Mechanisms of activity‐dependent increases in cerebral blood flow (CBF) were examined in rat cerebellar cortex using the laser Doppler flow technique and extracellular recordings of single unit activity and field potentials. Stimulation of the monosynaptic climbing fibre system evoked long‐lasting complex spikes in Purkinje cells, and extracellular field potentials with a characteristic profile that indicated contributions from both passive and active membrane mechanisms. The concomitant CBF increases were reproducible at fairly short intervals, and suggest that both synaptic activity and spikes may contribute to increased CBF. Stimulation of the disynaptic parallel fibre system inhibited the spiking activity in Purkinje cells, while the postsynaptic activity increased as indicated by the simultaneously recorded field potential. Nevertheless, CBF always increased. The inhibition of spike firing activity was partly dependent on GABAergic transmission, but may also relate to the intrinsic membrane properties of Purkinje cells. The CBF increases evoked by parallel or climbing fibre stimulation were highly correlated to the sum of neural activities, i.e. the negativity of field potentials multiplied by the stimulus frequency. This suggests a robust link between extracellular current flow and activity‐dependent increases in CBF. AMPA receptor blockade attenuated CBF increases and field potential amplitudes, while NMDA receptor antagonism did not. This is consistent with the idea that the CBF responses are of neuronal origin. This study has shown that activity‐dependent CBF increases evoked by stimulation of cerebellar parallel fibres are dependent on synaptic excitation, including excitation of inhibitory interneurones, whereas the net activity of Purkinje cells, the principal neurones of the cerebellar cortex, is unimportant for the vascular response. For the climbing fibre system, not only synaptic activity but also the generation of complex spikes from Purkinje cells contribute to the increases in CBF. The strong correlation between CBF and field potential amplitudes suggests that extracellular ion fluxes contribute to the coupling of brain activity to blood flow.

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Cortical spreading depression (CSD) is associated with release of arachidonic acid, impaired neurovascular coupling, and reduced cerebral blood flow (CBF), caused by cortical vasoconstriction. We tested the hypothesis that the released arachidonic acid is metabolized by the cytochrome P450 enzyme to produce the vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), and that this mechanism explains cortical vasoconstriction and vascular dysfunction after CSD. CSD was induced in the frontal cortex of rats and the cortical electrical activity and local field potentials recorded by glass microelectrodes, CBF by laser Doppler flowmetry, and tissue oxygen tension (tpO 2 ) using polarographic microelectrodes. 20-HETE synthesis was measured in parallel experiments in cortical brain slices exposed to CSD. We used the specific inhibitor HET0016 (N-hydroxy-NЈ-(4-n-butyl-2-methylphenyl)formamidine) to block 20-HETE synthesis. CSD increased 20-HETE synthesis in brain slices for 120 min, and the time course of the increase in 20-HETE paralleled the reduction in CBF after CSD in vivo. HET0016 blocked the CSD-induced increase in 20-HETE synthesis and ameliorated the persistent reduction in CBF, but not the impaired neurovascular coupling after CSD. These findings suggest that CSD-induced increments in 20-HETE cause the reduction in CBF after CSD and that the attenuation of stimulation-induced CBF responses after CSD has a different mechanism. We suggest that blockade of 20-HETE synthesis may be clinically relevant to ameliorate reduced CBF in patients with migraine and acute brain cortex injuries.

In a published paper we have provided two categories of information about the relationship between neuronal activity and functional increases in regional cerebellar blood flow (CeBF). First, we showed that activity-dependent increases in CeBF were not necessarily linked to increased spiking activity in the principal neurones of the region studied (Mathiesen et al. 1998). Second, a strong correlation was found between the maximal amplitude of the recorded field potentials and the maximal increase in CeBF (Mathiesen et al. 1998). The different time courses of the electrophysiological and vascular responses during activation precluded the existence of a simple relationship (Mathiesen et al. 1998): the CeBF response developed over tens of seconds while the electrophysiological response developed within milliseconds. The present study therefore examined the temporal correlation between the observed CeBF increases and neuronal activity. The hypothesis was that if the increases in CeBF were temporally coupled, albeit indirectly to neuronal activity, then it would be possible to model the time course of the vascular response by integrating the neuronal activity over time during all phases of the vascular response. The simplest mathematical approach to integrate neuronal activity over time was to obtain a running summation of the amplitudes of the evoked field potentials (runÓFP). The rat cerebellar cortex was used as a model since this brain region cannot generate the epilepsy that is common after stimulation of the cerebral cortex. The basic circuitry of the cerebellar cortex is organised around the Purkinje cells from which the final and only output from the cerebellar cortex originates (Fig. 1 and Eccles et al. 1967). The activity of Purkinje cells is influenced by two excitatory