Bernard Soumireu-Mourat

Effects of posttrial hippocampal stimulation on acquisition of operant behavior in the mouse

One-hundred male mice were used of which 70 were bilaterally implanted in dorsal hippocampus. Three experiments were performed: (1) The threshold of electrical seizure was measured by bilateral stimulation of hippocampus. (2) The animals were introduced in a Skinner box and allowed to press 8 times on a lever. Each press was reinforced by a food pellet. Thirty seconds after the eighth press, the two hippocampi were stimulated for 80 sec. The intensity of current was half of the threshold determined in the first experiment. Mice receiving hippocampal stimulation showed better retention of learning than not implanted and nonstimulated mice. (3) In the third experiment the temporal gradient of hippocampal stimulation was estimated; after a 600-sec delayed stimulation no improvement of performances was observed.

Time-dependent improvement of performance on appetitive tasks in mice

Three experiments were carried out to study improvement of performance with time on appetitive tasks in BALB/c mice. Experiments 1 and 2 showed that a 24-hr interval between learning sessions significantly improves performance. It seems that there was a curvilinear relationship between this improvement and the 1st learning session duration. Experiment 3 showed that improvement is time-dependent and occured between 1 and 12 hr after the end of the learning session. These results confirm the hypothesis according to which the consolidation phase should be more an elaborative process than a simple fixing.

Effects of seizure and subseizure posttrial hippocampal stimulation on appetitive operant behavior in mice.

Ninety-three BALB/c male mice were implanted bilaterally in the dorsal hippocampus. For each animal, the individual afterdischarge threshold intensity was determined for a 4-sec bilateral hippocampal stimulation train. Several days later, all animals underwent an initial 15 min session in a continuous reinforcement appetitive task (Skinner box). In Expt 1, all animals except a control group were stimulated 30 sec after the learning session either at half of the individual afterdischarge threshold value for 80 sec, or at the after-discharge threshold value for 4 or 80 sec. A 30-min second session took place 24 hr later. In the control group, an improvement of performance, the reminiscence effect, was found. In the subseizure stimulated group, the observed reminiscence effect was higher than that of the control group. Conversely, with the two durations of seizure stimulation, the reminiscence effect was erased. In Expt 2, the same learning paradigm was used, but the animals were stimulated at their individual afterdischarge threshold value at different times after the end of the first session (from 30 sec to 180 min). During the second session, 24 hr later, the impairment of the reminiscence effect for the different groups decreased as the learning-stimulation delay was increased. Thus, the effects of posttrial hippocampal stimulation on learning were studied. The duration of the long temporal amnesia gradient of seizure stimulation and of the shorter facilitatory gradient previously described at a half threshold value were discussed. The gradient duration in the first case could be related to the spread of afterdischarges from the hippocampus to other parts of the brain.

Behavioral effects of bilateral entorhinal cortex lesions in the balb/c mouse.

Balb/c mice with small bilateral electrolytic lesions of the entorhinal cortex were compared with both sham and nonoperated subjects in spontaneous and learned behaviors. The operated mice were hyperactive in an open field 1 week but not 6 weeks after surgery; locomotor activity was slightly increased during the night hours 2–3 weeks after lesion. Learning was tested between 2 and 6 weeks postsurgery. Retention of a CRF appetitive operant conditioning task was improved after the lesion, as well as retention of a one-way active avoidance task. In contrast, operated animals were disturbed in an operant task using a discriminative schedule and in passive avoidance conditioning. The observed increase in activity was not sufficient to explain the findings. Considering the connections of the entorhinal cortex with the hippocampal system, we suggest some kind of inhibitory function for this cortex. Because the entorhinal cortex provides multimodal sensory information to the hippocampus, it could be involved in postlearning modulation of information and thus in memory processes.

Intraventricular corticosterone injection facilitates memory of an appetitive discriminative task in mice.

The immediate post-training injection of corticosterone (1.0 μg) into the cerebral ventricles did not affect memory consolidation of a continuously reinforced operant conditioning task. Using a discriminative operant conditioning procedure with the same Skinner box, this treatment facilitated retention. These results suggest that memory consolidation of adaptive behaviors can be modulated through interaction between central nervous structures and the pituitary—adrenal axis.

Relationship between paradoxical sleep and time-dependent improvement of performance in BALB/c mice.

Abstract In order to study the relationship between the reminiscence phenomenon and the post-learning paradoxical sleep (PS), 13 BALB/c mice were submitted to 3 daily 15-min sessions in an operant CRF-conditioning procedure and sleep was measured for 6 h following each conditioning session. The results show a PS increase after the first and second learning sessions. Moreover, the temporal characteristics of the PS increases seem to be related to the kinetics of reminiscence: both phenomena appear about 3 h after learning. PS returns to the reference level after the learning curve asymptote.

Changes in hippocampal cholinergic activity following learning in mice

A short bar-press operant conditioning acquisition session with food reward on continuous reinforcement was shown to induce a decrease (13.5%) of hippocampal choline acetyltransferase activity in mice. Such an effect seems to be specific to this kind of learning since no change was observed in several control groups, including a group of mice submitted to another type of conditioning in the same apparatus. It is suggested that these enzymatic modifications might be responsible for the delayed improvement of performance observed on retention of this task.

Deep hypothermia in mice: Effect of the interval between learning and treatment

Abstract Mice received deep hypothermia at different intervals after the partial acquisition in a Skinner box and were tested 24 hr later. The deficit of performances was related to the time interval between acquisition and minimum temperature and no deficit was found when this interval reaches 3 hr. Consolidation duration and memory processes are discussed.

Hypothermie profonde chez la souris: Perturbations de l'activité electrique centrale et d'un apprentissage d'approche ☆

Resume Afin de preciser le role de lactivite electrique cerebrale dans les mecanismes dapprentissage, deux experiences ont ete entreprises. Dans lexperience 1, les effets de lhypothermie generale sont etudies sur lelectrocorticogramme et sur lactivite multiunitaire: ces deux activites disparaissent a environ 7°C (temperature corporelle) et evoluent dune facon parallele tout au long de lhypothermie. Dans lexperience 2 une acquisition partielle ou une retention en boite de Skinner sont suivies dune hypothermie a 3–4°C. Le controle est realise 24 hr plus tard en extinction. Lhypothermie entraine un deficit apres acquisition partielle, mais nen provoque pas apres retention. Dans les deux groupes, le niveau dactivite et la consommation alimentaire des animaux ne sont pas modifies. Ces resultats tendent a montrer que lhypothermie agit sur les processus de consolidation et les mecanismes possibles sont discutes.