Figure 3.

Gain control via dendritic inhibition. (A) It has been postulated that inhibition of active dendritic conductances might act divisively, altering the slope or the gain of the f/I curve whereas inhibition of passive dendrites or somas would be predicted to be subtractive (simply shifting the curve, i.e., raising the threshold; Holt and Koch, 1997). Several systems reliant on pyramidal neurons have confirmed this hypothesis. (B) In the electric fish, one pathway (nP) targets the perisomatic region whereas another, feedback pathway (EGp) targets their active dendrites. A clear difference can be measured in terms of divisive versus subtractive inhibition in this system. Adapted from (Mehaffey et al., 2005). (C) Similarly, in the mammalian hippocampus, synaptic input is segregated on the dendritic trees of CA1 neurons where dendritic targeting somatostatin-positive inhibitory neurons have been shown to perform gain modulation whereas more proximal parvalbumin-positive neurons have negligible effect on gain. Adapted from (Lovett-Barron et al., 2012). (D) A similar effect of dendritic targeting, somatostatin-positive, Martinotti neurons of the neocortex was found from recordings in anesthetized rats. Here, deep-layer Martinotti neurons activated in a feedback loop with local L5 pyramidal neurons, control the gain of the f/I curve (Murayama et al., 2009).