Biomedical subjects

Adriana Galvan

Publications and source records attributed to Adriana Galvan.

7 recordsLinked to original sources

PubMed2006

A shift from diffuse to focal cortical activity with development.

Recent imaging studies have suggested that developmental changes may parallel aspects of adult learning in cortical activation becoming less diffuse and more focal over time. However, while adult learning studies examine changes within subjects, developmental findings have been based on cross-sectional samples and even comparisons across studies. Here, we used functional MRI in children to test directly for shifts in cortical activity during performance of a cognitive control task, in a combined longitudinal and cross-sectional study. Our longitudinal findings, relative to our cross-sectional ones, show attenuated activation in dorsolateral prefrontal cortical areas, paralleled by increased focal activation in ventral prefrontal regions related to task performance.

Age Factors↗

PubMed2005

The role of ventral frontostriatal circuitry in reward-based learning in humans.

This study examined changes in behavior and neural activity with reward learning. Using an event-related functional magnetic resonance imaging paradigm, we show that the nucleus accumbens, thalamus, and orbital frontal cortex are each sensitive to reward magnitude, with the accumbens showing the greatest discrimination between reward values. Mean reaction times were significantly faster to cues predicting the greatest reward and slower to cues predicting the smallest reward. This behavioral change over the course of the experiment was paralleled by a shift in peak in accumbens activity from anticipation of the reward (immediately after the response), to the cue predicting the reward. The orbitofrontal and thalamic regions peaked in anticipation of the reward throughout the experiment. Our findings suggest discrete functions of regions within basal ganglia thalamocortical circuitry in adjusting behavior to maximize reward.

Adult↗

PubMed2005

GABAergic modulation of the activity of globus pallidus neurons in primates: in vivo analysis of the functions of GABA receptors and GABA transporters.

Neurons in the external and internal segment of the globus pallidus (GPe and GPi, respectively) receive substantial GABAergic inputs from the striatum and through axon collaterals of neighboring pallidal neurons. The effects of GABA on pallidal activity depend on the synaptic localization of GABA receptors and the distribution and activity of GABA transporters (GATs). To explore the contribution of GABA receptors and transporters to pallidal function, we recorded the activity of single neurons in GPe or GPi before, during, and after local microinjections of GABAergic compounds in awake rhesus monkeys. Activation of GABA(A) or GABA(B) receptors with muscimol or baclofen, respectively, inhibited pallidal activity. These effects were reversed by concomitant infusion of the respective GABA receptor antagonists, gabazine and CGP-55845. Given alone, the antagonists were without consistent effect. Application of the selective GAT-1 inhibitor, SKF-89976A, and the semiselective GAT-3 blocker, SNAP-5114, decreased pallidal activity. Both GAT inhibitors increased GABA levels in the pallidum, as measured by microdialysis. Electron microscopic observations revealed that these transporters are located on glial processes and unmyelinated axonal segments, but rarely on terminals. Our results indicate that activation of GABA(A) and GABA(B) receptors inhibits neuronal activity in both segments of the pallidum. GAT-1 and GAT-3 are involved in the modulation of endogenous GABA levels and may be important in regulating the extrasynaptic levels of GABA. Together with previous evidence that a considerable proportion of pallidal GABA receptors are located outside the synaptic cleft, our experiments strongly support the importance of extrasynaptic GABAergic transmission in the primate pallidum.

Action Potentials↗

PubMed2005

Changes in cerebral functional organization during cognitive development.

It has been just under a decade since contemporary neuroimaging tools, such as functional magnetic resonance imaging, were first applied to developmental questions. These tools provide invaluable information on how brain anatomy, function and connectivity change during development. Studies using these methods with children and adolescents show that brain regions that support motor and sensory function mature earliest, whereas higher-order association areas, such as the prefrontal cortex, which integrate these functions, mature later.

Cerebral Cortex↗

PubMed2004

An electron microscope immunocytochemical study of GABA(B) R2 receptors in the monkey basal ganglia: a comparative analysis with GABA(B) R1 receptor distribution.

Functional gamma-aminobutyric acid (GABA)(B) receptors are heterodimers made up of GABA(B) R1 and GABA(B) R2 subunits. The subcellular localization of GABA(B) R2 receptors remains poorly known in the central nervous system. Therefore, we performed an ultrastructural analysis of the localization of GABA(B) R2 receptor immunoreactivity in the monkey basal ganglia. Furthermore, to characterize better the neuronal sites at which GABA(B) R1 and GABA(B) R2 may interact to form functional receptors, we compared the relative distribution of immunoreactivity of the two GABA(B) receptors in various basal ganglia nuclei. Light to moderate GABA(B) R2 immunoreactivity was found in cell bodies and neuropil elements in all basal ganglia nuclei. At the electron microscope level, GABA(B) R2 immunoreactivity was commonly expressed postsynaptically, although immunoreactive preterminal axonal segments were also frequently encountered, particularly in the globus pallidus and substantia nigra, where they accounted for the third of the total number of GABA(B) R2-containing elements. A few labeled terminals that displayed the ultrastructural features of glutamatergic boutons were occasionally found in most basal ganglia nuclei, except for the subthalamic nucleus, which was devoid of GABA(B) R2-immunoreactive boutons. The relative distribution of GABA(B) R2 immunoreactivity in the monkey basal ganglia was largely consistent with that of GABA(B) R1, but some exceptions were found, most noticeably in the globus pallidus and substantia nigra, which contained a significantly larger proportion of presynaptic elements labeled for GABA(B) R1 than GABA(B) R2. These findings suggest the possible coexistence and heterodimerization of GABA(B) R1 and GABA(B) R2 at various pre- and postsynaptic sites, but also raise the possibility that the formation of functional GABA(B) receptors in specific compartments of basal ganglia neurons relies on mechanisms other than GABA(B) R1/R2 heterodimerization.

Animals↗

PubMed2004

Early development of subcortical regions involved in non-cued attention switching.

This study examined the cognitive and neural development of attention switching using a simple forced-choice attention task and functional magnetic resonance imaging Fourteen children and adults made discriminations among stimuli based on either shape or color. Performance on these trials was compared to performance during blocked trials requiring all color or all shape discriminations. Magnetic resonance echo planar images were acquired during performance of the task. Both children and adults showed robust bilateral activity of the caudate nucleus when switching attention between color and shape discriminations that correlated negatively with mean response latency on these trials. However, neither switching costs nor caudate activity correlated with age, suggesting early development of the underlying neural circuitry involved in switching between salient stimulus sets. Overall, children and adults differed in performance and patterns of brain activity on the task, with adults responding more accurately and faster than children, and recruiting more prefrontal and parietal regions. These results suggest an important role of subcortical regions (i.e. caudate nucleus) in non-cued attention switching, with increasing recruitment of cortical regions with age.

Adolescent↗

PubMed2003

Continuous monitoring of intracerebral glutamate levels in awake monkeys using microdialysis and enzyme fluorometric detection.

A technique for continuous on-line detection of glutamate using brain microdialysis in awake primates is described. The method is based on an enzymatic assay using fluorescence detection of glutamate. The time resolution of the continuous fluorescent readout compares favorably with that of most published studies, which have used standard high-pressure liquid chromatography detection methods for glutamate. Exposure of the system to other amino acids (GABA, aspartate, glutamine, ascorbate, taurine, valine, alanine, and D-glutamate) revealed that this method is highly specific for L-glutamate. In vitro, the system detects reliably glutamate levels as low as 0.5 micromol/l. In vivo testing in the striatum of Rhesus macaques showed that glutamate levels were enhanced after reverse microdialysis with a glutamate uptake blocker. Stimulation with high potassium increased substantially the levels of glutamate, an effect that was calcium-dependent. Glutamate levels were also increased when the microdialysis solution contained the blocker of voltage-gated potassium channels, 4-aminopyridine. This technique effectively detects short-term changes in glutamate levels evoked by physiologic or pharmacologic manipulations in the primate brain.

4-Aminopyridine↗