Impaired regulation of thalamic pacemaker channels through an imbalance of subunit expression in absence epilepsy.
The role of hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channel isoforms and hyperpolarization-activated cation current (Ih) 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 activity in vivo were prolonged during blockade of Ih by Cs+ and ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride). In vitro analyses revealed a hyperpolarizing shift of half-maximal Ih activation (Vh) in WAG/Rij (Vh = -93.2 mV) compared with nonepileptic controls [August x Copenhagen-Irish (ACI) (Vh = -88.0 mV)]. This effect is explained by a shift of the responsiveness of Ih to 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, Ih activation 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 shifting Ih activation 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 of Ih in TC neurons and resulting impairment to control the shift from burst to tonic firing, which, in turn, will prolong burst activity after recruitment of Ih during absence seizures.
Budde
Thomas
T
Institut für Experimentelle Epilepsieforschung, Westfälische Wilhelms-Universität Münster, D-48149 Münster, Germany.
Caputi
Luigi
L
Kanyshkova
Tatyana
T
Staak
Rainer
R
Abrahamczik
Christian
C
Munsch
Thomas
T
Pape
Hans-Christian
HC
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects; D000225:Adenine / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D015152:Blotting, Northern / Q000379:methods; D002540:Cerebral Cortex / Q000166:cytology*; D002586:Cesium / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D054815:Cyclic Nucleotide-Gated Cation Channels; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D004347:Drug Interactions; D004558:Electric Stimulation / Q000379:methods; D004569:Electroencephalography / Q000379:methods; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004832:Epilepsy, Absence / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007150:Immunohistochemistry / Q000379:methods; D017403:In Situ Hybridization / Q000379:methods; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D008845:Microinjections; D009474:Neurons / Q000378:metabolism*; D018408:Patch-Clamp Techniques / Q000379:methods; D015221:Potassium Channels; D020033:Protein Isoforms / Q000378:metabolism; D011743:Pyrimidines / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis; D051381:Rats; D011922:Rats, Mutant Strains; D017208:Rats, Wistar; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D013788:Thalamus / Q000166:cytology*
D002712:Chlorides; D054815:Cyclic Nucleotide-Gated Cation Channels; D004791:Enzyme Inhibitors; C578667:Hcn1 protein, rat; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D015221:Potassium Channels; D020033:Protein Isoforms; D011743:Pyrimidines; D012333:RNA, Messenger; C082246:ICI D2788; C017759:9-(tetrahydro-2-furyl)-adenine; D002586:Cesium; D015124:8-Bromo Cyclic Adenosine Monophosphate; C028019:cesium chloride; D000225:Adenine; D015056:1-Methyl-3-isobutylxanthine