Nitrendipine and omega-conotoxin modulate gonadotropin release and gonadotrope [Ca2+]i

Leptin increases L-type Ca2+ channel expression and GnRH-stimulated LH release in LbetaT2 gonadotropes

Leptin, a mediator of long-term regulation of energy balance, has been implicated in the release of adenohypophyseal gonadotropins by regulating gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus. However, a direct effect of leptin on hormone release from gonadotropes remains virtually unexplored. In the current report, we assessed the long-term (48 h) actions of leptin on voltage-gated channel activity and luteinizing hormone (LH) production in mouse pituitary gonadotrope LbetaT2 cells. Electrophysiological recordings showed that leptin treatment significantly increased whole-cell patch-clamp Ba(2+) current through L-type Ca(2+) channels. Quantitative RT-PCR analysis revealed increased levels of L-type (alpha(1D)) Ca(2+) channel mRNA. Likewise, radioimmunoassays using specific antibodies provided evidence that leptin alone had no effect on LH release but did enhance GnRH-induced secretion of the hormone. Leptin had no apparent effects on LH gene transcription in absence of GnRH, as measured by transient transfection assays using a LH promoter-reporter gene and real-time RT-PCR. These observations suggest that leptin might affect LH release by acting directly on the gonadotropes, favoring hormone production by enhancing responsiveness to GnRH as a result of increased Ca(2+) channel expression.

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

Relationship of reproductive hormones and neuromuscular disease of the gastrointestinal tract

Functional disorders of the gastrointestinal tract comprise a common but ill-defined group of diseases; they primarily afflict women. Although predominantly involving nerve and muscle, the cellular and molecular bases of the pathogenesis of these functional disorders are unknown. Clinical studies indicate that some result from neural dysfunction within the enteric nervous system, others may be due to muscular problems, and the causes of still others remain unknown. Laboratory studies have shown that ovarian products such as progesterone, luteinizing hormone, human chorionic gonadotropin, and relaxin (but not estrogen), are neural antagonists of gastrointestinal motility. The production and secretion of these ovarian substances are controlled by gonadotropin-releasing hormone (GnRH) released from the hypothalamus; they probably act on gamma-aminobutyric acid receptors and alter chloride influx into the cell. GnRH analogs are effective drugs that downmodulate the hypothalamic-pituitary-gonadal axis and inhibit the secretion of gonadal products involved in such hormone-dependent diseases as endometriosis and prostate cancer. Acting on the GnRH receptors (seven transmembrane domain receptors) on myenteric neurons, GnRH analogs are also effective neural modulators in such disorders as functional bowel disease. These analogs are a promising new group of compounds that may be used to treat difficult gastrointestinal problems.

L-type calcium channels mediate dynorphin neuropeptide release from dendrites but not axons of hippocampal granule cells

Granule cells in the guinea pig dentate gyrus release kappa opioid neuropeptides, dynorphins, from dendrites as well as from axon terminals. We have found that both L- and N-type calcium channel antagonists inhibited dendritic dynorphin release. In contrast, N-type but not L-type calcium channel antagonists inhibited axonal dynorphin release. Neither L- nor N-type channel antagonists directly altered the effects of kappa opioid receptor activation. By inhibiting dynorphin release, L-type channel antagonists also facilitated the induction of long-term potentiation of the perforant path-granule cell synapse. These studies establish that a single cell type can release a transmitter from two different cellular domains and provide new distinction between axonal and dendritic transmitter release mechanisms.

Testosterone sensitive dihydropyridine binding in the Harderian gland of the male hamster

It is well known that in different tissues, dihydropyridines bind at nanomolar concentrations to a receptor and block voltage-operated Ca2+ channels. In studies reported here, Harderian gland tissue homogenates from intact male hamsters exhibited significant dihydropyridine binding (Bmax = 1700 fmoles/mg protein) of high affinity (Kd = 1.1 nM). Tissue homogenates from female animals exhibited a similar Kd value (1.35 nM) but receptor density per mg protein was significantly reduced (Bmax = 270 fmoles). Dihydropyridine binding of Harderian gland tissue homogenates from castrated males was reduced greater than 80% (Bmax = 225 fmoles/mg protein). Treatment of castrated males with subcutaneous testosterone pellets resulted in significant restoration of dihydropyridine binding activity (approximately 80%, Bmax = 1630 fmoles/mg protein) with a comparable binding constant (Kd = 1.50 nM) as observed for noncastrated, control animals. Addition of testosterone (ex vivo) to homogenates from castrated hamsters did not restore dihydropyridine binding to control levels. These data indicate: (a) the Harderian gland from male hamsters exhibits significant dihydropyridine binding; (b) ligand binding is abolished following castration; and (c) significant restoration of dihydropyridine binding occurs following in vivo testosterone treatment. The dependence of dihydropyridine binding restoration upon in vivo steroid hormone administration suggests probable involvement of the steroid at the transcriptional level although non-genomic mechanisms such as the binding of testosterone to a receptor resident in the plasma membrane and subsequent activation of Ca2+ channels can not be ruled out.

Transient but not oscillating component of the calcium mobilizing response to gonadotropin-releasing hormone depends on calcium influx in pituitary gonadotrophs

Gonadotropin-releasing hormone (GnRH)-stimulated changes in the cytosolic free Ca2+ concentration ([Ca2+]i) were studied in gonadotrophs cultured from 3-week ovariectomized rat pituitaries. One animal was used per cell preparation. [Ca2+]i was monitored in individual gonadotrophs by dual emission microspectrofluorimetry, using Indo-1 as the intracellular fluorescent Ca2+ probe. A short stimulation with GnRH evoked a complex concentration-dependent Ca2+ response in individual gonadotrophs. 0.1-1 nM GnRH triggered a series of sinusoidal-like [Ca2+]i oscillations superimposed upon a modest slow [Ca2+]i rise--the oscillating response mode--while 10-100 nM GnRH caused a biphasic increase in [Ca2+]i consisting of a monophasic transient and oscillations--the transient/oscillating response mode. Despite the consistency of Ca2+ responses, an inter-preparation heterogeneity of [Ca2+]i oscillations frequency was noticed. Moreover, we observed that, within a given cell preparation, the frequency of [Ca2+]i oscillations was independent of GnRH concentration whereas both peak [Ca2+]i and area under the [Ca2+]i versus time curve were concentration-dependent. Thus, in gonadotrophs, the presence of the GnRH signal would lead to [Ca2+]i oscillations, while the amplitude of the [Ca2+]i responses would code for the concentration of agonist. Both transient and oscillating components of GnRH responses depended on releasing activity of Ca(2+)-sequestering pools in as much as GnRH responses were unaffected by brief removal of external Ca2+, but suppressed by chelating intracellular free Ca2+ with BAPTA. However, prolonged exposure to a Ca(2+)-free medium suppressed the transient component while leaving the oscillating component unaffected. We therefore propose that gonadotrophs employ Ca(2+)-sequestering pools, whose maintenance depends on a slow Ca(2+)-entry, to give an amplitude-coded Ca2+ rise in response to a short GnRH stimulation.

Smithkline Beecham Prize for Young Psychopharmacologists: A review of the relationship between calcium channels and psychiatric disorders

The symptoms and etiology of most major psychiatric disorders probably represent an underlying disturbance of neurotransmitter function. Understanding the mechanisms which control neurotransmitter function, and in particular neurotransmitter release, is therefore of considerable importance in determining the appropriate pharmacological treatment for these disorders. Calcium entry into neurons triggers the release of a wide range of neurotransmitters and recently our understanding of the mechanisms which control neuronal calcium entry has increased considerably. Neuronal calcium entry occurs through either voltage-sensitive or receptor-operated calcium channels. This article reviews the different subtypes of calcium channel, with particular reference to their structure; drugs which act upon them; and the possible function of the subtypes identified to date. In addition, it reviews the potential role of calcium channel antagonists in the treatment of a wide range of psychiatric disorders, and concludes that these drugs may have an increasing therapeutic role particularly in the treatment of drug dependence, mood disorders and Alzheimer's disease.