Brain Research Bulletin, Vol. 57, Nos. 3/4, pp. 427– 430, 2002
Copyright © 2002 Elsevier Science Inc.
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The nucleus subglomerulosus of the trout
hypothalamus is a link between chemosensory and
visual systems: A DiI study
Mónica Folgueira,1 Gema Huesa,1 Ramón Anadón2 and Julián Yáñez1*
1
Department of Cellular and Molecular Biology, University of A Coruña, A Coruña, Spain; and 2Department of
Fundamental Biology, University of Santiago de Compostela, Santiago de Compostela, Spain
ABSTRACT: We have studied the connections of the nucleus
subglomerulosus of the trout posterior tubercle. Main afferents
to the nucleus subglomerulosus come from the dorsal telencephalon and the visceral (gustatory) secondary nucleus, while
it projects to the optic tectum. In the light of the connections
observed, the nucleus subglomerulosus of trout (and probably
in other teleosts) appears to be involved in the modulation of
sensory-motor tectal processing by olfactory and visceral information. © 2002 Elsevier Science Inc.
hemispheres, the superficial pretectal nuclei, the preglomerular
complex, the SG, the diffuse nucleus of the lateral hypothalamic
lobes (DLI), the torus lateralis, the dorsal optic tectum (OT)
(dorsorostral, dorsomedial, and dorsocaudal parts), the torus semicircularis, the cerebellum, the nucleus lateralis valvulae, the nucleus isthmi, and the secondary visceral (gustatory) nucleus
(SVN). Transverse vibratome sections (50 ␮m thick) were examined and photographed with a fluorescence microscope equipped
with a rhodamine filter set. Negative films were scanned and
printed as positive.
KEY WORDS: Chemoreception, Optic tectum, Secondary gustatory nucleus, Connections, Teleosts.
RESULTS
The SG of trout is a characteristic nucleus located in the
dorsolateral region of the lateral hypothalamic recesses (Fig. 1).
Application of DiI to the SG and to a series of brain nuclei and
regions, showed that this nucleus have mainly telencephalic, tectal,
and isthmic connections. Application of DiI to the SG-labeled
neurons in the medial part (Dm) and in the ventrocaudal region of
the posterior part (Dp) of the dorsal telencephalic area (Fig. 2), and
fibers reaching Dm. As shown by DiI application to the OB, Dp
was the main target for secondary olfactory projections in the
dorsal telencephalon (Fig. 3), while no labeled fibers were observed in SG with this type of DiI application. DiI application to
Dp confirmed the ipsilateral projection from this region to the SG,
giving rise to a rather dense terminal field in SG and nearby
periventricular cell layer of the lateral hypothalamic recess (Fig.
4).
Application of DiI to the SG also labeled a number of perikarya
in the secondary visceral nucleus (Fig. 5) and fibers in the stratum
opticum of the rostral optic tectum. Although DiI application to the
nucleus isthmi produced intense labeling of numerous fibers in the
SG originating from a conspicuous tract (Fig. 6), this labeling
appears to be “en-passant” because SVN fibers cross this nucleus.
DiI application to the rostral OT labeled bilaterally a number of SG
perikarya (Fig. 7), whereas DiI crystals placed into the dorsomedial or dorsocaudal areas of the OT only labeled a few SG neurons
(Fig. 8).
Although some SG perikarya can be labeled after DiI application to the nucleus preglomerulosus or the magnocellular and
parvocellular superficial pretectal nuclei, these results appear to be
due to “en-passant” tract labeling, since DiI application to the SG
INTRODUCTION
The posterior tubercle of the diencephalon of trout and other
teleosts contains a characteristic U-shaped nucleus referred to as
nucleus subglomerulosus (SG) [1,2]. In the trout, the nucleus
consists of a layer of pear-shaped perikarya that surrounds a core
of neuropil [2]. Despite the outstanding appearance of this nucleus,
no data about its connections are available. With the aim to know
the afferent and efferent projections of SG, we have conducted
experiments in fixed brains using DiI as a tracer. DiI is a lipophylic
dye that diffuses along the plasma membrane of neurons in both
retrograde and anterograde directions.
MATERIALS AND METHODS
Young trouts (Oncorhynchus mykiss) were deeply anesthetized
and intracardially perfused with 4% paraformaldehyde in 0.1 M
phosphate buffer pH 7.4. Brains were then dissected out and
maintained in the same fixative. Two procedures of DiI application
were conducted as elsewhere [5]. In brief, for labeling superficial
nuclei and brain areas accessible externally, a small crystal of the
lipophylic tracer DiI (Eugene, OR, USA) placed on the tip of an
insect pin was directly applied under a stereomicroscope. For
labeling other nuclei and areas, brains were previously embedded
in agarose and sectioned on a vibratome at the required level, and
the tracer was applied as above. The cut surface was sealed with
melted agarose and brains were left for 2– 8 weeks in darkness at
37°C in frequently renewed fresh fixative. Sites of DiI application
were the olfactory bulbs (OB), several regions of the telencephalic
* Address for correspondence: Julián Yáñez, Department of Cellular and Molecular Biology, University of A Coruña, E-15071 A Coruña, Spain. Fax:
⫹34-981-167065; E-mail: juliany@udc.es
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FIG. 1. Nissl-stained transverse section through the trout hypothalamus
showing the rostral part of the nucleus subglomerulosus (SG). Scale bar: 70
␮m. See List of Abbreviations.
FIG. 2. Transverse section through the caudal telencephalon showing
retrograde labeled cells (arrowheads) in the ventrocaudal region of Dp after
DiI application to the nucleus subglomerulosus. Scale bar: 100 ␮m. See
List of Abbreviations.
FIG. 3. Transverse section through the caudal telencephalon showing
intensely labeled secondary olfactory projections in Dp after DiI application in the olfactory bulb. Scale bar: 100 ␮m. See List of Abbreviations.
FOLGUEIRA ET AL.
FIG. 4. Detail of a transverse section through the nucleus subglomerulosus
(SG) to show anterograde varicose fibers (arrows) after DiI application to
the Dp. Scale bar: 100 ␮m. See List of Abbreviations.
FIG. 5. Transverse section through the isthmus showing a dense group of
retrograde labeled cells (arrowhead) in the secondary visceral nucleus
(SVN) and their axons (arrows) crossing the nucleus isthmi (NI) after DiI
application to the SG. Scale bar: 100 ␮m. See List of Abbreviations.
FIG. 6. Transverse section through the hypothalamus showing anterogradely labeled fibres (arrow) in the SG after DiI application to the isthmus.
Scale bar: 100 ␮m. See List of Abbreviations.
CONNECTIONS OF THE NUCLEUS SUBGLOMERULOSUS
FIGS. 7 and 8. Transverse section through the SG after DiI application to
the rostro- (7) and caudo-dorsal (8) region of the optic tectum (OT)
showing retrogradely labeled perikarya (arrowheads). Scale bar: 60 ␮m.
See List of Abbreviations.
429
appear to be mainly telencephalic (Dp and Dm) and isthmal
(SVN). The afferent neurons located in Dp appear to overlap the
main terminal field of the secondary olfactory projections ([4],
present results). These results suggest that Dp transmits olfactory
information to the SG. Extratelencephalic projections originating
from Dp were not reported previously in teleosts (see [3]). The
SVN of trout projects mostly to the ipsilateral tertiary gustatory
nucleus of the hypothalamus [5]. The present results indicate that
the SG is also a specific target of the SVN, a connection that was
not previously reported in teleosts. Therefore, both tertiary visceral
(gustatory?) and olfactory projections appear to be the most significant afferents to the SG.
The OT is the main visual center of teleosts [4]. Interestingly,
it was the only target identified for the SG neurons. Our results also
indicate that many more SG neurons were labeled when DiI was
applied to the rostral OT than when it was applied to the caudal
tectum, which indicates some topographical organization of the
SG-tectal projection. Together, present experiments suggest that
the SG of trout may have a role in the modulation of sensory-motor
processing in the OT by both taste and olfactory inputs.
ACKNOWLEDGEMENTS
does not label terminal fields in these nuclei. DiI application to
other structures mentioned above did not label neurons or fibers in
the SG, with the exception of DLI.
This work was supported by a grant from the Xunta de Galicia
(PGIDT99BIO20002) and the Science and Technology Ministry
(BXX2000-0453-CO2-01 and O2).
DISCUSSION
REFERENCES
The present study demonstrates for the first time the connections of the SG. The afferents of the SG (summarized in Fig. 9)
1. Gómez Segade, P.; Anadón, R. Specialization in the diencephalon of
advanced teleosts. J. Morphol. 197:71–104; 1988.
FIG. 9. Schematic drawings of transverse sections (A–E) of the trout brain (levels are indicated in the inset) showing the main
connections of the SG. See List of Abbreviations.
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FOLGUEIRA ET AL.
2. Holmgren, N. Zur Anatomie und Histologie des Vorder- und Zwischenhirns der Knochenfischen hauptsächlich nach Untersuchungen an
Osmerus eperlanus. Acta Zool. (Stockh.) 1:137–153; 1920.
3. Matz, S. P. Connections of the olfactory bulb in the chinook salmon
(Oncorhynchus tshawytscha). Brain Behav. Evol. 46:108 –120; 1995.
4. Meek, J.; Nieuwenhuys, R. Holosteans and teleosts. In: Nieuwenhuys,
R.; ten Donkelaar, H. J.; Nicholson, C., eds. The central nervous system
of vertebrates, vol. 2. Berlin: Springer Verlag; 1998:759 –937.
5. Pérez, S. E.; Yáñez, J.; Marı́n, O.; Anadón, R.; González, A.; Rodrı́guez-Moldes, I. Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the adult trout, and tract-tracing observations on the connections of the nuclei of the isthmus. J. Comp.
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LIST OF ABBREVIATIONS
CB cerebellum
DLI diffuse nucleus of the lateral hypothalamic lobes
Dm medial part of the dorsal telencephalic area
Dp
ENT
H
HL
LR
NI
OB
OC
OT
PG
PON
PSP
SG
SVN
T
TS
VC
posterior part of the dorsal telencephalic area
entopeduncular nucleus
habenula
lateral hypothalamic lobes
lateral hypothalamic recess
nucleus isthmi
olfactory bulb
optic chiasm
optic tectum
nucleus preglomerulosus
preoptic nucleus
superficial pretectal nucleus, parvocellular part
nucleus subglomerulosus
secondary visceral nucleus
telencephalon
torus semicircularis
valvula cerebelli