Research
Interests
The long-term goals of my research are to understand the
relationship between neuronal structure and function, and to elucidate the
factors that affect neuronal morphology and function over the lifespan of the
mammal. Currently we are examining 1) the effects of synaptic activity on
neuronal development; 2) the effects of estrogen on neuronal morphology and on
learning and memory; and, 3) the effects of aging on neuronal structure and
function. We have focused our efforts on single neurons in the hippocampal
formation, a region that is critical for certain forms of learning and memory in
rodents and humans.
Although it has been known for some time that synaptic activity is necessary for maturation of the nervous system, the effects of synaptic activity on the early development of dendritic structure are not yet understood. We found that granule neurons in the dentate gyrus of the hippocampal formation undergo significant morphological changes over the first week of life in the rat (Jones et al., 2003) and that by day 7, at least some of these neurons exhibit synaptic plasticity in vivo, including long-term potentiation (LTP) and long-term depression (O'Boyle et al., 2004). Results also indicate that LTP at this age is blocked by an antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor, as it is in the adult. In addition, we tested the hypothesis that synaptic activity is necessary for the regression of immature dendritic features, and for the development of dendritic spines on these same neurons. Rats between the ages of 6 and 9 days were treated with an antagonist to the NMDA receptor. Results indicated that regression of immature dendritic features and the appearance of dendritic spines were attenuated, as compared to controls (Sanchez et al., 2001). Taken together, these data show that at least some neurons in the dentate gyrus are functional by day 7, much earlier than was previously thought, and that the development of dendritic trees in this region is dependent on synaptic activity. We are now interested in identifying specific genes that may be involved in mediating the effects of synaptic activity on early granule neuron development.
Women
undergoing natural or surgically-induced menopause are faced with the choice of
taking or forgoing estrogen replacement therapy. Given the conflicting results
about the risks and benefits of such therapy, the choice is a difficult one. We
have completed a series of experiments aimed at exploring the effects of
estrogen on brain structure, and on learning and memory in adult and aged
ovariectomized rats. In some experiments, we used either an oral delivery route
(comparable to the oral route used by most women) or an implanted, slow-release
capsule (comparable to the transdermal patch used by women), and either a high
or low dose of estrogen. First, we found that the effects of estrogen on
reference memory in the Morris water maze task, and on working memory in a
radial arm water maze vary with dose and route of administration in young adult
rats (Cantu et al., 2002; A. Garza, R. Cantu and B. Claiborne, unpublished
data). These results are important because they suggest that both the dosage and
the route of administration are critical variables in estrogen replacement
therapy. Second, we found that the effects of estrogen on working memory in the
radial arm water maze vary with age when estrogen is administered via an
implanted capsule (Garza-Meilandt et al., 2003). Data indicated that aged
estradiol-treated rats made fewer working memory errors than did aged
vehicle-treated animals, while young adult estradiol-treated and young adult
vehicle-treated rats performed better than comparable groups of aged animals, as
reported previously by others. Third, and of particular interest, data indicated
estrogen treatment and handling prior to behavioral testing interact to
influence performance and brain structure in young adult rats (Cantu et al.,
2001; Garza-Meilandt et al., 2002). Estrogen treatment improved performance in non-handled rats but impaired
performance in handled rats. In the non-handled rats that were treated with
estrogen, spine densities on pyramidal neurons in the hippocampus were
increased. In contrast, there was no effect of estrogen treatment on spine
densities in handled rats. These data are important because they suggest that
handling is a critical variable in experimental protocols aimed at deciphering
the effects of estrogen in rodents. These results may also explain the
difficulties that several labs have encountered in trying to replicate the
finding that estrogen increases spine densities on dendrites of CA1 pyramidal
neurons. Currently, in collaboration with Dr. Helen Scharfman (Helen Hayes
Hospital and Columbia University), we are exploring the possibility that the
effects of estrogen treatment on brain structure and performance may be mediated
by brain-derived neurotrophic factor (BDNF).
Learning and memory are impaired in some aged rodents and humans. Although previous studies indicate that there are physiological changes in neurons in aged rodents, we know very little about structural changes or about the potential interactions between dendritic morphology and biophysical parameters in the aged brain. We are interested in these interactions in neurons in the aging hippocampal formation, a region known to be critical for certain forms of learning and memory. Our preliminary data indicate that granule neurons in the dentate gyrus of the hippocampal formation increase in size in aged male mice (Rahimi et al., 2003). Specifically, the number of dendritic branches increases, leading to an increase in the total dendritic length per neuron. In pursuing these results, our overall goal is to test the hypotheses that such changes in dendritic structure impair neuronal function. The first specific aim of our current project is to verify that the dendritic length of hippocampal neurons increases in aged male mice, as suggested by our preliminary data. To quantify dendritic structure, we are using a novel line of transgenic mice that express green fluorescent protein in some hippocampal neurons, in addition to making use of conventional labeling with the fluorescent dye, DiI, in wild type mice. The second specific aim is to test the hypothesis that the density of excitatory synapses on hippocampal neurons decreases in the aged male mouse, while the density of inhibitory synapses does not change. Densities will be measured using electron microscopy. Our third specific aim is to test the hypothesis that morphological changes in aged male mice lead to less effective integration of synaptic currents, and hence impaired neuronal function. Physiological and morphological data from neurons in adult and aged mice will be combined to construct computational models (see Mainen et al., 1996; Carnevale et al., 1997). The models will then be used to test the hypothesis. In the future, we plan to examine the effects of synaptic activity on single neurons in aged brains, and to explore the molecular basis of such effects.