Cerebrospinal fluid beta-endorphin in models of hyperalgesia in the rat

Capsaicin Reduces Ethanol Consumption in C57BL/6 but not DBA/2 Mice

Objective

Capsaicin, the pungent analgesic substance of hot peppers which produces a burning sensation and pain is known to affect Substance P and central opioid activities. This experiment was designed to test the effect of capsaicin on alcohol consumption in C57BL/6 and DBA/2 mice. These two strains are known to differ in both their alcohol consumption and their endogenous opioid distribution and response to alcohol. It is hypothesized that this effect may be mediated by both increases Substance P and decreases beta-endorphin.

Methods

After i.p. administration of 0.01 and 0.001 mg/kg of capsaicin with a vehicle or the vehicle alone as the control for eight days in C57BL/6 and DBA/2 mice on limited access alcohol model, Capsaicin’s effects on 2-hour alcohol, 22-hours water, 24-hours food intake and body weight were studied.

Results

In this study, as expected, C57BL/6 mice drank significantly more alcohol than DBA/2 mice under baseline conditions. Capsaicin at both doses tested significantly reduced baseline alcohol consumption in C57BL/6 but not DBA/2 mice. These effects were selective for alcohol as capsaicin did not disrupt food or water consumption.

Conclusion

These results demonstrate that capsaicin differentially affects those mechanisms underlying alcohol consumption in two strains of mice known to differ in their preference for and consumption of alcohol. This effect is hypothesized to be related to differences in the response of the endogenous opioid system.

Effects of Spicy Stimulation and Spicy-Food Consumption on Human Pain Sensitivity: A Healthy Volunteer Study

Spicy-food intake has been shown to affect various human physiological systems and diseases. This study tested the analgesia effect caused by stimulation of a spicy sensation (spicy stimulation) and explored the effect of spicy-food consumption on human basal pain sensitivity. A total of 60 healthy undergraduates were included in the primary study. Placebo and sweet stimulation were used as reference interventions. Pressure and cold-pain thresholds were measured before and after taste stimulation. The frequency of spicy-food intake was also evaluated. An additional 100 subjects were recruited to validate the results. Compared to placebo stimulation, both pressure and cold-pain thresholds increased during spicy stimulation (P < .05). The increased thresholds remained, even when the taste stimulation residue was nearly eliminated (P < .05). The pressure (10.0 [2.1] vs 12.7 [3.0] kg/cm2, P < .001) and cold-pain (4.4 [1.6] vs 6.2 [2.7] seconds, P = .003) thresholds in subjects who consume spicy food ≥3 days/week were significantly lower than in those who consume it <3 days/week. In the validation population, the frequency of spicy-food intake was negatively associated with subjects' pressure (β = -.218, P = .013) and cold-pain (β = -.205, P = .035) thresholds. Spicy stimulation has an analgesia effect on adults that persists even after the taste stimulation stops. Conversely, a long-term spicy diet can reduce the human basal pain threshold. TRIAL REGISTRATION: The study protocol was approved by the Medical Ethics Committee of the Second Affiliated Hospital of Army Medical University, People's Liberation Army (identification No., 2017-023-01), and it was registered on the Chinese Clinical Trial Registry at www.chictr.org.cn (No. ChiCTR1800015053). PERSPECTIVE: This study directly examined the effects of stimulation of a spicy sensation on adult pain sensitivity and was the first to explore the relationship between long-term spicy-food intake and human pain sensitivity. The results provide evidence for future clinical pain intervention and individualized pain treatment.

Opioid and Endocannabinoid System in Orofacial Pain

Orofacial pain disorders are frequent in the general population and their pharmacological treatment is difficult and controversial. Therefore, the search for novel, safe and efficient analgesics is an important but still elusive goal for contemporary medicine. In the recent years, the antinociceptive potential of endocannabinoids and opioids has been emphasized. However, concerns for the safety of their use limit their clinical applications. the possibility of modulating the activity of endocannabinoids by regulation of their synthesis and/or degradation offers an innovative approach to the treatment of pain. A rat model of trigeminal pain, utilizing tongue jerks evoked by electrical tooth pulp stimulation during perfusion of the cerebral ventricles with various neurotransmitter solutions can be used in the pharmacological studies of nociception in the orofacial area. The aim of this review is to present the effects of pharmacological activity of opioids and endocannabinoids affecting the transmission of the sensory information from the orofacial area on the example of trigemino-hypoglossal reflex in rats.

Spicy Food Preference and Risk for Alcohol Dependence in Korean

OBJECTIVE

Previous studies have reported that both preference for spicy food and drinking behavior are associated with the activity of the opioid system in the central nervous system. The relationship between the preference for spicy food and the risk of alcohol dependence by comparing spicy food preference in alcohol-dependent patients vs. healthy controls was investigated. Also the association between the preference for spicy food and OPRM1 A118G was studied.

METHODS

A total of 150 Korean male patients with alcohol dependence and 100 normal male control subjects were included in this study. Preference for spicy food was measured using the Food Preference Scale (FPS). DNA analysis was conducted to detect the A118G polymorphism.

RESULTS

The mean FPS score was significantly higher in the alcohol-dependent patients (61.2±24.2) than in the normal control subjects (53.0±22.0). FPS scores differed significantly between alcohol-dependent patients and normal control subjects who had the G allele in OPRM1 A118G, but not between the two groups with the AA genotype.

CONCLUSION

A strong preference for spicy food can be assumed to be a risk factor for alcohol dependence, particularly in those carrying the G allele in OPRM1 A118G.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Acute effects of capsaicin on proopioimelanocortin mRNA levels in the arcuate nucleus of Sprague-Dawley rats

OBJECTIVE

Capsaicin, a noxious stimulant and main component of the hot flavor of red peppers, has an analgesic effect when administered to humans. We investigated the expression of proopioimelanocortin (POMC) mRNA in the arcuate nucleus of Sprague-Dawley (SD) rats after administering capsaicin, hypothesizing that administering capsaicin activates the central opioid system.

METHODS

SD rats were divided randomly into two groups; one group received a saline injection and the other received a capsaicin injection. The POMC mRNA level in the arcuate nucleus of the hypothalamus was measured by the reverse transcription-polymerase chain reaction at 0, 20, 40, 60, and 120 minutes after capsaicin administration.

RESULTS

Capsaicin administration resulted in a significantly increased POMC mRNA level, compared to that in saline-treated rats at the 20-minute time point (t=-4.445, p=0.001). However, no significant group differences were observed at other times (t=-1.886, p=0.089; t= -0.973, p=0.353; t=-2.193, p=0.053 for 40, 60, and 120 minutes, respectively).

CONCLUSION

The analgesic effect of capsaicin might be associated with increased activity of the cerebral opioid system. This finding suggests that capsaicin acted for nociception and analgesia and could affect alcohol-intake behavior, which might further imply that a food culture could affect drinking behavior.

The absence of endogenous beta-endorphin selectively blocks phosphorylation and desensitization of mu opioid receptors following partial sciatic nerve ligation

Phosphorylation of specific sites in the second intracellular loop and in the C-terminal domain have previously been suggested to cause desensitization and internalization of the mu-opioid receptor (MOP-R). To assess sites of MOP-R phosphorylation in vivo, affinity-purified, phosphoselective antibodies were raised against either phosphothreonine-180 in the second intracellular loop (MOR-P1) or the C-terminal domain of MOP-R containing phosphothreonine-370 and phosphoserine-375 (MOR-P2). We found that MOR-P2-immunoreactivity (IR) was significantly increased within the striatum of wild-type C57BL/6 mice after injection of the agonist fentanyl. Pretreatment with the antagonist naloxone blocked the fentanyl-induced increase. Furthermore, mutant mice lacking MOP-R showed only non-specific nuclear MOR-P2-IR before or after fentanyl treatment, confirming the specificity of the MOR-P2 antibodies. To assess whether MOP-R phosphorylation occurs following endogenous opioid release, we induced chronic neuropathic pain by partial sciatic nerve ligation (pSNL), which caused a significant increase in MOR-P2-IR in the striatum. pSNL also induced signs of mu opioid receptor tolerance demonstrated by a rightward shift in the morphine dose response in the tail withdrawal assay and by a reduction in morphine conditioned place preference (CPP). Mutant mice selectively lacking all forms of the beta-endorphin peptides derived from the proopiomelanocortin (Pomc) gene did not show increased MOR-P2-IR, decreased morphine antinociception, or reduced morphine CPP following pSNL. In contrast gene deletion of either proenkephalin or prodynorphin opioids did not block the effects of pSNL. These results suggest that neuropathic pain caused by pSNL in wild-type mice activates the release of the endogenous opioid beta-endorphin, which subsequently induces MOP-R phosphorylation and opiate tolerance.

Beta-endorphin in the brain. A role in nociception

We have known the endogenous opioid peptide beta-endorphin for 20 years. Surprisingly, our knowledge of the physiological role of this peptide and its receptors in modulation of pain perception is still fragmentary. Whereas most studies have tried to elucidate the physiological role of beta-endorphin by reversing evoked responses by the opioid antagonist naloxone, this review focuses on quantification of release of beta-endorphin in the brain as the approach to define physiological and pathophysiological roles of beta-endorphin in relation to nociception. Using a lateral ventricle-cisterna magna perfusion model in the anesthetized rat, it was shown that depolarization of neurons in the arcuate nucleus of the hypothalamus, where beta-endorphin in produced, was followed by release of beta-endorphin to the cerebrospinal fluid compartment. Intense activation of spinal nociceptive pathways by intrathecal capsaicin injections also led to beta-endorphin release. It is concluded that there may still be good reason to quantify beta-endorphin in human cerebrospinal fluid to elucidate the role of beta-endorphin in pain perception.

Release of beta-endorphin immunoreactivity into ventriculo-cisternal perfusate by lumbar intrathecal capsaicin in the rat

A model employing perfusion of artificial cerebrospinal fluid from the lateral ventricle to the cisterna magna in the halothane anesthetized rat was used to study beta-endorphin release in the brain. Injection of 75 micrograms capsaicin into the lumbar intrathecal space released beta-endorphin immunoreactivity into perfusate. The release was blocked by intrathecal pretreatment with 1.25 mg lidocaine and the capsaicin receptor antagonist capsazepine (92 micrograms), showing that the release is caused by binding of capsaicin to a spinal receptor. The release was also blocked by intrathecal pretreatment with the NMDA antagonist MK-801 (3 micrograms) and the NK-1 receptor antagonist CP96,345 (200 micrograms), whereas the AMPA receptor antagonist NBQX (6 micrograms) yielded no significant inhibition. Surprisingly, morphine (30 micrograms) and sufentanil (1.5 micrograms) did not prevent release of beta-endorphin immunoreactivity, although blocking the cardiovascular responses to a noxious heat stimulus. High performance liquid chromatography characterization of perfusates collected after capsaicin injection showed that all beta-endorphin immunoreactivity coeluted with authentic beta-endorphin1-31. beta-Endorphin immunoreactivity in plasma was increased 10 min, but not 25 min, after capsaicin injection. Capsaicin injection abolished the motor and cardiovascular responses to tail immersion in 52.5 degrees C water. Addition of MK-801 (10(-4) mol/l) to the lateral ventricle-cisterna magna perfusate blocked the capsaicin-induced beta-endorphin release, showing that our previous demonstration of an NMDA receptor regulating arcuate nucleus beta-endorphin neuron activity has functional significance. We conclude that in this in vivo, anesthetized preparation including three hot water tail immersions, beta-endorphin can be released into a ventriculo-cisternal perfusate, by activation of the central axons of small primary afferent neurons by capsaicin. These data support the idea that central beta-endorphin may be released in response to prolonged, intense noxious stimulation.