Neuroscience
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Loss of temporal processing is characteristic of age-related loss of speech understanding observed in the elderly. Inhibitory glycinergic circuits provide input onto dorsal cochlear nucleus (DCN) projection neurons which likely serve to modulate excitatory responses to time-varying complex acoustic signals. The present study sought to test the hypothesis that age-related loss of inhibition would compromise the ability of output neurons to encode sinusoidally amplitude modulated (SAM) tones. ⋯ Age-related decreases in the number of pauser-buildup response types and increases in wide-chopper types reported previously, could account, in part, for the observed loss of temporal coding of the aged fusiform cell. Age-related changes in SAM coding were similar to changes observed with receptor blockade of glycinergic inhibition onto fusiform cells and consistent with previously observed age-related loss of endogenous glycine levels and changes in normal adult glycine receptor function. DCN changes in SAM coding could, in part, underpin temporal processing deficits observed in the elderly.
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Comparative Study
Sound localization behavior in ferrets: comparison of acoustic orientation and approach-to-target responses.
Auditory localization experiments typically either require subjects to judge the location of a sound source from a discrete set of response alternatives or involve measurements of the accuracy of orienting responses made toward the source location. To compare the results obtained by both methods, we trained ferrets by positive conditioning to stand on a platform at the center of a circular arena prior to stimulus presentation and then approach the source of a broadband noise burst delivered from 1 of 12 loudspeakers arranged at 30 degrees intervals in the horizontal plane. Animals were rewarded for making a correct choice. ⋯ However, the final head bearing progressively undershot the target with increasing eccentricity and rarely exceeded 60 degrees to each side of the midline. In contrast to the approach-to-target responses, the accuracy of the head orienting responses did not change much with stimulus duration, suggesting that the improvement in percent correct scores with longer stimuli was due, at least in part, to re-sampling of the acoustical stimulus after the initial head turn had been made. Nevertheless, for incorrect trials, head orienting responses were more closely correlated with the direction approached by the animals than with the actual target direction, implying that at least part of the neural circuitry for translating sensory spatial signals into motor commands is shared by these two behaviors.
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Stains for acetylcholinesterase (AChE) and retrograde labeling with Fluorogold (FG) were used to study olivocochlear neurons and their dendritic patterns in mice. The two methods gave similar results for location and number of somata. The total number of medial olivocochlear (MOC) neurons in the ventral nucleus of the trapezoid body (VNTB) is about 170 per side. ⋯ DPO neurons, however, had more symmetric dendrites that projected into more dorsal parts of the trapezoid body, suggesting that this small group of olivocochlear neurons has very different physiological properties. Dendrites of both types of neurons were somewhat elongated rostrally, about 20% longer than those directed caudally. These results can be interpreted as extensions of dendrites of olivocochlear neurons toward their synaptic inputs: medially to meet crossing fibers from the cochlear nucleus that are part of the MOC reflex pathway, and rostrally to meet descending inputs from higher centers.
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We used an antibody to choline acetyltransferase (ChAT) to label cholinergic cells in guinea pig brainstem. ChAT-immunoreactive (IR) cells comprise several prominent groups, including the pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus, and parabigeminal nucleus, as well as the cranial nerve somatic motor and parasympathetic nuclei. Additional concentrations are present in the parabrachial nuclei and superior colliculus. ⋯ A few ChAT-IR cells are found in the cochlear nucleus and the ventral nucleus of the lateral lemniscus. The distribution of cholinergic cells in guinea pigs is largely similar to that of other species; differences occur mainly in cell groups that have few ChAT-IR cells. The results provide a basis for further studies to characterize the connections of these cholinergic groups.
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Genes that are highly expressed in the inner ear, as revealed by cDNA microarray analysis, may have a crucial functional role there. Those that are expressed specifically in auditory tissues are likely to be good candidates to screen for genetic alterations in patients with deafness, and several genes have been successfully identified as responsible for hereditary hearing loss. To understand the detailed mechanisms of the hearing loss caused by the mutations in these genes, the present study examined the immunocytochemical localization of the proteins encoded by Crym, KIAA1199 homolog, Uba52, Col9a3, and Col9a1 in the cochlea of rats and mice. ⋯ Uba52 protein was restrictedly localized within the surface of the marginal cells of the stria vascularis. Collagen type IX was found within the tectorial membrane as well as fibrocytes in the spiral ligament. The present results showed cell-specific localization of the encoded proteins of these highly expressed genes, indicating that the coordinated actions of various molecules distributed in different parts of the cochlea are essential for maintenance of auditory processing in the cochlea.