Predominantly linear summation of metabotropic postsynaptic potentials follows coactivation of neurogliaform interneurons

Summation of ionotropic receptor-mediated responses is critical in neuronal computation by shaping input-output characteristics of neurons. However, arithmetics of summation for metabotropic signals are not known. We characterized the combined ionotropic and metabotropic output of neocortical neurogliaform cells (NGFCs) using electrophysiological and anatomical methods in the rat cerebral cortex. These experiments revealed that GABA receptors are activated outside release sites and confirmed coactivation of putative NGFCs in superficial cortical layers in vivo. Triple recordings from presynaptic NGFCs converging to a postsynaptic neuron revealed sublinear summation of ionotropic GABA(A) responses and linear summation of metabotropic GABA(B) responses. Based on a model combining properties of volume transmission and distributions of all NGFC axon terminals, we predict that in 83% of cases one or two NGFCs can provide input to a point in the neuropil. We suggest that interactions of metabotropic GABAergic responses remain linear even if most superficial layer interneurons specialized to recruit GABA(B) receptors are simultaneously active.

Automated summary

Ionotropic receptor-mediated responses are critical in neuronal computation, while the arithmetic of summation for metabotropic signals is unclear. Experiments on neocortical neurogliaform cells revealed sublinear summation of ionotropic GABA(A) responses and linear summation of metabotropic GABA(B) responses. Model predictions suggest that one or two NGFCs can provide input to a point in the neuropil in 83% of cases and metabotropic GABAergic responses remain linear even with simultaneous activation of superficial layer interneurons specialized to recruit GABA(B) receptors.