Spinal cord dopamine D2/D3 receptors: in vivo and ex vivo imaging in the rat using ¹⁸F/¹¹C-fallypride

Objectives: The spinal cord is known to be innervated with dopaminergic cells with catecholaminergic projections arising from the medulla and pons and dopaminergic transmission in the spinal cord is vital for sensory and motor function. Our goal was to evaluate and compare the imaging capability of dopamine D2/D3 receptors in the rat spinal cord using PET ligands ¹⁸F-fallypride and ¹¹C-fallypride. Methods: Male Sprague–Dawley rats were used in all in vitro and in vivo studies. Spinal cord and brain sections were used for in vitro autoradiography and ex vivo autoradiography. For in vivo studies animals received a ¹⁸F-fallypride scan or a ¹¹C-fallypride PET scan. The spinal cord and the brain were then harvested, flash-frozen and imaged ex vivo. For in vivo analysis Logan plots with cerebellum as a reference was used to evaluate binding potentials (BP). Tissue ratios were used for ex vivo analysis. Drug effects were evaluated using clozapine, haloperidol and dopamine were evaluated on spinal cord sections in vitro. Results: In vitro studies showed ¹⁸F-fallypride binding to superficial dorsal horn (SDH), dorsal horn (DH), ventral horn (VH) and the pars centralis (PC). In the cervical section, the greatest amount of binding appeared to be in the SDH. Ex vivo studies showed approximately 6% of ¹⁸F-fallypride in SDH compared to that observed in the striatum. In vivo analysis of both ¹⁸F-fallypride and ¹¹C-fallypride in the spinal cord were comparable to that in the extrastriatal regions. Haloperidol and clozapine displaced more than 75% of the ¹⁸F-fallypride in spinal cord sections. Conclusions: Our studies showed ¹⁸F-fallypride and ¹¹C-fallypride binding in the spinal cord in vitro and in vivo. The binding pattern correlates well with the known distribution of dopamine D2/D3 receptors in the spinal cord