Direction tuning of inhibitory inputs to the turtle accessory optic system.

Neurons in turtle accessory optic system (basal optic nucleus, BON) were studied to compare excitatory and inhibitory visual inputs. Using a reduced in vitro brain stem preparation with the eyes attached, previous studies only showed a monosynaptic retinal input to the BON from direction-sensitive retinal ganglion cells that share a common preferred direction. Now using an intact brain stem preparation, not only did BON neurons display inhibitory postsynaptic potentials [IPSP(C)s] spontaneously, but IPSP(C)s were also evoked by visual pattern motion, they had their polarity reversed near the chloride equilibrium potential and they were blocked by the GABA(A) antagonist bicuculline. Because excitatory postsynaptic currents had reversal potentials >0 mV, BON cells were recorded using patch electrodes filled with QX-314 or Cs+ to measure the cell's direction tuning also at that higher reversal potential. For most of the BON neurons studied, their visual excitation and inhibition had a very similar preferred direction, indicating that both synaptic inputs were maximally active onto the same cell under the same stimulus conditions. These competing inputs may result from connections between the pretectum and accessory optic nuclei. Such synaptic interactions may serve a functional role in the visual processing necessary to create retinal slip signals for oculomotor control.