Association of two pertussis toxin-sensitive G-proteins with the D2-dopamine receptor from bovine striatum

The solubilized D2-dopamine receptor from bovine striatum exhibits high and low affinity states for dopaminergic agonists. Guanine nucleotides and pertussis toxin convert the solubilized receptor from a high affinity state to a low one. A D2-receptor preparation partially purified by affinity chromatography on a haloperidol adsorbent, exhibited agonist-stimulated GTPase activity. [32P]ADP-ribosylation by pertussis toxin of this receptor preparation resulted in the specific labeling of two protein bands corresponding to mol. wts of 39 and 41 kd, in SDS-PAGE. Association of these G-proteins with the receptor was specifically inhibited by Gpp(NH)p. Immunoblot analysis of these G-proteins indicated that the 41- and 39-kd protein bands are analogous to brain Gi and Go respectively. These experiments demonstrate that two distinct pertussis toxin-sensitive G-proteins are functionally associated with bovine striatum D2-dopamine receptor.

Presence of dopamine D-2 receptors in human tumoral cell lines

[125I] Iodosulpride binding was examined on eight human cell lines derived from lung, breast and digestive tract carcinomas, neuroblastomas and leukemia. Specific binding was detected in five of these cell lines. In the richest cell line N417, derived from small cell lung carcinoma, [125I] iodosulpride bound with a high affinity (Kd = 1.3 nM) to an apparently homogeneous population of binding site (Bmax = 1,606 sites per cell). These sites displayed a typical D-2 specificity, established with several dopaminergic agonists and antagonists selective of either D-1 or D-2 receptor subtypes. In addition, dopamine, apomorphine and RU 24926 distinguished high- and low-affinity sites, suggesting that the binding sites are associated with a G-protein. The biological significance and the possible diagnostic implication of the presence of D-2 receptors on these cell lines are discussed.

Synthesis and evaluation of optically pure [3H]-(+)-pentazocine, a highly potent and selective radioligand for sigma receptors

Tritium-labeled (+)-pentazocine ([3H]-1b) of specific activity 26.6 Ci/mmol was synthesized in 3 steps starting with (+)-normetazocine (2) of defined optical purity. [3H]-1b has been characterized as a highly selective ligand for labeling of sigma receptors. Competition data revealed that [3H]-1b could be displaced from guinea pig brain membrane preparations with a number of commonly used sigma receptor ligands. [3H]-1b exhibited saturable, enantioselective binding with a Kd of 5.13 +/- 0.97 nM and a Bmax of 1146 +/- 122 fmol/mg protein. Phencyclidine (PCP) displaced [3H]-1b with low affinity while MK-801 was inactive, thus indicating insignificant activity at the PCP-binding site; apomorphine failed to displace [3H]-1b indicating lack of dopamine receptor cross-reactivity. Since the affinity of [3H]-1b is about 6 times that of the two commonly employed sigma ligands ((+)-3-[3H]PPP and [3H]DTG) and since it is more selective for sigma receptors than the benzomorphan [3H]SKF-10,047, it represents the first example of a highly selective benzomorphan based sigma receptor ligand. [3H]-1b should prove useful for further study of the structure and function of sigma receptors.

Kinetic properties of the in vivo accumulation of 3H-(-)-N-n-propylnorapomorphine in mouse brain

(1) The influence of various dopamine (DA) receptor agonists and antagonists on the kinetic properties of the specific binding of 3H(-)-N-n-propylnorapomorphine (NPA) in the mouse striatum in vivo was studied. The specific binding of 3H-NPA, defined as the difference between the radioactivity in the striatum and cerebellum, was completely antagonized by the selective D-2 receptor antagonist raclopride but not by the selective D-1 antagonist SCH 23390, showing that the binding occurs exclusively to the D-2 receptors. (2) The selective D-2 receptor agonists pergolide and quinpirole inhibited the 3H-NPA binding biphasically at low doses, indicating that these DA receptor agonists have high affinities for a subfraction (10 to 30%) of the NPA binding sites. (3) Increasing the synaptic DA concentration by DA release [(+)-amphetamine] or uptake blockade (amfonelic acid and methylphenidate) inhibited the 3H-NPA binding in a competitive manner (unchanged Bmax, increased KD). Depletion of the DA in the synapses by gamma-butyrolactone or reserpine decreased the apparent KD value. (4) The possibility of estimating changes in the synaptic DA concentration from changes in the apparent KD is discussed. According to the results obtained, the normal concentration of DA in the synaptic cleft in mouse striatum in vivo is about 40 nmol/l and this concentration is increased 2 to 3 times by (+)-amphetamine and amfonelic acid in doses which evoke hyperactivity and stereotypic behaviour.

2-Haloaporphines as potent dopamine agonists

The synthesis of 2-amino- and 2-halo-substituted aporphines is described. The key step is the substitution of a hydroxy group in the 2-position with an amino group effected by a Smiles rearrangement reaction of the 2-methylpropanamide derivative 6. The affinity of the new compounds for the dopamine D-2 receptor in the anterior pituitary gland was evaluated. 2-Fluoroapomorphine was the most potent compound, being 1.5 times more potent than (-)-apomorphine. The structure-activity relationships are discussed in relation to a previously proposed receptor model.

Distribution of dopamine D-2 receptors in the rat striatal complex and its comparison with acetylcholinesterase

The distribution of D-2 dopamine receptors in the rat striatal complex was studied with autoradiography after specific in vivo labeling with the dopamine agonist [3H]N-n-propylnorapomorphine and subsequent irreversible fixation. This labeling technique allows the visualization of D-2 receptors at the cellular level by light microscopic emulsion autoradiography. During the preparation of emulsion autoradiograms, the recovery of the label was 75%, the specific and the aspecific label being equally affected. The distribution of label before and after the loss of radioactive label occurred, did not show differences. In rat neostriatum, dopamine D-2 receptors are not homogeneously distributed: in the caudate-putamen the density is laterally higher than medially. Moreover, there exists a mosaic-like pattern of receptor density. In the ventral striatum, comprising the fundus striati, nucleus accumbens septi and olfactory tubercle, the receptor density is lower than in the caudate-putamen, except for the core regions in the islands of Calleja and the rim of these islands, which contain high (as high as the lateral caudate-putamen) and a moderate density of receptors, respectively. In caudate-putamen and lateral nucleus accumbens it appeared that the intensity of acetylcholinesterase staining parallels more or less the distribution of dopamine D-2 receptors. In medial nucleus accumbens and in olfactory tubercle, the high intensity of acetylcholinesterase is not paralleled by a high D-2 receptor labeling density. This receptor labeling density does not seem to be matched by differences in densities of medium-sized neuronal cell bodies.

Neurotensin modulates the binding characteristics of dopamine D2 receptors in rat striatal membranes also following treatment with toluene

The effects of neurotensin in vitro (1-100 nM) on the binding characteristics of [3H]N-propylnorapomorphine ([3H]NPA) were analysed in striatal membrane preparations of the adult male rat. Subsequently, it was investigated whether the modulatory effects of 10 nM neurotensin on [3H]NPA binding were altered by treatment with toluene in vivo (80 p.p.m., 3 days, 6 h day-1) and in vitro (19 mumol ml-1). Displacement of [3H]NPA binding by raclopride (IC50 about 15 nM) and SCH 23390 (without effect) indicated that [3H]NPA labelled only D2 dopamine receptors in the present study. Neurotensin was found to reduce the affinity of D2 receptors with a maximum response at 10 nM. At this concentration the KD value was increased by 30-40% without any consistent changes in the number of binding sites. The modulatory effect of neurotensin remained intact also following toluene treatment in vivo and in vitro, although at a higher KD range, since toluene alone increased the KD value of [3H]NPA binding by 40-50%. Thus, the mechanisms mediating the effects of neurotensin and toluene on the D2 receptor are likely to be different. When neurotensin and toluene treatments were combined, the KD values of [3H]NPA binding were about twice as high as in non-treated controls. These additive effects may lead to a severely decreased efficiency of dopamine D2-mediated neurotransmission in vivo.

Muscarinic receptor-mediated regulation of OM-853-enhanced dopamine release in striatum of rat

The effect of OM-853, a new vincamine analogue, on cerebral dopaminergic neurons was investigated in male Wistar rats. The administration of OM-853 (200 mg/kg p.o.) induced facilitation of the metabolic turnover of dopamine (DA) in all brain areas except the cerebral cortex. Addition of OM-853 enhanced the release of [3H]DA from striatal slices; this release was antagonized by atropine (10(-7) M). However, pretreatment with scopolamine (0.5 mg/kg s.c.) inhibited the enhancement of striatal DA turnover induced by OM-853 administration. OM-853 (10(-4) M) inhibited [3H]quinuclidinyl benzilate binding to muscarinic cholinergic receptors in a striatal particulate fraction more potently than carbachol (10(-4) M). These results suggest that OM-853 may induce facilitation of striatal DA turnover by enhancing DA release via the stimulation of muscarinic cholinergic receptor.

Effect of buflomedil (4-(1-pyrrolidinyl)-1-(2,4,6-trimethoxy phenyl)-1-butanone hydrochloride) on the function of striatal dopaminergic neurons

Effect of buflomedil (4-(1-pyrrolidinyl)-1-(2,4,6-trimethoxy phenyl)-1-butanone hydrochloride) on the release and uptake of dopamine (DA) and the function of DA receptors in the striatum was investigated using male Wistar rats. In vitro addition of buflomedil (10(-5)-10(-8) M) had no effect on the uptake of [3H]-DA in striatal slices. On the other hand, buflomedil (10(-5)-10(-7) M) increased the spontaneous as well as high K+ (30 mM)-evoked releases of [3H]DA from striatal slices. Buflomedil inhibited the bindings of [3H]SCH23390, [3H]spiperone and [3H]apomorphine to striatal D1, D2 and D3 receptors only at a high concentration. On the other hand, buflomedil inhibited [3H]quinuclidinyl benzilate (QNB) binding to striatal muscarinic cholinergic receptors, which was similar to the action of carbachol. Pretreatment with scopolamine (0.5 mg/kg) in vivo inhibited the facilitation of striatal DA turnover induced by oral administration of buflomedil (300 mg/kg). In contrast, continuous oral administration of buflomedil (30 mg/kg x 7 days) to rats had no significant effect on the specific bindings of [3H]SCH23390, [3H]spiperone, [3H]apomorphine and [3H]QNB to synaptic membrane preparations obtained from the striatum. These results suggest that buflomedil may enhance striatal DA release by stimulating muscarinic cholinergic receptor and that DA receptors may not be involved in the enhancing effect of buflomedil on DA release.

Chronic estrogen treatment promotes a functional uncoupling of the D2 dopamine receptor in rat anterior pituitary gland

The effect of chronic estrogen treatment on the anterior pituitary D2 dopamine receptor was studied by treating rats with diethylstilbestrol (DES) over a 6-week period. DES treatment resulted in an increase in anterior pituitary weight and PRL content and serum PRL levels compared to those in sham-treated controls. The status of the anterior pituitary D2 dopamine receptor was evaluated using both radioligand binding and adenylate cyclase assays. [125I]N-(p-aminophenethyl)spiroperidol [( 125I]NAPS), a derivative of the D2-selective antagonist spiperone, was used to quantitate D2 receptors. Saturation analysis of [125I]NAPS binding indicated that DES treatment had no effect on the affinity or maximum binding capacity of the radioligand for the D2 receptor. Competition analysis with unlabeled D2 antagonists for [125I]NAPS binding also indicated that DES treatment did not affect antagonist interactions with the receptor. In contrast, the interactions of agonists with the D2 receptors from DES-treated rats were modified, as assessed through [125I]NAPS competition analysis. Using control tissue, agonist competition curves revealed both high and low affinity agonist binding states of the receptor. In the presence of guanine nucleotides, the high affinity agonist binding state is abolished, reflecting coupling of the receptor with a guanine nucleotide regulatory (G) protein. In DES-treated tissue, agonist competition curves indicated the presence of only low affinity agonist binding, with minimal effects of guanine nucleotides, suggesting uncoupling of receptor-G-protein interactions. The functionality of the D2 receptor was further assessed by examining dopaminergic inhibition of vasoactive intestinal peptide-stimulated adenylate cyclase activity. Although DES treatment resulted in a reduction of vasoactive intestinal peptide-stimulated enzyme activity itself, the ability of dopaminergic agonists to inhibit this activity was reduced by about 50%. These results suggest that estrogen is capable of attenuating the functional coupling of the D2 receptor with its biochemical effector system in the anterior pituitary gland.

Alkylation of striatal dopamine receptors abolishes stereotyped behavior but has no effect on dopamine stimulated adenylate cyclase activity

The role of dopamine stimulated adenylate cyclase (AC) activity in behaviours elicited by apomorphine stimulation of striatal dopamine receptors was investigated. Rats were treated with the irreversible dopamine receptor alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) alone or after pretreatment with either a D1 or D2 receptor antagonist and subsequently challenged with apomorphine. Animals that received no antagonist pretreatment showed significantly decreased striatal concentrations of D1 and D2 receptors and an abolition of apomorphine induced sniffing behaviour despite showing no change in striatal AC activity. In the groups which received antagonist pretreatment the reduction in the sniffing response paralleled the reduction in D2 receptor concentration whereas the incidence of vacuous oral movements was inversely related. In no case were the behavioural responses associated with changes in AC activity. We conclude that these behavioural effects observed in response to dopamine stimulation by apomorphine may be mediated through another second messenger system.

Species differences in hepatic microsomal drug-metabolizing enzymes

The activity of some metabolizing enzymes was assessed in the liver microsomes of Acomys cahirinus, mice and rats. The enzymatic studies were followed by the determination of cerebral level of apomorphine (APO), imipramine (IMI) and its metabolite desipramine (DMI) of animals treated with a single dose of APO or IMI. It was found that the level of cytochrome P-450 and the activity of IMI demethylase and glucuronyltransferase in the liver microsomes of rats was significantly higher than those in the liver microsomes of Acomys and mice. The brain levels of APO, IMI and DMI were different in investigated species and IMI and DMI levels in the brain of Acomys, mice and rats corresponded to the activity of IMI demethylase in the liver microsomes of these species.

6- and 8-hydroxy-3,4-dihydro-3-(dipropylamino)-2H-1-benzopyrans. Dopamine agonists with autoreceptor selectivity

The dopamine agonist profiles of 3,4-dihydro-3-(3-dipropylamino)-2H-1-benzopyran-6- and -8-ol (4 and 5, respectively) were examined. Both 4 and 5 exhibited greater relative affinity for receptors labeled with the dopamine agonist ligand [3H]propylnorapomorphine than for those labeled with the dopamine antagonist ligand [3H]haloperidol. Both compounds attenuated the stimulation of brain dopamine synthesis caused by gamma-butyrolactone (GBL) and decreased the firing rate of substantia nigra dopamine neurons in rats. This profile of activity, together with the ability of the dopamine antagonist haloperidol to reverse the inhibition of dopamine neuronal firing, indicate that both compounds are brain dopamine agonists.

Interaction of L-prolyl-L-leucyl glycinamide with dopamine D2 receptor: evidence for modulation of agonist affinity states in bovine striatal membranes

The role of the hypothalamic tripeptide L-prolyl-L-leucyl-glycinamide (PLG) in modulating the agonist binding to bovine striatal dopamine D2 receptor was investigated using a selective high-affinity agonist, n-propylnorapomorphine (NPA). PLG caused an enhancement in [3H]NPA binding in striatal membranes in a dose-dependent manner, the maximum effect being observed at 10(-7)-10(-6) M concentration of the tripeptide. The Scatchard analysis of [3H]NPA binding to membranes preincubated with 10(-6) M PLG revealed a significant increase in the affinity of the agonist binding sites. In contrast, there was no effect of PLG on the binding pattern of the antagonist [3H]spiroperidol. The antagonist versus agonist competition curves analyzed for agonist high- and low-affinity states of the receptor displayed an increase in the population and affinity of the high-affinity form of the receptor with PLG treatment. The low-affinity sites concomitantly decreased with relatively small change in the affinity for the agonists. Almost similar results were obtained when either NPA or apomorphine was used in the competition experiments. A partial antagonistic effect of PLG on 5'-guanylylimidodiphosphate [Gpp(NH)p]-induced inhibition of high-affinity agonist binding was also observed, as the ratio of high- to low-affinity forms of the receptor was significantly higher in the PLG-treated membranes compared to the controls. Direct [3H]NPA binding experiments demonstrated that PLG attenuated the Gpp(NH)p-induced inhibition of agonist binding by increasing the EC50 of the nucleotide (concentration that inhibits 50% of the specific binding). No effect of PLG on high-affinity [3H]NPA binding, however, could be observed when the striatal membranes were preincubated with Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of [125I]-N-(p-aminophenethyl)spiroperidol to the D-2 dopamine receptor in the neurointermediate lobe of the rat pituitary gland: a thermodynamic study

The novel iodinated ligand [125I]-N-(p-aminophenethyl)spiroperidol ([125I]NAPS) was used to identify the D-2 dopamine receptor in the intermediate lobe of the rat pituitary gland. The binding of [125I]NAPS was of high affinity and saturable, given that the dissociation constant and the maximal binding were 34.7 +/- 4.8 pM and 21.1 +/- 2.5 fmol/mg of protein, respectively. The ability of dopaminergic agonists and antagonists to compete with [125I]NAPS varied markedly with incubation temperature. The marked decrease of the molar potency associated with increasing incubation temperature in the competitive displacement curve suggested that the binding of five agonists, dopamine, (-)-apomorphine, (-)-n-propylnorapomorphine, N-0434, and LY-171555, to the D-2 dopamine receptor was enthalpy-driven, with a negative change in entropy. In contrast, the binding of three antagonists, fluphenazine, (+)-butaclamol, and domperidone, was entropy-driven, with positive change in entropy, suggesting less temperature-sensitive change in the molar potency. Several molecules gave unanticipated results; the molar potency of two dopamine agonists, bromocriptine and lisuride, was much less temperature-sensitive than the other agonists used in this study. The thermodynamic parameters for the atypical agonists indicated entropy-driven binding. Conversely, the molar potency of (+)-apomorphine, a dopamine receptor antagonist, was markedly affected by incubation temperature, indicating enthalpy-driven binding. Another antagonist, YM-09151-2, was affected by the inclusion of sodium chloride in the assay system: in the absence of sodium chloride, the drug was relatively weak and displayed enthalpy-driven binding; in the presence of sodium chloride, its molar potency was increased and its binding manner turned into entropy-driven.

The bindings of 3H-prazosin and 3H-yohimbine to alpha adrenoceptors in the guinea-pig stomach

Alpha adrenoceptor subtypes have been investigated by radioligand binding study in guinea-pig stomach using 3H-prazosin and 3H-yohimbine. The specific 3H-prazosin binding to guinea-pig stomach was saturable and of high affinity (KD = 1.4 nM) with a Bmax of 33 fmol/mg protein. Specific 3H-yohimbine binding to the tissue was also saturable and of high affinity (KD = 25.5 nM) with a Bmax of 150 fmol/mg protein. Adrenergic drugs competed for 3H-prazosin binding in order of prazosin greater than phentolamine greater than methoxamine greater than norepinephrine greater than clonidine greater than epinephrine greater than yohimbine. These drugs competed for 3H-yohimbine binding in order of yohimbine greater than phentolamine greater than clonidine greater than epinephrine greater than norepinephrine greater than prazosin greater than greater than prazosin greater than methoxamine. We also examined whether dopamine receptors exist in guinea-pig stomach, using radioligand binding study. Specific binding of 3H-spiperone, 3H-apomorphine, 3H-dopamine and 3H-domperidone was not detectable in the stomach. Dopaminergic drugs such as dopamine, haloperidol, domperidone and sulpiride competed for 3H-prazosin binding in order of haloperidol greater than domperidone greater than dopamine greater than sulpiride. Metoclopramide, sulpiride and dopamine competed for 3H-yohimbine binding in order of metoclopramide greater than sulpiride greater than dopamine. These results suggest that guinea-pig stomach has alpha 1 and alpha 2 adrenoceptors and has no specific dopamine receptors. It is also suggested that some dopamine receptor antagonists such as domperidone, haloperidol, sulpiride and metoclopramide have antagonistic actions on alpha adrenoceptors.

Ganglioside GM1 treatment prevents the effects of subacute exposure to toluene on N-[3H]propylnorapomorphine binding characteristics in rat striatal membranes

The effects of ganglioside GM1 treatment (10 mg/kg, i.p., 3 days, once daily) were analyzed on N-[3H]propylnorapomorphine ([3H]NPA) binding characteristics in striatal membrane preparations from air or toluene (80 ppm, 3 days, 6 h/day) exposed adult male rats. It was found that toluene exposure induced increased Kd and Bmax values for [3H]NPA binding. These changes could be blocked by GM1 treatment, which by itself had no significant effects on the binding characteristics of [3H]NPA. The addition of toluene (9.3 mumol/ml) in vitro had similar effects on [3H]NPA binding characteristics as toluene exposure in vivo. These results indicate that the effects of toluene exposure on dopamine receptors are due to an alteration of membrane fluidity which is prevented by GM1 treatment.

Further in vitro and in vivo studies with the putative presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The in vitro binding of the putative dopamine autoreceptor agonist [3H]DP-7-ATN to rat striatal membrane homogenates was investigated. The maximum number of binding sites Bmax was 497.5 +/- 50.2 fmol/mg protein and the affinity constant KD was 8.3 +/- 1.5 nM using 10 microM (+) butaclamol to define non-specific binding. Lesion of the left medium forebrain bundle by 6-hydroxydopamine resulted in an almost complete loss of dopamine in the striatum but did not affect the binding of [3H]DP-7-ATN. The binding of [3H]DP-7-ATN to the homogenates of the dopaminergic cell bodies in the substantia nigra revealed a Bmax of 542.4 +/- 40.1 fmol/mg protein and a KD of 11.1 +/- 1.3 nM. The pharmacological profile of the binding was characterized as being to D-2 receptors. No direct in vitro evidence could be found for a selective binding to DA autoreceptors. The dopamine uptake inhibitor GBR 12909 interacted in a noncompetitive manner with the in vitro binding of [3H]DP-7-ATN and the latter compounds uptake into isolated synaptosomes was not through the specific dopamine uptake system but rather through diffusion. GBR 12909 failed to reveal any agonistic or antagonistic activity in the GBL model but was able to antagonize the hypomotility in rats induced by 0.25 mg/kg DP-7-ATN. The inhibitory effect of DP-7-ATN on DA release was also demonstrated using in vivo brain dialysis in conscious rats. Based on the above results, the possibility is discussed that the release regulating DA autoreceptors, which might be coupled to the reuptake complex, and the DA biosynthesis regulating autoreceptors, are different entities.

Characterization of D2 dopamine receptors in dopamine-resistant prolactin-secreting rat pituitary tumors 7315a and MtTW15

We have investigated the structure of D2 receptors present in two prolactin-secreting, dopamine-resistant, transplantable rat pituitary tumors, 7315a and MtTW15. These receptors specifically bind with high affinity the dopamine antagonist [3H]spiroperidol when membrane bound or solubilized by [3-(3-cholamidopropyl)-dimethyl-ammonio]-1-propane sulfonate 10 mM and are pharmacologically characterized as D2 type. Target-size analysis by radiation inactivation indicated a molecular mass of approximately 100,000 and 200,000 daltons for receptors present respectively in 7315a and MtTW15 tumors either membrane bound or solubilized. The minimal size of the D2 binding site was evaluated at 94,000 daltons by photoaffinity labeling with [125I]azido-N-(p-aminophenethyl)-spiperone followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A guanine nucleotide had no effect on the displacing potency of the agonist N-propylnorapomorphine evaluated with membrane-bound or solubilized receptors obtained from either tumor. These results suggest the absence or inactivation of a guanine nucleotide binding protein in the receptorial complex of these tumors. Thus, our data indicate that a structural anomaly is present in the D2 receptorial complex of these prolactin-secreting rat pituitary tumors, which may be responsible for their resistance to the inhibitory effects of dopamine.

Pharmacological activities of the MIF-1 analogues Pro-Leu-Gly, Tyr-Pro-Leu-Gly and pareptide

The pharmacological activities of the related free acid analogues of MIF-1, Pro-Leu-Gly (PLG) and Tyr-Pro-Leu-Gly (YPLG), were investigated because of the possibility that they may be formed during the digestion of milk and wheat proteins in vivo. The amino acid sequences -Tyr-Pro-Leu-Gly- and -Pro-Leu-Gly- are present in proteins from these foods. Chronic administration of either PLG (0.25 mg/kg, SC, BID) or the control substance, pareptide (0.25 mg/kg, SC, BID), antagonized the development of tolerance to the cataleptic effects of haloperidol in mice. The effect of YPLG (0.25 mg/kg, SC, BID) on the development of this tolerance was borderline and not statistically significant. Nanomolar concentrations of PLG, YPLG, and pareptide each increased the in vitro binding of 3H-apomorphine to rat striatal receptors. In this in vitro system, bell-shaped dose response curves were observed for each peptide. The effects of these peptides on tolerance development and apomorphine binding are similar to those previously reported for MIF-1 and demonstrate that amidation at the carboxyl terminus is not required for biological activity.

C1- and C3-methyl-substituted derivatives of 7-hydroxy-2-(di-n-propylamino)tetralin: activities at central dopamine receptors

C1- and C3-methyl-substituted derivatives of the potent dopamine (DA) receptor agonist 7-hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT) have been synthesized, and their conformational preferences have been studied by use of NMR spectroscopy, X-ray crystallography, and molecular mechanics (MMP2) calculations. The compounds were tested for activity at central DA receptors, by use of biochemical and behavioral tests in rats. (1S,2R)-7-Hydroxy-1-methyl-2-(di-n-propylamino)tetralin [(+)-10] was demonstrated to be sevenfold less potent than (2R)-7-OH-DPAT as a DA receptor agonist. The other new compounds were of lower potency or inactive.

Alkylation of the bovine anterior pituitary D2-dopamine receptor: inconsistency with predictions of the ternary complex model

The ability of a high affinity, N-chloroethyl derivative to irreversibly occupy dopamine receptors has been used to probe the nature of the interaction of the bovine anterior pituitary D2-dopamine receptor with a putative G-protein. Conditions were developed that resulted in a significant 40-60% decrease in the total D2-dopamine receptor concentration without affecting the affinity of the receptor for either antagonists or agonists. Computer analyses of radioligand binding data for the pituitary D2-dopamine receptor have indicated that consistent with the decrease in total receptor concentration following alkylation there was a comparable decrease in the apparent agonist high-affinity state of the receptor as modelled according to independent classes of binding sites. These data were not consistent with the predictions of the ternary complex model. These results indicate that the present form of the model is not sufficient to account fully for the mechanism of ligand interactions with the pituitary D2-dopamine receptor and suggest, furthermore, that the receptor and the G-protein might be considered as a complex where ligand binding to either component regulates ligand interactions of the other, rather than enhancing the degree of association of the components.

Effect of long-term dosing with tiapride on brain dopamine receptors and metabolism in rats. Comparative study with sulpiride and haloperidol

The effects of long-term dosing with tiapride for 21 days on barin dopamine receptors and dopamine turnover were compared with those of sulpiride and haloperidol. Haloperidol caused an increase in both antagonist (3H-spiperone) labeled receptors and agonist (3H-N,n-propylnorapomorphine) labeled ones, whereas tiapride acted on the agonist binding sites and sulpiride acted on the antagonist binding sites. The increases induced by sulpiride were only observed in the striatum, while those induced by tiapride and haloperidol were observed in both the striatum and limbic area. Dopamine and dopamine metabolites in the brain tissues were measured at 2 hr and 3 days after long-term dosing with the drugs as an indicator of dopamine turnover. They were higher at 2 hr and lower at 3 days than those of the saline treated controls; however, the increase at 2 hr was much less than that after single acute dosing with drugs. This suggested that all drugs induced tolerance with regard to dopamine turnover. In these studies, tiapride and sulpiride were less active than haloperidol in the effects on brain dopamine receptors and dopamine turnover. This generally weaker activity of sulpiride and tiapride suggest that the benzamide drugs have fewer side effects such as a tardive dyskinesia, than does haloperidol, even after long-term dosing. Furthermore, a slight difference between the effects of tiapride and sulpiride on the dopamine receptor subtypes in the brain subdivision was suggested.

In vitro characterisation of dopamine receptors in the superior colliculus of the rat

In membrane preparations of superior colliculus of the rat, the binding of [3H]spiperone (0.15 nM) was displaced by the incorporation of (+)-butaclamol, haloperidol, apomorphine and (+/-)-sulpiride, but not by (-)-butaclamol, prazosin, propranolol, ketanserin or cinanserin. The Ki values for the displacement of [3H]spiperone by (+/-)-sulpiride, (+)-butaclamol and haloperidol were similar in tissue preparations from superior colliculus and striatum. Equilibrium analysis of the specific binding of [3H]spiperone (0.03-1.0 nM), defined by 10(-5) M (+/-)-sulpiride, to membrane preparations of the superior colliculus, showed the interaction to be saturable and of high affinity. However, the Bmax was only approximately 10% of that found in preparations of striatum; the apparent dissociation constant (KD) was the same in both preparations of the superior colliculus and striatum. Uptake of [3H]dopamine into synaptosomal preparations of the superior colliculus was approximately 20% of that found in synaptosomes from the striatum. In preparations of striatum nomifensine, but not desipramine or fluoxetine, inhibited the uptake of [3H]dopamine. However, in preparations from the superior colliculus, nomifensine, desipramine and fluoxetine were without effect on the uptake of [3H]dopamine. Dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) were present in small concentrations in the superior colliculus. Homovanillic acid (HVA) was present in larger concentrations and the HVA plus DOPAC/dopamine ratios were greater in the superior colliculus than in the striatum. The superior colliculus contained only small amounts of noradrenaline but 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were present in larger amounts.(ABSTRACT TRUNCATED AT 250 WORDS)