The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Abstract

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies about 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. Such molecules were shown to destabilize the locomotor output of spinal limb networks through 5-HT1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT1A receptor antagonist, whereas a 5-HT1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.

Walking with Salamanders: From Molecules to Biorobotics

“…V0V: ventral commissural excitatory neurons positive for Dbx1, Homeobox even-skipped homolog protein 1 (Evx1), negative for Paired box protein 7 (PAX7), and involved in slow swimming in zebrafish and in left-right hindlimb [47][48][49], and tail muscles [8]. These spinal circuits generate multiple motor patterns, including the standing and travelling waves of axial motoneuron activity [11] that underlie the walking and swimming movements observed in vivo [10] (Figure 2G,H). Single cell recordings were also carried out in the spinal cord during fictive locomotion in the axial network [50] and limb networks [51].…”

“…MLR Glut [14]; MLR GABA [112]; MLR ChAT [13,14,19]; RS Vglut2 [12]; RS GABA? [113]; axial Glut, Vglut2 [9,11]; axial ChAT [114]; axial GABA and Gly [9,50,115]; axial primary and secondary MNs [11,[115][116][117]; axial Rohon-Beard and Kolmer-Agduhr cells [115]; limb Glut [47,49]; limb GABA/Gly [48], limb MNs [47,55]; limb ChAT [118]; cutaneous [119,120] [75], the astrocytes. The salamander CNS is largely devoid of GFAP expressing cells intermingled with neurons both normally as well as after lesion, which may be one important reason behind the absence of injury responsive glial scar formation.…”

Serotonergic Modulation of Locomotor Activity From Basal Vertebrates to Mammals

“…This caudal group mainly sends descending projections to the spinal cord, local projections to the brainstem, and a limited amount of ascending projections to the forebrain (Hornung, 2010 ). Finally, 5-HT neurons in the spinal cord project locally, send branches rostrally or caudally, or toward the ventrolateral border, and occasionally in the central canal (lamprey: Harris-Warrick et al, 1985 ; zebrafish: Montgomery et al, 2018 ; salamander: Flaive et al, 2020 ; turtle: Fabbiani et al, 2018 ; mouse: Ballion et al, 2002 ; Figure 1C ). In recent years, the use of genetic tools has allowed researchers to refine the characterization of 5-HT nuclei.…”

“…Intriguingly, the effects of SSRI can be different from those of exogenous 5-HT. Whereas exogenous 5-HT increases rhythm stability, several studies reported that SSRI destabilizes fictive locomotion generated by the isolated spinal cord (lamprey: Christenson et al, 1989 ; mouse: Dunbar et al, 2010 ; zebrafish: Montgomery et al, 2018 ; salamander: Flaive et al, 2020 ). In some cases, in the same species, exogenous 5-HT decreases the frequency of fictive locomotor movements, whereas increasing the extracellular concentration of endogenous 5-HT increases the frequency (e.g., frog embryo: McLean and Sillar, 2004 ; salamander: Branchereau et al, 2000 ; Flaive et al, 2020 ).…”

“…The reasons why exogenous 5-HT and SSRI differentially affect fictive locomotion or the neurons of the locomotor circuitry are not fully resolved. From one study to another, the brainstem can be left attached to the spinal cord or removed from the preparation, possibly contributing to the variable outcomes of the experiments (Branchereau et al, 2000 ; Dunbar et al, 2010 ; Flaive et al, 2020 ). Another interesting possibility to consider is that different 5-HT receptors are recruited by exogenous or endogenous 5-HT (Dunbar et al, 2010 ; Cotel et al, 2013 ; for review Perrier and Cotel, 2015 ).…”