Microglial responses to dopamine in a cell culture model of Parkinson's disease

Abstract

Activated microglia appear to selectively attack dopamine (DA) neurons in the Parkinson's disease (PD) substantia nigra. We investigated potential mechanisms using culture models. As targets, human SH-SY5Y cells were left undifferentiated, or were differentiated with retinoic acid (RA) or RA plus brain-derived neurotrophic factor (RA/BDNF). RA/BDNF-treated cells were immunoreactive for tyrosine hydroxylase and the DA transporter, took up exogenous DA, and released DA after K + stimulation. Undifferentiated and RA-treated cells lacked these characteristics of a DA phenotype. Co-culture of target cells with human elderly microglia resulted in elevated toxicity in DA phenotype (RA/BDNF) cells. Lipopolysaccharide plus K + -stimulated DA release enhanced toxicity by 500-fold. DA induced microglial chemotaxis in Boyden chambers. Spiperone inhibited this effect. Cultured human elderly microglia expressed mRNAs for D1-D4 but not D5 DA receptors. The microglia, as well as PD microglia in situ, were also immunoreactive for D1-D4 but not D5 DA receptors. These findings demonstrate that activated microglia express DA receptors, and suggest that this mechanism may play a role in the selective vulnerability of DA neurons in PD.

Physiology of Microglia

“…The dopamine-induced chemotaxis was confirmed also for cultured elderly human microglia (555). Indeed, the D 1 , 2,3,4 (but not D 5 ) receptors were identified on trans-lational level in microglia in culture and visualized by immunostaining in substantia nigra from Parkinsonian brains (555), suggesting that activation of microglia in Parkinson's disease triggers expression of dopamine receptors, which may explain an increase in microglia-related toxicity towards dopamine-producing neurons (555). Incidentally, concentration of microglial cells in the substantia nigra (the brain region primarily affected in PD) is the highest in the brain: in mouse microglia amount to ϳ12% of total cells number in this region, compared with ϳ5% in the cortex and in the corpus callosum (498).…”

“…This indicates that dopamine does not mediate its effect by triggering adrenergic receptors, but provides evidence for microglial dopamine receptors expression (267). The dopamine-induced chemotaxis was confirmed also for cultured elderly human microglia (555). Indeed, the D 1 , 2,3,4 (but not D 5 ) receptors were identified on trans-lational level in microglia in culture and visualized by immunostaining in substantia nigra from Parkinsonian brains (555), suggesting that activation of microglia in Parkinson's disease triggers expression of dopamine receptors, which may explain an increase in microglia-related toxicity towards dopamine-producing neurons (555).…”

Neurotrophic Effects of Growth/Differentiation Factor 5 in a Neuronal Cell Line

“…Some authors have previously reported that TH is readily expressed by SH-SY5Y cells, while others reported that TH is not expressed in undifferentiated SH-SY5Y cells (Gomez-Santos et al 2002;Mastroeni et al 2008;McMillan et al 2007). In addition, GDF5 has previously been reported to induce DA differentiation of rat primary VM cultures (O'Keeffe et al 2004a).…”

“…SH-SY5Y cells have been reported by some groups to readily express tyrosine hydroxylase (TH, the ratelimiting enzyme in the synthesis of DA) (Gomez-Santos et al 2002;McMillan et al 2007), while others failed to demonstrate expression (Mastroeni et al 2008). To assess whether the neuroblastoma cell line model used here could recapitulate the results obtained using primary cultures, we analysed TH expression by RT-PCR, immuncytochemistry and immunoblotting in cultures maintained in the presence of 100 ng/ml GDF5 for 4 to 7 days.…”

De Novo Expression of Dopamine D2 Receptors on Microglia after Stroke

“…This is in line with results from cultured human elderly microglia that express DRD1, DRD2, DRD3, and DRD4 mRNAs. 23 Interestingly, a small fraction of human peripheral blood monocytes also express D2R. 24 The responses of cultured microglia to dopamine administration are heterogenous, suggesting that dopamine receptor expression may depend on functional and activation states.…”