The hippocampus receives cholinergic afferents and expresses neuronal nicotinic acetylcholine receptors. In particular, the alpha 7 and beta 2 nicotinic subunits are highly expressed in the hippocampus. There has been controversy about the location, distribution and roles of neuronal nicotinic acetylcholine receptors [Role L. W. and Berg D. K. (1996) Neuron 16, 1077-1085; Wonnacott S. (1997) Trends Neurosci. 20, 92-98]. Using immunocytochemistry and patch-clamp techniques. we examined the density and distribution of nicotinic receptors on rat hippocampal neurons in primary tissue culture. The density and distribution of alpha 7 subunits change with days in culture. Before 10 days in culture, alpha 7 expression, monitored immunocytochemically, is low and nicotinic currents are small or absent. In older cultures, about two-thirds of the neurons express nicotinic currents, and alpha 7 appears in small patches on the soma and out along the neuronal processes. These patches of alpha 7 subunits on the surface of the neuronal processes often co-localize with the presynaptic marker, synaptotagmin. The other most common nicotinic subunit, beta 2, stays confined mainly to the soma and proximal processes, and beta 2 is distributed more uniformly and is not specifically localized at presynaptic areas. The two subunits, alpha 7 and beta 2, have different expression patterns on the surface of the cultured hippocampal neurons. Taken together with previous physiological studies, the results indicate that alpha 7 subunits can be found at presynaptic terminals, and at these locations, these calcium-permeable channels may influence transmitter release. (C) 1998 IBRO. Published by Elsevier Science Ltd.
