Behavioral antagonism between lead and cadmium

Neurotoxicologyand Teratology, Vol. 12, pp. 99-104. ~ PergamonPress plc, 1990. Printed in the U.S.A.

0892-0362/90 $3.00 + .00

Behavioral Antagonism Between Lead and Cadmium J A C K R. N A T I O N , * C A T H Y A. G R O V E R , * G E R A L D R. B R A T T O N ~ " A N D J U A N A. S A L I N A S *

*Department of Psychology, Texas A & M University, College Station, TX 77843 ~Department of Veterinary Anatomy, Texas A & M University, College Station, TX 77843 R e c e i v e d 15 J u n e 1989

NATION, J. R., C. A. GROVER, G. R. BRATTON AND J. A. SALINAS. Behavioral antagonism between lead and cadmium. NEUROTOXICOL TERATOL 12(2) 99-104, 1990.--Adult male rats were exposed to one of four dietary conditions for a period of 60 days. Group Control-Diet received a diet containing no added lead or cadmium, group Lead-Diet received a diet that contained 500 ppm added lead, group Cadmium-Diet received a diet that contained 100 ppm added cadmium, and group Lead-Cadmium-Diet received a diet that contained both 500 ppm added lead and 100 ppm added cadmium. Subsequent to exposure, animals were tested in a Digiscan activity monitor. Animals were then sacrificed and metal concentrations were determined in blood and brain. The results from this experiment showed that lead alone increased movement and vertical activity. Cadmium alone decreased movement and increased rest time. Cotreatment with lead and cadmium failed to produce behavioral differences relative to controls; thus, it seems that the changes in activity caused by one metal are antagonized by the other. Results from the analyses of residues in tissues revealed that blood lead concentrations were lower in the cotreatment condition than the lead along condition. However, brain residue accumulations were not different for these two exposure conditions. There was no evidence that the presence of lead attenuated increases in cadmium residues in blood or brain. Overall, the residue data agree with a central, as contrasted with a peripheral, account of lead/cadmium interaction effects, at least as relates to behavior. Because lead and cadmium were additive with regard to producing decreased body weights, it seems that the toxic effect of these metals is antagonized by cotreatment in some instances, and augmented in others. Antagonism

Cadmium

Cotreatment

Lead

IN a recent study of fixed-interval (FI) performance, Nation et al. (17) observed that the joint effects of exposure to both cadmium and lead were less than the effects of either metal presented in isolation. Specifically, facilitated operant responding associated with recurrent dietary exposure to 500 ppm lead or 100 ppm cadmium was not evident in animals exposed to the same dose levels in combination. Moreover, similar antagonistic effects were observed with respect to toxicant-induced changes in neurochemistry. Increases in brain levels of dopamine (DA), serotonin [5-hydroxytryptamine (5-HT)], and their respective metabolites that were produced by exposure to lead or cadmium alone were attenuated by the cotreatment of the metals. And identical antagonism was evinced when lead and cadmium presented alone produced decreases in transmitter activity in selected brain regions; e.g., even though both lead and cadmium produced significantly lower turnover rates for 5-HT in the frontal cortex when they were presented alone, turnover rates associated with the combined treatment of the metals were not different from controls. As counterintuitive as these findings may seem, they are in agreement with an earlier in vitro report (8) which found that cadmium inhibited lead-related increases in the spontaneous release of peripheral acetylcholine. Speculative accounts of such phenomena point to the possible competition between lead and cadmium at calcium uptake sites that are integrally involved in both evoked transmitter release, and the ultimate concentrations of intracellular ionized calcium that modulate spontaneous release

[refer to (2) for a recent review of these putative chemical interactions]. And it may be that such changes in central neurotransmission underlie the behavioral and neurochemical anomalies observed by Nation et al. in their in vivo investigation of lead/cadmium interactions. But one must also consider that the metal antagonism may derive from peripheral rather than central influences. That is, it is possible that the locus of competition between lead and cadmium is at the level of the gastrointestinal tract rather than nervous system. Perhaps one metal blocks or antagonizes the intestinal absorption of the other and therein limits the distribution of the toxicant, thus decreasing residue accumulations in tissues. Because tissue concentrations of lead and cadmium were unavailable in the Nation et al. report, a peripheral rationale for the apparent antagonism cannot be ruled out. For peripheral blockade to be considered as a principal mechanism underlying lead/cadmium antagonism, it would need to be demonstrated that blood levels of lead/cadmium residues would be sharply reduced by joint exposure to the metals. Moreover, such diminished blood concentrations in the cotreatment condition should be translated into decreases in concentrations of lead and cadmium in the brain, otherwise arguments focusing on noncentral issues would be greatly compromised. The present investigation provides added information on the consequences of simultaneous exposure to lead and cadmium. In an effort to extend the range of behaviors affected by cotreatment manipulations, a general activity index was used to determine

99

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whether or not effects similar to those observed for schedulecontrolled responding would exist for such behavioral dimensions as vertical and horizontal activity, distance traveled, movement time, turns, and stereotypy. In addition, the issue of central versus peripheral antagonism was addressed in this study where blood and brain residues of lead and cadmium were assessed for animals exposed to control diets, diets containing either 500 ppm lead or 100 ppm cadmium, or a diet containing both 500 ppm lead and 100 ppm cadmium. METHOD

Animals The animals used in this study were 32 adult male SpragueDawley rats (Holtzman Company, Madison, WI) approximately 50 days old at the beginning of the experiment. Initial animal weights ranged between 180 and 200 g. Eight of the animals (group Control-Diet) were maintained ad lib on a diet of laboratory chow that contained no added chemicals. Eight other animals (group Lead-Diet) were placed on an identical feeding regimen that offered lab chow containing 500 ppm lead (as lead acetate). And eight animals (group Cadmium-Diet) were maintained on an ad lib diet that contained 100 ppm cadmium (as cadmium chloride). The remaining eight animals (group Lead-CadmiumDiet) were fed the same diet of lab chow as the other animals, but their food contained both 500 ppm lead and 100 ppm cadmium.

Preparation of Food For contaminant-treated food, pellets of semipurified Teklad Laboratory chow (Harlan Sprague-Dawley, Inc., Madison, WI) were ground in a small food mill then transferred to a large stainless steel food mixer in 10-kg batches. Two liters of distilled, deionized water containing the appropriate amount(s) of the contaminant(s) (lead acetate, cadmium chloride) were added to the mixer, and the mixing process was continued until the mixture appeared homogenous. Mixing was continued 20-30 rain to ensure complete distribution of lead and/or cadmium in the food. The food then was repelleted with a laboratory pelleter (Model CL Laboratory Pellet Mill, California Pellet Mill Co., San Francisco) and stored at <0°C. Control food was prepared in the same manner as treated food, but with only distilled water added. To control for possible cross-contamination between food batches, control food was mixed first, followed by lead, cadmium, and lead plus cadmium food, successively. The mixer and pelleter were cleaned thoroughly after each mixing.

Apparatus The behavioral apparatus used in this study involved an automated Digiscan-16 system. The system includes an optical beam activity monitor (Coulbourn E61-32) comprised of 16 vertical and 16 horizontal infrared sensors. The monitor surrounded an acrylic activity monitor cage (40 ×40 × 30.5 cm), completely enclosed with 0.5-cm air holes drilled in the top panel. An E61-58 multiplexer/analyzer located in an adjacent room monitored beam breaks from the optical beam activity monitor and tracked the simultaneous interruption of beams. The multiplexer/ analyzer updated the animal's position in the acrylic cage every 10 milliseconds using a 100% real time conversion system. Computerized integration of the data obtained from the monitor afforded the recording of the following behaviors: horizontal activity (number of beam breaks in horizontal sensors), total distance (cm), number of movements (number of times ambulatory activity was initiated for a period greater than 1 sec), movement time (sec),

rest time (sec), vertical activity (number of beam breaks in vertical sensors), number of vertical movements (number of rears where animals dropped below the vertical sensor for 1 sec), vertical time (sec), stereotypy (number of beam breaks during a period where the same beam is repeatedly interrupted), stereotypy time (sec), clockwise revolutions, anticlockwise revolutions, margin time (time in sec spent within 1 cm of the walls of the cage), center time (time in sec spent more than 1 cm away from the wails of the cage). Also, time spent in the four quadrants of the cage was recorded. A selector switch on the multiplexer/analyzer was set to print updated totals successively at 5-min intervals. Room lights were on during testing and in order to mask extraneous sounds, continuous white noise (range 45-55 dB) was present throughout testing.

Procedure Standard operating procedure in the laboratory required that the room lights in the animal-holding area be left on 24 hr daily. This procedure is designed to limit conditioned discriminations in a variety of experimental settings which might derive from separate start times for testing. While activity levels of rats are generally reduced during periods of illumination, at least relative to night cycle activity (11), disparate patterns of responding associated with different start times during the day (which were unavoidable in this study) were likely less of an issue because of the maintenance of constant light conditions. Throughout the exposure period of the experiment all animals were individually housed. Animal body weights, as well as total food consumption, were recorded weekly. Pretest. Pretest operations began on Day 60 of exposure to the respective control or contaminated diets. In an effort to permit animals to acclimate to the test cage conditions, each animal was placed in the acrylic test cage for a period of 30 min. Because the present study was concerned about general activity rather than neophobic reactions that are known to be produced by metal exposure (20,21), it was determined that this period of adaptation was necessary for an unconfounded assessment of the effects of the treatment manipulations on the aforementioned dependent measures. Test. Formal-testing operations were conducted on Day 61 of dietary exposure. Animals were run successively and one at a time. In an effort to control for the effects of different start times, the first animal from each group was run according to the following rotation: group Control-Diet, group Lead-Diet, group Cadmium-Diet, group Lead-Cadmium-Diet. Subsequently, the second animal from each group was run, and so on until completion. Each animal was placed in the test cage and approximately 15 sec later, a switch on the monitor was turned on and the 30-min recording period began. No observers were present in the room during testing. Once six successive 5-min data printouts were generated, the animal was removed from the test apparatus and returned to the home cage. The test cage was washed thoroughly with a soap solution following each animal's test.

Chemical Analyses Twenty-four hr after testing, animals were rendered unconscious in a bell jar with methoxyflurane (metofane, PitmanMoore, Washington Crossing, NJ) and blood samples were drawn via cardiac puncture. After the animal had been sacrificed, the whole brain was harvested. The concentration of lead and cadmium in blood and brain tissues was then measured via dry ashing and atomic absorption spectrophotometry, as described in detail previously (12,16).

LEAD AND CADMIUM ANTAGONISM

101

RESULTS

TABLE 1

Overt signs of toxicity such as ataxia, tremors, seizures, and paralysis were not observed for any animal during exposure or testing.

TOTAL GROUP MEANS FOR BEHAVIORALACTIVITY

Food Intake A 4 Groups (Control-Diet, Lead-Diet, Cadmium-Diet, LeadCadmium-Diet) x 10 Weeks (1-10) repeated measures analysis of variance (ANOVA) test performed on food intake (g) failed to show a significant Groups main effect, F(3,28) = 0.13, p > 0 . 0 5 , or a significant Groups × Days interaction effect, F(27,252) = 1.21, p > 0 . 0 5 . The main effect for Days was found to be significant, F(9,252) -- 77.97, p < 0 . 0 1 . Post hoc analyses (Tukey's) of weekly intake means indicated that all groups uniformly increased their consumption of food over the course of the experiment (ps<0.01).

Body Weight A 4 Groups (Control-Diet, Lead-Diet, Cadmium-Diet, LeadCadmium-Diet)x 11 Weeks (1-11) repeated measure A N O V A performed on weekly body weights (including initial weights) showed significant main effects for Groups, F ( 3 , 2 8 ) = 4 . 1 1 , p < 0 . 0 5 , and Days, F ( 1 0 , 2 8 0 ) = 6 0 1 . 4 9 , p < 0 . 0 1 . The interaction effect was not significant, F(30,280) = 1.46, p > 0 . 0 5 . Subsequent comparisons of group means revealed that group LeadCadmium-Diet animal body weights were lower ( p < 0 . 0 5 ) than the remaining three groups, which did not differ (ps>0.05).

Activity Measures The results from the analyses of the various activity measures showed that exposure to lead produced a behavioral pattern defined by a general increase in activity. Conversely, animals exposed to cadmium showed a decrease in behavioral activation relative to control animals. Of particular interest to the present study was the finding that animals exposed to both lead and cadmium exhibited behavioral profiles that were not significantly different from controls. Table 1 provides a summary of the group differences on the various activity measures. Statistical confirmation of the differences in group means was provided by a 4 Groups (Control-Diet, Lead-Diet, Cadmium-Diet, Lead-Cadmium-Diet) x 6 Intervals (1--6, successive five-minute test intervals) repeated measures ANOVA. The results indicated that group Lead-Diet exhibited greater numbers of movements, F(3,28) = 2.86, p < 0 . 0 5 , than the remaining three groups, which did not differ on this measure. On the measure of movement time, group Lead-Diet moved longer than the remaining three groups, and group Cadmium-Diet moved for a shorter time than controls, F(3,28) = 2.97, p < 0 . 0 5 . Consistent with this pattern of results, group Lead-Diet exhibited less rest time than the remaining three groups, and group Cadmium-Diet engaged in rest more than Controls, F ( 3 , 2 8 ) = 2 . 8 7 , p < 0 , 0 5 . It is important to note that group Lead-Cadmium-Diet was not different from group ControlDiet on any of the above measures. Group separation was also evident in terms of vertical responding. On the measure of vertical activity, group Lead-Diet had a higher mean value than the remaining three groups, F ( 3 , 2 8 ) = 3.91, p < 0 . 0 1 . Groups Control-Diet, Cadmium-Diet, and LeadCadmium-Diet were not different on this measure (ps>0.05). An identical pattern of results was obtained for the number of vertical movements measure, F ( 3 , 2 8 ) = 5.35, p < 0 . 0 0 5 , where increased activity was once again evident only for group Lead-Diet.

Lead and Cadmium Concentrations in Tissues The mean concentrations of lead and cadmium residues in

Group Activity

Control

Pb

Cd

Pb + Cd

p

Horizontal activity Total distance No. of movements Movement time (sec) Rest time (see) Vertical activity No. of vertical movements Vertical time (sec) Stereotypy count No. of stereotypy Stereotypy time (sec) Clockwise revolutions Anticlockwise revolutions Margin time (sec) Center time (sec) Time spent in comers: Left-front Right-front Left-rear Right-rear

7542 4451 196" 349* 1456" 696* 86*

9336 4899 371t 440t 1367t 911t 120t

7236 3291 289* 286~ 1519~ 604* 75*

7464 3996 299* 331"~t 1474"~ 635* 83*

--0.05 0.05 0.05 0.01 0.004

326 3036 208 259 13 12

397 3963 236 301 12 12

294 3179 218 307 8 10

316 3300 206 267 14 13

-------

1394 405

1356 444

1469 330

1361 438

---

24 98 263 90

32 36 66 155

31 21 179 133

25 45 165 170

-----

Note: Row means that do not have a common symbol are significantly different.

blood and brain are presented for each of the four groups in Table 2. With respect to the analyses performed on blood, a 4 Groups (Control-Diet, Lead-Diet, Cadmium-Diet, Lead-Cadmium-Diet) one-way A N O V A test of differences in lead residue concentrations reached an acceptable level for statistical significance, F ( 3 , 2 8 ) = 355.90, p < 0 . 0 0 0 1 . Subsequent individual comparisons of group means revealed that lead residues were greater for group Lead-Diet than for group Lead-Cadmium-Diet (p<0.01), and that lead residues in both of these groups were higher than those in groups Control-Diet and Cadmium-Diet (ps<0.01). A somewhat different pattern of results obtained for cadmium concentrations in blood. The A N O V A performed on these data indicated that the significant group separation, F ( 3 , 2 8 ) = 573.03, p < 0 . 0 0 0 1 , was due to increased concentrations of cadmium

TABLE 2 CONCENTRATIONS OF LEAD AND CADMIUMIN BLOOD AND BRAIN (ppm) Lead Concentration

Cadmium Concentration

Group

(Blood)

(Brain)

(Blood)

(Brain)

Control-Diet Lead-Diet Cadmium-Diet Lead-Cadmium-Diet

0.010" 0.243t 0.017" 0.1605

0.034* 0.180t 0.036* 0.175t

0.001" 0.001" 0.040t 0.036t

0.001" 0.002* 0.047t 0.046t

Note: Column means that do not have a common symbol are significantly different.

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NATION, GROVER, BRATTON AND SAL1NAS

residues in groups Cadmium-Diet and Lead-Cadmium-Diet relative to groups Control-Diet and Lead-Diet (ps<0.01). However, groups Cadmium-Diet and Lead-Cadmium-Diet were not different with respect to concentrations of cadmium in blood (ps>0.05). Because of the relatively higher dose of lead (500 ppm) compared to cadmium (100 ppm), it is not surprising that the concentrations of metal residues in blood were lower in animals exposed to cadmium compared to those exposed to lead. In any event, it is worth mentioning that the levels of metal residues reported here are consistent with previous lead (13,14) and cadmium (12, 18, 19) studies that have recorded behavioral changes following exposure according to the regimen employed here. Regarding the analysis performed on lead residue concentrations in the brain, the analysis showed significant group differences, F(3,28) = 152.12, p<0.0001. Comparisons of group means showed that groups Lead-Diet and Lead-Cadmium-Diet, which did not differ (p>0.05), were both higher (ps<0.01) in brain lead levels than groups Control-Diet and Cadmium-Diet, which did not differ (p>0.05). Finally, the ANOVA performed on brain cadmium levels revealed a significant Groups effect, F(3,28) = 1205.78, p<0.0001). As was the case for brain lead residues, cadmium concentration in brain was higher (ps<0.01) for the two relevant exposure conditions (groups Cadmium-Diet and Lead-CadmiumDiet) than for the other groups (groups Control-Diet and LeadDiet). Groups Cadmium-Diet and Lead-Cadmium-Diet were not different in terms of brain residues of cadmium (p>0.05). DISCUSSION The results from this investigation showed that recurrent exposure to a diet containing 500 ppm added lead was associated with increased movement, decreased rest time, and increased vertical activity, relative to a control condition. Exposure to cadmium (100 ppm) resulted in decreased movement time and increased rest time compared to controls. Of particular interest was the finding from this experiment that the behavioral perturbations occasioned by lead or cadmium contamination were attenuated by joint exposure to the metals. Indeed, animals exposed to 500 ppm lead and 100 ppm cadmium were not different from control animals on any index of activity. In addition to the behavioral results, the present investigation found that combined exposure to lead and cadmium reduced residue concentrations of lead in blood, relative to the lead alone condition. However, brain tissue concentrations of lead were equivalent in animals treated with lead only and animals cotreated with lead and cadmium. Finally, there was some evidence of additivity of metal toxicity on the measure of body weight in that combined exposure to lead and cadmium produced significantly lower body weights. The behavioral findings from this study of metal-induced changes in general activity are consistent with a previous investigation that examined lead/cadmium interactions within an operant framework (17). In that experiment, performance on a FI-1 reinforcement schedule (the first lever press after a 1-min interval elapsed produced food reward) was increased by treatment with lead alone, yet this facilitation effect was not produced by the same dose of lead when it was presented in combination with cadmium. Similarly, cadmium-induced increases in schedulecontrolled responding that occurred when cadmium was presented alone were not evident when cadmium and lead were presented together. An interesting difference between the two studies is that although cadmium antagonized lead-related increases in performance in both experiments, lead antagonized cadmium-induced increases in behavior [Nation et al., experiment (17)] and decreases in behavior (the present experiment). Evidently, behav-

ioral antagonism between the two metals occurs apart from the directionality of changes associated with chemical contamination. A point to be made at this juncture is that the behavioral antagonism observed here and in the previous operant investigation of lead/cadmium cotreatment (17) may be peculiar to these particular end points. Other response systems may reveal a quite different pattern of results. For instance, an unpublished study reported in a review of the neurobehavioral toxicity of lead and cadmium (22) failed to find additivity of lead and cadmium in an investigation of visual discrimination learning. Specifically, the degree of performance impairment on a modified Lashley task was essentially the same when lead and cadmium were administered alone or in combination. However, no evidence of antagonism was found in this experiment that perinatally presented lead only, cadmium only, or lead plus cadmium at doses much greater than those used here. Diverse experimental conditions make it difficult to compare across studies, of course, but such findings may prove instructive with respect to defining the range of conditions under which behavioral antagonism is likely to occur. Along other lines, the biochemical findings from this study are important in terms of determining the mechanisms for antagonism between lead and cadmium, and this may be true with respect to both behavioral issues and transmitter availability. As previously noted, it is possible that the source of competition between lead and cadmium lies outside the central nervous system. One candidate for consideration in this regard is the differential intestinal absorption of the metals when they are presented together or in isolation. Should one toxicant restrict the movement of the other across the intestinal wall, ultimately, systemic distribution would be affected and tissue burdens would be reduced by cotreatment, at least when compared to the situation where either metal is presented alone. Accordingly, prior records of the antagonistic effects between lead and cadmium on brain neurotransmission (17) may derive from a functional decrease in the presence of the metals in the cotreatment condition, rather than the competitive action of the two chemicals at neural membrane sites. Because the presence of cadmium in the cotreatment condition did reduce blood lead concentrations relative to the lead alone condition, there would appear to be prima facie support for the notion of gastrointestinal-based antagonism. However, when one considers that brain concentrations were increased uniformly for group Lead-Diet and Lead-Cadmium-Diet, the peripheral rationale seems less convincing. That is, although it is apparent that cadmium either interferes with the absorption of lead or increases clearance of lead from blood, the consequences are negligible with respect to residue accumulations in brain tissue. Perhaps the essentially equivalent accrual of brain lead in groups Lead-Diet and Lead-Cadmium-Diet derives from restricted uptake of the metal into brain tissue. That is, it is possible that blood lead concentrations above some undefined level fail to translate into further increases in brain residues because some maximum absorption point is reached. If so, then this maximum absorption point must have been reached by both Lead-Diet and Lead-CadmiumDiet animals. In any event, it is clear that whether lead is presented alone or in combination with cadmium, recurrent exposure ensures that substantial deposits of the metal will be sequestered in neural tissues. And one must further consider that there was no evidence here that lead reduced cadmium residues in blood or brain. Yet, lead does reverse the toxic effects of cadmium on behavior and neurotransmitter (DA and 5-HT) activity (17). Collectively, these data suggest more attention should be given to central (neural) mechanisms for lead/cadmium antagonism. Considering the complexity of the mammalian nervous system, and the multidimensional nature of most toxic disturbances provoked by heavy metal poisoning (9), determining the precise neural substrate for the antagonistic effects observed here and

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103

elsewhere is likely to prove problematic. Nonetheless, electrophysiological studies are beginning to provide a few clues about the effects of neurotoxicants such as lead and cadmium on extracellular and intracellular events that are involved in synaptic transmission [see Atchison (2) for a recent review]. A number of studies have shown that lead (3,8) and cadmium (7,8) suppress evoked transmitter release. One account of these effects is that the metals may block the influx of calcium, through membrane channels, into the nerve terminal following the action potential (2,10). Inasmuch as the elevation of free cytoplasmic calcium leads to a discharge of transmitter out of the nerve terminal (4), it is not surprising that decreased calcium influx caused by lead or cadmium would be associated with suppressed transmitter release. Nor would it be unexpected that the two metals might additively combine to produce even greater suppression of evoked transmission, a finding that has been reported (8). How, then, do we account for the antagonism between lead and cadmium? The answer may rest with intracellular events and spontaneous transmitter release. Although depolarized-induced (evoked) entry of calcium into the nerve terminal is quite rapid, calcium extrusion is relatively slow (5). Consequently, effective buffering mechanisms within the terminal must take up the excess calcium and slowly leak it back to the terminal cytoplasm at a level below that which is likely to cause exocytotic transmitter release. It is now suspected that lead acts either to disrupt this buffering of intracellular calcium or causes its discharge from storage sites (e.g., mitochrondria, smooth endoplasmic reticulum), thus increasing free cytoplasmic calcium and spontaneous transmitter release. Such interactive processes would explain lead-induced increases in brain DA and 5-HT activity (17) and perhaps corresponding increases in behavior (1, 15, 17). To understand the antagonistic role played by cadmium, one need only to recount that lead and cadmium compete with calcium at the channel site, and to realize

that lead and cadmium also compete with each other for entry through terminal membrane channels (6,7). Given that intracellular lead causes an increase in free cytoplasmic calcium, and ultimately an increase in neurotransmission via the aforementioned pathways, it follows that insofar as cadmium reduces lead influx, it should antagonize lead-induced changes in transmitter activity, as well as behavioral systems controlled by such transmitters. While such a rationale seems plausible, it should not go unnoticed that the majority of the reports cited above derive from in vitro preparations; thus, caution must be exercised in speculating about the in vivo case. Still, this line of reasoning does offer some direction for future investigations. Certainly the aforementioned neurochemical account of lead/cadmium interactions is consistent with what has been observed in this study and a previous investigation conducted in this laboratory (17). Yet, the account falls short as a comprehensive interpretive scheme, for it fails to address the antagonism of lead by cadmium. Future experiments on lead/cadmium interactions designed to clarify the mechanisms of neurobehavioral antagonisms are obviously needed. Finally, it should be recognized that while there was evidence of behavioral antagonism in this study, disturbances in body weight associated with the two metals were additive. That is, only in the case of cotreatment did lead and cadmium result in significantly lower body weights relative to controls. Such a finding, along with the fact that residue accumulations in tissues may be similar in insolated and joint exposure conditions, underscores the potential toxic impact of combined exposure to these toxicants. Even though the behavioral effects of one metal may be masked by the other, changes in histochemistry, cytotoxicity, etc., may occur commensurate with the total tissue burden. Thus, risk effects associated with cocontamination may ultimately be greater than would be the case should lead only or cadmium only poisoning occur.

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