Nicotinic acetylcholine receptors in the ground squirrel retina: Localization of the β4 subunit by immunohistochemistry andin situhybridization

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Abstract
Immunohistochemical andin situhybridization techniques were used to localize the β4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The β4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina.In situhybridization with a35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR β4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of β4 nAChR subunit.These results indicate that the β4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina. As the expression of that particular nAChR subunit appears to be very limited in the brain, the present data suggest that the retina might represent a useful model to study the function of nAChRs containing the β4 subunit.