Effects of transient, global, cerebral ischemia on striatal extracellular dopamine, serotonin and their metabolites

Rat striatal extracellular fluid levels of dopamine, serotonin, 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were measured before, during and after transient, global cerebral ischemia in awake rats using in vivo brain microdialysis. Before ischemia, extracellular levels of dopamine, DOPAC, HVA and 5-HIAA were detectable and consistent from sample to sample. During cerebral ischemia, there was a large increase in extracellular dopamine levels and a decrease in the extracellular levels of DOPAC, HVA, and 5-HIAA. During reperfusion, dopamine levels returned to normal as did those of DOPAC, HVA and 5-HIAA. Dialysate serotonin and 3-methoxytyramine concentrations were below detection limits except for samples collected during ischemia and early reperfusion.     

Amphetamine-Induced Dopamine Release in the Rat Striatum: An In Vivo Microdialysis Study

The effects of a number of biochemical and pharmacological manipulations on amphetamine (AMPH)-induced alterations in dopamine (DA) release and metabolism were examined in the rat striatum using the in vivo brain microdialysis method. Basal striatal dialysate concentrations were: DA, 7 nM; dihydroxyphenylacetic acid (DOPAC), 850 nM; homovanillic acid (HVA), 500 nM; 5-hydroxyindoleacetic acid (5-HIAA), 300 nM; and 3-methoxytyramine (3-MT), 3 nM. Intraperitoneal injection of AMPH (4 mg/kg) induced a substantial increase in DA efflux, which attained its maximum response 20-40 min after drug injection. On the other hand, DOPAC and HVA efflux declined following AMPH. The DA response, but not those of DOPAC and HVA, was dose dependent within the range of AMPH tested (2-16 mg/kg). High doses of AMPH (greater than 8 mg/kg) also decreased 5-HIAA and increased 3-MT efflux. Depletion of vesicular stores of DA using reserpine did not affect significantly AMPH-induced dopamine efflux. In contrast, prior inhibition of catecholamine synthesis, using alpha-methyl-p-tyrosine, proved to be an effective inhibitor of AMPH-evoked DA release (less than 35% of control). Moreover, the DA releasing action of AMPH was facilitated in pargyline-pretreated animals (220% of control). These data suggest that AMPH releases preferentially a newly synthesised pool of DA. Nomifensine, a DA uptake inhibitor, was an effective inhibitor of AMPH-induced DA efflux (18% of control). On the other hand, this action of AMPH was facilitated by veratrine and ouabain (200-210% of control). These results suggest that the membrane DA carrier may be involved in the actions of AMPH on DA efflux.     

Effect of chloral hydrate on in vivo KCl-induced striatal dopamine release in the rat

The release of striatal dopamine (DA) and its metabolites in response to locally-induced K⁺ depolarization was investigated in vivo in chloral hydrate-anesthetized and freely moving rats. KCl at concentrations of 30, 50, and 100 mM induced significant dose-dependent increases in extracellular DA overflow in both chloral hydrate-anesthetized and freely moving rats (P<0.05). Extracellular levels of dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were decreased. The DA overflow in response to 30 mM KCl stimulation in anesthetized rats was significantly greater than that in freely moving rats (P<0.05). In addition, chloral hydrate anesthesia resulted in a significant decrease in extracellular levels of DOPAC and significant increases in extracellular levels of HVA and 5-HIAA in comparison with freely moving rats (P<0.05). Furthermore, the basal level of extracellular HVA in chloral hydrateanesthetized rats was significantly higher than that in freely moving rats. These results suggest that chloral hydrate anesthesia could have significant effects on the pharmacological response of the striatal dopaminergic neurons.     

Effects of CGP 28014 on the in vivo release and metabolism of dopamine in the rat striatum assessed by brain microdialysis

The effects on rat striatal dopamine (DA) metabolism of systemic and local administration of CGP 28014, an inhibitor of catechol-O-methyl-transferase (COMT), were studied by in vivo microdialysis. CGP 28014 (30 mg/kg i.p.) significantly reduced the levels of homovanillic acid (HVA), but did not modify DA and 3,4-dihydroxyphenylacetic acid (DOPAC). The intrastriatal administration (via the microdialysis probe) of 5, 7.5, 10, and 20 mM of CGP 28014 elicited a concentration-dependent, several-fold increase in extracellular DA but did not alter the levels of HVA and DOPAC. Thus, the effects of CGP 28014 observed after i.p. injection (decrease in HVA levels) are different from those measured after intrastriatal administration (increase in DA release). Therefore, the inhibition of COMT is likely to be due to the action of a metabolite of CGP 28014 formed in the periphery and not in the brain.     

An In Vivo Study of Dopamine Release and Metabolism in Rat Brain Regions Using Intracerebral Dialysis

Intracerebral dialysis was used with a specifically designed HPLC with electrochemical detection assay to monitor extracellular levels of endogenous 3,4-dihydroxyphenylethylamine (dopamine, DA) and its major metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in brain regions of the halothane-anesthetized rat. Significant amounts of DA, DOPAC, and HVA were detected in control perfusates collected from striatum and n. accumbens whereas the medial prefrontal cortex showed lower monoamine levels. The ratio of DA in perfusate to DA in whole tissue suggests that in f. cortex, compared to n. accumbens and striatum, there is a greater amount of DA in the extracellular space relative to the intraneuronal DA content. The DOPAC/HVA ratio in control perfusates varied between regions in accordance with whole tissue measurements. This ratio was highest in n. accumbens and lowest in f. cortex. The monoamine oxidase inhibitor pargyline (100 mg/kg i.p.) caused an exponential decline in DOPAC, but not of HVA, in regional perfusates, an effect that was associated with an increase in DA. The data indicated a higher turnover of extracellular DOPAC in n. accumbens than in striatum and the lowest DOPAC turnover in f. cortex. The rate of decline in extracellular DA metabolite levels was slow compared to whole tissue measurements. In the perfusates there was no statistical correlation between basal amounts of DA in the perfusates and DOPAC and HVA levels or DOPAC turnover for any of the areas, indicating that measurement of DA metabolism in the brain under basal conditions does not provide a good index of DA release. In summary, this study shows clear regional differences in basal DA release and metabolite levels, metabolite patterns, and DOPAC turnover rates in rat brain in vivo.     

Dopamine metabolism and free-radical related mitochondrial injury during transient brain ischemia in gerbils

Regional extracellular release of dopamine (DA) and its metabolites, 3,4-dihydroxy-phenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) was measured in gerbils (with or without pargyline pretreatment) subjected to bilateral carotid artery occlusion (15 min) and various periods of recirculation (up to 6 hr), utilizing intracerebral microdialysis and high-performance liquid chromatography (HPLC) with electrochemical detection. Mitochondrial monoamine oxidase (MAO) and superoxide dismutase (SOD) activities andin vitro stimulated lipid peroxidation (TBARM) were determined in separate experimental groups of animals. The ischemically induced DA release, decrease of MAO-derived DA metabolites DOPAC and HVA, and accumulation of 3-MT were potentiated and prolonged by pargyline pretreatment. Mitochondrial MAO and SOD activities were significantly reduced during ischemia alone and up to 1 hr of reperfusion, whereas TBARM was enhanced during reflow only. The data suggest that reduced activity of mitochondrial antioxidative enzyme(s) but not DA metabolism by MAO may contribute to free radical-mediated injury of (mitochondrial) membranes.     

Effects of Manganese on Extracellular Levels of Dopamine in Rat Striatum: An Analysis <Emphasis Type="Italic">In vivo</Emphasis> by Brain Microdialysis

The aim of this study is to determine the effects of intrastriatal administration of MnCl_2, on the extracellular levels of dopamine (DA) and metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in basal conditions and stimulated by depolarization with KCl and pargyline administration. Also, we studied the effect of MnCl₂ on extracellular levels of l-Dopa in the presence of aromatic amino acid decarboxylase (AADC) inhibitor 3-hydroxybencilhydracine-HCl (NSD 1015). This study concluded that MnCl₂, reduced the basal and K⁺-stimulated DA-release in striatum, without notably affecting the DOPAC and HVA levels. Intraperitoneal injection of pargyline increased striatal DA levels, decreasing DOPAC and HVA levels. The infusion of MnCl₂ removed the increase in DA levels, without affecting DOPAC and HVA levels. Perfusion of NSD 1015 increased the extracellular levels of l-DOPA in striatum, and MnCl₂ increased the effect of NSD1015 on l-Dopa.     

Effect of N-Methyl-D-Aspartate and Potassium on Striatal Monoamine Metabolism in Immature Rat: An In Vivo Microdialysis Study

Effects of N-methyl-D-aspartate (NMDA) and potassium on 5-day-old rat's brain were examined. We measured extracellular striatal monoamines such as dopamine (DA), 3,4 dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindole-3-acetic acid (5-HIAA) using intracerebral microdialysis. After 3 h stabilization, pups received varying concentrations of NMDA (1–3 mM) and potassium (200–800 mM) by intrastriatal perfusion for 32 minutes. Increasing the concentration of NMDA and potassium induced a dose related DA increase (p < 0.001), whereas DOPAC, HVA, and 5-HIAA decreased significantly. Five days later the same animals were sacrificed and the weight reduction of their cerebral hemispheres was measured. The weight of the drug perfused side was significantly reduced compared with that of the contralateral one. We examined next the relationship between the level of maximum DA and the relative hemisphere weight reduction. The DA peak was highly correlated with the hemisphere weight reduction (r = 0.70, n = 52, p < 0.001 in the NMDA group, r = 0.83, n = 30, p < 0.001 in the potassium group, respectively). These data show that each treatment alter striatal monoamine metabolism in immature rat brain and that the extracellular DA peak is a potential early indicator to estimate brain injury.     

Effects of Single and Repeated Footshock on Dopamine Release and Metabolism in the Brains of Fischer Rats

Abstract: Changes in the tissue levels of 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and dopamine in the frontal cortex, hypothalamus, nucleus accumbens, and striatum were evaluated after 0.5-4 h of footshock (2 mA, for 3 s every 30 ± 5 s) in Fischer rats. 3-MT, DOPAC, and HVA levels in the four brain areas peaked at 0.5 h and in most cases returned to baseline values within 4 h. No changes were found in dopamine levels. Repeated footshock stress was evaluated by administering 10 footshock sessions (0.5 h, two per day for 5 days). At the end of the 10th footshock session, 3-MT levels were higher than at the end of the first footshock session in three of the four brain regions, indicating sensitization of dopamine release. No differences were found between the first and 10th footshock sessions in DOPAC and HVA levels. Fourteen days after the 10th footshock session, the levels of 3-MT, DOPAC, and HVA were the same as in control rats in all four brain regions. A 0.5-h footshock challenge presented 14 days after the 10th footshock session attenuated DOPAC levels in the hypothalamus and nucleus accumbens. In contrast, DOPAC and HVA levels in the frontal cortex showed sensitization after footshock challenge, and a similar trend was apparent for 3-MT levels. These results indicate that repeated footshock stress induces generalized sensitization of dopamine release and turnover in some areas of the brain of Fischer rats. This sensitization may persist in the cortical but not subcortical dopamine neurons after discontinuation of the treatment.     

In vivo release of endogenous dopamine, 5-hydroxytryptamine and some of their metabolites from rat caudate nucleus by phenylethylamine

The in vivo release of endogenous 3,4-dihydroxyphenylethylamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT), and of 5-hydroxytryptamine (5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), has been measured in the caudate nucleus of the anesthetized rat. A push-pull cannula was implanted into the brain, and the tissue perfused with artificial CSF or artificial CSF containing 5×10^–4 M phenylethylamine. The perfusate was collected and analyzed for DA, 5-HT and their metabolites by high performance liquid chromatography with electrochemical detection (HPLC-ECD). DA was released by phenylethylamine at rates significantly greater than its basal rate. 3-MT and 5-HT were undetectable in perfusates collected under basal conditions, but could be detected readlly during phenylethylamine stimulation. DOPAC, HVA and 5-HIAA concentrations were not significantly affected by phenylethylamine. The results suggest (1) that phenylethylamine may exert its behavioural effects through increased release of both DA and 5-HT, and (2) that in vivo measurements of the acid metabolites alone may not be indicative of the release of the amines.Special Issue Dedicated to Dr. Abel Lajtha.     

Further Studies on the Nature of Postsynaptic Dopamine Uptake and Metabolism in Rat Striatum: Sodium Dependency and Investigation of a Possible Role for Carrier-Mediated Uptake into Serotonin Neurons

The nature of postsynaptic sites involved in the uptake and metabolism of striatal 3,4-dihydroxyphenylethylamine (dopamine, DA) was investigated. The accumulation of [3H]DA (10(-7) M) into slices of rat striatum was found to be greatly dependent (greater than 99%) on the presence of sodium ion in the incubation medium. However, the formation of the [3H]dihydroxyphenylacetic acid (DOPAC) and [3H]homovanillic acid (HVA) was only partially reduced in the absence of sodium (DOPAC, 27% of control; HVA, 47% of control). Inhibition of carrier-mediated DA neuronal uptake with nomifensine (10(-5) M) significantly decreased DA accumulation (18% of control) and [3H]DOPAC formation (62% of control), but enhanced [3H]HVA production (143% of control). Inhibition of the 5-hydroxytryptamine (5-HT, serotonin) neuronal uptake system with fluoxetine (10(-6) M) or selective 5-HT neuronal lesions with 5,7-dihydroxytryptamine (5,7-DHT) had no effect on [3H]DOPAC or [3H]HVA formed from [3H]DA in the presence or absence of nomifensine. These results demonstrate that the uptake and subsequent metabolism of striatal DA to DOPAC and HVA is only partially dependent on carrier-mediated uptake mechanism(s) requiring sodium ion. These data support our previous findings suggesting a significant role for synaptic glial cell deamination and O-methylation of striatal DA. Further, experiments with fluoxetine or 5,7-DHT suggest that 5-HT neurons do not significantly contribute in the synaptic uptake and metabolism of striatal DA.     

Sodium diethyldithiocarbamate protects against the MPTP-induced inhibition of immune responses in mice

The effects of 1-methyl-4-phenyl - 1,2,3,6-tetrahydropyridine (MPTP) on immune parameters, and the restorative influence of sodium diethyldithiocarbamate (DTC) or deprenyl were evaluated in mice. The concentrations of dopamine (DA), 3-methoxytyramine (3-MT), 3-4-dihydroxyphenyl acetic acid (DOPAC), and homovanillic acid (HVA), were concomitantly measured in the striatum. MPTP depressed T-cell responses. DTC restored these responses as well as the concentration of striatal DA. Deprenyl had no effect on the concentrations of DA and its metabolites, yet it modified the immune responses alike MPTP. The findings suggest a dopamine pathway could be involved in the brain-controlled immunostimulation afforded by DTC.     

Propentophylline increases striatal dopamine release but dampens methamphetamine-induced dopamine dynamics: A microdialysis study

While there are currently no medications approved for methamphetamine (METH) addiction, it has been shown that propentofylline (PPF), an atypical methylxanthine, can suppress the rewarding effects of methamphetamine (METH) in mice. This experiment studied the interactions of PPF with METH in striatal dopaminergic transmission. Herein, the impact of PPF (10–40 mM, intrastriatally perfused (80 min) on the effect of METH (5 mg/kg, i.p.) on striatal dopamine (DA) release was evaluated using brain microdialysis in Sprague–Dawley adult rats. METH was injected at the 60 min time point of the 80 min PPF perfusion. The extracellular levels of DA and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined using high performance liquid chromatography with electrochemical detection (HPLC-ED). PPF induced a concentration-dependent increase in DA release beginning 30 min after the onset of PPF perfusion. DA peak levels evoked by 40 mM PPF were similar to those induced by 5 mg/kg METH i.p. Only the highest concentration of PPF decreased the METH-induced DA peak (circa 70%). The significant decreases in extracellular levels of DOPAC and HVA evoked by METH were partially blocked by 10 and 20 mM PPF. Although 40 mM of PPF also partially blocked the METH-induced DOPAC decrease, it completely blocked HVA depletion after a transient increase in HVA levels in METH-treated rats. Data indicates for the first time that while PPF increases presynaptic striatal DA dynamics it attenuates METH-induced striatal DA release and metabolism.     

Time Course of Adaptations in Dopamine Biosynthesis, Metabolism, and Release Following Nigrostriatal Lesions: Implications for Behavioral Recovery from Brain Injury

Alterations in neostriatal dopamine metabolism, release, and biosynthesis were determined 3, 5, or 18 days following partial, unilateral destruction of the rat nigrostriatal dopamine projection. Concentrations of dopamine and each of its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 3-methoxytyramine (3-MT) were markedly decreased in the lesioned striata at 3, 5, or 18 days postoperation. The decline in striatal high-affinity [3H]dopamine uptake closely matched the depletion of dopamine at 3 and 18 days postoperation. However, neither DOPAC, HVA, nor 3-MT concentrations were decreased to as great an extent as dopamine at any time following lesions that depleted the dopamine innervation of the striatum by greater than 80%. In these more severely lesioned animals, dopamine metabolism, estimated from the ratio of DOPAC or HVA to dopamine, was increased two- to four-fold in the injured hemisphere compared with the intact hemisphere. Dopamine release, estimated by the ratio of 3-MT to dopamine, was more increased, by five- to sixfold. Importantly, the HVA/dopamine, DOPAC/dopamine, and 3-MT/dopamine ratios did not differ between 3 and 18 days postlesioning. The rate of in vivo dopamine biosynthesis, as estimated by striatal DOPA accumulation following 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibition with NSD 1015, was increased by 2.6- to 2.7-fold in the surviving dopamine terminals but again equally at 3 and 18 days postoperation. Thus, maximal increases in dopamine metabolism, release, and biosynthesis occur rapidly within neostriatal terminals that survive a lesion. This mobilization of dopaminergic function could contribute to the recovery from the behavioral deficits of partial denervation by increasing the availability of dopamine to neostriatal dopamine receptors. However, these presynaptic compensations are not sufficient to account for the protracted (at least 3-week) time course of sensorimotor recovery that has been observed following partial nigrostriatal lesion.     

MK801 antagonism of the prolonged depletion of striatal dopamine by amphetamine in iprindole-treated rats.

The administration of amphetamine to rats pretreated with iprindole to inhibit the metabolism of amphetamine results in a long-lasting depletion of striatal dopamine and its metabolites, DOPAC and HVA. Pretreatment with MK801, a noncompetitive antagonist of the NMDA (N-methyl-D-aspartate) subclass of excitatory amino acid receptors, antagonized the depletion of striatal dopamine, DOPAC and HVA 3 days after a single dose of amphetamine in iprindole-treated rats. MK801 pretreatment was effective up to 4 hours but not at 8 or 24 hours in preventing amphetamine effects on striatal dopamine, DOPAC and HVA.     

Study by the intracerebral microdialysis method of the effects of atypical neuroleptics and anxiolytics on striatal release and metabolism of dopamine in awake rats]

Using brain microdialysis in awake rats effects of risperidone, ritanserin, buspirone, sulpiride and 5-methoxy-N,N-dimethyltryptamine (MeODMT) on striatal dopamine (DA) release and metabolism were studied. Risperidone, sulpiride and buspirone increased levels of DA, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Ritanserin failed to affect DA release, while increased DOPAC and HVA levels. MeODMT had no effect on striatal DA release and metabolism. Possible interaction between DA and serotonin systems is discussed.     

Dynamics of the striatal 3-MT pool in rat and mouse: species differences as assessed by steady-state measurements and intracerebral dialysis

Analysis of dopamine and 3-MT dynamics in the striatum and in striatal dialysates, after pargyline treatment, indicates that the 3-MT pool in the rat is smaller but more dynamic than that in the mouse. The fractional rate constants calculated for the extracellular 3-MT and dopamine pools also indicate that a larger proportion of released dopamine is metabolized to 3-MT in the rat, or alternatively, 3-MT is cleared more slowly from the mouse striatum. Our striatal dialysis data also support previous in vivo voltammetry studies which have demonstrated that the fractional rate constant of the extracellular dopamine pool is at least 10-fold greater than that of the total striatal dopamine pool. These data suggest that multiple striatal dopamine pools exist and that a minimum of 20 to 30% of the extracellular DA pool is metabolized to 3-MT.     

Increased Cerebrospinal Fluid Dopamine and 3,4-Dihydroxyphenylacetic Acid Levels in Huntington''s Disease: Evidence for an Overactive Dopaminergic Brain Transmission

Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), noradrenaline (NA), 3-methoxy-4-hydroxyphenylglycol (MHPG), and 5-hydroxyindoleacetic acid (5-HIAA) in the CSF of patients with Huntington's disease (HD) were measured by HPLC. CSF DA, DOPAC, and MHPG levels were found to be increased in HD patients. Levels of HVA, 5-HIAA, and NA in the CSF of HD patients did not differ from those of controls. Changes in CSF DA and DOPAC levels were consistent with previous findings of increased DA tissue content in some brain areas of patients with HD. These results suggest that CSF DOPAC levels could be a more reliable index of overactive dopaminergic brain systems in HD than CSF HVA levels.     

Effects of Catechol-O-Methyltransferase Inhibitors and L-3,4-Dihydroxyphenylalanine With or Without Carbidopa on Extracellular Dopamine in Rat Striatum

Abstract: The effects of two new catechol- O -methyltransferase (COMT) inhibitors, OR-611 and Ro 40-7592, in combination with L-3,4-dihydroxyphenylalanine (L-dopa) with or without carbidopa on extracellular levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3- O -methyldopa (3-OMD), and 5-hydroxyindoleacetic acid in rat striatum were studied. A dose of 10 mg/kg i.p. of Ro 40-7592 alone, in contrast to the same dose of OR-611, decreased the dialysate level of HVA and increased that of DOPAC; this dose was thus used to differentiate between the effects of central and peripheral COMT inhibition. L-Dopa (50 mg/kg i.p.) alone slightly increased extracellular levels of DA, DOPAC, and HVA. The effects of L-dopa were potentiated by carbidopa (50 mg/kg i.p.), and even 3-OMD levels in dialysate samples became detectable. Both OR-611 and Ro 40-7592 significantly further increased the DA and DOPAC efflux from striatum produced by L-dopa. This increase was more pronounced when carbidopa was added to the treatment. OR-611 did not modify the effect of L-dopa or carbidopa/L-dopa on dialysate HVA levels, whereas Ro 40-7592 markedly reduced those levels. Both OR-611 and Ro 40-7592 very clearly suppressed dialysate 3-OMD levels produced by carbidopa/L-dopa. Ro 40-7592 was more effective than OR-611 in potentiating the effects of L-dopa or carbidopa/L-dopa. These in vivo data show that the new COMT inhibitors markedly inhibit the O -methylation of L-dopa and increase its availability to brain, which is reflected as increased DA formation. A significant effect can be achieved even by inhibiting only the peripheral COMT activity. The data suggest that COMT inhibitors may be of clinical importance as adjuncts in the treatment of Parkinson's disease.     

Influence of a carrier transport process on in vivo release and metabolism of dopamine: dependence on extracellular Na+

In vivo microdialysis was utilized to evaluate the role of extracellular Na+ in regulating dopamine (DA) neurotransmission in the caudate-putamen of halothane-anaesthetized rats. Reduction of the extracellular Na+ concentration by introduction of a perfusion media containing 50mM Na+ (with choline replacement) produced an excessive release of DA that could be effectively blocked by nomifensine and Lu 19-005, potent inhibitors of an amine transport carrier. These results substantiate reports of a carrier-mediated efflux of DA from presynaptic terminals. Pretreatment with amphetamine, considered both a DA uptake inhibitor and releaser, did not, however, influence the efflux of DA induced by the low extracellular Na+ environment. Although the release of DA from an apparent non-granular cytosolic pool was greatly enhanced by the low extracellular Na+ environment, 3,4-dihydrophenylacetic acid (DOPAC) levels, which supposedly reflect metabolism of non-vesicular DA, were minimally effected. In contrast, homovanillic acid (HVA) was sensitive to extracellular Na+ and not directly related to extracellular levels of either DA or DOPAC, suggesting the possibility of a Na+-sensitive (carrier-mediated?) process involved in the formation of HVA. Overall, the results of this paper cannot be completely reconciled with the traditional concept of intracellular organization of DA pools and suggests the possibility of various non-granular pools being differentially sensitive to efflux and metabolism.