Fd Cosmel. Toxicol. Vol. IS. pp. 347-356. Pergamon Press 1977. Printed in Great Britain

Information See tion

ARTICLES OF GENERAL INTEREST

UNRAVELLING THE GLUTAMATE TANGLE

A flood of apparently conflicting reports leaves much to be established about the toxic effects of monosodium glutamate (MSG). A particular point of dispute has been a series of studies in neonatal ani- mals, the results of which could be interpreted as demonstrating that infants and children frequently consume in animal and vegetable proteins quantities of aspartate and glutamate very close to the levels claimed to be toxic to the neonates (Olney & Ho, Nature, Land. 1970, 227, 609; Bigwood, Fd Cosmet. Toxicol. 1975, 13, 300). This criticism has been coun- tered (Olney, ibid 1975, 13, 595) by the argument that there is a difference between ingesting amino acids bound to protein and ingesting free amino acids as food additives, since in the former instance absorption is inevitably more gradual and detoxication more effective.

Brain damage is probably the most frequently reported toxic effect of MSG. Studies of the appar- ently variable effect of the blood-brain barrier in pro- tecting animals against circulating glutamate have been described (Cited in F.C.T.’ 1971, 9, 717). More recent work on changes in the permeability of cere- bral tissue to trypan blue during convulsions induced in immature rats has thrown further light on this fac- tor (Nemeroff & Crisley, Bwir. Physiof. 1975, 5, 389). Trypan blue injected intra-arterially into rats younger than 10 days appeared in cerebral tissue when given simultaneously with an ip injection of 4g MSG/kg, which induced seizures, but the dye failed to penetrate when injected either before or after the MSG-induced seizures. When the experiment was repeated using rats older than 42 days, MSG injection did not induce any substantial seizure activity, and trypan blue did not penetrate into the brain. These findings suggest that MSG-induced seizures are associated with a rapidly reversible alteration in the permeability of the cerebrovascular system to plasma proteins able to bind with the dye, and are completely dependent upon the age of the animal. Characteristic lesions of the arcuate nucleus were found only in animals up to the age of 10 days and thus were not correlated with seizure activity, which continued up to 42 days of age.

Obey et al. (New Engl. J. Med. 1973, 289, 1374) found that guinea-pigs were susceptible to brain damage by 1 g MSG/kg injected SC. Guinea-pigs, like primates, have a well-developed nervous system at birth, and earlier work by the same group demon- strated brain lesions in infant monkeys (Cited in F.C.T. 1970,8, 590). In this species, large brain lesions were induced by high doses of MSG given SC and giving rise to blood-glutamate levels of 70-100 mg/100 ml, while relatively small lesions

resulted from the oral ingestion of doses pioducing blood-glutamate levels of 20mg/lOOml (Olney et al. J. Neuropath. exp. Neurol. 1972, 31, 464). Lemkey- Johnston & Reynolds (ibid 1974, 33, 74) gave doses of 4 g MSG/kg to neonatal mice by gastric intubation and found that oedema of astrocytic glia in the arcuate nucleus was detectable by electron micro- scopic examination after 15 minutes, while after 20 minutes lesions were also apparent by light micro- scopy. After 30 minutes the damaged area was filled with large dilated processes and some neurons appeared swollen, with vacuolated cytoplasm. Phago- cytosis began 3-6 hours after administration of the dose, with increased numbers of pyknotic nucl’ei and the appearance of peripheral hasophilic cells. After 24 hours the lesions appeared to be filled with phago- cytes and nuclei in advanced stages of pyknosis. Ultrastructurally, astrocytic processes were shown to have proliferated and to contain numerous phagocy- tic inclusions. The early distribution of lesions (15-60 minutes after dosage) involved the arcuate and/or preoptic region of the hypothalamus, while later the lesions spread over a much wider area. In general, the progression of the lesion suggested an inflow of the toxic agent from the cerebrospinal fluid. These workers also found lesions of the arcuate nucleus in adult mice given doses as low as 2 g MSG/kg. Although the lesions were less extensive in the adults, this indicates that susceptibility to MSG toxicity is not restricted to neonates.

It has been demonstrated that the rodent placenta, unlike that of the primate, is permeable to MSG (Reynolds et al. IX International Congress of Nutri- tion, 1972). Semprini et al. (Biomedicine 1974, 21, 398) failed to detect any lesions in the arcuate region of the hypothalamus in newborn and 15- and 30-day-old mice and rats from dams fed 1 or 2% MSG (equival- ent to 15--6g/kg brain in the mouse) from the start of pregnancy until the end of lactation. The neurons showed a normal pattern, without evidence of pyk- nosis and fragmentation. There was no discrepancy in cell numbers in the tissue between treated animals and controls. Semprini et al. (lot. cit.) suggest that earlier observations (Cited in F.C.T. 1969, 7, 682; ibid 1970,8, 225 & 590) on mice and monkeys may have been artefacts or that findings with injected MSG should not be considered relevant to the continuous ingestion of small amounts of the compound, a point that we have made in the past (Food and Cosmetics Toxicology 1970, 8, 590). Unfortunately their own results gave no indication of the blood-MSG levels of the neonates. Abraham et al. (Expl mol. Path. 1975, 23, 203) have added to the criticism by examining neonatal rhesus monkeys killed 3 or 24 hours or 8,

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348 Articles of general interest

15 or 30 days after a single intragastric or SC dose of 0.25, 1 or 4 g MSG/kg. They also examined mon- keys given oral doses of 250 mg or 1 mg MSG/kg for as long as 30 days. These animals showed no hypothalamic lesions when the tissue was examined by light or electron microscopy. The arcuate nuclei and all the components of the median eminence were not significantly different from those of controls. In both control and treated monkeys there were a few damaged or necrotic neuronal cells and oligoden- drocytes in the arcuate region of the hypothalamus.

Nevertheless, behavioural effects have been reported in rats fed 4g MSG/kg daily for the first 10 days of life and suggest minor brain damage (Berry et al. Deul Psychobiol. 1974, 7, 165). Rats given this MSG treatment were tested at 50 days of age in a swimming maze, where they were less able to learn the maze solution than littermates that had not received MSG. Test animals examined on day 10 of the course of MSG showed higher concentrations of aspartic and glutamic acids, taurine. urea and gluta- thione, but lower concentrations of tyrosine and histi- dine in brain, liver and blood, than did controls. Berry et al. (lot. cit.) suggest that these abnormalities may well have altered the structural development of the central nervous system and induced a lasting functional impairment of brain function at the time in the animals’ development when enzyme and trans- port systems were immature.

A single injection of 4 g MSG/kg given to 5-day-old chicks killed 18% within 46 hours and a further 2% within 48 hours (Snapir et al. Path. Eur. 1973, 8, 265). Hypothalamic damage, indicated by the development of a zone staining with thionine-Nissl stain after 2 hours and its persistence for the next 15 days, was located bilaterally near the midline, close to the third ventricle. During the first 6 hours, it extended dorsally to involve the subrotundus nuclei, laterally to involve the rotundus nuclei, and ventrally to the tractus opticus marginalis. There was a significant reduction in the number of neurons in the affected area, which was bordered by a zone of darkly stained condensed cells. Damage to the rotundus nuclei involved early neuronal swelling followed by a decrease in the number of nerve cells and, after 40 days, some vacuo- lation. Some birds given MSG also showed an abrupt diminution of nerve fibres and cells in the lateral fore- brain bundles.

Injections of MSG in newborn mice have been found to cause female sterility (Cited in F.C.T. 1969, 7, 682), although pregnant rats treated with MSG reproduced normally (ibid 1971, 9, 718). More recently, other reproduction studies in mice, rats and hamsters have been reported. The survival of second- generation newborn mice was increased by feeding

two successive generations on diets containing 1 or 2% MSG (1.3-6.2 g MSG/kg/day) together with a vitamin supplement at a level of 1 or 2% (Semprini et al. Nutr. Metabol. 1974, 16, 276). The average weight of the treated animals at weaning was higher than that of controls, but no detectable alteration in central nervous system histology resulted from MSG feeding. Trentini et al. (Fertil. Steril. 1974, 25, 478) treated rats with daily SC injections of MSG in doses that were increased gradually from 2.2 to 4.2 g/kg from day 2 to day 11. From the seventh injection, MSG induced brief seizures in 85% of the animals. Hair growth and eye opening were normal. In females, sexual development was premature but fer- tility was not impaired. Body weights and relative weights of pineal, pituitary, thyroid and adrenal glands were significantly reduced in treated females 105 days old, except in lactating animals. In the case of males, body weights and thyroid weights at 130 days were not significantly different from those of the controls, but pineal, pituitary, adrenal and testis weights were significantly decreased. No histological abnormalities of the endocrine organs or hypothala- mus were observed in either sex.

Neonatal hamsters were given daily SC injections of 4 or 8g MSG/kg on days 1-5, 6-10 or l-10 of life, and were killed on day 60 (Lamperti & Blaha, Biol. Reprod. 1976, 14, 362). Weights of the reproduc- tive organs and of the adrenal and pituitary glands were significantly lower in animals treated on days l-5 than in controls, and females on the higher dose had significantly fewer tubal ova. Lesions of the arcuate nucleus of the hypothalamus appeared only in hamsters given 8 g MSG/kg during days 610 or l-10. Females of these groups had ovaries with small follicles and no corpora lutea; males had atrophic seminiferous tubules, and showed little A5-3fl-steroid- dehydrogenase activity in their interstitial cells. Since treatment with chorionic gonadotrophin reversed the effect of MSG on testicular function and restored ovulation in hamsters, it seems evident that MSG affects that part of the hypothalamus that controls the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

Certainly most of the reports described above con- tain some sort of evidence of MSG toxicity. However, the critical influence of factors such as species vari- ation, developmental age, route of administration, number of doses and the timing of brain-tissue exam- inations leaves us with a mass of inconclusive evi- dence. More feeding studies are still needed, but with- out some change of direction, the question of MSG toxicity will remain a recurrent controversy.

[P. Cooper-BIBRA]

ERUCIC ACID IN RAPESEED OIL

The lipids of many cruciferous seeds are known high quantities of EA and these have been held re- to contain variable amounts of the mono-unsaturated sponsible for fatty deposits in heart muscle and as- fatty acid, cis-13-docosenoic acid, commonly referred sociated myocardial lesions in a number of experi- to as erucic acid (EA). In recent years, growing impor- mental animals (Cited in F.C.T. 1974, 12, 255). tance has been attached to the use of rapeseed as Although no research has as yet indicated similar a valuable oilseed crop, but the oil contains relatively effects on the human heart, several countries that