Cholinergic-receptor-independent modulation of intrinsic properties of subicular neurons through inhibition of hyperpolarization-activated cyclic nucleotide-gated channels

Abstract

Acetylcholine release is vital in tuning the hippocampal theta oscillations. Subiculum, the output region of the hippocampus endowed with different neuronal subtypes, also generates theta oscillations during arousal and REM sleep. Generation of theta rhythms in the hippocampus requires a combination of extrinsic cholinergic inputs from the medial septum and the intrinsic resonance in the hippocampal neurons. Hitherto, the effect of acetylcholine on the resonance of hippocampal neurons was thought to be mediated only via the activation of mAChRs. We studied the acetylcholine receptor-independent effect of cholinergic agents on the intrinsic properties of subiculum neurons and the underlying mechanism. We bath perfused acetylcholine in the presence of atropine on horizontal rat brain slices. Exogenously applied acetylcholine affected three groups of electrophysiologically characterized subicular neurons differently by reducing the resonance frequency and I h in bursting neurons, whereas these properties were unaffected in regular firing neurons and fast-spiking interneurons. This was further validated by studying the effect of endogenously released ACh by stimulating cholinergic fibers in septohippocampal brain slices. Endogenous ACh decreased the sag amplitude and resonance frequency and increased input resistance and tonic AP frequency, specifically in burst firing neurons among the two excitatory neurons. To further confirm the modulation of burst firing neurons properties, we applied nicotine, an agonist of nAChRs. Nicotine changed the intrinsic properties of burst firing neurons by decreasing the amplitude and increasing the activation time constant of Ih. The study suggests the active role of burst firing neurons over other neuronal populations that actively respond to acetylcholine through a shift in resonance frequency by partially inhibiting HCN current during high cholinergic inputs, which is independent of acetylcholine receptor activation. Direct modulation of a voltage-gated ion channel by the neurotransmitter ACh, was the unique observation that emerged from the study. This together with the action of ACh on its post- synaptic receptors may contribute to fine-tuning the rhythmic activity of neurons.