Involvement Of Astrocyte Aquaporin-4 (aqp-4) Deficiency In Spinal Nociception Processing

Pain is an unpleasant experience associated with actual or potential tissue damage, including a sensory-discriminative and an emotional-affective components. Threshold is one of the characteristics of the sensory system responding to stimulations. Peripheral nociceptors will be activated with proper stimulation upto the threshold. Then nociceptive signals will be transferred to dorsal root ganglion via primary sensory afferents. The signals reach nociceptive non-specific and specific neurons in the spinal cord dorsal horn. Among the neurons in the dorsal horn, a subset of neurons named wide-dynamic-range (WDR) neurons process fine temperature recognition in nociception processing.It is widely accepted that glia cells, especially astrocytes play very important roles in pain sensation and modulation. Activated astrocytes release lots of signal molecules such as glutamate, cytokines to regulate neurotransmission. Astrocyte dysfunction would abnormally modulate neurons' function including pain.Aquaporin-4 (AQP-4), a subtype of aquaporin, is specifically expressed in astrocytes in the central nervous system. Aquaporin-4 deletion down-regulates astrocyte function in water transport, astrocyte migration, and communication with neurons.Since astrocytes play important roles in the processing of pain, lacking of aquaporin-4 could possibly alter the pain modulation. In order to investigate the possible role of aquaporin-4 in the processing of pain, we carried out behavioral tests, electrophysiological recording, and immunohistochemical staining in AQP-4 knockout (KO) mice comparing to wild-type (WT) mice. For the first time, our work demonstrated the involvement of aquaporin-4 in pain processing. Firstly, pain thresholds of both WT and KO mice were measured by all kinds of behavioral tests including noxious heat induced pain tests, mechanical pain test, chemical pain test, inflammatory pain test, and formalin-induced conditional place avoidance (F-CPA) test. Thermal pain thresholds were significantly increased in KO mice compared to WT mice in radiant heat induced tail flick test, hot-water-immersion induced tail flick test, radiant heat induced paw withdrawal test, and hot-plate test, with no changes in both genders. Intraplantar injection of 2.5%formalin induced two phases were measured in both WT and KO mice. In the second phase, KO mice displayed significant attenuation of spontaneous pain behavior. The peak of the second phase was obviously delayed and significantly decreased. Since the locomotor activity and basal skin temperature was not changed in KO mice, all the results were considered as the dysfunction of astrocytes lacking aquaporin-4 water channel. F-CPA score showed no differences between KO and WT mice in nociception induced pain-related emotion. Taken together, we firstly demonstrated that aquaporin-4 played a role in physiological pain.To further investigate the cellular alternation in AQP-4 KO mice, we carried out extracellular recording in spinal dorsal horn. As the results shown, both numbers of nociceptive specific (NS) neurons and thermal-mechano-sensitive (TMS) wide-dynamic-range (WDR) neurons were significantly decreased. Moreover, five WDR neurons which had prolonged latencies were recorded in four KO mice. Also we recorded less low threshold (LT) neurons in AQP-4 KO mice compared to WT mice. The results suggested aquaporin-4 was not only involved in pain but also in touch.Whether aquaporin-4 is involved in analgesia or not is also followed by interest. Morphine analgesic effect was measured by noxious heat induced tail flick test and hot-plate test in both WT and KO mice after 10mg/kg morphine intraperitoneal injection. The results showed significant impaired analgesic effect of morphine in KO mice. Considering astrocytes playing very important roles in morphine analgesic tolerance, we investigated the role of aquaporin-4 in 70mg/kg morphine daily-injection-induced tolerance model. At the fourth day of continuously morphine injection, both KO and WT mice displayed significant morphine analgesic tolerance. After that, intraperitoneal injection of 5mg/kg naloxone, theμopioid receptor's antagonist, induced strongly withdrawal in WT mice. But no morphine withdrawal responses were observed in KO mice. The results suggested the involvement of aquaporin-4 in morphine induced analgesia and naloxone induced withdrawal but not in morphine analgesic tolerance.In conclusion, these results firstly suggested the involvement of AQP-4 in spinal nociception processing.