Figure 1.

Schematic of the spatial and temporal relationships between pyramidal cells and eight types of GABAergic interneuron in the CA1 area. Top: the main synaptic connections of pyramidal cells (red, middle), three types of CCK-expressing cells (basket cell, perforant path-associated cell, Schaffer collateral-associated cell), ivy cells and PV-expressing basket, axo-axonic, bistratified and O-LM interneurons. The other 13 defined types of interneuron are not shown; ACh, acetycholine. The firing probability histograms are averages from several cells of the same type recorded in anaesthetized rats; note different scales for the y-axis and schematic LFP oscillations (black). During theta and ripple oscillations, interneurons innervating different domains of pyramidal cells fire with distinct patterns. Their firing is coupled to field gamma oscillations to varying degrees (averages of several cells of each type). The soma and dendrites are innervated in parallel by both CCK- or PV-expressing cells, which show different temporal dynamics. One key feature is that during theta, pyramidal cell firing probability is lowest at the peak of the pyramidal layer LFP, when axo-axonic cells innervating the axon initial segment fire maximally and the sum of CCK- and PV-expressing basket cell firing probability is also maximal. The cooperative action of these three GABAergic cell types suppresses pyramidal cell firing at the peak of theta. The dendrite-innervating cells show similar theta phase coupling, approximately counter-phased with the combined impact of perisomatic innervating cells. Bistratified and ivy cells innervating basal and small oblique pyramidal cell dendrites show the highest gamma coupling of their firing and most of their dendrites are in the input zone from CA3 pyramidal cells which fire strongly phase coupled to gamma oscillations in CA3. During ripple oscillations, GABA release to the axon initial segments from axo-axonic cells is withdrawn, allowing maximal pyramidal cell discharge synchronized by PV-expressing basket and bistratified cells. Connections among interneurons are not shown for clarity. (Data from [15,37–40].)