Perforated synapses on double-headed dendritic spines: a possible structural substrate of synaptic plasticity.

Examination of axospinous synapses in serial sections obtained from the middle molecular layer of the rat dentate gyrus has revealed that some of them involve double-headed dendritic spines. Each spine head is apposed by a separate axon terminal with which it always forms a perforated synaptic contact distinguished by a discontinuous postsynaptic density. The number of perforated synapses on double-headed spines was estimated as a synapse-to-neuron ratio with the aid of the disector technique and found to be significantly increased in rats kindled via medial perforant path stimulation. These results support the notion that perforated synapses involving double-headed dendritic spines represent a structural modification related to enhanced synaptic efficacy.