LGN-projecting neurons of the cat's pretectum express glutamic acid decarboxylase mRNA

There have been conflicting reports on the chemical nature of the projection of the pretectal nuclei [nucleus of the optic tract and dorsal terminal nucleus of the accessory optic tract (NOT-DTN complex) and posterior pretectal nucleus] to the lateral geniculate nucleus and inferior olive. There is evidence that the pretecto-geniculate pathway is inhibitory. However, most attempts to verify the GABAergic nature of the projection neurons have failed. In order to answer this question, we employed a combination of retrograde transport and in situ hybridization. Rhodamine-labelled latex microspheres were injected into the electrophysiologically identified lateral geniculate nucleus. In addition, fluorescein-labelled latex microspheres were injected into the inferior olive. Retrograde axonal transport labelled large pretectal neurons. We then applied riboprobes specific for glutamic acid decarboxylase mRNA. We were able to demonstrate glutamic acid decarboxylase mRNA expression in up to 70% of lateral geniculate nucleus-projecting NOT-DTN and posterior pretectal nucleus neurons but in none of the pretecto-olivary projection neurons. The results suggest that the pretecto-geniculate projection is GABAergic in nature, which would confirm previous electrophysiological and morphological observations. The pretecto-olivary projection is not GABAergic.