Hebbian synapses in cortical and hippocampal pathways

Impaired associative learning in schizophrenia: behavioral and computational studies

Associative learning is a central building block of human cognition and in large part depends on mechanisms of synaptic plasticity, memory capacity and fronto-hippocampal interactions. A disorder like schizophrenia is thought to be characterized by altered plasticity, and impaired frontal and hippocampal function. Understanding the expression of this dysfunction through appropriate experimental studies, and understanding the processes that may give rise to impaired behavior through biologically plausible computational models will help clarify the nature of these deficits. We present a preliminary computational model designed to capture learning dynamics in healthy control and schizophrenia subjects. Experimental data was collected on a spatial-object paired-associate learning task. The task evinces classic patterns of negatively accelerated learning in both healthy control subjects and patients, with patients demonstrating lower rates of learning than controls. Our rudimentary computational model of the task was based on biologically plausible assumptions, including the separation of dorsal/spatial and ventral/object visual streams, implementation of rules of learning, the explicit parameterization of learning rates (a plausible surrogate for synaptic plasticity), and learning capacity (a plausible surrogate for memory capacity). Reductions in learning dynamics in schizophrenia were well-modeled by reductions in learning rate and learning capacity. The synergy between experimental research and a detailed computational model of performance provides a framework within which to infer plausible biological bases of impaired learning dynamics in schizophrenia.

Binding and transport in norepinephrine transporters. Real-time, spatially resolved analysis in single cells using a fluorescent substrate

Monoamine transporters, the molecular targets for drugs of abuse and antidepressants, clear norepinephrine, dopamine, or serotonin from the synaptic cleft. Neurotransmitters, amphetamines, and neurotoxins bind before being transported, whereas cocaine and antidepressants bind to block transport. Although binding is crucial to transport, few assays separate binding from transport, nor do they provide adequate temporal or spatial resolution to describe real-time kinetics or localize sites of active uptake. Here, we report a new method that distinguishes substrate binding from substrate transport using single-cell, space-resolved, real-time fluorescence microscopy. For these studies we use a fluorescent analogue of 1-methyl-4-phenylpyridinium, a neurotoxic metabolite and known substrate of monoamine transporters, to assess binding and transport with 50-ms, sub-micron resolution. We show that ASP(+) (4-(4-(dimethylamino)styrl)-N-methylpyridinium) has micromolar potency for the human norepinephrine transporter, that ASP(+) accumulation is Na(+)-, Cl(-)-, cocaine-, and desipramine-sensitive and temperature-dependent, and that ASP(+) competes with norepinephrine uptake. Using this method we demonstrate that norepinephrine transporters are efficient buffers for substrate, with binding rates exceeding transport rates by 100-fold. Furthermore, substrates bind deep within the transporter, isolated from both the bath and the lipid bilayer. Although transport per se depends on Na(+) and Cl(-), binding is independent of Na(+) and actually increases in low Cl(-). We further demonstrate that ASP(+) interacts with transporters not only in transfected cells but in cultured neurons. ASP(+) is also a substrate for dopamine and serotonin transporters and therefore represents a powerful new technique for studying the biophysical properties of monoamine transporters, an approach also amenable to high throughput assays for drug discovery.

Positron emission tomography: measurement of the activity of second messenger systems

Phosphoinositide turnover is closely connected to modulation of synaptic function and is part of an important second messenger-producing system. New radioligands for imaging second messenger systems by positron emission tomography have been developed: carbon-11-labeled 1,2-diacylglycerols. The theoretical background of second messenger imaging is described in detail and the relation between the biologically active compounds and potential tracers for imaging second messenger systems is discussed. We report informative findings on postsynaptic biological responses in the living human brain of healthy normal subjects and with various diseases.

Age-related and laminar-specific dendritic changes in the medial frontal cortex of the rat

Early hypotheses that normal brain aging involves widespread loss of neurons have been revised in light of accumulating evidence that, in most regions of the brain, the number of neurons is stable throughout adulthood and senescence. It is not clear, however, that all aspects of neuronal structure are similarly maintained, and anatomical changes are likely to contribute to age-related declines in cognitive function. The extent and pattern of dendritic branches is one likely target for age-dependent regulation since dendrites remain plastic into adulthood and since dendrites, as the site of most synapses, critically regulate neuronal function. This study quantified the dendritic extent and geometry of superficial and deep pyramidal neurons in the medial frontal cortex of Brown Norway rats from young adulthood through senescence. This region of cortex is of specific interest given its involvement in a variety of cognitive functions that change with age. In the present study, age-related changes in dendritic extent were found to occur with remarkable specificity. Superficial, but not deep, pyramidal neurons exhibited ongoing dendritic growth after 2 months-of-age and then dendritic regression after 18 months-of-age. Apical and basal dendrites were similarly regulated; in each arbor adult growth and regression were limited to terminal dendritic segments. The focal specificity of age-related changes suggests several possible regulatory mechanisms, including regional changes in trophic support and in neuronal activity. Although restricted to specific neuronal populations, dendritic regression in aged animals is likely to contribute to cognitive changes associated with senescence.

Regulation of hippocampal cell adhesion molecules NCAM and L1 by contextual fear conditioning is dependent upon time and stressor intensity

Cell adhesion molecules (CAMs) of the immunoglobulin superfamily, NCAM and L1, as well as the post-translational addition of alpha-2, 8-linked polysialic acid (PSA) homopolymers to NCAM (PSA-NCAM), have been implicated in the neural mechanisms underlying memory formation. Given that the degree of stress elicited by the training situation is one of the key factors that influence consolidation processes, this study questioned whether training rats under different stressor intensities (0.2, 0.4, or 1 mA shock intensity) in a contextual fear conditioning task might regulate subsequent expression of NCAM, PSA-NCAM and L1 in the hippocampus, as evaluated immediately after testing rats for conditioning at 12 and 24 h after training. Behavioural inhibition (evaluated as a 'freezing' index) at testing and post-testing plasma corticosterone levels were also assessed. The results showed that 12 h post-training, conditioned animals displayed reduced NCAM, but increased L1, expression. At this time point, the group trained at the highest shock intensity (1 mA) also presented decreased PSA-NCAM expression. Analyses performed 24 h post-training indicated that the 1 mA group exhibited increased NCAM and L1 expression, but decreased expression of PSA-NCAM levels. In addition, L1 values that presented a shock intensity-dependent U-shaped pattern were also increased in the group trained at the lowest shock condition (0.2 mA) and remained unchanged in the intermediate shock condition (0.4 mA). Freezing and corticosterone values at both testing times were positively related with shock intensity experienced at training. Therefore, our results show a complex regulation of CAMs of the immunoglobulin superfamily in the hippocampus that depends upon stressor intensity and time factors. In addition, the pattern of CAMs expression found in the 1 mA group (which is the one that shows higher post-training corticosterone levels and develops the stronger and longer-lasting levels of fear conditioning) supports the view that, after a first phase of synaptic de-adherence during consolidation, NCAM and L1 might participate in the stabilization of selected synapses underlying the establishment of long-term memory for contextual fear conditioning, and suggests that glucocorticoids might play a role in the observed regulation of CAMs.