Synaptic Plasticity: The New Explanation of Visceral Hypersensitivity in Rats with Trichinella spiralis Infection?

Background and Aim Synaptic plasticity plays an important role in affecting the intensity of visceral reflex. It may also be involved in the development of visceral hypersensitivity. The aim of this study was to investigate the role of synaptic plasticity on visceral hypersensitivity of rats infected by Trichinella spiralis. Methods Thirty male Sprague–Dawley (SD) rats were randomly divided into control, acute, and chronic infection groups, and were investigated at 1 week after adaptive feeding and at 2 and 8 weeks post infection (PI) by oral administration of 1 ml phosphate-buffered saline (PBS) containing 8,000 Trichinella spiralis larvae. Visceral sensitivity was evaluated by electromyography (EMG) recording during colorectal distension. Intestinal inflammation was observed by hematoxylin-eosin (HE) staining. Synaptic ultrastructure parameters, such as postsynaptic density (PSD) length, synaptic cleft, and number of synaptic vesicles, were examined by transmission electron microscopy (TEM). The expression of protein associated with synaptic plasticity, including postsynaptic density-95 (PSD-95), synaptophysin, calbindin-28 K, N-methyl-D-aspartate receptor-1 (NMDA-R1), α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R), and glial cell line-derived neurotrophic factor (GDNF), were analyzed by Western blot. Results (1) Visceral hypersensitivity was noted in the chronic infection group, although the inflammation was nearly eliminated (P < 0.05). Severe inflammation and downregulation of visceral sensitivity were observed in the acute infection group (P < 0.05). (2) There were many more synaptic vesicles and longer PSD in the chronic infection group than in the control group (P < 0.05, respectively). However, in comparison with control rats, disappearance of mitochondria cristae in the synapses, and decrease of synaptic vesicles and length of PSD were observed in the acute infection group. There was no significant difference in width of synaptic cleft among the three groups. (3) Compared with the control, the expression of proteins associated with synaptic plasticity was significantly upregulated during chronic infection phase (P < 0.05), and downregulated during acute infection phase. Conclusion Synaptic plasticity was observed in SD rats infected by Trichinella spiralis and was associated with visceral sensitivity, which suggests that it may play an important role in the formation of visceral hypersensitivity.