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 C-11-(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 C-11-(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 C-11-(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 K-D 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.