Rhythmic bursts of calcium transients in acute anterior pituitary slices

Modeling pulsativity in the hypothalamic-pituitary-adrenal hormonal axis

A new mathematical model for biological rhythms in the hypothalamic–pituitary–adrenal (HPA) axis is proposed. This model takes the form of a system of impulsive time-delay differential equations which include pulsatile release of adrenocorticotropin (ACTH) by the pituitary gland and a time delay for the release of glucocorticoid hormones by the adrenal gland. Numerical simulations demonstrate that the model’s response to periodic and circadian inputs from the hypothalamus are consistent with those generated by recent models which do not include a pulsatile pituitary. In contrast the oscillatory phenomena generated by the impulsive delay equation mode occur even if the time delay is zero. The observation that the time delay merely introduces a small phase shift suggesting that the effects of the adrenal gland are “downstream” to the origin of pulsativity. In addition, the model accounts for the occurrence of ultradian oscillations in an isolated pituitary gland. These observations suggest that principles of pulse modulated control, familiar to control engineers, may have an increasing role to play in understanding the HPA axis.

The GH-IGF-1 Axis in Circadian Rhythm

Organisms have developed common behavioral and physiological adaptations to the influence of the day/night cycle. The CLOCK system forms an internal circadian rhythm in the suprachiasmatic nucleus (SCN) during light/dark input. The SCN may synchronize the growth hormone (GH) secretion rhythm with the dimming cycle through somatostatin neurons, and the change of the clock system may be related to the pulsatile release of GH. The GH-insulin-like growth factor 1 (IGF-1) axis and clock system may interact further on the metabolism through regulatory pathways in peripheral organs. We have summarized the current clinical and animal evidence on the interaction of clock systems with the GH-IGF-1 axis and discussed their effects on metabolism.

Imaging of Endoplasmic Reticulum Ca2+ in the Intact Pituitary Gland of Transgenic Mice Expressing a Low Affinity Ca2+ Indicator

The adenohypophysis contains five secretory cell types (somatotrophs, lactotrophs, thyrotrophs, corticotrophs, and gonadotrophs), each secreting a different hormone, and controlled by different hypothalamic releasing hormones (HRHs). Exocytic secretion is regulated by cytosolic Ca²⁺ signals ([Ca²⁺]_C), which can be generated either by Ca²⁺ entry through the plasma membrane and/or by Ca²⁺ release from the endoplasmic reticulum (ER). In addition, Ca²⁺ entry signals can eventually be amplified by ER release via calcium-induced calcium release (CICR). We have investigated the contribution of ER Ca²⁺ release to the action of physiological agonists in pituitary gland. Changes of [Ca²⁺] in the ER ([Ca²⁺]_ER) were measured with the genetically encoded low-affinity Ca²⁺ sensor GAP3 targeted to the ER. We used a transgenic mouse strain that expressed erGAP3 driven by a ubiquitous promoter. Virtually all the pituitary cells were positive for the sensor. In order to mimick the physiological environment, intact pituitary glands or acute slices from the transgenic mouse were used to image [Ca²⁺]_ER. [Ca²⁺]_C was measured simultaneously with Rhod-2. Luteinizing hormone-releasing hormone (LHRH) or thyrotropin releasing hormone (TRH), two agonists known to elicit intracellular Ca²⁺ mobilization, provoked robust decreases of [Ca²⁺]_ER and concomitant rises of [Ca²⁺]_C. A smaller fraction of cells responded to thyrotropin releasing hormone (TRH). By contrast, depolarization with high K⁺ triggered a rise of [Ca²⁺]_C without a decrease of [Ca²⁺]_ER, indicating that the calcium-induced calcium-release (CICR) via ryanodine receptor amplification mechanism is not present in these cells. Our results show the potential of transgenic ER Ca²⁺ indicators as novel tools to explore intraorganellar Ca²⁺ dynamics in pituitary gland in situ.

BK Channels and the Control of the Pituitary

The pituitary gland provides the important link between the nervous system and the endocrine system and regulates a diverse range of physiological functions. The pituitary is connected to the hypothalamus by the pituitary stalk and is comprised primarily of two lobes. The anterior lobe consists of five hormone-secreting cell types which are electrically excitable and display single-spike action potentials as well as complex bursting patterns. Bursting is of particular interest as it raises intracellular calcium to a greater extent than spiking and is believed to underlie secretagogue-induced hormone secretion. BK channels have been identified as a key regulator of bursting in anterior pituitary cells. Experimental data and mathematical modeling have demonstrated that BK activation during the upstroke of an action potential results in a prolonged depolarization and an increase in intracellular calcium. In contrast, the posterior lobe is primarily composed of axonal projections of magnocellular neurosecretory cells which extend from the supraoptic and paraventricular nuclei of the hypothalamus. In these neuroendocrine cells, BK channel activation results in a decrease in excitability and hormone secretion. The opposite effect of BK channels in the anterior and posterior pituitary highlights the diverse role of BK channels in regulating the activity of excitable cells. Further studies of pituitary cell excitability and the specific role of BK channels would lead to a greater understanding of how pituitary cell excitability is regulated by both hypothalamic secretagogues and negative feedback loops, and could ultimately lead to novel treatments to pituitary-related disorders.

Variants in Ion Channel Genes Link Phenotypic Features of Bipolar Illness to Specific Neurobiological Process Domains

Recent advances in genome-wide association studies are pointing towards a major role for voltage-gated ion channels in neuropsychiatric disorders and, in particular, bipolar disorder (BD). The phenotype of BD is complex, with symptoms during mood episodes and deficits persisting between episodes. We have tried to elucidate the common neurobiological mechanisms associated with ion channel signaling in order to provide a new perspective on the clinical symptoms and possible endophenotypes seen in BD patients. We propose a model in which the multiple variants in genes coding for ion channel proteins would perturb motivational circuits, synaptic plasticity, myelination, hypothalamic-pituitary-adrenal axis function, circadian neuronal rhythms, and energy regulation. These changes in neurobiological mechanisms would manifest in endophenotypes of aberrant reward processing, white matter hyperintensities, deficits in executive function, altered frontolimbic connectivity, increased amygdala activity, increased melatonin suppression, decreased REM latency, and aberrant myo-inositol/ATP shuttling. The endophenotypes result in behaviors of poor impulse control, motivational changes, cognitive deficits, abnormal stress response, sleep disturbances, and energy changes involving different neurobiological process domains. The hypothesis is that these disturbances start with altered neural circuitry during development, following which multiple environmental triggers may disrupt the neuronal excitability balance through an activity-dependent molecular process, resulting in clinical mood episodes.

Intercellular communication within the rat anterior pituitary gland. XV. Properties of spontaneous and LHRH-induced Ca2+ transients in the transitional zone of the rat anterior pituitary in situ

In the transitional zone of the rat anterior pituitary, spontaneous and LHRH-induced Ca(2+) dynamics were visualized using fluo-4 fluorescence Ca(2+) imaging. A majority of cells exhibited spontaneous Ca(2+) transients, while small populations of cells remained quiescent. Approximately 70% of spontaneously active cells generated fast, oscillatory Ca(2+) transients that were inhibited by cyclopiazonic acid (10 μm) but not nicardipine (1 μm), suggesting that Ca(2+) handling by endoplasmic reticulum, but not Ca(2+) influx through voltage-dependent L-type Ca(2+) channels, plays a fundamental role in their generation. In the adult rat anterior pituitary, LHRH (100 μg/ml) caused a transient increase in the Ca(2+) level in a majority of preparations taken from the morning group rats killed between 0930 h and 1030 h. However, the second application of LHRH invariably failed to elevate Ca(2+) levels, suggesting that the long-lasting refractoriness to LHRH stimulation was developed upon the first challenge of LHRH. In contrast, LHRH had no effect in most preparations taken from the afternoon group rats euthanized between 1200 h and 1400 h. In the neonatal rat anterior pituitary, LHRH caused a suppression of spontaneous Ca(2+) transients. Strikingly, the second application of LHRH was capable of reproducing the suppression of Ca(2+) signals, indicating that the refractoriness to LHRH had not been established in neonatal rats. These results suggest that responsiveness to LHRH has a long-term refractoriness in adult rats, and that the physiological LHRH surge may be clocked in the morning. Moreover, LHRH-induced excitation and associated refractoriness appear to be incomplete in neonatal rats and may be acquired during development.

Intracellular and plasma membrane-initiated pathways involved in the [Ca2+]i elevations induced by iodothyronines (T3 and T2) in pituitary GH3 cells

The role of 3,5,3'-triiodo-l-thyronine (T3) and its metabolite 3,5-diiodo-l-thyronine (T2) in modulating the intracellular Ca(2+) concentration ([Ca(2+)](i)) and endogenous nitric oxide (NO) synthesis was evaluated in pituitary GH(3) cells in the absence or presence of extracellular Ca(2+). When applied in Ca(2+)-free solution, T2 and T3 increased [Ca(2+)](i), in a dose-dependent way, and NO levels. Inhibition of neuronal NO synthase by N(G)-nitro-l-arginine methyl ester and l-n(5)-(1-iminoethyl)ornithine hydrochloride significantly reduced the [Ca(2+)](i) increase induced by T2 and T3. However, while depletion of inositol trisphosphate-dependent Ca(2+) stores did not interfere with the T2- and T3-induced [Ca(2+)](i) increases, the inhibition of phosphatidylinositol 3-kinase by LY-294002 and the dominant negative form of Akt mutated at the ATP binding site prevented these effects. Furthermore, the mitochondrial protonophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone prevented the increases in both [Ca(2+)](i) and NO elicited by T2 or T3. Interestingly, rotenone blocked the early [Ca(2+)](i) increases elicited by T2 and T3, while antimycin prevented only that elicited by T3. Inhibition of mitochondrial Na(+)/Ca(2+) exchanger by CGP37157 significantly reduced the [Ca(2+)](i) increases induced by T2 and T3. In the presence of extracellular calcium (1.2 mM), under carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, T2 and T3 increased both [Ca(2+)](i) and intracellular Na(+) concentration; nimodipine reduced the [Ca(2+)](i) increases elicited by T2 and T3, but inhibition of NO synthase and blockade of the Na(+)/H(+) pump by 5-(N-ethyl-N-isopropyl)amiloride prevented only that elicited by T3; and CB-DMB, bisindolylmaleimide, and LY-294002 (inhibitors of the Na(+)/Ca(2+) exchanger, PKC, and phosphatidylinositol 3-kinase, respectively) failed to modify the T2- and T3-induced effects. Collectively, the present results suggest that T2 and T3 exert short-term nongenomic effects on intracellular calcium and NO by modulating plasma membrane and mitochondrial pathways that differ between these iodothyronines.

Coordination of calcium signals by pituitary endocrine cells in situ

The pulsatile secretion of hormones from the mammalian pituitary gland drives a wide range of homeostatic responses by dynamically altering the functional set-point of effector tissues. To accomplish this, endocrine cell populations residing within the intact pituitary display large-scale changes in coordinated calcium-spiking activity in response to various hypothalamic and peripheral inputs. Although the pituitary gland is structurally compartmentalized into specific and intermingled endocrine cell networks, providing a clear morphological basis for such coordinated activity, the mechanisms which facilitate the timely propagation of information between cells in situ remain largely unexplored. Therefore, the aim of the current review is to highlight the range of signalling modalities known to be employed by endocrine cells to coordinate intracellular calcium rises, and discuss how these mechanisms are integrated at the population level to orchestrate cell function and tissue output.

Molecular mechanisms of pituitary endocrine cell calcium handling

Endocrine pituitary cells express numerous voltage-gated Na(+), Ca(2+), K(+), and Cl(-) channels and several ligand-gated channels, and they fire action potentials spontaneously. Depending on the cell type, this electrical activity can generate localized or global Ca(2+) signals, the latter reaching the threshold for stimulus-secretion coupling. These cells also express numerous G-protein-coupled receptors, which can stimulate or silence electrical activity and Ca(2+) influx through voltage-gated Ca(2+) channels and hormone release. Receptors positively coupled to the adenylyl cyclase signaling pathway stimulate electrical activity with cAMP, which activates hyperpolarization-activated cyclic nucleotide-regulated channels directly, or by cAMP-dependent kinase-mediated phosphorylation of K(+), Na(+), Ca(2+), and/or non-selective cation-conducting channels. Receptors that are negatively coupled to adenylyl cyclase signaling pathways inhibit spontaneous electrical activity and accompanied Ca(2+) transients predominantly through the activation of inwardly rectifying K(+) channels and the inhibition of voltage-gated Ca(2+) channels. The Ca(2+)-mobilizing receptors activate inositol trisphosphate-gated Ca(2+) channels in the endoplasmic reticulum, leading to Ca(2+) release in an oscillatory or non-oscillatory manner, depending on the cell type. This Ca(2+) release causes a cell type-specific modulation of electrical activity and intracellular Ca(2+) handling.

Modulation of the tyrosine kinase receptor Ret/glial cell-derived neurotrophic factor (GDNF) signaling: a new player in reproduction induced anterior pituitary plasticity?

During gestation, parturition, and lactation, the endocrine axis of the dam must continually adapt to ensure the continual and healthy development of offspring. The anterior pituitary gland, which serves as the endocrine interface between the brain and periphery, undergoes adaptations that contribute to regulation of the reproductive axis. Growth factors and their receptors are potential candidates for intrapituitary and paracrine factors to participate in the functional and anatomical plasticity of the gland. We examined the involvement of the growth factor glial cell-derived neurotrophic factor (GDNF) and its receptor tyrosine kinase rearranged during transfection (Ret) in the physiological functional and anatomical plasticity of the anterior pituitary gland. We found that variations in both expression and subcellular localization of Ret during gestation and lactation are temporally correlated with changes in pituitary gland function. We showed that Ret/GDNF signaling could endorse two different functional roles depending on the physiological status. At the end of lactation and after weaning, Ret was colocalized with markers of apoptosis. We found that Ret could therefore act as a physiological dependence receptor capable of inducing apoptosis in the absence of GDNF. In addition, we identified the follicullostellate cell as a probable source for intrapituitary GDNF and proposed GDNF as a potential physiological modulator of endocrine cell function. During all stages studied, we showed that acute application of GDNF to pituitary slices was able to modulate both positively and negatively intracellular calcium activity. Altogether our results implicate Ret/GDNF as a potent pleiotropic factor able to influence pituitary physiology during a period of high plasticity.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Pituitary growth hormone network responses are sexually dimorphic and regulated by gonadal steroids in adulthood

There are well-recognized sex differences in many pituitary endocrine axes, usually thought to be generated by gonadal steroid imprinting of the neuroendocrine hypothalamus. However, the recognition that growth hormone (GH) cells are arranged in functionally organized networks raises the possibility that the responses of the network are different in males and females. We studied this by directly monitoring the calcium responses to an identical GH-releasing hormone (GHRH) stimulus in populations of individual GH cells in slices taken from male and female murine GH-eGFP pituitary glands. We found that the GH cell network responses are sexually dimorphic, with a higher proportion of responding cells in males than in females, correlated with greater GH release from male slices. Repetitive waves of calcium spiking activity were triggered by GHRH in some males, but were never observed in females. This was not due to a permanent difference in the network architecture between male and female mice; rather, the sex difference in the proportions of GH cells responding to GHRH were switched by postpubertal gonadectomy and reversed with hormone replacements, suggesting that the network responses are dynamically regulated in adulthood by gonadal steroids. Thus, the pituitary gland contributes to the sexually dimorphic patterns of GH secretion that play an important role in differences in growth and metabolism between the sexes.

Circadian timekeeping and multiple timescale neuroendocrine rhythms

The hormones secreted by the anterior pituitary gland regulate major functions such as reproduction, as well as body growth and metabolism. Their efficiency of action highly depends on their temporal profile of release in the blood stream. This review summarises the recent evidence suggesting that the circadian clock genes that pace our daily rhythms may also contribute to the regulation of pituitary pulsatility, even in the non 24-h range. This inter-relation between molecular circadian oscillators and endocrine rhythmicities is discussed in light of the longstanding literature that has considered the involvement of the central circadian pacemaker located within the suprachiasmatic nuclei. Other arguments that suggest a role for circadian clock genes outside the suprachiasmatic nuclei are also presented, with a special emphasis on endocrine pituitary cells and hypothalamic neuroendocrine neurones that directly pace pituitary secretion rates.

Pituitary somatostatin receptor signaling

Somatotropin-release inhibitory factor (SRIF) is a major regulator of pituitary function, mostly inhibiting hormone secretion and to a lesser extent pituitary cell growth. Five SRIF receptor subtypes (SSTR1-5) are ubiquitously expressed G-protein coupled receptors. In the pituitary, SSTR1, 2, 3 and 5 are expressed, with SSTR2 and SSTR5 predominating. As new SRIF analogs have recently been introduced for treatment of pituitary disease, we evaluate the current knowledge of cell-specific pituitary SRIF receptor signaling and highlight areas of future research for comprehensive understanding of these mechanisms. Elucidating pituitary SRIF receptor signaling enables understanding of pituitary hormone secretion and cell growth, and also encourages future therapeutic development for pituitary disorders.

The circadian clock components CRY1 and CRY2 are necessary to sustain sex dimorphism in mouse liver metabolism

In mammals, males and females exhibit anatomical, hormonal, and metabolic differences. A major example of such sex dimorphism in mouse involves hepatic drug metabolism, which is also a noticeable target of circadian timekeeping. However, whether the circadian clock itself contributes to sex-biased metabolism has remained unknown, although several daily output parameters differ between sexes in a number of species, including humans. Here we show that dimorphic liver metabolism is altered when the circadian regulators Cryptochromes, Cry1 and Cry2, are inactivated. Indeed, double mutant Cry1(-/-) Cry2(-/-) male mice that lack a functional circadian clock express a number of sex-specific liver products, including several cytochrome P450 enzymes, at levels close to those measured in females. In addition, body growth of Cry-deficient mice is impaired, also in a sex-biased manner, and this phenotype goes along with an altered pattern of circulating growth hormone (GH) in mutant males, specifically. It is noteworthy that hormonal injections able to mimic male GH pulses reversed the feminized gene expression profile in the liver of Cry1(-/-) Cry2(-/-) males. Altogether, our observations suggest that the 24-h clock paces the dimorphic ultradian pulsatility of GH that is responsible for sex-dependent liver activity. We thus conclude that circadian timing, sex dimorphism, and liver metabolism are finely interconnected.

Nitric oxide induces [Ca2+]i oscillations in pituitary GH3 cells: involvement of IDR and ERG K+ currents

The role of nitric oxide (NO) in the occurrence of intracellular Ca2+ concentration ([Ca2+]i) oscillations in pituitary GH3 cells was evaluated by studying the effect of increasing or decreasing endogenous NO synthesis with L-arginine and nitro-L-arginine methyl ester (L-NAME), respectively. When NO synthesis was blocked with L-NAME (1 mM) [Ca2+]i, oscillations disappeared in 68% of spontaneously active cells, whereas 41% of the quiescent cells showed [Ca2+]i oscillations in response to the NO synthase (NOS) substrate L-arginine (10 mM). This effect was reproduced by the NO donors NOC-18 and S-nitroso-N-acetylpenicillamine (SNAP). NOC-18 was ineffective in the presence of the L-type voltage-dependent Ca2+ channels (VDCC) blocker nimodipine (1 microM) or in Ca2+-free medium. Conversely, its effect was preserved when Ca2+ release from intracellular Ca2+ stores was inhibited either with the ryanodine-receptor blocker ryanodine (500 microM) or with the inositol 1,4,5-trisphosphate receptor blocker xestospongin C (3 microM). These results suggest that NO induces the appearance of [Ca2+]i oscillations by determining Ca2+ influx. Patch-clamp experiments excluded that NO acted directly on VDCC but suggested that NO determined membrane depolarization because of the inhibition of voltage-gated K+ channels. NOC-18 and SNAP caused a decrease in the amplitude of slow-inactivating (IDR) and ether-à-go-go-related gene (ERG) hyperpolarization-evoked, deactivating K+ currents. Similar results were obtained when GH3 cells were treated with L-arginine. The present study suggests that in GH3 cells, endogenous NO plays a permissive role for the occurrence of spontaneous [Ca2+]i oscillations through an inhibitory effect on IDR and on IERG.

Somatostatin receptor type 5 modulates somatostatin receptor type 2 regulation of adrenocorticotropin secretion

Somatostatin inhibits adrenocorticotropin (ACTH) secretion from pituitary tumor cells. To assess the contribution of somatostatin receptor subtype 5 (SST5) to somatostatin receptor subtype 2 (SST2) action in these cells, we assessed multipathway responses to novel highly monoreceptor-selective peptide agonists and multireceptor agonists, including octreotide and somatostatin-28. Octreotide and somatostatin-28 cell membrane binding affinities correlated with their respective SST2-selective peptide ligand. Although octreotide had similar inhibiting potency (picomolar) for cAMP accumulation and ACTH secretion as an SST2-selective agonist, somatostatin-28 exhibited a higher potency (femtomolar). Baseline spontaneous calcium oscillations assessed by fluorescent confocal microscopy revealed two distinct effects: SST2 activation reduced oscillations at femtomolar concentrations reflected by high inhibiting potency of averaged normalized oscillation amplitude, whereas SST5 activation induces brief oscillation pauses and increased oscillation amplitude. Octreotide exhibits an integrated effect of both receptors; however, somatostatin-28 exhibited a complex response with two separate inhibitory potencies. SST2 internalization was visualized with SST2-selective agonist at lower concentrations than for octreotide or somatostatin-28, whereas SST5 did not internalize. Using monoreceptor-selective peptide agonists, the results indicate that, in AtT-20 cells, SST5 regulates the dominant SST2 action, attenuating SST2 effects on intracellular calcium oscillation and internalization. This may explain superior somatostatin-28 potency and provides a rationale for somatostatin ligand design to treat ACTH-secreting pituitary tumors.

Cortisol rapidly suppresses intracellular calcium and voltage-gated calcium channel activity in prolactin cells of the tilapia (Oreochromis mossambicus)

Cortisol was previously shown to rapidly (10-20 min) reduce the release of prolactin (PRL) from pituitary glands of tilapia (Oreochromis mossambicus). This inhibition of PRL release by cortisol is accompanied by rapid reductions in (45)Ca(2+) and cAMP accumulation. Cortisol's early actions occur through a protein synthesis-independent pathway and are mimicked by a membrane-impermeable analog. The signaling pathway that mediates rapid, nongenomic membrane effects of glucocorticoids is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated and incubated in defined medium, we examined whether cortisol rapidly reduces intracellular free calcium (Ca(i)(2+)) and suppresses L-type voltage-gated ion channel activity in events that lead to reduced PRL release. Microspectrofluorometry, used in combination with the Ca(2+)-sensitive dye fura 2 revealed that cortisol reversibly reduces basal and hyposmotically induced Ca(i)(2+) within seconds (P < 0.001) in dispersed pituitary cells. Somatostatin, a peptide known to inhibit PRL release through a membrane receptor-coupled mechanism, similarly reduces Ca(i)(2+). Under depolarizing [K(+)], the L-type calcium channel agonist BAY K 8644, a factor known to delay the closing of L-type Ca(2+) channels, stimulates PRL release in a concentration-dependent fashion (P < 0.01). Cortisol (and somatostatin) blocks BAY K 8644-induced PRL release (P < 0.01; 30 min), well within the time course over which its actions occur, independent of protein synthesis and at the level of the plasma membrane. Results indicate that cortisol inhibits tilapia PRL release through rapid reductions in Ca(i)(2+) that likely involve an attenuation of Ca(2+) entry through L-type voltage-gated Ca(2+) channels. These results provide further evidence that glucocorticoids rapidly modulate hormone secretion via a membrane-associated mechanism similar to that observed with the fast effects of peptides and neurotransmitters.

Electrical activity in endocrine pituitary cells in situ: a support for a multiple-function coding

The anterior pituitary is an endocrine gland that controls basic body functions. Pituitary cell functioning depends on membrane excitability, which induces cytosolic calcium rises. Here, we reported the first identification of small-amplitude voltage fluctuations that controlled spike firing in endocrine cells recorded in situ. Three patterns of voltage fluctuations were distinguishable by their durations (1-100 s). These patterns could be ordered on top of each other, namely in response to secretagogues. Thus, pituitary endocrine cells express in situ a cell code in which small-amplitude voltage fluctuations lead to a multimodal arrangement of spike firing, which may finely tune calcium-dependent functions.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Dependence of pituitary hormone secretion on the pattern of spontaneous voltage-gated calcium influx. Cell type-specific action potential secretion coupling

In excitable cells, voltage-gated calcium influx provides an effective mechanism for the activation of exocytosis. In this study, we demonstrate that although rat anterior pituitary lactotrophs, somatotrophs, and gonadotrophs exhibited spontaneous and extracellular calcium-dependent electrical activity, voltage-gated calcium influx triggered secretion only in lactotrophs and somatotrophs. The lack of action potential-driven secretion in gonadotrophs was not due to the proportion of spontaneously firing cells or spike frequency. Gonadotrophs exhibited calcium signals during prolonged depolarization comparable with signals observed in somatotrophs and lactotrophs. The secretory vesicles in all three cell types also had a similar sensitivity to voltage-gated calcium influx. However, the pattern of action potential calcium influx differed among three cell types. Spontaneous activity in gonadotrophs was characterized by high amplitude, sharp spikes that had a limited capacity to promote calcium influx, whereas lactotrophs and somatotrophs fired plateau-bursting action potentials that generated high amplitude calcium signals. Furthermore, a shift in the pattern of firing from sharp spikes to plateau-like spikes in gonadotrophs triggered luteinizing hormone secretion. These results indicate that the cell type-specific action potential secretion coupling in pituitary cells is determined by the capacity of their plasma membrane oscillator to generate threshold calcium signals.

Gap junctions mediate electrical signaling and ensuing cytosolic Ca2+ increases between chromaffin cells in adrenal slices: A role in catecholamine release

In adrenal chromaffin cells, a rise in cytosolic calcium concentration ([Ca(2+)]i) is a key event in the triggering of catecholamine exocytosis after splanchnic nerve activation. Action potential- or nicotine-induced [Ca(2+)]i transients are well described in individual chromaffin cells, but whether they remain spatially confined to the stimulated cell or propagate to adjacent cells is not yet known. To address this issue, the spatiotemporal organization of electrical and associated Ca(2+) events between chromaffin cells was investigated using the patch-clamp technique and real-time confocal imaging in rat acute adrenal slices. Spontaneous or electrically evoked action potential-driven [Ca(2+)]i transients were simultaneously detected in neighboring cells. This was likely attributable to gap junction-mediated electrotonic communication, as shown by (1) the bidirectional reflection of voltage changes monitored between cell pairs, (2) Lucifer yellow (LY) diffusion between cells exhibiting spontaneous synchronized [Ca(2+)]i transients, and (3) the reduction of LY diffusion using the uncoupling agent carbenoxolone. Furthermore, transcripts encoding two connexins (Cx36 and Cx43) were found in single chromaffin cells. This gap junctional coupling was activated after a synaptic-like application of nicotine that mediated synchronous multicellular [Ca(2+)]i increases. In addition, nicotinic stimulation of a single cell triggered catecholamine release in coupled cells, as shown by amperometric detection of secretory events. Functional coupling between chromaffin cells in situ may represent an efficient complement to synaptic transmission to amplify catecholamine release after synaptic stimulation of a single excited chromaffin cell.

Folliculostellate cell network: a route for long-distance communication in the anterior pituitary

All higher life forms critically depend on hormones being rhythmically released by the anterior pituitary. The proper functioning of this master gland is dynamically controlled by a complex set of regulatory mechanisms that ultimately determine the fine tuning of the excitable endocrine cells, all of them heterogeneously distributed throughout the gland. Here, we provide evidence for an intrapituitary communication system by which information is transferred via the network of nonendocrine folliculostellate (FS) cells. Local electrical stimulation of FS cells in acute pituitary slices triggered cytosolic calcium waves, which propagated to other FS cells by signaling through gap junctions. Calcium wave initiation was because of the membrane excitability of FS cells, hitherto classified as silent cells. FS cell coupling could relay information between opposite regions of the gland. Because FS cells respond to central and peripheral stimuli and dialogue with endocrine cells, the form of large-scale intrapituitary communication described here may provide an efficient mechanism that orchestrates anterior pituitary functioning in response to physiological needs.

A secreted fluorescent reporter targeted to pituitary growth hormone cells in transgenic mice

In stable transfection experiments in the GH-producing GC cell line, a construct containing the entire signal peptide and the first 22 residues of human GH linked in frame with enhanced green fluorescent protein (eGFP), produced brightly fluorescent cells with a granular distribution of eGFP. This eGFP reporter was then inserted into a 40-kb cosmid transgene containing the locus control region for the hGH gene and used to generate transgenic mice. Anterior pituitaries from these GH-eGFP transgenic mice showed numerous clusters of strongly fluorescent cells, which were also immunopositive for GH, and which could be isolated and enriched by fluorescence-activated cell sorting. Confocal scanning microscopy of pituitary GH cells from GH-eGFP transgenic mice showed a markedly granular appearance of fluorescence. Immunogold electron microscopy and RIA confirmed that the eGFP product was packaged in the dense cored secretory vesicles of somatotrophs and was secreted in parallel with GH in response to stimulation by GRF. Using eGFP fluorescence, it was possible to identify clusters of GH cells in acute pituitary slices and to observe spontaneous transient rises in their intracellular Ca2+ concentrations after loading with Ca2+ sensitive dyes. This transgenic approach opens the way to direct visualization of spontaneous and secretagogue-induced secretory mechanisms in identified GH cells.