Synaptic plasticity in rat hippocampus associated with learning

Rewarding deep brain stimulation at the medial forebrain bundle favours avoidance conditioned response in a remote memory test, hinders extinction and increases neurogenesis

Intracranial Self-Stimulation (ICSS) at the medial forebrain bundle consistently facilitates learning and memory in rats when administered post-training or when administered non-concurrent to training, but its scope regarding remote memory has not yet been studied. The present work aims to test whether the combination of these two forms of ICSS administration can cause a greater persistence of the facilitating effect on remote retention and affect neurogenesis in the dentate gyrus (DG) of the hippocampus. Rats were trained in active avoidance conditioning and tested in two retention sessions (10 and 90 days) and later extinction. Subjects received an ICSS session after each of the five avoidance acquisition sessions (post-training treatment) and half of them also received ten additional ICSS sessions during the rest period between retention tests (non-concurrent treatment). All the stimulated groups showed a higher performance in acquisition and retention sessions, but only the rats receiving both ICSS treatments showed greater resistance to extinction. Remarkably, at seven months, rats receiving the non-concurrent ICSS treatment had a greater number of DCX-positive cells in the DG as well as a higher amount of new-born cells within the granular layer compared to rats that did not receive this additional ICSS treatment. Our present findings significantly extend the temporal window of the facilitating effect of ICSS on active avoidance and demonstrate a neurogenic effect of rewarding medial forebrain bundle stimulation.

Novel effects of Rosa damascena extract on memory and neurogenesis in a rat model of Alzheimer's disease

The number of older people who are suffering from memory impairment is increasing among populations throughout the world. Alzheimer's disease (AD) affects about 5% of people over 65 years old. The hippocampus, a brain area critical for learning and memory, is especially vulnerable to damage in the early stages of AD. Emerging evidence suggests that loss of neurons and synapses are correlated with dementia in this devastating disease. Therefore, neurogenesis and synaptogenesis in adulthood could serve as a preventive as well as a therapeutic target for AD. This study investigated the effect of Rosa damascena extract on neurogenesis and synaptogenesis in an animal model of AD. Molecular, cellular, and behavioral experiments revealed that this treatment could induce neurogenesis and synaptic plasticity and improve memory in AD. Our study suggests that R. damascena is a promising treatment for mild memory impairments and AD.

Ultrastructural synaptic changes associated with neurofibromatosis type 1: a quantitative analysis of hippocampal region CA1 in a Nf1(+/-) mouse model

Neurofibromatosis type 1 (NF1) is one of the most frequently diagnosed autosomal dominant inherited disorders resulting in neurological dysfunction, including an assortment of learning disabilities and cognitive deficits. To elucidate the neural mechanisms underlying the disorder, we employed a mouse model (Nf1(+/-) ) to conduct a quantitative analysis of ultrastructural changes associated with the NF1 disorder. Using both serial light and electron microscopy, we examined reconstructions of the CA1 region of the hippocampus, which is known to play a central role in many of the dysfunctions associated with NF1. In general, the morphology of synapses in both the Nf1(+/-) and wild-type groups of animals were similar. No differences were observed in synapse per neuron density, pre- and postsynaptic areas, or lengths. However, concave synapses were found to show a lower degree of curvature in the Nf1(+/-) mutant than in the wild type. These results indicate that the synaptic ultrastructure of Nf1(+/-) mice appears relatively normal with the exception of the degree of synaptic curvature in concave synapses, adding further support to the importance of synaptic curvature in synaptic plasticity, learning, and memory.

Both electrical stimulation thresholds and SMI-32-immunoreactive retinal ganglion cell density correlate with age in S334ter line 3 rat retina

Electrical stimulation threshold and retinal ganglion cell density were measured in a rat model of retinal degeneration. We performed in vivo electrophysiology and morphometric analysis on normal and S334ter line 3 (RD) rats (ages 84-782 days). We stimulated the retina in anesthetized animals and recorded evoked responses in the superior colliculus. Current pulses were delivered with a platinum-iridium (Pt-Ir) electrode of 75-μm diameter positioned on the epiretinal surface. In the same animals used for electrophysiology, SMI-32 immunolabeling of the retina enabled ganglion cell counting. An increase in threshold currents positively correlated with age of RD rats. SMI-32-labeled retinal ganglion cell density negatively correlated with age of RD rats. ANOVA shows that RD postnatal day (P)100 and P300 rats have threshold and density similar to normal rats, but RD P500 and P700 rats have threshold and density statistically different from normal rats (P < 0.05). Threshold charge densities were within the safety limits of Pt for all groups and pulse configurations, except at RD P600 and RD P700, where pulses were only safe up to 1- and 0.2-ms duration, respectively. Preservation of ganglion cells may enhance the efficiency and safety of electronic retinal implants.

Neuroplasticity in the mesolimbic system induced by natural reward and subsequent reward abstinence

BACKGROUND

Natural reward and drugs of abuse converge on the mesolimbic system, where drugs of abuse induce neuronal alterations. Here, we tested plasticity in this system after natural reward and the subsequent impact on drug responses.

METHODS

Effects of sexual experience in male rats on behavioral sensitization and conditioned place preference associated with d-amphetamine (AMPH) and Golgi-impregnated dendrites and spines of nucleus accumbens (NAc) cells were determined. Moreover, the impact of abstinence from sexual behavior in experienced males on these parameters was tested.

RESULTS

First, repeated sexual behavior induced a sensitized locomotor response to AMPH compared with sexually naive control subjects observed 1, 7, and 28 days after last mating session. Second, sexually experienced animals formed a conditioned place preference for lower doses of AMPH than sexually naive males, indicative of enhanced reward value of AMPH. Finally, Golgi-Cox analysis demonstrated increased numbers of dendrites and spines in the NAc core and shell with sexual experience. The latter two alterations were dependent on a period of abstinence of 7-10 days.

CONCLUSIONS

Sexual experience induces functional and morphological alterations in the mesolimbic system similar to repeated exposure to psychostimulants. Moreover, abstinence from sexual behavior after repeated mating was essential for increased reward for drugs and dendritic arbors of NAc neurons, suggesting that the loss of sexual reward might also contribute to neuroplasticity of the mesolimbic system. These results suggest that some alterations in the mesolimbic system are common for natural and drug reward and might play a role in general reinforcement.

Using non-invasive brain stimulation to augment motor training-induced plasticity

Therapies for motor recovery after stroke or traumatic brain injury are still not satisfactory. To date the best approach seems to be the intensive physical therapy. However the results are limited and functional gains are often minimal. The goal of motor training is to minimize functional disability and optimize functional motor recovery. This is thought to be achieved by modulation of plastic changes in the brain. Therefore, adjunct interventions that can augment the response of the motor system to the behavioural training might be useful to enhance the therapy-induced recovery in neurological populations. In this context, noninvasive brain stimulation appears to be an interesting option as an add-on intervention to standard physical therapies. Two non-invasive methods of inducing electrical currents into the brain have proved to be promising for inducing long-lasting plastic changes in motor systems: transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). These techniques represent powerful methods for priming cortical excitability for a subsequent motor task, demand, or stimulation. Thus, their mutual use can optimize the plastic changes induced by motor practice, leading to more remarkable and outlasting clinical gains in rehabilitation. In this review we discuss how these techniques can enhance the effects of a behavioural intervention and the clinical evidence to date.

Developmental periods of choline sensitivity provide an ontogenetic mechanism for regulating memory capacity and age-related dementia

In order to determine brain and behavioral sensitivity of nutrients that may serve as inductive signals during early development, we altered choline availability to rats during 7 time frames spanning embryonic day (ED) 6 through postnatal day (PD) 75 and examined spatial memory ability in the perinatally-treated adults. Two sensitive periods were identified, ED 12–17 and PD 16–30, during which choline supplementation facilitated spatial memory and produced increases in dendritic spine density in CA1 and dentate gyrus (DG) regions of the hippocampus while also changing the dendritic fields of DG granule cells. Moreover, choline supplementation during ED 12–17 only, prevented the memory decline normally observed in aged rats. These behavioral changes were strongly correlated with the acetylcholine (ACh) content of hippocampal slices following stimulated release. Our data demonstrate that the availability of choline during critical periods of brain development influences cognitive performance in adulthood and old age, and emphasize the importance of perinatal nutrition for successful cognitive aging.

Ultrastructural plasticity associated with hippocampal-dependent learning: A meta-analysis

In order to develop a profile of how individual synapses in the hippocampal formation alter their structure following learning experience, a meta-analysis synthesized the available literature on morphological change following hippocampal-dependent learning. Analysis of the 132 calculated effect sizes suggest a consistent profile of morphological change in the hippocampus following learning experience. Across the hippocampal formation, dendritic complexity, spine density, and the size of perforated postsynaptic densities showed consistent increases following training. Both the density of synapses in general and perforated synapses in particular showed unique responses to training, depending on the duration of training and/or different cell layers of the hippocampal formation. Most importantly, it seems that this profile, while consistent, is small and specific--only a select few of the morphological parameters typically measured in anatomical studies of plasticity showed significant change following training. Collectively, these data suggest that the distinct electrophysiological properties of neocortical versus hippocampal synapses may be at least partially mediated by distinct morphological cascades. That is, on the basis of theory, and with the support of the current data, it seems that synaptogenesis correlates with enduring neocortical plasticity, while structural changes correlate with more transient hippocampal plasticity. To be able to state these conclusions with conviction, however, more data are needed in several key areas for continued pursuit of the morphological correlates of hippocampal-dependent learning.

Pathophysiology of stroke rehabilitation: the natural course of clinical recovery, use-dependent plasticity and rehabilitative outcome

Even though the disruption of motor activity and function caused by stroke is at times severe, recovery is often highly dynamic. Recuperation reflects the ability of the neuronal network to adapt. Next to an unmasking of latent network representations, other adaptive processes, such as excitatory metabolic stress, an imbalance in activating and inhibiting transmission, leading to salient hyperexcitability, or the consolidation of novel connections, prime the plastic capabilities of the system. Rehabilitative interventions may modulate mechanisms of neurofunctional plasticity and influence the natural course after stroke, both positively, but potentially also acting detrimentally. Though routine rehabilitative procedures are an integral part of stroke care, evidence as to their effectiveness remains equivocal. The present review describes the natural course of motor recovery, focusing on ischemic stroke, and discusses use- and training-dependent adaptive effects. It complements a prior article which highlighted the pathophysiology of plasticity. Though the interaction between rehabilitation and plasticity remains elusive, an attempt is made to clarify how and to what extent rehabilitative therapy shapes motor recovery.

Retinal remodelling

Retinal degenerative diseases that progress through loss of photoreceptors initiate a sequence of events that culminates in negative remodelling of the retina. Initially, photoreceptor loss ablates glutamatergic signalling to the neural retina and eliminates coordinate Ca++-coupled homeostatic signalling. Retinal neurons react to this loss of glutamatergic input through retinal rewiring and migration of neurons throughout the axis of the retina. All diseases that kill photoreceptors trigger retinal remodelling as the final common pathway and cell death is a common feature. Retinal remodelling resembles CNS pathologic remodelling and constitutes a major challenge to all rescue strategies.

Retinal remodeling during retinal degeneration

Retinal degenerations, regardless of the initiating event or gene defect, often result in a loss of photoreceptors. This formal deafferentation of the neural retina eliminates the intrinsic glutamatergic drive of the sensory retina and, perhaps more importantly, removes coordinated Ca++-coupled signaling to the neural retina. As in other central nervous system degenerations, deafferentation activates remodeling. Neuronal remodeling is the common fate of all photoreceptor degenerations.

Olfactory learning-induced morphological modifications in single dendritic spines of young rats

Learning-related morphological modifications in single dendritic spines were studied quantitatively in the brains of young Sprague-Dawley rats. We have previously shown that olfactory discrimination rule-learning results in transient physiological and morphological modifications in piriform cortex pyramidal neurons. In particular, spine density along the apical dendrites of neurons from trained rats is increased after learning. The aim of the present study was to identify and describe olfactory learning-induced modifications in the morphology of single spines along apical dendrites of the same type of neurons. By using laser-scanning confocal microscopy, we show that 3 days after training completion spines on neurons from olfactory discrimination trained rats are shorter as compared to spines on neurons from control rats. Further analysis revealed that spine shortening attributed to olfactory discrimination learning derives from shortening of spine head and not from shortening of spine neck. In addition, detailed analysis of spine head volume suggests that spines with large heads are absent after learning. As spine head size may be related to the efficacy of the synapse it bears, we suggest that modifications in spine head dimensions following olfactory rule-learning enhance the cortical network ability to enter into a 'learning mode', in which memories of new odours can be acquired rapidly and efficiently.

Effects of active shock avoidance learning on hippocampal neurogenesis and plasma levels of corticosterone

Hippocampal granule neurons that are newly formed during adulthood might be involved in learning and memory processes. Experimental data suggest that only hippocampus-dependent learning tasks stimulate neurogenesis. To further address this issue, the effects of active shock avoidance (ASA) learning on hippocampal progenitor proliferation and survival of newly formed cells were investigated. ASA training, although considered as hippocampus-independent, is known to induce several neurobiological alterations in the hippocampus. Adult Wistar rats were trained in a shuttle box using a 1-day or 4-day paradigm and brains were analyzed for the mitotic marker Ki-67. Effects on survival of newly generated cells were examined by immunocytochemistry for 5-bromo-2-deoxyuridine (BrdU), which was injected 1 week before the training. Neither proliferation nor survival was affected by the ASA learning task. Because elevated glucocorticoid levels have a negative impact on hippocampal neurogenesis, blood samples were taken throughout the 4-day training paradigm. Both trained animals and control rats that were only placed in the shuttle box without receiving foot shocks showed a similar rise in corticosterone, enabling us to exclusively investigate the effects of ASA learning on neurogenesis without differential interference of stress between groups. On the other hand, the finding that ASA induced elevations in plasma corticosterone, but did not influence proliferation or survival of newly formed cells, indicates that this type of stress does not affect neurogenesis. The present study shows that, in line with the existing data on other hippocampus-independent learning tasks, ASA training has no effect on hippocampal neurogenesis.

Associative memory formation increases the observation of dendritic spines in the hippocampus

Dendritic spines are sources of synaptic contact that can be altered by experience and, as such, may be involved in memories for that experience. Here we tested whether the acquisition of new memories is associated with changes in the density of dendritic spines. Adult male rats were trained using the trace eyeblink conditioning paradigm, an associative learning task that requires the hippocampus for acquisition. Additional groups were exposed to the same number of stimuli presented in an explicitly unpaired manner or were naive. Twenty-four hours later, the density of dendritic spines was measured using Golgi impregnation. Trace conditioning was associated with an increase in the density of dendritic spines on the pyramidal cells of area CA1 of the hippocampus, an effect that was prevented by blocking acquisition of the learned response with a competitive NMDA receptor antagonist. Training with delay conditioning, a similar task that does not require the hippocampus, also produced an increase in spine density. The learning-induced increase in dendritic spine density was specific to basal dendrites of pyramidal cells in the CA1 region of the hippocampus. Changes did not occur on their apical dendrites or on cells in the dentate gyrus or somatosensory cortex. These results suggest that the formation and expression of associative memories increase the availability of dendritic spines and the potential for synaptic contact.

Differences in the expression of AMPA and NMDA receptors between axospinous perforated and nonperforated synapses are related to the configuration and size of postsynaptic densities

Axospinous synapses are traditionally divided according to postsynaptic density (PSD) configuration into a perforated subtype characterized by a complex-shaped PSD and nonperforated subtype exhibiting a simple-shaped, disc-like PSD. It has been hypothesized that perforated synapses are especially important for synaptic plasticity because they have a higher efficacy of impulse transmission. The aim of the present study was to test this hypothesis. The number of postsynaptic AMPA receptors (AMPARs) is widely regarded as the major determinant of synaptic efficacy. Therefore, the expression of AMPARs was evaluated in the two synaptic subtypes and compared with that of NMDA receptors (NMDARs). Postembedding immunogold electron microscopy was used to quantify the immunoreactivity following single labeling of AMPARs or NMDARs in serial sections through the CA1 stratum radiatum of adult rats. The results showed that all perforated synapses examined were immunopositive for AMPARs. In contrast, only a proportion of nonperforated synapses (64% on average) contained immunogold particles for AMPARs. The number of immunogold particles for AMPARs was markedly and significantly higher in perforated synapses than in immunopositive nonperforated synapses. Although all synapses of both subtypes were NMDAR immunopositive perforated synapses contained significantly more immunogold particles for NMDARs than nonperforated ones. Multivariate analysis of variance revealed that the mode of AMPAR and NMDAR expression is related to the complexity of PSD configuration, not only to PSD size. These findings support the notion that perforated synapses may evoke larger postsynaptic responses relative to nonperforated synapses and, hence, contribute to an enhancement of synaptic transmission associated with some forms of synaptic plasticity.

Corticosteroids: sculptors of the hippocampal formation

The influence of corticosteroids on hippocampus-dependent learning and memory processes is now indisputable. On the other hand, closer scrutiny of early studies together with interpretations from newer studies would suggest that the proposition that corticosteroid-induced hippocampal cell death accounts fully for the associated cognitive deficits is only partially correct. Firstly, it is now clear that a specific sub-population of hippocampal neurons, the granule cells of the dentate gyrus, is more sensitive to changes in the corticosteroid environment; this fact raises the interesting question of what might be the unique properties of granule cells that render them more vulnerable to these hormones, since virtually all hippocampal cells express corticosteroid receptors. Secondly, from a critical analysis of the available data, the picture that emerges is that corticosteroids, by acting through two distinct receptors, influence not only cell birth and death, but probably also cell differentiation. Mineralocorticoid receptor (MR) occupation appears to be essential for the survival of existing and newly generated granule neurons. In contrast, while glucocorticoid receptors (GR) can induce loss of neurons in the absence of MR activation, it appears that their occupation usually results in less drastic effects involving only dendritic atrophy and loss of synaptic contacts. This revised scheme of corticosteroid actions on hippocampal structure should explain earlier observations that many of the cognition- impairing effects of corticosteroids are reversible.

Genetic analysis of learning behavior-induced structural plasticity

It is well-documented that enriched environment and behavioral training can lead to improved learning and memory, as well as structural and morphological changes in the brain. It has been hypothesized that such experience-dependent behavioral improvement results from structural modifications that may represent some forms of possible memory substrates for these behavioral experiences. It was generally assumed until now that, like the activity-dependent structural plasticity observed in the developing brain, behavioral experience-induced structural plasticity would require the activation of the NMDA receptor, a molecular switch for learning and memory. Recent genetic and anatomical analyses reveal that behavioral experience-induced increases in spine and synapse density in the hippocampal CA1 region occur despite the deletion of the NMDA receptor in conditional knockout mice. Recent studies indicate that the molecular mechanism of behavioral experience-induced structural plasticity in the adult brain differs from that of the developing brain, and can be disassociated from the NMDA-mediated long-term potentiation (LTP) phenomenon. Deepening the understanding of the molecular mechanism of experience-induced structural plasticity should facilitate the study of the relationship between structural changes and memory formation. Using an integrated approach with genomic, genetic, and modern histological techniques should move us closer in this direction.

Brain plasticity and stroke rehabilitation. The Willis lecture

Neuronal connections and cortical maps are continuously remodeled by our experience. Knowledge of the potential capabilityof the brain to compensate for lesions is a prerequisite for optimal stroke rehabilitation strategies. Experimental focal cortical lesions induce changes in adjacent cortex and in the contralateral hemisphere. Neuroimaging studies in stroke patients indicate altered poststroke activation patterns, which suggest some functional reorganization. To what extent functional imaging data correspond to outcome data needs to be evaluated. Reorganization may be the principle process responsible for recovery of function after stroke, but what are the limits, and to what extent can postischemic intervention facilitate such changes? Postoperative housing of animals in an enriched environment can significantly enhance functional outcome and can also interact with other interventions, including neocortical grafting. What role will neuronal progenitor cells play in future rehabilitation-stimulated in situ or as neural replacement? And what is the future for blocking neural growth inhibitory factors? Better knowledge of postischemic molecular and neurophysiological events, and close interaction between basic and applied research, will hopefully enable us to design rehabilitation strategies based on neurobiological principles in a not-too-distant future.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Age-changes of brain synapses and synaptic plasticity in response to an enriched environment

Numerical synaptic density and synaptic vesicle density in rat frontal cortex were examined by electron microscopy as a function of age. The density of axospinous synapses, a major population of synapses, was found to peak at age 1 month, and to gradually decrease with aging. The synaptic vesicle density in axospinous synapses was shown to rapidly increase to a peak during the first 3 weeks and then decrease to the adult level, which remained unchanged in senescence. The time course of synaptic changes in aging is presented in this study. In a previous report (Saito et al. [1994] J. Neurosci. Res. 39:57-62), we showed that enriched rearing conditions restored the age-related decrease of synaptophysin contents. This might be due to increased numerical synaptic density or enhanced packing density of synaptic vesicles in synapses. The results of the present study support the latter explanation; that is, synaptic vesicle contents were increased without changes in synaptic density. Synaptic plasticity induced by environmental stimulation is shown to relate with synaptic strengthening, but not with the formation of new synapses.

Behavioral consequences of intracerebral vasopressin and oxytocin: focus on learning and memory

Since the pioneering work of David de Wied and his colleagues, the neuropeptides arginine vasopressin and oxytocin have been thought to play a pivotal role in behavioral regulation in general, and in learning and memory in particular. The present review focuses on the behavioral effects of intracerebral arginine vasopressin and oxytocin, with particular emphasis on the role of these neuropeptides as signals in interneuronal communication. We also discuss several methodological approaches that have been used to reveal the importance of these intracerebral neuropeptides as signals within signaling cascades. The literature suggests that arginine vasopressin improves, and oxytocin impairs, learning and memory. However, a critical analysis of the subject indicates the necessity for a revision of this generalized concept. We suggest that, depending on the behavioral test and the brain area under study, these endogenous neuropeptides are differentially involved in behavioral regulation; thus, generalizations derived from a single behavioral task should be avoided. In particular, recent studies on rodents indicate that socially relevant behaviors triggered by olfactory stimuli and paradigms in which the animals have to cope with an intense stressor (e.g., foot-shock motivated active or passive avoidance) are controlled by both arginine vasopressin and oxytocin released intracerebrally.

Hippocampal histamine receptors: possible role on the mechanisms of memory in the rat, II

In this paper it was studied the role of histamine and histamine receptors in the hippocampus of rats on an active avoidance response induced by an ultrasonic tone. The animals had to learn to walk through a swinging door into a safe compartment only after the conditioning ultrasonic tone was on in order to avoid an electric shock to their feet. Trained animals were implanted in the ventral hippocampus with microinjection cannulae and injected twice with 1 microliter of saline solution containing pyrilamine (PYR, H1-HA antagonist), ranitidine (RAN, H2-HA antagonist) or histamine. The histamine antagonists were applied in a dose of 65.5 nmol each while histamine was administered in a dose of 45 nmol. The two variables measured were the time in sec the rats take to present the conditioned avoidance response and the accumulated percentage of conditioned avoidance response (CAR). Results showed that histamine administration significantly increased the latency time to escape and decreased the % CAR. These effects were not blocked by the administration of RAN. However, administration of PYR completely counteracted the HA effects. Present findings confirm our previous findings about the inhibitory effect of histamine on the hippocampal retrieval mechanisms and give further support to the hypothesis that HA acts on the memory processes in the hippocampal formation, by activation of H1-histamine receptors.

An analysis of contemporary morphological concepts of synaptic remodelling in the CNS: perforated synapses revisited

Perforated synapses refer to a synaptic type found in the central nervous system. They are characterized by their large size and by a discontinuity of the postsynaptic density when viewed in transverse sections, and by a doughnut or horseshoe shape when viewed in en face views. Of recent morphological studies, one approach has followed their characteristics throughout development and maturity, while others have concentrated on their probable roles in activities including kindling, long-term potentiation, spatial working memory, differential rearing, and the functioning of neuroleptics. An assessment is made of the hypotheses and models that have proved determinative in the emergence of perforated synapses as being significant in synaptic plasticity. Their distribution and frequency are summarized, with emphasis on the importance of unbiased stereological procedures in their analysis. Using three-dimensional approaches various subtypes are recognized. Of these, a complex or fragmented subtype appears of especial significance in synaptic plasticity. Ideas regarding the life-cycle of perforated synapses are examined. The view that they originate from conventional, non-perforated synapses, enlarge, and subsequently split to give rise to a new generation of non-perforated synapses, is critically assessed. According to an alternative model, perforated and non-perforated synapses constitute separate populations from early in their development, each representing complementary forms of synaptic plasticity. An attempt is also made to discover whether synaptic studies on the human brain in normal aging and in Alzheimer's disease throw light on the role of perforated synapses in synaptic plasticity. The loss of synapses in Alzheimer's disease may include a loss of perforated synapses - of particular relevance for an understanding of certain neuropathological conditions.

Effects of localized histamine microinjections into the hippocampal formation on the retrieval of a one-way active avoidance response in rats

The possible role of histamine (HA) locally applied into the hippocampus on memory mechanisms of the rats was studied. The acquisition of a one-way active avoidance response to an ultrasonic 40 kHz sinus-wave tone anticipating an electric shock was used as experimental model. Learning sessions consisted in placing animals into a two compartment cage were they learnt to escape to the safe compartment after an ultrasonic tone anticipating an electric feet shock. After acquiring the conditioned avoidance response, animals were implanted with microinjection cannulae and injected with 1 microliter of saline, or increasing doses of histamine (9, 22.5, 45, and 90 nmol) into the hippocampus. In the experimental sessions, 4 trials before (PRE) and 4 trials afterward treatment (POST), the percentage of conditioned avoidance responses (% CAR) and the latency time to escape (LT) were measured. Results showed that HA increased significantly the LT and this effect was grossly dose-dependent. % CAR was also affected and the score was significantly inhibited by the imidazolamine administration. Results suggest that HA may be involved in memory retrieval processes in the hippocampus.