The functional architecture of the human dorsolateral prefrontal cortex (DLPFC), including its connections with other brain regions, appears to be specialized to mediate complex cognitive processes such as those that depend upon working memory--the ability to transiently retain and manipulate a limited amount of information in order guide a logical sequence of thoughts or behaviors. The normal organization of DLPFC neural circuitry and the underlying patterns of gene expression that subserve these functions are examined through a complementary set of research approaches utilizing the macaque monkey as a model system for the human brain. For example, tract-tracing, intracellular filling, immunocytochemical and electron microscopy techniques are combined to examine the connectivity among specific neuronal populations (Figures 1 and 2). In addition, the expression and localization of gene products within the specific cellular components of these circuits, and how these change in an activity-dependent fashion, are investigated using a range of approaches. For example, we are currently pursuing studies designed to determine the molecular profiles that distinguish separate populations of DLPFC pyramidal neurons based on their axonal projection target. Another major focus is the role of the cannabinoid receptor 1 (CB1) in regulating GABA neurotransmission in the prefrontal cortex (Figure 3), which may explain why the use of cannabis during adolescence increases the risk for the later development of schizophrenia. The functional properties of these neuronal populations and local circuits are examined through integrated anatomical-electrophysiological studies using an in vitro living slice preparation of monkey DLPFC (Figure 4). These studies are conducted in collaboration with Program members Drs. Guillermo Gonzalez-Burgos and Leonid Krimer.

Figure 1. Electron micrograph of biotinylated dextran amine (BDA)-labeled axon terminal that forms an asymmetric synapse (curved arrow) onto a dendritic spine in layer 3 of monkey DLPFC. The postsynaptic spine contains a well-defined spine apparatus (sa) and the spine neck and parent dendrite (d) are both apparent. (Melchitzky DS, Sesack SR, Pucak ML, Lewis DA: Synaptic targets and extrinsic projections of pyramidal neurons providing horizontal connections in monkey prefrontal cortex. J Comp Neurol 390:211-224, 1998.

Figure 2. Schematic diagram illustrating the morphological and biochemical features of subpopulations of cortical GABA neurons in the primate DLPFC. The diagram illustrates the calcium-binding proteins-parvalbumin (blue), calbindin (red) and calretinin (yellow)-and the locations of inhibitory synaptic inputs to a pyramidal neuron (green) by different morphological classes of cortical GABA neurons. The chandelier (Ch) and wide arbor (WA) or basket neurons provide inhibitory input to the axon initial segment (ais) and the cell body proximal dendrites, respectively, of pyramidal neurons. By contrast, the calbindin-expressing double bouquet (red DB), neurogliaform (Ng) and Martinotti (M) neurons tend to provide inhibitory inputs to the distal dendrites of pyramidal neurons. Finally, calretinin-expressing (yellow) DB and Cajal-Retzius cell (CRC) appear to target both pyramidal cell distal dendrites and other GABA (G) neurons. (Lewis DA, Hashimoto T, Volk DW: Cortical inhibitory neurons and schizophrenia. Nature Reviews Neuroscience 6:312-324, 2005.)

Figure 3. Photomicrographs illustrating CB1 immunoreactivity in sagittal (left) and coronal (right) sections through the macaque monkey brain at the levels shown in the upper left. Across the monkey neocortex, CB1-immunoreactive (IR) axons exhibited considerable heterogeneity in density and laminar distribution. Association regions, such as the prefrontal and cingulate cortices, demonstrated a higher density, and exhibited a unique laminar pattern of CB1-IR axons, compared to primary sensory and motor cortices. Similar regional and laminar distributions of CB1-IR axons were also present in the human neocortex. In electron microscopy studies of monkey prefrontal cortex, CB1 immunoreactivity was predominantly found in axon terminals that exclusively formed symmetric synapses. The high density, distinctive laminar distribution, and localization to inhibitory terminals of CB1 receptors in primate higher order association regions suggests that the CB1 receptor may play a critical role in the circuitry that subserves cognitive functions such as those that are disturbed in schizophrenia. (Eggan SM, Lewis DA: Immunocytochemical distribution of the cannabinoid CB1 receptor in the primate cerebral cortex: A regional and laminar analysis. Cereb Cortex in press.

Figure 4. Living tissue slices are prepared from the monkey DLPFC (A,B). Fast-spiking (FS) interneurons and pyramidal cells (PC) are visualized and recorded (C). Synaptically-connected presynaptic FS chandelier cell and a postsynaptic pyramidal cell were filled with biocytin during the electrophysiological recording (D). The pyramidal cell and chandelier cell axons are marked by the red and black arrows, respectively. In the partial reconstruction of the same cell pair shown in panel E, dendrites and axons are shown in thick and thin traces, respectively. Note that the chandelier cell axon is apposed to the pyramidal cell axon initial segment and appears to establish multiple synaptic contacts in this region. In both figures, the location of the putative synaptic contacts, as identified by differential interference contrast microscopy, is shown by the small arrows and circles. No putative appositions were observed at the proximal dendrites or soma of the pyramidal cell. The putative synapses formed a vertical arrangement or cartridge, typical of chandelier neuron axons. The distinctive firing properties of the fast-spiking chandelier cell are shown in panel F. (González-Burgos G, Krimer LS, Povysheva NV, Barrionuevo G, Lewis DA: Functional properties of fast spiking interneurons and their synaptic connections with pyramidal cells in primate dorsolateral prefrontal cortex. J Neurophysiology 93:942-953, 2005.)

• Translational Neuroscience Program • | Home | David A. Lewis, M.D. | Department of Psychiatry | University of Pittsburgh 3811 O'Hara Street, Biomedical Science Tower W1654 Pittsburgh, Pennsylvania 15213-2593 Phone: (412) 624-3934 - Fax: (412) 624-9910