Differential presynaptic and postsynaptic expression of m1-m4 muscarinic acetylcholine receptors at the perforant pathway/granule cell synapse.

A family of muscarinic acetylcholine receptor proteins mediates diverse pre- and postsynaptic functions in the hippocampus. However the roles of individual receptors are not understood. The present study identified the pre- and postsynaptic muscarinic acetylcholine receptors at the perforant pathway synapses in rat brain using a combination of lesioning, immunocytochemistry and electron microscopic techniques. Entorhinal cortex lesions resulted in lamina-specific reductions of m2, m3, and m4 immunoreactivity in parallel with the degeneration of the medial and lateral perforant pathway terminals in the middle and outer thirds of the molecular layer, respectively. In contrast, granule cell lesions selectively reduced m1 and m3 receptors consistent with degeneration of postsynaptic dendrites. Direct visualization of m1-m4 by electron microscopic immunocytochemistry confirmed their differential pre- and postsynaptic localizations. Together, these findings provide strong evidence for both redundancy and spatial selectivity of presynaptic (m2, m3 and m4) and postsynaptic (m1 and m3) muscarinic acetylcholine receptors at the perforant pathway synapse.