Evaluation of monoaminergic receptors in the genetically epilepsy prone rat

The role of neurotransmitters in glioblastoma multiforme-associated seizures

GBM, or glioblastoma multiforme, is a brain tumor that poses a great threat to both children and adults, being the primary cause of death related to brain tumors. GBM is often associated with epilepsy, which can be debilitating. Seizures and the development of epilepsy are the primary symptoms that have a severe impact on the quality of life for GBM patients. It is increasingly apparent that the nervous system plays an essential role in the tumor microenvironment for all cancer types, including GBM. In recent years, there has been a growing understanding of how neurotransmitters control the progression of gliomas. Evidence suggests that neurotransmitters and neuromodulators found in the tumor microenvironment play crucial roles in the excitability, proliferation, quiescence, and differentiation of neurons, glial cells, and neural stem cells. The involvement of neurotransmitters appears to play a significant role in various stages of GBM. In this review, the focus is on presenting updated knowledge and emerging ideas regarding the interplay between neurotransmitters and neuromodulators, such as glutamate, GABA, norepinephrine, dopamine, serotonin, adenosine, and their relationship with GBM and the seizures induced by this condition. The review aims to explore the current understanding and provide new insights into the complex interactions between these neurotransmitters and neuromodulators in the context of GBM-related seizures.

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.

Brain regional amino acid levels in seizure susceptible rats: Changes related to sound-induced seizures

Regional brain amino acid levels have been determined by HPLC, following microwave fixation, in seizure susceptible (University of Arizona, Audiogenic seizure-susceptible, AGS) and non-seizure susceptible Sprague-Dawley rats. Glutamine content is significantly lower in cerebellum, hippocampus, striatum, substantia nigra, colliculi and brain stem reticular formation in AGS rats. Aspartate levels are also reduced (by 33%) in striatum, substantia nigra and inferior colliculus, and glutamate is reduced in hippocampus and striatum. Other differences include a slight fall in taurine content (in striatum) and an increase in GABA content (in hippocampus). Measurements of amino acid levels in AGS rats during the course of a seizure induced by sound show increases in aspartate and glutamate content in some brain regions (including the inferior colliculus). Potassium-evoked [(3)H] d-aspartate release from hippocampal slices did not differ between the seizure-susceptible and seizure-resistant rat strains. It is proposed (i) that changes in the level and turnover of excitatory amino acid transmitters in AGS rats occur as a consequence of a primary biochemical defect that probably involves impaired neuronal membrane transport, and (ii) that altered function in excitatory synapses in the inferior colliculus, substantia nigra and reticular formation contributes importantly to the seizure susceptibility.

Rhythm and blues: animal models of epilepsy and depression comorbidity

Clinical evidence shows a strong, bidirectional comorbidity between depression and epilepsy that is associated with decreased quality of life and responsivity to pharmacotherapies. At present, the neurobiological underpinnings of this comorbidity remain hazy. To complicate matters, anticonvulsant drugs can cause mood disturbances, while antidepressant drugs can lower seizure threshold, making it difficult to treat patients suffering from both depression and epilepsy. Animal models have been created to untangle the mechanisms behind the relationship between these disorders and to serve as screening tools for new therapies targeted to treat both simultaneously. These animal models are based on chemical interventions (e.g. pentylenetetrazol, kainic acid, pilocarpine), electrical stimulations (e.g. kindling, electroshock), and genetic/selective breeding paradigms (e.g. genetically epilepsy-prone rats (GEPRs), genetic absence epilepsy rat from Strasbourg (GAERS), WAG/Rij rats, swim lo-active rats (SwLo)). Studies on these animal models point to some potential mechanisms that could explain epilepsy and depression comorbidity, such as various components of the dopaminergic, noradrenergic, serotonergic, and GABAergic systems, as well as key brain regions, like the amygdala and hippocampus. These models have also been used to screen possible therapies. The purpose of the present review is to highlight the importance of animal models in research on comorbid epilepsy and depression and to explore the contributions of these models to our understanding of the mechanisms and potential treatments for these disorders.

β-Adrenergic modulation of spontaneous spatiotemporal activity patterns and synchrony in hyperexcitable hippocampal circuits

A description of healthy and pathological brain dynamics requires an understanding of spatiotemporal patterns of neural activity and characteristics of its propagation between interconnected circuits. However, the structure and modulation of the neural activation maps underlying these patterns and their propagation remain elusive. We investigated effects of β-adrenergic receptor (β-AR) stimulation on the spatiotemporal characteristics of emergent activity in rat hippocampal circuits. Synchronized epileptiform-like activity, such as interictal bursts (IBs) and ictal-like events (ILEs), were evoked by 4-aminopyridine (4-AP), and their dynamics were studied using a combination of electrophysiology and fast voltage-sensitive dye imaging. Dynamic characterization of the spontaneous IBs showed that they originated in dentate gyrus/CA3 border and propagated toward CA1. To determine how β-AR modulates spatiotemporal characteristics of the emergent IBs, we used the β-AR agonist isoproterenol (ISO). ISO significantly reduced the spatiotemporal extent and propagation velocity of the IBs and significantly altered network activity in the 1- to 20-Hz range. Dual whole cell recordings of the IBs in CA3/CA1 pyramidal cells and optical analysis of those regions showed that ISO application reduced interpyramidal and interregional synchrony during the IBs. In addition, ISO significantly reduced duration not only of the shorter duration IBs but also the prolonged ILEs in 4-AP. To test whether the decrease in ILE duration was model dependent, we used a different hyperexcitability model, zero magnesium (0 Mg(2+)). Prolonged ILEs were readily formed in 0 Mg(2+), and addition of ISO significantly reduced their durations. Taken together, these novel results provide evidence that β-AR activation dynamically reshapes the spatiotemporal activity patterns in hyperexcitable circuits by altering network rhythmogenesis, propagation velocity, and intercellular/regional synchronization.

The role of the central noradrenergic system in behavioral inhibition

Although the central noradrenergic system has been shown to be involved in a number of behavioral and neurophysiological processes, the relation of these to its role in depressive illness has been difficult to define. The present review discusses the hypothesis that one of its chief functions that may be related to affective illness is the inhibition of behavioral activation, a prominent symptom of the disorder. This hypothesis is found to be consistent with most previous neuropsychopharmacological and immunohistochemical experiments on active behavior in rodents in a variety of experimental conditions using manipulation of neurotransmission at both locus coeruleus and forebrain adrenergic receptors. The findings support a mechanism in which high rates of noradrenergic neural activity suppress the neural activity of principal neurons in forebrain regions mediating active behavior. The suppression may be mediated through postsynaptic galaninergic and adrenergic receptors, and via the release of corticotrophin-releasing hormone. The hypothesis is consistent with clinical evidence for central noradrenergic system hyperactivity in depressives and with the view that this hyperactivity is a contributing etiological factor in the disorder. A similar mechanism may underlie the ability of the noradrenergic system to suppress seizure activity suggesting that inhibition of the spread of neural activation may be a unifying function.

Neurobehavioral consequences of stressor exposure in rodent models of epilepsy

Both normal, non-epileptic as well as seizure-prone rodents exhibit a spectrum of anxiogenic-like behaviors in response to stressor exposure. Comparative analysis reveals that the same set of emotionality dependent measures is sensitive to both stress reactivity in normal rodents as well as stress hyperreactivity typically seen in seizure-prone rodents. A variety of unconditioned, exploratory tasks reflect global sensitivity to stressor exposure in the form of behavioral inhibition of locomotor output. Moreover, well chosen stressors can trigger de novo seizures with or without a history of seizure incidence. Seizures may be elicited in response to stressful environmental stimuli such as noxious noises, tail suspension handling, or home cage disturbance. Stress reactivity studies in rodents with a genetic predisposition to seizures have yielded important clues regarding brain substrates that mediate seizure ontogeny and modulate ictogenesis. Brains of seizure susceptible rodents reflect elevated content of the stress-related neuropeptide, corticotropin-releasing factor (CRF) in several nuclei relative to non-susceptible controls and neutralization of brain CRF attenuates seizure sensitivity. Findings outlined in this review support a diathesis-stress hypothesis in which behavioral- and neuro-pathologies of genetically seizure susceptible rodents arise in part due to multifaceted hyperreactivity to noxious environmental stimuli.

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.

Affective disorder and epilepsy comorbidity: implications for development of treatments, preventions and diagnostic approaches

Concepts pertaining to affective disorder and epilepsy comorbidity are contributing appreciably to improvements in patient care. Several antiepileptic treatments have become important components of the management of bipolar affective disorder. In contrast, little progress has emerged in developing clinical applications of the anticonvulsant properties of the antidepressants in the treatment of the epilepsies. The slow onset of action of the antidepressants remains a major impediment to fully effective treatment of depressive episodes. Nevertheless, studies from experimental epileptology demonstrate that the anticonvulsant effects of the antidepressants occur rapidly and as a consequence of noradrenergic and/or serotonergic activation. These studies also demonstrate that adequate initial doses of the antidepressants are essential to rapid onset of anticonvulsant action. Pharmacokinetically valid loading dose paradigms are seemingly avoided with antidepressant drugs in humans because of potential toxicities and/or patient unacceptability. However, substantial progress has been made in reducing the adverse effect liability of the antidepressants. No longer is convulsive liability considered to stem from the therapeutic mechanisms of the anti-depressants. Rather, noradrenergic and serotonergic influences have demonstrable anticonvulsant properties. Other side effects may also be separable from the anticonvulsant and antidepressive effects of antidepressive treatments. The concept that the protracted process of antidepressant-induced beta-noradrenergic down-regulation is an essential prelude to the onset of mood benefit is no longer a sustainable premise. Nevertheless, increasing evidence underlies the possibility that knowledge of serotonergic and noradrenergic regulatory processes can be used to design strategies that will hasten the onset of antidepressive action. Similar optimism pervades efforts to determine the possibility that dual inhibition of serotonin and norepinephrine transporters will hasten onset of antidepressive action. Moreover, because noradrenergic and serotonergic systems are determinants of predisposition to seizures and to dysfunctional affective episodes, augmentation strategies may also be applicable to the use of antidepressant drugs in epilepsy and to the use of antiepileptic drugs such as carbamazepine in mood disorders. Recent studies have demonstrated that, in part, the therapeutic effectiveness of carbamazepine may stem from its marked capacity to elevate serotonin concentrations in the extracellular fluid of the brain via mechanisms that differ from those of the membrane reuptake inhibitors. Evidence suggests that the epilepsies and affective disorders may arise from a multiplicity of neurobiological abnormalities. A disorder in one individual may arise via different mechanisms than a phenomenologically similar disorder in another individual. Thus, diagnostic tools are needed to make mechanistic distinctions among individuals so that treatments can be appropriately developed and selected. In terms of epileptogenesis and affective disorder progression, neuroprotective paradigms for one individual may differ from those needed for another. Moreover, diagnostic technologies that are adequate to detect genetically and/or experientially determined vulnerability before the onset of a seizure or dysfunctional affective episode may be valuable steps toward achieving goals of prevention.

The role of catecholamines in seizure susceptibility: new results using genetically engineered mice

The catecholamines norepinephrine and dopamine are abundant in the CNS, and modulate neuronal excitability via G-protein-coupled receptor signaling. This review covers the history of research concerning the role of catecholamines in modulating seizure susceptibility in animal models of epilepsy. Traditionally, most work on this topic has been anatomical, pharmacological, or physiological in nature. However, the recent advances in transgenic and knockout mouse technology provide new tools to study catecholamines and their roles in seizure susceptibility. New results from genetically engineered mice with altered catecholamine signaling, as well as possibilities for future experiments, are discussed.

Effect of norepinephrine release on adrenoceptors in severe seizure genetically epilepsy-prone rats

The genetically epilepsy-prone rat (GEPR) seizure model is characterized by extensive abnormalities in brain noradrenergic function. Earlier studies had suggested that GEPRs might not regulate adrenoceptors in a normal fashion. The purpose of the present study was to determine if GEPR-9s are capable of up and down regulation of alpha(1)- and beta-adrenoceptors in response to increments or decrements in extracellular norepinephrine. Seizure induction has been shown to increase extracellular norepinephrine. Chronic sound or electroshock-induced seizures caused down regulation of beta-adrenoceptors in frontal cortex and in hippocampus from GEPR-9s. Similarly, chronic daily treatment with the norepinephrine reuptake inhibitor desmethylimipramine produced down regulation of beta-adrenoceptors in frontal cortex and in hippocampus from GEPR-9s. As is the case in neurologically normal animals, chronic electroshock-induced seizure did not cause down regulation of beta-adrenoceptors in 6-hydroxydopamine pretreated GEPR-9s. Chronic electroshock treatment also caused up-regulation of alpha(1)-adrenoceptors in frontal cortex but not in hippocampus. In 6-hydroxydopamine pretreated GEPR-9s, chronic electroshock treatment caused a further up-regulation of alpha(1)-adrenoceptors in frontal cortex but not in hippocampus. Taken together, these results indicate that GEPR-9s are capable of up and down regulation of alpha(1)- and beta-adrenoceptors in a manner that is qualitatively similar to the regulation of these receptors in normal animals. Whether the regulation of brain adrenoceptors is quantitatively different in GEPRs from normal animals remains to be established.

Enhancement of the anticonvulsant effect of fluoxetine following blockade of 5-HT1A receptors

Serotonin reuptake inhibitors, such as fluoxetine, have been shown to exert anticonvulsant effects in several animal models of epilepsy. In view of recent studies showing that 5-HT1A receptor antagonists (somatodendritic autoreceptor antagonists) enhance the increase in extracellular 5-hydroxytryptamine (5-HT, serotonin) produced by serotonin reuptake inhibitors, it was of interest to determine if these antagonists also enhance the anticonvulsant effect of fluoxetine in Genetically Epilepsy-Prone Rats (GEPRs). The 5-HT1A receptor antagonists (-)-pindolol and LY 206130 (1-[1-H-indol-4-yloxy]-3-[cyclohexylamino]-2-propanol maleate) were examined in the present study and both enhanced the anticonvulsant action of fluoxetine in severe seizure GEPRs (GEPR-9s). The latter effect of LY 206130 was found to be dose- and 5-HT-dependent. These findings provide further evidence that the increase in extracellular serotonin observed after administering fluoxetine in combination with a 5-HT1A receptor antagonist is physiologically important and that the anticonvulsant effect of fluoxetine in the GEPR is mediated through an increase in extracellular 5-HT.

Alterations in mRNA expression of systems that regulate neurotransmitter synaptic content in seizure-naive genetically epilepsy-prone rat (GEPR): transporter proteins and rate-limiting synthesizing enzymes for norepinephrine, dopamine and serotonin

Two models of genetically epilepsy-prone rat (GEPR) exist, the GEPR-3 and GEPR-9, GEPR-3 and GEPR-9 share a deficiency in presynaptic norepinephrine (NE) and serotonin (5HT) content in specific regions of the central nervous system (CNS). The presynaptic content of dopamine (DA) does not appear to be altered in either adult GEPR strain compared to Sprague-Dawley (SD) rats, the strain from which the GEPR was derived. Presynaptic content of monoamine neurotransmitters, such as NE, 5HT and DA, are maintained by several regulatory proteins which include: synthesis, re-uptake, release, degradation and vesicular transport. To further characterize the monoamine deficiency observed in the GEPR, the mRNA level of the rate limiting enzymes for the synthesis of NE, 5HT and DA and each of the neurotransporter proteins were measured in seizure-naive GEPR-3, GEPR-9 and SD rats. In the locus coeruleus (LC), the major noradrenergic locus, tyrosine hydroxylase (TH) mRNA level was significantly reduced only in GEPR-9 animals compared to SD rats and GEPR-3, while NE transporter (NET) mRNA was significantly elevated in GEPR-3 compared to SD rats and GEPR-9. TH and DA transporter (DAT) mRNA was measured in the dopaminergic neurons of the substantia nigra pars compacta (SNpc), ventral tegmental area (VTA) and zona incerta (ZI), DAT mRNA level was significantly reduced in all dopaminergic neurons in the GEPR-3 compared to SD rats and GEPR-9, while TH mRNA level was significantly elevated in the SNpc/VTA equally in GEPR-3 and GEPR-9 compared to SD rats. In the ZI, TH mRNA level was significantly reduced in GEPR-3 compared to SD rats and GEPR-9. In the dorsal raphe (DR), a major serotonergic locus, tryptophan hydroxylase (TRH) mRNA level was not significantly different from SD in either strain of GEPR; however, 5HT transporter (SERT) mRNA level was significantly reduced in GEPR-9 in the dorsal and lateral regions of the DR compared in SD rats and GEPR-3. These data indicate that two of the regulatory systems that maintain NE, 5HT and DA content are altered in a differential manner in seizure-naive GEPR-3 compared to seizure-naive GEPR-9, with GEPR-3 showing more alterations in dopaminergic neurons. It is uncertain at the present time how these alterations in mRNA level relate to the enhanced seizure susceptibility of these animals. It was apparent that a straightforward correlation between neurotransmitter loss to transcriptional changes in synthesizing enzymes mRNA or to re-uptake protein mRNA was not observed in noradrenergic and serotonergic neurons. Therefore, the decrease in presynaptic NE and 5HT tissue content in these animals may be due to posttranscriptional modification. In contrast, presynaptic DA tissue content which was unaltered in both strains of GEPR, shows an alteration in TH and DAT mRNA level compared to SD rats in all dopaminergic neurons examined. This indicates a possible involvement of DA in regulating the seizure susceptibility of these animals.

Neurochemical correlates of antiepileptic drugs in the genetically epilepsy-prone rat (GEPR)

The GEPR model is composed of two independently derived strains of rats each characterized by a broad-based seizure predisposition. Moderate seizure GEPRs (GEPR-3s) exhibit generalized clonus with loss of righting reflex in response to a standardized sound stimulus. The same stimulus in severe seizure GEPRs (GEPR-9s) produces a tonic-clonic convulsion much like that produced by supramaximal electroshock. The numeric descriptors (3 and 9) derive from the ordinal rating scale developed by Jobe and coworkers for evaluation of convulsion intensity. GEPRs experience an anticonvulsant effect in response to all established and many experimental antiepileptic drugs and distinctions between the classes of drugs can be made. Since serotonin plays an anticonvulsant role in nearly all animal seizure models, we examined the effects of antiepileptic drugs on serotonin using microdialysis. Among clinically effective anticonvulsants, carbamazepine, antiepilepsirine (used in China) and loreclezole produced dose-related anticonvulsant effects and increases in extracellular serotonin in GEPRs. Similarly, drugs known to block serotonin reuptake and increase extracellular serotonin (fluoxetine and sertraline) produce dose related anticonvulsant effects in GEPRs and other animal models. Accentuation of serotonin release by treating GEPRs with fluoxetine and 5-hydroxytryptophan enhances the anticonvulsant effect produced by fluoxetine. Depletion of serotonin greatly decreased the anticonvulsant effect produced by carbamazepine, antiepilepsirine and fluoxetine. Phenytoin produced a dose related anticonvulsant effect in GEPRs but did not increase extracellular serotonin. Depletion of serotonin did not diminish the anticonvulsant effect produced by phenytoin. Thus, serotonin appears to play a role in the anticonvulsant effect of several but not all anticonvulsant drugs.

Anticonvulsant properties of D-20443 in genetically epilepsy-prone rats: prediction of clinical response

D-20443 is an experimental antiepileptic drug. Its mechanism of antiepileptic action is unknown. We evaluated the anticonvulsant effectiveness of D-20443 against sound-induced seizures in genetically epilepsy-prone rats (GEPRs). This compound produced anticonvulsant effects against sound-induced seizures in moderate seizure GEPRs (GEPR-3s) at significantly lower doses than in severe seizure GEPRs (GEPR-9s). Based on these data and on the responses of GEPRs to other antiepileptic drugs, we predict that D-20443 will be a broad spectrum antiepileptic agent in humans. That is, we predict that D-20443 will suppress both tonic/clonic and absence seizures in humans.

The anticonvulsant effect of the broad spectrum anticonvulsant loreclezole may be mediated in part by serotonin in rats: a microdialysis study

Loreclezole is an experimental anticonvulsant drug. We found previously that several established anticonvulsants increase extracellular serotonin as measured by microdialysis. We have concluded that the increase in extracellular serotonin and the anticonvulsant effect produced by these anticonvulsant drugs are related in a cause and effect manner. To determine if anticonvulsant doses of loreclezole increase extracellular serotonin, we determined anticonvulsant dose-response relationships in genetically epilepsy-prone rats (GEPRs). Then, we administered ED99 doses of loreclezole to GEPRs and determined the effect on extracellular serotonin as measured by microdialysis in the striatum. We conclude that loreclezole produces a dose-related anticonvulsant effect in GEPRs and that anticonvulsant doses of loreclezole increase extracellular serotonin in these animals.

Noradrenergic terminal fields as determinants of seizure predisposition in GEPR-3s: a neuroanatomic assessment with intracerebral microinjections of 6-hydroxydopamine

The genetically epilepsy-prone rat (GEPR) and other mammals with genetically based epilepsy are characterized by an innate predisposition to seizures evoked by a wide variety of stimuli (including those of endogenous origin). The present investigation was undertaken to identify the anatomical location of the noradrenergic terminal fields responsible for regulation of seizure predisposition. In this study, audiogenic seizure severity was used as the index of seizure predisposition. The effect of widespread destruction of noradrenergic terminal fields was compared with the effect of destroying regionally distinct terminal fields. These lesions were produced by microinfusion of 6-hydroxydopamine (6-OHDA) into the locus ceruleus, the A1 noradrenergic area, the noradrenergic dorsal bundle, the cerebellar peduncles and spinal intrathecal space. Selective depletion of norepinephrine in the forebrain, the cerebellum, or the spinal cord failed to alter audiogenic seizure severity. An increase in seizure severity was always associated with marked depletion of norepinephrine in the midbrain excluding the inferior colliculus. Also a significant correlation existed between the seizure intensification and reduction of norepinephrine in this structure in all instances where a seizure intensification was observed. An association of seizure intensification also existed in all cases except one with depletion in the pons/medulla. The present findings support the hypothesis that the noradrenergic terminal fields of the midbrain excluding the inferior colliculus are determinants of seizure predisposition. Inasmuch as audiogenic seizures are a type of brainstem seizure, the present findings do not a priori pertain to the noradrenergic regulation of forebrain seizures.

Noradrenergic abnormalities in the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) has central nervous system noradrenergic deficits as compared to normal rats. It is possible that these deficits contribute to seizure predisposition because they are exhibited by seizure-naive as well as by seizure-experienced GEPRs. On the basis of pharmacological studies, it is hypothesized that there is an inverse relation between seizure predisposition and levels of noradrenergic activity in brain. Neurochemical studies indicate that deficits exist in areas innervated by both the locus ceruleus and the lateral tegmental noradrenergic systems. These deficits exist in GEPRs without seizure experience and are more pronounced in the severe seizure strain as compared to the moderate seizure strain. We review eight experimental steps undertaken to identify more precisely the anatomical location of noradrenergic determinants of seizure predisposition. These steps illustrate the theoretical bases for the studies and describe the specific experiments completed. Evidence supports the hypothesis that noradrenergic deficits in the superior colliculus and/or ventrally adjacent regions are determinants of seizure predisposition.

Anticonvulsant effects of intracerebroventricularly administered norepinephrine are potentiated in the presence of monoamine oxidase inhibition in severe seizure genetically epilepsy-prone rats (GEPR-9s)

Pharmacological and neurochemical evidence indicates that brain noradrenergic systems play an important role in the determination of audiogenic seizure severity in genetically epilepsy-prone rats (GEPRs). In earlier studies, intracerebroventricular (ICV) injections of norepinephrine suppressed convulsions in a now extinct moderate seizure GEPR colony. Also, ICV noradrenergic agonists are known to produce dose-related anticonvulsant effects in the extant moderate seizure GEPRs (GEPR-3s). The present experiments were undertaken to determine whether ICV norepinephrine also suppresses audiogenic seizures in the extant GEPR-3s and in the severe seizure genetically epilepsy-prone rats (GEPR-9s). Injections of norepinephrine or vehicle were made into the lateral ventricle through implanted guides. GEPR-9s were pretreated systemically either with the monoamine oxidase inhibitor pargyline or with saline. GEPR-3s received no pretreatment. In pargyline pretreated GEPR-9s, seizure severity fell and the fraction of animals exhibiting an anticonvulsant response increased progressively as the dose of norepinephrine was increased. In saline pretreated GEPR-9s, the anticonvulsant dose response curve for norepinephrine was shifted to a higher dose range. Accordingly, the anticonvulsant dose50 for norepinephrine was significantly greater in saline pretreated GEPR-9s than in pargyline pretreated animals. Moreover, the dose required to produce the anticonvulsant effect in GEPR-9s was approximately 10 fold greater than in the earlier studies in the extinct moderate seizure GEPRs. Also, the current experiment with extent GEPR-3s, showed that ICV norepinephrine was anticonvulsant in the same dose that was effective in the extinct colony of moderate seizure GEPRs. In general terms, these observations provide additional evidence that noradrenergic influences are anticonvulsant in the GEPR. The neurobiological factors responsible for reduced responsiveness of the GEPR-9 are presently unknown.

Norepinephrine-stimulated phosphatidylinositol metabolism in genetically epilepsy-prone and kindled rats

Genetically epilepsy-prone rats (GEPR-9) and kindled rats have reduced noradrenergic function. In the present study, norepinephrine-stimulated accumulation of inositol phosphates was reduced in cerebral cortex of GEPR-9 and kindled rats when compared to control and non-kindled rats, respectively. No such reduction was found in amygdala/pyriform cortex and hippocampus. These results support the hypothesis that cortical noradrenergic and associated second messenger systems are impaired in epilepsy.

Effect of norepinephrine depletion on audiogenic-like seizures elicited by microinfusion of an excitant amino acid into the inferior colliculus of normal rats

Infusions of an excitant amino acid, N-methyl-D-aspartate (NMDA) into the inferior colliculus (IC) render normal rats susceptible to audiogenic seizures (AGS) and/or spontaneous audiogenic-like seizures without tonic components. The excess excitant amino acid in the IC and the anticonvulsant effects of NMDA antagonists in genetically epilepsy-prone rats (GEPRs), along with innate norepinephrine (NE) deficits and anticonvulsant effects of NE agonists in these animals suggest a mutual role of excitant amino acids and NE in regulating AGS in GEPRs. Saline or 6-hydroxydopamine (6-OHDA, 4 micrograms/side in 2 microliters) was infused bilaterally into the locus coeruleus (LC) of normal male rats and guide cannulas were implanted into the IC. Two weeks later, NMDA was infused bilaterally into the IC (0.5 microliters; 10 nmol/side) and 10 min later the rats were subjected to an electric bell (110 db, 60 s) unless preceded by spontaneous tonic seizures. Tonic seizures were not observed in male rats following NMDA infusions in rats with LC infusions of saline. However, a marked increase in the incidence of tonic seizures was observed in the 6-OHDA-treated rats which were markedly depleted of brain NE as determined by HPLC. These findings indicate that a NE deficit greatly enhances the incidence of tonic convulsions and support the hypothesis that an excitant amino acid excess in the GEPR IC may act to initiate AGS, whereas the NE deficit may allow expression of the tonic components of AGS seen in some GEPRs.

Involvement of the noradrenergic system in the seizures of epileptic El mice

We studied the role of the noradrenergic system in the seizures of epileptic El mice. To this end, the anticonvulsant activity of adrenergic drugs was tested with a scoring method, and the binding of [3H]dihydroalprenolol, [3H]prazosin and [3H]yohimbine was evaluated in whole brains and various brain regions from stimulated and unstimulated El mice, and their maternal ddy mice. The seizures of El mice were inhibited by noradrenaline, phenylephrine, oxymetazoline, clonidine and yohimbine in a dose-dependent manner. These preventive effects of alpha-adrenoceptor agonists were antagonized by pretreatment with alpha-adrenoceptor antagonists. The preventive effect of yohimbine was reversed by pretreatment with clonidine or alpha-methyl-p-tyrosine, although the latter drug did not affect the anticonvulsant effect of clonidine. The binding of [3H]dihydroalprenolol was the same in the three groups of mice. More [3H]prazosin was bound in the cerebellum and striatum, and there were more [3H]yohimbine binding sites in the whole brain, cerebral cortex, hippocampus and brainstem of stimulated and unstimulated El mice than in the same areas of ddy mice. These findings suggest that up-regulated alpha 1- and alpha 2-adrenoceptors are involved in the inhibition of the seizures of El mice.

Prevention of ischemic neuronal damage by alpha 1-adrenoceptor agonist (methoxamine)

The effect of methoxamine, an alpha 1-adrenoceptor agonist, on ischemic neuronal damage was studied in the gerbil. The animals were subjected to 5 min of ischemia by bilateral common carotid arteries occlusion. Morphological changes and calcium accumulation were evaluated in the CA1 sector of the hippocampus after 7 days of survival. The degree of ischemic neuronal damage and calcium accumulation in the methoxamine-treated groups were significantly attenuated compared with the saline-treated ischemic group. The results suggest that alpha 1-adrenoceptor stimulation prevents ischemic neuronal damage.

Injections of noradrenergic and GABAergic agonists into the inferior colliculus: effects on audiogenic seizures in genetically epilepsy-prone rats

Genetically epilepsy-prone rats (GEPRs) which display tonic seizures (GEPR-9s) in response to acoustic stimulation were used in these studies. Other laboratories have shown that GEPR-9s have a reduced concentration of brain norepinephrine (NE). Previous reports have also indicated that audiogenic seizures (AGS) in these animals are inhibited by treatments that enhance noradrenergic (NA) neurotransmission. AGS in GEPRs are believed to be initiated in the inferior colliculus (IC) where GABA has been shown to exert inhibitory influences in GEPRs that display submaximal AGS. The present study examined whether the IC is a crucial site for NA suppression of tonic seizures by examining the effect of microinfusing NA agonists into the IC. The intracollicular effect of a GABA agonist, muscimol, on sound-induced tonic convulsions in GEPR-9s was also examined. Bilateral microinfusion of NE, phenylephrine, clonidine or isoproterenol failed to alter the AGS. In contrast, muscimol (30 or 60 ng/side) infused into the IC abolished the tonic and clonic components of the AGS in GEPR-9s. These findings suggest that enhancement of GABAergic neurotransmission in the IC markedly attenuates AGS in the GEPR, while augmentation of NA neurotransmission has little effect in this brain region.

Kainic acid-induced seizures: potentiation by alpha-methyl-p-tyrosine

We have investigated the influence of central noradrenergic and dopaminergic systems on the susceptibility of rats to seizures in the kainic acid (KA)-model of epilepsy. In the dose range of 0.75 to 10 mg/kg s.c., KA dose-dependently induced characteristic behavioural changes. Partial depletion of noradrenaline (NA) and dopamine (DA) in the brain by pretreatment with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT; 250 mg/kg, i.p.) markedly potentiated KA-induced epileptic symptoms. A low dose of KA (1.5 mg/kg s.c.), which was ineffective in normal rats, triggered in AMPT-pretreated rats a high incidence of wet dog shakes (WDS) and a seizure activity (seizure rating: 3.17 +/- 0.31) which was comparable in degree to that resulting from 10 mg/kg KA in rats with normal catecholamine synthesis (seizure rating: 3.33 +/- 0.28). In AMPT-pretreated rats a higher dose of KA (10 mg/kg) further enhanced seizure activity and was associated with a mortality rate of up to 80%. Within 6.5 h after AMPT-pretreatment the levels of NA and DA in amygdala/pyriform cortex declined from 0.56 +/- 0.02 (control) to 0.23 +/- 0.01 ng/mg tissue and from 0.21 +/- 0.03 to 0.05 +/- 0.01 ng/mg tissue, respectively. At a dose of 1.5 mg/kg KA was ineffective on the levels of NA and DA in normal rats, but further reduced these levels in AMPT-pretreated rats to 0.08 +/- 0.02 and 0.020 +/- 0.004 ng/mg tissue, respectively. Induction of seizure activity and decline in NA and DA levels in amygdala/pyriform cortex after AMPT/KA (1.5 mg/kg) treatment was antagonized by the alpha-adrenoceptor agonist clonidine (0.1 mg/kg, i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurobiology of seizure predisposition in the genetically epilepsy-prone rat

Seizure predisposition in the genetically epilepsy-prone rat (GEPR) is innately determined and these animals exhibit consistent and reproducible convulsive patterns. This epilepsy model is made up of 2 independently derived colonies of animals with each exhibiting a characteristic convulsive pattern. In response to a standardized acoustic stimulus, GEPR-3s exhibit moderate or clonic convulsions and GEPR-9s exhibit more severe tonic extensor convulsions. Besides exhibiting convulsions in response to sound stimulation, some GEPRs experience spontaneous and hyperthermic seizures. They are also abnormally sensitive to a number of seizure provoking stimuli that produce seizures in normal animals. The neurochemical basis for the seizure predisposition in GEPRs is increasingly well understood. Abnormalities in central nervous system norepinephrine and serotonin are widespread and may play a prominent role in regulation of seizures in the GEPR. Amino acid neurotransmitter systems are less well defined in the GEPR but abnormalities exist and may be, along with other documented deficiencies, responsible in part for the seizure predisposition that is characteristic of GEPRs.

Norepinephrine modulates seizures induced by quinolinic acid in rats: selective and distinct roles of alpha-adrenoceptor subtypes

We investigated in rats whether alterations in noradrenergic function caused by 6-hydroxydopamine or alpha- and beta-adrenoceptor agonists and antagonists would modify the susceptibility of the brain to electroencephalographic seizures induced by intrahippocampal infusion of quinolinic acid. 6-Hydroxydopamine depletion of norepinephrine facilitated the expression of seizures while alpha-adrenoceptor stimulation by clonidine had either proconvulsant (0.1 mg/kg) or anticonvulsant (from 0.5 to 2 mg/kg) effects. Clonidine's anticonvulsant activity (0.5 mg/kg) was mimicked by methoxamine given intrahippocampally (10 micrograms), and antagonized by prazosin (1 mg/kg), whereas both yohimbine (5 and 10 mg/kg) and piperoxane (5 mg/kg) had no significant effect. Seizure facilitation induced by clonidine (0.1 mg/kg) was blocked by yohimbine (10 mg/kg). Systemic (0.25 and 0.5 mg/kg) or intrahippocampal (10 and 20 micrograms) isoproterenol and propranolol (10 mg/kg) had no effect. Spiking activity and neurotoxicity induced by quinolinic acid were unaltered by treatments which protected against convulsions. Modulation of quinolinic acid-convulsive activity by alpha-adrenoceptor subtypes appears to be selective and complex, since alpha 1-type activation reduces seizures while alpha 2-type stimulation has proconvulsant effects.

Cerebral cortical GABA and benzodiazepine binding sites in genetically seizure prone rats

Adult male and female genetically seizure-prone rats were assessed for sound-induced seizures. Heterozygous control groups were compared with mild seizure (designated GEPR 3) and severe seizure animals (GEPR 9). Groups of animals were killed and crude synaptosome fractions (P2) prepared from freshly dissected cerebral cortices. Binding sites for gamma-aminobutyric acid (GABA) were assessed by [3H]-muscimol in the absence or presence of excess GABA and/or pentobarbital. Binding sites for benzodiazepines were assessed by [3H]-flunitrazepam in the presence or absence of clonazepam. Compared to controls, GEPR 3 animals had a modest increase and GEPR 9 animals a larger increase in Bmax for both high and low affinity GABA sites, with no change in Kd. Chloride-dependent, barbiturate-enhanced GABA binding (increased Bmax) was observed in all conditions and groups. Likewise benzodiazepine binding (Bmax) increased slightly in GEPR 9 animals. There were no observed changes in binding sites for a survey of biogenic amines. Seizure-prone animals appear to have compensatory denervation-like supersensitivity for their most prominent inhibitory receptor, which may or may not be linked to the seizure event.

Noradrenergic and serotonergic determinants of seizure susceptibility and severity in genetically epilepsy-prone rats

Pharmacological studies demonstrate a reciprocal relationship between both noradrenergic and serotonergic transmission and audiogenic seizure severity and susceptibility in the genetically epilepsy-prone rat (GEPR). In contrast, drug-induced changes in the neurochemical indices of dopaminergic activity do not result in alterations in seizure severity. These pharmacological investigations led to the hypothesis that both noradrenergic and serotonergic neurons are capable of regulating seizure severity in the GEPR. Pharmacological investigations also provided evidence that monoaminergic neurons serve as determinants of seizure susceptibility in these epileptic animals. The GEPR is susceptible to environmentally-induced seizures which cannot be precipitated in neurologically normal subjects. Drug studies suggest that monoaminergic decrements serve as one set of susceptibility determinants. However, non-monoaminergic abnormalities also play important roles in the seizure predisposition which characterizes the GEPR. Pathophysiological studies have confirmed and extended the concepts generated by the pharmacological investigations. Noradrenergic and serotonergic deficits do indeed characterize the seizure naive state of the GEPR. These studies have provided a basis for tentative identification of areas of the brain in which monoaminergic abnormalities regulate seizure severity and susceptibility. Monoaminergic defects in some areas such as the thalamus may regulate both susceptibility and severity. In other areas, defects may regulate only severity or susceptibility. In the striatum, noradrenergic defects do not appear to be present and probably are not determinants of the epileptic state of the GEPR.