Dopamine receptors in the retina may all be linked to adenylate cyclase

Abstract

Dopamine seems to be the principal catecholamine in the vertebrate retina, where it is found in two types of neurone¹. In most species, there is a subclass of amacrine cell which makes synapses on other amacrine cells in the inner plexiform layer². In teleost fish and New World monkeys, the dopaminergic neurones are interplexiform cells which make synapses in both plexiform layers on horizontal, bipolar and amacrine cells³. In other dopamine-rich areas of the central nervous system, there is evidence for multiple dopamine receptor types^4–7, and two sub-types have been suggested⁸. D-1 receptors are associated with an adenylate cyclase system whereas D-2 receptors are not. D-2 receptors are characterised by a high affinity for the buty-rophenones haloperidol⁹ and spiperone¹⁰, but these ligands also apparently bind to D-1 receptors¹¹. The butyrophenone domperidone also binds to central dopamine receptors¹², but it is a very weak antagonist of dopamine-sensitive adenylate cyclase¹³ (Table 1), suggesting that it may bind only to D-2 receptors. In the retina of many species, a dopamine-sensitive adenylate cyclase has been demonstrated^14,15, indicating the presence of D-1 receptors. We report here that there is no significant receptor-specific binding of ³H-domperidone to retinal membranes, thus suggesting the absence of D-2 receptors in this tissue.