Insulin-like growth factors decrease catecholamine content in PC12 rat pheochromocytoma cells.

Insulin-like growth factors (IGFs) stimulate proliferation and differentiation of PC12 rat pheochromocytoma cells and modulate catecholamine release in bovine adrenal medullary cells. Dexamethasone increases catecholamine synthesis in PC12 cells. We therefore studied the effects of IGFs and dexamethasone on catecholamine content in PC12 cells. Dopamine (DA) and norepinephrine (NE) content of PC12 cells were measured after incubation for 72 h with IGFs (100 ng/ml) and/or dexamethasone (500 nM). IGF-I (100 ng/ml) and IGF-II (100 ng/ml) decreased DA and NE content to approximately 35% and approximately 25% of control, respectively. [Leu27]IGF-II, which binds to the IGF-I receptor with markedly decreased affinity, did not reduce catecholamine levels, indicating that the effect is likely to be mediated by the IGF-I receptor. Dexamethasone (500 nM) increased levels of DA and NE to 173 +/- 20% and 331 +/- 48% of controls, respectively. Coincubation with IGFs did not significantly affect the stimulation of DA by dexamethasone, but abolished the rise in NE. Levels of tyrosine hydroxylase mRNA, protein and activity were increased following incubation with dexamethasone, but were unchanged by IGFs. These results indicate that IGFs decrease catecholamine content in PC12 cells via the IGF-I receptor. Complex regulation involving multiple synthetic and/or degradative steps is implicated in this process.     

Effects of Handling or Immobilization on Plasma Levels of 3,4-Dihydroxyphenylalanine, Catecholamines, and Metabolites in Rats

In conscious animals, handling and immobilization increase plasma levels of the catecholamines norepinephrine (NE) and epinephrine (EPI). This study examined plasma concentrations of endogenous compounds related to catecholamine synthesis and metabolism during and after exposure to these stressors in conscious rats. Plasma levels of 3,4-dihydroxyphenylalanine (DOPA), NE, EPI, and dopamine (DA), the deaminated catechol metabolites 3,4-dihydroxyphenylglycol (DHPG), and 3,4-dihydroxyphenylacetic acid (DOPAC), and their O-methylated derivatives methoxyhydroxyphenylglycol (MHPG) and homovanillic acid (HVA) were measured using liquid chromatography with electrochemical detection at 1, 3, 5, 20, 60, and 120 min of immobilization. By 1 min of immobilization, plasma NE and EPI levels had already reached peak values, and plasma levels of DOPA, DHPG, DOPAC, and MHPG were increased significantly from baseline, whereas plasma DA and HVA levels were unchanged. During the remainder of the immobilization period, the increased levels of DOPA, NE, and EPI were maintained, whereas levels of the metabolites progressively increased. In animals immobilized briefly (5 min), elevated concentrations of the metabolites persisted after release from the restraint, whereas DOPA and catecholamine levels returned to baseline. Gentle handling for 1 min also significantly increased plasma levels of DOPA, NE, EPI, and the NE metabolites DHPG and MHPG, without increasing levels of DA or HVA. The results show that in conscious rats, immobilization or even gentle handling rapidly increases plasma levels of catecholamines, the catecholamine precursor DOPA, and metabolites of NE and DA, indicating rapid increases in the synthesis, release, reuptake, and metabolism of catecholamines.     

Transport of dopamine and levodopa and their interaction in COS-7 cells heterologously expressing monoamine neurotransmitter transporters and in monoaminergic cell lines PC12 and SK-N-SH

The present study investigated the effects of levodopa, a precursor of dopamine (DA) therapeutically used for the treatment of Parkinson's disease, on DA transport in the two different systems, COS-7 cells heterologously expressing rat monoamine transporter cDNA and in monoaminergic cell lines PC12 and SK-N-SH. Levodopa enhanced uptake of [3H]DA and [3H]norepinephrine (NE) but not [3H]serotonin in the transfected COS-7 cells in a concentration-dependent manner. On the other hand, in PC12 and SK-N-SH cells where NET is functionally expressed, levodopa enhanced [3H]DA and [3H]NE uptake at low concentrations and inhibited the uptake at higher concentrations. The effects of levodopa on catecholamine transporters in the opposite direction suggest a different mechanism at the intra- and extracellular sites in a levodopa transport-dependent and independent manner.     

Effects of natriuretic peptides (ANP, BNP, CNP) on catecholamine synthesis and TH mRNA levels in PC12 cells

Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) are present in adrenal chromaffin cells, and are co-secreted with catecholamines suggesting that these natriuretic peptides (NPs) may modulate functions of chromaffin cells in an autocrine and/or paracrine manner. Therefore, we investigated the effects of NPs on tyrosine hydroxylase (TH: a rate-limiting enzyme in biosynthesis of catecholamine) mRNA in rat pheochromocytoma PC12 cells. It was also determined whether the cyclic GMP/cGMP-dependent protein kinase (cGMP/PKG) pathway was involved in theses effects. Finally, we examined the effects of NPs on intracellular catecholamine content to confirm increase of catecholamine synthesis following TH mRNA induction. NPs (0.1 microM) induced significant increases of the TH mRNA (ANP= BNP> CNP). Also, the effects of NPs on TH mRNA were mimicked by 8-bromo cyclic GMP (1mM), and were blocked by KT5823 (1 microM) (inhibitor PKG) or LY83583 (1 microM) (guanylate cyclase inhibitor). Moreover, NPs were shown to induce significant increases of intracellular catecholamine contents (ANP= BNP> CNP). These findings suggest that NPs induced increases of TH mRNA through cGMP/PKG dependent mechanisms, which, in turn, resulted in stimulation of catecholamine synthesis in PC12 cells.     

Catecholamines in larvae and juveniles of the prosobranch gastropod, Crepidula fornicata

We investigated roles of catecholamines in metamorphosis of the prosobranch gastropod, Crepidula fornicata. Levels of DOPA, norepinephrine (NE) and dopamine (DA) were measured by high-pressure liquid chromatography (HPLC) in competent larvae and juvenile siblings that metamorphosed in response to the natural adult-derived cue or to elevated K+. Competent larvae contained 1.58 +/- 0.26 (S.E.M.) x 10(-2) pmol DOPA, 0.91 +/- 0.45 x 10(-2) pmol NE, and 0.290 +/- 0.087 pmol DA (mean values per microg total protein, n = 4 batches of larvae). Levels of DA per individual were not different between larvae and juvenile siblings; levels of NE were higher in juveniles. The tyrosine hydroxylase (TH) inhibitor alpha-methyl-DL-m-tyrosine (alpha-MMT) depleted DOPA and DA to approximately half of control values without affecting levels of NE. Depletion of DOPA and DA was accompanied by inhibition of metamorphosis in response to the natural cue but not to elevated K+. The dopamine-beta-hydroxylase inhibitor diethyldithiocarbamate (DDTC) induced high frequencies of metamorphosis at concentrations of 0.1-10 microM. In juveniles induced by 10 microM DDTC, levels of both NE and DA averaged approximately 80% of those in control larvae. Catecholamines may function as endogenous regulators of metamorphosis in C. fornicata.     

Catecholamine level in cerebrospinal fluid of epileptics

We estimated catecholamine levels in CSF of 15 epileptics and 75 non-neurological patients utilizing a high performance liquid chromatograph with a highly sensitive fluorometer and found the following results: The dopamine (DA) levels in males were significantly higher than those in females, while norepinephrine (NE) levels in males were the same as in females. The DA levels were significantly lower and NE levels significantly higher in epileptics than in non-neurological patients. DA and NE in petit mal patients were on the average lower than in grand mal patients, but untreated grand mal patients had higher NE levels. These results suggest that epilepsy may be associated with a disturbance of DA and/or NE metabolism or release in the brain.     

Effect of 1-methyl-4-phenylpyridinium ion (MPP+) on catecholamine levels and activity of related enzymes in clonal rat pheochromocytoma PC12h cells

1-Methyl-4-phenylpyridinium ion (MPP+), a metabolite of a neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, was found to reduce dopamine (DA) level and the activity of enzymes related to its metabolism in clonal rat pheochromocytoma PC12h cells. After 6 days' culture in the presence of 1 mM and 100 microM MPP+, DA content in PC12h cells was reduced markedly, but with MPP+ at concentrations lower than 10 microM, DA levels in the cells did not change. The amounts of 3,4-dihydrophenylacetic acid (DOPAC), a metabolite of DA were reduced markedly in culture medium and in PC12h cells cultured with MPP+ at concentrations higher than 1 microM. MPP+ was found to reduce the enzyme activity of tyrosine hydroxylase (TH), monoamine oxidase (MAO) and aromatic L-aminoacid decarboxylase (AADC). In the presence of MPP+ at concentrations higher than 10 microM, reduction of TH activity in the cells was more pronounced than reduction of cell protein or of the activity of a non-specific enzyme, beta-galactosidase. With 1 mM and 100 microM MPP+, MAO activity was reduced to about 30% of that in control cells. Reduction was observed with MPP+ at concentrations higher than 1 microM. AADC was the most sensitive to MPP+ and its activity was reduced markedly in the cells cultured with 100 nM MPP+. These results indicate that MPP+ inhibits not only the biosynthesis of catecholamines, but also the enzyme participating in their catabolism in cells, and may thus perturb catecholamine levels in the brain.     

l-3,4-Dihydroxyphenylalanine Increases the Quantal Size of Exocytotic Dopamine Release In Vitro

Abstract: The catecholamine precursor l -3,4-dihydroxyphenylalanine ( l -DOPA) is used to augment striatal dopamine (DA), although its mechanism of altering neurotransmission is not well understood. We observed the effects of l -DOPA on catecholamine release in ventral midbrain neuron and PC12 pheochromocytoma cell line cultures. In ventral midbrain neuron cultures exposed to 40 m M potassium-containing media, l -DOPA (100 µ M for 1 h) increased DA release by >10-fold. The elevated extracellular DA levels were not significantly blocked by the DA/norepinephrine transport inhibitor nomifensine, demonstrating that reverse transport through catecholamine-uptake carriers plays little role in this release. In PC12 cells, where DA release from individual secretory vesicles can be observed, l -DOPA (50 µ M for 1 h) elevated DA release in high-potassium media by 370%. Amperometric measurements demonstrated that l -DOPA (50 µ M for 40–70 min) did not raise the frequency of vesicular exocytosis but increased the average size of quantal release to at least 250% of control levels. Together, these findings suggest that l -DOPA can increase stimulation-dependent transmitter release from DA cells by augmenting cytosolic neurotransmitter, leading to increased quantal size.     

Chronic ketamine administration modulates midbrain dopamine system in mice

Ketamine is an anesthetic and a popular abusive drug. As an anesthetic, effects of ketamine on glutamate and GABA transmission have been well documented but little is known about its long-term effects on the dopamine system. In the present study, the effects of ketamine on dopamine were studied in vitro and in vivo. In pheochromocytoma (PC 12) cells and NGF differentiated-PC 12 cells, ketamine decreased the cell viability while increasing dopamine (DA) concentrations in a dose-related manner. However, ketamine did not affect the expression of genes involved in DA synthesis. In the long-term (3 months) ketamine treated mice, significant increases of DA contents were found in the midbrain. Increased DA concentrations were further supported by up-regulation of tyrosine hydroxylase (TH), the rate limiting enzyme in catecholamine synthesis. Activation of midbrain dopaminergic neurons could be related to ketamine modulated cortical-subcortical glutamate connections. Using western blotting, significant increases in BDNF protein levels were found in the midbrain, suggesting that perhaps BDNF pathways in the cortical-subcortical connections might contribute to the long-term ketamine induced TH upregulation. These data suggest that long-term ketamine abuse caused a delayed and persistent upregulation of subcortical DA systems, which may contribute to the altered mental status in ketamine abusers.     

Plasma catecholamine responses to tyrosine hydroxylase inhibition and cold exposure

The acute effect of α-methyl-p-tyrosine (αMPT), a tyrosine hydroxylase inhibitor, on plasma levels of norepinephrine (NE), epinephrine (E), dopamine (DA), and adrenal cholesterol content in male rats at room temperature (24°C) and during acute cold exposure (4°C) was evaluated. Compared to saline-treated controls, αMPT: 1) significantly reduced plasma NE and DA in both normal and cold stress conditions, 2) significantly increased plasma E in both environments, and 3) stimulated the adrenal cortex. These findings suggest that tyrosine hydroxylase inhibition and consequent catecholamine synthesis blockade disrupts the homeokinesis of adrenergic processes and may present a significant stress to the intact animal.     

Dopamine effects on identified rat vagal motoneurons

Catecholaminergic neurons of the A2 area play a prominent role in brain stem vagal circuits. It is not clear, however, whether these neurons are noradrenergic or adrenergic, i.e., display tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DbetaH) immunoreactivity (-IR) or dopaminergic (i.e., TH- but not DbetaH-IR). Our aims were to investigate whether a subpopulation of neurons in the A2 area was dopaminergic and, if so, to investigate the effects of dopamine (DA) on the membrane of gastric-projecting vagal motoneurons. We observed that although the majority of A2 neurons were both TH- and DbetaH-IR, a small percentage of nucleus tractus solitarius neurons were TH-IR only, suggesting that DA itself may play role in these circuits. Whole cell recordings from thin brain stem slices showed that 71% of identified gastric-projecting motoneurons responded to DA (1-300 microM) with either an excitation (28%) or an inhibition (43%) of the membrane; the remaining 29% of the neurons were unresponsive. The DA-induced depolarization was mimicked by SK 38393 and prevented by pretreatment with SCH 23390. Conversely, the DA-induced inhibition was mimicked by bromoergocryptine and prevented by pretreatment with L741626. When tested on the same neuron, the effects of DA and NE were not always similar. In fact, in neurons in which DA induced a membrane depolarization, 77% were inhibited by NE, whereas 75% of neurons unresponsive to DA were inhibited by NE. Our data suggest that DA modulates the membrane properties of gastric-projecting motoneurons via D1- and D2-like receptors, and DA may play different roles than norepinephrine in brain stem vagal circuits.     

Regulation of nerve growth factor synthesis/secretion by catecholamine in cultured mouse astroglial cells

The nerve growth factor (NGF) synthesis/secretion by cultured mouse astroglial cells was modulated by catecholamine. In quiescent cells, epinephrine (EN) and dopamine (DA) markedly increased the NGF content in the conditioned medium (CM). Conversely, EN, DA, and norepinephrine (NE) decreased the NGF content in growing cells. Cholinergic agonists, metacholine and carbamylcholine, slightly increased the NGF content in quiescent cells, but showed no effects on growing cells. Other neurotransmitters tested had no effects on either growing or quiescent cells. These results suggest that catecholamine is one of the molecules responsible for regulation of NGF synthesis/secretion in the mouse brain.     

Glucocorticoids Increase Catecholamine Synthesis and Storage in PC 12 Pheochromocytoma Cell Cultures

Abstract: Glucocorticoids, cholera toxin and high plating density all increase the activity of tyrosine 3-monooxygenase (TH) in cultured PC12 pheochromocytoma cells. Glucocorticoids increase enzyme activity in cells treated with cholera toxin and in cells grown at high plating density. Glucocorticoids also increase the content of stored catecholamines in the cells. In cells cultured under routine conditions, glucocorticoids primarily increase the stores of dopamine. The addition of ascorbate to the culture medium increases the storage of norepinephrine in both steroid-treated and untreated cells. Incubation of the cells in media containing 56 n M K⁺ causes the release of the same percentage of stored dopamine from steroid-treated as from untreated cells. Steroid-treated cells contain more dopamine than do untreated cells and therefore, in response to high K⁺, the steroid-treated cells secrete more dopamine than do untreated cells. We conclude that the activity of tyrosine 3-monooxygenase in PC12 cells can be regulated by several distinct mechanisms; that glucocorticoids cause a coordinate increase in TH activity and in catecholamine storage; that steroids increase the storage of catecholamines in a releasable pool; and that the steroid-induced increase in catecholamine storage may result in increased secretion of catecholamines from steroid-treated cells.     

Release of dopamine and norepinephrine by hypoxia from PC-12 cells

We examined the effects of hypoxia on the release of dopamine(DA) and norepinephrine (NE) from rat pheochromocytoma 12 (PC-12) cellsand assessed the involvement ofCa²⁺ and protein kinases instimulus-secretion coupling. Catecholamine release was monitored bymicrovoltammetry using a carbon fiber electrode as well as by HPLCcoupled with electrochemical detection (ECD). Microvoltammetricanalysis showed that hypoxia-induced catecholamine secretion(PO₂ ofmedium ~40 mmHg) occurred within 1 min after the onset of thestimulus and reached a plateau between 10 and 15 min. HPLC-ECD analysisrevealed that, at any level of PO₂, therelease of NE was greater than the release of DA. In contrast, inresponse to K⁺ (80 mM), DA releasewas ~11-fold greater than NE release. The magnitude ofhypoxia-induced NE and DA releases depended on the passage, source, andculture conditions of the PC-12 cells. Omission of extracellularCa²⁺ or addition of voltage-gatedCa²⁺ channel blockers attenuatedhypoxia-induced release of both DA and NE to a similarextent. Protein kinase inhibitors, staurosporine (200 nM) andbisindolylmaleimide I (2 µM), on the other hand, attenuatedhypoxia-induced NE release more than DA release. However, proteinkinase inhibitors had no significant effect onK⁺-induced NE and DA releases.These results demonstrate that hypoxia releases catecholamines fromPC-12 cells and that, for a given change inPO₂, NErelease is greater than DA release. It is suggested that proteinkinases are involved in the enhanced release of NE during hypoxia.

     

Facilitation of dopamine and acetylcholine release by intermittent hypoxia in PC12 cells: involvement of calcium and reactive oxygen species.

We have investigated the effects of preconditioning pheochromocytoma (PC12) cells with intermittent hypoxia (IH) on transmitter release during acute hypoxia. Cell cultures were exposed to either alternating cycles of hypoxia (1% O(2) + 5% CO(2); 30 s/cycle) and normoxia (21% O(2) + 5% CO(2); 3 min/cycle) for 15 or 60 cycles or normoxia alone (control) for similar durations. Control and IH cells were challenged with either hyperoxia (basal release) or acute hypoxia (Po(2) of approximately 35 Torr) for 5 min, and the amounts of dopamine (DA) and acetylcholine (ACh) released in the medium were determined by HPLC combined with electrochemical detection. Hypoxia augmented DA (approximately 80%) but not ACh release in naive cells, whereas, in IH-conditioned cells, it further enhanced DA release (ranging from 120 to approximately 145%) and facilitated ACh release (approximately 30%). Hypoxia-evoked augmentation of transmitter release was not seen in cells conditioned with sustained hypoxia. IH-induced increase in DA but not IH-induced ACh release during hypoxia was partially inhibited by cadmium chloride (100 microM), a voltage-gated Ca(2+) channel blocker. By contrast, 2-aminoethoxydiphenylborate (75 microM), a blocker of inositol 1,4,5-trisphosphate (IP(3)) receptors, and N-acetyl-L-cysteine (300 microM), a potent scavenger of reactive oxygen species, either attenuated or abolished IH-evoked augmentation of transmitter release during hypoxia. Together, the above results demonstrate that IH conditioning increases hypoxia-evoked neurotransmitter release from PC12 cells via mechanisms involving mobilization of Ca(2+) from intracellular stores through activation of IP(3) receptors. Our findings also suggest that oxidative stress plays a central role in IH-induced augmentation of transmitter release from PC12 cells during acute hypoxia.     

Investigation of Dopamine Content, Synthesis, and Release in the Rabbit Retina In Vitro: II. Effects of High Potassium, Adenylate Cyclase Activators, and N-n-Propyl-3-(3-Hydroxyphenyl) Piperidine

The modulation of 3,4-dihydroxyphenylethylamine (dopamine, DA) synthesis and release in rabbit retina in vitro by high K+; adenylate cyclase activators such as forskolin, 2-chloroadenosine, vasoactive intestinal polypeptide (VIP); and the putative DA autoreceptor agonist N-n-propyl-3-(3-hydroxyphenyl) piperidine (3-PPP) has been investigated. Incubation of retinas in 50 mM K+ resulted in the activation of tyrosine hydroxylase (TH). Activation did not require the presence of extracellular Ca2+. K+ 50 mM also induced a Ca2+-dependent release of DA. Forskolin 50 microM stimulated TH but 100 microM 2-chloroadenosine and 650 nM VIP did not. Individually, (+)-3-PPP, (-)-3-PPP, and (+/-)-3-PPP reduced DA synthesis and increased its release. The effects of (+/-)-3-PPP were dose-dependent and did not require the presence of extracellular Ca2+. The activation of TH induced by 50 mM K+, but not that induced by 50 microM forskolin, was abolished by 100 microM (+/-)-3-PPP.     

PRIMARY CULTURES OF DISSOCIATED SYMPATHETIC NEURONS : II. Initial Studies on Catecholamine Metabolism

Initial studies are reported on the catecholamine metabolism of low-density cultures of dissociated primary sympathetic neurons. Radioactive tyrosine was used to study the synthesis and breakdown of catecholamines in the cultures. The dependence of catecholamine synthesis and accumulation on external tyrosine concentration was examined and a concentration which is near saturation, 30 µM, was chosen for further studies. The free tyrosine pool in the nerve cells equilibrated with extracellular tyrosine within 1 h; the total accumulation of tyrosine (free tyrosine plus protein, catecholamines, and metabolites) was linear for more than 24 h of incubation. Addition of biopterin, the cofactor of tyrosine hydroxylase, only slightly enhanced catecholamine biosynthesis by the cultured neurons. However, addition of reduced ascorbic acid, the cosubstrate for dopamine β-hydroxylase, markedly stimulated the conversion of dopamine (DA) to norepinephrine (NE). Phenylalanine, like tyrosine, served as a precursor for some of the DA and NE produced by the cultures, but tyrosine always accounted for more than 90% of the catecholamines produced. The DA pool labeled rapidly to a saturation level characteristic of the age of the culture. The NE pool filled more slowly and was much larger than the DA pool. The disappearance of radioactive NE and DA during chase experiments followed a simple exponential curve. Older cultures showed both more rapid production and more rapid turnover of the catecholamines than did younger cultures, suggesting a process of maturation.     

Catecholaminotropic effects of catecholamines in a teleost fish,Anguilla rostrata

Summary Injections of physiological and supraphysiological doses of epinephrine (E) into cardiaccannulated eels cause a dose-related increase of plasma dopamine (DA) and norepineprine (NE) within 3 min. Likewise, both exogenous DA and NE increase the plasma titers of the respective other two catecholamines (CAs). The baseline titers of NE and E are closely correlated. Lack of a correlation of the baseline titers of NE and E with that of DA appears to be due to a faster disappearance rate of DA from the circulation. E is strongly hyperglycemic, and the weaker glycemic action of NE may be mediated via E release. The effects of E seem to depend on a spurt-like increase rather than its titer per se. The ability of the eel to cope with very fast, excessive increases of plasma CAs raises the question of the underlying mechanisms.Abbreviations CA(s) catecholamine(s) - DA dopamine - NE norepinephrine - E epinephrine     

Synthetic analgesics and other phenylpiperidines: effects on uptake and storage of dopamine and other monoamines mouse forebrain tissue

The neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) can induce degeneration of dopamine (DA) and other central monoamine neurons, leading to Parkinson's disease-like effects in man, monkey, and mouse. MPTP and other substituted phenylpiperidines related to synthetic analgesics including alphaprodine and meperidine were evaluated for potency vs. uptake of 0.1 microM tritiated DA, norepinephrine (NE), or serotonin (5HT) in synaptosomal preparations of mouse striatum or cerebral cortex. The most potent inhibitor of the uptake of 3H-DA was N-methyl-4-phenylpyridinium ion (MPP+; IC50 = 1 microM, Ki = 0.4 microM), a metabolite of MPTP; its effect was competitive and reversible. Other analogs of MPTP: the N-ethylindole AHR-1709, N,N-dimethyl-MPTP, and N-methyl-4-phenylpiperidine were all more potent than MPTP against 3H-DA uptake. N-dealkylation and N-propyl substitution, as well as pyridine ring substitution, decreased affinity for DA uptake while 3',4'-dihydroxyphenyl substitution increased potency and selectivity for catecholamine uptake, and quarternarization of the pyridine ring also increased potency against DA uptake. Active compounds showed higher potency against the uptake of NE than of DA. MPP+ was also more potent than MPTP in releasing endogenous DA from striatal synaptosomes (EC50 = 3 vs. 30 microM), but did not release the cytoplasmic markers tyrosine hydroxylase and lactate dehydrogenase (LDH). In contrast to MPTP, synthetic phenylpiperidine analgesics, their potential metabolites and the experimental neuroleptic agent AHR-1709 all failed to deplete striatal DA in vivo, even if active in vitro against DA uptake.