Physiological Characteristics of Photoreceptors in the Lamprey, Lethenteron japonicum.

Lampreys are among the most basal vertebrates, and similar to jawed vertebrates, they have two types of photoreceptors: long photoreceptors (LP; putative cones) and short photoreceptors (SP; putative rods). It is intriguing to examine the physiological properties of vision in these animals. Although there is an accumulating body of histological and biochemical studies of photoreceptors of the lamprey Lethenteron japonicum, many physiological characteristics of this species have not been described. In the present study, single-cell recordings of photoreceptors in the upstream migrant lamprey were performed to investigate the physiological properties of SP and LP of the lamprey Lethenteron japonicum. It was found that the sensitivity in LP at 560 nm was 2000 photons µm-2, whereas that in SP at 520 nm was 67 photons µm-2, which is approximately a 30-fold difference. Moreover, the response kinetics of LP was remarkably faster than those of SP, which is consistent with previous studies of other Northern hemisphere lampreys. Unexpectedly, the amplitude of single-photon response in the lamprey SP was approximately 0.12 pA, less than 1% of the circulating current. The small amplitude in lamprey SP may degrade the ability to detect single photons of this species. The spectral sensitivity analysis revealed that approximately 30% of all the chromophores are composed of A2 retinal, which may account for the relatively low amplitude of single-photon response in SP.