Increases in neuronal activity are accompanied by local increases in cerebral blood flow that facilitate increased delivery of nutrients to meet the needs of the active neuropil. It has been suggested that astrocytes act as an intermediary stage in the process of flow-metabolism coupling by releasing vasodilator products of arachidonic acid metabolism from their perivascular end-feet (Harder et al. 2000).
In the present study we have investigated the dynamics of this process in cortical arterioles maintained in situ in brain slices. Coronal slices, 300-350 Ám thick were prepared from brains of urethane-anaesthetised (1.5 g kg-1 ip) male rats, 130-230 g body weight and maintained in artificial cerebrospinal fluid (ACSF) at 33°C. Images of parietal cortical arterioles (internal diameter (ID) 8.5-16.5 µm) were captured using a CCD camera and Openlab image analysis software (Improvision Ltd) as described previously (Lovick et al. 1999).
In 7 vessels inclusion of 75 nM U46619 in the ACSF decreased internal diameter -12.8 ± 2.5 %, mean ± S.E.M.) and induced rhythmic contractions (9.9 ± 1.4 min-1) of smooth muscle cells in the vessel wall (vasomotion). In these pre-constricted vessels, addition of AMPA (1 µM for 30 min) produced a 12.1 ± 1.5 % increase in ID (P < 0.001, Student’s unpaired t test) and the frequency of vasomotion decreased by 8.0 ± 1.4 contractions min-1 (P < 0.001). The response was maintained without decrement throughout the superfusion period and on washout, returned to control values within 20 min. In a further 7 vessels the response to AMPA was tested in the presence of the epoxygenase inhibitor miconazole (20 µM ). Addition of miconazole to resting vessels (no U46619 in ACSF) produced a 11.1 ± 2.7 % decrease in ID and vasomotion increased to 9.9 ± 2.3 min-1. These values are comparable to the pre-constriction produced by U46619. Addition of AMPA to miconazole-treated vessels produced an initial dilatation (9.8 ± 3.1 % increase in ID, P < 0.003) and reduction in vasomotion -4.6 ± 1.5 min-1, P < 0.02). However, the response was not maintained. Within 10 min the vessels began to constrict, vasomotion increased and by the end of the 30 min AMPA superfusion period the values had returned to control levels.
The results suggest that epoxygenase activity is not a pre-requisite for initiating activity-related dilatation in cortical arterioles even though it is clearly important for maintaining the response. Any contribution of epoxygenase products to the early phase of activity-related dilatation is likely to be due to release from pre-formed membrane stores.
This work was supported by the British Heart Foundation