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Effects of excitotoxic amino acids on pituitary hormone secretion in the rat
366
Brain Research, 289 (1'083)366-369 121scvicr
Effects of excitotoxic amino acids on pitu~ry hormone ~ e t i o n
in the rat
GEORGE A. MASON I, GARTH BISSETFE 1,* and CHARLES B. NEMEROFF 1.2,~ 1Biological Sciences Research Center, 2Department of Psychiatry, and 3The Neurobiology Program, University of North Carolina School of Medicine, Chapel Hill, NC27514 (U.S.A.) (Accepted August 30th, 1983) Key words: excitotoxic amino acids - - adenohypophyseal hormones
The acute effects of administration of 4 excitatory amino acids (n-methyl-DL-asparticacid (NMA), kainic acid (KA), ibotenic acid (IA) and quinolinic acid (QA)) on the serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH) and prolactin (PRL) were studied in the rat. NMA-treated rats exhibited increased serum LH and GH concentrations while KA-treated rats showed increases only in serum GH concentrations. Neither IA nor QA altered adenohypophyseal hormone levels. These endocrine alterations induced by NMA and KA are different from those previously reported after administration of glutamate, another excitatory amino acid. The finding that all of the excitatory amino acids studied did not produce identical effects on anterior pituitary hormone secretion may be due to differential permeability of these substances into the central nervous system or because they act at different subtypes of excitatory amino acid receptors. Administration of monosodium L-glutamate (MSG) or related excitatory amino acids to neonatal rodents results in destruction of neurons in the arcuate nucleus of the hypothalamus and other circumventricular organs6, ~1. As adults, these animals manifest a syndrome characterized by marked obesity, skeletal stunting, reproductive dysfunction and hypoplasia of the anterior pituitary, gonads and accessory sexual organs8Al,17; however, both the anterior pituitary and gonads of these animals respond appropriately to their natural hormonal stimuliS. Studies of the acute effects of MSG and related neurotoxic amino acids on neuroendocrine regulation were initiated by the work of Olney and his coworkerslL They reported a marked rise in luteinizing hormone (LH) and testosterone in adult male rats 15 min after subcutaneous (SC) administration of 1 g/kg of MSG. Nemeroff et al. 9 similarly treated rats with MSG and observed diminished serum levels of growth hormone ( G H ) and elevations of serum prolactin (PRL); however, no MSG-induced alterations in the concentration of L H or follicle-stimulating hormone (FSH) were observed. Olney, Price and their associates~3 also examined the acute effects of n-
methyl-DL-aspartate (NMA), kainic acid (KA) and DL-homocysteic acid on L H secretion in young male rats. They reported that N M A produced significant elevations of serum L H after SC administration i n doses below those required to produce hypothalamic lesions. These findings have been replicated by Schainker and Cicero~S and by Carrillo and Alcantura 1. This latter group observed that N M A failed to produce a simultaneous elevation in the levels of FSH. In the present study, we have measured the serum concentrations of L H , FSH, P R L and G H in rats after administration of N M A , K A , ibotenic acid (IA) or quinolinic acid ( Q A ) , all of which are excitatory amino acids (see ref. 2 for review). The same protocol was used in 4 separate experiments in which adenohypophyseal hormone levels were determined in rat serum following s.c. administration of different doses of either N M A (10--66 mg/kg), K A (2.3-21 mg/kg), I A (1-7.5 mg/kg) or Q A (100-300 mg/kg). N-methyl-DL-aspartic acid, KA, IA and Q A were purchased from Sigma Chemical Co. (St. Louis, MO). Adult male Sprague-Dawley rats (180-300 g), purchased from Charles River Laboratories (Wil-
Correspondence: George A. Mason, Biological Sciences Research Center 220H, University of North Carolina School of Medicine, Chapel Hill, NC 27514, U.S.A. * Present address: Duke University Medical Center, Department of Psychiatry, Durham, NC 27710, U.S.A. 0006-8993/83/$03.00 © 1983 Elsevier Science Publishers B.V.
367 No convulsions o r o t h e r unusual behaviors were o b s e r v e d in any of the animals receiving K A , N M A , I A o r Q A . The acute effects of the neurotoxic excitatory amino acids on the serum concentrations of anterior pituitary h o r m o n e s are illustrated in T a b l e I. A d m i n i s t r a t i o n of N M A resulted in elevations in the serum concentrations of both G H and L H (Table I A ) . Significant elevations in serum G H levels were observed after the i n t e r m e d i a t e (25 mg/kg) and highest (66 mg/kg) doses of N M A ; serum L H was elevated only after the highest dose. T h e serum concentrations of P R L and F S H were not altered by any of the 3 doses of N M A . The highest dose of K A tested (21 mg/kg) p r o d u c e d a significant elevation in serum concentrations of G H ; this effect was robust, producing a striking 6-fold increase over the basal level (Table IB). N o increases were o b s e r v e d in the o t h e r 3 anterior pituitary h o r m o n e s measured. N e i t h e r I A nor Q A , in the doses tested, p r o d u c e d any significant alteration in the serum concentrations of L H , F S H , G H or P R L (Table IC and ID). Perhaps our most salient finding is the previously u n r e p o r t e d elevation of serum G H elicited by s.c. administration of N M A or K A . This is of particular interest in view of the fact that M S G t r e a t m e n t has pre-
mington, M A ) , were h o u s e d 3--6 animals p e r cage for at least one w e e k prior to e x p e r i m e n t a t i o n in a controlled e n v i r o n m e n t animal facility on a 12 h light/ d a r k cycle and were fed l a b o r a t o r y chow and water ad libitum. B e t w e e n 10.00 and 12.00 h, rats received a single s.c. iniection of either vehicle (0.9% NaCl; 1 ml/kg) or an equivalent volume of vehicle containing one of several doses of either N M A , K A , I A , or Q A . The rats were killed by decapitation 71/2 rain post-injection with particular care t a k e n to minimize stress as previously described9. T r u n k b l o o d was collected and allowed to clot for 2-3 h at 4 °C. Serum was subsequently o b t a i n e d by centrifugation at 4 °C for 5 rain at 2000 g. Serum samples were then transferred to glass vials and stored at - - 2 0 °C. Pituitary hormones were m e a s u r e d by r a d i o i m m u n o a s s a y as previously describeda.9 using materials supplied by N I A M D D . E a c h t r e a t m e n t group consisted of 8 or 9 animals, with the exception of the I A e x p e r i m e n t in which availability of the substance restricted the group size to 5 animals. T h e d a t a from each experim e n t were analyzed using A N O V A and D u n n e t t ' s test for multiple comparisons 4. T h e null hypothesis was rejected when P < 0.05. TABLE I
The effects of excitotoxic amino acids on the serum concentrations of anteriorpituitary hormones in the rat Treatment
n
LH(ng/ml+ S.E.M.) FSH(ng/mI+ S.E.M.)
A, n-Methyl-nL-asparticacid (NMA) Vehicle 8 18.5 + NMA 10 mg/kg SC 8 29.1 + NMA 25 mg/kg SC 8 33.3 + NMA 66 mg/kg SC 8 49.9 +
4.9 3.5 6.0 8.0*
195.6 + 228.9 + 251.8 + 251.1 +
19.0 21.0 23.3 29.8
B, Kainicacid (KA) Vehicle KA 2.3 mg/kg KA 7.0 mg/kg KA 21.0 mg/kg
8 8 8 8
25.7 _ 2,8 28.7 + 3,9 26.5+3.0 27.1 + 2.9
355.9 + 18.9 357.6 + 21.9 356.6+20.8 328.3 + 19.4
C. lbotenic acid (IA) Vehicle IA 1.0 mg/kg IA 2.5 mg/kg IA 7.5 mg/kg
5 5 5 5
20.7 + 21.4 + 26.6 + 17.4 +
342.4 + 333.0 + 305.8 + 320.2 +
D. Quinolinic acid (QA) Vehicle 9 QA 100 mg/kg 9 QA 300mg/kg 9
2.9 2.7 5.3 2.2
15.1 + 2.7 18.5 + 1.2 21.2+4.5
* P < 0.05 when compared to vehicle-treated controls.
22.8 20.1 30.5 16.5
328 +_20 343 + 8 333+20
GH(ng/ml+ S.E.M.) PRL(ng/mI+_S.E.M.)
22.2 + 36.8 + 78.9 + 160.0 +
6.9 11.4 20.4* 46.5*
37.0 + 10.2 38.3 + 13.7 63.0+13.2 226.1 + 58.3* 18.1 + 11.0 + 39.7 + 33.0 +
9.5 2.6 13.8 15.2
35.6 + 4.7 35.2 + 3.0 45.1+4.9
8.8 + 11.0 + 9.0 + 9.3 +
2.5 2.4 2.6 2.0
10.6 + 3.0 7.9 +_ 1.6 11.9+3.1 10.7 + 2.2 9.5 + 6.3 + 7.7 + 8.9 +
3.8 1.0 1.8 2.7
11.2 + 3.6 5.0 + 0.8 7.9+1.9
368 viously been reported9,17 to markedly reduce G H levels. Furthermore, it is of interest that neither N M A , KA, I A or Q A had any acute effect on the secretion of P R L whereas MSG administration has been reported to produce marked increases in serum P R L concentrations9,17. Moreover, N M A , but none of the other excitatory amino acids, increased serum LH concentrations. The finding that all of the excitatory amino acids studied did not produce identical effects on anterior pituitary hormonal concentrations in serum is of particular interest. Thus MSG reduces G H secretion, N M A and K A increases G H secretion and I A and Q A had no effect on G H secretions. These data might, at least in part, be due to the fact that N M A and K A act at specific and selective receptors that are distinct from those acted upon by MSG, Q A or IA. Recent neurophysiological3 and receptor-binding studies 5 indicate that K A interacts with receptors distinguishable from those interacting with N M A or the glutamate analog, quisqualic acid. A n o t h e r important factor that might have contributed to these findings is the differential penetrability of each of these excitatory amino acids into the central nervous system. Previous studies1 have demonstrated no effect of N M A on anterior pituitary hormone secretion in vitro; its neuroendocrine action is therefore believed to be mediated at central nervous system sites. Ibotenic acid is thought to act at the n-methyl-Daspartate receptor3; however, unlike N M A , this amino acid, in the doses used in the present study, did not increase serum G H and L H levels. Recent neurophysiological data 10 indicate that I A acts not only at excitatory receptors but at an additional receptor which causes a hyperpolarization, resulting in inhibition. Zaczek and Coyle 20 suggested that the 1 Carrillo, A. J. and Alcantara, O., The effect of n-methyl aspartic acid (NMA) and monosodium glutamate on plasma LH levels in the ovariectomized estrogen-progesterone primed rat, Soc. Neurosci. Abstr., 108 (1981) 7. 2 Coyle, J. T., Excitatory amino acid neurotoxins. In L. L. Iversen, S. D. Iversen and S. H. Snyder (Eds.), Handbook of Psychopharmacology, Vol. 15. Plenum Press, New York, 1982, p. 237. 3 Davies, J., Evans, R. H., Francis, A. A. and Watkins, J. C., Excitatory amino acid receptors and synaptic excitation in the mammalian central nervous system, J. Physiol. (Paris), 75 (1979) 641. 4 Dunnett, C. W., New tables for multiple comparisons with a control, Biometrics, 20 (1964) 482.
lack of an expected epileptogenic action of IA was due to interaction of this amino acid with both excitatory and inhibitory receptors. The elevation of LH induced by N M A was statistically significant only at the highest dose tested; however, as discussed by Schainker and Cicero18, failure of the lower doses to produce significant increases in L H may have been due to the greater age of the rats in this study compared to those used in earlier studiesi4,15. Our observation of elevated serum L H without any concomitant elevation of serum FSH following N M A treatment confirms the results of a recent study by Carrillo and Alcantura 1. This finding is of particular interest because it addressed the still controversial issue of whether luteinizing hormone-releasing hormone is the only humoral modulator of secretion of both gonadotropins or alternatively, whether distinct LH- and FSH-releasing factors exist. The selective effect of N M A on L H release appears to support the latter hypothesis; however, other physiological factors such as the gonadal steroid environment must also be considered. In addition, in the rhesus monkey, Wilson and Knobi119 recently reported that NMA, administered intravenously, produces significant elevations in serum concentrations of LH, FSH and PRL. Thus species differences may also play a role in the differences in the effects of excitatory amino acids observed in the rat and monkey. This research was supported by N I C H H D H D 03110. We are grateful to Mr. Ossie Hatley for his excellent technical assistance in this research and to Mrs. Dorothy Yarbrough for her careful preparation of the manuscript.
5 London, E. D. and Coyle, J. T., Specific binding of [3H]kainic acid for receptor sites in rat brain, Motec. Pharmacol., 15 (1979) 492. 6 Nemeroff, C. B., Grant, L. D., Bissette, G., Ervin, G. N., Harrell, L. E. and Prange, A. J., Jr., Growth, endocrinological and behavioral deficits after monosodium L-glutamate in the neonatal rat: possible involvement of arcuate dopamine neuron damage, Psychoneuroendoerinology, 2 (1977) 179. 7 Nemeroff, C. B., Grant, L. D., Harrell, L. E., Bissette, G., Ervin, G. N. and Prange, A. J., Jr., Histoehemical evidence for the permanent destruction of arcuate dopamine neurons by neonatal monosodium L-glutamate in the rat, Soc. Neurosci. Abstr., 1 (1975) 434.
369 8 Nemeroff, C. B., Konkol, R. J., Bissette, G., Youngblood, W. W., Rone, M. S., Martin, J. B., Brazeau, P., Breese, G. R., Prange, A. J., Jr. and Kizer, J, S., Analysis of the disruption in hypothalamic-pituitary regulation in rats treated neonatally with monosodium L-glutamate (MSG): evidence for the involvement of tuberoinfundibular cholinergic and dopaminergic systems in neuroendocrine regulation, Endocrinology, 101 (1977) 613. 9 Nemeroff, C. B., Bissette, G., Greeley, G. H., Jr., Mailman, R. B., Martin, J. B., Brazeau, P. and Kizer, J. S., Effects of acute administration of monosodium L-glutamate (MSG), atropine or haloperidol on anterior pituitary hormone secretion in the rat, Brain Research, 156 (1978) 198. 10 Nistri, A., MacDonald, J. F. and Barker, J. L., Effects of ibotenic acid on amphibian and mammalian spinal neurons in vitro. In G. DiChara and G. L. Gessa (Eds.), Glutamate as a Neurotransrnitter, Raven Press, New York, 1981, p. 245. 11 Olney, J. W., Brain lesions, obesity and other disturbances in mice treated with monosodium glutamate Science, 164 (1969) 719. 12 Olney, J. W., Cicero, T. J., Meyer, E. R. and deGubareff, T., Acute glutamate-induced elevations in serum testosterone and luteinizing hormone, Brain Research, 112 (1976) 420. 13 Olney, J. W. and Price, M. T., Neuroendocrine interactions of excitatory and inhibitory amino acids, Brain Res.
Bull., 5 (1980) 361. 14 Price, M. T., Olney, J. W., Anglim, M. and Buchsbaum, S., Reversible action of n-methyl aspartate on gonadotropin neuroregulation, Brain Research, 196 (1979) 165. 15 Price, M. T., Olney, J. W. and Cicero, T. J., Acute elevations of serum luteinizing hormone induced by kainic acid, n-methyl aspartic acid or homocysteic acid, Neuroendocrinology, 26 (1978) 352. 16 Redding, T. W., Schally, A. V., Arimura, A. and Wakabayashi, I., Effects of monosodium glutamate on some endocrine functions, Neuroendocrinology, 8 (1971) 245. 17 Terry, L. C., Epelbaum, J. and Martin, J. B., Monosodium glutamate: acute and chronic effects on rhythmic growth hormone and prolactin secretion, and somatostatin in the undisturbed male rat, Brain Research, 217 (1981) 129. 18 Schainker, B. A. and Cicero, T. J., Acute central stimulation of luteinizing hormone by parenterally administered nmethyl-D,L-aspartic acid in the male rat, Brain Research, 184 (1980) 425. 19 Wilson, R. C. and Knobil, E., Acute effects of N-methylDL-aspartate on the release of pituitary gonadotropins and prolactin in the adult female rhesus monkey, Brain Research, 248 (1982) 177. 20 Zaczek, R. and Coyle, J. T., Excitatory amino acid analogues: neurotoxicity and seizures, Neuropharmacology, 21 (1982) 15.
Brain Research, 289 (1'083)366-369 121scvicr
Effects of excitotoxic amino acids on pitu~ry hormone ~ e t i o n
in the rat
GEORGE A. MASON I, GARTH BISSETFE 1,* and CHARLES B. NEMEROFF 1.2,~ 1Biological Sciences Research Center, 2Department of Psychiatry, and 3The Neurobiology Program, University of North Carolina School of Medicine, Chapel Hill, NC27514 (U.S.A.) (Accepted August 30th, 1983) Key words: excitotoxic amino acids - - adenohypophyseal hormones
The acute effects of administration of 4 excitatory amino acids (n-methyl-DL-asparticacid (NMA), kainic acid (KA), ibotenic acid (IA) and quinolinic acid (QA)) on the serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH) and prolactin (PRL) were studied in the rat. NMA-treated rats exhibited increased serum LH and GH concentrations while KA-treated rats showed increases only in serum GH concentrations. Neither IA nor QA altered adenohypophyseal hormone levels. These endocrine alterations induced by NMA and KA are different from those previously reported after administration of glutamate, another excitatory amino acid. The finding that all of the excitatory amino acids studied did not produce identical effects on anterior pituitary hormone secretion may be due to differential permeability of these substances into the central nervous system or because they act at different subtypes of excitatory amino acid receptors. Administration of monosodium L-glutamate (MSG) or related excitatory amino acids to neonatal rodents results in destruction of neurons in the arcuate nucleus of the hypothalamus and other circumventricular organs6, ~1. As adults, these animals manifest a syndrome characterized by marked obesity, skeletal stunting, reproductive dysfunction and hypoplasia of the anterior pituitary, gonads and accessory sexual organs8Al,17; however, both the anterior pituitary and gonads of these animals respond appropriately to their natural hormonal stimuliS. Studies of the acute effects of MSG and related neurotoxic amino acids on neuroendocrine regulation were initiated by the work of Olney and his coworkerslL They reported a marked rise in luteinizing hormone (LH) and testosterone in adult male rats 15 min after subcutaneous (SC) administration of 1 g/kg of MSG. Nemeroff et al. 9 similarly treated rats with MSG and observed diminished serum levels of growth hormone ( G H ) and elevations of serum prolactin (PRL); however, no MSG-induced alterations in the concentration of L H or follicle-stimulating hormone (FSH) were observed. Olney, Price and their associates~3 also examined the acute effects of n-
methyl-DL-aspartate (NMA), kainic acid (KA) and DL-homocysteic acid on L H secretion in young male rats. They reported that N M A produced significant elevations of serum L H after SC administration i n doses below those required to produce hypothalamic lesions. These findings have been replicated by Schainker and Cicero~S and by Carrillo and Alcantura 1. This latter group observed that N M A failed to produce a simultaneous elevation in the levels of FSH. In the present study, we have measured the serum concentrations of L H , FSH, P R L and G H in rats after administration of N M A , K A , ibotenic acid (IA) or quinolinic acid ( Q A ) , all of which are excitatory amino acids (see ref. 2 for review). The same protocol was used in 4 separate experiments in which adenohypophyseal hormone levels were determined in rat serum following s.c. administration of different doses of either N M A (10--66 mg/kg), K A (2.3-21 mg/kg), I A (1-7.5 mg/kg) or Q A (100-300 mg/kg). N-methyl-DL-aspartic acid, KA, IA and Q A were purchased from Sigma Chemical Co. (St. Louis, MO). Adult male Sprague-Dawley rats (180-300 g), purchased from Charles River Laboratories (Wil-
Correspondence: George A. Mason, Biological Sciences Research Center 220H, University of North Carolina School of Medicine, Chapel Hill, NC 27514, U.S.A. * Present address: Duke University Medical Center, Department of Psychiatry, Durham, NC 27710, U.S.A. 0006-8993/83/$03.00 © 1983 Elsevier Science Publishers B.V.
367 No convulsions o r o t h e r unusual behaviors were o b s e r v e d in any of the animals receiving K A , N M A , I A o r Q A . The acute effects of the neurotoxic excitatory amino acids on the serum concentrations of anterior pituitary h o r m o n e s are illustrated in T a b l e I. A d m i n i s t r a t i o n of N M A resulted in elevations in the serum concentrations of both G H and L H (Table I A ) . Significant elevations in serum G H levels were observed after the i n t e r m e d i a t e (25 mg/kg) and highest (66 mg/kg) doses of N M A ; serum L H was elevated only after the highest dose. T h e serum concentrations of P R L and F S H were not altered by any of the 3 doses of N M A . The highest dose of K A tested (21 mg/kg) p r o d u c e d a significant elevation in serum concentrations of G H ; this effect was robust, producing a striking 6-fold increase over the basal level (Table IB). N o increases were o b s e r v e d in the o t h e r 3 anterior pituitary h o r m o n e s measured. N e i t h e r I A nor Q A , in the doses tested, p r o d u c e d any significant alteration in the serum concentrations of L H , F S H , G H or P R L (Table IC and ID). Perhaps our most salient finding is the previously u n r e p o r t e d elevation of serum G H elicited by s.c. administration of N M A or K A . This is of particular interest in view of the fact that M S G t r e a t m e n t has pre-
mington, M A ) , were h o u s e d 3--6 animals p e r cage for at least one w e e k prior to e x p e r i m e n t a t i o n in a controlled e n v i r o n m e n t animal facility on a 12 h light/ d a r k cycle and were fed l a b o r a t o r y chow and water ad libitum. B e t w e e n 10.00 and 12.00 h, rats received a single s.c. iniection of either vehicle (0.9% NaCl; 1 ml/kg) or an equivalent volume of vehicle containing one of several doses of either N M A , K A , I A , or Q A . The rats were killed by decapitation 71/2 rain post-injection with particular care t a k e n to minimize stress as previously described9. T r u n k b l o o d was collected and allowed to clot for 2-3 h at 4 °C. Serum was subsequently o b t a i n e d by centrifugation at 4 °C for 5 rain at 2000 g. Serum samples were then transferred to glass vials and stored at - - 2 0 °C. Pituitary hormones were m e a s u r e d by r a d i o i m m u n o a s s a y as previously describeda.9 using materials supplied by N I A M D D . E a c h t r e a t m e n t group consisted of 8 or 9 animals, with the exception of the I A e x p e r i m e n t in which availability of the substance restricted the group size to 5 animals. T h e d a t a from each experim e n t were analyzed using A N O V A and D u n n e t t ' s test for multiple comparisons 4. T h e null hypothesis was rejected when P < 0.05. TABLE I
The effects of excitotoxic amino acids on the serum concentrations of anteriorpituitary hormones in the rat Treatment
n
LH(ng/ml+ S.E.M.) FSH(ng/mI+ S.E.M.)
A, n-Methyl-nL-asparticacid (NMA) Vehicle 8 18.5 + NMA 10 mg/kg SC 8 29.1 + NMA 25 mg/kg SC 8 33.3 + NMA 66 mg/kg SC 8 49.9 +
4.9 3.5 6.0 8.0*
195.6 + 228.9 + 251.8 + 251.1 +
19.0 21.0 23.3 29.8
B, Kainicacid (KA) Vehicle KA 2.3 mg/kg KA 7.0 mg/kg KA 21.0 mg/kg
8 8 8 8
25.7 _ 2,8 28.7 + 3,9 26.5+3.0 27.1 + 2.9
355.9 + 18.9 357.6 + 21.9 356.6+20.8 328.3 + 19.4
C. lbotenic acid (IA) Vehicle IA 1.0 mg/kg IA 2.5 mg/kg IA 7.5 mg/kg
5 5 5 5
20.7 + 21.4 + 26.6 + 17.4 +
342.4 + 333.0 + 305.8 + 320.2 +
D. Quinolinic acid (QA) Vehicle 9 QA 100 mg/kg 9 QA 300mg/kg 9
2.9 2.7 5.3 2.2
15.1 + 2.7 18.5 + 1.2 21.2+4.5
* P < 0.05 when compared to vehicle-treated controls.
22.8 20.1 30.5 16.5
328 +_20 343 + 8 333+20
GH(ng/ml+ S.E.M.) PRL(ng/mI+_S.E.M.)
22.2 + 36.8 + 78.9 + 160.0 +
6.9 11.4 20.4* 46.5*
37.0 + 10.2 38.3 + 13.7 63.0+13.2 226.1 + 58.3* 18.1 + 11.0 + 39.7 + 33.0 +
9.5 2.6 13.8 15.2
35.6 + 4.7 35.2 + 3.0 45.1+4.9
8.8 + 11.0 + 9.0 + 9.3 +
2.5 2.4 2.6 2.0
10.6 + 3.0 7.9 +_ 1.6 11.9+3.1 10.7 + 2.2 9.5 + 6.3 + 7.7 + 8.9 +
3.8 1.0 1.8 2.7
11.2 + 3.6 5.0 + 0.8 7.9+1.9
368 viously been reported9,17 to markedly reduce G H levels. Furthermore, it is of interest that neither N M A , KA, I A or Q A had any acute effect on the secretion of P R L whereas MSG administration has been reported to produce marked increases in serum P R L concentrations9,17. Moreover, N M A , but none of the other excitatory amino acids, increased serum LH concentrations. The finding that all of the excitatory amino acids studied did not produce identical effects on anterior pituitary hormonal concentrations in serum is of particular interest. Thus MSG reduces G H secretion, N M A and K A increases G H secretion and I A and Q A had no effect on G H secretions. These data might, at least in part, be due to the fact that N M A and K A act at specific and selective receptors that are distinct from those acted upon by MSG, Q A or IA. Recent neurophysiological3 and receptor-binding studies 5 indicate that K A interacts with receptors distinguishable from those interacting with N M A or the glutamate analog, quisqualic acid. A n o t h e r important factor that might have contributed to these findings is the differential penetrability of each of these excitatory amino acids into the central nervous system. Previous studies1 have demonstrated no effect of N M A on anterior pituitary hormone secretion in vitro; its neuroendocrine action is therefore believed to be mediated at central nervous system sites. Ibotenic acid is thought to act at the n-methyl-Daspartate receptor3; however, unlike N M A , this amino acid, in the doses used in the present study, did not increase serum G H and L H levels. Recent neurophysiological data 10 indicate that I A acts not only at excitatory receptors but at an additional receptor which causes a hyperpolarization, resulting in inhibition. Zaczek and Coyle 20 suggested that the 1 Carrillo, A. J. and Alcantara, O., The effect of n-methyl aspartic acid (NMA) and monosodium glutamate on plasma LH levels in the ovariectomized estrogen-progesterone primed rat, Soc. Neurosci. Abstr., 108 (1981) 7. 2 Coyle, J. T., Excitatory amino acid neurotoxins. In L. L. Iversen, S. D. Iversen and S. H. Snyder (Eds.), Handbook of Psychopharmacology, Vol. 15. Plenum Press, New York, 1982, p. 237. 3 Davies, J., Evans, R. H., Francis, A. A. and Watkins, J. C., Excitatory amino acid receptors and synaptic excitation in the mammalian central nervous system, J. Physiol. (Paris), 75 (1979) 641. 4 Dunnett, C. W., New tables for multiple comparisons with a control, Biometrics, 20 (1964) 482.
lack of an expected epileptogenic action of IA was due to interaction of this amino acid with both excitatory and inhibitory receptors. The elevation of LH induced by N M A was statistically significant only at the highest dose tested; however, as discussed by Schainker and Cicero18, failure of the lower doses to produce significant increases in L H may have been due to the greater age of the rats in this study compared to those used in earlier studiesi4,15. Our observation of elevated serum L H without any concomitant elevation of serum FSH following N M A treatment confirms the results of a recent study by Carrillo and Alcantura 1. This finding is of particular interest because it addressed the still controversial issue of whether luteinizing hormone-releasing hormone is the only humoral modulator of secretion of both gonadotropins or alternatively, whether distinct LH- and FSH-releasing factors exist. The selective effect of N M A on L H release appears to support the latter hypothesis; however, other physiological factors such as the gonadal steroid environment must also be considered. In addition, in the rhesus monkey, Wilson and Knobi119 recently reported that NMA, administered intravenously, produces significant elevations in serum concentrations of LH, FSH and PRL. Thus species differences may also play a role in the differences in the effects of excitatory amino acids observed in the rat and monkey. This research was supported by N I C H H D H D 03110. We are grateful to Mr. Ossie Hatley for his excellent technical assistance in this research and to Mrs. Dorothy Yarbrough for her careful preparation of the manuscript.
5 London, E. D. and Coyle, J. T., Specific binding of [3H]kainic acid for receptor sites in rat brain, Motec. Pharmacol., 15 (1979) 492. 6 Nemeroff, C. B., Grant, L. D., Bissette, G., Ervin, G. N., Harrell, L. E. and Prange, A. J., Jr., Growth, endocrinological and behavioral deficits after monosodium L-glutamate in the neonatal rat: possible involvement of arcuate dopamine neuron damage, Psychoneuroendoerinology, 2 (1977) 179. 7 Nemeroff, C. B., Grant, L. D., Harrell, L. E., Bissette, G., Ervin, G. N. and Prange, A. J., Jr., Histoehemical evidence for the permanent destruction of arcuate dopamine neurons by neonatal monosodium L-glutamate in the rat, Soc. Neurosci. Abstr., 1 (1975) 434.
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