GABAERGIC INHIBITION CONTRIBUTES TO NONLINEAR SUMMATION AMONG MULTIPLE FREQUENCY CHANNELS IN THE BARN OWL'S INFERIOR COLLICULUS

  Interaural time difference(ITD) is the primary azimuthal sound localization cue for the barn owl.  Since ITD is processed in separate frequency channels in the brainstem pathway, neurons therein cannot distinguish ITD from interaural phase difference.  Instead, they respond to multiple ITD's that give rise to the same phase difference ('phase ambiguity').  However, cells in the external nucleus and some cells in the lateral shell of the central nucleus (ICL type 2 cells of Fujita & Konishi, Soc.Neurosci.Abstr., 15:114, 1989) of the inferior colliculus solve the ambiguity by gathering inputs from different frequency channels.  We investigated the interaction among frequency channels in those cells, and examined the role of GABAA inhibition in the process by iontophoretic application of its specific antagonist, bicuculline methiodide (BMI).  Single units in the inferior colliculus of anesthetized barn owls were isolated with a multibarrel microelectrode.  Two tones ot different frequencies were presented binaurally.  ITD vs. response curves for simultaneous presentation of two tones were compared with those for separate presentation of the same tones.  Three types of interaction were noted; linear summation, non-linear facilitation, and non-linear suppression.  Some cells changed type of interaction depending on the frequency-pair used.  Iontophoretic application of 5mM BMI converted non-linear responses to a linear type to a varying degree.  These results demonstrate the complex nature of the non-linear interaction across frequencey channels in the inferior colliculus of the barn owl, as well as the involvement of GABAergic inhibition in this process.  (Supported by the Del E. Webb Foundation, the Uehara Memorial Foundation, NIH, and the Japanese Ministry of Education, Science and Culture)


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