In the mammalian retina, the dendritic arbors of cholinergic amacrine neurons have a unique starburst shape; these arbors lie in narrow sublaminae within the inner plexiform layer, where they provide input to a wide variety of ganglion cell types. Immunocytochemistry has been used to identify cholinergic cells in one poikilotherm, the goldfish (Tumosa et al., 1984), but there has been no description of the detailed dendritic morphology of these cells in the lower vertebrates. In the present study, cholinergic neurons have been characterized, by immunocytochemistry and dye filling, in the retina of the Pacific Coast dogfish, Squalus acanthias.
The inner nuclear layer contained two populations of choline acetyltransferase-immunoreactive amacrine cells, of different sizes (average soma diameters 12.2 vs. 16.3 μm); 70% of the immunoreactive cells were of the smaller type. Cholinergic dendrites from these two cell populations formed two narrow strata within the inner plexiform layer, at depths of 14% and 31%. In the ganglion cell layer, 40% of the cells were immunoreactive for choline acetyltransferase (ChAT); these cells were very homogeneous in size, had an average diameter of 12.6 μm, and appeared to represent a single class of cholinergic amacrine. The dendrites of these cells formed a single, narrow stratum within the inner plexiform layer, at a depth of 59%.
In living preparations, the smallest cell bodies in the ganglion cell layer were filled iontophoretically with Lucifer Yellow, under microscopic control. Such cells invariably had a stellate morphology; in many cases, they appeared quite similar to the starburst cholinergic amacrine cells described in rabbit and rat (Vaney, 1984; Voigt, 1986). Although double-label experiments failed to demonstrate ChAT immunoreactivity in specific dye-filled cells, the dendritic arbors of individual dye-filled stellate dogfish amacrines did co-stratify precisely with the proximal ChAT-immunoreactive sublamina of the inner plexiform layer. In addition, dye injection and ChAT immunocytochemistry appeared to label the same population of dogfish neurons, as suggested by the close structural similarity, and similar numerical proportion, of the cells identified with these two techniques.
Similarities between the displaced cholinergic amacrine neurons of the dogfish retina, and the cholinergic, “starburst” amacrine neurons of the rabbit retina, are discussed.