Metabolic Mapping of the Forelimb Motor System in the Rat: Local Cerebral Glucose Utilization Following Execution of Forelimb Movements Mainly Involving Proximal Musculature

Abstract

The present study was undertaken to establish a metabolic map of forelimb motor pathways under conditions of physiological activation. For that purpose, we used the [¹⁴C]2-deoxy-D-glucose (2-DG) method to identify 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 neocortical or subcortical centers. The largest left-right differences in local cerebral glucose utilization (LCGU) were found in a central region of the sensorimotor cortex composed of the caudal part of area 3 of the frontal cortex (Fr3; p < 0.01), the intermediate part of area 1 of Fr (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 left-right differences in LCGU were found (1) in the just-adjoining rostral-medial areas of the motor cortex involving the intermediate part of 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 (Pari; p < 0.01) and the caudal part of area 2 of Par (Par 2; p < 0.05), both corresponding to forelimb representation. Weak (not statistically significant) 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 cortical area, and in the caudal part of Par1 and the rostral part of Par2. In the remaining cortical areas (cingulate; agranular and granular retrosplenial; temporal; and occipital), there was practically no difference in left-right 2-DG uptake. In addition, increased 2-DG uptake was present contralaterally in several subcortical motor-related centers. In those centers in which a somatomotor map has been established (caudate putamen, ventral lateral and ventral posterolateral thalamic nuclei, and red nucleus), increased 2-DG uptake was found in regions corresponding to forelimb representation. Finally, increases in 2-DG uptake were also found in the rostral part of the reticular thalamic nucleus and in the parafascicular nucleus, the posterior thalamic nuclear group, the zona incerta, the reticular part of the substantia nigra, and the intermediate and deep layers of the superior colliculus.

Key Words:

• 2-deoxyglucose
• LCGU
• cortex
• forelimb area
• motor system
• rat