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Biology of Reproduction 64, 1160-1164 (2001)
© 2001 Society for the Study of Reproduction, Inc.

Regular Article

Fos-Induction in Gonadotropin-Releasing Hormone Neurons Receiving Vasoactive Intestinal Polypeptide Innervation Is Reduced in Middle-Aged Female Rats1

Kristine Krajnaka, Katherine L. Rosewellb, and Phyllis M. Wise2,b

a Department of Biology, West Virginia University, Morgantown, West Virginia 26506 b Department of Physiology, University of Kentucky Medical Center, Lexington, Kentucky 40536

ABSTRACT

A hallmark of reproductive aging in rats is a delay in the initiation and peak, and a decrease in the amplitude, of both proestrous and steroid-induced surges of LH and a decrease in the number of GnRH neurons that express Fos during the surge. The altered timing of the LH surge and the decline in Fos expression in GnRH neurons may be due to changes in the rhythmic expression of vasoactive intestinal polypeptide (VIP), a neuropeptide that carries time-of-day information from the circadian pacemaker, located in the suprachiasmatic nuclei (SCN), to GnRH neurons. The goals of our study were to determine if aging alters 1) the innervation of GnRH neurons by VIP and 2) the ability of VIP to activate GnRH neurons by examining the effects of aging on the number of GnRH neurons apposed by VIP fibers and the number of GnRH neurons that receive VIP input that express Fos. Immunocytochemistry for GnRH and VIP; or GnRH, VIP, and Fos was performed on tissue sections collected from young (2–4 mo), regularly cycling females and middle-aged (10–12 mo) females in constant estrus. The number of GnRH neurons, GnRH neurons apposed by VIP fibers, and GnRH neurons that express Fos and apposed by VIP fibers were counted in both age groups. Our results clearly demonstrate that aging does not alter the number of GnRH neurons that receive VIP innervation. However, the number of GnRH neurons that receive VIP innervation and coexpress Fos decreases significantly. We conclude that the age-related delay in the timing of the LH surge is not due to a change in VIP innervation of GnRH neurons, but instead may result from a decreased sensitivity of GnRH neurons to VIP input.

aging, circadian rhythm, gonadotropin-releasing hormone, hypothalamus, neuropeptides, suprachiasmatic nucleus

INTRODUCTION

In spontaneously ovulating mammals, synchronization of ovulation and sexual receptivity greatly enhance the chances of fertilization. On the day of proestrus or in animals that are ovariectomized and receiving estradiol and progesterone replacement, high circulating concentrations of steroid hormones trigger the synchronized activation of GnRH neurons [1]. This activation is characterized by an increase in the expression of the immediate early gene product, Fos, in GnRH neurons just prior to and during the surge (reviewed in [2]) and a bolus release of GnRH into the portal vasculature (reviewed in [3]). The synchronized activation of GnRH neurons stimulates preovulatory and steroid-induced surges of LH [2, 3]. The rise in circulating steroid hormone levels, the activation of GnRH neurons, and the surge of LH occur at a specific time in relation to environmental cues such as light/dark cycle [1, 4–6]. Several lines of evidence suggest that the precise timing of the events leading to the LH surge and the exact timing of the surge itself are regulated by a circadian pacemaker.

In mammals, the circadian pacemaker is located in the suprachiasmatic nuclei (SCN) of the hypothalamus (reviewed in [7]). The SCN generate circadian rhythms in behavior and physiological processes and entrain these endogenously generated rhythms to the light/dark cycle. Evidence indicating that the SCN regulate the timing of the preovulatory and steroid-induced LH surge include the following: 1) rodents that are ovariectomized and treated with estradiol have daily afternoon surges of LH that occur at a specific time relative to the light/dark cycle, indicating that the circadian signal is produced every day, but requires high estradiol levels to be expressed [4, 5]; 2) female hamsters housed in light/dark cycles of varying lengths (i.e., 21–24 h) also vary the length of their reproductive cycle such that a single estrous cycle is exactly four times the length of the light/dark cycle the animals are exposed to [8]; 3) female hamsters housed in the absence of any environmental light cues (i.e., constant darkness) continue to show estrous cycles with a period of approximately four times the endogenous, free-running rhythm of that animal [9]; and 4) lesions of the SCN abolish estrous cycles in gonadally intact females [10–12] and eliminate daily afternoon surges of LH in estrogen-treated ovariectomized females [13]. Thus, we can conclude that the rhythm that regulates the LH surge and estrous cycle is generated endogenously by a circadian pacemaker located in the SCN.

As females age they begin to display estrous cycles of irregular lengths (i.e., not equal to 4 or 5 days) and changes in the timing of various hormonal rhythms [14]. By the time females reach middle age (10–12 mo), both the initiation and timing of the peak of the LH surge are delayed by about an hour, and the amplitude of the surge is altered compared with that in young animals [14, 15]. The loss of the precise timing of the surge in relationship to the light/dark cycle is associated with the severe blunting in the expression of a number of neurotransmitters, neuropeptides, and receptor transcripts [16] (and reviewed in [17]) that have been shown to affect LH release. Because middle-aged females display changes in the expression of a number of different rhythms, it is likely that this more general loss of rhythmicity is due to changes in the functioning of the SCN.

One major neuropeptide of the SCN that transmits time-of-day information to efferent targets and that has been implicated in regulating the timing of LH surge is vasoactive intestinal polypeptide (VIP). VIP is synthesized by neurons in the ventrolateral portion of the SCN [18]. These neurons send axons to a number of different brain regions, including but not limited to, the organum vasculosum of the lamina terminalis (OVLT), preoptic area (POA), and lateral hypothalamus, where they synapse on GnRH neurons [19]. A number of studies indicate that VIP input to GnRH neurons may regulate the timing and amplitude of the LH surge, and it has been hypothesized that that age-related changes in this pathway are, in part, responsible for alterations in the timing and amplitude of the LH surge [16, 19–25].

The goals of these studies were to examine how aging alters the communication between the SCN (i.e., VIP) and GnRH neurons. Aging may alter the ability of the SCN and VIP to transmit timing information to GnRH neurons either by 1) decreasing the number of VIP inputs to GnRH neurons, or 2) decreasing the sensitivity of GnRH neurons to VIP input. Immunocytochemistry (ICC) was used to determine if aging results in a decrease in the number of GnRH neurons apposed by VIP fibers. A decrease in the number of VIP-GnRH appositions in aged animals would indicate that the pathway between the SCN is weakened, and thus, the transmission of the circadian signal timing the LH surge may not be as strong in aged animals. ICC was also performed to determine if Fos expression in GnRH neurons receiving VIP input was reduced in middle-aged animals. If Fos induction is specifically reduced in GnRH neurons receiving VIP input, it would suggest that GnRH neurons may be less sensitivity to VIP input.

MATERIALS AND METHODS

General Animal Housing and Care

All surgeries and procedures performed in these studies were approved by the Institutional Animal Care and Use Committee at the University of Kentucky and complied with the regulations set forth by the Guidelines for the Use of Animals in Research, published by the National Institutes of Health. Female Sprague-Dawley rats were obtained from Zivic-Miller (Penelope, PA). All females were maintained on a 14L:10D cycle (lights-on 0400 h) and were provided with food and water ad libitum. Estrous cycles were monitored by daily vaginal lavage. In both experiments, young females showing at least two consecutive 4-day estrous cycles (2–4 mo, n = 24, experiment 1; n = 8, experiment 2) and middle-aged females in constant estrus (10–12 mo, n = 24, experiment 1; n = 8, experiment 2) were used. On the first day of the experiment (Day 0), animals were ovariectomized under methoxyflurane anesthesia (Metofane; Pitman-Moore, Washington Crossing, NJ). Animals were allowed to recover for 1 wk (Day 7) before receiving a Silastic capsule implant (s.c.) containing 17ß-estradiol (180 µg/ml in sesame oil). Young animals received a single (20 mm) capsule and middle-aged animals received a single (30 mm) capsule. On Day 9 of the experiment, animals were injected (s.c.) with progesterone (1.5 mg/animal) at 1000 h. This hormonal regime has been shown to increase Fos expression in GnRH neurons [6] and induce an LH surge in young animals [26].

Experiment 1 Procedures

Animals Young and middle-aged rats were deeply anesthetized with ketamine/Promace (120 mg/kg and 8 mg/kg, respectively), and intracardially perfused with 50 ml of 0.9% saline containing 1000 U/ml heparin followed by 350 ml of 4% paraformaldehyde in 0.1 M PBS pH 7.4 containing 2.5% acrolein at 1100, 1400, 1600, and 1800 h (six young and six middle-aged animals per time point). Brains were removed and postfixed overnight at room temperature in 4% paraformaldehyde. Finally, brains were incubated at 4°C in 20% sucrose in 0.1 M PBS for at least 48 h before sectioning.

Immunocytochemistry Coronal (30 µm) sections starting at the level of the medial diagonal band (Bregma 0.48) and continuing through the arcuate nucleus/median eminence (Bregma -4.30) [27] were cut on a freezing microtome. Six sets of sections were collected from each animal, with each set containing every sixth section. Sections were stored in cryoprotectant [28] at -20°C until processed for ICC. ICC for VIP and GnRH was performed on a single set of tissue sections from each animal using the following protocol: On Day 1, sections were rinsed (3 x for 10 min) in 0.1 M PBS to remove the cryoprotectant, incubated for 20 min in 0.1% sodium borohydride to block endogenous peroxidase activity and deactivate the acrolein, rinsed in 0.1 M PBS (6 x for 10 min), and incubated in rabbit anti-VIP (1:10 000, Diasorin, Stillwater, MN) in 0.1 M PBS plus 0.4% Triton-X 100 (PBS-Tx) and 4% normal goat serum overnight at 4°C. On Day 2, sections were rinsed in 0.1 M PBS (3 x for 5 min), incubated in biotinylated anti-goat immunoglobulin G (IgG) diluted 1:200 in PBS-Tx for 1 h, followed by an incubation in avidin-biotin complex (ABC) diluted 1:150 in PBS-Tx for 1 h (IgG and ABC from Vectastain Kit, Vector Laboratories Inc., Burlingame, CA). Finally, all sections were incubated in 0.02% diaminobenzidine (DAB) in Tris-buffered saline pH 7.2 with 50 µl/ml of 500 mM nickel chloride and 0.5 µl/ml of 30% hydrogen peroxide for 10 min at room temperature. Each step was followed by rinses in PBS (3 x for 5 min). After the DAB reaction, sections were rinsed (6 x for 10 min) in PBS and then incubated in rabbit anti-GnRH (LR-1, Benoit, Montreal, Canada) diluted 1:10 000 in PBS-Tx and 4% normal goat serum overnight at 4°C. The following day, sections were treated exactly as described for Day 2 of the procedure. However, instead of using DAB with nickel chloride as the chromagen, DAB was used alone. Using this method, VIP fibers appeared to be dark blue or black, and GnRH neurons appeared brown. All sections were mounted on slides and coverslips were placed over them. To test for nonspecific binding or cross-reactivity of antibodies, separate tissue sections were processed deleting the secondary antibody or one of the primary antibodies.

Experiment 2 Procedures

Animals Animals were anesthetized and transcardially perfused between 1600 and 1800 h using procedures identical to those described in experiment 1. Animals were perfused between these specific times because this is when Fos expression in GnRH neurons is at its peak in young animals [1, 2, 13].

Immunocytochemistry Brains were prepared and sectioned as described in the procedures for experiment 1. One set of sections (every sixth section) was processed for GnRH, Fos, and VIP by ICC. The only differences in the ICC protocol from this experiment and that used in experiment 1 was that on Day 1, sections were coincubated in Fos and VIP antibodies. The Fos antibody, which recognizes Fos and other Fra-related antigens (Santa Cruz Biotechnology, Inc., Santa Cruz, CA), was used at a final dilution of 1:1000.

Analysis

Experiment 1 Sections containing the OVLT and POA were identified. We chose to examine these specific regions because previous work has shown that VIP innervates GnRH neurons located predominantly in these regions [19]. In experiment 1, the total number of GnRH immunopositive (GnRH+) neurons and the number of GnRH+ neurons apposed by VIP immunopositive (VIP+) fibers were counted in these regions (2500x total magnification). We used the same criteria as previous studies to determine whether a VIP+ fiber apposed a GnRH+ neuron; GnRH+ somas, and proximal axons that came into direct contact with VIP+ immunopositive varicosities were considered to be in direct apposition [19]. A 2(age) x 4(time of day) ANOVA was performed to determine if aging, time of day, or both altered the number of GnRH+ or percent of GnRH neurons receiving VIP input (GnRH+VIP+ neurons). Pairwise comparisons were made using a Tukey-HSD. Differences with a probability of P < 0.05 were considered significant.

Experiment 2 The number of GnRH+ neurons, GnRH neurons that were Fos immunopositive (GnRH+Fos+), GnRH neurons receiving VIP input (GnRH+VIP+), and GnRH neurons expressing Fos and apposed by VIP fibers (GnRH+Fos+VIP+) (Fig. 1) were counted in the OVLT and POA. To determine if there was a change in the sensitivity of GnRH neurons to VIP input, the percentage of GnRH neurons receiving VIP that expressed Fos (GnRH+Fos+VIP+/GnRH+VIP+) and the percentage of GnRH neurons receiving VIP input but not expressing Fos (GnRH+VIP+ with no Fos/GnRH+VIP+) in young and middle-aged females was analyzed using Student t-tests. Because these t-tests were not independent, we corrected for family-wise error by multiplying the chosen level of significance for a single t-test (i.e., P < 0.05) by the number of t-tests being performed (i.e., 6 t-tests). Thus, in this experiment, significant differences were those with P < 0.008.



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  		FIG. 1. This photomicrograph shows GnRH+Fos+VIP+ neurons in the preoptic area of a young female rat at 1600 h. Arrows designate direct appositions between VIP fibers and GnRH neurons

RESULTS

In experiment 1, there was no effect of age (F(1,41) = 0.190, P = 0.661) or time of day (F(3, 41) = 2.033, P = 0.128) on the number of GnRH+ neurons (Fig. 2A). There also was no effect of age or time of day on the percent of GnRH+VIP+ neurons (F(1, 41) = 2.900, P = 0.108; Fig. 2B). Time of day also did not alter the number of GnRH+VIP+ neurons in either age group (F(3, 41) = 0.679, P = 0.571). Thus, reproductive aging does not alter the total number of GnRH neurons or VIP innervation of GnRH neurons in middle-aged animals.



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  		FIG. 2. Number of GnRH+ neurons per section in young and middle-aged animals over time (A) and the percent of GnRH+VIP+ neurons in the OVLT and POA of young and middle-aged animals over time (B). Time and age did not significantly affect these measures

The number of immunopositive neurons within each category is presented in Table 1. The results of experiment 2 were consistent with those of experiment 1. Aging did not affect the total number of GnRH+ neurons (t(12) = 0.496, P = 0.686; Fig. 3A) or the percent of GnRH+VIP+ neurons (t(12) = 2.621, P = 0.022; Fig. 3B). However, the percent of GnRH+Fos+ neurons (t(12) = 4.59, P = 0.0006) and the percent of GnRH+Fos+VIP+ neurons (t(12) = 3.902, P = 0.002) was significantly reduced in middle-aged females compared with young females (Fig. 3, C and D). The age-related decline in the percent of GnRH+Fos+VIP+ neurons could be due to an overall reduction in Fos expression in GnRH neurons, a decrease in VIP innervation of GnRH neurons expressing Fos, or both. On the other hand, it could also be due to a decreased sensitivity of GnRH neurons to VIP input. To determine if the decline in Fos induction was specifically reduced in GnRH+ neurons receiving VIP input, Student t-tests were performed to compare the percent of GnRH+VIP+ neurons that did express Fos and the percent that did not express Fos in young and middle-aged animals. The percent of GnRH+VIP+ neurons expressing Fos was greater in young females than in middle-aged females (t(12) = 3.183, P = 0.0079; Fig. 4A). In contrast, the percent of GnRH+VIP+ neurons not expressing Fos was greater in middle-aged animals than in young animals (t(12) = 3.204, P = 0.0076; Fig. 4B), indicating that the decline in Fos expression in GnRH neurons may be due to a decreased sensitivity of GnRH neurons to VIP stimulation.


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  		TABLE 1. Mean ± SEM immunopositive neurons per section in the OVLT (2–3 sections per animal) and POA (3–4 sections per animal) of young and middle-aged female rats



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  		FIG. 3. Number of GnRH+ neurons per section (A), the percent of GnRH+VIP+ neurons (B), the percent of GnRH+Fos+ neurons (C), and the percent of GnRH+Fos+VIP+ neurons (D) in the OVLT and POA of young and middle-aged females during the peak of a steroid-induced LH surge. The percent of GnRH+Fos+ neurons and GnRH+Fos+VIP+ neurons was significantly reduced in middle-aged animals compared with young animals (t(12) = 4.59, P = 0.0006 and t(12) = 3.90, P = 0.002, respectively). However, aging did not alter the number of GnRH+ neurons per section or the percent of GnRH+VIP+ neurons



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  		FIG. 4. Percent of GnRH+VIP+ neurons that express Fos (A) and the percent of GnRH+VIP+ neurons that do not express Fos (B). Aging resulted in a decrease in the number of GnRH+VIP+ neurons that expressed Fos (t(12) = 3.183, P = 0.0079). Thus, as would be predicted, the percent of GnRH+VIP+ neurons that do not express Fos is higher in middle-aged females than in young females (t(12) = 3.204, P = 0.0076)

DISCUSSION

We tested the hypothesis that the age-related decrease in Fos expression in GnRH neurons prior to a steroid-induced surge is due to a decrease in VIP input to GnRH neurons. To test this hypothesis we examined the effects of aging on the number of GnRH neurons apposed by VIP fibers to determine if there were fewer VIP appositions on GnRH neurons in middle-aged animals. The results of this study reveal that aging does not significantly alter the number of VIP fibers apposing GnRH neurons. Thus, the age-related decline in VIP mRNA levels and the blunting of the VIP mRNA rhythm in the SCN [16] is not associated with a decrease in the number of detectable axons apposing GnRH neurons. Based on these results, we suggest that aging does not alter the number of VIP fibers projecting from the SCN to GnRH neurons. However, to show that aging does not alter VIP innervation of GnRH neurons, futures studies will use electron microscopic methods to assess the number of synapses between VIP fibers and GnRH neurons.

We also tested the hypothesis that the age-related decline in Fos expression in GnRH neurons just prior to the surge was due to changes in the ability of VIP to stimulate GnRH neurons. To assess GnRH neuronal sensitivity to VIP input, we examined the effects of aging on Fos expression in GnRH neurons receiving VIP input to determine if aging specifically reduced Fos expression in GnRH neurons receiving VIP input. Both the numbers of GnRH+Fos+ and GnRH+Fos+VIP+ neurons decreased significantly in middle-aged females compared with young females. By analyzing the number of GnRH+VIP+ neurons that did not express Fos we were able to show that there was an increase in the number of GnRH+VIP+ neurons not expressing Fos in middle-aged animals. Based on these results, we hypothesize that aging reduces the ability of VIP stimulate Fos induction in GnRH neurons.

This age-related reduction in the ability of VIP to stimulate Fos expression in GnRH neurons could be due to a number of age-related changes in the neuroendocrine system or in the output of the SCN. First, VIP release may be reduced in middle-aged animals, and this reduction in VIP release may result in lower levels of Fos induction in GnRH neurons. A number of studies suggest that this may be the case. First, intracerebroventricular infusions of VIP also result in increased Fos expression in GnRH neurons and an increase in LH release in young females [24], suggesting that VIP-induced Fos induction plays an important role in regulating LH release. Second, overall levels of VIP mRNA are reduced in middle-aged and old females compared with young females [16]. This reduction in mRNA could result in a decrease in VIP peptide levels. Finally, if VIP action is blocked in young animals either by administering antisense oligonucleotides to the SCN [4] or by intracerebroventricular administration of a VIP antibody [25], then steroid-induced LH surges are delayed or the amplitude is blunted. Based on the results of these studies, we suggest that VIP acts to increase Fos expression in GnRH neurons and stimulate GnRH activity in the presence of high levels of circulating estrogen.

It is also possible that GnRH neurons are less sensitive to VIP input in middle-aged females. We have observed VIP2 receptors that colocalize with a significant number of GnRH neurons [29]. In addition, published reports indicate that VIP acts directly on GnRH neurons. First, electron microscopic studies show that VIP terminals synapse on GnRH neurons [19]. Second, VIP2 receptors have been found in GT1-7 cells. Treating GT1-7 cells with VIP results in an increase in cAMP and an increased release of GnRH from these cells [30]. Thus, it is possible that aging results in a decrease in the density of VIP receptors on GnRH cells, thereby causing a decrease in sensitivity to VIP input. The coupling between VIP receptors and their cell signaling pathways (i.e., cAMP) may also be altered by aging, making VIP signaling less effective.

Finally, steroid gating of the VIP signal may be altered in middle-aged animals. The circadian signal regulating the timing of the LH surge is only expressed when circulating estrogen levels are high [4, 5]. VIP appears to be one of the outputs from the SCN, carrying the circadian signal that times the LH surge. The rhythmic expression of VIP mRNA within the SCN is different in males than it is in females, with females displaying a peak during the day, prior to the peak of the LH surge [21]. This daytime peak is present in proestrous, ovariectomized and estradiol-treated, and ovariectomized young females [21], as would be expected of a circadian signal. However, if aging alters the ability of estrogen to promote this circadian signal, changes in the timing of the LH surge certainly would be apparent.

In conclusion, the ability of VIP to induce Fos expression in GnRH neurons is altered by the time animals reach middle age. Because VIP plays a role in transmitting the circadian signal from the SCN to GnRH neurons, age-related alterations in the transmission of this signal may in part be responsible for the delay in the timing and blunting of the amplitude of the LH surge in middle-aged female rats.

FOOTNOTES

First decision: 21 August 2000.

1 Supported by National Institutes of Health grants AG 05755 to K.K. and AG 02224 to P.M.W. Back

2 Correspondence: Phyllis M. Wise, Department of Physiology, Medical Center, University of Kentucky, 800 Rose Street, Lexington, KY 40536. FAX: 859 323 1070; pmwise1{at}pop.uky.edu Back

Accepted: December 4, 2000.

Received: July 13, 2000.

REFERENCES

  1. Lee W-S, Smith MS, Hoffman GE. Progesterone enhances the surge of luteinizing hormone by increasing the activation of luteinizing hormone-releasing hormone neurons. Endocrinology 1990; 127:2604–2606.[Abstract/Free Full Text]
  2. Hoffman GE, Smith MS, Verbalis JG. c-Fos and related immediate early gene products as markers of activity in neuroendocrine systems. Front Neuroendocrinol 1993; 14:173–213.[CrossRef][Medline]
  3. Levine JE. New concepts of the neuroendocrine regulation of gonadotropin surges in rats. Biol Reprod 1997; 56:239–302.
  4. Legan SJ, Coon GA, Karsch FJ. Role of estrogen as initiator of daily LH surges in the ovariectomized rat. Endocrinology 1975; 96:50–56.[Abstract/Free Full Text]
  5. Legan SJ, Karsch FJ. A daily signal for the LH surge in the rat. Endocrinology 1975; 96:57–62.[Abstract/Free Full Text]
  6. Lloyd JM, Hoffman GE, Wise PM. Decline in immediate early gene expression in gonadotropin-releasing hormone neurons during proestrus in regularly cycling, middle-aged rats. Endocrinology 1994; 164:1800–1805.
  7. Miller JD, Morin LP, Schwartz WJ, Moore RY. New insights into the mammalian circadian clock. Sleep 1996; 19:641–667.[Medline]
  8. Carmichael MS, Nelson RJ, Zucker I. Hamster activity and estrous cycles: control by a single vs multiple circadian oscillator(s). Proc Natl Acad Sci U S A 1981; 78:7830–7834.[Abstract/Free Full Text]
  9. Stetson MH, Anderson PJ. Circadian pacemaker times gonadotropin release in free-running female hamsters. Am J Physiol 1980; 238:R23–R27.
  10. Gray GD, Sodersten P, Tallentire P, Davidson JM. Effects of lesions in various structures of the suprachiasmatic-preoptic region on LH regulation and sexual behaviour in female rats. Neuroendocrinology 1978; 25:174–179.[Medline]
  11. Stetson MH, Watson-Whitmyre M. Nucleus suprachiasmaticus: the biological pacemaker in the hamster? Science 1976; 191:197–199.
  12. Terasawa E, Weigand SJ, Bridson WE. A role for medial preoptic nucleus on afternoon of proestrous in female rats. Am J Physiol 1980; 238:E533–E539.
  13. Kawakami M, Arita J, Yoshioka E. Loss of estrogen-induced daily surges of prolactin and gonadotropins by suprachiasmatic nucleus lesions in ovariectomized rats. Endocrinology 1980; 106:1087–1092.[Abstract/Free Full Text]
  14. Wise PM. Alterations in proestrous, LH, FSH and prolactin surges in middle-aged female rats. Proc Soc Exp Biol Med 1982; 169:348–354.[CrossRef][Medline]
  15. Nass TE, LaPolt PS, Judd HL, Lu JHK. Alterations in ovarian steroid and gonadotrophin secretion preceding the cessation of regular oestrous cycles in aging female rats. J Endocrinol 1983; 100:43–50.[CrossRef]
  16. Krajnak K, Kashon ML, Rosewell KL, Wise PM. Aging alters the rhythmic expression of vasoactive intestinal polypeptide mRNA but not arginine vasopressin mRNA in the suprachiasmatic nuclei of female rats. J Neurosci 1998; 18:4767–4774.[Abstract/Free Full Text]
  17. Wise PM, Kashon ML, Krajnak KM, Rosewell KL, Cai A, Scarbrough K, Harney JP, McShane TM, Lloyd JM, Weiland NG. Aging of the female reproductive system: a window into brain aging. Recent Prog Horm Res 1997; 52:279–305.
  18. Card JP, Fitzpatrick-McElligott S, Gozes I, Baldino F Jr. Localization of vasopressin-, vasoactive intestinal polypeptide-, peptide histidine isoleucine- and somatostatin-mRNA in rat suprachiasmatic nucleus. Cell Tissue Res 1988; 252:307–315.[CrossRef][Medline]
  19. van der Beek EM, Horvath TL, Wiegant VM, van den Hurk R, Buijs RM. Evidence for a direct neuronal pathway from the suprachiasmatic nucleus to the gonadotropin-releasing hormone system: combined tracing and light and electron microscopic immunocytochemical studies. J Comp Neurol 1997; 384:569–579.[CrossRef][Medline]
  20. Harney JP, Scarbrough K, Rosewell KL, Wise PM. In vivo antisense antagonism of vasoactive intestinal peptide in the suprachiasmatic nuclei causes aging-like changes in the estradiol-induced luteinizing hormone and prolactin surges. Endocrinology 1996; 137:3696–3701.[Abstract]
  21. Krajnak K, Kashon ML, Rosewell KL, Wise PM. Sex differences in the daily rhythm of vasoactive intestinal polypeptide but not arginine vasopressin messenger ribonucleic acid in the suprachiasmatic nuclei. Endocrinology 1998; 139:4189–4196.[Abstract/Free Full Text]
  22. van der Beek EM, Wiegant VM, van der Honk HA, van den Hurk R, Buiks RM. Lesions of the suprachiasmatic nucleus indicate the presence of a direct vasoactive intestinal polypeptide-containing projection to gonadotropin-releasing hormone neurons in the female rat. J Neuroendocrinol 1993; 5:137–144.[Medline]
  23. van der Beek EM, van Oudheusden HJC, Buijs RM, van der Hurk HA, van der Honk R, Wiegant VM. Preferential induction of c-fos immunoreactivity in vasoactive intestinal polypeptide-innervated gonadotropin-releasing hormone neurons during a steroid-induced luteinizing hormone surge in the female rat. Endocrinology 1994; 134:2637–2644.
  24. van der Beek EM, Habing LJ, Swarts JJM. Changes in the activation of GnRH following central injections of VIP or VIP antiserum indicate a stimulatory role for central VIP in the regulation of LH release. Soc Neurosci Abstr 1999; 125(479.7):1187.
  25. van der Beek EM, Swarts HJM, Wiegant VM. Central administration of antiserum to vasoactive intestinal peptide delays and reduces luteinizing hormone and prolactin surges in ovariectomized, estrogen-treated rats. Neuroendocrinology 1999; 69:227–237.[CrossRef][Medline]
  26. Wise PM. Estradiol-induced daily luteinizing hormone and prolactin surges in young and middle-aged rats: correlations with age-related changes in pituitary responsiveness and catecholamine turnover rates in microdissected brain areas. Endocrinology 1984; 115:801–809.[Abstract/Free Full Text]
  27. Paxinos G, Watson C. The Rat Brain in Stereotaxic Coordinates, 4th ed. New York: Academic Press; 1997.
  28. Watson RE, Weigand SJ, Clough RW, Hoffman GE. Use of cryoprotectant to maintain long-term peptide immunoreactivity and tissue morphology. Peptides 1986; 7:155–159.[Medline]
  29. Smith MJ, Jennes L, Wise PM. Localization of the VIP2 receptor protein on GnRH neurons in the female rat. Endocrinology 2000; 141:4317–4320.[Abstract/Free Full Text]
  30. Olcese J, McArdle CA, Middendorff R, Greenland K. Pituitary adenylate cyclase-activating peptide and vasoactive intestinal peptide receptor expression in immortalized LHRH neurons. J Neuroendocrinol 1997; 9:937–943.[CrossRef][Medline]



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W. P. Williams III, E. M. Gibson, C. Wang, S. Tjho, N. Khattar, G. E. Bentley, K. Tsutsui, and L. J. Kriegsfeld
Proximate mechanisms driving circadian control of neuroendocrine function: lessons from the young and old
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G. S. Neal-Perry, G. D. Zeevalk, J. Shu, and A. M. Etgen
Restoration of the Luteinizing Hormone Surge in Middle-Aged Female Rats by Altering the Balance of GABA and Glutamate Transmission in the Medial Preoptic Area
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L. M. Gerhold and P. M. Wise
Vasoactive Intestinal Polypeptide Regulates Dynamic Changes in Astrocyte Morphometry: Impact on Gonadotropin-Releasing Hormone Neurons
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L. M. Gerhold, K. L. Rosewell, and P. M. Wise
Suppression of Vasoactive Intestinal Polypeptide in the Suprachiasmatic Nucleus Leads to Aging-Like Alterations in cAMP Rhythms and Activation of Gonadotropin-Releasing Hormone Neurons
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Y. Chaiseha, O. M. Youngren, and M. E. El Halawani
Expression of Vasoactive Intestinal Peptide Receptor Messenger RNA in the Hypothalamus and Pituitary Throughout the Turkey Reproductive Cycle
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P. M. Wise, M. J. Smith, D. B. Dubal, M. E. Wilson, S. W. Rau, A. B. Cashion, M. Bottner, and K. L. Rosewell
Neuroendocrine Modulation and Repercussions of Female Reproductive Aging
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