Upload
23980hcasdjkn
View
216
Download
0
Embed Size (px)
Citation preview
7/30/2019 Mercury Monkey Brain Delayed EffectAge-Related Increase in Auditory Impairment in Monkeys Exposed in Utero p…
http://slidepdf.com/reader/full/mercury-monkey-brain-delayed-effectage-related-increase-in-auditory-impairment 1/6
TOXICOLOGICAL SCIENCES 44, 191-196 (1998)
ARTICLE NO. T X9 82 48 7
Age-Related Increase in Auditory Impairment in Monkeys Exposedin Utero plus Postnatally to Methylmercury
Deborah C. Rice1
Toxicology Research Division, Bureau of Chemical Safety, Food Directorate, Health Protection Branch, Health Canada, Ottawa, Canada
Received January 26, 1998; accepted May 1, 1998
Age-Related Increase in Auditory Impairment in Monkeys Ex-posed in Utero plus Postnatally to Methylmercury. Rice, D. C.(1998). ToxicoL Sri. 44, 191-196.
Hearing impairment an d deafness have been reported as a resultof developmental and adult exposure to methylmercury; however,objective assessment of auditory function is generally lacking. Thisstudy extends previous research in our laboratory in which mon-keys exposed to methylmercury from birth to adulthood exhibitedhigh-frequency hearing impairment. Monkeys (Macaca fascicu-laris) were exposed throughout gestation and postnatally until 4years of age to 0, 10, 25, or 50 ftg/kg/day mercury as methylmer-curic chloride. When they were 11 and 19 years of age, pure-tonedetection thresholds for six frequencies between 0.125 and 31.5kHz were determined by means of a psychophysical (behavioral)procedure. At 19 years of age, all five methylmercury-exposedmonkeys exhibited elevated pure-tone thresholds compared withcontrols. Impairmen t was generally observed across the full rangeof frequencies. Comparisons of performance at 11 and 19 yearsrevealed relatively greater deterioration in function in treatedcompared with control monkeys. These results extend previously
reported evidence of deficits in auditory function produced bypostnatal methylmercury exposure, and describe a pa ttern of def-icit across frequencies different than that observed in the previousstudy. This study also provides evidence for development or ac-celeration of impairment of auditory function during aging as aconsequence of developmental methylmercury exposure, c IWS
Sod«7 of Toxicology.
Hearing deficits are a frequent result of methylmercury
poisoning in adults (Al-Damluji et al, 1976; Harada, 1968,
1977). Assessment of pure-tone detection thresholds, tested in
the range of speech frequencies (0.5-2 kHz), revealed hearing
impairment in half the ears tested (Ino and Mizukoshi, 1977).Severe hearing impairment, deafness, and delayed speech de-
velopment have been reported as a result of in utero exposure
to methylmercury (Marsh et al, 1980; Amin-Zaki et al, 1974,1979; Brenner and Snyder, 1980). However, the methodology
used to assess hearing in these studies was unspecified, and
results of assessments were not provided. More recently, a
1 Present address: 785 Main Rd., Islesboro, ME 04848. E-mail: ricekrwc®
midcoasLcom.
prospective study in a fish-eating population in the Seychelles
Islands reported deficits in auditory com prehension in children
with low exposure to methylmercury (Myers et al, 1995).
Since auditory function per se was not assessed, it is not known
whether this finding represents sensory impairment or some
other nervous system deficit.
In humans, developmental exposure to methylmercury pro-
duces a pattern of neuropathology different from that observedin adults (Takeu chi and Eto, 1975; Takeu chi, 1968). In adults,
deep sulci are preferentially damaged, while in utero exposure
results in damage that is much more uniform throughout the
brain. Infantile exposure results in a pattern of damage that is
intermediate between these patterns. The adult macaque mon-
key exhibits central nervous system pathology similar to that of
adult humans after exposure to methylmercury (Evans et al,
1977; Shaw et al, 1975; Garman et al, 1975). While the
auditory cortex has not been specifically examined after meth-
ylmercury exposure, temporal cortical damage has been ob-
served in both monkeys (Garman et al, 1975) and humans
(Takeuchi et al, 1962). Primary auditory cortex lies within the
fissure of the superior temporal gyrus and is thus a goodcandidate for preferential damage by methylmercury following
adult or infantile exposure (Takeuchi and Eto, 1975). In utero
exposure may be expected to produce diffuse damage to audi-
tory cortex.
There is evidence for delayed or accelerated neurotoxicity
during aging as a consequence of previous methylmercury
exposure (Rice, 1996). In an important study including more
than 90% of patients diagnosed with Minamata disease (MD)
20-30 years previously, it was reported that the ability to
independently eat, bathe, dress, wash the face, and use the
toilet decreased in an age-related manner in MD patients com-
pared with age-matched controls (Kinjo et al, 1993). In otherwords, there was an interaction between aging and previous
exposure to methylmercury. In a long-term follow-up study in
persons previously diagnosed with MD, it was reported that
auditory function had worsened (Harada, 1995). A "chronic"
MD was also identified that had a different pattern of signs than
early-onset MD and was characterized by a high incidence of
somatosensory impairment (glove and stocking hypoethesia)
and auditory impairment. Harada (1995, p. 14) differentiated
three patterns of worsening of signs of MD: "gradually pro-
191 1096-6080/98 $25.00
Copyright O 1998 by the Society of Toxicology.All rights of reproduction in any form reserved.
7/30/2019 Mercury Monkey Brain Delayed EffectAge-Related Increase in Auditory Impairment in Monkeys Exposed in Utero p…
http://slidepdf.com/reader/full/mercury-monkey-brain-delayed-effectage-related-increase-in-auditory-impairment 2/6
192 DEBORAH C. RICE
gressive type, delayed onset type, and escalator progressive
type with aging." In our laboratory, we observed overt toxicity
manifested as clumsiness at 13 years of age in a cohort of
monkeys exposed to methylmercury from birth to 7 years
(Rice, 1989a). These monkeys also developed additional signs
of methylmercury-induced toxicity as they approached 20
years of age that had not been present when they were younger
(Rice, 1996).We previously reported high-frequency hearing loss in the
cohort of monkeys dosed with methylmercury from birth to 7
years of age and tested at 14 years (Rice and Gilbert, 1992).
The current study extends this research by assessing auditory
function in monkeys exposed in utero to about 4 years of age
to the same or lower doses as those in the previous study.
Pure-tone detection thresholds were determined across most of
the hearing range of macaque monkeys using a psychophysical
(behavioral) procedure. All individuals were tested at 19 years
of age, while some individuals were also tested at 11 years,
allowing comparison of auditory function at middle age and
during aging.
METHODS
Subjects. Methylmercury-exposed monkeys (Macaca fascicularis) were
exposed in utero and continuing after birth until about the age of puberty. The
mothers of the infants were dosed three times per week with the equivalent of
10, 25, or 50 /xg/kg/day of mercury as methylmercunc chloride added to a
small amount of juice. When at least 90% of the estimated blood equilibrium
value based on a one-compartment model was reached (Rice, 1989a), females
were bred to untreated males. Infants were separated from their mothers at
birth and dosed with the same nominal dose their mothers had received,
administered 5 days a week. Dosing was discontinued when offspring were
3.5-4.5 years of age. Five infants were born in the high dose, two at the
intermediate dose, and one at the low dose. Two monkeys from the high-dose
group were alive at 11 years of age when auditory function was first assessed,as were the three monkeys at the lower doses. Infants were bom with blood
mercury concentrations about 1.8 times higher than those of their mothers,
which decreased with a half-life of 2-3 months for the monkeys in the current
experimen t to steady-state concentrations that were m aintained for the duration
of exposure. Modeling of blood mercury kinetics was carried out after termi-
nation of methylmercury exposure (Rice, 1989a), and included estimated
concentration at birth and under steady-state conditions and the half-life of
elimination following cessation of methylmercury exposure (Table 1). No
monkey ever received any known ototoxic agent including aminoglycoside
antibiotics. Overt signs of methylmercury toxicity such as nystagmus, obvious
visual impairment, tremor, clumsiness, or ataxia were not observed in any
monkey tested in this experiment.
Equipment and calibration procedure. Individual sine-wave frequencies
were generated by a Krohn-Hite (Avon, MA) oscillator (Model 4141R).
Sine-wave tones were fed through an attenuator to a rise-fall pulse shaper
(Model 584-04, Coulbourn Instruments, Lehigh Valley, PA), audio-mixer
amplifier (Coulbourn Model S82-24), and 40-dB attenuator to the earphone
(Sennheiser HD540 Reference Gold, Wedemark, Germany). Rise and fall time
was a 200-ms linear ramp. The linearity and absolute value of frequency and
amplitude were checked monthly. Headphone amplitude was calibrated using
Bruel and Kjaer (Naerum, Denmark) precision sound level meter Model 4230,
Bruel and K jaer sound measuring amplifier 26 10, Bruel and Kjaer artificial ear
Model 4153, and Bruel and Kjaer 0.5-in. condenser microphones 4134 (to
20,000 Hz) and 4136 (to 40,000 Hz). The calibration data were used to produce
a linear headphone response from 85 to 40,000 Hz.
TABLE 1
Subjects
Dose(ftg/kg/day)
50
25
10
0
Monkey No.
101102
116
118
104
117
120
Sex
FM
M
F
M
M
M
Mercury
concentration (ppm)
Birth
2.002.70
1.05
0.80
0.45
nd°
nd
Steady state
0.800.78
0.46
0.41
0.22
nd
nd
I
(days)
12.714.9
10.4
9.4
13.6
" Not detected.
Behavioral procedure. Monkeys were initially tested beginning at 11
years of age, at which time complete data were obtained from treated monkeys
102, 116, and 118 and control monkeys 117 and 120. High-dose monkey 101
failed to generate reliable data after almost a year of training. Low-dose
monkey 104 generated three reliable threshold s, at which time stereotypic
behavior made further testing unadvisable. Monkey 101 subsequently easilylearned a somatosensory task using the same psychophysical procedure as in
the auditory experiment, and 104 also performed well on the somatosensory
task with no behavioral problems. When monkeys were 19 years old, auditory
thresholds were redetermined. Individuals who had generated complete data at
11 years of age were retested in the right only, while 101 and 104 were tested
in both ears. The second assessment of all individuals was necessary for
comparisons to be age-matched. Reassessmen t also provided the opportunity to
compare possible degradation of auditory function over time in control and
methylmercury-trcated monkeys.
Schedule control and data acquisition were by means of a behavioral
notation language (Gilbert and Rice, 1979) run on a Nova 4 minicomputer
(Data General, Southboro MA ). Data were collected as interevent times so that
the session could be reconstructed from the raw data. The monkey sat in a
primate chair, restrained at the neck and the waist, inside a sound-attenuating
cubicle. Earpieces attached to the primate chair were precisely positioned overeach ear for delivery of the auditory signal. Different-size chairs were used for
individual monkeys as appropriate. The monkey initiated a trial by touching a
stainless-steel bar which completed a ground loop sensed by the computer.
This resulted in a signal light facing the monkey changing from red to green
and initiated a variable foreperiod of 2, 3, 5, or 7 s, chosen randomly within
each group of four trials. After the foreperiod, the tone to be detected was
delivered monaurally. The monkey's response indicated detection or lack of
detection of the stimulus (yes-no response paradigm). Release of the bar
within 1.5 s resulted in offset of the tone and delivery of 0.5 ml of apple juice
followed by a 3-s intertrial interval (111). Failure to release the bar within 1.5 s
resulted in offset of the tone, the signal light changing from green to white, and
initiation of a 10-s time-out (TO) period. Premature release of the bar before
tone onset resulted in a TO period and a repeat of the trial. Sessions comprised
100 (11 years) or 50 (19 years) completed threshold testing trials. Thirty-three
(11 years) or 16 (19 years) additional trials were catch trials with a 7-s
foreperiod in which a tone 12 dB above that of the previous trial was used asthe signal tone. Failure to release the bar within 5.0 s of the onset of the tone
resulted in a TO period; release resulted in reinforcement Monkeys were
tested one session per day, 5 days per week. Monkeys were given a specified
amount of water after each session, at least 50 ml/kg.
Only one ear and one frequency were tested within a session. No sound w as
delivered to the other car. The method of stimulus presentation was the
transformed up-down psychophysical method. The session began with a
high-intensity sound that was above the threshold for that individual. Starting
sound levels varied between 44 and 106 dB depending on the frequency being
tested and auditory sensitivity of the individual. The first 15 correct responses
7/30/2019 Mercury Monkey Brain Delayed EffectAge-Related Increase in Auditory Impairment in Monkeys Exposed in Utero p…
http://slidepdf.com/reader/full/mercury-monkey-brain-delayed-effectage-related-increase-in-auditory-impairment 3/6
AUDITORY IMPAIRMENT IN METHYLMERCURY-EXPOSED MONKEYS 193
CD
•oao
Xc/iLL J
a:
— HGHT19YW
—KHTIIWS
—uniitHS
117(COHTIK)l) 120(COtnKOQ
r-
4B 1 t» B Hi N S JU
FREQUENCY (kHz)
FI G . 1. Auditory threshold functions for control monkeys 117 and 120 in
which both ears were tested at 11 years of age and the right ear at 19 years.
Each point represents the mean ± SEM for the three sessions with the lowest
threshold.
each decreased the amplitude by 3.0 dB; subsequent correct responses de-
creased the amplitude by 3.0 dB with a probability of one-third. Each failure
to respond to the tone increased the amplitude for the next (noncatch) trial by
3.0 dB. This schedule results in threshold being estimated at 75% correctresponses.
A session was included for threshold determination if the monkey made a
premature release in fewer than 10% of noncatch trials and made either a
premature release or a failure to release on a total of fewer than 15% of catch
trials. Threshold was calculated by determining each crossing point for the last
60 (11 years) or 30 (19 years) noncatch trials of a session and determining the
median. A crossing point was defined as the mean decibel value between each
pair of changes of direction in am plitude; i.e., between w hen amplitude began
increasing as the result of an error and decreasing as the result of a correct
response according to the specified schedule criteria, or vice versa. A graph of
the amplitude across trials was also printed for each session. In almost all cases
the crossover points for the trials included for threshold determination were
characterized by a lack of slope, i.e., the threshold drifting neither up nor down
Whether the performance was stable was determined by visual inspection.
Sessions that did not meet these criteria were excluded. A threshold for eachfrequency for each ear was calculated as the mean of the lowest three daily
thresholds of five good (as defined above) sessions. Sessions w ere run at each
frequency until five sessions that met inclusion criteria were obtained. Fre-
quencies tested were 0.125, 1.0, 4.0, 10.0, 25.0, and 31.5 kHz. The order of
testing was such that adjacent frequencies were tested sequentially, beginning
at 4 kHz and moving up or down. This sequence was followed to maximize
practice effects. Treated and control monkeys at each of the two ages were
tested during the same period to ensure identical experimental conditions.
RESULTS
There were no differences between control and treated mon-
keys in measures of schedule control. Median reaction times
were 550-750 ms irrespective of foreperiod value.During assessment at 11 years of age, the two control
monke ys displayed psychophysical functions that are typical of
macaque monkeys, as discussed previously (Rice and Gilbert,
1992) (Fig. 1). Thresholds were elevated at the lowest and
highest frequencies, and generally flat in between. When re-
tested at 19 years of age, control monkey 117 had slightly to
moderately elevated thresholds in the right ear at all frequen-cies, ranging from about 3 to 17 dB (Fig. 1) (Table 2). In
contrast, control monkey 120 exhibited an 8.5-dB elevation in
threshold at 1 kHz w hile thresholds for most other freq uencies
were lower than at 11 years.
High-dose monkey 101 had extremely elevated thresholds in
both ears at all frequencies at 19 years of age (Fig. 2). The
shape of her curve was also abnormal, with thresholds gener-
ally decreasing slightly as a function of increasing frequency.
High-dose monkey 102's thresholds were elevated in both ears
compared with controls at all but the highest frequency at 11years of age; at 19 years of age, the three higher frequencies
were impaired compared with performance at 11 years. Mid-
dle-dose monkey 116 had an elevated threshold at 10 kHz in
the left ear at 11 years, while the threshold at 25 kHz in the
right ear was sufficiently elevated that 31.5 kHz was not tested.
At 19 years of age, high frequencies in the right ear were even
more elevated, compared with controls, than they had been at
11 years. For middle-dose monkey 118, thresholds in the right
ear appeared normal at 11 years, while the left ear had elevated
thresholds at the three lowest frequencies. When retested at 19
years of age, she exhibited elevated thresholds in the right ear
compared with the control monkeys at all but the highest
frequency. Low-dose monkey 104 exhibited elevated thresh-
olds in both ears at 19 years of age at all frequencies but the
highest, with the right ear being more impaired than the left.
The three data points collected w hen he w as 11 years of age, on
the other hand, were within the range of control values.
Comparison of the differences in auditory function between
the ages of 11 and 19 years provides some evidence for
differential impairment as a function of aging between control
and methylmercury-exposed monkeys (Table 2). Monkey 102
and, to a lesser extent, monkey 116 exhibited a selective
increase in impairment at higher frequencies, a pattern not
observed in controls. Monkey 118 had no increase in threshold
at 31.5 kHz between 11 and 19 years, while thresholds at mostother frequencies were relatively more elevated than those of
controls. Similarly, 104 had apparently normal auditory func-
tion for the frequencies tested at 11 years, while thresholds in
both ears were elevated compared with controls at 19 years,
with a differential in the right ear of 40 dB for the frequency
tested at both ages.
TABLE 2
Threshold Difference in Right Ear between 11 and 19 Years
Frequency (kHz)
Dose
(Hg/kg/day) Monkey No. 0.125 1 10 25 31.5
<dB difference (19-11 years)
50
25
10
0
102
116
118
104
117
120
25
2.0
2 6 . 7
8.8
- 1 . 0
- 3 . 0
8.5
19.0
17.0
8.5
1.0
- 6 . 0
13.8
5.5
- 5 . 5
2 9 . 0
3.5
7.5
4 0 . 0
16.5
0
3 0 . 5
11.5
2 8 . 0
8.5
- 8 . 0
1 4 . 0
- 1 . 0
2.5
- 7 . 0
7/30/2019 Mercury Monkey Brain Delayed EffectAge-Related Increase in Auditory Impairment in Monkeys Exposed in Utero p…
http://slidepdf.com/reader/full/mercury-monkey-brain-delayed-effectage-related-increase-in-auditory-impairment 4/6
194 D EBO RA H C. RICE
102(50) " 6 ( 2 S )
RIGHT 19 YRS
LEFT 19 YRS
RIGHT 11 YRS
LEFT 11 YRS
in i 4 « a JU
FREQUENCY (kHz)
FI G . 2. Auditory threshold functions for five monkeys exposed to methylmercury throughout gestation to 4 years of age. Each panel is identified with the
appropriate monkey number and dose group (/xg/kg/day) in parentheses. Individuals in which complete functions were generated in both ears at 11 years of age
were retested in the right ear only at 19 years. Monkey 104 completed two points in the left ear at 11 years and one point in the right ear at 10 kHz (displaced
along the x axis for clarity). Thresholds presented as in Fig. 1.
DISCUSSION
The threshold curves of the control monkeys at 11 years of
age are in agreement with data in macaques from other inves-
tigators (Stebbins and Rudy, 1978; Stebbins et al., 1966; Pf-
ingst et al, 1978), being elevated at high and low frequenciesand comprising a relatively flat function between 1 and 25 kHz.
At 19 years, there was a tendency for the threshold at 1 kHz to
be elevated relative to thresholds between 4 and 25 kHz.
Norm al m onkeys can hear at least an octave higher than normal
humans; below 8 kHz the detection levels of humans and
macaques are identical (Owren et al., 1988).
When tested at 19 years of age, all five methylmercury-
exposed monkeys exhibited elevated thresholds for pure tones
compared with controls, in some cases 50 dB or more. There
was a lso a tendency for the functions of the treated monkeys to
be relatively flat as a result of relatively greater impairment
across the range of middle frequencies. It is tempting to spec-
ulate that the extreme hearing impairment of high-dose mon-
key 101 was responsible for her apparent inability to learn the
behavioral task necessary for auditory testing at 11 years.
Exposure to the identical behavioral task used in the current
study to assess somatosensory function enabled her to subse-
quently transfer that experience to the auditory task, even
though the auditory stimuli provided less obvious cues during
training than they did to other individuals. In a similar situa-
tion, a monk ey with severely im paired som atosensory function
was unable to learn the psychophysical task for assessment of
vibration thresholds until a large vibratory stimulus applied to
his upper arm was used as the detection stimulus during train-
ing (Rice and Gilbert, 1995).
Comparison of changes in auditory function between 11 and
19 years also provides evidence for an increase in impairment
in methylmercury-exposed monkeys relative to controls. This
is readily apparent for middle-dose monkey 118, whose right-
ear function deteriorated from within control range to clearly
above it during the 8-year period. The data available for
low-dose monkey 104 also indicated normal pure-tone detec-
tion thresholds at 11 years, which was clearly not the case at 19
years. There was a suggestion in the three monkeys that were
tested at the higher frequencies at both ages that higher fre-
quencies were relatively more impaired at the older age com-
pared with their younger data, although the absolute frequen-
cies were not necessarily more elevated compared with
controls. The hearing deficits observed in the current studypresumably represent irreversible hearing loss, since they were
present 7-15 years after cessation of methylmercury exposure.
Since auditory function was not tested during the period of
dosing, it is not known whether this loss appeared during the
period of methylmercury exposure or after cessation of dosing
for those individuals with thresholds already elevated when
first tested. However, it is also clear that individuals at the
lower doses displayed normal thresholds at some frequencies at
7/30/2019 Mercury Monkey Brain Delayed EffectAge-Related Increase in Auditory Impairment in Monkeys Exposed in Utero p…
http://slidepdf.com/reader/full/mercury-monkey-brain-delayed-effectage-related-increase-in-auditory-impairment 5/6
AUDITORY IMPAIRMENT IN METHYLMERCURY-EXPOSED MONKEYS 195
11 years that were elevated at 19 years. The data suggest that
impairment in the high-dose monkeys, in addition to being
more severe, also had an earlier onset than in the lower-dose
individuals: high-dose monkey 101 presumably had severely
compromised auditory function at 11 years of age, and high-
dose monkey 102 also displayed greater impairment compared
with controls at 11 years than did the three monkeys at the
lower doses. The apparent delayed neurotoxicity in some in-dividuals, and relative acceleration of neurotoxic impairment
in others, is consistent with findings in Japan (Harada, 1995).
Persons with early MD as a result of relatively high exposure
exhibited a worsening of some signs years after exposure to
methylmercury ceased, while other individuals with lower ex-
posure developed sings of methylmercury poisoning years after
cessation of exposure. It also extends previous findings in our
laboratory in a cohort of methylmercury-exposed monkeys in
which overt clumsiness was first observed at 13 years of age,
6 years after cessation of methylmercury exposure (Rice,
1989b).
Average mercury levels in whole blood of unexposed per-sons (i.e., with no known occupational exposure and with little
fish consumption) are in the range 4-20 ppb in various studies
(WHO, 1990), clearly much lower than blood mercury con-
centrations of the monkeys in the current study. However,
maximum blood mercury concentrations in adults who con-
sume significant am ounts of fish were reported to be as high as
800 ppb in individuals in Japan and 650 ppb in Sweden (W HO ,
1990). The steady-state blood mercury concentrations of all
five monkeys in the current study were at or below the maxi-
mum observed in humans and therefore are representative of
individuals from fish-eating populations. Mercury concentra-
tions at birth were higher than steady-state levels for all mon-
keys, which is also observed in humans (Amin-Zaki et al,1974, 1976). Even these acute peak levels were below the
maximum in adults in the above studies for two of the mon-
keys. Brain:blood mercury ratios following chronic methyl-
mercury exposure in monkeys range from 2.5 to 3.0 (Bur-
bacher et al., 1990; Rice, 1989c). While the brain:blood ratio
has been reported to be greater than this in humans (Burbacher
et al., 1990), this may be an artifact of delayed sampling
(death) following cessation of methylme rcury ex posure and the
longer half-life of mercury in brain com pared with blood (Rice,
1989c). In any event, mercu ry levels in the target organ (brain)
in the monkeys in the current study during dosing were prob-
ably similar to those of highly exposed humans.
The site(s) of damage responsible for the deficits observed in
the current study is unknown. While hearing impairment and
even deafness have been observed as a consequence of meth-
yrmercury exposure in humans since the 1960s, pathological
assessment of human or monkey tissue has not included the
auditory system. While a few studies have been performed in
other animal models (Ramprashad and Ronald, 1977; Anniko
and Sarkady, 1978; Falk et al., 1974; Wassick and Yonovich,
1985), exposure was acute high dose, which is probably irrel-
evant to the current study or typical human exposure. The
primary auditory cortex of the macaq ue lies in the depths of the
sylvian fissure on the middle one-third of the superior temporal
plane, while much of the rest of the superior temporal gyrus is
also auditory. Removal of the primary and surrounding cortex
in macaq ues results in deficits in pure-ton e detection threshold
(Heffner and Heffner, 1990). Con sistent with the results of the
current study, deficits were found to be greatest throughout the
middle range of frequencies, with high (32 kHz) and lowfrequencies relatively spared. The pattern of hearing loss in the
current study differs somewhat from the results of a previous
study in our laboratory in which monkeys were exposed to 50
pig/kg/day mercury as methylmercury from birth to 7 years of
age (Rice and Gilbert, 1992). In that study, high-frequency
hearing was preferentially damaged when monkeys were tested
at 14 years of age. The fact that impairment was observed over
most frequencies in some individuals in the current study may
reflect the diffuse pattern of damage observed following in
utero exposure (Takeuchi and Eto, 1975; Takeuchi, 1968). In
addition, however, there was evidence of relatively greater
impairment of higher frequencies with increasing age in someindividuals, suggesting that there may be more than one neu-
rotoxic process underlying the observed deficits.
The monkeys in the current experiment also underwent
assessment of visual function at 4 - 6 years of age and somato-
sensory function at 15 years of age. High-dose monkey 102,
middle-dose monkey 116, and low-dose monkey 104 exhibited
extreme impairment in spatial visual function, with the other
two monkeys having normal spatial vision (Rice and Gilbert,
1990). All treated monkeys displayed elevated thresholds for
detection of vibration at the fingertips, with no indication of a
dose-related deficit (Rice and Gilbert, 1995; Rice, 1996). As in
the current study, there was no indication of a cognitive deficit
as assessed by the monkeys' ability to learn the task andproduce orderly, stable psychophysical data.
SUMMARY
Monkeys exposed to methylmercury in utero to 4 years of
age exhibited elevated detection thresholds for pure tones at 11
and/or 19 years of age compared with age-matched controls.
Deficits were observed at most frequencies tested, including
the range of frequencies used in human speech. While blood
mercury levels during the period of exposure were at or near
the upper boundary of persons ingesting large amounts of fish,
a recent study in children in which deficits in auditory proces s-
ing were observed suggests that subtle deficits in auditory
function may result from lower-level exposure. There was
evidence for delayed neurotoxicity in the auditory system
being manifested between the ages of 11 and 19 years in
low-dose individuals and acceleration of existing impairment
in higher-dose individuals when compared with controls.
These findings extend previous data from our laboratory in
which delayed manifestation of overt toxicity was observed in
another cohort of methylmercury-exposed monkeys, and are
consistent with findings in persons with Minamata disease.
7/30/2019 Mercury Monkey Brain Delayed EffectAge-Related Increase in Auditory Impairment in Monkeys Exposed in Utero p…
http://slidepdf.com/reader/full/mercury-monkey-brain-delayed-effectage-related-increase-in-auditory-impairment 6/6
196 DEBORAH C. RICE
ACKNOWLEDGMENTS
The author thanks Virginia Liston, Bruce Martin, Wendy Cherry, and
Michelle Warankie Sutherland for expert technical assistance and Virginia
Liston for generation of graphics.
REFERENCES
Al-Damluji, S. F., and the Clinical Committee on Mercury Poisoning (1976).Intoxication due to alkyl-mercury-treated seed—1971-1972 outbreak in
Iraq: Clinical aspects. Bull. WHO 53 (Suppl.), 6 5 - 8 1 .
Amin-Zaki, L., Elhassani, S., Majeed, M. A., Clarkson, T. W., Doherty, R. A.,
and Greenwood, M. (1974). Intra-uterine mercury poisoning in Iraq. Pedi-
atrics 54, 587-595.
Amin-Zaki, L., Elhassani, S., Majeed, M. A., Clarkson, T., Doherty, R. A.,Greenwood, M., and Giovanoh-Jakubezak, T. (1976). Perinatal methylmer-
cury poisoning in Iraq. Am. J. Dis. Child. 130, 1070-1076.
Amin-Zaki, L., Majeed, M. A., Elhassani, S. B., Clarkson, T. W., Greenwood,
M. R., and Doherty, R. A. (1979). Prenatal methylmercury poisoning. Am. J.
Dis. Child. 133, 172-177.
Anniko, M., and Sarkady, L. (1978). Cochlear pathology following exposure to
methylmercury. Ada OtolaryngoL 85, 213-224.
Brenner, R. P., and Snyder, R. D. (1980). Late EEG findings and clinical statusafter organic mercury poisoning. Arch. Neurol. 37, 282-284.
Burbacher, T. M., Rodier, P. M., and Weiss, B. (1990). Methylmercury
developmental neurotoxicity: A comparison of effects in humans and ani-
mals. Ncurotoxicol. Teratol. 12, 191.
Evans, H. L., Garman, R. H., and Weiss, B. (1977). Methylmercury exposure
and regional distribution as determinants of neurotoxicity in nonhuman
primates. ToxicoL Appl. Pharmacol. 41, 15-33.
Falk, S. Q., Klein, R., Haseman, J. K., Sanders, G. M., Talley, F. A., and Lim,
D. J. (1974). Acute methylmercury intoxication and ototoxicity in guinea
pigs. Arch. Pathol. 94, 297-305.
Garman, R., Weiss, B., and Evans, H. (1975). Alkylmercury encephalopathy in
the monkey (Saimiri sciureus and Macaca arctoides). Acta Neuropathol. 32 ,
61 -74 .
Gilbert, S. G., and Rice, D. C. (1979). NOVA SKED D: A behavioral notationlanguage utilizing the Data General Corporation real-time disk operating
system. Behav. Res. Methods Instnun. 11 , 71-73 .
Harada, Y. (1968). Infantile Minamata disease. In Minamata Disease, pp .
73—91. Study Group of Minamata Disease, Kumamoto University, Japan.
Harada, Y. (1977). Congenital Minamata disease. In Minamata Disease (T .
Tsubaki and K. Irukayama, Eds.), pp. 209-239. Elsevier, Amsterdam.
Harada, M. (1995). Minamata disease: Methylmercury poisoning in Japan
caused by environmental pollution. Crit. Rev. ToxicoL 25, 1—24.
Heffner, H. E., and Heffner, R. S. (1990). Effects of bilateral auditory cortex
lesions on absolute thresholds in Japanese macaques. J. Neurophysiol. 64 ,191-205.
Ino, H., and Mizukoshi, K. (1977). Otorhinolaryngological findings in intox-
ication by organomercury compounds. In Minamata Disease (T. Tsubaki
and K. Irukayama, Eds.), pp. 186-208. Elsevier, Amsterdam.
Kinjo, Y., Higashi, H., Nakano, A., Sakamoto, M., and Sakai, R. (1993).
Profile of subjective complaints and activities of daily living among current
patients with Minamata disease after 3 decades. Environ. Res. 63 , 241-251.
Marsh, D. O., Myers, G. J., Clarkson, T. W., Amin-Zaki, L., Tibriti, S., and
Majeed, S. A. (1980). Fetal methylmercury poisoning: Clinical and toxico-logical data on 29 cases. Ann. Neurol. 7, 348-353.
Myers, G. J., Davidson, P. W., Cox, C , Shamlaye, C. F., Tanner, M. A.,
Choisy, O., SIoane-Reeves, J., Marsh, D. O., Cemichiari, E., Choi, A.,
Berlin, M., and Clarkson, T. W. (1995). Neurodevelopmental outcomes of
Seychellois children sixty-six months after in utero exposure to methylmer-
cury from a maternal fish diet: Pilot study. Neurotoxicology 16, 639-652.
Ow ren, M. J., Hopp, S. L., Sinnott, J. M., and Peterson, M. R. (1988). Absolute
auditory thresholds in three Old World monkey species (Cercopithecus
aethiops, C. neglectus, Macaca fascicularis) and humans (Homo sapiens).
J. Comp. Psychol. 102, 99-107 .Pfingst, B . J., Laycock, J., Flam mino, F., L onsbury-Martin, B., and Martin, B.
(1978). Pure tone thresholds for the rhesus monkey. Hear. Res. 1, 43-47.
Ramprashad, F., and Ronald, K. (1977). A surface preparation study on the
effect of methylmercury on the sensory hair cell population in the cochlea of
the harp seal (Pagophilus groenlandicus Erxleben 1777). Can. J. Zool. 55 ,
223-230.
Rice, D. C. (1989a). Blood mercury concentrations following methylmercury
exposure in adult and infant monkeys. Environ. Res. 49, 115-126.
Rice, D. C. (1989b). Delayed neurotoxicity in monkeys exposed developmen-
tally to methylmercury. Neurotoxicology 10, 645—650.
Rice, D. C. (1989c). Brain and tissue levels of mercury after chronic methyl-
mercury exposure in the monkey. J. ToxicoL Environ. Health 27, 189-198.
Rice, D. C. (1996). Evidence for delayed neurotoxicity produced by methyl-
mercury. Neurotoxicology 17, 583-596.
Rice, D. C. (1997). Effects of lifetime lead exposure in monkeys on detection
of pure tones. Fundam. Appl. ToxicoL 36, 112-118.
Rice, D. C , and Gilbert, S. G. (1990). Effects of developmental exposure to
methylmercury on spatial and temporal visual function in monkeys. ToxicoL
Appl. Pharmacol. 102, 151-163.
Rice, D. C , and Gilbert, S. G. (1992). Exposure to methylmercury from birth
to adulthood impairs high-frequency hearing in monkeys. ToxicoL Appl.
Pharmacol. 115, 6-10 .
Rice, D. C , and Gilbert, S. G. (1995). Effects of developmental methylmer-
cury exposure or lifetime lead exposure on vibration sensitivity function in
monkeys. ToxicoL Appl. Pharmacol. 134, 161-169.
Shaw, C . M., Mottet, N. K., Body, R. L., and Lu schei, E. S. (1975). Variability
of neuropathologic lesions in experimental methylmercury encephalopathyin primates. Am. J. Pathol. 80, 451-469.
Stebbins, W. C , G reen, S., and Miller, F. L. (1966). Auditory sensitivity of the
monkey. Science 153, 1646-1647.
Stebbins, W. C, and Rudy, M. C. (1978). Behavioral ototoxicity. Environ.
Health Perspect. 26 , 43-51 .
Takeuchi, T. (1968). Pathology of Minamata disease. In Minamata Disease
(Organic Mercury Poisoning), pp. 141-252. Study Group of Minamata
Disease, Kumamoto University, Japan.
Takeuchi, T., and Eto, K. (1975). Minamata disease: Chronic occurrence from
pathological view points. In Studies on the Health Effects of Alkylmercury in
Japan, pp. 28-62. Environment Agency Japan.
Takeuchi, T., Morikawa, N., Matsumoto, H., and Shiraishi, Y. (1962). A
pathological study of Minamata disease in Japan. Acta NeuropathoL 2,
4 0 - 5 7 .Wassick, K. H., and Yonovich, A. (1985). Methylmercury ototoxicity in mice
determined by auditory brainsteam responses. Acta OtolaryngoL 99 , 35 -45 .
WHO (1990). Environmental Health Criteria 101: Methylmercury. Interna-
tional Program on Chemical Safety, World Health Organization, Geneva,
1990.