QUANTITATIVE ANATOMY, SYNAPTIC CONNECTIVITY AND PHYSIOLOGY OF AMACRINE CELLS WITH GLUCAGON-LIKE IMMUNOREACTIVITY IN THE TURTLE RETINA

Although a wide variety of neuropeptides have been localized in verteb rate retinas, many questions remain about the function of these peptid es and the amacrine cells that contain them. This is because many of t hese peptidergic amacrine cells have been studied using only immunocyt ochemical techniques. To address this limitation, the present study us ed a combination of quantitative anatomy, biochemistry and electrophys iology to examine amacrine cells in the turtle retina that contain the neuropeptide glucagon. In the turtle retina, there is a small populat ion of 2500 glucagonergic amacrine cells, which probably represents <1 % of the total number of amacrine cells. Circular distribution statist ics indicated that many of these tristratified amacrine cells had asym metric dendritic arborizations that were radially oriented toward the retinal periphery. The cells were found to have similar dendritic cove rage factors, to be distributed in a non-random arrangement in all reg ions of the retina, and to peak in density in the visual streak region . Electron microscopic studies indicated that glucagonergic amacrine c ells made synaptic contacts primarily with other amacrine cells, and s mall numbers of bipolar cells. The synaptic inputs and outputs were ba lanced in the inner strata of the inner plexiform layer, and were bias ed toward synaptic outputs in the outer strata of the inner plexiform layer. These contacts involved small unlabelled synaptic vesicles, and not the large labelled dense core vesicles also found in these neuron s. The biochemical studies indicated that glucagon could be released f rom the retina in a calcium dependent manner by high potassium stimula tion. The electrophysiology found no color opponency, and the glucagon ergic amacrine cells gave sustained hyperpolarizing responses to small stimulation spots and had antagonistic surrounds. The results of thes e studies suggest that there are significant regional specializations of glucagonergic amacrine cells, and that they may provide OFF-modulat ion in interactions between the ON- and OFF-centre visual pathways in the turtle retina.