Effects of physostigmine and calcium on acetylcholine efflux from the hippocampus of freely moving rats as determined by in vivo microdialysis and a radioimmunoassay

The effects varying the concentration of Ca2+ in perfused artificial cerebrospinal fluid ([Ca2+](csf)) on basal acetylcholine (ACh) efflux from the hippocampus of freely moving rats, in the presence and absence of the cholinesterase (ChE) inhibitor physostigmine, were investigated using in vivo microdialysis and a highly specific radioimmunoassay for ACh. In the absence of physostigmine, basal ACh efflux was 3.4 +/- 0.7 pg/30 min (mean +/- SEM) at [Ca2+](csf) = 1.26 mM. Stepwise increases in [Ca2+](csf) elicited a gradual increase in ACh efflux that was significant at [Ca2+](csf) = 5.04 mM. Inhibition of ChE by addition of 10 mu M physostigmine to the perfusate increased the efflux of ACh to 103.2 +/- 21.1 pg/30 min ([Ca2+](csf) = 1.26 mM), and the efflux was augmented still further by increasing [Ca2+](csf), a change that became significant at [Ca2+](csf) = 3.78. These results illustrate the sensitivity of basal ACh efflux from the hippocampus to changes in the extracellular Ca2+ concentration, and suggest that a more accurate picture of hippocampal cholinergic activity is obtained by microdialysis using normal artificial cerebrospinal fluid, under physiological conditions, rather than in the presence of a ChE inhibitor. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.