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Insulin-induced hypoglycemia does not impair the surge of luteinizing hormone secretion in the proestrous rat
Neuroscience Letters 256 (1998) 131–134
Insulin-induced hypoglycemia does not impair the surge of luteinizing hormone secretion in the proestrous rat Maiko Kawaguchi, Toshiya Funabashi, Fukuko Kimura* Department of Physiology, Yokohama City University School of Medicine, 3–9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan Received 10 March 1998; received in revised form 18 September 1998; accepted 18 September 1998
Abstract To know whether insulin-induced hypoglycemia (IIH), which has been shown to inhibit pulsatile luteinizing hormone (LH) secretion, also affects the surge of LH secretion, adult female rats were injected with insulin (5 units/rat) or saline intravenously at 1300 or 1600 h on the day of proestrus and serum concentrations of LH and blood glucose were determined during the period from 1100 to 2100 h. The injection of insulin at neither 1300 nor 1600 h affected the surge of LH secretion, but it significantly decreased the blood glucose. Together with our recent hypothesis that the pulsatile and surge of LH secretion are controlled by separate gonadotropin-releasing hormone (GnRH) mechanisms, we conclude that the GnRH surge generator is less sensitive to IIH than the GnRH pulse generator. 1998 Elsevier Science Ireland Ltd. All rights reserved
Keywords: Insulin; Hypoglycemia; Luteinizing hormone surge; Proestrus; Rat; Luteinizing hormone surge generator
Availability of metabolic fuels is known to affect luteinizing hormone (LH) secretion (see [17] for review). For example, insulin-induced hypoglycemia (IIH) has been shown to decrease pulsatile LH secretion in females [2,3,5,6]. In the monkey, IIH inhibits pulsatile LH secretion by reducing the electrical activity of the GnRH pulse generator [2]. We also recently found that electrical activity of the GnRH pulse generator in ovariectomized estrogenprimed rats was reduced by IIH and resulted in a reduction in the pulsatile LH secretion as well [6]. Furthermore, fasting decreased the number of Fos expressing GnRH neurons, suggesting that it attenuates the activity of GnRH neurons [1]. Interestingly, Morin [13] reported that, in the hamster, the decrease in LH secretion caused by fasting on the days of estrus and diestrus 1 during the estrous cycle resulted in a blockade of the next expected ovulation, but fasting on the days of diestrus 2 and proestrus did not [13,14]. The report suggested that the surge of LH secretion might occur in spite of fasting. In fact, Mangels et al. [12], suggested that the fasting did not inhibit the surge of LH secretion induced by * Corresponding author. Tel.: +81 45 7872579; fax: +81 45 7872509; e-mail: [email protected]
0304-3940/98/$ - see front matter PII S0304- 3940(98) 00783- 6
estrogen-priming in the ovariectomized hamster. Therefore, the pulsatile LH secretion seems to be much more sen-sitive to the metabolic state than the surge of LH secretion. On the other hand, we have hypothesized that two different patterns of LH secretion, i.e. the pulsatile LH secretion and the surge of LH secretion, are controlled by separate GnRH mechanisms, the GnRH pulse generator and the GnRH surge generator, respectively (See details in review by Kimura and Funabashi [11]). A simple interpretation of the data mentioned above is that the surge generator may not be as sensitive to the metabolic state as the pulse generator is. For this reason, we investigated whether IIH affected the surge of LH secretion in proestrous rats. In the present study, we show, for the first time in rats, that IIH does not affect the surge of LH secretion. Intact female Wistar rats were obtained from Charles River Laboratories (Yokohama, Japan) at 7–8 weeks of age, and were maintained under controlled environmental conditions (temperature, 24 ± 1°C; lights on at 0500–1900 h) with food and water available ad libitum. Vaginal smears were taken daily and those which exhibited at least two consecutive 4-day estrous cycles were used in the present study. In the afternoon of the day of diestrus 2 (1500–1700 h), the rats received an intraatrial cannula implantation
1998 Elsevier Science Ireland Ltd. All rights reserved
132
M. Kawaguchi et al. / Neuroscience Letters 256 (1998) 131–134
under ether anesthesia [9]. All animal housing and surgical procedures were in accordance with the guidelines of the Institutional Animal Care and Use Committee of the Yokohama City University School of Medicine (project No. 9773). In the first experiment (experiment 1), the effect of IIH just before the onset of the LH surge, i.e. 1300 h, proestrus, was evaluated. Blood samples (approximately 170 ml) were collected under freely moving conditions at 1-h intervals between 1100 and 2100 h, along with simultaneous measurements of the blood glucose level by means of an instant blood glucose assay apparatus (Advantage; Boehringer Mannheim, Tokyo, Japan). An equal volume of heparinized saline (0.9% NaCl) was replaced after each bleeding. Animals were injected with five units of insulin (Humulin; Eli Lilly Japan, Kobe, Japan) in 125 ml (insulin group, n = 7) or the same volume of saline (saline group, n = 7) at 1300 h through the cannula after bleeding. This dose of insulin was chosen because an injection of 1–5 units of insulin significantly decreased the blood glucose and effectively inhibited the pulsatile LH secretion in ovariectomized estrogenprimed rats in our laboratory [6]. In the second experiment (experiment 2), the effect of IIH during the continuing surge of LH secretion, i.e. 1600 h, was evaluated. All experimental procedures were the same as the first one except for the time of injection. The rats were injected with five units of insulin (insulin group, n = 7) or the same volume of saline (saline group, n = 9) at 1600 h through the cannula after bleeding. The experimental protocol was repeated on another group of rats by the same procedures (saline group, n = 5; insulin group, n = 6), since we thought that IIH might affect LH secretion to
some degree. These were referred to as experiments 2A and 2B. Serum concentrations of LH were measured by doubleantibody radioimmunoassay with materials supplied by the National Institute of Diabetes and Digestive and Kidney Disease (NIDDK) and Dr. K. Wakabayashi. The reference standard used was NIDDK rat LH-RP-3, but the amount of LH is expressed in terms of NIH LH-S1 for easy comparison of the data published by us. The hormone assay was done for each experiment, and thus a total of three assays were run. The mean ± SD minimally detectable amount of LH (95% confidence limits of buffer controls) in three assays was 0.25 ± 0.18 ng/ml. The intra- and inter-assay coefficients of variation at mean ± SEM LH values of 9.63 ± 1.03 ng/ml were 5.3–14.5% and 18.4%, respectively. To determine the effect of insulin on the LH secretion and blood glucose level, data were analyzed by two-way analyses of variance (ANOVA): one factor was treatment (saline or insulin), and the other factor was time. This was followed by post-hoc analysis with the Fisher protected least significant difference test. Mann–Whitney U-test was used to test the significance of difference between treatment groups in the peak time of LH surge. Two-tailed unpaired t-test was used for analysis of the amplitude of the LH surge. Significance was attained at P , 0.05. In experiment 1, although the blood glucose level was significantly decreased by insulin injection, all the animals exhibited a clear surge of LH secretion as ordinary observed in proestrous rats (Fig. 1, left). The mean peak time of the LH surge and the mean serum LH concentrations at the peak time, referred to as the amplitude of the LH surge, in the
Fig. 1. Changes in serum concentrations of LH (lower chart) and blood glucose levels (upper chart) in saline (open circle) and insulin groups (filled circle). Five units of insulin was injected at 1300 h (left) or 1600 h (middle and right). Points, mean; vertical lines, SEM; arrows, time of injection. Numbers in parentheses show the numbers of animals *P , 0.05 versus the saline group.
133
M. Kawaguchi et al. / Neuroscience Letters 256 (1998) 131–134 Table 1 Effects of insulin injection on the peak time and the amplitude of the LH surge Experiment
1 2A 2B
Time of injection (h) 13 16 16
Mean peak time (h)
Mean serum LH (ng/ml)
Saline
Insulin
P
Saline
Insulin
P
17.4 ± 0.5 (7) 18.1 ± 0.3 (9) 16.8 ± 0.4 (5)
17.6 ± 0.3 (7) 17.9 ± 0.4 (7) 16.3 ± 0.2 (6)
NS NS NS
61.8 ± 3.6 42.5 ± 6.8 51.4 ± 8.9
55.2 ± 4.3 39.7 ± 9.2 43.2 ± 7.6
NS NS NS
Data are the mean ± SEM. Numbers in parentheses show the number of animals. NS, not significant.
saline and insulin groups were not significantly different (Table 1). In experiments 2A and 2B, although the blood glucose level was significantly decreased as in the first experiment, all the animals exhibited a clear surge of LH secretion as well. There was no significant difference between the saline and insulin group in the LH surge (the middle and right panels in Fig. 1). There was no statistical difference between saline and insulin group in the peak times in experiments 2A and 2B (Table 1). Although the amplitude of the LH surge seemed lower in the insulin group than in the saline group, this difference was not statistically significant (Table 1). Contrary to the well-established theory that the pulsatile LH secretion is sensitive to IIH [2,3,5,6], it was observed in the present study that IIH due to five units of insulin affected neither the onset nor continuing of the surge of LH secretion in the afternoon of proestrus. As mentioned before, we observed that 1–5 units of insulin inhibited pulsatile LH secretion [6]. Therefore, the present results strongly suggest that the surge of LH secretion is resistant to IIH, unlike the pulsatile secretion. The results are consistent with those obtained by Mangels et al. [12] in hamsters which were ovariectomized and treated with estrogen, as well as those of Morin in rats during the late stage of the estrous cycle [13]. Although Berriman et al. [1] reported that two populations of GnRH neurons, one for pulsatile and the other for surge secretion, responded similarly to metabolic fuels, this was probably because they tested the effect of food deprivation in diestrus only and not in proestrus. It is possible that food deprivation in diestrus results in a delay in the development of follicles, leading to a delay in ovulation. Taking these findings together, it appears that the CNS mechanism for the surge of LH secretion, i.e. the GnRH surge generator, is resistant to IIH. This assumption is consistent with our hypothesis that both the GnRH surge generator and the pulse generator are composed of different GnRH neurons and interneurons [11]. Indeed, the hypothesis is based on experimental findings which indicate that the two patterns of LH secretion have different sensitivity to pentobarbital sodium, naloxone [10] and bicuculline [9]. Here we can add one more characteristic, i.e. the one associated with IIH sensitivity. It was not ascertained in the present study, however, why the GnRH surge generator could work almost normally
under considerably low levels of blood glucose. There seems general agreement concerning the theory that the spike activity of neurons causes an increase in glucose utilization (see review by Raichle [15]). It is assumed that the surge secretion of GnRH, probably conducted by the GnRH surge generator in the hypothalamus as we have hypothesized, is produced by the spike activity of GnRH neurons. There were a large number of experiments in which electrical stimulation of the septo-preoptico-hypothalamic area caused ovulation as well as LH secretion [4,7,16]. Furthermore, an increase in electrical activity was recorded in the preoptic area in the afternoon of proestrus in rats [8] and in the median eminence of the ovariectomized, estrogen and progesterone primed rhesus monkeys [18], in association with ovulation and the LH surge, respectively. At the moment, therefore, the only interpretation possible is that neurons involved in the GnRH surge generator require less glucose for the spike activity than does the pulse generator. We are grateful to Dr. K. Wakabayashi of Gunma University School of Medicine and the NIDDK for providing radioimmunoassay materials. The present study was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Science, Sport and Culture, Japan (09480231) to F.K. [1] Berriman, S.J., Wade, G.N. and Blaustein, J.D., Expression of fos-like proteins in gonadotropin-releasing hormone neurons of Syrian hamsters: effects of estrous cycles and metabolic fuels, Endocrinology, 131 (1992) 2222–2228. [2] Chen, M.D., O’Byrne, K.T., Chiappini, S.E., Hotchkiss, J. and Knobil, E., Hypoglycemic ‘stress’ and gonadotropin-releasing hormone pulse generator activity in the rhesus monkey: role of the ovary, Neuroendocrinology, 56 (1992) 666–673. [3] Clarke, I.J., Horton, R.J.E. and Doughton, B.W., Investigation of the mechanism by which insulin-induced hypoglycemia decreases luteinizing hormone secretion in ovariectomized ewes, Endocrinology, 127 (1990) 1470–1476. [4] Everett, J.W., Neurobiology of reproduction in the female rat: a fifty-year perspective, Monogr. Endocrinol., 32 (1989) 1– 133. [5] Heisler, L.E., Pallotta, C.M., Reid, R.L. and Van Vugt, D.A., Hypoglycemia-induced inhibition of luteinizing hormone secretion in the rhesus monkey is not mediated by endogenous opioid peptides, J. Clin. Endocrinol. Metab., 76 (1993) 1280– 1285. [6] Ka, K., Funabashi, T., Uemura, T., Minakuchi, K. and Kimura, F., Stimulatory effects of glucose on the attenuation of the GnRH pulse generator by insulin-induced hypoglycemia in the
134
[7]
[8]
[9]
[10]
[11]
[12]
[13]
M. Kawaguchi et al. / Neuroscience Letters 256 (1998) 131–134 estrogen-primed ovariectomized rat, Neurosci. Res., 21 (Suppl.)(1997) S2326. Kawakami, M. and Terasawa, E., Effect of electrical stimulation of the brain on ovulation during estrous cycle in the rats, Endocrinol. Japon, 17 (1970) 7–13. Kawakami, M., Terasawa, E., Kimura, F., Higuchi, T. and Konda, N., Changes in multiunit electrical activity (MUA) in rat brain during the estrous cycle and after administration of sex steroids, Prog. Brain Res., 39 (1973) 125–134. Kimura, F. and Jinnai, K., Bicuculline infusions advance the timing of luteinizing hormone surge in proestrous rats: comparisons with naloxone effects, Horm. Behav., 28 (1994) 424–430. Kimura, F., Jinnai, K. and Sano, A., LHRH pulse generator is stimulated by naloxone in the pentobarbital-blocked proestrous rat, J. Neuroendocrinol., 7 (1995) 917–922. Kimura, F. and Funabashi, T., Two subgroups of gonadotropinreleasing hormone (GnRH) neurons control gonadotropin secretion in rats, News Physiol. Sci., 13 (1998) 225–231. Mangels, R.A., Jetton, A.E., Powers, J.B. and Wade, G.N., Food deprivation and the facilitatory effects of estrogen in female hamsters: the LH surge and locomotor activity, Physiol. Behav., 60 (1996) 837–843. Morin, L.P., Environment and hamster reproduction: responses
[14]
[15]
[16]
[17]
[18]
to phase-specific starvation during estrous cycle, Am. J. Physiol., 251 (1986) 663–669. Nagatani, S., Bucholtz, D.C., Murahashi, K., Estacio, M.A.C., Tsukamura, H., Foster, D.L. and Maeda, K.-I., Reduction of glucose availability suppresses pulsatile luteinizing hormone release in female and male rats, Endocrinology, 137 (1996) 1166–1170. Raichle, M.E., Circulatory and metabolic correlates of brain function in normal humans. In V.B. Mountcastle, F. Plum and S.R. Geiger (Eds.), Handbook of Physiology, Vol. 5, Section 1, American Physiological Society, Bethesda, MD, 1987, pp. 643– 674. Terasawa, E. and Sawyer, C.H., Electrical and electrochemical stimulation of the hypothalamo-adenohypophysial system with stainless steel electrodes, Endocrinology, 84 (1969) 918–925. Wade, G.N. and Schneider, J.E., Metabolic fuels and reproduction in female mammals, Neurosci. Biobehav. Rev., 16 (1992) 235–272. Yeoman, R.R. and Terasawa, E., An increase in single unit activity of the medial basal hypothalamus occurs during the progesterone-induced luteinizing hormone surge in the female rhesus monkey, Endocrinology, 115 (1984) 2445–2452.
Insulin-induced hypoglycemia does not impair the surge of luteinizing hormone secretion in the proestrous rat Maiko Kawaguchi, Toshiya Funabashi, Fukuko Kimura* Department of Physiology, Yokohama City University School of Medicine, 3–9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan Received 10 March 1998; received in revised form 18 September 1998; accepted 18 September 1998
Abstract To know whether insulin-induced hypoglycemia (IIH), which has been shown to inhibit pulsatile luteinizing hormone (LH) secretion, also affects the surge of LH secretion, adult female rats were injected with insulin (5 units/rat) or saline intravenously at 1300 or 1600 h on the day of proestrus and serum concentrations of LH and blood glucose were determined during the period from 1100 to 2100 h. The injection of insulin at neither 1300 nor 1600 h affected the surge of LH secretion, but it significantly decreased the blood glucose. Together with our recent hypothesis that the pulsatile and surge of LH secretion are controlled by separate gonadotropin-releasing hormone (GnRH) mechanisms, we conclude that the GnRH surge generator is less sensitive to IIH than the GnRH pulse generator. 1998 Elsevier Science Ireland Ltd. All rights reserved
Keywords: Insulin; Hypoglycemia; Luteinizing hormone surge; Proestrus; Rat; Luteinizing hormone surge generator
Availability of metabolic fuels is known to affect luteinizing hormone (LH) secretion (see [17] for review). For example, insulin-induced hypoglycemia (IIH) has been shown to decrease pulsatile LH secretion in females [2,3,5,6]. In the monkey, IIH inhibits pulsatile LH secretion by reducing the electrical activity of the GnRH pulse generator [2]. We also recently found that electrical activity of the GnRH pulse generator in ovariectomized estrogenprimed rats was reduced by IIH and resulted in a reduction in the pulsatile LH secretion as well [6]. Furthermore, fasting decreased the number of Fos expressing GnRH neurons, suggesting that it attenuates the activity of GnRH neurons [1]. Interestingly, Morin [13] reported that, in the hamster, the decrease in LH secretion caused by fasting on the days of estrus and diestrus 1 during the estrous cycle resulted in a blockade of the next expected ovulation, but fasting on the days of diestrus 2 and proestrus did not [13,14]. The report suggested that the surge of LH secretion might occur in spite of fasting. In fact, Mangels et al. [12], suggested that the fasting did not inhibit the surge of LH secretion induced by * Corresponding author. Tel.: +81 45 7872579; fax: +81 45 7872509; e-mail: [email protected]
0304-3940/98/$ - see front matter PII S0304- 3940(98) 00783- 6
estrogen-priming in the ovariectomized hamster. Therefore, the pulsatile LH secretion seems to be much more sen-sitive to the metabolic state than the surge of LH secretion. On the other hand, we have hypothesized that two different patterns of LH secretion, i.e. the pulsatile LH secretion and the surge of LH secretion, are controlled by separate GnRH mechanisms, the GnRH pulse generator and the GnRH surge generator, respectively (See details in review by Kimura and Funabashi [11]). A simple interpretation of the data mentioned above is that the surge generator may not be as sensitive to the metabolic state as the pulse generator is. For this reason, we investigated whether IIH affected the surge of LH secretion in proestrous rats. In the present study, we show, for the first time in rats, that IIH does not affect the surge of LH secretion. Intact female Wistar rats were obtained from Charles River Laboratories (Yokohama, Japan) at 7–8 weeks of age, and were maintained under controlled environmental conditions (temperature, 24 ± 1°C; lights on at 0500–1900 h) with food and water available ad libitum. Vaginal smears were taken daily and those which exhibited at least two consecutive 4-day estrous cycles were used in the present study. In the afternoon of the day of diestrus 2 (1500–1700 h), the rats received an intraatrial cannula implantation
1998 Elsevier Science Ireland Ltd. All rights reserved
132
M. Kawaguchi et al. / Neuroscience Letters 256 (1998) 131–134
under ether anesthesia [9]. All animal housing and surgical procedures were in accordance with the guidelines of the Institutional Animal Care and Use Committee of the Yokohama City University School of Medicine (project No. 9773). In the first experiment (experiment 1), the effect of IIH just before the onset of the LH surge, i.e. 1300 h, proestrus, was evaluated. Blood samples (approximately 170 ml) were collected under freely moving conditions at 1-h intervals between 1100 and 2100 h, along with simultaneous measurements of the blood glucose level by means of an instant blood glucose assay apparatus (Advantage; Boehringer Mannheim, Tokyo, Japan). An equal volume of heparinized saline (0.9% NaCl) was replaced after each bleeding. Animals were injected with five units of insulin (Humulin; Eli Lilly Japan, Kobe, Japan) in 125 ml (insulin group, n = 7) or the same volume of saline (saline group, n = 7) at 1300 h through the cannula after bleeding. This dose of insulin was chosen because an injection of 1–5 units of insulin significantly decreased the blood glucose and effectively inhibited the pulsatile LH secretion in ovariectomized estrogenprimed rats in our laboratory [6]. In the second experiment (experiment 2), the effect of IIH during the continuing surge of LH secretion, i.e. 1600 h, was evaluated. All experimental procedures were the same as the first one except for the time of injection. The rats were injected with five units of insulin (insulin group, n = 7) or the same volume of saline (saline group, n = 9) at 1600 h through the cannula after bleeding. The experimental protocol was repeated on another group of rats by the same procedures (saline group, n = 5; insulin group, n = 6), since we thought that IIH might affect LH secretion to
some degree. These were referred to as experiments 2A and 2B. Serum concentrations of LH were measured by doubleantibody radioimmunoassay with materials supplied by the National Institute of Diabetes and Digestive and Kidney Disease (NIDDK) and Dr. K. Wakabayashi. The reference standard used was NIDDK rat LH-RP-3, but the amount of LH is expressed in terms of NIH LH-S1 for easy comparison of the data published by us. The hormone assay was done for each experiment, and thus a total of three assays were run. The mean ± SD minimally detectable amount of LH (95% confidence limits of buffer controls) in three assays was 0.25 ± 0.18 ng/ml. The intra- and inter-assay coefficients of variation at mean ± SEM LH values of 9.63 ± 1.03 ng/ml were 5.3–14.5% and 18.4%, respectively. To determine the effect of insulin on the LH secretion and blood glucose level, data were analyzed by two-way analyses of variance (ANOVA): one factor was treatment (saline or insulin), and the other factor was time. This was followed by post-hoc analysis with the Fisher protected least significant difference test. Mann–Whitney U-test was used to test the significance of difference between treatment groups in the peak time of LH surge. Two-tailed unpaired t-test was used for analysis of the amplitude of the LH surge. Significance was attained at P , 0.05. In experiment 1, although the blood glucose level was significantly decreased by insulin injection, all the animals exhibited a clear surge of LH secretion as ordinary observed in proestrous rats (Fig. 1, left). The mean peak time of the LH surge and the mean serum LH concentrations at the peak time, referred to as the amplitude of the LH surge, in the
Fig. 1. Changes in serum concentrations of LH (lower chart) and blood glucose levels (upper chart) in saline (open circle) and insulin groups (filled circle). Five units of insulin was injected at 1300 h (left) or 1600 h (middle and right). Points, mean; vertical lines, SEM; arrows, time of injection. Numbers in parentheses show the numbers of animals *P , 0.05 versus the saline group.
133
M. Kawaguchi et al. / Neuroscience Letters 256 (1998) 131–134 Table 1 Effects of insulin injection on the peak time and the amplitude of the LH surge Experiment
1 2A 2B
Time of injection (h) 13 16 16
Mean peak time (h)
Mean serum LH (ng/ml)
Saline
Insulin
P
Saline
Insulin
P
17.4 ± 0.5 (7) 18.1 ± 0.3 (9) 16.8 ± 0.4 (5)
17.6 ± 0.3 (7) 17.9 ± 0.4 (7) 16.3 ± 0.2 (6)
NS NS NS
61.8 ± 3.6 42.5 ± 6.8 51.4 ± 8.9
55.2 ± 4.3 39.7 ± 9.2 43.2 ± 7.6
NS NS NS
Data are the mean ± SEM. Numbers in parentheses show the number of animals. NS, not significant.
saline and insulin groups were not significantly different (Table 1). In experiments 2A and 2B, although the blood glucose level was significantly decreased as in the first experiment, all the animals exhibited a clear surge of LH secretion as well. There was no significant difference between the saline and insulin group in the LH surge (the middle and right panels in Fig. 1). There was no statistical difference between saline and insulin group in the peak times in experiments 2A and 2B (Table 1). Although the amplitude of the LH surge seemed lower in the insulin group than in the saline group, this difference was not statistically significant (Table 1). Contrary to the well-established theory that the pulsatile LH secretion is sensitive to IIH [2,3,5,6], it was observed in the present study that IIH due to five units of insulin affected neither the onset nor continuing of the surge of LH secretion in the afternoon of proestrus. As mentioned before, we observed that 1–5 units of insulin inhibited pulsatile LH secretion [6]. Therefore, the present results strongly suggest that the surge of LH secretion is resistant to IIH, unlike the pulsatile secretion. The results are consistent with those obtained by Mangels et al. [12] in hamsters which were ovariectomized and treated with estrogen, as well as those of Morin in rats during the late stage of the estrous cycle [13]. Although Berriman et al. [1] reported that two populations of GnRH neurons, one for pulsatile and the other for surge secretion, responded similarly to metabolic fuels, this was probably because they tested the effect of food deprivation in diestrus only and not in proestrus. It is possible that food deprivation in diestrus results in a delay in the development of follicles, leading to a delay in ovulation. Taking these findings together, it appears that the CNS mechanism for the surge of LH secretion, i.e. the GnRH surge generator, is resistant to IIH. This assumption is consistent with our hypothesis that both the GnRH surge generator and the pulse generator are composed of different GnRH neurons and interneurons [11]. Indeed, the hypothesis is based on experimental findings which indicate that the two patterns of LH secretion have different sensitivity to pentobarbital sodium, naloxone [10] and bicuculline [9]. Here we can add one more characteristic, i.e. the one associated with IIH sensitivity. It was not ascertained in the present study, however, why the GnRH surge generator could work almost normally
under considerably low levels of blood glucose. There seems general agreement concerning the theory that the spike activity of neurons causes an increase in glucose utilization (see review by Raichle [15]). It is assumed that the surge secretion of GnRH, probably conducted by the GnRH surge generator in the hypothalamus as we have hypothesized, is produced by the spike activity of GnRH neurons. There were a large number of experiments in which electrical stimulation of the septo-preoptico-hypothalamic area caused ovulation as well as LH secretion [4,7,16]. Furthermore, an increase in electrical activity was recorded in the preoptic area in the afternoon of proestrus in rats [8] and in the median eminence of the ovariectomized, estrogen and progesterone primed rhesus monkeys [18], in association with ovulation and the LH surge, respectively. At the moment, therefore, the only interpretation possible is that neurons involved in the GnRH surge generator require less glucose for the spike activity than does the pulse generator. We are grateful to Dr. K. Wakabayashi of Gunma University School of Medicine and the NIDDK for providing radioimmunoassay materials. The present study was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Science, Sport and Culture, Japan (09480231) to F.K. [1] Berriman, S.J., Wade, G.N. and Blaustein, J.D., Expression of fos-like proteins in gonadotropin-releasing hormone neurons of Syrian hamsters: effects of estrous cycles and metabolic fuels, Endocrinology, 131 (1992) 2222–2228. [2] Chen, M.D., O’Byrne, K.T., Chiappini, S.E., Hotchkiss, J. and Knobil, E., Hypoglycemic ‘stress’ and gonadotropin-releasing hormone pulse generator activity in the rhesus monkey: role of the ovary, Neuroendocrinology, 56 (1992) 666–673. [3] Clarke, I.J., Horton, R.J.E. and Doughton, B.W., Investigation of the mechanism by which insulin-induced hypoglycemia decreases luteinizing hormone secretion in ovariectomized ewes, Endocrinology, 127 (1990) 1470–1476. [4] Everett, J.W., Neurobiology of reproduction in the female rat: a fifty-year perspective, Monogr. Endocrinol., 32 (1989) 1– 133. [5] Heisler, L.E., Pallotta, C.M., Reid, R.L. and Van Vugt, D.A., Hypoglycemia-induced inhibition of luteinizing hormone secretion in the rhesus monkey is not mediated by endogenous opioid peptides, J. Clin. Endocrinol. Metab., 76 (1993) 1280– 1285. [6] Ka, K., Funabashi, T., Uemura, T., Minakuchi, K. and Kimura, F., Stimulatory effects of glucose on the attenuation of the GnRH pulse generator by insulin-induced hypoglycemia in the
134
[7]
[8]
[9]
[10]
[11]
[12]
[13]
M. Kawaguchi et al. / Neuroscience Letters 256 (1998) 131–134 estrogen-primed ovariectomized rat, Neurosci. Res., 21 (Suppl.)(1997) S2326. Kawakami, M. and Terasawa, E., Effect of electrical stimulation of the brain on ovulation during estrous cycle in the rats, Endocrinol. Japon, 17 (1970) 7–13. Kawakami, M., Terasawa, E., Kimura, F., Higuchi, T. and Konda, N., Changes in multiunit electrical activity (MUA) in rat brain during the estrous cycle and after administration of sex steroids, Prog. Brain Res., 39 (1973) 125–134. Kimura, F. and Jinnai, K., Bicuculline infusions advance the timing of luteinizing hormone surge in proestrous rats: comparisons with naloxone effects, Horm. Behav., 28 (1994) 424–430. Kimura, F., Jinnai, K. and Sano, A., LHRH pulse generator is stimulated by naloxone in the pentobarbital-blocked proestrous rat, J. Neuroendocrinol., 7 (1995) 917–922. Kimura, F. and Funabashi, T., Two subgroups of gonadotropinreleasing hormone (GnRH) neurons control gonadotropin secretion in rats, News Physiol. Sci., 13 (1998) 225–231. Mangels, R.A., Jetton, A.E., Powers, J.B. and Wade, G.N., Food deprivation and the facilitatory effects of estrogen in female hamsters: the LH surge and locomotor activity, Physiol. Behav., 60 (1996) 837–843. Morin, L.P., Environment and hamster reproduction: responses
[14]
[15]
[16]
[17]
[18]
to phase-specific starvation during estrous cycle, Am. J. Physiol., 251 (1986) 663–669. Nagatani, S., Bucholtz, D.C., Murahashi, K., Estacio, M.A.C., Tsukamura, H., Foster, D.L. and Maeda, K.-I., Reduction of glucose availability suppresses pulsatile luteinizing hormone release in female and male rats, Endocrinology, 137 (1996) 1166–1170. Raichle, M.E., Circulatory and metabolic correlates of brain function in normal humans. In V.B. Mountcastle, F. Plum and S.R. Geiger (Eds.), Handbook of Physiology, Vol. 5, Section 1, American Physiological Society, Bethesda, MD, 1987, pp. 643– 674. Terasawa, E. and Sawyer, C.H., Electrical and electrochemical stimulation of the hypothalamo-adenohypophysial system with stainless steel electrodes, Endocrinology, 84 (1969) 918–925. Wade, G.N. and Schneider, J.E., Metabolic fuels and reproduction in female mammals, Neurosci. Biobehav. Rev., 16 (1992) 235–272. Yeoman, R.R. and Terasawa, E., An increase in single unit activity of the medial basal hypothalamus occurs during the progesterone-induced luteinizing hormone surge in the female rhesus monkey, Endocrinology, 115 (1984) 2445–2452.