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.     

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.     

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.     

Inhibition of striatal dopamine release by CB1 receptor activation requires nonsynaptic communication involving GABA, H2O2, and KATP channels

The main psychoactive component of marijuana, Delta9-tetrahydrocannabinol (THC), acts in the CNS via type 1 cannabinoid receptors (CB1Rs). The behavioral consequences of THC or synthetic CB1R agonists include suppression of motor activity. One explanation for movement suppression might be inhibition of striatal dopamine (DA) release by CB1Rs, which are densely localized in motor striatum; however, data from previous studies are inconclusive. Here we examined the effect of CB1R activation on locally evoked DA release monitored with carbon-fiber microelectrodes and fast-scan cyclic voltammetry in striatal slices. Consistent with previous reports, DA release evoked by a single stimulus pulse was unaffected by WIN55,212-2, a cannabinoid receptor agonist. However, when DA release was evoked by a train of stimuli, WIN55,212-2 caused a significant decrease in evoked extracellular DA concentration ([DA]o), implicating the involvement of local striatal circuitry, with similar suppression seen in guinea pig, rat, and mouse striatum. Pulse-train evoked [DA]o was not altered by either AM251, an inverse CB1R agonist, or VCHSR1, a neutral antagonist, indicating the absence of DA release regulation by endogenous cannabinoids with the stimulation protocol used. However, both CB1R antagonists prevented and reversed suppression of evoked [DA]o by WIN55,212-2. The effect of WIN55,212-2 was also prevented by picrotoxin, a GABAA receptor antagonist, and by catalase, a metabolizing enzyme for hydrogen peroxide (H2O2). Furthermore, blockade of ATP-sensitive K+ (KATP) channels by tolbutamide or glybenclamide prevented the effect of WIN55,212-2 on DA release. Together, these data indicate that suppression of DA release by CB1R activation within striatum occurs via a novel nonsynaptic mechanism that involves GABA release inhibition, increased generation of the diffusible messenger H2O2, and activation of KATP channels to inhibit DA release. In addition, the findings suggest a possible physiological substrate for the motor effects of cannabinoid agonist administration.     

Distinct Muscarinic Receptor Subtypes Differentially Modulate Acetylcholine Release from Corticocerebral Synaptosomes

The effect of McN-A-343 and oxotremorine on acetylcholine (ACh) release and choline (Ch) transport was studied in corticocerebral synaptosomes of the guinea pig. The synaptosomes were preloaded with [3H]Ch after treatment with the irreversible cholinesterase inhibitor, diisopropyl fluorophosphate, and then tested for their ability to release isotope-labeled ACh and Ch in the presence and absence of these agents. The kinetics of release were determined at the resting state (basal release) and in the presence of 50 mM K+. Under either condition, McN-A-343 enhanced the release of isotope-labeled ACh, whereas oxotremorine inhibited the K(+)-evoked release but had no effect on the basal release. The enhancing effect of McN-A-343 on basal ACh release was fully blocked by the selective M1 muscarinic antagonist, pirenzepine (100 nM). In contrast to its enhancing effect on ACh release, McN-A-343 potently inhibited Ch efflux as well as Ch influx. These effects were not blocked by atropine, a nonselective muscarinic antagonist. Oxotremorine had no effect on Ch transport. Binding studies showed that McN-A-343 was 3.6-fold more potent in displacing radiolabeled quinuclidinyl benzilate from cerebral cortex muscarinic receptors (mostly M1 subtype) than from cerebellar receptors (mostly M2 subtype), whereas oxotremorine was 2.6-fold more potent in the cerebellum. The displacements of radio-labeled pirenzepine and cis-dioxolane confirmed the M1 subtype preference of McN-A-343 and the M2 subtype preference of oxotremorine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endogenous Dopamine Facilitates Striatalln Vivo Acetylcholine Release by Acting on D1 Receptors Localized in the Striatum

Intrastriatal application of the D1 antagonist SCH 23390 by two procedures, reverse dialysis (20 microM) and local injection (0.45 nmol per striatum), elicited a reduction in acetylcholine (ACh) release superimposable on that induced by systemic administration. The novel selective D1 antagonist SCH 39166 produced a similar decreasing effect on striatal ACh release on local injection (0.45 nmol per striatum). On the other hand, local application of SCH 23390 into the frontal cortices (0.45 nmol per side) failed to alter striatal ACh overflow, indicating that the drug does not diffuse out of its injection site to any significant extent. The dopamine release inducer d-amphetamine (2 mg/kg s.c.) and the dopamine uptake inhibitor cocaine raised ACh release like the D1 agonists. These effects were completely blocked by 10 microM SCH 23390 applied by reverse dialysis. The results suggest that D1 receptors regulating ACh release are located in the striatum.     

Selective stimulation of serotonin2c receptors blocks the enhancement of striatal and accumbal dopamine release induced by nicotine administration

The effects of acute and repeated nicotine administration on the extracellular levels of dopamine (DA) in the corpus striatum and the nucleus accumbens were studied in conscious, freely moving rats by in vivo microdialysis. Acute intraperitoneal (i.p.) injection of nicotine (1 mg/kg) increased DA outflow both in the corpus striatum and the nucleus accumbens. Repeated daily injection of nicotine (1 mg/kg, i.p.) for 10 consecutive days caused a significant increase in basal DA outflow both in the corpus striatum and the nucleus accumbens. Acute challenge with nicotine (1 mg/kg, i.p.) in animals treated repeatedly with this drug enhanced DA extracellular levels in both brain areas. However, the effect of nicotine was potentiated in the nucleus accumbens, but not in the corpus striatum. To test the hypothesis that stimulation of 5-HT (5-hydroxytryptamine, serotonin)(2C) receptors could affect nicotine-induced DA release, the selective 5-HT(2C) receptor agonist RO 60-0175 was used. Pretreatment with RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently prevented the enhancement in DA release elicited by acute nicotine in the corpus striatum, but was devoid of any significant effect in the nucleus accumbens. RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently reduced the stimulatory effect on striatal and accumbal DA release induced by an acute challenge with nicotine (1 mg/kg, i.p.) in rats treated repeatedly with this alkaloid. However, only the effect of 3 mg/kg RO 60-0175 reached statistical significance. The inhibitory effect of RO 60-0175 on DA release induced by nicotine in the corpus striatum and the nucleus accumbens was completely prevented by SB 242084 (0.5 mg/kg, i.p.) and SB 243213 (0.5 mg/kg, i.p.), two selective antagonists of 5-HT(2C) receptors. It is concluded that selective activation of 5-HT(2C) receptors can block the stimulatory action of nicotine on central DA function, an effect that might be relevant for the reported antiaddictive properties of RO 60-0175.     

Interaction between alpha-MSH and acetylcholinergic system upon striatal cAMP and IP(3) levels

The interaction between the neuropeptide alpha-MSH and the acetylcholinergic system as reflected by changes in cAMP and inositol 1-3-5 triphosphate(IP(3))production was investigated in an in vitro model of striatal slices. The possible involvement of D(1) receptors in cholinergic and alpha-MSH- stimulated cAMP and IP(3) production in slices of rat striatum was also examined, because it has been demonstrated that acetylcholinergic drugs induce endogenous dopamine release in the striatum. alpha-MSH, pilocarpine(PL) and the selective muscarinic M1 agonist McN-A-343 increased cAMP and IP(3) striatal levels, effects blocked by the D(1) antagonist SCH-23390, except for the effects of alpha-MSH on IP(3).The muscarinic M(2) antagonist gallamine (GL) brought about an increase in cAMP levels, an effect blocked by SCH-23390. The M(1) antagonist pirenzepine (Pz) induced a decrease both in cAMP and IP(3) content, and the nicotinic antagonist di-hydro-beta-eritroidine(DBE) only diminished cAMP production. When alpha-MSH and cholinergic agents were simultaneously added, cAMP and IP(3) levels were modified with respect to the values reached when these agents were added alone. An interaction between the acetylcholinergic system and alpha-MSH through M(1) and nicotinic receptors was also observed. These results suggest that the intracellular signaling pathways related to cAMP and IP(3) production gated by alpha-MSH and these cholinergic receptors are probably related. alpha-MSH striatum cAMP IP(3) muscarinic and nicotinic receptors an in vitro model.     

Mice transgenic for exon 1 of Huntington's disease: properties of cholinergic and dopaminergic pre-synaptic function in the striatum

In Huntington's disease (HD), neuronal loss is most prominent in the striatum leading to emotional, cognitive and progressive motor dysfunction. The R6/2 mice, transgenic for exon 1 of the HD gene, develop a neurological phenotype with similarities to these features of HD. In striatal tissue, electrically evoked release of tritiated acetylcholine (ACh) and dopamine (DA) were compared in wild-type (WT) and R6/2 mice. In R6/2 mice, the evoked release of ACh, its M2 autoreceptor-mediated maximum inhibition and its dopamine D2 heteroreceptor-mediated maximum inhibition was diminished to 51%, 74% and 87% of controls, respectively. Also, the activities of choline acetyltransferase and of synaptosomal high-affinity choline uptake decreased progressively with age in these mice. In the DA release model, however, electrical stimulation elicited equal amounts of [3H]-DA both in WT and R6/2 mice. Moreover, high-affinity DA uptake into striatal slices was similar in WT and R6/2 mice. In order to confirm these findings in vivo, intrastriatal levels of extracellular DA were measured by intracerebral microdialysis in freely moving mice: striatal DA levels were found to be equal in WT and R6/2 mice. In conclusion, in the transgenic R6/2 mice changes occur mainly in striatal cholinergic neurones and their pre-synaptic modulation, but not in the dopaminergic afferent terminals. Whether similar events also contribute to the pathogenesis of HD in humans has to be established.     

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.     

Effects of L-dopa and L-tyrosine on release of free and conjugated dopamine, homovanillic acid and dihydroxyphenylacetic acid from slices of rat striatum

Conjugation (presumably with sulfate) is a demonstrable metabolic pathway for 3, 4-dihydroxyphenylethylamine (dopamine, DA) in brain. Studies were done to determine whether conjugation becomes of increased significance in the presence of precursors of DA. The effects of 3, 4-dihydroxyphenylalanine (L-DOPA) and L-tyrosine on the efflux of free and conjugated DA, 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid from slices from striatum in rats were studied under quiescent conditions and during release evoked by 40 mM K+ or by 5 X 10(-5) M phenylethylamine (PEA). Conjugated DA was present in the basal efflux from striatal slices and the amounts present were increased during evoked release. More conjugated DA was present in superfusate during K+-evoked release than during PEA-evoked release. L-Tyrosine (5 X 10(-4) M or 5 X 10(-5) M) had little effect on the efflux of conjugated DA, but decreased the amounts of free DA released by PEA, and attenuated the increase in DOPAC that occurred during K+-evoked release of transmitter. L-DOPA (5 X 10(-5) M) increased the formation of conjugated DA, but to a lesser extent than that of free DA or of DOPAC. Thus even after the addition of precursors, conjugation remains a minor metabolic pathway for DA relative to O-methylation or oxidative deamination. The data also suggest that conjugation of DA occurs chiefly outside of the dopaminergic neurons in striatum.     

Modulation of dopaminergic neurotransmission in rat striatum upon in vitro and in vivo diclofenac treatment

Diclofenac (DCF) is a widely used non-steroidal anti-inflammatory drug, which also act as a mitochondrial toxin. As it is known that selective mitochondrial complex I inhibition combined with mild oxidative stress causes striatal dopaminergic dysfunction, we tested whether DCF also compromise dopaminergic function in the striatum. [3H]Dopamine ([3H]DA) release was measured from rat striatal slices after in vitro (2 h, 10-25 micromol/L) or in vivo (3 mg/kg i.v. for 28 days) DCF treatment. In vitro treatment significantly decreased [3H]DA uptake and dopamine (DA) content of the slices. H2O2 (0.1 mmol/L)-evoked DA release was enhanced. Intracellular reactive oxygen species production was not significantly changed in the presence of DCF. After in vivo DCF treatment no apparent decrease in striatal DA content was observed and the uptake of [3H]DA into slices was increased. The intensity of tyrosine hydroxylase immunoreactivity in the striatum was highly variable, and both decrease and increase were observed in individual rats. The H2O2-evoked [3H]DA release was significantly decreased and the effluent contained a significant amount of [3H]octopamine, [3H]tyramine, and [3H]beta-phenylethylamine. The ATP content and adenylate energy charge were decreased. In conclusion, whereas in vitro DCF pre-treatment resembles the effect of the mitochondrial toxin rotenone, in vivo it rather counteracts than aggravates dopaminergic dysfunction.     

Characterization and Pharmacological Responsiveness of Dopamine Release Recorded by Microdialysis in the Substantia Nigra of Conscious Rats

The extracellular concentration of dopamine (DA) and 3,4-dihydroxyphenylacetic acid in the substantia nigra (SN) and striatum was estimated by microdialysis. The dialysate content of DA from the SN was recorded during infusion of a DA uptake blocker (nomifensine; 5 mumol/L) dissolved in the perfusion fluid. Perfusion of tetrodotoxin (1 mumol/L) produced a virtually complete disappearance of nigral and striatal DA release. Dendritic as well as terminal release of DA was inhibited for several hours when the nerve impulse flow in dopaminergic neurons was blocked by systemic administration of gamma-butyrolactone (750 mg/kg, i.p.). The systemic administration (0.3 mg/kg, i.p.) as well as infusion (1 mumol/L) of the D2 agonist (-)-N-0437 [2-(n-propyl-N-2-thienylethylamino)-5-hydroxytetralin] produced a significant decrease in the release of DA in both the striatum and the SN. DA levels were recorded in the striatum both with and without addition of nomifensine to the perfusion fluid. The decrease in the striatum after (-)-N-0437 was suppressed in the presence of nomifensine. Infusion (1 mumol/L) as well as systemic administration (40 mg/kg) of sulpiride caused a similar increase in the release of striatal DA; this increase was, in both experiments, potentiated by nomifensine coinfusion. Sulpiride administration induced a small increase in the release of nigral DA. Infusion of (-)-N-0437 or (-)-sulpiride into the nigra caused a moderate decrease and increase, respectively, of striatal DA level.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of Striatal GABA Release by the Adenosine A2a Receptor Is Not Mediated by Increases in Cyclic AMP

Abstract: The adenosine A_2a receptor inhibition of potassium (15 m M )-evoked GABA release from striatal nerve terminals has been examined. High extracellular calcium concentrations (4 m M ) reduced the effect of the A_2a receptor agonist CGS-21680 (1 n M ). CGS-21680 inhibited GABA release in the presence of the L-type calcium channel blocker nifedipine, which itself inhibited evoked GABA release (by 16 ± 4%). ω-Conotoxin inhibited the evoked release by 45 ± 4% and prevented the action of CGS-21680. Forskolin and 8-bromo cyclic AMP both stimulated evoked GABA release at low concentrations, but at higher concentrations they abolished the inhibition by CGS-21680 without affecting the evoked release. The nonselective protein kinase inhibitor H-7 inhibited both the evoked release and the inhibition by CGS-21680, whereas the selective protein kinase A and G inhibitor HA-1004 had no effect on either evoked release or the action of CGS-21680. Pretreatment with pertussis toxin did not affect the A_2a receptor-mediated inhibition. Therefore, the effect of A_2a receptor stimulation was not mediated by protein kinases A or G but was inhibited by elevated cyclic AMP levels and mimicked by inhibitors of the N-type calcium channel and protein kinase C.     

Differential regulation of glutamate, aspartate and gamma-amino-butyrate release by N-methyl-D-aspartate receptors in rat striatum after partial and extensive lesions to the nigro-striatal dopamine pathway.

The in vivo microdialysis methodology was used to assess the effect of N-methyl-D-aspartate (NMDA) receptor ligands on glutamate (GLU), aspartate (ASP) and gamma-aminobutyrate (GABA) extracellular levels in the striatum of anaesthetized rats, after damage to the dopamine (DA) nigrostriatal pathway by injections of different doses of 6-hydroxydopamine (6-OH-DA) seven days earlier. The 6-OH-DA treated rats were divided into two groups, corresponding to animals with 20-80% (partial) and 85-99% (extensive) striatal DA tissue depletion, respectively. In rats with partial DA depletion, the striatal extracellular ASP levels significantly increased after intrastriatal dialysis perfusion with MK-801 (100 microM), an antagonist of NMDA receptors. In addition, a change in the pattern of local NMDA (500 microM)- induced efflux of ASP was observed in the striatum of these rats. However, in these partially DA-depleted striata no changes were found in basal extracellular levels of GLU, ASP and GABA or in NMDA- and MK-801-mediated effluxes of GLU and GABA relative to striata from sham rats. In contrast, rats with extensive striatal DA depletion exhibited a significant increase in ASP and GABA extracellular striatal levels, after intrastriatal dialysis perfusion with NMDA. In addition, the MK-801-mediated stimulation of extracellular ASP levels was accentuated along with the appearance of a MK-801 mediated increase in extracellular striatal GLU. Finally, basal extracellular levels of ASP, but not of GLU and GABA, were found to increase in extensive DA-depleted striata when compared to sham and partially DA-depleted striata. Thus, a differential regulation of basal and NMDA receptor-mediated release of transmitter amino acids occur seven days after partial and extensive DA-depleted striatum by 6-OH-DA-induced lesions of the nigrostriatal DA pathway. These findings may have implications as regards the participation of NMDA receptors in the compensatory mechanisms associated with the progress of Parkinson's disease, as well as in the treatment of this neurological disorder.     

Evoked dopamine overflow is augmented in the striatum of calcitriol treated rats

Calcitriol, the active metabolite of vitamin D, has been shown to have significant effects on the brain. These actions include reducing the severity of some central nervous system lesions, possibly by upregulating trophic factors such as glial cell line-derived neurotrophic factor (GDNF). GDNF has substantial effects on the nigrostriatal dopamine (DA) system of young adult, aged and lesioned animals. Thus, the administration of calcitriol may lead to significant effects on nigrostriatal DA neuron functioning. The present experiments were designed to examine the ability of calcitriol to alter striatal DA release, and striatal and nigral tissue levels of DA. Male Fischer-344 rats were administered vehicle or calcitriol (0.3, 1.0, or 3.0 μg/kg, s.c.) once daily for eight consecutive days. Three weeks later in vivo microdialysis experiments were conducted to measure basal and stimulus evoked overflow of DA from the striatum. Basal levels of extracellular DA were not significantly affected by the calcitriol treatments. However, the 1.0 and 3.0 μg/kg doses of calcitriol led to increases in both potassium and amphetamine evoked overflow of striatal DA. Although post-mortem tissue levels of striatal DA were not altered by the calcitriol injections, nigral tissue levels of DA and its main metabolites were increased by both the 1.0 and 3.0 μg/kg doses of calcitriol. In a separate group of animals GDNF levels were augmented in the striatum and substantia nigra after eight consecutive daily injections of calcitriol. These results suggest that systemically administered calcitriol can upregulate dopaminergic release processes in the striatum and DA levels in the substantia nigra. Increases in the levels of endogenous GDNF following calcitriol treatment may in part be responsible for these changes. The ability of calcitriol to lead to augmented DA release in the striatum suggests that calcitriol may be beneficial in disease processes involving dopaminergic dysfunction.     

In Vivo Microdialysis Evidence for Transient Dopamine Release by Benzazepines in Rat Striatum

Abstract: The effects of benzazepine derivatives on extracellular levels of dopamine (DA) and l -3,4-dihydroxyphenylacetic acid (DOPAC) in the dorsal striatum of freely moving rats were studied using in vivo microdialysis. Direct injection of SKF-38393 (0.5 or 1.5 µg/0.5 µl), a selective D₁ receptor agonist, into the striatum through a cannula secured alongside a microdialysis probe produced a rapid dose-dependent transient increase in striatal DA efflux and a more gradual reduction in efflux of DOPAC. The rapid increase in DA efflux was not affected by infusion of tetrodotoxin (TTX; 2 µ M ) or Ca²⁺-free Ringer's solution and occurred after either enantiomer of SKF-38393. A TTX-insensitive increase in DA level similar to that induced by SKF-38393 was also seen after other benzazepines acting as agonists (SKF-75670 and SKF-82958, each 1.5 µg in 0.5 µl) and antagonists (SCH-23390, 1.5 µg in 0.5 µl) at the D₁ receptor and after (+)-amphetamine. These effects were inhibited by infusion of nomifensine (100 µ M ). It is concluded that the transient increases in striatal DA efflux seen after intrastriatal injection of SKF-38393 and other benzazepines are not mediated by presynaptic D₁ receptors but by an amphetamine-like action on the dopamine transporter.     

Dopaminergic Modulation of Glutamate Release in Striatum as Measured by Microdialysis

Glutamate and aspartate are the primary neurotransmitters of projections from motor and premotor cortices to the striatum. Release of glutamate may be modulated by dopamine receptors located on corticostriatal terminals. The present study used microdialysis to investigate the dopaminergic modulation of in vivo striatal glutamate and aspartate release in the striatum of awake-behaving rats. Local perfusion with a depolarizing concentration of K+ through a dialysis probe into the rat striatum produced a significant increase in the release of glutamate, aspartate, and taurine. The D2 agonist LY171555 blocked the K(+)-induced release of glutamate and aspartate, but not taurine, in a concentration-dependent manner. The D1 agonist SKF 38393 did not alter K(+)-induced release of glutamate and taurine, but did significantly decrease aspartate release. Neither agonist had any effect on basal amino acid release. The D2 antagonist (-)-sulpiride reversed the inhibitory effects of LY 171555 on K(+)-induced glutamate release. These results provide in vivo evidence for a functional interaction between dopamine, the D2 receptor, and striatal glutamate release.     

Different effects of typical and atypical neuroleptics on K+-stimulated dopamine release from isolated rat striatum]

Effects of haloperidol (10(-7)-alpha 10(-5) M), trifluoperazine, metoclopramide, tiapride, sulpiride, thioridazine, clozapine remoxipride, raclopride, cis- and trans-isomers of carbidine, SCH 23390 (all at the 10(-6) M) on the K(+)-stimulated (28 mM) dopamine (DA) release from isolated rat striatum were studied. Haloperidol at the concentration of 10(-7) and 10(-6) M failed to affect, while at 10(-5) M the drug decreased the stimulated striatal DA release. Trifluoperazine, metoclopramide and tiapride were shown not to modify this process. Sulpiride, thioridazine, clozapine, remoxipride, raclopride, isomers of carbidine were found to increase significantly the stimulated striatal DA release. SCH 23390 failed to affect K(+)-stimulated release of DA in the striatum and also did not change K(+)-stimulated release enhancement produced by raclopride. It is suggested that the mechanism underlying observed effects of the drugs may contribute to pharmacological profile of atypical neuroleptics.     

Protein Kinases Are Involved in Prolonged Acetylcholine Release from Rat Hippocampus Induced by Thyrotropin-Releasing Hormone Analogue NS-3

Abstract: The effects of various protein kinase inhibitors on acetylcholine release from the rat hippocampus induced by the local application of NS-3 (montirelin hydrate, CG-3703), a thyrotropin-releasing hormone analogue, into the medial septum-diagonal band were examined using in vivo microdialysis. Perfusion of NS-3 (1 µ M ) into the medial septum-diagonal band for 20 min produced a pronounced and prolonged increase in the hippocampal acetylcholine efflux. Pretreatment of the medial septum-diagonal band with either K-252a, a nonselective protein kinase inhibitor, or selective protein kinase A inhibitor H-89 almost completely blocked the acetylcholine efflux evoked by NS-3, and selective protein kinase C inhibitor calphostin C inhibited the action of NS-3. On the other hand, NS-3 (0.1–10 µ M ) or TRH (1–100 µ M ) increased the cyclic AMP efflux from the medial septum-diagonal band in a concentration-dependent manner, as measured by microdialysis. These findings suggest that protein kinases A and C in the neurons of the medial septum-diagonal band are involved in the mechanism of the prolonged stimulation of acetylcholine release from the hippocampus induced by thyrotropin-releasing hormone and its analogue, NS-3.