Brain Research, 598 (1992) 307-310
307
© 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05.00 BRES 25455
Perinatal cocaine exposure and functional brainstem development in the rat Alan Salamy, Kathleen Dark, Mary Salfi, Shantilal Shah and Harman V.S. Peeke University of California, San Francisco, Brain Behavior Research Center, Sonoma Development Center, Eldridge, CA 95431 (USA)
(Accepted 15 September 1992)
Key words: Cocaine exposure; Auditory brainstem response; Brainstem transmission time; Early development
Rat pups exposed to cocaine via maternal intromission throughout gestation and lactation displayed significantlyprolonged auditory brainstem response component latencies and interwave intervals. Longitudinal analysis revealed that this effect was most pronounced on the 22nd postnatal day. Increasing the rate of stimulation further impaired neurosensory transmission in the caudal auditory pathway. These results indicate that both axonal and synaptic events may be affected to some degree and the timing (age) of optimum cocaine influence suggests that delayed myelination may be involved. The corresponding retardation in general development of the cocaine exposed pups further implicates maternal, fetal and postnatal utilization of nutritional resources as the basis for this outcome. Exposure to cocaine in utero has been shown to alter sensory transmission through the caudal acoustic pathway. Prolonged p e a k latencies and interwave intervals of the auditory brainstem response (ABR) have been reported in both animal and man 3'6't7'21. This effect appears to be transitory, dissipating in early postnatal life. In the human neonate, normalization of the A B R occurred by 3 months of age 17. However, the lack of precise information regarding the dose, duration and frequency of cocaine use, as well as the high potential for confounding from a variety of difficult to manage variables such as nutrition, prenatal care, socioeconomic factors, poly-drug abuse, etc. serve to limit interpretation of h u m a n cocaine results 9'17. Differences in the A B R ' s of juvenile rats prenatally exposed to cocaine and controls have also been reported. Significantly delayed responses observed around 60 days of age did not persist into adulthood despite the relatively large dosages of cocaine required to produce minimal effects 3. Taken together with human findings, these results suggest that prenatal cocaine exposure may disrupt or interfere with certain maturational processes underlying various latency measures of the ABR. The purpose of this research was to further explore brainstem function, as reflected in the
ABR, during the course of development following intrauterine and early postnatal cocaine exposure. Two female Long-Evans, hooded rats were randomly selected from an ongoing investigation of maternal and offspring variables associated with perinatal cocaine administration. They were separately housed in plastic tub cages with hardwood shavings. One animal received daily injections of cocaine hydrochloride dissolved in saline. Based on the work of others 4'7'22, we chose a dose of 50 m g / k g which was anticipated to produce maternal and fetal effects without causing severe congenital abnormalities or death. The drug regimen began 21 days prior to the introduction of a male, during mating and throughout gestation and lactation. With the exception of the cocaine injections, the experimental and control animals received equivalent amounts of handling and both had ad lib access to food and water. On the first day of drug administration and continuing until weaning, food and water intake as well as body weight was determined for each animal. U p o n delivery, the respective litters of 12 (drug exposed) and 13 (drug free) were culled to eight with preference given to four males and four females and secondarily to the elimination of very large or small pups.
Correspondence: A. Salamy, University of California, Brain Behavior Research Center, Sonoma Developmental Center, Eldridge, CA 95431,
USA.
308 At 15, 22 and 29 days of age, ABR's were recorded from needle electrodes inserted subcutaneously at the vertex of the scalp and the pinna of the left ear under ketamine anesthesia (87 mg/kg, 13 m g / k g xylazine). Animals were placed in an electrically shielded chamber (12" x 14" × 8") and temperature was maintained with a heating pad. Biopotentials were amplified (200,000), filtered (100-3,000 Hz) and digitized (20 kHz) using the Synap Averaging System (Gestron Engineering). The stimulus consisted of brief clicks (0.1 ms) at an intensity of 65 dB n H L delivered to a TDH-39 transducer positioned 12" above the head. Two averages of 500 trials were sequentially obtained at stimulus rates of 16, 54 and 92/s. The latency of the first four positive waves ( I - I V ) were measured, in milliseconds, from stimulus onset. An estimate of brainstem transmission was derived from the I V - I interwave interval. In addition, segmental transmission between the successive relay stations, i.e., II-I, III-I, H I - I I , IV-II and IV-III, were determined using the computer cursor routine. At birth and again at each ABR recording session the weight of the individual pups was recorded. Statistical treatment included two-way repeated measures analysis of variance (group by rate) with specific contrasts (t-tests for independent groups) for the absolute latencies and interwave intervals of the ABR as well as weight across age. In addition, t-tests were used to compare maternal body weight as well as food and water intake over time (before mating, during gestation and after parturition until weaning). Correlation coefficients were also computed for brainstem transmission time (IV-I), birthweight and the test weight of the pups at 15, 22 and 29 days.
The results of the analyses of variance (ANOVA) revealed that the preponderance of significant group effects occurred at 22 days of age. This finding held for virtually all interwave intervals (II-I, I I l - l , IV-I, I V II) and all of the absolute latencies except wave 1. There were essentially no statistical differences at 15 days, whereas only the IV-I, I V - I I and IV-III intervals distinguished the groups at 29 days. On all recording occasions, there was a significant rate effect but the interaction term (group-by-rate) exceeded the 5% level only at 22 days and exclusively for waves II and III and the I I - I and I I I - I intervals. Table I provides the mean, standard deviation and probability value for the cocaine exposed and control pups across stimulus rate at 22 days of age. Fig. 1 depicts brainstem transmission time (BTT) as measured by the I V - I interwave interval as a function of age at the rate of 16/s. Comparisons (ANOVA) of body weight between the drug exposed and drug free pups disclosed significant differences ( F = 55.05, P < 0.001) at birth, as well as on each test day with the experimental animals consistently smaller (i.e., 85, 70, 68, 78% of control weight at birth, 15, 22 and 29 days, respectively). BTT showed a moderate, inverse correlation with test weight only at 22 ( r = - 0 . 6 6 , P < 0 . 0 0 8 ) and 29 days ( r = - 0 . 7 4 , P < 0.002). From the outset, the cocaine-treated dam was slightly smaller than the control dam but both fell within 1 standard deviation of the population from which they were drawn. In the 21 day pre-pregnancy period the weight of the two animals progressively diverged (t = 15.89, P < 0.001, Fig. 2, top). During this phase of the study, the ratio of food and water intake to body weight revealed that the control dam con-
TABLE 1
Probability values based on overall F-ratio and independent groups t-test Wat~e
Subject
Stimulus rate 16/s
I II III IV II-I * III-1 * IV-I
control cocaine control cocaine control cocaine control cocaine control cocaine control cocaine control cocaine
54/s
92/s
Mean/S.D.
P
Mean/S.D.
P
Mean/S.D.
P
1.66 1.72 2.70 2.86 3.59 3.91 4.63 4.99 1.05 1.14 1.93 2.19 2.98 3.26
n.s.
1.68/0.05 1.75/0.08 2.76/0.22 3.01/0.13 3.74/0.26 4.11/0.16 4.87/0.25 5.29/0.22 1.09/0.24 1.26/0.10 2.06 '0.24 2.35/0.10 3.23/0.21 3.54/0.16
n.s.
!.69/0.04 1.75/0.08 2.88/0.24 3.18/0.15 3.76/0.29 4.24/0.17 5.14/0.32 5.47/0.23 1.19/0.21 1.43/0.21 2.07/0.26 2.49/0.10 3.43/0.29 3.72/0.16
n,s.
/0.05 /0.07 /0.21 /0.08 /0.21 /0.14 /0.16 /0.19 /0.12 /0.05 /0.20 /0.09 /0.12 /0.15
* Indicates significant rate by group interaction.
n.s. < 0.005 < 0.002 < 0.04 < 0.006 < 0.001
< 0.02 < 0.006 < 0.004 < 0.04 < 0.01 < 0.01 < 0.003
< 0.01 < 0.004 < 0.05 < 0.02 < 0.005 < 0.04
309 BRAINSTEM TRANSMISSION TIME 4.5
E
T
4.0
["o control °°c°'°°
3.5 c
T >
3.0
2.5 I
I
15 Days
I
22 Days
I
29 Days
AGE Fig. 1. Mean and standard deviation of the I V - I interwave interval (Brainstem transmission time) in cocaine exposed and control rat pups as a function of age. Only at 22 and 29 days were the groups significantly different ( F = 17.7, P < 0.001 and F = 5.5, P < 0.03, respectively).
Body Weight 450 400 550 300 250
Food
Consumption
,!
120 100
i gestation
80 60 40 20 ,
J
,
,
,
,
,
,
,
i
,,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
. . . .
Water Consumption
120 ~ IO0 8O
~ ]
6o
4O
i
20 .......... 0 10
~ i. . . . . . . . . ! .......... 20 30 40 50 60 Days
i I
Fig. 2. Body weight, food and water intake for cocaine treated and control dams 21 days before pregnancy, 21 days during gestation and 20 days following parturition. There were no differences in the f o o d / w e i g h t ratio from the onset of pregnancy until weaning.
sumed more food (t = 3.01, P < 0.002) whereas the cocaine dam ingested more water (t = 3.33, P < 0.001). The differential in water consumption persisted through gestation, otherwise there were no differences in food intake during pregnancy or following parturition until weaning (Fig. 2, middle, bottom). These findings were representative of 29 other rats assessed in this manner for other purposes. The sharp decrease in ABR latencies and interwave intervals in early postnatal life (Fig. 1) has been attributed to maturation of conductile processes, particularly that of myelogenesis15. Neuropathology or pharmacological manipulation resulting in impaired myelination has been shown to alter all ABR latency parameters 1'14'18. The 'quaking mouse', characterized by a specific disturbance in myelin metabolism, exhibits markedly prolonged BTT and absolute latencies 2°. A strong correlation between BTT and cerebroside concentration in the inferior colliculi has been demonstrated in the developing rat 19. It would appear that the link between ABR latency maturation and the deposition of myelin in brainstem structures is well established. Undernutrition during development has also been implicated in delayed evoked potential latency measures m~3. Retarded ABR's were evident in malnourished weanling rats between 14 and 22 days of age but not at 33 dayss. Severe food deprivation, however, was shown to influence the ABR out to 43 days". Interference with the proliferation of oligodendroglia, the cells responsible for the formation of myelin, has been postulated as the mechanism for impaired ABR ontogenesis associated with malnutritions. In the present study, cocaine exposure throughout gestation and the suckling period significantly increased BTI" and peak latencies. The effects were most pronounced at 22 days and to a lesser extent at 29 days (Fig. 1). It was noted that the eyes and ears for all pups were open by the 14th day. Although myelin may begin to accumulate in the rat brain as early as 12-15 days of age, the maximum rate of ensheathment occurs around the 20th postnatal day 1°. This is also about the time that the synthesis of cerebroside, which is exclusively a myelin lipid, approaches asymptote5. Cocaine exposure during pregnancy and lactation, at dosages similar to ours, reportedly reduced the myelin content in rat brain by 10% on the 20th day 23. These investigators point out that fetal cocaine exposure exerts a greater affect on postnatal myelination then does postnatal cocaine exposure. Since our experimental pups were significantly smaller from birth to weaning, the possibility of hypomyelination secondary to undernutrition cannot be ruled out. The finding that the cocaine
310
treated dam in this study ingested comparable quantities of food (food/weight ratio) during pregnancy and lactation relative to the control animal intimates that differential utilization of resources and not just the amount of nutrients per se must also be considered. There were no statistical differences in any ABR measures at 15 days. However, test weight was significantly correlated with BTT at 22 and 29 days, explaining approximately 44-55% of the variance. This relationship may reflect the continued influence of cocaine on neurophysiologic maturation through the suckling period in terms of either quality or quantity of available milk. The significant weight by age interaction lends support to this view by showing that the rate of growth was much slower for the experimental animals until weaning when some recovery began to take place (29 days). The greatest disparity in weight occurred at 22 days. Increasing the stimulus repetition rate has been said to 'stress' the system thereby modifying synaptic efficacy, i.e. available transmitter at pre- and post-synaptic terminals and synchronicity of quanta release 12'14'16. Changing the rate of stimulation from 16 to 54 and 92/s progressively lengthened all component latencies, except wave I, as well as segmental BTT (Table I). The interaction between rate and group (cocaine, control) reached significance only for the II-I and III-I interwave intervals indicating that cocaine may also act on certain synaptic elements in the lower brain stem (cochlea and olivary nuclei) during development. Our findings extend previous research in this area by taking into account maternal, fetal and neonatal nutritional factors during pregnancy and lactation and recording functional brainstem activity during the postnatal period most critical to brain development in the rat. This work was supported by Biomedical Research Support Grant No. S07-RR05755. 1 Amochaev, A., Johnson, R., Salamy, A. and Shah, S., Brainstem auditory evoked potentials and myelin changes in triethyltin-induced edema in young adult rats, Exp. Neurol., 66 (1979) 629-635. 2 Callison, D. and Spencer, J., Effect of chronic undernutrition a n d / o r visual deprivation upon the visual evoked potential from the developing rat brain, Dev. Psychobiol., 1 (1968) 196-204. 3 Church, M. and Overbeck, G., Prenatal cocaine exposure in the Long-Evans rat. III. Developmental effects on the brainstem auditory-evoked potential, Neurotoxicol. Teratol., 12 (1990) 345351.
4 Church, M., Overbeck, G. and Andrzejzak, A., Prenatal cocaine exposure in the Long-Evans rat. 1. Dose-dependent effects ~m gestation, mortality and postnatal maturation. Neurotoxi~ol. Tera tol., 12 (1990) 327 334. 5 Davison, A., The biochemistry of myelin sheath. In A. Davisun and A. Peters (Eds.), Myelination, Thomas Books, New York. 1970, pp. 80-161. 6 Church, M.W. and Overbeck, W.O., Sensorineural hearing loss as evidenced by the auditory brainstem response following prenatal cocaine exposure in the Long-Evans rat, Teratology, 43 (1991) 561-57(). 7 Fantel, A. and Macphail, B., The tetatogenicity of cocaine, Teratology, 26 (1982) 17-19. 8 Kawai, S., Nakamura, H. and Matsuo, T. Effects of early undernutrition on brainstem auditory evoked potentials in weanling rats, Biol. Neonatol., 55 (1989) 268-274. 9 Mayes, L., Granger, R., Bornstein, M. and Zuckerman, B., The problem of prenatal cocaine exposure, a rush to judgement, J. Am. Med. Ass., 267 (1992) 406-408. 10 Norton, W. and Poduslo, S., Myelination in rat brain: changes in myelin composition during brain maturation, 3. Neurochem., 21 (1973) 759-773. 11 Plantz, R., Willison, J. and Jewett, D., Effects of undernutrition on development of far field auditory brainstem responses in rat pups, Brain Res., 213 (1981) 319-326. 12 Pratt, H., Ben-David, Y., Peled, R., Podoshin, L. and Scbarf, B., Auditory brainstem evoked potentials: clinical promises of increasing stimulus rate, Electroencephalogr. Clin. Neurophysiol., 51 (19811 80-90. 13 Puthuraya, K., Nayar, U., Deo, M. and Manchanda, S., Effects of undernutrition on the visually evoked responses in rats during development, Det:. Neurosci., 3 (1980) 162-173. 14 Robinson, K. and Rudge, P., Abnormalities of the auditory evoked potentiation patients with multiple sclerosis, Brain, 10(1 (1977) 19-40. 15 Salamy, A., Eggermont, J. and Eldredge, L, Neurodevelopment and auditory function in preterm infants. In J. Jacobson (Ed.), Auditory Et,oked Potentials, Allyn and Bacon, Boston, in press. 16 Salamy, A. and Eldredge, L., Functional brain development in healthy and high risk infants. In P. Ackles, J. Jennings and M. Coles (Eds.), Aduances in Psychophysiology, Jai Press, Greenwich, 1985, pp. 219-257. 17 Salamy, A., Eldredge, L., Anderson, J. and Bull, D., Brain-stem transmission time in infants exposed to cocaine in utero, J. Pediatr., 117 (1990) 627-629. 18 Shah, S., Bhargava, V., Johnson, R. and McKean, C., Latency changes in brainstem auditory evoked potentials associated with impaired brain myelination, Exp Neurol., 58 (1978) 111-118. 19 Shah, S., Bhargava, V. and McKean, C., Maturational changes in early auditory evoked potentials and myelination of the inferior colliculus in rats, Neuroscience, 3 (19781 561-563. 20 Shah, S. and Salamy, A., Auditory evoked far field potentials in myelin-deficient mutant quacking mice, Neuroscience, 5 (1980) 2321-2323. 21 Shih, L., Cone-Wesson, B. and Reddix, B., Effects of maternal cocaine abuse on the neonatal auditory system, Int. J. Pediatr. Otorhinolaryngol., 15 (1988) 245-251. 22 Wiggins, R. and Ruiz, B., Development under the influence of cocaine. I. A comparison of the effects of daily cocaine treatment and resultant undernutrition on pregnancy and early growth in a large population of rats, Met. Brain Dis., 5 (1990) 85-99. 23 Wiggins, R. and Ruiz, B., Development under the influence of cocaine. II. Comparison of the effects of maternal cocaine and associated undernutrition on brain myelin development in the offspring, Met. Brain Dis., 5 (1990) 101-109.
307
© 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05.00 BRES 25455
Perinatal cocaine exposure and functional brainstem development in the rat Alan Salamy, Kathleen Dark, Mary Salfi, Shantilal Shah and Harman V.S. Peeke University of California, San Francisco, Brain Behavior Research Center, Sonoma Development Center, Eldridge, CA 95431 (USA)
(Accepted 15 September 1992)
Key words: Cocaine exposure; Auditory brainstem response; Brainstem transmission time; Early development
Rat pups exposed to cocaine via maternal intromission throughout gestation and lactation displayed significantlyprolonged auditory brainstem response component latencies and interwave intervals. Longitudinal analysis revealed that this effect was most pronounced on the 22nd postnatal day. Increasing the rate of stimulation further impaired neurosensory transmission in the caudal auditory pathway. These results indicate that both axonal and synaptic events may be affected to some degree and the timing (age) of optimum cocaine influence suggests that delayed myelination may be involved. The corresponding retardation in general development of the cocaine exposed pups further implicates maternal, fetal and postnatal utilization of nutritional resources as the basis for this outcome. Exposure to cocaine in utero has been shown to alter sensory transmission through the caudal acoustic pathway. Prolonged p e a k latencies and interwave intervals of the auditory brainstem response (ABR) have been reported in both animal and man 3'6't7'21. This effect appears to be transitory, dissipating in early postnatal life. In the human neonate, normalization of the A B R occurred by 3 months of age 17. However, the lack of precise information regarding the dose, duration and frequency of cocaine use, as well as the high potential for confounding from a variety of difficult to manage variables such as nutrition, prenatal care, socioeconomic factors, poly-drug abuse, etc. serve to limit interpretation of h u m a n cocaine results 9'17. Differences in the A B R ' s of juvenile rats prenatally exposed to cocaine and controls have also been reported. Significantly delayed responses observed around 60 days of age did not persist into adulthood despite the relatively large dosages of cocaine required to produce minimal effects 3. Taken together with human findings, these results suggest that prenatal cocaine exposure may disrupt or interfere with certain maturational processes underlying various latency measures of the ABR. The purpose of this research was to further explore brainstem function, as reflected in the
ABR, during the course of development following intrauterine and early postnatal cocaine exposure. Two female Long-Evans, hooded rats were randomly selected from an ongoing investigation of maternal and offspring variables associated with perinatal cocaine administration. They were separately housed in plastic tub cages with hardwood shavings. One animal received daily injections of cocaine hydrochloride dissolved in saline. Based on the work of others 4'7'22, we chose a dose of 50 m g / k g which was anticipated to produce maternal and fetal effects without causing severe congenital abnormalities or death. The drug regimen began 21 days prior to the introduction of a male, during mating and throughout gestation and lactation. With the exception of the cocaine injections, the experimental and control animals received equivalent amounts of handling and both had ad lib access to food and water. On the first day of drug administration and continuing until weaning, food and water intake as well as body weight was determined for each animal. U p o n delivery, the respective litters of 12 (drug exposed) and 13 (drug free) were culled to eight with preference given to four males and four females and secondarily to the elimination of very large or small pups.
Correspondence: A. Salamy, University of California, Brain Behavior Research Center, Sonoma Developmental Center, Eldridge, CA 95431,
USA.
308 At 15, 22 and 29 days of age, ABR's were recorded from needle electrodes inserted subcutaneously at the vertex of the scalp and the pinna of the left ear under ketamine anesthesia (87 mg/kg, 13 m g / k g xylazine). Animals were placed in an electrically shielded chamber (12" x 14" × 8") and temperature was maintained with a heating pad. Biopotentials were amplified (200,000), filtered (100-3,000 Hz) and digitized (20 kHz) using the Synap Averaging System (Gestron Engineering). The stimulus consisted of brief clicks (0.1 ms) at an intensity of 65 dB n H L delivered to a TDH-39 transducer positioned 12" above the head. Two averages of 500 trials were sequentially obtained at stimulus rates of 16, 54 and 92/s. The latency of the first four positive waves ( I - I V ) were measured, in milliseconds, from stimulus onset. An estimate of brainstem transmission was derived from the I V - I interwave interval. In addition, segmental transmission between the successive relay stations, i.e., II-I, III-I, H I - I I , IV-II and IV-III, were determined using the computer cursor routine. At birth and again at each ABR recording session the weight of the individual pups was recorded. Statistical treatment included two-way repeated measures analysis of variance (group by rate) with specific contrasts (t-tests for independent groups) for the absolute latencies and interwave intervals of the ABR as well as weight across age. In addition, t-tests were used to compare maternal body weight as well as food and water intake over time (before mating, during gestation and after parturition until weaning). Correlation coefficients were also computed for brainstem transmission time (IV-I), birthweight and the test weight of the pups at 15, 22 and 29 days.
The results of the analyses of variance (ANOVA) revealed that the preponderance of significant group effects occurred at 22 days of age. This finding held for virtually all interwave intervals (II-I, I I l - l , IV-I, I V II) and all of the absolute latencies except wave 1. There were essentially no statistical differences at 15 days, whereas only the IV-I, I V - I I and IV-III intervals distinguished the groups at 29 days. On all recording occasions, there was a significant rate effect but the interaction term (group-by-rate) exceeded the 5% level only at 22 days and exclusively for waves II and III and the I I - I and I I I - I intervals. Table I provides the mean, standard deviation and probability value for the cocaine exposed and control pups across stimulus rate at 22 days of age. Fig. 1 depicts brainstem transmission time (BTT) as measured by the I V - I interwave interval as a function of age at the rate of 16/s. Comparisons (ANOVA) of body weight between the drug exposed and drug free pups disclosed significant differences ( F = 55.05, P < 0.001) at birth, as well as on each test day with the experimental animals consistently smaller (i.e., 85, 70, 68, 78% of control weight at birth, 15, 22 and 29 days, respectively). BTT showed a moderate, inverse correlation with test weight only at 22 ( r = - 0 . 6 6 , P < 0 . 0 0 8 ) and 29 days ( r = - 0 . 7 4 , P < 0.002). From the outset, the cocaine-treated dam was slightly smaller than the control dam but both fell within 1 standard deviation of the population from which they were drawn. In the 21 day pre-pregnancy period the weight of the two animals progressively diverged (t = 15.89, P < 0.001, Fig. 2, top). During this phase of the study, the ratio of food and water intake to body weight revealed that the control dam con-
TABLE 1
Probability values based on overall F-ratio and independent groups t-test Wat~e
Subject
Stimulus rate 16/s
I II III IV II-I * III-1 * IV-I
control cocaine control cocaine control cocaine control cocaine control cocaine control cocaine control cocaine
54/s
92/s
Mean/S.D.
P
Mean/S.D.
P
Mean/S.D.
P
1.66 1.72 2.70 2.86 3.59 3.91 4.63 4.99 1.05 1.14 1.93 2.19 2.98 3.26
n.s.
1.68/0.05 1.75/0.08 2.76/0.22 3.01/0.13 3.74/0.26 4.11/0.16 4.87/0.25 5.29/0.22 1.09/0.24 1.26/0.10 2.06 '0.24 2.35/0.10 3.23/0.21 3.54/0.16
n.s.
!.69/0.04 1.75/0.08 2.88/0.24 3.18/0.15 3.76/0.29 4.24/0.17 5.14/0.32 5.47/0.23 1.19/0.21 1.43/0.21 2.07/0.26 2.49/0.10 3.43/0.29 3.72/0.16
n,s.
/0.05 /0.07 /0.21 /0.08 /0.21 /0.14 /0.16 /0.19 /0.12 /0.05 /0.20 /0.09 /0.12 /0.15
* Indicates significant rate by group interaction.
n.s. < 0.005 < 0.002 < 0.04 < 0.006 < 0.001
< 0.02 < 0.006 < 0.004 < 0.04 < 0.01 < 0.01 < 0.003
< 0.01 < 0.004 < 0.05 < 0.02 < 0.005 < 0.04
309 BRAINSTEM TRANSMISSION TIME 4.5
E
T
4.0
["o control °°c°'°°
3.5 c
T >
3.0
2.5 I
I
15 Days
I
22 Days
I
29 Days
AGE Fig. 1. Mean and standard deviation of the I V - I interwave interval (Brainstem transmission time) in cocaine exposed and control rat pups as a function of age. Only at 22 and 29 days were the groups significantly different ( F = 17.7, P < 0.001 and F = 5.5, P < 0.03, respectively).
Body Weight 450 400 550 300 250
Food
Consumption
,!
120 100
i gestation
80 60 40 20 ,
J
,
,
,
,
,
,
,
i
,,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
. . . .
Water Consumption
120 ~ IO0 8O
~ ]
6o
4O
i
20 .......... 0 10
~ i. . . . . . . . . ! .......... 20 30 40 50 60 Days
i I
Fig. 2. Body weight, food and water intake for cocaine treated and control dams 21 days before pregnancy, 21 days during gestation and 20 days following parturition. There were no differences in the f o o d / w e i g h t ratio from the onset of pregnancy until weaning.
sumed more food (t = 3.01, P < 0.002) whereas the cocaine dam ingested more water (t = 3.33, P < 0.001). The differential in water consumption persisted through gestation, otherwise there were no differences in food intake during pregnancy or following parturition until weaning (Fig. 2, middle, bottom). These findings were representative of 29 other rats assessed in this manner for other purposes. The sharp decrease in ABR latencies and interwave intervals in early postnatal life (Fig. 1) has been attributed to maturation of conductile processes, particularly that of myelogenesis15. Neuropathology or pharmacological manipulation resulting in impaired myelination has been shown to alter all ABR latency parameters 1'14'18. The 'quaking mouse', characterized by a specific disturbance in myelin metabolism, exhibits markedly prolonged BTT and absolute latencies 2°. A strong correlation between BTT and cerebroside concentration in the inferior colliculi has been demonstrated in the developing rat 19. It would appear that the link between ABR latency maturation and the deposition of myelin in brainstem structures is well established. Undernutrition during development has also been implicated in delayed evoked potential latency measures m~3. Retarded ABR's were evident in malnourished weanling rats between 14 and 22 days of age but not at 33 dayss. Severe food deprivation, however, was shown to influence the ABR out to 43 days". Interference with the proliferation of oligodendroglia, the cells responsible for the formation of myelin, has been postulated as the mechanism for impaired ABR ontogenesis associated with malnutritions. In the present study, cocaine exposure throughout gestation and the suckling period significantly increased BTI" and peak latencies. The effects were most pronounced at 22 days and to a lesser extent at 29 days (Fig. 1). It was noted that the eyes and ears for all pups were open by the 14th day. Although myelin may begin to accumulate in the rat brain as early as 12-15 days of age, the maximum rate of ensheathment occurs around the 20th postnatal day 1°. This is also about the time that the synthesis of cerebroside, which is exclusively a myelin lipid, approaches asymptote5. Cocaine exposure during pregnancy and lactation, at dosages similar to ours, reportedly reduced the myelin content in rat brain by 10% on the 20th day 23. These investigators point out that fetal cocaine exposure exerts a greater affect on postnatal myelination then does postnatal cocaine exposure. Since our experimental pups were significantly smaller from birth to weaning, the possibility of hypomyelination secondary to undernutrition cannot be ruled out. The finding that the cocaine
310
treated dam in this study ingested comparable quantities of food (food/weight ratio) during pregnancy and lactation relative to the control animal intimates that differential utilization of resources and not just the amount of nutrients per se must also be considered. There were no statistical differences in any ABR measures at 15 days. However, test weight was significantly correlated with BTT at 22 and 29 days, explaining approximately 44-55% of the variance. This relationship may reflect the continued influence of cocaine on neurophysiologic maturation through the suckling period in terms of either quality or quantity of available milk. The significant weight by age interaction lends support to this view by showing that the rate of growth was much slower for the experimental animals until weaning when some recovery began to take place (29 days). The greatest disparity in weight occurred at 22 days. Increasing the stimulus repetition rate has been said to 'stress' the system thereby modifying synaptic efficacy, i.e. available transmitter at pre- and post-synaptic terminals and synchronicity of quanta release 12'14'16. Changing the rate of stimulation from 16 to 54 and 92/s progressively lengthened all component latencies, except wave I, as well as segmental BTT (Table I). The interaction between rate and group (cocaine, control) reached significance only for the II-I and III-I interwave intervals indicating that cocaine may also act on certain synaptic elements in the lower brain stem (cochlea and olivary nuclei) during development. Our findings extend previous research in this area by taking into account maternal, fetal and neonatal nutritional factors during pregnancy and lactation and recording functional brainstem activity during the postnatal period most critical to brain development in the rat. This work was supported by Biomedical Research Support Grant No. S07-RR05755. 1 Amochaev, A., Johnson, R., Salamy, A. and Shah, S., Brainstem auditory evoked potentials and myelin changes in triethyltin-induced edema in young adult rats, Exp. Neurol., 66 (1979) 629-635. 2 Callison, D. and Spencer, J., Effect of chronic undernutrition a n d / o r visual deprivation upon the visual evoked potential from the developing rat brain, Dev. Psychobiol., 1 (1968) 196-204. 3 Church, M. and Overbeck, G., Prenatal cocaine exposure in the Long-Evans rat. III. Developmental effects on the brainstem auditory-evoked potential, Neurotoxicol. Teratol., 12 (1990) 345351.
4 Church, M., Overbeck, G. and Andrzejzak, A., Prenatal cocaine exposure in the Long-Evans rat. 1. Dose-dependent effects ~m gestation, mortality and postnatal maturation. Neurotoxi~ol. Tera tol., 12 (1990) 327 334. 5 Davison, A., The biochemistry of myelin sheath. In A. Davisun and A. Peters (Eds.), Myelination, Thomas Books, New York. 1970, pp. 80-161. 6 Church, M.W. and Overbeck, W.O., Sensorineural hearing loss as evidenced by the auditory brainstem response following prenatal cocaine exposure in the Long-Evans rat, Teratology, 43 (1991) 561-57(). 7 Fantel, A. and Macphail, B., The tetatogenicity of cocaine, Teratology, 26 (1982) 17-19. 8 Kawai, S., Nakamura, H. and Matsuo, T. Effects of early undernutrition on brainstem auditory evoked potentials in weanling rats, Biol. Neonatol., 55 (1989) 268-274. 9 Mayes, L., Granger, R., Bornstein, M. and Zuckerman, B., The problem of prenatal cocaine exposure, a rush to judgement, J. Am. Med. Ass., 267 (1992) 406-408. 10 Norton, W. and Poduslo, S., Myelination in rat brain: changes in myelin composition during brain maturation, 3. Neurochem., 21 (1973) 759-773. 11 Plantz, R., Willison, J. and Jewett, D., Effects of undernutrition on development of far field auditory brainstem responses in rat pups, Brain Res., 213 (1981) 319-326. 12 Pratt, H., Ben-David, Y., Peled, R., Podoshin, L. and Scbarf, B., Auditory brainstem evoked potentials: clinical promises of increasing stimulus rate, Electroencephalogr. Clin. Neurophysiol., 51 (19811 80-90. 13 Puthuraya, K., Nayar, U., Deo, M. and Manchanda, S., Effects of undernutrition on the visually evoked responses in rats during development, Det:. Neurosci., 3 (1980) 162-173. 14 Robinson, K. and Rudge, P., Abnormalities of the auditory evoked potentiation patients with multiple sclerosis, Brain, 10(1 (1977) 19-40. 15 Salamy, A., Eggermont, J. and Eldredge, L, Neurodevelopment and auditory function in preterm infants. In J. Jacobson (Ed.), Auditory Et,oked Potentials, Allyn and Bacon, Boston, in press. 16 Salamy, A. and Eldredge, L., Functional brain development in healthy and high risk infants. In P. Ackles, J. Jennings and M. Coles (Eds.), Aduances in Psychophysiology, Jai Press, Greenwich, 1985, pp. 219-257. 17 Salamy, A., Eldredge, L., Anderson, J. and Bull, D., Brain-stem transmission time in infants exposed to cocaine in utero, J. Pediatr., 117 (1990) 627-629. 18 Shah, S., Bhargava, V., Johnson, R. and McKean, C., Latency changes in brainstem auditory evoked potentials associated with impaired brain myelination, Exp Neurol., 58 (1978) 111-118. 19 Shah, S., Bhargava, V. and McKean, C., Maturational changes in early auditory evoked potentials and myelination of the inferior colliculus in rats, Neuroscience, 3 (19781 561-563. 20 Shah, S. and Salamy, A., Auditory evoked far field potentials in myelin-deficient mutant quacking mice, Neuroscience, 5 (1980) 2321-2323. 21 Shih, L., Cone-Wesson, B. and Reddix, B., Effects of maternal cocaine abuse on the neonatal auditory system, Int. J. Pediatr. Otorhinolaryngol., 15 (1988) 245-251. 22 Wiggins, R. and Ruiz, B., Development under the influence of cocaine. I. A comparison of the effects of daily cocaine treatment and resultant undernutrition on pregnancy and early growth in a large population of rats, Met. Brain Dis., 5 (1990) 85-99. 23 Wiggins, R. and Ruiz, B., Development under the influence of cocaine. II. Comparison of the effects of maternal cocaine and associated undernutrition on brain myelin development in the offspring, Met. Brain Dis., 5 (1990) 101-109.
