NMS-1286937 synthesis During pain signal transmission glutam

During pain signal transmission, glutamate is released into the synaptic cleft, which, in turn, causes activation of N-methyl-d-aspartate (NMDA) receptors and hence neurotransmission across the synapse occurs. The synaptic concentration and resultant activity of glutamate are controlled by excitatory amino NMS-1286937 synthesis transporters (EAATs) which transport glutamate from the synaptic cleft into the glial cells. In the spinal cord, EAAT1 and EAAT2 are the most efficient mediators of glutamate clearance at synaptic cleft (Danbolt, 2001, Holmseth et al., 2012), and their gene expression has an important role in visceral sensitivity (Lin et al., 2009, Gosselin et al., 2010). It has been shown that intrathecal administration of a glutamate transporter antagonist results in increased sensitivity to colorectal distension in rats (Gosselin et al., 2010). Moreover, transgenic mice overexpressing EAAT2 demonstrated a twofold increase in the glutamate uptake across these transporters and a significant reduction in the visceromotor response to colorectal distension (Lin et al., 2009). Furthermore, maternally separated rats have shown an increase in visceral sensitivity associated with a selective reduction in spinal EAAT1 expression (Gosselin et al., 2010). However, the majority of preclinical studies assessing the role of EAATs in pain have been carried out in male animals. Estrogen levels fluctuate during the phases of the menstrual cycle (humans) and estrous cycle (rodents) (Marcondes et al., 2001, Stricker et al., 2006) and were shown to alter the activity of glutamate receptors (Tang et al., 2008). Moreover, NMDARs showed higher activity in dorsal root ganglia (DRGs) in female rats as compared to males, and this effect was further potentiated in females by exogenous estrogen (McRoberts et al., 2007). Co-localization of estrogen receptor alpha (ER-α) and NR1 (a subunit of NMDAR) in the dorsal horn of the lumbosacral spinal cord suggests a direct modulation of glutamate receptor activity by estrogen through ER-α (Tang et al., 2008). Furthermore, it has been shown that the threshold for visceral pain is lower in high-estrogen proestrus phase (Ji et al., 2008).
Experimental procedures
Results
Discussion In this study, we have demonstrated that EAAT activity in the lumbosacral spinal cord differs between male and female rats. The activity of these transporters is decreased in spinal cord slices from male rats when exogenous estrogen is applied. Moreover, uptake via EAATs varies across the estrous cycle in female rats gradually decreasing from the diestrus to the estrus phase. This indicates that the gonadal hormone estrogen is capable of influencing the activity of these transporters and can be artificially manipulated. This provides insights into sex differences in visceral pain as the lumbosacral level of the spinal cord receives sensory signals from the gastrointestinal tract. Alterations in the expression of these transporters in this part of the spinal cord have been previously associated with visceral pain in an animal model of IBS (Gosselin et al., 2010), a disorder that is more prevalent in females. Hence, we proposed to assess in healthy rats if a difference in the activity of these transporters existed between the sexes that may help explain differences in pain that are contributed to by gonadal hormones. Here we also demonstrate that riluzole, which enhances EAAT function increases uptake of aspartate during the estrus phase in female rats only. This indicates that EAAT1 function in female rats may only be susceptible to manipulation during this stage of the cycle. As riluzole has been shown to have a therapeutic effect in chronic pain states (Gosselin et al., 2010, Nicholson et al., 2014), our study highlights the need to take fluctuating female hormones into account in pain management with riluzole or other EAAT enhancers. Our results correlate with other preclinical in vivo studies, which have shown that ovariectomy in rats (resulting in estrogen deficiency) significantly decreased the magnitude of visceromotor response to colorectal distension, and this effect was reversed by administration of estrogen to these animals (Ji et al., 2003).