Metabolic energy per synapse is approximately conserved during development in mammalian brains.

During mammalian development the cerebral metabolic rate correlates qualitatively with
synaptogenesis, and both often exhibit bimodal temporal profiles. Despite these non-monotonic
dependencies, it is found based on empirical data for different mammals that regional metabolic
rate per synapse is approximately conserved from birth to adulthood for a given species
(with a slight deviation from this constancy for human visual and temporal cortices during
adolescence). A typical synapse uses about 7000 glucose molecules per second in primate cerebral
cortex, and about 5 times of that amount in cat and rat visual cortices. A theoretical model
for brain metabolic expenditure is used to estimate synaptic signaling and neural spiking
activity during development. It is found that synaptic efficacy is generally inversely correlated
with average firing rate, and additionally, synapses consume a bulk of metabolic energy, roughly
50-90% during most of the developmental process (except human temporal cortex < 50%). Overall,
these results suggest a tight regulation of brain electrical and chemical activities during the
formation and consolidation of neural connections. This presumably reflects strong energetic
constraints on brain development.

References:
1) Karbowski J (2012) Approximate invariance of metabolic energy per synapse during development
    in mammalian brains. PLoS ONE 7(3): e33425. http://dx.doi.org/10.1371/journal.pone.0033425

2) Karbowski J (2009) Thermodynamic constraints on neural dimensions, firing rates, brain
    temperature, and size. J Comput Neurosci 27: 415-436. http://dx.doi.org/10.1007/s10827-009-0153-7