Metabolic and behavioral deficits following a routine surgical procedure in rats

Abstract

To test the hypothesis that functional metabolic deficits observed following surgical brain injury are associated with changes in cognitive performance in rodents, we performed serial imaging studies in parallel with behavioral measures in control animals and in animals with surgical implants. Memory function was assessed using the novel object recognition (NOR) test, administered 3 days prior to and 3, 7, 14 and 56 days after surgery. At each time point, general locomotion was also measured. Metabolic imaging with ¹⁸F-fluorodeoxyglucose ([¹⁸F]FDG) occurred 28 and 58 days after surgery. Animals with surgical implants performed significantly worse on tests of object recognition, while general locomotion was unaffected by the implant. There was a significant decrease in glucose uptake after surgery in most of the hemisphere ipsilateral to the implant relative to the contralateral hemisphere. At both time points, the most significant metabolic deficits occurred in the primary motor cortex (− 25%; p < 0.001), sensory cortex (− 15%, p < 0.001) and frontal cortex (− 12%; p < 0.001). Ipsilateral areas further from the site of insertion became progressively worse, including the sensory cortex, dorsal striatum and thalamus. These data was supported by a voxel-based analysis of the PET data, which revealed again a unilateral decrease in [¹⁸F]FDG uptake that extended throughout the ipsilateral cortex and persisted for the duration of the 58-day study. Probe implantation in the striatum results in a widespread and long-lasting decline in cortical glucose metabolism together with a persistent, injury-related deficit in the performance of a cognitive (object recognition) task in rats.

Introduction

In human subjects as well as in animal models, cognitive deficits are an extremely common finding in the aftermath of surgical, traumatic and ischemic brain injuries (Chen et al., 1991, Courtiere et al., 2005, Engstad et al., 2003, Funkiewiez et al., 2004, Hattori et al., 2000, Hauss-Wegrzyniak et al., 2000, Hauss-Wegrzyniak et al., 2002, Helmstaedter et al., 2004). Thousands of clinical neurosurgical procedures ranging from insertion of biopsy needles, shunts, catheters and electrodes to large resections and ablations are performed every day. In experimental animals, surgical procedures such as cannulation, electrode placement and microdialysis probe implants are performed routinely in studies of brain organization and function. Studies in our laboratory and others have shown that following these routine surgical procedures, animals undergo dramatic short-term and long-term neurochemical and metabolic changes (Biegon et al., 2004, Polikov et al., 2005, Schiffer et al., 2006, Stensaas and Stensaas, 1976, Yoshino et al., 1991).

Unlike injury-induced depressions that eventually recover in a matter of days or weeks, our previous observations of a persistent decrease in glucose metabolic rate as many as 25 days after the implant points toward a more enduring metabolic depression with a surgical implant. At a cellular level, it has been shown that activated, proliferating microglia appear around the implant site as early as 1-day post-implantation (Fujita et al., 1998, Giordana et al., 1994, Szarowski et al., 2003). These activated microglia are thought to gradually diminish both excess fluid and cellular debris 6–8 days after the implant, although necrotic tissue has been observed up to 42 days later (Stensaas and Stensaas, 1976, Turner et al., 1999). At later time points, typical inflammatory cells or hemorrhaging has not been observed (Yuen and Agnew, 1995), while others have documented neuronal cell loss and macrophages up to 112 days after the implant (Biran et al., 2005, Stensaas and Stensaas, 1976, Szarowski et al., 2003, Turner et al., 1999). As a testament to the transitory nature of this mechanically induced wound healing response, electrode tracks could not be found in animals after several months when the electrode was inserted and quickly removed (Biran et al., 2005, Csicsvari et al., 2003, Rousche et al., 2001, Yuen and Agnew, 1995). We have shown a similar effect with FDG, since glucose metabolism returned to normal levels within 7 days if the microdialysis probe was inserted and immediately removed (Schiffer et al., 2006). Taken together, it appears that injury alone produces a more transient metabolic response while the implant appears to induce a more enduring brain tissue response.

Specifically, brain energy metabolism after injury fluctuates such that immediately after injury a hyperactivated state lasting only 30 min (in rats) to a few hours (in humans) is followed by a metabolic depression lasting from 5 to 10 or 30 days in rats or humans, respectively (Bergsneider et al., 2001, Hovda et al., 1995, Moore et al., 2000, Yoshino et al., 1991). We have recently used serial small animal PET imaging to show a profound, persistent metabolic decline following microdialysis cannula implant that began as early as 2.0 h after the surgery, was at its lowest point 48 h after the surgery and persisted for the duration of the experiment, 25 days (Schiffer et al., 2006). In a related series of studies, we have also used autoradiography to show that mice subjected to closed head injury undergo dynamic changes in glutamate NMDA receptor activation and availability (Biegon et al., 2004). These studies demonstrated a transient increase in NMDA receptor function within the first hour of injury followed by long-lasting decrease (over 7 days).

Since cognitive impairments are extremely common and often long-lasting after brain injuries in humans (Hoofien et al., 2001), we sought to examine the presence and time course of cognitive deficits following surgical brain injury in rodents. In animal models, long-lasting cognitive deficits are observed not only after severe brain injury (Chen et al., 1991) but also after moderate (Piot-Grosjean et al., 2001) and even mild concussions which do not involve appreciable neuronal cell loss (Zohar et al., 2003). More recent studies in animals are beginning to report significant inflammation and neurodegeneration linked to minor surgery such as electrode implantation (Polikov et al., 2005), although to our knowledge the cognitive effects of a microdialysis probe cannulation have not been documented.

Many cortical brain regions are pivotal to the formation of memories and brain plasticity (Baker and Kim, 2002, Broersen, 2000, Kossel et al., 1997) and intracranial surgery usually involves cortical regions even if the target is subcortical. For example, a surgical implant targeting the striatum passes through the motor and sensory cortices of the rat, and we have shown that these areas as well as distal cortical areas show a persistent metabolic decline long after the cannulation. It follows that if an intracranial implant targeting the striatum damages cortical regions, it may also impact memory function. The striatum has also been implicated in learning and memory, adding to the potential cognitive impact of striatal implants (Chang and Gold, 2003, Chang and Gold, 2004, Sanberg et al., 1978). We therefore sought to extend our previous findings of metabolic functional deficits by assessing working memory function and general locomotion in rats following a unilateral microdialysis cannulation targeting the striatum. While our previous study characterized metabolic deficits immediately following the injury (from 2 h to 25 days), here we have extended these observations by scanning animals with [¹⁸F]FDG at 28 and 58 days after the procedure. Thus, while the overarching goal of these experiments was to determine whether or not the probe induced a measurable change in cognition in these animals, [¹⁸F]FDG scans were performed to ensure that the hypometabolic state described in our previous report not only existed in these animals but persisted for the duration of the 56-day behavioral experiment. Our results show that rats with chronic implants have deficits in a working memory task as well as widespread decreases in glucose uptake in brain regions ipsilateral to the implanted probe.