The results of our study showed that the distance and latency parameters of the water maze decreased more significantly in the group with preoperative chronic pain than in the simple operation group. The protein contents of IL–1β, IL–6 and TNF-α in the pain operation group increased on the 1st, 3rd and 7th days after the operation, indicating that preoperative chronic pain can significantly aggravate the neuroinflammatory reaction after operation. Preoperative chronic pain leads to neuronal apoptosis and increases the expression of pro-inflammatory cytokines in the hippocampus, which leads to a decrease in cognitive function after operation. Our study revealed that rats with preoperative pain are susceptible to cognitive impairment after surgical trauma. Postoperative cognitive impairment paralleled the increased expression of pro-inflammatory cytokines in the central nervous system.
In this study, we aimed to observe the effect of preoperative chronic pain on postoperative cognitive impairment, so we chose complete Freund’s adjuvant (CFA) to establish a chronic inflammatory pain model. CFA can precipitate antigens locally that releases slowly and plays an important role in stimulating antigens. It is an ideal model of chronic pain that is widely used. For our surgical intervention, we chose partial liver surgery. Distribution of painless nerves in the liver can minimize the impact of visceral pain on the central system during and after the operation and the interference between different pains. The liver acts as a machine. As the main metabolic organ of the body, the liver plays a key role in drug metabolism. However, there was no drug intervention in this study, so the metabolic activity of the liver had little effect on this experiment. In addition, we chose to remove 30% of the liver because such a procedure has no impact on the normal physiological function of the liver. Therefore, partial liver surgery was used as an intervention to study the effect of preoperative chronic pain on postoperative cognitive function.
Our initial results showed that, compared with those of the blank group and the simple pain group, the distance and latency parameters in the water maze test of the operation group were decreased; however, the rats with preoperative pain showed poorer cognitive ability than that of the rats that underwent simple operation, showing that the distance and latency parameters of the MWM experiment were prolonged. It has been suggested that preoperative chronic pain is an important risk factor for decreases in cognitive and learning abilities in rats after operation and that it prolongs the disease response. It is well known that preoperative pain activates the peripheral innate immune system. Peripheral immune attacks are transmitted to the brain through a variety of humoral and neurological pathways . This immune system-brain signal leads to the production of pro-inflammatory cytokines in the brain, which mainly involves the activation of glial cells . The activation of the peripheral innate immune system promotes the release of inflammatory cytokines, such as IL–1β, IL–6 and TNF-α, by directly or indirectly activating microglia . These neurotoxic inflammatory mediators can further change the function of neurons and lead to a decrease in cognitive function . Therefore, we evaluated the expression of pro-inflammatory cytokines in the hippocampus of the rats. The results showed that the expression of IL–1β, I L6 and TNFα in the hippocampus of the operation group was increased significantly(P<0.05) compared with that in the control group and pain group. In rats with preoperative pain, IL–1β, IL6 and TNFα were more highly expressed in the hippocampus of rats (P<0.05). This suggests that preoperative pain can cause a significant inflammatory response in the hippocampus after operation, so we further determined that preoperative pain is an important risk factor for postoperative cognitive impairment.
In our study, it was found that rats with preoperative chronic pain had longer reaction times than those that underwent simple surgery, which is an important phenomenon that was not mentioned in previous studies. We speculate that this may be related to the effect of acetylcholine. In some in vitro experiments, it has been shown that the inflammatory reaction of microglia can be inhibited by nicotine, while nicotine can reduce the local inflammatory reaction through the acetylcholine pathway and produce neuroprotective effects . Therefore, we further observed the content of ACh in the hippocampus. The results showed that, compared with the blank group and the simple pain group, the pain+operation group. exhibited significantly decreased ACh levels in the hippocampus on the 3rd and 7th days. ACh levels in the hippocampal region decreased only on the 1st and 3rd days after the operation in the operation group (P < 0.05), while the ACh levels in the hippocampus decreased significantly on the 3rd and 7th days after the operation (P < 0.05), and the levels in the operation group decreased only on the 1st and 3rd days after the operation (P < 0.05). According to the results, we speculate that, although some microglia may be activated in rats with chronic pain, there is no neuroinflammatory reaction, and cognitive function is not affected due to the anti-inflammatory pathway of acetylcholine. In rats that underwent simple operation, we observed that IL–1β, IL–6 and TNF-α expression increased to varying degrees, indicating that surgery is a risk factor for PND, which is consistent with the findings of previous studies. Like . However, as time passed after the operation, the expression of inflammatory factors in the rats in the simple operation group gradually decreased, indicating that the operation may be a transient stimulus that induces an increase of the expression of pro-inflammatory cytokines over a short period of time and that as time passes, acetylcholine increases, the anti-inflammatory effect increases, and cognitive function gradually recovers. However, there was no significant decrease in preoperative pain in rats under the same conditions, which indicates that the combined effects of preoperative chronic pain and surgery can induce a high level of neuronal apoptosis and prevent the body from repairing itself. Therefore, we think that preoperative pain is a major factor that leads to the prolongation of PND.
We further observed the activation of the acetylcholine pathway and its G protein coupling in the hippocampus. The results showed that the expression of cAMP, PKA, and GABAA in the hippocampus of rats that underwent partial hepatectomy was significantly lower than that in rats that did not undergo operation, and the content of AMP, PKA, and GABAA in the hippocampus of rats with preoperative pain was significantly lower than that of rats that underwent simple operation (P < 0.05). Additionally, the content of AMP, PKA, and GABAA in the hippocampus of rats with preoperative pain was significantly higher than that in rats that did not undergo operation (P < 0.05), and the content of PKA in the hippocampus of rats with preoperative pain was significantly lower than that of rats that did not undergo operation (P < 0.05); this was more obvious on the 7th day after the operation. The acetylcholine receptor can activate the cAMP-PKA pathway through G protein coupling to intracellular cAMP-activated PKA-phosphorylated CREB (p-CREB), and activated p-CREB can also interact with the targeted CRE region to regulate the transcription of many downstream genes, promote the development of synaptic connections, and form a long-term memory. This process is related to synaptic plasticity. Synaptic plasticity refers to the adjustment of the connection strength between nerve cells  An activity-dependent increase in synaptic transmission intensity may result in long-term potentiation (LTP), and memories are considered to be generated through the process of LTP, which is generally regarded as one of the main molecular mechanisms that form the basis of learning and memory. Interestingly, studies have shown that pro-inflammatory cytokines involved in immuno system-brain signal transduction are thought to destroy synaptic plasticity . Downstream of the cAMP-PKA pathway, GABAA can gate the GABAA receptor signal in synaptic transmission during LTP induced by high-frequency activity, modulate the frequency dependence of synaptic plasticity  and play a key role in learning and memory. Based on the above information, we speculate that GABAA is affected by the ACh-cAMP-PKA signaling pathway and that the role of the ACh-cAMP-PKA pathway is very important for the development of PND. Therefore, it can be inferred that preoperative chronic pain leads to inflammation in the central nervous system, activates the acetylcholine anti-inflammatory pathway, maintains the balance between the internal environment and synaptic plasticity, and does not induce obvious cognitive impairment. After surgical stimulation, neuroinflammation further increases, destroys the original balance, and leads to the release of a large number of central inflammatory, the induction of high levels of apoptosis in neuronal cells, and acetylcholine production disorder. The anti-inflammatory effect is weakened, and the ACh-cAMP-PKA signal is passed through the shadow pathway at the same time. Reducing GABAA results in the disruption of synaptic plasticity, which leads to an increase in PND incidence and prolongation of PND.
There are some shortcomings in this study. In our study, the behavior, apoptosis level of hippocampal cells, and expression of pro-inflammatory cytokines and acetylcholine pathway proteins were compared between rats with preoperative chronic in the presence and absence of surgery but not between rats with preoperative chronic pain, so it is impossible to determine whether the rats had preoperative chronic pain. The effective management of preoperative chronic pain has an effect on postoperative cognitive function, and whether the peripheral innate immune system is activated by preoperative chronic pain, especially whether “immune memory ability” occurs in this process, is important. At the same time, the effects of preoperative chronic pain and surgical trauma pain on the consequences were not eliminated. In the study of the cellular pathway, we observed changes in the expression of different proteins in the pathway, but unfortunately, we did not determine the specific proteins. At the same time, we did not elucidate the changes in endogenous and exogenous acetylcholine, and we did not inhibit the pathway to determine the effect of the pathway. Therefore, this study is only a preliminary observational study, and our follow-up study will compensate for the shortcomings.