When Less is More: Paradoxical Regulation of M1 ip-RGCs by Dopamine

Abstract

Purpose : Costratification of M1 ipRGC dendrites with those of dopaminergic amacrine cells suggests a role for dopamine in modulating photic signals conveyed to the SCN. As M1 ipRGCs are susceptible to depolarization block, it is conceivable that dopamine may act to mitigate these effects. Insights about the role of dopamine modulation on circadian rhythms has important therapeutic implications.

Methods : D₁ receptor expression patterns in ipRGCs were revealed by re-analysis of published RNAseq data. We investigated changes in M1 photocurrents and spiking activity following application of dopamine and D₁ antagonist, using both cell-attached and whole-cell patch clamp configurations. Photocurrents were elicited by a 100 ms flash of 480 nm light, ranging from 5x10⁶ to 10⁹ photons/µm². To survey a larger population of cells, we used a multi-electrode array (MEA). We generated a mouse line (3xPKA-Mut) where melanopsin is insensitive to PKA phosphorylation to study the effects of PKA phosphorylation on melanopsin-based phototransduction.

Results : D1 receptors are expressed in M1 ipRGCs. The magnitude of the melanopsin-based light response doubled when replacing dopamine with D₁ antagonist in 50% of M1 ipRGCs. Most M1 ipRGCs exhibited sustained spiking in dopamine and transient spiking in D₁ antagonist. Saturating light responses in M1 cells were ~ 400 pA in Opn4^+/+ retinas, ~ 200 pA in Opn4^3xMut/+ retinas, and < 20 pA in Opn4^3xMut/3xMut retinas. Immunofluorescence and confocal microscopy revealed a dramatic reduction in melanopsin expression in Opn4^3xMut/3xMut retinas compared to Opn4^+/+ and Opn4^3xMut/+ retinas. The remaining melanopsin was, however, sufficient to stimulate spike production in most M1 ipRGCs.

Conclusions : These results suggest that dopamine regulation is important for modulating the output of M1 ipRGCs. Although it reduces the magnitude of the melanopsin-based photocurrent, dopamine modulation may actually enhance M1 output by mitigating the effects of depolarization block, thereby allowing them to convey photic signals more effectively at high light intensities. Although our results with the Opn4^3xMut/3xMut mouse suggest that dopamine modulation plays an important role in ipRGC physiology and circadian behaviors, interpretation of these results is confounded by the dramatic reduction of melanopsin expression in these mice.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.