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MAILMAN RESEARCH CENTER

Program for Structural and Molecular Neuroscience

McLean Researcher Francine Benes Earns Prestigious Mentoring Award

The Program for Structural and Molecular Neuroscience was established in 1982 as the Laboratory for Structural Neuroscience. The research work of the lab is focused on learning how neural circuitry is altered in corticolimbic regions of schizophrenic and bipolar brain. This program uses a three-pronged translational approach involving cellular and molecular studies of postmortem brain tissue, rodent modeling for postmortem changes and studies of postnatal maturation of the brain during the equivalent of late adolescence when schizophrenia typically begins. Each of these strategies is summarized below:

Neurotransmitters

Interactions among three neurotransmitters may help us to better understand the schizophrenic brain

Studies of the Limbic System in Schizophrenia and Bipolar Disorder

For the past 20 years, the work in this laboratory has been systematically pursuing the question of whether there are alterations in the circuitry of the limbic lobe in schizophrenia (SZ) and, more recently, bipolar disorder (BD). By using a variety of cytochemical methods, we have demonstrated that there are preferential changes in the GABA system in layer II of the anterior cingulate cortex (ACCx-II) and sectors CA3 and CA2 of the hippocampus, two sites that receive a prominent innervation from the basolateral nucleus of the amygdala (BLa). The connections between the amygdala and the hippocampus may be particularly important for our understanding of the pathophysiology of neuropsychiatric disorders, Most prominent have been changes observed in the GABA system, but other alterations have also been observed in the glutamate and dopamine systems as well.

Most recently, we have used microarray-based gene expression profiling, together with laser microdissection, to "deconstruct" the trisynaptic pathway in the hippocampus of normal controls, schizophrenics and bipolars.   We have used a network association analysis to demonstrate that there is a cluster of approximately 30 genes that are involved in the regulation of GAD67, a key marker for GABA neurons.  In schizophrenics, there are 10 genes within this network that point to a defect in the epigenetic regulation of GAD67.  In bipolars, however, GAD67 expression appears to be regulated by transcription factors that play a role in the differentiation of GABA cells during embryogenesis.  Overall, these results are consistent with the idea that a common cell phenotype (the dysfunctional GABA cell) may have fundamentally different underlying endophenotypes that are presumably related to specific susceptibility genes for the respective disorders. 

Our GEP/LMD results have also demonstrated that striking changes in expression in GABA cells occur for genes associated with neurogenesis, cell cycle regulation, the DNA damage response and synaptic transmission.  As predicted from our studies of DNA damage, bipolars are probably unable to repair damaged DNA, while schizophrenics are less likely to have DNA damage or are better equipped to repair it.  An important aspect of these findings has been the observation that the genes showing changes in GABA neurons vary dramatically according to their location along the trisynaptic pathway within the hippocampus.  This probably means that gene expression profiles in neurons are influenced by their synaptic interactions with various afferent inputs they receive from within the hippocampus and from fiber systems originating outside of this region.

Rodent Modeling for Postmortem Abnormalities in Schizophrenia and Bipolar Disorder

Our postmortem studies have demonstrated a preponderance of abnormalities in two discrete loci within the limbic lobe.  These include layer II of the anterior cingulate cortex and sectors CA3/2 of the hippocampus.  Both of these sites receive a robust projection from the basolateral amygdala (BLA) and we have postulated that these latter fibers may play a role in the triggering abnormalities we have observed in our postmortem studies. To model for microscopic changes in ACCx-II and CA3/2 of the HIPP of schizophrenics, we have developed a model in which a GABA-A receptor antagonist picrotoxin is infused into the BLn of the amygdala to increase the outflow of excitatory glutamatergic activity from this region to the two termination sites. The results of our first study showed a decrease of GABAergic terminals with a similar subregional distribution to that seen in schizophrenia. More recently, we have extended this work by examining the effects of picrotoxin infusion 96 hrs later and when applied during the equivalent of late adolescence and early adulthood.  Using electrophysiological in vivo recording techniques, we have demonstrated that stimulation by BLA fibers results in a decrease of inhibitory postsynaptic potentials in projections neurons in sector CA3/2 of the hippocampus.  This latter study has suggested that the GABAergic abnormalities observed in our postmortem studies are probably associated with decreases of inhibitory modulation.  We are also using this model to systematically explore whether amygdalar activation influences the expression of genes linked to the regulation of GAD67 in GABAergic interneurons (see above).

Postnatal Development of the Limbic Lobe

In order to explore the question of whether normal postnatal developmental changes may "trigger" the onset of schizophrenia between 16 and 25 years of age, we have studied normal maturational changes in both human and rodent brain. Using quantitative techniques and a very large cohort, an earlier study from this laboratory demonstrated for the first time that there are pronounced increases in the amount of myelinated axons in the medial temporal lobe of human brain during adolescence. In parallel rodent studies, we have been able to assess the postnatal development of more specific neural systems, such as the GABA, dopamine (DA), serotonin (5HT) and glutamate systems; all of these  that have been implicated in schizophrenia and bipolar disorder. For example, the GABA system continues to mature until the equivalent of early adolescence, while DA fibers show a progressive infiltration of the medial prefrontal cortex (anterior cingulate region) until the start of the early adult period. When serotonergic fibers in the dorsal raphe nucleus are lesioned soon after birth, the DA system shows an opportunistic increase in the density of its connections with cingulate neurons. This suggests that these systems are highly plastic and subject to re-arrangements if perturbations of normal development occur.  More recent work has demonstrated that amygdalar fibers are also growing into the cingulate cortex at approximately the same time as DA fibers. Using anterograde tracing, we have been able to show a marked increase of BLn fibers in ACCx that continues into the early adult period.  The ingrowth of BLn fibers and its formation of functional interactions with GABA cells could be a "trigger" for the start of the schizophrenic syndrome during the equivalent of adolescence. 

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