Selective Dopamine D2 Receptor Agonist Research Articles

α4 nicotinic acetylcholine receptor modulated by galantamine on nigrostriatal terminals regulates dopamine receptor-mediated rotational behavior

Galantamine, an acetylcholine esterase (AChE) inhibitor used to treat dementia symptoms, also acts as an allosteric potentiating ligand (APL) at nicotinic acetylcholine receptors (nAChRs). This study was designed to evaluate the allosteric effect of galantamine on nAChR regulation of nigrostrial dopaminergic neuronal function in the hemiparkinsonian rat model established by unilateral nigral 6-hydroxydopamine (6-OHDA) injection. Methamphetamine, a dopamine releaser, induced ipsilateral rotation, whereas dopamine agonists apomorphine (a non-selective dopamine receptor agonist), SKF38393 (a selective dopamine D1 receptor agonist), and quinpirole (a selective dopamine D2 receptor agonist) induced contralateral rotation. When 6-OHDA-injected rats were co-treated with nomifensine, a dopamine transporter inhibitor, a more pronounced and a remarkable effect of nicotine and galantamine was observed. Under these conditions, the combination of nomifensine with nicotine or galantamine induced the ipsilateral rotation similar to the methamphetamine-induced rotational behavior, indicating that nicotine and galantamine also induce dopamine release from striatal terminals. Both nicotine- and galantamine-induced rotations were significantly blocked by flupenthixol (an antagonist of both D1 and D2 dopamine receptors) and mecamylamine (an antagonist of nAChRs), suggesting that galantamine modulation of nAChRs on striatal dopaminergic terminals regulates dopamine receptor-mediated movement. Immunohistochemical staining showed that α4 nAChRs were highly expressed on striatal dopaminergic terminals, while no α7 nAChRs were detected. Pretreatment with the α4 nAChR antagonist dihydroxy-β-erythroidine significantly inhibited nicotine- and galantamine-induced rotational behaviors, whereas pretreatment with the α7 nAChR antagonist methyllycaconitine was ineffective. Moreover, the α4 nAChR agonist ABT-418 induced ipsilateral rotation, while the α7 nAChR agonist PNU282987 had no significant effect on rotational behavior. These results suggest that galantamine can enhance striatal dopamine release through allosteric modulation of α4 nAChRs on nigrostriatal dopaminergic terminals.

Dopamine D1 receptor stimulation modulates the formation and retrieval of novel object recognition memory: Role of the prelimbic cortex

Previous studies have shown that dopamine D1 receptor antagonists impair novel object recognition memory but the effects of dopamine D1 receptor stimulation remain to be determined. This study investigated the effects of the selective dopamine D1 receptor agonist SKF81297 on acquisition and retrieval in the novel object recognition task in male Wistar rats. SKF81297 (0.4 and 0.8mg/kg s.c.) given 15min before the sampling phase impaired novel object recognition evaluated 10min or 24h later. The same treatments also reduced novel object recognition memory tested 24h after the sampling phase and when given 15min before the choice session. These data indicate that D1 receptor stimulation modulates both the encoding and retrieval of object recognition memory. Microinfusion of SKF81297 (0.025 or 0.05μg/side) into the prelimbic sub-region of the medial prefrontal cortex (mPFC) in this case 10min before the sampling phase also impaired novel object recognition memory, suggesting that the mPFC is one important site mediating the effects of D1 receptor stimulation on visual recognition memory.

Putative therapeutic targets for symptom subtypes of adult ADHD: D4 receptor agonism and COMT inhibition improve attention and response inhibition in a novel translational animal model

Prefrontal cortical dopamine plays an important role in cognitive control, specifically in attention and response inhibition; the core deficits in ADHD. We have previously shown that methylphenidate and atomoxetine differentially improve these deficits dependent on baseline performance. The present study extends this work to investigate the effects of putative therapeutic targets in our model. A selective dopamine D4 receptor agonist (A-412997) and the catechol-O-methyl-transferase (COMT) inhibitor; tolcapone, were investigated in the combined subtype of adult ADHD (ADHD-C). Adult female rats were trained to criterion in the 5C-CPT (5-Choice Continuous Performance Task) and then separated into subgroups according to baseline levels of sustained attention, vigilance, and response disinhibition. The subgroups included: high-attentive (HA) and low-attentive with high response disinhibition (ADHD-C). The ADHD-C subgroup was selected to represent the combined subtype of adult ADHD. Effects of tolcapone (3.0, 10.0, 15.0mg/kg) and A-412997 (0.1, 0.3, 1.0µmol/kg) were tested by increasing the variable inter-trial-interval (ITI) duration in the 5C-CPT. Tolcapone (15mg/kg) significantly increased sustained attention, vigilance and response inhibition in ADHD-C animals, and impaired attention in HA animals. A-412997 (1.0µmol/kg) significantly increased vigilance and response inhibition in ADHD-C animals only, with no effect in HA animals. This is the first study to use the translational 5C-CPT to model the adult ADHD-C subtype in rats and to study new targets in this model. Both tolcapone and A-412997 increased vigilance and response inhibition in the ADHD-C subgroup. D4 and COMT are emerging as important potential therapeutic targets in adult ADHD that warrant further investigation.

P.1.g.033 Neuroprotective activity of the novel antiparkinsonian drug hemantane

A-77636, ((1R,3S) 3-(1′-adamantyl)-1-aminomethyl-3,4-dihydro-5,6-dihydroxy-1H-2-benzopyran hydrochloride), is a selective dopamine D1 receptor agonist. In a battery of receptor binding assays, A-77636 shows the highest affinity (pKi = 7.40 ± 0.09;Ki = 39.8 nM for the dopamine D1 receptor. A-77636 is an agonist at the dopamine D1 receptors (pEC50 = 8.13; EC50 = 1.1 nM; intrinsic activity = 102% of dopamine and the rat caudate-putamen (pEC50 = 8.97; intrinsic activity = 134% of dopamine). The compound is functionally inactive at dopamine D2 receptors (EC50 > 10 μMM). In rats w unilateral 6-OHDA (6-hydroxydopamine) lesions of the nigro-striatal dopaminergic pathway, A-77636 elicits prolonged ( > 20 h) contralateral turning that is blocked by SCH 23390, a D1 receptor antagonist, but not by haloperidol at doses selective for the dopamine D2 receptor. Higher doses of A-77636 produce forelimb clonus in rats and mice. When tested in marmosets treated with MPTP to induce a parkinsonian-like state, A-77636 increases locomotor activity and decreases the severity of the parkinsonian-like symptoms: the compound is active after either subcutaneous or oral administration. A-77641, the optical antipode of A-77636, has a lower affinity towards the dopamine D1 receptor (pKi = 5.14, Ki = 7200 nM), is less potent as a dopamine D1 receptor agonist (EC50 = 5.65; EC50 = 2200 nM, fails to elicit turning in the 6-OHDA-lesioned rat, and lacks antiparkinsonian efficacy in the MPTP-treated marmoset. The possible utility of agonists for the dopamine D1 receptor in the treatment of idiopathic parkinsonism is discussed.

D1 receptor agonist improves sleep–wake parameters in experimental parkinsonism

Both excessive daytime sleepiness (EDS) and rapid eye movement (REM) sleep deregulation are part of Parkinson's disease (PD) non-motor symptoms and may complicate dopamine replacement therapy. We report here that dopamine agonists act differentially on sleep architecture in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine macaque monkey. Continuous sleep and wake electroencephalographic monitoring revealed no effect of the selective dopamine D2 receptor agonist quinpirole on EDS, whereas the selective dopamine D1 receptor agonist SKF38393 efficiently alleviated EDS and restored REM sleep to baseline values. The present results question the relevance of abandoning D1 receptor agonist treatment in PD as it might actually improve sleep-related disorders.

Selective dopamine D4 receptor agonist effects and their impact on regression of left ventricular hypertrophy in SH rats

Left ventricular hypertrophy is an adaptive response that involve a remodeling in heart tissues, however is associated with increased morbidity and mortality. In this study we characterized the effect of chronic administration of a selective dopamine D4 receptor agonist, the PD 168 099, in the left ventricular hypertrophy (LVH) in spontaneous hypertension rats (SHR). Male spontaneous hypertension rats (SHR), 2 months old, were distributed in 5 experimental groups, for daily drug oral administration during two months (metroprolol 100 mg/kg, losartan 50 mg/kg, bromocriptine 10 mg/kg, and PD 168 099 1 mg/Kg). After two months, systolic arterial blood pressure (SABP) by tail cut off occlusion method was recorder, after animals were sacrificed and hearts extracted, weighed and measured. In the sham groups, SABP, and ratio heart/body weight, were higher; 205±3 mmHg and 0.502±0.21, respectively. All drugs reduced the SABP, however the PD 168 099, was the most effective as losartan in decrease the ratio heart/body weight. Ours results demonstrate that PD 166 099 has a similar effect than losartan on the regression of left ventricular hypertrophy in the SH rats, suggesting that a selective dopamine D4 receptor agonist could be a good alternative as pharmacological treatment in LVH related to hypertension. Support from CONACYT

The dopaminergic stabilizer pridopidine increases neuronal activity of pyramidal neurons in the prefrontal cortex

The dopaminergic stabilizer pridopidine demonstrates state-dependent effects on locomotor activity, counteracting both hypo- and hyperactivity in rats. Pridopidine has been shown to display both functional dopamine D2 receptor antagonist properties and increase in biomarkers associated with NMDA-mediated glutamate transmission in the frontal cortex. To further characterise the effects of pridopidine on prefrontal cortex (PFC) neurons, a series of in vivo electrophysiological studies were performed in urethane-anaesthetised rats. Pridopidine, administered at doses from 10 to 60 mg/kg (i.v.), dose dependently increased pyramidal cell firing in the majority of the neurons tested. Pridopidine induced a significant increase of 162 % in mean firing activity of PFC neurons, versus initial basal firing activity as the cumulative dose of 30 mg/kg, i.v., was administered. This enhancement of activity was due to increased firing frequency of already spontaneously active neurons, rather than an increase in population activity. The increase was partially reversed or prevented by a sub-threshold dose of the dopamine D1 receptor antagonist SCH23390 (0.5 mg/kg, i.v.). Microiontophoretic application of pridopidine had only moderate activating effects. The selective dopamine D1 receptor agonist A-68930 also had limited effects when administered by microiontophoretic application, but exerted a dose dependent (0.2-3 mg/kg, i.v.) activation of firing in the majority of neurons tested (10/16). However, inhibition of firing by systemic administration of A-68930 was also observed in a subgroup of neurons (6/16). Both activation and inhibition of firing induced by systemic administration of A-68930 were reversed by the systemic administration of SCH23390. The present data suggests that pridopidine enhances pyramidal cell firing via an indirect dopamine D1 receptor-mediated mechanism. These effects of pridopidine may serve to strengthen the cortico-striatal communication and to improve motor control in Huntington's disease for which pridopidine is currently in development.

Persistent activation of the D1R/Shp-2/Erk1/2 pathway in l-DOPA-induced dyskinesia in the 6-hydroxy-dopamine rat model of Parkinson's disease

Prolonged l-3,4-dihydroxyphenylalanine (l-DOPA) administration, the gold standard therapy for Parkinson's disease (PD) is associated with serious motor complications, known as l-DOPA-induced dyskinesia (LID). One of the major molecular changes associated with LID is the increased activity of the extracellular signal-regulated kinases 1/2 (Erk1/2) signaling in the medium spiny neurons of the striatum induced by malfunctioning in the dopamine D1 receptor (D1R)-mediated transmission.We have previously established that in the striatum, activation of Shp-2, an intracellular tyrosine phosphatase associated with the D1R, is a requisite for the D1R to activate Erk1/2.In this study, we investigated the role of striatal D1R/Shp-2 complex in the molecular event underlying LID in the 6-OHDA-lesioned rat model of PD.We found that in hemiparkinsonian rats experiencing LID, the physiological interaction between D1R and Shp-2 in the striatum was preserved. In these animals, the chronic activation of D1R either by l-DOPA or by the selective D1R agonist SKF 38393 induced both dyskinesia and Shp-2/Erk1/2 activation. These effects were prevented by the selective D1R-antagonist SCH23390 suggesting the involvement of striatal D1R/Shp-2 complex, via Erk1/2 activation, in the molecular events underlying LID. Interestingly, we found that D1R-mediated Shp-2-Erk1/2 activation was persistently detected in the striatum of dyskinetic rats during l-DOPA washout, with a close correlation between LID severity and the extent of long term activation of both Shp-2 and Erk1/2.Taken together, our data show that in hemiparkinsonian rats developing dyskinesia, the aberrant phosphorylation of Shp-2 by D1R activation, represents an upstream molecular event leading to the persistent phosphorylation of Erk1/2 and therefore a novel therapeutic target to counteract LID development and maintenance during l-DOPA therapy.

The 5-HT1A-receptor agonist flibanserin reduces drug-induced dyskinesia in RGS9-deficient mice

Drug-induced dyskinesia is a major complication of dopamine replacement therapy in advanced Parkinson's disease consisting of dystonia, chorea and athetosis. Agonists at 5-HT1A-receptors attenuate levodopa-induced motor complications in non-human primates. Mice with increased dopamine D2 receptor (DRD2) signalling due to the lack of expression of the regulator of G-protein signalling 9 (RGS9) also develop dyskinesia following levodopa treatment. We investigated whether the 5-HT1A-receptor agonist flibanserin compared with buspirone reduces motor abnormalities induced by levodopa or quinelorane, a selective dopamine D2-receptor agonist. Following dopamine depletion via reserpine, 40 mice (20 wild-type and 20 RGS9 knock-out) were treated with flibanserin or buspirone in combination with levodopa or quinelorane. Motor behaviour was analysed using open field analysis. RGS9 knock-out mice displayed significantly more drug-induced dystonia (p<0.04; t test) than wild type. In quinelorane-treated wild-type mice flibanserin as well as buspirone significantly reduced dystonia (p<0.05). In RGS9 knock-out animals again both reduced quinelorane-induced dystonia. However, flibanserin was significantly more effective (p=0.003). Following reserpine pretreatment and administration of levodopa wild-type and RGS 9 knock-out mice showed mild to moderate dystonia. Surprisingly, 10mg/kg buspirone increased dystonia in both animal groups, whereas it was decreased by 10mg/kg flibanserin. However, compared with levodopa alone only the increase of dystonia by buspirone was significant (p<0.04). Flibanserin showed promising antidyskinetic effects in a model of drug-induced dyskinesia. Our data underline the possible benefit of 5-HT1A agonists in drug-induced dyskinesia.

D1‐dopamine receptor agonists increase phrenic nerve activity and synchronization with renal sympathetic nerve discharges in the adult rat in vivo

In adult cats in vivo, D1-dopamine receptor (D1R) agonists increase phrenic nerve discharge (PND) intensity and PND sensitivity to CO2. Our goals in the present investigation were to: One, determine if a highly selective D1R agonist, SKF 38393 (SKF), affects PND in the rat. Two, determine if SKF affects synchronization between PND and renal sympathetic nerve discharges (RSND). We performed experiments on anesthetized, vagotomized, paralyzed and mechanically ventilated adult rats. We gave SKF i.v. in graded doses and recorded raw and integrated PND and RSND, arterial blood pressure, end-tidal CO2 and rectal temperature. We also cross-correlated PND (as source) and RSND (destination). We observed two principal SKF effects: PND intensity increased after doses of 0.25–2 mg/kg without change in discharges/minute. Peak intensity increased 32.6 ± 3.9% (mean ± SE) at 1 mg/kg and reached a plateau or declined with larger (3–4 mg/kg) doses. RSND exhibited a rhythm synchronized to PND after all test doses of SKF, increasing in parallel with PND intensity. We suggest that SKF stimulates a common source of excitation and coupling between central respiratory and sympathetic motor outputs.

P.1.002 Drug interaction between fibrillar amyloid β and alpha7 nicotinic receptor in Alzheimer brain

Galantamine, an acetylcholine esterase (AChE) inhibitor used to treat dementia symptoms, also acts as an allosteric potentiating ligand (APL) at nicotinic acetylcholine receptors (nAChRs). This study was designed to evaluate the allosteric effect of galantamine on nAChR regulation of nigrostrial dopaminergic neuronal function in the hemiparkinsonian rat model established by unilateral nigral 6-hydroxydopamine (6-OHDA) injection. Methamphetamine, a dopamine releaser, induced ipsilateral rotation, whereas dopamine agonists apomorphine (a non-selective dopamine receptor agonist), SKF38393 (a selective dopamine D1 receptor agonist), and quinpirole (a selective dopamine D2 receptor agonist) induced contralateral rotation. When 6-OHDA-injected rats were co-treated with nomifensine, a dopamine transporter inhibitor, a more pronounced and a remarkable effect of nicotine and galantamine was observed. Under these conditions, the combination of nomifensine with nicotine or galantamine induced the ipsilateral rotation similar to the methamphetamine-induced rotational behavior, indicating that nicotine and galantamine also induce dopamine release from striatal terminals. Both nicotine- and galantamine-induced rotations were significantly blocked by flupenthixol (an antagonist of both D1 and D2 dopamine receptors) and mecamylamine (an antagonist of nAChRs), suggesting that galantamine modulation of nAChRs on striatal dopaminergic terminals regulates dopamine receptor-mediated movement. Immunohistochemical staining showed that α4 nAChRs were highly expressed on striatal dopaminergic terminals, while no α7 nAChRs were detected. Pretreatment with the α4 nAChR antagonist dihydroxy-β-erythroidine significantly inhibited nicotine- and galantamine-induced rotational behaviors, whereas pretreatment with the α7 nAChR antagonist methyllycaconitine was ineffective. Moreover, the α4 nAChR agonist ABT-418 induced ipsilateral rotation, while the α7 nAChR agonist PNU282987 had no significant effect on rotational behavior. These results suggest that galantamine can enhance striatal dopamine release through allosteric modulation of α4 nAChRs on nigrostriatal dopaminergic terminals.

Targeting Hypoxia-induced Inflammation

In the current issue of Anesthesiology, a paper by Kim et al. from the research laboratory of Dr. H. Thomas Lee investigates strategies to dampen ischemia-driven inflammation of the kidneys and subsequent multi-organ failure utilizing experimental models of acute kidney injury (AKI).1 In fact, one of the leading causes of morbidity and mortality in surgical patients is perioperative organ failure, such as occurs in the context of acute kidney injury, liver failure, intestinal or myocardial ischemia, stroke or acute lung injury. A common feature of these perioperative diseases is the presence of hypoxia-induced inflammation.2,3 The relationship between hypoxia and inflammation under these conditions is inter-dependent. For example, exposure to ambient hypoxia - as seen during high-altitude mountaineering - is associated with edema of the lungs or the brain, and systemic inflammatory responses in humans.4,5 Similarly, acute exposure of mice to ambient hypoxia (e.g. 8% of oxygen over 4–8h) leads to increased inflammatory cytokine levels and pulmonary edema.6 Moreover, prolonged donor organ exposure to ischemia during organ transplantation is known to enhance graft inflammation and early graft failure.7,8 For example, the endotoxin-receptor TLR4 is expressed in the donor kidney during kidney transplantation, and TLR4 expression levels increase with prolonged ischemia time. Donor kidneys with a loss-of-function mutation of the TLR4 receptor show attenuated kidney inflammation, and an increased rate of immediate graft function.7 Together, these studies indicate that hypoxia represents an inflammatory stimulus (Figure 1) and suggest that targeting hypoxia-elicited inflammation could represent an important therapeutic approach in perioperative medicine.

Exercise reduces oxidative stress but does not alleviate hyperinsulinemia or renal dopamine D1receptor dysfunction in obese rats

Impairment of renal dopamine D1 receptor (D1R)-mediated natriuresis is associated with hypertension in humans and animal models, including obese Zucker rats. We have previously reported that treatment of these rats with antioxidants or insulin sensitizers reduced insulin levels and oxidative stress, restored D1R-mediated natriuresis, and reduced blood pressure. Furthermore, the redox-sensitive transcription factor, nuclear factor-κB (NF-κB), has been implicated in impairment of D1R-mediated natriuresis during oxidative stress. In this study, we investigated the effect of exercise on insulin levels, oxidative stress, nuclear translocation of NF-κB, blood pressure, albuminuria, and D1R-mediated natriuresis. The exercise protocol involved treadmill exercise from 3 wk of age for 8 wk. Exercise reduced oxidative stress, nuclear translocation of NF-κB, and albuminuria. However, exercise did not reduce plasma insulin levels or blood pressure. Also, selective D1R agonist (SKF-38393)-mediated increases in sodium excretion and guanosine 5'-O-(3-thiotriphosphate) binding were impaired in obese rats compared with lean rats, and exercise did not restore this defect. We conclude that, while exercise is beneficial in reducing oxidative stress and renal injury, reducing insulin levels may be required to restore D1R-mediated natriuresis in this model of obesity and metabolic syndrome. Furthermore, this study supports previous observations that restoring D1R function contributes to blood pressure reduction in this model.

ChemInform Abstract: trans-Hexahydroindolo(4,3-ab)phenanthridines (“Benzergolines”), the First Structural Class of Potent and Selective Dopamine D1 Receptor Agonists Lacking a Catechol Group.

In contrast to the many selective dopamine (DA) D2 receptor agonists known, only two prototypes of selective D1 receptor agonists have been described; both show preference for the periphery due to their catechol partial structures. Our search for non-catechol, selective D1 agonists was based on the hypothesis that D1 selectivity could be conferred upon ergolines by annulation with a phenyl ring. The target molecules, trans-4,6,6a,7,8,12b-hexahydroindolo-[4,3-ab]phenanthridi nes (benzergolines), were efficiently synthesized by using the Ninomiya enamide photocyclization reaction. These compounds were found to be as active as the most potent D1 agonists in the adenylate cyclase D1 receptor model, but showed no activity in the ACh release D2 receptor assay. The acquired subtype selectivity of the novel structures was accompanied by an enhanced potency and efficacy as compared to the corresponding ergolines. This points to a D1 affinity enhancing, D2 receptor discriminating role for the additional phenyl group and provides further support for the existence of a D1 receptor specific accessory aryl binding site. Thus the benzergolines represent the first structural class of potent and selective D1 agonists lacking a catechol group which should allow an efficient central nervous system penetration. On the basis of these results, the D1 agonist pharmacophore has to be revised in the sense that potent activity requires neither a catechol function nor an orthogonal conformation of the aromatic rings.

Suppression of Prolactin Expression by Cabergoline Requires Prolactin Regulatory Element-binding Protein (PREB) in GH3 Cells

The prolactin regulatory element-binding protein (PREB) is a transcriptional factor that regulates prolactin (PRL) promoter activity in the anterior pituitary. Prolactinomas are the most common pituitary tumors. Administration of cabergoline, a selective dopamine D2-receptor agonist, has become the initial therapy of choice for most patients with prolactinomas. Although activation of the D2 receptor results in the inhibition of PRL synthesis, the details of the underlying mechanisms remain unknown. Samples of ten prolactinomas and ten nonfunctioning pituitary adenomas were analyzed by immunohistochemistry to detect the expression of PREB. The effect of cabergoline on PREB expression was assessed by western blotting and real-time polymerase chain reaction (PCR) analysis. Reporter gene analysis of PRL was employed to examine the role of PREB on cabergoline-induced suppression of PRL transcription. Immunohistochemical analysis revealed strong positive PREB expression in the prolactinoma tissue, but extremely weak or undetected expression in the nonfunctioning pituitary tumor tissue. Western blots probed with a PREB-specific antiserum revealed that the relative abundance of the PREB protein in the GH3 cells decreased in a dose-dependent manner in response to cabergoline treatment, as did the relative abundance of PREB mRNA. Although cabergoline inhibited the activity of the PRL promoter, mutation of PREB-binding site within the promoter abrogated the ability of cabergoline to inhibit the PRL promoter activity. We have demonstrated that PREB is expressed in prolactinomas and that the suppression of PRL expression by cabergoline requires the transcriptional factor PREB.

Activation of Dopamine D4 Receptors Is Protective Against Hypoxia/ Reoxygenation-Induced Cell Death in HT22 Cells

Several reports have shown that some dopamine receptor ligands modulate the ischemia–reperfusion injury in animal models; however, its underling mechanisms are still unclear. In this study, we sought to establish an in vitro experimental model of hypoxia/reoxygenation (H/R) using HT22 cells that originated from mouse hippocampal neurons and to examine protective the effect of dopamine-receptor ligands against H/R-induced cell injury. The treatment with hypoxia for 18 h followed by reoxygenation for 6 h induced the elevation of intracellular reactive oxygen species (ROS) and reduction of mitochondrial membrane potential; however, lactate dehydrogenase (LDH) release was not changed at this time point. LDH release was increased after reoxygenation for 18 h and longer, and this increase in LDH release was suppressed by dopamine receptor agonists such as apomorphine and apocodeine. The suppressive effects of these agonists were reversibly inhibited by L750667, a D4-receptor antagonist but not by D2- or D3-receptor antagonists. In addition, PD168077, a selective dopamine D4–receptor agonist, also protected against H/R-induced cell death. These results suggest that H/R causes oxidative stress–induced cell death and that the activation of dopamine D4 receptors protects against H/R-induced cell death in HT22 cells.

Selective dopamine D4 receptor agonist (A-412997) improves cognitive performance and stimulates motor activity without influencing reward-related behaviour in rat

Current therapies for attention deficit hyperactivity disorder comprise psychostimulants, which block the dopamine transporter and/or stimulate the release of dopamine, leading to a global elevation in extrasynaptic dopamine. These drugs are, however, associated with a series of unwanted side effects such as insomnia, anorexia, headache, stomach problems and potential drug abuse. Recent evidence suggests that the dopamine D4 receptor may represent a selective dopamine target that could mediate cognitive as well as striatal motor processes. In this study we compare the effects of a selective D4 receptor agonist, A-412997, with methylphenidate or amphetamine in preclinical models of efficacy versus abuse liability. Both methylphenidate and A-412997 improved a temporally induced deficit in the rat novel object recognition task at doses 10-fold lower than those stimulating activity. In both cases, procognitive doses were associated with elevated extracellular levels of dopamine and acetylcholine in the medial prefrontal cortex. In contrast to amphetamine, A-412997 did not mediate reward-related behaviour in the conditioned place preference paradigm, a preclinical rodent test used to assess potential abuse liability. Collectively, these data suggest that selective activation of the D4 receptor may represent a target for the treatment of attention deficit hyperactivity disorder without the potential drug abuse liability associated with current psychostimulant therapies.

Diethylation Labeling Combined with UPLC/MS/MS for Simultaneous Determination of a Panel of Monoamine Neurotransmitters in Rat Prefrontal Cortex Microdialysates

The primary challenge associated with the development of an LC/MS/MS-based assay for simultaneous determination of biogenic monoamine neurotransmitters such as norepinephrine (NE), dopamine (DA), serotonin (5-HT), and normetanephrine (NM) in rat brain microdialysates is to improve detection sensitivity. In this work, a UPLC/ MS/MS-based method combined with a diethyl labeling technique was developed for simultaneous determination of a panel of monoamines in rat prefrontal cortex microdialysates. The chromatographic run time is 3.5 min/ sample. The limits of detection of the UPLC/MS/MS-based method for NE, DA, 5-HT/ and NM, with/without diethyl labeling of monoamines, are 0.005/0.4 (30/2367 pM), 0.005/0.1 (33/653 pM), 0.005/0.2 (28/1136 pM), and 0.002/0.2 ng/mL (11/1092 pM), respectively. Diethyl labeling of amino groups of monoamines affords 20-100 times increased detection sensitivity of corresponding native monoamines during the UPLC/MS/MS analysis. This could result from the following: (1) improved fragmentation patterns; (2) increased hydrophobicity and concomitantly increased ionization efficiency in ESI MS and MS/MS analysis; (3) reduced matrix interference. This labeling reaction employs a commercially available reagent, acetaldehyde-d4, to label the amine groups on the monoamines via reductive amination. It is also simple, fast (approximately 25-min reaction time), specific, and quantitative under mild reaction conditions. Data are also presented from the application of this assay to monitor the drug-induced changes of monoamine concentrations in rat prefrontal cortex microdialysate samples followed by administration of SKF 81297, a selective D1 dopamine receptor agonist known to elevate the extracellular level of the neurotransmitters DA and NE in the central nervous system.

Efficacy of a dopamine-somatostatin chimeric molecule, BIM-23A760, in the control of cell growth from primary cultures of human non-functioning pituitary adenomas: a multi-center study

Dopamine D2 and somatostatin receptors (sstrs) were reported to affect non-functioning pituitary adenoma (NFPA) proliferation in vitro. However, the reported results differ according to the experimental conditions used. We established an experimental protocol allowing reproducible evaluation of NFPA cell proliferation in vitro, to test and compare the antiproliferative effects of dopamine and somatostatin analogs (alone or in combination) with the activity of the dopamine-somatostatin chimeric molecule BIM-23A760. The protocol was utilized by four independent laboratories, studying 38 fibroblast-deprived NFPA cell cultures. Cells were characterized for GH, POMC, sstr1-sstr5, total dopamine D2 receptor (D2R) (in all cases), and D2 receptor long and short isoforms (in 15 out of 38 cases) mRNA expression and for alpha-subunit, LH, and FSH release. D2R, sstr3, and sstr2 mRNAs were consistently observed, with the dominant expression of D2R (2.9+/-2.6 copy/copy beta-glucuronidase; mean+/-s.e.m.), when compared with sstr3 and sstr2 (0.6+/-1.0 and 0.3+/-0.6 respectively). BIM-23A760, a molecule with high affinity for D2R and sstr2, significantly inhibited [3H]thymidine incorporation in 23 out of 38 (60%) NFPA cultures (EC50=1.2 pM and Emax=-33.6+/-3.7%). BIM-23A760 effects were similar to those induced by the selective D2R agonist cabergoline that showed a statistically significant inhibition in 18 out of 27 tumors (compared with a significant inhibition obtained in 17 out of 27 tumors using BIM-23A760, in the same subgroup of adenomas analyzed), while octreotide was effective in 13 out of 27 cases. In conclusion, superimposable data generated in four independent laboratories using a standardized protocol demonstrate that, in vitro, chimeric dopamine/sstr agonists are effective in inhibiting cell proliferation in two-thirds of NFPAs.

Stimulation of dopamine receptors in the paraventricular nucleus of the hypothalamus of male rats induces penile erection and increases extra-cellular dopamine in the nucleus accumbens: Involvement of central oxytocin

The effect of a pro-erectile dose of apomorphine, a mixed dopamine receptor agonist, and of PD-168077 ( N-[4-(2-cyanophenyl)piperazin-1-ylmethyl]-3-methylbenzamide maleate), a selective dopamine D4 receptor agonist, injected into the paraventricular nucleus of the hypothalamus on the concentration of extra-cellular dopamine and its main metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the dialysate from the nucleus accumbens was studied in male rats. As expected, apomorphine (0.1 μg) and PD-168077 (0.1 μg) induced penile erection episodes, which occurred concomitantly to an increase in extra-cellular dopamine and DOPAC concentration in the dialysate from the shell of the nucleus accumbens, as measured by intracerebral microdialysis. When induced by apomorphine, these effects were reduced by 80% by raclopride, a selective D2/D3 receptor antagonist (1 μg) and only by 40–45% by L-745,870 (1 μg), a selective dopamine D4 receptor antagonist. When induced by PD-168077, these effects were reduced by more than 80% by L-745,870 (1 μg), but only by 35–40% by raclopride. Irrespective of the dopamine agonist used to induce penile erection, the pro-erectile effect and the concomitant increase in dopamine and DOPAC concentration in the nucleus accumbens dialysate were almost completely abolished by d(CH 2) 5Tyr(Me) 2-Orn 8-vasotocin(1 μg), a potent oxytocin receptor antagonist, given into the lateral ventricles. The present results suggest that stimulation of dopamine receptors (mainly of the D2 to D4 subtype) in the paraventricular nucleus induces the release of oxytocin in brain areas that influence the activity of mesolimbic dopaminergic neurons mediating the appetitive and reinforcing effects of sexual activity. This provides evidence for a role of oxytocin in neural circuits that integrate the activity of neural pathways controlling the consummatory aspects of sexual behaviour (e.g., penile erection) with those controlling sexual motivation and sexual arousal.