Kasandra L. Burgos

6th Year Graduate Student

Department: Cold Spring Harbor Laboratory

Graduate Program: Genetics

Advisor: Holly Cline

Abstract:

Title:  GABAergic neuron development in Xenopus Optic Tectum

GABAergic transmission is essential for the normal development of neuronal circuitry, however little is known about the developmental plasticity of GABAergic neurons in vivo, how they integrate into complex circuits, or whether these aspects of their development are regulated by sensory experience. The retinotectal projection of the albino X. laevis tadpole is a powerful system in which the structural and functional plasticity of neurons can be explored directly. 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 tadpole stages 40 to 42) to inhibitory (stages 47 to 48) by promoting glutamatergic synaptic maturation (Ackerman & Cline, 2006). Our aim for the current study was to increase our understanding of the activity driven aspects of the development of GABAergic transmission further in the developing visual system by manipulating the sensory environment.

In order to gain insight into the underlying differences associated with the shift in GABAergic transmission, we determined how GABAergic neurons arrange in the retinotectal projection of X. laevis as the circuit matures. We performed a descriptive analysis of the spatial 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 notably modified when comparing these two stages.

GABAergic projection pathways and local interneuronal systems play essential roles in visual function. In order to determine if the GABAergic system is regulated by sensory experience, we tested the effect of visual system activity on the levels of GABA. We analyzed GABA-IR under conditions of increased (4 h in a light chamber) or decreased (4 hours in a dark chamber) sensory drive, in stage 42 and 47 animals. We find that at stage 42, visual stimulation reduces the levels of GABA in the tectum compared to dark treated animals, whereas at stage 47, visual stimulation increases the levels of GABA-IR. The pattern of distribution of GABA-IR cells 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.