Abstracts

The ionotropic glutamate receptors NMDAr, AMPAr and KAr may participate in seizure-related progressive hippocampal and amygdalar injury. We used ATPA ((RS-2-amino-3-(3-hydroxy-5-tert-butylisoxazole-4-yl)propanoic acid), a specific KAr GluR5subtype agonist, to produce seizures after amygdalar injection in rats, and performed MRI scans to assess brain effects.
Rats anesthetized with ketamine / xylozine had MR-compatible cannulae placed stereotactically in basolateral amygdala. After several days rest, they were intubated, anesthetized with isoflurane, and paralyzed with pancuronium. Body core temperature was maintained at 37 degrees with a heated water pad. Lines were placed in femoral artery to monitor blood pressure, and femoral vein for drug and fluid administration. Blood gas was analysed at frequent intervals. MR was performed on a 7T Bruker Avance scanner. Rats were placed prone in a 72 mm radio frequency coil. Pilot images were obtained to locate the cannula site and center it in the field of view. Five 1 mm thick axial images delineated morphological details in the cannula centered region. The field of view was 3.2 cm (matrix size 256x128, NA = 4) yielding in plane resolution of 250 [mu]m. Regional blood flow was measured using arterial spin labeling. A 2mm axial slice containing the cannula was identified. A labeling pulse of 2 ms with power of 80 mG/cm was used at 2 cm cordal to the site of cannula to label arterial spins. The scanning parameters were: FOV = 3.2 cm, matrix 64x64, 3 echos, NA=2. Total time per scan = 4.5 mins). Two baseline image sets were acquired before infusion of 10 nanomoles ATPA (5uL) over 5 minutes. Spin labeled images were acquired continuously during and after infusion for an hour. T[sub]2[/sub] weighted and Diffusion weighted images were acquired with identical geometrical parameters as for morphology. Other T[sub]2[/sub] parameters were: TE = 10 ms, TR = 2000 ms) and for diffusion : NA = 1, TE = 31 ms TR = 2000 ms, [Delta] = 20 ms, Diffusion Gradient time = 5 ms and four b values ranging from 0 to 3000 G/ cm². These results were processed to calculate T[sub]2[/sub] maps and diffusion maps (ADC). At the end two more sets of spin labeled images were acquired approximately 2 hours after ATPA infusion.
ATPA did not lead to any significant changes in vital signs. The perfusion images showed variable increases in cerebral blood flow (CBF) beginning approximately 25 minutes after infusion, lasting up to 2 hours. T[sub]2[/sub] images and ADC maps showed increased relaxation times, and decreased ADC, restricted to the region of the infusion.
GluR5 activation may lead to amygdalar CBF activation, accompanied by increased T[sub]2[/sub] and decreased ADC. Increased T2 signal suggests initial tissue imjury, while decreased ADC is consistent with the absence of edema. Future experiments will determine whether the T[sub]2[/sub] signal increases show a persistent pattern consistent with neuronal injury.
[Supported by: NINDS NIH Division of Intramural Research]