Abstracts

This abstract has been invited to present during the Basic Science Poster Highlights poster session

Rationale: Over 400,000 people in the U.S. suffer from multi-drug resistant temporallobe epilepsy (TLE) and have limited options for seizure control. Recent data suggest that seizures are generated by more broadly distributed networks, highlighting a potential avenue for circuit-based therapies. The Papez circuit embeds the hippocampus in a broader recurrent network that includes the mammillary bodies, anterior thalamus, cingulum, and entorhinal cortex, and is a candidate for seizure interventions. Clinical trials demonstrate seizure control with deep brain stimulation of the anterior thalamus, however, very little is known about the upstream medial mammillary bodies (mMB). Here we used transgenic mice to gain selective access to the three main divisions of the mMB and studied their connectivity and functional contributions to hippocampal network activity under both physiological conditions and chronic TLE.

Methods: Experiments were done using adult male and female Nts, Calb1, and Pvalb Cre mice. To model chronic TLE, we injected kainate into the CA1 and waited >4 weeks for stable, spontaneous seizures. Both epileptic and control Cre mice underwent stereotactic injection of a channelrhodopsin expressing virus, AAV5-hChR2-eYFP, and implantation of an optic fiber dorsal to the mMB. After 3 to 4 weeks, mice were placed head-fixed on a ball and a silicon probe was lowered into the hippocampus to collect local field potential while a high-speed camera captured mouse behavior. Brief laser pulses that activate each mMB cell population were compared to spontaneous physiological events, including sharp-wave ripples, dentate spikes, and hippocampal theta, and to pathological events, including interictal spikes, high frequency oscillations, and seizures. After the experiment, the density of axons innervating the anterior thalamic nuclei were quantified as well as the anatomical position of cell types within the mMB.

Results: Nts-expressing cells were located in the medial aspect of the mMBs and selectively projected to the anteromedial thalamic nucleus, whereas Calb1- and Pvalb-expressing cells, located in the lateral and median aspects of the mMBS, respectively, primarily projected to the anteroventral nucleus. In line with the unique cortical projections of these anterior thalamic targets, we observed distinct hippocampal response profiles upon activation of ChR2 in Nts-expressing versus Calb1- or Pvalb-expressing cells. Preliminary experiments in epileptic mice indicate that these pathways remain anatomically intact and segregated with unique hippocampal response profiles.

Conclusions: Since the mMB are both upstream and downstream of the hippocampus, this area could be an important chokepoint for epileptiform activity and requires further investigation. Given the existence of distinct, parallel pathways emerging from mMB cell types, there may be considerable heterogeneity in pathways that conduct seizure activity and contribute to the manifestation of seizures.

Funding: This work was funded by a Stanford Medical Scholars Research Felllowship to M.A., a K99 award from the NIH to J.S.F., and grant numbers 5 U19 NS 104590–05 and 1 R01 NS 121106–01 to I.S. from the NIH.