Spatiotemporal order and interdependence of GABA receptor subtype expression in developing mouse retinal bipolar cells

Abstract

Purpose : Mouse retinal bipolar cells express distinct ionotropic GABA receptors on their axons and dendrites. ON bipolar cell axons predominantly express GABA_Aα1 and GABA_C receptors and relatively low levels of GABA_Aα3 receptors, whereas their dendrites express only GABA_Aα1 receptors (Hoon et. al. 2015). The developmental sequence in the appearance of these bipolar cell GABA receptor subtypes is unknown. Further, it is not known if the developmental regulation of the expression of these receptor subtypes is interdependent. This study tested the hypothesis that the developmental expressions of different GABA receptor subtypes on mouse retinal bipolar cells are regulated separately.

Methods : We immunostained GABA receptor subtypes in retinas of transgenic mice with fluorescently labeled ON bipolar cells before and after eye-opening, which occurs at around postnatal day (P) 14. To determine if the expression patterns of GABA receptor subtypes are regulated separately, we used two GABA_A receptor specific knockouts: The GABA_Aα3-subunit knockout (Yee et. al. 2005), and a line in which GABA_Aα1 subunit was conditionally knocked out in ON bipolar cells.

Results : GABA_Aα3 receptors were present on bipolar cell axon terminals at P7 and subsequently downregulated as GABA_Aα1 receptor expression emerged around P9. GABA_C receptors were expressed last, at around P12. ON bipolar cell dendrites never expressed GABA_Aα3 receptors, and GABA_Aα1 receptors appeared concurrently in the axon and dendrites. In the absence of GABA_Aα3 receptors, GABA_Aα1 receptors are much reduced in bipolar cell axons only. In contrast, the developmental profile of GABA_Aα3 receptors in bipolar cells is unchanged when GABA_Aα1 receptors are absent. Neither loss of GABA_Aα1 or GABA_Aα3 receptors affected GABA_C receptor expression.

Conclusions : There is a temporal order in the emergence of various bipolar cell GABA receptor subtypes. Expression of GABA_Aα1 receptors on bipolar cell axons, but not on dendrites, requires preceding GABA_Aα3 expression. However, the developmental downregulation of GABA_Aα3 receptors in bipolar cells is not dependent on GABA_Aα1 upregulation. GABA_C receptor expression in bipolar cell terminals is established independently of GABA_A receptors.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.