The neural circuit underlying fast spike interneuron enhanced gamma band oscillations (30-100 Hertz) has been relatively well defined. The fast spiking inhibitory interneurons release GABA onto GABA-A receptors on regular spiking pyramidal neurons. The time constant of GABA-A synaptic inhibition is a key factor controlling gamma rhythmicity. Nevertheless, there is more than one type of fast spiking interneuron in the brain that are often difficult to distinguish. In the recent paper “Parvalbumin-Containing Fast-Spiking Basket Cells Generate the Field Potential Oscillations Induced by Cholinergic Receptor Activation in the Hippocampus” (published November 10, 2010 in the Journal of Neuroscience) the authors set out to establish the identity of inhibitory interneurons responsible for helping to generate gamma oscillations.
In this paper the authors identified and focused on three types of interneurons. Two were parvalbumin containing fast spiking interneurons: fast spiking basket cells and axoaxonic cells (also known as chandelier cells). The other type of interneuron was the regular spiking basket cells. The data presented by the authors lead to the conclusion that fast spiking basket cells help generate gamma oscillations in the brain whereas axoaxonic cells and regular spiking basket cells do not. They presented the following evidence:
- Fast spiking basket cells fired more in synchrony with oscillations than did axoaxonic cells or regular spiking basket cells.
- Inhibitory postsynaptic currents of regular spiking basket cells were almost completely eliminated by the activation of acetylcholine receptors while inhibitory postsynaptic currents from fast spiking basket cells and axoaxonic cells were only reduced. Since the activation of acetylcholine receptors increased gamma oscillations, the fact that inhibition from regular spiking basket cells was nearly eliminated would suggest that these cells do not play an active role in the generation of oscillatory activity in the gamma band.
- A mu opioid receptor blocker selectively reduced the inhibitory currents of fast spiking basket cells but not of axoaxonic cells. Since the application of a mu opioid receptor blocker eliminated gamma oscillations, its selective block of the inhibitory currents of fast spiking basket cells suggests that the currents from this cell type are important in generating gamma oscillations.
These data add more evidence for a key role of fast spiking interneurons in the generation of gamma oscillations. More specifically, these data identify fast spiking basket cells as the drivers of gamma oscillations.
The data in this paper were obtained in hippocampal slices kept at room temperature. Therefore the observed oscillations were slower than gamma oscillations. Also, the activation of acetylcholine receptors was carried out using carbachol, which is a cholinergic agonist that induces oscillatory activity. Nevertheless, these data move us closer to establishing the detailed circuits and mechanisms underlying gamma oscillations in the brain.
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