Muscular heat and mechanical pain sensitivity after lengthening contractions in humans and animals.
Mechanical sensitivity of muscle nociceptors was previously shown to increase 2 days after lengthening contractions (LC), but heat sensitivity was not different despite nerve growth factor (NGF) being upregulated in the muscle during delayed-onset muscle soreness (DOMS). The discrepancy of these results and lack of other reports drove us to assess heat sensitivity during DOMS in humans and to evaluate the effect of NGF on the heat response of muscle C-fibers. Pressure pain thresholds and pain intensity scores to intramuscular injection of isotonic saline at 48°C and capsaicin were recorded in humans after inducing DOMS. The response of single unmyelinated afferents to mechanical and heat stimulations applied to their receptive field was recorded from muscle-nerve preparations in vitro. In humans, pressure pain thresholds were reduced but heat and capsaicin pain responses were not increased during DOMS. In rats, the mechanical but not the heat sensitivity of muscle C-fibers was increased in the LC group. NGF applied to the receptive field facilitated the heat sensitivity relative to the control. The absence of facilitated heat sensitivity after LC, despite the NGF sensitization, may be explained if the NGF concentration produced after LC is not sufficient to sensitize nociceptor response to heat.


CONCLUSIONS
This article presents new findings on the basic mechanisms underlying hyperalgesia during DOMS, which is a useful model to study myofascial pain syndrome, and the role of NGF on muscular nociception. This might be useful in the search for new pharmacologic targets and therapeutic approaches.
Department of Physical Therapy, College of Life and Health Sciences, Chubu University, Kasugai, Japan; Department of Neural Regulation, Graduate School of Medicine, Nagoya University, Nagoya, Japan.
Fernando
F
Queme
Toru
T
Taguchi
Kazue
K
Mizumura
Thomas
T
Graven-Nielsen
D000328:Adult; D000818:Animals; D005260:Female; D006358:Hot Temperature; D006801:Humans; D006930:Hyperalgesia / Q000503:physiopathology*; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000503:physiopathology; D063806:Myalgia / Q000503:physiopathology*; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology*; D020932:Nerve Growth Factor / Q000494:pharmacology; D010147:Pain Measurement; D058748:Pain Perception / Q000187:drug effects / Q000502:physiology*; D017288:Pain Threshold / Q000187:drug effects / Q000502:physiology; D010812:Physical Stimulation; D051381:Rats; D017207:Rats, Sprague-Dawley
D020932:Nerve Growth Factor
Muscle heat sensitivity; capsaicin; delayed-onset muscle soreness; lengthening contraction; nerve growth factor