Stress and Depression

Improved methods of assessment and research design have established a robust and causal association between stressful life events and major depressive episodes. The chapter reviews these developments briefly and attempts to identify gaps in the field and new directions in recent research. There are notable shortcomings in several important topics: measurement and evaluation of chronic stress and depression; exploration of potentially different processes of stress and depression associated with first-onset versus recurrent episodes; possible gender differences in exposure and reactivity to stressors; testing kindling/sensitization processes; longitudinal tests of diathesis-stress models; and understanding biological stress processes associated with naturally occurring stress and depressive outcomes. There is growing interest in moving away from unidirectional models of the stress-depression association, toward recognition of the effects of contexts and personal characteristics on the occurrence of stressors, and on the likelihood of progressive and dynamic relationships between stress and depression over time-including effects of childhood and lifetime stress exposure on later reactivity to stress.

Synaptic reorganization in the hippocampus induced by abnormal functional activity

Abnormal functional activity induces long-lasting physiological alterations in neural pathways that may play a role in the development of epilepsy. The cellular mechanisms of these alterations are not well understood. One hypothesis is that abnormal activity causes structural reorganization of neural pathways and promotes epileptogenesis. This report provides morphological evidence that synchronous perforant path activation and kindling of limbic pathways induce axonal growth and synaptic reorganization in the hippocampus, in the absence of overt morphological damage. The results show a previously unrecognized anatomic plasticity associated with synchronous activity and development of epileptic seizures in neural pathways.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

BDNF mRNA expression is increased in adult rat forebrain after limbic seizures: temporal patterns of induction distinct from NGF

We have localized brain-derived neurotrophic factor (BDNF) mRNA in rat brain and examined its regulation by seizure activity. In situ hybridization of BDNF 35S-cRNA most prominently labeled neurons in hippocampal stratum pyramidale and stratum granulosum, superficial olfactory cortex, pyramidal cell layers of neocortex, amygdala, claustrum, endopiriform nucleus, anterior olfactory nucleus, and ventromedial hypothalamus. Hybridization to BDNF mRNA was markedly increased in all of these regions after lesion-induced recurrent limbic seizures and within dentate gyrus granule cells following one electrically stimulated epileptiform afterdischarge. In contrast to seizure-elicited changes in nerve growth factor (NGF) mRNA expression, increases in BDNF mRNA occur in a greater number of different neuronal populations and develop several hours more rapidly in extrahippocampal loci. These results indicate that regulation by physiological activity may be an intrinsic property of this class of neurotrophic factor but that, in the recurrent seizure paradigm, different mechanisms mediate increased expression of mRNAs for BDNF and NGF outside hippocampus.

Circuit mechanisms of seizures in the pilocarpine model of chronic epilepsy: cell loss and mossy fiber sprouting

We used the pilocarpine model of chronic spontaneous recurrent seizures to evaluate the time course of supragranular dentate sprouting and to assess the relation between several changes that occur in epileptic tissue with different behavioral manifestations of this experimental model of temporal lobe epilepsy. Pilocarpine-induced status epilepticus (SE) invariably led to cell loss in the hilus of the dentate gyrus (DG) and to spontaneous recurrent seizures. Cell loss was often also noted in the DG and in hippocampal subfields CA1 and CA3. The seizures began to appear at a mean of 15 days after SE induction (silent period), recurred at variable frequencies for each animal, and lasted for as long as the animals were allowed to survive (325 days). The granule cell layer of the DG was dispersed in epileptic animals, and neo-Timm stains showed supra- and intragranular mossy fiber sprouting. Supragranular mossy fiber sprouting and dentate granule cell dispersion began to appear early after SE (as early as 4 and 9 days, respectively) and reached a plateau by 100 days. Animals with a greater degree of cell loss in hippocampal field CA3 showed later onset of chronic epilepsy (r = 0.83, p < 0.0005), suggesting that CA3 represents one of the routes for seizure spread. These results demonstrate that the pilocarpine model of chronic seizures replicates several of the features of human temporal lobe epilepsy (hippocampal cell loss, supra- and intragranular mossy fiber sprouting, dentate granule cell dispersion, spontaneous recurrent seizures) and that it may be a useful model for studying this human condition. The results also suggest that even though a certain amount of cell loss in specific areas may be essential for chronic seizures to occur, excessive cell loss may hinder epileptogenesis.

Kindling stimulation induces c-fos protein(s) in granule cells of the rat dentate gyrus

Alterations in neuronal gene expression have been proposed to account for permanent changes in brain function such as learning and memory. In particular, it has been suggested that protooncogenes such as c-fos may be rapidly induced in conditions that lead to neuronal plasticity and evoke permanent changes in the expression of effector genes. Concentrations of the c-fos proto-oncogene increase rapidly following depolarization-induced calcium influx in non-dividing neuronally differentiated PC 12 cells. Recently, the presence and induction of c-fos in the adult brain and spinal cord has been observed. Here we report that electrically-induced seizure activity, which leads to a permanent increase in the response of the brain to future seizures (kindling), rapidly and transiently increases c-fos protein-like immunoreactivity in the nuclei of granule cells in the rat dentate gyrus. These results suggest that c-fos protein is present within the nuclei of adult mammalian neurons, and could be involved in plastic changes in the nervous system associated with seizure activity.

Bridging the cleft at GABA synapses in the brain

A fragile balance between excitation and inhibition maintains the normal functioning of the CNS. The dominant inhibitory neurotransmitter of the mammalian brain is GABA, which acts mainly through GABAA and GABAB receptors. Small changes in GABA-mediated inhibition can alter neuronal excitability profoundly and, therefore, a wide range of compounds that clearly modify GABAA-receptor function are used clinically as anesthetics or for the treatment of various nervous system disorders. Recent findings have started to unravel the operation of central GABA synapses where inhibitory events appear to result from the synchronous opening of only tens of GABAA receptors activated by a saturating concentration of GABA. Such properties of GABA synapses impose certain constraints on the physiological and pharmacological modulation of inhibition in the brain.

Animal models of the epilepsies

The study of mechanisms of the epilepsies requires employment of animal models. Choice of a model system depends upon several factors, including the question to be studied, the type of epilepsy to be modelled, familiarity and convenience. Over 50 models are reviewed. Major categories of models are those for simple partial seizures: topical convulsants, acute electrical stimulation, cortically implanted metals, cryogenic injury; for complex partial seizures: kainic acid, tetanus toxin, injections into area tempesta, kindling, rodent hippocampal slice, isolated cell preparations, human neurosurgical tissue; for generalized tonic-clonic seizures: genetically seizure-prone strains of mouse, rat, gerbil, fruitfly and baboon, maximal electroshock seizures, systemic chemical convulsants, metabolic derangements; and for generalized absence seizures: thalamic stimulation, bilateral cortical foci, systemic penicillin, gamma-hydroxy-butyrate, intraventricular opiates, genetic rat models. The lithium-pilocarpine, homocysteine and rapid repetitive stimulation models are most useful in studies of status epilepticus. Key findings learned from each of the models, the model's strengths and weaknesses are detailed. Interpretation of findings from each of these models can be difficult. Do results pertain to the epilepsies or to the particular model under study? How important are species differences? Which clinical seizure type is really being modelled? In a model are behavior or EEG findings only similar superficially to epilepsy, or are the mechanisms comparable? The wealth of preparations available to model the epilepsies underscores the need for unifying themes, and for better understanding of basic mechanisms of the epilepsies.

Life stress, the "kindling" hypothesis, and the recurrence of depression: considerations from a life stress perspective

Major depression is frequently characterized by recurrent episodes over the life course. First lifetime episodes of depression, however, are typically more strongly associated with major life stress than are successive recurrences. A key theoretical issue involves how the role of major life stress changes from an initial episode over subsequent recurrences. The primary conceptual framework for research on life stress and recurrence of depression is the "kindling" hypothesis (R. M. Post, 1992). Despite the strengths of the kindling hypothesis, a review of the research literature reveals inconsistencies and confusion about life stress and its implications for the recurrence of depression. Adopting a life stress perspective, the authors introduce 3 major themes that resolve the inconsistencies in the current literature. They integrate these themes and extrapolate the ideas with available data to develop a preliminary framework for evaluating competing explanatory models and to guide research on life stress and the recurrence of depression.

Long-term potentiation phenomena in the rat limbic forebrain

Long-term potentiation (LTP) phenomena were investigated in several limbic forebrain pathways. With the possible exception of the lateral olfactory tract, LTP could be produced in all pathways tested. LTP effects tended to increase as the stimulation site moved caudally along the pyriform lobe. The largest effects were produced by stimulation of pathways into and out of the hippocampus. Target sites also differed, with the hippocampal sites showing the strongest and longest lasting LTP effects. The time course of LTP appeared to be best fitted by the sum of two exponential curves with time constants of about 1.5 h and 5 days respectively. We also looked at the potentiation effects produced by repeated epileptogenic (kindling) stimulations, and the effect of this potentiation on subsequent tests of short-term and long-term potentiation. In most cases, the short-term effects, and the first component of the LTP effect, were still intact after kindling. The longest lasting component, however, could no longer be produced with either amygdala or perforant path stimulation. This result indicates that the potentiation produced by kindling may be based upon the same mechanism as the LTP effect.

Glutamate, GABA and epilepsy

The nature and value of various animal models of epilepsy for the study and understanding of the human epilepsies are reviewed, with special reference to the ILAE classification of seizures. Kindling as a model of complex-partial seizures with secondary generalisation is treated in detail, dwelling principally on the evidence that the neurotransmitters glutamate and GABA are centrally involved in the kindling process. Kindling in the entorhinal cortex-hippocampus system and its relationship to LTP are analysed in detail. Changes in amino acid content in animal and human brain tissue following onset of the epileptic state are reviewed with special reference to glutamate and GABA. Studies of changes in the extent of basal and stimulus-evoked release of glutamate and GABA both in vivo (microdialysis) and in vitro (brain slices) are evaluated. This includes both kindling and other models of epilepsy, and microdialysis of human patients with epilepsy. Experiments which study the influence of pre-synaptic metabotropic glutamate receptors on glutamate release, and consequently on the extent of electrical kindling, are described. This pre-synaptic control of glutamate release can be studied using synaptosomes. The significance of the ability of focal intracerebrally injected glutamate and NMDA to cause (chemical) kindling and the strong sensitivity of this process to pre-treatment with NMDA receptor antagonists is analysed. Electrical and chemical kindling effects are additive, indicating the existence of mechanisms in common. They are both sensitive to NMDA antagonists and the common mechanism is probably NMDA receptor activation due to the presence of exogenous (chemical) or endogenous (electrically-released) extracellular glutamate. The participation of the NMDA receptor in the generation of the spontaneous hyperactivity which characterises the chronic epileptic state is reviewed. This includes the entry of Ca2+ to stimulate various post-synaptic phosphorylation processes, and possible modulation of NMDA receptor population size and sensitivity. The question of whether neurotransmitter glutamate is involved in initiation and/or spread of seizures is discussed.

NMDA receptors of dentate gyrus granule cells participate in synaptic transmission following kindling

In the mammalian central nervous system, receptors for the excitatory amino-acid neurotransmitters are divided into three subtypes depending on their sensitivity to three specific agonists: kainate, quisqualate and N-methyl-D-aspartate (NMDA). The ionophores operated by NMDA are gated by Mg2+ in a voltage-dependent manner and allow passage of several cations, including Ca2+ which may be important in plastic alterations of neuronal excitability. Indeed, specific antagonists of NMDA receptors effectively block spatial learning, long-term potentiation and some animal models of chronic epilepsy. Despite their abundance on central neurons, NMDA receptors, with a few noteworthy exceptions, do not generally seem to be involved in low-frequency synaptic transmission. Here we report for the first time that NMDA receptors of the dentate gyrus, where they do not normally contribute to the generation of synaptic potentials, become actively involved in synaptic transmission following long-lasting neuronal changes induced by daily electrical stimulation (kindling) of the amygdala or hippocampal commissures. In contrast to controls, the excitatory postsynaptic potentials (e.p.s.ps) of granule cells in hippocampal slices obtained from kindled animals displayed characteristics typical of an NMDA-receptor-mediated component. The involvement of NMDA receptors in synaptic transmission may underlie the long-lasting changes in neuronal function induced by kindling.

Cellular mechanisms of epilepsy: a status report

The cellular phenomena underlying focal epilepsy are currently understood in the context of contemporary concepts of cellular and synaptic function. Interictal discharges appear to be due to a combination of synaptic events and intrinsic currents, the exact proportion of which in any given neuron may vary according to the anatomic and functional substrate involved in the epileptic discharge and the epileptogenic agent used in a given model. The transition to seizure appears to be due to simultaneous increments in excitatory influences and decrements in inhibitory processes--both related to frequency-dependent neuronal events. A variety of specific hypotheses have been proposed to account for the increased excitability that occurs during epileptiform activity. Although each of the proposed mechanisms is likely to contribute significantly to the epileptic process, no single hypothesis provides an exclusive unifying framework within which all kinds of focal epilepsy can be understood. The spread of epileptic activity throughout the brain, the development of primary generalized epilepsy, the existence of "gating" mechanisms in specific anatomic locations, and the extrapolation of hypotheses derived from simple models of focal epilepsy to explain more complex forms of human epilepsy, all are not yet fully understood.

Increased production of the TrkB protein tyrosine kinase receptor after brain insults

The protein-tyrosine kinases Trk, TrkB, and TrkC are signal-transducing receptors for a family of neurotrophic factors known as the neurotrophins. Here we show that seizures induced by hippocampal kindling lead to a rapid, transient increase of trkB mRNA and protein in the hippocampus. TrkB is a component of a high affinity receptor for brain-derived neurotrophic factor (BDNF). No change was detected in mRNAs for Trk or TrkC, components of the high affinity nerve growth factor or neurotrophin-3 receptors, respectively. trkB mRNA was also transiently increased in the dentate gyrus following cerebral ischemia and hypoglycemic coma; these treatments had no effect on trk and trkC mRNAs. The increase in trkB mRNA and protein showed the same time course and distribution as the increase in BDNF mRNA. These data suggest that BDNF and its receptor may play a local role within the hippocampus in kindling-associated neural plasticity and in neuronal protection following epileptic, ischemic, and hypoglycemic insults.

The effects of kindling on GABA-mediated inhibition in the dentate gyrus of the rat. I. Paired-pulse depression

Double-pulse stimulation of the perforant path input to the dentate gyrus was used in the following experiments to produce paired pulse depression in that site. This effect provided an estimate of GABA-mediated recurrent inhibition. The depression was enhanced by drugs that facilitate or mimic GABA action and attenuated by drugs which block GABA transmission. Paired-pulse depression was significantly increased following amygdala kindling and was further enhanced to near maximal levels by subsequent kindling in the dentate. In addition, kindling did not increase the rate at which inhibition failed under conditions of excessive activation. Trains of 5 Hz stimulation, applied to the perforant path, caused paired-pulse depression to disappear and elicited a brief AD. Following kindling, the latency to AD onset tended to be increased rather than shortened, suggesting an enhanced resistance to inhibitory failure. These results indicated that kindling increased, rather than reduced, inhibition in the dentate gyrus.

Occipital lobe epilepsy: clinical characteristics, seizure spread patterns, and results of surgery

Twenty-five patients with occipital lobe seizure origin were retrospectively evaluated to determine clinical seizure characteristics and electroencephalographic manifestations. Certain symptoms and signs served to identify occipital lobe origin in 22 (88%). These included elementary visual hallucinations, ictal amaurosis, eye movement sensations, early forced blinking or eyelid flutter, and visual field deficits. Eye or head deviation, or both, was observed frequently and was contralateral to the side of seizure origin in 13, but 3 patients exhibited ipsilateral deviation in some or all their seizures. After the initial signs and symptoms, clinical seizure characteristics resembled those of seizures originating elsewhere. Seizures typical of temporal lobe origin with loss of contact and various types of automatic, semipurposeful activity occurred in 11 patients. Seizures in 3 patients exhibited asymmetrical tonic or focal clonic motor patterns characteristic of frontal lobe seizures. Eleven of the 25 patients had, on two occasions, two or more distinctly different seizure types. Scalp electroencephalographic findings were seldom helpful for occipital lobe localization and were frequently misleading. Intracranial electroencephalographic recording correctly identified occipital lobe seizure origin in most, but not all, patients who had such studies. Intracranial electroencephalic recording also proved the variability in clinical seizure characteristics was related to different seizure spread patterns, medially or laterally above and below the sylvian fissure, both ipsilateral and contralateral to the occipital lobe of seizure origin. Eighteen patients had occipital lobe lesions detected with computed tomographic or magnetic resonance imaging scans or both. Resection of the lesions in 16 patients produced excellent results in 14 (88%). Five patients had temporal lobectomies, with good results in 3, but poor results in 2. Two patients with unlocalized seizures had complete section of the corpus callosum, 1 with a good result and the other with a poor result.

Suppressed epileptogenesis in BDNF mutant mice

Kindling is an animal model of epilepsy in which repeated electrical stimulations lead to progressive and permanent amplification of seizure activity, culminating in generalized convulsions. Each brief period of seizure activity during kindling epileptogenesis causes a marked, transient increase of the synthesis of brain-derived neurotrophic factor (BDNF) in cortical and hippocampal neurons. We find that the development of kindling is markedly suppressed in mice heterozygous for a deletion of the BDNF gene. In contrast, the maintenance of kindling is unaffected. The mutant mice show lower levels of BDNF mRNA in cortical and hippocampal neurons after seizures than do wild-type mice. Hippocampal mossy fiber sprouting is augmented in BDNF mutants but there are no other morphological abnormalities. These results show that BDNF plays an important role in establishing hyperexcitability during epileptogenesis, probably by increasing efficacy in stimulated synapses.

Chronic alcohol neuroadaptation and stress contribute to susceptibility for alcohol craving and relapse

Alcoholism is a chronic relapsing disorder. Major characteristics observed in alcoholics during an initial period of alcohol abstinence are altered physiological functions and a negative emotional state. Evidence suggests that a persistent, cumulative adaptation involving a kindling/allostasis-like process occurs during the course of repeated chronic alcohol exposures that is critical for the negative symptoms observed during alcohol withdrawal. Basic studies have provided evidence for specific neurotransmitters within identified brain sites being responsible for the negative emotion induced by the persistent cumulative adaptation following intermittent-alcohol exposures. After an extended period of abstinence, the cumulative alcohol adaptation increases susceptibility to stress- and alcohol cue-induced negative symptoms and alcohol seeking, both of which can facilitate excessive ingestion of alcohol. In the alcoholic, stressful imagery and alcohol cues alter physiological responses, enhance negative emotion, and induce craving. Brain fMRI imaging following stress and alcohol cues has documented neural changes in specific brain regions of alcoholics not observed in social drinkers. Such altered activity in brain of abstinent alcoholics to stress and alcohol cues is consistent with a continuing ethanol adaptation being responsible. Therapies in alcoholics found to block responses to stress and alcohol cues would presumably be potential treatments by which susceptibility for continued alcohol abuse can be reduced. By continuing to define the neurobiological basis of the sustained alcohol adaptation critical for the increased susceptibility of alcoholics to stress and alcohol cues that facilitate craving, a new era is expected to evolve in which the high rate of relapse in alcoholism is minimized.

Is adenosine an endogenous anticonvulsant?

The anticonvulsant properties of adenosine were tested pharmacologically on amygdala-kindled seizure activity in rats. The adenosine analogue 2-chloroadenosine and the adenosine uptake blocker papaverine both increased the latency to behavioral clonus as well as reduced the duration and severity of the clonic motor convulsion. Both drugs, however, failed to alter the postkindling afterdischarge (AD) threshold. Theophylline, an adenosine antagonist, had the opposite effects, prolonging the AD and motor seizure durations and facilitating partially kindled seizures, but again not altering the prekindling or postkindling AD thresholds of amygdala-elicited seizures. In contrast, carbamazepine raised AD thresholds, suggesting that it does not produce its anticonvulsant effects through adenosine systems. Since endogenous adenosine can impede seizure spread and seizure continuation, but does not affect seizure initiation from the amygdala, perhaps endogenous adenosine has the special property of being brought into play as an anticonvulsant only by the seizure itself.

Kindling-induced neurogenesis in the dentate gyrus of the rat

Kindling, a form of neuronal plasticity produced by repeated low intensity electrical brain stimulation, leads to epileptic seizures. To address possible causes of this phenomenon, we have prepared amygdala-kindled animals and measured neurogenesis, by bromodeoxyuridine incorporation. Early, when focal seizures were present, there was no evidence of a change in the rate of hippocampal neurogenesis. In contrast, during the later phases of kindling, when secondary generalization was well established and motor seizures were present, neurogenesis was enhanced by 75-140%, depending on the hippocampal region. Double labelling with the neuron-specific marker TOAD-64 demonstrated the presence of numerous new-born granule neurons in the kindled animals. We propose that the newly-born neurons contribute to the cellular changes and behavioral symptoms associated with this type of epileptiform brain plasticity.

Activation of N-methyl-D-aspartate receptors parallels changes in cellular and synaptic properties of dentate gyrus granule cells after kindling

1. The cellular and synaptic properties of rat dentate gyrus granule cells (GCs) were examined using intra-/extracellular and Ca2+-sensitive microelectrode recordings following epilepsy induced by kindling of the hippocampal commissures or amygdala. 2. The recordings were made in hippocampal slices prepared from sham-stimulated controls and animals that have received daily stimuli to reach stage IV-V of kindling. The average number of stimulation trials (60 Hz/1 s, 100-150 microA) required to reach full motor seizures (stage V) was 23 +/- 2 for commissural kindling and 14 +/- 1 for amygdala kindling. 3. The resting membrane potential of GCs following kindling (RMP; -72 +/- 3 mV) was not significantly different from the RMP of control GCs (-70 +/- 2 mV). Similarly, action potential height and threshold were unaffected by kindling. However, kindling altered other cellular properties of GCs regardless of the site of stimulation (hippocampal commissures or amygdala), the stage of kindling reached (IV or V), or the time elapsed between the last kindling stimulus and preparation of the hippocampal slices (24 h-6 wk). The input resistance of kindled GCs (55 +/- 4 M omega) was significantly higher than that of controls (40 +/- 3 M omega). In contrast to most control GCs, the slope conductance (GS) of kindled neurons, measured with constant-amplitude current injections at various membrane potentials, generally increased at membrane potentials more negative than rest. Furthermore, other voltage-dependent ionic conductances (see below), that were not normally encountered in control GCs, were present in kindled neurons. 4. The intracellularly recorded monosynaptic excitatory postsynaptic potentials (EPSPs) of kindled GCs, evoked through the stimulation of the lateral perforant pathway, differed significantly from the EPSPs of control GCs. The amplitudes of control EPSPs increased upon hyperpolarizations and decreased following depolarizations of the membrane, as expected for conventional EPSPs without contribution from voltage-dependent conductances. In contrast, the EPSPs of kindled GCs invariably increased in amplitude and duration at membrane potentials 5-20 mV depolarized from rest, indicating the presence of a characteristic voltage-dependent component. Frequently, following the synaptically triggered action potentials, kindled GCs displayed depolarizing afterpotentials. 5. Perfusion of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 30 microM) had no effect on the EPSPs of control GCs, but consistently reduced the amplitude and duration of EPSPs in kindled GCs.(ABSTRACT TRUNCATED AT 400 WORDS)

Anticonvulsant and antiepileptogenic effects mediated by adeno-associated virus vector neuropeptide Y expression in the rat hippocampus

Neuropeptide Y (NPY) inhibits seizures in experimental models and reduces excitability in human epileptic tissue. We studied the effect of long-lasting NPY overexpression in the rat hippocampus with local application of recombinant adeno-associated viral (AAV) vectors on acute kainate seizures and kindling epileptogenesis. Transgene expression was significantly increased by 7 d, reached maximal expression by 2 weeks, and persisted for at least 3 months. Serotype 2 AAV vector increased NPY expression in hilar interneurons, whereas the chimeric serotype 1/2 vector caused far more widespread expression, also including mossy fibers, pyramidal cells, and the subiculum. EEG seizures induced by intrahippocampal kainate were reduced by 50-75%, depending on the vector serotype, and seizure onset was markedly delayed. In rats injected with the chimeric serotype 1/2 vector, status epilepticus was abolished, and kindling acquisition was significantly delayed. Thus, targeted NPY gene transfer provides a potential therapeutic principle for the treatment of drug-resistant partial epilepsies.

Increased seizure susceptibility of the immature brain

The ability of the CNS to generate seizures as a function of age was investigated utilizing the kindling model of epilepsy. Hourly electrical, low intensity stimulations of the amygdala induced kindling in adult rats, but stimulations delivered at 15 min intervals failed to or markedly retarded the development of kindled convulsions. In contrast, both types of stimulation induced consistent prolongation of the afterdischarges and repeated generalized seizures in suckling rat pups. The rate of development of the kindled convulsions in the pups was similar, irrespective of whether the stimulations were delivered at 15 or 60 min intervals, indicating that short (less than 15 min) seizure refractory periods exist in the immature brain. The data suggest that seizure susceptibility changes with age and is greater early in life.

Conditioning and sensitisation in the longitudinal course of affective illness

Few biological theories of manic-depressive illness have focused on the longitudinal course of affective dysfunction and the mechanisms underlying its often recurrent and progressive course. The authors discuss two models for the development of progressive behavioural dysfunction--behavioural sensitisation and electrophysiological kindling--as they provide clues to important clinical and biological variables relevant to sensitisation in affective illness. The role of environmental context and conditioning in mediating behavioural and biochemical aspects of this sensitisation is emphasised. The sensitisation models provide a conceptual approach to previously inexplicable clinical phenomena in the longitudinal course of affective illness and may provide a bridge between psychoanalytic/psychosocial and neurobiological formulations of manic-depressive illness.

Amygdala damage in experimental and human temporal lobe epilepsy

The amygdala complex is one component of the temporal lobe that may be damaged unilaterally or bilaterally in children and adults with temporal lobe epilepsy (TLE) or following status epilepticus. Most MR (magnetic resonance) imaging studies of epileptic patients have shown that volume reduction of the amygdala ranges from 10-30%. In the human amygdala, neuronal loss and gliosis have been reported in the lateral and basal nuclei. Studies in rats have more specifically identified the amygdaloid regions that are sensitive to status epilepticus-induced neuronal damage. These areas include the medial division of the lateral nucleus, the parvicellular division of the basal nucleus, the accessory basal nucleus, the posterior cortical nucleus, and portions of the anterior cortical and medial nuclei. Otherwise, other amygdala nuclei, such as the magnocellular and intermediate divisions of the basal nucleus and the central nucleus, remain relatively well preserved. Amygdala kindling studies in rats have shown that the density of a subpopulation of GABAergic inhibitory neurons that also contain somatostatin may be reduced even after a low number of generalized seizures. While analyses of histological sections and MR images indicate that in approximately 10% of TLE patients, seizure-induced damage is isolated to the amygdala, more often amygdala damage is combined with damage to the hippocampus and/or other brain areas. Moreover, recent data from rodents and nonhuman primates suggest that structural and functional alterations caused by seizure activity originating in the amygdala are not limited to the amygdala itself, but may also affect other temporal lobe structures. The information gathered so far on damage to the amygdala in epilepsy or after status epilepticus suggests that local alterations in inhibitory circuitries may contribute to a lowered seizure threshold and greater excitability within the amygdala. Furthermore, damage to select nuclei in the amygdala may predict impairment of performance in behavioral tasks that depend on the integrity of the amygdaloid circuits.