451
tegmental (VT) cells, and inferior olive (IO), all of which were found to contain
putative TH-ir terminals on both their somata and dendrites (Fig. 3d–h ). All of these
nuclei are consistent with previous studies that delineated hindbrain auditory circuitry by saccular backfi lls (Bass et al. 2000 ; Bass et al. 1994 ; Sisneros et al. 2002 ).
TH-ir fi bers and terminals are heavily concentrated throughout the longitudinal column of the dorsal hindbrain that includes dm and dorsolateral (dl) divisions of DO
and the lateral line recipient nucleus medialis (MED) as well as the magnocellular
octaval nucleus (MG) which sits adjacent to the entry of nerve VIII (Figs. 3a–c and
4a–c ). At rostral levels, TH-ir fi bers are highly concentrated within bilateral tracts
coursing through the ventral medial hindbrain which includes where SOv and VT
are located. TH-ir fi bers are also present but less dense within the cerebellar crest
(cc), and within central tracts of the posterior and anterior lateral line nerves (LL),
adjacent to dl (Fig. 3 ).
3.2.3 Octavolateralis Efferent Nucleus
A bulk saccular fi ll will also delineate the hindbrain octavolateralis efferent nucleus
(OE) which projects to all inner ear end organs and the lateral line system (Bass
et al. 2000 ; Bass et al. 1994 ; Chagnaud et al. 2011 ; Chagnaud and Bass 2013 ; Weeg
et al. 2005 ). These neurons are known to be cholinergic as they are labeled by choline acetyltransferase (ChAT)-ir (Brantley and Bass 1988 ; Forlano et al. 2015a ).
Qualitatively, the OE contains the most striking pattern of TH-ir innervation in the
brain (Figs. 4 and 5 ), and is also thus far the densest area of TH-ir terminals measured
quantitatively (Forlano et al. 2015a ). The large OE somata which lie at the midline of
the hindbrain just below the fourth ventricle have prominent ventrolaterally projecting dendrites and the distribution of TH-ir fi bers closely matches this dendritic fi eld
and forms terminals on both neurobiotin-fi lled somata and dendrites (Fig. 4e–h ). The
pattern is consistent in both the rostral and caudal divisions of OE which are also
lightly labeled by backfi ll of a single vocal nerve (Fig. 4d ; Bass et al. 1994 ;
Fig. 3 (continued) neurobiotin-labeled ( red ) octavolateralis efferent axons (ea) within the lateral
efferent bundle just caudal to where they join VIII. Arrowhead in a indicates TH-ir axon bundle
entering the lateral efferent tract just dorsal to the ventral secondary octaval nucleus (SOv). TH-ir
axons are seen descending within the efferent tract ( b ) as it turns to run longitudinally along the
midline ( c ) dorsal to internal arcuate fi ber tract (iaf) rostral to the octavolateralis efferent nucleus (OE,
see Fig. 4d ). Arrowhead in c shows TH-ir fi bers projecting dorsally into the rostral intermediate (ri)
and dorsal medial (dm) subdivisions of the descending octaval nucleus (compare with Fig. 5b ). ( d , e )
High magnifi cation of areas indicated by ( * ) in c showing TH-ir terminals and varicosities on somata
and dendrites ( arrowheads ) of dm and SOv neurons in d and e , respectively. ( f ) High magnifi cation
of area indicated by ( * ) in b showing TH-ir varicosities on dendrites of a ventral tegmental (VT)
neuron which lies just medial to the trigeminal motor nucleus (Vm). ( g , h ) TH-ir terminals and varicosities on fi lled cells in ri and inferior olive (IO), respectively. Images in g and h were selected from
sections outside of a – c . IO lies along the ventrolateral border of the caudal hindbrain. Scale bar = 200
μm in a – c , 16 μm in d – g , and 20 μm in h . From Forlano et al. ( 2014 )
Neuroanatomical Evidence for Catecholamines as Modulators of Audition…
tegmental (VT) cells, and inferior olive (IO), all of which were found to contain
putative TH-ir terminals on both their somata and dendrites (Fig. 3d–h ). All of these
nuclei are consistent with previous studies that delineated hindbrain auditory circuitry by saccular backfi lls (Bass et al. 2000 ; Bass et al. 1994 ; Sisneros et al. 2002 ).
TH-ir fi bers and terminals are heavily concentrated throughout the longitudinal column of the dorsal hindbrain that includes dm and dorsolateral (dl) divisions of DO
and the lateral line recipient nucleus medialis (MED) as well as the magnocellular
octaval nucleus (MG) which sits adjacent to the entry of nerve VIII (Figs. 3a–c and
4a–c ). At rostral levels, TH-ir fi bers are highly concentrated within bilateral tracts
coursing through the ventral medial hindbrain which includes where SOv and VT
are located. TH-ir fi bers are also present but less dense within the cerebellar crest
(cc), and within central tracts of the posterior and anterior lateral line nerves (LL),
adjacent to dl (Fig. 3 ).
3.2.3 Octavolateralis Efferent Nucleus
A bulk saccular fi ll will also delineate the hindbrain octavolateralis efferent nucleus
(OE) which projects to all inner ear end organs and the lateral line system (Bass
et al. 2000 ; Bass et al. 1994 ; Chagnaud et al. 2011 ; Chagnaud and Bass 2013 ; Weeg
et al. 2005 ). These neurons are known to be cholinergic as they are labeled by choline acetyltransferase (ChAT)-ir (Brantley and Bass 1988 ; Forlano et al. 2015a ).
Qualitatively, the OE contains the most striking pattern of TH-ir innervation in the
brain (Figs. 4 and 5 ), and is also thus far the densest area of TH-ir terminals measured
quantitatively (Forlano et al. 2015a ). The large OE somata which lie at the midline of
the hindbrain just below the fourth ventricle have prominent ventrolaterally projecting dendrites and the distribution of TH-ir fi bers closely matches this dendritic fi eld
and forms terminals on both neurobiotin-fi lled somata and dendrites (Fig. 4e–h ). The
pattern is consistent in both the rostral and caudal divisions of OE which are also
lightly labeled by backfi ll of a single vocal nerve (Fig. 4d ; Bass et al. 1994 ;
Fig. 3 (continued) neurobiotin-labeled ( red ) octavolateralis efferent axons (ea) within the lateral
efferent bundle just caudal to where they join VIII. Arrowhead in a indicates TH-ir axon bundle
entering the lateral efferent tract just dorsal to the ventral secondary octaval nucleus (SOv). TH-ir
axons are seen descending within the efferent tract ( b ) as it turns to run longitudinally along the
midline ( c ) dorsal to internal arcuate fi ber tract (iaf) rostral to the octavolateralis efferent nucleus (OE,
see Fig. 4d ). Arrowhead in c shows TH-ir fi bers projecting dorsally into the rostral intermediate (ri)
and dorsal medial (dm) subdivisions of the descending octaval nucleus (compare with Fig. 5b ). ( d , e )
High magnifi cation of areas indicated by ( * ) in c showing TH-ir terminals and varicosities on somata
and dendrites ( arrowheads ) of dm and SOv neurons in d and e , respectively. ( f ) High magnifi cation
of area indicated by ( * ) in b showing TH-ir varicosities on dendrites of a ventral tegmental (VT)
neuron which lies just medial to the trigeminal motor nucleus (Vm). ( g , h ) TH-ir terminals and varicosities on fi lled cells in ri and inferior olive (IO), respectively. Images in g and h were selected from
sections outside of a – c . IO lies along the ventrolateral border of the caudal hindbrain. Scale bar = 200
μm in a – c , 16 μm in d – g , and 20 μm in h . From Forlano et al. ( 2014 )
Neuroanatomical Evidence for Catecholamines as Modulators of Audition…