Specific Dopamine Receptor Agonist Research Articles

Simultaneous Measurements of Intracellular [Ca2+]i and [cAMP]i in Intact Islets to Study the Mechanism Underlying Dopaminergic Inhibition of Insulin Secretion

Pancreatic islets secrete multiple hormones, including insulin, that are required to maintain euglycemia while meeting the energy demand of the body during everyday activities. We are interested in understanding the interplay between molecular mechanisms that precisely regulate these secretions. One specific paracrine modulator, dopamine, functions as a negative regulator of insulin secretion in the context of the pancreatic islet. It is secreted by the insulin-producing β-cells, activates an autocrine negative feedback that decreases the frequency of glucose-stimulated [Ca2+]i oscillations, and in turn, inhibits insulin secretion. The G-protein coupled dopamine receptors are present in islet cells, but it is not clear how activation of the these receptors results in the observed changes in [Ca2+]i in intact pancreatic islet cells. We are using an mTurquoise-Based cAMP biosensor with an improved dynamic range (Klarenbeek, J.B., et al., PLoS One, 2011), along with organic and genetically encoded Ca2+-indicator dyes. Labeled cells are studied by live imaging using perfused pancreatic islet. Spectral unmixing is used to extract the fluorescence emissions. This experimental setup allows us to monitor the effect of specific dopamine receptor agonists and antagonists on the two main cellular second messengers. Also, islets from mice with a genetic target mutation of the DRD3 (D3-KO) are used to measure how the deletion of the dopaminergic feedback changes the second messenger dynamics. With the same approach we are measuring the effects of the overexpression of dopamine receptor D3 in wild-type and D3-KO islets. The information from these experiments will help elucidate the mechanism of dopamine signaling in the pancreatic islet.

Medial prefrontal cortex dopamine controls the persistent storage of aversive memories.

Medial prefrontal cortex (mPFC) is essential for initial memory processing and expression but its involvement in persistent memory storage has seldom been studied. Using the hippocampus dependent inhibitory avoidance learning task and the hippocampus-independent conditioned taste aversion paradigm together with specific dopamine receptor agonists and antagonists we found that persistence but not formation of long-term aversive memories requires dopamine D1/D5 receptors activation in mPFC immediately after training and, depending on the task, between 6 and 12 h later. Our results indicate that besides its well-known participation in retrieval and early consolidation, mPFC also modulates the endurance of long-lasting aversive memories regardless of whether formation of the aversive mnemonic trace requires the participation of the hippocampus.

The in vitro receptor profile of rotigotine: a new agent for the treatment of Parkinson’s disease

Rotigotine (Neupro) is a non-ergoline dopamine agonist developed for the once daily treatment of Parkinson's disease (PD) using a transdermal delivery system (patch) which provides patients with the drug continuously over 24 h. To fully understand the pharmacological actions of rotigotine, the present study determined its extended receptor profile. In standard binding assays, rotigotine demonstrated the highest affinity for dopamine receptors, particularly the dopamine D3 receptor (Ki=0.71 nM) with its affinities to other dopamine receptors being (Ki in nM): D4.2 (3.9), D4.7 (5.9), D5 (5.4), D2 (13.5), D4.4 (15), and D1 (83). Significant affinities were also demonstrated at alpha-adrenergic (alpha2B, Ki=27 nM) and serotonin receptors (5-HT1A Ki=30 nM). In newly developed reporter-gene assays for determination of functional activity, rotigotine behaved as a full agonist at dopamine receptors (rank order: D3>D2L>D1=D5>D4.4) with potencies 2,600 and 53 times higher than dopamine at dopamine D3 and D2L receptors, respectively. At alpha-adrenergic sites, rotigotine acted as an antagonist on alpha2B receptors. At serotonergic sites, rotigotine had a weak but significant agonistic activity at 5-HT1A receptors and a minor or nonexistent activity at other serotonin receptors. Thus, in respect to PD, rotigotine can be characterized as a specific dopamine receptor agonist with a preference for the D3 receptor over D2 and D1 receptors. In addition, it exhibits interaction with D4 and D5 receptors, the role of which in relation to PD is not clear yet. Among non-dopaminergic sites, rotigotine shows relevant affinity to only 5-HT1A and alpha2B receptors. Further studies are necessary to investigate the contribution of the different receptor subtypes to the efficacy of rotigotine in Parkinson's disease and possible other indications such as restless legs syndrome.

Prenatal ethanol preferentially enhances reactivity of the dopamine D 1 but not D 2 or D 3 receptors in offspring

Reports of prenatal ethanol (ETOH) effects on the dopamine system are inconsistent. In an attempt to clarify this issue, dams were given 35% ethanol-derived calories as the sole nutrient source in a liquid diet from the 10th through the 20th day of gestation (ETOH). Controls were pair-fed (PF) an isocaloric liquid diet or given ad libitum access to laboratory chow (LC). Prenatal exposure to both liquid diets reduced body weight of offspring relative to LC controls, more so for ETOH than for PF exposure. Prenatal ETOH also decreased litter size and viability, relative to both LC and PF control groups. On postnatal days 21–23, male and female offspring were given an injection of saline vehicle or one of eight specific dopamine receptor agonists or antagonists. Immediately after injection subjects were placed in individual observation cages, and over the following 30 min, eight behaviors (square entries, grooming, rearing, circling, sniffing, yawning, head and oral movements) were observed and quantified. No prenatal treatment effects on drug-induced behaviors were observed for dopamine D 2 (Apomorphine, DPAT or Quinpirole) or D 3 (PD 152255, Nafadotride, Apo or Quin effects on yawning) receptor agonists or antagonists, or for the vehicle control. In contrast, prenatal treatment effects were seen with drugs affecting the dopamine D 1 receptor. Both D 1 agonists (SKF 38393) and antagonists (SCH 23390 and high doses of spiperone) altered behaviors, especially oral and sniffing behaviors, in a manner which suggested enhanced dopamine D 1 drug sensitivity in both ETOH and PF offspring relative to LC controls. These results suggest that at this age, both sexes experience a prenatal undernutrition-linked increase in the behavioral response to dopamine D 1 agonists and antagonists, which can be intensified by gestational exposure to alcohol.

Dopamine inhibition of evoked IPSCs in rat prefrontal cortex.

Rat prefrontal cortex (PFC) receives substantial dopamine (DA) input. This DA innervation appears critical for modulation of PFC cognitive functions. Clinical and experimental studies have also implicated DA in the pathogenesis of a number of neurological and psychiatric disorders including epilepsy and schizophrenia. However, the actions of DA at the cellular level are incompletely understood. Both inhibitory interneurons and pyramidal cells are targets of DA and may express different DA receptor types. Our recent findings suggest that DA can directly excite cortical interneurons and increase the frequency of spontaneous inhibitory postsynaptic currents (IPSCs). The present study was undertaken to determine the effect of specific DA receptor agonists on evoked (e) IPSCs. Visually identified pyramidal neurons were studied using whole cell voltage-clamp techniques. Bath application of DA 30 microM reduced IPSC amplitude to 80 +/- 4% (mean +/- SE) of control without any significant change in IPSC kinetics or passive membrane properties. The D1-like DA receptor agonist SKF 38393 reduced IPSC amplitude to 71.5 +/- 8%, whereas the D2-like specific agonist quinpirole has no effect on amplitude (94.5 +/- 5%). The D1-like receptor antagonist SCH 23390 prevented DA inhibition of IPSC amplitude (98.2 +/- 4%), whereas IPSCs were still reduced in amplitude (79.7 +/- 4%) by DA in the presence of the D2-like receptor antagonist sulpiride. DA increased significantly paired-pulse inhibition, whereas responses to puff applied GABA were unaffected. Addition of the PKA inhibitor H-8 blocked the effect of DA on IPSCs. These results suggest that DA can decrease IPSCs in layer II-III PFC neocortical pyramidal cells by activating presynaptic D1-like receptors.

Modulation of brain catecholamine absorbing proteins by dopaminergic agents

Catecholamine absorbing proteins (CATNAPs) are localized in the brain and thus far have no known biochemical and pharmacological characteristics consistent with other receptor proteins or metabolic enzymes in the central nervous system. The oxidative metabolism of catecholamines in the brain, especially the catabolism of dopamine and its conjugation with metabolic brain proteins, results in the production of highly toxic free radicals. Since such processes are implicated in the pathophysiology of various neurodegenerative diseases, including parkinsonism, and since CATNAPs bind catecholamines with high affinity, there is a need to further investigate if these novel proteins could play a protective role against these harmful catecholamine metabolites. In this study, we demonstrate the purification, pharmacological characterization and modulation of CATNAPs, as the first steps necessary to elucidate the function of these proteins in the brain. First, CATNAPs were identified from tissues using [ 3H] N-n-propylnorapomorphine (a specific dopamine receptor agonist) and [ 125I]6-hydroxy-5-iodo[ N(N-2,4-dinitro-phenyl)-aminopropyl]1,2,3,4-tetrahydronaphthalene ([ 125I]DATN; a highly specific ligand synthesized in our laboratory). Three proteins, with molecular masses of 47, 40 and 26 kDa, were identified and purified, which allowed for the subsequent production of antibodies against each of these CATNAPs. The effects of in vivo chronic administration of several dopaminergic agents on CATNAPs were also examined by Western immunoblotting. l-3,4-Dihydroxyphenylalanine (L-DOPA) treatment in rats resulted in the increase of all of the three proteins, as compared to controls. Treatment in rats with the dopamine depleting agent, reserpine, produced a significant decrease in all of the three CATNAPs. In addition, the effects of direct administration of apomorphine, dopamine, epinephrine, isopropylnorepinephrine, norepinephrine, N-n-propylnorapomorphine and 6-hydroxydopamine on CATNAP levels in rats were examined. Interestingly, we observed an increase (as compared to control) of the 47, 40 and 26 kDa proteins in animals treated with dopamine, norepinephrine, N-n-propylnorapomorphine and apomorphine. In contrast, animals treated with 6-hydroxydopamine showed significant decreases in the levels of all three proteins. It is evident that as the concentration of catecholamines increases, there is a corresponding increase in the levels of CATNAPs in the brain. These results clearly demonstrate the pharmacological modulation of CATNAPs by dopaminergic agents and suggest their possible role in the cytoprotection against damage caused by free radicals generated by oxidative stress.

Hormonal Regulation of Parental Care Behavior in Nesting Male Bluegills: Do the Effects of Bromocriptine Suggest a Role for Prolactin?

The hormonal mechanisms underlying parental care behavior in fish are poorly understood. This study investigates the effects of bromocriptine, a specific dopamine receptor agonist that inhibits secretion of prolactin from the pituitary, on parental care behaviors of nesting male bluegills. Parental male bluegills (Lepomis macrochirus) exhibit a complex set of reproductive behaviors including the construction of nests in colonies and courtship of females. After spawning, these males then alone remain to fan and guard the eggs and larvae from predators. Compared with control males, males implanted with bromocriptine showed significantly less defense of their broods and spent significantly more time rim circling their nests, a behavior typical of prespawning males. Because there were no significant differences among male groups in the levels of 11-ketotestosterone (11KT) or testosterone (T), the observed behavioral alterations cannot be ascribed to androgenic effects of bromocriptine. Instead, we suggest that prolactin, mediated through the action of dopamine, is involved in the expression of parental care behavior in this species.

The role of dopamine and serotonin receptors in the mediation of the ethanol interoceptive cue.

The drug discrimination paradigm was used to evaluate the contribution of dopamine or serotonin receptors in the mediation of the stimulus properties of ethanol. Briefly, rats were trained to discriminate between ethanol (600 mg/kg, IP) and water vehicle. Dose-response relationships were observed for ethanol and rats were tested with various dopamine and serotonin receptor agonists and antagonists. The specific dopamine receptor agonists SKF 38393 (DA1) and LY 171555 (DA2) failed to produce appreciable ethanol-like stimulus effects. Furthermore, the dopamine receptor antagonists SCH 23390 (DA1) and haloperidol (DA2) did not affect ethanol-appropriate responding when administered in combination with the training dose of ethanol. A number of specific serotonergic receptor ligands were also tested. Quipazine, 5-MeODMT, buspirone, 8-OH-DPAT elicited intermediate ethanol-like stimulus properties in rats. The serotonin receptor blockers pizotifen, pirenperone and (-)propranolol were ineffective in blocking the interoceptive cue produced by 600 mg/kg ethanol. However, TFMPP produced strong ethanol-like discriminative properties and completely substituted for the training dose of ethanol. These results indicate that the stimulus properties of TFMPP are similar to those of a low dose of ethanol.

Effects of fenoldopam, a specific dopamine receptor agonist, on blood pressure and left ventricular function in systemic hypertension.

1. The effects of fenoldopam, an orally active, specific dopamine-1 receptor agonist, were studied in eleven patients with essential hypertension, using intra-arterial blood pressure recording and equilibrium gated radionuclide angiography. 2. A single dose of fenoldopam 100 mg produced a fall in blood pressure (BP) starting after 20 min. The maximum BP reduction (23/25 mm Hg) occurred after 50 min and was accompanied by a heart rate (HR) increase of 10 beats min-1. The acute effects on BP lasted for 130 min. 3. After 8 weeks of fenoldopam 100 mg, twice daily, only a small, statistically insignificant, hypotensive effect was still apparent after each dose of drug. The duration of the effect was too short to be clinically useful. Tilt-testing produced a BP fall of 24/14 mm Hg and a HR increase of 17 beats min-1. Three patients experienced symptoms of postural hypotension during the study. 4. The drug attenuated the blood pressure rise produced by dynamic cycle exercise and isometric hand grip. 5. Acute administration of fenoldopam increased the left ventricular ejection fraction from 61% to 71% (P less than 0.005) and increased the peak filling rate from 2.52 to 3.86 end diastolic vol s-1 (P less than 0.002). After chronic fenoldopam administration, the left ventricular ejection fraction was 65% (P = NS) pre-dose, rising to 69% (P less than 0.02) post-dose and the peak filling rate was increased from 2.7 to 3.38 end diastolic vol s-1 (P less than 0.01) 60 min post-dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological approaches to cocaine addiction

The involvement of central dopamine (DA) neurons in the rewarding action of cocaine is well established. Cocaine euphoria depends on DA activition, and cocaine increases DA neurotransmission by blocking synaptic reuptake. Chronic cocaine use, however, appears to deplete brain DA, and perhaps leads to cocaine craving and withdrawal states. Bromocriptine, a specific DA receptor agonist, appears to ameliorate cocaine craving. Possible clinical roles for bromocriptine, and for DA receptor antagonists, are outlined.

Drugs Five Years Later

Bromocriptine, a specific dopamine receptor agonist, has been used for the treatment of various hyperprolactinemic conditions and as adjunctive therapy for acromegaly (with or without concomitant hyperprolactinemia) and Parkinson's disease. Bromocriptine is extremely effective in suppressing prolactin secretion regardless of the cause, in restoring gonadal function and fertility, and in decreasing the size of prolactin-secreting pituitary tumors. Most patients with acromegaly have clinical improvement with this drug. When bromocriptine is added to a regimen of levodopa or carbidopa, patients with Parkinson's disease frequently have additional clinical improvement and, in most patients, the levodopa or carbidopa dose can be reduced. Withdrawal of bromocriptine therapy is associated in most patients with reversal of its beneficial effects--return of hyperprolactinemia, return of excess growth hormone secretion, and exacerbation of Parkinson's disease.

Pulmonary and Systemic Vascular Effects of SKF-82526-J, a New Specific Peripheral Dopamine Receptor Agonist, in Unanesthetized Neonatal Lambs

We examined the circulatory effects of SKF-82526-J, a new specific dopamine receptor agonist, using chronically instrumented neonatal lambs. Serial infusion of doses ranging from 1.4 to 337 micrograms/kg/min did not change cardiac output, heart rate, left atrial pressure, systemic arterial pressure (SAP) or vascular resistance (SVR). Pulmonary pressure (PAP) increased from 22.6 +/- 1.1 to 35.8 +/- 4.0 mm Hg at 337 micrograms/kg/min (p less than 0.005) and pulmonary resistance (PVR) changed from 0.099 +/- 0.01 to 0.15 +/- 0.03 mm Hg/ml/kg/min (p less than 0.025). After 3-12 min of recovery, SAP increased in all lambs (range 6-57 mm Hg), while PAP fell, without change in cardiac output. Thus, the ratio of PVR/SVR returned more quickly to baseline (4 min) than either PVR or SVR alone. We conclude, based on physiologic evidence, that the newborn lamb pulmonary circulation has no vasodilating dopamine receptors.

Cardiovascular dopamine receptors: physiological, pharmacological and therapeutic implications.

Dopamine receptor activation can lead to pronounced changes in cardiovascular function. The myriad of effects produced by dopamine receptor agonists results from the activation of dopamine receptors located at different anatomical sites in the cardiovascular system. Further basic research is required to better characterize these dopamine receptors so as to allow the development of more specific dopamine receptor agonists. Endogenous dopamine may be involved in the physiological control of fluid and electrolyte balance and continuing research efforts in this area should provide for a better understanding of the role of cardiovascular dopamine receptors in the maintenance of overall circulatory homeostasis. Cardiovascular dopamine receptor stimulation represents an important and promising approach for the development of novel therapeutic agents for the treatment of hypertension, angina pectoris, congestive heart failure and acute renal failure.

Effects of apomorphine and pimozide on temperature regulation during exercise in the rat.

The purpose of this study was to determine the effects of a specific dopamine receptor agonist (apomorphine) and antagonist (pimozide) on thermoregulation when the heat loss pathway was activated by the stress of exercise. Apomorphine or its control vehicle (0.9% wt/vol saline) was injected systemically (0.5, 1.0, 1.5 mg/kg ip) or within the preoptic-anterior hypothalamus (5, 10, 20 micrograms) immediately before the start of treadmill exercise at 21.5 m/min. Colonic, tailskin, and ambient temperatures were recorded each minute. Oxygen consumption was calculated from on-line measurements of percent O2 and CO2. Pimozide injected systemically (0.5 mg/kg ip) had no effect on resting colonic temperature, but caused a significant (P less than 0.05) hyperthermia during treadmill exercise compared to saline controls. Central and systemic injections of apomorphine caused a dose-dependent hypothermia that was blocked by pretreatment with pimozide. Oxygen uptake values during exercise following the central injection of apomorphine were virtually identical to those following the injection of saline, but colonic temperature was significantly (P less than 0.05) lower than saline controls, indicating that the hypothermia observed was not due to a reduction in metabolic rate. These data indicate that dopamine receptors in the preoptic-anterior hypothalamus of the rat participate in the mediation of heat dissipation when the animal is challenged with a heat stress.