Insulin action on the parameters of glutamatergic paired-pulse plasticity in cultured hippocampal neurons under hypoinsulinemia

Hypoinsulinemia is a pathological consequence of diabetes mellitus that can stimulate the development of neurodegenerative processes in the brain due to dysfunction of insulin signaling cascades into neurons and impaired synaptic plasticity properties. We used the whole-cell patch-clamp recording of evoked glutamatergic excitatory postsynaptic currents (eEPSCs) with the local stimulation of presynaptic axon to investigate the insulin (100 nM) effect on the paired-pulse plasticity at cultured hippocampal neurons under hypoinsulinemia (4 days insulin-free media incubation).At control normoinsulinemic conditions insulin enhances the paired-pulse facilitation (PPF) of eEPSCs (n=15) by glutamate release stimulation, as evidenced by increase eEPSCs amplitudes (ratio 1.1±0.02, P<0.005) and PPR from 1.15±0.01 to 1.25±0.01 (P<0.05) with unchanged CV2/CV1 ratio. Simple binomial analysis showed a significant increase of quantal content (m) without changing the probability of glutamate release (p). Under hypoinsulinemia insulin did not affect the parameters of PPF (n=17): PPR, CV2/CV1, p and m were not changed. However, in synapses with paired-pulse depression (PPD, n=16) insulin increased PPR (from 0.77±0.005 to 0.97±0.006; P<0.005) and decreased the CV2/CV1 (from 1.67±0.02 to 1.3±0,01; P<0,05), that may indicate an increase of synaptic release probability, since p were increased by 1.3 times, while m did not change.Thus, insulin modulates the paired-pulse plasticity in hippocampal synapses by stimulating the glutamate release due to quantal content increasing. Under hypoinsulinemia at synapses with PPD insulin can recovered the plasticity to the normoinsulinemic level, but has no effect on PPF synapses due to their probable insulin resistance.