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The short-term effect of Methylphenidate
on swim speed in zebrafish larvae
Dóra Dögg Kristófersdóttir
2014 BSc in Psychology
Author: Dóra Dögg Kristófersdóttir
ID number: 040183-7449
Supervisor: Jack Ernest James
Department of Psychology
School of Business
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 2
Abstract
Methylphenidate is a psychostimulant routinely prescribed for Attention Deficit Hyperactivity
Disorder (ADHD) early in life, with contradictory reports on optimal dosage sizes. Exhaustive
human research on it would be expensive and unethical, subjecting children or young adults to
random groups. Various animal studies using intravenous administrations are not applicable
to deduction on normative human use of Methylphenidate. The present study applied
zebrafish larvae, seven, ten and eleven days post-fertilization (dpf) swimming in four dosages
(zero, 4.63, 9.25 and 18.50) µmol Methylphenidate during day and night to see the change on
locomotor behavior in cm/s. The highest dose reduced locomotion compared to the other
groups during day and night, additionally there was a change between day and night with the
larvae inducing motion during night. The control, low, and medium groups were neither
different from each other nor between day and night. The highest dosing reduced locomotion
significantly, to such a degree that might imply a neurotoxic effect that neither completely
wash-out during the day nor night. The control group’s constant locomotion between day and
night might indicate the lighting malfunctioning or fasting affecting the larvae. More research
on Methylphenidate is needed.
Útdráttur
Methýlfenidat er geðörvandi lyf sem er venjubundið gefið ungu fólki með athyglisbrest með
ofvirkni (ADHD) og rannsóknir hafa almennt ekki verið sammála um skammtastærðir fyrir
hagstæðastan árangur. Ítarlegar manna-rannsóknir væru dýrar og það bryti gegn siðarreglum
að láta ungt fólk tilviljunarkennt í rannsóknarhópa. Dýrarannsóknir með lyfinu í sprautuformi
eru ekki yfirfæranlegar á eðlilega notkun manna. Umrædd rannsókn notaði sebrafiska-lirfur
sjö, tíu og ellefu dögum eftir frjóvgun og lét þær synda í fjórum skammtastærðum (núll, 4.62,
9.25 og 18.50) µmól Methýlfenidat yfir dag og nótt til að sjá breytingu á hreyfingu mældri í
cm/s. Hæðsti skammturinn dró marktækt úr hreyfingu miðað við hina hópana, ásamt því að
auka hreyfingu sína frá degi til nóttu. Það var hvorki marktækur munur á viðmiðunar hópnum,
lægsta og miðlungs skammtahópunum né milli dags og nætur. Hæsti skammtahópurinn dró
marktægt úr hreyfingu, sem gefur mögulega til kynna skaðleg áhrif á taugavirkni sem nær
hvorki að eyðast fyllilega yfir daginn né nóttina. Stanslaus hreyfing viðmiðunarhópsins frá
degi til nætur gæti gefið til kynna tækjabilun lýsingar eða að fasta lirfanna hafi haft áhrif á
hegðun þeirra. Þörf er á frekari rannsókum á efninu Methýlfenidat.
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 3
Foreword and acknowledgement
Submitted in partial fulfillment of the requirements of the BSc Psychology degree,
Reykjavík University, this thesis is presented in the style of an article for submission to a
peer-review journal.
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 4
The Short-term Effect of Methylphenidate on Swim Speed in Zebrafish Larvae
Methylphenidate and Attention Deficit Hyperactivity Disorder
The importance of adequate dosage size and administration method in animal studies
when comparing to human application is increasingly evident (Gerasimov et al., 2000). This
applies to Methylphenidate, a psychostimulant that is one of the principal active agents in
stimulants administered to attention-deficit/hyperactivity disorder (ADHD) patients of all age
groups (Faraone, Biederman, Spencer, & Aleardi, 2006; Geissler & Lesch, 2011; Kollins,
2008) as well as in other drug research. One of the most frequently prescribed medications
containing Methylphenidate is Ritalin (Volz, 2008), which is routinely prescribed for ADHD
symptoms (Kimko, Cross & Abernethy, 1999; Iversen & Iversen, 2007; Kokel & Peterson,
2008; Kollins, 2008), primarily for children and young adults for chronic use (Andersen &
Navalta, 2004).
ADHD is generally a childhood condition, although, it can last into adulthood, and
consists of behavioral, academic, and social disadvantage of inattention, disorganization,
and/or hyperactivity-impulsivity (American Psychiatric Association, 2013). Children with
ADHD make more errors of omission and commission, are more irritable, distracted, and
impulsive, have a longer response time, and perform worse than healthy controls on average
(Lecendreux & Konofal, 2000). Administration of Methylphenidate provides short-term relief
from the core symptoms of ADHD, enabling most to perform more like, if not identical to
healthy peers, (Faraone et al., 2006; Kimko et al., 1999). Stimulants such as Methylphenidate
work on neurosynaptic transmitters systems related to mobility, attention, inhibition, and
reward (Andresen, 2005)
Methylphenidate´s Effect on the Central Nervous System and Dosage
Attention, impulse control, and executive function are subserved in the prefrontal
cortex, and reward and locomotor processes are subserved by the nucleus accumbens. Both of
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 5
which contribute to ADHD symptoms when malfunctioning (Andersen, 2005). The
pharmacologic effect of Methylphenidate in the central nervous system enhances
catecholaminergic functions, either selectively through dopamine or noradrenaline, or by
effects from both of the neurotransmitters combined (Andersen, 2005; Heal & Pierce, 2006;
Levin et al., 2011; Volz, 2008), possibly causing Methylphenidate´s therapeutic efficacy as
well as the side-effects (Heal & Pierce, 2006; Kimko et al., 1999).
The average high-peak in short-term effect of Methylphenidate after oral intake varies
from one to four hours, which makes it a short-acting stimulant, and its concentration
following oral administration is strongest on average two hours after consumption (Kimko et
al., 1999). Dosage size has a parabolic function, with increased dosages optimizing short-term
relief of the symptoms, up to a certain plateau, after which the positive effects decrease with
more dosing (Andersen, 2005.; Repantis, Schlattmann, Laisney & Heuser, 2010; Kimko.,
1999; Stein et al., 2003). Ideal treatment dosage amount, however, has been debated, with
research inconsistent on dosing for optimal results (Anderson, 2005). With few well
controlled studies on the subject the inconsistency may be due to imprecise measuring
methods that can vary from subjective measures such as parent or teacher report, to varied
objective performance-tests and rating scales in diverse settings (Kimko et al., 1999).
Research conducted on healthy individuals comparing Methylphenidate with no drug has
revealed that it can improve cognitive performance short-term. Suggesting a possible overall
non-specific mechanism of arousal, improved attention, and vigilance measured in shortened
response time and decreased errors of omission (Andersen, 2005; Hermens et al., 2007;
Kimko et al., 1999). This has been debated, with inconsistent findings on exactly how
effective the drug is and what it enhances (Advokat, 2010; Repantis et al., 2010). The
cognitively enhancing effect has, however, been found in studies using various healthy
animal-models and in animals modified to present ADHD core symptoms (Andersen, 2005).
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 6
Methylphenidate is known to have negative short-term side-effects, some of the most
commonly reported being trouble sleeping and decreased appetite (Garland, Tripp & Taylor,
2010; Heal & Pierce, 2006; Kimko et al., 1999; Stein et al., 2003). Increased dosage amount
has been reported to intensify the negative side-effects (Heal & Pierce, 2006; Stein et al.,
2003). Reports contradict on the drug’s side-effects, when individuals conform to
Methylphenidate administration guidelines, both when sleep is measured, objectively and
subjectively. Some indicated beneficial sleep-efficacy (Repantis et al., 2010; Sobanski,
Schredl, Kettler & Alm, 2008) and others reduced sleep-efficacy (Galland et al., 2010). It has,
additionally, been demonstrated that recipients of placebo treatments have regularly reported
similar side-effect to Methylphenidate administration (Geissler & Lesch, 2011; Kimko et al.,
1999). With so many diverse and inconsistent reports on this subject, it is evident that further
research is needed (Barkley, McMurray, Edelbrock & Robbins, 1990; Repantis et al., 2010).
Normative Human Application of Methylphenidate
Prescribed drugs such as Methylphenidate are primarily administered orally to humans
(Heal & Pierce, 2006). The research literature on complex animal-models has largely reported
Methylphenidate intravenously applied, decreasing the predictive value for comparison except
for drug abuse (Andersen, 2005; Gerasimov et al., 2000; Heal & Pierce, 2006). Studies
consistently find evidence of reinforcing effects in intravenous Methylphenidate studies
inconsistent with results of oral administration of appropriate dosage-conforming individuals
(Andersen, 2005; Gerasimov et al., 2000; Kollins, 2008). More of the substance is digested
without reaching the brain after oral intake, with peak concentration after approximately two
hours, opposed to 15 minutes intravenously (Heal & Pierce, 2006; Gerasimov et al., 2000;
Kimko et al., 1999). This can be problematic when interpreting results for normative human
comparison (Anderson, 2005; Gerasimov et al., 2000). Animal studies that either use smaller
dosage sizes or administered orally have not found the same reinforcing effect (Gerasimov et
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 7
al., 2000; Koda, Ago, Cong, Takuma & Matsuda, 2010; Kollins, 2008) possibly as shorter
time from stimulus to response supplies more behaviour change (Tanno, Silberberg &
Sakagami, 2012).
Why Should Methylphenidate be Studied Further?
Defining long-term effects of Methylphenidate is problematic, and there is more
opportunity for confounding by extraneous variables (Kimko et al., 1999). Randomized
controlled long-term drug experiments on children are close to impossible to accomplish,
being both impractical and unethical to randomly assign children to a treatment plan and
sustain it over time (Vitiello, 2001). Animal and human research reveals possible long-term
effects, both in altering the brain and consequently behaviour (Andersen, 2005). Some
relating young patients with ADHD on stimulants with lower grade-retention rates and
comorbid psychological disorders subsequently in adulthood, both known risk-factors of
ADHD (Biederman, Monuteaux, Spencer, Wilens & Faraone, 2009). Others report that
ADHD, as a risk-factor for substance abuse, is less likely to lead to drug addiction when the
individuals are treated with Methylphenidate in childhood (Kollins, 2008). ADHD diagnosed
individuals treated with Methylphenidate in childhood have reported better self-esteem and
social skills later in life and in hindsight appraised their childhood more favorably than non-
treated individuals (Kimko et al., 1999).
Other research has revealed potential negative long-term effects of stimulants such as
Methylphenidate. Levin et al. (2011) indicated long-term dangers of early-life
Methylphenidate exposure on zebrafish larvae. They exposed zebrafish embryos 0-5 days post
fertilization to Methylphenidate doses that resulted in persistently altered behaviour in their
early adulthood after early developmental exposure. Their research and others directed at such
early exposure raise concern for women at childbearing age who use the drug when
conceiving and into their pregnancy as well as for the children prescribed Methylphenidate as
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 8
young as two years of age (Andersen & Navalta, 2004). The long-term effects are reported in
various animal studies across species, although, it is most consistent after stronger dosing than
applies to normative human use (Gerasimov et al., 2000; Vitiello, 2001). As stated previously,
another possible confounder may arise when the drug is administrated intravenously
(Gerasimov et al., 2000). It raises concern as research findings are used for over-
generalization purposes when the difference of the Methylphenidate concentration that
reaches the brain after oral opposed to intravenous application is undebated. While in practice
human intravenous administration of Methylphenidate is rare (Anderson, 2005; Gerasimov et
al., 2000; Grund, Lehmann, Bock, Rothenberger & Teuchert-Noodt, 2006; Vitello, 2001) with
one study reporting a level of 2% occurrence rate (Barrett, Darredeau, Bordy, & Pihl, 2005).
Despite minimal knowledge of Methylphenidates long-term effects (Kimko et al.,
1999) both positive and negative long-term effects may derive from the fact that individuals
are most likely to be chronically exposed in childhood and early adulthood (Andersen &
Navalta, 2004; Andersen, 2005). It is likely this stems from early- and mid-childhood being a
time where the dynamic brain develops rapidly with excessive production of synapses and
receptors (Andersen, 2005; Vitiello, 2001).
How to Study Methylphenidate Further?
Dosage size and normative administration of how human use of the drug must be
emulated when using non-humans as research models for direct human comparative relevance
(Andersen, 2005; Vitiello, 2001). Increased dosage size is reported to intensify the side-
effects (Stein et al., 2003) and researches are inconsistent on ideal dosage size, adding to the
importance of further study on the subject (Kimko et al., 1999). Long-term human research is
expensive and often impractical, especially if meant to analyse the impact over a lifetime, at
risk for various biases and unethical to conduct randomly and experimentally (Vitiello, 2001).
To find long-term biological and environmental consequences of Methylphenidate on
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 9
cognitive, genetic, molecular, and neural factors, preliminary studies on animals with a short
lifespan and cell models are essential (Andersen & Navalta, 2004; Geissler & Lesch, 2011).
After thorough animal research, long-term systematic experimental human research could
become a viable choice.
Zebrafish as an Animal Model for Human Comparison
Zebrafish have various benefits over other research animals making them suitable
simple vertebrate animal-models for human comparison (Hendricks, Sehgal & Pack, 2000).
Their early-life development is visible as they are see-through during their first days of life,
they are cheap in upkeep, easy to handle, procreate, and maintain. They are vertebrates with
relatively simple nervous system retaining fundamental circuits found in humans and have
been found repeatedly to have similar diurnal rhythm to humans (Appelbaum et al., 2009 ;
Carlson, 2010; Chiu & Prober, 2013; Emran & Dowling, 2010; Elbaz, Foulkes, Gothilf &
Appelbaum, 2013; Emran, Rihel, Adolph & Dowling, 2009; Hendricks, Sehgal & Pack. 2000;
Kishi, Slack, Uchiyama & Zhdanova, 2009; Saszik & Bilotta, 2001; Zhdanova, Wang,
Leclair & Danilova, 2001; Zimmerman, Naidoo, Raizen, & Pack, 2008). Sanchez and
Sanchez-Vazquez (2009) found contrary results on adult zebrafish circadian rhythm. They
revealed that in constant light; when fasting, or when free to receive food at own time-
preference in a self-feeding system, a flat locomotor activity occurred with the fish moving
congruently during the 24 hours.
Normal zebrafish lifespan in the wild is not known (Lawrence, 2007). In experimental
settings zebrafish have a parabolic lifespan consisting of initial extensive death rate, followed
by a higher survival rate, mean life expectancy of 42 month, after which death rate increases
until the maximum of 66 months (Gerhard et al., 2002). A useful way to study zebrafish is
measuring locomotion (Hurd, Debruyne, Straume & Cahyll, 1998; Saili et al., 2012) which
was applied in the present study, with swim speed in cm/s.
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 10
Aim of this study
The aim of this study was to record, measure, and analyse zebrafish larvae swim speed
cm/s during day and night for four separate dosages of Methylphenidate; zero (control), 4.63
(low), 9.25 (medium) and 18.50 (high) µmol of Methylphenidate. Particularly, the study
examined short-term effects of Methylphenidate on zebrafish larvae in order to identify the
suitable dosing amount.
Method
Participants
The subjects of this study were 604 Danio rerio zebrafish larvae where the University
of Oregon, Zebrafish International Resource Centre, provided the original fish of a wild stock
for breeding. The sample size was determined as the fish is known to have a naturally high
death rate over their first weeks and the experimental group’s early Methylphenidate exposure
likely to result in an increased death rate. Five out of eight recordings were successful, one
had 80 larvae that occupied the wells, three had 79 and one had 38 during recording (n=355).
Larvae at seven, ten and eleven- days post fertilization (dpf) were recorded successfully and
after exclusion, 324 (control n=58, low n=89, medium n=89, and high n=88) were used for
analyses.
Exclusion Criteria
If a larva was not tracked for minimum of 95% of the timeframe, it was excluded from
the statistical processing, as well as after various technical or procedural problems that could
affect and corrupt the recording of the larvae (n=280).
Statistical Analysis
The independent variables were two, the amount of drug and the phase. The mean
swim speed was assessed by two-way analyses of variance. With four levels of drug
manipulation, control (no drug), low Methylphenidate dose, medium Methylphenidate dose
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 11
and high Methylphenidate dose. There were two levels of the phase, measured in the
timeframe of light on (day) and light off (night). The dependent variable was the mean swim
speed in m/sec. The drug was a between-group comparison and the phase was a within-
subject comparison. All post hoc comparisons were subjected to Bonferroni corrections.
Significance threshold was set at p < .05 and T-tests were applied for comparison for each
condition day versus night.
Instruments
When the eggs were harvested Methylene blue was added to the tank containing the
eggs to minimize infection. After hatching the larvae were fed 0.25g larval feed per day,
every morning prior to the recording, and to remove the Methylene blue a 10 L multi-tank
constant flow system (Aquatic Habitats, Apopka, FL, USA) was used the day before
recording. During the recording larvae were not provided with feed, and the control group
only received water from the breeding tank. The three experimental groups, additionally,
received Methylphenidate, an active drug administered in three separate doses, zero (control),
4.63 (low), 9.25 (medium) and 18.50 (high) µmol of Methylphenidate to swim in. The larval
behavior, swim speed, was recorded and tracked by Ethovision version 7.0 (Noldus
Information Technology) at two dimensions with a Sony XC-E150 infrared camera (Sony) at
8.33 Hz with a 50-mm CCTV Pentax lens (Pentax). Microwell plate, with the ability to
accommodate 96 larvae in separate wells (.4cm inner dimension and .38ml solution), was held
by a custom-built transparent Plexiglass container with a constant water flow, to keep the
larvae at 27.9°C to 28.5°C. The Plexiglas container was placed under the monitor tracing the
larvae activity (Noldus Information Technology). Illumination under the container was 2551x
during the white-light phase (day) and 0lx during the infrared-dark phase (night) throughout
the recording.
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 12
Procedure
The new research stock was bred, the morning after the eggs were separated from the
adult fish and harvested, with Methylene blue added to the tank containing the eggs. When
hatched, the larvae were fed once daily with .025g larval food and 24h prior to recording a
constant 28.5°C water flow of filtered water, 10% volume per day, was added to the tank in a
10 L multi-tank constant flow system. Figure 1. reveals the procedure step by step.
Figure 1. Research Procedures
Figure 1. dpf = days post-fertilization.
Prior to each recording, solutions were measured and distributed per well one solution at a
time. Other concentrations were kept separate until it had been confirmed that a correct
concentration had been administered to the appropriate well to maintain secure separation
between drug conditions. The larvae were randomly selected from the stock, manually with
separate pipettes, and assigned a condition by a single researcher, with a fixed systematic
Breeding
new stock
• The adult fish randomly picked, between the time 20 - 21, from breeding-stock for breeding in a breeding-tank
Harvesting
• The eggs were harvested between 8 - 10 in the morning after, and methylene blue was added to the tank
Removing
methyline blue
• The previous day before the recording, filtered water flow was added to the breeding-tank to remove the methylene blue
Prepering recording
• Between 10:00 - 11:30 on the day of the recording, 80 wells of Microwell plates, filled with .38ml of each solution and larvae seven, ten and eleven dpf added to each of them
Recording
• Between 12:00 - 13:34 the recording was started and stopped 24 hours later, thereafter each recording was cut to same timeframe of 13:34 until 8:00 the morgning after
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 13
procedure for every recording of zebrafish swim speed in cm/s. First the high
Methylphenidate group was added to the wells, then the medium Methylphenidate group, next
the low Methylphenidate group and last the control group. Larvae were randomly assigned to
the groups seven, ten and eleven dpf. The microarray had 96 wells, however, technical
problems only allowed 80 wells per array to be tracked per recording.
The five separate successful recordings started at midday and ended at midday the
morning after. Three recordings were not usable because of system failure. The main lighting
in the experimental environment was twofold, white light-phase from the start of recordings
midday, until 22:00 when infrared dark-phase was turned on and visual lights were turned off.
Recordings ended at midday, after which the recorded period was cut so that all the
recordings were at the same real-time.
Results
The results showed valid recordings from 324 zebrafish larvae consisting of control
group with 58 zebrafish larvae, low Methylphenidate of 89, medium Methylphenidate of 89
and high Methylphenidate of 88. Table 1 is an ANOVA summary table that shows the main
and interaction effects between the four groups (N = 324). The repeated measures factor
Phase, of day and night, was subjected to Greenhouse-Geisser correction. There was a
significant main effect for condition, a significant main effect for Phase, and a significant
Conditions x Phase interaction effect.
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 14
Table 1.
Summary of Two-way Repeated-measures ANOVA.
Source df Mean square F
CONDIT1 (between groups) 3 2.714 16.462*
PHASE2 (within subjects) 1 0.003 22.777*
CONDIT x PHASE 3 0.001 8.138*
Error 320
Note. 1CONDIT - four experimental conditions involving swimming in the solution of 0
(control), 4.63 (low), 9.25 (medium) and 18.50 (high) µmol Methylphenidate.
2PHASE – lights on (DAY) and lights off (NIGHT).
* p < .001
In view of the significant main effect for group, post hoc tests were conducted with
Bonferroni correction. Testing the differences for the main effect between groups revealed
that the high Methylphenidate group differed from the other groups while the control, low
Methylphenidate and medium Methylphenidate groups were not statistically different. This
interaction can be seen in Figure 2. The cut-off for significance after Bonferroni correction
was .0125. T-tests were conducted to compare mean swim speed during day and night for
each group. The control group t(57) = 0.001,and the low Methylphenidate group t(88) =
0.984, were not significant, the medium Methylphenidate group t(88) = 2.415, p =.018, was
just below significance of .0125 after the correction, and the high Methylphenidate group was
significant at t(87) = 9.875, p < .001. The medium and high Methylphenidate dosed groups
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 15
were still slower than the control group during the night, although, all of the drugged groups
moved faster during the night than the day.
Figure 2. The Phase Day and Night for each Group
Figure 2. Shows larvae mean swim speed cm/s for each condition during the Day and Night.
MPH = Methylphenidate.
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
Day Night
Swim
sp
ead
in c
m/s
Phase
Control Group
Low MPH Group
Medium MPH Group
High MPH Group
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 16
Figure 3. Larvae Mean Swim Speed cm/s During the Day
Figure 3. Mean swim speed cm/s for each five-minute epoch. MPH = Methylphenidate.
Figure 3 shows recordings of larvae mean swim speed cm/s for each five-minute
epoch 13:40 until 21:20 for each condition. T-tests, as reported above, showed the three
groups, control, low Methylphenidate and medium Methylphenidate were not statistically
different from each other, while the larvae in the high Methylphenidate group moved less
during the timeframe.
The phase day with mean swim speed and standard deviation can be seen in Figure 4.
with the four conditions valid recordings, high Methylphenidate dose groups swim speed
lower than the other groups when looking at total mean swim speed during day.
,00
,01
,02
,03
,04
,05
,06
,07
,08
,09
13
:40
13
:55
14
:10
14
:25
14
:40
14
:55
15
:10
15
:25
15
:40
15
:55
16
:10
16
:25
16
:40
16
:55
17
:10
17
:25
17
:40
17
:55
18
:10
18
:25
18
:40
18
:55
19
:10
19
:25
19
:40
19
:55
20
:10
20
:25
20
:40
20
:55
21
:10
21
:25
Me
an S
wim
Sp
ee
d c
m/s
Time of recording
Larvae Velocity at Day
Control Low MPH dose Medium MPH dose High MPH dose
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 17
Figure 4. Mean Swim Speed during Day
Figure 4. Mean swim speed cm/s of the whole phase (day). MPH = Methylphenidate
Control group had the Mean (M) of 0,0693 and SD = 0,020, low Methylphenidate dosed
group had M = 0,0678 and SD = 0,0194, medium Methylphenidate dosed group had M =
0,0652 and SD = 0,0163 and the high Methylphenidate dosed group had M = 0,0524 and the
SD = 0,0122. The high Methylphenidate dosed group differed significantly from the control
group and the low and medium Methylphenidate groups.
Figure 5. reveals the locomotion during night with recordings for each condition
during five-minute bins from 22:20. until 8:00 where larvae behaviour change mean swim
speed was used for analysis.
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
Control Group Low MPH Group Medium MPH Group High MPH Group
Mea
n S
wim
Sp
ead
cm
/s
Larvae Mean Swim Spead Day
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 18
Figure 5. Mean Swim Speed cm/s at Night
Figure 5. Larvae mean swim speed every five minutes in cm/s in constant phase night from
22:20 until 08:00. MPH = Methylphenidate.
Figure 6 shows the mean swim speed at night during the recording with standard
deviation (SD). Control group had the M of 0,0693 and SD = 0,0158, low Methylphenidate
group had M = 0,0697 and SD = 0,0085, medium Methylphenidate group had M = 0,0689 and
SD = 0,0084 and the high Methylphenidate group had M = 0,0640 and the SD = 0,0090
,00
,01
,02
,03
,04
,05
,06
,07
,08
,09
22:2
0
22:4
0
23:0
0
23:2
0
23:4
0
00:0
0
00:2
0
00:4
0
01:0
0
01:2
0
01:4
0
02:0
0
02:2
0
02:4
0
03:0
0
03:2
0
03:4
0
04:0
0
04:2
0
04:4
0
05:0
0
05:2
0
05:4
0
06:0
0
06:2
0
06:4
0
07:0
0
07:2
0
07:4
0
08:0
0
Mea
n S
wim
Sp
eed
cm
/s
Time of recording
Larvae Swim Speed at Night
Control Low MPH dose Medium MPH dose High MPH dose
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 19
Figure 6. The Swim Speed of Zebrafish Larvae during Night
Figure 6. shows the mean swim speed during night with Standard Deviation bars.
MPH=Methylphenidate
Discussion
The purpose of the study was to analyse zebrafish larvae mean swim speed for four
dosages, zero, 4.62, 9.25 og 18.50 µmol of Methylphenidate to identify suitable dosing for
research on the species with this drug. The independent variables were two, phase and
condition. The phase had two stages, day and night, and the condition had four stages of
Methylphenidate exposure, low, medium and high. The dependent variable was the mean
swim speed cm/s during their first day and night of Methylphenidate exposure.
The larvae in the high Methylphenidate group moved slower than the other groups, so
slow that it could be interpreted as a status similar to an overdose or a toxic effect. The
control, low, and medium Methylphenidate groups were similar to each other both during day
and night, with no statistical difference. The high Methylphenidate group, however, showed
lack of movement compared to the other groups, both during day and night, with a slight rise
in movement during the night after being in the solution for the whole day, possibly as the
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
Control Group Low MPH Group Medium MPH Group High MPH Group
Mea
n S
wim
Sp
ead
cm
/s
Larvae Swim Spead at Night
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 20
drug in the solution was metabolized by the larvae. Looking at movement during the phases
for different conditions, their mean swim speed showed some tendency for bigger
Methylphenidate dosage to add to the increased motion. The high dosed group with high
statistical difference and the marginal effect for the medium dosed group and the low dosed
group closer to the control group. The larvae moved faster during the following night than the
previous day. The report by Galland et al. (2010) where Methylphenidate given during the
day deduces sleep efficiency during night might be supported by these findings. The high
Methylphenidate group, however, had the highest increase in motion from day to night and
was still not as fast at night as any of the other groups during the previous day, perhaps as the
high Methylphenidate group daytime swim speed was so low. It could indicate that the larvae
needed time to recuperate from toxic effects of the high Methylphenidate dose and that the
recovery was not complete.
The Levin et al (2011) research group studied zebrafish larvae with Methylphenidate
dosages about 26% stronger per group than the present study, given during the first five dpf
and examined three months subsequently. They found that the long-term effect was only
visible for the high Methylphenidate dosed group, perhaps indicating that the large dose is
neurotoxic. Their deduction was that humans should refrain from taking the drug while
reproducing. A valid question would then be: How much does result of a neurotoxic dose on
zebrafish larvae apply to normative research on human (regarding prescription and
application) were the present research finds that the 26% more diluted dose reviles such
reduction in locomotion.
The present research studied the larvae in the first day and night of Methylphenidate
exposure. The control group and the two Methylphenidate groups receiving the smaller
dosages were more like one group, with no statistical difference between them. It was
surprising that the control group had no statistical difference where the larvae move less
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 21
during the night when most other research suggest the zebrafish are a diurnal species (Elbaz et
al., 2013; Hurd et al., 1998). As the control group in the precent study did not respond as most
normal zebrafish larvae have been recorded to behave in the past, this research still needs to
be repeated and improved in order to clarify whether technical failures were responsible for
current results. The most plausible explanation would be that the lighting in this research was
not set correctly, as the various lighting in zebrafish environment has been proven to affect
their diurnal rhythm (Padilla, Hunter, Padnos, Frady, and MacPhail, 2011). The previous
research findings on zebrafish larvae is parallel to the study of Sanchez and Sanchez-Vazquez
(2009) on adult zebrafish where when fasting and in constant light the fish sustained their
locomotor activity sequence during the 24 hours. The locomotion was different when the fish
was fed and if the regular feeding time was at night the fish did not move as much as if
feeding was during day. Down to a cellular level, Cahill (2002) indicated that the zebrafish
circadian rhythm might derive from light-sensitive oscillators present in the peripheral tissues
with pacemakers distributed over the organism and entrained by light, independently of other
factors. Other research simply indicates zebrafish loose vision at night (Emran and Dowling,
2010; Emran et al., 2009) which could explain why most research have found that zebrafish
have similar diurnal rhythm to humans.
Additional explanations to the present research findings could be found in recent
research that has come out since the present research was conducted and points to critical
importance of the width and depth of the wells, and the post-fertilization age of the zebrafish
larvae, as vital influences on zebrafish larvae locomotor behaviour (Ingebretson and Masino,
2013; Padilla et al., 2011). Considering recent research, it would be informative to repeat the
present study to remove all doubt about lighting mechanisms malfunctioning and to study
further the relationship and effects of diverse lighting on zebrafish behaviour, especially
regarding diurnal rhythms when in constant lighting conditions. Since the present research
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 22
was conducted, Saili et al. (2012) have been able to alter zebrafish so that they revile ADHD
symptoms, which if applied to the present research, could improve it even further.
Furthermore, it might be informative to examine how gender is influenced by these
conditions, as the present study did not consider gender specifically.
Methylphenidate as a helpful solution to fundamental ADHD dysfunction is evident
but the lack of consistent research on the drug is a weakness that needs to be remedied.
Dosages, how and when to apply them, are all worthy questions when research continues to
be inconsistent. The present research showed indications that previous studies may not have
dosed equivalently to normative human application as short-term effect on humans after
normative oral administration has not been reported to be drastically reduced locomotion.
There have, however, been reports of more side-effects following higher dosing. If the short-
term effect of the substance on zebrafish larvae is to be applicable to studies on humans, the
dosage levels applied for further studies must fit the short-term effects on humans. If this
cannot be obtained, then applying zebrafish larvae as animal models for short- and long-term
effects of the drug might not be the best model. The question of how to apply animal drug
studies to deduction of human comparison is still left unanswered. If drug research on animals
is to be ethically conducted then results should further practical knowledge.
EFFECT OF METHYLPHENIDATE ON ZEBRAFISH 23
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