Effects of alaproclate, potassium channel blockers, and lidocaine on the release of 3H-acetylcholine from the guinea-pig ileum myenteric plexus

Subcutaneous accessory pain system (SAPS): A novel pain pathway for myofascial trigger points

Despite the accumulating neuro-physiological evidence of myofascial pain, many clinicians are skeptical about its existence as a separate disease entity. No single theory can fully explain the four cardinal features of MPS; taut bands, local tenderness, local twitching and the characteristic pattern of referred pain. Bridging the gap between basic and clinical knowledge mandates coupling the local trigger point changes with the clinically seen distant somatically innervated referred pain. The main question addressed by the present theory is why do trigger points behave differently in comparison to the surrounding muscle tissue and are trigger points the primary problem or secondary to a primary pathology. We propose that trigger points have an extra-innervation system that connect them with other spinal structures such as the facet, the annulus and other trigger points with a role for the subcutaneous fascia as part of trigger points pathogenesis or passage for the extra-innervation. The extra-innervation system is Subcutaneous accessory pain system (SAPS). The novel SAPS system connecting trigger points to the spinal segments via dorsal rami is presented. Individuals with this accessory pathway are prone to myofascial pain, trigger point activation and segmental referred somatic pain similar to other axial spinal structures. Despite the high prevalence of myofascial pain, the mechanism is not universally agreed upon. Why do the trigger points act differently from surrounding muscle tissue and are almost constant in location in different individuals is controversial. Why does myofascial pain and its two components, trigger points and referred pain, exist or are more prevalent in some individuals than in others is unexplained. The correlation between axial spinal structures pathology and the trigger points is not explored well. The existing theories about trigger point formation and referred pain is scientifically credible for each separate component and the SAPS novel system can provide the link between the two.

Mechanisms of Myofascial Pain

Myofascial pain syndrome is an important health problem. It affects a majority of the general population, impairs mobility, causes pain, and reduces the overall sense of well-being. Underlying this syndrome is the existence of painful taut bands of muscle that contain discrete, hypersensitive foci called myofascial trigger points. In spite of the significant impact on public health, a clear mechanistic understanding of the disorder does not exist. This is likely due to the complex nature of the disorder which involves the integration of cellular signaling, excitation-contraction coupling, neuromuscular inputs, local circulation, and energy metabolism. The difficulties are further exacerbated by the lack of an animal model for myofascial pain to test mechanistic hypothesis. In this review, current theories for myofascial pain are presented and their relative strengths and weaknesses are discussed. Based on new findings linking mechanoactivation of reactive oxygen species signaling to destabilized calcium signaling, we put forth a novel mechanistic hypothesis for the initiation and maintenance of myofascial trigger points. It is hoped that this lays a new foundation for understanding myofascial pain syndrome and how current therapies work, and gives key insights that will lead to the improvement of therapies for its treatment.

Alaproclate effects on voltage-dependent K+ channels and NMDA receptors: studies in cultured rat hippocampal neurons and fibroblast cells transformed with Kv1.2 K+ channel cDNA

The effects of alaproclate on voltage-dependent K+ currents and N-methyl-D-aspartate (NMDA) and gamma-aminobutyric acidA (GABAA) receptor currents were investigated in cultured rat hippocampal neurons using whole-cell voltage clamp recording techniques. Alaproclate produced a concentration-dependent block of the sustained voltage-dependent K+ current activated by depolarization from -60 to +40 mV (IC50, 6.9 microM). At similar concentrations alaproclate also blocked the sustained voltage-dependent K+ current in fibroblast cells transformed to stably express Kv1.2 K+ channels. Analysis of tail currents and the voltage-dependence of the alaproclate block suggested an open-channel blocking mechanism. Alaproclate also produced a potent block of NMDA receptor currents in hippocampal neurons (IC50, 1.1 microM), but did not affect GABAA receptor currents (concentrations up to 100 microM). The alaproclate block of NMDA receptors occurred predominantly by an open-channel mechanism, although the drug was also able to block closed NMDA channels at a much slower rate. The interaction of alaproclate with NMDA receptors (activated by 10 microM NMDA) appeared to be governed by a first order binding reaction with forward and reverse rate constants of 6.7 x 10(3) M-1 s-1, and 0.025 sec-1, respectively (at -60 mV). At depolarized potentials the alaproclate-induced block of the NMDA receptor current was strongly reduced, a result opposite to that seen with the voltage-activated K+ currents, suggesting that the K+ channel block may occur at a superficial internal site, whereas the NMDA receptor block occurs at a deep external site. (+)-Alaproclate was a more potent blocker of K+ currents than (-)-alaproclate, whereas a reversed stereoselectivity was observed for NMDA receptor current, supporting the view that alaproclate block of the two channel types occurs at structurally distinct binding sites.

Subtype classification of the presynaptic alpha-adrenoceptors which regulate [3H]-noradrenaline secretion in guinea-pig isolated urethra

1. The following experiments were carried out to investigate the presence and type of functional presynaptic receptors in adrenergic nerves of the guinea-pig urethra. 2. The urethra from male guinea-pigs was incubated with [3H]-noradrenaline and superfused with Tyrode solution in vitro. The fractional secretion of [3H]-noradrenaline evoked by 300 electrical pulses was measured. 3. The [3H]-noradrenaline secretion was positively frequency-dependent, yielding a half-maximal secretion at 8 +/- 5 Hz. Stimulation was usually applied at 5 Hz. 4. The [3H]-noradrenaline secretion was not altered by noradrenaline (1 or 100 microM), norephedrine (1 microM), isoprenaline (0.1 microM), 5-hydroxytryptamine (10 microM), oxotremorine (10 microM), adenosine (0.2 mM), propranolol (1 microM), atropine (1 microM) or 8-phenyltheophylline (10 microM). 5. The [3H]-noradrenaline secretion was enhanced by clonidine (3 microM), chlorpromazine (10 microM), metitepine (1 microM), 4-aminopyridine (0.5 mM), tetraethylammonium (2 mM), 3-isobutyl-1-methylxanthine (4 mM), 8-bromo cyclic AMP (1 mM) and forskolin (25 microM). 6. The alpha-adrenoceptor antagonists rauwolscine, yohimbine, phentolamine, prazosin and AR-C 239 maximally enhanced the [3H]-noradrenaline secretion to about 300% of control. The partial alpha-adrenoceptor agonist oxymetazoline maximally enhanced the secretion to about 200% of control. The order of apparent EC50 values was rauwolscine less than yohimbine less than phentolamine less than oxymetazoline less than prazosin less than AR-C 239.7. The enhancing effects of yohimbine (1 microM) with tetraethylammonium (2mM), 8-bromo cyclic AMP (1 mM), or forskolin (25,microM) were additive, but not those of yohimbine (1 microM) with prazosin (10 microM), 4-aminopyridine (0.5 mM), or 3-isobutyl-1-methylxanthine (4 mM).8. These results suggest that the [3H]-noradrenaline secretion in the guinea-pig urethra is regulated by presynaptic alpha2A-adrenoceptors which may, in a cyclic AMP-independent manner, be coupled to a 4-aminopyridine-sensitive potassium channel. The secretion is not influenced by compounds acting at beta-adrenoceptors, muscarinic cholinoceptors or adenosine receptors.