Chronic stress and memory: implication of the central cholinergic system

Involvement of muscarinic receptor mechanisms in antidepressant drug action

Conventional antidepressants typically require weeks of daily dosing to achieve full antidepressant response in antidepressant responders. A newly evolving group of compounds can engender more rapid response times in depressed patients. These drugs include the newly approved antidepressant (S)-ketamine (esketamine, Spravato). A seminal study by Furey and Drevets in 2006 showed antidepressant response in patients after only a few doses with the antimuscarinic drug scopolamine. Several clinical reports have generally confirmed scopolamine as a rapid-acting antidepressant. The data with scopolamine are consistent with the adrenergic/cholinergic hypothesis of mania/depression derived from clinical reports originating in the 1970s from Janowsky and colleagues. Additional support for a role for muscarinic receptors in mood disorders comes from the greater efficacy of conventional antidepressants that have relatively high levels of muscarinic receptor blocking actions (e.g., the tricyclic antidepressant amitriptyline vs the selective serotonin reuptake inhibitor fluoxetine). There appears to be appreciable overlap in the mechanisms of action of scopolamine and other rapid-acting antidepressants (ketamine) or putative rapid-acting agents (mGlu2/3 receptor antagonists) although gaps exist in the experimental literature. Current hypotheses regarding the mechanisms underlying the rapid antidepressant response to scopolamine posit an M₁ receptor subtype-initiated cascade of biological events that involve the amplification of AMPA receptors. Consequent impact on brain-derived neurotrophic factor and mTor signaling pathways result in the induction of dendritic spines that enable augmented functional connectivity in brain areas regulating mood. Two major goals for research in this area focus on finding ways in which scopolamine might best be utilized for depressed patients and the discovery of alternative compounds that improve upon the efficacy and safety of scopolamine.

The involvement of cholinergic and noradrenergic systems in behavioral recovery following oxotremorine treatment to chronically stressed rats

Chronic stress in rats has been shown to impair learning and memory, and precipitate several affective disorders like depression and anxiety. The mechanisms involved in these stress-induced disorders and the possible reversal are poorly understood, thus limiting the number of drugs available for their treatment. Our earlier studies suggest cholinergic dysfunction as the underlying cause in the behavioral deficits following stress. Muscarinic cholinergic agonist, oxotremorine is demonstrated to have a beneficial effect in reversing brain injury-induced behavioral dysfunction. In this study, we have evaluated the effect of oxotremorine treatment on chronic restraint stress-induced cognitive deficits. Rats were subjected to restraint stress (6 h/day) for 21 days followed by oxotremorine treatment for 10 days. Spatial learning and memory was assessed in a partially baited eight-arm radial maze task. Stressed rats exhibited impairment in performance, with decreased percentage of correct choices and an increase in the number of reference memory errors (RMEs). Oxotremorine treatment (0.1 or 0.2 mg/kg, i.p.) to stressed rats resulted in a significant increase in the percent correct choices and a decrease in the number of RMEs compared with stress as well as the stress+vehicle-treated groups. In the retention test, oxotremorine treated rats committed less RMEs compared with the stress group. Chronic restraint stress decreased acetylcholinesterase (AChE) activity in the hippocampus, frontal cortex and septum, which was reversed by both the doses of oxotremorine. Further, oxotremorine treatment also restored the norepinephrine levels in the hippocampus and frontal cortex. Thus, this study demonstrates the potential of cholinergic muscarinic agonists and the involvement of both cholinergic and noradrenergic systems in the reversal of stress-induced learning and memory deficits.

Effects of REM sleep deprivation on cholinergic receptor sensitivity and passive avoidance behavior in clomipramine model of depression

This study investigated the effects of REM (rapid eye movement) sleep deprivation (RSD) on the activity of central cholinergic receptors and passive avoidance retention in rats treated neonatally with clomipramine. Male rat pups treated with clomipramine (15 mg/kg, s.c.) twice daily from postnatal day 5 to 21 were subjected to RSD procedure at three months of age, for 4 days consecutively. In the post-RSD phase, RSD-control rats showed a significantly enhanced cholinomimetic-induced hypothermia and an improved retention in passive avoidance task. However, these measures were not significantly different in RSD-experimental group as compared to rats treated neonatally with saline. These results suggest that RSD reverses the sensitivity of central cholinergic receptors in rats given clomipramine neonatally, and this mechanism may be involved in mediating the antidepressant effects of RSD treatment in clomipramine model of depression.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

The effects of flumazenil on two way active avoidance and locomotor activity in diazepam-treated rats

The present study was undertaken to determine the effects of chronic flumazenil treatment alone and simultaneously with diazepam on acquisition performance in an active-avoidance task and on locomotor activity in rats. Flumazenil (5, 10 and 20 mg/kg) and diazepam (0,5, 1.0 and 2.0 mg/kg) were administered intraperitoneally to rats before each daily training session for 5 days. The baseline of avoidance performance was set to approximately 50% and responses were expressed as acquisition rate. Locomotor activity of the rats was simultaneously recorded but only following the first training session. Diazepam decreased acquisition rate between the dose range used. Flumazenil had no effect on the acquisition rate of naive rats but reversed low dose diazepam-induced learning and memory impairment. Diazepam induced locomotor depression within the same dose range that decreased acquisition rate. Flumazenil had no effect on locomotor activity, but reversed the locomotor depressant effect of diazepam. The striking contradiction with previous data that flumazenil has no effect on learning-memory processing is discussed.

The effects of acute and repeated restraint stress on the nociceptive response in rats

The effects of acute and repeated restraint stress on nociception, as measured by the tail-flick latency, were studied in adult male and female rats. After the exposure to a single restraint session, both male and female rats presented an increased latency in the tail-flick test. On the other hand, chronically stressed females presented a performance similar to the control group, whereas chronically stressed male rats responded to restraint with a decrease in the tail-flick latency. This response could be determined by the chronic treatment itself or by the restraint done just before the measurement. Thus, the effect of chronic stress upon basal tail-flick latency was evaluated. In male rats, this latency was significantly decreased in the stressed animals compared with the control group. In female rats, no difference between those groups was observed. Therefore, the results suggest that: (a) acute restraint stress induces an analgesic response in both male and female rats, and (b) there is a gender-specific nociceptive response induced by repeated restraint stress with a hyperalgesic effect in response to stress only in males.

Scopolamine blocks the effects of swim stress on memory retrieval in rats

This study examined whether application of swim stress improved retrieval of a passive avoidance memory and if pretreatment with the anticholinergic agent, scopolamine, blocked this effect on memory retrieval. Animals initially given a passive avoidance training session were subjected to either a two or four swim stress sessions (15 min each) with or without prior treatment of scopolamine (0.05 or 0.1 mg/kg). The retrieval performance in passive avoidance test and motor activity was assessed 24 hr after the last swim stress session. In an independent control experiment, the passive avoidance training and test were conducted respectively, 24 and 72 hr after the last of four swim stress sessions with or without prior injection of scopolamine (0.1 mg/kg). The results showed an enhanced performance for the passive avoidance task in rats subjected to four swim stress sessions in both experiments and scopolamine given 30 min prior to each stress session diminished this performance of animals in the passive avoidance test. Two swim stress sessions with or without scopolamine treatment caused no significant effects on the retrieval performance. Also, no significant difference was observed among the groups in motor activity following any of the stress treatments in the open field test. These results, thus suggested for the first time, a relationship among swim stress, cholinergic activity and avoidance memory processes.

Restraint stress reversibly enhances spatial memory performance

The effects of restraint stress on performance of a spatial memory task, the eight arm radial maze, was examined in rats. When stress was given for 6 h/day for 7 days and performance evaluated days 10-13 post stress, no effect on performance was noted; however, daily restraint stress for 13 days caused a small, but significant, enhancement of performance days 10-13 post stress. Stressed rats performed better than controls: their number of correct choices in the first 8 visits was higher than the controls, and stressed rats took fewer total choices to finish the maze than controls. Stress-dependent, enhanced performance does not appear permanent since further maze testing on days 14 and 15 post stress showed no differences between the groups. Performance of the stressed rats significantly correlated with their stress-induced, serum corticosterone levels measured after 6 h of restraint on the last day of restraint, day 13 (r = -0.63, P < 0.05); rats with higher levels of CORT took fewer choices to finish the task. Examination of hippocampal CA3c pyramidal neurons with Golgi techniques showed no effect of stress on the basal or apical dendritic arbors. Since our previous study showed that 21 days of restraint stress is associated with impaired spatial memory performance (10), these results suggest that the duration of stress may differentially affect learning/memory with shorter periods of stress serving an adaptive function while longer durations causing maladaptive changes.

Effects of central administration of arginine-vasopressin on aversive memory retrieval

This study examined whether arginine-vasopressin (A-VP), given before the test would produce an improved retrieval of aversive memory, in the same way as pre-exposure to inescapable footshocks, in rats. For this purpose animals conditioned in a T-maze with appetitive (10% sucrose) and aversive (2.0 mA footshock) events were administered (intracerebroventricular) a single dose of 2.5, 5, 10 or 20 ng/rat of A-VP, 20-min before testing. In the retention test conducted with the same training apparatus 72 h after conditioning, the peptide treated rats showed a dose-dependent increase in latencies to enter the previously shocked goalarm, with the absence of such a difference in responding to the non-shocked goalarm. This differential response was not observed in saline treated rats. This effect of peptide on memory retrieval was similar to that seen following inescapable footshock in rats. These results suggest the possible involvement of central vasopressinergic mechanisms in the differential enhancement of memory of helplessness condition.

Effects of neonatal clomipramine on cholinergic receptor sensitivity and passive avoidance behavior in adult rats

The effects of neonatal treatment with clomipramine on the sensitivity of cholinergic receptor and passive avoidance behavior were studied to examine the activity of the central cholinergic system. Rat pups were treated twice daily from postnatal day 5 to 21 with clomipramine (15 mg/kg, s.c.) and at 3 months of age the thermic responses to three different doses of oxotremorine were measured. One day following oxotremorine challenge study, the animals were subjected to passive avoidance training and retention was measured 24-hr later. Clomipramine treated animals showed an enhanced cholinomimetic-induced hypothermia and an increased latency in passive avoidance test. These findings may reflect an altered sensitivity of central cholinergic system in rats given clomipramine as neonates. The results were compared to other animal models of depression.

Effects of chronic mild stress on serum complement activity, saccharin preference, and corticosterone levels in Flinders lines of rats

Complement proteins and fragments participate in the induction and modulation of specific and nonspecific immune reactions. We have examined the effect of 4 weeks of chronic mild stress (CMS) on complement sheep red blood cell hemolytic activity measured in CH50 units in two selectively bred lines of rats, the Flinders resistant line (FRL) and the Flinders sensitive line (FSL), that differ in cholinergic sensitivity and behavioral characteristics. Additionally, CMS-induced hedonic deficit (decreased preference for 0.02% saccharin over water) and serum corticosterone levels were compared in FRL and FSL rats. CMS caused a significantly (p < 0.01) greater decline in CH50 responses in FSL (-15%) than in FRL (-7%) rats. This was accompanied by a significant (p < 0.01) suppression of saccharin preference over a 24 h period in both FRL and FSL rats. Both lines showed a similar, more than 2-fold (p < 0.01) increase in corticosterone levels following CMS. These results further confirm that CMS induces a depressive-like state in rats as well as the validity of the FSL rat as a genetic model of depression. They also indicate that the effect of stress on the immune system can be monitored by measuring the complement CH50 response.

Behavioral screening for cognition enhancers: from indiscriminate to valid testing: Part I

Preclinical efforts to detect and characterize potential cognition enhancers appear to have been dominated by a strategy of demonstrating a wide variety of apparently beneficial behavioral effects with little attention given to the specific psychological mechanisms underlying behavioral enhancement. In particular, the question of whether or not behavioral facilitation is based on relevant mnemonic mechanisms and is independent of the stimulus properties and/or the motivational and attentional components of a task is not often considered. As a result, an overwhelming number of compounds have failed to produce the clinical effects predicted for them on the basis of preclinical research. The available data suggest that a more successful approach requires deductive research strategies rather than the indiscriminate accumulation of apparently beneficial effects in a variety of behavioral tasks and animal models. The first step towards such an approach is a systematic and rigorous evaluation of the different aspects of validity for the models most frequently used in preclinical research. It is concluded that a combination of good construct validity and good face validity represents a necessary condition for screening tests with predictive validity, and that the most popular paradigms fail to fulfil these criteria. Future screening programs for cognition enhancers will probably be characterized by a depreciation of "fast and dirty tests" in favor of approaches focussing on the validity of the effects of potential cognition enhancers.