Spinal beta-endorphin analgesia involves an interaction with local monoaminergic systems

Monoamines as Drug Targets in Chronic Pain: Focusing on Neuropathic Pain

Monoamines are involved in regulating the endogenous pain system and indeed, peripheral and central monoaminergic dysfunction has been demonstrated in certain types of pain, particularly in neuropathic pain. Accordingly, drugs that modulate the monaminergic system and that were originally designed to treat depression are now considered to be first line treatments for certain types of neuropathic pain (e.g., serotonin and noradrenaline (and also dopamine) reuptake inhibitors). The analgesia induced by these drugs seems to be mediated by inhibiting the reuptake of these monoamines, thereby reinforcing the descending inhibitory pain pathways. Hence, it is of particular interest to study the monoaminergic mechanisms involved in the development and maintenance of chronic pain. Other analgesic drugs may also be used in combination with monoamines to facilitate descending pain inhibition (e.g., gabapentinoids and opioids) and such combinations are often also used to alleviate certain types of chronic pain. By contrast, while NSAIDs are thought to influence the monoaminergic system, they just produce consistent analgesia in inflammatory pain. Thus, in this review we will provide preclinical and clinical evidence of the role of monoamines in the modulation of chronic pain, reviewing how this system is implicated in the analgesic mechanism of action of antidepressants, gabapentinoids, atypical opioids, NSAIDs and histaminergic drugs.

Involvement of spinal cord opioid mechanisms in the acute antinociceptive effect of hyperbaric oxygen in mice

Earlier research has demonstrated that treatment with hyperbaric oxygen (HBO2) can elicit an antinociceptive response in models of acute pain. We have demonstrated that this antinociceptive effect is centrally-mediated and is dependent on opioid receptors. The purpose of the present study was to examine the role of endogenous opioid peptides and opioid receptors specifically in the spinal cord in the acute antinociceptive effect of HBO2 in mice. Male NIH Swiss mice were exposed to HBO2 (100% oxygen at 3.5atm absolute) for 11min and their antinociceptive responsiveness was determined using the glacial acetic acid-induced abdominal constriction test. HBO2-induced antinociception was sensitive to antagonism by intrathecal (i.t.) pretreatment with the κ- and μ-selective opioid antagonists norbinaltorphimine and β-funaltrexamine, respectively, but not the δ-selective antagonist naltrindole. The antinociceptive effect of HBO2 was also significantly attenuated by i.t. pretreatment with a rabbit antiserum against rat dynorphin1-13 but not antisera against β-endorphin or methionine-enkephalin. Based on these experimental findings, the acute antinociceptive effect of HBO2 appears to involve neuronal release of dynorphin and activation of κ- and μ-opioid receptors in the spinal cord.

No detectable analgesic effects in the formalin test even with one million bovine adrenal chromaffin cells

The present experiments were conducted to identify analgesic agents for transfection into immortalized adrenal chromaffin cell lines to maximize their analgesic potential. Analgesic agents known to be produced by adrenal chromaffin cells were infused intrathecally at a low dose (0.2 microg) which might conceivably be attained by adrenal chromaffin cell transplants. Numerous agents, administered individually and in two-factor combinations, produced significant analgesic effects in the formalin test. Before assessing the potential additive or synergistic effects of these analgesic agents with adrenal chromaffin cells, studies were conducted to demonstrate analgesic effects with adrenal chromaffin cells alone. Analgesic effects were previously reported in the literature with 80-100k intrathecal bovine adrenal chromaffin (BAC) cells; but in the present study 500k purified BAC cells failed to produce detectable analgesic effects. One million purified BAC cells also failed to produce analgesic effects in the formalin test. In a final study, even nicotine-stimulated release from one million purified BAC cells failed to produce analgesic effects in the formalin test. The fact that even one million nicotine-stimulated BAC cells failed to demonstrate therapeutic potential in these blinded experiments under conditions which were clearly sensitive to the analgesic agents produced by BAC cells, raises serious questions about the clinical utility of this experimental treatment.

Intrathecal endomorphin-1 produces antinociceptive activities modulated by alpha 2-adrenoceptors in the rat tail flick, tail pressure and formalin tests

It is known that spinal morphine produces antinociception that is modulated by alpha 2-adrenoceptors. Endomorphin-1, a newly-isolated endogenous opioid ligand, shows the greatest selectivity and affinity for the mu-opiate receptor of any endogenous substance found to date and may serve as a natural ligand for the mu-opiate receptor. We examined the antinociceptive effects of endomorphin-1 administered intrathecally (i.t.) in the rat tail flick, tail pressure and formalin tests. Intrathecal endomorphin-1 produced dose-dependent antinociceptive effects in the three tests. ED50 (CI95) values for antinociception of i.t. endomorphin-1 in the tail flick test and tail pressure test were 1.9 (0.96-3.76) nmol and 1.8 (0.8-4.2) nmol, respectively. ED50 (CI95) values for phase 1 and phase 2 in the formalin test were 12.5 (7.9-19.8) nmol and 17.5 (10.2-30) nmol, respectively. Pretreatment with i.t. beta-funaltrexamine (a mu-opioid receptor selective antagonist) significantly antagonized the antinociceptive effects of endomorphin-1 in the three tests. Beta-funaltrexamine alone had not effects on the three tests. The antinociceptive effects of endomorphin-1 were also antagonized by i.t. yohimbine (an alpha 2-adrenoceptor selective antagonist). The combination of ineffective doses of i.t. clonidine (an alpha 2-adrenoceptor agonist) and endomorphin-1 produced a significant antinociception in the three tests. The results showed that intrathecal endomorphin-1 produced antinociception in a dose-dependent manner in the rat tail flick, tail pressure and formalin tests, which was mediated by spinal mu-opioid receptors and modulated by alpha 2-adrenoceptors.

Differential influence of two serotonin 5-HT3 receptor antagonists on spinal serotonin-induced analgesia in rats

We tested the antinociceptive effect of intrathecal (i.t.) administration of 5-HT3 and the 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide (mCPBG), in rats submitted to a mechanical noxious stimulus and the influence of the 5-HT3 receptor selective antagonists, tropisetron and granisetron. Both 5-HT and mCPBG (0.01, 0.1, 1, 10, 20 micrograms/rat) produced a significant dose-dependent antinociception. The lowest active doses were 0.1 and 1 microgram for 5-HT and mCPBG, respectively. The effect, observed with 20 micrograms, was significantly lower with mCPBG (+33 +/- 6%) than with 5-HT (+63 +/- 7%). For 5-HT-induced antinociception, the minimal inhibitory doses were 0.001 micrograms/rat for tropisetron and 10 micrograms/rat for granisetron. In contrast, the same doses of the two antagonists (from 0.1 microgram/rat) similarly inhibited the effect of mCPBG. This study provides evidence that contrary to tropisetron, doses of granisetron able to inhibit the effect of a 5-HT3 receptor agonist failed to reduce that of 5-HT. This demonstrates a heterogeneity between 5-HT3 receptor antagonists and questions the true involvement of these receptors in spinal 5-HT-induced antinociception.

Interrelationships of neurochemicals, estrogen, and recurring headache

Recurring headache syndromes, such as migraine, are common problems for women throughout their adult lives. Headache symptoms often fluctuate over the years that they are present and, for most headache sufferers, these changes seem to occur randomly. For many women, however, chronic headache changes in predictable patterns in relation to alterations in hormonal states. Clinically, headache is often modified during menses, pregnancy, and menopause. Although sex hormones are changing with these clinical events, this paper will present the more important link between altered sex hormones and changes in neurochemicals believed to be responsible for recurring headache syndromes according to the neurobiological theory of migraine.

Opioid effects on spinal [3H]5-hydroxytryptamine release are not related to their antinociceptive action

Several opioid compounds were evaluated for an ability to modulate the K(+)-stimulated release of [3H]serotonin ([3H]5-hydroxytryptamine, [3H]5-HT) from rat spinal cord synaptosomal and tissue slice preparations. Selective kappa-opioid receptor agonists depressed K(+)-stimulated release of the radiolabelled transmitter from both tissue preparations, an effect which was reversed by norbinaltorphimine. Conversely, the selective mu- and delta-opioid receptor agonists [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO) and [D-Pen2,D-Pen5]enkephalin (DPDPE), respectively, enhanced the K(+)-stimulated release of [3H]5-HT. This effect was only seen using the tissue slice preparation. When used at concentrations near its reported Kd for mu-opioid receptors, the selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) blocked the action of DAMGO, but had no effect on the action of DPDPE. However, higher concentrations of CTOP, as well as all effective concentrations of selective delta-opioid receptor antagonists, blocked the action of both DAMGO and DPDPE. All agonist effects on spinal 5-HT release, regardless of the tissue preparation, were only seen at high (microM) concentrations. Moreover, effects of the opioid agonists were not consistent with the reported involvement of spinal 5-HT neurotransmission in the mediation of their antinociceptive action. Thus, the ability of opioids to modulate spinal 5-HT release appears to be of minimal physiological significance.

Age-related changes in the spinal antinociceptive effects of DAGO, DPDPE and beta-endorphin in the rat

These studies were designed to investigate how the aging process alters the spinal antinociceptive efficacy of mu (mu), delta (delta) and epsilon (epsilon) opioid receptor agonists administered intrathecally (i.t.) in rats. Various doses of the mu agonist DAGO, the delta agonist DPDPE or the putative epsilon beta-endorphin were injected i.t. in young (5-6-month-old), mature (15-16-month-old) and aged (25-26-month-old) Fischer 344 rats. Antinociception was measured using the rat tail-flick analgesiometric assay. The data demonstrated a decline in spinal opioid-induced antinociception as a function of age. For instance, the i.t. dose of DPDPE or beta-endorphin needed to produce antinociception in the 25-26-month-old rats was higher than that needed to elevate tail-flick latency in the young and mature animals. We also noted that the i.t. doses of the opioid agonists needed to produce 'antinociception' in the aged cohort were within a range of spinal doses that produced motor impairment. Apparently, the aging process alters the ability of opioid receptors to mediate antinociception. Perhaps an age-related decrease in the number and/or affinity of opioid receptor sites in the rat spinal cord accounts for these observations.

Effects of aging on spinal opioid-induced antinociception

Initial experiments were conducted to determine whether or not the aging process alters the ability of young, mature, or aged male Fischer 344 rats (5- to 6-, 15- to 16-, and 25- to 26-months-old, respectively) to respond to thermal nociceptive stimuli. Using the tail-flick analgesiometric assay, 25- to 26-month-old rats responded significantly faster to the heat source than 15- to 16-month-old animals, but no significant differences were noted between the 5- to 6-month-old and aged rats. Another series of investigations compared the effects of aging on the spinal antinociceptive properties of the mu opioid agonist [D-Ala2,N-methyl-Phe4,Gly5-ol] enkephalin (DAMPGO) and the delta agonist [D-Pen2,D-Pen5] enkephalin (DPDPE). In these studies, young, mature, and aged rats were injected intrathecally (IT) with different doses of DAMPGO or DPDPE, and opioid-induced antinociception was tested on the tail-flick test. All three age groups responded to IT DAMPGO in a dose-dependent manner but, for the most part, higher spinal doses were required to produce significant elevations in tail-flick latency in the aged cohort of rats. The spinal analgesic effects of DPDPE also declined with advanced age. The aging process apparently alters the pain-inhibitory function of mu and delta opioid receptors in the rat spinal cord.

Spinal dopaminergic system of the rat: light and electron microscopic study using an antiserum against dopamine, with particular emphasis on synaptic incidence

The mapping of the spinal dopaminergic innervation has been performed in the adult rat using an anti-dopamine antiserum. Immunoreactive fibers were detected with the light microscope in the dorsal horn (mainly in laminae III-IV), in the intermediolateral cell column (IML), in the peri-ependymal region and in the ventral horn. The ultrastructural analysis of dopaminergic innervation showed mainly axodendritic contacts and fewer axosomatic ones. In the ventral horn and the IML, the pattern of dopaminergic innervation exhibited a majority of classical synapses. In the dorsal horn, dopaminergic innervation was partly non-synaptic (at cervical level), whereas numerous axodendritic synapses were observed at thoraco-lumbar level. Previous studies described the non-synaptic organization of serotonergic and noradrenergic projections in the dorsal horn. It is thus hypothesized that the monoaminergic systems, involved in pain modulation within the dorsal horn, act partly through volume transmission. In contrast, these systems would modulate the motor and autonomic functions through classical synapses.

The noradrenergic component contributing to spinal fentanyl-induced antinociception is supraspinally mediated

1. Male Sprague-Dawley rats were fitted with intrathecal (i.t.) and intracerebroventricular (i.c.v.) catheters. Fentanyl was injected either i.t. or i.c.v., and the antinociceptive efficacy of fentanyl was evaluated using the tail-flick analgesiometric assay. 2. Fentanyl dose-dependently elevated tail-flick latency (TFL) following i.c.v. or i.t. administration. The antinociceptive effects of fentanyl were reversed by naltrexone. 3. Experiments were also designed to evaluate the effects of serotonin and alpha-adrenoceptor antagonists on i.t. or i.c.v. fentanyl-induced elevations in TFL. 4. Phentolamine administered i.t. reversed both the spinal and supraspinal antinociceptive effects of fentanyl, whereas i.t. methysergide did not significantly alter the i.t. or i.c.v. effects of the mu agonist. 5. These data suggest that fentanyl-induced antinociception does not rely on local serotonergic neuronal activation. Due to the highly lipophilic nature of fentanyl, it is possible that the noradrenergic component contributing to spinal fentanyl-induced analgesia is supraspinally-mediated.

A single restraint stress exposure potentiates analgesia induced by intrathecally administered DAGO

In rats, restraint exposure potentiates the magnitude and duration of analgesia following both the peripheral and intracerebroventricular administration of several opioid agonists as compared to non-stressed controls. It has been suggested that the site of action whereby restraint leads to potentiated opioid analgesia is located supraspinally. However, the possible contribution of spinal analgesic mechanisms also warrants investigation. Thus, the purpose of the present study was two-fold: (1) to determine whether a single exposure to restraint stress would result in the dose-dependent potentiation of analgesia following the intrathecal (i.t.) administration of the mu (mu)-receptor selective opioid agonist [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO) and (2) to quantify the degree of analgesia in restrained vs. non-restrained rats using the tail-flick and hot-plate analgesic assays. Using rats implanted with chronic i.t. cannula, dose- and time-course curves were observed following the i.t. administration of DAGO. The results demonstrate that both the duration and magnitude of analgesia was significantly potentiated in restrained rats compared to non-restrained controls. Restraint-treated rats receiving 0.15-0.6 micrograms of DAGO i.t. showed 1.3-1.5-fold potentiation of analgesia in the tail-flick assay and a 2.3-5.6-fold potentiation using the hot-plate assay. Restraint immobilization potentiated the magnitude and duration of DAGO-induced analgesia administered by the i.t. route as measured by the tail-flick and hot-plate assays. These data suggest that spinal analgesic mechanisms significantly contribute to the enhanced analgesic potency of opioids in subjects exposed to restraint stress.

The tail-flick inhibition induced by beta-endorphin administered intrathecally is mediated by activation of kappa- and mu-opioid receptors in the mouse

The inhibition of the tail-flick response induced by beta-endorphin given i.c.v. has been demonstrated to be mediated by the stimulation of epsilon- but not mu-, delta- or kappa-opioid receptors. beta-Endorphin given i.t. also inhibited the tail-flick response. The present studies were designed to determine what types of opioid receptors in the spinal cord were involved in i.t. beta-endorphin-induced tail-flick inhibition. Blockade of kappa-opioid receptors by coadministration of nor-binaltorphimine or Win 44,441-3 with beta-endorphin given i.t. dose dependently inhibited i.t. beta-endorphin-induced inhibition of the tail-flick response. Blockade of mu-opioid receptors by i.t. coadministration of D-Phe-Cys-Tyr-D-Try-Orn-Thr-Pen-Thr-NH2 with beta-endorphin blocked i.t. beta-endorphin-induced inhibition of the tail-flick response. I.t. injection of delta-opioid receptors antagonists, ICI 174,864 and naltrindole, or epsilon-opioid receptor antagonist, beta-endorphin-(1-27), did not affect inhibition of the tail-flick response induced by beta-endorphin given i.t. Blockade of alpha 2-adrenoceptors and 5-HT receptors by i.t. injection of yohimbine and methysergide, respectively, also did not affect inhibition of the tail-flick response induced by beta-endorphin given i.t. The results indicate that the inhibition of the tail-flick response induced by beta-endorphin given i.t. is mediated by the stimulation of kappa- and mu-opioid receptors but not delta- and epsilon-opioid receptors, alpha 2-adrenoceptors or 5-HT receptors.

Evaluation of the interactions of serotonergic and adrenergic drugs with mu, delta, and kappa opioid binding sites

Several serotonergic and adrenergic agents were tested for an ability to interact with mu, delta, and kappa opioid binding sites. Spiroxatrine interacted nearly equipotently with all three opioid subtypes, yielding Ki values near 110 nM. A number of other serotonergic and adrenergic agents interacted with affinities in the 1-50 microM range. Most of the other compounds tested in this study were found to compete for opioid binding to some degree, though not achieving a 50% inhibition of binding at concentrations up to 100 microM. If this interaction between monoaminergic agents and opioid receptors is found to have functional significance, it must be considered in the interpretation of results from studies using these agents to evaluate the contribution of monoaminergic systems to opioid-mediated events.

Serotonin contributes to the spinal antinociceptive effects of morphine

This study was designed to determine if morphine administered intrathecally (IT) interacts with serotonergic or noradrenergic nerve terminals in the spinal cord to produce analgesia on the spinally mediated tail-flick test. Male Sprague-Dawley rats were fitted with IT catheters. One week later, animals were spinally pretreated with receptor antagonists selective for opioid, serotonin or alpha-adrenoceptors, and the ability of these agents to alter spinal morphine-induced antinociception was assessed. Morphine dose-dependently elevated tail-flick latency in a naltrexone-reversible manner. The serotonin receptor antagonists spiroxatrine (5-HT1A), pindolol (5-HT1B), ritanserin (5-HT2) and ICS 205-930 (5-HT3) attenuated the spinal analgesic effects of morphine. In contrast, the alpha 1 and alpha 2-adrenoceptor antagonists prazosin and yohimbine, respectively, did not alter morphine-induced elevations in tail-flick latency. These data substantiate earlier reports that spinal morphine-induced antinociception relies on an opioid receptor-mediated component in addition to a local serotonergic component. The finding that the alpha-adrenoceptor antagonists did not alter the antinociceptive effects of IT morphine suggests that spinal norepinephrine does not contribute to the analgesic effects of the opiate.

Analgesic effects of serotonin and receptor-selective serotonin agonists in the rat spinal cord

1. Serotonin (5-HT) and selective 5-HT receptor agonists were administered intrathecally (i.t.) in rats, and the antinociceptive efficacy of these agents was assessed on the tail-flick and hot plate tests. 2. The 5-HT receptor agonists examined in this study included the 5-HT1A agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin (8-OH-DPAT), the 5-HT1B agonist m-trifluoromethylphenylpiperazine (TFMPP), the 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT3 agonist phenylbiguanide (PBG). 3. None of these agents produced significant elevations in tail-flick latency (TFL) at doses which produced elevations in hot plate latency (HPL). 4. In contrast, the i.t. dose of 5-HT which elevated TFL also produced analgesia on the hot plate test. 5. Serotonin-induced elevations in TFL were reversed by pindolol, ritanserin and ICS 205-930, suggesting that 5-HT interacts with more than one 5-HT site in the spinal cord to produce analgesia on the tail-flick test. 6. The finding that ritanserin reversed 5-HT-induced elevations in HPL suggests that the 5-HT2 site is primarily responsible for mediating the spinal antinociceptive effects of 5-HT on the hot plate test.

Supraspinal and spinal monoamine-modified function and the expression of opioid antinociception

Opioids are highly valuable clinical agents for the treatment of pain which are thought to act both at the spinal and supraspinal level. During the course of their actions, they have complex interactions with monoamine systems. These include 5-hydroxytryptamine (5-HT) and noradrenaline (NA), so this topic is discussed using these two transmitter systems, their locations and receptor sub-types, as prime candidates for modulating nociceptive and antinociceptive processes. Several classes of 5-HT receptors, as well as α(2)-adrenoceptors, appear to be clearly involved in antinociception and the functions of systems carrying these receptors may be modified using psychotropic agents. In particular, some antidepressants may acutely augment opioid antinociception and this property may be exploited to delay the onset of opioid tolerance in the sub-acute situation.

A comparative analysis of monoaminergic involvement in the spinal antinociceptive action of DAMPGO and DPDPE

The antinociceptive properties of intrathecally (i.t.) administered [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMPGO) and [D-Pen2, D-Pen5]enkephalin (DPDPE), selective opioid agonists for mu (mu) and delta (delta) sites, respectively, were compared in rats. DAMPGO and DPDPE elevated tail-flick latency (TFL) in a dose-dependent manner, and the spinal antinociceptive actions of both drugs were reversed by the opiate antagonist naloxone. These findings suggest that both DAMPGO and DPDPE interact with spinal opiate receptors to elevate TFL. Another set of experiments was done to determine the involvement of local spinal serotonin (5-HT) or norepinephrine (NE) in DAMPGO and DPDPE-induced spinal analgesia. Both the alpha 1 noradrenergic receptor antagonist WB-4101 and the alpha 2 blocker yohimbine failed to alter the antinociceptive actions of DAMPGO and DPDPE. Similarly, the 5-HT receptor antagonists pindolol, ritanserin and ICS 205-930 (selective for 5-HT1, 5-HT2 and 5-HT3 sites, respectively) failed to inhibit opioid-induced spinal analgesia. Thus, while DAMPGO and DPDPE produce antinociception via an interaction with spinal opioid receptors, apparently neither drug activates endogenous monoaminergic systems.