The A-current: how ubiquitous a feature of excitable cells is it?

Differential inhibition of glial K(+) currents by 4-AP

Spinal cord astrocytes express four biophysically and pharmacologically distinct voltage-activated potassium (K(+)) channel types. The K(+) channel blocker 4-aminopyridine (4-AP) exhibited differential and concentration-dependent block of all of these currents. Specifically, 100 microM 4-AP selectively inhibited a slowly inactivating outward current (K(SI)) that was insensitive to dendrototoxin (< or = 10 microM) and that activated at -50 mV. At 2 mM, 4-AP inhibited fast-inactivating, low-threshold (-70 mV) A-type currents (K(A)) and sustained, TEA-sensitive noninactivating delayed-rectifier-type currents (K(DR)). At an even higher concentration (8 mM), 4-AP additionally blocked inwardly rectifying, Cs(+)- and Ba(2+)-sensitive K(+) currents (K(IR)). Current injection into current-clamped astrocytes in culture or in acute spinal cord slices induced an overshooting voltage response reminiscent of slow neuronal action potentials. Increasing concentrations of 4-AP selectively modulated different phases in the repolarization of these glial spikes, suggesting that all four K(+) currents serve different roles in stabilization and repolarization of the astrocytic membrane potential. Our data suggest that 4-AP is an useful, dose-dependent inhibitor of all four astrocytic K(+) channels. We show that the slowly inactivating astrocytic K(+) currents, which had not been described as separate current entities in astrocytes, contribute to the resting K(+) conductance and may thus be involved in K(+) homeostatic functions of astrocytes. The high sensitivity of these currents to micromolar 4-AP suggests that application of 4-AP to inhibit neuronal A-currents or to induce epileptiform discharges in brain slices also may influence astrocytic K(+) buffering.

Effects of quinine on three different types of potassium currents in bullfrog sympathetic neurons

Whole-cell/voltage-clamp recordings were made from dissociated bullfrog sympathetic neurons to examine the sensitivity of potassium currents to a potassium channel blocker quinine (1-500 microM). Among three currents tested, a rapidly inactivating A-type current (I(A)) was the most sensitive to the block by quinine (IC50 approximately 22 microM). A non-inactivating M-type current (I(M)) was the least sensitive (IC50 approximately 445 microM), and the sensitivity of a slowly inactivating delayed rectifier-type current (I(K)) was in between (IC50 approximately 115 microM). Results suggest that the ability of quinine to block different types of potassium currents such as I(A) and I(M) with significantly different IC50 values would be of help for the potassium channel pharmacology in amphibian autonomic ganglion cells.

Diversity of firing frequency control by A-currents in vertebrate neurons: a modeling study

Experimental and modeling studies have accumulated strong evidence suggesting that A-currents control firing rates in invertebrate neurons. However, the direct demonstration of a similar role remains to be established in vertebrate neurons. We tested this possibility in a simulated neuron embedded with a generic model of vertebrate A-currents. Under simulated current-clamp protocols, the generic A-current produced a modest frequency reduction (15 Hz) that was constant within all firing frequencies. Modifications in steady-state properties of the A-current model within known physiological ranges annihilated or dramatically increased firing frequency reduction. These results suggest that the influence of A-currents on firing frequency should differ strongly among vertebrate neurons, and that modulations influencing A-currents provide a powerful control over the excitability of vertebrate neurons.

Development of a fast transient potassium current in chick cochlear ganglion neurons

Neurons of the cochlear ganglion are endowed with a set of voltage-gated ion channels that enable them to encode and transmit sound information from the cochlear receptors to the brain. The temporal expression pattern of the K+ currents in chick cochlear ganglion neurons during embryonic development was analyzed using whole-cell voltage clamp techniques. In acutely isolated neurons, slowly activating delayed rectifier K+ currents appear at embryonic day 7 (E7) and increase in amplitude during development. A fast activating, fast inactivating K+ current of the A type is first expressed at E10, increasing in amplitude thereafter. To investigate the possible role of neurotrophins in the induction of these K+ channels, neurons were grown in culture in the presence or absence of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). Neurons isolated at E8 and grown in culture for 1 day exhibit a high expression of A-current, together with the outgrowth of neurites. A-currents are not seen in acutely dissociated neurons from age-matched embryos (E9) which lack neurites, cut off by the isolation procedure. This suggests a preferential neuritic location of the channels carrying the A-current. However, the level of expression of the K+ currents was independent of BDNF or NT-3 application. Similarly, neurons isolated at E10 and grown in culture for up to 4 days maintain the amplitude of the K+ currents independently of the presence of the neurotrophins. These results indicate that BDNF and NT-3 may not directly regulate the expression of K+ channels in chick cochlear ganglion neurons. The notable expression of the fast inactivating A-current suggests that it plays a significant role in the modulation of synaptic efficacy and the encoding of auditory stimuli.

Inhibition of subfornical organ neuronal potassium channels by vasopressin

The subfornical organ is one of a specialized group of CNS structures devoid of a significant blood-brain barrier, collectively known as the circumventricular organs. While peptides are normally excluded from access to most regions of the CNS, the subfornical organ contains neurons with a high density of receptors for many circulating peptides, including vasopressin. There is a well-established role for the subfornical organ in stimulating the release of vasopressin, and recent evidence suggests that it may also play an important role in mediating the negative feedback actions of vasopressin. The aim of this study was to determine the direct effects of vasopressin on subfornical organ neurons through patch-clamp studies in a dissociated subfornical organ preparation. In current-clamp studies, bath application of 10 nM vasopressin caused depolarizations in 61%, hyperpolarizations in 11%, and no significant change in membrane potential in 28% of neurons tested. We then sought to determine the specific ion channels involved in regulating the vasopressin-induced excitability of subfornical organ neurons through voltage-clamp studies. Vasopressin (10 nM) decreased the peak outward current at +40 mV by 50% (n=7), which was blocked by pretreatment with a V1 receptor antagonist (n=5). Based on these findings, we carried out a systematic characterization of two subformical organ K+ channels, the delayed rectifier (I(K)) and the transient outward current (I(A)). Through voltage isolation of I(K), we found that vasopressin inhibited the steady-state current, by 33+/-7% (n=9). Vasopressin also inhibited the peak I(A) by 27+/-5% (n=8). These data provide the first evidence of a role for K+ channels in mediating the excitatory effects of vasopressin on subfornical organ neurons. The exact physiological roles and sources of vasopressin which may act on subfornical organ neurons are not completely understood at present.

Reduction in potassium currents in identified cutaneous afferent dorsal root ganglion neurons after axotomy

Potassium currents have an important role in modulating neuronal excitability. We have investigated the effects of axotomy on three voltage-activated K(+) currents, one sustained and two transient, in cutaneous afferent dorsal root ganglion (DRG) neurons. Fourteen to 21 days after axotomy, L(4) and L(5) DRG neurons were acutely dissociated and were studied 2-8 h after plating. Whole cell patch-clamp recordings were obtained from identified cutaneous afferent neurons (46-50 microm diam); K(+) currents were isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement and channel blockers. Separation of the current components was achieved on the basis of sensitivity to dendrotoxin or 4-aminopyridine and by the response to variation in conditioning voltage. Both control and injured neurons displayed qualitatively similar complex K(+) currents composed of distinct kinetic and pharmacological components. Three distinct K(+) current components, a sustained (I(K)) and two transient (I(A) and I(D)), were identified in variable proportions. However, total peak current was reduced by 52% in the axotomized cells when compared with control cells. Two current components were reduced after ligation, I(A) by 60%, I(K) by over 65%, compared with control cells. I(D) appeared unaffected after acute ligation. These results indicate a large reduction in overall K(+) current, resulting from reductions in I(K) and I(A), on large cutaneous afferent neurons after nerve ligation and have implications for excitability changes of injured primary afferent neurons.

Voltage-activated potassium outward currents in two types of spider mechanoreceptor neurons

We studied the properties of voltage-activated outward currents in two types of spider cuticular mechanoreceptor neurons to learn if these currents contribute to the differences in their adaptation properties. Both types of neurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts lasting several hundred milliseconds. We found that both neurons had two outward current components, 1) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current could be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 microM quinidine, but these blockers also reduced the amplitude of the noninactivating outward current. Charybdotoxin or apamin did not have any effect on the outward currents, indicating that Ca2+-activated K+ currents were not present or not inhibited by these toxins. The only significant differences between type A and type B neurons were found in the half-maximal activation (V50) values of both currents. The transient current had a V50 value of 9. 6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We conclude that, although differences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.

Primary and adaptive changes of A-type K+ currents in sympathetic neurons from hypertensive rats

The A-type K+ current (IA) of superior cervical ganglion neurons acutely isolated from spontaneously hypertensive (SHR) and age-matched Wistar-Kyoto (WKY) rats was compared under whole cell voltage clamp. Activation parameters were similar in each strain. Steady-state inactivation was shifted approximately -6 mV in SHR, where one-half inactivation occurred at -81 mV vs. -75 mV in WKY rats. The shift was not present in prehypertensive SHR but remained in adult enalapril-treated SHR and, therefore, may represent a primary alteration of IA properties. IA amplitudes evoked from physiological potentials were similar, despite inactivation of a greater fraction of the current in the SHR. Comparing maximal IA densities revealed that current density is elevated in the SHR, which compensates for the inactivation shift. Current density decreased with age in WKY neurons but did not significantly decline in SHR neurons unless hypertension was prevented with enalapril. Thus adult SHR neurons may retain a high IA density as an adaptive response to offset potential hyperexcitability resulting from the hyperpolarized IA inactivation.

Current clamp and modeling studies of low-threshold calcium spikes in cells of the cat's lateral geniculate nucleus

Current clamp and modeling studies of low-threshold calcium spikes in cells of the cat's lateral geniculate nucleus. All thalamic relay cells display a voltage-dependent low-threshold Ca2+ spike that plays an important role in relay of information to cortex. We investigated activation properties of this spike in relay cells of the cat's lateral geniculate nucleus using the combined approach of current-clamp intracellular recording from thalamic slices and simulations with a reduced model based on voltage-clamp data. Our experimental data from 42 relay cells showed that the actual Ca2+ spike activates in a nearly all-or-none manner and in this regard is similar to the conventional Na+/K+ action potential except that its voltage dependency is more hyperpolarized and its kinetics are slower. When the cell's membrane potential was hyperpolarized sufficiently to deinactivate much of the low-threshold Ca2+ current (IT) underlying the Ca2+ spike, depolarizing current injections typically produced a purely ohmic response when subthreshold and a full-blown Ca2+ spike of nearly invariant amplitude when suprathreshold. The transition between the ohmic response and activated Ca2+ spikes was abrupt and reflected a difference in depolarizing inputs of <1 mV. However, activation of a full-blown Ca2+ spike was preceded by a slower period of depolarization that was graded with the amplitude of current injection, and the full-blown Ca2+ spike activated when this slower depolarization reached a sufficient membrane potential, a quasithreshold. As a result, the latency of the evoked Ca2+ spike became less with stronger activating inputs because a stronger input produced a stronger depolarization that reached the critical membrane potential earlier. Although Ca2+ spikes were activated in a nearly all-or-none manner from a given holding potential, their actual amplitudes were related to these holding potentials, which, in turn, determined the level of IT deinactivation. Our simulations could reproduce all of the main experimental observations. They further suggest that the voltage-dependent K+ conductance underlying IA, which is known to delay firing in many cells, does not seem to contribute to the variable latency seen in activation of Ca2+ spikes. Instead the simulations indicate that the activation of IT starts initially with a slow and graded depolarization until enough of the underling transient (or T) Ca2+ channels are recruited to produce a fast, "autocatalytic" depolarization seen as the Ca2+ spike. This can produce variable latency dependent on the strength of the initial activation of T channels. The nearly all-or-none nature of Ca2+ spike activation suggests that when a burst of action potentials normally is evoked as a result of a Ca2+ spike and transmitted to cortex, this signal is largely invariant with the amplitude of the input activating the relay cell.

Contribution of potassium conductances to a time-dependent transition in electrical properties of a cockroach motoneuron soma

Contribution of potassium conductances to a time-dependent transition in electrical properties of a cockroach motoneuron soma. The cell body of the cockroach (Periplaneta americana) fast coxal depressor motoneuron (Df) displays a time-dependent change in excitability. Immediately after dissection, depolarization evokes plateau potentials, but after several hours all-or-none action potentials are evoked. Because K channel blockers have been shown to produce a similar transition in electrical properties, we have used current-clamp, voltage-clamp and action-potential-clamp recording to elucidate the contribution of different classes of K channel to the transition in electrical activity of the neuron. Apamin had no detectable effect on the neuron, but charybdotoxin (ChTX) caused a rapid transition from plateau potentials to spikes in the somatic response of Df to depolarization. In neurons that already produced spikes when depolarized, ChTX increased spike amplitude but did not increase their duration nor decrease the amplitude of their afterhyperpolarization. 4-Aminopyridine (4-AP) (which selectively blocks transient K currents) did not cause a transition from plateau potentials to spikes but did enhance oscillations superimposed on plateau potentials. When applied to neurons that already generated spikes when depolarized, 4-AP could augment spike amplitude, decrease the latency to the first spike, and prolong the afterhyperpolarization. Evidence suggests that the time-dependent transition in electrical properties of this motoneuron soma may result, at least in part, from a fall in calcium-dependent potassium current (IK,Ca), consequent on a gradual reduction in [Ca2+ ]i. Voltage-clamp experiments demonstrated directly that outward K currents in this neuron do fall with a time course that could be significant in the transition of electrical properties. Voltage-clamp experiments also confirmed the ineffectiveness of apamin and showed that ChTX blocked most of IK,Ca. Application of Cd2+ (0.5 mM), however, caused a small additional suppression in outward current. Calcium-insensitive outward currents could be divided into transient (4-AP-sensitive) and sustained components. The action-potential-clamp technique revealed that the ChTX-sensitive current underwent sufficient activation during the depolarizing phase of plateau potentials to enable it to shunt inward conductances. Although the ChTX-sensitive conductance apparently makes little contribution to spike repolarization, the ChTX-resistant IK,Ca does make a significant contribution to this phase of the action potential. The 4-AP-sensitive current began to develop during the rising phase of both action potentials and plateau potentials but had little effect on the electrical activity of the neuron, probably because of its relatively small amplitude.

Receptor potentials and electrical properties of nonspiking stretch-receptive neurons in the sand crab Emerita analoga (Anomura, Hippidae)

Receptor potentials and electrical properties of nonspiking stretch-receptive neurons in the sand crab Emerita analoga (Anomura, Hippidae). Four nonspiking, monopolar neurons with central somata and large peripheral dendrites constitute the sole innervation of the telson-uropod elastic strand stretch receptor in Emerita analoga. We characterized their responses to stretch and current injection, using two-electrode current clamp, in intact cells and in two types of isolated peripheral dendritic segments, one that included and one that excluded the dendritic termini (mechanosensory membrane). The membrane potentials of intact cells at rest (mean +/- SD: -57 +/- 4. 4 mV, n = 30), recorded in peripheral or neuropil processes, are similar to the membrane potentials of isolated dendritic segments and always less negative than membrane potentials of motoneurons and interneurons recorded in the same preparations. Ion substitution experiments indicate that the membrane potential is influenced strongly by Na+ conductance, probably localized in the mechanotransducing terminals within the elastic strand. The form of the receptor potential in response to ramp-hold-release stretch remains the same as stretch amplitude is varied and is not dependent on initial membrane potential (-70 to -30 mV) or recording site: initial depolarization (slope follows ramp of applied stretch), terminated by rapid, partial repolarization to a plateau (delayed depolarization) that is intermediate between the peak depolarization and the initial potential and sustained for the duration of the stretch. Responses to depolarizing current pulses are similar to stretch-evoked receptor potentials, except for small amplitude stimuli: an initial peak occurs only in response to stretch and probably reflects elastic recoil of the extracellular matrix surrounding the dendritic terminals. The rapid, partial repolarization depends on holding potential and is abolished by 4-aminopyridine (4-AP; 10 mM), implicating a fast-activating, fast-inactivating K+ conductance; TEA (60 mM) abolishes the remaining slow repolarization to the plateau. In intact cells, but not dendritic segments, regenerative depolarizations can arise in response to stretch or depolarizing current pulses; they are reduced by CdCl2 (10 microM) in the saline containing TEA and 4-AP and probably reflect current spread from Ca2+ influx at presynaptic terminals in the ganglion. We found no evidence for other voltage-activated conductances. Unlike morphologically similar "nonspiking" thoracic receptors of other species, E. analoga's nonspiking neurons are electrically compact and do not boost the analogue afferent signal by voltage-activated inward currents. The most prominent (only?) voltage-activated extra-ganglionic conductances are for potassium; by reducing the slope of the stretch-plateau depolarization curve, they extend each neuron's functional range to the full range of sensitivity of the receptor.

Properties of the transient K+ current in acutely isolated supraoptic neurons from adult rat

The transient outward current (ITO) in magnocellular neurosecretory cells was studied using whole cell patch clamp recordings made from supraoptic neurons acutely isolated from the adult rat. In the presence of tetrodotoxin, depolarizing steps applied from a negative holding potential evoked a rapidly activating and inactivating outward current followed by a slowly activating and sustained outward current. The ITO was unaffected by tetraethylammonium (TEA), but was blocked by 4-aminopyridine. The ITO evoked by depolarization from negative potentials in the presence of 40 mM TEA was not different from that revealed by digital subtraction of current traces recorded with and without a negative conditioning prepulse in control solutions. Tail current analysis during perfusion of media containing different external [K+] indicated that ITO is selective for K+ ions. Exposure to Ca(2+)-free solutions containing divalent inorganic cations such as Cd2+ and Ni2+ could cause shifts in voltage-dependency and, thereby, reduce the amplitude of ITO recorded at fixed submaximal potentials, The maximal ITO achievable under these conditions, however, was reduced compared to control. Moreover, the ITO was also reduced by application of organic Ca2+ channel antagonists such as nifedipine and omega-conotoxin GVIA, indicating that a component of the ITO is somehow dependent on Ca2+ influx.

Voltage-activated K+ currents of hypoglossal motoneurons in a brain stem slice preparation from the neonatal rat

Whole cell, patch-clamp recordings were performed on motoneurons of the hypoglossus nucleus in a brain stem slice preparation from the neonatal rat brain. The aim was to investigate transient outward currents activated by membrane depolarization under voltage clamp conditions. In a Ca2+-free medium containing tetrodotoxin and Cs+, depolarizing voltage commands from a holding potential of -50 mV induced slow outward currents (Islow) with 34 +/- 6 ms (SE) onset time constant at 0 mV and minimal decline during a 1 s pulse depolarization. When the depolarizing command was preceded by a prepulse to -110 mV, the outward current became biphasic as it comprised a faster component (Ifast), which could be investigated in isolation by subtracting the two sets of records. Ifast showed rapid kinetics (9 +/- 4 ms 10-90% rise time and 70 +/- 20 ms decay time constant at 0 mV) and strong voltage-dependent inactivation (half inactivation was at -92.9 +/- 0.2 mV) from which it readily recovered with a biexponential timecourse (4.4 +/- 0.6 and 17 +/- 2 ms time constants at -110 mV membrane potential). Islow was selectively blocked by TEA (10-30 mM) while Ifast was preferentially depressed by 2-3 mM 4-aminopyridine. Analysis of tail current reversal indicated that both Islow and Ifast were predominantly due to K+ with minor permeability to Na+ (92/1 and 50/1, respectively). These results suggest that membrane depolarization activated distinct K+ conductances that, in view of their largely dissimilar kinetics, are likely to play a differential role in regulating the firing behavior of hypoglossal motoneurons.

Early activation of inhibitory neurons in the thalamic reticular nucleus during focal neocortical seizures

The neurons of the thalamic reticular nucleus are among the main targets of corticothalamic projections and their vulnerability in pathological conditions is well established. The present experiments aimed at the description and immunocytochemical characterization of the neurons of the thalamic reticular nucleus activated in neocortical seizures. Focal seizures were induced by the topical application of isotonic, isohydric 4-aminopyridine solution to the sensorimotor neocortex of adult, anesthetized Wistar rats. The animals were perfused with fixative after 1 and 2 h of recorded seizure activity. Coronal plane vibratome sections were incubated with cocktails of polyclonal c-fos and monoclonal parvalbumin antisera. Labeled cells in the thalamic reticular nucleus were counted and related to total cell counts. Neurons and neuropil showed strong parvalbumin immunoreactivity. Double-stained sections revealed that 55.32% of the parvalbumin-positive cell population expressed c-fos protein in their cell nuclei at the end of the 1 h seizure period. This ratio decreased to 43.5% following 2 h seizure. Labeled cells, although less in number were also observed in the contralateral thalamic reticular nucleus. Since parvalbumin labels GABAergic cells, it is tempting to speculate that this activation of intrathalamic inhibiton might exert an important anticonvulsant protection on other thalamic nuclei.

Morphological and membrane properties of rat magnocellular basal forebrain neurons maintained in culture

Morphological and electrophysiological characteristics of magnocellular neurons from basal forebrain nuclei of postnatal rats (11-14 days old) were examined in dissociated cell culture. Neurons were maintained in culture for periods of 5-27 days, and 95% of magnocellular (>23 micron diam) neurons stained positive with acetylcholinesterase histochemistry. With the use of phase contrast microscopy, four morphological subtypes of magnocellular neurons could be distinguished according to the shape of their soma and pattern of dendritic branching. Corresponding passive and active membrane properties were investigated with the use of whole cell configuration of the patch-clamp technique. Neurons of all cell types displayed a prominent (6-39 mV; 6.7-50 ms duration) spike afterdepolarization (ADP), which in some cells reached firing threshold. The ADP was voltage dependent, increasing in amplitude and decreasing in duration with membrane hyperpolarization with an apparent reversal potential of -59 +/- 2.3 (SE) mV. Elevating [Ca2+]o (2.5-5.0 mM) or prolonging spike repolarization with 10 mM tetraethylammonium (TEA) or 1 mM 4-aminopyridine (4-AP), potentiated the ADP while it was inhibited by reducing [Ca2+]o (2.5-1 mM) or superfusion with Cd2+ (100 microM). The ADP was selectively inhibited by amiloride (0.1-0.3 mM or Ni2+ 10 microM) but unaffected by nifedipine (3 microM), omega-conotoxin GVIA (100 nM) or omega-agatoxin IVA (200 nM), indicating that Ca2+ entry was through T-type Ca2+ channels. After inhibition of the ADP with amiloride (300 microM), depolarization to less than -65 mV revealed a spike afterhyperpolarization (AHP) with both fast and slow components that could be inhibited by 4-AP (1 mM) and Cd2+ (100 microM), respectively. In all cell types, current-voltage relationships exhibited inward rectification at hyperpolarized potentials >/=EK (approximately -90 mV). Application of Cs+ (0.1-1 mM) or Ba2+ (1-10 microM) selectively inhibited inward rectification but had no effect on resting potential or cell excitability. At higher concentrations, Ba2+ (>10 microM) also inhibited an outward current tonically active at resting potential (VH -70 mV), which under current-clamp conditions resulted in small membrane depolarization (3-10 mV) and an increase in cell excitability. Depolarizing voltage commands from prepulse potential of -90 mV (VH -70 mV) in the presence of tetrodotoxin (0.5 microM) and Cd2+ (100 microM) to potentials between -40 and +40 mV cause voltage activation of both transient A-type and sustained delayed rectifier-type outward currents, which could be selectively inhibited by 4-AP (0.3-3 mM) and TEA (1-3 mM), respectively. These results show that, although acetylcholinesterase-positive magnocellular basal forebrain neurons exhibit considerable morphological heterogeneity, they have very similar and characteristic electrophysiological properties.

Density of transient K+ current influences excitability in acutely isolated vasopressin and oxytocin neurones of rat hypothalamus

1. The transient outward K+ current (ITO) was studied using whole-cell recording in immunocytochemically identified oxytocin (OT; n = 23) and vasopressin (VP; n = 67) magnocellular neurosecretory cells (MNCs) acutely isolated from the supraoptic nucleus of adult rats. 2. The peak density of ITO during steps to -10 mV was 26 % smaller in OT-MNCs (355 +/- 23 pA pF-1; mean +/- s.e. m.; n = 18) than in VP-MNCs (478 +/- 17 pA pF-1; n = 52). No differences were observed in the voltage dependence of activation or inactivation. 3. Kinetic analysis revealed two components of ITO inactivation in both OT-MNCs (tau1 = 9.2 +/- 0.4 ms and tau2 = 41.2 +/- 1.6 ms; n = 18) and VP-MNCs (tau1 = 12.4 +/- 0.4 ms and tau2 = 37.1 +/- 1.2 ms; n = 52). Although the density of the rapid component (tau1) was not different (275 +/- 13 versus 265 +/- 16 pA pF-1, respectively), the slow component (tau2) was markedly smaller in OT-MNCs (183 +/- 19 versus 331 +/- 16 pA pF-1 in VP-MNCs). 4. In unidentified MNCs, 0.5 mM 4-aminopyridine reduced ITO amplitude by 29% and decreased the latency to spike discharge by about 70% during depolarization from -70 mV. Latency to discharge from potentials less negative than -60 mV, where ITO is inactivated, was unaffected. 5. Comparison of latency to spike discharge in identified cells showed that OT-MNCs achieve spike threshold twice as fast as VP-MNCs when depolarized from -70 mV. The lower density of ITO in OT-MNCs, therefore, accelerates the rate at which excitation can occur in response to depolarizing stimuli and may facilitate the occurrence of higher frequency discharges in OT-MNCs during physiological activation.

Voltage-clamp analysis and computer simulation of a novel cesium-resistant A-current in guinea pig laterodorsal tegmental neurons

Increased firing of cholinergic neurons of the laterodorsal tegmental nucleus (LDT) plays a critical role in generating the behavioral states of arousal and rapid eye movement sleep. The majority of these neurons exhibit a prominent transient potassium current (IA) that shapes firing but the properties of which have not been examined in detail. Although IA has been reported to be blocked by intracellular cesium, the IA in LDT neurons appeared resistant to intracellular cesium. The present study compared the properties of this cesium-resistant current to those typically ascribed to IA. Whole cell recordings were obtained from LDT neurons (n = 67) in brain slices with potassium- or cesium-containing pipette solutions. A transient current was observed in cells dialyzed with each solution (KGluc-85%; CsGluc-79%). However, in cesium-dialyzed neurons, the transient current was inward at test potentials negative to about -35 mV. Extracellular 4-aminopyridine (4-AP; 2-5 mM) blocked both inward and outward current, suggesting the inward current was reversed IA rather than an unmasked transient calcium current as previously suggested. This conclusion was supported by increasing [K]o from 5 to 15 mM, which shifted the reversal potential positively for both inward and outward current (+17.89 +/- 0.41 mV; mean +/- SE). Moreover, recovery from inactivation was rapid (tau = 15.5 +/- 4 ms; n = 4), as reported for IA, and both inward and outward transient current persisted in calcium-free solution [0 calcium/4 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N', N'-tetraacetic acid; n = 4] and during cadmium-blockade of calcium currents (n = 3). Finally, the transient current was blocked by intracellular 4-AP indicating that adequate dialysis occurred during the recordings. Thus the Cs-resistant current is a subthreshold IA. We also estimated the voltage-dependence of activation (V1/2 = -45.8 +/- 2 mV, k = 5.21 +/- 0.62 mV, n = 6) and inactivation (V1/2 = -59. 0 +/- 2.38 mV, k = -5.4 +/- 0.49 mV, n = 3) of this current. Computer simulations using a morphologically accurate model cell indicated that except for the extreme case of only distal A-channels and a high intracellular resistivity, our parameter estimates were good approximations. In conclusion, guinea pig LDT neurons express subthreshold A-channels that are resistant to intracellular cesium ions. This suggests that these channels differ fundamentally in their ion permeation mechanism from those previously studied. It remains to be determined if Cs+ resistance is common among brain A-channels or if this property is conferred by known A-channel subunits.

Morphologically identified cutaneous afferent DRG neurons express three different potassium currents in varying proportions

Outward K+ currents were recorded using a whole cell patch-clamp configuration, from acutely dissociated adult rat cutaneous afferent dorsal root ganglion (DRG) neurons (L4 and L5) identified by retrograde labeling with Fluoro-gold. Recordings were obtained 16-24 h after dissociation from cells between 39 and 49 mm in diameter with minimal processes. These cells represent medium-sized DRG neurons relative to the entire population, but are large cutaneous afferent neurons giving rise to myelinated axons. Voltage-activated K+ currents were recorded routinely during 300-ms depolarizing test pulses increasing in 10-mV steps from -40 to +50 mV; the currents were preceded by a 500-ms conditioning prepulse of either -120 or -40 mV. Coexpression of at least three components of K+ current was revealed. Separation of these components was achieved on the basis of sensitivities to the K+ channel blockers, 4-aminopyridine (4-AP) and dendrotoxin (DTx), and by the current responses to variation in conditioning voltage. Changing extracellular K+ concentration from 3 to 40 mM resulted in a shift to the right of the I-V curve commensurate with K+ being the principal charge carrier. Presentation of 100 mM 4-AP revealed a rapidly activating K+ current sensitive to low concentrations of 4-AP. High concentrations of 4-AP (6 mM) extinguished all inactivating current, leaving almost pure sustained current (IK). On the basis of the relative distribution of K+ currents neurons could be separated into three distinct categories: fast inactivating current (IA), slow inactivating current (ID), and sustained current (IK); only IA and IK; and slow inactivating current and IK. However, IK was always the dominant outward current component. These results indicate that considerable variation in K+ currents is present not only in the entire population of DRG neurons, as previously reported, but even within a restricted size and functional group (large cutaneous afferent neurons).

Distinct functional stoichiometry of potassium channel beta subunits

Shaker-type potassium channels play important roles in determining the electrical excitability of cells. The native channel complex is thought to be formed by four pore-forming alpha subunits that provide four interaction sites for auxiliary modulatory Kvbeta subunits. Because Kvbeta subunits possess diverse modulatory activities including either up-regulation or down-regulation of potassium currents, differential assembly of the alpha-beta complex could give rise to diverse current properties. However, the detailed physical and functional stoichiometry of the alpha-beta complex remains unknown. Kvbeta1 subunits reduce potassium currents through inactivation, whereas Kvbeta2 subunits enhance potassium currents by inhibiting the Kvbeta1-mediated inactivation and at the same time by promoting the surface expression of certain potassium channels. In this report we show that Kvbeta1 and Kvbeta2 of the Shaker-type potassium channels display distinct functional stoichiometry to interact with the Kv1 alpha subunits, a subfamily of Shaker-type potassium channels. The interaction of Kvbeta1 subunits with alpha subunits is consistent with the alpha4betan model, where n equals 0, 1, 2, 3, or 4, depending upon the relative concentration of alpha and beta subunits. The alpha4betan stoichiometry allows for gradual changes of the Kvbeta1-mediated inactivation. In contrast, Kvbeta2 subunits self-associate to form oligomers and interact with the alpha subunits via alpha4beta4 stoichiometry, which permits effective multivalent associations with alpha subunits. Such distinct functional stoichiometry of Kvbeta1 and Kvbeta2 provides a molecular mechanism that is well suited to their contrasting activities of up-regulation or down-regulation of potassium currents.

Differential expression of Kv4 K+ channel subunits mediating subthreshold transient K+ (A-type) currents in rat brain

The mammalian Kv4 gene subfamily and its Drosophila Shal counterpart encode proteins that form fast inactivating K+ channels that activate and inactivate at subthreshold potentials and recover from inactivation at a faster rate than other inactivating Kv channels. Taken together, the properties of Kv4 channels compare best with those of low-voltage activating "A-currents" present in the neuronal somatodendritic compartment and widely reported across several types of central and peripheral neurons, as well as the (Ca2+-independent) transient outward potassium conductance of heart cells (Ito). Three distinct genes have been identified that encode mammalian Shal homologs (Kv4. 1, Kv4.2, and Kv4.3), of which the latter two are abundant in rat adult brain and heart tissues. The distribution in the adult rat brain of the mRNA transcripts encoding the three known Kv4 subunits was studied by in situ hybridization histochemistry. Kv4.1 signals are very faint, suggesting that Kv4.1 mRNAs are expressed at very low levels, but Kv4.2 and Kv4.3 transcripts appear to be abundant and each produces a unique pattern of expression. Although there is overlap expression of Kv4.2 and Kv4.3 transcripts in several neuronal populations, the dominant feature is one of differential, and sometimes reciprocal expression. For example, Kv4.2 transcripts are the predominant form in the caudate-putamen, pontine nucleus and several nuclei in the medula, whereas the substantia nigra pars compacta, the restrosplenial cortex, the superior colliculus, the raphe, and the amygdala express mainly Kv4.3. Some brain structures contain both Kv4.2 and Kv4.3 mRNAs but each dominates in distinct neuronal subpopulations. For example, in the olfactory bulb Kv4.2 dominates in granule cells and Kv4.3 in periglomerular cells. In the hippocampus Kv4.2 is the most abundant isoform in CA1 pyramidal cells, whereas only Kv4.3 is expressed in interneurons. Both are abundant in CA2-CA3 pyramidal cells and in granule cells of the dentate gyrus, which also express Kv4.1. In the dorsal thalamus strong Kv4.3 signals are seen in several lateral nuclei, whereas medial nuclei express Kv4.2 and Kv4.3 at moderate to low levels. In the cerebellum Kv4.3, but not Kv4.2, is expressed in Purkinje cells and molecular layer interneurons. In the cerebellar granule cell layer, the reciprocity between Kv4.2 and Kv4.3 is observed in subregions of the same neuronal population. In fact, the distribution of Kv4 channel transcripts in the cerebellum defines a new pattern of compartmentation of the cerebellar cortex and the first one involving molecules directly involved in signal processing.

Structural requirements and ionic mechanism of the Mytilus inhibitory peptides (MIPs) on Helix central neurons

1. The structural and ionic requirements for potent FVamide-induced inhibition were investigated using Helix aspersa central neurons under current or voltage clamp. 2. For potent FVamide inhibition the full hexapeptide sequence, GSPYFVamide, was required. The rank order of decreasing potency was GSPYFVamide >> SPYFVamide> PYFVamide > YFVamide. 3. GSPYFVamide inhibition involved an increase in conductance to both potassium and chloride ions as demonstrated by ion substitution experiments and addition of 4-aminopyridine, tetraethylammonium, 4,4-di-isothiocyanatostilbene-2,2'-disulfonic acid, 4,4-dinitrostilbene-2,2'-disulfonic acid disodium salt and furosemide to the solution bathing the preparation.

Voltage-activated potassium channels in mammalian neurons and their block by novel pharmacological agents

1. Electrophysiological studies have shown that a number of different types of potassium (K) channel currents exist in mammalian neurons. Among them are the voltage-gated K channel-currents which have been classified as fast-inactivating A-type currents (KA) and slowly inactivating delayed-rectifier type currents (KDR). 2. Two major molecular superfamilies of K channel have been identified; the KIR superfamily and the Shaker-related superfamily with a number of different pore-forming alpha-subunits in each superfamily. 3. Within the Shaker-related superfamily are the KV family, comprising of at least 18 different alpha-subunits that almost certainly underlie classically defined KA and KDR currents. However, the relationship between each of these cloned alpha-subunits and native voltage-gated K currents remains, for the most part, to be established. 4. Classical pharmacological blockers of voltage-gated K channels such as tetraethylammonium ions (TEA), 4-aminopyridine (4-AP), and certain toxins lack selectivity between different native channel currents and between different cloned K channel currents. 5. A number of other agents block neuronal voltage-gated K channels. All of these compounds are used primarily for other actions they possess. They include organic calcium (Ca) channel blockers, divalent and trivalent metal ions and certain calcium signalling agents such as caffeine. 6. A number of clinically active tricyclic compounds such as imipramine, amitriptyline, and chlorpromazine are also potent inhibitors of neuronal voltage-gated K channels. These compounds are weak bases and it appears that their uncharged form is required for activity. These compounds may provide a useful starting point for the rational design of novel selective K channel blocking agents.

Biophysical properties of descending brain neurons in larval lamprey

In the brains of larval lamprey, biophysical properties of reticulospinal (RS) neurons were determined by applying depolarizing and hyperpolarizing current pulses under current clamp conditions. In response to above threshold depolarizing current pulses, almost all RS neurons produced an initial relatively high spiking frequency (Fi) followed by a variable decay to a steady-state firing frequency (Fss). Spike-frequency adaptation (SFA), defined as [(Fi - Fss)/Fi] x 100%, was minimal at the lowest currents and more pronounced with larger applied current pulses. Some RS neurons, particularly those in the posterior rhombencephalic reticular nucleus (PRRN), either adapted very quickly, and stopped firing, or fired in short bursts during a constant depolarizing current pulse. Several types of RS neurons, including some Muller cells and unidentified neurons in the middle rhombencephalic reticular nucleus (MRRN), displayed delayed excitation (DE) in which spiking in response to a depolarizing current pulse was delayed if preceded by a hyperpolarizing prepulse. Very few neurons fired action potentials following a hyperpolarizing pulse, such as in the case of post-inhibitory rebound (PIR), and no neurons were found that displayed plateau potentials. The possible contributions of these properties to the descending activation of spinal locomotor networks is discussed.

Molecular cloning and tissue distribution of an alternatively spliced variant of an A-type K+ channel alpha-subunit, Kv4.3 in the rat

We describe here (1) the heterogeneous expression of Ca2+-independent transient (A-type) K+ channel alpha-subunits (Kv1.4, Kv3.3, Kv3.4, Kv4.2 and Kv4.3) in rat smooth muscle, heart and brain, (2) the molecular cloning and tissue distribution of a novel alternatively spliced variant of an A-type K+ channel alpha-subunit, Kv4.3, and (3) the functional expression of A-type K+ channels in HEK293 cells by the transfection with the novel splice variant of Kv4.3. A cDNA encoding this splice variant was identified from rat vas deferens by RT-PCR cloning. This cDNA clone contains a 1965 bp open reading frame that encodes for a protein of 655 amino acids. It has a 19 amino acid insertion in comparison with Kv4.3 previously reported in rat brain. RT-PCR analyses showed that the mRNAs of this longer variant are abundantly expressed in a number of smooth muscles of the rat, and that the mRNAs of the previously reported clones are absent. The longer splice variant is very weakly expressed in brain, but is the major product in heart.

Developmental changes of potassium currents of embryonic leech ganglion cells in primary culture

The expression of calcium-activated potassium currents (IK(Ca)), delayed outward rectifier potassium currents (IK(slow)), and transient outward currents (IA) was studied during the development of the nervous system of the leech using the whole-cell patch-clamp recording technique. Dissociated cells were isolated from leech embryos between stage E7 and E16 and maintained in primary culture. K+ currents were recorded at E7, when only few anterior ganglia had formed beneath the primordial mouth. IK(slow) was present in all cells tested, while IK(Ca) was expressed in only 67% of the cells studied. Even as early as E7, different types of IK(Ca) have been found. Neither frequency of occurrence nor the charge density of IK(Ca) showed significant changes between E7 and E16. The density of IK(slow), however, increased by a factor of two between E7 and E8, which resulted in a significant increase in the total K+ current of these cells. This rise in potassium outward current developed in parallel with the appearance of Na+ and Ca2+ inward currents (Schirrmacher and Deitmer: J Exp Biol 155:435-453, 1991) during early development, shaping the electrical excitability in embryonic leech neurones. I(A) could be separated by its voltage-dependence and pharmacological properties. The current was detected at stage E9, when all 32 ganglia are formed in the embryo. The frequency of occurrence of I(A) increased from 16% at E9 to 70% at E15. The channel density, steady state inactivation, and kinetics showed no significant changes during development.

Appearance of a fast inactivating voltage-dependent K+ currents in developing cerebellar granule cells in vitro

To elucidate the molecular mechanisms that regulate the maturation of action potential, we began by examining voltage-dependent K+ currents, known to contribute to the maturation of action potential, of developing granule cells in mouse cerebellar microexplant cultures. The migration of developing granule cells in this culture is reported to mimic the in vivo process, but their specific identification is still incomplete. In this study, we identified and characterized granule cells in this culture. Immunocytochemical analysis found that granule cells migrated radially out from explants and subsequently formed small clusters and also that their morphology changed from a bipolar to a T shape during migration. Moreover, in the electrophysiological study, the GABA response of granule cells in this culture clarified that the electrophysiological properties of granule cells were normally maintained. We therefore have concluded, that this culture system is a powerful tool for investigating the differentiation of cerebellar granule cells. Based on these findings, we recorded voltage-dependent K+ currents of developing granule cells in this culture, while concurrently observing their morphology. Our results show that voltage-dependent K+ currents of developing granule cells change from delayed rectifier to A current in parallel with their morphological changes from bipolar to T-shaped cells.

Conduction block in acute and chronic spinal cord injury: different dose-response characteristics for reversal by 4-aminopyridine

The effect of the potassium channel blocker, 4-aminopyridine (4-AP), on conduction of action potentials in injured guinea pig spinal cord axons was measured using isolated tracts in oxygenated Krebs' solution at 37 degrees C. The dose-response characteristics of acutely and chronically injured axons were compared. The maximal improvement of conduction occurred in acutely injured axons at a concentration of 100 microM 4-AP, but in chronically injured spinal cord at 10 microM. The threshold for significant response to 4-AP was between 0.5 and 1 microM in chronically injured cords, and between 1 and 10 microM following acute compression injury. The difference in susceptibility to potassium channel blockade may be related to underlying differences in the mechanism of conduction block at the two stages of injury. Initially, junctions between axons and myelin are acutely disrupted, altering primarily the leakage resistance of the myelin sheath and periaxonal space. In chronically injured cords, there is widespread but incomplete process of repair in the lesion site, which leaves many axons partially myelinated. The difference in sensitivity to 4-AP suggests there is also some modification of the accessibility of axonal potassium channel or a change in their affinity for the drug.

Outward potassium currents of supraoptic magnocellular neurosecretory cells isolated from the adult guinea-pig

1. Several types of whole-cell outward K+ current recorded from magnocellular neurosecretory cells (MNCs) dissociated from the supraoptic nucleus of the adult guinea-pig were identified on the basis of their voltage dependence, kinetics, pharmacology and Ca2+ dependence. 2. The predominant K+ current evoked from a holding potential of -40 mV was slowly activating, long-lasting, tetraethylammonium (TEA) sensitive and showed little steady-state inactivation. Also, this current was reduced by extracellular Cd2+. These data suggest that in supraoptic MNCs classical Ca(2+)-insensitive, delayed rectifier channels (KV) and Ca(2+)-sensitive, non-inactivating channels (KCa) both contribute to the sustained current. 3. A transient, low-threshold K+ current, which was 4-aminopyridine (4-AP) sensitive and showed significant steady-state inactivation, was evoked along with the sustained current from a holding potential of -90 mV. Based on these characteristics, this current corresponds to the A-current (IK(A)) described in other neurons. 4. IK(A) was activated when Ca2+ influx was blocked or when Ca2+ was absent from the extracellular medium, suggesting that Ca2+ influx is not necessary for activation of the current. 5. In many recordings, a transient 4-AP-insensitive outward current was evoked from a holding potential of -40 mV. This high-threshold transient K+ current was abolished by extracellular Cd2+ or TEA and was absent when extracellular Ca2+ was replaced by Sr2+, suggesting that it is a transient Ca(2+)-dependent K+ current. 6. We conclude that the presence of multiple types of K+ current may, in part, underlie the complex firing patterns of oxytocinergic and vasopressinergic MNCs.

Kvbeta2 inhibits the Kvbeta1-mediated inactivation of K+ channels in transfected mammalian cells

Cloned auxiliary beta-subunits (e.g. Kvbeta1) modulate the kinetic properties of the pore-forming alpha-subunits of a subset of Shaker-like potassium channels. Coexpression of the alpha-subunit and Kvbeta2, however, induces little change in channel properties. Since more than one beta-subunit has been found in individual K+ channel complexes and expression patterns of different beta-subunits overlap in vivo, it is important to test the possible physical and/or functional interaction(s) between different beta-subunits. In this report, we show that both Kvbeta2 and Kvbeta1 recognize the same region on the pore-forming alpha-subunits of the Kv1 Shaker-like potassium channels. In the absence of alpha-subunits the Kvbeta2 polypeptide interacts with additional beta-subunit(s) to form either a homomultimer with Kvbeta2 or a heteromultimer with Kvbeta1. When coexpressing alpha-subunits and Kvbeta1 in the presence of Kvbeta2, we find that Kvbeta2 is capable of inhibiting the Kvbeta1-mediated inactivation. Using deletion analysis, we have localized the minimal interaction region that is sufficient for Kvbeta2 to associate with both alpha-subunits and Kvbeta1. This mapped minimal interaction region is necessary and sufficient for inhibiting the Kvbeta1-mediated inactivation, consistent with the notion that the inhibitory activity of Kvbeta2 results from the coassembly of Kvbeta2 with compatible alpha-subunits and possibly with Kvbeta1. Together, these results provide biochemical evidence that Kvbeta2 may profoundly alter the inactivation activity of another beta-subunit by either differential subunit assembly or by competing for binding sites on alpha-subunits, which indicates that Kvbeta2 is capable of serving as an important determinant in regulating the kinetic properties of K+ currents.

Differential effects of low and high concentrations of 4-aminopyridine on axonal conduction in normal and injured spinal cord

Blockade of potassium channels with the drug 4-aminopyridine has been shown to effect recovery of action potential conduction in myelinated axons under a variety of pathological conditions, but the mechanism and significance of this phenomenon are not completely understood. This study examined the effects of a range of 4-aminopyridine concentrations on conduction in an experimental model of chronic spinal cord injury in guinea-pigs, using sucrose-gap recording from isolated spinal cord strips. The amplitude of the compound action potential increased in response to bath application of 4-aminopyridine, with a threshold between 0.5 and 1 microM and the peak response between 10 and 100 microns. Conduction was suppressed at concentrations of 1 and 10 mM. Uninjured white matter showed no effect on the compound potential of 4-aminopyridine below 1 mM, but there was a similar suppression at concentrations above 1 mM, accompanied by marked membrane depolarization. Peripheral nerve showed only slight action potential suppression and depolarization in the presence of 10 mM 4-aminopyridine. The sensitivity of injured axons to 1 microM 4-aminopyridine is consistent with the hypothesis that some beneficial effects of the drug seen in patients with spinal cord injury are related to improved conduction in myelinated axons, since cerebrospinal fluid levels of 4-aminopyridine should approach this concentration following clinical systemic doses, although it remains likely that synaptic effects also play a role. The blockade of action potential conduction produced by much higher levels of 4-aminopyridine in vitro is possibly a consequence of interference with the resting potential mechanism of the axon membrane, which appears to differ between central and peripheral nerve fibers.

Membrane and synaptic properties of mitral cells in slices of rat olfactory bulb

We have investigated the membrane properties and excitatory synaptic transmission of mitral cells in a slice preparation of rat olfactory bulb. In response to intracellular injection of depolarizing current, most mitral cells showed several distinct membrane properties: (1) delayed onset of firing (suggesting the presence of a type of potassium A current); (2) subthreshold oscillation of the membrane potential; and (3) repetitive firing of clustered action potentials during prolonged threshold stimulation. Olfactory nerve (ON) stimulation evoked a long-lasting EPSP in most of the mitral cells. This long EPSP was completely blocked by combined application of NMDA and non-NMDA receptor antagonists (20 microM CNQX and 100 microM APV), confirming that glutamate is the neurotransmitter at the synapses from ON to mitral cells. The ON-evoked EPSP was preceded by a prespike, which was resistant to membrane potential hyperpolarization at the soma. This fast prepotential may be indicative of an active response in the primary dendritic tufts of the mitral cells. Stimulation of the lateral olfactory tract evoked an antidromic pulse followed by a short EPSP, which could also be elicited independently of an antidromic spike in the recorded cell. Since the asymmetrical synapses so far observed on the mitral cells are all form the ON, this antidromically evoked EPSP may reflect self-excitation of a mitral cell by glutamate released from its own dendrites by antidromic impulse invasion, or/and lateral excitation by neighboring invaded dendrites.

Postnatal maturation of rat hypothalamoneurohypophysial neurons: evidence for a developmental decrease in calcium entry during action potentials

Action potentials and voltage-gated currents were studied in acutely dissociated neurosecretory cells from the rat supraoptic nucleus during the first three postnatal weeks (PW1-PW3), a period corresponding to the final establishment of neuroendocrine relationships. Action potential duration (at half maximum) decreased from 2.7 to 1.8 ms; this was attributable to a decrease in decay time. Application of cadmium (250 microM) reduced the decay time by 43% at PW1 and 21% at PW3, indicating that the contribution of calcium currents to action potentials decreased during postnatal development. The density of high-voltage-activated calcium currents increased from 4.4 to 10.1 pA/pF at postnatal days 1-5 and 11-14, respectively. The conductance density of sustained potassium current, measured at +20 mV, increased from 0.35 (PW1) to 0.53 (PW3) nS/pF. The time to half-maximal amplitude did not change. Conductance density and time- and voltage-dependent inactivation of the transient potassium current were stable from birth. At PW1, the density and time constant of decay (measured at 0 mV) were 0.29 nS/pF (n = 12) and 17.9 ms (n = 10), respectively. Voltage-dependent properties and density (1.1 nS/pF) of the sodium current did not change postnatally. During PW1, fitting the mean activation data with a Boltzmann function gave a half-activation potential of -25 mV. A double Boltzman equation was necessary to adequately fit the inactivation data, suggesting the presence of two populations of sodium channels. One population accounted for approximately 14% of the channels, with a half-inactivation potential of -86 mV; the remaining population showed a half-inactivation potential of -51 mV. A mathematical model, based on Hodgkin-Huxley equations, was used to assess the respective contributions of individual currents to the action potential. When the densities of calcium and sustained potassium currents were changed from immature to mature values, the decay time of the action potentials generated with the model decreased from 2.85 to 1.95 ms. A similar reduction was obtained when only the density of the potassium current was increased. Integration of the calcium currents generated during mature and immature action potentials demonstrated a significant decrease in calcium entry during development. We conclude that the developmental reduction of the action potential duration 1) is a consequence of the developmentally regulated increase in a sustained potassium current and 2) leads to a reduction of the participation of calcium currents in the action potential, resulting in a decreased amount of calcium entering the cell during each action potential.

Voltage-dependent potassium currents of sympathetic preganglionic neurons in neonatal rat spinal cord thin slices

Voltage-dependent potassium currents were analyzed in the visually identified sympathetic preganglionic neurons (SPNs) of neonatal rat spinal cord thin slices by the whole-cell patch-clamp technique. Some of the SPNs were identified by the presence of retrogradely transported fluorescent dye, DiI, injected into the superior cervical ganglion several days prior to experimentation. In a tetrodotoxin (TTX)-containing solution, a step depolarization from the holding potential of -72 mV generated a slow outward current that was suppressed by tetraethylammonium (TEA) and by Ca(2+)-free/2.5 ImM Co2+ solution. Ca(2+)-dependent current consisted of a transient and a sustained components. In a Ca(2+)-free (substituted with Mg2+) solution with TTX and TEA, a step depolarization from a hyperpolarized potential evoked a transient outward current that was blocked by 4-aminopyridine (4-AP). A step hyperpolarization evoked a voltage-dependent inward current, the conductance of which was dependent not only on the membrane potential, but also on the extracellular K+ concentration. Tail current analyses revealed that all of these currents were carried by K+ ions. These results indicate that SPN possesses at least five types of voltage-dependent K+ current, including the delayed rectifier current (IK), Ca(2+)-dependent transient current (IC), Ca(2+)-dependent sustained current (IAHP), A-current (IA) and inward rectifying current (Iu), which may be targets of putative transmitters released from various descending and segmental inputs impinging upon the SPN.

Interaction between low voltage-activated currents in reticular thalamic neurons in a rat model of absence epilepsy

A transient potassium (K+) outward current (IA) contributes to the distinctive patterns of low-threshold spike firing observed in various classes of thalamic neurons through a functional interaction with a calcium (Ca2+)-mediated inward current (IT). The present study was undertaken to investigate the properties of transient K+ currents and their interaction with IT in neurons of the reticular thalamic nucleus, and to compare these properties in reticular thalamic nucleus neurons from a rat model of absence epilepsy, designated the Genetic Absence Epilepsy Rat from Strasbourg (GAERS), with those from a Non-epileptic Control strain (NEC). This comparative approach appeared to be particularly important in view of the recent finding of a selective increase in IT in reticular thalamic nucleus neurons from GAERS. Neurons were acutely isolated from the reticular thalamic nucleus through enzymatic procedures, and identified by morphological and immunocytochemical criteria. Ionic currents were analysed using whole-cell patch-clamp techniques. Transient K+ currents in reticular thalamic nucleus neurons with properties indicative of IA activated at approximately -55 mV (with half-activation at -27 and -33 mV in NEC and GAERS respectively), declined rapidly with a voltage-dependent time constant (tau = 4 ms at +45 mV), were 50% steady-state-inactivated at -81 and -86 mV in the two strains of rats respectively, and rapidly recovered from inactivation with a monoexponential time course (tau = 31 and 37 ms respectively). No significant differences in IA properties or densities were found between reticular thalamic nucleus neurons from GAERS and NEC rats. Analysis of the interaction between IA and IT indicated a shift in the balance between the two opposing membrane conductances towards the generation of a low-voltage-activated inward current in reticular thalamic nucleus neurons from GAERS compared with NEC, and a lack of IA to functionally compensate for this shift, which in turn may contribute to pathological forms of low-threshold spike firing characterizing spike-and-wave discharges.

Different mechanisms underlying the repolarization of narrow and wide action potentials in pyramidal cells and interneurons of cat motor cortex

Two different types of action potentials were observed among the pyramidal cells and interneurons in cat motor cortex: the narrow action potentials and the wide action potentials. These two types of action potentials had similar rising phases (528.8 +/- 77.0 vs 553.1 +/- 71.8 mV/ms for the maximal rising rate), but differed in spike duration (0.44 +/- 0.09 vs 1.40 +/- 0.39 ms) and amplitude (57.31 +/- 8.22 vs 72.52 +/- 8.31 mV), implying that the ionic currents contributing to repolarization of these action potentials are different. Here we address this issue by pharmacological manipulation and using voltage-clamp technique in slices of cat motor cortex. Raising extracellular K+ concentration (from 3 mM to 10 mM), applying a low dose of 4-aminopyridine (2-200 microM) or administering a low concentration of tetraethylammonium (0.2-1.0 mM) each not only broadened the narrow action potentials, but also increased their amplitudes. In contrast, high K+ medium or low dose of tetraethylammonium only broadened the wide action potentials, leaving their amplitudes unaffected, and 4-aminopyridine had only a slight broadening effect on the wide spikes. These results implied that K+ currents were involved in the repolarization of both types of action potentials, and that the K+ currents in the narrow action potentials seemed to activate much earlier than those in the wide spikes. This early activated K+ current may counteract the rapid sodium current, yielding the extremely brief duration and small amplitude of the narrow spikes. The sensitivity of the narrow spikes to 4-aminopyridine may not be mainly attributed to blockade of the classical A current (IA), because depolarizing the membrane potential to inactivate IA did not reproduce the effects of 4-aminopyridine. Blockade of Ca2+ influx slowed the last two-thirds repolarization of the wide action potentials. On the contrary, the narrow action potentials were not affected by Ca(2+)-current blockers, but if they were first broadened by 4-aminopyridine or tetraethylammonium, subsequent application of Ca(2+)-free medium caused further broadening, suggesting that the narrow action potentials were too brief to activate the Ca(2+)-activated potassium currents for their repolarization. Therefore, the effects of low concentrations of tetraethylammonium on the narrow spikes appeared to be mainly due to blockade of an outward current that was different from the tetraethylammonium-sensitive Ca(2+)-activated potassium current (IC). In the neurons with the narrow spikes, voltage-clamp experiments revealed two voltage-gated outward currents that were sensitive to tetraethylammonium and 4-aminopyridine, respectively. Both currents were activated rapidly following the onset of depolarizing steps. Interestingly, the tetraethylammonium-sensitive current was a transient outward current that inactivated rapidly (tau < or = 5 ms), while the 4-aminopyridine-sensitive current was relatively persistent during maintained depolarization. The 4-aminopyridine-sensitive current did not show obvious inactivation even at membrane potential of -40 mV, which completely inactivated the transient tetraethylammonium-sensitive, current. The results indicate that different potassium currents are involved in the repolarization of the narrow and wide action potentials in cat motor cortex. A novel tetraethylammonium-sensitive transient outward current and a 4-aminopyridine-sensitive outward current are responsible for the short duration and small amplitude of the narrow action potentials in the interneurons and some of the layer V pyramidal cells. These two currents are voltage-gated and Ca(2+)-independent. For the wide action potentials that characterize most pyramidal neurons, a Ca(2+)-independent tetraethylammonium-sensitive outward current and a Ca(2+)-activated potassium current are the main contributors to their repolarization.

Modeling Hermissenda: I. Differential contributions of IA and IC to type-B cell plasticity

We developed a multicompartmental Hodgkin-Huxley model of the Hermissenda type-B photoreceptor and used it to address the relative contributions of reductions of two K+ currents, IA and IC, to changes in cellular excitability and synaptic strength that occur in these cells after associative learning. We found that reductions of [symbol: see text] C, the peak conductance of IC, substantially increased the firing frequency of the type-B cell during the plateau phase of a simulated light response, whereas reductions of [symbol: see text] A had only a modest contribution to the plateau frequency. This can be understood at least in part by the contributions of these currents to the light-induced (nonspiking) generator potential, the plateau of which was enhanced by [symbol: see text] C reductions, but not by [symbol: see text] A reductions. In contrast, however, reductions of [symbol: see text] A broadened the type-B cell action potential, increased Ca2+ influx, and increased the size of the postsynaptic potential produced in a type-A cell, whereas similar reductions of [symbol: see text] C had only negligible contributions to these measures. These results suggest that reductions of IA and IC play important but different roles in type-B cell plasticity.

NAB domain is essential for the subunit assembly of both alpha-alpha and alpha-beta complexes of shaker-like potassium channels

There are at least five subfamilies of Shaker-like K+ channels. The diverse function of K+ channels are thought to be further modulated by hydrophilic beta subunits. Here we report that Kv beta 1 inactivates RCK4 and Shaker B K+ channels of the Kv1 subfamily, but not Shal2 of the Kv4 subfamily. This correlates the subfamily-specific bindings of Kv beta 1 to the cytoplasmic N-terminal domains of Kv1 alpha subunits. We map the Kv beta 1-binding site to a region overlapping NABKv1, a domain that specifies different Kv1 alpha subunits to form heterotetramers. Using chimeric alpha subunits, we demonstrate that NABKv1 is essential for the Kv beta 1-mediated inactivation. These results suggest that Kv beta 1 modulates a subset of K+ channels through the specific assembly of alpha-beta complexes and reveal the dual function of the NAB domain in mediating the assembly of both alpha-alpha and alpha-beta complexes.

Higher-order associative processing in Hermissenda suggests multiple sites of neuronal modulation

Two important features of modern accounts of associative learning are (1) the capacity for contextual stimuli to serve as a signal for an unconditioned stimulus (US) and (2) the capacity for a previously conditioned (excitatory) stimulus to "block" learning about a redundant stimulus when both stimuli serve as a signal for the same US. Here, we examined the process of blocking, thought by some to reflect a cognitive aspect of classical conditioning, and its underlying mechanisms in the marine mollusc Hermissenda. In two behavioral experiments, a context defined by chemosensory stimuli was made excitatory by presenting unsignalled USs (rotation) in that context. The excitatory context subsequently blocked overt learning about a discrete conditioned stimulus (CS; light) paired with the US in that context. In a third experiment, the excitability of the B photoreceptors in the Hermissenda eye, which typically increases following light-rotation pairings, was examined in behaviorally blocked animals, as well as in animals that had acquired a normal CS-US association or animals that had been exposed to the CS and US unpaired. Both the behaviorally blocked and the "normal" learning groups exhibited increases in neuronal excitability relative to unpaired animals. However, light-induced multiunit activity in pedal nerves was suppressed following normal conditioning but not in blocked or unpaired control animals, suggesting that the expression of blocking is mediated by neuronal modifications not directly reflected in B-cell excitability, possibly within an extensive network of central light-responsive interneurons.

Whole-cell and perforated patch recordings of four distinct K+ currents in acutely dispersed coeliac-superior mesenteric ganglia neurons of adult rats

Properties and modulation of outward membrane currents in sympathetic neurons acutely dispersed from coeliac-superior mesenteric ganglia (C-SMG) of adult rats were examined using both the whole-cell variant of the patch-clamp technique and the perforated patch approach. Under voltage-clamp, four distinct outward currents were observed: a transient outward current (IA), a voltage-dependent sustained outward current consisting of a Ca(2+)-dependent component (IKCa) and a Ca(2+)-insensitive component (IKV), and a muscarinic agonist-sensitive outward current (IM). IA was isolated by digital subtraction, and characterized by very rapid activation at potentials more positive than -60 mV and by fast and complete voltage-dependent inactivation. Half inactivation potential (Vh) and slope factor (K) were -76 mV and 8.3 mV, respectively. IA was not affected by removal of external Ca2+, 1 mM tetraethylammonium ions, muscarinic agonists, or 8-bromo-cyclic AMP, but was suppressed by 4-aminopyridine (1 mM). Depolarizing pulses from of a holding potential of -50 or -60 mV to potentials more positive than -25 mV concomitantly activated two, independent sustained outward currents which decayed slowly; one exhibited voltage-dependent activation similar to the delayed rectifier current (IKV) and the other being triggered by Ca2+ influx into the cell (IKCa). The addition of tetraethylammonium ions (1 mM) strongly reduced the amplitude of the sustained outward currents. IM was characterized as a noninactivating time- and voltage-dependent outward current which activated at membrane potentials more positive than -60 mV and slowly turned off when the membrane was hyperpolarized back to -60 mV, and was suppressed by muscarinic agonists. The rank order of potency of the agonists tested was: oxotremorine > muscarine > bethanechol.

Differential effects of potassium channel blockers on extracellular concentrations of dopamine and 5-HT in the striatum of conscious rats

1. The selective Ca(2+)-activated K+ channel blocker apamin increased extracellular 5-hydroxytryptamine (5-HT) concentrations in the striatum when administered through the microdialysis probe at doses of 0.1 mM and 1 mM. Extracellular dopamine concentrations increased only at the highest dose administered (1 mM). 2. Mast cell degranulating peptide (MCDP), which blocks the dendrotoxin sensitive delayed rectifier (DR) current, increased extracellular concentrations of dopamine at dose of 10 microM-100 microM but had no effect on 5-HT. 3. The non selective K+ channel blocker tetraethylammonium (TEA) induced a dose-dependent (1 mM-10 mM) increase in extracellular dopamine concentrations and an increase in 5-HT which showed little or no dose-dependency. 4. 4-Aminopyridine (4-AP), a blocker with some similar characteristics to MCDP, increased extracellular dopamine concentrations at doses of 10 microM-1 mM, but had no effect on 5-HT. 5. These findings suggest that dopamine release may be modulated by DR-like current and/or A-current K+ channels. However, in view of the similar effects of MCDP and 4-AP at the concentrations used it is more likely that the dendrotoxin-sensitive DR-like current is involved. In contrast, 5-HT release appears to be modulated by Ca(2+)-activated K+ channels.

Properties of slow early potassium current in neurons of snail Helix pomatia

1. In isolated neurons of visceral ganglia of snail Helix pomatia a slow early outward current (IA) was studied using a two-microelectrode voltage clamp technique. 2. The time of activation and inactivation of IAS at -40 mV were 90-120 msec and 3-5 sec respectively. The removal of inactivation at -120 mV took 2-5 min. 3. The reversal potential of the IAS was about -80 mV in normal saline and was sensitive to the external potassium concentration, changing about 35 mV per fivefold change in potassium over the range from 4 to 20 mM. The results suggest that IA were due to K+. 4. The IA persisted in Ca2+ -free medium and in the presence of Ca2+ -channels blockers, e.g., Cd2+. 5. The IA were blocked by 1-10 microM extracellular 4-aminopyridine, 1 mM of tetraethylammonium ions, 1 mM of Ba2+, but one was resistant to 1 mM Cs+. 6. 4-aminopyridine had a dual effect on the IA. It blocked the normal current, and then appeared to increase the inactivated currents.

The potassium A-current, low firing rates and rebound excitation in Hodgkin-Huxley models

It is widely believed, following the work of Connor and Stevens (1971, J. Physiol. Lond. 214, 31-53) that the ability to fire action potentials over a wide frequency range, especially down to very low rates, is due to the transient, potassium A-current (IA). Using a reduction of the classical Hodgkin-Huxley model, we study the effects of IA on steady firing rate, especially in the near-threshold regime for the onset of firing. A minimum firing rate of zero corresponds to a homoclinic bifurcation of periodic solutions at a critical level of stimulating current. It requires that the membrane's steady-state current-voltage relation be N-shaped rather than monotonic. For experimentally based generic IA parameters, the model does not fire at arbitrarily low rates, although it can for the more atypical IA parameters given by Connor and Stevens for the crab axon. When the IA inactivation rate is slow, we find that the transient potassium current can mediate more complex firing patterns, such as periodic bursting in some parameter regimes. The number of spikes per burst increases as gA decreases and as inactivation rate decreases. We also study how IA affects properties of transient voltage responses, such as threshold and firing latency for anodal break excitation. We provide mathematical explanations for several of these dynamic behaviors using bifurcation theory and averaging methods.

Assembly of voltage-gated potassium channels. Conserved hydrophilic motifs determine subfamily-specific interactions between the alpha-subunits

Voltage-gated potassium (K+) channels are assembled by four identical or homologous alpha-subunits to form a tetrameric complex with a central conduction pore for potassium ions. Most of the cloned genes for the alpha-subunits are classified into four subfamilies: Kv1 (Shaker), Kv2 (Shab), Kv3 (Shaw), and Kv4 (Shal). Subfamily-specific assembly of heteromeric K+ channel complexes has been observed in vitro and in vivo, which contributes to the diversity of K+ currents. However, the molecular codes that mediate the subfamily-specific association remain unknown. To understand the molecular basis of the subfamily-specific assembly, we tested the protein-protein interactions of different regions of alpha-subunits. We report here that the cytoplasmic NH2-terminal domains of Kv1, Kv2, Kv3, and Kv4 subfamilies each associate to form homomultimers. Using the yeast two-hybrid system and eight K+ channel genes, two genes (one isolated from rat and one from Drosophila) from each subfamily, we demonstrated that the associations to form heteromultimers by the NH2-terminal domains are strictly subfamily-specific. These subfamily-specific associations suggest a molecular basis for the selective formation of heteromultimeric channels in vivo.

Characteristics of a fast transient outward current in guinea pig trigeminal motoneurons

A fast transient voltage dependent outward current (TOC) in trigeminal motoneurons (TMNs) was studied in guinea pig brainstem slices by use of sharp electrodes in combination with single electrode voltage clamp techniques. In solutions containing TTX, low Ca2+/Mn2+ and 20 mM TEA this current activated around -55 to -60 mV from holding potentials negative to resting potential, obtained its peak amplitude within 5 ms and decayed as a single exponential with a time constant of 6-8 ms. Half maximal values for inactivation and activation were -72 and -37 mV, respectively. Bath application of 5 mM 4-AP suppressed this current by approximately 90% and eliminated the early depolarizing transient membrane rectification observed in response to a constant depolarizing current pulse, prolonged the action potential duration, and reduced the threshold voltage and delay to onset of the action potential. It is suggested that this current resembles the typical A-current observed in many CNS neurons and, as a result of its voltage and time dependent properties, could contribute to control of motoneuronal discharge and timing of burst onset during rhythmical jaw movements. Therefore, any cellular models of masticatory activity should include the properties of this current.

Ionic currents and inhibitory effects of glibenclamide in seminal vesicle smooth muscle cells

1. Whole-cell voltage-clamp recordings were made from smooth muscle cells isolated from guinea-pig seminal vesicle. 2. When the recording pipette solution contained 130 mM KCl and a low concentration of EGTA (0.2 mM), a dominant outward current was elicited by depolarization to positive of -30 mV from a holding potential of -50 mV. The current was non-inactivating, stimulated by intracellular Ca2+ and blocked by bath-applied 1 mM tetraethylammonium but not 1 mM 3,4 diaminopyridine. 3. If 10 mM EGTA was added to the KCl pipette solution and the holding potential was -50 mV, or more negative, the major current elicited by depolarization to positive of -30 mV was an A-type K(+)-current. This current inactivated rapidly (within 100 ms) and was blocked by bath-applied 1 mM 3,4-diaminopyridine but not 10 mM tetraethylammonium. 4. An inward voltage-gated Ca channel current was observed on depolarization to positive of -30 mV with 1.5 mM Ca2+ or 10 mM Ba2+ in the bath solution and when Ca+ replaced K+ in the pipette. The Ba(2+)-current was shown to be abolished by bath-applied 100 microM Cd2+ and inhibited by 90% by 1 microM nifedipine, and thus appeared to be carried by L-type Ca channels. 5. High concentrations of glibenclamide (10-500 microM) inhibited A-type K(+)-current, Ba(2+)-current and contraction of the whole tissue induced by noradrenaline or electrical field stimulation. 6. From these data we suggest that seminal vesicle smooth muscle cells express Ca2+ -dependent K channels, A-type K channels and L-type Ca channels which are inhibited by tetraethylammonium,3,4-diaminopyridine and nifedipine, respectively. In addition, an unexpected relaxant effect of high concentrations of glibenclamide may be explained by inhibition of the Ca channels.

Structural versus functional modulation of the arterial baroreflex

Structural changes in large arteries are often considered the predominant mechanism responsible for decreased baroreflex sensitivity and baroreceptor resetting in hypertension, atherosclerosis, and aging. Recent work has demonstrated that "functional" mechanisms, both at the level of the peripheral sensory endings and within the central nervous system, contribute significantly to altered baroreflex responses. We have conducted both reductive studies of mechanoelectrical transduction in cultured baroreceptor neurons and integrative studies with in vivo recordings of the activity of baroreceptor afferent fibers and efferent sympathetic nerves. Results suggest that the primary mechanism of mechanical activation of baroreceptor neurons involves opening of stretch-activated ion channels susceptible to blockade by gadolinium. Baroreceptor nerve activity is modulated by the activity of potassium channels and the sodium-potassium pump and by paracrine factors, including prostacyclin, oxygen free radicals, and factors released from aggregating platelets. Endothelial dysfunction and altered release of these paracrine factors contribute significantly to the decreased baroreceptor sensitivity in hypertension and atherosclerosis. The central mediation of the baroreflex depends on the pulse phasic pattern of afferent baroreceptor discharge. Baroreflex-mediated inhibition of sympathetic nerve activity is well maintained during pulse phasic afferent activity. Continuous, nonphasic baroreceptor discharge or a rapid (> 1.5 Hz) pulse phasic discharge results in disinhibition of sympathetic activity. This disinhibition during continuous baroreceptor input is exaggerated with aging. Thus, a defect in central mediation of the baroreflex may be a major cause of the impaired baroreflex and sympathoexcitation in the elderly. In summary, functional neural mechanisms, in addition to structural vascular changes, contribute importantly to altered baroreflex responses in normal and pathophysiological states.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium currents in isolated CA1 neurons of the rat after kindling epileptogenesis

Daily tetanic stimulation of the Schaffer collaterals generates an epileptogenic focus in area CA1 of the rat hippocampus, ultimately leading to generalized tonic-clonic convulsions (kindling). Potassium currents were measured under voltage-clamp conditions in pyramidal neurons, acutely dissociated from the focus of fully kindled rats, one day and six weeks after the last generalized seizure. Their amplitude, kinetics, voltage dependence and calcium dependence were compared with controls. With Ca2+ influx blocked by 0.5 mM Ni2+, the sustained current (delayed rectifier) and the transient current (A-current) were not different after kindling. Calcium influx evoked an additional fast transient current component. This transient calcium-dependent current component was increased by 154%, but only immediately after the seizure. A second, slow calcium-dependent potassium current component was dependent on the intracellular calcium level, set by the pipette as well as on calcium influx. The peak amplitude of this slow calcium-dependent current was under optimal calcium conditions not different after kindling, but we found indications that either calcium homeostasis or the calcium sensitivity of the potassium channels was affected by the kindling process. In contrast to the previously described enhancement of calcium current, kindling epileptogenesis did not change the total potassium current amplitude. The minor changes that were observed can be related either to changes in calcium current or to changes in intracellular calcium homeostasis.

Modulation of acetylcholine release at mouse neuromuscular junctions by interaction of three homologous scorpion toxins with K+ channels

1. The effects of three scorpion toxins, charybdotoxin (CTX), iberiotoxin (IbTX), and noxiustoxin (NTX) have been studied on acetylcholine release and on K+ channels by means of twitch tension and electrophysiological recording techniques using isolated skeletal muscle preparations and by a radioligand binding assay using 125I-labelled dendrotoxin I (DpI) and rat brain synaptosomal membranes. 2. On chick biventer cervicis preparations, CTX and IbTX (125 nM) augmented the twitch responses to indirect muscle stimulation. Further, the increase (about 70-80% of control twitch height) was fast in onset, reaching a maximum within 25-30 min. NTX at 125 nM produced a slower augmentation of the twitch responses to indirect muscle stimulation, with the maximum response being seen after 40-50 min. 3. On mouse triangularis sterni preparations, CTX (300 nM after 35-40 min) and IbTX (100 nM after 15 min) increased quantal content of the evoked endplate potentials (e.p.p.) by about two fold. However, NTX (300 nM) caused only a small increase in e.p.p. amplitude, which was followed by repetitive e.p.ps in response to single shock nerve stimulation after 40-50 min. 4. Extracellular recording of nerve terminal current waveforms in triangularis sterni preparations revealed that CTX and IbTX (3-100 nM), but not NTX (100 nM), blocked the Ca(2+)-activated K+ current, IK-Ca. However, there was no major change in the portion of the nerve terminal waveform associated with voltage-dependent K+ currents, IKv. 5. In the radioligand binding assay, NTX potently displaced labelled [125I]-DpI, whereas CTX produced only partial displacement. However, IbTX did not displace [125I]-DpI from its binding sites on rat brain synaptosomal membranes.6. We conclude that these three structurally homologous scorpion toxins act on different K+ channels and that this leads to different patterns of facilitation of acetylcholine release. IbTX acts selectively on high conductance Ca2+-activated K+ channels, leading to an increase in the amplitude of e.p.ps without any other changes. NTX acts on voltage-dependent K+ channels that are sensitive to dendrotoxin and causes repetitive e.p.ps. CTX shares amino acid residues that exist in the structures of IbTX and NTX;CTX acts on both Ca2+- and voltage-dependent K+ channels.

Mechanisms of baroreceptor activation

The determinants of the nerve activity generated at the baroreceptor endings have been examined. 1) In the isolated carotid sinus, the placement of activated bovine aortic endothelial cells decreased baroreceptor activity (BRA) in a reversible manner. Both endothelin and nitric oxide (NO) suppress BRA, whereas prostacyclin (PGI2) increases activity. 2) The BRA in single units declines and often ceases during non-pulsatile increases in carotid sinus pressure sustained over several minutes. This "adaptation" is attenuated by the transient potassium channel (IA) blocker 4-aminopyridine (4-AP) and not by inhibition of the Na+/K+ pump. 3) In preliminary studies, mechano-electrical transduction was examined in isolated and cultured nodose ganglion neurons. Opening of stretch-activated (SA) channels by suction on the cell-attached patch was seen infrequently; however, probing the neurons consistently increased their intracellular calcium [Ca++]i measured with fura-2. This increase in [Ca++]i is blocked by gadolinium (Gd3+), a trivalent lanthanide reported to block SA channels. Gd3+ also blocks the BRA in the carotid sinus. We conclude that paracrine factors significantly modulate BR sensitivity, that selective ionic mechanisms (the 4-AP sensitive K+ channels) determine the degree of "adaptation" of BR to elevated pressure, and that SA channels sensitive to Gd3+ may be the mechano-electrical transducers in BR neurons.