Connexin26 (Cx26) is a member of the family of integral
membrane proteins that normally form intercellular gap
junctional channels. We have used Western blotting, immunofluorescence,
immunoelectron microscopy, and single-cell reverse-transcriptase
polymerase chain reaction amplification (RT-PCR) to analyze
the expression and cellular localization of Cx26 in the
carp retina. In the outer plexiform layer, strong clustered
Cx26 immunolabeling was concentrated at and restricted
to the terminal dendrites of horizontal cells. Single-cell
RT-PCR confirmed the expression of Cx26 in carp retinal
horizontal cells. 248-bp fragments amplified from cDNAs
of four different horizontal cells were cloned and each
nucleotide sequence encodes a protein fragment (AA 104-185)
with highly significant homology to rat and mouse Cx26.
Immunoelectron microscopy revealed that only the invaginating
dendrites of horizontal cells in intimate lateral association
with the presynaptic ribbon complex were labeled. No labeling
was found at the photoreceptor membrane and there was no
septalaminar structure, indicative of gap junctions, between
photoreceptors and horizontal cells. The focal location
of Cx26 at the membrane of the dendritic tips of horizontal
cells and the lack of gap junctional morphology suggests
that Cx26 might form hemichannels.