Patients with fibromyalgia commonly have symptoms of abdominal pain, suggesting that altered somatic afferent activity may influence visceral sensations. It is hypothesized that a noxious somatic stimulus increases input to the projection neurons in the dorsal horn, resulting in visceral hyperalgesia.
Two injections (100 microL, pH 4.0) were given unilaterally in the gastrocnemius muscle 2 days apart in male Sprague-Dawley rats. Paw withdrawal reflex (PWR) was measured to assess somatic pain. The control group received pH 7.2 saline injections. Similar injections (pH 4.0) were given in the front leg in a different group. Electromyography (EMG) from the external oblique muscle was recorded to graded colorectal distention at different time intervals. NMDA receptor antagonist (CGS-19755, 20 nmol) or AMPA/kainate receptor antagonist (NBQX, 20 nmol) was injected intrathecally before low-pH injections.
A bilateral decrease in PWR threshold occurred 72 hours after the second low-pH injection. There was no decrease in the threshold in rats injected with pH 7.2 saline. A significant increase in EMG to colorectal distention (> or =30 mm Hg) occurred at 72 hours and 2 weeks in the pH 4.0 group. No change in EMG was observed following 2 unilateral low-pH injections in the front leg. Both the visceral hyperalgesia and the decrease in somatic pain thresholds were prevented by prior intrathecal CGS-19755 or NBQX injections.
Noxious somatic afferent input from the hind limb facilitates visceral hyperalgesia, which is due to viscerosomatic convergence in the lower spinal cord. This can be blocked by ionotropic glutamate receptor antagonists.