Potentiation by WIN 55,212-2 of GABA-activated currents in rat trigeminal ganglion neurones

Aging is associated with sex-specific alteration in the expression of genes encoding for neuroestradiol synthesis and signaling proteins in the mouse trigeminal somatosensory input

Pain perception is influenced by sex and aging, with previous studies indicating the involvement of aromatase, the estradiol synthase enzyme, in regulating pain perception. Previous research has established the presence of aromatase in dorsal root ganglia sensory neurons and its role in modulating pain perception. The present study aims to explore the implications of aging and sex on the expression of aromatase and estrogen receptors in the trigeminal ganglion. The study examined mRNA levels of aromatase, ERs, and the androgen receptor (AR) in the trigeminal ganglion of 3-month-old and 27-month-old male and female mice, as well as 3-month-old mice from the four-core genotype (FCG) transgenic model. The latter facilitates the assessment of gonadal hormone and sex chromosome implications for sex-specific traits. Aromatase localization in the ganglion was further assessed through immunohistochemistry. Aromatase immunoreactivity was observed for the first time in sensory neurons within the trigeminal ganglion. Trigeminal ganglion gene expressions were detected for aromatase, ERs, and AR in both sexes. Aromatase, ERβ, and GPER gene expressions were higher in young males versus young females. Analyses of the FCG model indicated that sex differences depended solely on gonadal sex. The aging process induced an enhancement in the expression of aromatase, ERs, and AR genes across both sexes, culminating in a reversal of the previously observed gender-based differences. the potential impact of estrogen synthesis and signaling in the trigeminal ganglion on age and sex differences warrants consideration, particularly in relation to trigeminal sensory functions and pain perception.

Targeting Peripherally Restricted Cannabinoid Receptor 1, Cannabinoid Receptor 2, and Endocannabinoid-Degrading Enzymes for the Treatment of Neuropathic Pain Including Neuropathic Orofacial Pain

Neuropathic pain conditions including neuropathic orofacial pain (NOP) are difficult to treat. Contemporary therapeutic agents for neuropathic pain are often ineffective in relieving pain and are associated with various adverse effects. Finding new options for treating neuropathic pain is a major priority in pain-related research. Cannabinoid-based therapeutic strategies have emerged as promising new options. Cannabinoids mainly act on cannabinoid 1 (CB1) and 2 (CB2) receptors, and the former is widely distributed in the brain. The therapeutic significance of cannabinoids is masked by their adverse effects including sedation, motor impairment, addiction and cognitive impairment, which are thought to be mediated by CB1 receptors in the brain. Alternative approaches have been developed to overcome this problem by selectively targeting CB2 receptors, peripherally restricted CB1 receptors and endocannabinoids that may be locally synthesized on demand at sites where their actions are pertinent. Many preclinical studies have reported that these strategies are effective for treating neuropathic pain and produce no or minimal side effects. Recently, we observed that inhibition of degradation of a major endocannabinoid, 2-arachydonoylglycerol, can attenuate NOP following trigeminal nerve injury in mice. This review will discuss the above-mentioned alternative approaches that show potential for treating neuropathic pain including NOP.

Biological Actions of the Hsp90-binding Immunophilins FKBP51 and FKBP52

Immunophilins are a family of proteins whose signature domain is the peptidylprolyl-isomerase domain. High molecular weight immunophilins are characterized by the additional presence of tetratricopeptide-repeats (TPR) through which they bind to the 90-kDa heat-shock protein (Hsp90), and via this chaperone, immunophilins contribute to the regulation of the biological functions of several client-proteins. Among these Hsp90-binding immunophilins, there are two highly homologous members named FKBP51 and FKBP52 (FK506-binding protein of 51-kDa and 52-kDa, respectively) that were first characterized as components of the Hsp90-based heterocomplex associated to steroid receptors. Afterwards, they emerged as likely contributors to a variety of other hormone-dependent diseases, stress-related pathologies, psychiatric disorders, cancer, and other syndromes characterized by misfolded proteins. The differential biological actions of these immunophilins have been assigned to the structurally similar, but functionally divergent enzymatic domain. Nonetheless, they also require the complementary input of the TPR domain, most likely due to their dependence with the association to Hsp90 as a functional unit. FKBP51 and FKBP52 regulate a variety of biological processes such as steroid receptor action, transcriptional activity, protein conformation, protein trafficking, cell differentiation, apoptosis, cancer progression, telomerase activity, cytoskeleton architecture, etc. In this article we discuss the biology of these events and some mechanistic aspects.

Long-term application of cannabinoids leads to dissociation between changes in cAMP and modulation of GABAA receptors of mouse trigeminal sensory neurons

Antinociception caused by cannabinoids may have a partial peripheral origin in addition to its central site of action. In fact, we have observed that anandamide selectively and reversibly inhibits GABA_A receptors of putative nociceptive neurons of mouse trigeminal sensory ganglia via CB1 receptor activation to inhibit adenylyl cyclase and decrease cAMP with downstream posttranslational alterations. Since cannabinoids are often used chronically, we studied changes in cAMP levels and GABA-mediated currents of trigeminal neurons following 24 h application of anandamide (0.5 μM) or the synthetic cannabinoid WIN 55,212-2 (5 μM). With this protocol GABA responses were similar to control despite persistent fall in cAMP levels. Inhibition by WIN 55,212-2 of GABA effects recovered after 30 min washout and was not associated with changes in CB1 receptor expression, indicating lack of CB1 receptor inactivation and transient loss of negative coupling between CB1 receptors and GABA_A receptors. The phosphodiesterase inhibitor rolipram (100 μM; 24 h) enhanced cAMP levels and GABA-mediated currents, suggesting GABA_A receptors were sensitive to persistent upregulation via cAMP. While the adenylyl cyclase activator forskolin (1-20 μM) facilitated cAMP levels and GABA currents following 30 min application, this action was lost after 24 h in line with the drug limited lifespan. The PKA inhibitor PKI 14-22 (10 μM) increased cAMP without changing GABA currents. These data indicate that modulation of GABA_A receptors by intracellular cAMP could be lost following persistent application of cannabinoids. Thus, these observations provide an insight into the waning antinociceptive effects of these compounds.

Neuropathic orofacial pain: cannabinoids as a therapeutic avenue

Neuropathic orofacial pain (NOP) exists in several forms including pathologies such as burning mouth syndrome (BMS), persistent idiopathic facial pain (PIFP), trigeminal neuralgia (TN) and postherpetic neuralgia (PHN). BMS and PIFP are classically diagnosed by excluding other facial pain syndromes. TN and PHN are most often diagnosed based on a typical history and presenting pain characteristics. The pathophysiology of some of these conditions is still unclear and hence treatment options tend to vary and include a wide variety of treatments including cognitive behaviour therapy, anti-depressants, anti-convulsants and opioids; however such treatments often have limited efficacy with a great amount of inter-patient variability and poorly tolerated side effects. Analgesia is one the principal therapeutic targets of the cannabinoid system and many studies have demonstrated the efficacy of cannabinoid compounds in the treatment of neuropathic pain. This review will investigate the potential use of cannabinoids in the treatment of symptoms associated with NOP.

Potentiation of acid-sensing ion channel activity by the activation of 5-HT₂ receptors in rat dorsal root ganglion neurons

Acid-sensing ion channels (ASICs), as key sensors for extracellular protons, are expressed in nociceptive sensory neurons and contribute to signalling pain caused by tissue acidosis. ASICs are also the subject of various factors. Here, we further provide evidence that the activity of ASICs is potentiated by the activation of 5-HT₂ receptors in rat dorsal root ganglion neurons. A specific 5-HT₂ receptor agonist, α-methyl-5-HT, dose-dependently enhanced proton-gated currents with an EC₅₀ of 0.13 ± 0.07 nM. The α-methyl-5-HT enhancing effect on proton-gated currents was blocked by cyproheptadine, a 5-HT₂ receptor antagonist, and removed by intracellular dialysis of either GDP-β-S or protein kinase C inhibitor GF109203X. Moreover, α-methyl-5-HT altered acid-evoked membrane excitability of rat DRG neurons and caused a significant increase in the amplitude of the depolarization and the number of spikes induced by acid stimuli. Finally, α-methyl-5-HT increased nociceptive responses to injection of acetic acid in rats. These results suggest that α-methyl-5-HT up-regulates the activity of ASICs via 5-HT₂ receptor and protein kinase C dependent signal pathways in rat primary sensory neurons and this potentiation contributed to acid- mediated pain in tissue injury and inflammation.

Prokineticin 2 suppresses GABA-activated current in rat primary sensory neurons

Prokineticin 2 (PK2) is a newly identified regulatory protein, which is involved in a wide range of physiological processes including pain perception in mammals. However, the precise role of PK2 in nociception is yet not fully understood. Here, we investigate the effects of PK2 on GABA(A) receptor function in rat trigeminal ganglion neurons using whole-cell patch clamp technique. PK2 reversibly depressed inward currents produced by GABA(A) receptor activation (I(GABA)) with an IC₅₀ of 0.26 ± 0.02 nM. PK2 appeared to decrease the efficacy of GABA to GABA(A) receptor but not the affinity. The maximum response of the GABA dose-response curve decreased to 71.2 ± 7.0% of control after pretreatment with PK2, while the threshold value and EC₅₀ of curve did not alter significantly. The effects of PK2 on I(GABA) were voltage independent. The PK2-induced inhibition of I(GABA) was removed by intracellular dialysis of either GDP-β-S (a non-hydrolyzable GDP analog), EGTA (a Ca²+ chelator) or GF109203X (a selective protein kinase C inhibitor), but not by H89 (a protein kinase A inhibitor). These results suggest that PK2 down-regulates the function of the GABA(A) receptor via G-protein and protein kinase C dependent signal pathways in primary sensory neurons and this depression might underlie the hyperalgesia induced by PK2.

Histamine regulates activities of neurons in the ventrolateral preoptic nucleus

The neurons responsible for the onset of sleep are thought to be located in the ventrolateral preoptic nucleus (VLPO), which receives a dense histaminergic innervation from the tuberomammillary nucleus (TMN). Yet, the role of histamine in the VLPO remains unclear. Here we report that microinjection of histamine into the VLPO increases the motor activity of rats. Moreover, a bath application of histamine to acute brain slices inhibits the majority of VLPO neurons, which are also inhibited by noradrenaline. Histamine hyperpolarizes the membrane potential and lowers the firing rate. These effects are associated with an increase in the frequency but not in the amplitude of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic currents, and are blocked by gabazine or tetrodotoxin, indicating an indirect action. Conversely, on the noradrenaline-excited VLPO neurons, histamine depolarizes the membrane potential and increases the firing rate via activation of H(1) and H(2) subtype histamine receptors. Moreover, histamine-induced depolarization persists in the presence of gabazine or tetrodotoxin, indicating a direct action. Based on these findings, we propose that in the VLPO, noradrenaline-inhibited neurons may normally be under the inhibitory control of noradrenaline-excited neurons. By facilitating the inhibitory control of the noradrenaline-excited neurons, histamine may inhibit the noradrenaline-inhibited neurons, resulting in excitation of histamine-releasing neurons in the TMN through disinhibition. This effect of histamine in the VLPO may contribute to the maintenance of wakefulness.

Potentiation by WIN 55,212-2 of GABA-activated currents in rat trigeminal ganglion neurones

BACKGROUND AND PURPOSE

Although both natural and synthetic cannabinoid compounds have been shown to exert an antinociceptive effect on acute and persistent pain, the anatomical locus of the target of cannabinoid-induced analgesia has not been fully elucidated. Here, we investigated the effects of the cannabinoid agonist WIN 55,212-2 on GABA-activated currents (I(GABA)) in rat primary sensory neurones.

EXPERIMENTAL APPROACH

In the present study, experiments were performed on neurones freshly isolated from rat trigeminal ganglion (TG) by using whole-cell patch clamp and repatch techniques.

KEY RESULTS

GABA-evoked inward currents were potentiated by pretreatment with WIN 55,212-2 in a concentration-dependent manner (10(-10)-10(-8) M). WIN 55,212-2 shifted the GABA concentration-response curve upwards, with an increase of 30.3 +/- 3.7% in the maximal current response but with no significant change in the EC(50) (agonist concentration producing a half-maximal response) value. WIN 55,212-2 potentiated the responses to GABA in a manner independent of holding potential and in the absence of any change in the reversal potential of the current. This potentiation of I(GABA) induced by WIN 55,212-2 was almost completely blocked by AM 251 (3 x 10(-8) M), a CB(1) receptor antagonist, and, using the repatch technique, was found to be abolished after intracellular dialysis with the protein kinase A (PKA) activator cAMP or the PKA inhibitor H89.

CONCLUSIONS AND IMPLICATIONS

The potentiation by WIN 55,212-2 of I(GABA) in primary sensory neurones may help to elucidate the mechanism underlying the modulation of analgesia by cannabinoids in the spinal dorsal horn.