NICOTINE ENHANCES RECOGNITION MEMORY VIA INHIBITION OF VOLTAGE-DEPENDENT POTASSIUM CHANNELS IN THE MEDIAL PREFRONTAL CORTEX
Authors: Izumi *Shoma , Esaki Hirohito , Nishitani Naoya , Nagayasu Kazuki , Kaneko Shuji , Deyama Satoshi , Kaneda Katsuyuki 
Published: February 2025

Abstract

Background
We have previously reported that nicotine enhances recognition memory (Esaki et al. 2021), which may be mediated by increased spiking activity of medial prefrontal cortex (mPFC) layer V pyramidal cells via suppressing voltage-dependent potassium (Kv) channels. However, it remains unclear which subtype of Kv channels is associated with this effect of nicotine. Additionally, although nicotine has been suggested to affect synaptic transmission (Lambe et al. 2003), whether this effect is involved in recognition memory enhancement also remains undiscovered.

Aims & Objectives
We first investigated mechanisms of the nicotine-induced increased spiking in mPFC pyramidal cells. We next examined whether nicotine modulates synaptic transmission and whether this modulating effect of nicotine is involved in recognition memory enhancement.

Methods
Brain slices were prepared from male and female C57BL/6J mice (4 - 6 weeks old) and whole-cell patch- clamp recordings were made from mPFC layer V pyramidal cells. Spike responses were induced by depolarizing current injections. Excitatory postsynaptic potentials (eEPSPs) evoked by five repetitive stimuli at 50 Hz delivered in the vicinity of recording cells were recorded, and summation of eEPSPs was measured. In the novel object recognition test (NOR), male C57BL/6J mice (9 - 12 weeks old) were used. A 10-minute test session was performed 24 hours after a 10-minute training session. Discrimination index [DI = (novel object exploration time - familiar object exploration time)/total exploration time] in the test was calculated.

Results
Kv7 channels are expressed in soma and proximal axons, and involved in regulating spiking activity in the mPFC (Peng et al. 2017). Thus, we firstly investigated the involvement of Kv7 channels in the nicotine- induced increase in spiking activity in mPFC layer V pyramidal cells. Bath application of XE-991 (5 & micro;M), a Kv7 channel blocker, increased spike numbers. In the presence of XE-991, nicotine (1 & micro;M) did not increase spike numbers. In contrast, bath application of retigabine (5 & micro;M), a Kv7 channel opener, decreased spike numbers and additional nicotine application did not affect spike numbers.
We next examined whether nicotine affects excitatory synaptic transmission, summation of which is critical for regulating neuronal excitability. Repetitive stimuli induced eEPSP summation, and this summation was increased by nicotine. Since Kv4.3 channels are expressed in dendrites of pyramidal cells (Burkhalter et al. 2006), we investigated the involvement of the channels in the eEPSP summation.
Although bath application of NS5806 (30 & micro;M), a Kv4.3 channel activator, did not affect the eEPSP summation, it canceled the nicotine-induced increased summation. We finally investigated the involvement of Kv4.3 channels in recognition memory enhancement by nicotine using the NOR. Increased DI caused by nicotine administration (0.1 mg/kg, s.c.) before training session was inhibited by intra- mPFC administration of NS5806 (0.575 & micro;g/side). By contrast, miRNA-based mPFC pyramidal cell- specific Kv4.3 channel knockdown increased DI.

Discussion & Conclusion
Our results show that nicotine augments spiking activity and summation of excitatory synaptic transmission, respectively, via inhibition of Kv7 and Kv4.3 channels in mPFC pyramidal cells. These mechanisms may contribute to the nicotine-induced enhancement of recognition memory.

References
Esaki H, Izumi S, Fukao A, Ito S, Nishitani N, Deyama S, Kaneda K. (2021) Nicotine Enhances Object Recognition Memory via Stimulating α4β2 and α7 Nicotinic Acetylcholine Receptors in the Medial Prefrontal Cortex of Mice. Biol Pharm Bull, 44: 1007–1013.
Lambe EK, Picciotto MR, Aghajanian GK. (2003) Nicotine induces glutamate release from thalamocortical terminals in prefrontal cortex. Neuropsychopharmacology, 28: 216–225.
Peng H, Bian XL, Ma FC, Wang KW. (2017) Pharmacological modulation of the voltage-gated neuronal Kv7/KCNQ/M-channel alters the intrinsic excitability and synaptic responses of pyramidal neurons in rat prefrontal cortex slices. Acta Pharmacol Sin, 38: 1248–1256.
Burkhalter A, Gonchar Y, Mellor RL, Nerbonne JM. (2006) Differential Expression of IA Channel Subunits Kv4.2 and Kv4.3 in Mouse Visual Cortical Neurons and Synapses. J Neurosci, 97: 12274–12282.