Interbipolar contacts in the dorsal inner plexiform layer in the retina of Callionymus lyra L

Several types of direct contacts have been observed between the square pattern-arranged bipolar synaptic buttons in the P layers in the dorsal inner plexiform layer of the Callionymus retina. These contacts are not randomly occurring interconnections, but they are systematically formed throughout the whole of the dorsal retinal zone and they may be the site of direct synaptic interactions between similar as well as dissimilar bipolar axons. Reciprocal ribbon synapses have been found in the P2,2 layer. Morphologically mixed synapses occur in the P5,1 layer. These contact zones show morphologic characteristics of both chemical and electronic synapses. Morphologically mixed synaptic complexes, representing a triple coupling between two adjacent similar bipolar buttons and an amacrine cell process, have been observed in the P1,1, the P2,1 and the P5,2 layer. The morphology of the interbipolar contacts in the P3,2 layer does not clearly point to a synaptic function. Our results suggest a high degree of functional stratification in the inner plexiform layer.