Central GABA-ergic systems and analgesia

Antinociceptive Effect of the Citrus Flavonoid Eriocitrinon Postoperative Pain Conditions

BACKGROUND

Postoperative pain remains a major clinical problem as there are limited analgesic strategies that have been proven to be effective in preventing and relieving this type of pain. Natural products, including flavonoids, have distinct pharmacological properties and play an important role in the discovery of analgesic drugs.

MATERIALS AND METHODS

In this study, the flavonoid eriocitrin (eriodictyol 7-O-rutinoside), which is the main flavonoid in lemon fruit (Citrus limon), was mechanistically investigated for its prospective antinociceptive effect in a mouse model of postoperative pain. The antinociceptive property was evaluated by utilizing both tonic (acetic acid-induced writhing behavior) and phasic (hot-plate) nociception modalities. The hindpaw incisional surgery was performed and hyperalgesia was assessed using von Frey filaments.

RESULTS

The tested doses of eriocitrin significantly attenuated (P<0.01, P<0.001) the chemically-induced tonic visceral nociception (5, 10, 15, and 30 mg/kg) and acute phasic thermal nociception (10, 15, and 30 mg/kg). A significant dose-dependent reduction in the incisional nociceptive hyperalgesia was exhibited by eriocitrin, with a marked antinociception observed at doses of 15 mg/kg (P<0.05 during 30-60 minutes) and 30 mg/kg (P<0.05, P<0.01 during 30-120 minutes).

CONCLUSION

The antinociceptive effect of eriocitrin (30 mg/kg) was strongly blocked by the antagonists of the opioid receptor, naltrexone, and GABAA receptor, bicuculline, thereby suggesting the involvement of opioidergic and GABAergic mechanisms in the nociception, reducing proclivity of eriocitrin during transmission of incisional nociception. These results concluded that eriocitrin has a potent antinociceptive effect in postoperative pain conditions, probably mediated through opioid and GABAA receptors.

6-Methoxyflavanone attenuates mechanical allodynia and vulvodynia in the streptozotocin-induced diabetic neuropathic pain

BACKGROUND

Diabetic neuropathy is the most prevalent, persistent and debilitating complication of diabetes mellitus often coupled with vulvodynia that may present as an isolated symptom or as a part of constellation of other neuropathic abnormalities.

OBJECTIVE

Flavonoids have selective affinity for GABA receptors and 6-methoxyflavanone (6-MeOF) is a positive allosteric modulator of GABA responses at human recombinant GABA_A receptors. GABAergic and opioidergic system inhibition have been shown to facilitate neuropathic pain.

METHODS

6-MeOF was evaluated for analgesic effect in the hot plate test and streptozotocin-induced diabetic neuropathic pain in female rats using von Frey hairs. The possible involvement of opioidergic and GABAergic mechanisms was investigated using naloxone and pentylenetetrazole (PTZ) antagonists, respectively. The biodistribution of 6-MeOF in plasma and CNS was examined using a validated HPLC/UV analytical method. The binding affinity of 6-MeOF with opioid and GABA receptors was studied using molecular docking simulation approach.

RESULTS

6-MeOF (10 and 30mg/kg) attenuated the acute phasic thermal nociception in the hot plate test while in the case of streptozotocin-induced diabetic neuropathy model, 6-MeOF (10 and 30mg/kg) produced static/dynamic anti-allodynic (increased paw withdrawal threshold and latency) as well as static/dynamic anti-vulvodynic effects (increased flinching response threshold and latency), when compared to the vehicle and standard gabapentin (75mg/kg). In silico studies depicted the preference of 6-MeOF for the delta- and kappa-opioid and GABA_A receptors. Moreover, the pharmacokinetic profile revealed a quick appearance of 6-MeOF in the systemic circulation and brain areas with maximum concentration observed after 30min in the amygdala, brain stem and cerebral cortex.

CONCLUSION

6-MeOF readily crosses the blood brain barrier and may be effective in attenuating the diabetes-induced allodynia as well as vulvodynia, probably through interactions with the GABAergic and opioidergic systems.

Passiflora incarnata attenuation of neuropathic allodynia and vulvodynia apropos GABA-ergic and opioidergic antinociceptive and behavioural mechanisms

Background

Passiflora incarnata is widely used as an anxiolytic and sedative due to its putative GABAergic properties. Passiflora incarnata L. methanolic extract (PI-ME) was evaluated in an animal model of streptozotocin-induced diabetic neuropathic allodynia and vulvodynia in rats along with antinociceptive, anxiolytic and sedative activities in mice in order to examine possible underlying mechanisms.

Methods

PI-ME was tested preliminary for qualitative phytochemical analysis and then quantitatively by proximate and GC-MS analysis. The antinociceptive property was evaluated using the abdominal constriction assay and hot plate test. The anxiolytic activity was performed in a stair case model and sedative activity in an open field test. The antagonistic activities were evaluated using naloxone and/or pentylenetetrazole (PTZ). PI-ME was evaluated for prospective anti-allodynic and anti-vulvodynic properties in a rat model of streptozotocin induced neuropathic pain using the static and dynamic testing paradigms of mechanical allodynia and vulvodynia.

Results

GC-MS analysis revealed that PI-ME contained predominant quantities of oleamide (9-octadecenamide), palmitic acid (hexadecanoic acid) and 3-hydroxy-dodecanoic acid, among other active constituents. In the abdominal constriction assay and hot plate test, PI-ME produced dose dependant, naloxone and pentylenetetrazole reversible antinociception suggesting an involvement of opioidergic and GABAergic mechanisms. In the stair case test, PI-ME at 200 mg/kg increased the number of steps climbed while at 600 mg/kg a significant decrease was observed. The rearing incidence was diminished by PI-ME at all tested doses and in the open field test, PI-ME decreased locomotor activity to an extent that was analagous to diazepam. The effects of PI-ME were antagonized by PTZ in both the staircase and open field tests implicating GABAergic mechanisms in its anxiolytic and sedative activities. In the streptozotocin-induced neuropathic nociceptive model, PI-ME (200 and 300 mg/kg) exhibited static and dynamic anti-allodynic effects exemplified by an increase in paw withdrawal threshold and paw withdrawal latency. PI-ME relieved only the dynamic component of vulvodynia by increasing flinching response latency.

Conclusions

These findings suggest that Passiflora incarnata might be useful for treating neuropathic pain. The antinociceptive and behavioural findings inferring that its activity may stem from underlying opioidergic and GABAergic mechanisms though a potential oleamide-sourced cannabimimetic involvement is also discussed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12906-016-1048-6) contains supplementary material, which is available to authorized users.

Stress-induced hyperalgesia is associated with a reduced and delayed GABA inhibitory control that enhances post-synaptic NMDA receptor activation in the spinal cord

GABA and glutamate are both affected by stress and are involved in nociception. Thus, we determined whether stress-induced enhancement of inflammatory hyperalgesia is mediated by an imbalance between glutamate and GABA neurotransmission. Male rats were subjected daily to 10 to 20 minutes per day of either forced swimming (FS) or sham swimming for 3 consecutive days; nonconditioned rats served as controls. Some rats were treated i.p. with ketamine (5 mg/kg), diazepam (2 mg/kg), flumazenil (0.1 mg/kg), or vehicle (0.9% NaCl), 30 to 60 minutes before each conditioning session or nociception assessment. Pain behavior, spinal nociceptive neuronal activation and GABA and glutamate release were respectively evaluated by the formalin test, the expression of c-Fos and in vivo microdialysis of superficial laminae of the lumbar spinal cord, 48 hours after the last conditioning session. Nitric oxide metabolites (NO(x)) were determined as markers of post-synaptic NMDA receptor activation. FS stress enhanced formalin-induced hyperalgesia, increased pain-elicited c-Fos expression, decreased basal and delayed pain-induced GABA release, and increased basal and induced glutamate release. Hyperalgesia and c-Fos overexpression were blocked only by prestress treatment with diazepam and post-stress treatment with ketamine, whereas changes in GABA and glutamate release were reversed by prestress treatment with diazepam. Diazepam effects were blocked by flumazenil. NO(x) increased in lumbar spinal cord of FS rats by a mechanism antagonized by ketamine. Thus, stress-induced hyperalgesia is initiated by a decreased and delayed GABA release and GABA-A receptor activation, whereas it is maintained by increased glutamate release and NMDA glutamate receptor activation at the spinal level.

Reduced GABA neurotransmission underlies hyperalgesia induced by repeated forced swimming stress

We determined if cutaneous hyperalgesia and pain-induced c-Fos overexpression in the spinal cord produced by repeated forced swimming (FS) stress in the rat were related to changes in GABA neurotransmission by studying spinal release of GABA and the effect of positive modulation of GABA-A receptors with diazepam. Male rats were daily submitted to 10-20 min of either forced swimming or sham swimming (SS) for 3 consecutive days. Two days later, spinal GABA release was estimated by in vivo microdialysis. In other set of rats, either diazepam (2 mg/kg, i.p.) or saline was administered 1h before either SS or FS and inflammatory nociception was assessed with the formalin test; it was followed by removal of lumbar spinal cords for c-Fos immunocytochemistry. Basal and pain-evoked release of GABA in the spinal cord was lower in FS rats than in SS rats. In contrast, pain scores during formalin test late phase and pain-induced c-Fos expression in laminae I-VI of ipsilateral dorsal horn were significantly higher in FS rats than in SS rats. In FS rats, diazepam did not have effect on GABA release but reduced pain scores and overexpression of c-Fos whereas flumazenil (0.1 mg/kg, i.p.), an antagonist of the benzodiazepine binding site, reversed these effects. When diazepam was given only 1h before the formalin test, it slightly but significantly reduced pain scores during late phase in FS rats but not in SS rats. In conclusion, stress-induced reduction in GABA-A receptor activation is involved in the development of FS stress-induced hyperalgesia.

Participation of opiatergic, GABAergic, and serotonergic systems in the expression of copulatory analgesia in male rats

Copulation in the male rat provoked an abrupt and significant rise in the threshold to induce vocalization by electrical shock to the tail (copulatory analgesia, CA). The possible effect on CA of the intrathecal (IT) administration of receptor antagonists to neurotransmitters participating in nociception was ascertained in this study. CA was significantly reduced, though not abolished, by IT injections of either naloxone, picrotoxin, or methysergide, but not by strychnine or yohimbine. This analgesic effect was achieved without significantly altering copulatory behavior. Results suggest that both brain and spinal systems participate in the development of CA. Brain effects would be mediated by descending serotonergic fibers, although intrinsic spinal systems would involve both opiate and GABA interneurons.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

GABA-mediated inhibition in rostral ventromedial medulla: role in nociceptive modulation in the lightly anesthetized rat

Local microinjection of GABAA receptor agonists and antagonists was used to characterize the role of GABA-mediated inhibitory processes in the nociceptive modulatory functions of the rostral ventromedial medulla (RVM) in the lightly anesthetized rat. Microinjection of selective GABAA receptor antagonists bicuculline methiodide and SR95531 produced a significant increase in tail-flick (TF) latency. This antinociception was dose related, showed recovery and was attenuated by prior injection of the GABAA receptor agonist THIP at the same site. Microinjection of saline or the glycine receptor antagonist strychnine did not significantly affect TF latency. In contrast, administration of GABAA receptor agonists THIP and muscimol resulted in a significant decrease in TF latency. Microinjections at sites surrounding the RVM did not significantly affect TF latency. These results demonstrate that a GABA-mediated process within the RVM is crucial in permitting execution of the TF and, presumably, other spinal nociceptive reflexes.

Increased GABAergic transmission in medial hypothalamus facilitates lordosis but has the opposite effect in preoptic area

The role of gamma-aminobutyric acid (GABA) in mediation of lordosis in the rat has been unclear. We report here that GABA plays a dual role in the mediation of lordosis and has differential effects in the medial hypothalamus (MH) and preoptic area/anterior hypothalamus continuum (POA-AH). Bilateral infusion of the GABAA antagonist bicuculline into the MH of cannulated females primed with estradiol benzoate and progesterone (EB + P) resulted in a marked and transient inhibition of ongoing lordosis. A similar pattern of inhibition was seen in females treated with EB only. In contrast, infusion of the same dose of bicuculline into the POA-AH of sexually receptive females had no effect on lordosis whereas infusion of the GABAA agonist muscimol into this site resulted in a short-term inhibition of lordosis. Furthermore, when females were treated with subthreshold doses of EB + P to induce a low level of lordosis responding, infusion of muscimol into the MH resulted in a significant enhancement of lordosis; infusion of bicuculline into the POA-AH also enhanced lordosis responding as compared to saline-infused controls. These data indicate that increased GABAergic neurotransmission in the MH facilitates lordosis whereas increased GABAergic activity in the POA-AH inhibits this behavior.

Mechanism of the analgesic effect of neurotropin

Neurotropin, an extract from the inflamed skin of vaccinia virus-inoculated rabbits, has been observed clinically to be effective for treating pain in patients with lumbago, SMON and other neuropathies. In the present study, we examined the mechanism of the antinociceptive effect of neurotropin in mice in relation to administration routes, opioids, and noradrenergic or GABAergic drugs, by the tail pressure method. The antinociceptive effects of neurotropin were large when administered by the i.p. and intracisternal (i.cist.) routes, but comparatively small in the case of the intrathecal (i.th.) route. Neurotropin may thus act at the supraspinal level rather than on the spinal cord. The antinociceptive effect of neurotropin was not blocked by naloxone, and no cross-tolerance developed between neurotropin and morphine. The effect of neurotropin was blocked by phentolamine and reserpine, but not by atropine. Its effect was enhanced by GABA, muscimol, aminooxyacetic acid and diaminobutyric acid, but not by baclofen, and blocked by bicuculline methiodide. From these results, the antinociceptive action of neurotropin appears to be non-opioid in nature, and may possible be mediated by the noradrenergic and GABAergic systems, but unrelated to the cholinergic system.

Opioids and sexual behavior in the male rat

Naloxone in the doses of 4 or 16 mg/kg failed to affect copulatory behavior of testosterone-treated castrated male rats. Morphine 10 mg/kg, administered 60 min before behavioral observation, reduced the proportion of animals displaying sexual behavior. Doses of 2.5 or 5 mg/kg reduced the latency to the second ejaculation, whereas the few animals still copulating after morphine 10 mg/kg showed a reduced latency to the first ejaculation. The same doses of morphine administered 5 min before behavioral observation produced a dose-dependent reduction of mount, intromission and ejaculation percentages. However, those animals that did copulate showed a normal copulatory behavior. D-Ala2-Met5 enkephalinamide (DALA) infused into the left cerebral ventricle in a dose of 5 micrograms 5 or 60 min before tests had no effect. When the peptide was infused 30 sec after the first intromission, the number of intromissions as well as the latency to ejaculation were reduced. Opioids may facilitate ejaculatory mechanisms, perhaps as a consequence of their rewarding properties. Moreover, in animals treated with DALA after the first intromission, the number of intromissions and the latency to ejaculation were similar for the first and second copulatory series, while these parameters were much reduced upon the second ejaculation for control animals. It is possible that liberation of endogenous opioids is the cause of ejaculation-induced facilitation of subsequent sexual behavior.

Nociceptive responses to altered GABAergic activity at the spinal cord

GABA agonists and antagonists were injected intrathecally at the spinal cord, to determine their effect on nociceptive thresholds. Tactile stimulation, applied against the flank by a medium diameter von Frey fiber (5.5 g force), elicited distress vocalizations after, but not before injection of the GABA antagonists, bicuculline MI or picrotoxin (0.25 and 1 microgram dosages). Vocalization threshold to tail shock was significantly reduced by bicuculline MI or picrotoxin. Tail flick withdrawal latency from radiant heat was not altered by GABA antagonists. The GABA agonist, muscimol, significantly elevated vocalization threshold to tail shock at a 5 micrograms dose. At a lower dose level (1 microgram), muscimol significantly reduced vocalization threshold to tail shock. Tail flick latency was significantly prolonged by the 5 micrograms dose of muscimol; however, flaccid paralysis of the hind limbs was also evident. Nociceptive thresholds were not altered by GABA or saline injection. These findings indicate that GABAergic activity contributes to the tonic modulation of nociception at the spinal cord.

Reversal of the effect of centrally-administered diazepam on morphine antinociception by specific (Ro 15-1788 and Ro 15-3505) and non-specific (bicuculline and caffeine) benzodiazepine antagonists

Diazepam, injected into the lateral ventricles reduced the antinociceptive effect of morphine in rats, as measured by the tail-flick method. Specific antagonists of diazepam (Ro 15-1788 and Ro 15-3505) had no effect themselves but prevented inhibition by diazepam of morphine antinociception. Furthermore, the action of diazepam was partially reversed by intracerebroventricular injection of bicuculline or caffeine. These findings support the view that the depressant effect of diazepam on morphine antinociception is specific and GABAergic in nature and that some actions of diazepam are also mediated via the purinergic system.

GABAA and GABAB receptors on porcine pars intermedia cells in primary culture: functional role in modulating peptide release

A primary culture of porcine pars intermedia cells with particularly high yields has been developed. The cells, grown in monolayers, secrete the pro-opiomelanocortin-derived peptide alpha-melanocyte-stimulating hormone over several weeks. The patch-clamp technique has been used to demonstrate the presence of gamma-aminobutyrateA (GABAA) receptors on the cells. GABA or the selective GABAA receptor agonist isoguvacine produced a depolarizing increase in chloride conductance that desensitized rapidly. The response was antagonized by bicuculline and by the aminopyridazine derivative of GABA (SR 95103), a novel GABAA receptor antagonist. The effects of specific agonists for each receptor were tested on peptide release from cells maintained in a perfusion system. Isoguvacine (10 microM) potentiated Ba2+-evoked release of alpha-melanocyte-stimulating hormone, whereas (-)-baclofen (50 microM) decreased both basal and stimulated hormone release. This negative effect on peptide secretion was reproduced when GABA (50 microM) was perfused in the presence of bicuculline (10 microM) to block GABAA receptor activation. The possible mechanisms underlying these GABAA and GABAB effects on stimulus-secretion coupling in this neuroendocrine model are discussed.

Enhancement of brain [3H]flunitrazepam binding and analgesic activity of synthetic cannabimimetics

Novel, synthetic cannabimimetics and delta 9-tetrahydrocannabinol were found to enhance the binding of [3H]flunitrazepam to mouse brain in vivo. This property, suggestive of facilitation of binding to benzodiazepine receptors, is consistent with the potentiation of the anticonvulsant activity of diazepam against pentylenetetrazol by these compounds. The relative potencies of delta 9-tetrahydrocannabinol and the new cannabimimetics for enhancing [3H]flunitrazepam binding in vivo could also be correlated with their relative analgesic efficacies. Similar pharmacological stereospecificity was displayed for both binding enhancement and analgesic effects. The following order of decreasing potency was observed: N-methyllevonantradol and (-)-CP-55,244 greater than levonantradol, canbisol, CP-42,096 and (-)-CP-55,940 greater than 9-beta-normethyl-9-beta-hydroxyhexahydrocannabinol, nabilone and CP-47,497 greater than delta 9-tetrahydrocannabinol. Dextronantradol, (+)-CP-55,940 and (+)-CP-55,244 were considerably less active than the respective (-)-enantiomers; cannabidiol was inactive. Extensive investigation of structure versus activity led to N-methyllevonantradol and the 3-(2-hydroxyphenyl)cyclohexanols derivative, (-)-CP-55,244, which are approximately 1000-fold more potent than delta 9-tetrahydrocannabinol.

Ligand-binding studies on GABA receptors--relation to physiology and behavior

Some studies that have been conducted with ligand-binding methods support the contention that activation of cerebral GABA-receptors is involved in the control of certain behaviors. Further study of the changes in GABA-receptors that are associated with altered physiology and behavior in experimental animals and in man might lead to the development of new therapies for certain neuropsychiatric, cardiovascular, and endocrinological disorders.