AN ADAPTATION OF THE PUSH-PULL CANNULA METHOD TO STUDY THE IN VIVO RELEASE OF [3H]DOPAMINE SYNTHESIZED FROM [3H]TYROSINE IN THE CAT CAUDATE NUCLEUS: EFFECTS OF VARIOUS PHYSICAL AND PHARMACOLOGICAL TREATMENTS

Abstract— The release of [³H]dopamine ([³H]DA) continuously synthesized from l-[3,5–³H]tyrosine from the caudate nucleus of the cat was estimated in halothane anaesthetized or‘encéphale isolé’animals. For this purpose, an improved superfusion cannula, avoiding tissue damage, was used. The best localization for the tip of the superfusion cannula was found first by determining the topographical distribution of endogenous DA within the caudate nucleus. A rostro-caudal heterogenous distribution of the transmitter was detected. In perfusion experiments, l-[3,5–³H]tyrosine was introduced continuously at a rate of 33μl/min. [³H]DA was the only catecholamine found in serial 15 min fractions as revealed by cochromatography. The spontaneous release of [³H]DA was greater in anaesthetized than in ‘encéphale isolé’ cats; it represented 150 and 100 times the blank value, respectively. Depolarization by K⁺ (30 mm) applied locally in the striatum or by electrical or mechanical stimulation of the substantia nigra caused a transitory increase in [³H]DA release. Conversely, a decrease in nerve activity induced by tetrodotoxin (5 × 10^?-⁷ m) or by electrocoagulation of the substantia nigra was associated with a decline in the amounts of [³H]DA in superfusates. A temporary reduction in [³H]DA release could also be obtained by a short-lasting cooling block of the substantia nigra. As expected, d-amphetamine (10^?-⁵ m) and benzotropine(10^?-⁷ m) added to the superfusing medium increased [³H]DA release. These pharmacological results, as well as the changes in [³H]DA release observed after various manipulations of the activity of dopaminergic neurones, confirms the validity and the high sensitivity of this approach.     

Regional release of aromatic amines from tissues of the rat brain in vitro

Abstract— Radioactive hydroxylated phenylethylamines were released in vitro by electrical stimulation of minces of brain tissues from several anatomically discrete areas, while labelled urea and amphetamine were poorly released from all regions. Release of [³H]norepinephrine occurred in the order hypothalamus > caudate nucleus ≥ cerebral cortex, thus in parallel with the distribution of endogenous norepinephrine. In contrast, [³H]tyramine was poorly released from cortical tissues but readily released from minces of caudate nucleus or hypothalamus. [³H]Octopamine was released from all areas, but was most readily released from thecaudatenucleus. Results for cerebral cortex were similar to those for coronal slices or minces of whole brain; release occurred in the order: norepinephrine > octopamine > tyramine in all three preparations. We suggest that certain β-hydroxylated phenolic phenylethyl-amines may be released from norepinephrine- or dopamine-containing nerve endings in the brain, and that their non-β-hydroxylated congeners may be released from neurons in which endogenous amines are not β-hydroxylated.     

In vivo release of adenosine from cat basal ganglia—studies with a push pull cannula

The release of newly synthesized [³H]adenosine has been studied in vivo in cat caudate nucleus and substantia nigra, using a push pull cannula. In the presence of [³H]adenosine as precursor, spontaneously released [³H]adenosine was easily detectable in superfusates of the push pull cannula. In the caudate nucleus, potassium and veratridine caused a marked and reversible increase in [³H]adenosine release. The effect of veratridine was completely blocked by tetrodotoxin (TTX) although TTX had no action by itself. Ouabain as well as glutamate, also markedly increased the release of [³H]adenosine.The specific 5′ nucleotidase inhibitor α,β-methylene ADP, did not alter the increase in the amount of [³H]adenosine obtained by veratridine, although it diminished the spontaneous release of [³H]adenosine by about 20%.Push pull cannulae were also implanted simultaneously into the caudate nucleus and substantia nigra. Potassium applied into the caudate nucleus increased the local release of adenosine but did not change that observed in the substantia nigra. When potassium was applied into the substantia nigra, it also increased the local release of adenosine but did not change that observed in the caudate nucleus.The results are discussed in term of the possible existence of “purinergic neurons” and of the relation between the adenosine release and central nervous activity.     

THE RELEASE IN VIVO OF [3H]ACETYLCHOLINE FROM CAT CAUDATE NUCLEUS AND CEREBRAL CORTEX BY ATROPINE, PENTYLENETETRAZOL, K+-DEPOLARIZATION AND ELECTRICAL STIMULATION

Abstract— In the present experiments, the resting and stimulus evoked release of newly synthesized [³H]acetylcholine from the caudate nucleus, the cerebral cortex and the cerebellar cortex into the perfusate of the push-pull cannula was studied in the unanesthetized, midpontine, pretrigeminally transected cat following infusion at the push-pull site of [³H]choline. Separation of the metabolites in the perfusate revealed that after 20 min, approximately 20% of the recovered radioactivity in the sample was in a lipid fraction, about 10% was found to be phosphorylcholine and around 3% was observed to be incorporated into acetylcholine. The rest of the recovered radioactivity remained as choline. Electrical stimulation applied directly to the caudate nucleus, local potassium depolarization, atropine and pentylenetetrazol were all observed to result in a significant and stimulus dependent increase in the levels of [³H]acetyIchoIine, but not [³H]choline or [¹⁴C]urea in the effluent of the push-pull cannula located in the caudate nucleus. A similar release of newly synthesized [³H]acetylcholine was observed following atropine and potassium stimulation in the cerebral but not the cerebellar cortex. The specificity of this evoked increase in the levels of [³H]acetylchoiine is substantiated by obtaining the release with stimuli having different modes of action, by the absence of stimulus evoked changes in the levels of other water-soluble elements found in the perfusate and by the absence of an observable release of [³H]acetylcholine in perfusion experiments involving the cerebellum, a tissue not thought to have strong cholinergic innervation. The percentage increases in release of [³H] acetylcholine over baseline levels evoked by the various methods closely corresponded to those reported in the literature for authentic acetylcholine. This was taken to suggest that the neuronal pools containing endogenous acetylcholine and those containing newly synthesized acetylcholine, if not identical, were disposed to behave in the same manner following the activation of the synapse.     

Chronic Nicotine Administration Differentially Affects Neurotransmitter Release from Rat Striatal Slices

Abstract: The objective of these experiments was to determine whether the chronic administration of nicotine, at a dose regimen that increases the density of nicotine binding sites, alters the nicotine-induced release of [³H]dopamine ([³H]DA), [³H]norepinephrine ([³H]NE), [³H]serotonin ([³H]5-HT), or [³H]acetylcholine ([³H]ACh) from rat striatal slices. For these experiments, rats received subcutaneous injections of either saline or nicotine bitartrate [1.76 mg (3.6 µmol)/kg, dissolved in saline] twice daily for 10 days, and neurotransmitter release was measured following preloading of the tissues with [³H]DA, [³H]NE, [³H]5-HT, or [³H]choline. Chronic nicotine administration did not affect the accumulation of tritium by striatal slices, the basal release of radioactivity, or the 25 mM KCl-evoked release of neurotransmitter. Superfusion of striatal slices with 1, 10, and 100 µM nicotine increased [³H]DA release in a concentration-dependent manner, and release from slices from nicotine-injected animals was significantly (p < 0.05) greater than release from saline-injected controls; release from the former increased to 132, 191, and 172% of release from the controls following superfusion with 1, 10, and 100 µM nicotine, respectively. Similarly, [³H]5-HT release increased in a concentration-related manner following superfusion with nicotine, and release from slices from nicotine-injected rats was significantly (p < 0.05) greater than that from controls. [³H]5-HT release from slices from nicotine-injected rats evoked by superfusion with 1 and 10 µM nicotine increased to 453 and 217%, respectively, of release from slices from saline-injected animals. The nicotine-induced release of [³H]NE from striatal slices was also concentration dependent but was unaffected by chronic nicotine administration. [³H]ACh release from striatal slices could not be detected when samples were superfused with nicotine but was measurable when tissues were incubated with nicotine. The release of [³H]ACh from slices from nicotine-injected rats was significantly (p < 0.05) less than release from controls and decreased to 36, 83, and 77% of control values following incubation with 1, 10, or 100 µM nicotine, respectively. This decreased [³H]ACh release could not be attributed to methodological differences because slices from nicotine-injected rats incubated with nicotine exhibited an increased [³H]DA release, similar to results from superfusion studies. In addition, it is unlikely that the decreased release of [³H]ACh from striatal slices from nicotine-injected rats was secondary to increased DA release because [³H]ACh release from slices from hippocampus, which is not tonically inhibited by DA, also decreased significantly (p < 0.05) in response to nicotine; hippocampal slices from nicotine-injected rats incubated with 1 and 10 µM nicotine decreased to 42 and 70%, respectively, of release from slices from saline-injected animals. Results indicate that the chronic administration of nicotine increases the ability of nicotine to induce the release of [³H]DA and [³H]5-HT and decreases the ability of nicotine to evoke the release of [³H]ACh but does not alter the nicotine-induced release of [³H]NE from brain slices.     

Characterization of the effects of serotonin on the release of [3H]dopamine from rat nucleus accumbens and striatal slices

The effect of serotonin agonists on the depolarization (K⁺)-induced, calcium-dependent, release of [³H]dopamine (DA) from rat nucleus accumbens and striatal slices was investigated. Serotonin enhanced basal³H overflow and reduced K⁺-induced release of [³H]DA from nucleus accumbens slices. The effect of serotonin on basal³H overflow was not altered by the serotonin antagonist, methysergide, or the serotonin re-uptake blocker, chlorimipramine, but was reversed by the DA re-uptake carrier inhibitors nomifensine and benztropine. With the effect on basal overflow blocked, serotonin did not modulate K⁺-induced release of [³H]DA in the nucleus accumbens or striatum. The serotonin agonists, quipazine (in the presence of nomifensine) and 5-methoxytryptamine, did not significantly affect K⁺-induced release of [³H]DA in the nucleus accumbens. This study does not support suggestions that serotonin receptors inhibit the depolarization-induced release of dopamine in the nucleus accumbens or striatum of the rat brain. The present results do not preclude the possibility that serotonin may affect the mesolimbic reward system at a site which is post-synaptic to dopaminergic terminals in the nucleus accumbens.     

Modulation of GABA-augmented norepinephrine release in female rat brain slices by opioids and adenosine

GABA_A receptor activation augments electrically-stimulated release of norepinephrine (NE) from rat brain slices. Because this effect is not observed in synaptoneurosomes, GABA probably acts on inhibitory interneurons to disinhibit NE release. To determine whether opioids or adenosine influence GABA-augmented NE release, hypothalamic and cortical slices from female rats were superfused with GABA or vehicle in the presence and absence of 10 M morphine or 100 M adenosine. GABA augments [³H]NE release in the cortex and hypothalamus. Morphine alone has no effect on [³H]NE release, but attenuates GABA augmentation of [³H]NE release in both brain regions. Adenosine alone modestly inhibits [³H]NE release in the cortex, but not in the hypothalamus. Adenosine inhibits GABA-augmented [³H]NE release in both brain regions. The general protein kinase inhibitor H-7, augments [³H]NE release in both brain regions and may have additive effects with GABA in cortical slices. These results implicate opioid and adenosine interneurons and possibly protein kinases in regulating GABAergic influences on NE transmission.     

Release of endogenous norepinephrine and dopamine from rat brain regions in vitro

Using radioenzymatic assay procedures, we have measured picomolar amounts of endogenous norepinephrine (NE) and dopamine (DA) released $\text{in vitro}$. The release of NE and DA in response to KCl stimulation was examined in 6 brain regions: cortex, hippocampus, hypothalamus, striatum, combined accumbens-olfactory tubercle, and substantia nigra. NE release was detectable in all regions except striatum. Amounts of NE released by 55mM KCl (expressed as % control) were: cortex (313%), hippocampus (227%), hypothalamus (225%), accumbens-tubercle (278%), s. nigra (155%). KCl stimulated release of DA was detected in 3 regions: striatum (414%), accumbenstubercle (282%), and hypothalamus (312%). DA was measurable in filtrates from the s. nigra but levels in control and KCl stimulated samples were equal. Release of NE and DA was also measured in 12 brain regions after incubation of tissue $\text{in vitro}$ with 10^?4M d-amphetamine sulfate. d-Amphetamine stimulated NE outflow when compared to controls in all regions examined. DA outflow was markedly increased in most regions, especially striatum (287%), hypothalamus (387%) and accumbens-tubercle (670%). d-Amphetamine doubled endogenous DA outflow from the s. nigra.     

Effect of ethanol on [3H]Dopamine release in rat nucleus accumbens and striatal slices

Ethanol (10–200 mM) transiently increased tritium overflow from superfused rat nucleus accumbens slices previously incubated with [³H]dopamine (DA) and [¹⁴C]choline. The effect was greater in striatal tissue and did not appear to be a non-specific membrane effect since [¹⁴C]acetylcholine (ACh) release was not affected. Lack of antagonism by picrotoxin suggested that -aminobutyric acid (GABA) receptors were not involved. Calcium was not a requirement and the DA uptake blocker, nomifensine, was without effect. Ethanol appeared to be causing [³H]DA release into the cytoplasm. K⁺-stimulated release of [³H]DA and [¹⁴C]ACh from nucleus accumbens and striatal slices was not affected. Clonidine-mediated inhibition of the K⁺-evoked release of [³H]DA remained unaltered. Ethanol attenuated the isoproterenol-induced enhancement of [³H]DA release. Ethanol therefore appeared to interact with components of the DA terminal causing a transient increase in the release of neurotransmitter without impairing K⁺-evoked release but apparently interfering with the isoproterenol-induced effect.     

THE RELEASE IN VIVO OF DOPAMINE SYNTHESIZED FROM LABELLED PRECURSORS IN THE CAUDATE NUCLEUS OF THE CAT

Abstract— To study the release of dopamine (DA) evoked in vivo from the caudate nucleus, a push-pull cannula was inserted into the head of the caudate nucleus of cats anaesthetised with pentobarbitone sodium (Nembutal), and the tissue in the vicinity of the cannula tip was continuously irrigated with either l -[¹⁴C]tyrosine or DL-[¹⁴C]3,4-dihydroxyphenylala-nine (DOPA). The contents of [¹⁴C]DA and of the [¹⁴C]acidic metabolites in the perfusates were determined after separation from the labelled precursors by column chromatography, TLC and solvent partition. During perfusion with radioactive tyrosine, only small quantities of [¹⁴C]DA appeared in the effluent while the concentrations of the [¹⁴C]acidic metabolites gradually increased during the course of the experiment. When [¹⁴C]DOPA was substituted for [¹⁴C]tyrosine, the proportion of precursor that was converted to DA and released into the effluent as the amine or as its acidic metabolites was increased ten-fold. In an attempt to increase the resting release of [¹⁴C]DA, D-amphetamine, tropolone or pheniprazine were individually added to the perfusion fluid. Each drug increased the content of [¹⁴C]DA in the perfusate, but the enhanced release was maintained only when pheniprazine was added during perfusion with [¹⁴C]DOPA. Stimulation of the rostral substantia nigra (A5-5) and the medial forebrain bundle caused, in a majority of experiments, a two-to five-fold increase in the concentration of labelled DA in the effluent. Stimulation of the substantia nigra at A4-0 did not enhance the release of [¹⁴C]DA from the caudate nucleus but did enhance the release from the putamen. Since the increase in the output of [¹⁴C]DA was independent of changes in the output of labelled acidic metabolites, the evoked release was apparently not attributable to changes in extracellular fluid dynamics.     

[125I]MIBG uptake and release in different regions of the rat brain

Radioiodinated m-iodobenzylguanidine ([¹²⁵I]MIBG) and tritiated norepinephrine ([³H]NE]) uptake and release were compared, in different regions of the brain of the rat. The classification of the regions according to uptake was the same for both tracers: striatum > hypothalamus > hippocampus > cortex > brainstem. Tetrabenazine (TBZ), a granular monoamine uptake inhibitor reduced the uptake in the different regions. The inhibition rate was higher for [³H]NE uptake than for [¹²⁵I]MIBG. The spontaneous release was the same for [¹²⁵I]MIBG and [³H]NE and was the lowest in the striatum. The K⁺ stimulated release of [³H]NE was more complete than the release of [¹²⁵I]MIBG and was the most important in the striatum. From these results, it is inferred that MIBG enters the brain tissue via NE uptake mechanisms. It appears that MIBG is stored in the chromaffin granules, as NE, but also in the cytoplasm. A modified molecule derived from MIBG which would cross the blood-brain barrier, would then appear as a potential scintigraphic marker of monoamine uptake, storage and release.     

Development of transmitter secretory mechanisms by adrenergic neurons in the embryonic chick heart ventricle

Developmental changes in the function of adrenergic axons within the right ventricle of the chick embryo were assessed by measuring the ability of these axons (1) to release endogenous transmitter, and (2) to transport, retain, and release tritiated norepinephrine ([³H]NE). The release of endogenous catecholamines was assayed indirectly by measuring the increase in the twitch tension of ventricular muscle evoked by electrical stimulation of intramural nerves. The release of endogenous transmitter, which acted via β-adrenergic receptors, was first detected by this method on the 16th embryonic day. A cocaine-sensitive uptake of [³H]NE was first observed on the 12th embryonic day. At this time, elevated potassium first evoked a calcium-sensitive release of [³H]NE. Electrical stimulation of intramural axons first evoked a tetrodotoxin-sensitive release of [³H]NE on the 14th embryonic day. It is concluded that the axons of developing adrenergic neurons are capable of releasing transmitter soon after they contact their target tissue.     

EFFECTS OF DOPAMINERGIC AGONISTS AND ELECTRICAL STIMULATION OF THE MIDBRAIN RAPHÉ ON THE RELEASE OF 5-HYDROXYTRYPTAMINE FROM THE CAT BRAIN IN VIVO

—The cerebroventricular system of spinal-sectioned cats was perfused with artificial cerebrospinal fluid after the intraventricular administration of [³H]tryptophan, and the perfusate was analyzed for [³H]5-HT. At the end of the experiment the relative contents of [³H]5-HT in regions lining the cerebroventricular system (caudate nucleus, septum, hypothalamus) were essentially the same as the endogenous 5-HT contents. Electrical stimulation of the midbrain raphé did not alter the efflux of total radioactivity but significantly increased the efflux of [³H]5-HT. The addition of potassium to the perfusing CSF also increased the efflux of [³H]5-HT, while additions of d-amphetamine, apomorphine or l -DOPA were without effect.     

Increased dopamine D2 receptor-mediated inhibition of [14C]acetylcholine release in the dorsomedial part of the nucleus accumbens

Dopamine (DA) D2 receptor-mediated inhibition of the K⁺-stimulated release of [¹⁴C]acetylcholine (ACh) from prelabeled rat dorsomedial nucleus accumbens slices was found to be 1.7 times greater than that observed in dorsorostral and ventromedial slices. This observation is consistent with the 1.9 fold higher DA D2 receptor density found in the dorsomedial area. In contrast, there were no differences in the DA D2 receptor-mediated effects on [³H]DA release in these areas. In addition, DA D2 receptor-mediated effects on [³H]DA and [¹⁴C]ACh release could not be demonstrated in the ventrorostral part of the nucleus accumbens consistent with the fact that DA D2 receptors were barely detectable in this area. The results suggest that cholinergic terminals in the dorsomedial part of the nucleus accumbens are under greater inhibitory DA control than in other areas of the nucleus accumbens.     

Accumulation and metabolism of norepinephrine in rat hypothalamus after exhaustive stress

—Exhaustive stress in rats is followed by a temporary reduction of hypothalamic norepinephrine (NE) together with a persistent increase in turnover during recovery. To test for persistent alterations of NE storage and metabolism produced by stress, rats were subjected to 3 h of forced running and were then injected intraventricularly with [³H]NE or [³H]dopamine (DA). The hypothalamus was assayed for [³H]NE and its metabolites at various intervals after injection. The effects of stress were compared with those of reserpine (7·5 mg/kg) or α-methyltyrosine (AMT, 300 mg/kg) pretreatment. It was found that the stress-induced reduction of endogenous NE was not accompanied by a change in the accumulation of exogenous [³H]NE either 10 or 30 min after injection, whereas the NE depletions produced by reserpine or AMT were associated with decreased or increased accumulation, respectively. However, stress did produce an increased accumulation of [³H]NE endogenously synthesized from [³H]DA. These results indicate that exhaustive stress does not adversely affect the storage of NE. They also suggest that stores of NE depleted by stress are replenished chiefly with newly synthesized NE and not through an increased uptake and binding or decreased metabolism of extraneuronal NE. The latter factors may play a role in the maintenance of brain NE stores when biosynthesis is low, i.e. after AMT. The major metabolites of exogenous [³H]NE, at 30 min after injection, were identified as conjugates of 3,4-dihydroxyphenylglycol (DOPEG) and 3-methoxy-4-hydroxyphenylglycol (MOPEG) in approximately equal amounts. The finding of high levels of conjugated DOPEG confirms a recent report (Slgden and Eccleston , 1971) that this compound is a major metabolite of brain NE. Reserpine produced marked elevations of both conjugates; AMT slightly reduced each. Prior stress increased only conjugated MOPEG, an observation suggesting that CNS levels of this metabolite may reflect NE released by nervous activity.     

Enhancement in dopamine uptake and release induced by monosodium l-glutamate from caudate nucleus under in vitro conditions

l-Glutamate has an excitatory and cytotoxic effect on the central nervous system. It was shown previously that norepinephrine and dopamine uptake and release were affected by in vivo administration of glutamate to adult rats. The kinetic parameters, K_m and V_max of [¹⁴C]DA uptake and release were measured on synaptosomal and slices from caudate nucleus under in vitro conditions at different glutamate concentrations. Results showed an important increase in [¹⁴C]DA uptake on synaptosomal (> 100%) and slices by lower glutamate concentrations, the affinity for transport system was increased (100%) and its release of high potassium evoked was also increased at 0.5 μM of glutamate. The results suggest the possibility that glutamate may modify DA uptake and release interacting with the DA transporter complex at the synaptic level.     

N-type calcium channels are involved in the dopamine releasing effect of nicotine

Mouse striatum was incubated with [³H]dopamine ([³H]DA) and superfused with and the tritium efflux induced by nicotine, electrical stimulation, or simultaneous nicotine and electrical stimulation was measured, to characterize the role of different Ca²⁺ channels in the transmitter release. Nicotine stimulation and electrical stimulation exerted additive effects on tritium efflux. Separation of the released radioactivity on alumina columns indicated that nicotine or electrical stimulation increases the release of [³H]DA and that the outflow of³H-labeled metabolites was similar with the two different stimulation procedures. Removal of Ca²⁺ from the superfusate resulted in a marked reduction in the tritium release evoked by nicotine, whereas the electrical stimulation-evoked tritium release was completely dependent on external Ca²⁺. The L-and N-type calcium channel blockers omega-conotoxin GVIA and Cd²⁺ inhibited the tritium release from the striatum evoked by either nicotine or electrical stimulation, whereas the L-type and T-type channel blockers diltiazem and Ni²⁺ did not alter release of [³H]DA. We conclude that N-type voltage-sensitive Ca²⁺ channels participate in striatal dopamine release, and we speculate that nicotinic receptor-operated ion channels permeable to cations such as Ca²⁺ and N-type voltage-sensitive calcium channels may simultaneously open up, and they additively increase free intracellular Ca²⁺ concentration.     

Lack of effect of chronic desipramine treatment on dopaminergic activity in the nucleus accumbens of the rat

The binding of [³H]SCH 23390 to dopamine (DA) D₁-receptors was measured in the nucleus accumbens of rats treated chronically with desipramine for 14 days. DA D₁ — and D₂-receptor binding using [³H]SCH 23390 and [³H]spiperone, respectively as ligands, was determined in rats treated for 28 days. NeitherB _max norK _d values were influenced by chronic desipramine treatment. In addition, chronic desipramine treatment (28 days) did not influence the dose dependent, quinpirole (10–1000 nM)-mediated inhibition of the electrically stimulated release of [³H]DA and [¹⁴C]ACh from nucleus accumbens slices or the dose dependent increase in [³H]DA release and decrease in [¹⁴C]ACh release in the presence of 1 and 10 M nomifensine. Therefore, our results suggest that the effect of chronic antidepressant treatment cannot be attributed to changes in either DA D₁₁-or D₂-receptor binding or DA D₂-receptor function in the nucleus accumbens.     

Regional distribution of monoamines in the nucleus accumbens of the rat

Monoamine concentrations were low in the rostral area of the nucleus accumbens. Their distributions were not identical. Differences were observed in the medial area. DA concentrations were high in both medial and caudal areas. Noradrenaline (NA) and serotonin (5-HT) concentrations were considerably lower than the dopamine (DA) concentration. The NA concentration was highest in the caudal area of the nucleus accumbens and the (5-HT) concentration was highest in the ventrocaudal area. There was a rostrocaudal decrease in the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA and 5-hydroxyindole-3-acetic acid (5-HIAA)/5-HT ratios. Uptake of [³H]DA and [¹⁴C]choline was lowest in the rostral area. The K⁺-stimulated release of [¹⁴C]acetylcholine (ACh) was also lowest rostrally, but there was no rostrocaudal difference in the K⁺-stimulated release of [³H]DA. These results provide further evidence of the heterogeneity of the nucleus accumbens.     

Dopamine Transporter Loss in 6-OHDA Parkinson’s Model Is Unmet by Parallel Reduction in Dopamine Uptake

The dopamine transporter (DAT) regulates synaptic dopamine (DA) in striatum and modulation of DAT can affect locomotor activity. Thus, in Parkinson’s disease (PD), DAT loss could affect DA clearance and locomotor activity. The locomotor benefits of L-DOPA may be mediated by transport through monoamine transporters and conversion to DA. However, its impact upon DA reuptake is unknown and may modulate synaptic DA. Using the unilateral 6-OHDA rat PD model, we examined [³H]DA uptake dynamics in relation to striatal DAT and tyrosine hydroxylase (TH) protein loss compared with contralateral intact striatum. Despite >70% striatal DAT loss, DA uptake decreased only ∼25% and increased as DAT loss approached 99%. As other monoamine transporters can transport DA, we determined if norepinephrine (NE) and serotonin (5-HT) differentially modulated DA uptake in lesioned striatum. Unlabeled DA, NE, and 5-HT were used, at a concentration that differentially inhibited DA uptake in intact striatum, to compete against [³H]DA uptake. In 6-OHDA lesioned striatum, DA was less effective, whereas NE was more effective, at inhibiting [³H]DA uptake. Furthermore, norepinephrine transporter (NET) protein levels increased and desipramine was ∼two-fold more effective at inhibiting NE uptake. Serotonin inhibited [³H]DA uptake, but without significant difference between lesioned and contralateral striatum. L-DOPA inhibited [³H]DA uptake two-fold more in lesioned striatum and inhibited NE uptake ∼five-fold more than DA uptake in naïve striatum. Consequently, DA uptake may be mediated by NET when DAT loss is at PD levels. Increased inhibition of DA uptake by L-DOPA and its preferential inhibition of NE over DA uptake, indicates that NET-mediated DA uptake may be modulated by L-DOPA when DAT loss exceeds 70%. These results indicate a novel mechanism for DA uptake during PD progression and provide new insight into how L-DOPA affects DA uptake, revealing possible mechanisms of its therapeutic and side effect potential.