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Demonstration of competition between endogenous dopamine and [11C]raclopride binding in in vitro brain slices using a dynamic autoradiography technique

To elucidate the mechanism of in vivo binding competition between radioligand and endogenously released transmitter, we examined the influence of depolarization‐induced dopamine (DA) release on [11C]raclopride‐specific binding to D2 receptors in slices of living brain tissues using dynamic positron autoradiography. Rat brain slices were incubated in a chamber with [11C]raclopride in oxygenated medium at 34°C for 150 min. Two‐dimensional images of radioactivity in the slices were recorded on a storage phosphor screen and dynamic changes were measured. When the brain slices were exposed to the depolarization agents (25 mM K+, 50 mM K+, and 20 μM veratridine), the percentage inhibition of striatal [11C]raclopride‐specific binding was 22 ± 4%, 44 ± 8% and 54 ± 7% of the control, respectively. The percentage inhibition of [11C]raclopride‐specific binding during each depolarization treatment agreed proportionally with the amount of DA released into the medium. However, preexposure of brain slices to the same depolarization treatment (50 mM K+) did not affect the [11C]raclopride‐specific binding, suggesting that the reduction in receptor density and/or affinity was not involved in the decrease of [11C]raclopride‐specific binding. [11C]Raclopride‐specific binding decreased dose‐dependently in the presence of exogenously added DA (range 0.005–3mM). The synaptic DA concentration during each depolarization treatment estimated using three different methods.