Serotonin 5-HT2C agonists selectively inhibit morphine-induced dopamine efflux in the nucleus accumbens

In vivo microdialysis was used to compare the effects of serotonergic drugs on morphine- and cocaine-induced increases in extracellular dopamine (DA) concentrations in the rat nucleus accumbens (NAc). Systemic administration of the 5-HT_2A/2C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2.5 mg/kg, s.c.) prevented the increase in extracellular DA in the NAc produced by morphine (5 mg/kg, i.p.). In contrast, this dose of DOI had no effect on the ability of cocaine (10 mg/kg, i.p.) to increase extracellular DA concentrations in the NAc. A preferential 5-HT_2C receptor agonist, 6-chloro-2-[1-piperazinyl]-pyrazine (MK-212, 5 mg/kg, s.c.) also inhibited morphine-induced increases in extracellular DA concentrations in the NAc. Pretreatment of rats with the selective 5-HT_2A antagonist, amperozide, had no effect on morphine-induced elevation of NAc DA concentrations. In order to determine if inhibition of the firing of 5-HT neurons contributes to the serotonin agonist-mediated inhibition of morphine-induced accumbens DA release, rats were pretreated with the 5-HT_1A agonist, 8-OHDPAT. At a dose of 100 μg/kg (s.c.), 8-OHDPAT did not interfere with morphine's ability to increase DA concentrations in the NAc. These results suggest that the activation of 5-HT_2C receptors selectively inhibits morphine-induced DA release in the NAc in a manner which is independent of the inhibition of 5-HT neurons.     

Neurotensin injected into the nucleus accumbens blocks the psychostimulant effects of cocaine but does not attenuate cocaine self-administration in the rat

The neuropeptide neurotensin (NT) has been shown to modulate mesolimbic dopaminergic activity. Neurotensin injected into the VTA produces motor stimulation and release of dopamine in the nucleus accumbens. In contrast, when neurotensin is administered into the nucleus accumbens, it produces neuroleptic-like effects such as attenuation of the locomotor activity elicited by psychostimulants. In the present study, the hypothesis that neurotensin injected into the nucleus accumbens might modulate the psychostimulant and reinforcing actions of cocaine was tested. In experiment one, rats were trained to self-administer cocaine intravenously on an FR5 schedule of reinforcement. Following the establishment of baseline responding, rats were implanted with bilateral cannulae in the nucleus accumbens. One week later, rats were injected into the nucleus accumbens with various doses of neurotensin (4.2, 8.4 and 16.7 μg, total doses bilaterally) immediately prior to the self-administration session. No significant effects were found with any of the doses of neurotensin tested on the self-administration of cocaine. However, in experiment 2, neurotensin at doses of 4.2 and 16.7 μg injected into the nucleus accumbens significantly reduced the locomotor activation induced by an acute injection of cocaine (15 mg/kg i.p.) and a dose of 16.7 μg attenuated the locomotor activation induced by amphetamine (0.75 mg/kg i.p.). Thus, neurotensin in the nucleus accumbens appears to specifically modulate the acute locomotor activating properties of cocaine but not cocaine self-administration. Different mechanisms by which NT interacts with dopamine in the nucleus accumbens may provide a means of selectively altering psychostimulant motor actions without affecting psychostimulant reinforcement.     

Rats classified as low or high cocaine locomotor responders: A unique model involving striatal dopamine transporters that predicts cocaine addiction-like behaviors

Individual differences are a hallmark of drug addiction. Here, we describe a rat model based on differential initial responsiveness to low dose cocaine. Despite similar brain cocaine levels, individual outbred Sprague-Dawley rats exhibit markedly different magnitudes of acute cocaine-induced locomotor activity and, thereby, can be classified as low or high cocaine responders (LCRs or HCRs). LCRs and HCRs differ in drug-induced, but not novelty-associated, hyperactivity. LCRs have higher basal numbers of striatal dopamine transporters (DATs) than HCRs and exhibit marginal cocaine inhibition of in vivo DAT activity and cocaine-induced increases in extracellular DA. Importantly, lower initial cocaine response predicts greater locomotor sensitization, conditioned place preference and greater motivation to self-administer cocaine following low dose acquisition. Further, outbred Long-Evans rats classified as LCRs, versus HCRs, are more sensitive to cocaine's discriminative stimulus effects. Overall, results to date with the LCR/HCR model underscore the contribution of striatal DATs to individual differences in initial cocaine responsiveness and the value of assessing the influence of initial drug response on subsequent expression of addiction-like behaviors.     

Parametric analysis of the effects of cocaine and cocaine pretreatment on dopamine release in the nucleus accumbens measured by fast cyclic voltammetry

Fast cyclic voltammetry was used to measure dopamine (DA) release in the nucleus accumbens of anaesthetized rats, in response to electrical sine-wave stimulation of the ventral tegmental area. Voltammetric signals followed increases in either frequency (50–100 Hz), intensity (50–100 μA) or duration (0.5–5.0 s) of the stimulus. Cocaine administration (10 mg/kg) preferentially increased DA release by weak electrical stimuli. Cocaine pretreatment (3×10 mg/kg, two weeks earlier) preferentially increased DA release by stronger stimuli, and the effects of acute cocaine were potentiated in these animals. The effects of increasing stimulus duration conformed to first order kinetics. Cocaine pretreatment selectively increased the kinetic parameter representing maximal release, while acute cocaine administration preferentially decreased the parameter representing the stimulus duration eliciting half maximal release. The lack of statistical interaction between these two effects suggests that sensitization of the response to acute cocaine by cocaine pretreatment may simply reflect an increase in the size of the releasable pool of DA.     

Extracellular dopamine in the rat ventral tegmental area and nucleus accumbens following ventral tegmental infusion of cocaine

Rats were implanted with dual dialysis probes, one in the ventral tegmental area, and another one ipsilateral in the nucleus accumbens. Infusion of cocaine (10, 100, 1000 mM) into the ventral tegmental area gradually increased extracellular dopamine to 164, 329 and 991% of baseline in the ventral tegmental area, but reduced dopamine to 76, 47 and 38% of baseline in the nucleus acumbens. These results are consistent with cocaine-induced feedback regulation of dopamine cell activity involving somatodendritec impulse-regulating dopamine D₂ autoreceptors.     

Daily cocaine treatment produces a persistent reduction of [H]dopamine uptake in vitro in rat nucleus accumbens but not in striatum

Daily administration of cocaine (15 mg/kg i.p. × 3 days) led to a decrease in the total amount of 15 nM [³H]dopamine uptake in rat nucleus accumbens with no changes in uptake in the striatum when tested 24 h after the last injection. There was an increase in theK_m for dopamine uptake in the nucleus accumbens of cocaine-treated rats, with no change in theV_max. Furthermore, cocaine was a more potent inhibitor of [³H]dopamine uptake in vitro in the nucleus accumbens of rats treated with cocaine than in those which had received saline.     

Cholecystokinin-induced release of dopamine in the nucleus accumbens of the spontaneously hypertensive rat

Changes in dopamine neurotransmission in the nucleus accumbens of the spontaneously hypertensive rat (SHR) may be involved in the pathogenesis of hypertension. This investigation tested the hypothesis that the sulfated octapeptide cholecystokinin (CCK8S) induced release of dopamine is greater in the SHR than in its normotensive control, the Wistar-Kyoto rat (WKY). Dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were sampled using microdialysis in the caudal half of the nucleus accumbens of 10-week-old anesthetized SHRs and WKYs. Samples were collected in the following order: 3 baseline, 3 CCK8S (10 μmol/l), and 3 postdrug samples. The samples were then analyzed using high pressure liquid chromatography with electrochemical detection. CCK8S increased dopamine and DOPAC levels in both the SHR and WKY with a larger increase in basal dopamine in the SHR (greater than 200%). Perfusion of the nucleus accumbens with 1 μmol/l of CCK8S or the nonsulfated form of CCK8 (CCK8US, 10 μmol/l) produced no significant increase in the release of dopamine in the SHR. These results indicate that CCK8S-induced release of dopamine in the nucleus accumbens is greater in the SHR. Changes in CCK8S neurotransmission/receptor function may be responsible for the alterations in dopaminergic function of the SHR and the pathogenesis of hypertension.     

Behavioral sensitization to cocaine: modulation by the cyclic AMP system in the nucleus accumbens

We have previously observed that chronic cocaine administration increases levels of adenylyl cyclase and cAMP-dependent protein kinase (PKA) in the nucleus accumbens (NAc). In the present work we directly examined the involvement of the cAMP system at the level of the NAc in cocaine-induced locomotor activity and sensitization. Groups of rats were pretreated on 3 consecutive days with cocaine (10 mg/kg, i.p.) concurrently with intraaccumbens infusion of saline, 8-bromo-cAMP (2 μg/side; a membrane permeant analogue of cAMP which activates PKA), or RP-CPT-cAMP (20 nmol/side; which inhibits PKA). In a separate experiment, control animals received local infusion of either 8-bromo-cAMP or saline plus i.p. saline. All animals were tested for locomotor activity on pretreatment days, and following an additional cocaine challenge on a subsequent day. Over pretreatment days, animals given 8-bromo-cAMP showed greater cocaine-induced activity, while animals given RP-CPT-cAMP tended to be less active, compared to saline infused animals. When subsequently challenged with cocaine, animals pretreated with intraaccumbens 8-bromo-cAMP showed greater locomotor activity during the last 30 min of the 60 min test session than animals pretreated with saline or RP-CRT-cAMP. No differences in locomotor activity were evident between the two control groups on pretreatment or challenge days. These data suggest that PKA activation at the level of the NAc may have a facilitative role with respect to acute and long-term stimulant-induced locomotor activity.     

Involvement of nucleus accumbens dopamine in the motor activity induced by periodic food presentation: a microdialysis and behavioral study

Two experiments were undertaken to investigate the role of accumbens dopamine (DA) in food-related motor activities. Although presentation of large amounts of food elicits feeding behavior, periodic food presentation (PFP; e.g. a 45-mg pellet every 45 s) induces considerable locomotion, rearing and other motor activities in food-deprived rats. In the first experiment, in vivo microdialysis methods were used to study DA release and metabolism in the nucleus accumbens of behaving rats exposed to periodic food presentation. Four behavioral conditions were used: high rate of PFP (one pellet per 45 s), low rate of PFP (one pellet per 4 min), massed food presentation and food deprivation control. The rats that received a high rate of PFP showed substantial increases in locomotor activity, and also showed significant increases in extracellular DA and DA metabolites. Rats that received massed presentation of food pellets consumed large quantities of food, but showed no significant increases in locomotor activity or DA release. Although the group that received the high rate of PFP showed the highest motor activity and the largest increase in DA release, there was only a modest correlation (r = 0.34) between motor activity and increased DA release. In the second experiment, the neurotoxic agent 6-hydroxydopamine (6-OHDA) was injected into the nucleus accumbens in order to assess the effects of DA depletion of PFP-induced motor activity. DA depletion significantly reduced PFP-induced motor activity in the first week after surgery, but by the second week DA-depleted rats had recovered normal levels of motor activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

海洛因成瘾大鼠伏隔核毁损术前后觅药行为及多巴胺神经递质的变化

目的研究海洛因成瘾大鼠毁损伏隔核前后觅药行为及多巴胺神经递质的变化。方法建立海洛因成瘾大鼠模型，毁损大鼠双侧伏隔核，利用条件性位置偏好实验测定成瘾前、后，术前、术后成瘾大鼠觅药行为的变化，利用高效液相方法测定边缘系统多巴胺神经递质的变化。结果毁损大鼠双侧伏隔核能够完全消除条件性地点偏好。成瘾大鼠边缘系统多巴胺含量较对照组明显增高。毁损前后大鼠边缘系统多巴胺含量无明显变化。结论伏隔核是调节强化作用的重要位置，长期使用海洛因可使大鼠边缘系统多巴胺含量明显增高，但毁损前后无明显变化，与条件性位置偏好表现不同步。     

Decrease in basal dopamine levels in the nucleus accumbens shell during daily drug-seeking behaviour in rats

Accumbal dopamine (DA) is generally accepted to participate in the neural mechanisms underlying drug dependence. Recently the involvement of accumbal DA in drug-seeking behaviour has gained more experimental attention. To study an involvement of accumbal DA in drug-seeking behaviour within and between daily self-administration behaviour, changes in extracellular DA concentration in the nucleus accumbens (NAc) shell were measured during the daily dynamics of intravenous heroin and cocaine self-administration. Groups of drug naive rats were allowed to intravenously self-administer heroin (30 microg/infusion) and cocaine (30 microg/infusion) during five consecutive daily 3 h sessions. Extracellular DA concentrations in the NAc were measured before and after a single 3 h session (acute) and before and after 5 consecutive 3 h sessions (repeated). Following acute and repeated heroin and cocaine self-administration the extracellular DA concentration in the NAc shell was increased by two-fold to three-fold over baseline. These changes in DA concentrations are thought to reflect a direct effect of heroin and cocaine on DA neurotransmission in the NAC shell. Measurement of basal DA concentrations before the self-administration sessions revealed that just before the scheduled 5th self-administration session the (absolute) basal DA levels in the NAc in heroin or cocaine self-administering animals were decreased by approximately halve, as compared to drug-naive animals. It is assumed that just before a scheduled next session the (daily) desire for the drug is high. This decrease in basal DA neurotransmission in the NAc shell may, therefore, reflect an involvement of accumbal DA in drug-seeking behaviour during daily self-administration behaviour. The results demonstrate that initiation of i.v. heroin and cocaine self-administration is linked with changes in extracellular levels of DA in the NAc shell. Moreover, the present data suggest that accumbal DA might be involved in processes underlying the motivational aspects involved in daily drug-seeking behaviour, and that neuroadaptive changes in the mesolimbic DA system due to repeated drug intake lead to an tonic decrease in overall DA activity in the NAc.     

The in vivo microdialysis recovery of dopamine is altered independently of basal level by 6-hydroxydopamine lesions to the nucleus accumbens

The present study was designed to test the hypothesis that the active neurotransmitter processes of release and uptake affect the in vivo microdialysis recovery of dopamine (DA) in the nucleus accumbens (N ACC) of the rat. The in vivo recovery for DA was established for rats which had received either unilateral infusions of the neurotoxin 6-hydroxydopamine (6-OHDA, 8 μg) or vehicle (0.2 μg ascorbate). In the quantitative dialysis method used (point of no net flux method), DA is added to the perfusate at concentrations above and below the expected extracellular concentration (0, 5, 10 and 20 nM) and DA is measured in the dialysate from the brain to generate a series of points. A linear fit is performed, the slope of which is the in vivo recovery of the dialysis probe. The in vivo recovery of the 6-OHDA group was 30 ± 3% which was significantly lower (P < 0.002) than the in vivo recovery of the control group which was 60 ± 3% (mean ± SEM; n = 6/group). The zero intercept of this regression is the point of no net flux, which is the extracellular concentration of DA independent of the probe sampling characteristics. The extracellular DA concentration for the 6-OHDA group was 7.8 ± 1.1nM, which was not significantly different than the control group which was 6.9 ± 0.7nM. The tissue DOPAC/DA ratios of the 6-OHDA lesioned hemispheres were significantly higher than the contralateral hemispheres of the same animals (0.62 ± 0.1vs.0.27 ± 0.1; P < 0.02) while the DOPAC/DA ratios in the control group were not significantly different (0.24 ± 0.1vs.0.27 ± 0.1). The fractional DA efflux from the terminals in the 6-OHDA group was significantly higher than the fractional DA efflux of the control group (0.52 ± 0.08vs.0.03 ± 0.003; P < 0.0001), indicating that the remaining terminals have increased turnover of DA. Despite the increased turnover, however, the number of remaining release and uptake sites are not sufficient to maintain the high in vivo recovery observed in the control group.     

Effect of acute and daily cocaine treatment on extracellular dopamine in the nucleus accumbens

The behavioral stimulant effect of peripheral cocaine injection into rats is augmented following daily administration. In vivo dialysis in the nucleus accumbens of conscious rats was used to determine if the increased behavioral response following daily cocaine administration is associated with an increase in extracellular dopamine concentration. Acute injection of cocaine (15 mg/kg, ip) produced an elevation in extracellular dopamine concentration in the nucleus accumbens. Following daily pretreatment with cocaine (15 mg/kg, ip X 4 days), a subsequent acute injection of cocaine (15 mg/kg, ip) significantly elevated the extracellular dopamine levels compared to that produced by a single acute injection. Although the levels of extracellular dopamine metabolites was significantly lowered by both acute cocaine and daily cocaine, no difference between these two groups of animals was measured. The increase in extracellular dopamine following a single acute injection of cocaine was not correlated to the motor stimulant response. However, after daily pretreatment with cocaine the motor stimulant response to acute cocaine was positively correlated with the increased extracellular concentration of dopamine in the nucleus accumbens. These data demonstrate that enhanced dopamine release into the nucleus accumbens may mediate the behavioral sensitization produced by daily injections of cocaine, but that other neural systems are influential in mediating the acute motor stimulant effect of cocaine.     

Evidence for a tonic facilitatory influence of substance P on dopamine release in the nucleus accumbens

Microinjections of a monoclonal antibody substance P (SP) into the nucleus accumbens (NAS) increased the concentrations of dopamine (DA) and its metabolite 3,4-dihydroxyphenyllacetic acid (DOPAC) in the NAS but not neuroanatomically adjacent areas. SP immunoneutralization in the NAS also reduced the locomotor response to systemically administered -amphetamine. Microinjections of control antibody did not significantly alter either DA metabolism or -amphetamine-induced locomotion. These data are consistent with the hypothesis that endogenous SP modulates the release of DA in the NAS.     

Injections of -amphetamine into the ventral pallidum increase locomotor activity and responding for conditioned reward: a comparison with injections into the nucleus accumbens

The nucleus accumbens and ventral pallidum receive dopamine (DA) projections from the mesencephalon. Although DA inputs to the nucleus accumbens are implicated in both locomotion and reward processes, little is known of the behavioural significance of DA in the ventral pallidum. These studies examined the effects of -amphetamine injected into the nucleus accumbens or ventral pallidum on locomotor activity and responding for a conditioned reward (CR). In the nucleus accumbens -amphetamine dose dependently (1, 3 and 10 μg) increased locomotion within 5–10 min of injection. Intra-ventral pallidum microinjections of -amphetamine also increased activity in this dose range, but the effect occurred with a longer latency (5–20 min). The magnitude of the response evoked by ventral pallidum injections was lower than that evoked by nucleus accumbens injections. The GABA_A antagonist picrotoxin (0.1 μg) stimulated activity when injected into the ventral pallidum but not the nucleus accumbens, providing a pharmacological dissociation between the two injection sites. In the CR studies, -amphetamine injected into both sites potentiated responding for a CR previously paired with food delivery, without altering responding on an inactive lever. Picrotoxin injected into the ventral pallidum reduced responding and abolished the selectivity of responding for CR. The results show that DA release in the ventral pallidum enhances locomotion and responding for a CR, providing evidence that DA in the ventral pallidum plays a significant role in the mediation of the effects of -amphetamine. The failure of picrotoxin to elevate responding for CR despite increasing locomotor activity indicates that pharmacologically-induced blockade of GABA_A receptors in the ventral pallidum disrupts goal-directed responding.     

Blockade of glutamate reuptake in the rat nucleus accumbens increases locomotor activity

The effect on locomotor activity of blocking glutamate reuptake in the nucleus accumbens (NAcc) was investigated in the rat. Bilateral intracranial microinjections into the NAcc of the selective glutamate reuptake blocker, l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), were made in the freely moving rat and locomotor activity subsequently measured for 2 h. Different groups of rats injected with one of three doses of PDC (0.5, 5 or 10 nmole/0.5 microl/side) showed significant dose-dependent increases in both horizontal and vertical locomotor activity relative to control rats that received injections of the saline vehicle. These findings indicate that glutamate in the NAcc plays an important role in the production of locomotor behaviors.     

Basal extracellular dopamine is decreased in the rat nucleus accumbens during abstinence from chronic cocaine.

Rats were treated for 10 days with cocaine (20 mg/kg, i.p.) followed by either 1 or 10 days of abstinence. On the test day a microdialysis method was performed in which dopamine (DA) was added to the perfusate at concentrations above and below the expected extracellular concentration (0, 2.5, 5, and 10 nM) to generate a series of points that can be interpolated to determine the concentration of no net flux, which represents the extracellular DA concentration. The slope of the line generated by this method is the in vivo recovery of the dialysis probe. After 1 day of abstinence, there was no significant difference in basal DA levels in the nucleus accumbens (N ACC) between cocaine treated (4.1 +/- 0.3 nM; mean +/- SEM) and saline-treated (3.9 +/- 0.2 nM) groups. However, there was a significant increase in the slope of the cocaine-treated group (0.91 +/- 0.04 vs. 0.67 +/- 0.08; P greater than 0.03). After 10 days of abstinence, there were reduced basal extracellular levels of DA in the N ACC of the cocaine-treated group as compared with saline-treated controls (P less than 0.002). The basal extracellular DA concentration in the N ACC was 2.1 +/- 0.3 nM for the cocaine group and 3.9 +/- 0.2 nM for the control group. The slopes of the curves were not significantly different for the cocaine (0.63 +/- 0.07) and saline (0.64 +/- 0.09) groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of intra-amygdala dopaminergic grafts on methamphetamine-induced locomotor activity, extracellular dopamine and dopamine metabolite overflow: a comparison with the effect of intra-accumbens grafts

The effects of 6-hydroxydopamine lesions of the ventral tegmental area and following intra-amygdala or intra-accumbens dopaminergic (DAergic) grafts on methamphetamine (MAP)-induced locomotor activity were investigated in rats. Intra-accumbens DAergic grafts from rat embryos restored the locomotor hyperactivity response to MAP 5 weeks after grafting, while intra-amygdala grafts did not restore responses by 10 weeks after grafting. Biochemical measurements of extracellular DAergic activity in the amygdala (AMY) by in vivo microdialysis after grafting showed no significant change in the basal levels of dopamine (DA) and partial restoration of metabolite levels. MAP induced an increase of DA efflux and a decrease in dihydroxyphenylacetic acid without a significant change in homovanillic acid, which is the same pattern of response seen in control animals. These biochemical changes are similar to those seen previously after intra-accumbens grafts. The results show that restoration of DAergic activity in the AMY in the presence of DAergic denervation of accumbens does not have an effect on MAP-induced locomotion.     

Viral restoration of dopamine to the nucleus accumbens is sufficient to induce a locomotor response to amphetamine

Administration of amphetamine to mice evokes hyperlocomotion. Dopamine deficient (DD) mice, in which tyrosine hydroxylase (TH) has been specifically inactivated in dopaminergic neurons, have a blunted response to amphetamine, indicating that the hyperlocomotive response requires dopamine. Dopamine production can be restored to specific brain regions by using adeno-associated viruses expressing TH and GTP cyclohydrolase 1 (GTPCH1). Restoration of dopamine specifically to the nucleus accumbens (NAc) of DD mice completely restores the ability of these mice to respond to amphetamine. This response is specific to the dopamine production in the NAc, as restoration of dopamine production to the caudate putamen (CPu) does not fully restore the hyperlocomotive response to amphetamine. These data support previous studies in which accumbal dopamine is required for producing a normal locomotor response to amphetamine and further show that release of dopamine restricted to the NAc is sufficient for this response     

Simultaneous microdialysis and amphetamine infusion in the nucleus accumbens and striatum of freely moving rats: Increase in extracellular dopamine and serotonin

To test the effects of systemic and local amphetamine on dopamine and serotonin release in freely moving rats, guide cannulas were implanted in the nucleus accumbens and ventral striatum for removable 200 mu microdialysis probes. Comparing 45 min samples before and after IP amphetamine (2 mg/kg), dopamine (DA) in dialysate from the accumbens increased from a baseline of 3 pg/20 microliters to 11 pg/20 microliters whereas dopamine metabolites, DOPAC and HVA decreased. This was probably due to block of DA reuptake and inhibition of monoamine oxidase, MAO. Accumbens serotonin increased from a baseline of 8 to 11 pg/20 microliters. Changes in the ventral striatum were similar. In the second experiment, microdialysis was performed before and after local injection of amphetamine (4 micrograms) to reveal effects of amphetamine in the terminal area only instead of the whole brain. DA in the accumbens increased from 11 to 147 pg per sample; serotonin, from 11 to 107. The effect was even larger in the ventral striatum. DA increased from 12 to 409 pg/20 microliters; serotonin, 3 to 139. To avoid handling the rat or disturbing brain tissue in experiment three, amphetamine was infused via the microdialysis probe during ongoing dialysis sampling. Perfusate containing amphetamine was switched into the flow line which allowed roughly 10% or 4 micrograms to diffuse out into the extracellular space. During this 20 min, extracellular DA in the accumbens increased from a baseline of 10 pg/20 microliters to 300 pg/20 microliters sample. Serotonin increased from barely detectable to 60 pg/20 microliters. DOPAC decreased significantly; HVA and 5HIAA drifted lower.(ABSTRACT TRUNCATED AT 250 WORDS)