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Volume 17, Number 10, Issue of May 15, 1997 pp. 3766-3777
Copyright ©1997 Society for Neuroscience

A Monosynaptic GABAergic Input from the Inferior Colliculus to the Medial Geniculate Body in Rat

Received Jan. 21, 1997; revised Feb. 28, 1997; accepted March 6, 1997.

Daniel Peruzzi¹, Edward Bartlett², Philip H. Smith², and Douglas L. Oliver¹

¹ Department of Anatomy, University of Connecticut Health Center, Farmington, Connecticut, 06030-3405, and ² Department of Anatomy and the Neuroscience Training Program, University of Wisconsin, Madison, Wisconsin, 53706

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_B-mediated 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 lemniscus

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