Influence of intramuscular nerve growth factor injection on the response properties of rat masseter muscle afferent fibers.
OBJECTIVE
To investigate whether local administration of nerve growth factor (NGF) decreases the mechanical threshold (MT) of putative nociceptive masseter afferent fibers as part of its mechanism of mechanical sensitization.


METHODS
Electrophysiologic recordings were made from masseter afferents and a randomized, blinded approach was used to test the effects of intramuscular injection of NGF (2.5 or 25 microg/mL) into the rat masseter muscle on the MT of masseter afferents (n=65) and plasma protein extravasation.


RESULTS
The plasma protein extravasation data and electrophysiological recordings indicated that rat NGF injection was not inflammatory and did not evoke afferent discharge or induce mechanical sensitization (as reflected in a decreased MT) in masseter afferents in either male or female rats. To investigate whether the lack of effect of NGF injection might be due to differences between human and rat NGF, additional experiments with human NGF injection (25 microg/mL) were undertaken. Intramuscular injection of human NGF into the rat masseter muscle also failed to evoke afferent discharges; however, it did decrease the MT of masseter afferent fibers.


CONCLUSIONS
The finding that neither rat nor human NGF excited putative nociceptive masseter afferent fibers is consistent with a previous report that intramuscular NGF injections are not acutely painful in human subjects. The ability of human NGF injection into the rat masseter muscle to induce afferent mechanical sensitization suggests that this experimental approach may be useful for the study of peripheral mechanisms of myofascial pain and tenderness associated with temporomandibular disorders.
Mann
Mandeep K
MK
Faculty of Pharmaceutical Sciences, The University of British Columbia, Vancouver, British Columbia, Canada.
Dong
Xu-Dong
XD
Svensson
Peter
P
Cairns
Brian E
BE
D000344:Afferent Pathways / Q000187:drug effects; D000818:Animals; D001798:Blood Proteins / Q000032:analysis; D004558:Electric Stimulation; D004594:Electrophysiology; D005071:Evoked Potentials / Q000187:drug effects; D005260:Female; D006801:Humans; D007273:Injections, Intramuscular; D008297:Male; D008406:Masseter Muscle / Q000187:drug effects / Q000294:innervation*; D008465:Mechanoreceptors / Q000187:drug effects; D009412:Nerve Fibers / Q000187:drug effects*; D020932:Nerve Growth Factor / Q000008:administration & dosage / Q000494:pharmacology*; D009431:Neural Conduction / Q000187:drug effects; D009619:Nociceptors / Q000187:drug effects; D010812:Physical Stimulation; D011897:Random Allocation; D051381:Rats; D017207:Rats, Sprague-Dawley; D012684:Sensory Thresholds / Q000187:drug effects; D012737:Sex Factors; D016037:Single-Blind Method; D012668:Trigeminal Ganglion / Q000187:drug effects
D001798:Blood Proteins; D020932:Nerve Growth Factor