Post-hypoxic recovery of acetylcholine release: different sensitivity of guinea pig neocortical and striatal slices

Abstract

The release of endogenous acetylcholine (ACh) was measured in superfused guinea-pig cortical and striatal slices, kept at rest or electrically stimulated at different frequencies, before and during severe hypoxic conditions as well as after reoxygenation. In the cortex the basal release was unchanged by 30-60 min of hypoxia while it was inhibited in the striatum. The release evoked by short-term (2 min) stimulation at 0.5 Hz was moderately reduced (to 76%) by 30 min hypoxia in the cortex and in the striatum, but fully recovered after reoxygenation. The release evoked by continuous stimulation (from 5 to 10 to 20 Hz) was strongly inhibited (to 12-30%) in both areas after 30 min of hypoxia. After 30 min of reoxygenation, the recovery was complete in the cortex (mainly provided with cholinergic axons), but it was incomplete in the striatum (rich in cholinergic interneurones). The extent of the recovery in the latter area (i) was inversely related to stimulation frequency, (ii) did not depend on the depletion of neurotransmitter stores, because ACh tissue levels were fully restored by reoxygenation, and (iii) was consistently facilitated by excitatory aminoacid antagonists, slightly improved by the adenosine agonist R-phenylisopropyladenosine and unaffected by reducing the concentrations of radical species with catalase and superoxide dismutase or N omega-nitro-L-arginine. These results emphasize (i) the different vulnerability of the cortical and striatal cholinergic structures, (ii) the high sensitivity of the striatal interneurones to the frequency of stimulation during the posthypoxic recovery, and (iii) the relevant role played by endogenous glutamate on activity-dependent neurosecretory failure.