Vascular Endothelial Growth Factor Regulates The Release Of Nurotransmitters Via Voltage-dependent Calcium Channels In The Hippocampal Slices Of Neonatal Rats

Vascular endothelial growth factor (VEGF) was originally purified from endothelial cells. It has been demonstrated that VEGF widely expresses in the mammalian brain. Our previous studies have revealed that VEGF and its receptors locate at neurons in both immature and adult rat brains, and that VEGF inhibits the delayed outward potassium currents in hippocampal neurons via enhancing tyrosine phosphorylation of potassium channel kv 1.2 protein. In addition, a recent article reported that VEGF reduced the evoked postsynaptic potentials triggered by stimulating the excitatory pathways in hippocampal slices.To investigate whether VEGF regulates the neurotransmission, we used the whole-cell patch-clamp technique to record the glutamatergic spontaneous excitatory and GABAergic inhibitory postsynaptic currents (sEPSCs and sIPSCs) as well as the miniature inhibitory postsynaptic currents (mlPSCs) in the CAl pyramidal neurons of hippocampal slices from 14-day-old rats. The results are as follows:1. VEGF modulates the release of neurotransmitters in the hippocampus.(1) To study whether VEGF has any effect on neurotransmission, we separately recorded the glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) and the GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) in the hippocampal slices. We observed that VEGF significantly increased the frequency of sEPSCs and reduced the frequency and amplitude of sIPSCs. Among those recorded neurons, 30% and 50% of recorded neurons showed increase of sEPSCs and decrease of sIPSCs by VEGF, respectively. Then, we further analyzed whether the reduction of sIPSCs by VEGF was associated with the presynaptic or postsynaptic inhibition.(2) To study whether VEGF directly modulates the activities of postsynaptic GABA receptors, we recored the exogenous GABA-evoked postsynaptic chloride currents in hippocampal slices. The result showed that VEGF did not change the exogenous GABA-induced postsynaptic chloride currents, suggesting that VEGF-decreased sIPSCs does not relate to postsynaptic inhibition. Therefore, we further studied the effect of VEGF on the GABAergic miniature inhibitory postsynaptic currents (mlPSCs), of which frequency can reflect the release of GABA from presynaptic axonal terminals. We observed that VEGF dose-dependently reduced the mIPSC frequency. The results clearly demonstrated that the reduction of sIPSCs by VEGF was caused by inhibition of GABA release from the presynaptic axonal terminals.(3 ) To test the mechanism of the inhibition of GABA release by VEGF, we further studied the effect of VEGF on the release of GABA in the presence of barium, a general blocker of potassium channels, and cadmium, a nonspecific blocker of high-voltage-activated calcium channels (HVA calcium channels). The result showed that cadmium not barium prevented the reduction of the mIPSC frequency by VEGF. These results indicate that VEGF suppresses the release of GABA via HVA calcium channels. Furthermore, SU1498, a specific inhibitor of Flk-1, did not influence the efficacy of VEGF-inhibited mIPSCs, suggesting that Flk-1 is not involved in the VEGF's inhibition.Putting together, the results suggest that VEGF enhances the gluatmatergic neurotransmission, and suppresses the GABA release from presynaptic neuronal terminals via VDCCs.2. VEGF inhibits HVA calcium currents in the CAl pyramidal neurons of hippocampal slices.As mentioned above, the inhibition of GABA release by VEGF is associated with the activities of VDCCs, especially the HVA calcium channels. Therefore, we next studied the effect of VEGF on the HVA calcium currents in the CAl pyramidal neurons of hippocampal slices from 14-day-old rats. The results showed that VEGF rapidly, reversibly, dose-dependently decreased the HVA calcium currents, and that VEGF did not influence the voltage-dependent property of HVA calcium currents.Collectively, VEGF can directly and specifically reduce the activities of HVA calcium channels in the hippocampal CAl pyramidal neurons.In the present thesis, we provide the first evidence that VEGF can directly modulate the release of neurotransmitters from the presynaptic axonal terminals via voltage-dependent calcium channels.