significance of hippocampal dysfunction in low level lead exposure of rats

Neurotoxicology and Teratology, Vol. 10, pp. 245-253. © Pergamon Press plc, 1988. Printed in the U.S.A. 0892-0362/88 $3.00 • .oo

Significance of Hippocampal Dysfunction in Low Level Lead Exposure of Rats

C A R M E N M U N O Z , K U R T G A R B E , H E L L M U T H L I L I E N T H A L A N D G E R H A R D W l N N E K E 1

Medical Institute for Environmental Hygiene at the University o f Diisseldorf A u f m Hennekamp 50, D-4000 Diisseldorf 1, F .R.G.

Rece ived 18 March 1986

MUNOZ, C., K. GARBE, H. LILIENTHAL AND G. WlNNEKE. Significance ofhippocampal dysfunction in low level lead exposure of rats. NEUROTOXICOL TERATOL 10(3)245-253, 1988.--Previous reports have suggested a relationship between the neurotoxicity of lead and hippocampal dysfunction. Therefore, a comparison between the behavioral changes induced by lead exposure and by selective destruction of hippocampal neurons should help to clarify whether the intrinsic neurons of the hippocampus are directly influenced by lead. Rats maternally and permanently exposed to lead (750 ppm in the diet as lead acetate) were tested in a radial arm maze and compared with controls and rats with ibotenic acid-induced neuronal depletion in the dorsal hippocampus. Lead-exposed groups showed an impairment in the acquisition performance of the spatial task while hippocampally damaged animals did not. When they were retested 4 weeks after the end of the original acquisition, both groups of lead-exposed and ibotenic acid-treated rats showed a significant deficit in retention. These results suggest that this deficit produced by lead can be due to the damage of the hippocampal neurons but not the impairment observed in the acquisition. We propose that the neurotoxicity of lead is not entirely due to the dysfunction of the dorsal hippocampus and that other areas of the brain should be considered. Both maternally and permanently lead- exposed rats showed a similar degree of deficit in acquisition and retention, suggesting a long-lasting effect of early lead exposure.

Lead Hippocampus Ibotenic acid Radial arm maze Acquisition Retention

THE behavioral changes produced by early lead (Pb) exposure have been related to the limbic system dysfunction (e.g., [11, 13, 14, 22]) and several lines of evidence have pointed to the hippocampus as the specific target of lead. Summarizing the results found in several laboratories, first of all, it has been reported that lead accumulates preferen- tially in the hippocampus [14], being likely stored in the mossy fiber pathway [12]. Secondly the width and the length of the hippocampus and the length of the dentate gyrus and mossy fiber pathway have been found to be reduced in lead- exposed animals [1] as well as the dendritic branching and axonal development of the dentate cells [3]. In addition, it has also been reported that in infant rats, low levels of lead exposure reduced or retarded the development of the neuropil within the hilus of the dentate gyrus and reduced synaptogenesis in the mossy fiber zone [9]. In the adult rat, lead exposure increased the area number of glial cells of the stratum pyramidale and increased the numerical and areal density of the profiles of the mossy fiber buttons [8]. Fur- thermore, the maturation of the dentate gyrus occurs largely postnatally [38], usually when the animals are being exposed to lead; this makes it a particularly sensitive area for the neurotoxicological effects of the metal. A relationship between lead and the cholinergic system has also been suggested: neurochemical studies indicate a reduction of acetylcholine turnover in the hippocampus [41] and a reduced degree in hippocampal cholinergic plasticity following

1Requests for reprints should be addressed to Gerhard Winneke.

lead exposure [4]. Correlated with these data are behavioral changes after lead exposure, which show similarities with those behavioral effects seen after hippocampal damage [35].

The present investigation was designed to explore further the potential relationship between lead exposure and the hippocampal system. For that purpose, we have compared the behavior of rats with early low level lead exposure to rats which have suffered neuronal depletion in the dorsal hippocampus with ibotenic acid (IBO). This neurotoxin, when injected into a brain region, destroys intrinsic neurons but spares fibers of passage and afferents to the injection area [39]. The behavioral tests used were the acquisition and retention of a spatial task in a radial arm maze. The performance in this task is very sensitive to hippocampal destruction [29,30] and it involves elements of learning and memory, functions both reported to be affected by exposure to lead [6]. The results might indicate whether the neurons of the dorsal hippocampus, their intrinsic connections and their efferents are directly involved in the neurotoxicity of lead. By using two protocols of lead exposure, maternal and permanent, we have examined whether the behavioral effects of early lead exposure are long-lasting.

METHOD

Lead Exposure Protocol

The protocol of exposure to lead and the formation of

245

246 MUNOZ, GARBE, L I L I E N T H A L AND W l N N E K E

TABLE 1 LEAD EXPOSURE PROTOCOL AND FORMATION OF GROUPS

/ Females + Males J

/ (750 ppm Pb in (standard diet) food for 50 days)

/

Females + Males

/ (standard diet) (standard diet)

Postweaning exposure (750 ppm Pb in diet)

Postweaning standard diet

Control } Sham operated

Ibotenic acid injection

= 750group

= 750 Co group

= Control group

= IBO group

groups are schematized in Table I. Female adult Wistar rats were fed standard fortified laboratory diet (Altromin 1310; 0.32 mg Pb/kg) or the same diet containing 750 ppm of Pb as Pb-acetate (409,7 mg Pb/kg). In previous feeding studies in our laboratory, where the Pb concentration in the diet was controlled, a maximal deviation of 9.2 mg Pb/kg was found in excess or defect to the commercially reported value. After 50 days of feeding these diets, the animals were mated. Preg- nant animals were isolated and their offspring were weaned 16 days after birth (PN 16). The Pb-exposed mothers had continued the exposure to lead during mating, pregnancy and lactation periods. At weaning (PN 16) half of the litter of the Pb-exposed mothers received 750 ppm Pb in the diet (750 group, permanent lead exposure) until the beginning of the behavioral tests (PN 180) and the other half were fed standard laboratory diet without Pb addition (750 Co group, maternal lead exposure). For the experimental study the female offspring were used. The animals were randomly taken from 29 control litters and 29 litters from the lead-exposed mothers. All the rats drank tap water.

Surgery and Histology

Twenty rats from the control offspring were anesthetized with intraperitoneal (IP) injection of 45 mg/kg of ketamine hydrochloride (Ketanest, Parke Davis, 50 mg/ml) at PN 165. Ibotenic acid (Sigma Chemical Co., St. Louis, MO) was stereotaxically injected into the dorsal hippocampus through a 5/zl Hamilton syringe. The coordinates of the injection site were AP=4.0, L = --- 1.6, H = +2.5 according to the Pellegrino et al. atlas of the rat brain [34] in relation to the interaural line. The amount of IBO used was 10/zg in 0.5/zl phosphate buffer, pH=7.4 , in each hemisphere (IBO group). Ten sham operated controls received the same treatment without IBO injection. Nine additional unoperated controls were also used. There were no significant differences in any of the behavioral measures between the operated and unoperated control animals. The control data were therefore pooled (Co group).

After completion of the behavioral experiments, all the animals from the IBO group and some from the lead-exposed groups were killed with a lethal dose of pentobarbital and perfused transcardially with isotonic saline followed by 10% formol saline. After fixation, 50/zm frozen sections were cut through the entire brain. Every third section was stained with cresyl violet and examined by light microscopy to de- termine the extent of cell loss in the hippocampus and possibly occurring incidental damage to other areas of the brain.

Blood and Brain Lead Analysis

Blood samples were taken by heart puncture from a different group of same treated male and female pups at weaning (PN 16), and male rats at the age of about PN 110, 250 and 500. No differences in blood lead levels have been reported between sexes in rats and monkeys [16]. The blood lead levels were determined by electrothermal atomic absorption spectrophotometry according to the method of Stoeppler et al. [42] as described previously [7].

Lead was measured in homogenates of the whole brain. It was determined by electrothermal AAS after tissue digestion with an organic tissue solubilizer (Lumaton, Fa. KOrner Rosenheim, F.R.G.) . The procedure applied is a slightly modified version of the method described by Julshamm and Andersen [20].

Radial Maze Acquisition: Apparatus and Procedure

The apparatus was an automated 8 arm radial maze modified from the one described by Walsh et al. [43]. The maze consisted of a circular central arena (31 cm diameter) from which eight equally spaced alleys (30×7×7 cm) radiated. At the end of each arm was a transparent swinging door and behind it a 45 mg Noyes pellet. The time of each alley entry was detected by a Foto-Darlington transistor; there were 4 pairs of them in each alley, the first pair being placed directly at the entrance. The opening of the door to get the pellet was detected and registered by a microprocessor. The apparatus was made of colored transparent Plexiglas and was placed in a well illuminated room.

At the 15th day after the IBO and sham operations, all the animals from the five different groups were placed in a food deprivation schedule which reduced them to 85% of their free-feeding body weight. They were then placed in the maze without pellet reward and were allowed to explore it for 10 min. After this pretraining day, they received a daily session in the maze for 15 consecutive days. During each session, an animal was placed in the center of the maze enclosed in a 30 cm diameter cylinder made of opaque Plexiglas. Fifteen sec- onds later, the cylinder was removed and the program started. The rat had free access to the arms at which end was a nonreplaceable pellet. The session ended when all the reward were taken or 10 min had elapsed. A correct choice was defined when the animal entered an arm and took the reward, and an error when the rat entered an arm previously choosen. Using a microcomputer Rockwell AIM 65, the following parameters were measured and recorded: number and sequence of arms entered, time needed to enter an arm

HIPPOCAMPAL DYSFUNCTION AND LEAD EXPOSURE 247

TABLE 2A BLOOD AND BRAIN LEAD LEVELS AS WELL AS BRAIN AND BODY WEIGHT OF PUPS (PN 16) FROM MOTHERS EXPOSED TO STANDARD DIET (CONTROL) AND FROM MOTHERS EXPOSED TO THE SAME DIET WITH 750 ppm Pb ADDITION (750)

Lead Exposed Control (750 ppm)

Males Females Males Females N 9 8 5 4

/zg Pb/100 1 I 17.3 -+ 3.3 17.3 -+3.0 ml Blood

/~g Pb/100 1 1 7.3 -+ 2.7 7.3 -+ 1.3 g Brain

Brain Weight (g) t .17_+0.02 1.19_+0.09 1.20 -+ 0 . 1 3 1.20-+0.07 Body Weight (g) 27.9 -+2.3 27.0 -+3.6 34.8 -+ 1.5 34.3 -+2.5

Mean-+95% CL are given. (These values were collected in a separate feeding study run under the same

experimental conditions.)

and total time employed in making the 8 correct choices. When the animal left the arm where the last reward had been taken, the program stopped. The head of the rat inside the alley was considered as criterion of entrance. All the animals were fed a limited amount of control diet after each session.

At the end of the 15 days, the rats were kept in their home cages with control diet ad lib. Four weeks after the last day of training, the animals were reduced again to 85% of their free-feeding body weight and were retested in the maze for three consecutive days (retention test). The characteristics of the sessions and parameters measured were identical as those in the original acquisition. The behavioral tests were carded out in a completely blind fashion.

Statistical Evaluation

The data were analyzed by analysis of variance using the SAS package, or the Mann-Whitney U test. Post hoc comparisons were made following significant overall F tests.

RESULTS

Histology

The histological data from the 20 rats of the IBO group showed similar characteristics at the light microscope, and indicated complete bilateral depletion of neurons (granular and pyramidal) in the dorsal hippocampus (Fig. 1). The depletion extended from the most anterior part of the hippocampal formation (AP=5 IAL) caudally until AP= 1.6-1.4 IAL [34]. The ventral hippocampus, fimbria fornix and subiculum appeared intact. Careful study of the histological material failed to reveal neuronal depletion, detectable by light microscopy, in any other region of the brain. The damage to the thalamus adjacent to the injection site, if any, was minimal. No visible damage in any area of the brain, includ-

TABLE 2B BLOOD LEAD LEVELS OF CONTROL RATS (Co), RATS WITH

PERMANENT (750) AND MATERNAL (750 Co) LEAD EXPOSURE AT DIFFERENT POSTNATAL DAYS (PN)

PN

Groups 1 l0 Days 250 Days 500 Days

Control 1 1.5-0.2 1.3-+0.2 750 34.0-+3.4 39.0-+4.6 32.0-+5.0 750 Co 1 - - - -

--=Not measured.

ing the hippocampus, could be detected in lead-exposed animals.

General Observations and Tissue Lead Levels

The average size and weight of the litters of the Pb- exposed mothers (mean=9.8 and 52.5 g respectively) was not significantly different from those of the nonexposed mothers (mean=9.8 and 54.0 g). No significant differences were observed in the body weight of the pups at weaning (PN 16), although there was a tendency for the Pb-exposed animals to have increased values (Table 2A). The blood lead values of the Pb-exposed animals were about half of those of adult animals between ages 100 and 500 (Table 2B); this suggests reduced lead transfer from dams to offspring during lactation, and corresponds to findings of Mc Cauley et al. [25], who demonstrated decreasing Pb from birth to weaning. No differences were observed between sexes, neither for blood nor for brain lead levels. The blood-brain ratio is clearly higher in neonates as compared to adults. None of the

FOLLOWING PAGES

FIG. 1. Photomicrographs of cresyl violet stained 50/zm sections of the dorsal hippocampus of (A) a control rat, (B) a rat which received a bilateral injection of ibotenic acid into the dorsal hippocampus. The top row illustrates the dorsal hippocampus at low (2.3x) magnification and the second row, a 23x magnification of the granule cells of the dentate gyrus.

248 MUNOZ, GARBE, LILIENTHAL AND WINNEKE

250 MUNOZ, GARBE, LILIENTHAL AND Wl NNEKE

TABLE 3 ORIGINAL ACQUISITION IN THE RADIAL ARM M A Z E

NO. Correct

No. Diff. Choices No. Rats No. Days No. Days Arms in Before Reaching to Reach Missing the First No. Arms/ First

Group N Criterion Criterion Rewards 8 Choices Errors min Error

Co 19 19 7.1___1.3 1.9-+0.8 5.9---0.3 6 . 9 - + 0 . 9 5.7-+0.8t 5.2---0.3 IBO 20 19 7.0+1.2 1.6-+0.7 5.9-+0.1 6 . 4 - + 0 . 8 7.4+_1.0 * 5.1_+0.3 750 Co 10 6 10.7-3.1"t 3.7-1.9*t 5 . 5 - + 0 . 6 7 .1 - - -1 .7 4.9-+0.9"~ " 4.9-+0.4 750 10 8 10.2--- 2.6"t 4.0--- 2.4"t 5.5-+0.5 7.2-+ 1.2 4.4-+ 1. l*t 5.1 _+0.5

*p<0.05, significantly different from control values; tp<0.05, significantly different from IBO values. Performance by Control rats (Co), rats with permanent (750) and maternal (750 Co) lead exposure and rats with selective

neuronal depletion in the dorsal hippocampus (IBO). The values reported are means_+95% confidence interval.

animals from the Pb groups (permanent or maternal exposure) showed any overt sign of overreactivity when handled. ~"

Lead levels were below 3/zg/100 g brain for control and maternally-exposed animals (750 Co) at age 100 days. For the permanently lead-exposed group (750) the average value was 28---2.2/xg/100 g brain at age 100 days. Table 2B repre- ~" ~ - sents the Pb values in blood at different postnatal days. No significant differences between values were observed within the same group at different ages.

The ibotenic acid injection did not produce any visible ~ 300- debilitation, hyperexcitability or seizures. A small loss of o ~ body weight was observed during the first two days follow- ® ing the operation, which soon recovered. At the beginning of the behavioral tests, the body weight of the

200- a n i m a l s was constant and did not significantly differ among the groups. The means_S.E.M, expressed in grams were: Co=208.9_ 15.4, IBO=206.7 + 11.4, 750=212.6_+9.7 and 750 ~- Co=207.2---8.4. ~

100-

Behavioral Results in the Radial Maze

Acquisition. The performance of the rats in the maze is represented in Table 3. No significant differences between the groups in the number of arms visited before the first error or in the number of errors per session were observed. There was a tendency from Co and IBO groups to make fewer errors in the first 8 choices than the Pb-exposed groups, but it was statistically not significant.

Using as criterion of acquisition to make 8 correct choices on five consecutive days, the number of days needed to reach it was significantly (p<0.05, Mann-Whitney two tailed) increased in both Pb groups as compared to Co and IBO. From the control group all rats reached the criterion and from the IBO, 19 out of 20 did. Only 6 animals from the 750 Co and 8 from the 750 groups reached the criterion of acquisition. The number of days missing rewards, i.e., the days in which the animals did not complete the 8 correct choices during the 15 sessions, was also significantly higher (p<0.05, Mann-Whitney two tailed) in both Pb groups as compared to Co and IBO. The exploratory activity, measured as the number of arms visited per min during the pretraining session when no food was available in the maze, was significantly (p<0.05, Mann-Whitney two tailed) higher in the IBO than in the rest of the groups (mean_+95% confidence interval: Co=4.0__+0.5; IBO---5.0---0.5; 750 Co_3.9_0 .4 ;

+

I I I I

Co

7 1

750 Co 750

maternal I~'manent Wo-Exposure Po-Exposure

FIG. 2. Original acquisition in the radial arm maze. Mean (_+95% confidence interval) of the time needed by the different groups to make 8 correct choices per day. Co=control group (operated and unoperated), 750=permanent Pb exposure, 750 Co=maternal Pb exposure, IBO=rats with neuronal depletion in the dorsal hippocampus. *p<0.05, significantly different from control; +p<0.06, significantly different from IBO.

750=4.1-+0.4 arms per min). During the acquisition phase of the experiment the activity level of the IBO group remained elevated in comparison to all other groups (see Table 3). The activity levels of both lead-treated groups were significantly lower than the control level.

During the 15 days of training there were significant, F(3,55)=5.65, p<0.01, main effects as revealed by ANOVA with repeated measures. Post hoc comparisons showed that the animals with neuronal depletion in the dorsal hippocampus took significantly (p<0.05, Tukey) less time to make 8 correct choices daffy than controls and both groups of lead- exposed animals (Fig. 2).


TABLE 4 RETENTION IN THE RADIAL ARM MAZE

NO. No. Rats No. Diff. Correct Making Arms Choices

8 Correct No. Days in the Before Choices Missing First No. Arms/ First

Group N Dally Rewards 8 Choices Errors rain Error

Co 19 13 0.4-+0.3 5.4-+0.7 7.9-+2.3 3 .9_+0 .8 4.5-+0.6 IBO 19 9 0.9-+0.4 * 5.2-+0.6 10 .6-+2 .3 3 . 2 - + 1 . 5 4.8-+0.6 750 Co 6 3 1.0-+0.9" 5.7-+0.7 7.3-+1.5 3 . 0 - + 1 . 4 4.7-+1.3 750 8 2* 1.4-+0.7 5.6-+0.6 9.3-+2.9 4 .2_+2 .9 4.7-+0.9

*p<0.05, significantly different from Co values. Performance by Control rats (Co), rats with permanent (750) and maternal (750 Co) lead exposure and

rats with selective neuronal depletion in the dorsal hippocampus (IBO). The values reported are means_+95% confidence interval.

3!

2 - (D

i--

re-

1-

~t

7s° 750 Co

l

IB0

FIG. 3. Representation of the mean (-+95% confidence interval) time needed to make 8 correct choices per session in the retention test divided by the time needed in the original acquisition (ratio). *p<0.05, **p<0.002, significantly different from control.

Retention. Only those rats which reached the criterion of acquisition in the original task were retested in the maze four weeks after the end of the training. The percentage of control animals which completed the task (making 8 correct choices daily) was greater than that of IBO or Pb-exposed groups during the three days of the retest. This difference was only significant (/7<0.05 Chi 2) for the 750 group. The number of days where the task was not completed was significantly higher in IBO and Pb-exposed groups than in controls (p<0.05 Mann-Whitney one tailed for IBO and 750 Co; p<0.05 Mann-Whitney two tailed for 750). No significant differences were found in the number of different arms visited in the first 8 choices, number of errors or number of choices before the first error (Table 4).

When the time needed to make 8 correct choices during the three days was measured, there were no significant differences between the groups (mean time _+95% confidence interval in sec: Co=313.6_+35.1, IBO=364.4_+38.0, 750

Co=380.2-+75.9 and 750=440.0_+64.9. However, when this time was related to the time needed in the original acquisition, i.e., the behavioral baseline of each animal prior to the retention period, the hippocampally damaged and both groups of lead-exposed rats showed a significant (p<0.002 and p <0.05 respectively, Mann-Whitney) deficit in comparison to controls (Fig. 3). No significant differences were found between the Pb and IBO groups. The activity (measured as mean of arms visited per min) was slightly, but not significantly, higher in the IBO animals than in the rest of the groups (Table 4).

DISCUSSION

The results show that early lead exposure produces an impairment in the performance of a radial arm maze during the acquisition phase of the task, while neuronal depletion in the dorsal hippocampus does not. This impairment is re- flected by the fact that the lead maternal- and permanent- exposed animals (a) required more days to reach the criterion of acquisition and (b) they failed to perform consistently correct the eight daily choices in comparison to control and IBO animals. These performance deficits cannot be ex- plained in terms of malnutrition because there were no group differences as far as body weight was concerned. The lead- exposed animals did not advance in the arms enough to reach the reward area and had decreased activity levels compared to controls and IBO animals. Thus, it appears that the nature of the deficit lies in the emotional dimension rather than in the ability to use the specific cues to solve the task, since errors are not increased.

The observed impairment in the acquisition of the maze problem by the lead-exposed animals agrees with previous reports [2] using a method and performance criterion similar to ours, but much higher levels of lead exposure. No significant differences in the number of arms visited before the first error, number of errors or number of correct choices in the first 8 arms visited were found with controls or IBO. Al- though this finding agrees again with previous reports [2], the surprisingly low number of correct choices made by controls in the first eight attempts in our experiment (mean=5.9), most likely due to our stringent criterion of entrance, makes the interpretation of this particular result difficult. We could

252 MUNOZ, GARBE, LILIENTHAL AND WINNEKE

suggest that early lead exposure does not resemble the deficit in working memory or spatial mapping found after mechanical lesions in the hippocampus [28,32]. A study with a criterion of entrance more comparable with previous litera- ture [31] would be desirable.

Ibotenic acid-induced neuronal depletion in the dorsal hippocampus does not seem to produce the deficit in spatial behavior found in rats with mechanical damage in the hippocampal system. This finding agrees with previous reports [ 17-19] in which no impairment of spatial performance in rats with selective lesions in the field CA3, dentate gyrus or IBO-induced depletions in the dorsal hippocampus, was found. Our hippocampally damaged animals showed an increased exploratory activity when compared with control and lead groups, as previously reported for animals with mechanical or chemically-induced hippocampal lesions [21,26] and higher activity levels in the acquisition phase. All of the other performance measures were not different from controls. The performance deficits found in the Pb-exposed animals cannot then be due to a dysfunction of the neurons of the dorsal hippocampus.

The fornix has been considered to be the main cholinergic input to the hippocampus [24]. Acetylcholine has been impli- cated in learning and memory processes [5,15] and most concretely in spatial memory ([27,37], among others). There- fore, the spatial deficits in the maze reported after fornix lesions [19,30] may account for the spatial deficits found after hippocampal mechanical destruction, and can explain why the IBO-treated animals do not show these deficits. The only portion of the fornix destroyed would then be the con- tribution of the CA3 field, which is small when compared to the septo-hippocampal projection and the efferents from the subiculum, both spared in the ibotenic acid lesions.

In the retention phase of the problem, we can observe a significant decrease in the number of permanent lead- exposed animals which complete the 8 correct choices in the

daily sessions when compared with controls. Maternal- exposed and rats with neuronal depletion in the dorsal hippocampus show also a strong similar tendency to make fewer correct choices. One should bear in mind that only the animals which attained the criterion of acquisition were given the retention test. There is an increase in the number of days in which the hippocampally damaged and lead-exposed rats did not complete successfully the 8 choices.

These impairments from IBO and lead-exposed groups get emphasized when studying their daily performance in the maze. They needed more time in solving the problem than controls, when compared with the time needed during the acquisition phase. This performance deficit cannot be attributed to motor or activity alterations because the activity on the maze was not different between the groups.

We have found similarities in the performance of the retention of a radial arm maze task between early lead-exposed rats and rats with neuronal damage in the dorsal hippocampus, but not in the performance during the acquisition of the task. We therefore propose that the neurotoxicity of lead cannot exclusively be attributed to the dysfunction of the dorsal hippocampus. It should be remembered, however, that the lesions were carried out in adult animals, whereas deficits due to lead exposure can be attributed to the pre- and early postnatal period. The fact that we found no behavioral differences between rats with permanent and maternal lead exposure suggests that the changes produced by the early lead exposure are long-lasting. Lead has been found in the amygdala [ 13] and has been reported to produce pathological changes in the cerebellum [10] and permanent alterations in the cerebellar Purkinje neuron firing rates [33]. Also the thickness of the cerebral cortex has been found to be reduced with lead exposure [23,36]. Future investigations should include an examination of these brain structures to get more insight about the areas which might be responsible for the neurotoxicity of lead.

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significance of hippocampal dysfunction in low level lead exposure of rats

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