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Tail rotation, an early neurological sign of methylmercury poisoning in the rat
Tail Rotation, An Early Neurological Sign of Methylmercury Poisoning in the Rat
All of 95 Wistar rats on a diet containing 20 ppm of methylmercury and 0.1 ppm of namely. sustained vigorous circling movement of the selenium displayed “tail rotation,” tail when held up by the trunk, for 2 to 3 weeks prior to the onset of weight loss and of crossing and/or ataxia of the hind legs. With the onset of tail rotation morphological changes in the nervous system involved only the peripheral sensory nerves and their terminal corpuscles; mercury levels in the brain were two-thirds as high as those associated with crossing and ataxia of the hind legs. Ablation of visual and inner ear function did not seem to affect its directionality or velocity. This sign appears to be an earlier manifestation of peripheral sensory neuropathy induced by methylmercury.
INTRODUCTION
One of the tasks of experimental toxicology is the search for early clinical signs caused by the toxic substance of interest. Crossing and/or ataxia of the hind legs are well recognized neurological signs of murine methylmercury poisoning, and efforts have been made to seek earlier manifestations (Hunter et ~1.. 1940: Takeuchi et al., 1960: Miyakawa and Deshimaru, 1969; Suzuki and Miyama. 1971; Chang and Hartman, 1972; Klein et rzl., 1972: Somjen et al., 1973: Fehling et rrl., 197.5; Ohi et N/.. 197.5, 1976). Metabolic derangements in certain regions of the nervous system as well as discernible pathological changes in the peripheral sensory nerves predate the onset of these signs (Miyakawa et N/. , 1970; Cavanagh and Chen, 1971; Herman et trl., 1973). In addition, these signs become evident only when the general somatic condition of the animals has started displaying considerable deterioration (e.g., weight loss) (Chang and Hartman. 1972; Herman et rrl. I 1973; Somjen et c/l., 19731. Cavanagh and Chen (1971a) referred to this neurologically latent period (with concurrent biochemical and morphological changes) after methylmercury administrations as the “silent phase.” Another conspicuous neurological manifestation, vigorous sustained rotation of the tail. was first documented by Cavanagh and Chen (1971b). followed by Magos and Butler (1972, 19761 and was called, respectively, “circling movements of the tail” and ‘flailing reflex.” The latter authors believed the flailing reflex to be the earliest sign of ataxia. We report here our observations on the features of “tail rotation”: chronology of onset, reproducibility of the sign, directionality of rotation, associated pathology of the nervous system. and possible pathogenesis. ’ Currently at the Department of Tokyo. Hongo. Bunkyo-ku.
of Hygiene and Preventive Tokyo I13 Japan.
Medicine.
School
of Medicine,
University
354
OHI
MATERIALS
ETAL.
AND METHODS
Weanling Wistar rats were fed a diet containing 20 ppm of mercury as methylmercury chloride and water ad libiturn. They were weighed twice a week and observed daily for mortality and signs of morbidity such as roughened hair or neurological manifestations (gait disturbance on floor and a horizontal bar, righting reflex, response to touch, tail rotation, crossing of hind legs, and ability to hold and stay on a vertical rod). A morphological evaluation on both light and electron microscopic levels and chemical analysis for total mercury and methylmercury contents of the nervous system were conducted at the onset of each neurological manifestation. The preparation of the diet has been reported elsewhere (Ohi et u1., 1976) but an essential modification was the replacement of fish meal and wheat, which are rich in selenium, with casein and corn so that the dietary selenium level did not exceed 0.1 ppm. Rats anesthetized by ethyl ether were sacrificed by perfusion of saline followed by 2.5% glutaraldehyde. Blocks taken from various parts of the central and peripheral nervous system including the cutaneous receptor were postfixed in osmium tetroxide and embedded in epoxy resin. Thin sections were stained with paraphenylene diamine, and ultrathin sections doubly stained with uranyl acetate and lead citrate were examined with the JEM 100 C electron microscope. Conditions for analysis of total mercury and methylmercury contents were reported previously (Ohi rt nl., 1975); in essence the extraction of methylmercury followed the method of Westoo (1968), and the analysis for total mercury followed the procedure described by Deitz et crl. (1973). For selenium determination, the specimen was digested in nitric-perchloric acid, and selenium-diaminonaphthalene complex was measured by fluorometry (Watkinson, 1966). Statistical analysis was performed using Student’s t test. RESULTS
The rats showed signs of “tail rotation” 5 to 6 weeks after the start of the experiment, 2 to 3 weeks prior to weight loss and crossing of the hind legs (Table I). This sign is an active sustained circling movement of the tail, typically at a rate of 4 to 8 times/second. when the rat is grasped around the trunk and held up with the head facing the examiner and with the trunk held horizontally (Fig. 1). It was much less apparent if the animal was supported by something such as a strip of cloth; it was evokable until the very end, but became less TABLE EFFECTS
Series 1 2 3 (’ Days e Days
Number
Sex
18 29 48
M M F
OF ME.~HI-LMERCURY
Tail rotation” 38.9 2 4.1 37.3 -c 3.7 35.8 ? 3.8
I POISONING
Weight (mean
loss” only)
IK R~rs
Crossing and/or ataxia
56 49 48
from the start of methylmercury-treated diet (mean f SD). between the onset of tail rotation and that of crossing and/or
59.6 2 2.6 54.0 + 4.7 50.6 k 1.7
ataxia
Interval” 20.7 16.4 14.8
of the hind legs.
TAIL
FIG. 1. The rat rotates 52/second: F. 6.3; shutter
ROTATION
AND
355
METHYLMERCURY
its tail when grasped around the trunk. Camera conditions: speed, 0.25 second: film, TX; camera, Mamiya RB 67.
multistrobes.
conspicuous as the general condition of the rat deteriorated. With onset of tail rotation the rats became somewhat irritable but continued to gain weight. The direction of the rotation was such that when the trunk was slightly turned in a clockwise direction in relation to the long axis of the trunk, the tail also swirled clockwise; if it was turned counterclockwise, the tail swirled in a counterclockwise direction again with respect to the axis. Neither the direction nor the speed of rotation was affected when either or both of the inner ears had been destroyed beforehand and/or both eyes had been closed. Tail rotation was accompanied by
356
OH1
ETAL.
violent jerky movements of the hind legs and was usually sustained for several seconds but sometimes for more than 10 seconds; control rats occasionally rotated the tail in a similar manner but did not show a sustained rotation. Tail rotation was reproducible in every poisoned rat each time it was examined, while the crossing of the hind legs was reproducible only in up to four-fifths of a group at one time. Neurological performance (e.g., righting reflex, etc.) was otherwise well maintained until gross paralysis of hind legs became apparent. Morphological lesions of the nervous system of a rat just beginning to exhibit tail rotation were confined to the first sensory neuron. The fibers of the posterior roots, spinal ganglia, marginal zone of the posterior spinal column, and peripheral nerves showed various signs of degeneration described by other workers for the neurologically latent period (Fig. 2) (Miyakawa et al., 1970; Herman et al., 1973). At this stage the appearance of the cutaneous sensory receptor “Meissner-type corpuscles” was not remarkable on light microscopical viewing. However, electron microscopy disclosed abnormal organelles in the terminal axon: Numerous concentric lamellar bodies resembling myelin appeared to displace the normally existent mitochondria and neurotubules, but the cytoplasm of the lamellar cell was spared (Figs. 3a and b). Since this change is not present in either normal rats (Ide, 1976) or in Wallerian degeneration (unpublished data), it is most probable that methylmercury was responsible for the appearance. The chemical analysis of the brain revealed that both total and methylmercury concentrations at the onset of tail rotation were approximately by
FIG. 2. Marked posterior column.
disintegration PPD stain,
of the myelin x 200.
sheaths
of the posterior
root
and marginal
area of the
r.4IL
KOT.4TION
.4X-D
METHYLMERCL~RY
FIN. 3. (a) Cutaneous sensory receptor (Meissner-type electron-dense bodies and large vacuoles are found cytoplasmic plate of the lamellar cell: Epd. epidermis:
357
corpuscle) of the rat toe pad. (b) Numerous in the axon. la. lamellar cell: A. axon: cp. Ca. capsular cell. x 6400.
one-third lower than those associated with crossing and ataxia of the hind legs P < 0.005) (Table 2). A similar (total mercury, P < 0.01; methylmercury. accumulation tendency was noted in the spinal cord, but the statistical sigP < 0.025) nificance was not obvious (total mercury, P < 0.05: methylmercury, presumably because of the small sample size. DISCUSSION
Our observation on chronological relationship between the onset of “tail rotation” and weight loss, pathology. and mercurial levels in the brain essentially confirms the experiences of Magos and Butler (1972). They also noted the
358
OH1
ETAL.
TABLE TOTAL
AND
METHYLMERCURY
CONCENTRATIONS
Total
Organ
’ Each
8.2 30 58 122
2 t + +
0.3 5 15 12
is mean
12.4 59 62 142
k + + t
2.1 10 9 8
10.1 + 2.0
2 SD of three
ON WET
WEIGHT
BASIS)”
Methylmercury
Crossing and/or ataxia
7.5 -i- 1.2 value
(PPM
mercury
Tail rotation
Brain Liver Kidney Blood Spinal cord
2 IN THE ORGANS
Crossing andor ataxia
Tail rotation 8.3 30 53 123
+ -+ 2 k
0.5 3 9 15
13.0 59 48 144
7.3 2 0.7
+ 1.6 + 13 + 4 I3
10.3 -+ 1.7
rats.
“extensive” cortical lesions in the kidney when neurological manifestation was barely demonstrated; the proximal tubules are readily damaged in a chronic low-dose schedule where neurological manifestation is not expected to appear (Fowler, 1972). The mechanism of tail rotation is at present unclear, but evidence suggests its association with peripheral sensory disturbances: First, both morphological and electrophysiological changes due to methylmercury poisoning of rats, noted during the neurologically “silent phase,” were confined to the peripheral sensory nerves (Miyakawa rt al., 1970; Cavanagh and Chen, 1971a; Chang and Hartman, 1972; Herman et al., 1973; Somjen et al., 1973). Our morphological observation indicates involvement of the terminal corpuscles of the sensory nerve at this stage. Second, the obstruction of vision and destruction of the inner ears did not affect either directionality, velocity, or duration of tail rotation. Though the number of animals in these manipulations is small, this suggests that the reflexes mediated through those organs did not play a dominant role in evoking tail rotation. ACKNOWLEDGMENTS We are indebted to Professors discussions. We are also grateful for their technical assistance.
N. Imura. to Messrs.
T. Suzuki, K. Takahashi. H. Konno, K. Minowa,
K. Ueda, S. Osawa,
and 0. Wada for and K. Yunome
REFERENCES Cavanagh, J. B., and Chen. F. C. K. (1971a). Amino acid incorporation in protein during the “silent phase” before organo-mercury and p-bromophenylacetyl urea neuropathy in the rat. Acta Newopothol. 19, 216-224. Cavanagh, J. B., and Chen. F. C. K. (1971b). Effects of methylmercury-dicyandiamide on the peripheral nerves and spinal cord of rats. Ac,ta Neuropclthol. 19, 208-215. Chang. L. W., and Hartman, H. A. (1972). Ultrastructural studies of the nervous system after mercury intoxication I. Pathological changes in the nerve cell bodies. Acfn Neuropnthol. 20, 122- 138. Deitz. F. D., Sell, J. L.. and Bristol, D. (1973). Rapid sensitive method for determination of mercury in a variety of biological samples. J. Assoc. Qff. Ann/. Chem. 56, 378-382. M., Schutz, A., and Skerfving, S. (1975). MethylFehling, C., Abdulla. M., Brun. A., Dictor,
TAIL
ROTATION
AND
METHYLMERCURY
359
mercury poisoning in rat: A combined neurological, chemical and histopathological study. To.ricol. Appl. Pl~~rmrrcol. 33. 27-31. Fowler. B. A. (1972). Ultrastructural evidence for nephropathy induced by long-term exposure to small amounts of methylmercury. Science 175, 780-781. Herman. S. P.. Klein, R.. Talley, F. A., and Krigman, M. R. (1973). An ultrastructural study of methylmercury-induced primary sensory neuropathy in the rat. Lab. It~rvst. 28, 1046 118. Hunter. D.. Bomford, R. R.. and Russell. D. S. (1940). Poisoning by methylmercury compounds. Quart. J. Med. 9, 193-213. Ide. C. (1976). Fine structure of digital corpuscle of the mouse toe pad. with special reference to nerve fibers. Amer. J. Anat. 147. 329-335. Klein, R.. Herman. S. P., Brubaker, P. E.. Lucier. G. W.. and Krigman, M. R. (1972). A model of acute methylmercury intoxication in rats. Arch. P&ho/. 93, 408-418. Magos. L.. and Butler, W. H. (1972). Cumulative effects of methylmercury dicyandiamide given orally to rats. Food Cosmrt. Tosicol. 10, 51335 17. Magos, L.. and Butler, W. H. (1976). Kinetics of methylmercury administered repeatedly to rats. Arc,h. Twciwl. 35, 25-39. Miyakawa. T., and Deshimaru, M. (1969). Electron microscopical study of experimentally induced poisoning due to organic mercury compound. Acfu Nruroputhol. 14, 1266 136. Miyakawa, T.. Deshimaru. M., Sumiyoshi. S.. Teraoka, A., Udo, N., Hattori, E., and Tatetsu, S. ( 1970). Experimental organic mercury poisoning-Pathological changes in peripheral nerves. Attn. Nerrropnthol. 15, 45-55. Ohi. G.. Nishigaki, S., Seki. H.. Tamura, H.. Shimamura, Y., Maeda, H.. Ochiai, S., Yamada, H.. and Yagyu, H. (1975). Interaction of dietary methylmercury and selenium on accumulation and retention of these substances in rat organs. Toxicol. Appl. Pharmac~ol. 32, 527-533. Ohi. G.. Nishigaki. S.. Seki. H., Tamura, H., Shimamura. Y., Maeda. H., Ochiai. S., Yamada. H.. and Yagyu. H. (1976). Efficacy of selenium in tuna and selenite in modifying methylmercury poisoning. Environ. Rrs. 12. 49-58. Somjen. G. G.. Herman. S. P.. and Klein, R. (1973). Electrophysiology of methylmercury poisoning. J. PhclrmnccJl. Exp. Thor. 186. 579-592. Suzuki. T., and Miyama, T. (1971). Neurological symptoms and mercury concentration in the brain of mice fed with methylmercury salt. Ind. Heolrh 9, 51-58. Takeuchi. T.. Kambara, T.. Morioka, N.. Matsumoto. H.. Shiraishi, Y., Ito. H.. Sasaki, M., and Hirata. Y. (1960). J. Kumnmoto Med. Sot. 34, (Suppl. 3). 521-530 (in Japanese). Watkinson. J. H.. (1966). Fluorometric determination of selenium in biological material with 2,3diaminonaphthalene. Anul. Chrm. 38, 92-97. Westoo. G.. (1968). Determination of methylmercury salts in various kinds of biological material. Actcr. Chrm. Scund. 22. 2277-2280.
All of 95 Wistar rats on a diet containing 20 ppm of methylmercury and 0.1 ppm of namely. sustained vigorous circling movement of the selenium displayed “tail rotation,” tail when held up by the trunk, for 2 to 3 weeks prior to the onset of weight loss and of crossing and/or ataxia of the hind legs. With the onset of tail rotation morphological changes in the nervous system involved only the peripheral sensory nerves and their terminal corpuscles; mercury levels in the brain were two-thirds as high as those associated with crossing and ataxia of the hind legs. Ablation of visual and inner ear function did not seem to affect its directionality or velocity. This sign appears to be an earlier manifestation of peripheral sensory neuropathy induced by methylmercury.
INTRODUCTION
One of the tasks of experimental toxicology is the search for early clinical signs caused by the toxic substance of interest. Crossing and/or ataxia of the hind legs are well recognized neurological signs of murine methylmercury poisoning, and efforts have been made to seek earlier manifestations (Hunter et ~1.. 1940: Takeuchi et al., 1960: Miyakawa and Deshimaru, 1969; Suzuki and Miyama. 1971; Chang and Hartman, 1972; Klein et rzl., 1972: Somjen et al., 1973: Fehling et rrl., 197.5; Ohi et N/.. 197.5, 1976). Metabolic derangements in certain regions of the nervous system as well as discernible pathological changes in the peripheral sensory nerves predate the onset of these signs (Miyakawa et N/. , 1970; Cavanagh and Chen, 1971; Herman et trl., 1973). In addition, these signs become evident only when the general somatic condition of the animals has started displaying considerable deterioration (e.g., weight loss) (Chang and Hartman. 1972; Herman et rrl. I 1973; Somjen et c/l., 19731. Cavanagh and Chen (1971a) referred to this neurologically latent period (with concurrent biochemical and morphological changes) after methylmercury administrations as the “silent phase.” Another conspicuous neurological manifestation, vigorous sustained rotation of the tail. was first documented by Cavanagh and Chen (1971b). followed by Magos and Butler (1972, 19761 and was called, respectively, “circling movements of the tail” and ‘flailing reflex.” The latter authors believed the flailing reflex to be the earliest sign of ataxia. We report here our observations on the features of “tail rotation”: chronology of onset, reproducibility of the sign, directionality of rotation, associated pathology of the nervous system. and possible pathogenesis. ’ Currently at the Department of Tokyo. Hongo. Bunkyo-ku.
of Hygiene and Preventive Tokyo I13 Japan.
Medicine.
School
of Medicine,
University
354
OHI
MATERIALS
ETAL.
AND METHODS
Weanling Wistar rats were fed a diet containing 20 ppm of mercury as methylmercury chloride and water ad libiturn. They were weighed twice a week and observed daily for mortality and signs of morbidity such as roughened hair or neurological manifestations (gait disturbance on floor and a horizontal bar, righting reflex, response to touch, tail rotation, crossing of hind legs, and ability to hold and stay on a vertical rod). A morphological evaluation on both light and electron microscopic levels and chemical analysis for total mercury and methylmercury contents of the nervous system were conducted at the onset of each neurological manifestation. The preparation of the diet has been reported elsewhere (Ohi et u1., 1976) but an essential modification was the replacement of fish meal and wheat, which are rich in selenium, with casein and corn so that the dietary selenium level did not exceed 0.1 ppm. Rats anesthetized by ethyl ether were sacrificed by perfusion of saline followed by 2.5% glutaraldehyde. Blocks taken from various parts of the central and peripheral nervous system including the cutaneous receptor were postfixed in osmium tetroxide and embedded in epoxy resin. Thin sections were stained with paraphenylene diamine, and ultrathin sections doubly stained with uranyl acetate and lead citrate were examined with the JEM 100 C electron microscope. Conditions for analysis of total mercury and methylmercury contents were reported previously (Ohi rt nl., 1975); in essence the extraction of methylmercury followed the method of Westoo (1968), and the analysis for total mercury followed the procedure described by Deitz et crl. (1973). For selenium determination, the specimen was digested in nitric-perchloric acid, and selenium-diaminonaphthalene complex was measured by fluorometry (Watkinson, 1966). Statistical analysis was performed using Student’s t test. RESULTS
The rats showed signs of “tail rotation” 5 to 6 weeks after the start of the experiment, 2 to 3 weeks prior to weight loss and crossing of the hind legs (Table I). This sign is an active sustained circling movement of the tail, typically at a rate of 4 to 8 times/second. when the rat is grasped around the trunk and held up with the head facing the examiner and with the trunk held horizontally (Fig. 1). It was much less apparent if the animal was supported by something such as a strip of cloth; it was evokable until the very end, but became less TABLE EFFECTS
Series 1 2 3 (’ Days e Days
Number
Sex
18 29 48
M M F
OF ME.~HI-LMERCURY
Tail rotation” 38.9 2 4.1 37.3 -c 3.7 35.8 ? 3.8
I POISONING
Weight (mean
loss” only)
IK R~rs
Crossing and/or ataxia
56 49 48
from the start of methylmercury-treated diet (mean f SD). between the onset of tail rotation and that of crossing and/or
59.6 2 2.6 54.0 + 4.7 50.6 k 1.7
ataxia
Interval” 20.7 16.4 14.8
of the hind legs.
TAIL
FIG. 1. The rat rotates 52/second: F. 6.3; shutter
ROTATION
AND
355
METHYLMERCURY
its tail when grasped around the trunk. Camera conditions: speed, 0.25 second: film, TX; camera, Mamiya RB 67.
multistrobes.
conspicuous as the general condition of the rat deteriorated. With onset of tail rotation the rats became somewhat irritable but continued to gain weight. The direction of the rotation was such that when the trunk was slightly turned in a clockwise direction in relation to the long axis of the trunk, the tail also swirled clockwise; if it was turned counterclockwise, the tail swirled in a counterclockwise direction again with respect to the axis. Neither the direction nor the speed of rotation was affected when either or both of the inner ears had been destroyed beforehand and/or both eyes had been closed. Tail rotation was accompanied by
356
OH1
ETAL.
violent jerky movements of the hind legs and was usually sustained for several seconds but sometimes for more than 10 seconds; control rats occasionally rotated the tail in a similar manner but did not show a sustained rotation. Tail rotation was reproducible in every poisoned rat each time it was examined, while the crossing of the hind legs was reproducible only in up to four-fifths of a group at one time. Neurological performance (e.g., righting reflex, etc.) was otherwise well maintained until gross paralysis of hind legs became apparent. Morphological lesions of the nervous system of a rat just beginning to exhibit tail rotation were confined to the first sensory neuron. The fibers of the posterior roots, spinal ganglia, marginal zone of the posterior spinal column, and peripheral nerves showed various signs of degeneration described by other workers for the neurologically latent period (Fig. 2) (Miyakawa et al., 1970; Herman et al., 1973). At this stage the appearance of the cutaneous sensory receptor “Meissner-type corpuscles” was not remarkable on light microscopical viewing. However, electron microscopy disclosed abnormal organelles in the terminal axon: Numerous concentric lamellar bodies resembling myelin appeared to displace the normally existent mitochondria and neurotubules, but the cytoplasm of the lamellar cell was spared (Figs. 3a and b). Since this change is not present in either normal rats (Ide, 1976) or in Wallerian degeneration (unpublished data), it is most probable that methylmercury was responsible for the appearance. The chemical analysis of the brain revealed that both total and methylmercury concentrations at the onset of tail rotation were approximately by
FIG. 2. Marked posterior column.
disintegration PPD stain,
of the myelin x 200.
sheaths
of the posterior
root
and marginal
area of the
r.4IL
KOT.4TION
.4X-D
METHYLMERCL~RY
FIN. 3. (a) Cutaneous sensory receptor (Meissner-type electron-dense bodies and large vacuoles are found cytoplasmic plate of the lamellar cell: Epd. epidermis:
357
corpuscle) of the rat toe pad. (b) Numerous in the axon. la. lamellar cell: A. axon: cp. Ca. capsular cell. x 6400.
one-third lower than those associated with crossing and ataxia of the hind legs P < 0.005) (Table 2). A similar (total mercury, P < 0.01; methylmercury. accumulation tendency was noted in the spinal cord, but the statistical sigP < 0.025) nificance was not obvious (total mercury, P < 0.05: methylmercury, presumably because of the small sample size. DISCUSSION
Our observation on chronological relationship between the onset of “tail rotation” and weight loss, pathology. and mercurial levels in the brain essentially confirms the experiences of Magos and Butler (1972). They also noted the
358
OH1
ETAL.
TABLE TOTAL
AND
METHYLMERCURY
CONCENTRATIONS
Total
Organ
’ Each
8.2 30 58 122
2 t + +
0.3 5 15 12
is mean
12.4 59 62 142
k + + t
2.1 10 9 8
10.1 + 2.0
2 SD of three
ON WET
WEIGHT
BASIS)”
Methylmercury
Crossing and/or ataxia
7.5 -i- 1.2 value
(PPM
mercury
Tail rotation
Brain Liver Kidney Blood Spinal cord
2 IN THE ORGANS
Crossing andor ataxia
Tail rotation 8.3 30 53 123
+ -+ 2 k
0.5 3 9 15
13.0 59 48 144
7.3 2 0.7
+ 1.6 + 13 + 4 I3
10.3 -+ 1.7
rats.
“extensive” cortical lesions in the kidney when neurological manifestation was barely demonstrated; the proximal tubules are readily damaged in a chronic low-dose schedule where neurological manifestation is not expected to appear (Fowler, 1972). The mechanism of tail rotation is at present unclear, but evidence suggests its association with peripheral sensory disturbances: First, both morphological and electrophysiological changes due to methylmercury poisoning of rats, noted during the neurologically “silent phase,” were confined to the peripheral sensory nerves (Miyakawa rt al., 1970; Cavanagh and Chen, 1971a; Chang and Hartman, 1972; Herman et al., 1973; Somjen et al., 1973). Our morphological observation indicates involvement of the terminal corpuscles of the sensory nerve at this stage. Second, the obstruction of vision and destruction of the inner ears did not affect either directionality, velocity, or duration of tail rotation. Though the number of animals in these manipulations is small, this suggests that the reflexes mediated through those organs did not play a dominant role in evoking tail rotation. ACKNOWLEDGMENTS We are indebted to Professors discussions. We are also grateful for their technical assistance.
N. Imura. to Messrs.
T. Suzuki, K. Takahashi. H. Konno, K. Minowa,
K. Ueda, S. Osawa,
and 0. Wada for and K. Yunome
REFERENCES Cavanagh, J. B., and Chen. F. C. K. (1971a). Amino acid incorporation in protein during the “silent phase” before organo-mercury and p-bromophenylacetyl urea neuropathy in the rat. Acta Newopothol. 19, 216-224. Cavanagh, J. B., and Chen. F. C. K. (1971b). Effects of methylmercury-dicyandiamide on the peripheral nerves and spinal cord of rats. Ac,ta Neuropclthol. 19, 208-215. Chang. L. W., and Hartman, H. A. (1972). Ultrastructural studies of the nervous system after mercury intoxication I. Pathological changes in the nerve cell bodies. Acfn Neuropnthol. 20, 122- 138. Deitz. F. D., Sell, J. L.. and Bristol, D. (1973). Rapid sensitive method for determination of mercury in a variety of biological samples. J. Assoc. Qff. Ann/. Chem. 56, 378-382. M., Schutz, A., and Skerfving, S. (1975). MethylFehling, C., Abdulla. M., Brun. A., Dictor,
TAIL
ROTATION
AND
METHYLMERCURY
359
mercury poisoning in rat: A combined neurological, chemical and histopathological study. To.ricol. Appl. Pl~~rmrrcol. 33. 27-31. Fowler. B. A. (1972). Ultrastructural evidence for nephropathy induced by long-term exposure to small amounts of methylmercury. Science 175, 780-781. Herman. S. P.. Klein, R.. Talley, F. A., and Krigman, M. R. (1973). An ultrastructural study of methylmercury-induced primary sensory neuropathy in the rat. Lab. It~rvst. 28, 1046 118. Hunter. D.. Bomford, R. R.. and Russell. D. S. (1940). Poisoning by methylmercury compounds. Quart. J. Med. 9, 193-213. Ide. C. (1976). Fine structure of digital corpuscle of the mouse toe pad. with special reference to nerve fibers. Amer. J. Anat. 147. 329-335. Klein, R.. Herman. S. P., Brubaker, P. E.. Lucier. G. W.. and Krigman, M. R. (1972). A model of acute methylmercury intoxication in rats. Arch. P&ho/. 93, 408-418. Magos. L.. and Butler, W. H. (1972). Cumulative effects of methylmercury dicyandiamide given orally to rats. Food Cosmrt. Tosicol. 10, 51335 17. Magos, L.. and Butler, W. H. (1976). Kinetics of methylmercury administered repeatedly to rats. Arc,h. Twciwl. 35, 25-39. Miyakawa. T., and Deshimaru, M. (1969). Electron microscopical study of experimentally induced poisoning due to organic mercury compound. Acfu Nruroputhol. 14, 1266 136. Miyakawa, T.. Deshimaru. M., Sumiyoshi. S.. Teraoka, A., Udo, N., Hattori, E., and Tatetsu, S. ( 1970). Experimental organic mercury poisoning-Pathological changes in peripheral nerves. Attn. Nerrropnthol. 15, 45-55. Ohi. G.. Nishigaki, S., Seki. H.. Tamura, H.. Shimamura, Y., Maeda, H.. Ochiai, S., Yamada, H.. and Yagyu, H. (1975). Interaction of dietary methylmercury and selenium on accumulation and retention of these substances in rat organs. Toxicol. Appl. Pharmac~ol. 32, 527-533. Ohi. G.. Nishigaki. S.. Seki. H., Tamura, H., Shimamura. Y., Maeda. H., Ochiai. S., Yamada. H.. and Yagyu. H. (1976). Efficacy of selenium in tuna and selenite in modifying methylmercury poisoning. Environ. Rrs. 12. 49-58. Somjen. G. G.. Herman. S. P.. and Klein, R. (1973). Electrophysiology of methylmercury poisoning. J. PhclrmnccJl. Exp. Thor. 186. 579-592. Suzuki. T., and Miyama, T. (1971). Neurological symptoms and mercury concentration in the brain of mice fed with methylmercury salt. Ind. Heolrh 9, 51-58. Takeuchi. T.. Kambara, T.. Morioka, N.. Matsumoto. H.. Shiraishi, Y., Ito. H.. Sasaki, M., and Hirata. Y. (1960). J. Kumnmoto Med. Sot. 34, (Suppl. 3). 521-530 (in Japanese). Watkinson. J. H.. (1966). Fluorometric determination of selenium in biological material with 2,3diaminonaphthalene. Anul. Chrm. 38, 92-97. Westoo. G.. (1968). Determination of methylmercury salts in various kinds of biological material. Actcr. Chrm. Scund. 22. 2277-2280.