Endogenous levels of neurotransmitter candidates in photoreceptor cells of the turtle retina.

Abstract

Abstract—The neurotransmitter used by vertebrate photoreceptors has not been identified, although aspartic acid, glutamic acid and acetylcholine have all been suggested as possible candidates. In the present study, we have measured the endogenous levels of these substances in isolated photoreceptors of the turtle retina. Fractions containing over 80% of identified photoreceptors were obtained by enzymatic dissociation of the retina followed by velocity sedimentation at unit gravity. The endogenous levels of free amino acids in these fractions were measured by amino acid analysis and the ACh content was measured by a radiochemical assay. In addition, identified photoreceptors were drawn into a micropipet individually under visual observation and their ACh content was determined. Our results show that while the concentrations of free aspartic acid and glutamic acid in isolated photoreceptors are similar to those found in the retina, the concentration of ACh in cone photoreceptors (0.1–0.2 mM) is 2‐ to 3‐fold higher than that in the turtle retina. This result combined with the findings of others suggests that turtle cone photoreceptors may be cholinergic