Pharmacological relevance of peripheral type benzodiazepine receptors on motor nerve and skeletal muscle

Enhancement of mouse diaphragm contractility in the presence of antagonists of GABAA and GABAB receptors

NEW FINDINGS

What is the central question of this study? Do GABA receptors play any role at the neuromuscular junction? What is the main finding and its importance? In the presence of either ionotropic or metabotropic GABA receptor antagonists, diaphragm muscle force production elicited by stimulating the motor nerve at ≥50 Hz was increased. Our data indicate the presence of GABAergic signalling at the neuromuscular junction.

ABSTRACT

Despite the signalling role of GABA in the brain and spinal cord, the role of this molecule in the peripheral nervous system and, in particular, at the neuromuscular junction remains practically unexplored. In the present work, the force of mouse diaphragm contractions was measured in the presence of blockers of metabotropic GABA_B receptors (CGP 55845) and ionotropic GABA_A receptors (picrotoxin) with various patterns of indirect and direct stimulation of muscle by trains of 40 pulses delivered at 10, 20, 50 and 70 Hz. It was found that neither blocker affected the diaphragm contractility caused by indirect stimulation through the motor nerve at 10 and 20 Hz. However, when the stimulation frequency was increased to 50 or 70 Hz, the force of subsequent contractions in the train (when compared with the amplitude of contraction in response to the first pulse) was increased by both CGP 55845 and picrotoxin. With direct stimulation of the diaphragm, no significant changes in the contraction force were detected at any frequency used. The results obtained support the following conclusions: (i) pharmacological inhibition of GABA receptors increases the contractile activity of skeletal muscle; and (ii) frequency-dependent enhancement of GABA receptor activation takes place in the region of the neuromuscular junction.

Muscle power during intravenous sedation

Intravenous sedation is effective to reduce fear and anxiety in dental treatment. It also has been used for behavior modification technique in dental patients with special needs. Midazolam and propofol are commonly used for intravenous sedation. Although there have been many researches on the effects of midazolam and propofol on vital function and the recovery profile, little is known about muscle power. This review discusses the effects of intravenous sedation using midazolam and propofol on both grip strength and bite force. During light propofol sedation, grip strength increases slightly and bite force increases in a dose-dependent manner. Grip strength decreases while bite force increases during light midazolam sedation, and also during light sedation using a combination of midazolam and propofol. Flumazenil did not antagonise the increase in bite force by midazolam. These results may suggest following possibilities; (1) Activation of peripheral benzodiazepine receptors located within the temporomandibular joint region and masticatory muscles may be the cause of increasing bite force. (2) Propofol limited the long-latency exteroceptive suppression (ES2) period during jaw-opening reflex. Thus, control of masticatory muscle contraction, which is thought to have a negative feedback effect on excessive bite force, may be depressed by propofol.

The combined effects of midazolam and propofol sedation on muscle power

We performed a randomised, crossover study to investigate the effects of intravenous sedation on grip strength and bite force. Twenty male volunteers received a bolus intravenous injection of midazolam (0.02 mg.kg(-1)) together with a 30-min propofol infusion designed to achieve an effect-site concentration of 1.0 μg.ml(-1). Observed variables included bispectral index, observer's assessment of alertness/sedation, correct answer rate of Stroop colour-word test, grip strength and bite force. Grip strength decreased from a median (IQR [range]) of 483 (443-517 [380-586]) N to 358 (280-405 [108-580]) N (p < 0.001) during sedation and recovered following flumazenil administration, while bite force increased from 818 (593-1026 [405-1406]) N to 1377 (1243-1585 [836-2357]) N (p < 0.001) during sedation. Although bite force gradually returned to baseline following flumazenil administration, it remained increased throughout the experimental period. We conclude that bite force increased during intravenous sedation and that this may have clinical implications.

Translocator protein (Tspo) gene promoter-driven green fluorescent protein synthesis in transgenic mice: an in vivo model to study Tspo transcription

Translocator protein (TSPO), previously known as the peripheral-type benzodiazepine receptor, is a ubiquitous drug- and cholesterol-binding protein primarily found in the outer mitochondrial membrane as part of a mitochondrial cholesterol transport complex. TSPO is present at higher levels in steroid-synthesizing and rapidly proliferating tissues and its biological role has been mainly linked to mitochondrial function, steroidogenesis and cell proliferation/apoptosis. Aberrant TSPO levels have been linked to multiple diseases, including cancer, endocrine disorders, brain injury, neurodegeneration, ischemia-reperfusion injury and inflammatory diseases. Investigation of the functions of this protein in vitro and in vivo have been mainly carried out using high-affinity drug ligands, such as isoquinoline carboxamides and benzodiazepines and more recently, gene silencing methods. To establish a model to study the regulation of Tspo transcription in vivo, we generated a transgenic mouse model expressing green fluorescent protein (GFP) from Aequorea coerulescens under control of the Tspo promoter region (Tspo-AcGFP). The expression profiles of Tspo-AcGFP, endogenous TSPO and Tspo mRNA were found to be well-correlated. Tspo-AcGFP synthesis in the transgenic mice was seen in almost every tissue examined and as with TSPO in wild-type mice, Tspo-AcGFP was highly expressed in steroidogenic cells of the endocrine and reproductive systems, epithelial cells of the digestive system, skeletal muscle and other organs. In summary, this transgenic Tspo-AcGFP mouse model recapitulates endogenous Tspo expression patterns and could be a useful, tractable tool for monitoring the transcriptional regulation and function of Tspo in live animal experiments.

Population Pharmacokinetic Analysis for Simultaneous Determination of B max and K D In Vivo by Positron Emission Tomography

PURPOSE

Changes in GABA(A)-receptor density and affinity play an important role in many forms of epilepsy. A novel approach, using positron emission tomography (PET) and [C-11]flumazenil ([C-11]FMZ), was developed for simultaneous estimation of GABA(A)-receptor properties, characterized by B (max) and K (D).

PROCEDURES

Following an injection of [C-11]FMZ (dose range: 1-2,000 mug) to 21 rats, concentration time curves of FMZ in brain (using PET) and blood (using HPLC-UV) were analyzed simultaneously using a population pharmacokinetic (PK) model, containing expressions to describe the time course of the plasma concentration (including distribution to the body), the brain distribution, and the specific binding within the brain.

RESULTS

Application of this method in control rats resulted in estimates of B (max) and K (D) (14.5 +/- 3.7 ng/ml and 4.68 +/- 1.5 ng/ml, respectively).

CONCLUSIONS

The proposed population PK model allowed for simultaneous estimation of B (max) and K (D) for a group of animals using single injection PET experiments per animal.

The use of benzodiazepines in the treatment of chest pain: a review of the literature

Benzodiazepines, although not listed in the American Heart Association's guidelines for the treatment of chest pain, are often used to provide symptomatic relief to patients who experience chest pain. To investigate the utility of benzodiazepines in the treatment of chest pain, the pharmacologic actions and cardiovascular effects of benzodiazepines were reviewed. In addition, a literature search regarding the use of benzodiazepines to treat patients with chest pain was conducted. The results indicated that benzodiazepines reduce anxiety, pain, and cardiovascular activation. Benzodiazepines amplify gamma-aminobutyric acid (GABA) throughout the central nervous system, and act more peripherally to reduce catecholamines. In addition, preliminary evidence indicates that benzodiazepines may cause coronary vasodilatation, prevent dysrhythmias, and block platelet aggregation, though further study is needed. Both non-cardiac chest pain (associated with musculoskeletal, esophageal, neurologic, and psychiatric conditions) and cardiac chest pain (associated with acute and chronic myocardial ischemia) seem to be effectively treated with benzodiazepines. Benzodiazepines are safe and well tolerated when administered alone or in combination with other medications. Moreover, the risk of dependence is minimal when benzodiazepines are prescribed on a short-term basis. Further study of benzodiazepines in the treatment of acute chest pain is needed to confirm these favorable actions and better define their use in the acute medical setting.

Enhancement by benzodiazepines of the inhibitory effect of adenosine on skeletal neuromuscular transmission

1. Interactions of benzodiazepines with adenosine on the neuromuscular transmission were studied in mouse diaphragm preparations. 2. In tubocurarine (0.6-0.8 microM)-partially paralyzed preparations, diazepam (35 microM) and Ro 5-4864 (3-30 microM), a peripheral type benzodiazepine receptor agonist, potentiated the inhibitory effect of adenosine on indirect twitch responses. 3. The central type receptor agonist, clonazepam did not affect the inhibitory effect of adenosine. 4. The peripheral benzodiazepine receptor antagonist, PK11195 (1-10 microM) attenuated the adenosine inhibition and antagonized the potentiation by Ro 5-4864. 5. Ro 5-4864 failed to enhance further the inhibitory effect of adenosine in the presence of dipyridamole, an adenosine uptake inhibitor that also potentiated adenosine inhibition. 6. Neither Ro 5-4864 nor PK 11195 affected the inhibition produced by a stable adenosine analogue, 2-chloroadenosine, which is not a substrate for the adenosine uptake system. 7. Ro 5-4864 did not affect endplate potentials (e.p.ps) in the absence of adenosine, but reduced the amplitude of e.p.ps in the presence of adenosine without affecting miniature e.p.ps. 8. It is suggested that benzodiazepines potentiate the adenosine-effected presynaptic inhibition of neuromuscular transmission by an inhibition of adenosine uptake through activation of peripheral type benzodiazepine receptors.