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Vol. 282, Issue 2, 561-573, 1997
Department of Psychiatry, School of Medicine, University of California of San Diego at La Jolla, California
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Abstract |
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 Top
Abstract
Introduction
Methods
Results
Discussion
References
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Stimulant psychosis and addiction are most commonly associated with repeated, high-dose binges or runs, typically preceded by a more intermittent pattern of stimulant abuse. We previously reported that rats exposed to an escalating dose-run pattern of amphetamine administration exhibited changes in their behavioral response profile that differed both qualitatively and quantitatively from the response to either acute or intermittent daily treatment. To determine the generality of these effects and characterize further the nature of the behavioral and neurochemical changes of this treatment, rats received single daily injections of amphetamine (2.5 or 4.0 mg/kg s.c.) or equimolar doses of methamphetamine, followed by multiple runs (four daily injections at 2-hr intervals) with the pretreatment dose. This treatment resulted in a unique behavioral profile, including a profound increase in the relative expression of locomotion vs. stereotypy. The markedly enhanced poststereotypy locomotor activation was characterized by repeated "burst"-like episodes of ambulation. The number of runs required for the emergence of this behavior was dose dependent and was similar for the two drugs except that with methamphetamine, there also was a marked prolongation of the poststereotypy locomotor response during run exposures. During runs, both drugs produced a decline in the caudate but not the nucleus accumbens microdialysate dopamine response, whereas only methamphetamine produced a decline in the serotonin response that was apparent in both regions. The possible relationship between these behavioral and neurochemical changes and their implications for high dose patterns of stimulant abuse are discussed.
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Introduction |
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Abstract
Introduction
Methods
Results
Discussion
References
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Stimulant
addiction and the induction of a paranoid-like psychosis are most
commonly associated with a high-dose binge or run pattern of stimulant
abuse (Angrist, 1994b
; Davis and Schlemmer, 1980; Gawin and Khalsa,
1996), generally preceded by a relatively long-term, intermittent or
escalating dose exposure (Angrist, 1987, 1994a, 1994b; Gawin, 1991;
Gawin and Khalsa, 1996). Although numerous factors affect both the
pattern and outcome of chronic stimulant intake, it has been suggested
that an escalating dose-binge pattern of stimulant exposure in
experimental animals might provide valuable insight into the behavioral
and neurochemical alterations that occur most frequently during the
course of high-dose stimulant abuse (Gawin and Khalsa, 1996; Schmidt
et al., 1985b; Segal and Kuczenski, 1997).
Observations over the past decade have suggested that repeated SDI of
relatively high AMPH doses (
4.0 mg/kg) result in an altered
behavioral profile that includes elements of both tolerance and
sensitization (Eichler et al., 1980; Rebec and Segal, 1980; Segal, 1975; Segal et al., 1995). Furthermore, with multiple
daily injections, these changes became even more pronounced, and some qualitative alterations in the response also appeared to emerge primarily in the form of decreased variation in behavioral expression (Segal et al.,
1980).2 Recently, we
characterized these effects more systematically during the course of
exposure to escalating doses followed by repeated runs (Segal and
Kuczenski, 1997). For this study, we used the highest daily binge doses
that, after an escalating dose pretreatment, did not produce any
obvious physiological or behavioral toxicity under our experimental
conditions [i.e., four daily injections of AMPH (8.0 mg/kg)
every 2 hr for 
9 days.] The results revealed the emergence of a
unique behavioral profile that was characterized by a pronounced
increase in both the magnitude and duration of locomotor activation and
a decrease in continuous stereotypy. Furthermore, when observations
extended beyond the stereotypy phase, the animals appeared to be highly
agitated after multiple runs and engaged in burst-like episodes of
locomotion, which is uncharacteristic of acute or repeated intermittent
treatments.
In parallel studies, regional neurochemical analyses, using in vivo dialysis techniques, showed that the escalating dose-run treatment produced significantly different response profiles for DA, NE and 5-HT. Most notably, we observed a progressive decrease in caudate DA and 5-HT, both within and between successive runs. In contrast, the hippocampal NE response did not decline and, in fact, tended to increase during this treatment.
The present study was designed to confirm and extend these
observations. The binge effects of lower AMPH binge doses
(i.e., 2.5 and 4.0 mg/kg) were characterized to determine
whether the unique behavioral profile that we previously observed to
occur in response to binge exposure is restricted to relatively
high-doses. Similar findings with lower doses would extend the
potential relevancy of these observations to a wider range of stimulant
abuse patterns. The 2.5 mg/kg dose of AMPH was selected because it is
the lowest dose that, under our experimental conditions, produces a
multiphasic response profile, including a distinct, continuous phase of
stereotypy. The 4.0 mg/kg dose was included both because the stereotypy
it induces is qualitatively different from the lower dose (oral as opposed to repetitive head movements) and because its pattern of
behavioral alteration with repeated SDI, unlike the lower dose, includes elements of both sensitization and tolerance (Segal et al., 1995). Furthermore, to more closely simulate stimulant abuse patterns, exposure to both the pretreatment regimen and run phases were
prolonged, and both phases were interrupted by drug-free periods
(Gawin, 1991). In addition, to more accurately characterize the
apparent qualitative changes in the locomotor response, our observational ratings were extended beyond the continuous stereotypy phase to include the entire poststereotypy response. Furthermore, to
quantify the burst-like locomotion, rates of crossover activity were
determined.
In addition to AMPH, the effects of binges with METH were also
assessed to determine the generality of the AMPH-induced alterations. Although the two drugs have similar mechanisms of action, their acute
neurochemical profiles have been shown to differ (Kuczenski et
al., 1995), particularly with respect to their relative effects on
NE and 5-HT. Therefore, comparison of the responses to AMPH and METH
might provide insight into the role of these transmitters in the
behavioral profile that emerges with repeated run exposures.
The results of these studies indicated that the unique behavioral profile associated with the high-dose AMPH binges also occurred with lower doses of AMPH and METH. In addition, a number of differences were apparent between the drugs, most notably a marked prolongation of the locomotor response to METH during runs. These findings confirm and extend our original observations and provide further insight into possible underlying mechanisms.
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Methods |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Subjects.
Male Sprague-Dawley rats, weighing 325 to 350 g at the beginning of drug treatment, were housed for 1 week before
experimental manipulation in groups of two or three in wire-mesh cages
with ad libitum access to food and water in a temperature-
and humidity-controlled room and maintained on a 14-hr light (5:00 a.m.
to 7:00 p.m.)/10-hr dark cycle. Animals were obtained from Simonsen
Labs (Gilroy, CA).
Apparatus.
Behavior was monitored in custom-designed
activity chambers (Segal and Kuczenski, 1987). Briefly, each of the
chambers was located in a sound-attenuated cabinet maintained on a
14-hr/10-hr light-dark cycle with constant temperature (20°C) and
humidity (55 ± 5%). Each chamber consisted of two compartments:
an activity/exploratory compartment (30 × 20 × 38 cm) and a
smaller "home" compartment (14 × 14 × 10 cm) in which
food and water were available ad libitum. Movements of the
animal between quadrants within the activity/exploratory compartment
(crossovers) and rearings against the wall, as well as eating and
drinking and other vertical (e.g., contact with a hanging
stimulus) and horizontal movements (e.g,. intercompartment crossings) were monitored continuously by computer. Except where otherwise noted, crossovers and intercompartment crossings exhibited similar patterns. Therefore, these measures were combined to provide an
index of all horizontal movement and are presented in the figures as
"Crossings." In our previous binge/run treatment studies,
observations of animals during the locomotor phase suggested that the
enhanced locomotion was typically displayed in the form of burst-like
patterns, that is, during these burst episodes, most crossovers between quadrants were made in rapid succession of one another
(i.e., 0-2 sec). To obtain a quantifiable index of this
increased rate of movement, we added to our data collection system the
capability of separately monitoring the crossovers made within a 0- to
2-sec ICI (i.e., 0-2 sec ICI). We found that when this
measure is presented as a percentage of total crossovers, it provides
an accurate index of the occurrence of the burst patterns. In
particular, we have found that values between 50% and 70% for this
measure are obtained when the burst pattern is clearly observable. At
lower values (i.e., <35%), activity appears to be more
uniform and recurrent bursts of locomotion are not detectable.
Therefore, this measure represents a quantitative index that in
conjunction with our observational ratings (now extended to include the
poststereotypy locomotor period of the response) provides an accurate
reflection of the changes in the pattern of locomotion that emerge with
repeated run exposures.
8 hr to assess the qualitative features of the response during
both the stereotypy and poststereotypy phases. Raters who were unaware
of the specific experimental conditions subsequently rated the
videotapes on the basis of behavior ethograms and rating procedures
established previously (Segal and Kuczenski, 1987). Stereotypy was
assessed as the percentage of the observation interval during which the
animal displayed each specific behavior. The appearance of other
atypical responses or behavior patterns, undetectable by our automated
methods, were noted by the rater after each sampling interval. Because
of the magnitude of the experiment, it was not possible to videotape
sample behaviors from all animals; therefore, sets of rats
(n = 5-7) from the most relevant groups were randomly
selected for observational ratings.
Drugs. Amphetamine sulfate (NIDA, Rockville, MD) and methamphetamine hydrochloride (Sigma Chemical Co., St. Louis, MO) were dissolved in saline and administered subcutaneously (at 2 ml/kg to avoid local irritation, which might be produced by high concentrations). Doses represent the free base. For comparative purposes, equimolar doses of AMPH and METH were selected.
General procedures. Three days before the beginning of drug treatment, animals were placed in individual experimental chambers, where they remained for the duration of the experiment. For all experiments, there were 9 to 11 rats in each group. To facilitate habituation to the chambers and procedures, animals were handled and injected with saline at least once a day. During the remainder of the day and night, animals were not disturbed, and their behavior was continuously monitored. Preliminary studies revealed no significant differences in the responses to acute drug administration of cross-sectional control groups, indicating this 3-day period to be sufficient habituation for the prolonged residence in the chambers.
After the habituation period, groups of animals received 15 SDI of 2.5 or 4.0 mg/kg AMPH or 4.42 mg/kg METH (the METH dose selected is equimolar to 4.0 mg/kg AMPH). Drug was administered in cycles of 5 successive days, followed by 2 days of saline injections. Control groups received an equivalent number of saline injections. At 24 hr after the 15th drug injection (i.e., after the third drug cycle), the experimental and control groups received a challenge injection of the same doses of drug (day 20). Groups that continued on to the binge phase of this experiment received one injection of saline on day 21 and binge injections were initiated on day 22. In our earlier studies (Segal and Kuczenski, 1997), because of the high-dose of AMPH
used in the run (8 mg/kg), we used an escalating dose pretreatment
phase to enable tolerance to develop to the physiological and
behavioral toxicity (e.g., convulsions, ataxia, persistent
high-core temperature or lack of grooming) associated with these high
doses. In the present study, we administered lower doses during binges,
and pilot studies revealed that after pretreatment with as few as 6 SDI
of the same dose, there was no evidence of behavioral toxicity in
response to subsequent run exposures. During the run phase, animals
received the same dose every 2 hr, for four injections, beginning at
8:00 a.m. and ending at 2:00 p.m. Animals were exposed to this daily
binge regimen for three cycles, with each cycle consisting of 5 consecutive binge days, followed by 2 days of saline. Two days after
the third binge cycle, animals were challenged with a single injection
of the same dose of drug.
For dialysis studies, animals were stereotaxically implanted with guide
cannulae using procedures previously described in detail (Kuczenski and
Segal, 1989). Guide cannulae extended 2.6 mm below the surface of the
skull and were aimed at the caudate-putamen (1.0 mm anterior to bregma,
2.8 mm lateral and 6.2 mm below dura) or the nucleus accumbens (2.2 mm
anterior, 1.5 mm lateral and 7.8 mm below dura). After surgery, animals
were housed individually and allowed 1 week to recover before
receiving any treatment.
Groups of animals were pretreated with 6 SDI of 4.0 mg/kg AMPH or 4.42 mg/kg METH, and at 48 hr after the last injection were exposed to a
first binge with the same drug and dose. Pilot studies indicated that
the behavioral responses to a first binge after 6 or 15 SDI were
essentially identical. On the day before the binge (3:00-4:00 p.m.),
each rat was placed in an experimental chamber, and the dialysis probes
were inserted to allow for acclimation to the test environment and
adequate equilibration of the dialysis probes. The dialysis chambers
were essentially identical to the behavioral chambers described above,
with the exceptions that the "home" compartment and hanging
stimulus were removed to prevent interferences introduced by the
dialysis methodology. Concentric microdialysis probes were constructed
of Spectra/Por hollow fiber (MW cutoff 6000, o.d. 250 µm) as
previously described (Kuczenski and Segal, 1989). The length of the
active probe membrane was 3 mm for caudate-putamen and 1.5 mm for
nucleus accumbens. Probes were perfused with artificial cerebrospinal
fluid (147 mM NaCl, 1.2 mM CaCl2, 0.9 mM
MgCl2, 4.0 mM KCl) delivered by a microinfusion pump (1.5 µl/min) via 50 cm of Micro-Line ethyl vinyl
acetate tubing connected to a fluid swivel. Dialysate was collected
through glass capillary tubing into vials containing 20 µl of 25%
methanol and 0.2 M sodium citrate, pH 3.8. Under these conditions,
dialysate DA and 5-HT and metabolites were stable throughout the
collection and analysis interval. Samples were collected outside the
experimental chamber to avoid disturbing the animal. Individual probe
recoveries were estimated by sampling a standard DA solution in
vitro. Preliminary studies indicated that individual probe
recoveries for DA and 5-HT were similar. At the end of the experiment,
each animal was perfused with formalin for histological verification of
probe placements.
Dialysate samples were collected every 30 min and were assayed for DA,
3,4-dihydroxyphenylacetic acid, homovanillic acid, 3-methoxytyramine,
5-hydroxyindoleacetic acid and 5-HT. In all experiments, solutions of
standards revealed a clean separation between 3-methoxytyramine and
5-HT. The HPLC-EC consisted of a 100 × 4.6 mm
ODS-C18 3-µm column (Regis) maintained at
40°C. Mobile phase (0.05 M citric acid, 7% methanol, 0.1 mM
Na2EDTA and 0.2 mM octane sulfonate adjusted to
pH 4.0-4.5) was delivered at 0.6 to 0.8 ml/min by a Waters model 510 pump. Amines were detected with a Waters 460 detector with a glassy
carbon electrode maintained at +0.65 V relative to a Ag/AgCl reference
electrode. Concentrations were estimated from peak areas using a Waters
Maxima 820 data station. Substances in the dialysates were corrected
for individual probe recoveries to account for this source of
variability, and although the exact relationship between dialysate
concentration and actual extracellular transmitter content is not clear
(Benveniste et al., 1989; Church et al., 1987;
Stahle et al., 1991; Wages et al., 1986), values
are presented as dialysate concentration to allow meaningful
comparisons to other data in the literature.
Analyses of AMPH and METH in brain tissue were performed on the
trifluoroacetic anhydride derivatives using a deuterated internal standard (AMPH-d3) on a HP5971A GC/MS system with fused silica capillary column coated with methylsilicone as previously described (Melega et al., 1995).
Data analysis. Behavioral and neurochemical data were statistically analyzed using repeated-measures ANOVA and t tests with Bonferroni's corrections for specific group/time comparisons.
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Results |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Behavior.
With repeated SDI of the equimolar doses of AMPH
(4.0 mg/kg) and METH (4.42 mg/kg), both drugs exhibited a progressive
pattern of response alteration (figs.
1, 2, 3) typical of this dose range (for
recent reviews, AMPH: Rebec and Segal, 1980; Segal, 1975; METH:
Machiyama, 1992; Sato et al., 1992), and by the 16th
injection, there were no significant differences apparent between the
two drugs with respect to the magnitude or temporal features of the two
primary behavioral components of the response.
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; Segal et
al., 1995; Segal and Kuczenski, 1997). Therefore, only the first
half of this period (90-180 min) exhibited significant sensitization
by the fifth injection (day 1 vs. day 5: 145 ± 19 vs. 288 ± 37, t = 4.44, P < .01). In contrast, subsequent repeated exposure to daily runs resulted
in a relatively selective and pronounced increase in the magnitude of
poststereotypy locomotor activation, as evident in the locomotor
response pattern to the fourth injection of the 15th run (fig. 1).
(Except where otherwise noted, references to characteristics of the run
response refer to the fourth injection.) The duration of the continuous
stereotypy phase did not correspondingly increase with successive runs
and, in fact, was slightly shorter (fig. 3, top). Furthermore, although the intensity of stereotypy during the first run significantly increased compared with the last SDI (i.e., mild licking and
biting became a moderate-to-strong biting response; data not shown), the characteristics of the stereotypy did not change further during subsequent runs.
The videotaped observations of the animals did reveal a profound change
in the qualitative features of the poststereotypy locomotion. As
previously noted for higher dose runs (Segal and Kuczenski, 1997),
during this phase of the response, rats exhibited an "agitated"
behavioral profile, characterized by episodes of burst-like locomotion
during which crossovers were made within rapid succession of each other
(i.e., <2-sec ICI). Examination of crossovers during this
0- to 2-sec ICI as a percentage of total crossovers revealed that
during the course of SDI, the proportion of rapid rate crossovers
significantly increased from ~10% to 12% during the acute response
to ~20% to 25% after about five daily injections (fig. 1).
Observations indicated that this change reflected a relatively uniform
increase in the rate of movement (i.e., there were no
episodic bursts of activity). No further significant change in rapid
rate crossovers occurred during the remainder of the SDI or during the
initial exposure to runs. However, by run 15, >50% of the crossover
activity occurred within the 0- to 2-sec ICI (fig. 1), and this
activity was exhibited primarily in the form of discrete bursts.
Furthermore, unlike the response to SDI, during which animals often
paused between crossovers or engaged in behaviors such as grooming or
interaction with the hanging stimulus, the behavior between run-induced
bursting appeared to be more intense and considerably less varied
(i.e., primarily nose-poking).
A similar pattern of behavioral change resulted from multiple run
exposures to the equimolar dose of METH (4.42 mg/kg). The poststereotypy phase was characterized by both the enhanced magnitude and the predominance of burst-like locomotion (fig. 2)
as well as by the decrease in the duration of the continuous stereotypy phase relative to the acute response (fig. 3, bottom).
One pronounced difference, however, was that although the magnitude of
the poststereotypy response was comparable for the two drugs, the
duration of the METH response to the fourth injection of the run was
markedly prolonged by ~2 hr (crossings, 320-380-min interval; 4.0 mg/kg AMPH: 3.4 ± 1.9; 4.42 mg/kg METH: 109.3 ± 22.6;
t = 4.66, P < .001). This difference was apparent
during the first run and persisted through the course of subsequent run
exposures. Importantly, however, there was no corresponding increase in
the duration of the stereotypy phase during the METH runs (fig. 3).
The unique run-induced behavioral profile persisted in response to a
challenge injection of each drug for 3 days after the last run
exposure. Thus, the poststereotypy response to a challenge injection
was substantially augmented compared with the response to the 16th SDI,
for both AMPH (fig. 4, top) and METH
(fig. 4, bottom). Likewise, qualitative differences in the
poststereotypy locomotion, including the burst pattern of activity,
also distinguished these responses from the effect produced by SDI. In
addition, although the challenge response to AMPH was not significantly longer than during SDI, a significant prolongation was detected for
METH (250-280 min: F = 4.75; P < .01). In contrast,
the stereotypy responses to the 16th SDI and the challenge injection in
run-exposed animals were comparable in duration for both drugs (fig.
4).
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Neurochemistry.
We had previously shown that high-dose (8.0 mg/kg AMPH) multiple run exposure resulted in a depletion of
caudate-putamen DA of ~25% (Segal and Kuczenski, 1997). To assess
further the possible role of DA depletion in the run-induced behavioral
response, we examined caudate-putamen DA 3 days after 12 daily AMPH and
METH runs. Both higher-dose groups exhibited significant DA depletions of ~15% [control, 108 ± 4 pmol/g; 4.0 mg/kg AMPH, 86 ± 4 pmol/g (t = 3.73, P < .01); 4.42 mg/kg METH,
84 ± 4 pmol/g (t = 4.08, P < .01), whereas
neither lower-dose group exhibited a significant change in DA levels
(2.5 mg/kg AMPH, 97 ± 4 pmol/g; 2.76 METH, 100 ± 7 pmol/g).
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Discussion |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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We previously found (Segal and Kuczenski, 1997) that a unique
behavioral and neurochemical profile emerged in response to a stimulant
administration regimen that attempted to simulate the drug abuse
regimen most commonly associated with addiction and the induction of
various forms of psychopathology. The results of the present series of
studies support and extend these observations to repeated
drug-on/drug-off exposure cycles of lower doses of both AMPH and METH
and further characterize the behavioral and neurochemical changes
associated with this treatment exposure.
During the repeated SDI phase preceding the runs, the response to both
AMPH and METH exhibited patterns of change characteristic of the
respective doses, including elements of sensitization as previously
described (see Results and above for references). The locomotor
patterns did not display further significant change after ~5 to 10 SDI. These results, along with our previous findings (Segal and
Kuczenski, 1997), indicate that with SDI of doses ranging from 2.5 to
8.0 mg/kg, response alterations develop relatively fast and then remain
stable with further SDI.
However, during subsequent exposure to runs, further alterations in the response developed, with some changes apparent during the first run. For both doses of AMPH, although the temporal pattern of the locomotor response did not change from the last SDI, the stereotypy increased in intensity and exhibited a more rapid onset for the lower dose (2.5 mg/kg). The response to the first run of METH was similar to the equimolar dose of AMPH, except that the poststereotypy locomotor activity was markedly prolonged. This occurred without any corresponding increase in the duration of the stereotypy phase, as would be expected in response to increasing doses. The difference in response duration between AMPH and METH persisted throughout the run phase of the study. As with AMPH, however, the qualitative characteristics of the locomotor response during the first METH run did not differ significantly from its features during SDI.
For both drugs, with multiple runs, the magnitude and qualitative
characteristics of the poststereotypy locomotor activation were
significantly altered. In contrast, most features of the continuous
stereotypy phase either did not change or exhibited tolerance. Although
the rate of development of this unique behavioral profile appeared to
be dose dependent, the response ultimately achieved characteristics
similar to the profile we previously observed with a considerably
higher dose of AMPH (Segal and Kuczenski, 1997). With successive runs,
the most prominent change in the poststereotypy locomotion was an
increase in the magnitude of the locomotor response. In the present
study, observational ratings that were now extended beyond the
continuous stereotypy phase permitted a more complete characterization
of this behavior. These observations indicated that there was a
corresponding change in the qualitative features of the enhanced
locomotor response. Rather than the more uniform expression of
crossings associated with acute and single daily injections, after
multiple runs most of the locomotion occurred in the form of episodic
bursts of activity. Furthermore, although the locomotor response to SDI
often included rearing, grooming and even pauses, rats that received
successive run exposures appeared to be in a continuous state of
extreme agitation (i.e., even when not ambulating, their
heads and/or limbs were constantly moving). Between bursts, most
animals, irrespective of dose or drug, appeared to engage primarily in
nose-poking behavior, although at the higher doses, occasional biting
and licking were apparent, especially during the transition between the
stereotypy and locomotor phases of the response.
The burst-like behavior was also reflected quantitatively, in the increased proportion of rapid rate crossovers (i.e., crossovers within the 0-2-sec ICI). Our results indicated that within the 2.5 to 8.0 mg/kg AMPH dose range, only the pattern of drug administration altered this rate measure. That is, after acute drug injection, the proportion of crossovers within the 0- to 2-sec ICI was typically 10% to 15% of total crossovers, independent of dose and, therefore, of the total activity during the poststereotypy period. This rate measure increased to ~25% to 30% after SDI, with dose only influencing the number of injections required to achieve this level. During the first several run exposures, there were no further changes in the rate measures. However, corresponding to the appearance of bursting (between 5 and 15 runs, depending on dose), rapid rate crossovers increased to levels of ~50% to 60% of total crossovers for all doses of both AMPH and METH tested. These results are consistent with observations that after multiple runs, most of the locomotor activity occurred in a bursting pattern, with varying intervals between successive bursts. This behavioral change was apparent throughout the poststereotypy locomotor period produced by both AMPH and METH runs, indicating that the prolongation of the METH response likely involves mechanisms different from those responsible for the burst pattern of locomotion (see below).
It is important to emphasize that this unique behavioral pattern
requires multiple run exposures before it emerges; it does not occur in
response to acute or intermittent administration of any dose tested
(Segal, 1975; Segal and Kuczenski, 1997; present results), nor is it
displayed during the initial runs. Therefore, cumulative drug dose
achieved during the run is not a critical factor. Furthermore, although
these changes result from frequent episodes of multiple daily drug
injections, subsequent expression of the altered response does not
require run exposure. This is most convincingly demonstrated in the
response to challenge with an acute dose 3 days after the last run. Our
previous findings indicated that these effects persisted for 3 weeks
after the last 8.0 mg/kg AMPH run (Segal and Kuczenski, 1997). These
observations suggest that the run-induced changes in behavior reflect a
fundamental alteration in responsiveness to the drug.
The DA and 5-HT response profiles to successive injections of AMPH and
METH exhibited transmitter- and region-specific differences. We
previously reported that the caudate-putamen and nucleus accumbens DA
responses within the first 8 mg/kg AMPH run declined to near 50% of
the initial response (Segal and Kuczenski, 1997). A reduction in the
caudate-putamen DA response also occurred during the first run with 4.0 mg/kg AMPH or 4.42 mg/kg METH, although the decline was substantially
less (~20% of the initial peak response) than with the higher dose
[peak DA (nM) first injection vs. fourth injection: AMPH,
290 ± 42 vs. 220 ± 30; t = 2.67, P < .05; METH, 361 ± 38 vs. 283 ± 29;
t = 3.03, P < .01]. In contrast to the caudate-putamen, the nucleus accumbens DA responses exhibited no
significant decrement. These results are consistent with a dose-dependent tolerance/tachyphylaxis in the DA response within runs
and a region-dependent sensitivity to this treatment. In this regard,
we previously suggested that a shift in the balance between
mesostriatal vs. mesolimbic DA activation may contribute to
the progressive change in the relationship between the expression of
stereotypy and locomotion with multiple runs, and the present results
are supportive of this hypothesis.
The progressive decline in caudate-putamen DA responsivity within a run
may be related to the depletion of tissue levels of DA associated with
multiple high-dose runs (Clausing et al., 1995; Schmidt
et al., 1985a, 1985b; Segal and Kuczenski, 1997) because some evidence suggests that caudate-putamen DA is more sensitive to
depletion than is nucleus accumbens (Castañeda et al.,
1990; Ellison et al., 1978; Ellison and Eison, 1983; Paulson
and Robinson, 1995). If the decline in responsivity reflects DA
depletion, the absence of a significant decline in nucleus accumbens DA
may reflect this difference in sensitivity to depletion. However, it is
important to note that DA depletion probably does not play a critical
role in the behavioral profile associated with multiple runs because no
significant depletion was observed after multiple runs with the lowest
doses of AMPH and METH tested. Furthermore, the degree of depletion
that we did observe in the caudate-putamen is considerably less than
has been previously reported during runs with similar doses of METH
(O'Dell et al., 1991; Weihmuller et al., 1992)
indicating, as has previously been suggested (Schmidt et
al., 1985a, 1985b; Segal and Kuczenski, 1997; Stephans and
Yamamoto, 1996), that relatively low-dose METH pretreatment affords
significant protection against the depleting effects of subsequent
high-dose METH administration. In addition to protection against DA
depletion, as well as against the toxic sympathomimetic effects
(Angrist, 1994b; Fischman et al., 1985; Fischman and
Schuster, 1974; Schmidt et al., 1985b) of high-dose
stimulant administration, it is likely that lower-dose pretreatment
regimens alter the response of other neurochemical systems to
subsequent high-dose stimulant runs. For example, lower-dose METH
pretreatment substantially attenuated the rise in caudate-putamen extracellular glutamate associated with a binge pattern of high-dose METH administration, without altering the caudate-putamen DA response (Stephans and Yamamoto, 1996). These differential changes in
responsivity resulting from pretreatment exposure may have important
implications for the behavioral responses associated with binge
stimulant administration and their potential relevance to stimulant
abuse.
In contrast to the DA responses, the extracellular 5-HT response was
differentially affected by run exposures to the two drugs (i.e., both brain regions exhibited a decrease in 5-HT
response relative to the first injection with successive injections of METH, but neither region significantly changed with AMPH). Because our
previous results (Segal and Kuczenski, 1997) revealed a pronounced tolerance/tachyphylaxis of the 5-HT response in both brain regions with
8.0 mg/kg AMPH runs, the progressive decrement in the 5-HT response
appears to be dose dependent. Furthermore, the decrement in response
with multiple injections of METH but not the equimolar dose of AMPH may
reflect a depletion of a relevant pool of this transmitter because METH
appears to be more potent than AMPH in releasing 5-HT (fig. 8; see also
Kuczenski et al., 1995).
The relative AMPH- and METH-induced changes in extracellular 5-HT are
not consistent with our previous suggestions that stimulant-induced alterations in 5-HT play an important role, especially in the qualitative features of the stereotypy response (Kuczenski and Segal,
1989; Segal, 1976, 1977). That is, the drug-related differences in 5-HT
are most pronounced after the initial injection of the run and diminish
with subsequent drug injections, whereas the stereotypy profiles become
most distinguishable in terms of qualitative features with successive
injections (i.e., METH- but not AMPH-treated animals
developed self-directed oral behaviors; data not shown). Therefore, it
is presently unclear how the differences in the 5-HT response during
AMPH and METH runs contribute to their behavioral profiles.
Examination of the behavioral responses to AMPH and METH reveals
quantitative features, only some of which appear to parallel the
relative effects of these drugs on extracellular DA. For example, the
marked prolongation of the poststereotypy locomotor phase that occurs
only after METH run exposure is paralleled by higher extracellular DA
levels in both caudate-putamen and nucleus accumbens for the METH
compared with the AMPH responses. It is likely that pharmacokinetic
factors contribute to the prolongation of both the DA and the
behavioral responses because brain levels of METH and its active
metabolite, AMPH, were each ~2-fold higher than observed in response
to either an acute or a seventh SDI METH treatment (table 1). In
contrast, during AMPH run exposure, AMPH levels increased by only
~20% (Segal and Kuczenski, 1997). One possible explanation for this
difference is that METH metabolism and elimination may change during
the run because the METH levels after run exposure are substantially
higher than would be predicted (Benet et al., 1990) based on
the half-life of METH (Melega et al., 1995).
The role that differences in drug metabolism might play in the abuse of these drugs is difficult to extrapolate from the present studies. However, it is not likely that such pharmacokinetic changes are involved in the behavioral response that emerges with multiple daily runs because there were no further changes in METH metabolism after the first run (table 1), whereas the most profound changes in the behavioral response were not apparent until repeated run exposure.
In conclusion, the results of this study further support our hypothesis that relatively gradual exposure to stimulants (i.e., escalating doses or SDI, followed by repeated runs) results in a unique behavioral and neurochemical profile that cannot be produced by any acute or SDI dose tested. One of the most obvious features of the altered behavioral profile is the profound change in the relationship between the continuous stereotypy and locomotor components of the response. Because this behavior also appears to be qualitatively different from the enhanced poststereotypy locomotor response that develops after SDI, the further increase in the magnitude of the locomotion resulting from run exposures may not simply reflect a further augmented responsiveness (i.e., greater sensitization). It is also important to note that the altered behavioral profile can be elicited in response to a subsequent single injection challenge, indicating that these changes are not simply a function of cumulative dose.
On the basis of these results, we suggest that if stereotypy represents
a coping mechanism, as previously proposed (Mason, 1991; Mittleman
et al., 1991), the multiple run-induced behavioral profile
may reflect a state of stress or hyperarousal, relatively unattenuated
by the expression of response perseveration as a "calming"
mechanism (Davis and Schlemmer, 1980; Rylander, 1969, 1980; Schiorring,
1977). Therefore, rather than reflecting an enhanced responsiveness of
the neurochemical systems underlying the locomotor component of the
response, it is conceivable that mechanisms mediating response
perseveration are impaired as a function of multiple run exposures.
Although further research will be required to determine the likely
complex neurochemical and behavioral processes involved, our results
support and extend our previous findings (Segal and Kuczenski, 1997)
and provide additional evidence for the potential significance of these
profound alterations for the effects of high-dose stimulant abuse.
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Acknowledgments |
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The authors wish to thank Drs. Arthur K. Cho and William P. Melega for analysis of tissue levels of AMPH and METH. The authors also wish to express their appreciation to Brad Hirakawa and S. McCunney for assistance in executing the experimental protocol, Molly Roznoski and Joseph Higgins for their skills in performing the dialysis experiments, Julie Segal and Stefan Grafstein for their expert rating of videotapes and Pat Hermann for her efforts in preparing the manuscript.
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Footnotes |
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Accepted for publication April 15, 1997.
Received for publication November 20, 1996.
1 This work was supported in part by United States Public Health Service Grants DA-01568 and DA-04157. D.S.S. is the recipient of United States Public Health Service NIMH Career Scientist Award MH-70183.
2 D. S. Segal and R. Kuczenski, unpublished observations.
Send reprint requests to: Dr. David S. Segal, Psychiatry Department (0603), UCSD School of Medicine, La Jolla, CA 92093.
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Abbreviations |
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AMPH, amphetamine; METH, methamphetamine; DA, dopamine; NE, norepinephrine; 5-HT, serotonin; SDI, single daily injection(s); ICI, intercrossover interval; ANOVA, analysis of variance; AUC, area under the curve.
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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