[3H]Dihydrotetrabenazine, a New In Vitro Monoaminergic Probe for Human Brain

The monoamine transporter of dopamine (DA), noradrenaline, and 5-hydroxytryptamine synaptic vesicles was assayed in rat and human brain homogenates by in vitro binding of [3H]dihydrotetrabenazine. [3H]Reserpine, a second ligand of the vesicular monoamine transporter, could not be used. [3H]Dihydrotetrabenazine binding in rat brain was stable after 72 h at 22 degrees C postmortem. In major human brain regions, [3H]dihydrotetrabenazine binding was specific and saturable (KD, 2.7 nM). Displacement constants by substrates or inhibitors of vesicular monoamine uptake, and regional distribution in human brain were similar to those found in rodents. The highest densities of binding sites were observed in caudate nucleus, putamen, and accumbens nucleus. In caudate nucleus and in putamen from normal human subjects, [3H]dihydrotetrabenazine binding and homovanillic acid concentration were significantly or nearly significantly correlated. A weaker correlation was found between [3H]dihydrotetrabenazine binding and DA, in association with a higher variability of DA. [3H]Dihydrotetrabenazine binding in caudate nucleus and in putamen decreased significantly with age, unlike DA and homovanillic acid concentrations. The results establish [3H]dihydrotetrabenazine as a presynaptic monoaminergic ligand of interest for studies on postmortem human brain.     

Characterization of the vesicular monoamine transporter in cultured rat sympathetic neurons: persistence upon induction of cholinergic phenotypic traits

The binding of [3H]dihydrotetrabenazine ([3H]TBZOH), a specific ligand of the reserpine-sensitive monoamine transporter in brain and adrenal medulla storage vesicles, has been measured in cultured sympathetic neurons from newborn rat in relation to their neurotransmitter phenotype. As shown previously, neurons cultured in the absence of muscle-conditioned medium displayed high activities in catecholamine synthesizing enzymes and low levels of choline acetyltransferase, and neurons cultured in conditioned medium displayed the reverse pattern (J. P. Swerts, A. Le Van Thai, A. Vigny, and M. J. Weber, Dev. Biol. 100, 1-11, 1983). The density of [3H]TBZOH binding sites as well as their subcellular distribution were identical in both types of cultures. Two other structures rich in choline acetyltransferase, the electric organ of Torpedo and the ciliary ganglion of the chick embryo did not contain measurable amounts of [3H]TBZOH binding sites, suggesting that the monoamine transporter is not an ubiquitous component of cholinergic synaptic vesicles. These data suggest that the synthesis of the monoamine transporter in sympathetic neurons is not coregulated with the syntheses of the three norpinephrine synthesizing enzymes. It is proposed that the same population of synaptic vesicles can accumulate acetylcholine or catecholamine, depending only upon which neurotransmitter synthesizing enzymes are expressed by sympathetic neurons.     

Down''s Syndrome Individuals Begin Life with Normal Levels of Brain Cholinergic Markers

We measured the activities of the cholinergic marker enzymes choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in autopsied brains of seven infants (age range 3 months to 1 year) with Down's syndrome (DS), a disorder in which virtually all individuals will develop by middle age the neuropathological changes of Alzheimer's disease accompanied by a marked brain cholinergic reduction. When compared with age-matched controls cholinergic enzyme activity was normal in all brain regions of the individuals with infant DS with the exception of above-normal activity in the putamen (ChAT) and the occipital cortex (AChE). Our neurochemical observations suggest that DS individuals begin life with a normal complement of brain cholinergic neurons. This opens the possibility of early therapeutic intervention to prevent the development of brain cholinergic changes in patients with DS.     

Changes in the Turnover of Monoamines in Prefrontal Cortex of Rats Fed on Vitamin E-Deficient Diet

Turnover of noradrenaline (NA), dopamine (DA), serotonin (5-HT), and their metabolites has been measured after a 15-day vitamin E-deficient diet in adult rat prefrontal cortex. Turnover rates of 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxy-3-indoleacetic acid have been assayed from the disappearance rates after blocking by pargyline inhibition of monoamine oxidase. NA, DA, and 5-HT turnover rates have been measured as accumulation rates of NA, DA, and 5-HT after pargyline inhibition of monoamine oxidase. No change was found in the turnover rate of NA between control and experimental animals. In contrast, turnover rates of DA and homovanillic acid significantly increased in the animals fed on a low-vitamin E diet. However, the most striking results were found on the serotoninergic system. Levels of 5-HT and its main metabolite, 5-hydroxy-3-indoleacetic acid, and their respective turnover rates were lower in the vitamin E-deficient diet. These results could indicate that vitamin E is necessary for the normal functioning of the serotoninergic neurons in the rat prefrontal cortex. The involvement of vitamin E in preventing the formation of free radicals is well known. Therefore, this lack of protective effect after a 15-day vitamin E-deficient diet could be responsible for the neuronal damage to the serotoninergic system. The opposing results found in DA (increase) and 5-HT (decrease) turnover could provide further evidence for an inhibitory control of the serotoninergic ascending pathways to the dopaminergic system in the prefrontal cortex.     

Biochemical Evidence for Alteration of Neostriatal Dopaminergic Function by 5,7-Dihydroxytryptamine

Unilateral injection of 5,7-dihydroxytryptamine (DHT) into the rat neostriatum markedly reduced not only striatal tryptophan hydroxylase (TPH) activity but also striatal tyrosine hydroxylase (TH) activity and dopamine (DA) concentration measured 10--15 days later. The decrease in striatal TH activity was dose related over the range of 8--32 micrograms of DHT; a dose of 16 micrograms reduced striatal TH activity to 40--50% of control, DA concentration to 38% of control, and TPH activity to 5--20% of control. Intrastriatal injection of 16 micrograms of DHT reduced TH activity in the ipsilateral substantia nigra to 51% of control. Pretreatment with amfonelic acid, a potent DA uptake inhibitor, significantly reduced the effect of DHT on striatal and nigral TH activity and striatal DA concentration without affecting the DHT-induced decrease in striatal TPH activity. Desmethylimipramine (5 and 25 mg/kg) had no effect on the DHT-induced decrease in striatal TH activity. Striatal choline acetyltransferase and glutamic acid decarboxylase activities were not decreased by 16 micrograms of DHT. The results indicate that DHT can alter dopaminergic function in the rat neostriatum through a direct effect of the drug on DA neurons.     

EFFECTS OF BOTULINUM AND TETANUS TOXINS ON CHOLINE ACETYLTRANSFERASE ACTIVITY IN SKELETAL MUSCLE IN THE MOUSE

Abstract— The effects of botulinum and tetanus toxins on the activity of choline acetyltransferase present in the motor nerve terminals of fast and slow skeletal muscle in the mouse were investigated. There was no change in the activities of choline acetyltransferase in either muscle after the injection of botulinum toxin but tetanus toxin caused a rise in the activity of the enzyme in fast muscle. Botulinum toxin is known to inhibit the release of acetylcholine and whilst neuromuscular transmission is blocked the motor nerves sprout and form new end-plates. Tetanus toxin has been shown to cause hyperactivity of motor neurons. The nerve growth caused by the botulinum toxin did not result in increased choline acetyltransferase levels in the muscles, whereas the synaptic hyperactivity caused by tetanus was associated with increased enzyme levels.     

Changes in somatodendritic but not terminal dopamineregulation in aged rhesus monkeys

For these studies, young (8-9 years), middle-aged (14-17 years) and aged (23-28 years) rhesus monkeys were used as a model of normal aging in humans to investigate changes in dopamine (DA)-containing neurons in senescence. Aged monkeys exhibited significant age-related motoric declines as compared to the young animals. In vivo microdialysis studies showed that basal levels of the DA metabolites, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were diminished by 44% and 79%, respectively, in the substantia nigra (SN) of aged monkeys. In addition, d-amphetamine-evoked overflow of DA in the SN was diminished by 30% in the middle-aged animals and 67% in the aged monkeys. Post-mortem measures of DA and DA metabolites showed significant decreases in DA (20%), DOPAC (47%) and HVA (22%) levels in the putamen and a 25% decline in HVA tissue levels in the SN of the aged monkeys as compared to the young animals. Unbiased stereological cell counting of tyrosine hydroxylase (TH)-immunoreactive neurons in the SN showed a small (15-20%) but significant age-related decline in TH-positive neurons. In addition, there was a small (15-20%) but significant decline in TH-positive fiber density and TH-positive cell size. In comparison to the massive loss of DA neurons responsible for the movement dysfunctions seen in Parkinson's disease, pronounced functional changes in DA release in the SN and putamen may significantly contribute to the motoric dysfunctions characterizing normal aging in rhesus monkeys.     

RS-alpha-lipoic acid protects cholinergic cells against sodium nitroprusside and amyloid-beta neurotoxicity through restoration of acetyl-CoA level

The work presented here verifies the hypothesis that RS-alpha-lipoic acid may exert its cholinoprotective and cholinotrophic activities through the maintenance of appropriate levels of acetyl-CoA in mitochondrial and cytoplasmic compartments of cholinergic neurons. Sodium nitroprusside (SNP) and amyloid-beta decreased pyruvate dehydrogenase, choline acetyltransferase activities, acetyl-CoA content in mitochondria and cytoplasm, as well as increased fraction of non-viable, trypan blue positive cells in cultured differentiated cholinergic SN56 neuroblastoma cells. Lipoic acid totally reversed toxin-evoked suppression of choline acetyltrasferase and pyruvate dehydrogenase activities, as well as mitochondrial and cytoplasmic acetyl-CoA levels, and partially attenuated increase of cell mortality. Significant negative correlations were found between enzyme activities, acetyl-CoA levels and cell mortality in different neurotoxic and neuroprotective conditions employed here. The level of cytoplamic acetyl-CoA correlated with mitochondrial acetyl-CoA, whereas choline acetyltransferase activity followed shifts in cytoplasmic acetyl-CoA. Thus, we conclude that, in cholinergic neurons, particular elements of the pyruvate-acetyl-CoA-acetylcholine pathway form a functional unit responding uniformly to nerotoxic and neuroprotectory conditions.     

Effects of African Trypanosomiasis on Brain Levels of Dopamine, Serotonin, 5-Hydroxyindoleacetic Acid, and Homovanillic Acid in the Rabbit

Abstract: Serotonin (5-HT) levels fell by 21% in the mid-brain-thalamus-hypothalamus (MTH) region of the rabbit brain after chronic infection with the protozoan Trypanosoma brucei gambiense. 5-HT did not decrease significantly in the caudate/putamen (CP) or the pons/medulla (PM) region. 5-Hydroxyindoleacetic acid (5-HIAA) levels were unchanged in the MTH and caudate/putamen (CP) but increased by 17% in the pons/medulla (PM) after infection. Dopamine (DA) levels rose by 19% and homovanillic acid (HVA) by 33% in the PM during infection. DA and HVA tended to be lower in the CP of infected rabbits, but the apparent decreases were not statistically significant. DA and HVA levels in the MTH were also unchanged by infection. These neurochemical changes may be involved in the behavioral symptoms that frequently accompany this disease in man and cattle.     

THE EFFECT OF INTRAHIPPOCAMPAL KAINIC ACID INJECTIONS AND SURGICAL LESIONS ON NEUROTRANSMITTERS IN HIPPOCAMPUS AND SEPTUM

Local injection of kainic acid (2 μg) was accompanied by destruction of intrinsic neurons in the dorsal part of hippocampus. The lesion was accompanied by a 75% reduction in glutamate decarboxylase activity, a 60% reduction in the high affinity uptake of l -glutamate, a 40-60% reduction in the endogeneous levels of aspartate, glutamate and GABA and no changes in the activities of choline acetyltransferase or aromatic amino acid decarboxylase in the dorsal hippocampus. Unilateral destruction of neurons in the dorsal hippocampus was followed by a 20-40% reduction in the high affinity uptake of glutamate in lateral, but not in medial septum, on both sides. There was no reduction in choline acetyltransferase, glutamate decarboxylase or aromatic amino acid decarboxylase activities in the lateral or medial part of the septum. Transection of fimbria and superior fornix was accompanied by a severe reduction in choline acetyltransferase and aromatic amino acid decarboxylase activity in hippocampus, in the high affinity uptake of glutamate and in the endogenous level of glutamate in the lateral septum. The results are consistent with the concept that in the hippocampus kainic acid destroys intrinsic neurons and not afferent fibres. It seems therefore that all GABAergic fibres in the hippocampus belong to intrinsic neurons whereas glutamergic and aspartergic neurons belong partly to local neurons. The connection from the hippocampus to the lateral septum probably uses glutamate as a transmitter.     

Regional Effects of Sodium Valproate on Extracellular Concentrations of 5-Hydroxytryptamine, Dopamine, and Their Metabolites in the Rat Brain: An In Vivo Microdialysis Study

The effects of sodium valproate (VPA; 100, 200, and 400 mg/kg, i.p.) on ventral hippocampal and anterior caudate putamen extracellular levels of dopamine (DA) and 5-hydroxytryptamine (5-HT) were examined using in vivo microdialysis. VPA induced dose-related increases in dialysate DA, 3,4-dihydroxyphenylacetic acid, and 5-HT in the ventral hippocampus. Anterior caudate putamen dialysate 5-HT was also dose dependently elevated by the drug, whereas DA levels tended to decrease with increasing VPA dose. In contrast, VPA (200, 400, and 800 mg/kg, i.p.) produced no significant elevation of DA in posterior caudate putamen dialysates, although 5-HT levels were significantly elevated at the 400- and 800-mg/kg doses. In all three regions studied, dialysate concentrations of 5-hydroxyindoleacetic acid and homovanillic acid remained at basal levels following VPA treatments. The results are discussed with regard to the possible anticonvulsant mode of action of VPA.     

Abnormalities of neurotransmitter enzymes in Huntington's chorea

The activities ofl-glutamate decarboxylase (GAD), GABA-transaminase (GABA-T), choline acetyltransferase (CAT), and cysteic and cysteinesulfinic acids decarboxylase (CAD/CSAD) in putamen and frontal cortex in both Huntington's chorea and normal tissues were measured. The greatest difference between Huntington's and normal tissues occurred in putamen, in which the apparent CSAD activity was reduced by 85%, while no difference was observed in frontal cortex. GAD, CAD, and CAT activities were also reduced in putamen by 65%, 63%, and 42%, respectively (P<0.05). Slight reduction in the enzyme activities was also observed in frontal cortex. However, these reductions appeared to be statistically insignificant (P>0.05 in all cases). GABA-T showed little difference in both putamen and frontal cortex in Huntington's chorea and normal tissues. GAD and GABA-T from Huntington's tissues were indistinguishable from those obtained from normal tissues by double diffusion test and by microcomplement fixation test, which is capable of distinguishing proteins with a single amino acid substitution. Furthermore, the similarity of the complement fixation curves for GAD from Huntington's and normal tissues suggests that the decrease in GAD activity is probably due to the reduction in the number of GAD molecules, presumably through the loss of neurons, and not due to the inhibition or inactivation of GAD activity by toxic substances which might be present in Huntington's chorea.     

Biochemical effects of quipazine on rat striatal dopaminergic system

At high doses quipazine, a serotonergic agonist, induces a dose-dependent reduction of homovanillic acid (HVA) and of dihydroxyphenylacetic acid (DOPAC) levels in rat striatum, and reduces the conversion of tyrosine into dopamine. These effects are not mediated by a serotonergic-dopaminergic interaction as they are not antagonized by pretreatment with the serotonin antagonist methergoline. Neither are they caused by direct action on dopamine receptors as the drug does not antagonize the increase in HVA induced by haloperidol. 3-methoxytyramine (3MT), a DA metabolite which is the expression of DA present in the synaptic cleft, is high after quipazine treatment, but this is not because of monoamine oxidase inhibition. The increase in 3MT is already evident shortly after quipazine administration, while the effect on HVA and DOPAC levels appears later. The different effects of quipazine on DA metabolites and the temporal sequence of their appearance suggest that the lowered levels of acidic metabolites are an index of reduced DA turnover secondary to the increase in DA at the receptor sites caused by quipazine.     

Choline Acetyltransferase Activities in Single Spinal Motor Neurons from Patients with Amyotrophic Lateral Sclerosis

Activities of choline acetyltransferase (ChAT) were microassayed in individual cell bodies of motor neurons, isolated from freeze-dried sections after autopsy of lumbar spinal cords from four patients with sporadic amyotrophic lateral sclerosis (ALS) and four control patients with nonneurological diseases. Numerous large neurons were found in the anterior horn at the early degeneration stage of ALS, but the cell bodies atrophied and decreased in number at the late advanced stage. The small, atrophied neurons were very fragile and were easily destroyed during the isolation procedure with a microknife. The average activity, expressed on a dry weight basis, of 58 ALS neurons was lower than that of 67 control neurons. The large, well-preserved neurons at the early nonadvanced stage had markedly lower ChAT activities than control neurons. The specific activity gradually increased with the progress of atrophy but did not return to the control level.     

Marked disparity between age-related changes in dopamine and other presynaptic dopaminergic markers in human striatum

Because age-related changes in brain dopaminergic innervation are assumed to influence human disorders involving dopamine (DA), we measured the levels of several presynpatic DAergic markers [DA, homovanillic acid, tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AADC), vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT)] in post-mortem human striatum (caudate and putamen) from 56 neurologically normal subjects aged 1 day to 103 years. Striatal DA levels exhibited pronounced (2- to 3-fold) post-natal increases through adolescence and then decreases during aging. Similarly, TH and AADC increased almost 100% during the first 2 post-natal years; however, the levels of TH and, to a lesser extent, AADC then declined to adult levels by approximately 30 years of age. Although VMAT2 and DAT levels closely paralleled those of TH, resulting in relatively constant TH to transporter ratios during development and aging, a modest but significant decline (13%) in DAT levels was observed in only caudate during aging. This biphasic post-natal pattern of the presynaptic markers suggests that striatal DAergic innervation/neuropil appears to continue to develop well past birth but appears to become overelaborated and undergo regressive remodeling during adolescence. However, during adulthood, a striking discrepancy was observed between the loss of DA and the relative preservation of proteins involved in its biosynthesis and compartmentation. This suggests that declines in DA-related function during adulthood and senescence may be explained by losses in DA per se as opposed to DAergic neuropil.     

Dopamine turnover is upregulated in the caudate/putamen of asymptomatic MPTP-treated rhesus monkeys

In Parkinson's disease (PD) and experimental parkinsonism, losses of up to 60% and 80%, respectively, of dopaminergic neurons in substantia nigra, and dopamine (DA) in striatum remain asymptomatic. Several mechanisms have been suggested for this functional compensation, the DA-mediated being the most established one. Since this mechanism was recently challenged by striatal DA analysis in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, we present data on several DAergic parameters in three groups of rhesus monkeys: MPTP-treated asymptomatic animals; symptomatic MPTP-treated animals with stable parkinsonism; and untreated sex and age matched controls. We determined ratios of striatal and nigral 3,4-dihydroxyphenyl acetic acid (DOPAC) to DA levels and tyrosine hydroxylase (TH) enzyme activity to DA levels, in addition to the commonly used homovanillic acid (HVA)/DA ratios which, as such, might be less reliable under the conditions of partial denervation. We found that in the asymptomatic MPTP monkeys the DOPAC/DA ratios in putamen and caudate nucleus were shifted with high statistical significance 1.9-5.8-fold, as compared to controls, the shifting of the ratios being in the same range as the 2.6-5.4-fold shifts in the symptomatic animals. Also TH/DA ratios were significantly increased in both, the asymptomatic and the symptomatic MPTP-treated monkeys, with shifts in the putamen and caudate nucleus of 3- and 2.7-7.0-fold, respectively. In the substantia nigra, DOPAC levels and TH activity were strongly decreased after MPTP (-77 to -97%), but the ratios DOPAC/DA and TH/DA were not changed in this brain region. Collectively, our findings support the concept of DAergic compensation of the progressive striatal DA loss in the presymptomatic stages of the parkinsonian disease process.     

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.     

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.     

Synthesis and Release of Dopamine in Rat Brain: Comparison Between Substantia Nigra Pars Compacta, Pars Reticulata, and Striatum

Dopamine (DA) is synthesized and released not only from the terminals of the nigrostriatal dopaminergic neuronal pathway, but also from the dendrites in the substantia nigra. We have investigated the regulation of the DA turnover, the DA synthesis rate, and the DA release in the substantia nigra pars compacts (SNpc) and pars reticulata (SNpr) in vivo. As a measure of DA turnover, we have assessed the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid. As a measure of the DA synthesis rate, we have determined the 3,4-dihydroxyphenylalanine accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxybenzylhydrazine. As a measure of DA release, we have investigated the disappearance rate of DA after inhibition of its synthesis by alpha-methyl-p-tyrosine and the 3-methoxytyramine accumulation following monoamine oxidase inhibition by pargyline. Both the DA turnover and the DA synthesis rate increased following treatment with the DA receptor antagonist haloperidol and decreased following treatment with the DA receptor agonist apomorphine in the SNpc and in the SNpr, but the effects of the drugs were less pronounced than in the striatum. gamma-Butyrolactone treatment, which suppresses the firing of the dopaminergic neurons, increased the DA synthesis rate in the striatum (165%), but had no such effect in the SNpc or SNpr. Haloperidol, apomorphine, and gamma-butyrolactone increased, decreased, and abolished, respectively, the DA release in the striatum, but the drugs had no or only slight effects on the alpha-methyl-p-tyrosine-induced DA disappearance and on the pargyline-induced 3-methoxytyramine accumulation in the SNpc or SNpr. Taken together, these results indicate that the DA synthesis rate, but not the DA release, are influenced by DA receptor activity and neuronal firing in the SNpc and SNpr. This is in contrast to the situation in the striatum, where both the DA synthesis rate and the DA release are under such control.