Assessment of the Potential Role of Muscle Spindle Mechanoreceptor Afferents in Chronic Muscle Pain in the Rat Masseter Muscle

Abstract

BackgroundThe phenotype of large diameter sensory afferent neurons changes in several models of neuropathic pain. We asked if similar changes also occur in “functional” pain syndromes.Methodology/Principal FindingsAcidic saline (AS, pH 4.0) injections into the masseter muscle were used to induce persistent myalgia. Controls received saline at pH 7.2. Nocifensive responses of Experimental rats to applications of Von Frey Filaments to the masseters were above control levels 1–38 days post-injection. This effect was bilateral. Expression of c-Fos in the Trigeminal Mesencephalic Nucleus (NVmes), which contains the somata of masseter muscle spindle afferents (MSA), was above baseline levels 1 and 4 days after AS. The resting membrane potentials of neurons exposed to AS (n = 167) were hyperpolarized when compared to their control counterparts (n = 141), as were their thresholds for firing, high frequency membrane oscillations (HFMO), bursting, inward and outward rectification. The amplitude of HFMO was increased and spontaneous ectopic firing occurred in 10% of acid-exposed neurons, but never in Controls. These changes appeared within the same time frame as the observed nocifensive behaviour. Ectopic action potentials can travel centrally, but also antidromically to the peripheral terminals of MSA where they could cause neurotransmitter release and activation of adjacent fibre terminals. Using immunohistochemistry, we confirmed that annulospiral endings of masseter MSA express the glutamate vesicular transporter VGLUT1, indicating that they can release glutamate. Many capsules also contained fine fibers that were labelled by markers associated with nociceptors (calcitonin gene-related peptide, Substance P, P2X3 receptors and TRPV1 receptors) and that expressed the metabotropic glutamate receptor, mGluR5. Antagonists of glutamatergic receptors given together with the 2nd injection of AS prevented the hypersensitivity observed bilaterally but were ineffective if given contralaterally.Conclusions/SignificanceLow pH leads to changes in several electrical properties of MSA, including initiation of ectopic action potentials which could propagate centrally but could also invade the peripheral endings causing glutamate release and activation of nearby nociceptors within the spindle capsule. This peripheral drive could contribute both to the transition to, and maintenance of, persistent muscle pain as seen in some “functional” pain syndromes.

Vesicular Glutamate Transporter Immunoreactivity in the Periodontal Ligament of the Rat Incisor

“…Wu et al (2004) reported the presence of VGluT1 in the muscle spindles of rat triceps surae muscle. In the trigeminal system, the muscle spindles of the rat masseteric nerve displayed VGluT1 immunoreactivity that originated from the trigeminal mesencephalic nucleus neurons (Pang et al, 2006, Lund et al, 2010. Thus, it is safe to say that VGluT1 present in the nerve endings originates from medium-large neurons, because muscle spindles are innervated by medium-large sensory neurons.…”

“…In addition, the presence of VGluT has been documented in the peripheral endings of neurons in the dorsal root and trigeminal ganglion. In the trigeminal nervous system, muscle spindles of jaw closer display VGluT1 immunoreactivity (Pang et al, 2006;Lund et al, 2010). This originates from neurons in the trigeminal mesencephalic nucleus showing immunoreactivity for VGluT1 (Pang et al, 2006).…”

Muscle Pain and Muscle Spindles

“…Removal of substances which accumulate into the gel-filled periaxial space of the muscle spindle is a slow process. The sensory innervation of a muscle spindle consists of primary and secondary endings [13], and also IIIand IV-afferents [16][17][18][19]. Also, autonomic innervation has been observed [19,20].…”

“…Trigger points comprise inflamed and painful muscle spindles with overactive nociceptive afferents. There are somatic thin nerve axons inside the muscle spindle and in its capsule [19]. Thus, it is also conceivable that pain spots in routine EMG of healthy muscles [1] are in fact muscle spindles.…”

“…The spindle capsule at about the equatorial region is made up of fibrous tissue lamellae which usually number 5-7, and are rather rich in endothelial-like nuclei. Among the lamellae lie several small blood vessels [53] as well as thin somatic nerve axons [18,19]. The thick capsule on the equatorial area of the muscle spindle [14] may be felt as an increased resistance of the EMG needle resembling fascial planes [1,27].…”