Another debated issue is whether levodopa offers protection or is harmful to the remaining dopamine neurons. The latest studies in tissue culture show that when glia (the brain's supportive cells) tissue is present in addition to the nerve cells, the glia tissue becomes protective against any levodopa toxicity. Because glia tissue is present in brain, the argument has been made that levodopa should not be toxic in living brain tissue. A few studies have been carried out in animal models of PD. Two of these animal studies suggest levodopa is toxic to neurons, and two show that levodopa is not toxic and may actually have a protective effect. So there is no convincing or consistent evidence that levodopa damages dopamine neurons in humans or animal models of PD.