Amygdaloid kindling of rats increases preprosomatostatin mRNA and somatostatin without affecting glutamic acid decarboxylase (GAD) mRNA or GAD

Hyperexcitability: From Normal Fear to Pathological Anxiety and Trauma

Hyperexcitability in fear circuits is suggested to be important for development of pathological anxiety and trauma from adaptive mechanisms of fear. Hyperexcitability is proposed to be due to acquired sensitization in fear circuits that progressively becomes more severe over time causing changing symptoms in early and late pathology. We use the metaphor and mechanisms of kindling to examine gains and losses in function of one excitatory and one inhibitory neuropeptide, corticotrophin releasing factor and somatostatin, respectively, to explore this sensitization hypothesis. We suggest amygdala kindling induced hyperexcitability, hyper-inhibition and loss of inhibition provide clues to mechanisms for hyperexcitability and progressive changes in function initiated by stress and trauma.

Selective plasticity of hippocampal GABAergic interneuron populations following kindling of different brain regions

The vulnerability and plasticity of hippocampal GABAergic interneurons is a topic of broad interest and debate in the field of epilepsy. In this experiment, we used the electrical kindling model of epilepsy to determine whether seizures that originate in different brain regions have differential effects on hippocampal interneuron subpopulations. Long-Evans rats received 99 electrical stimulations of the hippocampus, amygdala, or caudate nucleus, followed by sacrifice and immunohistochemical or western blot analyses. We analyzed markers of dendritic (somatostatin), perisomatic (parvalbumin), and interneuron-selective (calretinin) inhibition, as well as an overall indicator (GAD67) of interneuron distribution across all major hippocampal subfields. Our results indicate that kindling produces selective effects on the number and morphology of different functional classes of GABAergic interneurons. In particular, limbic kindling appears to enhance dendritic inhibition, indicated by a greater number of somatostatin-immunoreactive (-ir) cells in the CA1 pyramidal layer and robust morphological sprouting in the dentate gyrus. We also found a reduction in the number of interneuron-selective calretinin-ir cells in the dentate gyrus of hippocampal-kindled rats, which suggests a possible reduction of synchronized dendritic inhibition. In contrast, perisomatic inhibition indicated by parvalbumin immunoreactivity appears to be largely resilient to the effects of kindling. Finally, we found a significant induction in the number of GAD67-cells in caudate-kindled rats in the dentate gyrus and CA3 hippocampal subfields. Taken together, our results demonstrate that kindling has subfield-selective effects on the different functional classes of hippocampal GABAergic interneurons. J. Comp. Neurol. 525:389-406, 2017. © 2016 Wiley Periodicals, Inc.

Which insights have we gained from the kindling and post-status epilepticus models?

Experimental animal epilepsy research got a big boost since the discovery that daily mild and short (seconds) tetanic stimulations in selected brain regions led to seizures with increasing duration and severity. This model that was developed by Goddard (1967) became known as the kindling model for epileptogenesis and has become a widely used model for temporal lobe epilepsy with complex partial seizures. During the late ninety-eighties the number of publications related to electrical kindling reached its maximum. However, since the kindling procedure is rather labor intensive and animals only develop spontaneous seizures (epilepsy) after hundreds of stimulations, research has shifted toward models in which the animals exhibit spontaneous seizures after a relatively short latent period. This led to post-status epilepticus (SE) models in which animals experience SE after injection of pharmacological compounds (e.g. kainate or pilocarpine) or via electrical stimulation of (limbic) brain regions. These post-SE models are the most widely used models in epilepsy research today. However, not all aspects of mesial temporal lobe epilepsy (MTLE) are reproduced and the widespread brain damage is often a caricature of the situation in the patient. Therefore, there is a need for models that can better replicate the disease. Kindling, although already a classic model, can still offer valid clues in this context. In this paper, we review different aspects of the kindling model with emphasis on experiments in the rat. Next, we review characteristic properties of the post-SE models and compare the neuropathological, electrophysiological and molecular differences between kindling and post-SE epilepsy models. Finally, we shortly discuss the advantages and disadvantages of these models.

Distribution of GABAergic neurons in the striatum of amygdala-kindled rats: An immunohistochemical and in situ hybridization study

A large body of experimental evidence suggests that the basal ganglia circuitry may be part of a remote control system modulating the spread of epileptic seizures. In the kindling model of temporal lobe epilepsy, this endogenous inhibitory control mechanism seems to be impaired. Neurochemical and neurophysiological studies have indicated that the activity of the GABAergic projection from the striatum to the substantia nigra pars reticulata is reduced in kindled rats, but the exact mechanisms involved in this observation are not known. Possible explanations include a kindling-induced loss of striatal GABAergic projection neurons to the substantia nigra or enhanced inhibition of these neurons by GABAergic interneurons. In the present experiments, the GABAergic system of the striatum (caudate-putamen) of amygdala-kindled rats and controls was studied immunohistochemically with a monoclonal antibody to GABA and with nonisotopic in situ hybridization with cRNA probes selective for glutamic acid decarboxylase 65 (GAD65) and GAD67, respectively. Compared to sham controls, an increased density of neurons heavily labeled for GAD67 mRNA was observed in the anterior striatum of kindled rats when cells were counted 6 weeks after the last kindled seizure. This subgroup of striatal GABAergic neurons has been suggested previously to correspond to the medium-sized aspiny interneurons in the striatum, indicating that kindling is associated with an increased activity of these neurons. Our previous finding of reduced GAD and GABA levels in synaptosomes isolated from the substantia nigra of kindled rats together with the present observation of increased density of GABAergic striatal interneurons in such rats suggest that kindling affects the regulation of the GABAergic projections from the striatum to the substantia nigra rather than directly damaging GABAergic neurons in the striatum.

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.

On the Role of Somatostatin in Seizure Control: Clues from the Hippocampus

The role of the hippocampal somatostatin (somatotropin release-inhibiting factor, SRIF) system in the control of partial complex seizures is discussed in this review. The SRIF system plays a role in the inhibitory modulation of hippocampal circuitries under normal conditions: 1) SRIF neurons in the dentate gyrus are part of a negative feedback circuit modulating the firing rate of granule cells; 2) SRIF released in CA3 interacts both with presynaptic receptors located on associational/commissural terminals and with postsynaptic receptors located on pyramidal cell dendrites, reducing excitability of pyramidal neurons; 3) in CA1, SRIF exerts a feedback inhibition and reduces the excitatory drive on pyramidal neurons. Significant changes in the hippocampal SRIF system have been documented in experimental models of temporal lobe epilepsy (TLE), in particular in the kindling and in the kainate models. SRIF biosynthesis and release are increased in the kindled hippocampus, especially in the dentate gyrus. This hyper-function may be instrumental to control the latent hyperexcitability of the kindled brain, preventing excessive discharge of the principal neurons and the occurrence of spontaneous seizures. In contrast, the hippocampal SRIF system undergoes damage in the dentate gyrus following kainate-induced status epilepticus. Although surviving SRIF neurons appear to hyperfunction, the loss of hilar SRIF interneurons may compromise inhibitory mechanisms in the dentate gyrus, facilitating the occurrence of spontaneous seizures. In keeping with these data, pharmacological activation of SRIF1 (sst2) receptors, i.e. of the prominent receptor subtype on granule cells, exerts antiseizure effects. Taken together, the data presented suggest that the hippocampal SRIF system plays a role in the control of partial complex seizures and, therefore, that it may be proposed as a therapeutic target for TLE.

Kindled seizure-evoked somatostatin release in the hippocampus: inhibition by MK-801

The aim of this study was to evaluate the contribution of ionotropic glutamate receptors to kindled seizure-evoked somatostatin release in the hippocampus, using a microdialysis approach. Basal and amygdala stimulation-evoked somatostatin-like immunoreactivity (-LI) release was significantly greater in kindled compared to naive rats. In naive rats, neither hippocampal perfusion with the selective AMPA/kainate receptor antagonist GYKI 52466 nor with the selective NMDA receptor antagonist MK-801 affected behavior, EEG, or somatostatin-LI release. In kindled rats, GYKI 52466 was still devoid of any effect, while MK-801 significantly decreased stimulus-evoked (but not basal) somatostatin-LI efflux. MK-801 produced identical effects when injected i.p. This study provides the first direct evidence that kindled seizure-evoked somatostatin release in the hippocampus is partly NMDA receptor dependent.

Somatostatin release in the hippocampus in the kindling model of epilepsy: a microdialysis study

Somatostatin biosynthesis in the hippocampus is activated during and following kindling epileptogenesis. The aim of this study was to investigate whether this phenomenon is associated with enhanced somatostatin release in vivo. Experiments have been run in awake, freely moving rats, implanted with a bipolar electrode in the right amygdala (for kindling stimulation), and with a recording electrode and a microdialysis probe in the left hippocampus. Basal somatostatin-like immunoreactivity (-LI) release was significantly greater in kindled than naive rats. In naive rats, a 2-min perfusion with 100 mM K(+) did not affect behavior and EEG recordings and nonsignificantly increased somatostatin-LI release; a 10-min K(+) perfusion evoked numerous wet dog shakes, electrical seizures (class 0; latency congruent with 8 min, duration congruent with 8 min), and somatostatin-LI release ( congruent with 350% of basal); and a single kindling after-discharge (4 +/- 3-s duration in the hippocampus) also evoked somatostatin-LI release ( congruent with 200% of basal). In kindled rats, a 2-min 100 mM K(+) perfusion evoked hippocampal discharges in three of seven animals (latency congruent with 2 min, mean duration congruent with 1.5 min) and increased somatostatin-LI release ( congruent with 250% of basal); a 10-min K(+) perfusion evoked behavioral seizures (class 1 to 5, latency congruent with 4 min, mean duration congruent with 12 min) with numerous wet dog shakes and robust somatostatin-LI release ( congruent with 350% of basal); and a kindling stimulation evoked generalized seizures (class 4 or 5, 77 +/- 15-s duration in the hippocampus) with remarkable somatostatin-LI release ( congruent with 300% of basal). These data demonstrate that hippocampal somatostatin release is increased in the kindling model in vivo.

Opioid peptide systems following a subconvulsant dose of pentylenetetrazol in rats

The development of epilepsy and a progressive increase in susceptibility to seizures may involve changes in inhibitory and excitatory systems from the beginning of the process. The present study was focused to analyze the opioid peptide changes induced by a chemical sub-convulsant stimulation. Experiments were carried out to determine opioid peptide release, mu receptor binding and proenkephalin expression in rat brain, as well as nociceptive responses, following the administration of a sub-convulsant dose of pentylenetetrazol (PTZ) (30 mg/kg, i.p.). Membrane binding experiments revealed reduced number of mu binding sites (Bmax) in cortex and amygdala, but not in striatum and hippocampus, an effect that was evident only 24 h, but not 28 days, after PTZ treatment. In situ hybridization experiments suggested a significant enhancement of proenkephalin mRNA expression in specific brain regions 24 h after PTZ treatment. Microdialysis combined with a universal opioid peptide radioimmunoassay revealed extracellular opioid peptide levels to be elevated in the amygdala (137%) 90 min after PTZ administration. Evaluation of nociceptive responses using the Randall-Selitto test showed an analgesic effect short term (30-90 min) after PTZ injection. Collectively, these data provide evidence for a significant activation of opioid peptide systems as a consequence of the administration of a sub-convulsant dose of PTZ. These neurochemical changes may play an important role in the progression of epileptogenesis.

Brain somatostatin: a candidate inhibitory role in seizures and epileptogenesis

Recent evidence shows that neuropeptide expression in the CNS is markedly affected by seizure activity, particularly in the limbic system. Changes in neuropeptides in specific neuronal populations depend on the type and intensity of seizures and on their chronic sequelae (i.e. neurodegeneration and spontaneous convulsions). This paper reviews the effects of seizures on somatostatin-containing neurons, somatostatin mRNA and immunoreactivity, the release of this peptide and its receptor subtypes in the CNS. Differences between kindling and status epilepticus in rats are emphasized and discussed in the light of an inhibitory role of somatostatin on hippocampal excitability. Pharmacological studies show that somatostatin affects electrophysiological properties of neurons, modulates classical neurotransmission and has anticonvulsant properties in experimental models of seizures. This peptidergic system may be an interesting target for pharmacological attempts to control pathological hyperactivity in neurons, thus providing new directions for the development of novel anticonvulsant treatments.

Regulation of the preprosomatostatin gene by cyclic-AMP in cerebrocortical neurons

The gene coding for preprosomatostatin (ppSom), the molecular precursor of somatostatin (Som), is regulated at the level of transcription by calcium ions and cyclic-AMP [F. Baldino, S. Fitzpatrick-McElligott, T. O'Kane, I. Gozes, Hormonal regulation of somatostatin, Synapse 2 (1988) 317-325; M.R. Montminy, M.J. Low, L. Tapia-Arancibia, Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells, J. Neurosci. 6 (1986) 1171-1176.], or by agents which increase intracellular levels of cAMP directly, such as forskolin [M.R. Montminy, M.J. Low, L. Tapia-Arancibia, Cyclic AMP regulates somatostatin mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells, J. Neurosci. 6 (1986) 1171-1176.]. Transcriptional induction of the ppSom gene as examined in PC12 cells, transfected fibroblasts and primary diencephalic cultures, requires the highly conserved cAMP response element (CRE), which confers gene responsiveness to cAMP [M. Comb, N. Mermod, S.E. Hyman, Proteins bound at adjacent DNA elements act synergistically to regulate human proenkephalin cAMP inducible transcription, EMBO J. 7 (1988) 3793-3805; T. Tsukada, J.S. Fink, G. Mandel, Identification of a region in the human vasoactive intestinal polypeptide gene responsible for regulation by cyclic AMP, J. Biol. Chem. 262 (1987) 8743-8747.]. The ppSom gene is subject to stringent regulation during cerebrocortical development in vivo; however, little information is available regarding ppSom gene regulation by neurotransmitters or second-messengers in cortical neurons. We used primary cerebrocortical cell cultures from fetal mice to examine the dose-response and time-course of ppSom gene expression in response to the cyclic-AMP analogs, dibutyrl-cAMP (dbcAMP), and 8-bromo-cAMP (8-BrcAMP). We report a dose-response for both analogs in the range of 0.1-10 mM. Dose-response studies using agents which directly stimulate intracellular cAMP synthesis (forskolin) or inhibit its breakdown (3-isobutyl 1-methyl xanthine) were also performed. We observed an apparent synergistic effect on ppSom expression when used in combination. An increase in ppSom mRNA levels was observed by 4 h, with a maximal response at 12-24 h. No change in ppSom mRNA levels was observed in response to phorbol myristate acetate (PMA). Our findings confirm the specificity of ppSom gene regulation by cAMP and Ca2+ ions, and demonstrate the utility of using primary cerebrocortical cultures for the study of somatostatin gene expression by neurotransmitters and second-messengers as a model of human neurologic disorders.

Time-dependent and regional expression of GABA transporter mRNAs following amygdala-kindled seizures in rats

To investigate the role played by GABA transporters in epileptic seizures, we examined time-dependent and regional changes in expression of GAT-1 and GAT-3 GABA transporter mRNA in amygdala-kindled rat brain using an in situ hybridization method. GAT-1 mRNA was significantly increased bilaterally in the hippocampal dentate gyrus (111-116%) at 1 h after kindled generalized seizures. GAT-1 mRNA was also significantly increased bilaterally in the hippocampal subfields (CA1-4 and dentate gyrus [110-117%]) at 4 h after kindled seizures. There were no significant changes in GAT-1 mRNA level in the amygdalar nuclei, pyriform cortex or cerebral cortex either ipsilaterally or contralaterally at any time after kindled seizures. In contrast, GAT-3 mRNA was significantly increased bilaterally in the amygdalar nuclei and in the contralateral pyriform cortex and cerebral cortex 1 h after seizures. Since all these changes returned to control levels by 8 or 24 h after kindled seizures, the increases in GABA transporter mRNA appeared to be transient responses to seizure activity. These findings indicate that GAT-1 subtype transporter is specifically involved in seizure activity in the hippocampus, while GAT-3 subtype transporter is mainly involved in seizure activity in the amygdalar nuclei and pyriform cortex following amygdala-kindled generalized seizures.

Decrease in somatostatin-immunoreactive neurons in the rat amygdaloid complex in a kindling model of temporal lobe epilepsy

In human temporal lobe epilepsy, seizures can begin in the hippocampus, amygdala, or surrounding cortical areas. Histologically, the seizure-induced selective neuronal damage and synaptic reorganization are best documented in the hippocampus. Little information is available about the damage in the other temporal lobe structures or whether the distribution of damage depends on the location of the primary seizure focus. We used an amygdala-kindling model of temporal lobe epilepsy to study whether seizures of amygdaloid origin cause damage to the amygdala and hippocampus. All rats experienced five class 5 generalized seizures. Neuronal damage was assessed by counting the density of GABA-immunoreactive (GABA-ir) and somatostatin-immunoreactive (SOM-ir) neurons in the amygdala and hilus of the dentate gyrus six months after the last seizure. We found that the density of GABA-ir neurons did not differ from that in controls in the contralateral amygdala. The density of SOM-ir neurons was, however, decreased in the lateral (69% of neurons remaining, P < 0.01), basal (67% remaining, P < 0.05), and accessory basal (68% remaining, P < 0.05) nuclei. In the hilus, the densities of GABA-ir and SOM-ir neurons were similar to that in controls. According to our data, a few seizures of amygdaloid origin may cause more severe damage to SOM-ir neurons in the amygdala than in the hilus. Such decrease in SOM-ir neurons which form one subpopulation of GABAergic inhibitory interneurons may increase the local excitability in the amygdala and, therefore, contribute to epileptogenesis.

Increased preproenkephalin mRNA and preprotachykinin mRNA in the striatum of Rolling mouse Nagoya

Although Rolling mouse Nagoya (RMN) has been considered to demonstrate cerebellar dysfunction, our previous metabolic and electrophysiological studies also revealed a dysfunction of the basal ganglia, with the presumable primary site of dysfunction being the striatum. In the present study, we investigated the neurochemical functions of the striatum. In RMN, both preproenkephalin mRNA and preprotachykinin mRNA increased significantly in the striatum, with unaltered GAD mRNA, [(3)H]spiperone binding, [(3)H]QNB binding and preprosomatostatin mRNA, thus indicating the dysfunction of striatal projection neurons. These findings support the hypothesis that the site of primary dysfunction in the basal ganglia is in the striatum of RMN.

Aberrant expression of neuropeptide Y in hippocampal mossy fibers in the absence of local cell injury following the onset of spike-wave synchronization

Stargazer mutant mice inherit a recessive neuronal excitability phenotype featuring frequent non-convulsive spike-wave seizures that arise from synchronous bursting in neocortical, thalamic and hippocampal networks. Immunocytochemistry reveals that granule cells in the mutant dentate gyrus aberrantly express neuropeptide Y (NPY) at multiple ages following the developmental onset of seizures. The ectopic NPY is selectively concentrated in the mossy fibers, co-localizing with the releasable dense core vesicle pool. The NPY content of native NPY+local circuit neurons is also elevated in the mutant CNS. There is no concurrent elevation of hippocampal 72 kDa heat shock protein (HSP72), glial fibrillary acidic protein (GFAP) or NADPH-diaphorase, three markers that are induced during cellular injury, and no evidence of granule cell loss. Since mossy fiber NPY expression appears after the developmental onset of spike-wave discharges and can be induced in wild type granule cells by electrical stimulation, the altered peptide phenotype is likely to reflect transynaptic gene induction triggered by synchronous bursting. These results link a specific pattern of repetitive synaptic input with selective molecular plasticity in dentate granule cells that may contribute to dynamic modifications in hippocampal network excitability.

Functional effects of D-Phe-c[Cys-Tyr-D-Trp-Lys-Val-Cys]-Trp-NH2 and differential changes in somatostatin receptor messenger RNAs, binding sites and somatostatin release in kainic acid-treated rats

In situ hybridization histochemistry for somatostatin receptors-1, -2, -3 and -4 section and receptor autoradiography using [125I]CGP 23996, [125I]somatostatin-28, [125I]seglitide and [125I]Tyr3 octreotide were carried out to determine the expression of somatostatin receptor messenger RNAs and binding sites in the hippocampus and cerebral cortex of rats 21 days following generalized limbic seizures induced by subcutaneous injection of 12mg/kg kainic acid. In control rats, somatostatin-1 to somatostatin-4 receptor messenger RNAs were found in the pyramidal layer and granule cell layer of the dentate gyrus. After kainate treatment, the CA1 subfield displayed a selective decrease in somatostatin-3 and somatostatin-4 receptor hybridization signals of 35 and 41%, respectively, whereas no changes were observed in the remaining hippocampal areas. Somatostatin-1 and somatostatin-2 receptor messenger RNA expression in the hippocampus remained unaffected by kainate treatment. No effect of kainate was observed in the expression of somatostatin receptor messenger RNAs in the cerebral cortex. In control rats, the selective somatostatin-2 receptor ligands, [125I]seglitide and [125I]Tyr3 octreotide and the non-selective somatostatin receptor ligands [125I]CGP 23996 and [125I]somatostatin-28, labelled preferentially the stratum oriens and radiatum CA1, the granule and molecular layers of the dentate gyrus and the deep layers of the cerebral cortex. [125I]somatostatin-28 and [125I]CGP 23996 labelled sites were selectively decreased by 32 and 39%, respectively, in the stratum radiatum CA1 after kainate treatment. [125I]CGP 23996 binding was also decreased by 35% in the stratum oriens CA1 and by 36% on average in the stratum oriens and radiatum CA3. [125I]seglitide and [125I]Tyr3 octreotide binding was not affected by kainate in any hippocampal region. The granule and molecular layers of the hippocampus and the layers IV-VI of the cerebral cortex did not show changes in binding sites for any of the radioligands analysed. A 18 and 35% decrease in the spontaneous and 50 mM KCl-induced somatostatin release from hippocampal slices was found two days after kainate, a likely reflection of neuronal cell loss. No differences in somatostatin release were observed 21 days after kainate treatment. At this latter time, the rats had an enhanced susceptibility to tonic-clonic seizures induced by intraperitoneal injection of 30 mg/kg pentylenetetrazol, a subconvulsant dose in naive rats. Bilateral infusion of 6 micrograms RC 160, a selective somatostatin-2 receptor agonist, in the dentate gyrus 21 days after kainate, significantly reduced (P < 0.05) the number of animals with tonic-clonic seizures induced by pentylenetetrazol.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological changes in the hippocampus in amygdaloid kindled mouse

To clarify the origin and maintenance of epileptogenesis, morphological changes in the hippocampus of amygdaloid-kindled mice were analyzed at different stages of kindling. The granule cell size in dentate gyrus and the pyramidal cell size in CA1 were clearly decreased depending on seizure stage. The cell size in CA2 was increased and density in dentate gyrus and CA2 was reduced, significantly. The morphological changes in hippocampus associated with kindling must be closely related to the acquisition and the maintenance of epileptogenesis. The results support the hypothesis that seizure-induced damage of neurons may lead to formation of new synaptic connections that produce abnormal hyperexitability and result in seizures.

Kindling induces transient changes in neuronal expression of somatostatin, neuropeptide Y, and calbindin in adult rat hippocampus and fascia dentata

Fully hippocampus-kindled rats were examined 1 day and 1 month after the last stimulation for changes in somatostatin (SS)-, neuropeptide Y (NPY)-, and calbindin (CaBP)-immunoreactivity (ir) and SS- and NPY-mRNA in situ hybridization (ISH). One day after the last stimulation, there was marked, bilateral increase in SS- and NPY-ir in the outer part of the dentate molecular layer. The cell bodies of dentate hilar SS- and NPY-containing neurons, known to project to this area, also appeared to display increased immunoreactivity as well as an increased ISH signal for SS and NPY mRNA. Bilateral de novo expression of NPY-ir in dentate mossy fiber projection to dentate hilus and CA3 was also evident, but we noted no corresponding NPY-mRNA signal in the parent cell bodies, the dentate granule cells. After 1 month, the levels of NPY-ir and ISH signal appeared essentially normal. In contrast, the levels of SS apparently were decreased, although not yet normal. CaBP-ir was markedly and selectively reduced in dentate granule cell bodies, dendrites, and mossy fibers 1 day after the last stimulation, but after 1 month CaBP-ir appeared essentially normal. Because kindling, once established, is a permanent phenomenon, the observed transient changes in SS, NPY, and CaBP in specific hippocampal terminal fields and neuronal populations cannot be associated specifically with kindling. Rather, they relate to the repeated high-frequency stimulations and may serve as protective measures against deleterious effects of such stimulations.

Vasopressin mRNA changes during kindling: the effects of kindling site and stage

Because of the many anatomical and functional links to the limbic system, the neuroendocrine system is often affected by limbic disturbances. Limbic seizures in humans and animals alter neuroendocrine function and hormone levels. We have shown that in an animal model for partial seizures, the amygdala kindled rat, plasma vasopressin levels are elevated and a sustained increase in vasopressin (VP) mRNA follows stage 5 kindled seizures. In the present experiments we sought to determine when during the course of amygdala kindling the VP mRNA increase occurs and whether specific anatomical pathways mediate this increase. Animals kindled to early seizure stages (stages 1, 2 or 3) had no consistent increase in VP mRNA in the supraoptic nucleus (SON) while animals kindled to generalized seizures, stages 4 or 5, invariably had increased VP mRNA relative to controls. Electrical kindling to stage 5 seizures from two other brain sites, the dorsal hippocampus and the anterior olfactory nucleus, consistently resulted in a significant increase in VP mRNA one week after completing kindling. In all experiments the increase in VP mRNA in the SON showed no differences related to the side or proximity of the electrodes used for kindling. Measures of water balance did not change following kindling. These results indicate that kindled seizure generalization is a prerequisite for the long-term increase in VP mRNA. Furthermore, the VP mRNA increase appears to involve polysynaptic pathways accessible from different limbic kindling sites. These studies support the hypothesis that changes in mRNA regulation may contribute to the neuroendocrine pathophysiology accompanying limbic seizures.

Kindled seizures induce a long-term increase in vasopressin mRNA

Neuroendocrine disturbances are among the significant problems associated with animal and human seizures. To investigate the mechanisms for these disturbances, we examined changes in the expression of vasopressin (VP) mRNA in the hypothalamic magnocellular neuroendocrine cells of rats after amygdala kindled seizures, a model for temporal lobe epilepsy. A prominent increase in VP mRNA was found in the supraoptic nucleus of kindled animals by one week after the last seizure which persisted for at least 4 months. The increase occurred bilaterally in the SON and remained unchanged despite the absence of further stimulation, seizures or change in body fluid homeostasis. Since the VP mRNA change after kindling correlated with the duration of afterdischarge but not the number of amygdala stimuli the change appears to be an effect of the seizure. This chronic increase in VP mRNA appears to reflect a change in neuroendocrine gene expression and may identify an important new mechanism of plasticity that contributes to the neuroendocrine disturbances accompanying epilepsy.

Somatostatin release is enhanced in the hippocampus of partially and fully kindled rats

The release of somatostatin (somatostatin-like immunoreactivity) from hippocampal slices during the development of hippocampal kindling in rats was measured under resting and depolarizing conditions. Preliminary experiments in naive rats showed that the spontaneous efflux of somatostatin (4.0 +/- 0.3 fmol/ml every 10 min) was independent of external Ca2+ but was reduced to 71.5 +/- 6% of baseline (P < 0.05) during 20 min incubation with 5 microM tetrodotoxin. Neuronal depolarization with 25, 50 and 100 mM KCl induced a Ca(2+)-dependent somatostatin release, respectively 4.3 +/- 0.4, 16.7 +/- 1.6 and 22.0 +/- 1.3 times baseline (P < 0.01). Veratridine caused a dose-dependent Ca2+ and tetrodotoxin (5 microM) sensitive release ranging from 6.5 +/- 0.1 to 13.0 +/- 1.4 times baseline at 1.4 microM and 50 microM respectively (P < 0.01). One week after the last of three consecutive stage 5 seizures (full seizure expression) or 48 h after the last stage 2 stimulation (preconvulsive stage), 50 mM KCl-induced somatostatin release was significantly higher (1.8 +/- 0.1, P < 0.01) than in shams (animals implanted with electrodes but not stimulated) in the stimulated and contralateral hippocampus. Somatostatin release measured under resting conditions was increased by 1.5 times in the stimulated hippocampus at stage 2 (P < 0.05) and by 2.2 and 1.7 times in both hippocampi at stage 5 (P < 0.01). Forty-eight hours after the induction of a single afterdischarge no significant changes were found in either spontaneous or 50 mM KCl-induced release of somatostatin.(ABSTRACT TRUNCATED AT 250 WORDS)

Persistence of reorganized synaptic connectivity in the amygdala of kindled rats

Kindling stimulation was applied to the basolateral amygdala of adult rats, and the density of dendritic synapses was examined under the electron microscope in the medial amygdaloid nucleus (MAN) contralateral to the site of stimulation, and an unfolding correction of biasedness was made. When generalized motor seizures had been induced for 5 days consecutively, the kindling was considered to be complete. The number of dendritic synapses, but not the number of somatic synapses, was markedly decreased in the MAN of the kindled rats. Reductions in numbers were marked in the case of both dendritic shaft and spine synapses. The reductions in numbers of shaft and spine synapses were similarly evident in the MAN of kindled rats 100 days after stimulation was discontinued. The numbers of dendritic synapses were similarly decreased in the rats that received additional bouts of stimulation subsequent to the completion of kindling. Thus, once the kindling was completed, the newly acquired synaptic connectivity was preserved in the MAN. These findings indicate that the remodeling of synaptic connectivity was a morphological correlate of the kindling in the MAN.

Temporal lobe epilepsy of the rat: differential expression of mRNAs of chromogranin B, secretogranin II, synaptin/synaptophysin and p65 in subfield of the hippocampus

We have investigated by in situ hybridization changes in the content of mRNAs encoding for chromogranin B, secretogranin II, synaptin/synaptophysin and p65 after kainic acid-induced seizures and pentylenetetrazol kindling. Kainic acid seizures resulted in marked but transient increases in secretogranin II mRNA concentrations in the granule cell layer and throughout the pyramidal cell layers of the hippocampus (by 100-500%) as well as in various areas of the cerebral cortex (by up to 900%) and the thalamus (up to 300%) 12 h after injection of the toxin. Chromogranin B mRNA concentrations were persistently increased in granule cells (but not in pyramidal cells) of the hippocampus (suprapyramidal blade, 450%) and in cortical areas (250%) at all time intervals after kainic acid injection (12 h to 60 days). Accordingly chromogranin B immunoreactivity was enhanced in the terminal field of mossy fibers and in the inner part of the molecular layer 30 days after kainic acid. Secretogranin II immunoreactivity was also markedly increased in CA1, the paraventricular thalamic nucleus and in the central amygdala. In rats kindled with pentylenetetrazol only chromogranin B (by 200%) but not secretogranin II mRNA was increased in dentate granule cells. In contrast to the mRNAs of these secretory proteins concentrations of mRNAs encoding synaptin/synaptophysin and p65, two membrane proteins of synaptic vesicles, were not altered in any of these brain structures. These data demonstrate that in brain the biosynthesis of chromogranin B and secretogranin II is regulated like that of neuropeptides which is consistent with a role of these secretory polypeptides as precursors of functional peptides. Activation of neurons induces an increased synthesis of neuropeptides but not a concomitant synthesis of membrane proteins of synaptic vesicle. This might lead to an increased quantal content available for transmission.

Functional changes in neuropeptide Y- and somatostatin-containing neurons induced by limbic seizures in the rat

The influence of sustained epileptic seizures evoked by intraperitoneal injection of kainic acid on the gene expression of the neuropeptides somatostatin and neuropeptide Y and on the damage of neurons containing these peptides was studied in the rat brain. Injection of kainic acid induced an extensive loss of somatostatin and, though less pronounced, of neuropeptide Y neurons in the inner part of the hilus of the dentate gyrus. Neuropeptide Y-immunoreactive neurons located in the subgranular layer of the hilus, presumably pyramidal-shaped basket cells, were spared by the treatment. Although neuropeptide Y messenger RNA was not detected in granule cells of control rats, it was found there after kainic acid seizures at all time intervals investigated (12 h to 90 days after injection of kainic acid). High concentrations of neuropeptide Y messenger RNA were especially observed 24 h after injection of kainic acid. At this time neuropeptide Y messenger RNA was also transiently observed in CA1 pyramidal cells. Neuropeptide Y synthesis in granule cells in turn gave rise to an intense immunoreactivity of the peptide in the terminal field of mossy fibers which persisted for the entire time period (90 days) investigated. In addition, neuropeptide Y messenger RNA concentrations were also drastically elevated in presumptive basket cells located at the inner surface of the granule cell layer, especially at the "late" time intervals investigated (30-90 days after kainic acid). These data support the concept that extensive activation of granule cells by limbic seizures contributes to the observed neuronal cell death in CA3 pyramidal neurons and interneurons of the hilus. Consecutively, basket cells containing neuropeptide Y and presumably GABA might be activated and participate in recurrent inhibition of granule cells. Neuropeptide Y-immunoreactive fibers observed in the inner molecular layer at "late" time intervals after kainic acid may result either from collateral sprouting of mossy fibers or from basket cells extensively expressing the peptide. It is speculated that neuropeptide Y synthesized and released at a high rate from granule cells and basket cells may exert a protective action against seizures.

Ziskind-Somerfeld Research Award 1992. Endogenous biochemical abnormalities in affective illness: therapeutic versus pathogenic

Examination of the neurobiology of psychiatric illness in general, and of affective disorders in particular, reveals a variety of associated biochemical abnormalities. These have generally been assumed to be part of the pathological process or secondary to it, and thus deserving of therapeutic efforts aimed at reversal. However, recent clinical and preclinical data suggest that some alterations occurring in the affective disorders may be compensatory and adaptive; that is, part of an endogenous therapeutic mechanism rather than part of the evolving disease process. For example, the symptom of sleep loss in depression seems to fall under this rubric inasmuch as sleep deprivation induces mood improvement in depressed patients. Preclinical data are presented that another primary pathological process--the occurrence of kindled seizures--can evoke endogenous compensatory processes that are either anticonvulsant in their own right, or enable the anticonvulsant effects of a drug such as carbamazepine. It may be that some biochemical abnormalities occurring in affective illness are similarly adaptive. As one example, increased thyrotropin-releasing hormone (TRH) has been reported in the cerebrospinal fluid (CSF) of depressed patients. This elevation of TRH and the resulting neuroendocrine profile may be part of an endogenous counter-regulatory process aimed at mood improvement. Again, preclinical seizure models are supportive in that TRH not only is induced following repeated seizures, but also exerts anticonvulsant effects on these same seizures. In an analogous fashion, TRH elevations in depressed patients may also exert ameliorating effects on depressive symptomatology. This formulation presents directly testable hypotheses that could importantly impact on our understanding of the pathophysiology of affective disorders, and suggests novel therapeutic strategies through the enhancement of endogenous compensatory mechanisms.

Effects of somatostatin and anti-somatostatin serum on picrotoxin-kindled seizures

The effects of somatostatin, administered into different areas of the brain were studied in preliminary picrotoxin-kindled rats. The injection of somatostatin into the lateral ventrical of the brain (i.c.v.) (1.8 nmol), the hippocampus (0.6 nmol) or the amygdala (0.6 nmol), resulted in a decrease in the severity of the picrotoxin-induced convulsions. Application of the peptide into the caudate-putamen or the substantia nigra reticulata did not alter the behavioural manifestations of the kindled seizures. The local injection of anti-somatostatin serum (1:5) into the hippocampus increased the severity of the kindled convulsions and blocked the anticonvulsive effect of somatostatin, given intraventricularly. Local administration of anti-somatostatin serum into the amygdala did not alter the kindled seizures and did not abolish the anticonvulsive action of somatostatin given intraventricularly. It is concluded that somatostatin could take part in endogenous control of seizures through a suppressant influence on limbic structures; the hippocampus could be a specific site for the antiepileptic action of somatostatin.

Cortical stimulation induces fos expression in striatal neurons

Electrical stimulation of a broad area of the frontoparietal cortex in the rat brain induces immunocytochemically detectable Fos in striatal neurons normally devoid of the protein. The vividness of labeling within striatal neurons was maximal at 0.5 h after the cessation of a 15-min-long stimulation period and became weaker by 3 h. Although Fos-reactive neurons were widely distributed in the striata of both hemispheres in an uneven pattern, those on the stimulated side were more numerous and more darkly stained. At no time-point were labeled neurons found in the globus pallidus, entopeduncular nucleus or substantia nigra. Destruction of the nigrostriatal dopamine projection with 6-hydroxydopamine did not induce Fos production and failed to prevent the induction of Fos by cortical stimulation. That many of the Fos-positive neurons project to the substantia nigra was confirmed by retrograde labeling with Fluoro-Gold. The data suggest that corticostriatal excitatory transmission may directly influence the genomic activity of striatal neurons by way of Fos.

In vivo microdialysis of amino acid neurotransmitters in the hippocampus in amygdaloid kindled rat

Extracellular concentrations of gamma-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) were determined by microdialysis in rat hippocampus during various amygdaloid kindled stages. The values of GABA and Glu were increased 3-4 times in C2-C3 stages in comparison with the values in control animals. After reaching the C5 stages, these values were increased 3-7 times. However, the concentration of Asp decreased depending on the kindling stage, reaching the lowest value of 33% in comparison with the normal value. The observed changes may be related to kindling induced seizures.

Increased preproneuropeptide Y mRNA in the rat hippocampus during the development of hippocampal kindling: comparison with the expression of preprosomatostatin mRNA

The levels of preproneuropeptide Y (ppNPY) mRNA and preprosomatostatin (ppSOM) mRNA were analyzed in different brain regions during the development of hippocampal kindling in rats. ppNPY mRNA levels were markedly elevated in the dorsal hippocampus bilaterally, two days after stage 2 (preconvulsive stage) and stage 5 (full seizure expression). The contents of ppSOM mRNA were slightly, although not significantly, increased in the dorsal hippocampus at stage 2 whereas a significant increase was observed in the ipsilateral hippocampus of fully kindled rats. ppNPY and ppSOM mRNA levels were unchanged in the cortex and striatum at both stages of kindling. These results show that an increased synthesis of somatostatin and neuropeptide Y, with a greater effect for the latter, occurs during hippocampal kindling in rats. The relative role of the two peptides in the development and expression of kindling phenomenon remains to be elucidated.

Alterations in somatostatin and proenkephalin mRNA in response to a single amygdaloid stimulation versus kindling

Previous studies have shown changes in both somatostatin (SS)- and proenkephalin(PE)-derived peptides in the brains of amygdaloid-kindled rats, suggesting possible roles for the peptides in the kindling process. In this study, we have extended this analysis by looking at the time course of changes in SS and PE mRNAs at various times after kindling, in comparison with a single non-convulsive stimulation. Blot analysis of total RNA showed increases in SS mRNA in striatum, frontal cortex and hippocampus of animals receiving only a single stimulation as well as kindled animals--the increase occurred 1-3 days following stimulation and levels were back to basal by 1 week. PE mRNA did not change. In situ hybridization analysis, one day after the last kindling stimulation, showed significant elevations of SS mRNA in CA1, CA2 and dentate gyrus of hippocampus and of PE mRNA in olfactory cortex that were specific to kindling. However, both a single stimulation and kindling increased PE mRNA in olfactory tubercle and arcuate nucleus. In contrast, a single electrical stimulus increased PE mRNA in ventral striatum and SS mRNA in cingulate cortex and olfactory tubercle. These data support the idea that changes of SS mRNA in hippocampus and of PE mRNA in olfactory cortex may be related to kindling, and point out the importance of using animals which receive a single electrical stimulus, rather than sham-operated animals, as controls.

Proto-oncogene transcription factors and epilepsy

Chemically and electrically induced seizures elicit the rapid transcriptional activation in neurons of a class of genes referred to as cellular immediate-early genes. Since the products of these genes include transcription factors and cytokines, they are proposed to be involved in coupling neuronal excitation to a complex, and poorly understood, programme of cellular responses that involves the regulation of gene expression. Products of two cellular immediate-early genes, c-fos and c-jun, are components of the transcription factor AP-1. In this review, Jim Morgan and Tom Curran discuss how these gene products have begun to reveal some of the molecular details of stimulus-transcription coupling in the nervous system following seizures. In addition, these genes have provided novel reagents and concepts for investigating the biochemical and cellular sequelae of seizure in the CNS, and point towards new avenues of research and potential therapeutic targets in epilepsy.

Enhanced rate of expression and biosynthesis of neuropeptide Y after kainic acid-induced seizures

Recent studies have shown marked increases in brain content of neuropeptide Y (NPY) after seizures induced by intraperitoneal injection of kainic acid and after pentylenetetrazole kindling in the rat. We have now investigated possible changes in the rate of biosynthesis of NPY after kainic acid treatment, by using pulse-labeling of the peptide and by determining prepro-NPY mRNA concentrations. For pulse labeling experiments, [3H]tyrosine was injected into the frontal cortex, and the incorporation of the amino acid into NPY was determined after purifying the peptide by gel filtration chromatography, antibody affinity chromatography, and reversed-phase HPLC. At 2 and 30 days after kainic acid treatment, the rate of tyrosine incorporation was enhanced by approximately 380% in the cortex. In addition, concentrations of pre-pro-NPY mRNA were determined in four different brain areas by hybridization of Northern blots with a complementary 32P-labeled RNA probe 2, 10, 30, and 60 days after kainic acid treatment. Marked increases were observed in the frontal cortex (by up to 350% of controls), in the dorsal hippocampus (by 750%), and in the amygdala/pyriform cortex (by 280%) at all intervals investigated. In the striatum only a small, transient increase was observed. The data demonstrate increased expression of prepro-NPY mRNA and an enhanced rate of in vivo synthesis of NPY as a result of seizures induced by the neurotoxin kainic acid.

Calcium binding protein (calbindin-D28k) and glutamate decarboxylase gene expression after kindling induced seizures

In order to determine whether calcium binding protein (calbindin-D28k or CaBP) and glutamate decarboxylase (GAD) may be involved in the process underlying the generation of seizure activity, changes in CaBP protein and mRNA and in GAD mRNA were examined in the kindling model of epilepsy. Following amygdaloid (AK) and commissure (CK) kindling significant decreases in the concentration of CaBP of 20% and 30%, respectively, were specifically observed in the hippocampal formation. However, using a cDNA specific to mammalian CaBP, Northern analysis of poly(A+) RNA and slot blot analysis of total RNA revealed no changes in the levels of CaBP mRNA in hippocampus, subcortical area (including amygdala, substantia nigra and striatum) or cerebellum of rats sacrificed 30 min, 1 h, 6 h or 24 h after the last kindled seizure. Similarly when these blots were reprobed with a cDNA specific to mammalian GAD, no changes in GAD gene expression were observed. However, fos gene expression was markedly enhanced at 1 h after seizure. We also tested whether changes in CaBP or GAD mRNA could be detected at any of the various stages of the kindling process. Slot blot analysis of cortex, subcortical structures and hippocampus revealed no changes in CaBP or GAD mRNA during the course of commissure kindling. In situ hybridization studies with GAD and CaBP 35S-labeled antisense probes also indicated no obvious changes upon visual analysis of autoradiographs. However, when silver grains were counted, significant changes in GAD mRNA in individual cells in hippocampus and substantia nigra were noted after kindling induced epilepsy. Our results indicate that, unlike fos gene expression, prominent alterations in GAD and CaBP mRNA in gross brain regions (as measured by slot blot and Northern blot analyses) are not observed in the kindling process. However, our in situ hybridization studies suggest that changes in GAD mRNA in individual cells may be involved in the process underlying kindling induced seizure activity.

Time-dependent pro- and anticonvulsant effects of cysteamine on the development and expression of amygdaloid kindled seizures

The time-dependent pro- and anticonvulsant effects of cysteamine, a depletor of somatostatin, were investigated on the development and expression of amygdaloid kindled seizures. Acute administration of cysteamine (25-400 mg/kg, i.p.) produced a dose-dependent potentiation of kindled seizures when evoked 4 h after the drug. However, the seizures initiated 1 day after drug administration were dose-dependently suppressed. Furthermore, elicitation of seizures 4 h after cysteamine enhanced its anticonvulsant effects at 1 day after the drug, causing a parallel left shift of the dose-response curve. Since it has been reported that somatostatin is released during generalized seizures, the seizures given 4 h after cysteamine may encourage the somatostatin depletion by cysteamine and thereby potentiate its later anticonvulsant effects. The repeated administration of cysteamine (100 mg/kg, i.p.) during kindling development strongly retarded the development of generalized seizures but not the development of focal seizures or of afterdischarges in the amygdala. In contrast to the acute experiments, kindling stimulation given 4 h after each cysteamine treatment did not augment the blocking effect on kindling development. These data indicate that chronic cysteamine treatment has a strong inhibitory effect on the development of amygdaloid kindling.

Effect of anticonvulsant treatment on kainic acid-induced increases in peptide levels

The influence of anticonvulsant treatment upon (1) chronically increased seizure susceptibility, (2) on late increases in peptide levels and (3) on seizure-induced brain damage was investigated during various stages of acute kainic acid (10 mg/kg i.p.)-induced seizures. The seizures were interrupted at various stages of the syndrome (50 min to 24 h after injection of the toxin) by injecting thiopental (50 mg/kg i.p.) or the excitatory amino acid antagonist, MK-801 (10 mg/kg i.p.). The increase in neuropeptide Y and somatostatin levels in the frontal cortex could be prevented by early injection of either anticonvulsant (up to 180 min after kainic acid). No protection against the increase in peptide levels was observed when the anticonvulsants were applied later. Kainic acid-induced neuronal damage in the amygdala, with glutamate decarboxylase as a neurochemical marker, was entirely prevented by interrupting seizures up to 2 h after kainic acid. Partial protection (about 40-50%) was even found when the anticonvulsant treatment was applied after the acute syndrome, as late as 8 h after kainic acid injection. Chronically increased seizure susceptibility induced by kainic acid was not prevented, even by early injection (90 min after kainic acid) of the anticonvulsant drugs. The data indicate that (1) the late increase in seizure susceptibility may be initiated early after injection of kainic acid. (2) the late increase in peptide levels may be related to the frequency of acute seizures rather than to a change in seizure threshold or brain damage and (3) even late anticonvulsant therapy may antagonize seizure-induced brain damage in the amygdala.

Preclinical and clinical studies with somatostatin related to the central nervous system

1. The tetradecapeptide somatostatin (SS) has a widespread, uneven distribution within several organs including the central nervous system (CNS), with particularly high concentration in the hypothalamus. 2. The SS-related peptides (SS28, SS28(1-12), SS28(15-28)) are originated from the precursor pre-prosomatostatin. 3. SS is suggested to be involved in a large number of CNS functions, locomotion, sedation, excitation, catatonia, body temperature, feeding, nociception, paradoxical sleep, self-stimulation, seizure, learning and memory. 4. SS influences central neurochemical processes. 5. It is possible that SS is related to various neurological and psychiatric illnesses, like Huntington's disease, multiple sclerosis, Parkinson's disease, epilepsy, eating disorders, Alzheimer's disease, schizophrenia and major depressive illness.

Neuropeptide Levels after Pentylenetetrazol Kindling in the Rat

Levels of several neuropeptides were measured in the frontal cortex, dorsal hippocampus, striatum, and amygdala/pyriform cortex in rats kindled for 5 weeks by daily injection of pentylenetetrazol (30 mg/kg, i.p.). Significantly increased concentrations (by 30 - 140%) were found in all examined brain areas for neuropeptide Y, somatostatin (except hippocampus) and neurokinin-like immunoreactivity 10 days after the last kindling session. Similar but less pronounced changes were also found 24 h after the last seizure. The increase in total neurokinin-like immunoreactivity was due to a marked increase in neurokinin B as revealed by HPLC analysis. Increases in peptide levels, however, were restricted to fully kindled animals. At the same time no changes in levels of substance P, vasoactive intestinal polypeptide and calcitonin gene-regulated peptide were observed. Cholecystokinin octapeptide was enhanced only in the hippocampus (by 46%). The increases in neuropeptide Y, somatostatin, and neurokinin-like immunoreactivity subsided after 3 months. A markedly decreased seizure threshold was observed 10 days and 2 months after the final kindling session. No nerve cell degeneration was observed in kindled rats 24 h or 10 days after the last pentylenetetrazol injection. Some animals (2 of 4), however, exhibited signs of blood - brain barrier damage when examined 24 h after the last kindling session which may reflect the preceding convulsions. No such changes were detected after 10 days. The increases in peptide levels may suggest increased activity of respective neurons which, at least to some degree, may be associated with gamma-aminobutyric acid. The changes in peptide levels may be more closely related to the kindling procedure itself than to the decreased seizure threshold of the animals.

Regulation of proenkephalin by Fos and Jun

Fos and Jun form a heterodimeric complex that associates with the nucleotide sequence motif known as the AP-1 binding site. Although this complex has been proposed to function as a transcriptional regulator in neurons, no specific target gene has yet been identified. Proenkephalin mRNA increased in the hippocampus during seizure just after an increase in c-fos and c-jun expression was detected. Fos-Jun complexes bound specifically to a regulatory sequence in the 5' control region of the proenkephalin gene. Furthermore, c-fos and c-jun stimulated transcription from this control region synergistically in transactivation assays. These data suggest that the proenkephalin gene may be a physiological target for Fos and Jun in the hippocampus and indicate that these proto-oncogene transcription factors may play a role in neuronal responses to stimulation.