![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Vol. 305, Issue 1, 323-330, April 2003
-Opioid Receptor Antagonists in
the Forced Swim Test in Rats
Behavioral Genetics Laboratory, Department of Psychiatry, Harvard Medical School, McLean Hospital, Belmont, Massachusetts (S.D.M., A.M.P., M.S.T., H.C.T., W.A.C.); and Department of Medicinal Chemistry, University of Minnesota, Minneapolis, Minnesota (Y.Z., W.C.S., R.M.J., P.S.P.)
 |
Abstract |
|---|
 Top
 Abstract
 Introduction
Materials and Methods
Results
Discussion
References
|
|---|
We showed previously that cAMP response element-binding protein (CREB)
within the nucleus accumbens (NAc) of rats regulates immobility in the
forced swim test (FST), an assay used to study depression. Because CREB
regulates expression of dynorphin (which acts at 
-opioid receptors)
in NAc neurons, these findings raised the possibility that
-receptors mediate immobility behaviors in the FST. Here, we report
that i.c.v. administration of the -antagonist
nor-binaltorphimine dose dependently decreased immobility in the
FST, suggesting that it has antidepressant-like effects. Implicating a
specific effect at -receptors, similar antidepressant-like effects
were seen after treatment with either of two novel, structurally dissimilar -antagonists: 5'-guanidinonaltrindole, which was
effective after i.c.v. but not systemic treatment, and
5'-acetamidinoethylnaltrindole (ANTI), which was potent and effective
after systemic treatment. The behavioral effects of the -antagonists
resembled those of tricyclic antidepressants (desipramine) and
selective serotonin reuptake inhibitors (fluoxetine and citalopram).
Conversely, systemic administration of the -agonist
[5
,7,8
]-N-methyl-N-[7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec8-yl]-benzenacetamide (U-69593) dose dependently increased immobility in the FST, consistent with prodepressant-like effects. The effects of the -ligands in the
FST were not correlated with nonspecific effects on locomotor activity.
Furthermore, the most potent and effective -antagonist (ANTI) did
not affect the rewarding impact of lateral hypothalamic brain
stimulation at a dose with strong antidepressant-like effects. These
findings are consistent with the hypothesis that CREB-mediated induction of dynorphin in the NAc "triggers" immobility behavior in
the FST. Furthermore, they raise the possibility that -antagonists may have efficacy as antidepressants, but lack stimulant or
reward-related effects.
| |
Introduction |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
The
neurobiology of depression is not understood. Because most
antidepressants with clinical efficacy act upon monoamines [primarily
norepinephrine (NE) and serotonin (5HT)], much research on depression
has focused upon interactions between these neurotransmitters and their
reuptake transporters and receptor proteins. However, recent research
has become progressively focused upon the intracellular mechanisms of
depression and antidepressant treatments (Manji et al., 2001
; Duman,
2002; Nestler et al., 2002), with the goal of developing novel
therapeutics that act faster, are more efficacious, and have fewer side
effects. This approach has led to the study of brain circuits typically
associated with reward-related processes, including the mesolimbic
dopamine (DA) system (Pliakas et al., 2001; Newton et al., 2002).
The mesolimbic DA system projects from the ventral tegmental area of
the midbrain to the nucleus accumbens (NAc) of the basal forebrain, and
is modulated directly and indirectly by noradrenergic and serotonergic
inputs (Pasquier et al., 1977). This circuitry contributes importantly
to the hedonic (rewarding) effects of food, sexual behavior, and
addictive drugs (Carlezon and Wise, 1996b; Kreek and Koob, 1998; Wise,
1998). It has been proposed that disruption of DA function within the
NAc causes anhedonia (reduced ability to experience reward) (Wise,
1982), a hallmark symptom of depression. Consistent with this notion,
withdrawal from chronic amphetamine in rats causes decreases in
extracellular concentrations of DA within the NAc that are accompanied
by behavioral depression (Paulson et al., 1991). Similarly, cocaine
withdrawal decreases glucose metabolism (Hammer et al., 1993) and
reduces sodium currents within this region (Zhang et al., 1998).
Chronic stress, a putative precipitator of depression, dramatically
alters DA transmission within the NAc (Di Chiara et al., 1999).
Together, these studies suggest that the NAc has an important role in
the neurobiology of depression.
Recently, experience-dependent molecular adaptations that affect DA
function within the NAc have been linked to the expression of
depression-like signs in rats. Stress elevates the activity of the
transcription factor cAMP response element-binding protein (CREB)
within the NAc shell (Pliakas et al., 2001). Elevated expression of
CREB within the NAc shell increases immobility in the forced swim test
(FST) (Pliakas et al., 2001), a rodent model often used to study
depression (Porsolt et al., 1977). The effects of CREB within the NAc
seem related to its ability to regulate transcription of dynorphin, an
endogenous -opioid receptor ligand (Carlezon et al., 1998). Indeed,
the -antagonist nor-binaltorphimine (norBNI) attenuates the
behavioral effects of elevated CREB expression within the NAc (Carlezon
et al., 1998; Pliakas et al., 2001), most likely by blocking -opioid
receptors that normally inhibit neurotransmitter release from
mesolimbic DA neurons (Shippenberg and Rea, 1997; Di Chiara and
Imperato, 1988; Svingos et al., 1999). Recent evidence suggests that
the actions of norBNI within the NAc itself are sufficient to cause an
antidepressant-like effect in the learned helplessness paradigm (Newton
et al., 2002). Taken together, these data raise the possibility that
CREB-mediated transcription of dynorphin within the NAc decreases DA
function, which triggers signs of depression.
Because of the possible connection between dynorphin and symptoms
of depression, the present study was designed to use pharmacological tools to determine whether -opioid receptors regulate
depression-like signs in the FST. One advantage of the FST is that it
identifies in rats treatments with antidepressant effects in humans
(Porsolt et al., 1977; Detke et al., 1995). We reported previously
(Pliakas et al., 2001) that high doses of norBNI, a well characterized -antagonist with long-lasting effects (Jones and Holtzman, 1992; Spanagel and Shippenberg, 1993), can increase the latency to become immobile in the FST, a putative indicator of antidepressant-like effects. A goal of the present studies was to extend this work by
examining the effects of norBNI over a more complete range of doses,
while using a more stringent and widely accepted method of scoring the
FST (behavioral sampling; Detke et al., 1995). Furthermore, to examine
whether the antidepressant-like effects of norBNI are associated with
specific actions at -receptors, we conducted similar studies using
two novel and structurally dissimilar -antagonists:
5'-guanidinonaltrindole (GNTI) (Jones and Portoghese, 2000; Jewett et
al., 2001; Negus et al., 2002) and 5'-acetamidinoethylnaltrindole
(ANTI) (Stevens et al., 2000). We also explored the possibility that
the -agonist
[5,7,8]-N-methyl-N-[7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec8-yl]-benzenacetamide (U-69593) would have the opposite (prodepressant-like) effects on
behavior in the FST. In parallel, we examined the effects of each agent
on locomotor activity. Finally, because systemic ANTI had strong
antidepressant-like effects in the FST, we examined whether it has
reward-related effects using intracranial self-stimulation (ICSS)
(Wise, 1998).
| |
Materials and Methods |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
Rats. A total of 591 male Sprague-Dawley rats (Charles River Laboratories, Inc., Wilmington, MA) were used in this study. Rats used for forced swim testing or locomotor activity testing were housed in groups of four and weighed 325 to 375 g at the time of testing, whereas those used for ICSS testing were housed singly and weighed 350 to 400 g at the time of stereotaxic surgery. All rats were maintained on a 12-h light (7:00 AM-7:00 PM)/12-h dark cycle with free access to food and water except during testing. Experiments were conducted in accordance with the 1996 Guide for the Care and Use of Laboratory Animals (National Institutes of Health) and McLean Hospital policies.
Drugs. Desipramine HCl (DMI), fluoxetine HCl (FLX), cocaine HCl (COC), norBNI, and U-69593 were obtained from Sigma-Aldrich (St. Louis, MO). Citalopram (CIT) was obtained from Forest Laboratories (New York, NY). GNTI and ANTI were synthesized at the University of Minnesota. Dosages of all drugs were based on their salt form. DMI, FLX, norBNI, and GNTI were dissolved in distilled water, and COC and CIT were dissolved in saline. U-69593 was dissolved in 0.1 N acetic acid diluted with distilled water. All drugs were administered in a volume of 1 ml/kg, except for FLX (and the FLX-associated vehicle groups), which was administered in a volume of 2 ml/kg because of poor solubility.
FST.
Three hundred seventy one rats were used for studies of
standard psychotropic agents (DMI, FLX, CIT, and COC) and -ligands (norBNI, GNTI, ANTI, and U-69593) in the FST. The FST is a 2-day procedure in which rats swim under conditions in which escape is not
possible. On the 1st day, the rats are forced to swim for 15 min. The
rats initially struggle to escape from the water, but eventually they
adopt a posture of immobility in which they make only the movements
necessary to keep their heads above water. When the rats are retested
24 h later, immobility is increased. Treatment with standard
antidepressant drugs within the 24-h period between the first exposure
to forced swimming and retesting can block facilitated immobility, an
effect correlated with antidepressant efficacy in humans (Porsolt et
al., 1977; Detke et al., 1995).
) of norBNI (1.25-20 µg) or GNTI
(5.0-20 µg) were administered in 2.0 µl over 10 min using a
Hamilton syringe with a 26-gauge needle (Pliakas et al., 2001). At the
dosages used, a single treatment with these agents has extended
efficacy: norBNI produces a -receptor-specific blockade in rats
(Jones and Holtzman, 1992) for >3 weeks (Spanagel and Shippenberg,
1993). GNTI has detectable -receptor-specific effects in monkeys for
~10 days (Negus et al., 2002), although its effects in rats do not
seem to endure as long as than those of norBNI (Jewett et al., 2001).
(ANTI was not tested in i.c.v. studies because it is a novel compound
with modifications to its chemical structure that made it
hypothetically more bioavailable by systemic injection than norBNI or
GNTI). Rats recovered from surgery for 2 days before the FST.
On the 1st day of the FST, rats were placed in clear, 65-cm-tall by
25-cm-diameter cylinders filled to 48 cm with 25°C water. After 15 min of forced swimming, the rats were removed from the water, dried
with towels, and placed in a warmed enclosure for 30 min. The cylinders
were emptied and cleaned between rats. Rats tested with the standard
psychotropic agents, ANTI, or U-69593 received three separate i.p.
injections at 1, 19, and 23 h after the first exposure to forced
swimming, a commonly used treatment regimen (Porsolt et al., 1977;
Detke et al., 1995; Carlezon et al., 2002). Rats given norBNI, GNTI, or
vehicle by i.c.v. microinjection did not receive systemic injections.
There were 7 to 15 rats per treatment; control (vehicle-treated) groups
had the highest numbers because each daily test session included four
rats, and each group of four rats contained at least one control rat.
At 24 h after the forced swim, rats were retested for 5 min (300 s) under identical swim conditions. Retest sessions were videotaped
from the side of the cylinders and scored using a behavioral sampling
method (Detke et al., 1995; Carlezon et al., 2002) by raters unaware of
the treatment condition. Rats were rated at 5-s intervals throughout
the duration of the retest session; at each 5-s interval, the
predominant behavior was assigned to one of four categories:
immobility, swimming, climbing, or diving. A rat was judged to be
immobile if it was making only movements necessary to keep its head
above water; climbing if it was making forceful thrashing movements
with its forelimbs directed against the walls of the cylinder; swimming
if it was actively making swimming movements that caused it to move
within the center of the cylinder; and diving if it swam below the
water, toward the bottom of the cylinder (data quantifying diving
behavior are not shown in the present report because it rarely
occurred, and it was not affected by any of the treatments tested).
This behavioral sampling method reportedly differentiates classes of
antidepressant drugs: for example, tricyclic antidepressants decrease
immobility and increase climbing without affecting swimming, whereas
selective serotonin reuptake inhibitors (SSRIs) decrease immobility and increase swimming without affecting climbing (Detke et al., 1995).
Data from each agent were analyzed separately. The number of
occurrences (to a maximum of 60) of each category of behavior was
analyzed using separate one-way (treatment) analyses of variance (ANOVAs). Significant effects were analyzed using post hoc Fisher's least significant difference (LSD) tests.
Locomotor Activity. Two hundred twelve rats were used to determine whether the treatments examined in the FST studies alter locomotor activity. These studies were conducted exactly as the FST studies had been conducted until the point of retesting, that is, all rats underwent the 1st day of the FST and were treated with the standard agents ANTI or U-69593 at the normal pretreatment times (1, 19, and 23 h after swimming). Rats given norBNI or GNTI received i.c.v. microinjections 2 days before the 1st day of the FST. At 24 h after the first exposure to forced swimming, the rats were placed for 1 h in automated 68 × 21 × 21 cm (length × width × height) activity chambers (MED Associates, St. Albans, VT) instead of being retested in the FST. There were 7 to 12 rats per treatment; control (vehicle-treated) groups had the highest numbers because each daily test session included four rats, and each group of four rats contained at least one control rat.
The total number of activity counts (photocell beam breaks) during the test session was quantified, and data for each agent were analyzed separately. For locomotor activity studies in which multiple doses of drug were tested (i.e., the standard psychotropic agents and U-69593), differences among treatment groups were analyzed using one-way (treatment) ANOVAs. Significant effects were analyzed further using post hoc Fisher's LSD tests. For studies involving norBNI, GNTI, and ANTI, only the highest active dose was tested, and differences from vehicle-treated rats were analyzed using Student's t tests.ICSS.
Each of eight rats was anesthetized as described above
and implanted with a monopolar, stainless steel electrode (0.250 mm in
diameter; Plastics One, Roanoke, VA) aimed at the left medial forebrain
bundle, at the level of the lateral hypothalamus (2.8 mm posterior to
bregma, 1.7 mm lateral from midsaggital suture, and 7.8 mm below dura;
Paxinos and Watson, 1986). The electrodes were coated with polyamide
insulation except at the flattened tip. Skull screws (one of which
served as the ground) and the electrode were secured to the skull with
dental acrylic.
). Each lever press earned a 0.5-s train of square-wave cathodal
pulses (0.1-ms pulse duration) at a set frequency of 158 Hz. The
delivery of the stimulation was accompanied by the illumination of a
2-W house light. Responses during the 0.5-s stimulation period did not
earn additional stimulation. The stimulation current (100-300 µA)
was adjusted gradually to the lowest value that would sustain a
reliable rate of responding (at least 40 rewards/min). Once an
appropriate current was found for each rat, it was held constant
throughout the remainder of the experiment.
Each rat was then adapted to brief tests with each of a descending
series of 15 stimulation frequencies. Each series comprised 1-min test
trials at each frequency. For each frequency tested, there was an
initial 5-s "priming" phase during which noncontingent stimulation
was given, followed by a 50-s test phase during which the number of
responses was counted. After the test phase, there was a 5-s time out
period during which no stimulation was available. The stimulation
frequency was then lowered by approximately 10% (0.05 log10 units), and another trial was started.
After responding had been evaluated at each of the 15 frequencies, the
procedure was repeated such that each rat was given six such series per day (90 min of training). During the training procedure, the range of
frequencies was adjusted for each rat so that only the highest five to
six frequencies would sustain responding. To characterize the functions
relating response strength to reward magnitude, a least-squares line of
best fit was plotted across the frequencies that sustained responding
at 20, 30, 40, 50, and 60% of the maximum rate using customized
analysis software (Steven Cabilio, Montreal, QC, Canada;
scabilio{at}hotmail.com). ICSS threshold was defined as the
frequency at which the line intersected the x-axis (
-0; Miliaressis et al., 1986). Drug testing started when mean ICSS thresholds varied by less than 10% over three consecutive training sessions.
For drug testing, three rate-frequency functions ("curves") were
determined for each rat immediately before drug treatment. The first
curve served as a warm-up period and was discarded because it tended to
be unreliable. The second and third curves were averaged to obtain the
baseline (threshold and maximal response rates) parameters. Each rat
then received an i.p. injection of drug or vehicle, and four more
15-min rate-frequency curves were obtained (1 h of testing). All rats
received the same treatments in a standardized order: the first test
was with vehicle, the second test was with cocaine (2.5 mg/kg i.p.),
the third test was with vehicle, and the fourth test was with ANTI (3.0 mg/kg i.p.). Vehicle treatments were used to ensure that the rats had
recovered from prior treatment with cocaine and to minimize the
possibility of conditioned drug effects. It has been established
previously that repeated treatment with cocaine (or other stimulants)
does not cause progressive changes in drug sensitivity in the ICSS
assay (for review, see Carlezon et al., 2001).
Effects of vehicle, COC, and ANTI on thresholds and maximal response
rates over the 60-min test period were evaluated using one-way ANOVAs
with repeated measures. The first and second tests with vehicle did not
differ and were combined into a single group. Significant effects were
analyzed further using post hoc Fisher's LSD tests.
Histology. Rats that received i.c.v. microinjections or ICSS electrodes were overdosed with pentobarbital (130 mg/kg i.p.) and perfused with 4% paraformaldehyde. The fixed brains were sliced in 40-µm sections for cresyl violet staining to confirm i.c.v. microinjection and electrode placements.
| |
Results |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
Each of the standard psychotropic agents (DMI, FLX, CIT, and COC)
reduced immobility in the FST, but effects on the occurrences of
swimming and climbing behaviors differed among drugs (Table 1). DMI (a tricyclic antidepressant) dose
dependently decreased occurrences of immobility
(F4,61 = 6.97, P < 0.01), did not affect occurrences of swimming, and increased
occurrences of climbing (F4,61 = 6.78, P < 0.01). In locomotor activity studies, DMI
decreased activity counts at the highest doses tested
(F4,42 = 9.21, P < 0.01). In contrast, FLX (an SSRI) dose dependently decreased
occurrences of immobility (F3,45 = 4.58, P < 0.01) and increased occurrences of swimming
behavior (F3,45 = 5.64, P < 0.01), but did not affect occurrences of climbing.
FLX also decreased locomotor activity at doses with efficacy in the FST
(F4,42 = 9.21, P < 0.01). CIT (an SSRI) had effects similar to those seen with FLX: it
dose dependently decreased occurrences of immobility
(F3,33 = 2.94, P < 0.05) and increased occurrences of swimming behavior
(F3,33 = 4.55, P < 0.01), without affecting occurrences of climbing. CIT began to decrease
locomotor activity at doses below those with efficacy in the FST
(F3,30 = 4.66, P < 0.01). Like each of the SSRIs, COC dose dependently decreased
occurrences of immobility (F3,36 = 7.06, P < 0.01) and increased occurrences of swimming behavior (F3,36 = 5.86, P < 0.01), without affecting occurrences of climbing.
Unlike any of the other standard antidepressant agents tested, COC did
not cause even nominal changes in locomotor activity at doses with
efficacy in the FST. Higher doses of COC were not tested in the FST
because they caused substantial increases in locomotor activity (5.0 mg/kg, 3479.3 ± 266.7; 10 mg/kg, 4971.8 ± 532.2) in small
numbers of rats (n = 4, each group).
|
The -antagonists (Fig. 1) produced
effects in the FST that were qualitatively similar to those seen with
the standard psychotropic agents. The i.c.v. microinjections of norBNI
dose dependently decreased occurrences of immobility
(F5,49 = 4.68, P < 0.01) and increased occurrences of swimming behavior
(F5,49 = 3.85, P < 0.01) (Fig. 2a). There was also a
statistically significant effect on climbing
(F5,49 = 2.74, P < 0.05), but post hoc analyses revealed that this was due to differences
between rats in the 1.25-µg norBNI and 10- and 20-µg norBNI groups
rather than any differences from vehicle-treated rats. There was no
effect of norBNI on locomotor activity at the dose with
antidepressant-like effects (20 µg) (Fig. 2b). Similarly, i.c.v.
microinjections of GNTI dose dependently decreased occurrences of
immobility (F3,35 = 3.00, P < 0.05) and increased occurrences of swimming
behavior (F3,35 = 3.04, P < 0.05), but did not affect climbing behavior (Fig.
3a). GNTI had no effect on locomotor
activity at the highest dose (20 µg) with antidepressant-like effects
in the FST (Fig. 3b). Systemic administration of GNTI (1.0-10 mg/kg
i.p.) had no effect in small numbers of rats (~5/group) (data not
shown). However, systemic (i.p.) administration of ANTI had efficacy in
the FST: it dose dependently decreased occurrences of immobility
(F4,44 = 2.89, P < 0.05), but unlike the other -antagonists, it increased occurrences
of climbing behavior (F4,44 = 2.88, P < 0.05) without affecting swimming behavior (Fig.
4a). Systemic administration of ANTI did
not affect locomotor activity at the highest dose (3.0 mg/kg) with
antidepressant-like effects in the FST (Fig. 4b).
|
|
|
|
Systemic (i.p.) administration of the -agonist U-69593 (Fig. 1) had
effects in the FST that were qualitatively opposite to those seen with
the -antagonists. U-69593 dose dependently increased occurrences of
immobility (F5,60 = 3.97, P < 0.01) and decreased occurrences of swimming
behavior (F5,60 = 4.31, P < 0.01), without affecting climbing behavior (Fig.
5a). U-69593 significantly decreased locomotor activity (F2,20 = 16.34, P < 0.01) at the highest dose (10 mg/kg i.p.) with
prodepressant-like effects in the FST, but had no effect at a lower
dose (3.0 mg/kg) that also affected immobility and swimming behaviors
(Fig. 5b).
|
In the ICSS studies, the effects on thresholds depended upon treatment
(F2,14 = 11.8, P < 0.01), whereas there were no significant effects on maximal response
rates. Treatment with vehicle tended to cause small, nonsignificant
increases in ICSS thresholds (Fig. 6a)
and decreases in maximum response rates (Fig. 6b) during the 1-h test
session. There were no differences between the effects of the first
(test 1) and second (test 3) injection with vehicle, so these two
sessions were combined into a single mean. The lowest dose of cocaine
(2.5 mg/kg) tested with antidepressant-like effects in the FST
significantly decreased ICSS thresholds without significantly affecting
maximal response rates. In contrast, the highest dose of ANTI (3.0 mg/kg) tested with antidepressant-like effects in the FST did not
affect ICSS thresholds or maximal response rates. ICSS electrodes were
located in the medial forebrain bundle at the level of the lateral
hypothalamus, and the placements were indistinguishable from those
depicted previously (Carlezon and Wise, 1996a; Carlezon et al., 2001).
|
| |
Discussion |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
Standard antidepressants (DMI, FLX, and CIT) reduced immobility in
the FST, a putative antidepressant-like effect (Porsolt et al., 1977;
Detke et al., 1995; Carlezon et al., 2002). Moreover, the behavioral
sampling scoring method (Detke et al., 1995) distinguishes among
classes of drugs: as expected, DMI decreased immobility and increased
climbing, whereas FLX and CIT each decreased immobility and increased
swimming. Together, these findings confirm the sensitivity of our
methodology to agents with antidepressant efficacy in humans.
Cocaine also reduced immobility in the FST, consistent with the
antidepressant and "mood-elevating" effects of the drug (Post et
al., 1974). Furthermore, cocaine increased swimming behaviors without
affecting climbing behaviors. This pattern suggests that the
antidepressant-like effects of cocaine resemble those of SSRIs more
closely than those of tricyclic antidepressants, although the
possibility that it reflects a dopaminergic mechanism cannot be
discounted. Cocaine is often regarded as a prototypical dopaminergic agent, but it actually has higher affinity for 5HT transporters than
for DA or NE transporters (Ritz and Kuhar, 1989). Additional work with
agents with improved selectivity is needed to determine whether
relatively specific increases in individual neurotransmitters are
associated with particular behavioral patterns in the FST.
A single i.c.v. treatment with the -antagonist norBNI dose
dependently decreased immobility in the FST, suggesting that this agent
has antidepressant-like effects. This effect is consistent with our
previous observations that norBNI decreases dysphoria likely associated
with acute cocaine withdrawal (Carlezon et al., 1998; Pliakas et al.,
2001). Indeed, antidepressants such as DMI reportedly reduce symptoms
of cocaine withdrawal in humans (Gawin et al., 1989) and rats (Markou
et al., 1992). Likewise, a single i.c.v. treatment with GNTI, a
structurally dissimilar -antagonist, had antidepressant-like
effects. GNTI was more potent than norBNI, consistent with previous
reports in which the effects of these agents were compared (Jones and
Portoghese, 2000; Jewett et al., 2001; Negus et al., 2002). The fact
that structurally dissimilar -antagonists have similar actions in
the FST suggests that the antidepressant-like effects of norBNI are
attributable to a specific blockade of -receptors rather than
nonspecific side effects. However, GNTI was not effective by systemic
administration at doses up to 10 mg/kg i.p., suggesting that the
chemical structure of this agent, specifically, the high
pKa of the guanidinium group, may
limit its bioavailability.
We also tested ANTI, a more lipophilic -antagonist, using the
repeated systemic (i.p.) administration regimen. ANTI is a potent and
selective -selective antagonist that contains an amidinium group,
which is less basic than the guanidinium group of GNTI. This
modification allows a greater percentage of nonionized ANTI to enter
the brain. ANTI also contains three additional hydrophobic groups (two
methylenes and one methyl) that increase its lipophilic character,
further contributing to improved access to the brain and greater
potency. Like norBNI and GNTI, ANTI reduced immobility in the FST,
providing additional evidence that antidepressant-like effects may be a
general attribute of -antagonists.
Effects on swimming and climbing behaviors differed among the
-ligands. Treatment with norBNI and GNTI increased swimming behaviors, whereas ANTI increased climbing behaviors. The significance of this observation is unclear. These data suggest that alterations in
the chemical structure of -antagonists can alter their effects on
monoamine function (Detke et al., 1995). However, differential effects
on the swimming and climbing measures may also involve factors other
than interactions among NE, 5HT, and DA (Carlezon et al., 2002).
Furthermore, systemic treatment with a -agonist had effects in the
FST that were opposite to those seen with the antagonists: U-69593
increased immobility and decreased swimming. These data suggest that
U-69593 has prodepressant-like effects in rats, which is consistent
with previous observations that -agonists produce dysphoria in
humans (Pfeiffer et al., 1986). Considering together the effects of the
-agonist and the antagonists, these findings raise the possibility
that -opioid receptors mediate signs of immobility in the FST.
One concern when using the FST is that nonspecific treatment effects on
activity levels could complicate data interpretation. If treatments
increase activity, they could seem to reduce immobility in the FST and
thus be incorrectly identified as antidepressants. Accordingly, we
conducted locomotion studies in parallel with the FST studies to
identify potentially confounding effects. Locomotion studies were
conducted exactly as the FST studies, except during the 2nd day of
testing the rats were placed in activity chambers rather than being
reexposed to forced swimming. If anything, the standard antidepressants
tended to decrease locomotor activity at the time of retesting in the
FST. It is unlikely, however, that the ability of an agent to decrease
locomotor activity is necessary to cause antidepressant-like actions in
the FST, because cocaine had antidepressant-like actions without
affecting activity. Furthermore, other treatments have been shown to
produce antidepressant-like (or prodepressant-like) effects in the FST
without affecting locomotor activity (Pliakas et al., 2001). The
observation that cocaine had antidepressant-like effects in the FST
without affecting locomotor activity suggests that its mood-enhancing
effects occur at lower doses than its stimulant effects. Similarly,
none of the -antagonists affected locomotor activity at the highest
doses with antidepressant-like effects in the FST. Although U-69593
decreased activity at the highest dose tested (10 mg/kg i.p.), it did
not affect activity at a lower dose (3.0 mg/kg i.p.) that also had
prodepressant-like effects. Because both antidepressant-like effects
and prodepressant-like effects can be observed after treatments that
decrease locomotion, it seems unlikely that our FST studies were
affected by nonspecific treatment effects on locomotor activity.
Although the mechanisms by which -receptor agents regulate behavior
in the FST are unknown, previous work suggests that altered DA function
may be involved. One possibility is that immobility is mediated by
-receptors located on the terminals of mesolimbic DA neurons that
project to the NAc (Svingos et al., 1999). These presynaptic
-receptors regulate DA release: administration of -agonists
directly into the NAc decreases local extracellular concentrations of
DA to ~50% of baseline (Di Chiara and Imperato, 1988;
Maisonneuve et al., 1994). Conversely, direct administration of
-antagonists into the NAc increases DA concentrations to ~175% of
baseline (Maisonneuve et al., 1994). These increases in concentrations of DA are modest compared with those observed after treatment with
psychostimulants such as cocaine (to 400-800% baseline) or amphetamine (to >1000% baseline) (Di Chiara and Imperato, 1988; Maisonneuve et al., 1994). Modest increases in DA concentrations within
the NAc may be sufficient to cause antidepressant-like effects in the
FST without stimulating locomotor activity.
Another possibility is that the efficacy of -antagonists in the FST
is related to their ability to block the effects of
experience-dependent elevations in endogenous dynorphin levels. Forced
swimming activates CREB within the NAc (Pliakas et al., 2001), and CREB
regulates dynorphin expression within the NAc and related tissues (Cole et al., 1995; Carlezon et al., 1998). Thus, forced swimming may trigger
CREB-mediated increases in dynorphin transcription within the NAc,
which contribute to the facilitated immobility normally observed during
the second exposure to the FST (Porsolt et al., 1977; Pliakas et al.,
2001). As such, -antagonists may have antidepressant effects by
blocking dynorphin-mediated reductions in extracellular concentrations
of DA within the NAc, an effect associated with symptoms of depression,
including anhedonia (Wise, 1982; Paulson et al., 1991, Pliakas et al.,
2001). Regardless of the exact mechanisms, recent evidence suggests
that -antagonist actions within the NAc itself are sufficient to
cause an antidepressant-like effect (Newton et al., 2002).
If -antagonists affect behavior through their ability to promote DA
transmission in the NAc then an obvious concern is that they may have
abuse liability (Di Chiara and Imperato, 1988). Drugs of abuse
facilitate ICSS in rats, reducing the amounts of stimulation required
to sustain responding (thresholds) (Wise, 1998). In the present study,
cocaine significantly reduced ICSS thresholds at a dose (2.5 mg/kg
i.p.) with antidepressant-like effects in the FST, confirming that its
"mood-elevating" effects are associated with reward-related effects
that contribute to abuse liability. In contrast, ANTI did not affect
ICSS thresholds at a dose with antidepressant-like effects in the FST.
Thus, the effects of ANTI seem similar to those of other antidepressant drugs (e.g., DMI and FLX), which, if anything, initially increase ICSS
thresholds (Hall et al., 1990; Markou et al., 1992; Lee and Kornetsky,
1998). The fact that effective doses of ANTI lack stimulant effects in
locomotor activity studies and rewarding effects in ICSS studies is
early evidence that this agent lacks abuse liability at therapeutic
concentrations. Studies examining the effects of ANTI and other
-ligands on ICSS behavior across a wide range of doses are in progress.
The present study raises the possibility that -antagonists may be a
new approach to the treatment of depressive disorders. A limitation of
norBNI and GNTI is low bioavailability after systemic administration.
Structural modifications designed to reduce basicity and increase
lipophilicity led to the design of ANTI (Stevens et al., 2000), which
was efficacious in the FST after systemic administration. Additional
modifications may yield -antagonists with increased potency or
efficacy and provide additional insight into the therapeutic potential
of this class of agents.
| |
Acknowledgments |
|---|
We thank Forest Laboratories for the gift of citalopram, Michael Detke for the use of behavioral sampling software, and Sonia Guediche for technical assistance.
| |
Footnotes |
|---|
Accepted for publication January 3, 2003.
Received for publication November 1, 2002.
This study was funded by Grant MH63266 from the National Institute of Mental Health (to W.C.), Grant DA01533 from the National Institute on Drug Abuse (to P.S.P.), an unrestricted gift from Johnson and Johnson (to W.C.), and donations by John A. Kaneb (to W.C.).
DOI: 10.1124/jpet.102.046433
Address correspondence to: Dr. Bill Carlezon, Department of Psychiatry, McLean Hospital, MRC 217, 115 Mill St., Belmont MA 02478. E-mail: carlezon{at}mclean.harvard.edu
| |
Abbreviations |
|---|
NE, norepinephrine;
5HT, 5-hydroxytryptamine;
DA, dopamine;
NAc, nucleus accumbens;
CREB, cAMP response
element-binding protein;
FST, forced swim test;
norBNI, nor-binaltorphimine;
GNTI, 5'-guanidinonaltrindole;
ANTI, 5'-acetamidinoethylnaltrindole;
U-69593, (5,7,8)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4.5]dec8-yl)-benzenacetamide;
ICSS, intracranial self-stimulation;
DMI, desipramine;
FLX, fluoxetine;
COC, cocaine;
CIT, citalopram;
SSRI, selective serotonin reuptake
inhibitor;
ANOVA, analysis of variance;
LSD, least significant
difference.
| |
References |
|---|
|
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
|
|---|
-opioid antagonist GNTI reduces U50,488-, DAMGO- and deprivation-induced feeding, but not butorphanol- and neuropeptide Y-induced feeding in rats.
Brain Res
909:
75-80[CrossRef][Medline].-opioid receptor blockade following nor-binaltorphimine.
Eur J Pharmacol
215:
345-348[CrossRef][Medline].-opioid receptor antagonist.
Eur J Pharmacol
396:
49-52[CrossRef][Medline].-opioid receptor agonist, attenuates cocaine-induced increases in extracellular dopamine in the nucleus accumbens of rats.
Neurosci Lett
181:
57-60[CrossRef][Medline].-antagonist 5'-guanidinonaltrindole (GNTI) in an assay of schedule-controlled behavior in rhesus monkeys.
Psychopharmacology
163:
412-419[CrossRef][Medline].-opiate receptors.
Science (Wash DC)
233:
774-776-opioid receptor agonists.
Pharmacol Biochem Behav
57:
449-455[CrossRef][Medline].-opioid systems in the rat.
Neurosci Lett
153:
232-236[CrossRef][Medline].-opioid receptor.
J Med Chem
43:
2759-2769[CrossRef][Medline].-opioid receptors in the rat nucleus accumbens shell.
J Neurosci
19:
1804-1813This article has been cited by other articles:
|
|
 |
|
  A. Kreibich, B. A. S. Reyes, A. L. Curtis, L. Ecke, C. Chavkin, E. J. V. Bockstaele, and R. J. Valentino Presynaptic Inhibition of Diverse Afferents to the Locus Ceruleus by {kappa}-Opiate Receptors: A Novel Mechanism for Regulating the Central Norepinephrine System J. Neurosci., June 18, 2008; 28(25): 6516 - 6525. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. V. Carr and S. D. Mague p38: The Link between the {kappa}-Opioid Receptor and Dysphoria J. Neurosci., March 5, 2008; 28(10): 2299 - 2300. [Full Text] [PDF]
|
|
|
|
|
|
B. B. Land, M. R. Bruchas, J. C. Lemos, M. Xu, E. J. Melief, and C. Chavkin The Dysphoric Component of Stress Is Encoded by Activation of the Dynorphin {kappa}-Opioid System J. Neurosci., January 9, 2008; 28(2): 407 - 414. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Beguin, D. N. Potter, J. A. DiNieri, T. A. Munro, M. R. Richards, T. A. Paine, L. Berry, Z. Zhao, B. L. Roth, W. Xu, et al. N-Methylacetamide Analog of Salvinorin A: A Highly Potent and Selective {kappa}-Opioid Receptor Agonist with Oral Efficacy J. Pharmacol. Exp. Ther., January 1, 2008; 324(1): 188 - 195. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. T. Knoll, E. G. Meloni, J. B. Thomas, F. I. Carroll, and W. A. Carlezon Jr. Anxiolytic-Like Effects of {kappa}-Opioid Receptor Antagonists in Models of Unlearned and Learned Fear in Rats J. Pharmacol. Exp. Ther., December 1, 2007; 323(3): 838 - 845. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. R. Bruchas, B. B. Land, M. Aita, M. Xu, S. K. Barot, S. Li, and C. Chavkin Stress-Induced p38 Mitogen-Activated Protein Kinase Activation Mediates {kappa}-Opioid-Dependent Dysphoria J. Neurosci., October 24, 2007; 27(43): 11614 - 11623. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. R. Bruchas, T. Yang, S. Schreiber, M. DeFino, S. C. Kwan, S. Li, and C. Chavkin Long-Acting {kappa} Opioid Antagonists Disrupt Receptor Signaling And Produce Noncompetitive Effects By Activating C-Jun N-Terminal Kinase J. Biol. Chem., October 12, 2007; 282(41): 29803 - 29811. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. J. Will, W. M. Vand |
