Functional polarity of dendrites and axons of primate A1 amacrine cells

The Al cell is an axon-bearing amacrine cell of the primate retina with a diffusely stratified, moderately branched dendritic tree (similar to 400 mu m diameter). Axons arise from proximal dendrites forming a second concentric, larger arborization (>4 turn diameter) of thin processes with bouton-like swellings along their length. Al cells are ON-OFF transient cells that fire a brief high frequency burst of action potentials in response to light (Stafford & Dacey, 1997). It has been hypothesized that Al cells receive local input to their dendrites, with action potentials propagating output via the axons across the retina, serving a global inhibitory function. To explore this hypothesis we recorded intracellularly from Al cells in an in vitro macaque monkey retina preparation. Al cells have an antagonistic center-surround receptive field structure for the ON and OFF components of the light response. Blocking the ON pathway with L-AP4 eliminated ON center responses but not OFF center responses or ON or OFF surround responses. Blocking GABAergic inhibition with picrotoxin increased response amplitudes without affecting receptive field structure. TTX abolished action potentials, with little effect on the sub-threshold light response or basic receptive field structure. We also used multi-photon laser scanning microscopy to record light-induced calcium transients in morphologically identified dendrites and axons of Al cells. TTX completely abolished such calcium transients in the axons but not in the dendrites. Together these results support the current model of Al function, whereby the dendritic tree receives synaptic input that determines the center-surround receptive field; and action potentials arise in the axons, which propagate away from the dendritic field across the retina.