Optogenetic Dissection of Entorhinal-Hippocampal Functional Connectivity

Introduction The mammalian space circuit contains several functionally specialized cell types, such as place cells in the hippocampus and grid cells, head direction cells, and border cells in the medial entorhinal cortex (MEC). The interaction between entorhinal cell types and hippocampal place cells is poorly understood. Hippocampal place fields are thought to be generated by transformation of spatial signals from the MEC, but which cell types contribute to this process remains elusive. Grid cells and border cells fire at similar latencies in response to local photostimulation. (Left) Spike rasters showing discharge in response to successive 3.5-ms light pulses for a grid cell and a border cell. Dots indicate spike times. (Right) Spike rasters showing color-coded normalized firing rate 0 to 50 ms after the light pulse for all grid cells and border cells. Methods We used a combined optogenetic-electrophysiological strategy to determine functional identity of entorhinal cells with output to place cells in the hippocampus. Channelrhodopsin-2 (ChR2) was expressed selectively in the hippocampus-targeting subset of entorhinal projection neurons by injecting retrogradely transportable ChR2-coding recombinant adeno-associated virus (rAAV) into the hippocampus. Hippocampus-projecting MEC cells could then be identified, after expression of ChR2 transgenes, as neurons that responded reliably, at a minimal latency, to photostimulation of either cell bodies in the MEC or axons of MEC cells in or near the hippocampus. Results Many light-responsive MEC cells were grid cells, but short-latency firing could also be induced in border cells and head direction cells, as well as neurons with irregular firing fields or no fields at all. In each cell group, the majority of neurons discharged at minimal response latencies, suggesting that they had direct projections to the hippocampus. The same cells could also be backfired, at slightly longer latencies, by photostimulation of the cells’ axons in the hippocampus. MEC cells could also be activated by photostimulation in the hippocampal CA1 subfield, but response latencies were more than 150% longer than with somatic or axonal stimulation, suggesting the activations was now synaptic. Discussion These findings suggest that place signals are generated by convergence of signals from a variety of entorhinal functional cell types, of which grid cells are the most abundant spatial cell type. A dual spatial input from grid cells and border cells is consistent with the idea that place cells have access to both self-motion and landmark-based information and raises the possibility that the spatial metric of the place-cell population originates from grid cells, whereas boundary- and landmark-induced firing is derived directly from border cells. The dual nature of the spatial input may explain the observation that place cells precede mature grid cells during ontogenesis of the spatial representation system and that place cells can maintain location specificity under conditions that reduce grid-cell periodicity in adult rats. Convergent input from a broad spectrum of entorhinal cell types may also enable individual place cells to respond dynamically, so that different types of input are favored in different behavioral states or behavioral circumstances. From Grid to Place Grid cells are considered one of the key sources for place-cell signals in the hippocampus. The entorhinal circuit also contains other functional cell types, but it is unclear which project to the place cells of the hippocampus. Zhang et al. (10.1126/science.1232627, see the Perspective by Poucet and Sargolini) addressed this question using optogenetics and in vivo multi-electrode electrophysiology. Hippocampal cells received input from a broad spectrum of entorhinal neuronal cell types. Grid cells represented the biggest group of spatial inputs, but border cells, head-direction cells, and a large fraction of nonspatial cells also provided inputs. Thus, hippocampal circuits have local mechanisms for processing specific types of functional input from the entorhinal cortex to generate place-specific signals. The place-cell population of the hippocampus receives direct input from a variety of medial entorhinal neurons. [Also see Perspective by Poucet and Sargolini] We used a combined optogenetic-electrophysiological strategy to determine the functional identity of entorhinal cells with output to the place-cell population in the hippocampus. Channelrhodopsin-2 (ChR2) was expressed selectively in the hippocampus-targeting subset of entorhinal projection neurons by infusing retrogradely transportable ChR2-coding recombinant adeno-associated virus in the hippocampus. Virally transduced ChR2-expressing cells were identified in medial entorhinal cortex as cells that fired at fixed minimal latencies in response to local flashes of light. A large number of responsive cells were grid cells, but short-latency firing was also induced in border cells and head-direction cells, as well as cells with irregular or nonspatial firing correlates, which suggests that place fields may be generated by convergence of signals from a broad spectrum of entorhinal functional cell types.