To date, the endogenous ligands described for cannabinoid receptors have been derived from membrane lipids. To identify a peptide ligand for CB 1 cannabinoid receptors, we used the recently described conformation-state sensitive antibodies and screened a panel of endogenous peptides from rodent brain or adipose tissue. This led to the identification of hemopressin (PVNFKFLSH) as a peptide ligand that selectively binds CB 1 cannabinoid receptors. We find that hemopressin is a CB 1 receptor-selective antagonist, because it is able to efficiently block signaling by CB 1 receptors but not by other members of family A G protein-coupled receptors (including the closely related CB2 receptors). Hemopressin also behaves as an inverse agonist of CB 1 receptors, because it is able to block the constitutive activity of these receptors to the same extent as its well characterized antagonist, rimonabant. Finally, we examine the activity of hemopressin in vivo using different models of pain and find that it exhibits antinociceptive effects when administered by either intrathecal, intraplantar, or oral routes, underscoring hemopressin's therapeutic potential. These results represent a demonstration of a peptide ligand for CB 1 cannabinoid receptors that also exhibits analgesic properties. These findings are likely to have a profound impact on the development of novel therapeutics targeting CB 1 receptors.

Motor cortex stimulation is generally suggested as a therapy for patients with chronic and refractory neuropathic pain. However, the mechanisms underlying its analgesic effects are still unknown. In a previous study, we demonstrated that cortical stimulation increases the nociceptive threshold of naive conscious rats with opioid participation. In the present study, we investigated the neurocircuitry involved during the antinociception induced by transdural stimulation of motor cortex in naive rats considering that little is known about the relation between motor cortex and analgesia. The neuronal activation patterns were evaluated in the thalamic nuclei and midbrain periaqueductal gray. Neuronal inactivation in response to motor cortex stimulation was detected in thalamic sites both in terms of immunolabeling (Zif268/Fos) and in the neuronal firing rates in ventral posterolateral nuclei and centromedian-parafascicular thalamic complex. This effect was particularly visible for neurons responsive to nociceptive peripheral stimulation. Furthermore, motor cortex stimulation enhanced neuronal firing rate and Fos immunoreactivity in the ipsilateral periaqueductal gray. We have also observed a decreased Zif268, δ-aminobutyric acid (GABA), and glutamic acid decarboxylase expression within the same region, suggesting an inhibition of GABAergic interneurons of the midbrain periaqueductal gray, consequently activating neurons responsible for the descending pain inhibitory control system. Taken together, the present findings suggest that inhibition of thalamic sensory neurons and disinhibition of the neurons in periaqueductal gray are at least in part responsible for the motor cortex stimulation-induced antinociception.

Motor cortex stimulation (MCS) has been used to treat patients with neuropathic pain resistant to other therapeutic approaches; however, the mechanisms of pain control by MCS are still not clearly understood. We have demonstrated that MCS increases the nociceptive threshold of naive conscious rats, with opioid participation. In the present study, the effect of transdural MCS on neuropathic pain in rats subjected to chronic constriction injury of the sciatic nerve was investigated. In addition, the pattern of neuronal activation, evaluated by Fos and Zif268 immunolabel, was performed in the spinal cord and brain sites associated with the modulation of persistent pain. MCS reversed the mechanical hyperalgesia and allodynia induced by peripheral neuropathy. After stimulation, Fos immunoreactivity (Fos-IR) decreased in the dorsal horn of the spinal cord and in the ventral posterior lateral and medial nuclei of the thalamus, when compared to animals with neuropathic pain. Furthermore, the MCS increased the Fos-IR in the periaqueductal gray, the anterior cingulate cortex and the central and basolateral amygdaloid nuclei. Zif268 results were similar to those obtained for Fos, although no changes were observed for Zif268 in the anterior cingulate cortex and the central amygdaloid nucleus after MCS. The present findings suggest that MCS reverts neuropathic pain phenomena in rats, mimicking the effect observed in humans, through activation of the limbic and descending pain inhibitory systems. Further investigation of the mechanisms involved in this effect may contribute to the improvement of the clinical treatment of persistent pain.

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