Figure 7.

Theta Firing-Phase Selectivity and Schematic Summary of the Place of GABAergic MS Teevra Cells in the Hippocampal Circuit

(A) Teevra cells (green) preferentially innervate CA3 axo-axonic (AAC, cyan) and putative CCK-expressing basket cells (CCK, violet), which in turn target the axon initial segment and somata of CA3 pyramidal cells, respectively. The entorhinal cortical (EC) input to CA1 and CA3 innervates pyramidal cells and those GABAergic cells, such as AACs, which have dendrites in the termination zone. Pyramidal cells (P) provide recurrent input to interneurons and to other pyramidal cells and project to other cortical and subcortical areas (arrows). The termination area and target cell selectivity of other GABAergic MS neurons (magenta, brown, and others) remain to be determined. One general GABAergic neuron is shown in CA1 (I, lilac). Dentate granule cells, other types of GABAergic interneurons, and cholinergic and glutamatergic MS cells are not shown.

(B) Theta-phase relationships of neurons shown in (A) from recorded data, referenced to dorsal CA1 pyramidal cell layer LFP. On average, Teevra cells (n = 12, current study) discharge maximally at the trough of CA1 theta oscillations inhibiting AACs (data from Viney et al., 2013, non-anesthetized rat, CA1 and CA2 AACs averaged) and putative CCK basket cells (Lasztóczi et al., 2011, anesthetized rat, CA3) leading to disinhibition of CA3 pyramidal cells, which provide the largest excitatory input to CA1 pyramidal cells (average firing probabilities from Mizuseki et al., 2009). The cell-type-specific temporal modulation of firing rates during theta cycles contributes to the implementation of oscillatory increases and decreases of excitability in pyramidal cell networks via subcellular compartment specific disinhibition.