0013-7227/91/1282-1100$03.00/0 Endocrinology Copyright © 1991 by The Endocrine Society

Vol. 128, No. 2 Printed in U.S.A.

Neonatal Treatment with Monosodium Glutamate: Effects of Prolonged Growth Hormone (GH)-Releasing Hormone Deficiency on Pulsatile GH Secretion and Growth in Female Rats* DOMINIQUE MAITERf, LOUIS E. UNDERWOOD, JOSEPH B. MARTIN*, AND JAMES I. KOENIG§ Department of Neurology (DM., J.B.M., J.I.K.), Massachusetts General Hospital and Harvard Medical School, Boston Massachusetts 02114; Department of Pediatrics, Division of Pediatric Endocrinology (L.E.U.), University of North Carolina, Chapel Hill North Carolina 27599

G

ABSTRACT. Administration of monosodium glutamate (MSG) to neonatal rodents produces permanent lesions of hypothalamic arcuate neurons that secrete GH-releasing hormone (GHRH). The present study was intended to determine the consequences of GHRH deficiency on the pulsatile GH secretory pattern and growth in MSG-treated female rats and to compare these effects with those observed in male littermates. Male and female rats were injected with MSG [4 mg/g body wt (BW), sc] or saline (controls) on days 2, 4, 6, 8, and 10 after birth. Immunoreactive GHRH concentrations were decreased in the hypothalamus (by 60%) and in the median eminence (by 95%) of adult male and female MSG-treated rats. In contrast, somatostatin concentrations were unaffected. BW and linear growth were severely impaired in male MSG-treated rats, but in MSGlesioned females BW was not different from controls, and the attenuation of longitudinal growth was less severe and the obesity more pronounced than in males. These sex differences occurred despite similar reductions (by 55%) in serum insulinlike growth factor I concentrations in male and female MSGtreated rats. MSG treatment also produced decreases in pituitary wt and GH content (by 60%), independent of sex. Pulsatile GH secretion was studied by serial blood sampling of chronically cannulated, freely moving rats. Plasma GH patterns were analyzed by the PULSAR program. Compared to controls, treatment with MSG led to a marked inhibition (by 90%) of GH

secretion in both sexes. Significant reductions in GH pulse amplitude (-95%) and pulse duration (-62%) were observed in males, whereas pulse amplitude (-85%), pulse frequency (—67%), and baseline GH concentrations (—80%) were markedly reduced in females. The GH responses to an iv bolus injection of rat GHRH (1 Mg/rat) was severely blunted in both male and female MSG-treated rats. This study demonstrates that GHRH deficiency in female rats results in a marked inhibition of GH pulses, as in males, but also causes severe and sex-specific reductions in GH basal secretion and pulse frequency. These observations suggest that hypothalamic GHRH secretion in female rats is more continuous than in males and is a determinant of the elevated interpulse secretion of GH. Moreover, body wt and linear growth are less severely affected by arcutite lesions in female animals, compared to males. These sex-related differences in growth rates may result in part from the tendancy of female MSG-lesioned rats to become more obese than males, and the development of obesity, in turn, may antagonize the factors that tend to slow linear growth. Additionally, the more deleterious consequences of GHRH deficiency in male animals may be due to loss of GH pulses, which appear to promote growth more effectively and which are normally moire prominent in males than females. (Endocrinology 128:1100-1106,1991)

H SECRETION is pulsatile and sexually differentiated in mature rats (1), and the GH secretory

pattern appears to be important for the promotion of growth and the regulation of several liver functions (1, 2). In male rats, high-amplitude pulses occur at regular 3-h intervals, being separated by low trough concentrations of GH (3). This male pattern is governed by the interplay between hypothalamic GH-releasing hormone (GHRH), which is secreted episodically into the hypophyseal-portal circulation and induces the GH pulses (4, 5) and somatostatin (SRIF), which is released out of phase with GHRH and maintains low interpeak GH secretion (5, 6). Secretion of GH is more continuous in female rats (1). The peaks are more frequent and of smaller amplitude, whereas basal GH values are higher

Received July 24,1990. Address all correspondence and requests for reprints to: Dr. Dominique Maiter, Unite de Diabetologie et Nutrition, University of Louvain 54.74, Avenue Hippocrate 54, B-1200 Brussels, Belgium. * This work was supported by NIH Grants DK-26252, DK-39251, and HD-08299. t Research Assistant of the National Foundation for Scientific Research, and recipient of NIH International Fogarty Award TW-03984. t Current address: Office of the Dean, School of Medicine, University of California, 513 Parnassus Avenue, San Francisco, California 94143. § Current address: Department of Physiology and Biophysics, Georgetown University School of Medicine, 3900 Reservoir Road NW, Washington, DC 20007.

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GHRH DEFICIENCY IN FEMALE RATS

compared to males. The patterns of hypothalamic GHRH and SRIF release leading to this female type of GH secretion have not been directly investigated. Previous observations suggest, however, that less GHRH and SRIF are secreted in female rats (7-9) and that these peptides are released in a more continuous fashion (1012) than in males. Because injection of monosodium glutamate (MSG) into neonatal rodents causes specific lesions in the hypothalamic arcuate nuclei (13, 14) and destroys most of the GHRH-producing neurones (15, 16), MSG-treated male rats have been used as a model to study GHRH deficiency (10, 17, 18). In the present work we have studied the effects of MSG in female rats, and we have determined in this gender the effects of a chronic GHRH deficiency on the pulsatile secretion of GH and growth. By comparing our results with those obtained from male rats, we infer the mode of GHRH secretion in female rats and its physiological importance.

Materials and Methods Animals and experimental procedures Neonatal Sprague-Dawley rats (Charles River, Wilmington, MA) received a sc injection of MSG [4 mg/g body wt (BW)] or hyperosmotic saline (0.01 ml 10% sodium chloride/g BW; controls), on days 2, 4, 6, 8, and 10 of life. At 23 days of age, the pups (8-10 per group) were weaned, sexed, and group-housed according to sex and treatment. The rats were maintained under controlled conditions of lighting (lights on from 07001900 h) and temperature (22 ±1C), with free access to standard rat chow pellets and tap water. BW were recorded weekly throughout the experiment. At 12 weeks of age, 6 animals of each group had a chronic intraatrial Silastic catheter inserted under pentobarbital anesthesia (males: 50 mg/kg, ip; females: 35 mg/kg, ip), as previously described (3). After surgery, the animals were housed individually, handled daily to minimize stress, and adapted to isolated blood sampling cages for 48 h before bleeding. Catheters were kept patent by flushing with 0.2 ml heparinized saline (50 IU/ml) every other day. After recovery of preoperative BW (6-8 days), blood samples (0.5 ml) were collected every 15 min from 1000-1600 h. Plasma, was separated immediately by centrifugation and stored at -20 C until being assayed for rat GH (rGH). The blood cells were resuspended in 0.3 ml heparinized saline (30 IU/ml) and returned to the animals after the next sampling. At the end of the sampling period (1600 h), 1 ng rat GHRH (rGHRH; Peninsula Laboratories, Belmont, CA) was injected iv, and blood samples were withdrawn 10, 20, and 30 min after the injection. Animals were killed by decapitation at 14 weeks of age. Tail length was measured from the anal margin to the tip except in those MSG-treated rats who showed evidence of self-mutilation. Tibial length was determined by x-rays of the right leg. Trunk blood was collected in glass tubes for subsequent determinations of serum insulin-like growth factor I (IGF-I) concentration. Brains were immediately removed from the skull and placed on wet ice for microdissection of the median eminence

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(ME) (7). The remaining brain tissue was frozen on dry ice, and the hypothalamus was dissected, using as boundaries the optic chiasm, the mammillary bodies, the hippocampal sulci, and the anterior commissure, at a depth of 2 mm from the ventral surface. Anterior pituitaries were also removed and placed in 1 ml ice-cold borate buffer, pH 9.0. Peptide extraction Our procedures for neuropeptide extraction have been described previously (7, 19). Briefly, hypothalami and ME were boiled for 20 min in 2.0 N acetic acid (1.0 and 0.2 ml, respectively). Aliquots of the supernatants were lyophilized and stored at -40 C until being reconstituted in assay buffer for measurement of GHRH and SRIF concentrations. Using this procedure, recoveries of the peptides exceeded 90% (7). Anterior pituitaries were sonicated in 1.0 ml borate buffer and frozen for later assay of rGH concentration. Assays Hypothalamic and ME concentrations of GHRH were determined by a specific RIA, described and validated previously (7, 19). The sensitivity of this RIA is 3 pg/tube and intra- and interassay coefficients of variation (CVs) are 6.7% and 12.5%, respectively. SRIF concentrations were measured by a RIA having a sensitivity of 4 pg/tube and intra- and interassay CVs of 3.5% and 8.7%, respectively. Measurements of plasma and pituitary rGH concentrations were made using materials provided by the National Hormone and Pituitary Program (NIH, Bethesda, MD). Results were expressed in terms of the RP-2 standard. Values above the range of the assay were reassayed with appropriate dilution. Serum IGF-I concentrations were measured by RIA on extracted serum samples, after removal of the binding proteins by chromatography on ODC silicic acid cartridges (Sep Pak, Waters, Milford, MA) (20). The standard for this assay was a purified human plasma-derived IGF-I (PS III) and intra- and interassay CVs were 6.0% and 6.2%, respectively. Tissue protein content was determined by the method of Lowry etal. (21). Statistical analyses Significant differences between groups were determined using two-way analysis of variance (ANOVA), followed by the Student-Newman-Keuls test when interaction between sex and MSG treatment was significant (22). The pulsatile patterns of GH secretion were analyzed using the PULSAR program (23). Exclusion criteria for pulse identification were 3.8, 2.6,1.9, 1.5, and 1.2 within-assay SD for pulses with a duration of 1, 2, 3, 4, and 5 time points, respectively. The within-assay SD was determined by assaying in quadruplets five samples with hormone concentrations distributed across the assay range. The linear regression equation relating the SD (y) to hormonal concentration (x) was: y = 0.11 + 0.08 x. The peak splitting cut-off value was assigned as 6 SD, in order to consider the large multiphasic GH surges as single secretory phenomena, rather than separate peaks. The amount of GH released after rGHRH injection was determined by calculation of the area under the curve. The parameters related to GH secretion were not normally distrib-

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GHRH DEFICIENCY IN FEMALE RATS

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uted, except for pulse frequency, and were log-transformed before statistical analysis. Values are shown as the mean ± SE, unless otherwise noted.

Results Treatment of male neonatal rats with MSG produced severe stunting in BW growth, becoming significant between 4-6 weeks of age and reaching a maximum of -17% after 12 weeks of age (Fig. 1). In contrast, BW of female MSG-treated rats was identical to that of the female controls throughout the experiment. Longitudinal growth was significantly impaired by neonatal MSG treatment in both sexes (Table 1). In our experiment, however, both tail length and tibial length were affected more severely in adult MSG-treated male rats (—15% and —11%, respectively) than in females (—11% and

—5%, respectively), as indicated by significant interactions between sex and MSG effects on tail length (P < 0.05) and tibial length (P < 0.01). MSG treatment caused profound reductions in hypothalamic and ME GHRH concentrations, and these reductions were identical in male and female rats (Table • o A A

400 -

Control males MSG-lreated males Conirol females MSG-treated females

/A"

S 300 i

C3 LU

i

200 -

i

Q

100 i

O CD

i

8

10

12

14

AGE (weeks) FIG. 1. BW curves of male and female rats injected with MSG (4 mg/ g) or hyperosmotic saline (controls) on days 2, 4, 6, 8, and 10 of life. Data are shown as the mean ± SD of 8-10 animals per group. *, P < 0.05; ***, P < 0.001, us. sex-matched controls. TABLE 1. Body wt, tail length, and tibial length in adult male and female rats treated neonatally with MSG or saline (controls) Groups Control males MSG-treated males Control females MSG-treated females

Body wta (g) 366 ± 304 ± 225 ± 224 ±

31 32C 15 31

Tail length' (mm) 194 ± 164 ± 176 ± 156 ±

3 9C 7 8C

Tibial length6 (mm) 39.2 ± 35.0 ± 34.6 ± 32.9 ±

1.0 1.2' 0.9 0.9c

Values are the mean ± SD of 8-10 animals per group. ° Values recorded at 12 weeks of age, before surgical catheter implantation. 6 Values recorded at the time of death (14 weeks of age). c P < 0.001 us. sex-matched controls.

Endo • 1991 Voll28«No2

2). The decrease of total GHRH content in the hypothalamus-ME axis reached 92% in 14-week-old male MSG-treated rats (0.40 ± 0.04 ng vs. 5.06 ± 0.32 ng in male controls; P < 0.001) and 90% in 14-week-old female MSG-treated rats (0.38 ± 0.03 ng vs. 3.97 ± 0.19 ng in female controls; P < 0.001). In both sexes, the GHRH depletion was more severe in the ME (—95%) than in the hypothalamic nuclei (—60%). As we have reported previously (7,19), GHRH concentrations were greater in both brain regions of male controls, compared to female controls. In contrast, hypothalamic and ME concentrations of SRIF were not affected by either sex or neonatal injections of MSG (Table 2). Typical pulsatile patterns of GH secretion were observed in male and female control rats (Fig. 2). Neonatal MSG treatment resulted in a marked inhibition of pulsatile GH secretion in both sexes at adulthood (Fig. 2), albeit with different characteristics, as indicated by PULSAR analysis of the individual GH secretory patterns in each experimental group (Fig. 3). In male MSGlesioned animals, the large multiphasic GH surges were reduced to monophasic spikes, with a 95% decrease in pulse height (increment over the baseline) and a 63% decrease in pulse duration (30 ± 10 vs. 80 ± 13 min in controls; P < 0.01). In females, in addition to reductions in pulse amplitude (by 85%) and pulse frequency (by 67%), a significant 80% decrease in baseline GH secretion was observed. Pulse duration was, however, not affected by MSG in female rats (30 ± 12 vs. 45 ± 10 min in controls; P > 0.05). Finally, a similar 85-90% inhibition of mean GH secretion resulted from MSG lesions, regardless of the sex of the animals. Pituitary GH concentrations were not significantly affected by MSG lesions, regardless of the sex of the animals (Table 3). The content of GH in the whole pituitary was decreased by 60% after neonatal MSG administration in both male (145 ± 20 vs. 430 ± 42 ng in controls; P < 0.001) and female rats (110 ± 19 vs. 281 ± 20 ng in controls; P < 0.001), paralleling decreases in anterior pituitary protein (Table 3). Furthermore, serum concentrations of IGF-I were reduced by 55% in both male and female MSG-treated rats (Table 3). These decreases seemed somewhat modest, in light of the severe attenuation of pulsatile GH secretion in these animals. Intravenous injection of rGHRH (1 /ig/rat) produced a rapid stimulation of GH release in control animals, with a greater response in males than in females (integrated GH response over 30 min: 4.34 ± 0.58 and 2.89 ± 0.75 fig/mm-ml, respectively; P < 0.01) (Fig. 4). After neonatal MSG, the GH response to GHRH was significantly blunted in both male and female adult animals (0.78 ± 0.33 and 0.52 ± 0.28 ^g/min-ml, respectively; P < 0.001 vs. sex-matched controls). Despite this severe reduction in absolute GH response, GHRH injection

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GHRH DEFICIENCY IN FEMALE RATS

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TABLE 2. Concentrations of GHRH and SRIF in the ME and hypothalamus (without the ME) of adult male and female rats treated neonatally with MSG or saline (controls) Groups

Hypothalamic GHRH concentration (pg/mg protein)

ME GHRH concentration (ng/mg protein)

Control males MSG-treated males Control females MSG-treated females

93 ± 8 35 ±2° 75 ± 7* 31 ±3°

269 ± 19 14 ± 2 " 226 ± 236 14 ± 2 "

Hypothalamic SRIF concentration (ng/mg protein) 6.2 ± 5.7 ± 5.9 ± 5.6 ±

ME SRIF concentration (ng/mg protein)

0.2 0.2 0.3 0.4

3420 ± 3090 ± 3910 ± 3450 ±

190 205 185 460

Values are the mean ± SE of 8-10 animals per group. ° P < 0.001 us. sex-matched controls. * P < 0.05 us. control males. GH PULSE HEIGHT (ng / ml)

MSG-treated male |

n

5

GH PULSE FREQUENCY ( nb / 6 hrs)

Controls MSG-treated

|MSG-lfcated lemale]

n

*

• MALES

10:00

13:00

16:00

10:00

13:00

FEMALES

FEMALES

16:00

CLOCK TIME (hours) 12.5

FIG. 2. Representative plasma GH profiles in individual 12-week-old male and female adult rats treated neonatally with saline (controls; left panels) or MSG (right panels) as explained in the legend of Fig. 1. GH

10.0

peaks identified by the PULSAR analysis are indicated by single or connected arrows. Plasma GH values greater than 100 ng/ml are shown in parentheses.

7.5

induced comparable 20- to 30-fold increases of GH over baseline levels in both MSG-treated and control animals.

Discussion Injection of MSG in neonatal rats produces selective lesions of the arcuate nuclei, resulting in a 70-90% destruction of neuronal cell bodies (13-16), including most of the GHRH-producing neurons (15-18). This causes almost complete disappearance of GHRH immunoreactive fibers in the ME of male MSG-treated animals (16) and markedly diminished GH secretion (17, 24). On the other hand, SRIF-producing neurons in the anterior hypothalamic periventricular nucleus are preserved in MSG-treated rats, as are the SRIF-containing nerve terminals in the ME (16, 25). Concentrations of SRIF in the hypothalamus of MSG-treated animals have been reported to be either normal (18, 26) or decreased (24). In keeping with these findings, we have observed marked reductions of GHRH immunoreactivity, but no significant change of immunoreactive SRIF concentra-

MEAN GH SECRETION (ng / ml)

BASELINE GH (ng / ml)

T 1

5.0 -

2.5 -

n

i

• ••

i

FIG. 3. Characteristics of the GH secretory pattern in 12-week-old male and female adult rats treated neonatally with saline (controls; open bars) or MSG-treated (hatched bars). Experimental details are as indicated in Fig. 1. Data were obtained from the PULSAR analysis of individual plasma GH profiles and are shown as the mean ± SE of 5-6 rats per group. Pulse frequency was calculated as the number (nb) of peaks occurring during the 6-h sampling period. *, P < 0.05; ***, P < 0.001, vs. sex-matched controls. A, P < 0.05; • • • , P < 0.001, vs. male controls.

tions, in both the hypothalamus and the ME of adult rats exposed neonatally to MSG. The more pronounced GHRH decrease in the ME nerve terminals (—95%) than in the hypothalamus (-60%) is probably due to the incompleteness of arcuate MSG lesions, preservation of GHRH-containing cell bodies in the dorso-lateral and

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GHRH DEFICIENCY IN FEMALE RATS

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TABLE 3. Pituitary protein content, pituitary GH concentrations, and serum IGF-I concentrations in adult male and female rats treated neonatally with MSG or saline (controls) Pituitary protein Pituitary GH Serum IGF-I content concentration concentration (ng/ml) (mg) (fig/mg protein)

Groups Control males MSG-treated males Control females MSG-treated females

1.55 ± 0.05 0.62 ± 0.05° 1.88 ± 0.08 0.75 ± 0.06°

276 ± 25 226 ± 16 149 ± 7 150 ± 16

1613 ± 129 742 ± 122° 1373 ± 191 613 ± 74°

Values are the mean ± SE of 8-10 rats/group. P < 0.001 us. sex-matched controls.

a

400 • A O

Control males Control females MSG-treated males MSG-treated females

TIME (minutes) FIG. 4. Plasma GH responses to a bolus iv injection of rGHRH (1 fig/ rat) in 12-week-old male and female adult rats treated neonatally with saline (controls) or MSG. Each point represents the mean ± SE of 5-6 values per experimental group.

ventral hypothalamus (16), and a predominant loss of GHRH neurones directing their axon toward the ME (and thus involved in GH control). Our results showing equivalent reductions of hypothalamic GHRH in male and female MSG-treated rats agree with another recent report (18) and validate the MSG-treated female rat as a model for the study of prolonged GHRH deficiency. Although decreased random plasma GH concentrations have been observed (26, 27), the precise effects of MSG on the GH secretory pattern have not been reported in female animals, nor compared with MSG effects in male littermates. Our study shows that chronic GHRH deficiency in females results in a marked inhibition of pulsatile GH secretion, in proportion similar to that observed in male animals. Sex differences were observed, however, in MSG-induced alterations of the GH secretory patterns. As in previous studies (17, 24), we found that the major alterations of GH secretion observed in male MSG-treated rats are reductions in pulse amplitude and duration. These observations support the concept that GH pulses in males are driven by episodic GHRH secretion, in agreement with the results of direct measurements of GHRH concentrations in the

Endo • 1991 Voll28«No2

portal blood (5), and with GHRH immunoneutralization studies in male rats (28). In contrast, in female MSGtreated animals, the diminished pulse amplitude was accompanied by major reductions in GH pulse frequency and in baseline GH values, accounting for a significant proportion of the overall 85% decrease in GH secretion. These findings suggest that the GHRH stimulatory tone contributes to the frequent GH peaks and high GH trough levels typically observed in female rats. Therefore, hypothalamic secretion of GHRH appears to be more continuous and less pulsatile in females compared to males. A continuous mode of GHRH release is also supported by prior observations in female rats that GHRH immunoneutralization of GHRH reduces interpulse GH concentrations (11) and that pulsatile GHRH infusion is able to induce a male-type GH secretory pattern (10). Several arguments mitigate against increased SRIF secretion being a de terminant of MSG-related inhibition of GH secretion. Administration of an anti-SRIF serum to MSG-treated male rats does not restore a normal GH secretion (17). Furthermore, decreased SRIF secretion would be expected in this model as a consequence of low circulating concentrations of GH and IGF-I and associated feedback mechanisms (29). Pituitary effects of hypothalamic MSG lesions have been well documented (26, 27, 30) and include the severe and parallel reductions in pituitary wt and GH content in both sexes, as we have observed here. These effects are likely to be the consequence of the prolonged GHRH deficiency, with loss of its potent trophic effects on anterior pituitary cells (31, 32). Atrophy of the somatotrophs probably accounts for the poor GH response to GHRH in MSG-treated animals. Similar effects of chronic GHRH deficiency have also been reported in humans (33, 34). In apparent contrast with our results, two previous reports have shown large GH responses to exogenous GHRH in MSG-lesioned animals (11, 35). However, no comparison with response in controls animals was provided, and experimental conditions, such as duration of GHRH depletion or timing of GHRH testing, were different. As expected, BW and linear growth were impaired in male MSG-treated rats (17, 27). In contrast, the wt gain was normal and the longitudinal growth was only modestly retarded in female animals. This phenomenon has been observed previously (27, 36), although not consistently (18, 30). These sex differences are explained in part by the more severe obesity that develops in female MSGtreated rats. Studies in mice have revealed that MSGassociated obesity does not result from overeating (13), but is the consequence of thermoregulation at a lower than normal body temperature during the night and early morning (36). Low physical activity also contributes to

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GHRH DEFICIENCY IN FEMALE RATS

the higher metabolic efficiency of MSG-treated rodents (37). Sex differences have been reported in the capability of MSG-treated mice to activate thermogenesis in their brown adipose tissue, and these differences may underlie the more pronounced development of obesity in female animals (36). Adrenalectomy appears to prevent obesity in MSG-treated mice, indicating that high corticosterone levels might induce the alterations of thermoregulation in these animals (38). Similar mechanisms might operate in glutamate-treated rats, which also exhibit increased corticosterone secretion (39). Although probably not involved in the development of obesity, hyperinsulinemia is present in MSG-treated female animals (38) and may antagonize the factors leading to growth retardation (40). Thus, in children becoming hyperphagic after surgery for craniopharyngioma, obesity is associated with growth, even in the complete absence of GH (41). On the other hand, much of the sex differences in the MSG effect on linear growth may be due to different consequences resulting from decreased pulsatile secretion of GH in male and female rats. Indeed, GH pulsatility is more efficient for promoting growth than continuous secretion (1, 2) and is normally less prominent in females than males. Therefore, the loss of GH pulses in MSG-treated females may produce less deleterious effects on growth than observed in males. The differences in growth delay observed between male and female MSG-lesioned rats are not likely to be mediated by circulating IGF-I, because serum concentrations of this growth factor were decreased equally in both sexes. Local production of IGF-I (42), however, may be more important for stimulating cell growth and multiplication, and our studies do not provide information on tissue concentrations of IGF-I. It is also notable that MSG-treated GHRH-deficient animals grow better and have higher levels of serum IGF-I than hypophysectomized rats (2). Residual GH secretion and/or the maintenance of other hormonal and nutritional factors critical to growth and IGF-I production (43) might account for these differences. We conclude from the present work that pulsatile secretion of GH is markedly inhibited in MSG-treated GHRH-deficient female rats, similar to MSG-treated male rats. Furthermore, interpulse plasma concentrations of GH are also dramatically reduced in MSGtreated female rats, suggesting a more continuous mode of GHRH secretion in females than males. Prolonged GHRH depletion has similar effects on pituitary growth and serum IGF-I in both sexes, but BW and linear growth are less affected in female MSG-treated animals compared to males. These sex differences may be explained, in part, by the more severe obesity and hyperinsulinemia that develop in female MSG-lesioned rats, but also by

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more deleterious consequences on growth resulting from the loss of pulsatile GH secretion in male rats.

Acknowledgments We wish to thank Ms. Carol Milbury and Eyvonne Bruton for expert technical assistance, and Ms. Nicole Amat-Peiro for her excellent secretarial work. We are also grateful to Dr. S. Raiti and the National Hormone and Pituitary Program for providing the materials used in the rGH RIA.

References 1. Jansson JO, Eden S, Isaksson O 1985 Sexual dimorphism in the control of growth hormone secretion. Endocr Rev 6:128-150 2. Maiter D, Underwood LE, Maes M, Davenport ML, Ketelslegers JM 1988 Different effects of intermittent and continuous growth hormone (GH) administration on serum somatomedin-C/insulinlike growth factor I and liver GH receptors in hypophysectomized rats. Endocrinology 123:1053-1059 3. Tannenbaum GS, Martin JB 1976 Evidence for an endogenous ultradian rhythm governing growth hormone secretion in the rat. Endocrinology 98:562-570 4. Frohman LA, Jansson JO 1986 Growth hormone-releasing hormone. Endocr Rev 7:223-253 5. Plotsky PM, Vale W 1985 Patterns of growth hormone-releasing factor and somatostatin secretion into the hypophysial-portal circulation of the rat. Science 230:461-463 6. Tannenbaum GS 1988 Somatostatin as a physiological regulator of pulsatile growth hormone secretion. Horm Res 29:70-74 7. Gabriel SM, Millard WJ, Koenig JI, Badger TM, Russell WR, Maiter DM, Martin JB 1989 Sexual and developmental differences in peptides regulating growth hormone secretion in the rat. Neuroendocrinology 50:299-307 8. Maiter D, Koenig JI, Martin JB, Kaplan LM, Sexually dimorphic expression of the growth hormone-releasing hormone (GHRH) gene in the rat hypothalamus. Program of the 72nd Annual Meeting of the Endocrine Society, 1990, Atlanta, GA, p 52 (Abstract) 9. Chowen-Breed JA, Steiner RA, Clifton DK 1989 Sexual dimorphism and testosterone-dependent regulation of somatostatin gene expression in the periventricular nucleus of the rat brain. Endocrinology 125:357-362 10. Clark RG, Robinson ICAF 1985 Growth induced by pulsatile infusion of an amidated fragment of human growth hormone releasing factor in normal and GHRF-deficient rats. Nature 314:281283 11. Ono M, Masunaga S, Miki N, The secretory pattern of growth hormone-releasing factor in the female rat. Program of the 70th Meeting of the Endocrine Society, New Orleans, LA, 1988, p 117 (Abstract) 12. Clark RG, Robinson ICAF 1985 Growth hormone responses to multiple injections of a fragment of human growth hormonereleasing factor in conscious male and female rats. J Endocrinol 106:281-289 13. Olney JW 1969 Brain lesions, obesity and other disturbances in mice treated with monosodium glutamate. Science 164:719-721 14. Simson EL, Gold RM, Standish LJ, Pellett PL 1977 Axon-sparing brain lesioning technique: the use of monosodium-L-glutamate and other amino-acids. Science 198:515-517 15. Acs Z, Antoni FA, Makara GB 1982 Corticoliberin and somatoliberin activity in the pituitary stalk median eminence of rats after neonatal treatment with monosodium glutamate. J Endocrinol 93:239-245 16. Bloch B, Ling N, Benoit R, Wehrenberg WB, Guillemin R 1984 Specific depletion of immunoreactive growth hormone-releasing factor by monosodium glutamate in rat median eminence. Nature 307:272-273 17. Millard WJ, Martin Jr JB, Audet J, Sagar SM, Martin JB 1982 Evidence that reduced growth hormone secretion observed in monosodium glutamate-treated rats is the result of a deficiency in

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Neonatal treatment with monosodium glutamate: effects of prolonged growth hormone (GH)-releasing hormone deficiency on pulsatile GH secretion and growth in female rats.

Administration of monosodium glutamate (MSG) to neonatal rodents produces permanent lesions of hypothalamic arcuate neurons that secrete GH-releasing ...
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