Insulin facilitates synaptic transmission via gap junctions between fast-spiking interneurons in the rat insular cortex.

Purpose: Inhibitory synaptic currents from fast-spiking neurons (FSNs), a typical gamma-aminobutyric acid (GABA)ergic interneuron in the cerebral cortex, to pyramidal neurons are facilitated by insulin. FSNs frequently show electrical synapses to FSNs, however, the effect of insulin on these electrical synapses is unknown. The aim of this study was to evaluate effects of insulin on electrical synaptic potentials between FSNs.

Methods: Electrical synaptic potentials via gap junctions between FSNs were recorded to examine how insulin modulates these potentials in the rat insular cortex (IC).

Results: Bath application of insulin (10 nM), which increases the spike firing rate of pyramidal neurons and unitary inhibitory postsynaptic currents recorded from FSN to pyramidal neuron connections, slightly but significantly increased electrical synaptic currents. The mean ratio of electrical synapses, the coupling coefficient that is obtained by postsynaptic voltage responses divided by presynaptic voltage amplitude, was 8.3 ± 1.1% in control and 9.2 ± 1.1% (n = 14) during 10 nM insulin application. Input resistance and voltage responses to large hyperpolarizing currents (-140 pA) were not changed by insulin.

Conclusion: These results suggest that insulin facilitates spike synchronization by increasing electrical synaptic currents via gap junctions of GABAergic FSNs in the IC.