The GABAergic nigro-collicular pathway is not involved in the inhibitory control of audiogenic seizures in the rat

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.

Endogenous opioid peptide-mediated neurotransmission in central and pericentral nuclei of the inferior colliculus recruits μ1-opioid receptor to modulate post-ictal antinociception

BACKGROUND

The aim of the present work was to investigate the involvement of the μ1-endogenous opioid peptide receptor-mediated system in post-ictal antinociception.

METHODS

Antinociceptive responses were determined by the tail-flick test after pre-treatment with the selective μ1-opioid receptor antagonist naloxonazine, peripherally or centrally administered at different doses.

RESULTS

Peripheral subchronic (24 h) pre-treatment with naloxonazine antagonised the antinociception elicited by tonic-clonic seizures. Acute (10 min) pre-treatment, however, did not have the same effect. In addition, microinjections of naloxonazine into the central, dorsal cortical and external cortical nuclei of the inferior colliculus antagonised tonic-clonic seizure-induced antinociception. Neither acute (10-min) peripheral pre-treatment with naloxonazine nor subchronic intramesencephalic blockade of μ1-opioid receptors resulted in consistent statistically significant differences in the severity of tonic-clonic seizures shown by Racine's index (1972), although the intracollicular specific antagonism of μ1-opioid receptor decreased the duration of seizures.

CONCLUSION

μ1-Opioid receptors and the inferior colliculus have been implicated in several endogenous opioid peptide-mediated responses such as antinociception and convulsion. The present findings suggest the involvement of μ1-opiate receptors of central and pericentral nuclei of the inferior colliculus in the modulation of tonic-clonic seizures and in the organisation of post-ictal antinociception.

Behavioral and EEG effects of GABAergic manipulation of the nigrotectal pathway in the Wistar audiogenic rat strain

The superior colliculus (SC), substantia nigra pars reticulata (SNPr), and striatum have been characterized as important structures involved in the modulation of seizure activity. In the current study, bicuculline (GABA(A) antagonist) and muscimol (GABA(A) agonist) were microinjected into the deep layers of either the anterior SC (aSC) or posterior SC (pSC) of genetically developed Wistar audiogenic rats. Behavior and EEG activity were studied simultaneously. Only muscimol microinjected into the pSC had behavioral and EEG anticonvulsant effects in Wistar audiogenic rats, eliciting EEG oscillation changes in both SNPr and pSC, primarily during tonic seizures. The SC of Wistar audiogenic rats thus comprises two functionally different subregions, pSC and aSC, defined by distinct behavioral and EEG features. The pSC has proconvulsant audiogenic seizure activity in Wistar audiogenic rats. Our data suggest that this phenomenon may be a consequence of the genetic selection of the Wistar audiogenic rat strain.

Role of the superior colliculus in the expression of acute and kindled audiogenic seizures in Wistar audiogenic rats

PURPOSE

The role of the superior colliculus (SC) in seizure expression is controversial and appears to be dependent upon the epilepsy model. This study shows the effect of disconnection between SC deep layers and adjacent tissues in the expression of acute and kindling seizures.

METHODS

Subcollicular transections, ablation of SC superficial and deep layers, and ablation of only the cerebral cortex were evaluated in the Wistar audiogenic rat (WAR) strain during acute and kindled audiogenic seizures. The audiogenic seizure kindling protocol started 4 days after surgeries, with two acoustic stimuli per day for 10 days. Acute audiogenic seizures were evaluated by a categorized seizure severity midbrain index (cSI) and kindled seizures by a severity limbic index (LI).

RESULTS

All subcollicular transections reaching the deep layers of the SC abolished audiogenic seizures or significantly decreased cSI. In the unlesioned kindled group, a reciprocal relationship between limbic and brainstem pattern of seizures was seen. The increased number of stimuli provoked an audiogenic kindling phenomenon. Ablation of the entire SC (ablation group) or of the cerebral cortex only (ctx-operated group) hampered the acquisition of limbic behaviors. There was no difference in cSI and LI between the ctx-operated and ablation groups, but there was a difference between ctx-operated and the unlesioned kindled group. There was also no difference in cSI between SC deep layer transection and ablation groups. Results of histologic analyses were similar for acute and kindled audiogenic seizure groups.

CONCLUSIONS

SC deep layers are involved in the expression of acute and kindled audiogenic seizure, and the cerebral cortex is essential for audiogenic kindling development.

EEG wavelet analyses of the striatum–substantia nigra pars reticulata–superior colliculus circuitry: Audiogenic seizures and anticonvulsant drug administration in Wistar audiogenic rats (War strain)

The importance of the substantia nigra pars reticulata (SNPr), striatum (STR) and superior colicullus (SC) in the blockade of experimental seizures is well known. But, in audiogenic seizures (brainstem tonic-clonic seizures), the anticonvulsant activity of these nuclei is still controversial. In the present study we aimed to analyze the STR-SNPr-CS circuitry in the audiogenic seizures of Wistar audiogenic rat (WAR). Behavioral and electroencephalographic (EEG) data were collected from WARs under no treatment or injection with systemic (phenobarbital) or intracerebral (intranigral) drugs (muscimol and phenobarbital). The main EEG frequency oscillation of STR, SNPr and SC seen before, during and after audiogenic seizures or during seizure protection, was determinated with wavelet spectral analyses. This method allows the association between behavior and EEG (video-EEG). Audiogenic seizures last only for half a minute in average, suggesting that the interruptions of seizures are probably not due to exhaustion. Systemic phenobarbital caused an acute and dose-dependent behavioral and EEGraphic anticonvulsant effect both in WARs. The dose of phenobarbital 15mg/kg protected animals almost completely, without side effects such as ataxia and sedation. In our data, this endogenous "natural" seizure blockade (or termination) seems to be similar to the "forced" seizure abolition, like the one caused by a systemic non-ataxic phenobarbital dose, because in both cases an intense decrease in the EEG main frequency oscillation can be seen in SNPr and SC. Intranigral phenobarbital or muscimol did not protect animals, and actually induced an increase in the main EEG frequency oscillation in SC. The main finding of the present study is that, in contrast to what is well believed about the incapacity to control audiogenic seizures by the striato-nigro-tectal circuitry, we collected here evidences that these nuclei are involved in the ability to block these seizures. However, the striato-nigro-tectal circuitry in WARs, a genetically developed strain, seems to have different functional mechanisms when compared with normal rats.

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.

A critical review on the participation of inferior colliculus in acoustic-motor and acoustic-limbic networks involved in the expression of acute and kindled audiogenic seizures

The main goal of this article is to review the key role that the inferior colliculus plays in the expression of acoustic-motor and acoustic-limbic integration involved, respectively, in acute and chronic audiogenic seizures. In order to put this in context, we will review the behavioral characterization of acute and chronic audiogenic seizures, neuroanatomical substrates, neurochemistry, neuropharmacology, electrophysiology, as well as the cellular and molecular mechanisms involved in their expression. Secondly, we will also correlate our results, collected from audiogenic seizures susceptible rats, before and after the genetic selection of our own audiogenic susceptible strain, and from those sensitized by lesions or drug microinjections, with those pertinent from the international literature. In brief, genetic or sensitized animals express acute audiogenic seizures as a wild running behavior preceding the onset of tonic-clonic seizures. The latter can have several presentations including opistotonus and fore- and hindlimb tonic hyperextensions, followed by clonic convulsions of fore- and hindlimbs. Chronic (kindled) audiogenic seizures change this behavioral expression, with similar patterns such as those present in temporal lobe epileptic seizures, intermingled with the original audiogenic seizure pattern, which is known to be dependent on brainstem networks.

Deep brain stimulation in epilepsy

Since the pioneering studies of Cooper et al. to influence epilepsy by cerebellar stimulation, numerous attempts have been made to reduce seizure frequency by stimulation of deep brain structures. Evidence from experimental animal studies suggests the existence of a nigral control of the epilepsy system. It is hypothesized that the dorsal midbrain anticonvulsant zone in the superior colliculi is under inhibitory control of efferents from the substantia nigra pars reticulata. Inhibition of the subthalamic nucleus (STN) could release the inhibitory effect of the substantia nigra pars reticulata on the dorsal midbrain anticonvulsant zone and thus activate the latter, raising the seizure threshold. Modulation of the seizure threshold by stimulation of deep brain structures-in particular, of the STN-is a promising future treatment option for patients with pharmacologically intractable epilepsy. Experimental studies supporting the existence of the nigral control of epilepsy system and preliminary results of STN stimulation in animals and humans are reviewed, and alternative mechanisms of seizure suppression by STN stimulation are discussed.

Inhibition of the substantia nigra suppresses absences and clonic seizures in audiogenic rats, but not tonic seizures: evidence for seizure specificity of the nigral control

GABAergic inhibition of the substantia nigra pars reticulata has been shown to suppress seizures in most models of epilepsy involving forebrain networks, such as absences or clonic seizures. No such antiepileptic effects were observed, however, in genetically audiogenic rats exhibiting tonic seizures generated in the brainstem. This suggests a constitutive dysfunction of the nigral GABAergic neurotransmission in this strain of rat or a selective action of the nigral control on specific networks. In the present study, we first confirmed that bilateral injection of muscimol (700 pmol/side) in the substantia nigra had no effect in Wistar rats with audiogenic seizures (Wistar AS). [3H]Muscimol autoradiography suggested a 40% reduced density of GABA(A) receptors in the substantia nigra of Wistar AS, whereas no change was observed in the cortex and the superior colliculus (superficial and intermediate layers), as compared to control animals. In Wistar AS where 40 repetitions of audiogenic stimulations progressively induced generalised convulsive seizures with both tonic and clonic components, bilateral injection of muscimol (350 pmol/side) in the substantia nigra suppressed the clonic component but had no effect on tonic seizures. In hybrid rats issued from cross-breeding between Wistar AS and rats with spontaneous absence seizures, bilateral injection of muscimol (18 pmol/side) in the substantia nigra abolished cortical spike-and-wave discharges, but had no effect on tonic audiogenic seizures at doses up to 700 pmol/side. These results show that despite a decreased number of GABA(A) receptors in the substantia nigra, inhibition of this structure in Wistar AS still leads to inhibition of seizures involving forebrain structures. These results confirm that GABAergic inhibition of the substantia nigra has antiepileptic effects through the control of forebrain circuits. They suggest that this control mechanism has no inhibitory effect on circuits underlying audiogenic tonic seizures.

Effect of precollicular transection on audiogenic seizures in genetically epilepsy-prone rats

Previous studies have demonstrated that generalized tonic-clonic seizures (GTCS) consisting of running/bouncing clonic and tonic extension can still be elicited in rats after brain transections which separate forebrain from brain stem, showing that forebrain circuitry is not required for GTCS. Inasmuch as sound-induced generalized tonic-clonic seizures in rodents are characterized by running-bouncing clonic and tonic convulsions, we have hypothesized that these are brain stem seizures that can occur independently of the forebrain. To test this hypothesis, we examined the response of two strains of genetically epilepsy-prone rats (GEPR-3s and GEPR-9s) to seizure-evoking auditory stimuli 3 h after a precollicular transection or sham surgery performed under ether anesthesia. In addition, the effect of a precollicular transection on audiogenic seizures was evaluated in normal rats made susceptible to such seizures by infusing NMDA into the inferior colliculus. Following the transection 58% of GEPR-9s displayed a sound-induced tonic-clonic convulsion and the remaining 42% exhibited a sound-induced seizure when subjected to stimulation 5 min after a subconvulsant dose of pentylenetetrazol (PTZ). While sham surgery and the precollicular transection both reduced sound-induced seizure severity in GEPR-3s, the full seizure response could be elicited by sound stimulation following a subconvulsant dose of PTZ. Moreover, the audiogenic seizures in normal rats rendered susceptible by NMDA were unaltered by the precollicular transection. These findings show that the anatomical circuitry required for generalized tonic-clonic seizures evoked by sound stimulation in rodents resides within the brain stem.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Neither intranigral fluoxetine nor 5,7-dihydroxytryptamine alter audiogenic seizures in genetically epilepsy-prone rats

Previous studies have shown that widespread depletion of brain 5-hydroxytryptamine (5-HT, serotonin) exacerbates audiogenic seizures in genetically epilepsy-prone rats (GEPRs), while elevations in brain 5-HT attenuate these seizures. However, the location of the central nervous system site(s) at which 5-HT exerts its anticonvulsant action on audiogenic seizures, remains unknown. The substantia nigra has been shown to exert modulatory actions over both brainstem and forebrain driven seizures in normal rats, and receives a rich serotonergic innervation. The present study was designed to determine if 5-HT exerts its modulatory effect on audiogenic seizures by an action in the substantia nigra. Microinfusion of 5,7-dihydroxytryptamine (4 micrograms/0.25 microliter bilateral) into the substantia nigra of GEPRs which display a moderate seizure (GEPR-3s) failed to alter the audiogenic seizure. Consistent with these findings, microinfusions of fluoxetine-HCl into the substantia nigra of severe seizure GEPRs (GEPR-9s) failed to alter any aspect of the audiogenic seizure. This effect was observed when fluoxetine was infused alone, or in combination with systemic administration of 5-hydroxytryptophan (75 mg/kg, i.p.). The present findings argue against a modulatory role of nigral 5-HT on audiogenic seizures in GEPRs.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.

The substantia nigra pars reticulata, seizures and Fos expression

The induction of the proto-oncogene c-fos has been used extensively to identify spatially distributed neural systems activated by seizures. The substantia nigra pars reticulata (SNpr) has been implicated as a critical structure in neural networks involved in the modulation of seizure expression, yet the SNpr has not been reported to express Fos following seizures induced in a variety of seizure paradigms. In this study we determined whether (1) the temporal characteristics of Fos induction in the SNpr were different than those of other brain areas following kindled seizures, (2) neurons in the SNpr possess the cellular machinery to express Fos, (3) Fos can be induced in SNpr by direct electrical stimulation, and (4) Fos expression is induced in the SNpr following kainate or pilocarpine-induced status epilepticus. Results indicate that Fos is not induced in SNpr at any time point (1-12 h) after kindled seizures, and that serum response factor, a constitutively expressed nuclear protein necessary for Fos expression, is present in SNpr neurons. Results further indicate that Fos expression in the SNpr is induced following either direct electrical stimulation or pilocarpine status, but not status elicited by kainate. We conclude that, in so far as the SNpr represents a critical structure for modulating seizure expression, seizure activity does not represent a sufficient stimulus to induce Fos in SNpr neurons. Further, the neural networks defined by Fos expression following seizure may be incomplete, and should be interpreted conservatively.

FG7142 causes opposite changes in [3H]GABA release from nigrocollicular regions

The activity of GABAergic neurons projecting from the striatum to the substantia nigra (SN) and from the SN to the superior colliculus (SC) may be involved in regulating seizure sensitivity such that striatonigral transmission is decreased and nigrocollicular transmission is increased in proconvulsant states. To test whether these changes occur in FG7142-treated rats, GABA transmission was assessed by measuring [3H]GABA release from superfused slices of the SN and SC and measuring [35S]TBPS binding to GABAA receptors throughout the brain. Nine daily injections of FG7142 (30 mg/kg IP) greatly increased myoclonic seizures in about one half of the animals. These animals exhibited a decrease in stimulated [3H]GABA release from the SN and an increase in both basal and stimulated release from the SC. Animals that were less sensitive to FG7142 treatment also had increased collicular release but not decreased nigral release. [35S]TBPS binding was unchanged by FG7142 treatment. Thus, decreased nigral GABA release may contribute to decreased striatonigral transmission after seizure occurrence whereas increased collicular GABA release may contribute to increased nigrocollicular transmission preceding multiple-seizure occurrence.

Neuroethological evaluation of audiogenic seizures and audiogenic-like seizures induced by microinjection of bicuculline into the inferior colliculus. I. Effects of midcollicular knife cuts

Audiogenic seizures (AS) are a model of generalized tonic-clonic seizures. The inferior colliculus (IC) and the GABAergic neurotransmission seems to be the most critical site and neurotransmitter system, respectively, of the auditory midbrain involved in AS origin and development. Thus, audiogenic-like seizures are evoked by GABAA antagonists such as bicuculline (BIC). Wistar audiogenic AS resistant (R) rats were sham-transected through the midcollicular line and microinjected with IC bicuculline (BIC; 80 ng/0.2 microliters) (n = 8); transected through the midcollicular line and microinjected with IC saline 0.9% (n = 8); transected through the cortex above the midcollicular line and microinjected with IC BIC (n = 3); transected through the midcollicular line up to 6.0 mm depth and microinjected with IC BIC (80 ng/0.2 microliters or 120 ng/0.3 microliters (n = 8). Wistar AS susceptible (S) rats were submitted to cortical transections (n = 8) and midcollicular transections (n = 7). Animals were studied by means of an ethological method before and after microinjections and/or transections in order to evaluate possible pathways in the AS-like evoked seizures. Bicuculline-evoked seizures were very similar to those evoked by acoustic stimulation, but lacked the tonic-clonic component. No modification in animal behavior was observed in the presence of sound, once the AS-like behavior was initiated. A small percentage of the animals, however, presented procursive behavior which was increased by sound. The IC BIC-evoked patterns were almost totally blocked by midcollicular but not cortical transections. Furthermore, midcollicular but not cortical transections blocked the tonic-clonic component of AS in genetically S animals without modifying the wild running component. These data suggest that the inferior colliculus-superior colliculus connection may be involved in the sensorimotor transduction necessary for AS-like behaviors.

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.

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.

Possible interaction between the inferior colliculus and the substantia nigra in audiogenic seizures in Wistar rats

Male Wistar rats were tested for sensitivity to audiogenic seizures (AS; 110 dB), using an audiogenic severity index (SI). Sensitive (S) animals were subjected to bilateral lesion of the inferior colliculus (IC) and/or the lateral lemniscus (LL). Resistant (R) animals were subjected to bilateral lesions of the IC, unilateral sequential lesions of the substantia nigra reticulata (SN) and/or IC (contralateral to one another), and unilateral thalamic and sham lesions. Bilateral lesions of the IC and LL abolish AS in S rats. Lesion of the SN resulted in more pronounced sensitivity to AS than unilateral lesion of IC, in R rats. When the SN lesion was contralateral to a previous IC lesion, the effect was not only an increase in the SI, but also a reversal of the asymmetry generated by IC lesion. Although the behavioral effects resulting from IC lesions are due to alterations in the primary structures involved in the origin of AS, unilateral SN lesions can alter critical substrates of sensorimotor integration involved in the control and expression of AS.

Anticonvulsant actions and interaction of GABA agonists and a benzodiazepine in pars reticulata of substantia nigra

In a previous study, infusion of flurazepam and midazolam, but not GABA or muscimol into the pars reticulata of substantia nigra (SNpr), blocked the tonus induced by a high dose of pentylenetetrazol (PTZ). It was hypothesized that the divergence of GABAA agonist and benzodiazepine (BZ) actions might be due to a complex action of the GABAA agonists in SNpr that is not shared by the BZs, and which is specific for certain experimental seizures. This was tested in the present study in which GABA, muscimol, midazolam, and combinations of midazolam with GABA and with muscimol were tested against seizures induced by PTZ (40 or 100 mg/kg, i.p.), bicuculline (0.5 mg/kg, i.v.) and maximal electroshock (MES). Intranigral midazolam was effective against PTZ, bicuculline and MES seizures. Intranigral GABA or muscimol, infused together with midazolam, prevented midazolam from blocking tonus induced by the high dose of PTZ. For bicuculline seizures, midazolam alone had a better overall anticonvulsant effect than did the combination of midazolam plus muscimol. Intranigral muscimol, midazolam and a combination of two were equally effective against the low dose PTZ (40 mg/kg, i.p.) and MES seizures. Since the effect of intranigral drugs was model-specific, it was suggested that different populations of nigral output neurons were involved in regulating the generalization of these seizures.