Sex and the substantia nigra: administration, teaching, patient care, and research

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.

Behavioral and EEG effects of GABAergic manipulation of the nigro-tectal pathway in the Wistar audiogenic rat (WAR) strain II: an EEG wavelet analysis and retrograde neuronal tracer approach

The role of the substantia nigra pars reticulata (SNPr) and superior colliculus (SC) network in rat strains susceptible to audiogenic seizures still remain underexplored in epileptology. In a previous study from our laboratory, the GABAergic drugs bicuculline (BIC) and muscimol (MUS) were microinjected into the deep layers of either the anterior SC (aSC) or the posterior SC (pSC) in animals of the Wistar audiogenic rat (WAR) strain submitted to acoustic stimulation, in which simultaneous electroencephalographic (EEG) recording of the aSC, pSC, SNPr and striatum was performed. Only MUS microinjected into the pSC blocked audiogenic seizures. In the present study, we expanded upon these previous results using the retrograde tracer Fluorogold (FG) microinjected into the aSC and pSC in conjunction with quantitative EEG analysis (wavelet transform), in the search for mechanisms associated with the susceptibility of this inbred strain to acoustic stimulation. Our hypothesis was that the WAR strain would have different connectivity between specific subareas of the superior colliculus and the SNPr when compared with resistant Wistar animals and that these connections would lead to altered behavior of this network during audiogenic seizures. Wavelet analysis showed that the only treatment with an anticonvulsant effect was MUS microinjected into the pSC region, and this treatment induced a sustained oscillation in the theta band only in the SNPr and in the pSC. These data suggest that in WAR animals, there are at least two subcortical loops and that the one involved in audiogenic seizure susceptibility appears to be the pSC-SNPr circuit. We also found that WARs presented an increase in the number of FG+ projections from the posterior SNPr to both the aSC and pSC (primarily to the pSC), with both acting as proconvulsant nuclei when compared with Wistar rats. We concluded that these two different subcortical loops within the basal ganglia are probably a consequence of the WAR genetic background.

Sexually dimorphic expression of KCC2 and GABA function

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate depolarizing effects, which result in activation of calcium-sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors acquire their classical hyperpolarizing signaling. The switch from depolarizing to hyperpolarizing GABA(A)-ergic signaling is triggered through the developmental shift in the balance of chloride cotransporters that either increase (i.e. NKCC1) or decrease (i.e. KCC2) intracellular chloride. The maturation of GABA(A) signaling follows sex-specific patterns, which correlate with the developmental expression profiles of chloride cotransporters. This has first been demonstrated in the substantia nigra, where the switch occurs earlier in females than in males. As a result, there are sensitive periods during development when drugs or conditions that activate GABA(A) receptors mediate different transcriptional effects in males and females. Furthermore, neurons with depolarizing or hyperpolarizing GABA(A)-ergic signaling respond differently to neurotrophic factors like estrogens. Consequently, during sensitive developmental periods, GABA(A) receptors may act as broadcasters of sexually differentiating signals, promoting gender-appropriate brain development. This has particular implications in epilepsy, where both the pathophysiology and treatment of epileptic seizures involve GABA(A) receptor activation. It is important therefore to study separately the effects of these factors not only on the course of epilepsy but also design new treatments that may not necessarily disturb the gender-appropriate brain development.

Abruptly Attenuated Terminal Ictal Pattern in Pediatrics

PURPOSE

We examined the ictal discharges at the end of pediatric seizures and categorized the various patterns. One particular pattern, termed "abruptly attenuated termination" was studied in detail.

METHODS

Ictal segments captured on video-EEG monitoring during a 26-month interval were analyzed for a variety of ictal termination patterns, including one that we rigorously defined as abruptly attenuated termination pattern (AAT). We studied the associations between AAT and the other ictal EEG and clinical features.

RESULTS

AAT was noted in 16 of 200 (8%) pediatric seizures. All 16 were immediately preceded by repetitive spikes or spike-waves. The presence of AAT also correlated with ictal spread pattern, initial ictal pattern, laterality of onset, seizure duration, age, and epilepsy etiology. AAT is more often noted in children older than 6 months and in children with idiopathic or cryptogenic forms of epilepsy.

CONCLUSIONS

The minority of pediatric seizures recorded in a tertiary epilepsy monitoring unit end with diffuse, synchronized abrupt attenuation. AAT probably is the result of an active process that is developmentally related. It appears to require some degree of mature and intact neurophysiology and may involve the thalamocortical circuit.

Sexo cerebral: um caminho que começa a ser percorrido

Fica cada vez mais claro que ocorre um dimorfismo sexual no cérebro de homens e mulheres, e experiências em animais têm mostrado que circuitos específicos se desenvolvem de acordo com o sexo do animal. Desde os trabalhos iniciais de Gorski em ratos, que descreviam o núcleo sexualmente dimórfico na área pré-óptica (SDN-POA), tem sido aceito que, por ação do estradiol, convertido localmente pela aromatase a partir de testosterona, faz-se o imprint para sexo masculino, inibindo-se a apoptose das células do SDN-POA e, portanto, levando a um núcleo anatomicamente maior em machos quando comparado ao de fêmeas. Outras regiões têm mostrado dimorfismo sexual e necessitamos de um marcador para que tais estruturas sejam diferenciadas e possam ser avaliadas na prática clínica. Este dado será de grande valia na atribuição de gênero a pacientes portadores de anomalias da diferenciação sexual, que nascem com ambigüidade genital. Têm havido muitas dúvidas na atribuição do gênero a alguns desses pacientes e não têm sido infreqüentes inadequações sexuais, com mudanças de opções sexuais em época puberal, com grandes traumas tanto para o paciente como para seus familiares. A evolução dos conhecimentos nessa área poderá nos trazer elementos muito importantes para nos auxiliar na atribuição do sexo de criação em vários estados intersexuais e é um caminho que, apesar de estar ainda no seu início, merece ser percorrido.

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.

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.