Abstract
Purpose :
Starburst amacrine cells (SACs) play important roles in refining visual circuits by initiating and propagating retinal waves during development. Although developing SACs have the ability to generate action potentials (Zhou and Fain, 1996), it remains unknown whether their spontaneous graded-depolarizations can induce action potentials. We sought to address this important question by examining electrical waves of ON- and OFF-SACs in the developing mouse retina.
Methods :
ChAT-Cre-tdTomato transgenic mice in which ON- and OFF-SACs are visualized by fluorescent microscopy were used for whole-cell current-clamp recordings in flat-mount retinas. Cholinergic and glutamatergic waves of SACs were recorded in the first and second postnatal weeks, respectively.
Results :
Immunostaining with a ChAT antibody confirmed that all tdTomato-labeled cells in ChAT-Cre-tdTomato mice are either ON- or OFF-SACs. ON-SACs at postnatal day 5 (P5) exhibited spontaneous rhythmic depolarizations that were blocked by hexamethonium, a nicotinic acetylcholine receptor antagonist. The depolarizations in 35% of cells recorded (9/26 cells) were accompanied by a burst of action potentials with a frequency of 5.70 ± 0.42 Hz (n=9, mean ± SE). This percentage of spiking cells was increased to 63% (5/8) at P3-4 but dropped to 33% (14/42) at P6-7. In contrast, no OFF-SACs recorded (n=17) exhibited graded-depolarization-induced action potentials during the first postnatal week. During the second postnatal week, spontaneous slow rhythmic depolarizations were also recorded in ON- and OFF-SACs. The depolarizations were blocked by glutamate receptor antagonists (CNQX and D-AP5). Again, OFF-SAC depolarizations did not induce any action potentials (n=14). The percentage of ON-SACs with graded-depolarization-induced action potentials was further dropped to 28% (11/39).
Conclusions :
Our results demonstrate for the first time that ON-SACs depolarize spontaneously with bursts of action potentials in an age-dependent manner. Lack of spiking OFF-SACs indicates that ON- and OFF-SACs could contribute to the initiation and propagation of retinal waves differently during development. We are currently seeking to identify the difference by examining the influence of action potentials on calcium waves across ON- or OFF-SACs.
This is a 2020 ARVO Annual Meeting abstract.