Central autonomic activation by intracisternal TRH analogue excites gastric splanchnic afferent neurons.
Intracisternal (ic) injection of thyrotropin-releasing hormone (TRH) or its stable analogue RX 77368 influences gastric function via stimulation of vagal muscarinic pathways. In rats, the increase in gastric mucosal blood flow evoked by a low ic dose of RX 77368 occurs via release of calcitonin gene-related peptide from capsaicin-sensitive afferent neurons, most probably of spinal origin. In this study, the effect of low ic doses of RX 77368 on afferent impulse activity in splanchnic single fibers was investigated. The cisterna magna of overnight-fasted, urethan-anesthetized Sprague-Dawley rats was acutely cannulated, and fine splanchnic nerve twigs containing at least one fiber responsive to mechanical probing of the stomach were isolated at a site immediately distal to the left suprarenal ganglion. Unit mechanoreceptive fields were encountered in all portions of the stomach, both superficially and in deeper layers. Splanchnic afferent unit impulse activity was recorded continuously during basal conditions and in response to consecutive ic injections of saline and RX 77368 (15-30 min later; 1.5 or 3 ng). Basal discharge rates ranged from 0 to 154 impulses/min (median = 10.2 impulses/min). A majority of splanchnic single units with ongoing activity increased their mean discharge rate by >/=20% after ic injection of RX 77368 at either 1.5 ng (6/10 units; median increase 63%) or 3 ng (19/24 units; median increase 175%). Five units lacking impulse activity in the 5-min before ic RX 77368 (3 ng) were also excited, with the onset of discharge occurring within 1.0-5.0 min postinjection. In units excited by ic RX 77368, peak discharge occurred 15.6 +/- 1.3 min after injection and was followed by a decline to stable activity levels +info)
Secretagogue-induced exocytosis recruits G protein-gated K+ channels to plasma membrane in endocrine cells.
Stimulation-regulated fusion of vesicles to the plasma membrane is an essential step for hormone secretion but may also serve for the recruitment of functional proteins to the plasma membrane. While studying the distribution of G protein-gated K+ (KG) channels in the anterior pituitary lobe, we found KG channel subunits Kir3.1 and Kir3.4 localized on the membranes of intracellular dense core vesicles that contained thyrotropin. Stimulation of these thyrotroph cells with thyrotropin-releasing hormone provoked fusion of vesicles to the plasma membrane, increased expression of Kir3.1 and Kir3.4 subunits in the plasma membrane, and markedly enhanced KG currents stimulated by dopamine and somatostatin. These data indicate a novel mechanism for the rapid insertion of functional ion channels into the plasma membrane, which could form a new type of negative feedback control loop for hormone secretion in the endocrine system. (+info
High concentration of thyrotropin-releasing hormone in pancreatic islets.
The concentration of thyrotropin-releasing hormone (TRH, thyroliberin) in rat islets of Langerhans is 30-fold higher than in whole rat pancreas, indicating that the islets are the main source of pancreatic TRH. The TRH extracted from islets is indistinguishable from synthetic TRH in its immunological and biological properties and in its inactivation by human serum. The physiologic function of islet TRH is unknown. However, because TRH is antagonistic to somatostatin in other systems, and somatostatin also is concentrated in islets in high concentrations, it is possible that islet TRH may serve a similar antagonistic function in the regulation of islet cell secretory activity. (+info
Somatostatin inhibits release of thyrotropin releasing factor from organ cultures of rat hypothalamus.
Somatostatin in concentrations of 10(-6) to 10(-8) M inhibited basal release of thyrotropin releasing factor in organ culture of rat hypothalamus. Norepinephrine in doses of 10(-4)--10(-6) M induced release of thyrotropin releasing factor which increased progressively with time and was temperature and dose dependent. This enhanced thyrotropin-releasing-factor release was inhibited by somatostatin at 10(-6)--10(-8) M. (+info
Purification of rat prolactin releasing factor by gel filtration.
Pregnant rat hypothalamic fragments were extracted with 30 mM Tris-HCl buffer at pH 7.8, subjected to enzymatic digestion, and applied to gel filtration on Sephadex G-25 for purification of the prolactin releasing factor. Effect of the eluted fractions on the release of prolactin were tested by the determination of serum and pituitary prolactin after the injection in lactating rats. Prolactin was estimated by radioimmunoassay. One fraction (tube number 61--73) eluted later than synthetic ACTH enhanced release of prolactin 30 min after injection, but other fractions had no effect on the release of prolactin. Prolactin releasing factor would be quantitatively predominant over prolactin inhibiting factor in pregnant rat. (+info
Paradoxical GH response to TRH during status epilepticus in man.
Information on GH in relation to epilepsy is sparse, and to our knowledge there is no information on GH levels during status epilepticus in man. We studied GH in serum in six patients during status epilepticus, and in a control group of six seizure-free patients with epilepsy, before and after injection of TRH. The baseline GH values before TRH administration were within the normal range in all patients. After injection of TRH all patients with status epilepticus showed a paradoxical peak-shaped increase of GH to at least twice their baseline levels within 45 min after the injection (median basal GH value 1.5 mU/l and median peak GH value 6. 5 mU/l, mean increase 330%). No uniform reaction to TRH was observed in the control group (median basal GH value 2.7 U/l and median of the highest value within 45 min 5.2mU/l). A paradoxical peak reaction of GH to TRH was significantly more frequent in the status epilepticus group compared with the control group (P=0.008, Fisher exact probability test). TRH is not considered a GH-releasing hormone in humans during normal conditions, but a paradoxical response of GH to TRH, similar to that observed during status epilepticus, has been reported in various other pathological conditions, such as acromegaly, liver cirrhosis, mental depression and hypothyroidism. Our results of GH release after TRH administration in patients with status epilepticus suggest an altered regulation of GH as a result of the long-standing epileptic activity. (+info
Evaluation of hypothalamic-pituitary-adrenal axis in amenorrhoeic women with insulin-dependent diabetes.
Diabetes is associated with a higher incidence of secondary hypogonadotrophic amenorrhoea. In amenorrhoeic women with insulin-dependent diabetes a derangement in hypothalamic-pituitary-ovary axis has been proposed. No data exist on hypothalamic-pituitary-adrenal function in these women. Gonadotrophin releasing hormone (GnRH), corticotrophin releasing hormone (CRH), metoclopramide and thyroid releasing hormone (TRH) tests were performed in 15 diabetic women, eight amenorrhoeic (AD) and seven eumenorrhoeic (ED). Frequent blood samples were taken during 24 h to evaluate cortisol plasma concentrations. There were no differences between the groups in body mass index, duration of diabetes, insulin dose and metabolic control. The AD women had lower plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin, oestradiol, androstenedione and 17-hydroxyprogesterone (17-OHP) than the ED women. The responses of pituitary gonadotrophins to GnRH, and of thyroid stimulating hormone (TSH) to TRH, were similar in both groups. The AD women had a lower prolactin response to TRH and metoclopramide, and lower ACTH and cortisol responses to CRH, than the ED women. Mean cortisol concentrations > 24 h were higher in the amenorrhoeic group. Significant differences in cortisol concentrations from 2400 to 1000 h were found between the two groups. Insulin-dependent diabetes may involve mild chronic hypercortisolism which may affect metabolic control. Stress-induced activation of the hypothalamic-pituitary-adrenal axis would increase hypothalamic secretion of CRH. This would lead directly and perhaps also indirectly by increasing dopaminergic tonus to inhibition of GnRH secretion and hence hypogonadotrophic amenorrhoea. Amenorrhoea associated with metabolically controlled insulin-dependent diabetes is a form of functional hypothalamic amenorrhoea that requires pharmacological and psychological management. (+info
Intracisternal TRH analog increases gastrin release and corpus histidine decarboxylase activity in rats.
Thyrotropin-releasing hormone (TRH) acts in brain stem nuclei to induce vagally mediated stimulation of gastric secretion. The effects of intracisternal injection of the TRH analog RX-77368 on plasma gastrin levels and corpus histidine decarboxylase (HDC) activity were studied in 48-h fasted conscious rats. RX-77368 (25-100 ng) increased plasma gastrin levels by threefold at 30 min, which remained significantly higher than control at 2 and 4 h postinjection. Corpus HDC activity began to increase at 2 h and reached a peak at 4 h postinjection with a 21-fold maximum response observed at 50 ng. Morphological changes in the appearance of corpus HDC-immunoreactive cells correlated well with HDC activity. Pretreatment with gastrin monoclonal antibody completely prevented RX-77368 stimulatory effects on HDC activity. Atropine significantly attenuated gastrin increase at 30 min by 26%. These results indicated that in conscious fasted rats, TRH analog acts in the brain to increase corpus HDC activity in the enterochromaffin-like cells, which involves gastrin release stimulated by central TRH analog. (+info