Fusaric acid in Fusarium moniliforme cultures, corn, and feeds toxic to livestock and the neurochemical effects in the brain and pineal gland of rats

Fusaric acid is produced by several species of Fusarium, which commonly infect corn and other agricultural commodities. Since this mycotoxin may augment the effects of other Fusarium toxins, a gas chromatography/mass spectrometry method of analysis in feeds was developed. Fusaric acid was analyzed as the trimethylsilyl-ester from F. moniliforme-cultures, -contaminated corn screenings, and feeds toxic to livestock. The mycotoxin was found in all samples and ranged from 0.43 to 12.39 micrograms/g sample. Also, fusaric acid was tested for its neurochemical effects in the brain and pineal gland of rats. Animals were dosed intraperitoneally (100 mg/kg body weight) 30 min prior to the onset of the dark phase (lights out) and the effects were studied at 1.5, 3.5, and 5.5 h after treatment. Brain serotonin (5HT), 5-hydroxyindoleacetic acid (5HIAA), tyrosine (TYRO), and dopamine (DA) were increased (P < 0.05) by fusaric acid, and norepinephrine (NEpi) was decreased (P < 0.05). Analogously, DA in the pineal gland increased and NEpi decreased (P < 0.05). Pineal N-acetylserotonin (NAc5HT) was increased (P < 0.05), whereas pineal 5HT and its two major metabolites 5HIAA and 5-hydroxytryptophol (5HTOL) decreased (P < 0.05). Elevated brain TYRO and brain and pineal DA, with decreased NEpi, may be consistent with fusaric acid's partial inhibitory effect on tyrosine-hydroxylase and its inhibitory effect on dopamine-beta-hydroxylase, respectively. Elevated pineal Nac5HT is consistent with decreased pineal 5HT and the increased pineal DA, and support the dopaminergic stimulatory activity of the enzyme responsible for the conversion of 5HT to NAc5HT. This is the first report of fusaric acid's in vivo effect on pineal DA, NEpi, 5HT, and NAc5HT in rats, and a relation for the effects on TYRO, 5HT, and 5HIAA in brain tissue. The results indicate fusaric acid alters brain and pineal neurotransmitters and may contribute to the toxic effects of Fusarium-contaminated feeds.

Blockade of noradrenergic neurotransmission with diethyldithiocarbamic acid decreases the mRNA level of gonadotropin-releasing hormone in the hypothalamus of ovariectomized, steroid-treated prepubertal rats

We have previously found that progesterone (P) augmented gonadotropin-releasing hormone (GnRH) mRNA levels in the hypothalamus of ovariectomized, estradiol-treated (OVX + E) prepubertal rats. In order to determine whether noradrenergic neurotransmission is involved in the stimulatory effect of P on GnRH gene expression, diethyldithiocarbamic acid (DDC, 500 mg/kg), a dopamine beta-hydroxylase inhibitor was administered i.p. 1 h before P (1 mg) injection into OVX + E treated rats, and the effect of DDC on the P-induced GnRH mRNA levels was examined. A single injection of P into OVX + E primed rats augmented norepinephrine (NE) content, while the administration of DDC effectively blocked the P-induced increase in NE content, along with the increase in dopamine content. Suppression of NE neurotransmission with DDC resulted in a marked decrease in the P-induced GnRH mRNA levels as well as GnRH release in vitro. These results clearly demonstrate that noradrenergic neurotransmission is involved in P-stimulated GnRH gene expression in the rat hypothalamus.

Ascorbate based novel high affinity alternate reductants and competitive inhibitors of dopamine beta-monooxygenase

A series of ascorbate derivatives has been used to examine the specificity and the chemistry of the reduction site of dopamine beta-monooxygenase (D beta M). Replacement of the 6-OH group of ascorbic acid with either bromine or hydrogen does not alter the enzyme reduction efficiency significantly. Unexpectedly, the 6-OH modified ascorbate derivatives, 6-S-phenyl-6-thio-L-ascorbic acid and 6-O-phenyl-L-ascorbic acid were found to have much higher affinity for the enzyme than the most effective known electron donor, ascorbic acid (AscH-). The affinity of 2-amino-6-S-phenyl-L-ascorbic acid was found to be similar to that of 2-amino-L-ascorbic acid. 6-Amino-6-deoxy-L-ascorbic acid is neither a substrate nor an inhibitor for the enzyme. Although glucoascorbic acid is an excellent substrate for the enzyme, imino glucoascorbic acid was found to be an extremely potent competitive inhibitor for the enzyme. The stereoelectronic properties and alternate binding modes of these molecules have been considered in explaining the observations.

In vivo inhibition of serum dopamine-beta-hydroxylase by CaNa2 EDTA injection

1. In humans CaNa2EDTA increases urinary excretion of lead as well as that of essential metals such as zinc, ferrum and manganese but not that of copper. 2. We studied the effect of CaNa2EDTA injection on serum dopamine-beta-hydroxylase, a copper-dependent enzyme, in three male lead welders hospitalized for suspected lead poisoning. 3. Injection of CaNa2EDTA (1000 mg i.v.) resulted in rapid reduction of serum dopamine-beta-hydroxylase activity which returned to normal level within 24 hours. 4. Copper sulphate (8 microns) but not ethylmaleimide (10 mM) restored in vitro serum dopamine-beta-hydroxylase activity to pre-dosing values which indicates that CaNa2EDTA inhibits dopamine-beta-hydroxylase by removing copper from the enzyme. 5. An inverse dose effect relationship between injected CaNa2EDTA and residual serum dopamine-beta-hydroxylase activity was observed. 6. In spite of the reported lack of changes in copper serum concentration, CaNa2EDTA significantly affects the activity of a copper-dependent enzyme in the serum. 7. As CaNa2EDTA is currently the drug of choice for the treatment of asymptomatic lead poisoning in children, the effect of CaNa2EDTA on copper-enzymes requires further investigation.

Extracellular catecholamine levels in rat hippocampus after a selective alpha-2 adrenoceptor antagonist or a selective dopamine uptake inhibitor: evidence for dopamine release from local dopaminergic nerve terminals

The effect of 6-chloro-2,3,4,5-tetrahydro-3-methyl-1-H-3-benzazepine (SKF 86466), a selective nonimidazoline alpha-2 adrenoceptor antagonist, on hippocampal release of norepinephrine and dopamine in conscious rats was investigated by in vivo microdialysis and high-pressure liquid chromatography. Additionally, extracellular concentrations of hippocampal dopamine (DA) and norepinephrine (NE), during infusion of selective monoamine uptake inhibitors, were determined in freely moving rats. The basal concentration of NE in the dialysate was 4.9 +/- 0.3 pg/20 microliters. Intravenous administration of 5 or 10 mg/kg of SKF 86466 was associated with a transient increase (30 min) of 2-fold (12 +/- 1 pg/20 microliters; P < .05) and 8-fold (39 +/- 3 pg/20 microliters; P < .05), respectively, in dialysate NE, whereas a 1-mg/kg dose had no effect. DA was not detected in basal dialysates, but after the administration of 5 or 10 mg/kg of SKF 86466, 3.9 +/- 0.4 and 6.4 +/- 0.6 pg/20 microliters, respectively, was present in the dialysates. The maximum increase in dialysate DA was reached 60 to 90 min after SKF 86466. The DA was not derived from plasma because plasma NE was elevated after the 5 mg/kg dose of SKF 86466 whereas no plasma DA was detected. In order to determine whether DA was present in noradrenergic nerve terminals, the dopamine beta-hydroxylase inhibitor SKF 102698 was administered (50 mg/kg i.p.). The inhibitor decreased dialysate NE but DA was still not detected in the dialysate.(ABSTRACT TRUNCATED AT 250 WORDS)

Recycling of a secretory granule membrane protein after stimulated secretion

Recycling of a secretory granule membrane protein, dopamine-beta-hydroxylase, was examined in primary cultures of bovine adrenal chromaffin cells. Cells were stimulated to secrete in the presence of antibodies directed against the luminal domain of dopamine-beta-hydroxylase. The location of the antibodies after various times of reincubation and after a second secretory stimulus was assessed using immunofluorescence microscopy. Stimulation led to the exposure of dopamine-beta-hydroxylase at the plasma membrane, which could be detected by a polyclonal antibody in living and fixed cells. The plasma membrane dopamine-beta-hydroxylase, either alone or complexed with antibody, was rapidly internalized after removal of the secretagogue. Internalized protein-antibody complex remained stable for at least 24 hours of reculture. Twenty four hours after stimulation the cells with internalized antibody could respond to further stimulation and some of the antibody was re-exposed at the plasma membrane. These findings were confirmed using FACS analysis. This suggests that the antibody-protein complex had returned to secretory granules that could respond to further secretagogue stimulation.

The dithiocarbamate fungicide thiram disrupts the hormonal control of ovulation in the female rat

Thiram has been reported to inhibit dopamine-beta-hydroxylase (D beta H), thereby affecting norepinephrine (NE) synthesis. Because NE is a neurotransmitter that is known to play an important role in the hypothalamic regulation of pituitary function, the acute effects of the thiram on the hormonal control of ovulation in the rat were investigated. Ovariectomized, estrogen-primed female rats were given a single injection of thiram (0, 6, 12, 25, 50, and 100 mg/kg, i.p.) at 1100 h and serum LH was measured in serial bleeds. Thiram at 100 and 50 mg/kg completely blocked the LH surge in all rats tested, while 12 and 25 mg/kg blocked the surge in 40 and 75% of the treated animals, respectively. Six mg/kg had no effect. Ovulation was then assessed in intact, proestrous females in response to thiram administration (0, 12, 25, or 50 mg/kg) at 0900, 1100, 1300, or 1800 h. Ovulation was blocked by 25 and 50 mg/kg at 1300 h in all rats, but when injected at 1100 h only the 50 mg/kg dose was effective. No such blockade was found with 50 mg/kg injected at 0900 and 1800 h. To assess the influence of thiram on the LH surge in intact rats, additional females were dosed at 1300 h on the day of proestrus and blood collected over that same day. Thiram at 50 mg/kg blocked the LH surge in all rats, while 25 mg/kg blocked the surge in 60% of the females tested. No effect occurred with 12 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

[Synthesis of 6-alkylaminopyridazine-3-carboxylic acid derivatives as dopamine beta-hydroxylase inhibitors]

A series of 6-alkylaminopyridazine-3-carboxylic acid derivatives was tested for dopamine beta-hydroxylase inhibitory activity in vitro according to the method of Kruse et al. Methyl 6-alkylaminopyridazine-3-carboxylates (4) were synthesized through the reaction of methyl 6-chloropyridazine-3-carboxylate (1) with ammonia followed by the condensation with primary amines, and by the methanolysis of the resulting 6-alkylaminopyridazine-3-carboxamides (3) in methanol in the presence of boron trifluoride etherate. Among tested compounds, 6-benzylaminopyridazine-3-carboxylic acid was found to have the most potent inhibitory activity, which was in the same level of the activity of fusaric acid.

Synthesis of substituted DL-3(5-benzazolyl)alanines as dopa and alpha-methyldopa analogs and their effects on dopamine beta-hydroxylase, tyrosinase and diphenoloxidase

The influence of bioisosteric replacement of catechol moiety of L-Dopa and alpha-Methyldopa with benzimidazole and benzotriazole ring has been examined on dopamine beta-hydroxylase and tyrosinase, in order to evidentiate an inhibitory activity on the synthesis of catecholamines and a possible antihypertensive action. The preliminary results obtained so far showed that inhibition of dopamine hydroxylase occurs at 5 x 10(-4) M concentration for the most active compounds bearing a trifluoromethyl group in the azole ring (2a,c). An analogous result was observed in the case of tyrosinase inhibition with compound 2c, while other compounds (2a,e) were equiactive (92% inhibition) at higher concentration (1 x 10(-3) M). Compound 2c was also the most active in inhibition of diphenoloxidase (83% at 6 x 10(-5) M concentration).

Propranolol reduces rat dopamine-beta-hydroxylase activity and catecholamine levels

Propranolol treatment (1 mg/kg i.p. twice daily for 8 days) reduced atrial dopamine-beta-hydroxylase (D beta H) activity by 60.5% and lung D beta H activity by 53.5% but did not alter ventricle D beta H activity. Propranolol also significantly reduced atrial noradrenaline (NA) by 66.3%, adrenaline (A) by 40%, dopamine (DA) by 72.4% and lung NA by 46.6% but did not change plasma or cardiac ventricle catecholamines. The addition of propranolol (10(-6) M) to cardiac and lung tissue homogenates in vitro did not inhibit tissue D beta H activities. The results suggest that chronic propranolol treatment inhibits NA synthesis and support the hypothesis of a centrally induced inhibition of sympathetic activity caused by beta-adrenoceptor blockade.

Inhibitory effect of neuropeptide Y (NPY) on the in vitro activity of dopamine-beta-hydroxylase

Neuropeptide Y (NPY) is found to be costored with norepinephrine (NE) in vesicles of the nerve terminals. An endogenous inhibitor of dopamine-beta-hydroxylase (DBH), the synthetic enzyme of NE, has been mentioned. In an attempt to clarify the effect of NPY on DBH activities, an in vitro assay is carried out using chromatographic analysis of NE formation from dopamine. NPY (20-80 pmol/ml) produced a dose-dependent depression of NE formation catalysed by the purified bovine adrenal DBH. Lineweaver-Burke plots (Km = 1.1 mM, Vmax = 10 pmol/min/mg protein) showed a non-competitive inhibition in NPY (30 pmol/ml, IC50)-treated samples. Moreover, failure of denatured NPY even at maximum concentration to influence the DBH activities suggested the essential of natural form of NPY. Participation of sulfhydryl compound seems also negligible, because N-ethylmaleimide did not overcome the effect of NPY. These results indicate that NPY has the ability to inhibit the catalytic action of DBH.

Mechanism of inhibition of dopamine beta-monooxygenase by quinol and phenol derivatives, as determined by solvent and substrate deuterium isotope effects

The mechanism of interaction of quinols and phenols with dopamine beta-monooxygenase (D beta M) has been investigated. The ratio of quinone formation (from catechol) to oxygen consumption rises from a value of 1 in the presence of phenethylamine substrate to 2 in the absence of substrate. These results implicate quinol oxidation at both the reductant- and substrate-binding sites of D beta M. In the presence of saturating ascorbate, catechol and p-hydroquinol behave as mechanism-based inhibitors of D beta M, with partitioning ratios of turnover to inactivation of 21:1 and 41:1, respectively. Phenol is found to inactivate the enzyme in a manner similar to p-cresol, suggesting that the methyl group of p-cresol is not an essential component of enzyme inhibition. Solvent isotope effects on inactivation and turnover have been measured for various inactivators. Although the majority of these inhibitors, including catechol, p-hydroquinol, aniline, phenethylenediamine, and benzylhydrazine, are characterized by relatively small solvent isotope effects (1.5-2.5) on the inactivation rate constant (ki), solvent isotope effects on ki for phenol and p-cresol are 5.7 and 7.4, respectively. By contrast, solvent isotope effects on the turnover of p-cresol are almost unity. Using p-cresol-d7 as substrate, we observe D(kcat) = 5.2 and D(kcat/Km) = 3.1, while isotope effects on inactivation are D(ki) = 0.95 and D(ki/Ki) = 0.59. These results lead us to propose that inhibitors fall into two mechanistic classes, involving either one-electron oxidation to form radical cation intermediates (quinols) or hydrogen atom abstraction (phenols).(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen receptor binding in regions of the rat hypothalamus and preoptic area after inhibition of dopamine-beta-hydroxylase

Previous studies have shown that administration of diethyldithiocarbamate (DDC), a dopamine-beta-hydroxylase inhibitor, results in a decreased concentration of estrogen receptors measured in the rodent hypothalamus and preoptic area. To determine if this modulation of receptor content is region-specific, in vitro estrogen binding assays were performed on cytosol and cell nuclear extracts of microdissected brain regions from female rats treated with DDC. For cytosol binding comparisons, ovariectomized (OVX) rats were treated with 550 mg DDC/kg b. wt. or the saline vehicle 12 h before sacrifice. For cell nuclear binding comparisons, OVX rats received a maximal dose of estradiol 12 h after DDC or saline treatment and 1 h before sacrifice. No region-specific decreases in estrogen binding were observed in either cytosol or nuclear extracts. To further examine possible regional specificity, quantitative autoradiographic analysis of the in vivo hypothalamic uptake of an iodinated analog of estradiol, 11 beta-methoxy-16 alpha-[125I]iodoestradiol (MIE2), in OVX rats treated with DDC was conducted. Animals received a saturating dose of [125I]MIE2 12 h after DDC or saline treatment and 1 h before sacrifice. DDC treatment resulted in higher background levels of radioactivity and a trend toward higher uptake levels in all brain regions, but with no evidence of marked regional specific effects in any area of the brain. In tissue uptake studies, DDC treatment resulted in higher levels of radioactivity recovered from serum and neural tissues of [125I]MIE2-injected rats, suggesting that DDC slows the clearance of MIE2.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of voluntary ethanol consumption by dopamine-beta-hydroxylase inhibition (FLA-57): mediated by changes in aversion, reinforcement or both

The dopamine-beta-hydroxylase inhibitor, FLA-57, was reported by several investigators to reduce voluntary ethanol consumption in rats. The nature of the effect of FLA-57 on this behavior had been attributed to its involvement in both the mediation of positive reinforcing and aversive processes. In the present study, the capacity of FLA-57 to induce a conditioned taste aversion (CTA) in both a forward and a "nominally backward conditioning" paradigms was investigated. This was done in an attempt to assess the possible contribution of a FLA-57-induced CTA to the previously observed reduction in ethanol intake in several drinking studies. Furthermore, the ability of FLA-57 to induce a CTA in a nonnovel situation, where the taste of the presented solution (ethanol or saccharin) was familiar to the animals, was also assessed. The inclusion of these specific conditions was necessitated by the attempt to create conditions similar to those prevalent in drinking studies. We found that FLA-57, in both conditioning paradigms, induced a significant CTA. Animals, naive and experienced with the taste of ethanol or saccharin, exhibited a CTA following the administration of FLA-57. However, the magnitude and rate of extinction of the observed CTAs did not resemble those observed in studies on the effects of FLA-57 on ethanol intake. The results of this study suggest that while it is possible that FLA-57 exerts its effect on ethanol intake, at least in part, through an aversive mechanism, such a mechanism is unlikely to be the exclusive process through which ethanol ingestion is attenuated.

Effect of the dopamine beta-hydroxylase inhibitor, SK&F 102698, on blood pressure in the 1-kidney, 1-clip hypertensive dog

The acute and chronic effects of a potent selective dopamine beta-hydroxylase inhibitor, SK&F 102698, were assessed in chronically instrumented 1-kidney, 1-clip Goldblatt hypertensive dogs. Blood pressure measured directly from either a carotid loop or from a vascular access port and cardiac output measured by impedence cardiography were monitored following acute (30 and 100 mg/kg, p.o.) and chronic (30 mg/kg/day for 4 days) administration of SK&F 102698. The data indicate that SK&F 102698 failed to alter blood pressure, cardiac output or total peripheral resistance after either acute or chronic administration. It is concluded that dopamine beta-hydroxylase inhibition with SK&F 102698 is not an effective antihypertensive agent in the 1-kidney, 1-clip Goldblatt hypertensive dog model.

Decreased norepinephrine and epinephrine contents in chromaffin tissue of rainbow trout (Oncorhynchus mykiss) exposed to diethyldithiocarbamate and amylxanthate

1. Rainbow trout (Oncorhynchus mykiss) were exposed to 0.5 or 5.0 microM of diethyldithiocarbamate (DDC) or amylxanthate (AX) for 24 hr. 2. Both DDC (0.5-5.0 microM) and AX (5.0 microM) significantly decreased norepinephrine and epinephrine levels in the head kidney as well as the quotients epinephrine/dopamine and/or norepinephrine/dopamine. 3. The results probably reflect an inhibition of dopamine-beta-hydroxylase, the enzyme responsible for the synthesis of norepinephrine and epinephrine from dopamine. 4. It is concluded that an exposure of fish to these complexing agents could disturb physiological processes controlled by catecholamines. 5. Diethyldithiocarbamate may prove to be a valuable pharmacological tool for the study of catecholamine function in fish.

Catecholamine synthesis inhibitors increase pineal adrenaline content by stimulating adrenal medullary activity

Estimation of noradrenaline and adrenaline utilization in the pineal gland of female rats was attempted using inhibitors of the enzymes that catalyse the catecholamine biosynthetic pathway. Treatment with FLA63, an inhibitor of dopamine beta-hydroxylase (10 mg/kg, 2 h before killing), induced depletion of noradrenaline and adrenaline in the preoptic area and median eminence (sites, respectively, inside and outside the blood-brain barrier) but, paradoxically, resulted in a significant increase (+77%) in the pineal content of adrenaline without affecting that of noradrenaline. Treatment with LY134046, an inhibitor of phenylethanolamine N-methyltransferase (40 mg/kg, 5 and 2 h before killing), induced depletion of adrenaline in the preoptic area and median eminence but, again, resulted in a paradoxical and large increase in pineal adrenaline (+224%); this increase was prevented by prior adrenalectomy. Blood samples taken from free-moving rats fitted with intravenous and intraperitoneal cannulae revealed a marked increase in plasma levels of adrenaline after each injection of LY134046. These results suggest that the adrenal medulla is the primary source for the increase in pineal adrenaline seen after administration of the enzyme inhibitors. The precise site of uptake and the biological implications of this phenomenon remain to be elucidated. Nevertheless, interpretation of in vivo experiments involving these catecholamine synthesis inhibitors should take this adrenal response into account.

3,4-Dihydroxyphenylacetic acid (DOPAC) as an index of noradrenaline turnover: effects of Hydergine and vincamine

Among the drugs commonly used in the treatment of memory disorders of the elderly, vincamine and hydergine have been shown to moderately increase the firing rate of noradrenergic locus coeruleus (LC) neurons. Since changes in electrical activity of noradrenergic neurons are generally reflected in corresponding alterations of the turnover of this transmitter, the effects of these drugs on the accumulation of 3,4-dihydroxyphenylacetic acid (DOPAC) and dopamine (DA) in the presence and absence of the dopamine-beta-hydroxylase inhibitor, FLA 63, were studied in the LC as well as in two of its projection areas, the hippocampus and the cerebellum. Characterization of this procedure with the alpha 2-adrenoceptor antagonist, idazoxan, the corresponding agonist, clonidine, the alpha 1-adrenoceptor antagonist prazosine, and haloperidol, suggested that--DOPAC changes are more suitable than those of DA or DOPAC/DA ratios in reflecting changes in noradrenaline (NA) turnover, inhibiting DBH is advantageous if NA turnover is to be measured in projection areas, but not in LC, and haloperidol and prazosine, in principle, did not affect NA turnover. Vincamine and hydergine at 10 mg/kg doses, at which they were reported to increase LC firing by 50%, did not induce a change in NA turnover in any of the areas. This, together with the data obtained with haloperidol, suggests that a minimal increase in the firing rate of LC cells (+140%) is required before it could influence the turnover of NA, as measured by DOPAC changes. Thus, the stimulating effect of nootropics on the central noradrenergic system may be more sensitively detected by electrophysiological techniques than by biochemical ones.

Dopamine beta-hydroxylase inhibition reveals a selective influence of endotoxin on catecholamine content of rat tissues

Endotoxin is known to affect sympathetic nervous system (SNS) activity. However, it is unknown whether selective activation occurs in different tissues. The present work assessed SNS activity in several tissues of endotoxin-treated rats by measuring the accumulation of dopamine following dopamine beta-hydroxylase inhibition with disulfiram. Twenty-four hours following endotoxin injection (1 mg/kg), body weight and food intake were significantly reduced. The weight of spleen and liver was significantly greater in endotoxin-injected rats compared to either control animals or rats pair-fed the same quantity of food ingested by the endotoxin-treated rats. Norepinephrine (NE) content of heart, liver, and brown adipose tissue (BAT) was significantly lower in endotoxin-injected animals, but it was unchanged in spleen and gastrocnemius muscle. Endotoxin caused a significant elevation in the dopamine content of heart, liver, BAT, and gastrocnemius muscle, whereas it had no effect on spleen. The results suggest that SNS activity is increased in heart, liver, BAT, and gastrocnemius muscle 26 h after endotoxin injection.

1-(Thienylalkyl)imidazole-2(3H)-thiones as potent competitive inhibitors of dopamine beta-hydroxylase

1-(2-Thienylalkyl)imidazole-2(3H)-thiones (5a-k) are competitive inhibitors of dopamine beta-hydroxylase (DBH) and demonstrate the utility of thiophene in the design of potent competitive inhibitors of this enzyme. The structure-activity relationships for these compounds are discussed and compared with those of 1-phenylalkyl-imidazole-2(3H)-thiones (1). With the aid of molecular modeling, an idealized active-site conformer is proposed and an explanation for the difference in activity between the phenyl (1) and thienyl (5) DBH inhibitors is presented. The difference in activity is consistent with our proposal that thiophene may not always be a bioisostere for phenyl. The inhibitor of most interest, 1-[2-(2-thienyl)ethyl]imidazole-2(3H)-thione (5g), was selected for study in the spontaneously hypertensive rat. The changes in dopamine and norepinephrine levels that resulted from oral administration of 5g correlated with the reduction of blood pressure.

In vivo monitoring of catecholaminergic metabolism in the C1 region of rat medulla oblongata: a comparative study by voltammetry and intracerebral microdialysis

In vivo voltammetry or microdialysis was used to monitor catecholaminergic metabolism in the C1 region of the ventrolateral medulla oblongata of anesthetized rats. In vivo voltammetry allowed the recording of a catechol oxidation current (CA.OC) peak in this region. This CA.OC was suppressed after inhibition of monoamine oxidase by pargyline or after inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine and was markedly increased after blockade of dopamine-beta-hydroxylase by FLA 63. Similar results were found when intracerebral microdialysis coupled with HPLC and electrochemical detection was used to measure the concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) in the dialysates obtained from the C1 region: The changes in CA.OC and DOPAC concentration in the dialysates exhibited very similar kinetic characteristics in the three pharmacological experiments. These results support the involvement of DOPAC as a major component of the electrochemical signal recorded by voltammetry in the C1 group of adrenergic neurons.

Noradrenergic system: effect of DSP4 and FLA-57 on ethanol intake in ethanol preferring rats

Ethanol preferring rats (male Long-Evans; n = 6) were selected as drinking rats (DR) and treated with DSP4 (50 mg.kg-1 IP) at the end of the preference selection. Two more groups received DSP4 (50 mg.kg-1 IP) + the inhibitor of dopamine beta-hydroxylase FLA-57 (1 mg.kg-1.d-1 during two weeks), IP (n = 5) or FLA-57 alone (1 mg.kg-1.d-1 during two weeks IP) (n = 5). The control DR group (n = 6) received NaCl 0.9%. 3H-Noradrenaline uptake was studied at the 17th day of treatment in DR, treated or not with DSP4, and in ethanol naive rats treated (n = 6) or not (n = 6) with DSP4 (50 mg.kg-1, IP) DSP4 does not modify ethanol intake in DR, and both treated groups (DR or ethanol naive rats). 3H-Noradrenaline uptake was decreased (about 60%), both in cortex and hippocampus. But the association of FLA-57 and DSP4 decreases both ethanol and fluid intakes. It was suggested 1) that the 40% of intact neurons was able to compensate the DSP4-induced noradrenergic neurons destruction, 2) that the destruction of noradrenergic pathways (FLA-57 + DSP4) is associated with a decrease in ethanol intake but also in fluid intakes, suggesting finally 3) that the modulation of ethanol intake by the noradrenergic system was partial or indirect.

Use of isotope effects to characterize intermediates in mechanism-based inactivation of dopamine beta-monooxygenase by beta-chlorophenethylamine

A mechanism for beta-chlorophenethylamine inhibition of dopamine beta-monooxygenase has been postulated in which bound alpha-aminoacetophenone is generated followed by an intramolecular redox reaction to yield a ketone-derived radical cation as the inhibitory species (Mangold, J.B., and Klinman, J.P. (1984) J. Biol. Chem. 259, 7772-7779). Based on the assumption that the ketone radical is the inhibitory intermediate, an analogous system was predicted and verified (Bossard, M.J., and Klinman, J.P. (1986) J. Biol. Chem. 261, 16421-16427). In the present study, the role of alpha-aminoacetophenone as the proposed intermediate in the inactivation by beta-chlorophenethylamine was examined in greater detail. From the interdependence of tyramine and alpha-aminoacetophenone concentrations, ketone inactivation is concluded to occur at the substrate site as opposed to potential binding at the reductant-binding site. Using beta-[2-1H]- and beta-[2-2H]chlorophenethylamine, the magnitude of the deuterium isotope effect on inactivation under second-order conditions has been found to be identical to that observed under catalytic turnover, D(kappa inact/Ki) = D(kappa cat/Km) = 6-7. By contrast, the isotope effect on inactivation under conditions of substrate and oxygen saturation, D kappa inact = 2, is 3-fold smaller than that seen on catalytic turnover, D kappa cat = 6. This reduced isotope effect for inactivation is attributed to a normal isotope effect on substrate hydroxylation followed by an inverse isotope effect on the partitioning of the enol of alpha-aminoacetophenone between oxidation to a radical cation versus protonation to regenerate ketone. These findings are unusual in that two isotopically sensitive steps are present in the inactivation pathway whereas only one is observable in turnover.

Dopamine-1 receptors in rat proximal convoluted tubule: regulation by intrarenal dopamine

Dopamine (DA) is synthesized in the renal proximal convoluted tubule (PCT) and may act as a paracrine substance at tubular DA-1 receptors to decrease sodium transport. Although DA-1 receptors have been identified in rabbit renal PCT by use of nonselective dopaminergic radioligands, DA-1 receptors have not been localized in specific nephron segments with the use of selective DA-1 radioligands. In these studies we used a novel DA-1 dopaminergic ligand 125I-SCH-23982, which has been shown to have a high affinity for brain and renal DA-1 receptors, to identify DA-1 receptors in the rat renal PCT and distal convoluted tubule (DCT). DA-1 receptors in the microdissected PCT and DCT were studied by a quantitative autoradiographic technique and by measuring adenylate cyclase (AC) activity. No specific 125I-SCH-23982 binding could be measured in the DCT indicating the absence of DA-1 receptors in this segment. Binding of 125I-SCH-23982 to PCT was saturable with time and radioligand concentration and was stereoselective. Saturation isotherm analysis in control rats yielded a dissociation constant (Kd) of 7.5 +/- 0.14 nM (n = 4) and a maximum receptor density (Bmax) of 0.69 +/- 0.04 pmol/mg protein (n = 4). The rank-order potency for agonist and antagonist displacement of 125I-SCH-23982 binding was consistent for DA-1 receptors: SCH-23390 greater than fenoldopam = SKF 38393 greater than SCH-23388. The stimulatory effect of the DA-1 agonist fenoldopam (10 microM) on AC activity was blocked by the DA-1 antagonist SCH-23390 (10 microM) but not by the beta-adrenergic antagonist (-)-propranolol (10 microM), indicating specificity. The DA-beta-hydroxylase blocker, SKF 102698, increased renal DA concentrations threefold, reduced the PCT DA-1 receptor Bmax by 33%, and abolished the stimulatory effect of 10 microM fenoldopam on AC activity in the PCT but had no effect on Kd. It is concluded that DA-1 receptors are present in rat PCT but not DCT and can be regulated by renal DA.

Interaction of dopamine-beta-monooxygenase with lipids: 1. Lipids as modulators of the catalytic activity of the enzyme

The interaction of the membrane-bound and soluble forms of dopamine-beta-monooxygenase with a variety of lipid analogues of the membrane of chromaffin granules were studied. The use of two independent methods to determine the kinetics of product formation and of oxidation of the electron donor substrates (two substrates, dopamine and tyramine, and two electron donors, ascorbate and ferrocyanide), showed that the enzyme is activated in the presence of either phosphatidylcholine or lysophosphatidylcholine, but is inhibited by phosphatidylserine, phosphatidylethanolamine, and phosphatidic acid. The magnitude of the effects observed was the same for both forms of the enzyme. The parameters Vmax and Km for the substrates and for the electron donors were determined in the presence of each of the lipids tested. The data lead to certain conclusions about the mechanism of action of the lipids upon the enzyme and about the possible physiological role of the effects observed.