NICOTINIC RECEPTOR-MEDIATED RESPONSES IN RELAY CELLS AND INTERNEURONSIN THE RAT LATERAL GENICULATE-NUCLEUS

We used the in vitro whole-cell recording technique to study the nicot inic responses of relay cells and interneurons in the adult rat dorsal lateral geniculate nucleus, the thalamic nucleus that conveys visual signals from the retina to the cortex. These geniculate relay cells an d interneurons were identified by their physiological and morphologica l properties. We found that, in the presence of a muscarinic antagonis t, atropine, acetylcholine induced a depolarization in relay cells. A similar depolarization was induced by application of nicotine. These d epolarizations were completely blocked by a nicotinic antagonist, hexa methonium, but were little affected by bath solution that contained te trodotoxin and/or low calcium concentration to block synaptic transmis sion. This suggests that the depolarization is mediated directly by ni cotinic receptors in relay cells. Application of nicotine also induced a depolarization in geniculate interneurons. The interneurons continu ed to exhibit a response to nicotine in the presence of synaptic block ade, although the time-course of the response was altered. The nicotin ic responses in relay cells and interneurons shared many similar prope rties. Both exhibited desensitization, although this characteristic wa s much more pronounced in the interneurons. In both cell types, the ni cotinic response activated a relatively linear conductance with a slig ht inward rectification. The reversal potential for the conductance wa s about -33 mV, which is consistent with a permeability to sodium and potassium ions. The reversal potential shifted negatively by 5-6 mV wh en the bath solution contained low calcium, which further suggests a p ermeability to calcium ions. Our results indicate that nicotinic recep tors are present in both geniculate relay cells and interneurons. The nicotinic depolarization in relay cells may serve to enhance transmiss ion of visual signals through the lateral geniculate nucleus as well a s to contribute to a voltage-dependent shift in the response mode of g eniculate relay cells from burst to tonic (single spike) firing. The n icotinic depolarization in interneurons may provide an explanation for reports that activation of the cholinergic system can enhance inhibit ory tuning in the lateral geniculate nucleus. (C) 1997 IBRO. Published by Elsevier Science Ltd.