Hélène Plamondon

Neuroprotection and functional recovery conferred by administration of kappa- and delta1-opioid agonists in a rat model of global ischemia

Studies that have evaluated the beneficial effect of pre-ischemic treatment of kappa-opioid receptor agonists have used short-term reperfusion intervals. We examined the long-term impact of the pre-ischemic peripheral injection of U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide), a selective kappa-opioid receptor agonist, on neuronal damage and behavioral deficits following global ischemia in rats. Four groups of ischemic rats were pretreated with various doses of U50,488H (i.p. 0, 5, 15, 30 mg/kg) 15 min prior to vessel occlusion. Two groups of sham-operated animals that received either saline or U50,488H (30 mg/kg) acted as controls. The injection of 30 mg/kg U50,488H led to a 65% increase in CA1 neuron survival 35 days post-ischemia. CA1 neuronal protection translated into significant improvement of ischemia-induced spatial memory deficits assessed in the 8-arm radial maze. However, there was no difference in activity in the open field. We also found that the pre-ischemic intracerebroventricular injection of 5 mug of the delta1-opioid receptor agonist DPDPE ([d-Pen(2,5)]-enkephalin) produced a 59% increase in CA1 neuron survival 7 days post-ischemia. Similar to U50,488H, DPDPE had no significant impact on locomotor activity. These findings support a role for kappa- and delta-opioid receptors in attenuation of ischemia-induced hippocampal damage and cognitive impairments.

Food restriction attenuates ischemia-induced spatial learning and memory deficits despite extensive CA1 ischemic injury

The purpose of the present study was to examine whether short-term food restriction (40% less food over a 3-month period) can attenuate ischemia-induced CA1 neuronal degeneration, and whether this attenuation translated into improved recovery of functional impairments following global ischemia. There was a significant loss of pyramidal CA1 neurons in ischemic compared to sham-operated rats but no difference between the ad lib and food-restricted ischemic animals. Although the diet did not influence neuronal damage in ischemic animals, the performance of food-restricted ischemic rats in spatial task such as the radial arm maze was significantly better than that of ad lib fed ischemic rats. Food-restricted ischemic rats made equivalent numbers of working memory errors as sham-operated animals and took the same time to complete a standard 8-arm radial arm maze task. They also displayed higher activity level in the open field compared to ad libitum fed ischemic rats, and spent considerably more time in the open arms of the elevated plus maze compared to the other groups, suggesting decreased anxiety in these ischemic rats. The relative sparing of spatial memory performance in food-restricted ischemic animals suggests that food restriction facilitates functional recovery.

Characterization of anxiety and habituation profile following global ischemia in rats

The purpose of the current study was to document the behavioral profile of ischemic rats in novel tasks including the elevated plus maze (EPM), the Vogel/conflict model of anxiety and novelty-induced feeding suppression paradigm as well as to further characterize using behavioral monitoring, the response of ischemic animals in existing paradigms such as the open field. Our findings revealed that ischemic animals spent significantly more time and made more entries in the open arm of the EPM as compared to sham animals, two behaviors indicative of decreased anxiety level. This anxiolytic effect appeared restricted to exploratory models of anxiety, as no differences in punished licking rate were observed between groups in the Vogel/conflict test. In the open field, behavioral monitoring revealed transient ischemia-induced hyperactivity, limited to the initial 15 min of a 30 min testing period. Increased activity in ischemic animals was primarily characterized by increased exploration and sniffing behavior with no significant alterations in rearing and grooming frequencies. Finally, using feeding behavior, our findings revealed a comparable rate of habituation to a novel environment in ischemic and sham rats. Taken together, these results suggest that ischemia-induced hyperactivity may involve a disinhibition to explore unfamiliar and/or mildly anxiogenic environments. However, the basis of such hyperactivity and the presence of habituation deficit following ischemia require further study and/or validation.

Chronic 17β-estradiol pretreatment and ischemia-induced hippocampal degeneration and memory impairments: A 6-month survival study

Exogenous administration of estrogen has been shown to significantly reduce ischemia-induced neuronal degeneration. However, the long-term impact of such treatment on neuronal protection and functional recovery remain largely unknown. The present study assessed the effects of a 15-day pretreatment with 17beta-estradiol on memory deficits and neuronal damage up to 6 months following a 10-min global ischemia in rats. Four groups of ovariectomized female rats [sham-operated and ischemic rats receiving a 15-day pretreatment of either the vehicle or 17beta-estradiol (100 microg/kg)] were tested. The 8-arm radial maze and object recognition tests served to evaluate the impact of 17beta-estradiol treatment on ischemia-induced spatial and recognition memory impairments, respectively. Testing in the radial maze was initiated at two distinct time intervals following reperfusion (7 and 120 days) to evaluate changes in memory functions over time. Our findings revealed long-lasting neuroprotective effects of 17beta-estradiol treatment on hippocampal CA1 pyramidal cells in ovariectomized ischemic rats (43.5% greater neuronal survival than observed in vehicle-treated ischemic animals). Importantly, this neuronal protection translated into significant improvements of recognition and spatial memory functions in estradiol-treated ischemic rats.

Time-dependent effects of global cerebral ischemia on anxiety, locomotion, and habituation in rats

Although changes in emotionality represent common features of post-ischemic recovery in humans, little is known about the effects of global cerebral ischemia on standard behavioral measures of emotionality in rodents. The present study investigated anxiety, locomotor activity, and habituation in test-naïve ischemic (subjected to 10 min global ischemia) and sham-operated rats tested 1, 5, 15, and 30 days post-reperfusion in the elevated plus-maze and the open-field. Although rats tested on day 1 post-reperfusion showed increased anxiety relative to sham-operated controls, they demonstrated decreased anxiety on day 5. Anxiety levels were normal on days 15 and 30 following ischemia. Similarly, time-dependent changes in locomotor activity were observed with ischemic rats showing increased activity level on days 1, 5, and 30 post-reperfusion. Surprisingly, locomotor activity was suppressed at day 15. Habituation deficits in the open-field were apparent only on day 1 despite the lack of CA1 neuronal degeneration at this time interval. These findings suggest that both the nature and extent of the effects of global ischemia on behavioral measures of emotionality, locomotion, and habituation in rats are time-dependent.

Repeated resveratrol administration confers lasting protection against neuronal damage but induces dose-related alterations of behavioral impairments after global ischemia.

Resveratrol, a naturally occurring polyphenol, has been shown to protect the heart and brain against ischemic injury. The current study investigated the effects of administration with either a 1 or 10-mg/kg dose of resveratrol on CA1 neuronal injury and behavioral/cognitive impairments after 10-min global ischemia in rats. The open-field, eight-arm radial maze and object recognition tests served to evaluate effects of resveratrol treatment on ischemia-induced locomotor activity, and spatial and recognition memory impairments, respectively. CA1 and CA3 neuronal injury was assessed upon completion of behavioral testing, 85 days postischemia. A separate series of groups served to assess neuronal injury at 7 days postischemia. Global ischemia (10 min) led to approximately 50% CA1 cell injury, which was prevented at both short (7 days) and long (85 days) postischemic intervals by resveratrol treatment. Importantly, despite comparable neuronal protection, the two resveratrol doses showed distinct behavioral effects. Thus, the 10-mg/kg resveratrol dose led to an enhanced locomotor activity in the open-field 4-days postischemia and an impaired spatial memory in the delayed nonmatching to sample and delayed matching to sample radial-maze tasks initiated on day 13 postischemia. These findings suggest independent actions of resveratrol on distinct physiological systems mediating cellular survival and functional recovery and dose-related actions of the polyphenol on behavioral and memory processes.

Food restriction induces long-lasting recovery of spatial memory deficits following global ischemia in delayed matching and non-matching-to-sample radial arm maze tasks

Food restriction has been shown to be beneficial for a number of brain processes. In the current study, we characterized the impact of food restriction on hippocampal damage 70 days following ischemia. We assessed memory and cognitive flexibility of ad libitum fed (AL) and food-restricted (FR) animals using complex delayed non-matching- and matching-to-sample tasks in the radial arm maze. Our findings demonstrate that food restriction led to significant improvement of ischemia-induced memory impairments. FR ischemic animals rapidly reached comparable performance as both AL and FR sham animals in delayed-non-matching (win-shift) and matching (win-stay) radial arm maze tasks. They also made considerably fewer microchoices in the retention trials than AL ischemic animals. In contrast, AL ischemic rats showed persistent spatial memory impairments in the same paradigms. Assessment of basal and stress-induced corticosterone (CORT) secretion revealed no significant differences in baseline levels in AL and FR rats prior to or following global ischemia. However, FR animals showed a more pronounced attenuation of CORT secretion 45 min following restraint. Both FR and AL ischemic rats had comparable cell loss within CA1 and CA3 subfields of Ammons horn (CA1 and CA3) at 70 days following reperfusion, although a trend toward increased CA3 cell survival was observed in FR ischemic rats. The functional sparing in the FR ischemic animals in the face of equivalent hippocampal cell loss suggests that food restriction somehow enhanced the efficacy of remaining hippocampal or extrahippocampal neurons following ischemia. In the current study, this phenomenon was not associated with diet- and or ischemia-related alterations of vesicular glutamate transporter 1 expression in various hippocampal regions although lower vesicular GABA transporter immunostaining was present in the CA1 stratum oriens and the CA3 stratum radiatum in FR sham and ischemic rats.

Time-dependent changes in CRH concentrations and release in discrete brain regions following global ischemia: effects of MK-801 pretreatment

The excitatory actions of corticotropin-releasing hormone (CRH) in the brain and the neuroprotective effects of CRH antagonists in models of ischemia suggest a role for this peptide in the cascade of events leading to cellular damage. The present study aimed to characterize endogenous activation of CRH in discrete brain regions following global ischemia. Time-dependent changes in CRH concentrations were assessed in 10 brain regions including hippocampal, parahippocampal, and hypothalamic regions as well as the amygdala and the frontal cortex at three post-ischemic intervals: 4, 24, and 72 h (Experiment 1). The impact of pretreatment with a neuroprotective dose of the NMDA antagonist (5R,10S)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; hydrogen maleate) on 24-h ischemia-induced CRH concentrations in the 10 brain regions was also determined (Experiment 2). In vivo microdialysis was used to assess dynamic fluctuations in CRH release at the dorsal hippocampus (CA1 pyramidal layer) and central nucleus of the amygdala (CeA; Experiment 3). Our findings revealed a rapid elevation of CRH concentrations at the piriform cortex (Pir) and hypothalamic nuclei following global ischemia. This was followed by decreased CRH concentrations at the amygdala, the frontal cortex (FC), the CA3, and the hypothalamus 24-h post-ischemia. MK-801 reversed the decreases in the hypothalamic nuclei but not in the other brain regions. Seventy-two hours post-ischemia, CRH levels returned to control values in all regions except the dentate gyrus (DG) where elevated CRH levels were observed. In vivo, a significant increase in CRH release in response to global ischemia was found at the CeA with no alterations at the CA1. These findings support brain region-specific ischemia-induced CRH alterations and suggest that CRH actions to mediate neuronal damage at the hippocampal CA1 layer may be indirect.

Impaired conditioned emotional response and object recognition are concomitant to neuronal damage in the amygdala and perirhinal cortex in middle-aged ischemic rats

The current study characterizes fear conditioning responses following global ischemia and evaluates neuronal damage affecting discrete extra-hippocampal areas susceptible to contribute to post ischemic emotional and memory impairments. Conditioned emotional response, Barnes Maze and object recognition tests were used to assess emotional, spatial and recognition memory, respectively. Behavioural testing was initiated in middle-aged animals (10-12 month old) 1 week following sham (n=16) or 4VO occlusion (n=18). Post-mortem cellular assessment was performed in the hippocampal CA1 layer, the perirhinal cortex and basolateral amygdala. Middle-aged ischemic animals showed impaired spatial memory in the initial three testing days in the Barnes Maze and deficit in recognition memory. Of interest, ischemic rats demonstrated a significant reduction of freezing and increased locomotion during the contextual fear testing period, suggesting reduced fear in these animals. Assessment of neuronal density 40 days following global ischemia revealed that CA1 neuronal injury was accompanied by 20-25% neuronal loss in the basolateral nucleus of the amygdala and perirhinal cortex in middle-aged ischemic compared to sham-operated animals. This study represents the first demonstration of altered conditioned fear responses following ischemia. Our findings also indicate a vulnerability of extra-hippocampal neurons to ischemic injury, possibly contributing to discrete emotional and/or memory impairments post ischemia.

Dietary PUFA supplements reduce memory deficits but not CA1 ischemic injury in rats

The purpose of the present study was to examine the impact of nutritional supplementation with essential omega 3 and 6 fatty acids on CA1 neuronal death and recovery of functional impairments following global ischemia. Groups of Wistar male rats were randomly assigned to four experimental conditions determined by the consumed diet and surgical condition. Rats either received a standard diet (SD; Purina 5012) or a 15% PUFA supplemented diet (FO+CO) prepared by adding 11.5% (w/w) fish oil from menhaden fish and 3.5% corn oil to standard rat chow. Diet conditions were initiated in 30 day old rats and maintained for an 18-week period (pre- and post-surgery). Sham or 8-min global ischemic surgeries occurred during the 13th feeding week and behavioral testing took place following reperfusion for an additional 4 weeks, after which all rats were euthanized. Our findings revealed significant loss of pyramidal CA1 neurons 31 days post ischemia in ischemic as compared to sham-operated rats but no difference between ischemic animals fed the SD or PUFA supplemented diets. In the radial arm maze, SD ischemic rats took longer time to complete the task and made significantly more working memory errors than PUFA ischemic and sham-operated animals. Independent of the diet, ischemic animals appeared less anxious in the elevated plus maze, spending considerably more time in the open arms as compared to sham-operated rats. Taken together, these results suggest that fish oil supplemented diet exerts beneficial effect on ischemia-induced spatial memory deficits despite protective effects on CA1 hippocampal neurons.