Angela C. Bauer-Dantoin

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.

Amplitude and frequency modulation of pulsatile luteinizing hormone-releasing hormone release

Summary1. A variety of neuroendocrine approaches has been used to characterize cellular mechanisms governing luteinizing hormone-releasing hormone (LHRH) pulse generation. We review recentin vivo microdialysis,in vitro superfusion, andin situ hybridization experiments in which we tested the hypothesis that the amplitude and frequency of LHRH pulses are subject to independent regulation via distinct and identifiable cellular pathways.2. Augmentation of LHRH pulse amplitude is proposed as a central feature of preovulatory LHRH surges. Three mechanisms are described which may contribute to this increase in LHRH pulse amplitude: (a) increased LHRH gene expression, (b) augmentation of facilitatory neurotransmission, and (c) increased responsiveness of LHRH neurons to afferent synaptic signals. Neuropeptide Y (NPY) is examined as a prototypical afferent transmitter regulating the generation of LHRH surges through the latter two mechanisms.3. Retardation of LHRH pulse generator frequency is postulated to mediate negative feedback actions of gonadal hormones. Evidence supporting this hypothesis is reviewed, including results ofin vivo monitoring experiments in which LHRH pulse frequency, but not amplitude, is shown to be increased following castration. A role for noradrenergic neurons as intervening targets of gonadal hormone negative feedback actions is discussed.4. Future directions for study of the LHRH pulse generator are suggested.

Testosterone-Dependent Effects of Galanin on Pituitary Luteinizing Hormone Secretion in Male Rats

Abstract Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion. The effects of galanin on pituitary LH secretion in male rats are relatively uncharacterized; thus, the present in vivo study was conducted 1) to examine the ability of galanin to affect basal or GnRH-stimulated LH secretion in male rats and 2) to determine whether the effects of galanin on LH secretion in male rats are testosterone-dependent. All three doses of galanin used (1, 5, and 10 μg/pulse) significantly enhanced GnRH-stimulated LH secretion in intact male rats. Only the highest dose of galanin directly stimulated LH secretion (without GnRH coadministration) in intact males. Galanin did not directly stimulate LH secretion or enhance GnRH-stimulated LH secretion in castrated male rats. In fact, the highest dose of galanin inhibited GnRH-stimulated LH secretion in castrated males. Upon testosterone replacement, the ability of galanin to directly stimulate LH secretion and to enhance GnRH-stimulated LH secretion was restored in castrated males. These results suggest a role for galanin in the regulation of LH release in male rats and demonstrate that testosterone upregulates the ability of the pituitary to respond to the stimulatory effects of galanin.

Gonadal Feedback Regulation of LHRH Release and Actions in the Rat

It is now well established that the intermittent discharge of luteinizing hormone releasing hormone into the hypophyseal portal vasculature serves as the primary neural determinant of pulsatile LH secretion (1–7). The endocrine (nonneural) regulation of pulsatile LH secretion, by contrast, is dominated by the actions of gonadal steroid hormones operating in both negative (males and females) and positive (females) feedback modes. Both forms of gonadal feedback regulation appear to be mediated by alterations in the amount or pattern of pulsatile LHRH release, as well as by modulation of pituitary responsiveness to the decapeptide. Given the central importance of these feedback loops in the maintenance of reproductive viability, it is not surprising that much attention has been focused on the nature of the neural and endocrine mechanisms that mediate both types of hormonal feedback. In the work summarized below, we have used a variety of approaches to measure, manipulate, and mimic LHRH and LH release patterns, so that we might analyze and compare the mechanisms by which positive and negative feedback actions may be manifest. The concept is advanced that negative and positive feedback regulations at the hypothalamic level differ not only in sign, but also in the fundamental mechanisms by which LHRH neurons may ultimately mediate these processes. Specifically, a case is made for frequency and amplitude modulation as the primary bases for negative and positive feedback regulation, respectively.