Inhibitory and modulatory inputs to the vocal central pattern generator of a teleost fish

Abstract

Vocalization is a behavioral feature that is shared among multiple vertebrate lineages, including fish. The temporal patterning of vocal communication signals is set, in part, by central pattern generators (CPGs). Toadfishes are well‐established models for CPG coding of vocalization at the hindbrain level. The vocal CPG comprises three topographically separate nuclei: pre‐pacemaker, pacemaker, motor. While the connectivity between these nuclei is well understood, their neurochemical profile remains largely unexplored. The highly vocal Gulf toadfish, Opsanus beta, has been the subject of previous behavioral, neuroanatomical and neurophysiological studies. Combining transneuronal neurobiotin‐labeling with immunohistochemistry, we map the distribution of inhibitory neurotransmitters and neuromodulators along with gap junctions in the vocal CPG of this species. Dense GABAergic and glycinergic label is found throughout the CPG, with labeled somata immediately adjacent to or within CPG nuclei, including a distinct subset of pacemaker neurons co‐labeled with neurobiotin and glycine. Neurobiotin‐labeled motor and pacemaker neurons are densely co‐labeled with the gap junction protein connexin 35/36, supporting the hypothesis that transneuronal neurobiotin‐labeling occurs, at least in part, via gap junction coupling. Serotonergic and catecholaminergic label is also robust within the entire vocal CPG, with additional cholinergic label in pacemaker and prepacemaker nuclei. Likely sources of these putative modulatory inputs are neurons within or immediately adjacent to vocal CPG neurons. Together with prior neurophysiological investigations, the results reveal potential mechanisms for generating multiple classes of social context‐dependent vocalizations with widely divergent temporal and spectral properties.

Serotonin distribution in the brain of the plainfin midshipman: Substrates for vocal‐acoustic modulation and a reevaluation of the serotonergic system in teleost fishes

“…Neuronal and fiber label for 5‐HT in the midshipman VPG resembles that found in recent immunohistochemical research in Gulf toadfish (Rosner et al, 2018). The Gulf toadfish VMN receives serotonergic processes, forming putative contacts, from cells situated ventral to it in the IRa, as well as 5‐HT‐ir neurons found within the VMN itself.…”

“…Vocal circuitry shows interconnectivity with thalamic and medullar auditory nuclei as well as higher order telencephalic regions (Goodson & Bass, 2002). A recent study in the vocal Gulf toadfish ( Opsanus beta ) described 5‐HT‐ir in the hindbrain VPG of that related species (Rosner, Rohmann, Bass, & Chagnaud, 2018). This study investigates the distribution and morphology of the serotonin system in the entire brain of the plainfin midshpman and with a focus on evolutionarily conserved vocal and auditory circuitry.…”

Feedback to the future: motor neuron contributions to central pattern generator function

“…Electrical coupling between motor neurons has been demonstrated by collision tests in which intracellularly evoked action potentials do not eliminate depolarizations following antidromic stimulation of the vocal nerve (Chagnaud et al, 2012). In another toadfish, Opsanus tau, the presence of gap junctions is supported by extensive co-labeling of neurobiotin-positive soma with an antibody that labels gap junctions (Cx35/36) in the vocal nuclei (Rosner et al, 2018). While the anatomical connections of vocal motor neurons within the CPG are well established in these fish (Fig.…”

Gap junction-mediated glycinergic inhibition ensures precise temporal patterning in vocal behavior

“…Recent studies of other motor systems provide a framework for Pappas and Bennett, 1966 observations by showing a role for electrotonic coupling between motoneurons and (inhibitory) premotoneurons in patterning vocalization in frogs ( Lawton et al, 2017 ) and locomotion in adult zebrafish ( Song et al, 2016 ) and larval flies ( Matsunaga et al, 2017 ). We also recently reported in Gulf toadfish a subset of glycinergic VPN neurons transneuronally labeled with the gap junction passable tracer neurobiotin, strongly suggestive of electrotonic coupling within the vocal network ( Rosner et al, 2018 ; see Figure 1f ). Could the electrotonically coupled glycinergic premotoneurons be involved in the patterning of motoneuron firing in the toadfish vocal pattern generator, as shown for other motor systems?…”

“…( f ) Photomicrographs of neurobiotin-labeled (red) VPN neurons and vocal tract (VoTr) (left), glycinergic-immunoreactive (green) neurons (somata indicated by white arrowheads) and fibers (middle), and overlay of both (right). A subset of glycinergic neurons in the VPN are co-labeled with neurobiotin (asterisks); see Rosner et al, 2018 for detailed quantification. The scale bar is 20 µm.…”