Effects of intrathecal or intracerebroventricular pretreatment with pertussis toxin on antinociception induced by beta-endorphin or morphine administered intracerebroventricularly in mice

Differential Modulatory Effects of Cholera Toxin and Pertussis Toxin on Pain Behavior Induced by TNF-a, lnterleukin-1β and Interferon- Injected Intrathecally

The present study was designed to characterize the possible roles of spinally located cholera toxin (CTX)- and pertussis toxin (PTX)-sensitive G-proteins in pro-inflammatory cytokine induced pain behaviors. Intrathecal injection of tumor necrosis factor-alpha (TNF-alpha; 100 pg), interleukin-1beta (IL-1beta; 100 pg) and interferon-gamma (INF-gamma; 100 pg) showed pain behavior. Intrathecal pretreatment with CTX (0.05, 0.1 and 0.5 mg) attenuated pain behavior induced by TNF-alpha and INF-gamma administered intrathecally. But intrathecal pretreatment with CTX (0.05, 0.1 and 0.5 microg) did not attenuate pain behavior induced by IL-1beta. On the other hand, intrathecal pretreatment with PTX further increased the pain behavior induced by TNF-alpha and IL-1beta administered intrathecally, especially at the dose of 0.5 microg. But intrathecal pretreatment with PTX did not affect pain behavior induced by INF-gamma. Our results suggest that, at the spinal cord level, CTX- and PTX-sensitive G-proteins appear to play important roles in modulating pain behavior induced by pro-inflammatory cytokines administered spinally. Furthermore, TNF-alpha, IL-1beta and INF-gamma administered spinally appear to produce pain behavior by different mechanisms.

Effects of spinally and supraspinally injected 3-isobutyl-1-methylxanthine, cholera toxin, and pertussis toxin on immobilization stress-induced antinociception in the mouse

The effects of intracerebroventricular (i.c.v.) and intrathecal (i.t.) 3-isobutyl-1-methylxanthine (IBMX), cholera toxin (CTX) and pertussis toxin (PTX) administration on immobilization-induced antinociception were studied in ICR mice. Antinociception was assessed by the tail-flick assay. Immobilization of the mouse increased inhibition of the tail-flick response for at least 1 h. The pretreatment with i.t. IBMX (0.01-1 ng), but not i.c.v. IBMX, significantly attenuated immobilization-induced inhibition of the tail-flick response. The pretreatments with i.c.v. PTX (0.05-0.5 microg) as well as i.t. CTX, but neither i.c.v. CTX (0.05-0.5 microg) nor i.t. PTX, potentiated the inhibition of the tail-flick response induced by immobilization stress. Our results suggest that spinally located phosphodiesterase appears to be involved in the production of immobilization stress-induced antinociception. In addition, inactivation of supraspinally located PTX-sensitive G-proteins and spinally located CTX-sensitive G-proteins may modulate immobilization stress-induced antinociception.

Antihistamine antinociception is mediated by Gi-protein activation

The effect of the i.c.v. administration of antisense oligodeoxynucleotides directed against the alpha subunit of different Gi-proteins (anti-Gi alpha(1), anti-Gi alpha(2), anti-Gi alpha(3)) on the antinociception induced by the H(1)-antihistamines was evaluated in the mouse hot-plate test. The administration of diphenhydramine (20 mg kg(-1) s.c.), pyrilamine (15 mg kg(-1) s.c.) and promethazine (6 mg kg(-1) s.c.) produced an increase of the pain threshold which peaked 15 min after injection. Pretreatment with anti-Gi alpha(1) (12.5 microg per mouse i.c.v.), anti-Gi alpha(2) (25 microg per mouse i.c.v.) and anti-Gi alpha(3) (25 microg per mouse i.c.v.), administered 24 and 18 h before test, prevented the antihistamine-induced antinociception. At the highest effective doses, none of the compounds used impaired motor coordination, as revealed by the rota rod test, nor modified spontaneous motility and inspection activity, as revealed by the hole board test. These results suggest an important role played by the Gi-protein pathway in the transduction mechanism involved in the enhancement of the pain threshold produced by H(1)-antihistamines.

Phosphatidylinositol kinase enzymes regulate the retrograde axonal transport of NT-3 and NT-4 in sympathetic and sensory neurons

Phosphatidylinositol 3-kinase (PI3-kinase) and phosphatidylinositol 4-kinase (PI4-kinase) enzymes are an important family of signaling molecules that have been implicated in the regulation of intracellular vesicle trafficking. It has previously been shown that PI3-kinase and PI4-kinase enzymes regulate neuronal survival and the retrograde axonal transport of nerve growth factor in sympathetic and sensory neurons. We have extended these studies to examine the role these enzymes play in the regulation of the retrograde axonal transport of neurotrophin-3 (NT-3) and neurotrophin-4 (NT-4) in sympathetic and sensory neurons in vivo. Wortmannin (0.1 nmol/eye), a PI3-kinase and PI4-kinase antagonist, reduced the amount of (125)I-NT-3 retrograde transport in sympathetic neurons by approximately 50% and (125)I-NT-4 in sympathetic neurons by approximately 40% and sensory neurons by approximately 20%. The PI3-kinase antagonist LY294002 (100 nmol/eye) reduced the retrograde axonal transport of (125)I-NT-4 in sympathetic and sensory neurons, and (125)I-NT-3 in sympathetic neurons. Phenylarsine oxide (PAO), a PI4-kinase antagonist, significantly inhibited (125)I-NT-4 retrograde axonal transport in sympathetic and sensory neurons. These results show that wortmannin-sensitive PI3-kinases and PI4-kinases may be involved in NT-3 and NT-4 retrograde axonal transport. The retrograde axonal transport of neurotrophic factors in sympathetic and sensory neurons in vivo appears to depend upon the activation of different receptors and second messenger cascades at the nerve terminal.

Pretreatment with cholera or pertussis toxin differentially modulates morphine- and beta-endorphin-induced antinociception in the mouse formalin test

The present study was designed to examine the possible involvement of supraspinal CTX- and PTX-sensitive G-proteins in an opioid-induced antinociception in the formalin test. Morphine (1 microg) and beta-endorphin (1 microg) given i.c.v. displayed near-maximal inhibitory effects against the formalin response in the first (0-5 min) and the second (20-40 min) phases. CTX (0.1-0.5 microg) pretreated i.c.v. produced antinociceptive effects in both phases of the formalin responses. Its effect was more pronounced in the first phase. However, PTX (0.05-0.5 microg) injected i.c.v produced the antinociceptive effect only in the first, but not the second, phase. Both CTX (0.5 microg) and PTX (0.5 microg), at the dose which had no intrinsic effect, significantly reversed the beta-endorphin-induced antinociceptive effect observed during the second, but not the first, phase. However, the antinociceptive effect by morphine failed to be affected by the same dose of treatment with CTX or PTX. Our results indicate that, at the supraspinal level, CTX- and PTX-sensitive G-proteins appear to be involved in the modulation of antinociception induced by supraspinally administered beta-endorphin, but not morphine, in the formalin pain model.

Differential roles of spinal cholera toxin- and pertussis toxin-sensitive G proteins in nociceptive responses caused by formalin, capsaicin, and substance P in mice

The aim of the present study is to characterize the roles of spinal cholera toxin (CTX)- and pertussis toxin (PTX)-sensitive G proteins in the regulation of various nociceptive responses. The effects of intrathecal (i.t.) pretreatments with CTX and PTX on the formalin (subcutaneous)-, capsaicin (i.t.)-, and substance P (SP; i.t.)-induced nociceptive behaviours were examined in mice. Pretreatment with CTX (i.t.; 24 h before) significantly and dose-dependently (0.05-0.5 microg) suppressed both the first and second phases of the formalin-induced nociceptive behaviour. On the other hand, pretreatment with PTX (i.t., 6 days before) at the same doses (0.05-N0.5 microg) did not affect the formalin-induced response. Capsaicin (i.t., 0.5 microg)- and SP (i.t., 0.7 microg)-induced nociceptive behaviours were attenuated by the pretreatment with CTX. In addition, SP-induced nociceptive response was also attenuated by the pretreatment with PTX. However, the capsaicin-induced nociceptive response was not influenced by PTX pretreatment. These findings suggest that, at the spinal cord level, CTX-sensitive G-proteins are involved in the formalin-, capsaicin-, and SP-induced nociceptive behavioural responses, whereas PTX-sensitive G proteins are involved in SP-induced nociceptive response.

Increased plasma corticosterone, aggressiveness and brain monoamine changes induced by central injection of pertussis toxin

The effects of intracerebroventricular (i.c.v.) injection of pertussis toxin, a specific inhibitor of G(i)/G(o) proteins, on plasma corticosterone levels, aggressiveness, and hypothalamic and hippocampal monoamines and their metabolites levels were examined in mice. Plasma corticosterone level was markedly increased at 3 h after pertussis toxin injection (0.03 and 0.2 microg/mouse), peaked at 6 h and was still increased for up to 6 days after injection. Mice injected with pertussis toxin (0.2 microg/mouse) did not show weight gain between day 0 and day 6 after injection. In addition, pertussis toxin (0.2 microg/mouse) induced a progressive increase in aggressiveness, i.e. a decrease in attack latency and an increase in number of attacks, on day 1 and 6 after injection. Brain monoamines and their metabolites levels were changed on day 1 and 6 after pertussis toxin injection (0.2 microg/mouse): in the hypothalamus, levels of dopamine and 3,4-dihydroxyphenylacetic acid were increased, norepinephrine level decreased, and 5-hydroxyindole acetic acid (5-HIAA) level was markedly increased, with no changes in 5-hydroxytryptamine (5-HT) level, whereas in the hippocampus, 5-HT level was significantly decreased, with no changes in 5-HIAA and catecholamines. These results suggest that signal transduction through G(i)/G(o) proteins in the brain is involved in the modulation of hypothalamo-pituitary-adrenal axis, aggressiveness, and monoamine levels in vivo.

Differential modulatory roles of cholera toxin and pertussis toxin in the regulation of pain responses induced by excitatory amino acids administered intrathecally in mice

The present study was designed to characterize the possible roles of spinally located cholera toxin (CTX)- and pertussis toxin (PTX)-sensitive G-proteins in excitatory amino acids induced pain response. Intrathecal (i.t.) injection of glutamate (20 microg), N-methyl-D-aspartic acid (NMDA; 60 ng), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA; 13 ng), and kainic acid (12 ng) showed pain response. Pretreatment with CTX (0.05 and 0.5 microg, i.t.) attenuated pain response induced by glutamate, NMDA, AMPA and kainic acid administered i.t. in a dose-dependent manner. On the other hand, i.t. pretreatment with PTX further increased the pain response induced by glutamate, NMDA, AMPA and kainic acid administered i.t., especially at the dose of 0.5 microg. Our results suggest that, at the spinal cord level, CTX- and PTX-sensitive G-proteins appear to play opposite roles in modulating the pain response induced by spinally administered. Furthermore, CTX- and PTX-sensitive G-proteins appear to modulate pain response induced by stimuli of both NMDA and non-NMDA glutamate receptors.

Calcitonin reverts pertussis toxin blockade of the opioid analgesia in mice

The aim of this paper is to study the influence of salmon calcitonin (SCT) on opioid analgesia when opioid transduction pathways are functionally uncoupled from Gi/o proteins by treatment with pertussis toxin (PTX). The antinociceptive effect of morphine and three selective opioid agonists, [D-Ala2,N-Me-Phe2,Gly5-ol]enkephalin (DAMGO) (OP(3-mu receptor agonist), [D-Pen2.5]-enkephalin (OP-1-delta receptor agonist) and trans-( +/- )-3,4-dichloro-N-methyl-N-[2-1(-pyrrolidinyl)-cyclohexyl]-benzene-acetam ide methane sulfonate (U-50, 488H) (OP1-kappareceptor agonist) was evaluated, using the tail flick test, in mice treated with PTX or with PTX and SCT. PTX blocked the antinociceptive effect of the opioids, being the antinociception similar in control animals and in mice treated with PTX and SCT. Thus, SCT prevents the effect of the blockade of Gi/o-proteins. From this it could be suggested that calcitonin activates alternative antinociceptive mechanisms that are not dependent on Gi/o-proteins.

Ginsenosides induce differential antinociception and inhibit substance P induced-nociceptive response in mice

Ginsenosides are main pharmacoactive molecules of ginseng. The antinociceptive activity of ginsenosides after intrathecal (i.t.) injection was examined in formalin test. We also investigated the effects of ginsenosides on substance P (SP) induced-pain behaviors by i.t. treatment using mice. Pretreatment of ginsenosides by i.t. induced the inhibition of biting and licking of hind paw injected with 1% formalin with dose-dependent manner. The ED50 was 23 (19-28, 95% C.I.) microg/mouse for acute phase and 15 (9-23, 95% C.I.) microg/mouse for tonic phase. Interestingly, cotreatment of ginsenosides with SP also inhibited SP-induced pain behaviors (scratching, licking or biting of hind portion of body) with dose-dependent manner. The ED50 for the inhibition of SP-induced pain behavior by ginsenosides was 30 (11-85, 95% C.I.) microg/mouse. These results suggest that ginsenosides have antinociceptive activity in formalin test and this effect is due to blocking of SP-induced nociceptive information to postsynaptic site(s) at the spinal level.

Evidence for the existence of the beta-endorphin-sensitive "epsilon-opioid receptor" in the brain: the mechanisms of epsilon-mediated antinociception

Recently, mu-, delta- and kappa-opioid receptors have been cloned and relatively well-characterized. In addition to three major opioid receptor types, more extensive studies have suggested the possible existence of other opioid receptor types that can be classified as non-mu, non-delta and non-kappa. Based upon anatomical and binding studies in the brain, the sensitive site for an endogenous opioid peptide, beta-endorphin, has been postulated to account for the unique characteristics of the opioid receptor defined as a putative epsilon-opioid receptor. Many epsilon-opioid receptors are functionally coupled to G-proteins. The functional epsilon-opioid receptors in the brain are stimulated by bremazocine and etorphine as well as beta-endorphin, but not by selective mu-, delta- or kappa-opioid receptor agonists. Epsilon-opioid receptor agonists injected into the brain produce profound antinociception. The brain sites most sensitive to epsilon-agonist-induced antinociception are located in the caudal medial medulla such as the nucleus raphe obscures, nucleus raphe pallidus and the adjacent midline reticular formation. The stimulation of epsilon-opioid receptors in the brain facilitates the descending enkephalinergic pathway, which probably originates from the brainstem terminating at the spinal cord. The endogenous opioid Met-enkephalin, released in the spinal cord by activation of supraspinal epsilon-opioid receptors, stimulates spinal delta2-opioid receptors for the production of antinociception. It is noteworthy that the epsilon-opioid receptor-mediated pain control system is different from that of other opioid systems. Although there appears to be no epsilon-selective ligand currently available, these findings provide strong evidence for the existence of the putative epsilon-opioid receptor and its unique function in the brain.

Effects of spinally and supraspinally injected 3-isobutyl-1-methylxanthine, cholera toxin, and pertussis toxin on cold water swimming stress-induced antinociception in the mouse

1. The cold (4 degrees C) water swimming stress (CWSS) for 3 min significantly increased the inhibition of the tail-flick response in ICR mice. 2. Pertussis toxin (PTX, 0.05-0.5 microgram) in mice pretreated intrathecally (IT) for 6 days attenuated the inhibition of the tail-flick response induced by CWSS. However, intracerebroventricular (ICV) pretreatment with PTX at the same doses did not affect CWSS-induced inhibition of the tail-flick inhibition. 3. 3-Isobutyl-1-methylxanthine (IBMX, 0.01-1 ng) in mice pretreated IT for 10 min dose-dependently attenuated the inhibition of the tail-flick response induced by CWSS. However, IBMX in mice ICV pretreated ICV at the same doses was not effective in attenuating the CWSS-induced inhibition of the tail-flick response. 4. Neither IT nor ICV pretreatment with cholera toxin (CTX, 0.05-0.5 microgram) for 24 hr affected the inhibition of the tail-flick response induced by CWSS. 5. The ICV or IT injection of PTX, CTX, or IBMX did not affect the basal tail-flick response latency. 6. It is concluded that spinal, but not supraspinal, PTX-sensitive G-proteins and cAMP phosphodiesterase may be involved in the antinociception produced by CWSS. However, neither spinal nor supraspinal CTX-sensitive G-proteins appear to be involved in mediating the antinociception induced by CWSS.

Pretreatment with protein kinase C activator phorbol 12,13-dibutyrate attenuates the antinociception induced by mu- but not epsilon-opioid receptor agonist in the mouse

The effects of pretreatment with a protein kinase C activator, phorbol 12,13-dibutyrate, on antinociception induced by i.c.v.-administered mu-opioid receptor agonist (D-Ala2, NMePhe4, Gly(ol)5) enkephalin (DAMGO) or morphine and epsilon-opioid receptor agonist beta-endorphin were studied in male ICR mice. The tail-flick responses were used for antinociceptive tests. I.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 30 or 60 but not 10 min attenuated antinociception induced by i.c.v.-administered DAMGO. I.c.v. pretreatment with phorbol 12,13-dibutyrate (10 and 50 pmol) for 60 min caused a dose-dependent attenuation of DAMGO (19.5 pmol)- or morphine (6.0 nmol)-induced antinociception. The dose-response curve for DAMGO-induced antinociception was shifted to the right by 7.3-fold by i.c.v. pretreatment with phorbol 12,13-dibutyrate (50 pmol) for 60 min. However, the i.c.v.-administered beta-endorphin-induced antinociception was not affected by the same pretreatment with phorbol 12,13-dibutyrate. The attenuation of i.c.v.-administered DAMGO- and morphine-induced antinociception by phorbol 12,13-dibutyrate was reversed by concomitant i.c.v. pretreatment with a selective protein kinase C inhibitor calphostin C. These results suggest that activation of protein kinase C by phorbol 12,13-dibutyrate leads to the desensitization of mu-, but not epsilon-opioid receptor-mediated antinociception. These findings also provide additional evidence for differential intracellular modulation on antinociceptive action of mu- and epsilon-opioid receptor agonists.

An examination of the relationship between mu-opioid antinociceptive efficacy and G-protein coupling using pertussis and cholera toxins

The hypothesis that mu-opioid agonists having low antinociceptive efficacy might be more susceptible to interference with G-protein coupling than mu-opioid agonists having higher antinociceptive efficacy was tested. Supraspinal antinociceptive efficacy for the three mu-opioid agonists morphine, [D-Ala2, NMePhe4, Gly5-ol]-enkephalin (DAMGO) and sufentanil in the mouse 55 degrees C warm-water tail-flick test was evaluated 18-24 h after intracerebroventricular (i.c.v.) administration of beta-funaltrexamine (beta-FNA). The beta-FNA pretreatment (0.2-2.0 nmol) attenuated antinociception in the order morphine > DAMGO > sufentanil, consistent with previous reports of their relative antinociceptive efficacy. The association of efficacy with G-protein coupling was then assessed by determining sensitivity to i.c.v. (0.1-3.0 micrograms) pertussis toxin (PTX) or cholera toxin (CTX). The effect of PTX on equiantinociceptive doses was in the inverse order of agonist efficacy. CTX augmented sufentanil-induced antinociception. Morphine- and DAMGO-induced antinociception were unaffected by CTX. These data suggest that: (i) highly efficacious mu agonists (viz., sufentanil) couple more efficiently to PTX-sensitive inhibitory Gi-proteins than do agonists of lower efficacy (viz., morphine, DAMGO) and (ii) highly efficacious mu agonists have greater capacity to utilize CTX-sensitive stimulatory Gs-proteins than do mu-agonists with lower efficacy.

Effect of pertussis toxin on morphine, diphenhydramine, baclofen, clomipramine and physostigmine antinociception

The effect of pretreatment with pertussis toxin at the doses of 0.25 and 0.50 micrograms per mouse i.c.v. on the analgesic effect produced by morphine (7 mg kg-1 s.c.), baclofen (4 mg kg-1 s.c.), diphenhydramine (20 mg kg-1 s.c.), clomipramine (25 mg kg-1 s.c.) and physostigmine (0.1-0.2 mg kg-1 s.c.) was investigated in the mouse hot-plate test. Seven days after a single injection of pertussis toxin, inhibition of morphine and diphenhydramine analgesia was observed, whereas 11 days after pertussis toxin pretreatment, baclofen- and clomipramine-induced antinociception was also reduced. By contrast, pertussis toxin had no effect on physostigmine-induced antinociception. The present results indicate that the activation of pertussis toxin-sensitive G-proteins represents an important transduction step in the central analgesia induced by opioids, antihistaminics, GABAB (gamma-aminobutyric acid B) agonists and tricyclic antidepressants, but not by cholinomimetics.

Differential effects of forskolin and phorbol-13-myristate injected intrathecally or intracerebroventricularly on antinociception induced by morphine or beta-endorphin administered intracerebroventricularly in the mouse

The effects of forskolin or phorbol-13-myristate (PMA) injected intrathecally (i.t.) or intracerebroventricularly (i.c.v.) on the inhibition of the tail-flick and hotplate responses induced by morphine or beta-endorphin administered i.c.v. were studied. Animals pretreated with forskolin (20 micrograms) i.t. for 10 min had an attenuated inhibition of the tail-flick response induced by i.c.v. administered morphine (2 micrograms) or beta-endorphin (1 microgram). However, i.t. pretreatment with PMA (100 ng) was not effective in reducing the inhibition of the tail-flick response induced by morphine or beta-endorphin administered i.c.v. In addition, i.t. pretreatment with either forskolin or PMA did not affect the inhibition of the hotplate response induced by morphine or beta-endorphin administered i.c.v. Forskolin pretreatment i.c.v. for 10 min attenuated the inhibition of the tail-flick and hotplate responses induced by i.c.v. administered morphine or beta-endorphin. However, i.c.v. pretreatment with PMA was not effective in reducing the inhibition of the tail-flick or hotplate responses induced by morphine or beta-endorphin administered i.c.v. Our results suggest that activation of adenylate cyclase located at both spinal and supraspinal sites appears to be involved in antagonizing antinociception induced by morphine and beta-endorphin administered supraspinally. However, spinal or supraspinal protein kinase C may not be involved in antagonizing antinociception induced by morphine or beta-endorphin administered supraspinally.

Pretreatment with pertussis toxin blocks morphine- but not beta-endorphin-induced antinociception in the mouse

We have previously demonstrated that the antinociception induced by morphine and beta-endorphin given intracerebroventricularly (i.c.v.) is mediated by the stimulation of respective mu- and epsilon-opioid receptors. The effects of i.c.v. pretreatment with pertussis toxin on the antinociception induced by morphine and beta-endorphin given i.c.v. were studied in male ICR mice. Antinociception was assessed by the tail-flick and hot-plate tests. Pretreatment with pertussis toxin (0.5 microgram) given i.c.v. 96 h earlier blocks the antinociception induced by i.c.v. administered morphine in both tail-flick and hot-plate tests. The same pretreatment did not affect the antinociception induced by i.c.v. administered beta-endorphin. Our results indicate that morphine-, but not beta-endorphin-induced antinociception is mediated by pertussis toxin sensitive G-proteins.

Multiplicative interaction between intrathecally and intracerebroventricularly administered morphine for antinociception in the mouse: effects of spinally and supraspinally injected 3-isobutyl-1-methylxanthine, cholera toxin, and pertussis toxin

1. Either intrathecal (i.t.) or intracerebroventricular (i.c.v.) administration of morphine alone at the dose of 0.2 microgram slightly increased inhibition of the tail-flick response. However, combined i.t. and i.c.v. injections of morphine at the same dose increased the inhibition of the tail-flick response in a synergistic manner. 2. Cholera toxin (CTX, 0.05 to 0.5 microgram) pretreated i.t. or i.c.v. for 24 hr or pertussis toxin (PTX, 0.05 to 0.5 microgram) for 6 days dose-dependently attenuated inhibition of the tail-flick response induced by combined i.t. and i.c.v. injection of morphine. 3. 3-Isobutyl-1-methylxanthine (IBMX, 0.001 to 0.1 ng) pretreated i.t. for 10 min dose-dependently attenuated the inhibition of the tail-flick response induced by combined i.t. and i.c.v. injections of morphine. However, IBMX pretreated i.c.v. for 10 min was not effective in attenuating the inhibition of the tail-flick response induced by combined i.t. and i.c.v. injections of morphine. 4. It is concluded that both spinal and supraspinal CTX- and PTX-sensitive G-proteins are involved in the antinociception produced by morphine-induced multiplicative interaction between spinal and supraspinal sites. However, only spinal but not supraspinal cAMP phosphodiesterase is involved in mediating antinociception induced by morphine-induced multiplicative interaction.

Differential effects of 3-isobutyl-1-methylxanthine injected intrathecally or intracerebroventricularly on antinociception induced by opioids administered intracerebroventricularly in the mouse

Various doses of 3-isobutyl-1-methylxanthine (IBMX), a cAMP phosphodiesterase inhibitor, injected intrathecally (i.t.) or intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. IBMX (0.01 to 1 ng) pretreatment i.t. for 10 min dose-dependently attenuated the inhibition of the tail-flick response induced by i.c.v. administered morphine (2 micrograms), beta-endorphin (1 microgram), and U50, 488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide), 60 micrograms. However, pretreatment with IBMX i.c.v. did not affect the inhibition of the tail-flick response induced by morphine, beta-endorphin, and U50, 488H administered i.c.v. Neither i.c.v. nor i.t. pretreatment with IBMX attenuated the inhibition of the tail-flick response induced by D-Pen2-D-Pen5-enkephalin (DPDPE; 10 micrograms) administered i.c.v. Our results suggest that spinal but not supraspinal cAMP phosphodiesterases are involved in mediating antinociception induced by morphine, beta-endorphin and U50, 488H administered supraspinally. However, neither spinal nor supraspinal cAMP phosphodiesterase is involved in mediating antinociception induced by DPDPE administered supraspinally.