Caffeine Protects Dopaminergic Neurons From Dopamine-Induced Neurodegeneration via Synergistic Adenosine-Dopamine D2-Like Receptor Interactions in Transgenic Caenorhabditis elegans

Abstract

Previous studies have suggested that caffeine reduces the risk of L-DOPA-induced dyskinesia. However, caffeine is also known to promote dopamine signaling, which seemingly contradicts this observed effect. To this end, the study aimed to clarify the mechanism of caffeine neuroprotection in vivo when excess dopamine is present. Transgenic Caenorhabditis elegans (UA57) overproducing dopamine was exposed to caffeine for 7 days and monitored by observing GFP-tagged dopaminergic (DA) neurons via fluorescence microscopy. Caffeine (10 mM) prevented neuronal cell loss in 96% of DA neurons, with a mean GFP intensity that is 40% higher than control (0.1% DMSO). To confirm if cAMP plays a role in the observed neuroprotection by caffeine, cAMP levels were elevated via forskolin (10 μM), an adenylyl cyclase activator. Forskolin (10 μM) exposure did not confer neuroprotection and was similar to control (0.1% DMSO) at the 7th day, suggesting that cAMP is not the sole secondary messenger utilized. Rotigotine (160 μM), a dopamine D2-like receptor (DOP2R) agonist, was not able to confer significant neuroprotection to the nematodes. This suggests that DOP2R activation is necessary but insufficient to mimic neuroprotection by caffeine. Lastly, co-administration of caffeine (10 mM) with olanzapine (160 μM), a DOP2R antagonist, eliminated neuroprotection. This suggests that the protective effect must involve both adenosine receptor antagonism and activation of DOP2Rs. Taken together, we show that caffeine protects DA neurons from dopamine-induced neurodegeneration and acts by modulating adenosine receptor-DOP2R interactions in C. elegans.

Caffeine reduces deficits in mechanosensation and locomotion induced by L-DOPA and protects dopaminergic neurons in a transgenic Caenorhabditis elegans model of Parkinson’s disease

“…This finding is independent of the neuronal profile, noting that nematodes exposed to 60LC20 had significant neurodegeneration compared with those exposed to 60LC10 (p < 0.05), yet the body-bending rates of both treatment groups were significantly less than that of caffeine or L-DOPA alone. Although these rates were significantly greater than in vehicle, these findings suggest that caffeine greatly reduces the rate of locomotion when coadministered with L-DOPA, which may point out to its effect on presynaptic autoreceptor dopamine D2-like receptors (DOP2Rs) that are inhibitory when dopamine release is in excess (Manalo and Medina 2018a).…”

“…Previously, we have shown that C. elegans strain UA57 exposed to 10 mM caffeine had less degeneration of the four cephalic and two anterior deirid neurons, which otherwise would have degenerated drastically in vehicle (0.1% DMSO). Further, we elucidated and proposed a mechanism thought to be conserved between C. elegans and mammals, that is, the close communication between adenosine receptor antagonism and activation of dopamine D2like receptors (DOP2Rs) in modulating excessive dopamine synthesis or release (Manalo and Medina 2018a). However, there is no clear-cut correlation between the neuroprotection conferred by caffeine and its effects on behaviour, which is probably more relevant to drug discovery and development for Parkinson's disease.…”

“…Briefly, five (5) treatment groups were used: (1) Negative control (0.1% DMSO) of the UA57 strain, (2) 60 mM L-DOPA, (3) 10 mM caffeine, (4) 60 mM L-DOPA þ 10 mM caffeine, and (5) 60 mM L-DOPA þ 20 mM caffeine. The concentration of caffeine (10 mM) was based on three studies: the highest mean lifespan extension of C. elegans at 20 C with the greatest delay in age-related pathology using 10 mM caffeine (Sutphin et al 2012); lifespan extension of C. elegans using 5 mM and 15 mM caffeine with significant developmental delay at 15 mM and reduced lifespan at 30 mM (Bridi et al 2015), and our previous study showing persistent protection of DAergic neurons in C. elegans at 10 mM (Manalo and Medina 2018a). These therefore demonstrate that the safe and efficacious concentration of caffeine is at 10 mM, and an excessive concentration is within 15-30 mM caffeine, which we chose to be 20 mM.…”

“…Age synchronisation and sampling were based on the protocol of Manalo and Medina (2018a), with slight modifications. Briefly, nematodes (N ¼ 15-20) at stage L4 to young-adulthood were transferred via worm-picking into prepared NGM plates seeded with Escherichia coli strain OP50 for all assays.…”

“…Thus, current research aims to further understand how the adverse effects of L-DOPA could be reduced or minimised. Previously, we have shown that in a transgenic strain (UA57) of Caenorhabditis elegans (Rhabditidae), caffeine can protect dopaminergic (DA) neurons in the presence of dopamine overproduction (Manalo and Medina 2018a). This result suggested that caffeine can potentially work hand-in-hand with L-DOPA not only to prevent neurodegeneration but also to restore bodily function.…”