Effects of pentylenetetrazol-induced brief convulsive seizures on spatial memory and fear memory

Previous studies have demonstrated that in the pentylenetetrazol (PTZ) kindling model, recurrent seizures either impair or have no effect on learning and memory. However, the effects of brief seizures on learning and memory remain unknown. Here, we found that a single injection of a convulsive dose of PTZ (50 mg/kg, ip) induced brief seizures in Sprague–Dawley rats. Administration of PTZ before training impaired the acquisition of spatial memory in the Morris water maze (MWM) and fear memory in contextual fear conditioning. However, the administration of PTZ immediately after training did not affect memory consolidation in either task. These findings suggest that brief seizures have different effects on acquisition and consolidation of spatial and fear memory.     

Dorsal hippocampal kindling selectively impairs spatial learning/short-term memory.

Kindling with electrical stimulation of the dorsal hippocampus has been shown to disrupt spatial task performance in rats. The present study investigated the specificity of this effect in terms of the possible contribution of nonmnemonic effects, the presence of a more general mnemonic deficit, and the involvement of learning/short-term memory and/or long-term memory processes. Rats were fully kindled with stimulation of the dorsal hippocampus and subsequently tested for acquisition, 7-day retention, and 28-day retention of a hidden platform (HP) location in the Morris water maze and an object discrimination problem in a modified water maze. To control for nonmnemonic behavioral impairments, testing on both tasks was preceded by training on visible platform control tasks. Kindling impaired acquisition of the HP location but spared performance on all other aspects of testing, indicating a specific impairment of spatial learning/short-term memory. These results suggest that epileptogenesis induced by hippocampal stimulation is indeed associated with a selective disruption of the mechanisms mediating spatial learning/short-term memory.     

Spatial learning in the guinea pig: cued versus non-cued learning, sex differences, and comparison with rats.

This paper provides the first report of spatial learning in guinea pigs using the Morris water maze (MWM). Male and female guinea pigs were trained for 5 consecutive days (8 trials/day; acquisition phase) in either the visible (cued) or the hidden (non-cued) platform version of the MWM. In both tests, guinea pigs learned to navigate to the escape platform, as indicated by a decrease in escape latency over the 5 training days. There were no sex differences in either test version. A comparison of guinea pigs and male Wistar rats showed that performance during acquisition training was not different for the two species in the visible platform test, but rats performed better during the early training days in the hidden platform test. A retention test (probe trial) was given 5 days after the last acquisition training day. Again, there was no sex difference, and no difference between guinea pigs and rats. Finally, acquisition of a new escape response to a shifted platform location was equivalent for rats and guinea pigs of both sexes. These results demonstrate that guinea pigs show robust cued and non-cued learning in the MWM. Both acquisition and retention performance in guinea pigs is similar to that in rats, even though rats appear to have a slight advantage in the acquisition of non-cued, spatial information in this test. We conclude that the MWM provides a valuable paradigm to assess behavior and learning/memory in the guinea pig.     

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.     

Lipopolysaccharide causes deficits in spatial learning in the watermaze but not in BDNF expression in the rat dentate gyrus.

We investigated the effects of a single injection and a daily injection of lipopolysaccharide (LPS) on spatial learning and brain-derived neurotrophic factor (BDNF) expression in the rat dentate gyrus. LPS is derived from the cell wall of Gram-negative bacteria and is a potent endotoxin that causes the release of cytokines such as interleukin-1 and tumour necrosis factor. LPS is thought to activate both the neuroimmune and neuroendocrine systems; it also blocks long-term potentiation in the hippocampus. Here, we examined the effects of LPS on a form of hippocampal-dependent learning-spatial learning in the water maze. Rats were injected with LPS intraperitoneally (100 microg/kg) and trained in the water maze. The first group of rats were injected on day 1 of training, 4 h prior to learning the water maze task. Groups 2 and 3 were injected daily, again 4 h prior to the water-maze task; group 2 with LPS and group 3 with saline. A number of behavioural variables were recorded by a computerised tracking system for each trial. The behavioural results showed a single injection of LPS (group 1) impaired escape latency in both the acquisition and retention phases of the study, whereas a daily injection of LPS did not significantly impair acquisition or retention. BDNF expression was analysed in the dentate gyrus of all animals. No significant differences in BDNF expression were found between the three groups.     

Effects of naloxone on the long-term potentiation of EPSPs from the pathway of Schaffer collateral to CA1 region of hippocampus in aged rats with declined memory

Morris water maze (MWM) was employed to distinguish the aged rats with declined memory to investigate the effect of naloxone on the synaptic plasticity of hippocampus in declined memory aged rats. After administration with naloxone for 7 days, LTP of excitatory post-synaptic potentials (EPSPs) from Schaffer collateral to CA1 region was recorded. The results showed that the maintenance of LTP of EPSPs from Schaffer collateral to CA1 subfield in isolate hippocampal brain slice was prolonged by naloxone with improved Morris water maze performance and reduced threshold of EPSPs. It is suggested that naloxone can improve learning and memory through enhancement of the synaptic plasticity of hippocampus in aged rats with declined memory.     

Effects of discrete kainic acid-induced hippocampal lesions on spatial and contextual learning and memory in rats

Substantial information is available concerning the influence of global hippocampal lesions on spatial learning and memory, however the contributions of discrete subregions within the hippocampus to these functions is less well understood. The present investigation utilized kainic acid to bilaterally lesion specific areas of the rat hippocampus. These animals were subsequently tested on a spatial orientation task using a circular water maze, and on an associative/contextual task using passive avoidance conditioning. The results indicate that both the dorsal CA1 and the ventral CA3 subregions play important roles in learning. Specifically, CA1 lesions produced a deficit in the acquisition of the water maze task and a significant memory impairment on the passive avoidance task. CA3 lesions also caused learning deficits in the acquisition of the water maze task, and produced even greater impairments in performance on the passive avoidance task. We conclude that CA1 and CA3 hippocampal subregions each play significant roles in the overall integration of information concerning spatial and associative learning.     

Brain lesions and delayed water maze learning deficits after intracerebroventricular spermine

The effects of spermine on the acquisition and retention of spatial learning in the Morris water maze were studied. Spermine 25 and 125 nmol i.c.v. did not alter the ability of rats to find a hidden platform in the water maze when administered before training over 5 days. However, the inhibitory effect of the benzodiazepine, diazepam (3 mg/kg i.p., 30 min prior to training), on path length to target was markedly potentiated by the higher dose of spermine, consistent with spermine acting as a functional antagonist at the NMDA receptor. This drug combination did not affect performance on visible platform trials. Administration of doses of 125 and 250 nmol (but not 62.5 nmol) of spermine i.c.v. in the week prior to training (daily for 5 days) dose-dependently inhibited subsequent learning of a platform position in the absence of drug. These higher doses of spermine produced neuronal loss and increased

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binding indicating microglial activation predominantly in the hippocampus and to a lesser extent in the striatum, septum, thalamus and amygdala. Spermine 125 nmol i.c.v. (daily for 7 days) also abolished retention of a previously learned platform position when administered in an interval between training and retention testing. The inhibitory effects of spermine 125 nmol i.c.v. (daily for 7 days) on subsequent spatial learning were not antagonised by concomitant administration of 30 nmol dizocilpine. These results demonstrate that spermine produces a delayed neurotoxic effect in particular neuronal populations in the brain that selectively impair spatial learning and recall.     

Evaluation of learning and memory dysfunction and histological findings in rats with chronic stage contusion and diffuse axonal injury

We previously reported a modified fluid percussion device capable of consistently producing experimental cortical contusion (CC) and diffuse axonal injury (DAI) in separate groups of rats by lateral and midline fluid percussion, respectively. The purpose of the present study was to compare the differences in learning acquisition and memory retention impairments between these two types of injured rats in the chronic stage using the Morris water maze technique. We also compared the histological differences between these two different types of traumatic brain injury. The results showed a statistically significant difference in learning acquisition impairment between the sham and CC rats and also between the sham and DAI rats. However, a significant difference in memory retention impairment was observed only between the sham and DAI rats. Histologically, the neuronal cell loss of CA3 pyramidal cells in the hippocampus was observed on the ipsilateral side in the CC and bilaterally in DAI. The neuronal cell loss was seen in bilateral entorhinal cortex layer II in DAI, but it was not seen in CC. From these results, we speculate that the marked cell loss in the hippocampus CA3 region in both CC and DAI rats was related to the impairment of spatial learning acquisition. The marked cell loss in entorhinal cortex layer II in DAI rats may be one of the important factors in the impairment of spatial memory retention.     

Long‐lasting modulation of synaptic plasticity in rat hippocampus after early‐life complex febrile seizures

A small fraction of children with febrile seizures appears to develop cognitive impairments. Recent studies in a rat model of hyperthermia‐induced febrile seizures indicate that prolonged febrile seizures early in life have long‐lasting effects on the hippocampus and induce cognitive deficits. However, data on network plasticity and the nature of cognitive deficits are conflicting. We examined three specific measures of hippocampal plasticity in adult rats with a prior history of experimental febrile seizures: (i) activity‐dependent synaptic plasticity (long‐term potentiation and depression) by electrophysiological recordings of Schaffer collateral/commissural‐evoked field excitatory synaptic potentials in CA1 of acute hippocampal slices; (ii) Morris water maze spatial learning and memory; and (iii) hippocampal mossy fiber plasticity by Timm histochemistry and quantification of terminal sprouting in CA3 and the dentate gyrus. We found enhanced hippocampal CA1 long‐term potentiation and reduced long‐term depression but normal spatial learning and memory in adult rats that were subjected to experimental febrile seizures on postnatal day 10. Furthermore, rats with experimental febrile seizures showed modest but significant sprouting of mossy fiber collaterals into the inner molecular layer of the dentate gyrus in adulthood. We conclude that enhanced CA1 long‐term potentiation and mild mossy fiber sprouting occur after experimental febrile seizures, without affecting spatial learning and memory in the Morris water maze. These long‐term functional and structural alterations in hippocampal plasticity are likely to play a role in the enhanced seizure susceptibility in this model of prolonged human febrile seizures but do not correlate with overt cognitive deficits.     

Selective neurodegeneration of hippocampus and entorhinal cortex correlates with spatial learning impairments in rats with bilateral ibotenate lesions of ventral subiculum

Rats with bilateral ibotenic acid lesions of ventral subiculum were tested in an eight-arm radial maze task for spatial learning and memory functions. The performance of the lesioned rats was severely impaired relative to control rats in both acquisition and retention of the spatial task. Following subicular lesions, profound neurodegeneration of the CA1 and CA3 sub sectors of hippocampus and entorhinocortical layers I, II, III, V and VI was observed. These results support the concept that neurons in the ventral subiculum are a part of the neural network along with the above neurons, which could be involved in the processing of spatial information.     

The effect of seizure type and medication on cognitive and behavioral functioning in children with idiopathic epilepsy

Antiepileptic drugs have been reported to have a variety of adverse effects on behavior and performance in children with epilepsy. Previous studies investigating these side effects, however, have not controlled for the baseline status of the child (e.g. underlying neurological condition, seizure type, socioeconomic status, family variables), making it difficult to determine whether changes in function are attributable to the use of medication. We investigated the cognitive and behavioral profiles of 43 children, aged from 4 to 16 years, with new onset, idiopathic seizures. Twenty-six of these children participated in a 6-month follow-up study, and 12 in a 12-month follow-up study, investigating the effects of antiepileptic medications on psychological functioning. The children were of average intelligence (mean 1Q108) and had not previously been treated with antiepileptic medication. Children were classified as having either generalized convulsive, generalized non-convulsive (absence), simple partial, or complex partial seizures. Prior to the initiation of treatment, children with partial seizures were found to perform better than children with generalized seizures on measures of cognitive functioning. Children with convulsive seizures obtained significantly higher cognitive scores than those with non-convulsive seizures. Children with generalized non-convulsive seizures had lower cognitive scores than subjects with other types of seizure. No differences were found between groups at baseline prior to the initiation of antiepileptic medications. Analysis of subjects' performance after 6 and 12 months of antiepileptic therapy showed no significant deterioration attributable to medication. The differences in cognitive performance of the four seizure groups at baseline were not apparent at the time of follow-up. These results indicate that intrinsic and environmental variables may play a more significant role in predisposing certain children to cognitive and learning problems than do antiepileptic medications.     

Impaired maze performance in aged rats is accompanied by increased density of NMDA, 5-HT1A, and alpha-adrenoceptor binding in hippocampus

Using quantitative receptor autoradiography, we assessed binding site densities and distribution patterns of glutamate, GABA(A), acetylcholine (ACh), and monoamine receptors in the hippocampus of 32-month-old Fischer 344/Brown Norway rats. Prior to autoradiography, the rats were divided into two groups according to their retention performance in a water maze reference memory task, which was assessed 1 week after 8 days of daily maze training. The animals of the inferior group showed less long-term retention of the hidden-platform task but did not differ from superior rats in their navigation performance during place training and cued trials. The decreased retention performance in the group of inferior learners was primarily accompanied by increased alpha(1)-adrenoceptors in all hippocampal subregions under inspection (CA1-CA4 and dentate gyrus), while elevated alpha(2)-adrenoceptor binding was observed in the CA1 region and DG. Furthermore, inferior learners had higher NMDA binding in the CA2 and CA4 and increased 5-HT(1A) binding sites in the CA2, CA3, and CA4 region. No significant differences between inferior and superior learners were evident with regard to AMPA, kainate, GABA(A), muscarinergic M(1), dopamine D(1), and 5-HT(2) binding densities in any hippocampal region analyzed. These results show that increased NMDA, 5-HT(1A), and alpha-adrenoceptor binding in the hippocampus is associated with a decline in spatial memory. The increased receptor binding observed in the group of old rats with inferior maze performance might be the result of neural adaptation triggered by age-related changes in synaptic connectivity and/or synaptic activity.     

A correlation between regional acetylcholinesterase activity in rat brain and performance in a spatial task

The acquisition and retention of a water maze task was examined in 12 intact, young Wistar rats. Acetylcholinesterase activity in 43 discrete brain regions was then measured in the same rats by quantitative histochemistry. Individual learning and retention indices were found to be significantly correlated with acetylcholinesterase (AChE) levels in specific regions, e.g. cholinergic nuclei; the ventral pallidum and nucleus basalis; and in the dentate gyrus of the hippocampus. High levels of AChE in these regions predicted poor performance in the water maze. Thus cholinergic activity in selected regions of the rat brain might be involved in the performance of spatial memory tasks.     

Open-field behaviors and water-maze learning in the F substrain of Ihara epileptic rats

PURPOSE: Genetically epileptic model rats, Ihara epileptic rat (IER/F substrain), have neuropathologic abnormalities and develop generalized convulsive seizures when they reach the age of approximately 5 months. Because the neuromorphologic abnormalities are centered in the hippocampus, we expected to observe spatial cognitive deficits. The present study aimed to evaluate emotionality and learning ability of the F substrain of IER. METHODS: To determine whether deficits are caused by inborn neuropathologic abnormalities or by repeated generalized convulsions, we tested nine 6- to 12-week-old IER/F rats that had not yet experienced seizures (experiment 1) and nine 7- to 9-month-old IER/F rats that had repeatedly experienced seizures (experiment 2) with identical tasks: an open-field test and the Morris water-maze place and cue tasks. RESULTS: Both groups of IER/Fs showed behaviors that were different from those of control rats in the open-field test, and extensive learning impairments were seen in both the place task, which requires spatial cognition, and the cue task, which does not require spatial cognition but requires simple association learning. Their impaired performance of the cue task indicates that their deficiency was not limited to spatial cognition. CONCLUSIONS: Because young IER/F rats without seizure experiences also showed severe learning impairments, genetically programmed microdysgenesis in the hippocampus was suspected as a cause of the severe learning deficits of IER/Fs.     

Postnatal phencyclidine-induced deficit in adult water maze performance is associated with N-methyl- -aspartate receptor upregulation

The N-methyl-D-aspartate (NMDA) receptor plays an important role in developmental plasticity. Earlier, we have shown that blocking the NMDA receptor with the non-competitive antagonist phencyclidine (PCP), during a brief postnatal period, disrupts the water maze performance in young juvenile rats (starting at 25 days of age). We now show the long-term effects of postnatal phencyclidine exposure on spatial learning and memory. Male and female rats were exposed to PCP (1 and 5mg/kg) or saline, from postnatal days 5-15, and their performance in the Morris water maze (MWM) was tested both as adolescents (starting on postnatal day (PD) 35) and as adults (starting on postnatal day 60). Separate groups of adult male and female postnatal PCP-treated and saline-treated rats were sacrificed and saturation [3H]MK-801 binding experiments were carried out in their hippocampi and frontal cortices; hippocampus and frontal cortex have high densities of NMDA receptors and both regions are important in spatial learning and memory. Postnatal PCP administration disrupted the water maze performance both in adolescent and adult rats of both sexes. Adult male and female rats treated postnatally with PCP had increased maximal [3H]MK-801 binding in the hippocampus and frontal cortex compared to same-sex saline-treated controls. Taken together, repeated postnatal PCP (RPP) administration impaired the acquisition of spatial learning in adolescent and adult male and female rats, and this cognitive deficit was associated with increased [3H]MK-801 labeled NMDA receptor in the hippocampus and frontal cortex. These findings are consistent with the hypothesis that PCP treatment during the postnatal period produces deficits in the water maze performance by disrupting the developing glutamatergic system.     

Serotonin depletion in rat hippocampus attenuates L-NAME-induced spatial learning deficits

Inhibition of nitric oxide (NO) synthesis has been found to produce learning deficits in spatial tasks. Recent studies also suggest a regulatory effect of endogenous NO on hippocampal serotonin (5-HT) release and have shown that NO-synthase (NOS) inhibitors increased extracellular levels of serotonin (5-HT) in the rat hippocampus. To clarify possible interactions between NO and 5-HT in the hippocampus on learning processes, the effect of selective hippocampal 5-HT depletion on NOS inhibition-induced spatial learning deficits was investigated. Rats received bilateral injections of 5,7-dihydroxytryptamine (5,7-DHT), a 5-HT neurotoxin, or its vehicle in the CA1 region of hippocampus following pretreatment with desipramine. Rats were subjected to 5 days of training in the Morris water maze (MWM); 4 days with the invisible platform to test spatial learning and the 5th day with the visible platform to test motivation and sensorimotor coordination. Nomega-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, was administered to either sham-operated or 5,7-DHT-lesioned groups 30 min before training each day. Results showed that L-NAME significantly impaired the ability of rats to locate the hidden platform. This impairment was reversed by co-administration of mole equivalent dose of L-arginine, the NO precursor. Although the 5,7-DHT-induced lesion had no effect by itself on rat performance in the MWM, it attenuated the memory impairment caused by L-NAME. The observed effect suggests an interaction between NO and 5-HT in the hippocampus on spatial memory formation; however, the mechanism of interaction is still unclear and requires further investigation.     

Intact spatial memory in mice with seizure-induced partial loss of hippocampal pyramidal neurons

We generated defined neuronal loss in hippocampus of genetically identical mice by pilocarpine injections and studied the impact of these seizures on the performance of mice in spatial learning and memory. The numbers of TUNEL-positive degenerating cells paralleled the severity of the seizures. When compared to the numbers found for not-seizured control mice, mild, moderate, and severe seizures produced significant increases in TUNEL-positive neurons in CA1 and CA3 regions by 19, 25, and 63%, respectively. Water maze learning was abolished after the severe seizures. However, spatial learning was normal after mild or moderate seizures. Therefore, there was no linear correlation between the impairment of learning and memory performance with the number of degenerating neurons in hippocampus. Our data suggest that normal spatial learning and memory can be achieved without the full number of hippocampal pyramidal neurons in partially lesioned hippocampus.     

Neonatal hippocampal damage in rats: Long-term spatial memory deficits and associations with magnitude of hippocampal damage

This study investigated the effects of neonatal hippocampal ablation on the development of spatial learning and memory abilities in rats. Newborn rats sustained bilateral electrolytic lesions of the hippocampus or were sham-operated on postnatal day 1 (PN1). At PN20–25, PN50–55, or PN90–95, separate groups of rats were tested in a Morris water maze on a visible “cue” condition (visible platform in a fixed location of the maze), a spatial “place” condition (submerged platform in a fixed location), or a no-contingency “random” condition (submerged platform in a random location). Rats were tested for 6 consecutive days, with 12 acquisition trials and 1 retention (probe) trial per day. During acquisition trials, the rat's latency to escape the maze was recorded. During retention trials (last trial for each day, no escape platform available), the total time the rat spent in the probe quadrant was recorded. Data from rats with hippocampal lesions tested as infants (PN20–25) or as adults (PN50–55 and PN90–95) converged across measures to reveal that 1) spatial (place) memory deficits were evident throughout developmental testing, suggesting that the deficits in spatial memory were long-lasting, if not permanent, and 2) behavioral performance measures under the spatial (place) condition were significantly correlated with total volume of hippocampal tissue damage, and with volume of damage to the right and anterior hippocampal regions. These results support the hypothesis that hippocampal integrity is important for the normal development of spatial learning and memory functions, and show that other brain structures do not assume hippocampal-spatial memory functions when the hippocampus is damaged during the neonatal period (even when testing is not begun until adulthood). Thus, neonatal hippocampal damage in rats may serve as a rodent model for assessing treatment strategies (e.g., pharmacological) relevant to human perinatal brain injury and developmental disabilities within the learning and memory realm. Hippocampus 7:403–415, 1997. © 1997 Wiley-Liss, Inc.     

Functional inactivation of orexin 1 receptors in CA1 region impairs acquisition, consolidation and retrieval in Morris water maze task

Orexin containing neurons in the lateral hypothalamic area (LHA) produce orexin-A (hypocretin-1) and orexin-B (hypocretin-2) and send their axons to the hippocampus, which predominantly expresses orexin 1 receptors (OX1Rs) showing a higher affinity to orexin-A. Recent studies have shown that central administration of orexin-A has an effect on learning and memory but literature concerning the role of orexinergic system in cognition remains controversial. Therefore, we examined the effect of pre-training, post-training and pre-probe trial intrahippocampal CA1 administration of a selective OX1R the orexin 1 receptor antagonist SB-334867-A (1.5, 3, 6 microg/0.5 microl) on acquisition, consolidation and retrieval in a single-day testing version of Morris water maze (MWM) task. Our results show that, SB-334867-A impaired acquisition, consolidation and retrieval of MWM task as compared with the control group. This drug had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous orexins, especially orexin-A, play an important role in spatial learning and memory in the rat.