High-resolution mapping of neuronal activity using the lipophilic thallium chelate complex TlDDC: Protocol and validation of the method

In neurons the rate of K+-uptake increases with increasing activity. K+-analogues like the heavy metal ion thallium (Tl+) can be used, therefore, as tracers for imaging neuronal activity. However, when water-soluble Tl+-salts are injected systemically only minute amounts of the tracer enter the brain and the Tl+-uptake patterns are influenced by regional differences in blood-brain barrier (BBB) K+-permeability. We here show that the BBB-related limitations in using Tl+ for imaging neuronal activity are no longer present when the lipophilic Tl+ chelate complex thallium diethyldithiocarbamate (TlDDC) is applied. We systemically injected rodents with TlDDC and mapped the Tl+-distribution in the brain using an autometallographic (AMG) technique, a histochemical method for detecting heavy metals. We find that Tl+-doses for optimum AMG staining could be substantially reduced, and regional differences attributable to differences in BBB K+-permeability were no longer detectable, indicating that TlDDC crosses the BBB. At the cellular level, however, the Tl+-distribution was essentially the same as after injection of water-soluble Tl+-salts, indicating Tl+-release from TlDDC prior to neuronal or glial uptake. Upon sensory stimulation or intracortical microstimulation neuronal Tl+-uptake increased after TlDDC injection, upon muscimol treatment neuronal Tl+-uptake decreased. We present a protocol for mapping neuronal activity with cellular resolution, which is based on intravenous TlDDC injections during ongoing activity in unrestrained behaving animals and short stimulation times of 5 min.