Compartment-specific tyrosine hydroxylase-positive innervation to AII amacrine cells in the rabbit retina

• Processes of TH+ ACs form rings around perykaria of both AII and non-AII ACs in stratum 1 of the IPL.
• TH+ ACs provide innervations to a series of AII ACs via both ring-like structures and en passant contacts.
• Besides AII AC somata, TH+ ACs preferentially target the main stalk area and rarely lobular or transversal AII dendrites.

Tyrosine-hydroxylase-positive (TH+) amacrine cells release dopamine in a paracrine manner and also form GABA-ergic contact sites with inner retinal neurons. The best known sites are formed by TH+ fibrous rings and AII amacrine cell somata in stratum 1 of the inner plexiform layer (IPL). An AII amacrine cell is a highly compartmentalized neuron with relatively large soma, a stout dendritic stalk and two sets of processes, one showing lobular appearance and extending horizontally in stratum 1 and a second transversally elongated group of fibers in strata 4 and 5. Although, all of these compartments have been reported as tic sites, it is uncertain if TH+ amacrine cell inputs are homogeneously distributed or they rather target specific AII cell compartments. In this study we investigated the TH+/AII cell system by immunohistochemistry to map the potential synaptic contacts in the rabbit retina. We found numerous intimate contacts between the two amacrine cell populations throughout the IPL. However, TH+ fibers favored the soma/main stalk region of AII amacrine cells and only contacted lobular appendages and transversal processes sporadically. In addition to the well-studied contacts between AII cell somata and TH+ rings in stratum 1 we found that the main stalk region in stratum 3 serves as a secondary major target for TH+ axons. These data thus clearly show that TH+ contacts to AII amacrine cells are highly compartment specific.

Dopamine-expressing (TH+) amacrine cells (green in pictures) of the mammalian retina provide compartment-specific inputs to AII amacrine cells (red in pictures).Figure optionsDownload high-quality image (164 K)Download as PowerPoint slide