Figure 2.

Types of dendritic inhibition. (A) Ionotropic, GABA_A inhibition shunts the dendrites and spines via conductance of Cl⁻ and HCO₃ ions. GABA_B receptors on the other hand, which operate metabotropically, have a range of actions and locations. Presynaptically, one isoform of the receptor tends to block Ca²⁺ channels responsible for triggering the release mechanism. Postsynaptically, a different isoform of the GABA_B receptor opens G-protein activated inwardly rectifying K⁺ channels and blocks Ca²⁺ and NMDA channels that control dendritic electrogenesis. GABA_B receptors are found extrasynaptically where they are involved in “volume transmission” and tonic inhibition. (B) Martinotti neurons have been shown to mediate disynaptic inhibition between neocortical pyramidal neurons. Their apical dendrite targets the dendritic initiation zones of nearby pyramidal neurons (Silberberg and Markram, 2007). (C) Top, repetitive activation of Martinotti neurons causes brief and small hyperpolarizing potentials in the dendrites of pyramidal neurons but Ca²⁺ spikes generated by local dendritic depolarization (middle) are still powerfully blocked by GABA_A-mediated dendritic inhibition (bottom) because of the profound block of the underlying mechanisms for Ca²⁺ spikes (Murayama et al., 2009). (D) Late-spiking neurogliaform neurons of layer 1 also target the dendrites of cortical pyramidal neurons (Chu et al., 2003). (E) Neurogliaform cells mediate a large fraction of their inhibitory action on the dendrites through GABA_B receptors (Oláh et al., 2007).