Homozygous park25 flies demonstrate climbing deficits as early as day 0 and maintain these deficits through day 20
Since our previous study analyzed climbing ability in heterozygous park25 flies in a simple negative geotaxis assay, we set out to fully characterize the climbing deficits in homozygous park25 flies in our new climbing assay [13]. The average number of climbs that reach 51 mm (the top of the climbing tube) was lower in 0-day-old park25 homozygous flies compared to 0-day-old control flies and this deficit is maintained through day 20 (all ages, P < 0.01) (Fig. 1). This indicates that park25 homozygotes do not climb to the top of the tube as much or as often as control flies. In agreement with this, the average height climbed in each climbing event was lower in park25 homozygotes versus control flies (all ages, P < 0.001). When the height climbed is totaled for the entire 20-minute period, the homozygous park25 flies were lower on days 5–20 (all three ages, P < 0.001), although it appears that even day 0 park25 flies climbed less, the difference was not significant. Control flies increase the number of climbing events from day 0 to day 10 (P < 0.05) and day 20 (P < 0.01). This same pattern is observed in the park25 mutants, in fact, there were no differences in the number of climbing events between the park25 and control flies (all ages, P > 0.99). This indicates that homozygous park25 flies make just as many attempts to climb as control flies. However, though the park25 homozygotes are climbing, they do not reach the top portion of the climbing tube as much as the control flies within the 20-minute assay. To measure this, the average time spent by the flies in the top third of the climbing tube (from 35 mm to 51 mm) was measured. On average, park25 homozygous flies spend a lower amount of time in the top portion of the climbing tube compared to control flies (all ages, P < 0.01). Conversely, the park25 homozygotes spend more time on day 0 and 5 in the lower third of the climbing vial when compared to the control flies (Supplemental Fig. 1, both P < 0.001). It appears that the park25 homozygotes try to climb, but fail in maintaining their climbing, spending more time in the bottom of the tube. In support of this, the average height reached in the climbing tube for each climbing event (average peak height) was lower at all ages in park25 mutants compared to controls (Supplemental Fig. 1, all ages, P < 0.001). Homozygous park25 flies are less active at all ages (all ages, P < 0.05), as measured by total movements. Though they move less, their ascending velocity is not different from controls (Supplemental Fig. 1, all ages, P > 0.12), possibly indicating that their physical ability to climb is not reduced. Though the ascending velocity is not different, the descending velocity of day 0 park25 flies is greater than control flies (Supplemental Fig. 1, P < 0.0001). Why these young flies descend faster, is unknown, but perhaps one explanation could be that instead of climbing down, they were actually falling down, which could lead to a faster velocity measurement. As a final measurement of climbing ability, the number complete ascents were determined. This is a measure of the ability of flies to climb the entire length of the climbing tube from the bottom to the top without reversing direction at any time in the climb. This pattern of climbing is likely what all flies desire to do but do the most infrequently. Except for the 10-day-old flies, the park25 mutants made less complete ascents compared to control flies (Supplemental Fig. 1, remaining three ages, P < 0.05). Overall, it is apparent that homozygous park25 flies have more significant climbing deficits compared to our previous study in heterozygous flies [13] that occur at earlier ages, typically right from eclosion.
Nicotine improves climbing deficits in homozygous park25 flies when given on day 0
We have previously determined that nicotine treatment at 3 and 4.5 µg/mL improved climbing ability in heterozygous park25 flies [13]. Therefore, we set out to determine if the same beneficial nicotine effects can be observed in homozygous park25 flies. In all previously measured climbing metrics, 9 µg/mL of nicotine, when given from day 0, improved the homozygous park25 mutant climbing deficit (Fig. 2 and Supplementary Fig. 2, all P < 0.01), except for total climbing events, and both ascending and descending velocity. In this set of experiments, the park25 mutants have a decreased number of climbing events compared to control flies (P < 0.0001), which was not observed in the 20-day-old flies (Fig. 1). Nevertheless, nicotine did not affect the number of climbing events, which again points to this climbing metric different from the rest. Indeed, this climbing metric may be more of a CNS phenomenon, measuring the flies’ motivation to climb.
While 6 µg/mL of nicotine did improve some of the climbing deficits in the park25 mutants, it was not as effective as 9 µg/mL. This is interesting because these doses are double the effective doses used in our previous heterozygous park25 study [13], reinforcing the idea that the homozygous park25 phenotypes are more severe and take a greater amount of nicotine to overcome. In agreement with this, 3 and 4.5 µg/mL of nicotine, which were equivalent to the previously used nicotine concentrations, had no effect in the park25 homozygotes except in complete ascents (Supplemental Fig. 3).
Another observed nicotine effect that replicated our previous findings was the overall negative effect of nicotine on the control flies. While nicotine tended to improve climbing performance in the park25 mutants, it had a detrimental effect in many of the climbing metrics in the control flies. This was even observed at the lower (3 and 4.5 µg/mL) amounts (Supplemental Fig. 3).
Nicotine improves flight ability in homozygous park25 flies
The vast majority (93%) of the untreated homozygous park25 flies were unable to fly in the flight assay, compared to the 88% of control flies that flew (Fig. 3; n: 99 park25, 154 control). Both 4.5 and 6 µg/mL concentrations of nicotine, when given from day 0, improved the flight ability of the park25 mutants (P < 0.05), although the maximal percentage of flying was only a modest 20% with 6 µg/mL of nicotine. Nicotine had a negative effect on the control flies, with flight being reduced by all concentrations above 3 µg/mL (P < 0.01).
Nicotine does not improve climbing deficits in park25 flies when given after day 0
Given the improvements in climbing performance due to nicotine given on day 0 post-eclosion, we sought to determine if delayed administration of nicotine could also provide these beneficial effects. Nicotine (9 ug/mL) was initiated on days 0–3 post-eclosion and then maintained until the climbing assay. When administration of nicotine was delayed by one or two days, all improvements in climbing were absent (Fig. 4 and Supplemental Fig. 4).
Nicotine improves flight deficits when given on days 0–5 post-eclosion
Since 4.5 µg/mL when given on day 0 improved flight performance in homozygous park25 flies, a time course of delayed initiation of nicotine was performed from days 0–8 post-eclosion in homozygous park25 and control flies. Nicotine improved flight performance in the first five days of the time course in the park25 mutants (Fig. 5). A similar effect by nicotine was also observed with the control flies, but in the opposite direction; nicotine decreased flight performance in the control flies through day six post-eclosion. The same overall results were observed when 3 µg/mL of nicotine was given to the flies across the eight-day initiation period, with the homozygous park25 flies benefitting and the control flies being negatively affected through day five by nicotine (Supplemental Fig. 5). Large numbers of flies were tested on days 6–8 to ensure that the effects of nicotine were truly lost by then (Supplemental Table 1).
Nicotine improves the quality of the descent in the park25 fly
We sought to determine a mechanism behind the improvements in climbing with nicotine treatment in the homozygous park25 flies. High speed video of ten flies (five park25 and five control) was captured while simultaneously performing the MBM climbing assay for 10 minutes. This allowed for the programmatic detection of two different types of descents in the climbing assay: falls and descending climbs (see Methods for more details). Analysis of the same climbing data from Fig. 2 reveals that control flies have more descending climbs than the park25 mutants (P < 0.0022), while they drop at equal rates (P = 0.1871) in the entire 20-minute climbing period (Fig. 6). Nicotine (9 µg/mL) treatment increased the number of descending climbs in the homozygous park25 flies (P < 0.0001). Nicotine had the opposite effect in control flies by decreasing the number of descending climbs (P = 0.0444) and increasing the number of drops (P = 0.0372). Similar results were observed in an overall percentage analysis of the descent data (data not shown), since there are descents that are not classifiable by the programming.