Jana Sawynok

Adenosine receptor activation and nociception

Adenosine and ATP exert multiple influences on pain transmission at peripheral and spinal sites. At peripheral nerve terminals in rodents, adenosine A1 receptor activation produces antinociception by decreasing, while adenosine A1 receptor activation produces pronociceptive or pain enhancing properties by increasing, cyclic AMP levels in the sensory nerve terminal. Adenosine A3 receptor activation produces pain behaviours due to the release of histamine and 5-hydroxytryptamine from mast cells and subsequent actions on the sensory nerve terminal. In humans, the peripheral administration of adenosine produces pain responses resembling that generated under ischemic conditions and the local release of adenosine may contribute to ischemic pain. In the spinal cord, adenosine A receptor activation produces antinociceptive properties in acute nociceptive, inflammatory and neuropathic pain tests. This is seen at doses lower than those which produce motor effects. Antinociception results from the inhibition of intrinsic neurons by an increase in K+ conductance and presynaptic inhibition of sensory nerve terminals to inhibit the release of substance P and perhaps glutamate. There are observations suggesting some involvement of spinal adenosine A2 receptors in pain processing, but no data on any adenosine A3 receptor involvement. Endogenous adenosine systems contribute to antinociceptive properties of caffeine, opioids, noradrenaline, 5-hydroxytryptamine, tricyclic antidepressants and transcutaneous electrical nerve stimulation. Purinergic systems exhibit a significant potential for development as therapeutic agents. An understanding of the contribution of adenosine to pain processing is important for understanding how caffeine produces adjuvant analgesic properties in some situations, but might interfere with the optimal benefit to be derived from others.

Topical and Peripherally Acting Analgesics

Acute nociceptive, inflammatory, and neuropathic pain all depend to some degree on the peripheral activation of primary sensory afferent neurons. The localized peripheral administration of drugs, such as by topical application, can potentially optimize drug concentrations at the site of origin of the pain, while leading to lower systemic levels and fewer adverse systemic effects, fewer drug interactions, and no need to titrate doses into a therapeutic range compared with systemic administration. Primary sensory afferent neurons can be activated by a range of inflammatory mediators such as prostanoids, bradykinin, ATP, histamine, and serotonin, and inhibiting their actions represents a strategy for the development of analgesics. Peripheral nerve endings also express a variety of inhibitory neuroreceptors such as opioid, α-adrenergic, cholinergic, adenosine and cannabinoid receptors, and agonists for these receptors also represent viable targets for drug development. At present, topical and other forms of peripheral administration of nonsteroidal anti-inflammatory drugs, opioids, capsaicin, local anesthetics, and α-adrenoceptor agonists are being used in a variety of clinical states. There also are some clinical data on the use of topical antidepressants and glutamate receptor antagonists. There are preclinical data supporting the potential for development of local formulations of adenosine agonists, cannabinoid agonists, cholinergic ligands, cytokine antagonists, bradykinin antagonists, ATP antagonists, biogenic amine antagonists, neuropeptide antagonists, and agents that alter the availability of nerve growth factor. Given that activation of sensory neurons involves multiple mediators, combinations of agents targeting different mechanisms may be particularly useful. Topical analgesics represent a promising area for future drug development.

The role of purines in nociception

The preceding review indicates that there is convincing evidence for the presence of adenosine in and release of adenosine from capsaicin-sensitive small diameter primary afferent neurons in the spinal cord (Fig. 1). Within the dorsal spinal cord, adenosine inhibits the transmission of nociceptive information, although details of mechanisms involved in this action remain to be established. In view of the antinociceptive actions of adenosine analogues, there has been some interest in the possibility of developing adenosine analogues as analgesic agents. However, this goal may be frustrated by this concomitant suppression of motor function, as well as the production of other side effects due to the diverse nature of pharmacological effects seen with adenosine analogues. Release of adenosine from small diameter primary afferent nerve terminals and subsequent activation of extracellular adenosine receptors in the dorsal horn of the spinal cord appears to contribute significantly to the spinal action of opioids. An understanding of spinal mechanisms of actions of adenosine therefore is an important prerequisite for our understanding of the action of this clinically important group of drugs. ATP may be a sensory neurotransmitter released from non-nociceptive large diameter primary afferent neurons (Fig. 1). The subsequent extracellular conversion of released ATP to adenosine may produce suppression of the transmission of noxious sensory information via small diameter primary afferent fibres, and contribute to the phenomenon of vibration induced analgesia. Clearly, the role of purines on spinal cord processing of nociceptive information merits considerable attention.

GABAergic mechanisms of analgesia: An update

Both directly acting (GABAA and GABAB agonists) and indirectly acting GABAergic agents (GABA uptake inhibitors and GABA-transaminase inhibitors) produce analgesia in a variety of animal test systems. Analgesia produced by GABAA agonists is probably due to a supraspinal action, although spinal sites may also play a role. GABAA agonist analgesia is insensitive to naloxone, bicuculline, picrotoxin and haloperidol, but is blocked by atropine, scopolamine and yohimbine suggesting a critical role for central cholinergic and noradrenergic pathways in this action. The lack of blockade by the GABAA antagonist bicuculline is difficult to explain. Both bicuculline and picrotoxin have intrinsic analgesia actions which may not necessarily be mediated by GABA receptors. The GABAB agonist baclofen produces analgesia by actions at both spinal and supraspinal sites. Baclofen analgesia is insensitive to naloxone, bicuculline and picrotoxin, and blockade by cholinergic antagonists occurs only under limited conditions. Catecholamines are important mediators of baclofen analgesia because analgesia is potentiated by reserpine, alpha-methyl-p-tyrosine, phentolamine, ergotamine, haloperidol and chlorpromazine. A role for serotonergic mechanisms is less well defined. Methylxanthines, which produce a clonidine-sensitive increase in noradrenaline (NA) turnover, increase baclofen analgesia by a clonidine-sensitive mechanism. Both ascending and descending NA pathways are implicated in the action of baclofen because dorsal bundle lesions, intrathecal 6-hydroxydopamine and medullary A1 lesions markedly decrease baclofen analgesia. However, simultaneous depletion of NA in ascending and descending pathways by locus coeruleus lesions potentiates baclofen analgesia suggesting a functionally important interaction between the two aspects. Baclofen analgesia within the spinal cord may be mediated by a distinct baclofen receptor because GABA does not mimic the effect of baclofen and the rank order of potency both of close structural analogs of baclofen as well as antagonists differs for analgesia and GABAB systems. The spinal mechanism may involve an interaction with substance P (SP) because SP blocks baclofen analgesia, and desensitization to SP alters the spinal analgesic effect of baclofen. GABA uptake inhibitors produce analgesia which is similar to that produced by GABAA agonists because it is blocked by atropine, scopolamine and yohimbine. Analgesia produced by GABA-transaminase inhibitors is similar to that produced by GABAA agonists because it can be blocked by atropine, but it is potentiated by haloperidol while THIP analgesia is not.(ABSTRACT TRUNCATED AT 400 WORDS)

Classification of adenosine receptors mediating antinociception in the rat spinal cord

1 Analogues of adenosine were injected intrathecally into rats implanted with chronic indwelling cannulae in order to determine a rank order of potency and hence characterize adenosine receptors involved in spinal antinociception. 2 In the tail flick test l‐N6‐phenylisopropyl adenosine (l‐PIA), cyclohexyladenosine (CHA) and 5′‐N‐ethylcarboxamide adenosine (NECA) produced dose‐related antinociception which attained a plateau level. NECA and CHA also produced an additional distinct second phase of antinociception. d‐N6‐Phenylisopropyl adenosine (d‐PIA) and 2‐chloroadenosine (CADO) had very little antinociceptive activity in this test. The rank order of potency in producing the plateau effect was l‐PIA > CHA > NECA > d‐PIA = CADO, while that for the second phase of antinociception was NECA >‐CHA. 3 Pretreatment with both theophylline and 8‐phenyltheophylline (8‐PT) antagonized antinociception produced by CHA, with 8‐PT being at least an order of magnitude more potent than theophylline. Both antagonists produced a significant hyperalgesia in the tail flick test. l‐PIA and CHA also produced methylxanthine‐sensitive antinociception in the hot plate test. 4 These results suggest that activation of A1‐receptors in the spinal cord can produce antinociception. Activation of A2‐receptors may produce an additional effect, but the relative activity of CHA in this component of activity is unusual.

Acute amitriptyline in a rat model of neuropathic pain: differential symptom and route effects.

The present study was designed to determine whether amitriptyline, a prototypical tricyclic antidepressant, could produce pain relieving properties in a rat model of neuropathic pain. Nerve injury was produced by tight ligation of the lumbar 5th and 6th dorsal roots and this resulted in persistent stimulus evoked neuropathic pain symptoms (tactile allodynia and thermal hyperalgesia). Thermal hyperalgesia was measured using a focused light beam directed at the ventral surface of the paw while tactile allodynia was determined using Semmes-Weinstein monofilaments applied to the ventral surface of the paw. Amitriptyline was administered systemically (intraperitoneal), spinally (intrathecal cannula), and locally (subcutaneously) via direct injection into the dorsal surface of the paw. Following systemic administration, amitriptyline completely reversed thermal hyperalgesia (10 mg/kg) in the injured paw. Spinal administration of amitriptyline (60 microg) also produced an antihyperalgesic effect. Interestingly, local administration of amitriptyline (100 nmol) had an immediate antihyperalgesic effect that persisted for 120 min following administration. Amitriptyline had no alleviating effect against mechanical allodynia regardless of the route of administration, but curiously, produced hyperaesthesia in the contralateral paw. These results indicate that in the rat model of spinal nerve ligation, amitriptyline is effective in alleviating thermal hyperalgesia (systemically, spinally and locally) but is ineffective against mechanical allodynia. The peripheral efficacy of amitriptyline suggests the possibility of the development of cream formulations that may be able to increase the local concentration of amitriptyline without increasing the systemic dose and the subsequent occurrence of side effects.

Pharmacological Rationale for the Clinical Use of Caffeine

SummaryCaffeine is widely consumed in beverages to obtain mild CNS stimulant effects. Long term use produces tolerance to some of the pharmacological effects. Withdrawal of caffeine, even from moderate intake levels, can produce symptoms such as headache, fatigue and anxiety. Caffeine is used therapeutically in combination with ergotamine for migraine headaches and in combination with nonsteroidal anti-inflammatory drugs in analgesic formulations. Caffeine alone is used as a somnolytic, to treat various headache conditions, respiratory depression in neonates, postprandial hypotension and obesity, and to enhance seizure duration in electroconvulsive therapy.In some headache and in pain paradigms, caffeine may produce direct adjuvant analgesic properties, while in other headache conditions (perioperative, post-dural puncture) caffeine may be effective by alleviating a manifestation of caffeine withdrawal. Other uses, such as to promote wakefulness, for respiratory stimulation and seizure prolongation, rely on central stimulant properties of caffeine. Effects of caffeine on the vasculature may contribute to the relief of some headaches and in postprandial hypotension. Blockade of methylxanthinesensitive adenosine receptors is the currently accepted mechanism of action of caffeine.

Formalin-induced nociceptive behavior and edema: involvement of multiple peripheral 5-hydroxytryptamine receptor subtypes

The role of 5-hydroxytryptamine and its receptor subtypes in the development of acute inflammation was investigated using the rat paw formalin test as a model for pain (measured by flinching behavior) and edema formation (measured by plethysmometry). The role of endogenously released 5-hydroxytryptamine was assessed using 5-hydroxytryptamine receptor subtype-selective antagonists co-injected with 2.5% formalin, while the receptor subtypes involved in the inflammatory process were further defined by co-injection of 5-hydroxytryptamine or 5-hydroxytryptamine receptor subtype-selective agonists with 0.5% formalin in anticipation of an augmented response. When co-administered with 2.5% formalin, propranolol, tropisetron or GR113808A, but not ketanserin, effectively blocked nociceptive behavior. In the presence of 0.5% formalin, 5-carboxamidotryptamine, 1-(m-chlorophenyl) biguanide or 5-methoxytryptamine, but not (+/-)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane, augmented the flinching response. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine3 and 5-hydroxytryptamine4 receptors in peripheral nociception. There may be some dissociation of nociception and edema formation, since no single 5-hydroxytryptamine receptor antagonist inhibited edema formation with 2.5% formalin; however, with 0.5% formalin, edema formation was enhanced by co-administration of 5-hydroxytryptamine, 5-carboxamidotryptamine, (+/-)-1-4-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane or 5-methoxytryptamine, but not 1-(m-chlorophenyl) biguanide. These data suggest involvement of 5-hydroxytryptamine1, 5-hydroxytryptamine2 and possibly 5-hydroxytryptamine4 receptors in edema formation. These results confirm the involvement of 5-hydroxytryptamine1 and 5-hydroxytryptamine3 receptor subtypes in peripheral nociception associated with acute inflammation and further suggest an involvement of the more recently characterized 5-hydroxytryptamine4 receptor in this process. There appears to be a dissociation in 5-hydroxytryptamine receptors involved in peripheral nociception and edema formation.

Antinociception by adenosine analogs and inhibitors of adenosine metabolism in an inflammatory thermal hyperalgesia model in the rat

&NA; The present study examined the spinal antinociceptive effects of adenosine analogs and inhibitors of adenosine kinase and adenosine deaminase in the carrageenan‐induced thermal hyperalgesia model in the rat. The possible enhancement of the antinociceptive effects of adenosine kinase inhibitors by an adenosine deaminase inhibitor also was investigated. Unilateral hindpaw inflammation was induced by an intraplantar injection of lambda carrageenan (2 mg/100 &mgr;l), which consistently produced significant paw swelling and thermal hyperalgesia. Drugs were administered intrathecally, either by acute percutaneous lumbar puncture (individual agents and combinations) or via an intrathecal catheter surgically implanted 7–10 days prior to drug testing (antagonist experiments). N6‐cyclohexyladenosine (CHA; adenosine A1 receptor agonist; 0.01–1 nmol), 2‐[p‐(2‐carboxyethyl)phenylethylamino]‐5′‐N‐ethylcarboxamidoadenosine (CGS21680; adenosine A2A receptor agonist; 0.1–10 nmol), 5′‐amino‐5′‐deoxyadenosine (NH2dAdo; adenosine kinase inhibitor; 10–300 nmol), and 5‐iodotubercidin (ITU; adenosine kinase inhibitor; 0.1–100 nmol) produced, to varying extents, dose‐dependent antinociception. No analgesia was seen following injection of 2′‐deoxycoformycin (dCF; an adenosine deaminase inhibitor; 100–300 nmol). Reversal of drug effects by caffeine (non‐selective adenosine A1/A2 receptor antagonist; 515 nmol) confirmed the involvement of the adenosine receptor, while antagonism by 8‐cyclopentyl‐1,3‐dimethylxanthine (CPT; adenosine A1 receptor antagonist; 242 nmol), but not 3,7‐dimethyl‐1‐propargylxanthine (DMPX; adenosine A2A receptor antagonist; 242 nmol), evidenced an adenosine A1 receptor mediated spinal antinociception by NH2dAdo. dCF (100 nmol), which was inactive by itself, enhanced the effects of 10 nmol and 30 nmol NH2dAdo. Enhancement of the antinociceptive effect of ITU by dCF was less pronounced. None of the antinociceptive drug regimens had any effect on paw swelling. These results demonstrate that both directly and indirectly acting adenosine agents, when administered spinally, produce antinociception through activation of spinal adenosine A1 receptors in an inflammatory model of thermal hyperalgesia. The spinal antinociceptive effects of selected adenosine kinase inhibitors can be significantly augmented when administered simultaneously with an adenosine deaminase inhibitor.

Peripheral antinociceptive action of amitriptyline in the rat formalin test: involvement of adenosine.

The present study determined (1) whether amitriptyline could produce a local antinociceptive action in the formalin test, (2) whether endogenous adenosine was involved in this action, and (3) which other systems might contribute to such an action. Coadministration of amitriptyline 10-100 nmol with 2.5% formalin produced a dose-related reduction in phase 1 (0-12 min) and phase 2 (16-60 min) flinching behaviours, as well as in phase 2 biting/licking time (no phase 1 expression). This action was not seen or only partially expressed at low concentrations of formalin (0.5%, 0.75%). Coadministration of caffeine with amitriptyline partially reversed the antinociceptive action of amitriptyline against both behaviours at 2.5% formalin. At 1.5% formalin, caffeine still produced only a partial reversal of effect; this appeared to be due to a block of adenosine A1 receptors, as it was also seen with the selective adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine. Using antagonists for a number of other systems, no evidence for an involvement of alpha-adrenergic, histamine, excitatory amino acid or opioid receptors in the action of amitriptyline was observed or inferred. A local anaesthetic action for amitriptyline remains a possibility for the residual action. These results indicate that amitriptyline can produce a local peripheral antinociceptive action which is mediated, in part, by an interaction with endogenous adenosine, most likely an inhibition of the cellular uptake of adenosine with a consequent activation of adenosine A1 receptors on sensory nerve terminals. Local application of amitriptyline by cream or gel might prove to be a useful method of drug delivery in inflammatory pain states.