Dopamine-stimulated Adenylate Cyclase Research Articles

Effect of ergotamine and dihydroergotamine on dopamine-stimulated adenylate cyclase in rat caudate nucleus.

Effect of ergotamine and dihydroergotamine on dopamine-stimulated adenylate cyclase in rat caudate nucleus.

Potent lipophilic substituted benzamide drugs are not selective D-1 dopamine receptor antagonists in the rat

The substituted benzamide drugs YM 09151-2 and clebopride potently inhibited apomorphine-induced stereotyped behaviour in the rat and caused displacement of the specific binding of [3H]spiperone to D-2 binding sites on striatal membranes in low nanomolar concentrations. Other substituted benzamide drugs including metoclopramide, sultopride and flubepride also inhibited stereotyped behaviour to a greater or lesser degree, but were less potent in displacing [3H]spiperone from D-2 sites. YM 09151-2 and clebopride only weakly displaced specific binding of [3H]piflutixol to D-1 sites on rat striatal membranes, and only weakly inhibited striatal dopamine stimulated adenylate cyclase activity, when compared with cis-flupenthixol. The other substituted benzamide drugs did not displace [3H]piflutixol or inhibit dopamine stimulation of adenylate cyclase activity in the concentration range used. Clebopride and YM 09151-2 were highly lipophilic with apparent partition coefficient (log P') values equivalent to those of classical neuroleptic compounds, such as cis-flupenthixol. In contrast, the other substituted benzamide drugs were markedly less lipophilic. A log P' value of approximately 2 was required before inhibition of adenylate cyclase activity or displacement of [3H]piflutixol binding occurred. However, in excess of this value there was no correlation between either inhibition of adenylate cyclase activity or displacement of [3H]piflutixol binding and the lipophilicity of the various compounds. We conclude that potent lipophilic substituted benzamide drugs, like other members of the substituted benzamide series, are selective D-2 receptor antagonists. Inherent steric factors within the drug series would appear to dictate activity at D-1 and D-2 sites, although lipophilicity may contribute to actions in these environments.

Paralemmin interacts with D3 dopamine receptors: Implications for membrane localization and cAMP signaling

Paralemmin is a novel lipid-anchored protein, which is highly expressed in neuronal plasma membranes. In this study, we demonstrate that paralemmin specifically interacts with the third intracellular loop of the D3 dopamine receptor. Utilizing co-immunoprecipitation and glutathione- S-transferase (GST) pulldown strategies, we demonstrate that paralemmin interacts exclusively with D3, but not D2 or D4 dopamine receptors or β-adrenergic receptors. Immunocytochemistry demonstrated co-localization of paralemmin and D3 receptor in vivo in hippocampus and cerebellum and in vitro in glial and neuronal cultures. Deletion mutational analysis indicates that amino acids 154–230 of paralemmin strongly interacted with amino acids 211–227 and 281–330 of the third intracellular loop of D3 receptor. The consequences of these interactions were investigated by co-expression in HEK293 cells. Cell surface biotinylation experiments demonstrate that paralemmin decreased D3 receptor concentration at the plasma membrane. Consistent with this observation, paralemmin expression decreased dopamine-stimulated adenylate cyclase activity. However, paralemmin also decreased basal, isoproterenol and forskolin-stimulated adenylate cyclase activity, suggesting a more general cellular function for paralemmin. Taken together, paralemmin has been implicated as a potent modulator of cellular cAMP signaling within the brain.

Antagonistic effects of isofloxythepin on dopamine D 1 and D 2 receptors and behaviors in rats

1. 1. In vitro, isofloxythepin competed for the binding of [ 3H]SCH 23390 to striatal D 1 receptors and for the binding of [ 3H]spiperone to striatal D 2 receptors with IC 50 values of 6.1 (±1.2) × 10 10M and 8.4 (±2.6) × 10 −10 M, respectively. Isofloxythepin further inhibited the D 1 receptor-mediated, dopamine-stimulated adenylate cyclase in the striatal tissue. 2. 2. Fifteen hours after rats were injected with a single dose of isofloxythepin (1 mg/kg, SC), the ex vivo binding curve of [ 3H]spiperone to striatal D 2 receptors was markedly inhibited, whereas the binding curve of [ 3H]SCH 23390 to striatal D 1 receptors was unaffected. 3. 3. Fifteen hours after isofloxythepin pretreatment, D 1 agonist SKF 38393 (15 mg/kg, IP)-stimulated grooming behavior was not affected, whereas the D 2 agonist quinpirole (3 mg/kg, IP)-stimulated stereotyped behavior was completely abolished. 4. 4. On the basis of the findings from in vivo studies, we conclude that, although isofloxythepin is found to have high affinity for both D 1 and D 2 receptors in vitro, it lacks D 1 antagonistic potency in vivo.

Stimulation of adenylate cyclase activity by benzazepine D-1 dopamine agonists with varying efficacies in the 6-hydroxydopamine lesioned rat—relationship to circling behaviour

The ability of benzazepine D-1 dopamine agonists with varying efficacies in stimulating adenylate cyclase and to induce contralateral circling was investigated in rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. In the 6-hydroxydopamine lesioned rats, the benzazepines SKF 38393 (7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine), SKF 75670 (3-CH 3 analogue), SKF 80723 (6-Br analogue), SKF 83959 (6-Cl, 3-CH 3, 3′-CH 3 analogue), SKF 83565 (6-Cl, 3-CH 3, 3′-Cl analogue) and SKF 82958 (6-Cl, 3-C 3H 5 analogue), all produced contralateral circling. The rank order of efficacies (maximal effect, E max) being, SKF 83565 = SKF 75670 = SKF 83959 = SKF 80723 > SKF 38393 ⪢ SKF 82958. In striatal slices from the intact hemisphere, dopamine, SKF 82958, SKF 80723 and SKF 75670 stimulated adenylate cyclase activity. The rank order of efficacies being SKF 82958 (109%) = dopamine (100%) = SKF 80723 (98%) > SKF 75670 (72%). Although, SKF 38393 (67%), SKF 83565 (64%) and SKF 83959 (59%) tended to stimulate adenylate cyclase activity, this effect did not reach statistical significance. In the 6-hydroxydopamine lesioned hemisphere, basal levels of adenylate cyclase activity were lower (−25%) than in the intact hemisphere. The maximal stimulation of adenylate cyclase activity (expressed as % basal levels) produced by dopamine and the benzazepines in the denervated striatum was greater than observed in the intact striatum. The rank order of efficacies in the dopamine denervated striatum being SKF 82958 (124%) > SKF 80723 (109%) = dopamine (100%) > SKF 38393 (82%) = SKF 83959 (77%) = SKF 83565 (70%) > SKF 75670 (55%). Moreover, dopamine stimulated adenylate cyclase activity in the denervated striatum with greater potency than in the intact side. The ability of the benzazepine derivatives to induce circling in the 6-hydroxydopamine lesioned rat is consistent with the general increase in the efficacies of dopamine and benzazepine stimulated adenylate cyclase activity in the dopamine denervated striatum. However, the maximal effects for inducing circling and stimulating adenylate cyclase activity do not correspond (e.g. SKF 82958 and SKF 75670). This discrepancy may reflect the involvement of other factors including a behavioural role for extrastriatal D-1 dopamine receptors and/or transduction systems other than adenylate cyclase.

Maternal ethanol consumption: effects on G proteins and second messengers in brain regions of offspring.

Previous work in this and other laboratories has demonstrated that in utero ethanol exposure adversely affects the development of the serotonergic, dopaminergic, cholinergic, and other neurotransmitter systems. In several of these systems, receptor number is significantly altered. To determine whether the altered number of two G protein-linked receptors is reflected in changes in cell function, we examined dopamine-stimulated adenylate cyclase in the striatum and cortex and carbachol-stimulated phosphoinositide (PI) hydrolysis in the cortex. Serotonin-stimulated cortical PI hydrolysis was assessed for comparison. We also studied G proteins that link adenylate cyclase and other second messenger systems to their receptors. The G proteins that were analyzed include the alpha-subunits for Gs, G0, Gi1, Gi2, and Gi3. G proteins were analyzed in the cortex and cortical regions, as well as in the brain stem. The results of these experiments demonstrated that dopamine-stimulated adenylate cyclase activity was comparable in the striatum of 5- and 19-day offspring of control and ethanol-fed rats and in the motor cortex of 19-day offspring. We also found that carbachol- and serotonin-stimulated hydrolysis of cortical phosphoinositides was unchanged in ethanol-exposed offspring on gestational day 19, and on postnatal days 5 and 19. G protein content was examined by Western blot analysis, using antibodies directed against the alpha-subunits of Gs, G0, and the Gi1/Gi2 and Gi3/G0 combinations. These investigations indicated that, with two minor exceptions (approximately 10% change in the proteins detected by antibodies against the alpha-subunits of the Gi1/Gi2 and Gi3/G0 combinations), there were no significant differences in the content of any of the G proteins analyzed.

Dopamine and opioids interactions in sleep deprivation

Paola Fadda, M. Cristina Martellotta, Gianluigi Gessa and Walter Fratta : Dopamine and Opioids interactions in sleep deprivation: Prog.Neuro-Psychopharmacol. & Biol. Psychiat. 1993, 17 :(2) 269–278. 1. 1. Sleep deprivation induced by the platform technique is considered to be a heavy stressful situation in rats. At the end of the sleep deprivation period (72 hrs) the rat displayed particular behavior characterized by wakefulness, a high degree of motor and exploratory activity,increased alertness and reactivity to enviromental stimuli. 2. 2. Our results indicate that this behavior is potently antagonized by the administration of D1 antagonist SCH 23390 and by the opioid antagonist naloxone. 3. 3. We also show that concomitantly to this behavior, an increased number of D1 receptors associated with an increased dopamine-stimulated adenylate cyclase activity is present in the limbic system but not in the striatum of these animals. On the contrary, a decreased Bmax of mu and delta opioid receptors was found in the same brain areas. 4. 4. These data suggest an active role of limbic dopamine and opioid system in the generation of arousal and insomnia related to sleep deprivation-induced stress.

Adenylate cyclase from sea urchin eggs is positively and negatively regulated by D-1 and D-2 dopamine receptors

The adenylate cyclase present in membranes prepared from sea urchin eggs is sensitive to dopamine stimulation. The receptor sites coupled to sea urchin adenylate cyclase were characterized by means of specific agonists and antagonists. The D-1 dopamine agonist SKF-38393 was able to stimulate enzyme activity, while the two D-1 dopamine antagonists, SCH-23390 and SKF-83566, suppressed the stimulatory effect of dopamine. In addition, the D-2 dopamine agonists, PPHT and metergoline, brought about a dose-dependent inhibition of dopamine-stimulated adenylate cyclase activity. These data show that: (i) in sea urchin eggs adenylate cyclase is regulated by dopamine receptors; (ii) these receptors share characteristics with D-1 and D-2 dopamine receptors present in the mammalian brain.

Dopamine D 1 and opioid receptor binding changes in the limbic system of sleep deprived rats

Sleep deprivation induced by the platform technique is considered to be a heavy stressful situation in rats. At the end of the sleep deprivation period (72 h) the rat displayed particular behavior characterized by wakefulness, a high degree of motor and exploratory activity, increased alertness and reactivity to environmental stimuli. Our previous results indicated that this behavior was potently antagonized by the administration of the D 1 selective antagonist SCH 23390 and by the opioid antagonist naloxone. In this paper we show that concomitantly to this behavior, an increased number of D 1 receptors associated with an increased dopamine-stimulated adenylate cyclase activity is present in the limbic system but not in the striatum of these animals. On the contrary, a decreased Bmax of mu and delta opioid receptors was found in the same brain area. These data suggest an active role of limbic dopamine and opioid systems in the generation of arousal and insomnia related to sleep deprivation-induced stress.

Autoradiographic identification of D 1 dopamine receptors labelled with [ 3H]dopamine: Distribution, regulation and relationship to coupling

On the basis of experiments made on striatal membranes, Leff and Creese [ Molec. Pharmac. (1985) 27, 184–192] have proposed that tritiated dopamine binds to a high-affinity agonist state of D 1 dopamine receptors (D 1h) which adopt this conformation when they are associated with the GTP-binding protein involved in the transduction process. Quantitative autoradiography was thus used to look for the distribution of these D 1h sites in the rat brain and to compare it with that of D 1 receptors labelled with [ 3H]7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine ([ 3H]SCH23390), a D 1 antagonist. The effects of unilateral 6-hydroxydopamine lesion of the ascending dopamine pathways on the density of [ 3H]dopamine D 1h and [ 3H]SCH23390 binding sites in the striatum and the nucleus accumbens were also analysed. In the striatum, when D 2 receptors were blocked by spiroperidol (20 nM), [ 3H]dopamine was found to bind specifically to dopamine receptors of the D 1 type. Complementary experiments made with dopamine uptake blockers indicated that high-affinity dopamine uptake sites were not labelled by [ 3H]dopamine under our experimental conditions. The anatomical distribution of [ 3H]dopamine D 1h binding sites was found to be markedly different from that of [ 3H]SCH23390 binding sites. This was particularly the case in the substantia nigra, some amygdaloid nuclei and the prefrontal cortex—structures in which the ratios between [ 3H]SCH23390 and [ 3H]dopamine binding sites were more than seven-fold higher than that observed in the striatum. [ 3H]SCH23390 binding was not significantly affected in either the striatum or the nucleus accumbens six weeks after a complete unilateral destruction of ascending dopamine pathways. In contrast, a marked decrease in [ 3H]dopamine D 1h binding sites was found in both structures, but this effect was lower in the medioventral (−60%) than in the laterodorsal (− 81%) part of the striatum, even though dopamine denervation was uniform throughout the structure. Preincubation of the sections with dopamine (0.5 μ M) led to a partial recovery (+ 126%) in the lesioned striatum and an increase of [ 3H]dopamine labelling in the control striatum (+68%). This suggests that the presence of dopamine stabilizes the D 1h state of D 1 receptors. The absence or low amounts of dopamine, either due to dopamine denervation or naturally occurring (prefrontal cortex), would then impair the [ 3H]dopamine D 1h binding. In addition, a lower coupling of D 1 receptors with adenylate cyclase was observed in the substantia nigra when compared to that in the striatum: this may explain the relatively weak [ 3H]dopamine binding in the substantia nigra. Similarly, the lower loss of [ 3H]dopamine binding sites seen specifically in the medioventral part of the striatum in the 6-hydroxydopamine-lesioned rats could be related to a supersensitive dopamine-stimulated adenylate cyclase activity found in this striatal area [Herve´ et al. (1989) J. Neurosci. 9, 3699–3707], In conclusion, the D 1h state of the D 1 receptors is dependent on the endogenous dopamine levels and on the intensity of the D 1 receptor coupling. In order to explain both influences, it is proposed that the D 1h binding sites observed in brain sections are generated by the dissociation of a ternary complex in which dopamine, D 1 receptor and GTP-binding protein are associated.

Effect of opioid agonists selective for three subclasses of opioid receptors and one sigma receptor agonist on dopamine stimulated adenylate cyclase in rat caudate nucleus.

The adenylate cyclase in rat caudate nucleus homogenate could be stimulated by dopamine and less potently by the dopamine D1 receptor specific agonist SKF38393. Agonists selective for mu[D-Ala2, MePhe4Gly(ol)5]enkephalin (DAGO) and delta opioid receptors [D-Pen2, D-Pen5]enkephalin (dPen-dPen), inhibited the dopamine but not the dopamine D1 stimulated adenylate cyclase. The kappa opioid agonist, U69593, had no effect, probably due to low kappa receptor contents in rat caudate nucleus. 10(-4) M of the sigma receptor specific agonist, 1,3-di-o-tolylguanidine (DTG), potentiated the dopamine as well as the dopamine D1 stimulated adenylate cyclase while lower concentrations of DTG had no effect.

Enhanced adenylate cyclase activity in neonatally dopamine lesioned rats is related to increased G s-protein coupling

It has previously been shown that neonatal selective lesions of the central dopamine system with 6-hydroxydopamine system with 6-hydroxydopamine lead to increased basal and dopamine-stimulated adenylate cyclase activity in striatum without any alterations of dopamine receptor binding characteristics. In the present study, it was shown that adenylate cyclase activity following G-protein stimulation by the GTP analogue 5-guanylimidodiphosphate (Gpp(NH)p) was increased in striatal preparations from neonatally 6-hydroxy-dopamine-lesioned rats, compared with control animals. No difference was seen in forskolin-stimulated enzyme activity between the two groups. These results indicate that neonatal dopamine lesions induce a selective functional supersensitivity at the D 1 receptor complex by enhancing the coupling efficiency of the G s protein to adenylate cyclase, without alternating the catalytic activity of the enzyme.

Dihydroergotoxine treatment improves active avoidance performance and increases dopaminestimulated adenylate cyclase activity, in young and aged rats

Dihydroergotoxine (DHET) improved the acquisition of a conditioned avoidance response in both young and old rats, although old animals, reached a lower percentage of conditioned responses than did young rats. DHET dose-dependently reduced locomotor activity in aged animals, while, in young rats, increasing doses caused a triphasic effect, inhibitory at low doses, stimulatory at intermediate doses and again inhibitory at high doses. In young rats, the stimulation of spontaneous movements by DHET was abolished by L-sulpiride; L-sulpiride had no effect in old animals. DHET treatment also potentiated both norepinephrine- and, to a greater extent, dopamine-stimulated frontal cortex adenylate cyclase activity, an effect more pronounced in young than in old rats. These data show that DHET effects both locomotor activity and active avoidance performance in young and aged rats, and modulates dopamine-sensitive adenylate cyclase activity.

Dopaminergic Unique Affinity of Tetrahydroberberine and <i>l</i>-Tetrahydroberberine-<i>d</i>-Camphor Sulfonate

l-Tetrahydroberberine-d-camphor sulfonate (THB-CS) possessed an inhibitory effect on apomorphine-induced chewing movement in a similar manner to that of tetrahydroberberine (THB). Both compounds enhanced barbiturate-induced hypnosis. They did not have an anticonvulsant effect on convulsive seizures induced by bicuculline, pentetrazole or strychnine. THB and THB-CS blocked dopamine-stimulated adenylate cyclase activity. These compounds showed almost equipotent affinities to dopamine D1 (3H-SCH-23390) and D2 (3H-spiperone) receptors but did not have significant affinity to mu-opioid, muscarinic and alpha 2-adrenergic receptors, and benzodiazepine binding sites. Furthermore, both compounds did not elicit cataleptogenic behavior, even at very high doses. These data suggest that THB and THB-CS have a central depressant effect through both D1 and D2 dopaminergic receptors and may have different modes of action from that of standard neuroleptics.

Sleep deprivation increases dopamine D 1 receptor antagonist [ 3H]SCH 23390 binding and dopamine-stimulated adenylate cyclase in the rat limbic system

Sleep deprivation induced by the platform technique is considered to be a heavy stressful situation in rats. At the end of the sleep deprivation period (72 h) the rats displayed particular behaviour characterized by wakefulness, a high degree of motor and exploratory activity, increased alertness and reactivity to environmental stimuli. Our previous results indicated that this behaviour was potently antagonized by the administration of the D 1-selective antagonist SCH 23390. In this paper we show that concomitantly to this behaviour, an increased number of D 1 receptors associated with an increased dopamine-stimulated adenylate cyclase activity is present in the limbic system but not in the striatum of these animals. These data suggest an active role of limbic D 1 receptors in the generation of arousal and insomnia related to sleep deprivation induced stress.

Desensitization of the dopaminergic system in bovine retina following incubation with high potassium

The effect of potassium depolarization on dopamine D1 receptor activity in bovine retina was investigated. Preincubation of bovine retinas in buffer containing high KCl (56 mM) as compared to a low KCl control buffer resulted in a significant decrease in dopamine-stimulated adenylate cyclase activity with no change in basal or GTP-stimulated adenylate cyclase activity. The apparent Vmax for dopamine was decreased from 102 +/- 15 pmol/min/mg protein in retinas preincubated in high KCl to 71 +/- 11 pmol/min/mg protein in control retinas (n = 5). The apparent Ka for dopamine stimulation of the enzyme did not change. The potassium-induced desensitization could be blocked by preincubation with the dopamine antagonist cis-flupenthixol suggesting that the desensitization was caused by the release of dopamine. The rapid desensitization was not accompanied by a change in D1 receptor density as assessed by binding of [3H]SCH23390 nor in agonist binding as assessed by competition of the selective D1 agonist, SKF38393, for [3H]SCH23390 binding. The potassium-induced desensitization was mimicked by preincubation of retinas in control medium containing isobutylmethylxanthine or dibutyryl cyclic AMP. Incubation of retinas in 56 mM KCl also led to a decrease in activation of adenylate cyclase by vasoactive intestinal polypeptide. These results strongly suggest that potassium depolarization leads to a very rapid heterologous desensitization of adenylate cyclase in bovine retinas.

Changes in the D 1 receptor-adenylate cyclase complex after priming

The D 1 agonist, SKF 38393, failed to induce contralateral turning in drug-naive rats lesioned unilaterally with 6-hydroxydopamine from 17 days. Priming with a dopamine agonist, such as the D 2 agonist, LY 171555, three days before, made SKF 38393 fully effective in inducing contralateral turning. Analysis of D 1 receptor binding in striata of drug-naive and primed rats showed no change in the B max and K d. In contrast, dopamine-stimulated adenylate cyclase showed a decrease in its K m for dopamine in the lesioned side of primed rats as compared with drug-naive rats. Thus, priming appears to elicit changes at the level of the transduction mechanism of D 1 receptors rather than in the D 1 recognition site itself.

Biochemical pharmacology of the atypical neuroleptic remoxipride

In vitro receptor ligand binding studies in the rat showed that remoxipride displaced different radioligands at the dopamine D2, but not the D1 receptor. Remoxipride did not block dopamine-stimulated adenylate cyclase activity in vitro suggesting that it did not directly interact with the dopamine D1 receptor. Like other antipsychotic compounds, it increased dopamine turnover in the dopamine-rich areas of the brain. It showed no affinity for a wide range of neurotransmitter receptors, with the exception of the opiate sigma receptor. The affinity of remoxipride for the D2 receptor was low in vitro, while in vivo, the affinity was relatively high. Remoxipride was far more potent in preventing [3H]raclopride-binding than [3H]spiperone-binding to the D2 receptor in vivo. When the D2 receptor was labelled with [3H]spiperone, remoxipride was shown to exert a preferential blockade of this binding in extrastriatal areas of the brain (for example, olfactory tubercle, septum, substantia nigra) in vivo. After the injection of high doses of remoxipride most if not all drug in the brain was identified as authentic remoxipride. After injection of [3H]remoxipride in smaller and larger doses, radioactivity was detected in all areas of brain examined, including cerebellum and neocortex. Most of the remoxipride-derived radioactivity was found in the choroid plexus and circumventricular organs, while smaller amounts were recovered in the striatum, olfactory tubercle, and substantia nigra. Taken together, these findings suggest that remoxipride acts at both the central D2 and sigma receptors and that its affinity for the D2 receptor is relatively low in vitro. A regional preference for D2 receptors can be observed in vivo depending upon the radioligand used.

Triadimefon, a triazole fungicide, induces stereotyped behavior and alters monoamine metabolism in rats

Triadimefon, a triazole fungicide, has been observed to increase locomotion and induce stereotyped behavior in rodents. The present experiments designed to characterize the stereotyped behavior induced by triadimefon used a computer-supported observational method, and tested the hypothesis that these observed effects involved central dopaminergic systems. Adult male and female Sprague-Dawley rats were injected with triadimefon (0, 50, 100, and 200 mg/kg) in corn oil (2 ml/kg ip) 4 hr prior to behavioral assessment. The two lowest doses of triadimefon increased the frequency of locomotion and rearing, while the highest dose induced highly stereotyped behaviors, including backward locomotion, circling, and head weaving. Immediately after behavioral testing, the rats were sacrificed, and the striata and olfactory tubercles, terminal fields of the nigrostriatal and mesolimbic dopamine systems, respectively, were removed. Steady-state concentrations of the monoamines dopamine and serotonin and their metabolites were determined by HPLC-EC. In independent experiments, the direct effects of triadimefon on dopamine (D 1 and D 2) receptor binding and dopamine-sensitive adenylate cyclase activity were assessed in vitro using rat striata. Dopamine concentrations were increased in olfactory tubercles, but decreased in striatum. Concentrations of 5-hydroxyindoleacetic acid (the major metabolite of serotonin) were increased only in striatum, and only in animals treated with 200 mg/kg triadimefon. In vitro, triadimefon neither competed with D 1 or D 2 dopaminergic radioligands nor affected dopamine-stimulated adenylate cyclase activity. Together these behavioral and biochemical data lend support to the hypothesis that triadimefon may have actions similar to those produced by indirect-acting dopamine agonists.

Denervation and hyperinnervation in the nervous system of diabetic animals: III. Functional alterations of G proteins in diabetic encephalopathy

G protein-mediated effects on cAMP production were evaluated in the corpus striatum of diabetic rats 5 and 14 weeks after alloxan injection by measuring both D1-receptor-induced stimulation and D2-receptor-mediated inhibition of adenylate-cyclase activity. At 5 weeks of diabetes, no obvious alterations of G protein functions were detected. Both dopamine-stimulated adenylate cyclase and bromocriptine-induced inhibition of enzyme activity were indeed similar in control and diabetic animals. Fourteen weeks after alloxan injection, profound alterations were observed. Dopamine-stimulated cAMP production was markedly increased in diabetic rats, whereas bromocriptine ability to reduce cAMP formation was almost abolished at this late stage of diabetes. Hypoactivity of Gi/Go proteins was also confirmed by the reduced ability of the GTP non-hydrolyzable analog GTP-gamma-S to inhibit forskolin-stimulation of adenylate cyclase. These results show an apparent functional imbalance between Gs and Gi/Go-mediated transduction mechanisms, with an increased efficacy of Gs activity likely due to the loss of Gi/Go inhibitory functions. Concomitantly with such transductional alteration detected in chronic diabetes, we observed a marked increase of the striatal content of met-enkephalin, which is known to utilize Gi/Go proteins for inhibition of adenylate cyclase. The measurement of other transmitters (vaso-active intestinal peptide, substance P, serotonin, noradrenaline, and dopamine) did not reveal any difference with respect to controls. The observed transductional defect in diabetic animals and the increased content and/or hyperinnervation by the metenkephalinergic system could be correlated as mutual compensatory mechanisms.