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Influence of developmental auditory deprivation on neuronal ultrastructure in the mouse anteroventral cochlear nucleus
304
Developmental Brain Research. 42 ( !9~,S)34)4-3~i8
BRD60276
Influence of developmental auditory deprivation on neuronal ultrastructure in the mouse anteroventral cochlear nucleus D e n n i s R. T r u n e 1'2 and Chris R. M o r g a n 1 Departments of 1CellBiology and Anatomy and 20tolaryngology, The Oregon Health Sciences University, Portland, OR 97201 (U.S.A.)
(Accepted 19 April 1988) Key words: Auditory deprivation; Cochlear nucleus; Neuronal organelle; Morphometry; Development:
Metabolism; Synaptic competition; Genetic expression
Developmental auditory deprivation caused mouse anteroventral cochlear nucleus neurons to have significantly fewer auditory nerve terminals and more non-auditory nerve terminals. This suggested that stimulation regulated the developmental arborization of auditory nerve terminals and competition for synaptic space. Intracellularly, mitochondria were smaller and darker in the deprived neurons and appeared less active metabolically. Interference with these neuronal processes may underlie the impaired development seen in auditory deprivation. Numerous studies have demonstrated that developmental auditory deprivation leads to smaller neurons in cochlear nucleus (CN), the termination site of auditory nerve fibers 1-3'11'12. Although this is evidence that some developmental process within these neurons is affected by deprivation, the cellular events involved are not known. Recent auditory deprivation studies have demonstrated that it is the cytoplasmic portion of the CN neuron that is smaller than normal 11. Furthermore, cochlear ablation in the chick leads to rapid changes in CN protein synthesis 9 and succinate dehydrogenase levels 4. These observations suggest that stimulation regulated cytoplasmic processes may be impaired by deprivation to interfere with normal development. It is also possible these impaired cytoplasmic processes may have corresponding organelle changes, although no studies to date have examined CN neuronal tiltrastructure following developmental auditory deprivation. Therefore, the present study was conducted to identify deprivation-induced structural changes to better understand the regulation of cellular mechanisms by auditory stimulation.
The anteroventral CN (AVCN) was examined in normal CBA/J mice and those that had been unilaterally sound-deprived throughout the period of hearing development H. Mice were anesthetized by hypothermia on day 3 and the blastemas of their right external auditory meati were removed. This prevented normal meatal opening on day 8 and created a complete meatal occlusion throughout hearing development (days 8-45). Six unilaterally deprived animals and 6 age-matched controls were sacrificed on postnatal day 45 and their right AVCN prepared for electron microscopy. Neuron types within the small spherical cell region 13 were differentiated according to previously established ultrastructurat criteria l°. All large neurons with visible nuclei were photographed and printed at 10,000x magnification for qualitative evaluations. Only one thin section per grid was photographed to assure that the same neuron was not photographed twice. Also, grids were separated by 20/~m to prevent the same neurons being examined on consecutive grids. Quantitative analyses were also performed to determine if ultrastructural features in the deprived and
Correspondence: D.R. Trune, Department of Cell Biology and Anatomy, The Oregon Health Sciences University, Portland, OR
97201, U.S.A. 0165-3806/88/$03.50 © 1988 Elsevier Science Publishers B.V. (BiomedicalDivision)
305 normal AVCN neurons were similar. All photographed convoluted nucleus-polysynaptic neurons from the unilaterally deprived (3 mice, 9 neurons) and control (4 mice, 9 neurons) A V C N were printed at 15,000x magnification and measured on a digitizer tablet interfaced with the Bioquant II Image Analysis System (R and M Biometrics). This was done blind so the person measuring the micrographs did not know their sources. Micrograph measurements were made of: (1) the area and density of mitochondria; (2) the length of rough endoplasmic reticulum; (3) the area and density of Golgi complexes; and (4) the area, synapse apposition length, and density of auditory and non-auditory nerve terminals. The means of each measurement were calculated for each AVCN and statistically compared (t-tests) to determine if the deprivation treatment had any effect on the morphological features measured. Mean differences with probabilities less than 5% were judged to be significant. Qualitative examination of the deprived AVCN showed all major neuronal types were affected by the developmental deprivation (Fig. 1). Cytoplasm was reduced and often limited to a narrow ring around the nucleus. The most striking organelle changes were seen in the mitochondria, which appeared smaller and less robust in the deprived neurons. Intramitochondrial appearance was always quite dense and dark staining was Observed, making internal features less discernible. Overall they appeared less active metabolically. Rough endoplasmic reticulum (RER) segments normally occurred as long, sometimes branching, channels with numerous attached ribosomes and frequently were seen as clumps of 3 - 5 short parallel rows (Nissl). On the other hand, R E R within the deprived neurons was more dispersed within the cytoplasm, seldom arranged in parallel layers, and frequently devoid of most ribosomes. Golgi complexes in deprived neurons did not appear to be qualitatively different in their structural features. The general appearance of the afferent nerve terminals on the deprived neurons was qualitatively similar to controls (Fig. 1). The auditory nerve endbulbs and boutons were present, as were the non-auditory nerve endings with either pleiomorphic or flat vesicles. The only difference seen in the deprived neurons was the relative numbers of these terminal
types. The auditory nerve endbulbs and boutons were seen less and non-auditory nerve boutons were more common. Also, the deprived polysynaptic neurons had segments of their soma membrane devoid of either terminal type. Results of the ultrastructural measurements of deprived and normal convoluted nucleus-polysynaptic neurons are summarized in Table I. Analyses of the afferent nerve terminals revealed fewer auditory nerve terminals (P < 0.005), but those present were of normal cross-sectional area and synapse apposition length (P > 0.50). Also, the auditory nerve terminals covered less of the deprived neuron membrane than normal (50% vs 28%; P < 0.05). On the other hand, non-auditory nerve terminals had normal size, but they were more abundant than normal (P < 0.01) and covered more of the soma membrane (P < 0.025). This dramatic shift is reflected in the ratio of auditory to non-auditory terminals, which changed from a normal value of 2.1 to 0.7 in the deprived neurons (P < 0.025). With regard to the total number of synapses, the normal neurons had 70% of the membrane covered by terminals, compared to 60% in the deprived neurons (P > 0.10). Thus, the increase in non-auditory nerve terminals was sufficient to make the total amount of membrane covered by all afferent terminals similar to normal. The quantitative analysis also demonstrated intracellular effects. Comparison of the average mitochondrion size for each AVCN revealed a trend for smaller mitochondria in deprived neurons, but this difference was slightly above the 5% probability (0.05 < P < 0.10). However, this analysis reduced hundreds of mitochondria measurements to just one mean value for each animal. If all mitochondria within each treatment group were evaluated together (control, 607; deprived, 541), the deprived mitochondria were significantly smaller (P < 0.005). The density of mitochondria within the cytoplasm was comparable in both groups (P > 0.05). The R E R segments within the deprived neurons were the same length as controls (P > 0.50), but they were slightly more concentrated (P < 0.025), presumably due to the reduction in cytoplasm. Similarly, the deprived Golgi profiles were of normal size (P > 0.50), but slightly higher density (P < 0.05). These analyses of AVCN neurons provide previously unreported details of the ultrastructural
3(){~
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A
%.
307 TABLE I Means of measured ultrastructural features in normal and developmentally deprived A VCN neurons 1.0 Standard error in parentheses; control, n = 4; deprived, n = 3. Neuronal feature Afferent terminals Auditory nerve endings Number/10/~m of membrane Apposition length (~m) Area (urn2) % Neuron membrane covered Non-auditory nerve endings Number/10/~m of membrane Apposition length (~m) Area ~ m 2) % Neuron membrane covered Overall % membrane covered Ratio auditory/non-auditory Rough endoplasmic reticulum (RER) Length ~m) Total RER length/~m 2 cytoplasm Number fragments//~m2 cytoplasm Golgi complex profiles Area ~ m 2) Number//~m2 cytoplasm Total area//~m2 cytoplasm Mitochondria Area (~m2) Number//~m2 cytoplasm Total area//~m2 cytoplasm All mitochondria combined: Mean area (,um2) Number
Normal
Deprived
Probability
2.8 (0.12) 1.8 (0.17) 1.6 (0.23) 50.0 (6.10)
1.6 (0.17) 1.7 (0.09) 1.9 (0.37) 28.0 (2.20)
<0.005 >0.50 >0.50 <0.05
1.4 (0.13) 1.5 (0.04) 1.3 (0.21) 20.0 (2.20) 70.0 (5.10) 2.1 (0.31)
2.4 (0.18) 1.4 (0.03) 1.0 (0.10) 32.0 (1.90) 60.0 (2.10) 0.7 (0.09)
<0.01 <0.10 <0.50 <0.025 >0.10 <0.025
0.78 (0.05) 0.41 (0.05) 0.53 (0.03)
0.82 (0.06) 0.57 (0.02) 0.72 (0.04)
>0.50 <0.10 <0.025
0.790 (0.110) 0.068 (0.004) 0.057 (0.007)
0.830 (0.080) 0.090 (0.006) 0.077 (0.003)
>0.50 <0.05 <0.10
0.195 (0.01) 0.645 (0.03) 0.125 (0.01)
0.153 (0.01) 0.783 (0.07) 0.116 (0.01)
<0.10 <0.25 >0.50
0.194 (0.007) 607
0.151 (0.005) 541
<0.005
changes associated with d e v e l o p m e n t a l auditory deprivation. This offers some insight into the cellular events that may be c o m p r o m i s e d by deprivation to lead to smaller soma sizes 1-3A2 and decreased cytoplasm n. The a b n o r m a l balance of axon terminals suggests there is considerable plasticity in auditory nerve terminal arborization. Because no loss of spiral ganglion neurons was seen following d e v e l o p m e n t a l deprivation 12, results here are assumed to r e p r e s e n t decreased branching of a n o r m a l n u m b e r of auditory nerve axons. R e c e n t studies have shown that the configuration of the auditory nerve e n d b u l b n o r m a l l y changes during d e v e l o p m e n t 5'6,8, progressing from a
chalis or cup-like structure to individual slender processes 8. The present study offers evidence that the e n d p o i n t of e n d b u l b r e m o d e l i n g m a y be regulated by activity of the auditory nerve itself. The c o n c o m i t a n t increase in n o n - a u d i t o r y nerve terminals also implies a competition for synaptic space because non-auditory nerve endings arrive at a p p r o x i m a t e l y the same time in CN d e v e l o p m e n t 6. Thus, a decrease in the n u m b e r of auditory nerve terminals could expose m o r e of the postsynaptic m e m b r a n e for contacts by n o n - a u d i t o r y nerve terminals and proliferation of the latter m a y be p r o p o r t i o n a l to the a m o u n t of membrane available.
Fig. 1. Normal (A) and deprived (B) spherical nucleus-polysynaptic neurons from the mouse AVCN. The large auditory nerve endbulbs (eb) and boutons (b) are less frequently seen on the deprived neurons. On the other hand, non-auditory nerve terminals, such as those with pleiomorphic vesicles (p), are more common. The mitochondria (m) within the deprived neurons are smaller than normal and darker, making intramitochondrial features less apparent. Rough endoplasmic reticulum (e) is prevalent within the deprived neurons, but multilayered segments (Nissl) are less frequently seen. Bar = 2 pm.
31~ Our previous studies have shown that cytoplasm is
would be affected. If this causes impaired production
reduced in developmentally deprived A V C N neurons I1. The present study supported this ultrastructurally and also demonstrated alterations in the mito-
of structural proteins during sensitive developmental phases, it would explain the smaller neuron sizes associated with deprivation ~ ~'1~12. Future studies of
chondria. A b n o r m a l l y small and dark mitochondria
gene expression alteration by auditory deprivation
also have been shown in unstimulated axon terminals in other neural pathways 7 and indicate that metabol-
will provide further information regarding this regu-
ic activity in the deprived neurons is reduced by auditory deprivation. This parallels observations of decreased leucine incorporation in developmentally de-
lation of brain development by hearing.
prived mouse CN neurons m and the deafferented
Ms. Jackie Gellatly provided technical assistance in electron microscopy and photography. This re-
chick CN following cochlear ablation u. Cochlear ab-
search was supported in part by The Deafness Re-
lation in the chick also affected CN levels of succinate dehydrogenase 4, a mitochondrial enzyme involved in metabolism. It naturally follows that if n e u r o n a l metabolism is impaired by auditory deprivation, then mitochondrial structure (this study) and function 4
search F o u n d a t i o n , The Medical Research Founda-
1 Blatchley, B.J., Williams, J.E. and Coleman, J.R., Agedependent effects of acoustic deprivation on spherical cells of the rat anteroventral cochlear nucleus, Exp. Neurol., 80 (1983) 81-93. 2 Coleman, J.R. and O'Connor, P., Effects of monaural and binaural sound deprivation on cell development in the anteroventral cochlear nucleus of rats, Exp. Neurol., 64 (1979) 553-566. 3 Conlee, J.W. and Parks, T.N., Age- and position-dependent effects of monaural acoustic deprivation in nucleus magnocellularis of the chicken, J. Comp. Neurol., 202 (1981) 373-384. 4 Durham, D. and Rubel, E.W., Afferent influences on brain stern auditory nuclei of the chicken: changes in succinate dehydrogenase activity following cochlea removal, J. Comp. Neurol., 231 (1985)446-456. 5 Jackson, H. and Parks, T.N., Functional synapse elimination in the developing avian cochlear nucleus with simultaneous reduction in cochlear nerve axon branching, J. Neurosci., 2 (1982) 1736-1743. 6 Neises, G.R., Mattox, D.E. and Gulley. R.L., The maturation of the end bulb of Held in the rat anteroventral cochlear nucleus, Anat. Rec., 204 (1982) 271-27911.
7 Prada, C., Effect of light deprivation upon the morphology of axon terminals in the dorsal lateral geniculate nucleus of mouse: an electron microscopical study using serial sections, Neurosci. Res., 4 (1987) 255-267. 8 Ryugo, D.K. and Fekete, D.M., Morphology of primary axosomatic endings in the anteroventral cochlear nucleus of the cat: a study of the endbulbs of Held, J. Comp. Neurol., 210 (1982) 239-257. 9 Steward, O. and Rubel, E.W., Afferent influences on brain stem auditory nuclei of the chicken: cessation of amino acid incorporation as an antecedent to age-dependent transneuronal degeneration, J. Comp. Neurol., 231 (1985)385-395. 10 Trune, D.R., Kiessling, A.A. and Planck, S.R., Altered protein synthesis in developmentally deprived cochlear nucleus neurons, Assoc. Res. Otolaryngol. Abstr., (1987) 66. 11 Trune, D.R. and Morgan, C.R,, Stimulation-dependent development of neuronal cytoplasm in mouse cochlear nucleus, Hear. Res., 33 (1988) 14t-15(I. 12 Webster, D.B., Auditory neuronal sizes after a unilateral conductive hearing loss, Exp. Neurol., 79 (1983) 130-140. 13 Webster, D.B. and Trune, D.R., Cochlear nuclear complex of mice, Am. J. Anat., 163 (1982) 103-13(I.
tion of Oregon, and B R S G S07 RR05412 Award by the Biomedical Research Support G r a n t Program, Division of Research Resources, National Institutes of Health.
Developmental Brain Research. 42 ( !9~,S)34)4-3~i8
BRD60276
Influence of developmental auditory deprivation on neuronal ultrastructure in the mouse anteroventral cochlear nucleus D e n n i s R. T r u n e 1'2 and Chris R. M o r g a n 1 Departments of 1CellBiology and Anatomy and 20tolaryngology, The Oregon Health Sciences University, Portland, OR 97201 (U.S.A.)
(Accepted 19 April 1988) Key words: Auditory deprivation; Cochlear nucleus; Neuronal organelle; Morphometry; Development:
Metabolism; Synaptic competition; Genetic expression
Developmental auditory deprivation caused mouse anteroventral cochlear nucleus neurons to have significantly fewer auditory nerve terminals and more non-auditory nerve terminals. This suggested that stimulation regulated the developmental arborization of auditory nerve terminals and competition for synaptic space. Intracellularly, mitochondria were smaller and darker in the deprived neurons and appeared less active metabolically. Interference with these neuronal processes may underlie the impaired development seen in auditory deprivation. Numerous studies have demonstrated that developmental auditory deprivation leads to smaller neurons in cochlear nucleus (CN), the termination site of auditory nerve fibers 1-3'11'12. Although this is evidence that some developmental process within these neurons is affected by deprivation, the cellular events involved are not known. Recent auditory deprivation studies have demonstrated that it is the cytoplasmic portion of the CN neuron that is smaller than normal 11. Furthermore, cochlear ablation in the chick leads to rapid changes in CN protein synthesis 9 and succinate dehydrogenase levels 4. These observations suggest that stimulation regulated cytoplasmic processes may be impaired by deprivation to interfere with normal development. It is also possible these impaired cytoplasmic processes may have corresponding organelle changes, although no studies to date have examined CN neuronal tiltrastructure following developmental auditory deprivation. Therefore, the present study was conducted to identify deprivation-induced structural changes to better understand the regulation of cellular mechanisms by auditory stimulation.
The anteroventral CN (AVCN) was examined in normal CBA/J mice and those that had been unilaterally sound-deprived throughout the period of hearing development H. Mice were anesthetized by hypothermia on day 3 and the blastemas of their right external auditory meati were removed. This prevented normal meatal opening on day 8 and created a complete meatal occlusion throughout hearing development (days 8-45). Six unilaterally deprived animals and 6 age-matched controls were sacrificed on postnatal day 45 and their right AVCN prepared for electron microscopy. Neuron types within the small spherical cell region 13 were differentiated according to previously established ultrastructurat criteria l°. All large neurons with visible nuclei were photographed and printed at 10,000x magnification for qualitative evaluations. Only one thin section per grid was photographed to assure that the same neuron was not photographed twice. Also, grids were separated by 20/~m to prevent the same neurons being examined on consecutive grids. Quantitative analyses were also performed to determine if ultrastructural features in the deprived and
Correspondence: D.R. Trune, Department of Cell Biology and Anatomy, The Oregon Health Sciences University, Portland, OR
97201, U.S.A. 0165-3806/88/$03.50 © 1988 Elsevier Science Publishers B.V. (BiomedicalDivision)
305 normal AVCN neurons were similar. All photographed convoluted nucleus-polysynaptic neurons from the unilaterally deprived (3 mice, 9 neurons) and control (4 mice, 9 neurons) A V C N were printed at 15,000x magnification and measured on a digitizer tablet interfaced with the Bioquant II Image Analysis System (R and M Biometrics). This was done blind so the person measuring the micrographs did not know their sources. Micrograph measurements were made of: (1) the area and density of mitochondria; (2) the length of rough endoplasmic reticulum; (3) the area and density of Golgi complexes; and (4) the area, synapse apposition length, and density of auditory and non-auditory nerve terminals. The means of each measurement were calculated for each AVCN and statistically compared (t-tests) to determine if the deprivation treatment had any effect on the morphological features measured. Mean differences with probabilities less than 5% were judged to be significant. Qualitative examination of the deprived AVCN showed all major neuronal types were affected by the developmental deprivation (Fig. 1). Cytoplasm was reduced and often limited to a narrow ring around the nucleus. The most striking organelle changes were seen in the mitochondria, which appeared smaller and less robust in the deprived neurons. Intramitochondrial appearance was always quite dense and dark staining was Observed, making internal features less discernible. Overall they appeared less active metabolically. Rough endoplasmic reticulum (RER) segments normally occurred as long, sometimes branching, channels with numerous attached ribosomes and frequently were seen as clumps of 3 - 5 short parallel rows (Nissl). On the other hand, R E R within the deprived neurons was more dispersed within the cytoplasm, seldom arranged in parallel layers, and frequently devoid of most ribosomes. Golgi complexes in deprived neurons did not appear to be qualitatively different in their structural features. The general appearance of the afferent nerve terminals on the deprived neurons was qualitatively similar to controls (Fig. 1). The auditory nerve endbulbs and boutons were present, as were the non-auditory nerve endings with either pleiomorphic or flat vesicles. The only difference seen in the deprived neurons was the relative numbers of these terminal
types. The auditory nerve endbulbs and boutons were seen less and non-auditory nerve boutons were more common. Also, the deprived polysynaptic neurons had segments of their soma membrane devoid of either terminal type. Results of the ultrastructural measurements of deprived and normal convoluted nucleus-polysynaptic neurons are summarized in Table I. Analyses of the afferent nerve terminals revealed fewer auditory nerve terminals (P < 0.005), but those present were of normal cross-sectional area and synapse apposition length (P > 0.50). Also, the auditory nerve terminals covered less of the deprived neuron membrane than normal (50% vs 28%; P < 0.05). On the other hand, non-auditory nerve terminals had normal size, but they were more abundant than normal (P < 0.01) and covered more of the soma membrane (P < 0.025). This dramatic shift is reflected in the ratio of auditory to non-auditory terminals, which changed from a normal value of 2.1 to 0.7 in the deprived neurons (P < 0.025). With regard to the total number of synapses, the normal neurons had 70% of the membrane covered by terminals, compared to 60% in the deprived neurons (P > 0.10). Thus, the increase in non-auditory nerve terminals was sufficient to make the total amount of membrane covered by all afferent terminals similar to normal. The quantitative analysis also demonstrated intracellular effects. Comparison of the average mitochondrion size for each AVCN revealed a trend for smaller mitochondria in deprived neurons, but this difference was slightly above the 5% probability (0.05 < P < 0.10). However, this analysis reduced hundreds of mitochondria measurements to just one mean value for each animal. If all mitochondria within each treatment group were evaluated together (control, 607; deprived, 541), the deprived mitochondria were significantly smaller (P < 0.005). The density of mitochondria within the cytoplasm was comparable in both groups (P > 0.05). The R E R segments within the deprived neurons were the same length as controls (P > 0.50), but they were slightly more concentrated (P < 0.025), presumably due to the reduction in cytoplasm. Similarly, the deprived Golgi profiles were of normal size (P > 0.50), but slightly higher density (P < 0.05). These analyses of AVCN neurons provide previously unreported details of the ultrastructural
3(){~
B
A
%.
307 TABLE I Means of measured ultrastructural features in normal and developmentally deprived A VCN neurons 1.0 Standard error in parentheses; control, n = 4; deprived, n = 3. Neuronal feature Afferent terminals Auditory nerve endings Number/10/~m of membrane Apposition length (~m) Area (urn2) % Neuron membrane covered Non-auditory nerve endings Number/10/~m of membrane Apposition length (~m) Area ~ m 2) % Neuron membrane covered Overall % membrane covered Ratio auditory/non-auditory Rough endoplasmic reticulum (RER) Length ~m) Total RER length/~m 2 cytoplasm Number fragments//~m2 cytoplasm Golgi complex profiles Area ~ m 2) Number//~m2 cytoplasm Total area//~m2 cytoplasm Mitochondria Area (~m2) Number//~m2 cytoplasm Total area//~m2 cytoplasm All mitochondria combined: Mean area (,um2) Number
Normal
Deprived
Probability
2.8 (0.12) 1.8 (0.17) 1.6 (0.23) 50.0 (6.10)
1.6 (0.17) 1.7 (0.09) 1.9 (0.37) 28.0 (2.20)
<0.005 >0.50 >0.50 <0.05
1.4 (0.13) 1.5 (0.04) 1.3 (0.21) 20.0 (2.20) 70.0 (5.10) 2.1 (0.31)
2.4 (0.18) 1.4 (0.03) 1.0 (0.10) 32.0 (1.90) 60.0 (2.10) 0.7 (0.09)
<0.01 <0.10 <0.50 <0.025 >0.10 <0.025
0.78 (0.05) 0.41 (0.05) 0.53 (0.03)
0.82 (0.06) 0.57 (0.02) 0.72 (0.04)
>0.50 <0.10 <0.025
0.790 (0.110) 0.068 (0.004) 0.057 (0.007)
0.830 (0.080) 0.090 (0.006) 0.077 (0.003)
>0.50 <0.05 <0.10
0.195 (0.01) 0.645 (0.03) 0.125 (0.01)
0.153 (0.01) 0.783 (0.07) 0.116 (0.01)
<0.10 <0.25 >0.50
0.194 (0.007) 607
0.151 (0.005) 541
<0.005
changes associated with d e v e l o p m e n t a l auditory deprivation. This offers some insight into the cellular events that may be c o m p r o m i s e d by deprivation to lead to smaller soma sizes 1-3A2 and decreased cytoplasm n. The a b n o r m a l balance of axon terminals suggests there is considerable plasticity in auditory nerve terminal arborization. Because no loss of spiral ganglion neurons was seen following d e v e l o p m e n t a l deprivation 12, results here are assumed to r e p r e s e n t decreased branching of a n o r m a l n u m b e r of auditory nerve axons. R e c e n t studies have shown that the configuration of the auditory nerve e n d b u l b n o r m a l l y changes during d e v e l o p m e n t 5'6,8, progressing from a
chalis or cup-like structure to individual slender processes 8. The present study offers evidence that the e n d p o i n t of e n d b u l b r e m o d e l i n g m a y be regulated by activity of the auditory nerve itself. The c o n c o m i t a n t increase in n o n - a u d i t o r y nerve terminals also implies a competition for synaptic space because non-auditory nerve endings arrive at a p p r o x i m a t e l y the same time in CN d e v e l o p m e n t 6. Thus, a decrease in the n u m b e r of auditory nerve terminals could expose m o r e of the postsynaptic m e m b r a n e for contacts by n o n - a u d i t o r y nerve terminals and proliferation of the latter m a y be p r o p o r t i o n a l to the a m o u n t of membrane available.
Fig. 1. Normal (A) and deprived (B) spherical nucleus-polysynaptic neurons from the mouse AVCN. The large auditory nerve endbulbs (eb) and boutons (b) are less frequently seen on the deprived neurons. On the other hand, non-auditory nerve terminals, such as those with pleiomorphic vesicles (p), are more common. The mitochondria (m) within the deprived neurons are smaller than normal and darker, making intramitochondrial features less apparent. Rough endoplasmic reticulum (e) is prevalent within the deprived neurons, but multilayered segments (Nissl) are less frequently seen. Bar = 2 pm.
31~ Our previous studies have shown that cytoplasm is
would be affected. If this causes impaired production
reduced in developmentally deprived A V C N neurons I1. The present study supported this ultrastructurally and also demonstrated alterations in the mito-
of structural proteins during sensitive developmental phases, it would explain the smaller neuron sizes associated with deprivation ~ ~'1~12. Future studies of
chondria. A b n o r m a l l y small and dark mitochondria
gene expression alteration by auditory deprivation
also have been shown in unstimulated axon terminals in other neural pathways 7 and indicate that metabol-
will provide further information regarding this regu-
ic activity in the deprived neurons is reduced by auditory deprivation. This parallels observations of decreased leucine incorporation in developmentally de-
lation of brain development by hearing.
prived mouse CN neurons m and the deafferented
Ms. Jackie Gellatly provided technical assistance in electron microscopy and photography. This re-
chick CN following cochlear ablation u. Cochlear ab-
search was supported in part by The Deafness Re-
lation in the chick also affected CN levels of succinate dehydrogenase 4, a mitochondrial enzyme involved in metabolism. It naturally follows that if n e u r o n a l metabolism is impaired by auditory deprivation, then mitochondrial structure (this study) and function 4
search F o u n d a t i o n , The Medical Research Founda-
1 Blatchley, B.J., Williams, J.E. and Coleman, J.R., Agedependent effects of acoustic deprivation on spherical cells of the rat anteroventral cochlear nucleus, Exp. Neurol., 80 (1983) 81-93. 2 Coleman, J.R. and O'Connor, P., Effects of monaural and binaural sound deprivation on cell development in the anteroventral cochlear nucleus of rats, Exp. Neurol., 64 (1979) 553-566. 3 Conlee, J.W. and Parks, T.N., Age- and position-dependent effects of monaural acoustic deprivation in nucleus magnocellularis of the chicken, J. Comp. Neurol., 202 (1981) 373-384. 4 Durham, D. and Rubel, E.W., Afferent influences on brain stern auditory nuclei of the chicken: changes in succinate dehydrogenase activity following cochlea removal, J. Comp. Neurol., 231 (1985)446-456. 5 Jackson, H. and Parks, T.N., Functional synapse elimination in the developing avian cochlear nucleus with simultaneous reduction in cochlear nerve axon branching, J. Neurosci., 2 (1982) 1736-1743. 6 Neises, G.R., Mattox, D.E. and Gulley. R.L., The maturation of the end bulb of Held in the rat anteroventral cochlear nucleus, Anat. Rec., 204 (1982) 271-27911.
7 Prada, C., Effect of light deprivation upon the morphology of axon terminals in the dorsal lateral geniculate nucleus of mouse: an electron microscopical study using serial sections, Neurosci. Res., 4 (1987) 255-267. 8 Ryugo, D.K. and Fekete, D.M., Morphology of primary axosomatic endings in the anteroventral cochlear nucleus of the cat: a study of the endbulbs of Held, J. Comp. Neurol., 210 (1982) 239-257. 9 Steward, O. and Rubel, E.W., Afferent influences on brain stem auditory nuclei of the chicken: cessation of amino acid incorporation as an antecedent to age-dependent transneuronal degeneration, J. Comp. Neurol., 231 (1985)385-395. 10 Trune, D.R., Kiessling, A.A. and Planck, S.R., Altered protein synthesis in developmentally deprived cochlear nucleus neurons, Assoc. Res. Otolaryngol. Abstr., (1987) 66. 11 Trune, D.R. and Morgan, C.R,, Stimulation-dependent development of neuronal cytoplasm in mouse cochlear nucleus, Hear. Res., 33 (1988) 14t-15(I. 12 Webster, D.B., Auditory neuronal sizes after a unilateral conductive hearing loss, Exp. Neurol., 79 (1983) 130-140. 13 Webster, D.B. and Trune, D.R., Cochlear nuclear complex of mice, Am. J. Anat., 163 (1982) 103-13(I.
tion of Oregon, and B R S G S07 RR05412 Award by the Biomedical Research Support G r a n t Program, Division of Research Resources, National Institutes of Health.