The goal was to investigate possible monosynaptic GABAergic projections from the inferior colliculus (IC) to thalamocortical neurons of the medial geniculate body (MGB) in the rat. Although there is little evidence for such a projection in other sensory thalamic nuclei, a GABAergic, ascending auditory projection was reported recently in the cat. In the present study, immunohistochemical and tract-tracing methods were used to identify neurons in the IC that contain GABA and project to the MGB. GABA-positive projection neurons were most numerous in the central nucleus and less so in the dorsal and lateral cortex. They were rare in the lateral tegmental system and brachium of the IC. The dorsal nucleus of the lateral lemniscus also contained GABA-positive projection neurons. In brain slices, stimulation of the brachium produced monosynaptic inhibitory postsynaptic potentials in morphologically identified thalamocortical relay neurons. The inhibitory potentials cannot originate locally, because they persisted when ionotropic glutamatergic transmission was blocked. Typically, brachium stimulation elicited a GABA A -mediated inhibitory potential followed by an excitatory potential and a longer latency GABA Bmediated inhibitory potential.We conclude that the GABA-containing neurons of the IC make short-latency, monosynaptic inputs to the thalamocortical projection neurons in the MGB. Such inputs may distinguish the main auditory pathway from indirect or tegmental auditory pathways as well as from other sensory systems. Monosynaptic inhibitory inputs to the medial geniculate may be important for the regulation of firing patterns in thalamocortical neurons. Key words: auditory pathway; retrograde tracing; brain slice preparation; immunohistochemistry; thalamus; midbrain; reticular formation; dorsal nucleus of the lateral lemniscusRecently, a novel and strong GABA-positive input from the inferior colliculus (IC) has been demonstrated in the cat medial geniculate body (MGB) . Thus, thalamocortical neurons in the MGB may receive monosynaptic, GABA-mediated inputs from the brainstem, in contrast to the neurons of other sensory thalamic nuclei. In most thalamic nuclei, inhibitory inputs come from local interneurons or intrathalamic neurons (for review, see Sherman and Koch, 1986;Steriade and Llinas, 1988). For example, in the lateral geniculate body, ascending inputs can excite directly the interneurons, which in turn inhibit thalamocortical neurons (Hirsch and Burnod, 1987;Lindstrom and Wrobel, 1990;Soltesz and Crunelli, 1992;Pape and McCormick, 1995). Such a scenario is less likely in the rat MGB. There are few local GABAergic interneurons, yet many GABA-positive axonal boutons are present Larue, 1988, 1996). This suggests that inhibitory influences arise outside the MGB. Our goal in this study was to identify and characterize one such inhibitory input in the rat.To test whether thalamocortical neurons of the MGB receive monosynaptic GABAergic input from the lower brainstem, we used both anatomical and electrophysiological...

AbstractÐDifferent type neurons in the inferior colliculus may have different functions. Recent intracellular studies of the inferior colliculus suggest that intrinsic electrical properties contribute to discharge patterns, but the intrinsic discharge patterns have not been fully characterized in the central nucleus, the main part of the inferior colliculus. Whether different types of neurons are related to different discharge patterns is unclear.We have used intracellular and whole-cell patch clamp-recording techniques in a brain slice preparation to better characterize discharge patterns and cell types in the central nucleus. Several types of discharge pattern were found in the inferior colliculus in response to long pulses of intracellular depolarizations. Rebound and buildup±pauser discharges, together, comprise neurons with a sustained response and are the majority of the neurons in the inferior colliculus. Both of these types of discharge pattern could be adapting or regular. Onset discharges distinguished another group of neurons. Onset neurons can also entrain to higher frequency stimuli than sustained neurons. Discharge patterns are correlated with distinctive current±voltage relationships and with some aspects of dendritic morphology. However, the morphological data demonstrates that the discharge patterns do not correspond simply to disc-shaped (¯at) or stellate (less-¯at) categories. This is the ®rst extensive analysis of electrophysiological properties of the central nucleus of the inferior colliculus in vitro. We suggest that there may be at least three functional classes of neurons and have implications for signal processing in the inferior colliculus. q 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.Key words: auditory pathways, temporal coding, rat, discharge patterns, neuronal morphology.What are the neuron types in the inferior colliculus (IC), the principal part of the auditory pathway in the midbrain? Morphology suggests the central nucleus of the IC should have two types of neurons based on the shape and orientation of the dendritic tree. 9,22,25,33,51 The principal neuron is discshaped (called the¯at neuron in rats) with dendrites that parallel the ®bro-dendritic laminae. A second, less common neuron with a different dendritic morphology is also found in all species studied (e.g. the less-¯at neuron in the rat).In contrast, physiology suggests three or more cell types are present based on responses to binaural acoustic stimuli and frequency-amplitude maps. 6,13,45 Morphological classi®cation based on dendritic shape or orientation is reconciled with some physiological characteristics and not others. For example, neurons with similar binaural properties often have heterogenous morphology, indicating a lack of correlation, whereas the presence of inhibitory side bands is well correlated with stellate morphology. 18 The discharge patterns in the IC may be a useful method to classify neurons. Onset, sustained, buildup, and pauser discharge patterns are well-documented in man...

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