METABOLIC MAPPING OF THE FORELIMB MOTOR SYSTEM IN THE RAT - LOCAL CEREBRAL GLUCOSE-UTILIZATION FOLLOWING EXECUTION OF FORELIMB MOVEMENTS MAINLY INVOLVING PROXIMAL MUSCULATURE

The present study was undertaken to establish a metabolic map of forel imb motor pathways under conditions of physiological activation. For t hat purpose, we used the [C-14]2-deoxy-D-glucose (2-DG) method to iden tify forebrain and midbrain centers showing an increase in 2-DG uptake in animals trained to execute specific lever-pressing movements with the right forelimb. Following repetitive execution of these movements, principally involving proximal (shoulder, elbow, and wrist) muscles, increases in 2-DG uptake were found contralaterally in several neocort ical or subcortical centers. The largest left-right differences in loc al cerebral glucose utilization (LCGU) were found in a central region of the sensorimotor cortex composed of the caudal part of area 3 of th e frontal cortex (Fr3; p < 0.01), the intermediate part of area 1 of F r (Fr1; p < 0.01), and the forelimb cortical area (p < 0.04). Fr3 was the brain center with the highest differences in left-right LCGU. This central region of the sensorimotor cortex seems to correspond closely to the caudal forelimb area of Neafsey et al. (1986). Intermediate le ft-right differences in LCGU were found (1) in the just-adjoining rost ral-medial areas of the motor cortex involving the intermediate part o f area 2 of Fr (Fr2; p < 0.01) and the rostral part of Fr1 (p < 0.04), and (2) in the rostral part of area 1 of the parietal cortex (Par1;p < 0.01) and the caudal part of area 2 of Par (Par 2; p < 0.05), both c orresponding to forelimb representation Weak (not statistically signif icant) left-right differences in LCGU were found in the rostral parts of Fr2 and Fr3, in the caudal parts of Fr2 and Fr1, in the hindlimb co rtical area, and in the caudal part of Par1 and the rostral part of Pa r2. In the remaining cortical areas (cingulate; agranular and granular retrosplenial; temporal; and occipital), there was practically no dif ference in left-right 2-DG uptake. In addition, increased 2-DG uptake was present contralaterally in several subcortical motor-related cente rs. In those centers in which a somatomotor map has been established ( caudate putamen, ventral lateral and ventral posterolateral thalamic n uclei, and red nucleus), increased 2-DG uptake was found in regions co rresponding to forelimb representation. Finally, increases in 2-DG upt ake were also found in the rostral part of the reticular thalamic nucl eus and in the parafascicular nucleus, the posterior thalamic nuclear group, the zona incerta, the reticular part of the substantia nigra, a nd the intermediate and deep layers of the superior colliculus.