Functional Interactions between Entorhinal Cortical Pathways Modulate Theta Activity in the Hippocampus

Theta oscillations organize neuronal firing in the hippocampus during context exploration and memory formation. Recently, we have shown that multiple theta rhythms coexist in the hippocampus, reflecting the activity in their afferent regions in CA3 (Schaffer collaterals) and the entorhinal cortex layers II (EC-II, perforant pathway) and III (EC-III, temporoammonic pathway). Frequency and phase coupling between theta rhythms were modulated by the behavioral state, with synchronized theta rhythmicity preferentially occurring in tasks involving memory updating. However, information transmission between theta generators was not investigated. Here, we used source separation techniques to disentangle the current generators recorded in the hippocampus of rats exploring a known environment with or without a novel stimulus. We applied analytical tools based on Granger causality and transfer entropy to investigate linear and non-linear directed interactions, respectively, between the theta activities. Exploration in the novelty condition was associated with increased theta power in the generators with EC origin. We found a significant directed interaction from the Schaffer input over the EC-III input in CA1, and a bidirectional interaction between the inputs in the hippocampus originating in the EC, likely reflecting the connection between layers II and III. During novelty exploration, the influence of the EC-II over the EC-III generator increased, while the Schaffer influence decreased. These results associate the increase in hippocampal theta activity and synchrony during novelty exploration with an increase in the directed functional connectivity from EC-II to EC-III.

Characterizing the hippocampal theta’s response to carbachol; using a complete septo-hippocampal preparation

The present study describes the pharmacological analysis of the effects of carbachol, a cholinergic agonist, on hippocampal theta activity. Knowing that this activity is critically related to cognitive function and altered in patients with neurodegeneration, pharmacological efforts aiming to directly modulate hippocampal theta activity becomes of central importance. In a recently developed complete septo-hippocampal preparation, carbachol elicited significant theta power enhancement with 1 μM. Concentrations under 1 μM and over 2 μM carbachol caused significant reduction in the power of hippocampal theta activity. Carbachol effects were completely blocked with the cholinergic antagonist scopolamine. At the experimental level, it is the first time the direct action of a cholinergic agonist is evaluated in the septo-hippocampal pathway completely isolated. However, carbachol as a cholinergic agonist is a drug with a certain level of nonspecific response. That is why to correct this experimental limitation, we used scopolamine (cholinergic antagonist) which allowed us to corroborate the effects on the cholinergic pathway. In summary, electrophysiological assays demonstrated an effective concentration range of carbachol specifically modulating hippocampal theta activity.

Characterizing the hippocampal theta’s response to carbachol; using a complete septo-hippocampal preparation

The present study describes the pharmacological analysis of the effects of carbachol, a cholinergic agonist, on hippocampal theta activity. Knowing that this activity is critically related to cognitive function and altered in patients with neurodegeneration, pharmacological efforts aiming to directly modulate hippocampal theta activity becomes of central importance. In a recently developed complete septo-hippocampal preparation, carbachol elicited significant theta power enhancement with 1 μM. Concentrations under 1 μM and over 2 μM carbachol caused significant reduction in the power of hippocampal theta activity. Carbachol effects were completely blocked with the cholinergic antagonist scopolamine. At the experimental level, it is the first time the direct action of a cholinergic agonist is evaluated in the septo-hippocampal pathway completely isolated. However, carbachol as a cholinergic agonist is a drug with a certain level of nonspecific response. That is why to correct this experimental limitation, we used scopolamine (cholinergic antagonist) which allowed us to corroborate the effects on the cholinergic pathway. In summary, electrophysiological assays demonstrated an effective concentration range of carbachol specifically modulating hippocampal theta activity.