Electrophysiological evidence for rod-like receptors in the gray squirrel, ground squirrel and prairie dog retinas.

Spectral sensitivities of the gray squirrel, Mexican and 13-line ground squirrel and prairie dog were determined by electroretinography under both dark- and light-adapted conditions. The dark-adapted spectral sensitivity function obtained from intact eyes of these species peaks between 515-525 nm; however, when corrected for lens absorption or recorded from the lensless eye, it peaks near 500 nm and closely matches in shape a rhodopsin nomogram curve (lambda max equals 502 nm). Upon light adaptation all these retinas become relatively more sensitive to long-wave stimuli (i.e., they show a small Purkinje shift). The light-adapted spectral sensitivity function is broader than that obtained from the dark-adapted eye, especially toward the longer wavelengths. Weconclude that in all these species the dark-adapted spectral sensitivity is mediated by a single, rhodopsin-like photopigment and that light-adapted sensitivity is mediated by two (or more) photopigments.