Transmitter-Like Release of Endogenous 3,4-Dihydroxyphenylalanine from Rat Striatal Slices

Biphasic electrical field stimulation (0.5-5 Hz, 2 ms, 25 V, 3 min) and high K+ (10-30 mM, 5 min) released endogenous 3,4-dihydroxyphenylalanine (DOPA) from superfused rat striatal slices. Characteristics of the DOPA release were compared with those of 3,4-dihydroxyphenylethylamine (dopamine, DA). Electrical stimulation at 2 Hz evoked DOPA and DA over similar time courses. alpha-Methyl-p-tyrosine (0.2 mM) markedly reduced release of DOPA but not of DA. Maximal release (0.3 pmol) of DOPA was obtained at 2 Hz and at 15 mM K+. The impulse-evoked release of DOPA and DA was completely tetrodotoxin (0.3 microM) sensitive and Ca2+ dependent and the 15 mM K+-evoked release was also Ca2+ dependent. On L-[3,5-3H]tyrosine (1 microM) superfusion, high K+ (15 and 60 mM) released DOPA and DA together with concentration-dependent decreases in tyrosine 3-monooxygenase (EC 1.14.16.2) activity as indicated by [3H]H2O formation, followed by concentration-dependent increases after DOPA and DA release ended. These findings suggest that striatal DOPA is released by a Ca2+-dependent excitation-secretion coupling process similar to that involved in transmitter release.     

Ca2+o-Independent Veratridine-Evoked Acetylcholine Release from Striatal Slices Is Not Inhibited by Vesamicol (AH5183): Mobilization of Distinct Transmitter Pools

The effect of 2-(4-phenylpiperidino)cyclohexanol (AH5183 or vesamicol), a compound known to block the uptake of acetylcholine (ACh) into cholinergic synaptic vesicles, on the release of endogenous and [14C]ACh from slices of rat striatum was investigated. ACh release was evoked either by electrical stimulation or by veratridine. The effect of electrical stimulation was entirely dependent on external Ca2+. By contrast, veratridine (40 microM) also enhanced ACh release in the absence of Ca2+. Indeed, with veratridine two components were clearly distinguished: one dependent on external Ca2+ and the other not. Vesamicol inhibited [14C]ACh release evoked by both veratridine and electrical stimulation in the presence of external Ca2+, provided it was added to the tissue prior to loading with [14C]choline. With the same treatment vesamicol only slightly affected the release of endogenous ACh. Under the same conditions the Ca2(+)-independent [14C]ACh release evoked by veratridine was not prevented by vesamicol. The differential responsiveness to vesamicol suggests that ACh pools involved in Ca2+o-dependent ACh release are different from those mobilized during Ca2+o-independent ACh release.     

Limited regulation of somatodendritic dopamine release by voltage-sensitive Ca channels contrasted with strong regulation of axonal dopamine release

The mechanism underlying somatodendritic release of dopamine (DA) appears to differ from that of axon-terminal release. Specifically, somatodendritic DA release in the substantia nigra pars compacta (SNc) persists in low extracellular Ca2+ concentrations that are insufficient to support axonal release in striatum, suggesting that limited Ca2+ entry is necessary to trigger somatodendritic release. Here, we compared the role of voltage-dependent Ca2+ channels in mediating DA release in striatum versus SNc using specific blockers of N-, P/Q-, T-, R- and L-type Ca2+ channels individually and in combination. Release of DA evoked by a single stimulus pulse in the dorsal striatum and SNc of guinea-pig brain slices was monitored in real time using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Single-pulse evoked DA release was shown to be independent of regulation by concurrently released glutamate or GABA acting at ionotropic receptors in both regions. Under these conditions, striatal DA release was completely prevented by an N-type channel blocker, omega-conotoxin GVIA (100 nm), and was decreased by 75% by the P/Q-type channel blocker omega-agatoxin IVA (200 nm). Blockade of T-type channels with Ni2+ (100 microm) or R-type channels with SNX-482 (100 nm) decreased axonal release in striatum by 25%, whereas inhibition of L-type channels with nifedipine (20 microm) had no effect. By contrast, none of these Ca2+-channel blockers altered the amplitude of somatodendritic DA release in the SNc. Even a cocktail of all blockers tested did not alter release-signal amplitude in the SNc, although the duration of the release response was curtailed. The limited involvement of voltage-dependent Ca2+ channels in somatodendritic DA release provides further evidence that minimal Ca2+ entry is required to trigger the release process, compared with that required for axon-terminal release.     

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

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

Stimulation of D-2 Dopamine Receptors Decreases the Evoked In Vitro Release of [3H] Acetylcholine from Rat Neostriatum: Role of K+ and Ca2+

Reportedly, stimulation of D-2 dopamine receptors inhibits the depolarization-induced release of acetylcholine from the neostriatum in a cyclic AMP-independent manner. In the present study, we investigated the role of K+ and Ca2+ in the D-2 receptor-mediated inhibition of evoked [3H]acetylcholine release from rat striatal tissue slices. It is shown that the D-2 receptor-mediated decrease of K+-evoked [3H]acetylcholine release is not influenced by the extracellular Ca2+ concentration. However, increasing extracellular K+, in the presence and absence of Ca2+, markedly attenuates the effect of D-2 stimulation on the K+-evoked [3H]acetylcholine release. Furthermore, it is shown that activation of D-2 receptors in the absence of Ca2+ also inhibits the veratrine-evoked release of [3H]acetylcholine from rat striatum. These results suggest that the D-2 dopamine receptor mediates the decrease of depolarization-induced [3H]acetylcholine release from rat striatum primarily by stimulation of K+ efflux (opening of K+ channels) and inhibition of intracellular Ca2+ mobilization.     

Conjugated Dopamine in Superfusates of Slices of Rat Striatum

Abstract: An acid-hydrolyzable conjugate of 3,4-dihydroxyphenylethylamine (dopamine, DA) was detected in superfusates from slices from rat striatum. The concentrations of endogenous free and conjugated DA, and of the acid metabolites (3,4-dihydroxyphenylacetic acid [DOPAC] and homovanillic acid [HVA]) in superfusates were measured using HPLC with electrochemical detection. Conjugated DA in superfusates represented 10–20% of the free DA under basal conditions and during release evoked by p -tyramine (5 × 10⁻⁶ M to 5 × 10⁻⁴ M ); much smaller amounts of conjugated DA overflowed into superfusate when DA was released by equimolar concentrations of β-phenylethyl-amine. Surprisingly, inhibition of monoamine oxidase by the inhibitors N -methyl- N -propargyl-3-(2,4-dichlorophenoxy)propylamine hydrochlo-ride (clorgyline) or N -methyl- N -2-propynylbenylamine (pargyline) had little effect on the amounts of conjugated DA present in superfusate. Under basal conditions, the amounts of conjugated DA in superfusate were always less than the amounts of DOPAC but quite similar to the amounts of HVA. However, during release of DA evoked by p -tyramine the concentrations of conjugated DA in superfusate showed much more pronounced increases than those of the acidic metabolites.     

Role of Excitatory Amino Acid Receptors in K+-and Glutamate-Evoked Release of Endogenous Adenosine from Rat Cortical Slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.     

The Tyrosine Kinase Inhibitor Genistein Increases Endogenous Dopamine Release from Normal and Weaver Mutant Mouse Striatal Slices

Abstract: Protein tyrosine kinases that are known to have major roles in the control of cell growth and transformation are abundant and have numerous phosphoprotein substrates in the adult CNS. Although less well characterized than serine/threonine kinases, tyrosine kinases are also concentrated in the synapse. The effect of genistein, a selective inhibitor of tyrosine kinase activity, was examined on the in vitro release of endogenous dopamine (DA) from superfused mouse striatal slices. Fractional release of DA was significantly increased over basal release levels by genistein (100 and 200 µ M ). The effect was concentration dependent and rapidly reversible on washout of the kinase inhibitor. No significant change from basal release levels was observed with two structural analogues of genistein that do not inhibit tyrosine kinase activity at the same concentration. We have previously described alterations in basal and evoked DA release from the striatum of the weaver ( wv/wv ) mutant mouse, and genotypic differences in fractional release were also observed with genistein stimulation. The total evoked release was 25–50% greater from the wv/wv striatum. These results suggest a modulatory role for tyrosine kinase activity in neurotransmitter release and perhaps an alteration of kinase-regulated mechanisms in the DA-deficient wv/wv striatum.     

Rapid Release of [3H]Dopamine from Median Eminence and Striatal Synaptosomes

Release of preaccumulated, tritium-labeled dopamine ([3H]DA) from preparations of isolated nerve terminals (synaptosomes) of rat median eminence (ME) and corpus striatum (CS) was examined over short time intervals (1-20 s). In both preparations, basal efflux of [3H]DA was linear with time. Depolarization with high K+ resulted in an initial rapid release of [3H]DA which stabilized by 20 s, whereas veratridine elicited an increased rate of release over basal levels that was linear over the first 20 s. The calculated rate constants of release for both the initial phase of K+- and the veratridine-stimulated release were approximately threefold greater in CS than in ME synaptosomes. The major component of the high K+-induced release of [3H]DA from both synaptosome preparations increased as a graded function of [Ca2+]o. However, a smaller component, independent of external Ca2+, existed in both ME and CS synaptosomes. Increasing the [Mg2+] in the external solution resulted in a right shift of both the [K+]o and the [Ca2+]o dose-response curves, consistent with actions of Mg2+ on screening surface membrane charges and blocking voltage-dependent Ca2+ channels. In all studies, steady-state uptake of the [3H]DA was about twofold greater into CS than into ME synaptosomes. Moreover, the fraction of incorporated [3H]DA released by stimulation from the CS was much greater than that released from ME synaptosomes. These data are consistent with differences between these two types of dopaminergic terminals with respect to packaging and/or distribution of the accumulated neurotransmitter in intraneuronal pools, as well as marked differences in the apparent kinetics of DA release.     

L-dopa administration enhances exocytotic dopamine release in vivo in the rat striatum.

Peripheral administration of L-3,4-dihydroxyphenylalanine (L-DOPA) methylester increased extracellular levels of DOPA and dopamine (DA) in the rat striatum monitored by in vivo brain microdialysis. The increase in DA levels persisted after inhibition of DA reuptake by nomifensine. Administration of blockers of voltage-dependent Na+ (tetrodotoxin) or Ca2+ (NKY-722) channels through the dialysis membrane completely eliminated the increase in DA levels. These results demonstrate that the L-DOPA-induced DA release is exocytotic in nature and hence, derived from neurons in the striatum.     

Characterization of the Participation of Sodium Channels on the Rise in Na+ Induced by 4-Aminopyridine (4-AP) in Synaptosomes

The participation of voltage-sensitive Na+ channels (VSSC) on the changes on internal (i) Na+, K+, Ca2+, and on DA, Glu, and GABA release caused by different concentrations of 4-AP was investigated in striatum synaptosomes. TTX, which abolished the increase in Na(i) (as determined with SBFI), induced by 0.1 mM 4-AP only inhibited by 30% the rise in Na(i) induced by 1 mM 4-AP. One millimolar 4-AP markedly decreased the fluorescence of the K+ indicator dye PBFI but 0.1 mM 4-AP did not. Like 1 mM 4-AP, ouabain decreased PBFI fluorescence and increased a considerable fraction of Na(i) in a TTX-insensitive manner. In contrast with the different TTX sensitivity of the rise in Na(i) induced by 0.1 and 1 mM 4-AP, the rise in Ca(i) (as determined with fura-2) induced by the two concentrations of 4-AP was markedly inhibited by TTX, as well as by omega-agatoxin in combination with omega-conotoxin GVIA, indicating that only the TTX-sensitive fraction of the rise in Na(i) induced by 4-AP is linked with the activation of presynaptic Ca2+ channels. It is concluded that the TTX-sensitive fraction of neurotransmitter release evoked by 4-AP is released by exocytosis, and the TTX insensitive fraction involves reversal of the neurotransmitters transporters. This contrasts with the exocytosis evoked by high K+ that is unchanged by TTX and with the neurotransmitter-transporter-mediated release evoked by veratridine, which is highly TTX sensitive and does not require activation of Ca2+ channels.     

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.     

Low-Na+ Medium Increases the Activity and the Phosphorylation of Tyrosine Hydroxylase in the Superior Cervical Ganglion of the Rat

Incubation of the rat superior cervical ganglion in Na+-free or low-Na+ medium increased the rate of synthesis of 3,4-dihydroxyphenylalanine (DOPA) in the ganglion fourfold and caused a concomitant stable activation of tyrosine hydroxylase. DOPA synthesis was half-maximal in medium containing about 20 mM Na+. Low-Na+ medium also increased the incorporation of 32Pi into tyrosine hydroxylase; the dependence of tyrosine hydroxylase phosphorylation on the Na+ concentration resembled that of DOPA synthesis. The stimulatory effects of low-Na+ medium on DOPA production and on tyrosine hydroxylase activity in vitro were dependent on extra-cellular Ca2+. The stimulation of DOPA synthesis in low-Na+ medium was inhibited by methoxyverapamil, an inhibitor of Ca2+ uptake, and was partially blocked by tetrodotoxin, but it was not affected by the cholinergic antagonists hexamethonium and atropine. Ionomycin, a calcium ionophore, stimulated DOPA synthesis to about the same extent as low-Na+ medium and also increased the incorporation of 32Pi into tyrosine hydroxylase. 8-Bromo cyclic AMP (1 mM) also stimulated DOPA production in the ganglion, and this stimulation was more than additive with that produced by low-Na+ medium. These data support the hypothesis that low-Na+ medium stimulates DOPA synthesis by raising intracellular Ca2+, which then promotes the phosphorylation of tyrosine hydroxylase.     

Multiple mechanisms of transmitter release evoked by "pathologically" elevated extracellular [K+]: involvement of transporter reversal and mitochondrial calcium

The release of [3H]GABA evoked by depolarization with various concentrations of KCl was studied using superfused rat cerebrocortex synaptosomes. Elevating [K+] produced release of [3H]GABA over basal which was increasingly less dependent on external Ca2+ but more sensitive to the GABA transporter blocker SKF 100330 A. Accordingly, the sensitivity to clostridial toxins of the depolarization-evoked amino acid release was inversely correlated to the concentration of KCl used. However, at 50 mM K+, one-third of the stimulated release remained which was external Ca2+-independent but insensitive to SKF 100330 A. This release was prevented by BAPTA, thapsigargin or dantrolene; it also was inhibited by blocking in mitochondria the ATP production with oligomycin, the H+-dependent Ca2+ uniporter with RU 360, the Na+/Ca2+ exchanger with CGP 37157 or by lowering extraterminal [Na+]. In fluorescence experiments with fura-2/AM, 50 mM K+ (in Ca2+ free medium) caused elevation of cytosolic [Ca2+] that was sensitive to thapsigargin or CGP 37157; these compounds produced partially additive effects. When exocytosis was monitored with the fluorescent dye acridine orange, the fluorescence elicited by 50 mM K+ was sensitive to thapsigargin or CGP 37157, which produced additive effects, and to low-Na+ media. To conclude, extracellular K+ concentrations occurring in the CNS in certain pathological conditions provoke GABA release by mechanisms different from classical exocytosis. These include carrier-mediated release and internal Ca2+-dependent exocytosis; in the latter, mitochondrial Ca2+ seems to play a primary role.     

YM-09151-2 but Not l-Sulpiride Induces Transient Dopamine Release in Rat Striatum via a Tetrodotoxin-Insensitive Mechanism

Abstract: The effects of the selective dopamine D₂ receptor antagonists YM-09151-2 and l -sulpiride on the in vivo release of dopamine (DA), l -3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in rat striatum were investigated. The drugs were injected into the striatum through a microinjection needle attached to a dialysis probe. YM-09151-2 (0.1 or 1.0 μg/0.5 μl) injected into the striatum produced a dramatic rapid-onset transient increase in striatal DA release in a dose-dependent manner. However, the DA increase induced by l -sulpiride (15 or 75 ng/0.5 μl) was small and of slower onset. An increase of DOPAC levels by YM-09151-2 was biphasic: The first peak occurred at 40 min, followed by a delayed-onset gradual increase. Slower-onset gradual increases were also found in DOPAC levels after l -sulpiride injection and in HVA levels after injections of both YM-09151-2 and l -sulpiride. The infusion of tetrodotoxin (TTX; 2 μM) revealed two different types of DA release mechanisms: The rapid-onset transient DA release induced by YM-09151-2 was TTX insensitive, whereas the slower-onset DA release induced by l -sulpiride was TTX sensitive. Moreover, the rapid-onset transient DA release was Ca²⁺ independent and was not affected by pre-treatment with l -sulpiride or nomifensine. Therefore, it is concluded that YM-09151-2 injected into the striatum produced a transient striatal DA release that is independent of D₂ receptors and the action potential.     

Modulation of Dendritic Release of Dopamine by N-Methyl-D-Aspartate Receptors in Rat Substantia Nigra

A superfusion system was used to study the effects of excitatory amino acids (EAA) on release of [3H]dopamine ([3H]DA) previously taken up by rat substantia nigra (SN) slices. The EAA tested (20-250 microM), with the exception of quisqualate and kainate, markedly evoked [3H]DA release from nigral slices when Mg2+ ions were omitted from the superfusion medium. The EAA receptor agonists exhibited the following relative potency in stimulating [3H]DA release: L-glutamate (L-Glu) greater than N-methyl-D-aspartate (NMDA) greater than NM(D,L)A greater than D-Glu much greater than quisqualate = kainate. D-2-Amino-5-phosphonovalerate (100-200 microM), an antagonist for NMDA receptors, substantially reduced [3H]DA release evoked by L-Glu or NMDA. In contrast, L-Glu diethyl ester (100-200 microM) produced a lesser blocking effect on [3H]DA release evoked by the EAA. Further experiments showed that the NMDA-mediated release of [3H]DA was totally suppressed by the omission of Ca2+ or by the addition of tetrodotoxin (0.1 microM) to the superfusion medium. In addition, strychnine, an antagonist for glycine (Gly) receptors, significantly decreased NMDA (100 microM)-evoked as well as glycine (100 microM)-evoked release of [3H]DA from nigral slices. The results shown support the idea that activation of NMDA subtype receptors in SN may trigger a Ca2+-dependent release of DA from dendrites of nigro-striatal DA-containing neurons. Furthermore, a transsynaptic mechanism that may partially involve Gly-containing interneurons is proposed to account for some of the events mediating NMDA receptor activation and DA release in SN.     

Serotonin Agonists Reduce Dopamine Synthesis in the Striatum Only when the Impulse Flow of Nigro-Striatal Neurons Is Intact

The effects of 5-methoxy-N, N-dimethyltryptamine (5-MeO-DMT) and m-chlorophenylpiperazine (CPP), two 5-hydroxytryptamine (5-HT, serotonin) agonists, on the accumulation of 3,4-dihydroxyphenylalanine (DOPA] were studied in the striatum of rats treated with gamma-butyrolactone (GBL). Unlike 2 mg/kg i.p. apomorphine, neither 5 mg/kg i.p. 5-MeO-DMT nor 2.5 mg/kg i.p. CPP significantly reduced the GBL-induced increase in DOPA accumulation in the striatum. 5-MeO-DMT and CPP significantly reduced DOPA accumulation in animals that had received the aromatic amino acid decarboxylase inhibitor Ro 4-4602 but not GBL. 5-HT (10 micrograms in 0.5 microliter) injected in the substantia nigra, pars compacta, like GBL, significantly increased Ro 4-4602-induced accumulation of DOPA in the striatum. The data indicate that 5-HT agonists can reduce 3,4-dihydroxyphenylethylamine (DA, dopamine) synthesis in the striatum of rats only when the impulse flow of DA neurons is intact. An indirect effect through mechanisms controlling DA synthesis in the striatum, for instance cholinergic and GABA-ergic neurons, is suggested.     

Influence of Potassium Concentration in Microdialysis Perfusate on Basal and Stimulated Striatal Dopamine Release: Effect of Ceruletide, a Cholecystokinin-Related Peptide

Abstract: The in vivo microdialysis method was used to study the effect of the cholecystokinin-related peptide, ceruletide, on extracellular levels of dopamine (DA) in the striatum following perfusion with various K⁺ concentrations. Increasing the K⁺ concentration in the perfusate from 4 to 15 or 17.5 m M did not change basal DA release or release evoked by electrical stimulation of the medial forebrain bundle (MFB). However, when the perfusing solution contained 20 or 30 m M K⁺, dose-dependent reductions of both basal and MFB-stimulated DA release occurred. Subcutaneous administration of ceruletide at 160 μg/kg had no influence on the basal or MFB-stimulated DA release with 4 or 15 m M K⁺ in the perfusate. However, after perfusion with 17.5 m M K⁺, ceruletide significantly attenuated the basal and MFB-stimulated DA release. Carbachol (10 μ M ) locally applied via the dialysis probe also attenuated MFB-stimulated DA release after perfusion with 17.5 m M K⁺. From these results, we conclude that under appropriate depolarization of striatal DA terminals, ceruletide induces further depolarization and inactivation of nigrostriatal DA terminals. The present data suggest that this effect may be mediated via intrinsic cholinergic neurons in the striatum.     

Role of Adenylate Cyclase in the Modulation of the Release of Dopamine: A Microdialysis Study in the Striatum of the Rat

In the present study, we have applied the brain microdialysis technique to investigate the effect of the stimulation of adenylate cyclase on the extracellular levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum of freely moving rats. Infusion of 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), 3-isobutyl-1-methylxanthine, or forskolin produced a significant increase in the release of DA. The effect of 8-Br-cAMP was tetrodotoxin, Ca2+, and dose dependent and was saturable. 8-Br-cAMP also caused an increase in the output of DOPAC and HVA. No effects were seen on the output of 5-HIAA, except at the highest 8-Br-cAMP concentration studied. Infusion of 8-Br-cAMP (25 microM, 1.0 mM, and 3.3 mM) together with infusion of (-)-sulpiride (1 microM) or systemic administration of (+/-)-sulpiride (55 mumol/kg i.p.) produced an additive effect on the release of DA. Infusion or peripheral administration of (-)-N-0437 (1 microM or 1 mumol/kg) both decreased the 8-Br-cAMP-induced increase in the release of DA. These results demonstrate that cyclic AMP may stimulate the release of DA, but it is unlikely that this second messenger is linked to presynaptic D2 receptors controlling the release of DA.     

3-Methoxytyramine Is the Major Metabolite of Released Dopamine in the Rat Frontal Cortex: Reassessment of the Effects of Antipsychotics on the Dynamics of Dopamine Release and Metabolism in the Frontal Cortex, Nucleus Accumbens, and Striatum by a Simple Two Pool Model

Abstract: 3-Methoxytyramine (3-MT) and 3,4-dihydroxyphenylacetic acid (DOPAC) rates of formation were used, respectively, to assess the dynamics of dopamine (DA) release and turnover in the rat frontal cortex, nucleus accumbens, and striatum. Assuming total (re)uptake and metabolism of released DA are relatively uniform among the three brain regions, a simplified two pool model was used to assess the metabolic fate of released DA. Under basal conditions, 3-MT formation was found to comprise >60% of total DA turnover (sum of 3-MT plus DOPAC rates of formation) in the frontal cortex, and not more than 15% in the nucleus accumbens and striatum. Haloperidol increased the 3-MT rate of formation to a greater extent in the frontal cortex than in the two other regions. Clozapine increased the 3-MT rate of formation in the frontal cortex and decreased it in the striatum. Both drugs increased DOPAC rate of formation in the frontal cortex and nucleus accumbens. It was elevated by haloperidol but not clozapine in the striatum. It is concluded that (1) O -methylation is a prominent step in the catabolism of DA in the frontal cortex under both physiological conditions and after acute treatment with antipsychotics, (2) 3-MT is the major metabolite of released DA in the frontal cortex and possibly also in the nucleus accumbens and striatum, (3) in contrast to the frontal cortex, most of the DOPAC in the nucleus accumbens and striatum appear to originate from intraneuronal deamination of DA that has not been released, (4) because presynaptic uptake and metabolism of DA give rise to DOPAC, whereas postsynaptic uptake and metabolism produced both DOPAC and 3-MT, the ratio of 3-MT to DOPAC rates of formation can be a useful index of reuptake inhibition.