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Retrograde axonal transport of cadmium in the rat hypoglossal nerve
Neuroseience Letters, 62 (1985) 45M9
45
Elsevier Scientific Publishers Ireland Ltd. NSL 03632
R E T R O G R A D E A X O N A L T R A N S P O R T OF C A D M I U M IN T H E RAT HYPOGLOSSAL NERVE
BJORN ARVIDSON Department ~?!Neurology. University o[ Uppsala. A kademiska .~jukhuset. S- 751 85 Uppsala f Sweden)
(Received J uly 4th, 1985: Revised version received August 19th, 1985: Accepted August 21 st, 1985)
Key words:
cadmium
axonal transport - hypoglossus autoradiography toxin
rat
A small volume of radioactively labelled cadmium was injected into the tongue of rats. Two weeks later, the rats were killed and the lower brainstem with the hypoglossal nuclei was dissected out and sectioned in a cryostat. Autoradiography of freeze-dried sections showed accumulation of cadmium in both hypoglossal nuclei. When unilateral nerve section was performed prior to the injection, only the contralatcral nucleus was labelled. The results are interpreted as strong evidence tbr retrograde axonal transport of cadmium in the hypoglossal nerve.
C a d m i u m c o m p o u n d s are used i n d u s t r i a l l y for several p u r p o s e s , including elect r o p l a t i n g o f iron, steel a n d c o p p e r , as p i g m e n t s in enamels, plastics a n d paint, and for the p r o d u c t i o n o f alkaline a c c u m u l a t o r s a n d c o n t r o l r o d s in nuclear reactors [5, 13, 14]. Toxic effects o f c a d m i u m on the h u m a n n e r v o u s system have been r e p o r t e d . Thus, i n h a l a t i o n o f c a d m i u m d u s t m a y cause a n o s m i a , which, it is t h o u g h t , is due to d a m a g e to the p e r i p h e r a l o l f a c t o r y r e c e p t o r s [1, 14]. S y m p t o m s from the central nervous system have been r e p o r t e d to o c c u r in w o r k e r s e x p o s e d to c a d m i u m in an accum u l a t o r f a c t o r y [25]. A s t r o n g negative c o r r e l a t i o n has been f o u n d between intelligence q u o t i e n t test p e r f o r m a n c e a n d hair c o n c e n t r a t i o n s o f lead a n d c a d m i u m in a s a m p l e o f public school c h i l d r e n [22]. In rodents, single toxic doses o f c a d m i u m c h l o r i d e m a y cause h e m o r r h a g i c necroses in certain regions o f b o t h the central a n d the p e r i p h e r a l n e r v o u s system, d e p e n d i n g on the age o f the a n i m a l [2, 3, 15, 16, 26, 27], P e r i p h e r a l n e u r o p a t h y was observed in rats e x p o s e d to low c o n c e n t r a t i o n s o f c a d m i u m c h l o r i d e in d r i n k i n g w a t e r for 1831 m o n t h s [21]. D e t a i l e d i n f o r m a t i o n on the v a r i o u s n e u r o t o x i c effects o f c a d m i u m is given in a recent review [3]. R e t r o g r a d e a x o n a l t r a n s p o r t is a p a t h w a y by which toxins a n d certain viruses m a y reach the nerve cells. H e a v y m e t a l s m a y also be t r a n s p o r t e d r e t r o g r a d e l y in axons, as shown p r e v i o u s l y for iron D e x t r a n [19], lead [6] a n d thallium [7]. The aim o f the present investigation was to find o u t w h e t h e r c a d m i u m u n d e r g o e s r e t r o g r a d e a x o n a l t r a n s p o r t . F o r this p u r p o s e , r a d i o a c t i v e l y labelled c a d m i u m was injected into the t o n g u e o f rats a n d the h y p o g l o s s a l nuclei were investigated with re03114-3940/85 $ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
46 gard to accumulation of the metal. The experiments were performed on 7 female Wistar rats weighing 190-210 g. The animals were housed individually in plastic-steel cages and had free access to a standard laboratory chow diet (Ewos AB, S6dertfilje, Sweden) and tap water. According to the manufacturer, the food contained <0.25 mg of cadmium and < 1.25 mg of lead per kg. Cadmium-109 (spec.act. 125 #Ci//~g) was obtained as cadmium chloride (l°9CdC12) in 0.1 M HCI from the Radiochemical Centre, Amersham, U.K. The solution was neutralized with 0.1 M N a O H before use. Under ether anesthesia, 5 rats were injected slowly into the tongue with 0.15 ml of the neutralized solution containing 10 pCi of J°gCdCI2. In two of the injected rats, the main branch of the right hypoglossal nerve was exposed where it passed under the jaw and approximately 5 mm of the nerve was resected one day before the injection. In two control rats, an equal amount of i°gCdCl2 was injected into the right gluteus muscle in order to study the possibility of a hematogenous spread of isotope to the brainstem. All rats were killed by an i.v. injection of sodium pentobarbital 150 mg/kg two weeks after the cadmium injection. The lower brainstem with the hypoglossal nuclei was dissected out and immediately frozen between two gold plates in liquid nitrogen. The brainstem was then cut into 20-#m serial sections in a cryostat. The sections were picked up on tape, and autoradiography was performed with the technique described by Ullberg [23, 24]. The sections were freeze-dried at - 2 0 ° C for 48 h, apposed to X-ray film (Agfa Structurix D7) and exposed for 16-18 weeks. After exposure, the film and the sections were separated and the film was developed in Kodak D19 developer for 5 min at 20°C, fixed and rinsed. The sections, attached to the tape, were stained with hematoxylin-eosin, dehydrated in a series of ethanol and mounted on glass slides in Euparal (GBI Laboratory Ltd., Manchester, U.K.). In the rats in which no nerve section was performed prior to injection, the autoradiograms of sections at the level of the hypoglossal nuclei showed strong labelling of the areas corresponding to both hypoglossal nuclei (Fig. 1). The area postrema and meninges were also labelled (Fig. 1) but not other regions of the nervous parenchyma. Sections from levels above and below the hypoglossal nuclei did not show any labelling of the nervous parenchyma, except for the choroid plexus of the fourth ventricle. In the rats in which unilateral nerve section was performed prior to the injection, only the contralateral hypoglossal nucleus was labelled. In all other respects, the findings were the same as for rats with both nerves intact. Autoradiograms of sections from the brainstem of control rats injected in the gluteus muscle did not show any labelling of the hypoglossal nuclei, but the area postrema, choroid plexus and meninges were labelled. This study has shown that cadmium injected into the tongue of rats is transported to the hypoglossal nuclei. The fact that unilateral section o f the hypoglossal nerve prevented accumulation of cadmium in the ipsilateral nucleus strongly suggests that
47
N,
"
Fig. I. Autoradiogram of a coronal section through the brainstem at the level of the hypoglossal nuclei. The level of the section according to a brain atlas [20] is approximately 4.8 mm behind the vertical plane passing through the interaural line. No nerve section was performed prior to the injection. The outer contours of the section and of the hypoglossal nuclei are outlined by dashed lines. Dark areas correspond to radioactivity. The area corresponding to both hypoglossal nuclei is labelled. The area postrema (ap) is also labelled.
the m e c h a n i s m is r e t r o g r a d e a x o n a l t r a n s p o r t . The c o n t r o l e x p e r i m e n t s p r o v i d e d no evidence for a h e m a t o g e n o u s s p r e a d o f c a d m i u m to the h y p o g l o s s a l nuclei. A u t o r a d i o g r a p h i c studies on the d i s t r i b u t i o n o f ~°gCd within the nervous system o f rats [3, 4] a n d mice [8] have n o t shown a n y u p t a k e o f c a d m i u m into the n e r v o u s p a r e n c h y m a , except in small regions with p e r m e a b l e vessels such as the c h o r o i d plexus, h y p o p h y s i s a n d pineal gland. R e t r o g r a d e a x o n a l t r a n s p o r t is, however, one way by which c a d m i u m and o t h e r n o x i o u s agents can b y p a s s the b l o o d - b r a i n b a r r i e r a n d cause d a m a g e to the n e u r o n s o f the b r a i n s t e m a n d spinal cord. Lead, a n o t h e r toxic h e a v y metal, is t r a n s p o r t e d r e t r o g r a d e l y in a x o n s o f the rat sciatic nerve [6] a n d in h u m a n s this m e c h a n i s m has been p o s t u l a t e d as a possible cause o f m o t o r n e u r o n deg e n e r a t i o n in a m y o t r o p h i c lateral sclerosis ( A L S ) [1 l]. Studies are in p r o g r e s s in o u r l a b o r a t o r y to find o u t w h e t h e r r e t r o g r a d e t r a n s p o r t o f c a d m i u m in the rat h y p o g l o s sal nerve causes d a m a g e to the c o r r e s p o n d i n g m o t o r neurons. The a c c u m u l a t i o n o f c a d m i u m in the h y p o g l o s s a l n e u r o n s implies t h a t the metal is taken up into nerve t e r m i n a l s at the n e u r o m u s c u l a r j u n c t i o n s o f the tongue. The
4~ detailed m e c h a n i s m of this uptake is n o t k n o w n at present, however. Theoretically, the metal might be taken up as ionic c a d m i u m or as a m e t a l - p r o t e i n complex, At 1 a n d 5 h after s.c, injection in rats, c a d m i u m was b o u n d to p l a s m a proteins [10, 17], but it might be anticipated that immediately after injection some c a d m i u m will be still present in ionic form. In vitro experiments o n the frog n e u r o m u s c u l a r j u n c t i o n have s h o w n effects that are t h o u g h t to be m e d i a t e d by e n t r y of c a d m i u m ions into the nerve terminals [l 2]. Deposits of lead, a divalent metal cation with m a n y properties similar to those of c a d m i u m , have been observed in the synaptic clefts a n d terminal axons in the a n t e r i o r tibial muscle of mice after i.m. injection o f a 41',;, a q u e o u s solution of lead citrate [18]. C e r t a i n substances which u n d e r g o retrograde axonal t r a n s p o r t leak from the b l o o d vessels after i.v. injection, a n d are t a k e n up into a x o n terminals at the neurom u s c u l a r j u n c t i o n s in different regions o f the body, to a c c u m u l a t e in the m o t o r neurons of the b r a i n s t e m a n d spinal cord. This has been c o n v i n c i n g l y d e m o n s t r a t e d for horseradish peroxidase [9]. The biological half-time of c a d m i u m is extremely long [15, 16] a n d even if only very small a m o u n t s o f c a d m i u m in the b l o o d enter the axon terminals for retrograde axonal t r a n s p o r t d u r i n g a limited period o f time, the neurons m a y a c c u m u l a t e considerable a m o u n t s o f the metal if the time period is extended over several years. The possible toxic effects o f such hypothetical slow intran e u r o n a l a c c u m u l a t i o n o f c a d m i u m have n o t yet been investigated. This study was s u p p o r t e d by grants from the Swedish Society of Medical Sciences a n d the Swedish Medical Research C o u n c i l (project No. B86-12X-07472-01A). I Adams, R.G. and Crabtree, N., Anosmia in alkaline battery workers, J. Industr. Med., 18 (1961) 216221. 2 Arvidson, B., Regional differences in severity of cadmium-induced lesions in the peripheral nervous system in mice, Acta Neuropathol., 49 (1980) 213 224. 3 Arvidson, B., Cadmium toxicity and neural cell damage. In I.E. Dreosti and R.M. Smith (Eds.), Neurobiology of the Trace Elements, Humana Press, Clifton, NJ, 1983, pp. 51-78. 4 Arvidson, B. and Tj~ilve,H., Distribution of ~°gCdin the nervous system of rats after intravenous injeclion, Acta Neuropathol. in press. 5 Aylett, B.J., The chemistry and bioinorganic chemistry of cadmium. In M. Webb (Ed.L The Chemistry, Biochemistry and Biology of Cadmium, Elsevier,Amsterdam, 1979, pp. 1~[4. 6 Baruah, K.B., Rasool, C.G., Bradley, W.G. and Munsat, T.L., Retrograde axonal transport of lead in rat sciatic nerve, Neurology, 31 (1981) 612-616. 7 Bergquist, J.-E., Edstr6m, A. and Hansson, P.A., Bidirectional axonal transport of thallium in frog sciatic nerve, Acta Physiol. Scand., 117 (1983) 513-518. 8 Berlin, M. and Ullberg, S., The fate of ~°~Cdin the mouse, Am. Med. Assoc. Environ. Health, 7 (1963) 686-691. 9 Broadwell, R.D. and Brightman, M.W., Entry of peroxidase into neurons of the central and peripheral nervous systems from extracerebral and cerebral blood, J. Comp. Neurol., 166 (1976) 257-284. 10 Chen, R.W., Whanger, P.D. and Weshig, P.H., Selenium-induced redistribution of cadmium binding to tissue proteins: a possible mechanism of protection against cadmium toxicity, Bioinorg. Chem., 4 (1975) 125- 133. 11 Conradi, S., Ronnevi, L.-O. and Vesterberg, O., Increased plasma levelsof lead in patients with amyotrophic lateral sclerosis compared with control subjects as determined by flamelessatomic absorption spectrophotometry, J. Neurol. Neurosurg. Psychiatr., 41 (1978) 389-393.
49 12 Cooper, G.P. and Manalis, R.S., Cadmium: effects on transmitter release at the frog neuromuscular junction, Eur. J. Pharmacol., 99 (1984) 251-256. 13 Friberg, L., Kjellstr6m, T., Nordberg, G.F. and Piscator, M., Cadmium. In L. Friberg, G.F. Nordberg and V.B. Vouk (Eds.), Handbook of the Toxicology of Metals, Elsevier, Amsterdam, 1979, pp. 355 382. 14 Friberg, L., Piscator, M., Nordberg, G.F. and Kjellstr6m, T., Cadmium in the Environment, CRC Press, Cleveland, 1974. 15 Gabbiani, G., Baic, O. and Deziel, C., Toxicity of cadmium for the central nervous system. Exp. Neurol., 18 (1967) 154-160. 16 Gabbiani, G., Gregory, A. and Baic, D., Cadmium-induced selective lesions of sensory ganglia, J Neuropathol. Exp. Neurol., 26 (1967) 498 506. 17 Gasiewicz, T.A. and Smith, J.C., Interactions of cadmium and selenium in rat plasma in vivo and in vitro, Biochim. Biophys. Acta, 428 (1976) 113-122. 18 Nakamura, T., On the affinity of the divalent metal cation for the neuromuscular junction, J. Electron Microsc., 28 (1979) 231 232. 19 Olsson, T. and Kristensson, K., A simple histochemical method for double labelling of neurons by retrograde axonal transport, Neurosci. Lett., 8 (1978) 265-268. 20 Paxinos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, Academic Press, New York, 1982. 21 Sato, K., Iwamasa, T., Tsuru, T. and Takeuchi, T., An ultrastructural study of chronic cadmium chloride induced neuropathy, Acta Neuropathol., 41 (1978) 185-190. 22 Thatcher, R.W., McAlaster, R. and Lester, M., Evoked potentials related to hair cadmium and lead in children, Ann. N.Y. Acad. Sci., 425 (1984) 384-390. 23 Ullberg, S., Studies on the distribution and fate of ~sS-labelled benzyl penicillin in the body, Acta Radiol., Suppl. 118 (1954) 1-110. 24 UIIberg. S., The technique of whole body autoradiography. Cryosectioning of large specimens, Science Tools, Special Issue, 2 (1977) 2 29. 25 Vorobjeva. R.S., Investigations of the nervous system function in workers exposed to cadmium oxide, Neuropath. Psikhiat., 57 (1957) 385 393. 26 Webster, W.A. and Valois, A.A., The toxic effecs of cadmium on the neonatal mouse CNS, J. Neuropathol. Exp. Neurol., 40 (1981) 247 257. 27 Wong, K.-L. and Klaassen, C.D., Neurotoxic effects of cadmium in young rats, Toxicol. Appl. Pharmacol.. 63 t 1982) 33(~ 337.
45
Elsevier Scientific Publishers Ireland Ltd. NSL 03632
R E T R O G R A D E A X O N A L T R A N S P O R T OF C A D M I U M IN T H E RAT HYPOGLOSSAL NERVE
BJORN ARVIDSON Department ~?!Neurology. University o[ Uppsala. A kademiska .~jukhuset. S- 751 85 Uppsala f Sweden)
(Received J uly 4th, 1985: Revised version received August 19th, 1985: Accepted August 21 st, 1985)
Key words:
cadmium
axonal transport - hypoglossus autoradiography toxin
rat
A small volume of radioactively labelled cadmium was injected into the tongue of rats. Two weeks later, the rats were killed and the lower brainstem with the hypoglossal nuclei was dissected out and sectioned in a cryostat. Autoradiography of freeze-dried sections showed accumulation of cadmium in both hypoglossal nuclei. When unilateral nerve section was performed prior to the injection, only the contralatcral nucleus was labelled. The results are interpreted as strong evidence tbr retrograde axonal transport of cadmium in the hypoglossal nerve.
C a d m i u m c o m p o u n d s are used i n d u s t r i a l l y for several p u r p o s e s , including elect r o p l a t i n g o f iron, steel a n d c o p p e r , as p i g m e n t s in enamels, plastics a n d paint, and for the p r o d u c t i o n o f alkaline a c c u m u l a t o r s a n d c o n t r o l r o d s in nuclear reactors [5, 13, 14]. Toxic effects o f c a d m i u m on the h u m a n n e r v o u s system have been r e p o r t e d . Thus, i n h a l a t i o n o f c a d m i u m d u s t m a y cause a n o s m i a , which, it is t h o u g h t , is due to d a m a g e to the p e r i p h e r a l o l f a c t o r y r e c e p t o r s [1, 14]. S y m p t o m s from the central nervous system have been r e p o r t e d to o c c u r in w o r k e r s e x p o s e d to c a d m i u m in an accum u l a t o r f a c t o r y [25]. A s t r o n g negative c o r r e l a t i o n has been f o u n d between intelligence q u o t i e n t test p e r f o r m a n c e a n d hair c o n c e n t r a t i o n s o f lead a n d c a d m i u m in a s a m p l e o f public school c h i l d r e n [22]. In rodents, single toxic doses o f c a d m i u m c h l o r i d e m a y cause h e m o r r h a g i c necroses in certain regions o f b o t h the central a n d the p e r i p h e r a l n e r v o u s system, d e p e n d i n g on the age o f the a n i m a l [2, 3, 15, 16, 26, 27], P e r i p h e r a l n e u r o p a t h y was observed in rats e x p o s e d to low c o n c e n t r a t i o n s o f c a d m i u m c h l o r i d e in d r i n k i n g w a t e r for 1831 m o n t h s [21]. D e t a i l e d i n f o r m a t i o n on the v a r i o u s n e u r o t o x i c effects o f c a d m i u m is given in a recent review [3]. R e t r o g r a d e a x o n a l t r a n s p o r t is a p a t h w a y by which toxins a n d certain viruses m a y reach the nerve cells. H e a v y m e t a l s m a y also be t r a n s p o r t e d r e t r o g r a d e l y in axons, as shown p r e v i o u s l y for iron D e x t r a n [19], lead [6] a n d thallium [7]. The aim o f the present investigation was to find o u t w h e t h e r c a d m i u m u n d e r g o e s r e t r o g r a d e a x o n a l t r a n s p o r t . F o r this p u r p o s e , r a d i o a c t i v e l y labelled c a d m i u m was injected into the t o n g u e o f rats a n d the h y p o g l o s s a l nuclei were investigated with re03114-3940/85 $ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.
46 gard to accumulation of the metal. The experiments were performed on 7 female Wistar rats weighing 190-210 g. The animals were housed individually in plastic-steel cages and had free access to a standard laboratory chow diet (Ewos AB, S6dertfilje, Sweden) and tap water. According to the manufacturer, the food contained <0.25 mg of cadmium and < 1.25 mg of lead per kg. Cadmium-109 (spec.act. 125 #Ci//~g) was obtained as cadmium chloride (l°9CdC12) in 0.1 M HCI from the Radiochemical Centre, Amersham, U.K. The solution was neutralized with 0.1 M N a O H before use. Under ether anesthesia, 5 rats were injected slowly into the tongue with 0.15 ml of the neutralized solution containing 10 pCi of J°gCdCI2. In two of the injected rats, the main branch of the right hypoglossal nerve was exposed where it passed under the jaw and approximately 5 mm of the nerve was resected one day before the injection. In two control rats, an equal amount of i°gCdCl2 was injected into the right gluteus muscle in order to study the possibility of a hematogenous spread of isotope to the brainstem. All rats were killed by an i.v. injection of sodium pentobarbital 150 mg/kg two weeks after the cadmium injection. The lower brainstem with the hypoglossal nuclei was dissected out and immediately frozen between two gold plates in liquid nitrogen. The brainstem was then cut into 20-#m serial sections in a cryostat. The sections were picked up on tape, and autoradiography was performed with the technique described by Ullberg [23, 24]. The sections were freeze-dried at - 2 0 ° C for 48 h, apposed to X-ray film (Agfa Structurix D7) and exposed for 16-18 weeks. After exposure, the film and the sections were separated and the film was developed in Kodak D19 developer for 5 min at 20°C, fixed and rinsed. The sections, attached to the tape, were stained with hematoxylin-eosin, dehydrated in a series of ethanol and mounted on glass slides in Euparal (GBI Laboratory Ltd., Manchester, U.K.). In the rats in which no nerve section was performed prior to injection, the autoradiograms of sections at the level of the hypoglossal nuclei showed strong labelling of the areas corresponding to both hypoglossal nuclei (Fig. 1). The area postrema and meninges were also labelled (Fig. 1) but not other regions of the nervous parenchyma. Sections from levels above and below the hypoglossal nuclei did not show any labelling of the nervous parenchyma, except for the choroid plexus of the fourth ventricle. In the rats in which unilateral nerve section was performed prior to the injection, only the contralateral hypoglossal nucleus was labelled. In all other respects, the findings were the same as for rats with both nerves intact. Autoradiograms of sections from the brainstem of control rats injected in the gluteus muscle did not show any labelling of the hypoglossal nuclei, but the area postrema, choroid plexus and meninges were labelled. This study has shown that cadmium injected into the tongue of rats is transported to the hypoglossal nuclei. The fact that unilateral section o f the hypoglossal nerve prevented accumulation of cadmium in the ipsilateral nucleus strongly suggests that
47
N,
"
Fig. I. Autoradiogram of a coronal section through the brainstem at the level of the hypoglossal nuclei. The level of the section according to a brain atlas [20] is approximately 4.8 mm behind the vertical plane passing through the interaural line. No nerve section was performed prior to the injection. The outer contours of the section and of the hypoglossal nuclei are outlined by dashed lines. Dark areas correspond to radioactivity. The area corresponding to both hypoglossal nuclei is labelled. The area postrema (ap) is also labelled.
the m e c h a n i s m is r e t r o g r a d e a x o n a l t r a n s p o r t . The c o n t r o l e x p e r i m e n t s p r o v i d e d no evidence for a h e m a t o g e n o u s s p r e a d o f c a d m i u m to the h y p o g l o s s a l nuclei. A u t o r a d i o g r a p h i c studies on the d i s t r i b u t i o n o f ~°gCd within the nervous system o f rats [3, 4] a n d mice [8] have n o t shown a n y u p t a k e o f c a d m i u m into the n e r v o u s p a r e n c h y m a , except in small regions with p e r m e a b l e vessels such as the c h o r o i d plexus, h y p o p h y s i s a n d pineal gland. R e t r o g r a d e a x o n a l t r a n s p o r t is, however, one way by which c a d m i u m and o t h e r n o x i o u s agents can b y p a s s the b l o o d - b r a i n b a r r i e r a n d cause d a m a g e to the n e u r o n s o f the b r a i n s t e m a n d spinal cord. Lead, a n o t h e r toxic h e a v y metal, is t r a n s p o r t e d r e t r o g r a d e l y in a x o n s o f the rat sciatic nerve [6] a n d in h u m a n s this m e c h a n i s m has been p o s t u l a t e d as a possible cause o f m o t o r n e u r o n deg e n e r a t i o n in a m y o t r o p h i c lateral sclerosis ( A L S ) [1 l]. Studies are in p r o g r e s s in o u r l a b o r a t o r y to find o u t w h e t h e r r e t r o g r a d e t r a n s p o r t o f c a d m i u m in the rat h y p o g l o s sal nerve causes d a m a g e to the c o r r e s p o n d i n g m o t o r neurons. The a c c u m u l a t i o n o f c a d m i u m in the h y p o g l o s s a l n e u r o n s implies t h a t the metal is taken up into nerve t e r m i n a l s at the n e u r o m u s c u l a r j u n c t i o n s o f the tongue. The
4~ detailed m e c h a n i s m of this uptake is n o t k n o w n at present, however. Theoretically, the metal might be taken up as ionic c a d m i u m or as a m e t a l - p r o t e i n complex, At 1 a n d 5 h after s.c, injection in rats, c a d m i u m was b o u n d to p l a s m a proteins [10, 17], but it might be anticipated that immediately after injection some c a d m i u m will be still present in ionic form. In vitro experiments o n the frog n e u r o m u s c u l a r j u n c t i o n have s h o w n effects that are t h o u g h t to be m e d i a t e d by e n t r y of c a d m i u m ions into the nerve terminals [l 2]. Deposits of lead, a divalent metal cation with m a n y properties similar to those of c a d m i u m , have been observed in the synaptic clefts a n d terminal axons in the a n t e r i o r tibial muscle of mice after i.m. injection o f a 41',;, a q u e o u s solution of lead citrate [18]. C e r t a i n substances which u n d e r g o retrograde axonal t r a n s p o r t leak from the b l o o d vessels after i.v. injection, a n d are t a k e n up into a x o n terminals at the neurom u s c u l a r j u n c t i o n s in different regions o f the body, to a c c u m u l a t e in the m o t o r neurons of the b r a i n s t e m a n d spinal cord. This has been c o n v i n c i n g l y d e m o n s t r a t e d for horseradish peroxidase [9]. The biological half-time of c a d m i u m is extremely long [15, 16] a n d even if only very small a m o u n t s o f c a d m i u m in the b l o o d enter the axon terminals for retrograde axonal t r a n s p o r t d u r i n g a limited period o f time, the neurons m a y a c c u m u l a t e considerable a m o u n t s o f the metal if the time period is extended over several years. The possible toxic effects o f such hypothetical slow intran e u r o n a l a c c u m u l a t i o n o f c a d m i u m have n o t yet been investigated. This study was s u p p o r t e d by grants from the Swedish Society of Medical Sciences a n d the Swedish Medical Research C o u n c i l (project No. B86-12X-07472-01A). I Adams, R.G. and Crabtree, N., Anosmia in alkaline battery workers, J. Industr. Med., 18 (1961) 216221. 2 Arvidson, B., Regional differences in severity of cadmium-induced lesions in the peripheral nervous system in mice, Acta Neuropathol., 49 (1980) 213 224. 3 Arvidson, B., Cadmium toxicity and neural cell damage. In I.E. Dreosti and R.M. Smith (Eds.), Neurobiology of the Trace Elements, Humana Press, Clifton, NJ, 1983, pp. 51-78. 4 Arvidson, B. and Tj~ilve,H., Distribution of ~°gCdin the nervous system of rats after intravenous injeclion, Acta Neuropathol. in press. 5 Aylett, B.J., The chemistry and bioinorganic chemistry of cadmium. In M. Webb (Ed.L The Chemistry, Biochemistry and Biology of Cadmium, Elsevier,Amsterdam, 1979, pp. 1~[4. 6 Baruah, K.B., Rasool, C.G., Bradley, W.G. and Munsat, T.L., Retrograde axonal transport of lead in rat sciatic nerve, Neurology, 31 (1981) 612-616. 7 Bergquist, J.-E., Edstr6m, A. and Hansson, P.A., Bidirectional axonal transport of thallium in frog sciatic nerve, Acta Physiol. Scand., 117 (1983) 513-518. 8 Berlin, M. and Ullberg, S., The fate of ~°~Cdin the mouse, Am. Med. Assoc. Environ. Health, 7 (1963) 686-691. 9 Broadwell, R.D. and Brightman, M.W., Entry of peroxidase into neurons of the central and peripheral nervous systems from extracerebral and cerebral blood, J. Comp. Neurol., 166 (1976) 257-284. 10 Chen, R.W., Whanger, P.D. and Weshig, P.H., Selenium-induced redistribution of cadmium binding to tissue proteins: a possible mechanism of protection against cadmium toxicity, Bioinorg. Chem., 4 (1975) 125- 133. 11 Conradi, S., Ronnevi, L.-O. and Vesterberg, O., Increased plasma levelsof lead in patients with amyotrophic lateral sclerosis compared with control subjects as determined by flamelessatomic absorption spectrophotometry, J. Neurol. Neurosurg. Psychiatr., 41 (1978) 389-393.
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