RELATIONS BETWEEN ACETYLCHOLINE-RELEASE AND ELECTROPHYSIOLOGICAL CHARACTERISTICS OF THETA-RHYTHM - A MICRODIALYSIS STUDY IN THE URETHANE-ANESTHETIZED RAT HIPPOCAMPUS

In urethane-anesthetized rats, recording electrodes were implanted in the left dorsal hippocampus and a dialysis probe was placed in the con tralateral dorsal or ventral hippocampus. Samples of extracellular ace tylcholine (ACh) levels were assessed at 10-min intervals over a perio d of 30 min using microdialysis with high-performance liquid chromatog raphy with electrochemical detection. EEG was recorded during the same period and amplitude, frequency, and duration of theta rhythm were ca lculated for each of the three 10-min intervals. Data were analyzed us ing the two-tailed Spearman rank-order correlation test, A positive an d high rank correlation coefficient (rho = 0.90, p < 0.01, n = 8) was seen between the average ACh outflow in the dorsal hippocampus and the average theta amplitude, both being calculated for the entire collect ion period. A lower but statistically significant positive correlation (rho = 0.59, p < 0.01) between dorsal hippocampus ACh output and thet a amplitude was also found when the couples of values collected for th e 30-min period were pooled (n = 20). In contrast, frequency and durat ion of theta were not significantly correlated with dorsal hippocampus ACh release. Also, no statistically significant correlation (p > 0.05 ) was found between ACh output in the ventral hippocampus and theta pa rameters. Because changes in hippocampal ACh outflow are believed to b e the reflection of changes in number and/or level of activity of chol inergic afferents to the dorsal hippocampus, our present findings supp ort the view that, at least in the dorsal hippocampus of the urethane- anesthetized rat, the septohippocampal cholinergic projection regulate s the theta amplitude but not frequency, Finally, the possibility that ACh outflow increase and tonic release in the hippocampus is not a su fficient condition to induce and maintain theta in the urethane-anesth etized rat is discussed. Copyright (C) 1997 Elsevier Science Inc.