Dopaminergic neurons in the cone-dominated ground squirrel retina: a light and electron microscopy study

Tyrosine hydroxylase (TH), the rate limiting enzyme in the conversion of tyrosine to DOPA, is a reliable marker for catecholaminergic (dopaminergic) neurons. To investigate the distribution of dopamine in the cone-dominated retina of the thirteen-lined ground squirrel (Spermophilus tridecemlineatus), retinal sections and wholemounts were incubated with an antiserum directed against TH and then processed using immunofluorescence and the avidin-biotin immunohistochemical method. TH-like immunoreactivity was exhibited by amacrine and interplexiform-like cells in the innermost portion of the inner nuclear layer (INL) and by cells we presume to be displaced amacrines in the ganglion cell layer (GCL). Their somata were 12 to 28 microns in diameter, with the majority measuring approximately 18 microns. In transverse sections the processes of the three types of neurons were seen to extend into lamina 1 of the inner plexiform layer (IPL). In horizontal sections 2-3 primary dendrites were seen to ramify and the branches extended for considerable distances, with overlap between the dendritic fields of neighboring TH-cells. A distance to the nearest neighbor analysis suggested the TH-neurons in the INL are distributed in a non-random fashion. The mean overall density of labeled amacrines is 15 cells/mm2, low when compared to the mean density reported in other species. EM results indicate that TH-labeled amacrines make synaptic contacts with unlabeled amacrines, although the possibility that they may contact ganglion cells can not be ruled out.