Growth inhibition of human glioma cells induced by 8-chloroadenosine, an active metabolite of 8-chloro cyclic adenosine 3':5'-monophosphate.

8-Chloroadenosine 3':5'-monophosphate (8ClcAMP) inhibits the growth of human glioma cell lines at much lower concentrations than more commonly used cyclic AMP analogues, without inducing morphological differentiation. The mechanism by which 8ClcAMP exerts this effect is not fully understood. We examined whether the growth-inhibitory effect of this compound is due to an active metabolite, using a sulforhodamine protein stain assay to determine the proliferation rate of the WF human glioma cell line. 8-Chloroadenosine, one of the metabolites, inhibited the proliferation of WF human glioma cells more potently than 8ClcAMP. In the presence of adenosine deaminase, which converts 8-chloroadenosine into 8-chloroinosine, 8-chloroadenosine no longer inhibited human glioma cell growth. Addition of adenosine deaminase also largely reduced the growth-inhibitory effect of 8ClcAMP, but not of 8-(4-chlorophenylthio)cAMP. High performance liquid chromatography analysis revealed that at least part of the 8ClcAMP in the culture medium is converted into 8-chloroadenosine. We concluded that 8ClcAMP exerts its growth-inhibitory effect through its active metabolite 8-chloroadenosine. Adenylate cyclase assays showed that 8-chloroadenosine does not affect the intracellular cAMP production through adenosine A1 or A2 receptor activation, which makes it unlikely that 8-chloroadenosine inhibits glioma cell growth by increasing the intracellular cyclic AMP concentration.

D-2 dopamine receptor-mediated inhibition of adenylate cyclase activity in the intermediate lobe of the rat pituitary gland requires guanosine 5'-triphosphate.

After treatment with cholera toxin, homogenates of intact intermediate lobe (IL) tissue of rat pituitary gland synthesized more cAMP than did homogenates of untreated IL tissue, and only in the presence of GTP did dopamine or apomorphine diminish the elevated adenylate cyclase activity in homogenates of cholera toxin-treated IL tissue. Furthermore, when tested on cholera toxin-treated IL tissue, 5'-guanylyl imidodiphosphate [Gpp(NH)p] and two other nonhydrolyzable analogs of GTP inhibited adenylate cyclase activity in the absence of either a dopaminergic agonist or GTP; GTP reversed the Gpp(NH)p-induced inhibition of adenylate cyclase activity. Apomorphine, a dopaminergic agonist, abolished the ability of GTP to reverse the inhibition by Gpp(NH)p; this effect of apomorphine was prevented by fluphenazine, a dopaminergic antagonist. Sodium fluoride inhibited adenylate cyclase activity to approximately the same level obtained with GTP and apomorphine. In addition, apomorphine decreased cAMP accumulation and alpha MSH release from dispersed IL cells pretreated with cholera toxin.

Differential sensitivity to hydrogen peroxide of dopaminergic and noradrenergic neurotransmission in rat brain slices.

Oxidative stress, induced by hydrogen peroxide, has been implicated in the pathogenesis of Parkinson's disease. Only scarce information is available if and how hydrogen peroxide, a side product of catecholamine (CA) breakdown, interferes with CAergic neurotransmission. Therefore, we investigated the effect of hydrogen peroxide on the release of [3H]dopamine (DA) and [3H]noradrenaline (NA) from rat striatal and cortical tissue slices, respectively. Hydrogen peroxide (0.01-1 mM) stimulated the spontaneous release of [3H]DA from striatal slices. Its effect on [3H]NA release from cortical slices, however, was much smaller than on DA release and occurred only in concentrations above 0.1 mM. Furthermore, only in concentrations of 1 mM or higher did a stimulation of spontaneous release of radioactivity from striatal slices incubated with [3H]choline occur. Omission of calcium significantly enhanced the effect on DA release, and an increase of calcium significantly reduced it. Blockade of vesicular storage with reserpine (0.3 microM) almost completely abolished [3H]DA release induced by hydrogen peroxide. Following incubation of striatal slices with [3H]NA in the presence of the NA (re)uptake blocker desmethylimipramine (0.3 microM), NA release was observed at a concentration (0.1 mM) at which no effect occurred in cortical slices. Moreover, under these conditions [3]NA and [3H]DA release from striatal slices reached comparable levels. Our results show that hydrogen peroxide induces a nonexocytotic release of DA and NA by interfering with the vesicular uptake and/or storage of these CAs. However, the striatal DA storage system, irrespective of the presence of either DA or NA, appeared to be substantially more sensitive to this effect than its cortical equivalent for storage of NA.

Astrocyte-enhanced neuronal survival is mediated by scavenging of extracellular reactive oxygen species.

The survival of cultured neurons is promoted by the presence of antioxidants or astrocytes. This indicates that extracellular reactive oxygen species (ROS) impair neuronal survival and suggests that astrocytes exert their survival-enhancing effect through inactivation of these toxicants. However, to our knowledge, data supporting this hypothesis are lacking. Previously, we showed that loss of the antioxidant glutathione abolishes the neuronal survival-stimulating action of astrocytes in cocultures, consisting of rat striatal astrocytes and mesencephalic, dopaminergic neurons. Using uptake of [3H]dopamine as marker of neuronal survival, we presently investigated whether this effect of glutathione depletion is mediated by extracellular ROS. For this purpose, we incubated glutathione-depleted cocultures with superoxide dismutase, catalase or both. Whereas superoxide dismutase had no effect and catalase only partially protected, addition of the enzymes together completely prevented the impairment of neuronal survival caused by glutathione loss. No change in neuronal survival occurred upon exposure of control cocultures to superoxide dismutase and/or catalase. These data strongly implicate scavenging of extracellular ROS in astrocyte-stimulated neuronal survival and moreover suggest a crucial role for glutathione in this process.

Opioid and D-2 receptor mediated inhibition of forskolin-stimulated cyclic AMP efflux from rat neostriatal slices.

Opioid and D-2 receptor agonists inhibit adenylate cyclase activity in neostriatal slices and homogenates. In the present study we used cyclic AMP efflux from rat neostriatal tissue as a parameter to estimate the effects of these drugs on cyclic AMP formation. Both the mu-opioid receptor agonist morphine and the D-2 dopamine receptor agonist LY 171555 were able to inhibit the forskolin-stimulated cyclic AMP efflux. The effects of morphine and LY 171555 could be reversed by naloxone and sulpiride, respectively. These data indicate that measurements of cyclic AMP efflux from brain slices is an accurate reflection of the effects of receptor stimulation on adenylate cyclase activity.

Muscarinic receptor activation attenuates D2 dopamine receptor mediated inhibition of acetylcholine release in rat striatum: indications for a common signal transduction pathway.

In the present investigations, we used a superfusion system to study the effect of simultaneous activation of D2 dopamine receptors and so-called muscarinic "autoreceptors" on the K(+)-evoked in vitro release of [3H]acetylcholine from rat striatal tissue slices. Activation of D2 receptors with the selective agonist LY 171555 (0.01-1 microM) clearly decreased the evoked release of [3H]acetylcholine. This effect was markedly attenuated in the presence of either the selective muscarinic receptor agonist oxotremorine (3 microM) or the cholinesterase inhibitor physostigmine (1 microM). Conversely, D2 receptor activation with LY 171555 (1 microM) completely abolished the muscarinic receptor mediated inhibition of evoked [3H]acetylcholine release induced by oxotremorine (0.03-10 microM). These results show that the inhibitory effects of D2 dopamine receptor and muscarinic receptor activation on striatal acetylcholine release are non-additive and therefore are interdependent processes. In addition, we investigated some aspects of the signal transduction mechanism by which the muscarinic receptor mediates inhibition of K(+)-evoked in vitro release of [3H]acetylcholine from rat striatal tissue slices. It appeared that the effect of muscarinic receptor activation was not significantly influenced either by a lowering of the extracellular Ca2+ concentration from the usual 1.2-0.12 mM or by an increase of the intracellular cyclic adenosine-3',5'-monophosphate content. However, increasing extracellular K+ strongly decreased the inhibition of evoked [3H]acetylcholine release mediated by activation of muscarinic receptors. This set of results indicates that the muscarinic "autoreceptor" mediates the decrease of depolarization induced [3H]acetylcholine release from rat striatum to a large extent through stimulation of K+ efflux (opening of K+ channels) in a cyclic adenosine-3',5'-monophosphate independent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

D-2 dopamine-receptors regulate the release of [3H]dopamine in rat cortical regions showing dopamine immunoreactive fibers.

Using an antibody raised against dopamine the occurrence of dopamine-containing fibers was demonstrated in the prefrontal cortex, anterior cingulate cortex, parietal neocortex, piriform cortex and entorhinal cortex. In extracts of these cortical regions significant amounts of dopamine, although approximately a 100-fold less than in the neostriatum or nucleus accumbens, were detected with high performance liquid chromatography. The release of [3H]dopamine from slices of all these cortical regions was studied in vitro in a superfusion system and desipramine was used to prevent the uptake of [3H]dopamine in noradrenergic nerve terminals. It appeared that the electrically evoked release of radioactivity was inhibited by drugs stimulating D-2 dopamine-receptors in all the regions studied. Cation-exchange column chromatography revealed that the radioactivity released consisted predominantly of [3H]dopamine, indicating that D-2 receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals. Likewise, in the neostriatum as well as in the nucleus accumbens D-2 receptor stimulation inhibits the release of [3H]dopamine. Therefore it is our conclusion that D-2 receptors regulate the release of dopamine from dopaminergic neurons originating in the ventral tegmental area as well as in the substantia nigra.

Iodine-123-N-omega-fluoropropyl-2beta-carbomethoxy-3beta-(4-iod ophenyl)tropane SPECT in healthy controls and early-stage, drug-naive Parkinson's disease.

UNLABELLED: The aims of this study were to investigate whether the loss of striatal dopamine transporters in early and drug-naive patients with Parkinson's disease could be demonstrated by means of 123I-N-omega-fluoropropyl-2beta-carbomethoxy-3beta-(4-iodoph enyl)tropane (123I-FP-CIT) SPECT in a 1-day protocol and whether the SPECT measures were correlated with disease severity. METHODS: Twenty-one early-stage and drug-naive Parkinson's disease patients (age range 42-73 yr; mean age 55.5 yr) and 14 healthy controls (age range 28-83 yr; mean age 53.6 yr) were examined. SPECT image acquisition was always performed at 3 hr postinjection. The ratio of specific to nonspecific striatal 123I-FP-CIT binding was used as the outcome measure. RESULTS: All striatal 123I-FP-CIT ratios were significantly lower in the Parkinson's disease group compared to those in the control group. The mean reduction in the putamen was 57% of the control mean, and that in the caudate nucleus was 29% of the control mean. Patients with unilateral Parkinson's disease showed a bilateral loss of striatal 123I-FP-CIT binding. Discriminant function analysis, using the 123I-FP-CIT SPECT data of the ipsilateral and contralateral putamen, predicted group membership in all cases; the contralateral putamen accounted for the greatest difference between the Parkinson's disease patients and the controls. In the control group, a clear decline in 123I-FP-CIT binding was found with aging, amounting to 9.6%/decade. Unexpectedly, in the Parkinson's disease group, regression analysis revealed that neither severity of disease nor age accounted for a significant part of the variance in striatal SPECT measures. CONCLUSION: Our findings indicate that 123I-FP-CIT SPECT is a reliable method to discriminate between early, drug-naive Parkinson's disease patients and healthy controls and to identify patients in the preclinical phase of Parkinson's disease. Possibly due to the relatively homogeneous group of Parkinson's disease patients and the use of a suboptimal outcome measure, no significant correlations were found between striatal 123I-FP-CIT binding ratios and disease severity, such as were established earlier with 123I-beta-CIT. Further research is necessary to interpret these findings.

Imaging of dopamine transporters with iodine-123-FP-CIT SPECT in healthy controls and patients with Parkinson's disease.

UNLABELLED: Several SPECT studies reported decreased striatal 123I-N-omega-fluoropropyl-2beta-carbomethoxy-3beta-(4-iodoph enyl)nortropane ([123I]FP-CIT) binding in patients with Parkinson's disease. For application in routine clinical studies, information on the reliability and reproducibility of the [123I]FP-CIT SPECT technique is critical. This study reports on the reliability and reproducibility of [I23I]FP-CIT SPECT in healthy control subjects and patients with Parkinson's disease using two different analysis protocols: the conventional region of interest (ROI) protocol and a newly developed, fully automatic, operator-independent volume of interest (VOI) protocol. METHODS: We performed repeated [123I]FP-CIT SPECT scans in 6 healthy control subjects and 10 patients with Parkinson's disease to measure scan-to-scan variations. Scintigraphic data were analyzed 3 hr after injection of the radiotracer. RESULTS: In controls, the mean test/retest for the ratio of the striatal-to-nonspecific [123I]FP-CIT uptake were (3.79 +/- 0.67/3.82 +/- 0.74) and (4.16 +/- 0.70/4.08 +/- 0.97) for the ROI and VOI technique, respectively. No significant differences were measured between test/retest studies. The mean test/retest variability for the ROI technique was low (7.25%) with excellent reliability (rho = 0.99). In addition, the mean test/retest variability for the VOI technique was also low (7.47%) with very high reliability (rho = 0.95). In Parkinson's disease patients, we found mean test/retest for the striatal-to-nonspecific [123I]FP-CIT ratio of (1.78 +/- 0.23/1.79 +/- 0.25) and (1.83 +/- 0.31/1.85 +/- 0.35) using the ROI and VOI technique, respectively. Also in patients, these results did not differ significantly between test/retest studies. The mean test/retest variability for the ROI technique was low (7.90%) with excellent reliability (rho = 1.00). In addition, the mean test/retest variability for the VOI technique was also low (7.36%) with high reliability (rho = 0.96). CONCLUSION: Reliable and reproducible results were obtained with the ROI, as well as the VOI technique, for the analysis of striatal dopamine transporters with [123I]FP-CIT SPECT in healthy controls and Parkinson's disease patients. The use of an operator-independent method will be a great advantage in routine clinical studies.

Restricted usefulness of tetraethylammonium and 4-aminopyridine for the characterization of receptor-operated K+-channels.

1. Recently, we suggested that the D2-dopamine receptor involved in the inhibition of evoked [3H]-acetylcholine release from rat striatum is coupled to K+-channels. 2. In the present study, an attempt was made to elucidate further the role of these K+-channels, using the K+-channel blocking agents tetraethylammonium and 4-aminopyridine. With a superfusion method, the effects of both drugs on the D2-dopamine receptor-mediated inhibition of the electrically evoked release of [3H]-acetylcholine from rat striatal tissue slices was investigated. 3. Both tetraethylammonium (30 mM) and 4-aminopyridine (0.1 mM) significantly stimulated the electrically evoked release of [3H]-acetylcholine and completely abolished the effect of the selective D2-receptor agonist LY 171555 (1 microM) on evoked acetylcholine release. In addition, tetraethylammonium (0.03-30 mM) and 4-aminopyridine (0.003-1 mM) strongly increased the basal (non-evoked) release of radioactivity in a concentration-dependent manner. The results suggest that the effect of the drugs on the basal release of radioactivity and on the electrically evoked release of acetylcholine cannot exclusively be explained by their action on K+-channels. 4. Furthermore, with the use of a receptor binding assay, data were obtained on the affinity of tetraethylammonium and 4-aminopyridine for D2-receptors and various other neurotransmitter recognition sites. At concentrations in which both drugs are known to block K+-channels, they were found to inhibit the specific binding of selective radioligands to their respective recognition sites. 5. It is concluded that due to their 'side-effects', both tetraethylammonium and 4-aminopyridine are of only limited value in the investigation of the alleged interaction between neurotransmitter receptors and K+-channels.

The antiproliferative effect of 8-chloro-adenosine, an active metabolite of 8-chloro-cyclic adenosine monophosphate, and disturbances in nucleic acid synthesis and cell cycle kinetics.

8-Chloro-adenosine, the dephosphorylated metabolite of the antineoplastic agent 8-chloro-cyclic AMP, has been proposed to act on the regulatory subunits of cyclic AMP-dependent protein kinase. 8-Chloro-adenosine has a growth-inhibitory effect, the mechanism of which is unclear. We investigated the effects of 8-chloro-cyclic AMP and 8-chloro-adenosine on nucleic acid synthesis and cell cycle kinetics in two human glioma cell lines. These effects were compared to those of the cyclic AMP analogue 8-(4-chlorophenyl)-thio-cyclic AMP (8-CPTcAMP), which is less susceptible to dephosphorylation. Whereas 8-CPTcAMP almost completely inhibited RNA and DNA synthesis, both 8-chloro-adenosine and 8-chloro-cyclic AMP only partly inhibited synthesis of RNA and DNA at growth-inhibitory concentrations, as demonstrated by using [5-1H] uridine and [14C]thymidine incorporation. Therefore, the growth-inhibitory effect of 8-chloro-cyclic AMP is not (or not completely) due to activation of cyclic AMP-dependent protein kinase nor to the inhibition of nucleic acid synthesis. Flow cytometric analysis revealed that 8-chloro-cyclic AMP and 8-chloro-adenosine probably block cell cycle progression at the G2M phase. The effects of 8-chloro-cyclic AMP on nucleic acid synthesis and cell cycle progression were largely prevented by adenosine deaminase, which inactivates 8-chloro-adenosine. This indicates that the effects of 8-chloro-cyclic AMP were at least in part due to its metabolite 8-chloro-adenosine. Incorporation of 8-chloro-adenosine into RNA and DNA might contribute to the disturbance of the cell cycle kinetics and growth-inhibitory effect of 8-chloro-adenosine.

Priming with L-DOPA differently affects dynorphin and substance P mRNA levels in the striatum of 6-hydroxydopamine-lesioned rats after challenge with dopamine D1-receptor agonist.

In unilaterally 6-hydroxydopamine-lesioned rats, potentiation of D1-agonist-induced turning behavior by priming with l-DOPA was correlated with changes in striatal neuropeptide mRNA levels. In non-primed rats, administration of the D1-agonist SKF-38393 markedly increased dynorphin and substance P mRNA levels in the lesioned striatum. Priming with l-DOPA dissociated the response of the two neuropeptides to the D1-agonist, with higher dynorphin and reduced substance P mRNA levels.

An evaluation of the young dopamine-lesioned rat as an animal model for minimal brain dysfunction (MBD).

Three main symptoms of minimal brain dysfunction (MBD), a common disorder in children, are hyperactivity, learning disabilities, and attention deficits. Drugs like amphetamine and methylphenidate have been demonstrated to produce a significant behavioral improvement in these children. The behavioral response of young rats (3--4 weeks), with selective lesioning of the central dopaminergic system, to a novel environment was analyzed. Both the frequencies and durations of eight mutually exclusive and complementary behavioral categories were scored. By analyzing the behavior in this way it appeared that considerable hyperactivity and learning disabilities could be demonstrated in these rats. Moreover, the bout length of some behavioral categories was somewhat shortened, which might be an indication of deficits in attention. However, treatment of the animals with amphetamine did not produce any "therapeutic" effect on the three symptoms. Since pharmacotherapeutic support is, in our opinion, a "conditio sine qua non" for the validity of the model, we do not believe that the young DA-lesioned rat is an appropriate animal model for MBD.

Sub-chronic administration of the dopamine D(1) antagonist SKF 83959 in bilaterally MPTP-treated rhesus monkeys: stable therapeutic effects and wearing-off dyskinesia.

RATIONALE: SKF 83959 acts as a D(1) antagonist in vitro but has been claimed to induce anti-parkinsonian effects after acute administration in MPTP-treated marmosets. OBJECTIVE: The aim of the present study was to evaluate the therapeutic and undesired effects of sub-chronic administration of SKF 83959 in bilaterally MPTP-treated rhesus monkeys and to compare these effects with the effects of l-dopa and the dopamine agonist SKF 82958. METHODS: MPTP was given in the left carotid artery (2.5 mg) and 6 weeks later, the right carotid artery (1.25 mg). The monkeys (n = 4) had previously been treated chronically with l-dopa (22 days, 10 mg/kg) and SKF 82958 (22 days, 1 mg/kg). Three months after the last administration of SKF 82958, SKF 83959 was given in a dose of 0.5 mg/kg from day 1 to day 15 and in a dose of 1.0 mg/kg from day 16 to day 18. RESULTS: SKF 83959 increased goal-directed limb movements in all animals, including those unresponsive to l-dopa. This therapeutic effect did not diminish during treatment. With respect to body displacement and undesired effects, a large variation in the response to SKF 83959 was found: a large increase in body displacement co-occurred with oro-facial dyskinesia (n = 2), whereas a small increase in body displacement co-occurred with dystonia (n = 2). In contrast to the undesired effects of l-dopa, the dyskinetic effects of SKF 83959 were primarily limited to the first treatment day. Unlike l-dopa and SKF 82958, SKF 83959 did not induce epileptoid behaviour. CONCLUSION: Sub-chronic administration of SKF 83959 induced both clear-cut therapeutic effects that remained stable in time, and a limited number of dyskinetic effects that wore off during the treatment. The dopamine D(1) antagonist SKF 83959 may be considered as an alternative treatment in Parkinson's disease, especially in those patients who do not respond to L-dopa.

Changes in the behavioral response to a novel environment following lesioning of the central dopaminergic system in rat pups.

During the third week of life, a hyperactive period for laboratory rat, the occurrence of 8 behavioral categories was recorded in individual littermates transferred to a novel environment. Neonatal destruction of the catecholaminergic system by intraventricular injection of 6-OH-DA resulted in increased motor activity during the third week of life. Selective lesioning of the dopaminergic system by the combined treatment of 6-OH-DA + desmethylimipramine also induced a significant increase in some active behavioral categories. It appeared that in contrasts to the gross behavioral sequence, as seen in controls, which compromised locomotion and rearing leads to grooming leads to sitting and lying down, the lesioned animals showed a prolonged phse of restless locomotion. These data are interpreted as a disability to habituate adequately to a novel environment after neonatal lesioning of the dopaminergic system.

Morphine and naltrexone modulate D2 but not D1 receptor induced motor behavior in MPTP-lesioned monkeys.

Interactions at the behavioral level between dopamine (DA) and opioid receptors in the mammalian brain have been amply demonstrated. Considering the pivotal role for DA receptors in the pharmacotherapy of Parkinson's disease (PD), these interactions might be clinically relevant. Therefore, in the present study the effects of the opioid antagonist naltrexone and agonist morphine on D1 and D2 receptor induced stimulation of motor behavior in the unilateral MPTP monkey model (n = 5) of PD were investigated. The results show that both naltrexone and morphine [0.1-1.0 mg/kg; intramuscular injection (IM)] inhibited D2 receptor stimulated contralateral rotational behavior and hand use induced by administration of quinpirole (LY 171555; 0.01 mg/kg, IM) in a dose-related way. However, no effects of these opioid drugs were observed on D1 receptor stimulated contralateral rotational behavior and hand use induced by administration of SKF 81297 (0.3 mg/kg, IM). Interestingly, the action of the alleged preferential mu-receptor antagonist naltrexone was mimicked by the selective delta-opioid antagonist naltrindole (0.5 mg/kg, IM). From this study it is concluded that in a non-human primate model of PD, alteration of opioid tonus leads to modulation of D2 receptor but not D1 receptor controlled motor behavior. The possible underlying mechanisms and clinical relevance of these findings are discussed.

Presence of glutathione immunoreactivity in cultured neurones and astrocytes.

Although glutathione (GSH) is considered an important antioxidant in the brain, its cellular localization is unclear. In general, neurones are supposed to contain considerably less GSH than astrocytes. We determined biochemically and immunocytochemically the presence of GSH in cultured neurones and astrocytes from the cortex, mesencephalon and striatum. Cortical neurones contained less GSH than astrocytes whereas GSH levels in neurones from the striatum and mesencephalon were comparable to those in astrocytes. Immunocytochemistry showed significant GSH staining in neurones. Fluorescent double staining of GSH and tyrosine hydroxylase revealed that dopaminergic neurones also contained GSH, although apparently at a lower level than other mesencephalic neurones.

Regional differences in the regulation of acetylcholine release upon D2 dopamine and N-methyl-D-aspartate receptor activation in rat nucleus accumbens and neostriatum.

The effect of D2 dopamine receptor activation on either the electrically, or N-methyl-D-aspartate induced release of radiolabeled acetylcholine (ACh) was investigated in different areas of the nucleus accumbens and the neostriatum of rats, by using a superfusion technique. Sequential slices of 100 microns were chopped along either a rostrocaudal, mediolateral or dorsoventral axis. In every slice the effect of a supramaximal concentration of the selective D2 receptor agonist quinpirole on the release of ACh was measured. In the entire neostriatum the release of ACh was reduced by approximately 70% in the presence of quinpirole. By contrast, in the nucleus accumbens, a gradual decrease in the inhibitory effect of quinpirole on the release of ACh was observed along both the rostral-to-caudal and the lateral-to-medial axes. Whereas in the rostrolateral part a 50% inhibition could be observed, in the caudomedial part no significant inhibition could be detected. Also the N-methyl-D-aspartate induced release of ACh was smaller in the caudomedial part as compared to the rostrolateral part of the nucleus accumbens. It is concluded that the nucleus accumbens is a very heterogeneous structure with respect to the regulation of the release of ACh by D2 dopamine and N-methyl-D-aspartate receptor activation.

Independent in vitro regulation by the D-2 dopamine receptor of dopamine-stimulated efflux of cyclic AMP and K+-stimulated release of acetylcholine from rat neostriatum.

Two types of dopamine receptors whose stimulation affect cAMP efflux (and by inference formation) could be identified in rat neostriatum. One type of receptor, called D-1 receptor, increased cAMP efflux whereas stimulation of a second type of dopamine receptor, called D-2 receptor, was followed by a reduction in cAMP efflux induced by stimulation with a D-1 receptor agonist. D-2 receptor agonists inhibited the effects of D-1 receptor agonists on cAMP efflux in a non-competitive way. These inhibiting effects of D-2 receptor agonists occurred also in the absence of Ca2+-ions which could imply that some of the D-2 receptors are located on cells possessing D-1 receptors. The dopamine receptor mediating inhibition of the release of radiolabeled acetylcholine (ACh) in the neostriatum appeared to have the same pharmacological characteristics as the D-2 dopamine receptor mediating the inhibition of the D-1 receptor agonist induced cAMP efflux. Selective D-2 receptor agonists like LY 141865 and RU 24926 stimulated this receptor while the D-1 receptor agonist SKF 38393 was inactive. Effects of the selective D-2 receptor agonists could be antagonized by (-)-sulpiride, a selective D-2 receptor antagonist. Although the pharmacological characteristics of the dopamine receptors mediating inhibition of both ACh release and (D-1 dopamine receptor agonist induced) cAMP efflux appeared to be similar, drugs stimulating cAMP efflux did not affect ACh release or LY 141865 induced inhibition of ACh release from rat neostriatum. Therefore it is still questionable whether the dopamine receptor mediating inhibition of both ACh release and cAMP efflux is one and the same functional entity.

Dopamine inhibits the release of immunoreactive beta-endorphin from rat hypothalamus in vitro.

Mediobasal hypothalamus tissue (MBH) from adult male rats was incubated in Krebs-Ringer bicarbonate medium (KRB). KRB was changed at 15 min intervals and the concentration of immunoreactive beta-endorphin (beta-ENDi) in the medium was measured by radioimmunoassay. Incubation of MBH tissue in normal KRB resulted in a constant release rate of beta-ENDi of approximately 1% of the tissue content per h. KRB containing 45 mM K+ causes a two fold increase in the release rate of beta-ENDi which was Ca2+ dependent. Dopamine (0.01-1.0 microM) inhibits both the spontaneous and the K+-stimulated release of beta-ENDi in a dose related manner. The dopamine receptor blocking agent haloperidol prevents this inhibitory effect of dopamine. The selective D-1 receptor agonist SKF 38393 does not affect the release rate of beta-ENDi; whereas the selective D-2 receptor agonist LY 141865 inhibits both the spontaneous and K+-stimulated release of beta-ENDi. The effects of LY 141865 can be blocked by (-)-sulpiride, a selective D-2 receptor antagonist. Norepinephrine only weakly inhibits the K+-stimulated release of beta-ENDi, an effect that can be blocked by haloperidol but not by the alpha-adrenoceptor blocker phentolamine. At concentrations tested (0.01-1.0 microM), isoproterenol, 5-hydroxytryptamine, carbachol and 8-Br-cAMP (1.0 microM) do not affect beta-ENDi release. It is concluded that dopamine can inhibit the release of beta-ENDi from hypothalamic neurons via a D-2 receptor mechanism.