Dopamine Uptake is Differentially Regulated in Rat Striatum and Nucleus Accumbens

Active uptake of 3,4-dihydroxyphenylethylamine (dopamine) is sodium- and temperature-dependent, strongly inhibited by benztropine and nomifensine, and present in corpus striatum and nucleus accumbens. In rat striatum dopamine uptake is related to a receptor that is specifically labelled by [3H]cocaine in the presence of Na+ and is located on dopaminergic terminals. The dopamine uptake is differentially affected in the two areas by single or repeated injections of cocaine. Cocaine inhibits dopamine uptake in slices of corpus striatum. Moreover Na+-dependent [3H]cocaine binding is not detectable in nucleus accumbens. Nomifensine inhibits [3H]dopamine uptake by interacting with low- and high-affinity sites in corpus striatum, but shows only low affinity for dopamine uptake in nucleus accumbens. The present data indicate that different mechanisms are involved in the regulation of dopamine uptake in corpus striatum and nucleus accumbens.     

Carrier-Mediated Release of Serotonin by 3,4-Methylenedioxymethamphetamine: Implications for Serotonin-Dopamine Interactions

Abstract: In vivo microdialysis was used to determine whether the 3,4-methylenedioxymethamphetamine (MDMA)-induced release of serotonin (5-HT) in vivo involves a carrier-mediated process and to investigate further the state-dependent interaction between 5-HT and dopamine. MDMA produced a dose-dependent increase in the extracellular concentration of 5-HT in the striatum and prefrontal cortex that was attenuated by treatment with fluoxetine but not by tetrodotoxin. Suppression by fluoxetine of the MDMA-induced release of 5-HT was accompanied by a suppression of the MDMA-induced release of dopamine. Administration of MDMA to rats treated with carbidopa and l -5-hydroxytryptophan resulted in a synergistic elevation of the extracellular concentration of 5-HT that was much greater than that produced by either treatment alone. The MDMA-induced release of dopamine by MDMA also was potentiated in 5-hydroxytryptophan-treated rats. These data are consistent with the view that MDMA increases the extracellular concentration of 5-HT by facilitating carrier-mediated 5-HT release, which can be enhanced greatly under conditions in which 5-HT synthesis is stimulated. Moreover, these data are supportive of a state-dependent, stimulatory role of 5-HT in the regulation of dopamine release.     

Differential Effect of Stress on In Vivo Dopamine Release in Striatum, Nucleus Accumbens, and Medial Frontal Cortex

Microdialysis was used to assess extracellular dopamine in striatum, nucleus accumbens, and medial frontal cortex of unanesthetized rats both under resting conditions and in response to intermittent tail-shock stress. The dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid also were measured. The resting extracellular concentration of dopamine was estimated to be approximately 10 nM in striatum, 11 nM in nucleus accumbens, and 3 nM in medial frontal cortex. In contrast, the resting extracellular levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid were in the low micromolar range. Intermittent tail-shock stress increased extracellular dopamine relative to baseline by 25% in striatum, 39% in nucleus accumbens, and 95% in medial frontal cortex. 3,4-Dihydroxyphenylacetic acid and homovanillic acid also were generally increased by stress, although there was a great deal of variability in these responses. These data provide direct in vivo evidence for the global activation of dopaminergic systems by stress and support the concept that there exist regional variations in the regulation of dopamine release.     

Effects of serotonin depletion byp-chlorophenylalanine,p-chloroamphetamine or 5,7-dihydroxytryptamine on central dopaminergic neurons: Focus on tuberoinfundibular dopaminergic neurons and serum prolactin

Three serotonin (5-HT) neurotoxins,p-chlorophenylalanine (PCPA, 125 and 250 mg/kg, i.p.),p-chloroamphetamine (PCA, 10 mg/kg, i.p.) and 5,7-dihydroxytryptamine (5,7-DHT, 200 µg/rat, i.c.v.) were used to examine whether depletion of central 5-HT has an effect on central dopaminergic (DA) neuronal activities or on prolactin (PRL) secretion. Adult ovariectomized Sprague-Dawley rats primed with estrogen (polyestradiol phosphate, 0.1 mg/rat, s.c.) were treated with one of three neurotoxins and then decapitated in the morning after 3–7 days. Blood sample and brain tissues were collected. The acute effect of PCA (from 30 to 180 min) was also determined. The concentrations of 5-HT, DA and their metabolites, 5-hydroxyindoleacetic acid and 3,4-dihydroxyphenylacetic acid, in the median eminence, striatum and nucleus accumbens were determined by HPLC-electrochemical detection. All three toxins significantly depleted central 5-HT stores by 11–20%. Except for PCPA, neither PCA nor 5,7-DHT had any significant effect on basal DA neuronal activities or PRL secretion. PCA also exhibited an acute effect on the release and reuptake of 5-HT and DA. In summary, depletion of central 5-HT stores to a significant extent for 3–7 days did not seem to affect basal DA neuronal activity and PRL secretion.     

Differential Effects of Forced Locomotion, Tail-Pinch, Immobilization, and Methyl-β-Carboline Carboxylate on Extracellular 3,4-Dihydroxyphenylacetic Acid Levels in the Rat Striatum, Nucleus Accumbens, and Prefrontal Cortex: An In Vivo Voltammetric Study

In vivo voltammetry with carbon fiber electrodes was used to assess extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) levels in striatum, nucleus accumbens, and anteromedial prefrontal cortex of freely moving rats subjected to altered motor activity or anxiogenic stimuli. Forced locomotion on a rotarod for 40 min caused an increase in extracellular DOPAC levels in the striatum and to a lesser extent in the nucleus accumbens but not in the prefrontal cortex. Subcutaneous injection of the anxiogenic agent methyl-beta-carboline carboxylate (10 mg/kg) increased extracellular DOPAC levels to a similar extent in prefrontal cortex and nucleus accumbens. Immobilization for 4 min augmented dopamine (DA) metabolism preferentially in the nucleus accumbens and to a lesser extent in the prefrontal cortex. Tail-pinch caused a selective activation of DA metabolism in the nucleus accumbens. None of these stimuli altered extracellular striatal DOPAC levels. These results confirm the involvement of dopaminergic systems projecting to the striatum and nucleus accumbens in motor function and suggest that mesolimbic and mesocortical dopaminergic systems can be specifically activated by certain kinds of anxiogenic stimuli; the relative activation of either of these latter systems could depend primarily on the nature (sensory modality, intensity) of the acute stressor.     

Extracellular Striatal Concentrations of Endogenous 3,4-Dihydroxyphenylalanine in the Absence of a Decarboxylase Inhibitor: A Dynamic Index of Dopamine Synthesis In Vivo

Abstract: Basal levels of endogenous 3,4-dihydroxyphenylalanine (DOPA) were detected by HPLC coupled with coulometric detection in dialysates from freely moving rats implanted 48–72 h earlier with transversal dialysis fibers in the dorsal caudate. Because decarboxylase inhibitor is absent in the Ringer's solution, this method allows monitoring of basal output of dopamine (DA) and 3,4-dihydroxyphenylacetic acid, as well as DOPA. Extracellular DOPA concentrations were reduced by the tyrosine hydroxylase inhibitor α-methylparatyrosine (200 mg/kg, i.p.) and by the dopaminergic agonist apomorphine (0.25 mg/kg, s.c.). The dopaminergic antagonist haloperidol (0.2 mg/kg, s.c.) stimulated DOPA output by about 60% over basal values. γ-Butyrolactone, at doses of 700 mg/kg, i.p., which are known to block dopaminergic neuronal firing and which reduce DA release, stimulated DOPA output maximally by 130% over basal values. Tetrodotoxin, which blocks DA release by blocking voltage-dependent Na⁺ channels, increased DOPA output maximally by 100% over basal values. The results indicate that basal DOPA can be detected and monitored in the extracellular fluid of the caudate of freely moving rats by transcerebral dialysis and can be taken as a dynamic index of DA synthesis in pharmacological conditions.     

Differences in Dopamine Clearance and Diffusion in Rat Striatum and Nucleus Accumbens Following Systemic Cocaine Administration

Acute cocaine administration preferentially increases extracellular dopamine levels in nucleus accumbens as compared with striatum. To investigate whether a differential effect of cocaine on dopamine uptake could explain this observation, we used in vivo electrochemical recordings in anesthetized rats in conjunction with a paradigm that measures dopamine clearance and diffusion without the confounding effects of release. When a finite amount of dopamine was pressure-ejected at 5-min intervals from a micropipette adjacent to the electrode, transient and reproducible increases in dopamine levels were detected. In response to 15 mg/kg of cocaine-HCl (i.p.), these signals increased in nucleus accumbens, indicating significant inhibition of the dopamine transporter. The time course of the dopamine signal increase paralleled that of behavioral changes in unanesthetized rats receiving the same dose of cocaine. In contrast, no change in the dopamine signal was detected in dorsal striatum; however, when the dose of cocaine was increased to 20 mg/kg, enhancement of the dopamine signal occurred in both brain areas. Quantitative autoradiography with [3H]mazindol revealed that the affinity of the dopamine transporter for cocaine was similar in both brain areas but that the density of [3H]mazindol binding sites in nucleus accumbens was 60% lower than in dorsal striatum. Tissue dopamine levels in nucleus accumbens were 44% lower. Our results suggest that a difference in dopamine uptake may explain the greater sensitivity of nucleus accumbens to cocaine as compared with dorsal striatum. Furthermore, this difference may be due to fewer dopamine transporter molecules in nucleus accumbens for cocaine to inhibit, rather than to a higher affinity of the transporter for cocaine.     

Estimating Extracellular Concentrations of Dopamine and 3,4-Dihydroxyphenylacetic Acid in Nucleus Accumbens and Striatum Using Microdialysis: Relationships Between In Vitro and In Vivo Recoveries

Abstract: It is common practice in microdialysis studies for probes to be “calibrated” in artificial CSF and in vitro recoveries determined for all substances to be measured in vivo. Dialysate concentrations of such substances are then “corrected” for in vitro recoveries to provide “estimates” of extracellular concentrations. At least for dopamine, in vitro and in vivo recoveries are significantly different and, therefore, an estimate of extracellular dopamine based on correction for in vitro recovery is likely to be erroneous. Generally, however, the relative relationships of such estimates among animals are of interest rather than the “true” extracellular values. Such relationships would be valid to the extent that estimated values are correlated with or predictive of true values. Using the “no net flux” procedure, the present study sought to determine, for both dopamine and its metabolite 3,4-dihydroxy-phenylacetic acid (DOPAC), whether in vitro and in vivo recoveries would correlate with each other as well as whether respective estimated and true (no net flux) values of these substances would correlate with each other. Probes (3 mm; BAS/CMed MF-5393), previously calibrated, were lowered into both the nucleus accumbens and striatum of freely moving rats the day before sample collection was begun. In vitro and in vivo recoveries were not significantly correlated (r= 0.1–0.3), for either dopamine or DOPAC. For both dopamine and DOPAC, however, there were significant correlations (r= 0.7–0.8) between estimated and true values. Surprisingly, when using these commercial probes, absolute dialysate levels for both substances were even better correlated (r = 0.9–0.95) with true values. This suggests that, with these probes, a direct comparison of dialysate concentrations can be used to determine relative changes in basal extracellular levels of dopamine and DOPAC when it is not practical to do no net flux studies (e.g., because of the time required to characterize a drug effect). The use of in vitro calibrations adjusts the values closer to the true values but also adds noise to each value and therefore should be avoided.     

Mazindol Attenuates the 3,4-Methylenedioxymethamphetamine-Induced Formation of Hydroxyl Radicals and Long-Term Depletion of Serotonin in the Striatum

The formation of hydroxyl radicals following the systemic administration of 3,4-methylenedioxymethamphetamine (MDMA) was studied in the striatum of the rat by quantifying the stable adducts of salicylic acid and D-phenylalanine, namely, 2,3-dihydroxybenzoic acid (2,3-DHBA) and p-tyrosine, respectively. The repeated administration of MDMA produced a sustained increase in the extracellular concentration of 2,3-DHBA and p-tyrosine, as well as dopamine. The MDMA-induced increase in the extracellular concentration of both dopamine and 2,3-DHBA was suppressed in rats treated with mazindol, a dopamine uptake inhibitor. Mazindol also attenuated the long-term depletion of serotonin (5-HT) in the striatum produced by MDMA without altering the acute hyperthermic response to MDMA. These results are supportive of the view that MDMA produces a dopamine-dependent increase in the formation of hydroxyl radicals in the striatum that may contribute to the mechanism whereby MDMA produces a long-term depletion of brain 5-HT content.     

Effects of Feeding and Drinking on Acetylcholine Release in the Nucleus Accumbens, Striatum, and Hippocampus of Freely Behaving Rats

Extracellular levels of acetylcholine (ACh) were measured in the nucleus accumbens (NAC), striatum (STR), and hippocampus (HIPP) using microdialysis in 30-min intervals before, during, and after free-feeding in 20-h food-deprived rats. The effects on ACh in the NAC and STR were also observed in response to water intake in 20-h water-deprived animals. Neostigmine was used in the perfusate to improve ACh recovery. Basal ACh was sensitive to tetrodotoxin and low calcium, and therefore largely neuronal in origin. Feeding caused a 38% increase in extracellular ACh in the NAC and no change in the STR or HIPP. Dopamine was also increased in the NAC (48%) and to a lesser extent in the STR (21%) following feeding. Drinking caused 18-20% increases in ACh release in both the NAC and STR. In a separate experiment, ACh release in the NAC was monitored in 10-min intervals during free-feeding; ACh increased in the interval immediately following maximal food intake. These results suggest a site-specific increase in ACh release following feeding that cannot be solely attributed to the activation associated with this behavior.     

Decarboxylation of Exogenous l-3,4-Dihydroxyphenylalanine in Rat Striatum as Studied by In Vivo Voltammetry

An in vivo voltammetric technique was used to determine whether striatal nondopaminergic neurons take up and decarboxylate exogenous L-3,4-dihydroxyphenylalanine (L-DOPA) and release it as dopamine. After the striatal serotonergic neurons of the rat had been destroyed by intraventricular injection of 5,7-dihydroxytryptamine, L-DOPA was administered intraperitoneally. It was found that changes in the dopamine concentration in the striatal extracellular fluid of the rat were the same as those in the nonlesioned rat. L-DOPA was also administered to the rat after the striatal perikarya had been destroyed by the intrastriatal injection of kainate. The striatal dopamine concentrations of the lesioned rat changed in parallel with 5,7-dihydroxytryptamine-lesioned rats, as well as the nonlesioned rats. Moreover, when normal rats were administered L-DOPA, the dopamine concentration was not increased in the cerebellum, where dopamine neurons do not exist. From these observations, it is concluded that exogenous L-DOPA is taken up, decarboxylated to dopamine, and released only in the striatal dopamine neurons.     

Clearance of Exogenous Dopamine in Rat Dorsal Striatum and Nucleus Accumbens: Role of Metabolism and Effects of Locally Applied Uptake Inhibitors

In vivo electrochemistry was used to investigate the mechanisms contributing to the clearance of locally applied dopamine in the dorsal striatum and nucleus accumbens of urethane-anesthetized rats. Chronoamperometric recordings were continuously made at 5 Hz using Nafion-coated carbon fiber electrodes. When a finite amount of dopamine was pressure-ejected at 5-min intervals from a micropipette adjacent to the electrode, transient and reproducible dopamine signals were detected. Substitution of L-a-methyldopamine, a substrate for the dopamine transporter but not for monoamine oxidase, for dopamine in the micropipette did not substantially alter the time course of the resulting signals. This indicates that metabolism of locally applied dopamine to 3,4-dihydroxyphenylacetic acid is not responsible for the decline in the dopamine signal. Similarly, changing the applied oxidation potential from ±0.45 to ±0.80 V, which allows for detection of 3-methoxytyramine formed from dopamine via catechol-O-methyltransferase, had little effect on signal amplitude or time course. In contrast, lesioning the dopamine terminals with 6-hydroxydopamine, or locally applying the dopamine uptake inhibitors cocaine or nomifensine before pressure ejection of dopamine, significantly increased the amplitude and time course of the dopamine signals in both regions. The effects of cocaine and nomifensine were greater in the nucleus accumbens than in the dorsal striatum. Local application of lidocaine and procaine had no effect on the dopamine signals. Initial attempts at modeling resulted in curves that were in qualitative agreement with our experimental findings. Taken together, these data indicate that (1) uptake of dopamine by the neuronal dopamine transporter, rather than metabolism or diffusion, is the major mechanism for clearing locally applied dopamine from the extracellular milieu of the dorsal striatum and nucleus accumbens, and (2) the nucleus accumbens is more sensitive to the effects of inhibitors of dopamine uptake than is the dorsal striatum.     

Effects of Cannabinoids on Dopamine Release in the Corpus Striatum and the Nucleus Accumbens In Vitro

Cannabinoid receptors are widely distributed in the nuclei of the extrapyramidal motor and mesolimbic reward systems; their exact functions are, however, not known. The aim of the present study was to characterize the effects of cannabinoids on the electrically evoked release of endogenous dopamine in the corpus striatum and the nucleus accumbens. In rat brain slices dopamine release elicited by single electrical pulses was determined by fast cyclic voltammetry. Dopamine release was markedly inhibited by the OP2 opioid receptor agonist U-50488 and the D2/D3 dopamine receptor agonist quinpirole, indicating that our method is suitable for studying presynaptic modulation of dopamine release. In contrast, the CB1/CB2 cannabinoid receptor agonists WIN55212-2 (10(-6) M) and CP55940 (10(-6)-10(-5) M) and the CB1 cannabinoid receptor antagonist SR141716A (10(-6) M) had no effect on the electrically evoked dopamine release in the corpus striatum and the nucleus accumbens. The lack of a presynaptic effect on terminals of nigrostriatal and mesolimbic dopaminergic neurons is in accord with the anatomical distribution of cannabinoid receptors: The perikarya of these neurons in the substantia nigra and the ventral tegmental area do not synthesize mRNA, and hence protein, for CB1 and CB2 cannabinoid receptors. It is therefore unlikely that presynaptic modulation of dopamine release in the corpus striatum and the nucleus accumbens plays a role in the extrapyramidal motor and rewarding effects of cannabinoids.     

Determination of Picomole Amounts of Dopamine, Noradrenaline, 3,4-Dihydroxyphenylalanine, 3,4-Dihydroxyphenylacetic Acid, Homovanillic Acid, and 5-Hydroxyindolacetic Acid in Nervous Tissue After One-Step Purification on Sephadex G-10, Using High-Performance Liquid Chromatography with a Novel Type of Electrochemical Detection

A determination of dopamine (DA), noradrenaline (NA), 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindolacetic acid (5-HIAA) in nervous tissue is described. The method is based on a rapidly performed isolation of DA, NA, DOPA, DOPAC, HVA, and 5-HIAA from one single nervous tissue sample on small columns of Sephadex G-10, followed by reverse-phase high-performance liquid chromatography with electrochemical detection. A new type of electrochemical detector based on a rotating disk electrode (RDE) was used. The rotating disc electrode was found to be a reliable and sensitive amperometric detector with several advantages over the currently used thin-layer cells. The detector appeared very useful for routine analysis. Practical details are given for the routine use of the RDE. Brain samples containing no more than 75-150 pg (DA, DOPA, DOPAC, HVA, and 5-HIAA) or 500 pg (NA) could be reproducibly assayed with high recovery (approx. 85%) and precision (approx. 5%), without the use of internal standards. Endogenous concentrations of DA, NA, DOPA, DOPAC, HVA, and 5-HIAA were determined in eight brain structures.     

Differential Effects of δ- and μ-Opioid Receptor Antagonists on the Amphetamine-Induced Increase in Extracellular Dopamine in Striatum and Nucleus Accumbens

Abstract: The specific opioid receptor antagonist naloxone attenuates the behavioral and neurochemical effects of amphetamine. Furthermore, the amphetamine-induced increase in locomotor activity is attenuated by intracisternally administered naltrindole, a selective δ-opioid receptor antagonist, but not by the irreversible μ-opioid receptor antagonist β-funaltrexamine. Therefore, this research was designed to determine if naltrindole would attenuate the neurochemical response to amphetamine as it did the behavioral response. In vivo microdialysis was used to monitor the change in extracellular concentrations of dopamine in awake rats. Naltrindole (3.0, 10, or 30 µg) or vehicle was given 15 min before and β-funaltrexamine (10 µg) or vehicle 24 h before the start of cumulative dosing, intracisternally in a 10-µl volume, while the rats were lightly anesthetized with methoxyflurane. Cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) followed pretreatment injections at 30-min intervals. Dialysate samples were collected every 10 min from either the striatum or nucleus accumbens and analyzed for dopamine content by HPLC. Amphetamine dose-dependently increased dopamine content in both the striatum and nucleus accumbens, as reported previously. Naltrindole (3.0, 10, and 30 µg) significantly reduced the dopamine response to amphetamine in the striatum. In contrast, 30 µg of naltrindole did not modify the dopamine response to amphetamine in the nucleus accumbens. On the other hand, β-funaltrexamine (10 µg) had no effect in the striatum but significantly attenuated the amphetamine-induced increase in extracellular dopamine content in the nucleus accumbens. These data suggest that δ-opioid receptors play a relatively larger role than μ-opioid receptors in mediating the amphetamine-induced increase in extracellular dopamine content in the striatum, whereas μ-opioid receptors play a larger role in mediating these effects in the nucleus accumbens.     

Reduced Clearance of Exogenous Dopamine in Rat Nucleus Accumbens, but Not in Dorsal Striatum, Following Cocaine Challenge in Rats Withdrawn from Repeated Cocaine Administration

Abstract: We investigated whether changes in the dopamine transporter in the nucleus accumbens or striatum are involved in cocaine-induced behavioral sensitization by using in vivo electrochemistry to monitor the clearance of locally applied dopamine in anesthetized rats. Rats were injected with cocaine-HCI (10 mg/kg i.p.) or saline daily for 7 consecutive days and then withdrawn for 7 days. Pressure ejection of a finite amount of dopamine at 5-min intervals from a micropipette adjacent to the electrochemical recording electrode produced transient and reproducible dopamine signals. After a challenge injection of cocaine (10 mg/kg i.p.), the signals in the nucleus accumbens of cocaine-treated animals became prolonged and the clearance rate of the dopamine decreased, indicating significant inhibition of the dopamine transporter. In contrast, simultaneous measurements in the dorsal striatum indicated a transient increase in both the amplitude of the signals and the clearance rate of the dopamine. The signals in both brain regions in the saline-treated animals given the cocaine challenge were similar to those in untreated animals given an acute injection of cocaine (10 mg/ kg i.p.) or saline. Behaviorally, not all of the cocaine- treated animals were sensitized; however, both sensitized and nonsensitized animals displayed similar changes in dopamine clearance rate. Quantitative autoradiography with [³H]mazindol revealed that the affinity of the dopamine transporter for cocaine and the density of binding sites were similar in cocaine- and saline-treated rats. The decrease in dopamine clearance rate observed in the nucleus accumbens of the cocaine-treated rats after a challenge injection of cocaine is consistent with increased do- paminergic transmission, but does not appear to be sufficient in itself for producing behavioral sensitization.     

In Vivo Assessment of Dopamine Uptake in Rat Medial Prefrontal Cortex: Comparison with Dorsal Striatum and Nucleus Accumbens

Abstract: In vivo electrochemistry was used to characterize dopamine clearance in the medial prefrontal cortex and to compare it with clearance in the dorsal striatum and nucleus accumbens. When calibrated amounts of dopamine were pressure-ejected into the cortex from micropipettes adjacent to the recording electrodes, transient and reproducible dopamine signals were detected. The local application of the selective uptake inhibitors GBR-12909, desipramine, and fluoxetine before the application of dopamine indicated that at the lower recording depths examined (2.5–5.0 mm below the brain surface), locally applied dopamine was cleared from the extracellular space primarily by the dopamine transporter. The norepinephrine transporter played a greater role at the more superficial recording sites (0.5–2.25 mm below the brain surface). To compare clearance of dopamine in the medial prefrontal cortex (deeper sites only), striatum, and nucleus accumbens, varying amounts of dopamine were locally applied in all three regions of individual animals. The signals recorded from the cortex were of greater amplitude and longer time course than those recorded from the striatum or accumbens (per picomole of dopamine applied), indicating less efficient dopamine uptake in the medial prefrontal cortex. The fewer number of transporters in the medial prefrontal cortex may be responsible, in part, for this difference, although other factors may also be involved. These results are consistent with the hypothesis that regulation of dopaminergic function is unique in the medial prefrontal cortex.     

Conversion of 3,4-Dihydroxyphenylalanine and Deuterated 3,4-Dihydroxyphenylalanine to Alcoholic Metabolites of Catecholamines in Rat Brain

Abstract: We have investigated the effects of 3,4-dihydroxyphenylalanine l -DOPA) and its deuterated analogue on the concentrations of alcoholic metabolites of catecholamines in rat brain by means of gas chromatography/mass spectrometry with selected-ion monitoring. Whole brain concentrations of the two neutral norepinephrine metabolites, 3-methoxy-4-hydroxyphenylethylene-glycol (MHPG) and 3,4-dihydroxyphenylethyleneglycol (DHPG), were significantly increased in a dose-dependent manner by a single intraperitoneal injection of l -DOPA. Both MHPG and DHPG, as well as the corresponding dopamine metabolites, reached a maximum 1 h after injection. Brain MHPG and DHPG concentrations were elevated by 78 and 134%, respectively, 1 h after injection of 150 mg/kg l -DOPA. Analyses of discrete brain regions revealed that concentrations of the norepinephrine metabolites were elevated uniformly in all regions, except that MHPG showed a greater increase in the cerebellum than in other regions. The latter result appeared to be explained by the finding that 52% of the total MHPG in the cerebellum was unconjugated (compared to 15% in the whole brain). l -DOPA caused a proportionately greater increase in free MHPG than in total MHPG in the cerebellum and brain stem. By using deuterated l -DOPA in place of l -DOPA and measuring both the deuterated and nondeuterated norepinephrine metabolites, we demonstrated that virtually all of the increases in MHPG and DHPG were due to the conversion of the exogenous l -DOPA to norepinephrine. Thus, the effects of norepinephrine metabolism need to be considered in attempts to understand clinical and behavioral effects of l -DOPA.     

Acute Effects of Typical and Atypical Antipsychotic Drugs on the Release of Dopamine from Prefrontal Cortex, Nucleus Accumbens, and Striatum of the Rat: An In Vivo Microdialysis Study

In vivo microdialysis has been used to study the acute effects of antipsychotic drugs on the extracellular level of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. (-)-Sulpiride (20, 50, and 100 mg/kg i.v.) and haloperidol (0.1 and 0.5 mg/kg i.v.) enhanced the outflow of dopamine in the striatum and nucleus accumbens. In the medial prefrontal cortex, (-)-sulpiride at all doses tested did not significantly affect the extracellular level of dopamine. The effect of haloperidol was also attenuated in the medial prefrontal cortex; 0.1 mg/kg did not increase the outflow of dopamine and the effect of 0.5 mg/kg haloperidol was of shorter duration in the prefrontal cortex than that observed in striatum and nucleus accumbens. The atypical antipsychotic drug clozapine (5 and 10 mg/kg) increased the extracellular concentration of dopamine in all three regions. In contrast to the effects of sulpiride and haloperidol, that of clozapine in the medial prefrontal cortex was profound. These data suggest that different classes of antipsychotic drugs may have distinct effects on the release of dopamine from the nigrostriatal, mesolimbic, and mesocortical terminals.     

Source and Physiological Significance of Plasma 3,4-Dihydroxyphenylalanine in the Rat

To elucidate the source and physiological significance of plasma 3,4-dihydroxyphenylalanine, the immediate product of the rate-limiting step in catecholamine biosynthesis, plasma 3,4-dihydroxyphenylalanine was quantified in conscious rats after administration of reserpine, desipramine, clorgyline, or forskolin, treatments that affect tyrosine hydroxylase activity. Plasma 3,4-dihydroxyphenylalanine was also examined during infusions of norepinephrine with or without clorgyline, reserpine, or desipramine pretreatment. After reserpine, the plasma 3,4-dihydroxyphenylalanine level decreased by 22% and then increased by 40%, a result consistent with modulation of tyrosine hydroxylase activity first by an increased axoplasmic norepinephrine content and then by depletion of norepinephrine stores. After desipramine, the plasma 3,4-dihydroxyphenylalanine level decreased by 20%, reflecting the depressant effect of neuronal uptake blockade on norepinephrine turnover. Forskolin increased the plasma 3,4-dihydroxyphenylalanine level by 30%, consistent with activation of tyrosine hydroxylase by cyclic AMP-dependent phosphorylation. Acute administration of clorgyline was without effect on the plasma 3,4-dihydroxyphenylalanine level. Norepinephrine infusions decreased the plasma 3,4-dihydroxyphenylalanine concentration, as expected from end-product inhibition of tyrosine hydroxylase. Pretreatment with desipramine prevented the norepinephrine-induced decrease in plasma dihydroxyphenylalanine content, indicating that inhibition of tyrosine hydroxylase required neuronal uptake of norepinephrine. Both reserpine and clorgyline augmented the norepinephrine-induced decrease in plasma 3,4-dihydroxyphenylalanine level, suggesting that retention of norepinephrine in the axoplasm--due to inhibition of norepinephrine sequestration into storage vesicles or catabolism--caused further inhibition of tyrosine hydroxylase. Changes in plasma 3,4-dihydroxyphenylalanine concentration during norepinephrine infusions were negatively correlated with those in plasma 3,4-dihydroxyphenylglycol level, a finding consistent with modulation of tyrosine hydroxylase activity by axoplasmic norepinephrine. In reserpinized animals, clorgyline and norepinephrine infusion together decreased the plasma 3,4-dihydroxyphenylalanine content by 50%, a result demonstrating that hydroxylation of tyrosine was depressed by at least half. The results indicate that quantification of plasma 3,4-dihydroxyphenylalanine can provide a simple and direct approach for examination of the rate-limiting step in catecholamine biosynthesis.