The Effect Of Stress And Acute Morphine Exposure On Synapic Plasticity Of Hippocampal CA1 Area And Spatial Memory In Rats Of Different Ages

Part I The Effect of Chronic Stress and Acute Morphine Exposure onSynaptic Plasticity of Hippocampal CA1 Area in Rats of Different Age inVivoObjectives: To study the effects of chronic stress and acutemorphine exposure on synaptic plasticity of hippocampal CA1 Area in4-or10-week-old rats. Methods: Fifty-one 4-week-old and 51 10-week-old male Wistarrats were randomly assigned to control group, chronic stress group,morphine group, chronic stress and morphine group, respectively. Eachgroup was then randomly assigned to high frequency stimulation (HFS)group and low frequency stimulation (LFS) group. Behavioral stress wasevoked by placing the rats on an elevated platform (EP, 10cm×10cm,high 1.6m) in the middle of a brightly lit room for 30min per time. Ratsreceiving chronic EP stress were placed on the EP twice per day at 9AMand 3PM for 7days (EP stress for 4-week-old rats was from 4th week to5th week;for 10-week-old rats from ninth week to 10th week).Experiments were carried out under pentobarbitone sodium (40-/60mg/kg, i.p) anesthesia. Electrode implantation sites were identified byusing stereotaxic coordinates. Recordings of field excitatory postsynapticpotential (fEPSP) were made from the CA1 stratum radiatum of thehippocampus in response to ipsilateral stimulation of the Schaffercollateral/commissural pathway similar to those described. The optimaldepth of the wire electrodes in the stratum radiatum of the CA1 area ofthe dorsal hippocampus was determined by using electrophysiologicalcriteria and was verified by post mortem examination. After 40 minutesstable baseline recording, HFS (200HZ) or LFS (1HZ) was given toinduce LTP or LTD, respectively. For the rats of chronic stress andmorphine group and morphine group, morphine (3mg/kg, i.p) was givenafter 40 minutes stable baseline recording,then HFS or LFS was given toinduce LTP or LTD after 90 minutes recording. Low frequencystimulation (LFS) consisted of 900 pulses at 1Hz. High frequencystimulation (HFS) consisted of 10 trains of 20 stimuli, inter-stimulusinterval 5 ms (200HZ), inter-train interval 2 s.Statistical comparisons were made using Student's t-test.Results: (1) The EPSP amplitude of LTP in 10-week-old rats withoutchronic EP stress was larger than that in 4-week-old rats (P<0.05). LTPwas impaired after chronic stress either in 10-week-old rats comparedwith control (P<0.05) or in 4-week-old rats compared with control(P<0.05). LTD was facilitated in 4-week-old stressed rats compared withcontrol (P<0.05), However low-frequency stimulation failed to induceLTD in 10-week-old stressed rats.(2) Acute morphine exposure (3mg/kg, i.p) facilitated the induction ofLTP in 10-week-old animals compared with control (P<0.05) andimpaired the induction of LTP in 4-week-old animals compared withcontrol (P<0.05);LTD was facilitated after acute morphine exposureeither in 10-week-old rats compared with control (P<0.05) or in4-week-old rats compared with control (P<0.05).(3) LTP was facilitated by acute morphine exposure (3mg/kg, i.p) in4-week-old stressed rats compared with control (P<0.05) and impaired in10-week-old stressed rats compared with control (P<0.05).Low-frequency stimulation (LFS) failed to induce LTD in 10-week-oldstressed rats but stable LTD was stilled induced and facilitated in4-week-old stressed rats after acute morphine exposure compared withcontrol (P<0.05).Conclusion: (1) chronic stress and morphine had different effects onLTP and LTD in 4-or10-week-old rats and it showed significantdifference between two ages. LTP and LTD were facilitated in chronicstress and morphine group of 4-week-old rats;LTP was impaired andLTD was completely inhibited in the same group of 10-week-old rats. Theresults also showed the difference between two ages.(2) The effect of acute morphine exposure on synaptic plasticity (LTPand LTD) after chronic stress was changed, either in 10-week-old rats orin 4-week-old rats.Part II Effects of Stress and Acute Morphine Exposure on SpatialMemory in Rats of Different AgeObjectives: To study the effects of acute and chronic stress andacute morphine exposure (2mg/kg) on spatial memory in 4-or10-week-old rats.Methods: 4-week-old Male Wistar rats (49) and 10-week-old MaleWistar rats (52) were randomly assigned to saline group (control group),chronic stress and saline group (chronic stress group), acute stress andsaline group (acute stress), chronic stress and morphine group, acutestress and morphine group and morphine group.Stress protocol and morphine injection: Behavioral stress wasevoked by placing the rats on an elevated platform (EP) in the middle of abrightly lit room for 30min per time. Rats receiving chronic EP stresswere placed on the EP twice per day at 9AM and 3PM,lasting 7days (EPstress for 4-week-old rats was from 4th week to 5th week;for10-week-old rats from ninth week to 10th week). Rats for chronic stressstill received EP stress one time before training and probe trial. Acutestress group received EP stress one time only before training and probetrial. Morphine was injected peritoneally (i.p) one time before probe trialat the dose of 2mg/kg, so did the same for saline injection.Training procedure: all the experiments were practiced in the Morriswater maze. Before training, a 180 s free swim trial was run in which theplatform was removed so that animals were adapted to temperature andenvironment of Morris water maze. During the training, a submergedPerspex platform (14cm×14cm) was placed 62.5cm away from the edgeof Morris water maze Which make animal to learn the location of theplatform that could be used to escape from swimming. The trainingsession consisted of eight trials (maximum trial duration=180s) with eightdifferent starting positions that were equally distributed around theperimeter of the maze. Total 16 trials were given as eight sessions with30min inter trial interval per day for consecutive two days.Probe trial: Thirty minutes after the end of the training session onday 2, a probe trial was given which consisted of a 180s free swim periodwithout the platform. Time spent in the Platform quadrant (targetquadrant) was calculated. It was given 30minute after the final trainingtrial. Swim path for all training and probe trial was monitored using anautomatic tracking system.Statistical comparison was made by using t test or one-way ANOVAfollowed by LSD (SPSS 10.0).Result(1) The effect of stress and acute morphine exposure on the spatialmemory retention in 4-week-old ratsRats of acute stress group, acute stress and morphine group andmorphine group spent significantly less time in the target quadrant thanthat of control group, respectively (P<0.05). Time in the target quadrantdid not differ in chronic stress group, chronic stress and morphine groupcompared with control (P>0.05).(2) The effect of stress and acute morphine exposure on the spatialmemory retention in 10-week-old ratsRats of chronic stress group and chronic stress and morphine groupspent significantly more time in the target quadrant than that of controlgroup (P<0.05);Rats of acute stress group spent significantly less time inthe target quadrant than that of control group (P<0.05). Time in the targetquadrant did not differ in acute stress and morphine group, morphinegroup compared with control group (P>0.05).(3) Comparison of the effect of stress and acute morphine exposure on thespatial memory retention between 4-week-old rats and 10-week-od rats.10-week-old rats in chronic stress group, chronic stress andmorphine group, morphine group, acute stress and morphine group spentsignificantly more time in the target quadrant than that of 4-week-old ratsin the same groups, respectively (P<0.05).Time in the target quadrant didnot differ in acute stress group and control group between 4-week-old ratsand 10-week-old rats (P>0.05).(4) Comparison of speed of groups in 4-or 10-week-old rats: Swimmingspeed did not differ in all groups of 4-or 10-week-old rats (P>0.05).Conclusion: (1) Acute EP stress induced impairment of memoryretention in 4-or10-week-old rats, respectively,but it did not differbetween 4-week-old rats and 10-week-old rats. Chronic EP stress inducedenhancement of memory retention in 10-week-old rats and had no effecton memory retrieval in 4-week-old rats. It showed difference betweentwo age groups.(2) Morphine at the dose of 2mg/kg induced impairment of memoryretention in 4-week-old rats and had no effect on memory retention in10-week-old rats. It showed difference between two age groups.(3) Acute EP stress and morphine induced impairment of memoryretention in 4-week-old rats and had no effect on memory in 10-week-oldrats. It showed difference between two age groups. Chronic stress andmorphine had no effect on memory retention in 4-week-old rats andinduced enhancement of memory retention in 10-week-old rats. It alsoshowed difference between two different ages.