The effects of dopamine agonists on fixed interval schedule-controlled behavior are selectively altered by low-level lead exposure

Brain gray matter volume of reward-related structures in Inuit adolescents pre- and postnatally exposed to lead, mercury and polychlorinated biphenyls

This study aimed to assess associations between prenatal and postnatal exposure to lead (Pb), mercury (Hg) and polychlorinated biphenyls (PCBs) and gray matter volume of key regions of the brain reward circuit, namely the caudate nucleus, putamen, nucleus accumbens (nAcc), the amygdala, the orbitofrontal cortex (OFC) and the anterior cingulate cortex (ACC). Structural magnetic resonance imaging (MRI) was conducted in 77 Inuit adolescents (mean age = 18.39) from Nunavik, Canada, who also completed the Brief Sensation Seeking Scale (BSSS-4) and Sensation Seeking - 2 (SS-2), two self-report questionnaires evaluating the tendency toward sensation seeking, which is a proxy of reward-related behaviors. Exposures to Pb, Hg and PCBs were measured in cord blood at birth, in blood samples at 11 years old and at time of testing (18 years old). Multivariate linear regressions were corrected for multiple comparisons and adjusted for potential confounders, such as participants' sociodemographic characteristics and nutrient fish intake. Results showed that higher cord blood Pb levels predicted smaller gray matter volume in the bilateral nAcc, caudate nucleus, amygdala and OFC as well as in left ACC. A moderating effect of sex was identified, indicating that the Pb-related reduction in volume in the nAcc and caudate nucleus was more pronounced in female. Higher blood Hg levels at age 11 predicted smaller right amygdala independently of sex. No significant associations were found between blood PCBs levels at all three times of exposure. This study provides scientific support for the detrimental effects of prenatal Pb and childhood Hg blood concentrations on gray matter volume in key reward-related brain structures.

Pre- and postnatal exposure to legacy environmental contaminants and sensation seeking in Inuit adolescents from Nunavik

Despite extensive evidence from cohort studies linking exposure to lead (Pb), mercury (Hg) and polychlorinated biphenyls (PCBs) to numerous cognitive outcomes in children and adolescents, very few studies addressed reward sensitivity, a key dimension of emotional regulation. The present study aimed to examine associations between pre- and postnatal exposure to these environmental neurotoxicants and sensation seeking, a behavioral feature of reward. A total of 207 Inuit adolescents (mean age = 18.5, SD = 1.2) from Nunavik, Canada, completed the Brief Sensation Seeking Scale (BSSS-4) and Sensation Seeking- 2 (SS-2), two self-report questionnaires assessing proneness to sensation seeking. Prenatal, childhood and adolescent exposure to Pb, Hg and PCBs were measured in cord blood at birth and blood samples at 11 years of age and at time of testing. Multiple linear regression models were performed, potential confounders including participants' sociodemographic characteristics and nutrient fish intake were considered. Results showed that higher child blood levels of Pb (b = -0.18, p = 0.01) and PCB-153 (b = -0.16, p = 0.06) were associated with lower BSSS-4 total scores, while cord and adolescent blood PCB-153 levels were significantly related to lower SS2 total scores (b = -0.15, p = 0.04; b = -0.24, p = 0.004). Such associations persisted after further adjustment for co-exposure to concurrent contaminants. These associations were influenced by self-report positive affect and marginally moderated by sex. Sex differences were only observed for child PCB exposure, with the association for risk-taking sensation seeking observed only in girls but not in boys. Further research is warranted to assess the extent to which reduced sensation seeking in chronically exposed individuals affects their behaviors, well-being, and emotional regulation.

Task-Dependent Effects of SKF83959 on Operant Behaviors Associated With Distinct Changes of CaMKII Signaling in Striatal Subareas

BACKGROUND

SKF83959, an atypical dopamine (DA) D1 receptor agonist, has been used to test the functions of DA-related receptor complexes in vitro, but little is known about its impact on conditioned behavior. The present study examined the effects of SKF83959 on operant behaviors and assayed the neurochemical mechanisms involved.

METHODS

Male rats were trained and maintained on either a fixed-interval 30-second (FI30) schedule or a differential reinforcement of low-rate response 10-second (DRL10) schedule of reinforcement. After drug treatment tests, western blotting assayed the protein expressions of the calcium-/calmodulin-dependent protein kinase II (CaMKII) and the transcription factor cyclic AMP response element binding protein (CREB) in tissues collected from 4 selected DA-related areas.

RESULTS

SKF83959 disrupted the performance of FI30 and DRL10 behaviors in a dose-dependent manner by reducing the total number of responses in varying magnitudes. Moreover, the distinct profiles of the behavior altered by the drug were manifested by analyzing qualitative and quantitative measures on both tasks. Western-blot results showed that phospho-CaMKII levels decreased in the nucleus accumbens and the dorsal striatum of the drug-treated FI30 and DRL10 subjects, respectively, compared with their vehicle controls. The phospho-CREB levels decreased in the nucleus accumbens and the hippocampus of drug-treated FI30 subjects but increased in the nucleus accumbens of drug-treated DRL10 subjects.

CONCLUSIONS

Our results provide important insight into the neuropsychopharmacology of SKF83959, indicating that the drug-altered operant behavior is task dependent and related to regional-dependent changes of CaMKII-CREB signaling in the mesocorticolimbic DA systems.

A Systematic Review on the Influences of Neurotoxicological Xenobiotic Compounds on Inhibitory Control

Background: Impulsive and compulsive traits represent a variety of maladaptive behaviors defined by the difficulties to stop an improper response and the control of a repeated behavioral pattern without sensitivity to changing contingencies, respectively. Otherwise, human beings are continuously exposed to plenty neurotoxicological agents which have been systematically linked to attentional, learning, and memory dysfunctions, both preclinical and clinical studies. Interestingly, the link between both impulsive and compulsive behaviors and the exposure to the most important xenobiotic compounds have been extensively developed; although the information has been rarely summarized. For this, the present systematic review schedule and analyze in depth the most important works relating different subtypes of the above-mentioned behaviors with 4 of the most important xenobiotic compounds: Lead (Pb), Methylmercury (MeHg), Polychlorinated biphenyls (PCB), and Organophosphates (OP) in both preclinical and clinical models.

Methods: Systematic search strategy on PubMed databases was developed, and the most important information was structured both in text and in separate tables based on rigorous methodological quality assessment.

Results: For Lead, Methylmercury, Polychlorinated biphenyls and organophosphates, a total of 44 (31 preclinical), 34 (21), 38 (23), and 30 (17) studies were accepted for systematic synthesis, respectively. All the compounds showed an important empirical support on their role in the modulation of impulsive and, in lesser degree, compulsive traits, stronger and more solid in animal models with inconclusive results in humans in some cases (i.e., MeHg). However, preclinical and clinical studies have systematically focused on different subtypes of the above-mentioned behaviors, as well as impulsive choice or habit conformations have been rarely studied.

Discussion: The strong empirical support in preclinical studies contrasts with the lack of connection between preclinical and clinical models, as well as the different methodologies used. Further research should be focused on dissipate these differences as well as deeply study impulsive choice, decision making, risk taking, and cognitive flexibility, both in experimental animals and humans.

Different Behavioral Experiences Produce Distinctive Parallel Changes in, and Correlate With, Frontal Cortex and Hippocampal Global Post-translational Histone Levels

While it is clear that behavioral experience modulates epigenetic profiles, it is less evident how the nature of that experience influences outcomes and whether epigenetic/genetic "biomarkers" could be extracted to classify different types of behavioral experience. To begin to address this question, male and female mice were subjected to either a Fixed Interval (FI) schedule of food reward, or a single episode of forced swim followed by restraint stress, or no explicit behavioral experience after which global expression levels of two activating (H3K9ac and H3K4me3) and two repressive (H3K9me2 and H3k27me3) post-translational histone modifications (PTHMs), were measured in hippocampus (HIPP) and frontal cortex (FC). The specific nature of the behavioral experience differentiated profiles of PTHMs in a sex- and brain region-dependent manner, with all 4 PTHMs changing in parallel in response to different behavioral experiences. These different behavioral experiences also modified the pattern of correlations of PTHMs both within and across FC and HIPP. Unexpectedly, highly robust correlations were found between global PTHM levels and behavioral performances, suggesting that global PTHMs may provide a higher-order pattern recognition function. Further efforts are needed to determine the generality of such findings and what characteristics of behavioral experience are critical for modulating PTHM responses.

Endocrine active metals, prenatal stress and enhanced neurobehavioral disruption

Metals, including lead (Pb), methylmercury (MeHg) and arsenic (As), are long-known developmental neurotoxicants. More recently, environmental context has been recognized to modulate metals toxicity, including nutritional state and stress exposure. Modulation of metal toxicity by stress exposure can occur through shared targeting of endocrine systems, such as the hypothalamic-pituitary-adrenal axis (HPA). Our previous rodent research has identified that prenatal stress (PS) modulates neurotoxicity of two endocrine active metals (EAMs), Pb and MeHg, by altering HPA and CNS systems disrupting behavior. Here, we review this research and further test the hypothesis that prenatal stress modulates metals neurotoxicity by expanding to test the effect of developmental As ± PS exposure. Serum corticosterone and behavior was assessed in offspring of dams exposed to As ± PS. PS increased female offspring serum corticosterone at birth, while developmental As exposure decreased adult serum corticosterone in both sexes. As + PS induced reductions in locomotor activity in females and reduced response rates on a Fixed Interval schedule of reinforcement in males, with the latter suggesting unique learning deficits only in the combined exposure. As-exposed males showed increased time in the open arms of an elevated plus maze and decreased novel object recognition whereas females did not. These data further confirm the hypothesis that combined exposure to chemical (EAMs) and non-chemical (PS) stressors results in enhanced neurobehavioral toxicity. Given that humans are exposed to multiple environmental risk factors that alter endocrine function in development, such models are critical for risk assessment and public health protection, particularly for children.

Developmental Lead and/or Prenatal Stress Exposures Followed by Different Types of Behavioral Experience Result in the Divergence of Brain Epigenetic Profiles in a Sex, Brain Region, and Time-Dependent Manner: Implications for Neurotoxicology

Over a lifetime, early developmental exposures to neurocognitive risk factors, such as lead (Pb) exposures and prenatal stress (PS), will be followed by multiple varied behavioral experiences. Pb, PS and behavioral experience can each influence brain epigenetic profiles. Our recent studies show a greater level of complexity, however, as all three factors interact within each sex to generate differential adult variation in global post-translational histone modifications (PTHMs), which may result in fundamentally different consequences for life-long learning and behavioral function. We have reported that PTHM profiles differ by sex, brain region and time point of measurement following developmental exposures to Pb±PS, resulting in different profiles for each unique combination of these parameters. Imposing differing behavioral experience following developmental Pb±PS results in additional divergence of PTHM profiles, again in a sex, brain region and time-dependent manner, further increasing complexity. Such findings underscore the need to link highly localized and variable epigenetic changes along single genes to the highly-integrated brain functional connectome that is ultimately responsible for governing behavioral function. Here we advance the idea that increased understanding may be achieved through iterative reductionist and holistic approaches. Implications for experimental design of animal studies of developmental exposures to neurotoxicants include the necessity of a 'no behavioral experience' group, given that epigenetic changes in response to behavioral testing can confound effects of the neurotoxicant itself. They also suggest the potential utility of the inclusion of salient behavioral experiences as a potential effect modifier in epidemiological studies.

Novel reinforcement learning paradigm based on response patterning under interval schedules of reinforcement

There is a need to develop cognitive tasks that address valid neuropsychological constructs implicated in disease mechanisms and can be used in animals and humans to guide novel drug discovery. Present experiments aimed to characterize a novel reinforcement learning task based on a classical operant behavioral phenomenon observed in multiple species - differences in response patterning under variable (VI) vs fixed interval (FI) schedules of reinforcement. Wistar rats were trained to press a lever for food under VI30s and later weekly test sessions were introduced with reinforcement schedule switched to FI30s. During the FI30s test session, post-reinforcement pauses (PRPs) gradually grew towards the end of the session reaching 22-43% of the initial values. Animals could be retrained under VI30s conditions, and FI30s test sessions were repeated over a period of several months without appreciable signs of a practice effect. Administration of the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 ((5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate) prior to FI30s sessions prevented adjustment of PRPs associated with the change from VI to FI schedule. This effect was most pronounced at the highest tested dose of MK-801 and appeared to be independent of the effects of this dose on response rates. These results provide initial evidence for the possibility to use different response patterning under VI and FI schedules with equivalent reinforcement density for studying effects of drug treatment on reinforcement learning.

Neurotoxicants, Micronutrients, and Social Environments

SUMMARY—Systematic research evaluating the separate and interacting impacts of neurotoxicants, micronutrients, and social environments on children's cognition and behavior has only recently been initiated. Years of extensive human epidemiologic and animal experimental research document the deleterious impact of lead and other metals on the nervous system. However, discrepancies among human studies and between animal and human studies underscore the importance of variations in child nutrition as well as social and behavioral aspects of children's environments that mitigate or exacerbate the effects of neurotoxicants.

In this monograph, we review existing research on the impact of neurotoxic metals, nutrients, and social environments and interactions across the three domains. We examine the literature on lead, mercury, manganese, and cadmium in terms of dispersal, epidemiology, experimental animal studies, effects of social environments, and effects of nutrition.

Research documenting the negative impact of lead on cognition and behavior influenced reductions by the Center for Disease Control in child lead-screening guidelines from 30 micrograms per deciliter (μg/dL) in 1975 to 25 μg/dL in 1985 and to 10 μg/dL in 1991. A further reduction is currently being considered. Experimental animal research documents lead's alteration of glutamate-neurotransmitter (particularly N-methyl-D-aspartate) activity vital to learning and memory. In addition, lead induces changes in cholinergic and dopaminergic activity. Elevated lead concentrations in the blood are more common among children living in poverty and there is some evidence that socioeconomic status influences associations between lead and child outcomes. Micronutrients that influence the effects of lead include iron and zinc.

Research documenting the negative impact of mercury on children (as well as adults) has resulted in a reference dose (RfD) of 0.1 microgram per kilogram of body weight per day (μg/kg/day). In animal studies, mercury interferes with glutamatergic, cholinergic, and dopaminergic activity. Although evidence for interactions of mercury with children's social contexts is minimal, researchers are examining interactions of mercury with several nutrients.

Research on the effects of cadmium and manganese on child cognition and behavior is just beginning. Experimental animal research links cadmium to learning deficits, manganese to behaviors characteristic of Parkinson's disease, and both to altered dopaminergic functioning.

We close our review with a discussion of policy implications, and we recommend interdisciplinary research that will enable us to bridge gaps within and across domains.

Brain hemispheric differences in the neurochemical effects of lead, prenatal stress, and the combination and their amelioration by behavioral experience

Brain lateralization, critical to mediation of cognitive functions and to "multitasking," is disrupted in conditions such as attention deficit disorder and schizophrenia. Both low-level lead (Pb) exposure and prenatal stress (PS) have been associated with mesocorticolimbic system-mediated executive-function cognitive and attention deficits. Mesocorticolimbic systems demonstrate significant laterality. Thus, altered brain lateralization could play a role in this behavioral toxicity. This study examined laterality of mesocorticolimbic monoamines (frontal cortex, nucleus accumbens, striatum, midbrain) and amino acids (frontal cortex) in male and female rats subjected to lifetime Pb exposure (0 or 50 ppm in drinking water), PS (restraint stress on gestational days 16-17), or the combination with and without repeated learning behavioral experience. Control males exhibited prominent laterality, particularly in midbrain and also in frontal cortex and striatum; females exhibited less laterality, and this was primarily striatal. Lateralized Pb ± PS induced neurotransmitter changes were assessed only in males because of limited sample sizes of Pb + PS females. In males, Pb ± PS changes occurred in left hemisphere of frontal cortex and right hemisphere of midbrain. Behavioral experience modified the laterality of Pb ± PS-induced neurotransmitter changes in a region-dependent manner. Notably, behavioral experience eliminated Pb ± PS neurotransmitter changes in males. These findings underscore the critical need to evaluate both sexes and brain hemispheres for the mechanistic understanding of sex-dependent differences in neuro- and behavioral toxicity. Furthermore, assessment of central nervous system mechanisms in the absence of behavioral experience, shown here for males, may constitute less relevant models of human health effects.

Influence of low level maternal Pb exposure and prenatal stress on offspring stress challenge responsivity

We previously demonstrated potentiated effects of maternal Pb exposure producing blood Pb(PbB) levels averaging 39microg/dl combined with prenatal restraint stress (PS) on stress challenge responsivity of female offspring as adults. The present study sought to determine if: (1) such interactions occurred at lower PbBs, (2) exhibited gender specificity, and (3) corticosterone and neurochemical changes contributed to behavioral outcomes. Rat dams were exposed to 0, 50 or 150ppm Pb acetate drinking water solutions from 2 mos prior to breeding through lactation (pup exposure ended at weaning; mean PbBs of dams at weaning were <1, 11 and 31microg/dl, respectively); a subset in each Pb group underwent prenatal restraint stress (PS) on gestational days 16-17. The effects of variable intermittent stress challenge (restraint, cold, novelty) on Fixed Interval (FI) schedule controlled behavior and corticosterone were examined in offspring when they were adults. Corticosterone changes were also measured in non-behaviorally tested (NFI) littermates. PS alone was associated with FI rate suppression in females and FI rate enhancement in males; Pb exposure blunted these effects in both genders, particularly following restraint stress. PS alone produced modest corticosterone elevation following restraint stress in adult females, but robust enhancements in males following all challenges. Pb exposure blunted these corticosterone changes in females, but further enhanced levels in males. Pb-associated changes showed linear concentration dependence in females, but non-linearity in males, with stronger or selective changes at 50ppm. Statistically, FI performance was associated with corticosterone changes in females, but with frontal cortical dopaminergic and serotonergic changes in males. Corticosterone changes differed markedly in FI vs. NFI groups in both genders, demonstrating a critical role for behavioral history and raising caution about extrapolating biochemical markers across such conditions. These findings demonstrate that maternal Pb interacts with prenatal stress to further modify both behavioral and corticosterone responses to stress challenge, thereby suggesting that studies of Pb in isolation from other disease risk factors will not reveal the extent of its adverse effects. These findings also underscore the critical need to extend screening programs for elevated Pb exposure, now restricted to young children, to pregnant, at risk, women.

CNS effects of developmental Pb exposure are enhanced by combined maternal and offspring stress

Lead (Pb) exposure and elevated stress are co-occurring risk factors. Both impact brain mesolimbic dopamine/glutamate systems involved in cognitive functions. We previously found that maternal stress can potentiate Pb-related adverse effects in offspring at blood Pb levels averaging approximately 40 microg/dl. The current study of combined Pb exposure and stress sought to extend those results to lower levels of Pb exposure, and to examine relationships among consequences in offspring for fixed interval (FI) schedule-controlled behavior, neurochemistry and corticosterone levels. Dams were exposed to maternal Pb beginning 2 months prior to breeding (0, 50 or 150 ppm in drinking water), maternal restraint stress on gestational days 16 and 17 (MS), or the combination. In addition, a subset of offspring from each resultant treatment group was also exposed intermittently to variable stressors as adults (MS+OS). Marked "Pb-stress"-related increases in response rates on a fixed interval schedule, a behavioral performance with demonstrated sensitivity to Pb, occurred preferentially in female offspring even at mean blood Pb levels of 11 microg/dl when 50 ppm Pb was combined with maternal and offspring stress. Greater sensitivity of females to frontal cortex catecholamine changes may contribute to the elevated FI response rates as mesocorticolimbic systems are critical to the mediation of this behavior. Basal and final corticosterone levels of offspring used to evaluate FI performance differed significantly from those of non-behaviorally tested (NFI) littermates, demonstrating that purported mechanisms of Pb, stress or Pb/stress effects determined in non-behaviorally trained animals cannot necessarily be generalized to animals with behavioral histories. Finally, the persistent and permanent consequences of Pb, stress and Pb+stress in offspring of both genders suggest that Pb screening programs should include pregnant women at risk for elevated Pb exposure, and that stress should be considered as an additional risk factor. Pb+stress effects observed in the absence of either risk factor alone (i.e., potentiated effects) raise questions about the capacity of current hazard identification approaches to adequately identify human health risks posed by neurotoxicants.

Response inhibition during Differential Reinforcement of Low Rates (DRL) schedules may be sensitive to low-level polychlorinated biphenyl, methylmercury, and lead exposure in children

BACKGROUND

Animal studies have shown that exposure to common, low-level environmental contaminants [e.g., polychlorinated biphenyls (PCBs), lead] causes excessive and inappropriate responding on intermittent reinforcement schedules. The Differential Reinforcement of Low Rates task (DRL) has been shown to be especially sensitive to low-level PCB exposure in monkeys.

OBJECTIVES

We investigated the relationships between prenatal PCB and postnatal Pb exposure performance on a DRL schedule in children. We predicted that a) prenatal PCB exposure would reduce interresponse times (IRTs) and reinforcements earned, and b) postnatal Pb exposure would reduce IRTs and reinforcements earned.

METHODS

We tested 167 children on a DRL20 (20 sec) reinforcement schedule, and recorded IRTs and the number of reinforced responses across the session. We measured prenatal PCB exposure (cord blood), methylmercury (MeHg) (maternal hair), and postnatal Pb exposure (venous blood), and > 50 potentially confounding variables.

RESULTS

Results indicated impaired performance in children exposed to PCBs, MeHg, and Pb. Children prenatally exposed to PCBs responded excessively, with significantly lower IRTs and fewer reinforcers earned across the session. In addition, exposure to either MeHg or Pb predicted statistically significant impairments of a similar magnitude to those for PCBs, and the associated impairments of all three contaminants (PCB, MeHg, and Pb) were statistically independent of one another.

CONCLUSIONS

These results, taken with animal literature, argue the high sensitivity of DRL performance to low-level PCB, MeHg, and Pb exposure. Future research should employ behavioral tasks in children, such as DRL, that have been demonstrably sensitive to low-level PCB, MeHg, and Pb exposure in animals.

Glutamate and dopamine in nucleus accumbens core and shell: sequence learning versus performance

This study sought to determine whether neurochemical changes associated with chronic postweaning lead (Pb) exposure, namely, enhanced dopamine (DA) activity and/or blockade of NMDA function in nucleus accumbens (NAC), underlie the learning impairments also associated with this Pb regimen, and whether core or shell subregions of nucleus accumbens would be more important to such effects. If so, then mimicking these neurochemical changes in normal (control) rats should reproduce these Pb-induced learning impairments. For this purpose, the effects of DA (20-80 microg), the non-competitive NMDA antagonist MK-801 (1.0-2.5 microg) or DA+MK-801 (40+1.0, 80+2.5 microg) were infused in core or shell of nucleus accumbens in normal rats and effects on a multiple schedule of repeated learning (RL) and performance (P) evaluated. In core, MK-801 mimicked the effects of Pb exposure, selectively reducing RL accuracy with no corresponding changes in P accuracy, an effect derived from an increased frequency of perseverative errors. DA produced non-specific changes, reducing accuracy levels in RL and P components. Accuracy and rate effects of DA could be reversed by concurrent administration of the higher MK-801 dose. In shell, MK-801, primarily the lower dose, reduced accuracy in both the RL and P components, while DA did not produce any systematic effects. Collectively, these results point to a greater importance of core as compared to shell in the mediation of learning of spatial sequences, and suggest that inhibition of glutamatergic NMDA function may play a critical role in the selective learning impairments associated with chronic low level Pb exposure.

Changes in mesocorticolimbic dopamine and D1/D2 receptor levels after low level lead exposure: a time course study

Chronic post weaning low-level lead exposure produces cognitive deficits associated with Pb-induced alterations of mesocorticolimbic dopamine (DA) function. This study examined Pb-induced changes in the temporal profile of D1/D2 receptor protein and DA levels in the nucleus accumbens (NAC), hippocampus (HIP), and the frontal cortex (FC). Male Long-Evans rats were exposed to 0 (n=16-20) and 50 ppm Pb (n=16-20) for 180 days. Blood Pb analysis by atomic absorption spectroscopy showed BPb<2 microg/dl in the control group and BPb>9 microg/dl in the Pb-exposed group. Brain DA levels were evaluated by high performance liquid chromatography; D1/D2 receptor expressions, by autoradiographic analysis. Pb exposure produced a transient hyperdopaminergic state, followed by a sustained decline in dopaminergic function within the NAC and a longer-lasting hyperdopaminergic condition within the HIP, whereas it decreased FC D1/D2 without significantly affecting FC DA levels. These findings indicate that time plays a critical, region-specific role in Pb's effects on the normal synaptic profile of the mesocorticolimbic dopaminergic system.

Low-level lead exposure modulates effects of quinpirole and eticlopride on response rates in a fixed-interval schedule

Exposure to low levels of lead (Pb) results in a wide range of behavioral changes. These behavioral deficits of lead are modified by duration of exposure, level of exposure, and stage of exposure. The mesoaccumbens dopamine (DA) system appears to be critically involved in these alterations; however, the precise mechanisms are not completely understood. This study investigated the effects of systemic administrations of the dopamine D(2)-like receptor agonist, quinpirole, and antagonist, eticlopride, on response rates of postweaning lead-exposed rats in a fixed-interval 1-minute (FI-1) schedule. Postweaning exposure to 50 ppm lead (lead acetate) resulted in increased response rates. The dopamine D(2)-like agonist, quinpirole (0.05, 1.0, 3.0 mg/kg), reversed the effects of lead by reducing the response rates. However the antagonist, eticlopride (0.01 and 0.05), did not produce any marked modulation of the response rates of the lead group. Rather, systemic injections of eticlopride attenuated the response rates of control rats. The effects suggest that lead-induced alterations in FI responding are modulated by dopamine D(2)-like mechanisms. Thus, postweaning, subchronic exposure to lead resulted in enhanced sensitivity to quinpirole administration and reduced sensitivity to eticlopride. These observations are consistent with attenuated dopaminergic activity.

Effects of chronic low-level oral lead exposure on prepulse inhibition of acoustic startle in the rat

Previous work has suggested that the behavioral effects of chronic low-level lead exposure on fixed interval (FI) operant behavior result from enhanced dopaminergic neurotransmission in the nucleus accumbens (Cory-Slechta et al., J Pharmacol Exp Ther 286: 794-805, 1998). The present studies were designed to further characterize the effects of chronic low-level oral lead exposure on another behavior that is modulated by dopaminergic neurotransmission in the nucleus accumbens. In these studies acoustic startle and the prepulse inhibition (PPI) of startle were studied in rats following chronic low-level oral lead exposure. Weanling male rats were treated for 5-6 weeks with lead via drinking water (250 ppm lead acetate; controls drank 250 ppm sodium acetate). Acoustic startle reactivity (95, 105, and 115 dB noise bursts) and PPI (prepulses of 1-8 dB over the 70-dB background) of startle were tested following lead exposure. Lead exposure did not affect body weight. Lead exposure also did not significantly affect baseline [i.e., no prepulse inhibition (NO-PPI)] acoustic startle as measured by 1) startle amplitude on the first startle trial (105 dB), 2) the average startle amplitude for the first ten trials (105 dB), or 3) the average startle amplitude for the NO-PPI trials during PPI testing (95, 105, and 115 dB). Lead exposure also did not affect the latency to onset for the startle response. In contrast, for both the 105 dB and 115 dB acoustic startle stimuli, chronic low-level oral lead exposure significantly attenuated the capacity of an acoustic prepulse to reduce the startle response. This effect was present whether the data were presented and analyzed as raw change from baseline or as the percentage of baseline startle. Given the strong link between the modulation of PPI and dopaminergic neurotransmission in the nucleus accumbens, the present data support the hypothesis that chronic low-level oral lead exposure facilitates dopamine neurotransmission in the nucleus accumbens.

Comparison of the stimulus properties of a pre- vs. a putative postsynaptic dose of quinpirole

Presynaptic D2-like receptors appear to mediate the stimulus properties of a low dose (0.05 mg/kg) of the D2-like agonist quinpirole (QUIN), because treatments decreasing dopamine (DA) release or blocking postsynaptic DA receptor activation produce QUIN-appropriate responding in a drug discrimination context, whereas treatments activating postsynaptic DA receptors evoke saline responding (28). This study examined the hypothesis that training to a presumably postsynaptic dose of QUIN (0.20 mg/kg) would produce the opposite pattern of effects. Using drug discrimination procedures, substitution for 0.05 mg/kg (28), but not 0.20 mg/kg QUIN, was produced by the D1 antagonist SCH23390, the catecholamine depleter alpha-methyl-paratyrosine and low doses of apomorphine (up to 0.25 mg/kg). The D2 agonist NPA substituted fully for 0.05 but only partially for 0.20 mg/kg QUIN. Cocaine and d-amphetamine (alone or with SCH 23390) substituted only minimally for either QUIN training dose. The putative D3 agonist 7-OH-DPAT engendered primarily saline responding when substituted for 0.20 QUIN. The 0.20 QUIN stimulus was antagonized by the D2 blocker haloperidol and partially blocked by the D1 antagonist SCH 23390. These data show a clear difference in the mediation of the stimulus properties of a low (0.05 mg/kg) vs. a high (0.20 mg/kg) dose of QUIN and are suggestive of a preferential postsynaptic D2 mediation of the 0.20 mg/kg QUIN dose.