Year : 1982 | Volume
: 30 | Issue : 1 | Page : 23--28
Axoplasmic transport in optic nerve
PK khosla, S Patra, Prem Prakash, KS Ratnakar
Dr Rajendra Prasad Centre for Ophthalmic Sciences, New Delhi, India
P K khosla
Dr. Rajendra Prasad Centre for Ophthalmic Sciences, New Deihi
|How to cite this article:|
khosla P K, Patra S, Prakash P, Ratnakar K S. Axoplasmic transport in optic nerve.Indian J Ophthalmol 1982;30:23-28
|How to cite this URL:|
khosla P K, Patra S, Prakash P, Ratnakar K S. Axoplasmic transport in optic nerve. Indian J Ophthalmol [serial online] 1982 [cited 2021 Jun 15 ];30:23-28
Available from: https://www.ijo.in/text.asp?1982/30/1/23/27912
Raised intraocular pressure is one of the cardinal features of glaucoma, although the exact mechanism of pressure induced damage is not yet fully known .
Protein turnover profile is important for the neuron at any given time and indicates the balance between synthesis and catabolism of materials transported in axoplasm i.e. axoplasmic transport. Its alteration by raised intraocular pressure in the glaucomatous process may provide a system for biochemical evaluation of the damage.
Experimental studies demonstrating the effect of elevated intraocular pressure on axoplasmic transport have been performed in several laboratories. Levy  established a reduction in slow axoplasmic transport into retrobulbar optic nerve in eyes with elevated intraocular pressure. Anderson and Hendrickson , Minckler et a1 , Quigloy and Anderson sub and Quigloy and Anderson [ 5] and Minckler et a1  recorded pressure induced blockade of rapid axoplasmic transport in primate optic nerve.
This study was undertaken to note the extent to which axoplasmic blockade induced by raised intraocular pressure in rabbits is reversible.
MATERIAL AND METHODS
Studies were undertaken in albino rabbits. Axoplasmic transport from retina into the optic nerve was studied by intravitreal injection of 0.1 ml Tritiated Leucine (100 microcurie, LCT.II; L Leucine T (G) Sp. Activity 8200 mci/mM) obtained from Bhabha Atomic Research Centre, Trombay (India).
For histo autoradiographic study the eye ball and the optic nerve to chiasma was obtained in each animal after opening the cranial vault. Paraffin embedded 6 micron thick sections of optic nerve on glass slides were dipped in Kodak NTE2 emulsion and exposed for 3 weeks. They were then developed in Kodak D-19 developer for five minutes followed by hematoxylin and eosin staining. The density of grains was designated as judged visually as follows (n-) less than normal grain, (n--) far less than normal grain (n} ) ,- more than the normal grain, (n++) far more than the normal grain where (n) normal grain density was taken as a mean of the findings in pilot study for that period. Masked evaluation of the autoradiographs was done to eliminate observer bias. Four normal eyes were subjected to histopathology to rule out any pseudograins.
In the pilot study (Group--P) the animals were sacrificed 1 hour, 3 hours, 5 hours, 9 hours and 11 hours respectively after intravitreal injection of the tracer without raising the intraocular pressure, to find out the time when the marker was seen in the chiasmal end of the optic nerve in sufficient quantities as to be evaluated by us. All these slides were evaluated by different observers to come to a conclusion about the mean normal (n) at various sites at a particular period.
Intraocular pressure was raised one hour _after the intravitreal injection by infusion technique (Hamasaki and Fujino, 1967) . In group H-I intraocular pressure was raised to 25 mm Hg (12 eyes) while in group H-II it was raised to 40 mm Hg (12 eyes). The raised intraocular pressure was maintained for 2 hours, 4 hours and 6 hours respectively (4 eyes in each sub-group) and these were designated as subgroups a, b and c. The animals were sacrificed at the end of this period which means that eyes were processed at the end of 3 hours in subgroup a, 5 hours in subgroup b and 7 hours in subgroup c from start of experiment comparable to the time period of the pilot study.
In Group RHI and RHII (l2 eyes in each group) we studied the reversibility of axoplasmic blockade, by bringing intraocular pressure to normal. The pressure was maintained as in Group H at a higher level (25 mm Hg or 40 mm Hg) for 2,4 and 6 hours respectively and then brought to normal and these subgroups were labelled a, b and c respectively (subgroup of 4 eyes each). The intraocular pressure was brought to normal and the animals were sacrificed only 4-hours after the maintenance of normal intraocular pressure. Hence the animals were sacrificed at 7, 9 and I 1 hours in subgroups a, b and c respectively after the start of experiment comparable to time period of pilot study.
In the pilot study retinal accumulation of grains was evident as early as one hour and was uniform [Figure 1]. However, there were no demonstrable grains at the optic nerve head, suggesting a time lag between the incorporation of the tracer in the retina and its accumulation at the optic nerve head sufficient in quantity to be detected. Grain patterns at lamina [Figure 2], and chiasmal end [Figure 3] of optic nerve appeared at 3 hours and the photographs show the mean grain density in these regions. Quantification of grain pattern at various intervals of time was done to come to the conclusion of average normal (n) for that period.
Retinal accumulation remained unchanged in Group HI and HII compared to pilot studies suggesting an unabated incorporation of the aminoacids into the proteins synthesized by the retinal ganglion cells. No changes were noticed in group RHI and RHII either. It indicates that raised intraocular pressure does not affect the process of incorporation of aminoacids in the retinal layers.
Averaged higher grain density indicative of increased accumulation of radioactive material in the nerve fibre bundles as they pass through the lamina cribrosa [Figure 4][Figure 5] was seen in Group HI and HII indicating the site of blockade in the axoplasmic transport. This was evident even in Group HI (a) i.e. after elevation of the intraocular pressure to 25 mm Hg for 2 hours. The same pattern persisted when the raised intraocular pressure was maintained upto 4 or 6 hours (Group HI b or c). The degree of accumulation was found to be directly-roughly proportional to the level of intraocular pressure, that is more in group HII (n++) than in group HI (n+) and not its duration [Table 1][Table 2]. Radioactivity was detected at the chiasmal end although the axoplasmic transport was significantly blocked at lamina cribrosa indicating that the blockade was only partial. However, the average density of grains was less than normal (n-) in Group HI and far less than normal (n --) in Group HlI [Figure 6][Figure 7] confirming that the blockade of axoplasmic flow was proportional to the level of intraocular pressure.
With reversal of intraocular pressure to normal and keeping it at that level for 4 hours (Group RHI and RHII), marked change was observed in the distribution of radioactivity in the optic nerve fibres. Normal density of grains (average values) was seen throughout the length of the optic nerve upto the chiasmal end in both the groups. It indicates that reversal to normal intraocular pressure for four hours even after elevation of intraocular pressure to 40 mm Hg for 6 hours (group RHIIc) restores the axoplasmic flow completely.
Axoplasmic transport of cellular constituents is the universal property of all nerve cells and it is known that the nerve cell body is capable of synthesizing the proteins. The study of axoplasmic transport in this work includes the events of incorporation of the tracer aminoacids (tritiated leucine) into the proteins synthesized by the retinal ganglion cells and their subsequent transport along its axons in the optic nerve. Many heterogenous compounds travel in the axon at distinctly different velocities and each of these velocities appear to contain a different group of cellular constituents. The protein transport has a "fast" component which was evaluated in this study.
The pilot study (Group P) was undertaken to see the time the tracer took to reach the chiasmal end of the optic nerve and the density of grains at various sites under normal intraocular pressure at various periods of time. This served as a control to assess the normal density of grains at various levels for comparison with experimental groups.
In Group P no radioactivity could be detected at the optic nerve head one hour after injection of the tracer while at 3 hours its presence in the chiasmal end suggested a time lag between injection of the tracer, its incorporation into the protein by the ganglion cells, and transport to that area in sufficient quantity to be detected by autoradiographic method. The presence of transported protein at chiasmal end at 3rd hour would suggest a velocity not less than 180 mm/day and would indicate that in this study we will be evaluating the fast component of axoplasmic transport.
To simulate clinical state of glaucoma, elevation of intraocular pressure was achieved -25 mm Hg in Group HI and 40 mm Hg in Group Hil. Accumulation of grains at laminar region in Group HI, when the intraocular pressure was raised for 2 hours, indicated blockade of axoplasmic flow at the lamina cribrosa region which occurred rapidly after the rise of intraocular pressure. Qualitatively, the density of grains in this region was more than the normal in group HI (n+) while far more than the normal in group HII (n+ +) suggesting a direct relation of rise of intraocular pressure and blockade of axoplasmic flow. The chiasmal end showed a reduced density of grains as compared to controls, less (n-) in group HI and far less than normal (n--) in Group HII, suggesting blockade to axoplasmic flow. The blockade was partial. It can either be due to a total blockade of some axons, leaving other axons uninterrupted or due to a partial effect on all axons.
Normal flow of the axonal transport was resumed quickly after normalisation of intraocular pressure for four hours. The labelled material which had accumulated at the optic nerve after the rise of intraocular pressure suggesting a blockade of the flow disappeared completely with reversal of intraocular pressure to normal. It was also evident by uniform distribution of grains of normal density throughout the optic nerve. This reversibility was seen even after the, maximum pressure (40 mm Hg) and duration (6 hours) in this study (Group RHII c) suggesting absence of permanent damage during this period of raised intraocular pressure.
The earliest change noted is at the laminar region. This region is a transitional zone where tissue pressure changes from intraocular to intracranial pressure. Transitional zone is always a favourable site for mechanical pressure to act. Permanent damage may occur if the intraocular pressure is high: and is kept at that level for a long time but is not the case in this study. Partial blockade can occur with mechanical pressure and the pressure induced blockade will be reversible with normalisation of intraocular pressure if the tension and the duration for which it is kept has not damaged the fibres completely. It may be concluded that pressure and duration in this study produced only reversible changes which may be partly of mechanical nature.
The exact pathogenic mechanism responsible for such pressure induced partial blockade of axoplasmic flow remains unclear. Whether this mechanism is vascular cannot be conclusively proved or denied. Reversal of blockade of axoplasmic flow after normalisation of intraocular pressure may suggest that mechanism involved is vascular anoxia which is only partial. If it was complete anoxia the change produced would have been permanent which is not the case. Hence it rules out total blockade of flow through some axons so partial ischaemia due to localised impairment of capillary circulation at this region may be a possibility.
The observations in this study indicate that the experimental model used in this study can help us to understand the changes of blockade occurring in axoplasmic transport with raised intraocular pressure and its reversal after the intraocular tension has been brought to normal.
Effect of rise of intraocular pressure on fast axoplasmic flow was studied by histo autoradiographic methods in rabbits.
Tritiated leucine was found to be a suitable tracer for labeling fast axoplasmic flow studied by histoautoradiography.
There was partial blockade of axoplasmic flow at laminar region with varying degree of rise of intraocular pressure maintained for varying periods of time. The amount of blockade was directly proportional to the level of intraocular pressure and not its duration.
Blockade of axoplasmic flow was completely reversible after intraocular pressure was reversed to normal and kept at that level for 4 hours.
Partial vascular anoxia has been suggested as a mechanism for the blockade of axoplasmic flow which was reversible under the experimental conditions.
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|2||Anderson, D.R. and Hendrickson, A., 1974, Invest. Ophthalmol. 13 : 771.|
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|4||Quigley, H.A. and Anderson, D.R., 1976, Invest. Ophthalmol. 15 : 606.|
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|6||Minckler, D.S., Bunt, A.H. and Johanson, G.W., 1977, Invest. Ophthalmol. 15 : 426.|
|7||Hamasaki, D.I. and Fujino, T., 1967, Arch, Ophthalmol. 78: 369.|