Ilmar Jurna

Mutual potentiation of antinociceptive effects of morphine and clonidine on motor and sensory responses in rat spinal cord

Clonidine and morphine depress nociceptive reflex responses when given alone; when given in combination, the effect of each is potentiated by the other. The present study was designed to test if activity in ascending axons evoked by electrical stimulation of afferent C-fibers in the sural nerve of the rat also exhibits potentiation of the depressant effects of clonidine and morphine when both drugs are administered in combination by intrathecal (i.t.) injection to the lumbar spinal cord. For comparison, experiments were also carried out on the tail-flick response in rats. The results show that clonidine produced a dose-dependent inhibition of the tail-flick response (Ed50 20 micrograms); a combination of ineffective doses of clonidine (0.3 microgram) and morphine (2 micrograms) significantly inhibited the tail-flick response; clonidine (35 micrograms) reduced spontaneous, C-fiber-evoked and, due to co-activation, A delta-fibre-evoked activity in ascending axons; and clonidine at a threshold (0.3 microgram) or higher (3 micrograms) dose administered together with morphine at a dose (2 micrograms) that caused only a moderate inhibition produced a supra-additive effect in significantly depressing spontaneous. A delta- and C-fiber-evoked ascending activity. The dose-response curve of depression by morphine alone of C-fiber-evoked activity (ED50 8 micrograms) is significantly shifted by clonidine to the left (ED50 0.9 microgram). Naloxone (0.2 mg/kg) injected intravenously did not affect the inhibition of ascending activity caused by clonidine at the highest dose (35 micrograms), but it reduced the depressant effect of combined i.t. administration of clonidine and morphine. The potentiation of the antinociceptive effects of clonidine and morphine given in combination are possibly due to actions of the two drugs at different sites between the nociceptive afferents and the neurons sending their axons to the brain.

Antinociceptive effect of centrally administered caerulein and cholecystokinin octapeptide (CCK-8)

Abstract When injected into the brain (periaqueductal gray (PAG), caudate nucleus, ventromedial thalamus) or subarachnoidal space of the lumbar spinal cord of rats, caerulein and CCK-8 produced a long-lasting (2 h) inhibition of the tail-flick response to thermal stimulation. The effective dose range for caerulin was from 1 to 5 ng per rat, for CCK-8 from 10 to 40 ng, and for morphine from 1 to 20 μg. On a molar basis and dependent upon the site of administration, caerulein was 4000–7000 times and CCK-8400-700 times more potent than morphine. Unsulphated caerulein (injected into the PAG) was less active than caerulin by a factor of 9. Sedation was seen after intracerebral, but not after spinal administration of the peptides; it was not observed after morphine or unsulphated caerulin. Naloxone (0.5 mg/kg i.p.) abolished the antinociceptive but not the sedative effect. It is concluded that caerulin and CCK-8 are very potent in reducing nociception by acting at both spinal and supraspinal levels.

Nociceptive threshold after neonatal capsaicin treatment

The nociceptive threshold as determined by the reaction time in the hot-plate and tail-flick tests was measured 3 to 4 months after pretreatment of young rats with capsaicin (50 mg kg-1 s.c.). The reaction time in the tail-flick test was prolonged in rats pretreated with capsaicin on the 2nd day of life. Capsaicin pretreatment up to the 10th day of life also resulted in a prolonged reaction time in the hot-plate test whereas capsaicin pretreatment after the 10th day of life was without effect. The elevation of the nociceptive threshold after neonatal capsaicin pretreatment most likely follows from degeneration of afferent nerve fibres activated by noxious stimuli.

Central effect of the non-steroid anti-inflammatory agents, indometacin, ibuprofen, and diclofenac, determined in C fibre-evoked activity in single neurones of the rat thalamus

&NA; This study aimed to investigate if the non‐steroid anti‐inflammatory agents, indometacin, ibuprofen and diclofenac, are capable of depressing sensory responses of the nociceptive system by a central action. For this purpose, experiments were carried out on rats under urethane anaesthesia in which activity was elicited by electrical stimulation of afferent C fibres in the sural nerve. Recordings were made ipsi‐ or contralaterally from single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus. The 3 drugs produced a dose‐dependent depression of the evoked activity which amounted to about 60% of the controls at the highest doses employed and lasted longer than 60 min. Their potency ranking, according to the ED50 values (in brackets), is: indometacin (5 mg/kg) > diclofenac (10.9 mg/kg) > ibuprofen (15.6 mg/kg). The results suggest that a central action might contribute to the analgesia produced by these non‐steroid anti‐inflammatory agents.

Depression by morphine and the non-opioid analgesic agents, metamizol (dipyrone), lysine acetylsalicylate, and paracetamol, of activity in rat thalamus neurones evoked by electrical stimulation of nociceptive afferents

&NA; Pyrazolone and salicylic acid derivatives and the aniline derivative, paracetamol, are often classified as peripherally acting analgesic agents, while morphine is a centrally acting analgesic agent. Since indications exist that the non‐opioid analgesic agents can also produce central effects, experiments were carried out on rats under urethane anaesthesia in which activity was recorded from single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus that was elicited by supramaximal electrical stimulation of nociceptive afferents in the sural nerve. In addition, activity was recorded in ascending axons of the spinal cord which was evoked by electrical stimulation of nociceptive afferents in the sural nerve. The substances studied were morphine, the pyrazolone derivatives, metamizol (dipyrone) and aminophenazone (‘Pyramidon’), lysine acetylsalicylate, and paracetamol. All drugs were found to depress dose‐dependently evoked activity in VDM neurones after intravenous (i.v.) injection. The ED50 of morphine in depressing evoked activity in VDM neurones is 0.05 mg/kg. Morphine also dose‐dependently reduced activity in ascending axons of the spinal cord, the ED50 being 1.7 mg/kg. The ED50 of metamizol in depressing evoked activity in VDM neurones is 120 mg/kg, and that of aminophenazone is 22.7 mg/kg. The 2 ED50 values differ significantly. It has been found previously that metamizol increased nociceptive activity in some ascending axons and aminophenazone increased this activity in all ascending axons tested. The ED50 of lysine acetylsalicylate in depressing evoked activity in VDM neurones is 74 mg/kg. The drug did not reduce nociceptive activity in ascending axons of the spinal cord. The ED650 of paracetamol in depressing evoked activity in VDM neurones is 19.0 mg/kg. Paracetamol did not depress nociceptive activity in ascending axons of the spinal cord at a dose as high as 150 mg/kg administered by intraperitoneal injection. Naloxone (0.2 mg/kg i.v.) abolished the depressant effects of morphine but failed to reduce those of the non‐opioid analgesic agents even at a high dose (1 mg/kg i.v.). Unlike morphine, the non‐opioid analgesic agents did not completely block evoked activity in VDM neurones but only partially blocked their activation. The results suggest that the non‐opioid analgesic agents tested can produce a central analgesic effect which, however, is weaker than that of morphine.

Activation of inhibition from the periaqueductal grey matter mediates central analgesic effect of metamizol (dipyrone)

&NA; The pyrazolone derivative, metamizol (dipyrone), possesses analgesic, antipyretic, anti‐inflammatory and spasmolytic properties. It is often classified as peripherally acting. To test the possibility that a central action of the drug contributes to its antinociceptive and analgesic effects, experiments were carried out in which the tail‐flick response to radiant heat, flexor reflex activity in the tibialis anterior muscle and activity in ascending spinal axons evoked by stimulation of afferent C fibres in the sural nerve and activity of neurones in the periaqueductal grey matter and the substantia nigra were assessed in rats. Metamizol administered by intraperitoneal (i.p.; 10, 20 and 40 mg/kg) or intrathecal (i.t.; 50 to 400 &mgr;g) injection to intact rats dose‐dependently prolonged the tail‐flick latency. Administration by i.t. injection to spinal rats was without effect. Intravenous (i.v.) injection of metamizol (140 mg/kg) reduced flexor reflex activity in intact animals, while an i.t. injection to spinal rats was ineffective at a low dose (100 &mgr;g) or enhanced the reflex activity at a higher dose (400 &mgr;g). Activity in ascending axons responding to afferent C fibre stimulation was mostly depressed by i.t. injection of metamizol (40, 80 and 140 mg/kg) in rats with an intact spinal cord. Ascending activity was increased by i.t. injection of the drug (100 and 200 &mgr;g) to spinal rats. Metamizol (140 mg/kg) i.v. increased the activity of neurones in the PAG and reduced that of neurones in the substantia nigra. Metamizol administered by microinjection into the PAG prolonged the tail‐flick latency (15–100 &mgr;g) and depressed C fibre‐evoked activity in ascending axons (100 &mgr;g). The results suggest that a central action is involved in the analgesic effect of metamizol and that this central action manifests itself by an activation of inhibition originating in the PAG.

The effect of morphine on the activity evoked in ventrolateral tract axons of the cat spinal cord.

SummaryThe effect of morphine on the activity in ventrolateral tract axons was studied in intercollicularly decerebrate cats with and without spinal section. Activity was elicited by electrical stimulation of Aδand C-fibres in the sural nerves. In spinal animals, morphine injected intravenously in a dose as low as 0.5 mg/kg reduced the post-stimulus discharge of impulses recorded in ventrolateral tract axons below the site of transection. The depression was not only abolished but reversed by levallorphan and naloxone. Pretreatment with reserpine did not diminish the effect of morphine. The effect of morphine was considerably weaker in decerebrate cats. Reversible block of the spinal cord produced by cold revealed that morphine reduced inhibition from the brain stem controlling the impulse transmission to ventrolateral tract axons.It is concluded that a spinal effect contributes to the analgesic action of morphine.

Intrathecal injection of acetylsalicylic acid, salicylic acid and indometacin depresses C fibre-evoked activity in the rat thalamus and spinal cord

&NA; It was aimed to assess if intrathecal (i.t.) injections of acetylsalicylic acid and salicylic acid depress C fibre‐evoked activity in the sensory part of the nociceptive system. In rats under urethane anaesthesia, activity was elicited in single neurones in the dorsomedial part of the ventral nucleus (VDM) of the thalamus and in ascending axons of the spinal cord by supramaximal electrical stimulation of the sural nerve. Acetylsalicylic acid and salicylic acid injected i.t. significantly reduced the activity evoked in thalamic neurones. The maximum depression amounted to about 50% of the activity evoked in the controls and was produced by acetylsalicylic acid at a dose of 50 &mgr;g (0.28 &mgr;mol/rat and by salicylic acid at a dose of 37.5 &mgr;g (0.27 &mgr;mol)/rat. Indometacin injected i.t. also reduced C fibre‐evoked activity in the thalamus in a dose‐dependent fashion, 100 &mgr;g producing a 50% depression. Salicylic acid (37.5 &mgr;g/rat i.e.) depressed C fibre‐evoked activity in ascending axons but had no effect on A&bgr; fibre‐evoked activity. It is concluded that i.t. injection of acetylsalicylic acid selectively inhibits nociceptive impulse transmission in the spinal cord by an action of the salicylic acid moiety. It is possible that prostaglandins are involved in the central action of salicylic acid.

The dependence of the anti-nociceptive effect of morphine and other analgesic agents on spinal motor activity after central monoamine depletion☆

Abstract The interference of central monoamine depletion with the anti-nociceptive effect of morphine, pethidine and aminophenazone was studied in rats with regard to the changes in spinal motor activity induced by reserpine, tetrabenazine and α-methyl-p-tyrosine. All three analgesic agents prolonged the time of the tail-flick reaction in intact rats. This effect was abolished by reserpine, which prolonged the reaction time. Bilateral lesioning of the substantiae nigrae with microinjections of 6-hydroxydopamine prolonged the reaction time and abolished the anti-nociceptive effect of morphine. In spinal rats, the time of the tail-flick reaction was prolonged by morphine and reduced by aminophenazone. Reserpine did not abolish the effect of morphine in spinal rats. Morphine, pethidine and aminophenazone inhibited the α-reflex discharges facilitated by conditioning stimulation in intact and spinal rats. Pethidine and aminophenazone, but not morphine, depressed the facilitation of α-reflex discharges produced by central monoamine depletion in intact rats. The increase in the amplitude of monosynaptic mass reflexes produced by reserpine in intact rats was not reduced by morphine, whereas the depression of polysynaptic mass reflexes after reserpine was antagonized by morphine. The three analgesic agents differ markedly in their action on spinal motor activity altered by monoamine depletion. It is concluded that the antagonizing effect of central monoamine depletion on the anti-nociceptive effect of morphine, pethidine and aminophenazone in the rat is due to a change in the basal motor activity on which the spinal nociceptive reflex is elicited.

Cyclic nucleotides and aminophylline produce different effects on nociceptive motor and sensory responses in the rat spinal cord

SummaryThe effect of intrathecal (i.t.) and systemic (i.p. and i.v.) administration of morphine, aminophylline, dibutyryl cyclic adenosine monophosphate (DBcAMP) and dibutyryl cyclic guanosine monophosphate (DBcGMP) on motor and sensory responses of the spinal nociceptive system was studied in rats. Motor responses were assessed in the tail-flick test performed on rats with an intact spinal cord, or as flexor reflex activity elicited in the electromyogram of the tibialis anterior muscle by supramaximal electrical stimulation of the sural nerve in rats in which the spinal cord was transected at the lower thoracic level. The sensory response consisted of activity in single ascending axons of the spinal cord evoked by electrical stimulation of afferent C fibres in spinal rats.Morphine (20 μg i.t. or 2 mg/kg i.p.) prolonged the tail-flick latency and aminophylline (25 mg/kg i.p. or 50 μg i.t.) prevented the antinociceptive effect of morphine. Aminophylline alone, administered by i.t. injection, reduced the tail-flick latency in a dose-dependent way. Morphine (2 mg/kg i.v. or 10 μg i.t.) reduced flexor reflex activity, and this reduction was abolished by aminophylline (25 mg/kg i.v. or 50 μg i.t.). Morphine (2 mg/kg i.v.) depressed spontaneous and evoked activity in single ascending axons responding to stimulation of afferent C fibres. This depressant effect of morphine was not abolished by aminophylline (50 μg i.t.); the depression was antagonized by naloxone (10 μg i.t.).DBcAMP (5 to 100 ng i.t.) dose-dependently prolonged the tail-flick latency. The antinociceptive effect of DBcAMP (50 ng i.t.) was prevented by aminophylline (50 μg i.t.) or naloxone (5 μg i.t.). DBcAMP (100 ng i.t.) reduced flexor reflex activity but facilitated activity in ascending axons responding to stimulation of afferent C fibres. DBcAMP (100 ng i.t.) did not affect ascending activity evoked by stimulation of afferent Aδ or Aβ fibres.DBcGMP (150 ng i.t.) depressed spontaneous and evoked activity in ascending axons responding to afferent Aδ and C fibre stimulation. It produced no effect on activity in ascending axons evoked by afferent Aβ fibre stimulation.These results demonstrate that (1) the depression by morphine of nociceptive motor and sensory responses of the rat spinal cord is based on different mechanisms in which nucleotides could (but need not) be involved, and (2) the two responses may divergently be influenced by drugs.