Low frequency neurons in the lateral central nucleus of the cat inferior colliculus receive their input predominantly from the medial superior olive

When iontophoretic injections of horseradish peroxidase were made in the vicinity of inferior colliculus units in the cat responding to low sound frequencies, retrograde labelling occurred in the ipsilateral medial superior olive. Both bipolar and multipolar cells of the medial superior olive participated in this projection, and the focus of labelling shifted topographically within the olive as the injection site best frequency varied. These observations confirm previous anatomical findings and link them to the tonotopic organization of the central nucleus of the inferior colliculus. However, the fact that the medial superior olive alone provides between 50 and 98% of labelled cells in the brain stem projecting to this region of the central nucleus is an unexpected observation. This study gives further support to an hypothesis of 'core zones' within the central nucleus that receive preferentially input from specific brain stem auditory nuclei.     

Coding of interaural time differences by some high-frequency neurons of the inferior colliculus: Responses to noise bands and two-tone complexes

The results of this experiment show that some high-frequency neurons of the central nucleus of the inferior colliculus are differentially responsive to interaural time differences when stimulated by noise bands and two-tone complexes. Since the stimuli were presented at low to moderate levels and in the presence of low-pass maskers, the interaural time sensitivity of the neurons is not attributable to low-frequency spectral components. The functional characteristics of these neurons provide a possible neurophysiological basis for the ability of human subjects to lateralize complex, high-frequency sounds by means of temporal cues.     

Responses of young and aged Fischer 344 rat inferior colliculus neurons to binaural tonal stimuli

The inferior colliculus (IC) is one nucleus of the central auditory system which displays age-related changes. Inputs to the IC use primarily the amino acid neurotransmitters glutamate and γ-aminobutryic acid (GABA). Neurochemical and anatomical studies of the Fischer 344 (F344) rat IC have shown decreases in GABA and GABA receptor levels (see Caspary et al., 1995 for review). GABA neurotransmission affects binaural response properties in the IC (Faingold et al., 1991a,b; Vater et al., 1992a; Park and Pollak, 1993, 1994). We hypothesized that aged F344 rats would show alterations in binaural IC neuronal response properties due to an imbalance in the relative levels of inhibition and excitation.

Extracellular recordings from 189 single units localized to the IC of anesthetized aged (24 month) F344 rats were compared to those obtained from 221 IC units in young adult (3 month) animals. Quantitative analyses were performed to determine the distribution of ipsilateral and binaural rate/intensity functions (RIFs) in the central nucleus of the IC and external cortex of the IC units.

The majority of IC units in both young and aged F344 rats were not responsive to monaural ipsilateral characteristic frequency tone bursts. Although there was some shift in the distribution of binaural RIF shapes with age, it was not statistically significant. The shift included a reduction in the percentage of units classified as E/I (excited by contralateral stimulation/ipsilaterally inhibited during binaural stimulation), but an increase with age in the percentage of units classified as E/f (excited by contralateral stimulation/ further facilitated by the addition of low intensity ipsilateral stimulation, but inhibited by higher intensity ipsilateral stimulation). Despite the role of inhibitory neurotransmission in binaural processing in the IC, age-related neurochemical deficits in the IC do not appear to result in a major deficit in the processing of simple binaural stimuli in F344 rats.     

Input from the inferior colliculus to medial olivocochlear neurons in the rat: A double label study with PHA-L and cholera toxin

The inferior colliculus provides a strong descending influence capable of modulating the excitability levels of olivocochlear neurons (Rajan, 1990). In an attempt to anatomically demonstrate this pathway in rats, an experimental paradigm was designed by which anterogradely transported Phaseolus vulgaris-leucoagglutinin (PHA-L), which delineates axonal arbors, and retrogradely transported cholera toxin B subunit alone (CT-B) or conjugated to horseradish peroxidase (CT-HRP), which delineate dendritic arbors, are visualized in the same brainstem sections. PHA-L was injected unilaterally into the central nucleus of the inferior colliculus of adult rats 5–9 days prior to injection of CT-B or CT-HRP into either the contralateral or the ipsilateral cochlea. Descending collicular axons labeled with PHA-L densely innervate the ventral nucleus of the trapezoid body (VNTB), which contains neurons of the medial olivocochlear system (MOCS), but do not enter the lateral superior olive, where the neurons of the lateral olivocochlear system (LOCS) are found. The collicular projection to VNTB is largely ipsilateral and supplies mostly the ventral half of the nucleus. Within VNTB, the collicular fibers intermingle with dendrites and, to a lesser extent, cell bodies of MOCS. Collicular boutons, predominantly of the en passant type, are often observed in close apposition to dendrites and, less frequently, cell bodies of both crossed and uncrossed MOCS. These light microscopic results suggest the existence of direct, synaptic contacts between descending collicular axons and ipsilateral crossed and uncrossed MOCS. Numerous collicular boutons were also seen at a distance from MOCS, suggesting that they establish synapses with other neuron types of the VNTB that do not send their axons to the cochlea.