Figure 1.
Moth pheromone-sensitive sensillum trichodeum in tip-recording conditions.
The sensillum is a small organ typically composed of 2 ORNs and 3 auxiliary cells (thecogen Th, trichogen Tr and Tormogen To), housed within a porous cuticular hair. The tight junctions between cells separate the ORN extracellular environment in two parts with different ionic compositions, the sensillar lymph bathing the outer dendritic segment (sensory) and the hemolymph bathing the inner dendrite and soma. In experimental conditions the pheromone is delivered close to the hair. The ORN electrical response is recorded extracellularly with an electrode slipped on the cut hair tip. Figures 2 and 3 give detailed views of the ORN membrane processes at the molecular level. Figure 6 gives an overview of the global electrical organization of the sensillum. Modified from [8].
Figure 2.
Extracellular (1–3) and early membrane (4–7) reactions involved in pheromone perireception and reception events.
1: Pheromone uptake from air (Lair) to sensillar lymph (L) and transport through sensillar lymph by PBP. 2: Deactivation (enzyme N) producing deactivated pheromone P. 3: Interaction with receptor R. 4: Activation of receptor (R*). 5: G-protein activation (G*). 6: Effector enzyme activation (E*). 7: Production of second-messengers (DAG and IP3). In the present work, all these reactions were modeled as previously described [36].
Figure 3.
Qualitative model of membrane and cytosol reactions in moth pheromone transduction.
Degradation of DAG and IP3, and deactivation of CaCaM and PKC* are not formally described in the present model (dotted arrows). All components are in the outer dendrite except the K+ channel and, possibly, the IP3-gated Ca2+ channel (see Discussion).
Table 1.
List of main assumptions in the model.
Figure 4.
Main biochemical reactions involving diffusible molecules.
The primary second messengers (DAG, IP3) come from their precursor (PIP2), the secondary messenger (Ca2+) comes from the sensillar lymph or intracellular stores. The two main modulators, Ca2+-calmodulin (CaCaM) and activated protein kinase C (PKC*), come from their precursors (CaM and PKC) in the presence of DAG and Ca2+. PA is phosphatidic acid. Reaction numbers same as in Figures 2 and 3.
Figure 5.
Plots of dose-conductance functions.
Illustrate eqs. (2) and (3) for (A) the DAG-gated cationic conductance Gcat and (B) the Ca2+-gated Cl− conductance GCl. The solid blue lines in A and B represent the conductance without inhibition. The EC50 of the cationic current, Kmcat = 0.01 µM of DAG, is reached at U = 10−4.25 µM/s, i.e. this current is most active at low pheromone uptakes. The EC50 of the cationic current, KmCl = 81 µM of Ca2+ corresponding to U = 50 µM/s, i.e. the Cl− current is most active at high uptakes. The dashed red lines represent the conductance at half-maximum inhibition (intermediate curve) and maximum inhibition (rightmost curve) by CaCaM (A) and PKC* (B). The PKC* inhibition of the Cl− current is very weak and practically negligible.
Figure 6.
Equivalent electrical circuit of the ORN within the sensillum trichodeum (cf. Figure 1).
Three main compartments are distinguished: ORN outer dendritic segment (circuit on the left with 5 conductances), ORN inner dendritic segment and soma (denoted “Soma”, circuit on the right with 2 conductances) and auxiliary cells (circuit on top with a single conductance). The experimentally recorded difference of potential (Ved) is between the sensillar lymph and the hemolymph.
Table 2.
Initial values of variables in the model.
Table 3.
Fixed parameters in the model.
Table 4.
Fitted parameters (10) of second messengers and diffusible modulators.
Table 5.
Fitted parameters (28) of ionic channels.
Figure 7.
Predicted kinetics of the main chemical species, currents and potential at various uptakes.
(A) Activated receptor R*. (B) Effector enzyme E*. (C) Second messengers DAG. (D) Ca2+. Major depolarizing currents (E) Icat and (F) ICl. (G) Major repolarizing current IK. (H) SP. Responses are shown for 2-s square pulses yielding different uptakes regularly spaced by 0.5 log units from 10−4.75 to 101.5 µM/s. Note that the scales of the time axes for DAG concentration (C) and cationic current (E) are not the same as for the other species and currents. Kinetics of IP3 (not shown) is identical to that of DAG (C).
Figure 8.
Comparison of dose-response characteristic of predicted and observed SPs.
(A) Height. (B) Rising time τrise. (C) Falling time τfall. (D) Definition of these characteristics shown on SP response to a 2-s square pulse at pheromone uptake U = 10−4 µM/s. Characteristics of predicted SP (solid red lines) compared to those of observed SP (dashed blue lines) at 26 uptakes from 10−4.75 µM/s to 32 µM/s. Characteristics of predicted RP are also shown (dashed red lines). Experimental data by courtesy of K.-E. Kaissling (see [23],[30]).
Figure 9.
Comparison of dose-response curves for height of the chemical species and currents.
Height (left column; see definition in Figure 8D) and relative height (right column) for chemical species (top row) and for depolarizing and repolarizing currents (bottom row). (A) Ca2+ (solid blue line) is the most abundant species (concentration divided 10 fold to be shown on the same scale as other species). (B) Responsiveness of all chemical species is much smaller than that of SP (curves shifted to the right of the SP curve shown as a solid red line), larger than that of effector enzyme E* (dashed red line) at low doses and smaller than E* at high doses. (C) Cl− (solid green), K+ (dashed red) and cationic (dashed blue) currents are the most intense currents. (D) Responsiveness of the Cl− and cationic currents is higher than that of the effector enzyme (cf. (B)) and the IP3-gated Ca2+ current (solid blue). In particular, the cationic current curve is close to that of SP (solid red curve, same as in B) and K+ (confounded with SP) at all doses, while the curve of the Cl− current (solid green) is on the right of the SP curve at low doses (smaller responsiveness) and close to it at high doses.
Figure 10.
Kinetics of the major currents and SP at different pheromone uptakes.
Uptakes separated by 2 log units from low to high, (A) 1.78×10−5, (B) 1.78×10−3, (C) 0.178 and (D) 17.8 µM/s. Insets show the rise of each current during the first 0.5 s (top) or 0.3 s (bottom). DAG-gated current Icat (dashed blue) is the main depolarizing current at low dose (A). Ca2+-gated current ICl (solid green) takes over the major role at high doses (C and D). The kinetic response of the repolarizing current IK (dashed red) is close to that of Icat at low dose (A) and close to that of ICl at high doses (C and D). As shown in the insets, IK closely follows Icat at the beginning of the rising phase.
Figure 11.
Normalized kinetics of the major currents and SP.
Same as Figure 10 except that currents and SP have been normalized with respect to their maxima for easier comparison of the rising and falling phases. At all doses, (A) 1.78×10−5, (B) 1.78×10−3, (C) 0.178 and (D) 17.8 µM/s, the DAG-gated cationic current Icat (dashed blue) rises faster than the K+ current IK (dashed red) and the Cl− current ICl (solid green), and IK closely follows ICl at intermediate and high uptakes.
Figure 12.
Phase portraits on the DAG-Ca2+ plane at different pheromone uptakes.
(A) 1.78×10−5, (B) 5.62×10−4, (C) 0.02, (D) 0.56 and (E) 17.78 µM/s. (F) Superimposition of the phase portraits for 15 stimuli from low to high uptakes. Uptakes are regularly separated by 1.5 log units, i.e. multiplied by 31.6 from one portrait to the next. The starting point at t = 0 is close to the origin (0, 10−3). The blue and red lines correspond to the rising and falling phases of DAG, respectively. The times at which DAG (cross) and Ca2+ (circle) reach their respective maxima are indicated (in s). Uptakes are regularly separated by 1.5 log units, i.e. multiplied by 31.6 from one portrait to the next.
Figure 13.
Phase portrait on the E*-SP plane at different pheromone uptakes.
(A) 1.78×10−5, (B) 5.62×10−4, (C) 0.02, (D) 0.56 and (E) 17.78 µM/s. (F) Superimposition of the phase portraits. The starting point at t = 0 is close to the origin (0, 0). Same representation as in Figure 12.
Table 6.
Amplification factors1 of each step at three different uptakes.