afni.nimh.nih.gov

BRIDGING THE GAP BETWEEN NEUROIMAGING AND NEURONAL PHYSIOLOGY by D. Kim and K.Ugurbil, Image Anal Stereol 2002;21:97-105
(www.stereologysociety.org/pdf/vol21no2/kim_04.pdf) addresses this issue (Question 4 on page 6) and gives a few recent references.
Their short answer:
"To the best of our knowledge, there is currently no evidence that the metabolic demands differ greatly between excitatory and inhibitory synapses."