Subjects
Sprague-Dawley rats (male, 200–250 g, 8–10 weeks old) for T-Maze task were purchased from SLACCAS (Shanghai, China). Two Sprague-Dawley rats (male, 300–350 g, 12 weeks old) for western blot were obtained from Jiangxi Province Key Laboratory of Laboratory Animal, Nanchang University. Rats were housed in plastic cages (2–3 rats per cage) at temperature controlled (24 ± 1 ºC) vivarium room on a 12/12 light/dark cycle. Food and water were available ad libitum before the behavior training period. All experimental procedures were approved by the Ethical Committee of Rat Experiments at Nanchang University (Nanchang, Jiangxi) and Fudan University (Shanghai, China).
Two adult female Rhesus monkeys (Macaca mulatta, 4.0 kg (#1) and 6.0 kg (#2), purchased from the Kunming Primate Laboratory Animal Center, Chinese Academy of Sciences, Kunming, Yunnan, China; License number: SCXK (Dian) 2008–0001 and SYXK (Dian) 2008–0001) were used in the present behavioral study. The animals were neighbor-housed individually in an established animal room with a 12 h/12 h light/dark cycle (light from 7:00 am to 7:00 pm) and a temperature-controlled (21 ± 1 ºC) environment. The animals were given free access to water and were fed with a diet of monkey chow (Kunming Gaoyun Biological Technology Co. Ltd., Kunming, Yunnan, China; License number: SCXK (Dian) 2009–0003) immediately after behavioral testing. The amounts of monkey chow were designed to keep monkey weights at 90% of the weights attained when food was freely available. Fresh skinless peanuts and raisins were used as food rewards during behavioral testing thus minimizing the need for dietary regulation. Animals were assigned to a single experimenter who trained them extensively before behavioral testing. The experimenter testing the animals was blind to treatment conditions. The experimental protocols were approved by the Laboratory Animal Supervision Committee of Yunnan, China.
All procedures were performed in accordance with the US National Institute of Health Guidelines for Use and Care of Laboratory Animals and the Use and Care of Laboratory Animals Guidelines established by China government.
Monkey brain tissues used for western blot were obtained from one male Rhesus monkey (Macaca mulatta, 7 years old) that provided by Guangdong blue-island biological technology Co. Ltd. The animal was sacrificed for other experimental purpose without affecting CNS.
Synaptosomes isolation and Western blotting
Animals were cardiac perfused with saline under anesthetization with pentobarbital sodium (40 mg/kg, i.p.). Brain tissues were obtained from rat mPFC or monkey principal sulcus, hippocampus (HIP) and visual cortex (VC) of both species, and then were used immediately or frozen under − 80°C. 50 mg tissue per sample was homogenized in a weight: volume ratio of 80:1000 using cold lysis buffer (1:50 PI and 1:100 PMSF). Homogenate was then centrifuged at 1000×g for 10 min at 4°C to separate nuclei (P1). The supernatant (S1) was then centrifuged at 10000×g for 15 min to separate the crude synaptosomes (P2) and supernatant S2. P2 was then lysed with TEVP buffer and spun at 25000×g for 20 min to get purified synaptosomes (LP1)[10].
Western blot was done with standard protocol. Proteins were separated by electrophoresis on SDS-PAGE with 200V voltage for 90 min. Proteins were then translocated to membrane with 300 mA current for 2 hr, and then incubated with blocking media (TBST containing 5% nonfat milk) for 2 hr at room temperature. The membranes were then incubated with primary NR2B (1:1000, #AB1557P, Merckmillipore), NR2A (1:1000, #AB1555P, Merckmillipore), PSD95 (1:1500) antibodies overnight at 4°C. After wash, HRP labeled secondary antibody (1:1500) was used to incubate for 2 hr at 4°C. ECL chemiluminescence substrate (Pierce) was used to detect HRP activity and developed onto x-ray films (Kodak).
Behavioral tasks
1. Delayed alteration task for rats
The T-maze paradigm was described previously[11]. In briefly, rats were subject to restrict diet to maintain at approximately 85% of normal body weight. After habituation to the T-maze until they voluntarily ate the reward food at the end of arms, the training was initiated. As shown in Fig. 3A, the animal was placed at the end of the start arm. The trial was initiated by removing the sliding door to allow the animal to explore the maze. In trial 0, both arms were baited, the animal can get reward by visiting either of them. The animal was then forced back to the start point without allowing to enter the other arm. After a certain interval (5 s delay), the formal trial started, and the animal was allowed to explore the maze again. The animal was rewarded by entering into the arm opposite to the previous trial. Error choice was followed by error-correction trials with the same inter trial interval (5 s) until the animal entered into the previous baited arm. Each session contained 10 formal trials. After the animals reached the criteria with 80% or higher correct response rate in two consecutive sessions, they were ready for the drug infusion test.
2. Delayed response task for monkeys
Behavioral testing was performed in a Wisconsin General Test Apparatus situated in a quiet room. Animals were tested at the same time of day immediately prior to feeding. Firstly, the animals were trained on the two-choice spatial delayed response task as previously described (Cai et al., 1993;Ou et al., 2001). The test board contained two food-wells (left and right) spaced 15 cm apart. An opaque screen could be lowered to separate the animal from the test board. Each trial of the delayed response task began with the experimenter putting the food reward into one of the two food-wells under the gaze of the animal (cue phase). Then, the food-wells were covered with identical plastic plaques (white squares) and the opaque screen was lowered between the animal and the test board for a specified delay (delay phase). At the end of this delay, the screen was raised, and the animal was allowed to choose one of the wells (response phase). There was an interval of 20 s between trials. Rewards were pseudo-randomly distributed between left and right over the 30 trials of a daily test session.
During the initial training phase, delays were held constantly during a daily session and were gradually increased from ‘‘0’’ s according to a stepwise procedure over the 1020 trials. After 1020 trials (34 sessions), the animals were ready for drug infusion. To observe the effect of drugs on memory capacity, the animals were trained on a variable delayed response task in which three different delays (delay a, delay b and delay c) were randomly distributed over the 30 trials. Delays were adjusted until the animal reached stable performance level of 27 trials correct out of 30 trials (90% correct) to make enough space to show the effects of Ro25-6981 or muscimol. Finally, the range of delays was ‘‘0’’ s (delay a), 20 s (delay b) and 40 s (delay c) for both monkeys. The ‘‘0’’ s delay consisted of lowering the screen and immediately raising it again.
3. Pattern discrimination task for monkeys
Once they had grasped the delayed response task, the animals were trained to learn another task, pattern discrimination task in the same apparatus. In the pattern discrimination task, the two food-wells were always covered by two opaque plaques, with an equilateral triangle or a crisscross pattern on the surface. Each trial began with the experimenter putting the food reward into one of the two food-wells and covering them with plaques (animals’ vision was blocked by the lowed screen). Then the screen was raised, and the animal was allowed to choose between the two patterns. The animals could get reward only by choosing the equilateral triangle pattern. And 20 s later (interval), a new trial began. There were 10 trials in a daily test session, and the trials (the two patterns) were quasi-randomly distributed between left and right. Once performance was demonstrated to be stable at 90% (at last 1 wrong trial) for more than 2 days, drug treatment was initiated.
4. Cannula implantation and drug infusion
4.1 Cannula implantation:
The animals were implanted with cannulae before drug infusion and behavioral test. For rats, they were anesthetized with pentobarbital sodium (40 mg/kg, i.p.). stainless steel guide cannulae (23 G) were bilaterally implanted into the mPFC (Bregma AP: 3.5 mm, ML: ±0.75 mm, DV: 1.5 mm from skull surface). The cannulae were then affixed in place with dental cement secured with sterile stainless-steel screws. Sterile stylets (24 gauge) were inserted into each guide cannula, at a point equal to the intracranial tip of the cannula, to prevent occlusion. After one-week recovery from surgery, the animals were re-trained on the T-maze task to insure the performance level of 80%.
For primates, they were pre-anesthetized with hydrochloric ketamine (5 mg/kg, intramuscularly). Then they were deeply anesthetized with sodium pentobarbital (35 mg/kg, intraperitoneally) after pretreatment with sulfate atropine (0.2 mg/kg, intramuscularly). Chronic guide cannulae were implanted using aseptic methods as following. An incision was made along the midline of the scalp, and 23 stainless-steel guide cannulae (20 gauge, 5 cannulae in a row, total 4 rows, see Fig. 1) were implanted into the dorsolateral PFC. The stereotaxic coordinates were derived from a monkey brain atlas (Paxinos, et al., 2008). The coordinates were ranged as: AP, + 25mm ~ + 33 mm from the interaural line; ML, ± 9 mm ~ ± 14 mm from the midline. The cannulae were lowered down to touching the dura without penetration and then affixed to the skull using dental cement secured with sterile stainless-steel screws. Sterile stylets (24 gauge) were inserted into each guide cannula, at a point equal to the intracranial tip of the cannula, to prevent occlusion. All the cannulae were enclosed in a plastic chamber (fixed to the skull) to avoid grasp and infection. Great care was taken to minimize pain and post-operative infection. Surgical incisions were smeared with analgetic antiphlogistine, and the animals were treated with penicillin on seven consecutive post-operative days. Animals were given at least 7 days to recover from surgery before behavioral testing.
4.2 Drug infusion
Animals were habituated to the infusion procedure by mock treatment. For the rats, the animal was gentle restricted with hand, the stylets were removed and replaced with infusion needles (24 gauge). The needle tips were inserted beyond the guide cannulae for 2.7 mm, i.e. 4.2 mm below the skull surface. 1 µl PBS or NR2B antagonist Ro25-6981 (0.5 mg/ml) was infused at 0.1 µl/min. After infusion, the needles were kept in place for additional 3 min. Then the needles were removed, and the stylets were inserted back, and the animals were returned to home cages for 15 min before the behavior test started.
For the primates, animals were seated in a restraint chair while the stylets were removed and replaced with sterile infusion needles (24 gauge with sharp tip for dura penetration) that the tip open point extended 1mm below the guide cannulae. Animals received bilateral intracranial infusions of either 0 (saline) or 7.5 µg muscimol (Sigma Chemical, USA) or 15µg Ro25-6981 (Tocris, UK) in 3 µl saline for each cannula. Infusion was driven by a microsyringe pump (Bioanalytical System Inc., West Lafayette, IN, USA) using 5 µl microsyringes at a speed of 1 µl/min. Needles remained in place for 1 min after completion of the infusion, then the stylets were immediately reinserted. Behavioral testing was performed 60 min after the infusion.
Histology
After behavioral testing, the positions of cannulae were examined. The animals were sacrificed by overdoses of sodium pentobarbital and a stainless-steel electrode (30 gauge) was put into the same position as the infusion needle to deposit ferric ion in the infusion point with a anodal current (6V, 10 s). Cadavers were treated by transcardial infusion of 0.9% (w/v) saline followed by formaldehyde (4%, w/v) solution (containing 1% (w/v) potassium ferrocyanide for the Prussian blue reaction with deposited ferric ion). The brain surface cannulae traces were photo recorded for position re-construction. Brains were stored in 4% (w/v) formaldehyde solution for several days and were then sectioned for histological verification of cannulae positions (centers of the sites stained by Prussian blue reaction).
Single unit recordings in dlPFC
Oculomotor delayed response (ODR) task
Studies were performed on two adult male rhesus monkeys (Macaca mulatta), cared for under the guidelines of the National Institutes of Health and the Yale IACUC. The monkeys were trained in the visuospatial ODR task that was described previously (Wang et al., 2013). The subject started a trial by fixating at the central spot and maintaining fixation for 0.5 seconds (fixation period), whereupon a cue was illuminated for a period of 0.5 seconds (cue period) at one of eight peripheral targets located at an eccentricity of 13°with respect to the fixation spot. After the cue was extinguished, a 2.5-second delay period followed. The subject was required to maintain central fixation throughout both the cue presentation and the delay period. At the end of the delay, the fixation spot was extinguished which instructed the monkey to make a memory guided saccade to the location where the cue had been shown prior to the delay period. A trial was considered successful if the subject’s response was completed within 0.5 seconds of the offset of the fixation spot and was within 2°around the correct cue location. The subject was rewarded with fruit juice immediately after every successful response.
Pharmacology, physiology and data acquisition
This study used iontophoresis to apply NR2B antagonist TCN237 (Tocris) near PFC neurons. Iontophoretic electrodes were constructed with a 20-µm-pitch carbon fiber (ELSI, San Diego, CA) inserted in the central barrel of a seven-barrel non-filamented capillary glass (Friedrich and Dimmock, Millville, NJ). A Neurophore BH2 iontophoretic system (Medical Systems Corp., Greenvale, NY) was used to control of the delivery of the drugs. The drug was ejected at currents that varied from 15–25 nA. Extra-cellular voltage was amplified using a custom low-noise preamplifier (SKYLAB) and band-pass filtered (180Hz-6Khz, 20dB gain, 4-pole Butterworth; Kron-Hite, Avon, MA). Signals were digitized (15 kHz, micro 1401, Cambridge Electronics Design, Cambridge, UK) and acquired using the Spike2 software (CED, Cambridge, UK). Neural activity was analyzed using waveform sorting by a template-matching algorithm, which made it possible to isolate more than one unit at the same recording site. Post-stimulus time histograms (PSTHs) and rastergrams were constructed online to determine the relationship of unit activity to the task. Unit activity was measured in spikes per second. If the rastergrams displayed task-related activity, the units were recorded further and pharmacological testing was performed.
Data were first collected from the cell under a control condition in which at least eight trials at each of 8 cue locations were obtained. Upon establishing the stability of the cells’ activity, this control condition was followed by iontophoretic application of drug(s). Dose-dependent effects of the drug were tested in two or more consecutive conditions. Drugs were continuously applied at a relevant current throughout a given condition. Each condition had ~ 8 (6–12) trials at each location for statistical analyses of effects. The cue epoch was used for Cue cells, the delay epoch for Delay cells, and the response epoch for Response cells.
Data analysis
Western blot band intensity for NR2B and NR2A were first normalized with each PSD95 band. For each batch of experiment, the NR2B/2A ratio for HIP was set as 1, and the ratios for PFC and VC were then normalized with the HIP value.
Delayed performance data in drug treated sessions were first separated as effective and noneffective sessions with 85% criteria, i.e., more than 25 hits in a 30-trials session was labeled as noneffective session. The effective data were then evaluated against the saline sessions using a two-way analysis of variance with repeated measures. The delay and drug treatment were within-subject factors. If a significant effect was noted, data were refined with the post hoc Tukey’s test by paired t-test (Tdep) or one-way analysis of variance.