Brain Research Bulletin 66 (2005) 361–364
Experimental study of the connections of the preglomerular nuclei and
corpus mamillare in the rainbow trout, Oncorhynchus mykiss
M. Folgueira a , R. Anadón b , J. Yáñez a,∗
b
a Department of Cellular and Molecular Biology, University of A Coruña, A Coruña 15007, Spain
Department of Cell Biology and Ecology, University of Santiago de Compostela, 15706 Santiago de Compostela, Spain
Available online 17 March 2005
Abstract
The preglomerular complex of trout consists of the anterior (aPGN) and medial (mPGN) preglomerular nuclei and the corpus mamillare
(CM). In order to improve knowledge on this complex, we applied a lipophilic neuronal tracer (DiI) to the three nuclei. These nuclei received
afferents from the medial part of the dorsal telencephalic area (Dm), the ventral part of the ventral telencephalic area (Vv), the preoptic
nucleus, the periventricular layer of the rostral optic tectum and the central posterior thalamic nucleus. The aPGN also received numerous
toral projections and, sent efferents to the anterior tuberal nucleus. In addition, both the aPGN and the mPGN nuclei gave rise to efferents
to the dorsal region of the dorsal telencephalic area (Dd), whereas the medial preglomerular nucleus and the CM sent fibers to the torus
lateralis and the diffuse nucleus, as confirmed by reciprocal labeling. A small mPGN/CM subgroup projected to the optic tectum. These
results suggest close functional inter-relationship between the trout preglomerular complex and two telencephalic regions (Dm and Vv). In
addition, all nuclei of the complex receive preoptic, tectal and dorsal thalamic afferents, whereas the aPGN and mPGN are related with
acoustic-lateral ascending pathways, and the mPGN and CM with the central region of the dorsal telencephalic area and visceral/gustatory
pathways.
© 2005 Elsevier Inc. All rights reserved.
Keywords: Posterior tubercle; Telencephalon; Projections; Teleost; Optic tectum; Hypothalamic lobes
1. Introduction
The posterior tubercle of teleosts is a well-developed structure located at the transition between the diencephalon and
mesencephalon, considered as a relay center of sensory information to the telencephalon from the acousticolateral and
gustatory systems (see [6]). The posterior tubercle consists of
Abbreviations: aPGN, anterior preglomerular nucleus; CG, central gray;
CM, corpus mamillare; Dc, central region of the dorsal telencephalon; Dd,
dorsal region of the dorsal telencephalon; Dm, medial region of the dorsal
telencephalon; fr, fasciculus retroflexus; HL, inferior hypothalamic lobe;
mPGN, medial preglomerular nucleus; mlf, medial longitudinal fascicle;
NAT, anterior tuberal nucleus; NI, nucleus isthmi; OT, optic tectum; PL,
posterior tubercular lobe; PO, preoptic region; R, superior raphe nucleus;
SR, superior reticular nucleus; TL, torus lateralis; TS, torus semicircularis;
Vd, dorsal region of the ventral telencephalon; Vv, ventral region of the
ventral telencephalon
∗ Corresponding author. Tel.: +34 981 167000; fax: +34 981 167065.
E-mail address: juliany@udc.es (J. Yáñez).
0361-9230/$ – see front matter © 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.brainresbull.2005.03.001
both non-migrated and migrated nuclei with no clear correspondence to any nuclei of other vertebrates [1,6]. At present,
the boundaries, nomenclature and subdivisions of the migrated posterior tubercle nuclei in different species are not
firmly established, showing substantial variations throughout the literature [6]. In the rainbow trout (Protacanthopterygii), the preglomerular complex is represented by a column
of migrated cells grouped in the anterior preglomerular nucleus, the medial preglomerular nucleus (=Holmgren’s nucleus pseudoglomerulosus), and the corpus mamillare. In percomorphs, the corpus mamillare appears clearly separated
from the caudal preglomerular nucleus [4], but in salmonids
such a distinction is not clear. Although most hodological
studies have considered the rostrolateral regions of the preglomerular complex as part of an ascending acoustic-lateral
telencephalic relay center [7,12–14], data about the connections of the corpus mamillare are limited to cypriniformes
[5,15] and one species of percomorphs [10,16]. The aim of
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M. Folgueira et al. / Brain Research Bulletin 66 (2005) 361–364
this hodological study is to expand our knowledge on the
preglomerular complex of salmonids.
2. Material and methods
Sixty-five juveniles of Oncorhynchus mykiss obtained
from a local fish farm were deeply anesthetized with MS-222
and transcardially perfused with cold 4% paraformaldehyde
in phosphate buffer 0.1 M, pH 7.4 (PB). Brains were then dissected out of the skull and maintained in the same fixative at
4 ◦ C until use. Two procedures of application of DiI (Molecular Probes, Eugene, OR, USA) were used as was previously
described [3]. In brief, in order to label superficial nuclei (externally and ventricular accessible brain areas), a small crystal
of DiI was placed on the tip of an electrolytically sharpened
insect pin and was then directly applied to the brain under
a stereomicroscope. The brain areas accessed by this procedure in this report were the anterior preglomerular nucleus
(aPGN). For labeling less accessible nuclei and areas, brains
were previously embedded in 3% agarose and sectioned on a
vibratome along the appropriate section plane to the required
level. The tracer was then applied as above. Using this procedure, DiI was applied to the medial preglomerular nucleus
(mPGN) and the corpus mamillare (CM). In order to confirm
connections, additional DiI applications to several regions of
the brain were made following the same procedures: different
regions of the dorsal and ventral telencephalic areas (Dm, Dd,
Vv), preoptic region, optic tectum, torus semicircularis, torus
lateralis, and inferior and lateral hypothalamic lobes. In both
procedures, the DiI application area was sealed with melted
agarose and brains were left in frequently renewed fixative for
Fig. 1. Photomicrographs of selected transverse sections through the trout brain showing labeled structures after application of DiI to the aPGN (A–E), mPGN
(F and G), CM (H–J and M), torus lateralis (K), and optic tectum (L). (A) Section through the telencephalon showing retrogradely labeled cells (arrowheads)
in Dm and anterogradely labeled fibers (arrow) reaching Dd. (B) Section through the preoptic region showing retrogradely labeled cells (arrowheads). Note
labeled fibers coursing in the lateral forebrain bundle (white star). (C) Section through the optic tectum showing retrogradely labeled cells (arrowheads) in the
stratum griseum periventriculare. (D) Detail of a retrogradely labeled cell of the optic tectum projecting to the preglomerular complex. (E) Section through
the mesencephalon showing retrogradely labeled perikarya (arrowheads) in the torus semicircularis. (F) Section through the isthmus showing retrogradely
labeled cells (arrowheads) in the superior reticular nucleus close to the lateral lemniscus (star). (G) Section through the hypothalamus showing a number of
retrogradely labeled cells (arrowheads) in the corpus mamillare and fibers (arrows) in the hypothalamic lobe. (H) Section through the telencephalon showing
retrogradely labeled cells (arrowheads) in the subpallium and labeled fibers in Dm (arrow). (I) Section through the telencephalon showing retrogradely labeled
cells in Dm (arrowhead) and in Dc (arrows). (J) Retrogradely labeled cells (arrowheads) in the central posterior nucleus of the dorsal thalamus. (K) Section
showing retrogradely labeled cells (arrowheads) in the corpus mamillare. (L) A small group of tectal projecting neurons closely associated to the ventral
mPGN-CM (arrowhead). (M) Section showing bilaterally labeled cells (arrowheads) and fibers (arrow) in the rhombencephalic central gray. For abbreviations,
see Abbreviations Section. Scale bars in (A, C and E): 125 ␮m; (B, F, G and J): 100 ␮m; (D): 80 ␮m; (H and M): 200 ␮m; (I and K): 180 ␮m; (L): 150 ␮m.
M. Folgueira et al. / Brain Research Bulletin 66 (2005) 361–364
2–8 weeks in darkness at 37 ◦ C. After this time, transverse or
sagittal sections (50 ␮m thick) were cut on a vibratome and
mounted on gelatin-coated slides with PB. Sections were examined and photographed with a Nikon E-1000 fluorescence
photomicroscope equipped with a rhodamine filter set using black-and-white negative film, which was then digitally
scanned and printed as positives.
3. Results
DiI application to each nucleus of the preglomerular complex always produced labeling (both fibers and cell bodies)
in the other nuclei of the complex, indicating close interrelationship between its different parts. DiI application to the
aPGN led to ipsilateral labeling of neurons in the medial region of the dorsal telencephalic area (Dm), the ventral telencephalic area (V), the preoptic region (PO), the central posterior thalamic nucleus, the stratum griseum periventriculare
of the optic tectum, and the torus semicircularis (Fig. 1A–E).
The main aPGN efferent projections confirmed by reciprocal experiments reach the ipsilateral dorsal part of the dorsal
telencephalic area (Dd) (Fig. 1A), Dm, and Vv. Efferent projections to the anterior tuberal nucleus were observed, though
not confirmed experimentally.
The afferent connections of the mPGN were similar to
those of the aPGN, except in that only a few afferent cells
from the torus semicircularis were found, and that labeled
cells were observed in the central region of the dorsal telencephalic area (Dc) and in the superior reticular nucleus
close to the lateral lemniscus (Fig. 1F). The mPGN efferents
confirmed by reciprocal experiments coursed to the torus lateralis and the lateral part of the hypothalamic lobe (Fig. 1G).
In some experiments, anterogradely labeled fibers in the nucleus subglomerulosus and retrogradely labeled neurons in
the secondary gustatory nucleus were also observed. The connections between these later structures and the mPGN could
not be confirmed by the reciprocal experiments (present results), labeling being probably produced via tertiary gustatory tracts running to the nucleus subglomerulosus close to
the mPGN [2].
DiI application to the CM led to retrograde labeling of
neurons in the telencephalon (Dm, Dc, Vv, PO) (Fig. 1H–I),
central posterior thalamic nucleus (Fig. 1J), and the optic
tectum but unlike the other preglomerular nuclei, no afferents from the torus semicircularis were observed. The efferent connections of the corpus mamillare confirmed by
reciprocal labeling were those to the Dm, Vv, torus lateralis, and inferior hypothalamic lobe (Fig. 1H and K).
DiI application to the optic tectum also led to labeling of
a small group of neurons closely associated to the ventral mPGN-CM (Fig. 1L). In addition, reciprocal connections were observed between the CM and the isthmic central gray (Fig. 1M), though no reciprocal experiment was
performed.
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4. Discussion
The connections of the trout preglomerular/CM complex
are summarized in Fig. 2A and B. The three parts of the
complex studied here (aPGN, mPGN, CM) are closely interrelated and show a rather similar pattern of telencephalic and
tectal connections. In this regard, they can be considered as
differentiated parts of the same system. The heavy reciprocal
connections between this complex and the Dm and Vv reported here suggest that these interconnections are essential
in their functions. In addition to the telencephalic connections, the preglomerular complex of the trout receives afferents from visual (optic tectum) and mechanosensory (torus
semicircularis) centers. The trout preglomerular/CM complex does not receive projections from the primary or secondary gustatory nuclei, or from any pretectal nuclei ([3],
present results). The hypothalamic tertiary gustatory nucleus
of trout (preglomerular tertiary gustatory nucleus of Yoshi-
Fig. 2. Summary diagrams of the trout brain in a lateral view showing the
afferent (A) and efferent (B) connections to the preglomerular complex.
Broken arrows, projections without confirmatory reciprocal experiments.
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M. Folgueira et al. / Brain Research Bulletin 66 (2005) 361–364
moto et al. [16]) appears to be part of a lemniscal system
projecting to the precommissural Dm [3]. Since it is neither
inter-related with the nuclei of the precommissural complex,
nor receives projections from the dorsal telencephalon, the
torus semicircularis or the optic tectum should not be considered a part of the preglomerular/CM complex.
There are striking differences between teleosts as regard
the connections of the corpus mamillare. Unlike cyprinids
[5,8,15], the corpus mamillare of trout does not receive afferents from the pretectal superficial nucleus pars magnocellularis or the secondary gustatory nucleus ([2,3], present
results). The connections of the CM were also different in
the percomorph Oreochromis [10]. In this species, the CM
receives a projection from the diffuse nucleus of the torus
lateralis and inferior hypothalamic lobe, and projects to the
ventromedial thalamus, optic tectum, and telencephalon (PO,
dDm). These remarkable differences between trout and the
other teleost species studied to date suggest that either these
CMs are not homologous or that they have highly different evolutionary histories from a generalized ancestor. By
its connections, the trout CM is clearly a caudal part of the
preglomerular complex, and it could correspond to the caudal commissural preglomerular nucleus of advanced teleosts,
which in these species lies just dorsal to the CM [4,10]. The
small group of neurons just ventral to the CM and projecting to the optic tectum observed in trout (present results)
might correspond to the neurons of the pars magnocellularis
dorsalis of the CM of Oreochromis giving rise to the tectal
projections [10]. If this hypothesis was correct, most parts of
the perciform CM would be absent in trout. Further hodological research in primitive groups of teleosts appears necessary
to understand evolutionary trends of this puzzling system.
Although the teleost preglomerular/CM complex and
the mammillary body of mammals are thought to be nonhomologous [1,11], it is interesting to note that both are
related with telencephalic centers involved in learning and
memory. The trout complex has bi-directional relationships
with Dm, which in the goldfish Dm appears involved in emotional learning [9], but not with the lateral part of the dorsal telencephalic area, which in goldfish is involved in spatial memory and has been proposed as the mammalian medial cortex homologue [9]. It could be hypothesized that the
preglomerular/CM complex and Dm of teleosts would form
part of a learning/memory circuitry analogous to the mammalian limbic system. Multisensory (visual, auditory/lateral
line, gustatory, . . .) inputs to the preglomerular/CM complex might be qualified in different teleosts by this learning/memory circuitry in an emotional context. In summary,
the preglomerular/corpus mamillare system of the rainbow
trout might be a multi-integrative center involved in higher
brain functions.
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