Involvement of GABA systems in acetylcholine release induced by 5-HT3 receptor blockade in slices from rat entorhinal cortex

The aim of the present study was to examine the role of 5-HT₃ receptors in spontaneous and K⁺-evoked acetylcholine (ACh) release from rat entorhinal cortex and striatal slices. The 5-HT3 receptor antagonists ondansetron and granisetron (0.01–10 μM) produced a concentration-dependent increase in both spontaneous and K⁺-evoked [³H]ACh release in the two brain regions studied. The release of ACh was Ca²⁺-dependent and tetrodotoxin-sensitive. 5-HT₃ receptor agonists, such as 2-methyl-5-HT and 1-phenylbiguanide, at concentrations up to 1 μM, did not show any intrinsic effect on [³H]ACh release in both rat brain regions. However, 2-methyl-5-HT, 1 μM, fully blocked the ondansetron-induced enhancement in both basal and K⁺-evoked ACh release, suggesting that 5-HT₃ through 5-HT₃ receptor activation, tonically inhibits ACh release. The possible implication of interposed inhibitory systems in ACh release after 5-HT₃ receptor blockade was subsequently analyzed. While the effect of ondansetron was not modified by haloperidol or naloxone, the GABA_A receptor antagonist bicuculline produced a marked potentiation of ACh release in the entorhinal cortex but not in the striatum. The results suggest that in this cortical area 5-HT activates 5-HT₃ receptors located on GABAergic neurons which in turn inhibit cholinergic function.     

Regulation of Dopamine D1 and D2 Receptors on Striatal Acetylcholine Release in Rats

The effects of dopamine (DA) D₁ and D₂ receptors on striatal acetylcholine (ACh) releases were investigated by in vivo microdialysis. All drugs were applied via dialysis membrane directly to the striatum. The levels of ACh release were increased by 10⁻⁴ M SKF38393, a D₁ receptor agonist. Although 10⁻⁴ M SCH23390, a D₁ receptor antagonist, exhibited an increase in the levels of ACh release, the agonist (10⁻⁴ M) induced-increase in the levels of ACh release was suppressed by coperfusion of the antagonist (10⁻⁴ M). In contrast, the levels of ACh release were decreased by the D₂ receptor agonist, N-434, in a dose-dependent manner (10⁻⁵ M to 10⁻⁷ M) and increased by the D₂ receptor antagonist, sulpiride, in a dose-dependent manner (10⁻⁵ M to 10⁻⁷ M). The agonist (10⁻⁵ M) induced-decrease in the levels of ACh release was suppressed by coperfusion of the antagonist (10⁻⁶ M). Coperfusion of D₁ (10⁻⁴ M) and D₂ (10⁻⁵ M) agonists blocked both effects of respective drug alone. In order to clarify the effect of endogenous DA, two drugs with different mechanisms for enhancing DA concentration in the synaptic cleft, the DA release-inducer methamphetamine, and the DA uptake inhibitor nomifensine were perfused separately. Both (10⁻⁴ M to 10⁻⁶ M) produced a dose- and a time-dependent decrease in the levels of ACh release. Significant higher levels of ACh release were observed in the striatum of the 6-hydroxydopamine (8

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)-treated rats with significant depletion of striatal DA content. These results suggest that in striatal DA-ACh interaction ACh release, as cholinergic interneuron's activity, is tonically inhibited via the D₂ receptor, mainly by dopaminergic input, and the D₁ receptor probably modifies the effect of the D₂ receptor indirectly.     

Synergistic interaction between an adenosine antagonist and a D1 dopamine agonist on rotational behavior and striatal c-Fos induction in 6-hydroxydopamine-lesioned rats

The interaction between adenosine and D₁ dopamine systems in regulating motor behavior and striatal c-Fos expression was examined in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. These results were compared to the synergistic interaction between D₁ and D₂ dopamine systems in 6-OHDA rats. Coadministration of the adenosine antagonist 3,7-dimethyl-1-propargylxanthine (DMPX: 10 mg/kg) and the D₁ dopamine agonist SKF38393 (0.5 mg/kg) to 6-OHDA-lesioned rats produced significant contralateral rotation and c-Fos expression in the ipsilateral striatum compared to 6-OHDA rats treated with either drug alone. However, the regional pattern of striatal c-Fos activation following treatment of 6-OHDA rats with SKF38393 and DMPX was different from the dorsolateral pattern of striatal c-Fos induction observed after coadministration of D₁ and D₂ dopamine agonists (SKF38393: 0.5 mg/kg+quinpirole: 0.05 mg/kg). These data are consistent with a functional interaction between D₁ dopamine and adenosine systems in the striatum, but suggest that activation of different subsets of striatal neurons underlie the behavioral synergy observed following combined adenosine antagonist-D₁ dopamine agonist and combined D₁ dopamine agonist–D₂ dopamine agonist treatment.     

Relationship between dopamine agonist stimulation of inositol phosphate formation and cytidine diphosphate–diacylglycerol accumulation in brain slices

Dopamine receptor-coupled stimulation of inositol phosphate formation has been characterized extensively, but little is known about the diacylglycerol arm of this dual-signaling pathway. This study examined several parameters of cytidine diphosphate–diacylglycerol (CDP–DG) accumulation as an index of agonist-stimulated DG formation. Rat brain slices pre-labeled with 5-[³H]cytidine were incubated with various test agents in the presence of LiCl and accumulated CDP–DG analyzed. Dopamine and SKF38393 significantly and dose-dependently stimulated CDP–DG accumulation. SKF38393 responses were inhibited by neomycin and reversed by myo-inositol or by exclusion of LiCl. Compared to inositol phosphate formation in 2-[³H]inositol-prelabeled slices, the CDP–DG responses were proportionately greater, while the agonist EC₅₀ values were similar between the two assays. The D₁-receptor antagonist SCH23390 inhibited SKF38393-mediated responses at 0.1–10 μM concentrations, whereas greater concentrations reversed the inhibition. SKF38393 effects were completely blocked by the DG kinase inhibitor R59022, thus precluding any role for phospholipase-D or de novo phosphatidate synthesis in the dopaminergic response. D609 which inhibits phosphatidylcholine-specific phospholipase-C (PLC), potently inhibited both CDP–DG accumulation and inositol phosphate formation. These findings demonstrate that the selective D₁-receptor antagonist SCH23390 is a partial agonist at the D₁-like dopamine receptor that couples to phosphoinositide signaling, that dopaminergic facilitation of phosphoinositide signaling is independent of de novo phosphatidate synthesis, and that the widely used enzyme inhibitor, D-609, is probably not selective for phosphatidylcholine-specific PLC in brain slice preparations. The greater sensitivity of the CDP–DG measurement presents this assay as a reliable and possibly superior index of dopamine receptor-coupled PLC activation in intact tissues.     

Desensitization of D1 dopamine receptors down-regulates the Gsα subunit of G protein in SK-N-MC neuroblastoma cells

The effect of long-term (72 h) treatment with dopamine D₁ receptor agonists, SKF 38393 and dopamine on D₁ dopamine receptor and G-protein (G_Sα) was investigated in SK-N-MC neuroblastoma cells. The prolonged treatment of cells with 10 μM SKF 38393 or 10 μM dopamine resulted in a decrease in dopamine D₁ receptor by 41 and 81%, respectively, as measured by specific antagonist [³H]SCH 23390 binding. Similary, the prolonged treatment of SK-N-MC cells with 10 μM SKF 38393 or 10μM dopamine resulted in a reduction of the level of G_Sα subunit of G-protein. The results indicate that agonist-induced down-regulation of D₁ dopamine receptor may also modulate G-proteins.     

Characterization of substance P release from the intermediate area of rat thoracic spinal cord

Substance P (SP) nerve terminals innervate the intermediolateral cell column (IML) of the thoracic spinal cord, where SP coexists with serotonin (5-HT), neurokinin A (NKA) and thyrotropin-releasing hormone (TRH). Neither the depolarization-induced release of SP nor the presence of other neurochemicals in the regulation of SP release has been directly studied in this system. In the present study, basal and K⁺-stimulated release of SP from the microdissected intermediate area (including the IML, intercalated nucleus and central autonomic nucleus) of the rat thoracic spinal cord, and the regulation of SP release by presynaptic autoreceptors and by coexisting neurochemicals (5-HT, NKA and TRH) were studied using an in vitro superfusion system. Potassium evoked a concentration- and extracellular Ca²⁺-dependent release of SP. In rats pretreated with the serotoninergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), both SP content and the absolute amount of SP released were decreased. However, the fraction of the remaining tissue content of SP released by K⁺ depolarization was not changed subsequent to 5,7-DHT treatment. Moreover, 5-HT, 5-HT_1B agonists (CGS-12066B and RU 24969) and a 5-HT₃ agonist (2-methyl-5-HT) did not alter the K⁺-evoked release of SP. These data demonstrate that SP is released from the intermediate area of the rat thoracic spinal cord and some of the SP released comes from serotoninergic nerve terminals. Although 5-HT coexists with SP in the IML, neither endogenous 5-HT nor 5-HT receptor ligands appear to regulate the release of SP. Other colocalized neuropeptides (NKA and TRH) are not involved in the regulation of SP release because neither NKA, a NK₂ agonist (GR 64349) nor a TRH analog (MK-771) changed the K⁺-evoked release of SP. A neurokinin-1 (NK₁) antagonist (GR 82334) dose-dependently (10^-9-10^-7 M) increased the K⁺ stimulated release of SP. These data suggest the presence of presynaptic inhibitory NK₁ autoreceptors. Whereas, NK₁ agonists, [GR 73632 (10^-9-10^-6 M) and [Sar⁹, Met (O₂)^1]] SP (10^-8-10^-6 M)], increased the basal and K⁺-stimulated release of SP, the excitatory effects of GR 73632 were not blocked by the NK₁ antagonist. Moreover, GR 73632 increased the efflus of SP to a greater extent in the absence of peptidase inhibitors. Thus, the effect of NK₁ agonists on the release of SP may be related to an inhibition of peptide degradation rather than activation of NK₁ autoreceptors. © 1996 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Dopamine D2 receptor agents, but not dopamine D1, modify brain glucose metabolism

The effects of recently described selective dopamine D₁ and D₂ agonists and antagonists on brain glucose metabolism were studied using the 2-[¹⁴C]deoxyglucose autoradiographic technique. The administration of LY-141865 or YM-09151-2, which behave as a specific D₂ agonist and antagonist respectively, modified brain glucose metabolism in a manner similar to that previously described for more classical dopaminergic agents, such as apomorphine and haloperidol. In contrast, the administration of SKF 38393 or SCH 23390, a specific D₁ agonist and antagonist respectively, was not followed by significant modifications of brain glucose metabolism in any of the brain regions studied. These results indicate that D₂ but not D₁ dopamine receptors are involved in the regulation of local brain glucose metabolism.     

Possible role of dopamine D1 receptor in the behavioural supersensitivity to dopamine agonists induced by chronic treatment with antidepressants

The effect of chronic treatment with antidepressants (ADs) on the behavioral responses to LY 171555, a selective D₂ receptor agonist, SKF 38393, a selective D₁ receptor agonist, and B-HT 920, a selective DA autoreceptor agonist, was studied in rats. In normal rats small, intermediate and high doses of LY 171555 produced hypomotility, hyperactivity and stereotypies, respectively. Chronic but not acute pretreatment with imipramine (IMI) greatly potentiated the motor stimulant effect of LY 171555, but failed to modify its stereotypic and sedative effect. The potentiation of the motor stimulant effect of LY 171555 was observed also after chronic, but not acute, treatment with desmethylimipramine (DMI), mianserin (MIA) or repeated electroconvulsive shock (ECS). Chronic treatment with IMI failed to modify the effect of SKF 38393 (motor stimulation, grooming and penile erection), but reversed the sedative effect of B-HT 920 into a motor stimulant response. The motor stimulant response to LY 171555 in IMI-pretreated animals was suppressed byl-sulpiride, a D₂ antagonist, and by a combination of reserpine with α-methyltyrosine (α-MT), but it was only partially antagonized by high doses of SCH 23390, a selective D₁ antagonist. The results indicate that chronic treatment with ADs potentiates the behavioural responses mediated by the stimulation of postsynaptic D₂ receptors in the mesolimbic system and suggest that this behavioural supersensitivity is due to enhanced neurotransmission at the D₁ receptor level.     

DOI, a 5-HT2A/2C receptor agonist, attenuates clozapine-induced cortical dopamine release

(±)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 1.25, 2.5 and 5 mg/kg), a serotonin (5-HT)_2A/2C agonist, produced an inverted U-shaped increase in DA release in rat medial prefrontal cortex (mPFC) with a significant effect only at 2.5 mg/kg. This effect was completely abolished by M100907 (0.1 mg/kg), a 5-HT_2A antagonist, and WAY100635 (0.2 mg/kg), a 5-HT_1A antagonist, neither of which when given alone affected dopamine release. DOI (2.5 mg/kg), but not the 5-HT_2C agonist Ro 60-0175 (3 mg/kg), attenuated clozapine (20 mg/kg)-induced mPFC dopamine release. These results suggest that 5-HT_2A receptor stimulation increases basal cortical dopamine release via 5-HT_1A receptor stimulation, and inhibits clozapine-induced cortical dopamine release by diminishing 5-HT_2A receptor blockade.     

Locomotor effects of amantadine in the mouse are not those of a typical glutamate antagonist

Summary Amantadine has been shown to displace [³H]MK 801 from its binding site on the NMDA receptor. We have therefore studied the motor effects of amantadine in normal and 24h reserpine-treated mice to determine whether the behavioural profile of this drug resembles that of other NMDA receptor antagonists (e.g. MK 801). In common with the latter, amantadine (5–40 mg/kg IP) produced a modest dose-dependent sedation in dopamineintact mice, with a reduction in locomotion and other species-typical behaviours (e.g. rearing and grooming), but with no signs of the hyperactivity, stereotypy, ataxia or loss of muscle tone commonly seen with MK 801. Amantadine (5–80 mg/kg IP) effected a small incrase in motility in akinetic reserpine-treated mice by itself, but this response was highly variable and not statistically significant. As with MK 801, amantadine significantly inhibited the locomotion induced by the selective D₂ agonist RU 24213 (5 mg/kg SC) and the mixed D₁/D₂ agonist apomorphine (0.5 mg/kg SC) in monominedepleted mice, without altering the animals' responsiveness to threshold doses of these drugs. However, amantadine did not facilitate the locomotion induced by threshold (3 mg/kg IP) or fully active doses (30 mg/kg IP) of the selective D₁ agonist SKF 38393, which distinguishes amantadine from other NMDA receptor blockers. Since the potentiation of dopamine D₁-dependent locomotion may be a major factor in the antiparkinson activity of MK 801 and other glutamate receptor antagonists the inability of amantadine to potentiate SKF 38393 in this study suggests the mechanism of its anti-akinetic activity differs, from that of conventional glutamate blocking drugs.     

The glycine site of the NMDA receptor contributes to neurokinin1 receptor agonist facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons

We have investigated the role of the glycine recognition site of the N-methyl-d-aspartate receptor (the Gly_NMDA site) in the facilitation of NMDA receptor agonist-evoked activity in rat dorsal horn neurons that is brought about by neurokinin ₁ (NK₁) receptor agonist and the contribution of protein kinase C (PKC) activation to this phenomenon. Ionophoresis of the selective NMDA receptor agonist 1-aminocyclobutane-ci's-1,3-dicarboxylic acid (ACBD) produced a sustained increase in the firing rate of single laminae III-V neurons recorded extracellularly using multibarrelled glass electrodes. The highly selective NK₁ receptor agonist acetyl-[Arg⁶, Sar⁹, Met(O₂)¹¹]-SP_6–11 (Sar⁹-SP) greatly facilitated this response, but under the present conditions had no effect when applied alone or with a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA receptor agonist) at the same current. In the presence of the Gly_NMDA site antagonists 2-carboxy-4,6-dichloro-(1H)-indole-3-propanoic acid (MDL 29951), 7-chloro-3-(cyclopropylcarbonyl)-4-hydroxy-2(1H)-quinoline (L701,252), 5,7-dinitroquinaxoline-2,3-dione (MNQX) or 7-chlorothiokynurenic acid (7-CTK), or the PKC inhibitors, chelery-thrine or GF109203X, the Sar⁹-SP-induced facilitation of ACBD-evoked activity was prevented, generally restoring activity to a level similar to that in the presence of ACBD alone, whilst an AMPA receptor antagonist, 6-nitro-7-sulfamoylbenzo (f) quinoxaline-2,3-dione (NBQX) did not inhibit the facilitation. At the same ionophoretic currents these compounds had no effect on ACBD-evoked activity in the absence of Sar⁹-SP but were inhibitory at significantly greater currents. To further substantiate the importance of the Gly_NMDA site in the interaction, the effects of NMDA receptor antagonists selective for alternative recognition sites on the NMDA receptor were investigated. MK-801, a non-competitive NMDA receptor antagonist and arcaine, a competitive inhibitor at the polyamine site, were applied to the facilitated activity seen in the presence of Sar⁹-SP and ACBD, and to ACBD-evoked activity alone. Unlike the Gly_NMDA site antagonists and PKC inhibitors, these compounds reduced both facilitated and ACBD-evoked activity at similar currents. Furthermore, like the NK₁ receptor agonist, a selective Gly_NMDA site agonist 1-aminocyclopropane carboxylic acid (ACPC) caused facilitation of ACBD-evoked activity which was also blocked by currents of L701,252 that did not alter activity evoked by ACBD alone. These data suggest that activation of the Gly_NMDA site (perhaps as a consequence of glycine release or modification of its influence by intracellular signalling cascades) is an essential component of the means by which NK₁ receptor activation results in facilitated responsiveness of dorsal horn neurons towards NMDA receptor agonists.     

Neurochemical correlates of behavioral sensitization following repeated apomorphine treatment: Assessment of the role of D1 dopamine receptor stimulation

Previous research has revealed a role of repeated D₁ dopamine receptor stimulation in the development of behavioral sensitization to the D₁/D₂ agonist apomorphine. The present experiments assessed the role of repeated D₁ receptor stimulation in neurochemical changes accompanying locomotor sensitization to apomorphine. To assess direct effects of D₁ stimulation on dopamine synthesis, rats were injected with the D₁ agonist SKF 38393 (8 mg/kg), followed by an injection with the 3,4-dihydroxyphenylalanine (DOPA) decarboxylase inhibitor, NSD-1015. DOPA accumulation, assessed in striatal, nucleus accumbens-olfactory tubercle (NAOT), and ventral mesencephalon (VM) tissue samples, was not affected by acute SKF 38393. In the second experiment, rats were treated with 10 daily injections of vehicle, apomorphine (5 mg/kg) or the D₁ agonist SKF 38393 (8 or 16 mg/kg). Daily measures of locomotor activity demonstrated a progressive increase in the apomorphine-treated rats, but not the SKF 38393-treated rats, across the 10 days. On day 11, all rats were injected with NSD-1015 for measurement of DOPA accumulation. Dopamine synthesis was enhanced in the striatum after repeated apomorphine treatment. In contrast, repeated SKF 38393 treatment resulted in either a small decrease or no change in DOPA accumulation in the different brain regions (striatum, NAOT, VM). In the third experiment, tissue levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and [³H]SCH 23390 binding to D₁ receptors were measured in rats treated with 10 daily injections of vehicle, apomorphine (5 mg/kg), or SKF 38393 (16 mg/kg). In the striatum and NAOT, none of the repeated drug treatments had an effect on DOPAC or dopamine levels. In the VM, DOPAC levels were enhanced following repeated apomorphine, but not repeated SKF 38393, whereas dopamine levels were not affected by either drug treatment. D₁ binding was not altered by the repeated drug treatments. Since repeated D₁ stimulation by SKF 38393 did not produce the same alterations in dopamine synthesis and DOPAC levels as repeated apomorphine, the neurochemical effects accompanying locomotor sensitization to apomorphine probably are not mediated by D₁ receptors. © 1993 Wiley-Liss, Inc.     

Modulation of acetylcholine release by D1, D2 dopamine receptors in rat striatum under freely moving conditions

Using in vivo brain dialysis under freely moving conditions, we have studied the effects of dopamine (DA) agonists and antagonists on acetylcholine (ACh) and DA release in rat striatum. The striatal infusion of the D1 DA receptor specific agonist, SKF38393, increased striatal ACh release in a dose-dependent manner (10(-6) to 10(-4) M), and 3 x 10(-5) M SKF38393 elicited a 60% augmentation in the level of ACh release. The level of ACh was increased with perfusion of 10(-4) M SCH23390, a D1 specific antagonist, but decreased with 10(-3) M SCH23390. The D2 specific agonist, LY171555, and the antagonist, sulpiride, slightly altered the level of ACh in the striatum. On the other hand the level of DA dramatically increased in a dose-dependent manner with SKF38393 or SCH23390 and decreased with LY171555. LY171555 inhibited the effect of 10(-4) M SKF38393 on ACh release, and enhanced the effect of SKF38393 on DA release. These results suggest that the D1 DA receptor mainly mediates ACh release and the D2 DA receptor modifies the effects of the D1 receptor.     

Acceleration of desipramine-induced changes on the dopamine receptor-coupled adenylate cyclase system by pertussis toxin

Summary Mice treated with reserpine (5 mg/kg IP), 24h beforehand, were completely akinetic. Fluent locomotion was reinstated with the D₁-selective agonist SKF 38393 (3–30 mg/kg IP), the D₂-selective agonist RU 24213 (0.5–5 mg/kg SC) and the mixed D₁/D₂ agonist apomorphine (0.025–0.5 mg/kg SC). Clonidine (0.03125–1 mg/kg IP) caused a dose-dependent sedation in dopamine-intact mice, but had no effect by itself on the locomotor activity of monoamine-depleted mice. In drug interaction experiments, clonidine did not modify the motor stimulant action of SKF 38393, but greatly enhanced the motor responses to RU 24213 and apomorphine. These results support the hypothesis that -adrenoceptor agonists facilitate dopamine D₂ but not dopamine D₁ motor responding in the reserpinetreated mouse model of Parkinson's disease.     

Evaluation of a Possible Role for the Dopamine D1 and D2 Receptors in the Steroid-Dependent Suppression of Luteinizing Hormone Secretion in the Seasonally Anoestrous Ewe

This study was undertaken to determine whether dopaminergic suppression of pulsatile luteinizing hormone (LH) secretion during seasonal anoestrus in the ewe is mediated via the dopamine D₁ or D₂ receptor. This was tested by 1) assessing the response to dopamine D₁ and D₂ antagonists during seasonal anoestrus, and 2) determining the ability of D₁ and D₂ agonists to suppress pulsatile LH secretion during the breeding season. In seasonally anoestrous ewes the D₂ antagonist pimozide increased LH pulse frequency although this effect did not reach significance (P = 0.07). The D₁ antagonist SCH 23390 had no effect on LH pulse frequency. LH pulse amplitude and mean LH were not affected by either treatment. During the breeding season, ovariectomized oestradiol-implanted ewes were injected intracerebroventricularly with vehicle, LY 171555 (dopamine D₂ agonist) and SKF 38393 (D₁ agonist) with each drug tested at 50 μg and 200 μg. At the higher dose, LY 171555 significantly (P<0.05) reduced LH pulse frequency in the 2 h period immediately after treatment. Mean LH declined at both doses but only in the first hour after treatment. SKF 38393 did not affect LH pulse frequency, pulse amplitude or mean LH. These results suggest that the D₁ receptor is not involved in the suppression of pulsatile LH secretion during seasonal anoestrus. Dopaminergic suppression of pulsatile LH secretion is mediated via the D₂ receptor but the significance of this neurotransmitter in the seasonal suppression of LH remains to be elucidated.     

D1 dopamine receptor stimulation inhibits firing activity of midbrain dopamine neurons in reserpine treated rats: An effect eliminated after hemitransection of diencephalon

It has been reported that systemic administration of the D₁ dopamine (DA) receptor agonist SKF 38393 inhibits the firing rate of substantia nigra pars compacta (SNC, A9) DA neurons after repeated reserpine treatment in locally anesthetized rats, although SKF 38393 induces little effect on the firing of midbrain DA neurons in normal rats. The present study found that local pressure microejection of SKF 38393 (10⁻² M, 20–100 nl) to SNC or substantia nigra pars reticulata (SNR) failed to influence the firing of SNC DA neurons in reserpinized rats (reserpine 1 mg/kg × 6 days, s.c.); subsequent intravenous (i.v.) injection of SKF 38393 (4 mg/kg), however, inhibited their firing and the inhibition was reversed by the D₁ receptor antagonist SCH 23390. Similarly, systemic administration of SKF 38393 (4 mg/kg, i.v.) inhibited the firing of ventral tegmental area (VTA, A10) DA cells in reserpinized rats, while local microejection of SKF 38393 (10⁻² M, 30–60 nl) did not affect their firing. Furthermore, the inhibitory effect of systemic SKF 38393 on firing rate of either SNC or VTA DA neurons in reserpinized rats was eliminated after hemitransection of diencephalon. These results suggest that repeated reserpine treatment renders midbrain DA neurons responsive to D₁ receptor stimulation and that D₁ receptor agonist-induced inhibition of midbrain DA cell firing in reserpinized rats may require the involvement of long-loop feedback pathways. © 1996 Wiley-Liss, Inc.     

Localization and Characterization of Dopamine D1 Receptors in Sheep Hypothalamus and Striatum

Dopamine receptors are pharmacologically grouped as D₁ and D₂ receptors. Previous research in the ewe has shown that central D₁ receptors may have a role in facilitating prolactin release. The aims of this study were therefore to localize and characterize D₁ binding sites in the hypothalamus of sheep. For comparison, a known D₁ receptor-rich tissue (striatum) was also studied. The bioactivities of several D₁ analogues were also assessed for their efficacy in sheep tissue. In vitro autoradiography with [¹²⁵I]-SCH23982 was used to localize D₁ binding sites. The ventromedial hypothalamic nucleus (VMH) displayed moderate levels of specific binding, localized to the medial portion of the nucleus. Low levels of specific binding were seen in the preoptic area, supraoptic nucleus and anterior hypothalamic area. The suprachiasmatic nucleus, median eminence and arcuate nucleus did not show specific binding. As expected the striatum displayed high levels of specific binding. The VMH, preoptic area, median eminence, striatum and anterior pituitary were examined with radioligand binding studies to quantify and characterize D₁ binding sites. Scatchard analysis gave K_D 1.04 nM and B_max 127.4 fmol/mg protein for VMH and K_D 1.99 nM and B_max 454.6 fmol/mg protein for striatum. While specific binding occurred in the preoptic area and median eminence this binding did not show saturation characteristics. Specific binding was not observed in the anterior pituitary. Affinities determined by competitive binding studies showed that the binding sites in both VMH and striatum have the characteristics of a D₁ receptor, that is, high affinity for the D₁ agonists and antagonists, low affinity for dopamine and the serotonergic antagonist ketanserin and extremely low affinity for the D₂ agonists and noradrenaline. Adenylate cyclase studies showed that in the striatum dopamine and the D₁ agonists, fenoldopam and SKF38393, were able to cause significant dose-dependent increases in adenylate cyclase activity. In contrast the D₁ agonist, SKF82958, was inactive in this system. The D₁ antagonists SCH23390 and SCH39166, but not SKF83566, abolished the adenylate cyclase response to 50 μM dopamine. In the VMH the D₁ agonist SKF38393, but not dopamine, stimulated adenylate cyclase activity. In conclusion, these results demonstrate that D₁ binding sites exist within the hypothalamus in the VMH and that these binding sites have the characteristics of D₁ receptors. These receptors are a potential site of action for dopamine in facilitating prolactin release. In addition, the results show that at least for some dopamine analogues, receptor binding affinity does not always correlate with biological activity.     

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 occured 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 efflux in the rat nucleus accumbens evoked by dopamine receptor stimulation in the entorhinal cortex is modulated by oestradiol and progesterone

This study compared the effects of dopamine receptor stimulation in the entorhinal cortex on dopamine release in the nucleus accumbens, measured by in vivo microdialysis in conscious Sprague-Dawley rats, with and without oestradiol and progesterone priming. Nonselective dopamine receptor stimulation with apomorphine reduced dopamine release in the nucleus accumbens, an effect which was prevented by injection of cis-flupenthixol into the entorhinal cortex. Selective D₁ receptor stimulation with SKF38393 increased dopamine release, whereas selective D₂ receptor stimulation with quinpirole did not affect dopamine release. Combined administration of oestradiol and progesterone potentiated the response to apomorphine and prevented the response to SKF38393. The effects of single hormone administration on the response to apomorphine suggested that the modulation was primarily due to oestradiol enhancing effects of progesterone. Experiments with high [K⁺] suggested these hormonal effects were exerted predominantly in the entorhinal cortex. The present experiments have demonstrated that dopaminergic modulation of transmission in a cortico-striatal loop linking temporal and prefrontal cortex is regulated by oestradiol and progesterone. Dysfunction in this system in humans may give rise to affective and cognitive symptoms which may, if initiated by a postpartum fall in oestrogen and progesterone concentrations, constitute the core pathophysiology of puerperal psychosis. Synapse 25:37–43, 1997. © 1997 Wiley-Liss, Inc.     

SCH 23390-induced behavioral supersensitivity is not related to striatal c-fos levels

Stereotyped behavior and striatal c-fos levels induced by chronic treatment with the D₁ dopamine antagonist SCH 23390 have been investigated in rats which received subsequent subacute dopamine agonist treatment. SCH 23390 treatment (0.5 mg/kg/day) for 21 days increased both apomorphine-induced orofacial stereotypies and striatal c-fos levels. Treatment with the D₁ dopamine agonist SKF 38393 (10 mg/kg/day) and the combination of SKF 38393 with the D₂ dopamine agonist quinpirole (1 mg/kg/day), for 5 consecutive days, attenuated apomorphine-induced stereotypies without changing c-fos levels in rats previously treated with SCH 23390. These findings suggest that SCH 23390-induced behavioral supersensitivity and the increased striatal c-fos levels are concomitant but unrelated phenomena.