Displaced cholinergic, GABAergic amacrine cells in the rabbit retina also contain adenosine

It is generally accepted that the purine nucleoside, adenosine, plays a neuromodulatory role in the central nervous system (CNS) (Daly et al., 1981; Phillis & Wu, 1983; Williams, 1986; Williams, 1987; Snyder, 1985). Adenosine is thought to exert its primary effects presynaptically, by inhibiting the release of neurotransmitters including gamma-aminobutyric acid (GABA) and acetylcholine (ACh) (Phillis & Barraco, 1985; Proctor & Dunwiddie, 1987). In mammalian retina, cell bodies that are strongly labeled for adenosine-like immunoreactivity (ALIR) have been localized to the ganglion cell layer (GCL) (Braas et al., 1987; Blazynski et al., 1989). Rabbit retinal cells that are labeled by markers for both ACh and GABA are located in the GCL and inner nuclear layer (INL) (Tauchi & Masland, 1984; Vaney & Young, 1988 b; Brecha et al., 1988). It is now demonstrated in the rabbit retina that approximately 50% of the cells labeled for ALIR within the GCL represent true ganglion cells, with the remainder presumed to be displaced cholinergic amacrine cells (DAPI accumulating). In addition, some of these same cells also demonstrate immunoreactivity to glutamate decarboxylase (GAD), involved in the biosynthesis of the neurotransmitter GABA. Thus, in a particular class of retinal neurons, two fast-acting neurotransmitters as well as a putative neuromodulator have been co-localized.