June 10, 2016

Activity-dependent processes govern place representation in hippocampus

IST Austria neuroscientists delve into activity-dependent plasticity of hippocampal place maps

Hippocampal excitatory cells fire in relation to space, and collectively these so-called place cells form an internal cognitive map of space. These hippocampal maps are updated during learning and in response to changes in the environment through activity-dependent synaptic plasticity. A research paper by IST Austria neuroscientists, published on June 10 in Nature Communications (DOI: 10.1038/ncomms11824) examines how changes in activity influence spatial coding in rats by using halorhodopsin-mediated, spatially-selective optogenetic silencing. As halorhodopsin is expressed in both place cells and inhibitory interneurons, optogenetic stimulation leads to light-induced suppression in many of these cells. However, some place cells were unaffected by light stimulation and some others increased their firing because they received less inhibition from the suppressed inhibitory interneurons.

In “Activity-dependent plasticity of hippocampal place maps” Professor Jozsef Csicsvari and two of his postdocs, Philipp Schoenenberger and Joseph O’Neill, find that place fields of the unaffected subpopulation remain stable before, during, and after the optogenetic stimulation. On the other hand, place fields of suppressed place cells are unstable, showing remapping across sessions before and after optogenetic inhibition. Disinhibited place cells have stable maps but sustain an elevated firing rate following the stimulation. Their findings suggest that place representation in the hippocampus is constantly governed by activity-dependent processes in which suppressing place cell activity can cause lasting changes in their spatial coding. By contrast, increasing the excitability of these cells can cause lasting upregulation of their place-specific firing rates.

»Download figure illustrating the optogenetic firing responses and the spatial firing of two cells: one suppressed while the other one disinhibited by the light. The panels on the left show how cells change their firing during light application. Black dots show the action potential times relative to light application marked by the green region. The intensity plots on the right illustrate the spatial firing of the cells before, during, and after the light application. Warm colours mark areas where the cells were active. During the light application only the green part of the circular environment was inhibited