Pentylenetetrazol-induced seizures are followed by a reduction in the multiunitary activity of hippocampal CA1 pyramidal neurons in adult rats.

Pentylenetetrazol (PTZ) blocks the inhibitory action of GABA, triggering a Glu-mediated hyperexcitation of the dendritic spines in hippocampal CA1 pyramidal neurons that leads to the generation of epileptiform seizures. The aim of this work was to determine the effect of PTZ on the electrical activity of the hippocampal pyramidal neurons in male rats. Bipolar electrodes were implanted stereotaxically in the right and left hippocampal CA1 fields of adults, and PTZ (65 mg/kg) was administered i.p. Simultaneous recordings of the field activity and the firing rate (multiunitary activity, MUA) were analyzed at 10, 20, and 30 min post-administration of PTZ. Only rats that presented tonic-clonic seizures during the first 1-5 min after PTZ treatment were included in the study. The recordings of the field activity were analyzed in 4 frequency bands. In both the right and left hippocampal CA1 fields, the relative power corresponding to the slow waves (4-7 Hz) increased, while in the bands 13-30 Hz and 31-50 Hz, it decreased at 10, 20, and 30 min post-PTZ. MUA recordings were analyzed at four levels. The highest levels corresponded to larger amplitudes of the action potentials in the pyramidal neurons. The firing rates of the PTZ-treated rats did not differ from baseline but presented a significant decrement at 10, 20, and 30 min post-PTZ. The decreased firing rate of the hippocampal CA1 pyramidal neurons after PTZ treatment could be associated with plastic changes of dendritic spines along with some microenvironmental adaptations at synaptic level, after neuronal PTZ-mediated hyperexcitation.

Modifications to cytoskeleton-associated proteins in dendritic spines underlie the adaptive plasticity involved in long term reference memory.

Spatial learning and memory enables individuals to orientate themselves in an external environment. Synaptic stimulation of dendritic spines on hippocampal place cells underlies adaptive cognitive performance, inducing plastic changes such as spinogenesis, pruning and structural interconversion. Such plastic changes are driven by complex molecular machinery that relies on several actin cytoskeleton-associated proteins (ACAP's), these interacting with actin filaments in the postsynaptic density to guide the conformational changes to spines in accordance with the synaptic information they receive. However, the specific dynamics of the plastic changes in spines driven by ACAP's are poorly understood. Adult rats exhibit efficient allocentric reference memory 30 days after training in a spatial learning paradigm in the Morris water maze. A Golgi study revealed this behavior to be associated with a reduction in both spine density and in mushroom spines, as well as a concomitant increase in thin spines. These changes were accompanied by the overexpression of mRNA encoding ß-actin, Spinophilin and Cortactin, whilst the expression of Profilin, α-actinin, Drebrin, Synaptopodin and Myosin decreased. By contrast, no changes were evident in Cofilin, Gelsolin and Arp2/3 mRNA. From this analysis, it appears that neither spinogenesis nor new mushroom spines are necessary for long-term spatial information retrieval, while thin spines could be potentiated to retrieve pre-learned spatial information. Further studies that focus on the signaling pathways and their related molecules may shed further light on the molecular dynamics of the plastic changes to dendritic spines that underlie cognitive performance, both under normal and pathological conditions.

Signaling mechanisms mediating the regulation of synaptic plasticity and memory by estradiol.

This article is part of a Special Issue "Estradiol and Cognition". Estradiol participates in the regulation of the function and plasticity of synaptic circuits in key cognitive brain regions, such as the prefrontal cortex and the hippocampus. The mechanisms elicited by estradiol are mediated by the regulation of transcriptional activity by nuclear estrogen receptors and by intracellular signaling cascades activated by estrogen receptors associated with the plasma membrane. In addition, the mechanisms include the interaction of estradiol with the signaling of other factors involved in the regulation of cognition, such as brain derived neurotrophic factor, insulin-like growth factor-1 and Wnt. Modifications in these signaling pathways by aging or by a long-lasting ovarian hormone deprivation after menopause may impair the enhancing effects of estradiol on synaptic plasticity and cognition.

Changes in the plastic properties of hippocampal dendritic spines underlie the attenuation of place learning in healthy aged rats.

Normal aging is characterized by slight impairments in spatial memory, and the modification of some electrophysiological parameters that underlie place learning and associated reference memory. However, the morphological mechanisms underlying these impairments remain unknown. In the present study, we analyzed the spine density and the proportion of thin, mushroom, stubby, wide, branched and double spines on pyramidal neuron dendrites in the hippocampal CA1 field of young and aged rats. These parameters were assessed both before and after evaluating place learning and reference memory in the Morris water maze. Aged rats adopted an egocentric strategy to resolve the task, swimming slower and further, and taking longer to locate the sunken platform. While probe trials revealed that aged animals could recall the platform position, these animals spent more time exploring incorrect quadrants than young rats. An increase in spine density was observed after task performance in both young and aged rats, but aging provoked a decrease in the density of thin spines. In addition, there was an increase in the density of mushroom and wide spines in aged animals after task performance as compared with the untested aged counterparts. Moreover, in aged animals there were fewer thin spines and more wide spines after task performance than in the young tested animals. These findings support the view that aging attenuates but does not abolish spatial memory, a process that may be associated with plastic changes in the type of dendritic spines on aged hippocampal CA1 neurons.

Estradiol-mediated modulation of memory and of the underlying dendritic spine plasticity through the life span.

The morphophysiology of the nervous system changes and adapts in response to external environmental inputs and the experiences of individuals throughout their lives. Other changes in the organisms internal environment can also contribute to nervous system restructuring in the form of plastic changes that underlie its capacity to adapt to emerging psychophysiological conditions. These adaptive processes lead to subtle modifications of the organisms internal homeostasis which is closely related with the activity of chemical messengers, such as neurotransmitters and hormones. Hormones reach the brain through the bloodstream, where they activate specific receptors through which certain biochemical, physiological, and morphological changes take place in numerous regions. Fetal development, infancy, puberty, and adulthood are all periods of substantial hormone-mediated brain remodeling in both males and females. Adulthood, specifically, is associated with a broad range of life events, including reproductive cycles in both sexes, and pregnancy and menopause in women. Events of this kind occur concomitantly with eventual modifications in behavioral performance and, especially, in cognitive abilities like learning and memory that underlie, at least in part, plastic changes in the dendritic spines of the neuronal cells in cerebral areas involved in processing cognitive information. Estrogens form a family that consists of three molecules [17ß-estradiol (E2), estrone, estriol] which are deeply involved in regulating numerous bodily functions in different stages of the life-cycle, including the modulation of cognitive performance. This review addresses the effects of E2 on the dendritic spine-mediated synaptic organization of cognitive performance throughout the life span.

Sexual experience induces a preponderance of mushroom spines in the medial prefrontal cortex and nucleus accumbens of male rats.

Sexual experience improves copulatory performance in male rats. Copulatory performance has been associated with dendritic spines density in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAcc), structures involved in the processing of sexual stimuli and the manifestation of sexual behavior. Dendritic spines modulate excitatory synaptic contacts, and their morphology is associated with the ability to learn from experience. This study was designed to determine the effect of sexual experience on the density of different types or shapes of dendritic spines in the mPFC and NAcc of male rats. A total of 16 male rats were used, half of them were sexually experienced while the other half were sexually inexperienced. After three sessions of sexual interaction to ejaculation, the sexually-experienced males presented shorter mount, intromission, and ejaculation latencies. Those rats presented a higher total dendritic density in the mPFC, and a higher numerical density of thin, mushroom, stubby, and wide spines. Sexual experience also increased the numerical density of mushroom spines in the NAcc. In both the mPFC and NAcc of the sexually experienced rats, there was a lower proportional density of thin spines and a higher proportional density of mushroom spines. Results show that the improvement in copulatory efficiency resulting from prior sexual experience in male rats is associated with changes in the proportional density of thin and mushroom dendritic spines in the mPFC and NAcc. This could represent the consolidation of afferent synaptic information in these brain regions, derived from the stimulus-sexual reward association.

From synaptic transmission to cognition: an intermediary role for dendritic spines.

Dendritic spines are cytoplasmic protrusions that develop directly or indirectly from the filopodia of neurons. Dendritic spines mediate excitatory neurotransmission and they can isolate the electrical activity generated by synaptic impulses, enabling them to translate excitatory afferent information via several types of plastic changes, including neoformation, disappearance, redistribution and changes in geometric shape. The fine line between normal and abnormal excitatory neurotransmission is mediated by the concerted action of glutamate-mediated stimulation and calcium ion entry into spines. Moreover, within the range of normal excitatory activity, dendritic spines undergo specific plastic changes to regulate different forms of afferent information that are often related to distinct modes of cognition-related electrophysiological stimulation, such as long-term potentiation or long-term depression.

Immune system in the brain: a modulatory role on dendritic spine morphophysiology?

The central nervous system is closely linked to the immune system at several levels. The brain parenchyma is separated from the periphery by the blood brain barrier, which under normal conditions prevents the entry of mediators such as activated leukocytes, antibodies, complement factors, and cytokines. The myeloid cell lineage plays a crucial role in the development of immune responses at the central level, and it comprises two main subtypes: (1) resident microglia, distributed throughout the brain parenchyma; (2) perivascular macrophages located in the brain capillaries of the basal lamina and the choroid plexus. In addition, astrocytes, oligodendrocytes, endothelial cells, and, to a lesser extent, neurons are implicated in the immune response in the central nervous system. By modulating synaptogenesis, microglia are most specifically involved in restoring neuronal connectivity following injury. These cells release immune mediators, such as cytokines, that modulate synaptic transmission and that alter the morphology of dendritic spines during the inflammatory process following injury. Thus, the expression and release of immune mediators in the brain parenchyma are closely linked to plastic morphophysiological changes in neuronal dendritic spines. Based on these observations, it has been proposed that these immune mediators are also implicated in learning and memory processes.

Selective estrogen receptor modulators regulate dendritic spine plasticity in the hippocampus of male rats.

Some selective estrogen receptor modulators, such as raloxifene and tamoxifen, are neuroprotective and reduce brain inflammation in several experimental models of neurodegeneration. In addition, raloxifene and tamoxifen counteract cognitive deficits caused by gonadal hormone deprivation in male rats. In this study, we have explored whether raloxifene and tamoxifen may regulate the number and geometry of dendritic spines in CA1 pyramidal neurons of the rat hippocampus. Young adult male rats were injected with raloxifene (1 mg/kg), tamoxifen (1 mg/kg), or vehicle and killed 24 h after the injection. Animals treated with raloxifene or tamoxifen showed an increased numerical density of dendritic spines in CA1 pyramidal neurons compared to animals treated with vehicle. Raloxifene and tamoxifen had also specific effects in the morphology of spines. These findings suggest that raloxifene and tamoxifen may influence the processing of information by hippocampal pyramidal neurons by affecting the number and shape of dendritic spines.

Rehabilitation on a treadmill induces plastic changes in the dendritic spines of spinal motoneurons associated with improved execution after a pharmacological injury to the motor cortex in rats.

Lesions to the corticospinal tract result in several neurological symptoms and several rehabilitation protocols have proven useful in attempts to direct underlying plastic phenomena. However, the effects that such protocols may exert on the dendritic spines of motoneurons to enhance accuracy during rehabilitation are unknown. Thirty three female Sprague-Dawley adult rats were injected stereotaxically at the primary motor cerebral cortex (Fr1) with saline (CTL), or kainic acid (INJ), or kainic acid and further rehabilitation on a treadmill 16 days after lesion (INJ+RB). Motor performance was evaluated with the the Basso, Beatie and Bresnahan (BBB) locomotion scale and in the Rotarod. Spine density was quantified in a primary dendrite of motoneurons in Lamina IX in the ventral horn of the thoracolumbar spinal cord as well as spine morphology. AMPA, BDNF, PSD-95 and synaptophysin expression was evaluated by Western blot. INJ+RB group showed higher scores in motor performance. Animals from the INJ+RB group showed more thin, mushroom, stubby and wide spines than the CTL group, while the content of AMPA, BDNF, PSD-95 and Synaptophysin was not different between the groups INJ+RB and CTL. AMPA and synaptophysin content was greater in INJ group than in CTL and INJ+RB groups. The increase in the proportion of each type of spine observed in INJ+RB group suggest spinogenesis and a greater capability to integrate the afferent information to motoneurons under relatively stable molecular conditions at the synaptic level.

Ibotenic acid induced lesions impair the modulation of dendritic spine plasticity in the prefrontal cortex and amygdala, a phenomenon that underlies working memory and social behavior.

The lesions induced by Ibotenic acid (IA) emulate some of the symptoms associated with schizophrenia, such as impaired working memory that is predominantly organized by the medial prefrontal cortex (mPFC), or difficulties in social interactions that aremainly organized by the amygdala (AMG). The plastic capacity of dendritic spines in neurons of the mPFC and AMG is modulated by molecules that participate in the known deterioration of working memory, although the influence of these on the socialization of schizophrenic patients is unknown. Here, the effect of a neonatal IA induced lesion on social behavior and working memory was evaluated in adult rats, along with the changes in cytoarchitecture of dendritic spines and their protein content, specifically the postsynaptic density protein 95 (PSD-95), Synaptophysin (Syn), AMPA receptors, and brain-derived neurotrophic factor (BDNF). Both working memory and social behavior were impaired, and the density of the spines, as well as their PSD-95, Syn, AMPA receptor and BDNF content was lower in IA lesioned animals. The proportional density of thin, mushroom, stubby and wide spines resulted in plastic changes that suggest the activation of compensatory processes in the face of the adverse effects of the lesion. In addition, the reduction in the levels of the modulating factors also suggests that the signaling pathways in which such factors are implicated would be altered in the brains of patients with schizophrenia. Accordingly, the experimental study of such signaling pathways is likely to aid the development of more effective pharmacological strategies for the treatment of schizophrenia.

Cortical Synaptic Reorganization Under Chronic Arsenic Exposure.

There is solid epidemiological evidence that arsenic exposure leads to cognitive impairment, while experimental work supports the hypothesis that it also contributes to neurodegeneration. Energy deficit, oxidative stress, demyelination, and defective neurotransmission are demonstrated arsenic effects, but it remains unclear whether synaptic structure is also affected. Employing both a triple-transgenic Alzheimer's disease model and Wistar rats, the cortical microstructure and synapses were analyzed under chronic arsenic exposure. Male animals were studied at 2 and 4 months of age, after exposure to 3 ppm sodium arsenite in drinking water during gestation, lactation, and postnatal development. Through nuclear magnetic resonance, diffusion-weighted images were acquired and anisotropy (integrity; FA) and apparent diffusion coefficient (dispersion degree; ADC) metrics were derived. Postsynaptic density protein and synaptophysin were analyzed by means of immunoblot and immunohistochemistry, while dendritic spine density and morphology of cortical pyramidal neurons were quantified after Golgi staining. A structural reorganization of the cortex was evidenced through high-ADC and low-FA values in the exposed group. Similar changes in synaptic protein levels in the 2 models suggest a decreased synaptic connectivity at 4 months of age. An abnormal dendritic arborization was observed at 4 months of age, after increased spine density at 2 months. These findings demonstrate alterations of cortical synaptic connectivity and microstructure associated to arsenic exposure appearing in young rodents and adults, and these subtle and non-adaptive plastic changes in dendritic spines and in synaptic markers may further progress to the degeneration observed at older ages.

Pentylenetetrazol-induced seizures in adult rats are associated with plastic changes to the dendritic spines on hippocampal CA1 pyramidal neurons.

Epilepsy is a chronic neurobehavioral disorder whereby an imbalance between neurochemical excitation and inhibition at the synaptic level provokes seizures. Various experimental models have been used to study epilepsy, including that based on acute or chronic administration of Pentylenetetrazol (PTZ). In this study, a single PTZ dose (60 mg/kg) was administered to adult male rats and 30 min later, various neurobiological parameters were studied related to the transmission and modulation of excitatory impulses in pyramidal neurons of the hippocampal CA1 field. Rats experienced generalized seizures 1-3 min after PTZ administration, accompanied by elevated levels of Synaptophysin and Glutaminase. This response suggests presynaptic glutamate release is exacerbated to toxic levels, which eventually provokes neuronal death as witnessed by the higher levels of Caspase-3, TUNEL and GFAP. Similarly, the increase in PSD-95 suggests that viable dendritic spines are functional. Indeed, the increase in stubby and wide spines is likely related to de novo spinogenesis, and the regulation of neuronal excitability, which could represent a plastic response to the synaptic over-excitation. Furthermore, the increase in mushroom spines could be associated with the storage of cognitive information and the potentiation of thin spines until they are transformed into mushroom spines. However, the reduction in BDNF suggests that the activity of these spines would be down-regulated, may in part be responsible for the cognitive decline related to hippocampal function in patients with epilepsy.

Prefrontal serotonergic denervation induces increase in the density of 5-HT2A receptors in adult rat prefrontal cortex.

The 5-HTergic system and particularly 5-HT(2A) receptors have been involved in prefrontal cognitive functions, but the underlying mechanisms by which the serotonin (5-HT) system modulates these processes are still unclear. In this work, the effects of prefrontal 5-HTergic denervation on the density and expression levels of 5-HT(2A) receptors were evaluated by immunohistochemical and molecular biology studies in the prefrontal cortex (PFC). The [(3)H]-Ketanserin binding study revealed an increase in the B(max), along with no change in the binding affinity (K(D)) for 5-HT(2A) receptors. The increase in PFC of 5-HT(2A) receptor density in response to denervation was accompanied by increase in 5-HT(2A) receptor mRNA and protein levels. This increase in the number of 5-HT(2A) receptors may be interpreted as an adaptive plastic change, i.e., hypersensitivity; resulting from the selective pharmacological lesion of the raphe-proceeding 5-HTergic fibers to the PFC. Based on previous evidence, this could be strongly related to the abnormal expression of short-term memory.

Long-term evaluation of cytoarchitectonic characteristics of prefrontal cortex pyramidal neurons, following global cerebral ischemia and neuroprotective melatonin treatment, in rats.

Global cerebral ischemia induces alterations of working memory, as evidenced in the eight-arm radial maze, in the absence of significant changes of pyramidal neuron population in the prefrontal cortex. These alterations can be prevented by a neuroprotective melatonin treatment. Thus, the cytoarchitectonic characteristics of the pyramidal neurons located at layers III and V in the prefrontal cortex of rats that had been submitted 120 days earlier to acute global cerebral ischemia (15 min four-vessel occlusion), and melatonin (10 mg/(kgh) for 6h, i.v.) or vehicle administration, starting 30min after the end of cerebral blood flow interruption, were evaluated in order to gain information on the changes of the neural substrate underlying disruption of prefrontocortical functioning. Soma size, rough length and number of bifurcations of basilar and apical dendrites, as well as spine density and proportions of the different types of spines in a 50 microm length segment of a secondary dendrite branching from the apical and the basilar dendrites, of pyramidal neurons of the dorsal medial prefrontal cortex, were evaluated in Golgi material. A significant reduction of soma size, apical and basilar dendrite length, number of dendritic bifurcations, and spine density were observed in pyramidal neurons at layers III and V after cerebral ischemia, while these alterations were prevented by melatonin treatment. These cytoarchitectural differences between groups seem to underlie the observed alterations in spatial working memory of ischemic, vehicle-treated rats in the absence of pyramidal neuron loss, as well as the better display of these functions long after ischemia and melatonin neuroprotection.

Atomoxetine prevents working memory loss in hyperactive rats, mediating plastic changes in prefrontal cortex pyramidal neurons.

Attention Deficit Hyperactivity Disorder (ADHD) causes impaired visuospatial working memory (VWM), which primarily maps to the prefrontal cortex. However, little is known about the synaptic processes underlying cognitive loss in ADHD, or those ultimately involved in the preventive effect observed through the clinical use of Atomoxetine (ATX). To investigate the plasticity underlying ADHD related cognitive loss, and that potentially involved in the preventive action of Atomoxetine, allocentric VWM was assessed, as well as the dendritic spine number and proportional density on pyramidal neurons in the prefrontal cerebral cortex layer III of neonatal 6-hydroxydopamine-lesioned rats. The effect of acute ATX treatment was also assessed at 28 days of age. 6-OHDA induced lesions produced increased motor activity and a loss of VWM, concomitant with a reduction in thin spine density. ATX administration reversed cognitive loss, in conjunction with a decrease in thin spines and an increase in mushroom spines. A reduction in the proportion of spines involved in learning in hyperactive animals could account for the loss in cognitive function observed. Considering thin spine density was also reduced after ATX administration, we hypothesized that the restoration in cognitive function recorded could be brought about by an increase in memory related mushroom spines.

Administration of an inhibitor of estrogen biosynthesis facilitates working memory acquisition in male rats.

Estradiol is known to improve performance of some working memory tasks in female animals and post-menopausal women. In females the main source of estradiol is the ovary. In addition, in both males and females estradiol is synthesized in extragonadal tissues. The role of non-ovarian estradiol synthesis on cognitive abilities has not been adequately explored. In the sent study we have assessed the effect of an inhibitor of aromatase, the enzyme that produces estradiol from testosterone, on egocentric working memory in male rats. Sprague-Dawley adult male rats received the intra-esophageal administration of the aromatase inhibitor letrozole (2.5 mg/kg), or vehicle. Rats treated with the aromatase inhibitor committed less errors than untreated animals or animals treated with vehicle, when tested in a cross-arms maze. Retention and retrieval stages were unaffected by aromatase inhibition. This finding indicates that aromatase activity is involved in egocentric working memory performance. The effect of the aromatase inhibitor on working memory may be due to the increase in testosterone levels resulting from aromatase inhibition or to modifications in the availability of estradiol in the brain.

Long-term study of dendritic spines from hippocampal CA1 pyramidal cells, after neuroprotective melatonin treatment following global cerebral ischemia in rats.

Melatonin reduces pyramidal neuronal death in the hippocampus and prevents the impairment of place learning and memory in the Morris water maze, otherwise occurring following global cerebral ischemia. The cytoarchitectonic characteristics of the hippocampal CA1 remaining pyramidal neurons in brains of rats submitted 120 days earlier to acute global cerebral ischemia (15-min four vessel occlusion, and melatonin 10mg/(kg h 6h), i.v. or vehicle administration) were compared to those of intact control rats in order to gain information concerning the neural substrate underlying preservation of hippocampal functioning. Hippocampi were processed according to a modification of the Golgi method. Dendritic bifurcations from pyramidal neurons in both the oriens-alveus and the striatum radiatum; as well as spine density and proportions of thin, stubby, mushroom-shaped, wide, ramified, and double spines in a 50 microm length segment of an oblique dendrite branching from the apical dendrite of the hippocampal CA1 remaining pyramidal neurons were evaluated. No impregnated CA1 pyramidal neurons were found in the ischemic-vehicle-treated rats. CA1 pyramidal neurons from ischemic-melatonin-treated rats showed stick-like and less ramified dendrites than those seen in intact control neurons. In addition, lesser density of spines, lower proportional density of thin spines, and higher proportional density of mushroom spines were counted in ischemic-melatonin-treated animals than those in the sinuously branched dendrites of the intact control group. These cytoarchitectural arrangements seem to be compatible with place learning and memory functions long after ischemia and melatonin neuroprotection.

Different patterns of motor activity induce differential plastic changes in pyramidal neurons in the motor cortex of rats: A Golgi study.

Rehabilitation is a process which favors recovery after brain damage involving motor systems, and neural plasticity is the only real resource the brain has for inducing neurobiological events in order to bring about re-adaptation. Rats were placed on a treadmill and made to walk, in different groups, at different velocities and with varying degrees of inclination. Plastic changes in the spines of the apical and basal dendrites of fifth-layer pyramidal neurons in the motor cortices of the rats were detected after study with the Golgi method. Numbers of dendritic spines increased in the three experimental groups, and thin, mushroom, stubby, wide, and branched spines increased or decreased in proportion depending on the motor demands made of each group. Along with the numerical increase of spines, the present findings provide evidence that dendritic spines' geometrical plasticity is involved in the differential performance of motor activity.