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

Yi Cheng,Nan Song,Yue Dai,Renkai Ge

doi:10.3389/fncir.2021.657445

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.

Highlights

Serotonergic (5-HT) neurons in the medulla play multiple functional roles and are associated with symptoms such as depression (Meera et al, 2003) and pain (Potrebic et al, 1994), or behaviors like locomotion (Liu and Jordan, 2005), respiration (Severson et al, 2003), and perception (Geyer and Vollenweider, 2008)
Consistent with that report, in this study we further showed that persistent inward currents (PICs) were widely expressed in 5-HT neurons of medulla in ePet-EYFP mice
PICs were observed in 77% of recorded 5-HT neurons (198/258) in both parapyramidal region (PPR) and midline raphe nuclei (MRN) of medulla (Figure 1E top)

Summary

INTRODUCTION

Serotonergic (5-HT) neurons in the medulla play multiple functional roles and are associated with symptoms such as depression (Meera et al, 2003) and pain (Potrebic et al, 1994), or behaviors like locomotion (Liu and Jordan, 2005), respiration (Severson et al, 2003), and perception (Geyer and Vollenweider, 2008). Previous study reported that electrical stimulation of 5HT neurons of the parapyramidal region (PPR) in medulla could initiate locomotion in isolated rat spinal cord (Liu and Jordan, 2005). Previous studies reported that serotonergic axons from the dorsal raphe (DR) nucleus are very fine and typically have small, pleomorphic varicosities. These fibers branch profusely in their vicinity areas and diffuse 5-HT through small varicosities (Colgan et al, 2012; Quentin et al, 2018). We discovered large amounts of small varicosities in the dendrites of 5-HT neurons in medulla It remains unknown about the implication of morphological characteristics for potential serotonergic modulation of 5-HT neurons. Preliminary data was published in abstract form (Cheng et al, 2019b; Dai and Cheng, 2019)

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CONCLUSION

ETHICS STATEMENT