Prostaglandin E2 receptor expression in the rat trigeminal-vascular system and other brain structures involved in pain

When pharmaceutical drugs become environmental pollutants: Potential neural effects and underlying mechanisms

Pharmaceutical drugs have become consumer products, with a daily use for some of them. The volume of production and consumption of drugs is such that they have become environmental pollutants. Their transfer to wastewater through urine, feces or rinsing in case of skin use, associated with partial elimination by wastewater treatment plants generalize pollution in the hydrosphere, including drinking water, sediments, soils, the food chain and plants. Here, we review the potential effects of environmental exposure to three classes of pharmaceutical drugs, i.e. antibiotics, antidepressants and non-steroidal anti-inflammatory drugs, on neurodevelopment. Experimental studies analyzing their underlying modes of action including those related to endocrine disruption, and molecular mechanisms including epigenetic modifications are presented. In addition, the contribution of brain imaging to the assessment of adverse effects of these three classes of pharmaceuticals is approached.

Plasma Lipolysis and Changes in Plasma and Cerebrospinal Fluid Signaling Lipids Reveal Abnormal Lipid Metabolism in Chronic Migraine

Background

Lipids are a primary storage form of energy and the source of inflammatory and pain signaling molecules, yet knowledge of their importance in chronic migraine (CM) pathology is incomplete. We aim to determine if plasma and cerebrospinal fluid (CSF) lipid metabolism are associated with CM pathology.

Methods

We obtained plasma and CSF from healthy controls (CT, n = 10) or CM subjects (n = 15) diagnosed using the International Headache Society criteria. We measured unesterified fatty acid (UFA) and esterified fatty acids (EFAs) using gas chromatography-mass spectrometry. Glycerophospholipids (GP) and sphingolipid (SP) levels were determined using LC-MS/MS, and phospholipase A₂ (PLA₂) activity was determined using fluorescent substrates.

Results

Unesterified fatty acid levels were significantly higher in CM plasma but not in CSF. Unesterified levels of five saturated fatty acids (SAFAs), eight monounsaturated fatty acids (MUFAs), five ω-3 polyunsaturated fatty acids (PUFAs), and five ω-6 PUFAs are higher in CM plasma. Esterified levels of three SAFAs, eight MUFAs, five ω-3 PUFAs, and three ω-6 PUFAs, are higher in CM plasma. The ratios C20:4n-6/homo-γ-C20:3n-6 representative of delta-5-desaturases (D5D) and the elongase ratio are lower in esterified and unesterified CM plasma, respectively. In the CSF, the esterified D5D index is lower in CM. While PLA₂ activity was similar, the plasma UFA to EFA ratio is higher in CM. Of all plasma GP/SPs detected, only ceramide levels are lower (p = 0.0003) in CM (0.26 ± 0.07%) compared to CT (0.48 ± 0.06%). The GP/SP proportion of platelet-activating factor (PAF) is significantly lower in CM CSF.

Conclusions

Plasma and CSF lipid changes are consistent with abnormal lipid metabolism in CM. Since plasma UFAs correspond to diet or adipose tissue levels, higher plasma fatty acids and UFA/EFA ratios suggest enhanced adipose lipolysis in CM. Differences in plasma and CSF desaturases and elongases suggest altered lipid metabolism in CM. A lower plasma ceramide level suggests reduced de novo synthesis or reduced sphingomyelin hydrolysis. Changes in CSF PAF suggest differences in brain lipid signaling pathways in CM. Together, this pilot study shows lipid metabolic abnormality in CM corresponding to altered energy homeostasis. We propose that controlling plasma lipolysis, desaturases, elongases, and lipid signaling pathways may relieve CM symptoms.

Effect of low-level laser therapy on tooth-related pain and somatosensory function evoked by orthodontic treatment

Low-level laser therapy (LLLT) may have an effect on the pain associated with orthodontic treatment. The aim of this study was to evaluate the effect of LLLT on pain and somatosensory sensitization induced by orthodontic treatment. Forty individuals (12–33 years old; mean ± standard deviations: 20.8 ± 5.9 years) scheduled to receive orthodontic treatment were randomly divided into a laser group (LG) or a placebo group (PG) (1:1). The LG received LLLT (810-nm gallium-aluminium-arsenic diode laser in continuous mode with the power set at 400 mW, 2 J·cm^–2) at 0 h, 2 h, 24 h, 4 d, and 7 d after treatment, and the PG received inactive treatment at the same time points. In both groups, the non-treated side served as a control. A numerical rating scale (NRS) of pain, pressure pain thresholds (PPTs), cold detection thresholds (CDTs), warmth detection thresholds (WDTs), cold pain thresholds (CPTs), and heat pain thresholds (HPTs) were tested on both sides at the gingiva and canine tooth and on the hand. The data were analysed by a repeated measures analysis of variance (ANOVA). The NRS pain scores were significantly lower in the LG group (P = 0.01). The CDTs, CPTs, WDTs, HPTs, and PPTs at the gingiva and the PPTs at the canine tooth were significantly less sensitive on the treatment side of the LG compared with that of the PG (P < 0.033). The parameters tested also showed significantly less sensitivity on the non-treatment side of the LG compared to that of the PG (P < 0.043). There were no differences between the groups for any quantitative sensory testing (QST) measures of the hand. The application of LLLT appears to reduce the pain and sensitivity of the tooth and gingiva associated with orthodontic treatment and may have contralateral effects within the trigeminal system but no generalized QST effects. Thus, the present study indicated a significant analgesia effect of LLLT application during orthodontic treatment. Further clinical applications are suggested.

Repeated applications of low-level laser therapy during and after orthodontic procedures could help reduce pain and discomfort. Many orthodontic patients experience pain in the hours and days following treatment, which impacts on treatment participation. Wenjing Chen at Nanjing Medical University and co-workers assessed whether low-level laser therapy (LLLT) can reduce pain and sensitivity in teeth and surrounding tissues. They divided 40 patients into two groups; one was given repeated LLLT in the hours and days following procedures, while the other group received a placebo course. They tested both groups for sensitivity to stimuli including heat and pressure, in the mouth and on the hands. LLLT significantly reduced pain in the mouth relative to the placebo group. No differences were found in tests on the hands, suggesting LLLT works as a targeted analgesia.

Effect of PGD2 on middle meningeal artery and mRNA expression profile of L-PGD2 synthase and DP receptors in trigeminovascular system and other pain processing structures in rat brain

BACKGROUND

Prostaglandins (PGs), particularly prostaglandin D₂ (PGD₂), E₂ (PGE₂), and I₂ (PGI₂), are considered to play a role in migraine pain. In humans, infusion of PGD₂ causes lesser headache as compared to infusion of PGE₂ and PGI₂. Follow-up studies in rats have shown that infusion of PGE₂ and PGI₂ dilate the middle meningeal artery (MMA), and mRNA for PGE₂ and PGI₂ receptors is present in rat trigeminovascular system (TVS) and in the brain structures associated with pain. In the present study, we have characterized the dilatory effect of PGD₂ on rat MMA and studied the relative mRNA expression of PGD₂ receptors and lipocalin-type of PGD₂ synthase (L-PGDS).

METHOD

Rat closed-cranial window (CCW) model was used to study the effect of the DP₁ receptor antagonist, MK-0524, on PGD₂-induced vasodilation of middle meningeal artery. The qPCR technique was used for mRNA expression analysis.

RESULTS

PGD₂ infusion evoked a dose-dependent dilation of the rat MMA. The calculated mean pED₅₀ value was 5.23±0.10 and E_max was 103±18% (n=5). MK-0524 significantly (∼61%, p<0.05) blocked the PGD₂-induced dilation of MMA. mRNA for the DP₁, DP₂ and L-PGDS were expressed differentially in all tested tissues. DP₁ receptor mRNA was expressed maximally in trigeminal ganglion (TG) and in cervical dorsal root ganglion (DRG).

CONCLUSIONS

High expression of DP₁ mRNA in the TG and DRG suggest that PGD₂ might play a role in migraine pathophysiology. Activation of the DP₁ receptor in MMA was mainly responsible for vasodilation induced by PGD₂ infusion.

Periaqueductal Grey EP3 Receptors Facilitate Spinal Nociception in Arthritic Secondary Hypersensitivity

Descending controls on spinal nociceptive processing play a pivotal role in shaping the pain experience after tissue injury. Secondary hypersensitivity develops within undamaged tissue adjacent and distant to damaged sites. Spinal neuronal pools innervating regions of secondary hypersensitivity are dominated by descending facilitation that amplifies spinal inputs from unsensitized peripheral nociceptors. Cyclooxygenase-prostaglandin (PG) E2 signaling within the ventrolateral periaqueductal gray (vlPAG) is pronociceptive in naive and acutely inflamed animals, but its contributions in more prolonged inflammation and, importantly, secondary hypersensitivity remain unknown. In naive rats, PG EP3 receptor (EP3R) antagonism in vlPAG modulated noxious withdrawal reflex (EMG) thresholds to preferential C-nociceptor, but not A-nociceptor, activation and raised thermal withdrawal thresholds in awake animals. In rats with inflammatory arthritis, secondary mechanical and thermal hypersensitivity of the hindpaw developed and was associated with spinal sensitization to A-nociceptor inputs alone. In arthritic rats, blockade of vlPAG EP3R raised EMG thresholds to C-nociceptor activation in the area of secondary hypersensitivity to a degree equivalent to that evoked by the same manipulation in naive rats. Importantly, vlPAG EP3R blockade also affected responses to A-nociceptor activation, but only in arthritic animals. We conclude that vlPAG EP3R activity exerts an equivalent facilitation on the spinal processing of C-nociceptor inputs in naive and arthritic animals, but gains in effects on spinal A-nociceptor processing from a region of secondary hypersensitivity. Therefore, the spinal sensitization to A-nociceptor inputs associated with secondary hypersensitivity is likely to be at least partly dependent on descending prostanergic facilitation from the vlPAG.

SIGNIFICANCE STATEMENT

After tissue damage, sensitivity to painful stimulation develops in undamaged areas (secondary hypersensitivity). This is found in many painful conditions, particularly arthritis. The periaqueductal gray (PAG) is an important center that controls spinal nociceptive processing, on which secondary hypersensitivity depends. Prostaglandins (PGs) are mediators of inflammation with pronociceptive actions within the PAG under normal conditions. We find that secondary hindpaw hypersensitivity in arthritic rats results from spinal sensitization to peripheral A-nociceptor inputs. In the PAG of arthritic, but not naive, rats, there is enhanced control of spinal A-nociceptor processing through PG EP3 receptors. The descending facilitatory actions of intra-PAG PGs play a direct and central role in the maintenance of inflammatory secondary hypersensitivity, particularly relating to the processing of A-fiber nociceptive information.

Metamizol Relieves Pain Without Interfering With Cerulein-Induced Acute Pancreatitis in Mice

OBJECTIVES

Animal models are essential to understand the pathogenesis of acute pancreatitis (AP) and to develop new therapeutic strategies. Although it has been shown that cerulein-induced AP is associated with pain in experimental animals, most experiments are carried out without any pain-relieving treatment because researchers are apprehensive of an interference of the analgetic agent with AP-associated inflammation. In light of the growing ethical concerns and the legal tightening regarding animal welfare during experiments, this attitude should be changed.

METHODS

Acute pancreatitis was induced by cerulein in the C57BL/6J and FVB/N mouse inbred strains. One group received vehicle only, and the other was treated with metamizol as analgetic agent. Pain sensation and parameters of AP were analyzed as well as the effect of metamizol in the pancreas and its actions in the brain.

RESULTS

We report that oral administration of metamizol protects cerulein-treated mice from abdominal pain without influencing the clinical and histopathological course of the disease. In addition, it could be shown that metamizol reduces the central pain response.

CONCLUSIONS

This study reveals that oral administered metamizol has no influence on the cerulein-induced AP and can be given as an analgesic to increase animal welfare in experiments with induced AP.

Caffeine and headache: specific remarks

Caffeine is the most widely used psychostimulant worldwide. Excessive caffeine consumption induces a series of both acute and chronic biological and physiological changes that may give rise to cognitive decline, depression, fatigue, insomnia, cardiovascular changes, and headache. Chronic consumption of caffeine promotes a pro-nociceptive state of cortical hyperexcitability that can intensify a primary headache or trigger a headache due to excessive analgesic use. This review offers an in-depth analysis of the physiological mechanisms of caffeine and its relationship with headache.

Prostaglandins in migraine: update

PURPOSE OF REVIEW

This review presents recent findings on the role of prostaglandins in migraine pathophysiology.

RECENT FINDINGS

Experimental studies have shown that prostaglandins are distributed in the trigeminal-vascular system and its receptors are localized in the trigeminal ganglion and the trigeminal nucleus caudalis. Prostaglandins were found in smooth muscles of cranial arteries, and functional studies in vivo showed that prostaglandins induced dilatation of cranial vessels. Human studies showed that intravenous infusion of vasodilating prostaglandins such as prostaglandin E₂ (PGE₂), prostaglandin I₂ (PGI₂) and prostaglandin D₂ (PGD₂) induced headache and dilatation of intra-cranial and extra-cranial arteries in healthy volunteers. In contrast, infusion of non-dilating prostaglandin F₂α (PGF₂α) caused no headache or any vascular responses in cranial arteries. PGE₂ and PGI₂ triggered migraine-like attacks in migraine patients without aura, accompanied by dilatation of the intra-cerebral and extra-cerebral arteries. A novel EP4 receptor antagonist could not prevent PGE₂-induced headache in healthy volunteers.

SUMMARY

Recent in-vitro/in-vivo data demonstrated presence and action of prostaglandins within the trigeminal pain pathways. Migraine induction after intravenous administration of PGE₂ and PGI₂ suggests a specific blockade of their receptors, EP and IP respectively, as a new potential drug target for the acute treatment of migraine.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.