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Adaptation in pheromone-sensitive trichoid sensilla of the hawkmoth Manduca sexta

Jan Dolzer^1,2, Karin Fischer² and Monika Stengl^1,2,*

¹ Biologie, Tierphysiologie, Philipps-Universität Marburg, D-35032 Marburg, Germany
² Institut für Zoologie, Universität Regensburg, D-93040 Regensburg, Germany

View larger version (30K): [in a new window]   Fig. 9. Adaptation of responses to long stimuli. At a stimulus duration of 1000 ms, the sensillar potential (SP) amplitude (A), the initial slope (B), the half time of the rising phase of the SP (t1/2 rise; C), the half time of the decline of the SP (t1/2 decline; D) and the action potential (AP) frequency (E) and latency (F) were dose-dependent (solid lines). After adaptation by a 1000 ms stimulus applied 60 s before the test stimuli, these dose–response curves were shifted to higher stimulus intensities (broken lines). As with stimuli of 50 ms duration (Fig. 7), the shift was larger for the AP response than for the SP response. Responses in the adapted state were recorded to a maximal dose of 100 µg bombykal. The data were normalized to the highest response during each recording, which is the largest numerical value for those variables positively correlated to the stimulus intensity. The AP latency and t1/2 rise, which are negatively correlated to the intensity, were inversely normalized to the smallest numerical value to focus on responses in the physiological dose range. C, control. Data represent means ± S.E.M. Sample sizes (N) range between 2 (10–2 µg; adapted) and 22 (1 µg; non-adapted). Asterisks indicate significant differences between the adapted and non-adapted state (*P<0.05; **P<0.01; Student's t-test).

View larger version (30K): [in a new window]   Fig. 7. Short-term adaptation of responses to short (50 ms) stimuli shifted the dose–response curves to higher stimulus intensities. While the sensillar potential (SP) amplitude (A) and the initial slope (B) encoded concentration changes at doses 10–2 µg bombykal, the half-time of the rising phase (t1/2 rise; C) showed no significant dose dependence. The decline of the SP back to baseline (t1/2 decline; D) was relatively slow, even at low doses. After adapting stimuli (dotted lines), the amplitude of the sensillar potential and its initial slope were reduced, while t1/2 rise was not affected. Adaptation accelerated the decline of the SP and reduced the action potential frequency computed from the first five interspike intervals (AP frequency), while the action potential latency (AP latency) increased. After adapting stimuli, the dose–response curves of those parameters that describe the initial phase of the SP were shifted to the right by approximately one log10-unit. Thus, 10 times more pheromone was needed to elicit the same response (A,B). The decline, in addition to a right-shift by more than one log10-unit, was accelerated even in the baseline region of the dose–response curve (D). The AP frequency and the AP latency were shifted by more than one log10-unit (E,F). The data were normalized to the highest response during each recording, which is the largest numerical value for those variables positively correlated to the stimulus intensity. The action potential latency and t1/2 rise, which are negatively correlated to the intensity, were inversely normalized to the smallest numerical value to focus on responses in the physiological dose range. C, control. Data represent means ± S.E.M. The stimulus duration was 50 ms. N=31 (A–D; unadapted), N=24 (E,F; unadapted) and N=10 (A–F; adapted). Asterisks indicate significant differences between the adapted and non-adapted state (*P<0.05; **P0.01; Student's t-test).

View larger version (30K): [in a new window]   Fig. 8. (A–J) Desensitization of pheromone responses during long pheromone stimuli. The action potential response to long bombykal (BAL) stimuli became more phasic with increasing stimulus intensity. Peri-stimulus-time-histograms (PSTHs) at a bin width of 10 ms were calculated for action potentials elicited by 1000 ms stimuli of BAL, which are indicated by the broken lines. While phasic–tonic responses occurred at all stimulus intensities, the phasic component was more prominent at high intensities. This indicates the presence of desensitization. Since the sample sizes (N) differed for the individual doses, all y-axes were scaled to 4/3xN. The considerable responses elicited by control stimulations with solvent-loaded filter papers (A) are discussed in the text.

View larger version (22K): [in a new window]   Fig. 10. Encoding of stimulus duration [20 ms (A), 200 ms (B) or 2000 ms (C)] in the bombykal cell. Neither the evaluated parameters of the sensillar potential nor the action potential response appeared to encode stimulus durations of 20 ms to 2 s. At long stimulus durations of 2000 ms, the duration of the plateau of the sensillar potential matched the stimulus duration without being reflected in the duration of the action potential response. The sensillum was stimulated locally with a dose of 1 µg bombykal. DC, unfiltered recording, AC, pseudo-highpass-filtered signal, St, stimulus signal, recorded by a pressure sensor. Above the AC traces, the initial portions of the action potential responses are shown as inserts at enlarged scale.

View larger version (26K): [in a new window]   Fig. 1. The pheromone response is characterized by six parameters. (A) An unfiltered response to a 50 ms stimulus of 1 µg bombykal (BAL). Action potentials are superimposed on the negative deflection of the transepithelial potential, the sensillar potential (SP). The maximal SP amplitude is measured between the baseline before the response and the negative peak during the response. The half-time of the rising phase (t1/2 rise) is determined between the onset of the SP and the time the potential has reached 50% of SP. The half-time of the decline phase (t1/2 decline) is measured between the end of the response, which at short stimulus duration coincides with the negative peak, and the time the potential has decayed to 50% SP. For the analysis of all parameters describing the SP, the responses are lowpass-filtered at 50 Hz or 70 Hz. (B) The initial phase of the response at an enlarged time scale. The initial slope is determined by dividing 0.5xSP by t1/2 rise. The action potential (AP) latency is measured between the onset of the SP and the peak of the first AP. (C) For the analysis of APs, the lowpass-filtered response is subtracted from the original trace, yielding a straight baseline. The initial AP frequency is computed from the first five interspike intervals.

View larger version (28K): [in a new window]   Fig. 2. The adaptation protocol for short-term adaptation. A conditioning stimulus of 10 µg bombykal (BAL) and 250 ms duration was applied 20 s before each test/control stimulus of 50 ms duration. Test stimuli of various BAL doses (measured in log10-units; filled columns) or the control (C; open columns) were applied in increasing order. To avoid accumulative adaptation, stimulus pairs were separated by at least 10 min. The time axis is not drawn to scale. For test stimuli of 1000 ms duration, a similar stimulation protocol was used. The conditioning stimuli were 1000 ms long, however, and applied 60 s before the test stimuli.

View larger version (10K): [in a new window]   Fig. 3. Cross-excitation of the non-bombykal cell occurred in response to a subset of the 10 µg bombykal (BAL) stimuli of 50 ms duration (see Table 2). While in (A) BAL stimuli elicited only large action potentials, in (B) small action potentials were also elicited at a lower frequency and after a longer latency. Responses of small action potentials were not elicited at BAL doses below 10 µg. The terms `large' and `small' action potentials refer to the amplitude of the first action potential of each class, before the stimulus-correlated amplitude reduction took place. AC: pseudo-highpass-filtered traces, obtained by subtracting lowpass-filtered responses (50 Hz Gaussian filter) from the original signals (DC).

View larger version (25K): [in a new window]   Fig. 4. At doses below 10 µg bombykal (BAL), only action potentials of the large class were affected by the stimulus-correlated amplitude reduction. (A) Response to a 50 ms stimulus of 1 µg BAL, pseudo-highpass-filtered as in Fig. 3. The amplitudes of the large action potentials were reduced after strong bombykal stimuli and regained their original amplitude in the course of several seconds (as illustrated by the broken line). Occasionally, spontaneous small action potentials occurred during (filled arrow) or after (open arrow) the response. The reduction of the amplitude during bursts of large (shaded arrow) and small (open arrow) action potentials was reported previously (Dolzer et al., 2001). (B) Enlarged view of the marked section in A. (C) Two small and two large spontaneous action potentials recorded before the stimulation, plotted at the same amplitude scaling as in B. The small action potential during the response was of the same amplitude as the small spontaneous action potentials.

View larger version (17K): [in a new window]   Fig. 5. Pheromone responses are dose-dependent. Stimulation with a solvent-loaded filter paper (control) did not elicit a measurable sensillar potential. The action potentials of the small class occurred at random. Stimulations with increasing doses of bombykal (BAL) elicited sensillar potentials of increasing amplitude, together with large action potentials at increasing frequency and shorter latency. At stimulus loads of 10–2 µg, the reduction of the action potential amplitude during the response became obvious. The stimulus duration was 50 ms and stimuli were applied in increasing order, separated by intervals of 60 s (dose ramp).

View larger version (21K): [in a new window]   Fig. 6. A strong conditioning stimulus caused adaptation of the sensillar potential and action potential responses. (A) Compared with a response without a preceding strong stimulus (upper trace), the sensillar potential amplitude and the action potential frequency were reduced (lower trace) after an adapting stimulus. (B) After lowpass-filtering and normalizing both responses to the same maximal amplitude, the faster decline (filled arrow) to baseline of the sensillar potential in the adapted state (broken line) became obvious. But the half-time of the rising phase (t1/2 rise; open arrow) was less affected. Stimulus duration was 50 ms. BAL, bombykal.

View larger version (24K): [in a new window]   Fig. 11. Encoding of stimulus duration in the bombykal cell was tested with cartridge stimulations at a dose of 10 µg bombykal (N=7). The sensillar potential (SP) amplitude (A), the half time of the rising phase of the SP (t1/2 rise; B), and the action potential (AP) frequency (E) did not encode stimulus durations from 20 ms to 5 s. But the declining phase of the SP, as reflected by t1/2 decline (C), and the total number of APs (D) distinguished stimulus durations between approximately 100 ms and 5 s.

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