Serotonin and acetylcholine are crucial to maintain hippocampal synapses and memory acquisition in rats

Molecules Affecting Brain Development and Nervous System

Brain development is the biological process through which neurons are produced [...].

Effects of Smart Drugs on Cholinergic System and Non-Neuronal Acetylcholine in the Mouse Hippocampus: Histopathological Approach

In recent years, people in the United States and other countries have been using smart drugs, called nootropic or cognitive enhancers, to improve concentration and memory learning skills. However, these drugs were originally prescribed for attention-deficit hyperactivity disorder and dementia, and their efficacy in healthy people has not yet been established. We focused on acetylcholine in the hippocampus, which is responsible for memory learning, and elucidate the long-term effects of smart drugs on the neural circuits. Smart drugs were administered orally in normal young mice for seven weeks. The hippocampus was sectioned and compared histologically by hematoxylin and eosin (HE) staining, immunohistochemistry for acetylcholine, and immunoelectron microscopy. There were no significant changes in acetylcholinesterase staining. However, in HE, we found perivascular edema, and choline acetyltransferase staining showed increased staining throughout the hippocampus and new signal induction in the perivascular area in the CA3, especially in the aniracetam and α-glyceryl phosphoryl choline group. Additionally, new muscarinic acetylcholine receptor signals were observed in the CA1 due to smart drug intake, suggesting that vasodilation might cause neuronal activation by increasing the influx of nutrients and oxygen. Moreover, these results suggest a possible new mechanism of acetylcholine-mediated neural circuit activation by smart drug intake.

Oral dysfunctions and cognitive impairment/dementia

With global increases in the aging population, the number of patients with dementia is greatly increasing, which has become a big social problem. Many studies have shown strong associations between oral disorders and systemic disorders, such as diabetes, arthritis, sepsis, aspiration pneumonia, arteriosclerosis, bacterial endocarditis, and other cardiovascular diseases. Similarly, numerous cross-sectional studies showed that patients with dementia usually have poor oral conditions and tooth loss. These have long been considered as a result of difficulty with oral care due to impaired cognitive function, memory, and physical ability in patients with dementia. Indeed, even in patients with mild cognitive impairment, oral care becomes insufficient owing to decreases in spontaneity of grooming and finger dexterity. However, recent studies have shown that tooth loss and occlusal dysfunction may affect brain function and trigger the onset of dementia found in neurodegenerative diseases including Alzheimer's disease. In this review, we highlight the relationships among aging, oral dysfunction, and the development of dementia. Increasing evidence suggests that oral dysfunction is not only a result of dementia in the elderly people, but could also be a causative factor for the onset of dementia.

Importance of Proactive Treatment of Depression in Lewy Body Dementias: The Impact on Hippocampal Neurogenesis and Cognition in a Post-Mortem Study

OBJECTIVE

To examine the impact of selective serotonin reuptake inhibitors (SSRIs) and depression on neurogenesis and cognition in dementia with Lewy bodies (DLB) and Parkinson disease dementia (PDD).

METHODS

Late-stage progenitor cells were quantified in the subgranular zone (SGZ) of the hippocampal dentate gyrus of DLB/PDD patients (n = 41) and controls without dementia (n = 15) and compared between treatment groups (unmedicated, SSRIs, acetyl cholinesterase inhibitors [AChEIs], combined SSRIs and AChEIs).

RESULTS

DLB/PDD patients had more doublecortin-positive cells in the SGZ compared to controls. The doublecortin-positive cell count was higher in the SGZ of patients treated with SSRIs and correlated to higher cognitive scores.

CONCLUSION

SSRI treatment was associated with increased hippocampal neurogenesis and preservation of cognition in DLB/PDD patients.

Hippocampal 5-HT Input Regulates Memory Formation and Schaffer Collateral Excitation

The efficacy and duration of memory storage is regulated by neuromodulatory transmitter actions. While the modulatory transmitter serotonin (5-HT) plays an important role in implicit forms of memory in the invertebrate Aplysia, its function in explicit memory mediated by the mammalian hippocampus is less clear. Specifically, the consequences elicited by the spatio-temporal gradient of endogenous 5-HT release are not known. Here we applied optogenetic techniques in mice to gain insight into this fundamental biological process. We find that activation of serotonergic terminals in the hippocampal CA1 region both potentiates excitatory transmission at CA3-to-CA1 synapses and enhances spatial memory. Conversely, optogenetic silencing of CA1 5-HT terminals inhibits spatial memory. We furthermore find that synaptic potentiation is mediated by 5-HT4 receptors and that systemic modulation of 5-HT4 receptor function can bidirectionally impact memory formation. Collectively, these data reveal powerful modulatory influence of serotonergic synaptic input on hippocampal function and memory formation.

Autonomic activity during sleep predicts memory consolidation in humans

Throughout history, psychologists and philosophers have proposed that good sleep benefits memory, yet current studies focusing on the relationship between traditionally reported sleep features (e.g., minutes in sleep stages) and changes in memory performance show contradictory findings. This discrepancy suggests that there are events occurring during sleep that have not yet been considered. The autonomic nervous system (ANS) shows strong variation across sleep stages. Also, increases in ANS activity during waking, as measured by heart rate variability (HRV), have been correlated with memory improvement. However, the role of ANS in sleep-dependent memory consolidation has never been examined. Here, we examined whether changes in cardiac ANS activity (HRV) during a daytime nap were related to performance on two memory conditions (Primed and Repeated) and a nonmemory control condition on the Remote Associates Test. In line with prior studies, we found sleep-dependent improvement in the Primed condition compared with the Quiet Wake control condition. Using regression analyses, we compared the proportion of variance in performance associated with traditionally reported sleep features (model 1) vs. sleep features and HRV during sleep (model 2). For both the Primed and Repeated conditions, model 2 (sleep + HRV) predicted performance significantly better (73% and 58% of variance explained, respectively) compared with model 1 (sleep only, 46% and 26% of variance explained, respectively). These findings present the first evidence, to our knowledge, that ANS activity may be one potential mechanism driving sleep-dependent plasticity.

Molecular and Functional Characterization of Bacopa monniera: A Retrospective Review

Over the last 50 years, laboratories around the world analyzed the pharmacological effect of Bacopa monniera extract in different dimensions, especially as a nerve tonic and memory enhancer. Studies in animal model evidenced that Bacopa treatment can attenuate dementia and enhances memory. Further, they demonstrate that Bacopa primarily either acts via antioxidant mechanism (i.e., neuroprotection) or alters different neurotransmitters (serotonin (5-hydroxytryptamine, 5-HT), dopamine (DA), acetylcholine (ACh), γ-aminobutyric acid (GABA)) to execute the pharmacological effect. Among them, 5-HT has been shown to fine tune the neural plasticity, which is a substrate for memory formation. This review focuses on the studies which trace the effect of Bacopa treatment on serotonergic system and 5-HT mediated key molecular changes that are associated with memory formation.

REM sleep rescues learning from interference

Classical human memory studies investigating the acquisition of temporally-linked events have found that the memories for two events will interfere with each other and cause forgetting (i.e., interference; Wixted, 2004). Importantly, sleep helps consolidate memories and protect them from subsequent interference (Ellenbogen, Hulbert, Stickgold, Dinges, & Thompson-Schill, 2006). We asked whether sleep can also repair memories that have already been damaged by interference. Using a perceptual learning paradigm, we induced interference either before or after a consolidation period. We varied brain states during consolidation by comparing active wake, quiet wake, and naps with either non-rapid eye movement sleep (NREM), or both NREM and REM sleep. When interference occurred after consolidation, sleep and wake both produced learning. However, interference prior to consolidation impaired memory, with retroactive interference showing more disruption than proactive interference. Sleep rescued learning damaged by interference. Critically, only naps that contained REM sleep were able to rescue learning that was highly disrupted by retroactive interference. Furthermore, the magnitude of rescued learning was correlated with the amount of REM sleep. We demonstrate the first evidence of a process by which the brain can rescue and consolidate memories damaged by interference, and that this process requires REM sleep. We explain these results within a theoretical model that considers how interference during encoding interacts with consolidation processes to predict which memories are retained or lost.

Sex differences in sleep-dependent perceptual learning

Sex differences in learning and memory suggest differences between men and women in mechanisms of neural plasticity. Such differences have been reported in a variety of explicit memory tasks, but implicit memory has not been studied in this context. We investigated differences between men and women in offline consolidation of perceptual learning (PL) of motion direction discrimination. Initially, discrimination thresholds were measured for two opposite directions of motion, followed by approximately 40 minutes of training on one of the directions. During a post-training consolidation period, subjects either took a nap or remained awake. Thresholds were then reassessed for both directions of motion. We found that rapid eye movement (REM) sleep facilitates consolidation of PL but that the pattern of specificity in the REM condition differed between men and women. PL for men whose naps contained REM sleep was highly specific to the trained direction of motion, whereas REM sleep in women resulted in generalized learning to the untrained direction as well as to a novel direction that was not previously tested. Moreover, for subjects in the REM condition, men exhibited greater PL than women for the trained direction. Our findings provide the first evidence of sex differences in the magnitude and specificity of PL and in the role of REM sleep in implicit learning. Our results have important implications for optimization of educational and training strategies designed for males and females.

The benefit of offline sleep and wake for novel object recognition

How do we segment and recognize novel objects? When explicit cues from motion and color are available, object boundary detection is relatively easy. However, under conditions of deep camouflage, in which objects share the same image cues as their background, the visual system must reassign new functional roles to existing image statistics in order to group continuities for detection and segmentation of object boundaries. This bootstrapped learning process is stimulus dependent and requires extensive task-specific training. Using a between-subject design, we tested participants on their ability to segment and recognize novel objects after a consolidation period of sleep or wake. We found a specific role for rapid eye movement (REM, n = 43) sleep in context-invariant novel object learning, and that REM sleep as well as a period of active wake (AW, n = 35) increased segmentation of context-specific object learning compared to a period of quiet wake (QW, n = 38; p = .007 and p = .017, respectively). Performance in the non-REM nap group (n = 32) was not different from the other groups. The REM sleep enhancement effect was especially robust for the top performing quartile of subjects, or "super learners" (p = .037). Together, these results suggest that the construction and generalization of novel representations through bootstrapped learning may benefit from REM sleep, and more specific object learning may also benefit from AW. We discuss these results in the context of shared electrophysiological and neurochemical features of AW and REM sleep, which are distinct from QW and non-REM sleep.

Vortioxetine, but not escitalopram or duloxetine, reverses memory impairment induced by central 5-HT depletion in rats: evidence for direct 5-HT receptor modulation

Depressed patients suffer from cognitive dysfunction, including memory deficits. Acute serotonin (5-HT) depletion impairs memory and mood in vulnerable patients. The investigational multimodal acting antidepressant vortioxetine is a 5-HT3, 5-HT7 and 5-HT1D receptor antagonist, 5-HT1B receptor partial agonist, 5-HT1A receptor agonist and 5-HT transporter (SERT) inhibitor that enhances memory in normal rats in novel object recognition (NOR) and conditioned fear (Mørk et al., 2013). We hypothesized that vortioxetine's 5-HT receptor mechanisms are involved in its memory effects, and therefore investigated these effects in 5-HT depleted rats. Four injections of the irreversible tryptophan hydroxylase inhibitor 4-chloro-dl-phenylalanine methyl ester hydrochloride (PCPA, 86mg/kg, s.c.) induced 5-HT depletion, as measured in hippocampal homogenate and microdialysate. The effects of acute challenge with vortioxetine or the 5-HT releaser fenfluramine on extracellular 5-HT were measured in PCPA-treated and control rats. PCPA's effects on NOR and spontaneous alternation (SA) performance were assessed along with the effects of acute treatment with 5-hydroxy-l-tryptophan (5-HTP), vortioxetine, the selective 5-HT reuptake inhibitor escitalopram, or the 5-HT norepinephrine reuptake inhibitor duloxetine. SERT occupancies were estimated by ex vivo autoradiography. PCPA depleted central 5-HT by >90% in tissue and microdialysate, and impaired NOR and SA performance. Restoring central 5-HT with 5-HTP reversed these deficits. At similar SERT occupancies (>90%) vortioxetine, but not escitalopram or duloxetine, restored memory performance. Acute fenfluramine significantly increased extracellular 5-HT in control and PCPA-treated rats, while vortioxetine did so only in control rats. Thus, vortioxetine restores 5-HT depletion impaired memory performance in rats through one or more of its receptor activities.

Galanthamine, an anticholinesterase drug, effects plant growth and development in Artemisia tridentate Nutt. via modulation of auxin and neutrotransmitter signaling

Galanthamine is a naturally occurring acetylcholinesterase (AchE) inhibitor that has been well established as a drug for treatment of mild to moderate Alzheimer disease, but the role of the compound in plant metabolism is not known. The current study was designed to investigate whether galanthamine could redirect morphogenesis of Artemisia tridentata Nutt. cultures by altering concentration of endogenous neurosignaling molecules acetylcholine (Ach), auxin (IAA), melatonin (Mel), and serotonin (5HT). Exposure of axenic A. tridentata cultures to 10 µM galanthamine decreased the concentration of endogenous Ach, IAA, MEL, and AchE, and altered plant growth in a manner reminiscent of 2-4D toxicity. Galanthamine itself demonstrated IAA activity in an oat coleotile elongation bioassay, 20 µM galanthamine showed no significant difference compared with 5 μM IAA or 5 μM 1-Naphthaleneacetic acid (NAA). Metabolomic analysis detected between 20,921 to 27,891 compounds in A. tridentata plantlets and showed greater commonality between control and 5 µM treatments. Furthermore, metabolomic analysis putatively identified coumarins scopoletin/isoscopoletin, and scopolin in A. tridentata leaf extracts and these metabolites linearly increased in response to galanthamine treatments. Overall, these data indicate that galanthamine is an allelopathic phytochemical and support the hypothesis that neurologically active compounds in plants help ensure plant survival and adaptation to environmental challenges.

Vortioxetine dose-dependently reverses 5-HT depletion-induced deficits in spatial working and object recognition memory: a potential role for 5-HT1A receptor agonism and 5-HT3 receptor antagonism

We previously reported that the investigational multimodal antidepressant, vortioxetine, reversed 5-HT depletion-induced memory deficits while escitalopram and duloxetine did not. The present report studied the effects of vortioxetine and the potential impact of its 5-HT1A receptor agonist and 5-HT3 receptor antagonist properties on 5-HT depletion-induced memory deficits. Recognition and spatial working memory were assessed in the object recognition (OR) and Y-maze spontaneous alternation (SA) tests, respectively. 5-HT depletion was induced in female Long-Evans rats using 4-cholro-DL-phenylalanine methyl ester HCl (PCPA) and receptor occupancies were determined by ex vivo autoradiography. Rats were acutely dosed with vortioxetine, ondansetron (5-HT3 receptor antagonist) or flesinoxan (5-HT1A receptor agonist). The effects of chronic vortioxetine administration on 5-HT depletion-induced memory deficits were also assessed. 5-HT depletion reliably impaired memory performance in both the tests. Vortioxetine reversed PCPA-induced memory deficits dose-dependently with a minimal effective dose (MED) ≤0.1mg/kg (∼80% 5-HT3 receptor occupancy; OR) and ≤3.0mg/kg (5-HT1A, 5-HT1B, 5-HT3 receptor occupancy: ∼15%, 60%, 95%) in SA. Ondansetron exhibited a MED ≤3.0μg/kg (∼25% 5-HT3 receptor occupancy; OR), but was inactive in the SA test. Flesinoxan had a MED ≤1.0mg/kg (∼25% 5-HT1A receptor occupancy; SA); only 1.0mg/kg ameliorated deficits in the NOR. Chronic p.o. vortioxetine administration significantly improved memory performance in OR and occupied 95%, 66%, and 9.5% of 5-HT3, 5-HT1B, and 5-HT1A receptors, respectively. Vortioxetine's effects on SA performance may involve 5-HT1A receptor agonism, but not 5-HT3 receptor antagonism, whereas the effects on OR performance may involve 5-HT3 receptor antagonism and 5-HT1A receptor agonism.

Impaired hippocampal acetylcholine release parallels spatial memory deficits in Tg2576 mice subjected to basal forebrain cholinergic degeneration

The Alzheimer's disease (AD) mouse model Tg2576 overexpresses an AD associated mutant variant of human APP and accumulates amyloid beta (Aβ) in an age-dependent manner. Using the selective cholinergic immunotoxin mu p75-saporin (SAP), we induced a partial basal forebrain cholinergic degeneration (BFCD) in 3 months old male Tg2576 mice to co-express cholinergic degeneration with Aβ overexpression as these characteristics constitutes key hallmarks of AD. At 9 months, SAP lesioned Tg2576 mice were cognitively impaired in two spatial paradigms addressing working memory and mid to long-term memory. Conversely, there was no deterioration of cognitive functioning in sham lesioned Tg2576 mice or wild type littermates (wt) receiving the immunotoxin. At 10 months of age, release of acetylcholine (ACh) was addressed by microdialysis in conscious mice. Scopolamine-induced increases in hippocampal ACh efflux was significantly reduced in SAP lesioned Tg2576 mice compared to sham lesioned Tg2576 mice. Intriguingly, there was no significant difference in ACh efflux between wt treatment groups. Following SAP treatment, choline acetyltransferase activity was reduced in the hippocampus and frontal cortex and the reduction was comparable between groups. Our results suggest that partial BFCD acts collectively with increased levels of Aβ to induce cognitive decline and to compromise cholinergic release. Tg2576 mice with BFCD may constitute a new and suitable AD mouse model to study the interrelations between cholinergic deficits and amsyloid deposition.

Brain enhancing ingredients from Āyurvedic medicine: quintessential example of Bacopa monniera, a narrative review

Āyurveda, the science (ved) of life (ayu), owing its origin to Veda, the oldest recorded wisdom of human civilization written in 3500 BCE, contains extensive knowledge of various diseases and their therapeutic approaches. It essentially relied on nature and the immune system of an individual, and therapeutic interventions were introduced only to augment the immune system. Āyurveda had eight specialties, including psycho-neuroscience (a combination of psychology, clinical psychology and psychiatry) and a unique promotive therapy encompassing nutrition, rejuvenation and geriatrics. The symptoms of various brain disorders, including memory disorder, were well defined. The goal of Āyurveda was to help an individual to achieve his cherished goal of leading a healthy life of 100 years. To achieve this, great emphasis was laid on nutrition, diet and a good conduct by the two great exponents of Āyurveda viz. Carak and Suśruta. By following these regimens, an individual could lead a less stressful life free from emotional disturbances. Both Carak and Suśruta had believed that these in combination with rasayana (rejuvenating) plants could enable an individual to lead a healthy life of 100 years.

c-Fos activity mapping reveals differential effects of noradrenaline and serotonin depletion on the regulation of ocular dominance plasticity in rats

The roles of the central noradrenergic and serotonergic system in the activity-dependent regulation of ocular dominance plasticity have been a contentious issue. Using c-Fos activity mapping, we have developed a new, straightforward method to measure the strength of ocular dominance plasticity: the number of c-Fos-immunopositive cells in layer IV of rat visual cortex (Oc1B), ipsilateral to the stimulated eye, is a sensitive and reliable measure of the effects of monocular deprivation. Applying this new method, here we studied the unique modification of the degree of c-Fos expression induced in the visual cortex, in that endogenous noradrenaline (NA) and serotonin (5HT) in the cortex were significantly reduced, respectively by specific pharmacological agents. Intraperitoneal injections of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) and p-chlorophenylalanine (pCPA) selectively impair NA- and 5HT-containing nerve terminals and fibers, respectively. In the visual cortex with strongly reduced NA, the number of c-Fos-immunopositive cells was found remaining significantly decreased in response to stimulation of the deprived eye, while by open eye stimulation the expected increase in c-Fos-immunoreactivity was strongly suppressed, showing values not different from those obtained by monocular stimulation in the normal rats. In contrast, in the visual cortex with strongly reduced 5HT no expected decrease was found in response to stimulation of the deprived eye, while, as is usually the case for the normal animals, a significant increase was still induced in response to open eye stimulation. These findings suggest that the noradrenergic and serotonergic system regulate ocular dominance (OD) plasticity differently: in the NA-depleted cortex the expected increase in c-Fos expression by open eye stimulation was not seen due to strong suppression, whereas in 5HT-depletion, the expected decrease in c-Fos expression was not materialized due to strong suppression. The present findings with c-Fos activity mapping method indicated a novel possibility of the differential regulation of OD plasticity by two types of common monoaminergic systems.

Attenuation of 1-(m-chlorophenyl)-biguanide induced hippocampus-dependent memory impairment by a standardised extract of Bacopa monniera (BESEB CDRI-08)

Bacopa monniera is a well-known medhya-rasayana (memory enhancing and rejuvenating) plant in Indian traditional medical system of Ayurveda. The effect of a standardized extract of Bacopa monniera (BESEB CDRI-08) on serotonergic receptors and its influence on other neurotransmitters during hippocampal-dependent learning was evaluated in the present study. Wistar rat pups received a single dose of BESEB CDRI-08 during postnatal days 15-29 showed higher latency during hippocampal-dependent learning accompanied with enhanced 5HT(3A) receptor expression, serotonin and acetylcholine levels in hippocampus. Furthermore, 5HT(3A) receptor agonist 1-(m-chlorophenyl)-biguanide (mCPBG) impaired learning in the passive avoidance task followed by reduction of 5HT(3A) receptor expression, 5HT and ACh levels. Administration of BESEB CDRI-08 along with mCPBG attenuated mCPBG induced behavioral, molecular and neurochemical alterations. Our results suggest that BESEB CDRI-08 possibly acts on serotonergic system, which in turn influences the cholinergic system through 5-HT(3) receptor to improve the hippocampal-dependent task.

Administration of serotonin inhibitor p-Chlorophenylalanine induces pessimistic-like judgement bias in sheep

Judgement bias has potential as a measure of affective state in animals. The serotonergic system may be one mechanism involved with the formation of negative judgement biases. It was hypothesised that depletion of brain serotonin would induce negative judgement biases in sheep. A dose response trial established that 40 mg/kg of p-Chlorophenylalanine (pCPA) administered to sheep for 3 days did not affect feeding motivation or locomotion required for testing judgement biases. Thirty Merino ewes (10 months old) were trained to an operant task for 3 weeks. Sheep learnt to approach a bucket when it was placed in one corner of the testing facility to receive a feed reward (go response), and not approach it when in the alternate corner (no-go response) to avoid a negative reinforcer (exposure to a dog). Following training, 15 sheep were treated with pCPA (40 mg/kg daily) for an extended duration (5 days). Treated and control sheep were tested for judgement bias following 3 and 5 days of treatment, and again 5 days after cessation of treatment. Testing involved the bucket being presented in ambiguous locations between the two learnt locations, and the response of the sheep (go/no-go) measured their judgement of the bucket locations. Following 5 days of treatment, pCPA-treated sheep approached the most positive ambiguous location significantly less than control sheep, suggesting a pessimistic-like bias (treatment × bucket location interaction F(1,124.6)=49.97, p=0.011). A trend towards a significant interaction was still evident 5 days after the cessation of pCPA treatment (p=0.068), however no significant interaction was seen on day 3 of testing (p=0.867). These results support the suggestion that judgement bias is a cognitive measure of affective state, and that the serotonergic pathway may be involved.

Cognitive dysfunction in neuropsychiatric disorders: selected serotonin receptor subtypes as therapeutic targets

The indolamine, serotonin (5-hydroxytryptamine-5-HT) was identified and initially characterized around the middle of the twentieth century and it is now known to participate in multiple physiologic processes in mammalians. As a neurotransmitter, 5-HT is well documented to play a significant role in the pathophysiology and treatment of a variety of psychiatric disorders including anxiety, depression, and schizophrenia. In addition, there is also some evidence to suggest that 5-HT function in the brain may be important (particularly in the behavioral disturbances) in various forms of dementia including Alzheimer's disease. While 5-HT is undoubtedly involved in cognitive function, its role in specific domains of cognition (attention, learning, and memory, etc.) is poorly understood. This understanding has been impeded to some extent by the many complex interactions between 5-HT neurons and other neuronal phenotypes, 5-HT receptor heterogeneity, and the conflicting results of some behavioral experiments in animals conducted to date. Through the combined use of modern molecular biology, transgenic animal models, and other more traditional research methods such as medicinal chemistry and classical pharmacology, a clearer picture of the role of serotonin and its receptor subtypes in mnemonic processes is beginning to emerge, however. Considerable data now support the argument that selective ligands at specific 5-HT receptor subtypes can serve as therapeutic agents designed to enhance cognitive function in psychiatric disorders such as schizophrenia as well as age-related neurodegenerative illnesses such as Alzheimer's disease. The purpose of this review is to provide a brief overview of these therapeutic targets within the 5-HT system and the pharmacologic approaches (including the most recently developed compounds) designed to enhance memory function.

Brain serotonin depletion impairs short-term memory, but not long-term memory in rats

Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) (150 microg; 4.5 microl/ventricle), a serotonergic neurotoxin, significantly decreased spontaneous alternation in Y-maze task and working memory in radial 8 arm-maze task, suggesting effects on short-term memory, without affecting long-term memory, explored by reference memory in radial 8 arm-maze task and step-through latency in multi-trial passive avoidance task. Parachlorophenylalanine (PCPA) (3 days treatment 200 microg, i.c.v.), a serotonin synthesis inhibitor, did not impair step-through-latency in multi-trial passive avoidance task, suggesting no effects on long-term memory. These results suggest that serotonin, among other neurotransmitters, play an important role in cognitive functions, especially short-term memory.

Identification of adrenoceptor subtype-mediated changes in the density of synapses in the rat visual cortex

Both serotonin and noradrenaline affect synapse formation and maintenance in the CNS. Although we previously demonstrated that serotonin regulates synaptic density via activation of serotonin(2A) receptor, it was still unclear which receptor subtype mediates the function of noradrenaline. In the present study we tried to identify the noradrenaline receptor (adrenoceptor) subtype, which could regulate the density of synapses in the rat visual cortex. Selective antagonists and/or agonists of adrenoceptor subtypes were administered to six weeks old rats. Changes in the density of axodendritic synapses were quantitatively examined in lamina I, where noradrenaline rather than serotonin is known to regulate the density of synapses. The alpha1 adrenoceptor antagonists (prazosin and 2-{[b-(4-hydroxyphenyl)ethyl]aminomethyl}-1-tetralone) decreased the number of synapses in a dose-dependent manner. In contrast, administrations of the alpha1-agonist (methoxamine) increased the density of synapses. The beta1 adrenoceptor antagonist (atenolol) had no effect on the density of synapses. The alpha2-antagonist (rauwolscine) increased synaptic density, whereas the beta2-antagonist (ICI-118,551) decreased synaptic density. Simultaneous treatments with the alpha1-antagonist and alpha1-agonist caused the alpha1-agonist to competitively block the effect of the alpha1-antagonist and recover the density of synapses to the control values. In addition, the alpha1-antagonist/agonist appeared to show a reverse effect on the changes in synaptic density following alpha2- or beta2-antagonist treatment by acting via the alpha1 receptor. Moreover, decreased synaptic density when a selective noradrenergic neurotoxin (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) was counterbalanced by the alpha1-agonist. These data suggest that noradrenaline regulates the density of synapses in the rat visual cortex primarily via the alpha1 receptor subtype. Both serotonin(2A) and alpha1 receptors are known to couple with phospholipase C, which has been shown to increase intracellular calcium. It may help us to understand the underlying mechanisms for synaptic plasticity in the CNS.

Electroacupuncture prevents cognitive deficits in pilocarpine-epileptic rats

Here we investigated the effects of electroacupuncture over the cognitive deficits in the pilocarpine model of epilepsy in rats. Acupuncture stimulation was provided at acupoints located in either the midline of the back and of the head [HD]: Gv-20 (Baihui), Gv-14 (Dazhui), Gv-2 (Yaoshu) and M-HN-3 (Yin Tang); or acupoints located in the limbs [LB]: St-36 (Zusanli) and Sp-6 (Sanyinjiao). In the elevated T-maze test, electroacupuncture at HD and LB acupoints produced an improvement in the acquisition and retention parameters. Retention in the inhibitory avoidance test was seen only in short-term retention and only for animals stimulated at HD. At histology it was found that electroacupuncture at HD acupoints abolished tissue shrinkage in dorsal hippocampus, basolateral nucleus of the amygdala, substantia nigra and perirhinal cortex, whereas stimulation of LB acupoints prevented tissue shrinkage in all of the above structures except dorsal hippocampus. Administration of p-chlorophenylalanine, a serotonergic releaser, abolished both behavioral and part of the histological changes in these animals. We conclude that electroacupuncture at HD and LB acupoints prevents atrophy of some limbic structures and improves cognitive deficits in pilocarpine-epileptic rats and that this effect is dependent on the serotonergic system.

Selective serotonin reuptake inhibitor treatment of early postnatal mice reverses their prenatal stress-induced brain dysfunction

Prenatal stress has long-lasting effects on cognitive function and on the hypothalamic-pituitary-adrenal response to stress. We previously reported that the serotonin concentration and synaptic density in the hippocampus were reduced following prenatal stress [Int J Dev Neurosci 16 (1998) 209]. Since serotonin plays a role in the formation and maintenance of synapses, we hypothesized that a neonatal reduction in hippocampal serotonin levels may lead to learning disabilities in prenatally stressed mice. To test this hypothesis, we treated prenatally stressed mice with a selective serotonin reuptake inhibitor in order to normalize their postnatal serotonin turnover levels. What we found was that the oral administration of a selective serotonin reuptake inhibitor to prenatally stressed mice during postnatal weeks 1-3 but not 6-8 normalized their corticosterone response to stress, serotonin turnover in the hippocampus, and density of dendritic spines and synapses in the hippocampal CA3 region. Concomitantly, such treatment partially restored their ability to learn spatial information.

Lamina-selective changes in the density of synapses following perturbation of monoamines and acetylcholine in the rat medial prefrontal cortex

The rat medial prefrontal cortex is known to have diverse brain functions such as learning and memory, attention, and behavioral flexibility. Although these functions are affected by monoamines (dopamine (DA), noradrenaline (NA) and serotonin (5-HT)) and acetylcholine (ACh), the detailed mechanisms remain unclear. These neuromodulators also have effects on synapse formation and maintenance, and regulate plasticity in the central nervous system (CNS). To clarify the effects of these neuromodulators on changes in the density of synapses in the rat medial prefrontal cortex, we separately administered a D1- or D2-antagonist, NA neurotoxin, 5-HT synthetic inhibitor, or muscarinic ACh antagonist for 1 week, and counted the number of synapses on electron microscopic photographs taken from the prelimbic area of the medial prefrontal cortex. The density of synapses in lamina I was regulated by DA via D1-like receptors, and that in laminae II/III was decreased by depletion of NA or ACh. However, 5-HT did not have a regulatory effect on the synaptic density throughout the layers in this brain region. The data in this study and our previous studies indicate that there are appreciable regional differences in the magnitude of biogenic amine-mediated synaptic plasticity in the rat CNS. These neuromodulators may have a trophic-like effect on the selected neuronal circuit to maintain synaptic contacts in the rat CNS. The synaptic density in the medial prefrontal cortex regulated by monoamines and ACh could be important not only for synaptic plasticity in this region but also for pharmacotherapeutic drug treatment.

Median raphe mediates estrogenic effects to the hippocampus in female rats

Subcortical regions such as the medial septum-diagonal band of Broca and supramammillary area have been shown to mediate indirect oestrogenic effects on hippocampal morphology and function. Here, the role of the median raphe (MR), a serotonergic subcortical structure, is studied. To this end, 17beta-estradiol-filled 30-gauge cannulae were implanted into the MR of female ovariectomized rats; cholesterol-filled cannulae served as controls. After seven days, using unbiased electron microscopic stereological calculations and semiquantitative analysis, the spine synapse density and surface density of glial fibrillary acidic protein-positive astrocyte processes, respectively, were determined in the stratum radiatum of the CA1 region of the hippocampus. Changes in the serotonergic innervation of the hippocampal CA1 region were determined by immunohistochemistry and subsequent morphometric analysis. In the stratum radiatum of the CA1 region, local estradiol application into the MR resulted in a 47% increase in spine synapse density. Simultaneously, the density of glial fibrillary acidic protein-positive fibers decreased by 16%. The density of serotonin (5-HT) innervation of the strata lacunosum moleculare and radiatum of the CA1 region of the hippocampus was reduced in response to estradiol, as shown by a decrease in the length of fibers (27.6 and 48.3% decrease, respectively) and the number of large varicosities (32.5 and 38.8% decrease, respectively). These observations suggest a major role of the MR in mediating oestrogenic effects on the hippocampus and an involvement of the serotonergic system.

Synaptic loss following depletion of noradrenaline and/or serotonin in the rat visual cortex: a quantitative electron microscopic study

Biogenic amines have a trophic-like role for the formation and the maintenance of synapses in the CNS. We examined the changes in the number of synaptic profiles in the developing and adult rat visual cortex following selective depletion of noradrenaline and/or serotonin. By the drug-induced decreases in levels of noradrenaline or serotonin between 1 and 2 weeks after birth, the number of synaptic profiles was decreased by 29-55% compared with that of control animals. The magnitude of reduction in the number of synaptic profiles was virtually the same following simultaneous depletion of both noradrenaline and serotonin compared with the depletion of noradrenaline or serotonin alone. Later in the developmental period, the function of noradrenaline and serotonin in facilitating synapse formation and maintenance became less prominent than that in younger animals. In the control animals, the number of axosomatic synapses was the highest at around 2 weeks after birth, and decreased with development. The number of axodendritic synapses was the highest between 2 and 7 weeks after birth, and decreased to 50% at 11 weeks after birth. These data demonstrate that synapses in the rat visual cortex are overproduced during the early developmental period. We suggest that both serotonin and noradrenaline are necessary for synapse formation during the early stages of development of the rat visual cortex.

FK960, a potential anti-dementia drug, increases synaptic density in the hippocampal CA3 region of aged rats

There is accumulating evidence suggesting that synapse formation in the adult brain is dynamically regulated, and that this regulation plays a role in cognitive function. A decrease in synaptic density is reportedly related to memory deficits in aged animals as well as in Alzheimer's patients. FK960 [N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate], a novel anti-dementia drug, has been shown to ameliorate experimental amnesia in rats and monkeys through activation of the somatostatinergic nervous system in the hippocampus. Furthermore, FK960 has been shown to be considerably more effective in a model of spontaneous amnesia in aged rats than cholinesterase inhibitors. In the present electron microscopy study, we demonstrated that the density of axodendritic and axosomatic synapses in the hippocampal CA3 region of aged rats was reduced compared to young rats, and that repeated treatment with FK960 for either 3 or 21 days dose-dependently reversed these deficits in aged rats. This FK960-induced increase in synaptic density was transient and density returned to basal levels at 8 days after the final dose. In contrast, FK960 did not alter synaptic density in the cingulate cortex or hippocampal CA1 region in aged rats, nor the CA3 region of young rats. Collectively, these results suggest that FK960 can selectively and reversibly increase synaptic density in the hippocampal CA3 region of aged rats, and that this activity may play a role in its cognitive-enhancing action.

Cajal's hypotheses on neurobiones and neurotropic factor match properties of microtubules and S-100 beta

Cajal described both the morphology and plasticity of neurons. He summarized the structure of neurons as composed of membrane, protoplasm, Golgi apparatus, nucleus, spongioplasm and neurofibrils (cytoskeleton). He initially considered the cytoskeleton as absorbing excitation energy and forming a "conductive pathway in the protoplasm" within the neuron. Later, he viewed the neurofibrillary threads as independent, living entities and called them neurobiones. Cajal recognized neuroplasticity in development, memory, sleep, injury and dementia, as well as after exposure to cold and starvation. He noted cytoskeletal changes during these events. However, he did not causatively connect the plastic changes in neurons with the changes in cytoskeleton. Finally, Cajal proposed a theory of chemoaffinity in 1892, and modified his neurotropic theory over the next 40 years. Today we accept that changes in the cytoskeleton produce changes in neuronal morphology. The properties of the cytoskeleton and neurobione as described by Cajal are similar to those of microtubules. These long intraneuronal neurofibrils are polymers of the protein tubulin and, whilst not being living entities, are highly dynamic, sensitive to environmental stimuli, and stabilized by microtubule associated proteins (MAPs). Furthermore, Cajal was very specific in his characterization of the neurotropic factor derived from Schwann cells. Initially, he thought the chemicals attracted the axonal fibers, but later he wrote that the factor was not attractant but rather was involved in assimilation, growth and ramifications. The neurotropic hypothesis described by Cajal in Degeneration and Regeneration in the Nervous System is more similar to a neurite extension factor (NEF) than to a neurotrophic growth factor or specific chemoaffinity (attractant) molecule. S-100 beta is the major NEF found in PNS Schwann cells and CNS astroglial cells. In summary, the views of Cajal on neuroplasticity, its frequency and function, agree with the modern hypothesis of neuronal instability. This concept states that MAPs regulate microtubule stability by a S-100 beta sensitive phosphorylation processes. Serotonin, by acting on the astroglial 5-HT1A receptor, releases S-100 beta and regulates neuronal morphology and apoptosis. This neuronal-glial connection provides a fresh view for linking neuroplasticity, mental illness, and memory with changes in the cytoskeleton.

Serotonin mediates CA1 spine density but is not crucial for ovarian steroid regulation of synaptic plasticity in the adult rat dorsal hippocampus

The activity of the serotonin (5-hydroxytryptamine, 5-HT) system is sensitive to estradiol and progesterone. During the ovarian cycle, dendritic spines on CA1 pyramidal neurons of the dorsal hippocampus are increased by estradiol and later decreased by progesterone. We sought to determine whether 5-HT is involved in maintaining CA1 spine density and/or in steroid regulation of synaptic plasticity in dorsal hippocampus. Ovariectomized rats were treated (sc) over 10 days with the tryptophan hydroxylase inhibitor parachlorophenylalanine (pCPA) to deplete 5-HT, followed by estradiol benzoate on days 10 and 11. A subset of animals received progesterone on day 12. The day after the last treatment, rats were perfused and brains were processed for Golgi impregnation. Separate groups were processed for radioimmunocytochemistry (RICC) for the spine-associated protein, spinophilin, or high-performance liquid chromatography (HPLC) for monoamine analysis. Golgi and RICC data indicate that CA1 apical spine density was significantly decreased by pCPA (17-20%). Estradiol increased spine density in both saline- and pCPA-treated rats compared to respective controls (30%); however, pCPA animals maintained significantly fewer spines. No differences in spine densities were observed between saline- and pCPA-treated rats given estradiol and progesterone. Depletion of 5-HT by pCPA was confirmed in the CA1 (-90%) and dorsal raphe (-80%) by HPLC analysis. While 5-HT depletion was associated with a 57% decrease in CA1 norepinephrine (NE), there was no difference in dorsal raphe NE. Thus, whereas 5-HT is involved in maintaining spine density in the adult female rat CA1, it is not crucial for steroid-mediated plasticity. 5-HT-regulated spines/synapses may represent distinct populations from those modulated by estradiol and progesterone in dorsal hippocampus.

An enriched environment increases noradrenaline concentration in the mouse brain

Exposure to an enriched environment has been shown to have many positive effects on brain structure and function. In the present study, we examined the effects of environmental enrichment on monoaminergic neurons in the mouse brain. After being exposed to an enriched environment for 40 days, noradrenaline content was increased significantly in the parieto-temporo-occipital cortex, the cerebellum and the pons/medulla oblongata. In contrast, no changes were observed in serotonin or dopamine levels in these same regions.

Altered dendritic spine density in animal models of depression and in response to antidepressant treatment

Olfactory bulbectomy, neonatal clomipramine administration, and maternal deprivation have been employed as animal models of depression. Each model is unique with respect to the experimental manipulations required to produce "depressive" signs, expression and duration of these signs, and response to antidepressant treatments. Dendritic spines represent a possible anatomical substrate for the enduring changes seen with depression and we have previously shown that chronic antidepressant drug exposure alters the density of hippocampal dendritic spines in an enduring fashion. The purpose of the present study was to determine whether persistent alteration of hippocampal spine density is a common element in each of these different models of depression and whether such alterations could be reversed with chronic antidepressant treatment. The results show that olfactory bulbectomy reduced spine density in CA1, CA3, and dentate gyrus compared to sham-operated controls. Chronic treatment with amitriptyline, a tricyclic antidepressant, reversed the bulbectomy- induced reduction in dendritic spine density in CA1, CA3, and dentate gyrus, whereas treatment with mianserin, an atypical antidepressant, reversed this reduction only in dentate gyrus. On the other hand, neither neonatal clomipramine administration nor maternal deprivation affected hippocampal dendritic spine density. Repeated neonatal handling, however, as a control or as part of the maternal deprivation procedure, elevated spine density in dentate gyrus. These data suggest that long-lasting alterations in hippocampal dendritic spine density contribute to the neural mechanism underlying the olfactory bulbectomy model of depression, but not the neonatal clomipramine or maternal deprivation models.

Neuronal instability: implications for Rett's syndrome

The maturational changes in the brain and spinal cord do not linearly proceed from immature in infants to mature in adults. Dendrites dynamically extend or retract as neurotrophic factors fluctuate. In certain cases mature neurons can be seen soon after birth, and in other cases immature neurons can be identified in the aged brain. Monoamine 'neurotransmitter'; such as serotonin (5-HT), dopamine and norepinephrine appear to function as Maintenance Growth Factors since they must be present in order to produce their maturational actions. Serotonin neurons contain TRK-B receptors and are sensitive to availability of the trophic factor, BDNF. 5-HT also functions by promoting the release of the glial extension factor, S-100beta. 5-HT and S-100beta can provide maturational signals to a variety of neurons, in both cortical and subcortical areas, and appear to be involved in regulating the maturation and release of acetylcholine and dopamine. We have shown that activation of the 5-HT1A receptor is particularly effective in inducing growth of stunted neurons. The mechanism of action of the 5-HT1A receptor involves both a direct inhibition on c-AMP and pCREB formation in postsynaptic neurons and a release of S-100beta from glial cells. Both these events are capable of stabilization and elaboration of the cytoskeleton of the neuron and inhibition of apoptosis. 5-HT1A receptors have been shown to effectively reverse stunted neurons and microencephaly produced in animal models of fetal alcohol syndrome and prenatal cocaine administration. I discuss the implications for regressive disorders such as Rett's syndrome and autism, and the feasibility of treatments with 5-HT1A agonists in children with developmental disorders.

A biogenic amine-synapse mechanism for mental retardation and developmental disabilities

Recent studies have demonstrated that biogenic amines have a function of facilitating formation and maintenance of synapses in diverse regions of the central nervous system in developing and adult animals. The normal number of synapses maintained by biogenic amines are crucial to acquire learning and memory. The level of biogenic amines was reported to decrease in the brain by several neurodevelopmental disorders associated with mental retardation and developmental disabilities such as Rett syndrome, autism and Down syndrome. Taken into consideration this fact together with the function of biogenic amines for synapses, the density of synapses appears to decrease considerably in the brains of patients suffered from the neurodevelopmental disorders. The synaptic overproduction during the critical period of development especially 1 year after birth has been considered as a background mechanism to provide plasticity for the developing brain. Synaptic overproduction does not appear to occur in the brains of patients suffered from the neurodevelopmental disorders, which they are observed mental retardation occurring in the first 1 year after birth. Along with the neurodevelopmental disorders, environmental factors (stress, drugs and nutrition) during pre- and post-natal critical developmental periods are known to change levels of biogenic amines in the brain. In fact, maternal stress has been shown to decrease the levels of serotonin and the density of synapses in the hippocampus of the offspring, and they showed developmental disabilities in the spatial learning and memory. A cascade appears to exist from either the child neurological disorders or the environmental factors to mental retardation and developmental disabilities by decreases in the levels of biogenic amines and synaptic density.

Effects of soft-diet feeding on synaptic density in the hippocampus and parietal cortex of senescence-accelerated mice

Some investigators have proposed that extracting of the teeth of rats or mice impairs their acquisition of spatial memory, implying that alterations of the neural networks in the brain result from a reduction of masticatory work. To evaluate numerical alterations of synapses in the cerebral cortex caused by reduced masticatory movements, two strains of the senescence-accelerated mouse, SAMR1 and SAMP8, were fed either a pelleted (hard-diet groups, R1-H and P8-H) or a powdered diet (soft-diet groups, R1-S and P8-S) after weaning. Radioimmunoassay using a monoclonal anti-synaptophysin antibody (SY38) revealed that the synaptophysin content in the whole cortex was significantly lower in P8-H compared with R1-H from 3 months to 12 months of age. The soft-diet feeding reduced the synaptophysin content in the cerebral cortex of both strains after 3 months of age. Immunohistochemistry and electron microscopy on the hippocampal formation and parietal cortex of 6-month-old mice showed that synaptic formation was significantly decreased in these areas in both R1-S and P8-S. The reduction rate of synaptic density due to soft-diet feeding was larger in the hippocampus than in the parietal cortex. The working memory of the four groups was tested at 6 months of age on an eight-arm radial maze. Performance significantly differed between R1-H and P8-H, between R1-H and R1-S, and between P8-H and P8-S. The results indicated that soft-diet feeding after weaning period reduces synaptic formation in the cerebral cortex and impairs the ability of spatial learning in adulthood.

Hormonal regulation of hippocampal spine synapse density involves subcortical mediation

It is well established that estrogen has positive effects on the density of pyramidal cell spines in the hippocampal CA1 subfield. This study explored whether afferent connections of the hippocampus that come from estrogen-sensitive subcortical structures, including the septal complex, median raphe and supramammillary area, play a role in this estrogen-induced hippocampal synaptic plasticity. These particular subcortical structures have major influences on hippocampal activity, including theta rhythm and long-term potentiation. The latter also promotes the formation of new synapses. All of the rats were ovariectomized; the fimbria/fornix, which contains the majority of subcortical efferents to the hippocampus, was transected unilaterally in each, and half of the animals received estrogen replacement. Using unbiased electron microscopic stereological methods, the CA1 pyramidal cell spine synapse density was calculated. In the estrogen-treated rats, contralateral to the fimbria/fornix transection, the spine density of CA1 pyramidal cells increased dramatically, compared to the spine density values of both the ipsilateral and contralateral hippocampi of non-estrogen-treated animals and to that of the ipsilateral hippocampus of the estrogen replaced rats. These observations indicate that fimbria/fornix transection itself does not considerably influence CA1 area pyramidal cell spine density and, most importantly, that the estrogenic effect on hippocampal morphology, in addition to directly affecting the hippocampus, involves subcortical mediation.

Impairment of maze learning in rats by restricting environmental space

We previously reported that the restriction of environmental space attenuates spontaneous locomotor activity and hippocampal acetylcholine release. To examine the effect of the restriction of environmental space on spatial learning function, male rats were individually housed in a cylindrical large cage (diameter=35 cm) or small cage (diameter=19 cm) for 5 days. Eight-arm radial maze performance was examined to evaluate spatial learning and memory functions. The task was performed once a day between 21:00 and 22:00 h in the dark phase. Although all rats learned and performed the task, those in the small cage had lower scores and took more trial time than those in the large cage. These results suggest that the restriction of environmental space impairs spatial learning in the dark phase in rats.

Chronic fluoxetine administration to juvenile rats prevents age-associated dendritic spine proliferation in hippocampus

The density of dendritic spines, the postsynaptic sites of most excitatory synapses, increases during the first 2 postnatal months in rat hippocampus. Significant alterations in hippocampal levels of serotonin and norepinephrine impact synaptic development during this time period. In the present study, dendritic spine density was studied in the hippocampus (CA1) and dentate gyrus of juvenile rats acutely and chronically exposed to antidepressant drugs that act on serotonin and norepinephrine. One group of 21-day-old rats was given a single injection of a serotonin specific re-uptake inhibitor (fluoxetine or fluvoxamine), a norepinephrine-specific re-uptake inhibitor (desipramine), or saline and killed after 24 h. A second group of rats was injected daily, beginning on postnatal day (PN) 21, for 3 weeks. This group was further subdivided into rats that were killed 1 day or 21 days after the last injection. Golgi analysis showed that a single injection of fluvoxamine produced a significant increase in dendritic spine density in stratum radiatum of CA1 and in the dentate gyrus. Further, acute treatment with all three antidepressants increased the total length of secondary dendrites in CA1, with fluoxetine and desipramine increasing the number of secondary dendrites as well. In fluoxetine-treated animals killed on days 42 or 62 (1 or 21 days post-treatment, respectively), dendritic spine density remained at levels present in CA1 at 21 days. These results show that acute antidepressant treatment can impact dendritic length and spine density, and raise the possibility that chronic fluoxetine treatment arrests spine development into young adulthood.

Lidocaine reversible inactivation of the median raphe nucleus has no effect on reference memory but enhances working memory versions of the Morris water maze task

Numerous studies in the past have dealt with the role of serotonergic system lesions in tasks aimed at measurement of cognitive behavior, but the literature concerning the role of serotonin in cognition remains controversial. Rats with electrolytic lesions of the median raphe nucleus (MRN) were found to display a profound impairment in both the acquisition and retention of spatial memory task. In this study, the lidocaine inactivation was employed to evaluate the involvement of the rat's median raphe nucleus in reference and working memory versions of the Morris water maze (MWM) task. Lidocaine (0.5 microl, 2%) was injected through a single cannula aimed at the MRN; control groups were treated in the same way with a 0.5 microl injection of saline. In Experiment 1, rats were trained in a reference memory version of the MWM with two blocks of four trials per day for three consecutive days, with intra-cerebral injection made 5 min before training. No significant difference was found. In Experiment 2, intra-cerebral injection was applied immediately after two blocks of four trials, and in Experiment 3, the drug was injected 5 min before retention test in rats that had received eight trials per day on three consecutive days. Again, no significant difference between control and treatment groups was found. These results indicate that MRN has no role in acquisition, consolidation and retrieval of spatial reference memory. In subsequent experiments, rats were trained in a working memory version of the MWM task to find a new target position in trial 1, and retrieval was tested 75 min later. MRN inactivation 5 min before (Experiment 4) and immediately after the acquisition trial (Experiment 5) enhanced spatial working memory. It is concluded that normal activity of the MRN has no role in formation and retrieval of reference memory, but it has an inhibitory role in working memory. Our results are confirmed with other studies suggesting that the serotonergic system has a different role in long-term and short-term memory. Interaction with other neurotransmitter systems like acetylcholine may be involved in this case.

Role of serotonin in memory impairment

As a result of its presence in various structures of the central nervous system serotonin (5-HT) plays a role in a great variety of behaviours such as food intake, activity rythms, sexual behaviour and emotional states. Despite this lack of functional specialization, the serotonergic system plays a significant role in learning and memory, in particular by interacting with the cholinergic, glutamatergic, dopaminergic or GABAergic systems. Its action is mediated via specific receptors located in crucial brain structures involved in these functions, primarily the septo-hippocampal complex and the nucleus basalis magnocellularis (NBM)-frontal cortex. Converging evidence suggests that the administration of 5-HT2A/2C or 5-HT4 receptor agonists or 5-HT1A or 5-HT3 and 5-HT1B receptor antagonists prevents memory impairment and facilitates learning in situations involving a high cognitive demand. In contrast, antagonists for 5-HT2A/2C and 5-HT4, or agonists for 5-HT1A or 5-HT3 and 5-HT1B generally have opposite effects. A better understanding of the role played by these and other serotonin receptor subtypes in learning and memory is likely to result from the recent availability of highly specific ligands, such as 5-HT1A, 5-HT1B, 5-HT2A receptor antagonists, and new molecular tools, such as gene knock-out mice, especially inducible mice in which a specific genetic alteration can be restricted both temporally and anatomically.

Age-associated changes in the densities of presynaptic monoamine transporters in different regions of the rat brain from early juvenile life to late adulthood

The binding parameters of highly selective ligands of serotonin (5-HT) transporters ([3H]paroxetine), noradrenaline (NE) transporters ([3H]nisoxetine), and of dopamine (DA) transporters ([3H]GBR-12935) were determined on membrane preparations from frontal cortex, striatum, midbrain and brain stem of Wistar rats on postnatal days 25, 50, 90 and 240, i.e., from the time of weaning till late adulthood. No age-dependent alterations in the affinity-parameters (K(D)-values) of all three monoamine transporters were observed. Age-associated changes in B(max)-values of the binding of all three specific ligands were most pronounced in the phylogenetically younger, late maturing brain regions (frontal cortex, striatum). Most likely, these changes reflect age-related changes in 5-HT, NE and DA-innervation densities. In the frontal cortex, 5-HT-transporter density increased steadily from weaning (day 25) till late adulthood, whereas the density of NE-transporters was highest at weaning, declined till puberty (day 50) and remained at this level until old age. DA-transporter density in the frontal cortex was not reliably measurable by [3H]GBR-binding assays. In the striatum, DA-transporter density increased till puberty and declined thereafter considerably and steadily to about one-fourth of the pubertal values at old age. No such age-associated changes in DA-transporter density were seen in the midbrain. Densities of 5-HT and NE remained at the level reached already at weaning until old age in the striatum, midbrain and brain stem. These findings provide the first comprehensive description of the normally occurring changes in the densities of all three presynaptically located monoamine transporters in the rat brain throughout the life span from weaning to late adulthood.

Increased numerical density of synapses in CA3 region of hippocampus and molecular layer of motor cortex after self-stimulation rewarding experience

Self-stimulation has been considered as an intensely rewarding behavioural experience, being perhaps even more influential than feeding or sexual behaviour. Our earlier studies have demonstrated a self-stimulation rewarding experience-induced increase in dendritic branching points, intersections and spine densities in CA3 hippocampal and layer V motor cortical pyramidal neurons. In the present study, we report self-stimulation-induced alterations in the numerical density of synapses in the hippocampus and motor cortex. A self-stimulation experience was provided 1 h daily for a period of 10 days through bipolar electrodes, implanted bilaterally in the lateral hypothalamus and substantia nigra-ventral tegmental area, stereotaxically. The results revealed a significant (P < 0.001) increase in the number of synapses in the CA3 region of hippocampus and the molecular layer of the motor cortex in self-stimulation-experienced rats. The increased synaptic number may be due to the activation of afferent pathways to the hippocampus and motor cortex following self-stimulation, which may lead to the induction of long-term potentiation. Long-term potentiation is known to cause structural changes by strengthening the existing synapses or resulting in the formation of new synapses. These changes may be related to the improved cognitive functions observed in self-stimulation-experienced rats.

Dopamine D1 and D2 receptor agents and their interaction influence the synaptic density of the rat prefrontal cortex

Recent findings indicate that some monoamines contribute to synaptic maintenance. We examined the synaptic density of the prefrontal cortex of rats with the single or combined administration of D1 and D2 antagonists and agonists. When each agent was administered individually, we observed a significant difference in synaptic density. This indicates that dopamine regulates synaptic maintenance. With the combined administration of antagonists or agonists at high dosages, a synergistic effect was observed. With the combined administration of a D1 antagonist and D2 agonist or a D1 agonist and D2 antagonist, a low dose of the D2 receptor drug enhanced the effect of the D1 receptor drug. The D1 receptor drug offset the effect of the high dose of D2 receptor drug, suggesting the possibility that the D1 receptor family is essential for synaptic maintenance.

Synaptic density of the prefrontal cortex regulated by dopamine instead of serotonin in rats

Recent findings indicate that monoamine contributes to synaptic plasticity. We examined the synaptic density of the prefrontal cortex and parietal cortex of rats using dopamine (DA) antagonists and agonists, as well as serotonin (5-HT) depleters and found a reduction in synaptic density in the prefrontal cortex lamina V-VI at a maximum of 20% with administration of a D1 antagonist (SCH23390) and at a maximum of 30% with a D2 antagonist (YM09151). Further, with the administration of D1+D2 antagonists there was a 27% decrease in synaptic density, which was a larger reduction than the total of the single dosages of each DA antagonist at equal levels. Increase in synaptic density was seen at a maximum of 8.5% with dosage of a D1 agonist (SKF38390) and 14.5% with dosage of a D2 agonist (PPHT). The dosage of D1+D2 agonists showed a 27.1% increase in synaptic density. There was no change in synaptic density of the parietal cortex with either DA antagonist or agonist administration. Administration of 5-HT depleter pCPA resulted in a 13.8% reduction of synaptic density in the parietal cortex, though there was no change identified in the synaptic density in the prefrontal cortex. Based on these results, it was suggested that the area of the brain with affected synaptic plasticity could differ, depending on the type of monoamine.