Abstract: Objective To observe the effect of IGF-1 on inhibitory synaptic transmission in cultured rat hippocampal neurons and explore the possible mechanism.Methods Hippocampal neurons of rat were cultured with serum-free medium and used for experiment at 10～14 days. The cells were divided into 2 groups in the electrophysiology experiment: the control group and the IGF-treated group(in this group neurons were treated with 10 μmol/L IGF-1 24 h before experiment). While, 3 groups were included in the immunocytochemistry section: the control group, the IGF-treated group and PD98059 (10 μmol/ L) pretreated group(PD98059 was added to the medium 1 h before the addition of IGF-1). We observed the effects of IGF-1 on inhibitory postsynaptic current(IPSC) with the whole-cell patch clamp recording method and on the ratio of GABAergic neurons with immunocytochemical method. The patch-pipette solution contained (in mmol/L): 140 KCl, 10 HEPES, 10 EGTA, 2 MgCl₂, 2 Na₂ATP, 1 CaCl₂, pH 7.3. The typical resistance of glass electrodes was 3～7 MΩ when filled with intracellular pipette solution. The range of the whole-cell series resistance is 10～15 MΩ. During experiments, culture dishes were rinsed and perfused with extracellular solution containing (in mmol/L):140 NaCl, 5 KCl, 1 MgCl₂ , 10 HEPES, 10 Glucose, 3 CaCl₂, pH 7.4. Synaptically spontaneous IPSCs were isolated by the application of 20 M 6, 7-dinitroquinoxaline-2, 3-dione(DNQX) and 50 μmol/L aminophosphonobutyrate(APⅤ), the antagonist to the excitatory-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid(AMPA) and N-methy-d-aspartate(NMDA) receptors, respectively.Results IGF-1 remarkably reduced the frequency of IPSC, while had little effect on the mean amplitude. IGF-1 decreased the ratio of GABAergic neurons, which could be blocked by the inhibitor of mitogen-actived protein kinase(MAPK).Conclusion IGF-1 depressed inhibitory synaptic transmission,which may prevent induction long-term potentiation. IGF-1 reduces the ratio of GABAergic neurons. IGF-1 may influence inhibitory synaptic transmission through MAPK signal transduction pathway. As the activation of GABA receptor may depress the LTP, IGF-1 may modulate the function of learning and memory by disinhibition of hippocampal neurons. IGF-1 may be a promising target for learning and memory.

Key words: insulin-like growth factor 1, hippocampal neurons, GABA, inhibitory postsynaptic current, whole-cell patch clamp