Effects of intraventricular injection of morphine and beta-endorphin on serotonin release from the spinal cord in rats

Oral morphine induces spinal 5-hydroxytryptamine (5-HT) release using an opioid receptor-independent mechanism

Morphine induces spinal 5-hydroxytryptamine (5-HT) release, but the role and mechanism of the spinal 5-HT release induced by morphine are not well understood. The purpose of this study was to define the role and mechanism of spinal 5-HT release induced by oral morphine. We also examined whether persistent pain affected the spinal 5-HT release induced by oral morphine. Spinal 5-HT release was measured using microdialysis of lumbar cerebrospinal fluid (CSF). Two opioids, morphine and oxycodone, were orally administered and 5-HT release was measured in awake rats. Naloxone and β-funaltrexamine (β-FNA) were used to determine whether the effect of morphine on 5-HT release was mediated by opioid receptor activation. To study persistent pain, a formalin test was used. At 45 min after oral morphine administration, the formalin test was started and spinal 5-HT release was measured. Oral morphine, but not oral oxycodone, increased 5-HT release at the spinal cord to approximately 4000% of the baseline value. This effect of morphine was not antagonized by either naloxone or β-FNA at a dose that antagonized the antinociceptive effect of morphine. Formalin-induced persistent pain itself had no effect on spinal 5-HT release but enhanced the oral morphine-induced spinal 5-HT release. Oral morphine-induced spinal 5-HT release was not mediated by opioid receptor activation. Spinal 5-HT induced by oral morphine did not play a major role in the antinociceptive effect of morphine in the hot plate test. Persistent pain increased oral morphine-induced spinal 5-HT release.

Third Ventricular Injection of CCL2 in Rat Embryo Stimulates CCL2/CCR2 Neuroimmune System in Neuroepithelial Radial Glia Progenitor Cells: Relation to Sexually Dimorphic, Stimulatory Effects on Peptide Neurons in Lateral Hypothalamus

Clinical and animal studies show maternal alcohol consumption during pregnancy causes in offspring persistent alterations in neuroimmune and neurochemical systems known to increase alcohol drinking and related behaviors. Studies in lateral hypothalamus (LH) demonstrate in adolescent offspring that maternal oral administration of ethanol stimulates the neuropeptide, melanin-concentrating hormone (MCH), together with the inflammatory chemokine C-C motif ligand 2 (CCL2) and its receptor CCR2 which are increased in most MCH neurons. These effects, consistently stronger in females than males, are detected in embryos, not only in LH but hypothalamic neuroepithelium (NEP) along the third ventricle where neurons are born and CCL2 is stimulated within radial glia progenitor cells and their laterally projecting processes that facilitate MCH neuronal migration toward LH. With ethanol's effects similarly produced by maternal peripheral CCL2 administration and blocked by CCR2 antagonist, we tested here using in utero intracerebroventricular (ICV) injections whether CCL2 acts locally within the embryonic NEP. After ICV injection of CCL2 (0.1 µg/µl) on embryonic day 14 (E14) when neurogenesis peaks, we observed in embryos just before birth (E19) a significant increase in endogenous CCL2 within radial glia cells and their processes in NEP. These auto-regulatory effects, evident only in female embryos, were accompanied by increased density of CCL2 and MCH neurons in LH, more strongly in females than males. These results support involvement of embryonic CCL2/CCR2 neuroimmune system in radial glia progenitor cells in mediating sexually dimorphic effects of maternal challenges such as ethanol on LH MCH neurons that colocalize CCL2 and CCR2.

5-HT receptors involved in opioid-activated descending inhibition of spinal withdrawal reflexes in the decerebrated rabbit

The role of 5-HT(1B/1D), 5-HT(2) and 5-HT(3) receptors in mediating descending inhibition of spinal reflexes activated by application of fentanyl to the fourth ventricle has been studied in rabbits decerebrated under N(2)O/isoflurane anaesthesia. In the control state, intraventricular fentanyl (3-30 microg kg(-1)) depressed, to an equal extent, short- and long-latency reflexes in the medial gastrocnemius muscle nerve evoked by electrical stimulation of all sural nerve afferents. Inhibition of reflexes resulted from a decreased base line excitability in the reflex pathway accompanied by a reduction in the rate of temporal summation of responses. Fentanyl-induced suppression of short- and long-latency reflexes was significantly reduced after intrathecal administration of the selective 5-HT(2)-receptor antagonist ICI 170,809 (300 microg). The same dose of the selective 5-HT(1B/1D) blocker GR 127,935 reduced inhibition from intraventricular fentanyl only for long-latency reflexes (i.e. those parts of the response for which the afferent drive is provided mainly by Adelta and C-fibre afferents). The 5-HT(3) antagonist tropisetron (also 300 microg intrathecal) did not significantly alter the descending inhibition of reflexes evoked by fentanyl. Both GR 127,935 and tropisetron reduced temporal summation of reflexes per se, effects that were reversed by intraventricular fentanyl. These data suggest that the descending pathway(s) activated by intraventricular fentanyl liberate 5-HT in the spinal cord to inhibit withdrawal reflexes by acting at 5-HT(2) and 5-HT(1B/1D), but not 5-HT(3) receptors. 5-HT(1B/1D), and to a lesser extent 5-HT(3) receptors also appear to have a role in modulating temporal summation of reflexes evoked by repetitive stimuli.

Involvement of 5-hydroxytryptamine(1A) receptors in the descending anti-nociceptive pathway from periaqueductal gray to the spinal dorsal horn in intact rats, rats with nerve injury and rats with inflammation

Studies have shown that 5-hydroxytryptamine (5-HT) plays an important role in the descending pathway of pain modulation from brainstem to the spinal cord. Using selective 5-HT receptor antagonists, the present study investigated which type of 5-HT receptor(s) in the spinal cord was involved in the morphine-induced anti-nociception in intact rats, in rats with nerve injury and in rats with inflammation. The hindpaw withdrawal latencies decreased significantly after sciatic nerve injury and hindpaw inflammation compared with intact rats. Intrathecal administration of 25 or 10 microg of the selective 5-HT(1A) recepter antagonist spiroxatrine, but not 1 microg of spiroxatrine, significantly blocked the increased hindpaw withdrawal latencies to thermal and mechanical stimulation induced by intra-periaqueductal gray injection of 1 microg of morphine in intact rats. Intrathecal injection of the 5-HT(2) receptor antagonist RS 102221 and the 5-HT(3) receptor antagonist MDL 72222 had no significant effects on the increased hindpaw withdrawal latencies to both noxious stimulations induced by intra-periaqueductal gray injection of morphine. Furthermore, intrathecal administration of spiroxatrine, but not RS 102221 nor MDL 72222, significantly attenuated the increased hindpaw withdrawal latencies induced by intra-periaqueductal gray administration of morphine in rats with nerve injury and in rats with inflammation. The results demonstrate that the 5-HT(1A) receptor, not 5-HT(2) nor 5-HT(3) receptor, plays an important role in the descending pathway of anti-nociception from the brainstem to the spinal cord in intact rats, in rats with nerve injury and in rats with inflammation.

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.

The role of 5-HT(1A)-receptors in fentanyl-induced bulbospinal inhibition of a spinal withdrawal reflex in the rabbit

The sural to gastrocnemius withdrawal reflex is inhibited after injection of the OP(3) (micro)-receptor-selective opioid fentanyl into the fourth ventricle of decerebrated rabbits. This effect is abolished by complete section of the spinal cord but not by the selective alpha(2)-adrenoceptor antagonist RX 821002 (Clarke RW, Parry-Baggott C, Houghton AK, Ogilvie J. The involvement of bulbo-spinal pathways in fentanyl-induced inhibition of spinal withdrawal reflexes in the decerebrated rabbit. Pain 1998;78:197-207). We have now investigated the role of 5-HT(1A) receptors in mediating the descending inhibition activated by intraventricular fentanyl. In the control state, intraventricular fentanyl (3-30 microgram/kg) inhibited gastrocnemius reflex responses to a median of 34% of pre-drug levels. After intrathecal administration of the selective 5-HT(1A) receptor antagonist WAY-100635 (100 microgram), fentanyl reduced reflex responses to 83% of pre-fentanyl values, significantly less inhibition than in the control state. In a separate group of experiments, intravenous fentanyl (0.3-30 microgram/kg) depressed the sural-gastrocnemius reflex to 17% of pre-drug controls. This inhibition was not affected by intrathecal WAY-100635 (100 microgram), but combined administration of the 5-HT(1A) antagonist with RX 821002 (100 microgram) significantly reduced the effectiveness of i.v. fentanyl. After the highest dose reflexes were 37% of pre-fentanyl levels. These data show that the bulbospinal inhibition activated by fentanyl is mediated, at least in part, by activation of spinal 5-HT(1A) receptors. That blockade of these receptors failed to influence the inhibition induced by i.v. fentanyl might be taken to mean that the brain-stem action of fentanyl does not contribute significantly to the systemic actions of this opioid. A more probable explanation is that, in the preparation used in the present study, the bulbospinal and direct spinal actions of fentanyl occlude each other to produce an overall inhibition that is less than the sum of the two effects.

Differential modulation by baclofen on antinociception induced by morphine and beta-endorphin administered intracerebroventricularly in the formalin test

Our previous studies have demonstrated that supraspinal GABAergic receptors are differentially involved in the antinociception induced by morphine and beta-endorphin given intracerebroventricularly (i.c.v.) in the tail-flick and hot-plate tests. These two models employed a phasic, thermal nociceptive stimulus. The present study was designed to examine the possible involvement of supraspinal GABAergic receptors in opioid-induced antinociception in the formalin test. Morphine (1 microg) and beta-endorphin (1 microg) given i.c.v. displayed the almost complete inhibitory effects against the hyperalgesic response in both phases. Muscimol (75-100 ng) and baclofen (5-10 ng) injected i.c.v. produced the hypoalgesic response in the both phases. The hypoalgesic response induced by muscimol and baclofen observed during the second phase was more pronounced than that observed during the second phase. Baclofen (2.5 ng), at the dose which did not affect the hyperalgesic response, resulted in a significant reversal of the i.c.v. administered beta-endorphin-induced hypoalgesic response observed during the second, but not the first, phase. However, the hypoalgesic response induced by i.c.v. administered morphine was not changed by the same dose of muscimol or baclofen injected i.c.v. Our results indicate that, at the supraspinal level, GABA(B)receptors appear to be involved in the modulation of antinociception induced by supraspinally administered beta-endorphin, but not morphine, in the formalin test model.

Differential effects of adenosine receptor antagonists injected intrathecally on antinociception induced by morphine and beta-endorphin administered intracerebroventricularly in the mouse

A previous study reported that beta-endorphin and morphine administered supraspinally produce antinociception by activating different descending pain inhibitory systems. The present study was designed to investigate the blocking effects of A1 or A2 adenosine receptors in the spinal cord on antinociception induced by supraspinally administered mu- and epsilon-opioid receptor agonists. The effects of 1,3-dipropyl-8-(2-amino-4-chloro-phenyl)-xanthine (PACPX; an A1 adenosine receptor antagonist) or 3,7-dimethyl-1-propargylxanthine (DMPX; an A2 adenosine receptor antagonist) on the antinociception induced by morphine (a mu-opioid receptor agonist) or beta-endorphin (an epsilon-opioid receptor agonist) administered intracerebroventricularly (i.c.v.) were studied. The antinociception was assayed by the tail-flick test. DMPX at doses of 1-40 micrograms (which administered intrathecally alone did not affect the latencies of tail-flick thresholds), attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered morphine (0.5 microgram) or beta-endorphin (1 microgram). PACPX at doses of 1-40 micrograms (which administered intrathecally alone did not affect the latencies of tail-flick thresholds), attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin but not morphine. These results suggest that A2 but not A1 adenosine receptors in the spinal cord may be involved in the antinociception induced by supraspinally administered morphine, while the antinociception induced by supraspinally administered beta-endorphin appears to be mediated by spinal A1 and A2 adenosine receptors. These results support the hypothesis that morphine and beta-endorphin administered supraspinally produce antinociception by different neuronal mechanisms.

Nicotine enhances morphine- and beta-endorphin-induced antinociception at the supraspinal level in the mouse

The effect of nicotine administered supraspinally on antinociception induced by supraspinally administered opioids was examined in ICR mice. The intracerebroventricular (i.c.v.) injection of nicotine alone at doses from 1 to 12 micrograms produced only a minimal inhibition of the tail-flick response. Morphine (0.2 micrograms), beta-endorphin (0.1 microgram), D-Pen2.5-enkephalin (DPDPE; 0.5 microgram), trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl] benzeocetamide (U50, 488H; 6 micrograms) caused only slight inhibition of the tail-flick response. Nicotine dose dependently enhanced inhibition of the tail-flick response induced by i.c.v. administered morphine (0.2 microgram) or beta-endorphin (0.1 microgram). The degree of enhancing effect of nicotine toward beta-endorphin-induced inhibition of the tail-flick response was greater than toward morphine-induced inhibition of the tail-flick response. However, i.c.v. administered nicotine at the same doses was not effective in enhancing the inhibition of the tail-flick response induced by DPDPE (0.5 microgram) or U50, 488H (6 micrograms) administered i.c.v. Our results suggest that stimulation of supraspinal nicotinic receptors may enhance antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) administered supraspinally. However, the activation of nicotinic receptors at supraspinal sites may not be involved in enhancing the antinociception induced by DPDPE (a delta-opioid receptor agonist) or U50, 488H (a kappa-opioid receptor agonist) administered supraspinally.

Effects of GABA receptor antagonists injected spinally on antinociception induced by opioids administered supraspinally in mice

The present study was designed to investigate the modulatory effects of blockade of spinal GABAA and GABAB receptors on antinociception induced by supraspinally administered mu- and epsilon-opioid receptor agonists. The effects of intrathecal (i.t.) injections with GABAA and GABAB receptor antagonists, SR 95531 [2-(3-carboxypropyl)-3-amino-6-(4-mehylphenyl)pyridazinium bromide] and 5-aminovaleric acid, respectively, on the antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. Antinociception was assayed using the tail-flick test. The i.t. injection of SR 95531 (0.04-0.16 nmol) and 5-aminovaleric acid (32.5-130 nmol), administered alone did not affect the latencies of the tail-flick response, but selectively antagonized the inhibition of the tail-flick response induced by muscimol (a GABAA receptor agonist) and baclofen (a GABAB receptor agonist), respectively. The i.t. injection of SR 95531 attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered morphine, without affecting the i.c.v. administered beta-endorphin-induced response. 5-Aminovaleric acid attenuated dose-dependently the inhibition of the tail-flick response induced by beta-endorphin, without affecting the response to i.c.v. administered morphine. Our results indicate that GABAA but not GABAB receptors located at the spinal cord appears to be involved in the antinociception induced by morphine administered supraspinally whereas GABAB but not GABAA receptors located at the spinal cord may be involved in the antinociception induced by supraspinally administered beta-endorphin, supporting further the hypothesis that morphine and beta-endorphin administered supraspinally produce their antinociception via the activation of different descending pain inhibitory systems.

The reduction of antinociceptive effect of morphine administered intraventricularly is correlated with the decrease of serotonin release from the spinal cord in streptozotocin-induced diabetic rats

1. The antinociceptive effect of morphine (25 micrograms) administered into the 3rd ventricle was significantly attenuated in streptozotocin-induced diabetic rats as measured by the tail-flick assay. 2. The release of serotonin (5-HT; 5-hydroxytryptamine) from the spinal cord caused by intraventricular injection of morphine (25 micrograms) was significantly reduced in streptozotocin-induced diabetic rats. 3. No differences of 5-HT contents of the spinal cord (lumbar cord) between streptozotocin-induced diabetic- and vehicle-treated rats were found. 4. It is concluded that the reduction of antinociception produced by intraventricular injection of morphine in streptozotocin-induced diabetic rats might be, at least partly, due to the decrease of 5-HT release from the spinal cord.

Inhibition of protein kinase C, but not of protein kinase A, blocks the development of acute antinociceptive tolerance to an intrathecally administered mu-opioid receptor agonist in the mouse

A specific protein kinase C inhibitor, calphostin C, which injected alone had no effect on the antinociception induced by intrathecal (i.t.) administration of a selective mu-opioid receptor agonist, [D-Ala2,NMePhe4,Gly(ol)5]enkephalin (DAMGO), dose-dependently attenuated the development of acute tolerance to the i.t. DAMGO-induced antinociception in male ICR mice. On the other hand, a selective protein kinase A inhibitor, KT5720, did not have any effect on the development of acute tolerance to DAMGO antinociception. These findings suggest that protein kinase C, but not protein kinase A, plays an important role in the development of acute tolerance to the mu-opioid receptor agonist-induced antinociception.