Insulin differentially modulates GABA signalling in hippocampal neurons and, in an age-dependent manner, normalizes GABA-activated currents in the tg-APPSwe mouse model of Alzheimer's disease.

Abstract

We examined if tonic γ-aminobutyric acid (GABA)-activated currents in primary hippocampal neurons were modulated by insulin in wild-type and tg-APPSwe mice, an Alzheimer's disease (AD) model. GABA-activated currents were recorded in dentate gyrus (DG) granule cells and CA3 pyramidal neurons in hippocampal brain slices, from 8 to 10weeks old (young) wild-type mice and in dorsal DG granule cells in adult, 5-6 and 10-12 (aged) months old wild-type and tg-APPSwe mice, in the absence or presence of insulin, by whole-cell patch-clamp electrophysiology. In young mice, insulin (1nmol/L) enhanced the total spontaneous inhibitory postsynaptic current (sIPSCT ) density in both dorsal and ventral DG granule cells. The extrasynaptic current density was only increased by insulin in dorsal CA3 pyramidal neurons. In absence of action potentials, insulin enhanced DG granule cells and dorsal CA3 pyramidal neurons miniature IPSC (mIPSC) frequency, consistent with insulin regulation of presynaptic GABA release. sIPSCT densities in DG granule cells were similar in wild-type and tg-APPSwe mice at 5-6months but significantly decreased in aged tg-APPSwe mice where insulin normalized currents to wild-type levels. The extrasynaptic current density was increased in tg-APPSwe mice relative to wild-type littermates but, only in aged tg-APPSwe mice did insulin decrease and normalize the current. Insulin effects on GABA signalling in hippocampal neurons are selective while multifaceted and context-based. Not only is the response to insulin related to cell-type, hippocampal axis-location, age of animals and disease but also to the subtype of neuronal inhibition involved, synaptic or extrasynaptic GABAA receptors-activated currents.