The proconvulsant effect of nigral infusions of THIP on flurothyl-induced seizures in rat pups

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.

Pharmacokinetic considerations in the treatment of childhood epilepsy

Organogenesis throughout childhood affects almost every aspect of pediatric pharmacotherapy. The antiepileptic drugs (AEDs) are particularly impacted since most elimination rates are diminished for the first 6 months of infancy, but quickly attain and supersede adult values. When children enter a hypermetabolic stage, large doses of AEDs may be necessary to maintain effective serum concentrations. Medication noncompliance is frequently confused as hypermetabolism, since both present with low serum drug concentrations. Amazingly, noncompliance among children with chronic illness approaches a similar incidence to that reported in the adult population. It is obviously important to include this in the differential diagnosis of the etiology of subtherapeutic serum AED concentrations. Maturational differences also affect gastrointestinal drug absorption. Intestinal transit time and absorptive surface area are both diminished in young children. Drug delivery systems suitable in adults may not deliver the total dosage in children. Differences in the composition of body compartments and protein binding can alter the volume of drug distribution and, consequently, serum concentrations. In addition to pathophysiologic changes, there is evidence to suggest differences between a mature and immature brain. These differences include quantitative and qualitative responses to neurotransmitters. Hence, it is understandable why seizure semiology is different in children compared with adults. This constellation of factors contributes to the challenges of caring for children with epilepsy.

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

The immature brain is most susceptible to the development of seizures. The substantia nigra may play a crucial role in the control of seizures as a function of age. In the adult substantia nigra pars reticulata (SNR), there are two regions that mediate opposing effects on seizures after infusions of GABA(A) agents. One region is located in the anterior SNR, and localized muscimol infusions mediate anticonvulsant effects. These anticonvulsant effects use a circuitry that may involve the ventromedial thalamic nucleus, the deep layer of the superior colliculus, or both. The second region is in the posterior SNR, and muscimol infusions produce proconvulsant effects, perhaps mediated by the striatum, the globus pallidus, the deep layer of the superior colliculus, or all three. In developing male rats, only the proconvulsant region is present up to the age of 21 days. In ongoing studies, it has been shown that, in the male rat, the transition from the immature to mature SNR-mediated seizure control occurs between the ages of 25 and 30 days, just before adolescence. In male rats castrated on the day of birth, the ensuing depletion of testosterone accelerates the development of the anterior SNR with its anticonvulsant features. Castration does not alter the development of the proconvulsant region. In the developing female SNR, muscimol infusions produce only anticonvulsant effects. The data indicate that gonadal hormones may have an important role in the maturation of systems involved in the containment of seizures.

An indirect method for absorption rate estimation: flurothyl-induced seizures

This paper develops a method to estimate a minimal amount of flurothyl necessary to induce the seizures (the seizure threshold). A simple mathematical model is proposed which permits one to determine the drug absorption rate from the amount which has been administered and from the measured latency to onset of seizure. Experimental animal (rats) were exposed to a continuous intake of flurothyl in two different situations: either being alone in the airtight chamber or sharing it in a pair. In the latter case, we assume that the two rats uniformly share the infused drug. Our calculations estimate that approximately 20 microliters of flurothyl is necessary to induce twitches, whereas 25 microliters of flurothyl is the dose required for the induction of clonic seizures. The model can be used to estimate the threshold amounts of any drug producing obvious behavioral changes irrespective of the route of administration.

Developmental regulation of regional functionality of substantial nigra GABAA receptors involved in seizures

GABAergic (gamma-aminobutyric acid) transmission in the substantia nigra pars reticulata is critical for seizure control. We tested the hypothesis that there is a differential regional distribution and functionality of nigral GABAA receptor sites that is developmentally regulated. In adult rats, we determined the effects on flurothyl seizures of (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid (ZAPA, a presumed agonist of the low-affinity GABAA receptor site), bicuculline (an antagonist of the low-affinity GABAA receptor site) and gamma-vinyl-GABA (a GABA-transaminase inhibitor), infused bilaterally in anterior or posterior substantia nigra pars reticulata. ZAPA infusions (8 micrograms) were anticonvulsant in anterior substantia nigra but proconvulsant in posterior substantia nigra. Bicuculline infusions (100 ng) were proconvulsant in anterior substantia nigra but ineffective in posterior substantia nigra. An anticonvulsant dose of gamma-vinyl-GABA, when infused in anterior substantia nigra, was proconvulsant when infused in posterior substantia nigra. In 15 day old rats, the effects of ZAPA, were biphasic: 2 micrograms was anticonvulsant while 8 micrograms was proconvulsant. There was no regional specificity. The data suggest that with maturation there is functional segregation of specific GABAA receptor subtypes involved in substantia nigra-mediated seizure control.

Age-related differences in the effects of GABAA agonists microinjected into rat substantia nigra: pro- and anticonvulsant actions

GABAergic transmission in the substantia nigra pars reticulata (SNR) has an important role in the control of experimental seizures. In the flurothyl seizure model, SNR microinjection of the selective GABAA receptor agonist muscimol results in a biphasic dose-response curve in adults: Intermediate doses are anticonvulsant, but high doses have proconvulsant effects. Another GABAA agonist, THIP (4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol), also produces anticonvulsant effects at lower doses, whereas higher doses tend to produce a proconvulsant effect. In 16-day-old rat pups, no anticonvulsant but only proconvulsant effects of muscimol occur, and at lower doses than in adults. These data suggest that the immature SNR is significantly more sensitive to the proconvulsant effects of GABAA receptor agonists than is the SNR of adults. We hypothesize that the age-related differences in nigral GABAergic response may be due to ontogenic changes in GABAA-sensitive neuronal circuits in the SNR.

The development of epilepsy in the paediatric brain

The immature central nervous system (CNS) is more susceptible to the development of seizures than its adult counterpart. Developmental studies of experimental seizures have suggested that young animals have unique behavioural seizure patterns, including the presence of bilateral, though asymmetric, convulsions. There are differences in the mechanisms responsible for the generation of seizures, propagation patterns and seizure arrest and recurrences. These differences are due to local factors as well as factors that affect neural systems consisting of long neuronal circuits. The substantia nigra, a site involved in the control of seizures, will be used as an example to demonstrate how evolving neurobiological processes modulates the suppression or exacerbation of seizures with age. Evidence will also be presented indicating that early in life, seizures may not produce hippocampal damage. An understanding of the age-related differences is important for the development of rational approaches to treating seizures and their consequences.

Maturation and segregation of brain networks that modify seizures

The mature brain is less susceptible to seizures than the immature brain. We demonstrate that in the mature substantia nigra (SN) there are two topographically discrete GABAA-sensitive regions which differ in the amount of mRNA expression of the GABAA receptor alpha 1 subunit. These two regions mediate separate anticonvulsant and proconvulsant effects and use divergent projection networks. By contrast, in the immature SN there is no special topography of mRNA expression of the alpha 1 subunit and only the proconvulsant network is present. The decreased seizure susceptibility of the mature brain may be related to postnatal segregation of GABAA-sensitive networks.

Developmental principles of antiepileptic drug therapy

Antiepileptic drug therapy in children can be challenging because seizure semiology and drug disposition change dramatically over time. Close monitoring of drug clearance is required in early life because of the rapid ontogenesis of drug-eliminating organs. Large variations in serum peak and trough antiepileptic drug concentrations increase the risk for both adverse drug effects and unsatisfactory seizure control. Antiepileptic drug adverse effects also differ, probably relative to different receptor sensitivities in children. Failure to appreciate the developmental differences between children and adults can result in medication noncompliance, breakthrough seizures, and unacceptable adverse effects.

Further evidence of involvement of substantia nigra GABAB receptors in seizure suppression in developing rats

To determine whether the substantia nigra GABAergic anticonvulsant effects depend on GABAB receptor activation, we tested the effects of intranigral CGP 35,348 (a GABAB receptor antagonist) alone or in combination with gamma-vinyl-GABA (GVG; a GABA-transaminase inhibitor) on flurothyl seizures in rat pups and adult rats. In pups, nigral infusions of CGP 35,348 decreased the thresholds for clonic and tonic seizures and attenuated the anticonvulsant effects of GVG. In adults, nigral infusions of CGP 35,348 did not alter seizure thresholds. The data suggest that, in rat pups, nigral GABAB receptors regulate, in part, flurothyl-induced seizures.

Baclofen prevents rapid amygdala kindling in adult rats

This study investigates the effect of the gamma-aminobutyric acid (GABAB) agonist, baclofen, on amygdala kindling in adult rats. Baclofen has been reported to be anticonvulsant in a variety of seizure models and prevents kindling in immature rats. These experiments describe the effects of baclofen (2, 5 and 10 mg/kg, i.p.) on the afterdischarge threshold and kindling rate. Baclofen, 10 mg/kg, significantly increased the afterdischarge threshold in the amygdala. Baclofen at 5 and 10 mg/kg, retarded the rate of kindling as measured by the number of stimuli required to advance to subsequent seizure stages. These results suggest that baclofen may decrease the local excitability of the amygdala and retard the rate of seizure spread (or generalization) throughout the brain. Baclofen, acting at GABAB receptors exerts an anticonvulsant effect on amygdala kindling in these experiments.