Difference between revisions of "Pain Pathophysiology"

===Adenosine (P1) and ATP (P2) Receptors{{Rosso inizio}}qui{{Rosso Fine}} ===

*At resting membrane potential, the receptor is blocked by extracellular Mg2+, and both agonist binding and depolarization are required for channel opening.

At the spinal level, NMDARs are involved in central sensitization, a state of hyperexcitability in the dorsal horns of the spinal cord characterized by sensory facilitation, expressed as hyperalgesia and allodynia. Increased NMDAR activity reflects an increased number of receptors and prolonged channel opening times due to transcriptional, translational, and post-translational modifications. PKC-mediated phosphorylation of NMDAR is of particular importance, reducing dependence on depolarization and Mg2+ block. PKC also regulates NMDAR function by activating the Src tyrosine kinase cascade, interacting with the cytoskeleton, and inducing up-regulation.   

Another protein that mediates many aspects of signal transduction is Ca2+-calmodulin-dependent protein kinase (CaMKII), which is persistently activated after NMDAR stimulation. It binds to the cytoplasmic domain of the receptor in a state that cannot be inactivated by phosphatases. Moreover, this kinase causes calmodulin entrapment, which may promote receptor down-regulation. Recent studies have shown that CaMKII is predominantly expressed in regions of the CNS involved in pain perception, such as the spinal lamina II and DRG, and undergoes up-regulation following inflammatory processes or peripheral lesions. This highlights the importance of the relationship between CaMKII and NMDARs in the development and maintenance of nociceptive hypersensitivity <ref>Dubner R et al. Towards a mechanism-based classification of pain? Pain 1998; 77: 227-230</ref> <ref>Ren K. WIND-UP and the NMDA receptor: from animal studies to humans. Pain 1994; 59: 157-158</ref>.

International literature reports the involvement of neurokinins in the genesis and transmission of pain, both peripherally and centrally, although they do not appear to play a significant role in neuropathic pain. However, their interactions with the endogenous opioid system, the glutamatergic system, molecular transduction mechanisms, and their involvement in wind-up phenomena warrant a brief discussion. Basbaum, in a 1999 review, noted that deletion of the preprotachykinin A (PPT-A) gene, the precursor of SP and NKA, leads to altered perception of acute pain (thermal, mechanical, and chemical) but not chronic pain, suggesting that these neuromediators do not participate in the genesis of neuropathic pain <ref>Przewlocki R, Przewlocka B. Opioids in chronic pain. Eur J Pharmacol 2001; 429: 79-91</ref>.

Hueda highlights peripheral and spinal molecular mechanisms closely related to the endogenous opioid system. In the periphery, chemical stimulation with BK or His induces Ca2+-mediated SP release; in the spinal cord, Orphanin FQ or kitorphin, through activation of the ORL1 receptor, trigger Gi1 activation and Ca2+ influx, resulting in SP release. SP, in turn, stimulates the activation of Gq/11, which activates PLC and the IP3 cascade, with prolonged calcium influx leading to NaV channel activation and depolarization <ref>Thompson SWN, Woolf CJ, Sivilotti LG. Small caliber afferent inputs produce a heterosynaptic facilitation of the synaptic responses evoked by primary afferent A-fibers in the neonatal rat spinal cord in vitro. J Neurophysiol 1993; 69: 2116-2128</ref>.

**CCK and Galanin**. It is well known that cholecystokinin (CCK) reduces the antinociceptive effects of opioids. The presence of CCK has been shown to overlap with the expression of opioids and their receptors throughout the CNS. CCK exerts its action via Gi/Go protein-coupled receptors, attenuating the action of endorphins and morphine. The level of CCK and its receptors, as well as its release, show considerable plasticity following nerve injury and inflammation, conditions associated with chronic pain. Such altered CCK release, along with receptor level changes in some cases, may be the cause of altered opioid sensitivity in various clinical pain conditions.

Neuropathic pain resulting from central or peripheral nervous system injury does not respond well to opioid treatment, likely due to increased activity in the endogenous CCKergic system. CCK receptor antagonists may, therefore, be useful as analgesics in combination with opioids for treating neuropathic pain <ref>Wiesenfeld-Hallin Z, Xu XJ, Hokfelt T. The Role of Spinal Cholecystokinin in Chronic Pain States. Pharmacol Toxicol 2002; 91: 398-403.</ref> <ref>Wiesenfeld-Hallin Z, Xu XJ. Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin. Eur J Pharmacol 2001; 429: 49-59.</ref>.

Galanin is a peptide involved in various functions, including pain perception, and it exerts its function through Gi/Go-coupled metabotropic receptors. It is normally expressed in small DRG neurons in rats, which also contain SP and CGRP, and it appears to be located in some neurons in lamina II. Electrophysiological and behavioral studies in rodents have shown that galanin produces complex effects on spinal pain perception, with a predominant inhibitory effect. Intrathecal administration of galanin enhances the analgesic effects of morphine, particularly when administered together with a CCK2 receptor inhibitor. Galanin reduces spinal hyperexcitability and the pain effects of SP. After peripheral nerve injury, a consistent increase in galanin expression and release can be observed in DRG, but not in dorsal horn interneurons of the spinal cord; additionally, there is no alteration in galanin receptor expression. The probable role of this peptide is tonic activation to suppress painful sensation in injured nerves, suggesting that low levels of galaninergic control may contribute to the development of neuropathic pain <ref>Wiesenfeld-Hallin Z, Xu XJ, Hokfelt T. The Role of Spinal Cholecystokinin in Chronic Pain States. Pharmacol Toxicol 2002; 91: 398-403.</ref>.

**Opioids**. The main groups of opioid peptides, enkephalins, dynorphins, and β-endorphins, are derived from proenkephalin, prodynorphin, and proopiomelanocortin, respectively. Recently, a new group of peptides called endomorphins (-1 and -2), with atypical structure and high selectivity for the μ-opioid receptor, has been discovered. Another group of endogenous opioids is derived from pronociceptin, acting on the ORL1 receptor. Three members of the opioid receptor family were cloned in the early 1990s: first the δ-receptor in mice (DOR1), followed by the μ (MOR1) and κ (KOR1) receptors. These three receptors belong to the seven-transmembrane domain superfamily, coupled to G proteins, and share significant structural similarities <ref>Zimmermann M. Central nervous mechanisms modulating pain-related information: do they become deficient after lesion of the peripheral or central nervous system? In: Casey, KL (Ed.) Pain and Central Nervous System Disease: The Central Pain Syndromes. Raven Press, New York, 1991; pp.183-199.</ref>.

These receptors and peptides are significantly involved in antinociception processes and are found in nociceptive pathways. Peripheral inflammation affects central structures and alters opioid activity; on one hand, it increases the activity of some receptor antagonists, while on the other hand, it increases the affinity and number of μ receptors, enhancing the analgesic potency of opioids. This is achieved by altering the expression of certain genes in the dorsal horn of the spinal cord <ref>Zimmermann M. Central nervous mechanisms modulating pain-related information: do they become deficient after lesion of the peripheral or central nervous system? In: Casey, KL (Ed.) Pain and Central Nervous System Disease: The Central Pain Syndromes. Raven Press, New York, 1991; pp.183-199.</ref>. 

Numerous studies have found evidence of the inefficient inhibition exerted by the endogenous opioid system in neuropathic pain and related hyperalgesia; indeed, both this system and the descending inhibitory system may be inadequate for controlling pain at the spinal level. The spinal pain transmission system is under continuous control from the basal nuclei, particularly the periaqueductal gray matter and the locus coeruleus. Zimmermann and colleagues have shown that in animals with neuropathy, although these inhibitory systems are still functioning, they provide less than 50% of the normal inhibition <ref>Zimmermann M. Central nervous mechanisms modulating pain-related information: do they become deficient after lesion of the peripheral or central nervous system? In: Casey, KL (Ed.) Pain and Central Nervous System Disease: The Central Pain Syndromes. Raven Press, New York, 1991; pp.183-199.</ref>; similarly, Porreca and colleagues have demonstrated tonic facilitation in pain transmission in the dorsal horn of the spinal cord in neuropathic animals, driven by neurons located in the ventromedial medulla <ref>Porreca et al. Inhibition of neuropathic pain by selective ablation of brainstem medullary cells expressing the μ-opioid receptor. J Neurosci 2001; 21: 5281-5288.</ref>. 

Although morphine is not able to exert its efficacy in neuropathic pain, a wide range of evidence suggests that it is not completely resistant, but only shows reduced sensitivity, and higher doses are needed to achieve the same response <ref>Mayer DJ, Mao J, Price DD. The development of morphine tolerance and dependence is associated with translocation of protein kinase C. Pain 1995; 61: 365-374.</ref>. Mayer and colleagues demonstrated that a nerve injury induced 8 days before the morphine test caused the dose-response curve to shift towards higher doses, by a factor of 6, meaning six times higher doses were needed to achieve the same response as in the control. Of particular interest is the fact that pretreatment with an NMDA receptor inhibitor (MK-801) prevents desensitization to morphine <ref>Mayer DJ, Mao J, Price DD. The development of morphine tolerance and dependence is associated with translocation of protein kinase C. Pain 1995; 61: 365-374.</ref>. 

Studies on the molecular processes of opioid regulation, tolerance, and dependence have shown that nitric oxide (NO) is closely linked to these mechanisms: not only do opioids influence NO release, but NO itself also participates in the processes of tolerance and dependence. In the first case, it has been shown that chronic pain activates NMDA receptors, allowing calcium entry, which activates Nitric Oxide Synthase (NOS) and, downstream, guanylate cyclase. The increase in cGMP causes thermal and mechanical hyperalgesia, and tactile allodynia. On the other hand, chronic activation of μ-opioid receptors causes PKC translocation, which phosphorylates NMDA receptors, increasing calcium levels and activating NOS; in this case, NO induces tolerance and dependence. This theory has been confirmed by studies in rodents that have demonstrated significant reductions in hyperalgesia, allodynia, and tolerance following the administration of NOS inhibitors such as Agmatine and N(G)-nitro-L-arginine methyl ester (L-NAME). Interestingly, these desensitization mechanisms do not occur with endomorphins, indicating the existence of different pathways for these molecules <ref>Mayer DJ, Mao J, Price DD. The development of morphine tolerance and dependence is associated with translocation of protein kinase C. Pain 1995; 61: 365-374.</ref>.

**BDNF: neuromodulator**

During development, brain-derived neurotrophic factor (BDNF) supports the survival of a neuronal population in both the central and peripheral nervous systems. In maturity, BDNF appears to act as an important modulator of synaptic plasticity. BDNF is synthesized by primary sensory neurons (presynaptic neurons) whose expression is regulated in models of inflammatory and neuropathic pain. The high-affinity receptor for BDNF, tropomyosine receptor kinase B (TrkB), is expressed by postsynaptic neurons in the dorsal horn of the spinal cord. Stimulation of presynaptic nociceptive afferent fibers induces the release of BDNF and the consequent activation of TrkB receptors, leading to postsynaptic excitability. Electrophysiological recordings in vitro show that BDNF increases discharge potential induced by stimulation of C fibers in ventral roots. Additionally, behavioral data indicate that BDNF exerts antagonism by attenuating both the second phase of hyperalgesia induced by formalin (in animals treated with NGF) and the thermal hyperalgesia induced by carrageenan antigen: this suggests that BDNF is involved in some aspects of central sensitization under conditions of peripheral inflammation. In conclusion, BDNF meets many of the criteria needed to be defined as a neurotransmitter/neuromodulator in small-diameter nociceptive neurons <ref>Pezzet S, Malcangio M, McMahon SB. BDNF: a neuromodulator in nociceptive pathways? Brain Res Rew 2002; 40: 240-249.</ref>.

**Wind-up and neuropathic pain**

** Nitric Oxide Synthase (NOS), Heme Oxygenase (HO), and Reactive Oxygen Species (ROS) activity**

Heme oxygenase is an enzyme that catalyzes the formation of biliverdin and iron and carbon monoxide monoxides through the heme structure. In humans, two isoforms have been identified: HO-1 and HO-2, the latter of which is present in both neurons and glial cells in the CNS <ref>Snyder SH, Jaffrey SR, Zakhary R. Nitric oxide and carbon monoxide: parallel roles as neural messengers. Brain Res Rev 1998; 26:167-175.</ref>.   

Similar results have been reported using various techniques: Liang and colleagues first used NOS and HO-2 inhibitors and then molecular biology techniques to demonstrate that these enzymes independently modulate the molecular changes that occur during chronic opioid exposure, tolerance, and the resulting behavioral alterations. Activation of the NOS system by chronic morphine stimulation limits the analgesic capacity of the opioid; additionally, NOS knock-out rodents exhibit reduced hyperalgesia. Similarly, HO-2 knock-out rodents show reduced mechanical allodynia after withdrawal from chronic morphine therapy; while wild-type rodents (not genetically altered) exhibit two- to three-fold increased expression of NOS, NMDAR, and prodynorphin. Morphine administration increases cGMP levels in spinal neurons, and cGMP analogs cause hyperalgesia. Administration of NOS and HO-2 inhibitors significantly reduces cGMP production induced by morphine and the resulting hyperalgesia <ref>Liang D et al. Heme oxygenase type 2 modulates behavioral and molecular changes during chronic exposure to morphine. Neuroscience 2003; 121: 999-1005.</ref> <ref>Li X et Clark JD. Spinal cord nitric oxide synthase and heme oxygenase limit morphine-induced analgesia. Molecular Brain Research 2001; 95: 96-102.</ref>.   

These data collectively demonstrate that NOS and HO alter opioid action and open new therapeutic strategies.