Intrathecal adenosine does not relieve allodynia-like behavior in spinally injured rats

Validity of acute and chronic tactile sensory testing after spinal cord injury in rats

Spinal cord injury (SCI) impairs sensory systems causing allodynia. Measuring the development of allodynia in rodent models of SCI is challenging due to spinal shock and marked motor impairments. Assessment of SCI-induced allodynia is not standardized across labs, making interpretation of results difficult. Therefore, we validated sensory threshold assessment after SCI and developed a novel assessment of allodynia prior to motor recovery in a rat SCI model. One hundred fifty-six Sprague-Dawley rats received T8 laminectomy or mild to moderate SCI using the OSU SCI device (0.3 mm to 1.3 mm cord displacement). To determine tactile thresholds, von Frey hairs (VFH) were applied in Up-Down or ascending order to the dorsal or plantar hindpaw. The most efficient and valid procedures that maintain high sensitivity and specificity were identified. Ten Up-Down VFH applications yielded stable thresholds; reducing the risk of threshold decay and unnecessary exposure to painful stimuli. Importantly, distraction of SCI-rats with food revealed differential decay of thresholds than when distraction is not provided. The new test uses dorsal VFH stimulation and is independent of trunk or hindlimb control. Acute dorsal VFH thresholds collected before recovery of hindlimb weight support accurately predicted plantar VFH thresholds measured at late timepoints (chi(2)=8.479; p<0.05). Thus, standardized testing early after SCI using the dorsal VFH test or later using 10 stimuli in the Up-Down test produces valid measures of tactile sensation across many SCI severities. Early detection of allodynia in experimental SCI will allow identification of mechanisms responsible for pain development and determine targets for therapeutic interventions.

Interaction of endogenous ligands mediating antinociception

It is well known that a multitude of transmitters and receptors are involved in the nociceptive system, some of them increasing and others inhibiting the pain sensation both peripherally and centrally. These substances, which include neurotransmitters, hormones, etc., can modify the activity of nerves involved in the pain pathways. Furthermore, the organism itself can express very effective antinociception under different circumstances (e.g. stress), and, during such situations, the levels of various endogenous ligands change. A very exciting field of pain research relates to the roles of endogenous ligands. Most of them have been suggested to influence pain transmission, but only a few studies have been performed on the interactions of different endogenous ligands. This review focuses on the results of antinociceptive interactions after the co-administration of endogenous ligands. The data based on 55 situations reveal that the interactions between the endogenous ligands are very different, depending on the substances, the pain tests, the species of animals and the route of administrations. It is also revealed that only a few of the possible interactions between endogenous ligands have been investigated to date, in spite of the fact that the type of antinociceptive interaction between different endogenous ligands could hardly be predicted. The results indicate that the combination of endogenous ligands should not be omitted from the pain therapy arsenal. Attention will hopefully be drawn to the complex interdependence of endogenous ligands and their potential use in clinical practice.

The Antinociceptive Potencies and Interactions of Endogenous Ligands During Continuous Intrathecal Administration: Adenosine, Agmatine, and Endomorphin-1

Recently, a series of endogenous ligands related to inhibition of sensory transduction of noxious stimuli at the spinal level has been described, including endomorphins, agmatine, and adenosine, which act on different receptors; however, little data exist concerning their effect during continuous administration or their interactions. In this study, we investigated the antinociceptive properties of continuously administered (for 60 min) adenosine and agmatine on carrageenan-induced thermal hyperalgesia by means of a thermal paw withdrawal test in awake rats. The possible interaction between endomorphin-1 and adenosine or agmatine was also determined. Continuous administration of adenosine (0.3-3 microg/min) did not influence the paw withdrawal latencies of the normal or inflamed paws during the infusion but in larger doses it resulted in a significant increase in latencies after the cessation of the infusion. Agmatine (0.3-3 microg/min) dose-dependently decreased the hyperalgesia, but the largest dose caused a temporary excitation in 50% of animals. The continuous administration of adenosine or agmatine (3 micro g/min) potentiated and prolonged the antinociceptive effect of endomorphin-1 (1 microg/min). Our results revealed that adenosine and agmatine have a small antinociceptive efficacy during continuous intrathecal administration but that both potentiate the effect of endomorphin-1. These data suggest that the combination of these endogenous ligands might represent novel targets for the therapeutic modulation of pain; however, the systematic examination of side effects is essential.

IMPLICATIONS

Adenosine and agmatine have little antinociceptive efficacy during continuous intrathecal administration, as shown by the inflammatory pain test in rats, but both potentiate the effect of endomorphin-1. These data suggest that the combination of these endogenous ligands might represent novel targets for the therapeutic modulation of pain; however, the systematic examination of side effects is essential.

Allosteric adenosine receptor modulation reduces hypersensitivity following peripheral inflammation by a central mechanism

Activation of adenosine A1 receptors by endogenous adenosine or synthetic agonists produces antinociception in animal models of acute pain and also reduces hypersensitivity in models of inflammatory and nerve-injury pain. Allosteric adenosine modulators facilitate and potentiate the action of adenosine agonists at the A1 receptors. The purpose of the current study was to examine the effect and site of action for an allosteric adenosine modulator, T62 [2-amino-3-(4-chlorobenzoyl)-5,6, 7,8-tetrahydrobenzothiophene], in rat models of acute pain and inflammation. Intrathecal (i.t.) T62 did not change the withdrawal latency or threshold of normal rats to acute heat or to acute paw pressure. In contrast, i.t. T62 reversed thermal hypersensitivity in carrageenin-inflamed rats. Subcutaneous (s.c.) injection of T62 into the inflamed paw had no such effect. To investigate a potential site of action on nociceptors, single-unit afferent activity to mechanical stimuli on A delta- and C-fibers was examined in normal or carrageenin-inflamed rats before and after intravenous (i.v.) T62 administration. Intravenous T62, 3 mg/kg, had no significant effect in either normal or inflamed conditions. These results support previous studies to suggest that adenosine receptor modulators lack efficacy to acute nociceptive stimuli in the normal condition, but reduce hypersensitivity during inflammation through a central mechanism.

Spinal noradrenergic activation mediates allodynia reduction from an allosteric adenosine modulator in a rat model of neuropathic pain

Activation of adenosine A1 receptors by endogenous adenosine or synthetic agonists produces anti-nociception in animal models of acute pain and also reduces hypersensitivity in models of inflammatory and nerve-injury pain. Allosteric adenosine modulators facilitate adenosine agonist binding to the A1 receptor. The purpose of the current study was to examine the effect, mechanisms of action, and interaction with noradrenergic systems of intrathecal (i.t.) or oral administration of the allosteric adenosine modulator T62 in a rat model of neuropathic pain. A spinal nerve ligation rat model (SNL; ligation of left L5 and L6 spinal nerve roots) was used. One week after SNL surgery, an i.t. catheter was inserted. Withdrawal threshold to mechanical stimulation of the left hind paw was determined before and after surgery, confirming mechanical hypersensitivity. Oral or i.t. T62 reduced hypersensitivity induced by SNL. The effects of i.t. T62 were inhibited by i.t. injection of an A1 receptor antagonist and by an 2-adrenergic antagonist but not by an A2 adenosine receptor antagonist. Anti-dopamine hydroxylase (DH)-saporin treatment reduce spinal norepinephrine content by 97%, accompanied by an almost complete loss of DH immunoreactive axons in the spinal dorsal horn and neurons in the locus coeruleus. The effect of T62 was completely lost in animals treated with anti-DH-saporin. These data support the hypothesis that activation of the A1 receptor by the allosteric modulator, T62, produces anti-nociception via spinal noradrenergic activation.

Effect of chronic and acute stress on ectonucleotidase activities in spinal cord

We have previously observed that, while acute stress induces analgesia, chronic stress causes a hyperalgesic response in male rats. No effect was observed in females. There is increasing evidence that both ATP and adenosine can modulate pain. Extracellular ATP and ADP are hydrolyzed by an apyrase in synaptosomes from the peripheral and central nervous systems. In the present study, we investigated the effect of chronic and acute stress on ATPase-ADPase and 5'-nucleotidase activities in spinal cord of male and female rats. Adult male and female Wistar rats were submitted to 1 h restraint stress/day for 1 day (acute) or 40 days (chronic) and were sacrificed 24 h later. ATPase-ADPase activities were assayed in the synaptosomal fraction obtained from the spinal cord of control and stressed animals. ADP hydrolysis was decreased 25% in chronically stressed males, while no change was observed on ATPase activity. There was an increase in the 5'-nucleotidase activity in the same group. No effect on ADPase, ATPase or on 5'-nucleotidase activity was observed in females with chronic stress, or after acute stress neither in males or females. Chronic stress reduced ADP hydrolysis and increased 5'-nucleotidase activity in the spinal cord in male rats.

Effects of intrathecal morphine, baclofen, clonidine and R-PIA on the acute allodynia-like behaviours after spinal cord ischaemia in rats

The present study assessed the efficacy and potency of intrathecal (i.t.) administration of the opiate morphine, the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, the alpha2-adrenoceptor agonist clonidine and the adenosine A1-receptor agonist R-phenylisopropyl-adenosine (R-PIA) on the acute allodynia-like behaviour after photochemically induced spinal cord injury (SCI) in rats. Rats displaying allodynia-like behaviours to brushing, von Frey hairs and cold stimulation 1-2 days after photochemically induced SCI were studied. In a cumulative dose regime, morphine (0.1-10 micrcog), baclofen (0.1-1 microg), clonidine (0.1-10 microg) and R-PIA (0.01-10 nmol) were administered i.t. through an implanted catheter at the lumbar spinal cord. All tested drugs dose-dependently reduced the brushing, von Frey hairs and cold stimulation-induced allodynia-like behaviour. No increase in adverse effects such as motor deficits was found for morphine, clonidine and R-PIA. There was a slight increase in motor impairments at the highest dose of baclofen. For the mechanical allodynia, morphine appeared to be most effective, whereas baclofen, clonidine and R-PIA only provided a partial alleviation. For the cold allodynia, morphine and baclofen were more effective than clonidine and R-PIA. In relieving acute mechanical and cold allodynia-like behaviours in rats 1-2 days after SCI, i.t. morphine and baclofen were superior to clonidine and R-PIA.

The effect of ABT-702, a novel adenosine kinase inhibitor, on the responses of spinal neurones following carrageenan inflammation and peripheral nerve injury

1. Adenosine (ADO) receptor activation modulates sensory transmission in the dorsal horn. Little is known about the circumstances underlying release of the purine. The present study was conducted to investigate the effect of a novel and potent non-nucleoside adenosine kinase (AK) inhibitor, ABT-702, on the responses of dorsal horn neurones to selected peripheral stimuli. ABT-702 is orally effective to reduce behavioural signs of nociception in models of acute, inflammatory, and neuropathic pain. 2. Electrophysiological recordings were made from wide dynamic range (WDR) neurones in halothane-anaesthetized rats. ABT-702 was given subcutaneously following either carrageenan inflammation or peripheral nerve injury (L5/L6 spinal nerve ligation). Comparisons were made between carrageenan and uninjected control animals, and similarly between spinal nerve ligated (SNL) and sham operated animals. 3. ABT-702 produced inhibition of the postdischarge, wind-up and C-fibre evoked responses in both carrageenan and nerve-injured animals. Furthermore, the mechanical and thermal evoked responses were similarly reduced in SNL rats. Overall, ABT-702 produced a significantly greater inhibition of these responses in SNL rats as compared to sham controls. Similarly ABT-702 tended to produce greater effects after carrageenan inflammation, however this did not reach significance. 4. Protection of endogenous adenosine by ABT-702 therefore produces a marked inhibition of the noxious evoked neuronal activity in inflamed and neuropathic rats. Our results demonstrate a plasticity in the endogenous adenosine-mediated inhibitory system following SNL and provide a possible basis for the use of this compound for the treatment of neuropathic and other persistent pain states.

Marked enhancement of anti-allodynic effect by combined intrathecal administration of the adenosine A1-receptor agonist R-phenylisopropyladenosine and morphine in a rat model of central pain

BACKGROUND

There is often no satisfactory treatment for chronic pain after spinal cord injury. We have previously reported that intrathecal (i.t.) administration of the adenosine A1-receptor agonist R-phenylisopropyl-adenosine (R-PIA) or the opioid morphine has anti-allodynic effects in a model of presumed chronic central pain after photochemically induced spinal cord injury in rats. In the present study, we set out to investigate the possible interaction between i.t. R-PIA and morphine in spinally injured rats.

METHODS

Sprague-Dawley rats displaying allodynia-like behaviors to mechanical and cold stimuli after photochemically induced spinal cord injury with minor motor deficits were used. R-PIA and morphine, either alone or in combination, were administered i.t. through an implanted catheter to lumbar spinal cord.

RESULTS

Cumulative doses of R-PIA or morphine dose-dependently reduced the mechanical allodynia-like behavior, with a threshold of 1 nmol and 1.5 nmol, respectively. When co-administrated, R-PIA and morphine produced marked suppression of mechanical allodynia at doses of 5 pmol and 7.5 pmol, respectively. The effect of i.t. co-administration of R-PIA and morphine on cold allodynia was comparable to i.t. R-PIA alone. The combination of R-PIA and morphine did not increase adverse effects such as motor deficits in comparison to either drug alone.

CONCLUSION

These results demonstrate a supra-additive interaction between the adenosine A1-receptor agonist R-PIA and morphine to reduce mechanical allodynia-like behavior in rats with chronic spinal cord injury. The combination of R-PIA and morphine administered spinally may be superior to R-PIA or morphine alone for treating such pain.