Biomedical subjects

D H Sanes

Publications and source records attributed to D H Sanes.

At least 19 recordsLinked to original sources

PubMed1996

Differential expression of MAG, MBP and L1 in the developing lateral superior olive.

The aim of this study was to investigate whether glial-associated molecules exhibit a pattern of expression that could influence oriented dendrite outgrowth in the gerbil lateral superior olive (LSO). In particular, we have previously noted that axon fascicles are oriented parallel to isofrequency laminae in the medial limb of the LSO, as are LSO dendrites, a phenotype that emerges postnatally. Therefore, we examined the immunocytochemical staining pattern of antibodies directed against three proteins that are found along axons: myelin basic protein (MBP), myelin-associated glycoprotein (MAG), and neuron-glia cell adhesion molecule (L1). MAG staining was first observed at postnatal day (P) 4 on the axon fibers surrounding the LSO. By P7 there was a differential pattern of MAG staining within the LSO, and immunopositive fibers were observed solely in the medial limb (e.g., high frequency projection region). Between P7 and P12, MAG staining was restricted largely to fascicles in the medial limb, and these were oriented parallel to the isofrequency axes. Few positive fibers of irregular orientation were observed in the lateral limb (e.g., low frequency projection region). Significant MAG-staining was not observed in the lateral limb until P15. The MAG immunoreactivity extended throughout the LSO by P21, although it was no longer restricted to axon fascicles. In contrast, MBP-positive fibers were uniformly distributed within the LSO by P12. Finally, L1 was found on oriented axon fascicles at P0, but became sparsely distributed throughout the LSO neuropil after P7, and was restricted to neuron cell bodies in the adult. Taken together, the results suggest that oriented axon fascicles bearing MAG and L1 may contribute to the developmental refinement of dendrite and axon arbors within the LSO.

Aging↗

PubMed1996

Regional distribution of neurotrophin receptors in the developing auditory brainstem.

Neuron survival and axonal regeneration become severely limited during early postnatal development. In conjunction with our recent organotypic analysis of regeneration in the auditory midbrain, we wished to determine whether neurotrophins could serve as a trophic substance during the postnatal period. Therefore, the current study examines the development of three neurotrophin receptor tyrosine kinases (TrkA, TrkB, and TrkC) in the gerbil auditory brainstem. Immunoreactivity to TrkA, the nerve growth-factor receptor, was observed in nonneuronal cells during the first two postnatal weeks. In the cochlear nucleus of mature animals, however, there was a TrkA-positive neuronal subpopulation. In contrast, immunoreactivity to TrkB and TrkC (the receptors for brain-derived neurotrophic factor and neurotrophin-3, respectively) displayed a widespread distribution in the auditory brainstem. At postnatal day 0, TrkB and TrkC staining was virtually absent from auditory nuclei, although immunopositive neurons were present in the mesencephalic trigeminal nucleus. By postnatal day 7, TrkB- and TrkC-positive neurons were present in most brainstem auditory nuclei. At postnatal day 15, TrkB immunoreactivity was observed throughout the inferior colliculus (IC), the cochlear nucleus, the medial and lateral nuclei of the trapezoid body, and the lateral superior olive, whereas TrkC labeled only a subpopulation of neurons within the central nucleus of the IC. The TrkB immunoreactivity was present on both neuronal somata and dendrites, whereas TrkC was generally restricted to cell bodies. At postnatal day 30, TrkB immunostaining was observed on most neurons of the IC. The medial and lateral nuclei of the trapezoid body displayed extremely strong TrkB staining, followed by the cochlear nucleus. In contrast, the TrkC immunostaining was decreased dramatically by postnatal day 21. Observations at the ultrastructural level confirmed a neuronal localization of TrkB and TrkC. Immunostaining for both receptors was restricted largely to the postsynaptic density of synaptic profiles in both dendrites and somata. In summary, this study illustrates a differential pattern of immunoreactivity between three neurotrophin receptors during development. The general increase of TrkB expression is well correlated with the onset of sound-evoked activity in this system, and its synaptic localization suggests that it may be involved in the modulation or maintenance of postsynaptic physiology.

Animals↗

PubMed1996

Developmental influence of glycinergic transmission: regulation of NMDA receptor-mediated EPSPs.

The influence of excitatory transmission on postsynaptic structure is well established in developing animals, but little is known about the role of synaptic inhibition. We addressed this issue in developing gerbils with two manipulations designed to decrease glycinergic transmission in an auditory nucleus, the lateral superior olive (LSO), before the onset of sound-evoked activity. First, contralateral cochlear ablation functionally denervated the glycinergic pathway from the medial nucleus of the trapezoid body (MNTB) to the LSO, while leaving the excitatory pathway intact. Second, continuous release of a glycine receptor antagonist, strychnine (SN), was used to decrease transmission. The strength of excitatory and inhibitory synapses was examined with whole-cell recordings from LSO neurons in a brain-slice preparation. The percentage of LSO neurons exhibiting MNTB-evoked IPSPs was reduced in both ablated and SN-treated animals. In those neurons displaying IPSPs, the amplitude was significantly reduced. This decrease was accompanied by an 8 mV depolarization in the IPSP equilibrium potential. In contrast, the ipsilaterally evoked EPSPs were of unusually long duration in experimental animals. These long-duration EPSPs were significantly shortened by hyperpolarizing the neuron to -90 mV or exposing them to aminophosphonopentanoic acid (AP-5), an NMDA receptor antagonist. Membrane hyperpolarization and AP-5 had little effect in control neurons. In addition, LSO neurons from ablated or SN-treated animals displayed broad rebound depolarizations after membrane hyperpolarization, and these were abolished in the presence of Ni2+. Because both cochlear ablation and SN-rearing were initiated before the onset of sound-evoked activity, the results suggest that spontaneous glycinergic transmission influences the development of postsynaptic properties, including the IPSP reversal potential, NMDA receptor function, and a Ca2+ conductance.

Aging↗

PubMed1996

Glycinergic transmission regulates dendrite size in organotypic culture.

We previously demonstrated that inhibitory synaptic transmission influences dendrite development in vivo. We now report an analogous finding in an organotypic culture of a glycinergic projection nucleus, the medial nucleus of the trapezoid body (MNTB), and its postsynaptic target, the lateral superior olive (LSO) of gerbils. Cultures were generated at 6-7 days postnatal and grown in serum containing medium with or without the glycine receptor antagonist, strychnine (SN), at 2 microM. LSO neurons were then labeled with biocytin, and the dendritic arbors were analyzed morphometrically. Compared to neurons form age-matched in vivo tissue, the neurons cultured in control media were somewhat atrophic, including decreases in dendritic branching and length. Incubation in strychnine led to a dramatic increase in dendritic branching and total dendritic length. Control neurons averaged 6.3 branches compared to 18 branches/neuron in SN-treated cultures. There was a similar increase in primary dendrites and total dendritic length. The physical elimination of MNTB cells did not mimic SN treatment, presumably because glycinergic LSO neurons generated intrinsic connections. In fact, the LSO soma area was significantly greater following MNTB removal, suggesting that these afferents provide a second signal to postsynaptic neurons. These results suggest that spontaneous glycinergic transmission regulates the growth of postsynaptic processes.

Animals↗

PubMed1995

Synaptically evoked prolonged depolarizations in the developing auditory system.

1. Although synaptic transmission is known to influence many aspects of neuronal development, activity rates are quite low at early ages. The present study describes a long-lasting postsynaptic response to brief periods of synaptic stimulation that may underlie such an influence. Whole-cell patch clamp recordings were made from the lateral superior olive (LSO) in a brain slice preparation from early postnatal gerbils. 2. Stimulation of the excitatory afferent pathway from the cochlear nucleus elicited a prolonged depolarization (PD) in approximately 60% of the LSO neurons tested. Low frequency stimulation (1 Hz) was as effective as tetanic stimulation in producing PDs. These synaptically evoked depolarizations ranged in amplitude from 3 to 32 mV and recovered spontaneously after 0.5-35 min. 3. The LSO neuron input resistance declined during every PD episode and remained significantly lower even after the membrane potential had recovered. These PDs were partially reversed by 2 mM Ni(+2), but 1 microM tetrodotoxin and 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were ineffective. The metabotropic glutamate receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (40 microM), produced depolarizations that outlasted the exposure period by an average of 20 min and were also partially repolarized by 2 mM Ni(+2). In contrast, the depolarizations produced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or N-methyl-D-aspartate decayed within a much shorter period of time. 4. To test whether in vivo discharge rates are, in fact, very low during development, spontaneous activity was recorded from neurons of the auditory midbrain in gerbils before and during the onset of sound-evoked responses. The average discharge rate of auditory neurons was quite low (X = 0.4 spikes/s), although many cells displayed brief periods of rapid discharge rate (X = 37 spikes/ s). Together, these results demonstrate a novel form of developmental plasticity elicited by low rates of glutamatergic transmission that may involve a metabotropic pathway and prolonged calcium influx.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed1995

Regeneration of the auditory midbrain intercommissural projection in organotypic culture.

The aim of this study was to determine whether postnatal mammalian central neurons retain the capacity for axonal regeneration across a lesion site in organotypic cultures of the auditory midbrain. Brain slices from the gerbil inferior colliculus (IC) were obtained from postnatal 6-8 d old animal and cultured for 6-15 d in vitro. IC explants containing an intact commissural projection exhibited robust axonal and dendritic morphologies as assessed with biocytin labelling. In transected explants, the two lobes of the inferior colliculus were cut at the midline and then reapposed to one another in vitro. There was a robust regeneration of commissural fibers across the lesion site in 78% of the biocytin-labeled explants. Massive axonal regeneration was also revealed by immunostaining explants for Tau (100% of sections), an axon-specific microtubule-associated protein. Ultrastructural analyses demonstrated that biocytin-labeled regenerating fibers established de novo synaptic profiles in the contralateral lobe of the inferior colliculus. Finally, the distribution of astrocytes and oligodendrocytes were assessed by staining for glial fibrillary acidic protein (GFAP) and myelin-associated glycoprotein (MAG), respectively. GFAP-positive astrocytes were more widely distributed than in vivo, and oligodendrocytes remained immature, and evenly distributed in all explants. Taken together, these data demonstrate that the postnatal mammalian auditory midbrain can be maintained in vitro, and that central axons are capable of regenerating across the site of injury without the aid of an artificial substrate.

Animals↗

PubMed1994

Structural and molecular heterogeneity of astrocytes and oligodendrocytes in the gerbil lateral superior olive.

The goal of this study was to determine the distribution and diversity of astrocytes and oligodendrocytes within the lateral superior olive of the gerbil. We used morphometric analyses and several immunocytochemical markers to assess differences in glial cell composition between the lateral (low-frequency projection) and the medial (high-frequency projection) limb of the lateral superior olive. Cell counts from Toluidine-stained semithin sections revealed a similar density of total astrocytes in both the lateral and the medial limbs. However, based on cytologic features, there was a prevalence of fibrous-like astrocytes in the lateral limb and protoplasmic-like astrocytes in the medial limb. In a similar manner, glial fibrillary acidic protein staining of astrocytes was intense in the lateral limb, but was largely restricted to the nucleus borders in the medial limb of the lateral superior olive. While glial fibrillary acidic protein was largely restricted to astrocytic processes, glutamine synthetase and S100 protein staining occurred, for the most part, in glial cell bodies. The density of glutamine synthetase positive cell bodies was homogeneous between the two limbs, while the density of S100-positive somata was significantly greater in the lateral limb. Cell counts obtained from semithin sections demonstrated a greater density of oligodendrocytes in the lateral limb than in the medial limb of the lateral superior olive. In a similar manner, there was a 40% greater density of carbonic anhydrase-positive somata in the lateral limb compared to the medial limb. Transferrin immunostaining was restricted to oligodendrocytes, but the density of labeled somata was identical in the lateral and medial limbs. 2',3'-Cyclic nucleotide 3'-phosphodiesterase and myelin-associated glycoprotein were also localized to the somata of oligodendrocytes, labeling both perisomatic and interfascicular cells. At the ultrastructural level, specialized contacts were found between pairs or clusters of oligodendrocytes. These results suggest that more than one type of astrocyte and oligodendrocyte is present within the gerbil lateral superior olive. Furthermore, glial cells were unevenly distributed, such that a greater density of oligodendrocytes and fibrous-like astrocytes were found in the low-frequency projection region. This heterogeneity is well correlated with known differences in the neuronal morphology within the lateral superior olive.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗

PubMed1994

Depression of developing neuromuscular synapses induced by repetitive postsynaptic depolarizations.

Effect of postsynaptic activity on the synaptic efficacy was studied in Xenopus nerve-muscle cultures. Repetitive postsynaptic depolarizations induced by injection of current pulses into singly innervated myocytes resulted in significant reduction in the frequency of spontaneous synaptic currents and the amplitude of nerve-evoked synaptic currents at the majority of synapses that showed immature synaptic properties. Repetitive hyperpolarizations and steady depolarizations of similar duration were without effect. The depolarization-induced synaptic depression appeared to result predominantly from a reduced ACh secretion from the presynaptic nerve terminal. Buffering the myocyte cytosolic Ca2+ at a low level with intracellular loading of a Ca2+ buffer, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA), significantly reduced the effect of the depolarizations. Thus postsynaptic electrical activity can regulate the synaptic efficacy of the developing neuromuscular synapases and the regulation may be mediated by retrograde transsynaptic interactions.

Action Potentials↗

PubMed1994

Synaptic inhibition influences the temporal coding properties of medial superior olivary neurons: an in vitro study.

The medial superior olive (MSO) functions as a coincidence detector for interaural time and phase differences by integrating excitatory synaptic inputs. Recent studies demonstrating glycinergic projections to MSO neurons suggest that coincidence detection results from the temporal integration of both EPSPs and IPSPs. We examined the impact of synaptic inhibition on the temporal coding properties of gerbil MSO neurons in vitro with intracellular recordings and electrical stimulation. For low-level bilateral electric stimulation, the EPSPs summated to produce an action potential in 73% of MSO neurons if they occurred within 50-500 microseconds of one another. Synaptic inhibition became more prominent at higher stimulus amplitudes in 73% of MSO neurons, and could block an evoked action potential if the stimuli to each pathway were delivered within 250 microseconds of one another. The glycine receptor antagonist strychnine influenced the response to simulated interaural time differences. In the presence of strychnine, interstimulus delays that originally resulted in full action potential suppression were sufficient to evoke an action potential. For trains of stimuli, as stimulus intensity increased (spatial summation), or as stimulus repetition rate increased to 100-500 Hz (temporal summation), there was a decrease in the number of stimulus pulses that evoked an action potential. In the presence of strychnine, MSO neurons generated a greater percentage of action potentials to the stimulus trains. When stimulus trains were delivered bilaterally, MSO neurons fired a greater number of action potentials at specific interstimulus time differences, and were selectively inhibited at other time differences.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Activity-dependent refinement of inhibitory connections.

Several lines of evidence suggest that excitatory synaptic transmission contributes to the maturation of precise neuronal connections. In the present study we determined whether the specific innervation pattern of single inhibitory arborizations was dependent upon neuronal activity during development. A homogeneous group of glycinergic inhibitory neurons in the central auditory system, the medial nucleus of the trapezoid body (MNTB), was functionally denervated in neonatal gerbils. The anatomical specificity of single MNTB terminal arborizations was subsequently measured along the tonotopic axis of a postsynaptic target, the lateral superior olive. Here we demonstrate that inhibitory terminal boutons spread a significantly greater distance along the frequency axis of the postsynaptic target following functional denervation. Although total arbor length remained unchanged, there was a significant increase in the number of branch points, suggesting de novo sprouting. The results indicate that normal inhibitory synaptic activity contributes to the developmental refinement of specific neuronal connections.

Acoustic Stimulation↗

PubMed1993

Bilateral inhibition by glycinergic afferents in the medial superior olive.

1. Coincidence-detection of excitatory synaptic potentials has long been considered to be the mechanism by which medial superior olivary (MSO) neurons compute interaural time differences. Here we demonstrate the contribution of synaptic inhibition in this circuit using a gerbil brain slice preparation. 2. Nearly all cells exhibited excitatory postsynaptic potentials (EPSPs) and action potentials (APs) after stimulation of either the ipsilateral or contralateral afferent pathway. In 44% of cells, the latency of APs depended on stimulus amplitude, exhibiting shifts of 0.25-2 ms. 3. Nearly all neurons (89%) exhibited stimulus-evoked synaptic inhibition. The inhibitory effects were enhanced at greater stimulus amplitudes and were usually able to block synaptically evoked APs. In addition, APs and EPSPs were reversibly blocked by delivering the inhibitory transmitter glycine in almost all tested cells (91%). 4. In the presence of the glycine antagonist strychnine, the effects of synaptic inhibition were suppressed. 5. The stimulus level-dependent inhibitory potentials influenced the probability that an MSO neuron would fire an AP, as well as the precise timing. Therefore, the present results have implications for the processing of interaural time differences by the MSO and at higher auditory centers.

Animals↗

PubMed1993

The development of synaptic function and integration in the central auditory system.

The development of inhibitory synaptic transmission is difficult to assess because the afferents usually arise from intrinsic neurons that are difficult to stimulate independently. The postnatal maturation of excitatory and inhibitory synaptic function was compared in the gerbil lateral superior olive (LSO), where it is possible to stimulate physically discrete afferent projections. Intracellular recordings obtained in a brain slice preparation revealed that transmission was prominent at birth. The EPSPs and IPSPs were up to 2 orders of magnitude longer than in more mature animals. Brief trains of electrical stimulus pulses led to a temporal summation of postsynaptic potentials (PSPs) in 1-14 d animals resulting in prolonged depolarizations or hyperpolarizations. In neonates, the depolarization could exceed 1 sec following a 70 msec stimulus train. The IPSPs in neonates were often of sufficient amplitude to evoke a rebound depolarization or action potential. The number of converging afferents was estimated from the quantized increases in PSP size. There was a significant decrease with age, suggesting that both inhibitory and excitatory afferents were eliminated during the first 3 postnatal weeks. The integration of action potentials with IPSPs was examined with conjoint stimuli to the two afferent pathways, and demonstrated that the effective IPSP duration decreased approximately 20-fold during the first 3 postnatal weeks. The magnitudes and durations of electrical stimulus-evoked PSPs suggest that spontaneous discharge of afferents to the LSO could have a substantial impact on their development, even prior to the response to airborne sound at 12 d. Furthermore, the synaptic responses obtained at 12-14 d postnatal indicated that both amplitude and temporal processing remain compromised. These immature synaptic properties would be expected to compound the inadequacies present in the cochlea and cochlear nucleus.

Animals↗

PubMed1992

The influence of inhibitory afferents on the development of postsynaptic dendritic arbors.

The growth and maintenance of dendritic form is dependent on normally functioning excitatory afferents. We have now examined the development of dendritic arbors in the gerbil lateral superior olive (LSO), following contralateral cochlear removal at postnatal day 7, a manipulation that substantially eliminates driven inhibitory transmission. Previous studies have demonstrated that the morphology of LSO dendritic arbors varies with tonotopic position and becomes more restricted with age. The presumed decrease of inhibitory transmission in the contralateral LSO resulted in a hypertrophic response. Quantification of Golgi-impregnated neurons revealed that dendrites had a significantly greater number of branch points, and their arbors were more spread out along the frequency axis compared to normal. This was especially apparent in the high frequency projection region where the glycine receptor density is known to be 4-fold higher than in the low frequency projection region. A measure of LSO nucleus size, cross-sectional area, was identical to control values, indicating no overt signs of degenerative phenomena. Cochlear ablation resulted in a significant atrophy of the ipsilateral LSO, with significant effects on dendritic structure. We conclude that decreased inhibitory transmission during development does not lead to a net degenerative response. Rather, the postsynaptic neurons exhibit a hypertrophic phenotype that may be due to the persistence of an immature state. These results indicate that activity-dependent morphogenetic events are a consequence of both excitatory and inhibitory synaptic transmission.

Afferent Pathways↗

PubMed1992

Refinement of dendritic arbors along the tonotopic axis of the gerbil lateral superior olive.

We have investigated the development of dendritic arbors in a central auditory nucleus in the Mongolian gerbil, the lateral superior olive (LSO). The morphology of these arbors has been shown to vary with tonotopic position in adults, with high frequency neurons having a more restricted field. In the present study, qualitative observations were made on horseradish peroxidase-filled neurons from animals 1-11 days postnatal, and quantitative results were obtained from Golgi-impregnated material from animals 10 days postnatal and older. The tonotopic position of each cell was computed as a percent of the total distance along the LSO. The dendritic arbors of high frequency neurons became spatially constrained along the frequency axis during the 3rd postnatal week, while those in the low frequency region retained a broader arborization into adulthood. This refinement was correlated with a decrease in total dendritic length and the number of branch points per neuron, particularly in the high frequency projection region. The distribution of octave bandwidths to which single LSO neurons responded in 13-16 day animals showed a similar course of maturation across the tonotopic axis: high frequency neurons responded to a larger number of octaves, and with greater variability, than those in adults. These data suggest that a specific alteration in dendrite morphology, which occurs after the onset of response to airborne sound, may contribute to adult frequency selectivity.

Animals↗

PubMed1992

Glycinergic transmission influences the development of dendrite shape.

We are interested in the trophic influence of inhibitory synaptic transmission during neuronal maturation. The morphology of dendrites in the gerbil lateral superior olive (LSO) was examined following pharmacological blockade of glycine receptors during postnatal development. The normally occurring change in dendrite morphology during the third postnatal week was prevented. Dendrites had a significantly greater number of branch points, and their arbors were more spread out along the frequency axis in the region of LSO where glycine receptor density is greatest. An identical result has recently been obtained when the glycinergic projection to the LSO was functionally denervated during development. We conclude that glycinergic transmission modulates the maturation of dendrite form during development.

Animals↗

PubMed1991

Applications of confocal microscopy to the study of myelin development and neuron structure.

Confocal laser scanning microscopy has been used to study the localization of myelin basic proteins expressed in nonglial cells, and to probe the three-dimensional structure of central auditory neurons in the lateral superior olive. The paper focuses on the techniques used to obtain the results. The key roles of confocal microscopy and computer image processing of the images obtained are emphasized as they relate to the discovery of essential structural information about these specimens.

Animals↗

PubMed1991

Development and specificity of inhibitory terminal arborizations in the central nervous system.

This study examined the morphological development of single inhibitory arborizations in the gerbil central auditory brain stem. Using a brain slice preparation, neurons of the medial nucleus of the trapezoid body (MNTB) were filled with horseradish peroxidase (HRP), and their complete arborizations were analyzed along the tonotopic axis of the lateral superior olive (LSO). The projections in neonatal animals displayed well-defined arbors that were ordered appropriately within the LSO. It was evident from the axonal pathways that the MNTB afferents could correct for projection errors after reaching the postsynaptic population. As development progressed, a number of arbors established diffuse or inappropriate projections within the LSO. These immature arborizations were no longer apparent by 18-25 days postnatal. The anatomical specificity of arbors at 12-13 and 18-25 days was quantified by measuring the distance that terminal boutons spread across the frequency axis. There was a significant reduction of this distance in older animals. In addition, there was a significant reduction in the mean number of boutons per arbor between 12-13 days and 18-25 days. The maximum nucleus cross-sectional area continued to increase through 15-16 days, indicating that the refined arbors occupied an even smaller fraction of the postsynaptic structure. Taken together, these observations suggest that central inhibitory arbors form exuberant contacts that must be eliminated during development.

Animals↗

PubMed1990

Dendritic morphology of central auditory neurons correlates with their tonotopic position.

We have investigated the morphology of dendritic arbors in a central auditory nucleus, the lateral superior olive, of the Mongolian gerbil. Morphometric observations were obtained directly from Golgi-impregnated material by using a microcomputer-based three-dimensional data acquisition system. In particular, measurements were made to determine the dendritic arborization across each of three axes: the tonotopic axis, the rostrocaudal axis, and the isofrequency axis (i.e., perpendicular to the tonotopic axis). The tonotopic position of each cell was computed on the basis of a topographic map that has been constructed for the gerbil LSO (Sanes et al.: J. Comp. Neurol. 279:436-444, 1989). It was found that the span of a dendritic arbor along the tonotopic axis was directly correlated with the neuron's tonotopic position: Low frequency neurons had much broader arborizations than high frequency neurons. Moreover, the distribution of frequency bandwidths to which single LSO neurons responded showed a striking similarity to dendritic arborizations across the tonotopic axis. Lower frequency neurons responded to a larger number of octaves than higher frequency neurons. There was no correlation between tonotopic position and dendritic arborization in the isofrequency or rostrocaudal axis. Nor was there any correlation between frequency and total dendritic length, number of primary dendrites, or soma area. However, there was a small but significant difference between the primary dendrite diameter of low and high frequency neurons. Low frequency neurons had significantly greater diameters. These results suggest that the frequency selectivity of central auditory neurons may employ, as one morphological substrate, the distance over which their dendrites arborize along the tonotopic axis.

Acoustic Stimulation↗