Enhanced NMDA Receptor-Dependent Thalamic Excitation and Network Oscillations in Stargazer Mice

Abstract

Disturbances in corticothalamic circuitry can lead to absence epilepsy. The reticular thalamic nucleus (RTN) plays a pivotal role in that it receives excitation from cortex and thalamus, and when strongly activated can generate excessive inhibitory output and epileptic thalamocortical oscillations that depend on post-inhibitory rebound. Stargazer mice (stg) have prominent absence seizures resulting from a mutant form of the AMPAR auxiliary protein stargazin. Reduced AMPAR excitation in RTN has previously been demonstrated in stg, yet the mechanisms leading from RTN hypoexcitation to epilepsy are unknown and unexpected, as thalamic epileptiform oscillatory activity requires AMPARs. We demonstrate hyperexcitability in stg thalamic slices and further characterize the various excitatory inputs to RTN using electrical stimulation and laser scanning photostimulation. Patch-clamp recordings of spontaneous and evoked EPSCs in RTN neurons demonstrate reduced amplitude and increased duration of the AMPAR-component with an increased amplitude NMDAR-component. Short 200 Hz stimulus trains evoked a gradual ~3-fold increase in NMDAR EPSCs compared to single stimuli in wild-type (wt), indicating progressive NMDAR recruitment, whereas in stg cells, NMDAR responses were nearly maximal with single stimuli. Array tomography revealed lower synaptic, but higher perisynaptic, AMPAR density in stg RTN. Increasing NMDAR activity via reduced [Mg2+]o in wt phenocopied the thalamic hyperexcitability observed in stg, while changing [Mg2+]o had no effect on stg slices. These findings suggest that, in stg, a trafficking defect in synaptic AMPARs in RTN cells leads to a compensatory increase in synaptic NMDARs, and enhanced thalamic excitability.

Rebound burst firing in the reticular thalamus is not essential for pharmacological absence seizures in mice

“…Consistent with our current observations, it has been reported that TRN neurons in epileptic genetic absence epileptic rats from Strasbourg (GAERS) rats displayed higher excitability, higher firing frequency, and regularity (34). In parallel with these results from GAERS rat, stargazer mice (another genetic model for absence seizures) showed hyperexcitable TRN neurons due to an enhanced NMDA receptor (35). Thus, several gene mutation models of absence seizures show altered TRN excitability.…”

Monogenic models of absence epilepsy

“…Similar to the GRIA4 knockout model, this synapse-specific mistrafficking of AMPA receptors in stargazer mice could lead to a loss of fast corticothalamic feedforward inhibition. However, while the faster AMPA receptor-mediated activation of the RTN is reduced (Menuz and Nicoll, 2008), the slower, NMDA receptor-mediated activation of the RTN is enhanced, providing a potential mechanism for recruitment of tonic inhibition in the thalamus (Lacey et al, 2012). Isolated stargazer cortical slices are also hyperexcitable (Di Pasquale et al, 1997), but whether there is also impaired feedforward inhibition or enhanced tonic inhibition in the cortex has not yet been determined.…”

Animal models of absence epilepsies: What do they model and do sex and sex hormones matter?

“…Experiments in commonly used spontaneous inbred genetic mouse models of absence seizures (tottering, lethargic, stargazer) were performed in both males and females without any reported sex-dependent differences in the frequency of absence seizures (Aizawa et al, 1997; Noebels and Sidman, 1979; Lacey et al, 2012). Moreover, studies of atypical absence epilepsy phenotypes generated in mice transgenically overexpressing GABA B receptor R1a or Ra1b subunits did not report sex-dependent effects (Aizawa et al, 1997; Lacey et al, 2012; Noebels and Sidman, 1979; Stewart et al, 2009; Wu et al, 2007). Pharmacologically-evoked absence seizures in mice have been produced through the systemic administration of GABA A R antagonists and GABA B R agonists.…”