Administration of a,ß-meATP (ATP) into neck muscles facilitates brainstem nociception in anesthetized mice. The unspecific nitric oxide synthase (NOS) blocker L-NMMA inhibits and reverses this facilitation. There are three NOS isoenzymes with different expression profiles and physiological functions. The neuronal NOS (nNOS) is constitutively expressed in nervous tissue and mediates cell communication. Due to the nNOS profile, the present study hypothesized involvement of nNOS isoenzyme in ATP-mediated neck muscle facilitation.

Bilateral infusion of ATP (1 mM, 20 ml, 20 ml/min) into both semispinal neck muscles (i.m.) was performed in anesthetized mice (n=25). Impact of neck muscle nociception on sensory brainstem processing was electrophysiologically monitored via the jaw-opening reflex (JOR) elicited by electrical tongue stimulation (0.1 Hz, 500 ms). The JOR integral was analyzed as the main reflex parameter and monitored for at least 90 min after i.m. ATP injection. Different dosages of nNOS inhibitor NPLA were intraperitoneally (i.p.) injected 30 min before i.m. ATP infusion (0.5, 1, 2 mg/kg, n=5 each). In control experiments possible effects of i.p. application of isotonic saline (0.9% NaCl) 30 min before local ATP injection were addressed (n=5 each).

After sole i.m. ATP infusion, JOR integral increased by 17922% (meansem, p<0.001). Systemic administration of NPLA had no modulatory effect on baseline JOR. Preceding NPLA with 1 or 2 mg/kg (4542%, -216%) prevented the facilitatory ATP effect for at least 90 min (p<0.05). Preceding injection of 0.5 mg/kg NPLA did not prevent JOR facilitation (12956%, n.s.).

ATP induces long-term facilitation of neck muscle nociception. Preceding NPLA application blocks this facilitation in a dose-dependent manner. As NPLA does not alter baseline brainstem nociception, nNOS participates in the induction of nociceptive facilitation in the brainstem triggered by ATP. In future experiments, subsequent administration of NPLA will clarify a putative involvement of nNOS in the maintenance of neck muscle nociceptive facilitation. A potential role of inducible or endothelial NOS isoenzymes remains to be investigated. These results point to a major role of nNOS isoenzyme in neck muscle nociception.