Iontophoretic Studies of Visual Neurons in Drosophila melanogaster

The physiology of single cells in the visual system of the fly Drosophila melanogaster is studied by intracellular recording. Iontophoresis or current passage from the micropipette is used in two distinct ways. The first of these is to yield a short injection of free calcium ion into the photoreceptor or primary sensory neuron. Iontophoresis of Ca++ is shown to have a pronounced but r eversible effect upon the receptor potential response to light input. The transient change is characterized by gain suppression and phase delay to sinusoidal stimuli. The phase delay is expressed at 10.0 Hz and higher. These changes are, however, dependent upon the genotype of the animal. Two single-gene mutants of this fly have been identified as having lesioned or altered physiology in the visual receptors. The location of the lesions is in the casual pathway after photon absorption by the photopigment and before ion permeability changes are effected at the plasma membrane. The norp AH52 mutant responds to Ca++ injection in a manner similar to that of wild type. The trp mutant, however, has a much prolonged recovery period. The recovery, when finally begun, is characterized by a transient phase advance with respect to the normal response. To this author's knowledge this is the first description of a phase advance in a visual cell potential that does not involve manipulation of the light stimulus. A physiological model for phototransduction is developed to account for the results of Ca++ injection and the new distinction in receptor function based upon genotype. This model incorporates both intracellular Ca++ and mitochondrial energy in the early events of the visual sensory process. A study of mathematical models developed to account for the time scale of the recovery to injection concludes that the photoreceptor sequesters free Ca++ following an injection. This conclusion is consistent with the physiological model for phototransduction. Iontophoresis is used in a second way in a study of membrane properties of both photoreceptors and second-order neurons of the visual system. Small currents are passed into the cells simultaneously with the application of Gaussian white noise light stimulation. The Wiener kernels obtained for the intracellular potential are used as sensitive assays of the dynamics of membrane resistance changes following light input. The photoreceptor membrane conductance is approximately a linear function of light intensity for the stimuli used. By contrast the monopolar neurons of the Drosophila visual system, which are proximal to the primary sensory cells, exhibit an active voltage dependence in the response.