Distribution of extracellular signal-regulated kinase 1- and 2-activated neurons in the rat periaqueductal gray matter after noxious stimulation

Loss of dopamine D1 receptors and diminished D1/5 receptor-mediated ERK phosphorylation in the periaqueductal gray after spinal cord lesion

Neuropathic pain resulting from spinal cord injury is often accompanied by maladaptive plasticity of the central nervous system, including the opioid receptor-rich periaqueductal gray (PAG). Evidence suggests that sensory signaling via the PAG is robustly modulated by dopamine D1- and D2-like receptors, but the effect of damage to the spinal cord on D1 and D2 receptor protein expression and function in the PAG has not been examined. Here we show that 21days after a T10 or C6 spinothalamic tract lesion, both mice and rats display a remarkable decline in the expression of D1 receptors in the PAG, revealed by western blot analysis. These changes were associated with a significant reduction in hindpaw withdrawal thresholds in lesioned animals compared to sham-operated controls. We investigated the consequences of diminished D1 receptor levels by quantifying D1-like receptor-mediated phosphorylation of ERK1,2 and CREB, events that have been observed in numerous brain structures. In naïve animals, western blot analysis revealed that ERK1,2, but not CREB phosphorylation was significantly increased in the PAG by the D1-like agonist SKF 81297. Using immunohistochemistry, we found that SKF 81297 increased ERK1,2 phosphorylation in the PAG of sham animals. However, in lesioned animals, basal pERK1,2 levels were elevated and did not significantly increase after exposure to SKF 81297. Our findings provide support for the hypothesis that molecular adaptations resulting in a decrease in D1 receptor expression and signaling in the PAG are a consequence of SCL.

ERK1/2: Function, signaling and implication in pain and pain-related anxio-depressive disorders

Despite the increasing knowledge regarding pain modulation, the understanding of the mechanisms behind a complex and pathologic chronic pain condition is still insufficient. These knowledge gaps might result in ineffective therapeutic approaches to relieve painful sensations. As a result, severe untreated chronic pain frequently triggers the onset of new disorders such as depression and/or anxiety, and therefore, both the diagnosis and treatment of patients suffering from chronic pain become seriously compromised, prompting a self-perpetuating cycle of symptomatology. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) are molecules strongly implicated in the somatic component of pain at the spinal cord level and have been emerging as mediators of the emotional-affective component as well. Although these molecules might represent good biomarkers, their use as pharmacological targets is still open to discussion as paradoxical information has been obtained. Here we review the current scientific literature regarding ERK1/2 signaling in the modulation of pain, depression and anxiety, including the emotional-affective spheres of the pain experience.

Extracellular signal-regulated kinase activation in the chronic constriction injury model of neuropathic pain in anaesthetized rats

BACKGROUND

The role of extracellular signal-regulated kinases (ERKs) in nociception has been explored in the last years. While in spinal cord their activation is frequently correlated with pain or acute noxious stimuli, supraspinally, this association is not so evident and remains unclear. This study aims to evaluate ERK1/2 activation in the spinal cord and brainstem nuclei upon neuropathy and/or an additional mechanical stimulus.

METHODS

Acute noxious mechanical stimulation was applied in the left hindpaw of anaesthetized SHAM-operated and chronic constriction injured (CCI, neuropathic pain model) rats. Other SHAM or CCI rats did not receive any stimulus. Immunohistochemistry against the phosphorylated isoforms of ERK1/2 (pERK1/2) was performed in lumbar spinal cord and brainstem sections to assess ERK1/2 activation.

RESULTS

In the spinal cord, stimulation promoted an increase in pERK1/2 expression in the superficial dorsal horn of SHAM rats. No significant effects were caused by CCI alone. At supraspinal level, changes in ERK1/2 activation induced by CCI were observed in A5, locus coeruleus (LC), raphe obscurus (ROb), raphe magnus, dorsal raphe (DRN), lateral reticular and paragigantocellularis nucleus. CCI increased pERK1/2 expression in all these nuclei, with exception of LC, where a significant decrease was verified. Mechanical noxious stimulation of CCI rats decreased pERK1/2 expression in ROb and DRN, but no further changes were detected in either SHAM- or CCI-stimulated animals.

CONCLUSION

ERK1/2 are differentially activated in the spinal cord and in selected brainstem nuclei implicated in nociception, in response to an acute noxious stimulus and/or to a neuropathic pain condition.

Activation of mitogen-activated protein kinase in descending pain modulatory system

The descending pain modulatory system is thought to undergo plastic changes following peripheral tissue injury and exerts bidirectional (facilitatory and inhibitory) influence on spinal nociceptive transmission. The mitogen-activated protein kinases (MAPKs) superfamily consists of four main members: the extracellular signal-regulated protein kinase1/2 (ERK1/2), the c-Jun N-terminal kinases (JNKs), the p38 MAPKs, and the ERK5. MAPKs not only regulate cell proliferation and survival but also play important roles in synaptic plasticity and memory formation. Recently, many studies have demonstrated that noxious stimuli activate MAPKs in several brain regions that are components of descending pain modulatory system. They are involved in pain perception and pain-related emotional responses. In addition, psychophysical stress also activates MAPKs in these brain structures. Greater appreciation of the convergence of mechanisms between noxious stimuli- and psychological stress-induced neuroplasticity is likely to lead to the identification of novel targets for a variety of pain syndromes.

The ERK 1 and 2 pathway in the nervous system: from basic aspects to possible clinical applications in pain and visceral dysfunction

The extracellular signal-regulated kinases 1 and 2 (ERK) cascade, member of the mitogen-activated protein kinases superfamily of signalling pathways, is one of the best characterized pathways as many protein interactions and phosphorylation events have been systematically studied. Traditionally, ERK are associated with the regulation of proliferation and differentiation as well as survival of various cell types. Their activity is controlled by phosphorylation on specific aminoacidic residues, which is induced by a variety of external cues, including growth-promoting factors.

In the nervous system, ERK phosphorylation is induced by binding of neurotrophins to their specific tyrosine kinase receptors or by neuronal activity leading to glutamate release and binding to its ionotropic and metabotropic receptors. Some studies have provided evidence of its importance in neuroplastic events. In particular, ERK phosphorylation in the spinal cord was shown to be nociceptive-specific and its upregulation, occurring in cases of chronic inflammatory and neuropathic pain, seems to be of the utmost importance to behavioural changes observed in those conditions. In fact, experiments using specific inhibitors of ERK phosphorylation have proved that ERK directly contributes to allodynia and hyperalgesia caused by spinal cord injury or chronic pain. Additionally, spinal ERK phosphorylation regulates the micturition reflex in experimental models of bladder inflammation and chronic spinal cord transection.

In this review we will address the main findings that suggest that ERK might be a future therapeutic target to treat pain and other complications arising from chronic pain or neuronal injury.

Identification of cholinergic and non-cholinergic neurons in the pons expressing phosphorylated cyclic adenosine monophosphate response element-binding protein as a function of rapid eye movement sleep

Recent studies have shown that in the pedunculopontine tegmental nucleus (PPT), increased neuronal activity and kainate receptor-mediated activation of intracellular protein kinase A (PKA) are important physiological and molecular steps for the generation of rapid eye movement (REM) sleep. In the present study performed on rats, phosphorylated cyclic AMP response element-binding protein (pCREB) immunostaining was used as a marker for increased intracellular PKA activation and as a reflection of increased neuronal activity. To identify whether activated cells were either cholinergic or noncholinergic, the PPT and laterodorsal tegmental nucleus (LDT) cells were immunostained for choline acetyltransferase (ChAT) in combination with pCREB or c-Fos. The results demonstrated that during high rapid eye movement sleep (HR, approximately 27%), significantly higher numbers of cells expressed pCREB and c-Fos in the PPT, of which 95% of pCREB-expressing cells were ChAT-positive. With HR, the numbers of pCREB-positive cells were also significantly higher in the medial pontine reticular formation (mPRF), pontine reticular nucleus oral (PnO), and dorsal subcoeruleus nucleus (SubCD) but very few in the locus coeruleus (LC) and dorsal raphe nucleus (DRN). Conversely, with low rapid eye movement sleep (LR, approximately 2%), the numbers of pCREB expressing cells were very few in the PPT, mPRF, PnO, and SubCD but significantly higher in the LC and DRN. The results of regression analyses revealed significant positive relationships between the total percentages of REM sleep and numbers of ChAT+/pCREB+ (Rsqr=0.98) cells in the PPT and pCREB+ cells in the mPRF (Rsqr=0.88), PnO (Rsqr=0.87), and SubCD (Rsqr=0.84); whereas significantly negative relationships were associated with the pCREB+ cells in the LC (Rsqr=0.70) and DRN (Rsqr=0.60). These results provide evidence supporting the hypothesis that during REM sleep, the PPT cholinergic neurons are active, whereas the LC and DRN neurons are inactive. More importantly, the regression analysis indicated that pCREB activation in approximately 98% of PPT cholinergic neurons, was caused by REM sleep. Moreover the results indicate that during REM sleep, PPT intracellular PKA activation and a transcriptional cascade involving pCREB occur exclusively in the cholinergic neurons.

Modulation of cardiovascular responses and neurotransmission during peripheral nociception following nNOS antagonism within the periaqueductal gray

Nitric oxide (NO) within the dorsal periaqueductal gray matter (dPAG) attenuated cardiovascular responses and changes in the concentrations of glutamate during both mechanical and thermal nociceptive stimulation [Ishide, T., Amer, A., Maher, T.J., Ally, A., 2005. Nitric oxide within periaqueductal gray modulates glutamatergic neurotransmission and cardiovascular responses during mechanical and thermal stimuli. Neurosci. Res. 51, 93-103]. Nitric oxide is synthesized from l-arginine via the enzyme, NO synthase (NOS), which exists in 3 isoforms: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). In this study, we examined the role of nNOS within the dPAG on cardiovascular responses and extracellular glutamate and GABA concentrations during mechanical and thermal nociception in anesthetized rats. The noxious mechanical stimulus was applied by a bilateral hindpaw pinch for 5 s that increased mean arterial pressure (MAP) and heart rate (HR) by 24+/-4 mm Hg and 41+/-7 bpm, respectively (n=10). Extracellular glutamate levels within the dPAG increased by 10.7+/-1.3 ng/mul while GABA concentrations decreased by 1.9+/-0.5 ng/microl. Bilateral microdialysis of a selective nNOS antagonist, 1-(2-trifluoromethylphenyl)-imidazole (TRIM; 10.0 microM), into the dPAG had no effect on MAP, HR, glutamate and GABA values (P>0.05) during a mechanical stimulation. In a separate set of experiments, a noxious thermal stimulus was generated by immersing the metatarsus of a hindpaw in a water-bath at 52 degrees C for 5 s (n=10). Glutamate, MAP, and HR increased by 14.6+/-2 ng/microl, 45+/-6 mm Hg, and 47+/-7 bpm, while GABA decreased by 2.1+/-0.6 ng/microl. Administration of TRIM into the dPAG significantly enhanced the cardiovascular responses and glutamate increases (P<0.05) but further attenuated GABA changes (P<0.05) during subsequent thermal nociception. These results demonstrate that nNOS within the dPAG plays a differential role in modulating cardiovascular responses and glutamatergic/GABAergic neurotransmission during thermal and mechanical nociception.