Suppression of spontaneous generalized non-convulsive seizures in the rat by microinjection of GABA antagonists into the superior colliculus

GABAergic circuits drive focal seizures

Epilepsy is based on abnormal neuronal activities that have historically been suggested to arise from an excess of excitation and a defect of inhibition, or in other words from an excessive glutamatergic drive not balanced by GABAergic activity. More recent data however indicate that GABAergic signaling is not defective at focal seizure onset and may even be actively involved in seizure generation by providing excitatory inputs. Recordings of interneurons revealed that they are active at seizure initiation and that their selective and time-controlled activation using optogenetics triggers seizures in a more general context of increased excitability. Moreover, GABAergic signaling appears to be mandatory at seizure onset in many models. The main pro-ictogenic effect of GABAergic signaling is the depolarizing action of GABA_A conductance which may occur when an excessive GABAergic activity causes Cl^- accumulation in neurons. This process may combine with background dysregulation of Cl^-, well described in epileptic tissues. Cl^- equilibrium is maintained by (Na⁺)/K⁺/Cl^- co-transporters, which can be defective and therefore favor the depolarizing effects of GABA. In addition, these co-transporters further contribute to this effect as they mediate K⁺ outflow together with Cl^- extrusion, a process that is responsible for K⁺ accumulation in the extracellular space and subsequent increase of local excitability. The role of GABAergic signaling in focal seizure generation is obvious but its complex dynamics and balance between GABA_A flux polarity and local excitability still remain to be established, especially in epileptic tissues where receptors and ion regulators are disrupted and in which GABAergic signaling rather plays a 2 faces Janus role.

The Superior Colliculus: Cell Types, Connectivity, and Behavior

The superior colliculus (SC), one of the most well-characterized midbrain sensorimotor structures where visual, auditory, and somatosensory information are integrated to initiate motor commands, is highly conserved across vertebrate evolution. Moreover, cell-type-specific SC neurons integrate afferent signals within local networks to generate defined output related to innate and cognitive behaviors. This review focuses on the recent progress in understanding of phenotypic diversity amongst SC neurons and their intrinsic circuits and long-projection targets. We further describe relevant neural circuits and specific cell types in relation to behavioral outputs and cognitive functions. The systematic delineation of SC organization, cell types, and neural connections is further put into context across species as these depend upon laminar architecture. Moreover, we focus on SC neural circuitry involving saccadic eye movement, and cognitive and innate behaviors. Overall, the review provides insight into SC functioning and represents a basis for further understanding of the pathology associated with SC dysfunction.

Divergent Effects of Systemic and Intracollicular CB Receptor Activation Against Forebrain and Hindbrain-Evoked Seizures in Rats

Cannabinoid (CB) receptor agonists are of growing interest as targets for anti-seizure therapies. Here we examined the effect of systemic administration of the CB receptor agonist WIN 55,212-2 (WIN) against audiogenic seizures (AGSs) in the Genetically Epilepsy Prone Rat (GEPR)-3 strain, and against seizures evoked focally from the Area Tempestas (AT). We compared these results to the effect of focal administration of the CB1/2 receptor agonist CP 55940 into the deep layers of the superior colliculus (DLSC), a brain site expressing CB1 receptors. While systemic administration of WIN dose-dependently decreased AGS in GEPR-3s, it was without effect in the AT model. By contrast, intra-DLSC infusion of CP 55940 decreased seizures in both models. To determine if the effects of systemic WIN were dependent upon activation of CB1 receptors in the DSLC, we next microinjected the CB1 receptor antagonist SR141716, before WIN systemic treatment, and tested animals for AGS susceptibility. The pretreatment of the DLSC with SR141716 was without effect on its own and did not alter the anti-convulsant action of WIN systemic administration. Thus, while CB receptors in the DLSC are a potential site of anticonvulsant action, they are not necessary for the effects of systemically administered CB agonists.

Descending projections from the substantia nigra pars reticulata differentially control seizures

Three decades of studies have shown that inhibition of the substantia nigra pars reticulata (SNpr) attenuates seizures, yet the circuits mediating this effect remain obscure. SNpr projects to the deep and intermediate layers of the superior colliculus (DLSC) and the pedunculopontine nucleus (PPN), but the contributions of these projections are unknown. To address this gap, we optogenetically silenced cell bodies within SNpr, nigrotectal terminals within DLSC, and nigrotegmental terminals within PPN. Inhibition of cell bodies in SNpr suppressed generalized seizures evoked by pentylenetetrazole (PTZ), partial seizures evoked from the forebrain, absence seizures evoked by gamma-butyrolactone (GBL), and audiogenic seizures in genetically epilepsy-prone rats. Strikingly, these effects were fully recapitulated by silencing nigrotectal projections. By contrast, silencing nigrotegmental terminals reduced only absence seizures and exacerbated seizures evoked by PTZ. These data underscore the broad-spectrum anticonvulsant efficacy of this circuit, and demonstrate that specific efferent projection pathways differentially control different seizure types.

It's a Storm, It's a Gale: Epilepsy Initiation From the Corticostriatal Circuit

[Box: see text].

Optogenetic activation of superior colliculus neurons suppresses seizures originating in diverse brain networks

Because sites of seizure origin may be unknown or multifocal, identifying targets from which activation can suppress seizures originating in diverse networks is essential. We evaluated the ability of optogenetic activation of the deep/intermediate layers of the superior colliculus (DLSC) to fill this role. Optogenetic activation of DLSC suppressed behavioral and electrographic seizures in the pentylenetetrazole (forebrain+brainstem seizures) and Area Tempestas (forebrain/complex partial seizures) models; this effect was specific to activation of DLSC, and not neighboring structures. DLSC activation likewise attenuated seizures evoked by gamma butyrolactone (thalamocortical/absence seizures), or acoustic stimulation of genetically epilepsy prone rates (brainstem seizures). Anticonvulsant effects were seen with stimulation frequencies as low as 5 Hz. Unlike previous applications of optogenetics for the control of seizures, activation of DLSC exerted broad-spectrum anticonvulsant actions, attenuating seizures originating in diverse and distal brain networks. These data indicate that DLSC is a promising target for optogenetic control of epilepsy.

Roles of the subthalamic nucleus and subthalamic HCN channels in absence seizures

Absence seizures consist of a brief and sudden impairment of consciousness. They are characterized by bilaterally synchronized spike and wave discharges (SWDs), which reflect abnormal oscillations in the thalamocortical loops. Recent studies have suggested that the basal ganglia are involved in generation of the SWDs, but their roles are poorly understood at the molecular and cellular levels. Here we studied the pathophysiological roles of the basal ganglia, using in vivo and in vitro measurements of tottering mice, a well-established model of absence epilepsy. We found that the membrane excitability in subthalamic nucleus (STN) neurons was enhanced in tottering mice, which resulted from reduced hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activity. Pharmacological blockade and activation of HCN channel activity in vitro bidirectionally altered the membrane excitability of the STN neurons. Furthermore, these pharmacological modulations of HCN channel activity in the STN in vivo bidirectionally altered the mean SWD duration. In addition, STN deep brain stimulation modulated SWDs in a frequency-dependent manner. These results indicate that STN is involved in the rhythm maintenance system of absence seizures.

Update on the role of substantia nigra pars reticulata in the regulation of seizures

The substantia nigra pars reticulata (SNR) represents an endogenous seizure suppressing system, which may be targeted to develop treatments for generalized or multifocal epilepsies. This review summarizes the region-, age-, and sex-specific features of the SNR-based seizure-controlling network.

Metabolic environment in substantia nigra reticulata is critical for the expression and control of hypoglycemia-induced seizures

Seizures represent a common and serious complication of hypoglycemia. Here we studied mechanisms of control of hypoglycemic seizures induced by insulin injection in fasted and nonfasted rats. We demonstrate that fasting predisposes rats to more rapid and consistent development of hypoglycemic seizures. However, the fasting-induced decrease in baseline blood glucose concentration cannot account for the earlier onset of seizures in fasted versus nonfasted rats. Data obtained with c-Fos immunohistochemistry and [14C]2-deoxyglucose uptake implicate a prominent involvement of the substantia nigra reticulata (SNR) among other structures in the hypoglycemic seizure control. This is supported by data showing that fasting decreases the SNR expression of K(ATP) channels, which link metabolism with activity, and is further confirmed with microinfusions of K(ATP) channel agonist and antagonist. Data obtained with whole-cell and perforated patch recordings from SNR neurons in slices in vitro demonstrate that both presynaptic and postsynaptic K(ATP) channels participate in the failure of the SNR to control hypoglycemic seizures. The results suggest that fasting and insulin-induced hypoglycemia can lead to impairment in the function of the SNR, leading thus to hypoglycemic seizures.

Circadian distribution of generalized tonic-clonic seizures associated with murine succinic semialdehyde dehydrogenase deficiency, a disorder of GABA metabolism

Human succinic semialdehyde dehydrogenase (SSADH) deficiency is an autosomal recessive disorder of GABA metabolism associated with motor impairment and epileptic seizures. Similarly, mice with targeted deletion of the Aldh5a1 gene (Aldh5a1(-/-)) exhibit SSADH deficiency and seizures early in life. These seizures begin as absence seizures the second week of life, but evolve into generalized convulsive seizures that increase in severity and become lethal during the fourth postnatal week. The seizures are alleviated and survival is prolonged when the mutant animals are weaned onto a ketogenic diet (KD). The persistence of spontaneous, recurrent, generalized tonic-clonic seizures in KD-treated adult Aldh5a1(-/-) mice allowed us to quantify their daily (circadian) distribution using a novel behavioral method based on the detection of changes in movement velocity. Adult KD-treated Aldh5a1(-/-) mice exhibited a seizure phenotype characterized by fits of wild running clonus accompanied by jumping and bouncing. These hypermotor seizures were largely spontaneous and occurred daily in a nonrandom pattern. The seizure rhythm showed a peak shortly after dark phase onset (2008 hours) with near-24-hour periodicity. Age-matched wild-type littermates showed no evidence of abnormal motor behavior. These new data suggest that generalized tonic-clonic seizures in Aldh5a1(-/-) mice are more frequent during a specific time of day and will provide useful information to clinicians for the treatment of seizures associated with human SSADH deficiency.

Activity of ventral medial thalamic neurons during absence seizures and modulation of cortical paroxysms by the nigrothalamic pathway

Absence seizures are characterized by bilaterally synchronous spike-and-wave discharges (SWDs) in the electroencephalogram, which reflect abnormal oscillations in corticothalamic networks. Although it was suggested that basal ganglia could modulate, via their feedback circuits to the cerebral cortex, the occurrence of SWDs, the cellular and network mechanisms underlying such a subcortical control of absence seizures remain unknown. The GABAergic projections from substantia nigra pars reticulata (SNR) to thalamocortical neurons of the ventral medial (VM) thalamic nucleus provide a potent network for the control of absence seizures by basal ganglia. The present in vivo study provides the first description of the activity of VM thalamic neurons during seizures in the genetic absence epilepsy rats from Strasbourg, a well established model of absence epilepsy. Cortical paroxysms were accompanied in VM thalamic neurons by rhythmic bursts of action potentials. Pharmacological blockade of excitatory inputs of nigrothalamic neurons led to a transient interruption of SWDs, correlated with a change in the activity of thalamic cells, which was increased in frequency and converted into a sustained arrhythmic firing pattern. Simultaneously, cortical neurons exhibited a decrease in their firing rate that was associated with an increase in membrane polarization and a decrease in input resistance. These new findings demonstrate that an inhibition of SNR neurons changes the activity of their thalamic targets, which in turn could affect cortical neurons excitability and, consequently, the generation of cortical epileptic discharges. Thus, the nigro-thalamo-cortical pathway may provide an on-line system control of absence seizures.

Anatomical evidence for an anticonvulsant relay in the rat ventromedial medulla

Pharmacological manipulation of the ventrolateral pontine reticular formation (vlPRF) of rats has an anticonvulsant effect in the maximal electroshock model of epilepsy. This study presents three anatomical experiments that determine the efferent projections from this region likely to mediate this anticonvulsant effect. In the first, the anterograde tracer biotinylated dextran amine (BDA) was injected into the vlPRF. A strong projection to the ventromedial medullary reticular formation (vmMRF) was revealed which continued only weakly to the spinal cord. In the second experiment, double-label procedures were used to indicate whether the BDA-labelled terminals from the vlPRF make contacts with neurons in vmMRF, retrogradely labelled with cholera-toxin B subunit from the lumbar spinal cord. Sections of the vmMRF were examined by: (i) light microscopy which showed significant overlap between terminals from vlPRF and retrogradely-labelled reticulospinal cells; (ii) confocal microscopy which showed labelled terminals in close association with reticulospinal cell bodies; and (iii) electron microscopy which showed vlPRF terminals making synaptic contact with reticulospinal neurons. Finally, immunohistochemical procedures in combination with anterograde tracing revealed that significant numbers of terminals labelled from vlPRF were also positive for markers of glutamatergic or GABAergic neurotransmission. This suggests that the projection from the vlPRF to the vmMRF is likely to include several different functional components. These connections could represent a final critical link of an anticonvulsant circuit that originates in the dorsal midbrain and projects via relays in the vlPRF and the vmMRF to interact with the low-level motor circuitry in the spinal cord.

Regional neural activity within the substantia nigra during peri-ictal flurothyl generalized seizure stages

Structures responsible for the onset, propagation, and cessation of generalized seizures are not known. Lesion and microinfusion studies suggest that the substantia nigra pars reticulata (SNR) seizure-controlling network could play a key role. However, the expression of neural activity within the SNR and its targets during discrete pre- and postictal periods has not been investigated. In rats, we used flurothyl to induce generalized seizures over a controlled time period and 2-deoxyglucose autoradiography mapping technique. Changes in neural activity within the SNR were region-specific. The SNRposterior was selectively active during the pre-clonic period and may represent an early gateway to seizure propagation. The SNRanterior and superior colliculus changed their activity during progression to tonic-clonic seizure, suggesting the involvement in coordinated regional activity that results in inhibitory effects on seizures. The postictal suppression state was correlated with changes in the SNR projection targets, specifically the pedunculopontine tegmental nucleus and superior colliculus.

Evidence for a role of the parafascicular nucleus of the thalamus in the control of epileptic seizures by the superior colliculus

PURPOSE

The aim of this study was to investigate whether the nucleus parafascicularis (Pf) of the thalamus could be a relay of the control of epileptic seizures by the superior colliculus (SC). The Pf is one of the main ascending projections of the SC, the disinhibition of which has been shown to suppress seizures in different animal models and has been proposed as the main relay of the nigral control of epilepsy.

METHODS

Rats with genetic absence seizures (generalized absence epilepsy rat from Strasbourg or GAERS) were used in this study. The effect of bilateral microinjection of picrotoxin, a gamma-aminobutyric acid (GABA) antagonist, in the SC on the glutamate and GABA extracellular concentration within the Pf was first investigated by using microdialysis. In a second experiment, the effect of direct activation of Pf neurons on the occurrence of absence seizures was examined with microinjection of low doses of kainate, a glutamate agonist.

RESULTS

Bilateral injection of picrotoxin (33 pmol/side) in the SC suppressed spike-and-wave discharges for 20 min. This treatment resulted in an increase of glutamate but not GABA levels in the Pf during the same time course. Bilateral injection of kainate (35 pmol/side) into the Pf significantly suppressed spike-and-wave discharges for 20 min, whereas such injections were without effects when at least one site was located outside the Pf.

CONCLUSIONS

These data suggest that glutamatergic projections to the Pf could be involved in the control of seizures by the SC. Disinhibition of these neurons could lead to seizure suppression and may be involved in the nigral control of epilepsy.

Role of the superior colliculus and the intercollicular nucleus in the brainstem seizure circuitry of the genetically epilepsy-prone rat

PURPOSE

The neuronal network responsible for the convulsive behavior associated with sound-induced seizures in genetically epilepsy-prone rats (GEPRs) is believed to include the inferior colliculus and other brainstem structures such as the deep layers of the superior colliculus (DLSC), periaqueductal gray, and pontine reticular formation. However, previous studies also suggested that the DLSC and the nearby intercollicular nucleus (ICN) are part of a midbrain anticonvulsant zone capable of suppressing tonic convulsions when activated with bicuculline. Our aim in this study was to investigate the role of the superior colliculus (SC) and the ICN in generalized tonic-clonic seizures (GTCSs).

METHODS

Bilateral lesions of the SC and the ICN as well as bicuculline infusions into the ICN were used to assess the role of this dorsal midbrain region in brainstem seizures induced by sound stimulation in GEPR-9s and GEPR-3s.

RESULTS

Lesions of the SC markedly attenuated audiogenic seizure (AGS) severity by abolishing all behavioral components except the wild running. Lesions of the ICN significantly reduced seizure severity in GEPR-9s, but were somewhat less effective than SC lesions. Bicuculline infusion into the deep layers of the SC and/or the ICN produced audiogenic-like seizures in GEPR-9s.

CONCLUSIONS

These findings support the hypothesis that the SC and ICN are important components of the brainstem seizure network, but suggest they are not necessary for the wild-running component of the seizure. The results further indicate that stimulation of the tectum facilitates GTCSs. Thus these findings suggest that the dorsal midbrain, when stimulated, is proconvulsant rather than anticonvulsant regarding brainstem seizures in GEPRs.

Suppression of absence seizures by electrical and pharmacological activation of the caudal superior colliculus in a genetic model of absence epilepsy in the rat

Activation of the superior colliculus has been shown to reproduce the antiepileptic effect of the inhibition of the substantia nigra reticulata. A circuit involving neurons of the caudal deep layers of the superior colliculus has been suggested to control brain stem convulsive seizures. The present study was designed to examine whether a similar circuit is also involved in the control of absence seizures. For this, activation of either the rostral or caudal parts of the deep and intermediate layers of the superior colliculus was applied in a genetic model of absence seizures in the rat (GAERS). Single-shock (5 s) electrical stimulation of the rostral and caudal superior colliculus interrupted ongoing spike-and-wave discharges at an intensity (antiepileptic threshold) significantly lower than the intensity inducing behavioral effects. At this intensity, no interruption of licking behavior was observed in water-deprived rats. Repeated stimulations (5 s on/5 s off) at the antiepileptic threshold reduced absence seizures only during the first 10 min. Bilateral microinjection of a GABA antagonist (picrotoxin, 33 pmol/side) significantly suppressed spike-and-wave discharges when applied in the caudal aspect of the superior colliculus. This antiepileptic effect appears dissociated from an anxiogenic effect, as tested in an elevated plus maze test. Finally, bilateral injection of picrotoxin (33 pmol/side) appeared more effective in the superficial and intermediate layers of the caudal superior colliculus, whereas such injections had only weak effects on absence seizures when applied in the deep layers. These results suggest that a specific population of neurons located in the intermediate and superficial layers of the caudal superior colliculus is involved in the inhibitory control of absence seizures. It may constitute an important relay for the control of absence seizures by the basal ganglia via the substantia nigra reticulata.

Mu-opioid receptors in seizure-controlling brain structures are altered by prenatal morphine exposure and by male and female gonadal steroids in adult rats

The present study used autoradiography to examine the effect of prenatal morphine exposure on mu-opioid receptor density in epileptic seizure-controlling brain structures including the substantia nigra pars compacta (SNC), substantia nigra pars reticulata (SNR), superior colliculus (SC), and subthalamic nucleus (STN) of adult male and female rats. The results demonstrate that prenatal morphine exposure increases the mu-opioid receptor density in the SNC and STN, but not in the SNR or in the SC of gonadally intact adult male rats. The density of mu-opioid receptors in the SNC and STN is, however, decreased following gonadectomy in morphine-exposed males, and testosterone treatment fails to restore this decrease to the level of gonadally intact males. Further, in the SC, the density of mu receptors was lower in both saline-exposed, gonadectomized (GNX) and GNX, TP-treated males and in morphine-exposed, GNX, TP-treated males relative to gonadally intact saline- and morphine-exposed males, respectively. In ovariectomized (OVX) female rats, the same prenatal morphine exposure increases the mu-opioid receptor density in the SNC and SNR, but decreases it in the STN. The density of mu-opioid receptors is also decreased in the SNC and SC of OVX estrogen-treated females and in the SNR and SC of OVX, progesterone-treated females. Thus, the present study demonstrates that mu-opioid receptors in seizure-controlling brain structures are sex-specifically altered by prenatal morphine exposure in adult progeny. Further, prenatal morphine exposure alters gonadal hormone effects on the density of mu receptors in adult, OVX females.

Superior colliculus firing changes after lesion or electrical stimulation of the subthalamic nucleus in the rat

Recent data have suggested a critical role for the basal ganglia in the remote control of epileptic seizures. In particular, it has been shown that inhibition of either substantia nigra pars reticulata or subthalamic nucleus as well as activation of the superior colliculus suppresses generalized seizures in several animal models. It was previously shown that high frequency stimulation of the subthalamic nucleus, thought to act as functional inhibition, stopped ongoing non-convulsive generalized seizures in rats. In order to determine whether high frequency stimulation of the subthalamic nucleus involved an activation of superior colliculus neurons, we examined the effects of subthalamic nucleus manipulation, by either high frequency stimulation or chemical lesion, on the spontaneous electrical activity of superior colliculus neurons. Acute high frequency stimulation of the subthalamic nucleus (frequency 130 Hz) induced an immediate increase of unitary activity in 70% of responding cells, mainly located within the deep layers, whereas a reduction was observed in the remaining 30%. The latter responses are dependent on the intensity and frequency of the stimulation. Unilateral excitotoxic lesion of the subthalamic nucleus induced a delayed and transient decrease of superior colliculus activity. Our data suggest that high frequency stimulation of the subthalamic nucleus suppresses generalised epileptic seizures through superior colliculus activation.

Modulation of the audiogenic seizure network by noradrenergic and glutamatergic receptors of the deep layers of superior colliculus

Recent studies suggest that the deep layers of superior colliculus (DLSC) play a role in the network for audiogenic seizures (AGS) in genetically epilepsy-prone rats (GEPR-9s). The present study examined the role of glutamatergic and noradrenergic receptors in DLSC in modulation of AGS susceptibility. The study examined effects of a competitive NMDA receptor antagonist [dl-2-amino-7-phosphonoheptanoic acid (AP7)] or an alpha1 noradrenergic agonist (phenylephrine) focally microinjected into DLSC as compared to effects in the inferior colliculus (IC) and pontine reticular formation (PRF), which are major established components of the AGS network. The results demonstrated that blockade of NMDA receptors in DLSC suppressed AGS susceptibility. AP7 microinjection was effective at relatively low doses in IC, but required higher doses in DLSC and PRF. The DLSC was relatively more sensitive to seizure reduction by the alpha1 noradrenergic agonist as compared to the IC and PRF. The anticonvulsant effect of AP7 was longer-lasting than phenylephrine in the DLSC and IC but not in the PRF. These data suggest that neurons in the DLSC are a requisite component for the neuronal network for AGS in GEPR-9s and that NMDA and alpha1 adrenoreceptors in this site may play important roles in the modulation of AGS propagation. The relatively greater sensitivity of DLSC to phenylephrine as compared to IC and PRF indicates that norepinephrine may be more important in the modulation of AGS in DLSC, which contrasts to the role of glutamate modulation. These data support recent neuronal recording data, which indicate that DLSC neurons play a critical role in AGS.

Neurons in the deep layers of superior colliculus play a critical role in the neuronal network for audiogenic seizures: mechanisms for production of wild running behavior

Recent investigations suggest that the deep layers of superior colliculus (DLSC) play a role in the neuronal network for audiogenic seizures (AGS). The present study examined DLSC neuronal firing and convulsive behavior simultaneously in freely-moving genetically epilepsy-prone rats (GEPR-9s) using chronically implanted microwire electrodes. An abrupt onset of acoustically-evoked firing at approximately 80-90 dB was observed in DLSC neurons of GEPR-9s, which was significantly above the normal threshold. DLSC neurons began to exhibit rapid tonic burst firing 1-2 s prior to the onset of the wild running behavior at the beginning of AGS. As the tonic phase of the seizure began, DLSC firing ceased, and only returned towards normal following post-ictal depression. These neuronal mechanisms may be relevant to other seizure models in which the DLSC is implicated. The temporal pattern of neuronal firing during AGS is specific to DLSC and differs markedly from those observed elsewhere in the AGS neuronal network. The temporal firing pattern suggests that the DLSC plays a primary role in the generation of the wild running phase of AGS. Previous studies indicate that the inferior colliculus is dominant during AGS initiation, and the pontine reticular formation is dominant during the tonic extension phase of AGS. Taken together these data suggest that the neurons in the neuronal network undergo a dominance shift as each specific convulsive behavior of AGS is elaborated.

The role of basal ganglia in the control of generalized absence seizures

During the last two decades, evidence has accumulated to demonstrate the existence, in the central nervous system, of an endogenous mechanism that exerts an inhibitory control over different forms of epileptic seizures. The substantia nigra and the superior colliculus have been described as key structures in this control circuit; inhibition of GABAergic neurons of the substantia nigra pars reticulata results in suppression of seizures in various animal models of epilepsy. The role in this control mechanism of the direct GABAergic projection from the striatum to the substantia nigra and of the indirect pathway, from the striatum through the globus pallidus and the subthalamic nucleus, was examined in a genetic model of absence seizures in the rat. In this model, pharmacological manipulations of both the direct and indirect pathways resulted in modulation of absence seizures. Activation of the direct pathway or inhibition of the indirect pathway suppressed absence seizures through disinhibition of neurons in the deep and intermediate layers of the superior colliculus. Dopamine D1 and D2 receptors in the nucleus accumbens, appear to be critical in these suppressive effects. Along with data from the literature, our results suggest that basal ganglia circuits play a major role in the modulation of absence seizures and provide a framework to understand the role of these circuits in the modulation of generalized seizures.

The dorsal midbrain anticonvulsant zone--I. Effects of locally administered excitatory amino acids or bicuculline on maximal electroshock seizures

Microinjections of bicuculline methiodide into the dorsal midbrain anticonvulsant zone, a region which includes the caudal deep layers of the superior colliculus, the adjacent mesencephalic reticular formation and the intercollicular nucleus, suppress tonic hindlimb extension induced by maximal electroshock. The purpose of the present experiments was to establish the most effective and convenient method for eliciting anticonvulsant properties from the dorsal midbrain using the electroshock model of epilepsy. A comparison of different injections of excitatory amino acids and bicuculline into the dorsal midbrain of the rat showed: (i) injections of kainate suppressed hindlimb extension but only at substantially larger doses (i.e. 200-400 pmol) than 50 pmol of bicuculline, which produced generally superior effects; (ii) quisqualate provided only weak protection against tonic seizures at doses that produced neurotoxic effects (2-40 nmol); (iii) N-methyl-D-aspartate was ineffective at doses which produced mild clonic seizure in their own right (2-4 nmol) and also produced some evidence of neurotoxicity; (iv) the suppression of hindlimb extension by bicuculline was dose related, and the lowest bilateral dose for producing reliable suppression was 50 pmol/400 nl per side; and (v) a unilateral injection of 100 pmol/400 nl also reliably suppressed hindlimb extension. The latter finding had important implications for the design and interpretation of the following lesion study. Injections of bicuculline into the dorsal midbrain also produced defence-like behavioural responses that included running and biting; the intensity of these responses correlated with the suppression of hindlimb extension.(ABSTRACT TRUNCATED AT 250 WORDS)

Inferior colliculus neuronal membrane and synaptic properties in genetically epilepsy-prone rats

Previous studies using single-unit recording techniques have shown that the inferior colliculus is critical for audiogenic seizure initiation in genetically epilepsy-prone rats (GEPR). In order to investigate cellular abnormalities that may be important in causing audiogenic seizure susceptibility, intracellular recordings were made from neurons of inferior colliculus dorsal cortex (ICd) in a GEPR variety that exhibits severe audiogenic seizures (GEPR-9). GEPR neuronal membrane and synaptic properties were compared to those of normal Sprague-Dawley rats (SD), the strain from which GEPR were derived. We found six electrophysiological differences between GEPR and normal SD ICd neurons, all of which could promote seizures in GEPR. (1) Input resistance was higher in GEPR than in normal ICd neurons. (2) Threshold for repetitive action potential firing was closer to resting membrane potential in GEPR ICd neurons. (3) GEPR neurons showed faster repetitive spike firing than normal SD neurons. (4) Anode break spikes occurred at the offset of a hyperpolarizing pulse more often in GEPR than in normal SD neurons. (5) Stimulation of the commissure of the inferior colliculus caused synaptic paired pulse inhibition in normal ICd neurons, but paired pulse facilitation was always observed in GEPR neurons. (6) In GEPR, a large epileptiform depolarizing event could be elicited by strong electrical stimulation of the commissure of the inferior colliculus. In normal SD rats, similar epileptiform activity was seen only after application of bicuculline or NMDA. Our results suggest that both abnormal neuronal membrane properties and altered synaptic transmission are likely to contribute to seizure predisposition and audiogenic seizure initiation in GEPR.

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.

Blockade of GABA receptors in superior colliculus protects against focally evoked limbic motor seizures

Blockade of GABA receptors in the rat superior colliculus (SC) has been shown to protect against maximal electroshock-induced tonic convulsions and spontaneous generalized non-convulsive seizures. In the present study, we determined that blockade of GABA receptors in SC could also protect against focally evoked limbic motor seizures. Limbic motor seizures were induced by the unilateral focal application of bicuculline methiodide into area tempestas (AT), an epileptogenic site in the deep prepiriform cortex. Control rats (receiving bilateral infusions of saline into SC) all exhibited convulsive seizures following bicuculline in AT. Rats pretreated (5 min before) with bicuculline (50 pmol) bilaterally in the deep layers of the SC, were protected against the AT-evoked convulsive seizures. Unilateral application of bicuculline in the deep SC or bilateral application in the superficial layers of the SC did not alter the convulsive response to bicuculline in AT. These results indicate that the anticonvulsant action of GABA blockade in SC is not limited to tonic convulsive seizures but extends to the clonic manifestations evoked by seizures originating in forebrain limbic circuits. Given that the deep layers of SC receive inputs from GABA neurons in substantia nigra and that suppression of the activity of nigral neurons is anticonvulsant against a variety of seizures (including those evoked from AT), it is likely that the anticonvulsant action of bicuculline in SC is due to interference with the influence of a nigrotectal GABAergic projection.

Role of GABA in the genesis of chemoconvulsant seizures

The direct or indirect interference with GABA-mediated neurotransmission results in convulsive seizure activity in humans and experimental animals. When this convulsant effect is experimentally analyzed, it turns out to be a product of discrete and restricted cerebral sites of drug action. Depending upon the brain circuitry affected, different convulsant patterns are produced. Acute interference with GABA transmission in convulsant trigger sites in the forebrain evokes convulsant seizures which can be clearly distinguished from those evoked by interference with GABA transmission in the hindbrain convulsant sites. While acute alterations of forebrain seizure susceptibility do not change hindbrain seizure susceptibility, chronic or repeated exposure to seizures may cause simultaneous "kindling" of both systems. In addition to the specific convulsant sites of action of GABA antagonists in the brain there are specific sites where GABA antagonists exert an anticonvulsant action. The ability of a chemical agent to evoke a convulsive seizure by interfering with GABA transmission depends upon the relative effect of the agent on GABA transmission in different brain areas as well as its effect on other excitatory and inhibitory neurotransmitters with which GABA interacts.

Infusion of bicuculline methiodide into the tectum: model specificity of pro- and anticonvulsant actions

Microinjection of drugs, such as muscimol, into the substantia nigra pars reticulata (SNpr) can inhibit several types of experimental seizures. Some findings suggested that this was a result of disinhibition of neurons receiving input from GABAergic nigrotectal cells. Indeed, it was reported that bicuculline methiodide (BMI), infused into the tectal region that was reported to receive nigral input, produced an anticonvulsant effect against maximal electroshock (MES) convulsion. Since previous work had suggested that the anticonvulsant effect of intranigral muscimol depended on the particular experimental seizure used, three different experimental seizures were used in the present study to evaluate the effects of BMI infusion into the tectum. Guide cannulas aimed at the tectal region receiving nigral innervation were stereotaxically implanted in rats a week before testing. Bilateral intratectal infusions of BMI (25 ng/side) had an anticonvulsant effect against MES convulsions, confirming a previous report. In contrast, the same BMI pretreatment worsened convulsions produced by either systemic pentylenetetrazol (40 mg/kg, i.p.) or bicuculline (0.5 mg/kg, i.v.). The effects of intratectal BMI were seizure model-dependent, suggesting different functional interconnections between tectum and those pathways responsible for generalization of MES as compared to PTZ or bicuculline convulsions.

Effects of neural transplantation on seizures in the immature genetically epilepsy-prone rat

To study the hypothesis that neural transplantations can alter seizure susceptibility in a genetic animal model of epilepsy, 93 pubescent genetically epilepsy-prone rats with stage 9 seizures received either bilateral inferior colliculi (N = 21) or lateral ventricle (N = 42) transplants or sham transplants (N = 30). The grafts consisted of embryonic locus ceruleus, neocortical, or cerebellar tissue. Starting 2 days after the transplantation the rats were subjected to audiogenic stimulations every other day for 61 days. Latency to the running and tonic phase, seizure severity score, and duration of the tonic and clonic phase were compared in the neural transplant and sham-operated controls. Rats that received transplants had a longer latency to the tonic phase and a shorter duration of the clonic phase than the controls. At age 110 days the rats had electrodes implanted bilaterally into the angular bundle and were kindled. No difference in kindling rate was found between the rats that received neural grafts and the sham-operated controls. Cerebrospinal fluid concentration of norepinephrine was not altered by the transplants. This study demonstrates that the anticonvulsant effects of neural transplants, using the genetically epilepsy-prone model of epilepsy, are mild.

Genetic absence epilepsy in rats from Strasbourg--a review

We have selected a strain of rats and designated it the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). In this strain, 100% of the animals present recurrent generalized non-convulsive seizures characterized by bilateral and synchronous spike-and-wave discharges accompanied with behavioural arrest, staring and sometimes twitching of the vibrissae. Spontaneous SWD (7-11 cps, 300-1,000 microV, 0.5-75 sec) start and end abruptly on a normal background EEG. They usually occur at a mean frequency of 1.5 per min when the animals are in a state of quiet wakefulness. Drugs effective against absence seizures in humans (ethosuccimide, trimethadione, valproate, benzodiazepines) suppress the SWD dose-dependently, whereas drugs specific for convulsive or focal seizures (carbamazepine, phenytoin) are ineffective. SWD are increased by epileptogenic drugs inducing petit mal-like seizures, such as pentylenetetrazol, gamma-hydroxybutyrate, THIP and penicillin. Depth EEG recordings and lesion experiments show that SWD in GAERs depend on cortical and thalamic structures with a possible rhythmic triggering by the lateral thalamus. Most neurotransmitters are involved in the control of SWD (dopamine, noradrenaline, NMDA, acetylcholine), but GABA and gamma-hydroxybutyrate (GHB) seem to play a critical role. SWD are genetically determined with an autosomal dominant inheritance. The variable expression of SWD in offsprings from GAERS x control reciprocal crosses may be due to the existence of multiple genes. Neurophysiological, behavioural, pharmacological and genetic studies demonstrate that spontaneous SWD in GAERS fulfill all the requirements for an experimental model of absence epilepsy. As the mechanisms underlying absence epilepsy in humans are still unknown, the analysis of the genetic thalamocortical dysfunction in GAERS may be fruitful in investigations of the pathogenesis of generalized non-convulsive seizures.

Pharmacological models of generalized absence seizures in rodents

A number of animal models of generalized absence seizures in rodents are described. These include absence seizures induced by gamma-hydroxybutyrate (GHB), low dose pentylenetetrazole, penicillin, THIP, and AY-9944. All of these models share behavioral and EEG similarity to human absence seizures and show pharmacologic specificity for antiabsence drugs such as ethosuximide and trimethadione. Moreover, the absence seizures induced by these agents are exacerbated by GABAergic agonists, a property unique to experimental absence seizures. These models are predictable, reproducible, and easy to standardize. They are useful both in studying mechanisms of pathogenesis of absence seizures as well as in screening for antiabsence activity of potential antiepileptic drugs.

The inhibitory control of the substantia nigra over generalized non-convulsive seizures in the rat

A system exerting inhibitory control over generalized epilepsies and involving neurons from the substantia nigra has been described by several authors in experimental models of convulsive seizures. In the present study, the existence of such a control system governing absence epilepsy was investigated using models of non-convulsive seizures in the rat. Activation of the GABAergic neurotransmission within the substantia nigra by local injection of GABA agonists (muscimol, THIP) or an inhibitor of GABA degradation (gamma-vinyl GABA) suppresses generalized non convulsive seizures, whether they are genetically determined or induced by systemic injections of gamma-butyrolactone (100 and 200 mg/kg), pentylenetetrazole (20 mg/kg) or THIP (7.5 mg/kg). The ascending dopaminergic nigral output or the GABAergic fibres to the ventromedial thalamus are not critically involved in this control system. By contrast, the GABAergic nigro-collicular pathway appears crucial: bilateral lesion of the superior colliculus abolishes the anti-epileptic effects of intranigral injection of muscimol and blockade of the GABAergic transmission within the superior colliculus results in a suppression of generalized non-convulsive seizures. Finally, activation of collicular cell bodies by low doses of kainic acid significantly suppresses absence seizures. These results suggest the existence of a control system inhibiting generalized non-convulsive seizures which is activated by the release of the tonic inhibition exerted by the nigral GABAergic fibres on collicular neurons. The similarities between this system and the control system described for convulsive seizures are discussed.

Lack of proconvulsant action of GABA depletion in substantia nigra in several seizure models

The effect of intranigral application of a gamma-aminobutyric acid (GABA) synthesis inhibitor, was examined in 3 different rat seizure models. Bilateral intranigral infusion of isoniazid (150 micrograms) did not potentiate the effect of subcutaneous administration of a threshold dose (1.5 mg/kg) of the GABA antagonist bicuculline. Similarly, following pretreatment with intranigral isoniazid, neither severity nor latency to onset of seizures elicited by systemic injection of kainic acid (9 mg/kg) were modified. In addition, convulsive seizures evoked by the focal injection of bicuculline methiodide (40 ng) in an epileptogenic site within the deep prepiriform cortex (area tempestas) were not potentiated by intranigral isoniazid. These results were in sharp contrast to the marked potentiating effect of intranigral isoniazid (150 or 85 micrograms) on seizures induced by systemic administration of a subconvulsant dose of pilocarpine (150 mg/kg). In addition, we attempted to evoke a proconvulsant action from striatum. The striatum, origin of GABAergic projections to substantia nigra, is a region in which application of GABA antagonists have been found to be anticonvulsant in several seizure models. We therefore examined the effect of bilateral intrastriatal infusion of the GABA agonist, muscimol (5 ng) on the convulsant effect of threshold doses of systemically administered bicuculline (1.5 mg/kg). As was true with intranigral isoniazid, no proconvulsant effect was found using intrastriatal muscimol. Our data demonstrate that whereas striatonigral GABA circuitry can be activated by exogenous treatments so as to produce anticonvulsant actions in most seizure models, suppression of this circuitry does not potentiate convulsant activity in many of the same models.

Involvement of the nigral output pathways in the inhibitory control of the substantia nigra over generalized non-convulsive seizures in the rat

Activation of GABAergic transmission within the substantia nigra has been shown to suppress several forms of generalized seizures in experimental models of epilepsy. More especially, such pharmacological manipulations suppress spontaneous and chemically-induced generalized non-convulsive seizures in the rat. The aim of the present study was to examine the role of the dopaminergic and GABAergic thalamic and collicular nigral outputs in this antiepileptic effect. For this purpose, we examined the effects of output destruction on the antiepileptic effect of intranigral injections of a GABA agonist or pharmacological blockade of the neurotransmission at the nerve terminal level in rats with spontaneous absence seizures. After selective destruction of dopaminergic neurons within the substantia nigra with 6-hydroxydopamine (5 micrograms/side) or hemisection of the ascending nigral output, bilateral intranigral injection of muscimol (2 ng/side) still significantly suppressed generalized non-convulsive seizures. Bilateral lesioning of the ventromedial nucleus of the thalamus did not abolish the antiepileptic effects of intranigral muscimol (2 ng/side) and the GABA antagonist, picrotoxin, when given into this thalamic nucleus (10 ng/side) also failed to induce suppression of spike and wave discharges. The antiepileptic effects of intranigral injection of muscimol (2 ng/side) was reversed by bilateral electrolytic lesions of the superior colliculus. Blockade of the GABAergic transmission at this level with picrotoxin (40 ng/side) significantly suppressed generalized non-convulsive seizures. Finally, excitation of collicular cell bodies with low doses of kainic acid (4 and 8 ng/side) also resulted in a suppression of spike and wave discharges. These results demonstrate that the GABAergic nigrocollicular pathway is critical for the inhibitory control of the substantia nigra over generalized non-convulsive seizures. The data further suggest that antiepileptic effects observed following potentiation of GABAergic transmission in the substantia nigra result from a disinhibition of collicular cell bodies.