Phenotypes of trpl mutants and interactions between the transient receptorpotential (TRP) and TRP-like channels in Drosophila

The trp and trpl genes are thought to encode two classes of light-activated ion channels in Drosophila. A previous report indicated that a null trpl m utant does not display any mutant phenotype. This lack of detectable mutant phenotypes made it difficult to suggest functions for the transient recept or potential-like (TRPL) channel in photoreceptor responses. Here, the prop erties of trpl photoreceptor responses were studied by using electroretinog ram (ERG) and intracellular recording techniques in combination with light stimuli of relatively long durations. Distinct mutant phenotypes were detec table under these conditions. These consisted of a reduced sustained compon ent, oscillations superimposed on the response, a poststimulus hyperpolariz ation, and altered adaptation properties to dim background light. Compariso n of photoreceptor responses obtained from wild type, trp, and trpl showed that the responses obtained from the trp and trpl null mutants did not sum up to that of the wild-type response. To explain the nonlinear summation at the peak of the response, Reuss et al. (1997) proposed that Ca2+ ions ente ring through the TRP channel modulate TRP and TRPL channel activities diffe rentially. However, nonlinear summation was present not only at the peak bu t throughout the duration of response. Two lines of evidence are presented to suggest that, in addition to the interaction proposed by Reuss et al. (1 997), there are other forms of interactions between TRP and TRPL channels, probably involving the channel proteins themselves.