Impaired Regulation of Thalamic Pacemaker Channels through an Imbalance of Subunit Expression in Absence Epilepsy

Abstract

The role of hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channel isoforms and hyperpolarization-activated cation current (I_h) for seizure-related burst firing in thalamocortical (TC) neurons was investigated in a rat genetic model of absence epilepsy [Wistar Albino Glaxo rats, bred in Rijswijk (WAG/Rij)]. Burst discharges in TC neurons locked to seizure activityin vivowere prolonged during blockade ofI_hby Cs⁺and ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride).In vitroanalyses revealed a hyperpolarizing shift of half-maximalI_hactivation (V_h) in WAG/Rij (V_h= -93.2 mV) compared with nonepileptic controls [August × Copenhagen-Irish (ACI) (V_h= -88.0 mV)]. This effect is explained by a shift of the responsiveness ofI_hto cAMP toward higher concentrations in TC neurons from WAG/Rij, as revealed by application of 8-bromo-cAMP and the phosphodiesterase inhibitor IBMX. During blockade of adenylyl cyclase activity,I_hactivation was similar in the two strains, whereas the difference in cAMP responsiveness persisted, thereby voting against different ambient cAMP levels between strains. Increasing the intracellular cAMP level and shiftingI_hactivation led to a change from burst to tonic firing mode in WAG/Rij but not in ACI rats. Furthermore, HCN1 expression was significantly increased on mRNA and protein levels, with no changes in HCN2-4 expression. In conclusion, there is an increase in HCN1 expression in the epileptic thalamus, associated with a decrease in cAMP responsiveness ofI_hin TC neurons and resulting impairment to control the shift from burst to tonic firing, which, in turn, will prolong burst activity after recruitment ofI_hduring absence seizures.