Cyclooxygenase-independent neuroprotective effects of aspirin against dopamine quinone-induced neurotoxicity.

Prostaglandin H synthase exerts not only cyclooxygenase activity but also peroxidase activity. The latter activity of the enzyme is thought to couple with oxidation of dopamine to dopamine quinone. Therefore, it has been proposed that cyclooxygenase inhibitors could suppress dopamine quinone formation. In the present study, we examined effects of various cyclooxygenase inhibitors against excess methyl L-3,4-dihydroxyphenylalanine (L-DOPA)-induced quinoprotein (protein-bound quinone) formation and neurotoxicity using dopaminergic CATH.a cells. The treatment with aspirin inhibited excess methyl L-DOPA-induced quinoprotein formation and cell death. However, acetaminophen did not show protective effects, and indomethacin and meloxicam rather aggravated these methyl L-DOPA-induced changes. Aspirin and indomethacin did not affect the level of glutathione that exerts quenching dopamine quinone in dopaminergic cells. In contrast with inhibiting effects of higher dose in the previous reports, relatively lower dose of aspirin that affected methyl L-DOPA-induced quinoprotein formation and cell death failed to prevent cyclooxygenase-induced dopamine chrome generation in cell-free system. Furthermore, aspirin but not acetaminophen or meloxicam showed direct dopamine quinone-scavenging effects in dopamine-semiquinone generating systems. The present results suggest that cyclooxygenase shows little contribution to dopamine oxidation in dopaminergic cells and that protective effects of aspirin against methyl L-DOPA-induced dopamine quinone neurotoxicity are based on its cyclooxygenase-independent property.

Toxicity of methyldopa (Aldomet) to mouse neuroblastoma cells in vivo.

The adrenergic blocking agent methyldopa (Aldomet) is toxic to C-1300 neuroblastoma cells in vivo. Four injections of Aldomet at a dose of 7.5 mg/injection were given over a period of 24 hr to C-1300 neuroblastoma-bearing mice. This treatment killed a significant proportion of the C-1300 neuroblastoma cells. Flow cytometric data suggest that sensitivity of tumor cells to Aldomet is not related to the cell cycle.

A kinetic analysis of Drosophila melanogaster dopa decarboxylase.

The kinetic mechanism of dopa decarboxylase (3,4-dihydroxy-L-phenylalanine carboxy-lyase, EC 4.1.1.28) was investigated in Drosophila melanogaster. Based on initial velocity and product inhibition studies, an ordered reaction is proposed for dopa decarboxylase. This kinetic mechanism is interpreted in the context of measured enzyme activities and the catecholamine pools in Drosophila. The 1(2)amd gene is immediately adjacent to the gene coding for dopa decarboxylase (Ddc) and determines hypersensitivity to alpha-methyldopa in Drosophila. Dopa decarboxylase does not decarboxylate alpha-methyldopa and hence does not generate a toxic product capable of inhibiting 1(2)amd gene function. We propose that the 1(2)amd gene is involved with an unknown catecholamine pathway involving dopa but not dopamine.

Toxicity studies of amphetamine sulfate, ampicillin trihydrate, codeine, 8-methoxypsoralen, alpha-methyldopa, penicillin VK and propantheline bromide in rats and mice.

Thirteen-week toxicity studies in F344/N rats and B6C3F1 mice were conducted to determine general toxicity and target organ toxicity with amphetamine sulfate, ampicillin trihydrate, codeine, 8-methoxypsoralen, alpha-methyldopa sesquihydrate, penicillin VK, and propantheline bromide. This paper discusses the toxicity observed; use of the toxicity data to set dose levels for subsequent 2-year studies; and comparison of dose levels administered to rodents with doses used in the treatment of human disease. Drugs were administered orally in the feed or by gavage. The lowest doses in the 13-week studies were comparable to therapeutic doses in man on a mg/m2 (body surface area) basis or 5-10 times doses used in man on a mg/kg body weight basis. Little toxicity was seen at the low dose level with ampicillin, penicillin VK, 8-methoxypsoralen or propantheline bromide. At higher doses, target organ toxicity seen included the urinary bladder in male rats after propantheline bromide; the glandular stomach in rats and mice after penicillin VK; the liver and adrenals in rats after 8-methoxypsoralen; and the kidney in mice and rats after alpha-methyldopa. After amphetamine, codeine, or ampicillin administration, no target organ toxicity was seen in rats or mice, even at doses which caused body weight gain depression. The high doses chosen for subsequent 2-year studies were within 10 times human dose levels when compared on a mg/m2 body surface area.

Reproductive toxicology of methyldopa in male F344/N rats.

Methyldopa, a widely used antihypertensive drug, was administered to male Fischer 344/N rats by gavage 5 days/week for 65 days at dosages of 0, 50, 100, 200, or 400 mg/kg. Decreased body weight was seen in treated animals. After mating to untreated female Fischer 344/N rats on days 57-61, the male rats were necropsied (days 65-67) and reproductive toxicity was measured by sperm count, sperm motility, organ weights, hormone levels and histologic evaluation of the testis. Decreased fertility was seen in males dosed with 200 or 400 mg/kg methyldopa. Decreases were also seen in sperm count, sperm motility, apparent number of late spermatids, and plasma testosterone levels in males in the 200 and 400 mg/kg groups. This alternation of reproductive function in male rats was found to be reversible after a 13-week recovery period without dosing. The marked decrease in circulating testosterone levels following methyldopa treatment at 200 or 400 mg/kg may have contributed to the reproductive toxicity of this drug.

Methyldopa: effects on the murine immune system.

Mice were treated for 7 weeks with doses of methyldopa somewhat exceeding those given to man, and mixed immunotoxic effects were observed. Daily subcutaneous injections of 5 mg (in 0.1 ml) methyldopa or saline (0.1 ml) did not generally alter body weights, except on day 19, when the methyldopa-treated mice weighed significantly less. During treatment, all mice were immunized twice with sheep red blood cells (SRBC) and bled four times. Anti-SRBC titers were not affected by methyldopa treatment, but leukocyte counts were dramatically decreased, and hematocrits to a lesser degree. Although in methyldopa-treated mice spleen and kidneys were increased in size, liver, lung, heart, and thymus size was not affected. These results are discussed in the context of other studies on the mode of action of methyldopa in eliciting an autoimmune anemia in man treated therapeutically with this drug.

Neurobehavioral teratogenic effects of clomipramine and alpha-methyldopa.

Neonatal treatment of rats with centrally acting drugs such as clomipramine was shown to affect adult body and brain weight, behavior and sleep. We made a further study of the effects of clomipramine and tested one dose of alpha-methyldopa. Male rats were treated twice daily with saline, 7.5 or 15 mg/kg clomipramine or 100 mg/kg alpha-methyldopa from postnatal day 2-14 and tested in adulthood for effects on acquisition of radial maze behavior, on problem solving behavior in Hebb-Williams mazes, sexual performance and sleep-wake patterns. Clomipramine-treated rats had reduced body weight. No effects of neonatal drug treatment were found on several measures in the two mazes. Ejaculating rats in all three treatment groups showed longer latencies for sexual behavior and clomipramine-treated rats showed fewer ejaculations. Clomipramine-treated rats spent more time sleeping than the controls during the 24 hr sleep-wake recordings in adulthood.

Influence of carbidopa, an aromatic amino acid decarboxylase inhibitor, on the development of autoimmune hemolytic anemia in NZB mice.

Causal relationships of carbidopa and its related drugs on the development of spontaneous autoimmune hemolytic anemia (AIHA) in NZB mice were studied, and the following results were obtained: 1) Long term treatment with carbidopa (3 mg/kg/day) and levodopa (30 mg/kg/day) neither accelerated nor suppressed the development of spontaneous AIHA in NZB mice. 2) In mice treated with carbidopa/levodopa (3/30 mg/kg/day), microhematocrit levels were lower than those in the control mice on and after 20 weeks of age and showed a significant decrease at 66 weeks of age (P less than 0.05). The average anti-RBC antibody titers reached the maximum level 8 weeks earlier than the control group. 3) Microhematocrit levels in the alpha-methyldopa (60 mg/kg/day)-treated group were higher than those in the control group, and at 66 weeks of age, they were decreased below that in the control group. The elevation of anti-RBC antibody titers was slower than that in the control group. As the reason for the weak effectiveness of alpha-methyldopa on the incidence of AIHA, it might be considered that the dosage employed was not sufficiently high enough and/or it may be due to the species difference between man and animals. Further studies are necessary in order to draw a conclusion on the AIHA-inducing ability of carbidopa.

Immunogenicity studies of a synthetic antigen of alpha methyl dopa.

Since the idiosyncratic liver toxicity of methyl dopa (L-alpha-methyl-3,4- dihydroxy-phenylalanine) may be due to immune mediated mechanisms, immunologic tools are needed to detect methyl dopa induced antibody and antigen. Hapten (methyl dopa)--carrier (albumin) conjugates were synthesized to generate antibodies reactive with this drug. Studies were also conducted to test the immunogenicity of this hapten-carrier conjugate and its cross reactivity with methyl catechol and levodopa. Methyl dopa (MD), levodopa (LD) or methyl catechol (MC) were conjugated to rabbit serum albumin (RSA) under high pH (base) conditions or by a tyrosinase (tyr) catalyzed reaction. Under the base conjugation conditions, MD-RSA, LD-RSA and MC-RSA conjugates were produced at higher hapten: carrier ratios than conjugates produced by the enzyme catalyzed reaction. Rabbits were subsequently immunized with either MD-RSA(base) or MD-RSA(tyr). Antibodies elicited by MD-RSA(base) had marked reactivity to the carrier protein, albumin, whereas antibodies elicited by MD-RSA(tyr) did not. In addition, reactivity of anti-MD antibody was equal to or greater with MC-RSA than reactivity with either MD-RSA or LD-RSA. This work suggests that the conjugation method using the tyr catalyzed reaction produces the optimal immunogen with minimal modification of the carrier protein. In addition, the catechol moiety of MD, MC and LD appears to be the immunogenic epitope on these haptens.

In vivo mutagenic effect of methyldopa. 1. Dominant lethal test in male mice.

The dominant lethal test was used to evaluate the mutagenic potential of methyldopa, an antihypertensive drug, in male mice. In the series treated, animals were exposed to 480 or 960 mg/kg of the drug as a suspension in 1% gum acacia via the oral route. A separate group of males served as control. Following treatment each male of the control, as as well as the treated series, was mated to two untreated virgin females each week for a period of 6 consecutive weeks. All mated females were sacrificed on the 10th day of separation and their ovaries and uterine contents were examined. Corpora lutea, implantations and resorptions were recorded. The drug treatment did not impair the mating capacity and fertility. Pre- and post-implantation losses were assessed. Higher incidences of post-implantation losses and abortion, and lower incidences of total implantations particularly at higher dose levels clearly reveal a dominant lethal effect of the drug.