Dulce A. Velázquez-Zamora

Dendritic Spine Abnormalities in Hippocampal CA1 Pyramidal Neurons Underlying Memory Deficits in the SAMP8 Mouse Model of Alzheimer's Disease

SAMP8 is a strain of mice with accelerated senescence. These mice have recently been the focus of attention as they show several alterations that have also been described in Alzheimers disease (AD) patients. The number of dendritic spines, spine plasticity, and morphology are basic to memory formation. In AD, the density of dendritic spines is severely decreased. We studied memory alterations using the object recognition test. We measured levels of synaptophysin as a marker of neurotransmission and used Golgi staining to quantify and characterize the number and morphology of dendritic spines in SAMP8 mice and in SAMR1 as control animals. While there were no memory differences at 3 months of age, the memory of both 6- and 9-month-old SAMP8 mice was impaired in comparison with age-matched SAMR1 mice or young SAMP8 mice. In addition, synaptophysin levels were not altered in young SAMP8 animals, but SAMP8 aged 6 and 9 months had less synaptophysin than SAMR1 controls and also less than 3-month-old SAMP8 mice. Moreover, while spine density remained stable with age in SAMR1 mice, the number of spines started to decrease in SAMP8 animals at 6 months, only to get worse at 9 months. Our results show that from 6 months onwards SAMP8 mice show impaired memory. This age coincides with that at which the levels of synaptophysin and spine density decrease. Thus, we conclude that together with other studies that describe several alterations at similar ages, SAMP8 mice are a very suitable model for studying AD.

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.

Plastic changes in dendritic spines of hippocampal CA1 pyramidal neurons from ovariectomized rats after estradiol treatment

Cognitive impairment or its recovery has been associated with the absence or reestablishment of estrogenic actions in the central nervous system of female experimental animals or women. It has been proposed that these cognitive phenomena are related to estrogen-mediated modulatory activity of synaptic transmission in brain structures involved in cognitive functions. In the present work a morphological study was conducted in adult female ovariectomized rats to evaluate estradiol-dependent dendritic spine sprouting in hippocampal pyramidal neurons, and changes in the presynaptic marker synaptophysin. Three or ten days after estradiol treatment (10 μg/day, twice) in the ovariectomized rats, a significant increase of synaptophysin was observed, which was coincident with a significant higher numerical density of thin (22%), stubby (36%), mushroom (47%) and double spines (125%), at day 3, without significant changes of spine density at day 10, after treatment. These results may be interpreted as evidence of pre- and postsynaptic plastic events that may be involved in the modulation of cognitive-related behavioral performance after estrogen replacement therapy.

Morphological development of dendritic spines on rat cerebellar Purkinje cells.

The posterior cerebellum is strongly involved in motor coordination and its maturation parallels the development of motor control. Climbing and mossy fibers from the spinal cord and inferior olivary complex, respectively, provide excitatory afferents to cerebellar Purkinje neurons. From post‐natal day 19 climbing fibers form synapses with thorn‐like spines located on the lower primary and secondary dendrites of Purkinje cells. By contrast, mossy fibers transmit synaptic information to Purkinje cells trans‐synaptically through granule cells. This communication occurs via excitatory synapses between the parallel fibers of granule cells and spines on the upper dendritic branchlets of Purkinje neurons that are first evident at post‐natal day 21. Dendritic spines influence the transmission of synaptic information through plastic changes in their distribution, density and geometric shape, which may be related to cerebellar maturation. Thus, spine density and shape was studied in the upper dendritic branchlets of rat Purkinje cells, at post‐natal days 21, 30 and 90. At 90 days the number of thin, mushroom and thorn‐like spines was greater than at 21 and 30 days, while the filopodia, stubby and wide spines diminished. Thin and mushroom spines are associated with increased synaptic strength, suggesting more efficient transmission of synaptic impulses than stubby or wide spines. Hence, the changes found suggest that the development of motor control may be closely linked to the distinct developmental patterns of dendritic spines on Purkinje cells, which has important implications for future studies of cerebellar dysfunctions.

Cytoarchitectural characteristics of hippocampal CA1 pyramidal neurons of rats, four months after global cerebral ischemia and progesterone treatment

PURPOSE To analyze the cytoarchitectural characteristics of the remaining pyramidal neurons in the hippocampal CA1 subfield of rats, four months after global cerebral ischemia (GCI) and progesterone treatment. METHODS Dendritic arborization, and density and shape of the dendritic spines of CA1 pyramidal neurons in brains of intact rats, or rats submitted 120 days earlier to GCI and treatment with progesterone (8 mg/kg) or its vehicle, at 15 min, and 2, 6, 24, 48, and 72 h after the onset of reperfusion, were analyzed in samples processed by a modified Golgi method. RESULTS Few impregnated CA1 pyramidal neurons were identified in the ischemic vehicle-treated rats, with a short apical dendrite devoid of bifurcations and dendritic spines. In contrast, the remaining CA1 pyramidal neurons sampled from ischemic progesterone-treated rats showed sinuously branched dendrites with similar number of bifurcations and whole density of spines, and higher proportional density of mushroom spines than those in the intact group. CONCLUSIONS These cytoarchitectural characteristics may underlie the long-term preservation of place learning and memory functions seen after ischemia and progesterone neuroprotective treatment, possibly compensating for the severe reduction in neuronal population.

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.

Damage and plasticity in adult rat hippocampal trisynaptic circuit neurons after neonatal exposure to glutamate excitotoxicity

Hippocampal vulnerability to excitotoxicity has been widely studied along with its implication to learning and memory. Neonatal glutamate excitotoxicity induces loss of CA1 pyramidal neurons in adult rats concomitantly with some plastic changes in the dendritic spines of surviving neurons. At least in part, these may underlie the place learning impairments seen in previous studies based on a similar excitotoxicity‐inducing model. In the present study, cytoarchitecture of dentate gyrus, CA3 and CA1 fields were evaluated in 120‐day‐old rats, after they had been neonatally treated with glutamate as monosodium salt. Dentate granule cells and CA1 pyramidal neurons were less than those counted in NaCl‐treated control animals. In addition, dentate granule cells had more dendrites as well as more branched spines. Spine density in CA1 pyramidal neurons was greater than in the controls. Additionally, thin and mushroom spines were proportionally more abundant in monosodium glutamate‐treated animals. No effects were seen in the hippocampal CA3 field. Our results strongly suggest a long‐term induction of plastic changes in the cytoarchitecture of the hippocampal trisynaptic circuit neurons after cell death provoked by the monosodium glutamate‐induced excitotoxicity. These plastic events as well as the aberrant expression of the glutamate NMDA receptors resulting from monosodium glutamate neonatal treatment could be strongly associated with the place learning impairments previously reported.

The motor learning induces plastic changes in dendritic spines of Purkinje cells from the neocerebellar cortex of the rat.

PURPOSE The presynaptic stimulatory activity of parallel fibers on the dendritic spines of cerebellar Purkinje cells (PC) has a strong influence on the organization of motor learning. Motor learning has been shown to modify the synapses established on PC dendritic spines but the plastic changes of the different spine types, possibly underlying motor learning, have not been studied. METHODS Adult male Sprague-Dawley rats were trained daily for 26 days using an acrobatic paradigm (AC), at the end of which dendritic spine density and the proportion of the different types of spines was assessed. RESULTS The learning curves of AC rats reflected a robust decrease in the latency for resolution and in the errors committed during the first week of training, which subsequently stabilized until the end of training. Dendritic spine density was greater in these AC rats, reflected in a larger proportion of thin, mushroom and stubby spines. CONCLUSIONS Since thin spines are associated with acquiring novel information whilst mushroom spines are associated with long-term information storage, there appears to be a strong relationship between AC motor learning and consolidation. The increase in stubby spines could be related to the regulation of excitatory stimulation underlying motor overactivity.

A Golgi study of the plasticity of dendritic spines in the hypothalamic ventromedial nucleus during the estrous cycle of female rats

Estradiol-induced plasticity involves changes in dendritic spine density and in the relative proportions of the different dendritic spine types that influence neurons and neural circuits. Such events affect brain structures that control the timing of neuroendocrine and behavioral processes, influencing both reproductive and cognitive functions during the estrous cycle. Accordingly, to investigate the dendritic spine-related plastic changes that may affect the neural processes involved in mating, estradiol-mediated dendritic spine plasticity was studied in type II cells situated in the ventrolateral portion of the ventromedial hypothalamic nucleus (VMN) of female, adult rats. The rats were assigned to four different groups (n=6) in function of their stage in the estrous cycle: proestrus, estrus, metaestrus, and diestrus. Dendritic spine density and the proportions of the different spine types on type II neurons were analyzed in the ventrolateral region of the VMN of these animals. Dendritic spine density on primary dendrites of VMN type II neurons was significantly lower in metaestrus than in diestrus, proestrus and estrus (with no differences between these latter stages). However, a significant variation in the proportional density of the different spine types was found, with a higher proportion of thin spines in diestrus, proestrus and estrus than in metaestrus. Likewise, a higher proportion of mushroom spines was seen in diestrus and proestrus than in metaestrus, and a higher proportion of stubby spines in estrus than in diestrus and metaestrus. Very few branched spines were found during proestrus and they were not detected during estrus or metaestrus. The different types of dendritic spines in non-projection neurons of the VMN could serve to maintain greater synaptic excitatory activity when receptivity and estradiol levels are maximal. However, they may also fulfill an additional functional role when receptivity and estradiol decline. To date specific roles of the different types of spines in neural hypothalamic activity during the estrous cycle remain unknown and they clearly deserve further study.

Administration of Anastrozole to Ovariectomized Rats Impairs Working Memoryin Association with Plastic Changes to Dendritic Spines on Prefrontal Third-layerPyramidal Neurons

Gonadal estrogens influence several neurobiological events related to synaptic plasticity underlying cognitive behavior. Likewise, estradiol synthesized in neurons affects aspects of brain organization associated with cognition. Here, plastic changes to dendritic spines on third-layer pyramidal neurons from the prefrontal cortex of ovariectomized, anastrozole-treated female rats were studied. Anastrozole treatment dampened the efficiency of rats in resolving a spatial working memory test in the “Y” maze when compared with ovariectomized control and proestrus female rats. In addition, the administration of Anastrozole led to an increase in dendritic spines and filopodia on the pyramidal cells studied. The thin, mushroom, stubby and wide spines remained unchanged. Since filopodia are considered to be the precursors of novel spines, the increase in dendritic spines is consistent with the increase in filopodia. However, this was clearly insufficient to drive proper working memory performance, despite the apparent stability in the translation of synaptic information suggested by the similar spine types evident in the ovariectomized controls and the increases observed. These findings show that brain-derived estradiol is necessary for prefrontal activity to account for working memory performance. Further studies will be needed to elucidate the mechanisms underlying such spine enhancement in the absence of estradiol-mediated modulation of plasticity.