Blockade by dendrotoxin homologues of voltage-dependent K+ currents in cultured sensory neurones from neonatal rats

Snake Venom: A Promising Source of Neurotoxins Targeting Voltage-Gated Potassium Channels

The venom derived from various sources of snakes represents a vast collection of predominantly protein-based toxins that exhibit a wide range of biological actions, including but not limited to inflammation, pain, cytotoxicity, cardiotoxicity, and neurotoxicity. The venom of a particular snake species is composed of several toxins, while the venoms of around 600 venomous snake species collectively encompass a substantial reservoir of pharmacologically intriguing compounds. Despite extensive research efforts, a significant portion of snake venoms remains uncharacterized. Recent findings have demonstrated the potential application of neurotoxins derived from snake venom in selectively targeting voltage-gated potassium channels (Kv). These neurotoxins include BPTI-Kunitz polypeptides, PLA2 neurotoxins, CRISPs, SVSPs, and various others. This study provides a comprehensive analysis of the existing literature on the significance of Kv channels in various tissues, highlighting their crucial role as proteins susceptible to modulation by diverse snake venoms. These toxins have demonstrated potential as valuable pharmacological resources and research tools for investigating the structural and functional characteristics of Kv channels.

Putative roles of SLC7A5 (LAT1) transporter in pain

Large amino acid transporter 1 (LAT1), also known as SLC7A5, is an essential amino acid transporter that forms a heterodimeric complex with the glycoprotein cell-surface antigen heavy chain (4F2hc (CD98, SLC3A2)). Within nociceptive pathways, LAT1 is expressed in the dorsal root ganglia and spinal cord. Although LAT1 expression is upregulated following spinal cord injury, little is known about LAT1 in neuropathic pain. To date, only circumstantial evidence supports LAT1/4F2hc's role in pain. Notably, LAT1's expression and regulation link it to key cell types and pathways implicated in pain. Transcriptional regulation of LAT1 expression occurs via the Wnt/frizzled/β-catenin signal transduction pathway, which has been shown to be involved in chronic pain. The LAT1/4F2hc complex may also be involved in pain pathways related to T- and B-cells. LAT1's expression induces activation of the mammalian target of rapamycin (mTOR) signaling axis, which is involved in inflammation and neuropathic pain. Similarly, hypoxia and cancer induce activation of hypoxia-inducible factor 2 alpha, promoting not only LAT1's expression but also mTORC1's activation. Perhaps the strongest evidence linking LAT1 to pain is its interactions with key voltage-gated ion channels connected to nociception, namely the voltage-gated potassium channels Kv1.1 and Kv1.2 and the voltage-gated sodium channel Nav1.7. Through functional regulation of these channels, LAT1 may play a role in governing the excitatory to inhibitory ratio which is altered in chronic neuropathic pain states. Remarkably, the most direct role for LAT1 in pain is to mediate the influx of gabapentin and pregabalin, two first-line neuropathic pain drugs, that indirectly inhibit high voltage-activated calcium channel auxiliary subunit α2δ-1. In this review, we discuss the expression, regulation, relevant signaling pathways, and protein interactions of LAT1 that may link it to the development and/or maintenance of pain. We hypothesize that LAT1 expressed in nociceptive pathways may be a viable new target in pain.

Nerve growth factor-dependent hyperexcitability of capsaicin-sensitive bladder afferent neurones in mice with spinal cord injury

NEW FINDINGS

What is the central question of this study? Nerve growth factor (NGF) is reportedly a mediator inducing urinary bladder dysfunction. Is NGF directly involved in hyperexcitability of capsaicin-sensitive C-fibre bladder afferent pathways after spinal cord injury (SCI)? What is the main finding and its importance? Neutralization of NGF by anti-NGF antibody treatment reversed the SCI-induced increase in the number of action potentials and the reduction in spike thresholds and A-type K⁺ current density in mouse capsaicin-sensitive bladder afferent neurones. Thus, NGF plays an important and direct role in hyperexcitability of capsaicin-sensitive C-fibre bladder afferent neurones attributable to the reduction in A-type K⁺ channel activity in SCI.

ABSTRACT

Nerve growth factor (NGF) has been implicated as an important mediator in the induction of C-fibre bladder afferent hyperexcitability, which contributes to the emergence of neurogenic lower urinary tract dysfunction after spinal cord injury (SCI). In this study, we determined whether NGF immunoneutralization using an anti-NGF antibody (NGF-Ab) normalizes the SCI-induced changes in electrophysiological properties of capsaicin-sensitive C-fibre bladder afferent neurones in female C57BL/6 mice. The spinal cord was transected at the Th8/Th9 level. Two weeks later, continuous administration of NGF-Ab (10 μg kg^-1  h^-1 , s.c. for 2 weeks) was started. Bladder afferent neurones were labelled with Fast-Blue (FB), a fluorescent retrograde tracer, injected into the bladder wall 3 weeks after SCI. Four weeks after SCI, freshly dissociated L6-S1 dorsal root ganglion neurones were prepared. Whole-cell patch-clamp recordings were then performed in FB-labelled neurones. After recording action potentials or voltage-gated K⁺ currents, the sensitivity of each neurone to capsaicin was evaluated. In capsaicin-sensitive FB-labelled neurones, SCI significantly reduced the spike threshold and increased the number of action potentials during membrane depolarization for 800 ms. These SCI-induced changes were reversed by NGF-Ab. Densities of slow-decaying A-type K⁺ (K_A ) and sustained delayed rectifier-type K⁺ currents were significantly reduced by SCI. The NGF-Ab treatment reversed the SCI-induced reduction in the K_A current density. These results indicate that NGF plays an important role in hyperexcitability of mouse capsaicin-sensitive C-fibre bladder afferent neurones attributable to a reduction in K_A channel activity. Thus, NGF-targeting therapies could be effective for treatment of afferent hyperexcitability and neurogenic lower urinary tract dysfunction after SCI.

Functional diversity of secreted cestode Kunitz proteins: Inhibition of serine peptidases and blockade of cation channels

We previously reported a multigene family of monodomain Kunitz proteins from Echinococcus granulosus (EgKU-1-EgKU-8), and provided evidence that some EgKUs are secreted by larval worms to the host interface. In addition, functional studies and homology modeling suggested that, similar to monodomain Kunitz families present in animal venoms, the E. granulosus family could include peptidase inhibitors as well as channel blockers. Using enzyme kinetics and whole-cell patch-clamp, we now demonstrate that the EgKUs are indeed functionally diverse. In fact, most of them behaved as high affinity inhibitors of either chymotrypsin (EgKU-2-EgKU-3) or trypsin (EgKU-5-EgKU-8). In contrast, the close paralogs EgKU-1 and EgKU-4 blocked voltage-dependent potassium channels (Kv); and also pH-dependent sodium channels (ASICs), while showing null (EgKU-1) or marginal (EgKU-4) peptidase inhibitory activity. We also confirmed the presence of EgKUs in secretions from other parasite stages, notably from adult worms and metacestodes. Interestingly, data from genome projects reveal that at least eight additional monodomain Kunitz proteins are encoded in the genome; that particular EgKUs are up-regulated in various stages; and that analogous Kunitz families exist in other medically important cestodes, but not in trematodes. Members of this expanded family of secreted cestode proteins thus have the potential to block, through high affinity interactions, the function of host counterparts (either peptidases or cation channels) and contribute to the establishment and persistence of infection. From a more general perspective, our results confirm that multigene families of Kunitz inhibitors from parasite secretions and animal venoms display a similar functional diversity and thus, that host-parasite co-evolution may also drive the emergence of a new function associated with the Kunitz scaffold.

Unveiling the nature of black mamba (Dendroaspis polylepis) venom through venomics and antivenom immunoprofiling: Identification of key toxin targets for antivenom development

The venom proteome of the black mamba, Dendroaspis polylepis, from Eastern Africa, was, for the first time, characterized. Forty- different proteins and one nucleoside were identified or assigned to protein families. The most abundant proteins were Kunitz-type proteinase inhibitors, which include the unique mamba venom components 'dendrotoxins', and α-neurotoxins and other representatives of the three-finger toxin family. In addition, the venom contains lower percentages of proteins from other families, including metalloproteinase, hyaluronidase, prokineticin, nerve growth factor, vascular endothelial growth factor, phospholipase A2, 5'-nucleotidase, and phosphodiesterase. Assessment of acute toxicity revealed that the most lethal components were α-neurotoxins and, to a lower extent, dendrotoxins. This venom also contains a relatively high concentration of adenosine, which might contribute to toxicity by influencing the toxin biodistribution. ELISA immunoprofiling and preclinical assessment of neutralization showed that polyspecific antivenoms manufactured in South Africa and India were effective in the neutralization of D. polylepis venom, albeit showing different potencies. Antivenoms had higher antibody titers against α-neurotoxins than against dendrotoxins, and displayed high titers against less toxic proteins of high molecular mass. Our results reveal the complexity of D. polylepis venom, and provide information for the identification of its most relevant toxins to be neutralized by antivenoms.

BIOLOGICAL SIGNIFICANCE

The black mamba, D. polylepis, is one of the most feared snakes in the world, owing to the potency of its venom, the severity and rapid onset of clinical manifestations of envenomings, and its ability to strike fast and repeatedly. The present study reports the first proteomic analysis of this venom. Results revealed a complex venom constituted predominantly by proteins belonging to the Kunitz-type proteinase inhibitor family, which comprises the dendrotoxins, and to α-neurotoxins of the three-finger toxin family. The proteins showing highest acute toxicity were α-neurotoxins, which induce post-synaptic blockade of the neuromuscular junctions, followed by dendrotoxins, which inhibit the voltage-dependent potassium channels. The combination of these two types of toxins in the venom underscores the presence of a dual strategy that results in a highly effective mechanism for prey subduction. This complex toxic arsenal is likely to provide D. polylepis with high trophic versatility. The rapid onset and severity of neurotoxic clinical manifestations in envenomings by D. polylepis demand the rapid administration of effective and safe antivenoms. Preclinical tests showed that an antivenom from South Africa and two antivenoms from India were effective in the neutralization of this venom, albeit differing in their potency. Moreover, ELISA immunoprofiling of these antivenoms against all venom fractions revealed that antivenoms have higher titers against α-neurotoxins than against dendrotoxins, thus underscoring the need to develop improved immunization strategies. The results of this investigation identified the most relevant toxins present in D. polylepis venom, which need to be targeted by antivenoms or other type of inhibitors.

Differential contribution of Kv4-containing channels to A-type, voltage-gated potassium currents in somatic and visceral dorsal root ganglion neurons

Little is known about electrophysiological differences of A-type transient K(+) (KA) currents in nociceptive afferent neurons that innervate somatic and visceral tissues. Staining with isolectin B4 (IB4)-FITC classifies L6-S1 dorsal root ganglion (DRG) neurons into three populations with distinct staining intensities: negative to weak, moderate, and intense fluorescence signals. All IB4 intensely stained cells are negative for a fluorescent dye, Fast Blue (FB), injected into the bladder wall, whereas a fraction of somatic neurons labeled by FB, injected to the external urethral dermis, is intensely stained with IB4. In whole-cell, patch-clamp recordings, phrixotoxin 2 (PaTx2), a voltage-gated K(+) (Kv)4 channel blocker, exhibits voltage-independent inhibition of the KA current in IB4 intensely stained cells but not the one in bladder-innervating cells. The toxin also shows voltage-independent inhibition of heterologously expressed Kv4.1 current, whereas its inhibition of Kv4.2 and Kv4.3 currents is voltage dependent. The swapping of four amino acids at the carboxyl portion of the S3 region between Kv4.1 and Kv4.2 transfers this characteristic. RT-PCRs detected Kv4.1 and the long isoform of Kv4.3 mRNAs without significant Kv4.2 mRNA in L6-S1 DRGs. Kv4.1 and Kv4.3 mRNA levels were higher in laser-captured, IB4-stained neurons than in bladder afferent neurons. These results indicate that PaTx2 acts differently on channels in the Kv4 family and that Kv4.1 and possibly Kv4.3 subunits functionally participate in the formation of KA channels in a subpopulation of somatic C-fiber neurons but not in visceral C-fiber neurons innervating the bladder.

Potassium channels in peripheral pain pathways: expression, function and therapeutic potential

Electrical excitation of peripheral somatosensory nerves is a first step in generation of most pain signals in mammalian nervous system. Such excitation is controlled by an intricate set of ion channels that are coordinated to produce a degree of excitation that is proportional to the strength of the external stimulation. However, in many disease states this coordination is disrupted resulting in deregulated peripheral excitability which, in turn, may underpin pathological pain states (i.e. migraine, neuralgia, neuropathic and inflammatory pains). One of the major groups of ion channels that are essential for controlling neuronal excitability is potassium channel family and, hereby, the focus of this review is on the K+ channels in peripheral pain pathways. The aim of the review is threefold. First, we will discuss current evidence for the expression and functional role of various K+ channels in peripheral nociceptive fibres. Second, we will consider a hypothesis suggesting that reduced functional activity of K+ channels within peripheral nociceptive pathways is a general feature of many types of pain. Third, we will evaluate the perspectives of pharmacological enhancement of K+ channels in nociceptive pathways as a strategy for new analgesic drug design.

Distinct cellular distributions of Kv4 pore-forming and auxiliary subunits in rat dorsal root ganglion neurons

AIMS

Dorsal root ganglia contain heterogeneous populations of primary afferent neurons that transmit various sensory stimuli. This functional diversity may be correlated with differential expression of voltage-gated K(+) (Kv) channels. Here, we examine cellular distributions of Kv4 pore-forming and ancillary subunits that are responsible for fast-inactivating A-type K(+) current.

MAIN METHODS

Expression pattern of Kv α-subunit, β-subunit and auxiliary subunit was investigated using immunohistochemistry, in situ hybridization and RT-PCR technique.

KEY FINDINGS

The two pore-forming subunits Kv4.1 and Kv4.3 show distinct cellular distributions: Kv4.3 is predominantly in small-sized C-fiber neurons, whereas Kv4.1 is seen in DRG neurons in various sizes. Furthermore, the two classes of Kv4 channel auxiliary subunits are also distributed in different-sized cells. KChIP3 is the only significantly expressed Ca(2+)-binding cytosolic ancillary subunit in DRGs and present in medium to large-sized neurons. The membrane-spanning auxiliary subunit DPP6 is seen in a large number of DRG neurons in various sizes, whereas DPP10 is restricted in small-sized neurons.

SIGNIFICANCE

Distinct combinations of Kv4 pore-forming and auxiliary subunits may constitute A-type channels in DRG neurons with different physiological roles. Kv4.1 subunit, in combination with KChIP3 and/or DPP6, form A-type K(+) channels in medium to large-sized A-fiber DRG neurons. In contrast, Kv4.3 and DPP10 may contribute to A-type K(+) current in non-peptidergic, C-fiber somatic afferent neurons.

Gastrodin inhibits allodynia and hyperalgesia in painful diabetic neuropathy rats by decreasing excitability of nociceptive primary sensory neurons

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus and adversely affects the patients' quality of life. Evidence has accumulated that PDN is associated with hyperexcitability of peripheral nociceptive primary sensory neurons. However, the precise cellular mechanism underlying PDN remains elusive. This may result in the lacking of effective therapies for the treatment of PDN. The phenolic glucoside, gastrodin, which is a main constituent of the Chinese herbal medicine Gastrodia elata Blume, has been widely used as an anticonvulsant, sedative, and analgesic since ancient times. However, the cellular mechanisms underlying its analgesic actions are not well understood. By utilizing a combination of behavioral surveys and electrophysiological recordings, the present study investigated the role of gastrodin in an experimental rat model of STZ-induced PDN and to further explore the underlying cellular mechanisms. Intraperitoneal administration of gastrodin effectively attenuated both the mechanical allodynia and thermal hyperalgesia induced by STZ injection. Whole-cell patch clamp recordings were obtained from nociceptive, capsaicin-sensitive small diameter neurons of the intact dorsal root ganglion (DRG). Recordings from diabetic rats revealed that the abnormal hyperexcitability of neurons was greatly abolished by application of GAS. To determine which currents were involved in the antinociceptive action of gastrodin, we examined the effects of gastrodin on transient sodium currents (I(NaT)) and potassium currents in diabetic small DRG neurons. Diabetes caused a prominent enhancement of I(NaT) and a decrease of potassium currents, especially slowly inactivating potassium currents (I(AS)); these effects were completely reversed by GAS in a dose-dependent manner. Furthermore, changes in activation and inactivation kinetics of I(NaT) and total potassium current as well as I(AS) currents induced by STZ were normalized by GAS. This study provides a clear cellular basis for the peripheral analgesic action of gastrodin for the treatment of chronic pain, including PDN.

Ion-channel modulators: more diversity than previously thought

Ion-channel function can be modified in various ways. For example, numerous studies have shown that currents through voltage-gated ion channels are affected by pore block or modification of voltage dependence of activation/inactivation. Recent experiments performed on various ion channels show that allosteric modulation is an important mechanism for affecting channel function. For instance, in K(Ca)2 (formerly SK) channels, the prototypic "blocker" apamin prevents conduction by an allosteric mechanism, while TRPV1 channels are prevented from closing by a tarantula toxin, DkTx, through an interaction with residues located away from the selectivity filter. The recent evidence, therefore, suggests that in several ion channels, the region around the outer mouth of the pore is rich in binding sites and could be exploited therapeutically. These discoveries also suggest that the pharmacological vocabulary should be adapted to define these various actions.

Ion channels set spike timing regularity of mammalian vestibular afferent neurons

In the mammalian vestibular nerve, some afferents have highly irregular interspike intervals and others have highly regular intervals. To investigate whether spike timing is determined by the afferents' ion channels, we studied spiking activity in their cell bodies, isolated from the vestibular ganglia of young rats. Whole cell recordings were made with the perforated-patch method. As previously reported, depolarizing current steps revealed distinct firing patterns. Transient neurons fired one or two onset spikes, independent of current level. Sustained neurons were more heterogeneous, firing either trains of spikes or a spike followed by large voltage oscillations. We show that the firing pattern categories are robust, occurring at different temperatures and ages, both in mice and in rats. A difference in average resting potential did not cause the difference in firing patterns, but contributed to differences in afterhyperpolarizations. A low-voltage-activated potassium current (I(LV)) was previously implicated in the transient firing pattern. We show that I(LV) grew from the first to second postnatal week and by the second week comprised Kv1 and Kv7 (KCNQ) components. Blocking I(LV) converted step-evoked firing patterns from transient to sustained. Separated from their normal synaptic inputs, the neurons did not spike spontaneously. To test whether the firing-pattern categories might correspond to afferent populations of different regularity, we injected simulated excitatory postsynaptic currents at pseudorandom intervals. Sustained neurons responded to a given pattern of input with more regular firing than did transient neurons. Pharmacological block of I(LV) made firing more regular. Thus ion channel differences that produce transient and sustained firing patterns in response to depolarizing current steps can also produce irregular and regular spike timing.

Bladder hyperactivity and increased excitability of bladder afferent neurons associated with reduced expression of Kv1.4 alpha-subunit in rats with cystitis

Hyperexcitability of C-fiber bladder afferent pathways has been proposed to contribute to urinary frequency and bladder pain in chronic bladder inflammation including interstitial cystitis. However, the detailed mechanisms inducing afferent hyperexcitability after bladder inflammation are not fully understood. Thus, we investigated changes in the properties of bladder afferent neurons in rats with bladder inflammation induced by intravesical application of hydrochloric acid. Eight days after the treatment, bladder function and bladder sensation were analyzed using cystometry and an electrodiagnostic device of sensory function (Neurometer), respectively. Whole cell patch-clamp recordings and immunohistochemical staining were also performed in dissociated bladder afferent neurons identified by a retrograde tracing dye, Fast Blue, injected into the bladder wall. Cystitis rats showed urinary frequency that was inhibited by pretreatment with capsaicin and bladder hyperalgesia mediated by C-fibers. Capsaicin-sensitive bladder afferent neurons from sham rats exhibited high thresholds for spike activation and a phasic firing pattern, whereas those from cystitis rats showed lower thresholds for spike activation and a tonic firing pattern. Transient A-type K(+) current density in capsaicin-sensitive bladder afferent neurons was significantly smaller in cystitis rats than in sham rats, although sustained delayed-rectifier K(+) current density was not altered after cystitis. The expression of voltage-gated K(+) Kv1.4 alpha-subunits, which can form A-type K(+) channels, was reduced in bladder afferent neurons from cystitis rats. These data suggest that bladder inflammation increases bladder afferent neuron excitability by decreasing expression of Kv1.4 alpha-subunits. Similar changes in capsaicin-sensitive C-fiber afferent terminals may contribute to bladder hyperactivity and hyperalgesia due to acid-induced bladder inflammation.

Bladder overactivity and hyperexcitability of bladder afferent neurons after intrathecal delivery of nerve growth factor in rats

Nerve growth factor (NGF) has been proposed as an important mediator inducing bladder overactivity under pathological conditions such as spinal cord injury, bladder outlet obstruction, or cystitis. We therefore examined the effects of chronic NGF treatment on bladder activity and the properties of bladder afferent neurons. In adult female rats, NGF (2.5 microg/microl) was infused continuously into the intrathecal space at the L6-S1 level of spinal cord for 1 or 2 weeks using osmotic pumps (0.5 microl/h). Bladder afferent neurons were labeled with axonal transport of Fast Blue injected into the bladder wall. After intrathecal injection of NGF, cystometrograms under an awake condition showed bladder overactivity revealed by time-dependent reductions in intercontraction intervals and voided volume. ELISA analyses showed significant increases in NGF levels in L6-S1 dorsal root ganglia of NGF-treated rats. In patch-clamp recordings, dissociated bladder afferent neurons exhibiting tetrodotoxin (TTX)-resistant action potentials from NGF-treated animals were larger in diameter and had significantly lower thresholds for spike activation compared with sham rats. In addition, the number of TTX-resistant action potentials during 600 ms depolarizing pulses was significantly increased time dependently after 1 or 2 weeks of NGF application. The density of slowly inactivating A-type K+ currents was decreased by 52% in bladder afferent neurons with TTX-resistant spikes after 2 week NGF treatment. These results indicate that increased NGF levels in bladder afferent pathways and NGF-induced reduction in A-type K+ current density could contribute to the emergence of bladder overactivity as well as somal hypertrophy and hyperexcitability of bladder afferent neurons.

Altered expression of potassium channel subunit mRNA and alpha-dendrotoxin sensitivity of potassium currents in rat dorsal root ganglion neurons after axotomy

Previous studies have raised the possibility that a decrease in voltage-gated K+ currents may contribute to hyperexcitability of injured dorsal root ganglion (DRG) neurons and the emergence of neuropathic pain. We examined the effects of axotomy on mRNA levels for various Kv1 family subunits and voltage-gated K+ currents in L4-L5 DRG neurons from sham-operated and sciatic nerve-transected rats. RNase protection assay revealed that Kv1.1 and Kv 1.2 mRNAs are highly abundant while Kv1.3, Kv1.4, Kv1.5 and Kv1.6 mRNAs were detected at lower levels in L4-L5 DRGs from sham and intact rats. Axotomy significantly decreased Kv1.1, Kv1.2, Kv1.3 and Kv1.4 mRNA levels by approximately 35%, approximately 60%, approximately 40% and approximately 80%, respectively, but did not significantly change Kv1.5 or Kv1.6 mRNA levels. Patch clamp recordings revealed two types of K+ currents in small-sized L4-L5 DRG neurons: sustained delayed rectifier currents elicited from a -40 mV holding potential and slowly inactivating A-type currents that was additionally activated from a -120 mV holding potential. Axotomy decreased both types of K+ currents by 50-60% in injured DRG neurons. In addition, axotomy increased the alpha-dendrotoxin sensitivity of the delayed rectifier, but not slow A-type K+ currents in injured DRG neurons. These results suggest that Kv1.1 and Kv1.2 subunits are major components of voltage-gated K+ channels in L4-L5 DRG neurons and that the decreased expression of Kv1-family subunits significantly contributes to the reduction and altered kinetics of Kv current in axotomized neurons.

Involvement of a Glutamatergic Mechanism in δ-Dendrotoxin-Induced Hippocampal Neuronal Cell Loss in the Rat

The epileptogenic and neurodegenerative effects of gamma-dendrotoxin, from Dendroaspis angusticeps, a specific blocker of a non-inactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats pretreated with CGP040116, a N-methyl-D-aspartate (NMDA) receptor antagonist, and in rats bearing a monolateral surgical lesion of the Schaffer collaterals whose terminals originate from CA3 pyramids and release glutamate in the CA1 hippocampal area. Administration of 35 pmol gamma-dendrotoxin elicited in all of the treated animals (n=8) bilateral EEG discharges and damage to the hippocampal formation. Quantitation of the damage revealed significant bilateral neuronal cell loss in the CA1, CA3 and CA4 pyramidal cell layers. The lowest dose (0.35 pmol; n=4) of the toxin used did not affect EEG activity and failed to cause significant hippocampal cell loss whereas the 3.5 pmol (n=6) dose caused EEG seizures and hippocampal cell loss limited to the CA1 area. Systematic intraperitoneal administration of CGP040116 (5mg/kg given 30 min. previously) delayed the onset of EEG seizures and reduced the number of epileptogenic discharges typically observed in rats receiving an injection of gamma-dendrotoxin (35 pmol) alone. Similarly, this treatment prevented the damage inflicted to the hippocampus by the toxin and in no instance was significant neuronal loss observed. Protection against seizures and hippocampal damage was also observed by a monolateral surgical lesion to the Schaffer collaterals. In conclusion, the present data suggest that an excitotoxic, glutamate-mediated, type of mechanism underlies seizures and hippocampal damage induced by gamma-dendrotoxin in rats.

Gamma-dendrotoxin blocks large conductance Ca2+-activated K+ channels in neuroblastoma cells

In N1E 115 neuroblastoma cells, gamma-dendrotoxin (DTX, 200 nM) blocked the outward K(+) current by 31.1 +/- 3.5% (n = 4) with approximately 500 nM Ca(2+) in the pipet solution, but had no effect on the outward K(+) current when internal Ca(2+) was reduced. Using a ramp protocol, iberiotoxin (IbTX, 100 nM) inhibited a component of the whole cell current, but in the presence of 200 nM gamma-DTX, no further inhibition by IbTX was observed. Two types of single channels were seen using outside-out patches when the pipette free Ca(2+) concentration was approximately 500 nM; a 63 pS and a 187 pS channel. The 63 pS channel was TEA-, IbTX- and gamma-DTX-insensitive, while the 187 pS channel was blocked by 1 mM TEA, 100 nM IbTX or 200 nM gamma-DTX. Both channels were activated by external application of ionomycin, when the pipet calcium concentration was reduced. gamma-DTX (200 nM) reduced the probability of openings of the 187 pS channel, with an IC(50) of 8.5 nM. In GH(3) cells gamma-DTX (200 nM) also blocked an IbTX-sensitive component of whole-cell K(+) currents. These results suggest that gamma-DTX blocks a large conductance Ca(2+) activated K(+) current in N1E 115 cells. This is the first indication that any of the dendrotoxins, which have classically been known to block voltage-gated (Kv) channels, can also block Ca(2+) activated K(+) channels.

Differential expression of three distinct potassium currents in the ventral cochlear nucleus

In the ventral cochlear nucleus (VCN), neurons transform information from auditory nerve fibers into a set of parallel ascending pathways, each emphasizing different aspects of the acoustic environment. Previous studies have shown that VCN neurons differ in their intrinsic electrical properties, including the K+ currents they express. In this study, we examine these K+ currents in more detail using whole cell voltage-clamp techniques on isolated VCN cells from adult guinea pigs at 22 degrees C. Our results show a differential expression of three distinct K+ currents. Whereas some VCN cells express only a high-threshold delayed-rectifier-like current (IHT), others express IHT in combination with a fast inactivating current (IA) and/or a slow-inactivating low-threshold current (ILT). IHT, ILT, and IA, were partially blocked by 1 mM 4-aminopyridine. In contrast, only ILT was blocked by 10-100 nM dendrotoxin-I. A surprising finding was the wide range of levels of ILT, suggesting ILT is expressed as a continuum across cell types rather than modally in a particular cell type. IA, on the other hand, appears to be expressed only in cells that show little or no ILT, the Type I cells. Boltzmann analysis shows IHT activates with 164 +/- 12 (SE) nS peak conductance, -14.3 +/- 0.7 mV half-activation, and 7.0 +/- 0.5 mV slope factor. Similar analysis shows ILT activates with 171 +/- 22 nS peak conductance, -47.4 +/- 1.0 mV half-activation, and 5.8 +/- 0.3 mV slope factor.

Kv1.1 channels of dorsal root ganglion neurons are inhibited by n-butyl-p-aminobenzoate, a promising anesthetic for the treatment of chronic pain

In this study, we investigated the effects of the local anesthetic n-butyl-p-aminobenzoate (BAB) on the delayed rectifier potassium current of cultured dorsal root ganglion (DRG) neurons using the patch-clamp technique. The majority of the K(+) current of small DRG neurons rapidly activates and slowly inactivates at depolarized voltages. BAB inhibited the whole-cell K(+) current of these neurons with an IC(50) value of 228 microM. Dendrotoxin K (DTX(K)), a specific inhibitor of Kv1.1, reduced the DRG K(+) current at +20 mV by 34%, consistent with an important contribution of channels incorporating the Kv1.1 subunit to the delayed rectifier current. To further investigate the mechanism of BAB inhibition, we examined its effect on Kv1.1 channels heterologously expressed in mammalian tsA201 cells. BAB inhibits the Kv1.1 channels with an IC(50) value of 238 microM, similar to what was observed for the native DRG current. BAB accelerates the opening and closing of Kv1.1, but does not alter the midpoint of steady-state activation. BAB seems to inhibit Kv1.1 by stabilizing closed conformations of the channel. Coexpression with the Kv beta 1 subunit induces rapid inactivation and reduces the BAB sensitivity of Kv1.1. Comparison of the heterologously expressed Kv1.1 and native DRG currents indicates that the Kv beta 1 subunit does not modulate the gating of the DTX(K)-sensitive Kv1.1 channels of DRG neurons. Inhibition of the delayed rectifier current of these neurons may contribute to the long-duration anesthesia attained during the epidural administration of BAB.

Platelet-activating factor-induced pulmonary edema is partly mediated by prostaglandin E(2), E-prostanoid 3-receptors, and potassium channels

Platelet-activating factor (PAF) is an important endogenous mediator of pulmonary edema in many models of acute lung injury. PAF triggers edema formation by simultaneous activation of two independent pathways; one is mediated by a cyclooxygenase metabolite, and the other is blocked by quinine. We examined the hypothesis that the cyclooxygenase-dependent part of PAF-induced edema is mediated by prostaglandin E(2) (PGE(2)). In isolated rat lungs, PAF administration stimulated release of PGE(2) into the venous effluate and increased lung weight as a measure of edema formation. Perfusion with a neutralizing PGE(2) antibody attenuated the PAF-induced edema formation. In vivo, E-prostanoid 3-receptor-deficient mice showed less pulmonary Evans blue extravasation in response to PAF injection than did mice deficient in EP1, EP2, or EP4 receptors. Perfusion of rat lungs with PGE(2) caused pulmonary edema, which was largely prevented by inhibition of voltage-gated potassium channels (25 nM beta-dendrotoxin), but not by blocking calcium-dependent potassium currents (100 micro M paxilline). In line with its effects on PGE(2)-induced edema formation, beta-dendrotoxin attenuated PAF-induced edema partly if given alone, and completely in combination with quinine. Our findings suggest that PAF-triggered edema is partly mediated by the release of PGE(2), activation of EP3 receptors, and activation of voltage-gated potassium channels.

Characteristics of brain Kv1 channels tailored to mimic native counterparts by tandem linkage of alpha subunits: implications for K+ channelopathies

Most neuronal Kv1 channels contain Kv1.1, Kv1.2 alpha, and Kvbeta2.1 subunits, yet the influences of their stoichiometries on properties of the (alpha)(4)(beta)(4) variants remain undefined. cDNAs were engineered to contain 0, 1, 2, or 4 copies of Kv1.1 with the requisite number of Kv1.2 and co-expressed in mammalian cells with Kvbeta2.1 to achieve "native-like" hetero-oligomers. The monomeric (Kv1.1 or 1.2), dimeric (Kv1.1-1.2 or 1.2-1.2), and tetrameric (Kv1.1-(1.2)(3)) constructs produced proteins of M(r) approximately 62,000, 120,000, and 240,000, which assembled into (alpha)(4)(beta)(4) complexes. Each alpha cRNA yielded a distinct K(+) current in oocytes, with voltage dependence of activation being shifted negatively as the Kv1.1 content in tetramers was increased. Channels containing 1, 2, or 4 copies of Kv1.1 were blocked by dendrotoxin k (DTX)(k) with similarly high potencies, whereas Kv(1.2)(4) proved nonsusceptible. Accordingly, Kv1.2/beta2.1 expressed in baby hamster kidney cells failed to bind DTX(k); in contrast, oligomers containing only one Kv1.1 subunit in a tetramer exhibited high affinity, with additional copies causing modest increases. Thus, one Kv1.1 subunit largely confers high affinity for DTX(k), whereas channel electrophysiological properties are tailored by the content of Kv1.1 relative to Kv1.2. This notable advance could explain the diversity of symptoms of human episodic ataxia I, which is often accompanied by myokymia, due to mutated Kv1.1 being assembled in different combinations with wild-type and Kv1.2.

Twenty years of dendrotoxins

Dendrotoxins are small proteins that were isolated 20 years ago from mamba (Dendroaspis) snake venoms (Harvey, A.L., Karlsson, E., 1980. Dendrotoxin from the venom of the green mamba, Dendroaspis angusticeps: a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Naunyn-Schmiedebergs Arch. Pharmacol. 312, 1-6.). Subsequently, a family of related proteins was found in mamba venoms and shown to be homologous to Kunitz-type serine protease inhibitors, such as aprotinin. The dendrotoxins contain 57-60 amino acid residues cross-linked by three disulphide bridges. The dendrotoxins have little or no anti-protease activity, but they were demonstrated to block particular subtypes of voltage-dependent potassium channels in neurons. Studies with cloned K(+) channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas toxin K from the black mamba Dendroaspis polylepis preferentially blocks Kv1.1 channels. Structural analogues of dendrotoxins have helped to define the molecular recognition properties of different types of K(+) channels, and radiolabelled dendrotoxins have also been useful in helping to discover toxins from other sources that bind to K(+) channels. Because dendrotoxins are useful markers of subtypes of K(+) channels in vivo, dendrotoxins have become widely used as probes for studying the function of K(+) channels in physiology and pathophysiology.

Effects of alpha-dendrotoxin and dendrotoxin K on extracellular excitatory amino acids and on electroencephalograph spectral power in the hippocampus of anaesthetised rats

Dendrotoxins, important pharmacological tools for studying K(+) channels, are potently convulsant in the central nervous system and evidence suggests that different members of the dendrotoxin family may act at pre- or post-synaptic sites. Using a combination of intrahippocampal infusion, microdialysis and electroencephalograph (EEG) recording, we have compared the effects of alpha-dendrotoxin and dendrotoxin K on extracellular levels of excitatory amino acids in anaesthetised rats. Our findings show that although infusion of 35 pmol of both peptides was associated with elevated extracellular aspartate and glutamate, these increased levels were more sustained with dendrotoxin K. Furthermore, there was EEG evidence of an associated transient functional change consistent with an action on pre-synaptic K(+) channels. In contrast, infusion of alpha-dendrotoxin produced only a brief effect on amino acid levels and no evidence of a functional consequence.

Energetic and structural interactions between delta-dendrotoxin and a voltage-gated potassium channel

Dendrotoxin proteins isolated from Mamba snake venom block potassium channels with a high degree of specificity and selectivity. Using site-directed mutagenesis we have identified residues that constitute the functional interaction surfaces of delta-dendrotoxin and its voltage-gated potassium channel receptor. delta-Dendrotoxin uses a triangular patch formed by seven side-chains (Lys3, Tyr4, Lys6, Leu7, Pro8, Arg10, Lys26) to block K(+) currents carried by a Shaker potassium channel variant. The inhibitory surface of the toxin interacts with channel residues at Shaker positions 423, 425, 427, 431, and 449 near the pore. Amino acid mutations that interact across the toxin-channel interface were identified by mutant cycle analysis. These results constrain the possible orientation of dendrotoxin with respect to the K(+) channel structure. We propose that dendrotoxin binds near the pore entryway but does not act as a physical plug.

Seizures and hippocampal damage produced by dendrotoxin-K in rats is prevented by the 21-aminosteroid U-74389G

The epileptogenic and neurodegenerative effects of dendrotoxin K (DTx-K), from Dendroaspis polylepsis, a specific blocker of a noninactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats pretreated with the 21-aminosteriod U-74389G, a scavenger of free oxygen radicals. Administration of 35 pmol DTx-K elicited in all of the treated animals (n = 6) motor seizures and bilateral electrocortical (ECoG) discharges after a latent period of approximately 5 min. At 24 h, histological examination of brain (n = 6) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation. Quantitation of damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer and dentate gyrus granule cell layer relative to the corresponding brain regions of rats (n = 6) injected with bovine serum albumin (300 ng), which per se was ineffective in all respects. DTx-K (35 pmol) also caused a significant loss of CA3 pyramidal neurons ipsilateral to the site of toxin injection. Systemic (i.p.) administration of U-74389G (5 mg/kg given 30 min beforehand) delayed the onset of motor and ECoG seizures and reduced the number of epileptogenic discharges typically observed in rats receiving an injection of DTx-K (35 pmol) alone. Similarly, this treatment prevented the damage inflicted to the hippocampus by the toxin and in no instance was significant neuronal loss observed. At variance with these results, pretreatment with U-74389G (up to 10 mg/kg i.p.) failed to prevent seizures and CA1 hippocampal damage evoked by intra-hippocampal injection of alpha-DTx (35 pmol), a DTx-K homologue which preferentially inhibits a slowly inactivating, voltage-dependent K+ conductance in nerve cells. In conclusion, the present data support a role for free oxygen radicals in mediating hippocampal damage induced by DTx-K, but not alpha-DTx, and confirm the original deduction that these DTx homologues are complementary neurobiological tools to study mechanisms of seizures and neuronal death.

The relative potencies of dendrotoxins as blockers of the cloned voltage-gated K+ channel, mKv1.1 (MK-1), when stably expressed in Chinese hamster ovary cells

1. The mKv1.1 voltage-gated K+ channel has been expressed stably in Chinese hamster ovary cells and whole-cell currents recorded by the patch-clamp method. 2. A range of structurally related peptide toxins (dendrotoxins) from the venom of green mamba (Dendroaspis angusticeps) and black mamba (Dendroaspis polylepis polylepis) snakes were tested for mKv1.1 channel blocking activity. Their potencies were compared based on EC50s derived from their respective concentration-inhibition relationships. 3. The rank order of potency, thus determined was: Toxin K > 7-dendrotoxin(7-Dtx) > delta-Dtx > Toxin I = alpha-Dtx > beta-Dtx. 4. Block was independent of voltage and no effects of the toxins on the kinetics of activation were observed. These results are consistent with a mechanism involving the block of closed channels. 5. A wide range of activity was observed even between toxins with an extremely high degree of sequence homology. Toxin K, in particular was an exquisitely potent blocker of the mKv1.1 channel, having an EC50 of 30 pM compared with 1.8 nM for delta-Dtx in spite of 95% sequence identity.

Recent studies on dendrotoxins and potassium ion channels

1. Dendrotoxins are small proteins isolated from mamba (Dendroaspis) snake venoms. They block some subtypes of voltage-dependent potassium channels in neurons. 2. Dendrotoxins contain 57-60 amino acid residues crosslinked by three disulfide bridges. They are homologous to Kunitz-type serine protease inhibitors, such as aprotinin, although they have little or no antiprotease activity. 3. Dendrotoxins act mainly on neuronal K+ channels. Studies with cloned K+ channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1 and Kv1.2 channels in the nanomolar range. In native cells, dendrotoxin appears preferentially to block inactivating forms of K+ current. 4. Dendrotoxins can induce repetitive firing in neurons and facilitate transmitter release. On direct injection to the CNS, dendrotoxins can induce epileptiform activity. 5. Radiolabeled dendrotoxins are useful markers of subtypes of K+ channels in vivo, and structural analogs help to define the molecular recognition properties of different types of K+ channels.

Effects of potassium channel inhibitors on nitrergic and adrenergic neurotransmission in lamina propria of the female rabbit urethra

Electrical field stimulation of strip preparations of the female rabbit urethral lamina propria induces a frequency-dependent adrenergic contraction or a non-adrenergic, non-cholinergic (NANC) relaxation, mediated by nitric oxide, depending on the prevailing tension. To study the role of potassium channels in these responses, the effects of inhibitors of voltage-dependent (dendrotoxin I, 4-aminopyridine), low (apamin) and high (iberiotoxin, charybdotoxin) conductance calcium-activated and ATP-sensitive (glibenclamide) potassium channels on the frequency-response relationship were examined. 4-Aminopyridine (1 mM), but none of the other inhibitors, augmented the NANC relaxation. The maximal response was, however, unaffected by 4-aminopyridine. The adrenergic contraction was enhanced by 4-aminopyridine (1 mM), dendrotoxin I (0.1 microM), iberiotoxin (0.1 microM) and charybdotoxin (0.1 microM), but not by apamin (0.1 microM) and glibenclamide (10 microM). Besides reducing the frequency eliciting half maximal contraction dendrotoxin and charybdotoxin also enhanced the maximal response. None of the inhibitors affected the relaxation induced by the nitric oxide donor 3-morpholinosydnonimine or the contraction elicited by noradrenaline. The results suggest that dendrotoxin-sensitive voltage-dependent and high conductance calcium-activated neuronal potassium channels participate in adrenergic, but not in nitrergic, neurotransmission in the lamina propria of the female rabbit urethra. This offers a possibility to selectively interfere with the adrenergic neuroeffector system with drugs acting on these K-channels.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors mediate seizures and CA1 hippocampal damage induced by dendrotoxin-K in rats

The epileptogenic and neurodegenerative effects of dendrotoxin K, from Dendroaspis polylepis, a specific blocker of a non-inactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats. Administration of 35 pmol dendrotoxin K elicited motor seizures and bilateral electrocortical discharges after a latent period (5.3 +/- 2.1 min), in all of the treated animals (n = 6). At 24 h, histological examination of brain (n = 5) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation which extended 300 microns rostral and caudal to the injection tract. Quantitation of the damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer relative to the corresponding brain regions of rats (n = 3) injected with bovine serum albumin (105 pmol), which per se was ineffective in all respects. Dendrotoxin K (35 pmol) also caused a significant loss of CA3 pyramidal neurons and dentate gyrus granule cells ipsilateral to the site of toxin injection. In one out of six rats, a lower dose (3.5 pmol) of dendrotoxin K produced convulsive behaviour and electrocortical seizures but after a longer latency and these were accompanied by significant neuronal loss in the CA1, CA3 and CA4 pyramidal cell layer ipsilateral to the injected side. The lowest dose (0.35 pmol; n = 6 rats) of dendrotoxin K used failed to induce seizures and did not cause hippocampal damage (n = 6 rats). Systemic (i.p.) treatment with dizocilpine maleate (3 mg/kg) or LY 274614 (5 mg/kg i.p.), two N-methyl-D-aspartate receptor antagonists (given 15 min beforehand), prevented dendrotoxin K (35 pmol)-induced motor seizures and electrocortical epileptogenic discharges in 100% of the animals (n = 6 per group) treated. Similarly, these antagonists minimized the damage typically produced in the rat hippocampus, with no significant neuronal loss being observed. By contrast, NBQX (30 mg/kg, i.p. given 15 min previously), a non-N-methyl-D-aspartate antagonist, failed to prevent seizures normally evoked by dendrotoxin K (35 pmol; n = 6 rats); also, this treatment was unable to abolish CA1 pyramidal cell loss but minimized the loss in hippocampal sectors distant to the site of dendrotoxin K injection. However, complete protection against motor and electrocortical seizures and hippocampal damage was afforded by GYKI 52466 (10 mg/kg i.p.; n = 6 rats), a more effective non-N-methyl-D-aspartate receptor antagonist. These findings differ from the reported lack of protection by N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonists to rats receiving intra-hippocampal injection of alpha-dendrotoxin; this difference may stem from the ability of alpha-dendrotoxin to block predominantly a slowly inactivating K+ current whereas dendrotoxin K inhibits a non inactivating variant. In conclusion, the present data on dendrotoxin K, together with the previously described pattern of neurotoxicity for alpha-dendrotoxin, show that these homologues act via different mechanisms and, thus, can be used effectively as complementary tools to study seizures and neuronal cell death.