NMR Study of Molecular Dynamics in a D ho Columnar Mesophase

In this paper we report the first molecular dynamics study combining fast field-cycling and conven-tional NMR techniques in a thermotropic liquid crystal of discotic molecules exhibiting an ordered columnar hexagonal mesophase. Using the association of these techniques we obtained proton T1 data over a very large domain of Larmor frequencies (ω/2π from 500 Hz to 85 MHz). The proton spin-lattice relaxation results were analysed considering the structure of the mesophase and the types of movements which are expected to influence significantly the relaxation rate, namely local molecu-lar rotational reorientations, inter-columnar self-diffusion and collective movements corresponding to bending and compression of the columns. We verified that these mechanisms dominate the relaxation respectively for high, medium and low Larmor frequencies.

Differential learning-stage dependent patterns of c-Fos protein expression in brain regions during the acquisition and memory consolidation of an operant task in mice

The present study analysed the effects of the stage of learning of an appetitive operant conditioning task on the spatial and temporal patterns of c-Fos protein levels in the brain of BALB/c mice. c-Fos levels were assessed by immunohistochemistry at either 60, 120 or 180 min after either the first, the second or the fifth daily training session and compared to sham animals. The results show an increase of c-Fos-positive nuclei in several subcortical and cortical brain regions, 60-min post-acquisition. Because these activations were a function of task mastery, the data indicate that they were specifically related to learning. Following the first acquisition session, significant increases in c-Fos-positive neurons were observed in the dorsal hippocampus (CA3), anterior cingulate, occipital and parietal cortices. Following the second daily training session, c-Fos was highly expressed in some subcortical regions, the hippocampus, the subiculum, the entorhinal, and posterior cingulate areas. Moreover, a significant correlation was found between the progression of performance from day 1 to day 2 and c-Fos expression on the hippocampal CA1 subfield. Following complete acquisition, no further task-dependent increases in c-Fos-labelled nuclei was observed in any brain region sampled, suggesting that the intervention of c-Fos-induced mechanisms in the consolidation process were terminated. The training stage-dependent changes in regional post-training c-Fos expression in the hippocampus and the connected limbic regions suggest that this neuronal network is actively engaged in memory consolidation processes.

Time-dependent reorganization of brain circuitry underlying long-term memory storage

Retrograde amnesia observed following hippocampal lesions in humans and animals is typically temporally graded, with recent memory being impaired while remote memories remain intact, indicating that the hippocampal formation has a time-limited role in memory storage. However, this claim remains controversial because studies involving hippocampal lesions tell us nothing about the contribution of the hippocampus to memory storage if this region was present at the time of memory retrieval. We therefore used non-invasive functional brain imaging using (14C)2-deoxyglucose uptake to examine how the brain circuitry underlying long-term memory storage is reorganized over time in an intact brain. Regional metabolic activity in the brain was mapped in mice tested at different times for retention of a spatial discrimination task. Here we report that increasing the retention interval from 5 days to 25 days resulted in both decreased hippocampal metabolic activity during retention testing and a loss of correlation between hippocampal metabolic activity and memory performance. Concomitantly, a recruitment of certain cortical areas was observed. These results indicate that there is a time-dependent reorganization of the neuronal circuitry underlying long-term memory storage, in which a transitory interaction between the hippocampal formation and the neocortex would mediate the establishment of long-lived cortical memory representations.

Sex-linked behavioural differences in mice expressing a human insulin transgene in the medial habenula

We previously reported that a human insulin transgene was specifically expressed in the medial habenula of the adult mouse brain, and that this expression was ascribed to the delta-168 transgene. The present study analyses the possible behavioural consequences of this insulin transgene expression using measures of food intake, spontaneous activity, emotional reactivity, learning and extinction performance of an operant task. The delta-168 transgenic mice did not differ from the C57BL/6 control mice as concerns food intake, behaviour in the open field, or emotional response in an elevated plus maze. On the other hand, measures of locomotor activity in a circular corridor revealed a significantly faster decline of spontaneous locomotor activity in male as compared to female delta-168 transgenic mice. Moreover, as compared to female transgenic mice, male transgenic mice exhibited a deficit in the rate of acquisition and an acceleration of the rate of extinction of a bar press response in a Skinner box. In contrast, the behaviour of female transgenic mice did not differ from either male or female C57BL/6 control mice. The results of the present study demonstrate that the behavioural modifications observed in delta-168 transgenic mice are sex-linked and suggest that these behavioural differences result from changes in the interaction (interface) between motivational and motor mechanisms mediated via the striato-habenulo-mesencephalic system.

Blockade of spontaneous posttraining performance improvement in mice by NMDA antagonists

We investigated the effects of immediate post-training systemic administration of gamma-L-glutamyl-L-aspartate (gamma-LGLA) and 3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP), antagonists at the N-methyl-D-aspartate receptor, in a lever-press task in two inbred strains of mice. When retention performance was tested in control animals 24 h after partial acquisition of the task. BALB/c mice exhibited a spontaneous performance improvement whereas C57BL/6J mice did not gamma-LGLA at doses of 2.5 and 25 mumol/kg and CPP at doses ranging between 0.025 and 2.5 mumol/kg blocked the spontaneous performance improvement found in BALB/c mice but had no apparent effects on the retention performance of C57BL/6J mice. These data suggest that retention impairment induced by CPP and gamma-LGLA in BALB/c mice results from an interference with posttraining memory processes.

Effects of radiofrequency versus neurotoxic cingulate lesions on spatial reversal learning in mice

Mice with radiofrequency (RF) lesions of the posterior (PC) or anterior (AC) cingulate cortex were trained on spatial discrimination reversal learning in a T-maze. The results were compared with those obtained in an earlier study after ibotenic acid (IBO) cingulate lesions. PC-RF lesions facilitated the initial discrimination and first reversal, whereas they retarded subsequent reversals; in contrast, PC-IBO lesions yielded a deficit on the initial discrimination and first reversal, but had no effect on subsequent reversals. AC-IBO, but not AC-RF lesions, precluded the formation of a learning set across reversals. These data suggest that cingulum transection, which accompanies RF but not IBO lesions, can mask or even antagonize the specific effects of cingulate damage. Consequently, inferences made from the effects of conventional lesions to assess and distinguish the functions of the two cingulate areas appear subject to caution.

Differential temporal evolution of post-training changes in regional brain glucose metabolism induced by repeated spatial discrimination training in mice: visualization of the memory consolidation process?

The present study analyses the effects of the stage of learning on the spatial patterns and time-course of [14C]glucose uptake in BALB/c mice brain regions produced by spatial discrimination training in an eight-arm radial maze. Our particular approach was designed to follow, during the post-training period, the level of functional activity in individual brain areas which may underlie the memory consolidation process. Regional mapping of relative [14C]glucose uptake was assessed at three post-training time intervals (5 min, 1 and 3 h) after either the first (Day 1), the fourth (Day 4) or the last (Day 9) daily training session of the discrimination task and compared with sham-conditioned animals placed in the same experimental environment. The results indicated that numerous subcortical and cortical brain regions exhibit metabolic alterations following the acquisition of the spatial discrimination task. These alterations, which were specifically related to learning since they did not appear in sham-conditioned animals, were functions both of the post-training interval studied and of the degree of mastery of the task. On Day 1, a progressive, time-dependent and sequential increase in labelling was found from subcortical (5 min post-training) to cortical regions (3 h post-training). On Day 4, a peak of cortical metabolic activation was identified at 1 h post-training. In contrast, on Day 9, maximum labelling was found 5 min post-training in all subcortical and cortical regions followed by a general monotonic decline at 1 and 3 h post-training. These findings, which show widely distributed changes of metabolic activity in the brain, are consistent with the hypothesis that learning involves distributed neural networks. The sequential activation from subcortical to cortical regions seems to indicate a general mechanism whose function would ultimately be to store cortical memory representations. The acquisition-dependent shifts in the patterns of post-training metabolic labelling observed as a function of task mastery may be taken to represent a visualization of the spatio-temporal evolution of the networks of brain structures actively engaged in the memory consolidation process. In particular, the present data suggest that the duration of post-acquisition memory processing is a function of the quantity of new information which has to be dealt with by the central nervous system.

Reduction of regional brain glucose metabolism following different durations of chronic ethanol consumption in mice: a selective effect on diencephalic structures

The effects of chronic alcohol consumption on regional brain glucose metabolism were examined in Balb/c mice using the [14C]2-deoxyglucose autoradiographic technique. Animals were given a solution of 12% v/v ethanol as their only source of fluid for either 6, 12 or 18 months and compared to control groups receiving either an isocaloric solution or saccharose or tap water. Alterations of cerebral brain glucose metabolism were assessed in mice who were returned to a non-alcoholic diet and allowed to freely explore a T-maze. The results showed that chronic ethanol consumption induced reductions of regional metabolic activity which were functions both of the duration of alcohol treatment and of the structure studied. Whereas a six month period of alcoholization did not induce any significant effects on metabolic activity, 12 months of treatment were necessary to induce the first observable and significant reductions in [14C]2-deoxyglucose labelling. These effects were mainly limited to diencephalic structures such as the lateral mammillary nuclei and the anterodorsal thalamic nuclei. The cerebellum was also affected but to a lesser degree. After 18 months of alcoholization, a generalized spread of the metabolic reduction to the entire mammillary complex (lateral, medial and posterior nuclei) and to the thalamic nuclei was observed. This same duration of treatment was necessary to induce the first detectable decrease of metabolic activity in the hippocampus. In agreement with data from human neuropathology, these findings confirm the particular vulnerability of diencephalic structures to ethanol and suggest that damage limited to diencephalic regions rather than to hippocampal or cortical areas could be primarily responsible for the memory disorders observed in Korsakoff's syndrome.

Differential time courses of c-fos mRNA expression in hippocampal subfields following acquisition and recall testing in mice

Spatio-temporal patterns of c-fos mRNA expression were studied in the mouse brain following the partial acquisition of an appetitive conditioning task in a Skinner box. We used two experimental situations: during the initial acquisition of the task (acquisition paradigm) and during the retention test (recall paradigm). In both paradigms the in situ hybridization signal was exclusively located in the hippocampal formation and the posterior cingulate cortex. However, the time-dependent pattern of expression was quite different according to the experimental situation: mRNA levels peaked at 90 min post-test in both paradigms but returned to basal (control) level by 180 min in the acquisition group, while in CA3 and DG subfields, high levels of mRNA expression were maintained at 180 min in the recall group. Taken together these results suggest that the IEG c-fos is implicated in the different phases of post-acquisition memory processes and involve a differential spatio-temporal regulation of its expression in hippocampal subfields.

Post-test apamin injection suppresses a Kamin-like effect following a learning session in mice

BALB/c mice were trained in a partial acquisition session of an appetitive bar-pressing task. They then received an immediate post-acquisition i.p. injection of either saline or apamin 0.2 mg kg-1. Each group was submitted to a retention test that was delayed either 25, 85 or 180 min after initial acquisition. In saline-injected groups retention of the original training was a U-shaped function of intersession interval with a significant drop in performance (Kamin-like effect) at the 85 min time interval. In contrast, at this same time, apamin injected subjects made significantly more reinforced responses than control animals. The suppression of the Kamin-like effect by apamin could be a consequence of an acceleration of the neuronal mechanisms implicated in consolidation and long-term memory storage processes.

Expression of a human insulin transgene in cholinergic neurons of the mouse medial habenula

We explored the possibility that an insulin gene deleted in its 5'-flanking region is expressed in adult mouse brain. We used three independent lines of mice carrying a human insulin transgene which included the insulin gene transcription unit flanked by 168 base pairs upstream and 5.5 kb downstream. Using a reverse transcription-polymerase chain reaction assay, human insulin mRNAs were detected in whole brain extracts. In all three lines, human insulin mRNAs were localized by in situ hybridization in a single cerebral site, the medial habenula. With a monoclonal antibody specific for human C-peptide and human proinsulin, labelling was restricted to a subset of habenular cholinergic neurons, with rare immunostained fibers. No labelling was observed in the projection fibers of the retroflexus fasciculus or in their axon terminals in the interpeduncular nucleus. Electron microscope studies suggested that the transgene expressing cells. These findings demonstrate that the human insulin transgene tested here includes a habenula specific promoter which could be useful for physiological and molecular studies on the habenula.

Relationships between septo-hippocampal cholinergic activation and the improvement of long-term retention produced by medial septal electrical stimulation in two inbred strains of mice

This experiment was designed to highlight the relationships between septo-hippocampal cholinergic activation and the processing of memory consolidation. For that purpose, we have analyzed the consequences of a medial septal electrical stimulation (100 Hz, 30 microA) applied soon after partial acquisition session of an appetitive operant conditioning task on in vivo hippocampal cholinergic activity on the one hand and on subsequent retention 24 h later on the other hand. For maximize our data base for such comparison we used two neurochemically and behaviorally distincts strains of mice, BALB/c and C57BL/6. In these conditions, our results showed that BALB/c mice evidenced better performance in retention than C57BL/6 mice after medial septal stimulation. On the other hand, the stimulation applied in resting conditions produced a moderate and similar hippocampal cholinergic activation in the two strains. Moreover, in BALB/c mice the only strain which exhibited good consolidation capacities the stimulation do not induced any additive effect on the substantial increase of the hippocampal cholinergic activity produced by the previous acquisition session. Finally, a correlative study realized in C57BL/6 mice seems to indicate that the higher the hippocampal cholinergic activity was the lower were the consolidation capacities. These results lead us to suggest that the improvement of memory consolidation induced by the medial septal stimulation is the consequence of the recruitment of non cholinergic elements located close to the electrode tip. Consequently the septo-hippocampal cholinergic activation is more likely to facilitate certain information processes prior to the consolidation mechanisms proper.

Memory processing and apamin induce immediate early gene expression in mouse brain

The present study analyses the effects of learning on the spatial pattern and the time-course of changes of immediate early gene messenger RNA's (c-fos and c-jun) in mouse brain produced by training in an appetitive bar-pressing task. Activation of c-fos and c-jun after training is strictly located in the hippocampal formation and is learning-dependent. Levels of both proto-oncogene mRNAs in the trained group were 4 to 5 times higher than in the sham-conditioned group. Injections of apamin, a bee venom neurotoxin that selectively blocks a class of Ca(2+)-activated K+ channels and improves learning and memory retention, produced as compared to untrained animals a 3- to 5-fold increase of expression of c-fos and c-jun with the same pattern as that observed in the trained animals. Post-training injection of 0.2 mg/kg apamin enhanced 1.4-fold the expression of both immediate early genes in CA1, CA3 and dentate gyrus as compared to trained saline-injected mice. All these results suggest that apamin-induced increase of immediate early gene expression might be related to the apamin-induced facilitation of learning.

Locomotor bias produced by intra-accumbens injection of dopamine agonists and antagonists

Several experiments have shown that the dopamine (DA) receptors in the nucleus accumbens control the intensity of locomotor activity; however, there are several contradictory results concerning the role of the accumbens in the regulation of the direction of locomotion. To further evaluate the contribution of dopaminergic function in the accumbens to the direction of locomotion, we first compared the effect on the direction of locomotor activity of unilateral intra-accumbens injections of the nonspecific DA antagonist haloperidol, the specific D-1 antagonist SCH-23390, the specific D-2 antagonist metoclopramide. In the second part of the experiment, we examined the effect on the direction of locomotor activity of unilateral intra-accumbens injections of the non-specific DA agonist apomorphine, the specific D-1 agonist SKF-38393, the specific D-2 agonist LY-171555, and the combination of SKF-38393 and LY-171555. Haloperidol, metoclopramide and to a lesser extent, SCH-23393 together with peripheral amphetamine injections produced a locomotor bias that resulted in ipsilateral turning. Apomorphine, LY-171555 or the combination of SKF-38393 and LY-171555 (but not SKF-38393 alone) produced a locomotor bias that resulted in contralateral turning. No significant locomotor bias was produced by intra-accumbens injection of the various vehicles. These results suggest that the bilateral DA organization thought to exist in the nigro-striatal pathway for the control of locomotion may also be true for the mesolimbic dopamine system.

Differential involvement of anterior and posterior cingulate cortices in spatial discriminative learning in a T-maze in mice

The contribution of the anterior and posterior cingulate cortical areas to spatial learning and memory was examined in mice using a behavioral paradigm based on a spatial discrimination task in a T-maze. Multiple injections of small amounts of ibotenic acid were used to produce fiber-sparing lesions of either the anterior (ACC) or the posterior (PCC) cingulate area. Mice with ACC lesions, though learning the initial acquisition and first reversal of the discrimination at about the normal rate, were impaired during the subsequent four reversal sessions. In contrast to control mice, they failed to improve their performance from the first to the last session. Nevertheless, when later required to repeatedly learn the same discrimination over several days (repetitive testing), animals with ACC lesions no longer exhibited any learning deficit. The converse pattern of results was found in mice with PCC lesions. These animals performed much more poorly than control animals during the acquisition and first reversal of the discrimination, but displayed remarkable improvement over the subsequent four reversal sessions, gradually overcoming their initial impairment. However, when later submitted to repetitive testing, these animals again showed a substantial learning deficit. Neither ACC nor PCC cingulate lesions significantly affected the animals' retention capacities as measured by single test-trials over a 24-h interval. Yet, mice with PCC lesions were retarded in reversal learning after a long intersession interval (10 days), indicating that PCC, but not ACC, lesions did interfere with some long-term retention processes. These results imply that the ACC, as a part of the medial frontal cortex, may play a crucial role in temporally ordering a series of spatial responses, whereas the PCC seems to contribute to the formation and retention of each individual spatial response, probably by transmitting information from limbic structures such as the anterior thalamus and hippocampal formation to posterior neocortical association areas.

Time-dependent sequential increases in [14C]2-deoxyglucose uptake in subcortical and cortical structures during memory consolidation of an operant training in mice

Previous results have suggested that memory processing may involve the sequential activation of subcortical and cortical structures. To study this phenomenon, we have examined the immediate (15 min) and delayed (220 min) metabolic changes produced in BALB/c mice by a partial training session in a bar-pressing appetitive task, using the [14C]-2-deoxyglucose (2-DG) relative glucose uptake method. These relative metabolic changes were compared to the ones produced in several control groups: untrained animals, sham-conditioned animals, overtrained animals, and animals forced to walk on a moving belt (immediate and delayed condition). Animals were given a single intrajugular injection (5 microCi) of 2-DG either 5 min before or 3 h (delayed condition) after the second training session. Forty minutes after the 2-DG injection, the animals were sacrificed and their brains processed for autoradiography. At the 15-min delay, a large 2-DG labeling increase was found in partially trained animals for various subcortical areas (septum, diagonal band, hippocampus, thalamus, and mammillary bodies) while a much smaller increase was found in four cortical areas (frontal, cingulate, parietal, and sensory motor cortices). At the 220-min delay, we observed a large 2-DG labeling increase in cortical (frontal, pyriform, and cingulate cortices) and subicular areas while a moderate 2-DG labeling increase was observed in entorhinal cortex and the diagonal band. These results show that, shortly after training, subcortical structures are preferentially activated while cortical structures are much less activated. Three hours later, at a time when retention performances have been shown to improve spontaneously in the same strain of mice and in the same task, cortical structures are highly activated.

Effect of apamin, a toxin that inhibits Ca(2+)-dependent K+ channels, on learning and memory processes

Apamin, a neurotoxin extracted from bee venom, specifically binds to a particular class of Ca(2+)-activated K+ channels which are involved in the slow afterhyperpolarization (S-AHP) that follows action potentials in many excitable cells. We tested in mice the effects of apamin on learning and memory processes. The results showed that pre-training injection of apamin accelerated the acquisition of a bar-pressing response but also increased the bar-pressing rates of the animals. This latter result suggests that apamin accelerated acquisition because it increased behavioral activity in general and the number of bar-presses in particular. Post-training apamin injection retroactively and non-contingently facilitated memory processes taking place shortly after training in a bar-pressing task. The lack of an effect of the delayed apamin injection showed that apamin did not act proactively on memory retrieval processes. These results suggest that apamin-sensitive KCa channels may contribute to memory processes.

Bidirectional potentiation between D1 and D2 dopamine agonists: effects of unilateral intra-accumbens injections on locomotor activity in mice

We tested the effect of a single unilateral injection of a specific D1 agonist into the nucleus accumbens on the behavioral response to a subsequent unilateral intra-accumbens injection of a selective D2 agonist ten days later. The effect of the inverse order of presentation (D2 agonist followed ten days later by a D1 agonist) was also tested. No significant differences between the locomotor effects of the intra-accumbens injection of either SKF-38393 (3.5 micrograms) or LY-171555 (10 micrograms) were observed during the first test. Ten days later, during the second test, intra-accumbens injection of either the LY-171555 and SKF-38393 increased the percentage of contralateral rotations relative to the first test while LY-171555 also increased the total number of rotations. Control injections showed that these effects of LY-171555 and SKF-38393 were not due to a conditioning process. Rather, the results suggested that the locomotor changes observed during the second test were the result of behavioral sensitization due to the initial acute injection of the agonists.

Memory-improving action of glucose: indirect evidence for a facilitation of hippocampal acetylcholine synthesis

The effect of a 3 g/kg glucose injection on the velocity of the sodium-dependent high-affinity choline uptake mechanism in the hippocampus was both measured in quiet control mice and in mice immediately after training in an operant bar pressing task. Glucose did not significantly change high-affinity choline uptake in resting animals. High-affinity choline uptake in the hippocampus was increased by training in the operant bar pressing task. Glucose significantly reduced the amplitude of the increase in high-affinity choline uptake observed in the trained animals. Similarly, a 3 g/kg glucose injection also attenuated the increase in high-affinity choline uptake observed in animals injected with 1 mg/kg scopolamine. Finally, a 3 g/kg glucose injection significantly attenuated the amnesia produced by a post-training 1 mg/kg scopolamine injection in mice trained for an operant bar pressing task. These results provide additional evidence for an action of glucose on hippocampal cholinergic activity under conditions of high acetylcholine demand. This action may be mediated via an increase in acetyl coenzyme A availability, one of the precursors of acetylcholine. This facilitative effect of glucose on hippocampal acetylcholine synthesis may constitute the physiological basis for its facilitative action on memory and its attenuation of scopolamine amnesia.

Quantitative [14C]2-deoxyglucose study of a functional dissociation between anterior and posterior cingulate cortices in mice

We have previously shown that lesion of the posterior cingulate cortex (CCP) but not of the anterior cingulate cortex (CCA), produced learning and memory deficits. As a first evaluation of the functional anatomical basis of this dissociation, we used the quantitative [14C]2-deoxyglucose method and electrical brain stimulation to determine the functional connections of the CCA and CCP in mice. CCP stimulation (but not CCA stimulation) produced significant metabolic increases in the hippocampal formation and in the subicular complex. This result is consistent with the hypothesis that learning and memory deficits following CCP lesion may be due to the disruption of functional neural pathways between the CCP and hippocampal structures.

Improvement of memory for an operant response by post-training glucose in mice

Previous experiments have shown that a post-training glucose injection can retroactively and non-contingently improve the retention of a previously learned association. To date, the memory-improving action of glucose has only been demonstrated in rats for negatively-motivated tasks. The present experiment sought to generalize these previous results by examining the effects in mice of post-training glucose injections on the retention of an operant bar-pressing response. The results show that post-training glucose can retroactively and non-contingently improve the retention of an appetitively motivated task in mice. There was a U-shaped relationship between the dose of glucose and the effect on memory similar to the ones already observed in rats using negatively motivated training. The implications of these results for an endogenous memory modulation mechanism are discussed.

Electrolytic but not ibotenic acid lesions of the posterior cingulate cortex produce transitory facilitation of learning in mice

The rate of acquisition of 12 Hebb-Williams mazes was studied after restricted bilateral lesions of the anterior (ANT) or posterior (POST) cingulate cortex in BALB/c mice. In a first experiment, animals with electrolytic lesions were tested with the different mazes at 3 time intervals between 19 and 48 days after surgery. The rate of acquisition in POST-lesioned mice was observed to be facilitated at the 2 first time intervals (between days 19-22 and 32-35), but this effect was reversed (impairment) when the test was carried out between 45-48 days postsurgery; no significant effects were observed in ANT-lesioned mice. In a second experiment, the same behavioral paradigm was used in mice with restricted ibotenic acid lesions of the POST cingulate cortex. These lesions had no significant effects on the acquisition of the mazes. A third experiment was carried out to test if the postoperative delay itself contributed to the long latency of the impairment observed in Expt. I. No impairment of acquisition was observed when POST cingulate lesioned animals underwent their first learning session between 45-48 days after surgery; in contrast, a significant facilitation of the performance was observed at this time. These results suggest an involvement of the posterior cingulate cortex, and in particular the cingulum bundle, both in acquisition and long-term memory processes.

Lesion of the temporo-ammonic perforant path facilitates self-stimulation of the lateral entorhinal cortex in mice

The effect of a lesion of the perforant path (PP) on self-stimulation (SS) of the lateral entorhinal cortex (LEC) was tested in mice between 8 and 21 days after surgery. The current intensities tested ranged between 0 and 80 microA (peak to peak 100 Hz sine-wave). The PP lesion led to a two-fold increase in SS rates at intensities above 30 microA without affecting the baseline SS rates (0 microA) and SS threshold (30 microA). The lesion also led to a significant increase in LEC after-discharge (AD) threshold and eliminated behavioral convulsions during SS testing. The suppression of AD by i.p. Na phenobarbital injection (10 mg/kg) led to a similar increase in SS rates in sham-lesioned mice; there was no difference in PP-lesioned animals. These results might be interpreted as evidence in favor of an independence of the neuronal processes mediating entorhinal and hippocampal reward-related behaviors.

Chronic administration of sulbutiamine improves long term memory formation in mice: possible cholinergic mediation

Thiamine deficiency in both man and animals is known to produce memory dysfunction and cognitive disorders which have been related to an impairment of cholinergic activity. The present experiment was aimed at testing whether, inversely, chronic administration of large doses of sulbutiamine would have a facilitative effect on memory and would induce changes in central cholinergic activity. Accordingly mice received 300 mg/kg of sulbutiamine daily for 10 days. They were then submitted to an appetitive operant level press conditioning test. When compared to control subjects, sulbutiamine treated mice learned the task at the same rate in a single session but showed greatly improved performance when tested 24 hr after partial acquisition of the same task. Parallel neurochemical investigations showed that the treatment induced a slight (+ 10%) but significant increase in hippocampal sodium-dependent high affinity choline uptake. The present findings and previous results suggest that sulbutiamine improves memory formation and that this behavioral effect could be mediated by an increase in hippocampal cholinergic activity.

Physostigmine reverses memory deficits produced by pretraining electrical stimulation of the dorsal hippocampus in mice

The aim of the present experiments was to test the validity of the hypothesis that presynaptic cholinergic activity has a functional significance for memory formation. The results show that electrical stimulation of the dorsal hippocampus delivered before learning in BALB/c mice which induces a decrease of about 40% in hippocampal choline acetyltransferase (ChAT) activity at the time of learning results in deficits in retention scores in two appetitive learning tasks (operant conditioning in the Skinner box or a spatial memory task using a 4-hole board). In both behavioral tasks intraventricular injection of 1 microgram of physostigmine 20 min before the acquisition session reverses the disruptive effect of pretraining hippocampal stimulation. Our results seem to indicate that the memory deficits produced by pretraining electrical stimulation of the hippocampus result from both a decrease in ChAT activity and a corresponding reduction of acetylcholine availability in the hippocampal formation.

Time-dependent effects of posttraining intrahippocampal injections of corticosterone on retention of appetitive learning tasks in mice

In previous studies we suggested that corticosterone may modulate hippocampal functioning during memory formation. To test this assumption, we studied the effects of posttrial administration of corticosterone (1 microgram) injected bilaterally in the hippocampus. The treatment was applied at different time intervals after the learning session and the retention session took place 24 h later. Using appetitive operant conditioning tasks in a Skinner box, we found that the posttrial treatment 1) did not affect the retention of a continuously reinforced schedule, 2) improved the retention of a successive discrimination learning task, and 3) was still effective when given 3 h after the acquisition of this task, but not after 6 h. Taken together, the results suggest that corticosterone modulates the hippocampal mechanisms involved in behavioral suppression during memory formation.

Late post-learning effect of entorhinal cortex electrical stimulation persists despite destruction of the perforant path

In an appetitive learning task in mice, stimulation of the lateral entorhinal cortex (LEC) 30 min after training produced an improvement in retention 24 h later, as well as faster extinction of conditioning. This effect persisted in animals with bilateral lesions of the perforant path. In addition, the threshold for hippocampal after-discharges produced by LEC stimulation was raised significantly in perforant-path lesioned animals. The results indicate a functional dissociation between hippocampal and cortical mechanisms involved in memory consolidation.

Dissociation of limbic structures by pharmacological effects of diazepam on electrical self-stimulation in the mouse

The effect of diazepam was tested on self-stimulation (SS) in 21 mice implanted with a bipolar electrode in the lateral hypothalamus (LH), the dorsolateral hippocampus (HPC) or the lateral entorhinal cortex (LEC). Diazepam, injected i.p. in doses of 0.5, 1 and 2 mg/kg, significantly increased SS rates with electrodes in LH while 4 and 8 mg/kg of diazepam had no significant effect. At low doses, similar increases were seen in mice with LEC electrodes but high doses produced a significant suppression. HPC animals showed an almost total suppression of SS beginning at 2 mg/kg of diazepam; lower doses had no significant effect. The results indicate that entorhinal and hippocampal SS are at least partly independent phenomena; in addition, the suppression of SS by moderate doses of diazepam remains specific to the HPC among the brain structures studied to date.