Sexually differentiated central pattern generators in Xenopus laevis

Abstract

Understanding the neural mechanisms that underlie the function of central pattern generators (CPGs) presents a formidable challenge requiring sophisticated tools and well-chosen model systems. In this article, we describe recent work on vocalizations of the African clawed frog Xenopus laevis. These behaviors are driven by sexually differentiated CPGs and are exceptionally well suited to this objective. In particular, a simplified mechanism of vocal production (independent of respiratory musculature) allows straightforward interpretations of nerve activity with respect to behavior. Furthermore, the development of a fictively vocalizing isolated brain, together with the finding of rapid androgen-induced masculinization of female vocalizations, provides an invaluable tool for determining how new behaviors arise from existing circuits.

Figure I

Temporal and spectral organization of advertisement calls and ticking. (a) Sound waveform of the male advertisement call that consists of alternating slow (∼35 Hz) and fast (∼70 Hz) trills. (b) Sound waveform of female ticking that consists of slow repetition of clicks (∼6 Hz). (c,d) Two dominant frequency bands (∼2 kHz) observed in male advertisement calls (c) are significantly higher than clicks produced by females (d).

Figure 1

Androgen-induced masculinization of female vocalizations. A female that produced only ticking before testosterone treatment (top) began to produce biphasic advertisement calls within 8 weeks of the treatment (middle). The click rates are slightly slower than those of male advertisement calls (bottom). Figure reproduced, with permission, from [11], (2005) American Physiological Society.

Figure 2

Laryngeal nerve activity as a direct readout of behavior. Simultaneous sound (top) and nerve recordings (middle) obtained from awake frogs show that each click is preceded by a compound action potential, indicating that the neuronal signals generated by the central vocal pathway are faithfully transduced into a series of clicks by the larynx. Serotonin-induced fictive vocalizations (bottom) are remarkably similar to nerve recordings obtained from awake frogs.

Figure 3

Brainstem vocal circuit of Xenopus laevis. (a) Dorsal view of the Xenopus CNS with key brainstem vocal nuclei. (b) Schematic diagram of the reciprocal connections between n.IX-X and DTAM. Four neuron populations – two motoneurons and two interneurons – have been identified in n.IX-X. Commissural neurons (magenta; IX-X_IX-X) project to glottal (dark blue) and laryngeal (red) motoneurons and DTAM-projecting interneurons (light blue; IX-X_DTAM). IX-X_DTAM cells send robust inputs to DTAM, and all four n.IX-X neurons receive inputs from DTAM. Bars represent known excitatory synapses; arrows represent synapses with currently unknown valence [19]. Abbreviations: DTAM = dorsal tegmental area of medulla; OB = olfactory bulb; OT = optic tectum; T = thalamus; TEL = telencephalon; n.IX-X = motor nucleus IX-X; N.IX-X = cranial nerve IX-X; GM = glottal motoneuron; LM = laryngeal motoneuron; IX-X_DTAM = DTAM-projecting n.IX-X interneuron; IX-X_IX-X = n.IX-X-projecting commissural interneuron; DTAM_IX-X = n.IX-X-projecting DTAM neuron.

Figure 4

Peripheral constraints on vocal masculinization. Top: vocal recordings from a female after 8 weeks of testosterone exposure. Bottom: fictive recordings from the brain of the same animal. Slow and fast trill segments of vocal and fictive recordings are marked. The fictive call resembles male advertisement calls more closely than the sound produced in vivo, which shows an abnormal pause between slow and fast trills. The vocal CPG is likely to be more ‘plastic’ and masculinizes more rapidly than the larynx. Consequently, the fully masculinized neuronal signal generated by the vocal CPG could be low-pass filtered by an incompletely masculinized larynx. Figure reproduced, with permission, from [3], (2007) Society for Neuroscience.