The release of adenosine during metabolic or traumatic insults to the mammalian CNS is regarded as an important neuroprotective strategy by virtue of the resultant adenosine A₁ receptor-mediated inhibition of glutamate release. We have previously shown, using an enzyme-based adenosine sensor, the production of adenosine coincident with the depression of glutamatergic synaptic transmission in s. radiatum of area CA1 of the rat hippocampus (Dale et al. 2000). We now present evidence that the release of adenosine in area CA1 during hypoxia is not uniform and instead shows regional variation.

Hippocampal slices were prepared from rat pups of either sex aged between 11 and 25 days as previously described (Dale et al. 2000). Briefly, rats were killed by cervical dislocation in accordance with Schedule 1 of the UK Animals (Scientific Procedures) Act, 1986. Hypoxia (33Ð34°C) was induced by switching from ACSF saturated with 95 % O₂ and 5 % CO₂ to ACSF saturated with 95 % N₂ and 5 % CO₂. Adenosine release was measured in real-time in s. radiatum and s. oriens with microelectrode sensors (25Ð50 mm diameter) either laid on the surface of each area or inserted through the entire depth of each region at a steep angle. Data are expressed as means ± S.E.M. and n = number of slices.

Hypoxia (5Ð10 min) reliably evoked adenosine release in both s. radiatum and oriens. In pups aged 11Ð15 days the adenosine release in s. radiatum and oriens was almost simultaneous. The difference in timing of release between the two areas (s. radiatum to oriens) was 12 ± 9 s (n = 7). In contrast, in pups 19 days or older the onset of adenosine release in s. oriens was delayed with respect to s. radiatum by 63 ± 16 s (n = 6). However, the amount of adenosine recorded in the two areas after 5 min of hypoxia was similar and did not alter with age (10 ± 4 and 8 ± 3 mM, s. radiatum and oriens, respectively). To test if adenosine is produced in both areas or only in s. radiatum and diffuses to s. oriens, we made a cut just above s. pyramidale, which enabled s. oriens to be moved away from s. radiatum. Under these conditions the amount of adenosine produced in s. radiatum after 5 min of hypoxia was 9 ± 2 mM while that in the displaced s. oriens was 0.2 ± 0.1 mM (n = 7). We conclude that, in contrast to s. radiatum, little adenosine is released in s. oriens during hypoxia. The adenosine detected in s. oriens probably arises through diffusion through the slice of adenosine released in s. radiatum or pyramidale. This surprising result challenges the assumption that a global signal such as hypoxia elicits a global, uniform release of adenosine.

All procedures accord with current UK legislation.