Endogenous Serotonin Acts on 5-HT_2C-Like Receptors in Key Vocal Areas of the Brain Stem to Initiate Vocalizations in Xenopus laevis

Abstract

Serotonin initiates various rhythmic behaviors in vertebrates. Previously we have shown that serotonergic neurons innervate the central vocal pathway in the African clawed frog (Xenopus laevis). We also discovered that exogenous serotonin applied to isolated brains in vitro activates fictive vocalizations by activating 5-HT(2C)-like receptors. In this study, we examined the location of 5-HT(2C)-like receptors and determined whether endogenously released serotonin also initiates vocalizations by activating 5-HT(2C)-like receptors in male Xenopus brains. To this end, we first identified the specific location of 5-HT(2C)-like receptors using immunohistochemistry. We next examined which of the populations of neurons that express 5-HT(2C)-like receptors are functionally relevant for initiating fictive vocalizations by applying a 5-HT(2C) receptor agonist to brains transected at various levels. Of four populations of immunopositive neurons, we showed that 5-HT(2C)-like receptors located in two areas of the brain stem vocal circuit, the raphe nucleus and motor nucleus IX-X, initiate fictive vocalizations. We next showed that endogenous serotonin can also activate fictive vocalizations by increasing the extracellular concentration of endogenous serotonin using a selective serotonin reuptake inhibitor (SSRI). The SSRI-induced vocal initiation is also mediated by activation of 5-HT(2C)-like receptors because blockade of these receptors prevents fictive vocalization. The results suggest that in vivo release of serotonin initiates male vocalizations by activating 5-HT(2C)-like receptors in the brain stem vocal nuclei.

Evolution of vocal patterns: tuning hindbrain circuits during species divergence

“…Because previous work has also shown that removal of the midbrain and forebrain does not disrupt fictive calling in X. laevis (Yu and Yamaguchi, 2010), we predicted that species-specific LFP wave patterns are generated autonomously within the hindbrain compartment that includes DTAM. To test this prediction, we combined a transection that eliminated LMN inputs with an additional transection just anterior to the cerebellum that removed inputs from the midbrain and forebrain in two X. laevis and three X. petersii brains.…”

NMDAR-Dependent Control of Call Duration in Xenopus laevis

“…Additional cool or warm agarose was added to the appropriate chamber to maintain the desired temperature difference during fictive vocal recordings. Because vocal nuclei anterior to DTAM are not required for vocal production (Yu and Yamaguchi 2010), we refer to cooling the anterior chamber as "selective DTAM cooling" because this is the only CPG component being cooled. Likewise, cooling of the posterior chamber is referred to as "selective n.IX-X cooling" because n.IX-X is the sole CPG nucleus in this part of the brain.…”

Harnessing vocal patterns for social communication

“…Fictive calling – a pattern of nerve compound action potentials (CAPs) that matches temporal features recorded during actual calling [32; Fig. 2B] – can be reliably evoked in an isolated brain by raising endogenous serotonin using reuptake inhibitors, by serotonin application or by stimulating the amygdala [33–35]. Vocal pattern generating circuitry is contained within the hindbrain and includes a more rostral, rhythmically active group of neurons located in the dorsal medulla at the level of cranial nerve V (DTAM, used as a proper noun) as well as interneurons and laryngeal motor neurons in the caudal hindbrain.…”