Introduction
In the adult brain cerebral energy metabolism has been functionally compartmentalized using the barbiturate thiopental into active part, associated with electroencephalographic (EEG) activity, and the balance, basal component, associated with the maintenance of neuronal viability. It has been shown that whole-brain estimation in adult monkeys showed an active:basal distribution of approximately 50:50 for CMRO2, 40:60 for CBF, and 60:40 for CMRGluc. However, similar studies have not been done in the immature brain. Moreover, we asked whether or not re-gional differences exist.
Methods
Experiments were performed on 14 newborn (age: 2–5 days) and 11 juvenile (age: 6–7 weeks) pigs. Animals were anesthetized and artificially ventilated. Several arterial, venous and sagittal sinus cathe-ters were inserted for taking blood pressure measurements and obtaining blood samples. ECoG recording was per-formed using 2 screw electrodes, inserted above the parietal cortex bilaterally. A left thoracotomy was performed through the third intercostal space and a catheter was inserted into the left auricle for injection of colored microspheres for regional CBF measurement. CMRO2 was calculated as forebrain CBF × avDO2. Regional CMRGluc was estimated by 18FDG and positron emission tomography. 7 newborn and 5 juvenile pigs served as sham-operated control. In remaining animals and after an initial measurement of CBF and CMRO2 under baseline conditions, burst suppression (BS) ECoG was induced by infusion of thiopental (TP, 50 mg/ml; 0, 2 ml/ min × kg b.w.). 30 min later 18FDG (100–250 MBq) was infused in all animals and regional CMRGluc was estimated for 60 min.
Results
Under baseline conditions, CBF of newborn pigs was increased by ∼20% (p<0.05), whereas CMRO2 was similar in both age groups. Regional CMRGluc, however, was markedly reduced in all brain regions between 22% and 30% in newborn piglets (p < 0.01). BS induced a similar reduction in CBF (by ∼50 %) and CMRO2 (∼56–61%) in both groups (p < 0.05). Juvenile pigs exhibited a compa-rable CMRGluc reduction in all brain regions studied, with the most pronounced suppression in forebrain regions (∼63%), whereas midbrain and cerebellum showed a diminished reduction of 73 and 80% of control (p < 0.05). In contrast, new-born piglets showed a BS-induced suppression of CMRGluc only in the forebrain regions, whereas brain stem and cere-bellum did not show changes in CMRGluc. In addition the amount of CMRGluc suppression in newborn piglets was markedly diminished in all brain regions compared with the older animals (p < 0.05).
Conclusion
There is a regional difference in active:basal distribution of brain cerebral energy metabolism even in the mature brain as estimated by regional CMRGluc under normal conditions vs. BS. Brain immaturity aggravates these regional differences, because brain stem and cerebellum does not respond with CMRGluc reduction during thiopental-induced BS. Thus, compartmentalization is less developed in early postnatal period, possibly due to an immature state of dendritic development and synapto-genesis. It can be speculated that neuroprotective procedures which influence prominently the basal component of brain energy metabolism may be preferred in the immature brain.