Objectives: Attention deficit hyperactivity disorder (ADHD) is linked to deficits in the midbrain dopaminergic system (i.e. hypodopaminergia). Yet the specific midbrain region(s) involved remain to be elucidated in mammals, particularly in relation to dopaminergic neuronal cell loss. If there is dopaminergic neuronal cell loss in one or more midbrain regions, then novel pharmacotherapies to target their rescue could be investigated.
Methods: A Sprague-Dawley rat model developed in the Oorschot laboratory was used. After exposure to repeated hypoxia during postnatal days 1 – 3, the animals were behaviourally characterized to be ADHD-like hyperactive and impulsive when older. The control repeated normoxic rats did not exhibit ADHD-like behaviour. To investigate midbrain anatomy in the same rats, they were anaesthetized at 18-months-of-age with ketamine/xylazine and perfused intracardially with 4% paraformaldehyde. The right midbrain was serially sectioned and immunostained with tyrosine hydroxylase to identify dopaminergic neurons in the ventral tegmental area, substantia nigra compacta ventral and dorsal tiers, and the retrorubral field. The absolute number of dopaminergic neurons in each midbrain region was measured stereologically using the Cavalieri and disector methods.
Results: Repeated hypoxia rats had a statistically significant specific loss of dopaminergic neurons in the right ventral tegmental area, providing new evidence of a site of pathology. No dopaminergic neuronal loss occurred in the three other midbrain regions. Fewer neurons in the ventral tegmental area correlated with increased ADHD-like hyperactivity and impulsivity.
Conclusion: Novel early intervention therapies to rescue dopaminergic neurons in the ventral tegmental area and potentially prevent ADHD-like hyperactivity/impulsivity can now be investigated.
Keywords: ADHD; hyperactivity/impulsivity; midbrain dopaminergic neurons; stereology; ventral tegmental area
Ethical statement: All animal procedures were approved by the Committee on Ethics in the Care and Use of Laboratory Animals at the University of Otago.
Funding statement: The research was funded by a University of Otago PhD Scholarship to Sarah Kohe.