Facilitatory effects of dexamethasone on neuromuscular transmission

Impact of intravenous dexamethasone on the initiation and recovery of atracurium in children: A double-blinded randomized controlled trial

BACKGROUND

Chronic steroid intake has been associated with attenuation of neuromuscular block. Despite some promising animal and adult studies, the effect of a single dose of intravenous dexamethasone on neuromuscular blockers is not well established. Thus, the present study aimed to demonstrate the effect of dexamethasone given at the time of induction for the prevention of PONV on the action of neuromuscular blockers in children undergoing elective surgery.

METHOD

After obtaining approval from the Institute Ethics Committee and written informed parental consent, 100 ASA I and II children aged 4-15 years undergoing elective surgery randomized to receive either: 0.15 mg/kg (maximum of 5 mg) of dexamethasone diluted to a total volume of 2 ml with 0.9% saline (n = 50) or 2 ml of 0.9% saline (n = 50) at the time of induction. The time interval between application of atracurium and maximum T1 depression, 25% twitch height recovery of T1, amid 25% and 75% twitch height recovery of T1, amid the 25% twitch height recovery of T1 and recovery of the neuromuscular block to a TOF ratio of 0.9, and in between the initiation of atracurium injection till the recovery of the neuromuscular block to a TOF ratio of 0.9 was defined as onset time, clinical duration, recovery index, recovery time, and total recovery period, respectively, and recorded.

RESULTS

The onset time and recovery index time were lower (1.96 ± 0.39, 8.04 ± 2.14, respectively) with dexamethasone in comparison with saline (2.01 ± 0.51, 8.9 ± 3.4, respectively) but not statistically significant. The clinical duration, recovery time, and total recovery period were similar.

CONCLUSION

Application of a single bolus dose (0.15 mg/kg) of dexamethasone during induction does not attenuate atracurium-induced neuromuscular blockade in children.

Dexamethasone concentration affecting rocuronium-induced neuromuscular blockade and sugammadex reversal in a rat phrenic nerve-hemidiaphragm model: An ex vivo study

BACKGROUND

The concentration range of dexamethasone that inhibits neuromuscular blockade (NMB) and sugammadex reversal remains unclear.

OBJECTIVE

To evaluate the effects of dexamethasone on rocuronium-induced NMB and sugammadex reversal.

DESIGN

Ex vivo study.

SETTING

Asan Institute for Life Sciences, Asan Medical Center, Korea, from July 2015 to November 2015.

ANIMALS

One hundred sixty male Sprague-Dawley rats.

INTERVENTIONS

We assessed the effect of four concentrations of dexamethasone [0, 0.5, 5 (clinical concentrations) and 50 μg ml (experimental concentration)] on partial NMB on 40 phrenic nerve-hemidiaphragm preparations (n=10 per concentration). Once the first twitch of train-of-four (TOF) had been depressed by 50% with rocuronium, dexamethasone was administered. To assess the effect of dexamethasone on sugammadex reversal, 120 phrenic nerve-hemidiaphragm preparations were used in three subexperiments (n=40 per experiment), using three administration regimens of rocuronium-equimolar sugammadex: a single dose, a split-dose (split and ) and a reduced split-dose (split and ). After complete NMB was achieved, dexamethasone and sugammadex were administered.

MAIN OUTCOME MEASURES

The change in the first twitch height, the recovery time to a TOF ratio at least 0.9, and the TOF ratio at 30 min were evaluated.

RESULTS

There were no significant differences in the first twitch height among groups (P = 0.532). With a single dose of sugammadex, dexamethasone did not affect the recovery time to a TOF ratio at least 0.9 (P = 0.070). After using a split-dose of sugammadex, the recovery time to a TOF ratio at least 0.9 was delayed only at a concentration of 50 μg ml of dexamethasone. With a reduced split-dose of sugammadex, the TOF ratio at 30 min was lowered only by a concentration of 50 μg ml of dexamethasone (P < 0.010).

CONCLUSION

Acute bolus administration of dexamethasone at clinical concentrations had no effect on NMB or on sugammadex reversal.

Effects of dexamethasone and hydrocortisone on rocuroniuminduced neuromuscular blockade and reversal by sugammadex in phrenic nerve-hemidiaphragm rat model

Background

The facilitator effects of steroids on neuromuscular transmission may cause resistance to neuromuscular blocking agents. Additionally, steroids may hinder sugammadex reversal of neuromuscular blockade, but these findings remain controversial. Therefore, we explored the effect of dexamethasone and hydrocortisone on rocuronium-induced neuromuscular blockade and their inhibitory effect on sugammadex.

Methods

We explored the effects of steroids, dexamethasone and hydrocortisone, in vitro using a phrenic nerve-hemidiaphragm rat model. In the first phase, an effective dose of rocuronium was calculated, and in the second phase, following sugammadex administration, the recovery of the train-of-four (TOF) ratio and T1 was evaluated for 30 minutes, and the recovery index was calculated in dexamethasone 0, 0.5, 5, and 50 μg/ml, or hydrocortisone 0, 1, 10, or 100 μg/ml.

Results

No significant effect of steroids on the effective dose of rocuronium was observed. The TOF ratios at 30 minutes after sugammadex administration were decreased significantly only at high experimental concentrations of steroids: dexamethasone 50 μg/ml and hydrocortisone 100 μg/ml (P < 0.001 and P = 0.042, respectively). There were no statistical significances in other concentrations. No differences were observed in T1. Recovery index was significantly different only in 100 μg/ml of hydrocortisone (P = 0.03).

Conclusions

Acute exposure to steroids did not resist the neuromuscular blockade caused by rocuronium. And inhibition of sugammadex reversal on rocuronium-induced neuromuscular blockade is unlikely at typical clinical doses of dexamethasone and also hydrocortisone. Conclusively, we can expect proper effects of rocuronium and sugammadex when dexamethasone or hydrocortisone is used during general anesthesia.

Effects of methylprednisolone on the duration of rocuronium-induced neuromuscular block: A randomized double-blind trial

BACKGROUND

We aim to investigate whether intraoperative use of methylprednisolone could affect the duration of rocuronium-induced neuromuscular blockade.

METHODS

A double blind, randomized, placebo-controlled trial was conducted. A total of 136 patients underwent gynecologic laparoscopic surgery were randomly divided into 3 groups: pregroup, receiving intravenous injection of methylprednisolone (40 mg) 30 minutes before induction of anesthesia; postgroup, receiving intravenous injection of methylprednisolone (40 mg) immediately after induction of anesthesia and intubation; and control group, receiving intravenous injection of normal saline. Patients were intravenously administrated with rocuronium 0.6 mg/kg, and changes in adductor policies evoked twitch responses were measured by ulnar nerve stimulator.

RESULTS

We found that all patients achieved maximum blockade effects, and there was no difference in onset time among the 2 groups. For time required to achieve train-of-four ratio (TOFR) 90%, pregroup (64.50 ± 10.52 minutes) and postgroup (65.29 ± 11.64 minutes) were significantly shorter than that of the control group (71.04 ± 10.55 minutes, P = .027), whereas clinical duration and total duration were significantly shorter in the 2 groups received methylprednisolone than the control group. However, there was no significant difference between the 2 treatment groups either in clinical duration and total duration of muscle relaxants, or time required to achieve TOFR 90%. No significant difference was found in recovery index among the 3 groups.

CONCLUSION

Our findings suggest that a single intravenous injection of methylprednisolone, no matter preoperatively or intraoperatively, could shorten the duration of rocuronium-induced neuromuscular blockade.

Dexamethasone Does Not Inhibit Sugammadex Reversal After Rocuronium-Induced Neuromuscular Block

BACKGROUND

Sugammadex is a relatively new molecule that reverses neuromuscular block induced by rocuronium. The particular structure of sugammadex traps the cyclopentanoperhydrophenanthrene ring of rocuronium in its hydrophobic cavity. Dexamethasone shares the same steroidal structure with rocuronium. Studies in vitro have demonstrated that dexamethasone interacts with sugammadex, reducing its efficacy. In this study, we investigated the clinical relevance of this interaction and its influence on neuromuscular reversal.

METHODS

In this retrospective case-control study, we analyzed data from 45 patients divided into 3 groups: dexamethasone after induction group (15 patients) treated with 8 mg dexamethasone as an antiemetic drug shortly after induction of anesthesia; dexamethasone before reversal group (15 patients) treated with dexamethasone just before sugammadex injection; and control group (15 patients) treated with 8 mg ondansetron. All groups received 0.6 mg/kg rocuronium at induction, 0.15 mg/kg rocuronium at train-of-four ratio (TOF) 2 for neuromuscular relaxation, and 2 mg/kg sugammadex for reversal at the end of the procedure at TOF2. Neuromuscular relaxation was monitored with a TOF-Watch® system.

RESULTS

The control group had a recovery time of 154 ± 54 seconds (mean ± SD), the dexamethasone after induction group 134 ± 55 seconds, and the dexamethasone before reversal group 131 ± 68 seconds. The differences among groups were not statistically significant (P = 0.5141).

CONCLUSIONS

Our results show that the use of dexamethasone as an antiemetic drug for the prevention of postoperative nausea and vomiting does not interfere with reversal of neuromuscular blockade with sugammadex in patients undergoing elective surgery with general anesthesia in contrast to in vitro studies that support this hypothesis.

Effect of dexamethasone on the onset time and recovery profiles of cisatracurium

BACKGROUND

The effect of dexamethasone injection on cisatracurium-induced neuromuscular block was compared according to different injection time points.

METHODS

One hundred seventeen patients were randomly assigned to three groups: 8 mg of dexamethasone injected intravenously 2-3 h before anesthesia (group A), just before anesthesia induction (group B), and at the end of surgery (control group). Three minutes after anesthesia induction, intubation was performed without neuromuscular blockers, and acceleromyography was initiated. All patients received 0.05 mg/kg cisatracurium; the onset time and recovery profiles were recorded.

RESULTS

Eighty patients were finally enrolled. The onset time (median [interquartile range], seconds) was significantly hastened in group A (520.0 [500.0-560.0], n = 30) compared to that in group B (562.5 [514.0-589.0], n = 22) (P = 0.008) and control group (586.5 [575.0-642.5], n = 28) (P < 0.001). The onset time in group B was faster than the control group (P = 0.015). The recovery time [mean (95% CI) minutes] was significantly hastened in group A [28.5 (27.3-29.6)] compared to that in group B [32.3 (31.0-33.6)] (P < 0.001) and control group [30.9 (29.9-31.8)] (P = 0.015). The total recovery time was significantly hastened more in group A [47.1 (45.5-48.6)] than group B [52.8 (51.6-54.0) minutes] (P < 0.001) and control group [50.5 (48.7-52.3) minutes] (P = 0.008).

CONCLUSIONS

A single dose of 8 mg of dexamethasone hastened the onset and total recovery times of cisatracurium-induced block by approximately 15 and 9%, respectively if administered 2-3 h prior to surgery.

Dexamethasone decreases the duration of rocuronium-induced neuromuscular block: a randomised controlled study

BACKGROUND

Several drugs influence the time course of neuromuscular block during general anaesthesia.

OBJECTIVE

To evaluate the effect of a single dose of dexamethasone 8 mg on the time course of a rocuronium-induced neuromuscular block.

DESIGN

A randomised controlled, unblinded, monocentre trial.

SETTING

Kreiskrankenhaus Dormagen, Dormagen, Germany.

PATIENTS

One hundred and eight adult patients scheduled for elective gynaecological laparoscopic surgery allocated to three groups.

INTERVENTIONS

Patients received dexamethasone 8 mg intravenously 2 to 3 h prior to surgery (Group A), during induction of anaesthesia (Group B) or after recovery of the neuromuscular block (Group C, control).

MAIN OUTCOME MEASURES

The time course of the neuromuscular block of rocuronium 0.3 mg kg was assessed using acceleromyography. The primary end point was the time from start of injection of rocuronium until recovery to a train-of-four ratio of 0.9.

RESULTS

The clinical duration was decreased in Group A (15.8 ± 4.5 min) compared with Group B (18.7 ± 5.8 min; P = 0.031). The recovery index was reduced in Group A (6.8 ± 1.8 min) compared with Group B (8.1 ± 2.6 min; P = 0.018) and Group C (8.3 ± 2.8 min; P = 0.01). The recovery to a train-of-four ratio of 0.9 was shorter in Group A (30.4 ± 6.9 min) than in Groups B (36.3 ± 10.7 min; P = 0.031) and C (36.8 ± 11.3 min; P = 0.02).

CONCLUSION

A single dose of dexamethasone 8 mg attenuated rocuronium-induced block by 15 to 20% if administered 2 to 3 h prior to induction of anaesthesia. However, the administration of dexamethasone during induction of anaesthesia did not influence the time course of the neuromuscular block.

TRIAL REGISTRATION

ClinicalTrial.gov Identifier: NCT01782820.

Amplification of neuromuscular transmission by methylprednisolone involves activation of presynaptic facilitatory adenosine A2A receptors and redistribution of synaptic vesicles

The mechanisms underlying improvement of neuromuscular transmission deficits by glucocorticoids are still a matter of debate despite these compounds have been used for decades in the treatment of autoimmune myasthenic syndromes. Besides their immunosuppressive action, corticosteroids may directly facilitate transmitter release during high-frequency motor nerve activity. This effect coincides with the predominant adenosine A2A receptor tonus, which coordinates the interplay with other receptors (e.g. muscarinic) on motor nerve endings to sustain acetylcholine (ACh) release that is required to overcome tetanic neuromuscular depression in myasthenics. Using myographic recordings, measurements of evoked [(3)H]ACh release and real-time video microscopy with the FM4-64 fluorescent dye, results show that tonic activation of facilitatory A2A receptors by endogenous adenosine accumulated during 50 Hz bursts delivered to the rat phrenic nerve is essential for methylprednisolone (0.3 mM)-induced transmitter release facilitation, because its effect was prevented by the A2A receptor antagonist, ZM 241385 (10 nM). Concurrent activation of the positive feedback loop operated by pirenzepine-sensitive muscarinic M1 autoreceptors may also play a role, whereas the corticosteroid action is restrained by the activation of co-expressed inhibitory M2 and A1 receptors blocked by methoctramine (0.1 μM) and DPCPX (2.5 nM), respectively. Inhibition of FM4-64 loading (endocytosis) by methylprednisolone following a brief tetanic stimulus (50 Hz for 5 s) suggests that it may negatively modulate synaptic vesicle turnover, thus increasing the release probability of newly recycled vesicles. Interestingly, bulk endocytosis was rehabilitated when methylprednisolone was co-applied with ZM241385. Data suggest that amplification of neuromuscular transmission by methylprednisolone may involve activation of presynaptic facilitatory adenosine A2A receptors by endogenous adenosine leading to synaptic vesicle redistribution.

Dexamethasone‑induced hyposensitivity to rocuronium in rat diaphragm associated with muscle‑fiber transformation

The aim of the current study was to investigate the effect of chronic dexamethasone (Dex) administration on rat diaphragm function and sensitivity to rocuronium and muscle‑fiber transformation. Adult male Sprague‑Dawley rats were randomized to receive a daily intraperitoneal injection of Dex to evaluate whether alterations in diaphragm function and susceptibility to rocuronium would be induced. In addition, diaphragm contractile properties, histopathology and isometric twitch tensions of nerve‑hemidiaphragm preparations were evaluated. Dex administration led to impaired diaphragm force generation, increased fatigue resistance and a prolonged half‑relaxation time, as well as time‑to‑peak tension. Dex treatment led to desensitization of the rat diaphragm to rocuronium, as demonstrated by a shift of the rocuronium concentration‑twitch tension curves to the right. Histochemical analysis of adenosine triphosphatase revealed that the distribution and cross‑sectional area of type II fibers were decreased in rats exposed to Dex. The present study indicates that chronic Dex treatment induced alterations in muscle function and that susceptibility to rocuronium is associated with muscle fiber‑type transformation, which may aid in directing future administration of muscle relaxants.

Opposing effects of dexamethasone, agrin and sugammadex on functional innervation and constitutive secretion of IL-6 in in vitro innervated primary human muscle cells

Neuromuscular junction development is the key process required for successful neuromuscular transmission and functional innervation of skeletal muscle fibres. Various substances can influence these processes, some of which are in common use in clinical practice. In the present study, the effects of the potentially new therapeutic agent agrin were followed, along with the widely used glucocorticoid dexamethasone. The in vitro experimental model used was functional innervation and constitutive interleukin 6 (IL-6) secretion of human muscle cells. Additionally, the selective relaxant binding agent sugammadex and its possible interaction with dexamethasone were followed. Dexamethasone impaired functional innervation while agrin had opposing effects. Furthermore, based on interference with IL-6 secretion, we show potential (chemical) interactions between dexamethasone and sugammadex. The physiological effects of this interaction should be taken into consideration under clinical conditions where these two drugs might be applied simultaneously.

Attenuation of a rocuronium-induced neuromuscular block in patients receiving prednisolone

BACKGROUND

This study tested the influence of continuous medication (more than 4 weeks) with prednisolone on a rocuronium-induced neuromuscular block.

METHODS

The time course of a rocuronium-induced neuromuscular blockade (0.3 mg/kg) was investigated in 40 patients with chronic inflammatory bowel disease undergoing elective abdominal surgery. The primary end point was the time from the start of injection of rocuronium until recovery of the TOF ratio to 0.9. Twenty patients received continuous medication with prednisolone (group A), and 20 were without glucocorticoid medication (group B). Additionally, another 20 patients without inflammatory bowel disease and without glucocorticoid medication served as control (group C).

RESULTS

The onset time was prolonged in group A [253 (51.2) s] compared with group B [187 (61.3) s]. Twitch height at the onset of the block was higher in group A [16.5 (0-61)%] than that in group B [5.0 (0-33)%]. The duration to 25% twitch height was shorter in group A [12.6 (0-20.7) min] compared with group B [16.7 (0-25.3) min] and group C [16.9 (0-29.3) min]. The recovery to a train-of-four ratio of 0.9 was reduced in group A [25.7 (23-34.3) min] compared with group B [34.7 (32.7-44.2) min] and group C [36.5 (31.7-42.3) min].

CONCLUSIONS

Prednisolone treatment in patients with inflammatory bowel disease is associated with a delayed onset and a shorter duration of action of rocuronium. The presence of an inflammatory bowel disease did not influence the neuromuscular block.

Influence of a continuous prednisolone medication on the time course of neuromuscular block of atracurium in patients with chronic inflammatory bowel disease

BACKGROUND

Corticosteroids interact with neuromuscular blocking agents. However, experimental data are contradictory: enhancement and attenuation of the neuromuscular block has been observed. This study tested the influence of long-term medication with prednisolone on atracurium-induced neuromuscular block.

METHODS

Sixty patients with chronic inflammatory bowel disease undergoing elective abdominal surgery were investigated. Thirty patients received a long-term medication with prednisolone (Group A) and 30 were without corticoid medication (Group B). Additionally, another 30 patients without inflammatory bowel disease and without corticoid medication served as control (Group C). The following parameters of an atracurium-induced neuromuscular block (0.25 mg kg(-1)) were measured: onset time, maximum block, recovery to 25% first twitch height, recovery index (time from 25% until 75% recovery of first twitch), duration to recovery to a train-of-four (TOF) rate of 0.7 and 0.9.

RESULTS

The groups did not differ with regard to onset time, maximum block, and recovery index. The duration to 25% twitch height was significantly lower in Group A [18.1 (0-30.7) min] compared with Group B [23.5 (0-36.7) min; P<0.05]. Duration to a TOF rate of 0.7 and 0.9, respectively, were significantly reduced in Group A [36.1 (7.9) and 40.9 (9.0 min)] compared with Group B [47.9 (7.6) and 53.4 (9.2) min; P<0.001].

CONCLUSIONS

Long-term medication with prednisolone resulted in a shorter duration of an atracurium-induced neuromuscular block in patients with Crohn's disease or ulcerative colitis. The presence of the inflammatory bowel disease did not influence the time course of the neuromuscular block.

Changes in the neuromuscular synapse induced by an antibody against gangliosides

In this study, we used a monoclonal IgM antibody from a patient with a pure motor chronic demyelinating polyneuropathy, which binds specifically to the complex gangliosides GM(2), GalNAc-GD(1a), and GalNAc-GM(1b), which appear to have a common epitope of -[GalNAcbeta1-4Gal(3-2alphaNeuAc)beta1]. This was done for the following reasons: (1) to localize these gangliosides in specific cellular components of the neuromuscular junction (NMJ), and (2) to describe the anti-ganglioside antibody-induced structural and functional changes in the NMJs to gain insight into the role of gangliosides in the synaptic function. Using immunofluorescence techniques, we found that these gangliosides are located only in the presynaptic component of the motor end-plates, both in nerve terminals and in Schwann cells. After 2 weeks of continued passive transfer of the IgM monoclonal antibody over the mouse levator auris longus muscle, electromyography showed an axonal or NMJ disorder. Morphology showed important nerve terminal growth and retraction changes. Using intracellular recording electrophysiology, we found neurotransmitter release alterations, including quantal content reduction and an immature expression of voltage-dependent calcium channels similar to what occurred during NMJ development and regeneration. These changes were complement independent. The results showed that these gangliosides were involved in the reciprocal Schwann cell-nerve terminal interactions, including structural stability and neurotransmission.

New evidence for a presynaptic action of prednisolone at neuromuscular junctions

The action of prednisolone at the neuromuscular junction was studied in mouse isolated phrenic nerve-diaphragm and rat external popliteal/sciatic nerve-tibialis anterior muscle preparations. Prednisolone (0.03 mM and 0.3 mM) did not alter the twitch-tension in phrenic nerve-diaphragm preparations after 120 min, but increased the frequency (170 +/- 4%) and amplitude (200 +/- 13%) of miniature end-plate potentials. Quantal content was not influenced by the glucocorticoid treatment. Prednisolone (400 microg/kg) did not change the twitch-tension in rat external popliteal/sciatic nerve-tibialis anterior muscle preparations. However, this steroid (0.3 mM) prevented the neuromuscular blockade by d-tubocurarine (1.45 microM) in mouse preparations by 70 +/- 10% (P < 0.05). A similar effect (82 +/- 6% protection, P < 0.05) occurred in rats treated with prednisolone (400 microg/kg) before d-tubocurarine (225 microg/kg). In phrenic nerve-diaphragm preparations, prednisolone (0.3 mM) increased (13 +/- 4%, p < 0.05) the twitch-tension in the presence of beta-bungarotoxin (1 microM), and prevented the blockade produced by this toxin (0.15 microM) in its third phase of action. This presynaptic facilitatory effect may contribute to the usefulness of prednisolone in myasthenia gravis.

Corticosteroid effects on diaphragm neuromuscular junctions

The effects of corticosteroid (CS) treatment (prednisolone continuously administered subcutaneously at a flow rate of 2.5 microl/h, daily dose 5.6 mg/kg, for 3 wk) on neuromuscular junction (NMJ) morphology and neuromuscular transmission in rat diaphragm muscle (Dimus) were compared with weight-matched (Sham) and ad libitum fed control (Ctl) groups. Fibers were classified on the basis of myosin heavy chain (MHC) isoform expression. CS treatment caused significant atrophy of fibers expressing MHC2X (type IIx), either alone or with MHC2B (type IIx/b). Fibers expressing MHCslow (type I) and MHC2A (type IIa) were unaffected by CS. The planar areas of nerve terminals and motor endplates at type IIx/b fibers were smaller in CS-treated Dimus compared with Sham and Ctl. However, CS-induced atrophy of type IIx/b fibers exceeded changes in NMJ morphology. Thus, when normalized for fiber diameter, NMJs were relatively larger in the CS-treated group compared with Ctl. Neuromuscular transmission failure, assessed in vitro by comparing force loss during repetitive (40 Hz) nerve vs. direct muscle stimulation, was less in CS-treated Dimus. These results indicate that alterations in NMJ morphology after CS treatment are dependent on fiber type and may contribute to improved neuromuscular transmission.

Chronic corticosterone treatment-induced ultrastructural changes at rat neuromuscular junction

BACKGROUND

Chronic exposure to glucocorticoids affects both the structure and function of vertebrate skeletal muscles. As little is known about the effects of such steroids on the neuromuscular junctions (NMJs) of different muscle fiber types, the influence of chronic corticosterone (CORT) administration on the ultrastructure of NMJs of soleus (SOL) and extensor digitorum longus (EDL) was studied.

METHODS

Ten Fischer 344 male rats, the same animals used previously, were either injected daily with 5-10 mg CORT or received vehicle as control animals for 3 months and were sacrificed at 5 months of age. Muscles were bathed in situ in 4% phosphate buffered glutaraldehyde for ten minutes, then removed and conventional electron microscopic procedures were followed. Qualitative and quantitative observations of nerve terminal ultrastructures were statistically treated with multivariate analysis of variance to determine differences between control and CORT-treated animals.

RESULTS

Fast-twitch EDL muscles were more affected by CORT-treatment than slow-twitch SOL muscles. Morphometric analysis of NMJ's in CORT-treated rats revealed significant decrease in fiber diameter, nerve terminal area and synaptic vesicle density, but a significant increase in synaptic cleft (P < 0.05). The NMJ's underwent partial denervation and reinnervation processes as demonstrated by large areas of presynaptic nerve terminal occupied by microtubules and electron dense granular material.

CONCLUSIONS

Chronic CORT-treatments induced degenerative changes which were more pronounced in fast-twitch EDL muscles than slow-twitch SOL muscles, suggesting that pattern or amount of activity affect the CORT-treatment outcome. These steroid-induced stress changes are similar to those observed in aging and disuse studies of NMJ. Thus, glucocorticoid hormones may play an etiological role in the homeostasis of the NMJ in response to various stimuli.

The regulation of quantal size

Quantal size can be altered experimentally by numerous treatments that seem to lack any common thread. The observations may seem haphazard and senseless unless clear distinctions are made from the outset. Some treatments shift the size of the entire population of quanta. These quanta are released by nerve stimulation. Other treatments add quanta of abnormal size or shape--monstrosities--to the population (4.0). Usually, perhaps even invariably, the monstrosities are not released by nerve stimulation. 6.1. POPULATION SIZE INCREASES. 6.1.1. Quantal size must be regulated. The size of the entire quantal population can be experimentally shifted to a larger size, with the mean rising two- or even four-fold. Before these observations, it was reasonable to suppose that quantal size was relatively fixed, with little room for maneuver. A logical picture is that synaptic vesicles have a maximum transmitter capacity, and usually they are filled to the brim. This picture is wrong. The quantity of transmitter packaged in the quantum must be regulated by the neuron, so depending on circumstances, quantal size can be increased or decreased. Figure 18 makes the case for regulation more strongly than words. We are beginning to identify some of the signals for up and down regulation, and the first steps have been made in discovering the signal transduction pathways, but we are far from a true understanding. This is hardly surprising, because our information about how transmitter molecules are assembled into quantal packages is still imperfect. Until we understand the engine, it may be difficult to picture the accelerator or the brake. 6.1.2. Signals that up regulate size. Stimulation of the presynaptic neuron increases quantal size at the NMJ, at synapses in autonomic ganglia and in hippocampus. The stimulus parameters necessary to elicit the quantal size increase have not been explored sufficiently in any of these cases, and all deserve further investigation. At both frog and mouse NMJs quantal size is roughly doubled following exposure to hypertonic solutions, which elevate the rate of spontaneous quantal release. This discovery, coupled with the increases caused by tetanic stimulation, suggested that the signal for up regulation is a period of greatly enhanced quantal output. The size increase takes about 15 min in hypertonic solution in mouse and about 60 min in frog. Highly hypertonic solutions do not increase the rate of quantal release in frog; they also do not increase quantal size. This supported the idea that quantal release rate is the signal for up regulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of chronic corticosterone treatment on the morphology of rat neuromuscular junctions

This study examined the effects of chronic corticosterone (CORT) treatment on the morphology of two physiologically different muscles. Nerve terminals from the slow twitch soleus (SOL) and fast twitch extensor digitorum longus (EDL) of Fischer 344 rats were stained using the zinc iodide osmium (ZIO) technique. Nerve terminal area, perimeter, and longitudinal extent length were measured using computer-aided morphometry. Slow and fast muscle fibers from animals which received 5-10 mg CORT per day for 3 months were atrophied compared with controls. Treated animals failed to gain weight during the study, while controls gained 37%. Adrenal weights in treated animals were 30% less than controls, after correction for body weight. Morphological parameters for SOL nerve terminals were generally larger in the CORT group, while EDL nerve terminals from the CORT group did not differ significantly from controls. Soleus nerve terminal area was 43% greater, perimeter 14% longer, and longitudinal extent length 18% longer than the control nerve terminals. This study demonstrates a greater effect of CORT treatment on slow twitch muscle than has been demonstrated in previous studies. Changes in the nerve terminal morphology of the SOL were also greater than in previous studies and suggest that a functional adaptation or remodelling may occur following CORT treatment to maintain the neuromuscular interface during the enhanced catabolic effects of the steroid. These steroid-induced stress changes are similar in some respects to those observed in aging and disuse studies of the vertebrate neuromuscular junction. This suggests that glucocorticoid hormones may play an etiological role in the homeostasis of the neuromuscular junction in response to various stimuli.