Stimulation of deep somatic tissue with capsaicin produces long-lasting mechanical allodynia and heat hypoalgesia that depends on early activation of the cAMP pathway.

Pain and hyperalgesia from deep somatic tissue (i.e., muscle and joint) are processed differently from that from skin. This study examined differences between deep and cutaneous tissue allodynia and the role of cAMP in associated behavioral changes. Capsaicin was injected into the plantar aspect of the skin, plantar muscles of the paw, or ankle joint, and responses to mechanical and heat stimuli were assessed until allodynia resolved. Capsaicin injected into skin resulted in a secondary mechanical allodynia and heat hypoalgesia lasting approximately 3 hr. In contrast, capsaicin injection into muscle or joint resulted in a long-lasting bilateral (1-4 weeks) mechanical allodynia with a simultaneous unilateral heat hypoalgesia. The pattern and degree of inflammation were similar when capsaicin was injected into skin, muscle, or joint, with peak increases 24 hr after injection. Heat hypoalgesia that occurs after injection into deep tissue was reversed by spinal blockade of adenylate cyclase or protein kinase A (PKA). Interestingly, mechanical allodynia was reversed if adenylate cyclase or PKA inhibitors were administered spinally 24 hr, but not 1 week, after injection of capsaicin. Spinally administered 8-bromo-cAMP resulted in a similar pattern, with heat hypoalgesia and mechanical allodynia occurring simultaneously. Thus, injection of capsaicin into deep tissues results in a longer-lasting mechanical allodynia and heat hypoalgesia compared with injection of capsaicin into skin. The mechanical allodynia depends on early activation of the cAMP pathway during the first 24 hr but is independent of the cAMP pathway by 1 week after injection of capsaicin.