Animal Studies

Visceral pain originating from chronic inflammation of the pancreas is often intractable and difficult to manage clinically. However, the pathogenesis of the central nervous system underlying visceral pain is still poorly understood. The aim of the present study was to investigate the role of the midbrain ventrolateral periaqueductal gray (vlPAG) in a rat model of chronic visceral pain induced by pancreatitis. In the present study, we used a well-established rat model of chronic pancreatitis induced by tail vein injection of dibutyltin dichloride (DBTC). To assess the DBTC-induced visceral pain, we examined the abdominal withdrawal by von Frey filament test. We further studied the synaptic transmission in the vlPAG by whole-cell patch-clamp electrophysiological recordings. Rats receiving DBTC injection exhibited a significantly increased withdrawal frequency to mechanical stimulation of the abdomen compared to rats injected with vehicle. Interestingly, compared to rats injected with vehicle, we found that neurons dissected from DBTC-treated rats exhibited a significantly decreased synaptic strength, which was revealed by a diminishedα-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/ N-methyl-D-aspartic acid (AMPA/NMDA) ratio in the vlPAG. Moreover, our results further demonstrated that neurons obtained from DBTC-treated rats displayed a higher paired-pulse ratio, as well as less frequent and smaller amplitudes of miniature excitatory postsynaptic currents in the vlPAG compared to rats injected with vehicle. Furthermore, intra-vlPAG microinjection of AMPA alleviated DBTC-induced abdominal hypersensitivity. Taken together, our findings suggest that diminished glutamatergic synaptic strength via both presynaptic and postsynaptic mechanisms in the midbrain vlPAG is associated with DBTC-induced abdominal hypersensitivity. In addition, activation of AMPA receptors in the vlPAG alleviates DBTC-induced abdominal hypersensitivity.

Group III/IV muscle afferents transduce nociceptive signals and modulate exercise pressor reflexes (EPRs). However, the mechanisms governing afferent responsiveness to dually modulate these processes are not well characterized. We and others have shown that ischemic injury can induce both nociception-related behaviors and exacerbated EPRs in the same mice. This correlated with primary muscle afferent sensitization and increased expression of glial cell line-derived neurotrophic factor (GDNF) in injured muscle and increased expression of GDNF family receptor α1 (GFRα1) in dorsal root ganglia (DRG). Here, we report that increased GDNF/GFRα1 signaling to sensory neurons from ischemia/reperfusion-affected muscle directly modulated nociceptive-like behaviors and increased exercise-mediated reflexes and group III/IV muscle afferent sensitization. This appeared to have taken effect through increased cyclic adenosine monophosphate (cAMP) response element binding (CREB)/CREB binding protein-mediated expression of the purinergic receptor P2X5 in the DRGs. Muscle GDNF signaling to neurons may, therefore, play an important dual role in nociception and sympathetic reflexes and could provide a therapeutic target for treating complications from ischemic injuries.

Physical exercise has been established as a low-cost, safe, and effective way to manage chronic pain, but exact mechanisms underlying such exercise-induced hypoalgesia (EIH) are not fully understood. Since a growing body of evidence implicated the amygdala (Amyg) as a critical node in emotional affective aspects of chronic pain, we hypothesized that the Amyg may play important roles to produce EIH effects. Here, using partial sciatic nerve ligation (PSL) model mice, we investigated the effects of voluntary running (VR) on the basal amygdala (BA) and the central nuclei of amygdala (CeA). The present study indicated that VR significantly improved heat hyperalgesia which was exacerbated in PSL-Sedentary mice, and that a significant positive correlation was detected between total running distances after PSL-surgery and thermal withdrawal latency. The number of activated glutamate (Glu) neurons in the medal BA (medBA) was significantly increased in PSL-Runner mice, while those were increased in the lateral BA in sedentary mice. Furthermore, in all subdivisions of the CeA, the number of activated gamma-aminobutyric acid (GABA) neurons was dramatically increased in PSL-Sedentary mice, but these numbers were significantly decreased in PSL-Runner mice. In addition, a tracer experiment demonstrated a marked increase in activated Glu neurons in the medBA projecting into the nucleus accumbens lateral shell in runner mice. Thus, our results suggest that VR may not only produce suppression of the negative emotion such as fear and anxiety closely related with pain chronification, but also promote pleasant emotion and hypoalgesia. Therefore, we conclude that EIH effects may be produced, at least in part, via such plastic changes in the Amyg.

Chronic pain has been shown to depend on nociceptive sensitization in the spinal cord, and while multiple mechanisms involved in the initiation of plastic changes have been established, the molecular targets which maintain spinal nociceptive sensitization are still largely unknown. Building upon the established neurobiology underlying the maintenance of LTP in the hippocampus, this present study investigated the contributions of spinal atypical PKC isoforms PKCι/λ and PKM and their downstream targets (p62/GluA1 and NSF/GluA2 interactions, respectively) to the maintenance of spinal nociceptive sensitization in male and female rats. Pharmacological inhibition of atypical PKCs by ZIP reversed established allodynia produced by repeated intramuscular (i.m.) acidic saline injections in male animals only, replicating previously demonstrated sex differences. Inhibition of both PKCι/λ and downstream substrates p62/GluA1 resulted in male-specific reversals of i.m. acidic saline-induced allodynia, while female animals continued to display allodynia. Inhibition of NSF/GluA2, the downstream target to PKM, reversed allodynia induced by i.m. acidic saline in both sexes. Neither PKCι/λ, p62/GluA1 or NSF/GluA2 inhibition had any effect on formalin response for either sex.This study provides novel behavioural evidence for the male-specific role of PKCι/λ and downstream target p62/GluA1, highlighting the potential influence of ongoing afferent input. The sexually divergent pathways underlying persistent pain are shown here to converge at the interaction between NSF and the GluR2 subunit of the AMPA receptor. Though this interaction is thought to be downstream of PKM in males, these findings and previous work suggest that females may rely on a factor independent of atypical PKCs for the maintenance of spinal nociceptive sensitization.

Descending nociceptive modulation from the supraspinal structures has an important role in cancer-induced bone pain (CIBP). Midbrain ventrolateral periaqueductal gray (vlPAG) is a critical component of descending nociceptive circuits; nevertheless, its precise cellular and molecular mechanisms involved in descending facilitation remain elusive. Our previous study has shown that activation of p38 MAPK in vlPAG microglia is essential for the neuropathic pain sensitization. However, the existence of potential connection between astrocytes and JNK pathway in CIBP has not yet been elucidated. The following study examines the involvement of astrocyte activation and up-regulation of p-JNK in vlPAG, using a CIBP rat model. Briefly, CIBP was mimicked by an intramedullary injection of Walker 256 mammary gland carcinoma cells into the animal tibia. A significant increase in expression levels of astrocytes in the vlPAG of CIBP rats was observed. Furthermore, stereotaxic microinjection of the astrocytic cytotoxin L-α-aminoadipic acid decreased the mechanical allodynia, as well as established and reversed the astrocyte activation in CIBP rats. A significant increase in expression levels of p-JNK in astrocytes in vlPAG of CIBP rats was also observed. Moreover, the intrathecal administration of JNK inhibitors SP600125 reduced the expression of GFAP, while microinjection of the SP600125 decreased the mechanical allodynia of CIBP rats. These results suggested that cancer-induced bone pain is associated with astrocyte activation in the vlPAG that probably participates in driving descending pain facilitation through the JNK MAPK signaling pathway. To sum up, these findings reveal a novel site of astrocytes modulation of CIBP.