Acetylcholine release in the hippocampus of the urethane anaesthetised ratpositively correlates with both peak theta frequency and relative power inthe theta band

The need to achieve a clearer understanding of relations between hippocampa l theta characteristics and cholinergic septohippocampal neuron activity, p rompted us to re-examine, in the urethane-anaesthetised rat, the statistica l relationships between the electrophysiological and neurochemical variable s using a procedure which is believed to enhance significantly the degree o f confidence with which parameters of theta recorded with classic macroelec trodes can be related to concomitant acetylcholine output measured by high- performance liquid chromatography with electrochemical detection. Firstly, the theta rhythm and the acetylcholine content were derived from the same h ippocampus. Secondly, the hippocampal electroencephalogram was quantified u sing spectral analysis which permits the more objective quantitative evalua tion of selected electroencephalogram samples. Thirdly, a larger number of rats than in our previous study was used here, thus enhancing the validity of statistical results. This procedure yielded, in our rime-course determin ation, two main findings. The first finding is th;lt acetylcholine release was positively correlated with frequency at the peak power of the theta ban d which reflects the frequency of the theta signal. This finding had not be en reported yet. The second finding is that hippocampal acetylcholine outfl ow also covaried with relative power of the theta band which reflects the a mplitude of the theta signal. This finding is consistent with our previous study in which EEG was quantified by means of a traditional method. These f indings suggest that the cholinergic component of the septohippocampal syst em, which is the main source of hippocampal acetylcholine, and neurophysiol ogical mechanisms involved in the modulation of both the amplitude and the frequency of theta are Functionally related. The possibility that, at least in the urethane-anaesthetised rat, hippocampal acetylcholine is involved i n these modulator mechanisms is discussed. (C) 2000 Elsevier Science BN. Al l rights reserved.