Dopamine, one of the essential brain neurotransmitters1,2, is mainly released by midbrain dopaminergic (DA) neurons. The majority of DA neurons reside in the ventral part of the mesencephalon2-6. Schematically, midbrain DA neurons can be divided in three anatomically and functionally distinct projection systems: mesostriatal, mesolimbic, and mesocortical pathways2,5. The nigrostriatal pathway is involved in motor behavior, the mesolimbic pathways play an important role in reinforcement, motivation, and learning, whereas the dopaminergic pathways projecting to the prefrontal cortex are implicated in cognition2.
DA neurons are involved in several human neurological disorders such as schizophrenia, attention deficit, hyper activity disorder, and Parkinson’s disease (PD)2,4. PD is characterized by a progressive and selective degeneration of DA neurons connecting substantia nigra pars compacta (SNc) to the striatum. The loss of nigro-striatal DA neurons results in severe dopamine depletion in the striatum that is responsible of the motor symptoms of PD (bradykinesia, resting tremor, and rigidity)7. The initial cause of the idiopathic PD has not been established and the current treatments are only symptomatic, aiming at restoring dopamine level in the striatum. The most prescribed drug is L-Dopa (Levodopa), the natural precursor of dopamine. Though administration of Levodopa compensates for the loss of dopamine for a certain time, motor complications occur after long-term treatments (dyskinesia and on/off states)8,9.
Research on dopaminergic neurons and PD is in constant progression and intense efforts are being made to develop treatments based on cell transplantation, gene therapy, or neuroprotective agents10,11. However, a major issue remains non-elucidated: what is the cause of the extreme vulnerability of DA neurons? Part of the answer can be found in the activity of DA neurons. A reduction in the electrical activity and of the excitability of DA neurons seems to augment their propensity to degenerate12. Nevertheless, the complexity of PD pathogenesis requires further studies to identify the mechanisms involved in DA neurons degeneration13-15.
Primary cultures are especially relevant to study DA neuron properties16-19 and to challenge these neurons to various stresses for evaluation of neuroprotective agents20-24. Rat culture models are most often used, as the dissection of rat embryo mesencephalon is easier, compared with the mouse, and higher amounts of neurons can be obtained in the rat. However, generation of transgenic mouse models of the disease25 has considerably increased the interest of the neuroscientist community for primary cultures from the mouse26-29. Although cultures prepared from newborn animals can be used, it is better to prepare them from embryos at the post-mitotic stage (E13.5 for mesencephalon neurons), when neurons have retained their capacity to differentiate. The following protocol presents isolated mesencephalon neurons in primary culture from mouse embryos (E13.5), which are the most difficult to prepare. Notably, we provide a protocol using serum-free culture medium for a better reproducibility. The two most critical steps in culture preparation (dissection and mechanical dissociation) will be carefully detailed in the associated video.