Limulus ventral eye. Physiological properties of photoreceptor cells in an organ culture medium.

Abstract

A B S TRA C T Ventral photoreceptor cells bathed in an organ culture medium typically have resting potentials of -85 mV and membrane resistances of 35 M12 and, when dark-adapted, exhibit large potential fluctuations (LPFs) of 60 mV and small potential fluctuations (SPFs) of <30 mV. LPFs appear to be regenerative events triggered by SPFs, the well-known quantum humps. In the dark, SPFs and LPFs occur spontaneously. At intensities near threshold, the rate of occurrence is directly proportional to light intensity, indicating that SPFs and LPFs are elicited by single photon events. At higher intensities, SPFs and LPFs sum to produce a receptor potential that is graded over approximately a 9-log-unit range of light intensity. Amplitude histograms of the discrete potential waves are bimodal, reflecting the SPF and LPF populations. Histograms of current waves are unimodal. SPFs and LPFs are insensitive to 1 /zM tetrodotoxin. I-V characteristics show initial inward currents of -15 nA for voltage clamps to -40 mV and steady-state outward currents for all clamp potentials. Photoreceptor cells bathed in organ culture medium retain these properties for periods of at least 75 days.

Limulus psychophysics: Increment threshold

“…Kaplan and Barlow (1975) noted, these values can be obtained from the steady-state intensity-response function by direct calculation if one uses Fechner's scaling assumption (namely, that equally discriminable light increments correspond to equal changes in the sensory signal). Such calculations were done for the behavioral results using Bayer and Barlow's (1978) integrated intensity-response data from the ventral eye." Since the behavioral data of the present experiment were mediated by the ventral eye, physiological data obtained from the ventral eye by Bayer and Barlow (1978) were used here for comparison whenever possible.…”

“…That was the purpose of the present investigation of Limu/us sensory behavior, which examined the increment threshold in Limu/us. This problem was chosen because physiological data exist that differentiate incremental responses mediated by the receptor integral from other possible codes: Following an earlier suggestion of duality in the Limu/us eye made by Wulff (1950), which was later reinforced by Dowling (1968), Barlow and his co-workers have shown that a clear-cut difference exists between the increment threshold mediated by the peak of the initial receptor transient as opposed to the increment threshold obtained by integrating the receptor response for a long period of time (Barlow & Kaplan, 1977;Bayer & Barlow, 1978;Kaplan & Barlow, 1975). The peak increment threshold is a linear function of log background intensity over a wide range.…”

“…Stimulus spectral distribution given as the quanta incident on each receptor per second per nanometer (nm) as a function of wavelength (in nm). Solid line describes the most intense stimulus used in the present work; dashed line, the stimulus used by Bayer and Barlow (1978). Their spectrum has an arbitrary vertical location.…”

“…Since a comparison of the results of this experiment with the work done by Bayer and Barlow (1978) on single cells required a conversion of radiance measures to quantal measures, the stimuli used in the present experiment were also calibrated spectroradiometrically. A J-6514 probe (whose spectral sensitivity was provided by Tektronix) was used with Zeiss (Jena) interference filters that had been individually calibrated in a Beckman DK-2 recording spectrophotometer.…”

“…The spectroradiometric data alone are not enough to compare our behavioral stimuli with the physiological stimuli used by Bayer and Barlow (1978), because the optics involved in the two experiments differed. They recorded from excised cells taken from the ventral eye of Limulus.…”