Age-induced alteration of neuromuscular transmission: effect of halothane

Molecular Adaptations of BDNF/NT-4 Neurotrophic and Muscarinic Pathways in Ageing Neuromuscular Synapses

Age-related conditions, such as sarcopenia, cause physical disabilities for an increasing section of society. At the neuromuscular junction, the postsynaptic-derived neurotrophic factors brain-derived neurotrophic factor (BDNF) and neurotrophin 4 (NT-4) have neuroprotective functions and contribute to the correct regulation of the exocytotic machinery. Similarly, presynaptic muscarinic signalling plays a fundamental modulatory function in this synapse. However, whether or not these signalling pathways are compromised in ageing neuromuscular system has not yet been analysed. The present study analyses, through Western blotting, the differences in expression and activation of the main key proteins of the BDNF/NT-4 and muscarinic pathways related to neurotransmission in young versus ageing Extensor digitorum longus (EDL) rat muscles. The main results show an imbalance in several sections of these pathways: (i) a change in the stoichiometry of BDNF/NT-4, (ii) an imbalance of Tropomyosin-related kinase B receptor (TrkB)-FL/TrkB-T1 and neurotrophic receptor p 75 (p75NTR), (iii) no changes in the cytosol/membrane distribution of phosphorylated downstream protein kinase C (PKC)βI and PKCε, (iv) a reduction in the M2-subtype muscarinic receptor and P/Q-subtype voltage-gated calcium channel, (v) an imbalance of phosphorylated mammalian uncoordinated-18-1 (Munc18-1) (S313) and synaptosomal-associated protein 25 (SNAP-25) (S187), and (vi) normal levels of molecules related to the management of acetylcholine (Ach). Based on this descriptive analysis, we hypothesise that these pathways can be adjusted to ensure neurotransmission rather than undergoing negative alterations caused by ageing. However, further studies are needed to assess this hypothetical suggestion. Our results contribute to the understanding of some previously described neuromuscular functional age-related impairments. Strategies to promote these signalling pathways could improve the neuromuscular physiology and quality of life of older people.

Age-related changes in the structure and function of mammalian neuromuscular junctions

As mammals age, their neuromuscular junctions (NMJs) change their form, with an increasingly complex system of axonal branches innervating increasingly fragmented regions of postsynaptic differentiation. It has been suggested that this remodeling is associated with impairment of neuromuscular transmission and that this contributes to age-related muscle weakness in mammals, including humans. Here, we review previous work on NMJ aging, most of which has focused on either structure or function, as well as a new study aimed at seeking correlation between the structure and function of individual NMJs. While it is clear that extensive structural changes occur as part of the aging process, it is much less certain how, if at all, these are correlated with an impairment of function. This leaves open the question of whether loss of NMJ function is a significant cause of age-related muscle weakness.

VAChT overexpression increases acetylcholine at the synaptic cleft and accelerates aging of neuromuscular junctions

BACKGROUND

Cholinergic dysfunction occurs during aging and in a variety of diseases, including amyotrophic lateral sclerosis (ALS). However, it remains unknown whether changes in cholinergic transmission contributes to age- and disease-related degeneration of the motor system. Here we investigated the effect of moderately increasing levels of synaptic acetylcholine (ACh) on the neuromuscular junction (NMJ), muscle fibers, and motor neurons during development and aging and in a mouse model for amyotrophic lateral sclerosis (ALS).

METHODS

Chat-ChR2-EYFP (VAChTHyp) mice containing multiple copies of the vesicular acetylcholine transporter (VAChT), mutant superoxide dismutase 1 (SOD1G93A), and Chat-IRES-Cre and tdTomato transgenic mice were used in this study. NMJs, muscle fibers, and α-motor neurons' somata and their axons were examined using a light microscope. Transcripts for select genes in muscles and spinal cords were assessed using real-time quantitative PCR. Motor function tests were carried out using an inverted wire mesh and a rotarod. Electrophysiological recordings were collected to examine miniature endplate potentials (MEPP) in muscles.

RESULTS

We show that VAChT is elevated in the spinal cord and at NMJs of VAChTHyp mice. We also show that the amplitude of MEPPs is significantly higher in VAChTHyp muscles, indicating that more ACh is loaded into synaptic vesicles and released into the synaptic cleft at NMJs of VAChTHyp mice compared to control mice. While the development of NMJs was not affected in VAChTHyp mice, NMJs prematurely acquired age-related structural alterations in adult VAChTHyp mice. These structural changes at NMJs were accompanied by motor deficits in VAChTHyp mice. However, cellular features of muscle fibers and levels of molecules with critical functions at the NMJ and in muscle fibers were largely unchanged in VAChTHyp mice. In the SOD1G93A mouse model for ALS, increasing synaptic ACh accelerated degeneration of NMJs caused motor deficits and resulted in premature death specifically in male mice.

CONCLUSIONS

The data presented in this manuscript demonstrate that increasing levels of ACh at the synaptic cleft promote degeneration of adult NMJs, contributing to age- and disease-related motor deficits. We thus propose that maintaining normal cholinergic signaling in muscles will slow degeneration of NMJs and attenuate loss of motor function caused by aging and neuromuscular diseases.

Age-related fragmentation of the motor endplate is not associated with impaired neuromuscular transmission in the mouse diaphragm

As mammals age, their neuromuscular junctions (NMJs) gradually change their form, acquiring an increasingly fragmented appearance consisting of numerous isolated regions of synaptic differentiation. It has been suggested that this remodelling is associated with impairment of neuromuscular transmission, and that this contributes to age-related muscle weakness in mammals, including humans. The underlying hypothesis, that increasing NMJ fragmentation is associated with impaired transmission, has never been directly tested. Here, by comparing the structure and function of individual NMJs, we show that neuromuscular transmission at the most highly fragmented NMJs in the diaphragms of old (26-28 months) mice is, if anything, stronger than in middle-aged (12-14 months) mice. We suggest that NMJ fragmentation per se is not a reliable indicator of impaired neuromuscular transmission.

Neuromuscular effects of sevoflurane in myasthenia gravis patients

BACKGROUND

Little information is available regarding the neuromuscular effects of sevoflurane in patients with myasthenia gravis (MG). We evaluated the neuromuscular effects of sevoflurane alone in patients with MG and in those with normal neuromuscular transmission.

METHODS

Sixteen patients with generalized type MG (MG group) and 12 otherwise healthy patients (control group) entered into this study. Anaesthesia was induced with propofol, fentanyl, and midazolam followed by nitrous oxide in oxygen. Neuromuscular monitoring was recorded from the adductor pollicis muscle using electromyography with train-of-four stimulation of the ulnar nerve. After a stabilization period, and before sevoflurane administration, baseline T4/T1 was obtained and MG patients were classified as non-fade MG group (baseline T4/T1 > or = 0.90) (n = 10) and fade MG group (baseline T4/T1 < 0.90) (n = 6). End-tidal sevoflurane concentration was kept constant at 1.7% for 30 min and doubled thereafter to 3.4% and maintained for a further 30 min.

RESULTS

Sevoflurane produced a concentration-dependent decrease in T1 and T4/T1 values. At 3.4% sevoflurane, T1 and T4/T1 decreased significantly from baseline values in all three groups. From baseline until the patient woke up from anaesthesia, the T4/T1 of the fade MG group was significantly lower than the other groups. At the end of anaesthesia, T4/T1 returned to values similar to the baseline in all three groups.

CONCLUSIONS

During sevoflurane anaesthesia, concentration-dependent inhibition of neuromuscular transmission was observed in MG and control patients. The inhibitory effects of sevoflurane were more prominent in MG patients with baseline T4/T1 <0.90.

Duration of exposure to sevoflurane affects dose-response relationship of vecuronium

The purpose of this study was to quantify the relationship between the dose-response curve of vecuronium and duration of exposure to an end-tidal concentration of 1.7% sevoflurane in 67% nitrous oxide and oxygen. Forty adult patients, in groups of 10, were allocated randomly to receive vecuronium by a cumulative dose method at intervals of 15 min (group 15), 30 min (group 30), 60 min (group 60) or 90 min (group 90) after starting inhalation of sevoflurane. Neuromuscular function was monitored by acceleromyographic train-of-four (TOF) responses of the adductor pollicis muscle to ulnar nerve stimulation. Dose-response curves were constructed by least-squares regression analysis and the effective doses of vecuronium (ED50, ED90 and ED95) were estimated and compared between groups. Mean (SEM) ED50, ED90 and ED95 were 16.8 (0.5), 32.6 (1.7) and 40.9 (2.4) micrograms kg-1, respectively, in group 15; 10.6 (1.0), 20.8 (1.7) and 26.2 (2.2) micrograms kg-1, respectively, in group 30; 11.2 (1.1), 21.7 (1.6) and 27.3 (1.8) micrograms kg-1, respectively, in group 60; and 11.0 (1.1), 21.7 (1.6) and 27.5 (1.9) micrograms kg-1, respectively, in group 90. The values obtained in group 15 were significantly higher than those in the other three groups (P < 0.05). The results indicate that the duration of sevoflurane anaesthesia influences the dose-response of vecuronium and 30 min inhalation of 1.7% end-tidal concentration is sufficient to achieve a stable potentiating effect.

A comparison of the effect of halothane on N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated excitatory synaptic transmission in the hippocampus

Halothane depresses synaptic transmission in the rat brain. First we determined the concentration of halothane which decreased the amplitude of the population spike recorded in the CA1 region of the hippocampus to 50% of the control value (105 +/- 4.9 micrograms/mL [0.53 mM] halothane). Hippocampal glutamate receptors are divided into N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionate (AMPA) and kainate (non-NMDA) subtypes. The NMDA and non-NMDA receptors were blocked with (+/-)-2-amino-5-phosphonopentanoic acid (AP5) (30 microM), and 6,7-dinitroquinoxaline-2,3-dione (DNQX) (10 microM), respectively, to allow observation of the effects of halothane on the NMDA and non-NMDA receptors, respectively. gamma-Aminobutyric acid type A (GABAA) receptors were blocked in all studies with picrotoxin (PTX) (40 microM). When the non-NMDA receptors were blocked a halothane concentration of 38.1 +/- 5.6 mg/mL was required to produce a further 50% decrease in population spike amplitude. When NMDA receptors were blocked with AP5 or only GABAA receptors were blocked the halothane concentrations needed to produce 50% block were higher than needed for the control (160.8 +/- 17.8 and 190.2 +/- 12.1 microgram/mL, respectively). These studies indicate that the NMDA receptors are more sensitive to the effects of halothane than the non-NMDA receptors.

Some effects of d-tubocurarine alone and combined with halothane or isoflurane on neuromuscular transmission

The mechanisms contributing to the neuromuscular block produced by nondepolarizing muscle relaxants and inhaled anesthetics include: 1) receptor blockade, 2) open or closed ion channel block, 3) decreased transmitter release, and 4) receptor desensitization. In this study we investigated the contributions of receptor and ion channel block. We used the two microelectrode voltage clamp to evaluate miniature end-plate currents (MEPCs) for amplitude and time constant of decay (tau) before and after the application of 1) d-Tubocurarine (DTC) (10(-7)-10(-6) M) alone, and then 2) the same concentration of DTC plus 0.5% or 1.0% halothane or isoflurane delivered by passing compressed air through a flow and temperature compensated vaporizer. The electrodes were maintained in the same cell for the entire experiment. DTC alone decreased MEPC amplitude to 91.3% +/- 4.5% and 65.1% +/- 5.6% of control at 10(-7) and 10(-6) M, respectively. MEPC amplitude with 10(-6) M DTC decreased further to 52.4% +/- 7.6% and 37.4% +/- 7.0% of control after the addition of 0.5% and 1.0% halothane, respectively. After the application of DTC 10(-7) M tau was 94.7% +/- 3.1% of control and decreased to 73.7% +/- 7.1% of control with 10(-6) M DTC. After the application of DTC 10(-6) M the addition of 0.5% and 1% halothane decreased tau to 52.4% +/- 7.6% and 30.0% +/- 5.9% of control. Isoflurane produced similar changes. This study provides evidence that at least some of the augmentation of nondepolarizing relaxants by inhaled anesthetics can be explained by the additive effect of nondepolarizing muscle relaxants and inhaled anesthetics on MEPC amplitude and tau.