Inferior Colliculus

Spatial release from masking (SRM), a factor in listening in noisy environments, is the improvement in auditory signal detection obtained when a signal is separated in space from a masker. To study the neural mechanisms of SRM, we recorded from single units in the inferior colliculus (IC) of barbiturate-anesthetized cats, focusing on low-frequency neurons sensitive to interaural time differences. The stimulus was a broadband chirp train with a 40-Hz repetition rate in continuous broadband noise, and the unit responses were measured for several signal and masker (virtual) locations. Masked thresholds (the lowest signal-to-noise ratio, SNR, for which the signal could be detected for 75% of the stimulus presentations) changed systematically with signal and masker location. Single-unit thresholds did not necessarily improve with signal and masker separation; instead, they tended to reflect the units' azimuth preference. Both how the signal was detected (through a rate increase or de...

Neurons in the inferior colliculus (IC) of the awake big brown bat, Eptesicus fuscus, were examined for joint frequency and latency response properties which could register the timing of the bat's frequency-modulated (FM) biosonar echoes. Best frequencies (BFs) range from 10 kHz to 100 kHz with 50% tuning widths mostly from 1 kHz to 8 kHz. Neurons respond with one discharge per 2-ms tone burst or FM stimulus at a characteristic latency in the range of 3–45 ms, with latency variability (SD) of 50 μs to 4–6 ms or more. BF distribution is related to biosonar signal structure. As observed previously, on a linear frequency scale BFs appear biased to lower frequencies, with 20–40 kHz overrepresented. However, on a hyperbolic frequency (linear period) scale BFs appear more uniformly distributed, with little overrepresentation. The cumulative proportion of BFs in FM1 and FM2 bands reconstructs a scaled version of the spectrogram of FM broadcasts. Correcting FM latencies for absolute BF latencies and BF time-in-sweep reveals a subset of IC cells which respond dynamically to the timing of their BFs in FM sweeps. Behaviorally, Eptesicus perceives echo delay and phase with microsecond or even submicrosecond accuracy and resolution, but even with use of phase-locked FM and tone-burst stimuli the cell-by-cell precision of IC time-frequency registration seems inadequate by itself to account for the temporal acuity exhibited by the bat.

The inferior colliculus is a critical structure for processing auditory information and receives ascending and descending synaptic auditory projections. In addition to GABAergic and glutamatergic innervations, other neurotransmitter systems are also reported in the inferior colliculus, including opioid peptides. In the present study, the relative distribution of each type of opioid receptor, mu (MOR), delta (DOR) and kappa (KOR) within GABAergic neurons in the inferior colliculus was examined. GABA immunoreactivity was expressed by small, medium and large neurons and distributed in the central nucleus and the pericentral nucleus of the inferior colliculus. Immunostaining for MOR, DOR and KOR receptors was found in both disc-shaped cells and stellate cells. Punctiform β-endorphin immunolabelling was observed in the proximity of GABA-positive neurons. Co-localization of GABA and MOR receptors was observed in neurons and nerve terminals in the central nucleus, dorsal cortex and external cortex of the inferior colliculus. Quantification of the co-localization patterns determined that a higher proportion of GABA neurons was associated with MOR receptors compared with KOR or DOR receptors.

As a part of a project concerning the development of hearing, some features of the morphological development of the inferior colliculus were studied in a marsuglial, the Northern quall or native cat (Dasyurus hallucatus). Marsuglials are of particular interest in developmental studies because much embryonic development occurs outside the uterus, in the pouch. Nissl-stained material was prepared from pouch-young at various ages between 11 and 81 days, and for a number of adults. Four pouch-young were injected with tritiated thymidine and killed later during pouch life.The inferior colliculus is first recognizable in pouch-young aged 23 days, when it is bordered by a cell-sparse ring of tissue. By this time, the labelling patterns following injections of tritiated thymidine made on days 7–9 suggest that migration of cells to the inferior colliculus from the ventricular germinal zone has been largely completed. At 81 days, close to the time when the young move out of the pouch, the adult cytoarchitecture—a central nucleus flanked by dorsal and lateral cortical regions—is clear.Cell areas expand monotonically as a function of age. The period of days 45–50 is associated with a large expansion of cell volume and a concomitant decrease in packing density. It is likely that functional connections are forming during this period, which may herald the onset of hearing in the quall.Total cell numbers increase to a peak at day 36, fall to a minimum at day 50, and rise again to the adult value. The second increase is likely to be a phase of glial proliferation, in part associated with the onset of myelination. This increase correlates with departure of the young from the pouch. © 1994 Wiley-Liss, Inc.

The barn owl (Tyto alba) is capable of capturing prey by passive hearing alone, guided by a topographic map of auditory space in the external nucleus of its inferior colliculus. The neurons of this auditory space map have discrete spatial receptive fields that result from the computation of interaural differences in the level (ILD) and time-of-arrival (ITD) of sounds. Below we review the synthesis of the spatial receptive fields from the frequency-specific ITDs and ILDs to which the neurons are tuned, concentrating on recent studies exploiting virtual auditory space techniques to analyze the contribution of ILD. We then compared the owl’s spatial discrimination, assessed behaviorally, with that of its space map neurons. Spatial discrimination was assessed using a novel paradigm involving the pupillary dilation response (PDR), and neuronal acuity was assessed by measuring the changes in firing rate resulting from changes in source location, scaled to the variance. This signal-detection-based approach revealed that the change in the position of the neural image on this map best explains the spatial discrimination measured using the PDR. We compare this result to recent studies in mammalian systems.

The inferior colliculus (IC) together with the dorsal periaqueductal gray (dPAG), the amygdala and the medial hypothalamus make part of the brain aversion system, which has mainly been related to the organization of unconditioned fear. However, the involvement of the IC and dPAG in the conditioned fear is still unclear. It is certain that GABA has a regulatory role on the aversive states generated and elaborated in these midbrain structures. In this study, we evaluated the effects of injections of the GABA-A receptor agonist muscimol (1.0 and 2.0 nmol/0.2 microL) into the IC or dPAG on the freezing and fear-potentiated startle (FPS) responses of rats submitted to a context fear conditioning. Intra-IC injections of muscimol did not cause any significant effect on the FPS or conditioned freezing but enhanced the startle reflex in non-conditioned animals. In contrast, intra-dPAG injections of muscimol caused significant reduction in FPS and conditioned freezing without changing the startle reflex in non-conditioned animals. Thus, intra-dPAG injections of muscimol produced the expected inhibitory effects on the anxiety-related responses, the FPS and the freezing whereas these injections into the IC produced quite opposite effects suggesting that descending inhibitory pathways from the IC, probably mediated by GABA-A mechanisms, exert a regulatory role on the lower brainstem circuits responsible for the startle reflex.

Using the nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) reaction with nitroblue tetrazolium, we provided a detailed investigation of the distribution, dimensional characteristics and morphology of NADPH-d-positive neurons in the three main subdivisions of the human inferior colliculus (IC): central nucleus, pericentral nucleus, and external nucleus. In accordance with their perikaryal diameter, dendritic and axonal morphology, these neurons were categorized as large (averaging up to 45 μm in diameter), medium (20–30 µm), small (13–16 µm) and very small (7–10 µm). Their morphological differences could contribute to varying functionality and processing capacity. Our results support the hypothesis that large and medium NADPH-d-positive cells represent projection neurons, while the small cells correspond to interneurons. Heretofore, the very small NADPH-d-positive neurons have not been described in any species. Their functions—and if they are, indeed, the smallest neurons in the IC of humans—remain to be clarified. Owing to their location, we posit that they are interneurons that connect the large NADPH-d-positive neurons and thereby serve as an anatomical substrate for information exchange and processing before feeding forward to higher brain centers. Our results also suggest that the broad distribution of nitric oxide (NO) synthesis in the human IC is closely tied to the neuromodulatory action of NO on collicular neurotransmitters such as GABA and glutamate, and to calcium-binding proteins such as parvalbumin. A deeper understanding of the relationship between NADPH-d-positive fibers in all IC connections and their co-localization with other neurotransmitters and calcium-binding proteins will assist in better defining the function of NO in the context of its interplay with the cerebral cortex, the sequelae of the aging process and neurodegenerative disorders.

Neuronal ion channels of different types often do not function independently but will inhibit or potentiate the activity of other types of channels, a process called cross-talk. The N-methyl-D-aspartate receptor (NMDA receptor) and the γ-aminobutyric acid type A receptor (GABAA receptor) are important excitatory and inhibitory receptors in the central nervous system, respectively. Currently, cross-talk between the NMDA receptor and the GABAA receptor, particularly in the central auditory system, is not well understood. In the present study, we investigated functional interactions between the NMDA receptor and the GABAA receptor using whole-cell patch-clamp techniques in cultured neurons from the inferior colliculus, which is an important nucleus in the central auditory system. We found that the currents induced by aspartate at 100 μmol L−1 were suppressed by the pre-perfusion of GABA at 100 μmol L−1, indicating cross-inhibition of NMDA receptors by activation of GABAA receptors. Moreover, we found that the currents induced by GABA at 100 μmol L−1 (I GABA) were not suppressed by the pre-perfusion of 100 μmol L−1 aspartate, but those induced by GABA at 3 μmol L−1 were suppressed, indicating concentration-dependent cross-inhibition of GABAA receptors by activation of NMDA receptors. In addition, inhibition of IGABA by aspartate was not affected by blockade of voltage-dependent Ca2+ channels with CdCl2 in a solution that contained Ca2+, however, CdCl2 effectively attenuated the inhibition of I GABA by aspartate when it was perfused in a solution that contained Ba2+ instead of Ca2+ or a solution that contained Ca2+ and 10 mmol L−1 BAPTA, a membrane-permeable Ca2+ chelator, suggesting that this inhibition is mediated by Ca2+ influx through NMDA receptors, rather than voltage-dependent Ca2+ channels. Finally, KN-62, a potent inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaMKII), reduced the inhibition of I GABA by aspartate, indicating the involvement of CaMKII in this cross-inhibition. Our study demonstrates a functional interaction between NMDA and GABAA receptors in the inferior colliculus of rats. The presence of cross-talk between these receptors suggests that the mechanisms underlying information processing in the central auditory system may be more complex than previously believed.