Allosteric regulation in enzymes is where a molecule binds at a site other than the active site and thus changes the tertiary structure of the active site, hence altering the binding of substrate and formation of enzyme-substrate complexes.
As I understand it, if this is extended to receptors, for example G protein-coupled receptors, which effectively causes a conformation change which is transmitted across the membrane and 'activates' a G protein which is associated with the receptor. The Gα subunit swaps a GDP for a GTP which causes the dissociation of the alpha from the beta-gamma coupled subunits and from the receptor. The dissociated Gα and Gβγ subunits then take part in a cascade process interacting with other molecules.
However, photoreceptors such as rods/cones in the eye respond to light. These photons tend to cause changes in the structure of the rhodopsin/iodopsin pigments and not the actual 'receptors' themselves. G proteins do seem to be involved in this process too.
Therefore, are photoreceptors allosteric? And if not, are they an anomaly in mammalian receptors, or are there many other examples of receptors that do not rely on allostery?