A proposed neural pathway for vocalization in South African clawed frogs,Xenopus laevis

Abstract

Vocalizations of South African clawed frogs are produced by contractions of laryngeal muscles innervated by motor neurons of the caudal medulla (within cranial nerve nucleus IX–X). We have traced afferents to laryngeal motor neurons in male and female frogs using retrograde axonal transport of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA). After iontophoretic injection of HRP-WGA into n. IX–X, retrogradely labelled neurons were seen in the contralateral n. IX–X, in rhombencephalic reticular nuclei, and in the pre-trigeminal nucleus of the dorsal tegmental area (DTAM) of both males and females. Injection of HRP-WGA into DTAM resulted in labelled cells in the striatum, preoptic area and thalamus. Posterior to DTAM, labelled cells were found in the rhombencephalic reticular nuclei as well as n. IX–X of males. Results in females were similar with the exception that n. IX–X labelled cells were only seen after very large injections of unconjugated HRP into DTAM and surrounding tegmentum. Thus, the projection of n. IX–X neurons to DTAM is not as robust in females as males. These anatomical studies revealed candidate brain nuclei contributing to the generation of vocal behaviors and confirmed some features of a model for anuran vocal behavior proposed by Schmidt (1976). Comparison of calling candidate brain nuclei to the location of steroid accumulating neurons (Kelley 1981) reveals that most calling nuclei contain hormone concentrating cells. Androgens may act to promote calling by influencing neural activity at multiple sites within the vocalization pathway.