Phenylazopyridines
are photoisomerizable compounds with high potential
to control biological functions with light. We have obtained a series
of phenylazopyridines with light dependent activity as negative allosteric
modulators (NAM) of metabotropic glutamate receptor subtype 5 (mGlu<sub>5</sub>). Here we describe the factors needed to achieve an operational
molecular photoisomerization and its effective translation into <i>in vitro</i> and <i>in vivo</i> receptor photoswitching,
which includes zebrafish larva motility and the regulation of the
antinociceptive effects in mice. The combination of light and some
specific phenylazopyridine ligands displays atypical pharmacological
profiles, including light-dependent receptor overactivation, which
can be observed both <i>in vitro</i> and <i>in vivo</i>. Remarkably, the localized administration of light and a photoswitchable
compound in the peripheral tissues of rodents or in the brain amygdalae
results in an illumination-dependent analgesic effect. The results
reveal a robust translation of the phenylazopyridine photoisomerization
to a precise photoregulation of biological activity