The results of the studies reported here suggest that [N
C]vorozole is a useful radiotracer for the non-invasive measurement of brain aromatase in humans, demonstrating anatomical and pharmacological specificity. Thus, the regional distribution pattern of the tracer was highly heterogeneous, with the highest levels found in distinct (dorsomedial, pulvinar and paraventricular) thalamic nuclei, followed by moderately high levels in amygdala, preoptic area, and medulla and low levels in cortex, putamen, cerebellum and cortical white matter. Tracer accumulation in all regions was reduced by oral pretreatment with the aromatase inhibitor letrozole (2.5 mg) given 2 hours before tracer injection. This is in line with published studies performed in healthy volunteers, where the same dose of letrozole resulted in a near-complete inhibition of aromatase, expressed by a >80% decrease in plasma estrogen levels (Iveson et al., 1993
). The time interval between pretreatment and tracer injection was also based on previous studies of peripheral inhibition of aromatase, where the effect of the drug peaked within 2-4 hours of administration and lasted for more than 24 hours. (Iveson et al., 1993
Previous PET studies with [11
C]vorozole (Lidstrom et al., 1998
; Takahashi et al., 2006
) did not report kinetic modeling of the tracer uptake but rather relied on region to cerebellum ratios at late time points. This approach, as well as a tissue reference model, was deemed inappropriate since all grey matter regions, including the cerebellum, appeared to contain displaceable [11
C]vorozole binding in our baboon studies (Kim et al., 2009
; Biegon et al in press
) as well as in the present studies of the human brain. A comparison of 1 and 2 compartment models to the model-independent graphical approach (Logan et al 2003
) demonstrated a better fit of the 2 compartment model, since the 1 compartment model consistently underestimated the volume of distributioin derived from the graphical analysis in baboons (Biegon et al in press
). Therefore, we have used the 2 compartment model to estimate the regional distribution volumes of [11
C]vorozole in the human brain. The regional rank order of VT was similar to the %ID/CC observed at late (>50 min) but not early times after tracer injection.
The regional distribution pattern of aromatase in the human brain is strikingly different from the distribution reported rodents and primates (baboon and rhesus), in which the highest levels were found in the amygdala and preoptic area while the thalamus and medulla were unremarkable (Lidstrom et al., 1998
, Takahashi et al., 2006
, Kim et al., 2009
). While difference between rodents and primates in brain receptor and enzyme distribution are common, primate and human brain usually show similar regional distribution patterns (e.g Osterlund et al., 2001
) making this an unexpected finding and suggesting a unique role for thalamic and medullar aromatase in humans.
Previous studies of human brain aromatase were conducted postmortem or on biopsy material and were confined to pre-selected, specific regions, such as the temporal cortex (Stoffler-Wagner et al., 1999) or specific hypothalamic and ventral forebrain nuclei (Ishunina et al., 2005
). Our results are in line with Sassano et al., (1998) who measured aromatase gene expression in thalamus as well as 8 other regions and found high levels of aromatase gene expression in thalamus, while confirming the presence of aromatase in all other regions investigated. Although the number of subjects in our study is too small for formal statistical analysis, we did not find higher levels in men compared to women, as expected from the findings of Sassano et al., (1998) Steckelbroeck et al., (1999)
, Stoffel-Wagner et al., (1999)
and Ishunina et al., (2005)
, who reported similar levels of brain aromatase activity and gene expression in men and women. Conversely, results from animal studies demonstrated higher levels of brain aromatase in males and suggested testosterone was a positive modulator of aromatase in the hypothalamic-preoptic area (Roselli et al., 1984
, Abdelgadir et al., 1994
). However, brain aromatase did not appear to be significantly regulated by the estrous cycle in rodents (Roselli et al., 1984
), matching our results in a small number of women imaged at opposite phases of the menstrual cycle. Taken together, our findings support the notion that brain aromatase expression is regulated in a species and region selective manner (Roselli et al., 1984
). Such specific regulation may be the result of tissue-specific aromatase promoters, which were identified in animal and human tissues (Golovine et al., 2003
, Jones et al., 2006
). Since other promoters besides the brain specific exon 1.f (Sasano et al., 1998
) are expressed in the human brain, this heterogeneity may provide the basis for brain region specific regulation and expression of aromatase in humans.
At present, we do not know which brain functions are served by the estrogen produced locally in the thalamus and inferior olive or the identity of the relevant estrogen receptor subtypes. Although the exact functions subserved by the human inferior olive are not clear, available information implicate this nucleus as well as specific thalamic nuclei in cognitive aspects of sensory and motor information processing, which may be modulated by estrogen (Ward et al., 2007
, Liu et al., 2008
As for the estrogen receptors, both ERα and ERβ receptors are expressed in a subtype-specific manner in the human brain, with ERα mostly restricted to hypothalamus and amygdala while ERβ expression is more widespread, and found also in hippocampus, cortex and thalamus (Osterlund et al., 2000
). It is also possible that estrogen synthesized in thalamus and medulla interacts with the more recently characterized membranal estrogen receptors (Toran-Allerand 2004, Qiu et al., 2008
), some of which are expressed in regions lacking classical ERα and ERβ such as striatum and medulla in rats (Brailoiu et al., 2007
). However, the regional distribution of membranal estrogen receptors in the human brain has not been reported to date.
In summary, [N-methyl-11C]vorozole is a useful tracer for aromatase in the human brain, showing fast brain penetration and a unique pattern of binding in specific brain regions. These studies set the stage for the non-invasive assessment of aromatase involvement in various physiological, pathological and pharmacological processes affecting the human brain.