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PubMed2001

Functional organization of the dorsal cochlear nucleus of the horseshoe bat (Rhinolophus rouxi) studied by GABA and glycine immunocytochemistry and electron microscopy.

Unique among mammals, the dorsal cochlear nucleus (DCN) of horseshoe bats consists of two functionally and anatomically distinct subdivisions: a laminated ventral portion that processes the frequency range below the constant frequency (CF) component of the echolocation signal and a nonlaminated dorsal portion that is specialized for processing the CF-signal range (76 kHz and higher). Using conventional transmission electron microscopy and postembedding immunocytochemistry for the inhibitory neurotransmitters GABA and glycine on semithin-alternating sections, we present further evidence that the ventral laminated subdivision of DCN conserves the main elements of microcircuitry and GABA/glycine labeling patterns typical for the mammalian DCN: (i) the main cell types and synaptic inventory of the granule cell/cartwheel cell system of the superficial layers are present as well as (ii) the tuberculoventral cell system of the deep layers. The nonlaminated dorsal subdivision lacks the granule cell/cartwheel cell system and is composed of a mixture of fusiform projection neurons with tuberculoventral cell analogues. Thus the inhibitory tuberculoventral system known to play an important role in temporal and spectral processing in VCN is conserved throughout the DCN of horseshoe bats, whereas functional components of cerebellar-like circuits are reduced in a specialized region that processes the dominant biosonar component.

Animals↗
PubMed1995

Recordings from slices indicate that octopus cells of the cochlear nucleus detect coincident firing of auditory nerve fibers with temporal precision.

Acoustic information in auditory nerve discharges is integrated in the cochlear nuclei, and ascends through several parallel pathways to higher centers. Octopus cells of the posteroventral cochlear nucleus form a pathway known to carry information in the timing of action potentials. Octopus cells have dendrites oriented to receive converging input from many auditory nerve fibers. In all 34 intracellular recordings from anatomically identified octopus cells in slices, shocks to the auditory nerve evoked brief, consistent, graded EPSPs. EPSPs were about 1 msec in duration. At all but the lowest shock strengths, the delays between shocks and the peaks of resultant EPSPs had SDs of 0.02 msec. Polysynaptic excitation, perhaps arising from the axon collaterals of octopus cells, was observed. No detectable glycinergic or GABAergic inhibition was evoked with shocks. The input resistances were low, around 10 M omega, voltage changes were rapid, with time constants of about 1 msec, and action potentials were small. The low input resistance resulted in part from a Cs(+)-sensitive conductance. In the presence of 10 or 15 mM extracellular Cs+ the time constants increased 20-fold in the hyperpolarizing voltage range. As several subthreshold inputs were required to produce suprathreshold responses, octopus cells detect the coincident firing of auditory nerve fibers. Under physiological conditions the low input resistance and resulting short time constant limit the time over which temporal summation of excitation from auditory nerve fibers can occur and thus provide temporal precision to electrical signaling.

Action Potentials↗
PubMed1999

Role of intrinsic conductances underlying responses to transients in octopus cells of the cochlear nucleus.

Recognition of acoustic patterns in natural sounds depends on the transmission of temporal information. Octopus cells of the mammalian ventral cochlear nucleus form a pathway that encodes the timing of firing of groups of auditory nerve fibers with exceptional precision. Whole-cell patch recordings from octopus cells were used to examine how the brevity and precision of firing are shaped by intrinsic conductances. Octopus cells responded to steps of current with small, rapid voltage changes. Input resistances and membrane time constants averaged 2.4 MOmega and 210 microseconds, respectively (n = 15). As a result of the low input resistances of octopus cells, action potential initiation required currents of at least 2 nA for their generation and never occurred repetitively. Backpropagated action potentials recorded at the soma were small (10-30 mV), brief (0.24-0.54 msec), and tetrodotoxin-sensitive. The low input resistance arose in part from an inwardly rectifying mixed cationic conductance blocked by cesium and potassium conductances blocked by 4-aminopyridine (4-AP). Conductances blocked by 4-AP also contributed to the repolarization of the action potentials and suppressed the generation of calcium spikes. In the face of the high membrane conductance of octopus cells, sodium and calcium conductances amplified depolarizations produced by intracellular current injection over a time course similar to that of EPSPs. We suggest that this transient amplification works in concert with the shunting influence of potassium and mixed cationic conductances to enhance the encoding of the onset of synchronous auditory nerve fiber activity.

4-Aminopyridine↗
PubMed1995

Effects of furosemide on distortion product otoacoustic emissions and on neuronal responses in the anteroventral cochlear nucleus.

1. The objective of this study was to precisely evaluate the relationship between the threshold of neurons in the anteroventral cochlear nucleus (AVCN) and the properties of distortion product otoacoustic emissions (DPOAEs). Response areas of multiunit clusters in the AVCN and DPOAEs in the ear canal were measured alternately in the adult gerbil during furosemide-induced changes of the endocochlear potential. Stimulus frequencies of the probe tones for DPOAE measurement were in the range of f1 = 1.7-7.6 kHz and f2 = 2.0-9.0 kHz; the ratio f2:f1 was always 1.18. Stimulus amplitudes were varied in 5-dB steps from 30 to 80 dB SPL, with either equal amplitudes (L1 = L2) or unequal, with L1 set 10 dB above L2. Multiunit response areas were determined from cluster responses to a series of 100-ms tone bursts presented with a pseudo-random sequence in frequency and intensity. 2. Changes in the multiunit discharge properties after 50-75 mg/kg furosemide injection were as follows: the best frequency (BF) threshold increased from initial values in the range of 20-30 dB SPL to 50-80 dB SPL at 10-20 min postinjection and then recovered fully by 60-90 min. The spontaneous discharge activity decreased to zero before any changes in the frequency threshold curve were observed and did not return to initial values for several hours. Likewise, total discharge rates of stimulus elicited responses were reduced and tended to stay reduced even after BF threshold had fully recovered. 3. From the DPOAE measurements, the changes observed in the cubic distortion tone (CDT, 2f1-f2) emission after furosemide injection were as follows: at high levels of the probe tones, changes in the emission intensities generally stayed within a 10-dB range. The CDT amplitudes for low stimulus levels, however, were typically reduced by up to 40 dB, but recovered (depending on the furosemide dosage) by approximately 60-90 min. 4. At low to moderate stimulus levels of 40-60 dB SPL, there was a near perfect, minute-by-minute covariation of the ear canal CDT amplitude and the BF threshold measured in the AVCN. A 10-dB increase in threshold was associated with a 5- to 7-dB decrease in the CDT emission. 5. The optimum stimulus parameter set for the noninvasive estimation of cochlear performance from the CDT response was for stimulus amplitudes L1 = 50, L2 = 40 dB SPL. 6. This experiment demonstrates that CDT emissions at low stimulus levels are very good predictors of the thresholds of cochlear afferents, but this validity is lost for BF thresholds greater than approximately 60-70 dB SPL. 7. The ear canal CDT amplitude is better correlated with the BF threshold sensitivity of neuronal response areas in the AVCN than with the spontaneous discharge rate or absolute above-threshold discharge rates.

Animals↗
PubMed1998

Effects of wide band inhibitors in the dorsal cochlear nucleus. II. Model calculations of the responses to complex sounds.

In a previous modeling study of signal processing in the dorsal cochlear nucleus [Reed and Blum, J. Acoust. Soc. Am 96, 1442-1453 (1997)] it was shown that inclusion of a wideband inhibitor (WBI) greatly improved the fit between model response maps and the experimental response maps of type IV units to pure tones. In this study we examine the effect of the WBI on the responses to complex sound stimuli such as broadband noise (BBN), notch noise, noise bands, and band/notch combinations. A new and more realistic model for auditory nerve (AN) response in the presence of different levels of noise is used. It is shown that one can explain and understand the qualitative features of virtually all the published data on type II and type IV unit responses to BBN, notch noise and noise bands. The monotone decreasing response of the maximum firing rate of type II units to noise bands of increasing width that is observed experimentally occurs in the model due to the increasing inhibition of type II cells by the WBI. Similarly, the various nonmonotone patterns of maximum firing rate of type IV units to noise bands of increasing width is shown to arise from the complex and highly nonlinear effects of inhibition from the type II to type IV and the WBI to type IV cells and the nonlinear direct excitation from the AN to the type IV cells. A number of experiments using double notches, double noise bands, or notch-noise band pairs are suggested which, by comparison with model results, would allow one to infer probable connectional patterns between type II and type IV units and between the WBI units and the type IV units.

Acoustic Stimulation↗
PubMed1994

Neural modeling of the dorsal cochlear nucleus: cross-correlation analysis of short-duration tone-burst responses.

A conceptual model of a portion of dorsal cochlear nucleus (DCN) neural circuitry has emerged over the past two decades. This model suggests that the response properties of the DCN's major projection neurons, called type IV units, are due, in part, to the behavior of local circuit inhibitory interneurons called type II units (Young and Brownell 1976). Cross-correlation studies of simultaneously recorded pairs of DCN units in decerebrate cat derived from 50-s best frequency (BF) stimuli are consistent with and have extended this conceptual model (Voigt and Young 1980, 1985, 1988, 1990). Interestingly, Gochin et al. (1989) found no signs of inhibition in the anesthetized rat DCN in cross-correlograms derived from 55-ms short-duration BF tone bursts. This seemingly contradictory result has motivated this study. Computer simulations were run using our network model of the intrinsic DCN neural circuitry. This model has previously been shown to reproduce the major features of both type II and type IV rate-level curves and the inhibitory trough (IT) observed in cross-correlograms derived from long-duration stimuli (Voigt and Davis 1994). The goal was to study the stimulus-duration-dependent strength of ITs in the cross-correlograms derived from short-duration BF tone-burst stimuli. The results suggest that ITs may not be detectable when the stimulus duration is 50 ms but may be detectable when the stimulus duration is 200 ms or greater. Furthermore, when the ITs are detected in cross-correlograms derived from 200-ms data sets, the strength of the IT, as measured by effectiveness, is comparable to the strength of ITs measured when the stimulus duration is 50 s.

Acoustic Stimulation↗
PubMed2003

Age-related synaptic changes in the anteroventral cochlear nucleus of Fischer-344 rats.

Previous studies have demonstrated age-related decreases in the transmitters glycine and glutamate in the cochlear nucleus (CN) of the Fischer-344 (F344) rat, along with declining levels of binding for glycine receptors. The purpose of this study was to evaluate structural correlates to the transmitter and receptor losses that accompany aging in the anteroventral CN (AVCN). Thin sections were obtained from the middle-frequency area of the right AVCNs from five 3-month-, four 19-month-, and five 28-month-old F344 rats. Montages were constructed from electron micrographs taken of several sites in each AVCN section. The presynaptic terminals were classified by vesicle type and postsynaptic target, and their perimeters and synaptic lengths were traced using morphometry software. The calibers of all dendritic profiles were also measured, and cell counts were performed on semi-thin sections. The data were compared among the three age groups using analysis of variance followed by Tukey's Honestly Significant Difference for pairwise comparisons. There were significant age-related decreases in the size of terminals contacting small-caliber (<2 microm) dendrites. Dendrites of this size comprised the largest percentage of dendrites in the AVCN. On these targets, round and pleomorphic-vesicle terminals were reduced in volume by nearly 44% and 24%, respectively, in 28-month olds when compared to the 3-month olds. On the other hand, the densities and numbers of synaptic terminals and dendritic profiles did not differ among age groups, and no neuronal losses were evident in the older animals. Also, there were no detectable changes in synaptic area among groups. The decrease in terminal size may be related to age-associated reductions in neurotransmitter levels previously described in the F344 CN. The observations presented here contrast with those previously described in the inferior colliculus (IC), in which there were significant age-related losses of synaptic terminals and dendrites, but no change in the size of synaptic terminals. The lack of synaptic and dendritic losses suggests that the structural connectivity of the rat AVCN remains relatively intact during aging, which is interesting in light of the synaptic and dendritic changes evident in the IC, a major target of its projections.

Age Factors↗
PubMed1998

Age-related glycine receptor subunit changes in the cochlear nucleus of Fischer-344 rats.

Previous studies have shown that levels of binding for the strychnine-sensitive glycine receptor in the cochlear nucleus (CN) of Fischer (F344) rats decrease with age. Given the major role glycine plays in normal CN function, changes in glycine-receptor activity may contribute to central presbycusis. To further evaluate the impact of age on glycine receptors, in situ hybridization was used to assess, in three age groups of F344 rats, changes in levels of gene expression for four of its subunits. When compared with the 3-month-old rats, expression of mRNAs for alpha1 and beta subunits in the anteroventral CN decreased significantly in the 18- and 27-month-old age groups, while mRNA expression for the alpha2 subunit increased. If protein expressions are similar, these subunit changes may alter the function of glycine receptors, thereby affecting binding to its ligands.

Aging↗
PubMedDate not supplied

Intrinsic oscillations and discharge regularity of units in the dorsal cochlear nucleus (DCN) of the barbiturate anesthetized gerbil.

Spike discharge patterns showing intrinsic oscillations (IOs) have been reported in units in the dorsal cochlear nucleus (DCN) of the decerebrate cat. These oscillations are related to the regularity of a unit's discharge rate and may be important for pitch perception. A DCN unit's regularity can be affected by several factors including: synaptic architecture, cell membrane properties, and the auditory nerve discharge rate. Responses to multiple presentations of short-duration tone bursts (200 ms duration, 1 s trial) at the unit's best frequency (BF) at 20 dB re threshold were recorded from 297 units in the DCN of the barbiturate-anesthetized gerbil. Comparisons of unit regularity properties and IO properties are shown. The relative power spectrum (Fourier transform of the autocorrelogram normalized by the average rate) was used to quantify IO properties. Most units (84%) exhibited IOs in their sustained discharge rate. With the exception of Onset units and most bursting units, the mean inter-spike interval was correlated with the IO frequency and the coefficient of variation was correlated with the IO magnitude. These results suggest that stimulus-encoding mechanisms utilizing IOs may depend on the temporal evolution of the units' regularity properties.

Animals↗
PubMed2000

Patterns of cell death in mouse anteroventral cochlear nucleus neurons after unilateral cochlea removal.

Developmental changes that influence the results of removal of afferent input on the survival of neurons of the anteroventral cochlear nucleus (AVCN) of mice were examined with the hope of providing a suitable model for understanding the cellular and molecular basis for these developmental changes in susceptibility. We performed unilateral cochlear ablation on wild-type mice at a variety of ages around the time of hearing onset to determine developmental changes in the sensitivity of AVCN neurons to afferent deprivation. In postnatal day 5 (P5) mice, cochlea removal resulted in 61% neuronal loss in the AVCN. By age P14, fewer than 1% of AVCN neurons were lost after this manipulation. This reveals a rather abrupt change in the sensitivity to disruption of afferent input, a critical period. We next investigated the temporal events associated with neuron loss after cochlea removal in susceptible animals. We demonstrate that significant cell loss occurs within 48 hours of cochlea removal in P7 animals. Furthermore, evidence of apoptosis was observed within 12 hours of cochlea removal, suggesting that the molecular events leading to cell loss after afferent deprivation begin to occur within hours of cochlea removal. Finally, we began to examine the role of the bcl-2 gene family in regulating afferent deprivation-induced cell death in the mouse AVCN. AVCN neurons in mature bcl-2 knockout mice demonstrate susceptibility to removal of afferent input comparable to neonatal sensitivity of wild-type controls. These data suggest that bcl-2 is one effector of cell survival as these cells switch from afferent-dependent to -independent survival mechanisms.

Afferent Pathways↗
PubMed1994

The electrotonic structure of regular-spiking neurons in the ventral cochlear nucleus may determine their response properties.

1. Intracellular recordings were obtained from neurons in parasagittal brain slices of the guinea pig ventral cochlear nucleus (VCN). The principal neurons of the VCN can be parceled into two categories. Regular-spiking (Type I) neurons have a linear current-voltage (I-V) relationship over a large range of intracellularly injected currents and fire tonically in response to suprathreshold depolarizing currents. Phasically spiking (Type II) neurons have a nonlinear I-V relationship and fire only phasically at the onset of a depolarizing current or offset of a hyperpolarizing current. Regular-spiking neurons have been shown to be of the stellate morphological type, whereas phasically spiking neurons have been shown to be bushy cells. 2. The electrotonic structure of regular-spiking neurons was studied by applying previously developed modeling techniques based on the somatic shunt model. In these techniques, physiological data are used to determine the set of parameters best describing the neuron. As predicted from previous theoretical investigations, the use of an anatomic constraint (somatic surface area) reduces the variance in estimates of model parameters, especially for the dendritic membrane time constant tau D. 3. Model representations of regular-spiking cells fall into two categories: those with (passive) somatic membrane properties that are nearly identical to those of the dendrite (8/15 cases), and those with a significant amount of somatic shunt (7/15). Estimates of tau D (mean = 7.7 ms) are lower than those often described in the literature. We argue that this low value of tau D may be related to the need of neurons in the auditory brainstem to operate at high firing rates and/or to encode audio-frequency temporal fluctuations. 4. Dendritic transfer functions were calculated as functions of synaptic location using somatic shunt representations of regular-spiking neurons. These transfer functions allow us to predict that mid-range auditory frequencies (approximately 1 kHz) are greatly attenuated, even for synapses near the soma. Thus it is suggested that the electrotonic architecture of regular-spiking cells creates sufficient low-pass filtering of synaptic inputs to reduce the synchronization of firing of these neurons to mid-frequency auditory stimuli.

Animals↗
PubMed1990

Dynamic temporal properties of effective connections in rat dorsal cochlear nucleus.

In a prior report we presented evidence that functionally connected dorsal cochlear nucleus (DCN) neurons in close proximity can show differing peristimulus time histograms (PSTHs) in response to the same stimulus. We wished to further investigate how interconnections between such neurons might participate in the PSTH patterns. Methodology has recently been developed which permits measurement of rapid changes in effective connectivity between neuron pairs: the normalized joint PSTH. Using this technique we have observed that rapid changes in effective connectivity do occur in the DCN. These observations demonstrate that the effects of one DCN neuron on another cannot necessarily be understood by sequential recordings from single units, even if anatomical connectivity can be established.

Acoustic Stimulation↗
PubMed1986

Encoding timing and intensity in the ventral cochlear nucleus of the cat.

Physiological response properties of neurons in the ventral cochlear nucleus have a variety of features that are substantially different from the stereotypical auditory nerve responses that serve as the principal source of activation for these neurons. These emergent features are the result of the varying distribution of auditory nerve inputs on the soma and dendrites of the various cell types within the nucleus; the intrinsic membrane characteristics of the various cell types causing different responses to the same input in different cell types; and secondary excitatory and inhibitory inputs to different cell types. Well-isolated units were recorded with high-impedance glass microelectrodes, both intracellularly and extracellularly. Units were characterized by their temporal response to short tones, rate vs. intensity relation, and response areas. The principal response patterns were onset, chopper, and primary-like. Onset units are characterized by a well-timed first spike in response to tones at the characteristic frequency. For frequencies less than 1 kHz, onset units can entrain to the stimulus frequency with greater precision than their auditory nerve inputs. This implies that onset units receive converging inputs from a number of auditory nerve fibers. Onset units are divided into three subcategories, OC, OL, and OI. OC units have extraordinarily wide dynamic ranges and low-frequency selectivity. Some are capable of sustaining firing rates of 800 spikes/s at high intensities. They have the smallest standard deviation and coefficient of variation of the first spike latency of any cells in the cochlear nuclei. OC units are candidates for encoding intensity. OI and OL units differ from OC units in that they have dynamic ranges and frequency selectivity ranges much like those of auditory nerve fibers. They differ from one another in their steady-state firing rates; OI units fire mainly at the onset of a tone. OI units also differ from OL units in that they prefer frequency sweeps in the low to high direction. Primary-like-with-notch (PLN) units also respond to tones with a well-timed first spike. They differ from onset cells in that the onset peak is not always as precise as the spontaneous rate is higher. A comparison of spontaneous firing rate and saturation firing rate of PLN units with auditory nerve fibers suggest that PLN units receive one to four auditory nerve fiber inputs. Chopper units fire in a sustained regular manner when they are excited by sound.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed1995

Discharge rate-level functions from dorsal cochlear nucleus single units in response to acoustic and electrical stimulation of the auditory nerve.

Discharge rate-level (I/O) functions possessed by dorsal cochlear nucleus (DCN) units were examined, in response to bipolar electrical stimulation of the cochlea of the barbiturate-anesthetized cat. Spontaneously active units usually possessed nonmonotonic functions with a minimum, and spontaneously inactive units usually possessed monotonic functions or nonmonotonic functions with a maximum (NM+). In response to acoustic high-pass filtered noise, the function relating discharge rate and cutoff frequency resembled the same unit's I/O function to electrical stimulation. The I/O functions to acoustic characteristic tones were usually monotonic or NM+. These results suggest that in the DCN, a prerequisite for the generation of acoustic-like responses with an electrical stimulus may be the matching of the cochlear place and spatial extent activated by each stimulus.

Acoustic Stimulation↗
PubMed1989

Coordinated activity of neuron pairs in anesthetized rat dorsal cochlear nucleus.

We have recorded from small groups of neurons in the dorsal cochlear nucleus of anesthetized rats in an effort to study neuronal interactions. Multi-unit recordings on each single electrode were sorted by waveform into spike trains from individual neurons using a principal components spike sorter. Pairs of such sorted spike trains were studied with cross-correlation analysis to detect excitatory and/or inhibitory interactions. In a few cases recordings were obtained from two electrodes simultaneously, thus allowing cross-correlation studies without the consequences of spike train waveform sorting. All neurons were characterized by their strongest response frequency (at a fixed sound pressure level) and peristimulus histogram responses to 55 ms tone bursts. Fifty-eight percent of the neuron pairs studied showed peaks in their cross-correlograms indicative of coordinated neural activity. Of these pairs, 86% showed peak configurations (i.e. correlograms with asymmetrically located peaks) consistent with the interpretation that one cell induced the other to discharge. The remaining correlograms contained symmetric peaks which were centrally located, possibly due to shared input to these neuron pairs. Latencies of asymmetric peaks in cross-correlograms were typically 2 ms; consequently, an intervening excitatory synapse may be involved. Similar results were obtained from at least one pair of neurons where each neuron was recorded by a separate electrode. Strongest response frequencies of each neuron pair, for which they could be determined, were within 0.17 log units. Peristimulus histograms from each neuron in these pairs revealed that it was common for adjacent cells to respond with differing time patterns under the same stimulus conditions. The variations in histogram patterns of interconnected neurons suggests some relatively complex integrative function for these circuits.

Acoustic Stimulation↗
PubMed1976

Responses to tones and noise of single cells in dorsal cochlear nucleus of unanesthetized cats.

1. Single-unit responses in the dorsal cochlear nucleus of unanesthetized, decerebrate cats have been divided into two categoreis. These have been differentiated on the basis of responses to best-frequency tones. Type IV units responded to best-frequency tones with excitation from threshold to about 20 or 30 dB above threshold; at higher levels, their response was inhibitory. In a few cases, the excitatory area near threshold was not seen and in a few others, the response became excitatory again at high levels. Type IV units could be divided into two groups based on the length of time that inhibition was maintained in response to long tones. Type IV units are not seen in anesthetized cats. 2. Type II/III units responded to best-frequency tones of all levels with excitation. Nonmonotonic rate versus level functions were seen in type II/III units, but they were of much less drastic character; the discharge rate of nonmonotonic type II/III units was still well above spontaneous rate for tones 50 dB above threshold. Type II/III units defined in this way were found to have, on the average, lower rates of spontaneous activity and higher thresholds than type IV units. 3. Type II/III units responded weakly to broad-band noise in comparison to auditory nerve fibers and many of them did not respond at all to noise. Type IV units, with best frequencies above 0.9 kHz, gave excitatory responses to noise. 4. The inhibitory response areas of type IV units could be divided into two areas: a central inhibitory area in the vicinity of best frequency where on- and off-discharges and afterdischarges were seen; and inhibitory side bands at higher and lower frequencies where simple inhibitory responses were seen. In four units, it was possible to show that the central inhibitory area was converted to an excitatory area after administration of an anesthetic dose of pentobarbital. 5. Most type II/III and type IV units could be excited or inhibited by stimuli in the contralateral ear. Broad-band noise was a more effective contralateral stimulus than tones at the ipsilateral best frequency. 6. On the basis of the properties of type II/III and type IV cells, it is suggested that type II/III responses are recorded from interneurons which provide a large share of the inhibitory imput to type IV cells.

Acoustic Stimulation↗
PubMed1989

New structural features of synapses in the anteroventral cochlear nucleus prepared by direct freezing and freeze-substitution.

The rostral anteroventral cochlear nucleus (AVCN) of the chinchilla provides a preparation in which neuronal cell bodies and synapses in the mammalian central nervous system can be examined after direct freezing and freeze-substitution of rapidly excised brain stem slices. Cell bodies and synapses in the freeze-substituted AVCN differed from those in perfusion-fixed AVCN in several interesting respects. Despite of these differences, four types of synaptic terminal were distinguished in freeze-substituted AVCN and correlated with the four well-known types of perfusion-fixed terminal. Since the transmitter at each of the four types of terminal has been tentatively identified, the structure of synaptic vesicles and junctions in the freeze-substituted terminals could be related to transmitter type. Synaptic vesicles were uniformly round, but their diameters, deployment, and related cytoskeletal elements near the synaptic junction differed in each chemical type of synapse; the synapses thought to be cholinergic, for instance, had only a few vesicles clustered at their presynaptic junctions while the rest of the vesicles were separated from the junction by a network of fine filaments. Two types of filamentous components, short vertical projections from the postsynaptic membrane and thin filaments protruding from these projections, comprised the basic structure of the postsynaptic specialization, but their sizes and distribution differed at each chemical type of terminal. For instance, the postsynaptic specialization at the glycine terminal was distinguished by numerous thin filaments which curved sideways to run parallel to the plasmalemma. Thus, freeze-substitution gives new information about structural differences between chemically different types of synapses, which may reflect differences in their transmitter storage, release, and reception. In addition, the AVCN preparation is of general interest in making cell bodies in the mammalian central nervous system available to the various structural and analytical techniques which depend on direct, rapid freezing.

Animals↗
PubMed1991

Temporal representation of rippled noise in the anteroventral cochlear nucleus of the chinchilla.

This paper describes the temporal responses of anteroventral cochlear nucleus (AVCN) units in the chinchilla to rippled noises. Rippled noise is generated when a wideband noise is delayed and added (cos+ noise) or subtracted (cos- noise) to the undelayed noise. Renewal densities were constructed to evaluate synchronous discharges at the delay. In response to rippled noise, AVCN units which show phase locking to best frequency (BF) tones gave renewal densities having major peaks at the delay for cos+ noise, but nulls at the delay for cos- noise. Most AVCN units which did not show BF phase locking gave renewal densities that did not contain features related to the rippled noise delay; a few of these nonphase-locked units did show peaks in renewal densities for both cos+ and cos- noises. Synchrony at the rippled noise delay was also demonstrated with evoked potential recording. Autocorrelation functions of the neurophonic potential showed peaks at the rippled noise delay for both cos+ and cos- noises. In addition, peaks could be observed in the autocorrelation functions of neurophonic potentials for rippled noises with delays as short as 1 ms; peaks were never observed in renewal densities of single units for ripple delays as short as 1 ms. The results show that a temporal representation of rippled noise delay does exist in the AVCN and are consistent with current hypotheses regarding functions of AVCN subsystems. The temporal representation of the delay is a presumptive neural code for the pitches of rippled noises.

Acoustic Stimulation↗