Conclusions
- Underlying circuit: The only modeling study of CCH-induced rhythms has focussed on theta (Traub et al 1992) elicited in high CCH concentrations (40-50mM). The model only accounts for the ‘waxing’ phase of the oscillation. Further work is required to account for the waning phase, and to explain the other rhythms found at lower CCH concentrations.
- Relevance to in vivo EEG. In vivo and in vitro delta, theta and gamma rhythms are possibly of different nature. However, the fact that a single neuromodulatory substance is capable of activating these 3 distinct rhythms in vitro is remarkable. We suggest that the hippocampus features a circuitry which is capable of ‘resonating’ at specific frequencies.
- The computational roles of these oscillatory modes are still largely unknown (but see Brad Wyble’s workshop).
Theta has been involved in induction and reversal of LTP or LTD (Barr et al 1995; Huerta Lisman 1995). Theta can be used to synchronize pyramidal cells (Cobb et al, 1995), and may play a role in learning (Liljenstrom and Hasselmo 1995; Hasselmo et al 1996) and memory buffering (Jensen et al 1997).
- We presented experimental evidence for 3 cholinergically induced oscillations in the hippocampal slices: Delta (.5-2 Hz), Theta (5-10Hz) and Gamma (50-90Hz). These rhythms can coexist in pairs.