Effects of 4-aminopyridine on statistical parameters of transmitter release at the neuromuscular junction

3,4-diaminopyridine reverses paralysis in botulinum neurotoxin-intoxicated diaphragms through two functionally distinct mechanisms

Botulinum neurotoxins (BoNTs) are exceedingly potent neurological poisons that prevent neurotransmitter release from peripheral nerve terminals by cleaving presynaptic proteins required for synaptic vesicle fusion. The ensuing neuromuscular paralysis causes death by asphyxiation. Although no antidotal treatments exist to block toxin activity within the nerve terminal, aminopyridine antagonists of voltage-gated potassium channels have been proposed as symptomatic treatments for botulism toxemia. However, clinical evaluation of aminopyridines as symptomatic treatments for botulism has been inconclusive, in part because mechanisms responsible for reversal of paralysis in BoNT-poisoned nerve terminals are not understood. Here we measured the effects of 3,4-diaminopyridine (DAP) on phrenic nerve-elicited diaphragm contraction and end-plate potentials at various times after intoxication with BoNT serotypes A, B, or E. We found that DAP-mediated increases in quantal content promote neurotransmission from intoxicated nerve terminals through two functionally distinguishable mechanisms. First, DAP increases the probability of neurotransmission at non-intoxicated release sites. This mechanism is serotype-independent, becomes less effective as nerve terminals become progressively impaired, and remains susceptible to ongoing intoxication. Second, DAP elicits persistent production of toxin-resistant endplate potentials from nerve terminals fully intoxicated by BoNT/A, but not serotypes B or E. Since this effect appears specific to BoNT/A intoxication, we propose that DAP treatment enables BoNT/A-cleaved SNAP-25 to productively engage in fusogenic release by increasing the opportunity for low-efficiency fusion events. These findings have important implications for DAP as a botulism therapeutic by defining conditions under which DAP may be clinically effective in reversing botulism symptoms.

Mechanisms of neuromodulation as dissected using Sr2+ at motor nerve endings

The use of binomial analysis as a tool for determining the sites of action of neuromodulators may be complicated by the nonuniformity of release probability. One of the potential sources for nonuniformity of release probability is the presence of multiple forms of synaptotagmins, the Ca2+ sensors responsible for triggering vesicular exocytosis. In this study we have used Sr2+, an ion whose actions may be restricted to a subpopulation of synaptotagmins, in an attempt to obtain meaningful estimates of the binomial parameters p (the probability of evoked acetylcholine [Ach] release) and n (the immediate available store of ACh quanta, whereby m = np). In contrast to results in Ca2+ solutions, binomial analysis of Sr2+-dependent release reveals a dramatically reduced dependence of n on extracellular Sr2+ concentrations. In Sr2+ solutions, blockade of potassium channels with 3,4-diaminopyridine increased m by an exclusive increase in p, whereas treatment with phorbol ester increased m solely by effects on n. The cyclic adenosine monophosphate (cAMP) analogue CPT-cAMP increased m by increasing both n and p. The effect of CPT-cAMP on p but not on n was blocked by protein kinase A (PKA) inhibitors, whereas the effect on n was mimicked by 8-CPT-2'-O-Me-cAMP, a selective agonist for exchange protein directly activated by cAMP, otherwise known as the cAMP-sensitive guanine nucleotide-exchange protein. The results demonstrate both the utility of the binomial distribution in Sr2+ solutions and the dual effects of cyclic AMP on both PKA-dependent and PKA-independent processes at the amphibian neuromuscular junction.

Pharmacologic inhibition of twin-pulse facilitation of release of transmitter quanta at the mouse neuromuscular junction

1. The frequency (F,s-1) of miniature endplate potentials and the quantal content (m) of endplate potentials were simultaneously measured intracellularly at mouse diaphragm endplates in a bath solution that contained 0.6 mM Ca2+ ions and 5 mM Mg2+ ions. 2. Twin pulses at 4-ms intervals gave the quantal contents of the first (m1) and second (m2) responses. The ratio of m2/m1 was taken as an indicator of the temporal facilitation of the release of transmitter. 3. Lead ions (Pb2+; 10 microM), bis (o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; loaded for 60 min at 200 microM), and chlortetracycline (CTC; loaded for 30 min at 80 microM) reduced the values of F and m2/m1. Pb2+ ions and CTC reduced the value of m, whereas BAPTA did not.omega-Agatoxin (omega AGT; 10 ng/ml) reduced the value of m without affecting F or m2/m1. 4. These results suggest that synaptic facilitation is modifiable by agents that can affect systems which buffer intracellular levels of Ca2+ ions.

Factors influencing the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction

1. The effects of several conditions and agents on the twin-pulse facilitation of the release of transmitter at the mouse neuromuscular junction in low-Ca2+ high-Mg2+ bathing solutions were examined. 2. Twin-pulses gave two endplate potential (epps) with first (m2) and second (m2) quantal contents. The ratio of m2/m1 was taken as a measure of the degree of facilitation. 3. The mean value of this ratio was > 1. Individual ratios fluctuated widely at junctions with smaller values of m1 but were focused around 1 at junctions with larger values of m1. Thus, some populations of junctions with smaller values of m1 contributed to an increment in the mean ratio. 4. The mean ratio was virtually constant irrespective of changes in the spontaneous and evoked release of transmitter at temperatures between 20 and 36 degrees C and at external concentrations of Ca2+ from 0.4 to 0.8 mM. 5. 4-Aminopyridine(4-AP) slightly but significantly increased this ratio with increases in m1 and m2 at temperatures of 24 and 36 degrees C. Ouabain slightly but significantly reduced the ratio, with increases in m1 and m2. The steadiness of the ratio was maintained in the presence of caffeine, high K+, neomycin or omega-conotoxin irrespective of changes in m1 and m2, except in the case of omega-conotoxin. 6. Spontaneous output at 36 degrees C increased in the presence of 4-aminopyridine, ouabain, caffeine, high K+ or neomycin. 7. These results indicate that maintenance of a stable value of the ratio of m2 to m1 is a dominant feature.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The effect of diaminopyridines on the activity recorded in the dorsal roots of isolated spinal cord

1. The diaminopyridines (DAP) increase the activity recorded in the dorsal roots of an isolated hamster spinal cord preparation. 2. The threshold for 3,4 DAP was 3 x 10(-6) M and the maximum effect was 10(-4) M. The threshold for 2,3 DAP was 3 x 10(-4) M and the maximum effect was at 10(-3) M. The threshold for 2,6 DAP was 10(-4) M and its maximum effect was at 3 x 10(-2) M. 3. When compared with the monoamino pyridines (AP) 4 AP was the most effective and the order of potency was 4 AP greater than 3,4 DAP greater than 2,3 DAP greater than 2 AP = 3 AP greater than 2,6 DAP. 4. Treatment with AP increased the activity simultaneously recorded in the dorsal and ventral roots, but whilst that in the dorsal root continued after washing away the amino pyridine, that in the ventral root ceased for a period of time. 5. There are indications that the amino pyridines may differently affect dorsal and ventral interneurones.

The effect of aminopyridines on the activity recorded in the dorsal roots of an isolated hamster spinal cord preparation

1. The aminopyridines (AP) increase the activity recorded on the dorsal roots of an isolated hamster spinal cord preparation. 2. The threshold for 4-AP was 4 X 10(-6) M and the maximum effect was 3 X 10(-5) M. The threshold for 2AP and 3AP was 3 X 10(-5) M and the maximum effect was at 10(-3) M. 3. It took longer for the preparation to recover on washing from treatment with 4-AP than from 2-AP or 3-AP. Washing often showed a transient increase in activity. 4. Treatment with AP changed the pattern of firing so that the activity after treatment and prolonged washing showed a bursting pattern. 5. The activity recorded in the dorsal roots is generated within the spinal cord and may be related to activity of central pattern generators.

Effects of synthetic omega-conotoxin, a new type Ca2+ antagonist, on frog and mouse neuromuscular transmission

A new type Ca2+ antagonist, synthetic omega-conotoxin (omega-CgTX) decreased the peak height of the endplate potential (EPP) in frog muscle but had no effect on the mouse neuromuscular junction. The reduction of endplate potential in frogs was due to a decrease in transmitter release, since the mean quantal content estimated by variance of EPPs (m) and from the peak heights of EPPs and miniature EPPs (m1) was reduced by omega-CgTX, but the postsynaptic sensitivity to ACh was unaltered. The decrease of mean quantal content caused by omega-CgTX was reversed by 4-aminopyridine, guanidine and Bay K 8644. Also, the effect of omega-CgTX was weakened in the presence of 10 mM Ca2+ or 12 mM Mg2+. Statistical analysis revealed that omega-CgTX decreased the number of quanta available (n) whereas the probability of release (p) remained unaffected.

4-Aminopyridine-mediated increase in long-term potentiation in CA1 of the rat hippocampus

The effect of 4-aminopyridine (4-AP) on long-term potentiation (LTP) was studied in the hippocampal slice preparation of the rat. Field excitatory postsynaptic potentials (EPSPs) were recorded and evoked in the stratum radiatum of the CA1. Both the low frequency EPSP and LTP of the EPSP were significantly increased by treatment with 4-AP. These effects were inhibited by increasing the magnesium concentration from 1 to 4 mM. Pretreatment with 20 microM DL-2-amino-5-phosphonovalerate antagonized only the increase in LTP produced by 4-AP. It is suggested that 4-AP enhances Ca influx either pre- or postsynaptically and thereby increases LTP.

Comparison of the pharmacological actions of some new 4-aminopyridine derivatives

The pharmacological actions of 2,4-diaminopyridine (2,4-DAP) and 3-[(dimethylamino)-carbonyl] amino 4-aminopyridine (LF-14) were examined and compared with those of 4-aminopyridine (4-AP) in anaesthetized rats and on isolated rat and guinea-pig tissues. Both compounds were more potent than 4-AP in reversing the neuromuscular block caused by pancuronium bromide. The ED50S of LF-14, 2,4-DAP and 4-AP were 100 micrograms/kg, 140 micrograms/kg and 450 micrograms/kg, respectively. LF-14 and 2,4-DAP were also more potent in their in vitro actions on the neuroeffector junctions in the ileum and the isolated heart. 2,4-DAP and LF-14 either did not facilitate or only slightly facilitated the recovery time from xylazine/ketamine anaesthesia which was used as a test for their central action; 4-AP significantly reduced the recovery time. We therefore conclude that both 2,4-DAP and LF-14 are stronger peripherally acting compounds with less central action, and that they may be possible replacements for 4-AP as antagonists of non-depolarizing muscle relaxants.

Influence of presynaptic receptors on neuromuscular transmission in rat

The presence and physiological significance of acetylcholine (ACh) receptors on motor nerve terminals was examined at the rat diaphragm neuromuscular junction. Intracellular recording techniques were used to monitor end-plate potentials (EPP), miniature end-plate potentials (MEPP), and resting potentials of the muscle fibers. Muscle action potentials were blocked by the cut-muscle technique. Quantal release was determined by the ratio EPP/MEPP, after correcting for nonlinear summation. Blockade of acetylcholinesterase with eserine and neostigmine was tested to determine the influence of residual ACh on transmitter release. Partial blockade of ACh receptors with curare was examined to further clarify the role of these presynaptic receptors. The experiments demonstrate that residual ACh inhibits transmitter release and that blockade of ACh receptors enhances transmitter release. It is concluded that presynaptic ACh receptors exist and that they serve an important physiological function. It is suggested that the presynaptic ACh receptors normally serve to limit transmitter release in a negative feedback pathway.

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.

4-Aminopyridine does not increase m.e.p.p. frequencies at junctions depolarized by potassium

4-aminopyridine (4-AP) is known to produce large increases in quantal acetylcholine release from stimulated motor nerve terminals. It has been suggested that the drug might act directly on Ca2+ channels to increase Ca2+ influx. This possibility was tested at frog neuromuscular junctions depolarized in elevated [K+]out. The 4-AP did not increase miniature end-plate potential frequencies. Also, 4-AP did not alter the increase in frequency that follows a rise in [Ca2+]out at a depolarized junction. Therefore, under these conditions, 4-AP does not appear to change Ca2+ entry into or elimination from the nerve terminal. The results support the hypothesis that 4-AP acts by lengthening the nerve terminal action potential.

Action of 4-aminopyridine on the cerebral circulation

4-Aminopyridine (4-AP) facilitates both inhibitory and excitatory synaptic activity in the central nervous system, and may, therefore, be a drug of potential therapeutic use in brain diseases with a disturbed synaptic transmission. In the present study the vasomotor effects upon isolated feline brain vessels, and regional cerebral blood flow and brain cortical metabolism in rats were examined. At high concentrations (above 10-6 M) a minor vasoconstriction was obtained of isolated pial vessels. Measurements of regional cerebral blood flow using the 14C-ethanol technique resulted in a significant increase in blood flow of caudate nucleus (93%), thalamus (74%) and cerebellum (82%). The arteriovenous oxygen difference of cortical tissue was reduced from 3.20 mmol O2/ml to 1.69 mmol O2/ml by 4-AP. This was not associated with an increase in cortical blood flow. Calculation of the cortical metabolic rate of oxygen, however, failed to demonstrate any significant change.