In neurons, activity dependent cAMP synthesis is primarily mediated by membrane bound Ca2+/calmodulin-stimulated adenylyl cyclases (ACs). So far, ten members of the adenylyl cyclase family have been identified . Of these, AC 1–9 isoforms are present in the brain (reviews, [2, 3]). Two of the adenylyl cyclases, AC1 and AC8, are activated by calcium through the calcium-binding protein calmodulin . These enzymes link activity-dependent increases in intracellular calcium to the production of intracellular cAMP. In addition, these neuronal adenylyl cyclases are viewed as coincidence detectors due to their specific interaction with NMDA receptors and voltage-dependent calcium channels at the neuronal membrane.
The role of the cAMP pathway in neurons along the pain pathway, including the anterior cingulate cortex (ACC) and the spinal dorsal horn were demonstrated [4, 5] and discussed [6, 7]. Peripheral injuries activate a series of signaling molecules downstream of adenylyl cyclases in neuronal populations. These molecules include pMAPK [8, 9], transcription factor pCREB [10–12], and the immediate early genes Egr-1 [13, 14] and Arc . Neuronal adenylyl cyclases were shown to contribute to NMDA receptor-dependent synaptic potentiation in the hippocampus [16, 17]. Recently, we showed that Ca2+/calmodulin-stimulated adenylyl cyclases are required to trigger synaptic potentiation in the ACC neurons  and synaptic facilitation in the spinal cord dorsal horn .
Chronic myofascial pain represents a considerable health problem . Muscle pain is a symptom of various disorders including fibromyalgia, metabolic myopathies, myositis and also a side effect of medications, especially the statin group of cholesterol lowering drugs [21, 22]. Injury to the muscle results in diffuse aching pain that is difficult to localize , spreading to regions outside the area of innervation . Inflammation induced by intramuscular injection of capsaicin or carrageenan results in long-lasting (over weeks) bilateral secondary mechanical hyperalgesia. On the other hand, injection of these agents into the skin causes only a short-lasting, unilateral secondary mechanical hyperalgesia [25, 26]. Although the role of adenylyl cyclases was shown to be important in behavioral sensitization associated with chronic subcutaneous inflammation and neuropathic pain [5, 7, 27], their role in inflammatory muscle pain has not been studied.
In the present work, we adapted a chronic inflammatory muscle pain model  and modified it to induce acute muscle pain by intramuscular injection of formalin. To overcome the non-selective action of adenylyl cyclase inhibitors (e.g. SQ 22536) used in previous studies [25, 27], the present investigation used AC1 knockout (KO), AC8 KO and AC1&8 double knockout (DKO) mice to study the behavioral nociceptive changes after inducing acute persistent pain (formalin injection) and chronic pain (carrageenan injection) in comparison to the wild-type mice.