D1-Class Receptor Antagonism Following Morphine Sensitization in Adolescent Pb-Exposed Mice

Lead (Pb) is a neurotoxin that can leak from aging pipes into drinking water, disproportionately harming disadvantaged communities and demanding stronger protective legislation.

Exposure to toxic Pb concentrations (300 mg/L) in C57BL/6 mice produces increases in locomotor behavior.

Repeated administration of morphine produces dopaminergic sensitization. The D1-class receptor antagonist, SCH 23390 HCl, blocks morphine-induced hyperactivity.

We hypothesized that adolescent Pb exposure would disrupt mice’s time spent in the center zone, locomotor response to initial morphine exposure, morphine sensitization, and D1-class receptor antagonism via SCH 23390 HCl.

Subjects: 32 C57BL/6J male (n=16) and female (n=16) mice were bred from breeding pairs purchased from The Jackson Laboratory, Bar Harbor, ME.  

Lead Exposure: Pb exposure via drinking water was used following protocols by Rangel-Barajas et al. (2020), Hernández et al. (2023), and Garcy and Boehm II (2023). At PND 21, the experimental group (n=16) was exposed continuously to 30 ppm Pb acetate dissolved in distilled water containing 0.5% glacial acetic acid until PND 42. The control group (n=16) received distilled water containing 0.5% glacial acetic acid. 

Habituation: After PND 42, the experimental group (n=16) and control group (n=16) were subjected to habituation in the open-field locomotor testing box for 60 minutes on Day 0 of testing.

Morphine Sensitization: The experimental group (n=16) and control group (n=16) were subjected to morphine sensitization. Days 2-8, the mice were injected with a daily 20 mg/kg IP morphine dose to induce sensitization.

SCH 23390 HCl D1-Class Receptor Antagonism: Half of the experimental group (n=8) and control group (n=8) received a 0.01 mg/kg IP injection of SCH 23390 HCl following morphine sensitization on Day 8. The other half of the mice received saline injections.

Behavioral Testing: Mice were divided into 9 cohorts (3–4 mice per cohort). Day 0 consisted of a 60-minute habituation test. Days 1, 2, 7, and 8, mice were tested in the open-field apparatus for 120 minutes.

Conclusion

Pb did not significantly affect baseline locomotion.

Pb did not significantly impact morphine sensitization but did reduce morphine-induced locomotion.

There did not appear to be a significant dysregulation of D1-class receptor activity in Pb mice.

Pb did not significantly influence total time mobile in any of the experiments.

Future Directions

Expose mice to a higher lead concentration (closer to 300 mg/L) to test whether the directional effect on locomotion depends on exposure dose.

Test a higher dose of SCH 23390 HCl to better assess the D1-class receptor activity contribution.

Use a higher concentration injection solution to limit total fluid volume injected.

Explore alternative injection sites beyond intraperitoneal (e.g. subcutaneous).

Investigate alternative mechanisms beyond D1-class antagonism, such as D2-class receptor involvement or glutamatergic signaling.

Power future studies specifically to detect sex-specific effects of Pb exposure.

Use other drugs like fentanyl, cocaine, or amphetamines to enhance locomotion sensitization differently from morphine to possibly better analyze the effect of SCH 23390 HCl.

Research supported by the National Institute on Drug Abuse and the Office of Research for Undergraduates at Miami University.