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OCR for page 159
Methods and Issues in Evaluating the
Neuroloxic Effects of Organic SoTvents
Beverly M. Kulig
The term "organic solvents" refers to a chemically heterogeneous
class of compounds and mixtures which are typically liquid between
0 and 250° C and are used industrially to extract, dissolve, or suspend
materials not soluble in water. Solvents are derived from many chemical
classes and are present in products such as paints, glues, adhesives,
coatings, and degreasing agents. Solvents are also used to manufac-
ture a wide variety of products including polymers, dyes, plastics,
textiles, printing inks, agricultural products, and pharmaceuticals (World
Health Organization, 1985~.
Because of the widespread application of these compounds in in-
dustrial products and processes, the volume of organic solvents pro-
duced and the number of workers exposed are considerable. In the
United States, for example, some 49 million tons is produced on an
annual basis, and the National Institute for Occupational Safety and
Health (NIOSH) estimates that approximately 9.8 million American
workers are potentially exposed (NIOSH, 1987~. Understandably,
much of the concern regarding the effects of organic solvents focuses
on the occupational setting where exposures can be relatively high.
As a result, recommended occupational exposure limits for many of
these compounds have been proposed (American Conference of Gov-
ernmental Industrial Hygienists, 1986; NIOSH, 1987~. Despite the
emphasis on the occupational setting, however, solvent exposure is
not limited to the workplace. Organic solvents are ubiquitous in the
159
OCR for page 160
160
BEVERLY M. KULIG
environment and are detectable in air, drinking water, and foodstuffs,
including human milk (Packard, 1985~.
Considerable controversy exists regarding the extent to which or-
ganic solvents are toxic to the nervous system and at what exposure
levels (Cranmer and Goldberg, 1986; Grasso et al., 1984~. For several
compounds (e.g., n-hexane, methyl n-butyl ketone, and carbon disul-
fide), a sufficiently large data base has been established whereby the
consequences of long-term overexposure can be evaluated (Spencer,
1985~. For the vast majority of organic solvents, however, very few
data regarding nervous system effects are available.
Until recently, the majority of animal studies examining the neuro-
toxicity of organic solvents concentrated either on the involvement of
the nervous system in the acute lethality of these compounds or on
the morphological changes that may result from chronic overexposure.
Of growing concern, however, is the possibility that long-term solvent
exposure is accompanied by subclinical changes which reflect a reduction
in the adaptive capabilities of the nervous system. Occupational studies,
for example, have suggested that long-term exposure may be associ-
ated with a number of neurobehavioral changes including psychomotor
slowing, attention and memory impairments, and changes in affective
behavior (Baker and Fine, 1986~. Although occupational studies are
an important source of information regarding the possible long-term
effects of overexposure, results from such studies are often difficult
to interpret. By applying animal neurobehavioral methods to the
study of solvents, the possibility of addressing many of the questions
regarding solvent neurotoxicity is becoming increasingly feasible. The
present chapter considers some of the interpretative issues that have
developed regarding the risks to human health associated with solvent
exposure and examines the possible use of animal behavioral methods
in this area.
ACUTE SOLVENT EFFECTS
The central nervous system (CNS) is a primary target organ for the
acute toxic effects of organic solvents. The acute toxicity of inhaled
solvent vapors is characterized, both in animals and in humans, by
reversible signs of CNS depression. At moderate levels of overexposure,
human subjects often complain about nausea, incoordination, and
feelings of intoxication. At sufficiently high exposure levels, uncon-
sciousness and death by respiratory arrest can occur (NIOSH, 1987~.
A number of factors contribute to the susceptibility of the CNS to
the acute effects of organic solvents. First, during the initial stages of
absor Cation of an inhaled dose of solvent vapors, distribution to different
OCR for page 161
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
161
body organs is related to regional blood flow, and as a result, entry
into the brain proceeds rapidly (Baker and Rickert, 1981~. Further,
because of their lipophilic nature, organic solvents accumulate in tissues
with a high lipid content, thus making brain and nerve potential
depots for these compounds and, in some cases, their toxic metabo-
lites (Bus et al., 1981~.
One experimental approach to determining behaviorally effective
levels is to expose human volunteers to controlled atmospheres of
solvent vapors and to examine changes in psychometric measures
designed to assess different aspects of motor, sensory, and cognitive
performance (Dick and Johnson, 1986~. In general, the results of
experimental exposure studies have demonstrated that one of the
most consistent effects of inhaled organic solvents in humans is a
slowing in behavioral performance evidenced by increased response
latencies in simple and complex reaction time tasks (Gamberale, 1985~.
However, the results of such studies also seem to indicate that the
level at which behavioral effects occur tends to be on the higher end
of the scale for human occupational exposures. As pointed out by
Dick and Johnson (1986), there are a number of difficulties in conducting
human experimental exposure studies. In addition to the narrow
dose-response range that can ethically be examined, the duration of
exposure which is acceptable to human volunteer subjects is often
less than that encountered in the occupational situation. Thus, human
exposure studies rarely employ an 8-hour exposure regimen even at
Threshold Limit Values (TLVs) and, in most instances, are restricted
to 2-4 hours of actual exposure. This may explain, in part, the mod-
est effects seen in human exposure studies and the limited dose-
response data obtained.
Application of Animal Behavioral Methods
Acute Effects of Single Exposures
Animal studies using operant techniques are being employed
increasingly to examine the effects of inhaled organic solvent vapors
on behavioral performance, and guidelines for the use of schedule-
controlled operant techniques for neurotoxicity evaluation have recently
been proposed by the U.S. Environmental Protection Agency (USEPA,
1985~. Because the aim of many of the studies employing operant
techniques has been to demonstrate the usefulness of these methods
in neurotoxicity screening, experimental protocols involving relatively
high-level, short-duration exposure schedules have typically been
employed (e.g., Glowa and Dews, 1983; Moser et al., 1985~. Although
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162
BEVERLY M. KULIG
such studies can provide highly reliable, quantitative information on
which to judge the relative potency of different compounds to affect
behavior in that animal test system, they also give the impression
that very high concentrations of inhaled solvents are necessary to
affect learned behavior in rodents. Perhaps as a result, very few
animal studies have examined occupationally relevant concentrations
and exposure durations for the purposes of risk assessment.
Although it is unclear at present just how sensitive measures of
learned behavior are to the acute effects of inhaled solvents, there is
some evidence that animal studies using low-level exposure may be
a worthwhile approach for estimating concentrations at which behavioral
effects can be expected to begin to occur in humans. Kishi and his
coworkers (1988), for example, have recently demonstrated that
inhalational exposure to toluene at 125 ppm for 4 hours produced
significant effects on signaled avoidance both during the initial stages
of exposure and for several hours following the end of exposure.
Results from acute exposure studies conducted in our laboratory
using positive reinforcement also indicate measurable effects of solvents
at occupationally relevant levels. In a recent study, for example, the
effects of low aromatic white spirits were examined in rats working
on a two-choice visual discrimination task for water reward. Rats
were first trained in operant chambers equipped with two levers,
two light panels located above each lever, and a pump for delivering
water reward in daily sessions consisting of 100 trials. The rat's task
was to depress the lever under the illuminated panel in order to
obtain a drop of water. Following stabilization of performance, rats
were randomly assigned to one of four groups and exposed by inha-
lation to low aromatic white spirits at O (controls), 1,200 mg/m (~200
ppm), 2,400 mg/m3 (~400 ppm), and 4,800 mg/m3 (~800 ppm) for 8
hours and tested immediately following the termination of exposure.
As the left panel in Figure 1 demonstrates, exposure to white spir-
its at these concentrations produced no observable effects on dis-
crimination accuracy. Speed of responding (right panel), however,
was affected at all concentrations tested, with exposure to 1,200 ma/
m3, 2,400 mg/m3, and 4,800 mg/m3 producing a mean increase in
trial response latency of 47, 96, and 156 percent, respectively. The
effects on two-choice response speed did not appear to be the result
of changes in motivation for water reward: All groups consumed the
same number of reinforcements and the latency to obtain reinforce-
ment following a correct trial response was similar for all groups.
Although only several human experimental exposure studies have
been conducted with white spirits and the length of exposure in the
human studies was considerably less than 8 hours, the data that are
OCR for page 163
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
100
90
80
Q
-
in
A
o
CL
in
40
G
o
70
60
30
20
10
o
_ 6
~ 41~
GOD All ~~ Amp
mg/m3
163
GOD `
mg/m3
FIGURE 1 Effects of a single 8-hour exposure to white spirits on accuracy (left panel)
and response speed (right panel) of rats performing on a two-choice visual discrimina-
tion task.
available indicate relatively good agreement between the levels of
exposure producing changes in learned performance in both types of
studies. For example, in studies conducted with young healthy male
volunteers, Gamberale and his coworkers (1975) reported that exposure
to white spirits at 4,000 mg/m3 for 50 minutes produced a small (i.e.,
10 ms) but significant increase in response latencies measured in a
simple reaction time task. Further, in a study employing a total
exposure duration of 7 hours and concentration levels of 34, 100, 200,
and 400 ppm, significant effects on response speed measures were
found at 100 ppm and higher, with effects on other cognitive and
motor tasks appearing at higher concentrations (Cohr et al., 1980~.
Thus, although the number of studies aimed at examining the acute
behavioral effects of low-level solvent exposure is limited, the data
available suggest that the rat may prove to be a more useful model
for estimating human observable effect levels than might be expected
on the basis of the results obtained in high-level, short-duration ex-
posure studies.
OCR for page 164
164
BEVERLY M. KULIG
Acute Effects in the Context of Chronic Exposures
Except for instances of accidental poisoning in which lethal sol-
vent concentrations are reached, all signs of CNS depression, even
those resulting from a single high-level intoxication, appear to be
readily reversible and no evidence exists to indicate that acute sol-
vent exposure is accompanied by neuropathological or persistent
neurofunctional sequelae. As a result, acute solvent effects are usually
mentioned only in passing in discussions of solvent neurotoxicity
(Baker and Fine, 1986; Grasso et al., 1984; Spencer, 1985~. Different
lines of evidence from the human literature, however, indicate that a
need exists for more careful consideration of acute solvent effects
particularly in the context of chronic exposure.
First, there is a considerable amount of evidence from the occupa-
tional literature demonstrating deficits in behavioral functioning in
exposed workers. Discussions of these effects often imply that because
neurobehavioral effects were measured in persons exposed on a chronic
basis, the effects themselves are chronic in nature. Behavioral testing
in occupational studies, however, is often conducted in workers who
have just left an acute exposure situation (i.e., the worksite) or in
workers tested within a day or two following an exposure period
(e.g., the work week). Thus, it is not unlikely that acute effects could
contribute to the changes in behavior often reported in the human
literature.
Further, there are indications from the human literature that sensitivity
to the acute effects of solvents can change in a repeated exposure
situation and that such changes may be important in monitoring the
potential hazards of these compounds. On one hand, there are both
anecdotal and experimental reports indicating that tolerance develops
to the effects of organic solvents, i.e., that acute effects are attenuated
with repeated exposures (Gotell et al., 1972~. On the other hand,
patients with suspected solvent-induced toxic encephalopathy often
complain of an increased sensitivity to the acute effects of solvents.
In such patients, signs of acquired intolerance are usually characterized
by dizziness and nausea when they are exposed to even very low
concentrations, despite the fact that they had been occupationally
exposed for years to higher concentrations without subjective symp-
toms (Gyntelberg et al., 1986~.
From the vast literature on the development of tolerance to ethyl
alcohol and other CNS depressants, a number of behavioral paradigms
are available for examining tolerance development to industrial solvents
as well. Himnan (1984), for example, demonstrated rapid development
of tolerance to the effects of repeated short-duration, high-level toluene
OCR for page 165
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
90
80
v 70
o
CO
CO
z
g
In
fir
o
a:
z
60
50
40
30
20
10 _
O
PreWk Day 1 Day 2
\\ ~ -
~,
. r
Exposure ~
Controls
- sty 100 ppm
O sty 340 ppm
~ - sty 1,22S ppm
165
Day 3 Post Post Wk
FIGURE 2 Effects of repeated inhalational exposure to styrene on the number of short-
latency two-choice responses of rats performing on a visual discrimination task.
exposures on measures of ataxia and rearing. However, the develop-
ment of tolerance appeared to depend on the behavioral functions
examined, with increased activity and head shakes showing a slowly
developing reverse tolerance.
In studies investigating the effects of styrene on learned discrimination
performance, we have also found a rapidly developing tolerance to
the effects of styrene on speed and accuracy measures of discrimina-
tion performance during the first week of exposure. As Figure 2 shows,
rats exposed to 100, 350, and 1,225 ppm of styrene for 18 hours a day
and tested on a visual discrimination task (described above) all showed
a reduction on Day 1 in the number of two-choice responses made
within 2 s of trial onset. However, as exposure continued, the effects
of styrene on short-latency responding, particularly in the highest
concentration group, showed a marked attenuation. Further, results
from a chronic exposure study (Kulig, 1988) indicated that the rapidly
developing tolerance to styrene seen in the first week of exposure
persisted throughout chronic exposure.
In contrast, results from studies examining the effects of trichloro-
ethylene (TCE) on discrimination indicated a quite different profile
OCR for page 166
66
100
80
o
In
Z
2
V)
m
IL 40
o
G
m
20
at
o
BEVERLY M. KULIG
A:,
_~
Ok
1 1 1
~ ~ iL~:,~?
Exposure ~
I
i
Controls
TCE 500
TCE 1 ,000
TCE 1,500 ppm
1 1 ~ I I 1 1 1
0 3 6 9 12 15 18 21 24
WEEK OF EXPERIMENT
FIGURE 3 Changes In the acute effects of trichloroethylene (TCE) on the number of
short-latency two-choice responses of rats during and following 18 weeks of exposure.
Redrawn with permission from Kulig, 1987, Pergamon Press PLC.
in the time course of effects during chronic exposure. Groups of rats
were exposed to TCE by inhalation at 0, 500, 1,000 and 1,500 ppm for
16 hours a day, 5 days per week, for 18 weeks (Kulig, 1987~. Similar
to styrene, exposure to TCE led to a within-week development of
tolerance to the effects of TCE on response speed of discrimination
performance but only during initial stages of exposure. As exposure
became chronic, within-week tolerance was lost, with the result that
the acute effects of TCE on response speed became more pronounced
as exposure continued (Figure 3~. Despite the very marked disturbances
seen in the behavior of rats chronically exposed to TCE, the effects
were apparently acute in nature because no evidence for a carryover
of effects into the postexposure period could be demonstrated.
Taken together, the results of our studies as well as those of other
investigators indicate that different profiles of tolerance and reverse
tolerance develop for different behavioral effects and for different
organic solvents. Given the indications from the human literature
that similar phenomena also occur in exposed workers, and the problems
associated with the interpretation of the nature of the deficits seen in
occupational behavioral studies, it appears that animal studies examining
OCR for page 167
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
167
acute effects in the context of the chronic exposure situation might
provide a worthwhile approach to resolving some of these issues.
Reinforcing Properties of Organic Solvents
Unlike heavy metals and pesticides, organic solvents possess a be-
havioral property that is not seen with other industrially used chemicals,
namely, the potential for abuse. Although it may be argued that
possible self-exposure to organic solvents for their euphoric effects
has little to do with evaluating the neurotoxicity of these compounds,
the reinforcing effects of these compounds and the possible cross-
tolerance with recreational drugs such as ethyl alcohol are important
considerations in evaluating the potential risk to human health posed
by these chemicals both in exposed workers and in the general population.
Similar to the effects of ethyl alcohol, acute high-level exposure to
solvents can produce feelings of euphoria, and intentional inhalational
solvent abuse has become a serious health problem, particularly among
children and adolescents Johnston et al., 1984~. The most commonly
abused products include glues, paints and paint thinners, gasoline,
lighter refills, and cleaning fluids (King, 1982~.
Although toluene-containing products appear to be particularly
popular, the chemical diversity of the different compounds which are
abused suggests that the potential for abuse is not limited to a single
compound. In addition to the social, psychological, and economic
consequences of any addiction, the long-term abuse of solvent-containing
products has been associated with a number of neurotoxic effects,
including psychosis, hallucinations, sensory and motor disturbances,
and convulsions. Although the acute encephalopathy produced by
solvent inhalant abuse appears reversible in most cases, reports indicate
that in some patients severe CNS effects may persist indefinitely (Boor
and Hurtig, 1977; Grabsky, 1961; King, 1982; Knox and Nelson, 1966;
Satran and Dodson, 1963; Weisenberger, 1977~. Moreover, although
children appear to be the high-risk group in the general population,
solvent abuse can occur in the occupational setting as well, and many
of the case studies reporting persistent CNS effects have involved
adults whose initial experience with the euphoric effects of organic
solvents occurred in the occupational setting.
~ . .. . . , . ~ . .
~ .
Systematic animal studies of the relative a ruse potential of differ-
ent organic solvents have not yet been conducted; however, animal
models for examining the stimulus properties of these chemicals have
been described. Nonhuman primates, for example, will self-administer
inhaled vapors (Wood, 1979~. Further, in studies using rodents, drug
discrimination procedures have been used to evaluate the stimulus
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168
BEVERLY M. KULIG
properties associated with acute intoxication (Overtop, 1984~. Rees et
al. (1987) have shown the similarity in stimulus properties of toluene
and barbiturates. If similar results are found with other solvents that
are abused by humans, it may be possible to develop a systematic
framework for evaluating the abuse potential of industrial and com-
mercial compounds.
The issue of abuse is so different from the usual concerns of toxi-
cology, neuropathology, and occupational medicine that one may question
whether this unique behavioral property of solvents belongs in the
toxicological picture of risk assessment. When one considers, how-
ever, that compounds producing acute euphoric effects may lead to
repeated near-lethal (and sometimes lethal) exposure situations that
result in irreversible brain damage, an evaluation of which solvents
possess the potential for abuse would seem warranted.
CHRONIC SOLVENT EFFECTS ON
THE PERIPHERAL NERVOUS SYSTEM
The Role of Animal Studies
The effects of organic solvents on the peripheral nervous system
provide some of the strongest evidence for the potential of these
compounds to produce irreversible nervous system damage, and animal
studies have played an important role in both helping to identify the
causative agent in outbreaks of human disease (Allen, 1980) and elu-
cidating the underlying mechanisms of action. In what has now be-
come an almost classic example of neurotoxicological detective work,
animal studies, initiated following an outbreak of peripheral neuropathy
in a plastics coating plant in Columbus, Ohio in 1973, not only helped
identify methyl n-butyl ketone (MnBK) as the causative agent, but
also demonstrated the role of the solvent methyl ethyl ketone in causing
the outbreak. Further, animal experimental studies helped clarify
the role of the gamma-dike/one pathway in the neurotoxicity of MnBK,
identified hexacarbons as a general class of potential neurotoxic agents,
and stimulated research into the pathological processes involved in
dying back neuropathies (see Spencer and Schaumberg, 1980~.
In addition, human data together with animal experimental studies
have identified solvents other than those involved in the gamma-
diketone pathway as toxic to peripheral nerve. Carbon disulfide, for
example, is a metabolic poison which produces a wide array of effects
including psychosis and peripheral neuropathy in man and lesions in
the brain and peripheral nerve in experimental animals (Wood, 1981~.
Exposure to trichloroethylene (TCE), a compound used extensively
OCR for page 169
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
169
in degreasing operations, was originally thought to be responsible
for causing cranial neuropathies. However, when TCE is exposed to
light or heat, it breaks down easily into dichloroacetylene. Animal
studies helped establish the neurotoxicity of dichloroacetylene, and it
is this compound that is now generally recognized as the causative
agent in producing TCE-induced trigeminal neuropathy (Spencer, 1985~.
More recently, 2-tert-butylazo-2-hydroxy-5-methylhexane (BHMH),
a solvent used in the manufacture of polyester-based plastics, was
removed from the market when central and peripheral nervous sys-
tem dysfunction developed in workers after several weeks of exposure
(Horan et al., 1985~. Although premarket testing demonstrated the
acute CNS toxicity of this compound in rodents, no chronic neurotoxicity
studies were carried out. Following the outbreak of human disease
at a manufacturing plant in Texas, animal studies were undertaken
which showed that dermal exposure to BHMH produced functional
signs of peripheral neuropathy within three weeks of exposure (Spencer
et al., 1985~. In addition, neuropathological studies indicated that
BHMH exposure produced axonal degeneration of the optic tracts,
ascending and descending spinal tracts, and peripheral nerve (Spen-
cer et al., 1985~. Although BHMH is a six-carbon straight-chain structure,
its decomposition does not seem to involve the gamma-dike/one pathway
(Horan et al., 1985) as with n-hexane or methyl n-butyl ketone. It is,
however, a potent neurotoxic agent both in animals and in humans.
What, of course, is particularly unfortunate in the case of BHMH is
that the human disease resulting from exposure to this compound
need not have occurred if adequate premarket animal testing had
been conducted.
Neurobehavioral Methods for Screening
Sensory and Motor Effects
Although morphological evidence of nervous system changes has
historically been the accepted endpoint in determining neurotoxicity,
there is increasing interest in quantitative functional measures that
could be used in conjunction with neuropathological evaluations for
screening purposes (Buckholtz and Panem, 1986~. In the United States,
for example, the U.S. Environmental Protection Agency, under the
Toxic Substances Control Act, has published guidelines for the use of
behavioral methods in neurotoxicity testing (USEPA, 1985), and at an
international level, the World Health Organization (WHO) is currently
sponsoring various activities in the field of neurotoxicity (WHO, 1986~.
Because of the well-documented potential of organic solvents and
other industrial compounds to affect sensory and motor function in
OCR for page 173
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
4
-
2
Go
1
o
173
Controls
CS2 7s
CS2 22s
CS2 700 ppm
rA
Exposure
0 6 12 18 24 30 36 42
WEEK OF EXPERIMENT
FIGURE 7 Effects of carbon disulfide on the latency of the compound nerve action
potental measured from the caudal nerve during and following 36 weeks of exposure.
was also affected beginning in Week 9 of exposure (Figure 5) and
was accompanied by deficits in coordinated movement (Figure 6~.
Furthermore, the behavioral evidence for disturbed peripheral nerve
function was supported by electrophysiological changes in peripheral
nerve conduction velocity (Figure 7~. For all measures, CS2-induced
changes persisted well beyond the end of the exposure period, indi-
cating that these effects were chronic in nature. In order to evaluate
possible structural changes in these animals, neuropathological ex-
aminations were conducted by I. B. Cavanagh at the University of
London at the termination of the study. Results indicated swollen
axons and nerve fiber degeneration in the 700-ppm group in the sci-
atic, tibial, and caudal nerves as well as in the spinocerebellar tracts
and the superior colliculus.
Taken together, these data demonstrate the ability of currently available
neurobehavioral methods to quantify the progressive development
of chemically induced changes in peripheral nerve function and to
study the relationship between neurofunctional changes and the
morphological changes. In addition to providing information regarding
the time course of effects, the repeated testing of chronically exposed
animals can also be used to operationally define appropriate time
OCR for page 174
174
BEVERLY M. KULIG
points for conducting morphological and chemical investigations. Further,
the ability to quantitate progressive changes in nervous system function
during a time in which no observable signs of dysfunction are evident
is also an important consideration in the use of these methods for
screening new compounds. Decisions regarding exposure schedules
and the total duration of exposure, especially in inhalational studies,
are often based on practical or rule-of-thumb considerations. How-
ever, there is nothing inherent in a 90-day exposure study with a
particular exposure schedule to ensure the frank expression of ob-
servable signs of neurotoxicity or easy-to-identify light-microscopical
changes. In such cases, small, but reliable, quantitative neurofunctional
changes may be the only indication that closer examination of the
compound using different exposure schedules and a longer duration
of exposure is warranted.
Despite the advantages of neurobehavioral testing, an examination
of control baseline performance in the CS2 study demonstrates some
of the considerations that must be taken into account in designing
neurobehavioral methods for use in chronic exposure experiments.
For example decreased levels of behavior such as that seen in tests of
spontaneous activity resulting from repeated testing or perhaps combined
with the effects of aging in nonexposed control animals can lead to a
floor effect and diminish the usefulness of the test in the latter stages
of prolonged studies. Conversely, difficulties with caudal nerve
conduction time measurements are more likely to occur during the
early stages of exposure. Caudal nerve conduction time improves
with age until 150-300 days after birth, and remains relatively stable
until an advanced age when it again shows signs of prolongation
(Schmelzer and Low, 1987~. Because most chronic exposure studies
begin when animals are young adults, the first months of exposure
correspond to the time of greatest improvement in conduction velocity,
making detection of compound-related effects during the early stages
of exposure difficult to detect. The need to consider age-related changes
is not unique to neurofunctional approaches to neurotoxicity evalua-
tion because morphological and neurochemical changes can also be
expected to occur. There is, however, a need in the further develop-
ment of neurobehavioral methods to evaluate the long-term operating
characteristics of any given test in order to better understand its strengths
and limitations in the chronic exposure situation.
CHRONIC TOXIC ENCEPHALOPATHY
In addition to their effects on peripheral nerve, organic solvents
have also been shown to produce irreversible effects on brain func-
OCR for page 175
NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
175
lion. Examination, for example, of CS2-poisoned workers conducted
during the first 40 years of this century when occupational exposures
were apparently very high, demonstrated the potential of this compound
to produce severe manic-depressive psychosis and other signs of CNS
dysfunction (see Wood, 1981~. In later studies using psychological
test instruments for quantifying the degree of psychological changes,
Hanninen (1971) demonstrated the ability of these techniques to describe
the range and pattern of cognitive and affective changes associated
with exposure in workers with symptoms of CS2 poisoning. In addition,
compared to nonexposed control subjects, exposed workers with no
subjective symptoms or clinical signs of overexposure also showed
changes in psychological test performance. The finding of psychological
changes in the absence of overt signs and symptoms together with
epidemiological studies indicating that occupational exposure to carbon
disulfide in the viscose rayon industry was associated with higher
rates of suicide (Mancuso and Locke, 1972), was particularly disturbing
because it raised the possibility that organic solvents as a general
class of industrial compounds could produce cognitive and affective
changes of a significant nature which were virtually undetectable by
clinical methods and unrecognizable by the exposed person himself
as being associated with chemical exposure.
As a consequence, an increasing number of studies were initiated
to examine the psychological functioning of workers in different industries
who were exposed to different types of organic solvents. In general,
results from these studies repeatedly have shown a higher incidence
of subjective complaints related to CNS effects in solvent-exposed
workers, changes in objective measures of psychological functioning,
and in some cases, a higher prevalence of EEG abnormalities and
reduced peripheral conduction velocities (see Baker and Fine, 1986;
WHO, 1985~. In a series of studies examining persons occupationally
exposed to mixed solvents, for example, painters and other occupational
groups have been identified as being at risk for developing irrevers-
ible changes in brain function based, at least in part, on the results of
behavioral evaluations (Arlien-Soborg et al., 1979; Bruhn et al., 1981~.
As a result of the growing number of cross-sectional occupational
studies demonstrating solvent-related neurofunctional changes and
the number of case reports of chronic encephalopathy produced by
solvent abuse, two workshops were convened to develop internationally
acceptable diagnostic criteria applicable to solvent-induced CNS dis-
ease (Cranmer and Goldberg, 1986; WHO, 1985~. Because toxic
encephalopathy produced by nervous system poisons has been recognized
as a clinical entity for many years, diagnostic criteria based on the
DSM-III classification of mental disorders (American Psychiatric As-
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76
BEVERLY M. KULIG
sociation, 1980) served as a basis for differentiating three different
levels of psychological impairment produced by chronic neurotoxic
overexposure. The mildest level of impairment, termed "organic af-
fective syndrome," was defined as one in which chronic chemical
exposure was accompanied by subjective complaints of fatigue, mild
memory and concentration difficulties, and affective changes. The
term used to describe the second level of impairment was "mild chronic
toxic encephalopathy"; it includes both subjective neurotoxic symp-
toms and sustained changes in personality or mood, as well as defi-
cits in performance on formal neuropsychological testing. Finally,
the third level of solvent-induced toxic encephalopathy refers to a
severe neuropsychiatric condition characterized by global deterioration
of intellectual and emotional functioning such as that described in
the turn of the century literature for carbon disulfide poisoning or in
present-day case studies of solvent abuse (Cranmer and Goldberg,
1986; NIOSH, 1987; WHO, 1985~.
Problems in the Interpretation of Human Studies
The ability of drugs and chemicals to produce toxic encephalopathy
is widely recognized, and there is little disagreement regarding the
potential of high-level exposure to lead, thallium, alcohol, or drugs
to produce severe signs of central nervous system poisoning that
may be irreversible or only slowly reversible. The potential of organic
solvents to produce persistent changes in brain function, however,
has become, for a number of reasons, an issue of considerable controversy.
First, from the discussion above, it is obvious that acute effects can
have important consequences for behavioral functioning. However, it
is often difficult or impossible to design occupational studies in which
the possibility of acute solvent effects contributing to changes in
psychological performance has been ruled out. Given the fact that
the test instruments sensitive to the effects of acute solvent exposures
in the experimental exposure situation are often the same as those
that are sensitive in detecting changes in psychological functioning
in cross-sectional occupational studies (Gamberale, 1985), a differen-
tiation of acute neurotoxicant effects from mild toxic encephalopathy
based on the selection of the test instrument does not seem feasible.
Moreover, occupational environments often contain many differ-
ent organic solvents, and workers are not necessarily aware of the
level and type of their present or previous occupational exposures.
Thus, conclusive proof as to the identification of the causative agents
producing psychological changes based on the results of these studies
is often difficult to obtain.
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NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
177
Another issue in both the conduct and the interpretation of occu-
pational behavioral studies is the selection of appropriate nonexposed
control subjects who are suitably matched on the basis of age, sex,
educational level, socioeconomic level, and other demographic vari-
ables known to affect psychological test performance. When such
variables are taken into account in the statistical analyses of group
differences of exposed and nonexposed workers, some studies have
indicated that originally large differences in psychological test results
can become borderline or disappear altogether (Baker et al., 1988;
Cherry et al., 1985~.
Problems stemming from the use of inappropriate control data are
illustrated by a recent report by Gade and his colleagues (Gade et al.,
1988~. In this study, solvent-exposed workers were examined with
clinical psychological tests and diagnosed as having solvent-induced
chronic toxic encephalopathy. However, the original evaluation of
these patients was apparently made without reference either to population
norms for the determination of within-subject profiles of cognitive
deficits or to estimates of premorbid levels of functioning, despite
the fact that both types of information are necessary for a valid
neuropsychological evaluation (Lezak, 1983~. When these patients
were retested at a later date and compared with nonexposed persons
of similar age and education, no differences in psychological test
performances could be seen. As a result, the authors were forced to
revise their earlier diagnoses of solvent-induced dementia and to conclude
that the poor test performance of these patients was related not to
solvent exposure, but to the lower level of intelligence and education
in their subject sample.
It is apparent from the discussion above that if appropriate con-
trols are used, well-designed neurobehavioral studies can be used to
evaluate acute neurotoxicant effects, to monitor the safety of workers
exposed to known neurotoxic agents, and to identify possible occupational
hazards. However, it is doubtful whether cross-sectional studies at
the human level can or should be used as screening tools for the
initial identification of compounds that possess CNS neurotoxic properties.
In all fairness to the behavioral toxicologists and neuropsychologists
working at the human level, they have received little help from their
counterparts at the animal level in addressing the issues surrounding
the possible adverse effects of long-term solvent exposures on cognitive
functioning. In part, this may be due to the difficulty most psychologists
working at the animal level have in evaluating and interpreting the
sometimes diffuse effects reported in the human literature. However,
it is more likely due to the lack of adequate test instruments for
examining the effects of chemical exposures on learning, memory,
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178
BEVERLY M. KULIG
and emotional functioning which can be applied to the chronic expo-
sure situation.
Animal Models of Cognitive Effects
Although the study of learning and memory has occupied the in-
terest of psychologists for many years, some of the paradigms developed
to study memory function, such as one-trial passive avoidance learning,
are obviously unsuitable for repeated evaluation of cognitive changes
in chronic exposure studies. There are, however, techniques described
in the literature which, with further study, may provide useful approaches
to examining those behavioral processes that would seem to be most
likely affected by exposure to centrally acting neurotoxic agents. Heise
(1983), for example, has described several discrete-trial operant procedures
involving delayed response and delayed comparison paradigms, which
can be easily acquired by normal rats and can be used repeatedly to
assess changes in memory. Recent studies employing radial arm
maze techniques (Peele and Baron, 1988) also indicate that repeated
acquisition paradigms may prove useful in assessing memory changes.
Even with the further development of behavioral methods to address
more fully the issue of possible cognitive changes accompanying solvent
exposure, it still remains to be seen whether these methods can provide
sufficiently stable control baselines such as those needed for long-
term studies. Moreover, if agent-related effects can be measured on
cognitive performance in the absence of clear-cut neuropathological
changes in appropriate brain structures, it will be necessary to seek
possible underlying mechanisms of action either at the neurochemical
level or with morphological techniques more sensitive than those
that are used routinely for neurotoxicity assessment.
One approach that may prove fruitful is the combined study of
neurobehavioral and neurochemical changes accompanying long-term
solvent exposure. Investigators at the Karolinska Institute examining
the effects of low-level (80 ppm) toluene exposure, for example, have
recently reported reductions in catecholamine turnover rates in rat
striatum and increased catecholamine levels in hypothalamus (Fuxe
et al., 1982), as well as changes in central receptor binding properties
(Fuxe et al., 1987) during subchronic exposure. Studies with styrene
have also demonstrated an effect on catecholamine function (Husain
et al., 1980), and a common mechanism at the neurochemical level
has been proposed (Mutt) and Franchini, 1987) based on the ability of
dopamine to condense nonenzymatically with solvent metabolites from
different chemical groups. Whether such neurochemical effects are
acute or chronic in nature and whether they can be directly related to
measurable neurofunctional changes have not yet been studied. However,
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NEUROTOXIC EFFECTS OF ORGANIC SOLVENTS
179
efforts to examine the role of possible changes in transmitter function
in producing central neurofunctional effects would appear to offer a
promising approach.
CONCLUSION
There seems to be a growing acceptance in toxicology of animal
neurofunctional methods for use in screening for neurotoxicity. The
development of neurobehavioral methods for assessing motor and
sensory function which has occurred in the last 10 years, the growing
empirical data base demonstrating both the sensitivity and the appli-
cability of these methods to chronic studies, and the increasing possi-
bility of designing studies to examine neurofunctional changes along
with the underlying neurochemical and neuromorphological changes
that accompany them, will continue to provide evidence for the im-
portance of neurobehavioral methods in the identification and further
understanding of the actions of chemicals on the nervous system.
There are, however, many industrially and commercially used or-
ganic solvents already on the market about which little or no infor-
mation regarding their potential effects on the nervous system is available
(McMillan, 1987~. Apparently, even the setting of occupational expo-
sure limits to avoid acute, intoxicating effects on the nervous system
has eluded an experimental basis. Moreover, psychologists working
at the human level have been virtually left on their own to identify
neurotoxic agents in the workplace and to sort out, as best they can,
the complex issues surrounding chronic human exposures. The sub-
ject matter of many of these issues is not the domain of classical
toxicology or neuropathology, it is uniquely behavioral in nature.
Despite the fact that the potential for chemicals to alter memory,
learning, and performance or to produce addiction may not be issues
of primary concern in toxicity screening, they are nonetheless important
considerations in evaluating the risks to human health associated
with long-term chemical exposures. With a better understanding of
the issues faced by investigators working at the human level and a
greater collaboration with scientists working at the cellular and sub-
cellular levels, behavioral toxicologists may be able to supply the
necessary methods and conceptual framework to bridge the rather
formidable gap that has evolved in neurotoxicity risk assessment.
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Representative terms from entire chapter:
neurotoxic effects
