Aminopyridines and synaptic transmission

4-Aminopyridine-induced increase in basal and stimulation-evoked [3H]-NA release in slices from rat hippocampus: Ca2+ sensitivity and presynaptic control

1. We have examined the mechanisms by which the K(+)-channel blocker 4-aminopyridine (4-AP) can dose-dependently increase both basal [3H]-noradrenaline ([3H]-NA) release and the [3H]-NA release evoked by electrical stimulation, but not the release of [3H]-acetylcholine ([3H]-ACh), from slices of rat hippocampus. 2. Both the electrically evoked and the 4-AP-induced release were blocked by tetrodotoxin (TTX) (3 microM). The Ca(2+)-dependence of the 4-AP-induced release (EC50 0.15 mM) was, however, different from that of the electrically evoked [3H]-NA release (EC50 0.76 mM). 3. The 4-AP-induced release could be inhibited by CdCl2(10 microM) and omega-conotoxin (30 nM), but not by nifedipine (1 microM). 4. Transmitter release evoked by 100 microM 4-AP could be blocked by the alpha 2-adrenoceptor agonist, UK 14,304 (0.1 microM) and by the A1-receptor agonist R-N6-phenylisopropyl adenosine (R-PIA, 1 microM) and increased by the alpha 2-adrenoceptor antagonist, yohimbine (1 microM), both in 0.25 and 1.3 mM Ca(2+)-containing medium. By contrast, the effect of alpha 2-adrenoceptor agonist and antagonists on transmitter release evoked by electrical stimulation was markedly reduced in the presence of 4-AP (100 microM). 5. The results suggest that 4-AP can depolarize some nerve endings in the central nervous system, leading to transmitter release that is dependent on nerve impulses and Ca2+. Furthermore, the fact that alpha 2-receptors and adenosine A1 receptor agonists can influence the release of NA evoked by 4-AP suggests that these drugs may have actions that are independent of blockade of aminopyridine-sensitive K(+)-channels.

Pharmacology and electrophysiology of a synchronous GABA-mediated potential in the human neocortex

Spontaneous synchronous field potentials of negative polarity (duration = 200-700 ms, inter-event interval = 9.1 +/- 2.9 s; n = 27 slices) were recorded, during application of 4-aminopyridine (50 microM), from the superficial/middle layers of slices of human neocortex obtained in the course of neurosurgery for the relief of intractable seizures. The negative-going field potential corresponded to an intracellular long-lasting (duration = 200-1600 ms) depolarization that could be preceded by preceded by an excitatory postsynaptic potential-hyperpolarizing inhibitory postsynaptic potential sequence and followed by a long-lasting hyperpolarization. This synchronous activity continued to occur following blockade of excitatory synaptic transmission by excitatory amino acid receptor antagonists, but was greatly reduced and eventually disappeared during application of the GABAA receptor antagonist bicuculline methiodide. Simultaneous extracellular recordings from three sites in the slice located along an axis parallel to the pia showed that successive synchronous field potentials could originate from any of the three areas. They invaded the other two sites in c. 35.5% of the cases, while propagation to another site only or no propagation at all was observed, respectively, in 44.4% and 20% of instances. The velocity of lateral propagation of the synchronous field potential was 7.9 +/- 2.5 mm/s (range = 4.5-11.8 mm/s, n = 6). The modalities of origin and propagation remained the same after blockade of excitatory amino acid receptors. Under these conditions, however, there was a higher incidence of non-propagation and the velocity was significantly lower than in control (5.6 +/- 1.9 mm/s; range = 2.8-7.7 mm/s, n = 6). These data indicate that, in the human neocortex, 4-aminopyridine can reveal a synchronous field potential that correlates with an intracellular long-lasting depolarization and is mainly due to the activation of postsynaptic GABAA receptors. The action of excitatory amino acid receptors is not necessary for the generation and propagation of these GABA-mediated potentials. We propose that this potential represents a novel mechanism for synchronization and spread of neuronal activity, including seizure-like discharges in the human neocortex.

Role of NMDA, AMPA and kainate receptors in mediating glutamate- and 4-AP-induced dopamine and acetylcholine release from rat striatal slices

Striatal slices, preincubated with [3H]dopamine and [14C]choline, were superfused continuously and subjected to electrical field stimulation (3 Hz) and perfused with amino acid analogues or 4-amino pyridine (4-AP). The released radioactivity was used to monitor release of the neurotransmitters dopamine (DA) and acetylcholine (ACh). Glutamate, NMDA (in the absence of Mg2+), AMPA, kainic acid, domoate and 4-AP all induced DA and ACh release in a concentration-dependent manner. The DA and ACh release induced by NMDA (15 microM) and glutamate (1 mM) was essentially abolished by Mg2+ (1.15 mM), whereas release induced by AMPA (100 microM), kainic acid (100 microM) or 4-AP (30 microM) was not reduced. Tetrodotoxin (1 microM) essentially abolished the effects of NMDA, markedly reduced the effects of glutamate, AMPA and 4-AP, whereas the effect of kainic acid was only modestly affected. MK-801 (30 nM) reduced NMDA-induced DA release by some 70% and ACh release by 30%. MK-801 reduced 4-AP-induced DA release by 40% but not ACh release. CNQX in a concentration (10 microM) that scarcely affected NMDA-induced ACh release, but blocked that induced by AMPA, kainic acid or domoate, reduced the ACh release induced by 4-AP. In summary, DA and ACh release from rat striatum can be stimulated by activation of NMDA and non-NMDA glutamate receptors, and this mechanism is activated by the potassium channel blocker 4-AP.

4-Aminopyridine-sensitive neurologic deficits in patients with spinal cord injury

4-Aminopyridine (4-AP) is a potassium channel blocking agent with the ability to restore conduction in demyelinated internodes of axons of the spinal cord. The present investigation sought to obtain electrophysiologic evidence of the effect of 4-AP in ameliorating central conduction deficits in a group of patients (n = 6) with spinal cord injury (SCI). The group was selected on the basis of having temperature-dependent central conduction deficits. 4-AP (24-25 mg total dose) was delivered intravenously at 6 mgh-1 or 15 mgh-1 while somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) were recorded as indices of central conduction. Two patients exhibited marked increases in the amplitude of cortical SEPs, and in one of these, 4-AP brought about a reduced central conduction time from L1 to cortex. Four patients revealed increased amplitude MEPs with concomitant reduction in latency indicative of enhanced conduction in corticospinal or corticobulbospinal pathways. Two of these patients demonstrated increased voluntary motor unit recruitment following 4-AP. Clinical examination revealed reduced spasticity (n = 2), reduced pain (n = 1), increased sensation (n = 1), improved leg movement (n = 3), and restored voluntary control of bowel (n = 1). These results support the hypothesis that 4-AP induces neurologic benefits in some patients with SCI. They are also consistent with the emerging concept that pharmaceutical amelioration of central conduction deficits caused by focal demyelination may contribute to the management of a select group of patients with compressive or contusive SCI.

Relaxation of the bovine retractor penis muscle by a small K+ excess and the role of K+ in its neurogenic relaxation

The mechanism of the relaxation of the bovine retractor penis muscle induced by 6.7 mM K+ as well as the role of K+ in the neurogenic relaxation of this muscle induced by nicotine, acetylcholine or electrical field stimulation, was studied. The relaxation induced by 6.7 mM K+ was, contrary to that induced by nicotine or electrical field stimulation, abolished by 10(-7) M ouabain. 15 min exposure to 10(-5) M NG-nitro-L-arginine, 3.2 x 10(-6) M tetrodotoxin, 5.0 x 10(-4) M hexamethonium, 5.3 x 10(-4) M methylene blue or hypoxia, all known to inhibit the neurogenic relaxation, did not affect the relaxation induced by 6.7 mM K+, which was also unaffected by 10(-5) M apamin, 3 x 10(-3) M 4-aminopyridine, 2.6 x 10(-2) M tetraethylammonium and 7.3 x 10(-4) M Ba2+. Exposure to K(+)-free solution reversibly abolished the neurogenic relaxations. The relaxations caused by 5.0 x 10(-7) M cromakalim and 2.0 x 10(-6) M pinacidil were totally blocked by 10(-5) M glibenclamide. Glibenclamide and apamin did not affect the tone of the muscle or its neurogenic relaxations. 4-Aminopyridine 4.0 x 10(-5) to 3.0 x 10(-3) M and tetraethylammonium 10(-4) to 2.6 x 10(-2) M raised the tone and enhanced the relaxations elicited by electrical field stimulation. The results indicate that the relaxation induced by 6.7 mM K+ is partly mediated by activation of Na(+)-K(+) ATPase and that its mechanism is thoroughly different from that of the neurogenic relaxations.(ABSTRACT TRUNCATED AT 250 WORDS)

Prejunctional effects of cromakalim, nicorandil and pinacidil on noradrenergic transmission in rat isolated mesenteric artery

The present study investigated the effects of cromakalim, nicorandil and pinacidil on resting and stimulation-induced (S-I) effluxes of radioactivity from rat isolated mesenteric artery preparations in which the noradrenergic transmitter stores had been radiolabelled with [3H]-noradrenaline. The efflux of radioactivity evoked by field stimulation of peri-arterial sympathetic nerves (pulses at 2 Hz frequency in trains of 60 s duration) was taken as an index of transmitter noradrenaline release. Cromakalim (1-100 microM) and nicorandil (1-1000 microM) produced minor effects on resting and S-I effluxes of radioactivity, but these did not exhibit concentration-dependency. Pinacidil (1-1000 microM) produced concentration-dependent increases, in both resting and S-I effluxes of radioactivity. With 1000 microM pinacidil, resting and S-I effluxes were increased to approximately 348% and 358% of their respective control values. The effects of pinacidil on resting and S-I effluxes were unaltered when the neuronal amine transport system was inhibited by desipramine (1 microM). Inhibition of monoamine oxidase with pargyline (100 microM) treatment markedly reduced the enhancement of resting efflux by 1000 microM pinacidil but did not alter its effect on S-I efflux. It is proposed that the enhanced resting efflux produced by pinacidil without pargyline treatment consists of deaminated [3H]-noradrenaline metabolites formed from [3H]-noradrenaline displaced from transmitter storage vesicles by pinacidil. The enhancement of S-I efflux by pinacidil does not appear to involve disruption of alpha 2-adrenoceptor auto-inhibition of transmitter release since equi-effective concentrations of phentolamine (1 microM) and pinacidil (1000 microM) produced additive effects on S-I efflux, whereas increasing the concentration of phentolamine from 1 to 2M produced no further increases in S-I efflux. In conclusion, this study has provided no evidence of a prejunctional inhibitory effect of the potassium channel openers cromakalim, nicorandil and pinacidil on transmitter noradrenaline release. However, the findings with pinacidil suggest that, in high concentrations, pinacidil displaces noradrenaline from transmitter stores, such that deaminated noradrenaline metabolites are released from the nerve terminals. Furthermore, pinacidil enhances S-I transmitter noradrenaline release, possibly by blocking neuronal potassium channels.

Hemodynamic effects of 3,4-diaminopyridine in a swine model of verapamil toxicity

STUDY HYPOTHESIS

3,4-Diaminopyridine (3,4-DAP) may reverse the hemodynamic effects of severe verapamil toxicity.

DESIGN

A nonblinded acute animal preparation.

INTERVENTIONS

Eighteen chloralose-anesthetized and instrumented swine were poisoned with verapamil at 10 mg/kg/hr for five minutes and then 5 mg/kg/hr until a systolic blood pressure of 55 mm Hg was achieved. Heart rate, lead II ECG, mean arterial pressure, left ventricular dP/dT max, and cardiac index were monitored. Nine control animals received 0.2 mL/kg/min infusion of normal saline, and nine treatment animals received similar volumes of 1 mg/kg/min 3,4-DAP until systolic blood pressure reached 100 mm Hg, an elapsed time of 30 minutes, or death.

RESULTS

Verapamil toxicity was produced in all animals following an average dose of 1.38 +/- 0.44 mg/kg verapamil, and resulted in diminished mean arterial pressure, dP/dT max, cardiac index, and heart rate to average values of 47%, 32%, 46%, and 88% of baseline values, respectively. Transient atrioventricular dissociation was noted in only 22% of cases. 3,4-DAP treatment (with an average dose of 14 +/- 5.6 mg/kg) resulted in significant increases in mean arterial pressure, dP/dT max, cardiac index, and heart rate to 136%, 298%, 149%, and 158% of baseline values, respectively. Mortality was unchanged (22% in both groups). 3,4-DAP treatment was complicated by muscle twitching, tachycardia (rate of more than 180) and hypertension (diastolic blood pressure of more than 110 mm Hg) each in 44% of cases.

CONCLUSION

Although 3,4-DAP reversed the hypotensive and negative inotropic effects of verapamil toxicity, it failed to improve survival and resulted in several complications including muscle twitching, tachycardia, and hypertension.

Prejunctional action of the venom from the Indian red scorpion Mesobuthus tamulus on adrenergic transmission in vitro

Venom (RSV) from the Indian red scorpion, Mesobuthus tamulus (Buthus tamulus), can cause increased peripheral sympathetic activity with consequent enhancement of adrenergic responses. We have therefore investigated the effects of RSV on adrenergic transmission in the rat isolated anococcygeus muscle. The effects of phentolamine (5 microM), tetrodotoxin (2 microM), guanethidine (5 microM), desipramine (1 microM) and reserpine pretreatment in vivo (5 mg/kg s.c. x 24 hr and 5 mg/kg i.p. x 3 hr) on the contractile responses of the rat anococcygeus muscle to RSV (1.5 microgram/ml), field stimulation, noradrenaline (NA, 1 microM or 3 microM) and tyramine (15 microM) were compared. The contractile responses to RSV and to field stimulation were completely blocked by phentolamine, tetrodotoxin, guanethidine and reserpine pretreatment, but the responses were potentiated by desipramine. The contractile responses to tyramine were completely blocked by phentolamine, reserpine pretreatment as well as desipramine. The responses to NA were completely blocked by phentolamine, but were potentiated by guanethidine, desipramine and reserpine. Relatively low concentrations (0.1 microgram/ml x 4) of RSV which did not produce any observable increase in tone of the anococcygeus muscle, potentiated the contractile response of the anococcygeus muscle to field stimulation, but not the responses to exogenous NA; 4-aminopyridine (25 microM x 2) also potentiated the muscle responses to field stimulation. HPLC measurements revealed only very low concentrations (0.10 +/- 0.03 mumol/g venom) of NA in RSV. Thus, the adrenergic agonist action of RSV in the rat isolated anococcygeus muscle can be attributed to the involvement of some prejunctional mechanism(s) of action that stimulates the release of neurotransmitter which differs from the indirect action mediated by tyramine.

Corticotropin-releasing factor release from the mediobasal hypothalamus of the rat as measured by microdialysis

Procedures were developed to permit the measurement of corticotropin-releasing factor in perfusate collected from microdialysis probes implanted in various brain areas of anesthetized and awake rats. Initially in vitro experiments were carried out to optimize the recovery of corticotropin-releasing factor and the radioimmunoassay conditions. Addition of a specific antiserum against corticotropin-releasing factor to the perfusion medium (artificial cerebrospinal fluid) increased the relative in vitro recovery over a range of different flow rates (1-10 microliters/min) using commercially available microdialysis probes with a membrane cutoff of 20,000 mol. wt. This procedure increased recovery from 3% to 6% at flow rate of 2.5 microliters/min, and from 4% to 8% at a flow rate of 5 microliters/min. In vivo experiments were performed with a flow rate of 3.3 microliters/min and 50-microliters fractions were used for radioimmunoassay. In each experiment, the standard curve of the radioimmunoassay was constructed from aliquots of the same medium used to perfuse the probe. Basal levels of corticotropin-releasing factor in dialysate collected from the mediobasal hypothalamus of anesthetized rats were estimated to be 0.75 +/- 0.07 fmol/50 microliters. Raising the concentration of potassium (60 mM) in the perfusate increased corticotropin-releasing factor levels to 2.04 +/- 0.37 fmol/50 microliters. Hypertonic stress induced by intraperitoneal injection of 1.5M NaCl (20 ml/kg) elevated the levels to 1.32 +/- 0.07 fmol/50 microliters. A marked increase of corticotropin-releasing factor levels was also produced by a 10-min pulse of the potassium-channel blocker 4-aminopyridine (10 mM) included in the perfusate. A second stimulation pulse with 4-aminopyridine, administered 2 h after the first pulse again increased the levels, with a mean ratio between the first and second pulse of 0.97. Corticotropin-releasing factor efflux produced by the second stimulation pulse was completely inhibited by perfusion with calcium-free medium containing calcium-chelating agent ethyleneglycol tetraacetic acid (10 mM). In separate experiments, microdialysis probes were implanted in several brain areas of anesthetized rats. Basal and potassium-evoked levels of corticotropin-releasing factor were measured in dialysate collected from the amygdala (1.20 +/- 0.22 and 2.05 +/- 0.48 fmol/50 microliters, respectively) and frontal cortex (0.51 +/- 0.10 and 1.64 +/- 0.15 fmol/50 microliters, respectively). Corticotropin-releasing factor levels in the dorsal part of the third ventricle and in the striatum were below the detection limits. In awake rats, corticotropin-releasing factor levels in the mediobasal hypothalamus were 0.98 +/- 0.03 fmol/50 microliters.(ABSTRACT TRUNCATED AT 400 WORDS)

On the usefulness of Fura-2 measurements of intrasynaptosomal calcium levels in rat cortical synaptosomes to study mechanisms of presynaptic function

Levels of [Ca2+]i in rat cortex synaptosomes were measured using the Ca2+ indicator Fura-2. Ca2+ influx was induced by veratridine in a concentration-dependent manner (1-10 microM). The resulting increase in [Ca2+]i was inhibited by tetrodotoxin (TTX). K+ (18 mM) increased the [Ca2+]i which was not influenced by TTX. K(+)-channel blockers such as 4-aminopyridine, alpha- and delta-dendrotoxin pre se were ineffective. The veratridine-induced Ca2+ influx in synaptosomes was reduced by L-type Ca(2+)-channel blockers, such as felodipine, nifedipine and PN-200-110, verapamil and diltiazem. omega-Conotoxin, and N-type Ca(2+)-channel blocker, did not inhibit the veratridine-stimulated [Ca2+]i increase. Bay K 8644, and L-channel agonist, stimulated an increase of [Ca2+]i in synaptosomes which was not sensitive to TTX. R-N6-Phenyl-isopropyl-adenosine (R-PIA) and clonidine, agonists at adenosine A1-receptors and alpha 2-adrenoceptors, respectively, did not influence the veratridine-stimulated [Ca2+]i increase. R-PIA did not interact with Bay K 8644-stimulated [Ca2+]i increase in synaptosomes. The results for all the substances used show major differences between the effects on Ca2+ influx in synaptosomes and on the electrically evoked neurotransmitter release in slice preparations. Thus, the synaptosome preparation is not a generally applicable experimental model for the study of Ca2+ mechanisms of presynaptic neuromodulation.

4-Aminopyridine causes reduction of cytochromes partly originated in glia with enhanced evoked potentials in the olfactory cortex slice

Enhanced electrical and metabolic activities with addition of 4-aminopyridine (4-AP) were investigated in the olfactory cortex slice by recording evoked potentials, the redox state of cytochromes and the rate of oxygen uptake. During perfusion with 4-AP (0.01-0.1 mM), the late N-wave was dose-relatedly elicited in the evoked potential, concomitant with reduction of cytochromes and acceleration of oxygen uptake. These enhanced responses by 4-AP (0.1 mM) depended on the extracellular Ca2+. When the slice was subject to 2-deoxy-D-glucose (2-DG) (10 mM), initiation of the late N-wave by 4-AP (0.1 mM) was delayed, and both the reduction of cytochromes and the raised oxygen uptake prompted by the same dose of 4-AP were also significantly attenuated. Nevertheless, the presynaptic potential and N-wave, which markedly diminished by 2-DG, became tolerant to glycopenia after addition of 4-AP. By prior exposure to fluoroacetate (1 mM), the N-wave was gradually increased, but the late N-wave was rapidly decreased in amplitude by adding 4-AP (0.1 mM). The cytochrome reduction and the accelerated oxygen uptake by 4-AP (0.1 mM) were suppressed in the presence of fluoroacetate (1 mM). These results indicate that cytochromes become reduced by 4-AP, concomitant with the enhanced evoked potentials and oxygen uptake in a Ca(2+)-dependent manner, and that glial cells probably contribute to the enhanced electrical and metabolic activities in the olfactory cortex slice.

Pituitary adenylate cyclase activating polypeptide (PACAP) is a potent relaxant of the rat ileum

The effect and mode of action of pituitary adenylate cyclase activating polypeptide (PACAP) were studied in rat ileal strips. PACAP relaxed, concentration dependently, rat ileum and was 50 times more potent than the structurally related vasoactive intestinal polypeptide (VIP). The inhibitory action of PACAP was not modified by TTX, omega-conotoxin, adrenergic, or ganglionic blockade, antagonists of adrenoreceptors and muscarinic receptors, indicating a direct myogenic effect probably through specific PACAP receptors. The lack of cross-tachyphylaxis between PACAP and VIP suggests that both peptides act by activation of distinct receptors. Structure-function analysis revealed that the N-terminal region of the PACAP molecule is crucial for biological activity.

4-Aminopyridine in chronic spinal cord injury: a controlled, double-blind, crossover study in eight patients

The potassium channel blocking drug 4-aminopyridine (4-AP) was administered to eight patients with chronic spinal cord injury, in a therapeutic trial based on the ability of the drug to restore conduction of impulses in demyelinated nerve fibers. The study was performed using a randomized, double-blind, crossover design, so that each patient received the drug and a vehicle placebo on different occasions, separated by 2 weeks. Drug and placebo were delivered by infusion over 2 h. An escalating total dose from 18.0 to 33.5 mg was used over the course of the study. Subjects were evaluated neurologically before and after the infusion. Two subjects returned for a second trial after 4 months and were examined daily for 3 to 4 days following drug infusion. Side effects were consistent with previous reports. Administration of the drug was associated with significant temporary neurologic improvement in five of six patients with incomplete spinal cord injury. No effect was detected in two cases of complete paraplegia and one of two severe incomplete cases (Frankel class B). Improvements in neurologic status following drug administration included increased motor control and sensory ability below the injury, and reduction in chronic pain and spasticity. The effects persisted up to 48 h after infusion of the drug, and patients largely returned to preinfusion status by 3 days. Compared with the more rapid elimination of the drug, these prolonged neurologic effects appear to involve a secondary response and are probably not a direct expression of potassium channel blockade.

Modulation by GABAB receptors of spontaneous synchronous activities induced by 4-aminopyridine in the rat hippocampus

Field potential recordings were made in the CA3 and/or CA1 subfields of rat hippocampal slices maintained in vitro during perfusion with medium containing the convulsant drug 4-aminopyridine (4AP, 50 microM). In this experimental condition, spontaneous interictal epileptiform discharges caused by the activation of excitatory amino acid receptors and synchronous, GABA-mediated potentials occurred regularly in both areas. Interictal discharges and GABA-mediated potentials were reduced and eventually abolished in both subfields by bath application of baclofen (0.5-100 microM), which is a selective agonist for the GABAB receptor. However, interictal epileptiform events disappeared in the presence of baclofen concentrations (i.e., 4.75 +/- 0.7 microM; IC50 = 3.4 microM; n = 9) that were lower than those required for abolishing the GABA-mediated potential (i.e., 96.1 +/- 19.4 microM; IC50 = 9.8 microM; n = 12). The effects of baclofen were antagonized by the GABAB receptor antagonists saclofen (1 mM; n = 3 slices) or CGP 35348 (0.2-1 mM; IC50 = 240 microM; n = 12 slices). Our findings indicate that activation of GABAB receptors by baclofen inhibits both types of 4AP-induced activities in the rat hippocampus although epileptiform discharges appear to be more sensitive than GABA-mediated potentials to this pharmacological procedure. Although our data do not indicate whether the action of baclofen is pre- or postsynaptic, they demonstrate that this mechanism is sensitive to available GABAB receptor antagonists.

Second messengers and ion channels in acetylcholine-induced chloride secretion in hen trachea

1. Hen tracheal epithelium can be stimulated by serosal application of acetylcholine (ACh) to secrete Cl- equal to approximately 60-90 microA/cm2. 2. Radio-ligand-displacement for IP3, cAMP and cGMP and ion channel selective drugs in voltage clamp set-ups were employed to characterize second messengers and Cl-, K+ and Ca2+ channels involved in the ACh response. 3. ACh induced a significant rise in IP3 in isolated tracheocytes, while ACh did not influence the production of cAMP in whole tissue, isolated tracheocytes or basolateral cell membrane vesicles. Further ACh desensitization did not effect cAMP level in tracheocytes. In addition neither ACh stimulation nor desensitization interfered with cAMP production in presence of 4.5 microM forskolin in tracheocytes, a level of forskolin rising base level cAMP by around five fold. 4. Around 35% of ACh Cl- secretion depends on Ca2+ mobilization from internal stores and about 65% on Ca2+ influx over basolateral membrane. The activated Ca2+ channel is insensitive to class I, II, III and IV Ca2+ antagonists. 5. A 23187 can mimic the ACh effect although 30% is indomethacin-sensitive demonstrating a prostaglandin activated adenylyl cyclase. 6. Two K+ channels are involved in ACh secretion, one sensitive to Ba2+ and quinine and both insensitive to 4-aminopyridine, apamin, charybdotoxin and TEA. 7. Flufenamate and triaminopyrimidine block a non-selective ion channel likely involved in the ACh response. An ACh activated apical Cl- channel is NPPB-sensitive.

Prejunctional actions of tacrine on autonomic neuroeffector transmission in rabbit isolated pulmonary artery and rat isolated atria

1. This study investigated the effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) on the resting and stimulation-induced (SI) release of radioactive substances from isolated preparations of rat atria and rabbit pulmonary artery in which the noradrenergic transmitter stores had been labelled with [3H]-noradrenaline, and from rat atrial preparations in which cholinergic transmitter stores had been labelled with [3H]-acetylcholine. In addition, the effect of tacrine on the uptake of [3H]-noradrenaline by noradrenergic nerves in rat atria was determined. 2. Tacrine produced concentration-dependent increases in the resting efflux of radioactivity from both the [3H]-noradrenaline-loaded artery and atrial preparations. Blockade of neuronal amine transport with desipramine reduced the release of radioactivity evoked by tacrine from atria but not that evoked from artery preparations. Inhibition of monoamine oxidase by pargyline pretreatment markedly reduced the tacrine-evoked release of radioactivity in both atrial and artery preparations. 3. The radioactivity released from [3H]-noradrenaline-labelled rat atrial preparations by 30 mumol/L tacrine consisted entirely of the deaminated metabolite [3H]-DOPEG. The evoked release of [3H]-DOPEG from atria was reduced by approximately 50% by desipramine (1 mumol/L). When atrial monoamine oxidase had been inhibited by pargyline treatment in vivo and in vitro, 30 mumol/L tacrine evoked the release of [3H]-noradrenaline instead of [3H]-DOPEG. However, the amounts of [3H]-noradrenaline released by tacrine when monoamine oxidase was inhibited were only about 25% of the amounts of [3H]-DOPEG released in untreated atria. 4. Tacrine, in concentrations of 1 and 10 mumol/L, enhanced the release of radioactivity evoked by field stimulation of [3H]-noradrenaline-loaded rabbit pulmonary artery preparations. This effect was unaltered by desipramine or pretreatment with pargyline. However, in artery preparations pretreated with pargyline, a high concentration of tacrine (100 mumol/L) markedly reduced SI efflux. In contrast to the findings with artery preparations, tacrine (1-30 mumol/L) did not alter SI efflux in rat atrial preparations. 5. It is concluded that tacrine displaces noradrenaline from intraneuronal transmitter stores of sympathetically-innervated tissues, and that the displaced amine is totally metabolized by monoamine oxidase before leaving the nerve terminals. When deamination of neuronal cytoplasmic noradrenaline is prevented, only a portion of the noradrenaline displaced from storage vesicles passes to the extracellular space. It is likely that the transfer of cytoplasmic noradrenaline out of the terminals is limited by the activity of the amine transport mechanism.

Effect of depolarizing agents on the Ca(2+)-independent and Ca(2+)-dependent release of [3H]GABA from sheep brain synaptosomes

The purpose of the present study was to compare the effects of several depolarizing agents on both the membrane potential and on the release of [3H] gamma-aminobutyric acid (GABA) from sheep brain cortex synaptosomes. We examined the effects of KCl, 4-aminopyridine (4-AP), veratridine, ouabain and tetraphenylphosphonium cation (TPP+) on Ca(2+)-independent (carrier-mediated) and Ca(2+)-dependent (exocytotic) release. We found that, in the absence of Ca2+, KCl at 40 mM releases 7.57 +/- 0.65%, veratridine at 50 microM releases 45.85 +/- 2.48%, ouabain at 1 mM releases 8.62 +/- 0.93% and TPP+ at 1 mM releases 4.09 +/- 0.37% of the total accumulated neurotransmitter, provided that the external medium contains Na+. These are about the maximal values of release obtained with each depolarizing agent in a Na+ medium and in the absence of Ca2+. Replacing external Na+ with choline blocks the release observed in the presence of the depolarizing agents in the absence of Ca2+, and this divalent ion can increase [3H]GABA release only for K+ or 4-AP. Synaptosomal depolarization requires Na+ except for K+ depolarization. Furthermore, although Ca2+ stimulates the release of [3H]GABA due to K+ depolarization (13.56 +/- 0.44%) or due to 4-AP (4.26 +/- 0.51%), it inhibits the release due to the other depolarizing agents. The amount of [3H]GABA released by 4-AP in Na+ medium (4.26 +/- 0.51%) is similar to that induced by KCl in the presence of Ca2+ in the absence of Na+ (3.39 +/- 0.29%) which represents only exocytotic release. This suggests that the Ca(2+)-dependent exocytotic release of [3H]GABA can be specifically induced by 4-AP in a Na+ medium, or by KCl in the absence of Na+, as reported by us earlier. The observation that Ca2+ inhibits the Ca(2+)-independent release is of interest because it suggests that Ca2+ may modulate the release of cytoplasmic GABA probably by inhibiting the carrier-mediated release of GABA. It is of interest as to whether Ca2+ regulation depends on intracellular Ca2+.

Ranitidine inhibits adrenergic transmission in the rat isolated anococcygeus muscle

1. The effects of ranitidine on adrenergic transmission in the rat isolated anococcygeus muscle were investigated. 2. Cumulative doses (2-8 mmol/L) or ranitidine produced a concentration-dependent inhibition of motor responses of the rat isolated anococcygeus muscle evoked by field stimulation (20-25 V, 10 Hz for 10 s, 1 ms pulse width) every 2 min, but also potentiated the contractile response to exogenous noradrenaline (5 mumol/L). The inhibited motor responses recovered rapidly and completely after washing out ranitidine. 3. 4-Aminopyridine (100 mumol/L) effectively reversed the partially inhibited (55% or greater) motor responses. 4. The results strongly suggest that ranitidine can inhibit adrenergic transmission in the anococcygeus muscle by a prejunctional mechanism with, presumably, consequent development of supersensitivity of the effector cells to noradrenaline.

Effects of anticonvulsant drugs on 4-aminopyridine-induced seizures in mice

The K+ channel blocker 4-aminopyridine (4-AP) causes epileptiform activity in in vitro preparations and is a potent convulsant in animals and man. In mice, 4-AP produces behavioral activation, clonic limb movements and wild running, followed by tonic hindlimb extension and death (ED97, 13.3 mg/kg, s.c.). We evaluated the ability of a series of anticonvulsant drugs to protect against 4-AP-induced seizures using lethality as the endpoint. Drugs with a phenytoin-like profile of activity were protective with ED50 values (all in mg/kg, i.p.) of 34.4 for phenytoin, 18.6 for carbamazepine, 26.9 for felbamate, and 41.5 for zonisamide. Phenobarbital and valproate also protected against 4-AP-induced seizures and lethality (ED50s, 30.6 and 301, respectively). In contrast the NMDA antagonists (+/-)-CPP and (+)-MK-801 were inactive as were the GABA enhancers diazepam, vigabatrin and tiagabine; the antiabsence drug ethosuximide; and the L-type Ca2+ channel blocker nimodipine. We conclude that drugs like phenytoin which block seizure spread are effective antagonists of seizures induced by K+ channel blockade. Drugs with specific actions on other cellular targets may be weak or inactive, presumably because they are unable to attenuate the spread of intense (non-NMDA receptor mediated) excitation evoked by 4-AP.

U-54494A reduces 4-AP-induced afterdischarges of CA1 pyramidal cells in the hippocampal slice of the rat

The anticonvulsant activity of U-54494A was studied in a 4-aminopyridine (4-AP) epilepsy model using extracellular recordings in in vitro hippocampal slices. Field potentials were evoked by stimulation of Schaffer collaterals, and recorded from the CA1 region of the hippocampus after infusion of 4-AP in the absence and presence of U-54494A. The number and the total area of after discharges (AD) in the presence of 4-AP were significantly decreased by increasing concentrations of U-54494A. In contrast, U-54494A did not significantly change the latency, duration, or area of the evoked PS in this paradigm. Phenytoin, a standard anticonvulsant, decreased the PS area without affecting either the PS latency or duration, or the AD number or area in the same paradigm. These present results provide more evidence that U-54494A is a novel and effective anticonvulsant that may be useful in the treatment of paroxysmal activity, without having generalized depressive effects.

Comparison of the effects of four dendrotoxin peptides, 4-aminopyridine and tetraethylammonium on the electrically evoked [3H]noradrenaline release from rat hippocampus

We have examined the effects of four dendrotoxin (DaTX) peptides, alpha-, beta-, gamma- and delta-DaTX, separated from the venom of the green mamba (Dendroaspis angusticeps), on field stimulation-evoked [3H]noradrenaline (NA) release from rat hippocampus and compared their effects with those of two other inhibitors of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP (10-300 microM) and TEA (0.1-5 mM) facilitated the evoked [3H]NA release in a concentration-dependent manner. The evoked [3H]NA release was reduced to about half by alpha 2-adrenoceptor stimulation (UK 14,304; 100 nM) and this reduction was antagonized by 4-AP (10-100 microM), whereas TEA even at 5 mM was a poor inhibitor of alpha 2-effects. alpha-DaTX (10-200 nM) mimicked 4-AP in increasing the electrically evoked [3H]NA release and diminishing the inhibitory effects of UK 14,304 in a concentration-dependent manner. delta-DaTX did not itself alter the electrically evoked [3H]NA release, but at 200 nM, it reduced the effects of alpha 2-receptor stimulation. beta- and gamma-DaTX (up to 200 nM) had no significant effects. 4-AP, 3,4-diaminopyridine (3,4-DAP), TEA and the four dendrotoxins displaced the binding of [3H]p-aminoclonidine ([3H]PAC) from alpha 2-receptors. The IC50 values were 6.6 x 10(-4), 1.42 x 10(-3), 5.6 x 10(-2) for 4-AP, 3,4-DAP and TEA, respectively, and 3.19 x 10(-6) M for alpha-DaTX. Thus, their potency as inhibitors of alpha 2-receptors is apparently too low to account alone for the antagonism by K+ channel inhibitors of alpha 2-effects on NA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of 4-aminopyridine-induced paroxysmal depolarizing shift in rat amygdaloid neurons by diltiazem

The effects of organic Ca2+ channel blocker, diltiazem, on the epileptiform activity induced by 4-aminopyridine (4-AP) were studied in rat amygdaloid slices using intracellular recording techniques. Application of 4-AP (0.5 mM) resulted in spontaneous and evoked epileptiform activity which consisted of an initial burst followed by a number of afterdischarges. The initial burst began with rapidly rising action potentials superimposed on a large depolarizing wave termed paroxysmal depolarizing shift (PDS). Diltiazem reversibly suppressed the amplitude and duration of PDS in a concentration-dependent manner. The IC50, estimated from the graph of the concentration-response relationship, was approximately 60 microM. These results demonstrate that a calcium current sensitive to diltiazem is involved in the generation of PDS and suggest that PDS is based on giant synaptic conductance as well as endogenous calcium currents.

Presynaptic mechanism of action of 4-aminopyridine: changes in intracellular free Ca2+ concentration and its relationship to B-50 (GAP-43) phosphorylation

Recently we have shown that 4-aminopyridine (4-AP), a drug known to enhance transmitter release, stimulates the phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat brain synaptosomes and that this effect is dependent on the presence of extracellular Ca2+. Hence, we were interested in the relationship between changes induced by 4-AP in the intracellular free Ca2+ concentration ([Ca2+]i) and B-50 phosphorylation in synaptosomes. 4-AP (100 microM) elevates the [Ca2+]i (as determined with fura-2) to approximately the same extent as depolarization with 30 mM K+ (from an initial resting level of 240 nM to approximately 480 nM after treatment). However, the underlying mechanisms appear to be different: In the presence of 4-AP, depolarization with K+ still evoked an increase in [Ca2+]i, which was additive to the elevation caused by 4-AP. Several Ca2+ channel antagonists (CdCl2, LaCl3, and diphenylhydantoin) inhibited the increase in B-50 phosphorylation by 4-AP. It is interesting that the increase in [Ca2+]i and the increase in B-50 phosphorylation by 4-AP were attenuated by tetrodotoxin, a finding pointing to a possible involvement of Na+ channels in this action. These results suggest that 4-AP (indirectly) stimulates both Ca2+ influx and B-50 phosphorylation through voltage-dependent channels by a mechanism dependent on Na+ channel activity.

Mast Cell Degranulating Peptide Increases the Frequency of Spontaneous Miniature Postsynaptic Currents in CA3 Rat Hippocampal Neurons

Mast cell degranulating peptide (MCDP) is a neurotoxic agent isolated from bee venom. It produces a long-term potentiation in the hippocampus. We now report that MCDP, at nanomolar concentrations, induces a reduction of a transient voltage-dependent potassium current (ID) in CA3 rat pyramidal neurons and a persistent (>30 min) enhancement of the frequency of spontaneous miniature excitatory and inhibitory postsynaptic currents (m.e.p.s.c.s. and m.i.p.s.c.s.). M.e.p.s.c.s. and m.i.p.s.c.s. were recorded in the presence of bicuculline (30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), respectively. The increased frequency of m.e.p.s.c.s. (408 +/- 60%) and m.i.p.s.c.s. (583 +/- 553%) was independent of the reduction of ID because 4-aminopyridine (4-AP, 30 microM - 2 mM) blocked ID but had no effects on m.e.p.s.c.s. and m.i.p.s.c.s. In the presence of the calcium channel blocker manganese (3 mM), MCDP still enhanced the frequency of m.e.p.s.c.s. (326 +/- 162%). It is concluded that MCDP augments the release of excitatory and inhibitory transmitter by an action, which is independent of calcium influx, through voltage-dependent channels.

Glutamate exocytosis evoked by 4-aminopyridine is inhibited by free fatty acids released from rat cerebrocortical synaptosomes

The Ca(2+)-dependent release of glutamate induced by 4-aminopyridine (4-AP) in rat cerebral cortical synaptosomes was reduced by removal of bovine serum albumin (BSA) from the incubation medium. The decrease in the glutamate release in the absence of BSA was consistent with a reduction in the rise in cytosolic free [Ca2+] after depolarization with 4-AP. Contrarily, neither the glutamate release nor the elevation in cytosolic free [Ca2+] after depolarization with 30 mM KCl was altered by the removal of BSA. The inhibitory action of the free fatty acids released during the incubation of synaptosomes was also observed when exogenous free fatty acids were added to the medium in the presence of BSA. The highest inhibition of 4-AP-induced release of glutamate was observed in the presence of arachidonic acid. The results strongly suggest an inhibitory action of free fatty acids by decreasing Ca2+ entry and glutamate release in rat cerebrocortical synaptosomes.

Cell excitation enhances muscarinic cholinergic responses in rat association cortex

The cerebral cortex receives a prominent cholinergic innervation which is thought to play an important role in regulating its normal function. Electrophysiological studies have shown that activation of cholinergic receptors results in a marked enhancement of excitatory stimuli onto cortical neurons and it has been suggested that this effect is secondary to the blockade of several voltage- and calcium-dependent potassium conductances in these cells. It is reported here that, in addition to these effects, activation of muscarinic receptors in the prefrontal cortex elicits the appearance of a slow calcium-dependent inward current in response to the generation of action potentials. This inward aftercurrent produces a slowly decaying depolarizing afterpotential which, when activated by stimulation of the cell, can summate with the carbachol-induced depolarization greatly increasing its magnitude. As a result the ability of muscarinic receptor to elicit a depolarization and excite cells in this region can be dramatically potentiated by evoked cell activation. This effect expands the range of mechanisms by which muscarinic receptors can facilitate excitatory inputs and provides a mechanism by which the association of brief excitatory stimuli to cholinergic stimulation can selectively enhance muscarinic responses among discrete cell populations in the cerebral cortex.

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampus: role of Na+ entry on Ca2+ pools and of protein kinase C

Slices of rat hippocampus, preincubated with [3H]noradrenaline [(3H]NA), were superfused continuously and stimulated by addition of 3,4-diaminopyridine (3,4-DAP; 100 microM) for 10 min to the superfusion medium. An overflow of 3H evoked by 3,4-DAP (representing [3H]NA release) was measurable not only in the presence but also in the absence of extracellular Ca2+. Both the protein kinase C (PKC) activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) and the PKC inhibitor polymyxin B, affected mainly the evoked release in the absence of extracellular Ca2+ in a facilitatory or inhibitory manner, respectively. Moreover, in the absence of extracellular Ca2+, both the 3,4-DAP-evoked [3H]NA release and the facilitatory effect of 4 beta-PDB were abolished in the presence of tetrodotoxin or in the absence of Na+ in the superfusion medium. Ruthenium red, a blocker of mitochondrial Ca2+ reuptake, potently increased 3,4-DAP-evoked [3H]NA release in Ca(2+)-free EGTA-containing medium. The facilitatory effects of ruthenium red and 4 beta-PDB were additive. From these and earlier observations we conclude (1) that the mechanism of 3,4-DAP-evoked [3H]NA release involves both Ca2+ influx into the nerve terminals and mobilization of intraneuronal Ca2+ pools. Most probably Ca2+ release from cytoplasmic Ca2+ stores (e.g. endoplasmic reticular pools or mitochondria) is induced by Na+ ions entering the nerve endings during 3,4-DAP-evoked repetitive action potentials. (2) The facilitatory effect of phorbol ester on 3,4-DAP-evoked NA release appears to be mediated not by changes in Ca2+ influx, but by enhancement of intraneuronal events distal to Na+ ion entry and increased intracellular Ca2+ availability.

Ion channel involvement in hypoxia-induced spreading depression in hippocampal slices

Rat hippocampal tissue slices were made hypoxic in control medium and in medium containing the ion channel blockers tetraethylammonium (TEA), 4-aminopyridine (4-AP), or tetrodotoxin (TTX). Postsynaptic evoked potentials, extracellular DC potential Vec, and in some experiments extracellular potassium concentration [K+]o were monitored in stratum pyramidale of the CA1 region. TEA (10 mM) decreased the latency of hypoxia-induced spreading depression (SD), and reduced the amplitudes of the changes in Vec and [K+]o. 4-AP (50 microM) also decreased the latency of SD but had no effect on the Vec shift. In most slices, TTX (1 microM) increased SD latency but had no effect on the Vec shift. In some slices, TTX blocked the occurrence of SD.

Long-term enhancement of synaptic responses in the songbird hippocampus

Electrophysiological recordings from the hippocampus were taken using brain slices from 1-year-old, female, song sparrows (Melospiza melodia). The evoked responses were smaller and less stable as compared with those obtained from the mammalian hippocampus. They consisted of two spikes. The first spike had low calcium dependency and represented mainly fiber potential. The second spike demonstrated a clear calcium dependency proving its synaptic origin. Paired-pulse facilitation showed inhibition of the second response at latencies below 20 ms, facilitation at 30 ms and no changes above 30 ms. Train stimulation with 20 Hz (each train consisted of 1 s stimulation with 10-s intervals repeated 3 times) evoked a stable increase in the size of the evoked response lasting approximately 2 h. These data indicate that the avian hippocampus possesses several neurophysiological properties that typify the mammalian hippocampus including the long-term enhancement of the synaptic response following certain patterns of stimulation.

K+ channel and 5-hydroxytryptamine1A autoreceptor interactions in the rat dorsal raphe nucleus: an in vitro electrophysiological study

Extracellular recordings were made from serotonergic neurons of the rat dorsal raphe nucleus in a slice preparation. In the presence of phenylephrine (3 microM) to restore the pacemaker activity of otherwise silent serotonergic neurons, superfusion with the 5-hydroxytryptamine1A agonist ipsapirone depressed the firing of these neurons with an IC50 of approximately 50 nM. Complete inhibition was achieved with 100-300 nM of the drug. Concomitant superfusion with the 5-hydroxytryptamine1A antagonists spiperone (100 nM) or propranolol (10 microM) markedly reduced the inhibitory effect of ipsapirone (100 nM). Superfusion with K+ channel blockers such as apamin (50-100 nM), charybdotoxin (100 nM) or Ba2+ (1 mM) did not induce any changes in the electrical activity of serotonergic neurons. However, 4-aminopyridine (0.1-1 mM) disrupted the regularity of their discharge without affecting the mean firing rate. The ipsapirone-induced inhibition was unchanged by apamin and charybdotoxin, but was markedly reduced by Ba2+ and 4-aminopyridine. Thus the IC50 of ipsapirone was shifted to approximately 150 nM in the presence of 1 mM of 4-aminopyridine. These results indicate that, in serotonergic neurons within the dorsal raphe nucleus, the K+ channel opened through the stimulation of 5-hydroxytryptamine1A autoreceptors is 4-aminopyridine-sensitive.

K+ and Ca2+ channel blockers may enhance or depress sympathetic transmitter release via a Ca(2+)-dependent mechanism "upstream" of the release site

Extracellular recording of the pre- and postjunctional electrical activity in guinea-pig or mouse vas deferens or rat tail artery was employed to study the mechanisms by which the K+ channel blockers, tetraethylammonium and 4-aminopyridine and the Ca2+ channel blockers, Cd2+, Mn2+ or nifedipine influence the nerve stimulation-induced release of adenosine 5'-triphosphate as a sympathetic co-transmitter. The K+ and Ca2+ channel blocking agents examined had no effect on the spontaneous quantal release of adenosine 5'-triphosphate. However, addition of tetraethylammonium and 4-aminopyridine inside the recording electrode broadened the nerve terminal action potential and caused it to become more resistant to local application of tetrodotoxin, and dramatically increased the magnitude and tetrodotoxin resistance of adenosine 5'-triphosphate release within the patch. Surprisingly, tetraethylammonium and 4-aminopyridine were equally effective when added outside the recording electrode; now they did not increase the duration of the nerve terminal action potential inside the patch but increased its resistance to locally applied tetrodotoxin and dramatically increased the magnitude as well as the tetrodotoxin resistance of adenosine 5'-triphosphate release from sites inside the patch. Both tetraethylammonium and 4-aminopyridine contributed to these effects, with a strong potentiating interaction. Nifedipine was without effect, but application of 1-100 microM Cd2+ or 1-5 mM Mn2+ either inside or outside the recording electrode blocked adenosine 5'-triphosphate release inside the patch. The results indicate: (i) that the nerve terminal action potential is generated by activation of voltage-gated, regenerative Na+ channels but also has a small component carried by influx of Ca2+ and that it is "normally" terminated by activation of voltage- as well as Ca(2+)-dependent K+ channels; (ii) that the release probability is tonically depressed by the resting K+ efflux, and promoted by the resting Ca2+ influx, "upstream" of the release sites; and (iii) that the upstream control of the release probability may involve both changes in properties of ionic channels in the nerve terminal membrane, and effects on the cytoskeleton leading to changes in the availability of releasable quanta in varicosities within the patch.

The effects of 4-aminopyridine on neurological deficits in chronic cases of traumatic spinal cord injury in dogs: a phase I clinical trial

A Phase I trial of 4-aminopyridine (4-AP) was carried out in 39 dogs referred to the veterinary teaching hospital with naturally occurring traumatic paraplegia or paraparesis. The rationale for the study was provided by the observation that 4-AP restores conduction in demyelinated nerve fibers in experimental spinal cord injury. Most injuries (77%) resulted from degenerative disk disease, occurring at or near the thoracolumbar junction, and producing chronic, complete paraplegia. Neurological examination of each dog was recorded on videotape before and at intervals after administration of 4-AP. The drug was administered systemically in total doses between 0.5 and 1 mg/kg body weight. Three areas of neurological status changed significantly at 15-45 minutes following administration of 4-AP: (a) striking improvements in hindlimb placing occurred in 18 animals; (b) increased awareness of painful stimuli to the hindlimb in 10 animals; (c) partial recovery of the cutaneus trunci muscle reflex of the back skin in 9 animals. These effects reversed within a few hours of administration. Other animals (36%) showed no change in neurological signs except a slight enhancement of hindlimb reflex tone. Significant side effects were seen in 6 dogs receiving higher intravenous doses, with elevation of body temperature and apparent anxiety, leading to mild seizures in 3 of the animals. These seizures were controlled with diazepam. The results indicate that conduction block may contribute significantly to functional deficits in closed-cord injuries and that potassium channel blockade may prove to be a valid, if limited approach to therapeutic intervention in chronic paraplegia and paraparesis.

Ion channels involved in the presynaptic hyperexcitability induced by herpes virus suis in rat superior cervical ganglion

Rat superior cervical ganglia infected with herpes virus suis (pseudorabies virus) display a spontaneous bursting activity of still unknown origin. Previous intracellular recordings from the ganglionic neurons combined with pharmacological studies showed that the postganglionic action potentials are induced by acetylcholine release spontaneously from the preganglionic nerve. In this study we investigated whether the acetylcholine release is caused by mechanisms which are dependent on action potentials spontaneously generated on the preganglionic nerve or by mechanisms which occur without any changes in the excitability of presynaptic fibers. Simultaneous intra- and extracellular recordings from the ganglion cells and from the preganglionic nerve, respectively, were performed 32-38 h after the inoculation of herpes virus suis (strain Aujeszky) into the anterior chamber of one eye of the rat. Tetrodotoxin, well known to prevent the generation of action potentials by blocking the fast sodium channels, completely and reversibly abolished, whereas the potassium channel blockers 4-aminopyridine and apamin, enhanced the spontaneous, bursting activity at pre- and postsynaptic levels. The nicotinic receptor antagonist hexamethonium abolished the postsynaptic discharges and reduced the preganglionic activity by 50%. Pre- and postsynaptic electrical activities were suppressed in low calcium Krebs' solution, demonstrating that extracellular calcium is required not only for acetylcholine release but also for triggering the presynaptic action potentials. It is concluded that in the infected ganglia the spontaneous acetylcholine release is due to the generation of action potentials in the preganglionic nerve. Voltage-gated sodium and calcium channels contribute to the presynaptic electrogenesis, while the latter appears to be damped by the activation of voltage- and calcium-dependent potassium channels. Possible factors as well as mechanisms inducing such an increase in excitability are discussed.

Mechanism of aminopyridine-induced release of [3H]dopamine from rat brain synaptosomes

1. Aminopyridines (APs) induced the release of [3H]dopamine (3H-DA) from rat synaptosomal preparations. 2. 4-AP and 3,4-DAP were of equal efficacy in inducing release of 3H-DA; 3-AP, 2-AP and 2,6-AP were less active; pyridine and pyridine-4-carboxylamide were inactive. 3. Cd2+ was more effective in inhibiting 4-AP-induced release of 3H-DA (IC50 approximately 4 microM) than Co2+ and Ni2+ (IC50s approximately 500 microM). 4. While 4-AP increased the 45Ca2+ content of whole synaptosomal preparations, no effect of 4-AP on 45Ca2+ content was observed in lysed synaptosomal preparations. 5. 4-AP-induced 45Ca2+ uptake was inhibited by Cd2+, Ni2+ and Co2+ in concentration ranges similar to those inhibiting 3H-DA release.

Purinergic modulation of hippocampal acetylcholine release involves alpha-dendrotoxin-sensitive potassium channels

Modulation of acetylcholine (ACh) release from superfused hippocampal slices was examined when the release of ACh was stimulated by exposure of slices to elevated K+ concentration. Evoked release was not sensitive to inhibition by 0.1 microM tetrodotoxin, but it could be inhibited in a dose-dependent manner by a muscarinic agonist (10-100 nM oxotremorine) and a purinergic agonist (10-100 nM 2-chloroadenosine). The alpha-dendrotoxin (100 nM), which selectively blocks voltage-gated inactivating K+ channels in nerve endings, did not affect the release of ACh under resting or depolarized conditions. However, alpha-dendrotoxin reduced the 2-chloroadenosine-induced inhibition of release, but did not alter the oxotremorine-induced inhibition. These results suggest that an alpha-dendrotoxin-sensitive K+ channel may be activated as an obligatory step in the modulation of ACh release by presynaptic purinergic receptor activation, but not in the modulation by presynaptic muscarinic receptors.

Facilitation by 3,4-diaminopyridine of regenerative acetylcholine release from mouse motor nerve

1. Effects of 3,4-diaminopyridine (DAP) on endplate potentials (e.p.ps) were studied in mouse phrenic nerve-hemidiaphragms. 2. In cut muscle preparations, low concentrations of DAP (2-20 microns) increased the amplitude of e.p.ps and shifted the curve relating Ca2+ concentration to e.p.p. amplitude leftward. 3. High concentration of DAP (40-4000 microns) prolonged the duration of e.p.ps dose-dependently up to one hundred fold (ca. 200 ms), yielding, in addition to the normal phasic e.p.p., a prolonged plateau depolarization component which was often preceded by an upstroke depolarization. During the plateau depolarization, nerve stimulations did not evoke any e.p.p. 4. The plateau component of prolonged e.p.ps was suppressed by tubocurarine, verapamil, nifedipine, Mn2+ and Cd2+ (but not by atropine) at low concentrations that had negligible effect on the amplitude of miniature e.p.ps or the phasic component of e.p.ps. Abolition of the plateau component by these agents restored the capability of the nerve terminal to evoke e.p.ps on nerve stimulation. 5. Low concentrations of neostigmine (0.01-0.02 microns) markedly lengthened DAP-prolonged e.p.ps. However, the regenerative endplate depolarization evoked in the presence of high concentrations of neostigmine (0.3-0.5 microns) was not prolonged by DAP. 6. Tetraethylammonium (1 mM) did not provoke prolonged e.p.ps but acted cooperatively with DAP to prolong the duration of plateau depolarization. At a high concentration (3 mM), tetraethylammonium depressed the amplitude of miniature e.p.ps and abolished DAP-prolonged e.p.ps. 7. In uncut muscle preparations, DAP apparently did not modify the time course and amplitude of miniature e.p.ps. Upon direct stimulation by current injection at endplate, DAP increased the muscle action potentials by only about 30%, but induced no prolonged depolarization. 8. These results suggest that the prolonged e.p.ps induced in the presence of DAP are due to a regenerative release of acetylcholine from motor nerve and the induction probably involves a presynaptic Ca2+ channel different from that for normal e.p.ps. It may be inferred that the regenerative acetylcholine release is recruited by Ca2 + channels modulated by nicotinic receptors and K+ channels.

Regulation of transmitter release at the squid giant synapse by presynaptic delayed rectifier potassium current

1. The three-microelectrode voltage clamp technique and pharmacological agents were used to examine the properties and functions of potassium currents in squid giant presynaptic terminals. 2. Outward currents consisted of two components: a slow component which activated over hundreds of milliseconds and was blocked by extracellular application of tetraethylammonium (TEA) ions and a more rapidly activating component which was relatively insensitive to extracellular TEA. 3. The more rapid component was studied in isolation by treating presynaptic terminals with extracellular TEA, as well as tetrodotoxin (to block sodium channel currents) and manganese (to block calcium channel currents). The magnitude of this current component was 1-2 mA cm-2 at 0 mV. Rates of activation and deactivation were voltage dependent and little evidence of inactivation was seen for depolarizations less than several seconds in duration. 4. The reversal potential of the current was -70 to -80 mV in normal saline and became more positive with elevated extracellular potassium concentrations, suggesting that potassium is the primary permeant ion. Accumulation of extracellular potassium appeared to be marked during depolarizations that produced significant activation of the current. 5. Extracellular application of 3,4-diaminopyridine (DAP) blocked the current with an apparent dissociation constant of 7 microM at 0 mV. Intracellular applications of DAP and TEA also were effective in reducing this current. These treatments, but not extracellular TEA application, broadened presynaptic action potentials and increased the magnitude and time-to-peak of postsynaptic currents elicited by the broadened presynaptic action potentials. Postsynaptic currents were a sensitive and linear function of action potential duration; a 30% increase in action potential duration increased postsynaptic current amplitude by 190%. 6. Estimation of the magnitude and time course of the presynaptic calcium current, based on previous measurements of calcium channel gating, indicated that action potential broadening produces a large increase in calcium current magnitude. These calculations predict that a 30% increase in presynaptic action potential duration will increase the peak amplitude of the calcium current by approximately 170% and the total amount of calcium entry by approximately 230%. This implies a linear relationship between transmitter release and calcium entry during an action potential and can be explained by assuming that calcium co-operatively triggers release within intracellular domains that do not overlap.(ABSTRACT TRUNCATED AT 400 WORDS)

Aminopyridines block an inactivating potassium current having slow recovery kinetics

The blocking action of aminopyridines on an inactivating K current (lKi) in GH3 pituitary cells was studied before and after altering the macroscopic decay of the current with N-bromoacetamide (NBA). The first depolarizing pulse delivered either seconds or minutes after beginning 4-aminopyridine (4AP) application, elicited a current with both a more rapid decay and a reduced peak amplitude. The rapid decay (or time-dependent block) was especially prominent in NBA-treated cells. With continued drug application, subsequent test pulses revealed a stable block of peak current, greater in NBA-treated than control cells. Recovery from block was enhanced by hyperpolarizing holding potentials and by the first depolarizing pulse delivered after prolonged recovery intervals. Unlike aminopyridine block of other K currents, there was no convincing evidence for voltage shifts in activation or inactivation, or for voltage and frequency-dependent unblock. Increasing the open probability of the channels did, however, facilitate the block. Although the behavior of currents in 4AP was suggestive of "open channel block," the block was not produced by 4-aminopyridine methiodide, a positively charged aminopyridine. Moreover, because partial block and recovery occurred without opening the channels we suggest that aminopyridines bind to, or near, this K channel, that this binding is enhanced by opening the channel, and that a conformational change is induced which mimics inactivation. Because recovery from block is enhanced by negative potentials, we suggest that aminopyridine molecules may become "trapped" by inactivation awaiting the slow process of reactivation to escape their binding sites.

The epileptiform activity induced by 4-aminopyridine in rat amygdala slices: antagonism by non-N-methyl-D-aspartate receptor antagonists

The effects of excitatory amino acid receptor antagonists kynuretic acid and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on epileptiform activity induced by 4-aminopyridine (4-AP) were studied in rat amygdala slices using intracellular recording techniques. Five to 10 min after switching to 4-AP-containing solution, spontaneous epileptiform bursts were observed in 35 out of 45 slices studied. The spontaneous epileptiform events consisted of an initial burst followed by a number of afterdischarges. Superfusion with kynuretic acid, a broad-spectrum excitatory amino acid receptor antagonist, reversibly reduced the duration of the spontaneous bursting discharges in a dose-dependent manner. The frequency of spontaneous bursting was also decreased. The IC50, estimated from the graph of the concentration-response relationship, was approximately 130 microM. In addition, CNQX which is a specific non-N-methyl-D-aspartate (NMDA) receptor antagonist blocked the spontaneous and evoked epileptiform bursting. In 11 out of 15 neurons tested, there was a residual depolarizing component remained. This depolarizing component was reversibly blocked by specific NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleate (D,L-APV). Relative to the CNQX-sensitive component, the D,L-APV-sensitive component is much smaller in amplitude and shorter in duration indicating that NMDA receptor plays only a minor role in this process. These data suggest that the generation or propagation of spontaneous epileptiform events induced by 4-AP in the neurons of basolateral amygdala is mediated by excitatory amino acids and that activation of non-NMDA receptors is of primary importance.

Concurrent fast and slow synchronized efferent phrenic activities in time and frequency domain

In urethane-anesthetized or decerebrated vagotomized rabbits efferent multifiber activity of the phrenic nerve was investigated for synchronized activities both in time and frequency domains. When respiratory drive was steadily increased by either an elevation of end-tidal CO2 concentration or i.v. administration of 4-aminopyridine, medium-frequency oscillations (MFO) first increased, then decreased and finally became absent. The power of high-frequency oscillations (HFO) steadily rose with increasing respiratory drive. In contrast to HFO which revealed a unimodal spectral peak of mostly small bandwidth, the MFO spectrum in most cases consisted of a broad complex. This complex in some cases was composed of two distinct peaks, i.e. MFO were heterogenous. The low- and high-frequency fractions of the MFO complex were related predominantly to the first and last third of inspiration, respectively. Examination of the on-going multifiber activity of the phrenic nerve with an expanded time scale revealed that lower frequency MFO probably result from synchronized ramp-like wave activity during early and mid-inspiration. The duration of the observed ramps well matched the corresponding MFO frequency. We suggest that these ramps might result from propagated synchronized waves of high-threshold phrenic motoneurons. During the last part of inspiration, however, MFO, like HFO, resulted from burst-like synchronized discharge of phenic motoneurons. Thus HFO are superimposed on ramp-like and burst-like activity of the MFO. It is assumed that the decline of MFO at high respiratory drive may be due to the increasing strength of HFO bursts which interrupt ramp activity in the MFO range and thus let MFO appear 'invisible' to the recording electrode. Both MFO and HFO were visually detectable in postinspiration.

Effects of tetrahydro-9-aminoacridine on cortical and hippocampal neurons in the rat: an in vivo and in vitro study

The effects of tetrahydro-9-aminoacridine (THA), an anticholinesterase drug, have been studied in the rat both in vivo (cerebral cortex) and in vitro (CA1 field of the hippocampus) and compared with those of physostigmine. In the cerebral cortex THA potentiated the excitatory effect of acetylcholine in most neurons, including cortical neurons recorded from chronic unanesthetized animals. In vitro, THA (but not physostigmine) had a depolarizing, atropine- and tetrodotoxin-insensitive effect. This effect is associated with an increase in membrane resistance which suggests a direct effect of THA on hippocampal neurons. In addition THA blocked the slow inhibitory postsynaptic potential. At the same concentration THA potentiated the slow cholinergic excitatory postsynaptic potential produced by electrical stimulation of the cholinergic afferents. Its potency was, however, about 10 times lower than that of physostigmine. These results show that THA: (1) is an anticholinesterase much less potent than physostigmine; but (2) has also direct effects on central neurons, not observed with physostigmine and unrelated to its anticholinesterase activity.

Regulation of the cytosolic free calcium concentration by Na+ spikes in immature cerebellar neurons with N-methyl-D-aspartate receptors

N-Methyl-D-aspartate and glycine increased the cytosolic free calcium concentration ([Ca]in) in medium-sized cerebellar neurons. Spontaneous changes in [Ca]in were occasionally observed in NMDA-responsive cells, but large increases in [Ca]in were triggered only through depolarizations by adding veratridine or K+ channel blockers in every cell examined. The [Ca]in increase was suppressed by voltage-dependent Na+ and Ca2+ channel blockers and by an inhibitory transmitter (GABA), suggesting that the generation of Na+ spikes is involved in the increase in [Ca]in.

Genetic correlations among inbred strain sensitivities to convulsions induced by 9 convulsant drugs

Inbred mouse strains differed significantly in sensitivity to convulsions induced by 9 convulsant drugs administered using a timed infusion procedure. Some strains (e.g. BALB/cJ, A/J) were generally seizure-susceptible, while some were generally seizure resistant (e.g. C57BL/6J, SWR/J). However, the overall pattern of strain sensitivities was complex, and depended upon drug and convulsant sign. Five of the drugs (bicuculline, DMCM, picrotoxin, TBPS and pentylenetetrazol (PTZ] produce convulsions, at least in part, through their interactions with the GABA receptor, while the other 4 (strychnine, CHEB, 4-aminopyridine and kainic acid) act through independent mechanisms. We predicted that responses to drugs with similar mechanisms of action would be genetically correlated. However, strains sensitive to picrotoxin-induced convulsions were not necessarily sensitive to convulsions elicited by PTZ or TBPS. Furthermore, different convulsant signs produced by a single drug were not always strongly correlated. Instead, genetic correlations were found among inbred strains for sensitivity to similar convulsant signs produced by different drugs. This suggests that genetic variation in sensitivity to these convulsant drugs arises primarily from variation in systems important for the expression of the convulsion.

Epileptiform activity induced by 4-aminopyridine in rat amygdala neurons: the involvement of N-methyl-D-aspartate receptors

The involvement of the N-methyl-D-aspartate (NMDA) receptor in the epileptiform activity induced by 4-aminopyridine (4-AP) was studied in rat amygdala slices using intracellular recording techniques. Stimulation of the ventral endopyriform nucleus evoked an excitatory postsynaptic potential (EPSP). After exposure to 4-AP (200 microM) the amygdala slices usually exhibited spontaneous and evoked epileptiform activity. The epileptiform events had an average duration of 522 +/- 78 ms with a frequency of 0.5-8.5 bursts/min. Superfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-NMDA receptor antagonist, practically abolished the epileptiform bursting. However, there remained a residual depolarizing component in 13 out of 18 neurons. This CNQX-resistant component was markedly enhanced both in amplitude and duration when extracellular Mg2+ was removed and could be reversibly blocked by the specific NMDA receptor antagonist, DL-2-amino-5-phosphonovaleate (DL-APV). Compared with the CNQX-sensitive component, the APV-sensitive component had a much smaller amplitude shorter duration. These data suggest that the NMDA receptor is likely to play only a minor role, and activation of the NMDA receptor may contribute to but is not required, for the generation of these bursts.