Neuroplasticity in Cholinergic Projections from the Basal Forebrain to the Basolateral Nucleus of the Amygdala in the Kainic Acid Model of Temporal Lobe Epilepsy.

The amygdala is a cerebral region whose function is compromised in temporal lobe epilepsy (TLE). Patients with TLE present cognitive and emotional dysfunctions, of which impairments in recognizing facial expressions have been clearly attributed to amygdala damage. However, damage to the amygdala has been scarcely addressed, with the majority of studies focusing on the hippocampus. The aim of this study was to evaluate epilepsy-related plasticity of cholinergic projections to the basolateral nucleus (BL) of the amygdala. Adult rats received kainic acid (KA) injections and developed status epilepticus. Weeks later, they showed spontaneous recurrent seizures documented by behavioral observations. Changes in cholinergic innervation of the BL were investigated by using an antibody against the vesicular acetylcholine transporter (VAChT). In KA-treated rats, it was found that (i) the BL shrunk to 25% of its original size (p < 0.01 vs. controls, Student's t-test), (ii) the density of vesicular acetylcholine transporter-immunoreactive (VAChT-IR) varicosities was unchanged, (iii) the volumes of VAChT-IR cell bodies projecting to the BL from the horizontal limb of the diagonal band of Broca, ventral pallidum, and subcommissural part of the substantia innominata were significantly increased (p < 0.05, Bonferroni correction). These results illustrate significant changes in the basal forebrain cholinergic cells projecting to the BL in the presence of spontaneous recurrent seizures.

Altered serotonin innervation in the rat epileptic brain.

Studies in animal models of epilepsy revealed compromised serotonin (5-HT) transmission between the raphe nuclei and the brain limbic system. The goal of the present study was to evaluate the effects of epilepsy on the structural integrity of the dorsal (DR) and median (MnR) raphe nuclei and on the morphology of serotonergic fiber terminals in the dentate gyrus (DG), infralimbic cortex (IL) and medial septum (MS). The study was performed in adult Wistar rats using the kainate (9.5 mg/kg) status epilepticus (SE) model. Four months post-SE, the brainstem sections of the animals were immunostained for 5-HT, whereas the forebrain sections were immunostained for serotonin transporter (SERT). Stereological analysis revealed that epileptic rats, as compared to controls, had approximately 30% less 5-HT-stained cells in the interfascicular part of the DR, but twice as many 5-HT-stained cells in the MnR. Another finding was the reorganization of the 5-HT fiber network in all target areas analyzed, as indicated by the rightward shift of the density-size distribution histograms of SERT-stained fiber varicosities. Nonlinear regression analysis of these histograms revealed that SERT-stained varicosities were represented by two subpopulations characterized by distinct cross-sectional areas. The areal density of the small-sized varicosities was decreased in the DG (hilus and molecular layer), IL cortex (layers II/III) and MS, while that of the larger-sized varicosities was increased. The present results support the hypothesis that chronic epilepsy can trigger profound structural reorganization of the ascending serotonergic pathways in the rat brain.

Partial depletion of septohippocampal cholinergic cells reduces seizure susceptibility, but does not mitigate hippocampal neurodegeneration in the kainate model of epilepsy.

The brain cholinergic system may undergo structural and functional alterations both in human epilepsy and in respective animal models, but the causal relationships between these alterations and epilepsy remain to be established. In this study, we attempted to examine how the inhibition of epilepsy-related cholinergic plasticity may be reflected in seizure susceptibility and/or in the development of chronic epilepsy and its neurological consequences. For this purpose, adult Wistar rats received intrahippocampal injections of low doses of 192-IgG-saporin (SAP) to produce a moderate, but significant loss of septohippocampal cholinergic cells and to suppress their plasticity. Then, animals were treated with kainic acid to induce status epilepticus, which leads to the development of chronic epilepsy later in life. It was found that SAP-pretreatment was associated with longer latency to the onset of status epilepticus and with reduced mortality rate, suggesting that increased activity of septal cholinergic cells may potentiate seizures. Interestingly, months later, a greater percentage of rats with intact septohippocampal cholinergic connections showed spontaneous seizures, when compared to SAP-pretreated rats. Treatment with kainic acid produced death of 40-50% of hippocampal neurons and this effect was not ameliorated by prior cholinergic depletion. Moreover, the kainate induced cognitive deficits were detected in both SAP-pretreated and sham-pretreated groups. These data suggest that seizure-induced plasticity of cholinergic cells may indeed enhance seizure susceptibility and contribute to epileptogenic processes. They do not support the hypothesis that epilepsy-related hypertrophy of cholinergic neurons may potentiate hippocampal cell loss and respective behavioral impairments.

The pedunculopontine and laterodorsal tegmental nuclei in the kainate model of epilepsy.

Prior studies showed that epilepsy can be associated with reorganization of the septohippocampal cholinergic fiber system. Using the kainate model of epilepsy, we wished to further examine the structural integrity of the mesopontine tegmental nuclei (pedunculopontine, PPN, and laterodorsal, LDT), which provide the cholinergic input to the thalamus. It was found that the total numbers of the PPN and LDT cells immunoreactive to the vesicular acetylcholine transporter did not differ between control and epileptic rats. However, the cholinergic cells had enlarged perikarya in epileptic rats. We further examined the effects of epilepsy on the distribution pattern of cholinergic fiber varicosities in the parafascicular nucleus, one of the principal thalamic targets of PPN projections. The density of cholinergic varicosities, represented by two distinct populations, was increased in epileptic rats. These data provide the first morphological evidence for structural alterations in mesopontine cholinergic neurons in experimental epilepsy. They suggest dysfunctional cholinergic transmission in the brainstem-thalamic pathway, which may partly account for various epilepsy-related neurological disturbances.

Serotonin depletion increases seizure susceptibility and worsens neuropathological outcomes in kainate model of epilepsy.

Serotonin is implicated in the regulation of seizures, but whether or not it can potentiate the effects of epileptogenic factors is not fully established. Using the kainic acid model of epilepsy in rats, we tested the effects of serotonin depletion on (1) susceptibility to acute seizures, (2) development of spontaneous recurrent seizures and (3) behavioral and neuroanatomical sequelae of kainic acid treatment. Serotonin was depleted by pretreating rats with p-chlorophenylalanine. In different groups, kainic acid was injected at 3 different doses: 6.5mg/kg, 9.0mg/kg or 12.5mg/kg. A single dose of 6.5mg/kg of kainic acid reliably induced status epilepticus in p-chlorophenylalanine-pretreated rats, but not in saline-pretreated rats. The neuroexcitatory effects of kainic acid in the p-chlorophenylalanine-pretreated rats, but not in saline-pretreated rats, were associated with the presence of tonic-clonic convulsions and high lethality. Compared to controls, a greater portion of serotonin-depleted rats showed spontaneous recurrent seizures after kainic acid injections. Loss of hippocampal neurons and spatial memory deficits associated with kainic acid treatment were exacerbated by prior depletion of serotonin. The present findings are of particular importance because they suggest that low serotonin activity may represent one of the major risk factors for epilepsy and, thus, offer potentially relevant targets for prevention of epileptogenesis.

Reorganization of the septohippocampal cholinergic fiber system in experimental epilepsy.

The septohippocampal cholinergic neurotransmission has long been implicated in seizures, but little is known about the structural features of this projection system in epileptic brain. We evaluated the effects of experimental epilepsy on the areal density of cholinergic terminals (fiber varicosities) in the dentate gyrus. For this purpose, we used two distinct post-status epilepticus rat models, in which epilepsy was induced with injections of either kainic acid or pilocarpine. To visualize the cholinergic fibers, we used brain sections immunostained for the vesicular acetylcholine transporter. It was found that the density of cholinergic fiber varicosities was higher in epileptic rats versus control rats in the inner and outer zones of the dentate molecular layer, but it was reduced in the dentate hilus. We further evaluated the effects of kainate treatment on the total number, density, and soma volume of septal cholinergic cells, which were visualized in brain sections stained for either vesicular acetylcholine transporter or choline acetyltransferase (ChAT). Both the number of septal cells with cholinergic phenotype and their density were increased in epileptic rats when compared to control rats. The septal cells stained for vesicular acetylcholine transporter, but not for ChAT, have enlarged perikarya in epileptic rats. These results revealed previously unknown details of structural reorganization of the septohippocampal cholinergic system in experimental epilepsy, involving fiber sprouting into the dentate molecular layer and a parallel fiber retraction from the dentate hilus. We hypothesize that epilepsy-related neuroplasticity of septohippocampal cholinergic neurons is capable of increasing neuronal excitability of the dentate gyrus.

Diverse firing properties and Aß-, Aδ-, and C-afferent inputs of small local circuit neurons in spinal lamina I.

Spinal lamina I is a key element of the pain processing system, which integrates primary afferent input and relays it to supraspinal areas. More than 90% of neurons in this layer are local circuit neurons, whose role in the signal processing is poorly understood. We performed whole-cell recordings in a spinal cord preparation with attached dorsal roots to examine morphological features and physiological properties of small local circuit neurons (n = 47) in lamina I. Cells successfully filled with biocytin (n = 17) had fusiform (n = 10), flattened (n = 4), and multipolar (n = 3) somatodendritic morphology; their axons branched extensively and terminated in laminae I-III. Intrinsic firing properties were diverse; in addition to standard tonic (n = 16), adapting (n = 7), and delayed (n = 6) patterns, small local circuit neurons also generated rhythmic discharges (n = 6) and plateau potentials (n = 10), the latter were suppressed by the L-type Ca(2+)-channel blocker nifedipine. The neurons received monosynaptic inputs from Aδ and C afferents and could generate bursts of spikes on the root stimulation. In addition, we identified lamina I neurons (n = 7) with direct inputs from the low-threshold Aß afferents, which could be picked up by ventral dendrites protruding to lamina III. Stimulation of afferents also evoked a disynaptic inhibition of neurons. Thus, small local circuit neurons exhibit diverse firing properties, can generate rhythmic discharges and plateau potentials, and their dendrites extending into several laminae allow broad integration of Aß-, Aδ-, and C-afferent inputs. These properties are required for processing diverse modalities of nociceptive inputs in lamina I and may underlie spinal sensitization to pain.

Altered taste preference and loss of limbic-projecting serotonergic neurons in the dorsal raphe nucleus of chronically epileptic rats.

Mood disorders and major depression are frequently comorbid with epilepsy. While the nature of this comorbidity is not fully understood, multiple lines of evidence suggest that changes in serotonin (5-HT) neurotransmission may be an underlying mechanism. In this study, we tested the hypothesis that chronic epilepsy in rats can be associated with loss of 5-HT neurons in the dorsal raphe (DR) nuclear complex, the main source of 5-HT projections to the cerebral cortex, which would help to explain respective behavioral deficits. Epilepsy was induced using the kainate model of status epilepticus in adult Wistar rats. After a 3-month recovery period, all kainate-treated rats that had experienced status epilepticus showed spontaneous seizures and reduced sucrose preference (anhedonia), a core symptom of depression. No changes in the forced swim test were detected. The total numbers of 5-HT immunoreactive cells were estimated in all DR subdivisions of control and epileptic rats. Interestingly, epilepsy-related loss of 5-HT neurons (approximately 35%) was observed only in the interfascicular part of the DR complex, which is known to innervate brain regions involved in depression. These findings support the notion that mental health impairments observed in epilepsy may be related to loss of a specific population of the DR 5-HT neurons projecting to limbic brain areas.

Loss of hippocampal neurons after kainate treatment correlates with behavioral deficits.

Treating rats with kainic acid induces status epilepticus (SE) and leads to the development of behavioral deficits and spontaneous recurrent seizures later in life. However, in a subset of rats, kainic acid treatment does not induce overt behaviorally obvious acute SE. The goal of this study was to compare the neuroanatomical and behavioral changes induced by kainate in rats that developed convulsive SE to those who did not. Adult male Wistar rats were treated with kainic acid and tested behaviorally 5 months later. Rats that had experienced convulsive SE showed impaired performance on the spatial water maze and passive avoidance tasks, and on the context and tone retention tests following fear conditioning. In addition, they exhibited less anxiety-like behaviors than controls on the open-field and elevated plus-maze tests. Histologically, convulsive SE was associated with marked neuron loss in the hippocampal CA3 and CA1 fields, and in the dentate hilus. Rats that had not experienced convulsive SE after kainate treatment showed less severe, but significant impairments on the spatial water maze and passive avoidance tasks. These rats had fewer neurons than control rats in the dentate hilus, but not in the hippocampal CA3 and CA1 fields. Correlational analyses revealed significant relationships between spatial memory indices of rats and neuronal numbers in the dentate hilus and CA3 pyramidal field. These results show that a part of the animals that do not display intense behavioral seizures (convulsive SE) immediately after an epileptogenic treatment, later in life, they may still have noticeable structural and functional changes in the brain.

Seizure-induced structural and functional changes in the rat hippocampal formation: comparison between brief seizures and status epilepticus.

Prolonged seizures produce death of hippocampal neurons, which is thought to initiate epileptogenesis and cause a disruption of hippocampally mediated behaviors. This study aimed to evaluate behavioral and neuroanatomical changes induced by brief seizures and to compare them with changes induced by prolonged seizures. Adult rats were administered 6 brief seizures, elicited by electroshock (ECS). Prolonged seizures (status epilepticus, SE) were induced by pilocarpine. Two months later, the rats' behavior was tested using the Morris water maze, passive avoidance and active avoidance tests. The number of neurons in the hippocampal formation was estimated using stereological methods. ECS seizures produced loss of neurons, ranging between 14% and 26%, in the dentate hilus, subiculum, presubiculum, parasubiculum, and entorhinal layers III and V/VI. However, the neuron loss caused by SE in the same structures, as well as in the hippocampal CA3 and CA1 fields, ranged between 34% and 50%. SE additionally killed many neurons in the dentate granular layer, postsubiculum and entorhinal layer II. ECS treatment caused mild impairments in spatial learning and passive avoidance, but it was not associated with spontaneous motor seizures. In contrast, SE produced a severe disruption of spatial learning, passive and active avoidance, and led to the development of spontaneous seizures. These data show that both prolonged seizure activity and brief seizures result in structural and functional alterations in the temporal lobe circuits, but those caused by prolonged seizures are considerably more severe. Hippocampal damage elicited by brief seizures does not necessarily lead to spontaneous motor seizures.

Chronic green tea consumption prevents age-related changes in rat hippocampal formation.

The hippocampal formation undergoes considerable structural and functional modifications during aging and oxidative stress emerges as a key player in the process. In the present study, we investigated whether prolonged consumption of green tea (GT), which contains large amounts of polyphenols, could interfere with age-related changes in this brain region using biochemical, morphological and behavioral approaches. Ten male Wistar rats aged 19 months were fed with GT since 12 months of age and results compared to those obtained from controls aged 19 months (C-19M). At 12 months of age, another group of rats was evaluated to provide baseline data. Oxidative stress markers (protein carbonyls and malondialdehyde) were quantified in hippocampal homogenates and stereological methods were applied to estimate the deposition of lipofuscin in hippocampal CA3 pyramidal neurons. Morris water maze was used to assess spatial learning and memory. Aging increased oxidative markers and lipofuscin accumulation and was associated with impaired memory acquisition. However, GT treatment protected proteins and lipids against oxidation and prevented the increase of lipofuscin deposition compared to age-matched controls. Furthermore, the spatial learning abilities of GT-treated rats were significantly improved when compared to those from C-19M group. Taken together, these findings confirm the neuroprotective ability of GT in the hippocampal formation probably due to the reduction of oxidative stress-related damage observed during aging.

Seizure-induced changes in neuropeptide Y-containing cortical neurons: Potential role for seizure threshold and epileptogenesis.

Seizure activity induces transient changes in the levels of neuropeptide Y (NPY) and somatostatin (SS) in various brain regions, but it remains unclear whether this effect can persist for long periods and whether it is relevant to epileptogenesis. We report that brief seizures evoked by electroshock produced an increase in the number of NPY neurons in the dentate hilus and retrosplenial cortex, an effect that lasted 10 weeks. The number of hilar SS neurons remained unchanged. However, the pentylenetetrazole seizure threshold was somewhat decreased in electroshock-treated rats. Despite this, no spontaneous seizures were detected in this group. In contrast, status epilepticus (pilocarpine model) produced loss of the hilar NPY and SS cells. Moreover, all rats with status epilepticus showed spontaneous behavioral seizures and their seizure threshold was markedly decreased. These findings support the notion that sustained NPY overexpression induced by brief seizures can be important in preventing epileptogenesis.

Effects of repeated electroconvulsive shock seizures and pilocarpine-induced status epilepticus on emotional behavior in the rat.

Affective symptoms are frequently observed in patients with epilepsy. Although the etiology of these behavioral complications remains unknown, it is possible that brain damage associated with frequent or prolonged seizures may contribute to their development. To address this issue, we examined the behavioral sequelae of repeated brief seizures evoked by electroconvulsive shock (ECS) and compared them with those resulting from prolonged status epilepticus (SE) induced with pilocarpine. Using the open-field and elevated plus-maze tests, we detected the presence of behavioral alterations indicative of elevated levels of anxiety in rats that were administered a course of ECS seizures. Fear conditioning was also enhanced in these animals. However, the rats that had experienced SE exhibited less anxiety-like behavior than controls and were severely impaired in fear conditioning. These results support the notion that brain lesions caused by either brief repeated seizures or SE is sufficient to induce some affective disturbances.

Retrosplenial granular b cortex in normal and epileptic rats: a stereological study.

Human temporal lobe epilepsy and experimental models of this disease are associated with loss of neurons and other structural alterations in several limbic brain structures including the hippocampal formation and adjacent parahippocampal cortical areas. The goal of this study was to test the hypothesis that seizure activity can produce damage to the retrosplenial granular b cortex (Rgb) which is known to be strongly connected with other limbic structures implicated in epilepsy. To test this hypothesis, we estimated, using stereological methods, the volumes and total neuronal numbers in Rgb cortex of rats that had experienced prolonged status epilepticus induced by pilocarpine (350 mg/kg), rats treated with six electroshock seizures (the first five seizures were spaced by 24-h intervals, whilst the last two were only 2 h apart), and control rats. Adult male Wistar rats were used in this experiment. Status epilepticus produced significant loss of neurons in Rgb cortical layers IV (22%) and V (44%), which was accompanied by volume reductions in layers I (17%), IV (11%), V (18%) and VI (24%). In electroshock-treated rats, the volume of Rgb cortical layer VI was reduced by 17% and the number of neurons estimated in layer V was smaller by 16% relative to control rats. Thus, the finding that status epilepticus and administration of brief generalized seizures both lead to degenerative morphological alterations in Rgb cortex provides the first experimental support for the hypothesis that this cortical area can be involved in seizure activity, as suggested by its anatomical connections.

Loss of synapses in the entorhinal-dentate gyrus pathway following repeated induction of electroshock seizures in the rat.

The goal of this study was to answer the question of whether repeated administration of electroconvulsive shock (ECS) seizures causes structural changes in the entorhinal-dentate projection system, whose neurons are known to be particularly vulnerable to seizure activity. Adult rats were administered six ECS seizures, the first five of which were spaced by 24-hr intervals, whereas the last two were only 2 hr apart. Stereological approaches were employed to compare the total neuronal and synaptic numbers in sham- and ECS-treated rats. Golgi-stained material was used to analyze dendritic arborizations of the dentate gyrus granule cells. Treatment with ECS produced loss of neurons in the entorhinal layer III and in the hilus of the dentate gyrus. The number of neurons in the entorhinal layer II, which provides the major source of dentate afferents, and in the granular layer of the dentate gyrus, known to receive entorhinal projections, remained unchanged. Despite this, the number of synapses established between the entorhinal layer II neurons and their targets, dentate granule cells, was reduced in ECS-treated rats. In addition, administration of ECS seizures produced atrophic changes in the dendritic arbors of dentate granule cells. The total volumes of entorhinal layers II, III, and V-VI were also found to be reduced in ECS-treated rats. By showing that treatment with ECS leads to partial disconnection of the entorhinal cortex and dentate gyrus, these findings shed new light on cellular processes that may underlie structural and functional brain changes induced by brief, generalized seizures.

Retrosplenial cortex lesions impair acquisition of active avoidance while sparing fear-based emotional memory.

There is strong evidence that the rat retrosplenial cortex (RC) is implicated in spatial navigation and in learning of both aversive and reward-based discrimination tasks. However, its involvement in other functions subserved by the limbic system to which it belongs has not yet been documented. We compared the performance of rats with bilateral excitotoxic damage to RC with that of control rats in a battery of conventional tests, including an open field, plus maze, fear conditioning, step-through passive avoidance, and two-way active avoidance techniques. In the open field, RC-lesioned rats showed somewhat decreased locomotion in the inner zone and increased defecation, which is suggestive of an anxiogenic effect. However, no differences between groups were detected in the plus-maze and inhibitory avoidance tests. Freezing scores recorded during fear conditioning, as well as during the context and tone tests, which were performed, respectively, 24 and 48 h after conditioning, did not differ between the groups. In contrast, acquisition of the active avoidance response was significantly impaired in rats with damaged RC, regarding both response latency and correctness. These data suggest that although the rat RC may play a role in the regulation of emotional responsiveness to new situations, it does not appear to contribute to emotional memory. They are also consistent with the idea that RC is a part of the limbic system that is involved in the compilation of motor programs for complex stereotyped movements such as approach and avoidance.

Impaired water maze navigation of Wistar rats with retrosplenial cortex lesions: effect of nonspatial pretraining.

Damage to the retrosplenial cortex (RC) impairs the performance of rodents on spatial learning and memory tasks, but the extent of these deficits was previously reported to be influenced by the lesion type, rat strain, and behavioral task used. The present study addressed the issue of whether or not cytotoxic damage to RC impairs place navigation of Wistar rats in the Morris water maze and, if so, whether this is merely attributable to spatial learning deficits or to impaired learning of general (nonspatial) behavioral strategies required to correctly perform this task or both. Behaviorally naive rats with bilateral lesions to RC were significantly impaired relative to sham-lesioned rats both during the period of initial learning of the task and during the later phases of training. In addition, these animals showed enhanced thigmotaxis, indicating that the lesion was associated with considerable abnormalities in nonspatial learning. In contrast, RC-lesioned animals that have been previously familiarized with general task rules in a series of shaping trials did not show more thigmotaxis than did their respective controls. Furthermore, although these rats were still impaired in the middle of the training process, their performance during the period of initial learning as well as by the end of training was found to now be normal. Our results confirm those of earlier studies indicating that RC is important for spatial navigation. The findings herein reported are also consistent with the notion that, in addition to spatial information processing, RC is involved in cognitive processes underlying the ability of subjects to properly respond to general task demands.

Timed hypocaloric food restriction alters the synthesis and expression of vasopressin and vasoactive intestinal peptide in the suprachiasmatic nucleus.

In mammals, the main circadian pacemaker is located in the suprachiasmatic nucleus (SCN) and its most potent synchronizer is the daily variation of the intensity of light. However, other nonphotic cues, such as timed food restriction, can induce changes in the circadian rhythms, leading also to the appearance of a food-entrained oscillator. The present study was designed to establish if the alterations of the circadian rhythms induced by timed hypocaloric food restriction are accompanied by structural changes in the SCN. Two groups of adult rats, both maintained on 12-h light/12-h dark cycles, were used; in one group, animals had permanent free access to food, whereas in the other they were subjected to a restricted hypocaloric early morning feeding during 7 months. Using stereological techniques and in situ hybridization, we have examined the structure of the SCN and the synthesis and expression of vasopressin (AVP) and vasoactive intestinal peptide (VIP). The volume of the SCN and the total number of neurons did not vary between the two groups. However, the total number of AVP- and VIP-immunoreactive neurons and the AVP and VIP mRNA levels were significantly decreased in timed hypocaloric food-restricted animals. The results indicate that timed hypocaloric food restriction has led to changes of AVP and VIP content of the neurons. They furthermore suggest the existence of a coupling between the food-entrainable oscillator and the light-entrainable pacemaker.

Selective loss of hilar neurons and impairment of initial learning in rats after repeated administration of electroconvulsive shock seizures.

Prolonged seizures induced by neurotoxins or intracranial electrical stimulation provoke death of hippocampal neurons, which results in conspicuous learning and memory deficits. We examined whether repeated brief seizures elicited by electroconvulsive shock (ECS) can also deteriorate hippocampal structure and function. Adult Wistar rats were administered six ECS seizures, the first five of which were 24 h apart, whilst the last two were spaced by a 2-h interval. Following a 2-month recovery period, the cognitive status of the animals was assessed using the water maze task. ECS-treated animals were incapable of learning the constant platform position version of this task during the first 4 days of training, but performed similarly to control rats throughout the rest of the acquisition period, on the probe trial, and on the variable platform position and visible platform tasks. The results of the morphological analysis showed that the total number of hippocampal pyramidal neurons and dentate gyrus granule cells were similar in control and ECS-treated rats. However, ECS treatment caused loss of approximately 17% of cells in the hilus of the dentate gyrus, which was accompanied by significant mossy fiber sprouting into the dentate inner molecular layer. In addition, we found that the ECS-induced decrease in the total number of hilar cells was not due to loss of inhibitory interneurons immunoreactive to somatostatin. These findings support the view that ECS-induced seizures can produce a number of morphological and functional changes in the rat hippocampal formation, which qualitatively resemble those previously described in other seizure models.

Nerve growth factor improves spatial learning and restores hippocampal cholinergic fibers in rats withdrawn from chronic treatment with ethanol.

The cholinergic septohippocampal pathway has long been known to be important for learning and memory. Prolonged intake of ethanol causes enduring memory deficits, which are paralleled by partial depletion of hippocampal cholinergic afferents. We hypothesized that exogenous supply of nerve growth factor (NGF), known to serve as a trophic substance for septal cholinergic neurons, can revert the ethanol-induced changes in the septohippocampal cholinergic system. Adult rats were given a 20% ethanol solution as their only source of fluid for 6 months. During the first 4 weeks after the animals were withdrawn from ethanol, they were intraventricularly infused with either NGF or vehicle alone via implanted osmotic minipumps. The vehicle-infused withdrawn animals showed impaired performance on a spatial reference memory version of the Morris water maze task, both during the task acquisition and on the retention test. In contrast, NGF-treated withdrawn rats were able to learn the task as well as controls, and significantly outperformed the vehicle-infused withdrawn rats. The histological analysis revealed that, in the latter group, the length density of fibers immunoreactive to choline acetyltransferase was reduced relative to control values by approximately 25%, as measured in the dentate gyrus and regio superior of the hippocampal formation. However, in NGF-treated withdrawn rats, the length density of these fibers was identical to that of control rats. These data provide support to the notion that NGF is capable of ameliorating memory deficits and restoring septohippocampal cholinergic projections following chronic treatment with ethanol.