Expression of GABA(A) receptor subunits in the rat central nucleus of the inferior colliculus

Asymmetric temporal interactions of sound-evoked excitatory and inhibitory inputs in the mouse auditory midbrain

In the auditory midbrain, synaptic mechanisms responsible for the precise temporal coding of inputs in the brainstem are absent. Instead, in the inferior colliculus (IC), the diverse temporal firing patterns must be coded by other synaptic mechanisms, about which little is known. Here, we demonstrate the temporal characteristics of sound-evoked excitatory and inhibitory postsynaptic currents (seEPSCs and seIPSCs, respectively) in vivo in response to long-duration tones. The seEPSCs and seIPSCs differ in the variability of their temporal properties. The seEPSCs have either early or late current peaks, and the early-peaked currents may be either transient or sustained varieties. The seIPSCs have only early-peaked sustained responses but often have offset responses. When measured in a single neuron, the seIPSC peaks usually follow early, transient seEPSCs, but the seIPSCs precede latest-peaking seEPSCs. A model of the firing produced by the integration of asymmetric seEPSCs and seIPSCs showed that the temporal pattern of the early-peaked sustained neurons was easily modified by changing the parameters of the seIPSC. These results suggest that the considerable variability in the peak time and duration of the seEPSCs shapes the overall time course of firing and often precedes or follows the less variable seIPSC. Despite this, the inhibitory currents are potent in modifying the firing patterns, and the inhibitory response to sound offset appears to be one area where the integration of excitatory and inhibitory synaptic currents is lacking. Thus, the integration of sound-evoked activity in the IC often employs the asymmetric temporal interaction of excitatory and inhibitory synaptic currents to shape the firing pattern of the neuron.

The level and distribution of the GABA(B)R1 and GABA(B)R2 receptor subunits in the rat's inferior colliculus

The type B γ-aminobutyric acid receptor (GABA(B) receptor) is an important neurotransmitter receptor in the midbrain auditory structure, the inferior colliculus (IC). A functional GABA(B) receptor is a heterodimer consisting of two subunits, GABA(B)R1 and GABA(B)R2. Western blotting and immunohistochemical experiments were conducted to examine the expression of the two subunits over the IC including its central nucleus, dorsal cortex, and external cortex (ICc, ICd, and ICx). Results revealed that the two subunits existed in both cell bodies and the neuropil throughout the IC. The two subunits had similar regional distributions over the IC. The combined level of cell body and neuropil labeling was higher in the ICd than the other two subdivisions. Labeling in the ICc and ICx was stronger in the dorsal than the ventral regions. In spite of regional differences, no defined boundaries were formed between different areas. For both subunits, the regional distribution of immunoreactivity in the neuropil was parallel to that of combined immunoreactivity in the neuropil and cell bodies. The density of labeled cell bodies tended to be higher but sizes of cell bodies tended to be smaller in the ICd than in the other subdivisions. No systematic regional changes were found in the level of cell body immunoreactivity, except that GABA(B)R2-immunoreactive cell bodies in the ICd had slightly higher optic density (OD) than in other regions. Elongated cell bodies existed throughout the IC. Many labeled cell bodies along the outline of the IC were oriented in parallel to the outline. No strong tendency of orientation was found in labeled cell bodies in ICc. Regional distributions of the subunits in ICc correlated well with inputs to this subdivision. Our finding regarding the contrast in the level of neuropil immunoreactivity among different subdivisions is consistent with the fact that the GABA(B) receptor has different pre- and postsynaptic functions in different IC regions.

Forebrain and midbrain distribution of major benzodiazepine-sensitive GABAA receptor subunits in the adult C57 mouse as assessed with in situ hybridization

In the adult brain, GABA is the major inhibitory neurotransmitter. Understanding of the behavioral and pharmacological functions of GABA has been advanced by recent studies of mouse lines that possess mutations in various GABA receptor subtypes and associated proteins. Genetically altered mice have become important tools for discerning GABAergic function. Thus detailed knowledge of the anatomical distribution of different GABA(A) subtype receptors in mice is a prerequisite for understanding the neural circuitry underlying changes in normal and drug-induced behaviors seen in mutated mice. In the current study, we used in situ hybridization histochemistry with [(35)S]UTP-labeled riboprobes to examine the regional expression pattern of mRNA transcripts for seven major GABA(A) receptor subunits in adjacent coronal brain sections (alpha 1, alpha 2, alpha 3, alpha 5, beta 2, beta 3, and gamma 2). Our results indicate that many of these GABAergic genes are co-expressed in much of the adult brain including the neocortex, hippocampus, amygdala, thalamus and striatum. However, each gene also shows a unique region-specific distribution pattern, indicative of distinct neuronal circuits that may serve specific physiological and pharmacological functions.

Neuronal organization of the rat inferior colliculus participating in four major auditory pathways

The central nucleus of the inferior colliculus (CNIC) contains different types of neurons and is a source of ascending projection to the medial geniculate body (MGB), commissural projection to the contralateral IC, direct descending projection to the cochlea nucleus (CN) and indirect projection to the CN via the superior olivary complex (SOC). Using a retrograde tracing technique, we examined what kind of neurons and what percentage of neurons of each type recognized in the CNIC participated in the above-mentioned four projection pathways. We also examined whether the individual CNIC neurons send the collateral to the MGB, the contralateral IC, the CN and the SOC. In the present study, we demonstrated that the neurons participating in the four projections could be morphologically classified into two types of neurons with soma size variation. The percentages of neurons of each type differed among the four projection pathways. Using a double-labeling technique, we found very few double-labeled neurons, indicating the collateral projections to the ipsilateral MGB and the contralateral IC. There were no double-labeled neurons in the collateral projections between the other combinations of targets. Therefore, we conclude that the ascending projection, the commissural projection and the descending projection to these targets arise from separate populations of neurons.

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

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.