Serotonin
reuptake inhibitors attenuate morphine
withdrawal syndrome in neonatal rats passively
exposed to morphine
Wu CC, Chen JY, Tao PL, Chen YA, Yeh
GC.
Department of
Anesthesiology, Taipei Medical University
Hospital,
Taipei Medical University,
Taiwan,
Previous investigations had shown that
inhibitor of serotonin reuptake transporter
(SERT) could attenuate morphine withdrawal
syndrome in adult animals. In the present study,
we determined whether postnatal injection of
serotonin reuptake inhibitors, fluoxetine,
clomipramine, or citalopram, is able to
attenuate the expression of the
naloxone-precipitated morphine withdrawal
syndrome in 5-day-old neonatal Sprauge-Dawley
rats born to dams rat that received morphine
injection since a week before mating till 5 days
after delivery. Withdrawal syndrome of morphine,
manifested as frequent abdominal stretching and
yawning, was generated by injection of naloxone
on postnatal day 5. Pre-injection with
fluoxetine, clomipramine, or citalopram,
significantly attenuated the
naloxone-precipitated syndrome in a
dose-dependent manner without apparent side
effect. The rank order of inhibitory potency is
citalopram=clomipramine>fluoxetine. This
result suggests that inhibitor of SERT may be of
potential in treating neonatal morphine
withdrawal syndrome.
1. Introduction
Infants born to mothers addicted to morphine
or heroin during pregnancy have great chance to
produce acute withdrawal syndrome after birth.
The withdrawal symptoms include jitteriness,
irritability, yawning, shrill crying,
hyperactivity-hypertonicity, poor feeding,
vomiting, diarrhea, sneezing, tachypnea, and
seizure (Volpe, 1995). Most of the newborns
presenting apparent features of withdrawal
syndrome require intensive care. In addition to
supportive therapy to stabilize the vital sign,
drug therapy with
paregoric, phenobarbital, chlorpromazine, or
diazepam is frequently used to minimize the
symptoms of the central nervous and
gastrointestinal systems. However, the side
effects of these applied medications in central
nervous system (CNS) require careful monitoring.
Thus, to improve the treatment outcome for
neonatal morphine withdrawal syndrome,
development of new effective and safety
medication is still needed.
Serotonin (5-hydroxytyramine, 5-HT), an
important biogenic amine, exerts its
neurotransmission via activating different 5-HT
subtype receptors in the CNS. Serotonin is
synthesized from tiyptophan through the action
of tryptophan hydroxylase, and is released by
pre-synaptic stimulation. Its action is
terminated by uptaking through presynaptic
serotonin reuptake transporter (SERT), which is
a transmembrane protein uptaking serotonin by a
sodium dependent mechanism. Serotonin once been
uptaked will be reused or degraded into
hyrodroxyindoleactic acid. Serotonm-mediated
neurotransmission has long been implicated in
the regulation of a wide variety of cortical
functions including modulation of appetite,
memory, mood, emotionality, thermoregulation,
and sexual behavior (Jacobs and Azmitia, 1992).
The forebrain and spinal serotonin pathways are
known to be involved in pain inhibition and
morphine analgesia (Brase, 1979).
It has been shown that deficiency in
serotonin-mediated neurotransmission contributes
to the expression of major depression and
chronic pain (Archer et al., 1986), and
therefore, selective SERT inhibitor, such as
citalopram, clomipramine, or fluoxetine, are
widely used in the treatment of depression,
anxiety disorder, eating disorder,
obsessive-compulsive disorder, and substance
abuse (Murphy, 1990; Singh et al., 2001).
Previous reports had shown that acute
administration of morphine could enhance brain
serotonin synthesis, release, and turn over rate
in adult animals (Boadle-Biber et al., 1987; Tao
and Auerbach, 1994). On the contrary, withdrawal
from long-term exposure to morphine profoundly
depressed serotonin level in many regions in CNS
(Tao et al., 1998). Thus, withdrawal-induced
reduction in brain serotonin level might be
responsible for somatic as well as subjective
symptoms of morphine withdraw.
This idea is supported by that
administrating SERT inhibitor significantly
attenuated the naloxone-precipitated
hyperactivity of noradrenergic locus coeruleus
neurons, an important brain substrate of opiate
withdrawal, and the behavior changes (Akaoka and
Aston-Jones, 1993; El-Kadi and Sharif, 1995;
Harris and Aston-Jones, 2001; Lu et al., 2001;
RafieianKopaei et al., 1995). However, all these
studies were performed in adult animals, yet no
similar investigation is applied on the neonatal
withdrawal syndrome. Therefore, in the present
study, we determined whether SERT inhibitors,
fluoxetine, clomipramine, and citalopram, could
attenuate naloxone-precipitated withdrawal
syndrome in 5-day-old rats, which had passively
exposed to morphine during the whole course of
pregnancy.
4. Discussion
The present study demonstrated that
inhibitor of SERT could effectively abolished
the morphine withdrawal syndrome elicited by
naloxone on the neonatal rats chronically and
passively exposed to morphine. We previously had
used the same animal model to demonstrate the
effectiveness of N-methyl-D-aspartate (NMDA)
receptor antagonist, MK801, and
dextromethorphan, in attenuating neonatal
morphine withdrawal syndrome (Yeh et al., 2002).
The major morphine withdrawal syndromes measured
in this study are abdominal stretching and
yawning. Although both symptoms have been
documented in adult rats (Chen et al., 2003;
Ramabadran, 1983), the expression of yawning is
not reported in the neonatal rats, including our
previous report (Tao et al., 2001; Yeh et al.,
2002). The reason for the lack of this
presentation in our previous study is not clear.
Nevertheless, yawning is one of apparent symptom
of morphine or heroin withdrawal found in human
including newborn baby (Bickel et al., 1988;
O'Brien, 1996; Ostrea et al., 1975).
There are two reasons for choosing the three
SERT inhibitors in this study. The first reason
is that they are all highly potent and selective
in inhibiting SERT. According to the ligand
binding analysis, their potencies in inhibiting
SERT are at least 100-fold higher than their
potencies in inhibiting norepinephrine reuptake
transporter (NET) or dopamine reuptake
transporter (DAT). Accordingly, the affmities
for these three compounds on SERT are proximally
0.75 nM for citalopram, 0.5 mM for clomipramine,
and 2 nM for fluoxetine, and the affinities on
NET are roughly 3000 nM for citalopram, 100-200
nM for clomipramine, and 500 nM for fluoxetine
(Millan et al., 2001; Owens et al., 1997). On
the other hand, their affinities on DAT are over
10 jiM. Thus, in general, their action on DAT
could be negligible in clinics. The second
reason is that these drugs are all currently
used in clinics, mainly used for
anti-depressant, and their safety and side
effects have been well documented.
This result revealed that the rank order of
potency for the three SERT inhibitors in
attenuating abdominal stretching or yawning is
citalopram=clomipramine>fluoxetine, and the
relative ratio of EC50 for
citalopram:clomipramine:fluoxetine is roughly
1:1:4(5-10 mg/kg for citalopram, 5-10 mg! kg for
clomipramine, 20-40 mg/kg for fluoxetine). Such
rank order and relative ratio of inhibitory
potency of these drugs are rather close to those
measured by ligand binding analysis on SERT, in
which the rank order of potency is citalopram
clomipramine>fluoxetine, and the affinity
ratio for citalopram:clomipramine:fluoxetine is
roughly 1:0.7:2.8 (0.75 nM for citalopram, 0.5
nM for clomipramine, and 2 nM for fluoxetine)
(Millan et al., 2001; Owens et al., 1997), and,
are apparently unlike to that on NET since the
rank
order of potency of these drugs binding to
NET is clomipramine>fluoxetine>citalopram,
and the relative ratio of affinity for
clomipramine:fluoxetine:citalopram is roughly
1:5:30 (100-200 nM for clomipramine, 500 nM for
fluoxetine, and 3000 nM for citalopram). This
comparison favors that the inhibitory effect of
these three drugs in naloxone-precipitated
withdrawal syndrome is through their inhibition
on SERT but not on NET.
Previous reports had proved that serotonin
system is one of the earliest neurotransmission
systems to be formed in the CNS (Lauder et al.,
1982). At birth, this system is well formed both
in structure and function and serotonin system
contributes to the development of brain,
especially for the cortex, in the early life
(Pranzatelli, 1994; Pranzatelli and Martens,
1992). More importantly, the existence of SERT
in the neonatal brain has been clearly
demonstrated (McGrath et al., 1997), and
application of SERT inhibitor, such as
clomipramine, did alter the function of the
serotoninmediated neurotransmission (Hansen and
Mikkelsen, 1998; Foguet et al., 1993). These
reports further support the notion that these
SERT inhibitors could act at SERT in neonatal
rat brain.
It appears that the potency of the three
examined SERT inhibitors in attenuating
naloxone-precipitated yawning behavior is
significantly higher than that in attenuating
abdominal stretching behavior since 20 mg/kg of
fluoxetine could have apparent effect in
suppressing yawning, but only have mild effect
on abdominal stretching, and, 2 mg/kg of
clomipramine or citalopram could completely
abolish the yawning behaviors, but 10 mg/kg of
these two drugs is required to abolish abdominal
stretching. The mechanism responsible for the
higher sensitivity of these three drugs in
abolishing yawing behavior is not clear at
present.
But, it is likely that such effectiveness of
these three drugs in yawning behavior is still
dependent on their blocking effect on the SERT
rather than on the NET or DRT since the rank
order of potency of these three drugs in
abolishing yawning is similar to that in
abdominal stretching, and, to block NET or DAT
requires much higher concentration of these
drugs than to block SERT. In addition, previous
report had found that activation of serotonin
receptor subtype, 5-HTIA receptor or 5-HT2
receptor, serves as an inhibitory mechanism for
yawning response (Argiolas and Meus, 1998).
However, the role of dopamine system or
norepinephrine system is not identified
yet.
The result of this study is consistent with
the study of SERT inhibitor on morphine
withdrawal syndrome in adult rats (El-Kadi and
Sharif, 1995; Lu et al., 2001; RafieianKopaei et
al., 1995), indicating that serotonin-mediated
transmission plays a significant role in the
expression of acute morphine withdrawal syndrome
in both adult and neonatal rats. Thus, either a
deficiency in the serotonin neurotransmission,
which contributes to the expression of
withdrawal syndrome, or an increase in serotonin
function, which counteracts the mechanisms
responsible for the expression of morphine
withdrawal syndrome, could explain the
therapeutic effect of the SERT inhibitors.
However, taking into account the finding
that NMDA receptor antagonists, like MK-801 or
dextromethorphan, are as effective as SERT
inhibitor in attenuating morphine withdrawal
syndrome in both adult and neonatal rats
(Tanganelli et al., 1991; Tiseo et al., 1994;
Tokuyama et al., 1996; Trujillo and Akil, 1991;
Yeh et al., 2002), it seems that the neural
pathway responsible for withdrawal syndrome is
far more complicated. Both glutamate and
serotonin neurotransmitter systems might
functionally converge in the neural network in
generation of the withdrawal syndrome. It might
be that the functions of these two systems are
altered in an opposite way during abstinence
state, in which the function of NMDA receptor
system is increased and the function of
serotonin system is decrease. Alternatively, the
functions of both systems might not be changed
at all in the abstinence state. Rather,
activation of NMDA receptor is required for the
expression of withdrawal syndrome, and enhanced
serotonin system in some way could counteract
directly or indirectly with it.
In summary, the present study supports that
SERT inhibitors are of potential in treating the
acute morphine withdrawal syndrome in newborn
baby. However, any therapeutics use of SERT
inhibitors in newborn stage still requires more
accurate evaluation of all possible adverse
effect and justification of the risk-benefit
ratio.
