Parallel auditory pathways: projection patterns of the different neuronal populations in the dorsal and ventral cochlear nuclei

Postnatal Expression of the Serotonin Transporter in Auditory Brainstem Neurons

To investigate the putative role of serotonin (5-HT) in auditory brainstem development, the expression of the 5-HT transporter (5-HTT) was evaluated in the normal mouse brainstem at 6 different postnatal ages. The brains of C3H/HeJ mice at birth (P0) and P1, P8-P9, P13, P21-P22, P35-P36 and P48-P50 were collected and processed immunohistochemically with an antibody raised against the 5-HTT. 5-HTT immunoreactivity (5-HTT-IR) was first observed in P8 mice and was localized to cell bodies in the ventral cochlear nucleus (VCN) and principal nuclei of the superior olivary complex, including the medial nucleus of the trapezoid body. Labeled neurons were found in similar regions in older mice except at P48-50, where labeled neurons were observed in the VCN only. 5-HTT-IR was especially prominent in VCN neurons at P21 and was observed in all of the brains examined at this age. These results indicate that auditory brainstem neurons of the normal inbred mouse express the 5-HTT postnatally. The presence of 5-HTT-IR in neurons located in the VCN indicates a regional expression of the 5-HTT that is related to the ascending auditory pathway. The timing of 5-HTT expression indicates that 5-HT may modulate developmental processes that rely on cochlear input.

Synaptic transmission mediated by ionotropic glutamate, glycine and GABA receptors in the rat’s ventral nucleus of the lateral lemniscus

The synaptic pharmacology of the ventral nucleus of the lateral lemniscus (VNLL) was investigated in brain slices obtained from rats of 14-37 days old using intracellular recording techniques. Excitatory and inhibitory synaptic potentials (EPSPs and IPSPs) were elicited by electrical stimulation of the lemniscal pathway and recorded from neurons with five types of intrinsic firing patterns (onset, pause, adapting, regular and bursting types). Synaptic receptors that mediated the EPSPs and IPSPs were identified using AMPA, NMDA, GABA(A) and glycine receptor antagonists. The early/short EPSPs were mediated by AMPA receptors. The late/long EPSPs, encountered only in neurons of younger animals, were mediated by NMDA receptors. The IPSPs in most neurons were mediated by glycine receptors. In some neurons the IPSPs were mediated by GABA(A) receptors or both glycine and GABA(A) receptors. The temporal dynamics of fast AMPA EPSPs and glycinergic IPSPs were very similar. AMPA EPSPs and glycinergic (and/or GABAergic) IPSPs could be encountered in a single neuron. The results suggest that the VNLL not only relays incoming signals rapidly from the lower brainstem to the inferior colliculus, but also integrates excitatory and inhibitory inputs to modify and process auditory information.

Intercollicular commissural projections modulate neuronal responses in the inferior colliculus

The right and left inferior colliculi (ICs) in the auditory midbrain are connected to one another by a bundle of fibres, the commissure of the IC. Previous studies show that this commissural projection connects corresponding frequency regions in the two sides and originates mainly from excitatory neurons, although some studies suggest a smaller number of GABAergic inhibitory neurons may also project via the commissure. Although the commissure of the IC is a major pathway connecting the most important nuclei of the auditory tectum, little is known about its functional significance. To investigate its role in auditory processing in the rat, we recorded sound-evoked responses of single neurons in one IC while injecting kynurenic acid into a corresponding region of the opposite IC. This procedure enabled us to block reversibly excitation of commissural projections to the recorded IC. The changes in the neural responses when input from the opposite IC was blocked are consistent with the commissural projection exerting both an excitatory and an inhibitory influence. The inhibition could be accounted for by monosynaptic or disynaptic connections. The responses to both monaural and binaural stimulation were affected, and the effects were proportionately greater at near-threshold sound levels. The results suggest that one function of the commissure of the IC may be to modulate the response gain of IC neurons to acoustic stimulation.

Targeted in vivo expression of proteins in the calyx of Held

The calyx of Held serves as a model for synaptic transmission in the mammalian central nervous system. While offering unique access to the biophysics of presynaptic function, studies addressing the molecular mechanisms of neurotransmitter exocytosis in this model have been mainly limited to pharmacological interventions. To overcome this experimental limitation we used stereotaxic delivery of viral gene shuttles to rapidly and selectively manipulate protein composition in the calyx terminal in vivo. Sindbis or Semliki Forest viruses encoding enhanced green fluorescent protein (EGFP) were injected into the ventral cochlear nucleus (VCN) of rats (postnatal days 7-21) and yielded bright fluorescence in cells of the VCN, including globular bushy cells with their axon and calyx terminal. Fluorescence imaging and three dimensional reconstructions visualized developmental changes in calyx morphology. Small cytoplasmic and synaptic vesicle proteins were successfully overexpressed in the calyx. We extended two-photon microscopy to obtain simultaneous fluorescence and infrared scanning gradient contrast images, allowing for efficient patch-clamp recordings from EGFP-labelled calyces in acute brain slices (postnatal days 9-14). Recordings of spontaneous miniature excitatory postsynaptic currents and short-term depression in synapses overexpressing EGFP or synaptophysin-EGFP revealed normal synaptic function. Thus, Sindbis and Semliki Forest virus-directed overexpression of proteins in the calyx of Held provides a new avenue for molecular structure-function studies of mammalian central synapses.

Acoustic stria: anatomy of physiologically characterized cells and their axonal projection patterns

The mammalian cochlear nucleus (CN) has been a model structure to study the relationship between physiological and morphological cell classes. Several issues remain, in particular with regard to the projection patterns and physiology of neurons that exit the CN dorsally via the dorsal (DAS), intermediate (IAS), and commissural stria. We studied these neurons physiologically and anatomically using the intra-axonal labeling method. Multipolar cells with onset chopper (O(C)) responses innervated the ipsilateral ventral and dorsal CN before exiting the CN via the commissural stria. Upon reaching the midline they turned caudally to innervate the opposite CN. No collaterals were seen innervating any olivary complex nuclei. Octopus cells typically showed onset responses with little or no sustained activity. The main axon used the IAS and followed one of two routes occasionally giving off olivary complex collaterals on their way to the contralateral ventral nucleus of the lateral lemniscus (VNLL). Here they can have elaborate terminal arbors that surround VNLL cells. Fusiform and giant cells have overlapping but not identical physiology. Fusiform but not giant cells typically show pauser or buildup responses. Axons of both cells exit via the DAS and take the same course to reach the contralateral IC without giving off any collaterals en route.

Auditory cortical projections to the cochlear nucleus in guinea pigs

We used anterograde tracing techniques to examine projections from auditory cortex to the cochlear nucleus in guinea pigs. Following injection of dextrans into the temporal cortex, labeled axons were present bilaterally in the cochlear nucleus. The distribution of boutons within the cochlear nucleus was similar on the two sides. The majority of boutons was usually located on the ipsilateral side. Most of the boutons were located in the granule cell areas, where many small boutons and a few larger, mossy-type endings were labeled. Additional small, labeled boutons were found in all layers of the dorsal cochlear nucleus, with the majority located in the fusiform cell layer. Labeled boutons were also present in the ventral cochlear nucleus, where they were located in the small cell cap as well as magnocellular parts of both posteroventral and anteroventral cochlear nucleus. Similar results were obtained with injections restricted to primary auditory cortex or to the dorsocaudal auditory field. The results illustrate direct cortical projections to the cochlear nucleus that are likely to modulate the activity in a number of ascending auditory pathways.

Neurobiological specializations in echolocating bats

Although the bat's nervous system follows the general mammalian plan in both its structure and function, it has undergone a number of modifications associated with flight and echolocation. The most obvious neuroanatomical specializations are seen in the cochleas of certain species of bats and in the lower brainstem auditory pathways of all microchiroptera. This article is a review of peripheral and central auditory neuroanatomical specializations in echolocating bats. Findings show that although the structural features of the central nervous system of echolocating microchiropteran bats are basically the same as those of more generalized mammals, certain pathways, mainly those having to do with accurate processing of temporal information and auditory control of motor activity, are hypertrophied and/or organized somewhat differently from those same pathways in nonecholocating species. Through the resulting changes in strengths and timing of synaptic inputs to neurons in these pathways, bats have optimized the mechanisms for analysis of complex sound patterns to derive accurate information about objects in their environment and direct behavior toward those objects.

Laminar inputs from dorsal cochlear nucleus and ventral cochlear nucleus to the central nucleus of the inferior colliculus: Two patterns of convergence

The central nucleus of the inferior colliculus is a laminated structure composed of oriented dendrites and similarly oriented afferent fibers that provide a substrate for tonotopic organization. Although inputs from many sources converge in the inferior colliculus, how axons from these sources contribute to the laminar pattern has remained unclear. Here, we investigated the axons from the cochlear nuclei that terminate in the central nucleus of the cat and rat. After characterization of the best frequency of the neurons at the injection sites in the cochlear nucleus, the neurons were labeled with dextran in order to visualize their axons and synaptic boutons in the central nucleus. Quantitative methods were used to determine the size and distribution of the boutons within the laminar organization. Two components in the laminae were identified: (1) a narrow axonal lamina that included the largest fibers and largest boutons; (2) a wide axonal lamina, surrounding the narrow lamina, composed of thin fibers and only small boutons. The wide lamina was approximately 30-40% wider than the narrow lamina, and it often extended more than 100 microm beyond the larger boutons on each side. The presence of both thick and thin fibers within the acoustic striae following these injections suggests that large and small fibers/boutons within these bands may originate from different neuronal types in the dorsal and ventral cochlear nucleus. We conclude that the narrow laminae that contain large fibers and boutons originate from larger cell types in the cochlear nucleus. In contrast, the wide lamina composed exclusively of small boutons may represent an input from other, perhaps smaller neurons in the cochlear nucleus. Thus, two types of inferior colliculus laminar structures may originate from the cochlear nucleus, and the small boutons in the wide laminae may contribute a functionally distinct input to the neurons of the inferior colliculus.

Neural processing of amplitude-modulated sounds

Amplitude modulation (AM) is a temporal feature of most natural acoustic signals. A long psychophysical tradition has shown that AM is important in a variety of perceptual tasks, over a range of time scales. Technical possibilities in stimulus synthesis have reinvigorated this field and brought the modulation dimension back into focus. We address the question whether specialized neural mechanisms exist to extract AM information, and thus whether consideration of the modulation domain is essential in understanding the neural architecture of the auditory system. The available evidence suggests that this is the case. Peripheral neural structures not only transmit envelope information in the form of neural activity synchronized to the modulation waveform but are often tuned so that they only respond over a limited range of modulation frequencies. Ascending the auditory neuraxis, AM tuning persists but increasingly takes the form of tuning in average firing rate, rather than synchronization, to modulation frequency. There is a decrease in the highest modulation frequencies that influence the neural response, either in average rate or synchronization, as one records at higher and higher levels along the neuraxis. In parallel, there is an increasing tolerance of modulation tuning for other stimulus parameters such as sound pressure level, modulation depth, and type of carrier. At several anatomical levels, consideration of modulation response properties assists the prediction of neural responses to complex natural stimuli. Finally, some evidence exists for a topographic ordering of neurons according to modulation tuning. The picture that emerges is that temporal modulations are a critical stimulus attribute that assists us in the detection, discrimination, identification, parsing, and localization of acoustic sources and that this wide-ranging role is reflected in dedicated physiological properties at different anatomical levels.