Muscarinic and dopaminergic receptor subtypes on striatal cholinergic interneurons

Unilateral stereotaxic injection of small amounts of the cholinotoxin, AF64A, caused minimal nonselective tissue damage and resulted in a significant loss of the presynaptic cholinergic markers [3H]hemicholinium-3 (45% reduction) and choline acetyltransferase (27% reduction). No significant change from control was observed in tyrosine hydroxylase or tryptophan hydroxylase activity; presynaptic neuronal markers for dopamine- and serotonin-containing neurons, respectively. The AF64A lesion resulted in a significant reduction of dopamine D2 receptors as evidenced by a decrease in [3H]sulpiride binding (42% reduction) and decrease of muscarinic non-M1 receptors as shown by a reduction in [3H]QNB binding in the presence of 100 nM pirenzepine (36% reduction). Saturation studies revealed that the change in [3H]sulpiride and [3H]QNB binding was due to a change in Bmax not Kd. Intrastriatal injection of AF64A failed to alter dopamine D1 or muscarinic M1 receptors labeled with [3H]SCH23390 and [3H]pirenzepine, respectively. In addition, no change in [3H]forskolin-labeled adenylate cyclase was observed. These results demonstrate that a subpopulation of muscarinic receptors (non-M1) are presynaptic on cholinergic interneurons (hence, autoreceptors), and a subpopulation of dopamine D2 receptors are postsynaptic on cholinergic interneurons. Furthermore, dopamine D1, muscarinic M1 and [3H]forskolin-labeled adenylate cyclase are not localized to striatal cholinergic interneurons.     

Reduced High-Affinity Agonist Binding at the M1 Muscarinic Receptor in Alzheimer's Disease Brain: Differential Sensitivity to Agonists and Divalent Cations

M(1) muscarinic acetylcholine receptor (M(1)AchR)-G protein coupling, as measured by high-affinity agonist binding, was examined in membranes prepared from postmortem human temporal cortex (Brodmann area 38) from individuals with Alzheimer's disease (AD, n = 8) and age-matched controls (n = 6). Binding competitions between the M(1)AchR-selective antagonist [(3)H]pirenzepine ([(3)H]PZ) and muscarinic agonists carbachol, acetylcholine, oxotremorine, and oxotremorine M were conducted. In the presence of 1 mM MgCl(2), the inhibition of [(3)H]PZ binding by carbachol, acetylcholine, or oxotremorine M was best described by a two-affinity state model for control and AD cases, while oxotremorine binding affinity was best fit to a single-state model. Although both control and AD groups had similar K(D) values for the high- and low-affinity agonist binding sites, the proportion of M(1)AchRs exhibiting high affinity for carbachol and acetylcholine was reduced by 48 and 33%, respectively, in AD membranes relative to controls (P < 0.05). No changes in the binding of the oxotremorine M or oxotremorine were noted. The nonhydrolyzable guanine nucleotide GppNHp (100 microM) reduced the proportion of M(1)AchRs with high affinity for agonists in both control and AD membranes. Substitution of 1 mM MnCl(2) for MgCl(2) restored high-affinity carbachol binding at the M(1)AchR in AD membranes similar to that seen in controls. In the presence of 1 mM MnCl(2), agonist binding in controls did not differ from 1 mM MgCl(2). In the absence of cations (1 mM EDTA), no differences between control and AD M(1)AchR carbachol binding were observed. Thus, the loss of high-affinity agonist binding at the M(1)AchR in AD is dependent on the agonist and cation studied.     

Autoradiographic studies in animal models of hemi-parkinsonism reveal dopamine D2 but not D1 receptor supersensitivity. I. 6-OHDA lesions of ascending mesencephalic dopaminergic pathways in the rat

The selective dopaminergic antagonist ligands [3H]SCH 23390 and [3H]sulpiride were used to reveal autoradiographically dopamine D1 and D2 receptors, respectively, in brain sections from rats which had received unilateral 6-hydroxydopamine (6-OHDA) injections destroying ascending nigrostriatal neurones. The binding of both ligands to striatal sections was first shown to be saturable, reversible and of high affinity and specificity [( 3H]SCH 23390: Bmax 2.16 pmol/mg protein, Kd 1.4 nM; [3H]sulpiride; Bmax 0.67 pmol/mg protein, Kd 10.7 nM). After unilateral stereotaxic 6-OHDA injections, rats rotated contralaterally when challenged with apomorphine (0.5 mg/kg), or specific D1 or D2 agonists, SKF 38393 (1.0-5.0 mg/kg) and LY 171555 (0.05-0.5 mg/kg), respectively. Loss of forebrain dopaminergic terminals was assessed autoradiographically using [3H]mazindol to label dopamine uptake sites. A loss of approximately 90-95% of uptake sites was reproducibly accompanied by an enhanced density of binding ipsilaterally for the D2 ligand, [3H]sulpiride, in all areas of the striatum, but most markedly in the lateral areas. An increase in the D2 binding site density was also seen in the ipsilateral nucleus accumbens and the olfactory tubercle. In contrast, in the same animals, the striatal D1 receptors were far less affected by dopaminergic denervation, with no consistent changes seen in the binding of [3H]SCH 23390. These results suggest that dopamine D2 receptors are more susceptible than D1 receptors to changes after dopaminergic denervation, which is expressed as an increase in the density of binding sites revealed here with [3H]sulpiride.     

Correlated asymmetries in striatal D1 and D2 binding: relationship to apomorphine-induced rotation

Long-Evans derived rats were tested for nocturnal, amphetamine-induced and apomorphine-induced rotation (circling behavior); the rats' left and right striata were subsequently dissected and D1 and D2 receptor densities (Bmax) were assayed in the same striatal homogenates using [3H]SCH-23390 and [3H]N-methylspiperone, respectively. D1 and D2 Bmax values were correlated (r = 0.68). Moreover, left-right asymmetries in D1 and D2 Bmax values were more highly correlated (r = 0.84). Although asymmetries in D1 and D2 binding were not by themselves related to rotational behavior, an asymmetry in the ratio or balance of D1 and D2 binding was associated with the direction of apomorphine-induced rotation: the D1/D2 ratio of Bmax values was significantly higher in the striatum ipsilateral to the preferred direction of apomorphine-induced rotation. These results suggest that normal variations in numbers of D1 and D2 receptors are determined by a common mechanism, that D1 and D2 receptors are functionally coupled, and that, with respect to activation of striatal receptors, D1 is inhibitory and D2 is excitatory. The effects of apomorphine, a mixed D1 and D2 agonist, appear to reflect the balance between D1 and D2 receptors.     

High-affinity binding of [3H]acetylcholine to muscarinic cholinergic receptors

High-affinity binding of [3H]acetylcholine to muscarinic cholinergic sites in rat CNS and peripheral tissues was measured in the presence of cytisin, which occupies nicotinic cholinergic receptors. The muscarinic sites were characterized with regard to binding kinetics, pharmacology, anatomical distribution, and regulation by guanyl nucleotides. These binding sites have characteristics of high-affinity muscarinic cholinergic receptors with a Kd of approximately 30 nM. Most of the muscarinic agonist and antagonist drugs tested have high affinity for the [3H]acetylcholine binding site, but pirenzepine, an antagonist which is selective for M-1 receptors, has relatively low affinity. The ratio of high-affinity [3H]acetylcholine binding sites to total muscarinic binding sites labeled by [3H]quinuclidinyl benzilate varies from 9 to 90% in different tissues, with the highest ratios in the pons, medulla, and heart atrium. In the presence of guanyl nucleotides, [3H] acetylcholine binding is decreased, but the extent of decrease varies from 40 to 90% in different tissues, with the largest decreases being found in the pons, medulla, cerebellum, and heart atrium. The results indicate that [3H]acetylcholine binds to high-affinity M-1 and M-2 muscarinic receptors, and they suggest that most M-2 sites have high affinity for acetylcholine but that only a small fraction of M-1 sites have such high affinity.     

3H]Ethynylbicycloorthobenzoate ([3H]EBOB) binding in recombinant GABAA receptors

Ethynylbicycloorthobenzoate (EBOB) is a recently developed ligand that binds to the convulsant site of the GABAA receptor. While a few studies have examined the binding of [3H]EBOB in vertebrate brain tissue and insect preparations, none have examined [3H]EBOB binding in preparations that express known configurations of the GABAA receptor. We have thus examined [3H]EBOB binding in HEK293 cells stably expressing human alpha1beta2gamma2 and alpha2beta2gamma2 GABAA receptors, and the effects of CNS convulsants on its binding. The ability of the CNS convulsants to displace the prototypical convulsant site ligand, [35S]TBPS, was also assessed. Saturation analysis revealed [3H]EBOB binding at a single site, with a K(d) of approximately 9 nM in alpha1beta2gamma2 and alpha2beta2gamma2 receptors. Binding of both [3H]EBOB and [35S]TBPS was inhibited by dieldrin, lindane, tert-butylbicycloorthobenzoate (TBOB), PTX, TBPS, and pentylenetetrazol (PTZ) at one site in a concentration-dependent fashion. Affinities were in the high nM to low microM range for all compounds except PTZ (low mM range), and the rank order of potency for these convulsants to displace [3H]EBOB and [35S]TBPS was the same. Low [GABA] stimulated [3H]EBOB binding, while higher [GABA] (greater than 10 microM) inhibited [3H]EBOB binding. Overall, our data demonstrate that [3H]EBOB binds to a single, high affinity site in alpha1beta2gamma2 and alpha2beta2gamma2 GABAA receptors, and modulation of its binding is similar to that seen with [35S]TBPS. [3H]EBOB has a number of desirable traits that may make it preferable to [35S]TBPS for analysis of the convulsant site of the GABAA receptor.     

Characterisation of dopamine receptors in insect (Apis mellifera) brain

In vitro binding experiments using the vertebrate D₁ dopamine receptor ligand [³H]SCH23390 and the vertebrate D₂ dopamine receptor ligand [³H]spiperone were conducted on membrane preparations of honey bee (Apis mellifera) brain. Specific binding of [³H]SCH23390 was saturable and reversible. Analysis of saturation data gave an apparent K_d of 6.3 ± 1.0 nM and B_max of 1.9 ± 0.2 pmol/mg protein for a single class of binding sites. The specificity of high affinity [³H]SCH23390 binding was confirmed in displacement experiments using a range of dopaminergic antagonists and agonists. The rank order of potency for antagonists was: R(+)-SCH23390 > cis-(Z)-flupentixol ≥ chlorpromazine > fluphenazine> S(+)-butaclamol > spiperone. R(±)-SKF38393 and dopamine were the most effective agonists tested. [³H]SCH23390 labels a site in bee brain that is similar, but not identical to the vertebrate D₁ dopamine receptor subtype. [³H]Spiperone also bound with high affinity to bee brain homogenates. Scatchard analysis of [³H]spiperone saturation data revealed a curvilinear plot suggesting binding site heterogeneity. The high affinity site had a apparent K_d of 0.11 ± 0.02 nM and B_max of 9.2 ± 0.5 fmol/mg protein. The calculated values for the low affinity site were a K_d of 19.9 nM and B_max of 862 fmol/mg protein. Kinetic analyses also indicated that [³H]spiperone recognises a heterogeneous population of sites in bee brain. Furthermore, agonist competition studies revealed a phenolaminergic as well as a dopaminergic component to [³H]spiperone binding in bee brain. The rank order of potency of dopaminergic antagonists in competing for [³H]spiperone binding was: spiperone > fluphenazine> S(+)-butaclamol > domperidone> R(+)-SCH23390 > S(−)-sulpiride.     

Effects of chronic administration of caffeine on adenosine A1 and A2 receptors in rat brain

Chronic administration of caffeine (75 mg/kg/day) to rats for 12 days increased [3H]R-PIA binding in the cerebral cortex and cerebellum and [3H]NECA binding to high affinity receptor sites in the striatum. The results indicate that both adenosine A1 and A2 receptor subtypes possess mechanisms of adaptation to chronic caffeine treatment. In addition, adenosine A1 receptor binding shows heterogenous neuroanatomical pattern indicating that the A1 response to caffeine treatment presents regional variation in the rat brain.     

Differential changes in presynaptic modulation of transmitter release during aging

The purpose of this study was to assess the functional role of presynaptic α2-autoreceptors in noradrenergic transmission in the hippocampus and dopamine-2 heteroreceptors in cholinergic transmission in the striatum in young, adult, and senescent rats. Male and female Wistar rats (4, 12, and 24 months old) were used and the release of radioactivity from striatal and hippocampal slices that had been loaded either with [³H]choline or with [³H]norepinephrine was measured at rest and in response to field stimulation (2 Hz, 360 shocks). The release was challenged by sulpiride, a selective dopamine-2 receptor antagonist, and CH-38083, a selective α2-adrenoceptor antagonist. The dissociation constant and the number of α2-adrenoceptors was also determined by binding studies using [³H]yohimbine as ligand in crude membrane preparations of frontal cortex. There were an age-related changes in α2-adrenoceptor-mediated negative feedback modulation of norepinephrine release and in the density and dissociation constant of α2-adrenoceptors. They were reduced in senescent rats. In contrast the presynaptic modulation of striatal cholinergic transmission by dopamine-2 receptors was not altered during aging, but the storage capacity of and the release of acetylcholine from cholinergic interneurons was significantly lower.     

β-adrenoceptor stimulation-induced increase in the number of α2-adrenoceptors of cerebral cortical membranes in rats

Effects of pre-treatment of synaptic membranes with β-adrenoceptor agonists and cholera toxin on [³H]clonidine and [³H]yohimbine binding were examined in rat cerebral cortex. Pre-incubation of cerebral cortical membranes with isoproterenol (10 or 200 μM) or dobutamine (1, 10 or 200 μM) at 37 °C for 40 min caused a significant elevation of specific [³H]clonidine binding but treatment with salbutamol (10 or 200 μM) did not. Scatchard analysis showed that 200 μM isoproterenol treatment resulted in a significant elevation of high affinity component of [³H]clonidine binding which was significantly decreased by the addition of 10μM GTP. A significant elevation in high affinity [³H]clonidine binding was observed by treatment with 100 μg/ml cholera toxin, while a significant decrease in low affinity one was by the treatment. Specific [³H]yohimbine binding was also elevated by 10 or 200 μM isoproterenol treatment. It is suggested that stimulation of β-receptors, presumably β₁-subtype could elevate the number of agonist and antagonist binding sites in α₂-receptors in synaptic membranes by partially mediated by stimulatory and/or inhibitory GTP binding regulatory proteins.     

Selectivity of pirenzepine in the central nervous system. I. Direct autoradiographic comparison of the regional distribution of pirenzepine and carbamylcholine binding sites

The binding capacities of the novel antagonist pirenzepine and the agonist carbamylcholine were examined autoradiographically to compare their abilities to reduce the binding of 1-[3H]quinuclidinyl benzilate ([3H]-1-QNB). This technique, which is applicable to any muscarinic ligand, permits a direct comparison between the binding of carbamylcholine and pirenzepine in the same assay. Analysis of the binding curves generated by standard scintillation counting of whole-brain slices indicated that the ligands bound heterogeneously to muscarinic receptors in the brain. Following apposition of slides to tritium-sensitive film, the binding profile for each ligand was examined visually and by microdensitometry. Regional analyses indicated that the agonist carbamylcholine displayed highest potency for thalamic nuclei, lower potency for cortical regions, and the lowest affinity for layers of the hippocampus. The M1-selective ligand pirenzepine displayed the highest potency for the dentate gyrus of the hippocampus, with lower inhibition levels in the cortex, and the lowest levels of inhibition found in the thalamus. The distribution of high affinity agonist sites was found to be distinct from the distribution of high-affinity antagonist binding sites. In a separate assay, the regional inhibition of pirenzepine and scopolamine was compared for the hippocampus and the forebrain. While scopolamine did not distinguish between muscarinic receptor sites in the hippocampus and cortex, pirenzepine inhibited [3H]-1-QNB labeling in the hippocampus significantly greater than in the cerebral cortex, providing additional evidence for the hypothesis that pirenzepine is a selective antagonist.     

Alteration of nicotinic cholinergic agonist binding sites in hippocampus after fimbria transection

Nicotinic cholinergic agonist binding sites were studied in rat hippocampus by the binding of [³H]acetylcholine in the presence of 1.5 μM atropine sulfate. Following transection of the fimbria/fornix there was a 49% increase in the binding of [³H]acetylcholine reflecting an increase in the affinity of the receptor binding site from K_d = 18.82 ± 3.6 nM in control animals to K_d = 9.06 ± 1.2 nM in experimental tissue. Chronic administration of the agonist nicotine (4 mg/kg/day) by osmotic minipumps produced an increased in the binding of 10 nM [³H]acetylcholine after 14 days (49% increase over control) and after 28 days (141% increase over controls). These data are consistent with the suggestion that [³H]acetylcholine labels a nicotinic cholinergic receptor in rat brain. Further they support the notion that some of the termination sites of the septal-cholinergic projection to the hippocampus are nicotinic.     

Alteration of alpha and muscarinic receptors in rat brain and heart following chronic nicotine treatment

Adrenergic and muscarinic binding sites in 4 brain regions (cerebral cortex, corpus striatum, hypothalamus/thalamus and brainstem) and in heart ventricles were measured in rats chronically treated with nicotine added to the drinking water in doses ranging from 6 to 8 mg/kg/day, for 4 weeks. Control rats received only tap water. The nicotine treatment led to increases in the specific binding of both [³H]prazosin and [³H]clonidine in the cerebral cortex. An increase in [³H]prazosin binding was also observed in the hypothalamus/thalamus of nicotine-treated rats. These changes were all due to an increase of about 23% in B_max. In the brainstem and heart left ventricle, respectively, an increase and a decrease in the affinity of [³H]quinuclidinyl benzilate binding were observed. There were no changes of the binding parameters for the 3 radioligands in other regions tested, and no alteration of [³H]dihydroalprenolol binding was detected in any region examined. These results indicate that chronic administration of nicotine causes an increase in the density of α₁-and α₂-binding sites in some brain regions and reciprocal changes of the affinity of muscarinic binding sites in the brain and in the heart.     

The interaction between β- and α2-adrenoceptors in cerebral cortical membranes: Isoproterenol-induced increase in [H]clonidine binding in rats

The pretreatment of rat cerebral cortical membranes with 10 or 100 μM isoproterenol at 37°C for 40 min caused a significant elevation of the B_max value of [³H]clonidine binding but pretreatment at 4°C did not affect the value. The isoproterenol-induced increase in the B_max value of the binding was higher in 50 mM Tris-HCl buffer (pH 7.7) than in Krebs-Ringer solution. In 50 mM Tris-HCl buffer (pH 7.7), treatment with isoproterenol reduced the B_max value of [³H]dihydroalprenolol binding but neitherK_d nor B_max of [³H]WB 4101 binding was affected by this treatment. Fitty μM propranolol or 100 μM GTP produced a significant reduction in isoproterenol-induced elevation of the B_max value of [³H]clonidine binding. In contrast, 100 μM cyclic AMP did not affect the control binding and 0.1 or 1 mM theophylline did not affect the isoproterenol-induced elevation of the binding. The only B_max value in high affinity binding of [³H]clonidine was increased by isoproterenol.It is suggested that isoproterenol increases the density of α₂-adrenoceptors in a temperature-dependent manner. The direct interaction between β- and α₂-receptor molecules and/or their indirect interaction, mediated by GTP regulatory proteins, would exist in the cerebral cortical membranes of rats.     

Local anesthetic properties of opioids and phencyclidines: Interaction with the voltage-dependent, batrachotoxin binding site in sodium channels

[³H]Batrachotoxinin-A 20-α-benzoate ([³H]BTX-B) binds specifically and with high affinity (K_d = 30 nM) to a site on voltage-dependent Na⁺ channels. Compounds with local anesthetic activity inhibit the binding of [³H]BTX-B by a mutually exclusive, allosteric mechanism. The potential local anesthetic potency of a series of 23 opioids and phencyclidine-like compounds has been estimated by their inhibition of [³H]BTX-B binding to Na⁺ channels in a preparation of synaptoneurosomes from guinea pig cerebral cortex. The potency of these compounds were also tested as inhibitors of the specific binding of [³H]phencyclidine ([³H]PCP) to a high affinity site on rat brain membranes. Opioids such as morphine and codeine show little affinity for the [³H]BTX-B binding site or for the [³H]PCP binding site. Other analgesics, many of the PCP-like compounds and dioxadrol derivatives are potent versus [³H]BTX-B binding and display both stereospecificity and high affinity towards the PCP-binding site. However, there was no correlation between local anesthetic potency assessed as antagonism of [³H]BTX-B binding and affinity towards the PCP site. Five classical local anesthetics had no affinity for the PCP-site, but did displace [³H]BTX-B from its binding site.     

Increased density of M1 receptors in the hippocampus of juvenile rats chronically deprived of NGF

Binding studies were used to assess the changes in affinity and/or number of M1 muscarinic receptors in hippocampi from juvenile rats chronically deprived of NGF. NGF deprivation was obtained by implanting into right ventricle at postnatal day 2 (P2) hybrydoma cells secreting high levels of monoclonal antibodies against NGF (αD11). Parenteral myeloma cells (P3U) were used as controls. Competition experiments were used to characterise the [³H]-PNZ binding sites in membrane preparations of hippocampi from rats sacrificed at P15. [³H]-PNZ bound M1 receptors both in P3U and αD11 group as shown by displacing potency order of antagonists: TLZ=4-DAMP>PNZ>p-F-HHSiD>MTC. The deprivation of NGF for two weeks significantly increased the number of M1 receptors without changing the K_i values of antagonists with exception of methoctramine which showed an increase in affinity in αD11 group. Similar changes in binding parameters were already observed after the first week of anti-NGF treatment. In contrast, a treatment for a week with implant at postnatal day 15 failed to produce any changes in M1 binding parameters. These results provide further physiological evidence for developmentally regulated modulatory role of NGF in the cholinergic function in the hippocampus.     

Effects of pertussis toxin on D2-dopamine receptor in rat striatum: Evidence for coupling of Ni regulatory protein with D2-receptor

It is well established that a pertussis toxin, islet activating protein (IAP), interacts directly with the Ni regulatory protein involved in the receptor-adenylate cyclase system. In this study we investigated the effect of the toxin on the dopaminergic function of the central nervous system in conjunction with the adenylate cyclase system. Direct bilateral microinjection of the toxin into rat striatum reduced the stereotyped behavior induced by apomorphine. This inhibitory effect was observed even 40 h after administration of the toxin, whereas the inhibition by haloperidol disappeared within 15 h. Toxin administration did not influence either the Bmax or affinity of specific binding of [3H]spiroperidol to the striatal membrane. However, it increased the IC50 value of apomorphine for the specific binding of [3H]spiroperidol. GTP (10(-4) M) had little effect on the apparent affinity of apomorphine to the [3H]spiroperidol binding sites in IAP-treated membrane, though an effect was clearly observed in untreated membrane. [3H]Spiroperidol binding sites were increased 34 +/- 8% (n = 4) on chronic treatment of haloperidol for 2 weeks. On the contrary, on long-term administration of IAP the ligand binding sites were decreased by 25 +/- 10% (n = 4). These results indicate that pertussis toxin can interact with the Ni protein coupled with striatal D2-dopamine receptor. Inhibition of the coupling between Ni protein and the D2-dopamine receptor attenuated the manifestation of rat stereotyped behavior. Furthermore, chronic administration of dopamine antagonist resulted in the up-regulation of the D2-dopamine receptor, while long-term inhibition of coupling between the D2-dopamine receptor and Ni regulatory protein reduced the amount of receptors.     

Interaction betweenα2- and β-adrenergic receptors in rat cerebral cortical membranes: clonidine-induced reduction in agonist and antagonist affinity for β-adrenergic receptors

The interaction betweenα₂- and β-adrenergic receptors was investigated in rat cerebral cortical membranes. Clonidine inhibition of [³H]dihydroalprenolol ([³H]DHA) binding resulted in biphasic competition curves with a mean Hill coefficient of 0.45. The addition of 1 μM yohimbine caused a rightward shift of the first portion of the clonidine inhibition curve. In the presence of 1 μM clonidine, the maximum concentration which did not inhibit [³H]DHA binding, inhibition curves of [³H]DHA binding by isoproterenol shifted to the right. A mean Hill coefficient increased from a control value of 0.63 to 0.76. Computer modeling analysis revealed that 1 μM clonidine decreased a β-adrenergic high-affinity state from 28% to 13%. However, the addition of 1 μM yohimbine completely prevented the clonidine-induced reduction in the β-adrenergic high-affinity state. In the presence of 200 μM GTP, the effect of clonidine was not observed. In addition,K_d andB_max values for[³H]p-aminoclonidine ([³H]PAC) binding were not significantly changed by the addition of 100 nM isoproterenol, the maximum concentration which did not inhibit [³H]PAC binding. Moreover, isoproterenol inhibition of [³H]PAC binding resulted in steep competition curves with a mean Hill coefficient of 0.97. The addition of 1 μM alprenolol did not affect the isoproterenol inhibition curve. These data demonstrated that clonidine caused a decrease in agonist and antagonist affinity for β-adrenergic receptors, while isoproterenol did not modulate the binding characteristics ofα₂-adrenergic receptors. Furthermore, these results suggest that regulation betweenα₂- and β-adrenergic receptors is not bidirectional, but is instead unidirectional fromα₂-adrenergic receptors to β-adrenergic receptors.     

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.