Effect of Kainic Acid Injection on the Circadian Rhythm of Aa–Nat Mrna and Dopamine in the Rat Retina

Abstract

Abstract: : Purpose: Earlier investigations have demonstrated that melatonin synthesis in the mammalian retina is under photic and circadian control. The anatomical location of the retinal circadian pacemaker driving melatonin synthesis is unknown. Recent studies have suggested that retinal dopaminergic neurons contain a circadian pacemaker and dopamine is the neurotransmitter that is driving the circadian rhythmicity in the mammalian retina. Previous work has established that intraocular injection of 200 nmol of kainic acid (KA) induces degeneration of neurons in the inner nuclear layer and in the ganglion cell layer, but does not affect photoreceptor cells. In the present study we investigated the effect of KA injection on the daily and circadian rhythms of dopamine and mRNA encoding arylalkylamine N– acetyltransferase (Aa–nat, the key regulatory enzyme in melatonin synthesis) mRNA and dopamine in the rat retina. Methods: Male rats of the Fisher strain (8–10 weeks old) were used in this study. Rats were injected with 200 nmol KA in the right eye and saline in the left eye at 4 different time points in light: dark (LD) cycles. Retinas were then collected 48 hours later. Aa–nat mRNA levels were measured using real–time quantitative RT–PCR; dopamine and DOPAC levels were assayed by HPLC. Results: Aa–nat mRNA levels showed a clear rhythm in LD and in DD in both saline– and KA– injected eyes (P < 0.01). The amplitude of Aa–nat mRNA rhythm was slightly reduced in the KA injected eyes. Dopamine and DOPAC levels were rhythmic in the saline injected eyes (P < 0.01) but not in the KA injected eyes (P> 0.5). Conclusions: Our results demonstrate that Aa–nat mRNA levels are rhythmic in the retina of rats treated with KA acid. Such a rhythm does not depend on rhythm in dopamine levels, since abolishing the circadian rhythm in dopamine does not abolish the rhythm in Aa–nat mRNA. These findings suggest that in rat – as in chicken and Xenopus – the photoreceptors cells contain a circadian pacemaker that drives the circadian rhythm in Aa–nat mRNA or that it is driven by non–dopaminergic input from other retinal cells.

Keywords: dopamine • melatonin • circadian rhythms