Nerve growth factor injection into semispinal neck muscle evokes sustained facilitation of the jaw-opening reflex in...

Nociceptive input from neck muscles probably plays a role in the pathophysiology of tension-type headache. In order to elaborate an animal model, the impact of noxious input from neck muscles on orofacial sensorimotor processing was investigated by electrophysiological means in anesthetized mice. Group IV muscle afferents of the semispinal neck muscle were excited by local injection of nerve growth factor (NGF, 0.8 microM, 20 microl). Orofacial sensorimotor processing was monitored by the jaw-opening reflex (JOR) elicited by electric tongue stimulation. After unilateral NGF injection into the right neck muscle (n = 10), JOR integral (+89%) and duration (+9%) increased and latency decreased (-5%) for at least 1 h. Bilateral injection of NGF (n = 10) into neck muscles induced an increase of JOR integral (+111%) and duration (+20%) and a reduction of latency (-9%). This facilitation of the JOR lasted for at least 90 min without any downward drift (n = 5). Electric JOR threshold diminished after NGF injection. After intramuscular injection of isotonic saline into the right semispinal neck muscle (20 microl), the JOR remained unchanged (n = 10). Local NGF injection into neck muscles evoked noxious input to the brainstem that induced a sustained central facilitation of the JOR for more than 1 h. This long-term facilitation of orofacial sensorimotor processing by a singular NGF injection possibly reflects plastic changes of nociceptive synaptic processing that may be involved in the pathophysiology of headache.