Morphology and distribution of dopaminergic neurons in the ground squirrel retina.

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 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 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 20 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 suggests the TH-neurons in the INL are distributed in a non-random fashion.