Aminopyridines and synaptic transmission

Antagonism of the ruthenium red-induced paralysis in mice by 4-aminopyridine, guanidine and lanthanum

4-Aminopyridine and guanidine were administered intraperitoneally to mice during the complete flaccid paralysis induced by treatment with ruthenium red (RuR). At 1-8 min after 4-aminopyridine or guanidine the animals had recovered completely from the paralysis, whereas the control mice injected only with RuR remained paralytic for at least 60 min. Intraperitoneal injections of LaCl3 had no apparent effects on animal motility and did not reverse the paralysis produced by RuR. However, when La3 + was administered 30 min prior to RuR the occurrence of flaccid paralysis was totally prevented. The results obtained are discussed in terms of the possible antagonist effects of the compounds used on acetylcholine release at neuromuscular junctions.

Genetics of the nervous system in Drosophila

The genetic analysis ofDrosophilaneurobiology has expanded into a considerable field. A host of genetic variants is now available for analyzing the development, structure, physiology, and chemistry of the fly's excitable cells and tissues and how they control the animal's behavior.

Presynaptic alpha-adrenoceptors and opiate receptors: inhibition of [3H]noradrenaline release from rat cerebral cortex slices by different mechanisms

The inhibitory effects of morphine and Cd2+ on electrically evoked [3H]noradrenaline release from superfused brain slices were unaffected when release was enhanced by increasing the pulse duration, while the inhibitory effect of noradrenaline and the enhancing effect of phentolamine were diminished. A similar enhancement of [3H]noradrenaline release by 4-aminopyridine reduced the modulatory effects of all drugs examined. Therefore there seem to be different mechanisms for the effect of presynaptic alpha-adrenoceptors and opiate receptors on the availability of Ca2+ for the stimulus-secretion coupling process in noradrenergic nerve terminals.

The effects of purified botulinum neurotoxin type A on cholinergic, adrenergic and non-adrenergic, atropine-resistant autonomic neuromuscular transmission

The twitch response observed during low frequency electrical stimulation of postganglionic cholinergic neurones supplying the longitudinal smooth muscle of the guinea-pig ileum was markedly reduced by incubation with an homogeneous preparation of botulinum type A neurotoxin (4.3-8.6 nM). This intoxication of the autonomic cholinergic neurones was long-lasting, irreversible by washing, but readily reversed by 4-aminopyridine (50-1000 microM). The noradrenergic motor response of the rat anococcygeus following field stimulation was partially antagonised by the neurotoxin. The non-adrenergic inhibitory response of the guinea-pig taenia coli, elicited by field stimulation, was not antagonised by botulinum toxin, suggesting that a source of a non-adrenergic inhibitory transmitter exists, other than intramural cholinergic neurones. However, the neurogenic excitatory responses of the guinea-pig bladder, elicited by field stimulation in the presence of atropine and guanethidine, were virtually abolished by botulinum toxin. It is suggested that the parasympathetic neurones which supply the smooth muscle of the guinea-pig urinary bladder co-release acetylcholine and a non-cholinergic excitatory transmitter; ATP or polypeptides are possible candidates.

Synaptic and non-synaptic components of the dorsal horn potential in isolated hamster spinal cord

Slow negative potentials, evoked by stimulation of the lumbar dorsal roots, have been demonstrated in the dorsal horn of an isolated, hemisected spinal cord preparation from golden hamsters. Paired stimuli revealed a period of partial suppression of this slow potential persisting for up to 2 s following the conditioning stimulus, but with high stimulation frequencies this effect was masked and above 20 Hz a tetanic train of stimuli produced a smoothly rising potential. The response evoked by tetanic stimulation was shown to consist of two components, a manganese-sensitive, synaptically generated component, and a manganese-resistant, frequency-dependent element. Treatment with 10(-4) M 4-aminopyridine blocked the manganese-resistant tetanic response but did not reduce the manganese-sensitive component. Bicuculline, picrotoxin and tubocurare had little effect upon the tetanic response, but 10(-3) M procaine blocked it completely. The possibility that the manganese-resistant response was due to the release of potassium ions is considered.

Electronic properties of motor nerve terminals

To obtain information about the electric membrane properties of frog motor nerve terminals we examined how depolarizing or hyperpolarizing current pulses of 2-8 ms duration to the preterminal, by electrotonic spread of potential, affected depolarization induced transmitter release. Sodium channels were blocked by tetrodotoxin. Under this condition a hyperpolarizing current pulse produced inhibition of release, followed by potentiation of release. Inhibition lasted more than 100 ms with a time constant of 50-150 ms. When, in addition, potassium channels were blocked by 3,4-diaminopyridine or tetraethylammonium a depolarizing current pulse potentiated transmitter release for a period up to 50 ms. The results imply that inward currents in the nerve terminal are carried mainly by sodium and calcium ions and outward currents by potassium ions while "leak" conductances are negligible. A low "leak" conductance and therefore a high specific membrane resistance facilitates the spread of electronic potentials and thereby explains the longlasting effects on transmitter release of brief current pulses to the preterminal.

4-aminoquinoline-induced 'giant' miniature endplate potentials at mammalian neuromuscular junctions

4-Aminoquinoline (4-AQ) in concentrations around 200 micrometers induces, within minutes of its application to isolated mouse or rat neuromuscular junctions, the appearance of a population of miniature endplate potentials (m.e.p.ps) with a larger than normal amplitude, so-called giant m.e.p.ps (g.m.e.p.ps). With amplitudes 2-12 times the modal value of m.e.p.p. amplitude, the population of g.m.e.p.ps varied between 15 and 45% of the total population of m.e.p.ps. There was no increase in the frequency of m.e.p.ps but a positive correlation between the frequency of g.m.e.p.ps and the total frequency of m.e.p.ps. In many instances the rise time and decay time of g.m.e.p.ps were prolonged compared to normal. Elevated extracellular calcium concentrations increased the frequency of m.e.p.ps but had no effect on g.m.e.p.p. frequency. High extracellular potassium concentrations markedly increased m.e.p.p. frequency but failed to influence g.m.e.p.p. frequency. Similar observations were made with ethanol 0.1 M, ouabain 200 micrometers or black widow spider venom. Botulinum toxin type A markedly reduced total m.e.p.p. frequency but 4-AQ still induced g.m.e.p.ps. Nerve stimulation failed to release quanta corresponding to the g.m.e.p.ps. G.m.e.p.ps seemed to originate from quantal acetylcholine release from the nerve terminal since they were abolished by surgical denervation and by the addition of d-tubocurarine to the medium. Blockade of voltage-sensitive calcium or sodium channels by, respectively, manganese ions or tetrodotoxin failed to affect the appearance and the frequency of g.m.e.p.ps. The electrophysiological findings and a statistical analysis of the characteristics of the m.e.p.ps indicate that they belong to two populations. One population is accelerated by the depolarization-release coupling mechanism responsible for evoked transmitter release and is characterized by an amplitude distribution and a process in time that indicate that they correspond to releases occurring at 'active zones' in the nerve terminal. The second population of m.e.p.ps is uninfluenced by nerve terminal depolarization and transmembrane calcium fluxes. This population apparently originates from sites dispersed in the nerve terminal membrane and outside the 'active zones'. 4-AQ increases the frequency of this second m.e.p.p. population without affecting the first population.

Enhancement of synaptic transmission by 4-aminopyridine in hippocampal slices of the rat

1. The effects of 4-aminopyridine (4-AP) on neurotransmission in hippocampal slices of the rat were studied in vitro. 2. Extracellular recordings of field potentials and intracellular recordings of excitatory and inhibitory post-synaptic potentials, action potentials and TTX-resistant spikes were obtained from the somatic and dendritic fields of CA 1 hippocampal neurones. 3. Constant perfusion with, or bolus injection of 4-AP in micromolar concentrations resulted in substantial and persistent enhancement of the amplitude of synaptic field potentials and population spikes with no alteration of the afferent input. Somatic positive waves (p-waves) were increased with local micro-application of 4-AP. 4. Intracellular recordings revealed an increase in the amplitude of e.p.s.p.s. and i.p.s.p.s. I.p.s.p.s were prolonged by 4-AP. 5. The pyramidal cells were, after an initial hyperpolarization, depolarized by up to 10 mV during exposure to 4-AP; the membrane conductance was not consistently changed. 6. Spontaneous shifts of the membrane potential were described as giant e.p.s.p.s and i.p.s.p.s. Spontaneous i.p.s.p.s were increased in frequency and amplitude. 7. Paired-pulse facilitation of e.p.s.p.s was reduced by 4-AP suggesting a presynaptic modulation of transmitter output. 8. It is concluded that 4-AP enhances transmitter release at both excitatory and inhibitory synapses in the hippocampus.

Presynaptic effects of 4-aminopyridine and streptomycin on the neuromuscular junction

Studies were done to assess the effects of 4-aminopyridine (4AP) and streptomycin (SM) on transmitter release parameters and extracellularly recorded presynaptic action potential. The application of 5 micrometer 4AP resulted in a marked increase in the mean quantal content (m1) associated with an increase in the total number of the store of available quanta (n) but had no effect on the probability of release (p) and the fractional release (P). Focal recording showed that 50 micrometer 4AP modified the shape of the presynaptic action potential from a triphasic configuration to a diphasic one. In contrast, prolongation of the muscle action potential was found only at higher concentrations (greater than 1 mM) of of 4AP. Thus, the increase in n with 4 AP was associated with prolongation of the presynaptic action potential evoked by blocking the K current. SM (172 micrometer mM) had no effect on p and P. Reduction of n by SM was completely reversed by 4AP. As the presynaptic action potential change induced by 4AP was not antagonised by SM, it may be that the decrease of n by SM followed a modification of the voltage-dependent Ca channel.

Effects of 4-aminopyridine on contractile response and action potential of rabbit papillary muscle

The effects of 4-aminopyridine (4-AP) were studied on isolated papillary muscles from the heart of reserpinized rabbits (at 37 degrees C). The preparations were paced to contract at 0.67 Hz under isometric conditions and the muscle length was adjusted to 95% of the length for optimum force production. Simultaneous recordings of isometric force and membrane potentials were performed. 4-AP (50 microM) increased peak force by approximately 20% of the control and prolonged the action potential by 20%. Higher concentrations of 4-AP (800 microM) resulted in further increments of force and action potential duration (60 and 70% of controls, respectively). Prolongation of the action potential and potentiation of the isometric force was still present one hour after withdrawal of the drug from the perfusate. The results are consistent with the view that 4-AP prolongs the action potential by inhibiting the late repolarizing potassium current. It is suggested that the calcium uptake by the ventricular cell during the prolonged action potential is increased and that this leads to the positive inotropic effect.

The effects of 4-aminopyridine on the isolated vas deferens and its effects on the inhibitory properties of adenosine, morphine, noradrenaline and gamma-aminobutyric acid

1 Adenosine, adenosine 5'-triphosphate (ATP), morphine, noradrenaline, gamma-aminobutyric acid (GABA) phentolamine and amyl nitrite were used to inhibit electrically-evoked contractions of the isolated superfused vas deferens of the mouse. 2 The inhibitory effects of adenosine ATP, morphine, noradrenaline and GABA, which are thought to be due to presynaptic action, were reduced by perfusion with media containing 4-aminopyridine (4AP) or tetraethylammonium (TEA) ions. The inhibitory effects of phentolamine and amyl nitrite were unaffected by 4AP or TEA. 3 Quinidine, which like 4AP and TEA produced some increase to twitch height, did not reduce responses to the various agonists, indicating that an increased muscle contraction was not itself responsible for the reduced responses. 4 It is concluded that antagonism between 4AP and adenosine is not a specific interaction, as had been suggested, but probably reflects an interaction with Ca2+ requiring processes in the presynaptic terminal.

Effect of taurine on seizures induced by 4-aminopyridine

The effect of intraperitoneally injected taurine against the convulsive activity induced by 4-aminopyridine (4-AP) was studied in 12- to 15-day-old mice. At a dose of 2.6 mg/kg, taurine increased the latency of clonic seizures from 7 to 20 minutes, reduced the incidence of tonic seizures from 92% to 30% and the postconvulsive mortality from 80% to 31%. The injection of EDTA prior to the administration of taurine prevented the protective effects of the amino acid. GABA and glycine at the same doses did not protect against 4-AP-induced seizures. 4-AP caused a small increase (19%) in 45Ca accumulation by mice brain synaptosomes incubated in a Krebs-HEPES medium containing low CaCl2 (0.1 mM) and also slightly potentiated the veratrine and potassium-induced increase in calcium accumulation. 4-AP at concentrations of 1-2 mM caused a marked increase (100%-500%) of 45 Ca accumulation by synaptosomes incubated in a Krebs-bicarbonate medium containing 2.5 mM CaCl2. This increase was completely antagonized by taurine but not by GABA of glycine. The present observations suggest that the anticonvulsant effect of taurine might be mediated by 4-AP-calcium-taurine interactions.