Behavioral paradigms

2.2.4.1. Climbing assay

The locomotor performance of flies was evaluated with a modified negative geo-taxis assay [Ganetzky & Flanagan, 1978; Feany & Bender, 2000; Friggi-Grelin et al., 2003]. A 10 ml serolocigal pipette (Carl Roth GmbH + Co KG; Karlsruhe, Germany) with an approximate length of 28 cm was divided into three parts. The lower part consisted of the lowest 7 cm region from the bottom of the pipette; the middle part was defined as ~14 cm between the lower and the upper part and the upper part was the upper 7 cm region until the top of the pipette. The lowest centimeter of the tip of the pipette was filled with cotton to prevent flies from getting stuck and was subsequently concealed with parafilm.

Flies were anesthetized after hatching and collected in groups of 10 in plastic vials containing food medium. After 6-8 days, the flies were transferred into the pipettes without additional anesthesia and left to rest for ten minutes. During this phase the pipette was already placed in the experimental box (see 2.1.9) at 60-80% humidity.

The temperature in the box was set to 32.0 - 33.0 ^◦C (24.5 - 25.5 ^◦C for control) beforehand. After the resting period, the pipettes were tapped on the ground to let the flies fall to the lower part of the pipette. Subsequently, the flies had one minute to climb the walls of the pipette before the number of flies per compartment was counted. A performance index was calculated with the following formula:

P I = 1

2 ∗ n_total+ (n_up−n_down) n_total

wheren_total is the total number of flies, n_up is the number of flies in the upper part andndown the number of flies in the lower part. The experiment was repeated three times and the average PI was used as a measurement of locomotor performance.

Subsequent data analysis was done with Origin 8.5G (OriginLab; Northampton, MA, USA). Normal distribution was tested with the Shapiro-Wilk normality test. Differ-ences in performance indices were tested for statistical significance with ANOVA and post hoc Bonferroni corrected t-tests. Visual presentation of the data was opti-mized with Adobe^R Illustrator (Adobe Creative Suite 5, Adobe Systems, Inc.; San Jose, CA, USA).

2. Materials and Methods 2.2.4.2. Olfactory learning

Figure 2.1. Photography of modified Tully-Quinn-Barrel for four parallel experi-ments.Different tubes can be attached to the machine: training tubes with an electrifi-able copper grid were used during training (right side) whereas test tubes made out of polyethylene were attached during the test phase (left side). In both pictures the odor cups are visible and attached to the lower tubes.

Groups of about 100 flies (5-9d) were trained in an associative olfactory learning paradigm as introduced by Tully & Quinn [1985] (see section 1.5). Four experiments were performed simultaneously in a modified learning apparatus described by Schwaerzel et al. [2002] (See figures 1.6 & 2.1). A constant air flow of ~167 ml/min in each tube assured a constant transport of odorant molecules inside the tubes.

1 2 3 4 5

0 0 1 2

Training Test

Time in minutes

Break to transfer flies

Figure 2.2. Time scheme for olfactory training.The training period lasts 5 minutes in which the flies are exposed to the CS+

and the CS-. The break be-tween training and test phase, in which the tubes are exchanged and the flies are transferred in-side the machine, ranged from 2 to 5 min. The test phase lasted 2 min.

All experiments were performed with Canton S wild type flies at 24-26^◦C and 60-80% relative humidity if not stated otherwise. The used odorants were diluted in mineral oil in the following dilutions:

3-Octanol 1:750

1-Octen-3-ol 1:500

4-Methylcyclohexanol 1:400

60 µl of the diluted odorant were pipetted into an odor cup with 5 mm diameter and placed into the odor cubes which can be attached to the training and test tubes of the learning apparatus. Olfactory train-ing started about one minute after transferrtrain-ing the flies into the tubes. Each odorant was presented for

1 min with a 1 min break between odorant applications. One odorant (conditioned stimulus +, CS+) was temporally paired with 12 electric shocks of 90 V (1.25 sec shock and 3.75 sec inter pulse interval) direct current (DC) applied through an elec-trifiable grid covering the inside of the tubes. The second odorant was presented without shock (conditioned stimulus -, CS-). After another minute, the flies were transferred to a T-maze part of the apparatus with both odorants presented from each side. The change of tubes and transfer of the flies lasted 1-5 min before the 2 min test for odorant preference started (Figure 2.2). Subsequently, the flies in each tube were counted and a preference index was calculated by subtracting the num-ber of flies on the side of the CS+(nCS+) from the number of flies on the side of the CS-(nCS−), divided by the total number of flies(ntotal):

P I = n_CS−−n_CS+

n_total

Negative PIs therefore represented an avoidance of the conditioned odorant, posi-tive scores represent an attraction. Learning indices were calculated by averaging two reciprocal experiments in which each odorant was used once as CS+ and once as CS-. Odor preference was obtained with the same protocol by presenting an odorant or mineral oil instead of the CS+ and the CS-.

Subsequent data analysis was done with Origin 8.5G (OriginLab; Northampton, MA, USA). Normal distribution of the data was tested with the Shapiro-Wilk normality test. Differences in odorant preference, preference indices and learning indices were tested for statistical significance using either ANOVA with post hoc Bonferroni corrected t-tests or the Two-sample t-test. Visual presentation of the data was optimized with Adobe^R Illustrator (Adobe Creative Suite 5, Adobe Systems, Inc.;

San Jose, CA, USA).

The olfactory learning experiments with wild type flies were performed together with Moritz Hermann during his Bachelor Thesis [Hermann, 2011]. The Thesis was co-supervised by the PhD candidate and the work during data acquisition was shared.

2. Materials and Methods

Shock avoidance was assayed in the T-maze. In contrast to the normal test situ-ation, one of the arms of the maze consisted of the training tube. During the test of one minute, 12 electric shocks of 90V DC (1.25 sec shock and 3.75 sec inter pulse interval) were administered to the training tube. The flies were placed in the elevator section in the middle of the Tully-machine. A subsequent counting of the flies on each side of the T-maze resulted in an avoidance index.

Odor preference was measured by placing the flies in the elevator section of the Tully-machine and a subsequent T-maze test. One arm of the maze was equipped with an odor cube with an odorant in the given dilution whereas the odor cube on the other arm was filled with the solvent, mineral oil. The flies had two minutes to decide which side of the T-maze to approach. After counting of the flies a preference index was calculated.

2.2.4.3. Silencing of synaptic transmission during olfactory learning

Inhibitory local interneurons in the antennal lobe were silenced using the shibire fly strain described by Kitamoto [2001] (see also section 1.3.2). Local interneu-rons of the LN1 type were targeted with the NP1227-GAL4 line [Seki et al., 2010;

Sachse et al., 2007]. To block synaptic transmission in type I inhibitory local in-terneurons,U AS−shi^ts; +;U AS−shi^ts flies and

y, w

⁻

;

^{N P}_CyO¹²²⁷

; +

were crossed.

The offspring of this combination was tested in the learning paradigm at differ-ent temperatures. During the subsequdiffer-ent counting, the flies were sorted accord-ing to the expression of the CyO marker. The offspraccord-ing without the CyO marker

(

_{U AS−shi}^y,w− ts

;

^{N P}₊¹²²⁷

;

_{U AS−shi}⁺ ts

)

expressed the temperature sensitive dynamin mu-tant shibire in inhibitory local interneurons of the LN1 type. Offspring expressing the marker did not express shibire as no GAL4 was produced and was used as heterozygous control

(

_{U AS}^y,w_−shi⁻ ts

;

^CyO₊

;

_{U AS−shi}⁺ ts

)

. The

y, w

⁻

;

^{N P}_CyO¹²²⁷

; +

parental line served as a control for the GAL4-driver.

Prior to the training procedure described above, all flies were transferred into empty vials and kept at the respective temperature next to the training apparatus for ~10 min to ensure complete blocking of synaptic transmission. Type II inhibitory local in-terneurons were targeted with the NP2426 GAL4-enhancer-trap line (N P2426; +; +).

In order to get appropriate heterozygous genetic controls for the combination of N P2426; +; +andU AS−shi^ts; +;U AS−shi^ts, the parental lines were crossed with W¹¹¹⁸ and the offspring used during the experiments.

Im Dokument Olfactory Perception and Physiology in Drosophila melanogaster (Seite 66-70)