The Interaction Between Synaptic Glutamatergic Receptors And GABAergic Receptors In Optic Tectum Of Juvenile Xenopus

Objective: During the early development of the visual system, many species of vertebrate have a transient critical period, when synaptic contacts are modifiable under the influence of the environment, namely synaptic plasticity. GABA and glutamate respectively as inhibitory and excitory neuro-transmitters exist commonly in the central nervous system (CNS). It has been found that the N-methyl-D-aspartate (NMDA) receptors underlie synaptic plasticity at early stages of development. Some studies have shown that r-amino-butyric acid (GABA) receptors have an effect on the synaptic transmission and play a role in the development and maturation of the central nervous system. The purpose of the study was to explore the interaction between synaptic glutamatergic receptors and GABAergic receptors in optic tectum of juvenile Xenopus.Methods: Using the blind technique, whole-cell patch-clamp recording was made from layer VI of a slice preparation of the juvenile Xenopus optic tectum in vitro. Spontaneous miniature postsynaptic currents (mPSCs) were recorded from the tectal neurons. The voltage-clamp was used and the membrane potential usually was held at –50mV. Test solutions were applied by bath perfusion. Spontaneous miniature events were digitally sampled and analyzed using MINI Analysis Software package. The minimal acceptable amplitude for a miniature event was 6 or 7 pA. Significant difference between two distributions was determined using the Kolmogotov-Smirnov test or a paired t test with a P-value <0.05 indicating significance.Results: 1 NMDA enhanced the frequency of miniature inhibitory postsynaptic currents (mIPSCs). The mIPSCs were recorded from the tectal neurons in the juvenile Xenopus. Glutamatergic receptors agonist NMDA could increase the frequency of mIPSCs. After NMDA (50-100M) was perfused, cumulative frequency distributions moved toward left (P<0.05, Kolmogorov-Smirnov test ). It was shown that the frequency of mIPSCs enhanced from 2.67±1.82Hz to 4.28±1.31Hz (paired t test, P=0.004, n=6), the value of mIPSCs frequency being 248.9% of control. 2 Effects of NMDA on the membrane current. It was also observed that the holding current appeared to change to inward delection and higher frequency of miniature excitory postsynaptic currents (mEPSCs) were evoked. Afterward, both mEPSCs and mIPSCs disappeared completely. Finally, the membrane current completely recovered to the beginning. 3 Effects of bicuculline (BM), an antagonist of GABAa receptor, on mPSCs. In some experiments, when the BM (100(M) was added to the bath solution, the frequency of mIPSCs fell down abruptly close to zero. Meanwhile, inward currents appeared, which were blocked after 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (50(M),an antagonist of glutamatergic receptors, was perfused . It had been shown that BM could inhibit mIPSCs and evoke mEPSCs or increase the frequency of mEPSCs.Conclusions: There are two kinds of synaptic receptors: excitory glutamatergic receptor and inhibitory GABAergic receptor in optic tectum of juvenile Xenopus. These receptors are interacted each other in the normal conditions, one kind of which may play a dominant role in the synaptic activities in various conditions. NMDA receptors are fundamental for neuronal plasticity and development in the central nervous system. In our experiments, there were also indications for a presynaptic terminals, and it was found that the presynaptic NMDA receptors modulated the release of GABA. Taken together, all spontaneous synaptic activities may participate in the interaction. The consistence of the synaptic activities contributes to acquire the stability in its structure and function. Futhermore, the nervous system as a kind of network may keep the stability and balance.