Proximity of excitatory and inhibitory axon terminals adjacent to pyramidal cell bodies provides a putative basis for nonsynaptic interactions

doi:10.1073/pnas.0900330106

Proximity of excitatory and inhibitory axon terminals adjacent to pyramidal cell bodies provides a putative basis for nonsynaptic interactions

^aDepartment of Anatomy I, School of Medicine, Universidad Complutense, 28040 Madrid, Spain;
^bInstituto Cajal, Consejo Superior de Investigaciones Científicas, Avenida Doctor Arce 37, 28002 Madrid, Spain; and
^cDepartment of Anatomy and Neurobiology, School of Medicine, University of California at Irvine, Irvine, CA 92697-1275

Edited by Edward G. Jones, University of California, Davis, CA, and approved April 28, 2009 (received for review January 12, 2009)

Abstract

Although pyramidal cells are the main excitatory neurons in the cerebral cortex, it has recently been reported that they can evoke inhibitory postsynaptic currents in neighboring pyramidal neurons. These inhibitory effects were proposed to be mediated by putative axo-axonic excitatory synapses between the axon terminals of pyramidal cells and perisomatic inhibitory axon terminals [Ren M, Yoshimura Y, Takada N, Horibe S, Komatsu Y (2007) Science 316:758–761]. However, the existence of this type of axo-axonic synapse was not found using serial section electron microscopy. Instead, we observed that inhibitory axon terminals synapsing on pyramidal cell bodies were frequently apposed by terminals that established excitatory synapses with neighbouring dendrites. We propose that a spillover of glutamate from these excitatory synapses can activate the adjacent inhibitory axo-somatic terminals.

Footnotes

¹To whom correspondence may be addressed. E-mail: defelipe{at}cajal.csic.es or amerchan{at}med.ucm.es
Author contributions: C.E.R. and J.D. designed research; A.M.-P. and J.-R.R. performed research; A.M.-P. and J.-R.R. analyzed data; and A.M.-P., C.E.R., and J.D. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0900330106/DCSupplemental.