Vocal Behavior and Vocal Central Pattern Generator Organization Diverge among Toadfishes

Abstract

Among fishes, acoustic communication is best studied in toadfishes, a single order and family that includes species commonly known as toadfish and midshipman. However, there is a lack of comparative anatomical and physiological studies, making it difficult to identify both shared and derived mechanisms of vocalization among toadfishes. Here, vocal nerve labeling and intracellular in vivo recording and staining delineated the hindbrain vocal network of the Gulf toadfish Opsanus beta. Dextran-biotin labeling of the vocal nerve or intracellular neurobiotin fills of motoneurons delineated a midline vocal motor nucleus (VMN). Motoneurons showed bilaterally extensive dendritic arbors both within and lateral to the paired motor nuclei. The motoneuron activity matched that of the spike-like vocal nerve motor volley that determines the natural call duration and frequency. Ipsilateral vocal nerve labeling with biocytin or neurobiotin yielded dense bilateral transneuronal filling of motoneurons and coextensive columns of premotor neurons. These premotor neurons generated pacemaker-like action potentials matched 1:1 with vocal nerve and motoneuron firing. Transneuronal transport further revealed connectivity within and between the pacemaker-motor circuit and a rostral prepacemaker nucleus. Unlike the pacemaker-motor circuit, prepacemaker firing did not match the frequency of vocal nerve activity but instead was predictive of the duration of the vocal nerve volley that codes for call duration. Transneuronally labeled terminal-like boutons also occurred in auditory-recipient hindbrain nuclei, including neurons innervating the inner ear and lateral line organs. Together with studies of midshipman, we propose that separate premotor populations coding vocal frequency and duration with direct premotor coupling to auditory-lateral line nuclei are plesiomorphic characters for toadfishes. Unlike in midshipman, transneuronal labeling in toadfishes reveals an expansive column of pacemaker neurons that is weakly coupled to prepacemaker neurons, a character that likely depends on the extent of gap junction coupling. We propose that these and other anatomical characters contribute to neurophysiological properties that, in turn, sculpt the species-typical patterning of frequency and amplitude-modulated vocalizations.

Comparative Neurobiology of Sound Production in Fishes

“…A VMN dedicated to sonic functions has been experimentally demonstrated in 22 species following the labeling of one or more vocal nerve roots with either horseradish peroxidase or a biotin derivative (neurobiotin, biocytin, dextran-biotin) (Table 2.1). Intracellular recording and staining further verifies the position of VMN in three species of Batrachoidiformes (plainfin midshipman; oyster toadfish; and Gulf toadfish, O. beta) and one species of Perciformes (northern sea robin, Prionotus carolinus) (Pappas and Bennett 1966;Bass andBaker 1990, 1991;Chagnaud et al 2012;Chagnaud and Bass 2014). As listed in Table 2.1, VMN is positioned along the rostral-caudal axis in the caudal hindbrain medulla and/or the rostral spinal cord in all species so far investigated.…”

“…The model was largely verified in midshipman fish by Bass and colleagues who provided the essential cellular level experiments (Bass and Baker 1990;Goodson and Bass 2000a;Kittelberger et al 2006). As recently shown by Chagnaud and colleagues (Chagnaud et al 2011Chagnaud and Bass 2014), the VPP and vocal pacemaker nuclei (VPN) discussed earlier code for call duration and fundamental frequency/PRR, respectively. VPP neurons receive PAG input (Kittelberger and Bass 2013) and project to the VPN-VMN region (Chagnaud et al 2011), acting as a link that gates forebrain/midbrain activation of hindbrain vocal sites (Kittelberger et al 2006).…”

“…For these groups of fish, as in midshipman and toadfish Chagnaud and Bass 2014), biocytin or neurobiotin labeling also showed connections between the vocal system and the central auditory system, including the octavolateralis efferent nucleus (OEN) that directly innervates the inner ear and lateral line organs. The activity of putative OEN axons projecting to the inner ear and lateral line system was correlated to call duration and frequency (Weeg et al 2005), as later verified via intracellular OEN recordings (Chagnaud and Bass 2013).…”

FoxP2 Expression in a Highly Vocal Teleost Fish with Comparisons to Tetrapods

“…Label was also robust in two ventral telencephalic nuclei, i.e., Vd and Vs, that have been compared, in part, to basal ganglia (Vd) and subpallial amygdala (Vs) of tetrapods [Mueller et al, 2008;Maximino et al, 2013], as well as in the preoptic area. Though label was not found in the neuronal populations comprising the hindbrain vocal CPG (VMN, VPP, and VPN) [Bass and Baker, 1990;Chagnaud et al, 2011Chagnaud et al, , 2012Chagnaud and Bass, 2014], robust or moderate label was identified in forebrain and midbrain sites that are major nodes in the central vocal and auditory networks ( Fig. 1 2000; Goodson and Bass, 2002;Bass et al, 2005;Kittelberger and Bass, 2013].…”

Vocal differentiation parallels development of auditory saccular sensitivity in a highly soniferous fish

“…However, these authors also reported that motoneurons, which directly establish the firing rate of sonic muscles and vocal features, establish their connections with the sonic muscle prior to establishing connections with the premotor neurons in the hindbrain. More recently, Chagnaud and Bass (2014) showed that the central pattern generator (CPG) anatomy of two batrachoidids (P. notatus and Opsanus tau; subfamilies Batrachoidinae and Porichthinae, respectively) differs at the levels of both the pacemaker-motoneuron circuit and the afferent pre-pacemaker neurons and this may contribute to the species-specific patterning of frequency and amplitude-modulated vocalisations. Together, this suggests that the maturation of CPG and patterns of connectivity between hindbrain vocal nuclei may underline the vocal differentiation observed in the Lusitanian toadfish.…”