The Mechanism Of Plasticity In The Spinal Pain-Sensitive Sensory Neurons Induced By Tetanically Sciatic Stimulation

Astrocytes are the most predominant glia in the mammalian central neural system (CNS). The traditional view is that astrocytes mainly provide metabolism and structural support for neurons. However, recent studies have demonstrated that astrocytes function as an active partner in synaptic transmission, and are considered as a third component of the synapse stucture.Astrocytes directly modulate synapse transmission by releasing gliotransmitters and bioactive substances. Also, astrocytes indirectly influence synapse transmission by glutamate uptake via excitatory amino acid transporters. It is known that astrocyte glutamate transporters are the primary scavengers of extracellular glutamate. Evidence has shown that inhibiton of glutamate transporters can regulate synapse transmission and synaptic plasticity.Mounting evidence has substantiated that astroyctes are implicated in the spinal nociceptive processing. Various treatments that lead to neuorpathic pain can activate spinal dorsal horn astrocytes. Drugs that block the pathological pain also block astrocyte activation. Furthermore, astrocyte activation is sufficient to result in hyperalgesia, which can be effectively alleviated by interruption of glial including astrocytic function. Particularly, it is suggested that astrocyte activation may be involved in the maintenance of neuropathic pain.It has been reported previously that spinal long-term potentiation (LTP) of C-fiber evoked field potentials can be induced by tetanically sciatic stimulation, natural noxious stimulation of peripheral tissues and nerve injury, which represents the plasticity of the spinal pain-sensitive sensory neurons. This LTP may underlie the central mechanism of spinal nociception. Our previous data have proved that tetanically sciatic stimulation, which can induce spinal LTP of C-fiber evoked field potentials, produces a long-lasting hyperalgesia in rats. Interruption of glial functionin the spinal dorsal horn can block the spinal LTP induced by tetanic stimulation, and inhibit the long-lasting mechanical allodynia following tetanic stimulation. The effects induced by interruption of glial function can be simulated by exogenous glutamate. These data suggest that glia can play important roles in tetanically sciatic stimulation-inducing LTP and behavioral hyperalgesia. However, it is still unclear that whether astrocytes are activated following tetanic stimulation of the sciatic nerve, and whether astrocyte activation is associated with the behavioral response following tetanically sciatic stimulation. Additionally, we also don't know whether astrocyte transporters are implicated in the induction of spinal LTP induced by tetanically scitatic stimulation.The present study aims to investigate the roles of astrocyte in the plasticity of the spinal pain-sensitive sensory neurons. The main results are as follows:1. Astrocyte activaton and sciatic neuropathy following tetanically sciatic stimulationThere is a long-lasting bilateral mechanical allodynia in rat hind paws following unilaterally tetanic stimulation of the sciatic nerve. At the time points of days 4 and 35, the GFAP staining density in the ipsilateral spinal dorsal horn increases obviously, suggesting astrocytes are activated. In addition, it is also observed that degenerative changes occur in the previously stimulated sciatic nerve. Changes in myelinated fibers occur on day 4. However, no changes of unmyelinated fibers are observed. On day 35, changes in both myelinated and unmyelinated fibers are observed. There is a temporal parallel between astrocyte activation, sciatic neuropathy and behavioral allodynia following tetanically sciatic stimulation. These data suggest that astrocyte activation and the ongoing activity of the injured nerves may play an important role in the maintenance of mechanical allodynia following tetanically sciatic stimulation via influencing the spinal pain-sensitive sensory neurons.2. Intrathecal GLT-1 inhibitor DHK blocks the induction of spinal LTPTetanically sciatic stimulation can induce spinal LTP of C-fiber evoked field potentials in the spinal dorsal horn. Intrathecal DHK can effectively inhibit (0.1 mM) or completely block (3.0 mM) the induction of this LTP, which may be attributed to extracellular increases of glutamate evoked by GLT-1 inhibition. To test thishypothesis, exogenous glutamate is given (4.5 mM). Intrathecal glutamate can also inhibit the induction of spinal LTP. Given that the predominant localization of spinal GLT-1 in astrocytes, it is reasonable to suggest that astroycte GLT-1 is implicated in the induction of spinal LTP of C-fiber evoked field potentials induced by tetanic stimulation of the sciatic nerve.3. Intrathecal DHK inhibits spinal Fos expression evoked by tetanically sciatic stimulationTetanically sciatic stimulation can evoke significant Fos expression in the ipsilateral spinal dorsal horn, which is mainly distributed in the superficial layers. Intrathecal DHK (3.0 mM) can significantly depress spinal Fos expression.Our previous study has reported that FC, an inhibitor of glial function, can disrupt spinal LTP. In this study, we further observe the effects of FC on spinal Fos expression. Intrathecal FC (1 nmol, 10μl) can also inhibit spinal Fos expression.The colocalization experiments of Fos, NeuN and GFAP display that Fos is mainly existed in neurons, no Fos immunoreactivity is seen in astrocytes. All these data suggest that the inhibitory effects of DHK (or FC) on Fos expression are not caused by suppressing glial Fos expression.In addition, considering that DHK can result in extracellular accumulation of glutamate and lead to glutamate excitotoxicity, we perform DNA fragmentation detection to investigate the possible involvement of excitotoxicity in DHK-inducing effects in the spinal dorsal horn. We don't observe significant apoptotic cells in the group of DHK combined with tetanic stimulation, compared with that of saline combined with tetanic stimulation.In general, astroycte activation and the ongoing activity of the injured nerves may contribute to the maintenance of pain hyperalgesia following tetanic stimulation of the sciatic nerve. Astrocyte glutamate transporter GLT-1 is involved in the regulation of spinal synaptic plasticity induced by tetanically sciatic stimulation.