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Melatonin, a neurohormone that binds to two G protein-coupled receptors MT1 and MT2, is involved in pain regulation, but the distinct role of each receptor has yet to be defined. We characterized the nociceptive responses of mice with genetic inactivation of melatonin MT1 (MT1−/−), or MT2 (MT2−/−), or both MT1/MT2 (MT1−/−/MT2−/−) receptors in the hot plate test (HPT), and the formalin test (FT). In HPT and FT, MT1−/− display no differences compared to their wild-type littermates (CTL), whereas both MT2−/− and MT1−/−/MT2−/− mice showed a reduced thermal sensitivity and a decreased tonic nocifensive behavior during phase 2 of the FT in the light phase. The MT2 partial agonist UCM924 induced an antinociceptive effect in MT1−/− but not in MT2−/− and MT1−/−/MT2−/− mice. Also, the competitive opioid antagonist naloxone had no effects in CTL, whereas it induced a decrease of nociceptive thresholds in MT2−/− mice. Our results show that the genetic inactivation of melatonin MT2, but not MT1 receptors, produces a distinct effect on nociceptive threshold, suggesting that the melatonin MT2 receptor subtype is selectively involved in the regulation of pain responses.

Several studies showed that chronic pain causes reorganization and functional alterations of supraspinal brain regions. The nociceptin-NOP receptor system is one of the major systems involved in pain control and much evidence also suggested its implication in stress, anxiety and depression. Therefore, we investigated the nociceptin-NOP system alterations in selected brain regions in a neuropathic pain murine model. Fourteen days after the common sciatic nerve ligature, polymerase chain reaction (PCR) analysis indicated a significant decrease of pronociceptin and NOP receptor mRNA levels in the thalamus; these alterations could contribute to the decrease of the thalamic inhibitory function reported in neuropathic pain condition. Nociceptin peptide and NOP mRNA increased in the anterior cingulate cortex (ACC) and not in the somatosensory cortex, suggesting a peculiar involvement of this system in pain regulating circuitry. Similarly to the ACC, an increase of nociceptin peptide levels was observed in the amygdala. Finally, the pronociceptin and NOP mRNAs decrease observed in the hypothalamus reflects the lack of hypothalamus-pituitary-adrenal axis activation, already reported in neuropathic pain models. Our data indicate that neuropathic pain conditions affect the supraspinal nociceptin-NOP system which is also altered in regions known to play a role in emotional aspects of pain.

Molecular mechanisms of adaptive transformations caused by alcohol exposure on opioid dynorphin and nociceptin systems have been investigated in the rat brain. Alcohol was intragastrically administered to rats to resemble human drinking with several hours of exposure: water or alcohol (20% in water) at a dose of 1.5 g/kg three times daily for 1 or 5 days. The development of tolerance and dependence were recorded daily. Brains were dissected 30 minutes (1- and 5-day groups) or 1, 3 or 7 days after the last administration for the three other 5-day groups (groups under withdrawal). Specific alterations in opioid genes expression were ascertained. In the amygdala, an up-regulation of prodynorphin and pronociceptin was observed in the 1-day group; moreover, pronociceptin and the kappa opioid receptor mRNAs in the 5-day group and both peptide precursors in the 1-day withdrawal group were also up-regulated. In the prefrontal cortex, an increase in prodynorhin expression in the 1-day group was detected. These data indicate a relevant role of the dynorphinergic system in the negative hedonic states associated with multiple alcohol exposure. The pattern of alterations observed for the nociceptin system appears to be consistent with its role of functional antagonism towards the actions of ethanol associated with other opioid peptides. Our findings could help to the understanding of how alcohol differentially affects the opioid systems in the brain and also suggest the dynorphin and nociceptin systems as possible targets for the treatment and/or prevention of alcohol dependence.

The opiate analgesic drugs currently represent the most powerful instrument in pain therapy and elicit their therapeutic action mimicking endogenous substances, the opioid peptides, natural ligands of the opioid receptors. They interact with specific receptors, physiologically present in the CNS and in periphery, where they modulate different functions. Beside the well known function related to nociceptive transmission modulation, this system is also involved in the regulation of gastrointestinal, endocrine and autonomic functions, as well as in the mechanisms underlying reward, addiction and in memory and learning processes. Opiates are more and more diffusing in pain therapy and physicians should be able to control pain before this becomes intractable. The recent discovery of new opioid endogenous systems, like the nociceptin one, the development of alternative administration routes and of new drugs with less abuse liability, will allow better scientific understanding, better targeted therapy and a safer use of opiate drugs for the pharmacological control of pain. The goals in this field of research are important and future knowledges will be able to help physicians to use therapeutic tools in the control of pain, before it becomes intractable.