The competition between endogenous transmitters and radiolabeled ligands for in vivo binding to neuroreceptors might provide a method to measure endogenous transmitter release in the living human brain with noninvasive techniques such as positron emission tomography (PET) or single photon emission computerized tomography (SPECT). In this study, we validated the measure of amphetamine-induced dopamine release with SPECT in nonhuman primates. Microdialysis experiments were conducted to establish the dose-response curve of amphetamine-induced dopamine release and to document how pretreatment with the dopamine depleter alpha-methyl-para-tyrosine (alpha MPT) affects this response. SPECT experiments were performed with two iodinated benzamides, [123I]IBZM and [123I]IBF, under sustained equilibrium condition. Both radio-tracers are specific D2 antagonists, but the affinity of [123I]IBZM (KD-0.4 nM) is lower than that of [123I]IBF (KD 0.1 nM). With both tracers, we observed a prolonged reduction in binding to D2 receptors following amphetamine injection. [123I]IBZM binding to D2 receptors was more affected than [123I]IBF by high doses of amphetamine, indicating that a lower affinity increases the vulnerability of a tracer to endogenous competition. With [123I]IBZM, we observed an excellent correlation between reduction of D2 receptor binding measured with SPECT and peak dopamine release measured with microdialysis after various doses of amphetamine. Pretreatment with alpha MPT significantly reduced the effect of amphetamine on [123I]IBZM binding to D2 receptors, confirming that this effect was mediated by intrasynaptic dopamine release. Together, these results validate the use of this SPECT paradigm as a noninvasive measurement of intrasynaptic dopamine release in the living brain.

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We used the reversibly binding D2 dopamine receptor radioligand [123I]IBZM (iodobenzamide) to test whether the endogenous neurotransmitter dopamine competes in vivo for radiotracer binding measured with single photon emission computed tomography (SPECT). In a series of nonhuman primate experiments (n = 27), the effects of temperature, amphetamine, haloperidol, and reserpine on brain uptake of [123I]IBZM were measured. Specific brain uptake of [123I]IBZM reached a peak by 100 min postinjection of radioligand and demonstrated a gradual, apparent "steady-state" washout over the next 2 hr. Brain uptake was temperature dependent, with rates of washout of specifically bound radioligand greater under normothermic conditions (26%/hr: core body temperature 35-37 degrees C) than under conditions of controlled hypothermia (11%/hr; 32-34 degrees C). Given the greater retention of radioactivity, low-temperature conditions were used in all other experiments. Administration of haloperidol (0.02 mg/kg IV) during the period of apparent steady state resulted in a dramatic increase in washout (60%/hr; p less than 0.0001), consistent with its potent D2 receptor antagonist properties. d-Amphetamine (1.0 mg/kg IV), which has negligible affinity for the D2 receptor but mediates the release of endogenous stores of dopamine, also enhanced washout (34%/hr; p less than 0.0005). Reserpine pretreatment at doses (1.0 mg/kg) sufficient to cause greater than 90% depletion of striatal dopamine levels blocked this amphetamine-enhanced washout (10%/hr; p less than 0.05). Reserpine did not block the increased washout induced by the direct-acting D2 receptor antagonist haloperidol. These results are consistent with the hypothesis that endogenous dopamine may effectively compete for radioligand binding in vivo in neuroreceptor imaging studies using PET and SPECT.