Nicotine exposure refines visual map topography through an NMDA receptor-mediated pathway

The precise mapping of one surface onto another is fundamental to visual system organization and depends upon adequate stimulation of postsynaptic targets to stabilize correctly placed synapses. As exogenous nicotine alters neuronal activity, we investigated whether it would affect the visual map created by retinal ganglion cell terminals in the frog optic tectum. Chronic exposure of the tectum to nicotine decreased the retinal area from which cells project to a given tectal site. This map refinement was also produced by exposure to either the alpha-bungarotoxin sensitive nicotinic receptor agonist, anatoxin-a or the alpha-bungarotoxin-insensitive nicotinic receptor agonist epiboxidine. Immunocytochemical studies using mAb306 and mAb22 demonstrated that alpha-bungarotoxin-sensitive and -insensitive nicotinic receptors, respectively, occupied different tectal sites. Choline acetyltransferase immunoreactivity overlapped with mAb306, but not mAb22, staining. The developing optic tectum was more sensitive to nicotine than the adult tectum and nicotine induced both map refinements and map disruptions in a concentration-dependent manner. Blockade of the N-methyl-D-aspartate (NMDA) receptor with D(-)-2-amino-5-phosphonopentanoic acid (D-APV) prevented nicotine from refining the map in the adult tectum. Exposure to the use-dependent NMDA antagonist MK801 alone had no effect on retinotectal topography but in combination with either NMDA or nicotine it disrupted the map. Exposure to NMDA alone produced refinement. We conclude that the map refinement induced by chronic nicotine treatment has as its basis an increase in the level of NMDA receptor activity. The data are consistent with a model whereby map topography can be bidirectionally affected by either increasing or decreasing NMDA receptor activity.