Functional characteristics of the luteinizing hormone-releasing hormone pulse generator in conscious unrestrained female rabbits: activation by norepinephrine

New insights of the role of β-NGF in the ovulation mechanism of induced ovulating species

The type of stimuli triggering GnRH secretion has been used to classify mammalian species into two categories: spontaneous or induced ovulators. In the former, ovarian steroids produced by a mature follicle elicit the release of GnRH from the hypothalamus, but in the latter, GnRH secretion requires coital stimulation. However, the mechanism responsible for eliciting the preovulatory LH surge in induced ovulators is still not well understood and seems to vary among species. The main goal of this review is to offer new information regarding the mechanism that regulates coitus-induced ovulation. Analysis of several studies documenting the discovery of β-NGF in seminal plasma and its role in the control of ovulation in the llama and rabbit will be described. We also propose a working hypothesis regarding the sites of action of β-NGF in the llama hypothalamus. Finally, we described the presence of β-NGF in the semen of species categorized as spontaneous ovulators, mainly cattle, and its potential role in ovarian function. The discovery of this seminal molecule and its ovulatory effect in induced ovulators challenges previous concepts about the neuroendocrinology of reflex ovulation and has provided a new opportunity to examine the mechanism(s) involved in the cascade of events leading to ovulation. The presence of the factor in the semen of induced as well as spontaneous ovulators highlights the importance of understanding its signaling pathways and mechanism of action and may have broad implications in mammalian fertility.

Gonadotrophin-releasing hormone (GnRH) release in marmosets I: in vivo measurement in ovary-intact and ovariectomised females

In vivo hypothalamic gonadotrophin-releasing hormone (GnRH) release was characterised for the first time in a New World primate. A nonterminal and repeatable push-pull perfusion (PPP) technique reliably measured GnRH in conscious common marmoset monkeys. Nineteen adult females (n = 8 ovary-intact in the mid-follicular phase; n = 11 ovariectomised) were fitted with long-term cranial pedestals, and a push-pull cannula was temporarily placed in unique locations within the pituitary stalk-median eminence (S-ME) 2 days prior to each PPP session. Marmosets underwent 1-3 PPPs (32 PPPs in total) lasting up to 12 h. Plasma cortisol levels were not elevated in these habituated marmosets during PPP, and PPP did not disrupt ovulatory cyclicity or subsequent fertility in ovary-intact females. GnRH displayed an organised pattern of release, with pulses occurring every 50.0 +/- 2.6 min and lasting 25.4 +/- 1.3 min. GnRH pulse frequency was consistent within individual marmosets across multiple PPPs. GnRH mean concentration, baseline concentration and pulse amplitude varied predictably with anatomical location of the cannula tip within the S-ME. GnRH release increased characteristically in response to a norepinephrine infusion and decreased abruptly during the evening transition to lights off. Ovary-intact (mid-follicular phase) and ovariectomised marmosets did not differ significantly on any parameter of GnRH release. Overall, these results indicate that PPP can be used to reliably assess in vivo GnRH release in marmosets and will be a useful tool for future studies of reproductive neuroendocrinology in this small primate.

Control of LH secretory-burst frequency and interpulse-interval regularity in women

Hypothalamic neurons generate discrete bursts of gonadotropin-releasing hormone (GnRH) and thereby pulses of luteinizing hormone (LH) at randomly timed intervals centered on a probabilistic mean frequency. We tested the hypothesis that physiological mechanisms govern not only the number but also the stochastic dispersion of the GnRH/LH pulse-renewal process in humans; for example, in young women in the early (EF) and late (LF) follicular and midluteal (ML) phases of the menstrual cycle (n = 18) and in postmenopausal individuals (PM, n = 16). To this end, we quantify stochastic interpulse variability by way of the order-independent, two-parameter Weibull renewal process (Keenan DM and Veldhuis J. Am J Physiol Regul Integr Comp Physiol 281: R1917-R1924, 2001) and the sequence-specific, model-free approximate-entropy statistic (ApEn) (Pincus SM. Proc Natl Acad Sci USA 88: 2297-2301, 1991). Statistical testing unveiled 1) reduced probabilistic mean LH secretory-burst frequency (lower lambda of the Weibull distribution) in ML compared with each of EF, LF, and PM (P < 0.001); 2) quantifiably more regular LH interburst-interval sets (elevated gamma of the Weibull density) in PM than in each of EF, LF, and ML (P < 0.01); 3) uniquely prolonged latency to maximal LH secretion within individual secretory bursts in ML (P < 0.01); and 4) comparably mean random, sequential LH interburst-interval and mass values (normalized ApEn) among the distinct hormonal milieus. From these data, we postulate that sex steroids and age determine daily LH secretory-burst number, quantifiable pulse-renewal variability, and secretory-waveform evolution.

Effects of environmental lighting on early semen production and correlated hormonal responses in turkeys

Recent work at our institution on lighting turkey males for semen production and correlated changes in plasma luteinizing hormone (LH) and testosterone (T) are summarized in this paper. In sexually mature males, both LH and T are secreted in pulses, with a pulse of LH about 10 min prior to a pulse of T. Pulses of LH and T occurred about every 2 h and were equally distributed between the light (L) and dark (D) portions of a 14 h L:10 h D d. The pattern of secretion and overall concentrations of LH and T were not affected by intermittent photoperiod lighting (1 L:2 D, 8 x d) in comparison to continuous photoperiod lighting (14 L:10 D) lighting. Pulses of LH or T were not entrained by L or D with the intermittent or continuous lighting treatment. To study the interaction of age and lighting treatment, males were exposed to one of two lighting treatments: long-day photoperiods (16 L:8 D) d(-1) from 10 to 12 or 29 wk of age (WOA) (Treatment LDLD) or short-day photoperiods (6 L:18 D d(-1) from 10 or 12 to 29 WOA, then long-day photoperiods (Treatment SDLD). Males in the LDLD treatment attained puberty earlier (25 WOA) than those in the SDLD treatment. In the later treatment, most of the males attained puberty after 29 WOA. Both LH and T were low until 18 WOA in the LDLD males, then both increased to adult levels over the next 2 to 3 wk. In the SDLD males, LH and T were lower than in the LDLD males until 48 h after switching to long-day photoperiods, when both were transiently higher before declining to lower adult levels by 35 WOA. Secretory patterns of LH and T were estimated at 13, 23, and 35 WOA, under both lighting treatments. At 13 WOA, LH and T were secreted in pulses, but levels of both hormones were low and not different between lighting treatments, and none of the birds (0/4) in either treatment were producing semen. At 23 WOA, LH and T were secreted in robust pulses, with the LDLD males having higher concentrations of LH and T than the SDLD males. At 23 WOA, most of the males in the LDLD group (3/4) but none in the SDLD group (0/4) were producing semen. At 35 WOA, 6 wk after photostimulation of the SDLD group, all males (4/4) in both groups were producing semen, and LH and T were at adult levels. However, fewer pulses of T were noted for males in the SDLD treatment.

Oestrogen upregulates noradrenaline release in the mediobasal hypothalamus and tyrosine hydroxylase gene expression in the brainstem of ovariectomized rhesus macaques

Noradrenaline plays a key role in the initiation of ovulation in nonprimate species. A similar noradrenaline role in the primate has not been established experimentally. We utilized the ovariectomized-oestrogen-supplemented (OVX + E) rhesus macaque to examine the effects of intravenous (i.v.) infusion of oestradiol-17beta (E2) on the activity of the brain noradrenaline system. Experiment 1 established the induction of a preovulatory surge-like release of luteinizing hormone in OVX + E monkeys by i.v. infusion of E2 (OVX + E + E2). In experiment 2, a marked increase in hypothalamic microdialysate noradrenaline concentrations occurred after identical E2 infusion into the OVX + E monkeys that were used in experiment 1. In experiment 3, tyrosine hydroxylase (TH) mRNA expression in the locus coeruleus of the brainstem increased at various times after E2 infusion as determined by semiquantitative in situ hybridization. The amount of TH mRNA in OVX + E + E2 animals was higher (P < 0.05) than that in either the OVX + E or OVX monkeys; no difference was found in the latter two groups. Moreover, selected locus coeruleus sections from E2-infused monkeys were examined for the localization of oestrogen receptors (ER) by in situ hybridization. Both ER-alpha and ER-beta mRNAs were expressed in the locus coeruleus, although the expression was greater for ER-alpha than for ER-beta. We conclude that i.v. infusion of E2, which induces a preovulatory surge-like release of LH, stimulates brain noradrenaline activity; this enhanced activity likely involves an ER-mediated process and is reflected by hypothalamic noradrenaline release and locus coeruleus TH mRNA expression. The results support the concept that noradrenaline can influence the E2-stimulated ovulation in nonhuman primates and that the brainstem is one of the components in this neuroendocrine process.

Neuroendocrine regulation of GnRH release in induced ovulators

GnRH is the key neuropeptide controlling reproductive function in all vertebrate species. Two different neuroendocrine mechanisms have evolved among female mammals to regulate the mediobasal hypothalamic (MBH) release of GnRH leading to the preovulatory secretion of LH by the anterior pituitary gland. In females of spontaneously ovulating species, including rats, mice, guinea pigs, sheep, monkeys, and women, ovarian steroids secreted by maturing ovarian follicles induce a pulsatile pattern of GnRH release in the median eminence that, in turn, stimulates a preovulatory LH surge. In females of induced ovulating species, including rabbits, ferrets, cats, and camels, the preovulatory release of GnRH, and the resultant preovulatory LH surge, is induced by the receipt of genital somatosensory stimuli during mating. Induced ovulators generally do not show "spontaneous" steroid-induced LH surges during their reproductive cycles, suggesting that the positive feedback actions of steroid hormones on GnRH release are reduced or absent in these species. By contrast, mating-induced preovulatory surges occasionally occur in some spontaneously ovulating species. Most research in the field of GnRH neurobiology has been performed using spontaneous ovulators including rat, guinea pig, sheep, and rhesus monkey. This review summarizes the literature concerning the neuroendocrine mechanisms controlling GnRH biosynthesis and release in females of several induced ovulating species, and whenever possible it contrasts the results with those obtained for spontaneously ovulating species. It also considers the adaptive, evolutionary benefits and disadvantages of each type of ovulatory control mechanism. In females of induced ovulating species estradiol acts in the brain to induce aspects of proceptive and receptive sexual behavior. The primary mechanism involved in the preovulatory release of GnRH among induced ovulators involves the activation of midbrain and brainstem noradrenergic neurons in response to genital-somatosensory signals generated by receipt of an intromission from a male during mating. These noradrenergic neurons project to the MBH and, when activated, promote the release of GnRH from nerve terminals in the median eminence. In contrast to spontaneous ovulators, there is little evidence that endogenous opioid peptides normally inhibit MBH GnRH release among induced ovulators. Instead, the neural signals that induce a preovulatory LH surge in these species seem to be primarily excitatory. A complete understanding of the neuroendocrine control of ovulation will only be achieved in the future by comparative studies of several animal model systems in which mating-induced as well as spontaneous, hormonally stimulated activation of GnRH neurons drives the preovulatory LH surge.

Possible involvement of norepinephrine transporter activity in the pulsatility of hypothalamic gonadotropin-releasing hormone release: influence of the gonad

Pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH) is suppressed by alpha-adrenergic antagonists in ovariectomized (OVX) rabbits, thus suggesting that initiation of GnRH pulses requires the presence of norepinephrine (NE) stimulation. Terminals of NE neurons are located in proximity with GnRH cells in the hypothalamus, including the arcuate nucleus-median eminence (AME) region. Synaptic NE molecules may be catabolized or transported back to NE terminals (i.e. reuptake) via specific NE transporter proteins (NET). Thus, the amount of synaptic NE acting on GnRH cells is a function of the rate of NE release, metabolism and reuptake. Hypothetically, the rise and fall of a GnRH pulse may be associated with the similar fluctuations of synaptic NE release and/or NET activity. To test this hypothesis, we examined the effects of AME administration of desipramine (DMI, a specific NET blocking drug) on GnRH release. First, we delivered 0.2-10 mM doses of DMI continuously for 1 h via an AME microdialysis (microD) system into intact male rabbits. We found that each AME-DMI infusion, between dosages of 1 mM and 10 mM, stimulated a GnRH pulse, and that the size of these GnRH pulses were proportional to the dosage of DMI. To confirm the specificity of DMI on NET, we measured catecholamine content in microD samples by HPLC. The temporal (60 min) DMI induced a pattern of NE release that included a rising limb within the first 20-30 min; although NE returned to baseline values within the period of DMI treatment. Neither epinephrine nor dopamine levels were changed by DMI. Second, a median dose of DMI (5 mM) was given by microD for 60 min in four separate rabbit models: gonadal intact females (F-INT), intact males (M-INT), gonadectomized females (F-GDX) and castrated males (M-GDX). Individual microD samples were measured for NE and GnRH. Regardless of gender or gonadal status, 5 mM of DMI concomitantly induced a pulse-like release of NE and GnRH. Furthermore, the response of GnRH to DMI was greater in GDX rabbits than in INT animals of both genders. Third, we administered DMI (5 mM) for 30 min via a push-pull perfusion (PPP) system during four repeated 90 min intervals, in either F-INT or ovariectomized (F-GDX) females, and measured GnRH in PPP samples. In both F-INT and F-GDX, each DMI challenge induced a GnRH pulse. In F-INT, all sequential DMI-induced GnRH pulses were nearly equal in size. In contrast, in F-GDX, the first DMI-induced GnRH pulse was greater than subsequent ones. Collectively, these observations are consistent with the concept of noradrenergic regulation of pulsatile GnRH release, and we conclude that the temporal activity of NET may be an integral part of the mechanism by which GnRH pulses operate.

Topographic comparison of the expression of norepinephrine transporter, tyrosine hydroxylase and neuropeptide Y mRNA in association with dopamine beta-hydroxylase neurons in the rabbit brainstem

In mammalian species, ovulation occurs following a massive release of hypothalamic gonadotropin-releasing hormone (GnRH). Several chemicals, including norepinephrine (NE) and neuropeptide Y (NPY), are responsible for the initiation and/or magnitude and duration of this pre-ovulatory GnRH surge. In the central nervous system, NE neural cell bodies are located in the brainstem; some are co-localized with NPY neurons and/or co-express the NE transporter (NET) gene which dictates NET protein production. The activity of NET at NE terminals is critical for synaptic NE function. In the rabbit, coitus induces a hypothalamic NE release which precedes the GnRH surge. We hypothesize that the coital stimulus is transmitted to the brainstem and transformed and integrated into GnRH-stimulating signals via NE, NET and/or NPY. However, very little is known about the distribution of cells expressing NET, NPY and tyrosine hydroxylase (TH, the rate-limiting enzyme of NE synthesis) in this species. Therefore, we utilized the sensitive in situ hybridization technique to identify the presence of these messages in conjunction with the location of NE cells, the latter being marked by dopamine beta-hydroxylase (DBH), the specific enzyme for NE synthesis. Three non-mated New Zealand White does were perfused with 4% paraformaldehyde and their brainstems were sectioned at 20-micron thick between 2 mm caudal to the obex and the rostral pons. Serial sections were immunohistochemically stained for DBH and hybridized with rabbit-specific TH and NET cRNAs and a human NPY probe. The data suggest that several DBH-positive areas in the medulla expressed one or more messages, i.e. the lateral tegmentum (A1) and the nucleus of the solitary tract (A2) expressed all three mRNAs, the area postrema (AP) contained NET and TH mRNAs but not NPY cells. In the pons, the locus coeruleus (LC), subnucleus of coeruleus (LCs) and lateral tegmental nuclei (A5) expressed NET and TH mRNAs but contained little or no NPY message. The distribution patterns of TH and NET appeared to be similar in the LC, LCs, A2 and AP.

Associated luteinizing hormone-releasing hormone and luteinizing hormone secretion in ovariectomized gilts

The secretion of luteinizing hormone-releasing hormone (LHRH) and its temporal association with pulses of luteinizing hormone (LH) was examined in ovariectomized prepuberal gilts. Push-pull cannulae (PPC) were implanted within the anterior pituitary gland and LHRH was quantified from 10 min (200 microliters) perfusate samples. Serum LH concentrations were determined from jugular vein blood obtained at the midpoint of perfusate collection. Initial studies without collection of blood samples, indicated that LHRH secretion in the ovariectomized gilt was pulsatile with pulses comprised of one to three samples. However, most pulses were probably of rapid onset and short duration, since they comprised only one sample. Greater LHRH pulse amplitudes were associated with PPC locations within medial regions of the anterior pituitary close to the median eminence. In studies which involved blood collection, LH secretion was not affected by push-pull perfusion of the anterior pituitary gland in most gilts, however, adaptation of pigs to the sampling procedures was essential for prolonged sampling. There was a close temporal relationship between perfusate LHRH pulses and serum LH pulses with LHRH pulses occurring coincident or one sample preceding serum LH pulses. There were occasional LHRH pulses without LH pulses and LH pulses without detectable LHRH pulses. These results provide direct evidence that pulsatile LHRH secretion is associated with pulsatile LH secretion in ovariectomized gilts. In addition, PPC perfusion of the anterior pituitary is a viable procedure for assessing hypothalamic hypophyseal neurohormone relationships.

Prostaglandin E2 mediates the stimulatory effect of methoxamine on in vivo luteinizing hormone-releasing hormone (LH-RH) release in the ovariectomized female rhesus monkey

Previously, we found that noradrenergic input through alpha 1-receptors modulates pulsatile release of luteinizing hormone-releasing hormone (LH-RH) in ovariectomized rhesus monkeys in the absence of estrogen. In the present study, the role of prostaglandin E2 (PGE2) in mediating alpha-adrenergic stimulation of LH-RH release is investigated. In the first experiment the effects of the alpha 1-adrenergic agonist methoxamine (MTX) on LH-RH and PGE2 release were examined. Push-pull perfusion of the stalk-median eminence (S-ME) was performed in conscious, ovariectomized monkeys, and perfusate samples were collected on ice. MTX (10(-5) M) was infused into the S-ME through the push cannula for 10 min at 90-min intervals, and LH-RH and PGE2 in aliquots of the same perfusate samples were measured by radioimmunoassay. Infusion of MTX significantly stimulated LH-RH release (n = 12; P less than 0.01) and PGE2 release (P less than 0.05). In the second experiment, the effect of PGE2 infusion on LH-RH release was tested. PGE2 (10(-7) M) was infused using the same protocol as above, and LH-RH was measured in the perfusates. Infusion of PGE2 through the push cannula significantly stimulated LH-RH release (n = 23; P less than 0.05). These results suggest that the stimulatory effect of MTX on LH-RH release is at least partly mediated by PGE2, since MTX stimulated not only LH-RH but also PGE2 release, and since PGE2 itself stimulated LH-RH release. Therefore, PGE2 may be an important endogenous mediator of alpha 1-adrenergic input stimulating pulsatile LH-RH release.(ABSTRACT TRUNCATED AT 250 WORDS)

An immunohistochemical study of the GnRH neuron morphology and topography in the adult female rabbit hypothalamus

The morphology and distribution of immunoreactive (GnRH) neural elements in the hypothalamus of the adult nulliparous female rabbit were examined. Approximately 1,000 GnRH cells (range 890-1136) were counted in the right half of the hypothalamus. Two distinct GnRH cell types were observed: GnRH cells with rough or spiny contours accounted for 64% of the total immunoreactive cells, and smooth-contoured cells represented 34% of the total. The majority of immunoreactive neural elements were found in the anterior hypothalamus. GnRH cells and processes were located primarily in the ventral and medial anterior hypothalamus forming an inverted V pattern. Processes were followed from the medial preoptic area and suprachiasmatic nucleus to the infundibular stem. Extrahypothalamic projections of GnRH cells were observed. Immunoreactive fibers were also found to contact the ependymal lining of the third ventricle. It is concluded that two morphologically distinct GnRH cell types exist and have a broad distribution in the rabbit hypothalamus. The functional significance of these cell types requires further study.

Neuroendocrine regulation of the luteinizing hormone-releasing hormone pulse generator in the rat

We have analyzed the mechanisms by which several known regulators of the LHRH release process may exert their effects. For each, we have attempted to determine how and where the regulatory input is manifest and, according to our working premise, we have attempted to identify factors which specifically regulate the LHRH pulse generator. Of the five regulatory factors examined, we have identified two inputs whose primary locus of action is on the pulse-generating mechanism--one endocrine (gonadal negative feedback), and one synaptic (alpha 1-adrenergic inputs) (see Fig. 29). Other factors which regulate LHRH and LH release appear to do so in different ways. The endogenous opioid peptides, for example, primarily regulate LHRH pulse amplitude (Karahalios and Levine, 1988), a finding that is consistent with the idea that these peptides exert direct postsynaptic or presynaptic inhibition (Drouva et al., 1981). Gonadal steroids exert positive feedback actions which also result in an increase in the amplitude of LHRH release, and this action may be exerted through a combination of cellular mechanisms which culminate in the production of a unique, punctuated set of synaptic signals. Gonadal hormones and neurohormones such as NPY also exert complementary actions at the level of the pituitary gland, by modifying the responsiveness of the pituitary to the stimulatory actions of LHRH. The LHRH neurosecretory system thus appears to be regulated at many levels, and by a variety of neural and endocrine factors. We have found examples of (1) neural regulation of the pulse generator, (2) hormonal regulation of the pulse generator, (3) hormonal regulation of a neural circuit which produces a unique, punctuated synaptic signal, (4) hormonal regulation of pituitary responsiveness to LHRH, and (5) neuropeptidergic regulation of pituitary responsiveness to LHRH. While an attempt has been made to place some of these regulatory inputs into a physiological context, it is certainly recognized that the physiological significance of these mechanisms remains to be clarified. We also stress that these represent only a small subset of the neural and endocrine factors which regulate the secretion or actions of LHRH. A more comprehensive list would also include CRF, GABA, serotonin, and a variety of other important regulators. Through a combination of design and chance, however, we have been able to identify at least one major example of each type of regulatory mechanism.

Luteinizing hormone releasing hormone (LHRH) neuroterminals mapped using the push-pull perfusion method in the rhesus monkey

In vivo luteinizing hormone releasing hormone (LHRH) release was measured in conscious, ovariectomized rhesus monkeys using a push-pull cannula inserted into the stalk-median eminence, and the relationship between sampling location and LHRH release was examined. Within an individual animal in which multiple experiments were conducted with different cannula placements, LHRH pulse frequency was consistent. In contrast, LHRH pulse amplitude and mean LHRH release varied with cannula tip location in a pattern which reflected the anatomical distribution of LHRH-immunoreactive fibers described for the rhesus monkey. These results suggest that our push-pull perfusion method is reliable for the in vivo measurement of LHRH and perhaps other neuropeptides and/or neurotransmitters, as well.

Uterotropic 6-methoxybenzoxazolinone is an adrenergic agonist and a melatonin analog

6-Methoxybenzoxazolinone (MBOA) is a compound isolated from grasses which has gonadotropic effects in a variety of animals. The weak beta-adrenergic agonist character of MBOA is shown by its in vitro stimulation of adenylate cyclase from several tissues. Tritiated MBOA bound specifically to particulate fractions from uterus is also displaced by alpha- and beta-adrenergic compounds. The adrenergic properties of MBOA suggest it may exert diverse effects including direct actions on gonadotropin synthesis and release. The mixed adrenergic agonist ephedrine and the antidepressant imipramine were also found to be uterotropic in the vole Microtus montanus following injection protocols used with MBOA. MBOA is structurally similar to melatonin (5-methoxy-N-acetyltryptamine); [3H]melatonin which binds to uterine and pineal membranes is displaced by MBOA and by other adrenergic agents. The fact that MBOA is a beta-adrenergic agonist and a melatonin analog can account for stimulatory and inhibitory effects of this compound on sexual development.

Release of hypothalamic neuropeptide Y and effects of exogenous NPY on the release of hypothalamic GnRH and pituitary gonadotropins in intact and ovariectomized does in vitro

The effect of NPY on the hypothalamic release of GnRH and pituitary release of gonadotropins was examined in intact and ovariectomized (OVEX) rabbits in a superfusion system. Exposure of mediobasal hypothalami (MBH) from intact rabbits to NPY (8 X 10(-8) M) resulted in a sustained stimulation of GnRH secretion into the medium. The same dose of NPY had no effect on MBH-GnRH release from OVEX rabbits. NPY also produced a sustained stimulation of LH and FSH release by pituitary fragments from intact rabbits, but NPY caused only a transient release of these hormones by pituitaries from OVEX does. Media samples from MBH superfusions were also measured for NPY concentrations. NPY was released episodically into the medium. The amplitude and frequency of NPY pulses in intact and OVEX rabbits did not differ; nor were mean levels of NPY significantly affected by castration. These results suggest that NPY has direct effects on both the hypothalamus and pituitary to modulate the the activities of GnRH neurons and gonadotropes. The pattern of GnRH and gonadotropin response to NPY exposure is determined by ovarian factors.

Superfused pituitary cell cultures: comparative responsiveness of cells derived from various stages of the estrous cycle to LHRH stimulation administered as short duration pulses

We have reinvestigated the question of maintenance of differential LHRH sensitivity in culture and further investigated the role of pulsatile LHRH in the in vitro release of pulsatile LH and FSH at different stages of the estrous cycle. Pituitaries were collected on each day of the 4 day cycle at 0800. In addition, pituitaries were also collected at 1500 and 1900 on proestrous. The cells were dispersed and exposed 48 hrs later to short duration 4 ng LHRH pulses; this dose was optimized for LH release and was applied at a frequency of 1 pulse/60 min. In terms of absolute magnitude of LH response, observed responsiveness was ranked in the following order: proestrous 1900 greater than estrous 0800 greater than diestrous 1 0800 greater than proestrous 1500 greater than diestrous 2 0800. Responsiveness was significantly greater at proestrous 1900 (p greater than 0.01), estrous 0800 (p greater than 0.05) and diestrous 1 0800 (p greater than 0.05) when compared to either of the other stages tested. The heightened LHRH sensitivity of proestrous was therefore maintained in cell culture indicating that the system should be valid for conducting studies on the control of gonadotropin secretion during this period. FSH did not respond in pulsatile manner to the LHRH levels employed further substantiating recent evidence that LHRH seems to function somehow less directly in FSH as compared to LH secretion.

Terminal nerve damage impairs the mating behavior of the male hamster

We examined the effects of bilateral terminal nerve (TN) transections (TNx) on the sexual behavior of male hamsters. These lesions produced a decrease in mating frequency and/or an increase in the number of intromissions required to reach ejaculation. Damage to the olfactory bulbs or rostral forebrain did not account for these effects. No amelioration of the behavioral impairments occurred over the mating sessions. Basal testosterone levels in the blood of male hamsters were not altered by TN damage. Hamsters with TNx retained their ability to detect odors, but demonstrated reduced attraction to vaginal odors as compared with unoperated animals. The reduced attraction to vaginal odors was most pronounced in sporadically mating TNx animals. These data suggest that the TN may facilitate odor-induced sexual excitation in the male hamster.

Neuroendocrinology of female reproduction: review, models, and potential approaches for risk assessment

The central role of the mammalian hypothalamic-pituitary axis in regulating female reproductive cycles is reviewed. A variety of animal models and techniques that offer increased sensitivity, speed, and flexibility over traditional reproductive toxicologic approaches for short term testing or screening are discussed, including the pivotal analysis of gonadotrophin releasing hormone (GnRH) pulse generator activity in vivo and in vitro. Other neuroendocrine techniques that require further development, but provide potential approaches to demonstrate specific sites or mechanisms of action for toxic effects on the hypothalamic-pituitary axis are suggested.