SPIKING AND NONSPIKING MODELS OF STARBURST AMACRINE CELLS IN THE RABBIT RETINA

The integrative properties of starburst amacrine cells in the rabbit r etina were studied with compartmental models and computer-simulation t echniques. The anatomical basis for these simulations was provided by computer reconstructions of intracellularly stained starburst amacrine cells and published data on dendritic diameter and biophysical proper ties. Passive and active membrane properties were included to simulate spiking and nonspiking behavior. Simulated synaptic inputs into one o r more compartments consisted of a bipolar-like conductance change wit h peak and steady-state components provided by the sum of two Gaussian responses. Simulated impulse generation was achieved by using a model of impulse generation that included five nonlinear channels (I-Na, I- Ca, I-A, I-K, I-K,I-Ca). The magnitude of the sodium channel conductan ce change was altered to meet several different types of impulse gener ation and propagation behaviors. We studied a range of model constrain ts which included variations in membrane resistance (R-m) from 4,000 O mega.cm(2) to 100,000 Omega.cm(2), and dendritic diameter from 0.1 to 0.3 mu m. In a separate series of simulations, we studied the feasibil ity of voltage-clamping starburst amacrine cells using a soma-applied, single-electrode voltage clamp, based on models with and without dend ritic and somatic spiking behavior. Our simulation studies suggest tha t single dendrites of starburst amacrine cells can behave as independe nt functional subunits when the R-m is high, provided that one or a sm all number of dendrites is synaptically co-activated. However, as the number of co-activated dendrites increases, the starburst cell behavio r becomes more uniform and independent dendritic function is less prev alent. The presence of impulse activity in the dendrites raises new qu estions about dendritic function. However, dendritic impulses do not n ecessarily eliminate independent dendritic function, because dendritic impulses commonly fail as they propagate toward the soma, where they contribute EPSP-like responses which summate with conventional synapti c currents.