XenopusVocalizations Are Controlled by a Sexually Differentiated Hindbrain Central Pattern Generator

Abstract

Male and female African clawed frogs (Xenopus laevis) produce rhythmic, sexually distinct vocalizations as part of courtship and mating. We found that Xenopus vocal behavior is governed by a sexually dimorphic central pattern generator (CPG) and that fictive vocalizations can be elicited from an in vitro brain preparation by application of serotonin or by electrical stimulation of a premotor nucleus. Male brains produced fictive vocal patterns representing two calls commonly produced by males in vivo (advertisement and amplectant call), as well as one call pattern (release call) that is common for juvenile males and females in vivo but rare for adult males. Female brains also produced fictive release call. The production of male calls is androgen dependent in Xenopus; to test the effects of androgens on the CPG, we examined fictive calling in the brains of testosterone-treated females. Both fictive male advertisement call and release call were produced. This suggests that all Xenopus possess a sexually undifferentiated pattern generator for release call. Androgen exposure leads to a gain-of-function, allowing the production of male-specific call types without prohibiting the production of the undifferentiated call pattern. We also demonstrate that the CPG is located in the brainstem and seems to rely on the same nuclei in both males and females. Finally, we identified endogenous serotonergic inputs to both the premotor and motor nuclei in the brainstem that may regulate vocal activity in vivo.

Central pattern generators for social vocalization: Androgen-dependent neurophysiological mechanisms

“…The recent study of fictive calling in the isolated brain preparation from Xenopus (see earlier section) revealed that serotonin could rapidly evoke both male, advertisement-like and female, release-like fictive calls from the brains of adult females treated with testosterone for either 8 weeks or 5 months ( Fig. 4B; Rhodes et al, 2007). In comparison to untreated females, the brains of testosterone-treated females, like those of males, showed a briefer delay between serotonin application and the onset of fictive calling.…”

“…Serotonin also evokes fictive bursts from respiratory-related glottal motoneurons, but these do not overlap with the fictive calls (see Fig. 4A also see Schmidt, 1992 for L. pipiens). Besides showing that the vocal CPG is sexually differentiated in Xenopus, Rhodes et al (2007) use a combination of brain transections and brief electrical microstimulation to show that the brain region containing DTAM is necessary for the induction of fictive calls by serotonin. The new results for fully aquatic Xenopus, together with the earlier ones for terrestrial frogs (Schmidt, 1992), support the general hypothesis that both DTAM-pretrigeminal nucleus and the more caudal region around and inclusive of motor nucleus IX-X are essential components of a vocal CPG in anuran amphibians (see Rhodes et al, 2007 andZornik andKelley, 2007 for further discussion).…”

“…Studies of androgen modulation of vocal patterning remain to investigate (also see Bass, in press;Remage-Healey and Bass, 2007;Rhodes et al, 2007) how androgen-induced plasticity is dependent on changes in synaptic and network properties (e.g., see White et al, 1999), as well as ligand (steroid)-dependent and independent (via second messenger pathways) events (e.g., see Blaustein, 2004;Ronnekleiv and Kelly, 2005). While multiple steroids (Table 1; also see Remage-Healey and Bass, 2004, 2006b, 2007 for rapid cortisol modulation) have rapid, but distinct, effects on fictive calling in batrachoidids, it remains to be shown if different steroids interact along a rapid time-course with each other.…”

“…Time scale and direction of polarity for all records are indicated. Fictive calling in X. laevis (adapted from Rhodes et al, 2007Rhodes et al, , copyright 2007 by the Society for Neuroscience). Laryngeal nerve recordings from males show that the temporal pattern of fictive advertisment-like calls recorded in an in vitro, isolated brain preparation closely compare to the nerve records obtained in vivo.…”

Regulation of Respiratory and Vocal Motor Pools in the Isolated Brain ofXenopus laevis

“…Because DTAM stimulation-induced CAPs are abolished by blocking glutamate receptors, we conclude that DTAM neurons project to laryngeal motor neurons monosynaptically using glutamate as an excitatory neurotransmitter; pharmacological evidence supports action via the AMPA class of receptors. The timing of clicks within the male advertisement call is highly stereotyped (Kelley and Tobias, 1999;Rhodes et al, 2007), a characteristic matched by the precision of DTAM-induced CAPs. The nature of the DTAM to LMN connection may be essential for producing this highly robust behavior.…”