corto plazo del método de la plataforma aislada sobre el nivel de corticosterona plasmática en ratas.

Plasma corticosterone was measured, in rats, after a 30 min, treatment either on small platforms used to induce paradoxical sleep deprivation, on large platforms utilized as control or in home cages ("Dry Control"group). The subjects on both platforms showed similar plasma corticosterone levels, those levels being significantly higher than the ones shown by "Dry Controls". It is concluded that the use of large platforms as control of the stress induced by the small platforms in those experiments aimed at determinating the relationship between short-term post-training paradoxical sleep deprivation and memory consolidation allow to discriminate the specifical effects induced by paradoxical sleep deprivation from the putative modulatory effects of learning exerted by the stress induced by the method.

Behavioral evaluation of the stress induced by the platform method for short-term paradoxical sleep deprivation in rats.

To evaluate whether the results of short-term PSD (Paradoxical Sleep Deprivation) using the platform method can be influenced by stress, changes in body weight and in behavioral indices (food and water intake and ambulation and defecation in an open field) were measured in rats after each of four 5-hour sessions of confinement to small or large platforms. The animals of the two platform groups when compared to animals kept in home cages showed a similar decrease in body weight which was significant only after the first day of treatment, while no changes in the other measures were observed. It is concluded that 1) the effects of stress induced by short-term confinement to platforms do not seem to be a remarkable confounding factor in short-term PSD studies and 2) large platforms can be used both as an adequate stress control for small platforms and as a means of adapting the animals to the method.