Abstracts

Rationale: The mediodorsal thalamus (MD) has been theorized to play a key role in seizure propagation (Bertram et al., 2008), in part due to its direct connections with key ictogenic structures including the hippocampus, piriform cortex and basolateral amygdala (BLA).  Indeed, upon stimulation of the BLA, the MD displays almost immediate electrographic discharges (Sloan et al., 2011). Consistent with these anatomical and neurophysiological findings, microinjection of the GABA-A receptor agonist muscimol into the MD attenuates seizures evoked from each of these areas. However, the functional role of specific efferent and afferent pathways linking the MD to these ictogenic regions have not been directly manipulated in the context of seizures. This level of circuit resolution, while unattainable with focal pharmacological manipulations, is now possible using new-generation techniques. We have recently reported that chemogenetic silencing of a large region of the midline and intralaminar thalamus was sufficient to block amygdala kindled seizures. Here, we employed chemogenetics to examine investigate the specific role of the MD and its efferent input from the BLA and the afferent output to the prefrontal cortex (PFC) in seizures originating from the BLA. Methods: Male, Sprague-Dawley rats were injected with virus (AAV-hSyn-hM4Di) and implanted with a stimulating electrode in the BLA. Groups of rats received either: (1) bilateral injection of virus in the BLA and bilateral cannulae in the MD, (2) bilateral injection of virus into the MD, (3) bilateral virus in the MD and bilateral cannulae in the PFC. Cannulae allowed for focal delivery of the DREADD agonist, clozapine-N-oxide (CNO). Animals were stimulated daily until fully kindled, and CNO was then delivered either systemically or focally, via microinjection. To directly target and manipulate the projection from the BLA to the MD or MD to PFC, 500 pmol of CNO in 0.2 µl was microinjected. Results: Following systemic administration of CNO (5 mg/kg), chemogenetic silencing of the MD significantly attenuated amygdala kindled seizures both with respect to severity (p=0.03) and duration (p < 0.001). Selective silencing of projections from BLA to MD likewise decreased seizure score (p=.02) and duration (p=0.005). A similar effect was seen following silencing the MD unilaterally (ipsilateral to the stimulating electrode): both seizure score (p=0.03) and duration (p=0.03) were reduced. Finally, selective bilateral silencing of the projections from the MD to PFC decreased seizure score and afterdischarge duration; unilateral silencing of this pathway was without effect. Conclusions: These data demonstrate and reinforce the notion that the MD plays a key role in limbic seizure propagation. Through circuit-level modulation of efferent and afferent projections of the MD we found that inputs from BLA to MD, and outputs from MD to PFC are necessary for the propagation and maintance of limbic seizures. This suggests that direct, rather than indirect, propagation of activity from BLA to MD and MD to PFC is critical in limbic ictogenesis. The specificity of projection-level targeting may have a translational utility and provides further opportunity to dissect the role of the MD in seizure propagation. Funding: R01NS09776201