Treadmill exercise enhances
passive avoidance learning in
rats: The role of down-regulated
serotonin system in the limbic
system
Chen et al 2007
Introduction

Treadmill running and running wheel exercise

Voluntary wheel running could not rule out
possible confounding effects of non-exercise
training factors


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Genetic differences between good runners and poor
runners
The effects of environment enrichment
The temporal relationship between the end of running
wheel exercise and the effect measurement is poordefined
(Huang et al 2006)

Treadmill running: a stressor to rodents?



Moderate exercise training with careful animal
handling and previous familiarization can
minimize the possible stress: acutely
upregulate rat hippocampal BDNF gene
expression and improve learning.
Strenuous exercise without familiarization
reduces the BDNF mRNA expression, similar to
that exposed to the immobilization stress.
(Huang et al 2006)
The treadmill-running: a mandatory exercise
paradigm with defined exercise intensity and
duration.

improves passive avoidance (PA) retention and
spatial learning.


Two avoidance tests
 Passive avoidance: the animal has to refrain from
executing a previous response, such as touching
food or water, stepping down or stepping through.
(Myhrer, 2003)
 Active avoidance: the animal has to emit a specific
response in order to escape a noxious stimulus.
(Cimadevilla et al., 2000)
Passive avoidance: the resulting increase in response
latency reflects the strength of the memory trace.
(Cimadevilla et al., 2000)



Serotonin (5-HT) influences a wide
range of physiological systems and
behavioral functions.
5-HT modulates cognitive processes:
fear learning and memory
5-HT1A receptors: 5-HT may impair
learning and memory performance via
activating 5-HT1A receptors.
Mainly 5-HT projections are showed in
brain areas important for memory
(Meneses and Perez-Garcia, 2007)



Regular exercise improves learning and
memory capabilities.
Exercise alters the 5-HT levels in different
brain regions.
Are the alterations in 5-HT metabolism or its
receptors in various brain regions involved in
chronic exercise-enhanced learning and
memory?
Materials and methods


Animals: male Sprague-Dawley rats
Treadmill exercise protocol


Familiarization: 9m/min for 10 min each day for 5
days
Control and exercise groups



Exercise: a leveled treadmill at an intensity of 70% of
maximal oxygen consumption for 60 min/d, 5d/week for
4 weeks, 12m/min – 15m/min
Control: placed in the treadmill without running for 10
min/d
No electric shock was applied
Passive avoidance apparatus



A sliding door
between two
compartments
A stainless-steel
floor
An illuminating
bulb

Behavioral tests

One-trail step-through passive avoidance test




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Exploration for 60s each without electric shock
The rat was placed in the lighted compartment facing
away from the closed door  it turned around  the
door was opened  once the rat had entered the dark
compartment, the door was closed and an electrical
current was delivered
Latency to cross into the dark compartment was
recorded. (pre-shock latency)
Retention of PA performance was tested 24h and 72h
afterwards. (testing latency)
The role of 5-HT1A receptor: sc injection of 8-OH-DPAT
(0.1mg/kg) or vehicle 15 min prior to PA training, PA
retention latencies were tested 24 h after training.

Pain sensitivity test




Rule out the effect of drug-induced alteration in
sensory input
Test was performed 15 min after sc. injection
Rats were individually placed on a hot plate
The latency to the first hind paw withdraw was
recorded as in index of nociceptive threshold to
thermal stimulation

Biochemical studies


Brain tissue dissection: prefrontal cortex,
hippocampus, amygdala
Determination of citrate synthase activity in soleus
muscles




Confirm the exercise training effect
Measurement of 5-HT and 5-HIAA by HPLC
Measurement pf 5-HT1A receptor and 5-HT2A
receptor and 5-HT transporter SERT by Western
blotting
Statistical analysis
Results

The exercise training effect

the citrate synthase enzyme activity was
significantly increased: 1.96±0.01 vs.
1.77±0.02 µmol/min/g wet weight for
exercise and control groups.
Passive avoidance performance


Pre-shock latency
was not different
between control and
exercise groups.
24-h testing retention
latency was
significantly increased
in the exercise group
compared to the
control group.





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No significant differences in
pre-shock training latencies
Exercise significantly
enhanced 24-h testing
retention latencies
The exercise effect was
abolished by the
administration of 8-OH-DPAT
8-OH-DPAT significantly
suppressed the PA
performance in both exercise
and control groups.
Rule out the drug influence
to the sensory input: 8-OHDPAT did not significantly
affect their pain sensitivity
The paw withdraw latency:


Saline group: 5.5±0.3s
8-OH-DPAT group: 6.1±0.7s
Levels of 5-HT and 5-HIAA in
different brain regions


The 5-HT level in the
hippocampus was
significantly reduced in
the exercise group at
various points after
exercise.
The 5-HIAA levels in
these three brain areas
were not significantly
affected by exercising
training.
Protein expression in different
brain regions

5-HT receptors

The protein
expression level of 5HT1A receptor in the
amygdala was
significantly reduced
in the exercise group
at all studied time
points

Treadmill exercise
training
significantly downregulated 5-HT2A
receptor in rat
amygdala, but not
in hippocampus or
frontal cortex.

5-HT transporter
SERT

Exercise did not
significantly alter
the protein
expression level of
SERT in these
three regions.
Discussion




Exercise Training can enhance aversive learning and
memory, possibly due to blunted serotonergic
inhibition in the brain limbic system.
The 5-HT system influenced by the exercise may
modulate multiple neurotransmitter systems:
cholinergic and glutamatergic etc.
Other 5-HT receptors may also modulate the
cognitive function: 5-HT1B, 5-HT2.
The exercise-decreased hippocampal 5-HT levels
were unlikely to be caused by changes in 5-HT
uptake or degradation.

The impact of stressful experiences?


Unlikely
Due to the separate groups of rats for PA
performance test or monitoring the 5-HTrelated parameters, the treadmill
familiarization, moderate exercise intensity
and the absence of electric shocks.
References
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1 Chen et al. (2007) Treadmill exercise enhances passive avoidance
learning in rats: The role of down-regulated serotonin system in the
limbic system. Neurobiology of Learning and Memory.
Doi:10.1016/j.nlm.2007.08.004
2 Huang et al. (2006) Compulsive exercise acutely upregulates rat
hippocampal brain-derived neurotrophic factor. Journal of Neural
Transmission. 113,803-811
3 Myhrer, T. (2003) Neurotransmitter systems involved in learning and
memory in the rat: a meta-analysis based on studies of four behavioral
tasks. Brain research reviews. 41, 268-287
4 Cimadevilla et al. (2000) Passive and active place avoidance as a
tool of spatial memory research in rats. Journal of neuroscience
methods. 192, 155-164
5 Meneses, A. and Perez-Garcia, G. (2007) 5-HT1A receptors and
memory. Neuroscience and biobehavioral reviews . 31, 705-727