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 [17β-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.

Spinogenesis in spinal cord motor neurons following pharmacological lesions to the rat motor cortex

INTRODUCTION

Motor function is impaired in multiple neurological diseases associated with corticospinal tract degeneration. Motor impairment has been linked to plastic changes at both the presynaptic and postsynaptic levels. However, there is no evidence of changes in information transmission from the cortex to spinal motor neurons.

METHODS

We used kainic acid to induce stereotactic lesions to the primary motor cortex of female adult rats. Fifteen days later, we evaluated motor function with the BBB scale and the rotarod and determined the density of thin, stubby, and mushroom spines of motor neurons from a thoracolumbar segment of the spinal cord. Spinophilin, synaptophysin, and β iii-tubulin expression was also measured.

RESULTS

Pharmacological lesions resulted in poor motor performance. Spine density and the proportion of thin and stubby spines were greater. We also observed increased expression of the 3 proteins analysed.

CONCLUSION

The clinical symptoms of neurological damage secondary to Wallerian degeneration of the corticospinal tract are associated with spontaneous, compensatory plastic changes at the synaptic level. Based on these findings, spontaneous plasticity is a factor to consider when designing more efficient strategies in the early phase of rehabilitation.

Spinogenesis and Plastic Changes in the Dendritic Spines of Spinal Cord Motoneurons After Traumatic Injury in Rats

BACKGROUND

Spinal cord injury (SCI) is highly incapacitating, and the neurobiological factors involved in an eventual functional recovery remain uncertain. Plastic changes to dendritic spines are closely related with the functional modifications of behavior.

AIM OF THE STUDY

To explore the plastic response of dendritic spines in motoneurons after SCI.

METHODS

Female rats were assigned to either of three groups: Intact (no manipulations), Sham (T9 laminectomy), and SCI (T9 laminectomy and spinal cord contusion).

RESULTS

Motor function according to a BBBscale was progressively recovered from 2 week through 8 week postinjury, reaching a plateau through week 16. Dendritic spine density was greater in SCI vs. control groups, rostral as well as caudal to the lesion, at 8 and 16 weeks postinjury. Thin and stubby/wide spines were more abundant at both locations and time points, whereas mushroom spines predominated at 2 and 4 months in rostral to the lesion. Filopodia and atypical structures resembling dendritic spines were observed. Synaptophysin expression was lower in SCI at the caudal portion at 8 weeks, and was higher at week 16.

CONCLUSION

Spinogenesis in spinal motoneurons may be a crucial plastic response to favor spontaneous recovery after SCI.

Motor learning induces plastic changes in Purkinje cell dendritic spines in the rat cerebellum

INTRODUCTION

The paramedian lobule of the cerebellum is involved in learning to correctly perform motor skills through practice. Dendritic spines are dynamic structures that regulate excitatory synaptic stimulation. We studied plastic changes occurring in the dendritic spines of Purkinje cells from the paramedian lobule of rats during motor learning.

METHODS

Adult male rats were trained over a 6-day period using an acrobatic motor learning paradigm; the density and type of dendritic spines were determined every day during the study period using a modified version of the Golgi method.

RESULTS

The learning curve reflected a considerable decrease in the number of errors made by rats as the training period progressed. We observed more dendritic spines on days 2 and 6, particularly more thin spines on days 1, 3, and 6, fewer mushroom spines on day 3, fewer stubby spines on day 1, and more thick spines on days 4 and 6.

CONCLUSION

The initial stage of motor learning may be associated with fast processing of the underlying synaptic information combined with an apparent "silencing" of memory consolidation processes, based on the regulation of the neuronal excitability.

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.

Guided motor training induces dendritic spine plastic changes in adult rat cerebellar purkinje cells

The simple cerebellar lobule is involved in several neuromotor processes and it is activated during guided exercise. Although guided exercises are essential for motor rehabilitation, the plastic events that occur in the simple cerebellar lobule during motor training remain unknown. In this study, normal adult rats were intensely trained on a motorized treadmill during a period of four weeks (IT group) varying both the velocity and the slope of the moving belt, and they were compared to a mildly trained (MC) group and an intact control group (IC). Dendritic spine density and proportions of the different spine types on Purkinje cells was assessed in the cerebellar simple lobule, as was drebrin A expression. Both dendritic spine density and drebrin expression increased in the MC and IT groups. Stubby spines were more abundant in the MC animals, while there was an increase in both stubby and wide spines in IT rats. In addition, mushroom spines were more numerous in the IT group. Increases in stubby and wide spines could be related to regulation of the excitability in Purkinje cells due to the motor training regime experienced by the MC and IT rats. Moreover, the observed increase in mushroom spines in the IT group could be related with the motor adjustments imposed by training.