Retina contains guanine nucleotide sensitive and insensitive classes of dopamine receptors.

Calf retina contains high affinity, stereospecific and saturable binding sites, characterized as dopaminergic, for 3H-spiropheridol (Bmax from Scatchard analysis = 38 +/- 4 fmoles/mg protein) and for 3H-ADTN (2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene). The 3H-ADTN sites include a discrete and saturable sub-set of sites sensitive to guanine nucleotide inhibition (Bmax = 50 +/- 6 fmoles/mg protein) and a separate saturable sub-set of sites insensitive to guanine nucleotides (Bmax = 63 +/- 7 fmoles/mg protein). The binding of agonists to 3H-spiroperidol sites is also inhibited by guanine nucleotides.