GABAergic transmission is essential for the normal development of neuronal circuitry, however little is known about the developmental plasticity of GABAergic neurons or whether these aspects of their development are regulated by sensory experience. Previous work in our lab has demonstrated that GABAergic transmission plays an essential role in the development of retinotectal circuitry as it shifts from excitatory at early stages to inhibitory at later stages by promoting glutamatergic synaptic maturation. Our aim for the current study was to increase our understanding of the activity driven aspects of this maturation by manipulating the sensory environment.

We first determined how GABAergic neurons arrange in the retinotectal projection of Xenopus laevis as the circuit matures. We determined the distribution of GABAergic neurons by immunoreactivity in stages before and after the shift from excitatory to inhibitory GABA neurotransmission. We found that both the number and distribution of GABA-immunoreactive neurons is modified when comparing these two stages.

Previous work has demonstrated that GAD67, the synthetic enzyme for GABA, is regulated by neuronal activity. To determine if the GABAergic system is regulated by sensory experience in Xenopus, we tested the effect of visual system activity on the levels of GABA immunoreactivity, as a readout of GAD67 activity. We performed a quantitative analysis of GABA immunoreactivity under conditions of increased or decreased sensory drive before and after the GABAergic transition from excitatory to inhibitory. We find that at the earlier stage, visual stimulation reduces the levels of GABA immunoreactivity in the tectum compared to dark treated animals, whereas at the older stage, visual stimulation increases the levels of GABA immunoreactivity, while the pattern of GABA immunoreactivity at the respective stages remains unmodified. Our findings describe the development of GABA immunoreactive neurons in the optic tectum as GABAergic transmission matures and suggest that the GABAergic system is differentially sensitive to sensory stimulation when it is excitatory and when it is inhibitory.

We went on to develop a tool to study GABAergic cells individually in vivo in the model system X. laevis. First we tested the murine GAD67 promoter for specific cell type specific expression. We then went on to adapt the Gal4-UAS binary system of inducible expression to transient transgenic technology and targeted expression.