Inhibitory and excitatory responses of the serotonin system: a simultaneous PET/fMRI study

Objectives Simultaneous PET/MR allows the synergistic combination of the molecular specificity of PET and the spatiotemporal resolution of fMRI within a single experimental setting. We recently demonstrated the utility of PET/MR to concurrently measure neurovascular responses using fMRI and receptor occupancy by PET following administration of dopamine receptor agonists and antagonists at graded doses. In the present work, we extend this earlier dopaminergic work in analogous studies of the serotonin (aka 5-hydroxytryptamine, or 5-HT) receptor system. Specifically, we use PET/MR to examine the excitatory effects of the 5-HT_2A receptor subtype and the inhibitory effects of the 5-HT_1A receptor subtype utilizing subtype specific agonist and antagonist drugs.

Methods Two rhesus macaques underwent a total of 16 scans on a simultaneous PET/MR (Siemens Trio with BrainPET insert). fMRI data were acquired with iron oxide contrast enhanced echo planar imaging to assess responses measured as change in cerebral blood volume (ΔCBV) elicited by subtype-specific agonists (5-HT_2A: NBOH-2C-CN, DOI 10-50 mcg/kg; 5-HT_1A: 8-OH-DPAT 2mg/kg) and antagonists (5-HT_2A: MDL100907 10 mcg/kg; 5-HT_1A: mefway 10 mcg/kg) at normal serotonin levels (‘normal-5HT’). Normal-5HT scanning sessions examining 5-HT_2A drugs included two PET/MR scans: a [¹¹C]MDL100907 baseline scan and a [¹¹C]MDL100907 plus 5-HT_2A drug competition scan allowing estimation of occupancy by 5-HT_2A drugs. Response from antagonist drugs were also examined under elevated 5-HT conditions (‘elevated-5HT’) using fenfluramine (5mg/kg i.v. infused over 5 min) to increase endogenous serotonin, thus increasing the amount of endogenous agonist transmitter signaling which can be silenced after displacement by an antagonist and enhancing the drug’s indirect effect on inhibition or excitation. In both animals, a fenfluramine-only scan was acquired for comparison with elevated-5HT scans having antagonist challenge administered 30 minutes after fenfluramine. We hypothesized that at low receptor occupancy, agonists would elicit pronounced CBV changes (excitatory increases for 5-HT_2A agonism and inhibitory decreases for 5-HT_1A agonism). Conversely, we posited that even at high occupancy, antagonists would produce modest or undetectable fMRI responses, but in the presence of fenfluramine-elevated serotonin levels these same drugs would generate conspicuous CBV changes (excitatory increases for 5-HT_1A antagonism and inhibitory decreases for 5-HT_2A antagonism) caused by muting of the heightened endogenous agonism.

Results The 5-HT_2A agonists DOI and NBOH-2C-CN led to strong cortical increases in ΔCBV (>30% peak response) whereas the 5-HT_1A agonist 8-OH-DPAT resulted in robust decreases in ΔCBV (<−20%). Under normal-5HT conditions, the 5-HT_2A antagonist MDL100907 exhibited undetectable ΔCBV (0%) even at PET measured peak occupancy >90%, however, under elevated-5HT conditions MDL100907 produced robustly negative ΔCBV responses (<−10%). Similarly, at normal-5HT conditions, there was no evident ΔCBV (0%) response due to the 5-HT_1A antagonist mefway at doses providing occupancies >90%, however, under elevated-5HT conditions mefway resulted in positive ΔCBV (>8%).

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Conclusions 5-HT_1A and 5-HT_2A agonists elicited inhibitory and excitatory responses, respectively, as predicted by the pharmacological characteristics of each receptor subtype. Antagonists provoked a measureable response only in the presence of elevated endogenous serotonin, and the responses were opposite those produced by the complementary agonists indicating the observed fMRI change caused by antagonists are due to blocking the access of serotonin at the specific receptor subtype. These findings support the validity of an fMRI construct sensitive to receptor-mediated signaling and pave the way toward joint PET/fMRI modeling of pharmacological action in the serotonergic system. Research Support: R01MH100350