Supraspinal Sites Research Articles

Inflammation-induced Fos protein expression in the rat spinal cord is enhanced following dorsolateral or ventrolateral funiculus lesions

Previous studies have shown an enhanced expression of Fos protein-like immunoreactivity in the lumbar spinal cord of rats with complete spinal transection following persistent hindpaw inflammation. To further locate the spinal pathways responsible for these effects, we compared the inflammation-evoked Fos expression in rats with bilateral lesions of the dorsolateral (DLFX) or ventrolateral (VLFX) funiculus, and with rats with a sham operation. The results indicate that the number of Fos-labeled neurons was significantly increased in all laminae of the dorsal horn ipsilateral to the inflamed hindpaw and in contralateral deep dorsal horn in both DLFX and VLFX rats compared to sham-operated rats. Moreover, when comparing DLFX and VLFX rats, in the ipsilateral spinal cord, DLFX resulted in more Fos expression in the deep dorsal horn; in contrast, a larger number of Fos-labeled cells in superficial laminae was observed in VLFX rats. These results suggest that modulatory systems, which descend in both DLF and VLF pathways, mediate the enhanced net descending nociceptive inhibition after persistent inflammation, although the supraspinal sites of origin of each pathway are likely functionally diverse.

Effects of ginsenosides injected intrathecally or intracerebroventricularly on antinociception induced by morphine administered intracerebroventricularly in the mouse

1. 1. Total saponin fraction at doses of 0.1 to 1.0 μg, which administered IT alone did not affect the latencies of the tail-flick threshold, dose dependently attenuated inhibition of the tail-flick response induced by ICV-administered morphine (2 μg). 2. 2. Total saponin fraction at doses of 1 to 20 μg, which administered ICV alone did not affect the latencies of the tail-flick response, did not affect ICV-administered morphine-induced antinociception. 3. 3. The duration of antagonistic action of the total saponin fraction against morphine-induced antinociception lasted for at least 6 hrs. 4. 4. Various doses of ginsenosides R b2, R c, R d and R g1, but not R b1 and R e, injected IT dose dependently attenuated antinociception induced by morphine administered ICV. 5. 5. In summary, ginsenosides R b2, R c, R d and R g1 injected spinally appear to have antagonistic action against the antinociception induced by supraspinally applied morphine. On the other hand, the total ginseng fraction, at supraspinal sites, may not have an antagonistic action against the antinociception induced by morphine administered supraspinally.

Ruthenium red and capsazepine antinociceptive effect in formalin and capsaicin models of pain in mice

This study investigates the antinociception caused by intradermal (i.d) or intracerebroventricular (i.c.v.) injection of the capsaicin receptor antagonist capsazepine (CPZ), and ruthenium red (RR) (a cation-selective antagonist coupled to vanilloid receptor of capsaicin), on the chemical nociception caused by i.d. injection of formalin (FM) and capsaicin (CAP) into the mouse paw. The i.d. injection of either CPZ or RR in association with FM or CAP, inhibited the early phase, and to a lesser extent the late phase, of the FM, as well as CAP-induced nociception. Given i.c.v., both CPZ and RR caused discrete antinociception in the FM (both phases), while producing graded inhibition of CAP. The actions of CPZ and RR were insensitive to i.p. injection of naloxone (5 mg/kg). These results indicate that i.d. injection of CPZ and RR produce marked antinociception in chemical models of neurogenic pain induced by CAP and FM in mice. However, administered by supraspinal site, both CPZ and RR were inactive in inhibiting FM, but prevented, in a graded manner, CAP-induced algesic response, suggesting the participation of distinct mechanisms in the nociception induced by FM and CAP. Thus, vanilloid selective antagonists seem to be useful tools for investigating the nociception elicited by CAP and FM.

Responses of T2-4 spinal cord neurons to irritation of the lower airways in the rat.

The aim of the study was to investigate the information processing in the thoracic spinal cord (T2-4) after chemical irritation of the lower airways. Experiments were performed in pentobarbital sodium-anesthetized and pancuronium-paralyzed male Sprague-Dawley rats. Balloon distension of the esophagus was used as the search stimulus. Ammonia and smoke were applied by means of a tracheal cannula; they produced excitatory, inhibitory, and biphasic responses in a concentration-related manner (ammonia 39/39; smoke 23/ 39). Inhaled irritant-responsive neurons exhibited a number of similarities that have been described for neurons responding to stimulation of other thoracic viscera. These similarities relate to the distribution of neurons in the deeper laminae of the thoracic spinal cord, the relatively small number of neurons receiving input from the lower airways, the extensive convergent input from the skin and other thoracic viscera, and the pattern of responses. In addition, both stimulus-induced responses and spontaneous activity are subject to modulation from supraspinal sites. On the basis of responses to inhaled irritants after either spinal cord or vagus nerve block/transection, these T2-4 spinal neurons are likely to receive spinal afferent input that is modulated by vagal-brain stem input.

Brain and spinal cord distribution of biphalin: correlation with opioid receptor density and mechanism of CNS entry.

Biphalin [(Tyr-D-Ala-Gly-Phe-NH)2] is a bivalent, opioid peptide containing two pharmacophores linked by a hydrazine bridge. When administered intracerebroventricularly, it has been shown to be more potent than morphine and etorphine at eliciting antinociception. Biphalin has also been shown to cross both the blood-brain and blood-cerebrospinal fluid barriers. To understand the basis of biphalin's potency, regional brain and spinal cord distribution studies with [125I-Tyr1]biphalin were performed 5, 20, and 40 min after intravenous bolus injections. A statistically greater amount of [125I-Tyr1]biphalin was detected in the nucleus accumbens compared with other brain regions (p < 0.05). This correlates with the high density of delta- and mu-opioid receptor mRNA and binding sites shown to be expressed in the nucleus accumbens. Also, a statistically greater amount of [125I-Tyr1] biphalin was detected in two other circumventricular organs, the choroid plexus and pituitary, when compared with other brain regions. These studies provide evidence that biphalin can reach not only brain sites, but also spinal sites to elicit antinociception. The overall CNS distribution of [125I-Tyr1]biphalin was decreased with naloxone, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2, or naltrindole pretreatment, showing that biphalin detected in the brain and spinal cord is binding to delta- and mu-opioid receptors. Additional in situ brain perfusion experiments identified a saturable component contributing to CNS entry of [125I-Tyr1]biphalin, which could be described by Michaelis-Menten kinetics with a Km of 2.6 +/- 4.8 microM, Vmax of 14.6 +/- 2.89 pmol(-1) x min(-1) x g(-1), and Kd of 0.568 +/- 0.157 microl x min(-1) x g(-1). Brain entry of [125I-Tyr1]biphalin was sensitive to 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid and L-phenylalanine, suggesting use of the large neutral amino acid carrier. This work provides evidence that biphalin is a promising, potent analgesic that has a unique mechanism for reaching both spinal and supraspinal opioid receptor sites.

Effects of clomipramine and desipramine on a C-fiber reflex in rats

A C-fiber nociceptive reflex evoked by electrical stimulation within the territory of the sural nerve, was recorded from the ipsilateral biceps femoris muscle in urethane anesthetized rats. Intravenously administered clomipramine and desipramine produced a dose-dependent depression of the C-fiber reflex. High doses of intrathecal desipramine also inhibited the C-fiber reflex, while similar intrathecal doses of clomipramine produced only a modest inhibition of the response. Intracerebroventricular administration of clomipramine decreased dose-dependently the C-fiber reflex whereas intracerebroventricular desipramine increased this reflex. These findings suggest that tricyclic antidepressants with noradrenergic selectivity, as desipramine, inhibit the spinal processing of C inputs by acting directly at the spinal cord level, while those with serotonergic spectra, as clomipramine, depress the C-fiber-evoked spinal reflex by acting at a supraspinal modulatory site.

Met]enkephalin in the spinal cord is involved in the antinociception induced by intracerebroventricularly-administered etorphine in the mouse

We have recently reported that the antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of both μ- and ϵ-opioid receptors and the activation of both monoaminergic and opioidergic descending pain control systems. [29]Since the opioid ϵ-receptor-mediated antinociception induced by β-endorphin is mediated by the release of [Met]enkephalin and subsequent stimulation of δ-opioid receptors in the spinal cord, the present studies were designed to determine if β-endorphin-like action is also involved in etorphine-induced antinociception. The tail-flick test was used to assess the antinociceptive response performed in male ICR mice. Etorphine at doses from 5–20 ng given i.c.v. produced a dose-dependent inhibition of the tail-flick response. The inhibition of the tail-flick response induced by etorphine given i.c.v. was antagonized by intrathecal pretreatment for 60 min with antiserum against [Met]enkephalin (10 μg), but not with antiserum against [Leu]enkephalin (10 μg) or dynorphin A (1–13) (10 μg). Desensitization of δ-opioid receptors in the spinal cord by intrathecal pretreatment with [Met]enkephalin (5 μg) for 60 min attenuated i.c.v. administered etorphine-induced tail-flick inhibition. However, intrathecal pretreatment with [Leu]enkephalin (5 μg) or dynorphin A (1–17) (0.1 μg) for 60 min did not attenuate i.c.v. administered etorphine-induced tail-flick inhibition. The results indicate that antinociception induced by etorphine given i.c.v. is mediated in part by the stimulation of the ϵ-opioid receptor at the supraspinal sites and by the release of [Met]enkephalin which subsequently stimulates δ-opioid receptors in the spinal cord.

Supraspinal administration of opioids with selectivity for μ-, δ- and κ-opioid receptors produces analgesia in amphibians

Previous results using an amphibian model showed that systemic and spinal administration of opioids selective for μ, δ and κ-opioid receptors produce analgesia. It is not known whether non-mammalian vertebrates also contain supraspinal sites mediating opioid analgesia. Thus, opioid agonists selective for μ (morphine; fentanyl), δ (DADLE, [ d-Ala 2, d-Leu 5]-enkephalin; DPDPE, [ d-Pen 2, d-Pen 5]-enkephalin) and κ (U50488, trans-3,4-dichloro- N-methyl- N-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide methanesulfonate; CI977, (5R)-(544 α,744 α,845 β)- N-methyl- N-[7-(1-pyrrolidinyl)-1-oxaspiro[4,5]dec-8yl]-4-benzofuranacetamide monohydrochloride) opioid receptors were tested for analgesia following i.c.v. administration in the Northern grass frog, Rana pipiens. Morphine, administered at 0.3, 1, 3 and 10 nmol/frog, produced a dose-dependent and long-lasting analgesic effect. Concurrent naltrexone (10 nmol) significantly blocked analgesia produced by i.c.v. morphine (10 nmol). ED 50 values for the six opioids ranged from 2.0 for morphine to 63.9 nmol for U50488. The rank order of analgesic potency was morphine>DADLE>DPDPE>CI977>fentanyl>U50488. These results show that supraspinal sites mediate opioid analgesia in amphibians and suggest that mechanisms of supraspinal opioid analgesia may be common to all vertebrates.

Hypothesis: inhaled anesthetics produce immobility and amnesia by different mechanisms at different sites.

Recent evidence supplies new insights regarding the two universal effects of inhaled anesthetics: 1) immobility in response to a noxious stimulus and 2) amnesia. We hypothesize that these two effects result from actions at separate molecular and anatomic sites and that they are produced by different mechanisms. We propose that inhaled anesthetics cause immobility in response to noxious stimuli by an action in the spinal cord at an interface between polar and nonpolar regions. Such a site might be an interfacial region adjacent to membranes or proteins. In contrast, we propose that production of amnesia occurs at a supraspinal site and occurs in a nonpolar environment. An example of such a nonpolar site could be the interior of a phospholipid bilayer or a hydrophobic pocket within a protein.

Inhibition of spinal nitric oxide synthase by Nω-nitro- l-arginine blocks the release of met-enkephalin and antinociception induced by supraspinally administered β-endorphin in the rat

The antinociception induced by β-endorphin given supraspinally has been demonstrated previously to be mediated by the release of Met-enkephalin acting on δ 2-opioid receptors in the spinal cord. The present study was designed to determine the role of nitric oxide in the spinal cord on β-endorphin-induced release of Met-enkephalin and antinociception. The experiments were performed in pentobarbital-anesthetized rats. The release of Met-enkephalin was performed using a spinal cord perfusion technique and the Met-enkephalin released in the spinal perfusates was measured by radioimmunoassay. Antinociception was assessed by the tail-flick test. β-Endorphin (2 μg) given intraventricularly induced the release of Met-enkephalin from the spinal cord. The release of Met-enkephalin was dose-dependently attenuated by N ω -nitro- l-arginine (0.1 nM–1 μM) added into spinal perfusates and the attenuation was reversed by intrathecally applied l-arginine. The stereoisomer N ω -nitro- d-arginine given intrathecally, however, did not inhibit the release of Met-enkephalin induced by intraventricularly administered β-endorphin. β-Endorphin (4 μg) given intraventricularly produced antinociception in rats pretreated intrathecally with saline. The antinociception induced by β-endorphin was blocked by intrathecally administered N ω -nitro- l-arginine (5 μg) and the blockade of antinociception was reversed by intrathecal injection of l-arginine (50 μg). N ω -Nitro- d-arginine (5 μg) given intrathecally did not block the intraventricularly administered β-endorphin-induced antinociception. N ω -Nitro- l-arginine (10 μg) given intraventricularly did not affect intraventricularly administered β-endorphin-induced Met-enkephalin release nor did it affect intraventricular β-endorphin-induced antinociception, indicating that the effect of N ω -nitro- l-arginine is not at supraspinal sites. Intrathecal pretreatment with N ω -nitro- l-arginine did not affect intrathecally administered [ d-Ala 2]deltorphin II-induced antinociception. Our results indicate that N ω -nitro- l-arginine given intrathecally attenuates intraventricular β-endorphin-administered inhibition of the tail-flick response by presynaptically inhibiting the release of Met-enkephalin.

PRIMARY MOTOR CORTEX INFLUENCES ON THE DESCENDING AND ASCENDING SYSTEMS

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck, trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles. © 1997 Elsevier Science Ltd. All Rights Reserved.

Nitric oxide synthase inhibition modulates the ventilatory depressant and antinociceptive actions of fourth ventricular infusions of morphine in the awake dog.

The role of nitric oxide (NO) production, at the brain-stem level, in ventilatory control and pain perception is poorly understood. Furthermore, it is not clear whether NO synthase (NOS) inhibition can affect morphine-induced ventilatory depression or analgesia. The central hypothesis of this investigation was that NO, at supraspinal sites, can influence ventilation and nociception and can modulate the ventilatory depressant and antinociceptive actions of morphine. Using drug delivery via the fourth cerebral ventricle, the authors examined the ventilatory and nociceptive effects of an NOS inhibitor and an NO donor in the presence or absence of morphine sulfate (MS). The studies were performed in awake dogs that were restrained in a stanchion using a fourth ventricle to cisterna magna perfusion system. The dogs were chronically prepared with fourth ventricle and cisterna magna guide cannulae, femoral arterial/venous catheters, and a tracheostomy. Agents were prepared in a temperature- and pH-controlled artificial cerebrospinal fluid, perfused at 1 ml/min through the fourth ventricle cannula, and permitted to flow out through the cisterna magna cannula. The authors measured PaCO2, ventilatory drive (inspiratory occlusion pressures during carbon dioxide rebreathing), and nociception (hindpaw withdrawal threshold to increasing electrical current). Study groups were organized according to the following perfusion sequences (40 min each step): (1) MS (1 microgram/ml)-->MS + the NOS inhibitor, nitro-L-arginine (L-NA; 10(-6), then 10(-5) M)-->MS + L-NA (10(-5) M) + the NO donor, S-nitroso-acetylpenicillamine (SNAP; 10(-4) M); (2) SNAP (10(-5) M)-->SNAP (10(-4) M); (3) L-NA (10(-6), then 10(-5) M)-->L-NA (10(-5) M) + MS (1 microgram/ml)-->L-NA (10(-5) M) + MS + SNAP (10(-4) M); (4) MS (1 microgram/ml)-->MS + SNAP (10(-4) M); and (5) continuous MS (1 microgram/ml) perfusion (time control). Each perfusion sequence was preceded by a 45- to 60-min perfusion with drug-free artificial cerebrospinal fluid, during which time baseline values for each measured variable were obtained. Nitro-L-arginine alone dose dependently and significantly reduced PaCO2 and increased the nociceptive threshold. S-nitroso-acetylpenicillamine alone did not change the ventilation or nociceptive threshold. Morphine sulfate elicited a marked increase in PaCO2, a decrease in ventilatory drive, and an increase in nociceptive threshold (P < 0.05 compared with baseline). With L-NA pretreatment (sequence 3), but not posttreatment (sequence 1), MS-induced ventilatory depression, relative to baseline, was significantly attenuated. For both the L-NA pre- and posttreatment protocols, combined MS/L-NA perfusions produced a significantly greater antinociceptive effect than seen when MS was given alone. The L-NA effects on MS-induced ventilatory depression and antinociception were reversed with SNAP coadministration. Endogenous NO, produced at supraspinal sites, acts as a ventilatory depressant and as a nociceptive mediator. When NOS is inhibited, the ventilatory depressant actions of morphine can be reduced and the antinociceptive actions of morphine can be potentiated. However, NOS inhibitor treatment is more effective in suppressing morphine-induced ventilatory depression when given before, rather than after, morphine administration. The specific mechanisms involved in these actions remain to be identified.

Nicotine enhances morphine- and β-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.

Orphanin FQ acts as a supraspinal, but not a spinal, anti-opioid peptide.

Orphanin FQ (OFQ), the endogenous ligand for the orphan opioid receptor, LC132, was recently isolated and characterized. The anti-opioid role of OFQ in supraspinal pain modulation was demonstrated by our previous observations that intracerebroventricular (i.c.v.) OFQ administration dose-dependently reverses systemic morphine antinociception and opioid-mediated stress-induced antinociception. The present study was designed to evaluate whether OFQ also modulates the antinociceptive actions of morphine in the spinal cord. Immediately after assessment of baseline nociceptive sensitivity on the 49 degrees C tail-withdrawal assay, mice of both sexes were given i.c.v. or intrathecal (i.t.) cocktails of morphine (0, 1, 10 or 50 micrograms [0-135 nmol]) and OFQ (0 or 10 nmol), and re-tested 15, 30 and 60 min later. OFQ alone did not affect nociceptive sensitivity when administered by either route. Following i.c.v. administration, the antinociception produced by 10 micrograms morphine was completely reversed by 10 nmol OFQ; antinociception induced by 50 micrograms morphine was significantly antagonized. In contrast, OFQ was completely ineffective against antinociception induced by i.t. morphine. These findings indicate that the anti-opioid actions of OFQ are restricted to supraspinal central nervous system sites.

Cerebrospinal fluid distribution of opioids after intraventricular & lumbar subarachnoid administration in sheep.

The study of opioid distribution in blood and cerebrospinal fluid (CSF) is required to understand pharmacokinetic-pharmacodynamic relationships following lumbar intrathecal (it) and intracerebroventicular (i.c.v.) administration, and to investigate the contributions of spinal or supraspinal sites of action. The sheep model developed for pharmacokinetic study of analgesics allows atraumatic sampling of plasma and CSF after drug administration by the intravenous (i.v.), i.c.v., and it routes in an unanesthetized animal. Five adult female sheep were prepared with femoral vascular catheters, lumbar it and epidural cannulae, i.c.v. cannulae, and cisterna magna cannulae. Hydromorphone, methadone, naloxone, and [14C] sucrose were injected and collected by two methods: 1) injection into the i.c.v. cannula with lumbar CSF samples collected via the lumbar cannula and 2) injection into the lumbar cannula and cisternal CSF samples collected via ventriculocisternal cannula. Hydromorphone, morphine, and [14C] sucrose were detected at 90-105 min in lumbar CSF after i.c.v. injection. Hydromorphone and [14C] sucrose were detected in i.c.v. cerebrospinal fluid at 50 min after lumbar i.t. injection. Methadone was not detected in i.c.v. cerebrospinal fluid after i.t. injection, nor was methadone significantly detected in lumbar CSF after i.c.v. injection. These data indicate that i.c.v. and i.t. administration of lipophilic opioids produces CSF distributions different from those of hydrophilic opioids. This suggests that lipophilic opioids such as methadone or naloxone exert their effects predominantly on tissues near the site of injection. The study of i.t. and i.c.v. opiate administration and CSF pharmacokinetics may therefore have direct clinical implications.

Pretreatment with pertussis toxin spinally, but not supraspinally, blocks the cold water swimming-induced antinociception in the mouse

Mice exposed to cold water swimming (4°C) for 3 min produced a marked antinociception. Experiments were designed to determine whether pretreatment with pertussis toxin given intrathecally (i.t.) or intracerebroventricularly (i.c.v.) attenuates cold water swimming-induced antinociception in male ICR mice. Antinociception was measured by the tail-flick test 7 min after cold water swimming. I.t. pretreatment with pertussis toxin at a dose of 0.5 μg for 24–96 h caused a time-dependent attenuation of cold water swimming-induced antinociception. Moreover, i.t. pretreatment with pertussis toxin at doses from 0.125 to 0.5 μg for 96 h attenuated cold water swimming-induced antinociception in a dose-dependent manner. However, i.c.v. pretreatment with pertussis toxin at doses from 0.125 to 0.5 μg for 24–96 h did not affect the cold water swimming-induced antinociception. The present results suggest that pertussis toxin-sensitive Gi/Go proteins in spinal cord, but not at the supraspinal sites, are involved in cold water swimming-induced antinociception.

Depression by morphine-6-glucuronide of nociceptive activity in rat thalamus neurons: comparison with morphine

To assess the contribution of the active metabolite of morphine, morphine-6-glucuronide (M6G), to the analgesic effect of systemically administered morphine, experiments were carried out on rats under urethane anesthesia in which nociceptive activity was evoked by electrical stimulation of afferent C fibers in the sural nerve and recorded from single neurons in the ventrobasal complex of the thalamus. Intravenous (i.v.) injections of morphine completely blocked the activity at doses of 500 and 1000 μg/kg, the ED,, being 44 μg/kg. M6G administered by i.v. injection reduced the evoked nociceptive activity only by about 40% at 80 and 160 μg/kg, the ED 50 being 6 μg/kg. After intrathecal (i.t.) injection, morphine produced maximum depression of 55% of the control activity at 20 μg the ED 50 is 18 μg. M6G injected i.t. produced maximum depression of 40% at doses ranging from 0.2 to 10 μg. The ED 50 of M6G i.t. is below 0.2 μg. The effects of morphine and M6G were reversed by naloxone (200 μg/kg i.v.). The results show that M6G is more potent than morphine, regardless of the route of administration, while morphine is more effective when injected i.v. Due to the low efficacy of M6G, it seems unlikely that this glucuronide contributes substantially to the analgesic effect of morphine when renal function is normal. The results also make evident that the maximum effect of morphine results from an action at spinal and supraspinal sites.

Differential effects of forskolin and phorbol-13-myristate injected intrathecally or intracerebroventricularly on antinociception induced by morphine or β-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 β-endorphin administered i.c.v. were studied. Animals pretreated with forskolin (20 μg) i.t. for 10 min had an attenuated inhibition of the tail-flick response induced by i.c.v. administered morphine (2 μg) or β-endorphin (1 μg). However, i.t. pretreatment with PMA (100 ng) was not effective in reducing the inhibition of the tail-flick response induced by morphine or β-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 β-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 β-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 β-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 β-endorphin administered supraspinally. However, spinal or supraspinal protein kinase C may not be involved in antagonizing antinociception induced by morphine or β-endorphin administered supraspinally.

How Far Should a Catheter Be Inserted into the Epidural Space?

In Response: We thank Dr. Kempen for his interest in and comments on our recent article [1]. We feel that we did indeed define the optimal distance that an epidural catheter should be threaded into the epidural space for the woman in labor receiving 13 mL 0.25% bupivacaine. We used epidural needles and catheters with centimeter markings (Perifix Registered Trademark; B. Braun Medical Inc., Bethlehem, PA) and were extremely careful about the amount of catheter threaded into the epidural space. The quotation from our paper, "no strict guidelines were met," was in regard to how much epidural catheter was initially threaded through the epidural needle. Obviously more catheter should initially be threaded through the needle if 7 cm will remain in the space than if only 3 cm has to remain in the space. All patients enrolled in the study were in active labor, with contractions at least once every 5 min, and were randomly assigned to their respective groups; the time of failure was documented 15 min after the last dose of local anesthetic. We felt that 15 min was an appropriate amount of time to wait prior to assessing the block. Although 0.25% bupivacaine is a slow-acting local anesthetic, Eisenach et al. [2] found the onset time of 10 mL of 0.25% bupivacaine to be 8.7 +/- 0.8 (mean +/- SD) min. Even if the onset time of bupivacaine is longer, the women were randomized into their respective groups and there was a statistically significant difference in failure rate among the three groups. Most of the other variables that Dr. Kempen mentioned were either considered and revealed or not felt to be important for the outcome of our study. The patients' age and parity may impact on the process and course of labor, but once the woman is in active labor they are no longer important variables. Whether the woman ultimately has a vaginal delivery or cesarean section is impossible to control for at the time one initiates the block. However, since this was a randomized study, any impact from this variable should be mitigated. We did not feel it necessary to document the dermatomal distribution of the block, as all blocks were bilateral, with the "missed segments" below the highest dermatomal level achieved. Perhaps this should have been more clearly stated in the paper. Although the visual analog scale (VAS) scale is a useful instrument to quantify pain in many situations, we did not feel that the VAS was appropriate for this particular study. If VAS scores are obtained, one may interpret a score of 10 or 20 mm as a success, especially if the woman started with scores of 70 or 80 mm. But this may not be the case. If a woman has a one-sided or partial block, she may have had a decrease in VAS score but the block is still inadequate from her point of view. Thus, we felt that asking the woman if she has pain was the most important end point, and VAS scores may cloud the picture. We felt that using bupivacaine without narcotics was the best way to perform our study. Narcotics diffuse into the subarachnoid space where they bind to (spinal) opioid receptors. Diffusion into the subarachnoid space is dependent on many factors including drug penetration of the dura and pia mater, absorption by epidural fat, and vascular uptake and redistribution to supraspinal sites. Analgesia from narcotics is probably not related to catheter position, so using narcotics in our study would have only confused the issue. Yaakov Beilin, MD Howard Bernstein, MD Barbara Zucker-Pinchoff, MD Department of Anesthesiology The Mount Sinai Medical Center New York, NY 10029

Paradoxical heat sensation in patients with multiple sclerosis. Evidence for a supraspinal integration of temperature sensation.

Temperature thresholds were determined in 16 patients with probable or definite multiple sclerosis, in six patients with possible but unconfirmed multiple sclerosis and in 34 healthy subjects, using the method of limits and the thermal sensory limen (TSL) of the MarStock technique. A significant proportion of the patients had thresholds outside the 2.5 SD range for normal subjects, both for warmth detection threshold and TSL. In addition, 10 patients with probable or definite multiple sclerosis and one patient with possible multiple sclerosis reported a paradoxical heat sensation, i.e. a sensation of warmth elicited by a cold stimulus. This illusion was almost exclusively observed with the alternating warm and cold stimuli of the TSL procedure. In contrast to experimental nerve block or peripheral demyelinating neuropathy, where paradoxical heat sensation has been described by various authors, in the patients with multiple sclerosis the demyelination sites were located in the central nervous system. The observation that multiple sclerosis patients had paradoxical heat sensation in addition to threshold abnormalities supports the view that supraspinal sites are important for the integration of temperature sensation.