Mechanisms governing dendritic γ-aminobutyric acid (GABA) release in the rat olfactory bulb

Mechanisms governing dendritic γ-aminobutyric acid (GABA) release in the rat olfactory bulb

Jeffry S. Isaacson*

Department of Neuroscience, University of California at San Diego School of Medicine, La Jolla, CA 92093-0608

Edited by Richard W. Tsien, Stanford University School of Medicine, Stanford, CA, and approved November 15, 2000 (received for review September 17, 2000)

Abstract

In the olfactory bulb, synaptic transmission between dendrites plays an important role in the processing of olfactory information. Glutamate released from the dendrites of principal mitral cells excites the dendritic spines of granule cells, which in turn release γ-aminobutyric acid (GABA) back onto mitral cell dendrites. Slow N-methyl-d-aspartate (NMDA) receptors on granule dendrites are particularly effective in driving this reciprocal dendrodendritic inhibition (DDI), raising the possibility that calcium influx through NMDA receptors may trigger GABA exocytosis directly. In this study, I show that NMDA receptor activation is not an absolute requirement and that DDI can be evoked solely by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors when granule cell excitability is increased or under conditions that slow AMPA receptor kinetics. In physiological extracellular Mg²⁺, DDI elicited by photolysis of caged calcium in mitral dendrites is blocked by cadmium and toxins to N- and P/Q-type voltage-gated calcium channels. DDI is largely unaffected after granule dendrites have been loaded with the slow calcium chelator EGTA, suggesting a tight coupling between the site of calcium influx and the release machinery governing GABA exocytosis. These results indicate that voltage-gated calcium channels play an essential role in dendritic GABA release during reciprocal feedback inhibition in the olfactory bulb.

Footnotes

↵ * To whom reprint requests should be addressed at: Dept. of Neuroscience, School of Medicine, University of California, San Diego, Basic Sciences Bldg., Rm. 3065, 9500 Gilman Drive, La Jolla, CA 92093-0608. E-mail: jisaacson{at}ucsd.edu.
This paper was submitted directly (Track II) to the PNAS office.
Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.021445798.
Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.021445798
Abbreviations:

GABA,

γ-aminobutyric acid;

DDI,

dendrodendritic self-inhibition;

NMDA,

N-methyl-d-aspartate;

NMDAR,

NMDA receptor;

AMPA,

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid;

AMPAR,

AMPA receptor;

TTX,

tetrodotoxin;

TEA,

tetraethylammonium;

4-AP,

4-aminopyridine;

EPSP,

excitatory postsynaptic potential;

EPSC,

excitatory postsynaptic current;

IPSC,

inhibitory postsynaptic current;

EPL,

external plexiform layer;

NBQX,

1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo[f]quinoxaline-7-sulfonamide;

APV,

d- amino-5-phosphovaleric acid;

CTZ,

cyclothiazide;

EGTA-AM,

EGTA acetoxymethyl ester;

PPF,

paired pulse facilitation