Pharmacologic evidence for abnormal thalamocortical functioning in GABA receptor beta3 subunit-deficient mice, a model of Angelman syndrome.
OBJECTIVE
gamma-Aminobutyric acid receptor (GABA(A)r) subunit beta3-deficient mice model Angelman syndrome by displaying impaired learning, abnormal EEG with interictal spikes and slowing, myoclonus, and convulsions. The beta3-subunit deficiency causes a failure of intrathalamic reticular nucleus inhibition, leading to abnormally synchronized thalamocortical oscillations. We postulated that this pathophysiology underlies the abnormal cortical EEG and triggers interictal spikes and seizures, but extrathalamic regions also contribute to interictal spikes and seizures, so that the EEG slowing should reveal an absence-like response profile, whereas spikes and seizures have dual responsiveness to absence and partial-seizure drugs.


METHODS
Recording electrodes were implanted over the parietal cortices of wild-type, heterozygotes, and homozygous null mice. In each experiment, EEG was recorded for 45 min, either drug or vehicle administered, and EEG recorded for another 3 h. Each EEG was scored for slow-wave activity, interictal spikes, and seizures by a reader blinded to treatments.


RESULTS
Interictal spiking and percentage of time in EEG slowing in heterozygotes were increased by the proabsence drug baclofen (GABA(B)-receptor agonist), whereas CGP 35348 (GABA(B)-receptor antagonist) had the opposite effect. The antiabsence drug ethosuximide markedly suppressed EEG slowing and interictal spiking in heterozygote and null mice. Broad-spectrum clonazepam and valproate were more effective on interictal spiking than on EEG slowing, and fosphenytoin suppressed only interictal spiking.


CONCLUSIONS
The results suggest that this model of Angelman syndrome, although not expressing typical absence seizures, is characterized by hypersynchronous thalamocortical oscillations that possess absence-like pharmacologic responsiveness and promote EEG slowing, interictal spikes, and convulsive seizures.
Handforth
Adrian
A
Department of Neurology, VA Greater Los Angeles Health Care System, Los Angeles, CA 90073, USA. charles.handforth@med.va.gov
Delorey
Timothy M
TM
Homanics
Gregg E
GE
Olsen
Richard W
RW
D017204:Angelman Syndrome / Q000188:drug therapy / Q000378:metabolism / Q000503:physiopathology*; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology / Q000627:therapeutic use; D001418:Baclofen / Q000494:pharmacology; D002540:Cerebral Cortex / Q000187:drug effects / Q000503:physiopathology*; D004195:Disease Models, Animal; D004567:Electrodes, Implanted; D004569:Electroencephalography / Q000187:drug effects* / Q000706:statistics & numerical data; D005013:Ethosuximide / Q000494:pharmacology; D005260:Female; D018755:GABA Agonists / Q000494:pharmacology*; D018756:GABA Antagonists / Q000494:pharmacology*; D006801:Humans; D008297:Male; D051379:Mice; D008817:Mice, Mutant Strains; D009434:Neural Pathways / Q000503:physiopathology; D009943:Organophosphorus Compounds / Q000494:pharmacology; D010296:Parietal Lobe / Q000503:physiopathology; D011963:Receptors, GABA-A / Q000172:deficiency / Q000187:drug effects* / Q000502:physiology*; D013788:Thalamus / Q000187:drug effects / Q000503:physiopathology*
D000927:Anticonvulsants; D018755:GABA Agonists; D018756:GABA Antagonists; D009943:Organophosphorus Compounds; D011963:Receptors, GABA-A; D005013:Ethosuximide; C066430:CGP 35348; D001418:Baclofen