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Topic

Characterization of Cholinergic mediated α7 and α4β2 Nicotinic Acetylcholine Receptor Responses in Layer1 Interneurons of the Medial Prefrontal Cortex

Division of Medical Sciences

Date & location

• Thursday, April 17, 2025
• 10:00 A.M.
• Medical Sciences Building, Room 160

Examining Committee

Supervisory Committee

• Dr. Raad Nashmi, Division of Medical Sciences, University of Victoria (Supervisor)
• Dr. Craig Brown, Division of Medical Sciences, UVic (Member)
• Dr. Kerry Delaney, Department of Biology, UVic (Outside Member)

External Examiner

• Dr. Brian Christie, Division of Medical Sciences, UVic

Chair of Oral Examination

• Dr. Neil Ernst, Department of Computer Science, UVic

Abstract

The medial prefrontal cortex (mPFC) is a brain region responsible for a variety of cognitive functions including attention and working memory. Cholinergic neurons, which release acetylcholine (ACh), are known to enhance attention and their pathophysiology is associated with disorders such as Alzheimer’s disease and epilepsy. Nicotinic acetylcholine receptors (nAChRs) are activated by the cholinergic system and modulate neuronal excitability. Therefore, understanding nAChR mediated synaptic neurotransmission will allow us to better understand how the activity of neurons is precisely controlled.

Using whole cell recordings of layer 1 neurons of mouse mPFC and optogenetic stimulation of ACh release resulted in two nAChR mediated currents -- one having a rapid rise and decay kinetics and sensitive to inhibition by the α7 nAChR antagonist MLA. The second nAChR current was long lasting and inhibited by the α4β2 nAChR antagonist DHβE. The α4β2 current was significantly inhibited by the calcium chelator EGTA-AM, while there was no effect on α7 currents. This suggests that ACh release eliciting α7 nAChR responses is mediated by tight coupling of the presynaptic calcium source and the calcium sensor, while that of α4β2 responses is a more distant coupling. Following stimulation of ACh release, there were delayed asynchronous miniature excitatory postsynaptic current (mEPSC) responses. DHβE eliminated both the α4β2 currents and the asynchronous current events. However, MLA eliminated α7 responses but did not impact the asynchronous events, thus confirming that the asynchronous responses were mediated by α4β2 nAChRs. The α4β2 mediated asynchronous activity was also inhibited by EGTA-AM. The spontaneous excitatory postsynaptic activity prior to light stimulation was insensitive to either nicotinic competitive antagonist DHβE or MLA but was inhibited by EGTA-AM and the L-type Ca2+ channel blocker nifedipine.

The substantial reduction in the number of spontaneous events after the application of nifedipine highlights a unique role for L-type calcium channels in regulating spontaneous activity. This could have implications for understanding presynaptic calcium dynamics and spontaneous neurotransmitter release mechanisms.