Downstream interaction by glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide agonism is required for synergistic effects on body weight

Objectives
Dual glucagon-like peptide-1 receptor and glucose-dependent insulinotropic polypeptide receptor agonists (GLP1RA and GIPRA, respectively) synergise to reduce body weight. Though this synergy depends on receptors within the brain, where and how this occurs is unclear.

Methods
We employed a combination of neuroanatomical approaches in the mouse to investigate access of the dual GLP1RA/GIPRA, tirzepatide, and study the central targets engaged by single agonist, dual agonist and combined agonist treatments. Genetic manipulations were then used to further investigate the functional significance of specific brain regions and distinct neuronal subtypes.

Results
We recorded penetration of fluorescently labelled tirzepatide limited mainly to circumventricular organs and confirmed the importance both GLP1R and GIPR in the dorsal vagal complex for the actions of systemically administered agonists. Receptor expression indicates GIPRA alone activates a distinct population of GABA neurons in the area postrema directly, but also neurotensin neurons in the central amygdala (Nts^CeA) indirectly. Disabling Nts^CeA neurons selectively reduces the synergistic effect of dual GLP1R/GIPR agonist administration on body weight.

Conclusions
As with selective GLP1RA, the actions of dual GLP1RA/GIPA appear to be dependent on the dorsal vagal complex for their action, probably most importantly by gaining access through the area postrema. Downstream targets include the central amygdala where signals following dual receptor agonism interact. Specifically, Nts^CeA neurons are required for the full synergistic effect of dual receptor agonism on body weight.