Methological advances in the examination of the dopamine system in brain

Abstract

The dopamine (DA) system in brain has attracted considerable attention in neuroscience due to its involvement in fundamental brain functions and its postulated role in the pathophysiology of several neuropsychiatric disorders. Radioligand binding techniques in vitro and in vivo, such as receptor binding autoradiography and positron emission tomography (PET) have substantially contributed to our understanding of the DA system.

With the aid of these methodologies the distribution, density and affinity of dopamine receptors can be examined in the brain at physiological conditions and in neuropsychiatric disorders. More recent aspects of research on DA receptors are the examination of coupling between DA receptors and G proteins, selective labelling of different DA receptor subtypes and in vivo examination of DA receptors in small regions of the human brain with minute receptor densities. Examination of the aforementioned aspects of the DA system necessitates the establishment of new techniques to study DA receptors.

The aim of the present thesis was to apply and evaluate new in vitro and in vivo imaging methods for the examination of dopamine-D2/D3 receptors in the brain with special attention to the following three topics: (i) examination of the interaction between D2/D3 receptors and G proteins in human brain in vitro, (ii) selective visualization of D3 receptors with PET, and (iii) improvement of the accuracy in PET imaging of D2/D3 receptors by correction for partial volume effects (PVEs).

In the first study agonist stimulated [35S]GTPgS binding autoradiography was established for the examination of D2/D3 receptors in the postmortem human brain. The functional response to DA, the physiological agonist, and quinpirole, a prototype D2/D3 agonist was described in human whole hemisphere cryosections. The stimulatory effect of DA was primarily mediated by D2/D3 receptors. Both DA and quinpirole stimulated [35S]GTPgS binding to the highest level in the striatum. Moderate to low stimulation was observed in other brain regions, such as substantia nigra, thalamus, amygdala, hippocampus and anterior cingulate. The results indicate that this method could be a suitable tool for examination of coupling between D2/D3 receptors and G proteins in neuropsychiatric diseases.

The aim of the next two studies was to develop a radioligand for selective in vivo labelling of D3 receptors by PET. As currently no D3-selective radioligands are available for in vivo examinations, the binding of the putative D3-selective radioligand, [11C]RGH-1756, was evaluated in the monkey brain. Despite the promising in vitro characteristics of the molecule, [11C]RGH-1756 yielded very low signal for specific D3 binding in the monkey brain. Pretreatment experiments with unlabelled RGH-1756 and raclopride showed some, albeit low, saturable binding of the radioligand. It has previously been suggested that endogenous DA occupies D3 receptors to a high degree, which could prevent binding of [11C]RGH-1756. To test this hypothesis the effect of reserpine induced DA depletion was examined on the binding of [11C]RGH-1756 in the monkey brain. Following reserpine treatment there was no consistent increase in specific binding of [11C]RGH-1756. This observation does not support the assumption that binding of [11C]RGH-1756 to D3 receptors is inhibited by high occupancy of D3 receptors by endogenous DA. The most likely reason for low specific binding of [11C]RGH-1756 is therefore the insufficient in vivo affinity of the radioligand.

In the final two studies the influence of PVE was estimated in PET studies using the D2/D3 selective radioligands, [11C]FLB 457 and [11C]raclopride in human subjects. Kinetic rate constants, binding potential (BP) and total volume of distribution (DVtot) were derived from the standard twotissue compartment model before and after PVE correction. The results demonstrated that underestimation of regional radioactivity concentration and contamination of time activity curves by spill-in of radioactivity from neighbouring regions have substantial effects on quantitative PET measurements with [11C]FLB 457 and [11C]raclopride. PVE correction can therefore contribute to the accuracy of quantitative PET measurements both by compensating for loss of activity (spill-out) and influence from neighbouring regions (spill-in). Based on the results initial recommendations were formulated for the application of PVE correction particularly in clinical PET studies on disorders with structural brain abnormalities.