Deborah A. Cory-Slechta
THE SCOPE AND AGENDA OF BEHAVIORAL TOXICOLOGY Behavioral toxicology can be generally conceptualized as that scientific discipline which strives to understand the mechanisms by which toxicants affect behavior. In this respect, it is similar to its counterpart, behavioral pharmacology, the goal of which is to delineate the mechanisms by which drugs modulate behavior. Since its inception, the scope of behavioral toxicology has expanded considerably, driven in part by the need to ascertain the role of existing environmental contaminants in producing functional impairment, as well as the need to develop procedures to preclude future introduction of neurotoxic chemicals. In this way, it differs from behavioral pharmacology which, instead, seeks to develop compounds or agents with specific behavioral actions for therapeutic purposes.
As the above implies, behavioral toxicology actually has dual, overlapping agendas. One derives from the growing recognition of the need to screen for performance impairment prior to the introduction of new chemicals into the environment, as well as to provide information relating to risk assessment based on neurotoxic endpoints. The second agenda involves the more traditional role of behavioral toxicology as the scientific discipline defined above, whose goal is to understand both the behavioral and the biological mechanisms by which toxicants impact behavioral function. It is primarily the latter agenda to which the comments herein are addressed.
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Despite recent advances in neurobiology and the obvious utility of in vitro approaches in the elucidation of biological substrates of behavior, it is difficult to conceive of any substitute for assessing the ultimate impact of a neurotoxicant on behavior, or of assessing behavioral mechanisms of toxicant action, other than in the whole organism. The links between molecular neurobiology or between neuropathological alterations and behavioral impairments are still obscure. Although the relationships between certain neurotransmitters and behavior have become increasingly evident, such as the role of dopamine in parkinsonism, other aspects of those relationships remain puzzling, e.g., the extensive dopaminergic depletions noted before any overt behavioral impairments appear. Thus, there are no substitute or alternative procedures for evaluating the functional impact of a toxicant. Put another way, behavioral toxicity cannot be reliably predicted from molecular events.
STATE OF DEVELOPMENT
Although the experimental capabilities for more precisely delineating behavioral and biological mechanisms of toxicant-induced performance impairment are generally at hand, the discipline remains largely at a characterization or descriptive stage of development. Much of its scientific literature attempts little more than to ascertain whether a particular toxicant alters a particular class of behavior, or to assess performance impairments produced by a toxicant across a range of behavioral endpoints; in some cases, only the barest approximation to a hypothesis may be invoked. Furthermore, little attempt may be made to rationalize the particular behavioral approach chosen, which may, instead, be based predominantly on available apparatus or technology in that laboratory. Nonetheless, owing more to the sheer magnitude of work with certain compounds, rather than to any systematic progression of studies within a laboratory, in certain areas these studies have begun to provide the prerequisite foundation from which more mechanistic approaches can now proceed.
Studies of performance impairments induced by lead exposure provide one example. Lead may be considered a prototypical behavioral toxicant and undoubtedly has been the most extensively studied of such compounds, both at the experimental animal and at the human level. The permanent mental retardation, which in some cases was the residual effect of acute high-dose lead exposure in children, resulted in a subsequent focus of these studies on issues of learning deficits at lower lead exposure levels. Human studies of environmental lead exposure in children have almost invariably focused on age-appropriate IQ
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most recent of such studies have documented decrements in IQ and similar psychometric measures at blood lead concentrations as low as 10 µg/dL (e.g., Bellinger et al., 1987; Fulton et al., 1987). However, even with the separation of verbal and motor subscales, IQ tests represent extremently global measures of performance that encompass a variety of different behavioral functions, as well as the involvement of multiple sensory systems, many of which may be marginally affected or others of which may be more dramatically affected. Such global measures always present the possibility that particular subtle deficits may be obscured by the sheer multiplicity of concurrently measured behaviors or may be clouded by a reserve capacity of the organism. The specific nature of the IQ decrements in humans thus remains unresolved.
Experimental animal studies can more readily address aspects of lead- induced changes in learning. Table 1 summarizes those studies that have assessed lead-induced changes in learning by using acquisition of a visual discrimination as a behavioral endpoint. The various studies are subdivided on the basis of both the type of visual cue utilized and the developmental period of lead exposure. Plus signs show those experiments reporting an impairment of visual discrimination learning as a result of lead exposure, whereas minus signs accompany those that found no change. As indicated by the preponderance of plus signs in each column, two types of visual discrimination paradigms emerge as those more sensitive to lead exposures: discriminations based on differences in brightness and on size of visual cues. Shape-form discrimination shows little obvious impact of lead. A within-laboratory comparison provides further support for this across-study conclusion. Winneke et al. (1977) reported that lead-treated rats required more trials to acquire a size discrimination than did control rats, but were not impaired in the acquisition of a form discrimination.
Although the effects noted with color-based discrimination are suggestive, they are, at present, based on a restricted data set.
The differential lead effects based upon visual cue emphasize the critical importance of the environmental context in modulating the behavioral effects of a toxicant such as lead. No generalized deficit in visual discrimination learning can be ascribed to lead; instead, such deficits depend upon environmental cues.
Studies of visual discrimination learning following lead exposure can direct future efforts aimed at understanding the behavioral and biological mechanisms that might explain such differential effects. With respect to behavioral mechanisms, the possibility of a generalized performance decrement
—for example, an increase in response bias or an alteration in motivation level
—obviously fails to accommodate the differential effects of visual cue. One explanation resides in the possibility of differential degrees of control exerted by the stimuli
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TABLE 1 Lead-Induced Changes in Visual Discrimination Learning Developmental
Period of Lead Exposure
Brightness Shape-Form Size Color
Prenatal +Brady et al.
(1975)a Rat +Zenick et
al. (1978) Rat +Carson et al. (1974) Sheep +Zenick et
al. (1978) Rat -Carson et al.
(1974)b Sheep +Winneke et al. (1977) Rat +Driscoll
and Stegner (1976) Rat
-Winneke et
al. (1977) Rat +Winneke et al. (1982) Rat
Postnatal -Brown
(1975) Rat -Carson
(1976) Sheep -Carson
(1976) Sheep +Bushnell and Bowman (1979) Monkey +Bushnell et
al. (1977) Monkey
-Overmann
(1977) Rat +Bushnell and Bowman (1979) Monkey
+Rice (1985) Monkey -Hastings et
al. (1977) Rat +Rice and Willes (1979) Monkey +Hastings et
al. (1979) Rat -Rice (1985) Monkey -Hastings et
al. (1979) Rat
Adult -Lanthorn
and Isaacson (1978) Rat
a Effect of lead reported.
b Absence of lead effect reported.
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over behaviors, which consequently exhibit differential behavioral sensitivity to disruption. Numerous behavioral pharmacology studies have shown that central nervous system (CNS) drugs may exhibit a much greater magnitude of effect on behavior that is under weak stimulus control (Laties, 1975), i.e., performances which exhibit relatively low overall accuracy levels or require an extensive number of trials to attain criterion performance. In such cases, the stimuli may be less salient, thus failing to generate strong control over performance and rendering it more easily disrupted by other factors.
The fact that lead effects on visual discrimination learning depend upon the type of visual cue, as shown in Table 1, might also reflect differential sensory effects of lead on the visual system. In regard to such a possibility, Fox et al. (1982) have reported persistent decreases in both visual acuity and spatial resolution in 90-day old rats that had been exposed from birth to weaning to 200 ppm of lead acetate via the dam. Although these, as well as other visual system deficits resulting from lead exposure have been reported (Bushnell et al., 1977;
Fox and Chu, 1988; Fox and Farber, 1988), their direct impact in mediating behavioral toxicity remains to be systematically investigated.
Experimental animal studies also reveal that lead impairs learning based on the acquisition of spatial discrimination and can be categorized on the basis of both the route and the developmental period of exposure. Table 2 illustrates several additional issues of importance
TABLE 2 Lead-Induced Changes in Spatial Discrimination Learning Developmental Period of Lead
Exposure Exposure Route
Oral Intraperitoneal
Pre- or postnatal +Snowdon (1973)a -Brown et al. (1971)b +Bushnell and Bowman
(1979a) +Klein et al. (1977)
+Bushnell and Bowman
(1979b) -Rosen et al. (1985)
-Overmann (1977) +Levin and Bowman (1983) +Laughlin et al. (1983)
Postweaning +Geist and Mattes (1979) -Brown et al. (1971)
Adult +Avery and Cross (1974) -Snowdon (1973)
+Lanthorn and Isaacson
(1978) -Ogilvie (1978)
+Ogilvie (1977) -Bullock et al. (1966)
±Dietz et al. (1979) -Penzien et al. (1982)
a Effect of lead reported.
b Absence of lead effect reported.
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in understanding the behavioral toxicity of lead, in particular, which may apply equally to other toxicants: for example, the critical importance of the kinetics of lead to its behavioral toxicity, the detrimental effects on spatial learning, and the consistency of the effect across a variety of different behavioral procedures.
Lead-induced impairment of spatial discrimination learning has been observed following postnatal, postweaning, and adult exposures, in contrast to brightness discrimination (Table 1) which appears most vulnerable in response to prenatal exposures. Thus, different behavioral performances may exhibit quite different critical periods of exposure to a toxicant, or the critical exposure period for behavioral effects produced by a toxicant may be determined at least partly by the sensitivity of the behavioral procedure.
Comparative changes in schedule-controlled behavior induced by lead exposure reveal additional aspects of its behavioral toxicity, aspects that in turn may impact on, or even underlie, other lead-induced performance effects. The most extensively studied of such reinforcement schedules has been the fixed- interval (FI) schedule, in which the reward for responding is temporally based, with the contingency stipulating that the first response occurring after a specified interval of time elapses produces reinforcement. Figure 1 presents the dose-effect function that summarizes the various studies of lead-induced changes in FI schedule-controlled behavior. It plots a parameter of FI performance (as a percentage of the corresponding control data) in relation to treatment dose.
In constructing this summary figure, the lead exposure dosage or concentration has been recalculated in milligrams per kilogram. Because not all experimenters used the same dependent variables, response rate was used where presented, but in other cases, the outcome was based on total number of reponses, median interresponse time (IRT), or group mean percentage of control reinforcements, all of which can be impacted upon by response rate changes.
The data were plotted from the session or sessions in which peak effects occurred, with the exception of data from our studies (Cory-Slechta and Thompson, 1979; Cory-Slechta et al., 1983, 1985), which were restricted to results from the first 30 experimental sessions so as to be comparable to the number of sessions used in most other studies. Prenatal and oral preweaning exposure studies could not be included in this summary figure because it was not possible to ascertain the dose to which the developing animals were exposed. Figure 1 reveals an inverse U-shaped function relating dose of lead to performance on the FI schedule of reinforcement. That is, exposure to lower concentrations or doses of lead produces response rate or output increases on the FI schedule; as the dose or exposure to lead increases, however, the rates of
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are depressed below control values. Plotting changes in FI response rate against the reported blood lead values in each study produces a similar function (Cory- Slechta, 1984).
Figure 1
Summary of studies investigating changes in fixed-interval performance (plotted as percent of the control group) as a function of lead dosage, taken from the session or sessions in which peak effects occurred. Data from studies involving prenatal or lactational exposures could not be included because it was not possible to ascertain the dose to which the developing organisms were exposed. Different experimental species are indicated by different symbols;
numbers refer to different studies: (1) Rice et al. (1979); (2) Cory-Slechta et al.
(1985); (3) Cory-Slechta (1989); (4) Cory-Slechta et al. (1983); (5) Cory- Slechta and Thompson (1979); (6) Van Gelder et al. (1973); (7) Barthalmus et al. (1977); (8) Angell and Weiss (1982); (9) Zenick et al. (1979); (10) Rice (1988).
SOURCE: Cory-Slechta (1984).
The potential generality of the dose-effect function is evidenced by the similarity of the lead-induced changes in rates of responding that have been described in other temporally based reinforcement schedules. For example, Nation et al. (1983) reported a dose-effect function for lead-induced changes in
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with steady-state blood lead of 11–13 µg/dL) on response rate or on the mean IRT value of monkeys responding on another temporally based schedule of reinforcement, a differential reinforcement of low rate (DRL 10 s or DRL 30 s) schedule. However, on both the DRL 10-and the DRL 30-s schedules, they did report an increase in the number of nonreinforced responses and a decline in the number of reinforcers received by lead-treated monkeys, an effect that would seemingly necessitate an increased overall rate of responding. This pattern of effects would be evident in the distribution of IRTs but might not have impacted the measured index, mean IRT, the value of which could be substantially influenced by a small number of very long IRTs.
The response rate-increasing properties of low-level lead exposure derive from a decrease in the time between successive responses (i.e., interresponse times). In particular, the frequency of short interresponse times (less than 0.5 s) during the fixed interval is increased by lead exposure, such that successive responses occur more rapidly in lead-exposed organisms than in controls; no consistent changes in postreinforcement pause time are noted. Figure 2 shows the proportion of short IRTs of control rats (left panels) and of rats treated with 25 ppm of lead acetate (right panels) over the course of 40 experimental sessions on an FI 60-s schedule of food reinforcement in two separate replications (top panel, Cory-Slechta et al., 1985; bottom panel, Cory-Slechta, 1989). As can be seen, the range of short IRTs exhibited by control and lead- exposed rats was actually quite comparable, but lead exposure yielded a shift of the distribution toward the upper extremes of the range (i.e., higher proportions) in both replications. Thus, control and lead-treated animals begin to respond at the same time during the fixed interval, but lead-exposed organisms then respond at much higher rates than control animals, engendering more responding per unit time.
In contrast, schedules of reinforcement based on number of responses (ratio based), rather than on temporal parameters, exhibit a different pattern of lead effects, another indication that its behavioral toxicity is dependent upon the environmental or behavioral context. Although high-level exposures to lead are reliably associated with decreases in response rate on ratio schedules, evidence for rate-enhancing effects at lower exposure levels is not compelling (Angell and Weiss, 1982; Barthalmus et al., 1977; Cory-Slechta, 1986; Padich and Zenick, 1977; Rice, 1988). Although Angell and Weiss (1982) reported shorter median IRTs on an FR schedule in rats exposed to lead prenatally only, the IRTs were actually not significantly different from those of nonexposed controls.
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