Relationship between hypothalamic noradrenergic neuronal activity and serum 3-methoxy-4-hydroxyphenylethylene glycol in the rat.

It has been suggested that the circulating levels of 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), a major end metabolite of noradrenaline (NA), may provide an index of central NA neuronal activity. The aim of this study was to examine in the rat the relationship between serum MHPG and hypothalamic NA neuronal activity during basal conditions, and when hypothalamic NA neuronal activity was stimulated or suppressed. Hypothalamic NA neuronal activity was assessed from the concentrations of the primary neuronal NA metabolite 3,4-dihydroxyphenylethyleneglycol (DHPG), MHPG, and the DHPG/NA and MHPG/NA ratios. Following 2-deoxyglucose (2DG) and cysteamine administration, hypothalamic NA neuronal activity and serum MHPG rose significantly. In contrast, hypothalamic NA neuronal activity and serum MHPG fell significantly in gentled rats. Serum MHPG correlated significantly with hypothalamic DHPG and the DHPG/NA ratio in control rats, and with hypothalamic DHPG, MHPG, and the DHPG/NA and MHPG/NA ratios in gentled, 2DG- and cysteamine-treated rats. In the latter two groups, serum MHPG also correlated significantly with serum glucose, which is itself closely related to hypothalamic NA neuronal activity. These studies demonstrate a significant relationship between serum MHPG and hypothalamic NA neuronal activity in the rat, so that serum MHPG provides an index of hypothalamic NA neuronal activity in the rat.

Tolbutamide increases hypothalamic serotonin activity in the rat.

Sulfonylureas are potent hypoglycemic agents; however, their mechanism of action remains incompletely understood. Recent data indicate that hypothalamic norepinephrine (NE) plays a major role in mediating the central neural regulation of blood glucose. We therefore examined whether the sulfonylurea tolbutamide might lower serum glucose via an effect on hypothalamic NE neuronal activity, and compared the effects with those of 2-deoxyglucose-induced neuroglycopenia and of chronic insulin administration. Serum glucose levels fell and serum insulin levels rose 10, 20, and 30 min after acute tolbutamide injection. Serum glucose concentrations were reduced after chronic tolbutamide administered in drinking water, but serum insulin did not change. Hypothalamic NE neuronal activity was increased 10 min after tolbutamide administration, but not at the later times, nor during chronic tolbutamide administration. However, consistent with a rise in serotonin (5-HT) neuronal activity, hypothalamic 5-hydroxyindoleacetic acid (5-HIAA) and the 5-HIAA/5-HT ratio rose 30 min after acute tolbutamide and during chronic tolbutamide administration. This rise was not due to neuroglycopenia per se, since hypothalamic NE neuronal activity was increased and hypothalamic 5-HT neuronal activity was reduced after 2-deoxyglucose-induced neuroglycopenia. Furthermore, the effect of chronic tolbutamide contrasted with that of chronic insulin administration where hypothalamic NE neuronal activity was increased, while hypothalamic 5-HT neuronal activity was unchanged. We conclude that tolbutamide does not lower serum glucose via a direct effect on hypothalamic NE neuronal activity; however, we note that tolbutamide specifically increases hypothalamic 5-HT neuronal activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid bidirectional effects of insulin on hypothalamic noradrenergic and serotoninergic neuronal activity in the rat: role in glucose homeostasis.

Glucose release from the liver is mediated by hypothalamic norepinephrine (NE) neuronal activity, but glucose itself (or a metabolite of it) exerts negative feedback effects on central NE activity. The aim of the present study was to investigate a possible role for insulin in these central glucose homeostatic mechanisms. Computerized gas chromatography-mass spectrometry was used to assess the neuronal activities of hypothalamic NE and serotonin [5-hydroxytryptamine (5-HT)] in rats after acute insulin (2 U/kg) administration. Medial basal hypothalamic NE neuronal activity was assessed by the ratio of 3,4-dihydroxyphenylethyleneglycol to NE. The ratio was suppressed (P less than 0.005) 10 min after insulin administration but rose again to be significantly higher (P less than 0.05) than in saline controls by 30 min and at 45 min post insulin was highly significantly elevated. Hypothalamic 5-HT neuronal activity was assessed by the ratio of 5-hydroxyindoleacetic acid to 5-HT and showed effects opposite to those on NE and was elevated (P less than 0.0005) 10 min post insulin. Significant changes in serum corticosterone and GH levels also occurred after insulin administration, and the changes in these two hormones were positively associated with the changes in hypothalamic neuronal activities of NE and 5-HT, respectively. Serum glucagon levels were found to be significantly elevated in association with the secondary rise in hypothalamic NE activity but did not fall in the 10 min postinsulin phase thus indicating stimulation of pancreatic glucagon release by central NE neuronal pathways. The hypothalamic NE and 5-HT neural responses to a bolus dose of insulin were unaffected by feeding or fasting. These results and evidence that brain glucose utilization is reduced in the immediate postinsulin period suggest that the rapid effects of insulin on hypothalamic NE and 5-HT neuronal activities is a direct one not mediated by stimulation of brain glucose uptake.

The central vs. peripheral effects of clonidine on ACTH, corticosterone and glucose release.

The role of central vs. peripheral actions of clonidine was investigated in the rat following the separate and combined administration of clonidine and 2-deoxy-D-glucose (2-DG). Clonidine or 2-DG alone stimulated serum glucose and corticosterone but hypothalamic noradrenaline neuronal activity and ACTH release were stimulated by 2-DG only. The stimulation of noradrenaline neuronal activity and ACTH by 2-DG were totally blocked by clonidine. It was concluded that the increases in serum glucose and corticosterone after clonidine administration were not mediated by central effects but that alpha 2-adrenoceptor agonism by clonidine was responsible for inhibition of hypothalamic noradrenaline neuronal activity and pituitary ACTH release.

Guanethidine blocks the 2-deoxy-D-glucose-induced hypothalamic noradrenergic drive to hyperglycemia.

To determine whether 2-deoxy-D-glucose (2-DG)-induced hyperglycemia is neurally mediated we administered guanethidine, an adrenergic neuron blocker, to 2-DG-treated rats. While 2-DG increased both medial basal hypothalamic noradrenergic neuronal activity (MBH NNA) and serum glucose, the rise in serum glucose was blocked by guanethidine while MBH NNA was even further increased. We conclude that 2-DG-induced hypothalamic noradrenergic drive to hyperglycemia is mediated by direct sympathetic nervous system activation of liver glucose output.

Hyperinsulinemia suppresses glucose utilization in specific brain regions: in vivo studies using the euglycemic clamp in the rat.

It has been suggested that insulin acts centrally by altering brain glucose uptake. Previous studies of the effect of insulin on brain glucose metabolism have been difficult to interpret due to lack of steady state conditions for glucose and/or insulin. To determine whether insulin per se alters brain glucose metabolism, we measured glucose utilization rates, using the deoxyglucose method, in the medial basal hypothalamus, locus coeruleus, and motor cortex of conscious, unrestrained rats undergoing 2-h euglycemic clamps (blood glucose, 4.1 +/- 0.1 mmol/liter) at increasing insulin infusion rates. Plateau insulin levels were 29 +/- 5 mU/liter (controls) and 48 +/- 4, 146 +/- 8, 670 +/- 40, and 7560 +/- 410 mU/liter (clamped). Glucose utilization rates in the medial basal hypothalamus fell significantly from 60 +/- 4 mumol/100 g X min (controls) to 46 +/- 3, 39 +/- 2, 35 +/- 2, and 39 +/- 3 mumol/100 g X min in respective insulin-infused animals (P less than 0.01 vs. controls). Similar falls of up to 39% and 48% were seen in the locus coeruleus and motor cortex, respectively. A significant inverse correlation was found between the glucose utilization rate in each brain region and the log plasma insulin level. The reduction in glucose utilization rate was associated with marked increases in serum corticosterone levels (995 +/- 157 vs. 91 +/- 31 nmol/liter in controls, P less than 0.001). Serum potassium was significantly lower in clamped animals (5.2 +/- 0.3 to 5.9 +/- 0.3 mmol/liter) than in controls (7.0 +/- 0.4 mmol/liter, P less than 0.01). However, the inverse correlation between regional brain glucose utilization and log plasma insulin was independent of changes in serum potassium, while there was no independent correlation with serum potassium. The data reveal reduced glucose utilization in specific brain regions in the presence of insulin levels both equal to and above those found in the postabsorptive state and support a direct effect of insulin in suppressing regional brain glucose utilization.

Relationships between brain noradrenergic activity and blood glucose.

Glucose is the principal energy substrate for the brain and studies have shown that the brain is able to increase glucose availability in the face of glucose starvation (neuroglycopaenia). The mechanisms, believed to be hypothalamic, that may be involved in a brain/blood glucose control system have not yet been identified. We have used novel techniques for assessing brain monoamine neuronal activity to investigate its relationship to blood glucose concentrations in the rat. We describe here two important relationships which emerge from these studies. One is that activation of hypothalamic noradrenaline (NA) activity following stress is associated with concurrent increases in plasma glucose concentrations. This relationship is linear and independent of the adrenal or pituitary glands. The second is an inverse relationship between plasma glucose concentration and hypothalamic NA neuronal activity--high blood glucose levels significantly inhibited the hypothalamic NA activity responses to stress, alpha 2-adrenergic blockade and adrenalectomy. Thus glucose (or a metabolite of it) seems to provide a negative feedback signal sensed by hypothalamic NA neuronal systems which, in turn, appear to stimulate liver glucose output by a neural mechanism.

Turnover rate of skeletal alkaline phosphatase in humans.

Two patients with Paget's disease of bone were subjected to plasmapheresis. Alkaline phosphatase activities of serum declined sharply, but returned to preplasmapheresis values within eight to 10 days. The biological half-life of circulating skeletal alkaline phosphatase, as calculated from these experiments, is between 1.12 and 2.15 days.

Paget's disease of bone: experiences with 100 patients treated with salmon calcitonin.

ONe hundred patients with Paget's disease of bone were treated with salmon calcitonin. Seventy per cent of patients who presented with pain reported improvement of symptoms. Side effects which occurred in 53 patients were more severe in women, necessitating the withdrawal of therapy in 30% of women as against 7.5% of men. Five of the patients with intolerable side effects were subsequently able to tolerate porcine calcitonin, while six were unable to tolerate salmon, porcine or human calcitonin. No audiological improvement occurred. Aortic valve lesions were detected in 11 patients. After a mean treatment period of 7.8 months, serum alkaline phosphatase level was reduced by 39.3%. Total 24-hour urinary hydroxyproline was reduced by 46.1% after 11.2 months of treatment.

A simplified assay for dihydroxylated vitamin D metabolites in human serum: application to hyper- and hypovitaminosis D.

We describe a simplified assay for 24,25-and 1.25-dihydroxyvitamin D in human serum. It involves two preparative steps, and normal chick intestine is used in preparing cytosol-binding protein. Our results for 24,25-dihydroxyvitamin D include a reference interval of 2.9--16 nmol/L (1.2--6.7 microgram/L), a mean of 6.7 nmol/L (2.8 microgram/L), an intra-assay CV of 11%, and an interassay CV of 22%. For 1,25-dihydroxyvitamin D, these data were 29--168 pmol/L (12--70 ng/L), 86 pmol/L (36 ng/L), 12%, and 22%, respectively. In hypoparathyroid patients with vitamin D intoxication, mean concentrations of 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D in serum were significantly above normal; the 1,25-dihydroxyvitamin D concentrations were significantly below normal. Patients with malabsorption and/or post-gastrectomy states had significantly subnormal values for both 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D in serum, and there was a significantly negative correlation between each of these biochemical values and the severity of osteomalacia. We also discuss cost effectiveness of assaying vitamin D metabolites in human serum.