The Effects of Operant Contingencies on Drug Sensitization and Tolerance to
Chronic Administration of Amphetamine
Cheryl Newbold, Student, Psychology Department
Abstract
Previous research has shown that drug sensitization, or the increase in the effect
of a drug with repeated exposure, is related to relapse to drug taking in animal models of
human behavior. Therefore, it is important to understand how the environment affects
the development of sensitization so that human drug abusers’ environments can be
modified such that relapse to drugs will be less likely to occur. Among other possible
explanations, sensitization has shown to be more likely to occur when drug-related
behaviors (i.e. locomotor activities) do not impede on an animal’s ability to obtain
important goals. We believe this shows evidence that the development of sensitization or
tolerance could be dependent upon operant contingencies, or different rates of
reinforcement available to the organism. Therefore, in the proposed study, rats that have
been given injections of d-amphetamine will respond for food reinforcers on two
different schedules. In one schedule, decreases in response rate will cause decreases in
the reinforcement rate. This is hypothesized to result in either tolerance or no
sensitization, since gaining reinforcers would be incompatible with drug-induced
locomotor activity. The theory is that the rat will learn to suppress those drug effects to
obtain reinforcers. In the other schedule, decreases in response rate will not decrease the
rate of reinforcement as substantially. This is hypothesized to result in sensitization,
since responding would not be incompatible with drug-related behaviors, and therefore
does not have to suppress those behaviors to gain reinforcers. This study is important in
order to determine if drug sensitization and tolerance can be a function of the
reinforcement contingencies in the organism’s environment. These results would not
only be helpful in treating and preventing drug abuse, but in the medical field as well, in
administering morphine or other painkillers to patients.
Literature Review
Drug sensitization is the increasing effect of a drug in response to a smaller
amount of the drug. Previous studies have suggested that drug sensitization is more
likely to result in relapse to drug taking after abstinence. Understanding how the
environment is related to sensitization is therefore important in studying drug abuse.
Several factors have been investigated and shown to be associated with the development
of sensitization in animal models, including the schedule of drug administration
(Robinson & Becker, 1986), individual differences of the subjects (Piazza, Deminiere, Le
Moal, & Simon 1989; Robinson & Becker 1986; Sax & Strakowski 2001) and the
novelty of the environment (Robinson & Berridge 2000). However, these results have
not given us a complete understanding of when tolerance or sensitization will occur after
chronic drug use.
Other studies have shown that rats are more likely to become sensitized to a drug
if it does not interfere with their goal-directed behaviors (Wolgin and Hertz 1995;
Pinkston and Branch 2003). Conversely, if drug-related behaviors (such as locomotor
activity) inhibit the rat’s ability to feed, they are more likely to show tolerance to the
drug. The most convincing evidence of this is Wolgin and Hertz (1995), in which the
development of tolerance or sensitization to drugs was determined by how compatible
feeding was with drug-related behaviors. Rats were given either acute or chronic
injections of cocaine, and fed either with a bottle or a cannula. Bottle-feeding involves
more movements and goal-directed behaviors than feeding with a cannula. At acute
doses, bottle-fed rats showed more stereotyped movements than cannula-fed rats.
However, at chronic doses, bottle-fed rats developed tolerance, and cannula-fed rats
developed sensitization. This suggests that rats will learn to suppress those drug-induced
behaviors that impede on their ability to gain food, since drinking from a bottle uses
movements and behaviors inconsistent with the locomotor activity caused by drugs. On
the other hand, if milk is injected into the rats’ mouth, there are no behaviors related to
feeding that are competing with drug-related behaviors.
Because it has been shown that drug sensitization can be determined by how the
drug effect interacts with the animal’s ability to feed, it is possible that sensitization can
be mediated by operant contingencies of food reinforcement. This was investigated by
Pinkston and Branch (2003) in which cocaine was given to pigeons in two different
groups. One group was exposed to a fixed-ratio 20 schedule of food reinforcement, in
which 3-second access to food was given after 20 consecutive key-pecking responses.
The second group did not receive exposure to any type of schedule. They found that the
first group exhibited tolerance, and the second group showed sensitization to cocaine.
These results are consistent with the idea that sensitization can be determined by the
animal’s ability to gain food; the first group had to suppress their drug-induced behaviors
to gain food, whereas the second group did not have that opportunity. However, there
have been few studies on how operant contingencies can affect reaction to drugs, and
further research should be done to better understand the factors that are more likely to
result in sensitization.
In the proposed study, rats will be used as the subjects, and it will use a withinsubjects design. There will be operant contingencies in both components to better
determine how behavior and rates of reinforcement interact to influence drug effects. It
is still unknown how behavior to different reinforcement rates in an animal’s
environment change drug effects, and the goal of this study is to investigate those effects.
This is a critical experiment to carry out, as it will be filling a gap in the literature.
Research Methods
Subjects
These experiments will use a “small-N” design. Each subject will experience all
components of the experiment, so its behavior in one component will be compared to its
behavior in the other component. These methods are standard for this discipline, and
minimize the number of animals needed for research while yielding sufficient data. Four
male Long-Evans rats (Charles River, Michigan, USA), will be approximately 90 days
old upon arrival to the behavior lab located in the Laboratory Animal Research Center
(LARC) at USU. All experiments and housing of the animals will take place in the
behavior lab and will be in compliance with the USDA Animal Welfare Act, PHS "Policy
for the Humane Care and Use of Animals,” U.S. Government Principles for the
Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training; and
the Animal Welfare Policies of the University. This study is approved by the
Institutional Animal Care and Use Committee (IACUC) under the advisor’s current
protocol (IACUC #1098). After free access to food and water for two weeks, food intake
will be restricted to reduce the rats to 85% of their free-feeding weight for an additional
two weeks. The rats will be maintained at this weight through post session feeding as
necessary. Rats will be housed individually with unrestricted access to water under a
12:12 reversed light-dark cycle. Sessions will be conducted daily at the same time.
Apparatus
Eight Coulbourn operant chambers (29 cm long, 24 cm wide and 29 cm high), in
sound-attenuating enclosures, will be used. The front panel has a houselight at the top
center, and two response levers centered 13 cm apart, above which are three 28 V DC
lamps. A solenoid-operated feeder, with a light, dispenses 45-mg food pellets. Control
of experimental events and data recording will be conducted with Med-Associates
interfacing and software using a personal computer located in an adjacent room.
Procedure
Rats will first be trained to take food pellets from the dispenser in the chamber by
delivering pellets after 20 seconds regardless of their behavior. When a pellet is
delivered, the house light will be extinguished and the feeder aperture will be lit for .4 s.
When a rat is reliably and quickly eating food pellets, lever press training will begin.
Rats will be trained to press the levers through an autoshaping procedure in which the
lights above the bar will be lit, and rats will put their front feet on the bar to investigate
the bar, which will result in reinforcement (food pellet delivery) after each bar press
response. Once a rat is reliably pressing the levers, the response requirement will
gradually increase to a variable ratio (VR) 60 schedule of reinforcement on the right
lever, which provides reinforcement after an average of 60 responses are made. The
response requirement on the left lever will gradually increase to a variable interval (VI)
60 second schedule, in which food is given after an average of 60 seconds. These
schedules are being used to examine how behavior in response to different rates of
reinforcement will interact with the drug effects. The VI and VR schedules and stimuli
signaling the schedules will be counterbalanced on the right and left levers for half of the
rats. Formal data collection will begin when responding is considered stable (showing no
systematic variation in daily response rates for at least 10 sessions, as determined by
visual assessment by the primary investigators).
During baseline, rats will be given intraperitoneal injections of saline 15 min prior
to the beginning of each session. In all phases, sessions will be preceded by a 15 min
chamber blackout immediately following injections. This is to allow time for drug
absorption when applicable. The rats will respond on a multiple VI 60 VR 60 schedule
for 30 sessions to determine response rates without the influence of d-amphetamine. The
duration of each session will be 30 min or 48 reinforcers, whichever occurs first. Rats
will then be given acute d-amphetamine injections 15 min prior to each session in varying
doses (0.3, 1.0, 3.0, 5.6, 10.0 mg/kg of d-amphetamine and saline, respectively) twice per
week to determine the dose-effect curve (see Fig. 1). D-amphetamine sulfate salt will be
ordered through the L.A.R.C.’s attending veterinarian. The same multiple VI 60 VR 60
schedule will be available as in baseline. Determinations of the effects of each dose will
be made twice, and a dose that causes at least a 25% change in response rate as compared
to control will be administered chronically each day under the same schedules of
reinforcement for 30 sessions.
After this chronic treatment phase, the rats will return to baseline conditions and
be tested with all doses again to determine the acute dose-effect curve again. If
predictions are correct, this dose-effect curve will shift to the left in the VI component,
showing sensitization, or an increase in the effect of the drug; and to the right in the VR
component, showing tolerance, or a decrease of the effects of amphetamine. For data
analysis, a two-way Analysis of Variance (ANOVA) will be used to find significant
differences between the components. The two factors in the ANOVA will be the drug
condition, acute or chronic, and the operant component, VI or VR. The dependent
variable will be the dose at which responding is decreased by at least 50% as compared to
control on the dose-effect curve. An alpha level of .05 will be used.
Hypotheses:
In the VR component, rats’ response rates are hypothesized to increase after
chronic drug administration as compared to their own response rates in the acute drug
phase. This shows tolerance to the drug. Conversely, rats’ response rates are
hypothesized to decrease in response to the VI component after chronic drug use. This
signifies sensitization to the drug.
Dose-effect curve
Responses per
minute
40
30
Con Sal
20
10
0.3
1
3
5.6
0
10
Amphetamine Dose (mg/kg)
Figure 1. Hypothetical data showing a decrease in response rates with increasing amphetamine doses.
Expected Timeline
March 6
March 20
April 3
April 17
May 15
June 19
July 18
August 22
Fall 2006
Order rats from Charles-River
Rats on free-feeding
Rats on food restriction to return to 85% of free-feeding weight
Bar press training begins
(Dependent upon when responding is stable) Initial injections (Baseline)
Chronic administration
Return to baseline, rats tested with all doses again
Experiment finished. Data analysis and create future research projects
Results presented to Student Advocates for Behavior Science (SABS)
meeting
September 15 Report results to Provost’s Office and receipts to Vice President for
Research Office
April 2007
Results presented at USU Student Showcase
May 25-29, 2007 Results to be presented as poster presentation at the Association for
Behavior Analysis 33rd Annual Convention in San Diego, California
Conclusion
This research is expected to not only contribute to the existing body of knowledge
in this discipline and help in understanding how the environment can affect an
organism’s reaction to drugs, but will also benefit me as a student. The experience I
would gain in the behavior lab would be a valuable asset in applying for graduate studies
in behavioral pharmacology. This is an excellent opportunity for me to learn injection
procedures on a rat, carry out experimental research, analyze data, and alter procedures as
necessary. I believe that this experience will make me a more desirable applicant to
graduate school and make the transition to graduate school easier as I would require less
training. The expenses described below are vital in carrying out this specific research.
Thank you for considering my project to be funded.
Budget
Item
Description
Costs
Rats
Four Male Long
Evans rats from
Charles River, 81-90
days old, 351-375 g
upon arrival
Four at $40.85 each =
$163.40 +
$22.73 (shipping) +
$10.70(crate charge)
= $196.83
D-Amphetamine
Sulfate Salt from
Sigma Aldrich
5 grams - $77.60 +
$60.00 (Admin fee) +
$19.43
(shipping/handling) =
$157.03
Amphetamine
Animal Care
Animal Care from
Laboratory Animal
Research Center at
Utah State University
4 rats, 6 months at
$.251/day/each =
$180.72
Food reinforcers
Bio-Serve dustless
precision pellets, 45
mg each
One box of 50,000
pellets = $77.00 +
15.00 (shipping) =
$92
Syringes
Syringes to inject
amphetamine from
Fisher
5 boxes at $20.18
each = $100.90 +
$10.00 (shipping) =
$110.90
Total Item Cost
$196.83
$157.03
$180.72
$92
$110.90
Total Cost
$737.48
Amount requested
from URCO funds
$368.74
Amount secured from
Psychology
department at USU
$368.74
References:
Piazza, P.V., Deminiere, J.M., Le Moal, M., & Simon, H. (1989). Factors that predict
invidiual vulnerability to amphetamine self-administration. Science, 245, 1511-1513.
Pinkston, J.W., and Branch, M.N. (2003). Sensitization to cocaine in pigeons: Interaction
with an operant contingency. Experimental and Clinical Psychopharmacology,
11, 102-109.
Robinson, T.E., & Becker, J.B. (1986). Enduring changes in brain and behavior
produced by chronic amphetamine administration: a review and evaluation of
animal models of amphetamine psychosis. Brain Research Reviews, 11, 157-198.
Robinson, T.E., & Berridge, K.C. (2000). The psychology and neurobiology of
addiction: an incentive-sensitization view. Addiction, 95, 91-117.
Sax , K.W., & Strakowski, S.M. (2001). Behavioral Sensitization in Humans. Journal of
Addictive Diseases, 20, 55-65.
Wolgin, D.L., & Hertz, J.M. (1995). Effects of acute and chronic cocaine on milk intake,
body weight, and activity in bottle- and cannula-fed rats. Behavioral
Pharmacology, 6, 746-753.