Information Section

ARTICLES OF GENERAL INTEREST

TRACKING GLUTAMATE LESIONS IN THE NEONATAL BRAIN

Brain damage is the most frequently reported toxic effect of monosodium glutamate (MSG) in neonatal animals (Cited in F.C.‘I 1977, 15, 347). The following papers present attempts to define the damage more fully and locate the susceptible areas of the hypo- thalamus.

Olney et al. (Brain Res. 1977, 120, 151) examined the vulnerability of the area postrema, a circumventri- cular area of the hypothalamus, to damage induced by glutamate. Tenday-old and adult HA-ICR mice were given a single SC injection of monosodium gluta- mate (MSG), the dose given being 1, 2, 3 or Qg/kg for infants and 2, 4 or 6 g/kg for adults. The animals were anaesthetized 35 hours later and brain slices were prepared. All animals given MSG showed a lesion of the area postrema essentially identical with that seen in the arcuate nucleus of mice given MSG. The characteristic findings were massive oedema and organelle degeneration of dendritic processes and neuronal somata, and rapid pyknotic changes in the nuclei of affected neurons. The reaction in adults was less severe than that in infants and involved a smaller number of neurons of the area postrema. Since this area adjoins the chemoreceptor trigger zone, stimu- lation of which gives rise to emesis, it is possible that small MSG doses may stimulate the neurons enough to induce vomiting while neuronal degeneration fol- lows larger doses. This view is supported by evidence from earlier studies showing that a sharp rise in plasma glutamate rapidly induces vomiting in dogs (Unna & Howe, Fedn Prot. Fedn Am. Sots exp. Biol. 1945, 4, 138), monkeys (Olney et al. J. Neuropath. exp. Neural. 1972, 31, 464) and man (Levey et a/. J. Lab. c/in. Med. 1949, 34, 1238).

Reynolds et al. (J. Toxicol. enuir. Hlth 1976, 2, 471) induced hypothalamic lesions in neonatal mice by oral doses of O.%kOmg MSG/kg or l-2 g aspar- tame/kg but found that lower doses did not have this effect. The hypothalamic lesions induced by MSG were much more severe than those induced by equal doses of aspartame. However, infant macaques dosed by stomach tube with up to 4 g MSG/kg or 2 g aspar- tame/kg did not develop any brain lesions detectable at either the microscopic or ultrastructural level. Thus, the susceptibility of the neonatal mouse brain to MSG is not shared by the primate brain. The pos- sibility that the resistance of the neonatal primate brain to MSG reflects the efficiency of either liver metabolism or the blood-brain barrier in these ani- mals remains an open question.

Holzwarth-McBride er al. (Anar. Rec. 1976, 186, 185) also treated newborn mice with MSG by the

SC route, giving doses of 2.5g/kg daily on days 5-10 after birth. Examination of the arcuate nucleus of the

hypothalamus of these animals showed an 80% de- crease in perikarya, and secondary effects of this lesion included endocrine deficiencies, a reduction in reproductive capacity, and obesity associated with small stature. The weights of the anterior pituitary, ovaries and testes were significantly reduced in MSG- treated animals, but adrenal weight was unaffected. No significant changes were seen in the concentration of nerve terminals or dense core vesicles in the con- tact zone of the median eminence of treated animals. This, taken in conjunction with the extensive destruc- tion of perikarya observed in the arcuate nucleus, sug- gests that afferent nerves from the arcuate nucleus form only a small portion of the total nerve popula- tion of the median eminence. The arcuate nucleus is known, however, to contain a small proportion of dopamine-producing neurons which project into the external zone of the median eminence, and in a further study by the same group (idem, ibid 1976, 186, 197) the effect of the MSG-induced lesion of the arcuate nucleus on catecholamines in that area and in the median eminence of the mouse hypothalamus was followed by means of a histofluorescence tech- nique. A green fluorescence produced by this method demonstrated the presence of the primary catechol- amines, dopamine and norepinephrine. The number of fluorescent perikarya in the arcuate nucleus was reduced by about 60% in newborns treated on six successive days with an SC injection of 2.5 g MSG/kg, and fluorescence in surviving neurons was markedly reduced. The intensity of fluorescence in the median eminence was also reduced. When the mice were pre- treated with 400mg pargyline/kg to inhibit mono- amine oxidase, fluorescence in both sites was greatly increased both in MSG-treated animals and in con- trols, although the number of fluorescing perikarya in the arcuate nucleus remained much smaller in the former group than in the latter. Thus, MSG adminis- tered to the neonatal mouse appears to destroy a large number of dopaminergic perikarya in the arcuate nucleus.

Contradicting this evidence of MSG neurotoxicity is the report by Heywood et al. (Toxicology Left. 1977, 1, 151) that when food containing 10% (w/w) MSG or drinking-water containing 5% (w/v), provid- ing very high mean daily intakes of 45.5 or 20.9 g MSG/kg, respectively, was consumed by weanling mice for 4 days (from 20 days of age) no lesions were induced in the hypothalamus. Determinations of plas- ma-glutamate concentrations immediately before the animals were killed showed that there was, neverthe- less, a two-fold increase when MSG was added to the food, and a 1.5-fold increase when it was added. to the drinking-water. It is concluded that a threshold

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84 Articles of general interest--Fd Cosrwr. Taxid. Vol. 17. No. 1

level for the neurotoxicity of MSG in the diet has mals, in spite of the demonstrated effects of MSG yet to be established in neonates and in mature ani- administered by SC injection or by gavage.

[P. Cooper-BIBRA]

CADMIUM AND THE KIDNEY

The mammalian kidney is one of the main targets of cadmium (Cd) toxicity. Nephrotoxic effects of Cd in animals and man were described in an earlier review (Grasso, Fd Cosmet. Toxicol. 1975, 13, 470). Much more of the recent work in this field comes from Japan where Cd pollution of water used in rice fields has been implicated as the cause of the painful bone disease, ‘itai-itai’ (Cited in F.C.T. 1977, 15, 478). Several of the papers cited below discuss the possible involvement of other food constituents in Cd toxicity. In the industrial field, there are many sources of Cd dust (e.6. in battery manufacture), and studies that indicate the early warning signs of Cd intoxication are especially valuable.

Human studies The effects of Cd exposure on solderers have been

reported by Welinder et al. (Br. J. ind. Med. 1977, 34, 221). In 21 solderers exposed in a typewriter fac- tory for periods of 1 month to 18 years to a mean concentration of 30nmol Cd/m3 in the form of par- ticles of less than 1 pm diameter, Cd concentrations ranged from less than 10 to 440nmol/litre in blood and from less than 05 to 27pmol/mol creatinine in urine. Individual blood Cd concentrations varied widely with time, whereas individual urinary levels showed less variation. Increasing the time of exposure to Cd significantly increased urinary excretion of Cd. Four other solderers who had received intermittent exposure for 8-20 years had blood levels of 45-150 nmol Cd/litre and urinary levels of 2-20 pmol Cd//mol creatinine, but showed no correlation between blood and urine levels. Most subjects when removed from Cd exposure showed a considerable fall in blood Cd; in 11 subjects the half-life of the element ranged from 25 to 146 days but was usually below 66 days, and blood Cd concentrations even- tually reached a plateau. Urinary concentrations of &microglobulin in these workers rose significantly with increasing urinary Cd concentration, when poss- ible age-effects had been eliminated, and might be taken as an early sign of possible renal tubular damage. Kjellstrijm et al. (Envir. Res. 1977, 13, 303) studied 240 workers in a Swedish battery factory who were exposed to mean levels of 53pg Cd/m3 mainly as oxide dust, and compared their urinary p,-micro- globulin excretion with that of a control group. The mean control excretion of /&-microglobulinuria was 84pg/Iitre and the upper 95% tolerance limit of 290&litre was chosen to give an operational defini- tion of proteinuria. The proportion of workers with &microglobulin levels above 290 pg/litre was 3.4% in the control group and increased with increasing exposure time to 19% of workers who had been employed for 6-12 years and 83% of workers who

had been employed for more than 34 years. However, possible age effects were not considered in this study. Smoking influences these figures, since the overall prevalence of excessive /I,-microglobulin excretion was almost three times higher in smokers than in non-smokers.

Living and farming in areas where Cd pollution by industry is high, as in some regions of Japan, car- ries a hazard no less than that of direct industrial exposure. Kjellstrijm et al. (ibid 1977, 13, 318) exam- ined 138 women aged 51-60 engaged in farming in such a region, where exposure to Cd came mainly from consumption of locally grown rice and river water. The mean urinary Cd level in such people was about twice as high as in controls. Cd concentration in blood and urinary excretion of /I,-microglobulin showed a correlation with the Cd concentration of food rice; /I,-microglobulin excretion also correlated with the period of residence in the polluted area and with the drinking of river water. Kojima er al. (ibid 1977, 14, 436) compared 156 farmers living in a Cd- polluted area of Japan with 93 living in a normal area. Since gastro-intestinal absorption of Cd is low (c. >loO/,) they used faecal Cd as a measure of Cd exposure and so avoided possible bias both from observers and participants. Mean daily faecal Cd con- tents were 41 pg in the controls and 146 pg in the exposed group. The mean urinary Cd excretion in the two groups was 2 and 7.5pg/litre respectively. Urinary excretion of &microglobulin in the controls was 86pg/Iitre. Defining tubular proteinuria as the condition when /3,-microglobulin excretion exceeds the mean plus two standard deviations, the prevalence of tubular proteinuria stood at 3% in the controls and 14% in the exposed subjects, and its incidence rose with greater age and longer exposure. Total pro- teinuria showed a lower ratio of incidence in the two groups than did tubular proteinuria.

Animal studies In an attempt to elucidate the relationship between

Cd-induced bone lesions and kidney lesions, Yoshiki et al. (Archs envir. Hlth 1975, 30, 559) fed young rats diets containing 10-300 ppm Cd as Cd& for 3 weeks in conjunction with an otherwise normal or a rachitic diet (low calcium (Ca) and no vitamin D). The groups fed 3OOppm Cd continued to be studied for 5-12 weeks. Osteoporotic bone changes particularly affect- ing the proximal end of the tibia appeared but no osteomalacia, the degree of osteoporosis being related to the Cd intake. These changes persisted until the end of the experiment at 12 weeks. In all groups fed a rachitic diet in addition to Cd the changes were similar to those in the groups fed normal diets with Cd, but were more severe. Metaphyseal trabecula for-