Interactions of lifetime lead exposure and stress: behavioral, neurochemical and HPA axis effects

Abstract

Lead (Pb) and stress co-occur as risk factors, share biological substrates and produce common adverse effects. We previously found that prenatal restraint stress (PS) or offspring stress (OS) could enhance maternal Pb-induced behavioral, brain neurotransmitter level and HPA axis changes. The current study examined how lifetime Pb exposure, consistent with human environmental exposure, interacts with stress. Dams were exposed to Pb beginning 2 mos prior to breeding (0, 50 or 150ppm in drinking water), PS on gestational days 16 and 17, or the combination. Offspring continued on the same Pb exposure as the dam. A subset of Pb+PS offspring also received 3 additional stress challenges (OS), yielding 9 exposure groups/gender: 0-NS, 0-PS, 0-OS, 50-NS, 50-PS, 50-OS, 150-NS, 150-PS and 150-OS. As with maternal Pb (Virgolini et al., 2008a), lifetime Pb and stress influenced Fixed Interval (FI) behavior primarily in females. Relative to 0-NS control, reductions in postreinforcement pause (PRP) times were seen only with combined Pb+PS (50-PS, 50-OS, 150-PS). Stress increased FI response rates when Pb alone was without effect (150-PS, 150-OS), but gradually mitigated rate increases produced by Pb alone (50-PS, 50-OS), effects that appear to be due primarily to PS, as they were of comparable magnitude in PS and OS groups. Individual subject data suggest that enhanced Pb and PS effects reflect increasing numbers of subjects shifting to the high end of the normal range of FI performance values, consistent with a dose-response type of Pb+stress additivity. Consistent with reports of cortico-striatal mediation of both interval timing (PRP) and FI rates, principal component analyses suggested potential mediation via altered frontal cortex norepinephrine, reduced nucleus accumbens dopaminergic control and enhanced striatal monoamine control. Altered FI performance, whether occurring through changes in response rate, PRP, or both, represent behavioral inefficiency and potentially sub-optimal or even dysfunctional resource/energy use.

Figure 1

Mean ± S.E. blood Pb levels (µg/dl) of dams (left) measured after 3 mos of Pb exposure (n=9–10) and at weaning (n=5/6 per stress group), and of female offspring (middle) and of male offspring (right) measured at 2 mos of age (n=2/stress condition) and following the termination of behavioral testing (n=5/group for females and n=3/group for males). * values from offspring at weaning (n=3–8/stress group for females and n=2–9/stress group for males) as reported in our previous studies [45; 48].

Figure 2

Mean ± S.E. corticosterone levels (ng/ml) of female (left) and male (right) offspring exposed to Pb (0, 50 or 150 ppm), and/or stress (none (NS), prenatal stress (PS) or prenatal followed by offspring stress (OS)) measured at approximately 2 mos of age (basal) and at the termination of behavioral testing (final; approximately 10 mos of age) in littermates of those offspring used in the 2 mos determinations. Solid horizontal line defines 0-NS control values. * signifies different from corresponding 0-NS control; # differs from corresponding Pb-NS value; + differs from 50-NS. Sample sizes: female basal corticosterone, n=5–10; male basal corticosterone, n=8–10; female final corticosterone, n=9–11; male final corticosterone, n=7–10.

Figure 3

Panel A. Group mean ± S.E. cytosolic (left column) and nuclear (right column) hippocampal glucocorticoid receptor density (units) in male (bottom row) and female (top row) behaviorally-tested offspring exposed to Pb, PS or OS in relation to Pb exposure level (ppm) as indicated. Dashed horizontal line depicts 0-NS control values. Pb=main effect of Pb, S=main effect of stress; PbxS= Pb by stress interaction in RMANOVA. Sample sizes: females: n=5–6; male: n=4–5. * differs from corresponding 0-NS control; # differs from 0-PS; +differs from corresponding Pb-NS group; $ differs from corresponding Pb-OS group; ^differs from 150-NS; ~ differs from 150-OS group. Panel B. Representative Western blots for female offspring for cytosolic (top row) and nuclear hippocampal (bottom row) glucocorticoid receptors at the designated 97Kd band.

Figure 4

Group mean ± S.E. overall response rates (top row), run rates (middle row) and postreinforcement pause time (bottom row) of female offspring exposed to 0 ppm Pb (left column), 50 ppm Pb (middle column) or 150 ppm (right column) alone or in conjunction with PS or OS across 25 blocks of 5 sessions each. Solid line flanked by dashed lines represents 0NS group mean ± S.E. values and are depicted across all Pb exposure groups. The break in the X axis shows sessions prior to and following the imposition of stress challenges in offspring (see Methods). Sample sizes, n=9–11 per group.

Figure 5

FI overall response rates (top row; responses per minute) and PRP times (bottom row; seconds) of individual 0 and 50 ppm Pb ± stress group subjects at session block 6 (left) and session block 15 (right).

Figure 6

Group mean ± S.E. overall response rates (top row), run rates (middle row) and postreinforcement pause time (bottom row) of male offspring exposed to 0 ppm Pb (left column), 50 ppm Pb (middle column) or 150 ppm (right column) alone or in conjunction with PS or OS across 25 blocks of 5 sessions each. Solid line flanked by dashed lines represents 0-NS group mean ± S.E. values and are depicted across all Pb exposure groups. The break in the X axis shows sessions prior to and following the imposition of stress challenges in offspring (see Methods). Sample sizes, n=9–11 per group.

Figure 7

Group mean ± S.E. values of frontal cortex levels of DA, DOPAC, DA TO, NE, 5HT and HIAA of female (top row) and male (bottom row) offspring in relation to Pb exposure and stress condition. Values are plotted as percent changes from corresponding group mean 0-NS values from littermates obtained at 2 mos of age. Solid gray horizontal lines depict the 100% of 2 mos values (no change from 2 mos of age). Sample sizes: females=6–11; males=8–11. Pb=main effect of Pb, S=main effect of stress; Pb x S = Pb by stress interaction in RMANOVAs. * differs from 0-NS control; # differs from 0-PS; + differs from 0-OS; $ differs from 50-NS; @ differs from corresponding Pb-NS group; ^ differs from corresponding 50 ppm group; & differs from 50-OS; % differs from 150-OS.

Figure 8

Group mean ± S.E. values of nucleus accumbens levels of DA, DOPAC, HVA, DA TO, NE, 5HT and HIAA of female (top row) and male (bottom row) offspring in relation to Pb exposure and stress condition. Values are plotted as percent changes from corresponding group mean 0-NS values from littermates obtained at 2 mos of age. Solid gray horizontal lines depict the 100% of 2 mos values (no change from 2 mos of age). Sample sizes: females=6–11; males=8–11. Pb=main effect of Pb, S=main effect of stress; Pb x S = Pb by stress interaction in RMANOVAs. * differs from 0-NS control; + differs from 0-OS; @ differs from corresponding Pb-NS group; & differs from 50-OS; % differs from 150-OS.

Figure 9

Group mean ± S.E. values of striatal levels of DA, DOPAC, HVA DA TO, NE, 5HT and HIAA of female (top row) and male (bottom row) offspring in relation to Pb exposure and stress condition. Values are plotted as percent changes from corresponding group mean 0-NS values from littermates obtained at 2 mos of age. Solid gray horizontal lines depict the 100% of 2 mos values (no change from 2 mos of age). Sample sizes: females=7–11; males=5–11. Pb=main effect of Pb, S=main effect of stress; Pb x S = Pb by stress interaction in RMANOVAs.

Figure 10

Relative strengths of female offspring on a scale from 0 to 1.0 for each neurotransmitter and FI overall response rate (y axis) from session block 10 derived from principal component analyses carried out for each Pb-stress group comparing 0-NS control values to all 50 ppm group values (panel A) and 0-NS control values to all 150 ppm group values (panel B). Open bars encompassing a specific neurotransmitter show systematic changes across the groupings.