Introduction: Absence seizures arise from disruptions of normal thalamocortical oscillations. Stargazer mice exhibit spontaneous absence seizures due to the loss of the stargazin protein. Stargazin plays a role in trafficking AMPA-type glutamate receptors to the synapse. We examined synaptic signaling in the neocortex of stargazer mice to investigate the apparent paradox of decreased synaptic glutamate receptors in the presence of hyperexcitability and seizures. We studied regular-spiking (RS, excitatory) cells and fast-spiking (FS, inhibitory) cells of layer IV, which both receive direct excitatory input from thalamocortical cells.
Methods: We performed whole-cell patch-clamp recordings in acute slices of somatosensory cortex from stargazer mutants and wild-type littermates using standard electrophysiological techniques. For the optogenetic experiments, we injected viruses carrying genes for yellow fluorescent protein and channelrhodopsin-2 into the thalamus of wild-type and stargazer mice in vivo 5-9 weeks prior to preparing slices.
Results: Spontaneous excitatory postsynaptic currents in FS cells are smaller and less frequent in stargazer mice than in wild-types. RS cells from stargazer mice do not exhibit similar changes. FS cells are known to mediate powerful feed-forward inhibition from the thalamus onto RS cells, which acts as a brake to prevent runaway cortical excitation. Since FS cells demonstrate specific changes, we directly measured feed-forward inhibition using optogenetic techniques. We selectively activated channelrhodopsin-2-expressing thalamocortical fibers with a blue laser, evoking a monosynaptic exctitatory current followed by a disynaptic inhibitory current in the RS cell. We discovered that the ratio of inhibitory to exctitatory charge transfer was decreased in stargazer mice.
Conclusions: In the stargazer mouse, we demonstrate a specific decrease in excitatory drive onto FS neurons, leading to decreased feedforward inhibition onto excitatory RS neurons. The loss of this powerful form of inhibition represents a novel mechanism by which a defect in AMPA-mediated glutamatergic signaling may contribute to the generation of absence seizures.
Patient Care: Elucidating the mechanisms leading to network hyperexcitability in various forms of epilepsy, including absence seizures, can lead to novel pharmacological and surgical treatments for this disease.
Learning Objectives: By the conclusion of this session, participants should be able to 1) describe the stargazer mouse model of absence seizures and 2) explain how a defect in glutamate receptors could nevertheless lead to hyperexcitability in the thalamocortical circuit