One contention within the field of neuroimaging concerns the character of the depicted activity: Does it represent neuronal action potential generation (i.e., spiking) or postsynaptic excitation? This question is related to the metabolic costs of different aspects of neurosignaling. The cerebellar cortex is well suited for addressing this problem because synaptic input to and spiking of the principal cell, the Purkinje cell (PC), are spatially segregated. Also, PCs are pacemakers, able to generate spikes endogenously. We examined the contributions to cerebellar cortical oxygen consumption (CMRO2) of postsynaptic excitation and PC spiking during evoked and ongoing neuronal activity in the rat. By inhibiting excitatory synaptic input using ionotropic glutamate receptor blockers, we found that the increase in CMRO2 evoked by parallel fiber (PF) stimulation depended entirely on postsynaptic excitation. In contrast, PC spiking was largely responsible for the increase in CMRO2 when ongoing neuronal activity was increased by γ-aminobutyric acid type A receptor blockade. In this case, CMRO 2 increased equally during PC spiking with excitatory synaptic activity as during PC pacemaker spiking without excitatory synaptic input. Subsequent inhibition of action potential propagation and neurotransmission by blocking voltage-gated Na+-channels eliminated the increases in CMRO2 due to PF stimulation and increased PC spiking, but left a large fraction of CMRO2, i.e., basal CMRO2, intact. In conclusion, whereas basal CMRO2 in anesthetized animals did not seem to be related to neurosignaling, increases in CMRO2 could be induced by all aspects of neurosignaling. Our findings imply that CMRO2 responses cannot a priori be assigned to specific neuronal activities.
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