Effects Of Orexin-A On The Recovery Of Aged Rats After Ketamine Anaesthesia And Its Mechanism

Postoperative cognitive dysfunction (POCD) is one of the post-anaesthesia complications. It can lead to increased mortality, delay of rehabilitation and increase of medical cost, even can seriously affect patients'postoperative life. Now the pathogenesis of POCD is unclear, but age and anesthetics were two main risk factors in the processes of POCD. As the global population aging , the medical and social problems caused by POCD would become serious. To prevent and cure POCD of the aged is an important task for the medical establishment.In 1998,two petides were described in the hypothalamus and named Orexin-A and Orexin-B. Orexin are mainly produced by the neurons within the lateral hypothalamus area. Their projections and orexin receptors are widely distributed in the brain. Accordingly, the peptides are implicated in regulation of many physiological functions. Much evidence has shown that orexins play an important part in promotion of wakefulness. Recently Experiment finding demonstrated Orexin-A elicited anesthetic arousal in terms of Electroencephalogram (EcoG) pattern and significantly decreased barbiturate anesthesia time. Moreover, recent studies have revealed Orexin-A was able to facilitate learning, to consolidate of learning and to retrieve processes in a passinve avoidance paradigm. However, the post-anesthesia effect of Orexin on the cognitive function has not be reported. In this research, animal model of aged rats on ketamine anesthesia was established. The behavior changes were judged by the duration of loss of righting reflex (LORR). ECoG recordings were used to evaluate the changes in activity of rat brain by comparing the pecrecent ofδwave in ECoG before and after application of drugs. The Morris water maze was used to test the ability of learning and spatial localization on rats. Immunocytochemistry and RT-PCR were used to evaluate the effect of Orexin-A on choline acetyltransferases in central cholinergic system. The mechanism of Orexin-A on the cognitive function after anesthesia was explored. The results showed as follows: 1.The effect of Orexin-A on the duration of LORRThe data showed that Orexin-A at the dose of 4 nmol induced significant decrease in the duration of LORR as compared with the control group(p<0.05), whereas Orexin-A at 1 nmol had no obvious effects on the duration of LORR(p<0.05). The experimental results indicated that a high dose of Orexin-A could decrease ketamine anesthesia time and promote the recovery of anesthesia.2.The effect of Orexin-A on the percent ofδwave in ECoGAfter LORR appeared, the percent ofδwave in ECoG was increased gradually. 4 nmol Orexin-A markedly reducedδactivities as compared with the model control group(p<0.05), whereas Orexin-A at the dose of 1 nmol had no obvious effect on theδactivities (P >0.05). The results indicated that high dose of Orexin-A could promote the recovery of anesthesia.3.The effect of Orexin-A on Morris water maze testWith the increase in training time, the escape latency of all were decreased rapidly. From the 3rd to 5rd ,compared with the control group, the escape latency of the group at the dose of 1 nmol Orexin-A and the model control group were more or much more longer (P < 0.01 or P < 0.05) in place navigation test, there is no significant difference with Orexin-A at the dose of 4 nmol group. Compared with the model control group, the escape latency of Orexin-A at the dose of 4 nmol group was shortened significantly after treatment (P < 0.05 or P < 0.01), and Orexin-A at low dose group was not changed significantly(P>0.05).(2) The time stay in platform quadrant and the number of rats crossing the platform decreased after anesthesia in spatial probe test. Model control group and Orexin-A at the dose of 1 nmol group changed significantly comparing with control group(P < 0.01), Orexin-A at the dose of 4 nmol group was no significant difference (P > 0.05). The time stay in platform quadrant and the number of rats crossing the platform increased significantly(P<0.05)after dealing with Orexin-A at the dose of 4 nmol, but Orexin-A at the dose of 1 nmol was no significant change (P>0.05). Analysis of spatial probe tracking chart, the strategy of space probe in rats on Orexin-A at the dose of 1 nmol group and model control group were lacked specific objectives, but the rats of Orexin-A at the dose of 4 nmol group and control group actived almost in the platform quadrant area.(3)The escape latency in four groups was not different from each other (P > 0.05) in visible platform test. The results showed: Learning and memory ability was impaired with ketamine injection. Orexin-A at 4 nmol acted as a protective factor but 1 nmol Orexin-A could not do.4.The effects of orxin-A on expression of ChAT in embryonic basal brain after ketamine anaesthesiaImmunohistochemistry results showed that, the expression of ChAT in embryonic basal brain of model control group and Orexin-A at 1 nmol group decreased significantly as compared with control group(p<0.01). Compared with model control group ,the expression of ChAT in embryonic basal brain in rat of Orexin-A at the dose of 4 nmol group increased significantly (p<0.01).5. The effects of Orxin-A on expression of ChAT mRNA in embryonic basal brain after ketamine anaesthesiaThe results showed that, compared with control group, the expression of ChAT mRNA in embryonic basal brain decreased significantly of rat in model control group and Orexin-A at the dose of 1 nmol group(p<0.05). Compared with model control group , the expression of ChAT mRNA in embryonic basal brain increased obviously(p<0.05).In summary , this research demonstrated: 1. A high dose of Orexin-A could decrease the depth and duration on aged rat of ketamine anesthesia ; A high dose of Orexin-A could promote the recovery of aged rat from ketamine anesthesia. 2. Learning and memory ability was impaired with ketamine injection. Orexin-A at the dose of 4 nmol acted as a protective factor but at the dose of 1 nmol did not act in the same way. 3. The expression of ChAT gene in embryonic basal brain of aged rat decreased after ketamine anesthesia. When ketamine is administered in aged rats, it can affect the spatial learning and memory behavior. The mechanism of damage might be related to the central cholinergic system. A high dose of Orexin-A could partly reverse this change.