Yolanda Zea-Ponce

Modulation of amphetamine-induced striatal dopamine release by ketamine in humans: implications for schizophrenia

BACKGROUND Recent brain imaging studies have indicated that schizophrenia is associated with increased amphetamine-induced dopamine release in the striatum. It has long been hypothesized that dysregulation of subcortical dopamine systems in schizophrenia might result from a failure of the prefrontal cortex (PFC) to adequately control subcortical dopaminergic function. The activity of midbrain dopaminergic neurons is regulated, in part, by glutamatergic projections from the PFC acting via glutamatergic N-methyl-D-aspartate (NMDA) receptors. The goal of this study was to test the hypothesis that a pharmacologically induced disruption of NMDA transmission leads to an increase in amphetamine-induced dopamine release in humans. METHODS In eight healthy volunteers, we compared striatal amphetamine-induced (0.25 mg/kg) dopamine release under control conditions and under sustained disruption of NMDA transmission induced by infusion of the noncompetitive NMDA antagonist ketamine (0.2 mg/kg intravenous bolus followed by 0.4 mg/kg/hour intravenous infusion for 4 hours). Amphetamine-induced dopamine release was determined with single photon emission computed tomography, as the reduction in the binding potential (BP) of the radiolabeled D(2) receptor antagonist [(123)I]IBZM. RESULTS Ketamine significantly enhanced the amphetamine-induced decrease in [(123)I]IBZM BP, from -5.5% +/- 3.5% under control conditions to -12. 8% +/- 8.8% under ketamine pretreatment (repeated-measures analysis of variance, p =.023). CONCLUSIONS The increase in amphetamine-induced dopamine release induced by ketamine (greater than twofold) was comparable in magnitude to the exaggerated response seen in patients with schizophrenia. These data are consistent with the hypothesis that the alteration of dopamine release revealed by amphetamine challenge in schizophrenia results from a disruption of glutamatergic neuronal systems regulating dopaminergic cell activity.

PET studies of binding competition between endogenous dopamine and the D1 radiotracer [11C]NNC 756

NNC 756 ((+)‐8‐chloro‐5‐(2,3‐dihydrobenzofuran‐7‐yl)‐7‐hydroxy‐3‐methyl‐2,3,4,5‐tetrahydro‐1H‐3‐benzazepine) is a new high affinity dopamine (DA) D1 receptor antagonist. Labeled with C‐11, it has been used as a PET radiotracer to visualize D1 receptors both in striatal and extrastriatal areas, such as the prefrontal cortex. The goal of this study was to evaluate several methods for derivation of D1 receptor binding potential (BP) with [11C]NNC 756 in baboons, and to use these methods to assess the vulnerability of [11C]NNC 756 binding to competition by endogenous DA. A three‐compartment model provided a good fit to PET data acquired following a single bolus injection. BP values obtained with this analysis were in good agreement with values derived from in vitro studies. BP values measured following injection of the potent DA releaser amphetamine (1 mg/kg, n = 2) were similar to values measured under control conditions. Kinetic parameters derived from single bolus experiments were used to design a bolus plus continuous infusion administration protocol aimed at achieving a state of sustained binding equilibrium. Injection of amphetamine during sustained equilibrium did not affect [11C]NNC 756 binding. Similar results were observed with another D1 radiotracer, [11C]SCH 23390. Doses of amphetamine used in this study are known to reduce by 20–40% the binding potential of several D2 receptors radiotracers. Therefore, the absence of displacement of [11C]NNC 756 by an endogenous DA surge may indicate important differences between D1 and D2 receptors in vivo, such as differences in proportion of high affinity states not occupied by DA at baseline. These findings may also imply that a simple binding competition model is inadequate to account for the effects of manipulation of endogenous DA levels on the in vivo binding of radiolabeled antagonists. Synapse 32:93–109, 1999.

Stability of [123I]IBZM SPECT measurement of amphetamine-induced striatal dopamine release in humans.

Binding competition between endogenous dopamine (DA) and the D2 receptor radiotracer [123I]IBZM allows measurement of the change in synaptic DA following amphetamine challenge with SPECT in the living human brain. Previous investigations using this technique in healthy subjects have shown that the magnitude of amphetamine effect on [123I]IBZM binding potential (BP) is small (range between 5 to 15% decrease), and that a large between‐subject variability in this effect is observed. Therefore, it was unclear how much of the apparent between‐subject variability was due to a low signal‐to‐noise ratio in the measurement, vs. true between‐subject differences in the magnitude of the response. The goals of this investigation were to test the within‐subject reproducibility and reliability of amphetamine‐induced decrease in [123I]IBZM BP with a test/retest paradigm, and to establish the presence or absence of tolerance or sensitization to single administration of i.v. amphetamine. Six healthy male subjects, never previously exposed to psychostimulants, twice underwent measurement of striatal amphetamine‐induced DA release (between‐measurement interval 16 ± 10 days) using SPECT and the [123I]IBZM constant infusion technique. Results demonstrated an excellent within‐subject reproducibility of amphetamine‐induced DA release: amphetamine‐induced decreases in [123I]IBZM BP were significant on each day, and had an intraclass correlation coefficient (ICC) of 0.89. Moreover, values from the second experiment were not significantly different from first experiment, suggesting the absence of either sensitization or tolerance to the effect of amphetamine on DA release in these experimental conditions. The subjective activation, as rated by the subjects on analog scales, was also highly reproducible. In conclusion, this scanning technique provides a reliable measurement of amphetamine‐induced reduction of [123I]IBZM BP and enables detection of between‐subject differences that appear stable over time. Synapse 31:302–308, 1999.

Pharmacokinetics and brain distribution in non human primate of R(-)[123I]DOI, A 5HT2A/2C serotonin agonist

Our goal was to synthesize with high specific activity R(-)-1-(2,5-Dimethoxy-4-[123I]iodophenyl)-2-aminopropane [R(-)[123I]DOI], an in vitro potent and selective 5-HT(2A/2C) serotonin agonist, and study in vivo its plasma pharmacokinetics and brain distribution in baboon by SPECT. The purpose was to evaluate this radiotracer as a potential tool in discerning the role of the agonist high affinity state of 5-HT(2) receptors in depression and other neurological disorders. The radiotracer was prepared by electrophilic radioiodination of the N-trifluoroacetyl precursor of R(-)-1-(2,5-Dimethoxyphenyl)-2-aminopropane [R(-)DMA-TFA] with high-purity sodium [123I]iodide in the presence of chloramine-T, followed by amino deprotection with KOH in isopropanol (labeling yield: 73%, radiochemical yield: 62%, radiochemical purity: 99%). In vivo studies in baboon showed high accumulation of radioactivity in thalamus, the frontoparietal cortex, temporal, occipital and the striatum regions, with slightly lower accumulation in the midbrain and cerebellum. Ketanserin did not displaced the radioactivity in any of these brain regions. Plasma metabolite analysis was performed using methanol protein precipitation, the methanol fractions contained from 68% to 92% of the mixture of a labeled metabolite and parent compound. The recovery coefficient of unmetabolized R(-)[123I]DOI was 68%. The percent parent compound present in the extracted fraction, measured by HPLC, decreased gradually with time from 99.8% to 0.3% still present after 4.7 hours post injection whereas the percentage of the only one detected metabolite increased conversely. Free fraction determination (f(1)), was 31 +/- 0.9% (n = 3). For comparison purposes, ex-vivo brain distribution, displacement and metabolite analysis was also carried out in rodents. Although R(-)[123I]DOI displayed good brain uptake and localized in serotonergic areas of the brain, its target to non target ratio and its insensitivity to ketanserin displacement suggest high nonspecific uptake, therefore non potentially useful as brain imaging radiotracer for visualization of the agonist high-affinity state of 5-HT(2A) receptors and for visualizing 5-HT(2C) receptors by SPECT.

Synthesis of [123I]IBZM: a reliable procedure for routine clinical studies

The single photon emission computed tomography (SPECT) D2/D3 receptor radiotracer [123I]IBZM, is prepared by electrophilic radioiodination of the precursor BZM with high-purity sodium [123I]iodide in the presence of diluted peracetic acid. However, in our hands, the most commonly used procedure for this radiosynthesis produced variable and inconsistent labeling yields, to such extent that it became inappropriate for routine clinical studies. Our goal was to modify the labeling procedure, to obtain consistently better labeling and radiochemical yields. The best conditions found for the radioiodination were as follows: 50 microg precursor in 50 microL EtOH mixed with buffer pH 2; Na[123I]I in 0.1 M NaOH (< 180 microL), 50 microL peracetic acid diluted solution, heating at 65 degrees C for 14 min. Purification was achieved by solid phase extraction (SPE) and reverse-phase high performance liquid chromatography (HPLC). Under these conditions, labeling yield average was 76 +/- 4% (n = 31); radiochemical yield was 69 +/- 4% and radiochemical purity was 98 +/- 1%. With larger volumes of the Na[123I]I solution the yields were consistent but lower. For example, for volumes between 417 and 523 microL the labeling yield was 61 +/- 5% (n = 21), radiochemical yield was 56 +/- 5% and radiochemical purity was 98 +/- 1%.

Protein precipitation: an expedient procedure for the routine analysis of the plasma metabolites of [123I]IBZM

Plasma metabolite analysis of the single photon emission computed tomography (SPECT) D2/D3 receptor radiotracer (S)(-)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2-hydroxy-3-[123I]iodo-6-metho xybenzamide ([123I]IBZM) is needed for the equilibrium analysis of the SPECT data, in brain imaging studies involving bolus plus constant infusion paradigm. The purpose of these experiments was to find an appropriate procedure to expedite this analysis during routine determinations. The procedure was applied to the plasma analysis of 22 human subjects. Each plasma sample was subjected to acetonitrile protein precipitation. After separation of the pellet, the acetonitrile fraction contained 91% +/- 2% (n = 88) of the mixture of labeled metabolites and parent compound. The recovery coefficient of unmetabolized [123I]IBZM determined with an standard plasma sample was 95% +/- 2% (n = 22). The percent parent compound present in the extracted fraction, measured by high performance liquid chromatography, was 16% +/- 9% (n = 85) and the percent metabolites was 84% +/- 9% (n = 85). Free fraction determination (f1, fraction of radiotracer unbound to protein), was 4% +/- 0.8% (n = 22). Free fraction of parent was 15% +/- 8% (n = 85). The results indicate that acetonitrile protein precipitation is an adequate method for the analysis of the [123I]IBZM plasma metabolites.