Cholinomimetics induce theta rhythm and reduce hippocampal pyramidal cell excitability

Area of Science:

• Neuroscience
• Pharmacology
• Neurophysiology

Background:

• Cholinergic system modulation of hippocampal activity is crucial for cognitive functions.
• Endogenous acetylcholine increases pyramidal cell excitability and influences theta rhythms.
• Exogenous cholinomimetics and cholinesterase inhibitors are used to study cholinergic pathways.

Purpose of the Study:

• To investigate the effects of cholinomimetics and physostigmine on hippocampal theta activity and pyramidal cell excitability in rats.
• To compare the actions of exogenous cholinergic agents with endogenous acetylcholine.
• To elucidate the central mechanisms underlying these effects.

Main Methods:

• Rats were pretreated with methylscopolamine and anesthetized with urethane.
• Intravenous administration of oxotremorine, arecoline, and physostigmine.
• Electrophysiological recordings of hippocampal theta activity and pyramidal cell excitability (population spike, dendritic EPSP).
• Spectral analysis of theta wave frequency.
• Lesion studies (septal lesions) and pharmacological blockade (scopolamine, methylscopolamine).

Main Results:

• Oxotremorine, arecoline, and physostigmine induced dose-dependent theta wave activity (3-6 Hz) in the hippocampus.
• Scopolamine blocked the induced theta activity.
• Arecoline-induced theta activity was abolished by septal lesions.
• Despite inducing theta activity, these agents decreased CA1 pyramidal cell excitability (reduced population spike and EPSP amplitude).
• Scopolamine attenuated these depressant effects, while methylscopolamine did not.
• Septal lesions attenuated the depressant effect of arecoline.

Conclusions:

• Exogenously administered cholinomimetics partially mimic, but do not fully replicate, the actions of endogenous acetylcholine in the hippocampus.
• Theta activity induction and pyramidal cell excitability changes by exogenous agents involve distinct, complex central mechanisms.
• The precise central sites of action for these exogenous cholinergic agents require further investigation.