Cholinergic Mechanisms in the Visual System of Rat

Abstract

The rich cholinergic innervation of subcortical visual structures is not associated with the primary optic projection (Bigl, V. and Schober, W., Exp. Brain Res., 27, 211–219, 1977) but ACh is regarded as the transmitter released from reticular connections to the lateral geniculate nucleus (LGN). A direct monosynaptic AChEcontaining pathway from the mesencephalic reticular formation (MRF) to the LGN has been described in the rat (Shute, C.C.D. and Lewis, P.R., Brain 90, 497–520, 1967), but could not be confirmed by recent horseradish peroxidase studies (Lüth, H.J., Seidel, J. and Schober, W., Acta Histochem., 60, 91–102, 1977). To provide quantitative data on the structural basis of cholinergic modulation of visual input, this communication describes biochemical and quantitative histochemical results on changes in ChAT and AChE activities in some visual structures after partial or total deafferentation. The results indicate: (i) The cholinergic fibres found within the optic nerve are partly of central and partly of retinal origin. (ii) About 30% of ChAT activity of the LGN seems to be related to afferents from the MRF. Lesions placed in this structure at the level of Nc. ruber induce bilateral loss, and lesions at the level of the sensory root of the Vth nerve, uni(ipsi)lateral loss of AChE and ChAT activity in the LGN (dorsal and ventral part) and in the superior colliculus. (iii) The large proportion of ChAT remaining in the LGN seems to be localized in intrinsic neurones. The facilitatory influence of MRF stimulation on synaptic transmission in the LGN has been shown to be due to disinhibition rather than to postsynaptic excitation (Singer, W., Physiol. Rev., 57, 386–420, 1977) but, on the other hand, LGN cells are excited by microiontophoretically applied ACh. The synaptic mechanism by which ACh acts in LGN transmission thus remains to be unravelled.