NMDAR-Dependent Control of Call Duration in Xenopus laevis

Abstract

. Many rhythmic behaviors, such as locomotion and vocalization, involve temporally dynamic patterns. How does the brain generate temporal complexity? Here, we use the vocal central pattern generator (CPG) of Xenopus laevis to address this question. Isolated brains can elicit fictive vocalizations, allowing us to study the CPG in vitro. The X. laevis advertisement call is temporally modulated; calls consist of rhythmic click trills that alternate between fast (ϳ60 Hz) and slow (ϳ30 Hz) rates. We investigated the role of two CPG nuclei-the laryngeal motor nucleus (n.IX-X) and the dorsal tegmental area of the medulla (DTAM)-in setting rhythm frequency and call durations. We discovered a local field potential wave in DTAM that coincides with fictive fast trills and phasic activity that coincides with fictive clicks. After disrupting n.IX-X connections, the wave persists, whereas phasic activity disappears. Wave duration was temperature dependent and correlated with fictive fast trills. This correlation persisted when wave duration was modified by temperature manipulations. Selectively cooling DTAM, but not n.IX-X, lengthened fictive call and fast trill durations, whereas cooling either nucleus decelerated the fictive click rate. The N-methyl-D-aspartate receptor (NMDAR) antagonist DAPV blocked waves and fictive fast trills, suggesting that the wave controls fast trill activation and, consequently, call duration. We conclude that two functionally distinct CPG circuits exist: 1) a pattern generator in DTAM that determines call duration and 2) a rhythm generator (spanning DTAM and n.IX-X) that determines click rates. The newly identified DTAM pattern generator provides an excellent model for understanding NDMARdependent rhythmic circuits. I N T R O D U C T I O NMany rhythmic motor behaviors consist of multiple simple rhythms woven into temporally and/or spatially intricate patterns. We sought to understand the neural mechanisms by which discrete rhythms are temporally organized. A major obstacle to understanding temporal patterning lies in the complexity of many behaviors. For example, the control of behaviors such as birdsong or vertebrate locomotion involves the coordination of many muscle groups in elaborate patterns of activation.In this study, we investigated the neural basis of temporal patterning of calling in the frog, Xenopus laevis. Xenopus vocalizations are generated by a simple mechanism of sound production. Calls are produced independent of respiratory movements (unlike most other vertebrate vocal mechanisms) by a single pair of laryngeal muscles. Despite this mechanistic simplicity, the most common male vocalization-advertisement call-is temporally complex, allowing us to explore how a tractable neuronal circuit generates elaborate temporal patterns.Each advertisement call consists of two click trills, fast (ϳ60 Hz) followed by slow (ϳ30 Hz), occasionally preceded by an introductory phase (ϳ20 -40 Hz; Fig. 1; Tobias et al. 1998Tobias et al. , 2004Yamaguchi et al. 2008). Fast and slow trills last close to ...

Coding Rate and Duration of Vocalizations of the Frog,Xenopus laevis

“…Accordingly, the total LLD durations correlated significantly with total fast-trill duration (R 2 ϭ 0.62; regression ANOVA F (1,24) ϭ 16.2, p Ͻ 0.0005). In a previous study (Zornik et al, 2010), we showed that LFP recordings obtained from DTAM contain a slow baseline wave that correlated with onset and offset of fast trills (see example in Fig. 7).…”

“…The rate and amplitude of CAPs during introductory phase are lower than those during fast trill, but the transition from introductory phase to fast trill is gradual. In a previous study, we developed a method for objectively defining the onset time of fast trill (Zornik et al, 2010). We first calculated the product of rate and amplitude (R*A) for each CAP interval (where the R is the reciprocal of the interval between two consecutive CAPs, and A is the amplitude of the second CAP making up that interval).…”

“…The results suggest that the summed LLDs across the FTN population may represent the cellular basis for the LFP wave recorded from DTAM, raising the possibility that FTNs control fast-trill duration in addition to the fast-trill rates. Given that NMDAR activation in DTAM is required for fast trill and LFP wave generation (Zornik et al, 2010), and FTNs may be the cellular basis of the wave, we predicted that FTNs may express the NMDARs that underlie generation of LLDs. Note the similarity in membrane potentials for two distinct current levels.…”

“…Previous local field potential (LFP) recordings from the dorsal tegmental area of medulla (DTAM) (a vocal premotor nucleus and homolog of the mammalian parabrachial nucleus) during fictive calling revealed activity correlated with both fast-trill rate and duration (Zornik et al, 2010). Also, NMDA receptor (NMDAR) activation in DTAM is necessary for fast-trill production (Zornik et al, 2010).…”

“…Also, NMDA receptor (NMDAR) activation in DTAM is necessary for fast-trill production (Zornik et al, 2010). Because fast-trill duration can be modified without changing click rate, we hypothesized that neurons in DTAM possessed distinct mechanisms that code fast-trill rate and duration.…”