Adenosine mediates spinal norepinephrine-produced antinociception as revealed by nociceptive discharges in parafascicular neurons in rats

ATP-sensitive potassium channels and endogenous adenosine are involved in spinal antinociception produced by locus coeruleus stimulation

The effects of locus coeruleus stimulation on nociceptive evoked discharges of thalamic parafascicular (PF) neurons were investigated in lightly urethane-anesthetized rats, aiming to study the mechanisms underlying these effects. Intrathecal (i.t.) administration of aminophylline (an adenosine antagonist), glibenclamide (an ATP-sensitive potassium [K+(ATP)] channels blocker), nicrorandil (Nico; an agonist of K+(ATP) channel and a K+(ATP) channel opener), and 5'-N-ethylcarboxamido-adenosine (NECA; an adenosine agonist) were used. The results showed that (1) locus coeruleus stimulation significantly inhibited the nociceptive evoked discharges of parafascicular neurons, (2) locus coeruleus stimulation-produced antinociception in PF neurons was blocked by both it. glibenclamide and i.t. aminophylline, (3) nociceptive discharges of PF neurons were also suppressed by both i.t. NECA and i.t. nicorandil, and (4) i.t. glibenclamide showed no effect on the suppression of nociceptive discharges induced by NECA, whereas aminophylline blocked the suppression of nociceptive discharges induced by nicorandil. These results suggest that (a) K+(ATP) channels and endogenous adenosine may be involved in the mediation of antinociception induced by norepinephrine, which is released in the dorsal horn by descending fibers originating from the locus coeruleus and (b) the opening of K+(ATP) channels may precede the release of endogenous adenosine in the process of suppressing nociceptive transmission at the spinal level.

Adenosine in the spinal cord and periphery: release and regulation of pain

In the central nervous system (CNS), adenosine is an important neuromodulator and regulates neuronal and non-neuronal cellular function (e.g. microglia) by actions on extracellular adenosine A(1), A(2A), A(2B) and A(3) receptors. Extracellular levels of adenosine are regulated by synthesis, metabolism, release and uptake of adenosine. Adenosine also regulates pain transmission in the spinal cord and in the periphery, and a number of agents can alter the extracellular availability of adenosine and subsequently modulate pain transmission, particularly by activation of adenosine A(1) receptors. The use of capsaicin (which activates receptors selectively expressed on C-fibre afferent neurons and produces neurotoxic actions in certain paradigms) allows for an interpretation of C-fibre involvement in such processes. In the spinal cord, adenosine availability/release is enhanced by depolarization (K(+), capsaicin, substance P, N-methyl-D-aspartate (NMDA)), by inhibition of metabolism or uptake (inhibitors of adenosine kinase (AK), adenosine deaminase (AD), equilibrative transporters), and by receptor-operated mechanisms (opioids, 5-hydroxytryptamine (5-HT), noradrenaline (NA)). Some of these agents release adenosine via an equilibrative transporter indicating production of adenosine inside the cell (K(+), morphine), while others release nucleotide which is converted extracellularly to adenosine by ecto-5'-nucleotidase (capsaicin, 5-HT). Release can be capsaicin-sensitive, Ca(2+)-dependent and involve G-proteins, and this suggests that within C-fibres, Ca(2+)-dependent intracellular processes regulate production and release of adenosine. In the periphery, adenosine is released from both neuronal and non-neuronal sources. Neuronal release from capsaicin-sensitive afferents is induced by glutamate and by neurogenic inflammation (capsaicin, low concentration of formalin), while that from sympathetic postganglionic neurons (probably as adenosine 5'-triphosphate (ATP) with NA) occurs following more generalized inflammation. Such release is modified differentially by inhibitors of AK and AD. Following nerve injury, there is an alteration in capsaicin-sensitive adenosine release, as spinal release now is less responsive to opioids, while peripheral release is less responsive to inhibitors of metabolism. Following inflammation, adenosine is released from a variety of cell types in addition to neurons (e.g. endothelial cells, neutrophils, mast cells, fibroblasts). ATP is released both spinally and peripherally following inflammation or injury, and may be converted to adenosine by ecto-5'-nucleotidase contributing an additional source of adenosine. Release of adenosine from both spinal and peripheral compartments has inhibitory effects on pain transmission, as methylxanthine adenosine receptor antagonists reduce analgesia produced by agents which augment extracellular levels of adenosine spinally (morphine, 5-HT, substance P, AK inhibitors) and peripherally (AK inhibitors, AD inhibitors). Increases in extracellular adenosine availability also may contribute to antiinflammatory effects of certain agents (methotrexate, sulfasalazine, salicylates, AK inhibitors), and this could have secondary effects on pain signalling in chronic inflammation. The purpose of the present review is to consider: (a). the factors that regulate the extracellular availability of adenosine in the spinal cord and at peripheral sites; and (b). the extent to which this adenosine affects pain signalling in these two distinct compartments.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Antinociceptive mechanisms of dipsacus saponin C administered intrathecally in mice

Dipsacus saponin C (DSC) administered intrathecally (i.t.) showed antinociceptive effect in a dose-dependent (from 3.75 to 30 microg) manner as measured by the tail-flick assay. The antinociception induced by DSC at the dose of 30 microg reached at peak 7.5 min and almost returned to the control level after 60 min. 5-Amino-valeric acid (5-AVA, a GABA(A) receptor antagonist, from 1 to 20 microg) and SR 95531 (a GABA(B) receptor antagonist, from 0.1 to 2 ng) dose-dependently attenuated i.t. administered DSC-induced increase of the inhibition of the tail-flick response. The i.t. injection of yohimbine (an alpha(2)-adrenergic receptor antagonist, from 1 to 20 microg) and methysergide (a serotonin receptor antagonist, from 1 to 20 microg), but not naloxone (from 2 to 8 microg), significantly attenuated inhibition of the tail-flick response induced by DSC (30 microg) administered i.t. Sulfated cholecystokinin (CCK, from 0.05 to 0.5 ng) injected i.t. significantly reduced the inhibition of the tail-flick response induced by DSC (30 microg) administered i.t. Our results suggest that DSC shows an antinociceptive effect when it is administered spinally and GABA(A), GABA(B), alpha(2)-adrenergic and serotonin receptors located at the spinal cord level, but not opioid receptors, may be involved in DSC-induced antinociception. Furthermore, CCK may play an important role for the modulation of i. t. injected DSC-induced antinociception.

Endogenous adenosine involved in the mediation of spinal antinociception produced by stimulating locus coeruleus

The focus of this study was to investigate whether spinal adenosine is involved in mediating descending nociceptive modulation by the locus coeruleus (LC). Nociceptive evoked responses in parafascicular (PF) neurons were studied before and after electrical stimulation of the LC as well as before and after intrathecal (i.t.) administration of phentolamine (Ph) or aminophylline (Aph), an adenosine receptor antagonist, and 5'ethylcarboxamidoadenosine (NECA), an adenosine agonist. The main results were as follows: (1) the nociceptive evoked responses recorded in PF neurons were suppressed by LC stimulation; (2) pretreatment with i.t., Ph (40 nmol) reversed the LC effects, i.e., the suppressive effect of LC stimulation on the PF nociceptive evoked responses was reversed in the presence of Ph; (3) smaller doses of i.t. Aph (120 nmol) blocked only the suppressive effect produced by LC stimulation, while larger doses (240 nmol) reversed the LC stimulation, i.e., the LC stimulation exerted a facilatatory effect; and (4) i.t. application of NECA, an adenosine agonist, suppressed the nociceptive discharges in PF neurons. The results suggest that spinal adenosine may be involved in the mediation of the spinal antinociceptive effect produced by LC stimulation.