Paradoxical effect of gonadotrophin-inhibiting hormone to negatively regulate neuropeptide Y neurones in mouse arcuate nucleus

Regulation of reproduction and energy homeostasis are linked, although our understanding of the central neural mechanisms subserving this connection is incomplete. Gonadotrophin-inhibiting hormone (GnIH) is a neuropeptide that negatively regulates reproduction and stimulates food intake. Neuropeptide Y (NPY) and products of the pro-opiomelanocortin (POMC) precursor (β-endorphin melanocortins) are appetite regulating peptides produced in the neurones of the arcuate nucleus; these peptides also regulate reproduction. In the present study, we determined the effects of GnIH on NPY and POMC neurones. Using brain slices from mice with transgenes for fluorescent tags in the two types of neurone and patch clamp electrophysiology, a predominant inhibitory effect of GnIH was observed. GnIH (100 nM) inhibited the firing rate in POMC cells, confirming the results of previous studies and consistent with the stimulatory effect of GnIH on food intake. Paradoxically (i.e. because both GnIH and NPY stimulate food intake), GnIH also had a predominantly inhibitory effect on action potential activity in NPY cells. GnIH also inhibited the secretion of NPY and α-melanocyte-stimulating hormone secretion in incubated hypothalamic blocks. GnIH (100 ng) injected into the cerebral ventricles of mice did not increase the number of NPY cells that were positively immunostained for c-Fos. Finally, dual label immunocytochemistry showed that 20% of NPY neurones had close contacts from GnIH fibres/varicosities. In conclusion, we confirm a negative effect of GnIH on POMC cells and demonstrate a paradoxical reduction of electrophysiological and functional activity in NPY cells.

Role of mitochondria in contraction and pacemaking in the mouse uterus

BACKGROUND AND PURPOSE

Uterine spontaneous contraction and pacemaking are poorly understood. This study investigates the role of the mitochondrial Ca(2+) store in uterine activity.

EXPERIMENTAL APPROACH

We investigated the effects of mitochondrial and sarco-endoplasmic reticulum (SER) inhibitors on contraction, membrane potential (Vm) and cytosolic Ca(2+) concentration ([Ca(2+) ](c) ) in longitudinal smooth muscle of the mouse uterus.

KEY RESULTS

The mitochondrial agents rotenone, carbonylcyanide-3-chlorophenylhydrazone (CCCP), 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157) and kaempferol decreased the force of contractions. The ATP synthase inhibitor oligomycin had no significant effect. The effects of these agents were compared with those of SER inhibitors cyclopiazonic acid (CPA), 2-amino ethoxyphenylborate (2-APB) and caffeine. All agents, except CPA and oligomycin, decreased contractile force. CPA and CCCP transiently increased contraction frequency, which returned to control levels, whereas rotenone, CGP37157, kaempferol and 2-APB decreased frequency and caffeine had no significant effect. Application of the mitochondrial agents when CPA functionally inhibited stores did not change contraction frequency but, with the exception of kaempferol, decreased force. CCCP caused depolarization and maintained increase in [Ca(2+) ](c) or depolarization/transient hyperpolarization and transient increase in [Ca(2+) ](c) for oestrus and di-oestrus tissues respectively. Rotenone caused hyperpolarization and maintained increase in [Ca(2+) ](c) . CGP37157 and kaempferol caused hyperpolarization but no measurable change in [Ca(2+) ](c) . Application of a range of K(+) channel blockers indicated a role of Ca(2+) -activated K(+) (K(Ca) ) channels in the CCCP- and CGP37157-induced actions.

CONCLUSIONS AND IMPLICATIONS

Mitochondria have a modulatory role on uterine contractions, with mitochondrial inhibition reducing contraction amplitude and pacemaker frequency by changes in Vm, [Ca(2+) ](c) and/or Ca(2+) influx.

Effects of birth asphyxia on neonatal hippocampal structure and function in the spiny mouse

Studies of human neonates, and in animal experiments, suggest that birth asphyxia results in functional compromise of the hippocampus, even when structural damage is not observable or resolves in early postnatal life. The aim of this study was to determine if changes in hippocampal function occur in a model of birth asphyxia in the precocial spiny mouse where it is reported there is no major lesion or infarct. Further, to assess if, as in human infants, this functional deficit has a sex-dependent component. At 37 days gestation (term=39 days) spiny mice fetuses were either delivered immediately by caesarean section (control group) or exposed to 7.5min of in utero asphyxia causing systemic acidosis and hypoxia. At 5 days of age hippocampal function was assessed ex vivo in brain slices, or brains were collected for examination of structure or protein expression. This model of birth asphyxia did not cause infarct or cystic lesion in the postnatal day 5 (P5) hippocampus, and the number of proliferating or pyknotic cells in the hippocampus was unchanged, although neuronal density in the CA1 and CA3 was increased. Protein expression of synaptophysin, brain-derived neurotrophic factor (BDNF), and the inositol trisphosphate receptor 1 (IP(3)R1) were all significantly increased after birth asphyxia, while long-term potentiation (LTP), paired pulse facilitation (PPF), and post-tetanic potentiation (PTP) were all reduced at P5 by birth asphyxia. In control P5 pups, PPF and synaptic fatigue were greater in female compared to male pups, and after birth asphyxia PPF and synaptic fatigue were reduced to a greater extent in female vs. male pups. In contrast, the asphyxia-induced increase in synaptophysin expression and neuronal density were greater in male pups. Thus, birth asphyxia in this precocial species causes functional deficits without major structural damage, and there is a sex-dependent effect on the hippocampus. This may be a clinically relevant model for assessing treatments delivered either before or after birth to protect this vulnerable region of the developing brain.

The Adam family metalloprotease Kuzbanian regulates the cleavage of the roundabout receptor to control axon repulsion at the midline

Slits and their Roundabout (Robo) receptors mediate repulsive axon guidance at the Drosophila ventral midline and in the vertebrate spinal cord. Slit is cleaved to produce fragments with distinct signaling properties. In a screen for genes involved in Slit-Robo repulsion, we have identified the Adam family metalloprotease Kuzbanian (Kuz). Kuz does not regulate midline repulsion through cleavage of Slit, nor is Slit cleavage essential for repulsion. Instead, Kuz acts in neurons to regulate repulsion and Kuz can cleave the Robo extracellular domain in Drosophila cells. Genetic rescue experiments using an uncleavable form of Robo show that this receptor does not maintain normal repellent activity. Finally, Kuz activity is required for Robo to recruit its downstream signaling partner, Son of sevenless (Sos). These observations support the model that Kuz-directed cleavage is important for Robo receptor activation.

Icm/dot-independent entry of Legionella pneumophila into amoeba and macrophage hosts

Legionella pneumophila, the causative agent of Legionnaires' disease, expresses a type IVB secretion apparatus that translocates bacterial proteins into amoeba and macrophage hosts. When stationary-phase cultures are used to infect hosts, the type IVB apparatus encoded by the icm/dot genes is required for entry, delay of phagosome-lysosome fusion, and intracellular multiplication within host cells. Null mutants with mutations in icm/dot genes are defective in these phenotypes. Here a new model is described in which hosts are infected with stationary-phase cultures that have been incubated overnight in pH 6.5 buffer. This model is called Ers treatment because it enhances the resistance to acid, hydrogen peroxide, and antibiotic stress beyond that of stationary-phase cultures. Following Ers treatment entry into amoeba and macrophage hosts does not require dotA, which is essential for Legionella virulence phenotypes when hosts are infected with stationary-phase cultures, dotB, icmF, icmV, or icmX. Defective host entry is also suppressed for null mutants with mutations in the KatA and KatB catalase-peroxidase enzymes, which are required for proper intracellular growth in amoeba and macrophage hosts. Ers treatment-induced suppression of defective entry is not associated with increased bacterial adhesion to host cells or with morphological changes in the bacterial envelope but is dependent on protein expression during Ers treatment. By using proteomic analysis, Ers treatment was shown to induce a protein predicted to contain eight tetratricopeptide repeats, a motif previously implicated in enhanced entry of L. pneumophila. Characterization of Ers treatment-dependent changes in expression is proposed as an avenue for identifying icm/dot-independent factors that function in the entry of Legionella into amoeba and macrophage hosts.

Early vitamin E supplementation attenuates diabetes-associated vascular dysfunction and the rise in protein kinase C-beta in mesenteric artery and ameliorates wall stiffness in femoral artery of Wistar rats

AIMS/HYPOTHESIS

The impact of early vitamin E supplementation on vascular function in diabetes remains unresolved. Therefore, we examined the effects of vitamin E on functional and structural parameters and on chemical markers that are disturbed in diabetes in mesenteric and femoral arteries.

METHODS

Segments of both arteries, taken from control and 8-week-old streptozotocin diabetic Wistar rats that were treated or not with vitamin E, were mounted on wire and pressure myographs, after which endothelium-dependent and -independent vasodilation was assessed. Passive mechanical wall properties and the localisation and levels of protein kinase C (PKC)-beta(2) and AGE were evaluated in these vessels.

RESULTS

Vitamin E supplementation was associated with improved endothelium-dependent and -independent vasodilatation in mesenteric arteries from diabetic rats. Impaired endothelium-dependent vasodilatation in diabetic mesenteric vessels was associated with PKC-beta(2) up-regulation and this was prevented by vitamin E supplementation. Increased AGE accumulation and plasma isoprostane levels in diabetic rats were not changed by vitamin E. In the femoral artery, vitamin E supplementation had no effect on endothelium-dependent or -independent vasodilatation, but did prevent the wall stiffening associated with diabetes.

CONCLUSIONS/INTERPRETATION

Early vitamin E supplementation has a beneficial effect on diabetes-induced endothelial dysfunction in resistance arteries. This benefit may arise from a direct effect on smooth muscle function, as a result of inhibition of the PKC-beta(2) isoform by vitamin E.

How important is stimulation of alpha-adrenoceptors for melatonin production in rat pineal glands?

The objective of this study was to determine the role of alpha-adrenoceptors in melatonin production by rat pineal gland. Pineal glands were isolated from adult male rats and maintained in organ baths. The perfusate was sampled every 5 min, stored, and later assayed for melatonin. Exposure to norepinephrine (10 microM) or the beta-adrenoceptor agonist orciprenaline (2-10 microM) increased the glands' production of melatonin. The time courses of melatonin production in response to these agonists were unaffected by the rats' pretreatment in vivo with the alpha-adrenoceptor antagonist prazosin (2 mg/kg i.p., three times). Rats that had had their superior cervical ganglia removed were primed with either orciprenaline (2 mg/kg i.p) or both orciprenaline and phenylephrine (1 mg/kg i.p) 1 hr before decapitation. Exposure of the pineal glands from these rats to orciprenaline evoked melatonin release that was similar in each group. These results lend weight to the suggestion that the marked potentiation by alpha-adrenoceptor agonists of the stimulation of cAMP and N-acetyltransferase (NAT) by beta-adrenoceptor agonists, demonstrated most readily in cultured glands or dispersed rat pinealocytes, does not carry over into significant augmentation of melatonin production in intact pineal glands.

Glycyrrhetinic derivatives inhibit hyperpolarization in endothelial cells of guinea pig and rat arteries

Glycyrrhetinic acid (GA) derivatives have been used to implicate gap junctions in vasorelaxation attributed to endothelium-derived hyperpolarizing factor (EDHF). The aim of this study was to assess whether GA compounds affect endothelial cell hyperpolarization. Membrane potentials were recorded from dye-identified endothelial and smooth muscle cells of guinea pig coronary and rat mesenteric arteries. GA derivatives had varied effects on the resting membrane potential: depolarization, hyperpolarization, or no effect, depending on the artery. 18alpha-GA (50 microM) had a small variable effect on ACh-induced hyperpolarizations in endothelial cells. 18beta-GA (30 microM) and carbenoxolone (100 microM) significantly reduced ACh-induced hyperpolarizations in both endothelial and smooth muscle cells. Smooth muscle action potentials in rat tail arteries were smaller and slower in the presence of 18beta-GA. Nerve-induced excitatory junction potentials were inhibited by 18beta-GA and carbenoxolone, whereas the time course of their decay initially increased and then decreased. In conclusion, the GA compounds had a range of effects. Their inhibition of the EDHF hyperpolarization and relaxation in the smooth muscle may stem from the inhibition of endothelial cell hyperpolarization.

EDHF is not K+ but may be due to spread of current from the endothelium in guinea pig arterioles

Endothelium-derived hyperpolarizing factor (EDHF)-attributed hyperpolarizations and relaxations were recorded simultaneously from submucosal arterioles of guinea pigs with the use of intracellular microelectrodes and a video-based system, respectively. Membrane currents were recorded from electrically short segments of arterioles under single-electrode voltage clamp. Substance P evoked an outward current with a current-voltage relationship that was well described by the Goldman-Hodgkin-Katz equation for a K+ current, consistent with the involvement of intermediate- and small-conductance Ca2+-activated K+ channels. 1-Ethyl-2-benzimidazolinone relaxed the arterioles and evoked hyperpolarizations that were blocked by charybdotoxin, but not by iberiotoxin. Application of K+induced depolarization under conditions in which EDHF evoked hyperpolarization. The Ba2+-sensitive component of the K+-induced current was inwardly rectifying, in contrast to the outwardly rectifying current evoked by substance P. EDHF-attributed hyperpolarizations in dye-identified smooth muscle cells were indistinguishable from those recorded from dye-identified endothelial cells in the same arterioles. These results provide evidence that EDHF is not K+ but may involve electrotonic spread of hyperpolarization from the endothelial cells to the smooth muscle cells.

K+ currents underlying the action of endothelium-derived hyperpolarizing factor in guinea-pig, rat and human blood vessels

Membrane currents attributed to endothelium-derived hyperpolarizing factor (EDHF) were recorded in short segments of submucosal arterioles of guinea-pigs using single microelectrode voltage clamp. The functional responses of arterioles and human subcutaneous, rat hepatic and guinea-pig coronary arteries were also assessed as changes in membrane potential recorded simultaneously with contractile activity. The current-voltage (I-V) relationship for the conductance due to EDHF displayed outward rectification with little voltage dependence. Components of the current were blocked by charybdotoxin (30-60 nM) and apamin (0.25-0.50 microM), which also blocked hyperpolarization and prevented EDHF-induced relaxation. The EDHF-induced current was insensitive to Ba2+ (20-100 microM) and/or ouabain (1 microM to 1 mM). In human subcutaneous arteries and guinea-pig coronary arteries and submucosal arterioles, the EDHF-induced responses were insensitive to Ba2+ and/or ouabain. Increasing [K+]o to 11-21 mM evoked depolarization under conditions in which EDHF evoked hyperpolarization. Responses to ACh, sympathetic nerve stimulation and action potentials were indistinguishable between dye-labelled smooth muscle and endothelial cells in arterioles. Action potentials in identified endothelial cells were always associated with constriction of the arterioles. 18beta-Glycyrrhetinic acid (30 microM) and carbenoxolone (100 microM) depolarized endothelial cells by 31 +/- 6 mV (n = 7 animals) and 33 +/- 4 mV (n = 5), respectively, inhibited action potentials in smooth muscle and endothelial cells and reduced the ACh-induced hyperpolarization of endothelial cells by 56 and 58 %, respectively. Thus, activation of outwardly rectifying K+ channels underlies the hyperpolarization and relaxation due to EDHF. These channels have properties similar to those of intermediate conductance (IKCa) and small conductance (SKCa) Ca2+-activated K+ channels. Strong electrical coupling between endothelial and smooth muscle cells implies that these two layers function as a single electrical syncytium. The non-specific effects of glycyrrhetinic acid precludes its use as an indicator of the involvement of gap junctions in EDHF-attributed responses. These conclusions are likely to apply to a variety of blood vessels including those of humans.

EDHF, NO and a prostanoid: hyperpolarization-dependent and -independent relaxation in guinea-pig arteries

The contribution of endothelium-derived hyperpolarizing factor (EDHF), nitric oxide (NO) and a prostanoid (PG) to endothelium-dependent hyperpolarization and relaxation were assessed in coronary and mammary arteries of guinea-pigs by integration of the responses evoked during discrete applications of acetylcholine (ACh). The results of this integration approach were compared with those using traditional peak analysis methods. N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM) and indomethacin (1 microM), alone or in combination, were without effect on peak hyperpolarizations or relaxations while they markedly reduced the integrated responses in both arteries. Integrated responses attributed to NO and PG were larger than those attributed to EDHF in the coronary artery (at 2 microM ACh, hyperpolarization (mV s): NO, 4200+/-91; PG, 5046+/-157; EDHF, 1532+/-94; relaxation (mN s mm(-1)): NO, 2488+/-122; PG, 2234+/-96; EDHF, 802+/-54). Integrated responses attributed to NO, PG and EDHF were similar in the mammary artery (at 2 microM ACh, hyperpolarization: NO, 347+/-69; PG, 217+/-49; EDHF, 310+/-63; relaxation: NO, 462+/-94; PG, 456+/-144; EDHF, 458+/-40). Gilbenclamide (1 microM) all but abolished the hyperpolarization attributable to NO and PG but not EDHF in both arteries allowing assessment of the role of the hyperpolarization in relaxation. Gilbenclamide was without effect on the integrated relaxation due to NO but significantly reduced the relaxation associated with PG in the two arteries. In conclusion, integration of the responses enabled a more complete assessment of the contribution of EDHF, NO and PG to endothelium-dependent responses, which were strikingly different in the two arteries. There is commonality in the role of hyperpolarization in relaxation in both arteries: EDHF-dependent relaxation is strongly dependent on hyperpolarization; hyperpolarization plays an important role in PG relaxation, whereas it has a small facilitatory role in NO-dependent relaxation.

Changes in the mechanisms involved in uterine contractions during pregnancy in guinea-pigs

1. The mechanisms involved in contraction in guinea-pig myometrium were compared at mid- and late pregnancy. Tension was recorded simultaneously with either membrane potential or cytoplasmic calcium ([Ca2+]i) in strips exposed briefly to prostaglandin F2alpha (PGF). 2. PGF-induced increases in tension were underpinned by action potentials followed by sustained depolarization and biphasic increases in [Ca2+]i at mid- (peak, 879 +/- 199 nM; sustained, 298 +/- 35 nM, n = 11) and late pregnancy (peak, 989 +/- 302 nM; sustained 178 +/- 33 nM, n = 8). 3. At mid- and late pregnancy, nifedipine (10-6 M) reduced (a) the PGF-induced increase in tension to 84 and 35 %, (b) the level attained during the depolarization by 2 and 12 mV and (c) the peak rise in [Ca2+]i to 42 and 17 %. The sustained rises in [Ca2+]i were resistant to nifedipine. 4. In Ca2+-free solution (containing 1 mM EGTA), PGF elicited an increase in tension that was 26 % of that in 2.5 mM Ca2+ and an increase in [Ca2+]i (24 % of the sustained level) at mid-pregnancy but no increase in tension or [Ca2+]i at term. 5. At both stages of pregnancy, PGF decreased the level of [Ca2+]i required to elicit increases in tension comparable to those evoked by high K+o. The slope of the tension-[Ca2+]i curves were steeper in mid- than in late pregnancy. 6. In conclusion, at mid-pregnancy, the contractile response of the guinea-pig myometrium to PGF involves Ca2+ influx through L-type voltage-operated Ca2+ channels (VOCCs) and by receptor-operated mechanisms, release of Ca2+ from intracellular stores, and an increase in the sensitivity of the contractile apparatus to Ca2+. At term the situation is different: a modest increase in the sensitivity of the contractile apparatus to Ca2+ persists and there is a major reliance on Ca2+ influx through VOCCs.

Contractile activity, membrane potential, and cytoplasmic calcium in human uterine smooth muscle in the third trimester of pregnancy and during labor

OBJECTIVE

This study was undertaken to investigate in human tissue samples the mechanisms underlying spontaneous and prostaglandin F(2)(alpha)-induced contractions during the final trimester of pregnancy and labor.

STUDY DESIGN

Membrane potential and cytoplasmic calcium were recorded simultaneously with contraction in uterine strips obtained from the lower segment during cesarean delivery.

RESULTS

Between week 28 of gestation and term there was a progressive increase in the frequency of spontaneous contractions and a decrease in the negative potential of the membrane. The response to prostaglandin F(2alpha) was biphasic. The initial excitatory component remained stable toward term. A later inhibitory component, which was underpinned by increased activity of the sodium-potassium adenosine triphosphatase pump, decreased at the time of labor.

CONCLUSIONS

There is a gradual increase in excitability in uterine muscle throughout the third trimester of human pregnancy. The initial component of the prostaglandin response is a large contraction that is kept brief by a subsequent inhibitory component of the response, which ensures that full relaxation occurs between contractions.

Hyperpolarization and slowing of the rate of contraction in human uterus in pregnancy by prostaglandins E2 and f2alpha: involvement of the Na+ pump

1. The effects of prostaglandins E2 (PGE) and F2alpha (PGF) on membrane potential and isometric tension and cytoplasmic free calcium concentration ([Ca2+]i) and tension were studied in strips of uterine smooth muscle obtained from women undergoing Caesarean delivery at term and during established labour. 2. Prostaglandins (PGs) evoked a biphasic response. The excitatory component consisted of depolarization of the membrane, which initiated spike action potentials, an increase in [Ca2+]i and tension development. The membrane remained depolarized at -19 +/- 1 mV for about 2 min, then repolarized abruptly, [Ca2+]i promptly returned to basal levels, and tension development ceased. 3. This component of the response to PGE or PGF was followed by a slow hyperpolarization which reached -85 +/- 2 mV (n = 22) at term and -70 +/- 2 mV (n = 9) during labour, and during which spontaneous action potentials and tension development did not occur. 4. Nifedipine (10-6 M) abolished spontaneous activity, abolished PG-induced action potentials and reduced the increase in [Ca2+]i (9 +/- 3 %, n = 6), the depolarization (10 +/- 1 mV, n = 14), the tension (2 +/- 1 %, n = 14) and the hyperpolarization (9 +/- 1 mV, n = 14, at term). 5. A variety of K+ channel blockers were without effect on the peak amplitude of the PG-induced hyperpolarization but the latter did not occur in the presence of ouabain (10-6 M) or in K+-free or low-Na+ solutions, suggesting an involvement of the Na+-K+-ATPase pump. 6. In conclusion, a substantial dependence on Ca2+ influx through voltage-operated Ca2+ channels accounts for the importance of membrane potential in regulating contractions in human uterine smooth muscle. The classical excitatory effect of PGE and PGF is followed by hyperpolarization involving the Na+-K+-ATPase pump. The hyperpolarization restricts the response to a single contraction and decreases the frequency of subsequent contractions. The amplitude of the hyperpolarization decreases during labour, allowing contraction frequency to increase. Its persistence at this time ensures complete relaxation between each single robust contraction, preventing spasm of the uterus that would restrict blood flow to the fetus during delivery.

Endothelium-dependent hyperpolarization in resting and depolarized mammary and coronary arteries of guinea-pigs

1. The membrane potential responses in guinea-pig coronary and mammary arteries attributable to endothelium-derived nitric oxide (NO), prostaglandin (PG) and hyperpolarizing factor (EDHF), and to exogenous NO and the prostacyclin analogue, iloprost, were compared at rest and when depolarized with the thromboxane analogue, U46619. 2. In the coronary artery, stimulation of the endothelium with acetylcholine (ACh) evoked hyperpolarization attributable to NO and a PG with similar pD2s at rest and in the presence of U46619. However, in depolarized tissues, the pD2 of the response attributed to EDHF required a 10 fold lower concentration of ACh compared with at rest. 3. In the mammary artery, lower concentrations of ACh were required to evoke NO- and EDHF-dependent hyperpolarizations in depolarized mammary artery compared with at rest, while PG-dependent hyperpolarization did not occur until the concentration of ACh was increased some 10 fold both at rest and in U46619. 4. The smooth muscle of the coronary artery of guinea-pigs was some 4 fold more sensitive to exogenous NO and iloprost than was the mammary artery. 5. In conclusion, the membrane potential response in arteries at rest, that is, in the absence of constrictor, may be extrapolated to events in the presence of constrictor when NO and PG are under study. However, the sensitivity to ACh and the magnitude of the hyperpolarization attributed to EDHF obtained in tissues at rest may underestimate these parameters in depolarized tissues.

Muscarinic receptor activation in guinea-pig chromaffin cells causes decreased membrane conductance and depolarization

Membrane potentials were recorded with conventional intracellular microelectrodes from chromaffin cells in isolated, bisected adrenal glands from guinea-pigs. The local pressure ejection of muscarinic agonists, acetylcholine (in the presence of hexamethonium) or bethanecol, caused a transient depolarization that was relatively slow (1-2 s) in onset compared with the depolarization associated with the activation of nicotinic receptors. Muscarinic receptor-induced depolarization was associated with an increase in input resistance and the firing of action potentials. Repetitive stimulation of splanchnic nerve fibers within the gland, in the presence of hexamethonium, caused a maintained depolarization that was slow in both onset and decay and in many cells caused the repetitive firing of action potentials. It is suggested that, in this species, the exocytosis of catecholamines caused by the activation of muscarinic receptors, described by others, may be due to the initiation of tetrodotoxin-sensitive action potentials and consequent opening of voltage sensitive Ca2+ channels.

Pregnancy-induced decrease in evoked excitatory junction potentials in guinea pig uterine artery

The effects of stimulating the intramural nerves on the membrane potential and tension in the uterine artery of virgin guinea pigs were compared with the responses during pregnancy. In all tissues the amplitude of the excitatory junction potential (EJP) increased as the stimulus voltage was increased. The rate of increase in EJP amplitude in tissues from virgin animals greatly exceeded that recorded in late pregnant tissues. EJPs were abolished by tetrodotoxin but were resistant to blockade by alpha-adrenoceptor antagonists. Stimulation of the nerves also evoked a slow depolarization and contraction which were abolished by both tetrodotoxin and alpha-adrenoceptor antagonists. The amplitudes of the depolarizations and contractions were not correlated. The role of EJPs and alpha-adrenoceptor activation in the control of vascular function is discussed. Fluorescence histochemistry revealed a decrease in the density of the catecholamine innervation that was correlated with a decrease in catecholamine content as pregnancy progressed. In addition, there appeared to be a difference in the arrangement of the fluorescent varicosities, with a shift from varicosities that were close to the outer layer of smooth muscle in virgin tissues to those that were more distantly dispersed in the adventitia during late pregnancy. The changes would be expected to reduce the effectiveness of vasoconstrictor drive to the uterine artery as pregnancy progresses.

The association between melatonin production and electrophysiology of the guinea pig pineal gland

Melatonin production by isolated pineal glands from guinea pigs was examined under conditions that affect membrane potential or the firing of action potential-like spikes. In glands from superior cervical ganglionectomized animals, depolarization resulting from increasing extracellular potassium concentration to 100 mM did not initiate melatonin production, and it delayed the response to the beta-adrenoceptor agonist orciprenaline. In glands from intact animals melatonin production was initiated by exposure to 100 mM potassium with a time-course similar to the response to orciprenaline. A proportion of this response was propanol resistant, suggesting that the normal control of melatonin production may involve a neurotransmitter in addition to norepinephrine. Exposure to verapamil or nifedipine, or removal of extracellular calcium, previously shown to eliminate action potential-like spikes, did not substantially affect the increase in melatonin production induced by orciprenaline. Phenylephrine, which stimulates spiking, produced only a slight increase in melatonin production. It is concluded that the depolarization and the spiking are not closely related to the stimulation of melatonin production, but may relate principally to the secretion of a substance other than melatonin.

Release of endothelium-derived hyperpolarizing factor (EDHF) by M3 receptor stimulation in guinea-pig coronary artery

1. The muscarinic receptor subtype(s) involved in the release of endothelium-derived hyperpolarizing factor (EDHF) were studied in the guinea-pig coronary artery by recording the membrane potential in the smooth muscle cells with intracellular microelectrodes. 2. Acetylcholine (ACh, pD2 6.68) was 10 times more potent than the M2 agonist, oxotremorine (pD2 5.65) and 500 fold more potent than the M1 agonist, McN-A-343 (pD2 3.95) in evoking the EDHF hyperpolarization. 3. The M3 muscarinic antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) was the most potent (pA2 9.5) in inhibiting the release of EDHF evoked by ACh, being more potent than pirenzepine (pA2 6.7), and AFDX-116 (pA2 6.1) which preferentially block M1 and M2 receptors, respectively. 4. These results suggest that EDHF is released from the endothelium of the guinea-pig coronary artery upon the activation of the muscarinic M3 receptor subtype.

Role of membrane potential in endothelium-dependent relaxation of guinea-pig coronary arterial smooth muscle

1. Membrane potential and tension were measured simultaneously in ring segments of main coronary artery of guinea-pigs. The synthetic thromboxane A2 analogue U46619 depolarized the tissues from -58 +/- 2 to -40 +/- 1 mV and increased tension by 12 +/- 1 mN mm-1. Nitric oxide (NO) and Iloprost, the stable analogue of prostacyclin, evoked hyperpolarization and relaxation. 2. The concentration of NO required to evoke half-maximal hyperpolarization (EC50 of 2 x 10(-5) M) was 40-fold higher than that which was required to induce relaxation (EC50 of 5 x 10(-7) M). The EC50 for Iloprost-induced hyperpolarization (3 x 10(-8) M) was similar to that for relaxation (4 x 10(-8) M). 3. Glibenclamide (10(-6) M) abolished the hyperpolarization in response to both NO and Iloprost but was without effect on the amplitudes of the relaxations over the complete concentration-response curves. 4. Acetylcholine evoked concentration-dependent hyperpolarization and relaxation in the presence of N omega-nitro-L-arginine methyl ester (NAME; 10(-5) M) and indomethacin (10(-6) M), and these responses were attributed to endothelium-derived hyperpolarizing factor (EDHF). The hyperpolarization produced by EDHF always preceded relaxation, and relaxation never occurred at concentrations of acetylcholine that were insufficient to evoke hyperpolarization. 5. The concentration-hyperpolarization and concentration-relaxation curves in response to acetylcholine were not affected by glibenclamide or barium (1-3 mM) but were shifted to the right 4- and 5-fold, respectively, by 1 mM tetraethylammonium. The hyperpolarization and relaxation evoked by acetylcholine were also reduced in a parallel manner when the potassium concentration in the superfusate was increased. 6. Hyperpolarizing current steps, applied to spiral strips of coronary artery denuded of endothelium and depolarized and constricted with U46619, caused relaxation. The relationship between hyperpolarization and relaxation evoked electronically was similar to that which was due to EDHF in intact tissues stimulated with acetylcholine. 7. It is concluded that the ability of NO or Iloprost to relax guinea-pig coronary artery does not depend upon hyperpolarization of the smooth muscle. In contrast, hyperpolarization is likely to play a major, if not the only, role in the relaxation in response to EDHF in this tissue.

Tetrodotoxin-sensitive action potentials in smooth muscle of mouse vas deferens

Action potentials were recorded during impalements of some but not all smooth muscle cells of mouse vas deferens in response to both nerve stimulation and intracellular current injection. They were resistant to blockade by nifedipine (0.1-1.0 microM) but were blocked by tetrodotoxin (TTX, 0.2-1.0 microM) when this was added in the presence of nifedipine. It is suggested that voltage-dependent sodium (Na+) channels are present in mouse vas deferens that function to amplify calcium (Ca2+) influx through voltage-dependent Ca2+ channels.

Currents caused by the spontaneous release of quanta of acetylcholine onto chromaffin cells in guinea-pig adrenal medulla

Membrane potentials were recorded from chromaffin cells in isolated bisected adrenal glands from guinea-pigs. Spontaneous excitatory synaptic potentials (SESPs) were recorded whose frequency was increased following brief (up to 10 s) periods of presynaptic nerve stimulation at 10-30 Hz. The single electrode voltage-clamp method was used to record the currents underlying all but the largest SESPs. Spontaneous excitatory synaptic currents (SESCs) decayed according to a single exponential with a time constant of about 8 ms at 30 degrees C. Thus the neuronal nicotinic receptor-channels giving rise to SESCs in chromaffin cells are probably very similar to those that are opened by quanta of acetylcholine in sympathetic ganglion cells.

Synaptic transmission from splanchnic nerves to the adrenal medulla of guinea-pigs

1. Membrane potentials were recorded with conventional intracellular microelectrodes from chromaffin cells in isolated, bisected adrenal glands from guinea-pigs. 2. All cells were electrically excitable and responded to depolarizing current with all-or-nothing action potentials that were blocked by tetrodotoxin. 3. Input resistance was 180 +/- 14 M omega and this was lower than that reported for isolated chromaffin cells using patch electrodes. 4. All cells responded to transmural stimulation with action potentials that arose from excitatory synaptic potentials in response to the excitation of one or more preganglionic fibres, many having strong synaptic action. Other fibres had weaker synaptic action but in all cases, maximal transmural stimulation caused depolarization well above threshold for action potential initiation. 5. Spontaneous excitatory synaptic potentials were observed whose frequency was greatly increased by repetitive stimulation at 10 or 30 Hz. 6. No evidence was found for the desensitization of nicotinic receptors in response to acetylcholine released from presynaptic nerve terminals. 7. These experiments show that there are many similarities between the responses to splanchnic nerve stimulation of guinea-pig chromaffin cells in situ and sympathetic ganglion cells from the same species.

Pilocarpine-induced relaxation of rat tail artery by a non-cholinergic mechanism and in the absence of an intact endothelium

1. The partial muscarinic agonist, pilocarpine, evoked concentration-dependent relaxation with an EC50 of 2.4 x 10(-3) M in isolated segments of rat tail artery that were constricted with phenylephrine (10(-8) to 2 x 10(-7) M). Acetylcholine also evoked concentration-dependent relaxation but was more potent than pilocarpine (EC50, 6.5 x 10(-7) M). 2. The concentration-relaxation curves for pilocarpine were not affected by the muscarinic antagonists, atropine (10(-9) M) or pirenzepine (5 x 10(-7) M), while the concentration-relaxation curves for acetylcholine-evoked relaxation of the same tissues were shifted some 10 fold to the right by these concentrations of atropine and pirenzepine. 3. Acetylcholine failed to evoke relaxation following removal of the endothelium. The smooth muscle of the rat tail artery was some 10 fold more sensitive to the relaxing action of pilocarpine following denudation of the endothelium. 4. The effects of pilocarpine and acetylcholine on membrane potential were studied in tissues that were depolarized to -39 +/- 1 mV with phenylephrine (5 x 10(-8) to 2 x 10(-7) M). In intact tissues, pilocarpine caused hyperpolarization, an effect that persisted in the presence of muscarinic antagonists. Acetylcholine also evoked hyperpolarization. 5. Following removal of the endothelium, pilocarpine (10(-5) to 10(-3) M) evoked hyperpolarization in 6 of 15 preparations and a decrease in the frequency of action potentials in the remainder. Both of these responses were associated with relaxation. 6. The effects of pilocarpine were compared with other agents that evoke endothelium-independent relaxation. The concentration-relaxation curves in response to pilocarpine and nitroprusside were shifted to the right by ferricyanide (10-5 M) and methylene blue (10-5 M). Glibenclamide (10-6 M) was without effect on the hyperpolarization and relaxation evoked by pilocarpine (10' to 10- M).7. Thus, pilocarpine evokes relaxation of rat tail artery independently of the cholinergic system and it is suggested that this is achieved by decreasing the frequency of action potentials in the smooth muscle.

Stretch revealed three components in the hyperpolarization of guinea-pig coronary artery in response to acetylcholine

1. Membrane potential was recorded with intracellular microelectrodes from the smooth muscle of coronary arteries of guinea-pigs, and the responses to endothelium-derived relaxants were studied under a variety of conditions. 2. Stimulation of the endothelium with brief applications of acetylcholine or substance P evoked concentration-dependent hyperpolarizations that were complex in nature. A transient component, which is likely to result from endothelium-derived hyperpolarizing factor (EDHF), was followed by a slow component that resulted from the production of nitric oxide (NO) and a prostaglandin. 3. The ability of exogenous and endogenous NO and prostacyclin to hyperpolarize the membrane depended upon the smooth muscle being under stretch. Unstretched preparations responded to acetylcholine with only the transient component of hyperpolarization; NO and prostacyclin were without effect. 4. In stretched preparations exogenous NO and prostacyclin, and its synthetic analogue methyl prostacyclin (Iloprost), evoked hyperpolarization, and the slow component of the response induced by acetylcholine appeared. The amplitudes of these responses reached maximum when the tissues were stretched to the equivalent of approximately 50 mmHg. 5. From a resting membrane potential of -61 +/- 0.6 mV, exogenous NO and Iloprost hyperpolarized the smooth muscle to around -80 mV. The EC50 values for NO- and Iloprost-induced hyperpolarization were 2.6 x 10(-6) and 1.3 x 10(-8) M, respectively. 6. Coronary arterial smooth muscles from rats, rabbits and sheep also hyperpolarized in response to exogenous NO, although their sensitivities were less than those of preparations obtained from guinea-pigs. Iloprost hyperpolarized tissues from rabbits and sheep but not those obtained from rats. 7. It is concluded that the endothelial lining of coronary arteries can release three factors, EDHF, NO and prostacyclin, all of which can hyperpolarize the membrane of the smooth muscle. The relative proportions and significance of each factor depends on the amount of stretch, on the artery and on the species of animal.

Response of the rat myometrium to phenylephrine in early pregnancy and the effects of 6-hydroxydopamine

1. The contractile responses of the longitudinal and circular muscle layers of the rat uterus to the alpha 1-adrenoceptor agonist phenylephrine were measured on days 3-6 of gestation. There was a progressive increase in sensitivity to phenylephrine in both muscle layers between days 3 and 6 of gestation. Overall, this amounted to a 13 and 9 fold increase in sensitivity in longitudinal and circular muscles, respectively. In longitudinal muscle the slope of the Hill plot was 2 on day 3 of pregnancy and was decreased to 1 thereafter. 2. The sympathetic nerve terminals innervating the smooth muscle of the uterus were destroyed by administration of 6-hydroxydopamine (2 x 50 mg kg-1) 4-7 days before testing with phenylephrine. Following this treatment there was a significant increase in sensitivity to phenylephrine on day 3 in both muscle layers. After day 4, the longitudinal muscle was less sensitive to phenylephrine. 3. In the longitudinal muscle there was a progressive increase in the contractile response to maximal concentrations of phenylephrine and to high potassium (100 mM) between days 3 and 6 of pregnancy. In the circular muscle the responsiveness to both phenylephrine and potassium remained unchanged between days 3 and 6 of gestation. 6-Hydroxydopamine had no effect on the maximal responses to phenylephrine or high potassium in either muscle layer. 4. In conclusion, denervation supersensitivity of uterine smooth muscle following injection of 6-hydroxydopamine is observed only on day 3 of pregnancy and appears to be replaced by subsensitivity by day 6. The decrease in the slope of the Hill plot in longitudinal muscle after day 3 may be explained by changes in events between activation of alpha 1-adrenoceptors and contraction.

Melatonin production by the guinea pig pineal gland in vitro

The fluid bathing pineal glands isolated from guinea pigs was collected serially and its melatonin content was estimated. Production was found to be high (500 pg/5 min) soon after isolation of the gland: it declined exponentially to a maintained lower level (50 pg/5 min) with a half-time of 16-20 min. A rapid increase could be produced by exposure of the gland to the beta-adrenoceptor agonist orciprenaline or by stimulating it electrically. The time course and the extent of the responses to either form of stimulation were similar in glands that had been taken in the morning, at dusk or in mid-dark: the prestimulation peak was lower from glands taken at dusk. Thus the pineal gland of the guinea pig is capable of responding rapidly to stimulation of its beta-adrenoceptors at any time. These responses parallel the depolarization observed intracellularly in this species when the pineal gland is stimulated electrically or exposed to beta-adrenoceptor agonists.

Hyperpolarization and relaxation of arterial smooth muscle caused by nitric oxide derived from the endothelium

Stimulation of the endothelial lining of arteries with acetylcholine results in the release of a diffusible substance that relaxes and hyperpolarizes the underlying smooth muscle. Nitric oxide (NO) has been a candidate for this substance, termed endothelium-derived relaxing factor. But there are several observations that argue against the involvement of NO in acetylcholine-induced hyperpolarization. First, exogenous NO has no effect on the membrane potential of canine mesenteric arteries. Second, although haemoglobin (believed to bind and inactivate NO (refs 11-15)) and methylene blue (which prevents the stimulation of guanylate cyclase) inhibit relaxation, neither has an effect on hyperpolarization. Finally, nitroprusside, thought to generate NO in vascular smooth muscle, relaxes rat aorta without increasing rubidium efflux. Nevertheless, nitrovasodilators, nitroprusside and nitroglycerin cause hyperpolarization in some arteries. NO might therefore be responsible for at least part of the hyperpolarization induced by acetylcholine. We now report that hyperpolarization and relaxation evoked by acetylcholine are reduced by NG-monomethyl-L-arginine, an inhibitor of NO biosynthesis from L-arginine. Thus NO derived from the endothelium can cause hyperpolarization of vascular smooth muscle, which might also contribute to relaxation by closing voltage-dependent calcium channels. Our findings raise the possibility that hyperpolarization might be a component of NO signal transduction in neurons or inflammatory cells.

Passively propagated spikes from the soma of cells in pineal gland of guinea pigs

The membrane properties of cells within the pineal gland of guinea pigs were studied using intracellular electrophysiological techniques. The electrotonic responses to intracellular current injection decayed with a single exponential in approximately 60% of cells but was preceded by a quicker component in the remainder. The membrane time constant was 2.8 ms. Depolarization beyond -29 mV activated an outward current that reversed at around the value of the resting potential. Hyperpolarization activated a slow inward current. Spikes occurred in response to activation of alpha-adrenoceptors. They were resistant to tetrodotoxin but were abolished by nifedipine and verapamil, suggesting that calcium carries the current during their upstroke. Spikes could not be evoked by depolarizing current pulses of 1-ms to 2-s duration. The responses to hyperpolarizing current steps or voltage-clamp steps applied during the peak of spikes evoked by nerve stimulation were indistinguishable from the responses to those applied between spikes. During nerve stimulation, fluctuations were observed in the current trace of cells under voltage clamp, indicating that the spikes could not be voltage clamped successfully. It is concluded that the spikes occurring in response to nerve stimulation are generated on the processes of the pinealocytes and are passive in the soma.

Ionic mechanisms underlying action potentials in myometrium

1. The ionic mechanisms underlying the simple spike action potential in longitudinal myometrium of pregnant rats and the complex action potential which occurs in the same layer of pregnant guinea-pigs are discussed. 2. The current during the upstroke of the simple spike is carried by calcium and repolarization results from inactivation of the calcium current and activation of a potassium current. 3. A slow inward current underlies the plateau component of the complex action potential and calcium is involved in carrying or regulating this current. 4. Single channel recordings from the longitudinal myometrium of pregnant guinea-pigs reveal large conductance (130-170 pS) potassium channels which are activated by depolarization of the membrane. The activation of these channels during the upstroke of the spike would contribute to the rapid termination of the spike. 5. The duration of the plateau component of the complex action potential closely correlates with the duration of contraction and it is suggested that sufficient calcium may enter the cell during the action potential to activate the contractile apparatus directly.

Gestational changes in the utilization of intracellularly stored calcium in the myometrium of guinea-pigs

1. The ability of oxytocin and prostaglandin F2 alpha (PGF) to induce contraction in guinea-pig myometrium in calcium-free solution was studied in an attempt to assess the extent to which intracellular calcium stores could be released by these two agonists. Both longitudinal and circular muscle layers were studied separately and the effects of gestational age were also examined. 2. In longitudinal strips, the responses to oxytocin and PGF in the absence of external calcium decreased progressively throughout gestation. Responses of circular strips to both agonists were unchanged throughout pregnancy, until day 64, when no response to PGF could be elicited. 3. Pre-treatment with high potassium (and normal calcium) increased the responses to the agonists in calcium-free medium while pre-treatment with beta-adrenoceptor agonists had no effect on responses to oxytocin or PGF. 4. Responses to both agonists decreased with time in calcium-free solution suggesting a loss of calcium from stores with a half-time of 3 min. The rate of the decline in the responses was the same in both muscle layers and did not change with gestational age. 5. In the presence of lanthanum contractions evoked by oxytocin, but not PGF, were augmented 2-3-fold. This potentiation of the response to oxytocin occurred in both muscle layers and throughout gestation. 6. Each agonist evoked only one response in calcium-free solution containing EGTA. The response to PGF in longitudinal strips following a challenge with oxytocin was reduced, compared with the response to PGF when applied first while the response to oxytocin in these strips was unchanged following exposure to PGF. In circular strips neither oxytocin- nor PGF-induced contractions were altered following prior exposure to the other agonist. 7. It is concluded that oxytocin and PGF operate via two distinct mechanisms to release intracellularly stored calcium in both longitudinal and circular components of the guinea-pig myometrium and a hypothesis to explain the results is presented.

Induction of prolonged excitability in myometrium of pregnant guinea-pigs by prostaglandin F2 alpha

1. Electrical and mechanical responses to prostaglandin F2 alpha (PGF) were studied in circular myometrium, with or without endometrium, during the first 3 weeks of gestation of the guinea-pig. 2. Muscle strips from which endometrium had been removed became inexcitable within 30-40 min of isolation from the animal such that action potentials and contraction could not be initiated by depolarizing current steps. Raising the concentration of potassium in the perfusing solution resulted in a small contraction. 3. In these preparations PGF induced complex action potentials that consisted of spikes and a prolonged plateau of depolarization. Each action potential was associated with a large phasic contraction. 4. Contractions induced by PGF are unlikely to result predominantly from release of calcium from intracellular stores since the ability of the agonist to evoke a response was reduced by some 97% in the absence of external calcium or in the presence of calcium channel blockers. 5. When preceded by a brief exposure to PGF, the contractile response to high potassium was enhanced to equal that in response to PGF. Enhancement persisted for approximately 30 min after removal of PGF and was not dependent upon the presence of external calcium. 6. Muscle strips with intact endometrium contracted spontaneously for hours. Each contraction was associated with a complex action potential, both of which were abolished by indomethacin. 7. It is concluded that PGF transforms inexcitable calcium channels in the membrane of the smooth muscle cells of the circular myometrium into excitable ones. The study also suggests that endogenous prostaglandin of endometrial origin may prevent the 'run-down' of channels in vivo.

Single channel Cl- and K+ currents from cells of uterus not treated with enzymes

Single channel currents were recorded from uterine smooth muscle cells of pregnant guinea-pigs. The cells were within small bundles of smooth muscle of low input resistance. Giga-ohm seals were formed between the patch clamp electrodes and the membrane of smooth muscle cells that had not been treated with enzymes. In cell-attached mode, outward current steps were observed that had a conductance of 130-170 pS, a reversal potential close to the potassium reversal potential, and the probability of a channel being in the open state increased e-fold per 8-11 mV depolarization. In the cell-free, inside-out patch mode, currents were recorded that had a large conductance, 420 pS, and smaller conductance levels. When the chloride concentration was changed the reversal potential shifted in a manner consistent with the Nernst equation indicating that the channels were permeable to chloride ions.

Two types of cells with central innervation in pineal gland of guinea pigs

Cells within pineal glands isolated from young, male guinea pigs were impaled with intracellular microelectrodes and their responses to stimulate the nerve supply to the gland were studied. Two types of cells were identified. The response of cells of type I was a depolarization on which spikes were superimposed. Blockers of alpha-adrenoceptors abolished the spikes while beta-adrenoceptor blockers reduced the depolarization to 27%, leaving a small tetrodotoxin-sensitive depolarization. After bilateral removal of the superior cervical ganglia (SCG) the beta-mediated depolarization was not observed while the spikes and the smaller depolarization persisted. The response of cells of type II was an initial large, transient depolarization followed by a smaller depolarization. Both components were reversibly blocked by tetrodotoxin. The only agents found to have any effect on these cells were oxytocin, vasopressin, and vasotocin. These peptides caused depolarization similar in amplitude to the larger response to nerve stimulation, although more prolonged. The large depolarization was not observed following ganglionectomy, but the smaller one persisted. It is concluded that cells of type I and II both receive inputs from nerves whose cell bodies lie in the SCG. Cells of both types are also innervated through another pathway.

Multiple sites for the initiation of action potentials in neurons of the inferior mesenteric ganglion of the guinea-pig

Responses to nerve stimulation were recorded with intracellular microelectrodes from neurons in the inferior mesenteric ganglion of the guinea-pig. An event was recorded which was subthreshold for a somatic action potential but which had a short rise time and a rapid initial repolarization that gave the event a small but well-defined peak. This event was termed a "partial spike." During repeated stimulation of the same nerve trunk, the fluctuations in the amplitude of the partial spike were small compared to those of evoked synaptic potentials. Stimulation of different nerve trunks evoked partial spikes of different amplitudes. When different nerve trunks were stimulated at short intervals between stimuli, one partial spike could occlude another partial spike. Antidromic responses could not be blocked by a preceding partial spike. This suggests that partial spikes are not initial segment spikes. Tubocurarine reversibly abolished partial spikes which is indicative of a synaptic origin. It is concluded that partial spikes result from action potentials initiated by synaptic potentials in the dendrites and which fail to generate somatic action potentials.