January 1, 2017;
Probing forebrain to hindbrain circuit functions in Xenopus.
The vertebrate hindbrain
includes neural circuits that govern essential functions including breathing, blood
pressure and heart
circuits also participate in generating rhythmic motor patterns for vocalization. In most tetrapods, sound production is powered by expiration and the circuitry underlying vocalization and respiration must be linked. Perception and arousal are also linked; acoustic features of social communication sounds-for example, a baby''s cry
-can drive autonomic responses. The close links between autonomic functions that are essential for life and vocal expression have been a major in vivo experimental challenge. Xenopus provides an opportunity to address this challenge using an ex vivo preparation: an isolated brain
that generates vocal and breathing patterns. The isolated brain
allows identification and manipulation of hindbrain
vocal circuits as well as their activation by forebrain
circuits that receive sensory input, initiate motor patterns and control arousal. Advances in imaging technologies, coupled to the production of Xenopus lines expressing genetically encoded calcium sensors, provide powerful tools for imaging neuronal patterns in the entire fictively behaving brain
, a goal of the BRAIN
Initiative. Comparisons of neural circuit activity across species (comparative neuromics) with distinctive vocal patterns can identify conserved features, and thereby reveal essential functional components.
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References [+] :
Vocal production in Xenopus. (a) Xenopus call while submerged. (b) Air travels through the glottis to the lungs. The glottis is closed during vocalization. Each sound pulse is generated by separation of paired arytenoids disks (AD), in response to contraction of laryngeal muscles (LM) driven by activity on the vocal nerve (VN). (c) Nerve activity patterns (lower traces) match the sexually differentiated patterns of sound pulses (upper traces) in the male advertisement call and the sexually unreceptive female ticking call
Fictive calling and breathing preparations in X. laevis. (a) The isolated (ex vivo) brain viewed from above (dorsally) from olfactory bulb rostrally (left) to the end of the hindbrain caudally (right). The most caudal rootlet of cranial nerve IX-X (indicated by the line) contains the axons of motor neurons that innervate glottal, laryngeal and heart muscles and comprises the vocal nerve. (b) Fictive advertisement calling (upper trace) can be recorded from the VN when the neuromodulator serotonin is bath applied to the ex vivo brain. The pattern of VN activity follows the pattern of a male advertisement call (lower trace). (c) Fictive ticking (upper trace) can be recorded from the VN when the neuromodulator serotonin is bath applied to the ex vivo brain. The pattern of VN activity follows the pattern of female ticking (lower trace). (from Rhodes et al., 2007). (d) The isolated (ex vivo) brain and innervated larynx viewed from above A branch of cranial nerve IX-X enters the larynx caudally and contains the axons of motor neurons that innervate glottal and laryngeal muscles. e Recordings from glottal muscles (lower panel; also see Figure 1b) reveal spontaneous bursts of activity that correspond to activity of hindbrain glottal motor neurons. This activity pattern produces the opening of the glottis that allows air to travel from the mouth through the larynx to the lungs and is thus termed fictive breathing
Initiation and production of vocal motor patterns in X. laevis. (a) The ex vivo brain (Figure 1a) now viewed from the side and illustrating subdivisions (hindbrain, midbrain, and forebrain) that include neural circuits participating in initiation of vocal patterns. In an adult male brain, nucleus ambiguus (NA) that includes glottal and laryngeal motor neurons (b) is ∼1mm from rostral to caudal. (b) A current view of brain nuclei that participate in vocal patterning. The activity of neurons in the inferior colliculus (ICo) of the midbrain is preferentially driven by sound pulse rates characteristic of specific calls (Elliott et al., 2011). The ICo projects via the central nucleus of the thalamus (CT) to the central amygdala (Ce) in the ventral forebrain (Brahic & Kelley, 2003). Stimulation of either the CeA or the adjacent bed nucleus of the stria terminalis (BNST; together the extended amygdala or EA) in vivo initiates fictive advertisement calling (Hall et al., 2013). (c) A current view (from Yamaguchi et al., 2016) of the forebrain and hindbrain connections and activity responsible for initiating and coordinating fictive calling. This schematic, bilateral view (from the top) extends from the EA rostrally (left) to NA caudally. The hindbrain includes separate pattern generator circuits for the two components of the male advertisement call: fast trill and slow trill. The slow trill pattern generator (in blue) is co-extensive with NA while the fast trill generator (in green) is distributed between NA and a more rostral nucleus DTAM (used as a proper name). Coordination between the two halves of the hindbrain is accomplished by contralateral connections at the level of DTAM (the anterior commissure or AC) and NA (the posterior commissure or PC) and by bilateral connections between DTAM and NA
Molecular identity of forebrain nuclei that contribute to vocal initiation. (a) and (b) are modified from “Evolution of the amygdaloid complex in vertebrates, with special reference to the anamnio-amniotic transition,” by Moreno and Gonzalez, 2007, Journal of Anatomy, 211, p 151. (c-e) are modified from “The Xenopus amygdala mediates socially appropriate vocal communication signals.” by Hall et al., 2013, The Journal of Neuroscience, 33, p 14534. (a) The anuran amygdaloid complex in a transverse hemisection of the caudal forebrain illustrating component nuclei, including descending projections to the brainstem (hindbrain); medial is to the right and dorsal is up. (b) A schematic hemisection corresponding to a and illustrating expression of a number of transcription factors (Isl1, Nkx2.1, Lhx7, Lhx1,Dll4), neurotransmitters or neuromodulators (GABA, TH,SP and ENK) and a calcium-binding protein (CR). Note the widespread distribution of the inhibitory neurotransmitter GABA within the CeA. Expression of these molecular markers corresponds to similar patterns in mammals (not included). (c) A fictive advertisement call recorded from the VN after microstimulation in the EA. (d) Effective (red, asterisks) and ineffective (black) stimulation sites for evoking fictive advertisement calls. (e) Comparison of temporal features (fast and slow trill) recorded from an advertisement calling male (in vivo), a fictively advertisement calling male brain (ex vivo), and following EA microstimulation (ex vivo EA stim) indicates that EA activity can initiate a specific fictive call pattern
Examples of vocal circuit comparisons across and within Xenopus clades. The S, L, and M clades use different genetic mechanisms for primary sex determination. Comparison of neural circuit elements—using a fictively calling preparation—across species from these clades should reveal common and diverged neural circuit mechanisms. Divergence time estimates of Silurana and Xenopus based on Canatella (2015). Phylogeny simplified from Evans et al. (2015)
Whole-brain functional imaging at cellular resolution using light-sheet microscopy.