On the role of a central adrenergic mechanism in morphine analgesic action

The emotion of pain and its chemistry

Pain is not an electrical impulse derived from tissue injury but an emotional experience arising when a nervous input is interpreted in the light of experience and emotional context as being 'painful'. Pain may thus signify tissue damage, vivid sensory experience or inner turmoil. Pain may be distressing but it can also be pleasurable. Whether a given stimulus provokes pain and whether that emotional feeling causes distress varies from individual to individual and from moment to moment. The brain possesses chemically mediated mechanisms that can exert control over the experience of 'pain'. An understanding of such mechanisms suggests new approaches to the relief of distressing pain (and to the artificial production of pleasurable pain).

Enhancement of morphine-induced analgesia and attenuation of morphine-induced side-effects by cocaine in rats

Effect of cocaine on morphine-induced analgesia and the accompanying respiratory depression, bradycardia and hypolocomotion/sedation was studied in rats. Cardiovascular and respiratory effects were studied under pentobarbitone-induced anaesthesia. Cocaine enhanced morphine-induced analgesia in the formalin test, hot plate test and heat-induced tail withdrawal test in intact rats. However, in spinal rats a similar combination of cocaine with morphine did not produce increased latencies in the tail withdrawal test. Of the three analgesic tests used, the formalin test was the most sensitive to the enhancement, as well as to the effects of morphine or cocaine alone. Morphine at the dose of 6 mg/kg produced complete analgesia in the formalin test, significant hypolocomotion/sedation, significant bradycardia and significant decrease in the respiratory rate. At an equianalgesic dose (complete analgesia in the formalin test) of morphine (3 mg/kg)-cocaine (5 mg/kg)-combination no significant changes in heart rate, respiratory rate or locomotion(/alertness) were observed. Changes in skin blood flow determined by the laser Doppler flow method were not significant in any of the experimental conditions. The results indicate that cocaine enhances morphine-induced analgesia, mainly due to supraspinal mechanisms. In contrast, the morphine-induced bradypnoea, bradycardia and hypolocomotion/sedation are attenuated by cocaine.

Dependency on the brain function of arginine vasopressin system of the development to and recovery from analgesic tolerance to morphine

Concomitant intracerebroventricular (i.c.v.) injection of anti-arginine vasopressin (AVP) antiserum dose-dependently suppressed the development of analgesic tolerance to daily morphine, 10 mg/kg, s.c., in mice. This suppressive effect of the antiserum was reduced by incubating the antiserum with AVP in vitro, before i.c.v. injection, suggesting that the antiserum inactivates brain AVP to result in the suppression of the development of tolerance in vivo. Similar to the antiserum, both AVP V1 and V2 antagonists given i.c.v., 10 ng and 20 ng/mouse, respectively, suppressed the development of morphine tolerance. Meanwhile, the administration of antiserum dose-dependently recovered morphine analgesia in morphine-tolerant mice and a complete recovery of analgesia was observed at the highest dose of antiserum following the second injection, and the effect of antiserum was maintained for 3 days after its withdrawal. Likewise, 10-100 ng/mouse of AVP V1 receptor antagonist given i.c.v. recovered morphine analgesia partially but significantly in a dose-dependent manner; however, AVP V2 receptor antagonist at the same doses partially recovered analgesic effect but the effect was neither significant nor dose-dependent. These findings suggest that the tolerance developed to morphine can be reversible when disturbing the function of brain AVP, but in addition to the different mechanisms of antiserum, V1 and V2 receptor antagonists, the V1 receptor-mediated mechanism may be more closely concerned in this phenomenon.

Ontogenetic homologous sensitization to the antinociceptive action of quinpirole in rats

Repeated postnatal treatment of rats with the dopamine receptor agonist, quinpirole results in exaggeration of selected behaviors that are induced by quinpirole in adulthood. To determine whether the antinociceptive response to quinpirole could be similarly enhanced, rats were treated daily from birth with quinpirole HCl (3.0 mg/kg per day i.p. x 28 days) and their response time in the hot plate analgesia test was determined at 4 months. An acute dose of quinpirole HCl (100 or 1000 micrograms/kg i.p.) produced an analgesic response in the neonatally primed rats and in the vehicle controls. More significantly, the effect was substantially greater in the quinpirole-primed group at each of these two doses of quinpirole. This effect of quinpirole was fully attenuated in both groups by treatment with the dopamine receptor antagonist, spiperone HCl (0.30 mg/kg i.p., 1 h before quinpirole). The analgesic effect of morphine sulfate (6.0 mg/kg i.p.) was not greater in the quinpirole-primed group. These findings demonstrate that the ontogenetic sensitization of quinpirole receptors results in enhanced antinociceptive responses to quinpirole in adulthood. This animal model may be useful for studying the involvement of dopamine systems in algesia and analgesia.

Potentiation of morphine analgesia by D-amphetamine is mediated by norepinephrine and not dopamine

Morphine will raise the threshold for escape from aversive electrical stimulation delivered to the mesencephalic reticular formation and this effect is potentiated by D-amphetamine. In order to study the roles which dopamine and norepinephrine play in modulating opiate analgesia, the effects of amfonelic acid, an indirect dopamine agonist, and nisoxetine, a selective norepinephrine reuptake blocker, were determined alone and in combination with morphine using this supraspinal model of analgesia. Amfonelic acid alone produced hyperalgesia and completely antagonized the analgesic effect of morphine. Nisoxetine had no effect by itself, however, it potentiated the analgesic effect of morphine when the two drugs were administered concomitantly. These findings suggest that norepinephrine and not dopamine plays a predominant role in the potentiation of opiate analgesia by D-amphetamine.

Stereospecific potentiation of opiate analgesia by cocaine: predominant role of noradrenaline

Cocaine hydrochloride (50 mg) pellets implanted subcutaneously in male Wistar rats potentiated the analgesia of morphine, levorphanol, methadone and buprenorphine as measured by the tail-withdrawal test. Potentiated opiate analgesia was abolished by naloxone and further enhanced by desipramine and phenoxybenzamine. Yohimbine, alpha-methyl p-tyrosine, haloperidol, zimelidine, methysergide, p-chlorophenylalanine produced no significant effect on potentiated opiate analgesia. Pseudo-cocaine (dextro-cocaine), which is several-fold less potent than cocaine as an inhibitor of noradrenaline and dopamine reuptake in the CNS, had no significant effect on opiate analgesia. Analgesia produced by low doses of baclofen, a GABA agonist, was also not potentiated by cocaine. This study suggests a predominant role for noradrenaline in the stereospecific potentiation of opiate analgesia by cocaine.

Morphine analgesia without development of tolerance in reserpinized mice

The relationship between the brain monoaminergic mechanism and morphine tolerance was examined in reserpinized mice. In parallel with the reduction of brain monoamine content, the analgesic effect of morphine was reduced in reserpinized animals. At the peak of the reserpine effect, 24 hr after a single dose of 2.5 mg/kg reserpine, i.p., the analgesic effect of morphine was lowered to about 45% of that in naive animals; and 5 days after reserpine treatment, it recovered to about 60% of the control activity. In these animals, the lowered effect of morphine was maintained at the same range during 6 daily repetitions, and the development of tolerance was suppressed. When daily morphine injection was started from 10 days after reserpine treatment, at the time when the brain level of monoamines was still reduced to 60 to 80% of the control, tolerance developed as rapidly as in control animals. On the other hand, daily treatment with a small dose of reserpine, 0.1 mg/kg, neither affected the brain level of norepinephrine and dopamine nor modified morphine analgesia, but completely blocked the development of tolerance. These results may suggest that suppression of the development of tolerance to morphine analgesia is not attributed to the reduction of brain norepinephrine and dopamine by reserpine. Morphine analgesia without development of tolerance in reserpinized mice may indicate the dissociation of the analgesic effect from tolerance liability.

Effects of peripheral antisympathetic treatments in the tail-flick, formalin and autotomy tests

The effects of peripheral adrenergic depleting agents on the threshold for non-damaging heat pain (tail-flick test), inflammatory pain associated with tissue injury (formalin test), and chronic pain or dysaesthesia associated with nerve lesions (autotomy test) were examined. Tail-flick latencies were increased by agents which deplete peripheral adrenergic transmitters--6-hydroxydopamine (6-OHDA) and guanethidine--as well as by an agent which prevents the synthesis of noradrenaline--FLA63. A combination of guanethidine and FLA63 also increased latencies, but no more than either treatment alone. Formalin pain scores were reduced by FLA63+ guanethidine and 6-OHDA, but not by guanethidine or FLA63 alone. The percentage of rats that exhibited autotomy was reduced minimally by 6-OHDA and guanethidine treatments which started the day of surgery, maximally by guanethidine treatments which started 4 days before surgery, and not at all by guanethidine treatments which started 4 days after surgery. The results obtained in the formalin and autotomy tests are interpreted in terms of the possible roles of adrenergic transmitters in stimulating and sensitizing damaged afferents immediately after injury. The tail-flick results, however, suggest that adrenergic transmitters also act in lowering thresholds of normal peripheral receptor-fibre units. The relevance of the findings to the development of chronic pain are discussed.

Role of prolactin-opiate interactions in the central regulation of pain threshold

Endogenous hyperprolactinaemia induced by anterior pituitary transplantation under the kidney capsule has been found to reduce the behavioural responsiveness to electrical footshock and to increase morphine-induced analgesia. The apparent analgesic effect of prolactin has been related to the stimulation of nigro-striatal dopaminergic transmission, as suggested by the increase in striatal dopamine turnover observed in hyperprolactinaemic rats. It seems likely that central opiate system is involved in the behavioural effects of prolactin. Thus, naloxone prevents the effects of hyperprolactinaemia on footshock responsiveness and heroin self-administration is decreased in hyperprolactinaemic rats.

Opposing effects of apomorphine on pain in rats. Evaluation of the dose-response curve

The effect of apomorphine on a supraspinally mediated response to pain was studied after subcutaneous administration of 10 different doses (25 micrograms/kg up to 10 mg/kg). Depending on the dose given, apomorphine was found to induce opposing effects on pain, so that low doses, 25-100 micrograms/kg, dose-dependently increased the sensitivity to pain. This effect then gradually declined in potency with increasing doses and high doses induced antinociception. The data therefore suggest that the net effect recorded involves the sum of responses from at least two functional systems. Using the Hill equation and the digital computer program NONLIN, we have dissociated the observed effect into two components, each having its particular dose-response characteristics: low doses having an ED50 value of 36 micrograms/kg produced hyperreactivity to pain, and high doses having an ED50 of 465 micrograms/kg (in the absence of hyperalgesia) induced antinociception.

Activation of striatal dopamine receptors induces pain inhibition in rats

In the rat, elevating dopamine content in corpus striatum with electrical stimulation of substantia nigra or direct administration of apomorphine (50-200 micrograms) into the lateral cerebral ventricle or apomorphine (2-10 microgram) into the caudate-putamen complex decreased pain sensitivity (as shown by an increase in the latency to hind-paw lick in the hot plate test). Furthermore, the decreased pain sensitivity after the central administration of apomorphine was antagonized by pretreatment with haloperidol (a dopamine antagonist). On the other hand, lowering dopamine content in corpus striatum with electrolytic destruction of substantia nigra and 6-hydroxydopamine lesions to the substantia nigra, as well as direct injection of haloperidol into the lateral cerebral ventricle or caudate-putamen complex increased pain sensitivity. The data indicate that activation of striatal dopamine receptors in rat brain induces pain inhibition.

Enhancement of sprouting and putative regeneration of central noradrenergic fibers by morphine

Treatment of newborn rats with 6-hydroxydopa (6-OHDOPA, 60 micrograms/g IP) increased the levels of norepinephrine in the adult cerebellum and hindbrain. Concurrent treatment with morphine sulfate (20 micrograms/g IP) potentiated the response to 6-OHDOPA in the cerebellum and pons-medulla. In addition, increased noradrenergic neurite density in 4 week cerebellar cortex (as observed with histofluorescent staining by glyoxylic acid) suggests that neonatal morphine increased the sprouting of noradrenergic neurons in the 6-OHDOPA treated rats.

Effect of pargyline on morphine tolerance and physical dependence development in mice

The effects of single and repeated pargyline administration on morphine antinociception in both naive and morphine-tolerant mice and on naloxone-precipitated withdrawal in morphine tolerant-dependent animals were investigated. Adult, male Swiss-Webster mice were rendered tolerant to and dependent on morphine by the s.c. pellet implantion technique. Morphine analgesia, as assessed by the tail-flick antinociceptive test, was potentiated in tolerant animals by acute adminstration of pargyline but antagonized by repeated pargyline administration; pargyline produced similar effects in non-tolerant mice and to the same relative degree. Repeated pargyline treatment during morphine pellet implantation enhanced the withdrawal jumping response precipitated by naloxone in dependent mice. Pargyline also, after a single injection, exacerbated jumping in mice undergoing abrupt withdrawal. Neither acute nor chronic pargyline administration altered the brain distribution of injected morphine in non-tolerant mice. It was concluded that pargyline may modify acute morphine actions and withdrawal without materially altering the process(es) involved in the development of tolerance and physical dependence.

The effects of acutely administered analgesics on the turnover of noradrenaline and dopamine in various regions of the rat brain

1 Noradrenaline and dopamine turnover rates were determined following blockade of synthesis by alpha-methyl-p-tyrosine. Morphine, pentazocine and methadone had no effect on steady state levels or on turnover of noradrenaline in whole brain and in the hypothalamus. Although morphine was without action on medulla-pons noradrenaline steady state levels, a drug-induced increase in turnover rate was observed which was antagonized by pretreatment with naloxone (5 mg/kg). Pentazocine and methadone failed to alter either the steady state level of noradrenaline in the medulla-pons or its turnover rate.2 Morphine accelerated the decline in striatal alpha-methyl-m-tyramine levels following subcutaneous injection of alpha-methyl-m-tyrosine 18 h previously.3 All three drugs increased the turnover of dopamine in whole brain and corpus striatum although the striatal effect was prevented by naloxone pretreatment. The minimum doses of morphine, pentazocine and methadone required to elicit a significant effect on striatal dopamine turnover were 10 mg/kg, 30 mg/kg and 10 mg/kg respectively.4 The possibility of a dopaminergic involvement in the antinociceptive effect of analgesics is discussed.

Pharmacological properties of centrally-administered agents which interfere with neurotransmitter function: a comparison with the central depressant effects of ouabain

1. Centrally administered sodium diethyldithiocarbamate (DDC) produced hypothermia, central nervous depression and potentiation of the antinociceptive effect of morphine. These effects resemble those seen with centrally administered ouabain. Furthermore, the interactions of (+)-amphetamine, desmethylimipramine and nialamide with DDC and ouabain were similar.2. 6-Hydroxydopamine by the same route also produced central nervous depressant effects including hypothermia, decreased locomotor activity and catalepsy but not ptosis.3. Both ouabain and chlorpromazine produced similar effects on behaviour and body temperature including selective abolition of a conditioned avoidance response.4. Although centrally administered tetrabenazine produced ptosis, decreased locomotor activity and catalepsy, it had no significant effect on body temperature. However, the hypothermia produced by peripherally administered reserpine was reversed by centrally administered dibutyryl cyclic 3',5'-adenosine monophosphate.5. Centrally administered cocaine and desmethylimipramine produced no depressant effects but an increased excitability and responsiveness were apparent in both cases.6. Although the observed behavioural depression and hypothermia can occur independently both seem to involve an interference with dopaminergic systems.

Monoamine mediation of the morphine-induced activation of mice

1. The dose-response relationship for hyperactivity in grouped mice following the injection of morphine sulphate has been established.2. The activation response can be modified by drugs which affect either catecholamines or indoleamines.3. The monoamine precursors L-DOPA and 5-hydroxytryptophan potentiate the response.4. The monoamine synthesis inhibitors alpha-methyl-p-tyrosine and p-chlorophenylalanine reduce the response.5. Inhibition of monoamine oxidase activity by pargyline caused a great increase in the response. The simultaneous administration of reserpine resulted in a further potentiation.6. Reserpine blocked the response whenever it was given alone, either before, with or after the injection of morphine.7. Blockade of alpha-adrenoceptors with phentolamine or phenoxybenzamine reduced the response.8. Blockade of tryptaminergic receptors with methysergide or cinanserin also antagonized the response.9. The major tranquillizers haloperidol and chlorpromazine reduced the response. Haloperidol was especially effective in this regard.10. The tricyclic antidepressant drug imipramine potentiated the response.11. The morphine antagonist nalorphine completely prevented the response.12. The anticholinergic agent atropine and the antihistaminic drug mepyramine did not affect the response.13. We conclude that dopamine, noradrenaline and 5-hydroxytryptamine are all involved in the normal activation response of grouped mice to morphine, with dopaminergic mechanisms being of primary importance.

Effects of alpha-methyl-p-tyrosine, p-chlorophenylalanine, l-beta-(3,4-dihydroxyphenyl)alanine, 5-hydroxytryptophan and diethyldithiocarbamate on the analgesic activity of morphine and methylamphetamine in the mouse

1. The analgesic activity of sympathomimetic drugs does not appear to involve a peripheral component.2. Drugs causing changes in morphine analgesia have similar effects on the analgesia produced by methylamphetamine.3. The analgesia produced by morphine and methylamphetamine is increased by drugs which increase the ratio of brain 5-hydroxytryptamine (5-HT) to dopamine.4. The analgesia is decreased by drugs causing a fall in brain 5-HT or a rise in dopamine relative to 5-HT.