Roles Of Hippocampal PKMζ In Central Sensitization Of Chronic Functional Visceral Pain

Objective In our previous studies, we found the significant enhancement of hippocampal synaptic transmission-LTP (long-term potentiation, LTP) in rats with chronic functional visceral pain. In this study, effects of hippocampal PKMζ on hippocampal LTP enhancement and visceral hypersensitivity was further investigated to clarify the potential mechanism underlying chronic functional visceral pain, which may provide a new target for its pharmaceutical treatment.Methods A rat model with chronic functional visceral pain was established by neonatal maternal separation for3h daily during post-natal days3-21.①Responses of the external oblique muscle of the abdomen to colorectal distention (CRD,20-80mm Hg) were measured to evaluate the sensitivity of visceral pain in rats when they were adult (8weeks).②The field potential of hippocampal CA1region was recorded in vitro to confirm whether hippocampal LTP was enhanced in rats with chronic functional visceral pain when compared with that in control rats.③The effect ZIP(0.1-2.5μM), a PKMζ inhibitor, on the inducement and maintenance of hippocampal LTP was tested to explore the roles of PKMζ on hippocampal LTP in rats with chronic functional visceral pain.④Expressions of hippocampal PKMζ and p-PKMζ were detected using Western blot method.⑤Effects of ZIP (1.0-5.0nM) on visceral hypersensitivity was examined after the intra-hippocampal injection to investigate the roles of PKMζ on visceral hypersensitivity in rats with chronic functional visceral pain.⑥Open field test and water maze experiment were used to examine whether ZIP has side effects on movement and learning memory. Results①The sensitivity of functional visceral pain significantly increased in rats with chronic functional visceral pain when compared with that in control rats (p <0.05).②LTP induced by high frequency stimulation (HFS) at SC-CA1synapses significantly enhanced in rats with chronic functional visceral pain when compared with that in control rats (p <0.05).③ZIP, a PKMζ inhibitor, had no effect on the inducement of CA1LTP (p>0.05), but dose-dependently inhibited the maintenance of CA1LTP in rats with chronic functional visceral pain when compared with that in control rats (p <0.05).④Expression of hippocampal p-PKMζ not PKMζ significantly increased in rats with chronic functional visceral pain when compared with those in control rats (p <0.05).⑤Intra-hippocampal injection of ZIP dose-dependently attenuated the visceral hypersensitivity in rats with chronic functional visceral pain when compared with that in control rats (p <0.05). The maximal inhibition was observed at the time point of30min and significant inhibition lasted for1.5-2h after ZIP application under20-80mm Hg CRD in rats with chronic functional visceral pain.⑥ZIP had no effect on the movement and learning memory in rats with chronic functional visceral pain (p>0.05), which suggested ZIP had no obvious side effects.ConclusionNeonatal maternal separation leaded to the formation of visceral hypersensitivity when grown up. Hippocampal LTP plays an important role in visceral hypersensitivity in rats with chronic functional visceral pain, and p-PKMζ may be the key factor.