Dopamine D2 receptors regulate the action potential threshold by modulating T‐type calcium channels in stellate cells of the medial entorhinal cortex

Abstract

Key pointsr Activation of axonal dopamine D2 receptors (D2Rs) increases action potential (AP) threshold, and thus decreases neuronal excitability in layer II stellate cells of medial entorhinal cortex.r Endogenous dopamine release increases the AP threshold of stellate cells by activating D2Rs. r Activation of D2Rs shifts the activation curve of T-type Ca 2+ channels in a positive direction in a protein kinase A-dependent manner.r Immunofluorescence staining reveals the presence of T-type Ca 2+ channels and D2Rs in the axon initial segments (AISs).r This research makes the pioneering discovery of D2R-induced AP threshold plasticity in AISs of stellate cells. The findings are likely to have significant implications for understanding the cellular processes by which dopamine influences neuronal intrinsic excitability.Abstract Stellate cells in the medial entorhinal cortex (MEC) are considered to constitute the largest population of grid cells, which provide spatial representation to support animal estimation of location. Although dopaminergic fibres from the ventral tegmental area and substantia nigra pars compacta innervate the majority of the cortex, including the MEC, little is known about how dopamine modulates the function of MEC stellate cells. Because dopamine D2 receptors (D2Rs) are involved in spatial cognition and MEC contains high levels of D2Rs, we investigated how D2R activation modulates the neuronal intrinsic excitability of stellate cells. Electrophysiological Xueqin Jin received her bachelor's degree in Pharmaceutical Science in 2014 from Jilin University. Currently, she is a PhD student in the School of Pharmaceutical Sciences at Peking University under the supervision of Prof. Zhuo Huang. She is now focusing on how neurotransmitters modulate neuronal functional plasticity in spatial cognition and working memory.3364 X. Jin and others J Physiol 597.13recordings, optogenetics and molecular biology experiments were performed to investigate the mechanism in mice. Activation of axonal D2Rs, not dendritic or somatic D2Rs, elevated the action potential (AP) threshold and decreased the intrinsic excitability of stellate cells, which was caused by shifting rightward the activation properties of T-type Ca 2+ channels in a D2R-protein kinase A-dependent manner without affecting their steady-state inactivation curve. In support, immunofluorescence assays revealed colocalization of D2Rs and Ca v 3.2 calcium channels within the axon initial segment. These findings are likely to have significant implications for understanding the cellular processes by which dopamine influences neuronal excitability and they may also be applicable to other hippocampal and cortical regions as dopaminergic fibres innervate wide brain regions. Taken together, these findings provide a novel cellular mechanism by which D2Rs modulate AP threshold of stellate cells through T-type Ca 2+ channels in MEC, indicating that D2Rs of MEC play a vital role in modulating the information processing of stellate cells.

Ca2+ entry through NaV channels generates submillisecond axonal Ca2+ signaling

“…The Ca V channel subtypes identified in axons are both cell type- and species-dependent and include the T-, P/Q- or N-type Ca V channels ( Callewaert et al, 1996 ; Bender and Trussell, 2009 ; Yu et al, 2010 ; Gründemann and Clark, 2015 ; Zhang and David, 2016 ). At the AIS in particular the T-type Ca 2+ channel mediates AP-dependent Ca 2+ influx ( Bender and Trussell, 2009 ; Martinello et al, 2015 ; Fukaya et al, 2018 ; Jin et al, 2019 ). However, in the prefrontal cortical pyramidal neuron AIS about 70% of the AP-evoked [Ca 2+ ] i remains following pharmacological block of T-type Ca V channels ( Clarkson et al, 2017 ).…”

Dopamine and action potential generation in the axon initial segment

“…In this issue of the Journal of Physiology , Jin et al . () report novel important findings on the role of dopamine (and in particular of D2Rs) with respect to the excitability of stellate cells of the MEC. Dopamine receptors are G‐protein coupled receptors (GPCRs) that can regulate the biophysical properties of ion channels and therefore the generation and propagation of the action potential (AP).…”

“…In their study, Jin et al . () demonstrate that MEC stellate cells direct activation of D2Rs by exogenous application of the agonist bromocriptine, and also that physiological activation of D2Rs by optical stimulation of ChR2 expressed in dopaminergic afferents reduces intrinsic excitability by increasing the threshold for activation of APs. These effects were exclusively mediated by those D2Rs expressed in the stellate cell axon initial segment (AIS).…”

“…Finally, while D2Rs and the T‐type VGCC subtype Cav3.2 appear to co‐localize in the stellate cell AIS, Jin et al . () provide evidence showing that the interaction between D2Rs and T‐type channels is regulated by protein kinase A. The results of their study and an explanation of the process are illustrated schematically in Fig.…”

“…Then, Jin et al . () show that blockade of T‐type voltage‐gated Ca 2+ channels (VGCCs) also increases the AP threshold, preventing the effect of bromocriptine, whereas activation of D2Rs shifts the activation curve of T‐type VGCCs in a more positive direction. Finally, while D2Rs and the T‐type VGCC subtype Cav3.2 appear to co‐localize in the stellate cell AIS, Jin et al .…”