摘要:

Sheep somatotroph-enriched cultures were obtained by means of enzyme dissociation and Percoll gradient separation. Nystatin-perforated-whole-cell recordings were performed on post-recording-identified somatotrophs after 4–14 days in vitro. Using Ca2+-free, tetrodotoxin-containing (1 µM) bath solution and K+ electrode solution, three types of voltage-dependent K+ currents were recorded as inward rectifying, outward transient and outward delayed rectifying K+ currents. The inward rectifying K+ current was very small at physiological extracellular K+ concentrations (5 mM) and enhanced by increasing the K+ concentration in the bath to 55 mM; it was blocked by tetraethylammonium (2 mM) but not by 4-aminopyridine (5 mM). A transient outward K+ current appeared at –50 mV and was selectively diminished by 4-aminopyridine (2 or 4 mM). A delayed rectifying outward K+ current was observed when the membrane potential was depolarized to -20 mV and was blocked by tetraethylammonium (2 mM) but not 4-aminopyridine (4 mM). Application of 4-aminopyridine but not tetraethylammonium (up to 5 mM) depolarized the cell membrane potential recorded under current clamp conditions and triggered action potentials when the bath solution contained Ca2+ (2 mM) but not tetrodotoxin. The intracellular Ca2+ concentration was increased by 4-aminopyridine as was growth hormone release. Therefore, the 4-aminopyridine-sensitive transient outward K+ current appears to be important in the determining the resting potential of ovine somatotrophs and plays a major role in regulating basal intracellular Ca2+ concentration and growth hormone secre

ISSN:0028-3835    

DOI:10.1159/000126631

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

GH4C1 cells, a clonal cell line from a rat pituitary tumor, have been widely used as a model to study the regulation of prolactin secretion. These cells, however, do not express dopamine D2 receptors and are therefore not suitable for exploring mechanisms involved in dopamine inhibition of prolactin secretion. The recent demonstration that epidermal growth factor (EGF) is able to induce functional expression of D2 receptors in GH3 cells, a parental clonal cell line, overcomes this difficulty. We have thus undertaken an electrophysio-logical study in order to check whether coupling of D2 receptors to K+ channels could be restored in that model. Effects of dopamine on the non-inactivating voltage-dependent outward K+ current (IK) were investigated both in control and in EGF-treated GH4C1 cells. The K+ current was not modified by EGF treatment alone. In control cells, IK measured before and during dopamine application was unchanged. In contrast, dopamine application markedly enhanced the K+ current in cells that had previously been exposed to EGF. The effect was mimicked by the specific D2 receptor agonist bromocriptine and blocked by sulpiride, a D2 receptor antagonist, thus indicating that the effect of dopamine was effectively due to the activation of D2 receptors. These results bring further evidence that EGF-induced D2 receptors in clonal strains from rat pituitary tumors are functional and are coupled to the delayed outward K+ current IK.

ISSN:0028-3835    

DOI:10.1159/000126632

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

A patch-clamp study was conducted on cultured frog pituitary melanotrophs, in order to investigate the effects of adrenaline on the electrical activity of these cells. In the whole-cell configuration, adrenaline (1 µM) caused hyperpolarization that was accompanied by a fall in membrane input resistance and a blockage of spontaneous action potentials. Under voltage clamp, adrenaline elicited a net-outward current. The hyperpolarization became undetectable at a command voltage of –100 mV which corresponded to the equilibrium potential of potassium ions. The effect of adrenaline on membrane potential and spontaneous activity was blocked by the α2-adrenergic receptor antagonist yohimbine (1-10 µM) but could not be mimicked by the α2-adrenergic agonist clonidine (1–10 µM). In the cell-attached configuration, exposure of the extra-patch membrane to adrenaline increased the occurrence of single-channel currents with a slope conductance of 100 pS. The deduced reversal potential of these currents corresponded to the equilibrium potential of potassium ions. These results suggest that frog melanotrophs display an α2-adrenergic receptor subtype coupled to potassium channels involved in hyperpol

ISSN:0028-3835    

DOI:10.1159/000126633

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

In the present work we investigated a possible role of an α1-noradrenergic (NA) pathway involving the hypothalamic paraventricular nucleus (PVN) in the central regulation of growth hormone (GH) release. A week after bilateral electrolytic lesions of the PVN, pulsatile GH-secretory patterns were monitored in unanesthetized, freely moving control or lesioned male rats. While the pulsatility of GH secretion was maintained, the amplitude of the pulses and the area under the curve during an 8-hour sampling period were twice as high in PVN-lesioned than in control rats. Trough levels of GH were similar in the two groups. Inactivation of PVN α1-receptors by local infusion of an α1-NA antagonist, prazosin (50 ng/rat), also induced an increase in GH release. In control animals, intravenous injection of the α1-NA agonist methoxamine (0.02 mg/100 g body weight) elicited a decrease in GH release but was ineffective when administered to PVN-lesioned rats. These data show that α1-NA receptors, mediating GH inhibition, are located in the PVN. In light of the analogous effects observed herein on PVN-lesioned animals and, previously, after locus coeruleus (LC) lesions it is suggested that GH inhibition by the LC is relayed by the PVN via a local α1-receptor popul

ISSN:0028-3835    

DOI:10.1159/000126634

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

Administration of various doses of clonidine increased plasma growth hormone levels in male Wistar rats. Chronic hydrocortisone treatment produced significant decreases in ACTH levels as well as a significant attenuation of clonidine-induced increases in growth hormone levels. Similarly, chronic treatment with 5-HT uptake-inhibiting antidepressants such as fluoxetine, clomipramine and imipramine also significantly attenuated clonidine-induced increases in growth hormone levels. On the other hand, chronic treatment with clorgyline (monoamine oxidase type A-inhibiting antidepressant) and 5-HT agonists, such as m-chlorophenylpiperazine, 1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane and 8-hydroxy-2-(di-n-propylamino)tetralin, did not have any significant effect on clonidine-induced increases in growth hormone levels. These findings suggest the development of functional subsensitivity of either α2-adrenergic heteroreceptors or 5-HT1C receptors mediating clonidine-induced growth hormone secretion following chronic treatment with glucocorticoids and 5-HT uptake inhibitors

ISSN:0028-3835    

DOI:10.1159/000126635

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

After bilateral superior cervical ganglionectomy (SCGx) of adult male rats, norepinephrine (NE) content of the medial basal hypothalamus (MBH) decreased significantly by 39-47% from 16 h to 7 days after surgery. During this time the levels of serum growth hormone (GH) and prolactin (PRL) and of MBH GH-releasing hormone (GRH), thyrotropin-releasing hormone (TRH) and somatostatin were measured by RIA. In sham-operated controls, serum PRL increased and serum GH decreased 16–24 h after surgery, attaining pre-surgical levels later on. In SCGx rats, significantly lower serum GH and PRL and higher MBH GRH and TRH content as compared to controls was observed 16-24 h after surgery, during the wallerian degeneration phase after SCGx. MBH somatostatin concentration decreased in SCGx rats 20 h after surgery. Two injections of the α1-adrenoceptor blocker prazosin 45 and 90 min before sacrifice, alone or together with the β-blocker propranolol, prevented the changes in MBH hypophysiotropic hormone content, as well as in serum GH and PRL levels, found in SCGx rats 20 h after surgery. Propranolol treatment did not affect hormone levels. Neither drug modified the decrease in MBH NE content observed after SCGx. The results argue in favor of the existence of physiologically relevant projections from superior cervical ganglion neurons to the MBH controlling hypophysiotropic hormone rele

ISSN:0028-3835    

DOI:10.1159/000126636

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

The role of the hypothalamic paraventricular nucleus (PVN) region in the control of seasonal prolactin (PRL) responses was investigated in female Siberian hamsters via disruption of PVN afferent connections from the region of the suprachiasmatic nucleus (SCN). Adult female hamsters received sham surgery or horizontal knife cuts placed ventral to the PVN so as to sever dorsally projecting fibers from the SCN and were either immediately transferred to a short-day photoperiod [10 h light: 14 h dark (10L: 14D); experiment (exp.) 1A] or returned to the long-day photoperiod (16L:8D; exp. 1B and 2). Serial blood samples were analyzed for determination of circulating PRL and follicle-stimulating hormone (FSH; exp. 1 A) levels at various time points after surgery. In exp. 1 A, sham-operated animals showed a steady decline in circulating levels of both PRL and FSH during exposure to 10L. Animals with knife cuts that passed through the extent of the SCN were prevented from showing declines in PRL and FSH during exposure to 10L. Animals with knife cuts located ventral to or through the PVN but dorsal to the SCN maintained high FSH levels during short-day exposure but showed a decline in PRL. Similarly, animals with knife cuts in exp. IB and 2 showed a decline in circulating PRL despite continued exposure to a stimulatory photoperiod. In exp. 2, the efficacy of a dopaminergic agonist (CB154) and an antagonist (pimozide) in altering circulating PRL under the 16L photoperiod was evaluated. CB154 induced declines in PRL in knife-cut but not sham-operated animals, whereas administration of pimozide elevated circulating PRL in both groups. These results suggest that disruption of afferent input to the PVN region may result in reductions in circulating PRL independently of the photoperiodic regulation of this hormone, and via mechanisms other than increased dopaminergic tone, i.e., possibly through disruption of a hypothalamic PRL-stimulating system.

ISSN:0028-3835    

DOI:10.1159/000126637

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

The prolactin-releasing effect of angiotensin II (AII) was studied in the developing female and male rat in vivo and in vitro. AII (50 and 100 µg/100 g b.w.) was injected intraperitoneally to female and male rats aged 4, 12, 20 and 28 days and males aged 38 days. AII (10–6M) was also tested in pituitaries incubated in vitro from animals of both sexes aged 12, 20 and 28 days. In addition, as two subtypes of AII receptors have been characterized on the basis of displacement with specific AII antagonists, we used the nonpeptide AII receptor antagonists losartan (ATI subtype) and PD 123319 (AT2 subtype) to determine the AII receptor subtype functionally involved in AII-induced prolactin secretion in vivo in 25-day-old male rats. The efficiency of the prolactin-releasing effect of AII in vivo increased with age, and first responses were observed at 20 days of age in both sexes. No sexual differences were encountered. On the other hand, AII-induced prolactin release from pituitaries incubated in vitro was first demonstrated at 12 days in females and at 20 days in males. The effect increased with age in both sexes, and, at 28 days, pituitaries from females released more prolactin in response to AII than those from males. Losartan (3 mg/kg) completely abolished AII (50µg/100g b.w.)-induced prolactin release in vivo, while PD 123319 (3 mg/kg) did not. This suggests that pituitary ATI receptors are functionally involved in the prolactin release induced by AII in vivo. In vivo, the integrity of the hypothalamic-hypophyseal unit is maintained, and therefore the inhibitory influence of the dopaminergic system, which is stronger in the female developing rat, could mask the sexual differences encountered in vitro. Finally, increasing efficiency of AII-induced prolactin release may be related to increasing prolactin titers in juvenile and prepubertal r

ISSN:0028-3835    

DOI:10.1159/000126638

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

It is well established that stress causes a rise in plasma prolactin (PRL) levels of male or cycling female rats. In lactating animals, the pituitary PRL response to stress is not well understood. Therefore, the purpose of the present study was to analyze this question in lactating rats having low or elevated prestress plasma PRL levels. The animals were exposed to ether, formalin or restraint, and plasma PRL and corticosterone levels were determined. In mothers continually together with their pups, plasma PRL levels decreased significantly after exposure to ether vapor or injection of formalin under the skin. At the same time, both agents caused a significant rise in blood corticosterone concentrations. Lactating rats isolated for 4 h had very low levels of PRL before application of stress. However, neither formalin nor restraint caused any elevation in their plasma PRL levels although both interventions increased blood corticosterone concentrations. Lactating mothers receiving formalin after a 30-min suckling stimulus preceded by 4 h isolation did not show appreciable changes in pituitary PRL secretion following the administration of formalin. For information on the mechanism of the effect of stress on PRL, lactating rats were pretreated with the dopamine receptor antagonist domperidone (injecting 80 µg/kg body weight) or were adrenalectomized 7 days prior to exposure to stress. The very high levels of PRL caused by domperidone decreased markedly in animals subjected to restraint stress. Administration of formalin to adrenalectomized lactating rats continually together with their litter caused a slight immediate decrease, followed by a transitory elevation and a subsequent small second decrease in blood PRL concentration. The depression was significantly less than in nonadrenalectomized animals receiving formalin. If formalin was administered 2 days after isolation of the mothers it caused an elevation of plasma PRL concentrations indicating that the PRL stress response pattern characteristic of nonlactating animals returns within 48 h of isolation. On the basis of the present findings we conclude that in lactating animals (a) stress causes an inhibition of PRL release from the pituitary; (b) the pituitary-adrenocortical system responds to stress, (c) the inhibition of stress-induced pituitary PRL release disappears within 48 h of separation of the mother from her litter, and (d) a dopaminergic mechanism is not involved, but the adrenocortical system may participate in the stress-induced inhibition of PRL release

ISSN:0028-3835    

DOI:10.1159/000126639

出版商:S. Karger AG    

年代:1994

数据来源: Karger

摘要:

Corticotropin-releasing hormone (CRH) plays a major role in activating the pituitary-adrenal axis in stress; its central application may therefore be expected to mimic stress. Since stress reportedly disrupts lactation, experiments were designed to study the effects of CRH administration upon the transfer of milk from rat mothers to their pups and to examine some of the possible underlying physiological mechanisms. CRH was administered intracerebroventricularly to primiparous rats on the 8th day of lactation immediately prior to being reunited with 8 of their overnight-separated pups. Changes in litter weights were measured for a period of up to 4 h as an index of milk procurement by the young (milk transfer); a qualitative assessment of maternal behaviour was also made. Treatment of dams with 0.1–1 nmol CRH resulted in a dose-dependent reduction in the amount of milk obtained by the pups. Conscious mothers treated with CRH initially showed intense behavioural activation; these events (mainly hyperlocomotion and grooming) in the mother resulted in reduced opportunities for nipple attachment by the pups and, thus, milk transfer. On the other hand, milk transfer was also significantly reduced in urethane-anaesthetized mothers treated with CRH, indicating that behavioural activation cannot have been the sole factor underlying the CRH-induced inhibition of milk transfer in awake dams. Although oxytocin (OT) release is stimulated by a variety of stressors, the possibility of an inhibitory effect of CRH upon OT secretion and/or disruption of the reflex arc serving milk ejection was considered. The peripheral administration of OT (100 mU/ rat s.c.) did not, however, surmount the inhibitory actions of CRH upon milk transfer. In view of other reports that CRH may in fact enhance OT secretion, it is tentatively proposed that the inability of exogenously administered OT to facilitate milk transfer might be related to the known effects of centrally applied CRH upon the autonomic nervous syste

ISSN:0028-3835    

DOI:10.1159/000126640

出版商:S. Karger AG    

年代:1994

数据来源: Karger