Input-output relationships of the dorsal nucleus of the lateral lemniscus: possible substrate for the processing of dynamic spatial cues.

One organizing principle of the auditory system is the progressive representation of best tuning frequency. Superimposed on this tonotopy are nucleotopic organizations, some of which are related to the processing of different spatial cues. In the present study, we correlated asymmetries in the outputs of the dorsal nucleus of the lateral lemniscus (DNLL) to the two inferior colliculi (ICs), with asymmetries in the inputs to DNLL from the two lateral superior olives (LSOs). The positions of DNLL neurons with crossed and uncrossed projections were plotted from cases with unilateral injections of retrograde tracers in the IC. We found an orderly dorsal-to-ventral progression to the output that recapitulated the tonotopy of DNLL. In addition, we found a nucleotopic organization in the ventral (high-frequency) part of DNLL. Neurons with projections to the ventromedial (high-frequency) part of the contralateral IC were preferentially located ventrolaterally in DNLL; those with projections to the ventromedial part of the ipsilateral IC were preferentially located ventromedially in DNLL. This partial segregation of outputs corresponded with a partial segregation of inputs from the two LSOs in cases which received closely matched bilateral injections of anterograde tracers in LSO. The ventral part of DNLL received a heavy projection medially from the opposite LSO and a heavy projection laterally from the ipsilateral LSO. The findings suggest a direct relationship in the ventral part of the DNLL between inputs from the two LSOs and outputs to the two ICs. Possible roles for this segregation of pathways in DNLL are discussed in relation to the processing of static and dynamic spatial cues.

Substrate for rapid feedforward inhibition of the auditory forebrain.

A sizeable, feedforward, GABAergic projection exists from the inferior colliculus to the medial geniculate body in cats. We compared the dimensions of GABA-immunoreactive and non-immunoreactive axons in the brachium of the cat inferior colliculus, and demonstrate that GABA-immunoreactive axons are among the largest of brachial axons. We propose that, based on their large size, GABAergic tectothalamic axons provide a substrate for rapid feedforward inhibition of geniculate neurons.

Patterns of gamma-aminobutyric acid and glycine immunoreactivities reflect structural and functional differences of the cat lateral lemniscal nuclei.

The three nuclei of the cat lateral lemniscus (dorsal, intermediate, and ventral) were distinguished by their immunoreactivities for the putative inhibitory transmitters, gamma-aminobutyric acid (GABA) and glycine. Each nucleus had a distinct pattern of somatic and perisomatic labeling. The dorsal nucleus contained mostly GABA-immunoreactive neurons (85%), with moderate numbers of GABA- and glycine-immunoreactive puncta along their somata. The remaining neurons were nonimmunoreactive (15%). The intermediate nucleus contained mostly nonimmunoreactive neurons (82%), and these had numerous glycine-immunoreactive and few GABA-immunoreactive perisomatic puncta. The remaining neurons were immunoreactive for GABA only (10%), glycine only (2%), or both (6%). The ventral nucleus contained mostly glycine-immunoreactive neurons (81%), and about half of these were also GABA-immunoreactive. The remaining neurons were either nonimmunoreactive (8%) or GABA-immunoreactive only (11%). Neurons in the ventral nucleus had fewer immunoreactive perisomatic puncta than neurons in either the dorsal or the intermediate nuclei. These differences in neuronal immunoreactivity and in the relative abundance of GABA-and glycine-immunoreactive perisomatic puncta among the three nuclei of the lateral lemniscus support connectional and electrophysiological evidence that each nucleus has a different functional role in auditory processing. In particular, this study demonstrates that the intermediate nucleus of the cat is cytochemically distinct from the dorsal and ventral nuclei in terms of the somatic and perisomatic immunoreactivity of its neurons for these two important inhibitory transmitters and may provide novel inputs to the inferior colliculus.

Ultrastructural localization of hydrogen peroxide in experimental autoimmune uveitis.

One of the most prominent features of S-antigen induced uveitis is the massive infiltration of polymorphonuclear leukocytes (PMNs) and mononuclear cells in the ocular tissues and fluids. These inflammatory cells generate reactive oxygen metabolites as microbicidal agents and release these oxidants into the surrounding tissues. Using the cerium perhydroxide method, we have localized subcellular hydrogen peroxide in various inflamed ocular tissues. Most notably, the positive electron-dense granules were seen in the plasma membranes of PMNs that were infiltrating in the retina and uvea. These deposits were noted also in PMNs located within the extravascular spaces. For the intravascular PMNs, the positive reaction products were seen in much lower concentrations. A direct demonstration of substantial concentrations of hydrogen peroxide in experimental autoimmune uveitis, therefore, suggests the possibility that this reactive metabolite is an inflammatory mediator in this condition.