Neuropathic pain syndromes—pain after a lesion or disease of the peripheral or central nervous system—are clinically characterized by spontaneous and evoked types of pain, which are underpinned by various distinct pathophysiological mechanisms in the peripheral and central nervous systems. In some patients, the nerve lesion triggers molecular changes in nociceptive neurons, which become abnormally sensitive and develop pathological spontaneous activity. Inflammatory reactions of the damaged nerve trunk can induce ectopic nociceptor activity, causing spontaneous pain. The hyperactivity in nociceptors induces secondary changes in processing neurons in the spinal cord and brain, so that input from mechanoreceptive A-fibers is perceived as pain. Neuroplastic changes in the central pain modulatory systems can lead to further hyperexcitability.
The time-dependent activation, the time to peak, measured over a range of test potentials from -65 mV to -30 mV. No significant difference was found The “window period” of T-type current is under neuropathic conditions has widened in comparison to control. The window period is the area were the activation and inactivation curves overlap. A window current can occur. Window current means the small range of voltage where T-type channels can open but not inactivate completely. There was a larger overlap area between activation and inactivation curves in the neuropathic cells. This indicated that more T-type calcium channels are available at physiological membrane potentials. Enhanced function of the T-type calcium channels under neuropathic condition.
Molecular Mechanisms of Pain. Part 1
Bogomoletz Institute of Physiology, Kiev, Ukraine [email_address] Dr. NANA VOITENKO AACIMP KIEV - 2011 Molecular Mechanisms of Pain part I
<ul><li>Pain is a clinical entity rather than a compilation of single disease states. </li></ul><ul><li>Molecular mechanisms of pain </li></ul><ul><li>Antisense oligonucleotides – a new strategy of pain treatment </li></ul>
Long term chronic pain, mainly inflammatory or neuropathic, afflicts about 25% of the general world population. More than 60% of people aged 65 plus complain of daily pains. This degree of disability has a huge economic toll in terms of loss of employment and disability payments but quality of life is equally compromised. Pain is thus a major medical issue but is not simply a sensation but an event that also triggers aversive and threatening psychological feelings. Intolerable pain is one of the main reason for euthanasia .
<ul><li>The treatment of persistent pain is still unsatisfactory, and a new hypothetical concept has been proposed, in which pain is analyzed on the basis of underlying mechanisms. </li></ul><ul><li>The increased knowledge of pain - generating mechanisms and their translation into symptoms and signs might eventually allow a dissection of the mechanisms that operate in each patient. If a precise clinical phenotypic characterization of the neuropathic pain is combined with a selection of drugs that act on those mechanisms, it should ultimately be possible to design optimal treatments for individuals. </li></ul><ul><li>The neuropathic pain to be considered as a clinical entity rather than a symptom of single disease. </li></ul>
Measurements of pain <ul><li>Hargreaves test – thermal hyper-/ hypoalgesia </li></ul><ul><li>Randall-Selitto test – mechanical hyper-/ hypoalgesia </li></ul><ul><li>Hot plate test – thermal allodynia </li></ul><ul><li>Von Frei test - mechanical / tactile allodynia </li></ul>
ASICs <ul><li>ASICs — Acid Sensing Ionic Channels, are neuronal voltage-insensitive cationic channels activated by extracellular protons. A ctivation threshold – pH 7.0; maximum current at pH 5.4. </li></ul><ul><li>In the last decade , increasing evidence has implicated the ASIC channels in an astounding range of physiological functions including pain , taste transduction, learning and memory. </li></ul><ul><li>ASICs have been proposed to be involved in the perception of pain in conditions associated with tissue acidosis such as ischemia, inflammation, tumors, or lesions. All these situations are associated with prolonged acidification, raising the question of the activation of these channels by slow and/or prolonged acidosis. </li></ul><ul><li>Acid-sensing ion channels are potential drug targets for treating a wide variety of conditions linked to both the CNS and PNS. </li></ul>“ Receptor for protons” (Krishtal and Pidoplichko, Neurosci. Lett. 1981) pH Oleg Krishtal
Lingueglia, E. J. Biol. Chem. 2007;282:17325-17329 The ASIC family of ion channels
ASICs knockouts: effects at the periphery ASIC3(-/-): <ul><li>light touch ↑ </li></ul><ul><li>noxious pinch ↓ </li></ul><ul><li>acid- and noxious heat ↓ </li></ul>( ↑↓ sensitivity increased/decreased) Price et al. , Neuron, 2001
<ul><li>P2X receptors are a family of cation-permeable ligand gated ion channels that open in response to the binding of extracellular adenosine 5'-triphosphate (ATP). They belong to a larger family of receptors known as the purinergic receptors. </li></ul><ul><li>In keeping with their wide distribution throughout the body, P2X receptors are involved in a variety of physiological processes,including: </li></ul><ul><li>- Modulation of cardiac rhythm and contractility </li></ul><ul><li>- Modulation of vascular tone </li></ul><ul><li>- Mediation of nociception </li></ul><ul><li>- Contraction of the smooth muscle cells </li></ul>1983 “Receptor for ATP in the membrane of mammalian sensory neurones” (Krishtal et al., Neurosci. Lett. 1983) Р2-( Purino )- receptors ATP Oleg Krishtal
Р2-( Purino )- receptors <ul><li>P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. </li></ul><ul><li>Seven genes encode P2X receptor subunits </li></ul><ul><li>Channels form as multimers of several subunits : h omomeric P2X1 - P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels </li></ul><ul><li>All P2X receptors are permeable to small cations such as Na+ and Ca2+. </li></ul><ul><li>The channel opening time is dependent upon the subunit makeup of the receptor. </li></ul>
Ionic currents through different subunits of P2X receptors P2X3 P2X2/3 P2X2 P2X3+P2X2/3
[Ca 2+ ] i increase under activation of P2X receptors
GEOFFREY BURNSTOCK 2007 Physiology and Pathophysiology of Purinergic Neurotransmission Mechanisms of ATP-induced sensitization
P2XRs knockouts in mice <ul><li>Reduced pain-related behaviour, urinary bladder hyporeflexia (Cockayne et at., 2000) </li></ul><ul><li>Formalin-induced pain behaviour is reduced, unable to code thermal stimuli (Souslova et al., 2000) </li></ul><ul><li>Enhanced thermal avoidance, enhanced induction of spinal c-FOS under heating (Shimizu et al., 2005) </li></ul><ul><li>Important contribution of heteromeric P2X2/3 receptors to nociceptive responses and mechanosensory transduction, especially within the urinary bladder (Cockayne et al., 2005) </li></ul>P2X3-/- P2X2/P2X3(Dbl-/-)
Modulation of pain by AppCH 2 ppG – antagonist of P2X3 receptor Viatchenko-Karpinski et al, 2011 in press
Effect of new P2X3 modulator: purotoxin1 (PT1, spider) Grishin et al., 2010 <ul><li>mechanical allodynia </li></ul><ul><li>thermal hyperalgesia </li></ul><ul><li>chemical pain behavior </li></ul>Pain symptoms
<ul><li>The first direct separation of HVA and LVA currents was performed on dorsal root ganglion (DRG) neurons at the Bogomoletz Institute of Physiology (Ukraine) in 1983 </li></ul>Low-voltage-activated calcium channels Veselovskii NS, Fedulova SA. 2 types of calcium channels in the somatic membrane of spinal ganglion neurons in the rat. Dokl Akad Nauk SSSR 1983; 268:747-50 (in Russian) N.S. Veselovsky
T-type calcium channels <ul><li>The T-type calcium channel is a type of voltage-gated calcium channel. "T" stands for transient referring to the length of activation. T-type calcium channel are known also as “low-voltage-activated” since they characterized by lower membrane potential as compare to other calcium channels for activation and fast inactivation. For activation they required hyperpolarisation. New evidence obtained using an array of techniques such as electrophysiological recordings, pharmacological behavioral experiments as well as molecular techniques strongly supports the role of peripheral T-type Ca 2+ channels in boosting nociceptive transmission in a variety of experimental pain models. Therefore, these channels in peripheral sensory neurons may be the important, although previously unappreciated, targets for novel pain therapies. </li></ul>P.Isope and T.H.Murphy 2005
Increasing of T-type window current and resting [Ca 2+ ] i in the C-fiber nociceptors of diabetic rats Voitenko group, under preparation <ul><li>thermal hyperalgesia </li></ul><ul><li>tactile allodynia </li></ul><ul><li>dynamic mechanical allodynia </li></ul><ul><li>punctate mechanical hyperalgesia </li></ul>
<ul><li>The best-known activators of TRPV1: </li></ul><ul><li>heat greater than 43°C, </li></ul><ul><li>capsaicin, the pungent compound in hot chili peppers </li></ul><ul><li>allyl isothiocyanate, the pungent compound in mustard and wasabi. </li></ul><ul><li>Endogenous activators: </li></ul><ul><li>low pH (acidic conditions), </li></ul><ul><li>endocannabinoid anandamide, </li></ul><ul><li>N-arachidonoyl-dopamine </li></ul>TRPV1 receptor Vaniloid receptor 1 (TRPV1, transient receptor potential vanilloid), also known as the capsaicin receptor, is a nonselective cation channel that may be activated by a wide variety of exogenous and endogenous physical and chemical stimuli. The activation of TRPV1 leads to painful, burning sensation. TRPV1 receptors are found mainly in the nociceptive neurons of the peripheral nervous system, but they have also been described in many other tissues, including the central nervous system. TRPV1 is involved in the transmission and modulation of pain (nociception), as well as the integration of diverse painful stimuli.
TRPV1 function under diabetic peripheral neuropathy Voitenko group, under preparation <ul><li>thermal allodynia ( TRPV1) </li></ul><ul><li>cold allodynia ( TRPM8) </li></ul><ul><li>thermal hyperalgesia ( TRPV1) </li></ul><ul><li>thermal hypoalgesia ( TRPV1) </li></ul>Diabetes, hypalgesia Diabetes, hyp er algesia Control vs control Kapsaicine vs control