Introduction
The aim of this study was to determine the influence of glutamatergic transmission for activity-dependent cerebellar glucose use (CeGU) and blood flow (CeBF). The experiments tested the hypothesis that increases in glucose in response to activation originate in postsynaptic cellular elements.
Materials and Methods
Experiments were carried out in 26 male á-chloralose-anesthetized Wistar rats (250–350 g). CeGU was quantitatively measured in all rats using the [14C]-2-deoxy-glucose (2-DG) autoradiographic technique. At end of experiment, brains were immediately removed, frozen in isopentane (−40 C) and cut into 20-μm coronal sections. Sections were collected on glass slides and autoradiographed together with [14C] standards on x-ray film for 5–6 days (Sokoloff et al., 1977). CeBF was recorded with a double-wavelength Laser Doppler Flowmetry probe using green laser light to record CeBF in the upper 0–250 μm of the cerebellar cortex and red laser light to record CeBF in deeper layers. Glutamatergic neurotransmission was increased by stimulation of cerebellar climbing fibres (CF) at the level of the inferior olive (IO) in the brain stem. Local field potentials (LFP) were recorded with glass microelectrodes. CeGU was measured in cerebellar lobules 5 and 6, i.e. at the same site at which CeBF and LFPs were recorded. CNQX, a selective AMPA receptor antagonist, was used to block the excitatory postsynaptic potentials. The protocol included 4 groups of rats: (i) sham rats (ii) CF stimulated rats (iii) CF stimulated rats + CNQX, (iv) sham rats + CNQX. The IO was continuously stimulated at 5 Hz for 45 min.
Results
Continuous IO stimulation evoked a sustained and robust CeBF increase (mean of 167% for the green probe, mean of 215% for the red probe) associated with increased LFP amplitudes (mean amplitude = 4.46 ± 0.62 mV), stable for the entire 45 min stimulation period. The AMPA receptor antagonist CNQX significantly attenuated CeBF increases evoked by IO stimulation by up to 80% and induced a marked decrease of LFP amplitudes by up to 90%. CNQX did not affect basal CeGU in sham + CNQX animals. The increase of postsynaptic activity of Purkinje cells was associated with a modest, heterogeneous change in CeGU that appeared to depend on the accompanying rise in CeBF. A large rise in CeBF was associated with a low change in CeGU value and vice versa. This relationship was lost when CNQX was applied.
Conclusion
Overall, our data indicate that CeGU changes do not seem to parallel CeBF variations and that postsynaptic excitatory activity is responsible for a functional link between CeBF and CeGU changes during activation of the CF system.