Dr Mikko Juusola: Photoreceptor Studies

Wild-type flies

We initially focused upon visual processing in the photoreceptors of wild type red-eyed flies, since these will form the control group for any future studies using mutations. Recordings were made from photoreceptors during prolonged periods of exposure to dynamically modulated light over a range of light intensities during recordings that lasted for over 1 hour. The performance of the photoreceptors in coding the light signal was quantitatively described by its sensitivity, signal-to-noise ratio (SNR) and frequency response. The information rate of the photoreceptors, a measure of the number of states a signalling system can transmit in a given time window, was calculated from the SNR providing accurate measures of the coding properties of the photoreceptors to which mutations could be compared (Juusola & Hardie, 2001a,b).

Shaker flies

Having obtained a detailed characterisation of the signalling properties of the wild-type cells we expanded our studies to include mutations in specific ion channels present in the membrane of the photoreceptors. One such mutation is in the Shaker gene, which encodes a voltage-activated K+-channel producing a rapidly inactivating A-type current. ShKS133 photoreceptors show a dramatic decrease in SNR of their voltage responses compared to those of wild-type photoreceptors resulting in an almost two-fold decrease in the information capacity of these cells in fully light-adapted conditions (Niven et al., 2003). By combining experiments with modelling, we showed that the inactivation of Shaker K+-channels amplifies voltage signals and enables photoreceptors to use their voltage range more effectively. Loss of the Shaker conductance attenuated the voltage signal and induced a compensatory drop in impedance. Our results showed that reduction in the information capacity in ShKS133 photoreceptors can be explained by the loss of Shaker conductance and suggested a compensatory increase in other conductances, and thus emphasised both the contribution of specific ion channel properties and the effects of compensatory mechanisms upon neuronal information processing (Niven et al. 2003a,b; Juusola et al. 2003).