Regional and age specific effects of zolpidem microinfusions in the substantia nigra on seizures

A disinhibitory nigra-parafascicular pathway amplifies seizure in temporal lobe epilepsy

The precise circuit of the substantia nigra pars reticulata (SNr) involved in temporal lobe epilepsy (TLE) is still unclear. Here we found that optogenetic or chemogenetic activation of SNr parvalbumin⁺ (PV) GABAergic neurons amplifies seizure activities in kindling- and kainic acid-induced TLE models, whereas selective inhibition of these neurons alleviates seizure activities. The severity of seizures is bidirectionally regulated by optogenetic manipulation of SNr PV fibers projecting to the parafascicular nucleus (PF). Electrophysiology combined with rabies virus-assisted circuit mapping shows that SNr PV neurons directly project to and functionally inhibit posterior PF GABAergic neurons. Activity of these neurons also regulates seizure activity. Collectively, our results reveal that a long-range SNr-PF disinhibitory circuit participates in regulating seizure in TLE and inactivation of this circuit can alleviate severity of epileptic seizures. These findings provide a better understanding of pathological changes from a circuit perspective and suggest a possibility to precisely control epilepsy.

The neural circuits through which the substantia nigra pars reticulata (SNr) exerts its role in epilepsy control are not known. Here the authors reveal that a long-range SNr-parafascicular nucleus disinhibitory circuit participates in regulating seizures in temporal lobe epilepsy and inhibition of this circuit can alleviate severity of epileptic seizures.

Limbic circuitry activation in ethanol withdrawal is regulated by a chromosome 1 locus

Physiological dependence and associated withdrawal episodes are thought to constitute a motivational force sustaining alcohol use/abuse and contributing to relapse in alcoholics. Although no animal model exactly duplicates alcoholism, models for specific factors, including the withdrawal syndrome, are useful for identifying potential genetic and neural determinants of liability in humans. We previously identified highly significant quantitative trait loci (QTLs) with large effects on predisposition to withdrawal after chronic and acute alcohol exposure in mice and mapped these loci to the same region of chromosome 1 (Alcdp1 and Alcw1, respectively). The present studies utilize a novel Alcdp1/Alcw1 congenic model (in which an interval spanning Alcdp1 and Alcw1 from the C57BL/6J donor strain [build GRCm38 150.3-174.6 Mb] has been introgressed onto a uniform inbred DBA/2J genetic background) known to demonstrate significantly less severe chronic and acute withdrawal compared to appropriate background strain animals. Here, using c-Fos induction as a high-resolution marker of neuronal activation, we report that male Alcdp1/Alcw1 congenic animals demonstrate significantly less alcohol withdrawal-associated neural activation compared to appropriate background strain animals in the prelimbic and cingulate cortices of the prefrontal cortex as well as discrete regions of the extended amygdala (i.e., basolateral) and extended basal ganglia (i.e., dorsolateral striatum, and caudal substantia nigra pars reticulata). These studies are the first to begin to elucidate circuitry by which this confirmed addiction-relevant QTL could influence behavior. This circuitry overlaps limbic circuitry involved in stress, providing additional mechanistic information. Alcdp1/Alcw1 maps to a region syntenic with human chromosome 1q, where multiple studies find significant associations with risk for alcoholism.

The role of the substantia nigra pars reticulata in kindling resistance in rats with genetic absence epilepsy

OBJECTIVE

Genetic Absence Epilepsy Rats from Strasbourg (GAERS) show a resistance to secondary generalization of focal limbic seizures evoked by kindling. The substantia nigra pars reticulata (SNR) is involved in the propagation and modulation of seizures in kindling. We first examined the role of the SNRanterior and SNRposterior subregions in the resistance to the development of kindling in GAERS. Subsequently, to determine whether kindling resistance relates to differential sensitivity of γ-aminobutyric acid γ-aminobutyric acid (GABA)ergic or dopaminergic SNR neurons to kindling, we studied the effects of kindling-inducing stimulations on parvalbumin (PRV; GABAergic neuron marker) or tyrosine hydroxylase (TH; dopaminergic neuron marker) immunoreactivity (ir), respectively, in GAERS and in nonepileptic control (NEC) Wistar rats that lack kindling resistance.

METHODS

Adult male GAERS were implanted with a stimulation electrode in the amygdala, and bilateral injection cannulas for lidocaine or saline injection (30 min before each kindling stimulation until the animals reached three stage 5 seizures or the 22 stimulations) into the SNRanterior or SNRposterior . In another experiment, PRV-ir in SNRanterior and SNRposterior and TH-ir in SNRposterior only were densitometrically compared in GAERS-SHAM, NEC-SHAM GAERS-STIM, and NEC-STIM animals (6 kindling stimulations).

RESULTS

Bilateral SNRposterior infusions of lidocaine eliminated the kindling resistance and resulted in stage 5 generalized motor seizures in all kindled rats. Bilateral lidocaine infusions in the SNRanterior failed to alter the kindling resistance in GAERS. PRV-ir in the SNRposterior was unaltered in GAERS-STIM but increased in NEC-STIM group. Cellular TH-ir in the SNRposterior significantly increased by kindling stimulations in both NEC-STIM and GAERS-STIM groups.

SIGNIFICANCE

The kindling resistance in GAERS is mediated by the SNRposterior in a lidocaine-sensitive manner. The insensitivity to kindling stimulation of PRV-ir in SNRposterior of GAERS but not NEC rats, implicate GABAergic SNRposterior neurons in kindling resistance. In contrast, the observed stimulation-specific increase in TH-ir in the SNRposterior is unrelated to kindling resistance.

Age- and sex-related characteristics of tonic GABA currents in the rat substantia nigra pars reticulata

Previous studies have shown that the pharmacologic effects of GABAergic drugs and the postsynaptic phasic GABAAergic inhibitory responses in the anterior part of the rat substantia nigra pars reticulata (SNRA) are age- and sex-specific. Here, we investigate whether there are age- and sex-related differences in the expression of the δ GABAA receptor (GABAAR) subunit and GABAAR mediated tonic currents. We have used δ-specific immunochemistry and whole cell patch clamp to study GABAAR mediated tonic currents in the SNRA of male and female postnatal day (PN) PN5-9, PN11-16, and PN25-32 rats. We observed age-related decline, but no sex-specific changes, in bicuculline (BIM) sensitive GABAAR tonic current density, which correlated with the decline in δ subunit in the SNRA between PN15 and 30. Furthermore, we show that the GABAAR tonic currents can be modified by muscimol (GABAAR agonist; partial GABACR agonist), THIP (4,5,6,7-tetrahydroisoxazolo (5,4-c)pyridin-3-ol: α4β3δ GABAARs agonist and GABACR antagonist), and zolpidem (α1-subunit selective GABAAR agonist) in age- and sex-dependent manner specific for each drug. We propose that the emergence of the GABAAR-sensitive anticonvulsant effects of the rat SNRA during development may depend upon the developmental decline in tonic GABAergic inhibition of the activity of rat SNRA neurons, although other sex-specific factors are also involved.

Sex dimorphism in seizure-controlling networks

Males and females show a different predisposition to certain types of seizures in clinical studies. Animal studies have provided growing evidence for sexual dimorphism of certain brain regions, including those that control seizures. Seizures are modulated by networks involving subcortical structures, including thalamus, reticular formation nuclei, and structures belonging to the basal ganglia. In animal models, the substantia nigra pars reticulata (SNR) is the best studied of these areas, given its relevant role in the expression and control of seizures throughout development in the rat. Studies with bilateral infusions of the GABA(A) receptor agonist muscimol have identified distinct roles of the anterior or posterior rat SNR in flurothyl seizure control, that follow sex-specific maturational patterns during development. These studies indicate that (a) the regional functional compartmentalization of the SNR appears only after the third week of life, (b) only the male SNR exhibits muscimol-sensitive proconvulsant effects which, in older animals, is confined to the posterior SNR, and (c) the expression of the muscimol-sensitive anticonvulsant effects become apparent earlier in females than in males. The first three postnatal days are crucial in determining the expression of the muscimol-sensitive proconvulsant effects of the immature male SNR, depending on the gonadal hormone setting. Activation of the androgen receptors during this early period seems to be important for the formation of this proconvulsant SNR region. We describe molecular/anatomical candidates underlying these age- and sex-related differences, as derived from in vitro and in vivo experiments, as well as by [(14)C]2-deoxyglucose autoradiography. These involve sex-specific patterns in the developmental changes in the structure or physiology or GABA(A) receptors or of other subcortical structures (e.g., locus coeruleus, hippocampus) that may affect the function of seizure-controlling networks.

The anticonvulsant response to valproate in kindled rats is correlated with its effect on neuronal firing in the substantia nigra pars reticulata: a new mechanism of pharmacoresistance

Resistance to antiepileptic drugs (AEDs) is a major problem in epilepsy treatment. However, mechanisms of resistance are only incompletely understood. We have recently shown that repeated administration of the AED phenytoin allows selecting resistant and responsive rats from the amygdala kindling model of epilepsy, providing a tool to study mechanisms of AED resistance. We now tested whether individual amygdala-kindled rats also differ in their anticonvulsant response to the major AED valproate (VPA) and which mechanism may underlie the different response to VPA. VPA has been proposed to act, at least in part, by reducing spontaneous activity in the substantia nigra pars reticulata (SNr), a main basal ganglia output structure involved in seizure propagation, seizure control, and epilepsy-induced neuroplasticity. Thus, we evaluated whether poor anticonvulsant response to VPA is correlated with low efficacy of VPA on SNr firing rate and pattern in kindled rats. We found (1) that good and poor VPA responders can be selected in kindled rats by repeatedly determining the effect of VPA on the electrographic seizure threshold, and (2) a significant correlation between the anticonvulsant response to VPA in kindled rats and its effect on SNr firing rate and pattern. The less VPA was able to raise seizure threshold, the lower was the VPA-induced reduction of SNr firing rate and the VPA-induced regularity of SNr firing. The data demonstrate for the first time an involvement of the SNr in pharmacoresistant experimental epilepsy and emphasize the relevance of the basal ganglia as target structures for new treatment options.

Amygdala-specific reduction of alpha1-GABAA receptors disrupts the anticonvulsant, locomotor, and sedative, but not anxiolytic, effects of benzodiazepines in mice

The heterogeneity and distribution of GABA(A) receptor subunits mediates differential roles in behavior. It is thought that particular behavioral responses to benzodiazepine (BZ) ligands might be associated with an action at a regionally defined receptor subtype. However, the role of specific GABA(A) receptor subtypes in particular brain regions is less clear. Such detailed knowledge of regional alpha1-GABA(A) receptor function will advance our understanding of the neural circuitry underlying the role of GABA(A) receptors and the effects of GABA(A)-modulating drugs on behavior. By combining inducible, site-specific alpha1 subunit deletion, using a lentivirus expressing Cre-recombinase in mice with the alpha1 subunit gene flanked by loxP sites, we examine baseline and pharmacological effects of deletion of amygdala alpha1-GABA(A) receptors. We find that amygdala-specific reduction of alpha1 receptor subunits does not affect mRNA or protein levels of amygdala alpha2 or alpha3 subunit receptors. Nor does this inducible reduction affect baseline locomotion or measures of anxiety. However, we also find that this inducible, site-specific deletion does disrupt the normal sedative-locomotor inhibition as well as the anticonvulsive effects, of two distinct BZ-site ligands, diazepam and zolpidem, which is relatively alpha1-subunit selective. These data, using inducible, region and subunit-specific deletion, combined with pharmacogenetic approaches, demonstrate that amygdala expression of the alpha1-GABA(A) receptor subunit is required for normal BZ effects on sedation, locomotion, and seizure inhibition, but not for anxiolysis.

Blockade of androgen receptors is sufficient to alter the sexual differentiation of the substantia nigra pars reticulata seizure-controlling network

The substantia nigra pars reticulata (SNR) controls seizures in a sex-specific manner. At postnatal day 15 (P15), SNR infusion of GABA(A) receptor agonist muscimol have proconvulsant effects in males but not in females. In males, administration of an androgen receptor antagonist flutamide between P0-P2 led to the disappearance of the proconvulsant muscimol effects at P15. Thus, activation of androgen receptors is important for the presence of proconvulsant SNR muscimol responses.

Modulation of synaptic GABAA receptor function by zolpidem in substantia nigra pars reticulata

AIM

The substantia nigra pars reticulata (SNr) constitutes one of the output centers of the basal ganglia, and its abnormal activity is believed to contribute to some basal ganglia motor disorders. Different lines of evidence revealed a major contribution of GABAA receptor-mediated synaptic inhibition in controlling the activity of SNr. The benzodiazepine binding site within the GABAA receptor is a modulation site of significant clinical interest. A high density of benzodiazepine binding sites has been reported in the rat SNr. In the present study, we investigate the effects of activating benzodiazepine binding sites in the SNr.

METHODS

Whole-cell patch-clamp recordings and motor behavior were applied.

RESULTS

Superfusion of zolpidem, a benzodiazepine binding agonist, at 100 nmol/L significantly prolonged the decay time of GABAA receptor-mediated postsynaptic currents. The prolongation on decay time induced by zolpidem was sensitive to the benzodiazepine antagonist flumazenil, confirming the specificity on the benzodiazepine site. Zolpidem at 1 micromol/L exerted a stronger prolongation on the decay time. A further experiment was performed on behaving rats. A unilateral microinjection of zolpidem into the rat SNr caused a robust contralateral rotation, which was significantly different from that of control animals receiving the vehicle injection.

CONCLUSION

The present in vitro and in vivo findings that zolpidem significantly potentiated GABA currents and thus inhibited the activity of the SNr provide a rationale for further investigations into its potential in the treatment of basal ganglia disorders.

The role of substantia nigra pars reticulata in modulating clonic seizures is determined by testosterone levels during the immediate postnatal period

GABAergic activation of substantia nigra pars reticulata (SNR) at postnatal day (PN) 15 has sex-specific features on seizure control in vivo and electrophysiological responses in vitro. In males, the GABA(A)-receptor agonist muscimol has proconvulsant effects and induces depolarizing responses. In females, muscimol has no effect on seizures and evokes hyperpolarizing responses. We determined the time period during which sex hormones must be present to produce the sex-specific muscimol effects on seizures and their influence on SNR GABA(A) receptor-mediated postsynaptic currents. Exposure to testosterone or its metabolites (estrogen or dihydrotestosterone) during PN0-2 in females or males castrated at PN0 was sufficient to produce proconvulsant muscimol effects but did not affect the in vitro GABA responses, which remained hyperpolarizing. The data suggest that the PN0-2 period is critical for the development of the seizure-controlling SNR system; the hormonal effect on seizure control is independent from their effect on GABA conductance.

Sex-specific control of flurothyl-induced tonic-clonic seizures by the substantia nigra pars reticulata during development

The substantia nigra pars reticulata (SNR) plays an important age- and sex-specific role in control of clonic seizures. Its involvement in control of tonic-clonic seizures is contradictory. We investigated the role of the SNR in the tonic-clonic seizures induced in male, female and neonatally castrated male rats using flurothyl. In adult female rats, vaginal impedance determined the changes in progesterone/estrogen ratio. Rats at various postnatal ages received infusions of muscimol or vehicle in the SNRanterior or SNRposterior. Furthermore, in 15-day-old (P15) and adult male rats, ZAPA (a GABA(A) receptor agonist) or AP7 (an NMDA receptor antagonist) was infused. The developmental profile of tonic-clonic seizure threshold differed between male and female rats possibly due to early postnatal testosterone surge in male rats. On the other hand, changing estrogen/progesterone ratio in cycling adult female rats had no effect on seizure threshold. Intranigral muscimol had proconvulsant effects on tonic-clonic seizures only in immature rats, and this effect was dependent on the perinatal testosterone surge. ZAPA had anticonvulsant effects in P15 rats but was not effective in adult rats. Only AP7 had anticonvulsant effects in both adult and P15 rats. Results indicate that thresholds for flurothyl-induced tonic-clonic seizures develop under the control of postnatal testosterone. Although GABAergic inhibition in the SNR affects tonic-clonic seizures in developing rats, only the NMDA antagonist had consistent anticonvulsant effects throughout development.

Anticonvulsant effects of GABA(A) modulators microinfused into area tempestas or substantia nigra in rats exposed to soman

Enhancement of GABAergic neurotransmission has anticonvulsant effects against nerve agent-induced seizures. However, systemic administration of drugs with GABA(A) agonist-like effects does not differentiate well between their anticonvulsant impact. In the present study, GABA(A) modulating drugs (1 microl) were microinfused bilaterally into the seizure controlling substrates, substantia nigra (SN) or area tempestas (AT), of rats subjected to seizures induced systemically by soman (100 microg/kg). The results showed that infusion of ethanol (0.47 micromol) and propofol (20 microg) in both SN and AT icrohad anticonvulsant effects (prevention of seizures or increased latency to seizures). Anticonvulsant effects were also obtained when muscimol (120 ng) was infused into AT or when diazepam (5 microg) was infused into SN. Pentobarbital (50 microg) did not attenuate soman-elicited seizures in any of the injection sites. Results from control experiments showed that the effects from the microinfusions were site-specific, and that the absence of effects of pentobarbital was not a result of too low dose of the drug. The microinfusion technique may allow a more detailed examination of anticonvulsant properties of drugs than by the use of systemic administration.

Seizures in the Developing Brain

PURPOSE

Development and sex hormones are important determinants of seizure susceptibility. Seizures develop in the immature brain more readily than in the mature brain. Male children experience a higher incidence of epilepsy or unprovoked seizures than do female children. Sex-specific differences in the development of seizure-suppressing neuronal networks may account, at least in part, for this increased age- and sex-related susceptibility to seizures. The control of seizures can be influenced by the substantia nigra pars reticulata (SNR) in an age- and sex-specific manner. In the adult male rat SNR, two topographically discrete regions (SNRanterior and SNRposterior) mediate distinct effects on seizures, by using divergent output networks in response to localized infusions of gamma-aminobutyric acid (GABA)A agents, such as muscimol. The GABAA-sensitive "anticonvulsant" region is located in the SNRanterior, whereas the GABAA-sensitive "proconvulsant region is in the SNRposterior. In immature postnatal day (PN)15-21 male rats, the SNR is not topographically segregated, and GABAAergic drug infusions produce similar effects when applied in the SNRanterior or SNRposterior. Only a GABAA-sensitive proconvulsant network is evident. By contrast, female SNR does not contain any region that mediates muscimol-related proconvulsant effects. As with the adult, immature female rats do not develop a proconvulsant SNR region at any age.

METHODS

We measured the effects of SNR muscimol infusions on seizures in male rats castrated at birth to better understand the effects of testosterone on the formation of age- and sex-specific features of the SNR.

RESULTS

Neonatal castration permanently alters the maturation of the muscimol-sensitive SNR effect on seizures. The SNR of neonatally castrated rats develops functionally like the "female" SNR. The "proconvulsant" SNR region does not develop in the absence of testosterone in the immediate postnatal period. The "male" type of SNR effects can be induced in neonatally castrated rats by restoration of testosterone levels or in female rats by artificially increasing testosterone levels. Dihydrotestosterone and estrogen, produced by the reduction and aromatization of testosterone, respectively, are the direct mediators of testosterone actions. At PN0, only beta estrogen receptors are equally expressed in the SNRs of males and females and may be responsible for testosterone-mediated effects in both sexes.

CONCLUSIONS

The phenotype of SNR GABAergic neurons, as characterized by GABAA-receptor subunit composition, by muscimol-induced electrophysiologic responses, and by connectivity of output networks each may be altered by the presence of testosterone. Higher KCC2 messenger RNA (mRNA) expression in female PN15 SNR neurons compared with males may be responsible for sex-related differences in muscimol-induced electrophysiologic responses. In summary, a growing body of compelling evidence identifying sex-related differences in the SNR implicates postnatal testosterone as a critical factor in the development of pro- or anticonvulsant circuits. The recognition of sex- and age-related features in the SNR holds the promise that these findings can be translated into the development of specific and effective treatments for seizure disorders.

Subregional changes in discharge rate, pattern, and drug sensitivity of putative GABAergic nigral neurons in the kindling model of epilepsy

The substantia nigra pars reticulata (SNr) is thought to act as a seizure-gating mechanism in kindling and other epilepsy models. We investigated whether the kindling process induces site-specific (anterior-posterior) and seizure-outlasting alterations in the activity of putative GABAergic SNr neurons and in their response to pharmacological manipulation. Female Wistar rats were kindled via the basolateral amygdala by daily stimulation. In vivo extracellular single unit recordings of SNr neurons were performed in kindled rats 1 day after a generalized seizure in order to examine activity changes that outlast the kindled seizures. Sham-kindled and naive rats served as controls. We found a significant and seizure-outlasting increase of discharge rates within the posterior but not within the anterior SNr of kindled rats when compared to controls. Furthermore, kindling resulted in seizure-outlasting burst-like firing pattern of SNr neurons. The antiepileptic drug valproic acid (VPA; 100 mg/kg i.v.) significantly reduced SNr discharge rates in all animal groups. Interestingly, neurons located in the anterior SNr of kindled rats were significantly less depressed by VPA compared to the reduction obtained in naive controls. The present data disclose kindling induced functional plasticity within basal ganglia regions. The findings are relevant for a better understanding of the mechanisms underlying the seizure-gating function of the SNr and might provide new targets for rational therapeutic manipulations, which aim to establish a remote control of epileptic seizures.

Modifications of local cerebral glucose utilization in thalamic structures following injection of a dopaminergic agonist in the nucleus accumbens—involvement in antiepileptic effects?

Dopaminergic transmission in the nucleus accumbens (NAcc) is implicated in different aspects of reward and motivational mechanisms. More recently, it has been suggested that this nucleus could also be involved in the modulation of generalized epileptic seizures. In particular, microinjection of dopaminergic agonists in the NAcc suppresses the occurrence of epileptic seizures in a model of absence seizures, the GAERS (generalized absence epileptic rats from Strasbourg). The aim of this study was to identify the structures involved in this effect. Local cerebral metabolic rates for glucose utilization (LCMRglc) were measured in different parts of the basal ganglia and output structures after apomorphine injection in the NAcc in GAERS and in the inbred non-epileptic rats (NE), concomitantly with seizure suppression. Apomorphine injection in the NAcc induced a significant increase of glucose intake in the anteromedial, mediodorsal and ventrolateral nuclei of the thalamus in NE rats, while no significant changes were observed in the basal ganglia structures (globus pallidus, subthalamic nucleus, substantia nigra). Furthermore, microinjections of muscimol (100 and 200 pmol/side) in the mediodorsal nucleus of the thalamus in GAERS rats suppressed seizures. These results suggest that the mediodorsal nucleus of the thalamus could be involved in absence seizures modulation. Along with data from the literature, our data suggest that this nucleus could participate in the control of the basal ganglia over generalized epileptic seizures.

GABA-B receptor activation in the rat globus pallidus potently suppresses pentylenetetrazol-induced tonic seizures

To determine the involvement of the globus pallidus in mediating epilepsy, the effects of microinjection of a GABA uptake blocker (tiagabine), a benzodiazepine agonist (zolpidem) and a GABA-B receptor agonist (baclofen) on pentylenetetrazol (PTZ)-induced tonic seizure were examined in adult rats. Administration of PTZ induced tonic seizures in all control animals, accompanied with a 100% mortality rate. Pretreatment with bilateral intrapallidal microinjection of tiagabine (1 mM) suppressed the incidence of tonic seizures to 67.7% and reduced the mortality rate to 16.7%. Furthermore, the latency to tonic seizures was 1,275 +/- 277 s, which was significantly longer than that of the corresponding control animals (319 +/- 225 s). On the other hand, administration of zolpidem (1 mM) was without significant effects on the mortality rate, the incidence and latency of the tonic seizure. In sharp contrast, microinjection of baclofen (1 mM) completely suppressed PTZ-induced tonic seizures and reduced the mortality rate to 0%. This effect was largely abolished by co-injection of the GABA-B receptor antagonist CGP55845. To elucidate the direct cellular action of baclofen, the excitability and membrane potential of globus pallidus neurons were studied by cell-attached and whole-cell patch-clamp recordings in the brain slice. Bath application of baclofen (50 microM) significantly reduced the firing of these neurons, and was often accompanied by a clear membrane hyperpolarization, in a CGP55845-sensitive manner. These data suggest that activation of GABA-B receptors in globus pallidus could significantly modulate PTZ-induced tonic seizures.

Behavioral effects of bicuculline microinjection in the dorsal versus ventral hippocampal formation of rats, and control of seizures by nigral muscimol

This work aims to describe behavioral/electroencephalographic (EEG) seizures induced by bicuculline microinjection intracerebroventricularly (ICV) and in the dorsal hippocampal formation (DHF) or ventral hippocampal formation/amygdala area (VHF-AMY). We also test if GABAergic manipulation in the substantia nigra pars reticulata (SNPR) is capable of controlling those seizures. ICV injection of bicuculline induced a progressive sequence of convulsive responses, jumps and escapes from the open-field. This effect was partially reached by bicuculline injection in the DHF or VHF-AMY injection. Also: muscimol injection, but not GABA uptake blockers (nipecotic acid or a spider venom neurotoxin FrPbA2), into the SNPR abolished seizures induced by bicuculline injection in the DHF. It was concluded that different neuronal circuitry in the hippocampal formation are modulated, at least partially by nigral GABAergic mechanisms.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Sex differences in androgen and estrogen receptor expression in rat substantia nigra during development: an immunohistochemical study

Gonadal hormones are important regulators of sexual differentiation of the CNS. Exposure to testosterone and estrogen during development causes permanent organizational differences between males and females. We previously described functional sex-related differences of the GABA(A)ergic circuits of the rat substantia nigra pars reticulata (SNR) involved in the control of flurothyl seizures. This sexual differentiation of the SNR is regulated by postnatal testosterone. To assess whether the organizing effects of testosterone in the SNR are mediated via the androgen receptor (AR) and/or estrogen receptors (ER), we used immunohistochemistry to study the ontogeny of AR, ERalpha and ERbeta expression in SNR and substantia nigra pars compacta (SNC) of male and female rats. Rats on the day of birth [postnatal day (PN) 0] and at PN1, PN5, PN15 and PN30 were used. AR- and ERbeta-immunopositive cells were present in SNR and SNC in both sexes and at all ages. ERalpha was not detected in male and female SNC at PN0-PN1. In both substantia nigra (SN) regions, there were developmentally regulated sex differences in AR, ERalpha and ERbeta immunoreactivity. In the SN, each receptor showed specific intracellular localization: AR was present in the nucleus, ERalpha and ERbeta were present both in nuclear and extranuclear compartments. ERalpha was detected also in processes. At PN0-PN1, quantitative analysis revealed sex and regional differences in the distribution of SN cells expressing AR and ERalpha, while ERbeta were equally present in both sexes. The presence of gonadal steroid receptors in the SN suggests that the biological effects of gonadal hormones in the CNS extend beyond reproduction-related functions and may affect and modify motor behaviors (including seizures) in a sex-specific manner. Based on the ontogeny of SNR ERbeta, we hypothesize that postnatal injections of testosterone may regulate the nigral GABA(A) system through the aromatization pathway and activation of ERbeta.

Electrical stimulation of substantia nigra pars reticulata is anticonvulsant in adult and young male rats

Electrical stimulation of deep brain structures has been used for pain relief and treatment of refractory Parkinson's disease. Recently, stimulation of the subthalamic nucleus or anterior nuclei of the thalamus was introduced for the treatment of refractory epilepsy when other treatments failed. The substantia nigra pars reticulata (SNR) is another crucial site involved in the control of seizures. We studied the effects of continuous electrical stimulation of the SNR as a function of age in male rats. Adult [postnatal day (PN) 60] and young (PN 15) rats with electrodes symmetrically implanted in the SNR were used. The rats were stimulated with continuous constant current pulses (130 Hz) and simultaneously challenged with flurothyl to induce seizures. Control rats had the electrodes implanted but were not stimulated. High-frequency electrical stimulation of the SNR had anticonvulsant effects in both age groups. However, we identified age-specific features: In PN 60 rats, both unilateral and bilateral stimulation of the anterior region of the SNR produced anticonvulsant effects against clonic seizures, while stimulation of the posterior region of the SNR was ineffective. Stimulation of either SNR region had no effects on tonic-clonic seizures. In PN 15 rats, irrespective of the stimulation site within the SNR, bilateral stimulations of the SNR produced anticonvulsant effects against both clonic and tonic-clonic flurothyl-induced seizures, while unilateral stimulation was without effect. The data suggest that the SNR may be a candidate site for deep brain stimulation for the treatment of epilepsy.

Lack of robust anticonvulsant effects of muscimol microinfusions in the anterior substantia nigra of kindled rats

The substantia nigra pars reticulata is thought to control the spread of seizures in various seizure models. Potentiation of gamma-aminobutyrate (GABA)-mediated transmission in this region by intranigral administration of drugs such as muscimol has been shown to inhibit seizure propagation in such models, including the kindling model of epilepsy. More recent studies have shown that the effects on seizures are site-specific within the substantia nigra pars reticulata. Using flurothyl to induce clonic seizures, it was reported that bilateral microinfusions of muscimol into the anterior substantia nigra pars reticulata were anticonvulsant, while similar infusions into the posterior pars reticulata were proconvulsant. This prompted us to reevaluate the effects of intranigral muscimol in the kindling model with particular emphasis on the anterior substantia nigra pars reticulata. In amygdala kindled rats, muscimol was bilaterally infused into the anterior pars reticulata at doses of either 60 or 120 ng. Thirty minutes later, the threshold for induction of afterdischarges in the amygdala and the threshold for generalized seizures were determined in each rat. Furthermore, severity and duration of seizures at threshold currents were recorded. Unexpectedly, muscimol failed to increase seizure thresholds or to significantly reduce seizure severity or duration of motor seizures, although there was a moderate reduction in motor seizure duration in several rats. The data indicate that, in contrast to flurothyl seizures, in kindled rats the anterior pars reticulata of the substantia nigra is not a site at which muscimol causes robust anticonvulsant effects.

Sexual dimorphism and developmental regulation of substantia nigra function

The substantia nigra is an important brain nucleus involved in the expression of movement disorders and seizures. The two most common movement disorders affecting the substantia nigra, Parkinson's disease and Tourette syndrome, show gender differences and age-related onset. To assess the substrates for the gender and age specificity of substantia nigra-related disorders, we determined the functional properties of the substantia nigra gamma-aminobutyric acid (GABAA) system along its anterior-posterior axis, using localized microinfusions of muscimol (a GABAA agonist) and susceptibility to motor seizures in rats. In the substantia nigra, there are sex-specific differences in the topographic segregation and functionality of GABAA systems. In mature male rats there are two distinct regions mediating opposite effects on seizures; in female rats there is only one region that can affect seizures. In the neonatal period, the presence of circulating testosterone is essential for the development of a substantia nigra region that exerts proconvulsant effects throughout the rat's life, a unique feature of the male substantia nigra. The final maturation of the substantia nigra occurs in the peripubertal period, and is in part regulated by testosterone as well. The recognition of the existence of distinct sex- and age-specific substantia nigra features can be translated into new cures of disorders affecting the substantia nigra.

Special considerations in treating children with epilepsy

The incidence of seizures is high in infants and children. Many epileptic syndromes have their onset early in life. The increase in seizure susceptibility of the immature brain may be due to several factors, including an imbalance between excitatory and inhibitory processes, age-specific differences in ionic transport and clearance systems, high incidence of epileptogenic stimuli early in life, and the age-specific expression of pre- and perinatal brain anomalies. All of these factors must be taken into account when developing safe and effective age-specific antiepileptic drugs (AEDs). The use of developmental epilepsy models, followed by clinical trials in children, may help identify such AEDs.

Substantia nigra pars reticulata lesion induces preconvulsive behavior and changes in glutamate receptor gene expression in the rat brain

The substantia nigra pars reticulata (SNpr) has been proposed to play an important role in the control of the propagation and/or the generation of epileptic seizures. Earlier studies have shown differential effects of the lesion of the SNpr on seizure genesis that demonstrated a regional difference in the anterior and posterior parts of the SNpr in preconvulsive behavior induced by unilateral reticulata injection of dopamine (DA). This study was aimed to investigate some of the underlying mechanisms of the preconvulsive behavior elicited by unilateral SNpr DA injection by the study of changes in the gene expression of glutamate receptor subunits (GluR1, GluR2 and NMDAR1) and of changes in animal behavior following coinfusion of DA and a DA D1 antagonist SCH 23390 into the SNpr. Unilateral injection of exogenous DA into the anterior region of the SNpr induced rapid and short lasting preconvulsive behavior up to wet dog shakes stage and a significant reduction of gene expression for GluR1, GluR2 and NMDAR1 subunits in rat hippocampal subfields including CA1 through CA4 and dentate gyrus (DG) at 1 day after nigral DA injection. The effect was long lasting and persisted for at least 3 weeks. Both preconvulsive behavior and downregulation of glutamate receptor subunit genes were completely blocked by simultaneous coinfusion of DA and SCH 23390. The results suggest, for the first time, that DA D1 receptor in the SNpr may mediate the nigral-involved seizure development. Glutamate desensitization, and/or selective early neuronal damage might be responsible for the downregulation of glutamate receptor subunits by transient preconvulsive activity.

Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Epileptic syndromes frequently start at childhood and therefore it is crucial to test new anticonvulsants at immature stages of the nervous system. We compared the effects of the gamma-aminobutyric acid (GABA) uptake inhibitor tiagabine [(R)-N-(4, 4-bis(3-methyl-2-thienyl)but)3-en-1-yl nipecotic acid] on low-Mg(2+)-induced epileptic discharges in brain slices from rat pups (p 5-8) and juvenile animals (p 15-20). In tissue from rat pups, tiagabine slightly reduced epileptiform activity in hippocampal area CA1 but had no effect in the entorhinal cortex. In juvenile rats, epileptiform discharges were unaffected in CA1 but suppressed by 60% in the entorhinal cortex. While tiagabine increases its efficacy with age, in-situ hybridisation and PCR analysis show that mRNA coding for the neuronal GABA-transporter GAT-1 is already present at p 5. We therefore conclude that the increasing efficacy of tiagabine during ontogenesis is due to functional maturation of GABAergic synapses rather than to up-regulation of GAT-1 expression.

The expression of GABA(A) receptor subunits in the substantia nigra is developmentally regulated and region-specific

The substantia nigra pars reticulata (SNR) controls the spread of seizures. GABA(A)ergic drug (agonist or antagonist) infusions into the SNR have age-specific and site-specific effects on flurothyl-induced seizures. Developmental and cell-specific regulation of GABA(A) receptor subunit expression may be responsible for these specific effects. To test this hypothesis, in situ hybridization was used to examine regional expression of alpha1 and gamma2L GABA(A) receptor subunit mRNAs in the SNR during development. Distinct temporal and spatial patterns of expression were observed. In rats at postnatal days (PN) 21-60, fewer neurons were labeled with probes directed to alpha1 and gamma2L subunits in SNRanterior compared with SNRposterior. In addition, neurons in SNRanterior contained higher amounts of hybridization grains than in SNRposterior. In PN 15 rats, the labeling of neurons was relatively diffuse throughout the anterior and posterior SNR regions with moderate amounts of hybridization grains for both subunits. The finding of age-related differential distribution of alpha1 and gamma2L subunit mRNAs in the SNR suggests that GABA(A) receptor heterogeneity may play a role in the age-specific and site-specific effects of GABA(A)ergic agents on seizures in the SNR.