Effects of intrathecally administered pentobarbital and naloxone on the activity evoked in ascending axons of the rat spinal cord by stimulation of afferent A and C fibres. Further evidence for a tonic endorphinergic inhibition in nociception

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Paradoxical Modulation of Nociception in Mice by Barbiturate Agonism and Antagonism: Is a GABA Site Involved in Nociception?

In a battery of four acute and chronic nociceptive tests, the GABA antagonist picrotoxin produces a uniform and sustained analgesia in mice. By contrast, barbiturates which have been presumed to act at the same receptor produce mixed and paradoxical actions. At a standard time of 10 min after drug administration a convulsant barbiturate [5-ethyl-5-(3'-methyl-but-2'-enyl)-barbituric acid] produced analgesia in three tests but had no effect in the fourth; a pure hypnotic barbiturate (amylobarbitone) produced hyperalgesia in three tests but analgesia in the fourth; while the mixed hypnotic-convulsant pentobarbitone produced hyperalgesia in two of the tests and was without any effect in the other two. There was no pattern in these results with respect to acute or chronic nociceptive tests. Surprisingly, with extended observation using the tail-flick test both pentobarbitone and the pure hypnotic (amylobarbitone) gave early hyperalgesia followed by analgesia; the convulsant barbiturate gave only analgesia. The results suggest a role for GABAA receptors in the transmission of nociceptive information; they also suggest that barbiturates act at quite a different receptor.

Chronic pain-related behaviors in spinally injured rats: evidence for functional alterations of the endogenous cholecystokinin and opioid systems

We have recently developed a rat model of chronic pain states after spinal cord injury. Thus, after severe, but incomplete, ischemic spinal cord injury, some rats chronically exhibited responses indicative of pain to innocuous mechanical stimuli (allodynia) in the rostral dermatomes involving the injured spinal segments. These responses have some characteristics in common with chronic central pain in patients with spinal cord injury. We now report that systemic CI988, a specific antagonist of the cholecystokinin (CCK) type B receptor, effectively relieved the allodynia-like symptom, an effect that was reversed by the opioid receptor antagonist naloxone. Furthermore, in rats which did not develop the allodynia-like symptom after spinal cord lesion, systemic naloxone induced typical allodynia. In contrast, naloxone failed to produce allodynia in normal animals. It is thus suggested that the abnormal sensory processing initiated by spinal cord ischemic lesion is under tonic opioidergic control and dysfunction of this control by the upregulated endogenous CCK system is responsible for the development of painful sensations in these rats.

Comparison of the antinociceptive effects induced by electroacupuncture and transcutaneous electrical nerve stimulation in the rat

The analgesic effects induced by two different kinds of peripheral conditioning stimulations, electroacupuncture (EA) and transcutaneous electrical nerve stimulation (TENS), were compared in the rat using the latency of radiant heat-evoked tail flick reflex as nociceptive index. The parallel elevations of withdrawal latency of tail flick were produced by EA and TENS administrations at the acupoints of S36 and Sp6 with low intensity (1-2-3 mA) and one of three different frequencies (2, 15 and 100 Hz). Analgesic effects of EA or TENS were characterized by slow-on and slow-off nature, and a significant linear correlation was found between both at any one of three frequencies. Systemic naloxone hydrochloride (2 mg/kg) almost completely and partially antagonized 2 and 15 Hz EA- or TENS-induced analgesia, respectively, but failed to affect those induced by 100 Hz EA or TENS. Tolerance to EA stimulation with one of three frequencies reduced the corresponding frequency TENS-induced analgesia and vice versa. These data indicate that: (a) there is no significant difference in producing antinociception for two different peripheral conditioning stimulations when applied at the same sites and (b) the common neural mechanisms most likely process the analgesic effects of EA and TENS. The involvement of (an) endogenous opiate mechanism in the management of different frequency EA and TENS analgesia is discussed in detail.

Electrophysiological evidence for a tonic activity of the spinal cord intrinsic opioid systems in a chronic pain model

The aim of this electrophysiological investigation was to evaluate the activity of the spinal endogenous opioid systems in a chronic pain model, the arthritic rat. The activity of nociceptive non-specific dorsal horn neurons (n = 23) were recorded in 23 spinal unanesthetized decerebrated rats. Naloxone (1 mg/kg i.v.) induced a highly significant increase in the spontaneous firing rate of these neurons. This observation is in favor of a tonic activity of spinal opioid endogenous systems in such a disease. In addition, the same dose of naloxone facilitates the transmission of noxious messages at the spinal level as revealed by the large enhancement of the responses of these neurons to C-fiber stimulation. These results are in good agreement with behavioral data showing that such a relatively high dose of naloxone induced well-reproducible hyperalgesia and with some biochemical observations showing an increase of levels and biosynthesis of endogenous opioids in the spinal cord of the arthritic rat.

Depression by flupirtine, a novel analgesic agent, of motor and sensory responses of the nociceptive system in the rat spinal cord

The analgesic agent, flupirtine, was tested on motor and sensory responses of the nociceptive system in rats. The motor response was determined in the tail-flick test with radiant heat. The sensory response was determined as activity evoked in ascending axons by electrical stimulation of nociceptive afferents in the sural nerve. The tail-flick latency was dose dependently increased by flupirtine administered by intraperitoneal (i.p.) injection (ED50 7.8 mg/kg), intrathecal (i.t.) injection (ED50 14.8 micrograms/rat) or bilateral microinjection into the periaqueductal grey (PAG; ED50 2.6 micrograms/rat). Naloxone reduced the effect of an i.p. injection of flupirtine but was ineffective against an i.t. injection of the drug. The activity in ascending axons responding to afferent C fibre stimulation was depressed by flupirtine administered by intravenous (i.v.) injection (7 mg/kg) under urethane anaesthesia with an intact spinal cord and brain, and by i.t. injection (14 micrograms/rat) to decerebrated spinal rats. Naloxone did not abolish the depressant effect of i.t. injections of flupirtine. Microinjection of flupirtine (1.7 micrograms/rat) made in the PAG did not reduce, but increased the spontaneous and C fibre-evoked activity in ascending axons. The results indicate that flupirtine selectively depresses responses of the nociceptive system by a spinal (motor and sensory responses) and a supraspinal (motor response) action in which opiate-like mechanisms play no or a minor role.

Opioid receptor systems and the endorphins: a review of their spinal organization

A review of the spinal organization of opioid receptor systems and endorphins is presented. The review is a consideration of the physiological mechanisms underlying the effect of spinal opioids, the pharmacology of the opioid receptors that moderate a variety of spinal processing systems, and the endorphin systems that act upon the spinal receptors.

Studies in suppression of nocifensive reflexes measured with tail flick electromyograms and using intrathecal drugs in barbiturate anesthetized rats

We report on the effects of somatic conditioning stimuli on the reflexive nocifensive tail flick electromyograms (TFEMGs) in the rat anesthetized with continuous pentobarbital infusions. Rather than using a reflexive tail movement as a measure of noxious responses, TFEMGs were recorded from the base of the rat's tail as this gave more reliable results in the anesthetized state. In order to demonstrate the compatibility of this model with previous tail flick studies, we demonstrated an inhibition of TFEMGs by intrathecal morphine which was reversed by intravenous naloxone. The TFEMG latencies were then shown to be increased by electroacupuncture. This effect was antagonized by intrathecal naltrexone pretreatment. All of these results under barbiturate anesthesia resembled those observed previously in awake rats. The constant pentobarbital infusion maintained a stable baseline state, as reflected by TFEMG latencies and blood pressure recordings during the prolonged time-course of each experiment (approximately 1 h). We feel that this method of studying electroacupuncture suppression of nocifensive reflexes is valuable for its relative simplicity and reliability. It also has the virtue of avoiding the pitfalls of such studies in awake animals.

Antinociceptive effect of systemic and intrathecal morphine in spinally transected rats

The antinociceptive effect of morphine on the tail withdrawal reflex was examined in spinally transected rats. The efficacy of systemically administered morphine was significantly reduced within 24 h after transection, and continued to decline during the first three posttransection weeks. In contrast to the diminished effect of systemic morphine, the efficacy of intrathecal morphine was not reduced during the first three weeks after a spinal transection. These data demonstrate a significant difference in the functional effect of systemic and spinal morphine in spinally transected rats. The results indicate that the direct antinociceptive effect of morphine on the spinal cord is not reduced after spinal transection.

Barbiturate-induced inhibition of a spinal nociceptive reflex: role of GABA mechanisms and descending modulation

The present study investigated the effect of systemically administered pentobarbital on the tail-flick (TF) reflex in rats, the neurochemical mechanism of action and the role of descending influences. Pentobarbital produced a clear inhibition of the TF response. Systemic administration of naloxone did not significantly alter this effect, thus it appears to be independent of endogenous opioid systems. Complete spinal transection resulted in a marked potentiation of pentobarbital-induced TF inhibition, demonstrating a spinal locus of action. Moreover, this observation suggests the existence of a tonic descending excitatory influence, opposing the pentobarbital-produced depression of nociceptive transmission in the intact animal. Intrathecal administration of pentobarbital caused a much more pronounced TF inhibition in transected than in intact animals, lending further support to this hypothesis. To identify the neurochemical mechanisms involved in pentobarbital-produced antinociception, the gamma-aminobutyric acid (GABA) antagonists bicuculline and picrotoxinin were administered intrathecally in spinalized animals. Both substances caused an attenuation of the pentobarbital effect, demonstrating the involvement of GABAergic transmission. The proposed descending excitatory system may act either presynaptically and cause a decreased release of GABA into the synapse or postsynaptically via endogenous GABA antagonistic neurotransmitters, which may change the conformation of the GABA-barbiturate receptor complex.

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.

Opioid-independent hyperalgesia induced in mice by pentobarbitone at low dosage

The abdominal constriction (writhing) test was used to investigate the effect of pentobarbitone on nociception in mice. At dosages of 10, 20 and 30 mg/kg, pentobarbitone produced marked increases in nociceptive scores. The extent of the enhancement of nociception was uninfluenced by naloxone or swim-stress. The swim-stress induced analgesia but, in contrast to previous results using other tests for nociception, the analgesia was wholly non-opioid with the abdominal constriction test.

A differential effect of naloxone on transmission of impulses in primary afferents to ventral roots and ascending spinal tracts

Ventral root reflexes and ascending volleys to stimulation of group I muscle afferents, large diameter cutaneous afferents and unmyelinated primary afferents were examined in barbiturate anaesthetized spinal cats. Intravenous naloxone (0.05-0.10 mg/kg) increased reflexes to stimulation of all primary afferent types but of the ascending volleys, only those to stimulation of unmyelinated primary afferents were increased. Thus it appears that opioid peptides have differential effects on transmission of primary afferent impulses to supraspinal areas, an action possibly relevant to analgesia, in contrast to a non-selective suppression of transmission to motoneurones.

Pharmacology of descending control systems

In the cat there is no convincing evidence that a particular compound mediates a supraspinal control of spinal transmission of nociceptive information. There is good evidence that opioid peptides are released segmentally in response to nociceptive input to the spinal cord and that this acts to inhibit motoneurons and to reduce transmission of nociceptive information to supraspinal areas. In the cat there is no evidence that stimulation at supraspinal sites producing analgesia results in a spinal release of opioid peptides. In the rat evidence for the latter has been obtained but there are no data from other species. Tonically present supraspinal inhibition of spinal transmission of nociceptive information in the cat does not involve opioid peptides. Indirect evidence favours a role for 5-hydroxytryptamine and noradrenaline in supraspinal control of spinal processing of nociceptive transmission. Peripheral antagonists of 5-HT have reduced spinal inhibition from stimulation at supraspinal sites but the site of action is unknown. Progress with noradrenaline involvement has been hindered by lack of a suitable antagonist. Although the amino acids, glycine and GABA are involved in segmental inhibition of transmission of nociceptive information, no convincing evidence has indicated their involvement in supraspinal controls.

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

The 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 micrograms i.t. or 2 mg/kg i.p.) prolonged the tail-flick latency and aminophylline (25 mg/kg i.p. or 50 micrograms 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 micrograms i.t.) reduced flexor reflex activity, and this reduction was abolished by aminophylline (25 mg/kg i.v. or 50 micrograms 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 micrograms i.t.); the depression was antagonized by naloxone (10 micrograms 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 micrograms i.t.) or naloxone (5 micrograms i.t.).(ABSTRACT TRUNCATED AT 250 WORDS)