Drug discrimination learning in lead-exposed rats

Measurement of the lowest dosage of phenobarbital that can produce drug discrimination in rats

RATIONALE

Accurate measurement of the threshold dosage of phenobarbital that can produce drug discrimination (DD) may improve our understanding of the mechanisms and properties of such discrimination.

OBJECTIVES

This study aimed to compare three methods for determining the threshold dosage for phenobarbital (D) versus no-drug (N) DD.

MATERIALS AND METHODS

Rats learned a D versus N DD in two-lever operant training chambers. A titration scheme was employed to increase or decrease dosage at the end of each 18-day block of sessions depending on whether the rat had achieved criterion accuracy during the sessions just completed. Three criterion rules were employed, all based on average percent drug lever responses during initial links of the last six D and six N sessions of a block. The criteria were: D% > 66 and N% < 33; D% > 50, and N% < 50; (D% - N%) > 33. Two squads of rats were trained, one immediately after the other.

RESULTS

All rats discriminated drug versus no drug. In most rats, dosage decreased to low levels and then oscillated near the minimum level required to maintain criterion performance. The lowest discriminated dosage significantly differed under the three criterion rules. The squad that was trained second may have benefited by partially duplicating the lever choices of the previous squad.

CONCLUSIONS

The lowest discriminated dosage is influenced by the criterion of discriminative control that is employed and is higher than the absolute threshold at which discrimination entirely disappears. Threshold estimations closer to absolute threshold can be obtained when criteria are employed that are more permissive of errors and that allow rats to maintain lever preferences.

Minimal behavioral effects from moderate postnatal lead treatment in rats

Developmental lead exposure continues to be a worldwide problem. This study investigated the behavioral effects resulting from developmental lead treatment in rats with corresponding physiological measures of lead exposure. Sprague-Dawley rats were treated with 350 ppm lead acetate from birth to weaning via the dam's drinking water. Behavioral measures assessed in the offspring included residential activity tests, complex maze performance, acoustic startle response, emergence behavior (light/dark preference), prepulse inhibition, and ethological assessments of play, dominance, and burrowing. Pb blood levels averaged 53 microg/dl in the dam at the time of offspring weaning and 46 microg/dl in weanling female offspring. Pb levels averaged 277 ng/g and 32 microg/g in the brain and bone, respectively, of female offspring at weaning. No behavioral assessment indicated any lead-related functional alterations nor were there any statistically significant differences when the lead-treated group was restricted to rats in those litters that were above the median Pb blood lead level at weaning. These results indicate that any lead-related functional alterations at this dose may be subtle and require a sufficient demand on the system for detection.

Lead-induced changes in NMDA receptor complex binding: correlations with learning accuracy and with sensitivity to learning impairments caused by MK-801 and NMDA administration

This study sought to further evaluate potential mechanistic relationships between Pb-induced alterations in glutamate neurotransmission and behavioral toxicity. It examined correlations between Pb-induced changes in [3H]MK-801 and [3H]CGP-39653 binding sites in 4 different brain regions (frontal cortex, dentate gyrus, CA1 and striatum) and (1) changes in learning accuracy on a multiple repeated acquisition and performance schedule, and (2) sensitivity to the accuracy-impairing effects of MK-801 and NMDA on this learning baseline. All data were obtained from a single population of rats that had been chronically exposed from weaning to 0, 50 or 250 ppm Pb acetate in drinking water and demonstrated selective learning impairments and altered sensitivity to the effects of MK-801 and NMDA on learning accuracy. Pb exposure decreased MK-801 binding and possibly increased CGP-39653 binding, effects statistically significant in some brain regions, but generally exhibiting similar trends across regions. At 0 ppm, higher levels, particularly of MK-801 binding, were associated with higher accuracy levels in the learning paradigm and with greater decrements in learning accuracy following MK-801 or NMDA administration. These linear correlations were negated and in some cases even reversed by 50 and 250 ppm Pb, an effect that might be attributable to an alteration of NMDA receptor complex subunit composition and thus, ligand binding. Of the 4 brain regions examined, striatal MK-801 binding proved to be the best predictor of learning accuracy levels. These data provide additional support for an involvement of the NMDA receptor complex in Pb-induced learning impairments. The fact that these effects were noted most frequently in striatum also raises the possibility that dopamine-glutamatergic interactions contribute to Pb's effects.

The neurobehavioural toxicology and teratology of lead

When comparing neurobehavioural observations from occupational lead-exposure of adults on the one hand, and environmental lead exposure of children on the other, it appears that the developing relative to the mature brain is more at risk. Neurobehavioural toxicity in occupational lead-exposure has typically not been observed at blood lead-concentrations (PbBs) below 400 micrograms/l, whereas ih environmentally exposed children such deficit has been reported to occur down to PbB of 100-150 micrograms/l and, perhaps, even below this range. Both cross-sectional and prospective studies have arrived at similar conclusions in this respect. The preferred endpoint in most such studies has been the IQ-measure, which has good psychometric qualities, is sufficiently well standardized to be comparable across studies, and exhibits attractive simplicity for the regulator in a public health context. At the same time, however, this IQ-focus has also interfered with systematic efforts to identify more specific lead-induced functional deficits by means of more detailed neurobehavioural analyses (Bellinger 1995). Metanalyses on both cross sectional and prospective studies in lead-exposed children have concluded that a typical doubling of PbB from 100 to 200 micrograms/l is associated with an average IQ-loss of 1-3 points (Pocock et al. 1994; WHO 1995), and no threshold has as yet been identified. Since, however, cause-effect contingencies necessarily remain doubtful in epidemiological studies if the observed effects are as subtle as these, experimental studies in animals have become helpful in supporting the causative role of lead to produce neurobehavioural deficit at steady-state PbB down to about 150 to 200 micrograms/l. Such deficit has been demonstrated by means of a variety of learning/memory models with positive and negative reinforcement contingencies in the rat--and in primates as well. It has also been shown in such studies that neurobehavioural deficit subsequent to early developmental exposure extends long into adulthood after cessation of exposure at weaning. It, therefore, appears that the neurobehavioural teratology of lead has more convincingly been demonstrated in animal models than in human exposure conditions, so far. A coherent theory to explain the particular vulnerability to lead of the developing brain is still lacking. Recent data do suggest, however, that Pb-induced disruption of calcium homeostasis in the immature brain might interfere with normal brain development.

Lead effects on the brain stem auditory evoked potential in monkeys during and after the treatment phase

Rhesus monkeys were pre- and postnatally exposed to either 0, 350, or 600 mg lead acetate/kg diet continuously until the age of about 9.75 years. At the age of 8-8.25 years (Experiment 1) and 9.25-9.5 years (Experiment 2) brain stem auditory evoked potentials (BAEPs) were recorded. Blood lead levels at the time of testing were about 5, 35, or 55 micrograms/dl for controls, the 350-mg group and the 600-mg group, respectively. There were no clinical signs of intoxications. Clicks varying in sound pressure level (SPL) and rate were used to elicit BAEPs. In addition, the influence of different levels of masking noise was explored in Experiment 1. Four early prominent waves were detected in accordance with other studies of the monkey BAEP. The most reliable wave was No. II. Latencies in the BAEP exhibited the known dependencies on parametric variation for SPL, stimulus rate, and masking level. The 600-mg group exhibited the longest latencies at all stimulus conditions. Analysis of wave II and IV latencies revealed a significant main effect for lead on wave II. At the rate condition there were also signs of latency decreases in the 350-mg group that did not reach significance. Therefore, repetition rate was varied on all SPLs in Experiment 2 to assess the reliability of this effect because similar observations were reported in lead-exposed children. There was no indication of reduced latencies using this extended design. In contrast, significant lead-induced increases in latencies of waves I, II, and IV were revealed by multivariate ANOVA. The purpose of Experiment 3 was to examine whether these results were dependent on current exposure or persisted after cessation of lead treatment. It started 18 months after the end of lead feeding, when blood lead levels had declined to nearly normal values. The same lead-related effects were detected as in the previous experiments. Taken together, these results indicate consistent prolongations of latencies in the BAEP due to subtoxic lead exposure that are not dependent on current treatment. The results are compared to the effects found in epidemiological studies in lead-exposed children.

Discriminative stimulus effects of diazepam and buspirone in normal volunteers

A within-subject design was used to characterize the effects of dose manipulations on discriminative and self-reported effects of oral diazepam and buspirone. Subjects were trained to discriminate diazepam (10 mg) versus placebo (n = 10), or buspirone (10 or 15 mg) versus placebo (n = 9). The compounds were identified to subjects by letter code before discrimination training began. In later sessions, correct identifications at 2 hr after the oral administration of drug earned money. All subjects showed accurate discrimination performance during the test-of-acquisition phase. In a low-dose generalization phase, diazepam and buspirone produced dose-related increases in drug identifications across a four-fold range of doses. In a subsequent low-dose training phase, in which subjects were trained to discriminate progressively lower drug doses, the median lowest discriminable dose of diazepam and buspirone was 2.5 and 7.5 mg, respectively. Dose-response functions for drug identifications were shifted leftward in the low-dose training phase relative to the low-dose generalization phase, suggesting that reinforcement of progressively lower doses enhances drug discriminability. The self-reported effects of diazepam and buspirone were similar (e.g., both drugs increased ratings of drug strength and clumsy/uncoordinated) and different (e.g., diazepam but not buspirone increased ratings of drowsy/sleepy; buspirone but not diazepam increased ratings of tense/nervous). This study demonstrates discriminative and self-reported effects of diazepam and buspirone at doses lower than previously shown to be behaviorally active, and suggests that at commonly used clinical doses, diazepam is relatively more discriminable than buspirone.

Drug discrimination training with low doses: maintenance of discriminative control

Procedures are reported that maintain control by the drug cue during and after drug discrimination training with lower doses that yield predominantly vehicle-appropriate choices. Twelve pigeons were trained to discriminate chlordiazepoxide (CDP) from saline using two-key (drug vs. vehicle) drug discrimination procedures. Intermixed within each block of 30 sessions were nine sessions of training with 8.0 mg/kg CDP, nine with one of seven lower training doses (4.0, 2.8, 2.0, 1.4, 1.0, 0.7, or 0.5 mg/kg CDP), and 12 with saline. The lower training dose was decreased across blocks. The three lowest training doses (1.0, 0.7, and 0.5 mg/kg CDP) yielded predominantly saline-appropriate choices but had no effect on discrimination of 8.0 mg/kg CDP or saline. Three doses (2.0, 1.4, and 1.0 mg/kg CDP) were retrained, and each yielded percentages of drug-appropriate choices nearly identical to those obtained during previous training. This drug discrimination procedure maintains control by the drug cue during and after training with vehicle-like doses of the training drug and may allow for repeated assessment of effects of low training doses.

Responsiveness to cocaine challenge in adult rats following prenatal exposure to cocaine

Adult rats that were gestationally exposed to cocaine and control offspring were examined for their sensitivity to challenge doses of cocaine. Offspring were derived from Sprague-Dawley dams that had received subcutaneous injections of 40 mg/kg per 3 cc cocaine hydrochloride daily on gestational days 8-20, pair-fed dams that were injected with saline, and nontreated control dams. In order to investigate the sensitivity to challenge doses of cocaine, offspring were assessed in adulthood for locomotor activity, cocaine drug discrimination, and the time course of cocaine in brain tissue following acute cocaine challenge. Adult offspring prenatally exposed to cocaine were observed to exhibit a reduced sensitivity to the discriminative stimulus effects of cocaine as evidenced by a significant shift to the right in the dose-response curve of cocaine discrimination. No prenatal treatment effects were observed in terms of the temporal patterns of cocaine discrimination or with regard to brain levels of cocaine. In addition, baseline locomotor activity and locomotor responses to challenge doses of cocaine were comparable across the prenatal treatment groups. Thus, prenatal cocaine exposure reduced sensitivity of offspring to the discriminative stimulus properties of cocaine without altering either the distribution of cocaine to the brain or the sensitivity of the offspring to the locomotor stimulant effects of cocaine.

Chronic lead exposure attenuates ethanol-induced hypoalgesia

Adult male rats were exposed to drinking fluid containing either 500 ppm lead acetate (group lead), or an equivalent concentration of sodium acetate (group control) for 61 days prior to pain reactivity testing using a tail-flick procedure. Rats were placed in restraining tubes for a 20 min acclimation period, and then baseline tail-flick latencies in response to a radiant heat source were measured. Subsequently, half the animals from each group were serially injected IP with either 1.0, 2.0, or 3.0 g/kg body weight of a 20% v/v ethanol solution, and the other half were injected with an equivalent volume of saline. Tail-flick latencies were reassessed at 20-min intervals over the next 2 h. Results indicated dose-dependent ethanol-induced hypoalgesia at all doses, but at the two higher doses the magnitude of the hypoalgesic response was significantly greater in the group control animals than in the group lead animals across the 2-h postinjection period. Results are discussed in terms of an attenuation of the pharmacological properties of ethanol by lead.

Drug discrimination by humans compared to nonhumans: current status and future directions

In drug discrimination (DD) procedures, behavior is differentially reinforced depending on the presence or absence of specific drug stimuli. The DD paradigm has been widely adopted by behavioral pharmacologists because of its specificity of stimulus control, concordance with drug action at cellular levels and its use as a preclinical model of subject-rated effects in humans. With the successful extension of DD to humans, a comparison of human and nonhuman DD will help place each in the context of the other. Twenty-eight studies of DD in humans are reviewed, including studies of amphetamine, opioid, benzodiazepine, caffeine, nicotine, marijuana and ethanol discriminative stimuli. Comparison of procedures between studies in humans and nonhumans reveals a common tradition, except the use of instructions appears to facilitate greatly DD acquisition in humans. Findings were qualitatively similar between humans and nonhumans. Potency relationships were quantitatively similar between humans and most, but not all, other species. Areas of human DD needing additional empirical evaluation include the influence of instructions, the effects of training dose and the effects of antagonists. Additionally, antihistamines, barbiturates, nicotine and marijuana are under-represented in human DD.

Chronic exposure to lead attenuates cocaine-induced behavioral activation

Adult, male rats were exposed to a diet containing 500 ppm (0.05%) lead for 105 days before testing for cocaine-related changes in activity using a Digiscan activity system. Behavioral testing occurred on 6 successive test days. Activity was recorded for 20 min prior to and 40 min after IP injections of either 10, 20, or 40 mg/kg cocaine HCl, with saline injections on the day preceding each drug test day. Cocaine-induced behavioral activation was evident in control diet animals for all three doses (10, 20, and 40 mg/kg). While 10 mg/kg cocaine HCl did not produce behavioral activation in lead-treated animals, both 20 and 40 mg/kg did result in increased activity comparable to that observed in control counterparts.

Postnatal lead exposure induces supersensitivity to the stimulus properties of a D2-D3 agonist

To examine the impact of lead (Pb) exposure during the ontogeny of dopaminergic (DA) systems on resultant DA function, rats were exposed postnatally (0-21 days of age) via the lactating dam to 0, 100 or 350 ppm Pb acetate in drinking water. At 2 months of age, the postnatally Pb-exposed rats were trained to discriminate the stimulus properties of either the D1 receptor agonist SKF38393 (6.0 mg/kg) or the D2-D3 receptor family subtype agonist quinpirole (0.05 mg/kg) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. In each training group, dose-effect curves describing drug lever responding to lower doses of the training drug and to preadministration of selective DA antagonists were obtained to examine Pb-induced changes in DA sensitivity, and doses of non-DA compounds were substituted to determine the specificity of any changes in DA sensitivity. In the D1/saline training condition, Pb exposure was not associated with any specific or consistent changes in DA sensitivity. In contrast, exposure to Pb was associated with D2-D3 receptor subtype supersensitivity as was indicated by significantly elevated levels of drug lever responding in the presence of quinpirole and haloperidol and to at least one dose of apomorphine. No differences in the dose-effect curves for either (+)-amphetamine or NMDA were observed in the D2-D3-trained control and Pb-exposed groups, but an increase in drug lever responding in the presence of pentobarbital was noted in the Pb-exposed group relative to control. Taken together, these findings are consistent with a Pb-induced functional D2-D3 supersensitivity possibly mediated via autoreceptors. Moreover, this functional D2-D3 supersensitivity necessarily represents a permanent effect of postnatal Pb exposure since both blood and brain Pb levels were negligible at the time drug discrimination training began.

Low-dose caffeine discrimination and self-reported mood effects in normal volunteers

A caffeine versus placebo discrimination procedure was used to determine the lowest caffeine dose that could produce discrimination and self-reported mood effects in normal volunteers. During daily sessions under double-blind conditions, caffeine-abstinent subjects orally ingested a capsule containing 178 mg caffeine or placebo. Before beginning discrimination training, the compounds were identified to subjects by letter codes. Fifteen, 30, and 45 min after capsule ingestion, subjects guessed the capsule's letter code. Correct guesses at 45 min earned money. After each session, subjects received a supplementary capsule containing caffeine or placebo to ensure that, within each phase of the study, subjects received the same daily dose of caffeine equal to the training dose. Five of the 15 subjects acquired the caffeine versus placebo discrimination within the first 20 sessions (greater than or equal to 75% correct); 6 other subjects acquired the discrimination with additional training. Nine subjects who acquired the discrimination were subsequently trained at progressively lower caffeine doses. In general, the lowest dose to produce discrimination (greater than or equal to 75% correct) was also the lowest dose to produce self-reported mood effects: 4 subjects showed discrimination and self-reported mood effects at 100 mg caffeine, 2 at 56 mg, 1 at 32 mg, and 1 at 18 mg. One of these subjects also showed self-reported mood effects at 10 mg. The present study documents discriminative stimulus and self-reported mood effects of caffeine at doses below those previously shown to affect any behavior in normal volunteers.

Tactile-kinesthetic system of rats as an animal model for minimal brain dysfunction

A previous study showed that rats exposed to methylmercury during development exhibit effects similar to those described for children with minimal brain dysfunction (MBD), namely, hyperactivity, altered locomotion structure, and unaltered learning ability, but reduced and more variable attention spans induced by increasing difficulties within an operant learning paradigm. Psychopathological studies suggest that behavioral disturbances of the MBD type may originate directly or indirectly from deficiencies in the tactile-kinesthetic system. This sensory modality is the main mechanism by which an individual organism assimilates reality. Deficiencies in the tactile-kinesthetic system impair the action of the equilibration processes (in Piaget's sense) which ensure that the stages of psychological development occur in an orderly sequence. The lack of this control over development may result in the behavioral characteristics of MBD. Problems with the tactile-kinesthetic system may also be the reason for the deficiencies of fine motor control in MBD children. In an attempt to extrapolate this interpretation of human psychopathological mechanisms to experimental animals, an operant paradigm was developed for the assessment of the tactile-kinesthetic system of rats, the schedule of "differential reinforcement of force range" (DRF). Rats were trained to press in discrete trials a force sensitive lever during at least 1 s between two force thresholds of 60 and 80 g without any feedback other than the rats' own tactile-kinesthetic perception. Offspring of rat dams exposed to 1.5 and 5 mg/l methylmercury-chloride in their drinking water from 2 weeks before pairing until weaning, exhibited a clearcut performance deficit (approximately 25% correct responses compared to approximately 50% of the controls).(ABSTRACT TRUNCATED AT 250 WORDS)

Low level lead exposure increases sensitivity to the stimulus properties of dopamine D1 and D2 agonists

To examine the impact of Pb exposure on dopaminergic (DA) function, weanling rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water. At 3 months of age, the rats were trained to discriminate the stimulus properties of either the D1 agonist SKF38393 (3.0 mg/kg i.p.; D1/sal) or the D2 agonist quinpirole (0.05 mg/kg i.p., D2/sal) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. Lead-exposed rats learned the discriminations faster than respective controls. Moreover, they exhibited greater levels of drug lever responding to lower doses of the training drugs (D1/sal and D2/sal), and to selected doses of other direct and indirect DA agonists (D2/sal only), including apomorphine, cocaine and (+)-amphetamine, and less blockade of drug lever responding by haloperidol (D2/sal). Taken together, these findings are consistent with a generalized DA supersensitivity. There were no differential Pb effects when non-DA compounds including morphine, pentobarbital and MK-801 were substituted for the training drugs, indicating the selectivity of the DA effects in the context of these experiments, and the improbability of a non-specific behavioral causation. Pb-exposed rats in the D2/sal group also showed a pronounced enhancement of drug lever responding when NMDA was substituted for quinpirole, suggesting the possibility of a Pb-induced NMDA supersensitivity as well.

Similarities and differences between behavioral control by drug-produced stimuli and by sensory stimuli

Drug-induced state-dependent learning and drug discriminations may be based on sensory stimuli induced by drug actions, or at least on events in the brain that have properties analogous to those of sensory stimuli. A variety of comparisons between behavioral control by drug-induced stimuli and by classically defined interoceptive and exteroceptive stimuli are possible. These allow inferences to be made about the nature of drug stimuli, the properties of behavioral paradigms that we use to investigate them, and the mechanisms by which drugs achieve behavioral control in drug discrimination and state-dependent learning paradigms. Each of these topics is selectively reviewed in this paper.

Effect of lead exposure on dopaminergic receptors in rat striatum and nucleus accumbens

Haloperidol- and sulpiride-displaceable [3H]spiroperidol binding and the dopamine-inhibited adenylate cyclase were measured in rats chronically exposed to lead acetate. Haloperidol-displaceable [3H]spiroperidol binding was unmodified while sulpiride-displaceable binding was increased in striatum and decreased in nucleus accumbens. In addition, the decrease of sulpiride-displaceable binding in nucleus accumbens was paralleled by a reduced ability of bromocriptine to inhibit cAMP formation in presence of the D1 receptor antagonist SCH 23390. The results support the concept that in vivo lead treatment affects dopaminergic receptors and that the binding sites labelled by [3H]spiroperidol displaced by haloperidol may be different from those which recognize sulpiride.

Effect of in vitro inorganic lead on dopamine release from superfused rat striatal synaptosomes

The effect of inorganic lead in vitro in several aspects of [3H]dopamine release from superfused rat striatal synaptosomes was examined. Under conditions of spontaneous release, lead (1-30 microM) induced dopamine release in a concentration-dependent manner. The onset of the lead-induced release was delayed by approximately 15-30 sec. The magnitude of dopamine release induced by lead was increased when calcium was removed from the superfusing buffer. Lead-induced release was unaffected in the presence of putative calcium, sodium, and/or potassium channel blockers (nickel, tetrodotoxin, tetraethylammonium, respectively). Depolarization-evoked dopamine release, produced by a 1-sec exposure to 61 mM potassium, was diminished at calcium concentrations below 0.254 mM. The onset of depolarization-evoked release was essentially immediate following exposure of the synaptosomes to high potassium. The combination of lead (3 or 10 microM) with high potassium reduced the magnitude of depolarization-evoked dopamine release. This depression of depolarization-evoked release by lead was greater in the presence of 0.25 mM than 2.54 mM calcium in the superfusing buffer. These findings demonstrate multiple actions of lead on synaptosomal dopamine release. Lead can induce dopamine release by yet unidentified neuronal mechanisms independent of external calcium. Lead can also reduce depolarization-evoked dopamine release by apparent competition with calcium influx at the neuronal membrane calcium channel.

Discriminative stimulus properties of amphetamine and other stimulants in lead-exposed and normal rats

The present study examined the discriminative stimulus properties of amphetamine (AMP) at progressively lower doses in lead-exposed and normal rats. In addition, generalization gradients of AMP, apomorphine, methylphenidate, and caffeine to both high and low training doses of AMP were determined in these rats. Under the high AMP training dose condition (1.0 mg/kg, IP) generalization gradients of AMP were similar for lead-exposed and control rats. When the training doses were progressively lowered, the lead-exposed rats tended to require a higher range of AMP doses (0.24-0.49 mg/kg) than did control rats (0.18-0.32 mg/kg) to maintain discriminative control. In parallel with this, the minimal discriminable doses tended to be higher for lead-exposed rats than for control rats. Methylphenidate generalization gradients were different for lead-exposed and control rats under the high AMP training condition but became similar under the low AMP training condition. No differences attributable to training dose or lead exposure were evident for apomorphine or caffeine.

Altered central monoamine response to D-amphetamine in rats chronically exposed to inorganic lead

Investigations of the mechanisms involved in the neurotoxicity resulting from chronic inorganic lead (Pb) exposure have centered on CNS biogenic amine function on the basis of behavioral and neurochemical findings. The following study examined the time course of the response of dopamine (DA) and 5-hydroxytryptamine (5-HT) neurons to d-amphetamine (AMPH) in rats chronically exposed to Pb from birth in order to further examine neurochemical mechanisms implicated by previous work. Offspring were exposed to 0.2% Pb acetate via the lactating dam and then weaned to the same drinking solution. At 120-140 days animals were injected with 1.0 mg/kg s.c. of the drug or with saline and sacrificed after various intervals. DA content in nucleus accumbens and corpus striatum in Pb-exposed animals was significantly higher than corresponding levels in controls at 20 minutes post-drug and remained significantly higher than baseline values at 80 minutes after the drug when DA concentrations in controls had returned to normal. These data suggest enhanced AMPH-induced DA synthesis in exposed rats. 5-Hydroxyindoleacetic acid (5-HIAA) content was significantly increased in three brain regions in exposed rats given AMPH compared to values in saline-injected exposed animals, indicating a compensation in these areas for the decreases in 5-HIAA values produced by Pb exposure alone. The results of this study reinforce the hypothesis that DA and 5-HT neurons are sensitive to relatively low levels of Pb exposure.

Influence of chronic inorganic lead exposure on regional dopamine and 5-hydroxytryptamine turnover in rat brain

The results of previous behavioral studies utilizing chronic exposure to low amounts of inorganic lead (Pb) have suggested alterations in the function of biogenic amine neuronal systems. The following study was performed to provide evidence for the possible bases of these changes in pharmacological responsiveness in exposed animals. Dams were administered 0.2% Pb acetate in drinking water to expose their offspring to Pb via the maternal milk. Males were weaned to the same drinking solution. At 120-140 days a tracer dose of 1.0 mCi L-[3H]2,6-tyrosine (3H-TYR) and 0.5 mCi L-[3H(G)]tryptophan (3H-TRP) was injected through an indwelling jugular catheter, and norepinephrine (NE), dopamine (DA), 5-hydroxytryptamine (5-HT) and their respective precursors and metabolites were quantified by liquid chromatography with electrochemical detection with column eluate collected for liquid scintillation counting. At this level of exposure (blood lead (PbB) at day 90 in exposed animals = 43.1 +/- 1.7 micrograms/dl) no changes were observed in concentration of NE or DA or DA metabolites in any brain region. However, DA turnover was decreased in Pb-exposed animals in nucleus accumbens and frontal cortex. No changes in 5-HT content and turnover were observed in any brain region, but 5-hydroxyindoleacetic acid (5-HIAA) levels were decreased in 6 of the 9 brain regions examined. These findings are consistent with observations of an attenuated behavioral responsiveness to d-amphetamine (AMPH) in exposed animals, and suggest that the changes in DA and 5-HT neurons noted by other workers at higher levels of exposure persist when PbBs are in the range of 40 micrograms/dl.

The effects of lead administration during development on lithium-induced polydipsia and dopaminergic function

Previous studies have demonstrated that postnatal (days 2-29 of life) administration of lead (200 mg/kg/day by gavage) to Long-Evans rats caused permanent increases in lithium-induced polydipsia (LIP). These lead-induced increases in LIP were apparently not of renal origin, did not occur in animals treated with lead after day 30, and persisted for at least 6 months. The present studies have narrowed the dose-time window for lead-induced increases in LIP. The first study showed that continuous administration of lead (200 mg/kg/day, p.o.) in the form of lead acetate during days 2-9 of life caused increases in LIP (P = 0.022). Although lead-induced increases in LIP were not statistically significant (P = 0.084) for the group administered lead from days 9 to 19, the lack of a significant difference between the 2-9- and 9-19-day groups suggested that lead treatment during either of these time periods would result in LIP increases. Lead administration between days 19 and 29 of life was not effective in increasing LIP (P = 0.8). In the second study, a single dose of lead (200 mg/kg/day) was administered either on day 5 or 15 of life. Concentrations of lead in the blood on day 30 of life averaged 23.2 micrograms/100 ml for treated rats versus 4.8 micrograms/100 ml for controls. When tested at approximately 90 days of age, both groups showed significant increases in LIP (P = 0.028). The rats from this second study were also examined for changes in nigrostriatal dopamine function, since this pathway is known to be essential for LIP.(ABSTRACT TRUNCATED AT 250 WORDS)