Abstracts

RATIONALE: Mutations in K+ channel subunits KCNQ2 and KCNQ3 are independently linked to benign familial neonatal convulsions (BFNC), an idiopathic epilepsy that occurs in newborns and spontaneously remits after several weeks or months. KCNQ2 and 3 underlie the M-current, a voltage-dependent sustained K+ current that is critical in regulating neuronal excitability. The mutations associated with BFNC lead to expression of an M-current that is hypofunctional. Developmental regulation of the M-current could be critical to the window of vulnerability to seizures associated with BFNC. The goal of this study was to determine whether changes in M-current expression and function during early postnatal development in rat neocortex and hippocampus could contribute to the transient expression of seizures found in BFNC. We focused on the second postnatal week, since cortical development at this time in rat approximates human neonates.
METHODS: We made hippocampal and neocortical slices from P10 to adult rats and used intracellular voltage-clamp recording techniques to record currents in hippocampal CA1 and somatosensory cortex layer V pyramidal neurons. Standard voltage protocols were used to record M-current and other sustained currents in the presence of tetrodotoxin. We characterized the pharmacological properties of the M-current using linopirdine, a selective blocker, and retigabine, a specific activator with demonstrated anti-epileptic properties. We also used immunohistochemistry and Western blotting to examine expression of KCNQ2 and 3 proteins during the first two postnatal weeks.
RESULTS: In hippocampus and neocortex, we saw an increase in M-current amplitude during the second postnatal week. In CA1 pyramidal neurons from P10-P15, M-current amplitudes were about half of that found in adults. Interestingly, M-currents recorded from immature hippocampus were much more sensitive to retigabine (2-10 [mu]M) than in adult. Linopirdine (10 [mu]M) blocked the M-current regardless of the developmental age. Concomitant to the functional increase in M-current, we also observed an increase in levels of KCNQ2 and KCNQ3 protein expression in hippocampus. In neocortex, in contrast to hippocampus, M-currents recorded from adult pyramidal neurons were more sensitive to retigabine than in immature.
CONCLUSIONS: Our results show that in CA1 and neocortex, a developmental ?switch? occurs in M-current pharmacology. In CA1, the M-current recorded from immature pyramidal neurons is more sensitive to retigabine than in adult, while the opposite holds true in neocortex. These changes could be a reflection of differential expression of KCNQ subunits or differences in post-translational modification. Thus, alterations in M-current expression and/or regulation could play a role in the transient nature of seizure vulnerability in neonates with hypofunctional M-current.
[Supported by: NIH grant NS 38633 and DA 13658.]