Effects of NDP-MSH, C-terminal ASIP and select chimera constructs on cAMP production at MC1R, which displays constitutive activity. NDP-MSH is an agonist elevating cAMP, whereas ASIP is an inverse agonist, as demonstrated by suppression of constitutive cAMP production. Interestingly, chimeras lacking a C-terminal loop (chi(+1 +1 0), chi(−1 +1 0) and chi(+1 −1 0)) are agonists at concentrations beyond 10 – 100 nM.