Transcript Morphine Reward in Dopamine

Morphine Reward in
Dopamine-deficient Mice
Hnasko TS, Sotak BN, Palmiter RD (2005). Nature 438:854-857.
Presented by Mattia M. Migliore
March 30, 2006
Introduction:
 First recorded reference to opium use
occurred around 300 B.C.
 Morphine is an analgesic first isolated
from opium in 1806.
 Morphine’s name came from
Morpheus, the Greek god of dreams.
 Opioids exert their effects by binding
to opioid receptors (µ, δ, and κ) which
then couple to G-proteins, inhibit
adenylate cyclase, activate K+
currents, and decrease Ca+2 currents.
 Much like other opioid analgesics,
morphine has the potential to cause
addiction.
http://www.heroin.org/papaver.jpg
http://www.sciencebase.com/images/
structure_of_morphine.jpg
Introduction (cont):
 Addiction can be defined as
uncontrolled, compulsive use of a
substance despite adverse consequences
resulting from its use.
 Addiction can result from repeated
exposure to a substance, which then
results in neurochemical adaptations in
the reward system of the brain.
 People can become addicted to drugs,
alcohol, tobacco, gambling, sex, and
even food.
 Addiction is extremely difficult to study
because no animal model of addiction
even comes close to the complexity of
the human condition.
www.hrmvideo.com/ resources/docs/2662.gif
Introduction (cont):
www.WHO.org
 Drug addiction (also
termed substance
dependence) affects
millions of people world
wide.
 The degree of drug
abuse ranges from just
occasional use to
compulsive use
ultimately resulting in
fatal consequences.
(Goldstein and Volkow et., 2002).
Neurobiology of Addiction:
 Drugs of abuse illicit a
feeling of euphoria or a
“high” by activating the
brain’s reward circuitry.
 Dopamine has been believed
to be responsible for feelings
of reward for the last 30 yrs,
and has been called the “feel
good neurotransmitter.”
 DA has long been implicated
in the development of
addiction.
 Most drugs of abuse have
been shown (via
microdialysis studies) to
increase extracellular DA
levels and/or DA cell firing in
the nucleus accumbens.
Goodman and Gilman’s 11th edition.
Synthesis of Dopamine:
http://web.indstate.edu/thcme/mwking/catecholaminesynthesis.jpg
Evidence supporting DA’s role in
reward:
 In 1954, Olds and Milner showed that direct electrical
stimulation of the brain had powerful rewarding
effects. Later, Olds et al. used intracranial selfadministration of various substances to try to identify
the neurotransmitters involved in reward.
 Studies showed that DA receptor antagonists can
inhibit the rewarding effects of food, and of
intracranial self-stimulation (Zhou and Palmiter,
1995).
 Dopamine agonists and drugs that inhibit the DAT
have been shown to cause animals to self administer
these agents, and to develop a conditioned place
preference (CPP) for these drugs.
Evidence supporting DA’s role in
reward (cont.):
 Bilateral 6-OHDA lesions result in a severe decrease
of activity, and the animals will refuse to eat or drink
(an obliteration of the natural reward cues).
 Schultz et al. showed that the anticipation of a reward
(juice) in monkeys caused an increase in firing, and a
change in the pattern of DA neuron firing.
 Maldonado et al. showed that D2 receptor knock out
mice do not show a CPP in response to morphine.
 Volkow et al. used neuroimaging studies in humans to
show that cocaine and methylphenidate increase brain
dopamine levels, and this increase was associated
with the feeling of a “high.”
Molecular Mechanism of
Drug Addiction:
(Nestler and Aghajanian, 1997)
Brain regions involved in drug
addiction:
(Golstein and Volkow, 2002).
(Volkow et al, 2003)
(Golstein and Volkow, 2002).
What makes some people become
substance dependent?
Hypothesis:
Dopamine is not an essential
component of opiate responses,
and that dopamine is not required
for opioid mediated reward.
Methods:
 Dopamine deficient mice: a
complete deletion of the tyrosine
hydroxylase (TH) encoding gene
results in a deficiency in both DA
and NE. In order to create only
DA deficient mice, Hnasko et. Al
used the TH encoding sequence
to target the dopamine ßhydroxylase (DBH) promoter in
embryonic stem cells. Then
DBH-TH +/- mice were crossed
with TH +/- mice to yield TH +/DBH-TH +/- which were then
crossed with TH +/- mice to yield
dopamine deficient mice capable
to still producing NE.
Zhou Q-YP, Richard D. (1995) Dopamine-deficient mice are
severely hypoactive, adipsic, and aphagic. Cell 83:1197-1209.
Synthesis of Dopamine:
http://web.indstate.edu/thcme/mwking/catecholaminesynthesis.jpg
Methods (cont.):



Mice required daily L-Dopa administration to induce them
to eat. Morphine was administered 18-24 hrs after L-Dopa .
Virally Rescued Dopamine Deficient Mice (vrDD): In
order to perform the behavioral tests, they used a viral gene
transfer to restore DA in the striatum (because DA deficient
mice are slow and hypoactive).
Behavioural tests:
1. Locomotor tests were done using photo-beam activity
cages. Morphine was administered IP at 0,0.25,2.5,12.5, and
25 mg/kg.
2. Tail flick tests were perfomed by using warm water
baths. Briefly, the animal’s tail was submerged 0.5-1 cm in
the water bath, and the latency to withdraw the tail was
recorded (with a cut off time of 15s). The animals were
tested three times/treatment and average used. Morphine
was administered 30 min. Prior to test IP at 0,3,6,12, and 24
mg/kg.
Methods (cont.):
(Cami and Farre, 2003).
3. Conditioned Place Preference (CPP)
was performed using clear plastic boxes
with 3 chambers (1 neutral grey
compartment in the middle, and 2
compartment with different colored
walls, different textured flooring, and
different scents). First, the mice were
administered caffeine (18-24 hrs after LDopa treatment) and placed in the center
and allowed to explore for 25 min. On
days 3-5 (conditioning phase), the
animals received saline SQ in the
morning and restricted to one
compartment for 25 min, and then
received morphine SQ and restricted to
the opposite compartment for 25 min in
the afternoons. Preference was tested on
the sixth day. In the L-Dopa rescue, LDopa was administered similarly to the
caffeine.
Results
Conclusions:
 Dopamine appears to be essential in the
development of locomotor response to
morphine.
 Dopamine appears to play an important role in
the level of analgesia experienced after
morphine administration.
 Dopamine may be required for reward seeking,
but does not appear to be indispensable for
morphine’s rewarding effects.