Catecholaminergic neurons projecting to the paraventricular nucleus of the hypothalamus are essential for cardiorespiratory adjustments to hypoxia

Brainstem catecholamine neurons modulate sensory information and participate in control of cardiorespiratory function. These neurons have multiple projections, including to the paraventricular nucleus (PVN), which contributes to cardiorespiratory and neuroendocrine responses to hypoxia. We have shown that PVN-projecting catecholaminergic neurons are activated by hypoxia, but the function of these neurons is not known. To test the hypothesis that PVN-projecting catecholamine neurons participate in responses to respiratory challenges, we injected IgG saporin (control; n = 6) or anti-dopamine β-hydroxylase saporin (DSAP; n = 6) into the PVN to retrogradely lesion catecholamine neurons projecting to the PVN. After 2 wk, respiratory measurements (plethysmography) were made in awake rats during normoxia, increasing intensities of hypoxia (12, 10, and 8% O2) and hypercapnia (5% CO2-95% O2). DSAP decreased the number of tyrosine hydroxylase-immunoreactive terminals in PVN and cells counted in ventrolateral medulla (VLM; -37%) and nucleus tractus solitarii (nTS; -36%). DSAP produced a small but significant decrease in respiratory rate at baseline (during normoxia) and at all intensities of hypoxia. Tidal volume and minute ventilation (VE) index also were impaired at higher hypoxic intensities (10-8% O2; e.g., VE at 8% O2: IgG = 181 ± 22, DSAP = 91 ± 4 arbitrary units). Depressed ventilation in DSAP rats was associated with significantly lower arterial O2 saturation at all hypoxic intensities. PVN DSAP also reduced ventilatory responses to 5% CO2 (VE: IgG = 176 ± 21 and DSAP = 84 ± 5 arbitrary units). Data indicate that catecholamine neurons projecting to the PVN are important for peripheral and central chemoreflex respiratory responses and for maintenance of arterial oxygen levels during hypoxic stimuli.

Human disposition, metabolism and excretion of etamicastat, a reversible, peripherally selective dopamine β-hydroxylase inhibitor

AIMS

Etamicastat is a reversible dopamine-β-hydroxylase inhibitor that decreases noradrenaline levels in sympathetically innervated tissues and slows down sympathetic nervous system drive. In this study, the disposition, metabolism and excretion of etamicastat were evaluated following [(14)C]-etamicastat dosing.

METHODS

Healthy Caucasian males (n = 4) were enrolled in this single-dose, open-label study. Subjects were administered 600 mg of unlabelled etamicastat and 98 µCi weighing 0.623 mg [(14)C]-etamicastat. Blood samples, urine and faeces were collected to characterize the disposition, excretion and metabolites of etamicastat.

RESULTS

Eleven days after administration, 94.0% of the administered radioactivity had been excreted; 33.3 and 58.5% of the administered dose was found in the faeces and urine, respectively. Renal excretion of unchanged etamicastat and its N-acetylated metabolite (BIA 5-961) accounted for 20.0 and 10.7% of the dose, respectively. Etamicastat and BIA 5-961 accounted for most of the circulating radioactivity, with a BIA 5-961/etamicastat ratio that was highly variable both for the maximal plasma concentration (19.68-226.28%) and for the area under the plasma concentration-time curve from time zero to the last sampling time at which the concentration was above the limit of quantification (15.82- 281.71%). Alongside N-acetylation, metabolism of etamicastat also occurs through oxidative deamination of the aminoethyl moiety, alkyl oxidation, desulfation and glucuronidation.

CONCLUSIONS

Etamicastat is rapidly absorbed, primarily excreted via urine, and its biotransformation occurs mainly via N-acetylation (N-acetyltransferase type 2), although glucuronidation, oxidation, oxidative deamination and desulfation also take place.

Chronic inhibition of dopamine β-hydroxylase facilitates behavioral responses to cocaine in mice

The anti-alcoholism medication, disulfiram (Antabuse), decreases cocaine use in humans regardless of concurrent alcohol consumption and facilitates cocaine sensitization in rats, but the functional targets are unknown. Disulfiram inhibits dopamine β-hydroxylase (DBH), the enzyme that converts dopamine (DA) to norepinephrine (NE) in noradrenergic neurons. The goal of this study was to test the effects of chronic genetic or pharmacological DBH inhibition on behavioral responses to cocaine using DBH knockout (Dbh −/−) mice, disulfiram, and the selective DBH inhibitor, nepicastat. Locomotor activity was measured in control (Dbh +/−) and Dbh −/− mice during a 5 day regimen of saline+saline, disulfiram+saline, nepicastat+saline, saline+cocaine, disulfiram+cocaine, or nepicastat+cocaine. After a 10 day withdrawal period, all groups were administered cocaine, and locomotor activity and stereotypy were measured. Drug-naïve Dbh −/− mice were hypersensitive to cocaine-induced locomotion and resembled cocaine-sensitized Dbh +/− mice. Chronic disulfiram administration facilitated cocaine-induced locomotion in some mice and induced stereotypy in others during the development of sensitization, while cocaine-induced stereotypy was evident in all nepicastat-treated mice. Cocaine-induced stereotypy was profoundly increased in the disulfiram+cocaine, nepicastat+cocaine, and nepicastat+saline groups upon cocaine challenge after withdrawal in Dbh +/− mice. Disulfiram or nepicastat treatment had no effect on behavioral responses to cocaine in Dbh −/− mice. These results demonstrate that chronic DBH inhibition facilitates behavioral responses to cocaine, although different methods of inhibition (genetic vs. non-selective inhibitor vs. selective inhibitor) enhance qualitatively different cocaine-induced behaviors.

[Disulfiram as a treatment for cocaine dependency]

BACKGROUND

Cocaine abuse and dependency have a considerable impact on society and health issues. Current treatment of cocaine dependency consists primarily of psychosocial and therapeutic interventions. There is a marked need for effective pharmacological treatments in addition to the currently available treatment options. Recently, there is some evidence that disulfiram (DSF) might reduce use of cocaine in patients presenting with cocaine dependency.

AIM

To explore the efficacy of disulfiram as a treatment for cocaine dependency.

METHOD

We performed a Medline search for randomised controlled trials (RCTs) relating to disulfiram as a treatment for cocaine dependency.

RESULTS

We found six RCT’s for this review. Disulfiram seemed to have a positive effect on the primary cocaine outcomes. In addition, a positive effect of disulfiram seemed to be independent of alcohol abuse. However, the quality of these studies was extremely variable both in terms of sample size and composition. A possible neurobiological explanation for this would be an inhibitory effect of disulfiram on dopamine beta-hydroxylase, thus generating lower levels of norepinefrine.

CONCLUSION

Disulfiram seems to be a valuable drug for the treatment of cocaine dependency. Due to the limited number and quality of the published studies, further research is needed in order to support the early positive results.

Structural insight of dopamine β-hydroxylase, a drug target for complex traits, and functional significance of exonic single nucleotide polymorphisms

Background

Human dopamine β-hydroxylase (DBH) is an important therapeutic target for complex traits. Several single nucleotide polymorphisms (SNPs) have also been identified in DBH with potential adverse physiological effect. However, difficulty in obtaining diffractable crystals and lack of a suitable template for modeling the protein has ensured that neither crystallographic three-dimensional structure nor computational model for the enzyme is available to aid rational drug design, prediction of functional significance of SNPs or analytical protein engineering.

Principal Findings

Adequate biochemical information regarding human DBH, structural coordinates for peptidylglycine alpha-hydroxylating monooxygenase and computational data from a partial model of rat DBH were used along with logical manual intervention in a novel way to build an in silico model of human DBH. The model provides structural insight into the active site, metal coordination, subunit interface, substrate recognition and inhibitor binding. It reveals that DOMON domain potentially promotes tetramerization, while substrate dopamine and a potential therapeutic inhibitor nepicastat are stabilized in the active site through multiple hydrogen bonding. Functional significance of several exonic SNPs could be described from a structural analysis of the model. The model confirms that SNP resulting in Ala318Ser or Leu317Pro mutation may not influence enzyme activity, while Gly482Arg might actually do so being in the proximity of the active site. Arg549Cys may cause abnormal oligomerization through non-native disulfide bond formation. Other SNPs like Glu181, Glu250, Lys239 and Asp290 could potentially inhibit tetramerization thus affecting function.

Conclusions

The first three-dimensional model of full-length human DBH protein was obtained in a novel manner with a set of experimental data as guideline for consistency of in silico prediction. Preliminary physicochemical tests validated the model. The model confirms, rationalizes and provides structural basis for several biochemical data and claims testable hypotheses regarding function. It provides a reasonable template for drug design as well.

Effect of food on the pharmacokinetic profile of etamicastat (BIA 5-453)

BACKGROUND

Etamicastat is a novel, potent, and reversible peripheral dopamine-β-hydroxylase inhibitor that has been administered orally at doses up to 600 mg once daily for 10 days to male healthy volunteers and appears to be well tolerated.

OBJECTIVE

The aim of this study was to investigate the effect of food on the pharmacokinetics of etamicastat.

MATERIAL AND METHODS

A single-center, open-label, randomized, two-way crossover study in 12 healthy male subjects was performed. Subjects were administered a single dose of etamicastat 200 mg following either a standard high-fat and high-calorie content meal (test) or 10 hours of fasting (reference). The statistical method for testing the effect of food on the pharmacokinetic parameters of interest was based upon the 90% confidence interval (CI) for the test/reference geometric mean ratio (GMR). The parameters of interest were maximum plasma concentration (C(max)), area under the plasma concentration-time curve (AUC) from time zero to the last measurable concentration (AUC(last)), and AUC from time zero to infinity (AUC(∞)). Bioequivalence was assumed when the 90% CI fell within the recommended acceptance interval (80, 125).

RESULTS

Etamicastat C(max), AUC(last), and AUC(∞) were 229 ng/mL, 1856 ng · h/mL, and 2238 ng · h/mL, respectively, following etamicastat in the fasting, and 166 ng/mL, 1737 ng · h/mL, and 2119 ng · h/mL, respectively, following etamicastat in the fed condition. Etamicastat test/reference GMR was 72.27% (90% CI 64.98, 80.38) for C(max), 93.59% (90% CI 89.28, 98.11) for AUC(last), and 96.47% (90% CI 91.67, 101.53) for AUC(∞). Time to C(max) was prolonged by the presence of food (p < 0.001). The C(max), AUC(last), and AUC(∞) values of the inactive metabolite BIA 5-961 were 275 ng/mL, 1827 ng · h/mL, and 2009 ng · h/mL, respectively, in the fasting, and 172 ng/mL, 1450 ng · h/mL, and 1677 ng · h/mL, respectively, in the fed condition. BIA 5-961 test/reference GMR was 62.42% (90% CI 56.77, 68.63) for C(max), 79.41% (90% CI 56.77, 68.63) for AUC(last), and 83.47% (90% CI 76.62, 90.93) for AUC(∞). A total of six mild to moderate unspecific adverse events were reported by four subjects. There was no clinically significant abnormality in laboratory assessments.

CONCLUSION

Etamicastat was well tolerated. The C(max) of etamicastat decreased 28% following oral administration of etamicastat in the presence of food, while AUC remained within the pre-defined acceptance interval. The delay in absorption and decrease in peak exposure of etamicastat is not clinically significant, and therefore etamicastat could be administered without regard to meals.

TRIAL REGISTRATION

EudraCT No. 2007-006530-33.

Central administration of p-hydroxyamphetamine produces a behavioral stimulant effect in rodents: evidence for the involvement of dopaminergic systems

RATIONALE AND OBJECTIVES

It is well-known that amphetamine induces increased locomotor activity in rodents. We previously found that intracerebroventricular (i.c.v.) administration of p-hydroxyamphetamine (p-OHA), an amphetamine metabolite, increases synaptic dopamine (DA) levels in the striatum. In the present study, we investigated the effect of p-OHA on locomotor activity in rodents.

RESULTS

In mice, i.c.v. administration of p-OHA significantly increased locomotor activity in a dose-dependent manner. p-Hydroxynorephedrine, another amphetamine metabolite, did not increase locomotor activity. This effect of p-OHA was inhibited by pretreatment with nomifensine, a dopamine-uptake inhibitor, but not by fluoxetine, a serotonin-uptake inhibitor, or diethyldithiocarbamate, a dopamine-beta-hydroxylase inhibitor. Furthermore, we tested the effects of microinjections of p-OHA into the rat nucleus accumbens (NAc) on locomotor activity. Local infusion of p-OHA into the NAc significantly increased locomotor activity. As in mice, the increased locomotor activity induced by p-OHA microinjection into the NAc in rats was inhibited by nomifensine.

CONCLUSIONS

These data suggest that dopaminergic systems in the NAc may play important roles in p-OHA-induced locomotor activity in rodents.

mechanisms of disulfiram-induced cocaine abstinence: antabuse and cocaine relapse

The anti-alcoholism drug disulfiram (Antabuse), which is an inhibitor of aldehyde dehydrogenase, induces an aversive reaction to alcohol consumption and thereby helps patients reduce alcohol intake. Recent clinical trials, initiated to investigate whether disulfiram could be used to treat individuals who abuse both alcohol and cocaine, have indicated that disulfiram effectively decreases cocaine consumption. Yet the ability of disulfiram to curb cocaine intake cannot be explained by the disruption of ethanol metabolism. Here, we synthesize clinical and animal data that point to dopamine beta-hydroxylase inhibition as a mechanism underlying the efficacy of disulfiram in the treatment of cocaine dependence.

Experimental dissociation of neural circuits underlying conditioned avoidance and hypophagic responses to lithium chloride

We previously reported that noradrenergic (NA) neurons in the nucleus of the solitary tract (NST) are necessary for exogenous CCK octapeptide to inhibit food intake in rats. To determine whether NST NA neurons also are necessary for lithium chloride (LiCl) to inhibit food intake and/or to support conditioned avoidance behavior, saporin toxin conjugated to an antibody against dopamine beta hydroxylase (DSAP) was microinjected bilaterally into the NST to ablate resident NA neurons. DSAP and sham control rats subsequently were tested for the ability of LiCl (0.15M, 2% body wt) to inhibit food intake and to support conditioned flavor avoidance (CFA). LiCl-induced hypophagia was significantly blunted in DSAP rats, and those with the most extensive loss of NST NA neurons demonstrated the most attenuated LiCl-induced hypophagia. Conversely, LiCl supported a robust CFA that was of similar magnitude in sham control and DSAP rats, including rats with the most extensive NA lesions. A terminal c-Fos study revealed intact LiCl-induced c-Fos expression in the lateral parabrachial nucleus and central amygdala in DSAP rats, despite significant loss of NST NA neurons and attenuated c-Fos activation of corticotropin-releasing hormone-positive neurons in the paraventricular nucleus of the hypothalamus (PVN). Thus, NST NA neurons contribute significantly to LiCl-induced hypophagia and recruitment of stress-responsive PVN neurons but appear to be unnecessary for CFA learning and expression. These findings support the view that distinct central nervous system circuits underlie LiCl-induced inhibition of food intake and conditioned avoidance behavior in rats.

Hypotensive hypovolemia and hypoglycemia activate different hindbrain catecholamine neurons with projections to the hypothalamus

To better understand the involvement of hindbrain catecholamine neurons in hypovolemia-induced secretion of AVP, we injected antidopamine beta-hydroxylase saporin (DSAP) or unconjugated saporin (SAP) control solution into the hypothalamic paraventricular nucleus (PVH) of anesthetized rats to retrogradely lesion catecholamine neurons innervating magnocellular areas of the hypothalamus. Subsequently, hypotensive hypovolemia was induced by remote blood withdrawal (4.5 ml, 1 ml/min) using an intra-atrial catheter. Blood was sampled at 2, 5, 20, and 50 min after onset of blood withdrawal. The AVP response was severely impaired by DSAP. Peak responses at 50 min were 51 pg/ml in SAP control and 17 pg/ml in DSAP-lesioned rats, indicating the importance of catecholamine neurons for this response. We also measured AVP responses to osmotic challenge induced by administration of hypertonic saline (1 M, 15 ml/kg, sc) and to insulin-induced hypoglycemia. Osmotic challenge increased AVP levels, but the response was not impaired by DSAP, indicating that AVP neurons were not damaged by the DSAP injection. Insulin-induced hypoglycemia did not increase AVP levels in either DSAP- or SAP-treated rats. However, the same dose of insulin increased food intake and corticosterone secretion in SAP controls, and these responses were profoundly impaired by DSAP. Thus catecholamine neurons are required for both the AVP response to hypotensive hypovolemia and for feeding and corticosterone responses to hypoglycemia. Lack of an AVP response to insulin-induced hypoglycemia in intact rats therefore indicates that responses to hypovolemia and hypoglycemia are mediated by different catecholamine neurons under distinct sensory controls.

Emerging pharmacological strategies in the fight against cocaine addiction

Cocaine addiction continues to be an important public health problem worldwide. At present, there are no proven pharmacotherapies for cocaine addiction. The studies reviewed here revealed a number of emerging targets for cocaine pharmacotherapy. First, disulfiram, a medication with dopaminergic effects, reduced cocaine use in a number of clinical trials. Second, GABA medications, tiagabine and topiramate, were found promising in clinical trials. Third, a beta-adrenergic blocker, propranolol, may be effective especially among cocaine-addicted individuals with high withdrawal severity. Fourth, treatment with a stimulant medication, modafinil, has reduced cocaine use. Last, a cocaine vaccine that slows entry of cocaine into the brain holds promise. These promising findings need to be further tested in controlled clinical trials.

Synthesis and Biological Evaluation of Novel, Peripherally Selective Chromanyl Imidazolethione-Based Inhibitors of Dopamine β-Hydroxylase

A novel series of dopamine beta-hydroxylase (DBH) inhibitors was designed and synthesized incorporating modifications to the core structure of nepicastat 3, with the principal aim of discovering potent DBH inhibitors exerting minimal effects on dopamine (DA) and noradrenaline (NA) levels in the central nervous system. This study resulted in the identification of a potent, peripherally selective DBH inhibitor, (R)-5-(2-aminoethyl)-1-(6,8-difluorochroman-3-yl)-1,3-dihydroimidazole-2-thione hydrochloride 54 (BIA 5-453). In experiments in mice and rats at T(max) (9 h after administration), 54 reduced NA levels in a dose-dependent manner in both the left atrium and the left ventricle, with the maximal inhibitory effect attained at a dose of 100 mg/kg. In contrast to that found in the heart, 54 failed to affect NA tissue levels in the brain. Compound 54 is thus presented as a candidate for clinical evaluation for the treatment of chronic heart failure and hypertension.

Effects of dopamine beta-hydroxylase genotype and disulfiram inhibition on catecholamine homeostasis in mice

RATIONALE

Dopamine beta-hydroxylase (DBH) converts dopamine (DA) to norepinephrine (NE), thus playing a critical role in catecholamine metabolism.

OBJECTIVES/METHODS

We examined the effects of Dbh gene dosage and the DBH inhibitor disulfiram in mice with zero, one, or two null Dbh alleles (+/+, +/-, and-/- mice).

RESULTS

DBH protein levels in adrenal and prefrontal cortex (PFC) and adrenal DBH activity were proportional to number of wild-type alleles. Adrenal DA was slightly increased in+/- mice and markedly increased (80-fold) in -/- mice compared to wild-type animals. While adrenal NE and epinephrine (EPI) were undetectable in -/- mice, adrenal concentrations of NE and EPI were similar in +/+ and +/- mice, suggesting that the increase in DA maintains the normal rate of beta-hydroxylation in Dbh +/- mice. Disulfiram had little effect on adrenal catecholamine levels, regardless of genotype or dose. NE was absent in the PFC of -/- mice, but only slightly reduced in +/- animals compared to wild-type animals. PFC DA was increased twofold in +/- mice and fivefold in -/- mice, and the NE to DA ratio was reduced ( approximately 35%) in +/- mice, compared to wild-type mice. Disulfiram significantly decreased PFC NE and increased DA in +/+ and +/- animals, with the disulfiram and genotype effects on the PFC NE to DA ratio apparently additive.

CONCLUSIONS

The data reveal potentially important and apparently additive effects of Dbh genotype and disulfiram administration on PFC catecholamine metabolism. These effects may have implications for genetic control of DBH activity in humans and for understanding therapeutic effects of disulfiram.

Picolinic Acids as Inhibitors of Dopamine ?-Monooxygenase: QSAR and Putative Binding Site

Dopamine beta-monooxygenase (DBM, EC 1.14.17.1) catalyzes the oxidation of dopamine into (R)-noradrenaline. DBM inhibitors may act as antihypertensive drugs. A series of 22 picolinic acids substituted in 4- and 5-position was previously synthesized and tested for inhibition of DBM from bovine adrenal medulla. The QSAR of these compounds were investigated by Hansch analysis and comparative molecular field analysis (CoMFA). The correlation of pI(50) values with electronic (nucleophilic substituent constant sigma(p) (-), oxygen net charges and highest occupied molecular orbital energy calculated by AMPAC-AM1), hydrophobic (pi values of R(4)) and steric descriptors (molar refraction and Sterimol parameters of R(5)) indicated that a more negatively charged carboxylate moiety, more lipophilic R(4) groups as well as wider bulk and higher molar refraction of 5-substituents increase DBM inhibition. The CoMFA approach generally reproduced these QSAR in terms of steric and electrostatic field variables, the latter restricted to the carboxylate area. To predict a putative binding site, dopamine and fusaric acid were docked into a partial homology model of DBM derived from a crystal structure of peptidylglycine alpha-hydroxylating monooxygenase (EC 1.14.17.3). The inhibitor is suggested to interact by its carboxylate group with the copper site CuB and the protonated amino group of dopamine according to the uncompetitive type of inhibition. R(4) points to a tyrosine side chain. R(5) protrudes into the fringe of the catalytic crevice. It may "freeze" to the solvated surface of polar amino acids and additionally contact an isoleucine residue. Taken together, the model explains the QSAR results by corresponding types of interaction.

Hindbrain noradrenergic lesions attenuate anorexia and alter central cFos expression in rats after gastric viscerosensory stimulation

Behavioral, autonomic, and endocrine outputs of the CNS are subject to important feedback modulation by viscerosensory signals that are conveyed initially to the hindbrain nucleus of the solitary tract (NST). In the present study, noradrenergic (NA) neurons [i.e., those that express the NA synthetic enzyme dopamine beta hydroxylase (DbH)] in the caudal NST were lesioned to determine their role in mediating anorexic responses to gastric stimulation and in conveying gastric sensory signals to the hypothalamus and amygdala. For this purpose, saporin toxin conjugated to an antibody against DbH was microinjected bilaterally into the caudal NST in adult rats. Control rats received similar microinjections of vehicle. Several weeks later, rats were tested for the ability of systemic cholecystokinin octapeptide (CCK) (0 or 10 microg/kg) to inhibit food intake. CCK-induced anorexia was significantly attenuated in toxin-treated rats. Rats subsequently were used in a terminal cFos study to determine central neural activation patterns after systemic CCK or vehicle and to evaluate lesion extent. Toxin-induced loss of DbH-positive NST neurons was positively correlated with loss of CCK-induced anorexia. Hypothalamic cFos expression was markedly attenuated in lesioned rats after CCK treatment, whereas CCK-induced neural activation in the parabrachial nucleus and amygdala appeared normal. These findings suggest that hindbrain NA neurons are an integral component of brainstem circuits that mediate CCK-induced anorexia and also are necessary for hypothalamic but not parabrachial or amygdala responses to gastric sensory stimulation.

A slow-tight binding inhibitor of dopamine beta-monooxygenase: a transition state analogue for the product release step

The steady-state kinetic data show that 3-hydroxy-4-phenylthiazole-2(3H)-thione (3H4PTT) is a potent tight-binding inhibitor for dopamine beta-monooxygenase (DbetaM) with a dissociation constant of 0.9 nM. Ackermann-Potter plots of the enzyme dependence of the inhibition revealed that the stoichiometry of the enzyme inhibition by 3H4PTT is 1:1. Pre-steady-state progress curves at varying inhibitor with fixed reductant and enzyme concentrations clearly show the slow binding behavior of the inhibitor. The observed kinetic behavior is consistent with the apparent direct formation of the tightly bound E x I* complex. The k(on) and k(off) for 3H4PTT which were determined under pre-steady-state conditions at variable inhibitor concentrations were found to be (1.85 +/- 0.07) x 10(6) M(-1) s(-1) and (1.9 +/- 0.6) x 10(-3) s(-1), respectively. The dissociation constant calculated from these rates was similar to that determined under steady-state conditions, confirming that 3H4PTT is a kinetically well-behaved inhibitor. The steady-state as well as pre-steady-state kinetic studies at variable DMPD concentrations show that the inhibition is competitive with respect to the reductant, demonstrating the exclusive interaction of 3H4PTT with the oxidized form of the enzyme. The kinetic behavior and the structural properties of 3H4PTT are consistent with the proposal that the E x 3H4PTT complex may mimic the transition state for the product (protonated) release step of the enzyme. Therefore, 3H4PTT could be used as a convenient probe to examine the properties of the E x P complex of the DbetaM reaction and also as an active site titrant for the oxidized enzyme.

Peripheral 5-HT(2A)-receptor-mediated formation of an inhibitor of atrial natriuretic peptide binding involves inflammation

A peripheral 5-HT(2A) receptor-mediated hemodynamic change prompted formation of indole-2,3-dione, an endogenous inhibitor of atrial natriuretic peptide (ANP) receptor binding and G protein-mediated intracellular signaling (IC(50): 0.4 microM). This effect was significantly suppressed by dexamethasone, indomethacin and the 5-HT(2) receptor antagonists, ketanserin or ritanserin. 5-HT(2A) receptor-mediated acute hemodynamic change was not modified significantly by indomethacin, prazosin or propranolol pretreatment. A tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine, but not a dopamine beta hydroxylase inhibitor, diethyldithiocarbamate, abolished the 5-HT(2A) receptor-mediated increase in indole-2,3-dione. Exogenous indole-2,3-dione induced a significant increase in plasma catecholamine levels and decrease in urine volume. A 5-HT(2A) receptor-mediated decrease in capillary flow may have caused an inflammatory process and peripheral sympathetic activation via ANP signaling inhibition. 3,4-dihydroxyphenylalanine (DOPA)/dopamine may contribute to the progression of inflammation or the generation of a precursor of indole-2,3-dione. The observation that indole-2,3-dione abolished angiotensin AT(1) receptor-mediated NADPH activation in both human umbilical vein endothelial cells and smooth muscle cells at 20 microM may suggest that sulfhydryl-reactive indole-2,3-dione could influence mitochondrial function and cellular redox states via flavoenzyme inhibition and/or regulation of dehydrogenase-oxidase conversion.

Long-term intrathecal catheterization in the rat

We report an intrathecal (i.t.) catheter system that permits repeated administration of volumes of 10 microl or more in the awake rat over many months. A small skin incision is made and a 32 ga polyurethane catheter is inserted in the sacral subarachnoid space using a modified 22 ga needle. The other end of the catheter is tunneled subcutaneously to the flank and exteriorized through a titanium port. The device is well tolerated, does not cause sensory or motor deficits, and does not interfere with behavioral testing. Rats equipped with this device can be housed with other rats. Over the 9 month observation period the function of the catheter was verified by repeated injection of 15 microl of 2% lidocaine that caused temporary paraplegia. Out of 12 implanted rats, the number of fully functional catheters was 10 at 3 months, seven at 6 months, and six at 9 months. At 3 months, i.t. injection of anti-dopamine-beta-hydroxylase antibodies conjugated to saporin (DBH-SAP, 5 microg/10 microl) resulted in noradrenergic denervation of the spinal cord in all rats (n=10). We propose that intrathecal catheterization is well suited for long term behavioral and pharmacological studies.

5-HT1 and 5-HT2 receptor agonists blunt +/- -alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-stimulated GH secretion in prepubertal male rats

OBJECTIVE

Excitatory amino acids, gamma-amino butyric acid (GABA), serotonin and catecholamines are involved in the control of GH secretion. The actions of these neurotransmitters are interconnected, and recently we showed that the stimulatory effect of GABA was blocked by MK-801, an antagonist of N-methyl-D-aspartate receptors. The present experiments were carried out to analyze the interrelationships between +/- -alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors and serotoninergic and catecholaminergic pathways in the control of GH secretion in prepubertal (16-23-day-old) male rats.

DESIGN AND METHODS

The GH response to AMPA was analyzed in animals pretreated with 5-hydroxytryptophan methyl ester (5-HTP) plus fluoxetine (a precursor of 5-hydroxytryptamine (5-HT) synthesis and a blocker of 5-HT re-uptake), R (+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT, an agonist of the 5-HT1 receptors), +/- -2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) and alpha-methyl-5-hydroxytryptamine (agonists of 5-HT2 receptors), I-phenylbiguanide (an agonist of 5-HT3 receptors), or alpha-methyl-p-tyrosine (alpha-MPT) and diethyldithiocarbamate (DDC) (blockers of catecholamine synthesis).

RESULTS

Basal GH secretion remained unchanged in prepubertal rats after activation of the serotoninergic system or blockade of catecholamine synthesis. The stimulatory effect of AMPA on GH secretion was blocked after activation of the serotoninergic system, through specific 5-HT1 and 5-HT2 receptor agonists. In contrast, activation of 5-HT3 receptors potentiated AMPA-stimulated GH secretion.

CONCLUSIONS

Serotoninergic receptors modulate the stimulatory effect of AMPA on GH secretion in prepubertal male rats.

Synthesis and characterization of immobilized dopamine beta-hydroxylase in membrane-bound and solubilized formats

Dopamine beta-hydroxylase (DBH) catalyzes the beta-hydroxylation of dopamine to norepinephrine. The enzyme in chromaffin granules occurs in a soluble form and a form confined to the surrounding membrane. DBH was noncovalently immobilized in the hydrophobic interface of an immobilized artificial membrane (IAM) liquid chromatographic stationary phase and the resulting DBH-IAM stationary phase was enzymatically active and was shown to mimic the membrane-bound form of the enzyme. DBH was also covalently immobilized onto a silica-based support containing, glutaraldehyde-P (Glut-P). The resulting DBH-Glut-P interphase was also enzymatically active, reproducible and shown to display characteristics of the solubilized enzyme. The results demonstrate that the different immobilization methods utilized for the enzyme can be used to quantitatively and qualitatively determine the enzyme kinetic constants associated with enzyme/substrate and enzyme/inhibitor interactions for the two distinct forms of the enzyme. These new entities can be used in basic biochemical studies as well as in high throughput screening of substances for DBH substrate/inhibitor properties.

Regulation of sympathetic tone and arterial pressure by rostral ventrolateral medulla after depletion of C1 cells in rat

1. In this study we examined whether the rostral ventrolateral medulla (RVLM) maintains resting sympathetic vasomotor tone and activates sympathetic nerve activity (SNA) after the depletion of bulbospinal C1 adrenergic neurones. 2. Bulbospinal C1 cells were destroyed ( approximately 84% loss) by bilateral microinjections (spinal segments T2-T3) of an anti-dopamine-beta-hydroxylase antibody conjugated to the ribosomal toxin saporin (anti-DH-SAP). 3. Extracellular recording and juxtacellular labelling of bulbospinal barosensitive neurones in the RVLM revealed that treatment with anti-DH-SAP spared the lightly myelinated neurones with no tyrosine hydroxylase immunoreactivity. 4. In rats treated with anti-DH-SAP, inhibition of RVLM neurones by bilateral microinjection of muscimol eliminated splanchnic SNA and produced the same degree of hypotension as in control rats. 5. Following treatment with anti-DH-SAP the sympathoexcitatory (splanchnic nerve) and pressor responses to electrical stimulation of the RVLM were reduced. 6. Treatment with anti-DH-SAP also eliminated the majority of A5 noradrenergic neurones. However, rats with selective lesion of A5 cells by microinjection of 6-hydroxydopamine into the pons showed no deficits to stimulation of the RVLM. 7. In summary, the loss of 84% of bulbospinal adrenergic neurones does not alter the ability of RVLM to maintain SNA and arterial pressure at rest in anaesthetized rats, but this loss reduces the sympathoexcitatory and pressor responses evoked by RVLM stimulation. The data suggest sympathoexcitatory roles for both the C1 cells and non-C1 cells of the RVLM and further suggest the C1 cells are critical for the full expression of sympathoexcitatory responses generated by the RVLM.

Effects of dopamine beta-hydroxylase inhibition with nepicastat on the progression of left ventricular dysfunction and remodeling in dogs with chronic heart failure

BACKGROUND

Inhibition of dopamine beta-hydroxylase (DBH) results in a decrease in norepinephrine synthesis. The present study was a randomized, blinded, placebo-controlled investigation of the long-term effects of therapy with the DBH inhibitor nepicastat (NCT) on the progression of left ventricular (LV) dysfunction and remodeling in dogs with chronic heart failure (HF).

METHODS AND RESULTS

Moderate HF (LV ejection fraction [LVEF] 30% to 40%) was produced in 30 dogs by intracoronary microembolization. Dogs were randomized to low-dose NCT (0.5 mg/kg twice daily, n=7) (L-NCT), high-dose NCT (2 mg/kg twice daily, n=7) (H-NCT), L-NCT plus enalapril (10 mg twice daily, n=8) (L-NCT+ENA), or placebo (PL, n=8). Transmyocardial (coronary sinus-arterial) plasma norepinephrine (tNEPI), LVEF, end-systolic volume, and end-diastolic volume were measured before and 3 months after initiating therapy. tNEPI levels were higher in PL compared with NL (86+/-20 versus 13+/-14 pg/mL, P:<0.01). L-NCT alone and L-NCT+ENA reduced tNEPI toward normal (28+/-4 and 39+/-17 pg/mL respectively), whereas HD-NCT reduced tNEPI to below normal levels (3+/-10 pg/mL). In PL dogs, LVEF decreased but was unchanged with L-NCT and increased with L-NCT+ENA. L-NCT and L-NCT+ENA prevented progressive LV remodeling, as evidenced by lack of ongoing increase in end-diastolic volume and end-systolic volume, whereas H-NCT did not

CONCLUSIONS

In dogs with HF, therapy with L-NCT prevented progressive LV dysfunction and remodeling. The addition of ENA to L-NCT afforded a greater increase in LV systolic function. NCT at doses that normalize tNEPI may be useful in the treatment of chronic HF.

Inhibitory effects of nitric oxide and nitrosative stress on dopamine-beta-hydroxylase

Dopamine-beta-hydroxylase (DbetaH) is a copper-containing enzyme that uses molecular oxygen and ascorbate to catalyze the addition of a hydroxyl group on the beta-carbon of dopamine to form norepinephrine. While norepinephrine causes vasoconstriction following reflex sympathetic stimulation, nitric oxide (NO) formation results in vasodilatation via a guanylyl cyclase-dependent mechanism. In this report, we investigated the relationship between NO and DbetaH enzymatic activity. In the initial in vitro experiments, the activity of purified DbetaH was inhibited by the NO donor, diethylamine/NO (DEA/NO), with an IC(50) of 1 mm. The inclusion of either azide or GSH partially restored DbetaH activity, suggesting the involvement of the reactive nitrogen oxide species, N(2)O(3). Treatment of human neuroblastoma cells (SK-N-MC) with diethylamine/NO decreased cellular DbetaH activity without affecting their growth rate and was augmented by the depletion of intracellular GSH. Co-culture of the SK-N-MC cells with interferon-gamma and lipopolysaccharide-activated macrophages, which release NO, also reduced the DbetaH activity in the neuroblastoma cells. Our results are consistent with the hypothesis that nitrosative stress, mediated by N(2)O(3), can result in the inhibition of norepinephrine biosynthesis and may contribute to the regulation of neurotransmission and vasodilatation.

Biochemical activities of extracts from Hypericum perforatum L. 5th communication: dopamine-beta-hydroxylase-product quantification by HPLC and inhibition by hypericins and flavonoids

Extracts from the herb "St. John's wort" (Hypericum perforatum L.) exhibit beneficial effects on patients suffering from mental depressions. Lack of catecholamine neurotransmitters may be one biochemical mechanism for this problem under discussion. It has been recently reported that alcoholic extracts from Hypericum perforatum inhibit dopamine-beta-hydroxylase (D-beta-H) with an I50 of 0.1 mumol/l on the basis of total hypericin content and with an I50 of 21 mumol/l with pure commercial hypericin. As test system polarographic determination of oxygen uptake with tyramine as a substrate analogue was used. In the present paper the quantification of the enzymatic activity and the potential influence of inhibitors are reported using dopamine as substrate and product (noradrenaline) quantification by HPLC. With this test system it could be shown that D-beta-H is strongly inhibited by pseudohypericin (I50 = approx. 3 mumol/l) and hypericin (I50 = approx. 5 mumol/l), whereas the I50-values of various flavonoids (quercitrin, isoquercitrin, hyperoside, rutin, quercetin, amentoflavone, kaempferol) are in the range of 50 mumol/l or higher.

Distribution of dopamine-immunoreactive fibers in the rat brainstem

We describe the distribution of axons immunoreactive for dopamine in pons and medulla oblongata of rat under normal conditions or after inhibition of monoamine oxidase or dopamine beta-hydroxylase. In the pons of non-treated animal, fairly dense plexuses of dopamine-immunoreactive varicose fibers were found in the locus coeruleus, dorsal parabrachial and dorsal raphe nuclei, central gray and reticular formation dorsal to the superior olive. In the medulla oblongata, the immunoreactive fibers were abundant in the dorsal vagal complex, lateral paragigantocellular nucleus, midline raphe nuclei and spinal trigeminal nucleus. Monoamine oxidase inhibition made it possible to increase the intensity of immunoreactivity and consequently the number of labeled fibers in these areas, indicating that dopamine is perpetually oxidized by monoamine oxidase, and consequently in low concentration under normal conditions. Sparse dopamine-immunoreactive fibers were observed in the pontine gray, motor trigeminal nucleus, inferior olive and major axon bundles such as the dorsal and ventral tegmental bundles, where numerous noradrenergic fibers have been reported. In axons of these areas, intense dopamine-immunoreactivity was seen only after inhibition of dopamine-beta-hydroxylase. It appears that dopamine is released and oxidized in response to autonomic changes such as hypoxia, hemorrhage, and cardiovascular variation in the caudal brainstem, as we have described elsewhere.

The regulation of the retrograde axonal transport of (125)I-beta nerve growth factor is independent of calcium

Calcium has been shown to play a major role in the regulation of endocytosis and exocytosis of synaptic vesicles and retrograde axonal transport of proteins. The role of calcium in the regulation of neurotrophin retrograde axonal transport is unknown. This study aimed to determine if calcium plays a role in the uptake and retrograde axonal transport of (125)I-beta nerve growth factor ((125)I-betaNGF) within sympathetic neurons innervating the iris by comparing it with (125)I-anti-dopamine beta hydroxylase (anti-DBH). The nonspecific voltage-sensitive calcium channel (VSCC) antagonists, cadmium (200 nmol/eye) and nickel (100 nmol/eye) reduced the amount of (125)I-anti-DBH retrograde axonal transport by 90 and 70%, respectively. In contrast, cadmium (200 nmol/eye) had no effect on (125)I-betaNGF retrograde axonal transport, while nickel (100 nmol/eye) caused a significant increase in the amount transported to the ganglia. The L-type VSCC antagonist nifedipine (10 nmol/eye) and N-type VSCC antagonist omega-conotoxin (1.5 nmol/eye) both had no effect on (125)I-anti-DBH retrograde axonal transport which suggests that these types of calcium channels are not involved in the exocytosis/endocytosis of anti-DBH containing vesicles. Thapsigargin (0.2 nmol/eye), an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPases also significantly inhibited (125)I-anti-DBH transport but had no effect on (125)I-betaNGF retrograde transport. This suggests that (125)I-anti-DBH and (125)I-betaNGF are internalized into different vesicle types and that the endocytosis and retrograde axonal transport of (125)I-betaNGF are not dependent upon calcium.

Biochemical activities of extracts from Hypericum perforatum L. 1st Communication: inhibition of dopamine-beta-hydroxylase

Extracts from the herb "St. John's wort" (Hypericum perforatum L.) are used for the treatment of mental depression, nervousness, sleeplessness and for their wound healing, diuretic and antirheumatic properties. As one biochemical mechanism for depression lack of catecholamine neurotransmitters has been discussed. The results of this investigation show that alcoholic extracts from Hypericum perforatum L. on the basis of total hypericin content inhibit dopamine-beta-hydroxylase with an IC50 of 0.1 mu mol/l; pure commercial hypericin inhibits with an IC50 of 21 mu mol/l. Enzymes involved in the synthesis of dopamine from tyrosine, namely tyrosinase and tyrosine decarboxylase, are not influenced by hypericin at concentrations from 1 up to 10 mu mol/l.

Regional distribution and control of tyrosine hydroxylase activity in the quail brain

Tyrosine hydroxylase (TH) activity, the rate-limiting step in the synthesis of catecholamines, was quantified in the preoptic area-hypothalamus of adult male Japanese quail by a new assay measuring the tritiated water production from 3,5-[3H]-L-tyrosine. Maximal levels of activity were observed at a 20-25 microM concentration of substrate, with more than 50% inhibition of the activity being recorded at a 100 microM concentration. TH activity was linear as a function of the incubation time during the first 20 min and maximal at a pH of 6.0. TH was heterogeneously distributed in the quail brain with highest levels of activity being found (in decreasing order) in the mesencephalon, diencephalon, and telencephalon. Given the large size of the telencephalon, this is the brain area that contains, as a whole, the highest level of enzyme activity. TH inhibitors that have been well-characterized in mammals, such as 3-iodo-L-tyrosine and L-alpha-methyl-p-tyrosine (AMPT) completely inhibited the enzyme activity at a 100 microM concentration. In mammals, the accumulation of catecholamines exerts a negative feedback control on TH activity. Similar controls were observed in the quail brain. Two inhibitors of the DOPA decarboxylase that should lead to accumulation of DOPA depressed TH activity by 60% or more, and the inhibitor of the dopamine beta-hydroxylase, fusaric acid that should cause an accumulation of dopamine, suppressed 90% of the TH activity. The addition of exogenous DOPA, dopamine, or norepinephrine to the brain homogenates also strongly inhibited TH activity, independently confirming the feedback effects of the enzyme products on the enzyme activity. These data demonstrate that TH activity in the quail brain is heterogeneously distributed and acutely regulated, as it is in mammals, by the accumulation of its products and of the derived catecholamines.

Dopamine-beta-hydroxylase inhibition: a novel sympatho-modulatory approach for the treatment of congestive heart failure

Pre-clinical and clinical studies suggest that chronic sympathetic activation in congestive heart failure (CHF) is a maladaptive response which accelerates the progressive worsening of the disease. Consequently, therapeutic interventions which inhibit sympathetic nerve function are likely to favorably alter the natural course of the disease. Indeed, recent clinical studies have shown that treatment with carvedilol, a beta-blocker, reduces mortality and the risk of death and hospitalization. The therapeutic value of beta-blockers, however, may be limited by their propensity to cause acute hemodynamic deterioration which results from abrupt withdrawal of sympathetic support. Thus, although the introduction of beta-blockers represents an important advance in the treatment of CHF, a better tolerated means of modulating the sympathetic nervous system would be highly desirable. An alternative strategy for directly modulating sympathetic nerve function is to inhibit the biosynthesis of norepinephrine (NE) via inhibition of dopamine-beta-hydroxylase (DBH), the enzyme which catalyzes the conversion of dopamine (DA) to NE in sympathetic nerves. This approach may have the following three merits over beta-blockade. First, this class of drugs would be expected to produce gradual modulation, as opposed to abrupt blockade, of sympathetic nerve function and, consequently, would not be associated with acute hemodynamic worsening thereby obviating the need for dose-titration. Second, from a theoretical standpoint, DBH inhibitors, at low doses, would preferentially inhibit NE release in the heart since the storage pool of NE in this organ is selectively depleted in CHF. Lastly, inhibition of DBH would augment the levels of DA which, via agonism of dopamine receptors, could have beneficial effects on renal function. Nepicastat is a novel, selective and potent (IC50 = 9 nM) inhibitor of DBH. Preclinical studies have shown that nepicastal produces gradual modulation of catecholamine levels (reduction in NE and elevation of DA and DA/NE ratio) in cardiovascular tissues and plasma, attenuates sympathetically-mediated cardiovascular responses and also has salutary effects on renal function. In a canine heart failure model, normalization of transmyocardial norepinephrine balance with nepicastat retards the process of ventricular dilation and prevents progressive worsening of cardiac function. Early short-term clinical studies in CHF patients have shown that nepicastat is well tolerated and produces significant and dose-dependent increases in plasma DA/NE concentrations.

[Noradrenaline synthesis during the first week of life and development of inherited stress-induced arterial hypertension in rats]

Increasing of the noradrenaline synthesis with daily i.p. administration of synthetic noradrenaline precursor DL-Threo on the 21-25th day of life of the rats with inherited stress-induced arterial hypertension (ISIAH) resulted in a drop of basal and stress-induced blood pressure in adult animals with no changes in response of the hypothalamic-pituitary-adrenocortical system (HPAS). Reduction of the noradrenaline synthesis with daily i.p. administration of dopamine-hydroxylase inhibitor FLA-57 in 21-25th day old Wistar rats induced no arterial hypertension in adults but decreased their adrenocortical response to emotional stress. Noradrenaline deficit in the brain structures on the 4th week of life in rats seems to be associated with arterial hypertension only in presence of genetic defect determining this pathology. Changes in adult HPAS function due to shortage of noradrenaline in the brain in the end of the 1st month of life do not depend on hypertension.

The local minima method (LMM) of pharmacophore determination: a protocol for predicting the bioactive conformation of small, conformationally flexible molecules

Software has been developed for potential energy surface analysis and the local minima method of pharmacophore determination. LMM is rigorous and systematic and employs multiple conformations which are the local minima from the potential energy surface of each compound in the data set. It produces a series of possible pharmacophores from a postulated set of pharmacophore elements. The best pharmacophore is then determined by performing a comparative molecular field analysis (CoMFA) on each one. The pharmacophore which produces the most self-consistent model is deemed the best. Local minima on the gas-phase potential energy surface are shown to be a reasonably close approximation to protein bound conformations, and these conformations can be found through systematic conformational searches followed by minimization of the local minima. LMM was used to develop a 3D-QSAR model for dopamine beta-hydroxylase (DBH) inhibitors which was highly predictive (predictive R2 = 0.71 and standard error of predictions = 0.41). The model predicted that the phenyl and thienyl series of inhibitors were acting as bioisosteres. Examination of compounds overlayed in the model indicated a possible hydrogen bond acceptor in the DBH active site. Three tyrosine residues previously labeled by mechanism based inhibitors may be acting as the acceptor and therefore represent excellent candidates for site-directed mutagenesis studies.

Cardiovascular effects of nepicastat (RS-25560-197), a novel dopamine beta-hydroxylase inhibitor

Nepicastat (RS-25560-197) is a novel, selective, and potent inhibitor of dopamine beta-hydroxylase, which modulates catecholamine levels (reduces norepinephrine and elevates dopamine) in cardiovascular tissues. This study was designed to evaluate the cardiovascular effects of nepicastat. Acute oral administration of nepicastat (0.3, 1, 3, 10, and 30 mg/kg) produced attenuation of the pressor and positive chronotropic responses to preganglionic sympathetic nerve stimulation (about twofold to sixfold shift in the frequency-response curve) in pithed spontaneously hypertensive rats (SHRs). In inactin-anesthetized SHRs, the antihypertensive effects of nepicastat (3 mg/kg, i.v.) were accompanied by a significant decrease in renal vascular resistance (38%), a tendency toward an increase in renal blood flow (22%), and no adverse effects on urine output and Na/K excretion. In conscious, unrestrained, telemetry-implanted SHRs, nepicastat (30 and 100 mg/kg/day for 30 days) produced dose-dependent decreases in mean arterial blood pressure (peak decrease of 20 and 42 mm Hg, respectively) without evoking reflex tachycardia. Long-term, concurrent administration of nepicastat (30 mg/kg/day, p.o.) and a subthreshold dose of enalapril (1 mg/kg/day, p.o.) produced greater antihypertensive effects than those produced by nepicastat alone. In normal dogs, nepicastat (5.0 mg/kg, p.o., b.i.d., for 4.5 days) blunted the positive chronotropic and pressor response to tyramine. These findings suggest that nepicastat functionally modulates sympathetic drive to cardiovascular tissues and may be of value in the treatment of cardiovascular disorders associated with overactivation of the sympathetic nervous system such as hypertension and congestive heart failure.

Three-dimensional quantitative structure-activity relationships from molecular similarity matrices and genetic neural networks. 2. Applications

Validation of a method that uses a genetic neural network with electrostatic and steric similarity matrices (SM/GNN) to obtain quantitative structure-activity relationships (QSARs) is performed with eight data sets. Biological and physicochemical properties from a broad range of chemical classes are correlated and predicted using this technique. Quantitatively the results compare favorably with the benchmarks obtained by a number of well-established QSAR methods; qualitatively the models are consistent with the published descriptions on the relative contribution of steric and electrostatic factors. The results demonstrate the general utility of this method in deriving QSARs. The implication of the importance of molecular alignment and possible methodological improvements are discussed.

Catecholamine modulatory effects of nepicastat (RS-25560-197), a novel, potent and selective inhibitor of dopamine-beta-hydroxylase

1. Inhibitory modulation of sympathetic nerve function may have a favourable impact on the progression of congestive heart failure. Nepicastat is a novel inhibitor of dopamine-beta-hydroxylase, the enzyme which catalyses the conversion of dopamine to noradrenaline in sympathetic nerves. The in vitro pharmacology and in vivo catecholamine modulatory effects of nepicastat were investigated in the present study. 2. Nepicastat produced concentration-dependent inhibition of bovine (IC50 = 8.5 +/- 0.8 nM) and human (IC50 = 9.0 +/- 0.8 nM) dopamine-beta-hydroxylase. The corresponding R-enantiomer (RS-25560-198) was approximately 2-3 fold less potent than nepicastat. Nepicastat had negligible affinity (> 10 microM) for twelve other enzymes and thirteen neurotransmitter receptors. 3. Administration of nepicastat to spontaneously hypertensive rats (SHRs) (three consecutive doses of either 3, 10, 30 or 100 mg kg-1, p.o.; 12 h apart) or beagle dogs (0.05, 0.5, 1.5 or 5 mg kg-1, p.o.; b.i.d., for 5 days) produced dose-dependent decreases in noradrenaline content, increases in dopamine content and increases in dopamine/noradrenaline ratio in the artery (mesenteric or renal), left ventricle and cerebral cortex. At the highest dose studied, the decreases in tissue noadrenaline were 47%, 35% and 42% (in SHRs) and 88%, 91% and 96% (in dogs) in the artery, left ventricle and cerebral cortex, respectively. When tested at 30 mg kg-1, p.o., in SHRs, nepicastat produced significantly greater changes in noradrenaline and dopamine content, as compared to the R-enantiomer (RS-25560-198), in the mesenteric artery and left ventricle. 4. Administration of nepicastat (2 mg kg-1, b.i.d, p.o.) to beagle dogs for 15 days produced significant decreases in plasma concentrations of noradrenaline and increases in plasma concentrations of dopamine and dopamine/noradrenaline ratio. The peak reduction (52%) in plasma concentration of noradrenaline and the peak increase (646%) in plasma concentration of dopamine were observed on day-6 and day-7 of dosing, respectively. 5. The findings of this study suggest that nepicastat is a potent, selective and orally active inhibitor of dopamine-beta-hydroxylase which produces gradual modulation of the sympathetic nervous system by inhibiting the biosynthesis of noradrenaline. This drug may, therefore, be of value in the treatment of cardiovascular disorders associated with over-activation of the sympathetic nervous system, such as congestive heart failure.

Dopamine-beta-hydroxylase activity is necessary for hypothalamo-pituitary-adrenal (HPA) responses to ether, and stress-induced facilitation of subsequent HPA responses to acute ether emerges as HPA responses are inhibited by increasing corticosterone (B)

To determine a role of norepinephrine (NE) in stress-induced HPA function, young male rats were treated with diethyldithiocarbamide (DDC) which inhibits dopamine-beta-hydroxylase, the enzyme that synthesizes NE from dopamine (DA). DDC injected 5 h prior to ether stress stimulated ACTH and corticosterone (B) during this time, and there was no further HPA response to ether. To control for elevated B feedback in DDC effects on HPA responses to ether, rats were adrenalectomized (Adx) and replaced with no (0% B), moderate (40% B) and high (80% B) levels of steroid 5 d prior to DDC or saline with ether stress 5 h later; Sham-Adx rats were included. In Adx rats increasing B inhibited thymus weight, median eminence CRF content, pituitary and plasma ACTH. In saline-treated rats, ether 5 h later caused increased CRF content and plasma ACTH in Sham-Adx and Adx, 0% B, increased ACTH in Adx, 40% B, and no response in Adx, 80% B. B treatment did not alter catecholamine content, and DDC treatment reduced NE content in the paraventricular nuclei by 50-60% in all groups. 5 h after DDC, pituitary ACTH was decreased in all rats with B and plasma ACTH was increased in sham-Adx and Adx, 40% B; thus DDC caused significant, prolonged stress which should facilitate subsequent HPA responses to acute stress. There was no HPA response to ether in Sham-Adx, Adx, 0% or 40% B groups, but there was a marked ACTH response to ether in the Adx, 80% B group treated with DDC. We conclude that: 1) the HPA response to ether stress is probably mediated by catecholamines; 2) DDC does not stimulate responses in the HPA axis in the absence of B; and, 3) facilitation of HPA responses to acute stress depends on increased steady-state B signals. Facilitated responses are probably not mediated by catecholamines. The consequence of facilitation is that under conditions of chronic stress and elevated B concentrations, as in depression or anorexia nervosa in man, or adjuvent-induced arthritis in rats, the HPA axis is continually responsive to new stimuli.

Chiral multisubstrate inhibitors of dopamine beta-monooxygenase: evidence for dual modes of interaction

The electronic and steric constraints of the dopamine beta-monooxygenase (DbetaM; E.C. 1.14.17.1) active site were studied using a series of chiral bisubstrate inhibitors. The (R) and (S) enantiomers of 5-phenyl-2-thiooxazolidone were apparent bisubstrate inhibitors for DbetaM with respect to tyramine and dioxygen, but with small enantiomeric selectivity. In contrast to the substrate specificity of the enzyme, N-methylation of both inhibitors increased the potency without altering the enantiomeric selectivity. The (S) C-4-methyl substitution was more detrimental toward the inhibition potency compared to (R) C-4-methyl substitution for both the (R) and (S) series, which was also opposite of the substrate specificity of the enzyme. The high inhibition potency and apparent bisubstrate behavior of 3-phenyl-1,5-bisthioglutarimide (XVI), a probe designed to mimic two distinct binding modes for the (R) and (S) inhibitors, suggested that they may interact with the enzyme by two different modes involving both coppers in the active site. Direct support for the interaction of the thione group(s) of XVI with the reduced DbetaM copper(s) is provided by the UV-vis spectroscopic studies. The complete disappearance of the characteristic UV absorption of XVI at 336 nm in the presence of stoichiometric amounts of reduced DbetaM demonstrate that it could be an active site titrant for reduced DbetaM. The ability of the enzyme to interact with these inhibitors by more than one mode suggests that the DbetaM active site possesses high steric and electronic tolerance.

Mechanism-based inactivation of dopamine beta-monooxygenase in adrenal chromaffin cells

Dopamine beta-monoxygenase (DBM, E.C. 1.14.17.1) is an attractive target point for possible modulation of adrenergic activity, and a variety of DBM-targeted pseudosubstrates and inhibitors have been developed in this laboratory and other laboratories. We now demonstrate the efficacy of a DBM-targeted mechanism-based inactivator, as well as enzymatic processing of two alternate DBM substrates, within functional adrenal chromaffin cells. When cultured adrenal medullary chromaffin cells were incubated with the mechanism-based inactivator 1-(4'-hydroxyphenyl)-1-(aminomethyl)-ethene (HOPAME), vesicular DBM activity was markedly decreased. Similarly, the alternate substrates 4'-hydroxyphenyl-2-aminoethyl sulfide and 4'-hydroxyphenyl-2-aminopropyl selenide each undergo uptake and DBM-catalyzed oxygenation within these cells. The simultaneous action of both the mechanism-based inactivator and an alternate substrate within functional chromaffin cells was also demonstrated. These results provide support for a direct mechanistic link between the enzymological properties of DBM-targeted adrenergic agents and their in-vivo pharmacological activities.

Catecholamine content and in vitro catecholamine synthesis in peripheral human lymphocytes

We studied the catecholamine (CA) content in peripheral human lymphocytes and the ability of these cells to synthesize CA in vitro. CA were separated by high performance liquid chromatography (HPLC) and determined in the supernatant by electrochemical detection as well as being determined after ultrasonic cell disruption in mononuclear leukocytes, adherent cells (monocytes/macrophages), total lymphocytes, and B- and T-cell enriched fractions. T lymphocytes contained L-Dopa and norepinephrine (NE), whereas B lymphocytes contained only L-Dopa. Lymphocytes seem to be able to synthesize NE from both L-tyrosine and L-Dopa added to the incubation medium in concentrations similar to the peripheral venous plasma (i.e. 5 x 10(-5) m and 10(-8) m, respectively). The addition of D-Dopa did not increase intracellular NE. alpha-methyl-p-L-tyrosine, benserazide, disulfiram, and fusaric acid (which are inhibitors of the enzymatic pathway) all decreased the synthesis of NE. After the addition of [3H]-L-Dopa (10(-8) m and 10(-7) m) to the incubation medium, [3H]-NE and [3H]-dopamine appeared. By increasing the concentration of L-Dopa in the medium (< 10(-6) m), CA were detected in the supernatant as well. These data show that peripheral human T lymphocytes contain and are able to synthesize CA from normal precursors in physiologic concentrations, i.e. a CA synthetic pathway is shown in nonneural cells. These data seem to support the hypothesis of autocrine and paracrine loops in the regulation of lymphocyte activity in lymphocytes taken from human cerebrospinal fluid (as suggested by other authors).

Influence of dopamine as noradrenaline precursor on the secretory function of the bovine corpus luteum in vitro

1. Dopamine is assumed to affect the ovary function after its conversion into noradrenaline (NA). 2. To study this bovine luteal slices from 11-14 days of the oestrous cycle were preincubated for 24 h to recover beta-receptors and next they were incubated for 1, 2 or 4 h with (a) different doses of dopamine; (b) dopamine together with a beta-antagonist (propranolol) or with a dopamine receptor blocker (droperidol); (c) dopamine with a dopamine-beta-hydroxylase inhibitor. 3. Dopamine stimulated the luteal content of oxytocin (OT) and progesterone. This effect was inhibited by propanolol but not by droperidol. 4. Dopamine added to the medium was followed by an increase of noradrenaline there. This rise was dose and time-dependent. 5. The dopamine-beta-hydroxylase inhibitor, inhibited the stimulating effect of dopamine on luteal progesterone and OT content. 6. Bovine corpus luteum can synthesize de novo NA from dopamine as a precursor.

Action of beta-phenylethylamine and related amines on nigrostriatal dopamine neurotransmission

The present paper describes the effect of beta-phenylethylamine and its metabolites phenylethanolamine, tyramine, acetyl-phenylethylamine and phenylacetaldehyde on the dopaminergic nigrostriatal system. The rotational behavioural response to the i.v. injection of these drugs was quantified in animals with a unilateral 6-hydroxydopamine lesion of the nigrostriatal dopamine system. Only beta-phenylethylamine and acetyl-phenylethylamine induced rotations ipsilateral to the side of the brain lesion. None of the compounds under study stimulated contralateral rotations. Acetyl-phenylethylamine was 90% less active than beta-phenylethylamine. After beta-phenylethylamine injection all animals (16/16) showed ipsilateral rotations. The dose-response curve showed that at doses as low as 1.75 mg/kg ipsilateral turns increase, with a dose-related rotational response between 1.75 mg/kg and 11.66 mg/kg, no differences being found at doses between 11.66 and 29.16 mg/kg. Rotations began a few seconds after beta-phenylethylamine injection. The highest response was found 30-60 s after the injection. The duration of the response was dose-related (4 min for the 3.5 mg/kg doses). The inhibition of dopamine-beta-hydroxylase activity with [1-3,5-difluorobenzyl)imidazole-2-thiol (SKF102698) did not modify the rotational response to beta-phenylethylamine. The inhibition of type B monoamine oxidase activity with l-deprenyl induced a slight increase in the ipsilateral rotational response to beta-phenylethylamine. The inhibition of tyrosine hydroxylase activity with alpha-methyl-p-tyrosine decreased the rotational response to beta-phenylethylamine. The dopamine receptor antagonist, haloperidol, completely blocked the ipsilateral rotational response to beta-phenylethylamine. The blocking of dopamine uptake into storage vesicles with reserpine increased the rotational action of beta-phenylethylamine. Taken together, the data suggest that, at low doses, beta-phenylethylamine stimulates the release of dopamine from the cytoplasmic pool and behaves as a dopamine receptor agonist with a very rapid and brief action.

Involvement of the brain catecholaminergic system in the regulation of dominant behavior

The role of the brain catecholaminergic system in establishing dominant-subordinate relationships in mice of different genotypes was studied using inhibitors of tyrosine hydroxylase (alpha-methyl-p-tyrosine) or of dopamine-beta-hydroxylase (FLA-57) or FLA-57 plus the dopamine precursor, DOPA. Demotion in all dominant and subdominant animals was associated with decreased noradrenaline levels, but the aggressive behavior of dominant male mice depended on the noradrenaline/dopamine ratio. Alterations in this relationship seem to have specific effects on social dominance in animals in the micropopulation, as drug-treated mice do not exhibit changes in their general activity. It can be concluded that brain catecholamines are of prime importance in maintenance of dominance.

The irreversible inactivation of two copper-dependent monooxygenases by sulfite: peptidylglycine alpha-amidating enzyme and dopamine beta-monooxygenase

Peptidylglycine alpha-amidating enzyme (alpha-AE) and dopamine beta-monooxygenase (D beta M), two copper-dependent monooxygenases that have catalytic and structural similarities, are irreversibly inactivated by sodium sulfite in a time- and concentration-dependent manner. Studies with alpha-AE show that the sulfite-mediated inactivation is dependent on the presence of redox active transition metals free in solution, with Cu(II) being the most effective in supporting the inactivation reaction. Sulfite inactivation of alpha-AE is specific for the monooxygenase reaction of this bifunctional enzyme and amidated peptides provide protection against the inactivation. Consequently, the sulfite-mediated inactivation of alpha-AE and D beta M most likely results from the transition metal-catalyzed oxidation of sulfite to the sulfite radical, SO3-.

Interaction of non-conjugated olefinic substrate analogues with dopamine beta-monooxygenase: catalysis and mechanism-based inhibition

The reaction of dopamine beta-monooxygenase (DBM; EC 1.14.17.1) with the prototypical non-conjugated olefinic substrate, 2-(1-cyclohexenyl)ethylamine (CyHEA) [see Sirimanne and May (1988) J. Am. Chem. Soc. 110, 7560-7561], was characterized. CyHEA undergoes facile DBM-catalysed allylic hydroxylation to form (R)-2-amino-1-(1-cyclohexenyl)ethanol (CyHEA-OH) without detectable epoxidation or allylic hydroxylation to form (R)-2-amino-1-(1-cyclohexenyl)ethanol (CyHEA-OH) without detectable epoxidation or allylic rearrangement, and with stereochemistry consistent with that of DBM-catalysed benzylic hydroxylation and sulphoxidation. The kcat. of 90 s-1 for CyHEA oxygenation is about 75% of the kcat. for tyramine, the substrate commonly used in assays of DBM activity. DBM-catalysed oxygenation of CyHEA also results in mechanism-based inactivation of DBM, with the inactivation reaction yielding kinact. = 0.3 min-1 at pH 5.0 and 37 degrees C, and a partition ratio of 16,000. Although both CyHEA turnover and inactivation exhibit normal kinetics, CyHEA processing also results in gradual depletion of copper from DBM; however, mechanism-based irreversible DBM inactivation occurs independent of this copper depletion when sufficient copper is present in the assay solution. A likely mechanism for turnover-dependent DBM inactivation by CyHEA involves initial abstraction of an allylic hydrogen to form a resonance-stabilized allylic radical, which can then either partition to product or undergo attack by an active-site residue. Acyclic, non-conjugated olefinic analogues exhibit diminished substrate activity toward DBM. Thus, kcat. for oxygenation of cis-2-hexenylamine, which also produces only allylic alcohol product, is only 14% of that for CyHEA. Similarly, kinact./KI for turnover-dependent inactivation by the acyclic olefin 2-aminomethyl-1-pentene is more than an order of magnitude smaller than that for benzylic olefins. Our results establish that DBM catalyses allylic oxygenation of a number of non-conjugated olefinic substrate analogues with neither epoxidation nor allylic rearrangement occurring. The absence of epoxide products from non-conjugated olefinic substrates implies an inability of the activated copper-oxygen species of DBM to effect radical cation formation from a non-conjugated olefinic moiety. The striking contrast between DBM and cytochrome P-450, which carries out both epoxidation and allylic oxidation with non-conjugated olefinic substrates, is probably a reflection of the differences in redox potential of the activated oxygen species operative for these two enzymes.

Changes in hypothalamic catecholamines, dopamine-beta-hydroxylase, and phenylethanolamine-N-methyltransferase in the catfish Heteropneustes fossilis in relation to season, raised photoperiod and temperature, ovariectomy, and estradiol-17 beta replacement

In Heteropneustes fossilis, contents and turnovers of hypothalamic catecholamines (CA) and activities of dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT) showed significant seasonal variations with significantly high day values. The seasonal pattern of dopamine (DA) on one hand and that of noradrenaline (NA) and adrenaline (A) on the other hand showed an inverse relationship, the former decreasing and the latter increasing during the progress of gonadal recrudescence. The DBH and PNMT levels were low in the resting phase and increased to the peak in the prespawning (DBH) and spawning (PNMT) phases. Maintenance of the fish under long photoperiods (16L:8D) and high temperature (28 +/- 2 degrees) stimulated the NA and A, and DBH and PNMT activities, and suppressed the DA mechanism, the changes being maximal in the raised temperature groups. In the resting phase (December), ovariectomy (OVX) or estradiol-17 beta (E2) replacement in 4-week ovariectomized fish did not produce any significant effects on the CA and enzyme activities. On the contrary, in the prespawning phase (May), OVX produced differential and biphasic responses on CA and the enzymes. The contents and turnovers of both NA and A increased significantly at 2-5 weeks and decreased in the sixth week. However, the reverse was true for DA. The DBH and PNMT activities (assayed only 3, 4, and 6 weeks after OVX) were elevated significantly in the third and fourth weeks but decreased in the sixth week. Plasma levels of gonadotropin (GTH) increased significantly at all durations of OVX in a bimodal pattern while the E2 levels decreased consistently. Supplementation with a low dose (0.1 microgram/g BW) of E2 restored the NA and A and enzyme activities while the higher doses (0.5, 1.0, and 5.0 micrograms/g BW) depleted them. The reverse was true for DA. The low dose of E2 restored the GTH level while the higher ones inhibited it significantly. These results indicate that both environmental photoperiod and temperature and E2-negative feedback act on the CA to modulate GTH secretion.