STATIC AND DYNAMIC MEMBRANE-PROPERTIES OF LARGE-TERMINAL BIPOLAR CELLS FROM GOLDFISH RETINA - EXPERIMENTAL TEST OF A COMPARTMENT MODEL

Capacitance measurements allow direct studies of exocytosis and endocy tosis in single synaptic terminals isolated from bipolar neurons of go ldfish retina. Extending the technique to intact bipolar cells, with t heir more complex morphology, requires information about the cells' el ectrotonic architecture. To this end, we developed a compartment model of bipolar neurons isolated from goldfish retina and tested the model experimentally. The isolated cells retained morphology similar to tha t of bipolar neurons in intact goldfish retina. In whole cell recordin gs, current relaxations in response to 10-mV hyperpolarizing voltage p ulses decayed with a biexponential time course. This suggests that the cells may be described by a two-compartment equivalent circuit with c ompartments corresponding to the soma/dendrites (6-10 pF) and synaptic terminal (2-4 pF), linked by the axial resistance (30-60 M Omega) of the axon. Four lines of evidence validate the equivalent circuit. 1) S imilar estimates of somatic/dendritic and terminal capacitance were ob tained whether the patch pipette was attached to the soma or to the sy naptic terminal. 2) Estimates of the capacitance of the two compartmen ts in intact cells were similar to estimates from somata and terminals that were isolated by cleavage of the connecting axon. 3) When curren t transients were generated from a more complete computer simulation o r a bipolar neuron, analysis of the simulated transients with the use of the simple two-compartment model yielded capacitance estimates simi lar to those used to set up the simulation. 4) In isolated cells, the model gave estimates of depolarization-evoked increases in capacitance of the synaptic terminal that were quantitatively similar to those me asured in terminals that were detached from the rest of the cell. Alth ough in previous studies researchers have attempted to apply a similar equivalent circuit to more geometrically complex cells, morphological correlates of the equivalent-circuit compartments have been elusive. Our results demonstrate that in dissociated bipolar cells, precise mor phological correlates cane assigned to the equivalent-circuit compartm ents. Additionally, the work shows that time-resolved capacitance meas urements of synaptic transmitter release are possible in intact, isola ted bipolar neurons and may also be feasible in intact tissue.