The synaptic organization of GABA-immunoreactive (GABA-IR) amacrine
cells in the inner plexiform layer (IPL) of salamander retina was
studied with the use of postembedding immuno-electron microscopy. A
total of 457 GABA-IR amacrine synapses, with identified postsynaptic
elements, were analyzed on photomontages of electron micrographs
covering 3,618 μm2 of the IPL. GABA-IR amacrine synapses
were distributed throughout the IPL, with a small peak at the proximal
margin of sublamina a. The majority of the output targets
(81%) were GABA(−) neurons. Most of the contacts were simple
synapses with one postsynaptic element identified as a process of an
amacrine cell (55%), bipolar cell (19%) or ganglion cell (26%), and
serial synapses were very rare. Of the 89 postsynaptic bipolar
terminals, 63% participated in a reciprocal feedback synapse with the
same presynaptic GABA-IR amacrine profile. There appeared to be no
preference between GABA-IR amacrine contacts with rod- or
cone-dominated bipolar cells (9.1% vs. 8.9%) or in the total
number of amacrine synapses in sublaminas a and b
(52% vs. 47%). The preponderance of amacrine cell input to
bipolar cells in the OFF layer was derived from GABA-IR cells. These
findings provide ultrastructural support to the existing physiological
studies regarding the functional roles of the GABAergic amacrine cells
in this species. Our results have added to the data base demonstrating
that, in contrast to mammals, GABA-IR amacrine cells in amphibians and
other nonmammals contact other amacrine cells more frequently,
suggesting greater involvement of GABAergic amacrine cells in
modulating lateral inhibitory pathways.