Membrane properties of principal neurons of the lateral superior olive

In the lateral superior olive (LSO) the firing rate of principal neurons is a linear function of inter-aural sound intensity difference (IID). The linearity and regularity of the "chopper response" of these neurons have been interpreted as a result of an integration of excitatory ipsilateral and inhibitory contralateral inputs by passive soma-dendritic cable properties. To account for temporal properties of this output, we searched for active time- and voltage-dependent nonlinearities in whole cell recordings from a slice preparation of the rat LSO. We found nonlinear current-voltage relations that varied with the membrane holding potential. Repetitive regular firing, supported by voltage oscillations, was evoked by current pulses injected from holding potentials near rest, but the response was reduced to an onset spike of fixed short latency when the pulse was injected from de- or hyperpolarized holding potentials. The onset spike was triggered by a depolarizing transient potential that was supported by T-type Ca(2+)-, subthreshold Na(+)-, and hyperpolarization-activated (I(H)) conductances sensitive, respectively, to blockade with Ni2+, tetrodotoxin (TTX), and Cs+. In the hyperpolarized voltage range, the I(H), was largely masked by an inwardly rectifying K+ conductance (I(KIR)) sensitive to blockade with 200 microM Ba2+. In the depolarized range, a variety of K+ conductances, including A-currents sensitive to blockade with 4-aminopyridine (4-AP) and additional tetraethylammonium (TEA)-sensitive currents, terminated the transient potential and firing of action potentials, supporting a strong spike-rate adaptation. The "chopper response," a hallmark of LSO principal neuron firing, may depend on the voltage- and time-dependent nonlinearities. These active membrane properties endow the LSO principal neurons with an adaptability that may maintain a stable code for sound direction under changing conditions, for example after partial cochlear hearing loss.

Metabotropic transmitter actions in auditory thalamus

Neurons in the ventral partition of the medial geniculate body (MGBv), the primary auditory thalamus, receive afferent input from the inferior colliculus via excitatory glutamate-ergic and inhibitory GABA-ergic input fibres. The feedback from the auditory cortex to the thalamic relay also is mediated via neuron systems using glutamate and GABA as transmitters. We studied effects on excitability mediated by these transmitters via G-protein coupled metabotropic receptors. In a slice preparation of rat thalamus we investigated the membrane responses of MGBv neurons using the whole cell recording technique. Application of a metabotropic glutamate receptor (mGluR) agonist, ACPD (5-100 microM), depolarized MGBv neurons. As a result, the burst mode of firing, which characterizes states of sleep at hyperpolarized potentials was replaced by the tonic mode, which is compatible with sound signal transmission during alertness. The depolarization was caused by an inward current (I(ACPD)) that persisted during blockade of Na+ channels with tetrodotoxin (TTX) and of Ca2+ channels with Cd2+. The I(ACPD) depended, however, on extracellular Na+, which could be replaced with Li+, excluding a major contribution of the Na+/Ca2+ exchange current. ACPD application also inhibited an inwardly rectifying K+ current at hyperpolarized potentials and activated an outward current in the depolarized range. Application of the GABA(B) agonist, baclofen (10 microM), hyperpolarized MGBv neurons by activation of an inwardly rectifying K+ current. The corresponding membrane conductance acted as a powerful shunt that reduced voltage responses and inhibited firing in both the tonic and burst modes. Thus, the effects of GABA(B) receptor activation would suppress auditory signal transfer, whereas mGluR activation enhances excitability, possibly accounting for the alerting effects of certain auditory stimuli.

Effects of metabotropic glutamate receptor activation in auditory thalamus

Metabotropic glutamate receptors (mGluRs) are expressed predominantly in dendritic regions of neurons of auditory thalamus. We studied the effects of mGluR activation in neurons of the ventral partition of medial geniculate body (MGBv) using whole cell current- and voltage-clamp recordings in brain slices. Bath application of the mGluR-agonist, 1S,3R-1-aminocyclopentan-1,3-dicarboxylic acid or 1S,3R-ACPD (5-100 microM), depolarized MGBv neurons (n = 67), changing evoked response patterns from bursts to tonic firing as well as frequency responses from resonance ( approximately 1 Hz) to low-pass filter characteristics. The depolarization was resistant to Na(+)-channel blockade with tetrodotoxin (TTX; 300 nM) and Ca(2+)-channel blockade with Cd(2+) (0.1 mM). The application of 1S, 3R-ACPD did not change input conductance and produced an inward current (I(ACPD)) with an average amplitude of 84.2 +/- 5.3 pA (at -70 mV, n = 22). The application of the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine (0.5 mM), reversibly blocked the depolarization or I(ACPD). During intracellular application of guanosine 5'-O-(3-thiotriphosphate) from the recording electrode, bath application of 1S,3R-ACPD irreversibly activated a large amplitude I(ACPD). During intracellular application of guanosine 5'-O-(2-thiodiphosphate), application of 1S, 3R-ACPD evoked only a small I(ACPD). These results implicate G proteins in mediation of the 1S,3R-ACPD response. A reduction of external [Na(+)] from 150 to 26 mM decreased I(ACPD) to 32.8 +/- 10. 3% of control. Internal applications of a Ca(2+) chelator, 1, 2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 10 mM), suppressed I(ACPD), implying a contribution of a Ca(2+) signal or Na(+)/Ca(2+) exchange. However, partial replacement of Na(+) with Li(+) (50 mM) did not significantly change I(ACPD). Therefore it seemed less likely that a Na(+)/Ca(2+) exchange current was a major participant in the response. A reduction of extracellular [K(+)] from 5.25 to 2.5 mM or external Ba(2+) (0.5 mM) or Cs(+) (2 mM) did not significantly change I(ACPD) between -40 and -85 mV. Below -85 mV, 1S,3R-ACPD application reversibly attenuated an inward rectification, displayed by 11 of 20 neurons. Blockade of an inwardly rectifying K(+) current with Ba(2+) (1 mM) or Cs(+) (2-3 mM) occluded the attenuation. In the range positive to -40 mV, 1S, 3R-ACPD application activated an outward current which Cs(+) blocked; this unmasked a voltage dependence of the inward I(ACPD) with a maximum amplitude at approximately -30 mV. The I(ACPD) properties are consistent with mGluR expression as a TTX-resistant, persistent Na(+) current in the dendritic periphery. We suggest that mGluR activation changes the behavior of MGBv neurons by three mechanisms: activation of a Na(+)-dependent inward current; activation of an outward current in a depolarized range; and inhibition of the inward rectifier, I(KIR). These mechanisms differ from previously reported mGluR effects in the thalamus.

Firing properties of chopper and delay neurons in the lateral superior olive of the rat

Neurons in the lateral superior olivary nucleus (LSO) respond to acoustic stimuli with the "chopper response", a regular repetitive firing pattern with a short and precise latency. In the past, this pattern has been attributed to dendritic integration of synaptic inputs. We investigated a possible contribution of intrinsic membrane properties using intracellular recording techniques in a tissue slice preparation. We found two electrophysiological classes of neurons in the LSO. Chopper neurons responded to depolarizing current pulses with a single onset spike at short, precise latency close to threshold and with repetitive, regular, but accommodating discharges at greater current intensities. An emphasis of response onset and subsequent rate accommodation resulted from the activation of a voltage- and time-dependent sustained outward rectification in a range depolarized from rest. Responses to hyperpolarizing pulses were characterized by an inward rectification, which caused a depolarizing voltage sag in a range negative to -65 mV. Peristimulus time histograms were multimodal, and discharge regularity was evident in narrow unimodal interspike interval time histograms and low coefficients of variation. The accommodation time course was usually fit best by two exponentials with time constants of tau1=3-8 ms and tau2=32-97 ms. Delay neurons responded with a regular repetitive firing to depolarization by current pulses. However, repetitive spike discharge occurred with a prolonged, variable delay of 25-180 ms. High current intensities evoked an additional onset spike with short, precise latency. Activation of a transient outward conductance in the depolarized voltage range caused an early repolarization, which terminated as a depolarizing ramp, reaching spike threshold after the delay. Flat peristimulus time histograms characterized the repetitive discharge in spite of narrow unimodal interspike interval time histograms and low coefficients of variation. Intracellular neurobiotin injections revealed morphological differences between these classes. Chopper neurons were large and fusiform, with a bipolar dendritic distribution oriented perpendicular to the curvature of the LSO. Delay neurons were small and spherical, with highly branched tortuous dendritic arbours of bipolar origin and variable orientation. Chopper and delay neurons are probably LSO principal cells and lateral olivocochlear efferent neurons, respectively. Our findings suggest that the pattern of firing activity of LSO neurons to sound, in vivo, is determined to a large extent by intrinsic membrane properties. Somato-dendritic integration of synaptic inputs are fundamental to the encoding of interaural sound differences, but membrane non-linearities play an important role in determining postsynaptic response patterns.

Modulation of frequency selectivity by Na+- and K+-conductances in neurons of auditory thalamus

In thalamic neurons, frequency-filter properties arise from intrinsic membrane properties which transform sensory inputs to thalamocortical signals. They also contribute to the tendency for the membrane to generate synchronized oscillations. We studied the frequency selectivities of thalamocortical neurons in the rat ventral medial geniculate body (MGBv) in vitro, using whole-cell recording techniques, sinewave (swept 'ZAP' or single) current inputs and pharmacological blockade of membrane currents. In a voltage range that was subthreshold to spike genesis, the frequency responses below 20 Hz were voltage-dependent, they exhibited lowpass characteristics at depolarized potentials and bandpass resonance (near 1 Hz) in the activation range (approximately -65 to -50 mV) of the low-threshold Ca2+-current (I(T)). A temperature increase of > 10 degrees C in 3 neurons did not change this voltage-dependence and increased the frequency of maximum resonance to 2 Hz. The removal of extracellular Ca2+, its equimolar substitution with Mg2+ or blockade of I(T) with Ni2+ (0.5 mM) completely blocked the resonance at hyperpolarized potentials or rest, as well as the low-threshold Ca2+-spike (LTS). Blockade of high threshold Ca2+-currents with Cd2+ (50 microM) did not affect the resonance. These data implied that, like the LTS, an activation of I(T) produced the membrane resonance. An increased ZAP-current input evoked action potentials near the resonant frequency as well as Cd2+-sensitive high-threshold Ca2+-spikes at depolarized membrane potentials and very low frequencies. By blocking a persistent Na+-current (I(NaP)), tetrodotoxin (300 nM) reduced the magnitude of the frequency response without affecting the frequency preference. The response was larger in amplitude, especially at frequencies lower than the maximum resonant frequency, when we used 4-aminopyridine (0.05-0.1 and 1-2 mM), Ba2+ (0.2 mM) or Cs+ (3 mM) to block voltage-dependent K+-currents. From these data, we suggest that A-type (I(A) and I(As)) and inwardly rectifying (I(KIR)) K+-currents modulate resonance, changing the quality of the lowpass filter function. We conclude that the generation of membrane resonance in MGBv neurons depends critically on I(T)-activation while the quality of the frequency response is subject to modulation by voltage-dependent conductances. The frequency selectivities in MGBv may contribute to lowpass filter functions for auditory transmission during wakefulness and oscillations observed during sleep.

Membrane properties that shape the auditory code in three nuclei of the central nervous system

OBJECTIVE

We investigated if auditory neurons have an intrinsic ability to radically transform auditory signals.

METHOD

We surveyed membrane properties that control coding by neurons, identified with intracellular staining or infrared-DIC videomicroscopy, in three stations of the auditory pathway. We used intracellular and patch-clamp techniques in slices, to study the voltage responses to current pulse injections and distinguished voltage-gated conductances with selective blockers.

RESULTS

First order spherical bushy cells in the anteroventral cochlear nucleus responded at a short, stable latency with single spikes, due to a perithreshold interaction of Na+ and Ca2+ conductances. Two K+ conductances suppressed firing after this onset-spike. Second-order principal neurons of the lateral superior olive use unspecified mechanisms to secure stable onset latencies but maintained a very regular tonic firing, resulting in a chopper pattern. Other intrinsic properties induced a marked accommodation in spike rate. When depolarized as during alert states, neurons in the medial geniculate body (MGB) of the thalamus fired with variable latencies in a tonic mode. At negative resting potentials characteristic of sleep states, they responded at the onset of a depolarization and the offset of a hyperpolarization with phasic bursts due to a transient low threshold Ca2+ current. In the phasic, but not tonic mode, MGB neurons produced high-threshold Ca2+ spikes that may couple signal transmission to the neuron's metabolism. The three neuron types exhibit analogue computing abilities that transform the same input into entirely different output patterns. Isoflurane anaesthesia induces a current shunt in MGB neurons, radically changing the properties and preventing normal responses. Thus, thalamocortical auditory codes are compromised under anaesthesia.

CONCLUSION

At all investigated stations of the auditory pathway, input signals are transformed by activation of voltage-controlled conductances and other intrinsic membrane properties.

Postnatal development of signal generation in auditory thalamic neurons

Using whole cell recording techniques, we distinguished immature from mature stages of development in auditory thalamic neurons of rats at ages P5 to P21. We compared voltage responses to injected currents and firing patterns of neurons in ventral partition of medial geniculate body (MGBv) in slices. Resting potential, input resistance and membrane time constant diminished to mature values between P5 and P14. Responses of young neurons to hyperpolarizing pulses showed delayed inward rectification; after P13, this was obscured by a rapid onset of another inward rectifier. All neurons possessed tetrodotoxin (TTX)-sensitive, depolarization-activated rectification, implying persistent Na+-current involvement. Despite a slightly higher voltage threshold for spiking, the current threshold was lower in younger neurons. Young neurons fired a short latency spike with afterhyperpolarization whereas older neurons exhibited a slow ramplike depolarization before tonic firing. Large currents caused continuous firing in all neurons. Before day P13, a high threshold Ca2+ spike (HTS) often was appended to action potentials. The low threshold Ca2+-spike (LTS) was too small in amplitude to evoke action potentials before P11 but produced a single spike at P12 and P13 and burst firing with HTS after P13. MGBv neurons have mature properties after P14, relevant for reactions to sound and the oscillations of slow-wave sleep.

GABA(B) receptor activation changes membrane and filter properties of auditory thalamic neurons

Inhibitory inputs from nucleus reticularis thalami and the inferior colliculus activate gamma-aminobutyric acid B (GABA(B)) receptors in auditory thalamic neurons. These metabotropic receptors have been implicated in the oscillatory behavior of thalamic neurons. We studied the effects of the GABA(B) receptor agonist, baclofen, on membrane and filter properties of neurons in the ventral partition of the medial geniculate body (MGBv) of the rat, using whole-cell patch-clamp recording techniques in a slice preparation. Application of baclofen caused a concentration-dependent and reversible hyperpolarization of MGBv neurons. An increase in membrane conductance shunted voltage signals. The shunt suppressed firing in both tonic and burst modes which normally characterize the neuronal excitation from depolarized and hyperpolarized potentials, respectively. The GABA(B) receptor antagonist, CGP 35348 (0.5 mM), completely and reversibly blocked the baclofen-evoked hyperpolarization and increase in conductance. In voltage-clamp and during blockade of synaptic transmission with tetrodotoxin and Cd2+, baclofen activated an inwardly rectifying outward K+ current, that was sensitive to blockade with Ba2+ (0.5 mM). Intracellular applications of GTPgammaS occluded the baclofen current whereas similar applications of GDPbetaS prevented it, suggesting that G-proteins mediated the baclofen current. We measured the impedance amplitude profile in the frequency domain with swept sinusoidal current injection. MGBv neurons normally have lowpass filter characteristics at depolarized potentials and resonance at approximately 1 Hz at hyperpolarized potentials. Baclofen application reduced the impedance below 20 Hz which lowered the membrane filter quality and abolished the resonance. Despite its hyperpolarizing effect, therefore, baclofen eliminated an intrinsic tendency to oscillate as well as the intrinsic frequency selectivity of MGBv neurons.

Intrinsic response properties of bursting neurons in the nucleus principalis trigemini of the gerbil

In trigeminal neurons, the spike rate, modulated by input parameters, may serve as a code for sensory information. We investigated intrinsic response properties that affect rate coding in neurons of nucleus principalis trigemini (young gerbils). Using the whole-cell recording technique and neurobiotin staining in slices, we found bursting behaviour in approximately 50% of the neurons. These neurons fired spike bursts, spontaneously, as well as at the onset of depolarizing, and offset of hyperpolarizing, current pulses. The spike rate within an initial burst was independent of stimulus strength, in contrast to single spike firing that occurred later in the response to current pulse injection. The spikes within a burst were superimposed on slow depolarizing humps. Under favourable conditions, these led to "plateau potentials", that lasted for hundreds of milliseconds at membrane potentials near approximately -20 mV. Occasionally, plateau potentials were spontaneous or evoked under control conditions: usually, they were evoked by current pulse injection during blockade of Ca2+ influx with Co2+ or Cd2+ in Ca(2+)-free extracellular media, or during blockade of K+ currents with tetraethylammonium. The plateau potentials recorded during internal Cs+ (132.5 mM) substitution of K+ had more positive amplitudes (near +20 mV). Despite relatively stable depolarization levels, the plateau potentials decreased in duration and decayed in amplitude during application of tetrodotoxin (0.6-1.8 nM). Higher tetrodotoxin concentrations (5-60 nM) eliminated the plateau potentials despite well-maintained, fast action potentials. A reduction of external [Na+] reduced the amplitudes of the spikes and plateau potentials. A hyperpolarization of long duration (> 3 s) followed a plateau potential, or a depolarizing response that was subthreshold for plateau generation. Tetrodotoxin application blocked this after-effect. We suggest that a persistent Na+ influx is a major contributor to the bursts and plateau potentials and that it mediates the hyperpolarization. Depending on Ca2+ influx, K+ conductances may regulate the amplitudes of these long-lasting depolarizations. A Ca2+ conductance, blockable by Ni2+, may support burst initiation in these neurons. In very young animals (P2-P9), we found only non-bursting neurons. Both bursting and non-bursting neurons with elongated dendritic fields showed inward rectification on hyperpolarization. The bursts in nucleus principalis trigemini neurons emphasize the onsets of stimulus transients, at the expense of using firing rate as a sensory code. Our studies describe neurons with a surprising ability to distort sensory signals, transforming depolarizing inputs into bursts of spikes by virtue of a Na(+)-conductance activation. The principal trigeminal nucleus also contains neurons with tonic firing ability, compatible with simple rate coding.

Modulation of bursts and high-threshold calcium spikes in neurons of rat auditory thalamus

Neurons in the ventral partition of the medial geniculate body are able to fire high-threshold Ca2+-spikes. The neurons normally discharge such spikes on low-threshold Ca2+-spikes after the action potentials of a burst. We studied membrane mechanisms that regulate the discharge of high-threshold Ca2+-spikes, using whole-cell recording techniques in a slice preparation of rat thalamus. A subthreshold (persistent) Na+-conductance amplified depolarizing inputs, enhancing membrane excitability in the tonic firing mode and amplifying the low-threshold Ca2+-spike in the burst firing mode. Application of tetrodotoxin blocked the amplification and high-threshold Ca2+-spike firing. A slowly inactivating K+ conductance, sensitive to blockade with 4-aminopyridine (50-100 microM), but not tetraethylammonium (2-10 mM), appeared to suppress excitability and high-threshold Ca2+-spike firing. Application of 4-aminopyridine increased the low-threshold Ca2+-spike and the number of action potentials in the burst, and led to a conversion of the superimposed high-threshold Ca2+-spike into a plateau potential. Application of the Ca2+-channel blocker Cd2+ (50 microM), reduced or eliminated this plateau potential. The tetrodotoxin sensitive, persistent Na+-conductance also sustained plateau potentials, triggered after 4-aminopyridine application on depolarization by current pulses. Our results suggest that high-threshold Ca2+-spike firing, and a short-term influx of Ca2+, are regulated by a balance of voltage-dependent conductances. Normally, a slowly inactivating A-type K+-conductance may reduce high-threshold Ca2+-spike firing and shorten high-threshold Ca2+-spike duration. A persistent Na+-conductance promotes coupling of the low-threshold Ca2+-spike to a high-threshold Ca2+-spike. Thus, the activation of both voltage-dependent conductances would affect Ca2+ influx into ventral medial geniculate neurons. This would alter the quality of the different signals transmitted in the thalamocortical system during wakefulness, sleep and pathological states.

Firing properties of spherical bushy cells in the anteroventral cochlear nucleus of the gerbil

In gerbils, spherical bushy cells (SBCs) encode low frequency sound signals into a temporal firing pattern. To investigate the support for the timing in this temporal code, we characterized the membrane electrical properties of visually identified SBCs in brainstem slices. A brief depolarizing subthreshold transient potential (TP) triggered, with relatively invariant latency, a single spike at the onset of a response to depolarizing current pulses. The activation of a subthreshold Na+-conductance, sensitive to blockade with tetrodotoxin, and a high threshold Ca2+-conductance, sensitive to blockade with Co2+ or Cd2+, accelerated the rising phase and amplified the TP. A K+-conductance, sensitive to blockade by 4-aminopyridine (4-AP, 50 microM), shaped the decay of the TP. Following a single spike, voltage-gated activation of transient and sustained K+-conductances suppressed any tendency to repetitively discharge. A reduction in either K+-conductance due to application of 4-AP or tetraethylammonium (TEA, 10 mM), converted the single spike mode to repetitive firing during the depolarizing pulses. A persistent, tetrodotoxin-sensitive Na+-conductance amplified steady-state depolarizing responses. A hyperpolarization-activated conductance, greatly decreased by extracellular Cs+ (3 mM) but resistant to Ba2+ (up to 1 mM), filtered the responses to hyperpolarizing current inputs. A depolarized membrane potential promoted repetitive firing in SBCs. This state, expected in pathophysiological conditions, would corrupt the temporal code.

Isoflurane attenuates resonant responses of auditory thalamic neurons

In thalamocortical neurons, sensory signals are transformed differently during various states of consciousness. We investigated the effects of a general anesthetic, isoflurane, on the frequency responses of neurons in the ventral medial geniculate body, the primary nucleus of the auditory thalamus. Using slice preparations, whole cell current-clamp recording techniques, and frequency-domain analyses with oscillatory inputs, we observed a resonance in the hyperpolarized voltage range, implying a frequency preference near 1 Hz in the subthreshold frequency responses of medial geniculate neurons. As in other thalamocortical neurons, an interaction of a T-type Ca2+ current with passive membrane properties generates the resonant responses. The frequency preference shapes the input-output signal transformation, coupling oscillatory inputs at preferred frequencies to firing. Thus resonance may contribute to the rhythmic synchronization of the output to the cortex. In a concentration range of 0.5-3%, isoflurane application reversibly decreased the resonant responses of medial geniculate neurons. Throughout the subthreshold voltage range, it reduced impedance at frequencies < 10 Hz. At depolarized potentials near -60 mV, isoflurane reduced the low-pass filter selectivity of the neuron membrane. At rest near -70 mV or at hyperpolarized potentials, isoflurane had a greater effect on resonance (centered at approximately 1 Hz), reducing the peak impedance more than the magnitudes at other frequencies. At concentrations of > or = 2%, isoflurane completely blocked the resonance peak, thereby imposing low-pass characteristics of poor quality throughout the subthreshold voltage range. Application of isoflurane reversibly increased membrane conductance and the current threshold for firing evoked by depolarizing pulses from potentials between -60 and -90 mV. The neurons discharged in a tonic pattern on depolarization from about -60 mV and in a phasic (burst) mode from potentials negative to about -70 mV. An increase in current amplitude compensated the suppression of tonic firing much more readily than that of the burst firing on a low-threshold Ca2+ spike. Although a reduction in T-type Ca2+ channel activation may occur during isoflurane application, the depression of resonance is consistent with an interaction of a greatly increased leak conductance with the low-threshold Ca2+ current and the membrane capacitance. In the intact animal, this would tend to disrupt synchronized neural oscillations and the transfer of auditory information.

Firing modes and membrane properties in lemniscal auditory thalamus

In neurons of the auditory thalamus, patterned sequences of action potentials encode the features of sound stimuli. The patterns vary with the membrane potential, characterizing states of wakefulness and sleep. We studied the dependence of the patterns on the membrane potential and specific voltage-gated conductances, using whole-cell patch-clamp recordings from neurons in the ventral medial geniculate body (MGBv) of in vitro slices. Thalamocortical neurons, identified with neurobiotin, exhibited different firing patterns to an excitatory input, depending on the initial membrane potential. From depolarized potentials, the neurons fired in a tonic mode. The delay to firing in this mode was regulated by a balance of persistent Na+ and A-type K+ conductances. When transiently depolarized from hyperpolarized holding potentials, the neurons fired brief phasic responses (burst mode). Phasic responses were induced by low threshold Ca2+ spikes (LTSs); the LTS-amplitude was controlled by Na+ and K+ conductances. Under favourable conditions, an LTS triggered more than one action potential and one or more high threshold Ca2+ spikes (HTSs). Consciously perceived sound signals are transmitted in the tonic mode. During sleep, alerting stimuli may interact with membrane non-linearities, converting hyperpolarized bursting MGBv neurons to the tonic mode.

Mechanisms for signal transformation in lemniscal auditory thalamus

1. During alertness, lemniscal thalamocortical neurons in the ventral medial geniculate body (MGBv) encode sound signals by firing action potentials in a tonic mode. When they are in a burst firing mode, characteristic of thalamic neurons during some sleep states, the same stimuli may have an alerting function, leading to conscious perception of sound. We investigated the intrinsic membrane properties of MGBv neurons in search of mechanisms that enable them to convert from burst to tonic firing modes, allowing accurate signal coding of sensory stimuli. 2. We studied thalamocortical relay neurons and identified neurons morphologically with injected N-(2-aminoethyl) biotinamide hydrochloride in in vitro slice preparations of young rats. With the use of the whole cell recording method, we examined the contributions of distinct conductances to voltage responses evoked by current pulses. The neurons (n = 74) displayed a narrow range of resting potentials (-68 +/- 4 mV, mean +/- SD) and an average input resistance of 226 +/- 100 M omega. The membrane time constant was 40 +/- 17.6 ms and the action potential threshold was -51.6 +/- 3 mV. 3. Injections of hyperpolarizing current pulses from rest revealed an inward rectification produced by two voltage-dependent components. A fast component, sensitive to blockade with Ba2+ (100-200 microM), was attributed to an inward rectifier, IIR. Such applications also increased input resistance and depolarized neurons, consistent with a blockade of various K+ conductances. Application of Ba2+ often unmasked another voltage-dependent rectification with a slower time course. The second component was sensitive to blockade with Cs+ (1.5 mM), reminiscent of a hyperpolarization-activated current, IH. 4. Depolarizing pulses from rest produced ramp-shaped voltage responses that led to delayed tonic firing. Blockade of Na+ conductances by tetrodotoxin (TTX, 300-600 nM), or extracellular replacement of Ca2+ with Mg2+ (with TTX present), reduced the slope of the ramp and the overall depolarizing response. Application of 4-aminopyridine (4-AP, 100 microM), a blocker of A-type K+ conductances, increased input resistance and the overall depolarizing response. The voltage ramp therefore represents a complex rectification due to voltage-dependent contributions of persistent Na-, Ca2+, and K+ conductances. 5. Depolarizing pulses from potentials of less than -75 mV evoked phasic burst responses, consisting of one to seven action potentials riding on a low-threshold spike (LTS). The LTS was absent in low extracellular Ca2+ conditions and was blocked by application of Ni2+ (0.6 mM), but not by Cd2+ (50 microM). Similar depolarization from less than -80 mV evoked several action potentials, often followed by a TTX-resistant high-threshold spike (HTS) of longer duration. Firing of HTSs always occurred during 4-AP (100 microM) application, inferring that, normally, A-type K+ conductances may control ability to fire an HTS. As in the LTS, a Ca2+ current is a major participant in the HTS because extracellular replacement of Ca2+ with Mg2+ or application of Cd2+ (50 microM) blocked its genesis. After TTX blockade of Na+ conductances, "tonic firing" of HTSs occurred during depolarization above -45 mV. 6. During tonic firing evoked by current pulses, the second and subsequent spikes were longer in duration than the initial action potentials. Low extracellular concentrations of Ca2+ or Cd2+ (50 microM) application reduced the durations of the nonprimary spikes, inferring a contribution of high-threshold voltage-dependent Ca2+ conductances to their repolarizing phase. Also, K+ conductances may contribute to spike repolarization, because 4-AP (100 microM) or tetraethylammonium (2 mM) application led to prolonged action potentials and the generation of plateau potentials. A fast afterhyperpolarization, likely mediated by a Ca(2+)-dependent K+ conductance, limited the tonic firing. Such conductances, therefore, may regulate the re

Intonation of musical intervals by musical intervals by deaf subjects stimulated with single bipolar cochlear implant electrodes

Some subjects with cochlear implants have been shown to associate electrical stimulus pulse rates with the pitches of musical tones. In order to clarify the role of these pitch sensations in a musical context, the present investigation examined the intonation accuracy achieved by implant subjects when adjusting pulse rates in the reconstruction of musical intervals. Using a method of adjustment, the subjects altered a variable pulse rate, relative to a fixed reference rate, on one electrode, in the tuning of musical intervals abstracted from familiar melodies. At low pulse rates, subjects generally tuned the intervals to the same frequency ratios which define tonal musical intervals in normal-hearing listeners, with error margins comparable to musically untrained subjects. Two subjects were, in addition, able to transpose these melodic intervals from a standard reference pulse rate to higher and lower reference rates (reference and target pulse rates with geometric means of the intervals ranging from 81 to 466 pulses/s). Generally, the intervals were adjusted on a ratio scale, according to the same frequency ratios which define analogous acoustical musical intervals. These results support the hypothesis that, at low pulse rates, a temporal code in the auditory nerve alone is capable of defining musical pitch.

Melody recognition and musical interval perception by deaf subjects stimulated with electrical pulse trains through single cochlear implant electrodes

The perception of musical pitch was investigated in postlinguistically deaf subjects with cochlear implants. Stimuli consisted of sequences of biphasic electrical pulse trains at rates which represented the tones of the equal-tempered musical scale, delivered at equalized comfortable loudness levels to selected single bipolar electrodes along the array of the Nucleus cochlear implant. Seventeen subjects correctly identified a mean of 44% of rhythmically intact familiar tunes, presented in an open-set paradigm. Three subjects were tested with a closed set of melodies without rhythmic cues. The results showed relatively higher recognition scores at lower pulse rates, although melody recognition remained possible up to rates of approximately 600-800 pulses per second. Stimulation of apical electrodes yielded higher recognition scores than of basal electrodes. The perception of musical intervals, defined as frequency ratios between two trains of stimulus pulse rates, was investigated in an interval intonation labeling experiment, for intervals ranging from a minor 3rd to a major 6th. Within a range of low pulse rates, subjects defined the intervals mediated by electrical pulse rate by the same ratios which govern musical intervals of tonal frequencies in normal-hearing listeners. It may be concluded that temporal cues are sufficient for the mediation of musical pitch, at least for the lower half of the range of fundamental frequencies commonly used in music.

Flow cytometric analyses of the subclasses of red cell IgG antibodies

We report on flow cytometric IgG subclass determinations of red cell antibodies using polyclonal FITC-labeled antibodies. The limit of detection of this method was 1 ng anti-D per 1 x 10(7) red cells. The inter- and intra-assay coefficients of variance were 8.2 and 2.3%, respectively. In 8 newborns with a positive direct antiglobulin test (DAT) in the gel centrifugation test (GCT), due to ABO antibodies, IgG1 was detected in all and IgG2 additionally in 4 of these cases. In 5 severe cases of hemolytic disease of the newborn (HDN) due to anti-D, large amounts of IgG1 were found, and in 3 of these 5, IgG3 in combination with IgG1. In 8 mild or moderate HDN cases (4 anti-D, 2 anti-E, 1 anti-Fya, 1 anti-Jka), phototherapy sufficed, and IgG1 was the only antibody. In 7 adult patients with malignant lymphoma and a positive DAT (GCT), only small amounts of IgG1 red cell autoantibodies could be demonstrated by flow cytometry. In 5 further patients with malignant lymphoma, a positive DAT, and severe hemolytic anemia, large amounts of IgG1 autoantibodies were found and IgG3 was also present in 3 of these cases. Flow-cytometric determination of IgG subclasses may be a useful tool in immunohematology, since subclass determinations were possible in all of these cases. This method is suited for clinical routine and offers the possibility of sufficient standardization.

Plasticity in human directional hearing

Interaural time difference (ITD), the main cue for localization of low-frequency sound in azimuth, is widely thought to be evaluated according to Jeffress' model. This theory proposes that each of an array of neurons detects coinciding input from both ears, conducted along axonal delay lines, with the azimuth angle corresponding to the activation of selected neurons. Thus, sound source localization is assumed to depend on axon conduction velocities, a relatively fixed parameter. Clinical experience suggests that directional hearing is adaptable. We investigated if sound localization in azimuth could adapt plastically to altered ITDs. We equipped binaural insert hearing aids with adjustable electronic delay lines. Subjects with normal hearing were required to wear these devices during all waking hours for several days. Localization of an invisible sound source was measured in an anechoic room before and at various intervals after introduction of a constant delay in one ear between 171 and 684 mus. Test sounds were high-pass, low-pass and broad-band noises. Introduction of a delay in one ear lead to an immediate displacement of the perceived sound location towards the opposite side. Within hours of exposure, the displacement was reduced, and further normalization of the perceived localization occurred over several days. After removal of the delays sound localization normalized rapidly. We conclude that ITD alterations can lead to plastic adaptation of directional hearing, which cannot rely exclusively on fixed axon conduction velocities. Our results suggest additional mechanisms for directional hearing on the basis ITD.

Electrical resonances in central auditory neurons

In the auditory pathway, signal processing depends on the filter functions of neurons. We used frequency analysis to investigate the contributions of intrinsic membrane properties to the input-output relationships in neurons. The whole-cell tight-seal recording technique in brain slices of chicks was used to study neurons at four levels of the auditory pathway. Neurons displayed resonant peaks in their voltage responses to injected sinusoidal currents that swept through a specified frequency range. Higher resonant frequencies tended to predominate at relatively lower stations in the auditory pathway (approximately 100 Hz in the nucleus magnocellularis, 24 Hz in the nucleus laminaris, 6 Hz in the nucleus ovoidalis). Field L neurons (cortex homologue) displayed low pass filter characteristics without resonance. We propose that the subthreshold membrane resonances amplify synaptic inputs at specific frequencies and contribute to the chick's ability to decode temporal sound parameters.

Connections of the superior olive in the chicken

The avian superior olive (OS) is known to be a station in the auditory pathway, although its anatomic connections remain uncertain. The afferent and efferent connections of OS neurons in the chicken were identified with wheat germ agglutinin conjugated to horse radish peroxidase (WGA-HRP) injected into the OS nucleus. Projections to the OS originate bilaterally in the cochlear nuclei (nucleus angularis) and the nucleus laminaris. Anterogradely labelled axon terminals were found in the ipsilateral nucleus magnocellularis, the contralateral intermediate nucleus of the lateral lemniscus, and the shell portion of the central nucleus of the inferior colliculus. Retrograde transport of [3H]-glycine from the OS was also charted. Glycine-transporting cells were found ipsilaterally in the nucleus angularis and the nucleus laminaris. Neuronal soma in a newly identified nucleus of the trapezoid body (NTB) were found to actively concentrate glycine, although the neurons probably do not synapse within the OS. Anatomically, the avian OS would appear to be part of the interaural intensity difference pathway; however, our data and published information are insufficient to establish a homology to the human lateral superior olive.

Retrograde labelling of auditory brainstem neurons following tritiated glycine injection into the inferior colliculus of the chicken, Gallus domesticus

We investigated retrograde labelling with tritiated glycine injected into the inferior colliculus of the chicken. Tracer deposits were placed in the central nucleus of the inferior colliculus at positions yielding unit activity with best frequencies between 0.4 and 4 kHz. Most neuron systems known to project to the inferior colliculus in birds were unlabelled, whereas strongly labelled cells were found in three nuclei, only on the ipsilateral side. The nucleus lemnisci lateralis pars ventralis contained numerous small glycine-transporting cells. The superior olivary nucleus contained few such cells of similar size in its peripheral region. The nucleus of the trapezoid body contained a group of larger labelled neurons. The observed specificity suggests that we labelled glycinergic neurons projecting to the inferior colliculus.

Mode of firing and rectifying properties of nucleus ovoidalis neurons in the avian auditory thalamus

1. We studied neurons of the nucleus ovoidalis, the principal auditory thalamic relay nucleus of the chicken, with tight-seal whole-cell recording techniques in in vitro slice preparations. Nucleus ovoidalis, marked by anterograde labeling of afferents from the inferior colliculus, consists of a clearly delineated group of densely packed, multipolar cells of approximately uniform diameter. We measured a wide range of non covarying resting potentials (-60 +/- 9 mV, mean +/- SD) and input resistances (277 +/- 168 M omega). All neurons discharged overshooting fast spikes. The observed electrophysiological properties may have a decisive role in the transfer of sensory signals. 2. We grouped neurons on the basis of their firing patterns, in response to intracellular injections of depolarizing current pulses from various membrane potentials. The majority of neurons (86%) displayed weakly adapting, tonic firing. A smaller group of neurons (14%) exhibited qualitative changes in firing modes. They fired repetitively when the stimulus pulse was superimposed on relatively depolarized levels, usually including the resting potential. DC-hyperpolarization led to burst responses consisting of fast action potentials on top of slow potentials. 3. In all neurons, application of 300 nM tetrodotoxin blocked the action potentials and reduced a depolarization-activated inward rectification, observed during 1-s current pulses in a range of membrane potentials depolarized from rest. This rectification is interpreted as a partial result of a persistent Na+ current. 4. During the applications of tetrodotoxin in neurons with burst firing capability, two other slow potentials were visible in isolation. Depolarizing current pulses evoked slow, transient depolarizations at the onset whereas rebound slow potentials occurred on termination of hyperpolarizing current pulses. The slow potentials were blocked by application of 0.5 mM Ni2+ and are likely a result of a low threshold Ca2+ current, such as a T-current. 5. A distinctive property of all ovoidalis neurons was a hyperpolarization-activated inward rectification. Application of Cs+ (3 mM) but not Ba2+ (3 mM), tetraethylammonium (10 mM), or 4-aminopyridine (4 mM) reversibly blocked the current that produced this rectification. The activation time constants for the current varied between approximately 50 and 400 ms and were voltage dependent in some neurons. Thus the hyperpolarization-activated current (IH), responsible for thalamic sleep mechanisms in mammals, also is present in a submammalian thalamus. 6. We suggest that the voltage and time dependencies of the persistent Na+ current and IH participate in generation of the sub- and suprathreshold temporal activity patterns in the neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Subthreshold frequency selectivity in avian auditory thalamus

1. We studied the frequency responses of neurons in the nucleus ovoidalis (OV), the principal thalamic auditory relay nucleus of the chicken, in the subthreshold range of membrane potentials. The frequency response is the impedance amplitude profile evident in the voltage response to a broadband stimulus. The stimulus was a deterministic periodic current input of small amplitude, sweeping through a specified frequency range. We used whole-cell, tight-seal recording techniques in slices to study the voltage responses and membrane properties in current and voltage clamp. 2. Generally, low-frequency resonant humps with peak impedances of approximately 6 Hz characterized the frequency responses of OV neurons. This resonance was the principal determinant for frequency selectivity in the majority of OV neurons expressing only a tonic mode of firing. 3. The 6-Hz resonance was voltage dependent and most distinct where the activation ranges of a hyperpolarization activated inward current (IH) and a persistent Na+ current tend to overlap. The potential range for optimal resonance often included the resting potential. 4. Application of the Na+ current antagonist, tetrodotoxin, blocked the persistent Na+ current and most of the resonant hump at depolarized levels but did not affect the resonant peak along the frequency axis. Thus the persistent Na+ current may serve to amplify the resonance. 5. Extracellular application of Cs+, but not Ba2+, blocked a voltage sag during pulsed hyperpolarization as well as the IH current. Application of Cs+ also eliminated the 6-Hz resonance. An IH seems, therefore, instrumental for the resonance. 6. A minority of neurons that expressed low-threshold Ca2+ spikes and burst firing at hyperpolarized states displayed voltage oscillations at 2-4 Hz, spontaneously or in response to pulsatile stimuli. Application of Ni2+ blocked the oscillations and the low-threshold spikes, presumably produced by a T-type Ca2+ current. The resonance at 6 Hz, however, was only slightly affected by Ni2+. A T-type current, therefore, is critical for the 2- to 4-Hz oscillations. 7. Membrane resonance may dominate the power spectrum of subthreshold potential fluctuations. The resonance demonstrated in vitro may be stabilized by experimental procedures; its frequency may be different and more variable in vivo. Resonances in thalamic neurons may play a role in auditory signal processing in birds.

Phrenic nerve reinnervation of the cat's larynx: a new technique with proven success

Reinnervation of the posterior cricoarytenoid muscle (PCA) should provide vocal cord abduction on inspiration, and passive adduction to enable phonation. Previous investigators have shown that reinnervation is possible, but results have not been clinically encouraging. When reinnervation was successful, the question remained whether it was provided by the transplanted nerve or by the ingrowth of adjacent nerves. In this study the phrenic nerve was transplanted directly into the PCA in a series of 12 cats. Fibrin glue was used to overcome nerve trauma and to prevent retraction of the nerve from the PCA. Laryngoscopy, electromyography, and retrograde labeling of the phrenic motoneurons provided evidence of functional reinnervation in 9 cats. Partial or complete failure in the remaining 3 was due to retraction of the nerve from the muscle. These results appear to justify trials of the procedure in humans.

A survey of auditory brainstem nuclei in the chicken (Gallus domesticus) with cytochrome oxidase histochemistry

The chicken's auditory brainstem nuclei from the cochlear nuclei to the nuclei of the lateral leminiscus were studied with cytochrome oxidase histochemistry. A strong reactivity in the cochlear and laminar nuclei was confirmed. Additional structures displaying high activity levels include the superior olive and both partitions of the nucleus intermedius lemnisci lateralis. Unilateral cochlea removal led to a strong reduction of activity in the cochlear nuclei and the nucleus laminaris, whereas there was no remarkable effect in higher brainstem centers. After bilateral cochlea extirpation auditory structures still displayed higher enzyme levels than most other nuclei. These observations point to the extraordinary metabolic activity in the ascending auditory pathway which is largely independent of sensory input from the auditory nerve.

A survey of the auditory midbrain, thalamus and forebrain in the chicken (Gallus domesticus) with cytochrome oxidase histochemistry

The chicken's central auditory nuclei from the inferior colliculus to field L in the forebrain were studied with cytochrome oxidase histochemistry. All stations of the ascending pathway displayed high activity levels, including the inferior colliculus, the nucleus ovoidalis of the thalamus, and field L1 to L3 and the hyperstriatum ventrale caudale which correspond to primary and secondary auditory cortex. In the inferior colliculus a moderately active external nucleus could be distinguished from a more intensely stained central and superficial nucleus. In the central nucleus there was a lateral shell displaying stronger neuropil reactivity than a central core. Unilateral cochlea removal caused no remarkable effect in tectum and thalamus. The auditory forebrain contralateral to the lesion displayed reduced CO reactivity compared with the ipsilateral side. After bilateral cochlea extirpation auditory structures still displayed higher enzyme levels than most other nuclei.

Frequency selectivity of central auditory neurons without inner ear

A group of neurons in the inferior colliculus (IC) of the chicken displayed frequency selectivity after surgical removal of both cochleae. Characteristic neuronal frequencies were evident in the following three measures. i) The cells fired spontaneously with discrete preferred interspike intervals; ii) Impulse responses of these neurons to electrical stimuli of cochlear nerves displayed oscillations at the preferred frequencies; iii) When the cochlear nerves were stimulated with a random pulse sequence, a reverse correlation analysis showed that the cells preferred the same frequencies in the stimulus input. Preferred frequencies observed thus far covered over 4 octaves of the auditory range. These "oscillating cells" were found only in a small rostromedial area in the IC. Neuronal frequency selectivity may serve a temporal analysis of sound and underlie sound identification with certain cochlear implants. It may also support interaural crosscorrelation necessary for directional hearing.

The distribution of neurons labelled retrogradely with [3H]-D-aspartate injected into the colliculus inferior of the cat

It is important to know if the transmission of sound signals through the inferior colliculus is mediated by the transmitters glutamate or aspartate because of pharmacological consequences for auditory perception. In order to identify candidate's neurons, the retrograde transport for [3H]-D-aspartate, injected into the left inferior colliculus, was studied in cats. Labelled cells were found in the dorsal and intermediate lateral lemniscal nuclei, mainly on the contralateral side. The cochlear nuclei, superior olivary nuclei and the auditory cortex were not labelled in brains containing other labelled neurons at greater distances from the injection site. Labelled cells were found in the reticular formation and adjacent nucleus coeruleus, the parabrachial nuclei, raphe nuclei (magnus, dorsalis and centralis superior), nucleus prepositus hypoglossi, lateral hypothalamus and hippocampal CA1.

Location of motoneurons innervating the middle ear muscle of the chicken, (Gallus domesticus)

The motoneuron pool for the musculus columellae, the avian equivalent to the m. stapedius, was identified by retrograde labeling with WGA-HRP. It consists of a discrete group of approximately 65 neurons located along the dorsolateral border in the ventral subnucleus of the facial nuclear complex. Other facial motoneurons were only labeled when diffusion of the tracer into neighbor structures was not excluded. The dorsal subnucleus of the facial nerve innervates the m. depressor mandibulae.

Sound delay lines in the nucleus laminaris of the chicken

Delays of neurophonic potentials (NP) induced by monaural sound stimuli were measured across the three dimensions in the nucleus laminaris (NL) of the anesthetized chicken. Peak latencies and delays in cross-correlograms changed with recording distance. An orderly delay line was observed across the NL thickness, that is, along dendritic trees of individual fusiform cells (FC), where phase lags increased dorso-ventrally during ipsi- and in the opposite direction during contralateral stimuli. Delays along isofrequency FC arrays were variable, with delay ranges being smaller for ipsilateral than for contralateral sound stimuli. Net delays for contralateral sounds were directed medio-laterally and differences between ipsi- and contralateral delays covered, roughly, intercochlear time differences (ITD). The observed delays are thought to contribute to sound localization and frequency analysis.

Cochlear efferent neurons projecting to both ears in the chicken, Gallus domesticus

Different retrograde neuroanatomical tracers were injected into each cochlea of adult chicken. The number of cells labeled in the cochlear efferent cell group found bilaterally within the caudal pontine reticular formation depended upon the tracer, with True Blue and Fluoro Gold yielding maximal average counts of 332 efferent neurons per injection. Double labeling of less than 1% of these cells was possible with the combination of True Blue and Diamidino Yellow. Thus the contribution of efferent neurons with axon collaterals projecting to both ears is not fundamentally different in birds and other vertebrates.

Can central neurons reproduce sound waveforms? An analysis of the neurophonic potential in the laminar nucleus of the chicken

Extracellular field potentials in response to pure tones, clicks and noise were recorded with microelectrodes from the laminar nucleus (NL) of the anesthetized chicken. Slow "on" and "off" potentials reversed polarity with recording depth, indicating that synaptically activated dendrites of fusiform cells were dorsal for ipsilateral and ventral for contralateral stimuli. Oscillations at sound frequency were found to be maintained for the duration of the stimulus (neurophonic potential, or NP). In contrast to slow "on" and "off" potentials, NPs were gradually shifted in phase as the electrodes penetrated the NL from dorsal to ventral. Neurophonic oscillation frequencies obtained with clicks and noise were equal to best pure tone frequencies yielding maximal NP amplitudes. Autocorrelation functions calculated from steady state NPs in response to pure tone stimuli indicated a presence of sine waves in noise, and power spectra typically consisted of single frequency components. NPs, slow "on" and slow "off" potentials were sharply tuned over similar frequency ranges and tuning tended to be sharper for higher frequencies. The NP represents an electrical replica of the sound waveform which may be present across the fusiform cell membrane.

Spectral response patterns of auditory cortex neurons to harmonic complex tones in alert monkey (Macaca mulatta)

1. The auditory cortex in the superior temporal region of the alert rhesus monkey was explored for neuronal responses to pure and harmonic complex tones and noise. The monkeys had been previously trained to recognize the similarity between harmonic complex tones with and without fundamentals. Because this suggested that they could preceive the pitch of the lacking fundamental similarly to humans, we searched for neuronal responses relevant to this perception. 2. Combination-sensitive neurons that might explain pitch perception were not found in the surveyed cortical regions. Such neurons would exhibit similar responses to stimuli with similar periodicities but differing spectral compositions. The fact that no neuron with responses to a fundamental frequency responded also to a corresponding harmonic complex missing the fundamental indicates that cochlear distortion products at the fundamental may not have been responsible for missing fundamental-pitch perception in these monkeys. 3. Neuronal responses can be expressed as relatively simple filter functions. Neurons with excitatory response areas (tuning curves) displayed various inhibitory sidebands at lower and/or higher frequencies. Thus responses varied along a continuum of combined excitatory and inhibitory filter functions. 4. Five elementary response classes along this continuum are presented to illustrate the range of response patterns. 5. "Filter (F) neurons" had little or no inhibitory sidebands and responded well when any component of a complex tone entered its pure-tone receptive field. Bandwidths increased with intensity. Filter functions of these neurons were thus similar to cochlear nerve-fiber tuning curves. 6. "High-resolution filter (HRF) neurons" displayed narrow tuning curves with narrowband widths that displayed little growth with intensity. Such cells were able to resolve up to the lowest seven components of harmonic complex tones as distinct responses. They also responded well to wideband stimuli. 7. "Fundamental (F0) neurons" displayed similar tuning bandwidths for pure tones and corresponding fundamentals of harmonic complexes. This response pattern was due to lower harmonic complexes. This response pattern was due to lower inhibitory sidebands. Thus these cells cannot respond to missing fundamentals of harmonic complexes. Only physically present components in the pure-tone receptive field would excite such neurons. 8. Cells with no or very weak responses to pure tones or other narrowband stimuli responded well to harmonic complexes or wideband noise.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiology of the electrically and mechanically damaged cochlea

Electrical and mechanical stimuli were used in an attempt to cause cochlear deafness in a preparation with a rich supply of afferent cochlear neurons. Hearing sensitivity was assessed by electrocochleography and neuron survival was estimated by evaluating electrically induced auditory brainstem responses (EABR). Charge balanced sinusoidal alternating currents between 1 and 30 kHz for up to 15 hours produced a limited high frequency hearing loss when applied through the intact round window. A similar permanent threshold shift (PTS) could be induced by mechanical irritation with a scala tympani electrode through a round window fenestration. There is a summation of electrical and mechanical damage; however, complete deafness never occurred and the EABR provided no evidence for a major retrocochlear damage. These results suggest that deafness associated with perilymph leakage or induced during certain types of ear surgery should not be accepted as inevitable.

Transmitter neurochemistry of the efferent neuron system innervating the labyrinth

It is likely that several mechanisms contribute to the efferent control of cochlear and vestibular function. Different effects are probably mediated by different neuronal transmitters. In spite of a number of transmitter candidates, it is still widely assumed that the entire efferent system can be globally characterized as cholinergic. We attempted to label retrogradely identified efferent neurons in the brainstem with a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine (ACh) synthesizing enzyme. Only a portion of the vestibular efferents could thus be shown to be cholinergic in the rat. Medial cochlear efferents, terminating under outer hair cells, may also be cholinergic since they stain intensely for acetylcholine esterase (AChE) after pre-treatment with the AChE inhibitor diisopropylfluorophosphate (DFP). The lateral cochlear efferents terminating under inner hair cells, as well as more than half of the vestibular efferent neuron population, reacted negatively with either method designed to identify cholinergic neurons. Half of the lateral olivo-cochlear neuron population filled retrogradely with tritiated gamma-amino butyric acid [( 3H]-GABA). These cells were similar in size and distribution to neurons staining for the GABA synthesizing enzyme glutamic acid decarboxylase (GAD). Retrograde transport of [3H]-aspartate from the inner ear to the brainstem was seen in half of the lateral olivocochlear population, as well as in part of the efferent vestibular population in group E and in the caudal pontine reticular nucleus (CPR). Since various peptides have also been located in efferent neurons, this system is chemically diversified. Several distinct mechanisms of efferent control with presumably differing functions must, therefore, exist.

Secondary vestibular and neck position signals in the vestibular nuclei of alert rhesus monkeys performing active head movements

Rhesus monkeys were trained to track a visual target with head and eye movements in order to study central vestibular neurons under natural conditions. Single unit recordings of cells in the vestibular nuclei were obtained during active head rotations in the horizontal plane, and also during passive copies of these self-induced movements. Most cells exhibited secondary responses immediately following the primary vestibular responses to active or passive rapid head movements. They were of opposite polarity to the primary responses, and generally rate enhancements of secondary responses were of greater amplitude than rate suppressions. In addition, vestibular nuclei cells also encoded tonic neck position. The corresponding signal consisted of a variation in the basal discharge rate as a function of neck, and thus head, position. These observations prove for the first time that dynamic and static response characteristics recorded earlier from lesioned brains and under anaesthesia are, at least qualitatively, representative for the normally behaving vestibulomotor system in primates.

Perception of the missing fundamental in nonhuman primates

In preparation for neurophysiological experiments aimed at mechanisms of pitch perception, four rhesus monkeys were trained to press a button when the fundamental frequencies (missing or present) of two complex tones in a tone pair matched. Both tones were based on a five-component harmonic series. Zero to three of the lowest components could be missing in the first tone, while the second (comparison) tone contained all five harmonics. The range of fundamentals tested varied from 200 to 600 Hz. Three monkeys learned to match tones missing their fundamentals to comparison harmonic complexes with the same pitch, whereas the fourth monkey required the physical presence of the fundamental. Consideration of several cues available to the monkeys suggests that the animals could perceive the missing fundamental.

Histopathology of chloroform-induced inner ear damage

Inner ear function loss was caused in guinea pigs and rats by injecting chloroform into the middle ear. After symptoms for cochlear and vestibular deficit had been registered, the animals were permitted to survive for one day to five months. Ear histopathology was then studied in celloidin sections. In both species, hair cells and afferent nerve fibers were intact at all survival times. The acute stage of functional loss in guinea pigs was associated with inner ears of normal histological appearance. Within days after chloroform injection a severe otitis media developed which led to fibrous occlusion of the round window and eventually to new bone growth in the middle ear space around the otic capsule. A secondary labyrinthitis was also observed, resulting in endolymphatic hydrops at longer survival times. Different histopathological changes were seen in rats. The tectorial membrane appeared swollen in all cases, the swelling being more severe in more apical turns at longer survival times. It is concluded that only secondary sequela of the initial functional insult can be detected by standard light microscopic histopathology. Chloroform does not cause a chemical labyrinthectomy as previously assumed, although it is severely ototoxic.

Retrograde transport of [3H]-D-aspartate label by cochlear and vestibular efferent neurons

[3H]-D-aspartic acid was injected into the inner ear of rats. After a six hour survival time, labeled cells were found at all locations known to contain efferent cochlear or vestibular neurons. Most labeled neurons were found in the ipsilateral lateral superior olivary nucleus (LSO), although both ventral nuclei of the trapezoid body (VTB), group E, and the caudal pontine reticular nucleus (CPR) just adjacent to the ascending limb of the facial nerve also contained labeled cells. Because not all efferent neurons in the rat could be previously shown to be cholinergic, aspartate and glutamate are efferent transmitter candidates.

Retrograde transport of [3H]-GABA by lateral olivocochlear neurons in the rat

Injection of [3H]-gamma aminobutyric acid (GABA) into the perilymphatic space of the rat's inner ear resulted in retrograde labeling of a portion of the small efferent olivocochlear neurons within the lateral superior olivary nucleus (LSO). These cells were of similar size as LSO neurons stained immunohistochemically with antibodies to the GABA synthesizing enzyme glutamic acid decarboxylase (GAD). They were of fusiform shape, but smaller than principal LSO cells, which did not stain for GAD and did not accumulate [3H]-GABA. Other efferent cochlear and vestibular neurons were not labeled.

Sound code for cochlear implants

This is a review of how various commercially available cochlear prostheses encode speech and complex tones. Arguments in favor of temporal coding through relatively few channels are presented. Advantages of analogue over pulsatile stimuli are explained. Parameters responsible for perception of vowels, consonants, musical pitch and loudness are didactically analyzed. Active rehabilitation is considered an integral part of the coding effort.

A complex tone code in the auditory cortex

In the search for an objective measure of a biologically meaningful encoding scheme for cochlear implants, we recorded responses to tones from the alert auditory cortex. Neurons in the primary auditory field (A1) are typically characterized by filter characteristics resembling those of eighth nerve afferent fibers (filter neurons). In contrast, a different class of neurons has been found thus far only outside of the cochleotopic A1 array. These cells are sharply tuned to pure tones, and corresponding fundamental frequencies of harmonic complexes (F0 neurons). Thus these cells are specific for various tones of the same pitch. Sharpened pure- and complex-tone tuning can be accounted for by lateral inhibition. Time patterns for tonal and noise responses can vary dramatically.

Electrophysiological evaluation of chloroform-induced inner ear damage

Local placement of chloroform in either the external or the middle ear has been previously reported to induce a chemical labyrinthectomy. In order to examine the value of this effect as a research tool, we injected chloroform into the middle ears of guinea pigs and rats. Cochlear damage was assessed by electrocochleography (ECochG) and auditory brainstem response (ABR) audiometry. Both species developed complete deafness within a few hours after instillation of the chloroform. The deafness was permanent in the guinea pigs, whereas there was a partial recovery of auditory function in the rats. The survival rate of the auditory nerve fibers was estimated by measuring the ABR evoked by electrical stimulation via the scala tympani (EABR). A normal EABR recruitment pattern suggested that the main chloroform effect was located peripheral to the afferent axons. In conclusion, chloroform must be considered a severely ototoxic agent when applied locally.

Vestibular nuclear neuron activity during active and passive head movement in the alert rhesus monkey

Responses of single neurons were recorded in the medial and descending vestibular nuclei (MVN and DVN) and in the deep cerebellar nuclei of three juvenile rhesus monkeys (Macaca mulatta). Neuronal activity was measured during both passive sinusoidal and nonsinusoidal whole body rotation (peak velocities were under 90 degrees/s) and during active head movements. Although the active head movements occasionally exceeded 300 degrees/s, most exhibited peak velocities of less than 200 degrees/s. A total of 133 units sensitive to horizontal head rotation were recorded, and of these, 38 were held for sufficient time to obtain both passive and active head movement data. Comparison of the neuronal firing patterns obtained during active and passive head movements revealed no apparent differences. Thus neurons that were observed to burst or pause during saccades with the head fixed continued to do so when the head was free. Both the sensitivity to head velocity and the "inferred" spontaneous firing rate were compared during active and passive head movements by plotting rate-velocity curves for both conditions. When the data points were fitted with linear regression lines, no statistically significant differences in either sensitivity or spontaneous rate were found. The present study provides no evidence that efferent vestibular activity alters the properties of afferent vestibular neurons during active head movements, as has previously been suggested (21). Furthermore, neurons in the rostral portions of the vestibular nuclei in primates encode head velocity based entirely on labyrinthine information. Neither neck proprioceptors nor an efference copy of the head movement motor program seem to contribute significantly to the firing patterns observed.

Projection of afferents from individual vestibular sense organs to the vestibular nuclei in the pigeon

The projection of individual labyrinthine sensory organs to the brain stem was studied by autoradiography, employing discrete [3H]leucine injections into the sensory epithelia. Within the vestibular nuclei, separate partly overlapping termination areas for each end organ were found in the superior and descending vestibular nuclei, whereas projection territories in the medial, ventrolateral and tangential nuclei overlapped extensively. A few lagenar fibres terminated in the external cuneate nucleus. Semicircular canals and utricular macula also project to the lateral cerebellar nucleus and the reticular formation. For each semicircular canal a projection system could be traced to distinct subgroups of the extraocular motoneuron pools.

Cholinergic innervation of the rat's labyrinth

Efferent vestibular and cochlear neurons were identified in the rat's brain stem by retrograde labelling with True Blue (TB) or wheat germ agglutinin - horseradish peroxidase (WGA-HRP) injected into the utricle. Such cells were found at the same locations described in 1983 by White and Warr (ipsilateral superior olivary nucleus (LSO), bilateral latero-ventral nucleus of the trapezoid body (LTz) bilateral group E medial and lateral to the genu facialis) and, in addition, bilaterally in the caudal pontine reticular nucleus (CPR) at the level of the descending facial nerve. Cholinergic neurons were identified by counterstaining sections containing TB filled perikarya for acetylcholinesterase (AChE) following pretreatment with diisopropylfluorophosphate (DFP) or choline acetyltransferase (ChAT), by immunohistochemistry with highly specific monoclonal antibodies. Many, but not all, vestibular efferent cell bodies located in group E were shown to be cholinergic. These and other recently published data suggest that the efferent octavus system may consist of a number of chemically distinct cell groups.

Bechterew decompensation

A right labyrinthectomy was performed in rats 5 months after a left labyrinthectomy. Spontaneous compensation of balance functions after both operations was assessed by observing nystagmus, rolling about the longitudinal axis, circular walking and head tilt. Decompensation, induced by brief halothane-NO anaesthesia, released predominantly symptoms characteristic for the period after the first labyrinthectomy. Bechterew symptoms could, however, also be decompensated. It is concluded that Bechterew compensation does not re-establish balance in the central vestibular system. Dysbalance and vertigo of vestibular origin is thus conceivable in patients after all peripheral vestibular function has been lost.