Hippocampal cholinergic blockade enhances hypothalamic-pituitary-adrenal responses to stress

Endogenous in-session cortisol during exposure therapy predicts symptom improvement: Preliminary results from a scopolamine-augmentation trial

The purpose of this study was to explore whether individual differences in glucocorticoid concentrations were associated with symptom improvement following exposure therapy for patients with social anxiety disorder. To do this, 60 participants with social anxiety disorder completed a randomized-controlled trial of exposure therapy, where participants were randomized to receive scopolamine-augmentation or placebo during their 7 exposure sessions. Scopolamine is an antimuscarinic which blocks the effects of acetylcholine and reduces autonomic arousal. During sessions 1, 4, 7, and during the post-treatment extinction assessment, participants provided up to 16 saliva samples (4 in each session). Pre-treatment, post-treatment, and at 1-month follow-up, participants completed the Liebowitz Social Anxiety Scale to monitor change in fear and avoidance symptoms. Elevated endogenous in-session cortisol during exposure sessions was associated with less symptom improvement from pre- to post-treatment and at 1-month follow-up. The association between elevated endogenous in-session cortisol and attenuated symptom change was not moderated by scopolamine treatment condition. Individuals with social anxiety disorder who have elevated neuroendocrine signaling may under-benefit from exposure therapy. This is the first study, to our knowledge, to examine whether endogenous in-session cortisol concentrations predict symptom changes following exposure therapy for the treatment of social anxiety disorder. More investigation of non-invasive and reliable biological markers that explain variability in responses to effective treatments are needed.

Selective decrease of cholinergic signaling from pedunculopontine and laterodorsal tegmental nuclei has little impact on cognition but markedly increases susceptibility to stress

The pedunculopontine tegmental nucleus (PPT) and laterodorsal tegmental nucleus (LDT) are heterogeneous brainstem structures that contain cholinergic, glutamatergic, and GABAergic neurons. PPT/LDT neurons are suggested to modulate both cognitive and noncognitive functions, yet the extent to which acetylcholine (ACh) signaling from the PPT/LDT is necessary for normal behavior remains uncertain. We addressed this issue by using a mouse model in which PPT/LDT cholinergic signaling is highly decreased by selective deletion of the vesicular ACh transporter (VAChT) gene. This approach interferes exclusively with ACh signaling, leaving signaling by other neurotransmitters from PPT/LDT cholinergic neurons intact and sparing other cells. VAChT mutants were examined on different PPT/LDT-associated cognitive domains. Interestingly, VAChT mutants showed no attentional deficits and only minor cognitive flexibility impairments while presenting large deficiencies in both spatial and cued Morris water maze (MWM) tasks. Conversely, working spatial memory determined with the Y-maze and spatial memory measured with the Barnes maze were not affected, suggesting that deficits in MWM were unrelated to spatial memory abnormalities. Supporting this interpretation, VAChT mutants exhibited alterations in anxiety-like behavior and increased corticosterone levels after exposure to the MWM, suggesting altered stress response. Thus, PPT/LDT VAChT-mutant mice present little cognitive impairment per se, yet they exhibit increased susceptibility to stress, which may lead to performance deficits in more stressful conditions.-Janickova, H., Kljakic, O., Rosborough, K., Raulic, S., Matovic, S., Gros, R., Saksida, L. M., Bussey, T. J., Inoue, W., Prado, V. F., Prado, M. A. M. Selective decrease of cholinergic signaling from pedunculopontine and laterodorsal tegmental nuclei has little impact on cognition but markedly increases susceptibility to stress.

Sexually diergic hypothalamic-pituitary-adrenal axis responses to selective and non-selective muscarinic antagonists prior to cholinergic stimulation by physostigmine in rats

Central cholinergic systems regulate the hypothalamic-pituitary-adrenal (HPA) axis differentially in males and females (sexual diergism). We previously investigated the role of muscarinic receptors in this regulation by administering physostigmine (PHYSO), an acetylcholinesterase inhibitor, to male and female rats pretreated with scopolamine (SCOP), a nonselective muscarinic antagonist. SCOP pretreatment enhanced adrenocorticotropic hormone (ACTH) and corticosterone (CORT) responses in both sexes, but males had greater ACTH responses while females had greater CORT responses. In the present study, we further explored the role of muscarinic receptor subtypes in HPA axis regulation by administering PHYSO to male and female rats following SCOP or various doses of either the M1 or the M2 selective muscarinic receptor antagonists, pirenzepine (PIREN) or methoctramine (METHO). Blood was sampled before and at multiple times after PHYSO. ACTH and CORT were determined by highly specific immunoassays. M1 antagonism by PIREN prior to PHYSO resulted in sustained, dose-dependent increases in ACTH and CORT: ACTH responses were similar in both sexes, and CORT responses were greater in females. M2 antagonism by METHO prior to PHYSO resulted in overall decreases in ACTH and CORT: ACTH and CORT responses were higher in females but lower in both sexes than the hormone responses following PIREN or SCOP pretreatment. Area under the curve analyses supported these findings. These results suggest that specific muscarinic receptor subtypes differentially influence the HPA axis in a sexually diergic manner.

NOAEL-dose of a neonicotinoid pesticide, clothianidin, acutely induce anxiety-related behavior with human-audible vocalizations in male mice in a novel environment

Neonicotinoids are novel systemic pesticides acting as agonists on the nicotinic acetylcholine receptors (nAChRs) of insects. Experimental studies have revealed that neonicotinoids pose potential risks for the nervous systems of non-target species, but the brain regions responsible for their behavioral effects remain incompletely understood. This study aimed to assess the neurobehavioral effects of clothianidin (CTD), a later neonicotinoid developed in 2001 and widely used worldwide, and to explore the target regions of neonicotinoids in the mammalian brain. A single-administration of 5 or 50mg/kg CTD to male C57BL/6N mice at or below the no-observed-adverse-effect level (NOAEL) induced an acute increase in anxiety during the elevated plus-maze test. In addition, mice in the CTD-administered group spontaneously emitted human-audible vocalizations (4-16kHz), which are behavioral signs of aversive emotions, and showed increased numbers of c-fos immunoreactive cells in the paraventricular thalamic nucleus and dentate gyrus of the hippocampus. In conclusion, mice exposed to NOAEL-dose CTD would be rendered vulnerable to a novel environment via the activation of thalamic and hippocampal regions related to stress responses. These findings should provide critical insight into the neurobehavioral effects of neonicotinoids on mammals.

The Role of Hippocampal NMDA Receptors in Long-Term Emotional Responses following Muscarinic Receptor Activation

Extensive evidence indicates the influence of the cholinergic system on emotional processing. Previous findings provided new insights into the underlying mechanisms of long-term anxiety, showing that rats injected with a single systemic dose of pilocarpine—a muscarinic receptor (mAChR) agonist—displayed persistent anxiogenic-like responses when evaluated in different behavioral tests and time-points (24 h up to 3 months later). Herein, we investigated whether the pilocarpine-induced long-term anxiogenesis modulates the HPA axis function and the putative involvement of NMDA receptors (NMDARs) following mAChRs activation. Accordingly, adult male Wistar rats presented anxiogenic-like behavior in the elevated plus-maze (EPM) after 24 h or 1 month of pilocarpine injection (150 mg/kg, i.p.). In these animals, mAChR activation disrupted HPA axis function inducing a long-term increase of corticosterone release associated with a reduced expression of hippocampal GRs, as well as consistently decreased NMDAR subunits expression. Furthermore, in another group of rats injected with memantine–an NMDARs antagonist (4 mg/kg, i.p.)–prior to pilocarpine, we found inhibition of anxiogenic-like behaviors in the EPM but no further alterations in the pilocarpine-induced NMDARs downregulation. Our data provide evidence that behavioral anxiogenesis induced by mAChR activation effectively yields short- and long-term alterations in hippocampal NMDARs expression associated with impairment of hippocampal inhibitory regulation of HPA axis activity. This is a novel mechanism associated with anxiety-like responses in rats, which comprise a putative target to future translational studies.

Ventral hippocampal nicotinic acetylcholine receptors mediate stress-induced analgesia in mice

Evidence suggests that various stressful procedures induce an analgesic effect in laboratory animals commonly referred to as stress-induced analgesia (SIA). The aim of the present study was to assess the role of ventral hippocampal (VH) nicotinic acetylcholine receptors (nAChRs) in SIA in adult male NMRI mice. The VHs of animals were bilaterally cannulated and nociceptive threshold was measured using infrared source in a tail-flick apparatus. Acute stress was evoked by placing the animals on an elevated platform for 10, 20 and 30 min. The results showed that exposure to 20 and 30 min acute stress produced analgesia, while exposure to 10 min stress had no effect on the pain response. Intra-VH microinjection of nicotine (0.001-0.1 μg/mouse), 5 min before an ineffective stress (10 min stress), induced analgesia, suggesting the potentiative effect of nicotine on SIA. It is important to note that bilateral intra-VH microinjections of the same doses of nicotine without stress had no effect on the tail-flick test. On the other hand, intra-VH microinjection of mecamylamine (0.5-1 μg/mouse) 5 min before 20-min stress inhibited SIA. However, bilateral intra-VH microinjections of the same doses of mecamylamine without stress had no effect on the tail-flick response. In addition, the microinjection of mecamylamine into the VH reversed the potentiative effect of nicotine on SIA. Taken together, it can be concluded that exposure to acute stress induces SIA in a time-dependent manner and the ventral hippocampal cholinergic system may be involved in SIA via nAChRs.

Acetylcholine receptor and behavioral deficits in mice lacking apolipoprotein E

Apolipoprotein E (apoE) is involved in the risk to develop sporadic Alzheimer's disease (AD). Since impaired central acetylcholine (ACh) function is a hallmark of AD, apoE may influence ACh function by modulating muscarinic ACh receptors (mAChRs). To test this hypothesis, mAChR binding was measured in mice lacking apoE and wild type C57BL/6J mice. Mice were also tested on the pre-pulse inhibition, delay eyeblink classical conditioning, and 5-choice serial reaction time tasks (5-SRTT), which are all modulated by ACh transmission. Mice were also given scopolamine to challenge central mAChR function. Compared to wild type mice, mice lacking apoE had reduced number of cortical and hippocampal mAChRs. Scopolamine had a small effect on delay eyeblink classical conditioning in wild type mice but a large effect in mice lacking apoE. Mice lacking apoE were also unable to acquire performance on the 5-SRTT. These results support a role for apoE in ACh function and suggest that modulation of cortical and hippocampal mAChRs might contribute to genotype differences in scopolamine sensitivity and task acquisition. Impaired apoE functioning may result in cholinergic deficits that contribute to the cognitive impairments seen in AD.

Self-reported motivation to smoke in schizophrenia is related to antipsychotic drug treatment

PURPOSE

The prevalence of smoking in schizophrenia has reliably been reported as being higher than for any other psychiatric disorder. While a number of theories have been proposed to account for such high rates of smoking, little is known about the subjective motivation for why schizophrenia patients smoke in comparison with those without the disease.

OBJECTIVES

The aim of the present study was to evaluate and compare smoking motivation in control subjects and schizophrenia patients, and determine if factors such as type of medication or access to cigarettes could contribute to self-reported motivation for smoking.

METHODS

We assessed motivation to smoke in 61 schizophrenia inpatients and 33 non-psychiatric health worker controls at a tertiary care psychiatric facility in a cross-sectional study. Nicotine dependency and smoking behavior were evaluated using the Fagerstrom Test for Nicotine Dependence and a validated questionnaire that assesses motivation for smoking along seven different dimensions.

RESULTS

Schizophrenia patients reported a stronger motivation to smoke than controls for reasons related to pleasure from the act of smoking, as well as a need for psychomotor stimulation. Scores on both these factors were significantly associated with daily antipsychotic drug dose. The sedative and anxiolytic effects of smoking were related to anticholinergic load of psychiatric medications.

CONCLUSION

The findings highlight important differences in self-reported motivation to smoke between schizophrenia patients and normals. Antipsychotic drugs may also influence aspects of motivation to smoke.

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.

Neuromodulators of LTP and NCAMs in the amygdala and hippocampus in response to stress

Possibly, at the onset of an emotional event the stress hormones permissively mediate plasticity. Specifically, CORT and NE stress hormones participate in modulation of memory consolidation processes in both the amygdala and the hippocampus. In addition, glucocorticoids and norepinephrin bound to adrenoceptors are also involved in modulating the regulation of NCAM polysialylation both in the amygdala and in the hippocampus. PSA-related synaptic remodeling is mobilized for memory formation in particularly challenging circumstances.

Analysis of direct hippocampal cortical field CA1 axonal projections to diencephalon in the rat

The hippocampal formation is generally considered essential for processing episodic memory. However, the structural organization of hippocampal afferent and efferent axonal connections is still not completely understood, although such information is critical to support functional hypotheses. The full extent of axonal projections from field CA1 to the interbrain (diencephalon) is analyzed here with the Phaseolus vulgaris-leucoagglutinin (PHAL) method. The ventral pole of field CA1 establishes direct pathways to, and terminal fields within, the anterior hypothalamic nucleus, ventromedial hypothalamic nucleus, lateral hypothalamic and lateral preoptic areas, medial preoptic area, and certain other hypothalamic regions, as well as particular midline thalamic nuclei. These results suggest that hippocampal field CA1 modulates motivated or goal-directed behaviors, and physiological responses, associated with the targeted hypothalamic neuron populations.

Natural drug extracts for a nutritive-tonic drink, promotes the induction of long-term potentiation in rat hippocampal dentate gyrus in vivo

We have shown previously that oral administration of a nutritive-tonic drink (NTD) improves scopolamine-induced memory impairment in the passive avoidance task and Morris water-maze in mice and that this action is attributable to the natural drug extracts, rather than synthetic drugs such as taurine and caffeine, in the NTD. In order to investigate the mechanism underlying the antiamnesic effects of the natural drug extracts, the effects of the extracts on the induction of long-term potentiation (LTP) was investigated in the dentate gyrus (DG) of normal and scopolamine-treated rats. Oral administration of natural drug extracts enhanced the induction of population spike amplitude induced by weak tetanic stimulation (30 pulses at 60 Hz). Scopolamine (0.3 mg/kg, i.p.) completely inhibited the induction of LTP induced by both weak and strong tetanic stimulation (100 pulses at 100 Hz). Natural drug extracts enhanced partially but significantly the induction of LTP by strong tetanus, but had a very weak effect on that induced by weak tetanus. These results suggest that LTP induced by strong tetanus is sensitive to natural drug extracts, and that the antiamnesic effect of the NTD is at least partly attributable to the LTP-improving effect of the natural drug extracts in the DG.

Central muscarinic blockade interferes with retrieval and reacquisition of active allothetic place avoidance despite spatial pretraining

Animal navigation to hidden goals (place navigation) ranks among the most intensively studied types of behaviour because it requires brain representations of environments in the form of cognitive maps, demonstrated to depend on hippocampal function. Intact function of muscarinic receptors in the brain was originally assumed to be crucial for place navigation, however, recent studies using non-spatial pretraining demonstrated that animals with central blockade of muscarinic acetylcholine receptors can also learn and retrieve spatial memory engrams. In the present study we addressed whether navigation in the active allothetic place avoidance (AAPA) task, which requires animals to separate spatial stimuli into coherent representations and navigate according to the representation relevant for the task, is dependent on intact muscarinic receptors in the brain. We studied the effect of three doses of scopolamine (0.5, 1.0 and 2.0mg/kg) administered 20 min prior to training on the retention of the AAPA and re-acquisition of the AAPA in a new environment. The dose of 2.0mg/kg was found to impair both AAPA retention and re-acquisition of the AAPA in a new environment, whereas the 1.0mg/kg dose only impaired the reinforced retention of AAPA. It is concluded that, unlike navigation in classic paradigms, efficient orientation in the AAPA task is critically dependent on muscarinic receptors in the brain.

The supramammillary area: its organization, functions and relationship to the hippocampus

The supramammillary area of the hypothalamus, although small in size, can have profound modulatory effects on the hippocampal formation and related temporal cortex. It can control hippocampal plasticity and also has recently been shown to contain cells that determine the frequency of hippocampal rhythmical slow activity (theta rhythm). We review here its organization and anatomical connections providing an atlas and a new nomenclature. We then review its functions particularly in relation to its links with the hippocampus. Much of its control of behaviour and its differential activation by specific classes of stimuli is consistent with a tight relationship with the hippocampus. However, its ascending connections involve not only caudal areas of the cortex with close links to the hippocampus but also reciprocal connections with more rostral areas such as the infralimbic and anterior cingulate cortices. These latter areas appear to be the most rostral part of a network that, via the medial septum, hippocampus and lateral septum, is topographically mapped into the hypothalamus. The supramammillary area is thus diffusely connected with areas that control emotion and cognition and receives more topographically specific return information from areas that control cognition while also receiving ascending information from brain stem areas involved in emotion. We suggest that it is a key part of a network that recursively transforms information to achieve integration of cognitive and emotional aspects of goal-directed behaviour.

Exposure to an elevated platform increases plasma corticosterone and hippocampal acetylcholine in the rat: reversal by chlordiazepoxide

There is evidence that the septohippocampal cholinergic system is activated in response to stressful stimuli. In addition, prior studies indicate that stimulating the hippocampal cholinergic neurotransmission increases open arm exploration in the elevated plus-maze. This raises the possibility that exposing the rat to an elevated platform, which would be similar to confining the animal to the open arms of the plus-maze, would alter hippocampal acetylcholine levels. Results from the present study suggest that an elevated platform can be used as an animal model of stress in that exposure to the platform significantly increased plasma corticosterone levels. Importantly, exposure to a platform significantly increased hippocampal acetylcholine efflux. Interestingly, the increase in plasma corticosterone and hippocampal acetylcholine levels upon exposure to an elevated platform could be prevented by chlordiazepoxide at a dose that had no effect on basal hippocampal acetylcholine or plasma corticosterone levels. However, the elevated platform-induced increase in hippocampal acetylcholine could not be blocked by prior administration of buspirone. These results provide direct evidence for the importance of the hippocampal cholinergic system in stress and provide validation for the elevated platform as a model of stress.

Pretraining or previous non-spatial experience improves spatial learning in the Morris water maze of nucleus basalis lesioned rats

Previous experiments have shown that infusions of ibotenic acid in the nucleus basalis magnocellularis (NBM) induce a strong impairment in spatial navigation for a hidden platform in the Morris water maze. This effect was initially attributed to a cholinergic deficit, but later studies showed that performance level did not correlate with the degree of cholinergic denervation. Therefore, this impairment is due to a combined cholinergic and non-cholinergic deficit. However, it is not clear in which particular processes the NBM is involved. In this study we have evaluated the origin of behavioural impairment in spatial navigation in the water maze after an ibotenic acid-induced lesion of NBM. In the first experiment, Wistar rats were trained preoperatively in an allocentric navigation task. Postoperatively, they were tested in the same task. All lesioned animals showed a performance level similar to controls. Lesions did not impede the acquisition of new positions in the water maze, nor did affect the ability of animals to remember new platform positions after an intertrial interval of 20s, even if animals had received only allocentric experience with the platform position, or allocentric and path integration information concurrently. Lesions also failed to affect the ability to locate a hidden platform in a new environment. However, hippocampal infusions of scopolamine (5 microg) produced a severe impairment in NBM-damaged animals, without impairing performance of controls. In the second experiment Wistar rats with the same lesion were first trained in a visual-guided task in the water maze, and subsequently evaluated in the spatial task. In both tasks lesioned animals were not different from controls. These results suggest that the NBM played an important role during acquisition phases but not in the execution of spatial navigation. Moreover, the excessive emotional response displayed by lesioned animals is postulated as a relevant cause for the impairment observed in spatial navigation after NBM damage.

Habituation to stress and dexamethasone suppression in rats with selective basal forebrain cholinergic lesions

Previous studies suggest a role for basal forebrain cholinergic neurons in enhancing the inhibitory influence of the hippocampus and medial prefrontal cortex (mPFC) on glucocorticoid stress responses mediated by the hypothalamic-pituitary-adrenocortical (HPA) axis. An inhibitory action of the basal forebrain cholinergic (BFC) system may occur through facilitation of stress-related information processing and maintenance of glucocorticoid receptor (GR) expression and negative feedback signaling in these target regions. The current study investigated the possibility that BFC input to the hippocampus contributes to habituation of the glucocorticoid response following repeated exposure to a stressor. Cholinergic lesions were made by microinjections of the immunotoxin 192 IgG-saporin into the medial septum/vertical limb of the diagonal band, and 3 weeks later rats were subjected to six daily sessions of restraint stress. Blood samples taken before, during and after acute stress revealed a significant increase in peak activation and protracted elevation of corticosterone in cholinergic lesioned rats. After 5 days of repeated stress, however, both groups habituated to the stressor, as indicated by similarly low corticosterone profiles throughout both the response and recovery period. Against that habituated background, rats were administered a dexamethasone challenge on day 6, so that feedback status could be examined. Dexamethasone-induced suppression of endogenous corticosterone before, during, and after stress was significantly attenuated in lesioned rats. The profile of dysfunction in glucocorticoid regulation after selective cholinergic lesions in young animals may be relevant to the adrenocortical hyperactivity and negative feedback deficits seen in conditions such as normal aging and Alzheimer's dementia, in which integrity of the basal forebrain cholinergic system is compromised.

Septal GABAergic and hippocampal cholinergic systems interact in the modulation of anxiety

According to Gray [(1982) The neuropsychology of anxiety: an enquiry into the function of the septo-hippocampal system. Oxford: Oxford University Press; (1991) Neural systems, emotion and personality. In: Neurobiology of learning, emotion, and affect (Madden J, ed), pp 273-306. New York: Raven Press; Gray JA, McNaughton N (2000) The neuropsychology of anxiety. Oxford: Oxford University Press], the septum and the hippocampus act in concert to control anxiety. In the present study we examined a possible interaction between septal GABAergic and hippocampal cholinergic systems in the shock-probe burying test, an animal model of anxiety. In experiment 1, we found that a 10-ng infusion of muscimol in the medial septum produced a significant suppression of burying behavior, whereas lower doses (2.5 and 5.0 ng) did not. In experiment 2, we found a significant suppression of burying behavior after a 20-microg infusion of physostigmine into the dorsal hippocampus, but not after lower-dose infusions (5 and 10 microg). In experiment 3, we infused combined sub-effective doses of physostigmine and muscimol in the hippocampus and medial septum respectively. The combination of sub-effective doses of physostigmine (5 microg) and muscimol (2.5 ng) significantly reduced burying of the shock probe. The results indicate that the hippocampal cholinergic and septal GABAergic systems act synergistically in the modulation of anxiety.

Mechanisms of amygdala modulation of hippocampal plasticity

Basolateral amygdala (BLA) activation by emotional arousal modulates memory-related processes in the hippocampus. We have shown (Akirav and Richter-Levin, 1999b) that activating the BLA before perforant path (PP) tetanization has a biphasic effect on hippocampal plasticity; priming the BLA immediately before PP tetanization results in the enhancement of dentate gyrus (DG) long-term potentiation (LTP) (an "emotional tag"), whereas stimulation in a spaced interval results in the suppression of DG-LTP. Here, we aimed to elucidate the mechanisms underlying BLA modulation of DG-LTP and specifically to examine whether the stress hormones norepinephrine (NE) and corticosterone (CORT) are main mediators of the BLA biphasic effects. We found that the BLA affects hippocampal plasticity in a complex manner; BLA priming enhanced DG-LTP, and both NE and CORT mediated this effect. Furthermore, we found that ipsilateral BLA spaced activation (2 hr before PP tetanization) suppressed DG-LTP and that this suppressive effect was also mediated by NE and CORT. Priming the contralateral BLA enhanced DG-LTP similarly to the ipsilateral enhancement, but neither NE nor CORT mediated this effect. The spaced activation of the contralateral BLA did not suppress DG-LTP. Taken together, these results suggest that differential mechanisms underlie the ipsilateral and contralateral BLA effects on hippocampal plasticity. Hence, the BLA modulates hippocampal memory processes, presumably via the mediation of the stress hormones NE and CORT, to establish a diverse memory of the experience. Possibly, at the onset of an emotional event the stress hormones permissively mediate plasticity. However, their prolonged presence in the system may suppress the cognitive response to stress.

Dorsal and ventral hippocampal cholinergic systems modulate anxiety in the plus-maze and shock-probe tests

There is emerging evidence that increased acetylcholine levels in brain reduce anxiety. More specifically there is evidence that some of these anxiolytic effects of acetylcholine are modulated by the hippocampus. In the present study we examined the roles of the cholinergic systems in the dorsal and ventral hippocampus in two animal models of anxiety: the elevated plus-maze and the shock-probe burying tests. We found that microinfusions (10 microg/0.5 microl) of the acetylcholinesterase inhibitor physostigmine in either the dorsal or the ventral hippocampus increased rats' open arm exploration in the plus-maze test, and decreased burying behavior in the shock-probe test. Interestingly, infusions in the ventral, but not the dorsal hippocampus also increased the number of contacts rats made with the shock-probe. Overall, the results suggest that cholinergic stimulation in the dorsal and ventral hippocampus modulate anxiety, but that only the ventral hippocampal cholinergic system is involved in the passive avoidance of painful stimuli.

Effects of estrogens on choline-acetyltransferase immunoreactivity and GAP-43 mRNA in the forebrain of young and aging male rats

1. Previous work demonstrated that estradiol (E2) treatment prevented the abnormal response to stress and the reduction of glucocorticoid receptors (GR) in hippocampus from aging male rats. The mechanisms originating these effects were unknown. 2. In the present work, we investigated the E2 effects on the cholinergic, growth-associated protein (GAP-43) expressing neurons of the medial septum (MS) and vertical limb of diagonal band of Broca (VDB). These areas project to the hippocampus, and may be involved in the mentioned E2 effects in aging animals. Therefore, the response to E2 of choline-acetyltransferase (ChAT) in neurons and cell processes and GAP-43 mRNA as a marker of neurite outgrowth was studied in young and old male rats. 3. Young (3-4 months) and old (18-20 months) male Sprague-Dawley rats remained untreated or were implanted s.c. with a 14 mg pellet of E2 benzoate during 6 weeks. We used immoucytochemistry to determine ChAT and isotopic in situ hybridization to analyze GAP-43 mRNA expression. 4. Aging males showed a reduction in the number and length of ChAT-immunoreactive cell processes, but not in the number of positive neurons in MS and VDB. E2 reverted both parameters in old rats to levels of young animals. Regarding basal levels of GAP-43 mRNA, they were similar in old and young animals, but E2 treatment up-regulated GAP-43 mRNA expression in MS and VDB of old animals only. 5. Our data suggest that prolonged E2 treatment may affect hippocampal function of aging male rats by regulating in part the plasticity of cholinergic, GAP-43 expressing neurones of the basal forebrain. Without discarding a direct E2 effect on the limbic tissue, effects on the cholinergic system may have a pronounced impact on the neuroendocrine and stress responses of the aging hippocampus.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

Glucocorticoid hypersecretion following intracerebroventricular injection of ethylcholine mustard aziridinium ion in rats

To investigate whether cholinergic hypofunctions in the brain influence hypothalamic-pituitary-adrenal activity, we examined the effects of cholinergic neurotoxin ethylcholine mustard aziridinium ion on basal and stress-induced levels of corticosterone in rats. Blood sampling from rats following intracerebroventricular injection of saline (5 microl, as a control) or this neurotoxin (5 nmol/5 microl) was performed over a day in one series, and was taken before, during and after an immobilization stress exposure in another series. Plasma levels of corticosterone and adrenocorticotropin were determined by the radioimmunoassay. The basal levels of plasma corticosterone and adrenocorticotropin over a day were significantly higher in the neurotoxin-treated rats, compared with the control rats (corticosterone, P<0.001; adrenocorticotropin, P<0.05). Further, relative adrenal gland weight of the neurotoxin-treated rats was significantly greater than that of the control rats (P<0.05). However, responses in plasma corticosterone level caused by the immobilization stress in the neurotoxin-treated rats were not different from those in the control rats. The present study demonstrated that damage to the cholinergic neurons in the brain increased hypothalamic-pituitary-adrenal activity over a day, probably due to freedom from inhibitory influences of the hippocampal cholinergic system, but that this damage did not influence stress-induced changes in plasma glucocorticoid level.

Sexual diergism in rat hypothalamic-pituitary-adrenal axis responses to cholinergic stimulation and antagonism

The hypothalamic-pituitary-adrenal (HPA) axis has differential physiological activity in male and female animals (sexual diergism). Central cholinergic systems stimulate this endocrine axis. In the present study we investigated muscarinic and nicotinic cholinergic influences on HPA axis activity in male and female rats by pretreatment with selective cholinergic receptor antagonists followed by stimulation with physostigmine (PHYSO), an acetylcholinesterase inhibitor. Hormonal measures were plasma arginine vasopressin (AVP), adrenocorticotropic hormone (ACTH), and corticosterone (CORT). Male rats had significantly greater AVP and ACTH responses to PHYSO alone than did females. Scopolamine (SCOP) enhanced the AVP response to PHYSO to a greater extent in males than in females. In contrast, mecamylamine (MEC) enhanced the AVP response in females but decreased it in males. SCOP potentiated, and MEC inhibited, the stimulatory effect of PHYSO on ACTH in both sexes, but SCOP potentiation was greater in males, and MEC inhibition was greater in females. Absolute CORT increases following PHYSO were greater in females, but percent increases over baseline were greater in males. Similar to their effects on ACTH responses, MEC attenuated, and SCOP enhanced, CORT responses to PHYSO. These results suggest that cholinergic receptor subtypes may influence HPA axis activity differentially in male and female rats.

Does the insulin-like growth factor system interact with prostaglandins and proinflammatory cytokines during neurodegeneration?

Prostaglandins and proinflammatory cytokines are implicated in the etiology of neurodegenerative diseases, such as Alzheimer's disease. Signaling cascades initiated by these factors may result in reactive oxygen species generation and cell death. The insulin-like growth factors (IGF) are ubiquitous polypeptides involved in all aspects of growth and development. Additionally, the IGF are regarded as survival factors that display potent antiapoptotic activity. Interfering with IGF production, distribution, or signaling may result in greater susceptibility to apoptotic stimuli. In neurodegenerative conditions, the IGF appear to be antagonized by prostaglandins and proinflammatory cytokines. In this review, the relationship among specific prostaglandins, the proinflammatory factors, tumor necrosis factor, interleukin-1, and interleukin-6, and the IGF system will be investigated.

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.

Changes in hippocampal theta following intrahippocampal corticotropin-releasing hormone (CRH) infusions in the rat

Hippocampal theta activity is a large amplitude, sinusoidal wave that occurs during attentive immobility and exploratory behaviour in the rat, and it is thought to be involved in memory formation. Recent reports suggest that corticotropin-releasing hormone (CRH) has pro-mnemonic effects in rodents. Because memory-enhancing substances/manipulations generally alter either theta frequencies or amplitudes, these variables were monitored in urethane-anaesthetised rats following intrahippocampal infusions of CRH. Adult male, Lister hooded rats were implanted with a hippocampal recording electrode and a guide cannula, both aimed at the dentate gyrus. When CRH was infused into the hippocampus, the main change in the hippocampal EEG was a slow onset increase in the amplitude of spontaneous theta and, paradoxically, a significant decrease in the amount of time spent displaying theta. These data suggest that CRH has the ability to modulate ongoing hippocampal theta, but, considering the slow effect, the involvement of hippocampal CRH receptors is suspect. Regardless of locus, the described electrophysiological changes suggest that hippocampal cholinergic systems may play a role in the memory-enhancing effects of CRH.

Regulation of hippocampal theta activity by corticosterone: opposing functions of mineralocorticoid and glucocorticoid receptors

Recently, we reported that intrahippocampal cholinergic blockade increased corticosterone (CORT) and adrenocorticotrophin (ACTH) secretion induced by restraint stress. These data suggested to us that CORT may modify hippocampal cholinergic function as part of the negative-feedback control of hypothalamic-pituitary-adrenal (HPA) axis activity. Hippocampal cholinergic theta is a rhythmic, sinusoidal waveform that occurs in alert, immobile rats presented with threatening stimuli and is reliably expressed in urethanized rats. We reasoned that if hippocampal cholinergic systems regulate HPA axis activity, perhaps CORT acts to modulate theta activity. In the present study we have examined the effects of blocking mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) on theta activity in urethane-anesthetized rats. Adult male, Lister hooded rats (n=15) were anesthetized with urethane, and a theta recording electrode was positioned in the hippocampus adjacent to an infusion cannula. A bipolar stimulating electrode was placed in the dorsomedial posterior hypothalamus (DMPH) to activate theta. Baseline recordings of DMPH-stimulated activity (0.1-0.5 mA) were obtained. Rats were then administered either the MR antagonist spironolactone or the GR antagonist RU 38486 (150 ng), and DMPH-stimulated activities were monitored for 45 min. Changes in theta frequency (Hz) and amplitude (mV; energy at peak theta frequency) were analyzed using analysis of variance (ANOVA) followed by Bonferroni t-tests. Neither drug affected hippocampal theta frequencies elicited by DMPH stimulation. However, GR blockade produced marked increases in theta amplitudes of approximately 100% above predrug levels. Alternatively, MR blockade produced exactly the opposite response, as amplitude values fell to approximately 50% of predrug levels. Hippocampal cholinergic theta activity is modulated by CORT acting through MR and GR, and the rapidity of the response suggests a nongenomic mechanism. These data raise the possibility that hippocampal cholinergic systems, and theta activity, are involved in CORT-mediated negative-feedback control of the HPA axis.