1. The question was addressed what effects do some neurotransmitters exert upon the ongoing compound activity of an organized invertebrate ganglion, such as the visceral ganglion of the gastropod mollusc, Helix, recorded with extracellular semimicroelectrodes in the neuropile, avoiding dominating single units. 2. Methods of analysis based on systems theory are used: the electrical activity of the snail brain is considered to be a summation of different measurable variables or subsystems (in this case, "frequency components"). Frequency components were analyzed by the Fourier transform and, for quantification of each component, root-mean-square (RMS)-voltage of digitally filtered signals. 3. Spontaneous discharge of the untreated ganglion shows RMS-voltage of less than 10μV in the 1-50 Hz range, most frequently 3-7μV. Energy extended far above 50 Hz, with less decline than in vertebrates, but was quantified only up to 80 Hz due to the wider fluctuations of RMS-voltage in the range > 50 Hz. 4. ACh (10-5-10-3M) induces strong, dose-related increase of activity with spontaneous frequency responses in a broad band between 2 and 20 Hz which accompany fluctuations of minor peakings frequently centering at 2-4, 4-8, 9-15 and 20 Hz. These frequency centers still remain but as an impression since a statistical study on the reality and position of the peaks has not yet been done. ACh-induced excitation seems to be rather transient: all frequency components except the 4-8 Hz component decrease within 4-6 min. 5. DA (10-4-10-3M) induces a dose-related increase in the low frequency range (1- 15 Hz) which reaches a maximum within 4 min and remains relatively unchanged for as long as 10 min. 6. NA (10-5-10-2M) mainly depresses the ganglionic activity, though moderately, reducing all components (-25%). The action of NA on the Helix ganglion seems to incline more towards inhibition of firing than towards excitation, especially depressing the 15-48 Hz activity, whereas that on the mammalian brain is known to be enhancement of alpha (8-13 Hz)- and beta (15-30 Hz)-activities. 7. The activities observed in the present investigation at several lower frequency power peaks are also commonly found in the electrograms of vertebrates and the EEG of higher animals. The manner, in which ACh and DA influence these peaks in Helix pomatia, are very similar to what has been reported on the EEG of mammals. 8. The descriptive efficiency of methods of systems theory made it possible to detect and analyze the EEG-like oscillatory phenomena in such a relatively simple cellular mass as the Helix ganglion. © 1992.
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