(R)-roscovitine prolongs the mean open time of unitary N-type calcium channel currents.

(R)-roscovitine (Ros) is a cyclin-dependent kinase inhibitor that also has been shown to have direct agonist and antagonist actions on Ca(v)2.1 (P/Q-type) and Ca(v) 2.2 (N-type) families of voltage-gated calcium channels. These kinase-independent effects represent a novel opportunity to advance our understanding of calcium channel function and calcium-triggered neurotransmitter release. Furthermore, such actions on calcium channels may direct the development of Ros derivatives as new therapeutic agents. We used patch clamp recordings to characterize mechanisms that underlie the agonist effects of Ros on unitary N-type calcium channel gating. We found that N-type channels normally gate with either a short or long mean open time, that Ros significantly prolonged the mean open time of the long gating component and increased the probability of observing channels that gated with a long open time, but had no effect on single channel conductance. Using Monte Carlo simulations of a single channel kinetic model and Ros interactions, we were able to reproduce our experimental results and investigate the model's microscopic dynamics. In particular, our simulations predicted that the longer open times generated by Ros were due to the appearance of a long open state combined with an increased amount of time spent in transitions between open states. Our results suggest a mechanism for agonist effects of Ros at the level of single channels, and provide a mechanistic explanation for previously reported agonist effects on whole cell calcium currents.

The temperature sensitivity of miniature endplate currents is mostly governed by channel gating: evidence from optimized recordings and Monte Carlo simulations.

The temperature dependence of miniature endplate current (MEPC) amplitude (A(c)), 20-80% rise time (t(r)), and 90-33% fall-time (t(f)) was determined for lizard (Anolis carolinensis) intercostal muscle using broadband extracellular (EC) and voltage clamp (VC) recordings. Voltage clamp methods were optimized for the fast MEPC rising phase using custom electronics. From 0-43 degrees C, A(c) increased by approximately 4.2-fold, while t(r) and t(f) decreased by approximately 3.6- and approximately 9.5-fold, respectively. Arrhenius plots were smoothly curved, with small apparent Q(10) (A(c)) or (Q(10))(-1) (t(r) and t(f)) values mostly well below 2.0. Nearly identical extracellular and voltage clamp results ruled out measurement artifacts, even for the shortest t(r) values (<60 microseconds). Monte Carlo simulation of MEPCs showed that a single underlying rate cannot determine the observed temperature dependence. To quantitatively reproduce the experimental t(f) results, a minimal model required activation energies of 46.0 (Q(10) approximately 2.0) and 63.6 (Q(10) approximately 2.5) kJ mol(-1) for channel opening and closing, respectively, and accounted for most of the observed changes in A(c) and t(r) as well. Thus, relatively large but offsetting temperature sensitivities of channel gating mostly govern and minimize the temperature dependence of MEPCs, preserving the safety factor for neuromuscular transmission. Additional temperature-sensitive parameters that could fine-tune the minimal model are discussed.

Absence of nerve-dependent conversion of rapidly degrading to stable acetylcholine receptors at adult innervated endplates.

It has been suggested that acetylcholine receptors newly inserted into adult innervated endplates have a rapid degradation rate, but are normally converted to a stable, slowly degrading form in a nerve-dependent fashion. Denervation therefore should eliminate conversion and cause pre-existing unconverted receptors to continue degrading rapidly. We tested this model of nerve-dependent conversion in mouse sternomastoid muscle, using quantitative electron microscopic autoradiography in order to specifically examine degradation of receptors at identified endplate membrane. Prior to denervation, we labelled the receptors with sequential alpha-bungarotoxin exposures, using conditions designed to maximize the predicted effect of denervation. However, we observed no difference in the rate of receptor degradation at innervated and denervated endplates up to seven days after denervation (at which time accelerated degradation of pre-existing stabilized receptors is known to begin in this muscle). The regulation of endplate acetylcholine receptor metabolic turnover is a complex and still largely undefined issue, related to many factors such as subunit composition, cytoskeleton and basement membrane composition, muscle activity, and neural influences. In particular, the nerve's influence on the normal stabilization of receptors at innervated adult endplates has been controversial. Our data indicate that slow degradation is probably an inherent property of newly inserted junctional receptors, and argue against nerve-dependent conversion and stabilization. Based on the present data, however, we cannot rule out the presence of a small nerve-independent subpopulation that degrades rapidly. The molecular mechanisms involved in establishing and maintaining a stable population of adult endplate acetylcholine receptors remain to be established.

Miniature endplate current rise times less than 100 microseconds from improved dual recordings can be modeled with passive acetylcholine diffusion from a synaptic vesicle.

We recorded miniature endplate currents (mEPCs) using simultaneous voltage clamp and extracellular methods, allowing correction for time course measurement errors. We obtained a 20-80% rise time (tr) of approximately 80 micros at 22 degrees C, shorter than any previously reported values, and tr variability (SD) with an upper limit of 25-30 micros. Extracellular electrode pressure can increase tr and its variability by 2- to 3-fold. Using Monte Carlo simulations, we modeled passive acetylcholine diffusion through a vesicle fusion pore expanding radially at 25 nm x ms(-1) (rapid, from endplate omega figure appearance) or 0.275 nm x ms(-1) (slow, from mast cell exocytosis). Simulated mEPCs obtained with rapid expansion reproduced tr and the overall shape of our experimental mEPCs, and were similar to simulated mEPCs obtained with instant acetylcholine release. We conclude that passive transmitter diffusion, coupled with rapid expansion of the fusion pore, is sufficient to explain the time course of experimentally measured synaptic currents with trs of less than 100 micros.

Acetylcholinesterase density and turnover number at frog neuromuscular junctions, with modeling of their role in synaptic function.

Acetylcholinesterase (AChE) density at the neuromuscular junction of frog cutaneous pectoris muscle was determined by electron microscope autoradiography and biochemistry to be approximately 600 sites micron-2 of postsynaptic area, approximately 4-fold lower than all previous reports (mouse), whereas the hydrolytic turnover number was 9,500 s-1, well within the range (2,000-16,000 s-1) for AChE from other species. Monte Carlo computer simulations of miniature endplate currents showed that for vertebrate neuromuscular junctions with different morphologies, an AChE density of only approximately 400 sites microns-2 and a turnover number of only approximately 1,000 s-1 are sufficient for normal quantal currents. Above these critical lower limits, miniature endplate currents were essentially insensitive to AChE density and turnover number values up to 5,000 sites microns-2 and 16,000 s-1, respectively.

Intracellular acetylcholinesterase of adult rat myofibers is more concentrated in endplate than non-endplate regions.

The intracellular distribution of acetylcholinesterase (AChE) was determined in adult rat anterior gracilis muscles. Echothiophate iodide (ECHO), a water-soluble cholinesterase inhibitor, was applied to muscles in situ to eliminate extracellular and/or extracellularly oriented enzyme. Control and ECHO-treated muscles were either cut into 1-mm segments and assayed for AChE activity or cytochemically stained for AChE. Subsequent analysis by light and electron microscopy showed that the AChE stain inside myofibers was highly localized and clearly visible only in the zone immediately underlying the point of nerve-muscle contact. Biochemical assay of muscle segments showed intracellular AChE to be most highly concentrated in regions containing large numbers of endplates (approximately twice the activity of endplate-free areas). Since such "endplate-rich" segments are in fact mostly extra-synaptic tissue, we conclude that intracellular AChE of adult rat gracilis myofibers, although present along the length of the cell, is more than two times as concentrated in sub-synaptic areas as compared to extra-synaptic areas. This result must be carefully considered when attempting to identify "endplate-specific" AChE activity of mammalian muscle, and further points to the importance of neural influences on AChE metabolism/regulation.

Skeletal muscle acetylcholinesterase molecular forms in amyotrophic lateral sclerosis.

Acetylcholinesterase (AChE) molecular forms in muscle biopsies from control and amyotrophic lateral sclerosis (ALS) patients were extracted under low (G: globular forms) and high (A: asymmetric forms) ionic strength conditions and were evaluated by velocity sedimentation analysis. Total AChE activity in endplate-containing ALS muscle sections was reduced by an average of 65% of control muscle levels. This decrement resulted from an almost complete disappearance of 9.5S (G4) and 8.0S (A4) AChE and significant decreases in the 3.8S (G1), 12.1S (A8), and 15.8S (A12) forms (66%, 9%, and 25% of control, respectively). In most of the ALS biopsies examined, ultrastructural-cytochemical analysis revealed large reductions in AChE reaction product of both synaptic infoldings (extracellular) and sarcoplasmic reticulum (intracellular) of the muscles' motor endplate regions. These data are compatible with the view that alterations observed in AChE forms from ALS muscles are related to disturbances in the normal "trophic" interactions between nerve and muscle.

Subcellular localization of acetylcholinesterase molecular forms in endplate regions of adult mammalian skeletal muscle.

The characterization of individual acetylcholinesterase (AChE) molecular form subcellular pools in adult mammalian skeletal muscle is a critical point when considering such questions as the origin, assembly, and neurotrophic regulation of these molecules. By correlating the results of differential extraction, in vitro collagenase digestion, and in situ pharmacologic probes of AChE molecular forms in endplate regions of adult rat anterior gracilis muscle, we have shown that: 1) 4.0S (G1) and 6.0S (G2) AChE are predominantly membrane-bound and intracellular; if an extracellular and/or soluble fraction of these forms exists, it cannot be adequately resolved by our methods; 2) 9-11S (globular) AChE activity is distributed between internal and external pools, as well as membrane-associated and soluble fractions; 3) 16.0S (A12) AChE is not an integral membrane protein and exists both intracellularly (25-30%) and extracellularly (70-75%).

Intra- versus extracellular recovery of 16S acetylcholinesterase following organophosphate inactivation in the rat.

Acetylcholinesterase (AChE) inhibitor treatments were used to study the temporal course of intra- versus extracellular 16S AChE recovery in endplate regions of adult rat anterior gracilis muscles previously exposed to a brief, in situ application of diisopropylfluorophosphate (DFP). Following such enzymatic inactivation (95-100%), extracellular 16S AChE recovery began significantly later than that of intracellular (onset at approximately 36 and 12 h, respectively) but, once begun, progressed at approximately the same rate (1.32%/h). The recovery of AChE molecular form activities subsequent to identical DFP-inactivation was blocked to a large extent (65-85%) by in vivo treatment with cycloheximide, a protein synthesis inhibitor. These results support the hypothesis that extracellular 16S AChE at mammalian skeletal muscle motor endplates is primarily derived from complete, previously assembled 16S molecules originating in myofibers.

Denervation induced changes in subcellular pools of 16S acetylcholinesterase activity from adult mammalian skeletal muscle.

Acetylcholinesterase (AChE) inhibitors which differ in lipid solubility and thus in their ability to penetrate cell membranes were utilized to study the effects of denervation on discrete pools of 16S AChE from endplate regions of adult rat anterior gracilis muscle. Such pools have been interpreted as extra- and intracellular fractions of endplate 16S AChE activity. Denervation caused an almost immediate decay of intracellular 16S AChE, and a later (12-18 h) but roughly parallel decrease in its extracellular counterpart. Thus, the level at which the motor nerve exerts its primary regulatory influence on adult mammalian skeletal muscle endplate 16S AChE activity appears to be within muscle cells. This influence may affect the molecule's synthesis, assembly, and/or degradation.