Long duration administration of isoflurane is usually used in medical procedures and in vivo neuroimaging researches. It has been reported that long-duration isoflurane administration would interfere with memory function in adult rats [1] and may cause brain cell death, and neurocognitive decline in immature rats [2]. In particular, anesthesia is a main cause of postoperative cognitive dysfunction in patients. As the alteration of default mode network (DMN) is closely associated with cognitive decline [3], we hypothesized DMN could be influenced by the long duration administration of isoflurane. In the present study, we examined the long-duration effect of isoflurane to DMN network of adult rhesus monkeys by using resting-state functional MRI (rsfMRI) techniques [4].Adult female rhesus monkeys (n=5, 7-11 years old) were anesthetized with ~1% isoflurane (~0.8 MAC maintenance dosage) mixed with oxygen for about 4 hours. O2 saturation, blood pressure, heart rate, respiration rate, body temperature and PaCO2, etc, were monitored continuously. All the physiological parameters were recorded and maintained in normal ranges. Isoflurane dosage was measured continuously with an anesthesia machine (GE Datex-ohmeda Cardiocap/5). MRI data were acquired using a gradient echo Echo Planar Imaging (EPI) sequence (TR/TE=2190 ms/25ms) and started ~15 minutes later after animals were moved into the scanner (Siemens 3T Trio with a Tx/Rx volume coil). The MRI parameters were: 150 volumes per scan, FOV = 96 mm × 96 mm, spatial resolution= 1.5×1.5×1.5mm3, 34 contiguous slices. Corresponding 3D T1 weighted images and field map images were also acquired. rsfMRI data were preprocessed firstly by applying field map for image distortion correction with FSL (http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FUGUE). Slice timing correction, rigid body registration, regressing out signal in white matter and cerebrospinal fluid (CSF) time series with a general linear model, temporal filtering with 0.009 Hz ~0.0237 Hz band-pass, spatial smooth by a Gaussian blur with 2.5-mm full width at half maximum was performed using a script from AFNI (http://afni.nimh.nih.gov) [5]. Anatomical regions of interest (ROI) corresponding to the whole posterior cingulate cortex (PCC), anterior cingulated cortex (ACC), and dorsal/media prefrontal cortex (DMPFC) were selected using AFNI and the monkey brain atlas [6] with T1-weighted images as reference. The averaged time courses of rsfMRI signal in PCC were used to perform seed-based correlation analysis. Z transformation was applied to the individual correlation maps to show normalized correlation maps. The averaged z values of connectivity between PCC and ACC or DMPFC were examined for statistical differences. All statistical analyses were performed in SPSS 21.0. P-values less than 0.05 were considered statistically significant.The correlation degree (z score) of PCC with either DMPFC or ACC was obviously decreased after 3 hours isoflurane administration (Fig. 1), but only the PCC-DMPFC connectivity decreased significantly (see Table 1). Mean arterial pressure (MAP) and heart rates were not showing significant changes during the 4-hour anesthesia although decreasing trends were seen during the 4-hour isoflurane administration (Fig. 2).

Vincent and colleagues [7] have demonstrated that DMN in human also exists in macaques by examining the macaque brain under isoflurane (0.8%-1.5%), and PCC has shown as a dominant region. Previous studies have demonstrated the deactivation of DMN is related with cognitive decline [3]. Our results indicate that PCC-ACC and PCC-DMPFC associations decreased significantly after 3 hour isoflurane exposure (Table 1). Probably it is because anesthetic results in breakdown of large-scale synchronization between brain regions [8, 9]; Also, it may be due to the fact that anesthetic causes a global loss of functional segregation/specialization, resulting in a decreased specificity. Therefore regions become more homogeneously connected to each other [9]. In addition, the decreased DMN activity is at least partly due to dysfunction of neuron and glia cell caused by long term isoflurane toxicity. Similar finding was also reported in another study [10]. Our finding of decrease in DMN of PCC is consistent with prior studies [11].

In conclusion, long-duration administration of isoflurane could cause decreased DMN activity, which may be associated with the cognitive decline seen in post anesthesia patients.