Enrique Saldaña

Connections of the Superior Paraolivary Nucleus of the Rat: II. Reciprocal Connections with the Tectal Longitudinal Column

The superior paraolivary nucleus (SPON), a prominent GABAergic center of the mammalian auditory brainstem, projects to the ipsilateral inferior colliculus (IC) and sends axons through the commissure of the IC (CoIC). Herein we demonstrate that the SPON is reciprocally connected with the recently discovered tectal longitudinal column (TLC). The TLC is a long and narrow structure that spans nearly the entire midbrain tectum longitudinally, immediately above the periaqueductal gray matter (PAG) and very close to the midline. Unilateral injections of biotinylated dextran into the SPON of the rat label abundant terminal fibers in the TLC of both sides, with an ipsilateral predominance. The SPON provides a dense innervation of the entire rostrocaudal extent of the ipsilateral TLC, and a relatively sparser innervation of the caudal and rostral portions of the contralateral TLC. SPON fibers reach the TLC by two routes: as collaterals of axons of the CoIC, and as axons that circumvent the ipsilateral IC before traveling in the deep layers of the superior colliculus (SC). The density of these projections identifies SPON as a significant source of input to the TLC. Other targets of the SPON discovered in this study include the deep layers of the SC and the PAG. The same experiments reveal numerous labeled cell bodies in the TLC, interspersed among the labeled SPON fibers. This observation suggests that the SPON is a significant target of TLC projections. The discovery of novel reciprocal connections between the SPON and the TLC opens unexpected avenues for investigation of sound processing in mammalian brainstem circuits.

Unbiased stereological estimates of neuron number in subcortical auditory nuclei of the rat.

The mammalian auditory system consists of a large number of cell groups, each containing its own complement of neuronal cell types. In recent years, much effort has been devoted to the quantitation of auditory neurons with common morphological, connectional, pharmacological or functional features. However, it is difficult to place these data into the proper quantitative perspective due to our lack of knowledge of the number of neurons contained within each auditory nucleus. To this end, we have employed unbiased stereological methods to estimate neuron number in the cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus and medial geniculate body. Additionally, we generated a three-dimensional model of the superior olivary complex. The utility of unbiased stereological estimates of auditory nuclei is discussed in the context of various experimental paradigms.

Colocalization of GABA and glycine in the ventral nucleus of the lateral lemniscus in rat: An in situ hybridization and semiquantitative immunocytochemical study

We have studied by in situ hybridization for GAD65 mRNA in thick sections and by semiquantitative postembedding immunocytochemistry in consecutive semithin sections, the expression of γ‐aminobutyric acid (GABA) and glycine in cell bodies and axosomatic puncta of the rat ventral nucleus of the lateral lemniscus (VNLL), a prominent monaural brainstem auditory structure. The in situ hybridization and the densitometric analysis of the immunostaining suggest that the rat VNLL contains two main populations of neurons. Approximately one‐third of neurons are unstained with either technique and are presumably excitatory; their cell bodies are enveloped by a large number of glycine‐immunoreactive puncta. Most if not all of the remaining two‐thirds colocalize GABA and glycine and are assumed to be inhibitory. These two populations show a complementary distribution within the VNLL, with inhibitory neurons located mainly ventrally and excitatory neurons dorsally. In scatterplots of gray values measured from cell bodies, the double‐labeled cells appear to form a single cluster in terms of their staining intensities for the two transmitter candidates. However, this cluster may have to be further subdivided because cells with extreme GABA/glycine ratios differ from those with average ratios with respect to location or size. The VNLL seems unique among auditory structures by its large number of neurons that colocalize GABA and glycine. Although the functional significance of this colocalization remains unknown, our results suggest that the VNLL exerts convergent excitatory and inhibitory influences over the inferior colliculus, which may underlie the timing processing in the auditory midbrain. J. Comp. Neurol. 432:409–424, 2001.

Morphology of cochlear root neurons in the rat

SummaryThe morphology of large neurons in the cochlear nerve root of albino rat has been studied with a variety of techniques including Nissl and cell-myelin staining, Golgi impregnation, horseradish peroxidase back-filling of severed axons, transmission electron microscopy, and morphometry. The cells, called root neurons, resemble the globular cells of the ventral cochlear nucleus in having an oval cell body, an eccentric nucleus, an axon that projects centrally via the trapezoid body, and in receiving many primary-like axosomatic boutons. The root neurons, however, are larger than globular cells, and they have at least two types of dendrites oriented, respectively, parallel and across the cochlear nerve fibres. The soma, moreover, has less finely dispersed Nissl material, is less completely covered with terminals, and receives a smaller proportion of presumably inhibitory synapses. So far, this particular type of neuron has been observed only in rat and mouse.

Projections of Cochlear Root Neurons, Sentinels of the Rat Auditory Pathway

In certain rodents, the root of the cochlear nerve contains a population of large neurons, known as cochlear root neurons (CRNs), an essential element of the primary acoustic startle pathway. To characterize the projections of the CRNs, we made stereotaxically guided, iontophoretic injections of biotinylated tracers into the cochlear nerve root of albino rats.

Connections of the superior paraolivary nucleus of the rat: projections to the inferior colliculus

GABAergic neurotransmission contributes to shaping the response properties of inferior colliculus (IC) neurons. In rodents, the superior paraolivary nucleus (SPON) is a prominent and well-defined cell group of the superior olivary complex that sends significant but often neglected GABAergic projections to the IC. To investigate the trajectory, distribution and morphology of these projections, we injected the neuroanatomical tracer biotinylated dextran amine into the SPON of albino rats. Our results demonstrate that: (1) the SPON innervates densely all three subdivisions of the ipsilateral IC: central nucleus (CNIC), dorsal cortex (DCIC) and external cortex (ECIC). The SPON also sends a sparse projection to the contralateral DCIC via the commissure of the IC. (2) SPON axons are relatively thick (diameter >1.2 microm), ascend to the midbrain tectum in the medial aspect of the lateral lemniscus, and, for the most part, do not innervate the nuclei of the lateral lemniscus. (3) SPON fibers ramify profusely within the IC and bear abundant en passant and terminal boutons. (4) The axons of neurons in discrete regions of the SPON form two laminar terminal plexuses in the ipsilateral IC: a medial plexus that spans the CNIC and DCIC parallel to the known fibrodendritic laminae of the CNIC, and a lateral plexus located in the ECIC and oriented more or less parallel to the surface of the IC. (5) The projection from SPON to the ipsilateral IC is topographic: medial SPON neurons innervate the ventromedial region of the CNIC and DCIC and the ventrolateral region of the ECIC, whereas more laterally situated SPON neurons innervate more dorsolateral regions of the CNIC and DCIC and more dorsomedial regions of the ECIC. Thus, SPON fibers follow a pattern of distribution within the IC similar to that previously reported for intracollicular and corticocollicular projections.

Anisotropic organization of the rat superior paraolivary nucleus.

In rodents, the superior paraolivary nucleus (SPON) is one of the major nuclei of the superior olivary complex that innervate the inferior colliculus. To analyze the intrinsic organization of the SPON and to gain further insight into its relationship with the inferior colliculus, the neuroanatomical tracers biotinylated dextran and horseradish peroxidase were unilaterally injected into different regions of the central nucleus of the inferior colliculus of adult albino rats. Both tracers resulted in retrograde labeling of SPON cell bodies. In addition, biotinylated dextran rendered excellent filling of dendri-tic and axonal processes within the nucleus. Our results confirm that the projection from the SPON to the central nucleus of the inferior colliculus is nearly exclusively ipsilateral and strictly topographic. Furthermore, our data show that virtually all SPON neurons participate in this projection. The labeling with biotinylated dextran reveals that typical SPON neurons are medium to large multipolar cells with four to seven thick, long, scarcely branched and smooth dendrites that extend over long distances within a nearly parasagittal plane and intermingle with similarly oriented axonal plexuses. Some of the neurons located ventrally within the nucleus possess dendrites that extend ventrally beyond the limits of the SPON to penetrate into the underlying ventral nucleus of the trapezoid body. The parallel arrangement of flattened dendritic and axonal fields within the SPON is reminiscent of the fibrodendritic laminae found in other mammalian auditory nuclei. This fact and the available data about the connectivity of the nucleus stress the similarities between the SPON and the principal nuclei of the superior olivary complex.

THE COCHLEAR ROOT NEURONS IN THE RAT, MOUSE AND GERBIL

The auditory portion of the eighth cranial nerve (the cochlear nerve) contains a neuronal population which has been well documented only in a few species of small rodents belonging to the Muridae family.1

The TLC: A Novel Auditory Nucleus of the Mammalian Brain

We have identified a novel nucleus of the mammalian brain and termed it the tectal longitudinal column (TLC). Basic histologic stains, tract-tracing techniques and three-dimensional reconstructions reveal that the rat TLC is a narrow, elongated structure spanning the midbrain tectum longitudinally. This paired nucleus is located close to the midline, immediately dorsal to the periaqueductal gray matter. It occupies what has traditionally been considered the most medial region of the deep superior colliculus and the most medial region of the inferior colliculus. The TLC differs from the neighboring nuclei of the superior and inferior colliculi and the periaqueductal gray by its distinct connections and cytoarchitecture. Extracellular electrophysiological recordings show that TLC neurons respond to auditory stimuli with physiologic properties that differ from those of neurons in the inferior or superior colliculi. We have identified the TLC in rodents, lagomorphs, carnivores, nonhuman primates, and humans, which indicates that the nucleus is conserved across mammals. The discovery of the TLC reveals an unexpected level of longitudinal organization in the mammalian tectum and raises questions as to the participation of this mesencephalic region in essential, yet completely unexplored, aspects of multisensory and/or sensorimotor integration.

Auditory response properties of neurons in the tectal longitudinal column of the rat.

The newly-discovered tectal longitudinal column (TLC) spans the paramedian region of the mammalian tectum. It has connections with several nuclei of the auditory system. In this report, we provide the first detailed description of the responses of TLC neurons to auditory stimuli, including monaural and binaural tones and amplitude modulated tones. For comparison, responses in the inferior colliculus (IC) were also recorded. Neurons in the TLC were sensitive to similar ranges of frequency as IC neurons, could have comparably low thresholds, and showed primarily excitatory responses to stimulation of the contralateral ear with either phasic or sustained response patterns. Differences of TLC compared to IC neurons included broader frequency tuning, higher average threshold, longer response latencies, little synchronization or rate tuning to amplitude modulation frequency and a smaller degree of inhibition evoked by stimulation of the ipsilateral ear. These features of TLC neurons suggest a role for the TLC in descending auditory pathways.