Geon 7. Local Synchronization via Interneurons MT

 

All forms of physiological gamma rhythm (30 - 80 Hz) are inhibition based (Whittington et al., 2011; Sohal et al., 2009). That is, the neurons (e.g., pyramidal cells) without electrical coupling can be synchronized by forming chemical synapses with an inhibitory GABAergic interneuron such as the parvalbumin (PV)-positive fast spiking (FS) cell which oscillates at the gamma frequency. Its mechanism is illustrated in Figure 1. Without inhibition, a group of pyramidal cells may oscillate at certain frequency with random phases. When the inhibitory interneuron fires, it can inhibit all connected pyramidal cells simultaneously. As the inhibition decays, the pyramidal cells will resume their oscillations. Consequently, all connected pyramidal cells will fire in nearly synchrony at the same frequency as the inhibitory interneuron (Gonzalez-Burgos et al., 2011).

Image

Figure 1. A model for the mechanism of synchronization mediated by interneurons.
Left panel: An inhibitory GABAergic interneuron targets a group of pyramidal cells.
Right panel: Black traces represent the membrane potential of the GABAergic interneuron which remains at rest before and after firing a sequence of four action potentials.
Red traces show the membrane potential simultaneously recorded from the postsynaptic pyramidal neurons, which are firing in response to a continuous excitatory input.
Note that, before the GABAergic interneuron starts firing, the pyramidal cells fire in an asynchronous manner. Shortly after GABAergic neuron spikes (indicated by black arrows), all pyramidal neurons are inhibited simultaneously. After the inhibition decays, the pyramidal neurons fire in nearly synchrony.
[Source: Gonzalez-Burgos et al., 2011]

There are two types of PV interneurons: basket cells that make synapses at the soma and proximal dendrite of target neurons and chandelier cells which target the axon initial segment (AIS) of pyramidal cells (Rudy et al., 2011). A basket cell may connect up to 1,000 pyramidal cells (Halasy et al., 1996). When a group of basket cells are synchronized by gap junctions (previous chapter), they will be able to recruit a large population of neurons into synchronization.

In addition to PV interneurons, there are two other classes of GABAergic interneurons: somatostatin (SOM or SST)-positive and cholecystokinin (CCK)-positive. CCK cells are also called 5HT3R interneurons because they express 5-HT3 receptors. SOM cells fire at 10-30 Hz with low threshold, thus also called low-threshold-spiking (LTS) interneurons (Mancilla et al., 2007). During theta oscillations, CCK cells fire at 8.8 ± 3.3 Hz on the ascending phase of theta waves (Klausberger et al., 2005). Note that SOM cells oscillate mainly in the beta band (13 - 30 Hz) while CCK cells oscillate largely in the alpha band (8 - 12 Hz). Hence, SOM and CCK cells could be employed to mediate beta and alpha synchronization, respectively.

 

Author: Frank Lee
First published: February 13, 2013
Last updated: April 15, 2019