Analgesia mediated by a direct spinal action of narcotics

Epidural morphine and post-operative pain relief

Epidural morphine was compared with placebo in two randomly selected but similar groups of patients undergoing upper abdominal operations. There was no quantitative or qualitative difference in the analgesia obtained in the two groups of patients but there were significantly diminished long term requirements of intramuscular morphine in those who had been previously given epidural morphine when compared with epidural placebo. The possible causes and implications of this difference are discussed.

Transfer of pethidine to CSF following intravenous administration

To help delineate the contributions to analgesia evoked by central and spinal opiate receptors, pethidine was administered intravenously to dogs by bolus followed by infusion. Plasma and cerebrospinal fluid were sampled and assayed for pethidine. Cerebrospinal fluid concentrations followed a similar time course to plasma but with lower concentrations. The average cerebrospinal fluid:plasma ratio was 0.4 and resembled the fraction of unbound pethidine in plasma. It was concluded that cerebrospinal fluid concentrations after intravenous administration were so low that spinal action would be unlikely.

Comparison of epidural and intramuscular pethidine for analgesia in labour

Analgesia mediated by a direct spinal action of narcotic drugs administered via the epidural route may have considerable advantages over conventional(conduction block) epidural analgesia in labour. The efficacy, mode of action and placental transfer of epidurally administered narcotics in labour has not yet been established. We have compared the systemic absorption, analgesia and other effects on the mothers and transfer of drug to the fetus in primigravidae who received epidural or intramuscular pethidine 100 mg in labour. The superior analgesia following epidural pethidine did not appear to be attributable to a selective spinal action but rather to higher plasma concentrations of pethidine together with a possible weak regional conduction block due to local anaesthetic action of 1% pethidine solution. Epidural pethidine is not an advantageous method for providing analgesia in labour.

The distribution of multiple opiate receptors in bovine brain

The distribution of mu and delta opiate receptors in bovine brain has been investigated using the selective radioligands [3H]morphine and D-[3H]Ala2, D-Leu5-enkephalin. Their distributions were found to vary independently through different brain areas with up to a 10-fold difference between the ratio of mu to delta binding sites for the substantia nigra and the dentate gyrus of the hippocampus.

Aminophylline differentiates between the depressant effects of morphine on the spinal nociceptive reflex and on the spinal ascending activity evoked from afferent C fibres

The action of aminophylline on anti-nociceptive effects of morphine in rats was tested on the tail-flick response to noxious heat and on the activity evoked in ascending axons of the spinal cord by stimulation of nociceptive afferents. The depression of the tail-flick response produced by an intraperitoneal (i.p.) injection of morphine 2 mg/kg in intact and spinal rats was abolished by an i.p. injection of aminophylline 25 mg/kg. The activity evoked in ascending axons of spinal rats by electrical stimulation of afferent C fibres of the sural nerve was depressed by an intravenous (i.v.) injection of morphine 2 mg/kg. Aminophylline 25 mg/kg injected i.v. after morphine produced a slight and transient increase in the ascending activity immediately after its administration but did not abolish the depressant effect of morphine. Naloxone 0.2 mg/kg administered after aminophylline antagonized the depressant effect of morphine on the ascending activity. It is suggested that morphine exerts its depressant effect on the two nociceptive responses (the motor and the sensory response) by different mechanisms, one being sensitive to aminophylline, the other being relatively resistant to the action of the purine derivative.

Immunohistochemical localization of substance-P, somatostatin, and methionine-enkephalin in the spinal cord and dorsal root ganglia of the North American opossum, Didelphis virginiana

The occurrence and distribution of substance-P (SP), somatostatin (SOM), and enkephalin (ENK) were studied in the North American opossum, a generalized marsupial. Substance-P immunoreactivity was present in the tract of Lissauer as well as within lamina I, the outer part of lamina II, the lateral portion of laminae III through VII, and lamina X at all spinal levels. Although present, it was very spare in the ventral horn. Substance-P was also localized within autonomic areas of the thoracolumbar cord. ENK immunoreactivity was present in laminae I-III and laminae VII-X, as well as within autonomic areas. There was more ENK immunoreactivity in the ventral horn than SP. The densest aggregates of ENK, however, were found within the outer part of lamina II. In contrast, SOM could not be localized within the spinal cord of the opossum. (See NOTE ADDED IN PROOF).

A re-evaluation of the neurochemical and antinociceptive effects of intrathecal capsaicin in the rat

The effect of intrathecal administration of capsaicin in the rat on thermal nociceptive thresholds and on the content of substance P, somatostatin and glutamic acid decarboxylase in the dorsal horn of the spinal cord was determined. The results suggest that the depletion of spinal cord substance P induced by capsaicin may not by itself be sufficient to explain the observed changes in noxious thermal thresholds, which may be related instead to non-specific damage to the spinal cord.

Analgesia following local injection of dibutyryl cyclic nucleotides at sites in the rat CNS

Tail flick latency (TFL) was elevated following microinjection of both dibutyryl cyclic AMP (dbcAMP) was dibutyryl cyclic GMP (dbcGMP) at sites in the caudal reticular formation. TFL was also elevated following injection of dbcAMP but not dbcGMP into the periaqueductal gray. TFL was unaltered following administration of either agent into the lumbar intrathecal space. These actions are discussed with respect to the possible involvement of cyclic nucleotides in the analgesic action of morphine.

Epidural morphine for postoperative pain relief

Thirty-three patients were randomly assigned to two groups to study the analgesic potency, duration of action and side effects of epidural and intramuscular morphine after hip surgery. Two milligrams of preservative-free morphine chloride in 10 ml of normal saline in the epidural space was compared to 10 mg of intramuscularly administered morphine. There was a more rapid onset of action after intramuscular morphine. However, the quality of pain relief was substantially higher and the duration of action markedly longer after epidural morphine. The total dose required in the epidural group was 3.6 mg and in the intramuscular group 41 mg during the 15-h observation period. The side effects of epidural morphine were few and mild, the most embarrassing being urinary retention (20%). Nausea and/or vomiting was less common after epidural morphine (20% versus 55%). Pruritus or respiratory depression which have been reported previously were not encountered. However, it is recommended that preservative-free solution are used to avoid itching and that the patients are monitored, as respiratory depression may occur long after administration of epidural opiate.

Postoperative pain control with intrathecal morphine

A clinical study of the use of intrathecal morphine in two groups of surgical patients, 32 in all, showed that the morphine provided very powerful and prolonged analgesia with little disturbance of cerebral function. When combined with a spinal block it did not interfere with the action of the spinal anaesthetic. When combined with a relaxant technique of anaesthesia some persistent curarisation was seen. The only complication to emerge from the study was respiratory depression which could be very delayed in its offset. The depression was sufficient to induce carbon dioxide narcosis in two patients. It is suggested that the dose of intrathecal morphine in the very elderly should be very severely restricted as they appeared to be unduly sensitive to the respiratory depressant effect of morphine when given intrathecally.

Immunohistochemical localization of leucine-enkephalin in the spinal cord of the cat: enkephalin-containing marginal neurons and pain modulation

This study examined the spinal cord distribution of the endogenous opioid peptide leucine-enkephalin in the cat using immunohistochemical techniques. The distribution of nerve processes was studied in untreated cats; colchicine was administered to study the distribution and morphology of spinal enkephalin-containing perikarya. Enkephalin immunoreactive processes were greatest in laminae I and II (marginal layer and substantia gelatinosa) of the superficial dorsal horn. In many sections, the outer substantia gelatinosa (SG), lamina IIa, was discernibly less immunoreactive than I or IIb. Laminae III and IV were relatively devoid of staining. Laminae V and VII had moderate enkephalin-immunoreactivity, lamina VI somewhat less. Enkephalin immunoreactivity in lamina X, around the central canal, was very dense. Enkephalin-containing beaded varicosities coursed throughout the ventral horn. Although previous studies in the rat emphasized the enkephalin-somata of the SG, we found that in the cat the majority of superficial dorsal horn enkephalin-somata are in the marginal layer. These enkephalin-containing marginal cells were morphologically similar to a population of marginal neurons which project to the brainstem and/or the thalamus. Some light staining small SG neurons were also identified; many were located at the lamina I-II border. Considerably more cells were found ventral to the SG, in lamina III, and at the IV-V border. These latter cells had dendrites coursing dorsally, toward the SG. Numerous immunoreactive cells were found in lamina VIII, in a band across the intermediate gray. These cells fused medially with cells of lamina X. Enkephalin cells were also found in the sacral autonomic nucleus and encircling the central cervical nucleus, Clarke's column, and stilling's nucleus. Although surrounded by labeled cells, the latter regions were devoid of enkephalin-immunoreactive processes. Many of these spinal enkephalin neurons are morphologically similar to and distributed in regions known to contain projection neurons. Thus it is suggested that many spinal enkephalin neurons, generally thought to be local circuit neurons, project rostrally, to other spinal levels and perhaps to brainstem and/or thalamus.

Pain II: new approaches to management

There is an intrinsic analgesia system in the brain with both endogenous opioid and biogenic amine links. This system provides a basis for understanding how different therapeutic approaches relieve pain. The present review critically evaluates the analgesic efficacy of the following therapies: placebo administration, transcutaneous electrical nerve stimulation, acupuncture, intrathecal and epidural opiate injection, electrical stimulation of the brain, and psychotropic drugs.

Analgesic tolerance produced by morphine pellets is facilitated by analgesic testing

Analgesic tolerance induced by morphine pellets was examined in rats using the nociceptive tail flick reflex. Analgesic responses of animals who received preliminary tail flick tests after morphine implantation were significantly lower than responses of naive, nontested animals. Previously tested animals were also significantly more tolerant to a morphine challenge than nontested animals. A dose response curve to morphine was not obtained, at the doses used here, from previously tested animals, where as naive animals responded to in a dose dependent manner. Environmental modulation of the tail flick represents an elementary form of behavioral plasticity which may prove useful for neural analyses of simple reflex systems.

The stereospecific effect of naloxone on rat dorsal horn neurones; inhibition in superficial laminae and excitation in deeper laminae

The effect of systemic naloxone on the activity evoked by C-fibre stimulation in dorsal horn neurones of the rat spinal cord has been investigated. Recordings were made in unanaesthetized, decerebrate spinalized rats. Fifteen units were recorded from laminae 4 and 5 of the dorsal horn, 11 of these units were excited by naloxone (0.2--1.0 mg/kg). The onset of this excitation was after 20 sec to 5 min and recovery to control levels occurred within 15--40 min. Of 17 units recorded in substantia gelatinosa of the dorsal horn, 13 were inhibited by the naloxone. The latency of onset of this inhibition was short (2--10 sec) and the effect persisted for 5--10 min. The effects were largely restricted to C-fibre evoked activity although sometimes A delta responses were similarly altered. Neurones stimulated by A beta-fibre threshold, or whose sole afferent input were A beta-fibres, were unaffected by the naloxone. The stereoisomer of naloxone, (+)naloxone which is inactive in opiate receptor binding tests, failed to produce the same changes found with (-)naloxone in 17 units. These results show a differential effect of naloxone on neurones in the dorsal horn which respond to C-fibre input. Units in the substantia gelatinosa are inhibited while units in deeper laminae are excited by naloxone. These effects are likely to be mediated by the blockade of endogenous opioids in the spinal cord.

Respiratory depression after intrathecal narcotics

Three out of six patients who had received 1 mg of morphine and 0.22 ml plain bupivacaine 0.5%/segment as a subarachnoid spinal anaesthetic developed serious and delayed respiratory depression on several occasions. This was reversed by intravenous naloxone. It is postulated that the morphine had diffused to the level of the cisterna magnum and thence through brain tissue around the fourth ventricle. Naloxone did not reverse the analgesia.

Intrathecal morphine in mice: a new technique

A simple, rapid technique for intrathecal injections by lumbar puncture in unanesthetized mice is described. Intrathecal [3H]morphine base was not found in significant quantities in either the midbrain or forebrain. Submicrogram quantities of morphine sulfate induced Straub tail response and tail-flick analgesia. These effects were dose related and antagonized by subcutaneous naloxone.

Regional distribution of a novel analgesic dipeptide kyotorphin (Tyr-Arg) in the rat brain and spinal cord

Using high performance liquid chromatography, we measured kyotorphin content in discrete regions of the rat brain and spinal cord in an attempt to clarify its physiological significance. Higher concentrations of the dipeptide were found in the midbrain, pons plus medulla oblongata and dorsal cord, which are the most sensitive sites for the microinjection of morphine and/or electrical stimulation-induced analgesia. Kyotorphin may play a physiological role in pain control in the central nervous system.

Epidural morphine analgesia in second-trimester induced abortion

Epidural administration of 2 mg of morphine to 16 patients who were undergoing induced abortion in the second trimester of pregnancy abolished labor pains in 10 of them within 10 to 20 minutes after treatment was begun. The pain did not recur until the abortion process started, sometimes hours later. In one patient, the relief of pain was achieved with an additional top up dose of 1 mg 15 minutes later. In the other five patients in whom the morphine had no appreciable effect, the addition of 4 ml of 0.5% bupivacaine hydrochloride, also injected epidurally, successfuly abolished pain. Because of the beneficial and prolonged action of morphine, as well as the lack of side effects, continuous epidural analgesia with low doses of it--supplemented, if necessary, with small quantities of bupivacaine--is effective for treatment of labor pains in induced abortion in the second trimester of pregnancy. The involvement of the anesthetist from the very beginning of the induction procedure is highly recommended.

Differential effects of intracerebrally microinjected enkephalin analogs on centrally versus peripherally induced pain, and evidence for a facial versus lower body analgesic effect

[D-Ala2]-Met-Enkephalin and [D-Ala2]-Met-Enkephalinamide were microinjected (10-30 microgram) into the midbrain ventrolateral central gray of rats. The opiate analogs produced profound analgesia in left and right facial areas, and on the hot plate test. The tail-flick test showed significant analgesia, but in a significantly smaller amount than that obtained with noxious face heating. All effects were blocked by naloxone pre-treatment. The drugs had no effect on thresholds for defense responses to high (200 Hz) and low (20 Hz) frequency aversive stimulation in midbrain areas associated with pain perception.

Continuous epidural infusion of fentanyl for postoperative analgesia

A continuous epidural infusion of fentanyl was used for control of postoperative pain in a series of 30 patients who had received an epidural anaesthetic for general surgical procedures. Fentanyl was originally administered at a maximal rate of 50 micrograms/hr but later reduced to 25 micrograms/hour or less. Satisfactory analgesia was provided in 24 patients, who required no other form of analgesia whatsoever. Three patients derived significant analgesia from the infusion, but did require occasional small supplementary doses of opiates. Three patients derived little or no benefit from the procedure. The only noticeable side effect was nausea and vomiting, which was less of a feature with the weaker solution. The method was used on general surgical wards with no extra demands being made on nursing staff. Many nursing procedures and physiotherapy could be carried out more conveniently than usual in the postoperative period.

Pharmacology of pain and analgesia

Physiological events involved in nociception and pain perception are examined. Substance P could be a primary afferent transmitter of certain nociceptive information. Transmission of this information can be modulated within the spinal cord by intrinsic and descending mechanisms. The intrinsic mechanism involves inhibitory opiate effects within substantia gelatinosa. Centres for descending systems are located in medulla and periaqueductal gray matter. They are activated by exogenous narcotic agonists, and by regional connections. Descending inhibitory pathways are serotonergic and noradrenergic. GABA and glycine are also possibly involved in antinociception. Narcotics have been shown to produce analgesia when administered to the intrathecal or epidural spaces of humans. These routes are still experimental. The place of clinical modification of transmitter system is discussed, but no conclusions or recommendations can be made at this early stage.

Spinal noradrenergic terminal system mediates antinociception

Intrathecal administration of norepinephrine (NE) into the lumbar subarachnoid space of rats and cats implanted with chronic spinal catheters produced a strong, dose-dependent, behaviorally defined analgesia. The effect appeared mediated by an alpha-receptor inasmuch as phenylephrine, but not isoproterenol produced the intrathecal effect. Moreover, the antinociceptive effect of NE was antagonized by the prior systemic or intrathecal administration of phentolamine (an alpha-blocker), but was unaffected by pretreatment with propranolol (a beta-blocker). The effect of intrathecal NE was significantly potentiated by prior administration of Lilly 51641 (a monoamine oxidase inhibitor) and protriptyline (a re-uptake inhibitor), and was not antagonized by the intrathecal administration of a non-specific vasodilator, papaverine. The antinociceptive effect of intrathecal NE showed tachyphylaxis following repeated injections. No cross-tolerance between intrathecal NE and morphine was observed, suggesting that the spinal action of morphine is not mediated by spinal noradrenergic terminals. Importantly, naloxone had no effect on the intrathecal NE effect. The present data provide further evidence for the modulatory role of a spinal noradrenergic system on the spinal processing of nociceptive transmission.

Dissociation of supraspinal and spinal actions of morphine: a quantitative evaluation

Opiate suppression of spinal withdrawal reflexes was tested in rats with lesions of several spinal funiculi to determine the relative contribution of supraspinal descending systems. The latency of tail-flick to noxious heat was used to assess "analgesia". The effect of lesions of dorsolateral funiculus (DLF), dorsal columns (DC) and ventral quadrant (VQ) were compared to that of sham operations. None of the lesions produced a change in baseline latency. Each animal was tested with varying doses of morphine sulfate over several weeks. Only DLF lesions consistently antagonized tail-flick suppression by morphine across the dose range studied (5-15 mg/kg i.p.), although VQ lesions were somewhat effective. The reduction of morphine's action was proportionally greater for lower doses. The results indicate that both spinal and supraspinal sites contribute significantly to the analgesia produced by systemic administration of opiates.

Epidural pethidine in labour. A study of dose requirements

The degree of pain relief and duration of action were assessed after various doses of epidural pethidine had been given to 12 primigravid mothers in labour. Evidence of sympathetic and motor blockade was sought. Twenty-eight doses were administered and of these 50% gave good pain relief and a further 43% of doses produced some relief. The duration of action was related to dose and varied between 55 min after 25 mg pethidine and 150 min after 100 mg. The blood pressure remained constant and there was no motor blockade. In the postpartum period five mothers expressed full satisfaction and seven were helped by the injection.

Morphine does not antagonize the substance P mediated excitation of dorsal horn neurons

Multibarrelled microelectrodes were used to test the effects of iontophoretically released substance P (SP), morphine, glutamate, and naloxone on spinal cord dorsal horn neurons. Cells excited by SP were also excited by noxious stimuli, a finding consistent with the hypothesis that SP is the neurotransmitter released by primary nociceptor afferents to excite dorsal horn neurons. Iontophoretic morphine failed to depress the SP-induced discharges. Indeed, iontophoretic morphine frequently potentiated the SP responses. In addition to potentiating SP-induced discharges, iontophoretic morphine frequently increased both the spontaneous activity of dorsal horn neurons and the activity evoked in these cells by noxious cutaneous heat and iontophoretic glutamate. Naloxone did not antagonize these excitatory effects. Intravenous morphine only depressed spontaneous discharges. Nevertheless, iontophoretic morphine still produced excitatory effects in spinal animals pretreated with analgesic doses of intravenous morphine. It is concluded that such excitatory effects are toxic actions indicative of supratherapeutic morphine concentrations in the vicinity of the neuron being studied. Intravenously administered morphine depressed the spontaneous activity of dorsal horn neurons of spinal cats, but failed to depress their responses to SP. Morphine also failed to antagonize SP's biological effects in peripheral systems (contraction of isolated guinea pig ileum, rabbit hypotensive effect, rat sialogogic response). It is concluded that morphine is not a substance P receptor antagonist. The results are discussed with respect to the hypotheses that (1) the spinal analgesic effects of systemically administered morphine occur on presynaptic terminals of sensory neurons, and (2) an SP antagonist might be a unique analgesic agent.

Failure of epidural morphine to relieve pain in labour

Lumbar epidural injections of 2 mg preservative-free morphine were given to 10 subjects in established labour. Assessment of pain using a visual analogue scoring system revealed no appreciable relief of pain 30 min after morphine infection. Analgesia was subsequently provided by epidural injection of 8 ml bupivacaine 0.375%. Failure of epidural 2 mg in the treatment of severe pain other than labour. Possible explanations for this discrepancy are discussed.

The potentiation of spinal analgesia by leucine enkephalin

In two strains of rats, intrathecal administration of morphine and met-enkephalin was found to produce a dose-dependent analgesic effect as measured by the tail-flick assay. Prior administration of leu-enkelphalin at a dose which had no analgesic effect was found to reduce significantly the analgesic ED50 of both morphine and met-enkephalin. Met-enkephalin pretreatment, at a similar sub-analgesic dose, was without effect. Thus, a differential interaction of the enkephalins with morphine is evident at spinal structures.

Effects of morphine of human spinal cord and peripheral nervous activities

The depressant effects of morphine on the evoked electrospinogram (EESG), evoked electromyogram (EEMG) and nerve action potential (NAP) were studied in surgical patients. The EESG was recorded with an epidural electrode in the posterior epidural space in the lumbar enlargement. A clinical dose of morphine (1 mg/kg) depressed the amplitude of all components (P1, N1, P2) of the EESG, elicited by both weak and strong stimuli to the tibial nerve. The depressant effects of morphine were most pronounced on the P2 component of EESG, which is believed to correspond to primary afferent depolarization. The H-relfex was augmented by the drug, while the M-wave and the NAP were unaffected. The changes induced by morphine in both the EESG and the H-reflex were reversed completely, partially or even potentiated by naloxone (0.1 mg/kg). These depressant effects of morphine on the EESG were minimized under nitrous oxide (75%) anaesthesia, indicating the existence of a degree of interaction between these two drugs. Thus, morphine in a clinical anaesthetic dose, affects both spinal cord function and afferent volleys along the roots in normal man.

Opiate analgesics and endorphins inhibit rat dorsal root potential in vitro

The effects of endorphins and opiate analgesics on the dorsal root potential (DRP) were studied in vitro using the isolated spinal cord of newborn rat. Bath-applied beta-endorphin and [D-Ala2]-Met-enkephalinamide (D-Ala) greatly depressed the DRP and the depressant effects were abolished by prior perfusion with naloxone. The potency of Met-enkephalin, Leu-enkephalin and alpha-endorphin was much weaker than that of D-Ala. Morphine and levorphanol depressed the DRP and these effects were also antagonized by naloxone. The isolated rat spinal cord appears to be a convenient in vitro preparation for analysing the effects of opiates on synaptic transmission in the central nervous system.

Profound analgesic effects of beta-endorphin in man

Profound and long-lasting analgesia (mean duration of pain relief 33.4 h, range 22.5--73.5 h) was produced by intrathecal administration of 3 mg synthetic beta-endorphin in all of 14 patients with intractable pain due to disseminated cancer. No respiratory depression, hypotension, hypothermia, or catatonia was observed.

The depressive effects of morphine on the C fibre response of dorsal horn neurones in the spinal rat pretreated or not by pCPA

(1) The effects of morphine upon the transmission of nociceptive messages at the spinal level have been investigated in the spinal rat. The responses of dorsal horn cells induced by the activation of C fibres were depressed in all cases in a dose-dependent fashion, this effect being reversed by the opiate antagonist naloxone. An estimation of the ED50 at the cellular level leads to the value of 6.3 mg/kg. The responses to A delta fibres were also depressed dose-dependently whereas the responses to A alpha fibres were unaffected. This is a confirmation in the rat of the differential effects of morphine on responses of convergent units elicited by the stimulation of different fibres, as previously described in the cat. (2) The hypothesis of the participation of serotonergic terminals in these effects has been checked by comparing the preceding results to those obtained in pCPA pretreated animals. Two populations of units were observed in the latter group: two-thirds of cells showed a dose-response curve similar to that of the non-pretreated group whereas the remaining one-third were unaffected either by morphine or naloxone. It is concluded that, at least, two mechanisms are involved in the depressive effects of morphine at the spinal level, serotonergic terminals being implicated in one of these. (3) The lowering of spinal cord serotonin content was associated with a decrease of both the size of the excitatory receptive field (34%) and the activities related to C fibre input (36%) of the recorded dorsal horn cells. This result is discussed with reference to the excitatory or sensitizatory effect of serotonin upon chemoreceptors related to pain.

Epidural analgesia with morphine a preliminary communcation

Morphine sulphate 2 to 4 mg was given by epidural injection for analgesia in eleven patients whose pain was ischaemic, traumatic or post-operative. Excellent analgesia was obtained in ten of the eleven patients without significant side effects. This technique appears to offer relief in post-operative and other types of severe pain and has been continued for up to five days.