We investigated the involvement of extracellular signal-regulated protein kinases (ERK) within spinal neurons in producing pain hypersensitivity. Within a minute of an intense noxious peripheral or C-fiber electrical stimulus, many phosphoERK-positive neurons were observed, most predominantly in lamina I and IIo of the ipsilateral dorsal horn. This staining was intensity and NMDA receptor dependent. Low-intensity stimuli or A-fiber input had no effect. Inhibition of ERK phosphorylation by a MEK inhibitor reduced the second phase of formalin-induced pain behavior, a measure of spinal neuron sensitization. ERK signaling within the spinal cord is therefore involved in generating pain hypersensitivity. Because of its rapid activation, this effect probably involves regulation of neuronal excitability without changes in transcription.

A cardinal feature of inflammation is heightened pain sensitivity at the site of the inflamed tissue. This results from the local release by immune and injured cells of nociceptor sensitizers, including prostaglandin E 2 , bradykinin, and nerve growth factor, that reduce the threshold and increase the excitability of the peripheral terminals of nociceptors so that they now respond to innocuous stimuli: the phenomenon of peripheral sensitization. We show here that the proinflammatory cytokine interleukin-1␤ (IL-1␤), in addition to producing inflammation and inducing synthesis of several nociceptor sensitizers, also rapidly and directly activates nociceptors to generate action potentials and induce pain hypersensitivity. IL-1␤ acts in a p38 mitogen-activated protein kinase (p38 MAP kinase)-dependent manner, to increase the excitability of nociceptors by relieving resting slow inactivation of tetrodotoxin-resistant voltage-gated sodium channels and also enhances persistent TTX-resistant current near threshold. By acting as an IL-1␤ sensor, nociceptors can directly signal the presence of ongoing tissue inflammation.

Although endocannabinoids constitute one of the first lines of defense against pain, the anatomical locus and the precise receptor mechanisms underlying cannabinergic modulation of pain are uncertain. Clinical exploitation of the system is severely hindered by the cognitive deficits, memory impairment, motor disturbances and psychotropic effects resulting from the central actions of cannabinoids. We deleted the type 1 cannabinoid receptor (CB 1 ) specifically in nociceptive neurons localized in the peripheral nervous system of mice, preserving its expression in the CNS, and analyzed these genetically modified mice in preclinical models of inflammatory and neuropathic pain. The nociceptor-specific loss of CB 1 substantially reduced the analgesia produced by local and systemic, but not intrathecal, delivery of cannabinoids. We conclude that the contribution of CB 1 -type receptors expressed on the peripheral terminals of nociceptors to cannabinoid-induced analgesia is paramount,