Activation of the cAMP pathway is an important mediator of chronic muscle pain. This study examined phosphorylation of the transcription factor cAMP-response-element-binding protein (p-CREB) and the NR1 subunit of the NMDA receptor (p-NR1) in the spinal cord. Bilateral mechanical hyperalgesia of the paw was induced by administering two injections of acidic saline, 5 days apart, into the gastrocnemius muscle of male Sprague–Dawley rats. The proportion of spinothalamic neurons that expressed p-NR1 or p-CREB did not change in the dorsal horn 24 h after the second intramuscular acid injection compared with animals that received pH 7.2 injections. This lack of change in spinothalamic neurons in the dorsal horn may be due to increases in individual spinothalamic neurons or increases in non-spinothalamic neurons. There was an increase in the proportion of spinothalamic neurons expressing p-NR1 in lamina X. These findings suggest that there are region-specific changes in spinothalamic neurons that express p-NR1 and lamina X may play an important role in the modulation of chronic muscle pain
