Is the dentate gyrus an independent generator of in vitro recorded theta rhythm?

In this study we investigated the ability of the dentate gyrus to independently generate cholinergically induced theta rhythm in vitro. Two different experiments were performed. In Experiment I, new laminar profiles of theta phase, amplitude, and current sources and sinks were constructed. In this experiment a gradual phase shift of theta waves in the CA1 stratum radiatum was observed. Simultaneously, two amplitude maxima were detected: the first in the CA1 stratum oriens, and the second in the CA1 stratum lacunosum-moleculare. Moreover, during the positive peak of theta in the CA1 stratum oriens, two large sinks were observed: the first localized in the stratum oriens and the second in the stratum lacunosum-moleculare. In Experiment II the EEG activity of three different transected hippocampal slices (CA1 transected slices, CA3c transected slices, and DG transected slices) was recorded. It was demonstrated that the dentate gyrus granular cell body layer was not able to independently produce in vitro theta rhythm. Data obtained in both experiments provide strong evidence that in cholinegically treated hippocampal formation maintained in vitro there is no independent generator of theta rhythm in the region of the dentate gyrus granular cell layer.

The effect of carbenoxolone on hippocampal formation theta rhythm in rats: in vitro and in vivo approaches.

The role of gap junction (GJ) coupling in the generation of hippocampal formation theta rhythm was investigated in vitro, with use of brain slices, and in vivo, with use of urethane anesthetized rats. Carbenoxolone, the succinyl ester of glycyrrhetinic acid, and GJ blocker reversibly abolished hippocampal formation theta rhythm recorded in slice preparations and urethane anesthetized rats. The present study yielded novel data which demonstrated that the pattern of delay in blockage of theta rhythm after carbenoxolone treatment, and the pattern of theta recovery after administration of this agent, require a specific time period (2-3h for delay and 8-12h for recovery), one that can be demonstrated using different experimental protocols.

Electrical coupling underlies theta rhythm in freely moving cats.

The role of gap junction coupling in the generation of theta rhythms in freely moving cats was investigated in a present study. Two gap junction blockers, carbenoxolone and quinine, were administered intraperitoneally and intrahippocampally; both gap junction blockers abolished or diminished (respectively) hippocampal formation theta. The inhibitory effect developed approximately 30 min after drug administration. This effect was found to be reversible. Our results provide the first direct in vivo evidence for the contribution of gap junction communication in mechanisms of neural synchrony, underlying the production of theta in in vivo conditions.

Firing cell repertoire during carbachol-induced theta rhythm in rat hippocampal formation slices.

One hundred and seven cells were recorded extracellularly in hippocampal formation (HPC) slices during carbachol-induced theta. The data obtained provided evidence of a population of HPC neurons which, when activated cholinergically, participate in the generation of in vitro theta. The activity patterns of in vitro recorded theta-related cells were shown to be similar to those of theta-related cells recorded in vivo and cells recorded in vitro during cholinergically induced theta, and non-theta intervals were successfully classified according to previously developed criteria for in vivo recorded theta-related cells. The current in vitro experiments showed that, in addition to theta-on and theta-off cells, the HPC contained cells that were probably involved in programming the appearance and duration of theta epochs and the intervals between theta epochs. These novel types of cells were termed 'gating cells'.

Electrical coupling underlies theta oscillations recorded in hippocampal formation slices.

The role of gap junction coupling in generation of carbachol-induced theta-like activity (TLA) in hippocampal formation (HPC) slices was investigated in this study. Two gap junction (GJ) blockers, carbenoxolone (100 microM) and quinine (100 microM), were tested. Both GJ blockers abolished cholinergically induced theta-like activity and related cell discharges. Abolishing effects were observed after 40-45 min of drug perfusion. These effects were found to be slowly and partially reversible. Our results provide evidence for the contribution of gap junction communication in mechanisms of neural synchrony, underlying the production of theta oscillations in limbic cortex maintained in vitro.

[Density of theta as a new parameter characterizing field activity in hippocampal formation of cats]. / Gestosc theta jako nowy parametr charakteryzujacy aktywnosc polowa formacji hipokampalnej u kotów.

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 stimulation 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.

[Electroencephalographic (EEG) activity recorded in brain tissue in vitro]. / Aktywnosc elektroencefalograficzna (EEG) rejestrowana w tkance mózgowej in vitro.

The generation of theta in the limbic cortex is a prime example of rhythmical activity involving central mechanisms of oscillations and synchrony. It was demonstrated that bath perfusion of hippocampal slices 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.

Septal cholinergic mediation of hippocampal theta in the cat.

The effects of intraseptally microinjected muscarinic (atropine sulfate, pirenzepine and gallamine) and nicotinic (hexamethonium) antagonists on spontaneous, sensory and electrically-induced hippocampal (HPC) theta EEG activity were investigated in the freely behaving cat. Administration of hexamethonium failed to elicit a detectable effect on HPC theta. Injections of atropine and pirenzepine abolished, whereas the injection of gallamine only reduced hippocampal theta. Moreover, a gradual recovery of theta amplitude and power was observed, while frequency recovered rapidly. Our data provide further evidence that the septal M1 and M2 muscarinic receptor subtypes mediate the HPC theta in this species. Intraseptal microinjection of cholinergic agonist (carbachol) produced almost a continuous HPC theta with increased amplitude and power. The contribution of the medial septal cholinergic projections to HPC theta frequency and amplitude was also discussed.