Medial Septal Neurons Regulate Hippocampal Local Field Potential Oscillations

Medial septum (MS) and hippocampus are reciprocally connected. Lesion of MS leads to vanish of local field potential (LFP) theta rhythm in hippocampus in rodents. Thus MS is considered to play an important role in the regulation of hippocampal LFP activities. There are different types of neurons in MS. Among them, the y-amino butyric acidergic (GABAergic) neurons are considered to be involved in the generation of theta oscillation in hippocampus. Activities of the cholinergic neurons in MS are related to not only hippocampal LFP oscillation but also hippocampus associated cognitive functions such as learning and memory. In this study, we used multi-channel in vivo recording techniques and optogenetics to investigate the firing properties of GABAergic neurons and cholinergic neurons in MS in C57/BL6 mice and choline acetyltransferase-channelrhodopsin2 (ChAT-ChR2) mice respectively, and how these neurons regulated LFP oscillatory activities in the hippocampal CA1 area.According to the extracellularly recorded spike waveform and mean firing rate, spikes of putative GABAergic neurons could be distinguished from spikes of the other recorded neurons. We recorded 114 putative GABAergic neurons in MS from 13 mice altogether. These neurons could be divided into two groups. Type Ⅰ GABAergic neurons fired periodically, and 54 neurons were categorized into this group. Firing of type Ⅱ GABAergic neurons didn’t show periodicity, and 60 neurons were categorized into this group. Burst analysis showed that type Ⅰ GABAergic neurons could be further divided into three subpopulations, type Ⅰ a, type Ⅰ b and type Ⅰc. During theta states, 98.3±1.0% of the spikes of typeⅠa neurons were intraburst spikes, and the number of spikes per burst was 11.2 ± 2.0. And 90.6 ± 3.1% of the spikes of type Ⅰ b neurons were intraburst spikes, the number of spikes per burst was 6.0 ± 0.8. The proportion of intraburst spikes in type Ⅰ c neurons was relatively smallest which was 67.2 ± 8.3%, and the number of spikes per burst was 4.8 ± 0.8. Further analysis indicated that firing of type Ⅰ a neurons showed strong periodicity for the peak/valley ratio was 4.6 ± 1.6 in autocorrelogram, were highly phase locked to hippocampal CA1 theta rhythm for the theta modulation depth was 0.56 ±0.10, displayed strong coherence which was 0.74 ± 0.08 with LFPs in theta band, and presented high Granger causality which was 0.48 ± 0.22 to LFP theta. Thus we presumed that type I a neurons may play an important part in hippocampal theta rhythm generation.Combining in vivo recording techniques and optogenetics, we recorded 10 neurons that responded to blue light stimulation in MS from ChAT-ChR2 mice. We speculated these neurons were cholinergic. Based on their in vivo firing patterns, these 10 neurons were divided into two groups. Mean firing rates of type I cholinergic neurons exceeded 4 Hz, which was never reported in previous studies. Mean firing rates of type II cholinergic neurons were always below 4 Hz, and firing patterns of these neurons were similar to the cholinergic neurons reported. Further analysis indicated that firing activities of type I cholinergic neurons were animal behavior-dependent. Across theta states such as active exploration and rapid eye movement sleep, this type of neurons were more active. Mean firing rates of these neurons were 7.7 ± 1.2 Hz during active exploration, and 3.1 ± 1.0 Hz during rapid eye movement sleep. Across non-theta states such as slow wave sleep, this type of neurons were less active. And Mean firing rate of these neurons was 0.4 ± 0.6 Hz. This phenomenon that activities of these neurons were related to theta rhythm coincided with previous reports that concentrations of acetylcholine in hippocampus were related to animal behavior.To study the functional role of cholinergic neurons in MS, we activated these neurons by 10 Hz light stimulation. We found that light stimulation suppressed LFP sharp wave-ripple in hippocampal CA1 area and increased the proportion of LFP power spectral density in theta band, which was blocked by atropine, an antagonist of muscarinic acetylcholine receptors. We speculated that high concentration of acetylcholine in hippocampus suppressed the transmission from CA3 to CA1, therefore suppressed sharp wave-ripple in CA1. Meanwhile, light evoked activation of cholinergic neurons decreased firing rates of ripple-associated interneurons in hippocampal CA1, altered the ripple-associated high-frequency firing pattern of these neurons.These results suggest that activities of GABAergic neurons and cholinergic neurons in MS are related to hippocampal LFP oscillation. GABAergic neurons which have theta rhythmical burst firing patterns may participate in regulating theta generation in hippocampus. During animal theta states, the activation of cholinergic neurons increases the concentration of choline in hippocampus, thus suppresses slow wave sleep associated sharp wave-ripple activities in CA1 area.