Simultaneous serotonin and dopamine monitoring across timescales by rapid pulse voltammetry with partial least squares regression

Many voltammetry methods have been developed to monitor brain extracellular dopamine levels. Fewer approaches have been successful in detecting serotonin in vivo. No voltammetric techniques are currently available to monitor both neurotransmitters simultaneously across timescales, even though they play integrated roles in modulating behavior. We provide proof-of-concept for rapid pulse voltammetry coupled with partial least squares regression (RPV-PLSR), an approach adapted from multi-electrode systems (i.e., electronic tongues) used to identify multiple components in complex environments. We exploited small differences in analyte redox profiles to select pulse steps for RPV waveforms. Using an intentionally designed pulse strategy combined with custom instrumentation and analysis software, we monitored basal and stimulated levels of dopamine and serotonin. In addition to faradaic currents, capacitive currents were important factors in analyte identification arguing against background subtraction. Compared to fast-scan cyclic voltammetry-principal components regression (FSCV-PCR), RPV-PLSR better differentiated and quantified basal and stimulated dopamine and serotonin associated with striatal recording electrode position, optical stimulation frequency, and serotonin reuptake inhibition. The RPV-PLSR approach can be generalized to other electrochemically active neurotransmitters and provides a feedback pipeline for future optimization of multi-analyte, fit-for-purpose waveforms and machine learning approaches to data analysis.

Dopamine-loaded nanoparticle systems circumvent the blood-brain barrier restoring motor function in mouse model for Parkinson's Disease

Parkinson's disease (PD) is a progressive and chronic neurodegenerative disease of the central nervous system. Early treatment for PD is efficient; however, long-term systemic medication commonly leads to deleterious side-effects. Strategies that enable more selective drug delivery to the brain using smaller dosages, while crossing the complex brain-blood barrier (BBB), are highly desirable to ensure treatment efficacy and decrease/avoid unwanted outcomes. Our goal was to design and test the neurotherapeutic potential of a forefront nanoparticle-based technology composed of albumin/PLGA nanosystems loaded with dopamine (ALNP-DA) in 6-OHDA PD mice model. ALNP-DA effectively crossed the BBB, replenishing dopamine at the nigrostriatal pathway, resulting in significant motor symptom improvement when compared to Lesioned and L-DOPA groups. Notably, ALNP-DA (20 mg/animal dose) additionally up-regulated and restored motor coordination, balance, and sensorimotor performance to non-lesioned (Sham) animal level. Overall, ALNPs represent an innovative, non-invasive nano-therapeutical strategy for PD, considering its efficacy to circumvent the BBB and ultimately deliver the drug of interest to the brain.

In vitro investigations on dopamine loaded Solid Lipid Nanoparticles

The progressive degeneration of nigrostriatal neurons leads to depletion of the neurotransmitter dopamine (DA) in Parkinson's disease (PD). The hydrophilicity of DA, hindering its cross of the Blood Brain Barrier, makes impossible its therapeutic administration. This work aims at investigating some physicochemical features of novel Solid Lipid Nanoparticles (SLN) intended to enhance DA brain delivery for PD patients by intranasal administration. For this aim, novel SLN were formulated in the presence of Glycol Chitosan (GCS), and it was found that SLN containing GCS and DA were smaller than DA-loaded SLN, endowed with a slightly positive zeta potential value and, remarkably, incorporated 81 % of the initial DA content. The formulated SLN were accurately characterized by Infrared Spectroscopy in Attenuated Total Reflectance mode (FT-IT/ATR) and Thermogravimetric Analysis (TGA) to highlight SLN solid-state properties as a preliminary step forward biological assay. Overall, in vitro characterization shows that SLN are promising for DA incorporation and stable from a thermal viewpoint. Further studies are in due course to test their potential for PD treatment.

A Novel Somatostatin-Dopamine Chimera (BIM23B065) Reduced GH Secretion in a First-in-Human Clinical Trial

CONTEXT

A somatostatin-dopamine chimera (BIM23B065) was under investigation to reduce GH secretion for the treatment of pituitary adenomas.

OBJECTIVE

To determine pharmacokinetics, safety, and tolerability and to monitor hormonal changes after single and multiple subcutaneous BIM23B065 administrations.

DESIGN

Randomized, double-blind, placebo-controlled, parallel-group design with five single and three 13-day multiple ascending-dose cohorts.

PATIENTS

A total of 63 healthy male white volunteers were enrolled (47 active, 16 placebo).

MAIN OUTCOME MEASURES

Pharmacokinetics, GH, prolactin (PRL), IGF-1, GH after GHRH administration, and general clinical safety criteria.

RESULTS

The maximum dosage of BIM23B065 administered in this study was 1.5 mg. BIM23B065 reduced the mean GH concentrations after 8 and 13 days of treatment. A decrease in GH release after GHRH administration indicated inhibition of the hypothalamic-pituitary-somatotropic axis. IGF-1 was not altered after single doses but showed a significant change from baseline after multiple dosing. PRL secretion was reduced in all subjects who were treated. Orthostatic hypotension and injection site reactions were commonly observed at high dosages. A 6-day uptitration period was included to successfully lower the cardiovascular effects in the multiple ascending dose part of the study.

CONCLUSIONS

Proof of pharmacology of BIM23B065 was shown by a reduction in GH, IGF-1, and PRL concentrations in healthy male volunteers, supporting activity of the somatostatin analog and dopamine agonist moieties. The safety and tolerability of the higher dosing regions was limited mainly by orthostatic hypotension.

Urinary Dopamine as a Potential Index of the Transport Activity of Multidrug and Toxin Extrusion in the Kidney

Dopamine is a cationic natriuretic catecholamine synthesized in proximal tubular cells (PTCs) of the kidney before secretion into the lumen, a key site of its action. However, the molecular mechanisms underlying dopamine secretion into the lumen remain unclear. Multidrug and toxin extrusion (MATE) is a H⁺/organic cation antiporter that is highly expressed in the brush border membrane of PTCs and mediates the efflux of organic cations, including metformin and cisplatin, from the epithelial cells into the urine. Therefore, we hypothesized that MATE mediates dopamine secretion, a cationic catecholamine, into the tubule lumen, thereby regulating natriuresis. Here, we show that [³H]dopamine uptake in human (h) MATE1-, hMATE-2K- and mouse (m) MATE-expressing cells exhibited saturable kinetics. Fluid retention and decreased urinary excretion of dopamine and Na⁺ were observed in Mate1-knockout mice compared to that in wild-type mice. Imatinib, a MATE inhibitor, inhibited [³H]dopamine uptake by hMATE1-, hMATE2-K- and mMATE1-expressing cells in a concentration-dependent manner. At clinically-relevant concentrations, imatinib inhibited [³H]dopamine uptake by hMATE1- and hMATE2-K-expressing cells. The urinary excretion of dopamine and Na⁺ decreased and fluid retention occurred in imatinib-treated mice. In conclusion, MATE transporters secrete renally-synthesized dopamine, and therefore, urinary dopamine has the potential to be an index of the MATE transporter activity.

In-Situ-Generated Vasoactive Intestinal Peptide Loaded Microspheres in Mussel-Inspired Polycaprolactone Nanosheets Creating Spatiotemporal Releasing Microenvironment to Promote Wound Healing and Angiogenesis

Vasoactive intestinal peptide (VIP) was reported to promote angiogenesis. Electrospun nanofibers lead to idea wound dressing substrates. Here we report a convenient and novel method to produce VIP loaded microspheres in polycaprolactone (PCL) nanofibrous membrane without complicated processes. We first coated mussel-inspired dopamine (DA) to nanofibers, then used strong adhesive DA to absorb the functional peptide. PCL membrane was then immersed into acetone to generate microspheres with VIP loading. We employed high pressure liquid chromatography to record encapsulation efficiency of (31.8 ± 2.2)% and loading capacity of (1.71 ± 0.16)%. The release profile of VIP from nanosheets showed a prolonged release. The results of laser scanning confocal microscope, scanning electron microscope and cell counting kit-8 proliferation assays showed that cell adhesion and proliferation were promoted. In order to verify the efficacy on wound healing, in vivo implantation was applied in the full-thickness defect wounds of BALB/c mice. Results showed that the wound healing was significantly promoted via favoring the growth of granulation tissue and angiogenesis. However, we found wound re-epithelialization was not significantly improved. The resulting VIP-DA-coated PCL (PCL-DA-VIP) nanosheets with spatiotemporal delivery of VIP could be a potential application in wound treatment and vascular tissue engineering.

Iptakalim attenuates self-administration and acquired goal-tracking behavior controlled by nicotine

Iptakalim is an ATP-sensitive potassium channel opener, as well as an α4β2-containing nicotinic acetylcholine receptor (nAChR) antagonist. Pretreatment with iptakalim diminishes nicotine-induced dopamine (DA) and glutamate release in the nucleus accumbens. This neuropharmacological profile suggests that iptakalim may be useful for treatment of nicotine dependence. Thus, we examined the effects of iptakalim in two preclinical models. First, the impact of iptakalim on the interoceptive stimulus effect of nicotine was evaluated by training rats in a discriminated goal-tracking task that included intermixed nicotine (0.4 mg/kg, SC) and saline sessions. Sucrose was intermittently presented in a response-independent manner only on nicotine sessions. On intervening test days, rats were pretreated with iptakalim (10, 30, 60 mg/kg, IP). Results revealed that iptakalim attenuated nicotine-evoked responding controlled by the nicotine stimulus in a dose-dependent manner. In a separate study, the impact of iptakalim on the reinforcing effects of nicotine was investigated by training rats to lever-press to self-administer nicotine (0.01 mg/kg/infusion) [Dosage error corrected]. Results revealed that pretreatment with iptakalim (1, 3, 6 mg/kg, IV) decreased nicotine intake (i.e., less active lever responding). Neither behavioral effect was due to a non-specific motor effect of iptakalim, nor to an ability of iptakalim to inhibit DA transporter (DAT) or serotonin transporter (SERT) function. Together, these finding support the notion that iptakalim may be an effective pharmacotherapy for increasing smoking cessation and a better understanding of its action could contribute to medication development.

Compact zwitterion-coated iron oxide nanoparticles for in vitro and in vivo imaging

We have recently developed compact and water-soluble zwitterionic dopamine sulfonate (ZDS) ligand coated superparamagnetic iron oxide nanoparticles (SPIONs) for use in various biomedical applications. The defining characteristics of ZDS-coated SPIONs are small hydrodynamic diameters, low non-specific interactions with fetal bovine serum, the opportunity for specific labeling, and stability with respect to time, pH, and salinity. We report here on the magnetic characterization of ZDS-coated SPIONs and their in vitro and in vivo performance relative to non-specific interactions with HeLa cells and in mice, respectively. ZDS-coated SPIONs retained the superparamagnetism and saturation magnetization (M(s)) of as-synthesized hydrophobic SPIONs, with M(s) = 74 emu g(-1) [Fe]. Moreover, ZDS-coated SPIONs showed only small non-specific uptake into HeLa cancer cells in vitro and low non-specific binding to serum proteins in vivo in mice.

Thromboresistant and endothelialization effects of dopamine-mediated heparin coating on a stent material surface

Heparinization of surfaces has proven a successful strategy to prevent thrombus formation. Inspired by the composition of adhesive proteins in mussels, the authors used dopamine to immobilize heparin on a stent surface. This study aimed to assess the thromboresistant and endothelialization effects of dopamine-mediated heparin (HPM) coating on a stent material surface. The HPM was synthesized by bonding dopamine and heparin chemically. Cobalt-chromium (Co-Cr) alloy disks were first placed in the HPM solution and applied to surface stability then underwent thromboresistant tests and human umbilical vein endothelial cells (HUVEC) cytotoxicity assays. The results showed not only thromboresistant activity and a stable state of heparin on the surfaces after investigation with toluidine blue and thrombin activation assay but also proliferation of HUVEC in vitro. Studies on animals showed that the HPM-coated stent has no obvious inflammation response and increasing of restenosis rate compared to the bare metal stent (BMS) indicating good biocompatibility as well as safety in its in vivo application. Moreover, improving the endothelial cell (EC) proliferation resulted in a higher strut-covering rate (i.e., endothelialization) with shuttle-shaped EC in the HPM-coated stent group compared to that of the BMS group. These results suggest that this facile coating approach could significantly promote endothelialization and offer greater safety than the BMS for its much improved thromboresistant property. Moreover, it may offer a platform for conjugating secondary drugs such as anti-proliferative drugs.

Increased uptake of [¹²³I]meta-iodobenzylguanidine, [¹⁸F]fluorodopamine, and [³H]norepinephrine in mouse pheochromocytoma cells and tumors after treatment with the histone deacetylase inhibitors

[¹³¹I]meta-iodobenzylguanidine ([¹³¹I]MIBG) is the most commonly used treatment for metastatic pheochromocytoma and paraganglioma. It enters the chromaffin cells via the membrane norepinephrine transporter; however, its success has been modest. We studied the ability of histone deacetylase (HDAC) inhibitors to enhance [¹²³I]MIBG uptake by tumors in a mouse metastatic pheochromocytoma model. HDAC inhibitors are known to arrest growth, induce differentiation and apoptosis in various cancer cells, and further inhibit tumor growth. We report the in vitro and in vivo effects of two HDAC inhibitors, romidepsin and trichostatin A, on the uptake of [(3)H]norepinephrine, [¹²³I]MIBG, and [(18)F]fluorodopamine in a mouse model of metastatic pheochromocytoma. The effects of both inhibitors on norepinephrine transporter activity were assessed in mouse pheochromocytoma (MPC) cells by using the transporter-blocking agent desipramine and the vesicular-blocking agent reserpine. HDAC inhibitors increased [(3)H]norepinephrine, [¹²³I]MIBG, and [(18)F]fluorodopamine uptake through the norepinephrine transporter in MPC cells. In vivo, inhibitor treatment resulted in significantly increased uptake of [(18)F]fluorodopamine positron emission tomography (PET) in pheochromocytoma liver metastases (19.1 ± 3.2% injected dose per gram of tumor (%ID/g) compared to liver metastases in pretreatment scans 5.9 ± 0.6%; P<0.001). Biodistribution analysis after inhibitors treatment confirmed the PET results. The uptake of [(123)I]MIBG was significantly increased in liver metastases 9.5 ± 1.1% compared to 3.19 ± 0.4% in untreated control liver metastases (P<0.05). We found that HDAC inhibitors caused an increase in the amount of norepinephrine transporter expressed in tumors. HDAC inhibitors may enhance the therapeutic efficacy of [(131)I]MIBG treatment in patients with advanced malignant pheochromocytoma and paraganglioma.

Glutamate-conjugated liposomes of dopamine hydrochloride for effective management of parkinsonism's

The blood-brain barrier restricts the brain uptake of many important hydrophilic drugs and limits their efficacy in the treatment of brain diseases because of the presence of tight junctions, high metabolic capacity, low pinocytic vesicular traffic, and efficient efflux mechanisms. In the present project, amino acid-coupled liposomes bearing dopamine HCl were prepared to deliver the drug to the brain utilizing receptor-mediated transcytosis. Uncoupled liposomes were prepared by cast film method using phosphatidylcholine and cholesterol, whereas coupled liposomes were prepared using phosphatidylcholine, cholesterol, and glutamate stearylamine conjugate in the film. These liposomes were characterized for entrapment efficiency, vesicle size, shape, in vitro drug release, and in vivo studies. The in vitro drug release was analysed by using dialysis membrane. The vesicle size was found to increase upon coupling of liposomes, whereas percent entrapment efficiency was reduced from 38.89 +/- 1.94% to 34.15 +/- 1.70% after coupling of liposomes. The in vitro percent cumulative drug release studies exhibited 51.6% drug release for uncoupled liposome and 37.9% for coupled liposome at the end of 24 h. These selected formulations were subjected for in vivo performance, which was assessed by periodic measurement of drug (chlorpromazine)-induced catatonia in albino rats (Wistar strain) and fluorescence microscopy studies of the rat brain. The results were compared with plain dopamine HCl solution. Studies revealed that dopamine HCl can be effectively delivered to brain via glutamate-coupled liposomes, and results clearly indicated the superiority of the coupled liposomal formulation over the uncoupled formulation.

Design, biometric simulation and optimization of a nano-enabled scaffold device for enhanced delivery of dopamine to the brain

This study focused on the design, biometric simulation and optimization of an intracranial nano-enabled scaffold device (NESD) for the site-specific delivery of dopamine (DA) as a strategy to minimize the peripheral side-effects of conventional forms of Parkinson's disease therapy. The NESD was modulated through biometric simulation and computational prototyping to produce a binary crosslinked alginate scaffold embedding stable DA-loaded cellulose acetate phthalate (CAP) nanoparticles optimized in accordance with Box-Behnken statistical designs. The physicomechanical properties of the NESD were characterized and in vitro and in vivo release studies performed. Prototyping predicted a 3D NESD model with enhanced internal micro-architecture. SEM and TEM revealed spherical, uniform and non-aggregated DA-loaded nanoparticles with the presence of CAP (FTIR bands at 1070, 1242 and 2926 cm(-1)). An optimum nanoparticle size of 197 nm (PdI=0.03), a zeta potential of -34.00 mV and a DEE of 63% was obtained. The secondary crosslinker BaCl(2) imparted crystallinity resulting in significant thermal shifts between native CAP (T(g)=160-170 degrees C; T(m)=192 degrees C) and CAP nanoparticles (T(g)=260 degrees C; T(m)=268 degrees C). DA release displayed an initial lag phase of 24 h and peaked after 3 days, maintaining favorable CSF (10 microg/mL) versus systemic concentrations (1-2 microg/mL) over 30 days and above the inherent baseline concentration of DA (1 microg/mL) following implantation in the parenchyma of the frontal lobe of the Sprague-Dawley rat model. The strategy of coupling polymeric scaffold science and nanotechnology enhanced the site-specific delivery of DA from the NESD.

Role of organic cation transporters in dopamine uptake across olfactory and nasal respiratory tissues

Dopamine has poor oral bioavailability and low permeability across the blood-brain barrier, and yet it has been reported to have good systemic and central nervous system (CNS) bioavailability following nasal administration. The aim of this study was to investigate the extent to which dopamine transport in the olfactory and respiratory mucosae results from the activity of organic cation transporters (OCTs) present in the nasal cavity. Transport studies were carried out to determine the mechanism of dopamine transport across bovine olfactory and nasal respiratory mucosa. Western blotting and immunohistochemistry were performed to determine the expression and localization of organic cation transporter-2, a major transporter of dopamine, in the nasal mucosa. Dopamine transport was found to be saturable across both tissues. Amantadine, an organic cation transporter-1 (OCT-1) and organic cation transporter-2 (OCT-2) mixed inhibitor, decreased dopamine flux to a greater extent than guanidine, a more specific organic cation transporter-2 inhibitor. Immunohistochemistry results showed that organic cation transporter-2 was localized in both the epithelial and submucosal regions of the nasal olfactory and respiratory mucosa. Dopamine transport across the olfactory and respiratory mucosae is partially mediated by organic cation transporters, including OCT-1 and OCT-2. Utilization of uptake transporters may provide the opportunity for improved systemic absorption and targeted CNS delivery of dopamine and other drug compounds following nasal administration.

Glycine stimulates the release of labeled acetylcholine but not dopamine nor glutamate from superfused rat striatal tissue

Glycine is known as an inhibitory neurotransmitter in the spinal cord and forebrain but its precise role in the forebrain is largely overlooked. This investigation evaluated whether glycine alters acetylcholine, glutamate or dopamine release from striatal tissue using an in vitro approach. We observed that while glycine induced a robust (3)H-acetylcholine release ((3)H-ACh) from superfused striatal tissue, it failed at releasing (3)H-glutamate or (3)H-dopamine. Glycine stimulated (3)H-ACh release in a dose- and calcium-dependent manner (EC(50)=69 microM). Tetrodotoxin (1 microM) inhibited about 75% of the release demonstrating a predominant dendritic and cell body location of glycine receptors. The prototypical glycine receptor antagonist strychnine at 10 microM completely abolished (3)H-ACh release. To further characterize the role of striatal glycine receptors in (3)H-ACh release we examined glycine effects after in vivo treatment with Haloperidol-decanoate (HD). Treatment for 30 days or more with HD decreased maximal glycine-stimulated release of (3)H-ACh suggesting a non-competitive inhibition. After 30 days of washout release parameters did not return to vehicle-treated levels. The glutamate agonist NMDA also stimulated acetylcholine release but showed slightly different behavior in HD-treated striatal tissue. These effects could be attributed to changes in chloride transporters expressed in the giant striatal cholinergic cell as well as glycine receptor subunit composition and finally, GABA/glycine co-release in this tissue.

Inhibitive effects of Fructus Psoraleae extract on dopamine transporter and noradrenaline transporter

A petroleum ether extract (FP) from Fructus Psoraleae, seeds of Psoralea corylifolia L. (Leguminosae), was found to strongly inhibit dopamine (DA) uptake by dopamine transporter (DAT) heterogeneously expressed cells (D8 cells) and noradrenaline (NE) uptake by noradrenaline transporter (NET) heterogeneously expressed cells, which, however, had no effect on gamma-aminobutyric acid transporter heterogeneously expressed cells and serotonin transporter heterogeneously expressed cells at the concentration up to 100 microg/ml. These inhibitory effects were also confirmed by experiments on SK-N-SH cell line and synaptosomes from rats' brains. In addition, FP showed a significantly mitigating effect on 1-methyl-4-pyridinium induced injury of D8 cells. Meanwhile, FP dose-dependently reduced the binding of tritium-labeled cocaine analog (-)-2beta-carbomethoxy-3beta-(4-fluorophenyl) tropane to DAT of D8 cells, which suggests that FP may inhibit DAT activity in the same way as cocaine does. Behavioral study showed FP had a long-lasting stimulant effects on the activity of intact mice and reserpinized mice. So FP is proposed as a kind of DAT and NET inhibitor and may be involved in the process of regulating the DA and NE system, and FP or its unknown bioactive compounds may be developed into new medicines for disorders such as Parkinson's disease, depression, Attention Deficit Hyperactivity Disorder (ADHD) or cocaine addiction.

Mechanism of 6-hydroxydopamine neurotoxicity: the role of NADPH oxidase and microglial activation in 6-hydroxydopamine-induced degeneration of dopaminergic neurons

Cell death induced by 6-hydroxydopamine (6-OHDA) is thought to be caused by reactive oxygen species (ROS) derived from 6-OHDA autooxidation and by a possible direct effect of 6-OHDA on the mitochondrial respiratory chain. However, the process has not been totally clarified. In rat primary mesencephalic cultures, we observed a significant increase in dopaminergic (DA) cell loss 24 h after administration of 6-OHDA (40 micromol/L) and a significant increase in NADPH subunit expression, microglial activation and superoxide anion/superoxide-derived ROS in DA cells that were decreased by the NADPH inhibitor apocynin. Low doses of 6-OHDA (10 micromol/L) did not induce a significant loss of DA cells or a significant increase in NADPH subunit expression, microglial activation or superoxide-derived ROS. However, treatment with the NADPH complex activator angiotensin II caused a significant increase in all the latter. Forty-eight hours after intrastriatal 6-OHDA injection in rats, there was still no loss of DA neurons although there was an increase in NADPH subunit expression and NADPH oxidase activity. The results suggest that in addition to the autooxidation-derived ROS and the inhibition of the mitochondrial respiratory chain, early microglial activation and NADPH oxidase-derived ROS act synergistically with 6-OHDA and constitute a relevant and early component of the 6-OHDA-induced cell death.

Low-dose dopamine in the intensive care unit

For much of the last four decades, low-dose dopamine has been considered the drug of choice to treat and prevent renal failure in the intensive care unit (ICU). The multifactorial etiology of renal failure in the ICU and the presence of coexisting multisystem organ dysfunction make the design and execution of clinical trials to study this problem difficult. However, in the last decade, several meta-analyses and one large randomized trial have all shown a lack of benefit of low-dose dopamine in improving renal function. There are multiple reasons for this lack of efficacy. While dopamine does cause a diuretic effect, it does very little to improve mortality, creatinine clearance, or the incidence of dialysis. Evidence is also growing of its adverse effects on the immune, endocrine, and respiratory systems. It may also potentially increase mortality in sepsis. It is the opinion of the authors that the practice of using low-dose dopamine should be abandoned. Other drugs and treatment modalities need to be explored to address the serious issue of renal failure in the ICU.

Inotropic, chronotropic, and arrhythmogenic effects of dopamine on the isolated working heart of rabbit

Some improvements of Kodama's method for perfusing the isolated rabbit heart in its working mode were made. Increases in the right and left atrium pressure, together with an increase in the pulmonary artery pressure, were observed to occur immediately after the start of venous return, and then all of the increased pressures were found to remain at each constant level. In these stable states, the administration of dopamine (DA) into the perfusate was found to produce dose-related increases in contractile activities. In the preparations denervated with reserpine or 6-hydroxydopamine, in which tyramine (Ty) produced no response, the inotropic effectiveness of DA did not differ from that in the normal ones. On the other hand, responses to noradrenaline (NA) were found to increase significantly after the denervation. DA produced a dose-related increase in heart rates in the normal preparation, and this effect was greatly suppressed in the denervated preparations, suggesting that the primary chronotropic effect of DA is an indirect one via the release of NA from the sympathetic nerve terminals. Arrhythmogenic effects of NA, Ty or DA were also observed in these preparations. At all the doses tested, the incidence rates by NA were as high as 50% or more, the type of arrhythmia being recognized as atrial or ventricular extrasystole from the ECG analysis. On the other hand, the rates by DA were relatively low, less than 34%. From a comparison of the incidence rates between the normal and denervated preparations, this effect of DA was considered to be primarily an indirect one.

Novel L-Dopa and Dopamine Prodrugs Containing a 2-Phenyl-imidazopyridine Moiety

PURPOSE

The aim of this study was to gain insight into the feasibility of enhancing the delivery of L-Dopa and dopamine to the brain by linking these neurotransmitters and L-Dopa ethyl ester to 2-phenyl-3-carboxymethyl-imidazopyridine compounds giving rise to the so-called Dopimid compounds.

MATERIALS AND METHODS

A number of Dopimid compounds were synthesized and both stability and binding studies to dopaminergic and benzodiazepine receptors were performed. To evaluate whether Dopimid compounds are P-gp substrates, [(3)H]ritonavir uptake experiments and bi-directional transport studies on confluent MDCKII-MDR1 monolayers were carried out. The brain penetration properties of Dopimid compounds were estimated by the Clark's computational model and evaluated by investigation of their transport across BBMECs monolayers. The dopamine levels following the intraperitoneal administration of the selected Dopimid compounds were measured in vivo by using brain microdialysis in rat.

RESULTS

Tested compounds were adequately stable in solution buffered at pH 7.4 but undergo faster cleavage in dilute rat serum at 37 degrees C. Receptor binding studies showed that Dopimid compounds are essentially devoid of affinity for dopaminergic and benzodiazepine receptors. [(3)H]ritonavir uptake experiments indicated that selected Dopimid compounds, like L-Dopa and dopamine hydrochloride, are not substrates of P-gp and it was also confirmed by bi-directional transport experiments across MDCKII-MDR1 monolayers. By Clark's model a significant brain penetration was deduced for L-Dopa ethyl ester and dopamine derivatives. Transport studies involving BBMECs monolayers indicated that some of these compounds should be able to cross the BBB. Interestingly, the rank order of apparent permeability (P (app)) values observed in these assays parallels that calculated by the computational approach. Brain microdialysis experiments in rat showed that intraperitoneal acute administration of some Dopimid compounds induced a dose-dependent increase in cortical dopamine output.

CONCLUSION

Based on these results, it may be concluded that some Dopimid compounds can be proposed as novel L-Dopa and dopamine prodrugs.

N-oleoyl-dopamine increases locomotor activity in the rat

N-oleoyl-dopamine (OLDA), a condensation product of oleic acid and dopamine (DA), is a bioactive lipid whose biological functions are not yet fully explored. The compound crosses the blood-brain barrier and might be considered as a carrier of DA into the brain. In this study we sought to determine whether OLDA would influence locomotor behavior and whether the central DA system would be involved in such influence. We addressed this issue by investigating horizontal locomotor activity in male Wistar rats after intraperitoneal administration of OLDA, 5-20 mg/kg, before and after pre-treatment with haloperidol, a D2 receptor antagonist. We found that OLDA caused a prompt stimulation of locomotor activity, with a bell-shaped dose-response. The maximum stimulatory effect was observed after 10 mg/kg of OLDA where the mean distance traveled by rats during a 2-hour test increased to 1213+/-196(SE) cm from the 403+/-89 cm in the vehicle-treated rats (P<0.05). This effect was dose-dependently antagonized by haloperidol (0.1-0.2 mg/kg). The results support the hypothesis that the OLDA-induced hyperlocomotion was mediated by the stimulation of DA systems. Using in vitro assays, we further demonstrated that OLDA is a stable compound that resists hydrolysis over a 2-hour period and thus the integral OLDA compound exerted DA-like effects. We conclude that OLDA is a potential brain modifier of motor behavior, the biological consequences of which remain to be explored.

Substrates and inhibitors display different sensitivity to expression level of the dopamine transporter in heterologously expressing cells

The use of heterologous expression systems for studying dopamine (DA) transporter (DAT) function has provided important information corroborating and complementing in situ obtained knowledge. Preliminary experiments with human embryonic kidney cells (HEK293) heterologously expressing varying amounts of DAT suggested fluctuations in the potency of cocaine in inhibiting DA uptake and led to the present systematic assessment of the impact of the density of DAT on its function. Transiently expressing intact HEK293 cells, transfected with increasing amounts of DAT cDNA, displayed increasing levels of surface DAT, binding of the cocaine analog [(3)H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane ([(3)H]CFT), and uptake of [(3)H]DA, [(3)H]N-methyl-4-phenylpyridinium ([(3)H]MPP(+)), [(3)H]norepinephrine, and [(3)H]serotonin. However, the amount of DAT cDNA and the DAT expression level required to produce 50% of maximal activity was threefold higher for CFT binding than for DA uptake. Increased DAT expression was accompanied by weakened potency in inhibiting [(3)H]DA uptake for cocaine, CFT, benztropine, and its analog JHW025, GBR 12909 and mazindol; their potency in inhibiting [(3)H]CFT binding was unaffected. Inhibition of uptake by the substrates DA, m-tyramine, d-amphetamine, or MPP(+) was also unaffected. Increasing DAT in stably expressing HEK293 cells by stimulation of gene expression with sodium butyrate also decreased the uptake inhibitory potency of a number of the above blockers without affecting the interaction between substrates and DAT. The present results prompt discussion of models explaining how factors regulating DAT expression at the plasma membrane can regulate DAT function and pharmacology.

Nicotine increases dopamine transporter function in rat striatum through a trafficking-independent mechanism

In previous in vivo voltammetry studies, acute nicotine administration increased striatal dopamine clearance. The current study aimed to determine whether nicotine also increases [(3)H]dopamine uptake across the time course of the previous voltammetry studies and whether dopamine transporter trafficking to the cell surface mediates the nicotine-induced augmentation of dopamine clearance in striatum. Rats were administered nicotine (0.32 mg/kg, s.c.); striatal synaptosomes were obtained 5, 10, 40 or 60 min later. Nicotine increased (25%) the V(max) of [(3)H]dopamine uptake at 10 and 40 min. To determine whether the increase in V(max) was due to an increase in dopamine transporter density, [(3)H]GBR 12935 (1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine dihydrochloride) binding was performed using rat striatal membranes; no differences were found between nicotine and saline-control groups at 5, 10 or 40 min post-injection, indicating that nicotine did not increase striatal dopamine transporter density; however, [(3)H]GBR 12935 binding assays determine both cell surface and intracellular dopamine transporter. Changes in cellular dopamine transporter localization in striatum were determined using biotinylation and subfractionation approaches; no differences between nicotine and saline-control groups were observed at 10 and 40 min post-injection. These results suggest that the nicotine-induced increase in dopamine uptake and clearance in striatum may occur via a trafficking-independent mechanism.

Metabolism of dopamine by the nasal mucosa

The nasal route of administration offers several advantages over oral and intravenous administration, including the ability to avoid hepatic first pass metabolism. Dopamine deficiency has been associated with several neurological disorders; it has been shown to have good systemic bioavailability and significant uptake into the CNS following intranasal administration. The purpose of these studies was to investigate the limiting role of mucosal metabolism of dopamine during nasal absorption. In vitro transport and initial rate studies were carried out using nasal mucosal explants to study dopamine permeability and metabolism. Dihydroxyphenylacetic acid (DOPAC) was the only metabolite detected. Monoamine oxidase (MAO), the enzyme responsible for DOPAC formation, was localized to the submucosal region of the nasal explants. The amount of DOPAC formed during the transport studies was less than 0.5% of the initial amount of dopamine placed into the system. Iproniazid, an MAO inhibitor, blocked DOPAC formation but had no effect on dopamine transport. The limited extent of dopamine metabolism compared to its mucosal transport demonstrates that nasal dopamine transport is not significantly reduced by mucosal metabolism and suggests that the nasal route may be promising for the efficient delivery of dopamine to the CNS.

Molecular mechanism involved in the transport of a prodrug dopamine glycosyl conjugate

We have previously demonstrated that dopamine conjugation to glucose allows it to induce therapeutic effects against Parkinson's disease after intravenous administration. In this paper we demonstrate that, unlike dopamine, the prodrug glu-dopamine is a transportable substrate of glucose transporters. Towards this, the effect of glucose-conjugation on the affinity and uptake of dopamine have been assessed in vitro, using human retinal pigment epithelium (HRPE) cells. Glucose transporter-mediated uptake was measured using [(3)H]3-O-methylglucose ([(3)H]3-O-MG) as the tracer. The uptake was found to be rapid and hyperbolically related to its concentrations (K(t)=7.8+/-1.2mM and V(max)=54+/-2 nmol/min mg protein). Inhibition experiments showed that dopamine was able to interact with glucose carriers only when conjugated to glucose (IC(50)=2.6+/-0.6mM). HPLC analysis of HRPE cell extracts showed that both dopamine and the prodrug permeate the cell, but only the uptake of the prodrug is inhibitable by glucose. This confirms that glucose transporters mediate the transport of the prodrug glu-dopamine, but not of dopamine. HRPE cells is therefore proposed as a promising model for in vitro studies involving the glucose transporter-mediated transport of drugs and their conjugates.

Gain of function mutants reveal sites important for the interaction of the atypical inhibitors benztropine and bupropion with monoamine transporters

Two atypical inhibitors of the dopamine transporter, benztropine, used in the treatment of Parkinson's disease, and bupropion, used as an antidepressant, show very different psychostimulant effects when compared with another inhibitor, cocaine. Taking advantage of the differential sensitivity of the dopamine and the norepinephrine transporters (DAT and NET) to benztropine and bupropion, we have used site-directed mutagenesis to produce gain-of-function mutants in NET which demonstrate that Ala279 in the trans-membrane domain 5 (TM5) and Ser359 in the TM7 of DAT are responsible for the higher sensitivity of DAT to both bupropion and benztropine. Substitution of these two DAT residues into the NET background does not alter the potency of NET-selective inhibitors, such as desipramine. The results from experiments examining the ability of DAT-selective inhibitors to displace [3H]nisoxetine binding in NET gain-of-function mutants suggest that Ser359 contributes to the initial binding of the inhibitor, and that Ala279 may influence subsequent steps involved in the blockade of translocation. Thus, these studies begin to identify residues that are important for the unique molecular interactions of benztropine and bupropion with the DAT, and that ultimately may contribute to the distinct behavioral actions of these drugs.

Low-dose dopamine: a physiologically based review

BACKGROUND

In an attempt to prevent or alter the course of acute renal failure, many surgeons continue to use low-dose dopamine. This article critically reviews the physiologic reasons why low-dose dopamine is not clinically efficacious.

METHODS

A critical review of English language literature.

RESULTS

The effect of dopamine on renal blood flow remains controversial. If dopamine does increase renal blood flow, the vascular anatomy of the kidney would limit its effectiveness. Rather than improving renal function, dopamine has been shown to impair renal oxygen kinetics, inhibit feedback systems that protect the kidney from ischemia, and may worsen tubular injury. Dopamine has not been proven useful in the prevention or alteration of the course of acute renal failure as a result of heart failure, cardiac surgery, abdominal aortic surgery, sepsis, and transplantation. Dopamine has been associated with multiple complications involving the cardiovascular, pulmonary, gastrointestinal, endocrine, and immune systems.

CONCLUSIONS

Based on the anatomy and physiology of the kidney, low-dose dopamine would not be expected to improve renal failure and this has been demonstrated by the lack of efficacy in clinical trials.

Wld(S) mice are protected against the Parkinsonian mimetic MPTP

The progressive loss of the nigrostriatal pathway is a distinguishing feature of Parkinson's disease. Because terminal field loss appears to precede cell body loss, we tested whether the mouse mutant Wld(S), which delays axonal degeneration in a variety of disorders, would ameliorate nigrostriatal degeneration following treatment with the Parkinsonian mimetic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The present findings show that the Wld(S) gene product enhances survival, prevents nigrostriatal axon degeneration, and attenuates neurotransmitter loss but does not rescue cell bodies. As MPTP is thought to impair mitochondrial energy production, these data suggest that disease pathology due to metabolic dysfunction could be improved by the Wld(S) gene product. These results suggest new therapeutic avenues for Parkinson's disease.

Topography of 3H-DHA, 3H-QNB, 3H-dopamine, and 3H-DAGO binding sites distribution in the central part of the sinoatrial node in rat heart

The topography of distribution of 3H-dihydroalprenolol, 3H-quinucledinyl benzilate, 3H-dopamine, and 3H-DAGO binding sites in the central part of the sinoatrial node in rat heart was studied by autoradiography after electrophysiological identification of the dominant pacemaker region location. Receptor asymmetry between the lateral and median regions of the central part of the sinoatrial node was shown. The dominant pacemaker region lay in the lateral area of the sinoatrial node; the number of binding sites for all four ligands was minimum in it. The number of binding sites gradually increased in the cranial and caudal directions from the dominant pacemaker region along the sinoatrial node artery (more smoothly in the caudal direction). The relative densities of bindings sites for 3H-dihydroalprenolol and 3H-dopamine were higher in the lateral region compared to the perinodal working myocardium, while the densities for 3H-quinucledinyl benzilate and 3H-DAGO were virtually the same. The distribution of binding sites along the artery in the median region of the sinoatrial node was even for 3H-quinucledinyl benzilate and 3H-DAGO. For 3H-DAGO these parameters were close to those in the perinodal atrial myocardium, for 3H-quinucledinyl benzilate somewhat lower. Curves presenting the distribution of binding site densities for 3H-dihydroalprenolol and 3H-dopamine in the median region of the sinoatrial node were similar, with a pronounced peak in the region contralateral to the dominant pacemaker region, and significantly higher binding parameters compared to those for the perinodal atrial myocardium. The difference consisted in higher density of 3H-dopamine binding sites in the median region of the sinoatrial node in comparison with the lateral region. Binding activity was maximum in the wall of the sinoatrial node artery. The distribution of binding sites for ligands to the main autonomic nervous system neurotransmitters in the rat heart sinoatrial node is heterogeneous.

Dopamine uptake and cocaine binding mechanisms: The involvement of charged amino acids from the transmembrane domains of the human dopamine transporter

The wild type human dopamine transporter (DAT) and five DAT mutants were transfected into COS-7 cells and their ability to uptake dopamine or to bind cocaine was examine three days later. In each mutant, a single charged amino acid, located in areas that initial hydrophobic analysis had indicated were DAT transmembrane domains was substituted by alanine. Mutants used in this study were lysines 257 and 525 (termed K257A and K525A), arginines 283 and 521 (termed R283A and R521A), and glutamate 491 (termed E491A). Dopamine affinity was significantly enhanced in the K257A and R283A mutants, and the IC(50) for displacement of the radioactive cocaine analog 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane (CFT) by cocaine was significantly elevated in the E491A mutant. All mutants displayed a reduction or complete loss of the maximal velocity (V(m)) of dopamine transport.

Chronic food restriction and dopamine transporter function in rat striatum

The present communication reports on DA uptake in rat striatum in a model of chronic food restriction. The K(m) for DA uptake was unaltered, but the V(max) was reduced by 32%, not supporting the idea that the enhanced behavioral sensitivity to cocaine or d-amphetamine upon chronic food restriction is due to a greater density of DAT at the plasma membrane for drug interaction. Chronic food restriction did not alter the potency of cocaine or D-amphetamine in inhibiting DA uptake in the striatum, suggesting that the enhanced behavioral sensitivity to these drugs upon chronic food restriction is not due to their enhanced affinity for DAT. These results point to factors other than DAT density or affinity underlying the sensitized response to psychostimulants in food restriction.

Effects of direct intravitreal dopamine injections on the development of lid-suture induced myopia in rabbits

BACKGROUND

Dopamine (DA) storage and release are reduced in form deprivation myopia (FDM) in a wide range of species, from chicks to primates. FDM can be prevented by treatment with DA agonists such as apomorphine, and paradoxically by the dopamine neurotoxin 6-hydroxydopamine. In this study, we increased the DA levels by direct intravitreal DA injections to learn if FDM can be suppressed in a rabbit model.

METHODS

Seven-day-old rabbits were deprived of pattern vision by the suturing the right eyelids after natural eye opening. In the first group (FD, n=20), the right eye received form deprivation (FD) alone. In the second group (DA-FD, n=16), the deprived eye of 7-day-old rabbits received four intravitreal injections of 20 microg dopamine every 5 days. In the third group (saline-FD, n=16), the deprived eye received saline injections with the same schedule. The contralateral eye remained untreated as a control. At the end of the 8-week deprivation period, the effects of DA on refractive error, corneal curvature and ocular dimensions were assessed by streak retinoscopy, keratometry and A-scan ultrasonography, respectively.

RESULTS

Eight weeks of FDM induced a myopic shift of -2.70+/-0.87 D (n=20) in treated eyes compared with contralateral eyes. The major structural correlate of the myopia appeared to be elongation of the vitreous chamber (0.7+/-0.3 mm, n=20) and axial elongation (0.9+/-0.3 mm, n=20), respectively. Repeated intravitreal injections of DA fully prevented the myopic shift (-0.06+/-0.37 D), elongation of the vitreous chamber (0.1+/-0.3 mm, n=16) and axial elongation (0.3+/-0.2 mm, n=16) due to lid suture, whereas saline injections had slight effect.

CONCLUSIONS

FD by suturing eyelids is an effective technique to induce a significant myopic shift, vitreous chamber and axial elongation in rabbits as a model of myopia development. These changes associated with FD were retarded by intravitreal injections of DA.

Discordant localization of 2-[18F]-fluoro-2-deoxy-D-glucose in 6-[18F]-fluorodopamine- and [123I]-metaiodobenzylguanidine-negative metastatic pheochromocytoma sites

BACKGROUND

Although the majority of pheochromocytomas (PHEO) are benign, a subset is malignant. Computed tomography (CT) and magnetic resonance imaging (MRI) localize PHEO with high sensitivity but, because of limited specificity, [(131)I]- or [(123)I]-metaiodobenzylguanidine ([(131)I]- or [(123)I]-MIBG) is often used as a complementary agent. 6-[18F]-fluorodopamine ([18F]-DA) has been developed as a radiopharmaceutical for the targeting of noradrenergic pathways, and has been shown to result in a better detection rate of PHEO sites than MIBG; however, [18F]-DA has shown a lack of accumulation in some patients with metastatic PHEO.

METHODS

Five patients with widespread metastatic PHEO who had CT and MRI evidence of metastatic disease (one man and four women; age range, 25-64 years), and who underwent imaging with [(123)I]-MIBG, [18F]-DA and 2-[18F]-fluoro-2-deoxy-D-glucose ([18F]-FDG), were evaluated retrospectively. Tomographic imaging was performed and positron emission tomography (PET) images were inspected visually and quantitatively.

RESULTS

All five patients had [(123)I]-MIBG scans that grossly underestimated the extent of disease when compared with conventional CT and MRI. All lesions seen on [(123)I]-MIBG scans were detected on [18F]-DA scans, which also detected additional lesions. Nonetheless, [18F]-DA also failed to detect numerous lesions seen on CT and MRI. In all of these cases, [18F]-FDG PET showed lesions that were not detected on either [(123)I]-MIBG or [18F]-DA scans.

CONCLUSIONS

When [(123)I]-MIBG or [18F]-DA fails to localize lesions seen on conventional imaging studies, [18F]-FDG may be recommended as an ancillary test for the diagnosis and localization of metastatic PHEO. This is particularly important in patients with aggressive PHEO.

Bupropion increases striatal vesicular monoamine transport

The vesicular monoamine transporter-2 (VMAT-2) is principally involved in regulating cytoplasmic dopamine (DA) concentrations within terminals by sequestering free DA into synaptic vesicles. This laboratory previously identified a correlation between striatal vesicular DA uptake through VMAT-2 and inhibition of the DA transporter (DAT). For example, administration of methylphenidate (MPD), a DAT inhibitor, increases vesicular DA uptake through VMAT-2 in a purified vesicular preparation; an effect associated with a redistribution of VMAT-2 protein within DA terminals. The purpose of this study was to determine if other DAT inhibitors, including bupropion, similarly affect VMAT-2. Results revealed bupropion rapidly, reversibly, and dose-dependently increased vesicular DA uptake; an effect also associated with VMAT-2 protein redistribution. The bupropion-induced increase in vesicular DA uptake was prevented by pretreatment with eticlopride, a DA D2 receptor antagonist, but not by SCH23390, a DA D1 receptor antagonist. We previously reported that MPD post-treatment prevents persistent DA deficits associated with multiple methamphetamine (METH) administrations. Although bupropion attenuated the METH-induced reduction in VMAT-2 activity acutely, it did not prevent the long-term dopaminergic toxicity or the METH-induced redistribution of VMAT-2 protein. The findings from this study demonstrate similarities and differences in the mechanism by which MPD and bupropion affect striatal dopaminergic nerve terminals.

Variants of the primate vesicular monoamine transporter-2

The vesicular monoamine transporter-2 (VMAT2) sequesters monoamine neurotransmitters into vesicles and prevents neurotoxicity. Human or monkey striatum generated three VMAT2 immunoreactive proteins of approximately 75 kDa, approximately 52-55 kDa, and approximately 45 kDa. The approximately 55-kDa band is considered the unglycosylated native protein. Deglycosylation of the VMAT2 from striatum or human VMAT2 expressed in HEK293 cells yielded a approximately 45-kDa, but not a 55-kDa immunoreactive band. We investigated this apparent mismatch between observed molecular size and predicted size.

A triple mutation in the second transmembrane domain of mouse dopamine transporter markedly decreases sensitivity to cocaine and methylphenidate

Previously, we reported that Phe105 in transmembrane domain 2 of the mouse dopamine transporter (DAT) is crucial for high-affinity cocaine binding. In the current study, we investigated whether other residues surrounding Phe105 also affect the potency of cocaine inhibition. After three rounds of sequential random mutagenesis at these residues, we found a triple mutant (L104V, F105C and A109V) of mouse DAT that retained over 50% uptake activity and was 69-fold less sensitive to cocaine inhibition when compared with the wild-type mouse DAT. The triple mutation also resulted in a 47-fold decrease in sensitivity to methylphenidate inhibition, suggesting that the binding sites for cocaine and methylphenidate may overlap. In contrast, the inhibition of dopamine uptake by amphetamine or methamphetamine was not significantly changed by the mutations, suggesting that the binding sites for the amphetamines differ from those for cocaine and methylphenidate. Such functional but cocaine-insensitive DAT mutants can be used to generate a knock-in mouse line to study the role of DAT in cocaine addiction.

Deficits in dopamine clearance and locomotion in hypoinsulinemic rats unmask novel modulation of dopamine transporters by amphetamine

Insulin affects brain reward pathways and there is converging evidence that this occurs through insulin regulation of the dopamine (DA) transporter (DAT). In rats made hypoinsulinemic by fasting, synaptosomal DA uptake is reduced. Interestingly, [3H]DA uptake is increased in hypoinsulinemic rats with a history of amphetamine self-administration. The possibility that amphetamine and insulin act in concert to regulate DAT activity prompted this study. Here we show that [3H]DA uptake, measured in vitro and clearance of exogenously applied DA in vivo, is significantly reduced in rats made hypoinsulinemic by a single injection of streptozotocin. Strikingly, amphetamine (1.78 mg/kg, given every other day for 8 days) restored DA clearance in streptozotocin-treated rats but was without effect on DA clearance in saline-treated rats. Basal locomotor activity of streptozotocin-treated rats was lower compared to control rats; however, in streptozotocin-treated rats, hyperlocomotion induced by amphetamine increased over successive amphetamine injections. In saline-treated rats the locomotor stimulant effect of amphetamine remained stable across the four amphetamine injections. These results provide exciting new evidence that actions of amphetamine on DA neurotransmission are insulin-dependent and further suggest that exposure to amphetamine may cause long-lasting changes in DAT function.

Theory for the impact of basal turnover on dopamine clearance kinetics in the rat striatum after medial forebrain bundle stimulation and pressure ejection

Although microdialysis measurements suggest that extracellular dopamine concentrations in the rat striatum are in the low nanomolar range, some recent voltammetry studies suggest that the concentration may be considerably higher, perhaps in the micromolar range. The presence of such high dopamine levels in the extracellular space has to be rationalized with the rapid, linear clearance of extracellular dopamine observed after electrical stimulation of the medial forebrain bundle. Kinetic analysis of dopamine clearance after evoked release suggests that the basal extracellular dopamine concentration is below the K(M) of dopamine uptake, which is near 0.2 microm. However, dopamine clearance after pressure ejection of dopamine into the rat striatum is slow and non-linear, which may alternatively be a sign that basal dopamine release is only slightly slower than the maximal velocity of dopamine uptake, Vmax. A high basal extracellular dopamine concentration would exist if basal dopamine release were only slightly slower than the Vmax of uptake. This report introduces a new kinetic analysis of dopamine uptake that sheds light on the possible source of the different clearance rates observed following evoked dopamine release and dopamine pressure ejection. Furthermore, the analysis rationalizes the rapid dopamine clearance after evoked release with the possibility that basal extracellular dopamine levels are above the K(M) of the transporter.

Selective vulnerability of dopaminergic neurons to microtubule depolymerization

Parkinson disease (PD) is characterized by the specific degeneration of dopaminergic (DA) neurons in substantia nigra and has been linked to a variety of environmental and genetic factors. Rotenone, an environmental PD toxin, exhibited much greater toxicity to DA neurons in midbrain neuronal cultures than to non-DA neurons. The effect was significantly decreased by the microtubule-stabilizing drug taxol and mimicked by microtubule-depolymerizing agents such as colchicine or nocodazole. Microtubule depolymerization disrupted vesicular transport along microtubules and caused the accumulation of dopamine vesicles in the soma. This led to increased oxidative stress due to oxidation of cytosolic dopamine leaked from vesicles. Inhibition of dopamine metabolism significantly reduced rotenone toxicity. Thus, our results suggest that microtubule depolymerization induced by PD toxins such as rotenone plays a key role in the selective death of dopaminergic neurons.

Estrogen mediated inhibition of dopamine transport in the striatum: regulation by G alpha i/o

In the current study, the interaction between estrogen priming and dopamine D2 receptor activation on dopamine uptake in the striatum of ovariectomized female rats was investigated. Basal ADP-[(32)P(i)]ribosylation of G(i/o) was examined in synaptosomal membranes prepared from ovariectomized, estrogen primed or N-p-(isothiocyanatophenethyl) spiperone (NIPS) treated rats. [(32)P(i)]-incorporation was significantly increased (141%) in tissue from NIPS treated animals but attenuated (57%) in tissue from estrogen primed animals. Dopamine uptake kinetics were measured in vivo following manipulation of the heterotrimeric G-protein by pertussis toxin (0.5 microg, 48 h). Pertussis toxin significantly inhibited dopamine uptake at all concentrations of dopamine examined. Co-treatment with estrogen and pertussis toxin resulted in a further attenuation of dopamine transport at high but not low dopamine concentrations. These data are consistent with an estrogen mediated alteration of G-protein activity and support the hypothesis that estrogen may alter transporter activity through a modulation of dopamine D2 autoreceptor/G alpha(i/o) protein coupling.

Loaded dopamine is preferentially stored in the halo portion of PC12 cell dense core vesicles

Large dense core vesicles in rat pheochromocytoma cells are morphologically distinct from dense core vesicles in mast and chromaffin cells in that the dense core occupies a much smaller fraction of the vesicular volume, allowing for a much larger vesicular clear space, or halo. In this work, we present evidence indicating that upon treatment with L-DOPA the majority of the dopamine loaded into these vesicles is preferentially compartmentalized into the halo portion of the vesicle. Amperometry was used to monitor release of loaded neurotransmitter from cells in both isotonic and hypertonic extracellular conditions, with the latter condition causing inhibition of dense core dissociation. In combination with this we have used transmission electron microscopy to determine the morphological characteristics of dense core vesicles before and after treatment with L-DOPA in solutions of varied osmolarity. The results provide a more complete understanding of the complex interaction of molecules within dense core vesicles, suggesting that newly loaded dopamine is located in the halo of the vesicle. This finding has fundamental significance for studies of neurotransmitter release from dense core vesicles, as the core appears to have a function involving more than simple storage of neurotransmitter and associated molecules, and the often overlooked vesicular halo appears to be an important storage compartment for neurotransmitter.

Behavioral and neurochemical studies on brain aging in galanin overexpressing mice

To study possible involvement of galanin in brain aging quality, we have investigated behavioral, neurochemical and morphological parameters in aged mice overexpressing galanin under the platelet-derived growth factor B promoter (GalOE mice) compared to wild-type littermates (WT mice). The behavioral analysis in the forced swim test showed that old GalOE animals spent more time in immobility compared to WT. In the activity cage test, galanin overexpression counteracted the age-induced decrease in exploratory behavior. The neurochemical analysis showed a 30% decrease in noradrenaline overflow in the cerebral cortex of WT old mice that was not present in age-matched GalOE mice. Our results indicate that overexpression of galanin can influence several behavioral and neurochemical parameters in old mice.

Interaction between a hydroxypiperidine analogue of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine and Aspartate 68 in the human dopamine transporter

Compound (+)-R,R-D-84 is an optically active trans-hydroxy-substituted derivative of 4-(2-benzhydryloxy-ethyl)-1-(4-fluorobenzyl)piperidine (D-164). As a hydroxypiperidine analog of GBR 12935, (+)-R,R-D-84 is a candidate dopamine transporter compound for the treatment of cocaine dependence. The present work addresses the functional activity of (+)-R,R-D-84 at monoamine transporters and its potential molecular mechanism involving acidic amino acids (D and E). The selectivity for the dopamine vs. serotonin transporter of (+)-R,R-D-84 was greater than that of (-)-S,S-D-83, its enantiomer, and the selectivity of both compounds was greater than that of GBR 12909 (diphenyl-fluorinated GBR 12935). Only (+)-R,R-D-84 displayed improved selectivity vs. the norepinephrine transporter. D313N or E215Q mutation did not alter the pattern of affinities (measured by membrane binding of the cocaine analog [3H]CFT) for the dopamine transporter of (+)-R,R-D-84, (-)-S,S-D-83, D-164 (non-hydroxylated analog), or GBR 12909. In contrast, D68N mutation specifically lowered the affinity of (+)-R,R-D-84, pointing to a role for D68 in the interaction with (+)-R,R-D-84, possibly through hydrogen bonding between the hydroxyl and the carboxyl group of D68 which is lacking in N68. The present results, combined with behavioral data, implicate D68 in the dopamine transporter in cocaine antagonist activity of (+)-R,R-D-84.

Evaluation of rat striatal L-dopa and DA concentration after intraperitoneal administration of L-dopa prodrugs in liposomal formulations

Parkinson's disease is a neurodegenerative disease and its symptoms are relieved by administration of L-dopa (LD), which is converted by neuronal aromatic L-aminoacid decarboxylase (AADC), restoring dopamine (DA) levels in surviving neurons. In order to minimize unfavourable side effects, we studied new dimeric LD derivatives, as potential prodrugs for Parkinson's therapeutic treatment. To improve the bioavailability of the synthesized prodrugs, they were encapsulated in unilamellar liposomes of dimiristoylphosphatidylcholine (DMPC) and cholesterol (CHOL). In vivo microdialysis was used to monitor the striatal LD and DA concentrations after i.p. administration of new delivery systems. Bioavailability evaluation was performed by means of the HPLC-EC method. The striatal levels of LD and DA were remarkably elevated after i.p. administration of liposomal formulation of prodrug (+)-1b ([(O,O-diacetyl)-L-dopa-methylester]-succinyldiamide). This formulation showed about 2.5-fold increase in the basal levels of DA in dialysate rat striatum, suggesting that liposomal formulation of (+)-1b significantly increases LD and DA concentrations with respect to equimolar administration of LD itself or free prodrug (+)-1b.

Age-dependent motor deficits and dopaminergic dysfunction in DJ-1 null mice

Mutations in the DJ-1 gene were recently identified in an autosomal recessive form of early-onset familial Parkinson disease. Structural biology, biochemistry, and cell biology studies have suggested potential functions of DJ-1 in oxidative stress, protein folding, and degradation pathways. However, animal models are needed to determine whether and how loss of DJ-1 function leads to Parkinson disease. We have generated DJ-1 null mice with a mutation that resembles the large deletion mutation reported in patients. Our behavioral analyses indicated that DJ-1 deficiency led to age-dependent and task-dependent motoric behavioral deficits that are detectable by 5 months of age. Unbiased stereological studies did not find obvious dopamine neuron loss in 6-month- and 11-month-old mice. Neurochemical examination revealed significant changes in striatal dopaminergic function consisting of increased dopamine reuptake rates and elevated tissue dopamine content. These data represent the in vivo evidence that loss of DJ-1 function alters nigrostriatal dopaminergic function and produces motor deficits.

Individual differences in cocaine- and amphetamine-induced activation of male Sprague-Dawley rats: contribution of the dopamine transporter

Previously we found that outbred male Sprague-Dawley rats can be classified as either low or high cocaine responders (LCRs or HCRs, respectively), based on their open-field locomotor response to acute cocaine (COC; 10 mg/kg, i.p.). Here, we extended this analysis to amphetamine (AMPH; 0.5, 1, and 5 mg/kg, i.p.) and found that the individual differences in behavioral activation were not as pronounced as with COC. This was confirmed with observational analysis of behaviors. Differences in drug-induced activation could involve differential dopamine transporter (DAT) function/trafficking. To address this possibility, we measured [3H]DA uptake into dorsal striatal synaptosomes prepared from rats injected 30 min earlier with saline, COC, or AMPH to determine DAT activity, and radioligand binding to determine the total number of DATs. Striatal [3H]DA uptake in COC-treated HCRs was significantly higher than in LCRs. Furthermore, regardless of LCR/HCR classification, uptake in individual COC-treated rats was significantly correlated with their locomotor behavior in the 30 min after drug administration. In contrast, AMPH-treated rats did not differ in uptake, nor were uptake and locomotor activity correlated. DAT number did not differ between LCRs or HCRs, or between AMPH-treated rats. In addition, when individual differences in COC-induced behavior were no longer detected in LCRs and HCRs 1 week after initial classification, uptake was also similar. Together, these results suggest that a difference in expression of functional DATs on the cell surface contributes to the individual differences observed in COC-induced, but not AMPH-induced, behavioral activation of rats.

Synthesis and biological evaluation of pentacyclo[5.4.0.0(2,6).0(3,10).0(5,9)]undecane derivatives as potential therapeutic agents in Parkinson's disease

In previous studies, the polycyclic cage amine 8-benzylamino-8,11-oxapentacyclo[5.4.0.0(2,6).0(3,10).0(5,9)]undecane (NGP1-01) and a number of its derivatives showed positive effects in neuroprotection studies with MPTP, in vivo. In view of these findings, we examined these compounds for their effects on [(3)H]dopamine ([(3)H]DA) release and uptake inhibition in murine striatal synaptosomes, as well as for inhibition of baboon liver monoamine oxidase (MAO) B. In order to assess specificity, initial experiments focused on compounds that blocked dopamine uptake without causing appreciable release (<40% at 100 microM) of the transmitter. NGP1-01 blocked the uptake of [(3)H]DA with an IC(50) of 57 microM, while another compound, 8-phenylethyl-8,11-oxapentacyclo[5.4.0.0(2,6).0(3,10).0(5,9)]undecane, blocked uptake at an IC(50) value of 23 microM. These values were comparable to that of another polycyclic cage amine, amantadine (IC(50); 82 micro), that is used in parkinsonian therapy. Structure-activity relationships of this series of compounds support the importance of geometric and steric, rather than electronic effects, in determining biological activity. MAO-B inhibition for this group was weak, with less than 50% inhibition at 300 microM for any of the compounds in the series. The present study suggests that blockage of the dopamine transporter may underlie, at least in part, their neuroprotective effects against MPTP-induced parkinsonism. These compounds may be considered as potential lead compounds for Parkinson's Disease therapy.

Dopaminergic properties and experimental anti-parkinsonian effects of IPX750 in rodent models of Parkinson disease

With a view toward improving the neural bioavailability of administered dopaminergic compounds, including dopamine, synthetic efforts have been directed toward enhancing the brain bioavailability of these compounds by accessing cellular sugar transport systems with stereoselective dopaminergic drugs. While synthesis and chemistry of the resultant class of compounds has recently been described in US Patent No. 6,548,484, the associated biologic properties have not previously been reported. One member of this new class, IPX-750, is a pro-drug dopamine-gluconamine designed to retain stereospecificity of binding at: glucose transporters (GLUT 1/GLUT 3 and intestinal Na/glucose co-transporters SGLT1), dopamine transporter (DAT); and, dopaminergic receptors of the D1/D2 families. Designed to be cleavable by tissue amidases, results reported here show that intact IPX-750 pro-drug retains dopaminergic agonist binding and biologic activities both in vitro and in vivo. IPX-750, like dopamine, exhibited predominant D5/D1 binding specificity with lower binding activity at D2. As expected, binding was highly stereo-specific, ie, IPX-760, a benzamide differing in just a hydrogen atom and keto oxygen from IPX-750, bound with 6-fold lower activity at D5. In cell culture, activation resulted from binding of IPX-750 at D1 or D5 in transfected cells was measured by increased intracellular cAMP. Interestingly, considering prior reported in vitro toxicity of dopamine oxidized and metabolic product dopamine, no evidence of in vitro toxicity was observed at up to 72 hrs in cell cultures at the EC50 of IPX-750 for increasing intracellular cAMP. IPX-750 was evaluated in the Parkinson's disease animal models, including MPTP mouse model, the 6-hydroxydopamine (6-OHDA) rat model and the Nurr1(+/-) knockout mouse model. In MPTP-lesioned and Nurr1+/- knockout mice, IPX-750 significantly increased Rota-rod time. In 6-OHDA-lesioned rats, IPX-750 significantly decreased apomorphine (APO)-induced rotation. Worthy of note, after cessation of IPX-750 treatments the anti-parkinsonian activity in MPTP-lesioned and Nurr1+/- mice required about 2 weeks to washout, suggesting a possible biologic reservoir of drug. In addition, after eight weeks of twice daily administration of 20 mg/kg IPX-750, mice did not show statistical difference in the total number of TH-positive neurons in substantia nigra (SN). These combined results suggest (i) that stereo-specific glycoconjugation may be an effective method to improve penetrability of drugs through the blood brain barrier; (ii) treatment with bioavailable IPX-750 in vitro did not show evidence for neurotoxicity; and, (iii) IPX-750 possesses dopaminergic properties and exerts anti-parkinsonian effects in three different PD rodent models, suggesting therapeutic potential for this new class of drugs in treating dopamine deficiency diseases.

Combinatorial synthesis of benztropine libraries and their evaluation as monoamine transporter inhibitors

A combinatorial synthesis of benztropine analogues is presented. Radical azidonation of 3-benzyloxy-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 3 to 3-(1-azidobenzyloxy)-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 4 was used as a key step in the synthesis. This step was optimized by adding 10% DMF to the reaction. Reaction of 4 with phenyl magnesium bromide followed by Boc removal and N-methylation gave benztropine 1. Reaction of five-component Grignard reagents with 4 was used to create a two-dimensional library of 25 N-normethylbenztropine analogues. Further reaction of this library with five alkyl bromides was carried out to create a three-dimensional library containing 125 compounds. Screening of the libraries towards binding and inhibition of uptake of the human dopamine (hDAT), serotonin (hSERT) and norepinephrine transporters (hNET) was carried out. None of the synthesized compounds were found to be stronger than benztropine, and none were selective for inhibition of binding over monoamine uptake.