SDZ GLC 756, a novel octahydrobenzo[g]quinoline derivative exerts opposing effects on dopamine D1 and D2 receptors

Summary SDZ GLC-756, a novel octahydrobenzo[g]quinoline derivative, is equipotent in displacing [³H]SCH23390 from dopamine D₁ receptors and [³H]205–501 from dopamine D₂ receptor binding sites. It blocks dopamine sensitive adenylate cyclase with the same potency as SCH23390, indicating antagonist properties at dopamine D₁ receptors. On the other hand, SDZ GLC 756 inhibits electrically evoked acetylcholine release from rat striatal slices with the same potency as the selective dopamine D₂ receptor agonist bromocriptine. This effect is blocked by spiperone suggesting that it is mediated by dopamine D₂ receptor activation. The opposing action of SDZ GLC 756 on dopamine D₁ and D₂ receptors is also evident in vivo. SDZ GLC 756, like SCH23390, blocks apomorphine-induced rearing in mice. On the other hand, it inhibits prolactin secretion and produces circling in unilateral 6-OHDA-lesioned rats, which is compatible with stimulant properties at dopamine D₂ receptors. This drug might be a new tool to study linkage between dopamine D₁ and D₂ receptors.     

Loss of D1/D2 dopamine receptor synergisms following repeated administration of D1 or D2 receptor selective antagonists: Electrophysiological and behavioral studies

Many effects resulting from D₂ dopamine (DA) receptor stimulation are manifest only when D₁ DA receptors are stimulated by endogenous DA. When D₁ receptor stimulation is enhanced by administration of selective D₁ receptor agonists, the functional effects of selective D₂ agonists are markedly increased. These qualitative and quantitative forms of D₁/D₂ DA receptor synergism are abolished by chronic DA depletion when both D₁ and D₂ DA receptors are supersensitive. Using both electrophysiological and behavioral methods, the present study examined the effects of selective D₁ and D₂ renaptnr supersensitivity, induced by repeated administration of selective D₁ or D₂ receptor antagonists, on the synergistic relationships between D₁ and D₂ receptors. Daily administration of the selective D₂ antagonist eticlopride (0.5 mg/kg, s.c.) for 3 weeks produced a selective supersensitivity of both dorsal (caudate-putamen) and ventral (nucleus accumbens) striatal neurons to the inhibitory effects of the D₂ agonist quinpirole (applied by microiontophoresis). This treatment also abolished the normal ability of the D₁ agonist SKF 38393 to potentiate quinpirole-induced inhibition, and relieved D₂ receptors from the necessity of D₁ receptor stimulation by endogenous DA (enabling), as indicated by significant electrophysiological and behavioral (sterotypy) effects of quinpirole in eticlopride-pretreated, but not saline-pretreated, rats that were also acutely depleted of DA. Daily administration of the selective D₁ receptor antagonist SCH 23390 (0.5 mg/kg, s.c.) caused supersensitivity of striatal neurons to the inhibitory effects of SKF 38393 and also abolished both the ability of SKF 38393 to potentiate quinpirole-induced inhibition and the necessity of D₁ receptor stimulation for such inhibition. However, both quinpirole-induced inhibition of striatal cells and stereotyped responses were also somewhat enhanced in SCH 23390-pretreated rats. When such Dl-sensitized rats were acutely depleted of DA, the behavioral effects of quinpirole were intermediate between saline-pretreated rats with acute DA depletion and SCH 23390-pretreated rats without acute DA depletion. Based upon these and related results, it is argued that the enhanced effects of quinpirole in D₁-sensitized rats are due to a heterologous sensitization of D₂ receptors rather than to enhanced enabling resulting from supersensitive D₁ receptors. It is suggested that supersensitivity of either D₁ or D₂ receptors can lead to an uncoupling of normal qualitative and quantitative D₁/D₂ synergisms and that the heterologous regulation of D₂ receptor sensitivity by D₁ receptors may be related to uncoupling of functional D₁/D₂ synergisms.     

D2L,D2S,and D3 dopamine receptors stably transfected into NG108-15 cells couple to a voltage-dependent potassium current via distinct G protein mechanisms

The D₂-like dopamine (DA) receptor family has continued to expand and now includes the D₂-short (D_2S) and D₂-long (D_2L) receptor isoforms and the D₃ and D₄ receptors. The D₂ receptor isoforms differ in length by 29 amino acids within the third cytoplasmic loop, a region of the receptor believed to be important for G protein coupling. This observation has led to the hypothesis that the two isoforms of the D₂ receptor may utilize different signal transduction pathways when present in the same cell. The D₂ and D₃ receptors, although mostly different, show some common amino acid sequences within the third cytoplasmic loop. Thus, it is possible that the D₂ and D₃ receptors may employ similar signal transduction pathways. To test these hypotheses directly, NG108-15 neuroblastoma-glioma hybrid cells were stably transfected to express either the D_2S, D_2L, or D₃ DA receptors. All transfected but not untransfected NG108-15 cells demonstrated a dose-dependent reduction in the peak whole-cell potassium (K⁺) current in response to receptor activation by DA or the DA receptor agonists quinpirole (QUIN) and apomorphine (APO). The modulation of K⁺ current by D_2S receptor stimulation was prevented by pretreatment of the cells with cholera toxin (20 μg/ml for 18 h), whereas pertussis toxin pretreatment (500 ng/ml for 4 h) completely blocked the effects of D_2L and D₃ receptor activation. These observations suggest that the signal transduction mechanisms involved in coupling the two isoforms of the D₂ receptor to the K⁺ current are different, whereas the D_2L and D₃ receptor coupling mechanisms may be similar. In direct support of this hypothesis, it was observed that the intracellular application of a polyclonal antibody that is specific for the G_0α subunit completely blocked the ability of D_2L and D₃ receptors to modulate outward K⁺ currents. In contrast, the D_2S-mediated modulation of K₊ currents was blocked by intracellular application of an antibody recognizing G_Sα but not G_0α. These findings demonstrate that D_2S and D_2L receptors are able to couple to a common effector in a cell via two G protein pathways. © 1996 Wiley-Liss, Inc.     

Effect of reserpine-induced depletion of synaptic dopamine on [11C]Raclopride binding to D2-dopamine receptors in the monkey brain

Positron emission tomography was used to examine the in vivo binding of [¹¹C]raclopride to D₂-dopamine (DA) receptors in the striatum of two Cynomolgus monkeys after a single dose of reserpine (1 mg/kg, i.v.). A Scatchard procedure was repeated five times to follow D₂ receptor density and apparent affinity for 7 weeks after reserpine. Reserpine-induced depletion of DA lead to a marked increase in [¹¹C]raclopride binding, which was still detectable 20 days after treatment. Scatchard analyses indicated that the measured increase in [¹¹C]raclopride binding reflected an increase in receptor affinity but no evident change in receptor density (B_max). Thus, the increase in [¹¹C]raclopride binding after reserpine should correspond to a reduced competition with endogenous DA for binding to D₂ receptors. The results were used to estimate the DA-induced D₂ occupancy to be about 40% at physiological conditions. Synapse 25:321–325, 1997. © 1997 Wiley-Liss, Inc.     

Neurotensin peptides antagonistically regulate postsynaptic dopamine D2 receptors in rat nucleus accumbens: a receptor binding and microdialysis study

Summary An in vitro receptor binding and in vivo microdialysis study was performed to further investigate the modulation of dopamine (DA) D₂ receptors by neurotensin (NT) peptides. Saturation experiments with the D₂ agonist [³H]NPA (N-propylnorapomorphine) showed that 10 nM of NT, 10 nM of neuromedin N (NN) and 1 nM of the C-terminal NT-(8–13) fragment significantly increased the K_D values by 125%, 181%, and 194%, respectively without significantly affecting the B_max value of the [³H]NPA binding sites in coronal sections of rat ventral forebrain mainly containing the nucleus accumbens (Acb) and the olfactory tubercle.In line with the previous findings that NT can increase GABA release in the Acb and that NT receptors are not found on DA terminals in this brain region, the present in vivo microdialysis study demonstrated that local perfusion of NT (1 nM) counteracted the D₂ agonist pergolide (2M) induced inhibition of GABA, but not of DA release in the rat Acb. This result indicates that NT counteracts the D₂ agonist induced inhibition of GABA release in the rat Acb, via an antagonistic postsynaptic NT/D₂ receptor interaction as also suggested by the inhibitory regulation of D₂ receptor affinity in the Acb by the NT peptides demonstrated in the present receptor binding experiments. Thus, the neuroleptic and potential antipsychotic profile of the NT peptides may involve an antagonistic NT/D₂ receptor regulation in the ventral striatum.Abbreviations Acb nucleus accumbens - DA dopamine - NPA N-propylnorapomorphine - NT neurotensin     

Decreased striatal D2 dopamine receptors in obese Zucker rats: changes during aging

The specific binding of [³H]YM-09151-2 was used to investigate the possible differences in age-associated changes in striatal D₂ dopamine (DA) receptor properties in genetically obese (fa/fa) Zucker rats and their lean3(Fa/?) littermates. The maximal binding sites (B_max) of D₂ DA receptors was found to decline with age in both obese and lean rats: the rate of decline in receptor B_max was slightly higher in lean than obese rats. However. the B_max of D₂ DA receptor in 6-, 12- and 18-month-old obese rats was significantly lower compared to the age-matched lean rats. These data indicate that obesity decreases the number of striatal D₂ DA receptors without affecting the rate at which receptor number decreases with age.     

Role of dopamine D1 receptors in the control of striatal acetylcholine release by endogenous dopamine

In order to determine the role of dopamine (DA) D₁ receptors in the control of striatal acetylcholine (ACh) transmission, we studied the effects of SCH 39166 (D₁ receptor antagonist), alone or in combination with quinpirole (D₂/D₃ agonist) or PD 128,907 (D₃ agonist) on ACh and DA release. Quinpirole reduced DA and ACh release; PD 128,907 decreased DA but not ACh relase. SCH 3916 stimulated DA and decreased ACh released. Pretreatment with quinpirole reduced or prevented (depending on the dose) the stimulation of DA release while potentiating the decrease of ACh relase elicited by SCH 39166. Similarly, SCH 39166 administered following PD 128,907 did not stimulate DA release, further decreasing ACh release. These results indicate that quinpirole or PD 128,907 affect the actions of SCH 39166 on DA and ACh relase in opposite manner, counteracting the increase of DA release and potentiating the reduction of ACh release. These data support the tenet that endogenous DA exerts a stimulatory input on striatal ACh neurotransmission mediated by D₁ receptors.     

B-HT 920 activates dopamine D2 receptors coupled to inhibition of adenylate cyclase activity

Summary In homogenates of female rat anterior pituitary, the azepine derivative B-HT 920 inhibited the forskolin-stimulated adenylate cyclase activity with an EC₅₀ value of 0.35 M. In male rat anterior pituitary, B-HT 920 curtailed the stimulation of adenylate cyclase activity by vasoactive intestinal peptide with an EC₅₀ of 0.20 M. In synaptic plasma membranes of rat striatum, B-HT 920 significantly reduced basal adenylate cyclase activity with an EC₅₀ of 0.68 M. Both in pituitary and striatum, the B-HT 920 inhibition was counteracted by the dopamine (DA) D₂ receptor antagonist 1-sulpiride, but not by the ₂-adrenergic antagonist yohimbine. These results indicate that B-HT 920 is capable of activating DA D₂ receptors negatively coupled to adenylate cyclase activity.     

Differential ontogeny of functional dopamine and muscarinic receptors mediating presynaptic inhibition of neurotransmitter release and postsynaptic regulation of adenylate cyclase activity in rat striatum.

To study the ontogeny of functional striatal dopamine (DA) D2 and muscarinic receptors we determined the first appearance of the inhibitory effects of activation of autoreceptors on neurotransmitter release and that of postsynaptic receptors on adenylate cyclase activity in striatal slices of rat foetuses and pups. On embryonic day 17 (E17), activation of D2 receptors with LY 171555 (1 microM) resulted in a 50% inhibition of the electrically evoked release of [3H]DA from superfused striata, indicating that D2 autoreceptors are functional at that time. Stimulation of adenylate cyclase activity with the Da D1 agonist SK&F 38393 could also be determined in the striatum on E17. In contrast, inhibition of D1-stimulated adenylate cyclase activity through activation of postsynaptic D2 receptors did not occur until postnatal day 14 (P14), whereas activation of postsynaptic muscarinic receptors with oxotremorine (1 microM) resulted in 30% inhibition already on E17. Endogenous activation of muscarinic receptors with physostigmine (1 microM) was ineffective in the prenatal period, but its inhibitory effect on D1-stimulated adenylate cyclase increased strongly between P7 and P21. Inhibition of striatal [3H]acetylcholine (ACh) release by activation of muscarinic receptors could not be determined until P7, because the release of the neurotransmitter was not measurable before that day. But on P7, oxotremorine and physostigmine (as well as the D2 receptor agonist LY 171555) reduced the electrically evoked release of [3H]ACh from striatal slices. Taken together, these data show that there is a marked time difference between the coupling of D2 receptors and that of the D1 and muscarinic receptors to adenylate cyclase in the developing striatum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sulpiride and the role of dopaminergic receptor blockade in the antipsychotic activity of neuroleptics

It is now generally recognized that dopamine receptors exist in the CNS as different subtypes: D₁ receptors, associated with adenylyl cyclase activity, and D₂ receptor, uncoupled to a cyclic AMP generating system. In order to understand the role of D₁ and D₂ receptors in the antipsychotic action of neuroleptics, we have performed subchronic treatment with haloperidol, a drug which acts on D₂ receptors, and sulpiride, a selective antagonist to D₂ receptors. Long-term treatment with haloperidol does not induce significant supersensitivity of the D₂ receptors. In fact under these conditions ³H-(-)-sulpiride binding, which is a marker of D₂ receptor function, does not increase in rat striatum, while the long-term administration of sulpiride itself produces supersensitivity of D₂ receptors. Moreover, sulpiride does not induce supersensitivity of the D₁ receptors, characterized by ³H-spiroperidol binding. These data suggest that both types of dopamine receptors may be involved in the clinical antipsychotic effects of neuroleptics. Unilateral lesion of the nigrostriatal dopaminergic pathway produces an increase of striatal dopaminergic receptors, measured either by ³H-spiroperidol and ³H-(-)-sulpiride binding. These findings suggest that D₁ and D₂ receptors are present in postsynaptic membranes while it is still not known whether they exist in the same cellular elements.     

Striatal dopaminergic D1 and D2 receptors after intracerebroventricular application of alloxan and streptozocin in rat

Summary Intracerebroventricular application of low, nondiabetogenic doses (500 g kg^–1) of alloxan and streptozocin is followed by alterations of the dopaminergic system in rat striatum. In this brain region the dopamine content significantly increased, while the density of dopaminergic D₁ receptors significantly decreased seven days after the intracerebroventricular application of betacytotoxics, as compared with the control group. The density of dopaminergic D₂ receptors in striatum remained unchanged. Dopaminergic D₁ and D₂ receptors operate through signalling mechanism of G proteins, but no changes of G_s and G_i proteins content have been found in rat striatum after the intracerebroventricular application of betacytotoxics. As intracerebroventricular, nondiabetogenic administration of betacytotoxics produces changes of the striatal dopamine content and D₁ receptor density similar to that produced by peripheral, diabetogenic administration of these drugs, the effect might be related not solely to pancreatic beta cells damage, but to alterations of the brain insulin system, as well.     

Dopamine β‐hydroxylase‐deficient mice have normal densities of D2 dopamine receptors in the high‐affinity state based on in vivo PET imaging and in vitro radioligand binding

In vitro, D₂ dopamine receptors (DAR) can exist in low‐ and high‐affinity states for agonists and increases of D₂ receptors in high‐affinity state have been proposed to underlie DA receptor supersensitivity in vivo. Deletion of the gene for dopamine β‐hydroxylase (DBH) causes mice to become hypersensitive to the effects of psychostimulants, and in vitro radioligand binding results suggest an increased percentage of D₂ receptors in a high‐affinity state. To determine whether DBH knockout mice display an increase of high‐affinity state D₂ receptors in vivo, we scanned DBH knockout and control mice with the agonist PET radioligand [¹¹C]MNPA, which is thought to bind preferentially to the high‐affinity state of the D₂ receptor. In addition, we performed in vitro binding experiments on striatal homogenates with [³H]methylspiperone to measure B_max values and the percentages of high‐ and low‐affinity states of the D₂ receptor. We found that the in vivo striatal binding of [¹¹C]MNPA was similar in DBH knockout mice and heterozygous controls and the in vitro B_max values and percentages of D₂ receptors in the high‐affinity state, were not significantly different between these two groups. In summary, our results suggest that DBH knockout mice have normal levels of D₂ receptors in the high‐affinity state and that additional mechanisms contribute to their behavioral sensitivity to psychostimulants. Synapse 64:699–703, 2010. © 2010 Wiley‐Liss, Inc.     

Intermittent morphine administration induces a long-lasting synergistic effect of corticosterone on dopamine D1 receptor functioning in rat striatal GABA neurons

Glucocorticoid receptor (GR)-mediated facilitation of striatal dopaminergic (DA) neurotransmission has been proposed to play a role in behavioral sensitization induced by intermittent exposure to drugs of abuse or stressors. Searching for possible common neuronal substrates acted upon by drugs of abuse and corticosterone, we addressed the question as to whether such a facilitatory effect is apparent (i.e., persists) in primary cultures of rat striatum subsequent to intermittent (prenatal) morphine administration. As previously observed in striatal slices of morphine-treated rats, intermittent morphine exposure in vivo caused a long-lasting increase in DA D1 receptor-stimulated adenylyl cyclase activity, that appeared to persist in primary cultures of rat striatal γ-aminobutyric acid (GABA) neurons. Subsequent in vitro exposure of these striatal neurons to corticosterone or dexamethasone, simultanously activating GR and mineralocorticoid receptors (MR), about doubled this adaptive effect of previous in vivo morphine administration. The selective MR agonist aldosterone was ineffective in this respect. Prior in vivo morphine treatment also enhanced the stimulatory in vitro effect of corticotropin releasing hormone (CRH) on adenylyl cyclase in cultured GABA neurons. However, the enhanced CRH receptor functioning was not potentiated by in vitro corticosterone exposure. Activation of GR by corticosterone did not facilitate the increase in D1 receptor efficacy induced by sustained activation of muscarinic receptors in cultured striatal neurons. These data indicate that previous intermittent morphine administration induces a long-lasting synergistic effect of corticosterone on enhanced striatal DA neurotransmission at the level of postsynaptic D1 receptors. Moreover, the induction of this neuroadaptation seems to display opioid receptor selectivity and its long-term expression may be confined to D1 receptors. Since exposure to drugs of abuse or stressors not only increase striatal DA release but also plasma corticosterone levels, we hypothesize that this adaptive phenomenon in DA-sensitive GABA neurons is involved in the expression of morphine-induced long-term behavioral sensitization to drugs of abuse and stressors. Synapse 25:381–388, 1997. © 1997 Wiley-Liss, Inc.     

Cerebellar and striatal dopamine receptors: Effects of reeler and weaver murine mutations

The presence and the binding characteristics of D₁ and D₂ receptors were investigated in normal-reeler and normal-weaver mutant mice utilizing [³H]spiperone (D₂ antagonist), [³H]SKF 38393 (D₁ agonist), and [³H]DA as ligands. Analysis of the binding data showed that in the cerebellum there are two binding components for all [³H]ligands. Comparison of the binding constants from cerebellum and striatum showed that in cerebellum the high affinity-low capacity component has similar affinity with that of striatum. The reeler and weaver mutations affected the binding of all ligands: In reeler, total cerebellar specific binding sites for [³H]spiperone and [³H]SKF 38393 decrease significantly (≈50% and ≈70%, respectively), while those for [³H]DA show a small (≈10–15%) but not significant decrease. In weaver, total cerebellar specific binding sites for [³H]spiperone, [³H]SKF 38393, and [³H]DA also decrease significantly (≈60%, ≈70%, and ≈50%, respectively). In reeler striatum [³H]SKF 38393 binding (B_max) is significantly decreased (≈24%), while [³H]spiperone and [³H]DA binding (B_max) is not affected. In weaver striatum, [³H]SKF 38393 binding is significantly increases significantly (≈40%), while [³H]DA binding (B_max) decreases significantly (≈70%). On the basis of the cytoarchitectural aberrations that characterize the cerebellum of these mutants and some well-established information regarding the dopaminergic system of the cerebellum, the above results indicate that in this region (a) D₁ receptors are mainly localized on granule cells and (b) D₂ receptors are localized postsynaptically on granule cells and presynaptically on the DA fibers innervating the cerebellum. © 1993 Wiley-Liss, Inc.     

In vivo imaging of dopamine receptors in a model of temporal lobe epilepsy

Purpose: Alterations in dopamine neurotransmission in animal models of epilepsies have been frequently demonstrated using invasive neuroscience or ex vivo techniques. We aimed to test whether corresponding alterations could be detected by noninvasive in vivo brain imaging with positron emission tomography (PET) in the chronic phase of the rat pilocarpine model of temporal lobe epilepsy. Methods: Six pilocarpine‐treated Wistar rats exhibiting spontaneous recurrent seizures and nine control rats were studied with PET using [¹⁸F]‐fallypride, a high‐affinity dopamine D_2/3 receptor ligand. Parametric images of [¹⁸F]‐fallypride specific binding were calculated using a reference tissue method, and the two groups were contrasted by whole‐brain voxel‐based analysis implemented in statistical parametric mapping (SPM5). Results: Dopamine D_2/3 receptor availability was 27% lower in the bilateral anterior caudate‐putamen of pilocarpine‐treated rats as compared to controls (p < 0.05), but binding was unaffected in other striatal or extrastriatal regions. Conclusions: The finding of substantially reduced availability of dopamine D_2/3 receptors in the anterior caudate‐putamen of rats during the chronic phase of the pilocarpine model is in agreement with results of invasive (microinjection, microdialysis) animal studies that have revealed increased dopamine tonus and a D_2/3 receptor–mediated anticonvulsant action of dopamine in the anterior segment of the rat striatum. The present PET approach could be prospectively applied for monitoring dopamine receptor changes longitudinally, that is, at different phases of the epileptogenic process, and opens perspectives for testing dopaminergic agents as potential antiepileptogenic drugs.     

Increased affinity of dopamine for D(2) -like versus D(1) -like receptors. Relevance for volume transmission in interpreting PET findings

Evidence indicates that dopamine (DA) mainly acts as a volume transmission (VT) transmitter through its release into the extracellular fluid where the D(1) -like and D(2) -like receptors are predominantly extrasynaptic. It was therefore of interest to compare the affinities of the two major families of DA receptors. [(3)H] raclopride /DA and [(3)H] SCH23390/DA competition assays compared the affinity of DA at D(2) -like and D(1) -like receptors in rat dorsal striatal membrane preparations as well as in membrane preparations from CHO cell lines stably transfected with human D(2L) and D(1) receptors. The IC(50) values of DA at D(2) -like receptors in dorsal striatal membranes and CHO cell membranes were markedly and significantly reduced compared with the IC(50) values of DA at D(1) -like receptors. These IC(50) values reflect differences in both the high and low affinity states. The K(iH) value for DA at [(3)H] raclopride-labeled D(2) -like receptors in dorsal striatum was 12 nM, and this can help explain PET findings that amphetamine-induced increases in DA release can produce an up to 50% decrease of [(11)C] raclopride binding in the dorsal striatum in vivo. These combined results give indications for the existence of striatal D(2) -like receptor-mediated DA VT at the local circuit level in vivo. The demonstration of a K(iH) value of 183 nM for DA at D(1) antagonist-labeled D(1) -like receptors instead gives a likely explanation for the failure of a reduction of D(1) -like receptor binding after amphetamine-induced DA release in PET studies using the D(1) -like antagonist radioligands [(11)C] SCH23390 and [(11)C] NNC. It seems difficult to evaluate the role of the extrasynaptic D(1) receptors in VT in vivo with the PET radioligands available for this receptor.     

Endogenously released dopamine inhibits the binding of dopaminergic PET and SPECT ligands in superfused rat striatal slices

Pharmacologically induced changes in synaptic levels of dopamine (DA) have been found, in some studies, to affect the in vivo binding of dopaminergic radioligands. In the present study we used a superfused brain slice preparation to examine the effect of synaptically released dopamine on the binding of some commonly used PET and SPECT radioligands under more controlled conditions than those present in vivo. The release of DA was evoked by electrical stimulation of striatal slices and the sensitivity of binding of the D₁ receptor ligand, [³H]SCH 23390, the D₂ receptor ligands [³H]raclopride and [¹²³I]epidepride, and the DA uptake transporter ligands, [³H]WIN 35,428 and [¹²³I]RTI-55, to the frequency of stimulation examined. Most affected by stimulation was the specific binding of [³H]SCH 23390, which was fully inhibited at 2.5 Hz. This was followed by [³H]raclopride and [¹²³I]epidepride, respectively, the binding of the latter showing only a 50% reduction at the highest frequency of 10 Hz. [³H]WIN 35,428 and [¹²³I]RTI-55 binding was unaffected by stimulation. The effects of stimulation on [³H]raclopride binding were prevented by reserpine pretreatment of the rat, when combined with inclusion of the dopamine synthesis inhibitor, α-methyl-p-tyrosine, in the superfusate medium. We conclude that, in brain slices, the binding of D₁ and D₂ receptor ligands but not that of DA uptake transporter ligands is readily inhibited by DA released into the synaptic cleft. Brain slices may prove to be a useful model system for the investigation of factors affecting competition between radioligand binding and endogenous neurotransmitters. © 1996 Wiley-Liss, Inc.     

Neurotensin-induced modulation of dopamine D2 receptors and their function in rat striatum: counteraction by a NTR1-like receptor antagonist

The present study investigated the neurotensin (NT) receptor subtype (NTR) involved in the antagonistic neurotensin modulation of striatal dopamine D2 receptors observed in vitro and in vivo. The NT induced increase of the IC50 values of dopamine (DA) competition for [125I]iodosulpiride binding sites was counteracted by the NTR1-like antagonist SR48692 in rat striatal slices. Intrastriatal perfusion of pergolide induced in the awake rat an inhibition of striatal DA release that was antagonized by NT. This action of NT was counteracted by co-perfusion with the NTR1 like antagonist SR48692. These data indicate that there exists in the striatum at the prejunctional level an intramembrane antagonistic NT receptor/DA D2 receptor-receptor interaction where NTR1 like receptor activation reduces the DA D2 autoreceptor function.     

Localization of ionotropic glutamate receptors in caudate-putamen and nucleus accumbens septi of rat brain: Comparison of NMDA,AMPA, and kainate receptors

Changes in binding of selective radioligands at NMDA ([³H]MK-801), AMPA ([³H]CNQX), and kainate ([³H]kainic acid) glutamate (GLU) ionotropic receptors in rat caudate-putamen (CPu) and nucleus accumbens (NAc) were examined by quantitative autoradiography following: 1) unilateral surgical ablation of frontal cerebral cortex to remove descending corticostriatal GLU projections, 2) unilateral injection of kainic acid (KA) into CPu or NAc to degenerate local intrinsic neurons, or 3) unilateral injections of 6-hydroxydopamine (6-OH-DA) into substantia nigra to degenerate ascending nigrostriatal dopamine (DA) projections. Cortical ablation significantly decreased NMDA receptor binding in ipsilateral medial CPu (20%), and NAc (16%), similar to previously reported losses of DA D₄ receptors. KA lesions produced large losses of NMDA receptor labeling in CPu and NAc (both by 52%), AMPA (41% and 45%, respectively), and kainate receptors (40% and 45%, respectively) that were similar to the loss of D₂ receptors in CPu and NAc after KA injections. Nigral 6-OH-DA lesions yielded smaller but significant losses in NMDA (17%), AMPA (12%), and kainate (11%) receptor binding in CPu. The results indicate that most NMDA, AMPA, and kainate receptors in rat CPu and NAc occur on intrinsic postsynaptic neurons. Also, some NMDA, but not AMPA or kainate, receptors are also found on corticostriatal projections in association with D₄ receptors; these may, respectively, represent excitatory presynaptic NMDA autoreceptors and inhibitory D₄ heteroceptors that regulate GLU release from corticostriatal axons in medial CPu and NAc. Conversely, the loss of all three GLU receptor subtypes after lesioning DA neurons supports their role as excitatory heteroceptors promoting DA release from nigrostriatal neurons. Synapse 30:227–235, 1998. © 1998 Wiley-Liss, Inc.     

Chronic lead treatment induces in rat a specific and differential effect on dopamine receptors in different brain areas

There is now evidence that two classes of dopaminergic receptors are present in CNS of the rat: D₁, associated, and D₂, not associated with adenylate cyclase activity. Drugs which interact specifically with D₂ receptor are more capable of antagonizing the hyperkinetic behavior induced by lead exposure in rat. They also have a beneficial effect in children with hyperkinetic disorders.We found that the dose of (−)sulpiride which causes sedation is lower in lead intoxicated animals than in controls. On the contrary, haloperidol produces sedation with the same potency in lead-treated and in control rats. The reported behavioral effects were found to be correlated woth biochemical changes. In fact, in lead exposed rats D₂ receptors, measured by (−)-[³H]sulpiride stereospecific binding, are altered, while D₁ receptors seem not to be affected. The alterations are different according to the area examined: D₂ receptor function is increased in the striatum and decreased in the nucleus accumbens. The impairment of D₂ receptor might explain the better capacity of substituted benzamides to improve the hyperkinetic behavior observed in lead exposed rats.