Study On Mechanisms By Which GABA Transmitter And Recepter Terminate The Critical Period Of Rat Visual Development

Objective: Patch-clamp whole cell recording and transmission electron microscope techniques were applied to investigate the electrophysiological and morphological property of layer IVneurons from visual cortex of the rats under different growth condition at different postnatal days and to explore the synaptic and cellular mechanism by which GABA transmitter and recepter involve in the ending of the critical period of rat visual development. Investigation of visual development will help interpret the principle of brain working, and provide theoretical basis for the visual reconstruction of adult visual damage and the treatment of infant-eye-disease.Methods: The visual cortex slices were prepared from normal, chondroitin sulphate proteoglycans (CSPGs) degradation and binocular form deprivation(BFD) rats of both genders. CSPGs degradation rats were obtained by injecting chABC into rat's visual cortex. Patch-clamp whole cell recording techniques were adopted. Pre-synaptic test stimulation was given at 0.07 Hz intervals through bipolar stimulating electrodes placed in white matter. PSCs of layer IV neurons were recorded by recording electrodes. Then GABA_A-IPSCs were isolated by adding AP-5 and CNQX into artificial cerebrospinal fluid (ACSF). Long-term potentiation (LTP) was induced by pairing 1Hz low frequency presynaptic stimulation with postsynaptic depolarization. During recording, 0.5% biocytin were injected into the cells. Cells were stained after recording for histological assessments. HE staining and transmission electron microscope techniques were also used for morphological study.Results: 1. Input resistance (IR), resting membrane potential (RMP) and the parameters of PSCs are not changed, while the peak value, 10-90% decaytime of GABA-IPSCs and ratio of IPSCs/PSCs increased with age from P3w to P8w in normal rat visual cortex, then reached the top at P5,6w and kept high level in adult. 2. Treatment with chABC had no effect on PSCs, IR and RMP, while peak value, 10-90% decaytime of GABA-IPSCs and ratio of IPSCs/PSCs were significantly decreased by chABC treatment.ChABC treatment also significantly decreased LTP incidence and amplitude of P35d rats. 3. BFD statistically decreased the peak value, 10-90% decaytime of PSCs and GABA-IPSCs and ratio of IPSCs/PSCs. There are also statistical differences of all the above parameters between BFD and chABC treatment group. 4. chABC had no effect on density of cells, cell body form, dentrite growth and ultrastructure, while BFD significantly changed those of layer IV interneurons in rat visual cortex.Summary: 1. Function of GABAa receptors of layer IV neurons in visual cortex is age dependent around the end of the critical period, reaching the top at P5,6w and keeping high level in adult. 2. The maturity of GABA circuits is involved in the end of critical period of visual development. The constant high level of GABAA receptors may result in the reduced plasticity in adult. 3. ChABC did not change PSCs, but changed GABAA-IPSCs through degradation of CSPGs, suggesting the maturity of GABA circuits is associated with the mechanism of inhibition of CSPGs for visual development plasticity. 4. CSPGs is suggested to be involved in LTP and long-term plasticity of layer IV in visual cortex. 5. Function of GABAa receptors of layer IV neuron in visual cortex is experience dependent. BFD affect inhibitory synaptic transmission through GABAa receptors. 6. Form information is crucial to the function development of visual cortical neurons. The kinetics basis of PSCs changes in BFD rats may lie in desensitization of postsynaptic receptors. 7. The changed morphological property and ultrastructure of layer IV intemeuron in visual cortex by BFD is the basis of the changed PSCs by BFD. 8. ChABC has no effect on morphological property and ultrastructure of layer IV intemeuron in visual cortex.