446
3.1 Organization of the Midshipman Ascending
Auditory System
Like most other teleosts, the saccule is the largest inner ear end organ (Fig. 1 ) and
thought to be the main peripheral sensory structure for hearing (Fay and EddsWalton 2008 ). The midshipman auditory system has been well delineated from multiple studies that have employed a variety of tract tracing techniques including bulk
fi lls of the saccular branch of the eighth nerve as well as isolation and characterization of single unit recordings of midbrain neurons followed by injections of biotin
compounds (Bass et al. 2000 ; Bass et al. 1994 ; Sisneros et al. 2002 ). In addition,
tracer injection into vocally active circuitry has demonstrated auditory-vocal integration sites in forebrain, midbrain, and hindbrain (Goodson and Bass 2002 ;
Kittelberger and Bass 2013 ). Due to their small molecular weight, biocytin and
neurobiotin compounds have been shown to be actively transported transynaptically, which has allowed for identifi cation of multiple levels of processing within
the hindbrain alone (Bass et al. 2000 ; Bass et al. 1994 ; Sisneros et al. 2002 ). The
most likely explanation for this degree of multi-synapse transport is via gap junctions (Bass et al. 1994 ). Primary afferents from the saccule synapse onto hindbrain
neurons of the descending octaval nucleus (DO) which are connected to secondary
octaval neurons (SO); both hindbrain groups project to the midbrain torus semicircularis (TS), which in turn projects to several diencephalic nuclei including the
anterior tuberal hypothalamus (AT), lateral division of nucleus preglomerulosus
(PGl) and central posterior thalamus (CP), as well as to ventral telencephalic nuclei.
The auditory thalamus (CP) also receives input from AT and relays information to
Fig. 1 (continued) analyzed in Forlano et al. ( 2015a ) (adapted from Forlano et al. 2014 ).
Abbreviations : ac anterior commissure, AT anterior tuberal nucleus, C cerebellum, CA cerebral
aqueduct, cc cerebellar crest, Cm molecular layer of the corpus of the cerebellum, Cg granular
layer of the corpus of the cerebellum, CP central posterior nucleus of the thalamus, Df diffuse
nucleus of the hypothalamus, dl dorsolateral division of the descending octaval nucleus, Dl lateral
zone of area dorsalis of the telencephalon, dm dorsomedial division of the descending octaval
nucleus, Dm medial zone of area dorsalis of the telencephalon, DO descending octaval nucleus, G
nucleus glomerulosus, Ha habenula, Hc central periventricular hypothalamus, Hd dorsal periventricular hypothalamus, Hv ventral periventricular hypothalamus, iaf internal arcuate fi ber tract, IV
fourth ventricle, LC locus coeruleus, ll lateral lemniscus, M midbrain, MED medial octavolateralis
nucleus, MFB medial forebrain bundle, MLF medial longitudinal fasciculus, OB olfactory bulb,
OE octavolateralis efferent nucleus, OT optic tract, PAG periaqueductal gray, Pe periventricular
cell layer of the torus semicircularis, PPa anterior parvocellular preoptic nucleus, PGl lateral division of nucleus preglomerulosus, Pit pituitary, PTN posterior tuberal nucleus, PTT paratoral tegmentum, PVO paraventricular organ, RF reticular formation, SE saccular epithelium of the inner
ear, SO secondary octaval nucleus, SR superior raphe, SV saccus vasculosus, T telencephalon, Te
midbrain tectum, TPp periventricular nucleus of the posterior tuberculum, TS torus semicircularis,
Vc central nucleus of area ventralis of the telencephalon, Vd dorsal nucleus of area ventralis of the
telencephalon, Vde descending tract of the trigeminal nerve, VIII eighth nerve, VL ventrolateral
nucleus of the thalamus, VM ventromedial nucleus of the thalamus, Vp postcommissural nucleus
of area ventralis of the telencephalon, Vs supracommissural nucleus of area ventralis of the telencephalon, vT ventral tuberal hypothalamus. Scale bar = 1.5 mm (dorsal brain); 500 μm (sections)
P.M. Forlano and J.A. Sisneros
3.1 Organization of the Midshipman Ascending
Auditory System
Like most other teleosts, the saccule is the largest inner ear end organ (Fig. 1 ) and
thought to be the main peripheral sensory structure for hearing (Fay and EddsWalton 2008 ). The midshipman auditory system has been well delineated from multiple studies that have employed a variety of tract tracing techniques including bulk
fi lls of the saccular branch of the eighth nerve as well as isolation and characterization of single unit recordings of midbrain neurons followed by injections of biotin
compounds (Bass et al. 2000 ; Bass et al. 1994 ; Sisneros et al. 2002 ). In addition,
tracer injection into vocally active circuitry has demonstrated auditory-vocal integration sites in forebrain, midbrain, and hindbrain (Goodson and Bass 2002 ;
Kittelberger and Bass 2013 ). Due to their small molecular weight, biocytin and
neurobiotin compounds have been shown to be actively transported transynaptically, which has allowed for identifi cation of multiple levels of processing within
the hindbrain alone (Bass et al. 2000 ; Bass et al. 1994 ; Sisneros et al. 2002 ). The
most likely explanation for this degree of multi-synapse transport is via gap junctions (Bass et al. 1994 ). Primary afferents from the saccule synapse onto hindbrain
neurons of the descending octaval nucleus (DO) which are connected to secondary
octaval neurons (SO); both hindbrain groups project to the midbrain torus semicircularis (TS), which in turn projects to several diencephalic nuclei including the
anterior tuberal hypothalamus (AT), lateral division of nucleus preglomerulosus
(PGl) and central posterior thalamus (CP), as well as to ventral telencephalic nuclei.
The auditory thalamus (CP) also receives input from AT and relays information to
Fig. 1 (continued) analyzed in Forlano et al. ( 2015a ) (adapted from Forlano et al. 2014 ).
Abbreviations : ac anterior commissure, AT anterior tuberal nucleus, C cerebellum, CA cerebral
aqueduct, cc cerebellar crest, Cm molecular layer of the corpus of the cerebellum, Cg granular
layer of the corpus of the cerebellum, CP central posterior nucleus of the thalamus, Df diffuse
nucleus of the hypothalamus, dl dorsolateral division of the descending octaval nucleus, Dl lateral
zone of area dorsalis of the telencephalon, dm dorsomedial division of the descending octaval
nucleus, Dm medial zone of area dorsalis of the telencephalon, DO descending octaval nucleus, G
nucleus glomerulosus, Ha habenula, Hc central periventricular hypothalamus, Hd dorsal periventricular hypothalamus, Hv ventral periventricular hypothalamus, iaf internal arcuate fi ber tract, IV
fourth ventricle, LC locus coeruleus, ll lateral lemniscus, M midbrain, MED medial octavolateralis
nucleus, MFB medial forebrain bundle, MLF medial longitudinal fasciculus, OB olfactory bulb,
OE octavolateralis efferent nucleus, OT optic tract, PAG periaqueductal gray, Pe periventricular
cell layer of the torus semicircularis, PPa anterior parvocellular preoptic nucleus, PGl lateral division of nucleus preglomerulosus, Pit pituitary, PTN posterior tuberal nucleus, PTT paratoral tegmentum, PVO paraventricular organ, RF reticular formation, SE saccular epithelium of the inner
ear, SO secondary octaval nucleus, SR superior raphe, SV saccus vasculosus, T telencephalon, Te
midbrain tectum, TPp periventricular nucleus of the posterior tuberculum, TS torus semicircularis,
Vc central nucleus of area ventralis of the telencephalon, Vd dorsal nucleus of area ventralis of the
telencephalon, Vde descending tract of the trigeminal nerve, VIII eighth nerve, VL ventrolateral
nucleus of the thalamus, VM ventromedial nucleus of the thalamus, Vp postcommissural nucleus
of area ventralis of the telencephalon, Vs supracommissural nucleus of area ventralis of the telencephalon, vT ventral tuberal hypothalamus. Scale bar = 1.5 mm (dorsal brain); 500 μm (sections)
P.M. Forlano and J.A. Sisneros