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

May 15, 1997The Journal of neuroscience : the official journal of the Society for Neuroscience

A direct inhibitory signal from the sound processing midbrain to the auditory relay area in rats

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Abstract

GABA-containing neurons in the inferior colliculus (IC) provide monosynaptic inputs to thalamocortical projection neurons in the medial geniculate body (MGB).

Projection neurons containing GABA were most abundant in the central nucleus of the IC.
Monosynaptic inhibitory postsynaptic potentials were observed in thalamocortical relay neurons following stimulation of the brachium.
The inhibitory potentials persisted even when glutamatergic transmission was blocked, indicating a non-local origin.
Stimulation typically induced a GABAA-mediated inhibitory potential followed by an excitatory potential and a longer latency GABAB-mediated inhibitory potential.
These GABAergic inputs may differentiate the primary auditory pathway from other sensory systems.

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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 GABAA-mediated inhibitory potential followed by an excitatory potential and a longer latency GABAB-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.

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

Abstract

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