The effect of stimulation of the laterodorsal tegmental nucleus (LDT) on the activity of single neurons in the dorsal lateral geniculate nucleus was studied in rats anesthetized with urethan. The LDT is the largest aggregation of cholinergic neurons in the brain stem that project to the thalamus, and in the rat is sufficiently compact to permit its localized stimulation. Position of stimulating electrodes was confirmed on histological sections processed with NADPH-diaphorase histochemistry, which in the rat brain stem selectively stains cholinergic neurons. Repetitive stimulation of the LDT at 200 Hz increased the firing rate of substantially all geniculate relay neurons and weakly depressed the activity of intrinsic interneurons. These effects usually occurred within several hundred milliseconds after the onset of stimulation and began to fade within a few seconds, despite continuing stimulation. The excitatory effects on relay neurons were blocked by scopolamine applied ionophoretically or intravenously, but not by noradrenergic antagonists, suggesting the cholinergic nature of LDT-induced excitation. During LDT stimulation the number of spikes evoked by photic stimulation of the receptive field of relay neurons usually increased, but it remained unchanged in a few cases. The increase was due to simple enhancement of photic responses or due to conversion of phasic type responses to tonic ones. As to the balance of background activity and photic responses, the effects of LDT stimulation varied from neuron to neuron. Even in a given neuron, the effects varied depending on its excitability level or the nature of the photic stimulation. These results show that the cholinergic projection from the LDT may be involved in the ascending reticular activating system, although the functional significance of the activating system in visual information processing in the geniculate nucleus remains to be clarified.
