Abstract
Purpose: :
Nob mice carry a mutation in the nyx gene, lack an ERG b-wave and have abnormal retinal waves post eye opening. Both spontaneous spiking activity (SA) and light evoked activity are abnormal in nob retinal ganglion cells (RGCs) and RGC axon terminals desegregate in the dorsal lateral geniculate nucleus after P12. While the ERG phenotype is identical to nob, RGCs in mice lacking Grm6 have normal SA. We hypothesized that altered nob RGC activity was related to abnormal formation and/or maintenance of retinal circuitry and evaluated SA and light evoked activity in nob RGCs to determine whether abnormal activity was related to synaptic input or intrinsic mechanisms.
Methods: :
SA and light evoked activity were evaluated from amacrine (AC) and RGCs in C57B6/J and nob flat-mounted retinas (P12-adult) using whole cell patch clamp. Pharmacological agents were used to block spiking activity or all synaptic input. Cell morphology and function were correlated using confocal microscopy.
Results: :
27/29 nob RGCs (>P17) had rhythmic SA (F1 = 3 Hz) as did 6/8P14-16 nob RGCs. WT RGCs had SA but none with a rhythmic component. Subthreshold oscillatory membrane potentials (OMPs) (F1 = 3 Hz) were found in 16/24 mature and 8/14 developing nob RGCs, but in 0/27 WT RGCs. In contrast, resting membrane potentials of nob RGCs were similar to WT. Nob ACs also exhibited OMPS. All OMPs were eliminated when synaptic input was blocked. Action potentials were abolished with TTX or QX314, although remnants of the OMPs remained. Nob RGCs light evoked responses were dominated by repetitive EPSCs (ECl) and IPSCs (Ecation) whose frequency matched the subthreshold OMPs. In current clamp, 7/22 nob RGCs were non-responsive (NR) to light and their dendritic arbors ramified in the ON sublaminae of the inner plexiform layer. In voltage clamp, NR nob RGCs lacked EPSCs at light onset and their IPSCs had peak times 2X faster than WT. 2/2 bistratified nob RGCs had responses to light onset that were significantly delayed compared to their response to light offset. All young WT RGCs responded to light with spiking activity, but only 6/14 nob RGCs exhibited light evoked responses and 3 of those responded with graded potentials.
Conclusions: :
A significant alteration in presynaptic input arises during development and is maintained in the mature nob retina. Abnormal SA is eliminated when synaptic transmission is blocked. Additionally, there are abnormal postsynaptic currents correlated with light responses. Since both ACs and RGCs are similarly affected, the most likely origin of abnormal input is the bipolar cells.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • retinal connections, networks, circuitry • development