TRANSNEURONAL TRANSPORT ~f THE VISUAL SYSTEM OF THE TOAD, BUFO hL4RINU$
LIDIA MAYNER
School of Biological Sciences, Flinders University of South Australia, Bedford Park, 5042 (South Australia)
(Received November 30th, 1979) (Accepted December 7th, 1979)
SUMMARY
Secondary optic projections were studied in the toad, Bufo marinu8 using the method of transneuronal transpox~ of triated proline and fucose. Following a sltwival of 14 days after eye injection auto~diographic label was trans- ported transneuronally to the contr~dateral nucleus isthmi, the deeper layers of the contralateral tectum, the pontine region, the ipsilateral tectum and to the contralateral thalamus. Toa&~ which survived 21 days or longer ex- hibited transneuronal transport of hJotope to the contralateral telencephalon. The labelled patch in the telenceph~don was located in the striatum, in the region of the lateral forebrain bundle.
The transneuronal transport of radioactive amino acids across synapses was first demonstrated in the mammalian visual system [ 2]. More recently transneuronal transport in the vis~,ud system has been described in other vertebrates: fish [ 14 ], frogs [ 10 ] ~md birds [ 15 ]. The present study reports on the transneuronal transport of :radioactive materials in the visual system of the cane toad, Bufo marinus, arid provides some new data on possible projections from the visual diencephalic centres to the optic tectum and the telencephalon.
Twelve cane toads received an eye injection of L-[5-SH]proline (frorc Amersham, spec. act. 24--29 Ci/mmol) or a combination of [SH]proline and D.[1-SH]fucose (from Amemham, spec. act. 1.3 Ci/mmol), with the radio- activity of the dose 30--150/~Ci. The survival times we~'e one day (two toads), 5 days (one), 14 days (one), 15 day.~ (two), 21 days (three), 28 days (two) and 49 days (one). The brains were fixed in Bouins fluid, embedded in paraffin and cut into 10.~m secCion.~. All sections were processed for autora- diography [ 1]. The slides were dipped in Ilford K~ emulsion, stored in the dark at 4°C for 4--9 weeks and dc~veloped in Kodak D-19 developer for 4 rain at 21°C. Sections were lightly stained with cresyl violet.
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In toads surviving 1--5 days following eye injection, the p]imary optic centres were labelled. Label was found on the contralateral side in the optic tectum and tegmentum and: bilaterally in the neuropfl of BeUonci, lateral geniculate body, posterior thalamic neuropil, pretecta! nucleus and uncinate field. Most of these primary optic terminations have been described previously in Bufo [17] and are similar to the primary optic centres in other anuran~ [9,13,18,19]. A recent study which used the cobalt filling technique ha~ reported retinal projections to several other regions in the frog, Rana esculenta [ 11 ]. These include the hypothalamus, ipsilateml rectum, ipsi- lateral te~jnentum and a number of areas in the contralateral diencephalon. ~ne present study did not provide any support for these observations when short term survivals were used following injections of one eye with [3H]- proline.
Fig. 1. Photomicrograph showing autoradiographic label in the contralateral optic tectum (TO) and nucleus isthmi (NI) in toad 133 which survived 49 days following an eye injec- tion of 60 ~Ci [3H]proline aud ~0 uCi [ 3H]fucose. Frontal section. Scale bar 0.5 ram.
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Toads which survived 14 days or longer showed transneuronal t ranspor t of isotope to a number of areas which are thought to receive projections from eithe:." the optic tec tum or the primary visual centres in the diencephalon. Heavy label was foand in the nucleus isthmi ipsilateral to the labelled tec tum (Fig. 1 ) [cf. 10,29}, in the deeper layers of the rec tum and in the pont ine region. In addition there was label in the t ec tum ipsilateral to the injected eye (Fig. 2) and izi the contralateral thalamus medial to the nucleus of BeUonci and lateral geniculate body. The nucleus isthmi, pontine region and thalamus receive a 9rojection from the tec tum [ 8,16]. The indirect retinal input to the ipsilateral tec tum probably arises in the thalamus [4 ,12,21] . There are se~reral re]g;orts of a direct tecto-tectal connect ion in anurans [ 4 ,12] , but the pa thway se~,.ms to involve only a few fibres and would therefore not t ransport significant quantit ies of isotope. Finally it seems unlikely that the label seen in the ipsilateral tecdum can be construed as evidence for a direct ipsilateral ret inotectal pa thway since long survival times were required to observe this label in Bufo.
Toads which survived 21 days or longer had a patch of label in the telen- cephalon (Fig. 3). The labelled region was within the division of the telen- cephalon known as the str iatum [6] and in the region of the lateral forebrain bundle. The visual input to thL area probably arises from the region of the nucleus of Bellonci and lateral geniculate body [ 5] and from the postero-
r ~
J
]
Fig. 2. Photomicrograph showing autoradiographic label ill the ipsilateral optic rectum in toad 12 which survived 28 days following an eye injection of 100 uCi [ ~ H ]proline. Scale bar 0.5 ram. Frontal section. The label is distributed over much of the ipsilateral rectum in layer 8. In the caudal part of the rectum the label is located mainly in the lateral half.
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•
Striatum
ON
1 mm
Fig. 3. Line drawing and photomicrograph showing the location of autoradiographic label in the contralateral te!encephalon in toad 12 which survived 28 days following an eye injection of 100/zCi [~H]proline. In the line drawing the ;abelled optic nerve (ON) and labelled pav~ of the striatum are indicated by stippling. Frontal section.
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lateral nucleus of the thalamus [ 3,7] which receives input from the tectum [18]. Microelectrode recordings have shown that single neurons of the stria- turn and lateral forebrain bundle are visual [ 3,4].
Two toads which survived 21 days following eye injection showed light label in the ipsilateral tegmentum. The primary visual centres in the dien- cephalon project to the ipsilateral tegmentum [5] and there is some evidence for a direct, but sparse, retinal projection to the ipsilateral tegmentum [11,12].
ACKNOWLEDGEMENTS
I thank Mrs. L. Pearson and Dr. K. Sanderson for assistance with this study and Miss Sue Hughes for typing the manuscript. The research was supported by a grant from the Flinders University Research Budget and from the Australian Research Grants Committee to Dr. Sanderson.
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