Attention and memory in aged rats: Impact of lifelong environmental enrichment.

Aged rodents exhibit memory and attention dysfunctions. Environmental enrichment (EE) attenuates memory impairments. Whether it may reduce attention deficits is not known. At the age of 1 month, Long-Evans female rats were placed in standard or EE conditions and tested after 3 (young), 12 (middle-aged) or 24 (aged) months of differential housing. Spatial reference memory was assessed in a water-maze task. Attention performance was evaluated in the five-choice serial reaction time (5-CSRT) task. EE improved spatial memory at all ages, but did not ameliorate 5-CSRT performance in young and middle-aged rats; it prevented, however, the degradation of attention performances detected in aged rats. The number of ChAT (+30 to +64%)- and p75(NTR)-positive (+35 to +44%) neurons was higher in the basal forebrain of aged enriched vs. standard rats, suggesting their EE-mediated protection. The weaker deficit of attention found in aged EE rats might be linked to a better survival in the very long term of neurons in the basalo-cortical system.

Hippocampal-dependent spatial memory functions might be lateralized in rats: An approach combining gene expression profiling and reversible inactivation.

The hippocampus is involved in spatial memory processes, as established in a variety of species such as birds and mammals including humans. In humans, some hippocampal-dependent memory functions may be lateralized, the right hippocampus being predominantly involved in spatial navigation. In rodents, the question of possible lateralization remains open. Therefore, we first microdissected the CA1 subregion of the left and right dorsal hippocampi for analysis of mRNA expression using microarrays in rats having learnt a reference memory task in the Morris water-maze. Relative to untrained controls, 623 genes were differentially expressed in the right hippocampus, against only 74 in the left hippocampus, in the rats that had learnt the hidden platform location. Thus, in the right hippocampus, 299 genes were induced, 324 were repressed, and about half of them participate in signaling and transport, metabolism, and nervous system functions. In addition, most differentially expressed genes associated with spatial learning have been previously related to synaptic plasticity and memory. We then subjected rats to unilateral (left or right) or bilateral reversible functional inactivations in the dorsal hippocampus; lidocaine was infused either before each acquisition session or before retrieval of a reference spatial memory in the Morris water maze. We found that after drug-free acquisition, right or bilateral lidocaine inactivation (vs. left, or bilateral phosphate buffered saline (PBS) infusions) of the dorsal hippocampus just before a delayed (24 h) probe trial impaired performance. Conversely, left or bilateral hippocampus inactivation (vs. right, or bilateral PBS infusions) before each acquisition session weakened performance during a delayed, drug-free probe trial. Our data confirm a functional association between transcriptional activity within the dorsal hippocampus and spatial memory in the rat. Further, they suggest that there could be a leftward bias of hippocampal functions in engram formation or information transfer, and a rightward bias in spatial memory storage/retrieval processes.

The behavioral effects of enriched housing are not altered by serotonin depletion but enrichment alters hippocampal neurochemistry.

To assess a possible role for serotonin in the mediation of the behavioral changes induced by enriched housing conditions (EC), adult female Long-Evans rats sustaining a serotonin depletion (150 microg of 5,7-dihydroxytryptamine, icv) and sham-operated rats were housed postoperatively for 30 days in enriched (12 rats/large cage containing various objects) or standard housing conditions (2 rats/standard laboratory cage). Thereafter, anxiety responses (elevated plus-maze), locomotor activity (in the home-cage), sensori-motor capabilities (beam-walking task), and spatial memory (eight-arm radial maze) were assessed. Monoamine levels were subsequently measured in the frontoparietal cortex and the hippocampus. Overall, EC reduced anxiety-related responses, enhanced sensori-motor performance and improved the memory span in the initial stage of the spatial memory task. Despite a substantial reduction of serotonergic markers in the hippocampus (82%) and the cortex (74%), these positive effects of EC were not altered by the lesion. EC reduced the serotonin levels in the ventral hippocampus (particularly in unlesioned rats: -23%), increased serotonin turnover in the entire hippocampus (particularly in lesioned rats: +36%) and augmented the norepinephrine levels in the dorsal hippocampus (+68% in unlesioned and +49% in lesioned rats); no such alterations were found in the frontoparietal cortex. Our data suggest that an intact serotonergic system is not a prerequisite for the induction of positive behavioral effects by EC. The neurochemical changes found in the hippocampus of EC rats, however, show that the monoaminergic innervation of the hippocampus is a target of EC.

Immunotoxic cholinergic lesions in the basal forebrain reverse the effects of entorhinal cortex lesions on conditioned odor aversion in the rat.

Conditioned odor aversion (COA) corresponds to the avoidance of an odorized-tasteless solution (conditioned stimulus, CS) previously paired with toxicosis. COA occurs only when the interstimulus interval (ISI) is kept short, suggesting that the memory trace of the odor is subject to rapid decay. Previous experiments have shown that the entorhinal cortex (EC) is involved in the acquisition of COA, since lesion of the EC rendered COA tolerant to long ISI. Because EC lesions induce a septo-hippocampal cholinergic sprouting, the present experiment investigated whether COA tolerance to long ISI may be linked to this sprouting reaction. In a first experiment, male Long-Evans rats subjected to bilateral excitotoxic EC lesions combined to intracerebroventricular infusions of the selective cholinergic immunotoxin 192 IgG-saporin were exposed to odor-toxicosis pairing using a long ISI (120 min). Results showed that EC-lesioned rats displayed COA with the long ISI but not the control groups. In rats with EC combined to 192 IgG-saporin lesions, histological analysis demonstrated no evidence for cholinergic septo-hippocampal sprouting. In a second experiment, animals with 192-IgG saporin lesion showed a marked COA with a short ISI (5 min). These results suggest that the COA with the long ISI found in rats with EC lesions might involve a functional activity related to the EC lesion-induced hippocampal cholinergic sprouting. As the injection of 192 IgG-saporin alone did not affect COA with a short ISI, our data also point to a possible role of hippocampal cholinergic neurons in the modulation of memory processes underlying COA.

Combined damage to entorhinal cortex and cholinergic basal forebrain neurons, two early neurodegenerative features accompanying Alzheimer's disease: effects on locomotor activity and memory functions in rats.

In Alzheimer's disease (AD), cognitive decline is linked to cholinergic dysfunctions in the basal forebrain (BF), although the earliest neuronal damage is described in the entorhinal cortex (EC). In rats, selective cholinergic BF lesions or fiber-sparing EC lesions may induce memory deficits, but most often of weak magnitude. This study investigated, in adult rats, the effects on activity and memory of both lesions, alone or in combination, using 192 IgG-saporin (OX7-saporin as a control) and L-N-methyl-D-aspartate to destroy BF and EC neurons, respectively. Rats were tested for locomotor activity in their home cage and for working- and/or reference-memory in various tasks (water maze, Hebb-Williams maze, radial maze). Only rats with combined lesions showed diurnal and nocturnal hyperactivity. EC lesions impaired working memory and induced anterograde memory deficits in almost all tasks. Lesions of BF cholinergic neurons induced more limited deficits: reference memory was impaired in the probe trial of the water-maze task and in the radial maze. When both lesions were combined, performance never improved in the water maze and the number of errors in the Hebb-Williams and the radial mazes was always larger than in any other group. These results (i) indicate synergistic implications of BF and EC in memory function, (ii) suggest that combined BF cholinergic and fiber-sparing EC lesions may model aspects of anterograde memory deficits and restlessness as seen in AD, (iii) challenge the cholinergic hypothesis of cognitive dysfunctions in AD, and (iv) contribute to open theoretical views on AD-related memory dysfunctions going beyond the latter hypothesis.

Whole-body exposure to 2.45 GHz electromagnetic fields does not alter anxiety responses in rats: a plus-maze study including test validation.

In a first phase of this investigation, a validation of our elevated plus-maze apparatus was performed in male Sprague-Dawley rats by testing anxiety response at various ambient light intensities (200, 30, 10 and 2.5 lux), as well as the effects of diazepam treatment (0.5 and 1.0 mg/kg, i.p. at 30 lux). Anxiety responses were found to decrease with decreasing light intensity and to be attenuated by diazepam treatment. Subsequently, a separate set of rats was exposed to 2.45 GHz EMFs (2 micros pulse width, 500 pulses per second, whole-body and time averaged of SAR 0.6 W/kg +/-2 dB, brain-averaged SAR of 0.9 W/kg +/-3 dB) for 45 min to assess whether EMF exposure altered anxiety responses in the same apparatus. As we made no a priori hypothesis on whether the effects would be anxiogenic or anxiolytic, part of the rats were tested under an ambient light intensity of 2.5 lux, the other one being tested at 30 lux. The low intensity level set the behavioural baseline for the detection of anxiogenic effects, while the higher one corresponded to the detection of anxiolytic effects. Sham-exposed and naive rats were used as controls. Whatever light intensity was used, EMF exposure failed to induce any significant effect on anxiety responses in the plus maze. The present experiment demonstrates that exposure to EMFs, which was previously found to increase the number of benzodiazepine receptors in the rat cortex [Lai H, Carino MA, Horita A, Guy AW. Single vs. repeated microwave exposure: effects on benzodiazepine receptors in the brain of the rat. Bioelectromagnetics 1992;13(1):57-66], does not alter anxiety responses assessed in the elevated plus maze.

No facilitation of amphetamine- or cocaine-induced hyperactivity in adult rats after various 192 IgG-saporin lesions in the basal forebrain.

Lesions of basal forebrain cholinergic neurons by intracerebroventricular (i.c.v.) injections of 192 IgG-saporin increased the locomotor response to 0.5 and 1.5 mg/kg of D-amphetamine in adult rats [A. Mattsson, S.O. Ogren, L. Olson, Facilitation of dopamine_mediated locomotor activity in adult rats following cholinergic denervation, Exp Neurol. 174 (2002) 96-108.]. In the present study, adult male rats were subjected to bilateral injections of 192 IgG-saporin either into the septum (Sp), the nucleus basalis magnocellularis (Nbm), both structures (SpNbm) or i.c.v. Locomotor activity was assessed in the home cage 23 days after surgery, and, subsequently, thrice after an intraperitoneal injection of D-amphetamine (1 mg/kg) and twice after an injection of cocaine (15 mg/kg). Analysis of AChE-stained material showed that Sp lesions induced preferentially hippocampal denervation, Nbm lesions induced preferentially cortical denervation, while both SpNbm and i.c.v. lesions deprived the hippocampus and the cortex of almost all AChE-positive reaction products. The spontaneous and drug-induced locomotor activity of all lesioned rats did not differ significantly from that of control rats, except in rats subjected to i.c.v. injections, in which the locomotor response was significantly increased after the second administration of cocaine. In addition, in Nbm and SpNbm rats, the locomotor reaction to cocaine was weaker right after the second injection. The present results do not confirm the report by Mattsson et al. on the potentiation of amphetamine-induced locomotion by i.c.v. injections of 192 IgG-saporin, but suggest that cocaine-induced locomotion can be increased by such lesions and, to some respect, attenuated by cholinergic damage in the Nbm.

Whole-body exposure to 2.45 GHz electromagnetic fields does not alter radial-maze performance in rats.

Mobile communication is based on utilization of electromagnetic fields (EMFs) in the frequency range of 0.3-300 GHz. Human and animal studies suggest that EMFs, which are in the 0.1 MHz-300 GHz range, might interfere with cognitive processes. In 1994, a report by Lai et al. [Bioelectromagnetics 15 (1994) 95-104] showed that whole-body exposure of rats to pulsed 2.45 GHz microwaves (2 micros pulse width, 500 pps, and specific absorption rate [SAR] 0.6 W/kg) for 45 min resulted in altered spatial working memory assessed in a 12-arm radial-maze task. Surprisingly, there has been only one attempt to replicate this experiment so far [Bioelectromagnetics 25 (2004) 49-57]; confirmation of the Lai et al. experiment failed. In the present study, rats were tested in a 12-arm radial-maze subsequently to a daily exposure to 2.45 GHz microwaves (2 micros pulse width, 500 pps, and SAR 0.6 W/kg) for 45 min. The performance of exposed rats was comparable to that found in sham-exposed or in naive rats (no contact with the exposure system). Regarding the methodological details provided by Lai et al. on their testing protocol, our results might suggest that the microwave-induced behavioral alterations measured by these authors might have had more to do with factors liable to performance bias than with spatial working memory per se.

Recovery from brain injury in animals: relative efficacy of environmental enrichment, physical exercise or formal training (1990-2002).

In the 1960s, it was shown for the first time that enriched housing enhances functional recovery after brain damage. During the 1970s and 1980s, many findings similar to this initial one have been reported, enlarging greatly its generality. Over the last 13 years, many different kinds of brain damage were modelled in animals or even directly studied in humans. Overall, these recent studies corroborated earlier findings, although occasional exceptions were reported. Other critical data, obtained mainly in intact animals, showed that enriched housing increases neurogenesis in the adult hippocampus. Recent evidence that this neurogenesis is involved in hippocampal-dependent learning supports the original interpretation of the enrichment effects as being the result of an accumulation of informal learning experiences (e.g., [. Heredity, environment, brain biochemistry, and learning. In: Current Trends in Psychological Theory. University of Pittsburgh Press, Pittsburgh, pp. 87-110;. Brain changes in response to experience. Sci. Am. 226, 22-29]). Other components of enriched environment, such as physical exercise, may have additive effects with those of training. The comparison of the relative effectiveness of enriched experience, of physical exercise and of training on structural and/or functional assessments of recovery, shows that training/learning is generally more effective than physical exercise and that enriched experience is a more potent therapy than either of these two other treatments. The combination of enriched experience with some other neurosurgical and/or neuropharmacological treatments may further improve its therapeutic effectiveness. Finally, other recent reports emphasize that the treatment parameters may be changed in order to approximate clinical/rehabilitation conditions and, nevertheless, remain effective.

Modulation of photic resetting in rats by lesions of projections to the suprachiasmatic nuclei expressing p75 neurotrophin receptor.

The suprachiasmatic nuclei of the hypothalamus (SCN) are the site of the master circadian clock in mammals. The SCN clock is mainly entrained by the light-dark cycle. Light information is conveyed from the retina to the SCN through direct, retinohypothalamic fibres. The SCN also receive other projections, like cholinergic fibres from basal forebrain. To test whether cholinergic afferents are involved in photic resetting, lesions of cholinergic projections were performed in rats with intracerebroventricular (i.c.v.) injections or intra-SCN microinjections of 192 IgG-saporin. When injected in the SCN, this immunotoxin destroys the cholinergic projections and retinohypothalamic afferents that express p75 low-affinity nerve growth factor (p75(NGF)) receptors. The extent of lesions in the basal forebrain and SCN was assessed by acetylcholinesterase histochemistry, p75(NGF) receptor, choline acetyl-transferase, calbindin-D28K and VIP immunocytochemistry. The intra-SCN treatment reduced light-induced phase advances by 30%, and induced a complete loss of forebrain and retinal afferents expressing p75(NGF) receptors within the SCN and a decrease of forebrain cholinergic neurons, most likely those projecting to the SCN. The i.c.v. treatment reduced light-induced phase advances by 40%, increased phase delays and led to extensive damage of forebrain p75(NGF)-expressing neurons, while sparing half of the fibres expressing p75(NGF) receptors (retinal afferents?) in the SCN. Because the integrity of forebrain p75(NGF)-expressing neurons appears to be critical in mediating the effects on light-induced phase advances, we therefore suggest that anterior cholinergic projections expressing p75(NGF) receptors modulate the sensitivity of the SCN clock to the phase advancing effects of light.

Intraseptal injection of the 5-HT1A/5-HT7 agonist 8-OH-DPAT and working memory in rats.

RATIONALE: In rats, 5-HT(1A) receptors are present in the septal region, e.g. on cholinergic neurons of the medial septum, where they might be a substrate for cognitively relevant interactions between cholinergic and serotonergic systems. OBJECTIVE: The present experiment assessed the effects of the stimulation of septal 5-HT(1A) receptors on spatial working memory. METHODS: Stimulation of septal 5-HT(1A) receptors was carried out by infusions targetting the medial septum of the 5-HT(1A)/5-HT(7) receptor agonist 8-hydroxy-2-(di-n-propyl-amino)-tetralin (8-OH-DPAT; 0.5 or 4 microg). Spatial memory was assessed in a water maze using a protocol placing emphasis on spatial working memory. The location of the hidden platform was changed every day and performance was assessed on two consecutive trials each day. RESULTS: In comparison to vehicle injections, the intraseptal infusion of 4 microg 8-OH-DPAT impaired performance significantly: rats treated with 8-OH-DPAT exhibited increased distances to reach the hidden platform on both trials 1 and 2. Rats infused with 0.5 microg showed similar changes that failed to be significant. Such effects were not observed when the platform was visible. CONCLUSIONS: These results extend those of a previous experiment which showed that intraseptal injections of 8-OH-DPAT impaired spatial reference memory. Based on the characteristics of the observed deficits, it is suggested that the 8-OH-DPAT-induced impairment, rather than being only the result of a true alteration of working memory, might reflect a more global cognitive deficiency in which alteration of general memory capacities may be biased by disrupted search strategies/exploration and/or dysfunctions of attention.

Baseline and 8-OH-DPAT-induced release of acetylcholine in the hippocampus of aged rats with different levels of cognitive dysfunction.

During aging, neurotransmission systems such as the cholinergic and serotonergic ones are altered. Using rats aged 3 or 24-26 months, this study investigated whether the well-described 8-OH-DPAT-induced increase of hippocampal acetylcholine release was altered in aged rats and whether it may vary according to the magnitude of age-related cognitive deficits. Long-Evans female rats aged 24-26 months were classified as good or bad performers on the basis of their reference-memory performance in a Morris water-maze task. Subsequently, the efficiency of 5-HT(1A) receptor agonist 8-OH-DPAT (0.5 mg/kg, s.c.) in triggering hippocampal acetylcholine release was evaluated by in vivo microdialysis and high performance liquid chromatography analysis. Besides a reduced baseline release in aged rats and a correlation between the baseline release and probe-trial performance in all rats, the results demonstrated that 8-OH-DPAT produced a significant increase of hippocampal acetylcholine release (peak value) in all rats, whether aged or young. While significant in bad performers (+56%), this increase did not reach significance in good performers (+32%). The results suggest that (i) some aspects of cognitive alterations related to aging might be linked to the baseline release of acetylcholine in the hippocampus, and (ii) the cholinergic innervation of the hippocampus of aged rats responds almost normally to systemic activation of 5-HT(1A) receptors, and (iii) differential alterations of cholinergic/serotonergic interactions assessed by determination of the 8-OH-DPAT-induced release of acetylcholine in the hippocampus could not be linked with clarity to the cognitive status of aged rats.

Selective immunolesions of CH4 cholinergic neurons do not disrupt spatial memory in rats.

Adult male Long-Evans rats were subjected to bilateral lesions of the cholinergic neurons in the nucleus basalis magnocellularis (NBM) by injection of 0.2 or 0.4 microg 192-IgG-saporin in 0.4 microl phosphate-buffered saline. Control rats received an equivalent amount of phosphate-buffered saline. Starting 2 weeks after surgery, all rats were tested for locomotor activity in their home cage, beam-walking performance, T-maze alternation rates (working memory), reference and working memory performance in a water-maze task, and memory capabilities in the eight-arm radial maze task using uninterrupted and interrupted (delay of 2 min, 2 h and 6 h after four arms had been visited) testing procedures. Histochemical analysis showed a significant decrease of acetylcholinesterase (AChE)-positive reaction products (30-66%) in various cortical regions at the 0.2-microg dose. At the dose of 0.4 microg, there was an additional, although weak, damage to the hippocampus (17-30%) and the cingulate cortex (34%). The behavioral results showed only minor impairments in spatial memory tasks, and only during initial phases of the tests (reference memory in the water maze, working memory in the radial maze). The behavioral effects of the dramatic cholinergic lesions do not support the idea of a substantial implication of cholinergic projections from the NBM to the cortex in the memory processes assessed in this study, but they remain congruent with an involvement of these projections in attentional functions.

Effects of 192 IgG-saporin on acetylcholinesterase histochemistry in male and female rats.

Sex hormones may exert neuroprotective effects in various models of brain lesions. Male and female Long-Evans rats were subjected to intracerebroventricular injections of 2 microg 192 IgG-saporin or vehicle. Starting 2 days before surgery, half the male rats were treated with estradiol for 7 days. Three weeks after surgery, they were sacrificed for histochemical staining of acetylcholinesterase (AChE) and densitometric evaluations. The lesion induced a substantial to dramatic decrease of the AChE-positive fiber density in the cingulate, somatosensory, piriform, retrosplenial and perirhinal cortices, and in the hippocampus. Weak effects were found in the striatum. There was no significant decrease in the dorsal thalamus. Sex had no significant effect on AChE-positive staining in any brain area. In males, estradiol treatment did not alter the effects of 192 IgG-saporin. These results show that sex or estradiol treatment in male rats does not interfere with the immunotoxic effects of intracerebroventricular injections of 192 IgG-saporin on cholinergic projections from the basal forebrain.

A comparison of the effects of fimbria-fornix, hippocampal, or entorhinal cortex lesions on spatial reference and working memory in rats: short versus long postsurgical recovery period.

Using a radial maze task and different postoperative recovery periods, this experiment assessed and compared the reference and working memory performances of adult Long-Evans male rats subjected to entorhinal cortex, fimbria-fornix, and hippocampus lesions. Sham-operated rats were used as controls. In order to see whether the duration of the postsurgical recovery period would influence acquisition of the complex radial maze task, training began 1 month following surgery (Delay 1) for half the rats in each group, while for the other half training was started 6.5 months following surgery (Delay 2). The results indicated that at both recovery periods the entorhinal cortex lesions failed to affect either working or reference memory in the spatial task. Conversely, both fimbria-fornix and hippocampus lesions impaired both reference and working memory. While the reference memory deficit was generally similar in both fimbria-fornix and hippocampal lesion groups, analysis of the results for working memory indicated that at the longer delay rats with fimbria-fornix lesions were still impaired but in animals that had the hippocampus removed, working memory did not differ from that of controls. These results suggest that there was some recovery in those rats with hippocampal lesions (e.g., on the working memory task) but both hippocampal and fimbria-fornix animals were still impaired compared to controls when training was delayed 6.5 months following the operations.