PET measurement of the in vivo affinity of ¹¹C-(R)-rolipram and the density of its target, phosphodiesterase-4, in the brains of conscious and anesthetized rats

A variety of phosphodiesterases hydrolyze and terminate the effects of the intracellular second messenger 3′,5′-cyclic adenosine monophosphate (cAMP). Phosphodiesterase subtype 4 (PDE4) is particularly abundant in the brain and has been imaged with ¹¹C-(R)-rolipram, a selective inhibitor of PDE4. We sought to measure in vivo both the binding site density (B _max) and the radioligand affinity (1/K_D) of ¹¹C-(R)-rolipram in the rat brain. We also studied 2 critical factors in small-animal PET scans: the influence of anesthesia and the difference in binding under in vivo and in vitro conditions. Methods: In vivo, B_max and K_D were measured in PET saturation experiments by the administration of ¹¹C-(R)-rolipram and various doses of carrier (R)-rolipram in conscious and isoflurane-anesthetized rats. The metabolite-corrected arterial input function was measured in each scan. To image conscious rats, the head of the rat was fixed in a holder and the animals were trained to comply with this apparatus. Bound and free (R)-rolipram levels were calculated under transient equilibrium conditions (i.e., at the time of peak specific binding). Results: The B_max and K_D of conscious rats were significantly greater than those of anesthetized rats, by 29% and 59%, respectively. In addition, the in vitro KD was 3-7 times greater than was the in vivo K_D, although the B_max was similar in both conditions. Conclusion: The in vivo B_max and K_D of (R)-rolipram were successfully measured in both conscious and anesthetized rats. K_D was affected to a greater extent than was B_max by the 2 conditions. That is, K_D was increased in the conscious rat, compared with in the anesthetized rat, and K_D was increased in vitro, compared with in vivo. The current study shows that the rat, a readily available species for research, can be used to measure in vivo both affinity and density of radioligand targets, which can later be directly assessed with standard in vitro techniques.