Transcript Sample Poster - University of Toronto Mississauga

ARTIFICIALLY REARED RATS SHOW
INCREASED SENSITIVITY TO
AMPHETAMINE
REVERSAL OF EFFECTS WITH TACTILE
STIMULATION
V. Lovic1, A.S. Fleming1, M.L. Smith1 & P.J. Fletcher1,2 *.
1Department of Psychology,
University of Toronto at Mississauga
2Centre for Addiction and Mental Health, Toronto, Canada
Introduction
Mammalian brain and behaviour plasticity is particularly evident during early
stages of development while the young organism is in the care of its mother.
Previously we have found that female rats that are raised without mothers, through
artificial rearing (AR), grow up to show deficits in the attentional-set shifting task (Lovic
& Fleming, 2004). These deficits were reversed in AR animals that were provided with
maternal-licking like stimulation during early postnatal period.
Rat attentional set-shifting task is an analogue of the Wisconsin Card Sorting
Task, a test of behavioural flexibility that is sensitive to changes in the DA system and
damage of the prefrontal cortex. It has been recently demonstrated in rats that
alteration of the DA system, through amphetamine sensitization, produces deficits in
the attentional set shifting task (Rizos, Lovic, Tann, Kapur & Fletcher, 2003).
We were interested in investigating whether artificial rearing produces
changes in the DA system. We decided to assess these changes indirectly by
observing changes in DA dependent behaviour (locomotor activity) in response to
treatment with two different DA agonists, amphetamine (dopamine releaser) and
methylphenidate (Ritalin – dopamine reuptake inhibitor). Furthermore, we were
interested in seeing whether maternal-licking like stimulation provided to some of the
AR pups would reverse alterations in the DA system associated with AR.
Methods
Subjects
Fifty-four, male Spraque-Dawley rats were used. Animals were reared at the
University of Toronto at Mississauga. Once they reached adulthood, they were
transported to Centre for Addiction and Mental Health where they were tested.
Artificial Rearing
On the day of birth (postnatal day – PND 0) litters were culled to 5 females and 8
males. On PND 3, three males from each litter received a cheek cannulea implant.
Two of the pups from each litter, that were implanted with cannulea were artificially
reared (AR) and the third one was returned to its mother and was mother reared
(MR-SHAM). The fourth male sibling was not manipulated and was designated as
control (MR-CON). Two AR pups were placed in two different conditions: AR-MIN
(AR with minimal amount of tactile stimulation – 2 x 30-sec/day) and AR-MAX (AR
with ‘maximal’ amount of stimulation 8 x 2-min/day).
Methods
Methods
Drugs
Amphetamine (AMPH) and Methylphenidate (METH) were dissolved in
0.9% saline and administered intraperitoneally, in a volume of 1 ml/kg. Each rat’s
activity was assessed after an injection of AMPH (0, 0.25, 0.5 and 1.0 mg/kg) or
METH (0, 2, 5 and 10 mg/kg). Order of injections was counterbalanced between
groups.
Locomotor Activity
Locomotor activity was assessed in four large, clear Plexiglas activity
chambers (Med Associates Inc.). The apparatus was a square box (WL 43cm x H
30cm) equipped with an array of 16x16 photoreceptors that were used to detect
locomotor activity. The boxes were connected to a computer capable of recording
distance traveled based on interruptions of photoreceptors. Distance traveled was
used as index of ambulatory activity and a dependent measure.
All rats were first habituated to the apparatus by placing them in the
activity chambers for 1-hour on 3 occasions, approximately a day apart. On the test
days, rats were placed in the activity chamber for a 30-min habituation period
followed immediately with an injection of one of four doses of AMPH or METH.
Following injection, locomotor activity was assessed during a 90-min period.
Results
AR Rats Showed Increased Activity in a Novel
Environment
Distance Traveled (mean + sem) in
cm.
Locomotor Acivity Across Three Habituation Sessions
3000
b
AR-MIN
a
AR-MAX
2500
MR
2000
1500
1000
500
0
Day 1
a - MR vs. AR-MIN
b – MR vs. AR-MAX
Day 2
Day 3
Results
Distance Traveled (mean + sem) in
cm.
AR-MIN Rats Showed a Dose Dependent Increase in
Amphetamine Sensitivity
Amphetamine Induced Locomotor Activity
14000
a
12000
10000
AR-MIN
AR-MAX
a
8000
MR
a
6000
4000
2000
0
0
a - MR vs. AR-MIN
0.25mg/kg
0.5mg/kg
1.0mg/kg
Results
Distance Traveled (mean + sem) in
cm.
There Were No Significant Group Differences in
Methylphenidate Sensitivity
Methylphenidate Induced Locomotor Activity
16000
AR-MIN
14000
AR-MAX
12000
MR
10000
8000
6000
4000
2000
0
0mg/kg
2mg/kg
5mg/kg
10mg/kg
Conclusions
AR rats showed hyperactivity in a novel environment. This effect dissipated
by the last habituation session, indicating that AR are not hyperactive in general, but
are rather hyperactive in response to novelty. Locomotor hyperactivity in response to
novelty is associated with alterations in the DA and excitatory amino acid
neurotransmission (Hooks & Kalivas, 1994).
AR rats also showed increased locomotor activity after injections of
amphetamine. This increased sensitivity was reversed by providing AR pups with
maternal-licking like stimulation as exemplified by the AR-MAX group. There were no
significant group differences in groups’ METH induced locomotion. These differences
between AMPH and METH effects could be explained by the mechanisms of action of
these two drugs (see figure). This would suggest that AR does not produce rats that
have chronic overactivity of the DA system, but rather rats that have the potential of
having a significantly more active DA system.
Conclusions
The mechanism of these
effects is unknow, however, previous
research has suggested that tactile
stimulation early in life changes the
levels of perinatal hormones, such as
thyroid hormone, that in turn produce
changes in the brain. We plan to
investigate the mechanism of action
underlying the changes in the
dopamine system associated with
early life treatments
Furthermore, we are
planning to investigate dopamineassociated behaviors of artificially
and mother reared animals,
specifically their reward system
(addiction) , as well as behavioral
flexibility and impulsiveness.
References
Hooks, M.S. & Kalivas, P.W. (1994). Involvement of dopamine and excitatory amino
acid transmission in novelty-induced motor activity. Journal of Experimental
Therapeutics, 269(3): 976-88.
Lovic, V. & Fleming, A.S. (in press). Artificially reared female rats show reduced
prepulse inhibition and deficits in the attentional set shifting task – reversal
of effects with maternal-like licking stimulation. Behavioural Brain Research.
Rizos, Z., Lovic, V., Tenn, C., Kapur, S. & Fletcher, P.J. (2003). Rats sensitized to
amphetamine, but not PCP, show deficits in an attentional set-shifting task.
Society for Neurscience Abstracts, New Oreleans, USA.
Abstract
Mammalian brain and behaviour plasticity is particularly evident during early stages
of development while the young organism is in the care of its mother. Previously we
have found that total maternal deprivation of rat pups, achieved through artificial
rearing (AR), leads to a number of behavioural and neurophysiological changes,
suggesting a change in the dopamine system. The purpose of this study was to
further investigate changes in the dopamine system, associated with AR, using two
indirect dopamine agonists, d-amphetamine (dopamine releaser) and
methylphenidate (Ritalin – dopamine reuptake inhibitor). Rats were raised with or
without mothers (AR, ‘pup-in-a-cup’ paradigm) and some of those raised without
mothers were provided with tactile, maternal-licking like stimulation. Rats’ locomotion
was measured after an injection of d-amphetamine (0, 0.25, 0.5 and 1.0mg/kg) or
methylphenidate (0, 2, 5, 10 mg/kg). While the AR produced increased sensitivity to
d-amphetamine in a dose-response fashion, as exemplified by increased locomotion,
there were no group differences in sensitivity to methylphenidate. Furthermore, AR
animals that were provided with maternal-licking like stimulation were not different
from the control, mother-reared animals. These data show that motherless rearing
can produce increased sensitivity to amphetamine and that this increase in sensitivity
can be reversed by providing motherless pups with tactile stimulation.