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Latency changes in brain stem auditory evoked potentials associated with impaired brain myelination
EXPERIMENTAL
NEUROLOGY
58,
111-l
18
(19%)
Latency Changes in Brain Stem Auditory Evoked Potentials Associated with Impaired Brain Myelination S. N. SHAH, Brain-Behavior
V. K. BHARGAVA, Research Nczcropsycl~iatric
R. C. JOHNSON,
AND C. M. MCKEAN
Cemter, University of California, Langley Institute, Sonoma State Hospital, Eldridge, California 95431 Received
July
l
Porter
29, 1977
Interference with myelin deposition and brain growth was produced in rats by administering p-chlorophenylalanine (PCPA), alone or with phenylalanine (PHE), from the fifth postnatal day. Treatment with PCPA + PHE until 20 days of age resulted in significantly greater developmental impairment than that produced by PCPA alone. The extent of the maturational arrest was reflected in a corresponding failure to achieve the progressive latency decrements observed in the brain stem evoked potentials of normally maturing animals. In rats treated until 3’0 days of age the metabolic and electrophysiologic abnormalities were attenuated. Synaptosomal content at either age and under both experimental conditions did not appear to differ from that of the control brains. Rats which received PCPA or PCPA + PHE for a brief period later in development (from 17 to 20 days of age) maintained relatively normal myelin deposition and brain growth. The latencies of their brain stem evoked potentials also remained normal despite the presence of low cerebral serotonin concentrations. These data suggst that this latency provides a general measure of brain development which seems particularly sensitive to changes in the rate of myelination.
INTRODUCTION There is now little doubt that brain stem activity can be reliably recorded from the surface of the scalp in animal and man, and that the various waves which make up the complex “far-field response” originate primarily from spatially separate structures in the classical auditory pathway (4, 7). Although it was suggestedthat somewaves arise from activation of Abbreviations : PCPA-fi-chlorophenylalanine; 1 This investigation was supported by National
PHE-phenylalanine. Institutes of Health Grant HD01823.
111 0014-4886/78/0581-0111$02.00/0 Co yright Q 1958 by AcademicPress,I&. All rigii ts of reproduction in any form reserved.
112
SHAH
ET
AL.
multiple local generators in algebraic summation (1, 7)) there is no question that they represent different levels of brain stem integration (18). This “far-field” technique has also been successfully applied to the assessment of subcortical maturation in the rat and cat (8)) as well as in the human [ (6, 12, 13) ; A. Salamy; C. M. McKean, P. G. Pettet, and T. Mendelson, unpublished observations]. At fixed intensity and rate of presentation, click stimuli produce short-latency evoked potentials that show a monotonic decrease in latency with increasing age. The purpose of the present study was to determine the correlation between impaired myelination and the latency decrements observed in brain stem evoked potentials during development. Treatment with p-chlorophenylalanine (PCPA) , either alone or with phenylalanine (PHE), is known to limit myelin deposition (5) and serotonin metabolism (10). We therefore investigated the effect of this treatment on latencies of the brain stem evoked potential and correlated the changes with reduction in cerebral myelin. METHODS Litters of Sprague-Dawley rats (10 to 12 pups in each litter) were divided into three groups of three or four rats each. Beginning on the fifth postnatal day the pups were injected intraperitoneally (ip) twice daily with p-chlorophenylalanine (PCPA, 50 mg/kg) alone, PCPA (50 mg/kg) + phenylalanine (300 mg/kg), or saline until they were either 20 or 30 days old. In one experiment, however, the injections were begun on the 17th day of postnatal age and terminated on the 20th day of age. Brain stem evoked potentials were recorded from lightly anesthetized animals at the 20th or 30th day of age (a-chloralose, 65 mg/kg, ip). The recordings were made using Grass E2 platinum needle ‘electrodes inserted subcutaneously at the vertex of the scalp while the reference was placed at the anterior border of the pinna of the stimulated ear. The auditory stimuli consisting of clicks (60 dB above experimenter’s threshold) produced by a Grass S4 stimulator were delivered monaurally at the rate of six per second through a miniature receiver (M-98 Radioear) connected to a plastic tube fitted snugly into the ear canal. Bioelectric activity was led to a Grass P54 preamplifier set at lo- to 3000-Hz bandpass. An average of 400 brain stem evoked potentials to acoustic stimuli were summated on a Digital PDP/SE computer and written out on an X-Y plotter. The latencies were computed from these tracings. Immediately after testing, the animals were killed and their brains were removed and weighed. The two hemispheres were dissected and one from each rat was frozen immediately. The remaining hemispheres from three or four rats of each group were pooled and homogenized for preparation of
EVOKED
POTENTIAL
LATENCY
AND
TABLE
CNS
1
Effect of p-Chlorophenylalanine (PCPA) and Phenylalanine Treatment on Body and Brain Weight in Rats Treatment
Age (days)
113
MYELINATION
Body weight
(g)
(PHE)
Brain weight
(g)
20
Saline (20)a PCPA (23) PCPA + PHE (20)
38.956 f 7.34 32.00” f 5.72 23.81d f 7.44
1.3433 * 0.0688 1.2731c f 0.0845 1.0044d f 0.1084
30
Saline (8) PCPA + PHE (10)
102.16 f 9.32 85.22d f 7.75
1.5433 f 0.0650 1.2138d f 0.0654
a Numbers in parentheses indicate the number of rats in each group. b Data represent mean f standard deviation. c Significantly different from saline-treated values, P < 0.05. d Significantly different from PCPA treatment, P < 0.05.
myelin and synaptosomesas described earlier (9). The serotonin content of the frozen hemisphereswas determined by the method of Barchas et al. (3). RESULTS At 20 days of age PCPA + PHE-treated rats had body weights that were approximately 60% those of the control animals; brain weights that were 75% those of controls; myelin contents that were 55 to 57% those of controls ; (Tables 1, 2). Rats treated with PCPA alone showed values TABLE
2
Effect of p-Chlorophenylalanine (PCPA) and Phenylalanine (PHE) Treatment on Myelin and Synaptosomal Content of Rat Brain Age (days) 20
Treatment
Saline PCPA PCPA + PHE
30
Saline PCPA + PHE
Expt. No. I II I II I II I II I II
Myelin content” (mg/g brain) 4.95 4.29 3.50 3.05 2.74 2.45 18.1 14.8 13.4 9.7
Percentage of control 100 100 71 71 55 57 100 100 74 66
Synaptosomal content (mg/g brain) 4.71 5.55 5.10 5.59 4.53 6.30 15.2 12.3 13.9 13.4
Percentage of control 100 100 108 100 96 113 100 100
91 109
a Values for myelin and synaptosomal content are from two independent experiments. In each experiment the fractions were prepared from brains pooled from three or four rats in each group.
Age
(15)b
(15)
+ PHE (13) + PHE
Saline
PCPA
PCPA Saline PCPA
Treatment
of +Chlorophenylalanine
(11)
(16)
(PCPA)
I
f
0.15
1.51s f 0.22 1.20 zk 0.18 1.33d f 0.20
1.36
0.14
Phenylalanine
1.29c f
and
3
f 0.20
0.16
of brain
Treatment
conduction.
zk 0.30 f 0.25 f 0.22
of central
2.73” 2.11 2.33d
2.S4d f
2.37
II
Latencies
(PHE)
TABLE
0 Difference in latencies of waves IV and I is taken as a measure 6 Numbers in parentheses indicate the number of animals. 6 Data represent mean f standard deviation. dSignificantly different from saline control, P < 0.05. 6 Significantly different from PCPA-treated values, P < 0.05.
30
20
(days)
Effect
3.67” 2.93 3.23d
3.47
3.34
stem
0.33
0.20
f 0.37 zk 0.28 dz 0.21
f
f
III
auditory
on Brain
Stem
5.546 4.11 4.45d
5.17d
4.80
potentials
Auditory
0.39 h 0.42 f 0.37 * 0.37
f
& 0.25
IV
(ms)
Potentials
4.086 2.83 3.08d
3.77d
3.52
0.22
st 0.46 f 0.16 i 0.36
zk 0.37
f
IV-1a
in Rats
F
3
ki
E!
(3) f PHE
PCPA PCPA
(4)
of Short-Term
1.83 1.79
f f
1.92c f
I
0.13 0.09
0.07
f
II 0.24
of brain
3.98 3.92
4.06
stem
f f
f
III
5.46 Z!I 0.10 5.50 f 0.27
0.21 0.09
0.14
stem
(ms)
5.59 *
IV
potentials
potentials
0.05 0.25
0.19
Phenylalanine Concentrations
auditory
3.63 f 3.64 f
3.66 f
IV-I
(PCPA) and and Serotonin
0.23
auditory
with p-Chlorophenylalanine Potentials, Brain iveight,
2.92 f 0.10 2.92 r!z 0.22
2.98
Latencies
(3-Day) Treatment Brain Stem Auditory
4
*The treatment of animals was begun on the 17th postnatal day and the brain b Numbers in parentheses indicate the number of animals in the group. c Data represent mean f standard deviation.
(3) b
Saline
Treatment&
Effect
TABLE
were
1.35 f 1.36 f
1.38 f
recorded
0.06 0.05
0.01
f f on the 20th
193.0 142.8
f
day.
24.04 28.06
15.21
serotonin (w/d
of
282.5
Brain
on Lntencies
Brain weight (g)
(PHE)
4 v1
$ u
Ic
F E
116
SHAH
ET
AL.
intermediate between the control and the PCPA + PHE-treated animals, The synaptosomal contents were similar for each experimental group of animals (Table 2). Brain stem evoked potential latencies were generally longer in both experimental groups than in the control group (Table 3). These abnormally long latencies correlated closely with deficiencies in cerebral myelin. In the PCPA + PHE-treated animals the latency of peak I, which is thought to reflect the maturation of acoustic structures peripheral to the brain stem, was about 17% longer than the control value. The latency difference between peaks I and IV (which presumably are generated by caudal and rostra1 brain stem structures, respectively) appears to reflect a relatively independent measure of brain stem maturation ( 13). This latency difference in PCPA + PHE-treated animals was 16% longer than the difference in controls. Predictably, the latencies of the group treated with PCPA alone were intermediate between the control and PCPA + PHE-treated animals. By 30 days of age the differences in myelin content between the control and experimental rats were attenuated with correspondingly smaller differences in brain stem evoked potential latencies. DISCUSSION In the course of normal rat brain maturation a striking increase in myelin deposition and synaptic proliferation occurs between 15 and 30 days of age. The systematic increase in synaptosomal content of rat brain during early development ( 11) closely parallels the rapid rise in numbers of synaptic junctions within the molecular layer of rat cortex (2). Although the PCPA + PHE treatment appeared to have no effect on this synaptosomal correlate of synaptogenesis, it produced a profound impairment of myelin deposition and brain growth. These structural impairments are closely associated with failure to achieve the normal decremental progression of brain stem evoked potential latency. The effect may be observed in the latencies of each potential. In contrast, short-term treatment with PCPA or PCPA + PHE (from 17 to 20 days of age), which lowered cerebral serotonin concentration more than 50% without changing myelin content or brain weight, left brain stem evoked potential latencies unaltered (Table 4). Taken together, these experiments suggest that the brain stem evoked potential latency parameter may provide a general measure of brain development which is particularly sensitive to changes in the rate of myelination. However, it should be pointed ,out that although the latency of peak I and the interpeak latency between peaks I and IV appear to represent anatomically distinct peripheral and central processes, respectively (13)) it is
EVOKED
POTENTIAL
LATENCY
AND
CNS
117
MYELINATION
virtually impossible to establish with certainty the extent of the peripheral contribution to “central” maturational changes (8). Nevertheless, this association between experimentally induced myelin deficiency and latency prolongation is consistent with brain stem evoked potential findings in patients with clemyelinating diseases. Characteristically, the components which manifest abnormally long latencies correspond to brain stem and thalamic generator-sources within that portion of the acoustic pathway which reveal significant loss of myelin at postmortem examination (14-18). REFERENCES 1. ACHOR,
L. J. 1976.Field-analysis of auditory brainstem
responses.
Neurosci.
Abstr.
2: 12. G. K., AND F. E. BLOOM. 1967.The formationof synapticjunctions in developingrat brain: A quantitative electronmicroscopicstudy. Brain Res.
2. AGHAJIANIAN,
6: 716-727. 3. BARCHAS, J., E. ERDELYI,
AND P. ANGWIP~. 1972.Simultaneousdeterminationof of indoleand catecholamines in tissuesusinga weakcation-exchange resin.Anal.
Biochem. 50 : 1-17. 4. BUCHWALD, J. S., AND C. M. HUANG. 1975.Far-field acousticresponse: Origins in the cat. Sciefzce 189 : 382-384. 5. GRUNDT, I., AND K. HOLE. 1974. +Chlorophenylalanine treatment in developing rats : proteins and lipids in whole brain and myelin. Brain Res. 74: 269-277. 6. HECOX, K., AND R. GALAMBOS. 1974. Brainstem auditory evoked responses in human infants and adults. Arch. 0 tolaryxgol. ,99 : 30-34. 7. JEWETT, D. L. 1970. Volume-conducted potentials in response to auditory stimuli as detected by averaging in the cat. Electroeweph. Clin. Nelbrophysiol. 28: 609618. 8. JEWETT, D. L., AND M. N. ROMANO. 1972. Neonatal development of auditory system potentials averaged from the scalp of rat and cat. Brain Res. 36: 101-115. 9. JOHNSON, R. C., C. M. MCKEAN, AND S. N. SHAH. 1977. Fatty acid composition of lipids in cerebral myelin and synaptosomes in phenylketonuria and Down’s Syndrome. Arch. Nezwol. 34 : 288-294. 10. Loo, Y. H. 1974. Serotonin deficiency in experimental hyperphenylalaninemia. J. Neurochm. 23 : 139-147. 11. RUBIOLO DE MACCIONI, A. A., AND R. CAPUTTO. 1968. Synthesis of gangliosides during development and its relation to the qualitative changes of subcellular particles of rat brain. J. Neurochem. 15 : 1257-1264. 12. SALAMY, A., C. M. MCKEAN, AND F. B. BUDA. 1975. Maturational changes in auditory transmission as reflected in human brainstem potentials. Brain Res. 96: 361-366. 13. SALAMY, A., AND C. M. MCKEAN. 1976. Postnatal development of human brainstem potentials during the first year of life. Electroerrceph. Clip. Neurophysiol. 40 : 418-425. 14. STARR, A., AND L. J. ACHOR. 1975. Auditory brainstem responses in neurological disease. Arch Newel. 32 : 761-766. 15. STARR, A., AND A. E. HAMILTON. 1976. Correlation between confirmed sites of neurological lesions and abnormalities of far-field auditory brainstem responses. Electroenceph.
Clin.
Neurophysiol.
41:
595-608.
118
SHAH
ET
AL.
16. STOCKARD, J. J., V. S. ROSSITER, W. C. WIEDERHOLT, AND R. M. KOBAYASHI. 1976. Brainstem auditory-evoked responses in suspected central pontine myelinolysis. Arch. Neural. 33 : 726728. 17. STOCKARD, J. J., AND V. S. ROSSITER. 1977. Clinical and pathological correlates of brainstem auditory response abnormalities. Neurology 27 : 316-325. 18. WIEDERHOLT, W. C., R. M. KOBAYASKI, J. J. STOCKAFCD, AND V. S. ROSSITER. 1977. Central pontine myelinolysis. Arch. Neural. 34 : 220-223.
NEUROLOGY
58,
111-l
18
(19%)
Latency Changes in Brain Stem Auditory Evoked Potentials Associated with Impaired Brain Myelination S. N. SHAH, Brain-Behavior
V. K. BHARGAVA, Research Nczcropsycl~iatric
R. C. JOHNSON,
AND C. M. MCKEAN
Cemter, University of California, Langley Institute, Sonoma State Hospital, Eldridge, California 95431 Received
July
l
Porter
29, 1977
Interference with myelin deposition and brain growth was produced in rats by administering p-chlorophenylalanine (PCPA), alone or with phenylalanine (PHE), from the fifth postnatal day. Treatment with PCPA + PHE until 20 days of age resulted in significantly greater developmental impairment than that produced by PCPA alone. The extent of the maturational arrest was reflected in a corresponding failure to achieve the progressive latency decrements observed in the brain stem evoked potentials of normally maturing animals. In rats treated until 3’0 days of age the metabolic and electrophysiologic abnormalities were attenuated. Synaptosomal content at either age and under both experimental conditions did not appear to differ from that of the control brains. Rats which received PCPA or PCPA + PHE for a brief period later in development (from 17 to 20 days of age) maintained relatively normal myelin deposition and brain growth. The latencies of their brain stem evoked potentials also remained normal despite the presence of low cerebral serotonin concentrations. These data suggst that this latency provides a general measure of brain development which seems particularly sensitive to changes in the rate of myelination.
INTRODUCTION There is now little doubt that brain stem activity can be reliably recorded from the surface of the scalp in animal and man, and that the various waves which make up the complex “far-field response” originate primarily from spatially separate structures in the classical auditory pathway (4, 7). Although it was suggestedthat somewaves arise from activation of Abbreviations : PCPA-fi-chlorophenylalanine; 1 This investigation was supported by National
PHE-phenylalanine. Institutes of Health Grant HD01823.
111 0014-4886/78/0581-0111$02.00/0 Co yright Q 1958 by AcademicPress,I&. All rigii ts of reproduction in any form reserved.
112
SHAH
ET
AL.
multiple local generators in algebraic summation (1, 7)) there is no question that they represent different levels of brain stem integration (18). This “far-field” technique has also been successfully applied to the assessment of subcortical maturation in the rat and cat (8)) as well as in the human [ (6, 12, 13) ; A. Salamy; C. M. McKean, P. G. Pettet, and T. Mendelson, unpublished observations]. At fixed intensity and rate of presentation, click stimuli produce short-latency evoked potentials that show a monotonic decrease in latency with increasing age. The purpose of the present study was to determine the correlation between impaired myelination and the latency decrements observed in brain stem evoked potentials during development. Treatment with p-chlorophenylalanine (PCPA) , either alone or with phenylalanine (PHE), is known to limit myelin deposition (5) and serotonin metabolism (10). We therefore investigated the effect of this treatment on latencies of the brain stem evoked potential and correlated the changes with reduction in cerebral myelin. METHODS Litters of Sprague-Dawley rats (10 to 12 pups in each litter) were divided into three groups of three or four rats each. Beginning on the fifth postnatal day the pups were injected intraperitoneally (ip) twice daily with p-chlorophenylalanine (PCPA, 50 mg/kg) alone, PCPA (50 mg/kg) + phenylalanine (300 mg/kg), or saline until they were either 20 or 30 days old. In one experiment, however, the injections were begun on the 17th day of postnatal age and terminated on the 20th day of age. Brain stem evoked potentials were recorded from lightly anesthetized animals at the 20th or 30th day of age (a-chloralose, 65 mg/kg, ip). The recordings were made using Grass E2 platinum needle ‘electrodes inserted subcutaneously at the vertex of the scalp while the reference was placed at the anterior border of the pinna of the stimulated ear. The auditory stimuli consisting of clicks (60 dB above experimenter’s threshold) produced by a Grass S4 stimulator were delivered monaurally at the rate of six per second through a miniature receiver (M-98 Radioear) connected to a plastic tube fitted snugly into the ear canal. Bioelectric activity was led to a Grass P54 preamplifier set at lo- to 3000-Hz bandpass. An average of 400 brain stem evoked potentials to acoustic stimuli were summated on a Digital PDP/SE computer and written out on an X-Y plotter. The latencies were computed from these tracings. Immediately after testing, the animals were killed and their brains were removed and weighed. The two hemispheres were dissected and one from each rat was frozen immediately. The remaining hemispheres from three or four rats of each group were pooled and homogenized for preparation of
EVOKED
POTENTIAL
LATENCY
AND
TABLE
CNS
1
Effect of p-Chlorophenylalanine (PCPA) and Phenylalanine Treatment on Body and Brain Weight in Rats Treatment
Age (days)
113
MYELINATION
Body weight
(g)
(PHE)
Brain weight
(g)
20
Saline (20)a PCPA (23) PCPA + PHE (20)
38.956 f 7.34 32.00” f 5.72 23.81d f 7.44
1.3433 * 0.0688 1.2731c f 0.0845 1.0044d f 0.1084
30
Saline (8) PCPA + PHE (10)
102.16 f 9.32 85.22d f 7.75
1.5433 f 0.0650 1.2138d f 0.0654
a Numbers in parentheses indicate the number of rats in each group. b Data represent mean f standard deviation. c Significantly different from saline-treated values, P < 0.05. d Significantly different from PCPA treatment, P < 0.05.
myelin and synaptosomesas described earlier (9). The serotonin content of the frozen hemisphereswas determined by the method of Barchas et al. (3). RESULTS At 20 days of age PCPA + PHE-treated rats had body weights that were approximately 60% those of the control animals; brain weights that were 75% those of controls; myelin contents that were 55 to 57% those of controls ; (Tables 1, 2). Rats treated with PCPA alone showed values TABLE
2
Effect of p-Chlorophenylalanine (PCPA) and Phenylalanine (PHE) Treatment on Myelin and Synaptosomal Content of Rat Brain Age (days) 20
Treatment
Saline PCPA PCPA + PHE
30
Saline PCPA + PHE
Expt. No. I II I II I II I II I II
Myelin content” (mg/g brain) 4.95 4.29 3.50 3.05 2.74 2.45 18.1 14.8 13.4 9.7
Percentage of control 100 100 71 71 55 57 100 100 74 66
Synaptosomal content (mg/g brain) 4.71 5.55 5.10 5.59 4.53 6.30 15.2 12.3 13.9 13.4
Percentage of control 100 100 108 100 96 113 100 100
91 109
a Values for myelin and synaptosomal content are from two independent experiments. In each experiment the fractions were prepared from brains pooled from three or four rats in each group.
Age
(15)b
(15)
+ PHE (13) + PHE
Saline
PCPA
PCPA Saline PCPA
Treatment
of +Chlorophenylalanine
(11)
(16)
(PCPA)
I
f
0.15
1.51s f 0.22 1.20 zk 0.18 1.33d f 0.20
1.36
0.14
Phenylalanine
1.29c f
and
3
f 0.20
0.16
of brain
Treatment
conduction.
zk 0.30 f 0.25 f 0.22
of central
2.73” 2.11 2.33d
2.S4d f
2.37
II
Latencies
(PHE)
TABLE
0 Difference in latencies of waves IV and I is taken as a measure 6 Numbers in parentheses indicate the number of animals. 6 Data represent mean f standard deviation. dSignificantly different from saline control, P < 0.05. 6 Significantly different from PCPA-treated values, P < 0.05.
30
20
(days)
Effect
3.67” 2.93 3.23d
3.47
3.34
stem
0.33
0.20
f 0.37 zk 0.28 dz 0.21
f
f
III
auditory
on Brain
Stem
5.546 4.11 4.45d
5.17d
4.80
potentials
Auditory
0.39 h 0.42 f 0.37 * 0.37
f
& 0.25
IV
(ms)
Potentials
4.086 2.83 3.08d
3.77d
3.52
0.22
st 0.46 f 0.16 i 0.36
zk 0.37
f
IV-1a
in Rats
F
3
ki
E!
(3) f PHE
PCPA PCPA
(4)
of Short-Term
1.83 1.79
f f
1.92c f
I
0.13 0.09
0.07
f
II 0.24
of brain
3.98 3.92
4.06
stem
f f
f
III
5.46 Z!I 0.10 5.50 f 0.27
0.21 0.09
0.14
stem
(ms)
5.59 *
IV
potentials
potentials
0.05 0.25
0.19
Phenylalanine Concentrations
auditory
3.63 f 3.64 f
3.66 f
IV-I
(PCPA) and and Serotonin
0.23
auditory
with p-Chlorophenylalanine Potentials, Brain iveight,
2.92 f 0.10 2.92 r!z 0.22
2.98
Latencies
(3-Day) Treatment Brain Stem Auditory
4
*The treatment of animals was begun on the 17th postnatal day and the brain b Numbers in parentheses indicate the number of animals in the group. c Data represent mean f standard deviation.
(3) b
Saline
Treatment&
Effect
TABLE
were
1.35 f 1.36 f
1.38 f
recorded
0.06 0.05
0.01
f f on the 20th
193.0 142.8
f
day.
24.04 28.06
15.21
serotonin (w/d
of
282.5
Brain
on Lntencies
Brain weight (g)
(PHE)
4 v1
$ u
Ic
F E
116
SHAH
ET
AL.
intermediate between the control and the PCPA + PHE-treated animals, The synaptosomal contents were similar for each experimental group of animals (Table 2). Brain stem evoked potential latencies were generally longer in both experimental groups than in the control group (Table 3). These abnormally long latencies correlated closely with deficiencies in cerebral myelin. In the PCPA + PHE-treated animals the latency of peak I, which is thought to reflect the maturation of acoustic structures peripheral to the brain stem, was about 17% longer than the control value. The latency difference between peaks I and IV (which presumably are generated by caudal and rostra1 brain stem structures, respectively) appears to reflect a relatively independent measure of brain stem maturation ( 13). This latency difference in PCPA + PHE-treated animals was 16% longer than the difference in controls. Predictably, the latencies of the group treated with PCPA alone were intermediate between the control and PCPA + PHE-treated animals. By 30 days of age the differences in myelin content between the control and experimental rats were attenuated with correspondingly smaller differences in brain stem evoked potential latencies. DISCUSSION In the course of normal rat brain maturation a striking increase in myelin deposition and synaptic proliferation occurs between 15 and 30 days of age. The systematic increase in synaptosomal content of rat brain during early development ( 11) closely parallels the rapid rise in numbers of synaptic junctions within the molecular layer of rat cortex (2). Although the PCPA + PHE treatment appeared to have no effect on this synaptosomal correlate of synaptogenesis, it produced a profound impairment of myelin deposition and brain growth. These structural impairments are closely associated with failure to achieve the normal decremental progression of brain stem evoked potential latency. The effect may be observed in the latencies of each potential. In contrast, short-term treatment with PCPA or PCPA + PHE (from 17 to 20 days of age), which lowered cerebral serotonin concentration more than 50% without changing myelin content or brain weight, left brain stem evoked potential latencies unaltered (Table 4). Taken together, these experiments suggest that the brain stem evoked potential latency parameter may provide a general measure of brain development which is particularly sensitive to changes in the rate of myelination. However, it should be pointed ,out that although the latency of peak I and the interpeak latency between peaks I and IV appear to represent anatomically distinct peripheral and central processes, respectively (13)) it is
EVOKED
POTENTIAL
LATENCY
AND
CNS
117
MYELINATION
virtually impossible to establish with certainty the extent of the peripheral contribution to “central” maturational changes (8). Nevertheless, this association between experimentally induced myelin deficiency and latency prolongation is consistent with brain stem evoked potential findings in patients with clemyelinating diseases. Characteristically, the components which manifest abnormally long latencies correspond to brain stem and thalamic generator-sources within that portion of the acoustic pathway which reveal significant loss of myelin at postmortem examination (14-18). REFERENCES 1. ACHOR,
L. J. 1976.Field-analysis of auditory brainstem
responses.
Neurosci.
Abstr.
2: 12. G. K., AND F. E. BLOOM. 1967.The formationof synapticjunctions in developingrat brain: A quantitative electronmicroscopicstudy. Brain Res.
2. AGHAJIANIAN,
6: 716-727. 3. BARCHAS, J., E. ERDELYI,
AND P. ANGWIP~. 1972.Simultaneousdeterminationof of indoleand catecholamines in tissuesusinga weakcation-exchange resin.Anal.
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