Anomalous rectification in neurons from cat sensorimotor cortex in vitro

Abstract

The ionic mechanisms underlying anomalous rectification in large neurons from layer V of cat sensorimotor cortex were studied in an in vitro brain slice. The anomalous rectification was apparent as an increase of slope conductance during membrane hyperpolarization, and the development of anomalous rectification during a hyperpolarizing current pulse was signaled by a depolarizing sag of membrane potential toward resting potential (RP). Voltage-clamp analysis revealed the time- and voltage-dependent inward current (IAR) that produced anomalous rectification. IAR reversal potential (EAR) was estimated to be approximately -50 mV from extrapolation of linear, instantaneous, current-voltage relations. The conductance underlying IAR (GAR) had a sigmoidal steady-state activation characteristic. GAR increased with hyperpolarization from -55 to -105 mV with half-activation at approximately -82 mV. The time course of both GAR and IAR during a voltage step was described by two exponentials. The faster exponential had a time constant (tau F) of approximately 40 ms; the slow time constant (tau S) was approximately 300 ms. Neither tau F nor tau S changed with voltage in the range -60 mV to -110 mV. The fast component constituted approximately 80% of IAR at each potential. Both IAR and GAR increased in raised extracellular potassium [( K+]o) and EAR shifted positive, but the GAR activation curve did not shift along the voltage axis. Solutions containing an impermeable Na+ substitute caused an initial transient decrease in IAR followed by a slower increase of IAR. Brain slices bathed in Na+-substituted solution developed a gradual increase in [K+]o as measured with K+-sensitive microelectrodes. We conclude that GAR is permeable to both Na+ and K+, but the full contribution of Na+ was masked by the slow increase of [K+]o that occurred in Na+ substituted solutions. Chloride did not appear to contribute significantly to IAR since estimates of EAR were similar in neurons impaled with microelectrodes filled with potassium chloride or methylsulfate, whereas, ECl (estimated from reversal of a GABA-induced ionic current) was approximately 30 mV more positive with the KCl-filled microelectrodes. Extracellular Cs+ caused a reversible dose- and voltage-dependent reduction of GAR, whereas intracellular Cs+ was ineffective. The parameters measured during voltage clamp were used to formulate a quantitative empirical model of IAR.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperpolarization-Activated Currents Regulate Excitability in Stellate Cells of the Mammalian Ventral Cochlear Nucleus

“…It has long been known that I h is sensitive to extracellular Cs ϩ (Maccaferri et al 1993;McCormick and Pape 1990b;Spain et al 1987). In the present experiments, 2 mM Cs ϩ blocked a conductance that was about sixfold greater than the voltage-sensitive g h but about half as great as that blocked by 50 M ZD7288 at -62 mV in T stellate cells.…”

“…In other cells, too, reversal potentials of this mixed cation current generally lie between -30 and -44 mV (Bal and Oertel 2000;Banks et al 1993;Chen 1997;Maccaferri and McBain 1996;McCormick and Pape 1990b;Mo and Davis 1997;Saitow and Konishi 2000;Spain et al 1987). This reversal potential reflects a permeability ratio P Na /P K of ϳ0.3 (Wollmuth and Hille 1992).…”

“…In many neurons, I h contributes to setting the resting potential (Bal and Oertel 2000;Banks et al 1993;Doan and Kunze 1999;Pape 1996) and to controlling the rate of spontaneous and evoked firing (Maccaferri and McBain 1996;McCormick and Pape 1990b;Spain et al 1987). In hippocampal cells, I h contributes to synaptic integration in dendrites (Magee 1998;Williams and Stuart 2000).…”

Variation of Input-Output Properties along the Somatodendritic Axis of Pyramidal Neurons

“…During tonic repetitive firing, the associated depolarization deactivates I h , thereby reducing the net excitatory drive (Spain et al, 1987;Magee, 1999). Again, this effect is enhanced in middle dendritic sites because of the higher density of I h , although boosting is not completely eliminated.…”

Morphology, Intrinsic Membrane Properties, and Rotation-Evoked Responses of Trochlear Motoneurons in the Turtle

“…IV. Such classification was based on the presence of anomalous rectification in a subset of recordings, revealed by a characteristic "sag" in membrane potential in response to hyperpolarizing current injection (Spain et al 1987). An example of a cell exhibiting such rectification is shown in Fig.…”