Differential effect of acute vs chronic desmethylimipramine on the rate of amygdala kindling in rats

Sumatriptan reduces severity of status epilepticus induced by lithium-pilocarpine through nitrergic transmission and 5-HT1B/D receptors in rats: A pharmacological-based evidence

Status epilepticus (SE) is a life-threatening neurologic disorder that can be as both cause and consequence of neuroinflammation. In addition to previous reports on anti-inflammatory property of the anti-migraine medication sumatriptan, we have recently shown its anticonvulsive effects on pentylenetetrazole-induced seizure in mice. In the present study, we investigated further (i) the effects of sumatriptan in the lithium-pilocarpine SE model in rats, and (ii) the possible involvement of nitric oxide (NO), 5-hydroxytryptamin 1B/1D (5-HT_1B/1D ) receptor, and inflammatory pathways in such effects of sumatriptan. Status epilepticus was induced by lithium chloride (127 mg/kg, i.p) and pilocarpine (60 mg/kg, i.p.) in Wistar rats. While SE induction increased SE scores and mortality rate, sumatriptan (0.001-1 mg/kg, i.p.) improved it (P < 0.001). Administration of the selective 5-HT_1B/1D antagonist GR-127935 (0.01 mg/kg, i.p.) reversed the anticonvulsive effects of sumatriptan (0.01 mg/kg, i.p.). Although both tumor necrosis factor-α (TNF-α) and NO levels were markedly elevated in the rats' brain tissues post-SE induction, pre-treatment with sumatriptan significantly reduced both TNF-α (P < 0.05) and NO (P < 0.001) levels. Combined GR-127935 and sumatriptan treatment inhibited these anti-inflammatory effects of sumatriptan, whereas combined non-specific NOS (L-NAME) or selective neuronal NOS (7-nitroindazole) inhibitors and sumatriptan further reduced NO levels. In conclusion, sumatriptan exerted a protective effect against the clinical manifestations and mortality rate of SE in rats which is possibly through targeting 5-HT_1B/1D receptors, neuroinflammation, and nitrergic transmission.

Mechanisms of Psychiatric Comorbidities in Epilepsy

Psychiatric illnesses, including depression and anxiety, are highly comorbid with epilepsy (for review see Josephson and Jetté (Int Rev Psychiatry 29:409-424, 2017), Salpekar and Mula (Epilepsy Behav 98:293-297, 2019)). Psychiatric comorbidities negatively impact the quality of life of patients (Johnson et al., Epilepsia 45:544-550, 2004; Cramer et al., Epilepsy Behav 4:515-521, 2003) and present a significant challenge to treating patients with epilepsy (Hitiris et al., Epilepsy Res 75:192-196, 2007; Petrovski et al., Neurology 75:1015-1021, 2010; Fazel et al., Lancet 382:1646-1654, 2013) (for review see Kanner (Seizure 49:79-82, 2017)). It has long been acknowledged that there is an association between psychiatric illnesses and epilepsy. Hippocrates, in the fourth-fifth century B.C., considered epilepsy and melancholia to be closely related in which he writes that "melancholics ordinarily become epileptics, and epileptics, melancholics" (Lewis, J Ment Sci 80:1-42, 1934). The Babylonians also recognized the frequency of psychosis in patients with epilepsy (Reynolds and Kinnier Wilson, Epilepsia 49:1488-1490, 2008). Despite the fact that the relationship between psychiatric comorbidities and epilepsy has been recognized for thousands of years, psychiatric illnesses in people with epilepsy still commonly go undiagnosed and untreated (Hermann et al., Epilepsia 41(Suppl 2):S31-S41, 2000) and systematic research in this area is still lacking (Devinsky, Epilepsy Behav 4(Suppl 4):S2-S10, 2003). Thus, although it is clear that these are not new issues, there is a need for improvements in the screening and management of patients with psychiatric comorbidities in epilepsy (Lopez et al., Epilepsy Behav 98:302-305, 2019) and progress is needed to understand the underlying neurobiology contributing to these comorbid conditions. To that end, this chapter will raise awareness regarding the scope of the problem as it relates to comorbid psychiatric illnesses and epilepsy and review our current understanding of the potential mechanisms contributing to these comorbidities, focusing on both basic science and clinical research findings.

Serotonin depletion effects on the pilocarpine model of epilepsy

The monoamine content in cerebral structures has been related to neuronal excitability and several approaches have been used to study this phenomenon during seizure vulnerability. In the present work, we have described the effects of serotonin (5-HT) depletion after the administration of 5,7-dihydroxytryptamine (5,7-DHT) into the median raphe nucleus in rats submitted to the pilocarpine model of epilepsy. Susceptibility to pilocarpine-induced status epilepticus as well as the spontaneous seizure frequency during the chronic period of the model was determined. Since the hippocampus is one of the main structures in the development of this epilepsy model, the 5-HT levels in this region were also determined after drug administration. Sixty-three percent of 5,7-DHT pre-treated rats (15/24) and only 33.4% of those receiving the control solution (9/24) progressed to motor limbic seizures evolving to status epilepticus, following the administration of pilocarpine. The frequency of seizures during the chronic period, in epileptic rats that received 5,7-DHT, showed a significant (58%) increase after the treatment, when compared with control group. Our data showed that serotonin may play an important role on seizure activity which seems to be exerted by its inhibitory action on the expression of overt behavior seizures departing from an established focus in the limbic system.

Cell therapy in models for temporal lobe epilepsy

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

The effects of chronic norepinephrine transporter inactivation on seizure susceptibility in mice

Epilepsy and depression are comorbid disorders, but the mechanisms underlying their relationship have not been identified. Traditionally, many antidepressants have been thought to increase seizure incidence, although this remains controversial, and it is unclear which medications should be used to treat individuals suffering from both epilepsy and depression. Since the neurotransmitter norepinephrine (NE) has both antidepressant and anticonvulsant properties, we speculated that NE transporter (NET) inhibitor antidepressants might be therapeutic candidates for comorbid individuals. To test this idea, we assessed the effects of chronic administration (via osmotic minipump) of the selective NET inhibitor reboxetine on flurothyl-induced seizures in mice. We found that reboxetine had both proconvulsant and anticonvulsant properties; it lowered both seizure threshold and maximal seizure severity. NET knockout (NET KO) mice essentially phenocopied the effects of reboxetine on flurothyl-induced seizures, and the trends were extended to pentylenetetrazole and maximal electroshock seizures (MES). Furthermore, reboxetine had no further effect in NET KO mice, demonstrating the specificity of reboxetine for the NET. We next tested the chronic and acute effects of other classes of antidepressants (desipramine, imipramine, sertraline, bupropion, and venlafaxine) on seizure susceptibility. Only venlafaxine was devoid of proconvulsant activity, and retained some anticonvulsant activity. These results suggest that chronic antidepressant drug treatment has both proconvulsant and anticonvulsant effects, and that venlafaxine is a good candidate for the treatment of epilepsy and depression comorbidity.

The role of norepinephrine in epilepsy: from the bench to the bedside

This article provides a brief review of the role of norepinephrine (NE) in epilepsy, starting from early studies reproducing the kindling model in NE-lesioned rats, through the use of specific ligands for adrenergic receptors in experimental models of epilepsy, up to recent advances obtained by using transgenic and knock-out mice for specific genes expressed in the NE system. Data obtained from multiple experimental models converge to demonstrate the antiepileptic role of endogenous NE. This effect predominantly consists in counteracting the development of an epileptic circuit (such as in the kindling model) rather than increasing the epileptic threshold. This suggests that NE activity is critical in modifying epilepsy-induced neuronal changes especially on the limbic system. These data encompass from experimental models to clinical applications as recently evidenced by the need of an intact NE innervation for the antiepileptic mechanisms of vagal nerve stimulation (VNS) in patients suffering from refractory epilepsy. Finally, recent data demonstrate that NE loss increases neuronal damage following focally induced limbic status epilepticus, confirming a protective effect of brain NE, which has already been shown in other neurological disorders.

Regional Forebrain Noradrenalin Release in Response to Focal and Generalized Seizures Induced by Hippocampal Kindling Stimulation

In vivo microdialysis was used to monitor noradrenalin (NA) release in the rat hippocampus, sensorimotor cortex and amygdala in response to seizures induced by electrical kindling stimulation in the hippocampus. Generalized seizures increased NA output in the hippocampus five-fold above baseline level (as assessed with 2-min sampling periods). The peak value was seen 2 - 4 min after onset of seizure activity and baseline was reached after another 6 - 8 min. In the sensorimotor cortex, there was a seven-fold increase showing a similar time-course. Focal hippocampal seizures gave rise to three-fold and 80% increases above baseline in the hippocampus and sensorimotor cortex, respectively. A unilateral knife transection of the dorsal noradrenergic bundle reduced hippocampal NA release induced by focal seizures by 53%. In animals subjected to 30 stimulus-evoked seizures with 5-min intervals ('rapid kindling'), maximal NA output was observed after the third seizure in both hippocampus (237% increase) and amygdala (122% increase). NA levels tapered off with repeated stimulation and reached baseline after nine stimulations in the hippocampus; in the amygdala, the NA output was still slightly elevated at the end of the stimulation period. These results indicate that there is a general activation of the locus coeruleus system during focal as well as generalized seizures, as evidenced by marked increases in transmitter release from noradrenergic terminals in all forebrain areas studied. NA output in areas exhibiting seizure activity is dependent on impulse flow in locus coeruleus neurons and probably also on local regulatory mechanisms active at the noradrenergic terminal level. The increase in inhibitory noradrenergic transmission in both epileptic and non-epileptic brain regions may dampen ongoing seizure activity as well as lessen its spread and generalization.

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

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

Seizure suppression in kindling epilepsy by intracerebral implants of GABA- but not by noradrenaline-releasing polymer matrices

Gamma-aminobutyric acid (GABA)-releasing polymer matrices were implanted bilaterally, immediately dorsal to the substantia nigra, in rats previously kindled in the amygdala. Two days after implantation, rats with GABA-releasing matrices exhibited only focal limbic seizures in response to electrical stimulation, whereas animals with control matrices devoid of GABA had generalized convulsions. GABA release from the polymer matrices was high during the first days after implantation, as demonstrated both in vitro and, using microdialysis, in vivo. The anticonvulsant effect was no longer observed at 7 and 14 days at which time GABA release was found to be low. In a parallel experiment, polymer matrices containing noradrenaline (NA) were implanted bilaterally into the hippocampus of rats with extensive forebrain NA depletion induced by an intraventricular 6-hydroxydopamine injection. No effect on the development of hippocampal kindling was observed, despite extracellular NA levels exceeding those of rats with intrahippocampal locus coeruleus grafts that have previously been shown to retard kindling rate. The results indicate that GABA-releasing implants located in the substantia nigra region can suppress seizure generalization in epilepsy, even in the absence of synapse formation and integration with the host brain. In contrast, the failure of NA-releasing polymer matrices to retard the development of seizures in NA-depleted rats suggests that such an effect can only be exerted by grafts acting through a well-regulated, synaptic release of NA.

Amphetamine and antidepressant drug effects on GABA- and NMDA-related seizures

Research has shown a synergistic relationship between amphetamine sensitization and limbic system kindling. To explore the role of GABA and NMDA receptor activity in modulating the positive effects of amphetamine on epileptogenesis, alterations in GABA- and NMDA-related convulsions were examined after acute and chronic amphetamine administration. A single injection of d-amphetamine (7.5 mg/kg) significantly decreased latencies to generalized motor seizures induced 12 h later by the noncompetitive GABAA receptor antagonist picrotoxin (10 mg/kg). The increased sensitivity to clonus was specific to acute amphetamine treatment and was not evident following withdrawal from chronic drug exposure. Seizures induced by NMDLA (1,000 mg/kg), on the other hand, were not modified by acute amphetamine injection; however, the latency to clonus was reduced substantially after NMDLA injection to mice chronically preexposed to amphetamine. The short- and long-term amphetamine effects on GABA- and NMDA-associated convulsive activity were not paralleled by similar drug treatment schedules involving acute (20 mg/kg) and chronic administration of desipramine, zimelidine, and buproprion. These results suggest that amphetamine may be acting on inhibitory and excitatory amino acid systems independently of its monoaminergic properties. The implications of these findings were discussed in relation to amphetamine sensitization of mesolimbic functioning.

Sensitization of mesolimbic brain stimulation reward after electrical kindling of the amygdala

The effects of partial kindling of the central amygdaloid nucleus on brain stimulation reward were evaluated. Intracranial self-stimulation (ICSS) rate intensity functions were determined using a two-hole nose-poke discrimination paradigm in rats implanted with electrodes in the A10 dopamine (DA) neuronal region of the ventral tegmental area (VTA), and in the medial forebrain bundle (MFB) at the level of the lateral hypothalamus. The kindling process did not influence ICSS baseline rates from either site. However, following stage 4 kindling, a low dose amphetamine challenge increased ICSS, resulted in a significant shift to the left of the rate intensity functions, and decreased reward thresholds. Analysis of the error scores which consisted of nonreinforced responses made during ICSS testing revealed that the interaction between kindling and amphetamine on ICSS was specific to changes in central reward processes, and could not be attributed to the influence of rate-enhancing performance variables. The kindling-elicited sensitization of mesolimbic DA reward functioning seen after amphetamine challenge was discussed in relation to the role of the central amygdala in the integration of stimulus-reward associations, and in the conditioning of affective emotional states.

Suppression of amygdala kindling with massed stimulation: effect of noradrenaline antagonists

Afterdischarge (AD) triggered by brief, daily stimulation of the amygdala progressively increases in complexity and duration and, over days, develops into generalized convulsions. This progression, called kindling, is delayed by noradrenaline (NA). When brief stimulation of the amygdala occurs too frequently (massed), there is a suppression of AD growth and little evidence of kindling. Previously we showed that depletion of NA before massed amygdala stimulation prevented the suppression of AD growth described above, and readily precipitated generalized seizures. In the present report, we examined the role of NA in maintaining this suppression of AD growth, after it was well established. We showed that suppression of AD development during the first 15 massed stimulations (interstimulus interval of 5 min) was reduced by subsequent injection of the NA alpha 2 antagonist, yohimbine, with most rats exhibiting occasional generalized convulsions. Conversely, rats exposed to the beta antagonist, propranolol, like controls, not only showed suppressed AD growth, but also elevated AD thresholds. Three weeks later, only a small positive transfer to daily kindling was observed in all groups. We conclude that alpha 2 NA receptors help maintain suppression of AD growth induced by massed stimulation of the amygdala, while beta receptors provide only a small proepileptic influence. These results and those from the 'rapid' kindling model (Lothman et al., Brain Research, 360 (1985) 83-91) are compared, and related to NA receptor subtype variations in the amygdala and hippocampus.

Amphetamine sensitization and amygdala kindling: pharmacological evaluation of catecholaminergic and cholinergic mechanisms

Chronic pharmacological experiments were conducted to evaluate the relationship between sensitization induced by repeated administration of amphetamine (AMPH) and electrical stimulation of the amygdala. While AMPH withdrawal did not influence the kindling process, AMPH administered during the kindling procedure increased the rate at which seizures evolved, and under these conditions withdrawal from chronic AMPH further facilitated the propensity to kindle. Haloperidol (HAL) treatment failed to block the stimulant-induced increase in kindling acquisition indicating that changes in dopamine (DA) are not necessary for the AMPH/kindling synergism to develop. Scopolamine dose-dependently retarded kindling evolution irrespective of prior AMPH pretreatment also ruling out a cholinergic mechanism in the kindling sensitization. Subsequent experiments assessed the interactive effects of AMPH and desipramine (DMI) on the kindling process. Animals chronically exposed to AMPH and switched to DMI treatment during the kindling procedure kindled faster than control subjects. In addition, withdrawal from DMI preexposure advanced the AMPH-induced increase in kindling rate. These results were discussed in terms of the role of norepinephrine-mediated inhibition of the kindling process, and were related to drug-elicited alterations in beta-adrenergic receptor functioning. Taken together, these findings implicate the amygdala as an important structure in the development of non-DA forms of AMPH sensitization.

Evidence for long-term reduction of noradrenaline release after kindling in the rat hippocampus

The in vivo microdialysis technique was used to monitor steady-state noradrenaline release in the rat hippocampus after hippocampal kindling. At 8 weeks after the last seizure, the noradrenaline release was reduced by 62% in the stimulated hippocampus in kindled animals as compared to non-kindled rats. The reduction was not due to the repeated handling of the animals as assessed in a separate experiment. The results suggest a decrease of inhibitory noradrenergic influence at the primary kindling site, which could play a role in kindling epileptogenesis.

Neonatal clonidine treatment results in long-lasting changes in noradrenaline sensitivity and kindling epileptogenesis

In the present experiment we tested the hypothesis that early interference with noradrenaline transmission can have permanent consequences for brain function in adulthood. Neonatal depletion of noradrenaline by daily subcutaneous injections of clonidine results in supersensitivity to noradrenaline in adult hippocampal CA1 cells as shown in our previous microiontophoretic study. These findings were confirmed and extended here with dose-response curves. Furthermore, we tested whether this form of neonatal interference with noradrenaline also permanently affects long-lasting plasticity as revealed in kindling epileptogenesis in adulthood. The initiation of the epileptic activity after the kindling stimulation was significantly delayed in the clonidine-treated group, and all measured parameters of seizure expression tended to be retarded in comparison with saline-treated control rats. This indicates that noradrenaline supersensitivity induced by neonatal clonidine treatment retards kindling development in adulthood.

Transient elevation of amygdala alpha 2 adrenergic receptor binding sites during the early stages of amygdala kindling

Enhanced noradrenergic neurotransmission retards but does not prevent the development of kindling. We previously reported that locus coeruleus (LC) alpha 2 adrenergic receptor binding sites are transiently elevated during the early stages of kindling development. Since the firing activity of LC noradrenergic neurons is partially regulated via an alpha 2 receptor-mediated recurrent inhibition, the transient elevation in LC alpha 2 receptors could decrease LC activity and consequently facilitate the development of kindling. Transient elevation of alpha 2 receptor binding sites during early stages of kindling may also occur on noradrenergic axon terminals projecting to forebrain sites. Using in vitro neurotransmitter autoradiography techniques, we investigated this hypothesis by measuring specific [3H]idazoxan binding in 5 different areas of rat forebrain at 2 different stages of kindling development. After 2 class 1 kindled seizures, specific [3H]idazoxan binding was elevated significantly in the amygdala, but not in other forebrain regions. No differences in specific [3H]idazoxan binding were observed in any of the 5 brain regions in rats kindled to a single class 5 kindled motor seizure. Saturation of binding experiments indicated that the increase in amygdala [3H]idazoxan binding, following 2 class 1 kindled motor seizures, was due to an increase in the total number of alpha 2 receptor binding sites without a change in the affinity of the binding sites for [3H]idazoxan. Thus, the transient increase in alpha 2 receptors that occurs in the LC in the early stages of kindling also occurs in the forebrain region in which the kindled seizure originates.

Behavioral characterization of intracranial self-stimulation from mesolimbic, mesocortical, nigrostriatal, hypothalamic and extra-hypothalamic sites in the non-inbred CD-1 mouse strain

A behavioral analysis of intracranial self-stimulation (ICSS) was provided for mesolimbic/mesocortical, nigrostriatal, hypothalamic and extrahypothalamic sites in the CD-1 mouse. Robust responding and rapid acquisition of mesocortical ICSS appeared dorsally along notably fluorescent sites in rostral and caudal planes. ICSS was diminished demonstrably in medial and ventral positions in posterior planes. Mesolimbic ICSS from the medial and ventral nucleus accumbens (Nas), was accompanied by significant elevations in locomotor activity, corresponding to regions of dopamine (DA) and cholecystokinin co-localization. Stimulation-induced seizures appeared from both the Nas as well as the mesocortex. ICSS from the ventral tegmental field (VTA) was evident along its medial, lateral and dorsal borders with longer pulse durations more likely to elicit responding. Seizure activity was absent from the VTA. Striatal ICSS was conspicuously poor in dorsal and medial locations; regions presumably devoid of tegmental innervation. ICSS emerged from both the ventrocaudal and anteromedial striatum; regions linked to innervation by the dorsolateral and ventromedial VTA. The red nucleus, a previously neglected self-stimulation site supported marked responding for ICSS. Regions supporting rubral ICSS were correlated with thalamic innervation sites; notably the ventrolateral thalamic nucleus and the parafascicular nucleus, regions found to support ICSS. The substantia nigra supported high rates of responding for ICSS when electrode placement was restricted to the dorsomedial portion of the pars compacta. Electrode deviations lateral and dorsal to the substantia nigra pars medialis induced a progressive decline in responding. Hypothalamic sites were found to support significant responding for ICSS, although such performance was frequently associated with seizure induction. Taken together these data (1) provide the first behavioral analysis of ICSS in mice responding from previously unexamined DA sites in the mesolimbic (e.g. VTA, Nas) and nigrostriatal systems (e.g. caudate, red nucleus) (2) suggest an anatomical reconsideration of the assumptions underlying the elicitation of ICSS from the frontal cortex (3) suggest that the neural circuitry underlying thalamic, caudate, rubral and frontal cortical ICSS are interrelated and (4) suggest that the Nas and the frontal cortex, like the hypothalamus, in the mouse appear to be particularly sensitive to stimulation-induced seizures.

Transient elevation of locus coeruleus alpha 2-adrenergic receptor binding during the early stages of amygdala kindling

Enhancement of noradrenergic neurotransmission retards, but does not prevent, the development of kindling. The firing activity of noradrenergic locus coeruleus (LC) neurons is partially regulated by axon collateral recurrent inhibition mediated via alpha 2-adrenergic receptors. We tested the hypothesis that LC autoinhibitory alpha 2-adrenergic receptors may change during the kindling process thereby altering LC excitability. Specific binding of the alpha 2-adrenergic receptor antagonist [3H]RX781094 (idazoxan) was measured in the LC of rats at 3 different stages of kindling development using in vitro neurotransmitter receptor autoradiography techniques. Specific [3H]RX781094 binding was elevated significantly in rats kindled to two Class 1 kindled motor seizures. No differences in binding were observed in animals kindled to Class 3 or Class 5 kindled motor seizures. Saturation of binding experiments indicated that the increase in binding following two Class 1 kindled motor seizures was due to an increase in the total number of alpha 2-receptors without a change in the affinity of the binding site for [3H]RX781094. The transient increase in number of LC alpha 2-adrenergic receptors is consistent with the idea that noradrenergic neurotransmission inhibits the early progress of kindling development, but then subsequently becomes ineffective in maintaining the inhibition during later stages of kindling development.

The effect of amygdala kindled seizures on locus coeruleus activity

Locus coeruleus (LC) neuronal activity was recorded in anesthetized and in awake behaving rats during the production of amygdala afterdischarges (AD's) using kindling protocol. Both LC multiple and single unit discharges were temporally correlated with the appearance of AD's in the amygdala. Seizures were manifested in single unit activity as significant increases in firing rate. In awake animals, a bursting pattern of discharges was observed. We postulate that the observed changes in the LC activity pattern may have a modulatory role in the development of kindled seizures.

Kindling-based status epilepticus: effect of norepinephrine depletion with 6-hydroxydopamine

In two experiments, involvement of norepinephrine in the development of status epilepticus was determined. Rats, pretreated with intraventricular 6-hydroxydopamine to deplete brain norepinephrine or with the saline vehicle alone, were implanted with electrodes in both amygdalae. In the first experiment, one amygdala was kindled to stage 5 levels and then 2 weeks later was stimulated continuously for 60 min in an effort to produce status epilepticus (SE), while in the second experiment such SE stimulation was applied to one amygdala without prior kindling. Although depletion of norepinephrine significantly facilitated amygdala kindling in experiment 1, it had no clear effect on the probability of developing SE (generalized, partial, or nonconvulsive) or on the distribution of gross brain pathology following spontaneous recovery from partial and nonconvulsive SE in either experiment. The significance of these results compared to other SE models was discussed.

Effects of chronic treatment of methamphetamine and imipramine on amygdaloid seizure's generation

We assessed the effects of chronic treatment of methamphetamine (1-2 mg/kg/day, i.p., 17 days) and imipramine (2-8 mg/kg/day, p.o., 17 days) on amygdala-generating seizures using the kindling method induced by low-frequency electrical stimulations. The number of stimulating pulses required for the triggering of epileptic afterdischarge (pulse-number threshold: PNT) is the indicator of seizure generating threshold. A PNT elevation followed by its reduction occurred, compared to the pretreatment level, during a 2 mg/kg/day chronic methamphetamine treatment. A reduction in the PNT and triggered afterdischarge durations occurred during a chronic imipramine treatment. These results indicate that both methamphetamine and imipramine reduced the seizure generating threshold by repeated applications. It is suggested that this finding might be related to the psychoactive potency and associated neurochemical changes which are known to be caused by these drugs.

Formamidine pesticides enhance susceptibility to kindled seizures in amygdala and hippocampus of the rat

Electrical kindling of the amygdala and hippocampus was used to evaluate the effects of two formamidines, chlordimeform (CDF) and amitraz (AMZ), upon seizures susceptibility in the rat. Male Long-Evans rats were implanted with electrodes in the amygdala or dorsal dentate gyrus, and injected IP daily with 40 mg/kg CDF, 50 mg/kg AMZ, or equal volumes of their respective vehicles. Afterdischarge (AD) thresholds were determined after the first injection. Animals were then stimulated twice daily, 2 and 4 hours postinjeciton, at a standard 200 microA stimulus intensity until three stage 5 generalized seizures ensued. Both CDF and AMZ significantly facilitated amygdaloid kindling rate, and CDF also facilitated hippocampal kindling rate. The effects of AMZ on hippocampal kindling were not assessed. AD durations were prolonged in the formamidine-treated groups, but there was no effect on AD thresholds. The alpha-2 adrenergic agonist and/or local anesthetic-like properties of these compounds may be responsible for these seizure enhancing effects.

Some aspects of stress and depression

1. The role of stress in depressive illness is discussed together with utility of the "learned helplessness" model and some neuropharmacological correlates of uncontrollable shock. 2. Similarities and differences between chronic antidepressant treatment and chronic stress treatment regimes are reviewed. 3. Finally the role of adaptive process in stress on antidepressant treatments is discussed.

Etiologic and preventive aspects of epilepsy in the child--bridging the gap between laboratory and clinic

Four broad categories of basic phenomena are pertinent to developing ways to prevent epilepsy. These include mechanisms of epileptogenesis, ictal initiation and temporary entrainment by the seizure discharge of normally functioning brain, seizure propagation, and control mechanisms that function both to restrain the cascade of epileptic events culminating in a seizure and to arrest the epileptic event and restore the interictal state. In newborns and children, hypoxia-ischemia is a major factor leading to epileptogenesis, and several schemes are proposed to classify, quantify, and prevent hypoxic-ischemic encephalopathy. Control mechanisms must be better understood in order to develop prophylactic recommendations for epilepsy, and an experimental model of "kindling antagonism" may increase our understanding of these. Programs of prevention of seizures in children will evolve only if basic researchers and clinicians work productively together to develop an adequate understanding of factors important in epileptogenesis and antiepileptogenic control mechanisms.

Activity of locus coeruleus neurons in amygdala kindled rats: role in the suppression of afterdischarge

Kindling is a model of epilepsy. The mechanisms of kindling development are unknown but may involve attenuation of noradrenergic neurotransmission. Single unit recordings, pharmacologic and lesion techniques were used to test the hypothesis that the increased seizure duration of kindled rats is the consequence of an inactivation of noradrenergic neurons in the locus coeruleus (LC). No difference was found between unkindled (naive) and kindled rats in the firing rates of recorded LC neurons either between or during seizures in the paralyzed, ventilated condition. Moreover, in naive rats, frank destruction of the LC did not lengthen seizure duration. We conclude that the lengthened seizure duration of kindled rats, in the paralyzed, ventilated conditions, is not the consequence of inactivation of the LC since the firing rate of recorded LC neurons in kindled rats was not decreased and since destruction of the LC in naive rats did not lengthen seizure duration. If attenuation of noradrenergic neurotransmission does contribute to the kindling phenomenon, then the LC is not likely to be the site at which this attenuation occurs.

Suppression of amygdala kindling with short interstimulus intervals: effect of norepinephrine depletion

The rate of development of generalized kindled convulsions was profoundly influenced by the interval between amygdala stimulations. With stimulation every 10 min, nearly complete interference with the progression of kindling was observed in most rats, and hourly stimulation precipitated kindling rates three times longer than did once per day. Depletion of norepinephrine (NE), as a result of intracerebroventricular pretreatment with 6-hydroxydopamine, virtually eliminated the interference with kindling development seen in the vehicle control rats. Such depletion of NE, however, had little influence on the generalized responses once developed. At this stage, interference with seizure provocation was observed as truncated electroencephalographic seizures which were usually devoid of motor correlates. This interference was more profound in the shorter interstimulus intervals and was independent of NE depletion. Finally, when changing from the short kindling intervals of 10 min and 1 h to the longer interval of 24 h, an unexpected interference with seizure provocation was observed. The implication of these results for the biochemical basis of kindling and kindling as a model of learning are discussed.

Alterations in beta-adrenergic receptor binding in partially and fully amygdala-kindled juvenile and adult rats

Twenty-eight-day-old (juvenile) rats were kindled with hourly stimulations to partial or fully kindled status. Adult rats were stimulated with hourly or daily stimulations. Alterations in [3H]dihydroalprenolol binding were determined 3 weeks after the last stimulation. We found that partially kindled, hourly stimulated juvenile rats showed a significant increase in the dissociation constant (Kd), with no change in maximal binding values. Fully kindled juvenile rats showed no change in Kd or Bmax. Partially kindled, hourly stimulated adult rats showed a significant decrease in Kd, with no change in Bmax. There was no change in Kd or Bmax values in fully kindled, hourly stimulated adult rats. Fully kindled, daily stimulated adult rats showed a decrease in maximal binding, with no change in Kd values. These findings indicate that kindling-induced beta-adrenergic receptor alterations were influenced by the age of the animal and the kindling parameters used, as well as the extent to which the animals were kindled.

The effects of chronic desmethylimipramine on entorhinal cortical kindling in rats

Chronic pretreatment of rats with desmethylimipramine (DMI) significantly slowed the rate of seizure generalization elicited by repeated electrical stimulation of the entorhinal cortex (kindling). An identical drug regimen administered to either fully kindled rats or rats partially kindled to early motor seizure stages failed to significantly alter kindling profiles in these animals. Under these latter conditions, in fact, there was a tendency for chronic DMI to exacerbate seizure activity. The effect of chronic DMI pretreatment to slow the development of kindled seizure generalization did not occur if a two-week delay was interposed between the end of drug treatment and the beginning of kindling trials. Results suggest that the retardation of entorhinal cortical kindling rate is dependent on DMI-induced CNS adaptations which recover within two weeks following treatment, and this effect is dependent on the presence of DMI-induced adaptations in a naive (unkindled) nervous system. Alterations of either the kindled state or the adaptational state produced by chronic drug eliminate the slowing of seizure generalization observed when both conditions are present.

Kindling antagonism: effects of norepinephrine depletion on kindled seizure suppression after concurrent, alternate stimulation in rats

The concurrent, alternate electrical stimulation of the entorhinal cortex and septal nucleus results in the development of fully generalized seizures at one electrode site and the suppression of seizure development at the other. We have labeled this phenomenon kindling antagonism. Selective, whole-brain depletion of norepinephrine (NE) virtually eliminates the development of kindling antagonism such that fully generalized seizures develop at both sites in a majority of animals. This effect occurs in the absence of appreciable changes in kindling characteristics of these animals compared with either untreated or vehicle-treated controls. These results suggest that the suppression of seizure development observed in the kindling antagonism model is normally maintained by a NE-dependent mechanism. Our results support those of earlier studies using single-site kindling paradigms in which NE depletion facilitates the rate of kindled seizure development. We suggest that the NE-dependent mechanism responsible for the seizure suppression observed to follow concurrent, alternate stimulation and the suppression of seizure development using single-site kindling paradigms may be the same. The nature of this NE-dependent seizure suppression mechanism and the anatomic locus or loci critical for this effect remain questions for future research.

Endogenous anticonvulsant substances

Epileptic seizures will normally arrest abruptly and spontaneously, and the brain will remain refractory to further seizures for some time thereafter. This paper reviews the possible mechanisms underlying this seizure arrest and refractoriness. The data suggests that neuronal fatigue is not involved in either of these processes, whereas the role of ions and excitatory systems are unclear. Rather, seizure arrest and refractoriness may come about by the seizure-induced release and/or activity of multiple endogenous anticonvulsant substances. The spontaneous arrest of the seizure may involve the purine adenosine, in addition to other unknown mechanisms. Seizure refractoriness involves multiple systems, the most important of which, on the available evidence, are prostaglandins and opioid peptides and possibly benzodiazepine systems, although other neuropeptides and the purines may also be involved. The implications of these conclusions to anti-epileptic drug development and status epilepticus are discussed.

Neuropharmacological modification of central catecholamines: effects on pinealectomy-induced convulsions

Removal of the pineal gland produces stereotyped tonic convulsions in parathyroidectomized rats. Inasmuch as central levels of norepinephrine (NE) are decreased in these animals, the purpose of this study was to investigate the effects of alterations in central catecholamine function on convulsions produced by pinealectomy in parathyroidectomized rats. The treatment of rats with alpha-methyl-p-tyrosine or FLA-63 produced large reductions in forebrain levels of both NE and dopamine or NE alone, respectively, which were not associated with facilitation of convulsions. However, the incidence of convulsions was increased by FLA-63 in rats pretreated with the catecholamine precursor L-dihydroxyphenylalanine. Reserpine, a monoamine depleter, had no effect on either the incidence or severity of convulsions. An acute injection of desipramine, an inhibitor of the reuptake of NE, however, significantly lowered the incidence of convulsions. Timolol, a beta-adrenergic receptor antagonist, reduced, in a dose-dependent manner, the average latency to onset of convulsions and increased the average number of convulsions each rat experienced. Clonidine, an alpha 2-adrenergic agonist, did not significantly alter convulsions. Thus presynaptic mechanisms such as synthesis and storage of both NE and DA appear to have little, if any, effect on pinealectomy-induced convulsions, whereas enhancing synaptic levels of NE by blocking its reuptake into adrenergic axons had an anticonvulsant effect. Further evidence suggesting a role for NE in modulating these convulsions is provided by the proconvulsant effect of blocking central beta-adrenergic receptors.

Amygdala kindling in juvenile rats following neonatal administration of 6-hydroxydopamine

The role of catecholamines in mediating the acquisition of amygdala-kindled seizures was investigated in juvenile rats administered intracisternal injections of 6-hydroxydopamine (6-OHDA) on postnatal days 1 and 2. Amygdala kindling was initiated on day 28, using stimulations delivered each hour through two consecutive stage V seizures. The 6-OHDA treatment resulted in a 53% increase in the overall rate of kindling in juvenile rats. This acceleration was confined primarily to the early phases of kindling in that the 6-OHDA-treated rats skipped the early kindling stages, and the later stages of kindling were unaffected. These findings support evidence from adult rats that catecholamines play a role in initially limiting the spread of seizure activity during kindled seizure acquisition; however, when the seizures have begun to generalize, the ability of catecholaminergic systems to inhibit seizure spread diminishes.

Chronic but not acute treatment with antidepressants enhances the electroconvulsive seizure response in rats

The effects of the chronic administration of antidepressants on threshold electroconvulsive (ECS) seizures were evaluated in rats. Initially, tonic-clonic seizures were induced in 90% of the animals. Of those animals responding with tonic-clonic seizures, 42% had hindlimb extension (extensors); the remainder showed only hindlimb flexion (flexors). No alteration in the pattern of seizures was observed 24 hr after a single oral dose of any of the antidepressant drugs. The rats were then treated with a total of 20 consecutive daily doses of antidepressants and threshold electroconvulsive seizure responses were evaluated 24 hr after the last dose. A significantly greater percentage of rats responded with extensor seizures after chronic treatment with amoxapine, chlorimipramine, parglyine and mianserin. There was no change in the pattern of seizures of the rats treated chronically with desipramine, but the duration of the clonic phase was reduced. After a 7 day period free of drugs a significantly greater percentage of animals had extensor seizures in the groups treated with amoxapine, chlorimipramine, pargyline and desipramine but not mianserin. In the light of evidence that chronic treatment with antidepressants reduces the activity of norepinephrine- or isoproterenol-sensitive adenylate-cyclase, and that the norepinephrine system is an important endogenous anticonvulsant factor in electroconvulsive seizures, these results suggest that the same mechanism may mediate both the therapeutic and proconvulsant effects of the chronic administration of antidepressants.

Monoaminergic and local anesthetic components of cocaine's effects on kindled seizure expression

Male Long-Evans rats were kindled via daily electrical stimulation of the left prepyriform cortex. The animals were then used in two experiments which examined the pharmacological basis of cocaine's effects on three mutually exclusive components of the kindled seizure, which were the following: (a) latency to clonus, (b) clonus duration, and (c) duration of AD outlasting clonus. The first experiment compared the effects produced by cocaine HCl (20 mg/kg, IP), lidocaine HCl (20 mg/kg, IP), and amphetamine sulfate (2.5 mg/kg, IP). The results indicated that both cocaine and lidocaine reduced the duration of kindled AD, latency to clonus, and duration of AD persisting beyond clonus, thus suggesting that these cocaine effects are mediated by local anesthetic mechanisms. Only cocaine reduced clonus duration, which suggests that this cocaine effect is not produced by a local anesthetic action. The second experiment examined the effects of cocaine following the administration of three dose levels of the monoamine antagonists haloperidol, prazosin, yohimbine, propranolol, or metergoline (selected for their ability to block dopamine, alpha-1-norepinephrine, alpha-2-norepinephrine, beta-norepinephrine, and serotonin receptors, respectively). The results of this experiment found no support for a monoaminergic contribution to cocaine's effect on clonus latency or AD after clonus. However, results for prazosin, which reduced clonus duration and exhibited an additive effect with cocaine on this variable, suggest that cocaine's norepinephrine action (especially on the alpha-norepinephrine systems) may modulate clonus duration.

Adenosine modulation of amygdala kindling

To test the hypothesis that kindling is restrained by the inhibitory neuromodulator, adenosine, the adenosine uptake blocker, papaverine, or the adenosine antagonist, aminophylline, were injected systemically into rats 20 min before each daily electrical stimulation of the amygdala. The effects on amygdala-triggered seizures of papaverine, adenosine, 2-chloroadenosine, and the adenosine antagonists, isobutylmethylxanthine and caffeine, were also investigated at seizure threshold. Papaverine inhibited kindling, whereas aminophylline accelerated kindling. The adenosine agonists had anticonvulsant effects on seizures, and the antagonists had proconvulsant effects which involved, primarily, the lengthening of afterdischarge duration. Aminophylline injected repeatedly, in the absence of electrical stimulation, induced seizures. These results support the hypothesis that adenosine can modulate kindling and affect the seizure process.

Attenuation of epileptogenesis: proactive effect of a single epinephrine injection of amygdaloid kindling

Repeated daily electrical stimulation of the amygdala can lead to a progressive increase in brain and behavioral seizures. This phenomenon, termed kindling, has been viewed as a model for epileptogenesis. The results reported here demonstrate that a single systemic epinephrine injection can significantly retard such epileptogenesis for a period of at least several days. These findings suggest that peripheral catecholamines, responding either to stress near the time of seizure initiation or to treatments administered at that time, may be important in regulating the development of epileptic states. In addition, the results indicate that an acute episode of high plasma epinephrine levels may result in a durable modification of brain function.

Long-term reduction in beta-adrenergic receptor binding after amygdala kindling in rats

The long-term effect of amygdala kindling on beta-adrenergic receptor binding was examined in three groups of Wistar rats. The animals in one of two kindled groups received six stage-5 generalized convulsions, one each day, and then were not stimulated for 23 days before being killed (23-day group). Animals in the other kindled group received only five generalized convulsions prior to 22 days of no stimulation but then received their sixth convulsion 1 day before being killed (1-day group). All animals including the operated controls experienced similar handling. A significant reduction in [3H]dihydroalprenolol binding in the anterior cortex and kindled amygdala was observed in the 23-day animals compared with control animals, whereas the 1-day animals exhibited intermediate values which were not different from either of the other two groups. Thus, amygdala kindling results in a long-lasting reduction in beta-adrenergic binding which, paradoxically, can be partially ameliorated by a single convulsion 1 day before killing.