The connexins are the protein subunits of the gap junction intercellular channels. In the present study a new rat connexin was cloned by degenerate reverse transcription-polymerase chain reaction and its gene isolated from a mouse genomic library. The nucleotide sequence encodes a protein of 321 amino acids (called Cx36) with highly significant homology to the members of the connexin family. In situ hybridization analysis of rat brain and retina showed the strongest expression in neurons of the inferior olive, the olfactory bulb, the CA3/CA4 hippocampal subfields and several brain-stem nuclei. An intense expression was also found in the pineal gland and in the retinal ganglion cell and inner nuclear layers. Experiments with neurotoxins, locally injected in the hippocampus or specifically acting on inferior olivary neurons, confirmed the neuronal localization of Cx36. It is the first connexin to be expressed predominantly in mammalian neurons and its identification paves the way for a molecular approach in the study of the role played by gap junctions in the physiology and the pathology of the mammalian brain.

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The expression and functional properties of connexin36 (Cx36) were examined in two communication-deficient cell lines (N2A-neuroblastoma and PC-12 cells) transfected with Cx36 and in hippocampal neurons that express the connexin endogenously. Transfected cells expressed the expected 2.9 kb Cx36 transcript and Cx36 immunoreactivity, whereas nontransfected cells were devoid of Cx36. The relationship between steadystate junctional conductance ( g j ) and transjunctional voltage was well described by a two-state Boltzmann equation. The half-inactivation voltage (V 0 ), the ratio of minimal to maximal g j ( g min /g max ), and the equivalent gating charge were Ϯ 75 mV, 0.55, and 1.75, respectively, indicating that Cx36 exhibits very low voltage sensitivity. Conductance of single Cx36 channels measured with patch pipettes containing 130 mM CsCl was 10-15 pS (n ϭ 15 cell pairs); despite this low unitary conductance, Cx36 channels were permeable to the dye Lucifer yellow. Hippocampal neurons expressed Cx36 both in vivo and in culture. The electrophysiological properties of channels in cultured hippocampal neurons were similar to those of the channels expressed by the transfected cell lines, and the neuronal channels were similarly permeable to Lucifer yellow. The unique combination of weak voltage sensitivity, small unitary conductance, and permeation by anions as large as second messenger molecules endows Cx36 gap junction channels with properties well suited for mediating flexible electrical and biochemical interactions between neurons.

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Previous studies have provided evidence for the transcripts of Cx43 and Cx45 within pancreatic islets. As of yet, however, it has proven difficult to unambiguously demonstrate the expression of these proteins by islet cells. We have investigated whether Cx36, a new connexin species recently identified in mammalian brain and retina, may also be expressed in pancreatic islets. Using probes that permitted the original identification of Cx36 in the central nervous system, we show that a transcript for Cx36 is clearly detectable in rat pancreatic islets. Using novel and affinity-purified polyclonal antibodies, we have found that Cx36 is actually expressed in pancreatic islets. Both in situ hybridization and immunolabeling indicated that this connexin is abundant in the centrally located insulin-producing