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G A Hedge

Publications and source records attributed to G A Hedge.

At least 37 records · Page 2Linked to original sources
PubMed1987

Immunoreactive prolactin in the rat hypothalamus: in vitro release and subcellular localization.

Immunocytochemical studies have identified immunoreactive prolactin (IR-PRL) in the hypothalamus and other areas of the rat brain. However, neither the release of IR-PRL from the hypothalamus nor its subcellular localization have been demonstrated. In this study, the release of IR-PRL from hypothalami obtained from female rats was examined using hypothalamic units incubated in vitro in Krebs-Ringer bicarbonate-glucose buffer. Hypothalamic tissue spontaneously released IR-PRL, and this release was increased by depolarizing concentrations of potassium by a calcium-dependent mechanism. Hypothalamic IR-PRL was also released from hypothalamic tissue obtained from hypophysectomized rats (14 days). The subcellular localization of IR-PRL was investigated using equilibrium-density centrifugation. Tissue homogenates from intact or hypophysectomized rats were centrifuged at 150 g at 4 degrees C for 10 min, and the supernatants were layered onto continuous sucrose gradients (1.00-1.27 g/ml) and centrifuged at 100,000 g (max.) for 16 h. IR-PRL in pituitary supernatants showed a high equilibrium-density peak with a modal density of 1.23 g/ml. Fractionation of the supernatant from ventral or dorsal hypothalamic tissue resulted in two high-equilibrium density peaks, a primary peak with a modal density of 1.23 g/ml and a smaller peak with a modal density of 1.10 g/ml. Both high-density peaks were maintained in tissue obtained from hypophysectomized rats and were disrupted by homogenization in hypo-osmotic medium. Together, these data suggest that hypothalamic IR-PRL is stored in membrane-bound particles which have densities similar to those of secretory granules and is released by a calcium-dependent mechanism when the tissue is depolarized.

Animals↗
PubMed1987

The pituitary TSH response to TRH is inversely related to the plasma TSH concentration and directly related to the pituitary TSH content during hypothyroidism in the rat.

We studied the effects of degree and duration of hypothyroidism on the pituitary TSH concentration and the pituitary TSH secretory response to TRH. Varying degrees of hypothyroidism were achieved in thyroparathyroidectomized rats (THYREX) by continuous sc infusion of T3 (0.2, 0.3, 0.4, or 0.5 microgram/100 g X day) or T4 (0.6, 1.2, or 1.8 microgram/100 g X day). While T3 was more potent than T4, both resulted in a dose-dependent suppression of the post-thyroidectomy rise in TSH. After 7 or 14 days of severe hypothyroidism (nonreplaced THYREX rats) the pituitary TSH secretory response to TRH (250 ng/100 g body weight, iv) was found to be decreased when compared to that of euthyroid rats. Decreasing the degree of hypothyroidism increased the pituitary secretory response to TRH and the pituitary TSH content. The results indicate that in the hypothyroid rat: severe hypothyroidism results in a blunted pituitary TSH response to TRH through 14 days after thyroidectomy, at 7 and 14 days after thyroidectomy the pituitary TSH response to exogenous TRH is inversely related to the basal plasma TSH concentration, the pituitary TSH concentration increases with the duration of hypothyroidism, the pituitary TSH content is increased by low rates of thyroid hormone replacement, and the pituitary TSH response to exogenous TRH is directly related to the pituitary TSH content.

Animals↗
PubMed1986

Neuropeptide control of thyroid blood flow and hormone secretion in the rat.

The effects of peptide HI (PHI), neuropeptide Y (NPY), and substance P (SP) on thyroid blood flow and hormone levels were studied in anesthetized rats. Regional blood flows were determined using radioactive microspheres. No change in heart rate or mean left ventricular pressure occurred during these neuropeptide infusions (0.625 micrograms iv over 2 min). PHI treatment resulted in a four-fold increase in thyroid blood flow. Blood flows to the pancreas and salivary gland also increased during PHI treatment. Infusions of NPY or SP did not significantly alter thyroid blood flow. However, SP decreased blood flow to the spleen and small intestine. These neuropeptides had no effect on blood flows to the adrenal, kidney, brain, heart, and adipose tissues. Following PHI, NPY, and SP infusions, plasma triiodothyronine and thyroxine levels were not different from values in saline-treated rats. This study demonstrates that PHI, like vasoactive intestinal peptide, is a potent thyroidal vasodilator at a dose that does not affect circulating thyroid hormone secretion.

Animals↗
PubMed1986

Subcellular localization of immunoreactive thyroid-stimulating hormone in the rat hypothalamus.

Hypothalamic tissue contains TSH-like material which is biologically, immunologically, and physicochemically similar to pituitary TSH. Immunoreactive thyroid-stimulating hormone (IR-TSH) is released from hypothalamic tissue in vitro by depolarizing concentrations of potassium or veratridine by a calcium-dependent mechanism. In the present study, we investigated the subcellular localization of IR-TSH using equilibrium density centrifugation. Tissue homogenates from intact, hypophysectomized or thyroidectomized rats were centrifuged at 150 g at 4 degrees C for 10 min and the supernatants were layered onto continuous sucrose gradients (1.00-1.27 g/ml) and centrifuged at 100,000 g (max) for 16 h. IR-TSH in pituitary supernatants from intact and thyroidectomized rats showed high equilibrium density peaks with a modal density around 1.2 g/ml. Fractionation of the supernatant from ventral or dorsal hypothalamic homogenates resulted in a bimodal distribution of IR-TSH. In supernatants from both tissues, IR-TSH containing particles were found at the top of the gradient in a low equilibrium density peak between 1.0 and 1.08 g/ml. In addition, IR-TSH containing particles were found in ventral and dorsal hypothalamic supernatants with modal densities at 1.16 and 1.25, respectively. These high density IR-TSH particles were present in tissue taken from hypophysectomized rats, and were not appreciably affected by thyroidectomy. Homogenization of the tissue in a hypo-osmotic medium disrupted the high density IR-TSH particles resulting in a single low density peak at the top of the gradient. These data suggest that hypothalamic IR-TSH is stored in membrane bound particles which have densities similar to that of secretory granules.

Animals↗
PubMed1986

Effects of vasoactive intestinal peptide on thyroid blood flow and circulating thyroid hormone levels in the rat.

The effects of vasoactive intestinal peptide (VIP) on thyroid blood flow and hormone levels were studied in rats. Tissue blood flow was determined from the distribution of radioactive microspheres after injection by cardiac puncture directly into the left ventricle of anesthetized rats. Initial results indicated that the systemic infusion of 6.25 micrograms VIP iv resulted in increased thyroid blood flow, but was also associated with hypotension, as measured by left ventricular pressure. In contrast, topical administration of VIP to the left of the thyroid increased blood flow to that lobe, but not to the right lobe, and produced no systemic cardiovascular effects. In a further set of experiments, graded doses of VIP were administered iv. Infusions of 6.25 and 0.625 micrograms VIP were associated with 2- to 3-fold increases in thyroid and pancreatic blood flows, but lower doses were ineffective. Blood flows to the adrenals, brain, small intestine, kidneys, and spleen were not altered by any dose of VIP. Mean left ventricular pressure was again reduced by the 6.25-micrograms dose of VIP, but was not affected by lower doses. The infusions of VIP had no effect on plasma TSH, T3, or T4 levels either 20 min or 2 h after treatment. These results suggest that thyroid blood flow is, in part, controlled by VIP and indicate that changes in thyroid blood flow can occur at doses of VIP that have no apparent effect on circulating thyroid hormone levels.

Animals↗
PubMed1985

Distribution and release of immunoreactive thyroid-stimulating hormone in the rat hypothalamus: effects of thyroidectomy, hypophysectomy and treatment with thyroid hormones.

Immunoreactive thyroid-stimulating hormone (IR-TSH) has been detected in the hypothalamus and is released in vitro by a calcium-dependent mechanism when the tissue is depolarized. Recently, immunocytochemical studies have revealed that IR-TSH is present in thyrotropes in the pars tuberalis. Therefore, because these thyrotropes are associated with the median eminence, the area with the highest concentration of IR-TSH, it is of interest to determine if 'hypothalamic' IR-TSH is from neural or pituitary cells. We addressed this issue by studying the effects of hypophysectomy, thyroidectomy, or chronic administration of triiodothyronine (T3) or thyroxine (T4) on the distribution and in vitro release of IR-TSH in the hypothalamus. We reasoned that, if hypothalamic IR-TSH is dependent on the thyrotropes of the pars tuberalis, then changes in hypothalamic IR-TSH concentration and release should be parallel to those measured in pituitary extracts. IR-TSH was measured in tissue extracted in ice-cold 2% NaCl, with a final pH of 4.5. For the in vitro studies, tissues were incubated for 20-min periods in Krebs-Ringer bicarbonate buffer at 37 degrees C. In untreated rats, the concentration of IR-TSH is greater in the ventral than the dorsal portion of the hypothalamus (39.3 +/- 8.2 vs. 4.0 +/- 1.5 ng/mg wet wt.). Upon finer dissection of the hypothalamus into median eminence and anterior, middle, and posterior portions of the remainder, IR-TSH was only detectable in the middle hypothalamus (5.3 +/- 1.5 ng/mg), and the median eminence (149 +/- 41 ng/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Effects of food deprivation on the feedback regulation of the hypothalamic-pituitary-thyroid axis of the rat.

We have studied the effects of food deprivation on the plasma concentrations of T4, T3, and TSH, the TSH MCR, the pituitary TSH content, and the pituitary TSH response to TRH in female rats. Two days before the beginning of the experiment all rats were prepared with chronic intraatrial catheters through which TRH (100 ng/100 g BW) or [125I]iodo-TSH was administered, and sequential blood samples were obtained. The plasma concentrations of T4 and T3 in food-deprived rats were significantly (P less than 0.05) reduced compared to those in control rats 6, but not 2, days after food removal. Despite the fall in plasma concentrations of T4 and T3, no compensatory rise in the plasma TSH concentration was observed in the food-deprived rats. Plasma TSH concentrations did not differ between groups at any time. The change in the plasma TSH concentration (delta plasma TSH) was significantly greater 15, 30, and 45 min after iv bolus injection of TRH in food-deprived rats than in control rats. To further evaluate the basal TSH secretion rate (MCR X plasma TSH concentration) and the amount of TSH secreted in response to TRH (MCR X area under delta plasma TSH curve), the MCR of TSH was determined in control and food-deprived rats from the plasma disappearance curves of injected [125I]iodo-TSH. No differences were found in the TSH MCR or the calculated basal plasma TSH secretion rate (micrograms per day/100 g BW) of control or 6-day food-deprived rats. However, the change in the amount of TSH secreted in response to TRH was significantly greater in food-deprived rats than in control rats (17.7 +/- 2.1 vs. 11.0 +/- 1.0 micrograms/100 g BW; P less than 0.05). In addition, pituitary weight was significantly decreased in food-deprived rats (8.3 +/- 0.3 vs. 10.8 +/- 0.5 mg; P less than 0.05), but the pituitary TSH concentration (micrograms per mg tissue) was unchanged. These results are consistent with the development of tertiary hypothyroidism (i.e. decreased TSH stimulatory input to the pituitary by TRH or some other agent) during food deprivation in the rat.

Animals↗
PubMed1984

The effects of the duration of severe hypothyroidism and aging on the metabolic clearance rate of thyrotropin (TSH) and the pituitary TSH response to TSH-releasing hormone.

We have studied the effects of the duration of severe hypothyroidism (thyroidectomy) and aging on the pituitary TSH response to exogenous TRH and the TSH MCR in female rats. Rats were thyroidectomized (THYREX) or sham operated (SHAM) 4, 14, or 68 days before experimentation. All rats were prepared with chronic intraatrial catheters through which TRH (0, 250, or 1000 ng/100 g BW) or [125I]TSH was administered, and sequential blood samples were obtained. The MCR of TSH was determined in each group from the plasma disappearance curves of injected [125I]iodo-TSH and were found to be decreased in 4-day-THYREX [0.23 +/- 0.01 (+/- SE) vs. 0.27 +/- 0.01 ml/min X 100 g; P less than 0.05], but not in 14-day-THYREX or 68-day-THYREX rats compared with age-matched SHAM controls. In addition, the TSH MCR was found to be decreased in 68-day-SHAM compared to 4- or 14-day-SHAM rats (0.15 +/- 0.01 vs. 0.27 +/- 0.01 ml/min X 100 g; P less than 0.05). The amount of TSH secreted (MCR X area under delta plasma TSH curve X BW) in response to each dose of TRH was calculated and was found to be decreased in 4-day-THYREX rats (4.5 +/- 1.5 vs. 30.0 +/- 4.5 micrograms; P less than 0.05), but not different in 14-day- or 68-day-THYREX rats compared with age-matched SHAM controls. The amount of TSH secreted in response to TRH was also reduced in 68-day-SHAM compared with 4- or 14-day-SHAM rats (16.0 +/- 1.0 vs. 30.0 +/- 4.5 micrograms; P less than 0.05). These results indicate that 1) the MCR of TSH is decreased by aging, but not by severe hypothyroidism in the rat; 2) the pituitary TSH response of the rat to exogenous TRH is decreased by aging, and 3) during severe hypothyroidism, the pituitary TSH response of the rat to a midrange or maximal dose of TRH is decreased (4-day-THYREX) or not different (14- and 68-day-THYREX) compared with that of age-matched euthyroid rats.

Aging↗
PubMed1984

Immunocytochemical studies of the peptidergic innervation of the thyroid gland in the Brattleboro rat.

An indirect immunofluorescence technique was used to study the peptidergic innervation of the thyroid gland in homozygous Brattleboro rats (DI) and normal Long-Evans rats (LE). The primary goal of this study was to determine whether the previously demonstrated decrease in thyroid responsiveness to TSH in DI might be due to an abnormality in the innervation of the thyroid. Thyroids from both types of rats were found to contain nerve fibers containing immunoreactivity for vasoactive intestinal peptide (VIP), substance P (SP), neuropeptide Y (NPY), and peptide HI (PHI). All four types of fibers were found in close association with both follicle cells and blood vessels. Well developed networks of fibers surrounding blood vessels were particularly apparent in the case of NPY. The density of fibers associated with follicle cells in DI was at least as great as that in LE in regard to SP, NPY, and PHI. Fibers containing VIP were found in greater abundance in DI than in LE. Additional studies revealed no evidence of thyroid fibers containing either somatostatin or neurophysin, which was used as a marker for vasopressin. We conclude that the reduced responsiveness of the thyroid in DI is not due to an inadequate supply of any of the neuropeptides included in this study. Since VIP is known to enhance thyroid secretion, we suggest that the apparent proliferation of VIP-containing fibers in DI may be a reflection of a neural mechanism attempting to compensate for a thyroid gland deficiency analogous to the humoral mechanism by which TSH secretion increases in response to thyroid deficiency.

Animals↗
PubMed1983

Factors involved in the attenuation of the TSH response to a second injection of TRH in the rat.

The attenuation of thyrotropin (TSH) responsiveness to a second injection of thyrotropin-releasing hormone (TRH) was studied in adult female rats. This attenuation occurs using submaximal doses of TRH and it is proportional to the dose of the first stimulus within the range of 0-1,250 ng TRH/100 g BW. The magnitude of the attenuation varies inversely with the interstimulus interval whether the TRH is administered intravenously or directly into the anterior pituitary. This diminished responsiveness is independent of stress or anesthesia, and it is not due to negative feedback by thyroid hormones released in response to the initial TRH stimulation. Exogenous rat TSH given to mimic the response to TRH also results in attenuation, and it is suggested that TSH is at least in part responsible for the attenuation observed after TRH administration.

Animals↗
PubMed1983

Direct pituitary stimulation of thyrotropin secretion by opioid peptides.

We have studied the effects of opioid peptides in vitro on the release of TSH from superfused anterior pituitary tissue. The opioid peptide beta-endorphin was found to increase TSH secretion in a dose-related manner from either dispersed pituitary cells or pituitary fragments. This effect of beta-endorphin was not blocked by a wide range of doses (10(-9)-10(-5) M) of naloxone. In contrast, somatostatin (10(-9) M) significantly decreased beta-endorphin-stimulated TSH secretion. In addition to beta-endorphin, the opioid peptides met-enkephalin, leu-enkephalin, dynorphin, and gamma-endorphin all increased, whereas des-Tyr-gamma-endorphin did not affect, TSH secretion. These results indicate that the opioid peptides may participate in the regulation of TSH secretion via a direct pituitary action.

Animals↗
PubMed1982

The roles of opioid peptides in controlling thyroid stimulating hormone release.

We have studied the role of the opioid peptides in controlling TSH secretion. Morphine sulfate significantly decreased, while naloxone had no effect on, basal plasma TSH levels of female rats. In contrast, naloxone blocked the stress-induced fall in plasma TSH. Microinjection of beta-endorphin into the third ventricle resulted in a fall in TSH while such injection of naloxone into the posterior hypothalamus increased TSH. Microinjection of beta-endorphin directly into the pituitary caused a rise in plasma TSH. It is concluded that opioid peptides probably play no role in basal TSH secretion, but are involved in the stress-induced fall in TSH. Furthermore, it appears that opioid peptides have a site of action in the hypothalamus to decrease TSH and a direct pituitary action to increase TSH.

Animals↗
PubMed1982

Diurnal variations of plasma thyrotropin, thyroxine, and triiodothyronine in female rats are phase shifted after inversion of the photoperiod.

The present study examined plasma TSH, T4, and T3 concentrations in rats throughout the day to determine if diurnal variations of these hormones occurred. Female rats on a 12-h light, 12-h dark cycle (onset of light, 0730 h) were sampled by cardiac puncture at 2-h intervals throughout the day on 2 days, 1 week apart. Significant diurnal variations of plasma TSH, T4, and T3 were detected (P less than 0.01), Peak TSH concentrations occurred soon after the onset of light, whereas T4 and T3 concentrations peaked 3-4 h later. After these variations were detected, the effect of inverting the photoperiod was examined. Female rats were placed on 12-h light,, 12-h dark cycles, with the onset of light at 0730 h (LD) or 1930 h (DL). After 3 weeks, rats from each group were killed by decapitation at 4-h intervals throughout the day, and trunk blood was collected. Diurnal variations in plasma TSH, T4, and T3 (P less than 0.01) were similar to those found with 2-h sampling intervals in the previous experiment, and plasma corticosterone and PRL rhythms peaked near the onset of darkness (P less than 0.05). Inversion of the photoperiod phase shifted all diurnal variations, such that they maintained a similar phase relationship to the light-dark cycle on both LD and DL photoperiods. These results indicated that diurnal variations of plasma TSH, T4, and T3 concentrations could be repeatedly detected with different sampling protocols. These variations were phase shifted by inverting the photoperiod, which indicated that some aspect of the light-dark cycle can act to set the phase of these diurnal variations in the pituitary-thyroid axis.

Animals↗
PubMed1982

Thyrotropin-like immunoreactivity in the pituitary and three brain regions of the female rat: diurnal variations and the effect of thyroidectomy.

A method for extracting and assaying TSH-like immunoreactivity (TSH-LI) in regions of the central nervous systems (CNS) of individual rats was developed and validated. Serial dilutions of tissue extracts paralleled dilutions of purified rat TSH standard in the RIA, and tissue TSH-LI comigrated electrophoretically with TSH standard. TSH-LI was measured in the pituitary (PIT), the medial basal hypothalamus (MBH), the remainder of the hypothalamus (HYPO), and the cerebral cortex (CC). Female rats were maintained on a normal 12-h light, 12-h dark cycle (LD; onset of light, 0730 h) or an inverted 12-h light, 12-h dark cycle (DL; onset of light, 1930 h) for 3 weeks and then killed at 4-h intervals throughout the day. Another group of rats on the LD photoperiod were either thyroidectomized or sham thyroidectomized and killed 3 weeks later at 1000 h. Diurnal variations of TSH-LI were present in the PIT, MBH, HYPO, and CC on the LD photoperiod (P less than 0.05). The acrophases of MBH, HYPO, and CC TSH-LI diurnal variations occurred soon after the onset of darkness, whereas the PIT TSH-LI variation peaked soon after the onset of light. Inverting the photoperiod inverted the PIT TSH-LI diurnal variation, but central nervous system TSH-LI variations were not phase shifted after 3 weeks on the inverted photoperiod. Thyroidectomy elevated plasma and MBH TSH-LI levels (P less than 0.05), but PIT, HYPO, and CC TSH-LI levels were not significantly different from those in sham-operated controls. These results indicated that brain TSH concentrations could change under different physiological states independently of changes in PIT or plasma TSH. Although brain TSH exhibited diurnal variations, the data suggested that the phase of these variations was not set by the light-dark cycle.

Animals↗
PubMed1982

Release of immunoreactive thyroid stimulating hormone from the rat hypothalamus in vitro.

Thyroid stimulating hormone (TSH) has been identified in the hypothalamus and other brain areas of the rat. However, the alteration of the release of brain TSH has not been demonstrated. Therefore, we examined the release of immunoreactive TSH (IR-TSH) in vitro from hypothalamic tissue obtained from hypophysectomized, thyroidectomized and intact control rats. Whereas TRH (10(-5) M) and PGE2 (10(-4) M) did not alter hypothalamic IR-TSH release, depolarizing concentrations of potassium (60 mM) or veratridine (5 mM) stimulated the release of IR-TSH from hypothalamic tissue from all groups. These data suggest that IR-TSH synthesized in the hypothalamus is stored in a releasable form.

Animals↗