A reciprocal connection between the ventral lateral geniculate nucleus and the pretectal nuclear complex and the superior colliculus: an in vitro characterization in the rat.

The ventral lateral geniculate nucleus (vLGN), the pretectal nuclear complex (PNC) and the superior colliculus (SC) are structures that all receive retinal input. All three structures are important relay stations of the subcortical visual system. They are strongly connected with each other and involved in circadian and/or visuomotor processes. However, the information transferred along these pathways is unknown and their possible functions are, therefore, not well understood. Here, we characterized multiple pathways between the vLGN, the PNC, and the SC electrophysiologically and anatomically in an in vitro study using acute rat brain slices. Using orthodromic and antidromic electrical stimulation, we first characterized vLGN neurons that receive pretectal input and those that project to the PNC. Morphological reconstructions of cells labeled after patch clamp recordings identified these neurons as geniculo-tectal neurons and as medium-sized multipolar neurons. We identified inhibitory connections in both pathways and we could show that inhibitory postsynaptic currents (IPSCs) evoked from the PNC in vLGN neurons are mediated only by GABAA receptors, while IPSCs evoked in PNC neurons by vLGN stimulation are either mediated by both, GABAA and GABAC receptors or by a GABA receptor with mixed GABAA and GABAC receptor-like pharmacology. Finally, retrograde double labeling experiments with two different fluorescent dextran amines indicated that pretectal neurons which project to the ipsilateral vLGN also project to the ipsilateral SC.