SOURCES OF GABAERGIC INPUT TO THE INFERIOR COLLICULUS OF THE RAT

We have studied the GABAergic projections to the inferior colliculus ( IC) of the rat by combining the retrograde transport of horseradish pe roxidase (HRP) and immunohistochemistry for gamma-amino butyric acid ( GABA). Medium-sized (0.06-0.14 mu l) HRP injections were made in the v entral part of the central nucleus (CNIC), in the dorsal part of the C NIC, in the dorsal cortex (DCIC), and in the external cortex (ECIC) of the IC. Single HRP-labeled and double (HRP-GABA)-labeled neurons were systematically counted in all brainstem auditory nuclei. Our results revealed that the IC receives GABAergic afferent connections from ipsi - and contralateral brainstem auditory nuclei. Most of the contralater al GABAergic input originates in the IC and the dorsal nucleus of the lateral lemniscus (DNLL). The dorsal region of the IC (DCIC and dorsal part of the CNIC) receives connections mostly from its homonimous con tralateral region, and the ventral region from the contralateral DNLL. The commissural GABAergic projections originate in a morphologically heterogeneous neuronal population that includes small to medium-sized round and fusiform neurons as well as large and giant neurons. Quantit atively, the ipsilateral ventral nucleus of the lateral lemniscus is t he most important source of GABAergic input to the CNIC. In the superi or olivary complex, a smaller number of neurons, which lie mainly in t he periolivary nuclei, display double labeling. In the contralateral c ochlear nuclei, only a few of the retrogradely labeled neurons were GA BA immunoreactive. These findings give us more information about the r ole of GABA in the auditory system, indicating that inhibitory inputs from different ipsi- and contralateral, mono- and binaural auditory br ainstem centers converge in the IC. (C) 1996 Wiley-Liss, Inc.