1. Using whole cell recording techniques, we studied subthreshold and suprathreshold voltage responses to oscillatory current inputs in neurons from the sensorimotor cortex of juvenile rats. 2. Based on firing patterns, neurons were classified as regular spiking (RS), intrinsic bursting (IB), and fast spiking (FS). The subthreshold voltage-current relationships of RS and IB neurons were rectifying whereas FS neurons were almost ohmic near rest. 3. Frequency response curves (FRCs) for neurons were determined by analyzing the frequency content of inputs and outputs. The FRCs of most neurons were voltage dependent at frequencies below, but not above, 20 Hz. Approximately 60% of RS and IB neurons had a membrane resonance at their resting potential. Resonant frequencies were between 0.7 and 2.5 Hz (24-26 degrees C) near -70 mV and usually increased with hyperpolarization and decreased with depolarization. The remaining RS and IB neurons and all FS neurons were nonresonant. 4. Resonant neurons near rest had a selective coupling between oscillatory inputs and firing. These neurons selectively fired action potentials when the frequency of the swept-sine-wave (ZAP) current input was near the resonant frequency. However, when these neurons were depolarized to -60 mV, spike firing was associated with many input frequencies rather than selectively near the resonant frequency. 5. We examined three subthreshold currents that could cause low-frequency resonance: IH, a slow, hyperpolarization-activated cation current that was blocked by external Cs+ but not Ba2+; IIR, an instantaneously activating, inwardly rectifying K+ current that was blocked by both Cs+ and Ba2+; and INaP, an quickly activating, inwardly rectifying persistent Na+ current that was blocked by tetrodotoxin (TTX). Voltage-clamp experiments defined the relative steady state activation ranges of these currents. IIR (activates below -80 mV) and INaP (activates above -65 mV) are unlikely to interact with each other because their activation ranges never overlap. However, both currents may interact with IH, which activated variably at potentials between -50 and -90 mV in different neurons. 6. We found that IH produces subthreshold response. Consistent with this, subthreshold resonance was blocked by external Cs+ but not Ba2+ or TTX. Application of Ba2+ enlarged FRCs and resonance at potentials below -80 mV, indicating that IK,ir normally attenuates resonance. Application of TTX greatly diminished resonance at potentials more depolarized than -65 mV, indicating that INaP normally amplifies resonance at these potentials. 7. The ZAP current input may be viewed as a model of oscillatory currents that arise in neocortical neurons during synchronized activity in the brain. We propose that the frequency selectivity endowed on neurons by IH may contribute to their participation in synchronized firing. The voltage dependence of the frequency-selective coupling between oscillatory inputs and spikes may indicate a novel mechanism for controlling the extent of low-frequency synch...

Passive and active (voltage- and time-dependent) membrane properties of trigeminal root ganglion neurons of decerebrate guinea pigs have been determined using frequency-domain analyses of small-amplitude perturbations of membrane voltage. The complex impedance functions of trigeminal ganglion neurons were computed from the ratios of the fast Fourier transforms of the intracellularly recorded voltage response from the neuron and of the input current, which had a defined oscillatory waveform. The impedance magnitude functions and corresponding impedance locus diagrams were fitted with various membrane models such that the passive and active properties were quantified. The complex impedances of less than one-quarter of the 105 neurons which were investigated extensively could be described by the complex impedance function for a simple RC-electrical circuit. In such neurons, the voltage responses to constant-current pulses, using conventional bridge-balance techniques, could be fitted with single exponential curves, also suggesting passive membrane behavior. A nonlinear least-squares fit of the complex impedance function for the simple model to the experimentally observed complex impedance yielded estimates of the resistance of the electrode, and of input capacitance (range, 56 to 490 pF) and input resistance (range, 0.8 to 30 M omega) of the neurons. The majority of trigeminal ganglion neurons were characterized by a resonance in the 50- to 250-Hz bandwidth of their impedance magnitude functions. Such neurons when injected with "large" hyperpolarizing current pulses using bridge-balance techniques showed membrane voltage responses that "sagged" (time-dependent rectification). Also, repetitive firing commonly occurred with depolarizing current pulses; this characteristic of neurons with resonance in their impedance magnitude functions was not observed in neurons with "purely" passive membrane behavior. A nonlinear least-squares fit of a five-parameter impedance fitting function based on a membrane model to the impedance locus diagram of a neuron with resonance yielded estimates of its membrane properties: input capacitance, the time-invariant part of the conductance, the conductance activated by the small oscillatory input current, and the relaxation time constant for this conductance. The ranges of the estimates for input capacitance and input resistance were comparable to the ranges of corresponding properties derived for neurons exhibiting "purely" passive behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

1. We obtained whole cell data from sensorimotor cortical neurons of in vitro slices (juvenile rats) and observed a low-frequency resonance (1-2 Hz) in their voltage responses. We constructed models of subthreshold membrane currents to determine whether a hyperpolarization-activated cation current (IH) is sufficient to account for this resonance. 2. Parameter values for a basic IH (BH) model were estimated from voltage-clamp experiments at room temperature. The BH model formed a component of a reduced membrane (RM) model. On numerical integration, the RM model exhibited voltage sags and rebounds to injected current pulses; maximal voltage responses to injected oscillatory currents occurred near 2 Hz. 3. We compared the experimentally measured frequency-response curves (FRCs) at room temperature with the theoretical FRCs derived from the RM model. The theoretical FRCs exhibited resonant humps with peaks between 1 and 2 Hz. At 36 degrees C, the theoretical FRCs peaked near 10 Hz. The characteristics of theoretical and observed FRCs were in close agreement, demonstrating that IH is sufficient to cause resonance. Thus we classified IH as a resonator current. 4. We developed a technique, the reactive current clamp (RCC), to inject a computer-calculated current corresponding to a membrane ionic current in response to the membrane potential of the neuron. This enabled us to study the interaction of an artificial ionic current with living neurons (electronic pharmacology or EP-method). Using the RCC, a simplified version of the BH model was used to cancel an endogenous IH (electronic antagonism) and to introduce an artificial IH (electronic expression) when an endogenous IH was absent. Antagonism of IH eliminated sags and rebounds, whereas expression of IH endowed neurons with resonance and the frequency-selective firing that accompanies resonance in neurons with an endogenous IH. Previous investigations have relied on the specificity of pharmacological agents to block ionic channels, e.g., Cs+ to block IH. However, Cs+ additionally affects other currents. This represents the first time an in vitro modeling technique (RCC) has been used to antagonize a specific endogenous current, IH. 5. A simplified RM model yielded values of the resonant frequency and Q (an index of the sharpness of resonance), which rose almost linearly between -55 and -80 mV. Resonant frequencies could be much higher than fH = (2 pi tau H) - 1 where tau H is the activation time constant for IH. 6. In the FRCs, resonance appeared as a hump at intermediate frequencies because of low- and high-frequency attenuations due to IH and membrane capacitance, respectively. Changing the parameters of IH altered the low-frequency attenuation and, hence, the resonance. Changes in the leak conductance affected both the low- and high-frequency attenuations. 7. We modeled an inwardly rectifying K+ current (IIR) and a persistent Na+ current (INaP) to study their effects on resonance. Neither current produced resonance in the absence of IH. We found that IIR attenuated...

By using thalamic brain slices from juvenile rats and the whole cell recording technique, we determined the effects of aqueous applications of the anesthetic isoflurane (IFL) on tonic and burst firing activities of ventrobasal relay neurons. At concentrations equivalent to those used for in vivo anesthesia, IFL induced a hyperpolarization and increased membrane conductance in a reversible and concentration-dependent manner (ionic mechanism detailed in companion paper). The increased conductance short-circuited the effectiveness of depolarizing pulses and was the main cause for inhibition of tonic firing of action potentials. Despite the IFL-induced hyperpolarization, which theoretically should have promoted bursting, the shunt blocked the low-threshold Ca2+ spike (LTS) and associated burst firing of action potentials as well as the high-threshold Ca2+ spike (HTS). Increasing the amplitude of either the depolarizing test pulse or hyperpolarizing prepulse or increasing the duration of the hyperpolarizing prepulse partially reversed the blockade of the LTS burst. In voltage-clamp experiments on the T-type Ca2+ current, which produces the LTS, IFL decreased the spatial distribution of imposed voltages and hence impaired the activation of spatially distant T channels. Although IFL may have increased a dendritic leak conductance or decreased dendritic Ca2+ currents, the somatic shunt appeared to block initiation of the LTS and HTS as well as their electrotonic propogation to the axon hillock. In summary, IFL hyperpolarized thalamocortical neurons and shunted voltage-dependent Na+ and Ca2+ currents. Considering the importance of the thalamus in relaying different sensory modalities (i.e., somatosensation, audition, and vision) and motor information as well as the corticothalamocortical loops in mediating consciousness, the shunted firing activities of thalamocortical neurons would be instrumental for the production of anesthesia in vivo.

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