Elsevier

Neuroscience

Volume 136, Issue 3, 2005, Pages 907-925
Neuroscience

The inferior colliculus of the rat: Quantitative immunocytochemical study of GABA and glycine

https://doi.org/10.1016/j.neuroscience.2004.12.030Get rights and content

Abstract

Both GABA and glycine (Gly) containing neurons send inhibitory projections to the inferior colliculus (IC), whereas inhibitory neurons within the IC are primarily GABAergic. To date, however, a quantitative description of the topographic distribution of GABAergic neurons in the rat’s IC and their GABAergic or glycinergic inputs is lacking. Accordingly, here we present detailed maps of GABAergic and glycinergic neurons and terminals in the rat’s IC. Semithin serial sections of the IC were obtained and stained for GABA and Gly. Images of the tissue were digitized and used for a quantitative densitometric analysis of GABA immunostaining. The optical density, perimeter, and number of GABA- and Gly immunoreactive boutons apposed to the somata were measured. Data analysis included comparisons across IC subdivisions and across frequency regions within the central nucleus of the IC.

The results show that: 1) 25% of the IC neurons are GABAergic; 2) there are more GABAergic neurons in the central nucleus of the IC than previously estimated; 3) GABAergic neurons are larger than non-GABAergic; 4) GABAergic neurons receive less GABA and glycine puncta than non-GABAergic; 5) differences across frequency regions are minor, except that the non-GABAergic neurons from high frequency regions are larger than their counterparts in low frequency regions; 6) differences within the laminae are greater along the dorsomedial–ventrolateral axis than along the rostrocaudal axis; 7) GABA and non-GABAergic neurons receive different numbers of puncta in different IC subdivisions; and 8) GABAergic puncta are both apposed to the somata and in the neuropil, glycinergic puncta are mostly confined to the neuropil.

Section snippets

Immunocytochemistry for GABA and Gly

Six adult Wistar rats of either sex (B.W., 250–300g) were anesthetized with sodium pentobarbital (60mg/kg, i.p.) and perfused transcardially with a cold (6°C) wash solution (40ml) composed of 2% dextran (MW 70,000) in 0.1M sodium phosphate buffer, pH 7.4 (PB), followed by 750ml of a fixative containing 1% paraformaldehyde and 2.5% glutaraldehyde in the same buffer at room temperature. The specimens were kept at 4°C overnight, and the next day the brains were removed. The brainstems were cut

Standard microscopic analysis of GABA and Gly immunoreactivity in the IC

Visual inspection of the immunostained tissue under the light microscope revealed that all parts of the IC show (1) dense punctate immunostaining of the neuropil to both GABA (Fig. 1, Fig. 2) and Gly (Fig. 2), (2) some IC neurons are GABA-IR, and (3) no IC neurons are Gly-IR.

GABA-IR neurons show a variable degree of immunostaining regardless of their topographical location. They are found in all subdivisions of the IC in slightly different proportions, and have cell bodies of variable sizes and

Discussion

The present account demonstrates that all neurons in the IC are under the influence of GABAergic and/or glycinergic inhibitory input. Furthermore, a quarter of the IC neurons in the rat are GABAergic and, in contrast to lower auditory centers, the IC lacks glycinergic cells.

Fig. 11 illustrates our main findings in schematic representations of the IC subdivisions (A) and of the CNIC lamina seen en face (B). The results may be summarized as follows: 1) GABAergic neurons are larger than

Acknowledgments

Dr. Ole P. Ottersen kindly provided GABA and glycine antibodies. We thank Ignacio Plaza for his excellent technical assistance, and Jack Kelly, Douglas Oliver, Kirsten Osen and Bruce Warr for their critical reading and comments on a previous version of the manuscript. This study was supported by the Spanish DGES; grant number: BFI-2000-1396, Spanish DGES; grant number BFI-2003-09147-02-01 to M.A.M. and M.S.M.; the Spanish JCYL-UE grant number: SA040/04 to M.S.M. and M.A.M.), and the Spanish

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    Present address: University Cayetano Heredia, Division of Neuroscience and Behavior, Lima, Peru.

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