The generation of theta in the limbic cortex is a prime example of rhythmical activity involving central mechanisms of oscillations and synchromy. It was demonstrated that bath perfusion of hippocampal slice with carbachol resulted in theta-like oscillations. A number of our experiments have provided a strong evidence that theta-like activity recorded in vitro replicates pharmacological and physiological properties of in vivo recorded theta.
Our experiments conducted in the last decade on freely moving cats, showed that commonly used theta parameters: frequency, amplitude and power are not sufficient for a precise characterization of theta field activity. It is specially seen in condition when the experimental protocol involves sensory or chemical stimalation in order to induce hippocampal theta. We described a novel theta parameter ? ?theta density?, that allows more adequate quantitative evaluation of this EEG pattern in freely moving cats.
The generation of EEG theta rhythm (RSA) in the hippocampal formation is a prime example of rhythmic activity involving central mechanisms of oscillation and synchrony. Cholinergic nature of the in vitro and the in vivo induced RSA has been undoubtedly established. Recently, we have demonstrated in vitro that the hippocampal formation theta rhythm resulted from interaction between the cholinergic and GABAergic systems. In the present study we have provided additional in vitro evidence that the hippocampal GABA-A receptors are actively involved in the mechanism of theta production. Specifically, we demonstrated that bicuculline - GABA-A antagonist significantly augmented carbachol induced theta response increasing amplitude and power of rhythmical slow waves. In separate experiments the carbachol+bicuculline induced RSA were studied in the presence of muscarinic M1 and M2 antagonists - pirenzepine and gallamine (respectively) and GABA-A agonist - muscimol. Both pirenzepine and muscimol antagonized induced theta oscillations and gallamine was found to be completely ineffective in blocking this EEG response. The results provided evidence for M1 cholinergic/GABA-Aergic interaction in mechanisms responsible for theta production.
The present paper reviews the experimental data concerning the use of trans-slice preparation technique in investigation of theta-like activity generation in the limbic cortex. Specyfic aspects of the technical arrangement of the hippocampal formation and entorhinal cortex microelectroencephalographic registration were emphasized.
The generation of EEG theta rhythm in the mammalian limbic cortex is a prime example of rhythmic activity that involves central mechanisms of oscillations and synchrony. This EEG pattern has been extensively studied since 1938, when Jung and Kornmller (1938) demonstrated the first theta recordings in the hippocampal formation of rabbits. In 1986 in collaboration with Drs. B.H. Bland, S.H. Roth and B.M. MacIver we demonstrated for the first time that bath perfusion of hippocampal slices with the cholinergic agonist, carbachol, resulted in theta-like oscillations. Since this initial demonstration of in vitro theta-like activity, we have carried out a number of experiments in an attempt to answer the basic question: what are the similarities between cholinergic-induced in vitro theta-like activity and theta rhythm which naturally occurs in the in vivo preparation. Thus far, our studies have provided strong evidence that theta-like activity recorded in vitro shares many of the physiological and pharmacological properties of theta rhythm observed in vivo. The question whether in vitro theta-like oscillations reflect features of epileptiform activity is also adressed in this review.
Rhythmical slow activity (theta) was mapped in the hypothalamic regionin freely moving cats. We recorded well synchronized and high amplitude theta rhythm in the medial part of hypothalamus posterior area. The EEG recordings made from lateral part of this hypothalamic region contained only irregular activity. These findings support earlier observations concerning the topography of hippocampal formation desynchrony and synchrony system. The observations of the present study also suggest that the hypothalamus posterior area is actively involved in the mechanisms responsible for generating theta oscillations in the cats.
The relationships between the entrohinal cortex (EC) and the hippocampal formation (Hipp) field potentials were examined in the present study. The detailed analyses of the signal let us group the patterns of theta appearance into three categories: (1) Theta rhythm dominating in both recordings from the EC and from Hipp (2) Theta rhythm dominating in the Hipp with irregular activity in the EC (3) Theta rhythm dominating in the EC with irregular activity in the Hipp. These findings provide the evidence for the intrinsic generator of theta rhythm to be localized in entorhinal cortex in cats.
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