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The Journal of Neuroscience, July 25, 2007, 27(30):8088-8100; doi:10.1523/JNEUROSCI.0702-07.2007

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Cellular/Molecular
Evidence against GABA Release from Glutamatergic Mossy Fiber Terminals in the Developing Hippocampus

Motokazu Uchigashima,¹ Masahiro Fukaya,¹ Masahiko Watanabe,¹ and Haruyuki Kamiya²

Departments of ¹Anatomy and ²Neurobiology, Hokkaido University School of Medicine, Sapporo 060-8638, Japan

Correspondence should be addressed to Haruyuki Kamiya, Department of Neurobiology, Hokkaido University School of Medicine, Sapporo 060-8638, Japan. Email: kamiya{at}med.hokudai.ac.jp

Hippocampal mossy fibers of young rodents have been reported to corelease inhibitory neurotransmitter GABA in addition to excitatory transmitter glutamate. In this study, we aimed at re-evaluating this corelease hypothesis of both inhibitory and excitatory transmitters in the hippocampus. Electrophysiological examination revealed that, in juvenile mice and rats of the two to 3 weeks old, stimulation at the granule cell layer of the dentate gyrus elicited monosynaptic GABAergic IPSCs in CA3 neurons in the presence of ionotropic glutamate receptor (iGluR) blockers, only when rather strong stimuli were given. The group II mGluR agonist (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclo-propyl)glycine (DCG-IV), which selectively suppresses transmission at the mossy fiber–CA3 synapse, abolished almost all postsynaptic responses elicited by the weak stimuli, whereas those by strong stimuli were inhibited only slightly. In addition, the minimal stimulation elicited GABAergic IPSCs in neonatal mice of the first postnatal week, whereas these responses are not sensitive to DCG-IV. Immunohistochemical examination revealed that mossy fiber terminals expressed GABA and the GABA-synthesizing enzyme GAD67, although the expression levels were much weaker than those in the inhibitory interneurons. Notably, the expression levels of the vesicular GABA transporter were much lower than those of GABA and GAD67, and almost below detection threshold. These results suggest that mossy fiber synapses are purely glutamatergic and apparent monosynaptic IPSCs so far reported are evoked by costimulation of inhibitory interneurons, at least in young mice and rats. Hippocampal mossy fiber terminals synthesize and store GABA, but have limited ability in vesicular release for GABA in the developing rodents.

Key words: hippocampus; mossy fiber; corelease; GABA; GAD; VGAT

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Received Feb. 15, 2007; revised June 13, 2007; accepted June 13, 2007.

Correspondence should be addressed to Haruyuki Kamiya, Department of Neurobiology, Hokkaido University School of Medicine, Sapporo 060-8638, Japan. Email: kamiya{at}med.hokudai.ac.jp

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				S. Sivakumaran, M. H. Mohajerani, and E. Cherubini At Immature Mossy-Fiber-CA3 Synapses, Correlated Presynaptic and Postsynaptic Activity Persistently Enhances GABA Release and Network Excitability via BDNF and cAMP-Dependent PKA J. Neurosci., February 25, 2009; 29(8): 2637 - 2647. [Abstract] [Full Text] [PDF]

				J.-L. Boulland, M. Jenstad, A. J. Boekel, F. G. Wouterlood, R. H. Edwards, J. Storm-Mathisen, and F. A. Chaudhry Vesicular Glutamate and GABA Transporters Sort to Distinct Sets of Vesicles in a Population of Presynaptic Terminals Cereb Cortex, January 1, 2009; 19(1): 241 - 248. [Abstract] [Full Text] [PDF]

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