Survival And Development Of Dopaminergic Neurons Affected By Dopamine Receptor Agonists

Apomorphine, the catechol-derived dopamine D1/D2 receptor agonist, is currently in use as an anti-parkinsonian drug. We have previously reported that apomorphine was able to elicit expression of the enzyme tyrosine hydroxylase, a marker for DA neurons, in the fetal rat cerebrocortical cultures whilst in the presence of brain-derived neurotrophic factor. The present study demonstrated that treatment of fetal rat ventral mesencephalic cultures with apomorphine caused marked increase in the number of dopaminergic neurons. The action of apomorphine can be mimicked by dopamine receptor (D1 and D2) agonists or blocked by pre-incubation with the receptor antagonists. Incubation of recipient mesencephalic cultures with the conditioned medium derived from apomorphine-stimulated donor mesencephalic cultures elicited 3.72-fold increase in the number of TH-positive neurons. Increased mRNA expression levels of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor were found in the apomorphine-treated mesencephalic cells and their protein expression levels were also elicited in the conditioned medium. Moreover, the trophic activity observed could be partially neutralized by antibodies against either brain-derived neurotrophic factor or glial cell line-derived neurotrophic factor. Cultured fetal striatal cells, but not hippocampal cells, also responded to apomorphine -treatment. The membrane filtration studies revealed that both < 30 kDa and >50 kDa fractions contained trophic activities. The latter characterization distinguishes themselves from most known neurotrophic factors. These results suggest that the apomorphine-modulated development of dopaminergic neurons may be mediated by activation of the dopamine receptor subtypes D1 and D2 thereby increasing the production of multiple growth factors.We also investigated the in influence of (-)-stepholidine, an effective dopamine D1 receptor agonist and D2 receptor antagonist, on the development of neural precursor cells. Incubation of striatal neural precursor cells with stepholidine resulted in significant increase in the number of proliferating precursor cell spheres when in the presence of fibroblast growth factor 2.This action can be blocked by application of haloperidol. Treatment with protein-2 immunoreactive cells in the cultures and promoted marked increases in tyrosine hydroxylase expression. These findings suggest that stepholidine is involved in the regulation of proliferation of precursor cells. The effect appears to be mediated by dopamine receptors. Stepholidine also promotes the differentiation of precursor cells, however, this action may be independent of Its effect on dopaminergic receptors.