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.

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.

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.

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.

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

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.

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.

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.