Interpreting functional imaging studies in terms of neurotransmitter cycling

Abstract

Functional imaging experiments, in particular positron-emission tomography and functional magnetic resonance imaging, can be analyzed either in psychological terms or on the basis of neuroscience. In the usual psychological interpretation, stimulations are designed to activate specific mental processes identified by cognitive psychology, which are then localized by the signals in functional imaging experiments. An alternate approach would be to analyze experiments in terms of the neurobiological processes responsible for the signals. Recent in vivo 13C NMR measurements of the glutamate-to-glutamine neurotransmitter cycling in rat and human brains facilitate a neuroscientific interpretation of functional imaging data in terms of neurobiological processes since incremental neurotransmitter flux showed a 1:1 stoichiometry with the incremental rate of glucose oxidation. Because functional imaging signals depend on brain energy consumption, a quantitative relationship can be established between the signal (S) and the specific neurochemical cerebral neurotransmitter activity (N) of glutamate-to-glutamine neurotransmitter cycling. The quantitation of neuronal activity proposed has implications for the psychological design and interpretation of functional imaging experiments. Measurements of the neurotransmitter cycling flux at rest in functional imaging experiments suggest that performing cognitive tasks and sensory stimulations increases neurotransmitter cycling by only 10-20%. Therefore it cannot be assumed that reference state activities are negligible, nor that they are constant during stimulation.

Figure 1

Schematic relations between the signal (S) obtained in functional imaging experiments and mental processes (M). In the usual experimental plan and interpretation, based on psychology, a direct relationship between S and M is assumed, as represented by the upper pathway. The definition of M is based on psychology, while the imaging experiment serves to localize and quantitate the brain activity identified with the process. The lower pathway, Neuroscience, assumes that M has a molecular and cellular basis, which is broken into three steps leading to S. The signal, S, in fMRI or PET experiments, is primarily a measure of the neurophysiological parameters (NP) of cerebral metabolic rate of glucose consumption (CMRglc), cerebral metabolic rate of oxygen consumption (CMRO₂) or CBF. PET methods have been developed for measuring each of these three parameters separately, while fMRI signals respond to differences in the changes of CBF and CMRO₂, whose quantitative relationships are being investigated. CMRO₂ and CMRglc measure cerebral energy consumption, while ΔCMRO₂ and ΔCMRglc measure its increment. The relation between (NP) the neurophysiological measure of energy consumption and neuronal activity (N) has been clarified by the ¹³C MRS experiments (–10). These recent findings allow measurements of S to be converted into measures of N, which places us squarely facing the unsolved “hard” problem of neuroscience, i.e., what is the relationship between M and N?

Figure 2

Proposed pathway of glutamate/glutamine neurotransmitter cycling between neurons and glia (22, 23), whose flux has been quantitated recently by ¹³C MRS experiments (9). Action potentials reaching the presynaptic neuron cause release of vesicular glutamate into the synaptic cleft, where it is recognized by glutamate receptors post-synaptically and is cleared by Na⁺ -coupled transport into glia. There it is converted enzymatically to glutamine, which passively diffuses back to the neuron and, after reconversion to glutamate, is repackaged into vesicles. The rate of the glutamate-to-glutamine step in this cycle (V_cycle), has been derived from recent ¹³C experiments (–10).

Figure 3

The experimental measure of V_Gln, the flux from glutamate to glutamine, includes, in addition to V_cycle, contributions from other pathways, as indicated by dashed lines, which have been shown to require small (≈10%) corrections to V_Gln to obtain V_cyle.

Figure 4

Experimental results of V_cycle and the rates of glucose oxidation measured simultaneously in each rat going from N₂O/morphine to α-chloralose to pentobarbital (least active), at graded anesthesia. The best fit line gives CMRglc(ox) = 1.04 V_cycle + 0.10. The slope shows that each mole of neurotransmitter glutamate cycling requires the oxidation of one mole of glucose. The awake, resting state has CMRglc(ox) ≈0.8 μmol/min/g, which means that under this condition ≈85% of the brain energy consumption is dedicated to V_cycle (9).

Figure 5

Schematic representation of the signals obtained in functional imaging differencing experiments. PET measurements of CMRglc and CMRO₂ directly measure S and (S + ΔS) under both conditions, although usually only the difference ΔS is used to localize and quantitate the task activation. On the other hand, fMRI signals include strong contributions in both conditions from pure imaging signals, so that in these experiments only ΔS is readily related to task activations.