Tail-flick Response Research Articles

The antinociceptive effects of endomorphin-1 and endomorphin-2 in diabetic mice

The antinociceptive effects of endomorphin-1 and endomorphin-2, endogenous μ-opioid receptor agonists, were examined using the tail-flick test in non-diabetic and diabetic mice. Endomorphin-1, at doses of 1 to 10 μg, i.c.v., and endomorphin-2, at doses of 3 to 30 μg, i.c.v., each dose dependently inhibited the tail-flick response in both non-diabetic and diabetic mice. There was no significant difference between the antinociceptive effects of endomorphin-1 in non-diabetic mice and diabetic mice. The antinociceptive effect of endomorphin-2 was greater in non-diabetic mice than in diabetic mice. In non-diabetic mice, the antinociceptive effects of endomorphin-1 and endomorphin-2 were significantly reduced by β-funaltrexamine, a μ-opioid receptor antagonist, and naloxonazine, a selective μ 1-opioid receptor antagonist, but not by naltrindole, a δ-opioid receptor antagonist, or nor-binaltorphimine, a κ-opioid receptor antagonist. In diabetic mice, the antinociceptive effect of endomorphin-2 was significantly reduced by β-funaltrexamine and naloxonazine. However, these μ-opioid receptor antagonists had no significant effect on the antinociceptive effect of endomorphin-1 in diabetic mice. The antinociception induced by endomorphin-1 in diabetic mice was significantly reduced by naltrindole and 7-benzylidenenaltrexon, a selective δ 1-opioid receptor antagonist, administered i.c.v. However, nor-binaltorphimine had no significant effect on the antinociceptive effects of endomorphin-1 and endomorphin-2 in diabetic mice. These results indicate that the antinociceptive effects of endomorphin-1 and endomorphin-2 in non-diabetic mice are mediated through the activation of μ 1-opioid receptors, whereas in diabetic mice, endomorphin-1 and endomorphin-2 may produce antinociception through different actions at δ 1- and μ 1-opioid receptors, respectively.

Strain Differences in the Antinociceptive Effect of Nitrous Oxide on the Tail Flick Test in Rats

To study strain differences in antinociceptive effects of nitrous oxide (N2O), we examined various outbred and inbred stains of rats by using tail flick latency response. All outbred strains, i.e., Sprague-Dawley from two different breeders, Wistar, and Long-Evans, showed a similar antinociceptive response. Namely, the peak response occurred after 30 min of exposure, and tolerance to N2O developed within 60 to 90 min. Each of the four inbred stains examined, i.e., Wistar-Kyoto, Brown-Norway, Fischer, and Lewis, displayed a unique pattern of antinociceptive response to N2O. Wistar-Kyoto and Brown-Norway strains showed somewhat similar patterns as those observed in outbred strains, apart from the fact that the Wistar-Kyoto displayed a more distinct development of tolerance, whereas, the Brown-Norway strain had a lower peak effect. The Fischer strain displayed the greatest antinociceptive response to N2O, and did not develop tolerance. The Lewis strain showed no antinociceptive response to N2O. These results indicate differences in the durability and the magnitude of the antinociceptive response to N2O among various strains of rats. Because of the variability that already exists, we recommend that animal studies examining the antinociceptive effects of nitrous oxide should be performed on inbred rat strains.

Pain relief caused by millimeter waves in mice: results of cold water tail flick tests

Purpose : To find out if millimeter waves can decrease experimental pain response in mice using cold water tail flick test. Materials and methods : Male Swiss albino mice (15 mice per group) were exposed to continuous millimeter waves at a frequency of 61.22 GHz with incident power densities (IPD) ranging from 0.15 to 5.0 mW/cm 2 for 15 min or sham exposed. Latency of tail withdrawal in a cold water (1 +/- 0.5°C) tail flick test was measured before the exposure (baseline) and then four times after the exposure with 15 min breaks. Results : The mean latency of the tail flick response in mice exposed to millimeter waves was more than twice that of sham-exposed controls (p < 0.05). This effect was proportional to the power of millimeter waves and completely disappeared at an IPD level of 0.5 mW/cm 2. Pretreatment of mice with the opioid antagonist naloxone (1 mg/kg i.p.) blocked the effect of millimeter waves. Conclusions : Results suggest that the antinociceptive effect of millimeter waves is mediated through endogenous opioids.

A peripheral, intracerebral, or intrathecal administration of an opioid receptor antagonist blocks illness-induced hyperalgesia in the rat.

We used the tail-flick response of rats to study the role of opioid receptors in illness-induced hyperalgesia. An intraperitoneal injection of lithium chloride (LiCl) produced hyperalgesia that was blocked in a dose-dependent manner by subcutaneous injection of the opioid antagonist naloxone. Neither hyperalgesia nor its blockade by naloxone were due to variations in tail-skin temperature induced by LiCl. Hyperalgesia was also blocked when opioid receptor antagonism was restricted to (a) the periphery, by intraperitoneal administration of the quaternary opioid receptor antagonist naloxone methiodide; (b) the brain, by intracerebroventricular microinjection of naloxone; or (c) the spinal cord, by intrathecal microinjection of naloxone. These results document a pain facilitatory role of opioid receptors in both the peripheral and central nervous systems and are discussed with reference to their analgesic and motivational functions.

Morphine antinociception is enhanced in mdr1a gene-deficient mice.

Previous studies have suggested that P-glycoprotein (P-gp) modulates opioid antinociception for selected mu-and delta-agonists. This study was undertaken to assess morphine antinociception in mice lacking the mdr1a gene for expression of P-gp in the CNS. Morphine (n = 4-5/group) was administered as a single s.c. dose to mdr1a(-/-) mice (3-5 mg/kg) or wild-type FVB controls (8-10 mg/kg). Tail-flick response to radiant heat, expressed as percent of maximum response (%MPR), was used to determine the antinociceptive effect of morphine. Concentrations in serum, brain tissue, and spinal cord samples obtained immediately after the tail-flick test were determined by HPLC with fluorescence detection. Parallel experiments with R(+)-verapamil, a chemical inhibitor of P-gp, also were performed to further investigate the effect of P-gp on morphine-associated antinociception. Morphine-associated antinociception was increased significantly in the mdr1a(-/-) mice. The ED50 for morphine was > 2-fold lower in mdr1a(-/-) (3.8+/-0.2 mg/kg) compared to FVB (8.8+/-0.2 mg/kg) mice. However, the EC50 derived from the brain tissue was similar between the two mouse strains (295 ng/g vs. 371 ng/g). Pretreatment with R(+)-verapamil produced changes similar to those observed in gene-deficient mice. P-gp does not appear to affect morphine distribution between spinal cord and blood, as the spinal cord:serum morphine concentration ratio was similar between gene-deficient and wild-type mice (0.47+/-0.03 vs. 0.56+/-0.04, p>0.05). The results of this study are consistent with the hypothesis that P-gp attenuates the antinociceptive action of morphine by limiting the brain:blood partitioning of the opioid.

Differential potentiative effects of gaba receptor agonists in the production of antinociception induced by morphine and β-endorphin administered intrathecally in the mouse

The effect of muscimol or baclofen injected intrathecally (i.t.) on the inhibition of the tail-flick response induced by morphine and β-endorphin administered i.t. Was studied in ICR mice. The i.t. Injection of muscimol (100 ng) or baclofen (10 ng) alone did not affect the basal inhibition of the tail-flick response. Morphine (0.2 μg) and β-endorphin (0.1 μg) caused only slight inhibition of the tail-flick response. Baclofen, but not muscimol, injected i.t. enhanced the inhibition of the tail-flick response induced by i.t. Administered morphine. Both muscimol and baclofen injected i.t. Significantly enhanced i.t. Injected β-endorphin-induced inhibition of the tail-flick response. Our results suggest that the GABA B, but not GABA A, receptors located in the spinal cord appear to be involved in enhancing the inhibition of the tail-flick response induced by morphine administered spinally. In addition, both GABA A and GABA B receptors are involved in enhancing the inhibition of the tail-flick response induced by β-endorphin administered i.t.

The modulatory effect of (+)-TAN-67 on the antinociceptive effects of the nociceptin/orphanin FQ in mice

To clarify the pharmacological properties of (+)2-Methyl-4 aα-(3-hydroxyphenyl)-1, 2, 3, 4, 4 a, 5, 12, 12 aα-octahydro-quinolino[2, 3, 3- g]isoquinoline ((+)-TAN-67), the effect of (+)-TAN-67 on the antinociception induced by the intrathecal (i.t.) administration of nociceptin/orphanin FQ was studied in mice using the tail-flick test and the formalin test. I.t. administration of (+)-TAN-67, at doses of 1 to 10 ng, facilitated the tail-flick response in a dose-dependent manner in mice. In addition, i.t. administration of (+)-TAN-67 (1 to 10 ng) in mice produced a marked pain-like aversive responses. I.t. pretreatment with d-Pro 9-[spiro-γ-lactam]-Leu 10-Trp 11-physalaemin(1–11) (GR82334, 0.1–1.0 nmol), a potent and selective tachykinin NK 1 receptor antagonist, dose-dependently blocked the reduction of the tail-flick response induced by (+)-TAN-67. Furthermore, (+)-TAN-67-induced facilitation of the tail-flick response was abolished in capsaicin-treated mice. On the other hand, (+)-TAN-67-induced flinching responses were dose-dependently and significantly reduced by i.t. pretreatment with GR82334 (0.1–1.0 nmol). The duration of i.t. (+)-TAN-67-induced flinching responses was significantly reduced in capsaicin-treated mice as compared with naive mice. I.t. administration of nociceptin/orphanin FQ (1–10 nmol) dose-dependently increased the tail-flick latency. I.t. administration of nociceptin/orphanin FQ (0.1–1.0 nmol) significantly and dose-dependently reduced the first-phase nociceptive response, but not the second-phase nociceptive response. I.t. pretreatment with (+)-TAN-67 (0.3–3.0 μg) for 30 min dose-dependently attenuated the antinociception induced by i.t. nociceptin (10 nmol) in the tail-flick test. Furthermore, the antinociceptive effect of nociceptin/orphanin FQ (1 nmol, i.t.) on the first-phase response in the formalin test was dose-dependently attenuated by s.c. pretreatment with (+)-TAN-67 (0.3–3.0 μg). (+)-TAN-67 (0.3–3.0 μg, i.t.), by itself, did not facilitate the tail-flick response or produce apparent behavioral changes. It is possible that (+)-TAN-67 has an antagonistic effect on nociceptin/orphanin FQ-induced antinociception.

Long-lasting antinociceptive effect of DAMGO chloromethyl ketone in rats

Previously, the opioid peptide Tyr- d-Ala-Gly-(NMe)Phe-CH 2Cl (DAMCK) has been shown to bind irreversibly to mu opioid receptors in vitro. In the present work, the antinociceptive effect of DAMCK has been evaluated. Rats treated systemically with DAMCK (1–100 pg/kg) displayed a dose-dependent increase in tail-flick analgesia that peaked by 15 min, then stayed about the same until 60 min, followed by some decrease over time. Higher doses of DAMCK (10 ng/kg–100 μg/kg) produced a near-maximal antinociceptive effect that remained stable for 4 h. Significant antinociception was still detected 8 h, but not 24 h postinjection. In comparison, the parent peptide DAMGO (Tyr- d-Ala-Gly-(NMe)Phe-Gly-ol) reached maximal effect by about 30 min, followed by a rapid cessation of its antinociceptive response. Naloxone administered before DAMCK antagonized the antinociceptive response of DAMCK, indicating that it was mediated via opioid receptors. Naloxone administered 45 min after DAMCK attenuated the tail-flick response to some extent, but a substantial part (40–60% depending on the peptide concentration and evaluation time) remained unaffected. Central administration of DAMCK also elicited time- and concentration-dependent, profound, opioid receptor mediated, apparently irreversible antinociception.

Studies on the anti-inflammatory and analgesic activity of Curatella americana L.

Curatella americana L. (Dilleneaceae) popularly known as ‘cajueiro-bravo’ and ‘sambaı́ba’ is used in Brazilian folk medicine for the treatment of inflammation and ulcer. The anti-inflammatory and analgesic tests were conducted with the hydroalcoholic extract (HAE) of the bark of the plant. The HAE inhibited mouse ear oedema induced by o-tetradecanoyl phorbol acetate (TPA) and by capsaicin. While the ID 50 values obtained for the HAE against these two irritants were 40.8±1.7 and 30±1.2 mg/kg i.p. (mean±S.E.M., n=6), respectively, the corresponding value for carrageenan induced paw oedema (3 h) was 21.8±2.1 mg/kg, i.p., n=6. In the established adjuvant-induced arthritis model, the HAE significantly inhibited the oedema in daily doses of 50 mg/kg, i.p. ( n=10). The HAE also inhibited acetic acid-induced writhing (ID 50 23.2±0.8 mg/kg, i.p., n=6) and the formalin-induced late phase paw licking response (ID 50 11.9±1.2 mg/kg, i.p., n=10) in the mice. However, the HAE was inactive in the formalin-induced initial paw licking response in mice or heat induced tail flick response in rats. The HAE has shown both anti-inflammatory and peripheral analgesic activities when administrated in the mouse by the intraperitoneal route in doses which are at least 12 times lower than its LD 50 dose of 647 mg/kg, i.p.

Experimental Realization of a Signal Transduction Algorithm

A theory of signal transduction specifies that in biological systems, any instantaneous input is appreciated by its departure from the mean past activity that occurred over a certain time window called the sample period; it also specifies that any input generates two so-called first- and second-order effects that are opposite in sign. We here report the detailed experimental realization of the algorithm that formalizes this signal transduction process; numerical simulations adequately predicted various intricate features of the (first order) analgesia and the paradoxical (second order) hyperalgesia which morphine produces. The data also offer a first estimation of the physiological sample period that may govern the classical rat tail flick response. The signal transduction process appears to operate ubiquitously and provides an unprecedented account of the paradoxical effects that have been observed with different signalling systems. This process may also operate with such phenomena as refractoriness, homeostasis, adaptation, sensitization, dependence, tolerance or resistance and neuronal plasticity.

The role of excitatory amino acid transmission within the rostral ventromedial medulla in the antinociceptive actions of systemically administered morphine

Two classes of neurons with distinct responses to opioids have been identified in the rostral ventromedial medulla (RVM), a region with a well-documented role in nociceptive modulation. `Off-cells' are activated, indirectly, by opioids, and are likely to exert a net inhibitory effect on nociceptive processing. `On-cells' are directly inhibited by opioids, and there is evidence that these neurons can, under various conditions, facilitate nociception. We showed previously that excitatory amino acid (EAA) neurotransmission is crucial to the nocifensor reflex-related on-cell burst, but plays little role in maintaining the ongoing activity of off-cells. The aim of the present study was to determine whether EAA transmission contributes to the activation of off-cells and the concomitant behavioral antinociception that follow systemic opioid administration. The non-selective EAA receptor antagonist kynurenate was infused into the RVM (1 nmol/200 nl) of lightly anesthetized rats prior to administration of morphine (1.5 mg/kg i.v). Off-cell, on-cell and neutral cell firing, as well as, tail flick response (TF) latencies were recorded. Kynurenate, significantly attenuated the characteristic opioid activation of off-cells. As a group, off-cells in kynurenate-treated animals did not become continuously active, and continued to exhibit tail-flick related pauses in firing. On-cell and neutral cell responses to opioid administration were unchanged. Opioid inhibition of the TF was also reduced, although baseline TF latency was unaffected, by RVM kynurenate. EAA-mediated activation of off-cells, thus has an important role in opioid analgesia. The present observations underscore the importance of excitatory interactions among opioid-sensitive nociceptive modulatory circuits for systemic morphine analgesia, suggesting that such interactions are a critical factor in the synergistic relationships which have been demonstrated among these sites.

Possible involvement of spinal protein kinase C in thermal allodynia and hyperalgesia in diabetic mice

We examined the tail-flick response to various heat intensities in diabetic and non-diabetic mice. Heat intensities were set to one of five values by adjusting the source voltage of a 50-W projection bulb to 25, 35, 50, 65 and 80 V. These heat intensities produced surface skin heating rates of 0.1, 0.4, 0.9, 3.0 and 7.3°C/s, respectively. Tail-flick latencies at source voltages of 35 and 50 V in diabetic mice were significantly shorter than those in non-diabetic mice. However, there were no significant differences in tail-flick latencies at 25, 65 and 80 V. In non-diabetic mice, tail-flick latencies were not affected by intrathecal (i.t.) pretreatment with capsaicin 24 h before testing. Tail-flick latencies at 35 and 50 V in diabetic mice were increased by pretreatment with capsaicin. Moreover, although tail-flick latencies in non-diabetic mice were not affected by i.t. pretreatment with calphostin C, a selective protein kinase C inhibitor, those at 35 and 50 V in diabetic mice were increased. However, i.t. pretreatment with (8 R, 9 S, 11 S)-(−)-9-hydroxy-9- n-hexyloxy-carbonyl-8-methyl-2, 3, 9, 10-tetrahydro-8, 11-epoxy-1 H, 8 H, 11 H-2, 7 b, 11 a-triazadibenzo [ a, g]cycloocta[ cde]-trinden-1-one (KT5720), a selective protein kinase A inhibitor, did not affect tail-flick latencies in either diabetic or non-diabetic mice. In non-diabetic mice, i.t. pretreatment with phorbol 12,13-dibutyrate (PDB), a protein kinase C activator, decreased tail-flick latencies at 35 and 50 V. Tail-flick latencies in diabetic mice were not affected by i.t. pretreatment with PDB 60 min before testing. Furthermore, the attenuation of tail-flick latencies induced by i.t. pretreatment with PDB in non-diabetic mice was reversed by i.t. pretreatment with capsaicin 24 h before testing. These results indicate that diabetic mice exhibit thermal allodynia and hyperalgesia. Furthermore, this thermal allodynia and hyperalgesia in diabetic mice may be due to the enhanced release of substance P followed by activation of protein kinase C in the spinal cord.

Effects of histamine receptor antagonists injected intrathecally on antinociception induced by opioids administered intracerebroventricularly in the mouse

The present study was designed to investigate the modulatory effects of blockade of spinal histamine receptors on antinociception induced by supraspinally administered μ-, ε-, δ-, and κ-opioid receptor agonists. The effects of intrathecal (i.t.) injections with cyproheptadine [a histamine-1 (H1) receptor antagonist], ranitidine (a H2 receptor antagonist), or thioperamide (a H3 receptor antagonist) injected i.t., on the antinociception induced by morphine (a μ-receptor antagonist), β-endorphin (an ε-receptor agonist), D-Pen2,5-enkephalin (DPDPE, a δ-receptor agonist) or trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohxyl] benzeocetamide (U50,488H, a κ-receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. The antinociception was assayed using the tail-flick test. The i.t. injection of cyproheptadine (from 0.31 to 62 nmole), ranitidine (from 0.28 to 56 nmole), or thioperamide (from 0.24 to 48 nmole) alone did not show any antinociceptive effect. The i.t. pretreatment with cyproheptadine or thioperamide dose-dependently attenuated the inhibition of the tail-flick response induced by i.c.v. administered morphine (0.6 nmole), b-endorphin (0.03 nmole), DPDPE (1.5 nmole), and U50,488H (130 nmole). In addition, the i.t. pretreatment with ranitidine dose-dependently attenuated the inhibition of the tail-flick response induced by morphine, b-endorphin and U50,488H without affecting DPDPE-induced response. Our results suggest that spinal histamine H1 and H3 receptors may involved in the production of antinociception induced by supraspinally applied morphine, b-endorphin, DPDPE and U50,488H. Spinal H2 receptors appear to be involved in supraspinally administered morphine, b-endorphin- and U50,488H-induced antinociception but not DPDPE-induced antinociception.

Effects of ginsenosides injected intrathecally or intracerebroventricularly on antinociception induced byβ-endorphin administered intracerebroventricularly in the mouse

The effect of total saponin fraction of ginseng injected intrathecally (i.t.) or intracerebroventricularly (i.c.v.) on the antinociception induced by β-endorphin administered i.c.v. was studied in ICR mice in the present study. The antinociception was assessed by the tail-flick test. Total saponin fraction at doses 0.1 to 1.0 μg, which administered i.t. alone did not affect the latencies of tail-flick threshold, attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered β-endorphin (1 μg). However, total saponin fraction at doses 1 to 20 μg, which administered i.c.v. alone did not affect the latencies of the tail-flick response, did not affect i.c.v. administered β-endorphiun (1 μg)-induced antinociception. The duration of antagonistic action of total saponin fraction against β-endorphin-induced antinociception lasted at least for 6 h. Various doses (from 0.1 to 1 μg) of ginsenoside Rc, but not Rb2, Rd, Rg1, Rb1and Reinjected i.t. dose-dependently attenuated antinociception induced by β-endorphin administered i.c.v. Our results indicate that total saponin fraction injected spinally appears to have antagonistic action against the antinociception induced by supraspinally applied β-endorphin. Ginsenoside Rcappears to be responsible for blocking i.c.v. administered β-endorphin-induced antinociception. On the other hand, total ginseng fraction, at supraspinal sites, may not exert an antagonistic action against the antinociception induced by supraspinally administered β-endorphin.

Antinociceptive effects of the ORL1 receptor agonist nociceptin/orphanin FQ in diabetic mice

The antinociceptive potency of nociceptin/orphanin FQ, an opioid-like orphan receptor agonist, was examined using the tail-flick test and the formalin-induced nociception test in diabetic mice. Nociceptin/orphanin FQ, at doses of 0.1 to 10 nmol, intrathecal (i.t.), produced a marked and dose-dependent inhibition of the tail-flick response in both non-diabetic and diabetic mice. The antinociceptive effect of nociceptin/orphanin FQ in the tail-flick test in diabetic mice was greater than that in non-diabetic mice. The antinociceptive effect of nociceptin/orphanin FQ was not antagonized by pretreatment with either β-funaltrexamine, a selective μ-opioid receptor antagonist, naltrindole, a selective δ-opioid receptor antagonist, or nor-binaltorphimine, a selective κ-opioid receptor antagonist. The antinociceptive effects of nociceptin/orphanin FQ in diabetic, but not in non-diabetic mice, were abolished when mice were pretreated with capsaicin i.t. 24 h before testing. In the formalin test, nociceptin/orphanin FQ also produced a marked and dose-dependent antinociceptive effect on the first-phase response, but not the second phase-response, in both diabetic and non-diabetic mice. Furthermore, nociceptin/orphanin FQ significantly and dose-dependently reduced the flinching responses to i.t.-administered substance P in diabetic mice, but not in non-diabetic mice. The results of the present experiments clearly indicate that the antinociceptive potency of nociceptin/orphanin FQ is significantly greater in diabetic mice than in non-diabetic mice. Furthermore, the results of this study suggest that the reduction of substance P-mediated nociceptive transmission in the spinal cord may be responsible for the antinociceptive effect of nociceptin/orphanin FQ.

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.

Spinal opioid mu receptor expression in lumbar spinal cord of rats following nerve injury

Previous studies in rats have shown that spinal morphine loses potency and efficacy to suppress an acute nociceptive stimulus applied to the tail or the paw following injury to peripheral nerves by tight ligation of the L5/L6 spinal nerves. Additionally, intrathecal (i.th.) morphine is ineffective in suppressing tactile allodynia at fully antinociceptive doses in these animals. The molecular basis for this loss of morphine potency and efficacy in nerve injury states is not known. One possible explanation for this phenomenon is a generalized, multi-segmental loss of opioid mu ( μ) receptors in the dorsal horn of the spinal cord after nerve injury. This hypothesis was tested here by determining whether nerve injury produces (a) a decrease in μ receptors in the lumbar spinal cord; (b) a decrease in the affinity of ligand–receptor interaction, (c) a decrease in the fraction of high-affinity state of the μ receptors and (d) a reduced ability of morphine to activate G-proteins via μ receptors. Lumbar spinal cord tissues were examined 7 days after the nerve injury, a time when stable allodynia was observed. At this point, no differences were observed in the receptor density or affinity of [ 3 H] DAMGO ( μ selective agonist) or [ 3 H] CTAP ( μ selective antagonist) in the dorsal quadrant of lumbar spinal cord ipsilateral to nerve injury. Additionally, no change in morphine's potency and efficacy in activating G-proteins was observed. In contrast, staining for μ opioid receptors using μ-selective antibodies revealed a discrete loss of μ opioid receptors localized ipsilateral to the nerve injury and specific for sections taken at the L6 level. At these spinal segments, μ opioid receptors were decreased in laminae I and II. The data indicate that the loss of μ opioid receptors are highly localized and may contribute to the loss of morphine activity involving input at these spinal segments (e.g., foot-flick response). On the other hand, the lack of a generalized loss of opioid μ receptors across spinal segments makes it unlikely that this is the primary cause for the loss of potency and efficacy of μ opioids to suppress multi-segmental reflexes, such as the tail-flick response.

Chronic desipramine treatment desensitizes the rat to anesthetic and antinociceptive effects of the alpha2-adrenergic agonist dexmedetomidine.

The effects of long-term administration of the tricyclic antidepressant agent desipramine on the hypnotic, antinociceptive, anesthetic-sparing, and central norepinephrine turnover suppressant action of short-term dexmedetomidine, a highly selective alpha2-adrenergic agonist, were studied in rats. Rats were given a 3- or 4-week course of twice daily administration of desipramine, 10 mg/kg, or saline. The effect of a hypnotic dose of dexmedetomidine, 250 microg/kg given intraperitoneally, on the duration of loss of righting reflex was determined. The tail flick latency response was determined before and after 50 microg/kg dexmedetomidine. The minimum anesthetic concentration of halothane and the central norepinephrine turnover rate were determined before and after administration of 30 microg/kg dexmedetomidine. Changes in the affinity and density of the alpha2-adrenergic receptor in locus coeruleus and spinal cord also were determined. Treatment with desipramine decreased dexmedetomidine-induced loss of righting reflex duration by 67% and eliminated the antinociceptive effect of dexmedetomidine. Dexmedetomidine produced a 55% decrease in minimum anesthetic concentration in the control group but no reduction in desipramine-treated rats. Desipramine did not change the receptor density or binding affinity of alpha2 receptors at the site for hypnotic (locus coeruleus) or antinociceptive (spinal cord) responses. No decrement in the central norepinephrine turnover rate was noted in the locus coeruleus of dexmedetomidine after 3 weeks of treatment with desipramine. The alpha1-adrenergic antagonist prazosin at 1 or 5 mg/kg completely (minimum anesthetic concentration reduction), almost completely (antinociceptive), or partially (hypnotic) restored responsiveness to normal. These data indicate that treatment with desipramine induces hyporesponsiveness to the hypnotic, analgesic, and minimum anesthetic concentration-reducing, but not to the suppression of central norepinephrine turnover, properties of dexmedetomidine. The hyporesponsiveness appears to involve an alpha1-adrenergic mechanism.

Antinociception produced by microinjection of morphine in the rat periaqueductal gray is enhanced in the foot, but not the tail, by intrathecal injection of α1-adrenoceptor antagonists

Antinociception produced by microinjection of morphine in the ventrolateral periaqueductal gray is mediated in part by α 2-adrenoceptors in the spinal cord dorsal horn. However, several recent reports demonstrate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the feet. More specifically, α 2-adrenoceptors appear to mediate the antinociception produced by morphine using the tail-flick test, but not that using the foot-withdrawal or hot-plate tests. The present study extended these findings and determined the role of α 1-adrenoceptors in mediating the antinociceptive effects of morphine microinjected into the ventrolateral periaqueductal gray using both the foot-withdrawal and the tail-flick responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of selective antagonists was used to determine whether the antinociceptive effects of morphine were modulated by α 1-adrenoceptors. Injection of the selective α 1-adrenoceptor antagonists prazosin or WB4101 potentiated the increase in the foot-withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray. In contrast, either prazosin or WB4101 partially reversed the increase in the tail-flick response latency produced by morphine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray modulates nociceptive responses to noxious heating of the feet by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the tail. More specifically, α 1-adrenoceptors mediate a pro-nociceptive action of morphine using the foot-withdrawal response, but in contrast, α 1-adrenoceptors appear to mediate part of the antinociceptive effect of morphine determined using the tail-flick test.

COMPARISON OF ANALGESIC RESPONSES TO INTRATHECAL ANALGESICS ADMINISTERED VIA PE-10 AND 32-GAUGE POLYURETHANE INTRATHECAL CATHETERS IN THE RAT

S288 Introduction: Intrathecal catheter implantation is routinely performed for the evaluation of both morphologic as well as pharmacologic effects of drugs. A recent study of the morphologic changes following intrathecal catheter implantation found that a 32 gauge polyurethane catheters (PU-32) produced less inflammatory reaction and reduced encapsulation compared to polyethylene (PE-10) tubing, while latencies to thermal challenges before and after normal saline injections did not differ at 7 days following implantation. [1] This reduction in the inflammatory response to the intrathecal catheter material might allow for a greater period for pharmacologic evaluation of intrathecally administered drugs.. However, no comparison of functional tests to analgesic drug administration via these different catheter systems has been reported. The purpose of this study was to compare analgesic testing following bolus drug analgesic administration via these two catheters over a period of 15 days in rats. Methods: Following IRB approval, 18 male Sprague-Dawley rats (260 +/- 43g) were studied. Catheters were cut from bulk PE-10 (Benton Dickinson, Sparks, MD) or PU-32 (Micor, Allison Park, PA) using a scalpel. To facilitate the bolus intrathecal injections, a 1-cm silastic extension catheter was attached to the proximal end of the PU-32 tubing using silicone adhesive. The coupling between these catheters was tested for leaks prior to insertion. Under pentobarbital anesthesia, the animals had catheters inserted via the cisterna magna and threaded 8.5 cm caudally so that the tip resided at the rostral portion of the lumbar enlargement. The catheters were flushed with saline and sealed with a stainless steel stylet. On the 5th and 15th day after implantation a 3 nmol bolus dose (8[micro sign]L) of morphine was administered followed by an 8[micro sign]L saline flush. Tail flick responses were evaluated at 30 minutes. At the end of the study the animals were perfused with a 10% formalin solution and position and integrity of the catheter examined. Tail flick responses (%MPE) were compared using repeated measures ANOVA. Post hoc testing was done using Bonferroni corrected t-tests. Data are presented as mean +/- SE. Results: Of the 11 PU-32 animals, 6 animals had significant resistance to injection by day 5. Tail flick responses in this group were 16.2 +/- 4.1. The analgesic responses (%MPE) on day 5 in the remaining 5 PU-32 animals was not different from the PE-10 group (Figure 1), but was significantly reduced compared to the PE-10 group on day 15. Examination of the catheter systems revealed material clogging in 60% of the PU-32 catheters.Figure 1Discussion: The findings of this study are somewhat surprising since the reduced inflammation and encapsulation previously reported with the PU-32 did not result in a prolonged period of utility compared to PE-10. The increased resistance to injection was likely a result of clogging of the catheters. These findings suggest that intrathecal PU-32 catheters may not be preferable to PE-10 when intermittent bolus administration is desired over many days.