DOPAMINERGIC AND GABAERGIC RETINAL CELL-POPULATIONS IN MAMMALS

A number of modern techniques now allow histologists to characterize s ubpopulations of retinal neurons by their neurotransmitters. The morph ologies and connections of these chemically defined neurons can be ana lyzed precisely at both light and electron microscope levels and lead to a better understanding of retinal circuitry. The dopaminergic neuro ns form a loose population of special wide-field amacrine cells bearin g intraretinal axons within the inner plexiform layer. One subtype, th e interplexiform cell,sends an axon to the outer plexiform and outer n uclear layers. The number of interplexiform cells is variable througho ut mammalian species. The GABAergic neurons form a dense and heterogen eous population of amacrine cells branching at all levels of the inner plexiform layer. The presence of GABA in horizontal cells seems to be species-dependent. Close relationships occur between dopaminergic and GABAergic cells. GABA antagonizes a number of dopaminergic actions by inhibiting both the release and synthesis of dopamine. This inhibitio n can be supported by GABA synapses onto dopaminergic cells, but GABA can also diffuse to its targets. Finally, GABA is also contained and s ynthesized in dopaminergic cells. This colocalization might be the bas is of an intracellular modulation of dopamine by GABA. (C) 1997 Wiley- Liss, Inc.