GABAergic neurons in inferior colliculus of the GAD67-GFP knock-in mouse: Electrophysiological and morphological properties
Introduction
Mammalian inferior colliculus (IC) is an integrative auditory processing center of the midbrain, receiving inputs from ascending, descending and intrinsic connections (Huffman and Henson, 1990, Saldana and Merchan, 1992, Casseday and Covey, 1996, Ehret, 1997, Oliver, 2000). IC is the richest source of γ-aminobutyric acid (GABAergic) neurons in the mammalian auditory tracts (gerbil, Roberts and Ribak, 1987; rat, Moore and Moore, 1987; cat, Oliver et al., 1994; bat, Winer et al., 1995), and about 20% of the central nucleus of IC neurons were reported to have GAD-immunoreactivity (Oliver et al., 1994, Winer et al., 1995). IC neurons receive GABAergic projections mainly from the dorsal nuclei of lateral lemniscus (DNLL) and intrinsic GABAergic neurons (Shneiderman et al., 1993, Oliver et al., 1994). Immunnohistochemical studies showed GABAergic axonal endings at dendrites or soma of IC neurons (Oliver and Beckius, 1992). IC neurons have a wide variety of response properties to sound stimulation (Casseday and Covey, 1996, Ehret, 1997). Previous studies have shown that GABAergic inhibitory inputs play an important role in shaping the response properties of IC neurons (Faingold et al., 1989, Faingold et al., 1991, Yang et al., 1992, Park and Pollak, 1993a, Park and Pollak, 1993b, Park and Pollak, 1994, Le-Beau et al., 1996). For example, iontophoretically applied bicuculline, a GABAA receptor blocker, broadened a receptive field of the tuning curve in IC of the bat (Yang et al., 1992, Jen et al., 2001). Electrophysiological studies in slice preparation also showed that a large populations of IC neurons have a synaptic response, which is sensitive to bicuculline (Moore et al., 1998, Reetz and Ehret, 1999). In spite of their functional significance, little is known about the firing characteristics of intrinsic IC GABAergic neurons. The difficulty in distinguishing GABAergic and non-GABAergic neurons in slice preapartion might have prevented an extensive electrophysiological study of the inhibitory neurons.
In our present study, we adopted glutamate decarboxylase-green fluorescent protein (GAD67-GFP) knock-in mouse (Yanagawa et al., 2001, Tamamaki et al., 2003) to elucidate the function of GABAergic neurons in IC. This knock-in mouse was made by the gene targeting of enhanced GFP (EGFP) to the locus of GAD67 gene, and the expression of EGFP is to be under the control of the endogenous GAD67 promoter. GABAergic neurons can be identified by the GFP-fluorescence in the knock-in mouse (Tamamaki et al., 2003). In this paper, we focused GFP-labeled neurons in IC to uncover the electrophysiological properties of the GABAergic neurons in it.
Section snippets
Knock-in mouse
Gene targeting of EGFP to the locus of GAD67 encoding lesion has been described elsewhere (Yanagawa et al., 2001, Tamamaki et al., 2003). Tamamaki et al. (2003) described two strains of transgenic mice, one is GAD67-GFP mouse, which retains a loxP-flanked neomycin-resistance cassette (PGK-Neo) and the other is GAD67-GFP mouse, which lacks the PGK-Neo cassette (Δneo). In this study we used the heterozygous transgenic mice, which were obtained by breeding the mice (PGK-Neo) with wild type ICR
GFP-positive neurons in IC
In this paper, we classified inferior colliculus (IC) into three parts, according to previous anatomical studies described in rats (Fay-Lund and Osen, 1985, Peruzzi et al., 2000). Three subdivisions are a dorsal cortex (DC), an external cortex (EC) and a central nucleus (CIC). We adopted this classification, because we observed almost same cytoarchitectonic areas and cortical structures in Nissl stained IC preparation in the mouse (Fig. 1A and B). Likely to rat (Fay-Lund and Osen, 1985), the
GABAergic neurons in IC
GABAergic neurons of the mouse IC can be roughly classified into two groups from their electrical properties: tonic and transient type neurons. Tonic type neurons showed regular and repetitive discharge pattern in response to a long depolarizing current pulse. And the firing frequency is almost proportional to the intensity of injected current before it reached to the maximum. Tonic type neurons are further divided into two subgroups of RS and BP neurons by the difference in firing pattern when
Acknowledgements
We appreciated Mr. M. Fukao for the technical assistance and Mr. K. Nakamura, Dr. T. Furuta, Professor. N. Tamamaki, Professor. H. Ohmori and Professor. T. Kaneko for helpful discussion. This work was supported by grants-in-aid from the ministry of education to K.K. (14580788), and Y.Y. (15500220, 16015315).
Reference (63)
- et al.
The descending auditory pathway and acousticomotor systems: connections with the inferior colliculus
Brain Res. Rev.
(1990) - et al.
Glutaminase-like immunoreactivity in the lower brainstem and cerebellum of the adult rat
Neuroscience
(1989) - et al.
Pharmacological evidence for two types of Ca2+-dependent K+-conductance in bullfrog sympathetic ganglion cells
Neurosci. Res.
(1985) - et al.
Voltage-gated ionic currents and their roles in timing coding in auditory neurons of the nucleus magnocellularis of the chick
Neurosci. Res.
(1996) Bursts as a unit of neural information: making unreliable synapses reliable
Trends. Neurosci.
(1997)- et al.
The inferior colliculus of the mouse. A Nissl and Golgi study
Neuroscience
(1986) Encoding of timing in the brain stem auditory nuclei of vertebrates
Neuron.
(1997)- et al.
Fine Structure of GABA-labeled axonal endings in the inferior colliculus of the cat: immunocytochemistry on deplasticized ultrathin sections
Neuroscience
(1992) - et al.
Inputs from three brainstem sources to identified neurons of the mouse inferior colliculus slice
Brain Res.
(1999) - et al.
Development of tonotopy in the inferior colliculus. I. Electrophysiological mapping in house mice
Dev. Brain Res.
(1990)