Blocking Retinal Chloride Co-Transporters KCC2 and NKCC: Impact on Direction Selective ON and OFF Responses in the Rat’s Nucleus of the Optic Tract

Katharina Spoida,Anne-Kathrin Trampe,Claudia Distler,Klaus-Peter Hoffmann,Samuel G Solomon

doi:10.1371/journal.pone.0044724

Abstract

In the present study we investigated in vivo the effects of pharmacological manipulation of retinal processing on the response properties of direction selective retinal slip cells in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN), the key visuomotor interface in the pathway underlying the optokinetic reflex. Employing a moving visual stimulus consisting of either a large dark or light edge we could differentiate direction selective ON and OFF responses in retinal slip cells. To disclose the origin of the retinal slip cells' unexpected OFF response we selectively blocked the retinal ON channels and inactivated the visual cortex by cooling. Cortical cooling had no effect on the direction selectivity of the ON or the OFF response in NOT-DTN retinal slip cells. Blockade of the retinal ON channel with APB led to a loss of the ON and, to a lesser degree, of the OFF response and a reduction in direction selectivity. Subsequent blocking of GABA receptors in the retina with picrotoxin unmasked a vigorous albeit direction unselective OFF response in the NOT-DTN. Disturbing the retinal chloride homeostasis by intraocular injections of bumetanide or furosemide led to a loss of direction selectivity in both the NOT-DTN's ON and the OFF response due to a reduced response in the neuron's preferred direction under bumetanide as well as under furosemide and a slightly increased response in the null direction under bumetanide. Our results indicate that the direction specificity of retinal slip cells in the NOT-DTN of the rat strongly depends on direction selective retinal input which depends on intraretinal chloride homeostasis. On top of the well established input from ON center direction selective ganglion cells we could demonstrate an equally effective input from the retinal OFF system to the NOT-DTN.

Highlights

In all mammals investigated so far a common pathway underlying the horizontal optokinetic reflex has emerged
Retinal slip neurons in the pretectal nucleus of the optic tract and the dorsal terminal nucleus of the accessory optic system (NOTDTN) represent the visuomotor interface linking the visual input from the retina and, in many mammals e.g. rat [1], rabbit [2], ferret [3], cat [4], guinea pig [5], monkey [6], but not in marsupials [7,8], the visual cortex with the motor output innervating the extraocular muscles via relays in the brainstem, the cerebellum, and the deep cerebellar nuclei [9, for a recent review see 10]
There is a wealth of data showing that ON center direction selective ganglion cells project to nuclei of the accessory optic system

Summary

Introduction

In all mammals investigated so far a common pathway underlying the horizontal optokinetic reflex (hOKR) has emerged. Anatomical, electrophysiological, and pharmacological studies have suggested that the other nuclei in the accessory optic system receive input only from ON-center direction selective ganglion cells in all vertebrates investigated so far (turtle [13, 14, 15, mouse [16], rat [17], rabbit [18,19], cat [20]). Starburst amacrine cells are characterized by radially symmetric dendritic trees that convey directionally selective GABAergic inhibition to direction selective ganglion cells [26,27,28]. These dendrites themselves are already direction selectively depolarised by movements from the soma to the tip of the dendrite. Direction selectivity in ganglion cells is generated by spatially asymmetric input-output relationships of amacrine dendrites, lateral inhibition between neighbouring amacrine cells, and local postsynaptic signal processing [28,30,31,33,34,35,36]

Methods

Results

Conclusion