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The goal of the present study was to provide a comprehensive and quantitative description of neurons immunoreactive for gamma-aminobutyric acid (GABA) in the inferior colliculus (IC) of the cat. Neurons were investigated with two different antisera and two different incubation methods. Free-floating frozen or vibratome-cut sections were incubated either with an antiserum to glutamic acid decarboxylase (GAD) or to GABA conjugated to protein with glutaraldehyde. Additional 1.5-microns-thick sections were incubated with the GABA antiserum after embedding and removal of the plastic. Quantitative data were obtained from much of this material. Despite the use of these different antisera and reaction methods, the results obtained were remarkably similar. The results show that GAD- or GABA-positive neurons represent a significant population of cells in the central nucleus of the IC, up to 20% of the neurons. Most of these neurons have large or medium-sized perikarya. In contrast, immunonegative neurons are medium-sized or small. Many GABA-positive neurons had proximal dendrites or somata oriented in parallel to the fibrodendritic laminae of the central nucleus and are presumed to be disc-shaped neurons. Other have an orthogonal orientation and are presumed to be stellate cells. Large GABA-positive neurons form two groups, those with many axosomatic endings and those with few. Collectively, these observations suggest that there are several types of GABAergic neuron in the central nucleus and, by extension, that these may participate in many types of inhibitory circuits.

A novel and robust projection from y-aminobutyric acid-containing (GABAergic) inferior colliculus neurons to the medial geniculate body (MGB) was discovered in the cat using axoplasmic transport methods combined with immunocytochemistry. This input travels with the classical inferior colliculus projection to the MGB, and it is a direct ascending GABAergic pathway to the sensory thalamus that may be inhibitory. This bilateral projection constitutes 10-30% of the neurons in the auditory tectothalamic system. Studies by others have shown that comparable input to the corresponding thalamic visual or somesthetic nuclei is absent. This suggests that monosynaptic inhibition or disinhibition is a prominent feature in the MGB and that differences in neural circuitry distinguish it from its thalamic visual and somesthetic counterparts.The thalamus is the gateway controlling the flow of sensory information reaching the cerebral cortex (1). The consensus is that input ascending to the primary thalamic nuclei for hearing (2), vision (3), and somesthesis (4) is entirely excitatory. We describe here a prominent and unexpected projection from y-aminobutyric acid-containing (GABAergic) cells in the inferior colliculus (IC) to the medial geniculate body (MGB) in the auditory tectothalamic (TT) pathway that constitutes an exception to this principle. This suggests that there is a convergence of classical excitatory and putatively inhibitory projections within the MGB and that an ascending pathway considered as neurochemically unitary in fact contains more than one channel. This striking difference in feedforward input between the auditory thalamus and other thalamic sensory nuclei is surprising since each has similar structural and intrinsic features (5-7), shared physiological properties (8-10), analogous cortical projection patterns (11-13), and a closely related neurochemical organization (14, 15). The present results suggest that, despite these parallels, there must be differences in circuitry and information processing regimes within the sensory thalamus.with isoflurane at a level that suppressed all nociceptive reflexes. All experimental procedures followed the applicable and approved institutional animal care and use protocols. Four unilateral penetrations were made using stereotaxic coordinates to target specific nuclei in various subdivisions. A total of seven, 50-nl deposits were made at 500-,um intervals along a track with a glass micropipet; the net volume was -1.5 ,lI.The animal was reanesthetized 3 days later and perfused with PBS followed by 2% paraformaldehyde and 3% glutaraldehyde in 0.12 M PB. Vibratome sections, 50-,m-thick, were collected in 0.1 M PB and placed in blocking serum (5% normal goat serum) for 60 min. Tissue was incubated overnight at 4°C in rabbit-anti-GABA (Incstar, Stillwater, MN) diluted 1:5000 and then reacted using avidin-biotin immunoperoxidase (Vectastain, Vector Laboratories, Burlingame, CA) with diaminobenzidine as the chromagen. WAHG labeling was intensified with silver (IntenSE-M...

The major excitatory, binaural inputs to the central nucleus of the inferior colliculus (ICC) are from two groups of neurons with different functions-the ipsilateral medial superior olive (MSO) and the contralateral lateral superior olive (LSO). A major inhibitory, binaural input emerges from glycinergic neurons in the ipsilateral LSO. To determine whether these inputs converge on the same postsynaptic targets in the ICC, two different anterograde tracers were injected in tonotopically matched areas of the MSO and the LSO on the opposite side in the same animal. The main findings were that the boutons from MSO axons terminated primarily in the central and caudal parts of the ICC laminae but that contralateral LSO terminals were concentrated more rostrally and on the ventral margins of the MSO inputs. In contrast, the ipsilateral LSO projection converged with the MSO inputs and was denser than the contralateral LSO projection. Consistent with this finding, retrograde transport experiments showed that the very low-frequency areas of the ICC with dense MSO inputs also received inputs from greater numbers of ipsilateral LSO neurons than from contralateral LSO neurons. The results suggest that different binaural pathways through the low-frequency ICC may be formed by the segregation of excitatory inputs to ICC from the MSO and the contralateral LSO. At the same time, the ipsilateral LSO is a major inhibitory influence in the target region of the MSO. These data support the concept that each frequency-band lamina in the ICC may comprise several functional modules with different combinations of inputs.

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