PREFERENTIAL INNERVATION OF IMMUNOREACTIVE CHOLINE-ACETYLTRANSFERASE SYNAPSES ON RELAY CELLS OF THE CATS LATERAL GENICULATE-NUCLEUS - A DOUBLE-LABELING STUDY

Relationships between cholinergic axons and GABA cells in the lateral geniculate and the perigeniculate nuclei were quantitatively assessed by combining pre-embedding immunocytochemical visualization of choline acetyltransferase with post-embedding immunogold labelling of GABA at the electron microscopic level. In the lateral geniculate nucleus, vesicle-containing profiles immunoreactive for choline acetyltransferase made synapses exclusively with dendritic profiles both within and outside synaptic glomeruli. Only 7.0% (seven of 100 dendrites) of the target dendrites tested were positive for GABA. Of these, the majority (85.7%; 6/7) were dendritic profiles containing synaptic vesicles. This is in contrast with the overall population of geniculate synapses where 21.3% of the targets were GABA-immunopositive, a three-fold significant difference. In the perigeniculate nucleus, immunoreactive choline acetyltransferase synapses targeted mainly dendritic profiles (93.9%; 31/33) and to a lesser extent somata (6.1%). In this nucleus, all unequivocally defined targets were GABA-immunopositive. Cholinergic inputs thus show a preference to target geniculate relay neurons without a strong innervation of GABA-immunoreactive interneurons. In the perigeniculate, however, cholinergic synapses provide a powerful input to GABAergic cells. This suggests that the facilitatory effects of acetylcholine on the excitability of relay cells would be mediated through (i) a direct innervation of the geniculate relay cells, and (ii) a cholinergic inhibitory innervation of the recurrent inhibition to the lateral geniculate nucleus via the perigeniculate nucleus.