PET imaging in rat brain shows opposite effects of acute and chronic alcohol exposure on phosphodiesterase-4B, an indirect biomarker of cAMP activity

Abstract

The cyclic adenosine monophosphate (cAMP) cascade is thought to play an important role in regulating alcohol-dependent behaviors, with potentially opposite effects following acute versus chronic administration. Phosphodiesterase 4 (PDE4) is the primary brain enzyme that metabolizes cAMP, thereby terminating its signal. Radioligand binding to PDE4 serves as an indirect biomarker of cAMP activity, as cAMP-protein kinase A (PKA)-mediated phosphorylation of PDE4 increases its affinity for radioligand binding ~10-fold. Of the four PDE4 subtypes, PDE4B polymorphisms are known to be strongly associated with alcohol and substance use disorders. This study imaged rats with the PDE4B-preferring positron emission tomography (PET) radioligand [¹⁸F]PF-06445974 following acute and chronic ethanol administration, aiming to explore the potential of PDE4B PET imaging for future human studies. Compared to the control group treated with saline, acute alcohol administration (i.p. ethanol 0.5 g/kg) significantly increased whole brain uptake of [¹⁸F]PF-06445974 as early as 30 minutes post-exposure. This effect persisted at 2 hours, peaked at 4 hours, and diminished at 6 hours and 24 hours post-exposure. In contrast, in a rat model of alcohol dependence, [¹⁸F]PF-06445974 brain uptake was significantly reduced at 5 hours post-exposure and was normalized by 3 days. This reduction may reflect long-term adaptation to repeated alcohol-induced activation of cAMP signaling with chronic exposure. Taken together, the results suggest that PET imaging of PDE4B in individuals with alcohol use disorder (AUD) should be considered in conjunction with ongoing trials of PDE4 inhibitors to treat alcohol withdrawal and reduce alcohol consumption.

Fig. 1. Brain and plasma levels of [¹⁸F]PF-06445974 and blood alcohol levels after acute alcohol exposure.

A Group-averaged standardized uptake value (SUV) images (averaged from 50 to 100 minutes) of saline and alcohol-exposed rats at 4 hours post-exposure. B The area under the time-activity curve (AUC) of the whole brain of saline- and alcohol-exposed rats at multiple time points after exposure. Whole brain radioactivity increased at 0.5 hours following acute alcohol exposure, peaked at 4 hours, and normalized by 6 hours. C A representative example of group-averaged whole brain time-activity curves at 4 hours post-saline or alcohol exposure. D Plasma level of parent radioactivity of two pairs of saline and alcohol exposed rats at 0.5 hours post-exposure; inserted window shows the later portion in linear scale. E Blood alcohol levels following alcohol administration in a group of three naïve rats. ^p < 0.06, *p < 0.05 difference to the saline group. EtOH: acute alcohol-exposed group.

Fig. 2. Brain and plasma levels of [¹⁸F]PF-06445974 in rats at 5 hours following chronic alcohol exposure.

Brain radioactivity decreased without altering the level of plasma parent. A Group-averaged standardized uptake value (SUV) images (averaged from 50 to 100 minutes). B Group-averaged whole brain time-activity curves of NON-DEP and DEP rats. C The area under the time-activity curve (AUC) of the whole brain, nucleus accumbens, and striatum. D Plasma level of parent radioactivity in two pairs of NON-DEP and DEP rats; inserted window shows the later portion in linear scale. *p < 0.001 between group differences. NON-DEP: non-dependent; DEP: dependent; WB: whole brain; NAc: nucleus accumbens; STR: striatum.

Fig. 3. Brain uptake of [¹⁸F]PF-06445974 in rats at 3 days and 2 weeks following chronic alcohol exposure.

A Group-averaged whole brain time-activity curves of NON-DEP and DEP rats at 3 days (top) and 2 weeks (bottom) post exposure. B The ratio of area under the time-activity curve (AUC) between DEP and NON-DEP groups at each time point. NON-DEP non-dependent, DEP dependent.

Fig. 4. Brain uptake of [¹⁸F]PF-06445974 in rats at 5 hours following chronic alcohol exposure pre- and post-efflux transporter inhibition by elacridar.

NON-DEP non-dependent, DEP dependent.

Fig. 5. Western blot analysis of rat brain tissues following chronic alcohol exposure.

A Western blot showing the PDE4B1/3, PDE4B2, and actin immunoreactivity from four pairs of rats. One set from the duplicate is shown for pair 3 and pair 4. B Averaged immunoreactivity relative to actin of NON-DEP and DEP brain tissues. NON-DEP non-dependent, DEP dependent.