Potassium currents in hippocampal pyramidal cells

Substance P and TRH share a common effector pathway in rat spinal motoneurones: an in vitro electrophysiological investigation

The actions of substance P and thyrotropin-releasing hormone (TRH) on neonatal rat spinal motoneurones in vitro were compared using intracellular current and voltage clamp techniques. Like TRH, substance P evoked a slowly-developing, persistent depolarisation plus an increase in input resistance under current clamp conditions. Under voltage clamp conditions, substance P elicited an inward current (mainly due to a conductance block) which peaked near -40 mV and reversed polarity close to the estimated EK. A distinct conductance increase (with a reversal potential near zero) also appeared to contribute to this response. The response to substance P at resting potential was suppressed by 1.5 mM Ba2+, but not by 20 mM tetraethylammonium, 2 mM 4-aminopyridine, 2 mM Cs+ and 0.2 mM Cd2+. In addition, co-application of TRH and substance P mutually occluded each other. Thus, it is suggested that substance P and TRH share a common effector mechanism, which primarily involves the suppression of IK(T), a persistent K+ current recently discovered in these neurones.

Cultured hippocampal neurons from trisomy 16 mouse, a model for Down's syndrome, have an abnormal action potential due to a reduced inward sodium current

Mouse trisomy 16 is an animal model for Down's syndrome (human trisomy 21). The whole-cell patch-clamp technique was used to compare passive and active electrical properties of trisomy 16 and diploid mouse 16 fetal hippocampal neurons maintained in culture for 2-5 weeks. There was no significant difference in any mean passive property, including resting potential, membrane resistance, capacitance and time constant. However, in trisomic neurons, the action potential had a 20% significantly slower rising phase and a 20% significantly smaller inward sodium current and inward sodium conductance than did control neurons. The outward conductance was not altered. The ratio of maximum inward conductance to maximum outward conductance was 30% less in the trisomy 16 cells. These results indicate that trisomy 16 hippocampal neurons have abnormal active electrical properties, most likely reflecting reduced sodium channel membrane density. Such subtle differences may influence elaboration of the hippocampus during development.

The binaural auditory pathway: membrane currents limiting multiple action potential generation in the rat medial nucleus of the trapezoid body

In this paper we describe the membrane currents of neurons in the medial nucleus of the trapezoid body (MNTB), which serves as an inverting relay in the binaural auditory pathway. In the following paper (Forsythe & Barnes-Davies (Proc. R. Soc. Lond. B 251, 151 (1993))) we describe the synaptic inputs to the MNTB and discuss the significance of these results for transmission through this nucleus, where the fidelity of information transfer will depend on the integration of synaptic responses with the intrinsic postsynaptic membrane properties. Whole-cell patch clamp recordings were made from MNTB neurons using a thin-slice preparation of the rat brain stem. Resting potentials were -70 mV with a neuronal input resistance of 250 M omega and a membrane time constant of 14 ms. Voltage-clamp studies showed that MNTB neurons possess an inward sodium current, an outward current similar to a delayed rectifier and an inward rectifier. In addition, a novel transient outward current exhibiting rapid kinetics and a sustained current are present, which are both blocked by micromolar concentrations of 4-aminopyridine (4AP). Current-clamp recording showed that MNTB neurons respond to depolarization with a single overshooting action potential (AP); 4AP blocked a fast after-hyperpolarization, increased AP duration, and converted the single AP response on depolarization to a train of action potentials.

Muscarinic inhibition of M-current and a potassium leak conductance in neurones of the rat basolateral amygdala

1. Voltage-clamp recordings using a single microelectrode were obtained from pyramidal neurones of the basolateral amygdala (BLA) in slices of the rat ventral forebrain. Slow inward current relaxations during hyperpolarizing voltage steps from a holding potential of -40 mV were identified as the muscarinic-sensitive M-current (IM), a time- and voltage-dependent potassium current previously identified in other neuronal cell types. 2. Activation of IM was voltage dependent with a threshold of approximately -70 mV. At membrane potentials positive to this, the steady-state current-voltage (I-V) relationship showed substantial outward rectification, reflecting the time- and voltage-dependent opening of M-channels. The underlying conductance (gM) also increased sharply with depolarization. 3. The reversal potential for IM was -84 mV in medium containing 3.5 mM K+. This was shifted positively by 27 mV when the external K+ concentration was raised to 15 mM. 4. The time courses of M-current activation and deactivation were fitted by a single exponential. The time constant for IM decay, measured at 24 degrees C, was strongly dependent on membrane potential, ranging from 330 ms at -40 mV to 12 ms at -100 mV. 5. Bath application of carbachol (0.5-40 microM) inhibited IM, as evidenced by the reduction or elimination of the slow inward M-current relaxations evoked during hyperpolarizing steps from a holding potential of -40 mV. The outward rectification of the steady-state I-V relationship at membrane potentials positive to -70 mV was also largely eliminated. The inhibition of IM by carbachol was dose dependent and antagonized by atropine. 6. Carbachol produced an inward current shift at a holding potential of -40 mV that was only partially attributable to inhibition of IM. An inward current shift was also produced by carbachol at membrane potentials negative to -70 mV, where IM is inactive. These effects were dose dependent and antagonized by atropine. They were attributed to the muscarinic inhibition of a voltage-insensitive potassium leak conductance (ILeak). 7. In most cells, carbachol reduced the slope of the instantaneous I-V relationship obtained from a holding potential of -70 mV so that it crossed the control I-V plot at the reversal potential for ILeak. This was found to be -108 mV in 3.5 mM K+ saline, shifting to -66 mV in 15 mM K+ saline.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation characteristics of a sustained, Ca(2+)-independent K+ current of rat hippocampal neurones in vitro

1. Current or voltage clamp recordings from CA3 neurones of the adult rat hippocampal slice were performed to study the inactivation properties of a slow outward K+ current identified as the delayed rectifier (IK). 2. In current clamp experiments, burst firing evoked from resting membrane potential by intracellular current injection was reduced or blocked by conditioning hyperpolarizing pre-pulses of 20-40 mV amplitude. This effect was inhibited by tetraethylammonium (TEA; 20 mM) but was unaffected by Cs+ (3 mM), 4-aminopyridine (4-AP; 2 mM), carbachol (30-50 microM), mast cell degranulating peptide (MCDP; 300 nM), thyrotrophin releasing hormone (TRH; 1 microM) or by a Ca(2+)-free solution containing Mn2+ or Co2+ (2 mM). 3. Single-electrode voltage clamp experiments were carried out on neurones superfused with Ca(2+)-free solution, containing tetrodotoxin (TTX; 1 microM), Mn2+ or Co2+ (2 mM), 4-AP (2 mM), Cs+ (3 mM) and carbachol (30 microM). Step depolarizations from a holding potential of -55 mV activated an outward current which reached a plateau after 200 ms, followed by an outward tail current. Such an outward current had the characteristics of IK. 4. The outward currents were significantly potentiated by conditioning hyperpolarizing pre-pulses suggesting the IK was reduced by a voltage-dependent inactivation process. Removal of inactivation was a function of the amplitude of the conditioning hyperpolarizing pre-pulse. At a holding potential of -55 mV removal of inactivation was time dependent with a time constant of 211 ms. High K+ (12.5 or 21.5 mM) solutions did not affect the inactivation characteristics of IK. 5. Tetraethylammonium (20 mM) or low concentrations of Ba2+ (0.1 mM) readily depressed the outward current without significantly affecting the inactivation process. Dendrotoxin (200 nM) also depressed such a slow current but, in addition, increased the inactivation process of IK. 6. It is suggested that removal of inactivation of IK by hyperpolarization can modulate cell excitability by fully restoring the ability of IK to inhibit burst firing of CA3 hippocampal neurones.

Evidence for a Slowly Inactivating K+ Current in Prefrontal Cortical Cells

The in vitro slice preparation of rat prefrontal cortical cells was used to analyse the presence and characteristics of a slowly inactivating outward current and its effect on the delayed integration of synaptic inputs. Pyramidal cells were identified as regular firing or bursting cells. In a fraction of these cells a depolarizing current pulse to -40 mV from a holding potential of -95 mV evoked the fast outward IA current followed by a slower outward current (IKs) which inactivated slowly during the 3-s pulse. This slowly inactivating outward current was completely inactivated at holding potentials near -40 mV and was fully deinactivated by large hyperpolarizing pulses of 1 s duration. It was sensitive to micromolar concentrations of 4-aminopyridine and to 10 mM tetraethylammonium. In current clamp experiments, when the cells were maintained at -80 mV, they responded to subliminal depolarizing current pulses by a slow rising depolarization which reached threshold for spike firing after a delay of several seconds. This delay was considerably reduced either by maintaining the cell at less hyperpolarized potentials or by bath application of 40 microM 4-aminopyridine, or by repeated application of depolarizing pulses. The inactivation of IKs by the last procedure also led to plateau depolarization of the cell. These results suggest that the activation of the slowly inactivating outward current IKs can shunt excitatory inputs, preventing the cell from reaching spike threshold as long as it is not largely inactivated.

Charybdotoxin, dendrotoxin and mast cell degranulating peptide block the voltage-activated K+ current of fibroblast cells stably transfected with NGK1 (Kv1.2) K+ channel complementary DNA

The blocking actions of the K+ channel toxins charybdotoxin, dendrotoxin and mast cell degranulating peptide were studied in B82 mouse fibroblast cells transformed to express NGK1 (Kv1.2) K+ channels. All three toxins were potent blockers of the K+ current in these cells, with KD values of 1.7, 2.8 and 185 nM, respectively. The toxin block exhibited a weak voltage-dependence with the degree of inhibition decreasing at positive membrane potentials. For charybdotoxin and dendrotoxin, reducing [K+]i did not increase the fractional block, demonstrating that the relief of block at positive membrane potentials is not due to displacement of the toxin molecules by outward flow of K+ ions. A voltage-jump protocol was used to determine the rates of binding and unbinding of dendrotoxin and mast cell degranulating peptide; binding of charybdotoxin was too rapid to be quantitatively evaluated in this manner. The binding rates (dendrotoxin, approximately 5 x 10(7)/M per s; mast cell degranulating peptide, approximately 0.8 x 10(7)/M per s) were largely voltage-independent, suggesting that association of the toxin molecules with the channel is diffusion limited. The rates of unbinding (dendrotoxin, approximately 0.3/s; mast cell degranulating peptide, approximately 3/s at +60 mV) of both toxins increased e-fold per approximately 40 mV change in membrane potential, thus accounting for the voltage-dependence of the equilibrium block. Internal perfusion with the three toxins failed to affect the K+ current (in contrast to internal tetraethylammonium which strongly blocked the current), indicating that the toxins exert their blocking action by binding to extracellular sites.

Tedisamil blocks a calcium-dependent potassium channel in cultured motoneurons

A calcium-dependent potassium channel K(Ca) has been isolated in mouse motoneurons. With physiological concentrations of potassium across inside-out patches, a 100 pS K(Ca) channel was activated when the bath solution of Ca2+ was in excess of 1 microM. Introduction of the drug tedisamil, a blocker of repolarizing potassium channels in cardiac cells, at concentrations in the range 0.2-10 microM, caused a dose-dependent decrease in the mean open times for K(Ca). The drug action was consistent with open channel block of K(Ca) with an onward (blocking) rate constant of 6 x 10(7) M-1 s-1. Tedisamil, at concentrations of 1 microM and 5 microM, also blocked the K(Ca) channel when applied to outside-out patches with a similar potency as found with internal application. A large conductance K(Ca) channel in hippocampal neurons is also blocked by a number of putative Class III antiarrhythmic drugs, including tedisamil; thus, these agents may have utility in the characterization of the properties of K(Ca) channels in various cells.

Ionic basis of membrane potential changes induced by anoxia in rat dorsal vagal motoneurones

1. The effects of anoxia on membrane properties of 119 dorsal vagal motoneurones (DVMs) were investigated in an in vitro slice preparation of the rat medulla. 2. Membrane potential was unaffected by anoxia in 11% of DVMs. An hyperpolarization accompanied by a decrease in input resistance occurred in 44% of DVMs; the remaining 45% depolarized with either an increase (60%) or decrease in input resistance (40%). TTX at a concentration of 0.3-1 microM did not significantly affect these responses. 3. Anoxic artificial cerebrospinal fluid (ACSF) containing 20 mM-TEA reversed the response of DVMs that hyperpolarized in standard ACSF to reveal a depolarization of 7.4 +/- 2.1 mV, and increased the anoxic depolarization from 5.0 +/- 0.7 to 8.7 +/- 1.4 mV. 4. Anoxic depolarization was converted to an hyperpolarization of 7.3 +/- 2.1 mV in ACSF containing 5 mM-4-aminopyridine (4-AP) and 1 microM-TTX. A residual depolarization of 4.5 +/- 3.5 mV was then observed in ACSF containing 5 mM-4-AP, 1 microM-TTX and 20 mM-TEA. Anoxic hyperpolarization was increased from 7.8 +/- 1.8 to 10.0 +/- 3.9 mV in 5 mM-4-AP and 1 microM-TTX and converted to a depolarization of 5.3 +/- 4.5 mV in 5 mM-4-AP, 1 microM-TTX and 20 mM-TEA. 5. In anoxic ACSF containing TEA, the action potential width was increased from 0.92 +/- 0.04 to 8.1 +/- 1.1 ms in hyperpolarizing DVMs, and from 0.85 +/- 0.01 to 2.4 +/- 1.0 ms in depolarizing DVMs. The increase in width was prevented by 2-3 mM-Mn2+. 6. The long after-hyperpolarization (AHP) of DVMs, which is contributed to by both an apamin-sensitive IK(Ca) and an apamin, charybdotoxin and TEA insensitive IK(Ca) was decreased in duration from 2.59 +/- 0.14 to 1.94 +/- 0.12 s during anoxia. 7. It is concluded that anoxia enhances the delayed rectifier current (IK(DR)) and an inward current, probably ICa, but suppresses the A currents (IA). In DVMs that hyperpolarize during anoxia, the increase in IK(DR) outweighs the increase in ICa and the decrease in IA. In depolarizing DVMs the decrease in IA and increase in ICa outweight the increase in IK(DR). The change in input resistance is determined by the relative sizes of current enhancement or suppression.

A burst-dependent hippocampal excitability defect elicited by potassium at the developmental onset of spike-wave seizures in the Tottering mutant

Hippocampal CA3 pyramidal neurons in the adult epileptic mutant mouse tottering (tg) show normal intrinsic membrane properties, yet fire abnormally prolonged paroxysmal depolarizing shifts (PDS) during in vitro exposure to elevated extracellular potassium solutions. Intracellular recordings in immature mutants reveal that this network burst abnormality is present during the developmental period that coincides with the onset of seizures in the mutant (19-20 postnatal days), and is significantly more pronounced at this age than at adulthood. These data are inconsistent with the hypothesis that the mutant PDS prolongation represents a secondary consequence of a prolonged history of repeated seizures and suggest that it may reflect a cellular epileptogenic phenotype more directly related to the primary neuropathological expression of the tg gene.

Synaptically triggered action potentials begin as a depolarizing ramp in rat hippocampal neurones in vitro

1. During just-suprathreshold synaptic activation of CA1 pyramidal cells in rat hippocampal slices in vitro the action potential begins as a slow depolarizing ramp, superimposed on the underlying EPSP and forming an integral part of the action potential. We call this ramp a synaptic prepotential (SyPP). 2. In order to examine the SyPP, a procedure for subtraction of the underlying EPSP was necessary. Because action potentials were only elicited by a subset of EPSPs with larger than average amplitude, a subtraction of the mean subthreshold EPSP would not give valid results. Instead, an EPSP to be subtracted was selected from an assemblage of subthreshold EPSPs, so that its amplitude matched the initial part of the spike-generating EPSP. 3. Virtually all action potentials started with a SyPP. Using an amplitude criterion of 1 S.D. of the mean of the matching subthreshold EPSPs, just-suprathreshold EPSPs gave prepotentials in 72-100% of all action potentials from fifteen randomly selected cells. With a criterion of 2 S.D.S, the frequency of occurrence ranged from 36 to 100%. 4. With a constant stimulus strength, there was a certain variability of the spike latencies. Shorter latency spikes had steeper, but smaller SyPPs than later spikes, suggesting that the slope of SyPP influenced the timing of the cell discharge. 5. The SyPP was best fitted by a single, exponentially rising curve, and was both smaller and slower than the large amplitude action potential. Its amplitude was 1-6 mV and the time constant 1-5 ms, which was 10-50 times slower than that of the upstroke of the action potential. 6. A properly timed hyperpolarizing current pulse could block the large amplitude action potential, thereby unmasking the SyPP as an initial depolarizing ramp. 7. The SyPP was more sensitive than the large amplitude action potential to intracellular injection of QX-314, a lidocaine derivative. At the concentrations used (10 or 30 mM) no detectable changes were seen in the large amplitude action potential. 8. Droplet application of a specific N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovaleric acid (1 mM), reduced both the EPSP and the firing probability, but did not change the SyPP. 9. The SyPP amplitude and time course depended upon the membrane potential at which the cell was activated. Depolarization enhanced and prolonged the SyPP, while hyperpolarization gave opposite effects. In part, the depolarization-induced amplitude increase could be attributed to membrane accommodation. 10. Antidromically evoked action potentials never started with a prepotential.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhancement of spatial learning in F344 rats by physical activity and related learning-associated alterations in hippocampal and cortical cholinergic functioning

The effects of physical activity on spatial memory performance and associated cholinergic function were examined in F344 rats. Cholinergic analysis included resting and depolarization-induced activation of high-affinity choline uptake and muscarinic receptor binding in the hippocampus, parietal cortex and frontal cortex. Rats that were physically trained, using chronic treadmill running, demonstrated significantly enhanced performance on the spatial learning task, both in second trial latency and first and second trial proximity ratio scores (P less than 0.002). Concomitant with enhanced behavioral performance were neurochemical changes of a reduction in hippocampal high-affinity choline uptake, an upregulation of muscarinic receptor density, and an increase in high-affinity choline uptake 24 h after spatial memory testing (P less than 0.05). Spatial memory tested rats demonstrated enhanced depolarization-induced activation of high-affinity choline uptake (P less than 0.001). Rats that were yoked for swim time to spatial memory tested rats did not show any spatial learning-induced alterations in high affinity choline uptake. These spatial learning- and physical activity-induced cholinergic alterations were observed only in the hippocampus, not in the parietal or frontal cortex. These data indicate that the chronic running-induced alterations in hippocampal high-affinity choline uptake and upregulation of muscarinic receptor density, in combination with enhancement of high-affinity choline uptake related to spatial learning, may contribute to the enhanced spatial learning performance of chronic-run rats.

Ionic conductance mechanisms contributing to the electrophysiological properties of neurons

Neurons have a multiplicity of ionic conductance mechanisms, the interactions of which determine in part the response of a neuron to chemical and electrical synaptic interactions, firing patterns, excitability, membrane potential and action-potential waveform. Several papers published in the past year have provided important new information on the role that ionic conductance mechanisms play in determining the electrophysiological properties of neurons, and how subtle differences can contribute to considerable variability of response.

Novel form of long-term potentiation produced by a K+ channel blocker in the hippocampus

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a widely studied model of memory processes. In the CA1 region, LTP is triggered by the entry of Ca2+ through N-methyl-D-aspartate (NMDA) receptor channels and maintained by the activation of Ca2(+)-sensitive intracellular messengers. We now report that in CA1, a transient block by tetraethylammonium of IC, IM and the delayed rectifier (IK) produces a Ca2(+)-dependent NMDA-independent form of LTP. Our results suggest that this new form of LTP (referred as to LTPK) is induced by a transient enhanced release of glutamate which generates a depolarization by way of the non-NMDA receptors and the consequent activation of voltage-dependent Ca2+ channels.