The discriminative stimulus properties of the D-1 agonist SK&F 38393 and the D-2 agonist (-)-NPA are mediated by separate mechanisms

The dopamine D-1 agonist SK&F 38393 (10 mg/kg) the D-2 agonist (-)-NPA (0.04 mg/kg) and d-amphetamine (1.0 mg/kg) were established as discriminative stimuli versus saline in rats. The stimulus induced by SK&F 38393 was stereoselective, since the R-(+)-, but not the S-(-)-enantiomer was effective. It was mimicked by two partial D-1 agonists with central effects, SK&F 75670 and Lu 24-040, but not by the peripheral agonist fenoldopam. D-2 agonists and d-amphetamine were ineffective. The effect of SK&F 38393 was antagonized by SCH 23390, but not by its inactive enantiomer SCH 23388 or by the D-2 antagonist YM 09151-2. The (-)-NPA stimulus was dependent on postsynaptic D-2 receptors: It was mimicked by quinpirole and pergolide in stimulant dosages, whereas the partial D-2 agonist (-)-3-PPP inhibited the effect of (-)-NPA. The dopamine synthesis inhibitor alpha-methyl-p-tyrosine did not antagonize the effect of (-)-NPA. Likewise, the above-mentioned D-1 agonists produced saline responding. D-amphetamine produced partial substitution to (-)-NPA. The (-)-NPA stimulus was blocked by YM 09151-2, but not by SCH 23390. In d-amphetamine-trained rats, quinpirole, (-)-NPA and pergolide produced generalization, whereas SK&F 38393 was ineffective. Both SCH 23390 and YM 09151-2 antagonized the effect of d-amphetamine. It is concluded that the cues induced by SK&F 38393 and (-)-NPA are mediated by separate D-1 and D-2 sites, whereas both sites contribute to the effect of d-amphetamine.     

Cerebellar GABAB receptors modulate function of GABAA receptors.

Interactions between GABAA and GABAB receptors were studied using muscimol-stimulated uptake of 36Cl- by membrane vesicles from mouse cerebellum. Baclofen inhibited muscimol-stimulated 36Cl- uptake and this action was more pronounced with longer flux times (30 vs. 3 s) and after predesensitization of GABAA receptors. Baclofen also inhibited 36Cl- flux by cortical membranes but was more effective with cerebellar preparations. The action of baclofen was stereoselective, calcium-dependent, and blocked by the GABAB receptor antagonist 2-OH-saclofen. It was mimicked by GTP-gamma-S but not by GDP-beta-S, which suggests that baclofen may be acting via a G protein. The action of baclofen was inhibited by U73122, an inhibitor of phospholipase C. However, the potassium channel blockers tetraethylammonium or Ba2+ did not affect the action of baclofen. The results show that activation of GABAB receptors can inhibit the function of GABAA receptors and suggest that this action involves either a nondesensitizing subtype of GABAA receptor or the rate or recycling of desensitized to nondesensitized receptors. We speculate that this action of baclofen results from activation of phospholipase C and phosphorylation of a subtype of GABAA receptor by protein kinase C.     

Ion and Temperature Effects on the Binding of γ-Aminobutyrate to Its Receptors and the High-Affinity Transport System

A set of procedures was developed to study the binding of gamma-[3H]aminobutyric acid ([3H]GABA) to GABAA and GABAB receptors, and to the Na(+)-dependent transport carrier, at 25 and 37 degrees C in the presence of physiological concentrations of Na+. The membrane preparation used in these procedures was not subjected to freeze-thawing or treatment with Triton X-100. Isoguvacine, (-)-baclofen, and (-)-nipecotate were used to block selectively the binding to GABAA receptors, GABAB receptors, and the transport site, respectively. Analysis of the binding characteristics of [3H]GABA to the GABAA receptor suggested the existence of high-(KD less than 30 nM), middle- (KD = 100-500 nM), and low-affinity (KD greater than 5 microM) binding sites. However, the binding data in the middle-affinity region (100-1,000 nM) were often indicative of cooperativity. The affinity between GABA and the GABAA receptor was reduced modestly by increases in temperature and by the presence of Cl- at physiological concentrations. Binding to the GABAB receptor required Ca2+ and Cl-. Apparent binding to the transport carrier required both Na+ and Cl-. A comparison of Bmax values in three brain regions revealed an inverse relationship between the high-affinity site of the GABAA receptor and the transport binding site.     

The role of the dopaminergic system and GABA-benzodiazepine- ionophore complex in the regulation of memory retrieval

The spontaneous forgetting model has been used to demonstrate the possibility of the memory forgetten trace extraction under the dopamine reuptake blockade by nomifensine and bupropion, increase of its quantity by amfonelic acid, activation of the postsynaptic dopaminergic receptors by (+)3-PPP, blockade of the latter by (-)3-PPP, and under the blockade of separate links of the GABA-benzodiazepine-ionophore complex by bicuculline, picrotoxin, flumazepil and R015-3505. Effectiveness of the neuropharmacological actions improving the memory forgotten trace retrieval is shown to depend upon the duration of the spontaneous forgetting process. The presynaptic receptors are involved in the retrieval process control--improvement of the conditioned habit performance after forgetting due to the activation of presynaptic dopaminergic receptors by specific agonist (-)3-PPP is clearly correlated with the initial retrieval level. The above facts underlie a hypothesis about the neurochemical forgetting mechanisms.     

Contribution of GABA receptors in extinction of memory trace in norm and depressive-like state

The dependence of activation and blockade of GABA receptors influences on extinction of passive avoidance response from a type of receptors and initial psychoemotional state of mice is shown. The activation of GABAA receptors by muscimol disrupted extinction in norm and did not influence on delay of this process at mice with "behavioral despair". The activation of GABAB receptors by baclofen accelerated extinction of fair memory at mice with depressive-like state. The blockade of GABAA receptors by bicuculline was ineffective in modification of extinction. The blockade of GABAB receptors by phaclofen promoted retention of fear expression at intact mice and facilitation of extinction at "depressive" mice.     

Supersensitivity of sigma receptors after repeated administration of cocaine.

We investigated the role of sigma receptors in the expression of behavioral sensitization induced by cocaine. Rats received intraperitoneal injections of either 20 mg/kg cocaine or saline once daily for 14 consecutive days. Cocaine-treated rats became sensitized. After a 5-day abstinence period, a challenge dose of (+)-3-[3-hydroxyphenyl]-N-(1-propyl)piperidine ((+)-3-PPP), a sigma receptor agonist, was administered. (+)-3-PPP at doses of 12 and 24 mg/kg induced significantly more frequent rearing and more potent stereotypy consisting of repetitive head movement and sniffing in cocaine-sensitized rats than in saline-pretreated rats. These enhanced responses to (+)-3-PPP lasted for at least a month. The enhanced responses to (+)-3-PPP were attenuated by 30 mg/kg BMY 14802, a putative sigma antagonist, and also attenuated by 100 mg/kg (+/-)-sulpiride, a D2 dopamine antagonist. These findings show that repeated administration of cocaine produces lasting supersensitivity of simga receptors, which may induce subsequent activation of dopaminergic transmission.     

Reproduction of a memory trace in amnesia and forgetting following alteration in the activity of pre- and postsynaptic dopamine receptors

The paper deals with analysis of the action of enantiomers 3-PPP on memory trace reproduction disturbed by amnestic effects and spontaneous forgetting in mice. A considerable antiamnestic effect is shown of (+)3-PPP and (-)3-PPP in 10 mg/kg doze changing the activity of postsynaptic dopamine receptors. The influence of drugs in 2 mg/kg doze changing the activity of presynaptic receptors consisted in recovery of conditioned habit only in situation of a weak amnestic effect and at forgetting, when the level of reproduction was like a weak amnesia. The range of enantiomers 3-PPP action on reproduction processes disturbed by amnesia or forgetting is determined by the possibility of specific activation of pre- and postsynaptic receptors at different depth of disturbances of memory trace reproduction causing differentiation of 3-PPP effects.     

Hydroxylated Analogues of 5-Aminovaleric Acid as 4-Aminobutyric AcidB Receptor Antagonists: Stereostructure-Activity Relationships

The (R) and (S) forms of 5-amino-2-hydroxyvaleric acid (2-OH-DAVA) and 5-amino-4-hydroxyvaleric acid (4-OH-DAVA) were designed as structural hybrids of the 4-aminobutyric acidB (GABAB) agonist (R)-(-)-4-amino-3-hydroxybutyric acid [(R)-(-)-3-OH-GABA] and the GABAB antagonist 5-aminovaleric acid (DAVA). (S)-(-)-2-OH-DAVA and (R)-(-)-4-OH-DAVA showed a moderately potent affinity for GABAB receptor sites in rat brain and showed GABAB antagonist effects in a guinea pig ileum preparation. The respective enantiomers, (R)-(+)-2-OH-DAVA and (S)-(+)-4-OH-DAVA, were markedly weaker in both test systems. All four compounds were weak inhibitors of GABAA receptor binding in rat brain, and none of them significantly affected synaptosomal GABA uptake. Based on molecular modeling studies it has been demonstrated that low-energy conformations of (R)-(-)-3-OH-GABA, (S)-(-)-2-OH-DAVA, and (R)-(-)-4-OH-DAVA can be superimposed. These conformations may reflect the shapes adopted by these conformationally flexible compounds during their interaction with GABAB receptors. The present studies emphasize the similar, but distinct, constraints imposed on agonists and antagonists for GABAB receptors.     

Differential effects of baclofen andγ-aminobutyric acid (GABA) on rat piriform cortex pyramidal neuronsin vitro

1. The effects of baclofen and GABA on rat piriform cortex neurons were investigated electrophysiologically using a brain slice preparation. 2. At resting potential GABA depolarized and baclofen hyperpolarized the cell, probably through activation of Cl and K conductances acting at GABAA and GABAB receptors, respectively. 3. The GABAA receptors were concentrated on the apical and basal dendrites near the cell body, while the baclofen-sensitive GABA receptors were concentrated particularly on the basal dendrites. 4. The different distributions of receptor localization must have functional consequences which remain to be elucidated.     

Lack of coupling of D-2 receptors to adenylate cyclase in GH-3 cells exposed to epidermal growth factor. Possible role of a differential expression of Gi protein subtypes.

Exposure of GH-3 cells to epidermal growth factor for 4 consecutive days induced the expression of both D-2(415) and D-2(444) dopamine-receptor isoforms. Epidermal growth factor also promoted a remarkable increase in the content of Gi3 protein, which is responsible for receptor-induced activation of potassium channels in GH-3 cells. D-2 receptors in this model apparently activate a specific transducing pathway, leading to opening of potassium channels and inhibition of prolactin release by cAMP-independent mechanisms. This is shown by: 1) the selective D-2 agonist quinpirole, while inactive on vasoactive intestinal peptide-induced prolactin release, strongly inhibited the hormone secretion induced by neurotensin; 2) quinpirole, up to 100 microM, did not inhibit cAMP production evoked by vasoactive intestinal peptide both in intact cells and in broken cell membrane preparations; and 3) quinpirole and other D-2 agonists strongly potentiated Rb+ efflux when measured in a nominally calcium-free reaction solution containing 100 mM potassium (voltage-dependent component), but did not modify Rb+ efflux if measured in a reaction solution containing 1 mM calcium and 5 mM potassium (calcium-activated, cAMP-dependent component).     

Effect of the activation of GABA A, benzodiazepine, and D2 dopamine receptors on the passive avoidance extinction in submissive and aggressive mice

The effect of activation of GABAA, benzodiazepine, and D2 dopamine receptors on extinction of passive avoidance and their dependence on the initial state of aggressive and submissive C57BL/6J mice were studied. It was found that in mice with the submissive stereotype of behavior produced by experience of defeats in daily agonistic confrontations, extinction of the conditioned reaction occurred faster than in control mice. The activation of D2 receptors by quinpirole and of benzodiazepine receptors by medasepam before training restored the retrieval of the memory trace. A prolongation of extinction was observed in aggressive mice in comparison with control and submissive animals, and activation of GABAA by muscimol and benzodiazepine receptors by medazepam led to acceleration of extinction. Activation of D2 receptors was ineffective. Thus, the difference in initial behavioral strategy determined both the development of extinction of the passive avoidance and variability of participation of D2, GABAA, and benzodiazepine receptors in the maintenance of availability of the memory trace to retrieval.     

Induction of wakefulness and inhibition of active (REM) sleep by GABAergic processes in the nucleus pontis oralis

The present study was undertaken to explore the role of brainstem GABAergic processes in the control of the behavioral states of sleep and wakefulness, and to compare the effects of GABAA agonists and antagonists with those of GABAB agonists and antagonists on these behavioral states. Accordingly, the following drugs were microinjected into the nucleus pontis oralis (NPO) in chronic, unanesthetized cats: muscimol (GABAA agonist), bicuculline (GABAA antagonist), baclofen (GABAB agonist) and phaclofen (GABAB antagonist). The percentage, latency, frequency and duration of each behavioral state were measured in order to quantify the effects of these microinjections on wakefulness and sleep. Microinjections of either muscimol or baclofen immediately induced wakefulness. There was a significant increase in the duration and the percentage of time spent in wakefulness as well as an increase in the latency to active (REM) sleep. These changes were accompanied by a decrease in the percentage of time spent in active and quiet sleep. In contrast, injections of bicuculline or phaclofen produced active sleep. The percentage of time spent in active sleep and the frequency of active sleep increased while the percentage of time spent in wakefulness and the latency to active sleep was significantly reduced. The effects of GABAA receptor agonists and antagonists on wakefulness and active sleep were comparable, but stronger than those of GABAB receptor agonists and antagonists. These data indicate that pontine GABAergic processes acting on both GABAA and GABAB receptors play a critical role in generating and maintaining wakefulness and in controlling the occurrence of state of active sleep.     

The molecular aspects of the functional heterogeneity of the GABA receptors

It is generally accepted that gamma-aminobutyric acid (GABA) is one of the main inhibitory transmitter in the mammalian brain. There are three types of GABA receptors in the vertebrata central nervous system: the GABAA, GABAB and GABAC receptors. The GABAA receptor is a GABA-gated Cl- channel and is the tetramer ore the pentamer made of some classes of subunit (alpha, beta, gamma, delta). GABAB receptors are not affiliated with Cl(-) ionophore. GABAB receptors appear to be coupled to Ca2+ and K+ channels of presynaptic membranes. It seems they regulate the release of neurotransmitters release. The structural and functional properties of GABA receptors are discussed.     

Striatal cholinergic function reflects differences in D-2 dopaminergic receptor activation

The ergot derivatives, bromocriptine, lisuride and quinpirole (Ly-171555), activators of D-2 receptors, increased striatal acetylcholine (ACh) content by about 40% and induced a 30% inhibition of ACh evoked release from striatal slices, similar to the effects of the dopaminergic agonist apomorphine. These actions were a consequence of dopaminergic activation since they were antagonized by pretreatment with the neuroleptic agent, pimozide. In contrast, pretreatment with L-sulpiride (100 mg/kg), a specific antagonist for the D-2 dopaminergic receptor only, prevented the rise of ACh levels induced by apomorphine or quinpirole but did not interfere with the lisuride- or bromocriptine- induced ACh increases. Similarly, inhibition of the ACh evoked release produced by lisuride (3ωM) was prevented by pimozide (1mg/kg) but not by pretreatment with L-sulpiride. Addition of L-sulpiride (5ωM) to the Krebs solution had no effect on the inhibition of ACh-evoked release induced by lisuride, but a lower concentration (1ωM) antagonized the inhibition induced by quinpirole. Lisuride and bromocriptine responses were both insensitive to sulpiride. These results are discussed in terms of different interaction with the dopaminergic D-2 receptors by the drugs studied.     

Interaction of (+)- and (-)-3-PPP with the dopamine receptor in the anterior pituitary gland

The interaction of the enantiomers of the novel dopamine agonist, 3-PPP (3-hydroxyphenyl)-N-n-piperidine) with the dopamine receptor in the anterior pituitary gland was examined. Both (+)- and (-)-3-PPP were effective in suppressing the elevation in serum prolactin (PRL) concentrations in rats treated with alpha-methyl-paratyrosine, an inhibitor of dopamine synthesis. The (+)-enantiomer was slightly more potent than the (-)-enantiomer in this regard. In addition, the secretion of PRL from anterior pituitary tissue under in vitro conditions was significantly inhibited by both isomers of 3-PPP, with (+)-3-PPP being approximately 10 times more potent than (-)-3-PPP. Both (+)- and (-)-3-PPP displaced 3H-(-)-N-n-propylnorapomorphine (3H-NPA) and 3H-spiperone from bovine anterior pituitary membranes. The Hill coefficients of (+)- and (-)-3-PPP for the displacement of 3H-spiperone were 0.6 and 0.7, respectively. These results are consistent with the view that the (+)- and (-)-enantiomer exhibit dopamine agonist effects at dopamine receptor sites in the anterior pituitary gland. However, (+)-3-PPP demonstrated marked differences in affinity for 3H-NPA- and 3H-spiperone labeled-sites, whereas (-)-)3-PPP showed the same order of affinity for these two sites. In view of these results and the fact that (-)-3-PPP has also been characterized as a dopamine antagonist at postsynaptic receptor sites in the striatum, (-)-3-PPP might be best described as a partial agonist at pituitary dopamine receptors. Moreover, these data are suggestive of a similarity, at least on a pharmacological basis, between dopamine autoreceptors and dopamine receptors in the anterior pituitary gland.     

Effects of the two partial D2 agonists (+)- and (-)-3-PPP on prolactin release in EEDQ treated male rats.

The effects of pretreatment with the irreversible inactivator of brain D2 receptors N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the suppression of prolactin (PRL) release induced by the two partial D2 agonists (+)- and (-)-3-(3-hydroxyphenyl)-N-n-propyl-piperidine [(+)-3-PPP, (-)-3-PPP] were investigated in gamma-butyrolactone (GBL) treated male rats. Pretreatment with a high dose of EEDQ (20 mg/kg, 24 h) did not influence the PRL response to (+)-3-PPP. In contrast, the effect of (-)-3-PPP was dose-dependently antagonized by EEDQ administration; thus, while pretreatment with EEDQ 2 mg/kg (24 h) failed to influence the efficacy of (-)-3-PPP, a complete antagonism of the PRL suppressive effect of the (-) enantiomer was obtained by administration of EEDQ 16 or 20 mg/kg (24 h). Moreover, in EEDQ (20 mg/kg, 24 h) treated rats (-)-3-PPP effectively antagonized the PRL inhibiting effect of the (+) enantiomer. Also, in EEDQ (20 mg/kg, 24 h) treated animals not receiving GBL (-)-3-PPP induced a dose-dependent increase in plasma levels of PRL. The data indicate that higher doses of EEDQ are required in order to reduce the responsiveness of lactotroph D2 receptors than that of various populations of brain D2 receptors. Also, the data lend support for the assumption that an altered receptor responsiveness may dramatically modify the response to a partial agonist with low intrinsic efficacy without affecting the response to a partial agonist with higher intrinsic efficacy.     

Dopamine receptor occupancy in vivo: measurement using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)

N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) inactivates a variety of monoamine neurotransmitter receptors. In this report, protection against EEDQ-induced inactivation of D-1 and D-2 DA receptors by DA antagonists and agonists was used to obtain a measure of occupancy of these receptors in vivo by such drugs. Rats were pretreated with drugs and then given EEDQ (10 mg/kg, i.p.). Twenty-four hours after the EEDQ injections, the animals were decapitated and the number of receptors remaining was measured using conventional receptor binding assays. The D-1 antagonist SCH 23390 potently protected D-1 sites from EEDQ-induced inactivation in a dose-dependent manner. Similarly, NO-756, another D-1 antagonist, selectively protected D-1 sites from inactivation. Conversely, haloperidol, a relatively selective D-2 antagonist, protected D-2 sites from inactivation. Likewise, a number of antipsychotic DA antagonists also protected D-2 sites from inactivation. Clozapine, fluperlapine, and (+) butaclamol were effective at protecting both D-1 sites and D-2 sites. In addition, the D-1 agonist SKF 38393 protected D-1 sites from EEDQ-induced inactivation, whereas the D-2 agonist quinpirole protected D-2 sites. (-) Apomorphine, a mixed D-1/D-2 agonist, protected both sites. Thus, this type of method provides a simple means of evaluating the occupation of DA receptors by DA antagonists and agonists in vivo.     

Effect of adrenoblockers on the analgesic effect of GABA-positive preparations

The experiments on rats have shown that selective alpha 1 and alpha 2 adrenoceptor blockers (prazosin and yohimbine) and an inhibitor of dopamine-beta-hydrolase FD-008 failed to change the antinociceptive effect of baclofen, a direct GABAB receptor agonist. The antinociceptive effect of THIP and depakin, acting predominantly on GABAA receptors, was significantly reduced by prazosin, FD-008 and yohimbine in vocalization test. In tail-flick test the analgetic effect of THIP and depakin was not altered by prazosin and FD-008, but was increased by yohimbine. The role of adrenergic mechanisms in GABAA and GABAB receptor-mediated analgesia is discussed.     

Blockade ofNMDA receptors differentially affectsD-1 andD-2 mediated turning behavior in the 6-hydroxydopamine model of Parkinson

Summary In rats with unilateral lesion of the nigrostriatal dopaminergic pathway, L-DOPA induces contralateral turning through activation of denervated D-1 and D-2 receptors. Blockade of N-methyl-D-aspartate (NMDA) receptors by the non-competitive antagonist (+)MK-801, potentiated the contralateral turning induced by L-DOPA as well as that induced by the D-1 agonist SKF 38393, while D-2 mediated turning was almost completely inhibited. Administration of the D-1 antagonist SCH 23390 blocked (+)MK-801-induced potentiation of L-DOPA contralateral turning, confirming the D-1 nature of the effects observed. Immunohistochemical studies on the early gene c-fos, which is known to be activated by stimulation of supersensitive D-1 receptors, revealed sparse c-fos positive nuclei in the lesioned CPu after SKF 38393, while after combined administration of (+)MK-801 and SKF 38393 dense labelling was obtained. Blockade of NMDA receptors, differentially affects D-1 and D-2 mediated turning behavior, suggesting that different neuronal pathways are involved in the mediation of D-1 and D-2 responses.     

Investigation of Dopamine Content, Synthesis, and Release in the Rabbit Retina In Vitro: II. Effects of High Potassium, Adenylate Cyclase Activators, and N-n-Propyl-3-(3-Hydroxyphenyl) Piperidine

The modulation of 3,4-dihydroxyphenylethylamine (dopamine, DA) synthesis and release in rabbit retina in vitro by high K+; adenylate cyclase activators such as forskolin, 2-chloroadenosine, vasoactive intestinal polypeptide (VIP); and the putative DA autoreceptor agonist N-n-propyl-3-(3-hydroxyphenyl) piperidine (3-PPP) has been investigated. Incubation of retinas in 50 mM K+ resulted in the activation of tyrosine hydroxylase (TH). Activation did not require the presence of extracellular Ca2+. K+ 50 mM also induced a Ca2+-dependent release of DA. Forskolin 50 microM stimulated TH but 100 microM 2-chloroadenosine and 650 nM VIP did not. Individually, (+)-3-PPP, (-)-3-PPP, and (+/-)-3-PPP reduced DA synthesis and increased its release. The effects of (+/-)-3-PPP were dose-dependent and did not require the presence of extracellular Ca2+. The activation of TH induced by 50 mM K+, but not that induced by 50 microM forskolin, was abolished by 100 microM (+/-)-3-PPP.