Different regulation of thyrotropin releasing hormone content and release in paraventricular nucleus (PVN) and median eminence (ME) of rat hypothalamus during in vitro incubation

Cold stress induces metabolic activation of thyrotrophin-releasing hormone-synthesising neurones in the magnocellular division of the hypothalamic paraventricular nucleus and concomitantly changes ovarian sympathetic activity parameters

Recent studies suggest thyrotrophin-releasing hormone (TRH) serves as a neurotransmitter and thereby provides a functional vegetative connection between the brain and the ovary. In the present study, magnocellular neurones of the paraventricular nucleus (PVN) in animals subjected to cold exposure were studied to determine the hypothalamic origin of the TRH involved in this pathway. In situ hybridisation analysis of hypothalamic tissue showed that cold exposure causes a two-fold increase in the total number of neurones expressing TRH mRNA in the PVN. Immunohistochemical studies showed that TRH peptide is localised to the magnocellular PVN and that the number of TRH immunoreactive cells increases two-fold following 64 h of cold exposure. Double-immunostaining for MAP-2 and TRH revealed that TRH peptide is localised in the perikarya of the magnocellular neurones. TRH release was measured in vivo from the magnocellular portion of the PVN using push-pull perfusion. Although controls exhibited a very low level of TRH release, animals subjected to cold showed a pulsatile-like TRH release profile with two different patterns of release: (i) low basal level with small bursts of TRH release and (ii) a profile with an up to seven-fold increase in TRH release compared to controls. The colocalisation of TRH with the specific somato-dendritic marker MAP-2 in processes of the magnocellular neurones suggested a local release of TRH. Additional studies demonstrated a reduction in ovarian noradrenaline content after 48 h of cold exposure, a feature indicative of nerve activation at the terminal organ. After 64 h of cold exposure, the ovarian noradrenaline returned to control values but the noradrenaline content of the coeliac ganglia was increased, suggesting a compensatory effect originating in the cell bodies of the sympathetic neurones that innervate the ovary. The correlation between the local release of TRH from dendrites within the magnocellular PVN in conditions of cold and the activation of the sympathetic nerves supplying the ovary raises the possibility that TRH contributes to the processing regulating sympathetic outflow and may thereby impact on the functional activity of the ovary.

The Effect of Swelling on TRH and Oxytocin Secretion From Hypothalamic Structures

1. Cell swelling induces exocytosis of material stored in secretory vesicles resulting in a secretory burst of peptidic hormones or enzymes from various types of cells including endocrine cells and neurons. We have previously shown that swelling-induced exocytosis possesses limited selectivity; hypotonic medium evokes TRH but not oxytocin release from hypothalamic paraventricular nucleus (PVN) and neurohypophysis (NH). 2. It is the aim of this study to ascertain whether the swelling-induced oxytocin secretion could be unmasked by the inhibition of specific osmotic response using Ca(2+)-free medium and GdCl(3), an inhibitor of stretch activated channels. 3. Oxytocin release from the PVN was stimulated by the hypotonic medium only in the presence of 50 or 100 microM GdCl(3.) Oxytocin release from supraoptic nucleus (SON) was also stimulated by the Ca(2+)-free hypotonic medium in the presence of GdCl(3). Oxytocin secretion from the NH was not stimulated even in the presence of GdCl(3), both in Ca(2+) containing and Ca(2+)-free medium. TRH response to swelling-inducing stimulus was not affected by the presence of GdCl(3). 4. An intranuclear oxytocin secretion to hyposmotic stimulation within the PVN and the SON could be unmasked by the inhibiting specific response by GdCl(3). At these conditions general secretory response to swelling-inducing stimuli emerged. Secretion of oxytocin from the NH was not affected by any of these treatments. 5. Peptides and proteins released after cell swelling can play an important role in the pathophysiology of ischemia and could be mediators of local or remote preconditioning. Disruption of mechanosensitive gating in magnocellular neurosecretory cells could result in an inadequate secretory response (e.g. stimulation instead of inhibition and vice versa) of hormones engaged in water and salt metabolism regulation.

Colchicine treatment differently affects releasable thyrotropin-releasing hormone (TRH) pools in the hypothalamic paraventricular nucleus (PVN) and the median eminence (ME)

1. Hypophysiotropic thyrotropin-releasing hormone (TRH) is synthesized in the hypothalamic paraventricular nucleus (PVN) and transported to the median eminence (ME) where it enters the hypophyseal portal blood. TRH in the ME is situated exclusively in nerve terminals, whereas TRH in the PVN and septum is of extrinsic (nerve terminals) as well as intrinsic (perikarya) origin. 2. To determine the source and possible differential regulation of TRH release from these structures, we blocked TRH axonal delivery by i.c.v. administration of colchicine into the lateral cerebral ventricle of euthyroid or hypothyroid rats in doses of 7.5 mug or 7.5, 75 and 100 microg, respectively, two days prior to the evaluation of the TRH secretion from the PVN, ME and the septum in vitro. 3. In euthyroid rats a low dose of colchicine did not significantly affect plasma TSH. The secretory response to both ethanol in an isosmolar medium and a high K+ in the ME as well as the PVN explants was well preserved. However, colchicine treatment resulted in the significant increase of basal secretion of TRH from the PVN. 4. Hypothyroidism induced by 200 mg/l methimazole in drinking water for two weeks resulted in growth arrest, elevated plasma thyrotropin and decreased TRH content in the PVN and the ME. Colchicine partially decreased elevated plasma thyrotropin and increased the TRH content in the PVN and its basal release in vitro which was independent of extracellular Ca2+. Interestingly, a TRH release from the PVN could not be further stimulated either by K+ membrane depolarization or by ethanol. TRH responsiveness to the stimulation remained unaffected in the ME. The effect of colchicine on the septal TRH secretion was intermediate between the effect observed in the PVN and the ME. 5. In conclusion, the absence of a TRH secretory response to stimuli in the PVN after colchicine disruption of the microtubules and Golgi system suggests that stimulated TRH release observed from the PVN explants in vitro occurs from nerve terminals projecting to the PVN from other brain regions. The independence from extracellular calcium implies that TRH released under the non-stimulating conditions occurs most likely via the constitutive secretory pathway from dendrites and/or perikarya. Regulation of septal TRH is markedly different from the hypophysiotropic one.

Cell swelling induced secretion of TRH by posterior pituitary, hypothalamic paraventricular nucleus and pancreatic islets: effect of L-canavanine

The aims of this study were to test if ethanol induces thyrotropin-releasing hormone (TRH) secretion in vitro from the posterior pituitary and hypothalamic explants by a mechanism involving cell swelling, and to characterize the pathway of stimulated secretion. Ethanol, at a concentration of 80 mM, stimulated the release of TRH from the posterior pituitary, the hypothalamic paraventricular nucleus, the median eminence, and the brain septum, when administered only in isosmolar but not in hyperosmolar medium. This indicates the involvement of a cell swelling-inducing mechanism. L-canavanine in a concentration of 3 mM, increased the basal and hyposmosis-induced TRH secretion from the posterior pituitary and the paraventricular nucleus, and both basal and ethanol-induced TRH secretion from isolated pancreatic islets. This indicates the presence of both constitutive and regulatory secretory pathways. Our results suggest that cell swelling induces exocytosis from clathrin coated granules.

Hypo-osmolarity stimulates and high sodium concentration inhibits thyrotropin-releasing hormone secretion from rat hypothalamus

The hypothalamic paraventricular nucleus, representing cell bodies in which thyrotropin-releasing hormone is synthesized, and the median eminence, representing nerve terminals, were incubated in vitro. Various hypo- and hyperosmotic solutions were tested to determine osmotic sensitivity of thyrotropin-releasing hormone secretion. High KCl (56 mM) causing membrane depolarization was used as a non-specific control stimulus to induce thyrotropin-releasing hormone secretion. A 30% decrease of medium osmolarity (from 288 to 202 mOsmol/l) increased thyrotropin-releasing hormone secretion from both the paraventricular nucleus and median eminence. A 30% decrease of medium NaCl content by its replacement with choline chloride did not affect basal thyrotropin-releasing hormone secretion. Increasing medium osmolarity with biologically inactive L-glucose did not affect basal or KCl-induced thyrotropin-releasing hormone secretion from either structure. Medium made hyperosmotic (350-450 mOsmol/l) by increasing the NaCl concentration resulted in a dose-dependent decrease of basal thyrotropin-releasing hormone secretion and abolished KCl-induced thyrotropin-releasing hormone secretion. If an osmotically equivalent amount of choline chloride was substituted for NaCl, there was no effect on thyrotropin-releasing hormone secretion, indicating a specific action of Na+. This study indicates a specific sensitivity to high concentrations of Na+ ions of both thyrotropin-releasing hormone-producing parvocellular paraventricular neurons and thyrotropin-releasing hormone-containing nerve terminals in the median eminence.

Chronic ethanol drinking and food deprivation affect rat hypothalamic-pituitary-thyroid axis and TRH in septum

Because chronic ethanol ingestion may perturb thyroid function, we evaluated the effect of 4-wk of oral 10% ethanol ingestion on the hypothalamic-pituitary-thyroid (HPT) axis and septal thyrotropin-releasing hormone (TRH) in 200-g male Wistar rats. Animals were divided into three groups: absolute control receiving tap water and food ad libitum; ethanol group receiving food ad libitum and 10% ethanol as the sole source of drinking fluid; pair-fed group receiving tap water and an amount of food corresponding to the consumption of ethanol group. After 4-wk of treatment, the body weight of the ethanol group was 7% and of the pair-fed rats 19% lower than that of the absolute controls. Both chronic ethanol treatment and food deprivation produced a decrease in plasma thyroid-stimulating hormone (TSH). Pair-fed rats also had a lower plasma T3. Type I iodothyronine 5'-deiodinase activity in the liver was increased in the pair-fed and even more in the ethanol-treated group. The content and secretion in vitro of TRH from the hypothalamic paraventricular nucleus and median eminence were unchanged. TRH content in the septum was increased in both the ethanol and pair-fed groups. TRH secretion from the septum in vitro was lower in the pair-fed, but unchanged in the ethanol group. These data suggest that 4-wk of peroral ethanol intake affects thyroid function mostly at the extrahypothalamic level and that there is a contribution of concomitant food deprivation. Both ethanol treatment and food deprivation increased TRH content in the septum.

Both iso- and hyperosmotic ethanol stimulate release of hypothalamic thyrotropin-releasing hormone despite opposite effect on neuron volume

Previous studies have indicated that isosmolar, but not hyperosmolar, ethanol induces in vitro gonadotropin-releasing hormone secretion from the basal hypothalamus, presumably by causing cell swelling. Moreover, ethanol reduces secretion of another hypothalamic neuropeptide vasopressin. We have studied the acute effect of ethanol on specific hypophysiotropic basal and K+-stimulated thyrotropin-releasing hormone secretion in vitro especially in relation to cell swelling. Isosmotic 40-160 mM ethanol increased thyrotropin-releasing hormone release from the hypothalamic paraventricular nucleus and median eminence in a dose-dependent manner. Both a 30% decrease of osmolarity and isosmotic 80 mM ethanol induced 12% swelling of hypothalamic neurons. Hyperosmotic 80 mM or 160 mM ethanol induced release of thyrotropin-releasing hormone from both hypothalamic structures but did not cause cell swelling (80 mM) or even induced cell shrinkage (160 mM). Depletion of medium Ca2+ did not affect thyrotropin-releasing hormone secretion caused by either isosmotic or hyperosmotic ethanol. Our data indicate that both iso- and hyperosmotic ethanol stimulated release of hypophysiotropic thyrotropin-releasing hormone despite opposite effects on neuron volume. The mechanism of ethanol action appears complex and variable depending on the type of cell and neuropeptide affected.