Sequential activation of multiple persistent inward currents induces staircase currents in serotonergic neurons of medulla in ePet-EYFP mice

Abstract

Persistent inward currents (PICs) are widely reported in rodent spinal neurons. A distinctive pattern observed recently is staircase-like PICs induced by voltage ramp in serotonergic neurons of mouse medulla. The mechanism underlying this pattern of PICs is unclear. Combining electrophysiological, pharmacological, and computational approaches, we investigated the staircase PICs in serotonergic neurons of medulla in ePet-EYFP transgenic mice (postnatal days 1–7). Staircase PICs induced by 10-s voltage biramps were observed in 70% of serotonergic neurons ( n = 73). Staircase PICs activated at −48.8 ± 5 mV and consisted of two components, with the first PIC of 45.8 ± 51 pA and the second PIC of 197.3 ± 126 pA ( n = 51). Staircase PICs were also composed of low-voltage-activated sodium PIC (Na-PIC; onset −46.2 ± 5 mV, n = 34), high-voltage-activated calcium PIC (Ca-PIC; onset −29.3 ± 6 mV, n = 23), and high-voltage-activated tetrodotoxin (TTX)- and dihydropyridine-resistant sodium PIC (TDR-PIC; onset −16.8 ± 4 mV, n = 28). Serotonergic neurons expressing Na-PIC, Ca-PIC, and TDR-PIC were evenly distributed in medulla. Bath application of 1–2 μM TTX blocked the first PIC and decreased the second PIC by 36% ( n = 23, P < 0.05). Nimodipine (25 μM) reduced the second PIC by 38% ( n = 34, P < 0.001) without altering the first PIC. TTX and nimodipine removed the first PIC and reduced the second PIC by 59% ( n = 28, P < 0.01). A modeling study mimicked the staircase PICs and verified experimental conclusions that sequential activation of Na-PIC, Ca-PIC, and TDR-PIC in order of voltage thresholds induced staircase PICs in serotonergic neurons. Further experimental results suggested that the multiple components of staircase PICs play functional roles in regulating excitability of serotonergic neurons in medulla. NEW & NOTEWORTHY Staircase persistent inward currents (PICs) are mediated by activation of L-type calcium channels in dendrites of mouse spinal motoneurons. A novel mechanism is explored in this study. Here we report that the staircase PICs are mediated by sequentially activating sodium and calcium PICs in serotonergic neurons of mouse medulla.

Three-Week Treadmill Exercise Enhances Persistent Inward Currents, Facilitates Dendritic Plasticity, and Upregulates the Excitability of Dorsal Raphe Serotonin Neurons in ePet-EYFP Mice

“…Previous studies have shown that PICs play an important role in regulating neuronal excitability and firing patterns in spinal motoneurons and interneurons (Theiss et al, 2007 ; Heckman et al, 2008 ). Recently, we found the presence of PICs in 5-HT neurons in the mesencephalic locomotor region and medulla of neonatal ePet-EYFP mice (Ge et al, 2019 ; Cheng et al, 2020 ). In this study, we further explored whether PICs presented in 5-HT neurons of DRN in juvenile mice as well as the effects of exercise training on PICs.…”

“…Both modeling and experiment studies suggested that these changes in Vth and AP amplitude could be produced by upregulating transient sodium channels (Dai et al, 2002 ; Power et al, 2012 ). Results from recent studies showed that riluzole, a Na-PIC blocker, significantly depolarized the Vth of 5-HT neurons (Svensson et al, 2017 ; Cheng et al, 2020 ), suggesting that persistent sodium channels modulate Vth (Dai et al, 2002 ; Power et al, 2012 ). In fact, transient sodium channels and persistent sodium channels are considered to originate from the same ion channels with different kinetics for activation and inactivation (Crill, 1999 ).…”

“…Additionally, activation of PICs amplifies the synaptic inputs at subthreshold voltages and facilitates spike initiation and repetitive firing, which are primarily mediated by persistent sodium channels (Crill, 1999 ). In addition to motoneurons, PICs are also found in interneurons (Theiss et al, 2007 ; Dai et al, 2009b ), hippocampal neurons (Muller et al, 2018 ), Renshaw cells (Boeri et al, 2018 ), and 5-HT neurons of midbrain and medullar (Ge et al, 2019 ; Cheng et al, 2020 ). For the first time, we revealed that exercise facilitated excitability of 5-HT neurons by enhancing Na-PIC and Ca-PIC.…”

“…The main membrane properties measured in current clamp included rheobase (Ith), voltage threshold (Vth), resting membrane potential (Em), input resistance (Rin), AP height and half width, and after-hyperpolarization (AHP) depth, and half-width and number of APs. The calculation of these parameter values had been described in our previous studies (Dai et al, 2009a ; Ge et al, 2019 ; Cheng et al, 2020 ). Briefly, the rheobase (Ith) was determined by step currents with a 0.5-s duration and 5 pA for each step.…”

“…Previous studies on PICs mainly focused on motoneurons. Recently, we discovered that PICs existed in 5-HT neurons in neonatal mice and played an essential role in regulating the excitability of 5-HT neurons (Ge et al, 2019 ; Cheng et al, 2020 ). However, little is known about exercise modulation of PICs in 5-HT neurons.…”