Richard L. Saint Marie

Neurotransmitter-specific uptake and retrograde transport of [3H]glycine from the inferior colliculus by ipsilateral projections of the superior olivary complex and nuclei of the lateral lemniscus

Neurotransmitter-specific uptake and retrograde axonal transport of [3H]glycine were used to identify glycinergic projections to the inferior colliculus in chinchillas and guinea pigs. Six h after injection of [3H]glycine in the inferior colliculus, autoradiographically labeled cells were found ipsilaterally in the ventral nucleus of the lateral lemniscus, the lateral superior olive and the dorsomedial periolivary nucleus. These 3 regions accounted for 95% of the labeled projection neurons, with the remainder scattered elsewhere in the ipsilateral superior olivary complex. No labeled cells were found contralaterally even after survival times as long as 24 h. Retrograde transport of HRP from the inferior colliculus in these same cases confirmed the presence of additional projections that did not accumulate [3H]glycine. These included ipsilateral projections from the medial superior olive and cochlear nucleus and contralateral projections from the inferior colliculus, dorsal nucleus of the lateral lemniscus, lateral superior olive, periolivary nuclei and cochlear nucleus. The results implicate uncrossed projections from the ventral nucleus of the lateral lemniscus, lateral superior olive, and dorsomedial periolivary nucleus as the principal sources of inhibitory glycinergic inputs to the inferior colliculus.

GABA‐ and glycine‐immunoreactive projections from the superior olivary complex to the cochlear nucleus in guinea pig

Retrograde transport of horseradish peroxidase was combined with immunocytochemistry to identify the origins of potential γ‐aminobutyric acid (GABA) ‐ergic and glycinergic inputs to different subdivisions of the cochlear nucleus. Projection neurons in the inferior colliculus, superior olivary complex, and contralateral cochlear nucleus were examined, but only those from the superior olivary complex contained significant numbers of GABA‐ or glycine‐immunoreactive neurons. The majority of these were in periolivary nuclei ipsilaterally, with a sizeable contribution from the contralateral ventral nucleus of the trapezoid body. Overall, 80% of olivary neurons projecting to the cochlear nucleus were immunoreactive for GABA, glycine, or both. Most glycine‐immunoreactive projection neurons were located ipsilaterally, in the lateral and ventral nuclei of the trapezoid body and the dorsal periolivary nucleus. This suggests that glycine is the predominant neurotransmitter used by ipsilateral olivary projections. Most GABA‐immunoreactive cells were located bilaterally in the ventral nuclei of the trapezoid body. The contralateral olivary projection was primarily GABA‐immunoreactive and provided almost half the GABA‐immunoreactive projections to the cochlear nucleus. This suggests that GABA is the predominant neurotransmitter used by contralateral olivary projections. The present results suggest that the superior olivary complex is the most important extrinsic source of inhibitory inputs to the cochlear nucleus. Individual periolivary nuclei differ in the strength and the transmitter content of their projections to the cochlear nucleus and may perform different roles in acoustic processing in the cochlear nucleus. J. Comp. Neurol. 381:500‐512, 1997.

Organization of binaural excitatory and inhibitory inputs to the inferior colliculus from the superior olive

The major excitatory, binaural inputs to the central nucleus of the inferior colliculus (ICC) are from two groups of neurons with different functions—the ipsilateral medial superior olive (MSO) and the contralateral lateral superior olive (LSO). A major inhibitory, binaural input emerges from glycinergic neurons in the ipsilateral LSO. To determine whether these inputs converge on the same postsynaptic targets in the ICC, two different anterograde tracers were injected in tonotopically matched areas of the MSO and the LSO on the opposite side in the same animal. The main findings were that the boutons from MSO axons terminated primarily in the central and caudal parts of the ICC laminae but that contralateral LSO terminals were concentrated more rostrally and on the ventral margins of the MSO inputs. In contrast, the ipsilateral LSO projection converged with the MSO inputs and was denser than the contralateral LSO projection. Consistent with this finding, retrograde transport experiments showed that the very low‐frequency areas of the ICC with dense MSO inputs also received inputs from greater numbers of ipsilateral LSO neurons than from contralateral LSO neurons. The results suggest that different binaural pathways through the low‐frequency ICC may be formed by the segregation of excitatory inputs to ICC from the MSO and the contralateral LSO. At the same time, the ipsilateral LSO is a major inhibitory influence in the target region of the MSO. These data support the concept that each frequency‐band lamina in the ICC may comprise several functional modules with different combinations of inputs. J. Comp. Neurol. 472:330–344, 2004.

The form and distribution of GABAergic synapses on the principal cell types of the ventral cochlear nucleus of the cat

The distribution of GABAergic endings was examined histochemically in the ventral cochlear nucleus (VCN) of the cat using an antibody to glutamate decarboxylase (GAD), the synthetic enzyme for GABA. Immunoreactive (GAD+) endings appeared in all subdivisions of the cat VCN. Each of the principal cell types had a characteristic labeling pattern, based on the size, concentration, and distribution of GAD+ endings on its soma. Spherical bushy cell somata were typically contacted by many small (less than 1.5 microns in diameter) and medium-sized (1.5-2 microns in diameter) endings, many of which aggregated into tight clusters. Globular bushy cells had a similar pattern, but the clusters of GAD+ endings were less tightly packed. Reactive endings on stellate cells were more evenly distributed. GAD+ endings on octopus cells were larger (up to 2.5 microns in diameter) than those on the bushy cells and tended to aggregate into small clusters or rows on the somata and dendrites. Reactive endings contained small pleomorphic vesicles and formed symmetrical synaptic contacts on each of the cell types examined. The patterns formed by GAD+ endings on each type of neuron resemble those of certain types of non-cochlear axons previously described with the Golgi methods as projecting from the dorsal cochlear nucleus and the trapezoid body.

Effects of stimulus frequency and intensity on c-fos mRNA expression in the adult rat auditory brainstem

Induction of the cellular fos gene (c‐fos) is one of the earliest transcriptional changes observed following neuronal excitation. Although not an activity marker in the strict electrophysiological sense, many neurons in the central nervous system increase their c‐fos expression after periods of sustained stimulation at physiological levels of intensity. In the present study, induction of c‐fos mRNA expression was examined in the auditory brainstem after 1 hour of continuous free‐field acoustic stimulation. Sprague‐Dawley rats were exposed to pure tones of 2, 8, 16, or 32 kHz or half‐octave noise bands centered on 2, 8, or 32 kHz at 80–120 dB SPL. Stimulation‐induced c‐fos mRNA expression was evident at all levels of the auditory brainstem, and this expression was intensity dependent. In some brain areas, induced expression manifested a clear tonotopic organization, i.e., in dorsal, posteroventral, and anteroventral cochlear nuclei, and in the medial nucleus of the trapezoid body. The inferior colliculus exhibited multiple tonotopic representations. The dorsal nucleus of the lateral lemniscus had a crude tonotopy. Although expression was present, tonotopy was not evident in periolivary nuclei or in the ventral or intermediate nuclei of the lateral lemniscus. Free‐field diotic stimulation did not induce c‐fos mRNA expression in the medial or lateral superior olivary nuclei. Expression was induced in the lateral superior olive by dichotic stimulation (after a unilateral cochlear ablation), and that expression was tonotopically organized. The results suggest that stimulation‐induced c‐fos mRNA expression can be an effective way of mapping neuronal activity in the central auditory system under both normal and pathological conditions. J. Comp. Neurol. 404:258–270, 1999.

Cochlear ablation alters acoustically induced c‐fos mRNA expression in the adult rat auditory brainstem

Expression of c‐fos mRNA was studied in the adult rat brain following cochlear ablations by using in situ hybridization. In normal animals, expression was produced by acoustic stimulation and was found to be tonotopically distributed in many auditory nuclei. Following unilateral cochlear ablation, acoustically driven expression was eliminated or decreased in areas normally activated by the ablated ear, e.g., the ipsilateral dorsal and ventral cochlear nuclei, dorsal periolivary nuclei, and lateral nucleus of the trapezoid body and the contralateral medial and ventral nuclei of the trapezoid body, lateral lemniscal nuclei, and inferior colliculus. These deficits did not recover, even after long survivals up to 6 months. Results also indicated that neurons in the dorsal cochlear nucleus could be activated by contralateral stimulation in the absence of ipsilateral cochlear input and that the influence of the contralateral ear was tonotopically organized. Results also indicated that c‐fos expression rose rapidly and persisted for up to 6 months in neurons in the rostral part of the contralateral medial nucleus of the trapezoid body following a cochlear ablation, even in the absence of acoustic stimulation. This response may reflect a release of constitutive excitatory inputs normally suppressed by missing afferent input or changes in homeostatic gene expression related to sensory deprivation. Instances of transient, surgery‐dependent increases in c‐fos mRNA expression in the absence of acoustic stimulation were observed in the superficial dorsal cochlear nucleus and the cochlear nerve root on the ablated side. J. Comp. Neurol. 404:271–283, 1999.

Non-Cochlear Projections to the Ventral Cochlear Nucleus: Are they Mainly Inhibitory?

Non-cochlear synapses in the ventral cochlear nucleus (VCN), originate from a variety of intrinsic and extrinsic sources. It has been suggested by some studies that these synapses may actually outnumber those of the primary afferents in some regions of the VCN. In this chapter we will review the evidence that most non-cochlear synapses may be inhibitory, examine their distribution on different cell types in the VCN, and attempt to identify their origins.

Spatial representation of frequency in the rat dorsal nucleus of the lateral lemniscus as revealed by acoustically induced c-fos mRNA expression

The conventional view, based largely on studies in cats, holds that the dorsal nucleus of the lateral lemniscus (DNLL) is tonotopically organized with a dorsal (low-frequency) to ventral (high-frequency) representation. Based on the topography of projections between the DNLL and inferior colliculus, it has been proposed that the rat DNLL has a concentric, inside-to-outside, tonotopic organization with high frequencies represented along the rind and low frequencies represented in the core. We used acoustic stimulation and c-fos mRNA expression to examine this issue. Results suggest that the rat DNLL does have a crude tonotopic organization and that this tonotopy has a concentric component. Following high-frequency stimulation, labeled neurons were found most frequently along the margins of DNLL, although they also tended to be more concentrated ventrally. Many fewer neurons labeled following middle-frequency stimulation, and these tended to be more uniformly distributed throughout the nucleus. Still fewer neurons labeled after low-frequency stimulation and these tended to be scattered mostly in the dorsal half of the nucleus. We conclude that: (i) many more neurons in the rat DNLL are responsive to high-frequency than to low-frequency acoustic stimulation; and (ii) that the frequency representation of the rat DNLL has both concentric and dorsal-to-ventral components.