Locus coeruleus promotes survival of dopamine neurons in ventral mesencephalon. An in oculo grafting study

Parkinson's disease is a neurodegenerative disorder where dopamine neurons in the substantia nigra of ventral mesencephalon undergo degeneration. In addition to the loss of dopamine neurons, noradrenaline neurons in the locus coeruleus degenerate, actually to a higher extent than the dopamine neurons. The interaction between these two nuclei is yet not fully known, hence this study was undertaken to investigate the role of locus coeruleus during development of dopamine neurons utilizing the intraocular grafting model. Fetal ventral mesencephalon and locus coeruleus were implanted either as single grafts or co-grafts, placed in direct contact or at a distance. The results revealed that the direct attachment of locus coeruleus to ventral mesencephalon enhanced graft volume and number of tyrosine hydroxylase (TH)-positive neurons in ventral mesencephalic grafts. Cell counts of subpopulations of TH-positive neurons also immunoreactive for aldehyde dehydrogenase 1-A1 (ALDH1) or calbindin, revealed improved survival of ALDH1/TH-positive neurons. However, the number of calbindin/TH-positive neurons was not affected. High density of dopamine-beta-hydroxylase (DBH)-positive innervation in the ventral mesencephalon placed adjacent to locus coeruleus was correlated to the improved survival. Ventral mesencephalic tissue, implanted at a distance to locus coeruleus, did not demonstrate improved survival, although DBH-positive nerve fibers were detected. In conclusion, the direct contact of locus coeruleus resulting in dense noradrenergic innervation of ventral mesencephalon is beneficial for the survival of ventral mesencephalic grafts. Thus, when trying to rescue dopamine neurons in Parkinson's disease, improving the noradrenergic input to the substantia nigra might be worth considering.