Heteromeric MT1/MT2 melatonin receptors modulate the scotopic electroretinogram via PKC zeta in mice

摘要

Melatonin plays an important role in the regulation of retinal functions, and previous studies have also reported that the action of melatonin on photoreceptors is mediated by melatonin receptor heterodimers. Furthermore, it has been reported that the melatonin-induced increase in the amplitude of the a- and b-wave is significantly blunted by inhibition of PKC. Previous work has also shown that PKC zeta is present in the photoreceptors, thus suggesting that PCK zeta may be implicated in the modulation of melatonin signaling in photoreceptors. To investigate the role PKC zeta plays in the modulation of the melatonin effect on the scotopic ERG, mice were injected with melatonin and with specific inhibitors of different PKC isoforms. PKC zeta knockout mice were also used in this study. PKC zeta activation in photoreceptors following melatonin injection was also investigated with immunocytochemistry. Inhibition of PKC zeta by PKC zeta-pseudosubstrate inhibitor (20 mu M) significantly reduced the melatonin-induced increase in the amplitude of the a- and b-wave. To further investigate the role of different PKCs in the modulation of the ERGs, we tested whether intra-vitreal injection of Enzastaurin (a potent inhibitor of PCK alpha, PKC beta, PKC gamma, and PKC epsilon) has any effect on the melatonin-induced increase in the a- and b-wave of the scotopic ERGs. Enzastaurin (100 nM) did not prevent the melatonin-induced increase in the amplitude of the awave, thus suggesting that PCK alpha, PKC beta, PKC gamma, and PKC epsilon are not involved in this phenomenon. Finally, our data indicated that, in mice lacking PKC zeta, melatonin injection failed to increase the amplitude of the a- and b-waves of the scotopic ERGs. An increase in PKC zeta phosphorylation in the photoreceptors was also observed by immunocytochemistry. Our data indicate that melatonin signaling does indeed use the PKC zeta pathway to increase the amplitude of the a- and b-wave of the scotopic ERG.