*L. R. JOHNSON1, A. AMIN1, A. KAMATH1, H. ALPHS1, M. HOU1, L. ALBERT JR1, J. RADLEY2, P. HOF3, J. E. LEDOUX1;
1Ctr. Neural Sci., NYU, New York, NY; 2Salk Inst., La Jolla, CA; 3Mt Sinai Sch. of Med., New York, NY

Symmetry is a fundamental property found in both the physical and natural worlds. Bilateral symmetry is also present in the organization of the brain, however the degree to which symmetry is also an organizing principal between and within the key constituent elements of the nervous system, neurons, is not known. We compared and contrasted the structural organization of principal neurons (PN) in the three subnuclei of the lateral amygdala (LA) of the rat and for comparison also from the infralimbic cortex (IL). The LA is a brain region previously shown to be crucial for Pavlovian fear conditioning mediated by structural plasticity (Lamprecht and LeDoux, 2004). We investigated PN of the lateral amygdala (LA) because these neurons have been extensively implicated in the formation of lasting emotional memories (LeDoux, 2000). We hypothesized that the branching pattern of the neuron dendrite is not completely random and in fact may be governed by specific rules. The LA is comprised of three subnuclei, the dorsal; ventral medial and ventral lateral (LAd, LAvm, LAvl). We electrophysiologically identified PN’s and compared and contrasted their structural organization using computerized reconstruction (Microbrightfield, VT). PN dendrites in the three subnuclei and in the IL cortex are different in spine density, length and number of segments per branch. Despite these differences, each subnucleus is symmetrical in its inter-branch ratio (IBR) for each measure. Moreover, we find that LA PN are also symmetrical in IBR of dendrite segment length and segment number. IL neurons are also found to be non variant from the LA PN IBR. We designate this novel form of natural symmetry dendrotaxic ratio symmetry. The identification of structural symmetry both between and within populations of neurons raises important questions for structural and functional homoeostasis in neuron networks. These results open the question of the extent to which neurons homeostatically compensate to maintain symmetry following changes in spine density, dendrite number and length in response to plasticity and hormonal regulation. Finally our findings of symmetry between dendrite branch number and length likely have important implication for synaptic integration. Future research into how stable is dendrite symmetry over conditions of plasticity and learning as well as identification of the molecular mechanisms driving dendrite symmetry should prove to be fruitful and powerful lines of enquiry.

Support Contributed By: R01 MH46516, R37 MH38774, K05 MH067048, P50 MH58911

Program No. 307.10/BBB23
Poster presentation:Sunday, Nov 04, 2007, 2:00 PM - 3:00 PM
Location: San Diego Convention Center: Halls B-H