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BEHAVIORAL PHARMACOLOGY
Department of Pharmacology, Emory University School of Medicine, Atlanta, Georgia
Received September 10, 2002; accepted October 25, 2002.
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Abstract |
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;
Blasig et al., 1973). Changes
in behavioral measures include suppression of operant responding
(Gellert and Sparber, 1977),
suppression of locomotor activity (Brady
and Holtzman, 1981), conditioned place aversion
(Mucha, 1987), and increased
thresholds for intracranial self-stimulation
(Schaefer and Michael, 1986).
These behavioral signs have been considered as operational measures of the
affective/motivational aspects of the opiate withdrawal syndrome
(Koob et al., 1989) and appear
to have neural substrates that are distinct from those of somatic signs
(Bozarth and Wise, 1984;
Koob et al., 1992).
Opioid antagonists can precipitate withdrawal-like signs and symptoms in
humans and animals pretreated with only a single dose of morphine, presumably
revealing a state of acute physical dependence
(Eisenberg, 1982;
Young, 1986;
Bickel et al., 1987;
Schulteis et al., 1997). For
example, naltrexone causes a number of somatic signs of withdrawal in rats
pretreated with a single dose of morphine 4 h earlier; although compared with
withdrawal from chronic morphine, there are fewer signs and a lower withdrawal
score (Easterling and Holtzman,
1999).
The assessment of withdrawal processes based on operant behaviors has
contributed significantly to the understanding of interoceptive and
motivational aspects of acute opioid dependence. That rats discriminating the
combination of acute morphine and naltrexone generalized dose dependently and
completely to naltrexone administered during chronic morphine treatment
provided evidence that withdrawal from acute and chronic dependence are
associated with similar interoceptive states
(Easterling and Holtzman,
1999). Other studies confirmed that a single dose of morphine
increased sensitivity to opioid antagonists in altering behaviors maintained
by food or brain stimulation in a fashion similar to that of chronic morphine
(Adams and Holtzman, 1990a;
Easterling and Holtzman,
1997).
Despite these findings, little research has been done on acute opioid
dependence and withdrawal using unconditioned behaviors, especially in
procedures that address the question of whether affective changes accompanying
acute withdrawal are similar to those of chronic withdrawal. The first
objective of this study was to determine whether auditory startle responses
and startle-induced ultrasonic vocalizations could be used as behavioral
measures of withdrawal from a single dose of morphine. Changes in these
behaviors provide objective and reliable measures of withdrawal from chronic
morphine (Vivian and Miczek,
1991; Mansbach et al.,
1992). Auditory startle amplitudes and startle-induced ultrasonic
vocalizations were evaluated in groups of rats undergoing withdrawal from
acute or chronic morphine to compare the magnitude of these changes across
different states of dependence. The second objective was to determine whether
somatic signs, increased urination, and defecation could provide objective
quantitative measures of withdrawal from acute dependence as they do
withdrawal from chronic dependence (Ho et
al., 1979; Pinelli and
Trivulzio, 1997). To determine an optimal pretreatment interval
for these somatic and behavioral indices of withdrawal, morphine pretreatment
times were varied between 2 and 6 h. In a recent drug discrimination study in
which rats were trained to discriminate acutely administered morphine followed
by naltrexone from saline followed by naltrexone, the maximal discriminative
effects occurred at 3 to 4 h (Easterling
and Holtzman, 1999). In contrast, the maximal potentiation of
naltrexone-induced drinking suppression by a single dose of morphine occurred
at a 2-h interval (White and Holtzman,
2001). Based on these findings, we hypothesized that the optimal
pretreatment time would vary across somatic or behavioral signs.
Materials and Methods
Animals. The subjects were adult male Sprague-Dawley rats (Charles
River Breeding Laboratories, Raleigh, NC) weighing between 220 and 300 g at
the start of the study. Animals were housed two to three per cage and
maintained in the Emory University Division of Animal Resources Care Facility
(Atlanta, Georgia). Food and water were always available. The colony room was
maintained on a 12-h light/dark cycle, with lights on at 7:00 AM. This study
was performed in accordance with the 1996 Guide for the Care and Use of
Laboratory Animals (National Academy of Sciences, Washington, DC), and the
research protocol was approved by the Institutional Animal Care and Use
Committee of Emory University.
Induction of Chronic Dependence. After a rat was anesthetized with
halothane, a 1.5-cm incision was made between the scapulae. A small cavity was
made using a blunt-ended hemostat and two Alzet osmotic pumps [model 2ML2 (14
day); Alza, Palo Alto, CA] were inserted. One group of rats (n = 24)
received osmotic pumps that delivered a total of approximately 20 to 22 mg/kg
morphine per day. Drug concentrations were based on the infusion rate of the
pump and on the weight of the animal. This treatment reliably produces
tolerance to and physical dependence upon morphine in the rat
(Adams and Holtzman, 1990b;
Easterling and Holtzman,
1997). Another group of rats (n = 17) received pumps
filed with saline. Half of the animals in each group were used in the
experiments on urination and defecation, and the other half were used in the
experiments on acoustic startle and startle-induced ultrasonic
vocalization.
To precipitate withdrawal, we injected naltrexone (0.01, 0.1, and 1.0 mg/kg) on days 7, 9, and 11 according to a Latin square design (animals tested for startle also received a saline injection on day 5). Immediately after the injection animals were placed into animal holders for a 20-min session (see below). We induced spontaneous withdrawal by removing the pumps on day 13 while the rats were anesthetized with halothane. The appropriate 20-min sessions were repeated in the same animals 3-, 6-, and 24-h after pump removal to determine the effects of spontaneous withdrawal from morphine.
Induction of Acute Dependence. An injection of either morphine (10 mg/kg; n = 42) or saline (n = 20) was given to otherwise drug-free rats. Two to six hours later all animals were given a single injection of saline and were immediately placed into animal holders for a 20-min session. The whole procedure was repeated at 2- to 3-day intervals, except naltrexone (0.01, 0.1 and 1.0 mg/kg) was administered instead of saline, according to a Latin square design.
Measurement of Urination and Defection. All measurements were taken inside the sound-attenuating cubicles (MED Associates, St. Albans, VT), with continuous 55-db background white noise maintained constantly. Rats were placed into animal holders (16 x 9 x 8 cm) fixed on the top of the platform (25 x 11 cm) that was used to measure the acoustic startle response; however, no startle stimuli were presented. All four sides of the animal holders were made of stainless steel bars placed 4 cm apart. Strips of Fisherbrand Absorbent Underpads (Fisher Scientific, Pittsburg, PA), weighed previously, were placed under each holder to collect urine and fecal boli. At the end of the 20-min session, fecal boli were counted and removed from the pad; each pad was then weighed again, and the difference in the weight of the pad before and after the session was used to estimate the amount of urine deposited on it.
Measurement of Auditory Startle Response. Rats were put into animal holders that were placed inside sound-attenuating cubicles, as described above. Startle was measured via recording the force applied by a rat onto a single point load cell (the startle-transducing platform). Startle-inducing auditory stimuli (white noise) were delivered through a speaker placed approximately at the level of the rat's ears. The startle response was recorded during the 500 ms following the onset of the auditory stimulus. Startle peak was defined as the first peak of the downward force that was produced by the rat on the platform, with a minimum latency of 20 ms, a minimum peak value of 50 (range 50–2047), and a minimum peak time of 30 ms. All events during the 20-min session were controlled and recorded by software from Med Associates (Startle Reflex, version 3).
Before the start of the experiment, all auditory stimuli were calibrated (precision of 0.5 db) in the four cubicles, with the aid of the Digital Sound Level Meter (model 840029; Sper Scientific, Scottsdale, AZ). Each startle platform was calibrated by using a constant force produced by a weight displaced around a 45° angle by a rotating-step motor. The four startle platforms varied by less than 2% in the startle calibration procedure. During the 20-min session, each animal was first habituated to the animal holder for 3 min. After that, a series of 18 (three blocks of six trials) discrete (30 ms) auditory pulses (0-ms rise/fall time) of the same intensity (105 db; intertrial interval of 30–45 s) was delivered. The responses of each rat were averaged over the 18 trials.
Measurement of Ultrasonic Vocalizations. Ultrasonic vocalizations (20–28 kHz) were recorded and transformed into an audible signal (0.2–10 kHz) with the aid of Mini-3 Bat Detectors (Ultra Sound Advice, London, UK). After passing through an audio filter (Noldus Information Technology, Inc., Sterling, VA), the signal was sent to a computer, where it was digitized and analyzed with the aid of the software UltraVox (Noldus Information Technology, Inc.). Ultrasonic distress vocalizations were recorded for the entire duration of each session for every rat in the study.
Drugs. Morphine sulfate (Penick Co., Nutley, NJ) and naltrexone hydrochloride (Sigma-Aldrich, St. Louis, MO) were prepared in 0.9% saline. Both drugs were injected s.c. in a volume of 1.0 ml/kg. In addition, morphine and saline were administered by continuous s.c. infusion via osmotic pumps, as described above. All doses are expressed as the free base.
Data Analysis. Bartlett's test for the homogeneity of variance was
applied to the interval scale data. Analysis of variance (ANOVA) was performed
on the interval scale data suitable for parametric analysis to evaluate the
main effect, followed by Student-Neuman-Keuls post hoc test when appropriate.
All proportion data (percentage of animals emitting ultrasonic vocalizations)
were analyzed with the 
-square test. The 
level chosen
was p 
0.05.
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Results |
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TopAbstractResultsDiscussionReferences |
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Acute Dependence. Naltrexone (0.01–1.0 mg/kg) had no effect on urination in animals pretreated with saline. In contrast, naltrexone stimulated urination in rats pretreated with a single dose of morphine, depending upon the pretreatment interval (Fig. 1c). An overall ANOVA revealed a significant main effect of pretreatment group (F3,56 = 28.78; p < 0.0001) and of dose (F2,56 = 6.21; p < 0.01); a pretreatment group x dose interaction approached, but did not reach statistical significance (F8,56 = 2.08; p = 0.07). All three doses of naltrexone stimulated urination after 2-h pretreatment with morphine, and no dose did after 6-h pretreatment (Fig. 1c). In 4-h pretreated animals, 0.01 and 1.0 mg/kg of naltrexone increased urination significantly (Fig. 1c). The peak effect of naltrexone, which was reached after as little as 0.01 mg/kg in 2-h pretreated rats, was almost twice the peak effect observed in rats chronically dependent upon morphine.
Defecation
Chronic Dependence. Following pump removal, the number of fecal boli
was significantly (F1,40 = 23.21; p = 0.0001) and
consistently higher in animals that had received morphine chronically than it
was in animals that had saline in their pump
(Fig. 2a). There was also a
significant main effect of time (F2,40 = 3.92; p
< 0.05), with the peak effect at 3 h after the pumps had been removed.
There was no significant treatment group x time interaction
(F1,40 = 2, 12; p > 0.05). In animals
chronically dependent on morphine, naltrexone produced robust dose-dependent
(F2,40 = 127.02; p < 0.0001) increases in
defecation (Fig. 2b). There was
a significant main effect of treatment group (F1,40 =
353.23; p < 0.0001; Fig.
2b) and a significant treatment group x dose interaction
(F2,40 = 139.96; p < 0.0001). Although all
doses of naltrexone stimulated defection in morphine-exposed rats, the two
higher doses (0.1 and 1.0 mg/kg) of naltrexone produced a particularly
dramatic effect (Fig. 2b).
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Acute Dependence. Naltrexone (0.01–1.0 mg/kg) had no effect on defecation in rats pretreated with saline but dose dependently increased the number of fecal boli in the three groups pretreated with morphine (Fig. 2c). An overall ANOVA revealed significant main effects of pretreatment group (F3,56 = 8.54; p < 0.001) and dose (F2,56 = 23.75; p < 0.001) as well as a significant pretreatment group x dose interaction (F6,56 = 3.00; p = 0.01). The highest dose of naltrexone (1.0 mg/kg) stimulated defection in all morphine-pretreated rats, the intermediate dose (0.1 mg/kg) increased defecation only in 6-h pretreated animals, and the lowest dose (0.01 mg/kg) had no effect in any group (Fig. 2c).
Auditory Startle Response
Chronic Dependence. Auditory startle amplitudes were significantly
(F1,36 = 8.96; p < 0.01) lower in animals
undergoing spontaneous withdrawal from chronic morphine than they were in
animals previously exposed to saline (Fig.
3a). There was also a significant main effect of time
(F2,36 = 3.96; p < 0.05) and a significant
treatment group x time interaction (F2,36 = 4.55;
p < 0.05). Compared with saline-exposed controls, animals
undergoing spontaneous withdrawal from chronic morphine had lower startle
amplitudes 3- and 24-h following removal of the pump
(Fig. 3a). Startle amplitude
was not affected by any dose of naltrexone (0.01–1.0 mg/kg) in rats that
had pumps containing only saline nor was it affected by the chronic morphine
treatment, as evidenced by the similar outcomes of the saline challenge to the
two groups (Fig. 3b, points
above Sal). It was, however, reduced more than 50% by 0.1 and 1.0 mg/kg of
naltrexone in animals that were physically dependent upon morphine
(Fig. 3b). ANOVA confirmed a
significant main effect of treatment group (F1,36 = 16.69;
p < 0.001) and of dose (F2,36 = 7.19;
p < 0.01) and revealed a significant treatment group x dose
interaction (F2,36 = 9.57; p < 0.001).
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Acute Dependence. The effect of naltrexone on startle amplitude in animals pretreated with a single dose of morphine was the opposite of that in animals chronically dependent upon morphine. Naltrexone (0.1 and 1.0 mg/kg) increased startle amplitudes in rats that had been injected with 10 mg/kg of morphine either 2 or 6 h earlier, and the lowest dose, 0.01 mg/kg, increased startle amplitude in the 2-h pretreatment group. ANOVA revealed a significant main effect of the pretreatment group (F2,54 = 4.73; p < 0.05) and of dose (F2,54 = 3.86; p < 0.05) as well as a significant pretreatment group x dose interaction (F4,54 = 4.01; p < 0.01). Naltrexone did not affect startle amplitudes in rats pretreated with saline (Fig. 3c), consistent with the outcome in rats that had received infusions of saline via osmotic pumps (Fig. 3b), nor did morphine alone at either pretreatment interval (Fig. 3c, points above Sal).
Startle-Induced Ultrasonic Vocalizations
Chronic Dependence. Animals undergoing spontaneous withdrawal from
morphine were less likely to vocalize 3 h after pump removal than were the
corresponding control animals (2 (1, n = 20) = 10.90;
p = 0.001; Fig. 4a).
There was no difference between the two groups in the percentage of animals
vocalizing at later time points (6- and 24-h;
Fig. 4a) or in the duration of
vocalization at any of the three time points
(Fig. 4d). Naltrexone dose
dependently decreased startle-induced ultrasonic vocalizations in
morphine-dependent animals (Fig. 4, b and
e). Compared with saline-exposed controls, morphine-dependent
animals were less likely to vocalize in response to 0.1 (2 (1,
n = 20) = 10.90; p = 0.001), and 1.0 mg/kg of naltrexone (1,
n = 20) = 16.15; p < 0.0001;
Fig. 4b). Furthermore, all
doses of naltrexone decreased the duration of vocalization
(Fig. 4e); there was a
significant main effect of treatment group (F1,36 = 42.57;
p < 0.0001) and dose (F2,36 = 5.83; p
< 0.01), but no significant treatment group x dose interaction. In
fact, the highest dose of naltrexone totally suppressed vocalizing. Naltrexone
had no effect on either measure of startle-induced vocalizing in rats that
were receiving a continuous infusion of saline via osmotic pump.
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Acute Dependence. Rats pretreated with 10 mg/kg of morphine 2 h
earlier and then given an injection of saline were less likely to vocalize
[2 (2, n = 29) = 8.86; p = 0.01] than were
rats treated with saline instead of morphine
(Fig. 4c). In addition, the
duration of vocalizing by this group was significantly shorter
(F2,27 = 4.83; p = 0.01) than that of
saline-pretreated animals, as well as of animals pretreated with morphine 6 h
earlier (Fig. 4f). The two
higher doses of naltrexone tended to increase vocalizing in rats pretreated
with 10 mg/kg of morphine regardless of the pretreatment interval. Compared
with saline-pretreated controls, rats pretreated with morphine 6 h earlier
were more likely to vocalize after 1.0 mg/kg of naltrexone [2
(2, n = 29) = 5.64; p = 0.05], and those pretreated 2 h
earlier were more likely to vocalize after 0.1 mg/kg naltrexone
[2 2, n = 29) = 6.00 p = 0.01]. For duration
of vocalization, there was a significant main effect of group
(F2,54 = 5.03; p = 0.01) and a significant group
x dose interaction (F2,54 = 9.01; p <
0.0001). The intermediate dose of naltrexone (0.1 mg/kg) increased duration of
vocalization in both of the morphine-pretreated groups relative to the group
pretreated with saline, whereas 1.0 mg/kg of naltrexone had this effect only
on 6-h pretreated animals. Naltrexone did not affect either measure of
vocalization in rats that were pretreated with saline instead with morphine
(Fig. 4, c and f).
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Discussion |
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TopAbstractResultsDiscussionReferences |
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;
Vivian and Miczek, 1991;
Mansbach et al., 1992;
Pinelli and Trivulzio, 1997).
These somatic and behavioral indices of withdrawal from chronic dependence
also were reliably present in animals treated with an opioid antagonist a few
hours after pretreatment with only a single dose of morphine, presumably
reflecting the state of acute physical dependence. Like withdrawal from
chronic morphine treatment, withdrawal from acute morphine treatment
stimulated urination and defecation in morphine pretreated animals. Withdrawal
from chronic morphine administration, however, affected auditory startle and
USV in the opposite manner from withdrawal from acute dependence. Naltrexone
had no affect on any of the dependent measures in the absence of morphine.
This is the first study to compare these components of the morphine withdrawal
syndrome during spontaneous withdrawal from chronic dependence and
antagonist-precipitated withdrawal from chronic and acute dependence.
Naltrexone produced robust increases in the number of fecal boli and the
amount of urine excreted by morphine-dependent animals. Potentiation of
defecation was particularly dramatic in rats chronically dependent upon
morphine; the number of fecal boli increased by more than 80-fold. The somatic
signs associated with spontaneous withdrawal from chronic morphine, as
expected, were qualitatively similar to those of naltrexone-precipitated
withdrawal, albeit milder. Signs of spontaneous withdrawal from chronic
morphine administration usually are less intense than are the withdrawal signs
precipitated by an opioid antagonist (Wei
and Way, 1975; Jasinski,
1977).
This is the first study that provides a quantitative evaluation of
urination and defecation in rats undergoing withdrawal from acute morphine. In
accord with the withdrawal from chronic dependence, naltrexone precipitated
dose- and time-dependent increases in urination and defecation in rats
pretreated with only a single dose of morphine. Thus, as in chronic opioid
dependence, increases in urination or defecation can be used as valid somatic
signs of acute physical dependence in the rat. The magnitudes of these
increases varied from sign to sign. For example, in chronically
morphine-dependent animals, increases in number of fecal boli were
particularly dramatic in response to the two higher doses (0.1 and 1.0 mg/kg)
of naltrexone. In animals pretreated with only a single dose of morphine, the
same doses of naltrexone produced more modest increases in number of fecal.
Unlike defecation, potentiation of urination was more robust in animals
undergoing withdrawal from acute rather than chronic dependence. Thus,
defecation and urination, like other signs of antagonist-precipitated
withdrawal from morphine in rats (Blasig et
al., 1973), vary quantitatively as a function of degree of
physical dependence and dose of antagonist.
In rats chronically-dependent on morphine, naltrexone precipitated
dose-dependent decreases in startle amplitude, the two higher doses decreasing
amplitude by 50 to 60%. These data are consistent with those from a study in
which naloxone dose dependently decreased auditory startle amplitude in
chronically morphine-dependent rats
(Mansbach et al., 1992). The
magnitude of startle decrease and effectiveness of the opioid antagonists in
the two studies were similar, even though different antagonists (naltrexone
versus naloxone) and different regimens of chronic morphine exposure (osmotic
pumps versus pellets) were used.
In accord with precipitated-withdrawal data, animals undergoing spontaneous
withdrawal from chronic morphine exhibited a time-dependent decrease in
acoustic startle responses. Compared with saline-exposed controls,
morphine-dependent rats exhibited significant (30%) decreases in startle
amplitudes as early as 3 h after pump removal. Startle amplitudes continued to
decrease with time; by 24 h after pump removal, they were 50% lower than those
of saline controls. In contrast to morphine withdrawal, withdrawal from
chronic exposure to psychostimulants, benzodiazepines, or alcohol is
associated primarily with increases in startle amplitudes
(Rassnick et al., 1992;
Miczek and Vivian, 1993;
Barros and Miczek, 1996).
Naltrexone dose and time dependently increased startle amplitudes in rats
pretreated with a single dose of morphine, in contrast to our expectations and
the results of withdrawal from chronic morphine treatment. Two-hour
pretreatment with 10 mg/kg morphine resulted in the largest effect, with all
doses of naltrexone producing similar potentiation of the startle response.
These increases in startle amplitude were not merely a consequence of
naltrexone reversing a depressant effect of morphine on this response because
morphine alone did not affect startle amplitude at either pretreatment
interval. Others also have found that startle amplitude is unaffected by
morphine (Davis, 1979;
Mansbach et al., 1992).
Similar to some other aversive stimuli, such as foot- or tail-shock
(Tonoue et al., 1986;
van der Poel et al., 1989),
social defeat (Tornatzky and Miczek,
1995), or cues that predict those stimuli
(Burgdorf et al., 2001),
startle-inducing auditory stimuli induce rats to emit USV in the 20- to 28-kHz
frequency range (Kaltwasser,
1990). There appear to be significant strain differences in the
capacity of auditory stimuli to induce USV. In response to 105-db acoustic
stimuli, up to 90% of drug-naive Sprague-Dawley male rats emitted USV; male
rats of other strains (56% Long-Evans; 40% Wistar) were less likely to
vocalize (Kaltwasser, 1990).
Naltrexone dose dependently decreased the occurrence and the total duration of
vocalization in rats chronically dependent on morphine. In response to the
highest dose of naltrexone (1.0 mg/kg), 80% of saline-exposed animals
vocalized, whereas vocalization was completely suppressed in the
morphine-dependent group. In accord with precipitated-withdrawal data, as
early as 3 h after pump removal animals undergoing spontaneous withdrawal from
chronic morphine were less likely to vocalize than were saline-exposed
controls. In contrast to our findings, rats that were made chronically
dependent upon morphine by pellet implantation had an increased total duration
of USV during spontaneous withdrawal
(Vivian and Miczek, 1991). In
that study, however, vocalizations were not emitted in response to
startle-inducing auditory stimuli, and rats vocalized only when tested in
dyads; potentiation of vocalization did not occur in animals without prior
social (agonistic or sexual) experience
(Vivian and Miczek, 1991).
Furthermore, that study was done with Long-Evans rats, which vocalize less in
response to startle-inducing auditory stimuli than do Sprague-Dawley rats
(vida supra).
Naltrexone precipitated time- and dose-dependent increases in ultrasonic
vocalization in rats treated with a single dose of morphine. Therefore, like
changes in the auditory startle response, changes in vocalization during
withdrawal from acute morphine dependence were in the opposite direction of
changes during withdrawal from chronic dependence. Animals pretreated with a
single dose of morphine 2 h (but not 6 h) before testing were less likely to
vocalize and vocalized for a shorter time than did saline-pretreated controls.
In accord with these findings, morphine dose dependently attenuated USV
induced by electric foot shocks (Tonoue et
al., 1986) or by social defeat
(Vivian and Miczek, 1993).
As we hypothesized, the maximal effect of naltrexone in animals injected
with only a single dose of morphine occurred at different pretreatment
intervals across different somatic and behavioral signs. The maximal
potentiation of urination and acoustic startle by naltrexone was observed with
2-h morphine pretreatment. With longer pretreatment intervals (4–6 h),
potentiation of these signs was milder (urination) or absent (acoustic
startle). In accord with these findings, potentiation of naltrexone-induced
drinking suppression by a single dose of morphine also occurred at a 2-h
pretreatment interval (White and Holtzman,
2001). Other somatic and behavioral signs, such as increases in
defecation and USV appear to require longer (e.g., 6 h) pretreatment
times.
There is large body of evidence suggesting that the state of acute opioid
withdrawal is qualitatively similar to the state of chronic opioid withdrawal
(see Introduction). There is some evidence, however, that mechanisms
underlying chronic and acute opioid dependence are not identical and, in some
cases, can be opposite to each other. For example, morphine given acutely
inhibits adenylyl cyclase, leading to a reduction of cAMP in the cell, whereas
chronically administered morphine up-regulates this second messenger system
(Nestler and Aghajanian,
1997). The opposite changes in auditory startle response and USV
during withdrawal from acute and chronic morphine complement these findings.
Thus, it appears that the phenotypic expression and the underlying mechanisms
of withdrawal from acute dependence and chronic dependence can be similar or
different, depending upon the neural substrates that mediate a particular
component of the syndrome. Nonetheless, our results demonstrate that changes
in urination, defecation, auditory startle response amplitude, and
startle-induced USV are objectively quantifiable components of the withdrawal
syndrome from acute morphine dependence as well as of the syndrome of
withdrawal from chronic morphine dependence.
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Acknowledgements |
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Footnotes |
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ABBREVIATIONS: USV, ultrasonic vocalizations; ANOVA, analysis of variance.
1 Current address: Schizophrenia and Bipolar Disorders Research, Psychiatry
CEDD, GlaxoSmithKline plc., 3rd Ave., Harlow CM195AW, UK. 
Address correspondence to: Dr. Stephen G. Holtzman, Department of Pharmacology, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Road, Atlanta, GA 30322. E-mail: sholtzm{at}emory.edu
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