Jaundiced Gunn rats have increased synaptic delays in the ventral cochlear nucleus

Recordings were made in vitro from cochlear nuclei of Gunn rats, a strain with a recessive mutation that predisposes rats to hyperbilirubinemia at birth. Delays between shocks to the auditory nerve and earliest synaptic responses of the cochlear nuclear neurons were on average longer in Gunn rats than in heterozygotes. Injections of sulfonamide further increased average synaptic delays in jaundiced rats. Responses to injected current in rats were like those in mice.

Morphology and physiology of cells in slice preparations of the dorsal cochlear nucleus of mice

Horseradish peroxidase (HRP) was injected into cells from which intracellular recordings were made in slices of the dorsal cochlear nucleus (DCN) in order to correlate physiology with morphology. In general, the morphology of cells labeled intracellularly with HRP corresponded to those made with Golgi impregnations in mice and other mammals. The following cells were labeled: one granule cell, four cartwheel cells, eight fusiform cells, two other cells in the fusiform cell layer, and two tuberculoventral association cells in the deep layers of the DCN. The axon of the granule cell runs parallel to isofrequency laminae with collaterals branching perpendicularly and running along the tonotopic axis. The cartwheel cells have dendrites in the molecular layer that are densely covered with spines. The axon of one cell terminates just dorsally to the cell body. Fusiform cells have the characteristic spiny, apical and smooth, basal dendrites. The basal dendrites are conspicuously oriented parallel to isofrequency laminae. Axons of the fusiform cells exit through the dorsal acoustic stria without branching. The two tuberculoventral association cells in the deep DCN have axons that terminate both in the deep DCN, within the same isofrequency lamina that contains the cell body, and in the ventral cochlear nucleus (VCN). Intracellular recordings from 11 of these cells show that they cannot be distinguished on the basis of their responses to intracellularly injected current. All cell types fired large action potentials that were followed by a fast and a slower undershoot, distinguishing them from cells of the VCN but not from one another. Most cells responded to shocks of the auditory nerve root with early EPSPs and later IPSPs. The latencies of EPSPs show that some were monosynaptic and others polysynaptic. That there was no systematic relationship between the latencies of EPSPs and the cell types from which they were recorded shows that shocks to the nerve root may have activated more than just the large, myelinated, auditory nerve fibers.

Auditory nerve neurotransmitter acts on a kainate receptor: evidence from intracellular recordings in brain slices from mice

Intracellular recordings from neurons in brain slice preparations of the mouse ventral cochlear nucleus (VCN) were used to examine the actions of excitatory amino acid agonists and antagonists. Synaptic responses to electrical stimulation of the auditory nerve root were partially blocked by kynurenic acid, an antagonist that is specific for glutamate receptors. The antagonists specific for N-methyl-D-aspartate (NMDA), DL-2-amino-5-phosphonovalerate (APV) and Mg2+, did not affect the response, arguing against a role for NMDA receptors at the VIIIth nerve synapse. To test postsynaptic sensitivity to excitatory amino acid agonists, responses to bath applications were measured in VCN neurons while synaptic transmission was blocked by the removal of Ca2+ from the bath or by the addition of tetrodotoxin. Neurons in the VCN were 500-1000 times more sensitive to kainate than to glutamate or aspartate. In the absence of Mg2+, they were also sensitive to NMDA. The responses to kainate and glutamate were increased by the removal of calcium from the bath. These results imply that VCN neurons have both kainate and NMDA receptors and that synaptic transmission between auditory nerve fibers and neurons in the cochlear nuclear complex could be mediated by a substance related to kainate.

Tonotopic projection from the dorsal to the anteroventral cochlear nucleus of mice

To understand how auditory information is processed in the cochlear nuclei, it is crucial to know what circuitry exists and how it functions. In slice preparations, horseradish peroxidase (HRP) injections into the anteroventral cochlear nucleus (AVCN) reveal two circuits: a connection between the dorsal cochlear nucleus (DCN) and AVCN and a local circuit confined to the AVCN. Extracellular injection in the AVCN labels a band of cells in the DCN. The labeled cells in the DCN lie within a band of auditory nerve fiber terminals that are labeled by the same injection, showing that the connection from the DCN to the AVCN is frequency specific. The injections into the AVCN also labeled a cluster of neurons in the AVCN dorsal to the injection site. These cells may be interneurons that relay information from areas encoding higher frequencies to areas encoding lower frequencies within the AVCN. In the parasagittal plane, the AVCN is organized along two orthogonal axes that are indicated with HRP labeling of fibers and cell bodies. The tonotopic axis runs approximately dorsoventrally; the isofrequency axis runs approximately rostrocaudally. The axons of labeled DCN neurons and the cluster lie along the tonotopic axis, whereas the labeled auditory nerve fibers define the isofrequency axis. Where they cross is where HRP is taken up by the fibers. The area of uptake is small and lies in the middle of the darkly stained injection site.

Intrinsic properties of neurones in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices of the cochlear nuclear complex. Probably the larger and most frequent cells were impaled. 2. The steady-state current-voltage (I-V) properties of all cells impaled were nonlinear. The I-V curve was steepest in the voltage range depolarized from the resting potential and most shallow when the cell was hyperpolarized from rest by more than about 10 mV. Thus, the inwardly rectifying I-V characteristics of cells in the DCN distinguish them from those of ventral cochlear nuclear neurones (Oertel, 1983). 3. When depolarized with current, most cells fired trains of large, all-or-none action potentials. The undershoot after single spikes comprised an initial, fast component followed by a second, slower wave. A few cells (15%) generated bursts of smaller, graded spikes in addition to the large ones. 4. Repetitive firing evoked by depolarizing pulses of current was followed by an after-hyperpolarization whose magnitude depended on the strength and duration of the preceding current pulse. 5. Blocking the large action potentials with tetrodotoxin (TTX) revealed Ca2+-dependent spikes in all cells examined. 6. The steady-state I-V relationship became linear in the presence of TTX, suggesting that a persistent Na+ conductance probably mediates the inward rectification seen above the resting potential. 7. Muscarine at micromolar concentrations excited cells and increased their input resistance.

Synaptic connections in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices that contained the root of the auditory nerve and parts of the dorsal and ventral cochlear nuclei. Probably the largest and most common cells were impaled. 2. Weak shocks to the nerve usually evoked an excitatory postsynaptic potential (EPSP) that lasted about 90 ms and whose latency was often less than 1.2 ms, indicating monosynaptic input. 3. Stronger shocks elicited a larger EPSP and a later train of inhibitory postsynaptic potentials (IPSPs). Increasing the stimulus voltage shortened the latency of the train of IPSPs and increased its efficacy so that at large stimulus strengths inhibition dominated the synaptic response. 4. To determine whether any of the neuronal circuitry which generated the synaptic responses involved the ventral cochlear nucleus, recordings were made from slices containing only the dorsal nucleus. Synaptic responses to stimulation of the pial surface of the isolated DCN resembled those driven from the nerve root. That is, weak shocks evoked long-lasting, monosynaptic EPSPs and stronger stimuli elicited a larger EPSP followed by trains of IPSPs. The DCN, therefore, contains intrinsic inhibitory interneurones. 5. The parallel fibres of the DCN course superficially, near the stimulating electrodes, whereas the axons of the auditory nerve terminate in deeper areas. Thus, the monosynaptic EPSPs evoked from the pial surface are probably generated by parallel fibres. Apparently the inhibitory interneurones are also excited by a circuit including parallel fibres. 6. The putative neurotransmitter of parallel fibres, glutamate, excited all neurones tested. 7. Cells were sensitive both to glycine and to gamma-aminobutyric acid (GABA). Only strychnine, however, not picrotoxin or bicuculline, blocked IPSPs.

Maturation of synapses and electrical properties of cells in the cochlear nuclei

Auditory nerve fibers carry impulses from the cochlea to the cochlear nuclei. There the temporal firing patterns of auditory nerve fibers are preserved by some cells and altered by others. The two factors which govern how firing patterns are shaped are (1) the intrinsic electrical properties of cells that determine the size and time course of voltage changes caused by synaptic currents and (2) the synaptic circuitry between cells. The electrical properties of cells were measured by recording the responses to current injected intracellularly into brain slice preparations. The synaptic responses to electrical shocks of the auditory nerve were used to determine the functional properties of synaptic connections. The three distinct types of electrical properties of cells that can be distinguished electrophysiologically in similar preparations of mature tissue, bushy and stellate cells in the ventral cochlear nucleus [(1984) J. Neurosci. 4, 1577-1588] and cells in the dorsal cochlear nucleus [Hirsch and Oertel (1987) (submitted); Oertel et al. (1987) In: Functions of the Auditory System, Editor: S. Hassler. J. Wiley and Sons (in press)] can be differentiated at least as early as 7 days after birth. Young cells, however, have higher input resistances and lower input capacitances than mature cells, and they cannot sustain high firing rates. Bushy and stellate cells in the ventral cochlear nucleus respond to electrical stimulation of the auditory nerve with both excitatory and inhibitory postsynaptic potentials as early as 4 days after birth. The synaptic potentials occur with longer and more variable latencies than in mature cells and synapses fatigue more easily, however. Cells of the dorsal cochlear nucleus also receive both excitatory and inhibitory synaptic inputs 4 days after birth, upon stimulation of the auditory nerve. No systematic changes were detected in these synaptic responses as a function of age but this may have been because the variability in the shape and timing of synaptic responses was large even in mature tissue.

Inhibitory circuitry in the ventral cochlear nucleus is probably mediated by glycine

Intracellular recordings from brain slice preparations of the ventral cochlear nuclei (VCN) of mice have shown that both the major cell types, stellate and bushy cells, distinguishable by their responses to intracellularly injected current (Oertel, 1983; Wu and Oertel, 1984), receive late inhibitory as well as early excitatory inputs when the auditory nerve is stimulated electrically. When the extracellular Cl- concentration was lowered or when the intracellular Cl- was raised, the reversal potential of IPSPs became more positive; the reversal potentials were independent of the extracellular K+ concentration. Therefore, IPSPs result from increases in Cl- permeability. To determine whether either or both GABA or glycine might mediate the inhibition, the sensitivity of cells to bath-applied putative neurotransmitters was tested. All cells responded to applications of 0.1-10 mM GABA and glycine with large drops in input resistance; these drops were Cl- dependent. To determine which of these 2 substances was more likely to mediate the IPSPs, antagonists specific to GABA and glycine were tested for their ability to block the IPSPs. All IPSPs were eliminated by 1 microM strychnine, a blocker of glycine-mediated inhibition; they were not consistently blocked by 100 microM bicuculline or by 100 microM picrotoxin, blockers of GABA-mediated inhibition. These results indicate that the inhibition is likely to be mediated by glycine. A simple interpretation of the finding that IPSPs have latencies (1.2-4 msec) at least 2X as long as EPSPs (0.6-0.9 msec) is that cells in the VCN are excited monosynaptically by auditory nerve fibers, and that they are inhibited disynaptically through interneurons within the VCN. To test physiologically whether EPSPs and IPSPs are, respectively, monosynaptic and polysynaptic, 500-700 microM sodium pentobarbital was applied to the preparation. Pentobarbital raised the thresholds of all impaled cells and their synaptic inputs. EPSPs could be evoked in the presence of pentobarbital by raising the stimulus strength, as expected when thresholds are raised in a monosynaptic circuit; even if the thresholds of IPSPs were lower than those of EPSPs in normal saline, they were raised above those of EPSPs in the presence of pentobarbital. The finding that the thresholds of IPSPs are raised more than those of EPSPs supports the interpretation that IPSPs are mediated through a polysynaptic pathway, and this may explain why inhibition has been detected inconsistently in vivo.

Cells in the anteroventral cochlear nucleus are insensitive to L-glutamate and L-aspartate; excitatory synaptic responses are not blocked by D-alpha-aminoadipate

Auditory nerve fibers transmit signals from the cochlea to the 3 regions of the cochlear nuclear complex, the anteroventral (AVCN), posteroventral, and dorsal cochlear nucleus in the brainstem. It has been suggested that the amino acids L-aspartate and L-glutamate might serve as a neurotransmitter in auditory nerve fibers. The sensitivity of postsynaptic cells in the cochlear nuclei to these amino acids has been tested by iontophoretic techniques. One difficulty with these experiments is that responses were recorded only extracellularly. A second difficulty is that the concentrations needed to affect cells could not be determined. To avoid these difficulties a brain slice preparation was used to test the sensitivity of cells in the AVCN to bath applied L-glutamate and L-aspartate at concentrations ranging from 10(-5) to 10(-2) M. All cells that were tested in the cochlear nuclear complex were insensitive at all concentrations used; the resting potentials and the input resistances remained unchanged and the synaptic responses to electrical stimulation of the auditory nerve were not desensitized. All cells that were tested in the hippocampus, however, depolarized in the presence of 10(-4) M L-glutamate and L-aspartate. The synaptic responses to electrical stimulation of the auditory nerve were not blocked by D-alpha-aminoadipate, an amino acid which has been shown to block excitation of cells in the cochlear nuclei by auditory nerve fibers. The results are not consistent with L-glutamate and L-aspartate serving as neurotransmitters in the AVCN.

Intracellular injection with horseradish peroxidase of physiologically characterized stellate and bushy cells in slices of mouse anteroventral cochlear nucleus

Nissl-stained tissue from brain slice preparations of the anteroventral cochlear nucleus of the mouse resembles tissue fixed in situ. Multipolar, spherical, globular, and granule cells can be distinguished after intracellular injection with horseradish peroxidase (HRP). Stellate cells have relatively large dendritic fields; their axons have collaterals which terminate within the cochlear nuclear complex. Bushy cells have smaller dendritic fields; where they can be seen, axons have no collaterals. Granule cells have few short dendrites; their very fine axons branch close to the cell body and could be followed only for short distances. Intracellular recordings from six stellate cells labeled by intracellular injection of HRP revealed that they have linear current-voltage relationships around the resting potential and that they respond to suprathreshold depolarization with large, regularly firing action potentials. Intracellular recordings from four bushy cells, also labeled by injection of HRP, showed that these cells have nonlinear current-voltage relationships around the resting potential and that they respond to suprathreshold depolarizations with only one or two small action potentials. The anatomical and physiological features of bushy cells reduce summing in time and space and make bushy cells well suited to preserve the firing patterns of auditory nerve inputs. The anatomical and physiological features of stellate cells, in contrast, allow summing in time and space.

Synaptic responses and electrical properties of cells in brain slices of the mouse anteroventral cochlear nucleus

Intracellular recordings were made from cells in brain slices of the anteroventral cochlear nucleus. Responses to electrical stimulation of the stump of the auditory nerve were: (1) all-or-none, following the stimulus with no delay, and insensitive to the removal of extracellular Ca2+, probably representing the firing of directly stimulated auditory nerve fibers, or (2) graded, excitatory postsynaptic potentials, with synaptic delays of about 0.7 msec, which were often followed by graded, inhibitory synaptic potentials with synaptic delays of 1.2 msec or longer. The excitatory and inhibitory synaptic potentials were abolished by the removal of extracellular Ca2+. The result that delays of inhibitory synaptic responses were at least 2 times as long as those of excitatory ones indicates that probably an additional synapse was interposed. Responses to intracellularly injected current pulses show that cells in the anteroventral cochlear nucleus have one of two types of electrical characteristics. Type I properties are characterized by linear current-voltage relationships in the subthreshold voltage range and large, regularly firing action potentials in the suprathreshold range. Type II properties are characterized by nonlinear current-voltage relationships; suprathreshold depolarization elicits only one or two small action potentials. Type II characteristics are particularly well suited for maintaining the information contained in the timing and patterns of firing of the auditory nerve fibers.

Physiological response properties of cells labeled intracellularly with horseradish peroxidase in cat ventral cochlear nucleus

To determine the correspondence between anatomical and physiological cell types in the ventral cochlear nucleus of the cat, intracellular injections of horseradish peroxidase were made into cells whose extracellular and intracellular responses to sound had been studied. Three identified cells responded to a short tone burst at their characteristic frequencies with an onset pattern. This pattern is characterized by a strong response to the onset of the stimulus. One was an octopus cell. The second cell, located in the octopus-cell area, was a giant cell with a few somatic spines and thin tapering dendrites; the intracellular record revealed that even in the absence of sound it received continuous synaptic input, while tones at characteristic frequency produced a sustained depolarization. A third cell, which had an onset response at low intensities and a chopper response at high intensities, was a stellate cell located in the intermediate acoustic stria with dendrites oriented parallel to the fiber tract. This cell had an unusually broad dynamic range in response to changes in intensity. Two cells with transient chopper response patterns were stellate cells in the posteroventral cochlear nucleus with many branched and beaded dendrites. Three cells with more sustained chopper response patterns were stellate cells in the anteroventral cochlear nucleus with fewer, less-branched, smooth dendrites. Two cells with primarylike responses to tones were bushy cells with numerous short, thin, highly branched dendrites in the posterior division of the anteroventral cochlear nucleus. Intracellular responses to tones at the characteristic frequency consisted of large brief depolarizations, which were not sustained. Another cell, which responded to tones in a phase-locked fashion, was also located in the anteroventral cochlear nucleus. It was a small, stellate cell with relatively few, smooth dendrites. The labeled cells largely support previous attempts at physiological-morphological correlations: (1) bushy cells exhibit primarylike pattern; (2) stellate cells exhibit chopper patterns; and (3) octopus cells exhibit an onset pattern. It was also demonstrated that more than one cell type can exhibit a particular response pattern.

Physiological response properties of cells labeled intracellularly with horseradish peroxidase in cat dorsal cochlear nucleus

The physiology and morphology of fusiform cells in the dorsal cochlear nucleus were studied using extracellular and intracellular recording and intracellular injection of horseradish peroxidase. Fusiform cells displayed a variety of responses to tone pips presented at the characteristic frequency; most often these cells exhibited the pauser/buildup pattern defined in earlier studies. The response pattern of each neuron was dependent on frequency and sound-pressure level. Tone pips evoked short-lasting depolarizations of about 10 mV and long-lasting hyperpolarizations of about 10 mV in cells whose resting potentials were -50 to -65 mV. The time courses of both the excitation and the inhibition depended on frequency and sound-pressure level. Generally the depolarization was sustained for the duration of the tone pip, whereas the hyperpolarization could last as long as 600 ms after the end of the tone pip. Often a neuron exhibited a sustained chopper pattern after microelectrode impalement. This was probably a result of a decrease in membrane potential which altered the relative effectiveness of the excitatory and inhibitory inputs. The large, bitufted fusiform cells had many apical dendrites, which branched one to five times and were covered with spines, and fewer basal dendrites, which exhibited little branching and had few appendages. The morphology of fusiform cells varied systematically as a function of location within the dorsal cochlear nucleus. Response patterns for tone pips were not exclusive to individual cell types as two nonfusiform cells were found to exhibit a buildup pattern. Axons of injected neurons left the nucleus via the dorsal acoustic stria and 14 of 15 had collaterals within the dorsal cochlear nucleus.

Transformation of signals by interneurones in the barnacle's visual pathway

1. The photoreceptors of the median eye of the giant barnacle drive decrementally-conducting neurones in the supraoesophageal ganglion termed ;inverting cells' (I-cells) which in turn drive impulse-producing neurones termed ;amplifying cells' (A-cells). Using intracellular recording techniques we have studied the role of I-cells in visual processing.2. Horseradish peroxidase injections show that I-cells are interneurones whose processes are confined to the regions of the photoreceptor terminals on both sides of the bilaterally symmetrical ganglion.3. In the dark, I-cell membrane potentials (-45 mV) are considerably less negative than those of other ganglion cells (-60 to -70 mV). At the onset of a maintained light, I-cells undergo a transient peak hyperpolarization which declines to a steady-state response. Both response components are graded with light intensity.4. The reversal potential of the peak of the I-cell light response depends on the external K(+) concentration more strongly than does the dark resting potential (3-30 mm-K(+)). This evidence indicates that the hyperpolarization results from an increase in the cell's permeability to K(+) ions.5. At the offset of light an I-cell undergoes a transient depolarization that overshoots the dark membrane potential. Dimming of a background light can also cause the I-cell membrane potential to overshoot its dark resting value. This overshoot is associated with a large depolarizing synaptic potential in A-cells.6. An overshoot of the dark resting potential can also be elicited by the break of a hyperpolarizing pulse of current injected into an I-cell. The amplitude of this overshoot increases with pulse duration over a time course of seconds.7. In the presence of external tetraethylammonium ion (TEA) and tetrodotoxin, (TTX), the break of a hyperpolarizing pulse or the onset of a depolarizing pulse can evoke in an I-cell an action potential whose rate of rise and amplitude depend on the external Ca concentration. This action potential can be maintained by replacement of external Ca with Ba, or blocked by addition of 15 mm-Co to the saline. These observation's indicate that depolarizing potential changes in this cell activate a voltage-sensitive Ca conductance.8. When hyperpolarizing current pulses are injected into an I-cell, the voltage during the pulse sags back slowly towards the dark resting potential. Thus, during hyperpolarization with light or current an I-cell's membrane properties change over a time course of seconds.9. The onset of a depolarizing pulse or the offset of a hyperpolarizing pulse of current injected into an I-cell leads to a transient depolarization of a simultaneously impaled A-cell. Synaptic transmission occurs when the I-cell is depolarized to the vicinity of the dark resting potential. The amplitude of the response in an A-cell depends on the rate of change of the I-cell voltage.

A potassium conductance activated by hyperpolarization in paramecium

Voltage clamp studies show that the wild-type membrane of Paramecium tetraurelia contains a conductance component which is sensitive to hyperpolarization. This component manifests itself as "anomalous", or "inward going", rectification of membrane voltage in response to applied constant current pulses and as a "hyperpolarizing spike" when no K is added to the external solution (Y. Satow, C. Kung, 1977. J. Comp. Physiol. 119:99). Like the conductances which underlie anomalous rectification in other cells, the hyperpolarization-sensitive conductance in Paramecium is specific for K, and the magnitude of the voltage-dependent conductance change depends not only on voltage but also on external potassium concentration. The internal potassium ion concentration of Paramecium is calculated to be between 17 and 18mM.

Neuronal properties underlying processing of visual information in the barnacle

Generation of a transient, amplified response to the dimming of light in the visual system of the barnacle involves two synaptic stages. It is accomplished primarily by decrementally conducting neurones that are similar to bipolar cells of the vertebrate retina.

Separation of membrane currents using a Paramecium mutant

The net membrane current of the electrically excitable membrane of Paramecium during step depolarisations was measured using voltage-clamp techniques. The mutant, pawn B, lacks a functional Ca channel. Thus the difference between the total current measured in the wild type and the leakage and rectification currents measured in the pawn mutant is the Ca current. These findings were confirmed by ion substitution experiments. The possibility that inactivation may be apparent only, and caused by a hypothetical, superposed, Ca-induced, rectifying K+ current was eliminated.

Neural excitation of the larval firefly photocyte: slow depolarization possibly mediated by a cyclic nucleotide

1. In firefly larvae, extracellular recordings from the light organ nerve show that a volley of action potentials elicits a glow of an intact animal. 2. Intracellular recordings from the photocytes show that they respond to nerve stimulation with a slow, graded depolarization which precedes light emission. The depolarization begins about 0-5 s after the nerve is stimulated; it peaks about 1 s after stimulation; and subsides about 2-5 s after the stimulus. The glow increases fastest when the photocyte depolarization is at its peak and lasts 5–15 s. 3. Photocyte depolarization is associated with a decrease in the input resistance of the cell. 4. Adrenergic receptors in the light organ are pharmacologically similar to vertebrate alpha-receptors. 5. Phophodiesterase inhbitors, aminophylline and theophylline, cause the light organ to glow, suggesting that cyclic nucleotides may mediate the effect of the adrenergic nerve transmitter.

Ultrastructure of the larval firefly light organ as related to control of light emission

The firefly larva has a pair of light organs consisting of a layer of interdigitating, light emitting cells, covered dorsally with a layer of opaque, white cells. Each light organ is ventilated by one large and several smaller tracheal branches and is innervated by a branch of the segmental nerve containing two axons. These axons branch profusely in the photocyte layer so that several nerve profiles are seen around any photocyte. Nerve terminals contain large dense-core vesicles and small light-core vesicles. Clusters of light-core vesicles surrounding irregularly shaped membrane densifications, presumably the synapses between nerve and photocyte, are common in nerve terminals. Light emitting cells in insects characteristically contain photocyte vesicles. In the larva there are both full and empty photocyte vesicles; the full vesicles contain a matrix with tubular membrane invaginations in contrast to the empty vesicles which contain amorphous membrane invaginations.