During afterdischarges in the young rat in vivo extracellular potassium is not elevated above adult levels

Anticonvulsant 4-aminobenzenesulfonamide derivatives with branched-alkylamide moieties: X-ray crystallography and inhibition studies of human carbonic anhydrase isoforms I, II, VII, and XIV

Aromatic amides comprising branched aliphatic carboxylic acids and 4-aminobenzenesulfonamide were evaluated for their inhibition of carbonic anhydrase (CA) isoforms. Of the most anticonvulsant-active compounds (2, 4, 13, 16, and 17), only 13, 16, and 17 were potent inhibitors of CAs VII and XIV. Compounds 9, 14, and 19 inhibited CA II, while 10 and 12 inhibited all isoforms. Structural studies suggest that differences in the active sites' hydrophobicity modulate the affinity of the inhibitors.

Brain development and susceptibility to damage; ion levels and movements

Responses of immature brains to physiological and pathological stimuli often differ from those in the adult. Because CNS function critically depends on ion movements, this chapter evaluates ion levels and gradients during ontogeny and their alterations in response to adverse conditions. Total brain Na(+) and Cl(-) content decreases during development, but K(+) content rises, reflecting shrinkage of the extracellular and increase in the intracellular water spaces and a reduction in total brain water volume. Unexpectedly, [K(+)](i) seems to fall during the first postnatal week, which should reduce [K(+)](i)/ [K(+)](e) and result in a lower V(m), consistent with experimental observations. Neuronal [Cl(-)](i) is high during early postnatal development, hence the opening of Cl(-) conduction pathways may lead to plasma membrane depolarization. Equivalent loss of K(+)(i) into a relatively large extracellular space leads to a smaller increase in [K(+)](e) in immature animals, while the larger reservoir of Ca(2+)(e) may result in a greater [Ca(2+)](i) rise. In vivo and in vitro studies show that compared with adult, developing brains are more resistant to hypoxic/ischemic ion leakage: increases in [K(+)](e) and decreases in [Ca(2+)](e) are slower and smaller, consistent with the known low level of energy utilization and better maintenance of [ATP]. Severe hypoxia/ischemia may, however, lead to large Ca(2+)(i) overload. Rises in [K(+)](e) during epileptogenesis in vivo are smaller and take longer to manifest themselves in immature brains, although the rate of K(+) clearance is slower. By contrast, in vitro studies suggest the existence of a period of enhanced vulnerability sometime during the developmental period. This chapter concludes that there is a great need for more information on ion changes during ontogeny and poses the question whether the rat is the most appropriate model for investigation of mechanisms of pathological changes in human neonates.

Perinatal seizures preferentially protect CA1 neurons from seizure-induced damage in prepubescent rats

Neonatal seizures may increase neuronal vulnerability later in life. Therefore, status epilepticus was induced with kainate (KA) during the first and second postnatal (P) weeks to determine whether early seizures shift the window of neuronal vulnerability to a younger age. KA was injected (i.p.) once (1x KA) on P13, P20 or P30 or three times (3 x KA), once on P6 and P9, and then either on P13, P20 or P30. After 1x KA, onset to behavioral seizures increased with age. Electroencephalography (EEG) showed interictal events appeared with maturation. After 3 x KA, spike number, frequency, spike amplitude, and high-frequency synchronous events and duration were increased at P13 when compared to age-matched controls. In contrast, P20 and P30 rats had decreases in EEG parameters relative to P20 and P30 rats with 1x KA despite that these animals had the same history of perinatal seizures on P6 and P9. In P13 rats with 1x KA, silver impregnation, hematoxylin/eosin and TUNEL methods showed no significant hippocampal injury and damage was minimal with 3 x KA. In contrast, P20 and P30 rats with 1x KA had robust eosinophilic or TUNEL positive labeling and preferential accumulation of silver ions within inner layer CA1 neurons. After 3 x KA, the CA1 but not CA3 of P20 and P30 rats was preferentially protected following 3 or 6 days. Although paradoxical changes occur in the EEG with maturation, the results indicate that early perinatal seizures do not significantly shift the window of hippocampal vulnerability to an earlier age but induce a tolerance that leads to long-term neuroprotection that differentially affects endogenous properties of CA1 versus CA3 neurons.

Regulation of extracellular pH in the developing hippocampus

The increased propensity of the immature brain to have seizures may be due, in part, to an inability to regulate the extracellular ionic environment. Here, we tested the regulation of extracellular pH in the developing hippocampus during and after intense neuronal activity induced by 20 Hz electrical stimulation in vivo. In CA1, stimulus trains to the CA3 region elicited an early alkaline shift associated with a later slow acid shift in all ages tested (2 weeks old, 3 weeks old and adult). In the dentate gyrus in adults, stimulation only elicited acidification. An initial alkalinization was only observed in the dentate gyrus in 2-week-old animals. The rate of recovery of the extracellular pH after termination of the stimulation was slower in the younger animals compared to adults in both CA1 and the dentate gyrus. The results indicate that the extracellular pH is regulated by mechanisms that undergo developmental changes that parallel the development of the dentate gyrus and development of the regulation of extracellular potassium.

Regulation of extracellular potassium in the developing hippocampus

It has long been felt that one of the reasons that the immature brain has an increased propensity to seize is due to an inability to regulate the extracellular potassium ([K+]o). However, the data supporting this hypothesis is controversial. Here, we tested the regulation of [K+]o in urethane-anesthetized juvenile and adult rats during and after synchronized epileptiform activity induced by 20 Hz stimulus trains to the CA3 region. The regulation of [K+]o in CA3 was compared to the same measurements in CA1. Across all age groups, there was no difference between CA1 and CA3. There was a slight decrease in the peak level of [K+]o reached during stimulation only in the youngest age groups tested (PN9-11). There was an age-dependent change in the rate of recovery of [K+]o from the elevated levels. This recovery was slowest in the PN9-11 age group. In all the animals in the PN9-11 group and 3/7 animals in the PN14-15 group, there was a secondary increase in the [K+]o during the afterdischarge. This secondary rise was never observed in animals over 15 days of age. These data confirm that there is altered regulation of [K+]o in the developing brain that takes two forms: (1) the ability to recover from elevated [K+]o levels, suggesting maturation of regulatory, or uptake, mechanisms; and (2) that which is related to mechanisms of synchronization and initiation of afterdischarges and the level of [K+]o that is produced during the discharge.