Since DPP4 is found mainly in the meninges and blood vessels, whereas DPP8 and DPP9 are ubiquitously distributed in the brain, it has been hypothesized that NPY is cleaved to the truncated NPY 3-36 in the periphery by soluble DPP4 or membrane-bound DPP4
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Acta Physiol (Oxf) (2018) 222(4):e13007
These mechanisms include changes in microbial composition, immune activation, vagus nerve signaling, and the production of specific microbial neuroactive metabolites (197)
doi: 10.3390/antiox12040782 199 WuTZhangCShaoTChenJChenD
Curiously, they also seemed to suppress appetite, and some people taking them lost a modest amount of weight