Abstracts

Rationale: The septohippocampal axis - connecting the medial septum (MS) and hippocampus - has been described as a pacemaker for the generation and maintenance of the hippocampal theta rhythm. Theta, in turn, is associated with the prevention or cessation of epileptic activity. Modulating hippocampal theta could prove a viable method of controlling seizures. Glutamatergic neurons of the MS have been proposed as a theta pacemaker due to their direct projections to hippocampal pyramidal neurons and tendency to fire at theta frequencies. To functionally investigate these connections in vivo, we explored cell-type specific optogenetic activation of the MS and the resulting oscillatory local field potential (LFP) activity in the dorsal hippocampus. Methods: Sprague-Dawley rats were infected in the MS with AAV2-CaMKIIα-ChR2, targeting glutamatergic neurons; AAV5-hSyn-ChR2, non-specifically expressing in all neurons; or a control virus. After two weeks, each rat was anesthetized and implanted with an optical fiber targeting the MS and a 16-channel microwire multielectrode array (Tucker-Davis Technologies) simultaneously targeting CA3 and CA1 of the hippocampus. Animals underwent 473 nm blue laser stimulation across numerous parameters, including frequencies from 7-42 Hz and pulse widths of 1- 10 ms. Electrode and optical fiber location was confirmed histologically and cell-specific transgene expression was immunohistochemically determined. Recorded electrophysiological data was spectrographically analyzed using custom-written and Chronux Matlab scripts. Results: Optogenetic control of cell-type specific neurons of the MS in awake and behaving rats drove activity locally as well as in the dorsal hippocampus. At beta (15-35Hz) and gamma (40+Hz) frequencies, increase in stimulus frequency-specific power in the hippocampal LFP was observed with either neuronal target, but not in ChR2-negative controls. Notably, frequency-specific power generated in CaMKIIα-ChR2 rats was less than that generated by hSyn-ChR2 rats. In anesthetized animals (reduced theta state), only hSyn-ChR2 rats generated corresponding hippocampal LFP oscillations with theta stimulation. In awake and behaving non-epileptic animals, neither hSyn-ChR2 rats nor CaMKIIα-ChR2 stimulation could increase theta power, but hSyn-ChR2 rats could phase-lock extant hippocampal theta oscillations to the optical stimulus. Conclusions: We have developed a system for optogenetic stimulation and multielectrode recording, and used it to begin dissecting the neural circuitry of the septohippocampal axis. We demonstrate support for the hypothesis that the medial septum functionally modulates hippocampal activity. Furthermore, our results suggests that the glutamatergic neurons of the medial septum are not the drivers of theta activity, due to their inability to modulate the hippocampal theta rhythm in awake, behaving animals, and must play some other role in the septohippocampal axis.