Streptozotocin Impaired Consolidation And Reinstatement Of Conditioned Fear Memory In Rats

Objectives:Streptozotocin (STZ) is a glucosamine derivative of nitrosourea.Intracerebroventricular (i.c.v.) injection of STZ has been described as an appropriateanimal model for sporadic Alzheimer’s disease (SAD). The animal model causes aprolonged impairment of brain glucose and energy metabolism, along with progressivedeterioration of memory (i.e., impaired hippocampus-dependent spatial learning andmemory). However, no reports are available on the SAD animal model accompanying withdeterioration of emotion learning and memory. In the current study, we provide fearconditioning paradigm to evaluate the effects of intracerebroventricular streptozotocinadministration on emotion learning and memory, and to seek to the possible mechanismsabout the effects.Methods:1. STZ (3mg/kg) was injected intracerebroventricularly when rats were anaesthetized.The same dose of STZ was repeated after48h of first dose. The control group also receivedi.c.v. injection of Citrate buffer solution as the same volume of STZ on day1and3.Peripheral blood glucose levels were measured on the first day before injection and thetenth day after injection.2. Morris water maze test was performed on day15from the1st STZ injection. Allrats were given four consecutive training days for place navigation test, a daily session of4trials per day. Different starting position was used on each trial. Escape latency to reach thehidden platform was recorded. During the spatial probe test, rats were placed into the tankat the opposite side of the platform position. The time spending in the target quadrant wasrecorded. 3. The test for a freezing response to contextual cues was performed on day15fromthe1st STZ injection. There were five experimental procedures, including habituation,contextual fear conditioning, extinction, reinstatement, and reinstatement test. On the firstday, rats were allowed to freely explore chamber A for10min. On the2nd day, each ratwas placed in the conditioning chamber A where they received five presentations of a footshocks unconditioned stimulus. Extinction training was performed for20min without anystimulus for4consecutive days from the third day. On the7th day, rats were given2footshocks for reinstatement. Rats were back to the conditioning chamber A for reinstatementtest on the8th day. The time of the freezing response to contextual was recorded.4. The test for a freezing response to tone cues was performed on day14from the1stSTZ injection. There were five experimental procedures, including habituation, cue fearconditioning, extinction, reinstatement, and reinstatement test. On the first day, rats wereallowed to freely explore chamber A and chamber B for10min, respectively. On the2ndday, rats were attained by pairing of five tones with co-terminated foot shocks for cue fearconditioning. From the third day, rats were subjected to12trials with a single tone forextinction for3consecutive days. On the6th day, rats were given2single foot shocks forreinstatement. On the7th day, rats were back to the conditioning chamber B forreinstatement test along with12tone trials. The time of the freezing response to tone cueswas recorded.5. Spontaneous locomotor activity was assessed on day14from the1st STZ injection.Rats were placed into a test chamber and allowed to explore it for30min. The distancetraveled the arena was recorded.6. Foot shocks sensitivity was tested after24h of the cue fear conditioning. Rats wereback to the conditioning chamber. The minimum electric current was recorded when ratsfirst notice, flinch and vocalize.7. Elevated plus maze test was performed on day14from the1st STZ injection. Thetime taken in open arm, the times entered the open and closed arm in the5minutes wererecorded. 8. After behavioral studies completed, the whole brain was quickly removed anddissected into hippocampus and amygdale for measuring biochemical estimations ofSynaptophysin (SYP), phosphorylated cyclic AMP response element binding protein(p-CREB), total cyclic AMP response element binding protein (CREB) and nucleartranscription factor kappa B (NF-κB) by western blotting.Results：1. In STZ treated group, there was no significant difference in peripheral bloodglucose levels as compared to control group.2. Escape latency of STZ treated group was significantly higher than that of controlgroup. The time spending in the target quadrant was significantly less than that of controlgroup.3. During the first day of extinction training period and the reinstatement test, percentfreezing of STZ treated group was significantly lower than that of control group. However,all rats exhibited comparable levels of behavioral freezing during the other periods in thecontextual fear conditioning paradigm.4. During the first day of extinction training period and the reinstatement test, percentfreezing of STZ treated group was significantly lower than that of control group. However,all rats exhibited comparable levels of behavioral freezing during the other periods in thecued fear conditioning paradigm.5. The distance traveled the arena had no significant difference between control andSTZ treated group.6. The minimum electric current had no significant difference between control andSTZ treated group when rats first notice, flinch and vocalize.7. Percentage of time spent in open arm and percentage of times entered open orclosed arm had no significant difference between control and STZ treated group.8. There were lower levels of SYP, p-CREB and NF-κB expression in hippocampusand amygdale in STZ treated rats. Conclusion：Intracerebroventricular injection of STZ could impair consolidation and reinstatementof conditioned fear memory, the possible mechanisms maybe relatively about thedecreased expression of SYP, p-CREB and NF-κB.