MODULATION OF ELECTRICAL SYNAPTIC TRANSMISSION IN ZEBRAFISH RETINAL HORIZONTAL CELLS

Electrical synaptic transmission is widespread in the vertebrate CNS a nd its modulation plays a critical role in altering the properties of coupled neural networks. In order to define further the mechanisms of electrical synaptic plasticity in the vertebrate retina, the electroph ysiological characteristics of solitary horizontal cells and horizonta l cell pairs from the zebrafish (Brachydanio rerio) were examined by w hole-cell patch-clamp recordings from cells in primary cell culture. I n solitary cells, the current-voltage relation exhibited inward curren t at potentials negative to -60 mV, a linear region of high resistance from -50 mV to 0 mV, and outward current positive to + 20 mV. The inw ard current at negative potentials was blocked by substituting Cs+ for K+ in the extracellular medium, while the outward current at positive potentials was blocked by substitution of Cs+ for K+ in the pipette s olution. Measurements of gap junctional conductance from electrically coupled cell pairs revealed that zebrafish horizontal cells expressed a mean junctional conductance of considerably smaller magnitude than o ther teleost retinal horizontal cells. Junctional conductance was foun d to be voltage dependent, exhibiting time-dependent closure with incr easing transjunctional voltage. Voltage sensitivity was symmetrical ab out 0 mV junctional potential. At +/-90 mV the ratio of steady state t o peak current was approximately 0.5 and the time constant for inactiv ation of the junctional current was approximately 120 msec. Junctional conductance was also modulated by dopamine and cAMP. Pairs of horizon tal cells responded to puff application of dopamine with a two- to thr eefold reduction in junctional conductance, but there was no discernib le effect on extrajunctional conductances. The action of dopamine on c oupling was mimicked by application of the dopamine agonist (+/-)-6,7- dihydroxy-2-amino-tetralin (ADTN) and the membrane permeable cAMP anal og 8-bromo-cAMP. The selective D, dopamine receptor antagonist SCH2339 0 blocked uncoupling by dopamine. These data provide a primary descrip tion of the electrophysiological characteristics of solitary horizonta l cells and the electrical coupling between pairs of horizontal cells dissociated from the zebrafish retina. They indicate that zebrafish ho rizontal cells are distinct from the horizontal cells of other teleost s in their coupling characteristics. The results suggest that zebrafis h horizontal cells exhibit differences in the regulation of synaptic a ssembly and maintenance that have important implications for the funct ion of the zebrafish horizontal cell network in vivo.