Summary. Horseradish peroxidase (HRP) was injected or iontophoretically ejected in various thalamic nuclei in 63 adult cats. In 11 other animals. HRP was ...
Exp Brain Res 40, 405-418 (1980)
9
1980
The Vestibulothalamic Projections in the Cat Studied by Retrograde Axonal Transport of Horseradish Peroxidase N. Kotchabhakdi 2, E. Rinvik 1, F. Walberg 1, and K. Yingchareon2 a Anatomical Institute, University of Oslo, Karl Johansgt. 47, Oslo-1, Norway z Laboratory of Neurobiology, Mahidol University, Bangkok, Thailand
Summary. Horseradish peroxidase (HRP) was injected or iontophoretically ejected in various thalamic nuclei in 63 adult cats. In 11 other animals HRP was deposited outside the thalamic territory. The number and distribution of labelled cells within the vestibular nuclear complex (VC) were mapped in each case. To a varying degree all subgroups of VC appear to contribute to the vestibulothalamic projections. Such fibres are distributed to several thalamic areas. From the present investigation it appears that generally speaking, there exist three distinct vestibulothalamic pathways with regard to origin as well as to site of termination of the fibres. One projection appears to originate mainly in caudal parts of the medial (M) and descending (D) vestibular nuclei and in cell group z. This pathway terminates chiefly in the contralateral medial part of the posterior nucleus of the thalamus (POm) including the magnocellular part of the medial geniculate body (Mgmc), the ventrobasal complex (VB) and the area of the ventral lateral nucleus (VL) bordering on VB. A second projection originates mainly in the superior vestibular nucleus (S) and in cell group y and terminates mainly in the contralateral nucleus centralis lateralis (CL) and the adjoining nucleus paracentralis (Pc). A third, more modest, pathway originates chiefly in the middle M and D, with a minor contribution from S and cell group y, and terminates in the contralateral ventral nucleus of the lateral geniculate body (GLV). There is some degree of overlap between the origin of these three vestibulothalamic pathways. Key words: Vestibular complex - Thalamus - Horseradish peroxidase
Offprint requests to: Prof. E. Rinvik (address see above)
Cortically evoked responses following stimulation of the vestibular nerve or organ have been repeatedly reported in various species (for a review and references, see Fredrichson et al. 1974). By analogy with other major sensory pathways it would be expected that impulses which ascend from the vestibular nuclei were relayed in the thalamus. However, the existence of a vestibulothalamic projection has been a matter of considerable controversy, being in the earlier literature described by some authors (Hassler 1948, 1956, 1972; Carpenter and Hanna 1962; Carpenter and Strominger 1965) but denied by other investigators (Brodal and Pompeiano 1958; Brodal et al. 1962; Tarlov 1969). With the introduction during the last years of axoplasmic transport techniques the existence of a vestibulothalamic projection appears to have become well established (Raymond et al. 1976; Cond6 and Cond6 1978; Wagner et al. 1978; Lang et al. 1978; Magnin and Kennedy 1979). However, these reports differ considerably with regard to details concerning the major sites of termination and also as concerns the precise localization of the cells of origin of the vestibulothalamic fibres. These discrepancies are readily explained by the fact that in the anterograde and retrograde tracing studies the injections covered only certain parts of the vestibular nuclei and thalamic complex, respectively (Raymond et al. 1974, 1976; Cond6 and Cond6 1978; Wagner et al. 1978; Lang et al. 1979; Magnin and Kennedy 1979). The present study was therefore undertaken with the primary aim of unravelling in greater detail the localization of the cells of origin of the vestibulothalamic fibres by using the technique of retrograde transport of horseradish peroxidase (HRP). During the course of this investigation it became apparent that fibres ascending from the vestibular nuclear complex are distributed to more thalamic areas than is generally assumed. 0014-4819/80/0040/0405/$ 2.80
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N. Kotchabhakdi et al.: Vestibulothalamic Projections
Table 1, summarizing the main pattern in the distribution of labelled cells in various nuclei of the vestibular complex following injections of HRP in different thalamic areas. For details, particularly concerning the differential distribution of labelled cells within M and D following HRP injections in VL, VP and Pc, the reader should consult the text M
D
Contra Ipsi
S
Contra Ipsi
CL Pc/CL
*
(*)
(*)
Pc/CeM
(*)
L
z
Contra Ipsi
Contra Ipsi
**
(*)
Contra Ipsi
sv Contra Ipsi
f Contra Ipsi
(*)
Y Contra Ipsi (*)
*
(*)
*
CM/Pf
(*)
(*)
VL/LMI
***
vc
.
VP
.
.
GLV
*
(*)
VM
(*)
POm
.
(*) .
.
(*)
***
(*)
. .
(*)
(*)
(*)
(*)
.
*
.
(*)
.
*
* (*)
(*) (*)
.
(*)
(*)
(*)
(*) .
.
.
.
(*)
(*) labelled cells occasionally present. * not more than 20 cells in the nucleus. ** 20-50 labelled cells in the nucleus. *** more than 50 labelled cells in the nucleus. Contra: Contralateral to the thalamic injection. Ipsi: Ipsilateral to the thalamic injection
Material and Methods
Altogether 74 adult cats of both sexes were used. In 63 animals HRP was either pressure injected or iontophoretically ejected in the various thalamic nuclei. In the remaining 11 cats the HRP was deposited in telencephalic or diencephalic regions outside the thalamus. 0.2 gl of a 25% suspension in physiological saline of HRP (Serva Feinbiochemica, Heidelberg) with dimethylsulfoxide (DMSO) added to make a final solution of 2% DMSO (Keefer 1978) was used for the injections. In cases of iontophoretic ejections a 4% suspension of HRP (Serva) in 2 M KC1 was used. The ejections were achieved by using a constant current of 5 ~A for a time ranging between 5-15 min. The animals were deeply anaesthetized 2-3 days postoperatively and perfused with a mixture of 1% paraformaldehyde and 1.25% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4). At the end of this perfusion-fixation several of the animals were in addition perfused with 1 1 of a 10% solution of sucrose in phosphate buffer. The brains of these latter animals were immediately dissected free, divided in appropriate blocks and cut in serial frontal sections at 50 lxm on the freezing microtome. In the former animals the dissected brain was postfixed for an additional 12-24 h before trimmed blocks were transferred to a 30% solution of sucrose in phosphate buffer for another 24 h, and subsequently cut in serial sections of 50 ~tm. Serial transverse sections of the diencephalon and the brain stem were activated for the detection of HRP according to the method described by Graham and Karnovsky (1966) or by Mesulam (1976, 1978). In some cases alternative sections were treated according to all three methods, The sections incubated with diaminobenzidine (DAB) (Graham and Karnovsky 1966) were weakly stained with
cresyl violet, whereas those incubated with benzidinedihydrochloride (BDHC) or tetramethylbenzidine (TMB) (Mesulam 1976, 1978) were stained with neutral red. The localization and extent of the injected HRP were mapped by the aid of a projector and checked under the microscope. The various groups of thalamic nuclei were identified according to the descriptions of Jasper and Ajmone-Marsan (1954), Rinvik (1968, 1972) and Robertson and Rinvik (1973). The presence of HRP labelled cells in the vestibular nuclear complex was studied by means of bright- and dark-field as well as interference contrast (Normarski) microscopy and entered in a standard diagram adapted from Brodal and Pompeiano (1957).
Results
F o l l o w i n g i n j e c t i o n s o f H R P in c e r t a i n a r e a s o f t h e t h a l a m u s l a b e l l e d cells a r e f o u n d in v a r i o u s s u b d i v i sions o f t h e v e s t i b u l a r n u c l e a r c o m p l e x ( V C ) , i n c l u d ing t h e s u p e r i o r (S), t h e m e d i a l ( M ) , t h e l a t e r a l (L) a n d t h e d e s c e n d i n g ( D ) v e s t i b u l a r n u c l e i , in t h e cell g r o u p s y, z a n d f, a n d in t h e s u p r a v e s t i b u l a r n u c l e u s (sv). I n all cases l a b e l l e d cells in t h e V C w e r e f o u n d m a i n l y c o n t r a l a t e r a l to t h e i n j e c t e d H R P , b u t in some instances an ipsilateral labelling of a minor n u m b e r o f cells w a s also e v i d e n t ( T a b l e 1).
N. Kotchabhakdi et al.: Vestibulothalamic Projections
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