Studies on the involvement of dopamine D-1 and D-2 receptors in the anticonvulsant effect of dopamine agonists in various rodent models of epilepsy

Dopamine agonists with different selectivity for dopamine D-1 and D-2 receptors in the brain were tested for their effects: on thresholds for maximal electroshock seizures in mice and rats and for pentylenetetrazol-induced clonic seizures in mice; on seizures induced by air blast stimulation in gerbils, and on seizures induced by amygdala-kindling in rats. The mixed D-1/D-2 agonist apomorphine exerted anticonvulsant effects in all models except kindling. In gerbils and mice, the anticonvulsant action of apomorphine could be antagonized by the D-2 selective dopamine antagonist sulpiride. When injected alone, sulpiride exerted no significant effect on seizure activity. The preferential D-2 receptor agonists lisuride and (+)-PHNO [+)-4-propyl-9-hydroxynaphthoxazine) differed in their profile of action. Both compounds displayed anticonvulsant efficacy in gerbils, while only lisuride proved capable of reducing kindled seizure severity. (+)-PHNO increased the threshold for electroconvulsions in mice while lisuride was ineffective in this respect or even decreased the threshold. The reverse was obtained in regard to electroshock seizures in rats. The threshold for seizures induced by pentylenetetrazol in mice was increased significantly by lisuride but not by (+)-PHNO. The selective dopamine D-1 receptor agonist SKF 38393-A exerted no anticonvulsant effect in any seizure test except a moderate increase of the electroconvulsive threshold in mice. In contrast, the dopamine precursor L-DOPA (injected after pretreatment with carbidopa) proved capable of reducing seizure activity in all models. In mice, the increase in the threshold for maximal electroshock seizures induced by L-DOPA was significantly reduced by sulpiride, which also attenuated the anticonvulsant effect of L-DOPA in gerbils. Collectively, the data indicate that dopamine D-2 receptors mediate the anticonvulsant effect of dopamine agonist and, at least in part, of L-DOPA whereas D-1 receptors seem not to be involved.     

Aporphine enantiomers. Interactions with D-1 and D-2 dopamine receptors

The R(-) and the S(+) enantiomers of apomorphine (APO) and N-n-propyl norapomorphine (NPA) interact with both the D-1 and the D-2 dopamine receptors. R(-)-APO, as well as R(-)- and S(+)-NPA, stimulates the D-1 dopamine receptor in carp retina; S(+)-APO blocks this dopamine receptor. Similarly, R(-)-APO, as well as R(-)- and S(+)-NPA, stimulates the D-2 dopamine receptor in the intermediate lobe of the rat pituitary gland; S(+)-APO blocks the intermediate lobe D-2 receptor. The interactions between these aporphine enantiomers and the D-1 and the D-2 dopamine receptors exemplify several manifestations of the previously described "n-propyl phenomenon." Because S(+)-APO is distinguished from the other tested aporphines by its ability to antagonize either the D-1 or the D-2 dopamine receptors, it is hypothesized that the presence of an N-methylated tertiary amine in a molecule of appropriate configuration can confer dopamine receptor antagonist activity to the molecule.     

Permissive role of D-1 receptor stimulation by endogenous dopamine for the expression of postsynaptic D-2-mediated behavioural responses. Yawning in rats

Low doses of BHT 920, LY 171555 and (+)3PPP, three dopamine agonists selective for D-2 receptors, induced yawning in rats. This effect was reduced by the selective D-1 antagonist SCH 23390 but the antagonism did not exceed a 50% reduction from the control values. In contrast, the selective D-2 antagonist (-)sulpiride completely abolished agonist-induced yawning. A 6 h reserpine pretreatment (5 mg/kg i.p.), which depletes brain dopamine (DA) by about 95%, reduced agonist-induced yawning by an extent similar to SCH 23390; in the reserpinized rats, SCH 23390 completely lost the property of blocking agonist-induced yawning while (-)sulpiride retained it. Two 5HT receptor antagonist, ketanserin and metergoline failed to influence agonist-induced yawning. The reportedly selective D-1 agonist, SKF 38393, failed to induce yawning in normal rats as well as in rats pretreated with reserpine 6 or 16 h earlier. If one excludes that SCH 23390 and the D-2 agonists interact with the same DA-receptors, the data are consistent with the possibility that stimulation of D-1 receptors by endogenous DA plays a permissive-facilitatory role for the behavioural expression of D-2 receptor activation.     

Topography of dopamine D-1 and D-2 receptor-mediated rotation after intrastriatal injections of dopamine-related drugs in normosensitive rats

Naive rats were challenged systematically with apomorphine after receiving unilateral dorsal or ventral intracaudate injections of the dopamine D-1 receptor antagonist, SCH23390, or the D-2 receptor antagonist, sulpiride. Sulpiride injections into both the dorsal and ventral striatum induced a robust ipsilateral rotation, while SCH23390 elicited a weaker ipsilateral rotation only on injection into the ventral striatum. Both drugs were ineffective in saline-treated rats, although sulpiride injections into the ventral striatum after systemic saline elicited a small ipsilateral preference. The results from this rotational model mediated by normosensitive receptors indicate that only dopamine D-1 receptors in the ventral striatum mediate rotation while D-2 receptors in both striatal regions mediate rotation. A functional dichotomy between these two neostriatal regions is thus proposed.     

Dopamine D-2 receptor imaging radiopharmaceuticals: synthesis, radiolabeling, and in vitro binding of (R)-(+)- and (S)-(-)-3-iodo-2-hydroxy-6-methoxy-N- [(1-ethyl-2-pyrrolidinyl)methyl]benzamide

In developing central nervous system (CNS) dopamine D-2 receptor imaging agents, enantiomers, R-(+) and S-(-) isomers, of 3-[125I]iodo-2-hydroxy-6-methoxy-N-[(1-ethyl-2- pyrrolidinyl)methyl]benzamide, [125I]IBZM, were synthesized, and their in vitro binding characteristics were evaluated in rat striatum tissue preparation. The (S)-(-)-[125I]IBZM showed high specific dopamine D-2 receptor binding (Kd = 0.43 nM, Bmax = 0.48 pmol/mg of protein). Competition data of various ligands for IBZM binding displayed the following rank order of potency: spiperone greater than (S)-(-)-IBZM greater than (+)-butaclamol much greater than (R)-(+)-IBZM greater than (S)-(-)-BZM greater than dopamine greater than ketanserin greater than SCH23390 much greater than propanolol. The results indicate that [125I]IBZM binds specifically to the dopamine D-2-receptor with stereospecificity. The [123I]IBZM is potentially useful as an imaging agent for the investigation of dopamine D-2 receptors in humans.     

Biochemical characterization of the enantiomers of the novel ergoline dopamine agonist 6-ethyl-9-oxaergoline (EOE)

The enantiomers of the novel oxaergoline dopamine (DA) agonist, 6-ethyl-9-oxaergoline (EOE) were compared with the parent recemate for their interactions with dopamine and other monoamine receptor systems in mammalian and teleost nervous tissue. The (?)enantiomer of EOE was over 100 times more active in displacing [³H]apomorphine than the (+)isomer, and 70 times more active in displacing [³H]spiperone. The (?)isomer also exhibited greater binding affinity at serotonin-1 and -2 receptors than the (+)isomer. In contrast, however, the α₁- and α₂-adrenoceptor activity of the compound resided in the (+)isomer, the latter being some 32 times more active at α₂ than α₁ adrenoceptors. Examination of EOE in the DA-sensitive adenylate cyclase system of carp retina showed that while the (?)enantiomer was a partial agonist some 30 times less active than dopamine, the (+)enantiomer was devoid of activity in this D-1 coupled system at concentrations up to 300 μM.     

Role of D-1 and D-2 receptors in apomorphine-induced pecking in chicks

The possible involvement of subtypes of dopamine (DA) receptors in pecking induced by apomorphine (APO) in chicks was studied. D-1/D-2 agonist APO dose-dependently induced pecking in chicks. The APO response was decreased in animals pretreated with either the D-2 receptor antagonist sulpiride or the D-1 receptor antagonist SCH 23390. The inhibitory effects of both antagonists were also dose dependent. The pecking induced by APO was completely inhibited in animals pretreated with a combination of SCH 23390 and sulpiride and was potentiated with reserpine. Single administration of D-2 agonist quinpirole or D-1 agonist SKF 38393 did not induced pecking, although quinpirole, but not SKF 38393 caused considerable response in reserpine or reserpine + -methyl-p-tyrosine (AMPT)-treated animals. When quinpirole was administered with SKF 38393, a slight pecking response was shown. This was also potentiated in reserpine or reserpine + AMPT-treated chicks. The results may indicate that both D-1 and D-2 DA receptors are involved in pecking induced by APO, and reserpine treatment caused the sensitization of the D-2 receptors for the induction of pecking in chicks.     

Role of dopamine D-1 and D-2 receptors in the regulation of neurotensin systems of the neostriatum and the nucleus accumbens

Activation of dopamine D-1 receptors with multiple administrations of SKF 38393 significantly increased the level of neurotensin-like immunoreactivity in the striatum and the nucleus accumbens. However, a similar treatment with the D-2 receptor-selective agonist, LY 171555, decreased the same in both structures; when the two drugs were administered concureently, their individual effects were blocked. These results suggest that dopamine D-1 and D-2 receptors antagonistically regulate neurotensin systems of the striatum and nucleus accumbens. On the other hand, blockade of D-2 receptors (with sulpiride) elevated, while D-1 receptors blockade (with SCH 23390) caused no change in the level of neurotensin in both these structures. Dopamine D-1 receptors did not appear to contribute to the sulpiride-mediated effect as concurrent administration of SCH 23390 did not alter the response.     

Effects of dopamine receptor antagonists on gastrin and vomiting responses to apomorphine

Summary Apomorphine (0.05 mg/kg intravenously) was given to conscious dogs, and gastrin levels were measured in peripheral venous blood with a radioimmunoassay. Apomorphine induced an increase of gastrin levels which peaked at 5 min. The peripheral dopamine D-2/DA₂ receptor antagonist domperidone (0.2 mg/kg), but not halopemide (0.1–1 mg/kg) nor the D-1 /DA₁ receptor antagonist SCH 23390 (0.1 mg/kg), blocked the gastrin response to apomorphine. Both domperidone and halopemide, but not SCH 23 390, blocked the Apomorphine-induced vomiting. These results suggest that apomorphine increases gastrin levels by an action at D-2/DA₂ receptors, which are situated outside the blood brain barrier and differ from the receptor inducing the vomiting. Send offprint requests to M. Goiny at the above address     

Blockade of both D-1 and D-2 dopamine receptors may induce catalepsy in mice.

1. The catalepsy induced by dopamine antagonists has been tested and the possible dopamine subtypes involved in catalepsy was determined. 2. Dopamine antagonist fluphenazine, D-1 antagonist SCH 23390 or D-2 antagonist sulpiride induced catalepsy. The effect of fluphenazine and sulpiride was dose-dependent. Combination of SCH 23390 with sulpiride did not induce catalepsy potentiation. 3. D-1 agonist SKF 38393 or D-2 agonist quinpirole decreased the catalepsy induced by fluphenazine, SCH 23390 or sulpiride. 4. Combination of SKF 38393 with quinpirole did not cause potentiated inhibitory effect on catalepsy induced by dopamine antagonists. 5. The data may indicate that although D-2 receptor blockade is involved in catalepsy, the D-1 receptor may plan a role.     

Effect of D-1 or D-2 receptor stimulation on memory retrieval in mice

Mice were trained in one-way active avoidance procedure and retention was tested at 4, 8, 16 and 24 h after training of animals and compared with non-shocked or untrained animals. The effect of drugs was tested on retrieval 24 h after training in other groups of mice. High doses of apomorphine or bromocriptine impaired, while low doses of the drugs improved, retrieval of avoidance. High doses of sulpiride reversed the impairment induced by high doses of these dopamine agonists. Low doses of sulpiride antagonized the improvement of retrieval induced by low doses of apomorphine. SKF 38393 treatment of animals also improved the retrieval. The retrieval impairment induced by higher doses of apomorphine or the improvement induced by different doses of SKF 38393 was antagonized by SCH 23390 pre-treatment. Single administration of SCH 23390 or low doses of sulpiride also impaired retrieval. It is concluded that stimulating post-synaptic D-2 dopamine receptors impairs retrieval whilst activation of pre-synaptic D-2 or post-synaptic D-1 receptors improves memory retrieval.     

Repeated administration of (-)sulpiride and SCH 23390 differentially up-regulate D-1 and D-2 dopamine receptor function in rat mesostriatal areas but not in cortical-limbic brain regions

We studied the possible functional modifications of both D-1 and D-2 dopamine (DA) receptor subtypes following repeated administration of DA antagonists that act selectively on a single class of DA receptors. The functional state of D-1 and D-2 DA receptors in particular was evaluated by measuring SKF 82526-stimulated and bromocriptine-inhibited adenylate cyclase activity in different brain regions of rats treated with saline, SCH 23390, or (-)sulpiride for 21 days. The results indicate that chronic blockade of D-1 DA receptors in striatum, nucleus accumbens, and substantia nigra by SCH 23390 induced up-regulation of the D-1 receptors without changing the functional activity of D-2 receptors. Likewise, chronic blockade of D-2 DA receptors by (-)sulpiride caused up-regulation of D-2 but not D-1 DA receptors in striatum, nucleus accumbens, substantia nigra and pituitary. SCH 23390 or (-)sulpiride did not modify the functional activity of either D-1 or D-2 DA receptors located in frontal cortex and hippocampus. In conclusion, these results indicate that chronic treatment with selective D-1 or D-2 DA receptor blockers induces a receptor-specific up-regulation which involves the DA receptors located in the nigrostriatal system and pituitary but not those in the limbic-cortical areas.     

N-0437: a selective D-2 dopamine receptor agonist in in vitro and in vivo models

The selectivity of the potent dopamine D-2 agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) was examined in a series of in vivo and in vitro pharmacological models. In radioligand binding assays, N-0437 showed high potency (Ki = 0.69 nM) and selectivity for D-2 receptors as compared to its potency and selectivity at various other neuronal receptors (Ki in nM): D-1 (678) dopamine, alpha 1-(534) and alpha 2-(195) adrenoceptor, S1-(6940) and S2-(5900) serotonin and muscarine (2660). Very low activity (Ki greater than 10(-5) M) was seen at the beta-adrenoceptor, A1-adenosine, GABAA and benzodiazepine receptors. Furthermore, N-0437 inhibited the calcium-dependent release of [3H]dopamine (IC50: 4 nM) and [3H]acetylcholine (IC50: 6.3 nM) from rabbit striatal slices in the nanomolar range. These effects of N-0437 were mediated through activation of D-2 dopamine autoreceptors and D-2 dopamine heteroreceptors, respectively. Presynaptic dopaminergic activity in vivo was measurable as an inhibition of the locomotor activity of mice, and in this model N-0437 was more effective than apomorphine. Moreover, the effect of N-0437 could be antagonized by sulpiride but not by yohimbine. N-0437 was equipotent with apomorphine in inducing circling behaviour in 6-OHDA-lesioned rats. N-0437 had almost no serotonergic activity in vivo. The results show that N-0437 is a selective dopamine D-2 agonist, and thus, that it is a new ligand of choice for studies on the D-2 receptor.     

Effects of the dopamine D-1 antagonist SCH 23390 and the D-2 antagonist sulpiride on saline acceptance-rejection in water-deprived rats

Separate groups of water-deprived rats were familiarized with drinking water or one of a range of NaCl solutions (0.45-2.7%) in a 30 min test. The substituted benzamide, sulpiride, a selective dopamine D-2 receptor antagonist, significantly increased the consumption of water and hypotonic saline at 30 mg/kg. In contrast, the selective dopamine D-1 receptor antagonist, SCH 23390 (0.01-0.1 mg/kg SC) significantly reduced the intake of water and of saline at different concentrations in a dose-dependent manner. Consumption of water and 0.45% saline were most sensitive to the antidipsogenic effect of SCH 23390. These results suggest that previously-reported antidipsogenic effects of neuroleptics may depend, to at least some degree, on dopamine D-1 receptor blockade. The increase in drinking produced by sulpiride indicates that dopamine may act at D-2 receptors to inhibit the consumption of water and hypotonic saline, but not of stronger salt solutions.     

Changes in apomorphine-induced stereotypy as a result of subacute neuroleptic treatment correlates with increased D-2 receptors, but not with increases in D-1 receptors

Administration of haloperidol (5 mg/kg i.p.), cis-flupenthixol (2.5 mg/kg i.p.) or sulpiride (2 X 100 mg/kg i.p.) daily for 21 days followed by a 3-day drug withdrawal period caused equivalent cerebral dopamine receptor supersensitivity as judged by enhanced apomorphine-induced stereotypy. These treatments also produced equivalent rises in the number of adenylate cyclase-independent dopamine receptors (D-2) in both striatal and mesolimbic tissue as assessed by specific [3H]spiperone and [3H]N,n-propylnorapomorphine (NPA) binding. No change in the dissociation constant (KD) was apparent in response to neuroleptic treatment. However, only repeated administration of cis-flupenthixol caused an increase in the number of adenylate cyclase-linked dopamine receptors (D-1) in striatum as assessed by enhanced [3H]piflutixol binding and increased dopamine-stimulated cyclic AMP formation. The dissociation constant for [3H]piflutixol binding was unchanged by cis-flupenthixol administration. No change in D-1 receptor numbers or dopamine stimulation of adenylate cyclase occurred in mesolimbic tissue. Repeated treatment with sulpiride or haloperidol was without effect on either [3H]piflutixol binding to D-1 receptors or cyclic AMP formation. In conclusion, increased apomorphine-induced stereotypy following subacute neuroleptic treatment correlates with changes in D-2 receptor numbers, but not with changes in D-1 receptors.     

Dopamine D-1 and D-2 receptor agonists and antagonists and neuropeptide-induced excessive grooming

The administration of the dopamine D-1 receptor antagonist, SCH 23390, but not of the dopamine D-2 receptor antagonist, sulpiride, suppressed the excessive grooming induced by a new environment or by various neuropeptides. In addition, administration of the dopamine D-1 agonist, SK & F 38393, induced excessive grooming but that of the dopamine D-2 agonist, quinpirole, did not. It is suggested that dopamine D-1 rather than D-2 receptor stimulation is an important mechanism underlying novelty-induced as well as neuropeptide-induced excessive grooming.     

Stereoisomers of apomorphine differ in affinity and intrinsic activity at presynaptic dopamine receptors modulating [3H]dopamine and [3H]acetylcholine release in slices of cat caudate

We investigated the effects of R(?)-apomorphine and S(+)-apomorphine on dopamine receptors modulating electrically evoked [³H]dopamine and [³H]acetylcholine release from slices of cat caudate nucleus. R(?)-Apomorphine inhibited the release of both [³H]dopamine and [³H]acetylcholine with an IC₅₀ of 20 nM, while S(+)-apomorphine was without inhibitory action on the electrically evoked release of either neurotransmitter at concentrations up to 1 μM. At a concentration of 1 μM, however, S(+)-apomorphine antagonized the inhibition by R(?)-apomorphine, producing a parallel five-fold shift to the right in the concentration-response curve to R(?)-apomorphine. These results indicate that S(+)-apomorphine is devoid of intrinsic activity to stimulate presynaptic dopamine receptors modulating the electrically evoked release of dopamine and acetylcholine. In addition, S(+)-apomorphine has an approximately ten-fold lower affinity for presynaptic dopamine receptors compared to R(?)-apomorphine.     

Functional characterization of a rat dopamine D-2 receptor cDNA expressed in a mammalian cell line

We recently cloned a complementary DNA for the rat dopamine D-2 receptor, making it possible to create cell lines expressing this receptor. A cell line (LZR1) was created by transfecting the D-2 cDNA (RGB-2) into mouse fibroblast Ltk- cells. LZR1 cells, previously described as L-RGB2Zem-1 cells, express a high density of D-2 receptors, whereas the wild-type cells do not. A number of agonists competitively and stereoselectively inhibited the binding of [3H]spiroperidol to the expressed D-2 receptors in a GTP-sensitive manner. The potency of dopamine was decreased by the addition of GTP. NaCl and GTP together caused a further decrease in potency and increased the Hill slope for inhibition of radioligand binding by dopamine almost to 1.0. Pretreatment of cells with pertussis toxin inhibited high affinity binding of dopamine and prevented further inhibition of binding by GTP. The NaCl-induced decrease in affinity was not prevented by pertussis toxin treatment. Dopamine reduced forskolin-stimulated adenylate cyclase activity by 27% in membranes prepared from LZR1 cells. Inhibition by dopamine was blocked by (+)-butaclamol or prior treatment of intact cells with pertussis toxin. Other dopamine receptor agonists stereoselectively inhibited adenylate cyclase activity. These data indicate that the RGB-2 cDNA directs the expression of a dopamine D-2 receptor capable of interacting with guanine nucleotide-binding proteins and inhibiting adenylate cyclase activity. Furthermore, the RGB-2 cDNA provides a means of creating many cell lines that will be useful tools for the biochemical and pharmacological characterization of dopamine D-2 receptors.     

The enantiomers of SK&F 83566, a new selective D-1 dopamine antagonist, stereospecifically block stereotyped behaviour induced by apomorphine and by the selective D-2 agonist RU 24213

The non-selective dopamine receptor agonist apomorphine and the selective D-2 receptor agonist RU24213 each induced a typical syndrome of stereotyped behaviour characterised by sniffing and locomotion. The syndrome and its associated behaviours induced by each agonist were dose-dependently antagonised by pretreatment with the R- but not with the S-enantiomer of the benzazepine SK&F 83566. R- but not S-SK&F 83566 stereoselectively blocks D-1 receptors with high affinity, while the enantiomers have a very weak affinity for D-2 receptors which shows negligible stereoselectivity. Therefore, enantioselective blockade of brain D-1 receptors appears able to influence the expression of behaviours initiated by D-2 receptor stimulation.     

Differential behavioural interactions between the dopamine D-1 antagonist SCH 23390 and the dopamine D-2 antagonists metoclopramide and sulpiride in nonhabituated mice

This study investigated the effects of the selective D-1 antagonist SCH 23390, when administered alone and in combination with a typical (metoclopramide) or atypical neuroleptic (sulpiride), on species-typical behaviours in nonhabituated mice. When tested singly, all three compounds caused a progressive dose-dependent inhibition of locomotion, rearing and grooming, though their potencies varied widely. Mixtures of a threshold dose of 0.01 mg/kg SCH 23390 and metoclopramide (0.05-1.25 mg/kg) interacted synergistically to promote hypomotility and to decrease rearing, but did not affect grooming. By contrast, combinations of 0.01 mg/kg SCH 23390 and sulpiride (2-10 mg/kg) resulted in a marked potentiation of grooming, but only additive reductions in horizontal and vertical movements, consistent with sulpiride and metoclopramide occluding different populations of D-2 receptors. The results show that blockade of D-1 receptors and certain populations of D-2 receptors can interact positively to modify animal motor behaviour, and add a fresh perspective to the concept that these two types of dopamine receptor normally work interdependently to control movements of the body.