Immunolesioning of basal forebrain cholinergic neurons facilitates hippocampal kindling and perturbs neurotrophin messenger RNA regulation

Alzheimer's disease and epilepsy: Research hotspots for comorbidity in the era of global aging

Neurological conditions such as Alzheimer's disease (AD) and epilepsy share a significant clinical overlap, particularly in the elderly, with each disorder potentiating the risk of the other. This interplay is significant amidst an aging global demographic. The review explores the classical pathologies of AD, including amyloid-beta plaques and hyperphosphorylated tau, and their potential role in the genesis of epilepsy. It also delves into the imbalance of glutamate and gamma-amino butyric acid activities, a key mechanism in epilepsy that may be influenced by AD pathology. The impact of age of onset on comorbidity is examined, with early-onset AD and Down syndrome presenting higher risks of epilepsy. The review suggests that epilepsy might precede cognitive symptoms in AD, indicating a complex interaction. Sleep modulation is highlighted as a factor, with sleep disturbances potentially contributing to AD progression. The necessity for cautious medication management is emphasized due to the cognitive effects of certain antiepileptic drugs. Animal models are recognized for their importance in understanding the relationship between AD and epilepsy, though creating fully representative models presents a challenge. The review concludes by noting the efficacy of medications such as lamotrigine, levetiracetam, and memantine in managing both conditions and suggests the ketogenic diet and cannabidiol as emerging treatment options, warranting further investigation for comprehensive patient care strategies.

Long-term nucleus basalis cholinergic depletion induces attentional deficits and impacts cortical neurons and BDNF levels without affecting the NGF synthesis

Basal forebrain cholinergic neurons (BFCNs) represent the main source of cholinergic innervation to the cortex and hippocampus and degenerate early in Alzheimer's disease (AD) progression. Phenotypic maintenance of BFCNs depends on levels of mature nerve growth factor (mNGF) and mature brain-derived neurotrophic factor (mBDNF), produced by target neurons and retrogradely transported to the cell body. Whether a reciprocal interaction where BFCN inputs impact neurotrophin availability and affect cortical neuronal markers is unknown. To address our hypothesis, we immunolesioned the nucleus basalis (nb), a basal forebrain cholinergic nuclei projecting mainly to the cortex, by bilateral stereotaxic injection of 192-IgG-Saporin (the cytotoxin Saporin binds p75ntr receptors expressed exclusively by BFCNs) in 2.5-month-old Wistar rats. At six months post-lesion, Saporin-injected rats (SAP) showed an impairment in a modified version of the 5-Choice Serial Reaction Time Task (5-choice task). Post-mortem analyses of the brain revealed a reduction of Choline Acetyltransferase-immunoreactive neurons compared to wild-type controls. A diminished number of cortical vesicular acetylcholine transporter-immunoreactive boutons was accompanied by a reduction in BDNF mRNA, mBDNF protein levels, markers of glutamatergic (vGluT1) and GABAergic (GAD65) neurons in the SAP-group compared to the controls. NGF mRNA, NGF precursor and mNGF protein levels were not affected. Additionally, cholinergic markers correlated with the attentional deficit and BDNF levels. Our findings demonstrate that while cholinergic nb loss impairs cognition and reduces cortical neuron markers, it produces differential effects on neurotrophin availability, affecting BDNF but not NGF levels.

Direct Septum-Hippocampus Cholinergic Circuit Attenuates Seizure Through Driving Somatostatin Inhibition

BACKGROUND

Previous studies indicated the involvement of cholinergic neurons in seizure; however, the specific role of the medial septum (MS)-hippocampus cholinergic circuit in temporal lobe epilepsy (TLE) has not yet been completely elucidated.

METHODS

In the current study, we used magnetic resonance imaging and diffusion tensor imaging to characterize the pathological change of the MS-hippocampus circuit in 42 patients with TLE compared with 22 healthy volunteers. Using optogenetics and chemogenetics, combined with in vivo or in vitro electrophysiology and retrograde rabies virus tracing, we revealed a direct MS-hippocampus cholinergic circuit that potently attenuates seizure through driving somatostatin inhibition in animal TLE models.

RESULTS

We found that patients with TLE with hippocampal sclerosis showed a decrease of neuronal fiber connectivity of the MS-hippocampus compared with healthy people. In the mouse TLE model, MS cholinergic neurons ceased firing during hippocampal seizures. Optogenetic and chemogenetic activation of MS cholinergic neurons (but not glutamatergic or GABAergic [gamma-aminobutyric acidergic] neurons) significantly attenuated hippocampal seizures, while specific inhibition promoted hippocampal seizures. Electrophysiology combined with modified rabies virus tracing studies showed that direct (but not indirect) MS-hippocampal cholinergic projections mediated the antiseizure effect by preferentially targeting hippocampal GABAergic neurons. Furthermore, chemogenetic inhibition of hippocampal somatostatin-positive (rather than parvalbumin-positive) subtype of GABAergic neurons reversed the antiseizure effect of the MS-hippocampus cholinergic circuit, which was mimicked by activating somatostatin-positive neurons.

CONCLUSIONS

These findings underscore the notable antiseizure role of the direct cholinergic MS-hippocampus circuit in TLE through driving the downstream somatostatin effector. This may provide a better understanding of the changes of the seizure circuit and the precise spatiotemporal control of epilepsy.

Acute Down-regulation of BDNF Signaling Does Not Replicate Exacerbated Amyloid-β Levels and Cognitive Impairment Induced by Cholinergic Basal Forebrain Lesion

Degeneration of basal forebrain cholinergic neurons (BFCNs) precedes hippocampal degeneration and pathological amyloid-beta (Aβ) accumulation, and underpins the development of cognitive dysfunction in sporadic Alzheimer’s disease (AD). We hypothesized that degeneration of BFCNs causes a decrease in neurotrophin levels in innervated brain areas, which in turn promotes the development of Aβ pathology and cognitive impairment. Here we show that lesion of septo-hippocampal BFCNs in a pre-symptomatic transgenic amyloid AD mouse model (APP/PS1 mice) increases soluble Aβ levels in the hippocampus, and induces cognitive deficits in a spatial memory task that are not seen in either unlesioned APP/PS1 or non-transgenic littermate control mice. Furthermore, the BFCN lesion results in decreased levels of brain-derived neurotrophic factor (BDNF). However, viral knockdown of neuronal BDNF in the hippocampus of APP/PS1 mice (in the absence of BFCN loss) neither increased the level of Aβ nor caused cognitive deficits. These results suggest that the cognitive decline and Aβ pathology induced by BFCN loss occur independent of dysfunctional neuronal BDNF signaling, and may therefore be directly underpinned by reduced cholinergic neurotransmission.

Cholinergic Basal Forebrain Lesion Decreases Neurotrophin Signaling without Affecting Tau Hyperphosphorylation in Genetically Susceptible Mice

Alzheimer's disease (AD) is a progressive, irreversible neurodegenerative disease that destroys memory and cognitive function. Aggregates of hyperphosphorylated tau protein are a prominent feature in the brain of patients with AD, and are a major contributor to neuronal toxicity and disease progression. However, the factors that initiate the toxic cascade that results in tau hyperphosphorylation in sporadic AD are unknown. Here we investigated whether degeneration of basal forebrain cholinergic neurons (BFCNs) and/or a resultant decrease in neurotrophin signaling cause aberrant tau hyperphosphorylation. Our results reveal that the loss of BFCNs in pre-symptomatic pR5 (P301L) tau transgenic mice results in a decrease in hippocampal brain-derived neurotrophic factor levels and reduced TrkB receptor activation. However, there was no exacerbation of the levels of phosphorylated tau or its aggregation in the hippocampus of susceptible mice. Furthermore the animals' performance in a hippocampal-dependent learning and memory task was unaltered, and no changes in hippocampal synaptic markers were observed. This suggests that tau pathology is likely to be regulated independently of BFCN degeneration and the corresponding decrease in hippocampal neurotrophin levels, although these features may still contribute to disease etiology.

Unexpected Effects of Acetylcholine Precursors on Pilocarpine Seizure- Induced Neuronal Death

BACKGROUND

Choline alfoscerate (α-GPC) and Cytidine 5'-diphosphocholine (CDPCholine) are both acetylcholine precursors and are considered to act as pro-cholinergic nootropic agents. Acetylcholine precursors have also recently found frequent use in the neurology clinic. Stroke and many types of dementia have been shown to respond favorably after treatment with these agents, not only in terms of cognitive dysfunction but also behavioral and psychological symptoms. The primary mechanisms of Acetylcholine precursors are the following: 1) Acetylcholine precursors themselves are used in the biosynthesis of acetylcholine and 2) byproducts like glycerophosphate have protective functions for neuronal phospholipids. However, whether acetylcholine precursors have a similar effect in treating cognitive impairment in patients with epilepsy remains controversial.

METHODS

Our previous studies investigating acetylcholine precursors in seizure-experienced animals have produced variable results that were dependent on the timing of administration.

RESULTS

Early administration of CDP-choline immediately after seizure increased neuronal death, blood-brain barrier (BBB) disruption and microglial activation in the hippocampus. However, administration of α-GPC starting 3 weeks after seizure (late administration) improved cognitive function through reduced neuronal death and BBB disruption, and increased neurogenesis in the hippocampus.

CONCLUSION

These seemingly contradictory results may be attributed to both epileptogenic features and neuroprotective functions of several acetylcholine precursors.

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.

Increased BDNF may not be associated with cognitive impairment in heroin-dependent patients

A growing number of evidence suggests that brain-derived neurotrophic factor (BDNF) plays an important part in modulating the activities on the basis of hippocampus neural plasticity, such as learning and memory. Heroin addiction has a series of cognitive impairments that may be associated with BDNF. In this study, we explored the association of BDNF with cognitive function in heroin-dependent patients.We enrolled 86 heroin-dependent patients and 238 normal control subjects and examined their cognition by the repeatable battery for the assessment of neuropsychological status (RBANS) and serum BDNF levels in 2 groups.BDNF levels were significantly higher in patients than controls (P < .001). Cognitive scores of the RBANS showed that attention and language index (P < .05) were significantly lower in heroin-dependent patients than control groups. Unfortunately, we found no positive association between BDNF and cognitive function in patients, except that BDNF was positively associated with visuospatial/constructional index in control groups.Our findings suggest that BDNF may not be involved in the pathophysiology of heroin dependence, but more studies about cognitive impairment in heroin addiction are needed.

Pre- and postsynaptic twists in BDNF secretion and action in synaptic plasticity

Overwhelming evidence collected since the early 1990's strongly supports the notion that BDNF is among the key regulators of synaptic plasticity in many areas of the mammalian central nervous system. Still, due to the extremely low expression levels of endogenous BDNF in most brain areas, surprisingly little data i) pinpointing pre- and postsynaptic release sites, ii) unraveling the time course of release, and iii) elucidating the physiological levels of synaptic activity driving this secretion are available. Likewise, our knowledge regarding pre- and postsynaptic effects of endogenous BDNF at the single cell level in mediating long-term potentiation still is sparse. Thus, our review will discuss the data currently available regarding synaptic BDNF secretion in response to physiologically relevant levels of activity, and will discuss how endogenously secreted BDNF affects synaptic plasticity, giving a special focus on spike timing-dependent types of LTP and on mossy fiber LTP. We will attempt to open up perspectives how the remaining challenging questions regarding synaptic BDNF release and action might be addressed by future experiments. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Alzheimer's disease and epilepsy: insight from animal models

Alzheimer's disease (AD) and epilepsy are separated in the medical community, but seizures occur in some patients with AD, and AD is a risk factor for epilepsy. Furthermore, memory impairment is common in patients with epilepsy. The relationship between AD and epilepsy remains an important question because ideas for therapeutic approaches could be shared between AD and epilepsy research laboratories if AD and epilepsy were related. Here we focus on one of the many types of epilepsy, temporal lobe epilepsy (TLE), because patients with TLE often exhibit memory impairment, depression and other comorbidities that occur in AD. Moreover, the seizures that occur in patients with AD may be nonconvulsive, which occur in patients with TLE. Here we first compare neuropathology in TLE and AD with an emphasis on the hippocampus, which is central to both AD and TLE research. Then we compare animal models of AD pathology with animal models of TLE. Although many aspects of the comparisons are still controversial, there is one conclusion that we suggest is clear: some animal models of TLE could be used to help address questions in AD research, and some animal models of AD pathology are bona fide animal models of epilepsy.

Revisiting the cholinergic hypothesis in the development of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia affecting the elderly population today; however, there is currently no accurate description of the etiology of this devastating disorder. No single factor has been demonstrated as being causative; however, an alternative co-factors theory suggests that the interaction of multiple risk factors is responsible for AD. We have used this model, in combination with the original cholinergic hypothesis of AD to propose a "new" cholinergic hypothesis that we present in this review. This new version takes into account recent findings from the literature and our reports of removal of medial septum cholinergic projections to the hippocampus reduces both behavioural and anatomical plasticity, resulting in greater cognitive impairment in response to secondary insults (stress, injury, disease, etc.). We will first summarize the experimental results and discuss some potential mechanisms that could explain our results. We will then present our 'new' version of the cholinergic hypothesis and how it relates to the field of AD research today. Finally we will discuss some of the implications for treatment that arise from this model and present directions for future study.

Cell therapy in models for temporal lobe epilepsy

For patients with refractory epilepsy it is important to search for alternative treatments. One of these potential treatments could be introducing new cells or modulating endogenous neurogenesis to reconstruct damaged epileptic circuits or to bring neurotransmitter function back into balance. In this review the scientific basis of these cell therapy strategies is discussed and the results are critically evaluated. Research on cell transplantation strategies has mainly been performed in animal models for temporal lobe epilepsy, in which seizure foci or seizure propagation pathways are targeted. Promising results have been obtained, although there remains a lot of debate about the relevance of the animal models, the appropriate target for transplantation, the suitable cell source and the proper time point for transplantation. From the presented studies it should be evident that transplanted cells can survive and sometimes even integrate in an epileptic brain and in a brain that is subjected to epileptogenic interventions. There is evidence that transplanted cells can partially restore damaged structures and/or release substances that modulate existent or induced hyperexcitability. Even though several studies show encouraging results, more studies need to be done in animal models with spontaneous seizures in order to have a better comparison to the human situation.

Up-regulation of cation-independent mannose 6-phosphate receptor and endosomal-lysosomal markers in surviving neurons after 192-IgG-saporin administrations into the adult rat brain

The cation-independent mannose 6-phosphate receptor (CI-MPR) is a single transmembrane domain glycoprotein that plays a major role in the trafficking of lysosomal enzymes from the trans-Golgi network to the endosomal-lysosomal (EL) system. Because dysfunction of EL system is associated with a variety of neurodegenerative disorders, it is possible that the CI-MPR may have a role in regulating neuronal viability after toxicity/injury. In the present study, we report that 192-IgG-saporin-induced loss of basal forebrain cholinergic neurons causes a transient up-regulation of CI-MPR protein levels in surviving neurons of the basal forebrain and frontal cortex but not in the brainstem region, which was relatively spared by the immunotoxin. This was accompanied by a parallel time-dependent increase in other EL markers, ie, cathepsin D, Rab5, and LAMP2 in the basal forebrain region, whereas in the frontal cortex the levels of cathepsin D, and to some extent Rab5, were increased. Given the critical role of the EL system in the clearance of abnormal proteins in response to changing conditions, it is likely that the observed increase in the CI-MPR and components of the EL system in surviving neurons after 192-IgG-saporin treatment represents an adaptive mechanism to restore the metabolic/structural abnormalities induced by the loss of cholin-ergic neurons.

CCK-8 induces NGF and BDNF synthesis and modulates TrkA and TrkB expression in the rat hippocampus and septum: Effects on kindling development

In our previous studies, we demonstrated that intraperitoneal (i.p.) injections with the neurotransmitter/neuromodulatory peptide Cholecystokinin-8 (CCK-8) stimulate the synthesis of the neurotrophin nerve growth factor (NGF) resulting in the structural and functional recovery of neuronal damage. This neurotrophin-mediated neuroprotective action of CCK-8 has opened a new perspective for a better understanding of the CCK neurobiological and pharmacological properties. To explore the possible beneficial effects of the CCK-induced increase of neurotrophin availability in brain, we compared the effects of i.p. CCK-8 in healthy rats and in a chemical kindling model using a subconvulsive dose of pentylenetetrazol (PTZ). Behavioural changes were monitored during treatment and classified according to a six-point scale. After 3 weeks of treatment (12 trials), the PTZ group of rats manifested generalized clonic-tonic seizures (Class 5 behaviour). For this reason, this time point was chosen to compare the effects of CCK-8 treatment on the expression of NGF, the brain derived neurotrophin factor (BDNF) and their receptors in the septum and hippocampus. We found that repeated i.p. injections with CCK-8 in adult rats result in: (1) an increase of NGF and BDNF protein and mRNA levels in the septum and hippocampus; (2) a down-regulation of TrkA and p75NTR and an up-regulation of TrkB; (3) reduced susceptibility to develop chemical kindling; (4) recovery of the PTZ-induced changes in the expression of neurotrophin receptors in the septal and hippocampal tissues. This data clearly indicates that CCK-induced variation of neurotrophin synthesis in brain is able to influence the susceptibility to develop seizures in adult rats most probably by counteracting the progressive neuronal dysfunction and/or damage.

Susceptibility to seizure-induced injury and acquired microencephaly following intraventricular injection of saporin-conjugated 192 IgG in developing rat brain

To study the role of neurotrophin-responsive neurons in brain growth and developmental resistance to seizure-induced injury, we infused saporin-conjugated 192-IgG (192 IgG-saporin), a monoclonal antibody directed at the P75 neurotrophin receptors (p75(NTR)), into the ventricles of postnatal day 8 (P8) rat pups. 7-10 days after immunotoxin treatment, loss of p75(NTR) immunoreactivity was associated with depletion of basal forebrain cholinergic projection to the neocortex and hippocampus. Kainic acid (KA)-induced seizures on P15 resulted in hippocampal neuronal injury in the majority of toxin-treated animals (13/16), but only rarely in saline-injected controls (2/25) (P < 0.001). In addition, widespread cerebral atrophy and a significant decrease in brain weight with preserved body weight were observed. Volumetric analysis of the hippocampal hilar region revealed a 2-fold reduction in perikaryal size and a 1.7-fold increase in cell packing density after 192 IgG-saporin injection. These observations indicate that neurotrophin-responsive neurons including basal forebrain magnocellular cholinergic neurons may be critical for normal brain growth and play a protective role in preventing excitotoxic neuronal injury during development.

Forebrain acetylcholine regulates adult hippocampal neurogenesis and learning

Hippocampus-mediated learning enhances neurogenesis in the adult dentate gyrus (DG), and this process has been suggested to be involved in memory formation. The hippocampus receives abundant cholinergic innervation and acetylcholine (ACh) plays an important role in learning and Alzheimer's disease (AD) pathophysiology. Here, we show that a selective neurotoxic lesion of forebrain cholinergic input with 192 IgG-saporin reduces DG neurogenesis with a concurrent impairment in spatial memory. Conversely, systemic administration of the cholinergic agonist physostigmine increases DG neurogenesis. We find that changes of forebrain ACh levels primarily influence the proliferation and/or the short-term survival rather than the long-term survival or differentiation of the new neurons. We further demonstrate that these newly born cells express the muscarinic receptor subtypes M1 and M4. Our data provide evidence that forebrain ACh promotes neurogenesis, and suggest that the impaired cholinergic function in AD may in part contribute to deficits in learning and memory through reductions in the formation of new hippocampal neurons.

Focal treatment for refractory epilepsy: hope for the future?

Despite advances in anti-epileptic drug therapy and epilepsy surgery in recent years, intractable epilepsy remains a large clinical problem. Surgical resection, which can have an excellent outcome, is appropriate for only a minority of patients in whom an identifiable focus in non-eloquent brain can be identified. Systemic drug delivery is inevitably limited by the potential for unwanted side effects, due to actions both outside the CNS and in non-epileptic brain regions. Thus for a substantial number of patients novel treatment approaches are urgently needed. Both focal drug delivery and neuronal stem cell grafting have been evaluated in a variety of experimental epilepsy models in recent years, targeting either the seizure focus or key propagation pathways. The literature in this field is critically reviewed and considered in a clinical context. Studies in both areas are hampered by the limitations of available animal models, and by uncertainties in discerning which changes in the epileptic brain directly promote seizures, and which are compensatory. However, in many cases promising, though short-term, results have been obtained. Before such studies could be considered in humans further investigations that include long-term seizure and behavioural outcomes, in clinically relevant experimental models, are required. However, the current literature does provide proof in principle for a focal treatment approach, which may offer hope for many currently intractable patients for whom drug developments and surgical advances have proved disappointing.

Effects of lesions of basal forebrain cholinergic neurons in newborn rats on susceptibility to seizures

The cholinergic system modulates cerebral excitability. We recently reported that immunolesions of the basal forebrain (BF) cholinergic neurons in adult rats increase the susceptibility to generalized seizures. In this study we investigated the effects of lesions of the BF cholinergic neurons in neonatal rats on seizure susceptibility and cognitive function. Neonatal rats at postnatal day (P) 7 received intracerebroventricular (i.c.v.) injections of 192 IgG-saporin (SAP) or phosphate-buffered saline. Following 3 weeks after the injection the first group of rats was implanted with hippocampal electrodes for electroencephalogram (EEG) recordings while the second group of rats was tested for visual spatial memory using the hidden platform version of the water maze test. The first group of rats was then tested for seizure susceptibility using flurothyl 1 week after the electrode implantation. Rats that received immunolesions of the BF cholinergic neurons at P7 had significantly shorter latencies to onset of myoclonic jerks and tonic-clonic seizures than controls. However, no significant differences were found in the duration of seizures, or EEG ictal duration. No significant deficits in spatial learning were found between rats that received i.c.v. injections of SAP at P7 and controls. As in adult rats, lesions of the BF cholinergic system in rat pups result in subsequent increase in seizure susceptibility.

Hippocampal brain-derived neurotrophic factor gene regulation by exercise and the medial septum

Brain-derived neurotrophic factor (BDNF) enhances synaptic plasticity and neuron function. We have reported that voluntary exercise increases BDNF mRNA levels in the hippocampus; however, mechanisms underlying this regulation have not been defined. We hypothesized that medial septal cholinergic and/or gamma amino butyric acid (GABA)ergic neurons, which provide a major input to the hippocampus, may regulate the baseline gene expression and exercise-dependent gene upregulation of this neurotrophin. Focal lesions were produced by medial septal infusion of the saporin-linked immunotoxins 192-IgG-saporin or OX7-saporin. 192-IgG-saporin produced a selective and complete loss of medial septal cholinergic neurons with no accompanying GABA loss. Baseline BDNF mRNA was reduced in the hippocampus of sedentary animals, but exercise-induced gene upregulation was not impaired, despite complete loss of septo-hippocampal cholinergic afferents. OX7-saporin produced a graded lesion of the medial septum characterized by predominant GABA neuron loss with less reduction in the number of cholinergic cells. OX7-saporin lesion reduced baseline hippocampal BDNF mRNA and attenuated exercise-induced gene upregulation, in a dose-dependent manner. These results suggest that combined loss of septal GABAergic and cholinergic input to the hippocampus may be important for exercise-dependent BDNF gene regulation, while cholinergic activity on its own is not sufficient. These results are discussed in relation to their implications for aging and Alzheimer's disease.

Lack of effect of moderate Purkinje cell loss on working memory

192 immunoglobulin G-saporin (192-sap) is an immunotoxin which targets the cholinergic basal forebrain after injection into either the ventricular system or the parenchyma of the rat brain. When injected by the i.c.v. route, 192-sap kills some cerebellar Purkinje cells in addition to its more extensive killing of the cholinergic basal forebrain. Behaviorally, i.c.v. injections of 192-sap result in impaired performance in a variety of experimental paradigms of learning and memory including a working memory task in the radial maze. The current study examined the contribution, if any, of immunotoxin-induced Purkinje cell loss to impaired performance in the radial maze. To meet this aim, we used i.c.v. injection of another immunotoxin, OX7-saporin (OX7-sap), at a dose that produced Purkinje cell loss of similar extent to that produced by i.c.v. 192-sap. We then compared these OX7-sap-injected rats with 192-sap-injected rats in a radial maze working memory task. We found a working memory impairment only in the 192-sap-injected rats. These data show that moderate Purkinje cell loss alone is insufficient to impair working memory. Furthermore, the data are consistent with the idea that the working memory deficit observed in 192-sap-injected animals is likely due to lesioning of the cholinergic basal forebrain.

Functional switch between motor tracts in the presence of the mAb IN-1 in the adult rat

Fine finger and hand movements in humans, monkeys, and rats are under the direct control of the corticospinal tract (CST). CST lesions lead to severe, long-term deficits of precision movements. We transected completely both CSTs in adult rats and treated the animals for 2 weeks with an antibody that neutralized the central nervous system neurite growth inhibitory protein Nogo-A (mAb IN-1). Anatomical studies of the rubrospinal tracts showed that the number of collaterals innervating the cervical spinal cord doubled in the mAb IN-1- but not in the control antibody-treated animals. Precision movements of the forelimb and fingers were severely impaired in the controls, but almost completely recovered in the mAb IN-1-treated rats. Low threshold microstimulation of the motor cortex induced a rapid forelimb electromyography response that was mediated by the red nucleus in the mAb IN-1 animals but not in the controls. These findings demonstrate an unexpectedly high capacity of the adult central nervous system motor system to sprout and reorganize in a targeted and functionally meaningful way.

Septal cholinergic neurons suppress seizure development in hippocampal kindling in rats: comparison with noradrenergic neurons

Widespread lesions of forebrain cholinergic or noradrenergic projections by intraventricular administration of 192 IgG-saporin or 6-hydroxydopamine, respectively, accelerate kindling epileptogenesis. Here we demonstrate both quantitative and qualitative differences between the two lesions in their effects on hippocampal kindling in rats. Epileptogenesis was significantly faster after noradrenergic as compared to cholinergic denervation, and when both lesions were combined, kindling development resembled that in animals with 6-hydroxydopamine lesion alone. Furthermore, whereas the 192 IgG-saporin lesion promoted the development only of the early stages of kindling, administration of 6-hydroxydopamine or both neurotoxins accelerated the late stages also. To investigate the contribution of different subparts of the basal forebrain cholinergic system to its seizure-suppressant action in hippocampal kindling, 192 IgG-saporin was injected into medial septum/vertical limb of the diagonal band of Broca or nucleus basalis magnocellularis, leading to selective hippocampal or cortical cholinergic deafferentation, respectively. The denervation of the hippocampus facilitated kindling similar to the extensive lesion caused by intraventricular 192 IgG-saporin, whereas the cortical lesion had no effect. These results indicate that although both noradrenergic and cholinergic projections to the forebrain exert powerful inhibitory effects on hippocampal kindling epileptogenesis, the action of the cholinergic system is less pronounced and occurs specifically prior to seizure generalization. In contrast, noradrenergic neurons inhibit the development of both focal and generalized seizures. The septo-hippocampal neurons are responsible for the antiepileptogenic effect of the cholinergic system in hippocampal kindling, whereas the cortical projection is not significantly involved. Conversely, we have previously shown [Ferencz I. et al. (2000) Eur. J. Neurosci., 12, 2107-2116] that seizure-suppression in amygdala kindling is exerted through the cortical and not the hippocampal cholinergic projection. This shows that, depending on the location of the primary epileptic focus, i.e. the site of stimulation, basal forebrain cholinergic neurons operate through different subsystems to counteract seizure development in kindling.

Is there nicotinic modulation of nerve growth factor? Implications for cholinergic therapies in Alzheimer's disease

Studies on the neurobiology of nerve growth factor (NGF) reveal a diverse range of actions. Through alterations in gene expression, NGF is important in maintaining and regulating the phenotype of neurons that express the high-affinity receptor, trkA. Nerve growth factor also has a rapid action, revealed by its role in pain signaling in bladder and in skin. In the central nervous system (CNS), NGF has an intimate relationship with the cholinergic system. It promotes cholinergic neuron survival after experimental injury but also maintains and regulates the phenotype of uninjured cholinergic neurons. In addition to these effects mediated by gene expression, NGF has a rapid neurotransmitter-like action to regulate cholinergic neurotransmission and neuronal excitability. Consistent with its actions on the cholinergic system, NGF can enhance function in animals with cholinergic lesions and has been proposed to be useful in humans with Alzheimer's disease (AD); however, the problems of CNS delivery and of side effects (particularly pain) limit the clinical efficacy of NGF. Drug treatment strategies to enhance production of NGF in the CNS may be useful in the treatment of AD. Nicotine is one such agent, which, when administered directly to the hippocampus in rats, produces long-lasting elevation of NGF production.

The role of cytokines and growth factors in seizures and their sequelae

Although the neuropathological changes caused by severe or repeated seizures have been well characterized, many questions about the molecular mechanisms involved remain unanswered. Neuronal cell death, reactive gliosis, enhanced neurogenesis, and axonal sprouting are four of the best-studied sequelae of seizures. In vitro, each of these pathological processes can be substantially influenced by soluble protein factors, including neurotrophins, cytokines, and growth factors. Furthermore, many of these proteins and their receptors are expressed in the adult brain and are up-regulated in response to neuronal activity and injury. We review the evidence that these intercellular signaling proteins regulate seizure activity as well as subsequent pathology in vivo. As nerve growth factor and brain derived neurotrophic factor are the best-studied proteins of this class, we begin by discussing the evidence linking these neurotrophins to epilepsy and seizure. More than a dozen additional cytokines, growth factors, and neurotrophins that have been examined in the context of epilepsy models are then considered. We discuss the effect of seizure on expression of cytokines and growth factors, and explore the regulation of seizure development and aftermath by exogenous application or antagonist perturbation of these proteins. The experimental evidence supports a role for these factors in each aspect of seizure and pathology, and suggests potential targets for future therapeutics.

Increased susceptibility to generalized seizures after immunolesions of the basal forebrain cholinergic neurons in rats

We investigated whether basal forebrain cholinergic neurons influence the expression of generalized seizures. Animals received intracerebroventricular injections of saporin (lesioned) or saline (controls) and were tested for susceptibility to flurothyl- or pentylenetetrazole-induced seizures. Lesioned rats had significantly shorter latencies to onset of generalized tonic-clonic seizures than controls. Our findings suggest that basal forebrain cholinergic neurons may participate in the modulation of generalized seizures.

Basal forebrain neurons suppress amygdala kindling via cortical but not hippocampal cholinergic projections in rats

Intraventricular administration of the immunotoxin 192 IgG-saporin in rats has been shown to cause a selective loss of cholinergic afferents to the hippocampus and cortical areas, and to facilitate seizure development in hippocampal kindling. Here we demonstrate that this lesion also accelerates seizure progression when kindling is induced by electrical stimulations in the amygdala. However, whereas intraventricular 192 IgG-saporin facilitated the development of the initial stages of hippocampal kindling, the same lesion promoted the late stages of amygdala kindling. To explore the role of various parts of the basal forebrain cholinergic system in amygdala kindling, selective lesions of the cholinergic projections to either hippocampus or cortex were produced by intraparenchymal injections of 192 IgG-saporin into medial septum/vertical limb of the diagonal band or nucleus basalis, respectively. Cholinergic denervation of the cortical regions caused acceleration of amygdala kindling closely resembling that observed after the more widespread lesion induced by intraventricular 192 IgG-saporin. In contrast, removal of the cholinergic input to the hippocampus had no effect on the development of amygdala kindling. These data indicate that basal forebrain cholinergic neurons suppress kindling elicited from amygdala, and that this dampening effect is mediated via cortical but not hippocampal projections.

Changes in the gene expression of GABAA receptor subunit mRNAs in the septum of rats subjected to pentylenetetrazol-induced kindling

Chemical kindling was induced in rats by long-term administration of pentylenetetrazol (PTZ) (30 mg/kg three times a week for 9 weeks). The effects of such kindling on the abundance of transcripts encoding subunits of the gamma-aminobutyric acid type A (GABAA) receptor in the brain were measured by RNase protection assay. Kindled rats were examined either 3 or 30 days after discontinuation of PTZ treatment. The amounts of gamma2L and gamma2S subunit mRNAs were significantly increased in the hippocampus and cerebral cortex of kindled rats 3 and 30 days after treatment discontinuation, compared with those observed in control rats, and these effects were prevented by the concomitant administration of the anticonvulsant abecarnil. In contrast, the amounts of alpha1 and beta2 subunit mRNAs in these two brain regions did not differ significantly between kindled and control rats. The abundance of alpha1, beta2, gamma2L and gamma2S subunit mRNAs was decreased in the septum of rats 3 or 30 days after discontinuation of treatment with PTZ either alone or in combination with abecarnil. The amounts of none of the four subunit mRNAs measured differed significantly between the striatum or frontal cortex of kindled rats and control rats 3 days after drug discontinuation. Immunohistochemical analysis with antibodies to choline acetyltransferase revealed a marked decrease in the number of cholinergic neurons in the septum of kindled rats 30 days after discontinuation of PTZ treatment; this effect was not prevented by the administration of abecarnil. These results suggest that long-term treatment with PTZ induces a loss of GABAA receptors in the septum.

Choline and selective antagonists identify two subtypes of nicotinic acetylcholine receptors that modulate GABA release from CA1 interneurons in rat hippocampal slices

Neuronal nicotinic receptors (nAChR) are known to control transmitter release in the CNS. Thus, this study was aimed at exploring the diversity and localization of nAChRs present in CA1 interneurons in rat hippocampal slices. The use of a U-tube as the agonist delivery system was critical for the reliable detection of nicotinic responses induced by brief exposure of the neurons to ACh or to the alpha7 nAChR-selective agonist choline. The present study demonstrated that CA1 interneurons, in addition to expressing functional alpha7 nAChRs, also express functional alpha4beta2-like nAChRs and that activation of both receptors facilitates an action potential-dependent release of GABA. Depending on the experimental condition, one of the following nicotinic responses was recorded from the interneurons by means of the patch-clamp technique: a nicotinic whole-cell current, depolarization accompanied by action potentials, or GABA-mediated postsynaptic currents (PSCs). Responses mediated by alpha7 nAChRs were short-lasting, whereas those mediated by alpha4beta2 nAChRs were long-lasting. Thus, phasic or tonic inhibition of CA1 interneurons may be achieved by selective activation of alpha7 or alpha4beta2 nAChRs, respectively. It can also be suggested that synaptic levels of choline generated by hydrolysis of ACh in vivo may be sufficient to control the activity of the alpha7 nAChRs. The finding that methyllycaconitine and dihydro-beta-erythroidine (antagonists of alpha7 and alpha4beta2 nAChRs, respectively) increased the frequency and amplitude of GABAergic PSCs suggests that there is an intrinsic cholinergic activity that sustains a basal level of nAChR activity in these interneurons.

Hippocampal neurotrophin and trk receptor mRNA levels are altered by local administration of nicotine, carbachol and pilocarpine

Cholinergic receptor agonists nicotine (nicotinic), carbachol (nicotinic/muscarinic) and pilocarpine (muscarinic) were administered into the hippocampus and mRNA levels of neurotrophins and their receptors determined using in situ hybridisation. Drug doses were carefully chosen to avoid the potentially confounding effects of seizure and cell death. Nicotine caused a long-lasting increase in nerve growth factor (NGF) mRNA in all subfields of the hippocampus. The increase was evident from 24 h up to 72 h after drug administration. This increase was dependent on excitatory amino acid neurotransmission as it was blocked by administration of an AMPA or NMDA receptor antagonist. In contrast, carbachol and pilocarpine produced a transient increase in NGF mRNA levels present 4-8 h after drug administration. Pilocarpine caused a transient increase in hippocampal brain-derived neurotrophic factor (BDNF) levels, with carbachol and nicotine showing the same trend. Nicotine and carbachol caused transient decreases in NT-3 mRNA levels in dentate gyrus and CA2 with pilocarpine showing a similar trend. Increases in mRNA encoding full-length trkB were seen 8 h after nicotine, with nicotine also causing elevations in a mRNA encoding a truncated isoform (trkB.T2). TrkC mRNA was not altered by any of the conditions used. The study suggests that muscarinic and nicotinic receptor activation in the hippocampus causes transient changes in all of the neurotrophins, but that NGF levels are selectively up-regulated by nicotinic receptor stimulation. The reciprocal interaction between NGF and ascending cholinergic systems may be a component of the cognitive enhancing effects of nicotine.

Exercise-induced regulation of brain-derived neurotrophic factor (BDNF) transcripts in the rat hippocampus

Previous results from our laboratory indicate that two nights of voluntary wheel running upregulates brain-derived neurotrophic factor (BDNF) mRNA expression in the hippocampus. In order to investigate the time-course of the BDNF response and to examine how physical activity preferentially activates particular transcriptional pathways, the effects of 6 and 12 h of voluntary wheel running on BDNF and exons I-IV mRNA expression were investigated in rats. Hippocampal full-length BDNF mRNA expression was rapidly influenced by physical activity, showing significant increases in expression levels as soon as 6 h of voluntary wheel running. Moreover, there was a strong positive correlation between distance run and BDNF mRNA expression. Exon I mRNA expression was significantly upregulated after 6 h of running and was maintained or enhanced by 12 h of voluntary running. Exon II had a slower time-course and was significantly upregulated after 12 h, selectively in the CA1 hippocampal region. Exon III and Exon IV showed no significant increase in expression level after 6 or 12 h of running in the paradigm studied. It is significant that the rapid neurotrophin response is demonstrated for a physiologically relevant stimulus, as opposed to the extreme conditions of seizure paradigms. Furthermore, exercise-induced upregulation of BDNF may help increase the brain's resistance to damage and neurodegeneration that occurs with aging.

Amelioration of spatial navigation and short-term memory deficits by grafts of foetal basal forebrain tissue placed into the hippocampus and cortex of rats with selective cholinergic lesions

Impairments in learning and memory, induced by surgical or excitotoxic lesions of the septo-hippocampal or basalo-cortical pathways, can be ameliorated by grafts of cholinergic-rich foetal basal forebrain tissue into the hippocampus and/or neocortex. However, the effects of such grafts have been only partial, which may be due to the non-specific nature of the lesioning procedures used in these studies, known to destroy both cholinergic and non-cholinergic neuronal projections. In the present study, we have explored the effects of cholinergic-rich grafts in rats subjected to selective cholinergic lesions, induced by intraventricular injections of the immunotoxin 192 IgG-saporin. This lesion, which selectively destroyed 85-95% of the cholinergic neurons in both the septal-diagonal band and nucleus basalis, produced a long-lasting, substantial impairment in both the acquisition of spatial reference memory in the Morris water maze task and delay-dependent short-term memory performance, as seen in a delayed matching-to-position test. Foetal cholinergic grafts (but not control grafts of cerebellar tissue) implanted at multiple sites into both the hippocampus and fronto-parietal neocortex, bilaterally, completely reversed the acquisition deficit in place navigation in the water maze, to an extent that greatly exceeded that previously seen in animals with non-selective lesions. Most notably, however, the impairment in short-term memory was only partially and inconsistently affected, and only at the longest delay times. The morphological analysis, performed at about 7 months after transplantation, showed that the grafts had re-established a close to normal cholinergic innervation in the initially denervated cortical and hippocampal territories. It is proposed that the differential effects of cholinergic-rich transplants on different aspects of cognitive performance may define intrinsic limitations to the functional capacity of the ectopically placed grafts, which may be due to incomplete integration of the grafted cholinergic neurons into functional regulatory circuitries normally available to the basal forebrain cholinergic system.

Contribution of nicotinic receptors to the function of synapses in the central nervous system: the action of choline as a selective agonist of alpha 7 receptors

The alpha 7-nicotinic receptor (nAChR)-selective agonist choline and nAChR-subtype-selective antagonists led to the discovery that activation of both alpha 7 and alpha 4 beta 2 nAChRs located in CA1 interneurons in slices taken from the rat hippocampus facilitates the tetrodotoxin (TTX)-sensitive release of gamma-aminobutyric acid (GABA). Experiments carried out in cultured hippocampal neurons not only confirmed that preterminal alpha 7 and alpha 4 beta 2 nAChRs modulate the TTX-sensitive release of GABA, but also demonstrated that evoked release of GABA is reduced by rapid exposure of the neurons to acetylcholine (ACh, 10 microM-1 mM) in the presence of the muscarinic receptor antagonist atropine (1 microM). This effect of ACh, which is fully reversible and concentration-dependent, is partially blocked by superfusion of the cultured neurons with external solution containing either the alpha 7-nAChR-selective antagonist methyllycaconitine (MLA, 1 nM) or the alpha 4 beta 2-nAChR-selective antagonist dihydro-beta-erythroidine (DH beta E, 100 nM). A complete blockade of ACh-induced reduction of evoked release of GABA was achieved only when the neurons were perfused with external solution containing both MLA and DH beta E, suggesting that activation of both alpha 7 and alpha 4 beta 2 nAChRs modulates the evoked release of GABA from hippocampal neurons. Such mechanisms may account for the apparent involvement of nAChRs in the psychological effects of tobacco smoking, in brain disorders (e.g., schizophrenia and epilepsy), and in physiological processes, including cognition and nociception.

Dynamic changes of brain-derived neurotrophic factor protein levels in the rat forebrain after single and recurring kindling-induced seizures

Regional levels of brain-derived neurotrophic factor protein were measured in the rat brain using enzyme immunoassay following seizures evoked by hippocampal kindling stimulations. One stimulation, which induced a brief, single episode of epileptiform activity in hippocampus and piriform cortex but not in parietal cortex or striatum, gave rise to a transient increase of brain-derived neurotrophic factor levels in dentate gyrus and CA3 region and a decrease in piriform cortex. After 40 rapidly recurring seizures, with epileptiform activity also involving parietal cortex and striatum, increases were observed in dentate gyrus, CA3 and CA1 regions, piriform cortex and striatum. Maximum levels were reached at 2-24 h and brain-derived neurotrophic factor then returned to baseline except in dentate gyrus, where elevated protein content was sustained for four days. The differential regulation of brain-derived neurotrophic factor protein levels in various forebrain structures, which only partly correlates to messenger RNA changes, could indicate regional differences in protein release, antero- or retrograde transport, or brain-derived neurotrophic factor promotor activation. The dynamic changes of brain-derived neurotrophic factor levels in regions involved in the generation and spread of seizure activity may regulate excitability and trigger plastic responses in the post-seizure period.

Suppression of kindling epileptogenesis in rats by intrahippocampal cholinergic grafts

Selective immunolesioning of the basal forebrain cholinergic system by 192 IgG-saporin, which leads to a dramatic loss of the cholinergic innervation in cortical and hippocampal regions, facilitates the development of hippocampal kindling in rats. The aim of the present study was to explore whether grafted cholinergic neurones are able to reverse the lesion-induced increase of seizure susceptibility. Intraventricular 192 IgG-saporin was administered to rats which 3 weeks later were implanted with rat embryonic, acetylcholine-rich septal-diagonal band tissue ('cholinergic grafts') or cortical tissue/vehicle ('sham grafts') bilaterally into the hippocampal formation. After 3 months, the grafted animals as well as non-lesioned control rats were subjected to daily hippocampal kindling stimulations. In the animals with cholinergic grafts, which had reinnervated the hippocampus and dentate gyrus bilaterally, there was a marked suppression of the development of seizures as compared with the hyperexcitable, sham-grafted rats. This effect was significantly correlated to the density of the graft-derived cholinergic innervation of the host hippocampal formation. The kindling rate in the rats with cholinergic grafts was similar to that in non-lesioned controls. These results provide further evidence that the intrinsic basal forebrain cholinergic system dampens kindling epileptogenesis and demonstrate that this function can be exerted also by grafted cholinergic neurones.

Effects of cholinergic denervation on seizure development and neurotrophin messenger RNA regulation in rapid hippocampal kindling

Intraventricular 192 IgG-saporin was used to induce a selective lesion of basal forebrain cholinergic neurons in rats. When subjected to 40 rapid hippocampal kindling stimulations with 5-min intervals, these animals exhibited increased number of generalized seizures and a higher mean seizure grade in response to the first five stimulations, and required fewer stimuli to develop focal behavioural seizures, as compared to non-lesioned rats. In contrast, both groups showed similarly enhanced responsiveness when test stimulated four weeks later. Using in situ hybridization, cholinergic denervation was found to cause a significant decrease of basal brain-derived neurotrophic factor messenger RNA levels in the hippocampal formation and piriform cortex, whereas gene expression for nerve growth factor, neurotrophin-3, and TrkB and TrkC was unchanged. Four weeks after rapid kindling stimulations, basal levels of brain-derived neurotrophic factor messenger RNA in the dentate granule cells were restored to normal in the lesioned rats, whereas neurotrophin-3 messenger RNA levels were decreased. No differences in the seizure-evoked levels of neurotrophin and Trk messenger RNAs were detected, except in the dentate granule cell layer, which had significantly higher brain-derived neurotrophic factor messenger RNA expression in the lesioned animals at 2 h. In conclusion, the basal forebrain cholinergic system (i) dampens the severity of recurring seizures induced by rapid hippocampal kindling stimulations, but has no effect on the subsequent delayed phase of epileptogenesis; and (ii) exerts a tonic stimulation of basal brain-derived neurotrophic factor messenger RNA levels in the hippocampal formation and piriform cortex. The findings also indicate that the cholinergic lesion does not affect neurotrophin and Trk gene expression after recurring seizures, and that the kindling process leads to long-term changes in basal brain-derived neurotrophic factor and neurotrophin-3 messenger RNA levels in the denervated animals.

Cholinergic deafferentation exacerbates seizure-induced loss of somatostatin-immunoreactive neurons in the rat hippocampus

The loss of somatostatin-immunoreactive neurons and the sprouting of mossy fibers are typical histopathological abnormalities in the hippocampus in experimental and human temporal lobe epilepsy. To investigate whether the development of seizure-induced alterations is regulated by the subcortical afferent pathways to the hippocampus, we lesioned cholinergic, noradrenergic or serotonergic afferent pathways in rats two days after seizures were induced with kainate. Two months later, somatostatin-immunoreactive neurons were counted in the hilus to assess the severity of neuronal damage. Mossy fiber sprouting was analysed from adjacent Timm-stained sections. Kainate-induced seizures caused a loss of hilar somatostatin-immunoreactive neurons in the septal end of the hippocampus, where 63% of the somatostatin-immunoreactive neurons survived. Even more severe damage was found in the temporal end of the hippocampus (only 21% surviving). Cholinergic deafferentation of the hippocampus (using 192-IgG saporin) decreased the overall number of hilar somatostatin-immunoreactive neurons. In control rats that did not receive kainate, 87% (septal end) and 74% (temporal end) of the hilar somatostatin-immunoreactive neurons remained after cholinergic deafferentation. Moreover, seizure-induced damage to hilar somatostatin-immunoreactive neurons was further exacerbated by 192-IgG-saporin, with only 35% of the neurons remaining in the septal end and 14% in the temporal end of the hippocampus. Noradrenergic [using N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] or serotonergic (using 5,7-dihydroxytryptamine) lesions did not affect the number of hilar somatostatin-immunoreactive neurons either in control or in kainate-treated rats. The severity and distribution of seizure-induced mossy fiber sprouting were also not affected by any of the lesions. These data suggest that various subcortical afferent pathways may differentially modulate seizure-induced damage to the hippocampus. Damage to cholinergic neurons results in the loss of hilar somatostatin-immunoreactive neurons and exacerbates the seizure-induced loss of somatostatin-immunoreactive neurons.

Potentiation of glutamatergic EPSPs in rat CA1 hippocampal neurons after selective cholinergic denervation by 192 IgG-saporin

A complete and selective destruction of the basal forebrain cholinergic neurons projecting to the cerebral cortex and the hippocampus was induced in the rat by the toxin 192 IgG-saporin. Using electrophysiologic techniques, we have investigated the consequences of this cholinergic denervation on inhibitory and excitatory synaptic responses of CA1 pyramidal cells in rat hippocampal slices ex vivo. Histochemical experiments were performed in slices from control and 192 IgG-saporin-treated rats to check the efficacy of the intracerebroventricular injection of the immunotoxin. Stimulation of stratum radiatum elicits a glutamatergic excitatory postsynaptic potentials followed by a biphasic GABAergic inhibitory postsynaptic potential (IPSP). No significant change in IPSP was observed in 192 IgG-saporin-treated rats. By contrast, the N-methyl-D-aspartate (NMDA) and to a lesser extent the non-NMDA components of the glutamatergic response were potentiated in these animals. The possible pre- and postsynaptic mechanisms of this potentiation were discussed.

Moderate lesion of the rat cholinergic septohippocampal pathway increases hippocampal nerve growth factor synthesis: evidence for long-term compensatory changes?

Moderate lesions of the septohippocampal pathway by intraventricular infusions of ethylcholine aziridinium (AF64A) induced a dose-dependent decrease of hippocampal choline acetyltransferase (ChAT) activity, which partially recovered between 1 and 5 weeks after treatment. The cholinergic deficit was associated with an increase in nerve growth factor (NGF) mRNA only within the hippocampal dentate gyrus and hilus by maximally 51% and 111% 3 and 7 weeks after AF64A treatment, respectively, whereas no changes in brain-derived neurotrophic factor- and neurotrophin-3 mRNA were observed. The content of NGF protein transiently increased in the ventral part of the hippocampus 3 weeks after AF64A infusion but returned to control levels at 5 weeks. At that time, however, NGF content as well as ChAT activity were significantly increased in the septum, suggesting an increased utilization of NGF by the remaining cholinergic neurons. Thus, the present data provide correlative evidence for a critical role of endogenous NGF in neuroregeneration and plasticity of the cholinergic basal forebrain in case of incipient damage.

Reversal of a selective decrease in hippocampal acetylcholine release, but not of the persistence of kindling, after discontinuation of long-term pentylenetetrazol administration in rats

The time course of the effect of pentylenetetrazol (PTZ)-induced kindling on acetylcholine release in the hippocampus of freely moving rats was investigated with the transversal microdialysis technique. The basal extracellular concentration of acetylcholine in the hippocampus was reduced significantly (-29%, P < 0.05) after 3 weeks, and the effect was maximal (-52%, P < 0.01) after 4 weeks and remained essentially unchanged during the remaining 4 weeks of PTZ treatment (30 mg/kg, i.p., 3 times/week), relative to vehicle-treated rats. The basal release of acetylcholine in the prefrontal cortex and in the striatum of kindled rats was unchanged compared with that of vehicle-treated rats. The specific binding of [3H]quinuclidinyl benzilate, a non-selective ligand of muscarinic receptors, was significantly increased (+29%, P < 0.01) in hippocampal membrane, but not in membranes prepared from the prefrontal cortex or striatum, of PTZ-kindled rats. Thirty days after discontinuation of PTZ treatment, both hippocampal acetylcholine output and the density of muscarinic receptors had returned to values characteristic of vehicle-treated rats, whereas seizure susceptibility did not differ significantly from that apparent 4 days after PTZ administration. These results suggest that the selective and transient decrease in acetylcholine output and the parallel increase in the density of postsynaptic muscarinic receptors in the hippocampus may play a role in facilitating the development of kindling rather than in the maintenance of the kindled state.

Seizure-induced differential expression of messenger RNAs for neurotrophins and their receptors in genetically fast and slow kindling rats

Levels of messenger RNAs for brain-derived neurotrophic factor, nerve growth factor and neurotrophin-3, and their high-affinity receptors, TrkB and TrkC, were analysed in the brains of genetically fast and slow kindling rats using in situ hybridization. Basal expression of neurotrophins and Trk messenger RNAs in the hippocampal formation, amygdala, frontoparietal and piriform cortices did not differ between the two strains. At 2 h after the third generalized grade 5 seizure, induced by kindling stimulations in the amygdala, increased expression of brain-derived neurotrophic factor messenger RNA was detected in the dentate gyrus granule cell layer, amygdala, frontoparietal and piriform cortices of the fast kindlers. Similar seizure-evoked increases of brain-derived neurotrophic factor messenger RNA levels were also observed in the amygdala and piriform cortex of slow kindlers. However, in these animals, brain-derived neurotrophic factor messenger RNA expression was not significantly altered by the seizures in the dentate gyrus granule cell layer and frontoparietal cortex. Furthermore, the seizure-induced increase of nerve growth factor, TrkB and TrkC messenger RNAs and decrease of neurotrophin-3 messenger RNA levels in the dentate gyrus granule cell layer was only observed in fast, but not in slow, kindlers. The neurotrophins are believed to regulate synaptic plasticity and efficacy and to facilitate long-term potentiation and kindling epileptogenesis. The present data suggest that the slow and fast kindling rates in the two strains studied here might partly be due to differences in seizure-evoked neurotrophin and Trk synthesis.