Phenotypes of trpl Mutants and Interactions between the Transient Receptor Potential (TRP) and TRP-Like Channels inDrosophila

Inability oftrpl³⁰² to adapt to a dim background illumination. A, V–logI curves for wild type (a) andtrpl³⁰²(b) determined at four different intensities of background illumination. V–log I curves relate the response amplitudes to relative stimulus intensities, which are given in log units. Both the test and background stimuli were white lights. Before each test stimulus a background light of 1 min duration was turned on first, and the 2 sec test stimulus was presented at the very end of the 1 min background. For each cell that was examined, all responses were normalized with respect to the maximal peak amplitude obtained in that cell by using the brightest stimulus (logI/I₀= 0) in the absence of background illumination (n = 5). The average maximal peak amplitudes obtained from wild type andtrpl³⁰² were 28 ± 4.2 mV (n = 5) and 26.1 ± 3.3 mV (n = 5), respectively. Dark BG, No background illumination (open squares); −5 BG, background illumination attenuated by five log units (open diamonds); −4 BG, attenuated by four log units (open circles); −3 BG, attenuated by three log units (open triangles). Unlike in wild type, the V–log I curve oftrpl³⁰² obtained at −5 BG is indistinguishable from that obtained in Dark BG. B, Receptor potentials obtained from wild type (a) and trpl³⁰²(b) by using maximum intensity white test stimuli at different background intensities and by using the protocol described in A. Intrpl³⁰² the receptor potentials recorded in dark and −5 log backgrounds are very similar in amplitude and waveform.

Comparison of the refractory periods and response latencies of the receptor potentials obtained from wild type, trpl³⁰², andtrp^P343. The stimulus protocol is shown at the top. After a 2 min dark adaptation two 2 sec stimuli (S1 andS2) were presented 20 sec apart, and the corresponding responses (R1 and R2) were recorded.A, The responses, R1 and R2, obtained in the above protocol are shown superimposed to allow for a comparison of amplitudes: a, wild type; b,trpl³⁰²; c,trp^P343. The term “refractory period” refers to the time required for the second response (R2) to attain a response amplitude similar to that of the first (R1). R2s of both wild type andtrpl³⁰² have amplitudes similar to those of R1s. R2 oftrp^P343, however, is much smaller than that of R1. The stimulus intensity was attenuated by one log unit (log I/I₀ = −1). B, The initial 120 msec of the responses shown inA are presented at a higher sweep speed than inA to allow for a comparison of latencies:a, wild type; b,trpl³⁰²; c,trp^P343.Arrows indicate the beginning of light stimuli. The response latency is defined as the time between the beginning of stimulus and the onset of the response. C, Histogram showing the response latencies of R1 and R2 obtained from wild type,trpl³⁰², andtrp^P343(n = 10). In both wild type andtrpl³⁰² the response latencies of R1 and R2 are similar in magnitude, and in both the R2 latency is significantly shorter than that of R1. Intrp^P343, on the other hand, both R1 and R2 latencies are much longer than those of the other genotypes; moreover, the R2 latency is significantly longer than that of R1. D, Summing thetrpl³⁰² andtrp^P343responses does not reproduce the wild-type response. Theshadedarea represents the summation of the trpl³⁰² andtrp^P343 responses. The summed response has a larger peak amplitude but a smaller sustained component than the wild-type response.

A, Western blot analysis showing the relative quantity of the TRP protein in wild type,trpl³⁰²,trpl³⁰²InaD^P215, and InaD^P215. The amount of TRP intrpl³⁰² is indistinguishable from that in wild type. Although the amount of TRP inInaD^P215 is reduced only slightly, that oftrpl³⁰²InaD^P215is <10% of the wild-type amount. B, Confocal micrograph showing normal-looking rhabdomeres of thetrpl³⁰²InaD^P215double mutant. The rhabdomeres were visualized by staining F-actin with phalloidin.

Comparison of the properties of the receptor potentials obtained fromInaD^P215 andtrp^P301.A, Representative receptor potentials recorded from wild type, InaD^P215, andtrp^P301 by using prolonged, unattenuated white light stimuli. Although bothInaD^P215 andtrp^P301 receptor potentials decay toward the baseline, theInaD^P215 receptor potential decays more slowly than that oftrp^P301.B–D, The differences in the properties ofInaD^P215 andtrp^P301 responses revealed in the two-stimuli protocol described in Figure 3. As in Figure 3, A and B, the R1 and R2 responses are shown superimposed in B andC. In B, the R2 amplitude is a significantly larger fraction of the R1 amplitude inInaD^P215 than intrp^P301 at any point in the response time course so that the time integral of R2 is a significantly larger fraction of that of R1. C, The initial 130 msec portions of the responses ofInaD^P215 andtrp^P301 inB are presented at a higher sweep speed to allow for a comparison of latencies. D, Histogram comparing the response latencies of R1 and R2 from analysis of records similar to those in C (n = 10) obtained from wild type, InaD^P215, and trp^P301. The response latency of R2 is shorter than that of R1 in wild type, approximately the same as that of R1 inInaD^P215, and longer than that of R1 intrp^P301.