Removal of melatonin receptor type 1 increases intraocular pressure and retinal ganglion cells death in the mouse

Abstract

Previous studies have demonstrated that melatonin is effective in lowering intraocular pressure and that it may also protect ganglion cells. We have recently reported that, in mice lacking the melatonin receptors type 1, 25-30% ganglion cells die out by 18months of age, suggesting that these receptors might be important for ganglion cells survival. In this study we show that the loss of ganglion cells is specific for melatonin receptors type 1 knock-out since mice lacking the melatonin receptors type 2 did not show any significant change in the number ganglion cells during aging. Furthermore, we report that melatonin receptors type 1 knock-out mice have higher intraocular pressure during the nocturnal hours than control or melatonin receptors type 2 knock-out mice at 3 and 12months of age. Finally, our data indicate that administration of exogenous melatonin in wild-type, but not in melatonin receptors type 1 knock-out, can significantly reduce intraocular pressure. Our studies indicate that the decreased viability of ganglion cells observed in melatonin receptors type 1 knock-out mice may be a consequence of the increases in the nocturnal intraocular pressure thus suggesting that intraocular pressure levels at night and melatonin signaling should be considered as risk factor in the pathogenesis of glaucoma.

Figure 1

A) Photomicrograph of the superior central retinal of MT₂^−\− at the 18 months of age. Although a clear reduction in the number of cell nuclei is present in the outer nuclear layer (ONL) no changes are detectable in the inner nuclear layer (INL) or in the ganglion cell layer (GCL). B) Number of cells in the RGL in the central superior retina of WT, MT₁^−\− and MT₂^−\− mice at different ages. A significant difference in the number of cells in the RGL is present between WT and MT₁^−\− and MT₁^−\− and MT₂^−\− at 18 months of age. Each bar represents the mean +/− SEM; n = 4–6. *, P< 0.05 (One- Way ANOVA followed by Holm-Sidak tests). Data for the WT and MT₁^−\− have been redrawn from [4].

Figure 2

IOPs in WT, MT₁^−\− and MT₂^−\− mice at 3 and 3 (A), 3 and 12 (B) and 18 (C) months of age. MT₁^−\− showed a significant increase in the IOP during the night at 12 months of age. A significant increase in the IOP was also observed in aged WT and MT₂^−\− (18 months) mice with respect to the values measured at 3 or 12 months (One-Way ANOVA followed by Holm-Sidak tests). Each point represents the mean +/− SEM. N = 4–6 for each point* < 0.05. The animals were maintained in a 12 light: 12 dark cycles. Light on at 06:00 am (ZT0) light off at 06:00 pm (ZT12). ZT=Zeitgeber Time.

Figure 3

Effect of melatonin (i.p. 1 mg/kg) injection on IOP in WT, MT₁^−\− and MT₂^−\− mice. No changes in the IOP were observed in WT mice injected at ZT6 (A, t-test, P > 0.1). Melatonin significantly suppressed IOP in WT mice when injected at ZT12 (B, One-Way ANOVA followed by Holm-Sidak test, P < 0.05) but no changes were observed in MT₁^−\− or MT₂^−\− mice (B). Each point represent the mean +/− SEM. N = 6–8 for each point * < 0.05