You’ve heard the expression, “good cop, bad cop.”

How about “good dopamine, bad dopamine?”

Dopamine is an essential neurotransmitter that falls into short supply in Parkinson’s patients. Without it, people develop severe problems with paralysis and tremors.

But Columbia University Medical Center scientists think too much dopamine in the wrong places may be lethal to brain cells called neurons.

In a healthy brain, dopamine is manufactured inside of neurons, then stuffed into compartments and shipped to the edges of the cell, where it is released.

But according to the new theory, if dopamine escapes those compartments before it’s released, it mixes with benign chemicals swimming in the cell to produce a lethal soup.

Everybody has heard of one of the chemicals … calcium.

The other is a protein called alpha-synuclein.

When these three tangle, the result is toxic.

Calcium channels somehow promote dopamine, and excess dopamine reacts with alpha-synuclein to create deadly waste products.

And the cell dies.

The good news is neurons will likely survive if any of the trio of elements can be contained or controlled.

That’s a ray of hope for drugs that block calcium channels, which have already reached clinical trials.

But the better treatment is probably many years away.

If dopamine can be stuffed back into compartments, perhaps through some kind of gene therapy, researchers say it would no longer be toxic, and the cell would have more dopamine to safely release.

Then dopamine would be all good.