[Frontiers in Bioscience 2, d88-125, March 1, 1997]

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Bianca Marchetti

Department of Pharmacology, Medical School, University of Catania, 95125 Catania, Laboratory of Biotech. Neuropharmacology, OASI Institute for Research and Care (IRCCS) on Mental Retardation and Brain Aging (IRCCS) Troina, (EN), Italy.

Received 8/2/96; Accepted 2/20/97; On-line 3/1/97

7. The LHRH Neuron-Astroglia Network of Signals

7.1. LHRH as the Primum Movens in the Neuro-Endocrine-Immune Reproductive Axis

Neuro-endocrine-immunomodulation (NEI) represents a significant means whereby hormones, growth factors, neuroactive substances and soluble immune mediators convey and translate information to the different neuronal and non-neuronal elements of the CNS. Indeed, evidence, accumulated in the last decades, has clearly documented the vital importance of interacting neuroendocrine-immune networks in the regulation of physiological homeostatic mechanisms (for review see 129-141). In particular, from the early studies of Calzolari (142) almost a century ago, followed by subsequent intuitions of Besedowski (144) and Pierpaoli (143, 144) and more recently others (133, 134, 137-139, 145-153), the brain-pituitary-reproductive axis and the brain thymus-lymphoid axis have been shown to communicate via an array of internal mechanisms of communication that use similar signals (neurotransmitters, peptides, growth factors, hormones) acting on similar recognition targets (the receptors). Moreover, such communication networks form the basis for the controls of each step and every level of reproductive physiology. One such conveying signal is luteinizing hormone-releasing hormone (LHRH), the key reproductive hormone coordinating the major features of mammalian reproduction (Fig. 8).

Figure 8. Schematic representation of the possible interactions between the hypothalamus hypophyseal-gonadal axis and the thymus, with LHRH serving as a major channel of communication. Hypothalamic LHRH governs the release of the pituitary gonadotropins LH and FSH, responsible for gonadal production of the sex steroids. The gonadal hormones in turn, feed back information to the thymus and hypothalamus. At the thymus level, sex steroids act on specific receptors present on the reticulo-epithelial matrix, and induce both up/down regulation of target genes involved in the control of T-cell response. On the other hand, the sex steroid background alters the production of thymic peptides (thymosins) and neuropeptides such as LHRH, with autocrine/paracrine regulatory influence within the thymic microenvironment. The direct neural pathways innervating immune and endocrine organs together with the modulatory influence of glucocorticoids and catecholamines, are also indicated.

Luteinizing hormone-releasing hormone (LHRH), a decapeptide manufactured by highly specialized neuroendocrine cells, is the key regulator of the hypothalamic-hypophyseal-gonadal axis and is essential for reproductive competence (see 137-139, 153, 154). This hormone regulates the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the gonadotropic cells of the anterior pituitary gland (155).

Hypothalamic LHRH, released into portal capillaries that perfuse the anterior pituitary drives the menstrual cycle by stimulation of pituitary LH and FSH (see 155). Pituitary gonadotropin secretion is finely modulated by classical aminergic neuro-transmitters, the aminoacids, and the neuropeptides (for comprehensive review see 156), regulating the secretion of the "trigger" for the preovulatory surge of pituitary LH secretion on proestrus (Fig. 9).

Figure 9. Schematic representation of hypothalamic peptidergic and aminergic signals together with integrating environmental factors, glial and hypophyseal-mediated mechanisms in the control of the episodic discharge of LHRH. The model includes the LHRH pulse generator, the neural elements (the clock) regulating directly the activity of this generator, and those elements involved in its indirect regulation via the negative feedback action of gonadal steroids. A modulatory influence is represented by the action of sex steroids (estrogens, E2) impinging in this circuitry at both central and peripheral (hypophyseal level) via estrogen receptors, as well as by modifications in the number of pituitary LHRH receptors responsible for alterations in the sensitivity of the gonadotropes to LHRH. Gonadal steroids may also influence astroglial cells to produce and release GFs impinging on the LHRH secretory machinery. The concomitant production of other peptides (i.e. galanine, GAL) together with LHRH and its influence in stimulating the proestrus LH surge (156) is also illustrated.

The episodic manner of LHRH secretion, an intrinsic property of LHRH neuronal networks (157-159) is adjusted by a local hypothalamic network composed of diverse signals including opiates, N-methyl-D-aspartate, t-aminobutyrate and a-adrenergic inputs, the intensity of which may vary according to the sex steroid priming (see 156) (Fig. 9). A further level of control is represented by the ability of the decapeptide to directly modulate its own secretion via an ultra-short feedback mechanism, by exerting both stimulatory and inhibitory actions in LHRH neuronal cells depending on its concentration and duration (153, 159).

7.2. The LHRH Neuronal System within the Central Framework of Immune Signaling Systems

The powerful interaction between the immunologically-derived soluble products (cytokines) and the LHRH system, at the CNS level (140, 141, 160-166) coupled to the immunomodulatory properties of LHRH and its potent analogs (see 147-150, 167-180), lend support to the notion that a commonality of signaling mechanism(s) exists between the immune and neuroendocrine cells. A number of cytokines have been shown to affect LHRH release from the medio-basal-hypothalamus (MBH) either in vivo or in vitro. In particular, interleukin 1 (IL-1), one of the key mediators of immunological responses to stress, infection and antigenic challenge (see 140, 162, 181), has been shown to interfere powerfully with the hypothalamic-hypophyseal-gonadal axis (HHGA). At the CNS level (see 160-162, 164-166), when administrated in an acute fashion, IL-1 has been shown to decrease plasma LH levels, a phenomenon attributed to the inhibition of hypothalamic secretion of LHRH and LHRH gene expression. That IL-1 represents an extremely potent factor inhibiting the activity of the HHGA is supported by several different lines of evidences. Interleukin-1a inhibits pulsatile release of LH via a direct action on the LHRH neurons by suppressing the release of prostaglandin E2 (PGE2) from the MBH (see 164). Moreover, IL-1 administration inhibited of the physiological or experimentally induced afternoon proestrus LH surge follows (161), together with expression early c-fos gene which occurs within the LHRH cell nuclei during this same period of the cycle (166). The ability of endotoxin to induce release of IL-6 from the MBH has been demonstrated by Spangelo and coworkers (182). Moreover, hypothalamic LHRH neurons in culture spontaneously secrete IL-6, and in turn exogenous IL-6 is able to stimulate LHRH release in a dose- and time-dependent fashion (163). The intermediacy of nitric oxide in IL-1-a control of LH in vivo and vitro has been recently established (165). In addition, it was demonstrated that when HHGA is chronically exposed to icv infusion of IL-1ß, a complete disruption of the estrous cycle, decreased biosynthesis/release of hypothalamic LHRH and gonadotropins was accompanied by a block in luteolisis of newly formed corpora lutea (CL) (166).

Interleukin-1 has been shown to be present in the cerebrospinal fluid, IL-1 mRNA is detected in normal brain and IL-1ß-like immunoreactivity in both hypothalamic and extrahypothalamic sites in human brain have been identified (for review see 140, 181). A major compartment of cytokine production is, however, represented by astroglial and microglial cells. It would, then, appear that according to the stage of the estrous cycle, the peptidergic and aminergic background, a number of potential interactions between the cytokines and the central LHRH system, may be envisaged (Fig. 9).

Further evidence for an interaction between LHRH and a central immune network came from the studies of Silverman and collaborators (183) that demonstrated the presence of a population of non-neuronal cells, recognized by LHRH-like immune material present in large numbers in the medial habenula of the ring dove, which presented all the features of mast cells. Therefore, it is possible that mast cell secretion into the brain (and other peripheral organs) may represent an additional delivery system for biologically active substances such as LHRH (183). In many regions, including the CNS, mast cells are innervated or in close proximity to nerve terminals, and can be stimulated to release their granular content by neuropeptides. Of particular interest, is the clinical observation that histamine secretion from mast cells and cutaneous anaphylaxis can be induced with LHRH and LHRH-agonists and antagonists, and that LHRH-agonists (LHRH-A) binding sites are present in mast cells (184, 185).

7.3. Growth Factor and Steroid Sensitivity of LHRH Neurons

The work of Ojeda and other authors, has clearly established a prominent role of polypeptide growth factors with neurotrophic activity in the developmental regulation of the hypothalamus (for extensive review see 186-190). These authors have postulated that the initiation of puberty involves the trans-synaptic stimulation of LHRH neurons by excitatory neurotransmitter system(s) and the facilitatory effects of GFs, that are suspected to act indirectly via activation of glial function (188). TGF-alpha mRNA levels increase gradually in both preoptic area and the MBH after the anestrous phase of puberty, reaching peak values on the afternoon of the first proestrus (186-188). Since parallel changes in hypothalamic astroglial IGF-I like immunoreactivity have been detected (107, 108) an interdependence of the two mechanisms has been suggested. Such findings coupled to the gender differences in astroglial IGF-I immunoreactivity and the reported fluctuations associated with the estrous cycle clearly underline the participation of this growth factor of astroglial origin in the hypothalamic control of sexual maturation (98, 99). Interestingly, an up- and down-regulation of LHRH release from the GT1-1 cell line has been recently measured following activation of growth factor receptors (43). This work suggested that the signaling pathways activated by different GFs (EGF, IGF-I and Ins) may influence the LHRH machinery, possibly via a crosstalk between the protein tyrosine kinase (PTK) and protein kinase C (PKC) transducing pathways.

The protein kinase A (PKA) and the PKC have been implicated in LHRH biosynthesis and secretion (191-197). LHRH release is affected by a number of neurotransmitters that act through the PKA and PKC/calcium second messenger systems. In fact, cAMP and LHRH levels in the hypothalamus vary in concert during the estrous cycle, and both are highly responsive to estrogens (195). Neurotransmitters can stimulate cAMP accumulation, or calcium influx, and their effects can be blocked in the absence of calcium. Moreover, the effects of these neurotransmitters can be mimicked by direct application of PKA or PKC activators (see 195, 196). Forskolin and phorbol esters, such as phorbol myristate acetate (PMA), activators of the PKA and PKC pathways respectively, have been shown to enhance LHRH mRNA steady state levels in the hypothalamus (194-196). In particular, PMA stimulated LHRH release from the GT1 cell line while inhibiting transcription of the pro-LHRH gene and suppressing LHRH messenger RNA (mRNA) levels. Using the GT1-7 immortalized LHRH cell line, Wetsel and coworkers have recently (195) shown that forskolin can produce changes in neuronal morphology while phorbol esters induced decreased neurite formation and cell-cell adhesion (195). Translational efficiency of LHRH mRNA has been also shown to be negatively regulated by phorbol esters in the GT1 cell line (197).

The nitric oxidergic pathway is importantly involved in the dynamic regulation of LHRH expression and peptide release (see 165, 198-199). The source of NO and NO synthase (see 199-201) have been claimed to reside in proximity or in the LHRH neuron itself. For instance, NO has been shown to maintain pulsatile LHRH release, to be involved in NE-induced stimulation of LHRH release, and in cytokine-induced inhibition of LHRH release with the participation of PGE2 (see 140, 165).