Characterization of glucagon-expressing neurons in the chicken retina.

We recently identified large glucagon-expressing neurons that densely ramify neurites in the peripheral edge of the retina and regulate the proliferation of progenitors in the circumferential marginal zone (CMZ) of the postnatal chicken eye (Fischer et al. [2005] J Neurosci 25:10157-10166). However, nothing is known about the transmitters and proteins that are expressed by the glucagon-expressing neurons in the avian retina. We used antibodies to cell-distinguishing markers to better characterize the different types of glucagon-expressing neurons. We found that the large glucagon-expressing neurons were immunoreactive for substance P, neurofilament, Pax6, AP2alpha, HuD, calretinin, trkB, and trkC. Colocalization of glucagon and substance P in the large glucagon-expressing neurons indicates that these cells are the "bullwhip cells" that have been briefly described by Ehrlich et al. ([1987] J Comp Neurol 266:220-233). Similar to the bullwhip cells, the conventional glucagon-expressing amacrine cells were immunoreactive for calretinin, HuD, Pax6, and AP2alpha. Unlike bullwhip cells, the conventional glucagon-expressing amacrine cells were immunoreactive for GABA. While glucagon-immunoreactive amacrine cells were negative for substance P in central regions of the retina, a subset of this type of amacrine cell was immunoreactive for substance P in far peripheral regions of the retina. An additional type of glucagon/substance P-expressing neuron, resembling the bullwhip cells, was found in far peripheral and dorsal regions of the retina. Based on morphology, distribution within the retina, and histological markers, we conclude that there may be four different types of glucagon-expressing neurons in the avian retina.