Cephalopod vision involves dicarboxylic amino acids: D-aspartate, L-aspartate and L-glutamate

In the present study, we report the finding of high concentrations of D-Asp (D-aspartate) in the retina of the cephalopods Sepia officinalis, Loligo vulgaris and Octopus vulgaris. D-Asp increases in concentration in the retina and optic lobes as the animal develops. In neonatal S. officinalis, the concentration of D-Asp in the retina is 1.8 +/- 0.2 mu mol/g of tissue, and in the optic lobes it is 5.5 +/- 0.4 mu mol/g of tissue. In adult animals, D-Asp is found at a concentration of 3.5 +/- 0.4 mu mol/g in retina and 16.2 +/- 1.5 mu mol/g in optic lobes (1.9-fold increased in the retina, and 2.9-fold increased in the optic lobes). In the retina and optic lobes of S. officinalis, the concentration of D-Asp, L-Asp (L-aspartate) and L-Glu (L-glutamate) is significantly influenced by the light/dark environment. In adult animals left in the dark, these three amino acids fall significantly in concentration in both retina (approx. 25 % less) and optic lobes (approx. 20 % less) compared with the control animals (animals left in a diurnal/nocturnal physiological cycle). The reduction in concentration is in all cases statistically significant (P = 0.01-0.05). Experiments conducted in: S. officinalis by using D-[2,3-H-3]Asp have shown that D-Asp is synthesized in the optic lobes and is then transported actively into the retina. D-aspartate racemase, an enzyme which converts L-Asp into D-Asp, is also present in these tissues, and it is significantly decreased in concentration in animals left for 5 days in the dark compared with control animals. Our hypothesis is that the dicarboxylic amino acids, D-Asp, L-Asp and L-Glu, play important roles in vision.