Neuroscience-Net - Pharmacology Section Article #10008

<head><title>Neuroscience-Net-Pharmacology Section Article #00008</title></head> <body bgcolor="ffffff"> <a name="top"> <table border=3> <tr><td><IMG SRC="../images/h_phar1.gif" ALT="Pharmacology"></td> </table><p> <table> <tr><td valign="top" align=left>© Neuroscience-Net<br> Volume 1, Article #10008 <td width=500 align=right>Received July 6, 1996<br> Accepted for Publication September 18, 1996<br> Published October 2, 1996<br> </table><br> <A HREF="../../pharmogy.html"><IMG SRC="../images/b_back.gif" ALT="Back" Border=0></A> <center> <font size=+1><b>CHANGES IN EXCITATORY AMINO ACID TRANSMISSION IN THE NUCLEUS ACCUMBENS ASSOCIATED WITH BEHAVIORAL SENSITIZATION TO COCAINE DURING EARLY WITHDRAWAL </b></font><p> </center> R. Christopher Pierce, Patricia Duffy and Peter W. Kalivas<BR> Alcohol and Drug Abuse Program<BR> Washington State University<BR> Pullman, WA 99164-6520<BR> <BR><BR> <CENTER><STRONG>Key Words:</STRONG> Glutamate, Cocaine, Dopamine, Nucleus Accumbens, Sensitization</CENTER><P> <HR> <ul> <li><a href="index.html" TARGET="_parent">Abstract</a><br> <li><a href="#intro">Introduction</a><br> <li><a href="#materials">Materials and Methods</a><br> <li><a href="#results">Results</a><br> <li><a href="#discuss">Discussion</a><br> <li><a href="#acknow">Acknowledgments</a><br> <li><a href="#legends">Legends</a><br> <li><a href="#references">References</a><br> </ul> <HR> <A NAME="intro"> <font size=+2><b>INTRODUCTION</b></font><br><br> Repeated administration of amphetamine-like psychostimulants results in a progressive and enduring augmentation in behavioral hyperactivity known as behavioral sensitization (Segal and Schuckit, 1983; Robinson and Becker, 1986; Post and Weiss, 1988; Kalivas and Stewart, 1991). A growing body of evidence indicates that the nucleus accumbens plays an important role in the expression of psychostimulant-induced behavioral sensitization (Robinson and Becker, 1986; Kalivas and Stewart, 1991; Nestler, 1992). After extended withdrawal periods, many laboratories report an increase in the capacity of amphetamine or cocaine to augment extracellular dopamine in the nucleus accumbens following repeated treatment with psychostimulants (Robinson et al., 1988; Akimoto et al., 1989; Pettit et al., 1990; Kalivas and Duffy, 1990, 1993; Patrick et al., 1991; Wolf et al., 1993; Pierce and Kalivas, 1995). While cocaine-induced behavioral sensitization becomes more robust following several weeks of withdrawal from repeated drug administration, augmented behavior is present during the first few days after the cessation of cocaine treatment (Hitzemann et al., 1977; Kolta et al., 1985; Paulson et al., 1991; Kalivas and Duffy, 1993; Hooks et al., 1994). In contrast, the augmentation in the releasability of dopamine in the nucleus accumbens is not consistently observed during early withdrawal (Segal and Kuczenski, 1992a,b; Kalivas and Duffy, 1993; Pierce and Kalivas, 1995). These latter results suggest that the behavioral augmentation produced by repeated cocaine early in withdrawal may be independent of augmented dopamine release in the nucleus accumbens. <BR><BR> Based upon the finding that systemic administration of a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists blocked the expression of behavioral sensitization to psychostimulants, Karler et al. (1991, 1994) proposed that alterations in excitatory amino acid (EAA) transmission may also contribute to the long-term expression of behavioral sensitization. This postulate was supported by a recent study by Pierce et al. (1996) who found that at two to three weeks after discontinuing daily cocaine treatments, administration of the AMPA antagonist, CNQX, into the nucleus accumbens inhibits the expression of behavioral sensitization. Moreover, behavioral stimulation following intra-accumbens infusion of AMPA was augmented among animals that develop behavioral sensitization to daily cocaine. Finally, using microdialysis Pierce et al. (1996) demonstrated that a systemic cocaine injection increased extracellular glutamate levels in the nucleus accumbens only in rats that had developed behavioral sensitization to repeated cocaine. <BR><BR> Considering the fact that augmented dopamine transmission is more consistently measured a few weeks after discontinuing daily cocaine treatments than after only a few days (Kalivas and Duffy, 1993; Wolf et al., 1993; Heidbreder et al., 1996), the present study was designed to determine if the changes in glutamate transmission found by Pierce et al. (1996) at two to three wks after discontinuing cocaine were also present during the first three days. All rats were pretreated with daily injections of cocaine or saline, and three experiments were conducted at one to three days after discontinuing the daily injections. 1) AMPA was microinjected into the nucleus accumbens and motor activity was monitored. 2) Since the response to AMPA was augmented in rats developing behavioral sensitization to cocaine, it was determined if the increased response to AMPA was associated with an enhanced capacity for AMPA to elevate extracellular dopamine in the nucleus accumbens. 3) The effect of a cocaine challenge on extracellular glutamate in the nucleus accumbens was examined in cocaine- and saline-pretreated rats. <BR><BR> <HR> <A NAME="materials"> <font size=+2><b>MATERIALS AND METHODS</b></font><br><br> <B><U>Animal housing and surgery:</B></U> Male Sprague-Dawley rats (Simmonsen Laboratories, Gilroy, CA) were individually housed with food and water available ad libitum. A 12/12 hr light/dark cycle was used with the lights on at 7:00 a.m. All cocaine injections, behavioral testing and microdialysis were performed during the light cycle. <BR><BR> Prior to surgery, rats weighing 250 to 350 g were anesthetized with Equithesin (3.0 ml/kg) and mounted in a stereotaxic apparatus. For microdialysis and behavioral experiments, cannulae (12 mm, 20 gauge stainless steel for dialysis; 14 mm, 26 gauge for behavior) were implanted bilaterally 2 mm dorsal to the nucleus accumbens core or shell (9.0 mm A/P; 0.8-1.8 mm M/L; 0.0 mm relative to the interaural line, Pellegrino et al., 1979) and cemented in place by affixing dental acrylic to three stainless steel screws tapped into the skull. <BR><BR> <B><U>Repeated cocaine or saline treatment:</B></U> In both behavior and microdialysis experiments, subjects were assigned to either the cocaine or saline treatment groups following at least 7 days of post-operative recovery. The day prior to the start of the experiment, all animals were habituated to the photocell boxes (Omnitech Electronics, Columbus, OH) for 3 hours. On the first treatment day, animals were habituated to the photocell boxes for 1 hour. Following habituation, the rats were injected with either cocaine (15 mg/kg, ip; donated by the National Institute of Drug Abuse) or saline (1.0 ml/kg, ip) and behavior was monitored for 2 hr. On days 2 through 6, cocaine rats received daily injections of cocaine (30 mg/kg, ip) while control animals received saline in their home cages. On the seventh day, all animals were again habituated to the photocell boxes for 1 hr followed by the administration of cocaine (15 mg/kg, ip) or saline and behavior was monitored for 2 hr post-injection. Thus, half of the subjects received 7 administrations of cocaine while the other half were injected daily with saline. <BR><BR> <B><U>AMPA microinjection and behavior:</B></U> One day after discontinuing daily saline or cocaine, rats were microinjected with saline into the nucleus accumbens. During the next two days, rats were given one of two doses of AMPA (0.03 or 0.1 痢; RBI, Natick, MA) in random order and motor activity was monitored for 2 hr. Following a one hr adaptation to the photocell cell apparatus (Omnitech Electronics), the obturators were removed from the microinjection guide cannulae and were replaced by an injection needle (33 ga stainless steel) which extended 2 mm below the tip of the guide cannulae into the nucleus accumbens. Bilateral infusions were made over 60 sec in a volume of 0.5 microliter/side. Twenty sec later the injector was removed and the rat was returned to the photocell cage. <BR><BR> <B><U>Microdialysis and measurement of extracellular glutamate and dopamine:</B></U> The dialysis probes were constructed as described by Robinson and Wishaw (1988), with approximately 2.0 mm of active dialysis membrane exposed at the tip. The probes were inserted through the guide cannulae into the nucleus accumbens the night prior to the experiment. The next day, dialysis buffer (5 mM KCl, 120 mM NaCl, 1.4 mM CaCl<sub>2</sub>, 1.2 mM MgCl<sub>2</sub>, 5.0 mM glucose, plus 0.2 mM phosphate-buffered saline to give a pH value of 7.4 and a final sodium concentration of 120.7 mM) was advanced through the probe at a rate of 1.9 microliter/min via a syringe pump (Harvard Instruments, Boston, MA) for 1 hr. Baseline samples were collected for 100 min prior to beginning drug treatment. <BR><BR> In order to assess the effects of AMPA on extracellular dopamine levels in the nucleus accumbens, 24 to 48 hrs after discontinuing repeated cocaine or saline injections four concentrations of AMPA (0.1, 1.0, 10.0 and 100.0 micromolar) were administered sequentially in ascending order through the dialysis probe for 100 min each. Both prior to and during AMPA administration, samples were taken every 20 min. <BR><BR> For the measurement of extracellular dopamine, samples were collected into microfuge tubes containing 20 痞 of mobile phase (0.1 M citric acid, 75mM Na<sub>2</sub>HPO<sub>4</sub>, 1.5 mM heptane sulfonic acid, 0.1 mM EDTA, 15 % methanol, v/v, pH=4.2) plus 2.0 pmol of dihydroxybenzylamine as the internal standard. Following collection, all samples were frozen at -80<sup>o</sup> C until analyzed. The samples were subsequently thawed and placed in an autosampler (Gilson Medical supplies, Middleton, WI) connected to an HPLC system with electrochemical detection. The dopamine was separated using a 25 cm C-18 reversed phase column (Bioanalytical Systems, West LaFayette, IN) and oxidized/reduced using coulometric detection (ESA Inc., Bedford, MA). Three electrodes were used: a preinjection port guard cell (+0.4 V) to oxidize the mobile phase, an oxidation analytical electrode (+0.3 V) and a reduction analytical electrode (-0.14 V). Peaks were recorded on a chart recorder and compared to an external standard curve (10-1000 fmol). <BR><BR> In a separate group of animals, microdialysis was conducted in the nucleus accumbens to measure the levels of extracellular glutamate on the first day after discontinuing repeated cocaine or saline. After collecting 100 min of baseline data, rats were challenged with saline (1.0 ml/kg, ip) followed 80 min later by cocaine (15 mg/kg, ip). Samples were collected for 180 min (once every 20 min) after cocaine administration before terminating the experiment. <BR><BR> For the measurement of extracellular glutamate, samples were collected into 10 痞 of mobile phase (0.1 M Na<sub>2</sub>HPO<sub>4</sub>, 0.13 mM Na<sub>2</sub>EDTA, 25% methanol, v/v, pH = 6.2) plus 2.0 pmol homoserine as an internal standard. Precolumn derivatization with o-pthaldehyde was performed by an autosampler, and the chromatography was conducted as described above for dopamine, except a 10 cm column was used and the electrode currents were set as follows: preinjection port guard cell=+0.7 V; reduction analytical electrode=-0.25 V; oxidation analytical electrode=+0.65 V. <BR><BR> <B><U>Histology:</B></U> Following dialysis and behavioral experiments, rats were given an overdose of pentobarbital (>100 mg/kg, ip) and perfused intracardially with phosphate-buffered saline followed by 10% formalin. The brain was removed and stored in 10% formalin for at least 1 week. The brains were blocked and coronal sections (100 痠) were taken at the level of the nucleus accumbens with a vibratome. The sections were mounted on gelatin-coated slides and stained with Cresyl violet. Probe and cannula placements were determined according to the atlas of Paxinos and Watson (1986) by an individual unaware of the rats' behavioral or neurochemical response. <BR><BR> <B><U>Data analysis:</B></U> Mixed factors analyses of variance (ANOVAs) were performed on the totals (horizontal counts summed across 120 min) of the behavioral data (see figure legends for specific statistical information). For time course analysis of behavioral data, a two-way ANOVA with repeated measures over time was conducted. For neurochemistry, the dopamine or glutamate content was expressed as both the raw values (pmol/sample) and percent change from baseline. Based on the average of the last two dialysis samples at each AMPA concentration, dopamine data were evaluated using a two-way ANOVA with repeated measures over dose. For the glutamate data, a two-way ANOVA with repeated measures over time was performed. In all cases, pairwise comparisons were made using a least significant difference test for repeated measures ANOVAs. <BR><BR> Based upon the study by Pierce et al. (1996), rats receiving daily cocaine were divided into two groups. Animals in the sensitized group demonstrated a >20% increase in horizontal photocell counts on the last day of daily cocaine administration compared with the first day. The remaining animals were grouped as nonsensitized. Separating rats into these two groups was found by Pierce et al. (1996) to clearly distinguish animals in which repeated cocaine elevated glutamate transmission in the nucleus accumbens at three weeks of withdrawal. The majority of rats fall into the sensitized group, and in experiments 2 and 3 the nonsensitized group included less than 3 animals (see Table 1). Therefore, for experiments 2 and 3 the data from the nonsensitized group were excluded from the statistical analyses. <BR><br> <HR> <A NAME="results"> <font size=+2><b>RESULTS</b></font><br><br> <B><U>Sensitization to repeated cocaine:</B></U> Table 1 shows the behavioral response to cocaine or saline on the first and last days of daily administration. As described by Pierce et al. (1996), the daily cocaine treated group was divided into a sensitized and a nonsensitized subgroup. The criterion used to define a rat as sensitized was an increase of at least a 20% in the number of horizontal photocell counts on the last cocaine injection (day 7) compared to the first (day 1). Between 63 and 100% of all rats developed behavioral sensitization by this criterion. The response to repeated saline, in contrast, was not significantly augmented after 7 daily injections. <BR><BR> <B><U>Experiment 1: Microinjection of AMPA.</B></U> <A HREF="#figure1">Figure 1A</A> shows that following microinjection of 0.1 痢 AMPA into the nucleus accumbens there was a significant increase in horizontal photocell counts in all three groups of animals. However, the sensitized animals showed an enhanced behavioral response to AMPA relative to the saline and nonsensitized groups. There was no statistical difference in the response to AMPA between the nonsensitized and saline pretreated rats. This effect is more readily seen in the time course of the behavioral response to AMPA shown in <A HREF="#figure1">figure 1B</A>. The behavioral response to 0.1 痢 AMPA was significantly greater in sensitized, relative to saline pretreated and nonsensitized animals at the 45 and 60 min time points after microinjection. At no time point did the behavioral response of the nonsensitized animals differ from the saline group. <BR><BR> <B><U>Experiment 2: AMPA-induced dopamine release.</B></U> In this experiment only two rats failed to meet the criterion for the development of sensitization (<A HREF="#table1">see Table 1</A>). Thus, the nonsensitized animals were excluded from the data analysis. The perfusion of AMPA through a microdialysis probe in the nucleus accumbens produced a dose-dependent increase in extracellular dopamine in the nucleus accumbens of saline and sensitized animals (<A HREF="#figure2">see figure 2</A>). However, there were no significant differences in the dopamine responses between the sensitized and saline groups when the data were expressed as either percent baseline (<A HREF="#figure2">figure 2A</A>) or as the raw data (<A HREF="#figure2">figure 2B</A>). The baseline values for dopamine did not differ significantly among the treatment groups (F(1,14)=0.037, p=0.8498; saline=0.071�0.02 pmol/sample, sensitized=0.076�0.018). <BR><BR> <B><U>Experiment 3: Extracellular glutamate after a cocaine challenge.</B></U> In this experiment all of the animals assigned to the repeated cocaine group met the criterion for sensitization. <A HREF="#figure3">Figure 3A</A> shows that although the average basal levels of glutamate did not differ significantly between the treatment groups, the levels in the sensitized animals tended to be lower than the saline pretreated rats (t(12)=2.09, p=0.058; saline=32.6�7.3 pmol/sample, sensitized=13.4�3.9). Following the challenge injections of saline and cocaine (15 mg/kg, ip), the analysis of the absolute levels of extracellular glutamate revealed a significant main effect of time. Likewise, when the data were examined as percent change from baseline (<A HREF="#figure3">figure 3B</A>) there was significant effect over time However, in neither data format was there a significant difference between the saline and sensitized groups. <BR><BR> <B><U>Histology:</B></U> <A HREF="#figure4">Figure 4</A> (top panel) is a micrograph showing the location of the ventral tip of the microinjection cannulae in the nucleus accumbens for the behavioral experiments from experiment 1. For this experiment, the cannulae were distributed between the core and shell regions of the nucleus accumbens. <A HREF="#figure4">Figure 4</A> (bottom panel) depicts a micrograph of dialysis probe placements in the nucleus accumbens core from an animal used in experiment 3 (all placements from experiments 2 and 3 were similarly located in the core). In some cases the microdialysis probe placement included portions of the ventral nucleus accumbens shell or the neostriatum. Note in both micrographs that there was no neurotoxicity produced by AMPA beyond the mechanical tissue destruction resulting from insertion of the injectors or dialysis probes. <BR><br> <HR> <A NAME="discuss"> <font size=+2><b>DISCUSSION </b></font><br><br> The expression of behavioral sensitization to psychostimulants is associated with enhanced extracellular dopamine levels in the nucleus accumbens beginning at least one week after discontinuing repeated psychostimulant administration (Kalivas and Duffy, 1993; Wolf et al., 1993; Hooks et al., 1994; Heidebreder et al., 1996). However, while the sensitized behavioral response is observed early in the withdrawal period, the enhanced releasability of dopamine in the nucleus accumbens is reduced or absent during the first week after discontinuing repeated drug treatments (Hurd et al., 1989; Kalivas and Stewart, 1991; Segal and Kuczenski, 1992a,b; Wolf et al., 1993; Kalivas and Duffy, 1993; Hooks et al., 1994; Pierce and Kalivas, 1995). Similar to extracellular dopamine in the nucleus accumbens, Pierce et al. (1996) recently reported that three weeks after discontinuing daily cocaine, glutamate transmission is enhanced in the nucleus accumbens of rats developing behavioral sensitization and the present report indicates that this augmentation in extracellular glutamate in cocaine-sensitized rats is absent during the first three days after discontinuing daily cocaine. <BR><BR> Pierce et al. (1996) also reported that three weeks after discontinuing daily cocaine injections, the behavioral response elicited by microinjection of AMPA into the nucleus accumbens was augmented. In contrast to the augmented levels of extracellular glutamate and dopamine, the increased behavioral responsiveness to AMPA was present at two and three days after discontinuing repeated cocaine, albeit somewhat reduced compared with three weeks of withdrawal (see Pierce et al., 1996). The presence of an augmented behavioral response to AMPA may result, in part, from the lower levels of basal glutamate measured at both 1 and 21 days after discontinuing daily cocaine injections. Thus, lower levels of synaptic glutamate may result in an upregulation of AMPA receptor density or transduction. The findings by Fitzgerald et al. (1996) that neither the GluR1 nor GluR2 subunits are altered in the nucleus accumbens after repeated cocaine argues that receptor density is not altered even though GluR2 mRNA is increased after repeated cocaine (personal communication, Behnam Ghasemzadeh, Ph.D., Washington State University). Likewise, the finding of White et al. (1995) that the excitatory response to iontophoretic glutamate in the nucleus accumbens is blunted after repeated cocaine treatment is difficult to reconcile with upregulated AMPA receptor density or transduction. <BR><BR> Similar to Pierce et al. (1996), we found that the augmented behavioral response to AMPA occurred only in rats that developed behavioral sensitization to criterion (>20% increase in photocell counts on the last, compared to the first day of daily cocaine administration). The predictability of this criterion supports a causal association between the development of behaivoral sensitization and enhanced responsiveness to AMPA receptor stimulation in the nucleus accumbens. However, regression analysis between the percent difference in photocell counts on the first and last cocaine injection and total horizontal photocell counts following AMPA (0.1 nmol) microinjection into the nucleus accumbens did not reveal a significant correlation (r= .207; p= 0.476). While the lack of correlation indicates that augmented AMPA receptor responsiveness is not the primary variable regulating the expression of behavioral sensitization, it may still contribute to the augmented behavioral profile. <BR><BR> The fact that AMPA receptor responsivenes was not enhanced in rats that received daily cocaine and did not develop behavioral sensitization (i.e. the nonsensitized group) demonstrates that the effect on AMPA receptors does not arise purely as a pharmacological effect of cocaine binding to the dopamine transporter. Rather, the augmented AMPA response is regulated by a combination of cocaine administration and other factors. One factor making a contribution to the expression of behavioral sensitization is the development of learned associations with cocaine administration (Tilson and Rech, 1973; Post et al., 1981; Stewart and Vezina, 1988; Weiss et al., 1989; Pert et al., 1992; Badiani et al., 1995). It is possible that the enhanced responsiveness of AMPA receptor stimulation may arise in combination with learned associations the animal makes with the daily cocaine injection. Consistent with this hypothesis, rats repeatedly administered cocaine in a distinct environment displayed a greater response to AMPA administration in that environment than did rats given daily cocaine in the home cage (Bell and Kalivas, in press). These results suggest that the contextually conditioned aspect of behavioral sensitization to psychostimulants may be mediated in part by changes in AMPA receptor sensitivity in the nucleus accumbens. <BR><BR> In summary, the current results, combined with those reported by Pierce et al. (1996) indicate that, similar to changes in dopamine transmission associated with behavioral sensitization, the changes in glutamate transmission are blunted at early compared to late withdrawal times. Nonetheless, intra-accumbens administration of either AMPA or D1 dopamine antagonists blocks the expression of behavioral sensitization to cocaine (Pierce et al., 1996; unpublished observations). Thus, while both dopamine and glutamate transmission in the nucleus accumbens are necessary for the expression of behavioral sensitization to repeated cocaine, other factors are also playing a critical role. <BR><br> <HR> <A NAME="acknow"> <font size=+2><b>ACKNOWLEDGMENTS</b></font><br><br> This research was supported in part by the Washington State Alcohol and Drug Abuse Program and U.S. Public Health Service grants MH-40817 and DA-03906, Research Career Development Award DA-00158 (P.W.K.) and National Research Service Award DA-05589 (R.C.P.). <BR><br> <HR> <A NAME="legends"> <font size=+2><b>FIGURE LEGENDS</b></font><br><br> <TABLE BORDER=0> <TR VALIGN=TOP><TD><A HREF="afb09965.html"><IMG SRC="../images/afb09965s.gif" BORDER=0></A></TD> <TD><A NAME="table1"><A HREF="afb09965.html">Table 1</A>: Effect of seven daily administrations of cocaine or saline on horizontal activity in the rat. Day 1 corresponds to the first, and day 7 the last, daily injection. Saline refers to rats receiving 7 daily saline administrations; sensitized to the subpopulation of cocaine rats showing a greater than 20% increase in photocell counts between day 1 and day 7; nonsensitized corresponds to cocaine animals demonstrating a less than 20% increase. Data are presented as the mean (standard error) horizontal photocell counts over the first 120 min after cocaine injection. <BR><BR></TD></TR> <TR VALIGN=TOP><TD><A HREF="abp09961.html"><IMG SRC="../images/abp09961s.jpg" BORDER=0></A></TD> <TD><A NAME="figure1"><A HREF="abp09961.html">Figure 1</A>: Behavioral effect of AMPA microinjection into the nucleus accumbens of saline, sensitized and nonsensitized groups one to three days after discontinuing daily treatments with saline or cocaine. The data are represented as mean (� standard error) horizontal photocell counts. The upper panel shows total photocell counts (over the 120 min recording period after saline or AMPA microinjection); these data were evaluated with a 2-way ANOVA (treatment X AMPA dose) with repeated measures over dose. This analysis revealed significant main effects of treatment [F(2,72)=2.97, p=0.056] and AMPA dose [F(2,72)=20.98, p=0.0001]. The lower panel shows the time course of the effect of 0.1 痢 AMPA in the three treatment groups. These data were evaluated with a 2-way ANOVA (treatment X time) with repeated measures over time. This analysis revealed a significant main effect of time [F(7,168)=41.43, p=0.0001] and a significant interaction between treatment and time [F(14,168)=2.3, p=0.0065].<BR> *p < 0.05, comparing treatments to saline-saline using a least significant difference test (Milliken and Johnson, 1984).<BR> +p < 0.05, comparing sensitized and nonsensitized to saline-0.1 痢 AMPA. <BR><BR></TD></TR> <TR VALIGN=TOP><TD><A HREF="afb09962.html"><IMG SRC="../images/afb09962s.jpg" BORDER=0></A></TD> <TD><A NAME="figure2"><A HREF="afb09962.html">Figure 2</A>: Effect of AMPA administration through the dialysis probe in the nucleus accumbens on extracellular dopamine content. The experiment was conducted at 1 or 2 days after discontinuing repeated cocaine or saline. These data were calculated by averaging the last two samples taken at baseline and each AMPA concentration and were analyzed via a 2-way ANOVA (treatment X dose) with repeated measures over dose. When the data were expressed as percent baseline (panel A) this analysis revealed a significant main effect of AMPA dose [F(4,40)=16.33, p<0.0001]. Similarly, the analysis of the raw data resulted only in a significant effect of dose [F(4,40)=9.32, p<0.0001]. <BR><BR></TD></TR> <TR VALIGN=TOP><TD><A HREF="afb09963.html"><IMG SRC="../images/afb09963s.jpg" BORDER=0></A></TD> <TD><A NAME="figure3"><A HREF="afb09963.html">Figure 3</A>: Effect of saline and cocaine (15 mg/kg, ip) injections on extracellular glutamate in the nucleus accumbens of saline- and cocaine-pretreated rats one day after discontinuing daily injections. The data are presented as the mean (� standard error) percent change from baseline glutamate. The data were evaluated with a 2-way ANOVA (treatment X time) with repeated measures over time. The analysis of the percent baseline transformation (panel A) and the raw data (panel B) revealed significant main effects of treatment [panel A: F(15,180)=2.02, p=0.016; panel B: F(15,180)=2.63, p=0.001]. The open arrow indicates the administration of saline (after 100 min) and the closed arrow indicates the injection of cocaine (after 180 min). <BR><BR></TD></TR> <TR VALIGN=TOP><TD><A HREF="afb09964.html"><IMG SRC="../images/afb09964s.jpg" BORDER=0></A></TD> <TD><A NAME="figure4"><A HREF="afb09964.html">Figure 4</A>: The upper photomicrograph is representative of an animal microinjected with AMPA in experiment 1. The arrows point to the injection sites. The lower micrograph is from an animal used in experiment 3 and shows the placement of the dialysis probe in the core region of the nucleus accumbens. The arrows point to the extreme ends of the active region of the dialysis probe. In both micrographs, note the lack of neurotoxicity outside of the mechanical damage produced by the injection cannulae or probe following perfusion of AMPA through the dialysis probe. Bars= 1 mm. ac=anterior commisure. <BR><BR></TD></TR> </TABLE> <HR> <A NAME="references"> <font size=+2><b>REFERENCES</b></font><br><br> Akimoto, K., Hamamura, T., Kazahaya, Y., Akiyama, K. and Otsuki, S. (1990) Enhanced extracellular dopamine level may be the fundamental neuropharmacological basis of cross-behavioral sensitization between methamphetamine and cocaine - an in vivo dialysis study in freely moving rats. Brain Res., 507:344-346. <BR><BR> Badiani, A., Browman, K.E. and Robinson, T.E. (1995) Influence of novel versus home environments on sensitization to the psychomotor stimulant effects of cocaine and amphetamine. Brain Res., 674:291-298. <BR><BR> Bell, K.B. and Kalivas, P.W. 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(1993) Differential development of autoreceptor subsensitivity and enhanced dopamine release during amphetamine sensitization. J. Pharmacol. Exp. Ther., 264:249-255. <BR><BR> <CENTER> <table> <tr> <td align=center><A HREF="index.html"> <img src="../images/b_back.gif" border=0 ALT="Back"></A> <td align=center><a href="#top"><img src="../images/b_top.gif" border=0 ALT="Top"></a> <tr> <td><a href="../../articles.html" TARGET="_top"><img src="../images/b_ita1.gif" border=0 ALT="Index to Articles"></a> <td><a href="../../index.html" TARGET="_top"><img src="../images/b_home1.gif" border=0 ALT="Return Home"></a><br> </table><p> </CENTER> </body>