Local anaesthetic activity of vesamicol in the electric organ of Torpedo

The fate of newly synthesized hormone from neuroendocrine cells of Aplysia

The neuroendocrine bag cells from Aplysia control reproductive functions through the regulated release of egg-laying hormone (ELH). The purposes of this study were to investigate (1) if synthesis of ELH in bag cells compensates for the release and degradation of the hormone, (2) the ratio of released to stored ELH, (3) the fate of newly synthesized ELH and its relative contribution to the overall pattern of ELH secretion, and (4) the consequences of blocking protein synthesis in bag cells on the pattern of ELH secretion and subsequent reproductive function. Excised bag cells were incubated with [3H]leucine to radiolabel newly synthesized ELH and were then electrically stimulated to trigger secretion of the hormone on 4 successive days, mimicking bag cell activities during the breeding season. Samples of bag cell secretion were collected; total (new and old) and radiolabeled (new) ELH levels were determined. The results showed that ELH synthesis could not keep up with the daily loss of ELH through release and degradation, but the ratio of released to stored ELH remained constant over the 4-day study. In addition, releasable ELH was secreted preferentially within 24 hr after it was synthesized. Blocking protein synthesis resulted in a 50% decrease in total ELH released, suggesting that newly synthesized ELH contributes to half of the total ELH secreted. Furthermore, this lowered level of total ELH, i.e., older ELH alone, was not sufficient to stimulate egg laying in Aplysia, indicating that the contribution of newly synthesized ELH was physiologically relevant.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of tetrodotoxin, Ca2+ absence, d-tubocurarine and vesamicol on spontaneous acetylcholine release from rat muscle

1. Rat hemidiaphragms were incubated in a physiological low-K+ medium without stimulation and the amount of acetylcholine (ACh) released was measured radioenzymatically. Cholinesterases were inhibited by paraoxon. 2. In the presence of 1 microM tetrodotoxin (TTX), the amount of ACh released during a 2 h incubation was lowered by 40%. A similar decrease was observed in the absence of Ca2+ and in the presence of 10 microM-d-tubocurarine (dTC). The effects of TTX combined with Ca2+ removal, and of TTX combined with dTC were no greater than those of TTX, dTC or Ca2+ removal alone. TTX and dTC had no effect on the release of ACh from diaphragms 4 days after denervation. 3. The reduction of spontaneous ACh release observed in the presence of TTX or dTC or in the absence of Ca2+ is best interpreted on the assumption that about 40% of the ACh release was due to the impulse activity known to be generated in intramuscular motor nerve branches by the ACh which accumulates after the inhibition of cholinesterases. 4. In the presence of 1 and 10 microM vesamicol (AH5183, 2-(4-phenylpiperidino)-cyclohexanol), the release of ACh was also diminished by approximately 40%. Vesamicol did not augment the inhibition of release produced by TTX or by the omission of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Mobilization of the readily releasable pool of acetylcholine from a sympathetic ganglion by tityustoxin in the presence of vesamicol

The present experiments tested whether preganglionic stimulation and direct depolarization of nerve terminals by tityustoxin could mobilize similar or different pools of acetylcholine (ACh) from the cat superior cervical ganglia in the presence of 2-(4-phenylpiperidino)cyclohexanol (vesamicol, AH5183), an inhibitor of ACh uptake into synaptic vesicles. In the absence of vesamicol, both nerve stimulation and tityustoxin increased ACh release. In the presence of vesamicol, the release of ACh induced by tityustoxin was inhibited, and just 16% of the initial tissue content could be released, a result similar to that obtained with electrical stimulation under the same condition. When the impulse-releasable pool of ACh had been depleted, tityustoxin still could release transmitter, amounting to some 10% of the ganglion's initial content. This pool of transmitter seemed to be preformed in the synaptic vesicles, rather than synthesized in response to stimuli, as tityustoxin could not release newly synthesized [3H]ACh formed in the presence of vesamicol, and hemicholinium-3 did not prevent the toxin-induced release. In contrast to the results with tityustoxin, preganglionic stimulation could not release transmitter when impulse-releasable or toxin-releasable compartments had been depleted. Our results confirm that vesamicol inhibits the mobilization of transmitter from a reserve to a more readily releasable pool, and they also suggest that, under these experimental conditions, there might be some futile transmitter mobilization, apparently to sites other than nerve terminal active zones.

The effects of vesamicol on trains of endplate currents and on focally recorded nerve terminal currents at mammalian neuromuscular junctions

1. The effects of vesamicol, an inhibitor of vesicular acetylcholine (ACh) storage, were studied on trains of endplate currents (e.p.cs) in the cut rat hemidiaphragm nerve-muscle preparation and on trains of focally recorded nerve terminal current waveforms in the mouse triangularis sterni nerve-muscle preparation. 2. In the rat, 0.1 and 1 microM (-)-vesamicol produced an enhancement of the rundown of e.p.c. amplitudes during trains of high frequency (50 Hz) nerve stimulation. However, 1 microM (+)-vesamicol had no effect on the rundown of e.p.c. amplitudes. 3. In the mouse, high concentrations of (-)-vesamicol (10-100 microM) produced a concentration- and stimulation-dependent decrease in the amplitude of the second negative-going deflection of focally recorded nerve terminal current waveforms. 4. At 1 mM, (-)-vesamicol produced a stimulation-independent decrease in the amplitude of the first negative-going deflection of the nerve terminal current waveforms, an increase in signal delay and evidence of nerve conduction failure. These all indicate a local anaesthetic-like block of nodal Na(+)-channels. 5. In contrast to its effects on trains of e.p.cs, the effects of vesamicol on the nerve terminal current waveforms were not stereoselective, the (+)-isomer being equipotent with the (-)-isomer. 6. Low concentrations of the Na(+)-channel blocking toxin, tetrodotoxin (15-60 nM), produced similar changes in the focally recorded nerve terminal current waveforms to those seen with vesamicol. 7. It is concluded that the stereoselective rundown of e.p.c. amplitudes produced by (-)-vesamicol is due to an effect, either direct or indirect, on ACh mobilization within motor nerve terminals. Furthermore, in mammalian species, the inhibitory effects of vesamicol on nodal Na+-channels which are seen at high concentrations do not contribute to the principal neuromuscular effects of the compound.

Inhibition of neurotransmitter and hormone transport into secretory vesicles by 2-(4-phenylpiperidino)cyclohexanol and 2-bromo-alpha-ergocryptine: both compounds act as uncouplers and dissipate the electrochemical gradient of protons

2-(4-Phenylpiperidino)cyclohexanol (AH-5183) and 2-bromo-alpha-ergocryptine, known inhibitors of the transport of acetylcholine and L-glutamate, respectively, into synaptic vesicles, inhibited the ATP-dependent uptake of dopamine in parallel with the dissipation of the electrochemical gradient of protons in chromaffin granule membrane vesicles. These compounds induced the release of accumulated dopamine from the vesicles. They also inhibited the ATP-dependent formation of the electrochemical gradient of protons in liposomes reconstituted with chromaffin H(+)-ATPase without affecting the activities for ATP hydrolysis, and ATP-dependent uptakes of dopamine, gamma-aminobutyrate, and glutamate into synaptic vesicles. These results indicated that 2-(4-phenylpiperidino)cyclohexanol and 2-bromo-alpha-ergocryptine acted as uncouplers in the secretory vesicles.