Download A Matching Law Analysis of the Effect of Amphetamine

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BELKE AND NEUBAUER
increase in the release of dopamine in the mesolimbic pathway that is
associated with the biological basis of reward (Lambert, 1992; Watson,
Trujillo, Herman, & Akil, 1989). The purpose of the present study was to
investigate this dopamine hypothesis by observing the effects of a
dopamine agonist on indices of motor performance and reinforcement
efficacy derived from a matching law analysis of the relationship
between response and reinforcement rates in a context where
responding was reinforced by the opportunity to run.
Lambert (1992) proposed that the pharmacological basis of the
rewarding effects of running is a function of a dopamine rather than an
opiate mechanism. Specifically, "after moderate running, increased
mesolimbic dopaminergic activity provides physiological reinforcement,
which sustains maladaptive running in the face of low food consumption"
(Lambert, 1992, p. 27). Evidence cited in support of this hypothesis
comes from studies of the effects of dopamine agonists and antagonists
on running. Dopamine agonists such as amphetamine and cocaine
increase running (Evans & Vaccarino, 1986; Glavin, Pare, Vincent, &
Tsuda, 1981; Jakubczak & Gomer, 1973; Tainter, 1943) whereas
dopamine antagonists such as pimozide and chlorpromazine decrease
running (Beninger & Freedman, 1982; Routtenberg, 1968; Routtenberg
& Kuznesof, 1967). However, this evidence must be viewed with a
degree of skepticism because it fails to distinguish between motor and
motivational effects of the drugs. That is, a decrease in running under
the influence of a drug may be the result of either a decrease in
motivation to run or an impairment of motor behavior.
One procedure that offers the potential to discriminate between motor
and motivational changes in reinforced responding due to the effect of a
drug is based on Herrnstein's (1970, 1974) matching law (Hamilton, Stellar,
& Hart, 1985; Heyman, 1983, 1992; Heyman & Monaghan, 1987, 1990;
Heyman, Kinzie, & Seiden, 1986; Heyman & Seiden, 1985). Herrnstein
(1970) formulated an elementary matching law equation for the case where
there is only a single measured source of reinforcement and a single
measured response rate. The relationship between response and
reinforcement rates in this case generally takes the form of a negatively
accelerated monotonic function that is described by the following equation:
81 -
k R1 .
R1 + Re
(1 )
In Equation 1, 81 is response rate, R1 is reinforcement rate, and k
and Re are estimated parameters. Specfically, k refers to the asymptotic
rate of responding (i.e., the maximal response rate) and Re is the rate of
reinforcement associated with one half the asymptotic rate of
responding. Re describes how quickly response rate rises toward
asymptote as reinforcement rate increases and at one half the
asymptotic response rate (kI2), the value of Re can be estimated
because R1 and Re are equivalent.
AMPHETAMINE AND RUNNING
485
The k parameter in Equation 1 has been interpreted as an index of
the motor aspects of a reinforced response (Hamilton et aI., 1985;
Herrnstein, 1974; Heyman, 1983). This interpretation derives from
empirical observations that showed that when k changed, but Re
remained stable, the experimenter changed some aspect of the
response requirement (Heyman & Monaghan, 1987). For example, the
value of k changed independent of Re with a change in response
topography when the response manipulandum for pigeons was changed
from a pecking key to a treadle (McSweeney, 1978) and with a change in
response force on the same manipulandum when the force required to
make a response was increased (Belke & Heyman, 1994; Heyman &
Monaghan, 1987). In addition, Porter and Villanueva (1988) found a
relation between response duration and the value of the k parameter.
Specifically, as response duration increased, the value of k decreased.
Thus, k appears to index motor aspects of performance.
In contrast, the Re parameter has been interpreted as an index of
the motivational component of a reinforced response (Heyman &
Monaghan, 1987). This interpretation follows from the observation that in
studies where Re changed, while k remained stable, experimenters
changed either reinforcement magnitude, reinforcement quality, or
deprivation level (Heyman & Monaghan, 1987). For example, the value
of Re decreased when the body weight of subjects responding for
sucrose reinforcement was decreased (Bradshaw, Szabadi, Ruddle, &
Pears, 1983) or when the concentration of sucrose reinforcement was
increased (Heyman & Monaghan, 1994). In both cases, as motivation to
obtain the scheduled reinforcement increased (i.e., with greater
deprivation for food and with an increase in the sweetness of the
reinforcement), response rate rose more quickly toward asymptote and
the value of Re decreased. In contrast, if motivation to obtain the
scheduled reinforcement decreased, response rate would rise less
quickly toward asymptote and the value of Re would increase. Thus, Re
appears to index a motivational aspect of performance.
Belke and Heyman (1994) extended the matching law approach to
the study of the relationship between response and reinforcement rates
when the opportunity to run was scheduled as the reinforcing
consequence for lever pressing. Manipulations of the force required to
make a response and access to the wheel demonstrated that the
empirical interpretations of the k and Re parameters held when running
functioned as a reinforcing consequence. When the force required to
make a response was increased by 26 grams, the mean value of k
decreased from 61 to 37 responses/minute while the Re remained
constant at approximately 93 reinforcers/hour. When access to the wheel
was limited by placing the subjects in the locked wheel for 45 minutes
prior to a session, the average value of Re decreased from 73 to 51
reinforcers/hour while k remained constant at 57 responses/minute.
Previous research using the matching law procedure to study the
effects of drugs on reinforcement efficacy and motor performance has
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BELKE AND NEUBAUER
shown that dopamine antagonists such as chlorpromazine and
pimozide decrease the efficacy of water (Heyman et aI., 1986) and
food reinforcement (Heyman, 1983; Willner, Sampson, Phillips, &
Muscat, 1990) at low doses. In contrast, dopamine agonists such as
amphetamine increase the efficacy of food reinforcement to maintain
behavior at low doses (Heyman, 1983, 1992; Heyman & Seiden,
1985). However, this procedure remains to be extended to the
investigation of the effects of dopamine agonists on responding
maintained by nonappetitive wheel-running reinforcement.
In the present study, subjects were exposed to a series of tandem
fixed ratio 1 variable-interval schedules of wheel-running
reinforcement and Herrnstein's (1970, 1974) matching law was fit to
the obtained response and reinforcement rates. After 60 sessions,
subjects were exposed to amphetamine to observe the effects of
amphetamine on responding reinforced by the opportunity to run. If
amphetamine increases the reinforcing efficacy of running (i.e.,
motivation to run) then the matching law analysis should reveal that
Re decreases while k remains unchanged. This result would be
consistent with Lambert's (1992) dopamine hypothesis. Alternatively, if
k changes while Re remains unchanged, then the matching law
analysis would suggest that amphetamine alters motoric rather than
motivational aspects of reinforced responding and this result would
not support Lambert's (1992) hypothesis.
Method
Animals
Twelve male Wistar rats (Charles River Breeding Laboratories,
Que.) were individually housed in polycarbonate cages (20 x 24 x 40
cm) in a holding room on a 12-hr light/dark cycle (lights on 8:00 am).
Subjects were maintained at 80% of an initial free-feed body weight
with free access to distilled water in the home cage at all times. Under
food deprivation conditions, body weights ranged from 288 to 298g.
Apparatus
Subjects were tested in standard activity wheels (3 Wahmann & 5
LaFayette Instruments Model #86041 A) located in soundproof shells.
Each wheel was 35.5 cm in diameter. A solenoid-operated brake was
attached to the base of each wheel. When the solenoid was operated,
a rubber tip attached to a metal shaft contacted the wheel and caused
the wheel to stop. A retractable lever (Med Associates ENV-112) was
mounted at the opening of each wheel so that the lever would
protrude into the wheel chamber when extended. A 24-V DC light was
mounted on each side of the frame of the wheel to illuminate the
interior of the wheel and the area of the lever. Control of experimental
events and recording of data were handled by IBM personal
computers.
AMPHETAMINE AND RUNNING
487
Procedure
Initially, all rats were given free access to a running wheel for 30 min
each day for 10 days. After 10 days, subjects were placed in a standard
operant conditioning chamber following the session in the wheel and
shaped to press a lever. Each lever press produced 0.1 ml of a 15%
sucrose solution. When subjects reliably pressed the lever, the schedule
of reinforcement was shifted from a continuous reinforcement schedule
through a series of variable-ratio (VR) schedules, including VR 3, VR 6,
VR 9, and VR 15. Each schedule was in effect for approximately four
sessions.
Throughout the period of lever training, subjects continued to run in
wheels daily for a single 30-min session. When four sessions of lever
pressing for sucrose solution on the VR 15 schedule were completed,
lever pressing for sucrose solution was discontinued. At this time, the
retractable lever in the wheel chamber was made operative and the
opportunity to run for 60 s was made contingent upon a single lever
press. A session consisted of 30 opportunities to run.
Training then proceeded through the following steps. The schedule
of reinforcement was successively shifted through the following
sequence: fixed ratio (FR) 1 response, VR 3 response, VR 5 response,
VR 9 response, and VR 15 response. Subjects remained on each
schedule for 3 days before advancing to the next schedule. During this
period of training on the series of VR schedules, 8 rats that ran at higher
rates and completed all scheduled reinforcements within the shortest
duration were chosen to continue with the study. There were two
reasons for this selection. First, animals with low response rates and
long session durations were not likely to complete all components of a
mixed schedule within the drug's half-life. Interpretation of a drug effect
using the matching law depends upon all reinforcement schedules being
completed while the animal is under the influence of the drug. Note that
the half-life for amphetamine varies with the pH of urine and may be as
short as 7 hours (McKim, 1991). Second, animals that ran at higher
rates typically tended to respond at higher rates (Belke, 1996) and, thus,
were more likely to develop response-reinforcement relationships
sufficiently well defined so as to permit clear discrimination of motor and
motivational effects of the drug treatments (Belke & Heyman, 1994).
Following the 3rd day on the VR 15 schedule, the session was changed to
a sequence of four tandem FR 1 variable-interval schedules (VI 5, VI 15, VI 30,
VI 60) and the reinforcement period was shifted from 60 s to 30 s. With the
change from a response-based (VR) reinforcement requirement to a
response-initiated time-based (VI) reinforcement requirement, the operation of
the schedule was modified so that a reinforcement interval did not start timing
until the first response was made. Thus, after termination of a reinforcement, a
new interval was selected, but it did not begin to time out until the first lever
press. When the reinforcement interval elapsed, the first lever press caused
the lever to retract and the brake to release. The wheel was free to turn for 30
s before the brake was engaged and the lever was extended for the next
488
BELKE AND NEUBAUER
reinforcement interval. The programmed interreinforcement intervals for the
variable interval component of the tandem FR 1 VI schedules approximated
an exponential distribution (Fleshier & Hoffman, 1962) and the order of
intervals was randomized across sessions.
Within a session, successful completion of 10 reinforcers in each
component on a given schedule was followed by a 1-min blackout time
during which the lights were turned off and the brake was engaged. When
the blackout period expired, the lever extended, the lights were turned on,
and the animal was given the opportunity to obtain another 10 reinforcers
on a different reinforcement schedule. A session consisted of completion of
each of the four schedules of reinforcement for a total of 40 reinforcement
periods. Each animal was presented with the same sequence of schedules
across all sessions, however, sequences were varied across rats (see Table
1). After each session, animals were weighed and fed a measured amount
of food to maintain 80% body weight level.
Animals were given 60 sessions under these conditions to allow for
differentiation between response rates on the different schedules, before
drug testing commenced. Subjects were then administered damphetamine sulfate by intraperitoneal injection 20 min prior to a
session at doses of 0.25, 0.5, and 1.0 mg/kg. Each dose was
administered three times and the order of dose administration was
randomized across rats. Between drug administrations, baseline and
saline-vehicle injection sessions occurred.
Dependent measures taken for each session were total lever
presses, time spent pressing the lever, postreinforcement latency
(latency to press following reinforcement), and wheel revolutions. Local
response rates were calculated as the number of lever presses divided
by the time spent lever pressing exclusive of latency to respond and
expressed as responses/minute. Data obtained from all rats were
included in the analysis of drug effects on local response rate, total
Table 1
Data from the Last 5 Sessions Prior to Drug Testing for Each Rat
Rat
X09
X10
X11
X12
X13
X15
X18
X24
Schedule Order
30 60 15
5
5 30 60 15
5 60 30 15
30
5 15 60
60 15
5 30
60
5 30 15
15 60
5 30
15 30 60
5
k
83.0
31.4
38.4
53.7
42.7
95.7
40.2
48.0
Re
193.5
29.4
33.7
84.4
70.9
71.6
52.1
59.9
%VAC
94.2*
55.6
69.1
83.8*
93.8*
85.3*
64.6
43.1
Revs
655
610
716
668
882
820
736
522
Res
28.1
23.3
27.1
28.1
23.2
54.3
24.9
30.2
Lat
474
1156
802
2329
522
908
1379
1410
Note. The schedule order for each rat is shown followed by the parameters and goodness of fit
percentages from Equation 1 fit to the data for each rat averaged over the last five sessions of
the baseline condition prior to drug testing. Asterisks denotes rats selected for analysis of drug
effects using Equation 1 based on criteria stated in the methods. U%VAC" refers to the
percentage of variance in response rates. Also shown are the mean revolutions (Revs),
response rate (Resp) (responses/minute), and cumulative latency to respond following the
termination of reinforcement (Lat) (seconds) from the last five baseline sessions.
489
AMPHETAMINE AND RUNNING
100
X12
X09
80
--.
Q)
+oJ
~
60
s:::::
40
""'V)
20
·E
Q)
V)
c::
0
c.
V)
0
X13
Q)
Lo-
-..Q)
80
+oJ
ro
Lo-
Q)
V)
60
s:::::
0
a.
V)
Q)
0
•
•0
40
C
0:::
20
200 400 600 800
0
Baseline
0.0 mg/kg
0.25 mg/kg
0.5 mg/kg
1.0 mg/kg
200 400 600 800 1000
Reinforcement rate (reinforcers/hour)
Figure 1. Response rates plotted as a function of obtained reinforcement rates for the
baseline, 0.0 mg/kg, 0.25 mg/kg, 0.5 mg/kg, and 1.0 mg/kg doses of amphetamine. Curves
were obtained by fitting Equation 1 to the data. Note that the data and curve for the 1.0
mg/kg amphetamine dose for Rat X15 was not plotted because of the negative value of
Re estimated for this condition (see Table 2).
wheel revolutions, and total postreinforcement latency. A repeated
measures analysis of variance (ANOVA) was performed to test for dose
effects and Dunnett's t tests were performed to compare means.
In addition, lever presses, time, and latency to press were recorded
for each reinforcement to examine within-session effects on response
rates. Wilkinson's (1961) method of estimating the parameters of a
hyperbolic function was used to generate k and Re values. A-priori
criteria were established for analysis of data from rats that showed
systematic differentiation of response rates as a function of
reinforcement rates. First, and most importantly, when Equation 1 was fit
to response and reinforcement rates averaged over the last five sessions
of the baseline condition at least 75% of the variance in response rates
had to be accounted for by variance in reinforcement rates (Willner et aI. ,
1990). Second, estimates of k and Re for each session over the last five
consecutive sessions could be neither the highest nor the lowest
490
BELKE AND NEUBAUER
Table 2
Effects of Amphetamine on k and Re Estimates
Condition
k
Re
SE k
SE Re
Rat X09
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
71
79
76
93
65
153
168
117
119
48
9.1
12.2
9.9
8.4
4.5
45.7
57.3
39.0
29.2
13.7
94
92
91
95
92
Rat X12
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
58
57
55
37
43
101
92
62
40
42
8.7
4.4
3.1
4.2
3.5
38.8
20.6
12.9
19.5
15.4
84
96
96
79
89
Rat X13
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
31
32
31
31
27
59
44
22
24
11
3.0
3.2
1.7
1.8
0.9
19.4
18.8
8.0
8.7
4.2
90
84
85
84
79
Rat X15
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
101
96
88
85
79
62
54
55
8.3
9.3
9.0
9.1
20.4
21.4
19.7
20.9
95
89
85
85
%VAC
Note. Parameters and goodness of fit percentages from Equation 1 fit to the data for the
rats that met the inclusion criteria. Conditions were baseline, 0.0 mg/kg, 0.25 mg/kg, 0.50
mg/kg, and 1.0 mg/kg doses of amphetamine. U%VAC" refers to the percentage of variance
in response rates accounted for by variance in reinforcement rates. Standard errors for the
estimates of k and Re are given.
observed under the baseline condition. The rationale for this selection
procedure is that if the relationship between response and reinforcement
rates is not adequately described by Herrnstein's equation, then the
estimated parameters k and Re are unreliable and interpretation of drug
effects based on these parameters should not be undertaken. Although
all rats met the second criterion, Table 1 shows that only 4 of the 8 rats
met the first criterion.
Results
The effect of amphetamine on responding within sessions was
evaluated through a matching law analysis of the relationship between
response and reinforcement rates. This analysis was conducted on data
from all animals; however, interpretation of the effects of amphetamine
492
BELKE AND NEUBAUER
mg/kg dose, tJ12) = 3.60, P < .05; the 0.5 mg/kg dose, tJ12) = 3.97, P <
.01; and the 1.0 mg/kg dose, tJ12) = 7.42, P < .01.
With respect to the effect of amphetamine on local response rate,
cumulative latency to respond, and total revolutions over the entire
session, no significant effects were observed. For this analysis, the data
from all 8 rats were used. Relative to baseline, local response rates
increased on average 1.6, 6.6, and 15.5% for the 0.0, 0.25, and 0.5
mg/kg doses, but decreased by 3.0% for the 1.0 mg/kg doses. None of
these changes attained significance, F(4, 28) = 1.48, p> .10. Cumulative
latency to respond increased on average by 7.7,8.1, 11.9, and 18.8% for
the 0.0, 0.25, 0.5, and 1.0 mg/kg doses, respectively. As with response
rates, there was no significant drug effect, F(4, 28) = 0.67, P > .10.
Finally, total revolutions increased by 1.3% for the 0.0 mg/kg dose, but
decreased by 6.3, 1.7, and 8.1 % for the 0.25, 0.5, and 1.0 mg/kg doses,
respectively. Analysis revealed a drug effect, F(4, 28) = 3.24, P < .05;
however, no dose produced a significant effect relative to baseline. Note
that restricting the analysis of local response rate, cumulative latency,
and total revolutions to only the data from the 4 rats that met the criteria
for the matching law analysis also revealed no significant drug effects;
F(4, 12) = 1.02, p> .10, F(4, 12) = 0.16, p> .10, F(4, 12) = 0.90. p> .10,
respectively.
Finally, mean session durations averaged across all animals for the
baseline, 0.0 mg/kg, 0.25 mg/kg, 0.50 mg/kg, and 1.0 mg/kg conditions
were 4238, 4397, 4478, 4495, and 4838 s, respectively. Session
durations in all conditions were within the half-life of amphetamine.
Discussion
In the present study, the effect of amphetamine on responding
reinforced by the opportunity to run for a brief period of time was
investigated using Herrnstein's (1970, 1974) matching law equation.
Changes in the relationship between response and reinforcement rates
under the influence of amphetamine were consistent with the
interpretation that amphetamine increased the reinforcing efficacy of
running. Specifically, amphetamine decreased Re while the value of k
remained relatively unchanged. The empirical basis for this interpretation
comes from studies that have shown that decreases in Re independent
of changes in k occur when reinforcement quality, reinforcement
magnitude, and deprivation level were manipulated (Bradshaw, Ruddle,
& Szabadi, 1981; Bradshaw, Szabadi, & Bevan, 1978; Bradshaw et aI.,
1983; Conrad & Sidman, 1956; Guttman, 1954; Heyman & Monaghan,
1987, 1994, deVilliers & Herrnstein, 1976, analyzed results for studies
prior to 1976). Furthermore, the results were consistent with previous
studies using a matching law analysis that have shown that dopamine
agonists (Le., amphetamine, methylphenidate) increase the reinforcing
efficacy of the experimentally arranged reinforcement (Le., milk, sucrose
solution) (Heyman, 1983, 1992; Heyman & Seiden, 1985).
AMPHETAMINE AND RUNNING
493
Paradoxically, although the reinforcing efficacy of running, as
indexed through the relationship between response and reinforcement
rates, increased, running did not increase. On the contrary, revolutions
decreased across the dose range that increased the reinforcing value of
running, though not significantly so. However, this paradox may be more
apparent than real. Previous research has shown that as the duration of
the opportunity to run decreases, rate of running increases toward an
asymptotic level (Belke, in press). Thus, the brief duration used as
reinforcement in the present study to ensure that session duration fell
within the drug's half-life probably induced near asymptotic rates of
running. Consequently, with running near a behavioral ceiling, it is
unlikely that a psychomotor stimulant such as amphetamine would
increase running.
Finally, the results lend tentative support to Lambert's (1992) hypothesis
that the pharmacological basis for the reinforcing value of running is
dopaminergic. The present study represents an advance over previous
studies that investigated the effects of drugs, such as amphetamine, directly
on running. Increases or decreases in running under these conditions could
represent either a motivational or a motor effect. In the present study,
operant responding maintained by the opportunity to run represented an
index of the reinforcing efficacy of running and changes in the relationship
between response and reinforcement rates provided a means to distinguish
changes in the motor and motivational components of a reinforced
response. Further substantiation that the effect is motivational would come
with future research negating the possibility that the observed changes were
caused by either central or peripheral adrenergic effects of amphetamine.
The finding that the pharmacological basis of the rewarding
properties of running may involve dopamine is also consistent with
previous research showing that dopamine plays an important role in
preparatory behavior (Blackburn, Phillips, & Fibiger, 1987; Blackburn,
Phillips, Jakubovic, & Fibiger, 1989). Ethologists distinguish between
preparatory and consummatory behavior. With respect to food,
consummatory behaviors occur when animal has made contact with a
food item and result in the ingestion of the food. Examples of
consummatory behaviors are biting, chewing, and swallowing. In
contrast, preparatory behaviors refer to "appetitive acts that typically lead
to, or make possible, consummatory behavior" (Blackburn et aI., 1987, p.
352) that are elicited by biologically significant events known as
"incentive stimuli." Examples of preparatory behaviors are foraging and
hoarding. It is possible that the pharmacological basis of the reinforcing
value of running is related to the role of activity in foraging for food, as
such, it would be an example of a preparatory behavior. Although it is
important to note that in the context of the present study, there was no
explicit incentive stimulus (i.e., food item) that would elicit running as a
preparatory behavior. However, it may be the case that interoceptive
stimulation associated with food deprivation may serve this function
(Davidson, 1993).
494
BELKE AND NEUBAUER
In sum, although the widely known opiate hypothesis for "runner's
high" would imply that the reinforcing properties of running are a function
of endogenous opiates, results from the present study suggest
otherwise. Changes in the response-reinforcement rate functions under
the influence of amphetamine in the present study suggest that
amphetamine enhanced the reinforcing efficacy of the opportunity to run.
Therefore, the results from the present study, although not negating the
opiate hypothesis, lend support to the alternative dopamine hypothesis.
Further support for the dopamine hypothesis may come with future
investigation of the effects of selective dopamine antagonists on
responding reinforced by the opportunity to run.
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Appendix
Parameters and goodness of fit percentages from Equation 1 fit to the data for
the rats that did not meet the inclusion criteria. Conditions were baseline, 0.0
mg/kg, 0.25 mg/kg, 0.50 mg/kg, and 1.0 mg/kg doses of amphetamine. Standard
errors for the estimates of k and Re are given. Note that estimates of variance
accounted for (%VAC) are low while standard errors for the estimates of Re
tend to be larger than the estimates. With the exception of the 0.50 mg/kg
condition, the data for Rat X11 is adequately described; however, inclusion of
data for this rat would not alter the findings of the study.
Rat X10
Condition
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
k
25
21
24
29
20
Re
11
6
25
19
44
SEk
3.0
5.7
4.9
1.6
5.7
SERe
13.9
32.3
26.9
7.5
39.2
%VAC
23
-26
22
82
21
497
AMPHETAMINE AND RUNNING
Rat X11
Condition
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
k
38
35
39
28
35
Re
37
30
32
7
10
SEk
3.1
4.4
3.4
3.5
0.4
SERe
14.1
20.2
13.9
14.0
1.5
%VAC
86
68
82
7
97
Rat X18
Condition
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
k
41
33
29
36
29
Re
55
32
25
26
34
SEk
12.5
7.9
7.8
3.0
9.8
SERe
61.3
37.7
38.1
12.8
41.8
%VAC
29
29
10
77
-10
Rat X24
Condition
Baseline
0.00 mg/kg
0.25 mg/kg
0.50 mg/kg
1.00 mg/kg
k
46
51
36
36
13
Re
93
114
66
20
-6
SEk
14.9
14.6
5.5
4.6
2.3
SERe
73.6
72.0
33.0
18.3
14.2
%VAC
34
48
77
39
-4
498
BELKE AND NEUBAUER