Serotonergic Modulation of Persistent Inward Currents in Serotonergic Neurons of Medulla in ePet-EYFP Mice

Abstract

Serotonergic (5-HT) neurons in the medulla play multiple functional roles associated with many symptoms and motor activities. The descending serotonergic pathway from medulla is essential for initiating locomotion. However, the ionic properties of 5-HT neurons in the medulla remain unclear. Using whole-cell patch-clamp technique, we studied the biophysical and modulatory properties of persistent inward currents (PICs) in 5-HT neurons of medulla in ePet-EYFP transgenic mice (P3–P6). PICs were recorded by a family of voltage bi-ramps (10-s duration, 40-mV peak step), and the ascending and descending PICs were mirrored to analyze the PIC hysteresis. PICs were found in 77% of 5-HT neurons (198/258) with no significant difference between parapyramidal region (n = 107) and midline raphe nuclei (MRN) (n = 91) in either PIC onset (−47.4 ± 10 mV and −48.7 ± 7 mV; P = 0.44) or PIC amplitude (226.9 ± 138 pA and 259.2 ± 141 pA; P = 0.29). Ninety-six percentage (191/198) of the 5-HT neurons displayed counterclockwise hysteresis and four percentage (7/198) exhibited the clockwise hysteresis. The composite PICs could be differentiated as calcium component (Ca_PIC) by bath application of nimodipine (25 μM), sodium component (Na_PIC) by tetrodotoxin (TTX, 2 μM), and TTX- and dihydropyridine-resistance component (TDR_PIC) by TTX and nimodipine. Ca_PIC, Na_PIC and TDR_PIC all contributed to upregulation of excitability of 5-HT neurons. 5-HT (15 μM) enhanced the PICs, including a 26% increase in amplitude of the compound currents of Ca_PIC and TDR_PIC (P < 0.001, n = 9), 3.6 ± 5 mV hyperpolarization of Na_PIC and TDR_PIC onset (P < 0.05, n = 12), 30% increase in amplitude of TDR_PIC (P < 0.01), and 2.0 ± 3 mV hyperpolarization of TDR_PIC onset (P < 0.05, n = 18). 5-HT also facilitated repetitive firing of 5-HT neurons through modulation of composite PIC, Na_PIC and TDR_PIC, and Ca_PIC and TDR_PIC, respectively. In particular, the high voltage-activated TDR_PIC facilitated the repetitive firing in higher membrane potential, and this facilitation could be amplified by 5-HT. Morphological data analysis indicated that the dendrites of 5-HT neurons possessed dense spherical varicosities intensively crossing 5-HT neurons in medulla. We characterized the PICs in 5-HT neurons and unveiled the mechanism underlying upregulation of excitability of 5-HT neurons through serotonergic modulation of PICs. This study provided insight into channel mechanisms responsible for the serotonergic modulation of serotonergic neurons in brainstem.

Chronic exercise increases excitability of lamina X neurons through enhancement of persistent inward currents and dendritic development in mice

“…We classified these patterns into three types based on differences in onset and offset voltages of PICs. In fact, six patterns of PICs have been described previously in locomotor‐activated spinal interneurons (Dai & Jordan, 2010) and four patterns in the brainstem serotonergic neurons (Cheng et al., 2021; Ge & Dai, 2020). PICs have been shown to be responsible for variable patterns of repetitive firing induced by current bi‐ramps (Harvey et al., 2006; Lee & Heckman, 1998, 2001; Powers & Binder, 2003; Powers et al., 2008).…”

“…Incomplete space clamp is a problem for almost all studies using whole cell patch‐clamp techniques (Cheng et al., 2021; Dai & Jordan, 2011). Any voltage‐dependent current could be contaminated by the unclamped currents.…”

“…The liquid conjunction potential was calculated as 10.4 mV with pH value adjusted to 7.3 by KOH, osmolarity adjusted to 310 mOsm by sucrose, and the presence of 10 μM TEA in the recording solution. This value was not corrected in this study, in order to make our data comparable to those from our previous study of PICs (Cheng et al, 2021;Dai & Jordan, 2010).…”

The effects of membrane potential oscillations on the excitability of rat hypoglossal motoneurons

“…Neuronal excitability is generally characterized not only by voltage threshold, but also by recruitment (I rec ) and decruitment (I dec ) currents (e.g., Cheng et al, 2020 ; Cheng et al, 2021 ). In the present study, we defined the recruitment current as the current ramp at which the first spike was initiated ( Figures 3A1,A2 , red dots) and decruitment current as the current ramp at which the last spike was elicited ( Figures 3B1,B2 ).…”

Exercise-induced adaptation of neurons in the vertebrate locomotor system

“… 15 , 16 Previous studies have reported that persistent sodium channels regulate Vth and repetitive firing in spinal motoneurons, 36 interneurons, 76 hippocampal neurons, 77 and brainstem 5-HT neurons. 78 In addition, persistent sodium channels are critical in regulating pacemakers and locomotion speed. 79 Recent studies have discovered that 3 weeks of treadmill training increases neuronal excitability through the enhancement of persistent sodium channels in spinal interneurons 15 and midbrain 5-HT neurons in mice.…”