Effects of endogenous dopamine on measures of [18F]N-methylspiroperidol binding in the basal ganglia: comparison of simulations and experimental results from PET studies in baboons.

The effect of endogenous dopamine on PET measures of radioligand binding is important to the measurement of receptor density (or availability) and neurotransmitter interactions in vivo. We recently reported that pretreatment with amphetamine, a drug which stimulates dopamine release, significantly reduced NMS binding in the baboon brain as determined by the product lambda k3 derived from the graphical analysis method for irreversible systems (lambda is the ratio of the forward to reverse plasma to tissue transport constants and k3 is proportional to receptor density) (Dewey et al.: Synapse 7:324-327, 1991). The purpose of this work is twofold: to evaluate the sensitivity and stability of the analysis method used for the NMS data and from simulation studies which include the competitive effects of dopamine on NMS binding to predict the effect of dopamine on the in vivo PET experiment. Using a measured plasma [18F]-NMS input function from a control study in a baboon, simulation data was numerically generated explicitly allowing competition between NMS and dopamine in the calculation. This data was analyzed using the same techniques as used for the experimental data and the results were compared to in vitro calculations. The following conclusions were reached: 1) The effect of dopamine on specific binding was found to be greater in vivo than in vitro because the in vitro equilibrium experiment is controlled only by the relative Kd's of tracer and dopamine while the in vivo experiment also depends upon the halftime of tracer in tissue which is controlled by the tissue-to-plasma transport constant; 2) Experimental evidence from rodent studies (Seeman et al.: Synapse 3:96-97, 1989) and the agreement between PET studies (Wong et al.: Science 234:1558-1563, 1986a) and postmortem human studies (Seeman et al.: Science 225:728-731, 1984) in schizophrenics suggest that NMS is not likely to be affected by normal levels of endogenous dopamine. From the calculations reported here the effective in vivo Kd of dopamine for the NMS binding site would have to be on the order of or greater than 100 nM, assuming a synaptic dopamine concentration of 20 nM, in order that this concentration of dopamine have little effect on NMS binding.