I was digging in a very positive Berberine article: Berberine: A Promising Treatment for Neurodegenerative Diseases 2022
When I came across this: "Meanwhile, there are several studies offer some important insights into the neurotoxic effects of berberine. It is reported that in the Parkinson’s disease model rats induced by 6-OHDA, berberine aggravate the degeneration of dopaminergic neuron in the substantia nigra of rats (Shin et al., 2013). In addition, berberine can aggravate the cytotoxicity induced by 6-OHDA in PC12 cells and aggravate of dopaminergic neuron death (Kwon et al., 2010)."
So... these are the referenced articles:
Neurotoxic effects of berberine on long-term L-DOPA administration in 6-hydroxydopamine-lesioned rat model of Parkinson's disease 2013 pubmed.ncbi.nlm.nih.gov/235... "However, both concentrations of berberine in 6-OHDA-lesioned groups treated with L-DOPA aggravated the numbers of TH-immunopositive neurons in the substantia nigra and the levels of dopamine, norepinephrine, DOPAC and HVA in the striatum as compared to rats not treated with berberine. These results suggest that berberine leads to the degeneration of dopaminergic neuronal cells in the substantia nigra in the rat model of PD with chronic L-DOPA administration. Long-term L-DOPA therapy that may involve possibly neurotoxic isoquinoline agents including berberine should involve monitoring for adverse symptoms."
Effects of berberine on 6-hydroxydopamine-induced neurotoxicity in PC12 cells and a rat model of Parkinson's disease 2010 pubmed.ncbi.nlm.nih.gov/208... "In addition, treatment with berberine (5 and 30mg/kg, i.p.) for 21 days in 6-OHDA-lesioned rats markedly depleted tyrosine hydroxylase-immunopositive cells in the substantia nigra as compared to berberine-untreated rats. Further, the levels of dopamine and norepinephrine were also significantly decreased by berberine administration (5 and 30mg/kg) in the striatal regions of 6-OHDA-lesioned rats. These results suggested that berberine aggravated 6-OHDA-induced cytotoxicity in PC12 cells, and led to the degeneration of dopaminergic neuronal cells in the substantia nigra of 6-OHDA-lesioned rats. It is, therefore, suggested that the use of long-term l-DOPA therapy with isoquinoline derivatives including berberine may need to be examined for the presence of adverse symptoms."
I will keep digging.