Synaptic connectivity of two types of recoverin-labeled cone bipolar cellsand glutamic acid decarboxylase immunoreactive amacrine cells in the innerplexiform layer of the rat retina

We investigated the synaptic connectivity of two populations of recoverin-l abeled bipolar cells and GABAergic neurons in the inner plexiform layer (IP L) of the rat retina. Two types of cone bipolar cells, type 2 and type 8, w ere stained with anti-recoverin antibodies, and GABAergic neurons were stai ned with anti-glutamic acid decarboxylase (GAD) antibodies. Type 2 cone bip olar axone received synaptic input from amacrine cell processes in 177 case s; among these amacrine cell processes, 92 processes (52.0%) were CAD-like immunoreactive. A total of 159 amacrine cell processes, which are presynapt ic to type 8 cone bipolar cells, were observed. Among these processes, 117 processes (73.6%) were CAD-like immunoreactive. The postsynaptic elements a t the ribbon synapses of recoverin-labeled cone bipolar cells were observed in 482 processes. In both type 2 and type 8 cone bipolar cells, the major output was to amacrine cell processes. At the ribbon synapses of the type 2 cone bipolar cells, 224 of the postsynaptic profiles were amacrine cell pr ocesses, 97 processes (43.3%) were CAD-like immunoreactive. In type 8 cone bipolar cells, 45 processes (30.2%) of 149 amacrine cell processes were GAD -like immunoreactive. Our results provide morphological evidence that GABA is a major transmitter involved in the visual processing of type 2 and 8 co ne bipolar cells and GABA may have a stronger influence on type 8 cone bipo lar cells than type 2 cone bipolar cells in the IPL of the rat retina.