Elsevier

Neuroscience

Volume 133, Issue 3, 2005, Pages 819-829
Neuroscience

Sensory system
Properties of external plexiform layer interneurons in mouse olfactory bulb slices

https://doi.org/10.1016/j.neuroscience.2005.03.008Get rights and content

Abstract

In the external plexiform layer (EPL) of the main olfactory bulb, apical dendrites of inhibitory granule cells form large numbers of synapses with mitral and tufted (M/T) cells, which regulate the spread of activity along the M/T cell dendrites. The EPL also contains intrinsic interneurons, the functions of which are unknown. In the present study, recordings were obtained from cell bodies in the EPL of mouse olfactory bulb slices. Biocytin-filling confirmed that the recorded cells included interneurons, tufted cells, and astrocytes. The interneurons had fine, varicose dendrites, and those located superficially bridged the EPL space below several adjacent glomeruli. Interneuron activity was characterized by high frequency spontaneous excitatory postsynaptic potential/currents that were blocked by the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and largely eliminated by the voltage-sensitive Na+ channel blocker, tetrodotoxin. Interneuron activity differed markedly from that of tufted cells, which usually exhibited spontaneous action potential bursts. The interneurons produced few action potentials spontaneously, but often produced them in response to depolarization and/or olfactory nerve (ON) stimulation. The responses to depolarization resembled responses of late- and fast-spiking interneurons found in other cortical regions. The latency and variability of the ON-evoked responses were indicative of polysynaptic input. Interneurons expressing green fluorescent protein under control of the mouse glutamic acid decarboxylase 65 promoter exhibited identical properties, providing evidence that the EPL interneurons are GABAergic. Together, these results suggest that EPL interneurons are excited by M/T cells via AMPA/kainate receptors and may in turn inhibit M/T cells within spatial domains that are topographically related to several adjacent glomeruli.

Section snippets

Recording methods

Juvenile and mature 22–34 day old C57BL/6J mice from Jackson Laboratories (Bar Harbor, ME, USA) and transgenic mice reared at the University of Maryland, Baltimore, MD, USA, were used. The transgenic mice expressed enhanced green fluorescent protein (GFP) under control of the regulatory region of the 65 kDa mouse glutamic acid decarboxylase (GAD) 65 gene (Erdélyi et al 2002, Galarreta et al 2004). Horizontal slices (400μm thick) were obtained using a vibratome (Series 1000, Ted Pella Inc.,

Morphology

Voltage- and current-clamp recordings were obtained from 20 histologically verified interneurons: 12 in slices from C57BL/6J mice and eight in slices from GAD65-GFP mice. All 20 of the interneurons were distinguished by their varicose processes (Fig. 1A, B, and D). The cell bodies were round-to-oval in shape, with average major and minor dimensions of 12.6±0.6μm (mean±S.E.M.) and 9.1±0.4μm, respectively. Both the GL and mitral cell layer (MCL) borders were visible in sections containing 19 of

Discussion

This study provides new information about the poorly understood interneurons found in the EPL of the main olfactory bulb. Biocytin staining shows that the mouse interneurons resemble the VG and parvalbumin-IR multipolar interneurons described in the hamster and rat using other staining methods (Schneider and Macrides 1978, Kosaka et al 1994). Like many of the hamster and rat interneurons, the mouse interneurons do not appear to bear axons. The axons of EPL interneurons might only be

Acknowledgments

We thank Jason Aungst, Phil Heyward, Sergei Karnup, Stephanie Parrish-Aungst, and Adam Puche for technical assistance, Frank Margolis for the GAD65-GFP mice, Faith Scipio for genotyping, and Michael Shipley and the Department of Anatomy and Neurobiology for sabbatical leave support for K. A. Hamilton at the University of Maryland, Baltimore. Support contributed by NIH grants DC00347, DC03112, DC03195, DC06356, DC36940, and the Biomedical Research Foundation of Northwest Louisiana.

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    Present address: Department of Anatomy, Howard University College of Medicine, Washington, DC 20059, USA (T. Heinbockel); Department of Anatomy and Neurobiology, University of Tennessee Health Science Center, Memphis, TN 38163, USA (M. Ennis, A. Hayar).

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