Abstracts

Rationale: The medial septum is a hypothesized pacemaker and modulator of the hippocampal theta rhythm. Theta, in turn, is associated with the prevention or cessation of epileptic activity in animal models and humans. Modulating hippocampal theta could therefore prove an effective method for seizure control. We hypothesize that medial septal glutamatergic neurons modulate hippocampal oscillatory activity via direct projections to hippocampal pyramidal neurons, while slow-firing cholinergic neurons provide control over theta power. We explored cell-type specific optogenetic activation of these neuron populations in the medial septum to functionally investigate their influence on hippocampal oscillatory local field potential (LFP) and single unit activity.Methods: Sprague-Dawley rats were injected in the medial septum with AAV2-CaMKII -ChR2, targeting glutamatergic neurons; AAV5-hSyn-ChR2, non-specifically expressing in all neurons; or a control virus. Cholinergic neurons were targeted using transgenic Long-Evans Chat-CRE rats and an AAV5-EF1a-DIO-ChR2 virus. After two weeks, each rat was anesthetized and implanted with an optical fiber targeting the medial septum and a 16-channel microwire multielectrode array (Tucker-Davis Technologies) recording from hippocampal layers CA1 and CA3. Animals underwent 465 nm blue LED stimulation across frequencies from 0.1-50 Hz and pulse widths of 1-50 ms. Electrode, optical fiber, and transgene expression locations were confirmed histologically. Recorded electrophysiological data was spectrographically analyzed using custom-written Chronux Matlab scripts.Results: Optogenetic excitation of subpopulations of the medial septum altered hippocampal neural activity. At beta (15-35Hz) and gamma (40+Hz) frequencies, increase in stimulus frequency-specific power in the hippocampal LFP was observed with hSyn and CaMKII driven channels, but not in ChAT-ChR2 or ChR2-negative controls. In anesthetized animals (reduced theta state), hSyn-ChR2 rats generated corresponding hippocampal LFP oscillations with theta stimulation, which were not seen in glutamatergic or cholinergic stimulation. Anesthetized hSyn-ChR2 and ChAT-ChR2 stimulation produced significant alterations in hippocampal single-unit firing rate, but with different temporal profiles. In awake and behaving non-epileptic animals, neither hSyn-ChR2 rats nor CaMKII -ChR2 stimulation could increase theta power, but hSyn-ChR2 rats could phase-lock extant hippocampal theta oscillations to the optical stimulus.Conclusions: We used cell-type specific optogenetic stimulation and multielectrode recording to begin functionally dissecting the neural circuitry of the septohippocampal axis. Our results suggest that the glutamatergic and cholinergic neurons of the medial septum are not pacing theta activity, due to their inability to modulate the hippocampal theta rhythm in awake, behaving animals. This suggests that these cell types play some other role in the septohippocampal axis, and that perhaps the GABAergic neurons of the medial septum are serving as pacemaker for the antiepileptic hippocampal theta rhythm.